Category Archives: Biofuels and Bioenergy

Combustion/Incineration

Combustion or incineration technology is used for a very wide range of waste to reduce its volume and hazardous characteristics as well as to generate heat and electricity. It is most widely applied and can be implemented on a large as well as small scale. Waste generally contains organic matter, minerals, met­als, and water. Metals are separated before incineration. During incineration, flue gases are generated that contain energy which can be transformed into heat and electricity. The organic waste burns when it has reached the ignition temperature and come into contact with oxygen. The overall oxidative pro­cess of combustion is highly exothermic, although it occurs in the stages of drying, degassing, pyrolysis, gasification, and combustion; not all of which are exothermic in nature. Initially, heat may be needed to start the process. However, once the chain reaction of the combustion process is started [11], no external heat or additional fuel is required. Although the stages of the com­bustion process are inseparable, furnace design, air distribution, and control system can affect these stages and reduce pollutant emissions.

Typical Reaction Conditions and Products from Pyrolysis, Gasification, Incineration, and Plasma-Based Processes

TABLE 6.3

Pyrolysis

Gasification

Combustion

Plasma

Treatment

Temperature [°C]

250-900

500-1,800

800-1,450

1,200-2,000

Pressure [bar]

1

1-45

1

1

Atmosphere

Inert/nitrogen

Gasification

Air

Gasification

Stoichiometric

0

agent: O2, H2O

<1

>1

agent:

O2, H2O Plasma gas: O2, N2, Ar <1

ratio

Products from the process: Gas phase

H2, CO, H2O,

H2, CO, CO2,

CO2, H2O, O2,

H2, CO, CO2,

N,

CH4, H2O, n2

n2

CH4,H2O, n2

Solid phase

Hydrocarbons Ash, coke

Slag, ash

Ash, slag

Slag, ash

Liquid phase

Pyrolysis oil

Some tar

and water

Source: From Helsen and Bosmans. 2010. Waste to energy through thermochemical processes: Matching waste with process, Conference Proceedings on Enhanced Landfill Mining and Transition to Sustainable Materials Management, Molenheide (Houthalen-Heichteren, Belgium), October 4-6; Kolb and Seifert. 2002, Thermal waste treatment: State of the art — A summary. Waste Management 2002: The Future of Waste Management in Europe, October 7-8, Strasbourg (France), Edited by VDI GVC (Dusseldorf, Germany), and ETC/RWM. 2007. Environmental Outlooks: Municipal Waste, Working Paper no. 1/2007, European Topic Centre on Resource and Waste Management, Retrieved July 27, 2010, from http://waste. eionet. europa. eu/publications.

A typical set of reaction conditions for the combustion process is outlined in Table 6.3 [7, 9]. The table also compares the typical operating conditions for combustion with those of pyrolysis, gasification, and plasma treatment (i. e., other important thermochemical processes). Air in the combustion pro­cess can be replaced by oxygen, and it is the only thermochemical process where air (or oxygen) is in stoichiometric excess. In fully oxidative combus­tion, the flue gas contains water vapor, nitrogen, carbon dioxide, and oxygen. However, depending upon the nature of the waste and the operating condi­tions, smaller amounts of CO, HCl, HF, HBr, HI, NOx, SO2, VOCs, PCDD/F, PCBs, and heavy metal compounds can be a part of flue gas [7]. The pres­ence of these compounds can cause environmental issues, and they should be removed before the flue gas is emitted to the environment. Depending upon the combustion temperature, heavy metals and inorganic matters (salts) can end up in the flue gas or fly ash. The amount and composition of solid residue during combustion depends on the nature of waste, combustion temperature,

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and the detailed process design. In MSW incineration, the bottom ash is about 25 to 30% by weight of solid waste input. Fly ash can be 1 to 5% by weight of the waste input [12]. Additional treatment of the bottom ash can allow its use in the concrete and other construction industries. Fly ash needs to be immobi­lized or vitrified to make it environmentally safe for landfill disposal.

For an effective incineration, an excess of air or oxygen (generally 1.2 to

2.5 times stoichiometric requirement) is necessary. This number depends upon the nature of fuel (gas, liquid, or solid) and the incinerator design. In principle, the incinerator generates excess energy from MSW combustion to produce heat and electricity. The incinerator size may vary depending on the availability of waste. In Europe, the average capacity is about 193 k tons per year [7, 11]. A typical schematic of the incineration plant is shown in Figure 6.4. Such a plant includes (a) waste reception, storage, and preparation unit; (b) a combustor; (c) boiler or other type of energy recovery unit; (d) flue gas cleaning and emission monitoring and control system; (f) waste water control and management system (e. g., from site drainage, flue gas treatment, and waste storage); and (g) residue management and discharge system which includes bottom ash management and treatment system as well as solid resi­due discharge and disposal system [13]. The plant design also depends on the nature of the waste being treated (its chemical composition and physical and thermal characteristics), types and quantities of residues which in turn depend on the installation design, its operation, and the waste input.

Process stability and optimization depend significantly upon the variabil­ity in the waste input. Although a narrow variability of input will give more stability and better environmental performance, this may require expensive waste pretreatment and the selective collection of waste. On the other hand, gas cleaning equipment is generally 15 to 35% of total capital investment, so its optimization is also essential [7]. Thus, a prudent optimization of the overall waste management system is often needed.

There are basically three types of incinerators used in commercial opera­tions: grate incinerators, rotary kilns, and fluidized beds [7].

Energy Integration and Energy Efficiency Enhancement

A key to the overall success of the biomass program depends upon its energy efficiency of the overall technology implementation. The net energy value (NEV) of corn ethanol technology is often a matter of concern and a sub­ject of debate, for example. In all of the biomass processes, ingenious energy integration schemes are adopted, to list a few, drying of biomass feed using hot effluent stream, combustion of biomass residues for reheating the heat transfer medium such as sand, generation and utilization of process steam, efficient process alignment for combined heat and power, autothermal refor­mation, and more.

Calorific Value (CV) or Heating Value (HV)

The lower calorific value (LCV) or lower heating value (LHV) of biodiesel is 37.37 MJ/kg, whereas that for low sulfur diesel is 42.612 MJ/kg. The higher calorific value (HCV) or higher heating value (HHV) of biodiesel is 40.168 MJ/kg, whereas that for low sulfur diesel is 45.575 MJ/kg. Both heating val­ues of biodiesel are approximately 12% lower than those of low-sulfur diesel. Variations in the heating values for biodiesel are mainly from the variability of the biodiesel feedstock, that is, the source of triglycerides. A downside of biodiesel in comparison to petrodiesel is its lower calorific value, even though the difference is not very substantial. However, it has been claimed by fuel engineers that the ultimate fuel efficiency of biodiesel is comparable to that of petrodiesel, despite its lower energy density, thanks to several com­pensating factors including more complete combustion and better lubricity. The lower heating value of biodiesel is attributable to its oxygenated molec­ular structure in contrast to the nonoxygenated hydrocarbon structures of petrodiesel. Highly oxygenated structures of biodiesel adversely affect the
cold flow properties of biodiesel, which are represented by the fuel’s cloud point, pour point, and cold filter plugging point [47].

Manufacture of Industrial Alcohol

Industrial alcohol can be produced (1) synthetically from ethylene, (2) as a by­product of certain industrial operations, or (3) by the fermentation of sugars, starch, or cellulose. There are two principal processes for the synthesis of alcohol from ethylene. The original method (first carried out in the 1930s by Union Carbide) was the indirect hydration process, alternately referred to as the strong sulfuric acid-ethylene process, the ethyl sulfate process, the ester­ification hydrolysis process, or the sulfation hydrolysis process. The other synthetic process, designed to eliminate the use of sulfuric acid, is the direct hydration process, where ethanol is manufactured by directly reacting ethyl­ene with steam. The hydration reaction is exothermic and reversible; that is, the maximum conversion is limited by chemical equilibrium.

CH2 = CH2(g) + H2O(g} ^ CH3CH2OH(g) (-AH2098) = 45 kJ / mol

Only about 5% of the reactant ethylene is converted into ethanol per each pass through the reactor. By selectively removing the ethanol from the equilibrium product mixture and recycling the unreacted ethylene, it is possible to achieve an overall 95% conversion. Typical reaction conditions are: 300°C, 6-7 MPa, and employing phosphoric (V) acid catalyst adsorbed onto a porous support of silica gel or diatomaceous earth material. This catalytic process was first utilized on a large scale by Shell Oil Company in 1947.

In addition to the direct hydration process, the sulfuric acid process, and fermentation routes to manufacture ethanol, several other processes have also been suggested [11-14]. However, none of these has been successfully implemented on a commercial scale.

Thermal and Thermochemical Conversion of Biomass

There are five thermal or thermochemical approaches that are commonly used to convert biomass into an alternative fuel/energy: direct combus­tion, gasification, liquefaction, pyrolysis, and partial oxidation. These five modes of conversion are also applicable to the conventional utilization of coal. When biomass is heated under oxygen-deficient conditions, it gener­ates bio-oil and bio gas that consists primarily of carbon dioxide, meth­ane, and hydrogen. When biomass is gasified at a higher temperature with an appropriate gasifying medium, synthesis gas is produced, which has similar compositions to that of coal syngas. This syngas can be directly burned or further processed for other gaseous or liquid fuel products. In this sense, thermal or thermochemical conversion of biomass is very similar to that of coal [5]. Figure 5.1 shows a variety of process options of biomass treatment and utilization as well as their resultant alternative fuel products.

Of a variety of thermochemical conversion options of biomass, this chapter is mainly focused on fast pyrolysis and gasification of biomass due to their technological significance, and other options of thermochemical conversion are discussed whenever deemed relevant.

Supercritical Water Gasification

Hydrothermal treatment of waste materials in supercritical conditions has gained a great deal of momentum ever since the pioneering work of Modell and coworkers from M. I.T. in the late 1970s. As indicated earlier, Figure 6.8 illustrates the region of supercritical water gasification for waste treatment in the pressure — temperature diagram. The three regions shown in the diagram take advantage of substantial changes in the properties of water that occur in the vicinity of its critical point at 374°C (Tc) and 22 MPa (Pc). The behavior of the important prop­erties of water such as density, ion dissociation constant, and dielectric constant with respect to temperature is illustrated in Figure 6.9 [76]. In supercritical con­ditions, more chemically and energetically favorable pathways to gaseous and liquid fuels can be achieved by better control of the rate of hydrolysis and phase partitioning and solubility of components in supercritical water.

Water at ambient conditions (25°C and 0.1 MPa) is a good solvent for elec­trolytes because of its high dielectric constant [76], whereas most organic matter is sparingly soluble [76]. As water is heated, the H-bonding starts weakening, allowing dissociation of water into acidic hydronium ions (H3O+) and basic hydroxide ions (OH-). The structure of water changes significantly near the critical point because of the breakage of infinite networks of hydro­gen bonds, and water exists as separate clusters with a chain structure. In fact, the dielectric constant of water decreases considerably near the critical

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point, which causes a change in the dynamic viscosity and also increases the self-diffusion coefficient of water [76].

Supercritical water has liquid-like density and gaslike transport properties, and behaves very differently than water at room temperature. For example, it is highly nonpolar, permitting complete solubilization of most organic compounds. The resulting single-phase mixture does not have many of the conventional transport limitations that are encountered in multiphase reac­tors. However, the polar species present, such as inorganic salts, are no longer soluble and start precipitating. The physicochemical properties of water, such as viscosity, ion product, density, and heat capacity, also change dramatically in the supercritical region with only a small change in the temperature or pressure, resulting in a substantial increase in the rates of chemical reactions.

It is important to mention that the dielectric behavior of 200°C water is similar to that of ambient methanol, 300°C water is similar to ambient ace­tone, 370°C water is similar to methylene chloride, and 500°C water is simi­lar to ambient hexane [76]. In addition to the unusual dielectric behavior, the transport properties of water are significantly different than the ambient water as shown in Table 6.6.

Supercritical processes are often not considered to be economical because of high capital costs associated with high-pressure equipment and high operating costs associated with the compression or pumping of supercriti­cal media, however, the above-described unique properties of supercritical fluids offer some very interesting possibilities. Furthermore, in recent years the prices of high-pressure equipment has come down. In conjunction with catalysis, a supercritical fluid can dissolve unwarranted hydrocarbons from the catalyst surface into the supercritical fluid phase. Supercritical fluids have a better capacity to handle heat due to high capacities. The adsorption/ desorption phenomena can be better handled in a supercritical fluid due to higher solubility. The oligomeric coke precursors or sulfur species can be easily dissolved by the supercritical fluids.

An excellent review of supercritical water (SCW) gasification of bio­mass and organic wastes was recently published by Guo, Cao, and Liu [91]. Numerous studies have examined supercritical water partial oxidation [92,

TABLE 6.6

Comparison of Ambient and Supercritical Water

Ambient Water Supercritical Water

Dielectric constant

78

<5

Solubility of organic compounds

Very low

Fully miscible

Solubility of oxygen

6 ppm

Fully miscible

Solubility of inorganic compounds

Very high

~0

Diffusivity (cm2 s-1)

10-5

10-3

Viscosity (g cm-1 s-1)

10-2

10-4

Density (g cm-3)

1

0.2-0.9

93]. These and other studies found that the yields of H2O and CO increased with increasing water density. Yields of H2 were 4 times better with NaOH and 1.5 times better with ZrO2 compared to reaction without a catalyst. Supercritical fluids gave increased pore accessibility, enhanced catalyst abil­ity to coking, and increased desired product selectivity.

An extensive amount of work on supercritical water gasification of organic wastes has been reported in the literature [78, 79, 94-96]. The studies have shown that gasification generally produces a hydrogen and carbon dioxide mixture with simultaneous decontamination of waste. The homogeneous solution of waste and water makes it easy to pump to the high-pressure reactor without pretreatment. Xu and Antal [97] studied gasification of 7.69 wt% digested sewage sludge in supercritical water and obtained gas that largely contained H2, CO2, a smaller amount of CH4, and a trace of CO. Other waste materials show similar behavior. The equilibrium yields as functions of temperature and pressure for SCWG of 5% sawdust reported by Guo et al. [91] indicate the main products to be hydrogen, carbon diox­ide, and methane at low temperatures and hydrogen and carbon dioxide at high temperatures.

An increase in pressure significantly decreases the product concentration of carbon monoxide and slightly decreases the product concentration of the hydrogen. The pressure change has very little effect on the product concen­trations of carbon dioxide and methane. In addition to temperature and pres­sure, other parameters that affect the gas yield are feedstock concentration, oxidant, reaction time, feedstock composition, inorganic impurities in the feedstock, and biomass particle size. Several catalysts such as alkali (NaOH, KOH, Na2CO3, K2CO3, Ca(OH)2), an activated carbon, metal oxide, and met­als also affect the conversion and gas yields. The last two are important for reforming under supercritical conditions. Although high-temperature super­critical water gasification produces hydrogen and carbon dioxide, Sinag, Kruse, and Schwarzkopf [98] showed that a combination of two technologies, supercritical water and hydropyrolysis on glucose in the presence of K2CO3, produces phenols, furfurals, organic acids, aldehydes, and gases.

The generation of hydrogen from waste has long-term and strategic impli­cations inasmuch as hydrogen is the purest form of energy and is very useful for product upgrading, fuel cells, and many other applications. Hydrogen can be produced from waste via numerous high-temperature technologies such as conventional or fast pyrolysis (e. g., olive husk, tea waste, crop straw, etc.), high-temperature or steam gasification (e. g., bionutshell, black liquor, wood waste, etc.), supercritical fluid extraction (e. g., swine manure, orange peel waste, crop grain residue, petroleum basis plastic waste, etc.), and supercritical water gasification (e. g., all types of organic waste, agricultural and forestry waste, etc.), as well as low-temperature technologies such as anaerobic digestion and fermentation (e. g., manure slurry, agricultural resi­due, MSW, tofu wastewater, starch from food waste, etc.).

For high-temperature technologies, supercritical water gasification gener­ates more hydrogen at a lower temperature than pyrolysis or gasification [99, 100]. Supercritical water gasification also does not require drying, sizing, and other methods of feed preparation thereby requiring less expense for the overall process. The temperature of the pyrolysis and gasification processes can be reduced if the gases coming from them are further steam reformed. This, however, adds to the overall cost. The rates for the low-temperature processes such as anaerobic digestion and fermentation can be enhanced with the use of suitable microbes and enzymes. The development of a future hydrogen economy will require further research in the improvement of these technologies.

Use of Vegetable Oil for Biodiesel Manufacture

Triglycerides, which are the principal ingredients of vegetable oils and algae oils, are chemically converted into biodiesel via catalytic transesteri­fication reaction under mild reaction conditions. The effects of transesteri­fication on selected fuel properties have been examined by Bello, Magaji, and Agge [15]. Their results showed that the density reduces by 7-9%, and the cetane number (CN) increases by 60-78% by transesterification. They also found that transesterification overall tends to make the properties of vegetable oils close to those of petrodiesel [15]. Major parts of ensuing sections of this chapter are devoted to the processing and manufacture of biodiesel and its precursor feedstocks.

Corn Ethanol Production Technologies

3.2.1.1 Dry Mill Process versus Wet Mill Process

Ethanol production facilities can be classified into two broad types: wet mill­ing and dry milling operations. As the term "dry" implies, the dry milling process first grinds the entire corn kernel into flour which is referred to as "meal" or "corn meal." Dry mills are usually smaller in size (capacity) and are built primarily to manufacture ethanol only. The remaining stillage from ethanol purification undergoes a different process treatment to produce a highly nutritious livestock feed. Wet mill facilities are called "corn refiner­ies," and also produce a list of high-valued coproducts such as high-fruc­tose corn syrup (HFCS), dextrose, and cornstarch. Both wet and dry milling operations are currently used to convert corn to ethanol. Wet milling is usu­ally a larger and more versatile process, and could be valuable for coping with volatile energy markets. Wet milling can be used to produce a greater variety of products such as cornstarch, corn syrup, ethanol, dry distillers grains, artificial sweeteners like Splenda®, and more. Although wet milling is a more versatile process and offers a more diverse product portfolio than dry milling, when producing fuel ethanol, dry milling has higher efficiency and lower capital and operating costs than wet milling. Most of the recent ethanol plants built in the United States are based on dry milling operations [13]. As of the end of 2008, a total of 86% of corn ethanol in the United States was commercially produced using the dry mill process using a total of 150 dry milling plants [14].

Enzyme System

There are several different kinds of cellulases, and they differ mechanisti­cally and structurally. Each cellulolytic microbial group has an enzyme sys­tem unique to it. The enzyme capabilities range from those with which only soluble derivatives of cellulose can be hydrolyzed to those with which a cel­lulose complex can be disrupted. Although it is a usual practice to refer to a mixture of compounds that have the ability to degrade cellulose as cellulase, it is actually composed of a number of distinctive enzymes. Based on the specific type of reaction catalyzed, the cellulases may be characterized into five general groups, namely,

1. Endocellulase cleaves internal bonds to disrupt the crystalline struc­ture of cellulose and expose individual cellulose polysaccharide chains.

2. Exocellulase detaches two or four saccharide units from the ends of the exposed chains produced by endocellulase, resulting in disac­charides or tetrasaccharides, such as cellobiose. Cellobiose is a disac­charide with the formula [HOCH2CHO(CHOH)3]2O. There are two principal types of exocellulases, or cellobiohydrolases (CBH): (a) CBH-I works processively from the reducing end of cellulose and (b) CBH-II works processively from the nonreducing end of cellulose.

In this description, processivity is the ability of an enzyme to con­tinue repetitively its catalytic function without dissociating from its substrate. By an active enzyme being held onto the surface of a solid substrate, the chance for reaction is significantly enhanced.

3. Beta-glucosidase or cellobiase hydrolyzes the exocellulase products, disaccharides and tetrasaccharides, into individual monosaccharides.

4. Oxidative cellulases depolymerize and break down cellulose mole­cules by radical reactions, as in the case with a cellobiose dehydro­genase (acceptor), which is an enzyme that catalyzes the chemical reaction of

cellobiose + acceptor ^ cellobiono-1,5-lactone + reduced acceptor by which cellobiose is dehydrogenated and the acceptor is reduced.

5. Cellulose phosphorylases depolymerize cellulose using phosphates instead of water.

In most cases, the enzyme complex breaks down cellulose to beta-glu­cose. This type of cellulase enzyme is produced mainly by symbiotic bac­teria. Symbiotic bacteria are bacteria living in symbiosis (close and long-term interaction) with another organism or each other. Enzymes that hydro­lyze hemicellulose are usually referred to as hemicellulase and are still com­monly classified under cellulases. Enzymes that break down lignin are not classified as cellulase, strictly speaking. Along with diverse types of enzymes, it must be clearly pointed out that a principal challenge in hydrolytic degradation of biomass into fermentable sugars is how to make these different enzymes work together as a synergistic enzyme system. For example, cellulases and hemicellulases are secreted from a cell as free enzymes or extracellular cellulosomes (complexes of cellulolytic enzymes created by bacteria). The collective activity of these enzymes in a system is likely to be more active than, or at least quite different from, the individual activity of an isolated enzyme.

The enzymes described above can be classified into two types: progressive (also known as processive) and nonprogressive (or, nonprocessive) types. Progressive cellulase will continue to interact with a single polysaccharide strand, whereas nonprogressive cellulase will interact once, disengage, and then engage another polysaccharide strand.

Based on the enzymatic capability, cellulase is characterized into two groups, namely, Q enzyme or factor and CX enzyme or factor [41]. The Q factor is regarded as an "affinity" or prehydrolysis factor that transforms highly ordered (crystalline) cellulose, (i. e., cotton fibers or Avicel) into linear and hydroglucose chains. The Q factor has little effect on soluble deriva­tives. Raw cotton is composed of 91% pure cellulose. As such, it serves as an essential precursor to the action of the CX factor. The CX (hydrolytic) factor breaks down the linear chains into soluble carbohydrates, usually cellobiose (a disaccharide) and glucose (a monosaccharide).

Microbes rich in Q are more useful in the production of glucose from the cellulose. Moreover, because the Q phase proceeds more slowly than the subsequent step, it is the rate controlling step. Among the many microbes, Trichoderma reesei surpasses all others in the possession of Q complex. Trichoderma reesei is an industrially important cellulolytic filamentous fun­gus and is capable of secreting large amounts of cellulases and hemicel — lulases [44]. Recent advances in cellulase enzymology, cellulose hydrolysis (cellulolysis), strain enhancement, molecular cloning, and process design and engineering are bringing T. reesei cellulases closer to being a commer­cially viable option for cellulose hydrolysis [45]. The site of action of cellulo­lytic enzymes is important in the design of hydrolytic systems (CX factor). If the enzyme is within the cell mass, the material to be reacted must diffuse into the cell mass. Therefore, the enzymatic hydrolysis of cellulose usually takes place extracellularly, where the enzyme is diffused from the cell mass into the external medium.

Another important factor in the enzymatic reaction is whether the enzyme is adaptive or constitutive. A constitutive enzyme is present in a cell at all times. Adaptive enzymes are found only in the presence of a given substance, and the synthesis of the enzyme is triggered by an inducing agent. Most of the fungal cellulases are adaptive [15, 41].

Cellobiose is an inducing agent with respect to Trichoderma reesei. In fact, depending on the circumstances, cellobiose can be either an inhibitor or an inducing agent. It is inhibitory when its concentration exceeds 0.5 to 1.0%. Cellobiose is an intermediate product and is generally present in concentra­tions low enough to permit it to serve as a continuous inducer [46].

A milestone achievement (2004) accomplished by the National Renewable Energy Laboratory in collaboration with Genencor International and Novozyme Biotech is of significance in making effective cellulase enzymes at substantially reduced costs, as mentioned in an earlier section.

4.5.2 Enzymatic Processes

All enzymatic processes basically consist of four major steps that may be combined in a variety of ways: pretreatment, enzyme production, hydroly­sis, and fermentation, as represented in Figure 4.7.

image37

FIGURE 4.7

Fungal enzyme hydrolysis process. (Modified from Wright, 1988. Ethanol from biomass by enzymatic hydrolysis, Chem. Eng. Prog, 84: 62-74.)

4.5.3.1 Pretreatment

It has long been recognized that some form of pretreatment is necessary to achieve reasonable rates and yields in the enzymatic hydrolysis of biomass. Pretreatment has generally been practiced to reduce the crystallinity of cel­lulose, to lessen the average degree of polymerization of the cellulose and the lignin-hemicellulose sheath that surrounds the cellulose, and to allevi­ate the lack of available surface area for the enzymes to attack. A typical pretreatment system consists of size reduction, pressure sealing, heating, reaction, pressure release, surface area increase, and hydrolyzate/solids sep­aration [47].

Mechanical pretreatments such as intensive ball milling and roll milling have been investigated as a means of increasing the surface area, but they require exorbitant amounts of energy. The efficiency of the chemical process can be understood by considering the interaction between the enzymes and the substrate. The hydrolysis of cellulose into sugars and other oligomers is a solid phase reaction in which the enzymes must bind to the surface to catalyze the reaction. Cellulase enzymes are large proteins, with molecular weights ranging from 30,000 to 60,000 and are thought to be ellipsoidal with major and minor dimensions of 30 to 200 A°. The internal surface area of wood is very large, however, only about 20% of the pore volume is accessible to cellulase-sized molecules. By breaking down the tight hemicellulose-lig — nin matrix, hemicellulose or lignin can be separated and the accessible vol­ume can be greatly increased. This removal of material greatly enhances the enzymatic digestibility.

The hemicellulose-lignin sheath can be disrupted by either acidic or basic catalysts. Basic catalysts simultaneously remove both lignin and hemicel- lulose, but suffer large consumption of the base through neutralization by ash and acid groups in the hemicellulose. In recent years, attention has been focused on the acidic catalysts. They can be mineral acids or organic acids generated in situ by autohydrolysis of hemicellulose.

Various types of pretreatments are used for biomass conversion. The pre­treatments that have been studied in recent years are steam explosion auto­hydrolysis, wet oxidation, organosolv, and rapid steam hydrolysis (RASH). The major objective of most pretreatments is to increase the susceptibility of cellulose and lignocellulose material to acid and enzymatic hydrolysis. Enzymatic hydrolysis is a very sensitive indicator of lignin depolymeriza­tion and cellulose accessibility. Cellulase enzyme systems react very slowly with untreated material; however, if the lignin barrier around the plant cell is partially disrupted, then the rates of enzymatic hydrolysis are increased dramatically.

Most pretreatment approaches are not intended to actually hydrolyze cel­lulose to soluble sugars, but rather to generate a pretreated cellulosic residue that is more readily hydrolyzable by cellulase enzymes than native biomass. Dilute acid hydrolysis processes are currently being proposed for several
near-term commercialization efforts until lower-cost commercial cellulase preparations become available. Such dilute acid hydrolysis processes typi­cally result in no more than 60% yields of glucose from cellulose.

Biosyngas

Depending upon the compositions of resultant gaseous products, syngas may be classified as (i) balanced syngas whose H2:CO molar ratio is close to 2:1, (ii) unbalanced syngas whose H2:CO molar ratio is substantially lower than 2:1, (iii) CO2-rich syngas whose CO2 molar concentration exceeds 10%, and so on. The balanced syngas is also referred to as hydrogen-rich syngas, whereas unbalanced syngas is called CO-rich syngas. The gaseous product can also be classified based on its heating value (HV) as (i) high BTU gas, (ii) medium BTU gas, and (iii) low BTU gas. Alternately, syngas may be classi­fied based on its origin as: (i) natural gas-derived syngas, (ii) coal-derived syngas, (iii) biomass syngas, and (iv) coke oven gas. These descriptions are commonly used for all types of syngas derived from a variety of feedstock including natural gas, coal, and biomass. The first category of classification has been used widely in the synthesis of clean liquid fuels such as methanol and dimethylether (DME) synthesis, whereas the second category has come from the classical design of coal gasifiers [5].

For further clarification of the terminology, biogas usually stands for a gas produced by anaerobic digestion of organic materials, and is largely com­prised of methane (about 65% or higher) and carbon dioxide. Therefore, the term "biogas" is not interchangeable with the term "biomass syngas." The methane-rich biogas is a high BTU gas and is also called "marsh gas," "land­fill gas," or "swamp gas." As the name implies, swamp gas is produced by the same anaerobic processes (where oxygen is absent and unavailable for biological conversion) that take place during the underwater decomposition of organic matter in wetlands. Anaerobic digestion is a series of biochemical processes in which micro-organisms break down biodegradable material in the absence or serious deficiency of oxygen. The biochemical processes car­ried out by micro-organisms consist of four principal stages, viz., (i) hydro­lysis, (ii) acidogenesis, (iii) acetogenesis, and (iv) methanogenesis. Therefore, the biogas obtained from anaerobic digesters is not classified as a thermo­chemical intermediate of biomass conversion.