In the pioneering work describing this method, variety of catalysts was tested for deoxygenation of stearic acid at 300OC and 0.6 MPa of helium [9]. Catalyst screening was performed for Pd, Pt, Mo, Ni, Ru, Rh, Ir, and Os metals, bimetallic Pd-Pt catalyst as well as Raney nickel and oxides Ni-Mo/Al2O3. It was shown that Pd and Pt active metals on carbon as a support have the highest activity and selectivity to hydrocarbons (Table 6.2).

Reaction conditions: mstearic acid = 4.5 g, mdodecane = 86 g, mcatalyst = 1 g, T = 300o C, p 0.6 MPa, Vcarrier gas = 25 mL/min (He). Adopted from Ref. [14]. The metal loading is in wt%. a Conversion of stearic acid and selectivities towards products after 6 h of reaction b Selectivity to C35 symmetrical ketones

c Crack denotes cracking products consisted of shorter fatty acids, C10-C17 acids and shorter hydrocarbons, C13-C16 hydrocarbons

d Heavy denotes dimeric products formed via unsaturated acids and olefins e Other denotes unidentified products

Furthermore, different catalyst supports were used such as Al2O3, SiO2, Cr2O3, active carbon, and zeolites, from which active carbon was the most suitable for decarboxylation/decarbonylation of fatty acids. The zeolite-supported metal cata­lysts show high activity, but low selectivity to long-chain hydrocarbons, which was caused by cracking of the feedstock. Therefore, high acidity of the support is not suitable for obtaining fuel with high cetane number. On the other hand, basicity of the supports is not desired as well due to very low selectivity to hydrocarbons. The Ru/MgO catalyst with a basic support used for deoxygenation of stearic acid, converted 99% of fatty acids into ketones containing 35 carbons (Table 6.2).

The bimetallic metal oxides Ni-Mo/Al2O3 show low activity and selectivity towards hydrocarbons at the temperature of 300°C and 0.6 MPa helium pressure. This result is in agreement with the other works (Table 6.1) which show low activity of Ni-Mo oxide catalyst at temperatures around 300°C.

Since fatty acids, their esters, and triglycerides are relatively large molecules it could be beneficial to use catalysts support with its mesoporous structure. Sibunit carbon was used as a support for deoxygenation of dodecanoic acid [8]. The advantage of Sibunit over activated carbon is mesoporous structure (pores larger than 2 nm) and higher thermal and mechanical strength which are more suitable for industrial application of the catalyst. Despite change of the properties of Sibunit compared to active carbon the activity and selectivity of palladium sup­ported on Sibunit are still the same.

The influence of mesoporous support structure was shown in deoxygenation of stearic acid and ethyl stearate over palladium on mesocellular silica foam support [26]. The catalysts proposed for deoxygenation of fatty acids have amorphous cell and window of 37 and 17 nm, respectively. High porosity of the material ensured good accessibility of substrates to the palladium nanoparticles, thus decreasing internal diffusion limitations.

The effect of metal dispersion was studied over 1% Pd/C at temperature of 300°C with pressure 1.75 MPa of hydrogen in argon [27]. The catalysts with metal dispersion between 18 and 72% were used for deoxygenation of stearic and pal­mitic acid. The optimum results for deoxygenation of fatty acids were achieved with the catalysts with palladium dispersion between 47 and 65%. In the case of the catalyst with the lowest palladium dispersion (18%), the metallic surface area is too low to provide sufficient deoxygenation activity. Moreover, extensive cat­alyst deactivation occurs. For the catalyst with the most dispersed palladium on the surface (72%), activity was lower than for catalysts with dispersion of 65 and 47%. This result could be explained by deposition of the smallest palladium particles in the micropores which could be easily blocked by coke deposits.

Microorganisms capable of converting syngas into ethanol and other bioproducts are predominantly mesophilic (Table 9.9). The most favorable operational tem­perature for mesophilic microorganisms is between 37 and 40°C whereas for thermophilic, the temperature varies between 55 and 80°C. Some thermophilic microbes, however, can operate at a higher temperature. The most favourable pH range for efficient microbial activity varies between 5.8 and 7.0, employed to conduct the fermentation, depending upon the species.

The combustion process comprises four basic phases: heating and drying, distil­lation of volatile gases, combustion of these volatile gases, and combustion of the residual fixed carbon. Prior to the actual combustion process, the biomass is first subjected to pelletizing and/or briquetting in order to increase the density of the biomass and simultaneously reduce the moisture content. This also increases the calorific value of the biomass and increases the easy handling of the biomass during transportation and processing. The following steps are involved in pellet­izing of biomass [9]:

1. Drying. The biomass is dried to a moisture content of about 8-12% (weight basis) before pelletizing.

2. Milling. Size reduction of the biomass is done in hammer mills.

3. Conditioning. Conditioning of the biomass is done by addition of steam, whereby the particles are covered with a thin liquid layer to improve adhesion.

4. Pelletizing. Flat die or Ring die pelletizers are used to convert the above material into compact pellets.

5. Cooling. The temperature of the pellets increases during the densification process. Therefore, careful cooling of the pellets is required before the pellets leave the press, to ensure high durability of the pellets.

Pelletization is expensive compared to briquetting where the biomass is com­pressed and extruded in heavy duty extruders into solid cylinders. This pelletized or briquetted biomass is subjected to heat, which breaks down the plant cells. The volatile matter is driven off from the compacted biomass and instead of being released directly into the atmosphere, it is made to pass through a high temperature zone (above 630°C) in presence of secondary air. Here, the gases are combusted and release more heat. A carbonaceous residue called char, containing the mineral components is left behind.

After briquetting or pelletizing, the biomass is fed into the combustion furnace after which combustion proceeds in four phases [7]:

Phase 1: Heating and drying

Moisture in the biomass varies from 10 to 50% of the total weight (wet basis). This moisture reduces the dry heat value of the biomass and slows down the heating and drying process. It is therefore essential to remove this moisture in order to increase the efficiency of the combustion process. The size of the feed particles is also important because most biomass is woody in nature and wood is a poor conductor of heat. The larger the particle size, the lower the rate of heat transmission through the feed bed. The biomass is hence reduced in size so that the maximum distance from the center of the particle of the feed to the surface does not exceed 20-30 mm. Thus, wood chips, sawdust, shredded straw, and pulverized biomass fuels such as bagasse are preferred.

Phase 2: Distillation of volatiles

After the evaporation of moisture is complete, the heat supplied gets used in volatilization of the liquid constituents present in the biomass. This occurs between 180 and 530°C. Distillation occurs during this phase. The gases released comprise complex saturated and unsaturated organic compounds such as paraffin, phenols, esters, and fatty acids. These distill at different distillation temperatures thus making the concept of ‘‘biorefinery’’ possible.

Phase 3: Combustion of volatiles

Ignition of the volatilized components takes place at temperatures between 630 and 730°C. This involves an exothermic reaction between the volatilized gases and oxygen, as a result of which, heat is produced and CO2 and water vapor are released. The flame temperature in this phase depends on the amount of excess air present and the amount of moisture initially present in the biomass (because this evaporated moisture is present as water vapor in this gas phase). Here, supply of excess oxygen in the form of secondary air supply is essential because this will maintain high temperatures during this phase. In absence of this, incomplete combustion will result in lower process efficiency . The unburnt carbonaceous part is called soot. This soot absorbs volatile components which condense in the cooler parts of the furnace and form an oily product called tar.

Phase 4: Combustion of residual fixed carbon

After the moisture and volatiles have been removed, the fixed carbon component of the biomass remains as char. This char begins to burn as oxygen is now available, and carbon monoxide is released which, in the presence of oxygen gets converted into CO2. This CO2 is finally emitted from the furnace.

Careful planning is required for biomass production, which consists of integration of different techniques and improved methods. The general sequence for biomass production is the integration of different techniques and improved methods starting from site survey, nursery techniques, transplanting techniques and maintenance of the plantation. The production techniques include:

• Site survey

• Planting site selection

• Species selection

• Preparation of the planting site

• Preparation of the soil mixture

• Sowing of seed

• Method of sowing

• Transplanting of seedling into containers

• Transport of seedlings to the planting site

• Maintenance of the plantations

After successful plantation of biomass it is harvested by various methods such as:

• Coppicing

It is one of the most widely used harvesting methods in which the tree is cut at the base, usually between 15 and 75 cm above the ground level. New shoots develop from the stamp or root. These shoots are sometimes referred to as sucker or sprouts. Management of sprouts should be carried out according to use. For fuel wood the number of sprouts allowed to grow, should depend on the desired sizes of fuel wood. If many sprouts are allowed to grow for a long period, the weight of the sprouts may cause the sprouts to tear away from the main trunk. Several rotations of coppicing are usually possible with many species. The length of the rotation period depends on the required tree products from the plantation. It is a suitable method for production of fuel wood. Most eucalyptus species and many species of the leguminous family, mainly naturally accessing shrubs can be harvested by coppicing.

• Pollarding

It is the harvesting system in which the branches including the top of the tree are cut, at a height of about 2 m above the ground and the main trunk is allowed to stand. The new shoots emerge from the main stem to develop a new crown. This results into a continuous increase in the diameter of the main stem although not in height. Finally, when the tree loses its sprouting vigor, the main stem is also cut for use as large diameter poles. An advantage of this method over coppicing is that the new shoots are high enough off the ground so that they are out of reach of most grazing animals. The neem tree (Azadirachta indica) is usually harvested in this manner. The branches may be used for poles and fuel wood.

• Lopping

In this method most of the branches of the tree are cut. The fresh foliage starts sprouting from the bottom to the top of the denuded stem in spite of severe defoliation, surprisingly quickly. The crown also re-grows and after a few years, the tree is lopped again. The lopped trunk continues to grow and increases in height, unless this is deliberately prevented by pruning it at the top.

• Pruning

It is a very common harvesting method. It involves the cutting of smaller branches and stems. The clipped materials constitute a major source of biomass for fuel and other purposes, such as fodder mulching between tree rows. It is also often required for the maintenance of fruit and forage trees, alley cropping and live fences. The process of pruning also increases the business of trees and shrubs for bio fencing. Root pruning at a required distance from the hole is effective to reduce border tree competition with crops for water and nutrients.

• Thinning

It is a traditional forestry practice and in fuel wood plantation, it can also be of importance. The primary objectives of thinning are to enhance diametric growth of some specific trees through early removal of poor and diseased trees to improve the plantation by reducing the competition for light and nutrients. Depending on initial plant density, initial thinning can be used for fuel wood or pole production.

The increasing research attention onto the utilization of biomass as feedstock for the production of sustainable materials and chemicals has been directed toward an in-depth understanding of plant cell wall natural structures and their constituents, which are consisted mainly with cellulose, hemicellulose, and lignin. A better understanding of these issues, on one hand, can guide the development of new efficient pretreatment technologies and robust catalysts for the catalytic separation and conversion of biopolymers; on the other hand, can provide new avenues to rationally designing of bio-energy crops with improved processing properties by either reducing the amounts of lignin present or providing a lignin that is easier to degrade. Traditionally, the elucidation of wood structure (lignin) usually follows the destruction-analysis process (e. g. Klason method) due to the insolubility of lignocellulose in conventional organic solvents, and the information obtained does not represent the natural structure of lignin.

Lignin is a complex aromatic chemical polymer present most commonly in wood. As an integral part of the secondary cell walls of plants, it is one of the most abundant organic polymers on the earth, exceeded only by cellulose. In 2007, Jiang et al. investigated the solubility of lignin in different ionic liquids, and the results showed that the order of lignin solubility for varying anions was: [MeSO4]- > Cl — > Br- ^ [PF6]-. This result indicated that the solubility of lignin is principally influenced by the anions of ILs [36]. Further 13C NMR analysis of lignin and lignin model compounds presented that 13C signals using ILs as the solvent is shifted up-field by d 0.1-1.9 ppm in comparison to 13C NMR data acquired using dimethyl sulfoxide (DMSO) as the solvent.

The full dissolution of lignocellulosic materials in ILs provided a new homo­geneous media without degradation of their components for the structural analysis of plant cell wall and lignin. Kilpelainen et al. demonstrated that the fully acet — ylated Norway spruce in ionic liquids was soluble in CDCl3, which allowed the first recording of the solution state 1H NMR spectra of intact acetylated wood. The careful integration of P-O-4 signals for lignin in the 1H NMR spectrum yielded a value of 7.3%, which was in good agreement with the anticipated value of 8% [21]. Further in situ quantitative 31P NMR analysis of spruce dissolved in ionic liquids showed the presence of 13.3 mmol/g hydroxyl groups. This value was close to the theoretically calculated value of 15.7 mmol/g based on traditional methods [37]. Analysis of different pulverization degrees provided semi-empirical data to chart the solubility of Norway spruce in IL [amim] Cl, and further method refinement afforded an optimized method of analysis of the lignin phenolic functionalities, without prior isolation of the lignin from the wood fiber [38, 39].

ILs not only can be used as solvents for catalysis and biomass dissolution, but also can be used as solvents for nuclear magnetic resonance analysis directly. Ragauskas et al. synthesized a series of perdeuterated pyridinium ILs for the direct dissolution and NMR analysis of plant cell walls. Due to the high melting point of pyridinium salts, a co-solvent DMSO-d6 was used to reduce the viscosity of the resulting mixtures, for example, a mixture of 1:2 [Hpyr] Cl-d6/DMSO-d6 was able to dissolve Poplar up to 8 wt% at 80°C in 6 h. Further in situ 1H NMR and 13C NMR analysis showed the full structural map of signals from cellulose, hemi- cellulose, and lignin. For example, the signals at d 61.5, 74.1, 75.8, 76.9, 80.1, and

103.0 ppm were in part attributed to cellulose. Whereas, the lignin methoxyl group corresponding to the signals at d 57 ppm and d 58-88 ppm could be attributed, in part, to Cp in b-O-4, Cc/Ca in P-O-4, P-5, and b-p. The signal at d 106 ppm was attributed to C2/6 resonance of syringyl-like lignin structures, and between 110 and 120 ppm to C2, C5, and C6 resonance of guaiacyl-like lignin structures. The properties and easy preparation of perdeuterated pyridium molten salt [Hpyr]Cl-d6 offer significant benefits over imidazolium molten salts for NMR analysis of plant cell walls; furthermore, the use of non-ball-milled samples in this study can pro­vide a more efficient and accurate characterization of lignin in the plant cell walls compared with the results from traditional methods [40]. Although lignin can provide a renewable source of phenolic polymers, a high lignin content has proved to be a major obstacle not only in the processing of plant biomass to biofuels, but also in other processes such as chemical pulping and forage digestibility. There­fore, precise analytic techniques for efficient lignin content assessment of a large number of samples are in high demand. Further study from Ragauskas’s group reported a linear extrapolation method for the measurement of lignin content by the addition of a specific amount of isolated switchgrass lignin to the biomass solution, and the integration ratio changes could be measured in the quantitative 1H NMR spectra with non-deuterated DMSO as the internal standard. The results showed comparable lignin contents as the traditional Klason lignin contents. They demonstrated that this direct dissolution and NMR analysis of biomass provided a new venue for rapidly assessing the lignin contents in large numbers of ‘‘new’’ plants in biofuel research [41].

In addition to the development of new IL-resistant enzymes, a variety of methods have been developed to stabilize enzymes in ILs [136]. Most of them have been applied to cellulases. One of these methods is the preparation of microemulsions with ILs that can reduce the dehydration effect of ILs on enzymes. To reduce toxicity and deactivation, microemulsions of water in [BMIM][PF6] were stabi­lized using the surfactant Triton X-100. Lignin peroxidases from Phanerochaete chrysosporium and laccases from Trametes versicolor could oxidize 2,6-dime — thoxyphenol and o-phenylenediamine in these microemulsions. The highest activities for lignin peroxidases and laccases were 13 and 33 qmol/l/min, respectively. In contrast, the same enzymes had negligible catalytic activity in pure or water-saturated [BMIM][PF6] [128].

Another approach to stabilize cellulases is their immobilization on a substrate. For example, immobilization on a poly(ethylene glycol) substrate increased activity of cellulases from Trichoderma reesei in 0.05 M citrate buffer and [BMIM][Cl], compared to the free enzyme [121]. Cellulase from Trichoderma reesei immobilized on 150 qm particles suffered no inhibition in 20 vol.% of [MMIM][DMP], N, N-dimethylethanolammonium lactate, and N, N-dimethylet- hanolammonium acetate. In contrast, reducing sugar yields decreased in 20 vol.% of [MMIM][MeSO4], [BMIM][Cl], [BMIM][PF6], and [BMPy][Cl], by 36, 28, 37, and 34%, respectively [44].

Celluclast immobilized onto a polymeric support (Amberlite XAD4) was coated with the hydrophobic ILs 1-butyl-3-methylimidazolium bis(tri — fluoromethylsulfonyl)imide ([BMIM][Tf2N]) or butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([N1114][Tf2N]). The activity of these coated cellulases were assessed as a function incubation time in [BMIM][Cl] and in [N1114][Tf2N]/[BMIM][Cl] mixtures at different molar ratios. The hydrophobic IL coating slowed the deactivation effect from [BMIM][Cl]. It was believed that the hygroscopicity of water-immiscible ILs can keep the enzymes hydrated and prevent their unfolding. The polymeric support may act as a water reservoir to preserve the cellulase activity. The cellulase activity decreased with increasing [N1114][Tf2N] concentration, most likely due to the restricted access of cellulose to the coated enzyme [137].

Butanol inhibition is one of the most crucial problems for developing industrial scale production of butanol. Butanol-producing bacteria can rarely tolerate more than 2% butanol in broth [70]. More precisely, 1% exposure of butanol caused a

20-30% increment in the fluidity of cell membrane [71, 72]. C. acetobutylicum was found sensitive to the higher concentration of butanol than 12-13 g l-1 [43, 73]. Various attempts are being made at the organism and process level for reducing the butanol inhibition. One attractive development in process is as an integrated system of fermentation and recovery processes, which allows simulta­neous production and selective removal of solvents illustrated very significant results at laboratory scale studies. The common butanol recovery techniques are adsorption, liquid-liquid extraction, perstraction, reverse osmosis, pervaporation, and gas stripping, which can be integrated with ABE fermentation for online removal of products (Table 7.3) [74].

Gas stripping (Fig. 7.3) comprises the most advantageous characteristics such as simple and economical process (no need of expensive equipments), does not harm the culture, does not remove the nutrients and reaction intermediates, and reduces butanol inhibition effectively [39, 40]. At laboratory scale, gas stripping showed the significant results as integrated with various kinds of fermentation processes, batch [39, 40, 53], fed-batch [53, 75], and continuous [40]. While nitrogen [40] and gases produced in fermentation (hydrogen and carbon dioxide) [39, 53, 75] are used for stripping purpose, whereas utilization of nitrogen gas reflected more effective recovery than other gases.

Genetical engineering techniques have been applied to increase substrate range in microorganisms such as S. cerevisiae and Z. mobilis that help to maximize ethanol production like that in E. Coli. It also supplies other important traits for conversion of lignocellulose into ethanol. Since the molecular basis for ethanol and inhibitor tolerance is not fully understood, random mutagenesis and evolutionary engi­neering have also been applied to improve those traits. Moreover, as a result of technological developments, systems biology approaches have recently been applied to characterize the functional genomics of microorganisms and to evaluate the impact of metabolic and evolutionary engineering strategies. This advanced characterization (genomics, transcriptomics, proteomics, metabolomics) is already contributing to better understand that physiological responses and to identify crucial targets for metabolic engineering [14, 90, 189].

Anaerobic digestion involves the microbial fermentation of cellulosic/lignocellu — losic biomass in the absence of oxygen for about 2-8 weeks. A similar process involving municipal solid waste, which may, in addition to cellulosic components, contain polymeric substances, fats, proteins, etc. is termed as anaerobic digestion. This is the most commonly used, commercially viable process under the bio­chemical methods of biomass conversion. The product of anaerobic fermentation comprises 65-70% methane, 30-35% carbon dioxide, and traces of other gases such as H2S and hydrogen. This product has a heating value of 26 MJ/m3 [23].

Lignocellulose is very refractory in nature and requires harsh pretreatment procedures before it can be used for fermentation. The pretreatment procedures usually used are described in ‘‘Forest Biorefinery’’. Following the pretreatment procedure, lignocellulose breaks down to cellulose, hemicellulose, and lignin. The cellulose and hemicellulose subsequently undergo anaerobic, or alcoholic fer­mentation to give biogas or ethanol. Alcoholic fermentation is discussed in the following section.

Anaerobic fermentation/digestion is also called biomethanation as methane is the major end product of the process. It can be carried out according to a two-stage scheme or a four-stage scheme. The two-stage scheme originated in the 1930s and involves two major metabolic groups of bacteria- the acid forming and the methane forming bacteria. The acid forming bacteria are a complex species of bacteria which hydrolyze the primary substrate polymers such as polysaccharides, proteins, and lipids, and ferment these to mainly fatty acids and other organic acids, alcohols, ammonia, sulfide, carbon dioxide, and methane. The methane forming bacteria involve a group of bacteria which degrade the products of the first stage to methane and carbon dioxide.

The current processes used (Fig. 1.19), involve four stages which are executed by four different groups of organisms, and end up with different end products [24].

The first phase involves facultative, or strictly anaerobic bacteria such as Streptococcus, Peptococcus, Micrococcus, and Clostridium (a thermophilic spe­cies). These convert the polymers/polysaccharides into the biomass to small monomers or oligomers such as glucose, cellobiose, amino acids, short chain fatty acids, and glycerol. Carbon dioxide and hydrogen are released in the process. In

other words, in this phase, the biomass is hydrolyzed to give smaller fragments, which are then processed further by other groups of microbes. This process is catalyzed by extracellular hydrolases such as cellulase, xylanase, protease, and lipase. This stage is relatively slow and may often be the rate limiting stage in the process.

The second phase is the acidogenesis phase which again involves facultative and strictly anaerobic microbes such as Bacteriodes, Clostridium, Butyribacteri — um, Propionibacterium, Pseudomonas, and Ruminococcus. Some of these are hydrolytic and others, non-hydrolytic. The products of this phase are short chain fatty acids such as formate, acetate, butyrate, isobutyrate, and succinate. Among these, acetate is the major short chain fatty acid. Small quantities of alcohols (methanol, ethanol, glycerol, and butanol) and acetone may also be produced depending on the nature of the initial feedstock and the anaerobic digestion

process used. This stage of fermentative acidogenesis is rapid and often leads to accumulation of the short chain fatty acids. If this happens, it usually leads to reduced methane formation, followed by methanation failure in the subsequent methanation stage. This usually happens when the feedstock contains large amounts of readily fermentable carbohydrates, and when the loading rate is high.

The third phase involves strictly anaerobic bacteria, which are syntrophic acetogens—Syntrophomonas wolfei and Syntrophobacteri wolinii. They are called syntrophs because they are in close proximity to methanogens, the group of microbes that are responsible for methanogenesis, which is the fourth phase of anaerobic digestion (the syntrophy occurs through interspecies hydrogen transfer). The syntrophic acetogens convert substances such as ethanol, and esters such as propionate, butyrate, valerate, and other short chain fatty acids containing three or more than three carbons to acetate, hydrogen, and carbon dioxide. The hydrogen produced is rapidly consumed by the methanogens, which reduces the partial pressure of hydrogen and takes the processes forward toward generation of methane in the next phase. Syntrophic acetogens grow very slowly (require more than one week to grow to sufficient numbers), making this stage another possible rate limiting step (in addition to the first hydrolytic stage).

In case of inefficiency of this acetogenesis phase, accumulation of the non­acetic acid small chain fatty acids, i. e., the products of the second phase occurs, which reduces the pH of the fermentation reactor. This inhibits the methanogens, thus reducing the formation of methane, which eventually ceases completely.

The fourth phase involves a group of microbes called the methanogens that are completely different from bacteria and belong to a class called Archaea. These are strict anaerobes, which require a very narrow range of environmental conditions of pH and temperature and a reduction potential of <300 mV. These are present naturally in anaerobic environments such as swamps and wetlands and grow slowly. Hence, this is a third possible rate limiting step in the process of anaerobic digestion. These microbes use acetate, hydrogen, carbon dioxide (the products of the third phase), in addition to methanol, formate, methylamines, and methyl sulfides as their substrate. Depending on the substrate specificity and methano — genesis pathway, these microorganisms are classified into two categories—the Hydrogenotrophic methanogens and the Acetoclastic or acetotrophic methano — gens. The Hydrogenotrophic methanogens use hydrogen produced by both the acidogens, as well as the syntrophic acetogens to reduce carbon dioxide to methane or to convert methanol, methylamines and methyl sulfides to methane. A wide diversity of species is found in this category. Acetoclastic or acetotrophic methanogens convert acetate to methane. As acetate is the major product of the preceding stage of anaerobic digestion, two-thirds of the methane produced is by this class of microorganisms. Only two genera in this category have been identi — fied—Methanosaeta (formerly called methanothrix) and Methanosarcina. The latter is both acetotrophic as well as hydrogenotrophic. All these processes are endothermic processes. The yield of the anaerobic digestion processes depend on the nature of feedstock used. Yu Zhongtang et al. [24] have listed the Biochemical methane potential (BMP) of a variety of feedstock such as livestock manure, food­processing wastes, municipal solid wastes, crop residues, and energy crops. From among the livestock manure evaluated, poultry manure had the highest BMP of 460 m3/dry ton, whereas beef and dairy cattle manure had a BMP of 148-250 m3/dry ton. Among the food-producing wastes, fresh fruit and vegetable waste had a BMP of 228-495 m3/dry ton and municipal solid (organic fraction) waste had a BMP of 300-550 m3/dry ton. Agricultural residue such as corn stover and wheat straw had a BMP of 250 and 161-241 m3/dry ton, respectively. Energy crops such as sugar beet and grass silage had a BMP of 380 and 390 m3/dry ton respectively. The technologies currently used for anaerobic fermentation/ digestion, along with their salient features are shown in Table 1.7.

In the screw press technology, the biomass is extruded continuously by a screw through a taper die which is heated externally to reduce the friction.

Merits and Demerits of this Technology

1. The output is continuous and the briquette is uniform in size.

2. The outer surface of the briquette is partially carbonized facilitating easy ignition and combustion. This also protects the briquettes from ambient moisture.

3. A concentric hole in the briquette helps in combustion because of sufficient circulation of air.

4. The machine runs very smoothly without any shock load.

5. The machine is lightweight compared to the piston press because of the absence of reciprocating parts and flywheel. The machine parts and the oil used in the machine are free from dust or raw material contamination.

6. The power requirement of the machine is high compared to that of piston press.

At present, screw press and piston press technologies are becoming more important commercially. As the piston press technology is comparatively older than the screw press technology, more piston presses are operating today. How­ever, the screw press technology is also rapidly gaining in importance. The lack of basic research to improve the piston press and the manufacturers’ inability to understand the technology are the two prime reasons that these presses are not performing satisfactorily on a commercial basis [6]. Entrepreneurs face many problems due to frequent wear in the ram and the die. The life of the ram has been observed from 33 to 300 h. This is the most frequently used briquetting equipment and is manufactured throughout the world. It consists of a flywheel that operates a piston, which presses the material through a tapered die where the briquette is formed. But piston presses have not been successful due to a lack of understanding of the characteristics of raw material which in turn affects machine design parameters like flywheel size and speed, crank shaft size and piston stroke length. The feeding mechanism also needs to be perfected, in this case according to the bulk density of the raw material.