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 BIOMASS |
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
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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. |
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.
Utilisation of biomass as the energy source in the world.
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.
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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:
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).
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.
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.
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.
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.
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.
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.
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).
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.
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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. |
Timber processing is a further source of wood residues. Dry sawdust and waste produced during the processing of cut timber make very good fuel. The British furniture industry is estimated to use 35 000 tonnes of such residues a year, one third of its production, providing 0,5 PJ of space and water heating and process heat (FOE, 1991). In Sweden, where biomass already provides nearly 15% of primary energy, forestry residues and wood industries contribute over 200 PJ/yr., mainly as fuel for CHP plant.
AGRICULTURAL 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.
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.
BIOMASS FUELS IN DEVELOPING
COUNTRIES
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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.
WOOD
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.
MANUALLY-FIRED BOILERS
BURN-THROUGH
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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
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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
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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.
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.
WOOD PELLETS AND WOOD CHIPS IN AUTOMATICALLY-FIRED BOILERS
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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
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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
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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. |
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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.
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.
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.
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.
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.
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.
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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.
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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.
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.
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OPEN FIRE used for cooking in the millions of rural homes transfers heat to a pot poorly. As little as 10 percent of the heat goes to the cooking utensil; the rest is released to the environment. |
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 (and 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).
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.
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.
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CERAMIC JIKO increases stove efficiency by addition of a ceramic insulating liner (the brown element), which enables 25 to 40 percent of the heat to be delivered to the pot. From 20 to 40 percent of the heat is absorbed by the stove walls or else escapes to the environment. In addition, 10 to 30 percent gets lost as flue gases, such as carbon dioxide. |
 |
The traditional metal stove that the ceramic Jiko replaces delivers only 10 to 20 percent of the heat generated to a pot, METAL STOVE , a traditional cooking implement, directs only 10 to 20 percent of the heat to a pot. From 50 to 70 percent of the heat is lost through the stove's metal sides, and another 10 to 30 percent escapes as carbon monoxide, methane and other flue gases. |
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.
| ADVATAGES OF CHARCOAL:Charcoal can be
produced from nearly any kind of plant-derived biomass material. Biomass can be
converted to charcoal with conversion yields of 40% to 60% compared to
current yields of 25% to 35%.High conversion
efficiencies mean less feedstock is required to produce the same amount of
charcoal, or conversely more charcoal is produced from the same amount of
feedstock. Charcoal can be
produced in 1 to 2 hours compared to days with conventional systems. |
 |
Wood
Gasification Basics
Wood gasification is also called producer gas generation and destructive
distillation. The essence of the process is the production of flammable gas
products from the heating of wood. Carbon monoxide, methyl gas, methane,
hydrogen, hydrocarbon gases, and other assorted components, in different
proportions, can be obtained by heating or burning wood products in an isolated
or oxygen poor environment. This is done by burning wood in a burner which
restricts combustion air intake so that the complete burning of the fuel cannot
occur. A related process is the heating of wood in a closed vessel using an
outside heat source. Each process produces different products. If wood were
given all the oxygen it needs to burn cleanly the by-products of the combustion
would be carbon dioxide, water,
some small amount of ash, (to account for the inorganic components of wood)
and heat. This is the type of burning we strive for in wood stoves. Once burning
begins though it is possible to restrict the air to the fuel and still have the
combustion process continue. Lack of sufficient oxygen caused by restricted
combustion air will cause partial combustion. In full combustion of a
hydrocarbon (wood is basically a hydrocarbon) oxygen will combine with the
carbon in the ratio of two atoms to each carbon atom. It combines with the
hydrogen in the ratio of two atoms of hydrogen to one of oxygen. This produces
CO2 (carbon dioxide) and H2O (water). Restrict the air to combustion and the
heat will still allow combustion to continue, but imperfectly. In this
restricted combustion one atom of oxygen will combine with one atom of carbon,
while the hydrogen will sometimes combine with oxygen and sometimes not combine
with anything. This produces carbon monoxide, (the same gas as car exhaust
and for the same reason) water, and hydrogen gas. It will also produce a lot of
other compounds and elements such as carbon which is smoke. Combustion of wood
is a bootstrap process. The heat from combustion breaks down the chemical bonds
between the complex hydrocarbons found in wood (or any other hydrocarbon fuel)
while the combination of the resultant carbon and hydrogen with
oxygen-combustion-produces the heat. Thus the process drives itself. If the air
is restricted to combustion the process will still produce enough heat to break
down the wood but the products of this inhibited combustion will be carbon
monoxide and hydrogen, fuel gases which have the potential to continue the
combustion reaction and release heat since they are not completely burned yet.
(The other products of incomplete combustion, predominately carbon dioxide and
water, are products of complete combustion and can be carried no further.) Thus
it is a simple technological step to produce a gaseous fuel from solid wood.
Where wood is bulky to handle, a fuel like wood gas (producer gas) is convenient
and can be burned in various existing devices, not the least of which is the
internal combustion engine. A properly designed burner combining wood and air is
one relatively safe way of doing this. so this water is available to play a part
in the destructive distillation process. Wood also contains many other chemicals
from alkaloid poisons to minerals. These also become part of the process.
As a general concept, destructive distillation of wood will produce methane
gas, methyl gas, hydrogen, carbon dioxide, carbon monoxide, wood alcohol,
carbon, water, and a lot of other things in small quantities. Methane gas might
make up as much as 75% of such a mixture. Methane is a simple hydrocarbon gas
which occurs in natural gas and can also be obtained from anaerobic bacterial
decomposition as “bio-gas” or “swamp gas”. It has high heat value and is
simple to handle. Methyl gas is very closely related to methyl alcohol (wood
alcohol) and can be burned directly or converted into methyl alcohol (methanol),
a high quality liquid fuel suitable for use in internal combustion engines with
very small modification. It’s obvious that both of these routes to the
production of wood gas, by incomplete combustion or by destructive distillation,
will produce an easily handled fuel that can be used as a direct replacement for
fossil fuel gases (natural gas or liquefied petroleum gases such as propane or
butane). It can be handled by the same devices that regulate natural gas and it
will work in burners or as a fuel for internal combustion engines with some very
important cautions.
Producer Gas Generators
The simplest device is a tank shaped like an inverted cone (a funnel). A hole
at the top which can be sealed allows the user to load sawdust into the tank.
There is an outlet at the top to draw the wood gas off. At the bottom the point
of the “funnel” is opened and this is where the burning takes place. Once
loaded (the natural pack of the sawdust will keep it from falling out the
bottom) the sawdust is lit from the bottom using a device such as a propane
torch. The sawdust smoulders away. The combustion is maintained by a source of
vacuum applied to the outlet at the top, such as a squirrel cage blower or an
internal combustion engine. Smoke is drawn up through the porous sawdust, being
partly filtered in the process, and exits the burner at the top where it goes on
to be further conditioned and filtered. The vacuum also draws air in to support
the fire. This burner is crude and uncontrollable, especially as combustion
nears the top of the sawdust pile. This can happen rapidly since there is no
control to assure that the sawdust burns evenly. “Leads” of fire can form in
the sawdust reaching toward the top surface. Once the fire breaks through the
top of the sawdust the vacuum applied to the burner will pull large amounts of
air in supporting full combustion and leaning out the value of the producer gas
as a fuel. This process depends on the poor porosity of the sawdust to control
the combustion air so chunk wood cannot be used since its much greater porosity
would allow too much air in and user would achieve full combustion at very high
temperatures rather than the smouldering and the partial combustion needed. Such
a burner is unsatisfactory for prolonged gas generation but it is cheap to build
and it will work with a lot of fiddling. For prolonged trouble free operation of
a wood gas generator the burner unit must have more complete control of the
combustion air and the fuel feed. There are various ways to do this. For
example, if the point of above mentioned original funnel shaped burner is
completely enclosed then control over the air entering the burner can be
achieved. This configuration will successfully burn much larger amount of wood.
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.
Digestible Property of Organic Matter
When organic raw materials are digested in an airtight container only a
certain percentage of the waste i
