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Strona główna digital Encyclopedia of Applied Physics || Biomass Energy
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BIOMASS ENERGY DONALD 1. 2. 2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.4 2.4.1 2.4.2 2.5 3. 3.1 3.2 K. WALTER,* Severna Park, Maryland, U.S.A. Introduction Energy Consumption The Resource Conventional Wood and Wood Wastes Agricultural Crops, Wastes, and Residues Crops Wastes Excess Corn Municipal Wastes Residential Wastes Commercial, Institutional, and Industrial Wastes Sludge Recycling Biogas Energy Crops Terrestrial Crops Aquatic Crops International Considerations .. Conversion Technologies Preparation Thermochemical Conversion .. 473 473 474 474 474 474 474 474 474 475 475 475 475 475 475 475 476 476 477 477 477 INTRODUCTION Biomass converted to fuel (biofuels) has had a significant role in the supply of energy throughout history. Until the middle of the 19th century, it was the predominant source of energy providing heat, cooking, and light. During the late 19th century, the industrial revolution demanded more energy-dense fuels to produce steam. Coal filled that need until it was supplanted by petroleum, particularly as the demand for liquid transportation fuel arose in the early 1900s. During the 1930s and 40s, natural gas, then considered a waste product from oil wells, entered the *Mr. Walter is the Director of the Biofuels and Municipal Waste Division, U.S. Department of Energy, Washington, D.C. Any opinions expressed are those of the author and not those of the Department of Energy. 3.2.1 3.2.2 184.108.40.206 18.104.22.168 3.2.3 3.3 3.3.1 184.108.40.206 220.127.116.11 220.127.116.11 188.8.131.52 184.108.40.206 3.3.2 Combustion Gasification Reactors Utilization Liquefaction Biochemical Conversion Fermentation Future Ethanol Production Pretreatment Hydrolysis Xylose Fermentation Lignin Anaerobic Digestion 477 479 480 480 480 481 481 481 482 482 482 482 482 4. Biomass Energy and the Environment Agricultural Sector Forestry Sector Greenhouse Gases Acid Gases Other Emissions Glossary Works Cited Further Reading 483 483 483 483 485 485 485 486 487 4.1 4.2 4.3 4.4 4.5 marketplace as a source of fuel and chemical fe; edstock. With the increase in environmental awareness in the 1960s, biomass began a resurgence and today provides about 4% of the United States' energy needs. In addition, it supplies as much as 30% of some lesser developed countries' energy needs. With further utilization of existing and development of the resource and/or new improved conversion technologies, it can be a major energy supply source that has positive impact on domestic energy supply, the environment, the forest products industry, farm crops, and local job markets. 1. ENERGY CONSUMPTION Today the United States has three principal sources of primary energy: coal, petroleum, 474 Biomass Energy Table 1. Energy consumption by source in the United States (in EJ). Source Consumption as primary energy Oila Natural gasa Coala Nuclear electricitya Hydroelectric3 Other3 Biomassb 35.79 19.60 19.82 5.98 2.76 0.30 3.69 a From Energy Information Administration (1988). b From Energy Information Administraton (1989) and National Wood Energy Association (1988). and gas. Other sources are nuclear energy, hydropower, biomass, geothermal, and other renewables such as wind and photovoltaics. Table 1 lists the consumption of energy by source. Further reading on energy consumption in the United States can be found in Energy Information Administration (1988, 1989) and National Wood Energy Association (1988). 2. THE RESOURCE There is a broad range of materials which make up the biomass resource. Table 2 lists biomass resources by resource measurement category and current consumption. 2.1 Conventional Wood and Wood Wastes This resource includes the standing forests of the United States and the wastes from various forestry and wood product operations. It has a moisture content of 50% when green and physical characteristics dependent on the source. Examples of wood waste include the limbs and slash left in the forest during log- ging, sawdust from woodworking and sawmill operations, and bark and sludge from paper making. The principal energy use today is to cogenerate steam and electricity for the paper and lumber industries. The second largest use is for home heat in the form of logs, wood waste, and pellets. Wood averages about 19.5 MJ/kg on a moisture- and ash-free basis. 2.2 Agricultural Crops, Wastes, and Residues This is a diverse resource including residues from food crops, wastes from food processing, and manures. They are more moist than wood and are more diverse physically. 2.2.1 Crops. Crops such as plant oils from seeds (sunflower, rapeseed, safflower, etc.) can be used as diesel fuel, particularly for on-farm vehicles. Engine trials indicate severe coking of diesel injectors unless the oils are modified by transesterification with methanol or ethanol. Then they become suitable diesel substitutes. Certain species such as guayule and euphorbia produce latexes that can be converted to liquid fuels. 2.2.2 Wastes. Wastes such as corn cobs and stover, wheat straw, and rice husks are used to generate heat for drying, steam for industry, and electricity for the grid. Manures and food-processing wastes are used to produce biogas for boiler and engine fuel. 2.2.3 Excess Corn. Excess corn and other grains, molasses, and food-processing wastes are used to produce fuel ethanol. 2.3 Municipal Wastes These are the wastes of residential, commercial, and institutional activities and the mixed industrial wastes that are discarded in Table 2. Biomass resources (in quads). Resource Total Accessible Reserves Current Forests & wood wastes Agricultural wastes & manures Municipal & industrial wastes Recycling Energy crops Aquatic crops 442 10.5 2.4 0.8 55 5 20 5.2 1.8 0.4 28.0 1 12.5 1.1 1.6 0.4 9.2 0 2.5 0.2 0.2 0.2 0.0 0 Biomass Energy sanitary landfills. Municipal wastes are burned directly for heat and/or electricity, used to produce refuse-derived fuel (RDF), or recycled. Municipal waste, as are most wastes, is a unique fuel. Typically, it is delivered free. Further, a disposal or tipping fee is paid to the owner or operator of the energy plant. This fee is based on the cost of disposing of the waste by other means. For example, the cost of disposing of waste in a specific Long Island community is $150 per ton (1988) whether sent to the local waste-to-energy plant or hauled off the island. The negative cost of waste as a fuel is increasing as the environmental costs of landfill as an alternative disposal system are recognized and internalized. 2.3.1 Residential Wastes. Residential wastes are heterogeneous with roughly 55% organic matter (mixed paper, food and yard wastes, plastic, textiles, and urban wood), 25% moisture, and 20% inorganics (iron, glass, aluminum, other metals, dirt, and grit). They average about 21.8 MJ/kg on a moisture- and ash-free basis. 2.3.2 Commercial, Institutional, and Industrial Wastes. Commercial, institutional, and industrial wastes tend to be more uniform, drier, and of high combustible content. However, they are closer to wood and average about 19.5 MJ/kg on a moisture- and ash-free basis. 2.3.3 Sludge. Sludge from sewage treatment plants is included in the municipal wastes. Sewage sludge is 92%-95% moisture as produced, and depending on the source may be dewatered mechanically to 20%-40% solids. The energy content is about 25.5 MJ/kg on a moisture- and ash-free basis. 2.3.4 Recycling. There are several components of municipal solid waste that are recyclable, including paper and plastics, aluminum and iron, and glass. The best recyclable component is aluminum beverage containers. They have a positive market value in part because each kilogram conserves 220 MJ when compared to production from ore. For the last few years, aggressive recovery activities by aluminum companies and a purchase price of $600-$ 1000 per ton have resulted in 55%-60% of all aluminum beverage containers being source separated and recycled. By 475 tonnage, waste paper has been the principal recycled material. Manufacturing waste newsprint conserves about 4.6 MJ/kg. However, because of oversupply, collectors have had to pay to dispose of separated newsprint on the East Coast for the year 1989. Other paper such as corrugated boxes requires as much energy to recycle as to produce from virgin material. Further reading on the energy aspects of several recyclable materials can be found from United States Department of Energy (1982). 2.3.5 Biogas. Biogas is recovered from landfills and sewage treatment plants. It is used as a boiler fuel, as a fuel for engine generator sets, as natural gas after the carbon dioxide is scrubbed, and in a few instances compressed as an automotive fuel. 2.4 Energy Crops A significant increase in the availability of biomass resources will require the development of new and improved crops. These crops can be suited for terrestrial or aquatic environments. Energy crops are at varying stages of development. 2.4.1 Terrestrial Crops. Terrestrial energy crops can be either trees or grasses depending on the conversion process and the land to be utilized. There are two broad categories of trees, coniferous or softwood and deciduous or hardwood. Softwoods are used commercially for pulp and dimension lumber. Research has improved their growth from 2.5 to 7.5 metric tons per hectare and their fiber yield. Hardwoods have been used for high value specialty purposes such as furniture and veneers. The natural supply has been sufficient to meet demand and no improvement was undertaken until their value as energy crops was recognized. Hardwoods are preferred as an energy crop since certain species grow faster than softwoods and they coppice when cut. Further, second and third growth cycles are proving to be more productive than the first cycle. Because of the broad diversity of land forms, soils, and climate, several species are required. Six species have been selected for development from 140 surveyed based on growth rate, survivability in adverse conditions, nutrient requirements, and ability to coppice. The principal genus in development 476 Biomass Energy is Populus (poplar, cottonwood, etc.). The Populus species are candidates for growth in every region of the United States. The other species are more limited in range. Black locust is a nitrogen-fixing tree with range limited to the midwest and southeast. Silver maple has been selected for the midwest, great plains, and northeast; American sycamore for the southeast, sweet gum for the southeast, and eucalyptus for subtropical regions. In nature, these trees produce up to 7.5 dry metric tons per hectare per year (DT/H). However, by selection of fast-growing clones, hybridization, and biotechnological manipulation, growth rates of over 30 DT/H have been reached and an average United States growth of 25 DT/H is reasonably achievable. Since the nutrients concentrate in the leaves and energy wood is harvested in the winter while dormant, fertilizer requirements are minimal. Since the trees are closely spaced (2 m or less), the leaves and branches shade the ground and weeds are controlled. As with wood species, there are two broad categories of agricultural crops, the annuals which die and require replanting each year, and the perennial crops which retain viable root mass each year. The perennials are preferred as energy crops since annual reestablishment is not required and the living root system reduces erosion. Selection of target species is still ongoing in the United States; however, the thick-stemmed perennial grasses (e.g., napier grass) are favored. The annuals such as sorghum are being considered because of their fast growth rates. Energy cane (a variety of sugar cane) has produced 150 DT/H in Puerto Rico. The availability of land and the potential for competition between food and fiber and energy and chemical uses are of concern. However, world economies are learning to grow food. Two major importers of food in the past, India and China, now export food. Exports of large United States surpluses are decreasing. The federal government is subsidizing farmers to keep land out of production. Twelve million hectares of land are in the Conservation Reserve Program which removes highly erodible land from agriculture practice. Trees are approved for planting on these lands to stabilize the soil. The federal government pays to keep 20 to 30 million hectares of land fallow to reduce surplus crops. The Agricultural Research Service has identified a need for alternative crops for 60 million hectares of current farm land by 2010. Further, the Department of Agriculture has identified 62 million hectares of land, now in pasture and range, that are suited for farmland. This total acreage at an average of 25 DT/H could provide 58 EJ (55 quads) of energy per year. 2.4.2 Aquatic Crops. There are three potential aquatic biomass resources: the emergents or marsh crops such as cattails, macroalgae such as giant kelp grown in the open sea, and microalgae. Emergents have a resource base limited primarily to the marsh land of the upper midwest. They are difficult to harvest and to match to a conversion process which must have a year-round supply of feedstock. Macroalgae are difficult to grow and harvest although they have some valuable energy-crop characteristics such as the 45% sugar content of kelp. Microalgae are single-cell organisms that live in either fresh or saline water, grow rapidly, and under stress fix large quantities of lipid (plant oil). These oils are typically hydrocarbons in the C18 to C22 range and are similar to the plant oils from seeds. The same transesterification will be needed to convert them to suitable diesel fuels. 2.5 International Considerations Worldwide, there is broad interest in the use of biomass as an energy resource. Many of the developing countries generate as much as 30% of their primary energy from biomass. In many cases the resource is overused and actions are necessary to reverse the trend through improved efficiency and replanting. For example, inefficient stoves in the sub-Saharan Desert region are being replaced and new trees planted to halt desertification of the area. The major food-raising countries all have an interest in the improved use of agricultural and wood wastes. Under the auspices of both internal and international agencies there is increasing use of bagasse from sugar cane and rice husks for the cogeneration of steam and electricity for refinery operation. Many countries are considering the infrastructure and legal changes necessary to supply electricity from wastes to the nations' Biomass Energy electric grid. The Brazilians have embarked on a major program to use ethanol from cane sugar as a gasoline replacement. The industrialized nations are more advanced than the United States in the productive use of municipal solid waste. While we use perhaps 20% for energy and recycling other countries make use of up to 75% of their waste. Worldwide there is increasing interest in the growth of energy crops. As noted, Brazil produces ethanol from cane sugar. In addition, they have a great deal of expertise in fast-growing eucalyptus trees. Now they are engaging in research to produce ethanol from those trees. England, Sweden, Canada, Finland, and the United States cooperate under the auspices of the International Energy Agency. Belgium maintains a bank of over 3000 Populus species clones. In fact, several of the United States commercial plots have been planted with clones from that bank. India and the United States have a bilateral agreement in part to develop energy crops. The Indians are the preeminent researchers in species of Populus for alkaline soil conditions. However, statistics are not readily available for the worldwide potential for energy crops. 477 plex is the separation of municipal solid waste (MSW) into various components. A recent innovation is a combustion system that can burn whole trees. The preparation of biomass has two aims: provision of a more uniform fuel and separation of the fuel into components. Wood generally is used as chunk wood (5- to 15-cm pieces), hog fuel (2.5 to 5 cm), and sawdust. Finer particles may be compressed into pellets or briquettes. Screens are used to remove rocks and other inerts, and magnets to remove tramp metal such as saw teeth, barbed wire, etc. MSW is used as received or processed to produce refuse-derived fuel (RDF), materials for recycling, and inerts. MSW is prepared by screens of various designs for size separation, hammermills, and flail mills for size reduction and a series of technologies to separate specific components, such as magnets to recover ferrous metals. Refusederived fuel and magnetic metals have been recovered successfully. The technology to recover glass and aluminum has been developed. MSW separation is an art unto itself. Further reading can be found in Diaz et at (1982) and Hasselriis (1984). 3.2 Thermochemical Conversion 3. CONVERSION TECHNOLOGIES There are two broad categories for the conversion of biomass to a fuel form, thermochemical and biochemical. The former uses heat and is typified by fast reaction times (seconds to minutes) and harsh conditions (high temperatures from 300 °C to over 1000 °C and pressures of up to 200 atm). The products tend to be variable. The biochemical systems use enzymes (natural catalysts) and are typically slow (reaction times of hours to days) and mild (temperatures from room to the boiling point of water, and atmospheric pressure). The products are specific. 3.1 Preparation The design and operation of any biofuels system depends on the form of the feedstock. In general, smaller and more uniform particles favor conversion. However, obtaining such fuel can be expensive. The simplest form of preparation is drying, and the most com- As biomass is heated, the long chains of molecules that comprise cellulose, hemicellulose, and lignin begin to break into simpler molecules and become modified. The final form of those molecules depends on the rate at which heat is applied, on the length of heat application, on the final temperature, on the external oxygen available, and to a lesser extent on the pressure. High heat rates, short residence times, and limited oxygen can favor the retention of long-chain molecules that are liquid at standard temperature and pressure. As reaction time increases, heat rate decreases, final temperature increases, and oxygen levels increase, the molecules become shorter until with sufficient time and oxygen, the fuel is completely oxidized. 3.2.1 Combustion. The most common technology for the use of biomass as a fuel today is combustion. There are thousands of facilities that burn biomass and MSW to provide industrial steam and heat, electricity, 478 Biomass Energy and district heating. In addition, over 2 million homes use wood stoves as a principal means of heat. During combustion, fuel is first dried to drive off free water, then the long chains of molecules are broken until essentially the carbon reacts with oxygen to form carbon dioxide. The general chemical equation is Cc(H20)>,+.*;02 -> *CO2+.yH2O. (1) In an ideal combustion process only the stoichiometric amount of oxygen is added. In practice, excess air (oxygen) must be added to maximize the oxidation of the carbon and minimize the formation of carbon monoxide. Figure 1 is a typical graph of carbon monoxide versus oxygen. Since these graphs are a function of the specific fuel and conditions of combustion, no units of measurement have been added. As the amount of excess air is increased, the combustion process is cooled, carbon monoxide production increased, and overall efficiency reduced. A modern biofuels combustion system can reduce carbon monoxide emissions to 30 parts per million (ppm) and achieve efficiencies in the 65%-75% range. Here, efficiency is defined as the steam produced per unit of total-energy input. For comparison a modern natural gas plant is 80% efficient, fuel oil 85%, and coal 90%. The differences are principally related to the amount of water produced per joule of fuel burned both from the conversion of fuel hydrogen to water and from free and fuelbound water. The success of the combustion of biomass is a function of the design of the system. The majority of biomass combustion facilities have a grate to retain fuel for complete com- STOICHIOMETRIC OXYGEN (percent) FIG. 1. Carbon monoxide vs oxygen. bustion and to distribute part of the combustion (underfire) air. Grates vary from a simple slab of metal with small air distribution holes (pinhole grate) for sawdust to complex reciprocating grates for raw MSW. The Energy Research and Development Administration (1977) provided sketches of a series of MSW grate systems. In addition to the primary air delivered through the grate, secondary air is delivered above the grate to promote turbulence and to ensure complete combustion. The grate is contained in a firebox which may have walls constructed from either refractory fire brick or a membrane of welded tubes (waterwall). In the latter case the tubes are filled with water which removes both radiant and sensible heat. Waterwall systems have integrated superheaters, economizers, and feedwater heaters to recover steam. Refractory systems have a separate waste heat boiler. Because radiant heat of the fire is captured, waterwalled systems tend to be more efficient than refractory systems; however, the refractory systems tend to have lower maintenance and repair costs. Further reading on design and a more complete description of boilers can be found in Robinson (1986), Chap. 13. The quality (temperature and pressure) of the steam recovered depends on its end use and the overall system function. For example, industrial processes frequently require 1 MPa, saturated steam while electric generation is most efficient at high steam pressures and degrees of superheat. Particularly in MSW fired units, steam temperature is technically limited by the wall temperature of the superheater tubes. As the tubes are heated, they are subjected to two distinct corrosion patterns. First, there is a low-temperature peak of corrosion at about 150 °C followed by a high-temperature peak at around 450 °C. Corrosion at high temperatures is not well understood although it is most likely caused by a complex sulfur, iron, and chlorine reaction. Overall pressure is not a limiting factor. See Plumley (1977) for diagrams and further discussion on corrosion regimes. An emerging thermal technology is the fluid bed combustor. The biomass fuel is combusted in a chamber that is partially full of sand. Air is injected below the bed and the sand acts as a fluid. The large mass of turbulent hot sand ensures very rapid heat transfer Biomass Energy and complete combustion of the fuel. Heat is recovered by in-bed and above-bed tubes, waterwall tubes, and integrated boiler banks. If limestone is substituted for sand, the bed also captures chlorides and sulfur oxides. Fluid bed combustion units require less excess air than other biomass combustion designs and are more efficient in the production of steam. There are two variants of the fluid bed combustor. The atmospheric fluid bed uses low air velocities to retain sand in the combustion chamber. The circulating fluid bed uses high enough air velocities to elutriate sand. Hot sand and char are removed in a hot cyclone and recycled to the main bed. There are a significant number of fluid bed combustion units operating in the wood products industry, most providing hot air for drying and curing products. Pollution-control systems for combustion systems include particulate and acid-gas controls. Table 3 has proximate and ultimate analyses for wood, MSW waste, and coal. Because of low-acid gas-forming components, wood-burning systems require only particulate control. Modern MSW combustion facilities generally are required to have sophisticated acid-gas controls. The acid-gas controls available include wet scrubbers, dry scrubbers, and direct lime injection into the combustion chamber. Wet scrubbers condense acid gases with a water spray. They leave an acid solution that is difficult to manage. Dry scrubbers spray a slurry of lime into the combustion gas stream and collect dry reagent in Table 3. Proximate and ultimate analysis (percentage by weight). From Domalski et al. (1987). Wood MSW Coal Proximate analysis (as received) Moisture Volatile Fixed Ash 50.0 40.3 9.7 0.02 25.1 43.8 6.1 25.0 13.1 27.9 51.1 7.9 Ultimate analysis (dry) Carbon Hydrogen Oxygen Nitrogen Chlorine Sulfur Ash 48.9 5.9 43.3 0.03 0.01 0.01 0.02 39.5 5.0 29.5 0.5 0.4 0.1 25.0 74.3 4.9 7.8 1.4 NA 3.7 7.9 479 a bag house or electrostatic precipitator. They reduce chlorine emissions by 90% and sulfur by 70%. Direct injection of lime into the combustion chamber costs 10% that of dry scrubbing with 10% degradation of efficiency of removal. The control of acid gases in fluid beds is achieved by the selection of lime or limestone as the bed material. There are several particulate control systems including cyclones, baghouses, and electrostatic precipitators. Cyclones act as a centrifuge. They are relatively inefficient and not used in modern pollution-control systems. Baghouses pass combustion gas through finely woven bags that capture particulates. As a layer of particulate builds, efficiency of capture increases. When the pressure drop across the bag exceeds a set limit the bag is taken off line and cleaned with a reverse flow of clean air. They are favored in modern plants since they are more efficient in the removal of fine respirable particulate. Electrostatic precipitators generate an electric charge on particulates before passing the combustion gas between plates with the opposite charge to attract and hold the particulate until the unit is taken off line and the plates are cleaned. Both baghouses and electrostatic precipitators are more than 99% efficient. 3.2.2 Gasification. As the available oxygen is reduced in a combustion system, the carbon is no longer completely oxidized and combustible gases containing carbon monoxide, hydrogen, and other higher hydrocarbons are produced. Continued decrease of oxygen finally results in unreacted char that is mostly carbon and ash. The gasification reaction is endothermic; therefore, an external source of heat is required. The source of heat may be the combustion of char with air or oxygen, or an external heat source. When char is burned with air in the reactor, the product gas is diluted with nitrogen and has a heating value of 1.8-7.2 MJ per standard cubic meter (MJ/SCM) (low-energy gas). If pure oxygen is used, the diluting nitrogen is removed and a product gas with a heating value of 10.9-14.5 MJ/SCM (medium-energy gas) is produced. Finally, if the reactor is heated externally, the production of hydrocarbons such as methane is favored over carbon dioxide. The gas produced has a heating value of 12.7-19.9 MJ/ SCM (medium-energy gas). The chemical reaction is 480 Biomass Energy Cx{H2O)y+heal-+yCO+yH2 + {x- (2) Since there is no way to exclude oxygen and free water from the reactor completely, a series of other reactions occur, and Eq. (2) is modified. At least part of the CO is shifted to hydrogen via the water-gas shift reaction [Eq. (3)], and if partial combustion is the entire source of heat, the carbon char is burned to carbon dioxide [Eq. (4)]. Other reactions and incomplete reactions combine to produce higher hydrocarbons such as methane and long-chain molecules that are solid or liquid at atmospheric conditions: CO+H2O «-• CO2+H2 (water-gas shift), (3) C+O 2 -> CO2. (4) 126.96.36.199 Reactors. There are a series of types of gasification reactors. In the updraft gasifier, a descending bed of feedstock is first dried, then gasified, and the char is burned (either in air or oxygen) to produce a rising stream of hot gas to drive the reactions. The downdraft gasifier partially combusts the surface of a bed of feedstock. An induced draft pulls the hot product gas down through the glowing bed of coals. As a result the majority of the long-chain molecules are cracked and tars are reduced to produce gas. This product gas is cleaner and more adaptable to the internal combustion and gas turbine engines. Also, the fluid bed reactor has been adapted to gasification. The high thermal mass of the bed with its rapid heat transfer and relatively long residence time is well adapted to gasification. Fluid bed reactors can operate with the source of external heat from partial combustion of the char in the bed, from fire tubes in the bed, or by external heating of the bed material. Further reading on gasification can be found in Reed and Das (1988). 188.8.131.52 Utilization. Low-energy gas cannot be transported economically over distances of more than a kilometer. Therefore, its use is mostly viewed as a fuel for a boiler or, where tar and ash are low, as an engine fuel. Most of the 40 or so applications to date have been as a boiler fuel although at least one gasifier-engine-generator system has been permitted. In addition to fuel uses, medium-energy gas is a suitable chemical feedstock. Again, low tars and ash are desirable characteristics. In addition, the CO/H2 ratio may be adjusted by the water-gas shift reaction [Eq. (3)]. For example, if methanol (CH30H) is the end product, then the CO/H2 ratio must be adjusted to 1/2. 3.2.3 Liquefaction. As reaction time and final temperature are decreased, more long-chain molecules survive when compared to gasification. These molecules condense at standard conditions to a viscous, oxygenated fuel called biocrude. Biocrude might be considered as a substitute for fuel oil but because of instability and corrosiveness, will more likely be upgraded to other liquids such as gasoline. One of the early, large-scale attempts to produce a liquid biofuel used MSW refined to a fine (less than 30 mesh) powder which was pyrolyzed while entrained in a hot gas stream. The unreacted char was removed and burned to provide the reaction gas stream. The liquid product was condensed to a viscous, relatively unstable, corrosive oil that was to be used as a partial replacement for fuel oil in a utility boiler. The combustible gas produced was used for drying the feedstock and for reactor heat. The risk of the technology and the lack of government funds to support further testing led the industrial partner to abandon the technology when there was a major refractory failure in the reactor. The high level of processing and the limited use of the product oil were contributing factors. A second system was constructed based on the high-pressure liquefaction of wood in a carrier of hot wood oil. This system successfully produced many barrels of biocrude, but was uneconomic. Any of the reactors discussed under gasification could be adapted to liquefaction. However, only two systems are in active research. The first is based on fine wood in a carrier of product oil at high temperature and pressure. The biocrude oil produced is relatively low in oxygen and can be hydrotreated to produce first run (i.e., 72 octane) gasoline. The chemistry of the technology is complex and varies with the feedstock. The second technology uses a vortex reactor that transfers heat rapidly from a hot wall to fine biomass. Unreacted wood and char are removed by a cyclone and recycled. The product gas is passed through a zeolite catalyst to produce gasoline. More development is necessary before either technology can be fully assessed for commercial viability. Current models indicate that Biomass Energy when fully developed the technology could produce gasoline for a wholesale cost of $1.00 per gallon. 481 phate) is modified by another enzyme into the second (glyceraldhyde 3-phosphate). A second phosphate group is added and a hydrogen ion removed. Next, one of the phosphate groups is replaced by a hydroxyl (OH) radi3.3 Biochemical Conversion cal. Then the phosphate group is moved to a Biochemical conversion takes advantage new location in the atom and a hydroxyl radof enzymes to convert biomass to other more ical and a hydrogen ion are removed from useful products. The two processes in use different locations in the molecule. The secwhich are significant to energy are fermentaond phosphate group is removed to produce tion to produce ethanol and anaerobic digespyruvate, which is reduced to carbon dioxide tion to produce methane. In the fundamental and acetaldehyde. Finally, a hydrogen ion is operation of life processes, genes, which are added to acetaldehyde to produce ethanol. segments of DNA, produce enzymes which Further information on this reaction and miconvert organic molecules to useful products. crobiology, chemistry, and ethanol producEnzyme production is self-regulated. A spetion is provided in Solar Energy Research Incific promoter initiates the production of enstitute (1981). zyme. When either enough product is available or no more feedstock is present a reA beer with 8%-10% alcohol is produced. pressor gene turns off the production of enAny remaining unconverted organic material zyme. For example, in human nutrition, is filtered and the water removed by distillaorganisms in the mouth produce alpha-amytion to the azeotrope of a mixture of 93% ethlase, an enzyme which reduces starch to anol and 7% water. The remainder of the wasugar. Food more difficult to digest is reduced ter is separated by chemical alteration of the to sugar in the stomach by dilute acid. The combined boiling points or by selective absugar is absorbed in the bloodstream. Further sorption. reading on biotechnology and enzyme proThe current fuel ethanol market uses duction can be found in Watson et ah (1983). starch or sugar as its feedstock. In the United States over 85% of the fuel-grade ethanol is 3.3.1 Fermentation. Fermentation promade from corn starch. In 1988, about 330 cesses have been used for thousands of years bushels of excess corn were converted to 850 to produce beverage alcohol. More recently million gallons of fuel-grade ethanol. The rewith the oil crises of 1973 and 1978, ethanol mainder is made from other grains, molasses, has been produced as a transportation fuel. and industrial waste. In Brazil the majority is Starch is reduced to glucose by commercial made from sugar. However, the United States alpha-amylase enzymes [Eq. (5)]. The glucose is then fermented by a yeast such as Saccha- and Brazilian industries require subsidies to romyces cerevisiae to ethanol and carbon di- survive. The United States industry is supoxide [Eq. (6)]: ported by a rebate of 6$ of the 9?? federal gasoline excise tax if 10% ethanol is blended (C6H10O5)w+«H2O +yeast -> wC6H12O6, (5) with the gasoline (equivalent to a 60?? per gallon of ethanol rebate) or a 60?? per gallon of C6H12O6 -> 2C2H5OH+2CO2+heat. (6) ethanol income tax credit (rarely used). In adThe conversion of glucose to ethanol is not dition, there are state tax incentives equivaa simple rupturing of appropriate atomic lent to 10??-20?? per gallon of ethanol in 20 bonds in the glucose atom. The pathway was states. In Brazil, where all gasoline is a blend first described by Embden and Meyerhoff with 20% ethanol and a significant number of (Solar Energy Research Institute, 1981). new cars are designed for pure ethanol, the There are 12 distinct steps that are catalyzed industry is supported by a tax on gasoline. by different enzymes. First, a phosphate 184.108.40.206 Future Ethanol Production. group is added, then that compound is Sugar and starch prices are based on food changed to fructose and another phosphate and feed value. They are unlikely to produce group is added. This compound is cleaved economic ethanol until the current price of into two different isomers of three carbon gasoline doubles. Cellulosic crops such as compounds, each with a phosphate group. One of the isomers (dihydroxyacetone phoswood or grasses could provide an economic 482 Biomass Energy feedstock. These might be wastes or specific energy crops (see earlier section on terrestrial crops). 18.104.22.168 Pretreatment. To use crops, their three major components (cellulose, hemicellulose, and lignin) must be separated and the former two hydrolyzed to sugar. The paper industry hydrolyzes lignin with a strong alkaline solution to leave the cellulose and hemicellulose fibers intact. For ethanol production, the hydrolysis of the hemicellulose into five-carbon sugars (primarily xylose) is preferable. A number of technologies including weak acid, organosolv, ammoniafreeze-dry explosion, and steam explosion are used to hydrolyze the hemicellulose. 126.96.36.199 Hydrolysis. Cellulose is crystalline and difficult to hydrolyze. Acid digestion will reduce the cellulose molecule to glucose; however, control is difficult and the acid will degrade glucose to furfural. Acid hydrolysis was used in the United States and Canada to produce industrial ethanol in World War II. That practice stopped as soon as ethylene became available from oil after the war. The Russians still operate over 40 acid hydrolysis plants to convert wood to ethanol. Also, cellulose can be hydrolyzed enzymatically. Certain yeasts produce a consortium of enzymes that degrade cellulose specifically to glucose. The enzyme endoglucosidase breaks the long- chain cellulose molecule at interior bonds. Exoglucosidase removes a disaccharide from the end of the cellulose chain. The third, beta-glucosidase, completes the reduction of the disaccharide to hexose which may then be fermented to ethanol. Betaglucosidase is the rate-limiting enzyme. Finally, hexose is fermented to ethanol via Eq. (6). 22.214.171.124 Xylose Fermentation. The hemicellulose that was hydrolyzed to xylose in the pretreatment step represents 25%-35% of the total feedstock. Its use is essential if the overall ethanol production system is to be economic. There is no known organism or enzyme that economically will ferment xylose at efficiencies comparable to glucose. However, an enzyme, xylose isomerase, produced by E. Coli naturally converts xylose to a different five-carbon sugar isomer, xylulose. Xylulose is fermented to ethanol by commercial enzymes. Normally, E. Coli does not produce large quantities of xylose isomerase. Biotechnology has been used to modify the appropriate repressor gene of E. Coli to increase xy- lose isomerase production. Today 70% of the hemicellulose can be fermented to ethanol. Incremental improvements are expected to increase that to 90%: (C5H8O4)W+«H2O -> wC5H10O5, (7) 3C 5 H 10 O 5 +enzymes=5C 2 H 5 OH+5CO 2 . (8) The actual conversion path is from 3 xylulose to 1 fructose and 1 glyceraldhyde 3-phosphate. These then follow the pathway briefly described above for glucose fermentation. 18.104.22.168 Lignin. Lignin, the third component of plant matter, is not readily biodegradable, and therefore cannot be readily converted to ethanol at this time. However, it is a useful fuel that can be used to produce steam and electricity to power the overall plant. Today, lignin that is hydrolyzed in paper making is burned for energy, to recover chemicals, and for disposal. In addition, lignin can be converted to an aryl ether thermochemically. The process is one of hydrogenation and then reaction with either methanol or ethanol to produce methyl (MAE) or ethyl aryl ether (EAE). MAE and EAE are high-octane liquids that can be used in transportation fuel blends. 3.3.2 Anaerobic Digestion. Anaerobic digestion is a common occurrence in nature. It is likely to have been the source of today's natural gas. As biomass died in swamps and was submerged 50 to 250 million years ago, it was digested into methane and carbon dioxide. Some of the gas was trapped and, as time passed, water dissolved the carbon dioxide, leaving natural gas. Like ethanol fermentation, anaerobic digestion is a complex process that relies on a host of organisms. First, carbohydrates are hydrolyzed to simple organic acids. These are converted to acetic acid by acetogenic bacteria: (C 6 H 10 O 5 ) w +7wH 2 O+enzymes - 6«CH3OOH. (9) Then the strict anaerobic methanogens reduce acetic acid to methane and carbon dioxide: CH3OOH+enzymes -> CH4 and CO2. (10) Also in the methane production pathway is the reduction of carbon dioxide with hydrogen via Biomass Energy CO2 + 4H2 _> CH4+2H2O. (11) A description of these pathways was given in National Research Council (1977). Chapter 19 of Robinson (1986) contains design parameters. 4. BIOMASS ENERGY AND THE ENVIRONMENT Biofuels impact on many environmental sectors including the agricultural, forestry, greenhouse, energy conversion, and energy use sectors. The overall impact is a function of the specific fuel pathway; for example, the impact of an energy crop is different from the impact of a corn crop. Similarly, the impact of a combustion system is different from that of a fermentation process. 4.1 Agricultural Sector The principal concerns of the agricultural sector are erosion and chemical releases to the environment. Erosion is caused by the lack of a stabilizing vegetation layer on the land. In general, it is associated with an annual row crop such as corn. Energy crops such as fast growing trees or perennial grasses leave viable root systems to hold soil in place. Currently, trees are approved for the purpose of controlling erosion on United States land which has been taken out of service under the Soil Conservation Reserve program. Chemical pollution results in surface runoff and ground-water contamination. Excess fertilizer causes algae blooms which cause oxygen depletion in receiving waters. Herbicides and pesticides may be poisonous to aquatic life. Energy crops require smaller applications of fertilizer and chemicals. For example, while an annual farm crop requires annual fertilizer applications of 260 kg/ hectare, energy trees require 225 kg/hectare in the first year, and 110 kg/hectare in the second year, and no further applications until harvest. In addition, some fast-growing trees (e.g., black locust) are nitrogen-fixing species that might be intermingled to provide fertilizer to a plot. Bacteria living symbiotically in root structures actually provide for nitrogen fixation. Biotechnology may be able to transfer these characteristics to other fast-growing energy species. Herbicide application is re- 483 quired annually for agricultural crops but only in the first and second years after establishment for an energy crop. Then the crown closes and shade provides weed control. Pesticide applications are not well understood but the first energy tree clone biotechnologically altered for disease control has been planted in test plots. Overall, energy crops are expected to improve the agricultural environment. 4.2 Forestry Sector The principal concern in the conventional forestry sector is the air pollution caused by the open burning of slash and waste developed in the harvesting of a timber stand. The development of uses and collection systems for these materials plus environmental regulation will lead to their recovery and use as energy feedstocks. In addition, the sawdust and wastes of the forest product industry increasingly are being used as energy sources rather than being landfilled or incinerated with poor combustion control. Today, the forest products industry is over 50% selfsufficient in energy production. Within 20 years, it expects to be 100% self-sufficient. 4.3 Greenhouse Gases Incoming short-wave solar radiation is absorbed by the earth, clouds, and the atmosphere and reflected from the earth and clouds, and backscattered by air. Long-wave surface and atmospheric radiations are emitted to space. Some of this radiation is absorbed by CO2, ozone, and water vapor from which it is reflected back to earth. Several gases are increasing in concentration in the atmosphere and may upset the normal balance of radiation so as to cause the earth's temperature to rise. The greenhouse gases of concern are CO2, CH4, NO^, ozone, and CFC's (chlorofluorocarbons such as Freon). Biofuels can affect the release of the first three and generally are not involved in the generation of the last two. Figure 2, adapted from Green (1989), displays the global average earth energy balance. The buildup of CO2 and NO^ is directly related to the combustion of fossil fuels while the increase in CH4 is related to food production and waste disposal (see AIR POLLUTION: COMPONENTS, CAUSES, AND CURES). 484 Biomass Energy SPACE INCOMING SOLAR RADIATION OUTGOING RADIATION Shortwave Longwave 100 ATMOSPHERE Net Emission by Water Vapor, CO 2 ,O 3 Absorption by Clouds Water Vapor, 1 0 6 CO 2 ,O 3 Absorbed by Water Vapor, Dust, O3 Latent Heat Flux Sensible Heat Flux LONGWAVE RADIATION 100 OCEAN, LAND FIG. 2. Earth's energy balance. trial revolution and currently is about 350 ppm (up from 280 ppm in 1850). The uncontrolled burning of biomass fuel releases both carbon dioxide and methane. However, when a biomass waste is burned, no new net carbon dioxide is added to the atmosphere since the biomass will decay in a relatively short period. Further, when an energy crop is grown for energy, the carbon dioxide absorbed in the growth is released in the utilization process. Thus the carbon dioxide is recycled. Methane is present in significantly lesser Carbon dioxide is produced during the combustion of any fuel. Two important fuel characteristics are the heating value per pound and the hydrogen-carbon ratio. Figure 3 is adapted from Green (1989) and gives these values for several common fossil and biofuels. The higher the hydrogen-carbon ratio, the less carbon dioxide produced per unit of energy. Thus the best fuel is pure hydrogen and the worst, anthracite coal. The quantity of carbon dioxide in the atmosphere has been increasing since the beginning of the indus1 0 1 10 1 1 l 1 20 30 lr uir 1 40 S | 6 AB B L i f 6 8 12 10 15 SOLIDS A B | 10 C 1 50 I 60 Ru 4 2 0 Re 6W 2 HV 14 14 25 20 LIQUIDS m 32 60 36 Kcal/g 62 MBTU/lb GASESg b p e 1 H / C LEGEND: Upper Scale, HEATING VALUE; Lower Scale, HYDROGEN/CARBON RATIO. Symbols denote the following: A-anthracite, B-Low volatile bituminous, G-Garbage, H-High volatile bituminous, h-Hydrogen, L—Lignite, M-Methane, Re-Refuse, Ru-Rubbish, S-Sub-bituminous, T-Trash, W-Wood, Wd-Dry wood, Ww-Wet wood, 2-No. 2 oil, 6-N0. 6 oil. FIG. 3. Fuel characteristics. Biomass Energy amounts than CO2; however, methane is a much more reactive gas than CO2 in the upper atmosphere. While expert opinion varies, recently the EPA has published the relative reactivity of methane as being 30 times that of carbon dioxide as a greenhouse gas. Further reading on greenhouse gas issues can be found in Walter et al (1989). The specific impact of greenhouse methane is particularly important in the utilization of wastes. Municipal waste decays anaerobically in landfills and in certain circumstances biomass may also decay anaerobically. The equations of this decay are the same as Eqs. (9) and (10), that is, for every molecule of CO2, a molecule of CH4 is produced. Each molecule of methane is estimated to have 30 times [United States Environmental Protection Agency (1990)] the greenhouse effect of a molecule of carbon dioxide. Therefore, productively using organic waste not only reduces the relative CO2 effect from 31 to 2, but also displaces CO2 that would have been produced from fossil fuel. 4.4 Acid Gases Another environmental issue is the release of acid gases and more specifically sulfur oxides into the atmosphere. The conversion processes generally collect much of the acid-gas component of the primary feedstock, e.g., refining crude oil to motor fuel collects the majority of sulfur that is naturally present in oil. Despite this, gasoline still contains about 0.4% sulfur and diesel fuel up to 3% sulfur. In contrast, bioalcohols are sulfur free. The direct combustion processes oxidize the available sulfur compounds to SO^. This produces acid rain which, in certain locations, is believed to be the cause of damage to forests and lakes. The amount of acid rain produced by any combustion process is proportional to the amount of fuel-bound sulfur. Table 3 lists the fuel-bound sulfur for biomass, municipal waste, oil, and coal. For comparison, the chlorine contents of biomass and municipal waste are also listed since it produces HC1 when burned. Both sulfur and chlorine emissions are being controlled by wet or dry scrubbers, particularly in the case of municipal waste combustion. Nitrogen oxides are produced both from fuel nitrogen and from the oxidation of nitro- 485 gen in the air during combustion. The amount of NO^ produced by the fuel is proportional to the nitrogen in the fuel. Table 3 lists the fuel nitrogen for biomass, municipal solid waste, oil, and coal. Nitrogen oxidized from the air is a function of the temperature of combustion. It is lower for biomass and municipal solid waste which tend to burn at lower temperatures because of relatively high moisture contents. 4.5 Other Emissions The principal products of combustion are carbon dioxide and water. Sulfur oxides, nitrogen oxides, and hydrogen chloride are discussed above. Carbon monoxide and other trace gases also are produced. These are controlled to meet current emission standards and are no higher than the similar emissions from coal and oil plants. In addition, particulates are discharged from the stack. Compared to the combustion of coal and No. 6 oil, particulates are controlled to the same or more stringent standards, particularly for municipal waste. GLOSSARY Accessible Resource: A subset of the total resource base that can be acquired by current technology or technology that will be available in the near future. Anaerobic Digestion: A biological reaction that occurs in the absence of air. Azeotrope: A mixture of two liquids such that the vapor at the boiling point has the same composition as the liquid, and thus the boiling point does not change as the evaporation proceeds. Biogas: The gas produced by anaerobic digestion. Stoichiometrically it is 50% methane and 50% carbon dioxide. Biomass: Any natural organic material or waste of recent origin excluding fossil fuels. Carbohydrate: A compound made up of carbon, hydrogen, and oxygen with the hydrogen and oxygen in the atomic ratio of 2 to 1. Cellulose: One of the four main components of plant matter. It is a long, crystalline, straight-chain carbohydrate primarily made 486 Biomass Energy up of glucose. The chemical formula of cellulose is complex but may be simplified to (C6(H2O)5)tt. Coppice: The regrowth of a tree from the stump. Disaccharide: Two monosaccharides bound together with an 0H~ radical removed from one and a H + radical from the other. The most common is sucrose which has one glucose and one fructose molecule combined. The chemical formula is C^H^On. Enzyme: A natural biochemical catalyst. They are responsible for specific chemical reactions and are not consumed during the reactions. Extractive: One of the four main components of plant matter. They are various chemicals and components such as sap. They carry nutrients throughout the tree, and provide disease and pest resistance. Of the extractives, sugar and starch are the main components of interest for biochemical conversion processes and resin is the main component of interest for thermal conversion processes. Hemicellulose: One of the four main components of plant matter. It is a long, straight chain polysaccharide primarily composed of xylose (a five-carbon sugar). The chemical formula of hemicellulose is complex but may be simplified to (C5(H2O)4)W. Hexose: A monosaccharide with six carbon atoms. Hydrolysis: The process of splitting an atom by the addition of water with one hydrogen molecule incorporated into one fragment and the hydroxyl radical into the other. Lignin: One of the four principal components of plant matter. It is an amorphous polymer primarily composed of phenol molecules. Monosaccharide: A simple carbohydrate. The two most common are glucose and fructose, both of which have the formula C6H12O6; however, the atoms are organized in different ways and they have different chemical characteristics. The corresponding pentose is xylose with the formula C5H10O5. Pentose: A monosaccharide with five carbon atoms. Photosynthesis: The system by which plants use sunlight to convert C02 and water to plant matter and oxygen. Although the actual chemical composition of plant matter is very complex, frequently it is simplified as C^(H2O)r The values of x and y vary, al- though typical values are x=l, y=0.72 to 1. The general formula for photosynthesis is rcCO2+JcH2O+sunlight - Cn(H2O)x+nO2. Polysaccharide: More than two linked monosaccharide molecules. Quad: A measure of energy equal to 1 quadrillion Btu or 1.054 EJ. Reserve: That subset of the accessible resource that can be economically and legally used to yield useful energy. Resource Measurement Categories: Standardized definitions from the mineral industry to describe a resource. Further reading on the categories and quantities of renewable resources can be found from the Meridian Corporation (1989). Short Rotation Hardwood: A fastgrowing wood species that coppices and is grown on excess agricultural land. Short rotation wood was developed for energy but is being commercialized as a fiber and energy resource. Starch: A folded chain of monosaccharides with the general formula of (C^H^O)^!)),,. Sugar: A simple carbohydrate. There are three basic classes: monosaccharide, disaccharide, and polysaccharide. Total Resource Base: The combination of undiscovered and identified, subeconomic and economic concentrations of a resource. List of Works Cited Domalski, E. S., Jobe, T. L., Milne, T. A. (1987), Thermodynamic Data for Biomass Materials and Waste Components, New York: American Society of Mechanical Engineers. Energy Information Administration (1988), Annual Energy Review, Washington, DC: U.S. EIA, Report No. DOE/EIA-0384(88). Energy Information Administration (1989), Es- timates of Biofuels Consumption in the United States During 1987, Washington, DC: U.S. EIA , Report No. CNEAF/NAFD 89-03. Energy Research and Development Administration (1977), Waste-to-Energy Systems: An Overview, Washington, DC: U.S. Department of Energy, Report No. CONS/2103-6. National Technical Information Service, Springfield, VA. Green, A. E. S. (1989), Overview, Summary and Afterword, Vol. 7. New York: American Society of Mechanical Engineers. National Wood Energy Association (1988), State Biomass Statistical Summary, Arlington, VA: National Wood Energy Assoc. Biomass Energy Solar Energy Research Institute (1981), Ethanol Fuel: Use, Production and Economics, Golden, CO: Solar Energy Research Institute. United States Environmental Protection Agency (1990), Facts: Ethanol as a Fuel, Washington, DC: U.S. GPO. Further Reading Diaz, L. F., Savage, G. M., Golueke, C. G. (1982), Resource Recovery from Municipal Solid Wastes, Boca Raton, FL: CRC. Hasselriis, F. (1984), Refuse Derived Fuel Processing, Boston, MA: Butterworth. Meridian Corporation (1989), Characterization of Energy Resources and Reserves, Alexandria, VA: Meridian Corp. Neenan, B., Feinberg, D., Hill, A., Mclntosh, R., Terry, K. (1986), Fuels from Microalgae: Technology Status, Potential, and Research Requirements, Golden, CO: Solar Energy Research Institute. Plumley, A. (1977), Present Status and Research Needs in Energy Recovery from Wastes, Proceedings on the 1976 Conference, New York: American Soci- 487 ety of Mechanical Engineers. Reed, T. R., Das, A. (1988), Handbook of Downdraft Gasifier Engine Systems, Golden, CO: Solar Energy Research Institute. Robinson, W. D. (1986), The Solid Waste Handbook, A Practical Guide, New York: Wiley. Solar Energy Research Institute (1989), Biofuels Technical Information Guide, Golden, CO: Solar Energy Research Institute. National Research Council (1977), Methane from Human, Animal and Cellulosic Wastes, Washington, DC: National Academy of Sciences. United States Department of Energy (1982), A Review of the Comparative Energy Use in Materials Potentially Recoverable from Municipal Solid Waste, Washington, DC: U.S. GPO, Report No. CE20167-12. Walter, D. K., Murphy, W. E., Richards, R. (1989), Biofuels and Greenhouse Mitigation, Vol. 7. New York: American Society of Mechanical Engineers. Watson, J. D., Tooze, J., Kurtz, D. T. (1983), Recombinant DNA, a Short Course, New York: Free-


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Planning your Orlando Theme Park Holiday? Stay on Florida's Space Coast!
The Orlando theme parks spare no expense in decking the halls to bring holiday cheer to the millions of vacationers who flood the parks in December. With premier Christmas events and tons of exciting attractions to be enjoyed at Walt Disney World, Universal Orlando and SeaWorld, the theme park capital of the world is always full of energy and excitement during the holiday season.
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