A short discussion is also in order regarding the biochemical pathways to the polysaccharides (celluloses and hemicelluloses), which are the dominant organic components in most biomass, and the lignins, proteins (polypeptides), and triglycerides (lipids or fats) that are found in biomass. Most biomass on

a dry basis contains about 50 wt % celluloses. The other components are present in lower concentrations. It is evident that since the simple sugars are the initial products of photosynthesis, they are the primary precursors of all the organic components in biomass. More details on the composition of biomass and the chemical structures of the major components are presented later in Section III.

Celluloses and Hemicelluloses

The pathways to the high-molecular-weight polysaccharides involve successive condensations of the monosaccharides, mainly the hexoses to yield celluloses and starches, and mainly the pentoses to yield hemicelluloses. Celluloses are composed of /З-glucosidic units in the polymer chain, and starches are com­posed of a-glucosidic units. Glucose is the dominant immediate precursor of the celluloses. Similarly, the dominant repeating unit in the hemicelluloses is the pentoses, which are intermediates in the photosynthetic pathways to glu­cose of C3 and C4 plants. Since the celluloses always occur in terrestrial biomass together with the hemicelluloses, it is likely that some of the C5 intermediates are shunted from the glucose pathway to form the hemicelluloses.

There are basically two types of municipal waste that offer opportunities for combined waste disposal and energy recovery—municipal solid waste (MSW, urban refuse, garbage) and biosolids (sewage, sludge). Each has its own distinc­tive set of characteristics as a biomass energy resource.

A. Municipal Solid Waste

Abundance

As the populations of urban areas grow, the production of MSW increases, sometimes in a disproportionate way. To illustrate, the generation of MSW in the United States increased from about 80 million tonnes in 1960 to 180 million tonnes in 1990 and shows no sign of reaching a plateau. During this same period, the corresponding per-capita generation of MSW in 10-year increments was 1.23 kg/person-day in 1960, 1.49 in 1970, 1.65 in 1980, and 1.97 in 1990. The associated difficulties of MSW disposal have become serious problems that do not bode well for future generations of city dwellers and areas that have high population densities. Governments often mandate the use of more environmentally acceptable methods of MSW disposal while limiting and sometimes phasing out some of the more traditional disposal methods. The collection and disposal costs increase and proper disposal becomes more difficult to achieve with the passage of time. The average “tipping fees” of MSW in the United States, for example, increased from about $11 per tonne in 1982 to about $32 per tonne in 1992. In some highly populated areas, the tipping fee is over $90 per tonne. At the same time, the loss of natural resources in the MSW occurs if no effort is made to recover them. The opportunities for combined waste disposal and energy recovery are evident.

Table 5.1 is a detailed summary of relevant data on MSW generation, disposal, and recovery in the United States from 1960 to 1993. Several conclu­sions can be reached from examination of this data. In the 1960s and 1970s, combined disposal-energy recovery systems did not exist to any significant extent even though 20 to 30% of the MSW generated was disposed of by burning. No effort was made to recover the heat evolved on combustion of the MSW. Since then, energy recovery systems have been incorporated into some of the disposal processes so that by the mid-1990s, about 15% of the MSW generated and disposed of by combustion includes energy recovery operations. Throughout this period, the bulk of MSW continued to be disposed of by landfilling. This process will be discussed in some detail in later chapters, but suffice it to say at this point that a medium-energy fuel gas containing about 50 mol % methane is emitted by MSW landfills. The recovery of this gas over long periods of time from many landfills is a well-established commer­cial technology.

It is apparent from the compositional data on the raw MSW in Table 5.1 that the combustible materials make up the bulk of the MSW on a weight percentage basis, about 85 wt % in the 1990s. The amount of the individual MSW components recovered since 1960, presumably for sale of marketable components, has increased to about one-fifth of the total amount generated. The largest components by weight in the recovered material include paper and paperboard and noncombustible metals and glass. Much of this material is recycled.

The moisture content of green biomass can be quite high and can adversely affect the combustion process. If the moisture content is excessive, the combus­tion process may not be self-sustaining and supplemental fuel must be used, which could defeat the objective of producing energy by biomass combustion for captive use or market. High moisture can also cause incomplete combustion, low overall thermal efficiencies, excessive emissions, and the formation of products such as tars that interfere with operation of the system. Predrying of the fuel or blending it with dry fuel to reduce the equivalent moisture content before combustion may be necessary in these cases. Woody biomass fuels containing 10 to 20 wt % moisture are generally preferred for conventional biomass combustion systems. This moisture content range permits a close approach to complete combustion without incurring the costs of further bio­mass drying and allows temperatures in the combustion chamber to reach 750 to 1000°C. As already mentioned, lower moisture contents in biomass fuels can facilitate attainmant of even higher combustion temperatures.

Another factor in biomass combustion is fuel particle size and particle size distribution. The furnace design often determines the optimum ranges of these parameters. But in general, the smaller the fuel particles, the more rapid and complete the combustion process. The larger particles require longer residence times in the combustion chamber at a given temperature. In commercial sys­tems, the capital and operating costs of fuel particle size reduction and predry-

ing are weighed against their beneficial effects on combustion and furnace design and costs.

The processes listed in Table 9.4 that are reported to be used commercially to supply synthesis gas for methanol production are the Lurgi process, the

Winkler process, the Koppers-Totzek process, and the Texaco process. Down­stream adjustment and treatment of the raw product gases is required when these processes are used to supply feedstock or cofeedstock to a typical low — pressure methanol process operating at 220 to 270°C and 5.066 to 10.132 MPa (50 to 100 atm). A few of the operating details of these and other commercial coal gasification processes are presented here.

Dry Ash Lurgi Process

This process is a fixed-bed process that gasifies crushed, dried coal at 620 to 760°C, 2.43 to 3.14 MPa, and residence times of about 1 h. The raw product gas exits the gasifier at 370 to 590°C and contains tar, oil, naphtha, phenols, ammonia, sulfides, and fines. Quenching with oil removes tar and oil. Catalytic shifting and scrubbing of the quenched product gas provides a gas that can be methanated to produce substitute natural gas, or the equivalent of pipeline gas. The process is limited to noncaking coals.

The need for energy and fuels is one of the common threads throughout history and is related to almost everything that man does or wishes to do. Energy, in its many useful forms, is a basic element that influences and limits our standard of living and technological progress. It is clearly an essential support system for all of us. In the twentieth century, the subject did not receive much attention until well into the middle of the century, that is, the fossil fuel era, and then usually only in crisis situations of one kind or another. Until we were confronted with energy and fuel shortages that affected our daily lives, most of us assumed that the petroleum, natural gas, and electric power industries would exist forever. A bountiful supply of energy in whatever forms needed was taken for granted.

An energy corollary to the economic law of supply and demand gradually evolved. In the early 1970s, the law’s first derivative might legitimately have been called the law of energy availability and cost. The oil marketing policies of the Organization of Petroleum Exporting Countries initiated the so-called First Oil Shock in 1973-1974 and changed, probably forever, the international oil markets and the energy policies of most industrialized nations. Oil prices increased dramatically, seemingly overnight. Markets were disrupted and short­ages developed. Crash programs to develop alternatives to petroleum-based fuels began in earnest in many parts of the world. Many of these programs continue today.

Intensive research programs were started to develop renewable energy re­sources such as active and passive solar energy, photovoltaic, wind, and ocean power systems, and biomass—the only indigenous renewable energy resource capable of displacing large amounts of solid, liquid, and gaseous fossil fuels. As a widely dispersed, naturally occurring carbon resource, biomass was a logical choice as a raw material for the production of a broad range of fossil

fuel substitutes. Environmental issues such as air quality and global climate change that many believe are related to fossil fuel consumption also began to come to the fore. The world appeared ready to resurrect biomass as a major indigenous energy resource for industrialized nations, as it had been up to the end of the nineteenth century. It now appears that biomass energy will displace increasingly larger amounts of fossil fuels as time passes.

This book addresses biomass energy technologies and the development of virgin and waste biomass as renewable, indigenous, energy resources for the production of heat, steam, and electric power, as well as solid, liquid, and gaseous fuels that are suitable as substitutes for fossil fuels and their refined products. Biomass is defined as nonfossil, energy-containing forms of carbon and includes all land — and water-based vegetation and such materials as munici­pal solid wastes, forestry and agricultural residues, municipal biosolids, and some industrial wastes. In other words, biomass is all nonfossil organic materi­als that have an intrinsic chemical energy content. The history, status, and future expectations of biomass research, development, and deployment efforts are examined from the standpoint of the role of biomass in our global and national energy economy, the impact of biomass energy use on the environ­ment, its potential to replace fossil fuels, and the commercial systems already in place. The development of advanced technology and improved biomass growth and conversion processes and environmental issues are also discussed. One chapter is also devoted to organic commodity chemicals from biomass.

Because of the special organization of most chapters, this book should serve as an introduction to the subject for the student and professional who wish to become knowledgeable about the production and consumption of biomass energy and its potential long-range impact. This book is also useful for energy professionals interested in some of the technical details of and references for specific biomass energy applications. One special feature of the book that will become apparent to the reader is that it is multidisciplinary in content and treatment of the subject matter, because many scientific and engineering disci­plines are directly or indirectly involved in the development of biomass energy. For example, the biological gasification of biomass is described in terms of its microbiology and biochemistry, but the practical use of this information for the design and operation of combined waste disposal—methane production processes for feedstocks such as municipal solid waste is also discussed. An­other example of the multidisciplinary nature of the book is the treatment given to biomass production. The study and selection of special strains of hybrid trees for use as biomass resources, as well as the advanced agricultural practices used for growth and harvesting in short-rotation biomass plantations, are discussed. An important feature of this book is the effort to discuss barriers that hinder biomass energy utilization and what must be done to overcome them. An example of one of the barriers is net energy production. Given that the prime objective of a biomass energy system is to replace fossil fuels in a specific application, the system cannot effectively attain this objective if the net energy output available for market is less than the total of nonrenewable energy inputs required to operate the system.

Throughout this book, the International System of Units, or le Systeme International d’Unites (SI), is used. SI is a modern metrication system of mea­surement consisting of coherent base and derived units and is used by many scientists, engineers, and energy specialists. Most major technical associations and publishers require that SI be used in their publications. Because of the United States’ position as the world’s largest energy-consuming country, com­monly used U. S. energy units, several of which are somewhat unusual, such as quads of energy (1 X 1015 Btu/quad) and barrels of oil equivalent (BOE), and their conversion factors are presented in Appendix A along with the definitions and conversion factors for SI units. This makes it possible for the reader who is familiar only with U. S. units to readily convert them to SI units and to convert the SI units used in the text to common U. S. units. In the text, common U. S. units are sometimes cited in parentheses after the SI units for clarification.

I. INTRODUCTION

The manufacture of synfuels or energy products from virgin biomass requires that suitable quantities of biomass chosen for use as energy crops be grown, harvested, and transported to the end user or conversion plant. For continuous, integrated biomass production and conversion, provision must be made to supply sufficient feedstock to sustain conversion plant operations. Since at least 250,000 botanical species, of which only about 300 are cash crops, are known in the world, it would seem that biomass selection for energy could be achieved rather easily. This does not necessarily follow simply from the multiplicity of biomass species that can be considered for energy usage. Com­pared to the total known botanical species, a relatively small number are suitable for the manufacture of synfuels and energy products. The selection is not easily accomplished in some cases because of the discontinuous nature of the growing season and the compositional changes that sometimes occur on biomass storage. Many parameters must be studied in great detail to choose the proper biomass species or combination of species for operation of the system. They concern such matters as growth area availability; soil type, quality, and topography; propagation and planting procedures; growth cycles; fertilizer, herbicide, pesticide, and other chemical needs; disease resistance of monocul­tures; insolation, temperature, precipitation and irrigation needs; preharvest management, crop management, and harvesting methods; storage stability of the harvest; solar drying in the field versus in-plant drying in connection with conversion requirements; growth area competition for food, feed, fiber, and other end uses; the possibilities and potential benefits of simultaneous or sequential growth of two or more biomass species for synfuels and foodstuffs; multiple end uses; and transport to the conversion plant gate or end-use site.

As mentioned in earlier chapters, biomass chosen for energy applications, in the ideal case, should be high-yield, low-cash-value species that have short growth cycles and that grow well in the area in which the biomass energy system is located. Fertilization requirements should be low and possibly nil if the species selected fix ambient nitrogen, thereby minimizing the amount of external chemical nutrients that have to be supplied to the growth areas. In areas having low annual rainfall, the species grown should have low con­sumptive water usage and be able to utilize available precipitation at high efficiencies. For terrestrial energy crops, the requirements should be such that they can grow well on low-grade soils so that the best classes of agricultural or forestry land are not needed. After harvesting, growth should commence again without the need for replanting by vegetative or coppice growth. Surpris­ingly, several biomass species meet many of these idealized characteristics and appear to be quite suitable for energy applications. This chapter addresses the important factors that affect biomass production for energy applications.

Waste biomass contributes a substantial amount of energy to primary demand in the United States (see Table 2.9). The energy consumption pattern is similar in many other industrialized countries. The energy potentials and availabilities discussed here for the United States are summarized in Table 5.6. It is evident that additional contributions to primary energy demand can be realized. At least 4 EJ/year of incremental energy usage appears to be feasible based on the energy potential and available energy estimates. This is equivalent to displacing about 1.9 million BOE/day in oil consumption. At a market price

TABLE 5.6 Energy Potentials and Availabilities from Waste Biomass in United States

“The energy potentials of agricultural crops have been adjusted to take adverse soil impacts into account (Table 5.5).

of $20/bbl, this corresponds to about $13.61 billion/year in avoided cost for imported oil.

The new, incremental energy contributions that can be obtained from waste biomass will depend on future government policies, on the rates of fossil fuel depletion, and on extrinsic and intrinsic economic factors, as well as the availability of specific residues in areas where they can be collected and utilized. Environmental regulations will affect how the producers dispose of waste biomass and whether energy applications can be justified. Extrinsic economic factors include the costs of competitive energy resources, the costs of existing disposal methods, the costs of any mandated disposal methods, and in some cases, the markets for recyclables. The intrinsic economic factors include the costs of collection and transport of the waste biomass to end-use site or market, and conversion costs to energy or fuel. All of these factors should be examined in some detail to evaluate the development of incremental energy contributions from waste biomass.

CHAPTER

Many dehydration, cracking, isomerization, dehydrogenation, aromatization, coking, and condensation reactions and rearrangements occur during pyrolysis. The products are water, carbon oxides, other gases, charcoal, organic com­pounds (which have lower average molecular weights than their immediate precursors), tars, and polymers. When cellulose is slowly heated at about 250 to 270°C, a large quantity of gas is produced consisting chiefly of carbon dioxide and carbon monoxide. Table 8.1 illustrates how the carbon oxides, hydrocarbons, and hydrogen in the product gas vary with increasing tempera­ture as the slow, dry distillation of wood progresses (Nikitin et aL, 1962). Initially, small amounts of hydrogen and hydrocarbon gases and larger amounts of carbon oxides are emitted. The hydrocarbons in the product gas then increase with further temperature increases until hydrogen is the main product. The carbon oxides and most other products owe their formation to secondary and further reactions.

Pyrolysis of cellulose yields the best-known of the 1,6-anhydrohexoses, /J-glucosan or levoglucosan (1,6-anhydro-jS-D-glucopyranose), in reasonably good yields (Shafizadeh, 1982) (Fig. 8.1). A novel technique based on flash devolatilization of biomass and direct molecular-beam, mass-spectrometric analysis has shown that levoglucosan is a primary product of the pyrolysis of pure cellulose (Evans and Milne, 1987a, 1987b, 1988). However, the yield of levoglucosan on pyrolysis of most biomass is low even though the cellulose

TABLE 8.1 Composition of Gases from the Slow, Dry Distillation of Wood”

Process (°С)

Elimination of water 155-200

Evolution of carbon oxides 200-280

Start of hydrocarbon evolution 280-380

Evolution of hydrocarbons 380-500

Dissociation 500-700

Evolution of hydrogen 700-900 “Nikitin et al. (1962). “HCs” are hydrocarbons.

content is about 50 wt %. Also, when pure cellulose is treated with only a small amount of alkali, levoglucosan formation is inhibited and a different product slate composed of furan derivatives is produced.

Levoglucosan is also obtained directly on pyrolysis of glucose and starch. The compound has the same empirical formula as the monomeric building block of cellulosic polymers, (C6HI0O5). Some investigators suggest that these observations support a mechanism wherein the initial pyrolysis reaction yields glucose as an intermediate. This is equivalent to the sequential hydrolysis of cellulose by addition of water to form glucose, and elimination of water by dehydration of glucose to form the anhydride. It seems more probable that if levoglucosan is the initial intermediate, a thermally induced, depolymerization — internal displacement reaction occurs to form the pyranose directly by a con­certed mechanism.

In early work on the mechanisms and kinetics of biomass pyrolysis, measure­ment of the weight change as a function of time over a 1000-h period during the pyrolysis of pure cellulose at temperatures up to 260°C in a vacuum led to a multistep mechanism consistent with the experimental data (Broido, 1976). A two-path mechanism was proposed in which one involved depolymerization and led to completely volatile products, and the other involved a sequence of steps leading to char formation. Most investigators now generally recognize at least two pathways for cellulose pyrolysis (Fig. 8.2). One involves dehydra­tion and charring reactions via anhydrocellulose intermediates to form chars, tars, carbon oxides, and water, and one involves depolymerization and volatil­ization via levoglucosan intermediate to form chars and combustible volatiles (с/. Zaror and Pyle, 1982; Antal and Varhegyi, 1995). The first pathway would be expected to occur at lower temperatures where dehydration reactions are dominant. The second pathway results in the formation of oligomeric species as well as their degradation products, which immediately enter the vapor phase (Antal et ai, 1996). If permitted to quickly escape the reactor, the vapors form condensed oils and tars. If held in contact with the solid biomass undergoing devolatilization within the reactor, the vapors degrade further to form chars, various gases, and water. The two competitive pathways help to explain the effects of pyrolysis conditions on product yields and distributions. Note that although these pathways may be dominant, there are undoubtedly many other pathways that are operative with actual biomass species. Thermal treatment converts hemicelluloses to furanoses and furans, the lignins to mononuclear and condensed aromatic and phenolic compounds, and the proteins to a wide range of nitrogen-containing aliphatic and heterocyclic compounds.

In the 1970s and early 1980s, about 40 companies worldwide offered to build biomass gasification plants for different applications. Since then, many of the smaller companies and some of the larger ones have gone out of business, discontinued biomass gasification projects, or emphasized established biomass combustion technologies. The problems encountered in first-of-a-kind biomass gasification plants and the low prices of petroleum and natural gas all had an adverse impact on the marketability of biomass gasification technologies. Sev­eral of the plants built in North America in the 1970s and 1980s have been

shut down, dismantled, or placed on standby. A survey of commercial thermal biomass gasification showed that few gasifiers have been installed in the United States (Miles and Miles, 1989). Most of the units in use are retrofitted to small boilers, dryers, and kilns. The majority of the existing units operate at rates of 0.14 to 1.0 t/h of wood wastes on updraft moving grates. In the United States, many purveyors of biomass gasification technologies have gone out of business or are focusing their marketing activities in other countries or on other conversion technologies, particularly combustion for power generation, in states where combined federal and state incentives make the economic factors attractive. Some existing gasification installations have also been shut down and placed in a standby mode until natural gas prices make biomass gasification competitive again.

Examination of state-of-the-art thermal biomass gasification technology shows that moving-bed gasifiers have been studied and extensively tested

(Babu and Whaley, 1992). Nine atmospheric-pressure updraft gasifiers were commercialized from 1982 to 1986 in Europe and have been successfully operated with wood and peat. Six plants were placed in operation for close — coupled district heating purposes in Finland, while three plants were built in Sweden for district heating and drying wood chips (Kurkela, 1991). In general, the moving-bed systems require close control of feedstock size and moisture content and appropriate means to handle the high tar content of the raw product gases.

The Winkler fluid-bed coal gasifier was successfully scaled up to gasify 25 dry t/h of peat in 1988 by Kemira Oy in Finland (Babu and Whaley, 1992). The product gas was used for the manufacture of ammonia. Major mechanical and process modifications included improvements to the peat lockhopper feeding system, and the control of naphthalene formation by using higher gasifier temperatures and the addition of a benzene scrubber for naphthalene removal. The application of fluid-bed gasifiers to wood and other types of biomass has been commercialized in North America by Omnifuel in Canada and by Southern Electric International, Inc., in Florida, both of which are described later, and Energy Products of Idaho, Inc. The largest and most successful fluid-bed biomass gasification plants to date have been attributed to the Ahlstrom and Gotavarken circulating, fluid-bed gasifiers employed in close-coupled operation with lime kilns in Sweden, Finland, and Portugal (Babu and Whaley, 1992). The gasifiers are about 2 m in diameter and range in height from 15 to 22 m. They are operated at near atmospheric pressure at about 700°C with circulating limestone and are capable of handling mixtures of sawdust, screening residues, and bark. A large-scale circulating fluid-bed gasifier was built in 1992 by Studsvik AB for gasifying RDF (refuse-derived fuel) (Rensfelt, 1991).

These biomass gasifiers are representative low-pressure technologies, which when combined with current state-of-the-art gas-cleanup systems render them­selves suitable for close-coupled operation with lime kilns, furnaces, boilers, and probably advanced, combined-cycle power systems. However, from the standpoint of producing methanol, gasification under elevated pressure and temperature is preferred because the equipment size is reduced for the same throughput, the cost of recompression before methanol synthesis should be less, and the noncondensable hydrocarbons and tars are only present in low concentrations in the spent water. The opposing effects of temperature and pressure on CrC4 light hydrocarbon yields can be optimized to afford low yields of these products by careful selection of operating temperature and pressure.

At least five industrial-scale biomass gasifiers were available commercially from U. S. manufacturers in the 1990s. A two-stage stirred-bed gasifier is avail­able from Producers Rice Mill Energy. The company built three gasifiers of 11 to 18 t/day capacity in Malaysia for rice hull feedstocks. Sur-Lite Corporation built small-scale, fluid-bed gasifiers of up to 10 GJ/h capacity for cotton-gin trash in California and Arizona, for rice husks in Indonesia, and for wood and coal in White Horse, Canada. Morbark Industries, Inc. supplies two-stage, starved-air gasification-combustion systems. The units in operation include a 4-GJ/h system for a nursing home in Michigan and a 1-GJ/h system for heating private facilities. Energy Products of Idaho supplies fluid-bed gasifiers to pro­duce low-energy gas. The company has constructed a 57-GJ/h plant in Califor­nia to fuel a boiler, a 99-GJ/h plant in Missouri to fuel a dryer, and an 85-GJ/ h plant in Oregon to generate 5 MW of power from steam. Southern Electric International, a subsidiary of The Southern Company, coordinated the design and construction of a 264-GJ/h, fluid-bed, wood gasification plant in Florida, which has since been dismantled and moved to Georgia. It is described later.

A few representative biomass gasification processes that have been commer­cialized or that are near commercialization are described here to illustrate some of the details of gasifier designs and the operating results. The biomass pyrolysis plants described in Chapter 8 are not discussed here because the major products are liquids and charcoals, and the by-product gases are used for plant fuel.

Pyrolysis and Partial Oxidation with Air of MSW in a Rotating Kiln

Monsanto Enviro-Chem Systems, Inc., developed an MSW pyrolysis process called the Landgard process through the commercial stage (U. S. Environmental Protection Agency, 1975; Klass, 1982). A full-scale, 1050-t/day plant was built in Maryland and placed in operation in the mid-1970s. The plant was designed to operate for 10 h/day and to accept residential and commercial solid waste typical of U. S. cities. MSW disposal was the primary objective of the plant, not energy recovery. Large household appliances, occasional tires, and similar materials were acceptable feeds; automobiles and industrial wastes were ex­cluded. The process included several operations: shredding of the MSW from storage in 900-HP hammer mills to provide particles small enough (4-cm diameter) to fall through the grates, storage of the shredded MSW which had a heating value of 10.7 MJ/kg, feeding of the shredded MSW to the pyrolysis reactor by twin hydraulic rams, pyrolysis, gas processing, and gas utilization in two waste heat boilers which generated 90,700 kg/h of steam, and processing of the ungasified residue to remove ferrous metals. Pyrolysis took place in a refractory-lined, horizontal, rotary kiln, which was 5.8 m in diameter and 30.5 m long. The kiln was rotated at 2 r/min. The heat required for pyrolysis was provided by burning the MSW with 40% of the theoretical air needed for complete combustion, and supplemental fuel (No. 2 fuel oil) was supplied at a rate of 24.4 L/t of waste. The fuel oil burner was located at the discharge end of the kiln. Pyrolysis gases moved countercurrent to the waste and exited at the feed end of the kiln. The gas temperature was controlled to 650°C, and the residue was kept below 1100°C to prevent slagging. The product gas on a dry basis had a heating value of 4.7 MJ/m3 (n) and consisted of about 6.6 mol % hydrogen, 6.6 mol % carbon monoxide, 11.4 mol % carbon dioxide, 2.8 mol % methane, 1.7 mol % ethylene, 1.6 mol % oxygen, and 69.3 mol % nitrogen. The plant was shut down in January 1981 and was scheduled to be replaced by a direct combustion system. Cost and reliability were cited as the reasons for the change.

A. Biospheric Carbon Fluxes

Most global studies of the transport and distribution of the earth’s carbon eventually lead many analysts to conclude that the continuous exchange of carbon with the atmosphere and the assumptions and extrapolations that must be employed make it next to impossible to eliminate large errors in the results and uncertainty in the conclusions. Only a very small fraction of the immense mass of carbon at or near the earth’s surface is in relatively rapid circulation in the earth’s biosphere, which includes the upper portions of the earth’s crust, the hydrosphere, and biomass. There is a continuous flow of carbon between the various sources and sinks. The atmosphere is the conduit for most of this flux, which occurs primarily as C02.

Some of the difficulties encountered in analyzing this flux are illustrated by estimating the C02 exchanges with the atmosphere (Table 2.1). Despite the possibilities for errors in this tabulation, especially regarding absolute values, several important trends and observations are apparent and should be valid for many years. The first observation is that fossil fuel combustion and industrial operations such as cement manufacture emit much smaller amounts of C02 to the atmosphere than biomass respiration and decay, and the physical exchanges between the oceans and the atmosphere. The total amount of C02 emissions from coal, oil, and natural gas combustion is also less than 3% of that emitted by all sources. This is perhaps unexpected because most of the climate change literature indicates that the largest source of C02 emissions is fossil fuel combustion. Note that human and animal respiration are projected to emit more than five times the C02 emissions of all industry exclusive of energy-related emissions. Note also that biomass burning appears to emit almost as much C02 as oil and natural gas consumption together.

One of the C02 sources not listed in Table 2.1 that can result in significant net C02 fluxes to the atmosphere is land cover changes such as those that result from urbanization, highway construction, and the clear-cutting of forestland for agricultural purposes. It has been estimated that the net flux of C02 to the atmosphere in 1980, for example, was 5.13 Gt, or 1.40 Gt of carbon, because of land cover changes (Houghton and Hackler, 1995). Land cover changes are usually permanent, so the loss in atmospheric carbon-fixing capacity and annual biomass growth are essentially permanent also. It has been estimated from the world’s biomass production data that losses of only 1% of standing forest biomass and annual forest biomass productivity correspond to the ulti­mate return of approximately 27 Gt of C02 to the atmosphere, and an annual loss of about 1.22 Gt in atmospheric C02 removal capacity (cf. Klass, 1993).

Overall, the importance of the two primary sinks for atmospheric C02— terrestrial biota and the oceans—is obvious. No other large sinks have been identified. It is evident that only small changes in the estimated C02 uptake and release rates of these sinks determine whether there is a net positive or negative exchange of C02 with the atmosphere. A small change in either or both carbon fixation in biomass by photosynthesis or biomass respiration estimates tends to cause a large percentage change in the arithmetic difference

“The fossil fuel, human, and animal emissions were estimated by the author (Appendix C). Most of the other exchanges are derived from exchanges that have been reported in the liter­ature (с/. Boden, Marland, and Andres, 1995) or they are based on assumptions that have generally been used by climatologists. It was assumed that 50% of the terrestrial biomass car­bon fixed by photosynthesis is respired and that an equal amount is emitted by the soil. The total uptake and emission of carbon dioxide by the oceans were assumed to be 104 and 100 Gt C/year (Houghton and Woodwell, 1989), and biomass respiration was assumed to emit 50% of the carbon fixed by photosynthesis. The carbon dioxide emissions from cement pro­duction and other industrial processes are process emissions that exclude energy-related emis­sions; they are included in the fossil fuel consumption figures.

between them. And the impact of the assumptions is very large. The assumption that live biomass respires about 50% per year of the total carbon that is photochemically fixed results in a substantial calculated addition of CO2 to the atmosphere, far more than that from fossil fuel combustion. The other assumption incorporated in most biospheric carbon budgets concerns the annual emission of CO2 from soils by microbial action and the oxidation of dead biomass, namely that the emission of C02 occurs at an annual rate approximately equal to 50% of the gross annual photosynthetic carbon uptake. This assumption has little experimental support. The end result of the use of these assumptions with respect to terrestrial biomass, the soils, and the oceans is that they are almost neutral factors in the scenarios generally published on carbon exchanges with the atmosphere and the buildup of atmospheric C02; that is, about the same amount of C02 is emitted as is taken up each year, as shown in the tabulation. This conclusion can be subject to major error when attempting to quantify carbon exchanges with the atmosphere. The largest reservoir of biomass carbon resides in live forest biomass, as will be shown later, and unless this biomass is removed or killed, it fixes atmospheric C02 with the passage of time during most of its life cycle. Tо sustain the environmen­tal benefits of biomass growth as a sink for the removal of C02 from the atmosphere, it is evident that biomass growth should be sustained and ex­panded. The large-scale use of virgin biomass for energy will not adversely affect these benefits if it is replaced at the same or a greater rate than the rate of consumption.