A. Fundamentals

Reduction in physical size is often required before biomass is used as a fuel or feedstock. Size-reduction techniques are employed to prepare biomass for direct fuel use, fabrication into fuel pellets, cubes, and briquettes, or conversion. Smaller particles and pieces of biomass reduce its storage volume, facilitate handling of the material in the solid state and transport of the material as a slurry or pneumatically, and sometimes permit ready separation of components such as bark and whitewood. The size of the pieces or particles can be critical when drying is used because the exposed surface area, which is a function of physical size, can determine drying time and the methods and conditions needed to remove moisture. There are a few exceptions where size reduction is not needed, such as in whole-tree burning.

The physical dimensions of the feedstock are also related to the conversion method that is used. Particle size should satisfy the requirements of supplying feedstock to the conversion reactor and of the conversion process itself. For combustion systems, the combustion chamber and heat exchanger designs, the operating conditions, and the methods of delivering solid fuel and removing the ash determine the optimum size characteristics of the fuel. For thermal gasification and liquefaction processes, particle size and size distribution can influence the rate of conversion, the operating conditions of the process, and product yields and distributions. Biological processes are also affected by the physical size of the feedstock. In general, the smaller the substrate particles, the higher the reaction rate because more surface area is exposed to the enzymes and microorganisms that promote the process.

If the particle size of the biomass fuel or feedstock is not predetermined by its history, as is the case for sawdusts, nutshells, and a few other waste biomass materials, size reduction is usually carried out with one or more units that make up the “front end” of the total processing system. Many different kinds of machines are employed. Generally, the size of the feed is reduced by grinding, cutting, or impact mechanisms. Not all of the designs are suitable for biomass energy applications because the equipment is customized for certain uses or the cost of size reduction is excessive. Agricultural crops and woody biomass are also usually processed by different types of machines. A brief review of the basic types of machines that are or have been used for biomass follows to illustrate the variety of size-reduction equipment and their biomass applications.

A. Hardware

Pyrolysis systems are as varied as combustion systems. The ancient process of making charcoal by the slow pyrolysis of a pile of wood covered with earth is still used today in some developing countries. Pyrolysis times are several days and charcoal is the main product. Before fossil fuels became the preferred feedstocks for chemical production in the early part of the twentieth century, biomass pyrolysis reactors in industrialized countries consisted of various types of ovens and horizontal and vertical steel retorts, essentially all of which were operated in the batch mode. Provision was made for charcoal recovery, pyroligneous acid refining, by-product recovery, and gas recovery and usage. Modern pyrolysis reactor configurations include fixed beds, moving beds, suspended beds, fluidized beds, entrained-feed solids reactors, stationary vertical-shaft reactors, inclined rotating kilns, horizontal shaft kilns, high- temperature (1000 to 3000°C) electrically heated reactors with gas-blanketed walls, single and multihearth reactors, and a host of other designs.

Commercial systems consisting of a close-coupled gasifier and combustor are manufactured by CHIPTEC Wood Energy Systems and are widely used in the Northeast to supply hot water and steam to schools and commercial buildings (Bravakis, 1996). The plants are fueled with wood chips that are conveyed to the refractory-lined gasification chamber by an automated feeder. The fuel can contain up to 45% moisture. An induced draft fan draws air into the crossdraft gasifier, and the resulting low-energy product gas, which is produced under oxygen-deficient conditions, is passed to the combustion chamber. High tem­perature combustion, a 20:1 turn-down ratio, refractory heat storage, and controlled air allow the gasifier to respond quickly to boiler demand. Gasifier outputs range in size from 0.5 to 10.5 GJ/h, and the corresponding firing rates are 50.8 to 965 kg/h with wood chips having a 35% moisture content. The smaller systems have stationary grates and the larger systems are equipped with moving grates.

I. INTRODUCTION

Although many questions remain to be answered regarding the complex chem­istry of biomass growth, the reactions that occur when carbon dioxide (C02) is fixed in live green biomass are photochemical and biochemical conversions that involve the uptake of C02, water, and the solar energy absorbed by plant pigments. Carbon dioxide is reduced in the process and water is oxidized. The overall process is called photosynthesis and is expressed by a simple equation that affords monosaccharides as the initial organic products. Light energy is converted by photosynthesis into the chemical energy contained in the biomass components:

6C02 + 6H20 + light -> C6H1206 + 602.

Hexose

The inorganic materials, C02 and water, are converted to organic chemicals, and oxygen is released. The initial products of a large group of biochemical reactions that occur in the photosynthetic assimilation of ambient C02 are
sugars. Secondary products derived from key intermediates include polysaccha­rides, lipids, proteins and a wide range of organic compounds, which may or may not be produced in a given biomass species, such as simple low-molecular — weight organic chemicals (e. g., acids, alcohols, aldehydes, ethers, and esters), and complex alkaloids, nucleic acids, pyrroles, steroids, terpenes, waxes, and high-molecular-weight polymers such as the polyisoprenes. When biomass is burned, the process is reversed and the energy absorbed in photosynthesis is liberated along with the initial reactants.

The fundamentals of photosynthesis are examined in this chapter, with emphasis on how they relate to biomass production and its limitations. The compositions of different biomass species and the chemical structures of the major components are also examined in the context of biomass as an energy resource and feedstock.

A large variety of virgin biomass feedstock developments for the production of energy, biofuels, and chemicals is in the research stage in Canada, the

United States, and many other countries. Research is progressing to develop and select special species and clones of trees and herbaceous crops and to develop advanced growth and management procedures for dedicated energy crops. This work is being done in the laboratory and in the field and is aimed at reducing the cost of biomass and increasing the efficiency of production. Research on short-rotation tree growth methods and the screening of woody and herbaceous biomass continues, generally on small-scale test plots. The North American effort has focused on hybrid poplar, willow, switchgrass, and a few other species. The emphasis in South America is on species such as Eucalyptus that grow well in semitropical and tropical climates. Larger scale field trials in which dedicated biomass production is integrated with conversion are beginning to evolve in the United States from the research done with small systems. But most of the continuing research in the United States on the selection of suitable biomass is limited to laboratory studies and small-scale test plots. Many of the research programs on feedstock development were started in the 1970s and early 1980s. Based on the research data accumulated in this work, some of the herbaceous and woody biomass species that appear to be good models for energy feedstock production are shown by region in Fig. 4. 9 for the continental United States and Hawaii (Wright, 1994).

A. Definition

Complete combustion (incineration, direct firing, burning) of biomass consists of the rapid chemical reaction (oxidation) of biomass and oxygen, the release of energy, and the simultaneous formation of the ultimate oxidation products of organic matter—C02 and water. Chemical energy is released, usually as radiant energy and thermal energy, the amount of which is a function of the enthalpy of combustion of the biomass. In the idealized case, stoichiometric amounts of biomass and oxygen are present and react so that perfect combus­tion occurs; that is, each reactant is totally consumed and only C02 and water are formed. Under normal conditions, such combustion does not occur with most carbon-containing solid fuels, including biomass.

When biomass is combusted under normal conditions, a flame is produced as visible radiation, provided oxidation occurs at a sufficient rate. By use of thermodynamic data, the theoretical temperature at which the products of combustion form under adiabatic, reversible conditions can be calculated. The theoretical flame temperature for the combustion of wood of various moisture contents with excess air is shown in Fig. 7.1 (Tewksbury, 1991). Green wood generally contains about 50% moisture by weight in the field, and excess air is used to promote complete combustion. It is apparent from Fig. 7.1 that both high fuel moisture levels and excess air significantly reduce the theoretical flame temperature. It is also apparent that to achieve maximum flame tempera­ture, dry fuel and small amounts of excess air are required. In actual practice, however, combustion is not adiabatic and the reactions that occur are irrevers­ible, so the actual flame temperature is less than the theoretical value.

Coal gasification is reviewed here to provide a foundation for more detailed discussion of biomass gasification.

A. Brief History

Coal gasification to produce gas for a variety of applications such as fuels, chemicals, and chemical intermediates has been known for many years. The largest application of coal gasification by far has been for manufactured fuel gas production by pyrolytic and partial oxidation processes in which the primary fuel components in the product gas are hydrogen, carbon monoxide, and methane. The first manufactured gas (town gas) plant was built in England in 1812 by London and Westminster Chartered Gas, Light and Coke Company, although the first record of experimental manufactured gas production from coal dates back to seventeenth-century England (cj. Environmental Research and Technology and Koppers Co., 1984; Srivastava, 1993). North America’s first manufactured gas plants were built in Baltimore in 1816, in Boston in 1822, and in New York in 1825 (Rhodes, 1974). The early processes involved the carbonization or destructive distillation of bituminous coal at temperatures of 600 to 800°C in small cast-iron retorts to yield “coal gas” (Villaume, 1984). It has been estimated that more than 1500 manufactured gas plants were in operation in the United States during the nineteenth century and the first half

“The standard enthalpies of formation used for the calculations are from Stull, Westrum, and Sinke (1987) and Daubert and Danner (1989). The standard enthalpy of formation of cellulose was calculated from its heat of combustion. The monomeric unit of cellulose is C6H10O5. The enthalpies are listed in kj/g-mol of monomeric unit gasified.

of the twentieth century. The gasification processes used in these plants af­forded water gas, producer gas, oil gas, coke oven gas, and blast furnace gas (Liebs, 1985; Remediation Technologies, Inc., 1990).

Natural gas displaced most manufactured gas for municipal distribution in industrialized countries after World War II. In the 1960s and 1970s, interest in developing advanced coal gasification processes was rekindled when it was believed that natural gas reserves would become insufficient in a few years to meet demand. This activity has since declined, but several coal gasification processes developed during this period have been commercialized and are used for production of fuel and synthesis gas.

Many economic analyses of biomass gasification for low — and medium-energy gas, synthesis gas, and methanol production have been performed after biomass gasification developments started to increase in the 1970s. The basic approach to many of these analyses is illustrated here by focusing on the manufacture of methanol. For stand-alone methanol plants using biomass feedstocks, the se­quence of operations has generally consisted of gasification to a low — or medium — energy gas, steam reforming to essentially all hydrogen and carbon oxides, water gas shift to produce a gas with a molar ratio of hydrogen: carbon monoxide of 2:1, acid gas scrubbing to remove carbon dioxide, and methanol synthesis. The gas compositions that would ideally be obtained from each step, using Bailie’s indirectly heated steam-gasification process as the source of the synthesis gas, are shown in Table 9.12. Analysis of the cost of synthesis gas production alone, which was reported in the early 1980s for this process (Bailie, 1980,1981), re­sulted in a projected capital cost of $22,050/dry t ($20,000/dry ton) of biomass feedstock capacity per day, and a synthesis gas cost of $3.04 to $3.39/GJ ($3.21 to $3.57/MBtu) at a feedstock cost of $31.58/dry t ($28.64 dry ton), or $1.70/GJ ($1.79/MBtu). At that time, the posted prices of natural gas and methanol were $3.16/GJ ($3.00/MBtu) and $10.54/GJ ($10.00/MBtu), or $0.145/L ($0.56/gal). Average capital costs for the steam gasification of biomass in mid-1990 nominal dollars range from about $55,000 to $88,000/dry t ($50,000 to $80,000/dry ton)

of biomass feedstock capacity per day depending upon plant size and other fac­tors, so the impact of time on equipment costs is evident. The feedstock and operating costs are also higher.

A plethora of economic projections has appeared in North America on the production of synthesis gas and methanol from biomass since this early work. Governmental regulations regarding motor fuel compositions and the use of oxygenates are undoubtedly responsible in part for this renewed interest. The details of a comparative economic analysis that compared the capital, operating, and methanol production costs of Wright-Malta’s steam gasification process, Battelle’s steam-air gasification process, IGT’s steam-oxygen RENUGAS pro­cess, and Shell Oil’s coal gasification process as applied to the steam-oxygen gasification of biomass are summarized in Tables 9.13 and 9.14 (Larson and Katofsky, 1992).

“Larson and Katofsky (1992).

kThis process produces about 2 to 3 mol % C;+, but is not included in the analysis. CHHV of product gas/Sum of HHVs gasifier (and combustor in Battelle case).

According to this analysis, the capital, operating, and methanol production costs from a plant supplied with 1650 dry t/day of wood feedstock ranges from $204 to $338 million, $35.3 to $43.7 million/year, and $0.17 to $0.27/L, respectively. The feedstock cost was assumed to range from $38.19 to $41.88/dry t. At a 90% capacity factor, methanol production ranges from 293 to 417 million L/year depending on the process. Production is highest with the Wright-Malta process and lowest with the Battelle process because a substantial portion of the feedstock is used as fuel to the combustors for the latter process. A generally conservative approach was used for this economic assessment. All unit operations with the exception of biomass gasification were established, commercial technologies when the analysis was performed. The overall cost of methanol is more attractive for the two indirectly heated steam gasification processes (Wright-Malta and Battelle) compared to the methanol cost estimated for the directly heated gasification processes (IGT and Shell). The cost of the oxygen plant is a major contributor to product cost for the directly heated processes. Also, a few of the assumptions made by the analysts appear to disproportionately and adversely affect the cost of methanol from the Wright- Malta process, which when adjusted would provide still lower cost methanol. The utilities and purchased energy costs for this process seem to be excessive because only a small amount of purchased energy would be necessary, as already mentioned in the discussion of the reported autothermal nature of this process. In addition, the requirement for 17 gasification kilns operating in parallel to achieve a target plant capacity of 1650 t/day because of the kilns’ low throughput capacity contributed significantly to product cost for the Wright-Malta process. Nevertheless, this type of comparative analysis illus­trates the various facets of such economic assessments that should be examined and emphasizes where improvements might be made in the economics of each process.

The enthalpies of formation of biomass are quite useful for thermodynamic calculations. The heats of specific reactions that utilize biomass feedstocks can be estimated from the standard enthalpies of formation at 298 К of the combustion products (in kcal/g-mol: C02, -94.05; liquid H20, —68.37; N02, 8.09; S02, —70.95), the elemental analyses of the biomass being examined, and its HHV. The enthalpy of formation of a particular biomass sample is equal to the sum of the heats of formation of the products of combustion

minus the HHV. It is assumed that the ash is inert. For example, a sample of giant brown kelp has the empirical formula C2.6iH4.e3N0.10S0.01O2.23 (dry basis), which is derived from the elemental analysis, and a HHV of 296.1 kcal/g-mol (12.39 MJ/kg) at an assumed molecular weight of 100, including the ash. The stoichiometry for calculating the enthalpy of formation is

2.61C + 2.315H2 + 0.05N2 + 0.01S + 1.11502

—* C2.6iH463No. ioSo. oi02.23Ash267 (AHf — 107.5).

The enthalpy of formation is —107.5 kcal/g-mol (—4.50 MJ/kg) including the ash for this particular biomass sample. An example of the utilization of this information is illustrated by applying it to the biological gasification process under anaerobic conditions. The stoichiometry of the process is

C2.6iH4.63O2.23 (s) + 0.337H2O(l) -> І. З26СН4 (g)

+ 1.283C02 (g) (ДН -13.85).

The enthalpy of the process is estimated to be —13.85 kcal/g-mol (—0.58 MJ/ kg) of kelp reacted (Klass and Ghosh, 1977).

It is assumed in these calculations that the inorganic components are carried through the process unchanged, and that the nitrogen and sulfur can be ignored since their concentrations are small. For each kilogram of kelp reacted, the feedstock energy input is 12.39 MJ, and the energy output is 11.81 MJ as methane (0.8903 MJ/g-mol at 298 K). The calculations indicate the process is slightly exothermic. About 95% of the feed energy resides in product methane, and about 5% is lost as heat of reaction.

Forestry residues consist of slash left on the forest floor following logging operations; stems, stumps, tops, foliage, and damaged trees that are not mer­chantable; and wood and bark residues accumulated at primary wood manufac­turing plants during production of lumber. Underground tree roots can also be included in the list of forestry residues. The difficulty of accurately assessing the amounts of forestry residues that are and can be realistically collected and utilized as waste biomass for an entire country has been encountered by almost all who have embarked on the task. In the United States, many federal, state, and regional forestry offices and many of the companies in the logging and lumber manufacturing businesses do not keep and maintain detailed records of residue production and its disposition. Surveys are done only periodically and they vary widely from state to state. The survey of an entire country for a given time period is therefore subject to considerable error. Nevertheless, such assessments provide valuable information and an overall indication of the energy potential of forestry residues as waste biomass.