A biomass fuel is an energy source derived from living organisms. Most commonly it is plant residue, harvested, dried and burned, or further processed into solid, liquid, or gaseous fuels. The most familiar and widely used biomass fuel is wood. Agricultural waste, including materials such as the cereal straw, seed hulls, corn stalks and cobs, is also a significant source. Native shrubs and herbaceous plants are potential sources. Animal waste , although much less abundant overall, is a bountiful source in some areas.
Wood accounted for 25% of all energy used in the United States at the beginning of this century. With increased use of fossil fuels , its significance rapidly declined. By 1976, only 1–2% of United States energy was supplied by wood, and burning of tree wastes by the forest products industry accounted for most of it. Although the same trend has been evident in all industrialized countries, the decline has not been as dramatic everywhere. Sweden, for instance, still meets 8% of its energy needs with wood, and Finland, 15%.
Globally, it is estimated that biomass supplies about 6 or 7% of total energy, and it continues to be a very important energy source for many developing countries. In the last 15–20 years, interest in biomass has greatly increased even in countries where its use has drastically declined. In the United States rising fuel prices led to a large increase in the use of wood-burning stoves and furnaces for space heating. Impending fossil fuel shortages have greatly increased research on its use in the United States and elsewhere. Because biomass is a potentially renewable resource, it is recognized as a possible replacement of petroleum and natural gas .
Historically, burning has been the primary mode for using biomass, but because of its large water content it must be dried to burn effectively. In the field, the energy of the sun may be all that is needed to sufficiently lower its water level. When this is not sufficient, another energy source may be needed.
Biomass is not as concentrated an energy source as most fossil fuels even when it is thoroughly dry. Its density may be increased by milling and compressing dried residues. The resulting briquettes or pellets are also easier to handle, store, and transport. Compression has been used with a variety of materials including crop residues, herbaceous native plant material, sawdust, and other forest wastes.
Solid fuels are not as convenient or versatile as liquids or gases, and this is a drawback to the direct use of biomass. Fortunately, a number of techniques are known for converting it to liquid or gaseous forms.
Partial combustion is one method. In this procedure, biomass is burned in an environment with restricted oxygen. Carbon monoxide and hydrogen are formed instead of carbon dioxide and water. This mixture is called synthetic gas or "syngas." It can serve as fuel although its energy content is lower than natural gas (methane ). Syngas may also be converted to methanol , a one carbon-alcohol that can be used as a transportation fuel. Because methanol is a liquid, it is easy to store and transport.
Anaerobic digestion is another method for forming gases from biomass. It uses microorganisms ,in the absence of oxygen, to convert organic materials to methane. This method is particularly suitable for animal and human waste. Animal feedlots faced with disposal problems may install microbial gasifiers to convert waste to gaseous fuel used to heat farm buildings or generate electricity.
For materials rich in starch and sugar, fermentation is an attractive alternative. Through acid hydrolysis or enzymatic digestion, starch can be extracted and converted to sugars. Sugars can be fermented to produce ethanol ,a liquid biofuel with many potential uses.
Cellulose is the single most important component of plant biomass. Like starch, it is made of linked sugar components that may be easily fermented when separated from the cellulose polymer. The complex structure of cellulose makes separation difficult, but enzymatic means are being developed to do so. Perfection of this technology will create a large potential for ethanol production using plant materials that are not human foods.
The efficiency with which biomass may be converted to ethanol or other convenient liquid or gaseous fuels is a major concern. Conversion generally requires appreciable energy. If an excessive amount of expensive fuel is used in the process, costs may be prohibitive. Corn (Zea mays ) has been a particular focus of efficiency studies. Inputs for the corn system include energy for production and application of fertilizer and pesticide , tractor fuel, on-farm electricity, etc., as well as those more directly related to fermentation. A recent estimate puts the industry average for energy output at 133% of that needed for production and processing. This net energy gain of 33% includes credit for co-products such as corn oil and protein feed as well as the energy value of ethanol. The most efficient production and conversion systems are estimated to have a net energy gain of 87%. Although it is too soon to make an accurate assessment of the net energy gain for cellulose-based ethanol production, it has been estimated that a net energy gain of 145% is possible.
Biomass-derived gaseous and liquid fuels share many of the same characteristics as their fossil fuel counterparts. Once formed, they can be substituted in whole or in part for petroleum-derived products. Gasohol , a mixture of 10% ethanol in gasoline , is an example. Ethanol contains about 35% oxygen, much more than gasoline, and a gallon contains only 68% of the energy found in a gallon of gasoline. For this reason, motorists may notice a slight reduction in gas mileage when burning gasohol. However, automobiles burning mixtures of ethanol and gasoline have a lower exhaust temperature. This results in reduced toxic emissions, one reason that clean air advocates often favor gasohol use in urban areas.
Biomass is called as a renewable resource since green plants are essentially solar collectors that capture and store sunlight in the form of chemical energy. Its renewability assumes that source plants are grown under conditions where yields are sustainable over long periods of time. Obviously, this is not always the case, and care must be taken to insure that growing conditions are not degraded during biomass production.
A number of studies have attempted to estimate the global potential of biomass energy. Although the amount of sunlight reaching the earth's surface is substantial, less than a tenth of a percent of the total is actually captured and stored by plants. About half of it is reflected back to space. The rest serves to maintain global temperatures at life-sustaining levels. Other factors that contribute to the small fraction of the sun's energy that plants store include Antarctic and Arctic zones where little photosynthesis occurs, cold winters in temperate belts when plant growth is impossible, and lack of adequate water in arid regions. The global total net production of biomass energy has been estimated at 100 million megawatts per year per year. Forests and woodlands account for about 40% of the total, and oceans about 35%. Approximately 1% of all biomass is used as food by humans and other animals.
Soil requires some organic content to preserve structure and fertility. The amount required varies widely depending on climate and soil type. In tropical rain forests, for instance, most of the nutrients are found in living and decaying vegetation. In the interests of preserving photosynthetic potential, it is probably inadvisable to remove much if any organic matter from the soil. Likewise, in sandy soils, organic matter is needed to maintain fertility and increase water retention. Considering all the constraints on biomass harvesting, it has been estimated that about six million MWyr/yr of biomass are available for energy use. This represents about 60% of human society's total energy use and assumes that the planet is converted into a global garden with a carefully managed "photosphere."
Although biomass fuel potential is limited, it provides a basis for significantly reducing society's dependence on non-renewable reserves. Its potential is seriously diminished by factors that degrade growing conditions either globally or regionally. Thus, the impact of factors like global warming and acid rain must be taken into account to assess how well that potential might eventually be realized. It is in this context that one of the most important aspects of biomass fuel should be noted. Growing plants remove carbon dioxide from the atmosphere that is released back to the atmosphere when biomass fuels are used. Thus the overall concentration of atmospheric carbon dioxide should not change, and global warming should not result. Another environmental advantage arises from the fact that biomass contains much less sulfur than most fossil fuels. As a consequence, biomass fuels should reduce the impact of acid rain.
[Douglas C. Pratt ]
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