The average net annual productivities of dry organic matter on good growth sites for terrestrial and aquatic biomass are shown in Table 4.12. With the exception of phytoplankton, which generally has lower net productivities,

Average Net Yield

(dry t/ha-year) Climate Ecosystem type Remarks

’Westlake (1963).

aquatic biomass seems to exhibit higher net organic yields than most terrestrial biomass. Aquatic biomass species that are considered to be the most suitable for energy applications include the unicellular and multicellular algae, freshwater plants, and marine species.

U. S. standards for biomass pellet fuels have been developed and recommended by the Pellet Fuels Institute in the United States; they are shown in Table 6.5. The older standards included recommendations for moisture content and heating value, but these do not. Instead, it is recommended that the heating value be certified by the pellet manufacturer, so whatever the pellet material

and its moisture content, the consumer should be able to estimate the energy cost. The national standards in Table 6.5 make it possible for the manufacturers of pellet stoves, most newer versions of which auger-feed the pellets from the top, to produce units designed to accept the standardized pellets.

B. Economic Factors

The wholesale cost in the United States of wood waste pellets is in the range of $85 to $140/t (mid-1997). This cost range effectively precludes their use as feedstocks for most conversion processes, and it limits residential fuel applications. The production cost exclusive of biomass cost is estimated to be about 30 to 60% of the wholesale cost and depends on production rates and the amount of processing needed. For example, in Spain, the increase in electric energy consumption required to mill wood wastes to 5- to 8-mm sizes is almost totally compensated for by the decrease in electric energy consumption during densification (Ortiz and Gonzalez, 1993). Exclusive of wood cost, the cost of manufacturing densified wood residues in small units operated by one person is about $22/t at a production rate of 1250 t/year. Smaller particles in the 2-mm size range can increase production rates by 50% or more, but the energy cost is excessive. Industrial manufacturing costs in Spain of densified wood wastes exclusive of wood cost are about $32 to $48/t at production rates of 1.0 t/h (Ortiz, Miguez, and Granada, 1996). In Finland, the cost of producing straw fuel pellets on farms in small, portable pelletizers is estimated to be about $54 to $84/t (Wilen et ah, 1987). Note that the hardware cost can be a major factor in the cost of producing densified biomass. Biotruck 2000, described earlier, for producing pellets or briquettes from agricultural wastes in Europe has a production rate of about 8 t/h in the field and costs about $400,000 (700,000 DM) (Sutor, 1995).

A. Aqueous and Non-aqueous Non-pyrolytic Conversion

Note that the transformations described here, which take place in an aqueous or non-aqueous liquid medium that may or may not react with the biomass feedstock, are termed nonpyrolytic processes, as compared to pyrolysis pro­cesses in which the biomass feedstocks are directly heated.

The direct conversion of cellulosic materials to liquids by heating in aqueous systems has been known for more than 100 years (с/. Ostermann, Bishop, and Rosson, 1980). Pure cellulose is liquefied at 300°C and below at a pressure of 19.3 MPa in less than 1 h with or without added sodium carbonate catalyst up to a concentration of 0.8% in the medium (Molton et al, 1981). A wide range of aliphatic and aromatic alcohols, phenols, hydrocarbons, substituted furans, and alicyclic compounds is formed. The presence or absence of a gaseous atmosphere of carbon monoxide had no effect on the results, which contrasts somewhat to the results obtained from demonstration of a similar conversion system (PERC process) described later. The experimental results support a degradation mechanism that forms acetone, acrolein, and acetoin intermediates, which recondense under alkaline conditions to yield the ob­served products. Model compound experiments indicated that the ketones and furans are formed by aldol condensations and Michael reactions of carbonyl intermediates. The formation of aromatic hydrocarbons and phenol from these molecules under these conditions appears to involve a variant of the aldol condensation. In other experiments at temperatures of 268 to 407°C, pressures of 21 to 35 MPa, and reaction times of 20 or 60 min, the average liquid yield was 25 wt % of the cellulose converted to acetone-soluble oil (Miller, Molten, and Russell, 1981). About 10 wt % charcoal was produced along with gaseous by-products. The heating of poplar wood chips in water alone at about 330°C in autoclaves affords acetone-soluble liquid oils containing about 20 to 35% oxygen at yields up to about 50 wt % of the feedstock (Boocock et al, 1985, 1987). Little or no char is produced, and the physical breakdown of the chips is believed to occur by water absorption, swelling, and disruption and liquefaction of the matrix, whereupon the absorbed water is regenerated. When poplar chips are used, about one-half the oil is phenolic and one-fourth is phenol itself. The phenol yield is 6.5 wt % on a dry wood basis or 25 wt % based on the lignin content. The relatively high yield of phenol was suppressed under alkaline conditions. It was concluded that the steam formed in the process at the self-generated pressure of about 15.9 MPa was responsible for disruption of the chips. In subsequent experiments with steam injection at 350°C into a downflow reactor, the oil yields from poplar wood chips were in excess of 40%. The oils softened and flowed just above 100°C and their oxygen content was in the low 20% range.

Another interesting catalytic liquefaction method involves the reaction of biomass-water slurries (LBL process) or biomass-recycle oil slurries (PERC process) with sodium carbonate and carbon monoxide gas at elevated tempera­ture and pressure to form heavy liquid fuels. Biomass and the combustible fraction of wastes have been converted at weight yields of 40 to 60% at temperatures of 250 to 425°C and pressures of 10 to 28 MPa. Lower viscosity products are generally obtained at higher reaction temperatures and solid or semisolid products are obtained when the reaction temperature is below 300°C. However, the high nitrogen and oxygen contents and the boiling characteristics and high viscosity range of the liquid products make it difficult to classify them as petroleum substitutes. They would have to be upgraded by other processes. The original PERC process consisted of a sequence of steps: drying and grinding wood chips to a fine powder, mixing the powder with recycled product oil (10% wood powder to 90% recycle oil), blending the mixture with water containing sodium carbonate, and treating the slurry with synthesis gas at about 27,579 kPa and 370°C. The modified LBL process consists of partially hydrolyzing the wood in dilute sulfuric acid and treating the water slurry containing dissolved sugars and about 20% solids with synthesis gas and sodium carbonate at 27,579 kPa and 370°C on a once-through basis. The resulting oil product yield is about 1 bbl/400 kg of chips and is approximately equivalent to No. 6 grade boiler fuel. It contains about 50% phenolics, 18% high-boiling alcohols, 18% hydrocarbons, and 10% water.

The evaluation of pressurized wood-slurry liquefaction by the LBL process of wood-water slurries was performed mainly in small-scale equipment (Ergun, 1981; Davis, 1983). Oils similar to those produced by the PERC process were obtained, but at lower yields. In the late 1970s and early 1980s, the PERC process was evaluated in a PDU (Thigpen and Berry, 1982). The purpose of this work was to demonstrate the direct, continuous, thermochemical liquefac­tion of biomass. In this process, a synthesis gas mixture is reacted with wood slurried in recycled oil in the presence of 5% sodium carbonate at pressures of 20.7 MPa and temperatures of about 270°C for 1 to 1.5 h. It was suggested by the researchers who developed this process in the laboratory that carbon monoxide reacts with sodium carbonate in the presence of water to form sodium formate, which in turn deoxygenates the biomass feedstock to yield oil.

Study of the mechanism of this complex reduction-liquefaction process led to the suggestion that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the cellulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (Appell et al, 1975). The following reactions were suggested:

Na2C03 + H20 + CO —» 2HC02Na + C02 QH10O5 + HC02Na -» C6H10O4 + NaHCOj (Wood) (Oil)

NaHC03 + CO —» HC02Na + C02 HC02Na + H20 NaHC03 + H2 H2 + C6H10O5 * C6H10O4 + H20.

The approximate stoichiometry of the process developed from the data (Thig­pen and Berry, 1982) corresponded to

C6H9840414 + 3.55CO + 2.14H2—» 0.877C6H762O066 + 3.99CO, + 0.22 CO

+ 4.36H2.

(Wood) (Oil)

In view of the complex nature of the reactants and products, it is likely that a complete understanding of all of the chemical reactions that occur in the PERC process will not be developed unless detailed mechanistic studies are carried out.

In the PDU tests of the PERC process, a crude product oil comparable to No. 6 fuel oil was produced in barrel quantities at yields of about 53 wt % of the feedstock. It had a heating value of up to 34.5 MJ/kg, a specific gravity of 1.1, a viscosity of 0.20 Pa-s at 99°C, and an oxygen content of 12.3%. The distillate from the crude oil had a heating value of 40.4 MJ/kg, a viscosity of 0.01 Pa-s, and an oxygen content of 6.2%. Its characteristics were similar to those of No. 2 fuel oil. These results were obtained from the longest sustained run with Douglas fir; 4988 kg of crude oil was obtained from 9953 kg of feedstock. These data suggest that the PERC process yields what much of the research on the direct liquefaction of biomass has been unable to achieve—a one-step, direct liquefaction process using woody feedstocks that yields a crude

product oil similar to a petroleum fuel oil. Unfortunately, operation of the PDU was terminated before several key questions could be answered. Is sodium carbonate necessary? Is synthesis gas necessary, and if so, how much? If one or both of these reactants is eliminated, what is the effect on the crude product oil’s composition and yield? Eventually, these questions will be resolved, if and when the process is developed further. But it seems evident from the PDU data that the PERC process is capable of overcoming some of the problems encountered in other direct biomass liquefaction processes.

The RENUGAS process was developed by the Institute of Gas Technology (Evans et ah, 1987; Trenka et ah, 1991; Trenka, 1996). After tests in a 9.1-t/ day PDU, a demonstration plant for 91 t/day of wood or 63 t/d of bagasse feedstock was constructed by the Pacific International Center for High Technol­ogy Research in Hawaii at the Hawaiian Commerce and Sugar Company. Bagasse, whole-tree chips, and possibly RDF are being tested in this plant. The gasifier has an inside diameter of about 1.2 m and is fed by a lockhopper and a live-bottom feed hopper. The development work was done in a 0.3-m inside diameter, 9.1-t/day PDU, so the scale-up factor is less than 10. For 92 to 96% carbon conversion, the oxygen requirement ranges from 0.24 to 0.34 kg/kg of wood feed, the dry fuel gas yield ranges from 1 to 1.2 m3/kg of wood feed, and the heating value of the gas is about 11.8 to 13.5 MJ/m3 (n). A typical run with whole-tree chips consisted of a feed rate of 321 kg/h with wood containing 9 wt % moisture, 0.69 kg of steam/kg of wood, 0.26 kg of oxygen as air/kg of wood, and gasification at 910°C and 2189 kPa. The heating value of the raw gas on a dry, nitrogen-free basis was 13.6 MJ/m3 (n) and contained 16 mol % carbon monoxide, 38 mol % carbon dioxide, 17 mol % methane, 1 mol % higher hydrocarbons, and 28 mol % hydrogen. The yield of this gas was 1.04 m3 (n)/kg of wood (wet). This plant is being operated at pressures up to 2027 kPa over a range of steam/oxygen ratios. The objectives are to demonstrate medium-energy gas production for power generation, hot — gas cleanup, and synthesis gas production. A special system for gas cleanup is being tested in both the 9.1-t/day PDU and the 91-t/day plant (Wiant et al, 1993).

This group of biomass components is unique in that it occurs mainly in woody biomass and cellulosic, terrestrial biomass as aromatic polymers containing phenyl propane units in which the benzene rings are substituted by methoxyl and hydroxyl groups. The linkages in the polymers occur directly between the rings, between the propane units, and through ether linkages via the hydroxyl groups. About 25 wt % of dry wood consists of lignins; slightly more of which is usually contained in softwoods than hardwoods. The precursors of the lignins appear to be C9 compounds such as p-hydroxyphenylpyruvic acid, which can be derived through a series of condensation reactions starting with glucose. Amino acids seem to play a role in this biochemical pathway. One pathway proposed for the formation of spruce lignin involves the biochemical conversion of glucose, which is transformed into the lignins by a series of reactions via shikimic acid (3,4,6-trihydroxy-cyclohexene-l-carboxylic acid) and coniferin, a glycoside of coniferyl alcohol (3-(4-hydroxy-3-methoxy- phenyl)-2-propene-1 — ol) (Freudenberg, 1957). Note that because of the chemi­cal complexity of the lignins and the variety of specific lignin structures formed by different biomass, numerous biochemical pathways have been proposed to explain how lignins are formed from sugars. Coniferyl alcohol is a precursor in many of these pathways.

Proteins

Proteins are polymers composed of natural amino acids that are bonded to­gether through peptide linkages. They are formed via condensation of the acids through the amino and carboxyl groups by abstraction of water to form polyamides and are widely distributed in biomass as well as animals. Indeed, although they are present in some systems at concentrations approaching 0 wt %, they are found in all living systems because the enzyme catalysts that promote the various biochemical reactions are proteins. The apparent precursors of the proteins are the amino acids in which an amino group, or an imino group in a few cases, is bonded to the carbon atom adjacent to the carboxyl group. Many amino acids have been isolated from natural sources, but only about 20 of them are used for protein biosynthesis. This does not mean that all 20 of these amino acids appear in each polypeptide molecule. The number of amino acids used and the possible sequences in the polymeric chains correspond to an infinite number of potential polypeptide structures. Natural selection controls these parameters.

Regarding the precursors of the amino acids, several biochemical intermedi­ates and various nitrogen sources are utilized. The amino acids have been divided into five families: glutamate (glutamine, arginine, proline), aspartate (asparagine, methionine, threonine, isoleucine, lysine), aromatic (tryptophan, phenylalanine, tyrosine), serine (glycine, cysteine), and pyruvate (alanine, valine, and leucine) (Stanier et at, 1986). The corresponding precursors for these families are a-ketoglutarate, oxalacetate, phosphoenolpyruvate and erythrose-4-phosphate, 3-phosphoglycerate, and pyruvate. The biosynthesis of histidine uses an isolated pathway and requires a phosphoribosylpyrophos — phate intermediate. All of these precursors are either biochemical intermediates or are derived from them. The nitrogen source is ambient nitrogen for many biomass species and ammonia, urea, or ammonium salts for most cash crops.

MSW is collected for disposal by urban communities in all industrialized countries, so there is no question regarding its physical availability as a waste biomass feedstock in centralized locations in these countries. The question is how best to utilize this material if it is regarded as an “urban ore” rather than an urban waste. The data in Table 5.1 show that in the mid-1990s, a large portion of the MSW generated in the United States was available as feedstock for additional energy recovery processing. As indicated above, landfilled MSW can provide energy as fuel gas for heat, steam, and electric power production over long time periods. Surface-processing of MSW can also provide energy for the same end uses when MSW is used as a fuel or a feedstock.

Energy Potential

At a higher heating value of 12.7 MJ/dry kg of MSW (Table 3.3), the energy potentially available from the MSW generated in the United States in the 1990s is in the range of 2.5 EJ/year. Presuming the total combustibles in the recovered

‘Adapted from Franklin Associates (1994). Sums of individual figures may not equal totals because of rounding. The data in this table are for postconsumer residential and commercial MSW, which makes up the major portion of typical collections. Excludes mining, agricultural, and industrial processing, demolition and construction wastes, municipal biosolids, and junked autos and equipment wastes. Based on material-flows estimating procedure and wet weight as generated. Other wastes are predominantly foodstuffs, leather, rubber, textiles, and wood.

fractions of MSW are utilizable and that the energy recovery systems in opera­tion continue to be used, the data for the United States indicate that about 60 to 65% of the MSW generated could have supplied up to an additional 1.6 EJ/year in the mid-1990s. New energy recovery plants supplied with MSW feedstock could also provide an additional benefit by increasing the life of landfills. Only the unrecyclable inorganic materials in the ash would be land­filled if thermal processing of the MSW is employed. Some of the ash itself could be used as material of construction, such as in roadbeds and other applications.

A. Hardware

The purposes of solid fuel-burning equipment are to proportion and mix the fuel and air, to initiate and maintain ignition, to volatilize the fuel, to position the flames in areas of useful heat release, and to supply fuel and air at the proper rates and pressures to facilitate each of these functions (Reed, 1983). The specific equipment appropriate for most biomass combustion and energy recovery systems depends on the types, amounts, and characteristics of the biomass fuel; the ultimate energy form desired (heat, steam, electric or cogener­ated power); the relationship of the system to other systems in the plant (independent, integrated); whether recycling or co-combustion is practiced; the disposal methods needed for residues; and environmental factors. The design of efficient, large-scale biomass combustion systems requires detailed analysis of many parameters and hardware components. Among them are the numerical values and variability of moisture, volatile matter content, ash content, composition, and energy content of the biomass fuel; biomass han­dling, drying, and grinding equipment; the furnace design and associated heat transfer requirements and materials of construction; combustion and emissions controls; the amounts, composition, fusion temperature, agglomerating charac­teristics, and disposal of ash; and flue gas compositions and treatment that may be needed to meet emissions limitations.

In conventional biomass combustion equipment, combustion of the solid fuel takes place on horizontal or inclined steel grates or in shallow suspension above the grate. The grate is a stationary, vibrating, reciprocating, or traveling platform, and the fuel is supplied in the batch, semicontinuous, or continuous mode. Many furnace designs have been used such as pile-burning systems (Dutch ovens), fixed — and moving-bed furnaces, multiple hearth furnaces, stationary and rotating horizontal and inclined kilns, overfeed, underfeed, and spreader stokers, and pulverized fuel burners. The principal difference between conventional solid fuel-burning equipment and liquid-fuel — or gas-burning equipment is that furnaces for solid fuels must allow for additional fuel resi­dence time for the slower burning chars to combust after all gases and volatile liquids have been driven off. One of the principal methods of expediting this process is by burning smaller fuel particles. Advanced combustion designs such as fluid-bed and cyclonic combustors further improve biomass combustion and are discussed in Part E.

The differences in the furnaces suitable for biomass combustion reside mainly in the design of the combustion chambers, the operating temperatures, and the heat transfer mechanisms. Refractory-lined furnaces operating at about 1000°C were standard until the introduction a few years ago of water-wall incinerators. Ash buildup can occur rapidly in refractory-lined furnaces, and excess air must be introduced to limit the wall temperature. The water-wall incinerator has combustion chamber walls containing banks of tubes through which water is circulated, thereby reducing the amount of cooling air needed. Heat is transferred directly to the tubes to produce steam. There are numerous configurations, but the basic concept has not changed for many years, apart from operating conditions and materials improvements to improve heat transfer and thermal efficiencies. Considerable advancements have been made, how­ever, in ancillary hardware designs to control the combustion process and reduce emissions, to remove ash, and to remove flyash and emissions from the stack gas. Improvements have also been made in the methods used to recover sensible heat from the stack gases and heat from the condensate and boiler blowdown. Other overall efficiency improvements have resulted from advances in predrying hardware for moisture reduction in the incoming bio­mass fuel.

This process incorporates advancements into Lurgi’s dry-ash gasifier that con­vert the system to a slagging gasifier, reduce the steam requirement to about 15% of that required by the dry-ash gasifier, provide a raw gas with higher carbon monoxide and lower methane, carbon dioxide, and moisture, and improve the capability to use caking coals and a significant amount of fines. The process affords increased gas yields by limiting the net hydrocarbon liquids to naphtha and phenols.

Winkler Process

This process converts crushed coal, oxygen, and steam at 820 to 1000°C and near-atmospheric pressure in a fluid-bed gasifier. After passage of the raw gas through a waste heat recovery section, flyash is removed by cyclones, wet scrubbers, and electrostatic precipitators. Further processing, depending on end use, yields a gas suitable as synthesis gas or pipeline gas.

High-Temperature Winkler Process

This process uses a fluid-bed unit that is especially designed for gasification of brown and hard coals, peat, and biomass. In the case of brown coal, predried feed at 12 wt % moisture is fed along with oxygen and steam to the reactor which operates at 750 to 800°C and 2.53 MPa.

TABLE 9.4 Comparison of Operating Temperatures and Pressures and Typical Product Gas Heating Values and Compositions of Selected Coal Gasification Processes Process type Process Conditions HHV, MJ/mJ (n) Raw gas composition, dry mol %" (°С) (MPa) Steam-02 Steam-air H2 CO CH4 co2 n2 Others Fixed bed Lurgi (dry ash)1′ 621-760 2.43-3.14 11.86 40 21 10 28 1 Lurgi (dry ash)“ 621-760 2.43-3.14 7.08 25 16 5 14 39 1 Lurgi (slagging)11 1296-1371 2.53 14.25 28 61 8 2 2 Ruhr 100c 1927 10.13 13.31 32 22 16 28 2 Wellman-Galusha1* 1315 30.40 6.60 15 29 3 3 50 Fluid bed Winkler1 816-982 0.10-0.61 10.80 42 33 3 21 1 0.3 Winkler* 816-982 0.10-0.61 4.63 13 21 1 7 58 0.2 Synthanef 982 3.55-7.09 15.90 28 17 24 29 0.8 1.3 C02 Acceptor® 871 1.01-1.52 17.28 54 17 21 7 0.2 1.4 U-Gas* 1038 2.43 5.89 13 19 5 10 52 0.7 Entrained Bi-Gas’ 927-1482 2.03 14.92 24 44 16 14 0.6 1.4 Koppers-TotzekJ 1816 0.10 11.78 37 56 0 6 1.1 0.3 Texaco* 1093-1371 2.74 11.78 45 45 1 9 Shell Oil Co. 1482 High 11.78 31 67 2

"Lurgi Mineraloltechnik Gmbh; commercial.

bLurgi Mineraloltechnik Gmbh, 10-min residence time; commercial. cRuhrgas AG; demonstrated.

^Wellman Engineering Company; 4-h residence time; commercial.

"Davy Powergas, Inc.; 30-min residence time; commercial; technology owned by Davy International Corp.

Aj. S. Bureau of Mines; demonstrated.

^Consolidated Coal Company; demonstrated. hIGT; commercial.

‘Bituminous Coal Research, Inc.; two-stage system with upper stage at 927°C and lower slagging stage at 1482°C; residence time of the order of seconds; demon­strated.

■’Heinrich Koppers Gmbh; 1-s residence time; commercial.

^Texaco, Inc.; commercial.

‘Applied Technology Corporation; uses molten iron; demonstrated. mM. W. Kellogg Company; uses molten sodium carbonates; demonstrated.

"Atomics International; uses molten sodium carbonates; hydrogen and carbon monoxide not differentiated; demonstrated.

"Raw gas compositions are rounded figures; raw gas from these processes usually contains small amounts of tars, oils, phenols, ammonia, sulfides, light hydrocarbons, and fines from the ash.

I would like to take this opportunity to thank several groups and individuals who helped formulate my thinking on the subject of this book. It may seem unusual to some, but the Institute of Gas Technology, an education and research institute that specializes in the fossil fuel natural gas, is where I first became interested in biomass after spending several years in the petroleum industry. IGT’s policy, which was very close to one of academic freedom, and my association with colleagues in both research and education were invaluable in encouraging and stimulating me to structure and sustain a biomass research program. I also had the opportunity to develop the conference “Energy from Biomass and Wastes” that was started almost simultaneously with the renewal of interest in biomass R&D in North America in the 1970s. The conference was presented annually until I retired from IGT in 1992. Literally hundreds of researchers and project developers presented the results of their efforts at this conference. I learned much about biomass energy research and commer­cialization from these meetings. The exchange of new ideas and information always inspired in me fresh approaches to new projects. My association with the directors and many of the members of the Biomass Energy Research Association (BERA) and direct contact with the Washington scene as a result of this affiliation since the early 1980s had the same stimulatory effect. I thank all of my colleagues, many of whom are still involved in biomass energy development, for sharing their thoughts and expertise with me. Without their contributions to my “data bank” over a period of three decades, it would have been impossible for me to prepare this book. Finally, I want to extend a special thank you to Dr. DonJ. Stevens, a director of BERA and consultant with Cascade Research, Inc. I invited him to review the manuscript of this book. He accepted and performed a superb job of providing me with an objective assessment and numerous suggestions.

CHAPTER

The biomass species selected as energy crops and the climate must be compati­ble to sustain operation of the energy or fuel farm under human-controlled conditions. Wild stands of biomass are also amenable to harvesting as energy crops and are still the primary sources of virgin biomass feedstocks because large-scale energy and fuel farms in which dedicated biomass energy crops are grown have not yet been established. The few attempts that have been made to design, build, and operate such farms have not been too successful. The compatibility of biomass and climate is, nevertheless, essential to ensure that these systems can ultimately be operated at a profit on a commercial scale. The three primary climatic factors that have the most influence on the produc­tivity and yields of an indigenous or transplanted biomass species are insolation, precipitation, and temperature. Natural fluctuations of these factors remove them from human control, but the information compiled over the years in

meteorological records and from agricultural and forestry practice supplies a valuable data base from which biomass energy systems can be conceptualized and developed. Of these three factors, precipitation has the greatest impact because droughts can wreak havoc on biomass growth. Fluctuations in inso­lation and temperature during normal growing seasons do not adversely affect biomass growth as much as insufficient water. Ambient carbon dioxide (C02) concentration and the availability of macronutrients and micronutrients are also important factors in biomass production.