Biomass as the solar energy stored in chemical form in plant and animal materials is among the most precious and versatile resources on earth. It provides not only food but also energy, building materials, paper, fabrics, medicines and chemicals. Biomass has been used for energy purposes ever since man discovered fire. Today, biomass fuels can be utilised for tasks ranging from heating the house to fuelling a car and running a computer.
The chemical composition of biomass varies among species, but plants consists of about 25% lignin and 75% carbohydrates or sugars. The carbohydrate fraction consists of many sugar molecules linked together in long chains or polymers. Two larger carbohydrate categories that have significant value are cellulose and hemi-cellulose. The lignin fraction consists of non-sugar type molecules. Nature uses the long cellulose polymers to build the fibers that give a plant its strength. The lignin fraction acts like a “glue” that holds the cellulose fibers together.

Carbon dioxide from the atmosphere and water from the earth are combined in the photosynthetic process to produce carbohydrates (sugars) that form the building blocks of biomass. The solar energy that drives photosynthesis is stored in the chemical bonds of the structural components of biomass. If we burn biomass efficiently (extract the energy stored in the chemical bonds) oxygen from the atmosphere combines with the carbon in plants to produce carbon dioxide and water. The process is cyclic because the carbon dioxide is then available to produce new biomass.
In addition to the aesthetic value of the planet’s flora, biomass represents a useful and valuable resource to man. For millennia humans have exploited the solar energy stored in the chemical bonds by burning biomass as fuel and eating plants for the nutritional energy of their sugar and starch content. More recently, in the last few hundred years, humans have exploited fossilized biomass in the form of coal. This fossil fuel is the result of very slow chemical transformations that convert the sugar polymer fraction into a chemical composition that resembles the lignin fraction. Thus, the additional chemical bonds in coal represent a more concentrated source of energy as fuel. All of the fossil fuels we consume - coal, oil and natural gas - are simply ancient biomass. Over millions of years, the earth has buried ages-old plant material and converted it into these valuable fuels. But while fossil fuels contain the same constituents - hydrogen and carbon - as those found in fresh biomass, they are not considered renewable because they take such a long time to create.
Environmental impacts pose another significant distinction between biomass and fossil fuels. When a plant decays, it releases most of its chemical matter back into the atmosphere. In contrast, fossil fuels are locked away deep in the ground and do not affect the earth’s atmosphere unless they are burned.
Wood may be the best-known example of biomass. When burned, the wood releases the energy the tree captured from the sun’s rays. But wood is just one example of biomass. Various biomass resources such as agricultural residues (e.g. bagasse from sugarcane, corn fiber, rice straw and hulls, and nutshells), wood waste (e.g. sawdust, timber slash, and mill scrap), the paper trash and urban yard clippings in municipal waste, energy crops (fast growing trees like poplars, willows, and grasses like switchgrass or elephant grass), and the methane captured from landfills, municipal waste water treatment, and manure from cattle or poultry, can also be used.
Biomass is considered to be one of the key renewable resources of the future at both small- and large-scale levels. It already supplies 14 % of the world’s primary energy consumption. But for three quarters of the world’s population living in developing countries biomass is the most important source of energy. With increases in population and per capita demand, and depletion of fossil-fuel resources, the demand for biomass is expected to increase rapidly in developing countries. On average, biomass produces 38 % of the primary energy in developing countries (90 % in some countries). Biomass is likely to remain an important global source in developing countries well into the next century.
Even in developed countries, biomass is being increasingly used. A number of developed countries use this source quite substantially, e.g. in Sweden and Austria 15 % of their primary energy consumption is covered by biomass. Sweden has plans to increase further use of biomass as it phases down nuclear and fossil-fuel plants into the next century.
In the USA , which derives 4 % of its total energy from biomass (nearly as much as it derives from nuclear power), now more than 9000 MW electrical power is installed in facilities firing biomass. But biomass could easily supply 20% more than 20 % of US energy consumption. In other words, due to the available land and agricultural infrastructure this country has, biomass could, sustainably, replace all of the power nuclear plants generate without a major impact on food prices. Furthermore, biomass used to produce ethanol could reduce also oil imports up to 50%.

* Total mass of living matter (including moisture) - 2000 billion tonnes
* Total mass in land plants - 1800 billion tonnes
* Total mass in forests -1600 billion tonnes
* Per capita terrestrial biomass - 400 tonnes
* Energy stored in terrestrial biomass 25 000 EJ
* Net annual production of terrestrial biomass - 400 000 million tonnes
* Rate of energy storage by land biomass - 3000 EJ/y  (95 TW)
* Total consumption of all forms of energy - 400 EJ/y (12 TW)
* Biomass energy consumption - 55 EJ/y ( 1. 7 TW)

Despite its wide use in developing countries, biomass energy is usually used so inefficiently that only a small percentage of its useful energy is obtained. The overall efficiency in traditional use is only about 5-15 per cent, and biomass is often less convenient to use compared with fossil fuels. It can also be a health hazard in some circumstances, for example, cooking stoves can release particulates, CO, NOx formaldehyde, and other organic compounds in poorly ventilated homes, often far exceeding recommended WHO levels. Furthermore, the traditional uses of biomass, i.e., burning of wood is often associated with the increasing scarcity of hand-gathered wood, nutrient depletion, and the problems of deforestation and desertification. In the early 1980s, almost 1.3 billion people met their fuelwood needs by depleting wood reserves.
Share of biomass on total energy consumption.
Nepal 95 %
Malawi 94 %
Kenya 75 %
India 50 %
China 33 %
Brazil 25 %
Egypt 20 %

There is an enormous biomass potential that can be tapped by improving the utilization of existing resources and by increasing plant productivity. Bioenergy can be modernized through the application of advanced technology to convert raw biomass into modern, easy-to-use carriers (such as electricity, liquid or gaseous fuels, or processed solid fuels). Therefore, much more useful energy could be extracted from biomass than at present. This could bring very significant social and economic benefits to both rural and urban areas. The present lack of access to convenient sources limits the quality of life of millions of people throughout the world, particularly in rural areas of developing countries. Growing biomass is a rural, labour-intensive activity, and can, therefore, create jobs in rural areas and help stem rural-to-urban migration, whilst, at the same time, providing convenient carriers to help promote other rural industries.

A major criticism often levelled against biomass, particularly against large-scale fuel production, is that it could divert agricultural production away from food crops, especially in developing countries. The basic argument is that energy-crop programmes compete with food crops in a number of ways (agricultural, rural investment, infrastructure, water, fertilizers, skilled labour etc.) and thus cause food shortages and price increases. However, this so-called “food versus fuel” controversy appears to have been exaggerated in many cases. The subject is far more complex than has generally been presented since agricultural and export policy and the politics of food availability are factors of far greater importance. The argument should be analysed against the background of the world’s (or an individual country’s or region’s) real food situation of food supply and demand (ever-increasing food surpluses in most industrialized and a number of developing countries), the use of food as animal feed, the under-utilized agricultural production potential, the increased potential for agricultural productivity, and the advantages and disadvantages of producing biofuels.
The food shortages and price increases that Brazil suffered a few years ago, were blamed on the ProAlcool programme. However, a closer examination does not support the view that bioethanol production has adversely affected food production since Brazil is one of the world’s largest exporters of agricultural commodities and agricultural production has kept ahead of population growth: in 1976 the production of cereals was 416 kg per capita, and in 1987 - 418 kg per capita. Of the 55 million ha of land area devoted to primary food crops, only 4.1 million ha (7.5 per cent) was used for sugarcane, which represents only 0.6 per cent of the total area registered for economic use (or 0.3 per cent of Brazil’s total area). Of this, only 1.7 million ha was used for ethanol production, so competition between food and crops is not significant. Furthermore, crop rotation in sugarcane areas has led to an increase in certain food crops, while some byproducts such as hydrolyzed bagasse and dry yeast are used as animal feed. Some experts (Goldemberg,1992) believe that “In fact, the potential for producing food in conjunction with sugarcane appears to be larger than expected and should be explored further,”. Food shortages and price increases in Brazil have resulted from a combination of policies which were biased towards commodity export crops and large acreage increases of such crops, hyper-inflation, currency devaluation, price control of domestic foodstuffs etc. Within this reality, any negative effects that bioethanol production might have had should be considered as part of the overall problem, not the problem.
It is important to mention that developing countries are facing both food and fuel problems. Adoption of agricultural practices should, therefore take into account this reality and evolve efficient methods of utilising available land and other resources to meet both food and fuel needs (besides other products), e.g., from agroforestry systems.

Biomass differs fundamentally from other forms of fuels since it requires land to grow on and is therefore subject to the range of independent factors which govern how, and by whom, that land should be used. There are basically two main approaches to deciding on land use for biomass. The “technocratic” approach starts from a need for, then identifies a biological source, the site to grow it, and then considers the possible environmental impacts. This approach generally had ignored many of the local and more remote side-effects of biomass plantations and also ignored the expertise of the local farmers who know the local conditions. This has resulted in many biomass project failures in the past. The “multi-uses” approach asks how land can best be used for sustainable development, and considers what mixture of land use and cropping patterns will make optimum use of a particular plot of land to meet multiple objectives of food, fuel, fodder, societal needs etc. This requires a full understanding of the complexity of land use.
Generally it can be said that biomass productivity can be improved since in many place of the world is low, being much less than 5 t/ha/yr. for woody species without good management. Increased productivity is the key to both providing competitive costs and better utilisation of available land. Advances have included the identification of fast-growing species, breeding successes and multiple species opportunities, new physiological knowledge of plant growth processes, and manipulation of plants through biotechnology applications, which could raise productivity 5 to 10 times over natural growth rates in plants or trees.
It is now possible with good management, research, and planting of selected species and clones on appropriate soils to obtain 10 to 15 t/ha/yr. in temperate areas and 15 to 25 t/ha/yr. in tropical countries. Record yields of 40 t/ha/yr. (dry weight) have been obtained with eucalyptus in Brazil and Ethiopia. High yields are also feasible with herbaceous (non-woody) crops where the agro-ecological conditions are suitable. For example, in Brazil, the average yield of sugarcane has risen from 47 to 65 t/ha (harvested weight) over the last 15 years while over 100t/ha/yr are common in a number of areas such as Hawaii, South Africa, and Queensland in Australia. It should be possible with various types of biomass production to emulate the three-fold increase in grain yields which have been achieved over the past 45 years although this would require the same high levels of inputs and infrastructure development. However, in trials in Hawaii, yields of 25 t/ha/yr. have been achieved without nitrogen fertilizers when eucalyptus is interplanted with nitrogen fixing Albizia trees (De Bell et al, 1989).

Biomass (when considering its energy potential) refers to all forms of plant-derived material that can be used for energy: wood, herbaceous plants, crop and forest residues, animal wastes etc. Because biomass is a solid fuel it can be compared to coal. On a dry-weight basis, heating values range from 17,5 GJ per tonne for various herbaceous crops like wheat straw, sugarcane bagasse to about 20 GJ/tonne for wood. The corresponding values for bituminous coals and lignite are 30 GJ/tonne and 20 GJ/tonne respectively (see tables at the end). At the time of its harvest biomass contains considerable amount of moisture, ranging from 8 to 20 % for wheat straw, to 30 to 60 % for woods, to 75 to 90 % for animal manure, and to 95 % for water hyacinth. In contrast the moisture content of the most bituminous coals ranges from 2 to 12 %. Thus the energy density for the biomass at the point of production are lower than those for coal. On the other side chemical attributes make it superior in many ways. The ash content of biomass is much lower than for coals, and the ash is generally free of the toxic metals and other contaminants and can be used as soil fertiliser.
Biomass is generally and wrongly regarded as a low-status fuel, and in many countries rarely finds its way into statistics. It offers considerable flexibility of fuel supply due to the range and diversity of fuels which can be produced. Biomass energy can be used to generate heat and electricity through direct combustion in modern devices, ranging from very-small-scale domestic boilers to multi-megawatt size power plants electricity (e.g. via gas turbines), or liquid fuels for motor vehicles such as ethanol, or other alcohol fuels. Biomass-energy systems can increase economic development without contributing to the greenhouse effect since biomass is not a net emitter of CO2 to the atmosphere when it is produced and used sustainably. It also has other benign environmental attributes such as lower sulphur and NOx emissions and can help rehabilitate degraded lands. There is a growing recognition that the use of biomass in larger commercial systems based on sustainable, already accumulated resources and residues can help improve natural resource management.

Energy contents comparison table.
Content of water %
Oak- tree
Rape oil
Hard coal
Brown coal
Heating oil
Bio methanol
Sewer gas
Wood gas
Biogas from cattle dung
Natural gas

Rural economic development in both developed and developing countries is one of the major benefits of biomass. Increase in farm income and market diversification, reduction of agricultural commodity surpluses and derived support payments, enhancement of international competitiveness, revitalization of retarded rural economies, reduction of negative environmental impacts are most important issues related to utilisation of biomass as energy source. The new incomes for farmers and rural population improve the material welfare of rural communities and this might result in a further activation of the local economy. In the end, this will mean a reduction in the emigration rates to urban environments, which is very important in many areas of the world.
The number of jobs created (for production, harvesting and use) and the industrial growth (from developing conversion facilities for fuel, industrial feedstocks, and power) would be enormous. For instance, the U.S. Department of Agriculture estimates that 17,000 jobs are created per every million of gallons of ethanol produced, and the Electric Power Research Institute has estimated that producing 5 quadrillion Btu’s (British Thermal Units) of electricity on 50 million acres of land would increase overall farm income by $12 billion annually (the U.S. consumes about 90 quadrillion Btu’s annually).  By providing farmers with stable income, these new markets diversify and strengthen the local economy by keeping income recycling through the community.
Improvement in agricultural resource utilisation has been frequently proposed in EU. The development of alternative markets for agricultural products might result in more productive uses of the cropland, currently under-utilised in many EU countries. In 1991, the EU planted 128 million ha of land to crops. Approximately 0,8 million ha were removed from production under the set aside program. A much greater amount is planned to remain idled in future. It is clear that reorientation of some of these lands to non-food utilisation (like biomass for energy) might avoid misallocation of agricultural resources. European agriculture relies on the production of a limited number of crops, mainly used for human and livestock food, many of which are at present on surplus production. Reduced prices  have resulted in low and variable income for many EU farmers. The cultivation of energy crops could reduce surpluses. New energy crops may be more economically competitive than crops in surplus production.
The use of biomass energy has many unique qualities that provide environmental benefits. It can help mitigate climate change, reduce acid rain, soil erosion, water pollution and pressure on landfills, provide wildlife habitat, and help maintain forest health through better management.

Climate change is a growing concern world-wide.  Human activity, primarily through the combustion of fossil fuels, has released hundreds of millions of tons of so-called ‘greenhouse gases’ (GHGs) into the atmosphere. GHGs include such gases as carbon dioxide (CO2) and methane (CH4).  The concern is that all of the greenhouse gases in the atmosphere will change the Earth’s climate, disrupting the entire biosphere which currently supports life as we know it.  Biomass energy technologies can help minimize this concern.  Although both methane and carbon dioxide pose significant threats, CH4 is 20 times more potent (though shorter-lived in the atmosphere) than CO2. Capturing methane from landfills, wastewater treatment, and manure lagoons prevents the methane from being vented to the atmosphere and allows the energy to be used to generate electricity or power motor vehicles.  All crops, including biomass energy crops, sequester carbon in the plant and roots while they grow, providing a carbon sink. In other words, the carbon dioxide released while burning biomass is absorbed by the next crop growing. This is called a closed carbon cycle.  In fact, the amount of carbon sequestered may be greater than that released by combustion because most energy crops are perennials, they are harvested by cutting rather than uprooting.  Thus the roots remain to stabilize the soil, sequester carbon and to regenerate the following year.

Acid rain is caused primarily by the release of sulphur and nitrogen oxides from the combustion of fuels.  Acid rain has been implicated in the killing of lakes, as well as impacting humans and wildlife in other ways.  Since biomass has no sulphur content, and easily mixes with coal, “co-firing” is a very simple way of reducing sulphur emissions and thus, reduce acid rain. “Co-firing” refers to burning biomass jointly with coal in a traditionally coal-fired power plant or heating plant.

Biomass crops can reduce water pollution in a number of ways. Energy crops can be grown on more marginal lands, in floodplains, and in between annual crops areas. In all these cases, the crops stabilize the soil, thus reducing soil erosion. They also reduce nutrient run-off, which protects aquatic ecosystems. Their shade can even enhance the habitat for numerous aquatic organisms like fish. Furthermore, because energy crops tend to be perennials, they do not have to be planted every year. Since farm machinery spends less time going over the field, less soil compaction and soil disruption takes place.  Another way biomass energy can reduce water pollution is by capturing the methane, through anaerobic digestion, from manure lagoons on cattle, hog and poultry farms.  These enormous lagoons have been responsible for polluting rivers and streams across the country. By utilizing anaerobic digesters, the farmers can reduce odour, capture the methane for energy, and create either liquid or semi-solid soil fertilisers which can be used on-site or sold.

Plants are the most common source of biomass. They have been used in the form of wood, peat and straw for thousands of years. Today the western world is far less reliant on this high energy fuel. This is because of the general acceptance that coal, oil and electricity are cleaner, more efficient and more in keeping with modernisation and technology. However this is not really the right impression. Plants can either be specially grown for energy production, or they can be harvested from the natural environment. Plantations tend to use breeds of plant that are to produce a lot of biomass quickly in a sustainable fashion. These could be trees (e.g. willows or Eucalyptus) or other high growth rate plants (such as sugar cane or maize or soybean).

Wood can be, and usually is, removed sustainably from existing forests world-wide by using methods such as coppicing. It is difficult to estimate the mean annual increment  (growth) of the world’s forests. One rough estimate is 12,5x109 m3/yr with an content of 182 EJ equivalent to 1,3 times the total world coal consumption. The estimated global average annual wood harvests in the period 1985-1987 were 3,4x 109 m3/yr (equivalent to 40 EJ/yr.), so some of the unused increment could be recovered for energy purposes while maintaining or possibly even enhancing the productivity of forests.
Operations such as thinning of plantations and trimming of felled trees generate large volumes of forestry residues. At present these are often left to rot on site - even in countries with fuelwood shortages. They can be collected, dried and used as fuel by nearby rural industry and domestic consumers, but their bulk and high water content makes transporting them for wider use uneconomic. In developing countries where charcoal is an important fuel, on-site kilns can reduce transport costs. Mechanical harvesters and chippers have been developed in Europe and North America over the last 15 years to produce uniform 30-40 mm wood chips which can be handled, dried and burned easily in chip-fired boilers. The use of forest residues to produce steam for heating and/or power generation is now a growing business in many countries. American electricity utilities have more than 9 000 MW (output of 9 nuclear power plants) of biomass-fired generating plant on line, much of it constructed in the last ten years. Austria has about 1250 MW of wood-fired heating capacity in the form of domestic stoves and district heating plant, burning waste wood, bark and wood chips. Most of these district heating systems are of 1-2 MW capacity, with a few larger units (around15 MW) and a number of small-scale CHP systems.
Timber processing is a further source of wood residues. Dry sawdust and waste produced during the processing of cut timber make very good fuel. The British furniture industry is estimated to use 35 000 tonnes of such residues a year, one third of its production, providing 0,5 PJ of space and water heating and process heat (FOE, 1991). In Sweden, where biomass already provides nearly 15% of primary energy, forestry residues and wood industries contribute over 200 PJ/yr., mainly as fuel for CHP plant.

Agricultural waste is a potentially huge source of biomass. Crop and animal wastes provide significant amounts of energy coming second only to wood as the dominant biomass fuel world-wide. Waste from agriculture includes: the portions of crop plants discarded like straw, whether damaged or surplus supplies, and animal dung. It was estimated, for example, that 110 Mt of dung and crop residues were used as fuel in India in 1985, compared with 133 Mt of wood, and in China the mass of available agricultural residues has been estimated at 2.2 times the mass of wood fuel.
Every year, millions tonnes of straw are produced world-wide with usually half of it surplus to need. In many countries this is still being burned in the field or ploughed back into the soil, but in some developed countries environmental legislation which restrict field burning has drawn attention to its potential as an energy resource
Effort to remove crop residues from soils and to use them for energy purposes leads to a central question:  how much residue should be left and recycled into soil to sustain production of biomass ? According to the experience from developed countries around 35% of crop residues can be removed from soil without adverse effects on future plant production.
Industrial waste that contains biomass may be used to produce energy. For example the sludge left after alcohol production (known as vinasse) can produce flammable gas. Other useful waste products include, waste from food processing and fluff from the cotton and textiles industry.

Biomass can be also be produced by so-called short-rotation plantation of trees and other plants like grasses (sorghum, sugarcane, switchgrass). All these plants can be used as fuels like wood with the main advantage of their short span between plantation and harvesting – typically between three and eight years. For some grasses harvesting is taking place every six to 12 months. Recently there are about 100 million hectares of land utilised for tree plantation world-wide. Most of these trees are used for forest products markets.
Parameters which are important in evaluating species for short rotation plants include availability of planting stock, ease of propagation, survival ability under adverse conditions and the yield potential measured as dry matter production per hectare per year (t/ha/y). Yield is a measure of a plant’s ability to utilize the site resources. It is the most important factor when considering biomass production due to the need to optimize/maximize yield from a given area of land within a given time frame at the least possible cost. High yielding species are therefore preferred for biomass energy systems.
Some plant communities have shown superiority in dry matter production compared to others grown under similar conditions. Although reported dry matter production of different tree species varies over a wide range depending on soil types and climate, certain species stand out. For Eucalyptus species, yields of up to 65 t/ha/y have been reported, compared to 30 and 43 t/ha/y in Salix and Populus species respectively.
Despite the fact that biomass plantation can be of great importance for most developed countries experience has shown it is unlikely to be established on a large scale in many developing countries, especially in poor rural areas, so long as biofuels (particularly wood) can be obtained at zero or near zero cost.

3.6.1 Fuelwood
The term fuelwood describe all types of fuels derived from forestry and plantation. Fuelwood accounts for about 10 per cent of the total used in the world. It provides about 20 % of all used in Asia and Latin America, and about 50 % of total used in Africa. However, it is the major source of, in particular for domestic purposes, in poor developing countries: in 22 countries, fuelwood accounted for 25 to 49 %, in 17 countries, 50-74 %, and in 26 countries, 75-100 % of their respective national consumption.
More than half of the total wood harvested in the world is used as fuelwood. For specific countries, for example in Tanzania, the contribution can be as high as 97% . Although fuelwood is the major source of for most rural and low-income people in the developing world, the potential supply of fuelwood is dwindling rapidly, leading to scarcity of and environmental degradation. It is estimated that, for more than a third of the world population, the real crisis is the daily scramble to obtain fuelwood to meet domestic use.
Several studies on fuelwood supply in developing countries have concluded that fuelwood scarcities are real and will continue to exist, unless appropriate approaches to resource management are undertaken. The increase of fuelwood production through efficient techniques, can, therefore, be considered as one of the major pre-requisites for attaining sustainable development in developing countries.

3.6.2 Charcoal
The main expansion in the use of charcoal in Europe came with the industrial revolution in England in the 17th and 18th centuries. In Sweden, charcoal consumption for iron making grew through most of the 19th century, and was the basis of the good quality tradition of Swedish steel. Today charcoal is an important household fuel and to a lesser extent, industrial fuel in many developing countries. It is mainly used in the urban areas where its ease of storage, high content (30 MJ/kg as compared with 15 MJ/kg in fuelwood), lower levels of smoke emissions, and, resistance to insect attacks make it more attractive than fuelwood. In the United Republic of Tanzania, charcoal accounts for an estimated 90 per cent of biofuels consumed in urban centres.

3.6.3 Residues
Agricultural residues have an enormous potential for production. In favourable circumstances, biomass power generation could be significant given the vast quantities of existing forestry and agricultural residues - over 2 billion t/yr. world-wide. This potential is currently under-utilized in many areas of the world. In wood-scarce areas, such as Bangladesh, China, the northern plains of India, and Pakistan, as much as 90 per cent of household in many villages covers their energy needs with agricultural residues. It has been estimated that about 800 million people world-wide rely on agricultural residues and dung for cooking, although reliable figures are difficult to obtain. Contrary to the general belief, the use of animal manure as an source is not confined to developing countries alone, e.g., in California a commercial plant generates about 17.5 MW of electricity from cattle manure, and a number of plants are operating in the Europe.
There is 54 EJ of biomass energy theoretically available from recoverable residues in developing countries and 42 EJ in industrialized regions. The amount of potentially recoverable residues includes the three main sources: forestry, crops and dung. The calculations assume only 25 per cent of the potentially harvestable residues are likely to be used. Developing countries could theoretically derive 15 per cent of present energy consumption from this source and industrialized countries could derive 4 per cent.
Sugarcane residues (bagasse, and leaves) - are particularly important and offer an enormous potential for generation of electricity. Generally, residues are still used very inefficiently for electricity production, in many cases deliberately to prevent their accumulation, but also because of lack of technical and financial capabilities in developing countries.
Depending on the choice of the gas turbine technology and the extent to which cane tops and leaves can be used for off-season generation, according to some estimates (Williams  1989) amount of electricity that can be produced from cane residues could be up to 44 times the on-site needs of the sugar factory or alcohol distillery. For each litre of alcohol produced a BIG/STIG unit would be able to produce more than 11 kWh of electricity in excess of the distillery’s needs (about 820 kWh/t). Another estimate of bagasse in condensing-extraction steam turbines puts the surplus electricity values at 20-65 kWh per ton of cane, and this surplus could be doubled by using barbojo for generation during the off-season. The cost of the generated electricity is estimated to be about $US 0.05/kWh. Revenues from the sale of electricity co-produced with sugar could be comparable with sugar revenues, or alternatively, revenues from the sale of electricity co-produced with ethanol could be much greater than the alcohol revenues. In the latter instance, electricity would become the primary product of sugarcane, and alcohol the by-product.
In India alone, electricity production from sugarcane residues by the year 2030 could be up to 550 TWh/year (the total electricity production from all sources in 1987 was less than 220 TWh (Ogden et al, 1990). Globally, it has been estimated that about 50,000 MW could be supported by currently produced residues. The theoretical potential of residues in the 80 sugarcane-producing developing countries could be up to 2800 TWh/yr., which is about 70 per cent more than the total electricity production of these countries from all sources in 1987. Studies of the sugarcane industry indicate a combined power capability in excess of 500 TWh/yr. Assuming that a third of the global residue resources could economically and sustainably be recovered by new energy technology, 10 per cent of the current global electricity demand (10.000 TWh/yr.) could be generated.
Obviously, to achieving such goals, these are theoretical calculations with country- and site specific problems. They do however emphasize the potential which many countries have to provide a substantial proportion of their from biomass grown on a sustainable basis.

Nearly all types of raw biomass decompose rather quickly, so few are very good long-term energy stores; and because of their relatively low energy densities, they are likely to be rather expensive to transport over appreciable distances. Recent years have therefore seen considerable effort devoted to the search for the best ways to use these potentially valuable sources of energy.
In considering the methods for extracting the energy, it is possible to order them by the complexity of the processes involved:
* Direct combustion of biomass.
* Thermochemical processing to upgrade the biofuel. Processes in this category include pyrolysis, gasification and liquefaction.
* Biological processing. Natural processes such as anaerobic digestion and fermentation which lead to a useful gaseous or liquid fuel.

The immediate ‘product, of some of these processes is heat - normally used at place of production or at not too great a distance, for chemical processing or district heating, or to generate steam for power production. For other processes the product is a solid, liquid or gaseous fuel: charcoal, liquid fuel as a petrol substitute or additive, gas for sale or for power generation using either steam or gas turbines.

The technology of direct combustion as the most obvious way of extracting energy from biomass is well understood, straightforward and commercially available. Combustion systems come in a wide range of shapes and sizes burning virtually any kind of fuel, from chicken manure and straw bales to tree trunks, municipal refuse and scrap tyres. Some of the ways in which heat from burning wastes is currently used include space and water heating, industrial processing and electricity generation. One problem with this method is its very low efficiency. With an open fire most of the heat is wasted and is not used to cook or whatever.
Combustion of wood can be divided into four phases:
* Water inside the wood boils off. Even wood that has been dried for ages has as much as 15 to 20% of water in its cell structure.
* Gas content is freed from the wood. It is vital that these gases should burn and not just disappear up the chimney.
* The gases emitted mix with atmospheric air and burn at a high temperature.
* The rest of the wood (mostly carbon) burns. In perfect combustion the entire energy is utilised and all that is left is a little pile of ashes.

Three things are needed for effective burning:
* high enough temperatures;
* enough air, and
* enough time for full combustion.

If not enough air gets in, combustion is incomplete and the smoke is black from the unburned carbon. It smells terrible, and you get soot deposited in the chimney, with the risk of fire. If too much air gets in the temperature drops and the gases escape unburned, taking the heat with them. The right amount of air gives the best utilisation of fuel. No smell, no smoke, and very little risk of chimney fires. Regulation of the air supply depends largely on the chimney and the draught it can put up.
Direct combustion is the simplest and most common method of capturing the energy contained within biomass. Boiling a pan of water over a wood fire is a simple process. Unfortunately, it is also very inefficient, as a little elementary calculation reveals.
The energy content of a cubic metre dry wood is 10 GJ, which is ten million kJ. To raise the temperature of a litre of water by 1 degree Celsius requires 4,2 kJ of heat energy. Bringing a litre to the boil should therefore require rather less than 400 kJ, equivalent to 40 cubic centimetres of wood - one small stick, perhaps. In practice, with a simple open fire we might need at least fifty times this amount: a conversion efficiency no better than 2%.
Designing a stove or boiler which will make rather better use of valuable fuel requires an understanding of the processes involved in the combustion of a solid fuel. The first is one which consumes rather than produces energy: the evaporation of any water in the fuel. With reasonably dry fuel, however, this uses only a few percent of the total energy. In the combustion process itself there are always two stages, because any solid fuel contains two combustible constituents. The volatile matter is released as a mixture of vapours or vaporised tars and oils by the fuel as its temperature rises. The combustion of these produces the little spurts of pyrolysis.
Modern combustion facilities (boilers) usually produce heat, steam (used in industrial process) or electricity. Direct combustion systems vary considerably in their design. The fuel choice makes a difference in the design and efficiency of the combustion system. Direct combustion technology using biomass as the fuel is very similar to that used for coal.  Biomass and coal can be handled and burned in essentially the same fashion. In fact, biomass can be “co-fired” with coal in small percentages in existing boilers. The biomass which is co-fired are usually low-cost feedstocks, like wood or agricultural waste, which also help to reduce the emissions typically associated with coal. Coal is simply fossilized biomass heated and compressed over millions of years. The process which coal undergoes as it is heated and compressed deep within the earth, adds elements like sulphur and mercury to the coal. Burning coal for heat or electricity releases these elements, which biomass does not contain.

Pyrolysis is the simplest and almost certainly the oldest method of processing one fuel in order to produce a better one. A wide range of energy-rich fuels can be produced by roasting dry wood or even the straw. The process has been used for centuries to produce charcoal. Conventional pyrolysis involves heating the original material (which is often pulverised or shredded then fed into a reactor vessel) in the near-absence of air, typically at 300 - 500 °C, until the volatile matter has been driven off. The residue is then the char - more commonly known as charcoal - a fuel which has about twice the energy density of the original and burns at a much higher temperature. For many centuries, and in much of the world still today, charcoal is produced by pyrolysis of wood. Depending on the moisture content and the efficiency of the process, 4-10 tonnes of wood are required to produce one tonne of charcoal, and if no attempt is made to collect the volatile matter, the charcoal is obtained at the cost of perhaps two-thirds of the original energy content.
Pyrolysis can also be carried out in the presence of a small quantity of oxygen (‘gasification’), water (‘steam gasification’) or hydrogen (‘hydrogenation’). One of the most useful products is methane, which is a suitable fuel for electricity generation using high-efficiency gas turbines.
With more sophisticated pyrolysis techniques, the volatiles can be collected, and careful choice of the temperature at which the process takes place allows control of their composition. The liquid product has potential as fuel oil, but is contaminated with acids and must be treated before use. Fast pyrolysis of plant material, such as wood or nutshells, at temperatures of 800-900 degrees Celsius leaves as little as 10% of the material as solid char and converts some 60% into a gas rich in hydrogen and carbon monoxide. This makes fast pyrolysis a competitor with conventional gasification methods (see bellow), but like the latter, it has yet to be developed as a treatment for biomass on a commercial scale.
At present, conventional pyrolysis is considered the more attractive technology. The relatively low temperatures mean that fewer potential pollutants are emitted than in full combustion, giving pyrolysis an environmental advantage in dealing with certain wastes. There have been some trials with small-scale pyrolysis plants treating wastes from the plastics industry and also used tyres - a disposal problem of increasingly urgent concern.

The basic principles of gasification have been under study and development since the early nineteenth century, and during the Second World War nearly a million biomass gasifier-powered vehicles were used in Europe. Interest in biomass gasification was revived during the “energy crisis” of the 1970s and slumped again with the subsequent decline of oil prices in the 1980s. The World Bank (1989) estimated that only 1000 - 3000 gasifiers have been installed globally, mostly small charcoal gasifiers in South America.
Gasification based on wood as a fuel produces a flammable gas mixture of hydrogen, carbon monoxide, methane and other non flammable by products. This is done by partially burning and partially heating the biomass (using the heat from the limited burning) in the presence of charcoal (a natural by-product of burning biomass). The gas can be used instead of petrol and reduces the power output of the car by 40%. It is also possible that in the future this fuel could be a major source of energy for power stations.
A gasifier which uses oxygen rather than air can produce a gas consisting mainly of H2, CO and C02, and the interesting potential of this lies in the fact that removal of the C02 leaves the mixture called synthesis gas, from which almost any hydrocarbon compound may be synthesised. Reacting the H2 and CO is one way to produce pure methane. Another possible product is methanol (CH3OH), a liquid hydrocarbon with an energy density of 23 GJ per tonne. Producing methanol in this way involves a series of sophisticated chemical processes with high temperatures and pressures and expensive plant, and one might wonder why it is of interest. The answer lies in the product: methanol is that valuable commodity, a liquid fuel which is a direct substitute for gasoline. At present the production of methanol using synthesis gas from biomass is not a commercial proposition, but the technology already exists, having been developed for use with coal as feedstock - as a precaution by coal-rich countries at times when their oil supplies were threatened.

Fermentation of sugar solution is the way how ethanol (ethyl alcohol) can be produced. Ethanol is a very high liquid energy fuel  which can be used as the substitute for gasoline in cars. This fuel is used successfully in Brazil. Suitable feedstocks include crushed sugar beet or fruit. Sugars can also be manufactured from vegetable starches and cellulose by pulping and cooking, or from cellulose by milling and treatment with hot acid. After about 30 hours of fermentation, the brew contains 6-10 per cent alcohol, which can be removed by distillation as a fuel.
Fermentation is an anaerobic biological process in which sugars are converted to alcohol by the action of micro-organisms, usually yeast. The resulting alcohol is ethanol (C2H3OH) rather than methanol (CH3OH), but it too can be used in internal combustion engines, either directly in suitably modified engines or as a gasoline extender in gasohol: gasoline (petrol) containing up to 20% ethanol.
The value of any particular type of biomass as feedstock for fermentation depends on the ease with which it can be converted to sugars. The best known source of ethanol is sugar-cane - or the molasses remaining after the cane juice has been extracted. Other plants whose main carbohydrate is starch (potatoes, corn and other grains) require processing to convert the starch to sugar. This is commonly carried out, as in the production of some alcoholic drinks, by enzymes in malts. Even wood can act as feedstock, but its carbohydrate, cellulose, is resistant to breakdown into sugars by acid or enzymes (even in finely divided forms such as sawdust), adding further complication to the process.
The liquid resulting from fermentation contains only about 10% ethanol, which must be distilled off before it can be used as fuel. The energy content of the final product is about 30 GJ/t, or 24 GJ/m3. The complete process requires a considerable amount of heat, which is usually supplied by crop residues (e.g. sugar cane bagasse or maize stalks and cobs). The energy loss in fermentation is substantial, but this may be compensated for by the convenience and transportability of the liquid fuel, and by the comparatively low cost and familiarity of the technology.

Nature has a provision of destroying and disposing of wastes and dead plants and animals. Tiny micro-organisms called bacteria carry out this decay or decomposition. The farmyard manure and compost is also obtained through decomposition of organic matter. When a heap of vegetable or animal matter and weeds etc. die or decompose at the bottom of back water or shallow lagoons then the bubbles can be noticed rising to the surface of water. Some times these bubbles burn with flame at dusk. This phenomenon was noticed for ages, which puzzled man for a long time. It was only during the last 200 years or so when scientists unlocked this secret, as the decomposition process that takes place under the absence of air (oxygen). This gas, production of which was first noticed in marshy places, was and is still called as ‘Marsh Gas’. It is now well known that this gas (Marsh Gas) is a mixture of Methane (CH4) and Carbon dioxide (CO2) and is commonly called as the ‘Biogas’. As per records biogas was first discovered by Alessandro Volta in 1776 and Humphery Davy was the first to pronounce the presence of combustible gas Methane in the Farmyard Manure in as early as 1800. The technology of scientifically harnessing this gas from any biodegradable material (organic matter) under artificially created conditions is known as biogas technology.
Anaerobic digestion, like pyrolysis, occurs in the absence of air; but in this case the decomposition is caused by bacterial action rather than high temperatures. It is a process which takes place in almost any biological material, but is favoured by warm, wet and of course airless conditions. It occurs naturally in decaying vegetation on the bottom of ponds, producing the marsh gas which bubbles to the surface and can even catch fire.
Anaerobic digestion also occurs in situations created by human activities. One is the biogas which is generated in concentrations of sewage or animal manure, and the other is the landfill gas produced by domestic refuse buried in landfill sites. In both cases the resulting gas is a mixture consisting mainly of methane and carbon dioxide; but major differences in the nature of the input, the scale of the plant and the time-scale for gas production lead to very different technologies for dealing with the two sources.
The detailed chemistry of the production of biogas and landfill gas is complex, but it appears that a mixed population of bacteria breaks down the organic material into sugars and then into various acids which are decomposed to produce the final gas, leaving an inert residue whose composition depends on the type of system and the original feedstock. Biogas
is a valuable fuel which is in many countries produced in purpose built digesters filled with the feedstock like dung or sewage. Digesters range in size from one cubic metre for a small ‘household’ unit to more than thousand cubic meters used in large commercial installation or farm plants. The input may be continuous or in batches, and digestion is allowed to continue for a period of from ten days to a few weeks. The bacterial action itself generates heat, but in cold climates additional heat is normally required to maintain the ideal process temperature of at least 35 degrees Celsius, and this must be provided from the biogas. In extreme cases all the gas may be used for this purpose, but although the net energy output is then zero, the plant may still pay for itself through the saving in fossil fuel which would have been needed to process the wastes. A well-run digester will produce 200-400 m3 of biogas with a methane content of 50% to 75% for each dry tonne of input.
A large proportion of ordinary domestic refuse - municipal solid wastes - is biological material and its disposal in landfills creates suitable conditions for anaerobic digestion. That landfill sites produce methane has been known for decades, and recognition of the potential hazard led to the fitting of systems for burning it off; however, it was only in the 1970s that serious attention was paid to the idea of using this ‘undesirable’ product.
The waste matter is more miscellaneous in a landfill than in a biogas digester, and the conditions neither as warm nor as wet, so the process is much slower, taking place over years rather than weeks. The end product, known as landfill gas, is again a mixture consisting mainly of CH4 and CO2. In theory, the lifetime yield of a good site should lie in the range 150-300 m3 of gas per tonne of wastes, with between 50% and 60% by volume of methane. This suggests a total energy of 5-6 GJ per tonne of refuse, but in practice yields are much less.
In developing a site, each area is covered with a layer of impervious clay or similar material after it is filled, producing an environment which encourages anaerobic digestion. The gas is collected by an array of interconnected perforated pipes buried at depths up to 20 metres in the refuse. In new sites this pipe system is constructed before the wastes start to arrive, and in a large well-established landfill there can be several miles of pipes, with as much as 1000 m3 an hour of gas being pumped out.
Increasingly, the gas from landfill sites is used for power generation. At present most plants are based on large internal combustion engines, such as standard marine engines. Driving 500 kW generators, these are well matched to typical gas supply rates of the order of 10 GJ an hour.

3.8.1 Heat production with wood firing boilers
Most common process of biomass combustion is burning of wood. In developed countries replacing oil or coal-fired central heating boiler with a wood burning one can save between 20 and 60% on heating bills, because wood costs less than oil or coal. At the same time wood burning units are eco-friendly. They only emit the same amount of the greenhouse gas CO2 as the tree absorbed when it was growing. So burning wood does not contribute to global warming. Since wood contains less sulphur than oil does, less sulphate is discharged into the atmosphere. This means less acid rain and less acid in the environment.

Small wood burning boilers are frequently used for heating houses. There are approx. 70,000 small boilers burning firewood, wood chips, or wood pellets in Denmark alone. Such a boiler gives off its heat to radiators in exactly the same way as e.g. an oil-fired one. In this it differs from a wood burning stove, which only gives off its heat to the room it is in. In other words a wood burning boiler can heat whole house and provide hot water. For a single family home, a hand-fired wood burning boiler is usually the best and most economical investment. In larger places such as farms the saving from burning wood is often so great that it pays to install an automatic stoker unit burning wood pellets.
Many of small boilers are manually fired with storage tank for wood. Distinctions should be made between manually fired boilers for fuelwood and automatically fired boilers for wood chips and wood pellets. Manually fired boilers are installed with storage tank so as to accumulate the heat energy from fuel. Automatic boilers are equipped with a silo containing wood pellets or wood chips. A screw feeder feeds the fuel simultaneously with the output demand of the dwelling.
Great advances have been made over the recent 10 years for both boiler types in respect of higher efficiency and reduced emission from the chimney (dust and carbon monoxide). Improvements have been achieved particularly in respect of the design of combustion chamber, combustion air supply, and the automatics controlling the process of combustion. In the field of manually fired boilers, an increase in the efficiency has been achieved from below 50% to 75-90%. For the automatically fired boilers, an increase in the efficiency from60% to 85-92% has been achieved.

The principal rule is that manually fired boilers for fuelwood only have an acceptable combustion at the boiler rated output (at full load). At individual plants with oxygen control, the load can, however, be reduced to approx. 50% of the nominal output without thereby influencing neither the efficiency nor emissions. By oxygen control, a lambda probe measures the oxygen content in the flue gas, and the automatic boiler control varies the combustion air inlet.
The same system is used in cars. In order for the boiler not to need feeding at intervals of 2-4 hours a day, during the coldest periods of the year, the fuelwood boiler nominal output is selected so as to be up to 2-3 times the output demand of the dwelling. This means that the boiler efficiency figures shown in Figure 15 and 16 should be multiplied by 2 or 3 in the case of manually fired boilers. Boilers designed for fuelwood should always be equipped with storage tank. This ensures both the greatest comfort for the user and the least financial and environmental strain. In case of no storage tank, an increased corrosion of the boiler is often seen due to variations in water and flue gas temperatures.

Despite an often simple construction, most of the automatically fired boilers can achieve an efficiency of 80-90% and a CO emission of approx. 100 ppm (100 ppm = 0.01 volume %). For some boilers, the figures are 92% and 20 ppm, respectively. An important condition for achieving these good results is that the boiler efficiency during day-to-day operation is close to full load. For automatic boilers, it is of great importance that the boiler nominal output (at full load) does not exceed the max. output demand in winter periods. In the transition periods (3-5 months) spring and autumn, the output demand of the dwelling will typically be approx. 20-40% of the boiler nominal output, which means a deteriorated operating result. During the summer period, the output demand of the dwelling will often be in the range of 1-3 kW, since only the hot water supply will be maintained. This equals 5 -10% of the boiler nominal output. This operating method reduces the efficiency - typically 20-30% lower than that of the nominal output - and an increased negative effect on the environment. The alternative to the deteriorated summer operating is to combine the installation with a storage tank and solar collectors.

Nearly all old-fashioned cast iron stoves act on the burn-through principle: air comes in from below  and  passes upwards through the fuel. In burn-through boilers the wood burns very quickly. The gases do not burn very well, since the boiler temperature is low. Most of the gas goes up the chimney, and the energy with it. The flue gases have a very short space in which to give off their heat to the boiler in the convection section. By and large, burn-through furnaces are unsuitable for wood. The useful effect of a burn-through boiler is typically under 50%.

Underburn boiler is very different from a burn-through one. The air is not drawn through all the fuel at once, but only through part of it. Only the bottom layer of wood burns; the rest dries out and gives off its gases very slowly. Adding extra air (so-called “secondary air”) direct to the flames burns the gases more effectively. In modern underburning boilers the combustion chamber is ceramic lined, which insulates well and keeps the heat in. This gives a high temperature of combustion, burning the gases most effectively. An underburning boiler typically has a useful effect of 65-75%.

In reverse combustion too, air is only added to part of the fuel. As in underburning, the gases leave the fuel slowly and are burnt efficiently. Secondary air is also led into an earthenware-lined chamber, giving a high temperature of combustion. The flue gas has to pass  through  the entire boiler, giving it plenty of time to give up its heat. The useful effect is typically of the order of 75-85%. Some reverse combustion boilers have a blower instead of natural draught. Such boilers often have slightly better combustion, with less soot and pollution than ones with natural draught, but their useful effect is not significantly better.

How good a boiler is partially depends on the proportion of the energy in the fuel that it transfers to the central heating system. This proportion is called the “efficiency”. The efficiency of a boiler is defined as the relationship between the energy in the hot water and that in the wood: the higher the efficiency, the more of the energy in the fuel is transferred to the water in the boiler. Good boilers have a efficiency of the order of 80-90%.
The a wood consumption in reverse burning boiler is typically between 4 kg/hour for 18 kW boiler to 18 kg/hr for 80 kW boiler. In Central European condition an average single family house (150 m2) need cca 12 m3 of wood for the whole heating season. Typical boilers can burn wood logs up to 80 cm long.  More technical data for Central European condition see the table bellow.
Power output (kW)
Wood consumption  (kg/hr) 
Wood consumption in heating season  (m3)
Wood heating value 15-18 MJ/kg.

It almost always pays to buy a storage tank when installing a wood burning boiler. A storage tank holds water that has been heated up by the boiler. The extra cost repays itself very quickly, and it is easier to fire properly. Shortly after lighting up, combustion is clean and the boiler starts producing masses of heat. Without a storage tank to take up the heat, the water will rapidly get too hot and the damper will have to be shut to stop it boiling. The reduced amount of air leads to smoky, incomplete combustion.
But with a hot water tank you can fire away and store the heat. The water in the boiler cannot overheat because it goes into the tank. The damper remains open and combustion continues at high efficiency. When you need heat in the radiators, it comes from the storage tank. The size of the storage tank depends on the amount of heat the house needs and the efficiency of the boiler.

If you do decide to install a wood burning unit, it is recommended also to consider putting in solar heating. The wood burning boiler and the solar panels can frequently use the same storage tank, reducing the cost of the system as a whole. Make sure first that the storage tank is suitable for the purpose. At the same time it makes it unnecessary to have a fire going in summer just to get hot water. And it is cheaper to “burn” solar energy than wood!

Whatever fuel you decide to use, it must be dry. Newly felled timber has a water content of about 50%, which makes it uneconomical to burn. This is because a proportion of the energy in the wood goes to evaporating the water off, giving less energy for heat. So wood has to be dried before it can be burnt. The best thing to do is to leave the wood to dry for at least a year, and preferably two. It is easiest to stack it in an outdoor woodshed so that the rain cannot get at it.
Never burn wood that has been painted or glued, since toxic gases are formed on combustion. Nor should one burn refuse such as waxed paper milk cartons and that sort of thing. You can also burn wood briquettes. They are made of compressed sawdust and wood shavings, about 10 or 20 cm long and 5 cm in diameter. Because they are compressed and have a low water content they have a higher energy density than ordinary wood, so they need less storage space.

Chimney is responsible for the draught going through the boiler. The difference in the density of the air between the top of the chimney and the outlet on the boiler is what creates the draught. So the height of the chimney, the insulation, and thus the temperature of the smoke all contribute to the draught. Bends and horizontal bits of piping reduce the draught. They create resistance, which the hot air has to overcome. So the idea is to have as few horizontal flues and bends as possible. Some boilers have a built-in blower, ensuring a proper draught at all times.

A boiler must be installed and maintained properly. This increases its life and your safety. Most countries have regulations about siting: in some places boilers have to be put in a separate room. The chimney will need sweeping at least once a year. This reduces the risk of fire. Too much soot may mean you are not letting enough air through.

The automatic boiler is connected to the central heating system in exactly the same way as an oil-fired one. The heat of combustion is transferred to water, which is heated up and carried round the house to the radiators. The automatic boiler thus supplies heat to all the radiators in the house, unlike a wood burning stove, which really only heats the room it is in. Pellets and wood-chips are of a size and shape that make them ideal for automatic boilers, since they can be fed in directly from a bunker. This makes it much easier to stoke, since the bunker only needs filling up once or twice a week. In hand-fired units like wood burning boilers, one has to stoke up several times a day - though they are usually cheaper to buy than automatic ones.

Wood pellets are a comparatively new and attractive form of fuel. When you burn wood pellets, you are utilising an energy resource that would otherwise have gone to waste or been dumped in a landfill. Pellets are usually made out of waste (sawdust and wood shavings), and are used in large quantities by district heating systems. The pellets are made in presses, and come out 1-3 cm long and about 1 cm wide. They are clean, pleasant smelling and smooth to touch. Wood pellets have a low moisture content (under 10% by weight), giving them a higher combustion value than other wood fuels. The fact that they are pressed means they take up less space, so they have a higher volume energy (more energy per cubic meter). The burning process is highly combustible and produces little residue. Some countries have exempted pellet appliances from the smoke emission testing requirements.
There are different kinds of pellets. Some manufacturers use a bonding agent to extend the life of the pellets; others make them without it. The bonder used often contains sulphur, which goes up the chimney on burning. Sulphate pollution contributes to acid rain and chimney corrosion, so it is best to buy pellets without a bonding agent.
Wood pellets characteristics:
Diameter : 5 - 8 mm
Length  : max. 30 mm
Density : min. 650 kg/m3
Moisture content : max. 8% of weight
Energy value : 4,5 - 5,2 kWh/kg
2 kg pellets = 1 litre of heating oil

There are many advantages in using pellets as the fuel of choice. No trees are cut to make the pellets - they are only made from leftover wood residue. Burning pellet fuel actually helps reduce waste created by lumber production or furniture manufacturing. There are no additives put into the pellets to make them burn longer or more efficiently. Pellet fuel does not smoke or give off any harmful fumes. Using this fuel reduces the need for fossil fuels which are known to be harmful for the environment.
The cost of pellet fuel may depend on the geographic region where it is sold, and the current season. Whether you live in a condominium in the city or a home in the country, pellet fuel is among the safest, healthiest way to heat. This technology is also valuable for non-residential buildings such as hotels, resorts, restaurants, retail stores, offices, hospitals, and schools. Pellets are recently used in over 500 000 homes in North America.

Wood-chips are made of waste wood from the forests. Trees have to be thinned to make room for commercial timber (beams, flooring, furniture). Wood-chips are thus a waste product of normal forestry operations.  Wood is cut up in mechanical chippers. The size and shape of the chips depends on the machine, but they are typically about a centimetre thick and 2 to 5 cm long. The water content of newly felled chips is usually about 50% by weight, but this drops considerably on drying. In many countries like in Denmark wood-chips currently produced are burnt in wood-chip fired district heating stations. They are usually delivered by road, so there must be facilities for storing at least 20 m3 of chips under cover if they are to be used in an automatic burner.

In the table bellow you can find a comparison of different wood burning systems for single family house 150 m2 (12 kW heat load). Data are coming from Austria.
Investment costs
Fuel consumption in heating season
From 80 000 ATS
12 m3
Fuel input 1-2 times a day
Wood chips
From 150 000 ATS
28 m3
Fuel input 1-2 times a year
Wood Pellets
From 80 000 ATS
7,5 m3
 Note 14 ATS = 1 USD

Automatic furnaces come in three types :
* Compact units in which the boiler and bunker are in one.
* Stoker-fired units, with separate boiler and bunker.
* Boilers with built-in pre-furnace.

In compact units the fuel is fed into the fire from the bunker by an automatic feeder. The rate at which fuel is fed in is determined by a thermostat, which puts less in when the water is hot and more in when it is cold. Compact units are excellent for wood pellets, but not for wood-chips. This is due to the lower volume energy of chips, so that stoking has to be more frequent. In addition, the water content of wood-chips is often so high that compact units do not combust them properly.

In stoker-fired units too, the fuel is automatically fed into the boiler. This is a helical conveyor which conveys the fuel from the bunker to the boiler. The fuel is fed in at the bottom of the grate, where it burns. As in compact units, feed-in is thermostatically controlled. Wood pellets are best for stoker-fired units, but chips can also be used if the unit is designed for them. The chips must not be too moist, so they need drying first. The best way of doing this is to leave the trees outside to dry until they are put through the chipper. Chips can also be dried under cover after being cut up. If wood-chips are used, they need drying under cover for at least two months. They also need a lot of storage space.

In the third type of unit most of the combustion takes place at high temperature in a pre-furnace. The pre-furnace is earthenware-lined, allowing high temperatures to be maintained. A pre-furnace-mounted boiler is therefore highly suitable for burning wet wood-chips. Heat comes in from the pre-furnace and is transferred to the water in the boiler. Any gases not combusted in the pre-furnace are burnt off in the boiler. Boilers fitted with pre-furnace are designed for burning wood-chips. Some can also burn pellets, though others would be damaged by the heat generated by the dry fuel. Ask the manufacturer before buying.

It costs more to buy an automatic stoker unit than a hand-fired one, because there are more bits and pieces in it. Usually they can be economical if there is a need for a lot of heat during the year. In EU countries it means to have a need to burn the equivalent of at least 3,000 litres of oil a year. If the homeowner use less, it is better to buy a hand-fired unit burning firewood. If the house is already equipped with a boiler that works well and the homeowner is thinking of buying an automatic unit, the cheapest thing is to invest in a separate stoker. In Denmark this sort of thing costs about DKK 20-25,000 to install. A compact unit, a stoked unit or a pre-furnace boiler cost at least DKK 50,000. Despite this a wood burning unit pays in the long run, because the saving on fuel is of the order of DKK 2,000 for each 1,000 litres of oil replaced.

Maintenance is very important, otherwise there is a risk of chimney fires and carbon monoxide poisoning. A properly maintained fire utilises fuel better and gives better value for money. The working life of the unit also depends on maintenance.

Straw has a heating value which is similar to that of wood and can be used as a fuel in boilers. Nevertheless there are some difficulties which make straw a fuel source utilised only in large boilers usually connected to district heating systems and agriculture sector .
Straw is a difficult type of fuel. It is difficult to handle and to feed into a boiler because it is inhomogeneous, relatively moist, and bulky in proportion to its energy content: its volume is approx. 10-20 times that of coal. Moreover 70% of the combustible part of the straw is contained in the gases emitted during heating, the so called volatile components. Such a high content of volatile gases makes special demands on the distribution and mixing of the combustion air and to the design of the burner and the combustion chamber. Straw also contains many chlorine compounds which may cause corrosion problems, particularly with high surface temperatures. The softening and melting temperatures of straw ash are relatively low due to a large content of alkali metals. As a consequence, slugging problems may occur at low surface temperatures. District heating systems
Despite all problems with the straw there is a huge number of straw-fired district heating plants all around the world. Only in Since 1980 more than 70 such plants have been built in Denmark alone. Their output power range from 0,6 MW to 9 MW and the average size is 3,7 MW. These plants use mostly so called Hesston bales of straw with the dimensions 2,4x1,2x1,3 m and a weight of 450 kg. It is common to have a back up system based on oil or gas-fired boiler which can cover required output during peak load situations, repairs and breakdowns. Thus the straw-fired boiler is usually dimensioned for 60-70 % of maximum load which makes it easier to operate at low summer load level.
Straw-firing plants are made up of the same main components :
* Straw storage building
* Straw weighing device
* Straw crane
* Conveyor (feeding unit)
* Feeding system
* Boiler
* Flue gas cleaning
* Stack

The conveyor carries the straw into the bottom of the boiler which consists of a sturdy iron grate. This is the place where the combustion takes place. The grate is usually divided into several combustion zones with separate blowers supplying combustion air through the grate. Combustion can be controlled individually in each zone , thus an acceptable burn-out of the straw can be obtained. Most of the energy content of the straw is represented by volatile gases (approx. 70%) which are released during heating and are burned off in the combustion chamber above the grate. In order to provide combustion air for the gases, secondary air is supplied through nozzles located in the boiler walls. From the combustion chamber, the flue gases are led to the convection section of the boiler where most of the heat is transferred through the boiler wall to the circulating boiler water. The convector is usually made up of rows of vertical pipes through which the flue gases pass. Most existing plants have an economiser , i.e. a heat exchanger installed after the convector. In this unit , the flue gases transmit more heat to the boiler water, resulting in an increased efficiency of the system.

The straw supplied to the plants must conform to certain requirements in order to reduce the risk of operating problems during various processes of energy production. Storage, handling, dosing, feeding, combustion, and the environmental consequences of those processes are all potential causes of problems. The moisture content of the straw is the most important quality criteria for the this fuel. Moisture content varies between 10-25% but in some cases it may be even higher. The calorific value (energy content per kg) of the straw is directly proportional to the moisture content from which the price is calculated.
All heating plants specify a maximum acceptable moisture content in straw supplied. A high water content may cause storing problems and plant malfunction as well as reduced capacity and increased generating costs during handling, dosing and feeding (and possibly a reduction in boiler efficiency). The maximum acceptable moisture content varies from plant to plant but it is usually 18-22% water. Different types of straw behave very differently during combustion. Some types burn almost explosively, leaving hardly any ash, whereas other types burn very slowly, leaving almost complete skeletons of ash on the grate. Experience from straw-fired district heating plants is not always identical from plant to plant, and the different combustion conditions can rarely be explained on the basis of ordinary laboratory examinations. Heating plants smaller than 1 MW
This type of plant differs technically from district heating plants and is used mostly in agriculture. The use of straw for energy production in the agricultural sector as we know it today started in the 1970’s as a result of the “energy crisis” and the resulting subsidies for the installation of straw-fired boilers. During the past 10-15 years, the concept of burning straw has developed from small primitive and labour-demanding boilers with batch firing and considerable smoke problems into large boilers emitting little smoke which are either batch-fired or automatic with fuel being supplied only 1-2 times per day.

Earlier, the market was dominated by boilers for small bales. Today, however, most of the batch-fired boilers are designed for big bales (round bales, medium-sized bales or Hesston bales).The big bale boilers are well suited for an annual heating requirement corresponding to at least 10,000 litres of oil. The boilers are available in different sizes, holding from 1 round bale (200-300 kg) to 2 Hesston bales ( 1,000 kg). The boiler is fired with 1 bale at a time. A tractor fitted with a grab or a fork introduces the bale through a feeding gate at the front of the boiler. In order to ensure proper combustion and minimize particle emission from flue gases, air velocity and supply may be regulated through gradually changing between the upper and lower section of the boiler and by adjusting the air volume.
Batch-fired boilers used to cause many problems when fed with straw of inferior quality and the supply of combustion air was difficult to control. In recent models, however, the control problem has eventually been solved but the water content of the straw must still be kept below 15- l8 %. Today, an efficiency of 75% and a CO content below 0.5% is possible in batch-fired boilers. About l0 years ago, the efficiency was only 35%.

Interest in automatically fired boilers is due to the large amount of labour needed when operating small bale boilers with batch firing which used to be very popular. Several types of automatic boiler plants have been developed but they all include a dosing device which automatically feeds the straw into the boiler continuously. The dosing device may be designed for whole bales, cut straw or straw pellets.

Units consisting of a scarifier/cutter have been developed which separate the bales, parting them into pieces of varying sizes. The bales are fed into this unit on a conveyor. The volume of straw treated is often regulated by merely modifying the velocity of the conveyor. The straw is transported from the scarifier/cutter by worm conveyors or blowers. If blowers are used, the distance to the boiler can be greater than with worms but this equipment also consumes more energy.
The scarifier does not actually cut or shred the straw but it separates the straw into the segments it was compacted into by the piston of the baler. In order to ensure a steady flow of straw through the transport system, the scarifier usually has a retaining device. Most scarifiers have knives to loosen the straw without creating large lumps.
In automatically fired boilers, combustion takes places as the straw is fed into the boiler. The air supply is adapted to the straw volume by means of an adjustable damper on a blower. This ensures a good combustion, a significantly improved utilization factor, and a corresponding reduction of particle emission problems as compared with the first manually fired boilers without air regulating devices. Straw ignites easily in an automatic boiler because fresh straw is supplied continuously.

The use of straw pellets for energy production has aroused some interest in recent years.
Until now, only small quantities of straw pellets have been produced. Of interest is the homogeneous and handy nature of this fuel which makes it perfect for transport in tankers and for use in automatic heating plants.
There are, however, still unsolved slag problems when the pellets are used in small boilers. The possibility of establishing a sales network for rural districts and villages is being considered in some developed countries.
Pellet-fed plants are usually intended for domestic heating and they consist of a boiler and a closed magazine for fuel (straw pellets). A stoker worm feeds the fuel into a hearth located in the boiler.
When the plant is operating, the stoker worm works intermittently and the feeding capacity is regulated by adjusting its on/off intervals. Combustion air is supplied by a blower. The amount of ash from a small straw-fired boiler is typically 4% by weight of the straw used.

For more than a third of the world’s people, the real energy crisis is a daily scramble to find the wood they need to cook dinner. Their search for wood, once a simple task, has changed as forests recede, to a day’s labour in some places. Reforestation, use of alternative fuels and fuel conservation through improved stoves are the three methods which offer possible solutions to the firewood crisis. Reforestation programs have been started in many countries, but the high rate of growth in demand means that forests are being cut much faster than they are being replanted. Alternative fuels like biogas and solar energy can be one part of solution. Another part consists of utilisation of efficient wood burning techniques like improved cook stoves. Fuel-efficient cook stoves
The most immediate way to decrease the use of wood as cooking fuel is to introduce improved wood- and charcoal-burning cook stoves. Simple stove models already in use can halve the use of firewood. A concerted effort to develop more efficient models might reduce this figure to 1/3 or ¼, saving more forests than all of the replanting efforts planned for the rest of the century. Using simple hearths such as those used in India, Indonesia, Guatemala and elsewhere, one-third as much wood would provide the same service. These clay “cookers” are usually built on the spot with a closed hearth, holes in which to place the vessels to be heated, and a short chimney for the draught. Their energy yield varies, depending on the model, between approximately 15 and 25%. If these “cookers” were used throughout the Sahel, firewood consumption would be reduced by two-thirds: 0,2 m3 instead of 0,6 m3 per person per year. There are clear benefits of improved cook stoves to the individual family, the local community, the nation and the global community.  In brief, they include:
* Less time spent gathering wood or less money spent on fuel,- less smoke in the kitchen; lessening of respiratory problems associated with smoke inhalation,- less manure used as fuel, releasing more fertilizer for agriculture,- little initial cost compared to most other kinds of cookers, - improved hygiene with models that raise cooking off the floor, - safety: fewer burns from open flames; less chance of children falling into the fire or boiling pots; if pots are securely set into the stove, less chance of children pulling them down on themselves,- cooking convenience: stoves (an be made to any height and can have work space on the surface, - the fire requires less attention, as stoves with damper control can be easier to tend.
* Stove building may create new jobs,- potential for using local materials and- potential for local innovations,- money and time saved can be invested elsewhere in the community.
* Lowered rate of deforestation improves climate, wood supply and hydrology; decreases soil erosion,- potential for reducing dependence on imported fuel.

In regions where much of the daily cooking involves a long simmering period (required for many beans, grains, stews and soups) the amount of fuel needed to complete the cooking process can be greatly reduced by cooking with retained heat. This is a practice of ancient origin which is still used in some parts of the world today.
In some areas a pit is dug and lined with rocks previously heated in a fire. The food to be cooked is placed in the lined pit, often covered with leaves, and the whole is covered by a mound of earth. The heat from the rocks is retained by the earth insulation, and the food cooks slowly over time.
Another version of this method consists of digging a pit and lining it with hay or another good insulating material. A pot of food which has previously been heated up to a boil is placed in the pit, covered with more hay and then earth, and allowed to cook slowly with the retained heat.

This latter method is the direct ancestor of the Haybox Cooker, which is simply a well insulated box lined with a reflective material into which a pot of food previously brought to a boil is placed. The food is cooked in 3 to 6 hours by the heat retained in the insulated box. The insulation greatly slows the loss of conductive heat, convective heat in the surrounding air is trapped inside the box, and the shiny lining reflects the radiant heat back into the pot.
Simple haybox style cookers could be introduced along with fuel-saving cook stoves in areas where slow cooking is practised. How these boxes should be made, and from what materials, is perhaps best left to people working in each region. Ideally, of course, they should be made of inexpensive, locally available materials and should fit standard pot sizes used in the area.

There are several principles which should be kept in mind in regard to the construction of a haybox cooker:
* Insulation should cover an six sides of the box (especially the bottom and lid). If one or more sides are not insulated, heat will be lost by conduction through the uninsulated sides and much efficiency will be lost.
* The box should be airtight. If it is not airtight, heat will be lost through warm air escaping by convection out of the box.
* The inner surfaces of the box should be of a heat reflective material (such as aluminium foil) to reflect radiant heat from the pot back to it.
A simple, lightweight haybox can be made from a 60 by 120 cm sheet of rigid foil-faced insulation and aluminium tape. Haybox cookers can also be constructed as a box-in-a-box with the intervening space filled with any good insulating material. The required thickness of the insulation will vary with how efficient it is (see below).
Good Insulating Materials
Suggested Wall Thickness
5 cm
Polystyrene sheets/pellets/drinking cups 
5 cm
10 cm
Sawdust/wood shavings
10 cm
10 cm
Fiberglas/glass wool
10 cm
Shredded newspaper/cardboard
10 cm
Rice hulls/nut shells
15 cm
The inner box should have a reflective interior: aluminium foil, shiny aluminium sheeting, old printing plates, other polished sheet metal’ or silver paint will all work. The box can be wooden, or a can-in-a-can, or cardboard, or any combination; a pair of cloth bags might also work. Be inventive. Always be sure the lid is air tight.

There are some adjustments involved in cooking with haybox cookers:
* Less water should be used since it is not boiled away.
* Less spicing is needed since the aroma is not boiled away.
* Cooking must be started earlier to give the food enough time to cook at a lower temperature than over a stove.
* Haybox cookers work best for large quantities (over 4 lifers) as small amounts of food have less thermal mass and cool faster than a larger quantity. Two or more smaller amounts of food may be placed in the box to cook simultaneously.
* The food should boil for several minutes before being placed in the box. This ensures that all the food is at boiling temperature, not just the water.

The boxes perform best at low altitudes where boiling temperature is highest. They should not be expected to perform as well at high altitudes. One great advantage of haybox cookers is that the cook no longer has to keep up a fire or watch or stir the pot once it’s in the box. In fact, the box should not be opened during cooking as valuable heat is lost. And finally, food will never burn in a haybox.

The Lorena system involves building a solid sand/clay block, then carving out a firebox and flue tunnels. The block is an integral sand/clay mixture which, upon drying, has the strength of a weak concrete (without the cost). The mixture contains 2 to 5 parts of sand to 1 part of clay, though the proportions can differ widely.
Pure clay stoves crack badly because the clay shrinks as it dries and expands when it is heated. Sand/clay stoves are predominantly sand, with merely enough clay to glue the sand together. The mix should contain enough clay to bind the sand grains tightly together. The sand/clay mixture is strong in compression, but resists impact poorly. It is adequately strong in tension if thin walls are avoided. Unlike concrete, which works well as a thin shell, the sand/clay mixture relies upon mass for tensile strength.
* Sand and clay are available in most places, and cheap.
* The material is versatile; it can be used to build almost any size or shape of stove.
* The tools required are simple.
* Construction of the stoves requires simple skills.
* Stoves are easy to repair or replace.

* Construction relies on heavy materials that are not always available at the building site and are difficult to transport.
* The stoves are not transportable.
* Sand/clay stoves are not waterproof.
* Stove construction can require several days of hard work.
* Efficiency of the stoves relies on the quality of the workmanship in their construction. Normally, they can be expected to work well for at least a year, after which they may need to be repaired.

One of the most successful urban stove projects in the world is the Kenya Ceramic Jiko (KCJ) initiative. Over 500,000 stoves of this new improved design have been produced and disseminated in Kenya since the mid-1980s (Davidson and Karekezi, 1991). Known as the Kenya Ceramic Jiko, KCJ for short, the improved stove is made of ceramic and metal components and is produced and marketed through the local informal sector. One of the key characteristics of this project was its ability to utilize the existing cook stove production and distribution system to produce and market the KCJ. Thus, the improved stove is fabricated and distributed by the same people who manufacture and sell the traditional stove design.
Another important feature of the Kenya stove project is that the KCJ design is not a radical departure from the traditional stove. The KCJ is, in essence, an incremental development from the traditional all-metal stove. It uses materials that are locally available and can be produced locally. In addition, the KCJ is well adapted to the cooking patterns of a large majority of Kenya’s urban households. In many respects, the KCJ project provides an ideal case study of how an improved stove project should be initiated and implemented. CHARCOAL PRODUCTION - PYROLYSIS
The production of charcoal spans a wide range of technologies from simple and rudimentary earth kilos to complex, large-capacity charcoal retorts. The various production techniques produce charcoal of varying quality. Improved charcoal production technologies are largely aimed at attaining increases in the net volume of charcoal produced as well as at enhancing the quality characteristics of charcoal.
Typical characteristics of good-quality charcoal:
Ash content : 5 per cent
Fixed carbon content : 75 per cent
Volatiles content : 20 per cent
Bulk density : 250-300 kg/m3
Physical characteristics : Moderately friable

Efforts to improve charcoal production are largely aimed at optimising the above characteristics at the lowest possible investment and labour cost while maintaining a high production volume and weight ratios with respect to the wood feedstock.
The production of charcoal consist of six major stages:
1. Preparation of wood
2. Drying - reduction of moisture content
3. Pre-carbonization - reduction of volatiles content
4. Carbonization - further reduction of volatiles content
5. End of carbonization - increasing the carbon content
6. Cooling and stabilization of charcoal

The first stage consists of collection and preparation of wood, the principal raw material. For small-scale and informal charcoal makers, charcoal production is an off-peak activity that is carried out intermittently to bring in extra cash. Consequently, for them, preparation of the wood for charcoal production consists of simply stacking odd branches and sticks either cleared from farms or collected from nearby woodlands. Little time is invested in the preparation of the wood. The stacking may, however, assist in drying the wood which reduces moisture content thus facilitating the carbonization process. More sophisticated charcoal production systems entail additional wood preparation, such as debarking the wood to reduce the ash content of the charcoal produced. It is estimated that wood which is not debarked produces charcoal with an ash content of almost 30 per cent. Debarking reduces the ash content to between 1 and 5 per cent which improves the combustion characteristics of the charcoal.
The second stage of charcoal production is carried out at temperatures ranging from 110 to 220 degrees Celsius. This stage consists mainly of reducing the water content by first removing the water stored in the wood pores then the water found in the cell walls of wood and finally chemically-bound water.
The third stage takes place at higher temperatures of about 170 to 300 degrees and is often called the pre-carbonization stage. In this stage pyroligneous liquids in the form of methanol and acetic acids are expelled and a small amount of carbon monoxide and carbon dioxide is emitted.
The fourth stage occurs at 200 to 300 degrees where a substantial proportion of the light tars and pyroligneous acids are produced. The end of this stage produces charcoal which is in essence the carbonized residue of wood.
The fifth stage takes place at temperatures between 300 degrees and a maximum of about 500 degrees. This stage drives off the remaining volatiles and increases the carbon content of the charcoal.
The sixth stage involves cooling of charcoal for at least 24 hours to enhance its stability and reduce the possibility of spontaneous combustion.
The final stage consists of removal of charcoal from the kiln, packing, transporting, bulk and retail sale to customers. The final stage is a vital component that affects the quality of the finally-delivered charcoal. Because of the fragility of charcoal, excessive handling and transporting over long distances can increase the amount of fines to about 40 per cent thus greatly reducing the value of the charcoal. Distribution in bags helps to limit the amount of fines produced in addition to providing a convenient measurable quantity for both retail and bulk sales.

3.8.6 Wood Gasification Basics
Wood gasification is also called producer gas generation and destructive distillation. The essence of the process is the production of flammable gas products from the heating of wood. Carbon monoxide, methyl gas, methane, hydrogen, hydrocarbon gases, and other assorted components, in different proportions, can be obtained by heating or burning wood products in an isolated or oxygen poor environment. This is done by burning wood in a burner which restricts combustion air intake so that the complete burning of the fuel cannot occur. A related process is the heating of wood in a closed vessel using an outside heat source. Each process produces different products. If wood were given all the oxygen it needs to burn cleanly the by-products of the combustion would be carbon dioxide, water,
some small amount of ash, (to account for the inorganic components of wood) and heat. This is the type of burning we strive for in wood stoves. Once burning begins though it is possible to restrict the air to the fuel and still have the combustion process continue. Lack of sufficient oxygen caused by restricted combustion air will cause partial combustion. In full combustion of a hydrocarbon (wood is basically a hydrocarbon) oxygen will combine with the carbon in the ratio of two atoms to each carbon atom. It combines with the hydrogen in the ratio of two atoms of hydrogen to one of oxygen. This produces CO2 (carbon dioxide) and H2O (water). Restrict the air to combustion and the heat will still allow combustion to continue, but imperfectly. In this restricted combustion one atom of oxygen will combine with one atom of carbon, while the hydrogen will sometimes combine with oxygen and sometimes not combine with anything. This produces carbon monoxide,  (the same gas as car exhaust and for the same reason) water, and hydrogen gas. It will also produce a lot of other compounds and elements such as carbon which is smoke. Combustion of wood is a bootstrap process. The heat from combustion breaks down the chemical bonds between the complex hydrocarbons found in wood (or any other hydrocarbon fuel) while the combination of the resultant carbon and hydrogen with oxygen-combustion-produces the heat. Thus the process drives itself. If the air is restricted to combustion the process will still produce enough heat to break down the wood but the products of this inhibited combustion will be carbon monoxide and hydrogen, fuel gases which have the potential to continue the combustion reaction and release heat since they are not completely burned yet. (The other products of incomplete combustion, predominately carbon dioxide and water, are products of complete combustion and can be carried no further.) Thus it is a simple technological step to produce a gaseous fuel from solid wood. Where wood is bulky to handle, a fuel like wood gas (producer gas) is convenient and can be burned in various existing devices, not the least of which is the internal combustion engine. A properly designed burner combining wood and air is one relatively safe way of doing this. so this water is available to play a part in the destructive distillation process. Wood also contains many other chemicals from alkaloid poisons to minerals. These also become part of the process.
As a general concept, destructive distillation of wood will produce methane gas, methyl gas, hydrogen, carbon dioxide, carbon monoxide, wood alcohol, carbon, water, and a lot of other things in small quantities. Methane gas might make up as much as 75% of such a mixture. Methane is a simple hydrocarbon gas which occurs in natural gas and can also be obtained from anaerobic bacterial decomposition as “bio-gas” or “swamp gas”. It has high heat value and is simple to handle. Methyl gas is very closely related to methyl alcohol (wood alcohol) and can be burned directly or converted into methyl alcohol (methanol), a high quality liquid fuel suitable for use in internal combustion engines with very small modification. It’s obvious that both of these routes to the production of wood gas, by incomplete combustion or by destructive distillation, will produce an easily handled fuel that can be used as a direct replacement for fossil fuel gases (natural gas or liquefied petroleum gases such as propane or butane). It can be handled by the same devices that regulate natural gas and it will work in burners or as a fuel for internal combustion engines with some very important cautions. Producer Gas Generators
The simplest device is a tank shaped like an inverted cone (a funnel). A hole at the top which can be sealed allows the user to load sawdust into the tank. There is an outlet at the top to draw the wood gas off. At the bottom the point of the “funnel” is opened and this is where the burning takes place. Once loaded (the natural pack of the sawdust will keep it from falling out the bottom) the sawdust is lit from the bottom using a device such as a propane torch. The sawdust smoulders away. The combustion is maintained by a source of vacuum applied to the outlet at the top, such as a squirrel cage blower or an internal combustion engine. Smoke is drawn up through the porous sawdust, being partly filtered in the process, and exits the burner at the top where it goes on to be further conditioned and filtered. The vacuum also draws air in to support the fire. This burner is crude and uncontrollable, especially as combustion nears the top of the sawdust pile. This can happen rapidly since there is no control to assure that the sawdust burns evenly. “Leads” of fire can form in the sawdust reaching toward the top surface. Once the fire breaks through the top of the sawdust the vacuum applied to the burner will pull large amounts of air in supporting full combustion and leaning out the value of the producer gas as a fuel. This process depends on the poor porosity of the sawdust to control the combustion air so chunk wood cannot be used since its much greater porosity would allow too much air in and user would achieve full combustion at very high temperatures rather than the smouldering and the partial combustion needed. Such a burner is unsatisfactory for prolonged gas generation but it is cheap to build and it will work with a lot of fiddling. For prolonged trouble free operation of a wood gas generator the burner unit must have more complete control of the combustion air and the fuel feed. There are various ways to do this. For example, if the point of above mentioned original funnel shaped burner is completely enclosed then control over the air entering the burner can be achieved. This configuration will successfully burn much larger amount of wood.

3.8.7 FERMENTATION - Conversion of biomass into ethanol
Alcohol can be used as a liquid fuel in internal combustion engines either on their own or blended with petroleum. Therefore, they have the potential to change and/or enhance the supply and use of fuel (especially for transport) in many parts of the world. There are many widely-available raw materials from which alcohol can be made, using already improved and demonstrated existing technologies. Alcohol have favourable combustion characteristics, namely clean burning and high octane-rated performance. Internal combustion engines optimized for operation on alcohol fuels are 20 per cent more energy-efficient than when operated on gasoline, and an engine designed specifically to run on ethanol can be 30 per cent more efficient. Furthermore, there are numerous environmental advantages, particularly with regard to lead, CO2, SO2, particulates, hydrocarbons and CO emissions.
Ethanol as most important alcohol fuel can be produced by converting the starch content of biomass feedstocks (e.g. corn, potatoes, beets, sugarcane, wheat) into alcohol. The fermentation process is essentially the same process used to make alcoholic beverages. Here yeast and heat are used to break down complex sugars into more simple sugars, creating ethanol. There is a relatively new process to produce ethanol which utilizes the cellulosic portion of biomass feedstocks like trees, grasses and agricultural wastes. Cellulose is another form of carbohydrate and can be broken down into more simple sugars. This process is relatively new and is not yet commercially available, but potentially can use a much wider variety of abundant, inexpensive feedstocks.
Currently, about 6 billion litres of ethanol are produced this way each year in the U.S. World-wide, fermentation capacity for fuel ethanol has increased eightfold since 1977 to about 20 billion litres per year. Latin America, dominated by Brazil, is the world’s largest production region of bioethanol. Countries such as Brazil and Argentina already produce large amounts, and there are many other countries such as Bolivia, Costa Rica, Honduras and Paraguay, among others, which are seriously considering the bioethanol option. Alcohol fuels have also been aggressively pursued in a number of African countries currently producing sugar - Kenya, Malawi, South Africa and Zimbabwe. Others with great potential include Mauritius, Swaziland and Zambia. Some countries have modernized sugar industry and have low production costs. Many of these countries are landlocked which means that it is not feasible to sell molasses as a by-product on the world market, while oil imports are also very expensive and subject to disruption. The major objectives of these programmes are: diversification of the sugarcane industry, displacement of energy imports and better resource use, and, indirectly, better environmental management. These conditions, combined with relatively low total demand for liquid transport fuels, make ethanol fuel attractive. Global interest in ethanol fuels has increased considerably over the last decade despite the fall in oil prices after 1981. In developing countries interest in alcohol fuels has been mainly due to low sugar prices in the international market, and also for strategic reasons. In the industrialized countries, a major reason is increasing environmental concern, and also the possibility of solving some wider socio-economic problems, such as agricultural land use and food surpluses. As the value of bioethanol is increasingly being recognized, more and more policies to support development and implementation of ethanol as a fuel are being introduced.
Since ethanol has different chemical properties than gasoline, it requires slightly different handling.  For example, ethanol changes from a liquid to a gas (evaporates) less readily than gasoline. This means that in neat (100%) ethanol applications, cold starts can be a problem. However, this issue can be resolved through engine design and fuel formulation.  Changes in engine design will also allow for greater efficiency.  Although a litre of ethanol has about two-thirds of the energy content of a litre of gasoline, tuning the engine for ethanol can make up as much as half the difference. Furthermore, since ethanol is an organic product, should there be a spill, it will biodegrade more quickly and easily than gasoline.
Using ethanol even in low-level blends (e.g. E10 - which is 10% ethanol, 90% gasoline) can have environmental benefits. Tests show that E10 produces less carbon monoxide (CO), sulphur dioxide (SO2) and carbon dioxide (CO2) than reformulated gasoline (RFG).  These blends have helped clean up carbon monoxide problems in cities like Denver and Phoenix.  However E10 produces more volatile organic compounds (VOC), particulates (PM), and nitrogen oxide (NOx) emissions than RFG.  Higher blends (E85, which is 15% gasoline), or even neat ethanol-E100 - burn with less of virtually all the pollutants mentioned above.
The production of ethanol by fermentation involves four major steps: (a) the growth, harvest and delivery of raw material to an alcohol plant; (b) the pre-treatment or conversion of the raw material to a substrate suitable for fermentation to ethanol; (c) fermentation of the substrate to alcohol, and purification by distillation; and (d) treatment of the fermentation residue to reduce pollution and to recover by-products. Fermentation technology and efficiency has improved rapidly in the past decade and is undergoing a series of technical innovations aimed at using new alternative materials and reducing costs. Technological advances will have, however, less of an impact overall on market growth than the availability and costs of feedstock and the cost-competing liquid fuel options.
The many and varied raw materials for bioethanol production can be conveniently classified into three types: (a) sugar from sugarcane, sugar beet and fruit, which may be converted to ethanol directly; (b) starches from grain and root crops, which must first be hydrolysed to fermentable sugars by the action of enzymes; and (c) cellulose from wood, agricultural wastes etc., which must be converted to sugars using either acid or enzymatic hydrolysis. These new systems are, however, at the demonstration stage and are still considered uneconomic. Of major interest are sugarcane, maize, wood, cassava and sorghum and to a lesser extent grains and Jerusalem artichoke. Ethanol is also produced from lactose from waste whey; for example in Ireland to produce potable alcohol and also in New Zealand to produce fuel ethanol. A problem still to be overcome is seasonability of crops, which means that quite often an alternative source must be found to keep a plant operating all-year round.
Sugarcane is the world’s largest source of fermentation ethanol. It is one of the most photosynthetic efficient plants - about 2,5 % photosynthetic efficiency on an annual basis under optimum agricultural conditions. A further advantage is that bagasse, a by-product of sugarcane production, can be used as a convenient on-site electricity source. The tops and leaves of the cane plant can also be used for electricity production. An efficient ethanol distillery using sugarcane by-products can therefore be self-sufficient and also generate a surplus of electricity. The production of ethanol by enzymatic or acid hydrolysis of bagasse could allow off-season production of ethanol with very little new equipment.

Methanol is another alcohol fuel which can be obtained from biomass and coal. But methanol is currently produced mostly from natural gas and has only been used as fuel for fleet demonstration and racing purposes and, thus, will not be considered here. In addition, there is a growing consensus that methanol does not have all the environmental benefits that are commonly sought for oxygenates and which can be fulfilled by ethanol. Brazil
Brazil first used ethanol as a transport fuel in 1903, and now has the world’s largest bioethanol programme. Since the creation of the National Alcohol Programme (ProAlcool) in 1975, Brazil has produced over 90 billion litres of ethanol from sugarcane. The installed capacity in 1988 was over 16 billion litres distributed over 661 projects. In 1989, over 12 billion litres of ethanol replaced about 200,000 barrels of imported oil a day and almost 5 million automobiles now run on pure bioethanol and a further 9 million run on a 20 to 22 per cent blend of alcohol and gasoline (the production of cars powered by pure gasoline was stopped in 1979). From 1976 to 1987 the total investment in ProAlcool reached $6,970,000 million and the total savings equivalent in imported gasoline was $12,480,000 million.
Apart from ProAlcool’s main objective of reducing oil imports, other broad objectives of the programme were to protect the sugarcane plantation industry, to increase the utilization of domestic renewable-energy resources, to develop the alcohol capital goods sector and process technology for the production and utilization of industrial alcohols, and to achieve greater socio-economic and regional equality through the expansion of cultivable lands for alcohol production and the generation of employment. Although ProAlcool was planned centrally, alcohol is produced entirely by the private sector in a decentralized manner.
The ProAlcool programme has accelerated the pace of technological development and reduced costs within agriculture and other industries. Brazil has developed a modem and efficient agribusiness capable of competing with any of its counterparts abroad. The alcohol industry is now among Brazil’s largest industrial sectors, and Brazilian firms export alcohol technology to many countries. Another industry which has expanded greatly due to the creation of ProAlcool is the ethanol chemistry sector.
Ethanol-based chemical plants are more suitable for many developing countries than petrochemical plants because they are smaller in scale, require less investment, can be set up in agricultural areas, and use raw materials which can be produced locally.

Rural job creation has been credited as a major benefit of ProAlcool because alcohol production in Brazil is highly labour-intensive. Some 700,000 direct jobs with perhaps three to four times this number of indirect jobs have been created. The investment to generate one job in the ethanol industry varies between $12,000 and $22,000, about 20 times less than in the chemical industry for example.

Environmental pollution by the ProAlcool programme has been a cause of serious concern, particularly in the early days. The environmental impact of alcohol production can be considerable because large amounts of stillage are produced and often escape into waterways. For each litre of ethanol produced the distilleries produce 10 to 14 litres of effluent with high biochemical oxygen demand (BOD) stillage. In the later stages of the programme serious efforts were made to overcome these environmental problems, and today a number of alternative technological solutions are available or are being developed, e.g., decreasing effluent volume and turning stillage into fertilizer, animal feed, biogas etc. These have sharply reduced the level of pollution and in Sao Paulo. The use of stillage as a fertilizer in sugarcane fields has increased productivity by 20-30 per cent.

Despite many studies carried out on nearly all aspects of the programme, there is still considerable disagreement with regard to the economics of ethanol production in Brazil. This is because the production cost of ethanol and its economic value to the consumer and to the country depend on many tangible and intangible factors making the costs very site-specific and variable even from day to day. For example, production costs depend on the location, design and management of the installation, and on whether the facility is an autonomous distillery in a cane plantation dedicated to alcohol production, or a distillery annexed to a plantation primarily engaged in production of sugar for export. The economic value of ethanol produced, on the other hand, depends primarily on the world prices of crude oil and sugar, and also on whether the ethanol is used in anhydrous form for blending with gasoline, or used in hydrous forte in 100 per cent alcohol-powered cars.
The costs of ethanol were declining at an annual rate of 4 per cent between 1979 and 1988 due to major efforts to improve the productivity and economics of sugarcane agriculture and ethanol production. The costs of ethanol production could be further reduced if sugarcane residues, mainly bagasse, were to be fully utilized. With sale credits from the residues, it would be possible to produce hydrous ethanol at a net cost of less than $0.15/litre, making it competitive with gasoline even at the low early-1990 oil prices. Using the biomass gasifier/intercooled steam-injected gas turbine (BIG/STIG) systems for electricity generation from bagasse, they calculated that simultaneously with producing cost-competitive ethanol, the electricity cost would be less than $0.0451kWh. If the milling season is shortened to 133 days to make greater use of the barbojo (tops and leaves) the economics become even more favourable. Such developments could have significant implications for the overall economics of ethanol production.
Despite all the problems ProAlcool is an outstanding technical success that has achieved many of its aims, its physical targets were achieved on time and its costs were below initial estimates. It has enabled the sugar and alcohol industries to develop their own technological expertise along with greatly increased capacity. It has increased independence, made significant foreign-exchange savings, provided the basis for technological developments in both production and end-use, and created jobs. Overall, Brazil’s success with implementing large-scale ethanol production and utilization has been due to a combination of factors which include: government support and clear policy for ethanol production; economic and financial incentives; direct involvement of the private sector; technological capability of the ethanol production sector; long historical experience with production and use of ethanol; co-operation between Government, sugarcane producers and the automobile industry; an adequate labour force; a plentiful, low-priced sugarcane crop with a suitable climate and abundant agricultural land; and a well established and developed sugarcane industry which resulted in low investment costs in seeing up new distilleries. In the specific case of ethanol-fuelled vehicles, the following factors were influential: government incentives (e.g., lower taxes and cheaper credit); security of supply and nationalistic motivation; and consistent price policy which favoured the alcohol-powered car. Zimbabwe
Zimbabwe is an example of a relatively small country which has begun to tackle its import problem while fostering its own agro-industrial base. An independent and secure source of liquid fuel was seen as a sensible strategy because of Zimbabwe’s geographical position, its politically vulnerable situation and foreign-exchange limitations, and for other economic considerations. Zimbabwe has no oil resources and all petroleum products must be imported, accounting for nearly $120 million per annum on average in recent years which amounted to 18 per cent of the country’s foreign-exchange earnings. Since1980 Zimbabwe pioneered the production of fuel ethanol for blending with gasoline in Africa. Initially a 15-per cent alcohol/gasoline mix was used, but due to increased consumption, the blend is now about 12 per cent alcohol. This is the only fuel available in Zimbabwe for vehicles powered by spark-ignition engines. Annually, production of 40 million litres has been possible since 1983.

3.9 Low Cost Practical Designs of Biogas Technology
There are two basic type of decomposition or fermentation: natural and artificial aerobic decomposition. Anaerobic means in the absence of Air (Oxygen). Therefore any decomposition or fermentation of organic material takes place in the absence of air (oxygen) is known as anaerobic decomposition or fermentation. Anaerobic decomposition can also be achieved in two ways namely, (i) natural and (ii) artificial.

3.9.1 Digestible Property of Organic Matter
When organic raw materials are digested in an airtight container only a certain percentage of the waste is actually converted into Biogas and Digested Manure. Some of it is indigestible to varying degree and either gets accumulated inside the digester or discharged with the effluent. The digestibility and other related properties of the organic matter are usually expressed in the following terms:

This is the weight of water lost upon drying of organic matter (OM) at 100 degrees Celsius (0,10 degrees Celsius (220 deg.F). This is achieved by drying the organic matter for 48 hours in an oven until no moisture is lost. The moisture content is determined by subtracting the final (dried) weight from the original weight of the OM, taken just before putting in the oven.

Total Solids (TS)
The weight of dry matter (DM) or total solids (TS) remaining after drying the organic matter in an oven as described above. The TS is the “Dry Weight” of the OM (Note: after the sun drying the weight of OM still contains about 20% moisture). A figure of 10% TS means that 100 gm of sample will contain 10 gm of moisture and 90 gm of dry weight. The Total Solids (TS) consists of Digestible Organic (or Volatile Solids-VS) and the indigestible solid (Ash).

Volatile Solids (VS)/ Volatile Matter (VM)
The weight of burned-off organic matter (OM) when “Dry Matter-DM” or “Total Solids-TS” is heated at a temperature of 550 degrees Celsius(0,50 degrees Celsius  or 1000 deg. F) for about 3 hours is known as Volatile Solids (VS) or Volatile Matter (VM). Muffle Furnace is used for heating the Dry Matter or Total Solids of the OM at this high temperature after which only ash (inorganic matter) remains. In other wards the Volatile Solids (VS) is that portion of the Total Solids (TS) which volatilizes when it is heated at 550 degrees Celsius and the inorganic material left after heating of OM at this temperature is know as Fixed Solids or Ash. It is the Volatile Solids (VS) fraction of the Total Solids (TS) which is converted by bacteria (microbes) in to biogas.

Fixed Solids (FS) or Ash
The weight of matter remaining after the sample is heated at 550 degrees Celsius is known as Fixed Solids (FS) or ash. The Fixed Solids is biologically inert material and is also known as Ash.

3.9.2 Biogas Production System
The biogas (mainly mixture of methane and carbon dioxide) is produced/generated under both, natural and artificial conditions. However for techno-economically-viable production of biogas for wider application the artificial system is the best and most convenient method. The production of biogas is a biological process which takes place in the absence of air (oxygen), through which the organic material is converted in to, essentially Methane (CH4) and Carbon dioxide (CO2) and in the process gives excellent organic fertilizer and humus as the second by-product. The one essential requirement in producing biogas is an airtight (air leak-proof) container. Biogas is generated only when the decomposition of biomass takes place under the anaerobic conditions, as the anaerobic bacteria (microbes) that live without oxygen are responsible for the production of this gas through the destruction of organic matter. The airtight container used for the biogas production under artificial condition is known as digester or reactor.

3.9.3 Composition of Biogas
Biogas is a colourless, odourless, inflammable gas, produced by organic waste and biomass decomposition (fermentation). Biogas can be produced from animal, human and plant (crop) wastes, weeds, grasses, vines, leaves, aquatic plants and crop residues etc. The composition of different gases in biogas :
Methane (CH4) : 55-70%
Carbon Dioxide (CO2) : 30-45%
Hydrogen Sulphide (H2S) : 1-2%
Nitrogen (N2) : 0-1%
Hydrogen (H2) : 0-1%
Carbon Mono Oxide (CO) : Traces
Oxygen (O2) : Traces

3.9.4 Property of Biogas
Biogas burns with a blue flame. It has a heat value of 500-700 BTU/Ft3 (4,500-5,000 Kcal/M3) when its methane content is in the range of 60-70%. The value is directly proportional to the amount of methane contains and this depends upon the nature of raw materials used in the digestion. Since the composition of this gas is different, the burners designed for coal gas, butane or LPG when used, as ‘biogas burner’ will give much lower efficiency. Therefore specially designed biogas burners are used which give a thermal efficiency of 55-65%.
Biogas is a very stable gas, which is a non-toxic, colourless, tasteless and odourless gas. However, as biogas has a small percentage of Hydrogen Sulphide, the mixture may very slightly smell of rotten egg, which is not often noticeable especially when being burned. When the mixture of methane and air (oxygen) burn a blue flame is emitted, producing large amount of heat energy. Because of the mixture of Carbon Dioxide in large quantity the biogas becomes a safe fuel in rural homes as will prevent explosion.
A 1 M3 biogas will generate 4,500-5,500 Kcal/m2 of heat energy, and when burned in specifically designed burners having 60% efficiency, will give out effective heat of 2,700-3,200 Kcal/m2. 1 Kcal is defined as the heat required to raise the temperature of 1 kg (litre) of water by 1 degrees Celsius. Therefore this effective heat (say 3,000 Kcal/m2 is on an average), is sufficient to bring approx. 100 kg (litre) of water from 20 degrees Celsius to a boil, or light a lamp with a brightness equivalent to 60-100 Watts for 4-5 hours.

3.9.5 Mechanics of Extraction of Biogas
The decomposition (fermentation) process for the formation of methane from organic material (biodegradable material) involves a group of organisms belonging to the family- ‘Methane Bacteria’ and is a complex biological and chemical process. For the understanding of common people and field workers, broadly speaking the biogas production involves two major processes consisting of acid formation (liquefaction) and gas formation (gasification). However scientifically speaking these two broad process can further be divide, which gives four stages of anaerobic fermentation inside the digester-they are (i) Hydrolysis, (ii) Acidification, (iii) Hydrogenation and (iv) Methane Formation. At the same time for all practical purposes one can take the methane production cycle as a three stage activity- namely, (i) Hydrolysis, (ii) Acidification and (iii) Methane formation.
Two groups of bacteria work on the substrate (feedstock) inside the digester-they are (i) Non-methanogens and (ii) Methanogens. Under normal conditions, the non-methanogenic bacteria (microbes) can grow at a pH range of 5.0-8.5 and a temperature range of 25-42  deg. ;whereas, methanogenic bacteria can ideally grow at a pH range of 6.5-7.5 and a temperature range of 25-35 degrees Celsius. These methanogenic bacteria are known as ‘Mesophillic Bacteria’ and are found to be more flexible and useful incase of simple household digesters, as they can work under a broad range of temperature, as low as 15 degrees Celsius to as high as 40 degrees Celsius. However their efficiency goes down considerably if the slurry temperature goes below 20 degrees Celsius and almost stop functioning at a slurry temperature below 15 degrees Celsius. Due to this Mesophillic Bacteria can work under all the three temperature zones of India, without having to provide either heating system in the digester or insulation in the plant, thus keeping the cost of family size biogas plants at an affordable level.
There are other two groups of anaerobic bacteria-they are (i) Pyscrophillic Bacteria and (ii) Thermophillic Bacteria. The group of Pyscrophillic Bacteria work at low temperature, in the range of 10-15 degrees Celsius but the work is still going on to find out the viability of these group of bacteria for field applications. The group of Thermophillic Bacteria work at a much higher temperature, in the range of 45-55 degrees Celsius and are very efficient, however they are more useful in high rate digestions (fermentation), especially where a large quantity of effluent is already being discharged at a higher temperature. As in both the cases the plant design needs to be sophisticated therefore these two groups of Bacteria (Pyscrophillic & Thermophillic) are not very useful in the case of simple Indian rural biogas plant.

3.9.6 Biogas Plant (BGP)
Biogas Plant (BGP) is an airtight container that facilitates fermentation of material under anaerobic condition. The other names given to this device are ‘Biogas Digester’, ‘Biogas Reactor’, ‘Methane Generator’ and ‘Methane Reactor’. The recycling and treatment of organic wastes (biodegradable material) through Anaerobic Digestion (Fermentation) Technology not only provides biogas as a clean and convenient fuel but also an excellent and enriched bio-manure. Thus the BGP also acts as a miniature Bio-fertilizer Factory hence some people prefer to refer it as ‘Biogas Fertilizer Plant’ or ‘Bio-manure Plant’. The fresh organic material (generally in a homogenous slurry form) is fed into the digester of the plant from one end, known as Inlet Pipe or Inlet Tank. The decomposition (fermentation) takes place inside the digester due to bacterial (microbial) action, which produces biogas and organic fertilizer (manure) rich in humus & other nutrients. There is a provision for storing biogas on the upper portion of the BGP. There are some BGP designs that have Floating Gasholder and others have Fixed Gas Storage Chamber. On the other end of the digester Outlet Pipe or Outlet Tank is provided for the automatic discharge of the liquid digested manure. Components of Biogas Plant (BGP)
The major components of BGP are - (i) Digester, (ii) Gasholder or Gas Storage Chamber, (iii) Inlet, (iv) Outlet, (v) Mixing Tank and (vi) Gas Outlet Pipe.

It is either an under ground Cylindrical-shaped or Ellipsoidal-shaped structure where the digestion (fermentation) of substrate takes place. The digester is also known as ‘Fermentation Tank or Chamber’. In a simple Rural Household BGP working under ambient temperature, the digester (fermentation chamber) is designed to hold slurry equivalent to of 55, 40 or 30 days of daily feeding. This is known as Hydraulic Retention Time (HRT) of BGP. The designed HRT of 55, 40 and 30 days is determined by the different temperature zones in the country- the states & regions falling under the different temperature zones are already defined for India. The digester can be constructed of brick masonry, cement concrete (CC) or reinforced cement concrete (RCC) or stone masonry or pre-fabricated cement concrete blocks (PFCCB) or Ferro-cement (ferroconcrete) or steel or rubber or bamboo reinforced cement mortar (BRCM). In the case of smaller capacity floating gasholder plants of 2 & 3 M3 no partition wall is provided inside the digester, whereas the BGPs of 4 M3 capacity and above have been provided partition wall in the middle. This is provided for preventing short-circuiting of slurry and promoting better efficiency. This means the partition wall also divides the entire volume of the digester (fermentation chamber) into two halves. As against this no partition wall is provided inside the digester of a fixed dome design. The reason for this is that the diameter of the digesters in all the fixed dome models are comparatively much bigger than the floating drum BGPs, which takes care of the short-circuiting problems to a satisfactory level, without adding to additional cost of providing a partition wall.

In the case of floating gas holder BGPs, the Gas holder is a drum like structure, fabricated either of mild steel sheets or ferro-cement (ferroconcrete) or high density plastic (HDP) or fibre glass reinforced plastic (FRP). It fits like a cap on the mouth of digester where it is submerged in the slurry and rests on the ledge, constructed inside the digester for this purpose. The drum collects gas, which is produced from the slurry inside the digester as it gets decomposed, and rises upwards, being lighter than air. To ensure that there is enough pressure on the stored gas so that it flows on its own to the point of utilisation through pipeline when the gate valve is open, the gas is stored inside the gas holder at a constant pressure of 8-10 cm of water column. This pressure is achieved by making the weight of biogas holder as 80-100 kg/cm2. In its up and down movement the drum is guided by a central guide pipe. The gas formed is otherwise sealed from all sides except at the bottom. The scum of the semidried mat formed on the surface of the slurry is broken (disturbed) by rotating the biogas holder, which has scum-breaking arrangement inside it. The gas storage capacity of a family size floating biogas holder BGP is kept as 50% of the rate capacity (daily gas production in 24 hours). This storage capacity comes to approximately 12 hours of biogas produced every day.
In the case of fixed dome designs the biogas holder is commonly known as gas storage chamber (GSC). The GSC is the integral and fixed part of the Main Unit of the Plant (MUP) in the case of fixed dome BGPs. Therefore the GSC of the fixed dome BGP is made of the same building material as that of the MUP. The gas storage capacity of a family size fixed dome BGP is kept as 33% of the rate capacity (daily gas production in 24 hours). This storage capacity comes to approximately 8 hours of biogas produced during the night when it is not in use.

In the case of floating biogas holder pipe the Inlet is made of cement concrete (CC) pipe. The Inlet Pipe reaches the bottom of the digester well on one side of the partition wall. The top end of this pipe is connected to the Mixing Tank.
In the case of the first approved fixed dome models (Janata Model) the inlet is like a chamber or tank-it is a bell mouth shaped brick masonry construction and its outer wall is sloppy. The top end of the outer wall of the inlet chamber has an opening connecting the mixing tank, whereas the bottom portion joins the inlet gate. The top (mouth) of the inlet chamber is kept covered with heavy slab. The Inlet of the other fixed dome models (Deenbandhu and Shramik Bandhu) has Asbestos Cement Concrete (ACC) pipes of appropriate diameters.

In the case of floating gas holder pipe the Outlet is made of cement concrete (CC) pipe standing at an angle, which reaches the bottom of the digester on the opposite side of the partition wall. In smaller plants (2 & 3 M3 capacity BGPs) which has no partition walls, the outlet is made of small (approx. 2 ft. length) cement concrete (CC) pipe inserted on top most portion of the digester, submerged in the slurry.
In the two fixed dome (Janata & Deenbandhu models) plants, the Outlet is made in the form of rectangular tank. However, in the case of Shramik Bandhu model the upper portion of the Outlet (known as Outlet Displacement Chamber) is made hemi-spherical in shape, designed to save in the material and labour cost. In all the three-fixed dome models (Janata, Deenbandhu & Shramik Bandhu models), the bottom end of the outlet tank is connected to the outlet gate. There is a small opening provided on the outer wall of the outlet chamber for the automatic discharge of the digested slurry outside the BGP, equal to approximately 80-90% of the daily feed. The top mouth of the outlet chamber is kept covered with heavy slab.

This is a cylindrical tank used for making homogenous slurry by mixing the manure from domestic farm animals with appropriate quantity of water. Thoroughly mixing of slurry before releasing it inside the digester, through the inlet, helps in increasing the efficiency of digestion. Normally a feeder fan is fixed inside the mixing tank for facilitating easy and faster mixing of manure with water for making homogenous slurry.

The Gas Outlet Pipe is made of GI pipe and fixed on top of the drum at the centre in case of floating biogas holder BGP and on the crown of the fixed dome BGP. From this pipe the connection to gas pipeline is made for conveying the gas to the point of utilisation. A gate valve is fixed on the gas outlet pipe to close and check the flow of biogas from plant to the pipeline.

3.9.7 Functioning of a Simple India Rural Household Biogas Plants (BGPs)
The fresh organic material (generally in a homogenous slurry form) is fed into the digester of the plant from one end, known as Inlet. Fixed quantity of fresh material fed each day (normally in one lot at a predetermine time) goes down at the bottom of the digester and forms the ‘bottom-most active layer’, being heavier then the previous day and older material. The decomposition (fermentation) takes place inside the digester due to bacterial (microbial) action, which produces biogas and digested or semi-digested organic material. As the organic material ferments, biogas is formed which rises to the top and gets accumulated (collected) in the Gas Holder (in case of floating gas holder BGPs) or Gas Storage Chamber (in case of fixed dome BGPs). A Gas Outlet Pipe is provided on the top most portion of the Gas Holder (Gas Storage Chamber) of the BGP. Alternatively, the biogas produced can be taken to another place through pipe connected on top of the Gas Outlet Pipe and stored separately. The Slurry (semi-digested and digested) occupies the major portion of the digester and the Sludge (almost fully digested) occupies the bottom most portion of the digester. The digested slurry (also known as effluent) is automatically discharged from the other opening, known as Outlet, is an excellent bio-fertilizer, rich in humus. The anaerobic fermentation increases the ammonia content by 120% and quick acting phosphorous by 150%. Similarly the percentage of potash and several micro-nutrients useful to the healthy growth of the crops also increase. The nitrogen is transformed into Ammonia that is easier for plant to absorb. This digested slurry can either be taken directly to the farmer’s field along with irrigation water or stored in a Slurry Pits (attached to the BGP) for drying or directed to the Compost Pit for making compost along with other waste biomass. The slurry and also the sludge contain a higher percentage of nitrogen and phosphorous than the same quantity of raw organic material fed inside the digester of the BGP. Type of Digestion
The digestion of organic materials in simple rural household biogas plants can be classified under three broad categories. They are (i) Batch-fed digestion (ii) Semi-continuous digestion and (iii) Semi-batch-fed digestion.

In batch-fed digestion process, material to be digested is loaded (with seed material or innouculam) into the digester at the start of the process. The digester is then sealed and the contents left to digest (ferment). At completion of the digestion cycle, the digester is opened and sludge (manure) removed (emptied). The digester is cleaned and once again loaded with fresh organic material, available in the season.

This involves feeding of organic mater in homogenous slurry form inside the digester of the BGP once in a day, normally at a fixed time. Each day digested slurry; equivalent to about 85-95% of the daily input slurry is automatically discharged from the outlet side. The digester is designed in such a way that the fresh material fed comes out after completing a HRT cycle (either 55, 40 or 30 days), in the form of digested slurry. In a Semi-continuous digestion system, once the process is stabilized in a few days of the initial loading of the BGP, the biogas production follows a uniform pattern.

A combination of batch and semi-continuous digestion is known as Semi-batch fed Digestion. Such a digestion process is used where the manure from domestic farm animals is not sufficient to operate a plant and at the same time organic waste like, crop residues, agricultural wastes (paddy & weed straw), water hyacinths and weeds etc, are available during the season. In as Semi-batch fed Digestion the initial loading is done with green or semi-dry or dry biomass (that can not be reduced in to slurry form) mixed with starter and the digester is sealed. This plant also has an inlet pipe for daily feeding of manure slurry from animals. The Semi-batch fed Digester will have much longer digestion cycle of gas production as compared to the batch-fed digester. It is ideally suited for the poor peasants having 1-2 cattle or 3-4 goats to meet the major cooking requirement and at the end of the cycle (6-9 months) will give enriched manure in the form of digested sludge. Stratification (Layering) of Digester due to Anaerobic Fermentation
In the process of digestion of feedstock in a BGP many by-products are formed. They are biogas, scum, supernatant, digested slurry, digested sludge and inorganic solids. If the content of Biogas Digester is not stirred or disturbed for a few hours then these by-products get formed in to different layers inside the digester. The heaviest by-product, which is Inorganic Solids will be at the bottom most portion, followed by Digested Sludge, and so on and so forth as shown in the three diagrams for three different types of digester.

Biogas is a combustible gas produced from the anaerobic digestion of organic matter. Comprising 55-70% Methane, 30-45% Carbon Dioxide, 1-2% of Hydrogen Sulphide and traces other gases.
Gas BIOGAS Combustible gas
Fibrous SCUM Fertilizer
Liquid SUPERNATANT  Biologically Active
Semi Solid DIGESTED SLUDGE Fertilizer
 Diagram 1. By-Product of Batch Fed Digester
Gas BIOGAS Combustible gas
Fibrous SCUM Fertilizer
Liquid DIGESTED SLURRY Fertilizer
Diagram 2. By-Product of Semi-Continuous Fed Digester
Gas BIOGAS Combustible gas
Fibrous SCUM Fertilizer
Semi solid DIGESTED SLUDGE Fertuilizer
Diagram 3. By-Product of Semi-batch Fed Digester

Mixture of coarse fibrous and lighter material that separates from the manure slurry and floats on the top most layer of the slurry is called Scum. The accumulation and removal of scum is sometimes a serious problem. In moderate amount scum can’t do any harm and can be easily broken by gentle stirring, but in large quantity can lead to slowing down biogas production and even shutting down the BGPs.

The spent liquid of the slurry (mixture of manure and water) layering just above the sludge, in case of Batch-fed and Semi Batch-fed Digester, is known as Supernatant. Since supernatant has dissolved solids, the fertiliser value of this liquid (supernatant) is as great as that of effluent (digested slurry). Supernatant is a biologically active by-product; therefore must be sun dried before using it in agricultural fields.

The effluent of the digested slurry is in liquid form and has its solid content (total solid-TS) reduced to approximately 10-20% by volume of the original (Influent) manure (fresh) slurry, after going through the anaerobic digestion cycle. Out of the three types of digestion processes mentioned above, the digested slurry in effluent-form comes out only in semi-continuous BGP. The digested slurry effluent, either in liquid-form or after sun drying in Slurry Pits makes excellent bio-fertilizer for agricultural and horticultural crops or aquaculture.

In the batch-fed or semi batch-fed digester where the plant wastes and other solid organic materials are added, the digested material contains less of effluent and more of sludge. The sludge precipitates at the bottom of the digester and is formed mostly of the solids substances of plant wastes. The sludge is usually composted with chemical fertilizers as it may contain higher percentage of parasites and pathogens and hookworm eggs of etc., especially if the semi-batch digesters are either connected to the pigsty or latrines. Depending upon the raw materials used and the conditions of the digestion, the sludge contains many elements essential to the plant life e.g. Nitrogen, Phosphorous, Potassium plus a small quantity of Salts (trace elements), indispensable to the plant growth- the trace elements such as boron, calcium, copper, iron, magnesium, sulphur and zinc etc. The fresh digested sludge, especially if the night soil is used, has high ammonia content and in this state may act like a chemical fertiliser by forcing a large dose of nitrogen than required by the plant and thus increasing the accumulation of toxic nitrogen compounds. For this reason, it is probably best to let the sludge age for about two weeks in open place. The fresher the sludge the more it needs to be diluted with water before application to the crops, otherwise very high concentration of nitrogen my kill the plants.
In village situation the floor of the animals shelters are full of dirt, which gets mixed with the manure. Added to this the collected manure is kept on the unlined surface which has plenty of mud and dirt. Due to all this the feed stock for the BGP always has some inorganic solids, which goes inside the digester along with the organic materials. The bacteria can not digest the inorganic solids, and therefore settles down as a part of the bottom most layer inside the digester. The Inorganic Solids contains mud, ash, sand, gravel and other inorganic materials. The presence of too much inorganic solids in the digester can adversely affect the efficiency of the BGP. Therefore to improve the efficiency and enhance the life of a semi-continuous BGP it is advisable to empty even it in a period of 5-10 years for thoroughly cleaning and washing it from inside and then reloading it with fresh slurry.

3.9.8 Classification of Biogas Plants (BGPs)
The simple rural household BGPs can be classified under the following broad categories- (i) BGP with Floating Gas Holder, (ii) BGP with Fixed Roof, (iii) BGP with Separate Gas Holder and (iv) Flexible Bag Biogas Plants. Biogas Plant with Floating gas Holder
This is one of the common designs in India and comes under the category of semi-continuous-fed plant. It has a cylindrical shaped floating biogas holder on top of the well-shaped digester. As the biogas is produced in the digester, it rises vertically and gets accumulated and stored in the biogas holder at a constant pressure of 8-10 cm of water column. The biogas holder is designed to store 50% of the daily gas production. Therefore if the gas is not used regularly then the extra gas will bubble out from the sides of the biogas holder. Fixed Dome Biogas Plant
The plants based on Fixed Dome concept was developed in India in the middle of 1970, after a team of officers visited China. The Chinese fixed dome plants use seasonal crop wastes as the major feed stock for feeding, therefore, their design is based on principle of ‘Semi Batch-fed Digester’. However, the Indian Fixed Dome BGPs designs differ from that of Chinese designs, as the animal manure is the major substrate (feed stock) used in India. Therefore all the Indian fixed dome designs are based on the principle of ‘Semi Continuous-fed Digester’. While the Chinese designs have no fixed storage capacity for biogas due to use of variety of crop wastes as feed stock, the Indian household BGP designs have fixed storage capacity, which is 33% of the rated gas production per day. The Indian fixed dome plant designs use the principle of displacement of slurry inside the digester for storage of biogas in the fixed Gas Storage Chamber. Due to this in Indian fixed dome designs have ‘Displacement Chamber(s)’, either on both Inlet and Outlet sides (like Janata Model) or only on the Outlet Side (like Deenbandhu or Shramik Bandhu Model). Therefore in Indian fixed dome design it is essential to keep the combined volume of Inlet & Outlet Displacement Chamber(s) equal to the volume of the fixed Gas Storage Chamber, otherwise the desired quantity of biogas will not be stored in the plant. The pressure developed inside the Chinese fixed dome BGP ranges from a minimum of 0 to a maximum of 150 cm of water column. And the maximum pressure is normally controlled by connecting a simple Manometer on the pipeline near the point of gas utilisation. On the other hand the Indian fixed dome BGPs are designed for pressure inside the plant, varying from a minimum of 0 to a maximum of 90 cm of water column. The Discharge Opening located on the outer wall surface of the Outlet Displacement Chamber and automatically controls the maximum pressure in the Indian design. Biogas Plant with Separate Gas Holder
The digester of this plant is closed and sealed from the top. A gas outlet pipe is provided on top, at the centre of the digester to connect one end of the pipeline. The other end of the pipeline is connected to a floating biogas holder, located at some distance to the digester. Thus unlike the fixed dome plant there is no pressure exerted on the digester and the chances of leakage in the Main Unit of the Plant (MUP) are not there or minimised to a very great extent. The advantage of this system is that several digesters, which only function as digestion (fermentation) chambers (units), can be connected with only one large size gas holder, built at one place close to the point of utilisation. However, as this system is expensive therefore, is normally used for connecting a battery of batch-fed digesters to one common biogas holder. Flexible Bag Biogas Plant
The entire Main Unit of the Plant (MUP) including the digester is fabricated out of Rubber, High Strength Plastic, Neoprene or Red Mud Plastic. The Inlet and Outlet is made of heavy duty PVC tubing. A small pipe of the same PVC tubing is fixed on top of the plant as Gas Outlet Pipe. The Flexible Bag Biogas Plant is portable and can be easily erected. Being flexible, it needs to be provided support from outside, up to the slurry level, to maintain the shape as per its design configuration, which is done by placing the bag inside a pit dug at the proposed site. The depth of the pit should as per the height of the digester (fermentation chamber) so that the mark of the initial slurry level is in line with the ground level. The outlet pipe is fixed in such a way that its outlet opening is also in line with the ground level. Some weight has to be added on the top of the bag to build the desired pressure to convey the generated gas to the point of utilisation. The advantage of this plant is that the fabrication can be centralised for mass production, at the district or even at the block level. Individuals or agencies having land and some basic infrastructure facilities can take up fabrication of this BGP with small investment, after some training. However, as the cost of good quality plastic and rubber is high which increases the comparative cost of fabricating it. Moreover the useful working life of this plant is much less, compared to other Indian simple Household BGPs, therefore inspite of having good potential, the Flexible Bag Biogas Plant has not been taken up seriously for promotion by the field agencies.

3.9.9 Common Indian Biogas Plant (BGP) Designs
The three of the most common Indian BGP design are- (i) KVIC Model, (ii) Janata Model and (iii) Deenbandhu Model, which are briefly described in the subsequent paragraphs: KVIC Model
The KVIC Model is a floating biogas holder semi continuous-fed BGP and has two types, viz. (i) Vertical and (ii) Horizontal. The vertical type is more commonly used and the horizontal type is only used in the high water table region. Though the description of the various components mentioned under this section are common to both the types of KVIC models (Vertical and Horizontal types) some of the details mentioned pertains to Vertical type only. The major components of the KVIC Model are briefly described below:

It is a compact base made of a mixture of cement concrete and brick ballast. The foundation is well compacted using wooden ram and then the top surface is cemented to prevent any percolation & seepage.
Digester (Fermentation Chamber)
It is a cylindrical shaped well like structure, constructed using the foundation as its base. The digester is made of bricks and cement mortar and its inside walls are plastered with a mixture of cement and sand. The digester walls can also be made of stone blocks in places where it is easily available and cheap instead of bricks. All the vertical types of KVIC Model of 4 M3 capacity and above have partition wall inside the digester.

The biogas holder drum of the KVIC model is normally made of mild steel sheets. The biogas holder rests on a ledge constructed inside the walls of the digester well. If the KVIC model is made with a water jacket on top of the digester wall, no ledge is made and the drum of the biogas holder is placed inside the water jacket. The biogas holder is also fabricated out of fibre glass reinforced plastic (FRP), high-density polyethylene (HDP) or Ferroconcrete (FRC). The biogas holder floats up and down on a guide pipe situated in the centre of the digester. The biogas holder has a rotary movement that helps in breaking the scum-mat formed on the top surface of the slurry. The weight of the biogas holder is 8-10 kg/m2 so that it can stores biogas at a constant pressure of 8-10 cm of water column.

The inlet pipe is made out of Cement Concrete (CC) or Asbestos Cement Concrete (ACC) or Pipe. The one end of the inlet pipe is connected to the Mixing Tank and the other end goes inside the digester on the inlet side of the partition wall and rests on a support made of bricks of about 1 feet height.

The outlet pipe is made out of Cement Concrete (CC) or Asbestos Cement Concrete (ACC) or Pipe. The one end of the outlet pipe is connected to the Outlet Tank and the other end goes inside the digester, on the outlet side of the partition wall and rests on a support made of bricks of about 1 feet height. In the case KVIC model of 3 M3 capacity and below, there is no partition wall, hence the outlet pipe is made of short and horizontal, which rest fully immersed in slurry at the top surface of the digester.

The Biogas Outlet Pipe is fixed on the top middle portion of the biogas holder, which is made of a small of GI Pipe fitted with socket and a Gate Valve. The biogas generated in the plant and stored in the biogas holder is taken through the gas outlet pipe via pipeline to the place of utilisation.

3.9.10 Janata Model
The Janata model consists of a digester and a fixed biogas holder (known as Gas Storage Chamber) covered by a dome shaped enclosed roof structure. The entire plant is made of bricks and cement masonry and constructed underground. Unlike the KVIC model, the Janata model has no movable part. A brief description of the different major components of Janata model is described below:

The foundation is well-compacted base of the digester, constructed of brick ballast and cement concrete. The upper portion of the foundation has a smooth plaster surface.

The digester is a cylindrical tank resting on the foundation. The top surface of the foundation serves as the bottom of the digester. The digester (fermentation chamber) is constructed with bricks and cement mortar. The digester wall has two small rectangular openings at the middle, situated diametrically opposite, known as inlet and outlet gate, one for the inflow of fresh slurry and the other for the outflow of digested slurry. The digester of Janata BGP comprises the fermentation chamber (effective digester volume) and the gas storage chamber (GSC).

Gas Storage Chamber (GSC)
The Gas Storage Chamber (GSC) is also cylindrical in shape and is the integral part of the digester and located just above the fermentation chamber. The GSC is designed to store 33% (approx. 8 hours) of the daily gas production from the plant. The Gas Storage Chamber (GSC) is constructed with bricks and cement mortar. The gas pressure in Janata model varies from a minimum of 0 cm water column (when the plant is completely empty) to a maximum of up to 90 cm of water column when the plant is completely full of biogas.

Fixed Dome Roof
The hemi-spherical shaped dome forms the cover (roof) of the digester and constructed with brick and cement concrete mixture, after which it is plastered with cement mortar. The dome is only an enclosed roof designed in such a way to avoid steel reinforcement. (Note: The gas collected in the dome of a Janata plant is not under pressure therefore can not be utilised. It is only the gas stored in the Gas Storage Chamber (GSC) portion of the digester and that is under pressure and can be said as utilisable biogas).

Inlet Chamber
The upper portion of the Inlet Chamber is in the shape of bell mouth and constructed using bricks and cements mortar. Its outer wall is kept inclined to the cylindrical wall of the digester so that the feed material can flow easily into the digester by gravity. The bottom opening of the Inlet Chamber is connected to the Inlet Gate and the upper portion is much wider and known as Inlet Displacement Chamber (IDC). The top opening of the inlet chamber is located close to the ground level to enable easy feeding of fresh slurry.

Outlet Chamber
It is a rectangular shaped chamber located just on the opposite side of the inlet chamber. The bottom opening of the Outlet Chamber is connected to the Outlet Gate and the upper portion is much wider and known as Outlet Displacement Chamber (ODC). The Outlet Chamber is constructed using bricks and cement mortar. The top opening of the Outlet Chamber is located close to the ground level to enable easy removal of the digested slurry through a discharge opening. The level of the discharge opening provided on the outer wall of the outlet chamber is kept at a somewhat lower level than the upper mouth of the inlet opening, as well as kept lower than the Crown of the Dome ceiling. This is to facilitate easy flow of the digested slurry out the plant in to the digested slurry pit and also to prevent reverse flow, either in the mixing tank through inlet chamber or to go inside the gas outlet pipe and choke it.

Biogas Outlet Pipe
The Biogas Outlet Pipe is fixed at the crown of the dome, which is made of a small length of GI Pipe fitted with socket and a Gate Valve. Deenbandhu Model
The Deenbandhu Model is a semi continuous-fed fixed dome Biogas plant. While designing the Deenbandhu model an attempt has was made to minimise the surface area of the BGP with a view to reduce the installation cost, without compromising on the efficiency. The design essentially consists of segments of two spheres of different diameters joined at their bases. The structure thus formed comprises of (i) the digester (fermentation chamber), (ii) the gas storage chamber, and (iii) the empty space just above the gas storage chamber. The higher compressive strength of the brick masonry and concrete makes it preferable to go in for a structure that could be always kept under compression. A spherical structure loaded from the convex side will be under compression and therefor, the internal load will not have any effect on the structure.
The digester of the Deenbandhu BGP is connected with the Inlet Pipe and the Outlet Tank. The upper part (above the normal slurry level) of the outlet tank is designed to accommodate the slurry to be displaced out of the digester (actually from the gas storage chamber) with the generation and accumulation of biogas and known as the Outlet Displacement Chamber (ODC). The Inlet Pipe of the Deenbandhu BGP replaces the Inlet Chamber of Janata Plant. Therefore to accommodate all the slurry displaced out from the Gas Storage Chamber (GSC), the volume of the Outlet Chamber of Deenbandhu model twice the volume of the Outlet Tank of the Janata BGP of the same capacity.
Being a fixed dome technology, the other components and their functions are same as in the case of Janata Model BGP and therefore are not elaborated here once again. Shramik Bandhu Model
This new BRCM biogas plant model which is also a semi-continuous hydraulic digester plant was designed by the author and christened as SHRAMIK BANDHU (friend of the labour). Since then, three more models (rural household plants) in the family of SHRAMIK BANDHU Plants have also been developed. The second one, a semi-continuous hydraulic digester, works on the principle of semi-plug flow digester (suitable for use as a Night Soil based or Toilet attached plant). The third one uses simple low cost anaerobic bacterial filters, designed for possible application as a Low Cost and low Maintenance Wastewater Treatment System. The fourth one is a semi-batch fed hydraulic digester, ideally suitable for the regions where plenty of seasonal crop wastes and waste green biomass are available and population of domestic farm animals are less, for producing the desired quantity of biogas from it alone. For this reason the first model which is the simplest and cheapest in the family of Shramik Bandhu plants, is christened as SBP-I Model. The other three models, yet to be field evaluated, are, SBP-II, SBP-III and SBP-IV, respectively.
The family of SHRAMIK BANDHU biogas plants designs uses the fixed dome concepts as in the case of pervious two most popular Indian fixed dome plants, namely, Janata and Deenbandhu models. In other words, all the four Models of the family of SHRAMIK BANDHU Plant have both, (i) the Gas Storage Chamber (GSC) and (ii) the Dome shaped Roof. However, in this section, the description about Shramik Bandhu plants relates to SBP-I model only.

The SHRAMIK BANDHU Plant is made of Bamboo Reinforced Cement Mortar (BRCM), by pre-fabricated bamboo shells, using the correct size mould for a given capacity SBP-I model- Thus, completely replacing the bricks. These bamboo shells are made by weaving bamboo strips (weaving of which can be done in the village itself) for casting a BRCM structure. The BRCM structures on the one hand are used for giving the right shape to this plant, while on the other hand acts as the reinforcement to the cement mortar plaster as it is casted more or less like the ferro-cement structure. In order to protect the bamboo strips from microbial attack, they are pre-treated by immersing them in water mixed with prescribed ratio of Copper Sulphate (CuSO4) for a minimum of 24 hours before weaving of shell structure is done. As a further safety measure DPC powder in appropriate quantity is mixed while doing second layer (coat) of smooth plastering on the Main Unit of the Plant (MUP), Outlet Chamber (OC); as well as other BRCM components and sub-components, to make the entire structure of SBP-I moisture proof. The Shramik Bandhu plant made from BRCM would be much stronger because it has both higher tensile, as well as compressive strength, as compared to either First Class Bricks or Cement Concrete (CC) or Cement Mortar (CM), when used alone. The reason for this is that the bamboo shell structures used (for both reinforcement and shape of the plant) for the construction of Shramik Bandhu plant is made by weaving strips [only the outer harder surface (skin) and not the softer inner part of bamboo] from seasoned (properly cured) bamboo. Therefore, the entire structure (body) of the SBP-I model would be very strong, durable and have long useful working life. The two previous fixed dome models, namely Janata and Deenbandhu model have no reinforcement and are made of Bricks and Cement Mortar only, therefore, while they are very strong under compression but cannot withstand high tensile force. The hemi-spherical shell shaped (structure) of SHRAMIK BANDHU (SBP-I) model loaded from top on its convex side will be under compression. However, due to comprehensive strength provided by both cement mortar, as well as the reinforcement provided by the woven bamboo shell will ensure that the internal and external load will not have any residual effects on the structure. The bamboo reinforcement will provide added strength (both tensile and compressive) to make the entire structure of SHRAMIK BANDHU (SBP-I) model very sound, as compared to the previous two fixed dome Indian models (Janata & Deenbandhu), referred above.
The digester of SBP-I model is connected to the slurry mixing tank with inlet pipe made of 10 cm or 100 mm (4”) diameter Asbestos Cement Concrete (ACC) pipe, for feeding the slurry inside the plant.
The Outlet Displacement Chamber (ODC) is designed to accommodate the slurry to be displaced out of the digester with the generation & accumulation of biogas. The Outlet Displacement Chamber (ODC) of SBP-I model is also kept hemi-spherical in shape to reduce it’s surface area for a given volume (to save in building materials and time taken for construction)- The ODC is also made of BRCM, using a hemi-spherical shaped woven bamboo shell structure.
A Manhole opening of about 60 cm or 600 mm (2.0 Ft) diameter is provided on the crown of the hemi-spherical shaped ODC. The Manhole is big enough for one person to go inside and come out, at the same time small enough to be able to easily close it by a same size Manhole Cover, which is also made of BRCM.

The Shramik Bandhu (SBP-I) Model is made of two major components and several minor components and sub-components. They are categorized as, (a) Main Unit OF The Plant (MUP), (b) Outlet Chamber (OC) and (c) Other Minor Components. These major and minor components are further divided into sub-components, as given below:
Main Unit Of the plant (MUP)
The Main Unit of the Plant (MUP) is one of the major components of Shramik Bandhu (SBP-I) Model. The MUP has following six main “Sub-Components”:
(i). Digester {or Fermentation Chamber (FC)}
(ii). Gas Storage Chamber (GSC)
(iii). Free Space Area (FSA), located just above the GSC
(iv). Dome (Roof of the Plant-entire area located just above the FSA); and
(v). The following three other sub-components:
[{(e)-(i) the Foundation of the MUP & (e)-(ii)} the Ring Beam for MUP (these two have also been considered here as the two sub-components of the MUP} and {the third is (e)-(iii) the Gas Outlet Pipe (GIP), for better explanation & understanding of the constructional aspects of SBP-I Plant].

Outlet Chamber
The Outlet Chamber (OC)) is the second major component of Shramik Bandhu (SBP-I) Model. The OC has the following four main “Sub-Components”:
(i). Outlet Tank (OT)
(ii). Outlet Displacement Chamber (ODC)
(iii). Empty Space Area (ESA) above the ODC- though for all practical purpose the ODC includes the Empty Space Area (ESA) above it; however, from the designing point of view, the effective ODC of SBP-I model is considered up to the starting of discharge opening located on its outer wall
(iv). Discharge Opening (DO)

Minor Components of the SBP-I Plant
The Minor Components of the Shramik Bandhu (SBP-I) Model are as follows:
(i). Inlet Pipe (IP)
(ii). Outlet Gate (OG)
(iii). Mixing Tank (MT) or Slurry Mixing Tank (SMT)
(iv). Short Inlet Channel (SIC)
(v). Gas Outlet Pipe (GOP)
(vi). Grating (made of Bamboo Sticks)
(vii). Manhole Cover (MHC) for ODC

Being a fixed dome technology, the components and their functions are same as in the case of Janata and Deenbandhu Model BGP and therefore not elaborated here once again.

3.10 Conversion of biomass into electricity
Historically one of the earliest alternatives to fossil fuels is a wood fired boiler producing steam which powers an engine driving a generator. This, unfortunately is about the only advantage. But the steam power has all the disadvantages of an engine/generator and even several more. The wood must be chopped and carried, cured, split, and fed, just as for any wood stove. Ashes must be handled and hauled. The entire installation requires constant control while it is running. Due to compounds in some of the feedstocks, “slagging and fouling” can occur.  Slagging is accumulation of solid residues on parts of the combustion system.  Fouling is simply the accumulation of liquid or semi-liquid residue. This is an important aspect of plant operation and operators need to understand how biomass differs from more commonly used fuels.

3.10.1 Gasification
Usually, electricity from biomass is produced via the condensing steam turbine, in which the biomass is burned in a boiler to produce steam’ which is expanded through a turbine driving a generator. The technology is well-established, robust and can accept a wide variety of feedstocks. However, it has a relatively high unit-capital cost and low operating efficiency with little prospect of improving either significantly in the future. There is also the inherent danger in steam. Steam occupies about 1200 times the volume of water at atmospheric pressure (known as “gage” pressure). Producing steam requires heating water to above boiling temperature under pressure. Water boils at 100° C at sea level. By pressurizing the boiler it is possible to raise the boiling temperature of water much higher. Elevating steam temperature has to be done to use the generated steam for any useful work otherwise the steam would condense in the supply lines or inside the cylinder of the steam engine itself.
Gasification is the newest method to generate electricity from biomass.  Instead of simply burning the fuel, gasification captures about 65-70% of the energy in solid fuel by converting it first into combustible gases.  This gas is then burned as natural gas is, to create electricity, fuel a vehicle, in industrial applications, or converted to synfuels-synthetic fuels.  Since this is the latest technology, it is still under development.
A promising alternative is the gas turbine fuelled by gas produced from biomass by means of thermochemical decomposition in an atmosphere that has a restricted supply of air. Gas turbines have lower unit-capital costs, can be considerably more efficient and have good prospects for improvements of both parameters.
Biomass gasification systems generally have four principal components:
(a) Fuel preparation, handling and feed system;
(b) Gasification reactor vessel;
(c)  Gas cleaning, cooling and mixing system;
(d) Energy conversion system (e.g., internal-combustion engine with generator or pump set, or gas burner coupled to a boiler and kiln).
When gas is used in an internal-combustion engine for electricity production (power gasifiers), it usually requires elaborate gas cleaning, cooling and mixing systems with strict quality and reactor design criteria making the technology quite complicated. Therefore, “Power gasifiers world-wide have had a historical record of sensitivity to changes in fuel characteristics, technical hitches, manpower capabilities and environmental conditions”.
Gasifiers used simply for heat generation do not have such complex requirements and are, therefore, easier to design and operate, less costly and more energy- efficient.. All types of gasifiers require feedstocks with low moisture and volatile contents. Therefore, good quality charcoal is generally best, although it requires a separate production facility and gives a lower overall efficiency.
In the simplest, open-cycle gas turbine the hot exhaust of the turbine, is discharged directly to the atmosphere. Alternatively, it can be used to produce steam in a heat recovery steam generator. The steam can then be used for heating in a cogeneration system; for injecting back into the gas turbine, thus improving power output and generating efficiency known as a steam-injected gas turbine (STIG) cycle; or for expanding through a steam turbine to boost power output and efficiency - a gas turbine/steam turbine combined cycle (GTCC) (Williams & Larson, 1992). While natural gas is the preferred fuel, limited future supplies have stimulated the expenditure of millions of dollars in research and development efforts on the thermo-chemical gasification of coal as a gas-turbine feedstock. Much of the work on coal-gasifier/gas-turbine systems is directly relevant to biomass integrated gasifier/gas turbines (BlG/GTs). Biomass is easier to gasify than coal and has a very low sulphur content. Also, BIG/GT technologies for cogeneration or stand-alone power applications have the promise of being able to produce electricity at a lower cost in many instances than most alternatives, including large centralized, coal-fired, steam-electric power plants with flue gas desulphurization, nuclear power plants, and hydroelectric power plants.
Gasifiers using wood and charcoal (the only fuel adequately proved so far) are again becoming commercially available, and research is being carried out on ways of gasifying other biomass fuels (such as residues) in some parts of the world. Problems to overcome include the sensitivity of power gasifiers to changes in fuel characteristics, technical problems and environmental conditions. Capital costs can still sometimes be limiting, but can be reduced considerably if systems are manufactured locally or use local materials. For example, a ferrocement gasifier developed at the Asian institute of Technology in Bangkok had a capital cost reduced by a factor of ten. For developing countries, the sugarcane industries that produce sugar and fuel ethanol are promising targets for near-term applications of BIG/GT technologies.
Gasification has been the focus of attention in India because of its potential for large scale commercialization. Biomass gasification technology could meet a variety of energy needs, particularly in the agricultural and rural sectors. A detailed micro- and macroanalysis by Jain (1989) showed that the overall potential in terms of installed capacity could be as large as 10,000 to 20,000 MW by the year 2000, consisting of small-scale decentralized installations for irrigation pumping and village electrification, as well as captive industrial power generation and grid fed power from energy plantations. This results from a combination of favourable parameters in India which includes political commitment, prevailing power shortages and high costs, potential for specific applications such as irrigation pumping and rural electrification, and the existence of an infrastructure and technological base. Nonetheless, considerable efforts are still needed for large- scale commercialization.

3.10.2 CO-FIRING
Co-firing of biofuels (e.g. gasified wood) and coal seems to be the way how to reduce emissions from coal firing power plants in many countries. In 1999 a new co-firing system - biomass and coal - started its operation in Zeltweg (Austria). A 10 MW biomass gasification unit was installed in combination with an existing coal fired power station. The gasifier needs 16 m3 woody biomass (chips and bark) per hour. The calorific value of the gas ranges between 2,5 - 5 MJ/Nm3. The project named “Biococomb” is an EU demonstration project. It was realised by the “Verbund” company together with several other companies from Italy, Belgium, Germany and Austria and co-financed by the European Commission.

3.10.3 COGENERATION Biomass-Fired Gas Turbine
A current trend in industrialized countries is the use of increasing number of smaller and more flexible biomass based plants for cogeneration of heat and electricity. A newly developed biomass cogeneration plant in Knoxville, Tennessee, USA, is at the cutting edge of one of the promising technologies behind this development. The plant combines a wood furnace with a gas turbine. A hot, pressurized flue-gas filter cleans the exhaust gas from the furnace before it drives the power turbine. The plant can run on fresh cut sawdust (40% humidity), and produces 5.8 MW of electricity, while consuming 10 tons sawdust/hour, and delivering heat as hot exhaust gas at 370°C. This gives an electric efficiency of about 19% and overall efficiency of up to about 75%. The exhaust gas can be used in a steam turbine, increasing electric output to 9.6 MW, and electricity efficiency to over 30%. The plant in Knoxville has been operating since spring 1999.

3.11 Guideline for Estimation of Biomass Potentials, Barriers and Effects
3.11.1 Unused Forest Energy Potential & Fuelwood
Most commercial forests in Europe have an unused energy potential, which can be used without endangering their role in the natural eco-systems. Beside this, most forests already have a production of firewood. Mountain forests and other less commercial forests can in certain cases also deliver wood for energy, but only after due environmental consideration.

The available forest residues are generally branches with diameters smaller than 7 cm. Generally, leaves and roots should be left in the forest to preserve a healthy forest environment. They are also more difficult to use for energy than branches.

It is not enough to use more firewood, the efficiency needs to be increased as well: Traditional ovens and furnaces have in many cases efficiencies as low as 30%, compared with about 80% for efficient furnaces. Increased efficiency can thus more than double the energy outcome of wood burning, without using more wood. For larger installations, flue-gas condensation can raise efficiency further. For larger applications, wood furnaces can be replaced with wood gasifiers + gas motors or steam boilers + turbines, for cogeneration of electricity and heat.

Energy content
The energy content in totally dry wood is apr. 5.2 kWh/kg. In normally dry firewood (20% humidity) the energy content is apr. 4.2 kWh/kg (lower heating value). In most statistics, wood is measured in cubic meter solid wood (with or without bark). The density of dry wood varies from 800 kg/m3 for hard leafy wood (e.g. beech) to 600 kg/m3 for coniferous (e.g. pine). This gives energy contents of respectively 3400 and 2500 kWh/m3 for beech and pine (lower heating value, 20% humidity).
For furnaces with flue-gas condensers, the energy output can be 80-90% of the higher heating value, which is respectively apr. 4% and 10% above lower heating values for wood with 20% and 40% humidity.

Resource estimation
The available amount of wood can be estimated from forest statistics as the difference between annual growth (in m3, including bark) and the annual wood extraction for timber and other non-energy purposes. Bark can be estimated to 20% of wood exclusive bark. Often the statistics provide only commercial extraction, to which should be added an estimate of non- commercial use. The non-commercial use is often in the form of firewood-gathering by local inhabitants, and could thus be included in the energy potential. In reality the resource might be lower than this estimate due to problems of extracting all branches and/or due to the need of leaving some branches in the forest for ecological reasons. These two factors can reduce the resource with as much as 50% even in commercial forests.

If forest statistics are incomplete or unreliable, simplified estimates can be made:
* if only figures for commercial use is available, the potential for wood residues can be estimated as a fraction of the commercial use. Danish experience is that wood for wood-chips (branches smaller 7 cm in diameter) is equivalent to 25% of the timber production including bark or 31% of the timber exclusive bark.
* if only forest area is known, a first estimate can be made based on area of commercial forest. An estimate from Germany (BUND) gives an annual growth of forests of 10-15 tonnes/ha with an energy content of 150 - 225 GJ/ha (42 - 63 MWh/ha). If 3/4 of this is used for timber, the available residues has an energy content of 40-60 GJ/ha (11 - 16 MWh/ha). An estimation of residues from forests on the Danish island Bornholm gives practical usable residues smaller than 7 cm in diameter of 1.7 tons/ha, equivalent to 18 GJ/ha (5 MWh/ha) with 40% humidity or 25 GJ/ha (7 MWh/ha) with 20% humidity. These estimates do not take into account the important factors of climate and soil for the actual wood production.

Use of firewood for heating does not in general pose barriers. The efficient use of firewood, however, requires efficient ovens and basic knowledge of the users. Using wood-chips requires equipment for producing the wood- chips, storaging, drying, and feeding into an appropriate boiler. This production-chain should be set up locally for successful use of wood-chips for heating. Wood-chips are most suitable in larger boilers, above 100 kW. Often wood-chips have high humidity (40 - 60%), and boilers with flue-gas condensation should be preferred.

Effects on economy, environment and employment
Use of firewood and wood-chips are based on a local resource, requires minimal transport/import and is therefore quite inexpensive in comparison to fossil fuels.
Price estimates, excluding transport & profits (of leafy trees, density 760 kg/m3):
* Denmark: 240 DKK/m3 equal to 0.11 DKK/kWh (0.0203 $/kWh)
* Danish example with Czech wages: 513 Csk/m3 equal to 0.24 CsK/kWh (0.011 $/kWh)
Of the Danish price 2/3 is wages, while the rest is fuel and machine costs. Of the Czech price 1/3 is wages.

Use of wood replacing fossil fuels reduces net CO2 emissions, because the forest absorbs the same quantity of CO2, which is released in the later combustion of the wood. The energy to process the wood is in the order of a few percent of its heating value.
Wood combustion emits very little sulphur (SO2) compared with coal and oil. NOx emissions depend on the combustion process and often the lower combustion temperature leads to lower emissions than for coal and oil combustion. Emissions of particulate and unburned hydrocarbons are totally dependent on the combustion processes, and can be a problem in small and badly designed furnaces. Ashes from the combustion can often be used as fertilizer.
It is important that the extraction of wood is done in a sustainable manner, with adequate re-planting etc.

According to French experience, utilizing of excess energy from forests requires 450 jobs/TWh with the degree of mechanization that is normal for Western Europe.

Each ha of forest on good soil in Central Europe grows 10 tons/ha of wood. If 25% of this is available as waste-wood for energy, the output for energy is 2.5 tons or 11 MWh (20% humidity).

3.11.2 Residues from wood industry
In saw-mills, pulp mills and all wood processing industries, residues are made that can be used for energy purposes. From saw-mills is mainly bark and saw-dust. From pulp-mills (cellulose and paper production) is black and sulphite liquors as well as wood and bark residues. From sawmills comes edgings, chips, sawdust, bark and other residues. Some of these residues are used for pulping, and particle-and fibreboard. Analysis of 7 countries shows that 30-70% of wood industry residues are used for these non-energy purposes.
The residues in forms of larger pieces can be made into wood- chips for wood-chip boilers, while sawdust can be burned in special furnaces or compressed into wood pellets of brickets, that can be used in smaller furnaces and ovens.
Often wood industry uses their wood residues to meet own energy demands for heating, steam and eventually electricity.

Energy content
The energy content for wood residues are about 4.2 kWh/kg (lower heating value, 20% humidity), equivalent to 3400 and 2500 kWh/m3 for beech and pine respectively. See also previous chapter.

Resource Estimation
Evaluation of wood residues can be based on trade-statistics of non-energy wood and wood-products compared with total extraction from forests. The difference is available for energy purposes, and is probably to some extent already used as such in wood industries.
As a simple estimate can be used that residues in general are 25-35% of total forest removals (e.g. Poland 29%, Canada 29%, Finland 33%, Sweden 36%, USA 37% from Biofuels). If a larger part of forest removals are exported without processing, the figure will be lower.

This resource has in general the fewest barriers of all renewable energies. An efficient utilization requires, however, investments in new boilers, or at least in a pre-combustion furnace, that can be attached to an existing (good) boiler.

Effect on economy, environment and employment
When the residues from industry are treated as waste without commercial value, the economy of using them for energy is almost always cost-effective, and has a better economy than wood residues from forests.

Environmental effects are equal to wood residues from forests, as long as combustion of chemically treated and painted wood residues is avoided. Such wood-residues should be treated as municipal waste or chemical waste depending on the treatment.

The direct employment of using industrial wood waste is low because the waste has to be handled anyway. Indirectly it gives considerable employment because it turns unused materials into a valuable product (energy).

3.11.3 Combustible waste from agriculture
Straw, prunings of fruit trees and wine and olive oil residues are all residues from agriculture that can be used for energy purposes. Straw harvest is depending on weather conditions and vary considerably from year to year. The straw surplus has also large variations from year to year. If a large part of the surplus is used, an alternative fuel should be considered for years with little surplus straw. Such an alternative fuel could be wood-chips forest residues, that can be used alternatively with straw in many boilers. The forest residues can stay several years in the forests before usage. Straw surplus can be ploughed into the field for enriching the humus layer of the field. When this is needed for a sustainable agriculture, the surplus straw for energy will be lower.

Energy Content
The energy content of straw is 4.9 kWh/kg of dry matter (high heating value). With a typical of 15% humidity the lower heating value is 4.1 kWh/kg.
The energy in 1 m3 of densely compressed straw bales is 500 kWh (density 120 kg/m3).
The average efficiency for 22 straw-fired heating stations in operation in Denmark is 80-85%, not including flue-gas condensation.

Resource Estimation
Estimations of straw production can be obtained from agricultural statistics. This value should be reduced with agricultural consumption of straw for animal fodder and bedding. The agricultural consumption is very dependent on the type of stables used. In Denmark the average available surplus for energy is estimated to 59% of which 1/5 is already used, mainly for heating (Straw). In Eastern Bohemia, this surplus is estimated to about 35%. As a general, conservative estimate for Europe 25% of the straw production can be used for energy. The straw production varies +/- 30% from average years to years with high respectively low straw harvest.

If straw production is not available from statistics, relatively good estimates can be made from statistics of grain production. As a rough estimate the amount in tons of straw can be equalled to the amount of grain in tons. In the Czech Republic the average ratio between straw and grain is found to:
* wheat 1.3 tons straw/tons grain
* barley 0.8 tons straw/tons grain
* rye 1.4 tons straw/tons grain
* oat 1.1 tons straw/tons grain

A rough estimate can be made based on agricultural area and a straw harvest of 4-7 tons/ha depending on soil, type of grain and weather.

Limited experience and funds for the necessary investments are often the largest barriers to use straw for energy. Other barriers can be:
* the need to develop a market for straw with attractive prices for users as well as suppliers,
* pesticides can in certain situations give unwanted chlorine compounds in the straw. This can be reduced by leaving the straw for a period at the field before collection, so called wilting.
* use of straw in inadequate and polluting boilers can give straw a bad reputation.

Effect on economy, environment and employment
In Denmark, straw-prices vary from 0.085 DKK/kWh (1.2 EURO cent or 1.2 US cent) to 0.12 DKK/kWh  for baled straw delivered at a straw-firing station. In Czech Republic the prices for straw collected at the farm has been quoted at 0.043 Csk/kWh (0,15 EURO cent) for loose straw and 0.054 Csk/kWh (0.19 EURO cent) for baled straw.
Costs, average for 16 straw-fired installations in Denmark are per kWh heat produced:
 Danish average  Estimate for Czech Republic
Fuel 1,9 EURO cent 0,26 EURO cent
Electricity* 0,12 EURO cent 0,12 EURO cent
O&M, administr. 1,3 EURO cent 0,26 EURO cent
Capital costs  1,5 EURO cent 1,5 EURO cent
TOTAL 4,8 EURO cent 2,14 EURO cent
* Electricity consumption is in average 2.3% of heat produced

The environmental impact of using agricultural residues are, as for wood, reduced CO2-emission, reduced sulphur emissions, compared with coal and oil.  Emissions of particulate, NOx and volatile organic compounds (VOC) depend on furnaces and flue-gas treatment. Chlorine components in straw gives emission of HCl as mentioned above. Danish experience from 13 straw-fires heating stations shows the following emissions (all plants have particulate filters):
Average Emission
g/kWh straw
Variation of emissions
g/kWh straw
1-10 ng
* PAH = Polyaromatic Hydro-Carbons. This is the carcinogenic part of VOCs.
** Dioxin figures are based on only two measurements, figures given in nanogram,
   10-9 g.

The direct employment of harvesting straw in a fully mechanized agriculture in Denmark is estimated to 350 jobs/TWh. This is for technologies with large straw-bales (500 kg each). For a system based on smaller bales (10-20 kg), the employment is larger.

3.11.4 Energy Crops
It is estimated that 20-40 million hectares of land in the EU will be surplus to conventional agricultural requirement. The same situation (agricultural overproduction and setting the land aside) can be expected in Central Europe as well. This set aside land can be used for different purposes, one of them is energy crop production.

Promising crops which can be planted for energy purposes in Europe are short rotation trees (coppice of various willows and poplars), Miscanthus and Sweet Sorghum. These crops can be utilized by direct combustion for heat and electricity production. Other promising energy crops are plants for liquid fuels as rape seeds for bio-oil.

Energy Contents and Yields
The following table gives an overview of the expected yields and energy contents for three of the promising plants for solid fuel production.
Energy content
(GJ/dry tonne)
Energy Yields
Salix (Willow)*
Miscanthus (Elephant grass)
Sweet Sorghum
*Increment of Salix is 2-3 meters in one year (2-3 cm per day in the summer), harvest every third year.

Another promising plant is hemp, which has yields up to 24 tonnes/hectare in approximately 4 month. Hemp plantation is illegal in many countries, even though some variants has very little content of cannabis.

Resource Estimation
The energy potentials can be estimated from the area of land which is set aside in the country/region and can be used for energy plantation and the expected outcome of the above crops under the actual climate and soil conditions. In most countries, national estimates exists of the different yields of the plants. Using excess farm land and ecologically degraded land should be the priority.

Important feature in estimation of potential is input : output ratio. If the bagasse of Sweet Sorghum (2/3 of its energy content) and the sugar (1/3 of its energy content) are utilised for energy purposes the input : output (I/O) energy ratio will reach 1:5 . This means that five times more energy is recovered from  crop (on fuel basis) in comparison with energy utilised for the seeding, fertilisers and pesticides treatment, harvesting, transport and conversion into useable fuels. Usually the input : output ratio is larger than 1:5 for trees and smaller for plants for liquid biofuels.

Short rotation crops may require as much fertilization as traditional crops and degraded land must be regenerated before cultivation using fertilization. For tree crops these drawbacks may be offset by the fact that they retain an active root system throughout the year. Wood ash would be an effective fertilizer for biofuels plantation, reducing the problems caused by the leaching of fertilizers into ground water.

Effect on Economy, Environment and Employment
Economy, Costs
Production costs for Sweet Sorghum are 50 ECU per dry tonne.
Production cost of Salix are 70 ECU (500 DKK) / tonne of dry matter in Denmark (Hvidsed).

Electricity generation cost for biomass (Sweet sorghum ) fuelled system (1992) and improved systems (2000).
Small facility : 0,16 EURO/kWh
Large facility : 0,08 EURO/kWh
Small improved : 0,07 EURO/kWh
Large improved : 0,05 EURO/kWh

An important feature for Salix is that it can be used for water purification - it is possible to grow Salix in purification systems and in the same time harvest the Salix for energy (10-20 tonnes of sludge can be used on each hectare every year). Other benefits of biomass for energy plantation includes forest fire control, improved erosion control, dust absorption, and used as replacement for fossil fuels: no sulphur emission and lower NOx emissions.

For Sweet Sorghum production cost 50% is manpower cost. Production of about 500 tonnes of dry biomass per year justifies the creation of one new job. Other new jobs could be created in related industries such as composting, pulp for paper, service organisation etc.

Hand Rule
Sweet Sorghum output for trials in different locations of Central and Southern Europe:
Annually 90 tonnes of fresh material = 25 tonnes of dry matter per hectare = 450 GJ or 11 tonnes of oil equivalent can be produced. 1/3 as ethanol from sugars and 2/3 of fuel from bagasse. This corresponds to the  absorption of 30-45 tonnes of CO2 per hectare and per year.
Average yearly electricity consumption of a West European person can be met by growing poplar on 0.25 hectare.

3.11.5 Biogas
The largest potential for biogas is in manure from agriculture. Other potential raw-materials for biogas are:
* sludge from mechanical and biological waste-water treatment (sludge from chemical waste-water treatment has often low biogas potential)
* organic household waste
* organic, bio-degradable waste from industries, in particular slaughter-houses and food-processing industries
Care should be taken not to include waste with heavy metals or harmful chemical substances when the resulting sludge is to be used as fertilizer. These kinds of polluted sludge can be used in biogas plants, where the resulting sludge is treated as waste and e.g. incinerated.

Another biogas source is landfills with large amounts of organic waste, where the gas can be extracted directly from drillings in the landfill, so called landfill gas. Such drillings will reduce uncontrolled methane emission from landfills.

Energy Content
The biogas-production will normally be in the range of 0.3 - 0.45 m3 of biogas (60% methane) per kg of solid (total solid, TS) for a well functioning process with a typical retention time of 20-30 days at 32oC. The lower heating value of this gas is about 6.6 kWh/m3. Often is given the production per kg of volatile solid (VS), which for manure without straw, sand or others is about 80% of total solids (TS).
A biogas plant have a self-consumption of energy to keep the manure warm. This is typically 20% of the energy production for a well designed biogas plant. If the gas is used for co-generation, the available electricity will be 30-40% of the energy in the gas, the heat will be 40-50% and the remaining 20% will be self-consumption.

Resource Estimation
For manure, the available data is often the numbers of livestock. From this can be made an estimation of available manure. While the amount of manure produced from animals depends on amount and type of fodder, some average figures are made for most countries.
The following table shows the figures for Denmark :
Kind of animal 
Manure type
Solid amount (kg/day)
Biogas per animal (m3/day)*
Energy per animal
Yearly energy output is for biogas plant with 20% average self-consumption and 360 working days. When animals are not in stables around the year, the figure will be smaller. The figures are for milking cows and for sows with breeding pigs under 5 kg.
*biogas with 65% methane

To make an estimation of the yearly production, it should be evaluated how many days per year the animals are in stables. For large poultry farms and pig-farms it is often the whole year, while cows are in stables from a few months a year to the whole year.

To estimate amount of manure from calfs, pigs and chicken, the following estimates can be used:
* calfs 1-6 month: 25% of milking cows
* other cattle ( calfs > 6 months, cattle for meet, pregnant cows): 60% of milking cows
* small pigs, 5-15 kg: 28% of sows with pigs
* fattening pigs > 15 kg: 52% of sows with pigs
* fattening chicken: 75% of hens

A number of barriers hold back a large scale development of biogas plants in CEEC:
* commercial technology for agriculture (the largest resource base) is not available and have to be developed from existing prototypes or imported.
* it is difficult to make biogas plants cost-effective with sale of energy as the only income. The most likely applications are when other effects of the sludge-treatment has a value. This can e.g. be better hygiene, easier handling, reduced smell, and treatment of industrial waste.
* little knowledge on biogas technology among planners and decision-makers.

Effect on economy, environment and employment
The economy of a biogas plant consists of large investments costs, some operation and maintenance costs, mostly free raw materials, and income from sale of biogas or electricity and heat. Sometimes can be added other values e.g. for improved value of sludge as a fertilizer.
In an example from Czech Republic the price for a Czech plant is estimated to about 70,000 US $ for a plant for treatment of manure from 100 cows. This plant will produce about 220 MWh/year + energy for its own heating. This gives an investment of 0.32 US $ per kWh/year. New Danish biogas plants have similar investment figures. It is estimated that a joint-venture of Czech and Danish technology could reduce prices by about 40% (to about 0.2 US $ per kWh/year); but this has not been shown in practice.
Operating and maintenance (O&M) will normally per year be 10-20% of investment costs, but it vary much with organization, wages, type of plant and eventual transport of sludge. If O&M is 10% of investment costs, simple pay-back requirement is 10 years and no price can be set to increased value of the sludge, the resulting energy price will be 0.04-0.06 US $/kWh or 0.03 - 0.045 ECU/kWh (based on the above examples from Czech Republic).

The environmental effects of biogas plants are:
* production of energy that can replace fossil fuels, reducing CO2 emissions
* reduce smell and hygiene problems of sludge and manure
* treatment of certain kinds of organic waste that would otherwise pose an environmental problem
* reduce potential methane emissions from uncontrolled anaerobic degradation of the sludge.
* easier handling of sludge, which can increase the fraction used as fertilizer and facilitate a more accurate use as fertilizer

The direct employment of biogas plants are for Denmark estimated to 560 jobs/TWh, of which 420 jobs/TWh are operating and maintenance, while 140 job/TWh are construction (2000 man-years to construct plants producing 1 TWh and with lifetime of 14 years). This estimate will be valid for mechanized systems with some degree of centralization: some of the manure is transported to the biogas plant from nearby farms.

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