As mentioned earlier, a typical biomass has three main components: (1) cel­lulose, (2) hemicellulose, and (3) lignin. These constituents have different rates of degradation and preferred temperature ranges of decomposition.

Cellulose

Decomposition of cellulose is a complex multistage process. A large number of models have been proposed to explain it. The Broido-Shafizadeh model (Bradbury et al., 1979) is the best-known and can be applied, at least qualita­tively, to most biomass (Bridgwater et al., 2001).

Figure 3.8 is a schematic of the Broido-Shafizadeh model, according to which the pyrolysis process involves an intermediate prereaction (I) followed by two competing first-order reactions:

• Reaction II: dehydration (dominates at low temperature and slow heating

rates)

• Reaction III: depolymerization (dominates at fast heating rates)

Reaction II involves dehydration, decarboxylation, and carbonization through a sequence of steps to produce char and noncondensable gases like water vapor, carbon dioxide, and carbon monoxide. It is favored at low tem­peratures, of less than 300 °C (Soltes and Elder, 1981, p. 82), and slow heating rates (Reed, 2002, p. II-113).

Reaction III involves depolymerization and scission, forming vapors includ­ing tar and condensable gases. Levoglucosan is an important intermediate product in this path (Klass, 1998, p. 228), which is favored under faster heating rates (Reed, 2002, p. II-113) and higher temperatures of over 300 °C (Soltes and Elder, 1981, p. 82).

The condensable vapor, if permitted to escape the reactor quickly, can condense as bio-oil or tar. On the other hand, if it is held in contact with biomass within the reactor, it can undergo secondary reactions (reaction IV), cracking the vapor into secondary char, tar, and gases (Figure 3.8). Reactions II and III are preceded by reaction I, which forms a very short-lived intermediate product called active cellulose that is liquid at the reaction temperature but solid at room temperature (Boutin and Lede, 2001; Bradbury et al., 1979; Bridgwater et al., 2001).

There are diverse views on the existence of reaction I, as this unstable species is not seen in the final product in most pyrolysis processes. It is, however, apparent in ablative pyrolysis, where wood is dragged over a hot metal surface to produce the feeling of smooth lubrication due to the presence of the intermediate liquid product, "active cellulose.”

Depolymerization (reaction III) (Figure 3.8) has activation energies higher than those of dehydration (reaction II) (Bridgwater et al, 2001). Thus, a lower

temperature and a longer residence time favor this reaction, producing primarily char, water, and carbon dioxide. On the other hand, owing to its higher activa­tion energy, reaction III is favored at higher temperatures, fast heating rate, and longer residence times, yielding mainly gas. Moderate temperature and short vapor residence time avoid secondary cracking, producing mainly condensable vapor—the precursor of bio-oil, which is of great commercial importance. For cellulose pyrolysis, Table 3.3 gives some suggested reaction rate constants for reactions I, II, III, and IV.

The Broido-Shafizadeh model, though developed for one biomass compo­nent (cellulose), can be applied to the pyrolysis of an entire biomass such as wood. If a log of wood is heated very slowly, it shows glowing ignition, because reaction II predominates under this condition, producing mostly char, which ignites in contact with air without a yellow flame. If the wood is heated faster, it burns with a yellow flame, because at a higher heating rate, reaction III pre­dominates, producing more vapors or tar, both of which burn in air with a bright flame.

Hemicellulose

Hemicellulose produces more gas and less tar but also produces less char in comparison to cellulose. However, it produces the same amount of aqueous product of pyroligneous acid (Soltes and Elder, 1981, p. 84). Hemicellulose undergoes rapid thermal decomposition (Demirbas, 2000), which starts at a temperature lower than that for cellulose or lignin. It contains more combined moisture than lignin has, and its softening point is lower as well. The exother­mic peak of hemicellulose appears at a temperature lower than that for lignin (Demirbas, 2000). In slow pyrolysis of wood, hemicellulose pyrolysis begins at 130 to 194 °C, with most of the decomposition occurring above 180 °C (Mohan et al., 2006, p. 126).

Lignin

Pyrolysis of lignin typically produces about 55% char (Soltes and Elder, 1981), 15% tar, 20% aqueous components (pyroligneous acid), and about 12% gases. It is more difficult to dehydrate lignin than cellulose or hemicellulose (Mohan, 2006, p. 127). The tar produced from it contains a mixture of phenolic com­pounds, one of which, phenol, is an important raw material of green resin (a resin produced from biomass). The aqueous portion comprises methanol, acetic acid, acetone, and water. The decomposition of lignin in wood can begin at 280 °C, continuing to 450 to 500 °C, and can reach a peak rate at 350 to 450 °C (Kudo and Yoshida, 1957).

The polymeric composition of the cell walls and other constituents of a biomass vary widely (Bergman et al., 2005a), but they are essentially made of three major polymers: cellulose, hemicellulose, and lignin.

Cellulose

Cellulose, the most common organic compound on Earth, is the primary structural component of cell walls in biomass. Its amount varies from 90% (by weight) in cotton to 33% for most other plants. Represented by the generic formula (C6H10O5)„, cellulose is a long chain polymer with a high degree of

polymerization (-10,000) and a large molecular weight (-500,000). It has a crystalline structure of thousands of units, which are made up of many glucose molecules. This structure gives it high strength, permitting it to provide the skeletal structure of most terrestrial biomass (Klass, 1998, p. 82). Cellulose is primarily composed of d-glucose, which is made of six carbons or hexose sugars (Figure 2.7).

Cellulose is highly insoluble and, though a carbohydrate, is not digestible by humans. It is a dominant component of wood, making up about 40 to 44% by dry weight.

Hemicellulose

Hemicellulose is another constituent of the cell walls of a plant. While cellulose is of a crystalline, strong structure that is resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength (Figure 2.8). It is a group of carbohydrates with a branched chain structure and a lower degree of polym­erization (-100-200), and may be represented by the generic formula (C5H8O4)„ (Klass, 1998, p. 84). Figure 2.8 shows the molecular arrangement of a typical hemicellulose molecule, xylan.

There is significant variation in the composition and structure of hemicel­lulose among different biomass. Most hemicelluloses, however, contain some simple sugar residues like d-xylose (the most common), d-glucose, d-galactose, l-ababinose, d-glucurnoic acid, and d-mannose. These typically contain 50 to 200 units in their branched structures.

Hemicellulose tends to yield more gases and less tar than cellulose (Milne, 2002). It is soluble in weak alkaline solutions and is easily hydrolyzed by dilute acid or base. It constitutes about 20 to 30% of the dry weight of most wood.

Lignin

Lignin is a complex highly branched polymer of phenylpropane and is an integral part of the secondary cell walls of plants. It is primarily a three­dimensional polymer of 4-propenyl phenol, 4-propenyl-2-methoxy phenol, and 4-propenyl-2.5-dimethoxyl phenol (Diebold and Bridgwater, 1997) (Figure 2.9). It is one of the most abundant organic polymers on Earth (exceeded only by cellulose). It is the third important constituent of the cell walls of woody biomass.

Lignin is the cementing agent for cellulose fibers holding adjacent cells together. The dominant monomeric units in the polymers are benzene rings. It is similar to the glue in a cardboard box, which is made by gluing together papers in special fashion. The middle lamella (Figure 2.5), which is composed primarily of lignin, glues together adjacent cells or tracheids.

Lignin is highly insoluble, even in sulphuric acid (Klass, 1998, p. 84). A typical hardwood contains about 18 to 25%, while a softwood contains 25 to 35% by dry weight.

In this process, biomass particles are fed into the bottom of a rotating cone (360-960 rev/min) together with an excess of heat-carrier solid particles (see Figure 3.9(e)). Centrifugal force pushes the particles against the hot wall; the particles are transported spirally upward along the wall. Owing to its excellent mixing, the biomass undergoes rapid heating (5000 K/s) and is pyrolyzed within the small annular volume. The product gas containing bio-oil vapor leaves through another tube, while the solid char and sand spill over the upper rim of the rotating cone into a fluidized bed surrounding it, as shown in Figure 3.9(e). The char burns in the fluidized bed, and this combustion helps heat the cone as well as the solids that are recycled to it to supply heat for pyrolysis. Special features of this reactor include very short solids residence time (0.5 seconds) and a small gas-phase residence time (0.3 seconds). These typically provide a liquid yield of 60 to 70% on dry feed (Hulet et al., 2005). The absence of a carrier gas is another advantage of this process. The complex geometry of the system may raise some scale-up issues.

In biochemical conversion, biomass molecules are broken down into smaller molecules by bacteria or enzymes. This process is much slower than thermo­chemical conversion, but does not require much external energy. The three principal routes for biochemical conversion are:

• Digestion (anaerobic and aerobic)

• Fermentation

• Enzymatic or acid hydrolysis

The main products of anaerobic digestion are methane and carbon dioxide in addition to a solid residue. Bacteria access oxygen from the biomass itself instead of from ambient air.

Aerobic digestion, or composting, is also a biochemical breakdown of biomass, except that it takes place in the presence of oxygen. It uses different types of microorganisms that access oxygen from the air, producing carbon dioxide, heat, and a solid digestate.

In fermentation, part of the biomass is converted into sugars using acid or enzymes. The sugar is then converted into ethanol or other chemicals with the help of yeasts. The lignin is not converted and is left either for combustion or for thermochemical conversion into chemicals. Unlike in anaerobic digestion, the product of fermentation is liquid.

Fermentation of starch and sugar-based feedstock (i. e., corn and sugarcane) into ethanol is fully commercial, but this is not the case with cellulosic biomass because of the expense and difficulty in breaking down (hydrolyzing) the mate­rials into fermentable sugars. Ligno-cellulosic feedstock, like wood, requires hydrolysis pretreatment (acid, enzymatic, or hydrothermal) to break down the cellulose and hemicellulose into simple sugars needed by the yeast and bacteria for the fermentation process. Acid hydrolysis technology is more mature than enzymatic hydrolysis technology, though the latter could have a significant cost advantage. Figure 1.6 shows the schemes for fermentation (of sugar) and acid hydrolysis (of cellulose) routes.

The heating value of biomass is relatively low, especially on a volume basis, because its density is very low.

Higher Heating Value

Higher heating value (HHV) is defined as the amount of heat released by the unit mass or volume of fuel (initially at 25 °C) once it is combusted and the products have returned to a temperature of 25 °C. It includes the latent heat of vaporization of water. HHV can be measured in a bomb calorimeter using ASTM standard D-2015 (withdrawn by ASTM 2000, and not replaced). It is also called gross calorific value (GCV). In North America the thermal effi­ciency of a system is usually expressed in terms of HHV, so it is important to know the HHV of the design fuel.

Lower Heating Value

The temperature of the exhaust flue gas of a boiler is generally in the range 120 to 180 °C. The products of combustion are rarely cooled to the initial tempera­ture of the fuel, which is generally below the condensation temperature of steam. So the water vapor in the flue gas does not condense, and therefore the latent heat of vaporization of this component is not recovered. The effective heat available for use in the boiler is a lower amount, which is less than the chemical energy stored in the fuel.

The lower heating value (LHV), also known as the net calorific value (NCV), is defined as the amount of heat released by fully combusting a speci­fied quantity less the heat of vaporization of the water in the combustion product.

The relationship between HHV and LHV is given by

where LHV, HHV, H, and M are lower heating value, higher heating value, hydrogen percentage, and moisture percentage, respectively, on an as-received basis. Here, hg is the latent heat of steam in the same units as HHV (i. e., 970 BTU/lb, 2260 kJ/kg, or 540 kCal/kg).

Many European countries define the efficiency of a thermal system in terms of LHV. Thus, an efficiency expressed in this way appears higher than that expressed in HHV (as is the norm in many countries, including the United States and Canada), unless the basis is specified.

Bases for Expressing Heating Values

Similar to fuel composition, heating value (HHV or LHV) may be also expressed on any of the following bases:

• As-received basis (ar)

• Dry basis (db), also known as moisture-free basis (mf)

• Dry ash-free basis (daf), also known as moisture ash-free basis (maf)

If Mf kg of fuel contains Q kJ of heat, Mw kg of moisture, and Mash kg of ash, HHV can be written on different bases as follows:

ННУаг = Q kJ/kg

Estimation of Biomass Heating Values

Experimental methods are the most reliable means of determining the heating value of biomass. If these are not possible, empirical correlations like the Dulong-Berthelot equation, originally developed for coal with modified coef­ficients for biomass, may be used. Channiwala and Parikh (2002) developed the following unified correlation for HHV based on 15 existing correlations and 50 fuels, including biomass, liquid, gas, and coal.

HHV = 349.1C + 1178.3H +100.55 -103.40 — 15.Ш — 21.1ASH kJ/kg (2.32)

where C, H, S, O, N, and ASH are percentages of carbon, hydrogen, sulfur, oxygen, nitrogen, and ash as determined by ultimate analysis on a dry basis. This correlation is valid within the range:

• 0 < C < 92%; 0.43 < H < 25%

• 0 < O < 50; 0 < N < 5.6%

• 0 < ASH < 71%; 4745 < HHV < 55,345 kJ/kg

Ultimate analysis is necessary with this correlation, but it is expensive and time consuming. Zhu and Venderbosch (2005) developed an empirical method to estimate HHV without ultimate analysis. This empirical relationship between the stoichiometric ratio (SR) and the HHV is based on data for 28 fuels that include biomass, coal, liquid, and gases. The relation is useful for preliminary design:

HHV = 3220 x Stoichiometric ratio, kJ/kg (2.33)

where the stoichiometric ratio is the theoretical mass of the air required to burn 1 kg fuel.

Stoichiometric Amount of Air for Complete Combustion Noting that dry air contains 23.16% oxygen, 76.8% nitrogen, and 0.04% inert gases by weight, the dry air required for complete combustion of a unit weight of dry hydrocarbon, Mda, is given by

Mda = [0.1153 C + 0.3434 (H — 0/8) + 0.0434 S ] kg/kg dry fuel (2.34)

where C, H, O, and S are the percentages of carbon, hydrogen, oxygen, and sulfur, respectively, on a dry basis.

Syngas provides the feedstock for many chemical reactions, including methanol synthesis (Eq 1.11). Methanol (CH3OH) is a basic building block of many products, including gasoline.

CO + 2H2(catalysts)^ CH3OH (1.11)

1.6.2 Ammonia synthesis

Ammonia (NH3) is an important feedstock for fertilizer production. It is pro­duced from pure hydrogen and nitrogen from air.

3H2 + N2 (catalysts) ^ 2NH3 — 92,000 kJ/kmol (1.12)

1.6.3 Fischer-Tropsch Reaction

The Fischer-Tropsch synthesis reaction can synthesize a mixture of CO and H2 into a range of hydrocarbons, including diesel oil.

(2n +1) H2 + nCO (catalyst) —— CnH(2 n+2) + nH2O (1.13)

Here, C„H(2„+2) represents a mixture of hydrocarbons ranging from methane and gasoline to wax. Its relative distribution depends on the catalyst, the tempera­ture, and the pressure chosen for the reaction.

To optimize the process parameters and maximize desired yields, knowledge of the kinetics of pyrolysis is important. However, it is very difficult to obtain reliable data of kinetic rate constants that can be used for a wide range of biomass and for different heating rates. This is even more difficult for fast pyrolysis as it is a nonequilibrium and nonsteady state process. For engineering design purposes, a “black-box” approach can be useful, at least for the first approximation. The following discussion presents a qualitative understanding of the process based on data from relatively slow heating rates.

Kinetic models of the pyrolysis of ligno-cellulosic fuels like biomass may be broadly classified into three types (Blasi, 1993):

One-stage global single reaction. The pyrolysis is modeled by a one-step reaction using experimentally measured weight-loss rates.

One-stage, multiple reactions. Several parallel reactions are used to describe the degradation of biomass into char and several gases. A one-stage simplified kinetic model is used for these parallel reactions. It is useful for determination of product distribution.

Two-stage semiglobal. This model includes both primary and secondary reactions, occurring in series.

One-Stage Global Single-Reaction Model This reaction model is based on a single overall reaction:

Biomass ^ Volatile + Char

The rate of pyrolysis depends on the unpyrolyzed mass of the biomass. Thus, the decomposition rate of mass, mb, in the primary pyrolysis process may be written as

Here, mc is the mass of char remaining after complete conversion (kg), k is the first-order reaction rate constant (T1), and t is the time (s).

The fractional change, X, in the mass of the biomass may be written in nondimensional form as

x = (3.3)

(m0 — mc)

where m0 is the initial mass of the biomass (kg).

Substituting fractional conversion for the mass of biomass in Eq. (3.2),

— = к (1 — X) (3.4)

dt

Solving this equation we get

X = 1 — A exp (-kt) (3.5)

where A is the pre-exponential coefficient, E is the activation energy (J/mol), R is the gas constant (J/mol. K), and T is the temperature (K).

Owing to the difficulties in extracting data from dynamic thermogravimetric analysis, reliable data on the pre-exponential factor A and the activation energy E are not easily available for fast pyrolysis (Reed, 2002, p. II-103). However, for slow heating we can obtain some reasonable values. If the effect of second­ary cracking and the heat-transfer limitation can be restricted, the weight-loss rate of pure cellulose during pyrolysis can be represented by an irreversible, one-stage global first-order equation.

Table 3.4 lists values of the activation energy E and the pre-exponential factor A, according to the one-step global reaction model for the pyrolysis of various biomass types at a relatively slow heating rate.

Other models are not discussed here, but details are available in several publications, including Blasi (1993).

Classification is an important means of assessing the properties of a fuel. Fuels belonging to a particular group have similar behavior irrespective of their type or origin. Thus, when a new biomass is considered for gasification or other thermochemical conversion, we can check its classification, and then from the known properties of a biomass of that group, we can infer its conversion potential.

There are three methods of classifying and ranking fuels using their chemi­cal constituents: atomic ratios, the ratio of ligno-cellulose constituents, and the ternary diagram. All hydrocarbon fuels may be classified or ranked according to their atomic ratios, but the second classification is limited to ligno-cellulose biomass.

A vacuum pyrolyzer, as shown in Figure 3.9(f), comprises a number of stacked heated circular plates. The top plate is at about 200 °C while the bottom one is at about 400 °C. Biomass fed to the top plate drops into successive lower plates by means of scrapers. The biomass undergoes drying and pyrolysis while moving over the plates. No carrier gas is required in this pyrolyzer. Only char is left when the biomass reaches the lowest plate. Though the heating rate of the biomass is relatively slow, the residence time of the vapor in the pyrolysis zone is short. As a result, the liquid yield in this process is relatively modest, about 35 to 50% on dry feed, with a high char yield. This pyrolyzer design is complex, especially given the fouling potential of the vacuum pump.

In thermochemical conversion, the entire biomass is converted into gases, which are then synthesized into the desired chemicals or used directly (Figure 1.7). The Fischer-Tropsch synthesis of syngas into liquid transport fuels is an example of thermochemical conversion. Production of thermal energy is the main driver for this conversion route that has four broad pathways:

• Combustion

• Pyrolysis

• Gasification

• Liquefaction

Table 1.2 compares these four thermochemical paths for biomass conversion. It also shows the typical range of their reaction temperatures.

Combustion involves high-temperature conversion of biomass in excess air into carbon dioxide and steam. Gasification, on the other hand, involves a chemical reaction in an oxygen-deficient environment. Pyrolysis takes place at a relatively low temperature in the total absence of oxygen. In liquefaction, the large feedstock molecules are decomposed into liquids having smaller mole­cules. This occurs in the presence of a catalyst and at a still lower temperature.

Table 1.3 presents a comparison of the thermochemical and biochemical routes for biomass conversion (see page 13). It shows that the biochemical route for ethanol production is more commercially developed than the thermochemi­cal route, but the former requires sugar or starch for feedstock; it cannot use ligno-cellulosic stuff. As a result, a larger fraction of the available biomass is not converted into ethanol.

For example, in a corn plant only the kernel is used for production. The stover, stalk, roots, and leaves, which are ligno-cellulosic, are left as wastes. Even though the enzymatic or biochemical route is more developed, this is a batch process and takes an order of magnitude longer to complete than the thermochemical process. In the thermochemical route, the biomass is first converted into syngas, which is then converted into ethanol through synthesis or some other means.

Combustion

Combustion represents perhaps the oldest utilization of biomass, given that civilization began with the discovery of fire. The burning of forest wood taught humans how to cook and how to be warm. Chemically, combustion is an

exothermic reaction between oxygen and the hydrocarbon in biomass. Here, the biomass is converted into two major stable compounds: H2O and CO2. The reaction heat released is presently the largest source of human energy consump­tion, accounting for more than 90% of the energy from biomass.

Heat and electricity are two principal forms of energy derived from biomass. Biomass still provides heat for cooking and warmth, especially in rural areas. District or industrial heating is also produced by steam generated in biomass — fired boilers. Pellet stoves and log-fired fireplaces are as well a direct source of warmth in many cold-climate countries. Electricity, the foundation of all modern economic activities, may be generated from biomass combustion. The most common practice involves the generation of steam by burning biomass in a boiler and the generation of electricity through a steam turbine. In some places, electricity is produced by burning combustible gas derived from biomass through gasification.

Biomass is used either as a standalone fuel or as a supplement to fossil fuels in a boiler. The latter option is becoming increasingly common as the fastest and least-expensive means for decreasing the emission of carbon dioxide from an existing fossil fuel plant (Basu et al., 2009). This option is called co­combustion or co-firing.

Pyrolysis

Unlike combustion, pyrolysis takes place in the total absence of oxygen, except in cases where partial combustion is allowed to provide the thermal energy needed for this process. Pyrolysis is a thermal decomposition of the biomass into gas, liquid, and solid. It has three variations:

• Torrefaction, or mild pyrolysis

• Slow pyrolysis

• Fast pyrolysis

In pyrolysis, large hydrocarbon molecules of biomass are broken down into smaller hydrocarbon molecules. Fast pyrolysis produces mainly liquid fuel, known as bio-oil; slow pyrolysis produces some gas and solid charcoal (one of the most ancient fuels, used for heating and metal extraction before the discov­ery of coal). Pyrolysis is promising for conversion of waste biomass into useful liquid fuels. Unlike combustion, it is not exothermic.

Torrefaction, which is currently being considered for effective biomass utilization, is also a form of pyrolysis. In this process (named for the French word for roasting), the biomass is heated to 230 to 300 °C without contact with oxygen. The chemical structure of the wood is altered, which produces carbon dioxide, carbon monoxide, water, acetic acid, and methanol.

Torrefaction increases the energy density of the biomass. It also greatly reduces its weight as well as its hygroscopic nature, thus enhancing the commercial use of wood for energy production by reducing its transportation cost.

Gasification

Gasification converts fossil or nonfossil fuels (solid, liquid, or gaseous) into useful gases and chemicals. It requires a medium for reaction, which can be gas or supercritical water (not to be confused with ordinary water at subcritical condition). Gaseous mediums include air, oxygen, subcritical steam, or a mixture of these.

Presently, gasification of fossil fuels is more common than that of nonfossil fuels like biomass for production of synthetic gases. It essentially converts a potential fuel from one form to another. There are three major motivations for such a transformation:

• To increase the heating value of the fuel by rejecting noncombustible com­ponents like nitrogen and water.

• To remove sulfur and nitrogen such that when burnt the gasified fuel does not release them into the atmosphere.

• To reduce the carbon-to-hydrogen (C/H) mass ratio in the fuel.

In general, the higher the hydrogen content of a fuel, the lower the vaporiza­tion temperature and the higher the probability of the fuel being in a gaseous state. Gasification or pyrolysis increases the relative hydrogen content (H/C ratio) in the product through one the followings means:

Direct: Direct exposure to hydrogen at high pressure.

Indirect: Exposure to steam at high temperature and pressure, where hydro­gen, an intermediate product, is added to the product. This process also includes steam reforming.

Pyrolysis: Reduction of carbon by rejecting it through solid char or CO2 gas.

Gasification of biomass also involves removal of oxygen from the fuel to increase its energy density. For example, a typical biomass has about 40 to 60% oxygen by weight, but a useful fuel gas contains only a small percentage of oxygen (Table 1.4). The oxygen is removed from the biomass by either dehy­dration or decarboxylation. The latter process, which rejects the oxygen through CO2, increases the H/C ratio of the fuel so that it emits less greenhouse gas when combusted:

Dehydration: CmHnOq ^ CmH„_2q + qH2O O2 removal through H2O (1.1)

Decarboxylation: CmH„Oq ^ Cm-q/2Hn O2 removal through CO2

+ qCO2 ( . )

Hydrogen, when required in bulk for the production of ammonia, is pro­duced from natural gas (~CH4) through steam reforming, which produces

syngas (a mixture of H2 and CO). The CO in syngas is indirectly hydrogenated by steam to produce methanol (CH3OH), an important feedstock for a large number of chemicals. These processes, however, use fossil fuels, which are not only nonrenewable but are responsible for the net addition of carbon dioxide (a major greenhouse gas) in the atmosphere.

Biomass can deliver nearly everything that fossil fuels provide, whether fuel or chemical feedstock. Additionally, it provides two important benefits that make it a viable feedstock for syngas production. First, it does not make any net contribution to the atmosphere when burnt; second, its use reduces depen­dence on nonrenewable and often imported fossil fuel.

For these reasons, biomass gasification into CO and H2 provides a good base for production of liquid transportation fuels, such as gasoline, and syn­thetic chemicals, such as methanol. It also produces methane, which can be burned directly for energy production. Gasification is carried out generally in one of the three major types of gasifiers:

• Moving bed (also called fixed bed)

• Fluidized bed

• Entrained flow

Downdraft and updraft are two common types of moving-bed gasifier. A survey of gasifiers in Europe, the United States, and Canada shows that down­draft gasifiers are the most common (Knoef, 2000). It shows that 75% are downdraft, 20% are fluidized beds, 2.5% are updraft, and 2.5% are of various other designs.

Liquefaction

Liquefaction of solid biomass into liquid fuel can be done through pyrolysis, gasification as well as through hydrothermal process. In the latter process, biomass is converted into an oily liquid by contacting the biomass with water at elevated temperatures (300-350 °C) with high (12-20 MPa) for a period of time. There are several other means including the supercritical water process (Chapter 7) for direct liquefaction of biomass. Behrendt et al. (2008) present a review of these processes.