The gradual change in the energy consumption pattern of the United States from 1860 to 1990 is illustrated in Fig. 1.1. In the mid-1800s, biomass, princi-

FIGURE 1.1 Historical energy consumption pattern for United States, 1860-1990.

pally woody biomass, supplied over 90% of U. S. energy and fuel needs, after which biomass consumption began to decrease as fossil fuels became the preferred energy resources. For many years, a safe illuminant had been sought as a less expensive substitute for whale oils. By the mid-1800s, distillation of coal oils yielded naphthas, coal oil kerosines, lubricants, and waxes, while liquid fuels were manufactured by the distillation of petroleum, asphalt, and bituminous shales. Coal slowly displaced biomass and became the primary energy resource until natural gas and oil began to displace coal. In 1816, the first gas company was established in Baltimore, and by 1859, more than 300 U. S. cities were lighted by gas. Natural gas was no longer a curiosity, but illuminating gas manufactured from coal by thermal gasification processes still ruled the burgeoning gas industry. Natural gas did not come to the fore until manufactured gas was widely adopted for cooking, space heating, water heating, and industrial uses. Installation of a nationwide pipeline grid system after World War II for transmission of natural gas eventually made it available in most urban areas.

After the first oil well was drilled in 1859 in Titusville, Pennsylvania, for the specific purpose of bringing liquid petroleum to the surface in quantity, produc­ing oil wells were drilled in many states. The installation of long-distance pipelines for transport of oil from the producing regions to the refineries and the natural gas pipeline grid signaled the end of coal’s dominance as an energy resource in the United States. As shown in Fig. 1.1, the percentage contributions

to total primary U. S. energy demand in the 1990s were about 70% for petroleum and natural gas and 20% for coal. Biomass, hydroelectric power, and nuclear power made up the balance. It is noteworthy that since the advent of nuclear power, its overall contribution to U. S. energy demand has remained rela­tively small.

Over the period 1860 to 1990, U. S. fossil fuel consumption correlated well with the growth in population (Fig. 1.2), but more revealing is the trend over the same period, in annual and per-capita U. S. energy consumption (Fig. 1.3). As technology advanced, the efficiency of energy utilization increased. Less energy per capita was consumed even though living standards were dramati­cally improved. Large reductions in per-capita energy consumption occurred from over 600 GJ/capita-year (102 BOE/capita-year) in 1860 to a level of about 200 GJ/capita-year in 1900. Per-capita energy consumption then remained relatively stable until the 1940s when it began to increase again. In the 1970s, energy consumption stabilized again at about 350 GJ/capita-year (59 ВОЕ/ capita-year). This is undoubtedly due to the emphasis that has been given to energy conservation and the more efficient utilization of energy and because of improvements in energy-consuming processes and hardware.

Because of the increasing efficiency of energy utilization, the energy con­sumed per U. S. gross national product dollar exhibited substantial reductions also over the period 1930 to the early 1990s (Fig. 1.4). The U. S. gross national

POPULATION, MILLION EJ/YEAR

300 100

GJ/CAPITA-YEAR

700

YEAR
-x — EJ/YEAR — s- GJ/CAPITA-YEAR

FIGURE 1.3 Annual and per-capita energy consumption for United States, 1860-1990.

GNP, TRILLION DOLLARS MJ/$ GNP

7 30

GNP TRILLION DOLLARS — s — MJ/ $ GNP

FIGURE 1.4 U. S. gross national product and energy consumption per dollar of GNP, 1994dollars.

product increased more than sixfold in 1994 dollars over this period, while energy consumption per dollar of gross national product decreased from about 26 MJ/$ GNP to 14 MJ/$ GNP.

Many studies show that higher concentrations of C02 than normally present in air will promote more carbon fixation and increase biomass yields. In confined environmentally controlled enclosures such as hothouses, carbon dioxide-enriched air is often used to stimulate growth. In large-scale, open — air systems such as those envisaged for biomass energy farms, this is not practical. For aquatic biomass production, C02 enrichment of the water phase may be a potentially attractive method of promoting biomass growth if C02 concentration is a limiting factor, since biomass growth often occurs by uptake of C02 from both the air and liquid phase near the surface.

For some high-growth-rate biomass species, the C02 concentration in the air among the leaves of the plant is often considerably less than that in the surround­ing atmosphere. Photosynthesis may be limited by the C02 concentrations under these conditions when wind velocities are low and insolation is high.

Q I 1 J I I I

20 25 30 35 40 4 5 50 55 60

TEMPERATURE, °С

FIGURE 4.8 Effect of temperature on net photosynthesis for sorghum and cotton leaves.

B. Nutrients

All living biomass requires nutrients other than carbon, hydrogen, and oxygen to synthesize cellular material. Major nutrients are nitrogen, phosphorus, and

potassium; other nutrients required in lesser amounts are sulfur, sodium, magnesium, calcium, iron, manganese, cobalt, copper, zinc, and molybdenum. The last five nutrients, as well as a few others not listed, are sometimes referred to as micronutrients because only trace quantities are needed to stimulate growth. For terrestrial biomass, all of these elements are usually supplied by the soil, so eventually the soil’s natural nutrients are depleted if they are not replaced through fertilization. Some biomass species, such as the legumes, are able to meet all or part of their nitrogen requirements through fixation of ambient nitrogen. Marine biomass such as giant brown kelp use the natural nutrients in ocean waters. Freshwater biomass such as water hyacinth is often grown in water rich in nutrients such as municipal wastewaters. The growth of the plant is stimulated and at the same time, the influent wastewater is stabilized because its components are taken up by the plant as nutrients. So — called luxuriant growth of water hyacinth on biosolids, in which more than the needed nutrients are removed from the wastewater, can be used as a substitute wastewater treatment method.

Whole plants typically contain 2 wt % N, 1 wt % K, and 0.5 wt % P, so at a yield of 20 t/ha-year, harvesting of the whole plant without return of any of the plant parts to the soil corresponds to the annual removal of 400 kg N, 200 kg K, and 100 kg P per hectare. This illustrates the importance of fertiliza­tion, especially with these macronutrients, to maintain soil fertility. Indeed, biomass growth is often nutrient-limited and yield correlates with increased dose rates. An example is U. S. corn production from 1945 to 1970. Nitrogen fertilizer applications were increased from 8 kg/ha to 125 kg/ha over this period; the corresponding edible corn yields increased from 2132 to 5080 kg/ ha (Krummel, 1976). Average nitrogen fertilizer applications for produc­tion of wheat, rice, potato, and brussels sprouts are about 73, 134, 148, and 180 kg/ha, respectively, in the United States (Krummel, 1976). Much of the success of the “green revolution” is claimed to be the result of greater fertilizer applications. Estimates of balanced fertilizers needed to produce various land biomass species are shown in Table 4.5 (Roller et al, 1975). Note that alfalfa does not require added nitrogen because of its nitrogen-fixing ability. It is estimated that this legume can fix from about 130 to 600 kg of elemental nitrogen per hectare annually (Evans and Barber, 1977).

Normal weathering processes that occur in nutritious soils release nutrients, but they are often not available at rates that promote maximum biomass yields. Fertilization is usually necessary to maximize yields. Since nitrogenous fertilizers are currently manufactured from fossil fuels, mainly natural gas, and since fertilizer needs are usually the most energy intensive of all the inputs in a biomass production system, a careful analysis of the integrated biomass production-conversion system is necessary to ensure that net energy produc­tion is positive. Trade-offs between synfuel outputs, nonsolar energy inputs,

and biomass yields are required to operate a system that produces only en­ergy products.

Dewatering methods are available for most high-water-content virgin and waste biomass. This suggests that the moisture content of such feedstocks can be readily adjusted before conversion. This is not the case, however, because it is often difficult to reduce moisture content to the level desired at reasonable cost.

The equipment used for dewatering includes filters and screening devices of various types, centrifuges, hydrocyclones, extrusion and expression presses, water extractors, and thickening, clarifying, and flotation hardware. The pro­cessing methods encompass a broad range of water-removal techniques. They
can also incorporate the use of chemical flocculants and surfactants and high- and low-temperature treatments. The drawbacks to the dewatering of high — water-content biomass by most of these methods are numerous. Direct physical separation of the occluded moisture in aquatic species by dewatering is nor­mally not feasible unless the biomass is subjected to physical processes that disrupt the cell walls. Solar drying in open air is a low-cost option for moisture reduction as already pointed out, but most high-water-content biomass species begin to decompose, some quite rapidly and often with a relatively large loss in carbon and energy content, when dried under these conditions. In contrast, municipal biosolids are often dewatered to 5 to 20 wt % solids content, and some of the advanced dewatering methods are capable of increasing the solids content to as high as 50 wt % or more. The drying methods used commercially in wastewater treatment plants facilitate final disposal, but they are costly and afford products that are still far from the preferred moisture content range of feedstocks for thermochemical conversion.

Strict physical processing of high-water-content biomass for partial removal of moisture can sometimes be accomplished by combined use of shearing or cutting devices and mechanical pressing. Some of the dewatered products produced by these techniques can sustain their own combustion, can be com­bined with low-moisture feedstock for thermochemical conversion, or can be fabricated into briquettes or pellets for use as fuels. Overall consideration of the difficulties of dewatering high-water-content biomass suggests that microbial conversion processes should be used so the feedstock does not have to be dewatered or dried and can be used as such.

Although it is relatively costly, one drying method deserves special mention because it is used commercially for several high-water-content waste biomass streams such as brewery and fermentation industry wastes, food and dairy industry wastes, and primary and secondary municipal biosolids. The technique is based on the equivalent of multiple-effect evaporation and vapor recompres­sion so that most of the water exits the process as liquid, except in the last effect, to avoid losing the latent heat of vaporization. Several advanced processes have been developed to separate water from solids at lower energy inputs than conventional, single-effect drying systems. With the advent of large centrifugal compressors in the 1960s, it became possible to mechanically recompress the water vapor from an evaporative stage to drive that same stage rather than another as in multiple-effect evaporation. Using standard technology, the dewa­tering of high-water-content biomass can utilize mechanical vapor recompres­sion to raise the solids concentration to near 30 wt %, followed by multiple — effect evaporation to raise the solids concentration to near 50 wt %, followed by a rotary dryer if desired for further moisture reduction (с/. Crumm and Crumm, 1984). The efficiency of the segment of evaporation that mechanical vapor recompression accomplishes is very high. At an energy equivalent of 10.5 MJ/kWh (10,400 Btu/kWh), mechanical vapor recompression can vapor­ize 1 kg of water for less than 0.46 MJ (1.0 lb for less than 200 Btu). The Carver-Greenfield process is based on combining mechanical vapor recompres­sion with multiple-effect evaporation to dry high-water-content biomass and other solid suspensions. Many full-scale units have been placed in operation since the first facility was installed in 1961. One unit was used at the Hyperion wastewater treatment plant in Los Angeles from 1987 to early 1995 to dry 40 t/day of biosolids wetcake to 99+% total solids content (Haug, Moore, and Harrison, 1995). The process has since been replaced by rotary steam dryers because it was not possible to reach the design capacity of the unit.

Chars, gases, light and heavy liquids, and water are formed in varying amounts on pyrolysis of biomass. The yields depend particularly on the feed composi­tion, dimensions of the feed particles, heating rate, temperature, and reaction time. When hardwoods are heated in the absence of air, they decompose and are converted into charcoal and a volatile fraction that partly condenses on cooling to a liquor called pyroligneous acid, which separates into a dark heavy oil as the lower layer in about 10 wt % yields, and an upper aqueous layer. Dry distillation of softwoods such as pine yield similar products in about the same amounts as well as lighter pine oils and terpene liquids such as turpen­tines. The supernatant layer contains methanol, acetic acid, traces of acetone, allyl alcohol, and other water-soluble compounds. Methanol is formed from the lignin components that bear methoxyl groups. The heavy oil contains tars, higher viscosity pitches, and some char. The wood tars and pitches are complex mixtures in which hundreds of organic compounds have been identified, pri­marily acidic and heterocyclic compounds.

The data in Table 8.3 show how the gas, pyrolytic oil, and char yields vary with pyrolysis temperature and different biomass feedstocks (Epstein, Kostrin, and Alpert, 1978). Extensive depolymerization of the celluloses starts at about 300°C and usable charcoal formation (carbon content about 75 wt %) starts at about 350°C (Zaror and Pyle, 1982). Higher temperatures and longer residence times promote gas production, while higher char yields are obtained at lower temperatures and slow heating rates. The product slate is similar for each feedstock at a given temperature, although the yields of gas, pyrolytic oil, and char can be quite different. The cellulosics and hemicellulosics are the main sources of volatiles in biomass feedstocks, but yield only about 8 to 15% of their weight as charcoal under conventional pyrolysis conditions (i. e., slow heating rate, atmospheric pressure, and maximum temperatures of 400 to 450°C). The lignins yield nearly 50% of their weight as charcoal under these conditions (Zaror and Pyle, 1982). A more detailed distribution of specific products on long-term pyrolysis of three woody biomass feedstocks, birch, pine, and spruce wood, to a final temperature of 400°C is shown in Table 8.4 (Nikitin et al, 1962; Bagrova and Kozlov, 1958). Both the individual product distributions and the yields of carbon, pyroligneous distillate, and gases are similar for each wood species. It is evident that the product mixture is complex and that selectivities for specific chemicals are low. The order of decreasing yield on a weight basis by product group from highest to lowest is pyroligneous distillate, charcoal, and gaseous products. This might be expected because of the relatively low pyrolysis temperatures and the 8-h period over which these experiments were performed. However, if water is excluded from the yield calculations, the order of decreasing yield is charcoal, pyroligneous distillate, and gaseous products.

The pyrolysis of the combustible fraction of MSW at higher temperatures is illustrated by the data in Table 8.5. These data show how temperature affects product yields and gas and char compositions on pyrolysis at temperatures up to 900°C (Hoffman and Fitz, 1968). Gas yield increases as the temperature is increased from 500 to 900°C. Although the heating value of the product gas remains about the same, significant increases in gas yield on a weight percent and energy yield basis and in hydrogen occur with increasing tempera­ture. Interestingly, as the temperature increases, the char yields and volatile matter content of the chars decrease as expected, but the energy value of the chars is relatively constant.

"Epstein, Kostrin, and Alpert (1978). The feedstock was pyrolyzed in a 0.5-m ID fluid-bed reactor containing sand and an inert gas generated from compressed air-natural gas combustion with a slight excess of air (about 0.2 to 0.6%). The fluidizing velocities were 0.3 to 1 m/s. The products were low-energy gas (3.89-11.78 MJ/mJ (n)), pyrolytic oil (23.3-27.9 MJ/kg), and charcoal. Feed rates were 50-200 kg/h. The moisture contents of the feedstocks were not specified. The balance of the yield for each feedstock is water.

Since biomass pyrolysis product mixtures are very complex and selectivities are low for specific products, considerable effort has been devoted to improving selectivities. Selectivities can sometimes be increased by addition of coreactants or catalysts, or by changing the pyrolysis conditions (cf. Nikitin et al, 1962). For example, the pyrolysis of maplewood impregnated with phosphoric acid increased the yield of methanol to 2.2 wt % of the wood as compared to 1.3 wt % obtained on dry distillation of the untreated wood. Addition of sodium carbonate to oak and maple increased the yield of methanol by 100 and 60%, respectively, compared to pyrolysis yields without sodium carbonate. Other weakly alkaline reagents exhibited a similar effect. Pyrolysis of wood in a stream of benzene, xylene, or kerosine increased the yields of acetic acid, aldehydes, and phenols and reduced the yield of tars. Optimization of pyrolysis conditions will be shown later to have large effects on product distributions and yields.

The first commercial, pressurized, air-blown, fluid-bed process for wood feed­stocks was developed by Omnifuel Gasification Systems Ltd. and was installed in a plywood mill in Ontario, Canada in 1981 (Bircher, 1982). The unit was an 84-GJ/h gasifier that was supplied with 5.9 t/h of wood chips and wood dust. It operated at about 760°C and 35.5 kPa gauge. The low-energy product gas was used on-site as boiler fuel. Air was introduced at the bottom of the bubbling bed of sand particles and maintained the bed in constant motion as it passed up through the bed. Some of the air caused combustion of feed to maintain the temperature in the desired range, and some reacted with char to yield additional gas. A typical wet gas analysis was 12.3 mol % carbon monox­ide, 4.6 mol % methane, 1.6 mol % C2+ hydrocarbons, 7.8 mol % hydrogen, and 73.7 mol % nitrogen, carbon dioxide, and water. Carbon conversion effi­ciencies of the order of 99% were obtained, and tar production was very low, of the order of 0.1 to 0.2%. Ash entrained in the product gas was removed by cy­clones. Some difficulty was encountered with gas combustion equipment be­cause of the large variation in gas quality and the plant has been shutdown (cf. Klass, 1985). This was attributed to the large range of wood feedstock moisture which varied between 5 and 50 wt %. The heating values of the product gas ranged from 3.1 to 7.9 MJ/m3 (n). Operation with oxygen at 1420 kPa was projected to produce a gas with a heating value of 11.8 to 15.7 MJ/m3 (n).

A. U. S. Markets

As mentioned in the introduction to this chapter, biomass energy is already a substantial contributor to commercial primary energy demand. Market pene­tration is significant and is expected to increase. A comparison of U. S. consump­tion of biomass energy in 1990 with projections for 2000 (Table 2.9) (Klass, 1994) shows that consumption in 2000 is expected to be about 50% greater. This assessment is based on the following assumptions: Noncrisis conditions prevail; the U. S. tax incentives in place continue and are not changed; no

TABLE 2.9 Consumption of Biomass Energy in United States in 1990 and Projected for 2000“

1990 2000

Resource EJ BOE/day EJ BOE/day

“Klass (1994) and U. S. Department of Energy (1990) for 1990; Klass (1990, 1994) for 2000. hither estimates range from 3.5 to 5.8 EJ/year in 2000 (с/. Hohenstein and Wright, 1994).

legislative mandates to embark on an off-oil campaign via fossil carbon con­sumption taxes or related disincentives to use fossil fuels, such as those in place in certain parts of Europe, are enacted; and total energy consumption in 2000 is 92 EJ (87 quad).

In 1990, industrial and residential utilization of biomass energy as wood and wood wastes was responsible for almost 84% of total biomass energy consumption, while MSW contributed about 10%. When these figures are compared with the estimated recoverable amounts of biomass energy available in the United States in 2000 (Table 2.7), it is evident that biomass energy consumption can be substantially increased. The development of large-scale biomass energy plantations in which system designs incorporate total replace­ment of virgin biomass resources as utilized could provide much larger in­creases in biomass energy consumption beyond these estimates. At an average U. S. wellhead price of petroleum of $20/bbl in 1990, total biomass consumption in 1990 was equivalent to about $27.4 million per day retained in the country and not expended on fossil fuels. There are clearly strong beneficial economic impacts of biomass energy consumption on U. S. trade deficits, a good portion of which is caused by oil imports.

A few comments are in order regarding the utilization of fuel ethanol, most of which is manufactured from corn in the United States. Fuel ethanol is used in motor gasoline blends as an octane enhancer and as an oxygenate to reduce emissions. The Clean Air Act Amendments of 1990 (U. S. Public Law 101­549) mandate the use of oxygenates in reformulated gasolines, and the market for ethanol from biomass is therefore expected to exhibit substantial growth as time passes, provided the tax incentives in place for fuel ethanol from biomass continue or fossil fuel consumption taxes are implemented to attempt to reduce atmospheric pollution. As will be shown in later chapters, advanced technologies may eventually make it possible for fuel ethanol to be manufac­tured from low-grade cellulosic biomass feedstocks and to be economically competitive with motor gasolines without the need for tax incentives. In the mid-1990s, the production capacity for fuel ethanol from biomass was about 4.2 billion L/year, or 0.088 EJ. Total U. S. production of fuel ethanol has increased by more than an order of magnitude since it was first marketed in modern times in the United States as a gasoline extender and octane enhancer in 1979. Fuel ethanol is a major biomass energy commodity, the production of which is expected to increase by another 2.3 billion L/year as the Clean Air Act Amendments are fully implemented. But note that the U. S. motor gasoline market in the mid-1990s was more than 379 billion L/year (100 billion galJ yr), so fuel ethanol only displaced about 1% by volume of petroleum gasolines.

The estimate of U. S. biomass energy usage in 2000 (Table 2.9) indicates that the largest contributions are still expected to come from wood and wood wastes in the industrial and residential sectors, or about three-quarters of total estimated U. S. biomass energy consumption. Because of the technical and economic problems associated with solid waste disposal, the increasing amounts of MSW generated by increasing urban populations, and the phase­out of sanitary landfilling as a preferred method of MSW disposal, the contribu­tion of MSW to biomass energy usage is expected to double by 2000.

A projection of biomass energy consumption for the United States is shown for the years 2000, 2010, 2020, and 2030 by end-use sector in Table 2.10 (U. S. Dept, of Energy, 1990). This particular analysis is based on a national premiums scenario which assumes specific market incentives are applied to all new renewable energy technology deployment and continue to 2030. The premiums are 2<t/kWh on electricity generation from fossil fuels, $1.90/GJ ($2.00/106 Btu) on direct coal and petroleum consumption, and $0.95/GJ ($1.00/106 Btu) on direct natural gas consumption. This scenario depends on the enactment of federal legislation that is equivalent to a fossil fuel consump­tion tax. Any incentives over and above those assumed for the assessment in Table 2.9 can be a strong stimulus to increase biomass energy consumption.

The market penetration of synthetic fuels from virgin and waste biomass in the United States depends on several basic factors such as demand, price, performance, competitive feedstock uses, government incentives, whether an established fuel is replaced by a chemically identical fuel or a different fuel, and the cost and availability of other fuels such as oil and natural gas. Many detailed analyses have been performed to predict the market penetration of biomass energy over the next 10 to 50 years. There seems to be a range from about 4 to 20 quads per year that characterize the growth of biomass energy consumption. All of these projections of future market penetrations for biomass energy in the United States should be viewed in the proper perspective.

The potential of biomass energy is easily demonstrated as shown in this chapter, but the necessary infrastructure does not exist to realize this potential without large investments by industry. Government incentives will probably be necessary too. U. S. capacity for producing virtually all biofuels manufac­tured by biological or thermal conversion of biomass would have to be dramati­cally increased to approach the potential contributions of virgin and waste biomass. For example, an incremental quad per year of methane from biomass feedstocks in the United States requires about 200 times the biological methane production capacity in place, and an incremental quad per year of fuel ethanol requires about 13 to 14 times the existing plant capacity to manufacture fermentation ethanol. Given the long lead times necessary to design and con­struct large biomass conversion plants, it is unrealistic to assume that sufficient capacity and the associated infrastructure could be placed on-line in the near term to satisfy quad-blocks of energy demand. This is not to say that plant capacities cannot be rapidly increased if a concerted effort is made by the private sector to do so.

Conversely, the upside of any assessment of virgin biomass feedstocks is that energy and fuel markets are very large and expand with the population, so there should be no shortage of demand for economically competitive energy supplies in the foreseeable future. Systems that offer improved waste disposal together with efficient energy recovery are also expected to fare quite well.

Projections of market penetrations and contributions to primary energy demand by biomass can contain significant errors. It is important, therefore, to keep in mind that even though some of these projections may turn out to be incorrect, they are still necessary to assess the future role and impact of renewable energy resources. They are also of great help in deciding whether a potential renewable energy resource should be developed and commercialized.

An example of the detailed production costs in the mid-1990s of two commercial herbaceous crops grown without irrigation in the Corn Belt of the U. S. Midwest, the perennial alfalfa and the annual corn, is shown in Table 4.14 (University of Illinois Urbana-Champaign FaRM Lab, 1995). The economics are shown for the maintenance and harvesting of established alfalfa. The cost of planting in the first year is therefore excluded. For corn, no-till, no-rotation planting is used. This technique affords the lowest production cost, although attention must eventually be given to counteract any adverse effects on soil chemistry. Alfalfa has been proposed as a dedicated energy crop and corn is a commercial feedstock for fuel ethanol production. The analysis showed that the annual loss in nominal dollars was about $115/ha for alfalfa and $101/ha for corn. It is evident that at the production costs, reported yields, and market prices at that time, production of either crop could have led to a significant loss for the farmer. It is also evident that the major variable cost factors are chemicals and harvesting labor, and the major fixed cost is land rent.

It is immediately apparent from this assessment that situations can exist that would make alfalfa and corn production profitable. If the land is rented at much lower cost than indicated in Table 4.14 or is owned by the farmer with no outstanding debt, or the crops are grown on one or more family farms where resident labor is available, the economics can be quite different and favorable. Many scenarios can be envisaged that will improve the net return. The point is that what may appear to be uneconomic at first is subject to change when the details are analyzed and appropriate actions can be taken to improve profitability. The difficulty of accurately predicting market prices is another factor that complicates matters further. Indeed, it was only a few months after this analysis that the market price of corn began to increase at a rapid rate and to reach an all-time high of over $5.00/bu, which effectively

“Adapted from University of Illinois Urbana-Champaign FaRM Lab (1995). bOne bushel (0.03524 m3) of corn is approximately 25.4 kg (56 lb).

doubled the farmer’s revenue. Careful consideration of all cost factors is obvi­ously necessary, but there is no approach to the elimination of all risk when growing a dedicated energy crop, or any other crop for that matter.

An economic analysis of the delivered costs of virgin biomass energy in 1990 dollars has been performed for candidate virgin herbaceous and woody biomass for different regions of the United States (Fraser, 1993). The analysis was done for each decade from 1990 to 2030 for Class I and И lands, but only the results for biomass grown on Class II lands for the years 1990 and 2030 are shown in Table 4.15. The total production costs for biomass were projected with discounted cash flow models, one for the herbaceous crops switchgrass, napier grass, and sorghum, and one for the short-rotation production of syca­more and hybrid poplar trees. The delivered costs are shown in Table 4.15 in 1990 $/dry t and 1990 $/MJ and are tabulated by region and biomass species.

"Adapted from Fraser (1993). Discounted cash-flow models account for the use of capital, income taxes, time value of money, and operating expenses. Real after-tax return is assumed to be 12.0%. Short-rotation model used for sycamore and poplar. Herbaceous model used for other species. The costs are in 1990 dollars. The yields in 1990 are on Class II lands. The average total field yields are for the entire region on prime to good soil, less harvesting and storage losses. The yields in 2030 are assumed to be attained through research and genetic improvements. Short — rotation woody crops (hybrid poplar and sycamore) are grown on 6-year rotations on six indepen­dent plots. Net income is negative for first 5 years for each SRWC plot.

The yield figures for 1990 were obtained by the analysts from the literature and the projected yields for 2030 were assumed to be achievable from continued research. The annual, dry biomass yields per unit area have a great influence on the final estimated costs, as would be expected. This analysis indicates that the lowest-cost energy crop of those chosen can be different for different regions of the country. A few of the biomass-region combinations appear to come close to providing delivered biomass energy near the U. S. Department of Energy cost goal. But realizing that there are many differences in the method­ologies and assumptions used to compile the 1990 costs for delivered fossil fuels in Table 4.13 and delivered virgin biomass energy in Table 4.15, it appears that many of the biomass energy costs are competitive with those of fossil fuels in several end-use sectors, even without incorporating the yield improve­ments that are expected to evolve from continued research on biomass en­ergy crops.

However, it is essential to recognize several other factors in addition to the basic cost of virgin biomass and its conversion when considering whether the economics are competitive with the costs of other energy resources and fuels. Some potential biomass energy feedstocks have negative values; that is, waste biomass of several types such as municipal biosolids, municipal solid wastes, and certain industrial and commercial wastes that must be disposed of at additional cost by environmentally acceptable methods. These biomass feed­stocks will be discussed in the next chapter, but suffice it to say at this point that many generators of waste biomass will pay a service company for removing and disposing of the wastes, and many of the generators will undertake the task on their own. These kinds of feedstocks often provide an additional economic benefit and revenue stream that can support commercial use of biomass energy.

Another factor is the potential economic benefit that may be realized from the utilization of both waste and virgin biomass as energy resources due to current and future environmental regulations. If carbon taxes are ever imposed on the use of fossil fuels in the United States as they have been in a few other countries to help reduce undesirable automobile and power plant emissions to the atmosphere, additional economic incentives will be available to stimulate development of new biomass energy systems. Certain tax credits and subsidies are already available for commercial use of specific types of biomass energy systems (Klass, 1995).

Solvent extraction of biomass, its derived ash, or biomass parts such as the seeds has been or is currently used commercially to isolate and separate certain chemicals or groups of related compounds that are present. Inorganic salts are found in some biomass species at concentrations that may justify extraction and purification (Chapter 3). Aqueous extraction of the ash from giant brown kelp and the spent pulp of sugar beet and fractional crystallization of the extract, for example, were commercial processes for the manufacture of potassium compounds in the early 1900s. Examples of some of the organic compounds that are extracted with solvents are triglycerides, terpenes, and lignins. Water and water in mixtures with polar solvents have been used for extraction of several of the low-molecular-weight, water-soluble sugars. Some detail on the extraction of lignins illustrates how solvent extraction processes might be developed.

Aqueous organic solvents are effective for the selective extraction of lignins in biomass. Lignins can also be extracted from biomass by use of dilute aqueous alkali under mild conditions (с/. Lawther, Sun, and Banks, 1996), but aqueous alcohols alone such as 50% ethanol solubilize lignins in wood, leaving relatively pure undecomposed cellulose (Aronovsky and Gortner, 1936; Nikitin et aL, 1962). Deciduous trees are delignified by aqueous ethanol extraction to a greater extent than conifers. Lignin is also readily extracted by mixtures of butanol or amyl and isoamyl alcohols with water. Separation of the lignins from the extracts yields tarlike substances that become brittle on cooling. Since one of the prime objectives of producing chemical pulps from wood is delignification without changing the cellulosic fibers, the data accumulated on the solvent extraction of wood suggests that high-quality paper pulps could be manufactured by solvent extraction of hardwoods and softwoods as well as other biomass species. The lignins in the extracts might provide the starting point for the production of new lignin derivatives and polymers.

Solvent extraction of biomass under relatively mild conditions to remove lignins by a strictly physical process without the addition of other chemicals would seem to offer several advantages over chemical pulping methods. Solvent recoveries approaching 100% should permit solvent recycling with minimal losses. A continuous process for the pulping of wood with aqueous n-butanol, which was found to be the most effective solvent, has been proposed for the pulping of wood and the separation of the lignins (Hansen and April, 1981).

This type of process, which would be expected to be environmentally benign, does not seem to have been commercialized to any extent by the pulp industry (с/. U. S. Dept, of Energy, 1995).

I. INTRODUCTION

In Chapters 7 and 8, the thermal conversion of biomass to energy by combus­tion and to liquid fuels by pyrolysis and a few nonpyrolytic liquefaction pro­cesses was examined. In this chapter, the subject of thermal conversion will be expanded further by addressing biomass gasification. Biomass gasification processes are generally designed to produce low — to medium-energy fuel gases, synthesis gases for the manufacture of chemicals, or hydrogen. More than one million small-scale, airblown gasifiers for wood and biomass-derived charcoal feedstocks were built during World War II to manufacture low-energy gas to power vehicles and to generate steam and electric power. Units were available in many designs. Thousands were mounted on vehicles and many were retrofit­ted to gas-fired furnaces. Sweden alone had over 70,000 “GENGAS” trucks, buses, and cars in operation in mid-1945 (Swedish Academy of Engineering, 1950). Research continues to develop innovative biomass gasification processes in North America, and considerable research has also been conducted in Europe and Asia. The Swedish automobile manufacturers Volvo and Saab have ongoing programs to develop a standard gasifier design suitable for mass production for vehicles. Much effort has been devoted to the commercialization of biomass gasification technologies in the United States since the early 1970s. A significant number of biomass gasification plants have been built, but many have been closed down and dismantled or mothballed.

There is abundant literature on the thermal gasification of biomass. Informa­tion and data carefully chosen from this literature are discussed in this chapter. Information on coal gasification is also included because of its relevancy to the commercialization of biomass gasification; large-scale coal gasifiers have been in commercial operation for several years. This is not the case for most biomass gasifiers. Some of the coal gasification processes are also suitable for biomass feedstocks. Since the conditions required for coal gasification are more severe than those needed for biomass, some coal gasifiers can be operated on biomass or biomass-coal feedstock blends. Indeed, some gasifiers that were originally designed for coal gasification are currently in commercial use with biomass feedstocks.

The pyrolytic gasification of biomass has been interpreted to involve the decomposition of carbohydrates by depolymerization and dehydration fol­lowed by steam-carbon and steam-carbon fragment reactions. So the chemis­tries of coal and biomass gasification are quite similar in terms of the steam — carbon chemistry and are essentially identical after a certain point is reached in the gasification process. Note, however, that biomass is much more reactive than most coals. Biomass contains more volatile matter than coal, and the pyrolytic chars from biomass are more reactive than pyrolytic coal chars.

II. FUNDAMENTALS

A. Definition

Basically, there are three types of biomass gasification processes—pyrolysis, partial oxidation, and reforming. As discussed in Chapter 8, if the temperature is sufficient, the primary products from the pyrolysis of biomass are gases. At high temperatures, charcoal and liquids are either minor products or not present in the product mixture. Partial oxidation processes (direct oxidation, starved-air or starved-oxygen combustion) are those that utilize less than the stoichiometric amounts of oxygen needed for complete combustion, so partially oxidized products are formed. The term “reforming” was originally used to describe the thermal conversion of petroleum fractions to more volatile prod­ucts of higher octane number, and represented the total effect of many simulta­neous reactions, such as cracking, dehydrogenation, and isomerization. Exam­ples are hydroforming, in which the process takes place in the presence of hydrogen, and catalytic reforming. Reforming also refers to the conversion of hydrocarbon gases and vaporized organic compounds to hydrogen-containing gases such as synthesis gas, a mixture of carbon monoxide and hydrogen. Synthesis gas can be produced from natural gas, for example, by such processes as reforming in the presence of steam (steam reforming). For biomass feed­stocks, reforming refers to gasification in the presence of another reactant. Examples of biomass gasification by reforming are steam reforming (steam gasification, steam pyrolysis), and steam-oxygen and steam-air reforming. Steam reforming processes involve reactions of biomass and steam and of the secondary products formed from biomass and steam. Steam-oxygen or steam-air gasification of biomass often includes combustion of residual char from the gasifier, of a portion of the product gas, or of a portion of the biomass feedstock to supply heat. The processes can be carried out with or without catalysis.

Under idealized conditions, the primary products of biomass gasification by pyrolysis, partial oxidation, or reforming are essentially the same: The carbon oxides and hydrogen are formed. Methane and light hydrocarbon gases are also formed under certain conditions. Using cellulose as a representative feedstock, examples of some stoichiometries are illustrated by these equations:

Pyrolysis: C6Hw05 —> 5CO + 5H2 + C

Partial oxidation: C6Hi0O5 + 02^> 5CO + C02 + 5H2

Steam reforming: C6H10O5 + H20 —* 6CO + 6H2.

The energy content of the product gas from biomass gasification can be varied. Low-energy gases (3.92 to 11.78 MJ/m3(n), 100 to 300 Btu/SCF) are generally formed when there is direct contact of biomass feedstock and air. This is due to dilution of the product gases with nitrogen from air during the gasification process. Medium-energy gases (11.78 to 27.48 MJ/m3 (n), 300 to 700 Btu/SCF) can be obtained from directly heated biomass gasifiers when oxygen is used, and from indirectly heated biomass gasifiers when air is used and heat transfer occurs via an inert solid medium. Indirect heating of the gasifier eliminates dilution of the product gas with nitrogen in air and keeps it separated from the gasification products. High-energy product gases (27.48 to 39.26 MJ/m3 (n), 700 to 1000 Btu/SCF) can be formed when the gasification conditions promote the formation of methane and other light hydrocarbons, or processing subsequent to gasification is carried out to increase the concentra­tion of these fuel components in the product gas. Methane is the dominant fuel component in natural gas and has a higher heating value of 39.73 MJ/m3 (n) (1012 Btu/SCF).

This process was developed by Manufacturing and Technology Conversion International, Inc. (Durai-Swamy, Colamino, and Mansour, 1989; Durai — Swamy et a I., 1990). Biomass is reacted with steam in an indirectly heated fluid-bed gasifier at a temperature of 590 to 730°C. This process uses pulse — enhanced, gas-fired, Helmholtz pulse combustors consisting of compact, multi­ple resonance tubes which serve as the in-bed heat transfer surface. The pulsed heater generates an oscillating flow in the heat transfer tubes that results in turbulent mixing and enhanced heat transfer. Higher heat transfer coefficients than those available in conventional fire-tube configurations were estimated for this process. A medium-energy gas is produced at steam-to-biomass ratios of about 1.0. Based on carbon, the dry gas, char, and tar and oil yields were typically 90%, 4 to 8%, and 1 to 3%, respectively. Dry gas compositions from a wide variety of biomass (wood chips at 20 wt % moisture, pistachio shells and rice hulls at 9 wt % moisture, and recycled waste paper with plastic) ranged from 19 to 24 mol % carbon monoxide, 20 to 28 mol % carbon dioxide, 8 to 12 mol % methane, and 35 to 50 mol % hydrogen. The C2-C5 hydrocarbons ranged from a low of about 0.5 mol % to a high of about 6 mol % depending on the feedstock. The higher heating values of the product gas ranged from 12.9 to 15.9 MJ/m3 (n). This work was conducted in a reactor shell 2.9 m in height; the overall height was 4.6 m including the plenums. The biomass feed rates were about 9 to 13.6 kg/h. Pilot tests in different scales of reactors from 0.2 to 68 t/day with different feedstocks have been carried out. A 15-t/day demonstration unit has been constructed and operated on waste cardboard feedstocks in California, and after relocation to Maryland, the plant was oper­ated on wood chips, straw, and coal (Mansour, Durai-Swamy, and Voelker, 1995). A 109-t/day plant for processing black liquor has been built in North Carolina, and a similar plant has been built in India for processing spent distillery waste. Several cogeneration plants ranging in size from 5 to 50 MW are envisaged for more than 500 sugar mills in India.