Starting in the early 1980s, PRM Energy Systems, Inc., began to market gasifi­cation technology for converting biomass to low-energy fuel gases (Bailey and Bailey, 1996). Several commercial plants based on PRM’s air-blown updraft designs for the gasification of rice hulls have been built and operated in Australia, Costa Rica, Malaysia, and the United States. High-silica ash is a salable by-product. An example of this technology is the plant installed in Mississippi in 1995 for the gasification of 300 t/day of rice hulls. The system converts unground rice hulls to fuel gas (121 GJ/h) for an existing boiler — power island which supplies electric power (5 MW capacity) and 6800 kg/h of process steam for parboiling rice. In operation, feedstock is metered into the gasifier by a water-cooled screw conveyor that discharges into the drying and heating zone of the gasifier. The gasifier is a refractory-lined, cylindrical steel shell that is equipped with a fixed grate at the bottom and is mounted in a vertical position. The gasification process is automatically controlled to maintain a preset first-stage gasification zone temperature. Almost all of the ash is removed from the bottom of the gasifier. The low particulate concentration in the product gas makes it possible to direct-fire a boiler without the use of emission control equipment. Total particulate emissions in the boiler exhaust of this plant were determined to be 0.103 kg/GJ.

In addition to a suitable environment, appropriate pigments, whose cumulative light-absorbing properties determine the range of wavelengths over which photosynthesis occurs, a reaction center where the excited pigments emit
electrons, and an electron transfer chain that generates the high-energy phos- phorylating agent adenosine triphosphate (ATP) by photophosphorylation are necessary for ambient C02 reduction. The pigment chlorophyll absorbs light and is oxidized by ejection of an electron. The electron is accepted by ferredoxin (Fd), a nonheme iron protein, to form reduced ferredoxin (Fd+2), which through other electron carriers generates ATP and the original oxidized ferre­doxin (Fd+3). Chlorophyll functions as both a light absorber and a source of electrons in the excited state, and as the site of the initial photochemical reaction. Accessory pigments function to absorb and transfer light energy to chlorophyll.

Two photochemical systems are involved in these “light reactions”: photo­systems II (PS II) and I (PS I). PS II consists of the first series of reactions that occur in the light phase of photosynthesis during which the excited pigment participates in the photolysis of water to liberate free oxygen, protons, and electrons. PS I is the second series of reactions that occur in the light phase of photosynthesis; they result in the transfer of reducing power to nicotinamide adenine dinucleotide phosphate (NADP) for ultimate utilization by C02. The light reactions yield ATP, and the reduced form of nicotinamide adenine dinucleotide phosphate (NADPFI2), both of which facilitate the dark reactions that yield sugars. Hydrogen is transferred by NADPH2. The low — energy adenosine diphosphate (ADP) and NADP produced in the dark reactions are reconverted to ATP and NADPH2 in the light reactions.

The chemistry of the light reactions was elucidated in 1954 by the U. S. biochemist Daniel Arnon and co-workers (Arnon, Allen, and Whatley, 1954). Light energy is absorbed by the chlorophyll pigments in plant chloroplasts and transferred to the high-energy bonds in ATP, which is produced in noncy­clic and cyclic photophosphorylation reactions. Noncyclic photophosphoryla­tion occurs in the presence of light, requires Fd catalyst, and yields ATP and oxygen. NADP is then reduced by Fd+2 in the absence of light:

4Fd+3 + 2ADP + 2P, + 2H20 -> 4Fd+2 + 2ATP + 02 + 4H+

4Fd+2 + 2NADP + 4H+ -* 4Fd+3 + 2NADPH2.

For each molecule of oxygen evolved, two molecules each of ATP and NADPH2 are formed. Cyclic photophosphorylation requires Fd catalyst and produces ATP only:

ADP 4- P, —» ATP.

This process provides the additional molecule of ATP needed for assimilation of one molecule of C02.

PS I appears to promote cyclic photophosphorylation and proceeds best in light of wavelengths greater than 700 nm, whereas PS II promotes noncyclic photophosphorylation and proceeds best in light of wavelengths shorter than
700 nm. PS II and PS I operate in series in the chloroplast membranes and transfer reducing power from water to Fd and NADP by an electron chain that includes plastoquinone and three proteins in chloroplasts—cytochrome b cytochrome f, and the copper protein plastocyanin. To overcome the potential difference between the carbon dioxide-glucose couple and the water-oxygen couple, two photons are absorbed, one by PS II and one by PS I. In the traditional “Z scheme” concept of photosynthesis first proposed in 1960 (Hill and Bendall, 1960), and which is now well accepted, the strong oxidizing agent, oxygen, is at the bottom, and the strong reducing agent, reduced ferredoxin, is at the top (Fig. 3.1) (Bassham, 1976). By transfer of electrons from water to ferredoxin, the chemical potential for reduction of C02 is created. PS II is depicted as absorbing one photon to raise an electron to an intermediate energy level, after which the electron falls to an intermediate lower energy level while generating ATP. PS I then absorbs the second photon and raises the electron to a still higher intermediate energy level, and subsequently generates NADPH2 via reduced ferredoxin, after which C02 is reduced to yield sugars. In effect, the Z scheme transfers electrons from a low chemical potential in water to a higher chemical potential in NADPH2, which is necessary to reduce C02.

It has been discovered, however, that both photosystems do not seem to be necessary as depicted in the Z scheme to reduce C02. PS II seems to be adequate alone to generate the chemical potential for reduction, at least in

biomass species under certain conditions. PS I is absent in a mutant of the alga Chlamydomonas reinhardtii, but the organism has been found to be capable of photoautrophic assimilation of C02 and the simultaneous evolution of oxygen and hydrogen (Greenbaum et ah, 1995). The investigators interpre­ted their results to mean that a single-photon light reaction has the potential of increasing the efficiency of photosynthesis by overcoming the thermodynamic limitations of converting light energy into chemical energy. This will be referred to later in the discussion of photosynthesis efficiency and biomass yield. In any case, this observation tends to verify Amon’s argument that water oxidation and NADP reduction can be driven by PS II alone (Barber, 1995).

In summary, ambient C02 fixation by photosynthesis involves the photo­chemical decomposition of water to form oxygen, protons, and electrons; the transport of these electrons to a higher energy level via PS II and I and several electron transfer agents; the concomitant generation of NADPH2 and ATP; and reductive assimilation of C02 to monosaccharides. The initial process is the absorption of light by chlorophyll, which promotes the decomposition of water. The ejected electrons are accepted by the oxidized form of Fd. The reduced Fd then starts a series of electron transfers to generate ATP from ADP and inorganic phosphate, and NADPH2 in the light reactions. The stoichiometry, including the reduction in the dark reactions of 1 mol C02 to carbohydrate, represented by the building block (CH20), is illustrated in simplified form as follows:

2H20(1) -> 4H+ + 4e + 02(g)

4Fd+3 + 4e -» 4Fd+2 3ADP 4- ЗР, 3ATP

4Fd+2 -> 4Fd+3 + 4e 2NADP + 4H+ + 4e ^ 2NADPH2 C02(g) + 3ATP + 2NADPFP, -> (CH20) + 3ADP + 2NADP + 3P,

+ H20(1)

Overall: C02(g) + H20(1) -► (CH20) + 02(g).

For each of the two light reactions, one photon is required to transfer each electron; a total of eight photons is thus required to fix one molecule of C02. Assuming C02 is in the gaseous phase and the initial product is glucose, the standard Gibbs free energy change at 25°C is +0.48 MJ (+114 kcal) per mole of C02 assimilated and the corresponding enthalpy change is +0.47 MJ (+112 kcal).

Research to develop trees as energy crops in the United States via short-rotation intensive culture made significant progress in the 1980s and 1990s. Projections indicate that yields of organic matter can be substantially increased by coppic­ing techniques and genetic improvements. Advanced designs of whole-tree harvesters, logging residue collection and chipping units, and automated plant­ers for rapid planting have been developed to the point where prototype units have been evaluated in the field and some are being manufactured for commercial use. It is expected that several additional devices will be offered for commercial use. The on going research is also leading to significant changes in forestry harvesting techniques. Clear-cutting is being phased out and partial harvesting or thinning operations are being phased in. New thinning technolo­gies have been proposed for testing in the forests of the Northwest after successful tests in California. The California research data show that the thin­ning of overgrown stands reduces tree mortality, provides healthier stands, and may offer biomass fuels at a cost that make it possible to operate wood — fueled power plants on a stand-alone basis at a profit in competition with market prices for electric power.

Some of the tree species that have been targeted for continuing research are red alder, black cottonwood, Douglas fir, and ponderosa pine in the Northwest; Eucalyptus, mesquite, Chinese tallow, and the leucaena in the West and South­west; sycamore, Eastern cottonwood, black locust, catalpa, sugar maple, poplar, and conifers in the Midwest; sycamore, sweetgum, European black alder, and loblolly pine in the Southwest; and sycamore, poplar, willow, and sugar maple in the East. Generally, tree growth in research plots is studied in terms of soil type and the requirements for site preparation, planting density, irrigation, fertilization, weed control, disease control, and nutrients. Harvesting methods are equally important, especially in the case of coppice growth for SRWC hardwoods. Although three species native to the region are usually included in the experimental designs, nonnative and hybrid species have often been tested in research plots as well. Advanced biochemical methods and techniques such as tissue culture propagation, genetic transformation, and somaclonal variation are being used in this research to clonally propagate individual geno­types and to regenerate genetically modified species.

After an intensive research effort over about 10 years, SRWC yields in the United States, based on accumulated data are projected to be 9, 9, 11, 17, and 17 dry t/ha-year in the Northeast, South/Southeast, Midwest/Lake, Northwest, and Subtropics, respectively (Wright, 1992). The corresponding research goals are 15, 18, 20, 30, and 30 dry t/ha-year. Hybrid poplar, which grows in many parts of the United States, and Eucalyptus, which is limited to Hawaii, Florida, southern Texas, and part of California, have shown the greatest potential thus far for attaining exceptionally fast growth rates. Both have achieved yields in the range of 20 to 43 t/ha-year in experimental trials with selected clones. Continuing research indicates that other promising species are black locust, sycamore, sweetgum, and silver maple.

Research on hybridizing techniques seem to be leading to super trees that have short growth cycles and that yield larger quantities of biomass. Fast­growing clones are being developed for energy farms in which the trees are ready for harvest in as little as 10 years and yield up to 30 m3/ha-year. Genetic and environmental manipulation has also led to valuable techniques for the fast growth of saplings in artificial light and with controlled atmospheres, humidity, and nutrition. The growth of infant trees in a few months is equiva­lent to what can be obtained in several years by conventional techniques.

Chemical injections into pine trees have been found to have stimulatory effects on the natural production of resins and terpenes and may result in high yields of these valuable chemicals. Combined oleoresin-timber production in mixed stands of pine and timber trees is under development, and it appears that when short-rotation forestry is used, the yields of energy products and timber can be substantially higher than the yields from separate operations.

One of the largest research projects on SRWC in the Western World, LEBEN or the Large European Bioenergy Project, was reported to be scheduled for initiation in the Abruzzo region of Italy in the mid-1980s and to be established near the end of that decade (Grassi, 1987; Klass, 1987). This project integrates SRWC production, the production of herbaceous energy crops and residues, and biomass conversion to biofuels and energy. About 400,000 t/year of biomass, consisting of 260,000 t/year of woody biomass from 700 ha and 120,000 t/year of agricultural residues from 700 ha of vineyards and olive and fruit orchards, will be used. Later, 110,000 t/year of energy crops from 1050 ha will be utilized. The energy products include liquid fuels (biomass —

derived oil), charcoal, 200 million kWh/year of electric power, and waste heat for injection into the regional agroforestry and industrial sectors. This project is still in the start-up stages in the mid-1990s.

One of the largest demonstration programs in the United States was started in 1993 in Minnesota where hybrid poplar is grown under short-rotation conditions on a few sites that total 2000 ha. As the results of this program are reported, a much more rigorous analysis of the potential of SWRC for energy will be possible. The ultimate approach to perfecting this technology, however, is to integrate large-scale biomass production with conversion. Little research of this type has been done. The assumptions and projections that have been made to evaluate the technology are based primarily on small-scale laboratory results, what others have reported as research results, or predictions about individual steps that make up the overall system. But this situation is starting to change as government-industry support of integrated biomass production and conversion research make it possible to examine the sustain­ability of these systems in detail. In the United States, several research projects in which virgin biomass production is integrated with conversion have been selected for field demonstration in plots that are expected to be a minimum of 405 ha (1000 ac) in size (Klass, 1996). This research will provide first-hand experience in operating integrated systems on a sustained basis in which a dedicated biomass feedstock is supplied to a conversion plant. The first group of biomass energy technologies to be scaled up consist of alfalfa production integrated with a gasifier-combined-cycle power plant in Minnesota, switch — grass production integrated with a power plant in Iowa in which biomass and coal are со-fired, hybrid willow production integrated with a power plant in New York in which biomass and coal are со-fired, and an innovative whole tree production system integrated with a power plant in Minnesota (Spaeth and Pierce, 1996). As these projects are implemented, others are expected to be added to the program.

During combustion, the chemically bound carbon and hydrogen in the various organic components of biomass are oxidized. Incomplete combustion can result in excessive emissions of particulate matter and partially oxidized derivatives, some of which are toxic. Chemically bound nitrogen and sulfur that may be present in the biomass are oxidized to nitrogen and sulfur oxides—mostly sulfur dioxide, S02, but some sulfur trioxide, S03; and mostly nitric oxide, NO, but some nitrogen dioxide, N02. Air is the usual source of the oxidant, oxygen, for biomass combustion. Small amounts of nitrogen in the air are
also converted to nitrogen oxides at combustion temperatures according to the reactions

N2 + 02 —» 2NO 2NO + 02 2N02.

The equilibrium concentrations of NO formed from equimolar amounts of nitrogen and oxygen at various temperatures are shown in Table 7.1. It is evident that the higher the combustion temperature, the higher the NO concen­tration. The concentrations of chemically bound nitrogen and sulfur are zero to very low in most woody biomass species, but some biomass can contain relatively large amounts of these elements (Chapter 3). Elements such as chlorine, which can be present at relatively high concentrations in biomass such as MSW and RDF, but which are present in very small concentrations in woody biomass, are converted to chlorine compounds such as hydrogen chloride. Most of the chlorine derivatives are considered to be pollutants. Carbon monoxide, acid gases, unburned hydrocarbons, partially oxidized or­ganic compounds, polycyclic aromatic derivatives, trace metal oxides, nitrogen and sulfur oxides, chlorine derivatives, particulate carbon, and flyash are found in the flue gases of poorly controlled systems. The amounts of ash formed on oxidation of the metallic elements in biomass can be minor or major combus­tion products, depending on the composition of the biomass fuel. Biomass combustion systems should be designed to approach complete combustion under controlled conditions as closely as possible to extract the maximum amount of thermal energy, minimize undesirable emissions, and meet environ­mental regulations.

The stoichiometric amount of oxygen is the minimum amount needed for complete combustion of the fuel. A limited amount of excess oxygen is often

TABLE 7.1 Thermodynamic Equilibrium Concentrations of Nitric Oxide from Equimolar Amounts of Nitrogen and Oxygen”

43

89

251

500

1000

1630 5460

‘Tewksbury (1991).

used with solid fuels to promote complete combustion. Since ambient air contains about 79 mol % nitrogen and is the usual source of oxygen under normal conditions, nitrogen is a major constituent of the flue gas. The tempera­ture attained in the combustion chamber depends on the rate of heat release, its dissipation and transfer, and the quantity of combustion gases. So the increase in combustion temperature is substantially less with air as the oxygen source compared to pure oxygen because of dilution of the combustion gases with nitrogen. The air-to-biomass mass ratio is therefore an important parame­ter because it affects the rate of combustion and the final temperature of the combustion gases. Oxygen-enriched air and the use of small fuel particles or powders have been employed to maximize combustion temperatures.

Coal gasifier designs are almost as numerous as the many different types and ranks of coal. The basic configurations, hardware, and operations that have been considered are described here because several of them are applicable to biomass gasification (cf. National Academy of Engineering, 1973, and accompa­nying references).

Modern coal gasification processes consist of a sequence of operations: coal crushing, grinding, drying, and pretreatment, if necessary; feeding the coal into the gasifier; contacting the coal with the reacting gases for the required time in the gasifier at the required temperature and pressure; removing and separating the solid, liquid, and gaseous products; and treating the products downstream to upgrade them and to stabilize and dispose of solid and liquid wastes, dust, fines, and emissions. A large number of solids-feeding devices have been developed for low-pressure, atmospheric gasifiers. These include

screws and star valves. For gasifiers operating at elevated pressures, lockhop — pers and slurry pumping are the two leading solids-feeding devices. Lockhop — pers are operated in an intermittent fashion so that coal fills the hopper vessel at atmospheric pressure. The vessel is pressurized with gas; the coal then flows to the gasifier at elevated pressure, and the lockhopper is restored to atmospheric pressure. If one lockhopper is used, the flow of coal to the gasifier is intermittent; two or more can be used for continuous feeding. The ash is

withdrawn from the gasifiers as a slurry or by lockhopper. If the ash is molten as in slagging gasifiers, it is ordinarily quenched in water to solidify and break it up before disposal.

Gasifier operating temperatures range from 500 to 1650°C and pressures range from atmospheric to 7.6 MPa. The feedstocks are lump coal or pulverized coal. Processes using moving beds of lump coal can operate at temperatures up to about 980°C if the ash is recovered as a dry solid. Higher temperatures are possible if the ash is removed in a molten state. The methods of contacting

the solid coal feed with reactant gases include reactors that contain a descending bed of solids with upflowing gas, a fluidized bed of solids, entrained flow of solids in gas, or molten baths of gasifying media. Modern processes generally utilize fixed-bed reactors operated under nonslagging or slagging conditions, circulating or bubbling fluid-bed reactors with ash recovered from the bed in either a dry or agglomerated form, entrained-flow reactors with pulverized coal suspended in the gas stream wherein gasification is completed before the gas containing the ash leaves the gasifier, or molten bath reactors.

Fixed-bed gasifiers, which are also called moving-bed gasifiers, are usually counterflow systems. Coal is fed at the top of the gasifier and air or oxygen along with steam is generally injected near the bottom. The maximum temperature, normally 930 to 1430°C, occurs at the bottom, and the residence time in the gasifier is 1 to 2 h. The fixed-bed gasifier involves countercurrent flow in which large particles of coal move slowly down the bed and react with gases moving up through the bed. Various processes occur in different zones of the reactor. At the top of the gasifier, the coal is heated and dried while cooling the exiting product gas. The temperatures of the exit gas range from 315°C for high-moisture lignites to 550°C for bituminous coal. As the coal descends through the gasifier, sequential heating, drying, devolatilization, carbonization, and gasification take place. Fixed-bed coal gasifiers are characterized by lower gasification and product gas temperatures, lower oxygen requirements, lower tar and oil production, higher methane content in the product gas, and limited ability to handle caking coals and coal fines.

Fluid-bed gasifiers generally require coal in the 10- to 100-mesh size range, and the maximum bed temperature is determined by the fusion point of the ash, which is usually 815 to 1040°C. Operation below the fusion temperature avoids formation of sticky, molten slag. Fresh coal feed is well mixed with the particles of coal and char already undergoing gasification. Steam and oxygen or air is usually injected near the bottom of the bed. Some unreacted coal and char particles are reduced in size during gasification and are entrained in the hot exit gas. This material is recovered for recycling. The ash is removed at the bottom of the bed and is cooled by heating the incoming feed gas and recycle gas. Fluid-bed gasifiers generally utilize significant recycle of flyash, operate at moderate and constant temperatures, and are limited in their ability to convert high rank coals. Agglomerated ash operation, which can be achieved by incorporation of a hot-ash agglomerating zone in the bottom of the reactor so that the ash particles stick together and grow in size until separated from the unreacted char, improves the ability of the process to gasify high rank and caking coals.

Entrained-flow gasifiers use pulverized coal, about 70% of which is smaller than 200 mesh, and have high feedstock flexibility. The coal particles are entrained in the steam-oxygen feed and the recycled gas stream and gasified at residence times of a few seconds, after which the product gas is separated from the ash. The lower residence times can offer potentially higher through­puts at elevated pressures. Entrained-flow gasifiers can be operated at lower temperatures to maintain the ash as a dry solid, or at temperatures well above the ash fusion point in the slagging mode so that the ash is removed as a molten liquid. Operation at higher temperature results in little or no tars and oils in the product gas.

In molten bath processes, crushed coal is passed with reacting gases into the liquid bath, where gasification occurs. The ash can become part of the liquid bath or can be separated. The media include liquid iron and liquid sodium carbonate.

Low-, medium-, and high-energy gases can be produced in coal gasification processes. The important parameters are essentially the same as those for

biomass gasification systems. The higher heating values of the combustible gases commonly formed in coal-derived gases are listed in Table 9.2. As in the case of biomass gasification, the primary combustible components in low — energy product gases are carbon monoxide and hydrogen. In gasifiers where the coal particles are in direct contact with the oxygen-containing gas, nitrogen is a major component in the product gas if air is used as a coreactant instead of oxygen. Medium-energy gases are usually formed with oxygen and contain a higher percentage of combustibles, in addition to hydrogen and carbon monoxide, such as methane. High-energy gases approaching heating values of 39.3 MJ/m3 (n) (1000 Btu/SCF), the approximate higher heating value of pure methane, are produced at lower temperature conditions with oxygen instead of air, to maximize methane concentration. Further processing is necessary to methanate residual carbon monoxide and to separate noncombustible gases to provide a high-energy gas.

I. INTRODUCTION

The conversion of solar radiation into chemical energy via photosynthesis results in the growth of woody, herbaceous, and aquatic biomass and the formation of many organic compounds in situ, each of which has an intrinsic energy content. The lower the oxygenated state of the fixed carbon in these compounds, the higher the energy content. As discussed in previous chapters, a-cellulose, or cellulose as it is more commonly known, is usually the chief structural element and principal constituent of many biomass species, particu­larly woody biomass, but is not always the dominant carbohydrate, especially in aquatic species. The lignins and hemicelluloses comprise most of the remaining organic components. In addition, other polymers and a large variety of nonpoly­meric organic solids are formed naturally, although not equally, in biomass. Many of these chemicals are or have been used in specialty applications such as pharmaceuticals and industrial formulations. Natural products continue to be discovered, and many have been found to have useful applications. Hundreds of biomass species have also been found to produce low-molecular-weight organic liquids, several of which are used or proposed for use as transportation

fuels for vehicles driven by spark — or compression-ignition engines. These liquids are glycerides and terpenes. The glycerides, which are the primary members of a group of organic compounds called lipids, are mainly triglyceride esters of long-chain fatty acids and the triol glycerol. Lipid is a general name for plant and animal products that are structurally esters of higher fatty acids, but certain other oil-soluble, water-insoluble substances are also called lipids. The fatty acids are any of a variety of monobasic acids such as palmitic, stearic, and oleic acids. Among more than 50 fatty acids found in nature, almost all are straight-chain acids containing an even number of carbon atoms. A few biomass species produce esters of fatty alcohols and acids. Certain glycerides are essential components of the human diet and are obtained or derived from animal fats and vegetable oils. In addition to cooking and food uses, many natural glycerides have long been used as lubricants and as raw materials for the manufacture of soaps, detergents, cosmetics, and chemicals. Some are directly useful as motor fuels as formed or can be converted to suitable fuels after relatively simple upgrading using established processes. The derivatives formed on transesterification (alcoholysis) of several natural glycerides with low-molecular-weight alcohols are useful as neat diesel fuels or in diesel fuel blends or as diesel fuel additives. An example is the ester formed on methanol — ysis of soybean oil.

The term terpenes originally designated a mixture of isomeric hydrocarbons of molecular formula Cl0Hi6 occurring in turpentine obtained from coniferous trees, especially pine trees. Today, the term refers to a large number of naturally occurring hydrocarbons that can be represented as isoprene adducts having the formula (C5H8)n, where n is 2 or more, and to an even larger number of derived terpenoids in various states of oxidation and unsaturation. Terpenes are widely distributed in many biomass species and are often found in biomass oils, resins, and balsams. They are classified according to the number of iso­prene units contained in the empirical formula: for example, Ci0Hi6, monoter — penes; C15H24, sesquiterpenes; C20H32, diterpenes; C30H48, triterpenes; and (C5H8)X, polyisoprenes. The terpenes thus range from relatively simple hydro­carbons to large polymeric molecules. The lower molecular weight terpenes are usually liquid at room temperature at n = 2 or 3 and are mainly alicyclic structures. Terpenes are monocyclic, bicyclic, tricyclic, etc.; open-chain acyclic terpenes are also known. Examples of terpenes are the dienes limonene (mono­cyclic monoterpene) and cadinene (bicyclic sesquiterpene), the monoenes a — and /З-pinene (bicyclic monoterpenes), the triene myrcene (acyclic monoter­pene), the hexaene squalene (acyclic triterpene), and natural rubbers, which are high-molecular-weight polymers of isoprene.

In this chapter, the sources of natural biochemical liquids potentially suit­able as motor fuels, their basic properties and conversion chemistry, and their process economics are examined.

Knowledge of the chemical structures of the major organic components in biomass is quite valuable in the development of processes for producing derived fuels and chemicals. Information on the chemical structures can often lead to methods of improving existing processes and to development of advanced conversion methods. Somewhat more detailed information on the chemical structures of the major components in biomass is presented here.

Alpha cellulose is a polysaccharide having the generic formula (C6H10O5)„ and an average molecular weight range of 300,000 to 500,000. Complete hydrolysis established that the polymer consists of D-glucose units. Partial hydrolysis yields cellobiose (glucose-/3-glucoside), cellotriose, and cellotetrose. These results show that the glucose units in cellulose are linked as in cellobiose (Fig. 3.6). Cotton is almost pure a-cellulose, whereas wood cellulose, the raw material for the pulp and paper industry, always occurs in association with hemicelluloses and lignins. Cellulose is insoluble in water, forms the skeletal structure of most terrestrial biomass, and constitutes approximately 50% of the cell wall material. Carefully purified wood cellulose contains a few carboxyl groups which are believed to be esterified in the natural state.

Starches are polysaccharides that have the generic formula (СбН10О5)п. They are reserve sources of carbohydrate in some biomass, and are also made up of D-glucose units as shown by the results of hydrolysis experiments. But in contrast to the structure of cellulose, the hexose units are linked as in maltose, or glucose-a-glucoside (Fig. 3.6), as indicated by the results of partial hydroly­sis. Another difference between celluloses and starches is that the latter can

ОНО I неон I HOCH I неон I неон I CH2OH D-glucose (aldehydic form)

он

СН2ОН ‘ — о. .он

но

СН2ОН

он

Cellulose

CHO I неон I HOCH I неон I неон I CH2OH D-glucose (aldehydic form)

Starch Chains

CHO I неон I HOCH I неон I CH2OH D-xylose (aldehydic form)

OH OH OH Xylan Chains

CH30

CH30 Some structural units of lignin

o II ch2ocr I 0 I II CHOCR’ I 0 I II CH2OCR" Triglyceride

FIGURE 3.6 Chemical structures of some biomass components.

be separated into two fractions by treatment with hot water: a soluble compo­nent called amylose (10 to 20%) and insoluble amylopectin (80 to 90%). Amylose and amylopectin have molecular weight ranges of 10,000 to 50,000 and 50,000 to 1,000,000, respectively. Both fractions yield glucose or maltose on hydrolysis, but amylopectin is believed to consist of branched chains. Starches occur in the form of minute granules in seeds, tubers, and other plant parts and are important constituents of corn, beans, potatoes, rice, wheat, and other biomass foodstuffs.

Hemicelluloses are complex polysaccharides that occur in association with cellulose in the cell walls. But unlike cellulose, hemicelluloses are soluble in dilute alkali and consist of branched structures, which vary significantly among different woody and herbaceous biomass species. Many have the generic for­mula (C5H804)„. They are termed pentosans and yield mainly pentoses instead of hexoses on hydrolysis. Some hemicelluloses, however, contain hexose units. Hemicelluloses usually consist of 50 to 200 monomeric units and a few simple sugar residues. The most abundant one, xylan, consists of D-xylose units linked in the 1- and 4-positions (Fig. 3.6). Xylan is closely related to polyglucuronic acid with which it is associated in the natural state, and from which it can be produced by decarboxylation. Other hemicelluloses include the glucomannans, which consist of D-glucose and D-mannose units in the polymeric chains in ratios of about 30: 70, and galactoglucomannans, which consist of D-galactose, D-glucose, and D-mannose in the polymeric chains in ratios of about 2:10:30. The pentosans can occur in large amounts (20 to 40%) in corncobs and corn stalks and in biomass straws and brans. The xylans have been found in soft­woods and hardwoods up to about 10% and 30% of the dry weight of the species, respectively, whereas mannans are generally present at about 15% of the dry weight in softwoods and only a few percent by weight in hardwoods.

The lignins are highly branched, substituted, mononuclear aromatic poly­mers in the cell walls of certain biomass, especially woody species, and are often bound to adjacent cellulose fibers to form what has been called a lignocellulosic complex. This complex and the lignins alone are often quite resistant to conversion by microbial systems and many chemical agents. The complex can be broken and the lignin fraction separated, however, by treatment with strong sulfuric acid, in which the lignins are insoluble. The precise structures of the polymers have not been determined because of their diverse nature and complexity. The dominant monomeric units in the polymers are benzene rings bearing methoxyl, hydroxyl, and propyl groups that can be attached to other units (Fig. 3.6). The lignin contents on a dry basis in both softwoods and hardwoods generally range from 20 to 40% by weight, and from 10 to 40% by weight in various herbaceous species such as bagasse, corncobs, peanut shells, rice hulls, and straws.

As previously mentioned, the triglycerides found in biomass are esters of the triol, glycerol, and fatty acids (Fig. 3.6). These water-insoluble, oil-soluble esters are common in many biomass species, especially the oilseed crops, but the concentrations are small compared to those of the polysaccharides and lignins. Many saturated fatty acids have been identified as constituents of the lipids. Surprisingly, almost all the fatty acids that have been found in natural lipids are straight-chain acids containing an even number of carbon atoms. Most lipids in biomass are esters of two or three fatty acids, the most common of which are lauric (C12), myristic (C14), palmitic (Ci6), oleic (C!8), and linoleic (C18) acids. Palmitic acid is of widest occurrence and is the major constituent (33 to 45%) of the fatty acids of palm oil. Lauric acid is the most abundant fatty acid of palm-kernel oil (52%), coconut oil (48%), and babassu nut oil (46%). The monounsaturated oleic acid and polyunsaturated linoleic acid comprise about 90% of sunflower oil fatty acids. Linoleic acid is the dominant fatty acid in corn oil (55%), soybean oil (53%), and safflower oil (75%). Saturated fatty acids of 18 or more carbon atoms are widely distributed, but are usually present in biomass only in trace amounts, except in waxes.

Other classes of organic materials, such as alkaloids, pigments, resins, ste­rols, terpenes, terpenoids, and waxes, and many simple organic compounds are often present in various biomass species, but are not discussed here because they are usually present in very small amounts. The peptides present in herba­ceous biomass are also not discussed here because, although the nitrogen and sulfur contents of the biomass should be assessed for certain microbiological processes, the amino acids that make up the proteins are generally not impor­tant factors in conversion processes.

Industry uses more than one-third of all energy consumed in the United States. Sizable amounts of waste are generated, but only the pulp and paper industry generates large quantities of waste biomass. The food processing industry also generates waste biomass. It is not discussed here because of the difficulty of compiling the amounts and disposition of the residues produced by a disaggregated industry composed of many small, medium, and large companies involved in diverse activities. However, the utilization of solid residues and some aqueous wastewaters as a captive energy resource by many of the food processing companies is well known.

I. INTRODUCTION

As discussed in Chapter 7, the final products of biomass combustion are C02, water, and energy. This is the case, of course, for the combustion of all organic matter. It is not known how much time passed after the utility of biomass combustion was discovered by man until biomass pyrolysis was discovered. But when it was, it is probable that a new era in biomass usage evolved, and quite rapidly. Biomass pyrolysis can be described as the direct thermal decomposition of the organic components in biomass in the absence of oxygen to yield an array of useful products—liquid and solid derivatives and fuel gases. Eventually, pyrolysis processes were utilized for the commercial production of a wide range of fuels, solvents, chemicals, and other products from biomass feedstocks. Improvements continue to be made today to perfect the technology. (It is important to note at the outset that any organic material can be pyrolyzed. Indeed, the pyrolysis of coal has been in commercial use for many years, and still is in several areas of the world for the production of fuel gases, cokes, tars, and chemicals.)

Knowledge of the effects of various independent parameters such as biomass feedstock type and composition, reaction temperature and pressure, residence time, and catalysts on reaction rates, product selectivities, and product yields has led to development of advanced biomass pyrolysis processes. The accumu­lation of considerable experimental data on these parameters has resulted in advanced pyrolysis methods for the direct thermal conversion of biomass to liquid fuels and various chemicals in higher yields than those obtained by the traditional long-residence-time pyrolysis methods. Thermal conversion processes have also been developed for producing high yields of charcoals from biomass.

In this chapter, the basic chemistry of the direct pyrolysis of biomass and the state-of-the-art systems that have been or are expected to be commercialized are discussed. Pyrolysis in the presence of hydrogen (hydropyrolysis) and methane (methanolysis) are also addressed. Energy recovery in the form of liquid fuels and chars is emphasized. Another group of processes for direct thermal conversion of biomass employs a liquid medium for conversion of biomass to liquid fuels. For convenience, these processes are grouped together in this chapter as miscellaneous thermal liquefaction methods. Some discussion of biomass gasification is included in this chapter when needed to clarify pyrolysis chemistry. High-temperature pyrolysis, which yields gaseous fuels and feedstocks such as low — and medium-energy fuel gases, hydrogen, and synthesis gases, is discussed in more detail in Chapter 9.

II. FUNDAMENTALS

Basically, biomass gasifiers can be categorized into several reactor design groups: a descending bed of biomass, often referred to as a moving or fixed bed, with countercurrent gas flow (updraft); a descending bed of biomass with cocurrent gas flow (downdraft); a descending bed of biomass with crossflow of gas; a fluidized bed of biomass with rising gas; an entrained-flow circulating bed of biomass; and tumbling beds. Many reactor designs have been evaluated under a broad range of operating conditions. The designs include fixed-bed, moving-bed, suspended-bed, and fluid-bed reactors; entrained-feed solids reac-

tors; stationary vertical-shaft reactors; inclined rotating kilns; horizontal-shaft kilns; high-temperature electrically heated reactors with gas-blanketed walls; single and multihearth reactors; ablative, ultrafast, and flash pyrolysis reactors; and several other designs. There are clearly numerous reactor designs and configurations for biomass gasification, probably more than in the case of coal gasification systems because of the relative ease of thermal biomass conversion.

Fixed-bed, updraft gasifiers are simple to construct and can consist of carbon steel shells equipped with a grate at the bottom fed by a process air manifold, a lockhopper at the top to feed material, and a manifold to remove gas at the top (с/. Miller, 1987). These units are simple to construct and operate and are relatively inexpensive. The gas exiting the gasifier tends to be cool because it has percolated up through the bed and therefore usually contains a fair fraction of lower molecular weight hydrocarbons. Much of the sensible heat has been lost, the feeds are limited to wood chips, and the size is usually not more than 50 million GJ/h. Fixed-bed, downdraft gasifiers consist of two concentric shells. The inner shell holds the material on the grate; the outer shell is used to transport the gas. The gas is drawn out from under the grate through the outer shell to the outside of the system. The gas exits at the combustion zone, and because it is hot, it contains few longer chain hydrocarbons and particulates. The system, however, is more expensive to construct than fixed-bed, updraft gasifiers, and is also limited to sizes up to about 10 to 20 GJ/h and chip feeds. Fluid-bed systems afford more efficient gasification because hot spots are eliminated, diverse feedstocks can be charged, the exit gas has a high sensible heat content, and the gasifiers are capable of scale-up to relatively large sizes. However, the units are more expensive to construct and product gas quality must be carefully monitored because of its higher particulate content.