1 . INTRODUCTION

Energy is an integral part of a society and plays a pivotal role in its socio-economic development by raising the standard of living and the quality of life. The state of economic development of any region can be assessed from the pattern and consumption quality of its energy. Energy demand increases as the economy grows bringing along a change in the consumption pattern, which in turn varies with the source and availability of its energy, conversion loss and end use efficiency. Through the different stages of development, humankind has experimented with various sources of energy ranging from wood, coal, oil and petroleum to nuclear power. But indiscriminate exploitation of resources and unplanned developmental activities has led to serious ecological and environmental problems.

Industrialisation during the nineteenth century witnessed dependence on resources such as coal, oil, etc. But in the early seventies, oil crisis shifted the focus of planners to renewable sources of energy and energy conservation aspects. With environmental concern catching up, sustainable and equitable developments have become critical issues in most parts of India. After food, the most pressing concern in the foreseeable future will be to provide energy for both subsistence as well as economically productive activities.

The burgeoning population coupled with developmental activities based on ad-hoc decisions have led to resource scarcity in many parts of India. A judicious choice of energy utilisation is required to achieve growth in a sustainable manner. With 70% population still in rural areas, there is tremendous demand on resources such as fuelwood, agricultural residues, etc. to meet the daily fuel requirements. Dependence on bioresources to meet the daily requirement of fuel, fodder, etc. in rural areas is more than 85% while in urban area the demand is about 35%.

In many developing countries biomass is the major source of energy nearly accounting for 33 % of the country's energy use reaching levels as high as 75-90 % for countries like Bangladesh, Kenya and Paraguay (UNEP, 1993). This component is much higher in rural areas with dominant use of fuel wood (56%) for cooking and heating purposes. When scarce, fuel wood is substituted by crop residues and animal dung. The quantitative energy consumption and its pattern reveal a sharp contrast between rural and urban energy systems . The urban systems largely depend on commercial energy sources, while the rural system is primarily dependent on non-commercial energy sources like fuel wood, cowdung etc. Biomass fuels meet 85-90% of domestic energy demand and 75% of the rural energy demand (Natarajan, 1985). Firewood is the primary energy source for cooking used by rural households - 78% (TERI, 1999). Commercial fuels like LPG have achieved little penetration into the domestic sector, with only 1.3% of households using the fuel for cooking in rural India (TERI, 1998).

Biomass use in rural areas continues to increase. An increased dependence on fuel wood in rural areas has been indicated with the share of fuel wood in cooking increasing from 56% in 89/90 to nearly 62% in 1994/95 (TERI, 1999). In Karnataka State, non-commercial energy constitutes 53.2%, met mainly by sources like firewood, agricultural residues, charcoal and cowdung, while commercial energy's share is 46.8%, met mainly by electricity, oil, coal etc.

Availability of bioresource is highly diversified as it is influenced by a host of factors like edaphic, meteorological, geographic and also to certain extent the socio-economic status of the people. The agro-climatic conditions are an important factor influencing the use of biofuels in rural areas and availability of bioresources. Regions with similar geographic, edaphic and meteorological characteristics have been grouped and are termed agro-climatic zones and Karnataka State has 10 zones based on agro-climatic conditions. Agro-climatic zones along with taluks and demographic details are listed in Annexure I. Inventorying based on agro climatic zones helps in identifying the reasons responsible for their inequality in distribution, availability and demand. This will also help serious energy planners to incorporate conservation measures for the resources during policy interventions. Implementation of sound management strategies is a prerequisite for sustainable utilisation of resources. This endeavour presents biomass energy status for Karnataka State based on talukwise agro-climatic conditions, and reviews techno-economic aspects of feasible technologies. To augment the resources viable options viz. the utilisation of the neglected and less productive land to meet the bioresource requirement of a region are explored.

1.1 Biomass

The fuel crisis in early seventies and consequent oil price hike necessitated exploring for ecologically sound, economically viable and technically feasible energy alternatives. In India, focus was shifted on the choice of renewable resource, which meets the demand of the most. Rural population depends on bioresources to meet the daily requirement of fuel, food and fodder.

Land and water based vegetation; organic wastes and photosynthetic organisms are generally termed as biomass. It is also defined as the weight of all the living organisms in a given population, area, volume or other units being measured (Johnson, 1986). The reaction of this organic carbon with oxygen releases bioenergy. The feedstock for bioenergy includes agriculture and forest residues, domestic, commercial and industrial wastes, and special energy crops like sorghum, sugarcane, maize and oilseed, and wood from sustainable forests.

The primary step in the build up of biomass is photosynthesis. In this process, sunlight is absorbed by chlorophyll in the chloroplasts of green plant cells and is utilized by the plant to produce carbohydrates from water and carbondioxide (Johansson et al., 1993). The process can be represented by the following equation.

6CO2 + 6H2O		C6 6H12O6 + 6O2
	Sunlight

As per an estimate, globally photosynthesis produces 220 billion dry tonnes of biomass per year with 1% conversion efficiency (Johansson et.al, 1993). To enhance their usage efficiency, these carbon reserves are converted by conversion technologies to more flexible forms like heat, steam or other solid, liquid or gaseous fuels. These fuels due to their biological origin are also often referred as biofuels. Literatures indicate that the stored energy in biomass is equivalent to ten times the world's energy consumption. Importance of biomass is enhanced considerably owning to its local resource availability. The most significant potential sources of biofuels are residues, wood resources from natural forests and biomass from managed plantations. Biomass residues are the organic by-products of food, fibre and forest production. The energy value of residues generated worldwide by the forest-products industry and in selected agricultural activities is estimated to be about 111 exajoules per year (Johansson et.al, 1993). For developing countries, the corresponding residue production rate is 69 exajoules per year. But not all the residues can be utilized for energy purposes as most of them have alternate uses. But, in villages, such residues can account for up to 90 percent of household energy. It has been estimated by FAO that about 800 million people worldwide rely on crop residues and dung energy. Residues are also burnt by some industries like sugar factories and alcohol distilleries to run their operation.

Biomass Resource

Biomass can be categorised broadly as woody, non-woody and animal wastes.

•  Woody biomass: comprises of forests, agro industrial plantations, bush trees, urban trees and farm trees. Wood, bark, branches and leaves constitute the above ground woody biomass. Woody biomass is generally a high valued commodity and has diverse uses such as timber, raw material for pulp and paper, pencil and matchstick industries, and cooking fuel.

•  Non-woody biomass: comprises of crop residues like straw, leaves and plant stems (agro wastes), processing residues like saw dust, bagasse, nutshells and husks, and domestic wastes (food, rubbish, sewage). They are harvested at the village level and are essentially used either as fodder or cooking fuel.

•  Animal wastes : constitute the wastes from the animal husbandry.

All these are considered primary biofuel and derived forms such as ethanol; biogas and charcoal are secondary fuels.

Forest residues are obtained normally by collecting branches, tops after primary harvest, or by whole tree harvesting, which is not desirable, unless sustainable. According to FAO statistics, about half of the world's round wood (the recoverable trunks and large branches of the harvested trees) is used for industrial products and half for fuel wood and charcoal production. Industrialized countries account for three-fourths of the industrial round wood, while developing countries account for five-sixths of the round wood used for fuel wood and charcoal. Beyond logging residues, existing forests also provide additional biomass for energy. With a reliable mean annual increment data, the potential for utilising wood for energy can be estimated.

The amount of crop residues available for energy purposes will depend on the cultivation practices, recovery and storage technologies. The recovery and delivery costs of these residues to bioenergy will vary considerably, depending on the crop, lignin and cellulose content, climate, topography, cost of labour as well as the opportunity costs associated with using the biomass for energy instead of other purposes.

Biomass feedstock can also be generated through short rotation intensive-culture energy plantation. To make these resources viable, it is necessary to sustain higher yields over larger areas and long periods. This requires formulation of long-term strategies such as site establishment, species selection, soil fertility, pests and diseases, erosion, water pollution and the biological diversity and productivity of the plantation and its environs.

Animal dung is a potentially large biomass resource and dried dung has the same energy content as wood. When burned for heat, the efficiency is only about 10%. About 150 Million tonne (dry) of cow dung are used for fuel each year across the globe, 40% of which is in India (UNEP, 1980). But dung is readily recoverable only from confined livestock or in settings where the labour costs associated with gathering dung are modest. The efficiency of conversion of animal residues could be raised to 60% by digesting anaerobically (to produce biogas). Biogas production will also resolve the conflict between energy recovery and nutrient utilisation as the effluent from the digester could be returned to the fields.

Composition of Biomass

The chemical composition of biomass is very important from the energy standpoint as it influences the various conversion processes. Mostly, alpha cellulose is the principal structural element of many biomass types. Fat and protein content contribute to a lesser extent on a percentage basis than the carbohydrate component. Though fat has the highest energy content, its percentage composition is less in most of the biomass types. Table 1 lists the carbon percentage and the energy contribution of some of the biomass components.

Table 1: Carbon percentage and energy contribution of some biomass components

(Wiley Encyclopaedia of energy and environment, 1997)

Wood is most commonly exploited for fuel purposes since it can be used without any treatment or modification except that of being cut into small pieces. This is because of its high volatility, high char reactivity, and low sulphur and ash content. It is largely used as an energy source for cooking and for mechanical energy in both rural and urban areas and for industrial thermal energy. The fuel wood characteristics of wood are attributed to its anatomical, physical and chemical properties (Tillman et.al, 1981). The fuel implication of the anatomical structure is that wood can both absorb and adsorb moisture into the traechids and lumina. The moisture trapped in these structures is difficult to remove and the process is energy intensive. Even when subjected to combustion, the anatomical features decide the migration of the moisture pathway, thereby altering the heat transfer properties of the fuel particles. Among the physical properties affecting fuel characteristics are moisture content, specific gravity and void volume.

The moisture content is variable and depends on the extent to which the wood is dried. Fuel wood has a variable, but low energy value ranging from 10.9-21.3 GJ per ton, with an average of 16.9 GJ for oven-dried wood (moisture content of 0 percent). A ton of air-dried wood (average 20% moisture content) has an energy value of approximately 13.5 GJ (Susan Bogach, 1985).

The chemical structure and composition of wood determines its combustion efficiency, as combustion is a series of chemical reactions. The major chemical compositions of wood are cellulose, hemicellulose and lignin. Extractives are also present though in minor quantities in most of the species. Depending on the composition wood is grouped as either hardwood or softwood. Generally softwoods have 40-45% cellulose, 24-37% hemicellulose and 25-30% lignin. The hardwoods contain approximately 40-50% cellulose and 22-40% hemicellulose. Lignin acts like glue, holding the carbohydrate (holocellulose) fraction of the wood together. The precursor of lignin is phenylalanine, and it accounts for some of the nitrogen content of the wood. Lignin is more abundant and has a higher degree of polymerisation in softwoods than in hard woods. Woods having higher lignin content and plenty of extractives will have a higher heating value. Cellulose and hemi-cellulose contain only around 17.5 MJ/kg while lignin has about 26.5 MJ/kg and extractives can approach 35 MJ/kg (Shafizadeh and DeGroot, 1976).

There are also inorganic fractions of wood, which form ash on charring. Of the various components of wood, it is the sugar and lignin content that affects the process technology and process economics. The composition of intact wood, as per cent of oven dry weight is given in Table 2.

Table 2: Composition of intact wood (Oven dry weight percentage)

(Egneus and Ellegard, 1984)

Wood has the flexibility of being modified into various forms that are convenient to use like charcoal, liquid fuels like methanol and ethanol and producer gas (carbon monoxide and nitrogen). Charcoal is mainly made of carbon and is obtained by the destructive distillation of wood. It has a relatively high-energy value of 28.9 GJ/ton. Methanol and ethanol can be produced largely from organic matter, including woody biomass. They have an energy content of about one half that of gasoline. Producer gas is obtained by the burning of carbon in a supply of air insufficient to convert it to charcoal. Cellulose and hemicellulose constitute 45-70% of the dried plant residues, which vary according to the age and maturity of the plant when harvested (Sloneker, 1976). About 20% of the total carbohydrate in the plant tissue is composed of xylose, arabinose, mannose and galactose, which are released upon acidic or enzymatic hydrolysis of the crop residues (Chahal, 1991). The ultimate analysis of different plant residues in % of dry matter is given in Table 3. The elemental composition is important as it helps in evaluating the combustion characteristics (Hall et.al, 1987). It is seen that the ash content varies considerably among the residues. The carbon content is typical of organic substrates, being between 40% and 50%. Hydrogen content at 5-6% and oxygen content mainly below 40% are nearly uniform. Nitrogen content is a measure of the protein content of the residue. The nitrogen content is in most cases far below 1%, indicating that the residues have low protein content.

Table 3: Analysis of plant residues (Percentage of dry matter)

(Source: Hall and Overend, 1987)

Biomass resources are gaining popularity as energy fuels as they are readily available, cleaner and can be produced sustainably. Modern bioenergy programmes provide a basis for rural development and employment. Another added advantage is that its production and use leads to no net build-up of carbondioxide in the atmosphere. Biomass is more reactive than coal, making it an especially attractive feedstock for thermochemical gasification. The greater reactivity of biomass relates to its chemical structure. Neglecting minor chemical constituents, a typical biomass feedstock can be represented chemically as CH_1.45O_0.7 compared to CH_0.8O_0.08 for coal. Thus, biomass has nearly twice as much of hydrogen and nearly an order of magnitude more oxygen per carbon atom than coal. The calorific value and heat utilisation efficiency of various fuels are given in Table 4.

Table4: Calorific value and heat utilisation efficiency of various fuels

(Veena, 1988)

Secondary forest fuels are produced as a result of conversion of primary woody material to more valuable fuels by carbonisation, distillation, gasification, etc. These secondary fuels could be charcoal, producer gas, synthetic gas, methanol and synfuel, besides briquettes. Under Indian conditions, a useful process is the conversion through carbonisation to charcoal. Approximately 4.7 Mkcal is contained in a tonne of oven dry wood and 7.1 Mkcal in a tonne of charcoal.

1.2 Estimation of Bioenergy Potential

Assessment of available bioresources is helpful in revealing its status and helps in taking conservation measures and ensures a sustained supply to meet the energy demand. Assessment of bioenergy potential can be theoretical, technical or economic. Natural conditions that favor the growth of biomass determine the theoretical potential. Technical potential depends on the available technologies that can be exploited for the conversion of biomass to more flexible forms and so is subjected to change with time. Of all the three potential estimates, the economic potential is subjected to high variability, as economic conditions fluctuate drastically over space and time.

1.3 Bioresource Inventorying

Bioresource inventory helps in describing the quality, quantity, change, productivity and condition of bioresources in a given area. These inventories may be for regional or national level assessments.

Forest Inventory: For the assessment of forest biomass, forest inventory is most commonly used and it differs depending on scope and purpose. According to Cunia (1983), forest inventory is a systematic procedure used to collect mensurational data on forest biomass and the land on which it grows, process and analyse these data and present them for management use. Recently, inventories are being designed to obtain information on other uses of the forest like recreation, grazing, wildlife and water conservation. The forest biomass inventory is one of the new types of forest inventory. It is designed to measure forest biomass rather than or in addition to traditional volume. Table 5 lists some forest inventories.

Table5: Forest Inventories

(Hall and Overend, 1987)

Generally the forest is managed for timber production but, increasingly, other uses are considered as well. The inventory area is usually one or more management units, each ranging in size from a few hundred to many thousand hectares. Each unit may be divided into forest-based strata or administrative sub-populations for which separate estimates are required. The attribute of primary interest is merchantable wood volume, with stem frequency. Basal area data is of secondary importance. These attributes are usually given by tree size classes and by a number of forest and administrative classes that are described in a classification system such as the following:

•  Total inventory area is divided into land and water

•  Land is divided into forest and non-forest

•  Forest is divided into productive forest and unproductive forest

•  Productive forest is classified by ownership and status into forest and cover type, and by stand density, height, age, and site quality classes.

The information required for management inventories are obtained from the existent base maps, soil maps, and geological maps, narrative descriptions of the area and its history, aerial photographs which are used to obtain information about individual stands, and field samples, from which detailed volume data are obtained through sampling procedures.

The management inventory includes the following four general steps:

•  Determine the study area from the base map by delineating the population.

•  Define sub-populations using aerial photos, etc.

•  Obtain detailed volume data using field-sampling procedures. The general approach is to establish sample plots, measure individual trees within the sample plots, apply equations to estimate tree volumes and summarise the volumes by species and size classes. The plots are then combined with other plot totals for individual strata, sub-populations or other desired classes, and average values and precision estimates are calculated.

• The area data from (2) are combined with the averages from (3) to yield estimates (e.g. total volume) of individual strata, sub-populations, and the whole population. The data are then summarized and presented with maps.

Above ground standing biomass of trees is the weight of trees above ground, in a given area, if harvested at a given time. The change in standing biomass over a period of time is called productivity. The standing biomass helps to estimate the productivity of an area and also gives information on the carrying capacity of land. It also helps in estimating the biomass that can be continuously extracted. The standing biomass is measured using the harvest method or by using biomass estimation equations. In the harvest method, vegetation in the selected sample plots are harvested and their weight is estimated in fresh and dry form to measure biomass. For trees, this method is inappropriate, as it requires their felling or destructive sampling.

Biomass is reported in kilograms for individual trees, and in tonnes for stands and other area based measurements. For biomass measurements the tree is segregated to major components: stem wood, stem bark, branches and foliage. In the same way as tree measurements in the sample plots are applied to tree volume equations to obtain tree, plot, and stand volumes; measurements can be applied to tree biomass equations to obtain tree, plot, and stand biomass values. The main difference here is that, while only one (volume) equation for each species is required to estimate volume, a whole set of biomass equations (by components) is required for each species to estimate biomass. Tree biomass equations are very similar to tree volume equations. Both include DBH as an independent variable, often with tree height (H) and other variables, and both use models of similar types. These equations are based on the height, girth at breast height (GBH) and basal areas of trees. These parameters serve as good indicators of volume or weight of the tree. Using height, the standing biomass of a tree is determined using the equation,

Standing biomass in kg = b + (aD²H)

Where D is the diameter at breast height, H is the height of the tree, a and b are constants. Equations involving the basal area are used for all tree species and therefore are used to estimate the standing biomass of mixed forests. The advantage of this equation is that it does not use the height measurements, which are difficult to estimate in dense forests. Tree biomass equations are additive, i.e. the sum of biomass of components is equal to the total tree biomass for a given species (Shailaja & Sudha, 1997)

Productivity, which is the increase in weight or volume of any biomass over a period of time, can be estimated when the standing biomass estimates are available for two consecutive years. It can also be calculated by knowing the age of the forest stand. Productivity = Standing biomass per hectare/age of a tree or the trees per forest stand. Productivity estimates are important as they help to calculate the extent of biomass that can be extracted for fuel purposes.

The special type inventory is a catch all for a diverse group. Some of the methods are designed for specific purposes, e.g. regeneration surveys, others are developed for specific kinds of forests, and yet others are trials of different methodology, particularly those related to field sampling.

The crop residue inventory involves the measurement of both crop yields and crop residues to allow the development of residue-yield ratio estimators as well as area-based estimates of residue yields. The ASF (Area Sampling frame) methodology provides a very efficient basis for estimating crop yields (Houseman, 1975). This methodology involves the delineation of permanent or long-term sampling segments from aerial photos or satellite imagery. These are then used as sampling frames for subsequent agricultural surveys. The crop residues are surveyed during both the Kharif and Rabi season. Field sampling is carried out within one week before harvest to ensure that crop yield and residue measurements are related to fully mature crops.

For arid areas Acacia tortilis is an outstanding multipurpose tree. Its introduction in India saw it being raised in large quantities exclusively for fuelwood purposes in many States. Other species of Acacia chosen for fuelwood plantations are A. auriculiformis, A. cyclops, A. cambagei, A. ligulata, A. aneura and A. nilotica . The other leguminosae genus suitable for fuelwood plantings is Prosopis. The Genus includes P. alba, P. chilensis, P.juliflora, P.pallida and P. tamarugo. P.cineraria grows naturally in dry parts of India. Albizia lebbek is another popular fuelwood species. Albizia falcataria is one of the fastest growing species known, reaching a height of 7m in a year, with a MAI of 25-40 m³/ha in a 8-12 year rotation. Among the non-leguminous species having root nodules with nitrogen-fixing actinomycetes, Casuarina equisetifolia is the most favoured with a calorific value of 20.7 MJ/kg. Another popular Genus of the arid and semi-arid region is Eucalyptus, which belongs to the family of myrtaceae. Some of the species are E. globulus, E. grandis, E. gomphocephala, E. occidentalis, E. astringens, E. sargentii, E. teriticornis, E. astringens and E. microtheca. In the arid tropics, Azadirachta indica is preferred to Eucalyptus. Apart from trees, fast growing shrub and bushes are also used. Eg: Calliandra calothyrsus and Sesbania bispinosa . Some of the fuelwood species for the tropics are: Acacia auriculiformis, Calliandra calothyrsus, Casuarina equisetifolia, Derris indica, Gliricidia sepium, Gmelina arborea, Guazuma ulmifolia, Leucaena leucocephala, Mangroves , Mimosa scabrella, Muntingia calabura, Sesbania bispinosa, Sesbania grandiflora, Syzygium cumini, Terminalia catappa, Trema species, Ailanthus altissima, Albizia lebbek, Alnus acuminata, Cajanus cajan, Cassia siamea, Eucalyptus species , Grevillea robusta and Pithecellobium dulce.

Fuelwood species for the tropical highlands are: Acacia mearnsii, Ailanthus altissima, Alnus acuminata, Alnus nepalensis, Alnus rubra, Eucalyptus grandis, Eucalyptus globulus, Grevillea robusta, Inga vera, Casuarina oligodon, Eucalyptus camaldulensis, Gliricidia sepium, Mimosa scabrella, Pinus halepensis, Sesbania bispinosa, Syzygium cumini and Terminalia catappa.

Fuelwood species for arid and semi-arid regions are Acacia brachystachya, Acacia cambagei, Acacia cyclops, Acacia nilotica, Acacia saligna, Acacia senegal, Acacia tortilis, Adhatoda vasica, Albizzia lebbek, Anogeissus latifolia, Azadirachta indica, Cajanus cajan, Cassia siamea, Eucalyptus citriodora, Pinus halepensis, Prosopis chilensis, Eucalyptus occidentalis, Pinus halepensis, Prosopis cineraria, Prosopis tamarugo, Tamarix aphylla, Zizyphus mauritania, Eucalyptus grandis, Acacia auriculiformis, Leucaena leucocephala, Terminalia catappa, Tectona grandis, Anogeissus leiocarpus, Pithecellobium and Guazuma ulmifolia. The name of some of the fuelwood species and their calorific values are given in Table 6.

Table 6: Calorific Values of some Fuelwood species

(Khosla, 1982)

For maximum yield of biomass, intensive crop management with fertilizers, weed control, and irrigation would be necessary. It is necessary that energy plantations require substantial resources like good land, moisture, nutrients and other direct and indirect energy inputs.

The utilisation and regeneration of existing resources in energy plantation can be combined with agroforestry involving plantation of trees having nitrogen fixing potential, waste land reclamation and social forestry.

Planting of trees and shrubs can provide firewood for use in bioresource scarce regions in a couple of years. The agricultural output can also be increased by releasing cowdung and by providing soil conservation. Planting of fodder trees, combined with more scientific management of cattle, can result in direct supplement to rural income (Nautiyal and Chowdhary, 1979).

1.4 Conversion Technologies

Besides satisfying the rural domestic energy requirements (cooking and water heating), biomass also finds use in the manufacture of construction materials like bricks, lime and tiles, and in agro-processing such as curing of tobacco, preparation of crude sugar etc. Cooking energy requirements are also met from cattle dung, leaf biomass from energy plantations and crop residues.

In comparison to the fossil fuels, fresh biomass has certain drawbacks like, high moisture content that reduces its combustion efficiency, low bulk density and lack of a homogeneous physical form. Biomass conversion helps to improve the characteristics of the material as a fuel. The conversion processes largely involve the reduction of the water content and improving the handling characteristics of the material. The energy so obtained can be used for domestic purposes, in agriculture, small scale industries like jaggery making, sericulture activities, coffee/tea processing, paper making, paddy drying etc. To exploit the energy content, the biomass feedstock is subjected to one of the three conversion processes, physical, biological and thermochemical, each one detailed below.

Physical processes

These processes essentially alter the physical state of the biomass feedstock that improves its fuel characteristics which is required if it is to be used directly as a fuel. Some of these methods are given below:

i. Particle size reduction: Physical modification like fabrication into pellets helps easy storage and transportation. The commercial machines used for the size modifications are wet and dry shredders.

 ii. Separation: Sometimes, separation of feedstock becomes desirable if its components are to be used for different applications. Eg: separation of agricultural biomass into food component and residues that may serve as fuel or as raw material for synfuel manufacture. Separation also h elps to exploit those components that have a higher fuel value.

 iii. Drying: It refers to the vaporisation of all or part of the water in the feedstock. Open air solar drying is one of the cheapest methods. Spray dryers, drum dryers or conventional ovens can be used for materials that are less stable towards solar drying. Drying reduces the process energy consumption to a large extent.

 iv. Fabrication: Manufacturing pellets by extrusion techniques compacts the feedstock. If necessary, binding agents like thermoplastic resins can be used. Compaction results in uniform combustion of the biomass material.

Biological processes

The biological conversion processes are anaerobic digestion (biomethanation) and fermentation-ethanol production. Biological conversion processes are the ones that are mediated by microorganisms. An example of anaerobic fermentation is the conversion of biomass to methane and carbondioxide in a roughly 2:1 volumetric ratio. The residual sludge is rich in nitrogen and can be used to yield good quality fertiliser. The reaction temperature can be in the mesophilic region of 35-37 ° C or at 55 ° C for faster thermophillic rates and the fermentation may be batch, semi-continuous, or continuous. To obtain high methane output the carbon/nitrogen ratio of the input slurry feed must be approximately 30:1. The process is marginally exothermic and the ideal pH for rapid methanogenesis is 7-7.2. The primary step in methanogenesis is acidogenesis in which the input feed (principally carbohydrate) is hydrolysed and fermented to organic acids (chiefly acetic) and hydrogen. The acids are then converted by methanogenic bacteria into dissolved methane and carbondioxide, which finally undergo transition from liquid to gaseous phase. About 70% of the methane is obtained from acetate. The reaction rates are very slow. Presuming cellulose to be the principal component, the chemical reactions involved in the process can be summarised as follows (UN, 1980).

(C6H10O6) x + xO2		x C6H12O6
	Hydrolysis
	Acidification
x C6H10O6		3x CH3COOH
	Methanation
3x CH3COOH		3x CH4 + 3x CO22

In alcoholic fermentation certain types of starchy biomass such as corn and high sugar crops are readily converted into ethanol under anaerobic fermentation conditions in the presence of certain yeast ( Sacchoromyces cerevisiae ) and other organisms. Wood and municipal wastes that contain high concentration of cellulose are converted to sugar concentrates by acid or enzyme catalysed hydrolysis. The reactions are summarised below.

C6H10O5 + xH2O		xC6H12O6
	Hydrolysis
	Fermentation
xC6H12O6		2xC2H5OH + 2x CO2

In the tropics the bacterium Zymomonas mobilis is used for fermenting alcoholic beverages. It gives higher ethanol yields and lower biomass production than yeast does.

Thermochemical processes

Thermochemical conversion refers to the alteration of the physical and chemical nature of biomass through heating. Biomass feedstock has unique properties that make them ideal for thermochemical conversions which convert 85-95% of the organic material in the feedstock to liquid and gases with high efficiency and relatively little sensitivity to variations in the feed material (Schiefelbein et al., 1984). Depending on the conditions used (primarily the temperature reached, the oxygen to fuel ratio, the residence time and temperature) biomass can be altered very slightly, or be completely changed. It is these three variables that define the conditions for pyrolysis, gasification and combustion. Selection of treatment conditions permits a variety of outcomes of importance to the production of bioenergy products. These products differ in the proportions of solid, gaseous or liquid forms. Combustion, pyrolysis and gasification are the three key thermo chemical processes. They are described in detail below:

• Combustion: The objective of direct and complete combustion is heat. Air is supplied in excess to afford combustion of biomass in the production of fully oxidised, permanent gases. Ash and highly resistant carbon products are formed. Liquid products even if present occur as traces of condensates. The carbon and hydrogen in the residue undergo exothermic reaction with oxygen to form carbon dioxide, water and heat. The process is oxygen demanding because of problems in air-fuel mixing. A major constraint in direct combustion of biomass is the moisture content. The higher the water content, more is the energy required for the evaporation of water. Water vapour at an elevated temperature like 200° C occupies nearly 2000 times the volume of liquid water. This in turn increases the stack volume of the gas, increasing the gas velocity resulting in losses of heat to stack gases. Since there are physical feed problems, as well as lower energy content and lower efficiency in use of wood for combustion, direct substitution involves high costs for burners, which have to be changed, and energy installations need to be enlarged. Several problems associated with the direct burning of biomass can be overcome by the prior conversion of biomass, via gasification or pyrolysis into better fuels. Dried wood chips, cereal straw and organic refuses, with heat contents of 18.6-20.9, 16-17 and 10.5 MJ/kg respectively, are all biomass candidates for combustion.

• Pyrolysis: If thermo chemical conversion of biomass is conducted at elevated temperatures still lower than those for gasification (below 600° C), and in insufficient or absence of air, all three primary products - char, tar and gas can be recovered. The reaction conditions can be so adjusted to favour one of these products. For instance, higher heating rates generally favour tar production at the expense of char and gas. The pyrolysis gas has a low heating value of 3.9-15.7 MJ/m³ at normal conditions. It contains carbondioxide, carbon monoxide, hydrogen, ethane, ethylene and minor amounts of higher gaseous organics and water vapour. The most common form of biomass pyrolysis is carbonisation, i.e. to produce char. Depending on the pyrolysis temperature, char fraction contains inorganic materials ashed to varying degrees, any unconverted organic solids and fixed carbon residues produced on thermal decomposition of the organics. Char is superior as a fuel compared to wood or agricultural residues, as its production can be afforded in very simple systems. Chars, besides being easily produced is more energy dense than their parent biomass forms, and has considerable advantages. Being essentially smokeless fuels, they are mostly used as fuels for cooking and water heating purposes in developing countries.

Pyrolysis can also be used for tar production, which is favoured by conditions of shorter residence time at higher temperatures (generally not greater than 400° C) under oxygen deficient or inert atmosphere. Tars have undesirable physical properties. Reported attempts at tar utilization relate to direct combustion, although tars are poor fuels. They are viscous at ambient temperatures, not completely volatile, exhibit high oxygen content, gummy, corrosive, and carcinogenic and do not mix with conventional fuels. Fixed bed, moving bed or fluidised bed reactors are used for pyrolysis.

• Gasification: At elevated temperatures just short of those required for combustion, but in the presence of limited amount of oxygen or air, biomass can be primarily converted into a mixture of carbon monoxide, hydrogen and volatile hydrocarbons. This process is called gasification and the objective is gaseous fuel. Providing sufficient residence time at conversion temperature maximises the production of gaseous fuel product, by minimising the other two primary conversion products, tar and char. It is necessary to ensure complete combustion, else some tar/char is always formed. Tar is undesirable as it is sticky and being corrosive creates problems in handling and disposal, although staged gasification/combustion system solves this problem. Wood and agricultural residues can be used as feedstock for gasification. A typical composition of the gas obtained from wood gasification on volumetric basis is given in Table 7.

Table 7: Typical Composition of Gas obtained from Wood Gasification

(TERI, 1993)

The calorific value of this gas is about 900-1200 kcal/Nm³ and the gas can be used for generation of motive power either in dual fuel engines or in diesel engines with some modifications (Kishore, 1993). The gasification of solid fuels containing carbon is accomplished in an air sealed, closed chamber under slight vacuum or pressure relative to the ambient pressure. The fuel column is ignited at one point and exposed to the air blast. The gas is drawn off at the other location. Depending upon the positions of air inlet and gas withdrawal with reference to the fuel bed movement, three broad types of gasifiers have been designed and operated. They are updraft, downdraft and cross draft gasifiers. The department of non-conventional energy sources has installed more than 300 small wood gasifier systems, mainly in Karnataka and Gujarat in the last few years. Besides woody biomass, agro and mill residues offer a large scope of utilization for gasification purposes.

The status of biomass energy in Karnataka State, based on talukwise agro-climatic conditions, and techno-economic aspects of feasible technologies will be presented in the following pages.

Overview of biomass conversion routes (Jensen, Sorensen 1984) :