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, producing electicity or fuelling a car.iomass as the solar energy stored in chemical form in plant and animal materials is among the most precious and versatile resources on earth. It provides not only food but also energy, building materials, paper, fabrics, medicines and chemicals. Biomass has been used for energy purposes ever since man discovered fire. Today, biomass fuels can be utilised for tasks ranging from heating the house to fuelling a car and running a computer.

THE CHEMICAL COMPOSITION OF BIOMASS

The chemical composition of biomass varies among species, but plants consists of about 25% lignin and 75% carbohydrates or sugars . The carbohydrate fraction consists of many sugar molecules linked together in long chains or polymers. Two larger carbohydrate categories that have significant value are cellulose and hemicellulose. The lignin fraction consists of non-sugar type molecules. Nature uses the long cellulose polymers to build the fibers that give a plant its strength. The lignin fraction acts like a “glue” that holds the cellulose fibers together.

WHERE DOES BIOMASS COME FROM?
Carbon dioxide from the atmosphere and water from the earth are combined in the photosynthetic process to produce carbohydrates (sugars) that form the building blocks of biomass. The solar energy that drives photosynthesis is stored in the chemical bonds of the structural components of biomass. If we burn biomass efficiently (extract the energy stored in the chemical bonds) oxygen from the atmosphere combines with the carbon in plants to produce carbon dioxide and water. The process is cyclic because the carbon dioxide is then available to produce new biomass. 

In addition to the aesthetic value of the planet’s flora, biomass represents a useful and valuable resource to man. For millennia humans have exploited the solar energy stored in the chemical bonds by burning biomass as fuel and eating plants for the nutritional energy of their sugar and starch content. More recently, in the last few hundred years, humans have exploited fossilized biomass in the form of coal. This fossil fuel is the result of very slow chemical transformations that convert the sugar polymer fraction into a chemical composition that resembles the lignin fraction. Thus, the additional chemical bonds in coal represent a more concentrated source of energy as fuel. All of the fossil fuels we consume - coal, oil and natural gas - are simply ancient biomass. Over millions of years, the earth has buried ages-old plant material and converted it into these valuable fuels. But while fossil fuels contain the same constituents - hydrogen and carbon - as those found in fresh biomass, they are not considered renewable because they take such a long time to create. Environmental impacts pose another significant distinction between biomass and fossil fuels. When a plant decays, it releases most of its chemical matter back into the atmosphere. In contrast, fossil fuels are locked away deep in the ground and do not affect the earth’s atmosphere unless they are burned.

Wood may be the best-known example of biomass. When burned, the wood releases the energy the tree captured from the sun’s rays. But wood is just one example of biomass. Various biomass resources such as agricultural residues (e.g. bagasse from sugarcane, corn fiber, straw and even 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.

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 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 world biomass content - 1880 billion tonnes
Total mass in tropical forests -1030 billion tonnes
Total mass in temperate forests, savanna and tundra - 790 billion tonnes
Total marine biomass content - 4 billion tonnes
Per capita terrestrial biomass - 310 tonnes
Energy stored in terrestrial biomass - 25 000 EJ
Net annual production of of terrestrial biomass - 400 billion 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)

often used 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.

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. 

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 bellow). 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.

Energy contents comparison table.

FUEL	Content of water %	MJ/kg	kW/kg
Oak- tree	20	14,1	3,9
Pine-tree	20	13,8	3,8
Straw	15	14,3	3,9
Grain	15	14,2	3,9
Rape oil	-	37,1	10,3
Hard coal	4	30,0-35,0	8,3
Brown coal	20	10,0-20,0	5,5
Heating oil	-	42,7	11,9
Bio methanol	-	19,5	5,4

FUEL	MJ/Nm3	kWh/Nm3
Sewer gas	16,0	4,4
Wood gas	5,0	1,4
Biogas from cattle dung	22,0	6,1
Natural gas	31,7	8,8
Hydrogen	10,8	3,0

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 2001, 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 like coal.  Acid rain has been implicated in the killing of lakes, as well as impacting humans and wildlife in other ways.  Since biomass has practically 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 energy fuel. This is because of the general acceptance that coal, oil, natural gas 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 12,5 billion 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 is around 3,4 billion m3/yr. (equivalent to 40 EJ/yr.), so some of the unused increment could be recovered for energy purposes while maintaining or possibly even enhancing the productivity of forests.

Operations such as thinning of plantations and trimming of felled trees generate large volumes of forestry residues. At present these are often left to rot on site - even in countries with fuelwood shortages. They can be collected, dried and used as fuel by nearby rural industry and domestic consumers, but their bulk and high water content makes transporting them for wider use uneconomic. In developing countries where charcoal is an important fuel, on-site kilns can reduce transport costs. Mechanical harvesters and chippers have been developed in Europe and North America over the last 15 years to produce uniform 30-40 mm wood chips which can be handled, dried and burned easily in chip-fired boilers.

The use of forest residues to produce hot water or steam for heating and/or power generation is now a growing business in many countries. American electricity utilities have more than 9 000 MW (output of 9 nuclear power plants) of biomass-fired generating plant on line, much of it constructed in the last ten years. Austria has about 1250 MW of wood-fired heating capacity in the form of domestic stoves and district heating plant, burning waste wood, bark and wood chips. Most of these district heating systems are of 1-2 MW capacity, with a few larger units (around 15 MW) and a number of small-scale CHP systems.

Timber processing is a further source of wood residues. Dry sawdust and waste produced during the processing of cut timber make very good fuel. The British furniture industry is estimated to use 35 000 tonnes of such residues a year, one third of its production, providing 0,5 PJ of space and water heating and process heat. 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 million tonnes of dung and crop residues is  used as fuel in India, 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

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 (World health Organisation) 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. Recently almost 1,3 billion people is meeting their fuelwood needs by depleting wood reserves.

Share of biomass on total energy consumption:

Nepal    95 %
Malawi  94 %
Kenya   75 %
India      50 %
China     33 %
Brazil     25 %
Egypt     20 %

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

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 favorable 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 a 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. 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, according to some estimates 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 technology unit could 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 2004 was around 530 TWh . 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. 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 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 energy needs 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 heating purposes, chemical processing or  to generate steam for power production. For other processes the product is a solid, liquid or gaseous fuel: charcoal, liquid fuel like ethanol 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 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 coal 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 CO2, and the interesting potential of this lies in the fact that removal of the CO2 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 mainly 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.

BIOGAS
Biogas is a valuable fuel which is in many countries produced in purpose built digesters filled with the feedstock like manure or sewage. Digesters range in size from one cubic meter 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 can produce 200-400 m3 of biogas with a methane content of 50% to 75% for each dry tonne of input.

Digestors - outside view.	Digestor from inside.

Biogas plant with integrated gas holder.	Biogas plant with separate gas holder.

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. Landfill sites that 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.

TECHNOLOGY EXAMPLES

Heating buildings with biomass

• utilising of a stove to heat an individual room, or
• a boiler that will be connected to the central heating and hot water systems in the house.

 Stand-alone woodburning stoves can usually produce 6-12kW of heat and can generally be fuelled by either logs or pellets. There are also an automatic-feed stoves on the market for which only pellets are suitable as the fuel. Usually wood-burning stoves are placed in the living room of a house where they can provide extra heating. Although many only serve for one room heater, higherpower types can also be connected to their own back boiler to provide hot water.

Boilers burning wood or pellets are larger systems providing heat for the whole house and therefore need to generate higher levels of heat than stoves. A typical domestic system has the capacity of 15kW and more. There are lots of different systems on the market.Log-fuelled boilers have to be filled up manually and will need large storage space for the logs. Automatic-feed boilers can take either wood chips or pellets – or sometimes both – and come with their own storage system where thefuel is stored.

District heating systems
District heating system means the central biomass plant which provides heat to several public, commercial, and residential buildings located around in the area near the heating plant. These heating systems offer a many advantages over the use of individual heating boilers in houses. Efficiency and the level of automation are the most important ones. Higher efficiency in central heating plants leads to lower fuel consumption, labour requirements and also emissions. On the other hand district heating systems may have some limitation in order to accomodate new consumers when theete is not sufficient capacity (output power, piping etc).

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 for hot water 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. Boilers designed for fuelwood should always be equipped with storage tank for hot water. 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

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

UNDERBURN BOILERS

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

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.  For more technical data for Central European conditions see the table bellow.

Power output (kW)	Wood consumption  (kg/hr)	Wood consumption in heating season  (m3)
18	4	10
25	6	15
32	7	20
50	13	30
80	18	50

Wood heating value 15-18 MJ/kg.

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

BURNING WOOD COMBINED WITH SOLAR HEATING

If the building is equipped with the 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. 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 use solar energy than to burn the wood!

FUEL CHOICE
Whatever fuel is chosen for 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 the 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 that there is not enough air flowing through. 

WOOD PELLETS AND WOOD CHIPS IN AUTOMATICALLY-FIRED BOILERS

The automatic boiler is connected to the central heating system in exactly the same way as e.g. 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 only heats the room it is in. Pellets and wood-chips are of a size and shape that make them ideal for automatic boilers, since they can be fed in directly from a bunker. This makes it much easier to stoke, since the bunker only needs filling up once or twice a week. In hand-fired units like wood burning boilers, one has to stoke up several times a day - though they are usually cheaper to buy than automatic ones.

WOOD PELLETS

Wood pellets are a comparatively new and attractive form of fuel. When people burn wood pellets, they 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.

Large boiler (2,5 MW) for wood pellets or chips is used in district heating systems.

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 users 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 20.000 buildings in Germany (3% of all boilers) and in over 500 000 homes in North America.  

Pellets are delivered to the custumer at the begining of the heating season.

Annual pellet consumption in selected countries in 2006.

WOOD CHIPS

Wood-chips are made of waste wood from the forests. Trees have to be thinned to make room for commercial timber (beams, flooring, furniture). Wood-chips are thus a waste product of normal forestry operations.  Wood is cut up in mechanical chippers. The size and shape of the chips depends on the machine, but they are typically about a centimeter 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. Careful processing and drying enable optimal storage and trouble-free operation of heating systems with negligible production of ash and low emissions.

Wood chiper.	Wood briquettes.

Wood chips are usually locally produced and environmentally friendly energy source which are not subject to crises. Their utilisation also secure domestic jobs. It is mainly local farmers who are involved with the production and sales of wood chips.  And for their production not one single tree more has to be cut down. Every year, more wood grows in forrests than is used thus all kinds of waste wood material is suitable for use as wood chips including wood damaged by storms. In many countries like in Denmark wood-chips currently produced are burnt in wood-chip fired district heating stations. For the utilisation in single family houses 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.

Wood logs, chips and pellets
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 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.

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

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

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. Despite all problems with the straw there is a huge number of straw-fired district heating plants all around the world.

In Denmark alone there was 60 district straw burning heating plants 10 combined heat and power production plants and more than 10 000 straw fired boilers for farms. Annual straw harvest in this country reached 6 million tonnes in the year 2000 and approx. 24% (1,5 mikl. tonnes) was used for energy production.

The output power of larger straw burning systems 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.

Batch-fired boilers used to cause many problems when fed with straw of inferior quality and the supply of combustion air was difficult to control. In recent models, however, the control problem has eventually been solved but the water content of the straw must still be kept below 15- l8 %. Today, an efficiency of 75% and a CO content below 0.5% is possible in batch-fired boilers. About l0 years ago, the efficiency was only 35%.

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

BOILERS FOR BALES OF STRAW
Units consisting of a scarifier/cutter have been developed which separate the bales, parting them into pieces of varying sizes. The bales are fed into this unit on a conveyor. The volume of straw treated is often regulated by merely modifying the velocity of the conveyor. The straw is transported from the scarifier/cutter by worm conveyors or blowers. If blowers are used, the distance to the boiler can be greater than with worms but this equipment also consumes more energy.
The scarifier does not actually cut or shred the straw but it separates the straw into the segments it was compacted into by the piston of the baler. In order to ensure a steady flow of straw through the transport system, the scarifier usually has a retaining device. Most scarifiers have knives to loosen the straw without creating large lumps. 

In automatically fired boilers, combustion takes places as the straw is fed into the boiler. The air supply is adapted to the straw volume by means of an adjustable damper on a blower. This ensures a good combustion, a significantly improved utilization factor, and a corresponding reduction of particle emission problems as compared with the first manually fired boilers without air regulating devices. Straw ignites easily in an automatic boiler because fresh straw is supplied continuously.

BOLLERS FOR STRAW PELLETS
The use of straw pellets for energy production is growing 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.

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 1/4, 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. 
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 longer cooking times (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).

Good Insulating Materials	Suggested Wall Thickness
Cork	5 cm
Polystyrene sheets/pellets/drinking cups	5 cm
Hay/straw/rushes	10 cm
Sawdust/wood shavings	10 cm
Wool/fur	10 cm
Fiberglas/glass wool	10 cm
Shredded newspaper/cardboard	10 cm
Rice hulls/nut shells	15 cm

The inner box should have a reflective interior: aluminium foil, shiny aluminium sheeting, old printing plates, other polished sheet metal’ or silver paint will all work. The box can be wooden, or a can-in-a-can, or cardboard, or any combination; a pair of cloth bags might also work. Be inventive. Always be sure the lid is air tight.

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.
 

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 is actually converted into Biogas and Digested Manure. Some of it is indigestible to varying degree and either gets accumulated inside the digester or discharged with the effluent. The digestibility and other related properties of the organic matter are usually expressed in the following terms:

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

Total Solids (TS)
The TS is the “Dry Weight” of the OM (Note: after the sun drying the weight of OM still contains about 20% moisture). A figure of 10% TS means that 100 grams of sample will contain 10 grams of moisture and 90 grams of dry weight. The Total Solids (TS) consists of Digestible Organic (or Volatile Solids) and the indigestible solid (Ash).

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

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

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

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

Property of Biogas
Biogas burns with a blue flame. It has a heat value of 23 MJ/m3) when its methane content is in the range of 60-70%. The value is directly proportional to the amount of methane content and this depends upon the nature of raw materials used in the digestion. Since the composition of this gas is different, the burners designed for coal gas, butane or LPG when used, as ‘biogas burner’ will give much lower efficiency. Therefore specially designed biogas burners are used which give a thermal efficiency of 55-65%.
Biogas is a very stable gas, which is a non-toxic, colourless, tasteless and odourless gas. However, as biogas has a small percentage of Hydrogen Sulphide, the mixture may very slightly smell of rotten egg, which is not often noticeable especially when being burned. When the mixture of methane and air (oxygen) burn a blue flame is emitted, producing large amount of heat energy. Because of the mixture of Carbon Dioxide in large quantity the biogas becomes a safe fuel in rural homes as will prevent explosion.
A 1 m3 biogas, when burned, will generate heat energy sufficient to bring approx. 100 litre of water from 20 degrees Celsius to a boil, or light a lamp with a brightness equivalent to 60-100 Watts for 4-5 hours.

Mechanics of Extraction of Biogas
The decomposition process for the formation of methane from organic material (biodegradable material) involves a group of organisms belonging to the family - ‘Methane Bacteria’ and is a complex biological and chemical process. The biogas production involves two major processes consisting of acid formation (liquefaction) and gas formation (gasification). However scientifically speaking these two broad process can further be divide, which gives four stages of anaerobic fermentation inside the digester-they are (i) Hydrolysis, (ii) Acidification, (iii) Hydrogenation and (iv) Methane Formation. At the same time for all practical purposes one can take the methane production cycle as a three stage activity- namely, (i) Hydrolysis, (ii) Acidification and (iii) Methane formation.
Two groups of bacteria work on the substrate (feedstock) inside the digester-they are (i) Non-methanogens and (ii) Methanogens. Under normal conditions, the non-methanogenic bacteria (microbes) can grow at a pH range of 5,0 - 8,5 and a temperature range of 25-42  deg. ;whereas, methanogenic bacteria can ideally grow at a pH range of 6.5-7.5 and a temperature range of 25-35 degrees Celsius. These methanogenic bacteria are known as ‘Mesophillic Bacteria’ and are found to be more flexible and useful in case of simple household digesters, as they can work under a broad range of temperature, as low as 15 degrees Celsius to as high as 40 degrees Celsius. However their efficiency goes down considerably if the slurry temperature goes below 20 degrees Celsius and almost stop functioning at a slurry temperature below 15 degrees Celsius. Due to this Mesophillic Bacteria can work under all the three temperature zones of e.g. India, without having to provide either heating system in the digester or insulation in the plant, thus keeping the cost of family size biogas plants at an affordable level.

There are other two groups of anaerobic bacteria-they are (i) Pyscrophillic Bacteria and (ii) Thermophillic Bacteria. The group of Pyscrophillic Bacteria work at low temperature, in the range of 10-15 degrees Celsius but the work is still going on to find out the viability of these group of bacteria for field applications. The group of Thermophillic Bacteria work at a much higher temperature, in the range of 45-55 degrees Celsius and are very efficient, however they are more useful in high rate digestions (fermentation), especially where a large quantity of effluent is already being discharged at a higher temperature. As in both the cases the plant design needs to be sophisticated therefore these two groups of Bacteria (Pyscrophillic & Thermophillic) are not very useful in the case of simple Indian rural biogas plant.

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

Components of Biogas Plant
The major components of BGP are - (i) Digester, (ii) Gasholder or Gas Storage Chamber, (iii) Inlet, (iv) Outlet, (v) Mixing Tank and (vi) Gas Outlet Pipe.

DIGESTER
It is either an under ground Cylindrical-shaped or Ellipsoidal-shaped structure where the digestion (fermentation) of substrate takes place. The digester is also known as ‘Fermentation Tank or Chamber’. In a simple Rural Household BGP working under ambient temperature, the digester (fermentation chamber) is designed to hold slurry equivalent to of 55, 40 or 30 days of daily feeding. This is known as Hydraulic Retention Time (HRT) of BGP. The designed HRT of 55, 40 and 30 days is determined by the different temperature zones in country like  India. The digester can be constructed of brick masonry, cement concrete or reinforced cement concrete or stone masonry or pre-fabricated cement concrete blocks or Ferro-cement (ferroconcrete) or steel or rubber or bamboo reinforced cement mortar. In the case of smaller capacity floating gasholder plants of 2 & 3 m3 no partition wall is provided inside the digester, whereas the BGPs of 4 m3 capacity and above have been provided partition wall in the middle. This is provided for preventing short-circuiting of slurry and promoting better efficiency. This means the partition wall also divides the entire volume of the digester (fermentation chamber) into two halves. As against this no partition wall is provided inside the digester of a fixed dome design. The reason for this is that the diameter of the digesters in all the fixed dome models are comparatively much bigger than the floating drum BGPs, which takes care of the short-circuiting problems to a satisfactory level, without adding to additional cost of providing a partition wall.

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

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

OUTLET
In the case of floating gas holder pipe the Outlet is made of cement concrete pipe standing at an angle, which reaches the bottom of the digester on the opposite side of the partition wall. In smaller plants (2 & 3 m3 capacity BGPs) which has no partition walls, the outlet is made of small (approx. 50 cm length) cement concrete pipe inserted on top most portion of the digester, submerged in the slurry.
In the two fixed dome (Janata & Deenbandhu models) plants, the Outlet is made in the form of rectangular tank. However, in the case of Shramik Bandhu model the upper portion of the Outlet (known as Outlet Displacement Chamber) is made hemi-spherical in shape, designed to save in the material and labour cost. In all the three-fixed dome models (Janata, Deenbandhu & Shramik Bandhu models), the bottom end of the outlet tank is connected to the outlet gate. There is a small opening provided on the outer wall of the outlet chamber for the automatic discharge of the digested slurry outside the BGP, equal to approximately 80-90% of the daily feed. The top mouth of the outlet chamber is kept covered with heavy slab.

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

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

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

Type of Digestion
The digestion of organic materials in simple rural household biogas plants can be classified under three broad categories. They are:
1. Batch-fed digestion
2. Semi-continuous digestion and
3. Semi-batch-fed digestion.

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

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

SEMI-BATCH FED DIGESTION
A combination of batch and semi-continuous digestion is known as Semi-batch fed Digestion. Such a digestion process is used where the manure from domestic farm animals is not sufficient to operate a plant and at the same time organic waste like, crop residues, agricultural wastes (paddy & weed straw), water hyacinths and weeds etc, are available during the season. In as Semi-batch fed Digestion the initial loading is done with green or semi-dry or dry biomass (that can not be reduced in to slurry form) mixed with starter and the digester is sealed. This plant also has an inlet pipe for daily feeding of manure slurry from animals. The Semi-batch fed Digester will have much longer digestion cycle of gas production as compared to the batch-fed digester. It is ideally suited for the poor peasants having 1-2 cattle or 3-4 goats to meet the major cooking requirement and at the end of the cycle (6-9 months) will give enriched manure in the form of digested sludge.

Stratification (Layering) of Digester due to Anaerobic Fermentation
In the process of digestion of feedstock in a BGP many by-products are formed. They are biogas, scum, supernatant, digested slurry, digested sludge and inorganic solids. If the content of Biogas Digester is not stirred or disturbed for a few hours then these by-products get formed into different layers inside the digester. The heaviest by-product, which is Inorganic Solids will be at the bottom most portion, followed by Digested Sludge.

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

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

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

SLUDGE
In the batch-fed or semi batch-fed digester where the plant wastes and other solid organic materials are added, the digested material contains less effluent and more of sludge. The sludge precipitates at the bottom of the digester and is formed mostly of the solids substances of plant wastes. The sludge is usually composted with chemical fertilizers as it may contain higher percentage of parasites and pathogens and hookworm eggs of etc., especially if the semi-batch digesters are either connected to the pigsty or latrines. Depending upon the raw materials used and the conditions of the digestion, the sludge contains many elements essential to the plant life e.g. Nitrogen, Phosphorous, Potassium plus a small quantity of Salts (trace elements), indispensable to the plant growth- the trace elements such as boron, calcium, copper, iron, magnesium, sulphur and zinc etc. The fresh digested sludge has high ammonia content and in this state may act like a chemical fertiliser by forcing a large dose of nitrogen than required by the plant and thus increasing the accumulation of toxic nitrogen compounds. For this reason, it is probably best to let the sludge age for about two weeks in open place. The fresher the sludge the more it needs to be diluted with water before application to the crops, otherwise very high concentration of nitrogen my kill the plants.

INORGANIC SOLIDS
In village situation the floor of the animals shelters are full of dirt, which gets mixed with the manure. Added to this the collected manure is kept on the unlined surface which has plenty of mud and dirt. Due to all this the feed stock for the BGP always has some inorganic solids, which goes inside the digester along with the organic materials. The bacteria can not digest the inorganic solids, and therefore settles down as a part of the bottom most layer inside the digester. The Inorganic Solids contains mud, ash, sand, gravel and other inorganic materials. The presence of too much inorganic solids in the digester can adversely affect the efficiency of the BGP. Therefore to improve the efficiency and enhance the life of a semi-continuous BGP it is advisable to empty even it in a period of 5-10 years for thoroughly cleaning and washing it from inside and then reloading it with fresh slurry.

Classification of Biogas Plants
The simple rural household BGPs can be classified under the following broad categories- (i) BGP with Floating Gas Holder, (ii) BGP with Fixed Roof, (iii) BGP with Separate Gas Holder and (iv) Flexible Bag Biogas Plants.

Biogas Plant with Floating gas Holder
This is one of the common designs in India and comes under the category of semi-continuous-fed plant. It has a cylindrical shaped floating biogas holder on top of the well-shaped digester. As the biogas is produced in the digester, it rises vertically and gets accumulated and stored in the biogas holder at a constant pressure of 8-10 cm of water column. The biogas holder is designed to store 50% of the daily gas production. Therefore if the gas is not used regularly then the extra gas will bubble out from the sides of the biogas holder.

Fixed Dome Biogas Plant
The plants based on Fixed Dome concept were developed in India in the middle of 1970, after a team of officers visited China. The Chinese fixed dome plants use seasonal crop wastes as the major feed stock for feeding, therefore, their design is based on principle of ‘Semi Batch-fed Digester’. However, the Indian Fixed Dome BGPs designs differ from that of Chinese designs, as the animal manure is the major substrate (feed stock) used in India. Therefore all the Indian fixed dome designs are based on the principle of ‘Semi Continuous-fed Digester’. While the Chinese designs have no fixed storage capacity for biogas due to use of variety of crop wastes as feed stock, the Indian household BGP designs have fixed storage capacity, which is 33% of the rated gas production per day. The Indian fixed dome plant designs use the principle of displacement of slurry inside the digester for storage of biogas in the fixed Gas Storage Chamber. Due to this in Indian fixed dome designs have ‘Displacement Chamber(s)’, either on both Inlet and Outlet sides (like Janata Model) or only on the Outlet Side (like Deenbandhu or Shramik Bandhu Model). Therefore in Indian fixed dome design it is essential to keep the combined volume of Inlet & Outlet Displacement Chamber(s) equal to the volume of the fixed Gas Storage Chamber, otherwise the desired quantity of biogas will not be stored in the plant. The pressure developed inside the Chinese fixed dome BGP ranges from a minimum of 0 to a maximum of 150 cm of water column. And the maximum pressure is normally controlled by connecting a simple Manometer on the pipeline near the point of gas utilisation. On the other hand the Indian fixed dome BGPs are designed for pressure inside the plant, varying from a minimum of 0 to a maximum of 90 cm of water column. The Discharge Opening located on the outer wall surface of the Outlet Displacement Chamber and automatically controls the maximum pressure in the Indian design.

Biogas Plant with Separate Gas Holder
The digester of this plant is closed and sealed from the top. A gas outlet pipe is provided on top, at the centre of the digester to connect one end of the pipeline. The other end of the pipeline is connected to a floating biogas holder, located at some distance to the digester. Thus unlike the fixed dome plant there is no pressure exerted on the digester and the chances of leakage in the Main Unit of the Plant are not there or minimised to a very great extent. The advantage of this system is that several digesters, which only function as digestion (fermentation) chambers (units), can be connected with only one large size gas holder, built at one place close to the point of utilisation. However, as this system is expensive therefore, is normally used for connecting a battery of batch-fed digesters to one common biogas holder.

Flexible Bag Biogas Plant
The entire Main Unit of the Plant including the digester is fabricated out of Rubber, High Strength Plastic, Neoprene or Red Mud Plastic. The Inlet and Outlet is made of heavy duty PVC tubing. A small pipe of the same PVC tubing is fixed on top of the plant as Gas Outlet Pipe. The Flexible Bag Biogas Plant is portable and can be easily erected. Being flexible, it needs to be provided support from outside, up to the slurry level, to maintain the shape as per its design configuration, which is done by placing the bag inside a pit dug at the proposed site. The depth of the pit should as per the height of the digester (fermentation chamber) so that the mark of the initial slurry level is in line with the ground level. The outlet pipe is fixed in such a way that its outlet opening is also in line with the ground level. Some weight has to be added on the top of the bag to build the desired pressure to convey the generated gas to the point of utilisation. The advantage of this plant is that the fabrication can be centralised for mass production, at the district or even at the block level. Individuals or agencies having land and some basic infrastructure facilities can take up fabrication of this BGP with small investment, after some training. However, as the cost of good quality plastic and rubber is high which increases the comparative cost of fabricating it. Moreover the useful working life of this plant is much less, compared to other Indian simple Household BGPs, therefore inspite of having good potential, the Flexible Bag Biogas Plant has not been taken up seriously for promotion by the agencies.

Common Indian Biogas Plant (BGP) Designs
The three of the most common Indian BGP design are:
1. KVIC Model,
2. Janata Model and
3. Deenbandhu Model

The KVIC Model is a floating biogas holder semi continuous-fed BGP and has two types (i) Vertical and (ii) Horizontal. The vertical type is more commonly used and the horizontal type is only used in the high water table region. Though the description of the various components mentioned under this section are common to both the types of KVIC models (Vertical and Horizontal types) some of the details mentioned pertains to Vertical type only. The major components of the KVIC Model are briefly described below:

FOUNDATION
It is a compact base made of a mixture of cement concrete and brick ballast. The foundation is well compacted using wooden ram and then the top surface is cemented to prevent any percolation & seepage.

Digester (Fermentation Chamber)
It is a cylindrical shaped well like structure, constructed using the foundation as its base. The digester is made of bricks and cement mortar and its inside walls are plastered with a mixture of cement and sand. The digester walls can also be made of stone blocks in places where it is easily available and cheap instead of bricks. All the vertical types of KVIC Model of 4 M3 capacity and above have partition wall inside the digester.

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

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

OUTLET PIPE
The outlet pipe is made out of Cement Concrete or Asbestos Cement Concrete or Pipe. The one end of the outlet pipe is connected to the Outlet Tank and the other end goes inside the digester, on the outlet side of the partition wall and rests on a support made of bricks of about 30 cm height. In the case KVIC model of 3 m3 capacity and below, there is no partition wall, hence the outlet pipe is made of short and horizontal, which rest fully immersed in slurry at the top surface of the digester.

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

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

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

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

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

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

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

Biogas Outlet Pipe
The Biogas Outlet Pipe is fixed at the crown of the dome, which is made of a small length of pipe fitted with socket and a Gate Valve.

Deenbandhu Model
The Deenbandhu Model is a semi continuous-fed fixed dome Biogas plant. While designing the Deenbandhu model an attempt was made to minimise the surface area of the BGP with a view to reduce the installation cost, without compromising on the efficiency. The design essentially consists of segments of two spheres of different diameters joined at their bases. The structure thus formed comprises of (i) the digester (fermentation chamber), (ii) the gas storage chamber, and (iii) the empty space just above the gas storage chamber. The higher compressive strength of the brick masonry and concrete makes it preferable to go in for a structure that could be always kept under compression. A spherical structure loaded from the convex side will be under compression and therefor, the internal load will not have any effect on the structure.
The digester of the Deenbandhu BGP is connected with the Inlet Pipe and the Outlet Tank. The upper part (above the normal slurry level) of the outlet tank is designed to accommodate the slurry to be displaced out of the digester (actually from the gas storage chamber) with the generation and accumulation of biogas and known as the Outlet Displacement Chamber. The Inlet Pipe of the Deenbandhu BGP replaces the Inlet Chamber of Janata Plant. Therefore to accommodate all the slurry displaced out from the Gas Storage Chamber, the volume of the Outlet Chamber of Deenbandhu model is twice the volume of the Outlet Tank of the Janata BGP of the same capacity.
Being a fixed dome technology, the other components and their functions are same as in the case of Janata Model BGP and therefore are not elaborated here once again.

Shramik Bandhu Model
This new biogas plant model which is also a semi-continuous hydraulic digester plant was designed by the author and christened as SHRAMIK BANDHU. Since then, three more models (rural household plants) in the family of SHRAMIK BANDHU Plants have also been developed. The second one, a semi-continuous hydraulic digester, works on the principle of semi-plug flow digester (suitable for use as a Night Soil based or Toilet attached plant). The third one uses simple low cost anaerobic bacterial filters, designed for possible application as a low cost and low maintenance wastewater treatment system. The fourth one is a semi-batch fed hydraulic digester, ideally suitable for the regions where plenty of seasonal crop wastes and waste green biomass are available and population of domestic farm animals are less, for producing the desired quantity of biogas from it alone.
The family of SHRAMIK BANDHU biogas plants designs uses the fixed dome concepts as in the case of pervious two most popular Indian fixed dome plants, namely, Janata and Deenbandhu models. In other words, all the four Models of the family of SHRAMIK BANDHU Plant have both, (i) the Gas Storage Chamber (GSC) and (ii) the Dome shaped Roof. However, in this section, the description about Shramik Bandhu plants relates to SBP-I model only.

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

Gasification is the newest method to generate electricity from biomass.  Instead of simply burning the fuel, gasification captures about 65-70% of the energy in solid fuel by converting it first into combustible gases.  This gas is then burned as natural gas is, to create electricity, fuel a vehicle, in industrial applications, or converted to synfuels-synthetic fuels.  Since this is the latest technology, there is still a lot of researc development going on.
A promising alternative is the gas turbine fuelled by gas produced from biomass by means of thermochemical decomposition in an atmosphere that has a restricted supply of air. Gas turbines have lower unit-capital costs, can be considerably more efficient and have good prospects for improvements of both parameters.
Biomass gasification systems generally have four principal components:
(a) Fuel preparation, handling and feed system;
(b) Gasification reactor vessel;
(c) Gas cleaning, cooling and mixing system;
(d) Energy conversion system (e.g., internal-combustion engine with generator or pump set, or gas burner coupled to a boiler and kiln).

When gas is used in an internal-combustion engine for electricity production (power gasifiers), it usually requires elaborate gas cleaning, cooling and mixing systems with strict quality and reactor design criteria making the technology quite complicated. Therefore, power gasifiers world-wide have had a historical record of sensitivity to changes in fuel characteristics, technical hitches, manpower capabilities and environmental conditions.
Gasifiers used simply for heat generation do not have such complex requirements and are, therefore, easier to design and operate, less costly and more energy- efficient. All types of gasifiers require feedstocks with low moisture and volatile contents. Therefore, good quality charcoal is generally best, although it requires a separate production facility and gives a lower overall efficiency.
 

In the simplest, open-cycle gas turbine the hot exhaust of the turbine, is discharged directly to the atmosphere. Alternatively, it can be used to produce steam in a heat recovery steam generator. The steam can then be used for heating in a cogeneration system; for injecting back into the gas turbine, thus improving power output and generating efficiency known as a steam-injected gas turbine (STIG) cycle; or for expanding through a steam turbine to boost power output and efficiency - a gas turbine/steam turbine combined cycle (GTCC). While natural gas is the preferred fuel, limited future supplies have stimulated the expenditure of millions of dollars in research and development efforts on the thermo-chemical gasification of coal as a gas-turbine feedstock. Much of the work on coal-gasifier/gas-turbine systems is directly relevant to biomass integrated gasifier/gas turbines (BlG/GTs). Biomass is easier to gasify than coal and has a very low sulphur content. Also, BIG/GT technologies for cogeneration or stand-alone power applications have the promise of being able to produce electricity at a lower cost in many instances than most alternatives, including large centralized, coal-fired, steam-electric power plants with flue gas desulphurization, nuclear power plants, and hydroelectric power plants.
Gasifiers using wood and charcoal (the only fuel adequately proved so far) are again becoming commercially available, and research is being carried out on ways of gasifying other biomass fuels (such as residues) in some parts of the world. Problems to overcome include the sensitivity of power gasifiers to changes in fuel characteristics, technical problems and environmental conditions. Capital costs can still sometimes be limiting, but can be reduced considerably if systems are manufactured locally or use local materials. For example, a ferrocement gasifier developed at the Asian institute of Technology in Bangkok had a capital cost reduced by a factor of ten. For developing countries, the sugarcane industries that produce sugar and fuel ethanol are promising targets for near-term applications of BIG/GT technologies.

CO-FIRING

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

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

Unused Forest Energy Potential & Fuelwood

Most commercial forests e.g. in Europe have an unused energy potential, which can be used without endangering their role in the natural eco-systems. Beside this, most forests already have a production of firewood. Mountain forests and other less commercial forests can in certain cases also deliver wood for energy, but only after due environmental consideration.

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

Energy content
The energy content in totally dry wood is apr. 5,2 kWh/kg. In normally dry firewood (20% humidity) the energy content is apr. 4,2 kWh/kg (lower heating value). In most statistics, wood is measured in cubic meter solid wood (with or without bark). The density of dry wood varies from 800 kg/m3 for hard leafy wood (e.g. beech) to 600 kg/m3 for coniferous (e.g. pine). This gives energy contents of respectively 3400 and 2500 kWh/m3 for beech and pine (lower heating value, 20% humidity).
For furnaces with flue-gas condensers, the energy output can be 80-90% of the higher heating value, which is respectively apr. 4% and 10% above lower heating values for wood with 20% and 40% humidity.

Resource estimation

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

If forest statistics are incomplete or unreliable, simplified estimates can be made:

• if only figures for commercial use is available, the potential for wood residues can be estimated as a fraction of the commercial use. Danish experience is that wood for wood-chips (branches smaller 7 cm in diameter) is equivalent to 25% of the timber production including bark or 31% of the timber exclusive bark.
  
• If only forest area is known, a first estimate can be made based on area of commercial forest. An estimate from Germany gives an annual growth of forests of 10-15 tonnes/ha with an energy content of 150 - 225 GJ/ha (42 - 63 MWh/ha).If 3/4 of this is used for timber, the available residues has an energy content of 40-60 GJ/ha (11 - 16 MWh/ha). An estimation of residues from forests on the Danish island Bornholm gives practical usable residues smaller than 7 cm in diameter of 1,7 tons/ha, equivalent to 18 GJ/ha (5 MWh/ha) with 40% humidity or 25 GJ/ha (7 MWh/ha) with 20% humidity. These estimates do not take into account the important factors of climate and soil for the actual wood production.
  

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

Effects on economy, environment and employment
Economy

Utilization of firewood and wood-chips is based on a local resource, requires minimal transport/import and is therefore quite inexpensive in comparison to fossil fuels.

Price estimates, excluding transport & profits (of leafy trees, density 760 kg/m3):
Denmark: 240 DKK/m3 equal to 0,11 DKK/kWh (1 EUR = 7,4 DKK). Danish example with Czech wages: 513 CZK/m3 equal to 0,24 CZK/kWh (1EUR=27 CZK).

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

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

Hand-rules

Each ha of forest on good soil in Central Europe grows 10 tons/ha of wood. If 25% of this is available as waste-wood for energy, the output for energy is 2,5 tons or 11 MWh (20% humidity).

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

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

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

Effect on economy, environment and employment

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

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

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

Combustible waste from agriculture

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

Energy Content
The energy content of straw is 4,9 kWh/kg of dry matter (high heating value). With a typical of 15% humidity the lower heating value is 4,1 kWh/kg. The energy in 1 m3 of densely compressed straw bales is 500 kWh (density 120 kg/m3).
The average efficiency for 22 straw-fired heating stations in operation in Denmark is 80-85%, not including flue-gas condensation. 

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

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

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

Effect on economy, environment and employment
Economy
In Denmark, straw-prices vary from 0,085 DKK/kWh to 0,12 DKK/kWh (1EUR=7,4 DKK) for baled straw delivered at a straw-firing station. In Czech Republic the prices for straw collected at the farm has been quoted at 0,043 CZK/kWh (1EUR=27 CZK)for loose straw and 0,054 CZK/kWh  for baled straw.

Costs, average for 16 straw-fired installations in Denmark are per kWh heat produced:

	Danish average	Estimate for Czech Republic
Fuel	1,9 EURO cent	0,26 EURO cent
Electricity*	0,12 EURO cent	0,12 EURO cent
O&M, administr.	1,3 EURO cent	0,26 EURO cent
Capital costs	1,5 EURO cent	1,5 EURO cent
TOTAL	4,8 EURO cent	2,14 EURO cent

* Electricity consumption is in average 2.3% of heat produced

The environmental impact of using agricultural residues are, as for wood, reduced CO2-emission, reduced sulphur emissions, compared with coal and oil.  Emissions of particulate, NOx and volatile organic compounds (VOC) depend on furnaces and flue-gas treatment. Chlorine components in straw gives emission of HCl as mentioned above. Danish experience from 13 straw-fires heating stations shows the following emissions (all plants have particulate filters):

Emission	Average Emission  g/kWh straw	Variation of emissions  g/kWh straw
Particulate	0,14	0,01-0,3
CO	2,2	0,4-4
NOx	0,32	0,14-0,5
SO2	0,47	0,4-0,6
HCl	0,14	0,05-0,3
PAH*	0,6	0,4-1
Dioxin**		1-10 ng

* PAH = Polyaromatic Hydro-Carbons. This is the carcinogenic part of VOCs.
** Dioxin figures are based on only two measurements, figures given in nanogram.

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

ENERGY CROPS

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

Willows.
Energy Contents and Yields
The following table gives an overview of the expected yields and energy contents for three of the promising plants for solid fuel production.

	Yields  (tonnes/ha/year)	Energy content  (GJ/dry tonne)	Energy Yields  (GJ/ha/year)
Salix (Willow)*	15	16	240
Miscanthus (Elephant grass)	20	17	340
Sweet Sorghum	25	18	450

*Increment of Salix is 2-3 meters in one year (2-3 cm per day in the summer), harvest every third year.
Miscanthus.

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

Resource Estimation

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

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

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

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

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

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

Hand Rule

BIOGAS
The largest potential for biogas is in manure from agriculture. Other potential raw-materials for biogas are:
sludge from mechanical and biological waste-water treatment (sludge from chemical waste-water treatment has often low biogas potential)
organic household waste
organic, bio-degradable waste from industries, in particular slaughter-houses and food-processing industries

Care should be taken not to include waste with heavy metals or harmful chemical substances when the resulting sludge is to be used as fertilizer. These kinds of polluted sludge can be used in biogas plants, where the resulting sludge is treated as waste and e.g. incinerated.

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

Energy Content
The biogas-production will normally be in the range of 0,3 - 0,45 m3 of biogas (60% methane) per kg of solid (total solid, TS) for a well functioning process with a typical retention time of 20-30 days at 32 deg.C. The lower heating value of this gas is about 6,6 kWh/m3. Often is given the production per kg of volatile solid (VS), which for manure without straw, sand or others is about 80% of total solids (TS).
A biogas plant have a self-consumption of energy to keep the manure warm. This is typically 20% of the energy production for a well designed biogas plant. If the gas is used for co-generation, the available electricity will be 30-40% of the energy in the gas, the heat will be 40-50% and the remaining 20% will be self-consumption.

Resource Estimation
For manure, the available data is often the numbers of livestock. From this can be made an estimation of available manure. While the amount of manure produced from animals depends on amount and type of fodder, some average figures are made for most countries.
The following table shows the figures for Denmark :

Kind of animal	Manure type	Amount (kg/day)	Solid amount (kg/day)	Biogas per animal (m3/day)*	Energy per animal  (kWh/yr)
Cow	Slurry	51	5,4	1,6	3400
Cow	Dry	32	5,6	1,6	3400
Sow	Slurry	16,7	1,3	0,46	970
Sow	Dry	9,9	2,9	0,46	970
Hen	Dry	0,66	0,047	0,017	36

*biogas with 65% methane.
Yearly energy output is for biogas plant with 20% average self-consumption and 360 working days. When animals are not in stables around the year, the figure will be smaller. The figures are for milking cows and for sows with breeding pigs under 5 kg.

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

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

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

Effect on economy, environment and employment
Economy

The economy of a biogas plant consists of large investments costs, some operation and maintenance costs, mostly free raw materials, and income from sale of biogas or electricity and heat. Sometimes can be added other values e.g. for improved value of sludge as a fertilizer.

In an example from Czech Republic the price for a Czech plant is estimated to about 70,000 US $ for a plant for treatment of manure from 100 cows. This plant will produce about 220 MWh/year + energy for its own heating. This gives an investment of 0,32 US $ per kWh/year. New Danish biogas plants have similar investment figures. It is estimated that a joint-venture of Czech and Danish technology could reduce prices by about 40% (to about 0,2 US $ per kWh/year); but this has not been shown in practice.
Operating and maintenance (O&M) will normally per year be 10-20% of investment costs, but it vary much with organization, wages, type of plant and eventual transport of sludge. If O&M is 10% of investment costs, simple pay-back requirement is 10 years and no price can be set to increased value of the sludge, the resulting energy price will be 0,03 - 0,045 Euros/kWh (based on the above examples from Czech Republic).

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

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