Typical proximate analyses and higher heating values (product water in liquid state, HHV) of representative biomass types and species illustrate the wide range of some parameters such as moisture and ash contents and the relatively limited range of heating values (Table 3.3). The physical moisture contents of biomass are generally determined experimentally by drying a sample at 100 to 105°C at atmospheric pressure or at lower temperature and reduced pressure. In a few cases, some organic compounds may be lost by these procedures because of volatilization and/or steam distillation, but generally the results are suitable for biomass characterization. The moisture contents listed in Table 3.3 range from a low of 2 to 3 wt % for the biomass derivatives char and paper to a high of 98 wt % for primary biosolids (primary sewage sludge). Green wood in the field before drying usually contains about 50 wt % moisture, whereas primary biosolids contain only a few percent suspended and dissolved solids in water. Similarly, the marine biomass giant brown kelp (Macrocystis pyrifera) and most other aquatic biomass contain only a few percent organic matter when first harvested; the main component is intracellular water.

Total organic matter is estimated by difference between 100 and the ash percentage that is experimentally determined by ashing the biomass samples at elevated temperature using standard methods (cf. annual volumes of ASTM Standards, American Society for Testing and Materials; Methods for the Examina­tion of Water and Wastewater, American Public Health Association). The chemi­cal reactions that occur during ashing result in the uptake of oxygen and the formation of metal oxides, so the experimental ash content is not identical to the inorganic mineral matter in the original sample. Ideally, all carbon in the original sample is eliminated on ashing, the metals are not oxidized, and none of the metals is lost. Such is not the case for some ashing procedures, particu­larly when samples contain high alkali metal concentrations. The loss of mate­rial due to the volatility of some alkali metal oxides at the ashing temperature causes errors in the analysis. Adjustments are sometimes made to the experi­mental ash determinations so that they correspond more closely to the inor­ganic matter present in the unashed samples. Nevertheless, subtraction of the experimental ash values in percent dry weight of the biomass from 100 to obtain the percent organic matter is adequate for most purposes.

Detailed chemical analyses of the components in the ash from two woody and one herbaceous biomass samples (Table 3.4) show that many metal oxides

’The ash, organic matter, and heating values were obtained from Boley and Landers (1969), Bowerman (1969),Chow et al. (1995), Hodgman (1949), Jerger et al. (1982), Klass (1980, 1984), Monk et al. (1984), Paisley et al. (1993), Pober and Bauer (1977), Tillman (1978), Wen et al. (1974). The moisture content ranges of biomass in the field were measured, estimated, or obtained from various literature sources.

are present, but that the distribution of the metallic elements is quite different in each sample analyzed. The oxides of calcium and potassium are dominant in hybrid poplar ash; the oxides of calcium and silica are dominant in pine ash; and the oxides of potassium and silica are dominant in switchgrass ash. As will be shown in later chapters, the distribution of the metals in biomass and the compositions of the ash are important in the development of certain types of biomass conversion processes because they can affect process perfor­mance. Also, some biomass species that have an unusually large amount of a specific metal have been harvested and used as a commercial source of that material during times of shortages.

It is evident from the data in Table 3.3 that the organic matter content and the HHV are affected by the ash, which in almost all cases has no energy value. The higher the ash value, the lower the organic matter and the HHV, as expected.

Intuitively, it might also be expected that the composition of biomass would vary over a broad range because there are so many different types and species.

The elemental compositions summarized in Table 3.5 support this hypothesis. In this table, typical ultimate and proximate analyses and the HHVs of land — and water-based biomass (pine wood, Kentucky bluegrass, giant brown kelp) and waste biomass (cattle feedlot manure, municipal solid waste, primary biosolids) are compared with those of cellulose, peat, and bituminous coal. On a dry basis, the ash values for these particular samples range from 0.5 wt % for pine wood to about 39 wt % for giant brown kelp. Also, on a dry basis, the total organic matter and the elemental analyses for carbon and hydrogen do not vary quite as much as the moisture and ash contents. Pure cellulose, a representative primary photosynthetic product, has a carbon content of 44.4%. Most of the renewable carbon sources listed in Table 3.5 have carbon contents near this value. When adjusted for moisture and ash contents, it is seen that with the exception of the biosolids sample, the carbon contents are slightly higher than that of cellulose, but span a relatively narrow range. It is also evident from the data in Table 3.5 that the HHVs per unit mass of carbon are quite close. Even those for reed sedge peat and Illinois bituminous coal are close to those calculated per unit mass of biomass carbon. As will be shown below, a reasonably good correlation exists between the carbon content of biomass and its energy content.

One of the analyses not included in the compositional information presented here on biomass is the percentage of so-called fixed carbon. This subject will be discussed in Chapter 8 under pyrolysis because there is no fixed carbon as such in biomass.

Abundance

Intuitively, high populations of specific animals would be expected to offer the greatest opportunity to serve as sources of waste biomass because waste generation is maximized. Because of the relationship of waste productivity and animal size, this is not always the case as will be shown here. Domestic farm animals and those confined to feedlots are appropriate choices. In addition, commercial poultry production systems, some of which have bird populations over 200,000, would be expected to provide large accumulations of manures in one location. The animals that produce large, localized quantities of excreta are cattle, hogs and pigs, sheep and lambs, and poultry. U. S. populations of these animals in the mid-1990s, the estimated total, annual manure production for each species, and the human population equivalents in terms of solid waste generation are shown in Table 5.2. Several observations can be derived from

TABLE 5.2 Livestock and Poultry Manures Generated in the United States and Their Human Population Equivalent”

Human

population

Manure production equivalent

aU. S. Dept, of Agriculture (1995) for population data. Populations of cattle, hogs and pigs, and sheep and lambs are for 1995; remaining populations are for 1994. With the exception of the commercial broiler population, other populations are assumed to be steady-state values because the variations are relatively small for each of the preceding 10 years. Commercial broiler production was approximately 20% higher in 1995 than in 1990, and 57% higher than in 1985. Daily manure production factors on a dry basis include ash and were calculated from Stanford Research Institute (1976). The factors for converting animal populations to human population equivalents in terms of waste generation are from Wadleigh (1968).

these data assuming that they represent reasonably steady-state conditions. With the exception of the commercial broiler population, the animal popula­tions are assumed to be steady-state values because the variations are relatively small for each of the preceding 10 years. Commercial broiler production was approximately 20% higher in 1995 than in 1990, and 57% higher than in 1985. But it is assumed that the population is relatively constant for purposes of this assessment because the population increases each year. This should tend to eliminate some of the fluctuations in manure production throughout the year because of animal growth and marketing cycles.

Commercial broilers had the highest population, about 7 billion, and they produced the second largest amount of manure; cattle, which had a population of about 100 million, produced the largest amount. No differentiation was made in this assessment between dairy and beef cattle. The daily production rate per head of dry manure solids used was the arithmetic average of dairy cattle and beef cattle, since the dairy-to-beef cattle ratio of the reported manure production rates is about 1.45 (Stanford Research Institute, 1976). Some assess­ments indicate that the ratio of productivities is 3.1 (Jaycor, 1990). The effects of daily manure production per animal are evident as shown in Table 5.2. When published waste production factors are used for conversion of the animal populations to human equivalents in terms of solid waste generation as shown in the table, the total human population equivalent of these animals is estimated at almost 3 billion people. This is approximately 12 times the U. S. population in the mid-1990s, a ratio considerably less than reported in 1968, when it was estimated to be 20 times that of the human population (American Chem­ical Society, 1969). But the latter ratio included liquid wastes as well. The ratio of total animal excreta to total municipal biosolids generation calcu­lated here, both of which exclude liquid wastes, is about 36 on a mass basis (307.1 dry t/8.6 million dry t).

Other advanced combustion systems for solid biomass fuels also offer consider­able advantages over conventional designs and are in commercial use or under development. A few of them are described here.

Combustion of waste biomass is often employed not for energy recovery, but for waste disposal purposes. One of the most difficult of biomass solids to combust is municipal biosolids (sewage). Its high moisture content of 95% or more and its chemical and physical properties require special dewatering techniques and furnace designs when combustion is used as the primary disposal method. Supplemental fuels are usually required, but it is possible to use dewatered biosolids for self-sustained combustion. In one plant, thickened biosolids at a concentration of about 4% solids is dewatered to about 38% solids, and then combusted in a six-hearth incinerator (U. S. Environmental Protection Agency, 1985). The dewatered material contains about 70% volatile solids, but only has a net heating value of 1.7 MJ/kg. Yet stable autogenous combustion is obtained by automatically controlling the injection of primary air into the bottom stage of the furnace to take advantage of the draft effect that changes according to the load to the furnace and the biosolids properties. The temperature in the hottest hearth is held between 700 and 900°C. The control measures used prevent unstable combustion, high air-fuel ratios, and discharge of unburned biosolids; they also minimize clinker and slag formation. Autogeneous combustion was attained with a small amount of heavy oil at a rate of 8 L/t of dewatered biosolids. Oil consumption is commonly 170 L/t of biosolids.

The disposal of waste automobile tires is a major problem. In the United States, it is estimated that more than 200 million tires per year are disposed of in some form or recycled for retreading or reuse. About 75% are disposed of in landfills. Combustion of whole tires and tire chips is already being practiced to provide supplemental fuel for the combustion of high-moisture wood residue fuels. But emissions of metal oxides, volatile organic compounds, and sulfur oxides from the tires have precluded the use of high ratios of tire fuel in conventional combustors. The ability to handle the high steel wire concentrations, which can be as much as 10% of the total weight of the tires, has limited waste tire usage as fuel. A circulating fluidized-bed combustion system has been designed to combust tires with nearly 100% conversion of the carbon, good emissions characteristics, and the capability of separating the wire (Murphy, 1988). Carbon monoxide levels of 25 ppm in the flue gases have been readily maintained with excess air. Sulfur oxide capture with limestone in the fluidized bed and ash recycle can be as high as 80%. The sand is withdrawn from the bottom of the unit and after the temperature is reduced to about 315°C, the material is passed over a rotating drum magnet for wire removal. The dewired sand is screened to remove any oversized particles before return to the combustor.

Fluid-bed combustion has been given a great deal of attention in recent times because of its advantages, particularly in large-scale systems (cf. Murphy, 1991). Typically, combustion takes place in a cylindrical vessel in which air is dispersed through an orifice plate at the bottom of the unit. The air then passes through a bed of an inert refractory, pieces and particles of fuel, and ash and residual inorganic particles remaining from combustion, thereby caus­ing the effective volume of the bed to increase and the bed to become “fluidized.” Small particles burn rapidly above the fluidized bed while larger particles filter into the bed where they are dried and gasified. Most of the residual char is burned in the fluidized bed while volatiles burn both in and above the bed. The fuel is fed to this rapidly mixed bed, where flameless combustion occurs at about 650°C. This temperature can be substantially below flame temperature. Because of the lower heat input requirements, many high-moisture-content fuels can be combusted without supplemental fuel. Materials such as limestone are often added to the bed to minimize pollutants in the flue gases. The constant motion of the fluidized bed ensures good mixing and intimate contact of the air and fuel, improves combustion, reduces emissions, and makes it possible to combust a wide range of fuels having different shapes, sizes, moisture contents, and heating values. Excellent heat transfer rates to boiler tubes or materials immersed in the bed can be obtained. Bubbling and circulating fluidized-bed designs are the principal hardware configurations.

The combination of fluidized-bed technology and cyclonic combustion has led to the development of innovative two-stage systems for disposal of waste biomass with heat recovery (Rehmat and Khinkis, 1991). The first stage is a sloped-grid, agglomerating fluidized-bed reactor that can operate under either substoichiometric or excess air conditions. When municipal biosolids are burned, the noncombustibles are agglomerated to form a vitrified, glassy matrix that is removed from the bottom of the fluidized bed. The inert agglomerate can be safely used in construction applications and is reported to meet leachability standards in landfills. The amount of supplemental fuel required to maintain temperatures of about 815 to 1100°C in the bed depends on the heating value of the fuel. The second stage is a cyclonic combustor where flue gas from the fluidized bed is further combusted. The cyclonic combustor provides sufficient residence time at operating conditions to oxidize all carbon monoxide and organic compounds to C02 and water. The combined system is reported to have a destruction and removal efficiency for organic materials greater than 99.99%. The system is used mainly for waste disposal, but can be operated in the autogenous mode with dry waste biomass feedstocks.

Direct-fired gas turbines are another innovative development in biomass combustion (McCarroll and Partanen, 1995). The compressor section of the gas turbine provides pressurized combustion air to burn biomass in an external, pressurized combustor capable of operating at pressures required by the gas turbines. Hot combustion gases are ducted through a cyclonic separator into the hot section of the gas turbine to drive a generator. Hot exhaust gas from the turbine at about 480°C can be either used directly as a source of thermal energy or fed into a heat recovery steam generator to produce process steam. Full utilization of both types of energy in the cogeneration mode is expected to allow system efficiencies in excess of 70%. This type of direct-fired turbine is believed suitable for small and medium-sized industrial and commercial applications up to 5 MW in capacity. Low-ash, debarked wood particles less than 0.3 cm long and containing less than 15% moisture are the preferred fuel, but other processed biomass can also be used. A similar 3-MW, direct — fired, gas turbine system used dried sawdust fuel containing 12 to 25% moisture as it entered the combustor (Hamrick, 1987). This system has been modified and upgraded, and a 5-MW commercial plant was built in Tennessee to demon­strate the technology (Rizzie, Picker, and Freve, 1996). The power will be sold to the Tenessee Valley Authority. The plant is fueled with fresh sawdust from local sawmills, and will later be used with other biomass fuels. Fine — tuning of this plant is expected to produce a net output of up to 6.6 MW in the open cycle mode at a heat rate of 14.2 MJ/kWh.

Pulsed combustion is another advanced technology under development for biomass (Buchkowski and Kitchen, 1995). A pulse combustor consists of a combustion chamber in the form of a short pipe with an air and fuel admitting valve at one end and a length of reduced-diameter pipe at the opposite end. The valve, which allows flow in only one direction, admits air from a blower to the combustion chamber, where it mixes with the fuel to form an explosive mixture. Ignition is provided by a spark plug and a rapid increase in pressure follows. The gases are driven out through the small-diameter tail pipe. A vacuum follows the explosion and a new charge of fuel and air are drawn into the combustion chamber. The cycle is repeated many times per second. Although fuel gases are suitable fuels, pulverized hog fuel and sawdust with less than 15% moisture may be suitable alone as fuels after the system is operational. A wood fuel feed auger was employed for the initial studies. Pulsed combustion was achieved momentarily, which indicates that a practical design is possible. Pulsed combustion is reported to offer high heat transfer rates, efficient combustion, low nitrogen oxide emissions, and a source of kinetic energy for providing the motive force for a drying system.

An innovative approach to large-scale biomass combustion for power gener­ation is the whole-tree-burning concept in which whole trees, including branches, are supplied directly to the combustion chamber using conveyors and rams (Ostlie and Drennen, 1989). The whole trees are stored in large piles in drying buildings for 30 days before combustion. Condenser waste heat supplies dry, heated air to these buildings. The combustion chamber is a two — stage combustion unit. In the first stage, a water-cooled grate supports the pile of trees. Burning releases gases which combust above the pile at temperatures reported to be as high as 1480°C. Temperatures within the pile are reported to be 100°C. The second stage of combustion occurs below the bed as char falls through openings in the grate. Ash collects at the bottom of the second stage for removal through an ash discharge. Underfire air at approximately 340°C enters the secondary combustion chamber and is used for control. Raising or lowering the flow rate and the temperature of the air raises and lowers the combustion rate of the trees and the release of volatiles. Introduction of secondary air above the pile assures complete burning of the volatiles, while the boiler sections installed above the primary combustion chamber ensure maximum steam production.

Pyrolytic Gasification

The primary products of biomass pyrolysis under conventional pyrolysis condi­tions are gas, oil, char, and water. As the reaction temperature increases, gas yields increase. It is important to note that pyrolysis may involve green or predried biomass, and that product water is formed in both cases. Water is released as the biomass dries in the gasifier and is also a product of the chemical reactions that occur, even with bone-dry biomass. Unless it is rapidly removed from the reactor, this water would be expected to participate in the process along with any added feedwater or steam. As will be shown later, the exothermic heat from the steam gasification of woody biomass under certain conditions appears to be sufficient to eliminate the need for an external heat source or the use of oxygen. Self-sustained steam gasification can effectively be carried out with biomass feedstocks, according to some investigators.

One of the more innovative pyrolytic gasification processes is an indirectly heated, fluid-bed system (cf. Alpert et al, 1972; Bailie, 1981; Paisley, Feldmann, and Appelbaum, 1984). This system uses two fluid-bed reactors containing sand as a heat transfer medium. Combustion of char formed in the pyrolysis reactor takes place with air within the combustion reactor. The heat released supplies the energy for pyrolysis of the combustible fraction in the pyrolysis reactor. Heat transfer is accomplished by flow of hot sand from the combustion reactor at 950°C to the pyrolysis reactor at 800°C and return of the sand to the combustion reactor (Fig. 9.5). This configuration separates the combustion

and pyrolysis reactions and keeps the nitrogen in air separated from the pyrolysis gas. It yields a pyrolysis gas that can be upgraded to a high-energy gas (substitute natural gas, SNG) by shifting, scrubbing, and methanating without regard to nitrogen separation. The pyrolysis gas with hybrid poplar feedstocks typically contains about 38 mol % carbon monoxide, 15 mol % carbon dioxide, 15 mol % methane, 26 mol % hydrogen, and 6 mol % C2’s. This is a medium-energy gas having a higher heating value of about 19.4 MJ/ m3 (n). The projected gas yields are about 670 m3 (n) of pyrolysis gas, or about 200 m3 (n) of methane per dry ton of feed if SNG is produced.

The relationship of gross national product per capita to energy consumption per capita for most countries of the world correlates very well with the status of economic and technological development. The World Bank defines devel­oping countries as low — and middle-income countries for which the annual gross national product is $5999 or less per capita (World Bank, 1989; U. S. Congress, 1991). With the exceptions of Brunei, Bahrain, Japan, Kuwait, Qatar, Saudi Arabia, Singapore, and the United Arab Emirates, it includes all countries in Africa, Asia, Latin America, and the Middle East, and Bulgaria, Greece, Hungary, Papua New Guinea, Poland, Turkey, and the former Yugoslavia. All of the developing countries that have annual gross national products of less than $5999 per capita also consume less than 25 BOE/capita-year (3300 kg of oil equivalent/capita-year). In fact, there is a good correlation between the magnitude of annual energy consumption per capita and the correspond­ing gross national product per capita for both the developing and developed countries (Fig. 1.5).

Annual global energy consumption statistics by region show that although fossil fuels supply the vast majority of energy demand, the developing areas of the world consume more biomass energy than the developed or more industrialized regions (Tables 1.1 and 1.2). More than one-third of the energy consumed in Africa, for example, is supplied by biomass. But examination of the energy consumption and population statistics in modern times of the world’s 10 highest energy-consuming countries reveals some interesting trends that may not generally be intuitively realized. Excluding biomass energy con­sumption, these countries consumed about 65% of the world’s primary energy demand in 1992 and contained about one-half of the world’s population (Table 1.3). The industrialized countries and some of the more populated countries of the world are responsible for most of the world’s primary energy consump­tion (65%) and for most of the fossil fuel consumption. One extreme, however, is represented by the United States, which has only about 5% of the world’s population, and yet consumes about one quarter of the total global primary energy demand. Coal, oil, and natural gas contributed 23, 41, and 25%, respec­tively, to total U. S. energy demand in 1992, about 80% of which was produced within the United States. Oil has been the single largest source of energy for many years. The U. S. per-capita energy consumption in 1992, 56.3 BOE/capita, was second only to that of Canada, 69.8 BOE/capita, in this group of countries.

Energy consumption per capita, kg oil equivalent/cap-year FIGURE 1.5 Gross national product vs energy consumption of selected countries, 1990.

Another extreme is represented by China and India, which rank first and second in population. Their respective per-capita energy consumptions were 4.4 and 1.7 barrels of oil equivalent in 1992, the smallest in this group of countries. Of the three fossil fuels—coal, oil, and natural gas—coal contributed 78 and 60% to energy demand in China and India, while natural gas contributed only 2 and 6%, respectively. This suggests that the indigenous reserves of coal are large and those of natural gas are small in these countries.

Globally, total energy consumption exhibited an almost exponential increase from 1860 to 1990. Total consumption increased from 16 to 403 EJ, or by a

“Adapted from United Nations (1992). The sums of individual figures may not equal the totals because of rounding.

^Europe includes the former U. S.S. R.

‘Solids are hard coal, lignite, peat, and oil shale. Liquids are crude petroleum and natural gas liquids. Gases are natural gas.

^Electricity includes hydro, nuclear, and geothermal sources, but not fossil fuel-based electricity, which is included in fossil fuels.

‘Biomass includes fuelwood, charcoal, bagasse, and animal, crop, pulp, paper, and municipal solid wastes, but does not include derived biofuels.

^Estimated by the author: 2.95 EJ for the U. S.A., 0.5 EJ for Canada, and 0.3 EJ for Mexico. More details are presented in Chapter 2.

factor of about 25 (Klass, 1992) (Fig. 1.6). The world’s population exhibited about a fivefold increase to 5.3 billion people over this same period. From 1860 to the mid-1930s, the world’s population, total fossil fuel consumption,

“Derived from Table 1.1.

kDoes not include derived biofuels such as ethanol or methane.

“Energy consumption data adapted from U. S. Department of Energy (1994). Population data are for mid-1994 (U. S. Bureau of the Census, 1994). Sums of individual figures may not equal totals because of rounding.

EJ/YEAR

500 і——————-

and per-capita fossil fuel consumption gradually increased, but then increased much more rapidly after the beginning of World War II (Figs. 1.7 and 1.8).

Since the 1940s, fossil energy resources have clearly become the world’s largest source of energy. Interestingly, the average overall per-capita fossil fuel consumption by the world’s population started to level off in the range of 60 GJ/capita-year (10 BOE/capita-year) in 1970 (Fig. 1.8). Meanwhile, the contribution of biomass energy, which was over 70% of the world’s total energy demand in 1860, decreased to about 7% of total demand in the early 1990s.

A. Land Areas

The availability of land suitable for production of terrestrial biomass can be estimated by several techniques. For the United States, one method relies on a land capabilities classification scheme in which land is divided into eight classes (Table 4.6) (U. S. Dept, of Agriculture, 1966). Classes I to III are suit­able for cultivation of many kinds of crops; Class IV is suitable only for lim­ited production; and Classes V to VIII are useful only for permanent vegetation such as grasses and trees. The U. S. Department of Agriculture surveyed nonfederal land usage for 1987 in terms of these classifications and arrived at the breakdown shown in Table 4.7 (U. S. Dept, of Agriculture, 1989). Out of about 568 million ha, which corresponds to about 60% of the 50-state area, about 43% of the land (246.4 million ha) was in Classes I to III, 13% (75.6 million ha) was in Class IV, and 43% (246.3 million ha) was in Classes V to VIII. The actual usage of this land at the time of the survey is shown in Table 4.8 (U. S. Dept, of Agriculture, 1989). This table shows that of all the land judged suitable for cultivation in Classes I to III, only about 58% of it was

TABLE 4.6 Land Capability Classification by United States Department of Agriculture*

Class Description

I Few limitations that restrict use.

II Moderate limitations that reduce the choice of plants or require moderate conservation practice.

III Severe limitations that reduce the choice of plants or require special conservation

practices, or both.

IV Very severe limitations that reduce the choice of plants or require very careful

management, or both.

V Not likely to erode, but other limitations, impractical to remove, that limit use largely to pasture, range, woodland, or wildlife habitat.

VI Severe limitations that make soils generally unsuited to cultivation and limit their use largely to pasture or range, woodland, or wildlife habitat.

VII Severe limitations that make soils unsuited to cultivation and that restrict use largely to pasture or range, woodland, or wildlife habitat.

VIII Limitations that preclude use for communical plants and restrict use largely to recreation, wildlife habitat, water supply, or to esthetic purposes.

“U. S. Dept, of Agriculture (1966).

“Adapted from U. S. Dept, of Agriculture (1989). Data are for the 48 contiguous states, Hawaii, and the Caribbean area. bThis area is 72.35% of the land surveyed, or 60.55% of the total 50-state area excluding the outlying areas (938.50 million ha). The federal land, water, and developed land areas are 17.43, 2.14, and 3.34% of the total 50-state area, respectively.

"Adapted from U. S. Dept, of Agriculture (1989). Totals may not be precise summations because of rounding. Cropland is land used for production of crops for harvest alone or in rotation with grasses and legumes. Pastureland is land used for production of adapted, introduced, or native species in a pure stand, grass mixture, or a grass-legume mixture. Rangeland is land on which the vegetation is predominantly grasses, grass-like plants, forbs, or shrubs suitable for grazing or browsing. Forest is land that is at least 10% stocked by trees of any size or formerly having had such tree cover and not currently developed for nonforest use. Other land is land such as farmsteads, strip mines, quarries, and lands that do not fit into the other land use category.

actually used as cropland, the locations of which are shown in Table 4.9 (U. S. Dept, of Agriculture, 1989). Also, the combined areas of pasture, range, and forestlands made up about 66% of the total nonfederal lands. This survey suggests that there is ample opportunity to produce biomass for energy appli­cations on nonfederal land that is not used for foodstuffs production. Large areas of land in Classes V to VIII not suitable for cultivation would appear to be available also for biomass energy applications, and sizable areas in Classes I to IV that are not being used for crop production seem to be available. Land now used for crop production could also be considered for simultaneous or sequential growth of biomass for foodstuffs and energy. Portions of the federally owned lands, which are not included in this survey, might also be dedicated to biomass energy applications. Careful design and management of land-based biomass production areas could very well result in improvement or upgrading of lands to higher land capability classifications.

The mechanisms of water uptake by trees suggest several methods of drying terrestrial biomass. The most obvious method is to expose biomass to circulat­ing, low-humidity air that is heated. Open-air solar drying meets these require­ments and has been used for hundreds of years to season or cure woods and grasses. The final moisture content of the air-dried biomass is usually in the 35-wt % range or less. The advantage of this partial drying method is that it is low in cost. The disadvantages are several. The process is slow and depends on the local climate. Some labor is required to arrange the freshly harvested biomass in suitable piles or windrows to facilitate exposure to sunlight and air circulation. Periodic turning of the windrows may be necessary to allow drying of plant parts in direct contact with the soil and to prevent fungal infection of wet biomass. Natural precipitation may require excessive drying times. Forage crops have traditionally been partially dried in open air to this moisture level so they can be removed from the field and stored without significant deterioration and loss of nutrient value. Solar drying also facilitates densification of hay by baling.

In a field study in Florida of the tall grasses elephantgrass (Pennisetum purpureum) and energycane (Saccharum spontaneum L.), which are good candi­dates as biomass energy crops, the air drying in windrows of mature crops of 2- to 4-cm stem diameters required about 7 to 10 days without rainfall to reach moisture levels of 15 to 20 wt % (Mislevy and Fluck, 1993). The seasoning of freshly harvested mature trees by air drying requires longer time periods to reduce the moisture level to about 25 to 35 wt % because of the larger diameter trunks and pieces. Decay fungi that may be present progress rarely, if ever, at moisture contents below 25 wt %. Green wood chips can be air dried in less time because of their smaller size. In a study of the use of hybrid willow harvested at З-year rotations as fuel for a direct wood-fired, gas turbine power plant, it was projected that air-dried willow bundles would reach 30

to 35 wt % moisture at the same cost as green wood chips at 50 wt % moisture content (Ismail and Quick, 1991). The cost is the same because what is saved in not chipping the wood is spent on bundling and storage for 6 months to air-dry the bundles.

Kiln drying under controlled conditions is commonly employed to improve the stability and physical characteristics of lumber products used as materials of construction or for manufacturing furniture, whereas open-air drying is traditionally employed for the curing or seasoning of tree parts and roundwoods to be used as fuel. Kiln drying promotes the removal of moisture by circulating heated air by natural draft or with fans or blowers through the wood, which is carefully piled in the kiln to promote the drying process. Heat is transferred from hot air heated by steam coils supplied by a boiler, or from hot stack gases heated by the burning of waste biomass or other fuels through manifolds. In the batch-drying of large volumes of wood, the temperature of the air can be gradually increased; the final temperatures and humidities are usually near 90°C and 15%. Kiln drying is rapid compared to the rate of open-air solar drying, but it is too slow for some continuous, thermochemical conversion processes unless the dryers and storage facilities are sized to handle the demand for predried feedstock. The continuous drying of wood chips, wood chunks, and hog fuel with industrial dryers or in drying ducts installed prior to the conversion unit is the approach that is often used when predrying is judged to be sufficiently beneficial. Continuous, direct-heat drying, in which hot air or stack gas contacts the biomass as it is fed to the conversion reactor, and indirect-heat drying, in which heat is transferred by convection and radiation from conducting surfaces to the biomass, can be utilized. Many commercial drying ovens and dryers such as rotary drum dryers, which have been effectively used for many years for drying wood and other biomass, are available. The use of superheated steam for drying rather than burning some of the feedstock as a heat source may allow further improvements in efficiency (cf. Wiltsee, McGowin, and Hughes, 1993). The direct-heat systems are generally lower in cost than the indirect-heat systems if commercial drying units are used. Thermochemical conversion reactors can also be designed so that incoming fresh feed is dried to the desired level by heat transfer from the hot reaction products. The simple addition of enclosed drying tunnels for passage of hot air or stack gases over and through incoming fresh feed can sometimes suffice to reduce moisture to the desired level and preheat the feed without the need to install industrial driers.

Note, however, that stack gases from biomass-fired boilers contain about 15 wt % moisture, and that at temperatures below 250°C, only a small amount of additional moisture can be absorbed before the gas becomes fully saturated. This is evident from the following equation (Routly, 1991):

WG = (2940 M)/T, — T0 where WG = drying gas weight, kg/h M = water evaporated, kg/h T, = temperature of drying gas entering, °С T0 = temperature of drying gas leaving, °С

This equation indicates that large fans and motors are required for circulation of the drying gases when low-temperature gas is used as the drying medium. To obtain sufficient heat for drying purposes, some of the stack gas may have to be extracted upstream of the boiler heat recovery equipment, which can have an adverse effect on steam generation. Stack gas drying should therefore be evaluated for each application to determine whether it is technically and economically feasible. For most thermochemical conversion systems that pro­cess green biomass, a balance is usually struck among the optimum moisture range needed for conversion, the feedstock demand rate, the drying require­ments, the size of the feedstock storage facility, feedstock stability on storage, and the cost of supplying predried feedstock.

The transpirational drying in open air of whole trees felled in the forest has been evaluated, but has not been widely adopted (McMinn, 1986). How­ever, the drying of whole trees has been incorporated as part of the whole — tree-burning concept for power production (Chapters 7 and 14; also see Ostlie and Drennan, 1989). Whole trees including branches are dried in large build­ings equipped with heat exchangers supplied with warm water at temperatures up to 50°C. Additional higher temperature waste heat is available from the power plant for peaking. Fans along the base of the drying buildings draw outside air over the heat exchangers and circulate it through piles of whole trees. The resulting warm, moist air is drawn out of the buildings through vents. For optimal drying conditions, the relative humidity levels are kept below 35%. After approximately 30 days of storage in the drying buildings, the moisture content of the whole trees is reduced to 25 wt % or less. Experi­mental testing of whole tree drying provided several interesting and perhaps unexpected results. The two tree species tested, aspen and eastern cottonwood, dried significantly faster with the leaves intact than without the leaves. It was also found that logs do not appear to dry more quickly than whole trees, and that the branches of the trees tested were drier than the corresponding trunks.

Whenever it is necessary to remove moisture from virgin or waste biomass feedstocks, air drying, mechanical dewatering, and drying with waste heat or stack gases should be evaluated first. The lower costs of these methods com­pared to the costs of thermal drying in which external fuel or a portion of the feedstock supplies heat may justify their use.

Processes categorized as fast pyrolysis systems are continuously operated at temperatures generally in the range of 400 to 650°C and residence times of a few seconds to a fraction of a second. Manipulation of these parameters permits the bulk product yields to be changed from those of conventional pyrolysis systems within a wide range, but the products are still chars, liquids, and gases plus water. Fast pyrolysis is characterized by high heating rates and rapid quenching of the liquid products to terminate additional conversion of the products downstream of the pyrolysis reactor. The selectivity for specific chemi­cals is usually low, as in the case of conventional pyrolysis. Very rapid heating of biomass results in the fragmentation of the polymeric components in biomass to afford 60 to 70 wt % primary vapor products composed of oxygenated monomers and polymer fragments (Diebold et ah, 1987). Rapid, efficient quenching of the product streams and short residence times tend to “freeze the product compositions so that they correspond more closely with the chemicals formed initially on biomass pyrolysis. More details are presented in Section III on fast pyrolysis.

A. Fixed Carbon

The carbonaceous residues from biomass pyrolysis are in the charcoal fraction. These residues are called “fixed carbon” by most energy specialists. The gener­ally accepted definition of fixed carbon was originally promulgated by coal chemists. It is the amount of combustible material remaining in a sample of coal, coke, or bituminous material after removal of moisture, volatile matter, and ash, and is expressed as a percentage of the original material. American Society for Testing and Materials (ASTM) procedures have been developed for determination of each of these parameters: moisture (ASTM D 3173; 104- 110°C for 1 h), volatile matter (ASTM D 3175; 950°C for 7 min), and ash (ASTM D 3174; gradual heating to redness and finishing ignition at 750°C). Fixed carbon is the difference between 100 and the sum of these determinations (ASTM D 3172) and is essentially the elemental carbon in the original coal sample plus the carbonaceous residue formed on heating the coal sample at 950°C for 7 min. As the temperature rises above 300°C, coals emit volatile matter that consists of gases, oils, and tars. The residues contain elemental carbon, some of the higher molecular weight polynuclear aromatic hydrocar­bons formed in the process, and a few other high-molecular-weight compounds that are also formed in the process. Peat, which is derived from biomass, is categorized by some specialists as “young coal” and does contain some elemen­tal carbon. In contrast, all carbon in biomass is fixed carbon in the same sense that organic nitrogen is fixed nitrogen, but elemental carbon is not present in biomass. The photosynthetic fixation of C02 results in the formation of fixed carbon. But since there is no elemental carbon as such in biomass, its fixed carbon content can also be considered to be zero. In other words, the terminol­ogy “fixed carbon” in biomass is a misnomer.

If biomass is subjected to the ASTM D 3172 procedure for determination of fixed carbon, chemical transformation of a portion of the organic carbon in biomass into carbonaceous material occurs as described here. All of the fixed carbon determined by the ASTM procedure is therefore generated by the analytical method. Furthermore, the amount of fixed carbon generated depends on the heating rate used to reach biomass pyrolysis temperatures and the time the sample is subjected to these temperatures. Nevertheless, such analyses are valuable for the development of thermal conversion processes for biomass feedstocks. But application of the ASTM procedures to biomass might more properly be called a method for determination of pyrolytic carbon or coking yields. In the petroleum industry, the Conradson carbon (ASTM D 189, differ­ential heating with a gas burner for total of 30 min to final temperature of cherry-red crucible) and the Ramsbottom carbon (ASTM D 524, 549°C for 20 min) procedures are used to determine the coking tendency on pyrolysis of petroleum products. Use of these procedures with biomass would be ex­pected to give somewhat different results for fixed carbon than ASTM D 3172.

Table 8.6 is a tabulation of the fixed carbon, volatile matter, and ash analyses of selected biomass species, biomass derivatives, and coals as determined by the ASTM D 3172 procedure. The data for the wood species, wood barks, and herbaceous biomass species show that significant quantities of pyrolytic carbon are produced by this method. The pyrolytic chars listed, which already contain substantial amounts of elemental carbon because of the nature of the pyrolysis process, contain more fixed carbon than the coal samples listed in this table. In contrast, MSW and the papers in MSW, which are high in celluloses, contain considerably less fixed carbon suggesting that the lignins in biomass contribute more to production of fixed carbon than the other components. This is expected because of the nature of the chemical structures of the lignins, and the fact that papers are low in lignins. The times and temperatures used for the ASTM D 3172 procedure coupled with the data in Table 8.6 suggest that for maximum yields of noncarbonaceous products to be obtained on biomass pyrolysis, short reaction times should be used at relatively low pyrolysis temperatures. These conditions would be expected to yield smaller amounts of charcoal, tars, and gases, and larger amounts of liquid products. As will be discussed later, opti­mum conditions have been developed for separately maximizing charcoal and liquid product yields in biomass pyrolysis.

Because of the amounts of sample, labor, and time required to perform the ASTM D 3172 procedure, it is recommended that thermogravimetry (TG) and differential thermogravimetry (DTG) be used for moisture and proximate analysis of biomass and the rapid estimation of their thermal conversion charac­teristics. Application of these techniques shows that the proximate analyses of standard coal samples agree closely or match the values obtained with the ASTM procedure (с/. Kumar and Pratt, 1996). Thermogravimetric procedures are used to determine the thermal stabilities and properties of inorganic and organic materials and can be carried out with small samples in laboratory equipment. The results are reasonably accurate and reproducible. TG and DTG employ sensitive thermobalances and automated data processors to measure weight loss and the rate of weight loss of the samples as a function of tempera­ture, respectively. A TG curve (thermogram) records the weight of the sample with time at a preset temperature or a programmed heating rate in an inert or reactive gaseous atmosphere. Some laboratory instrumentation has also been designed to operate at elevated pressures (cf. Johnson, 1979). Differentiated TG data with time or temperature provides the rate of weight loss. A TG curve is used for moisture and proximate analyses, and a DTG curve can be used to

“Adapted from Graboski and Bain (1979) and Jenkins and Ebeling (1985). The data on barks and pyrolytic chars are from Howlett and Gamache (1977). The data on MSW are from Mass and Ghosh (1973). The data on coals are from Bituminous Coal Research (1974). The data on herbaceous biomass are from Jenkins and Ebeling (1985). The data on woods are from Howlett and Gamache (1977) and Jenkins and Ebeling (1985).

examine combustion, pyrolysis, and gasification characteristics. The kinetics of conversion and the conditions for process optimization can also be estimated using TG and DTG. Analysis of selected biomass components by these tech­niques indicates that pyrolysis is initiated at 150-350° C for hemicelluloses, 275-350°C for celluloses, 250-500°C for lignins, 500-620°C for latex, and 550- 900°C for high-molecular-weight resins and oils (cf. Kumar and Pratt, 1996). The combustion of fixed carbon and ash in representative samples of biomass occurs at 900°C according to these studies.

The data presented in Table 8.7 illustrate the pyrolysis characteristics ob­tained by TG and DTG analysis of several biomass species and parts. This type of data and derived data can be used to project the utility of a given biomass feedstock for thermal conversion. For example, the organic material emitted in the temperature range 320 to 500°C was assumed to be potential tar-forming volatiles (Grover, 1989). Those biomass species having lower emissions of volatiles in this temperature range were judged to be more suitable for conver­sion to tar-free gases on pyrolysis.

In the mid — to late 1980s, a 258-GJ/h, fluid-bed wood gasification plant was built in Florida by Alternate Gas, Inc., for Southern Electric International (Miller, 1987; Makansi, 1987; Bulpitt and Rittenhouse, 1989). Each twin gasifier was 2.44 m in diameter and converted wood chips at the rate of 15.4 t/h into 129 GJ/h of low-energy gas. Hardwood, whole-tree chips, and sawmill residues were the feedstock. Before gasification, the feedstock was predried to 25% moisture in a triple-pass dryer equipped with burners that could burn either product gas or natural gas. About 10 to 20% of the wood charged was combusted in the refractory-lined gasifiers with 25% of the stoi­chiometric air required to provide the heat needed for gasification, which takes place at 790 to 815°C at 34.5 kPa gauge or less. The product gas was cleaned in two stages of cyclones to remove particulates and was then used as fuel for clay dryers. The gas had a heating value of 5.9 to 7.1 MJ/m3 (n). The product char after separation from the ash was sold to a charcoal briquette manufacturer. The plant was operated successfully for more than a year and then dismantled and moved to a new location in Georgia by Southern International.

The United Nations estimate of global biomass energy consumption was about 6.7% of the world’s energy consumption in 1990 (Table 1.2). Biomass energy continues to be a major source of energy and fuels in the developing regions of the world—Africa, South America, and Asia. The markets for biomass energy and biofuels as replacements and substitutes for fossil fuels are obviously large, but have only been developed to a limited extent.

There are still major barriers that must be overcome to permit biomass energy to have a truly large role in displacing fossil fuels. Among these are developing large-scale biomass energy plantations that can supply sus­tainable amounts of low-cost feedstocks; developing integrated biomass production-conversion systems that are capable of producing quad blocks of energy at competitive prices; developing nationwide biomass energy distribu­tion systems that simplify consumer access and ease of use; and increasing the availability of capital for financing biomass projects in the private sector. Niche markets for biomass energy will continue to expand, and as fossil fuels either are phased out because of environmental issues or become less available and uneconomical because of depletion, biomass energy is expected to acquire an increasingly larger share of the global energy market.