Generally, the high viscosity of the IL affects negatively the overall efficiency of the pretreatment [7]. The IL viscosity depends on the IL chemical composition and temperature. For example, [AMIM][Cl] has a lower viscosity than [BMIM][Cl], which enabled wood dissolution at a lower temperature (80°C instead of 110°C). Dissolution in ILs with aromatic substituents, such as [BzMIM][Cl] and 1-methyl — 3-m-methoxylbenzylimidazolium chloride, required higher temperatures (130°C) to achieve the same wood solubility, which was attributed to their higher melting temperatures and viscosities [7]. However, the viscosity of the wood/IL mixture also increases with the wood dissolution over time with the accumulation of extracted products and generation of by-products [4]. The viscosity of cellulose solutions in [EMIM][OAc] or [BMIM][Cl] was found to increase with the cellu­lose concentration [51, 52].

One way to decrease the viscosity of the wood/IL mixture is to increase the temperature, but this solution is energy-intensive and can accelerate the degra­dation of the IL [22]. Another method consists in the addition of a co-solvent with lower viscosity. The viscosity of a wood/[BMIM][Cl] mixture was reduced by the addition of deuterated dimethyl sulfoxide, which had no noticeable effect on the wood dissolution efficiency [4]. In another study, [BzMIM][Cl] was blended with [AMIM][Cl] to reduce its viscosity without significant efficiency loss. Biomass dissolution could occur at a lower temperature and even at room temperature in the less viscous [AMIM][Cl] [7].

During wood dissolution in [BAIM] [Cl] and [MAIM][Cl] with AlCl3 as a catalyst, increased acidity led to a decrease in viscosity, which was attributed to the formation of AlCl — and Al2Cl- that weakens the hydrogen bonds in ionic liquids [16].

Pyrolysis is the thermal heating of materials in the absence of oxygen, which results in the production of three categories: gases, pyrolytic oil and char [22, 23].

Fructose ♦ Furans —- ♦ Biofuels

w

Dimcthymiran

—- ► Biochemials

Lcvulinic Acid

♦ Biomaterials

Fig. 5.2 Effect of catalysts on biomass conversion

Pyrolytic oil, also known as ‘‘tar or bio-oil’’, cannot be used as transportation fuels directly due to the high oxygen (40-50 wt%) and water contents (15-30 wt%) and also low H/C ratios. However, pyrolytic oil is viscous, corrosive, relatively unstable and chemically very complex [1, 24-26]. To use bio-oil as a conventional liquid transportation fuel, it must be catalytically upgraded [31]. Catalytic pyro­lysis (Fig. 5.2) is an acceptable method for improving the quality of pyrolytic oil such as removal of oxygen, increasing calorific value, lowering the viscosity and improving stability. Many researches have been carried out on upgrading pyro­lytic oil in the presence of different catalysts such as HZSM-5, MCM41, Al2O3, Al2O3/B2O3, Na2CO3, NaOH, NaCl, Na2SiO3, TiO2, Fe/Cr, etc. [27-30]. Upgrading of the gaseous products from pyrolysis can also be achieved by reacting the vapors directly with a catalyst (in situ pyrolysis).

ZSM-5 is an aluminosilicate zeolite with a high silica and low aluminum content. Its structure is based on channels with intersecting tunnels (Fig. 5.3). The aluminum sites are very acidic. The substitution of Al3+ in place of the tetrahedral Si4+ silica requires the presence of an added postive charge. When this is H+, the acidity of the zeolite is very high. The reaction and catalysis chemistry of the ZSM-5 is due to this acidity.

Zeolite catalysts added into the pyrolysis process can convert oxygenated compounds generated by pyrolysis of the biomass into gasoline-range aromatics. Using zeolite catalysts in pyrolysis, Carlson et al. [31] reported that gasoline-range aromatics can be produced from solid biomass feedstock in a single reactor at short residence times (less than 2 minutes) and at intermediate temperatures (400- 600°C). In fact, acidity of an ideal catalyst for biomass pyrolysis should be manupilated by various methods such as ion exchange with alkalis. Silica-alumina containing catalysts (weak acids) might also be given as an example.

Mobile crystalline material (MCM-41) is one of the most used catalysts for the conversion of biomass to value-added products during pyrolysis (Fig. 5.4).

Fig. 5.4 The hexagonal pore structure of molecular sieve MCM-41 (red oxygen, blue silicon, light blue hydrogen, brown carbon)

Pore size of MCM-41 is relatively narrow and this catalyst has a large surface area (>1000 m2 g-1). MCM-41 type mesoporous catalysts converted the pyrolysis vapors into lower molecular weight products, and hence, more desired bio-oil properties could be achieved. The catalytic properties of MCM-41 materials can be significantly improved when specific transition metal cations or metal complexes are introduced into the structure. Pore enlargement allows the processing of larger molecules. Different pore sizes were obtained by altering the chain length of the

template and by applying a spacer. Due to the activity of the catalysts, the product distribution of pyrolysis vapors changed significantly. In accordance with pub­lished reports, higher coke and water formation was observed during the reaction in the presence of the catalysts. The various catalysts showed different influences on the product distribution, and the greatest difference was achieved by using the unmodified Al-MCM-41 catalyst [32].

Stefanidis et al. [33] recently investigated the catalytic activity of Silicalite, ZSM-5, MCM41 and Al2O3 for the pyrolysis of beech wood. The results are given in Table 5.1. They found that the use of strongly acidic zeolite H-ZSM-5 leads to a decrease in the total liquid yield (bio-oil) while decreasing the organic phase of bio-oil and increasing its water content, accompanied by an increase of gases and formation of coke on the catalyst.

According to this study, it was found that zeolite silicalite with very low number of acid sites and the mildly acidic Al-MCM-41 induced similar effects with those of H-ZSM-5 but to a less extent, except of the significantly higher coke that was deposited on Al-MCM-41. With regard to the composition of bio-oil, all the catalysts and mostly the strongly acidic H-ZSM-5 zeolite reduced the oxygen content of the organic fraction, mainly by decreasing the concentration of acids, ketones and phenols.

V. K. Joshi, Abhishek Walia and Neerja S. Rana

9.1 Introduction

Ethanol, a solvent, extractant, and antifreeze, is used for synthesis of many sol­vents in the preparation of dyes, pharmaceuticals, lubricants, adhesives, detergents, pesticides, explosives, and resins for the manufacture of synthetic fibers and liquid fuel [163]. Ethanol is a major solvent in industries and ranks second only to water [152].

It is also employed as a solvent for resins, cosmetics and household cleaning products. The ethanol obtained from biomass-based waste materials or renewable sources is called as bioethanol and can be used as a fuel, chemical feedstock, and a solvent in various industries. Besides ethanol, biofuels containing butanol, pro­panol, 2-methyl 1-butanol, isobutanol, isopropanol, etc. are also employed. Bio­ethanol produced by fermentation is rapidly gaining popularity all over the world. The US, Brazil, Japan, France, U. K., Italy, Belgium, and The Netherlands are among the few countries widely using bioethanol for various uses [98]. It has certain advantages as petroleum substitutes, viz., alcohol can be produced from a number of renewable resources, alcohol as fuel burns cleaner than petroleum which is environmentally more acceptable. It is biodegradable and thus, checks pollution. It is far less toxic than fossil fuels. It can easily be integrated to the

V. K. Joshi (H)

Department of Food Science and Technology,

Dr. Y. S. Parmar University of Horticulture and Forestry, Nauni, Solan, Himachal Pradesh, India e-mail: vkjoshipht@rediffmail. com

A. Walia • N. S. Rana Department of Basic Sciences,

Dr. Y. S. Parmar University of Horticulture and Forestry, Nauni, Solan, Himachal Pradesh, India e-mail: sunny_0999walia@yahoo. co. in

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_9, © Springer-Verlag Berlin Heidelberg 2012 existing transport fuel system, i. e., up to 5% bioethanol can be blended with conventional fuel without the need for modification.

Gasohol (mixture of gasoline and alcohol) is widely used to run vehicles in developed countries. The use of alcohol as fuels is gaining vast popularity day-by­day and gasohol program is encouraged throughout the world. By encouraging bioethanol use, the rural economy could also receive a boost by growing the necessary crops. New technologies are being developed that are economically and strategically superior.

The interest in bioethanol as a fuel in response to petroleum price increase is the most significant factor influencing the world ethanol market. Recent oil shortage and escalating oil prices have led scientists to develop alternative energy sources to substitute petroleum. Global warming alerts and threats are on the rise due to the over utilization of fossil fuels. Alternative fuel sources like bioethanol and biodiesel are being produced to combat these threats. Bioethanol production from plant biomass has received considerable attention recently in order to mitigate global warming and demands for petroleum not from a finite resource and is a greenhouse gas emission. The road transport network accounts for 22% of all greenhouse gas emissions, and through the use of bioethanol as some of these emissions will be reduced as the fuel crops absorb CO2. Also, blending bioethanol with petrol will help extend the life of diminishing oil supplies and ensure greater fuel security, avoiding heavy dependence on oil producing nations.

Biofuel obtained from renewable sources can be classified on the basis of their production techniques as given below:

• First-generation fuels refer to biofuels made from plants rich in oil and sugar.

• Second-generation biofuels (Biomass to liquid) are made from organic materi­als, such as straw, wood residues, agricultural residues, reclaimed wood, saw­dust, and low value timber.

• Biofuels of third generation are produced from algae by using modern gene and nanotechnologies.

• Fourth-generation biofuels are produced from vegetable oil by using hydrolytic conversion/deoxygenating.

Tables 9.1 and 9.2 show biofuels of the four generations, their substrates and technological processes of their production.

It is apparent that different types of substrates can be employed to produce bioethanol. Accordingly, modification in its production technology has been made. The replacement of ethanol by ethylene is reversed in less industrialized nations. In Brazil and India, ethylene and its chemical derivates are produced by catalytic dehydration of fermentative ethanol [5].

The USA and Brazil are currently the primary producers of fuel ethanol, pro­ducing 49.6 and 38.3% of the 2007 global production, respectively. US bioethanol production is almost entirely from maize (corn) starch, which is converted into fermentable glucose by the addition of amylase and glucoamylase enzymes. In 2007, 24.6 billion L of ethanol was produced in the USA, that comprised of only 3.2% of gasoline consumption on an energy-equivalent basis [188].

Source [172]

The production costs of various chemicals from ethanol and petroleum feed­stocks are compared in Table 9.3. Clearly, the production of bioethanol from first generation is economically unreasonable because of discarding cellulose and hemicellulose which constitute the majority of carbon resources of plants. Fur­thermore, the biofuels of this generation also compete with food products intended for human consumption. Thus, second-generation bioethanol production is important as it allows improved CO2 balance and make use of cheap, waste source which does not compete with human food products.

In brief, the use of ethanol as a biofuel is gaining increasing popularity. Although it is produced from several sources but the technologies using the waste material for its production is most attractive as it does not interfere with food particular substrates needed for the ever increasing world population. Different types of waste materials, their composition, biochemistry, microbiology, and the technology involved in bioethanol production have been reviewed in this chapter. New strategies and future thrust has also been briefly highlighted.

Fermentation by-products or non-metabolized feed components can inhibit the ethanol production and yeast growth. These secondary components become more concentrated when used and this limits the recycling process of distillery residue.

Acetate and lactate are the most important inhibitory fermentation by-products

[125] . Certain inhibitors are high in a few substances, e. g., sulfite waste liquor may be high in sulphurous acid and furfural. Blackstrap molasses may contain high concentrations of calcium salts. High temperature, sugar concentration, and ster­ilization in the presence of salts (especially phosphates) and proteins can produce components toxic to yeast [22].

When important individual inhibitors are not present, a combination of inhib­itors or generalized osmotic pressure effects shall be the limiting factors. High salt concentrations also encourage the production of undesirable by-products such as

glycerol [193]. A 16-20% non-fermentable dissolved solids content sets the practical upper limit for most yeasts in the absence of toxic inhibitors [52].

A biorefinery using forest residues as its feedstock is called a forest biorefinery. The enormous scope of using biomass generated from forests for energy genera­tion has been excellently highlighted by Klass [45] by way of the statistics pre­sented in his book titled ‘‘Biomass for renewable energy, fuels and chemicals’’. According to this statistics, forests cover only about 9.5% of the earth’s surface or about 32% of the total land area but account for 89.3% of the total standing biomass and 42.9% of total annual biomass production. In terms of energy, forests alone could produce 1,030 quadrillion BTU/year which is equivalent to more than double the world’s total primary consumption of about 460 quadrillion BTU in 2005. Thus, forest biomass can be considered to be a very important source for feedstock of a biorefinery. Forest biomass can be categorized into two categories as shown in Table 1.10.

The use of forest biomass for energy generation had taken a back seat until a few years ago due to the depletion of forests as a result of felling trees for pro­ducing forest products like lumber, paper, and other items. However, this excellent source of fuel has again gained importance since the development of biorefinery concept as it is now being recognized as an attractive alternative for pulp and paper mills which see in it, the incentive of increasing their revenue by producing biofuels and other chemicals in addition to their core products, as also the forest waste can be processed in a typical forest biorefinery to yield a number of valuable products including biofuels, without jeopardizing the forest vegetation.

Forest biomass is mainly lignocellulosic in nature. Lignocellulose consists of cellulose (40-47%), hemicellulose (25-35%), lignin (16-31%), and other extrac­tives (2-8%), where the polysaccharides cellulose and hemicellulose are tightly cross-linked with lignin via ester and ether linkages with the purpose of providing structural rigidity to higher plants and trees and protecting the cell walls of plants from various external physical and chemical hazards. Therefore lignocellulose, in its native form, is highly refractory in nature and resistant to most hydrolytic processes which aim at extracting cellulose for further hydrolyzing it to fermen­table sugars which can be converted to biofuels. Hence, pretreatment of lig — nocellulose is essential before it can be used for other conversion processes in a biorefinery. Table 1.11 gives a list of the types of pretreatment that can be done on lignocellulose before it can be used in a biorefinery [46, 47].

The pretreatment of lignocellulosic feed stock serves the basic purpose of converting the native lignocellulosic biomass into a form where hydrolysis can be effectively achieved. Among these, the biological methods of pretreatment have a number of advantages in that, the equipment requirement is modest, no environ­mentally damaging waste products are generated, and hazardous chemicals and conditions are avoided. All this results in a significant amount of cost saving. However, the total pretreatment time required is very lengthy and there are chances of degradation of polysaccharide which may reduce the total fermentable substrate. Newer methods of pretreatment aim at not only improving hydrolysis but also carrying out fractionation, where the lignocellulosic biomass is converted to its core components—cellulose, hemicelluloses, and lignin. A lot of work still needs to be done, however, to carry out fractionation of lignocelluloses in a manner that is technically feasible and at the same time, economically viable. The conceptual ideas for the purpose have been proposed by a number of researchers. FitzPatrick et al. [47] have reviewed these techniques at length.

A majority of lignocellulosic biomass including forest biomass is used in a kraft mill where the lignocellulosic feedstock is processed to paper pulp, which serves as an important intermediate material for the generation of a variety of paper products. Figure 1.26a and b shows a schematic of a typical Kraft mill and how the biorefinery concept can be integrated into such a kraft mill to get multiple products including energy products and other value-added products [48].

The key requirement of integrating the biorefinery concept into a kraft mill is the recovery of hemicellulose which will enable its conversion into ethanol and/or other products. Mao et al. [49] introduced a ‘‘near neutral’’ process prior to pul­ping, for extraction of hemicellulose which otherwise ends up in the black liquor. However, this ‘‘near neutral’’ process modifies the energy balance of the kraft pulp mill. During the pretreatment process, approximately 10% of hemicellulose and lignin is extracted. This reduces the calorific value of the black liquor which is used for production of steam. Thus, less steam is produced whereas the steam consumed in the extraction process is greater. Marinova et al. [48] studied the effect of introducing a hemicellulose extraction and conversion stage into a Canadian hardwood Kraft pulp mill on the energy supply and demand of the mill. On the basis of their studies, they have proposed process optimization methods

have been shown to reduce the steam consumption in a Kraft mill by 5.04 GJ/Adt, thus making the process more cost-effective and economical.

The lignin, which remains after processing has the potential to serve as an important precursor for a wide variety of products. The US DOE gives a com­prehensive data regarding the possible products that can be obtained out of pro­cessing of this residual lignin [37]. Presently, very few Kraft mills separate and use lignin for producing other products.

Biorefinery Based on Industry (Process Residues and Leftovers), and Municipal Solid Waste

Residues and wastes comprise of the following:

— Municipal solid waste

— Municipal sewage sludge

— Animal waste

— Crop residues

— Industrial waste

— Forest waste.

Use of crop residues and forest waste for biomass conversion has already been discussed in the earlier sections. This section will focus specifically on the major source of biorefinery feedstock viz. municipal solid waste and industrial waste. Municipal soild waste also includes municipal green waste such as tree trimmings and gardening wastes, waste wood, and paper component of domestic rubbish.

Municipal solid waste can be defined as a combination of domestic, light industrial, and demolition solid waste generated within a community [6]. There are established priorities so far as disposal of municipal solid waste is concerned. Recycling, if it is economical to do so, enjoys the first priority. The green com­ponent of MSW can be separated out and used for compost or as mulch. The use of municipal green waste for energy production has the advantage that it reduces the waste load to municipal landfills which consequently reduces GHG methane arising from its decomposition. The anaerobic conversion process used for such conversions has already been discussed in ‘‘Anaerobic Digestion’’ The biorefinery integration into the waste conversion process is described in Fig. 1.27 which shows a current waste biorefinery and how an advanced future biorefinery could be developed to maximally tap the potential of waste-to-energy technology.

The residence time in a digester varies with the amount and type of feed material, the configuration of the digestion system and whether it be one-stage or two-stage. In the case of single-stage thermophilic digestion residence times may be in the region of 14 days, which compared to mesophilic digestion is relatively fast. The plug-flow nature of some of these systems will mean that the full degradation of the material may not have been realized in this timescale. In two-stage mesophilic digestion, residence time may vary between 15 and 40 days. In the case of mes­ophilic UASB digestion hydraulic residence times can be (1 h-1 day) and solid retention times can be up to 90 days. In this manner the UASB system is able to separate solid and hydraulic retention times with the utilization of a sludge blanket.

Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mix the contents enabling the bacteria and the food to be in contact. They also allow excess material to be continuously extracted to maintain a reasonably constant volume within the digestion tanks.

Numerous ILs cause the swelling of native biomass, which was seen as a good indicator of biomass solubility. Swelling occurs even at room temperature in poplar exposed to [EMIM][OAc]. The cross-sectional area of poplar cell walls expanded by 60 to 100% in 3 h in [EMIM][OAc]. After rinsing with deionized water, the wood structure contracted almost immediately [70, 71]. Significant swelling was also observed in Miscanthus switchgrass heated at 100°C in [EMIM][Cl] for 20 h [26]. The magnitude of wood swelling depended on the IL used in the pretreatment.

The swelling of pine (Pinus radiata) sapwood chips (dimensions 10 x 10 x 5 mm3) was studied in ILs consisting of the [BMIM] cation and several different anions. Little swelling was observed in 1-butyl-3-methylimida — zolium trifluoromethanesulfonate ([BMIM][CF3SO3]) even at 120°C. The ILs 1-butyl-3-methylimidazolium dicyanamide ([BMIM][N(CN)2]) and [BMIM] [MeSO4] led to a swelling along the tangential direction (tangent to tree rings) of about 8%, which is larger than with just water (5%). The most dramatic swelling was observed in the tangential direction with [BMIM][Me2PO4] and [BMIM] [OAc] and the magnitude was temperature-dependent, from 15% at 90°C to 20% at 120°C [37]. Little expansion was observed in pine chips along the radial direction. [BMIM][N(CN)2] and [BMIM][MeSO4] caused the expansion along the axial direction, while 1-butyl-3-methylimidazolium dimethylphosphate ([BMIM] [Me2PO4]) and [BMIM][OAc] caused the reduction in the axial direction, most likely due to partial dissolution. The different swelling rates among ILs were attributed to the temperature-dependent viscosity [37].

However, swelling induced by the IL does not necessarily mean that the IL is a good solvent for cellulose and biomass. The swelling and dissolution of pine wood pulp fibers were studied in [BMIM][Cl], 1-allyl-3-methylimidazolium bromide ([AMIM][Br]), and butenylmethylimidazolium bromide. While the fibers swelled and dissolved in [BMIM][Cl], they swelled homogeneously in [AMIM][Br] and butenylmethylimidazolium bromide without dissolution. Both ILs penetrated the fibers quickly but did not disrupt the hydrogen bonding [90].

Transformation of fatty acids over palladium catalysts is highly selective to pro­duce hydrocarbons with n-1-carbon number, where n is carbon number of fatty acid substrate. The formation of gaseous products, CO, and CO2, indicates that transformation of fatty acids occurs via decarboxylation and decarbonylation [9]. Hydrogen is not needed for the reaction to occur but low amounts are beneficial for catalyst stability. The reaction pathway was proposed for deoxygenation of stearic acid over Pd/C catalyst (Fig. 6.4) [9]. The pathways have been updated from the original work of [14] by adding the intermediate steps of the formation of aldehyde and its hydrogenation to alcohol based on our recent data [28], where it was shown that catalytic pathway in deoxygenation of fatty acids depends on the hydrogen content in gas atmosphere. In hydrogen-free conditions the main reaction is decarboxylation whereas in hydrogen-rich conditions decarbonylation dominates. The latter reaction proceeds through an aldehyde intermediate, which is trans­formed at high rate to (n-1) hydrocarbon. The alcohol intermediate can be formed in these conditions via hydrogenation of aldehyde; however, further dehydroxy — lation to the corresponding hydrocarbon does not proceed, while alcohol is

I_ TI I (isomerization) ,1 1————————

С17Н36 |-*—————————- *—— ► saturated Cl 7

+/-H2

(dimenzation) ^

Fig. 6.4 Reaction pathway for deoxygenation of stearic acid over a Pd/C catalyst. Adopted from Ref. [9] decomposed over palladium via an alkyl intermediate to (n-1) hydrocarbon [28]. Despite numerous of reactions that can occur on the palladium catalyst the selectivity toward by-products is very low. The 95 mol% of stearic acid was converted to n-heptadecane and 3 mol% to n-heptadecenes, over 5 wt% Pd/C (Table 6.2).

The desired products of deoxygenation are long-chain hydrocarbons. By-products of the reaction can by created by cracking (not observed over Pd/C catalyst) and due to transformation of the unsaturated products (olefins) or unsaturated fatty acids. Olefins can be transformed to cycloalkanes by cyclization reaction which goes via dehydrogenation forming aromatics (e. g. benzyl unde­cane) [29], while unsaturated fatty acids can form dimers by Diels-Alder reaction [30].

Deoxygenation of fatty acid esters over palladium catalyst occurs via formation of fatty acid followed by decarboxylation/decarbonylation reaction [31, 32]. Aromatic C17 hydrocarbons were also found as by-products in small extents.

In the case of unsaturated fatty acids the reaction pathway is extended by isomerization reaction of the feedstock (Fig. 6.5). In the studies of linoleic and oleic acid deoxygenateon over Pd/C catalyst, it was proven that cis/trans izomerization can occur [33] together with migration of the double bond through aliphatic chain giving high number of fatty acid isomers [29].

In the first work describing the deoxygenation of fatty acids over noble metal catalysts [9] it was shown that decarboxylation is the main reaction giving CO2 as a gas product. Recently it was proven that during reaction, the ratio between

Heavy byproducts (dimers, aromatics)

OMOUirCH]

Hep lade carte

Fig. 6.5 Reaction scheme of oleic acid deoxygenation over a Pd/C catalyst. Taken from Ref. [33]

decarboxylation and decarbonylation can change. Deoxygenation over a fresh Pd/C catalyst starts with high selectivity toward decarboxylation. However, in time the selectivity toward decarboxylation can decrease followed by an increase of decarbonylation reaction [34]. This phenomenon could occur due to accumulation of carbon monoxide in the reaction atmosphere (see Sect. 6.2.2.3).

Hydrogen per se is not needed for decarboxylation/decarbonylation reaction to occur. Its presence can, however, influence reaction pathway. It was shown that increase of hydrogen content in the reaction atmosphere is increasing hydroge­nation rate of carboxylate group. Thereafter, an instant decarbonylation of the created aldehyde species occurs [28].

9.4.1 Fermentation of Carbohydrates

Carbohydrates serve as the chief source of energy in all heterotrophs with sup­plementation by proteins and fats. The metabolic sequence of energy generation from these major groups of nutrients suggests that carbohydrates are the source of

energy in the primitive form of life. In the following section, the degradation of carbohydrates, especially polysaccharides that are generally the source of energy liberated either by fermentation or through other metabolic processes, will be discussed.

The design of a combustion system is important for achieving optimum efficiency from the process. During the combustion process, slagging and fouling of the furnace and the boiler occurs. This is more serious when biomass contains a high proportion of alkali metals. The alkalis volatilize during combustion and condense as alkali metal salts on the relatively cool furnace walls. These elements react with other compounds to form a sticky lining on the furnace and boiler wall surface. Regular cleaning of these deposits is required which usually involves process shut­down, reducing the efficiency of the process. The design of the combustion equipment should be such that a minimum of fouling takes place. A number of different designs of combustion systems have evolved in an attempt to get max­imum combustion efficiency with minimum fouling. These are summarized along with the salient features of each design in Table 1.3.

Fixed-bed combustion

In this type of combustion system, the biomass is fed in the form of a bed on grates at the bottom of a furnace. The grates may be either inclined or horizontal. Air is passed through the grate (on which the fuel is present) at a restricted rate such that the fuel is not stirred and there is no relative movement of the fuel solids. The stokers used for feeding the fuel may be either overfeed stokers or spreader stokers.

The overfeed stokers were originally designed for firing coal. These feed the fuel by gravity onto the moving grate at one end. The grate travels slowly across the furnace, carrying the fuel along, as combustion takes place. The residual ash and slag are continuously discharged at the opposite end.

Table 1.3 Designs of combustion systems [7, 8] Combustion method Salient features Fixed bed Florizontal grate: Overfeed Grate is level and moving in different manners. Biomass is fed by gravity onto the moving combustion — Forward moving grate — Reverse moving grate stokers grate at one end. It ignites and burns as surface combustion. Residual ash and slag is continuously discharged at the opposite end. — Reciprocating grate Spreader Grate is level and moving in different manners. Stokers distribute the comminuted — Step grate — Louvre grate Inclined grate stokers biomass onto the furnace above an ignited fuel bed on an air cooled travelling grate. Suspension firing occurs partially. Fine particles tend to bum in suspension while larger particles fall onto the travelling grate where they are burnt. Most common design selected for biomass combustion systems. Biomass is fed at the upper part of the grate. Pre-drying of fuel occurs at the upper part of the furnace after which it slowly tumbles down under gravity onto a reciprocating grate lower in the furnace where combustion takes place. The grate is either water cooled or air cooled. Suitable for biomass fuels with lower ash contents. Fluidized Bubbling fluidized bed combustion Circulation fluidized bed combustion Finely comminuted biomass particles fed onto a bed of sand at the bottom of the furnace and subjected to an evenly upward flow of air which fluidizes the biomass. Initial drying followed by ignition takes place. Rotary hearth furnace combustion Kiln furnace Suitable for combustion of high moisture fuel such as liquid organic sludge and food residue. Burner combustion Burner Used for burning wood powder and fine powder such as bagasse and pith.

Spreader stokers distribute the comminuted and dried biomass fuel over an ignited fuel bed on an air cooled traveling grate. These stokers can be made responsive to heat load changes by automatic adjustment of grate travel speed, fuel feed rate, and air intake. A major disadvantage with this type of a system is that an ash layer needs to be maintained on the grate in order to protect it from thermal degradation. Biomass ash may have a high silica content which may cause a greater abrasion of the grate, resulting in a higher maintenance cost of the grate. Another disadvantage with this type of a combustion design is that there can be a significant amount of fly ash and unburned carbon in the flue gas, resulting in lower combustion and boiler efficiencies and higher costs of emission controls.