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3 BIOMASS
3.1 INTRODUCTION
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
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.
WHERE DOES BIOMASS COME FROM?
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%.
BIOMASS - SOME BASIC DATA
* 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)
BIOMASS IN DEVELOPING COUNTRIES
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.
FOOD OR FUEL?
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.
LAND AVAILABILITY
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).
3.2 ENERGY VALUE
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.
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3.3 BENEFITS OF BIOMASS AS ENERGY SOURCE
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.
3.4 ENVIRONMENTAL BENEFITS
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.
3.4.1 CLIMATE CHANGE
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.
3.4.2 ACID RAIN
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.
3.4.3 SOIL EROSION & WATER POLLUTION
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.
3.5 BIOMASS FUELS
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).
3.5.1 WOOD RESIDUES
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.
3.5.2 AGRICULTURAL RESIDUES
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.
3.5.3 SHORT ROTATION PLANTS
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 BIOMASS FUELS IN DEVELOPING COUNTRIES
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.
3.7 METHODS OF GENERATING ENERGY FROM BIOMASS
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.
3.7.1 COMBUSTION
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.
3.7.2 PYROLYSIS
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.
3.7.3 GASIFICATION
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.
SYNTHETIC FUELS
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.
3.7.4 FERMENTATION
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.
3.7.5 ANAEROBIC DIGESTION
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.
3.7.5.1 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.
LANDFILL GAS
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 TECHNOLOGY EXAMPLES
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 BOILERS
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.
MANUALLY FIRED BOILERS
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.
AUTOMATICALLY FIRED BOILERS
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.
3.8.2 MANUALLY-FIRED BOILERS
BURN-THROUGH
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 BOILERS
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%.
REVERSE COMBUSTION BOILERS
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.
THE EFFICIENCY OF THE BOILER
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.
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BURNING WOOD COMBINED WITH SOLAR HEATING
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!
FUEL CHOICE
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
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.
BOILER MAINTENANCE
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.
3.8.3 WOOD PELLETS AND WOOD CHIPS IN AUTOMATICALLY-FIRED BOILERS
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
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
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.
FUEL CONSUMPTION AND INVESTMENT COST
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.
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BOILER TYPES FOR WOOD PELLETS AND WOOD CHIPS
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.
COMPACT UNITS
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.
STOKER-FIRED UNITS
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.
BOILERS WITH PRE-FURNACE
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.
COSTS
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
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.
3.8.4 STRAW FIRING BOILERS
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.
3.8.4.1 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
BOILER
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.
QUALITY REQUIREMENTS TO THE STRAW
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.
3.8.4.2 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.
BATCH-FIRED BOILERS
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%.
AUTOMATICALLY FIRED BOILERS
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.
BOILERS FOR BALES OF STRAW
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.
BOLLERS FOR PELLETS
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.
3.8.5 EFFICIENT WOOD BURNING TECHNIQUES FOR DEVELOPING COUNTRIES
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.
3.8.5.1 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.
COOKING WITH RETAINED HEAT
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.
THE HAYBOX COOKER
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.
BUILDING INSTRUCTIONS
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).
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INSTRUCTIONS FOR USE
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.
SAND/CLAY STOVES: THE LORENA SYSTEM
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.
Advantages:
* 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.
Disadvantages:
* 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.
KENYA STOVE
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.
3.8.5.2 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.
3.8.6.1 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
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.
3.8.7.1 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.
SOCIAL DEVELOPMENT
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 IMPACTS
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.
ECONOMICS
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.
3.8.7.2 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
DECOMPOSITION
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:
Moisture
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.
3.9.6.1 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.
DIGESTER
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.
GAS HOLDER OR GAS STORAGE CHAMBER
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.
INLET
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.
OUTLET
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.
MIXING TANK
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.
GAS OUTLET PIPE
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.
3.9.7.1 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.
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.
SEMI-CONTINUOUS DIGESTION
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.
SEMI-BATCH FED DIGESTION
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.
3.9.7.2 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
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.
| LAYERING | USEFUL FRACTIONS | |
| Gas | BIOGAS | Combustible gas |
| Fibrous | SCUM | Fertilizer |
| Liquid | SUPERNATANT | Biologically Active |
| Semi Solid | DIGESTED SLUDGE | Fertilizer |
| Solid | INORGANIC SOLIDS | Waste |
| LAYERING | USEFUL FRACTIONS | |
| Gas | BIOGAS | Combustible gas |
| Fibrous | SCUM | Fertilizer |
| Liquid | DIGESTED SLURRY | Fertilizer |
| Liquid | SLURRY IN DIFFERENT STAGES OF FERMENTATION | Biologically Active |
| Solid | INORGANIC SOLIDS | Waste |
| LAYERING | USEFUL FRACTIONS | |
| Gas | BIOGAS | Combustible gas |
| Fibrous | SCUM | Fertilizer |
| Liquid | DIGESTED SLURRY (EFFLUENT) | Fertilizer |
| Liquid | MIXTURE OF SUPERNATANT AND SLURRY IN DIFFERENT STAGES OF FERMENTATION | Biologically Active |
| Semi solid | DIGESTED SLUDGE | Fertuilizer |
| Solid | INORGANIC SOLIDS | Waste |
SUPERNATANT
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.
DIGESTED SLURRY (EFFLUENT)
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.
SLUDGE
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.
INORGANIC SOLIDS
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.
3.9.8.1 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.
3.9.8.2 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.
3.9.8.3 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.
3.9.8.4 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:
3.9.9.1 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:
FOUNDATION
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.
GAS HOLDER
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.
INLET PIPE
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.
OUTLET PIPE
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.
BIOGAS OUTLET PIPE
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:
Foundation
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.
Digester
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.
3.9.10.1 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.
3.9.10.2 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.
COMPONENTS OF SHRAMIK BANDHU (SBP-I MODEL) BIOGAS PLANT (BGP)
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
3.10.3.1 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.
Barriers
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
Economy
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.
Environment
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.
Employment
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.
Hand-rules
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.
Barriers
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.
Barriers
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
Economy
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 |
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):
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Employment
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.
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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.
Barriers
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
Environment
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.
Employment
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 :
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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
Barriers
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
Economy
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
Employment
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.
3.12 LITERATURE - BIOMASS
Ackley WB, Crandall PC, Russel TS (1958). The use of linear measurements
in estimating leaf areas. American Society for Horticultural Sciences 72:
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