| Energy research in developing countries |
|Volume 8: Bioenergy|
This paper reviews a number of different sources and processes for producing biomass energy. The important link between biomass energy and agriculture is emphasized. There is justification for a less developed country (LDC) to use agriculture to produce fuel if this stimulates the agricultural economy. A detailed description of ethanol production from carbohydrates and cellulose is provided, and anaerobic digestion is explained. Current research efforts to find plants suitable for the extraction of vegetable oil and hydrocarbon fuels from biomass are examined. Environmental, sociocultural, and economic considerations associated with different bioenergy sources and production methods are highlighted.
Effective use of biomass for energy in LDCs is pressing because of the increasing demand for energy to fuel economic growth and the decreasing supplies of fossil fuels. There is an inextricable link between biomass energy and agriculture because the agricultural sector is a major source of supply. In LDCs, biomass can be used for energy production if it directly stimulates the agricultural economy. This stimulation may help to reduce migration to urban areas and may provide the energy needed to improve agricultural productivity. Whether current productivity is increased or new lands are brought under cultivation, major investments will be required. In Brazil, government incentives have induced the expansion of sugarcane agriculture at the expense of food crops. Therefore, food crops may be dislocated to marginal lands, and this may cause an increase in food prices. A solution to the food-fuel issue can be the cultivation of energy crops that do not compete with food sources. These crops include
· Multipurpose crops with both food and fuel potential (for example, maize, soybean, cotton, pines, cane, sorghum, and cassava),
· Cash crops grown between the harvesting and sowing of the main crop (for example, fodder, kale, barley, and short-rotation grasses),
· Marshland crops (for example, cattails, reeds, water hyacinths, and marine crops such as giant kelp and other seaweeds), and
· Marginal-land crops (for example, bracken).
Environmental, Technical, and Economic Impact. of Biomass Production
The most negative environmental impacts of biomass production are erosion and water pollution. The inherent problems of monocultures may also appear if only energy crops are raised. Deforestation to establish agricultural land results in soil erosion, the loss of habitats, and the disruption of watersheds.
However, alternative biomass technology exists that could have positive impacts. The expansion of biogas units has raised health standards in China. These units also function as a system of waste disposal for pig manure in the Philippines. Domestic wastes and sewage provide substrates for anaerobic digestion and can be used for ethanol fermentation. Water hyacinths can also be used for water purification. Finally, ethanol production and biomethanation dispose of wastes that would otherwise be dumped or burned.
Energy supplies are beneficial only to the extent that their use is practical and economical. When developing alternative technologies for biomass conversion, one needs to consider several conditions.
Equipment should be appropriate for local conditions and users. A supportive institutional infrastructure that includes training and maintenance facilities should be established. Local participation should be included to assist technology transfer.
The evaluation of options for fuel supply involves a range of economic, sociocultural, and political considerations. Traditional philosophies of Western economic development continue to be applied to the evaluation of biomass technologies despite the fact that cost-benefit studies should include environmental, social, and political concerns. Biomass projects should be evaluated on an individual basis (in the same way as coal and gas projects).
The development of alternative energy supplies is not a goal in itself, but a means of achieving a better standard of living. Therefore, a project must be investigated for its social impact. Biomass systems may exacerbate the social imbalances of LDCs when related subsidies favour the rich. Large-scale projects may eliminate small farms (the case in Brazil). In addition, changes in energy production may limit the availability of traditional fuels that are available for free collection and use by the poorest segment of the population.
Other factors to consider in the establishment of a successful bioenergy program are the supply of reliable labour, the integration of women in the program, the impact of cultural customs and taboos, and patterns of settlement. Ultimately, the decision to implement a biomass project is a political one that requires the support of the government.
Production of Ethanol from Carbohydrates
Because the market price of feedstock is the most important factor in the economics of ethanol production, the discovery of new crop varieties and the development of hybrids are integral to energy research. For example, a high-yield variety of sweet sorghum is being examined as an ethanol feedstock. Improvements in cultivation techniques and the discovery of nonconventional crops that grow well on marginal lands will help to reduce the cost of feedstocks. Wastes can also be used as feedstocks, but their use is limited because of storage problems and the lack of year-round availability.
Pretreatment is usually required for ethanol production whether the feedstock contains sugar or starch. Traditionally, mills are used to pretreat the sugar feedstocks to release the juices that contain the sugar. Developments that eliminate the need for roller mills make small-scale operations more competitive (for example, the injection of low-pressure steam into coarsely chopped feedstock, and solid-phase fermentation).
The pretreatment of feedstocks that contain starch requires the reduction of starch to glucose partly by milling and grinding and partly by chemical means. In chemical pretreatment, enzymes are used to liquefy the starch and to carry out the saccharification. Research is being undertaken to discover faster acting thermophilic amylases for liquefaction. A simultaneous saccharification and fermentation process, which lowers costs, has been developed by a Japanese company.
After pretreatment, the mash is diluted to a suitable sugar concentration for fermentation. Although yeasts possess many of the characteristics of a good fermenting organism, bacteria may be superior because they have shorter doubling times and are easier to handle and manipulate. Research is directed toward the discovery of new microorganisms that can withstand high temperatures and high alcohol concentrations and can ferment at high rates.
Ethanol is recovered from the aqueous solution by distillation, which accounts for over half of the total energy consumed by the distillery. Improvements in the energy efficiency of distillation could reduce costs. Alternative methods of alcohol separation (for example, membrane isolation, solvent extraction, and reverse osmosis) have not been tested on an industrial scale. If essentially anhydrous ethanol is required, the 4% water remaining from simple distillation can be removed by distillation with a water-immiscible solvent.
Ethanol production yields a number of marketable by-products. Their value is important in assessments of the cost of production. Bagasse from sugarcane and sorghum allows their fermentation to be nearly self-sufficient in energy supply. Excess bagasse can be used for crop drying, electricity generation, biogas or ethanol production, or conversion to paper or animal feed. By-products of the fermentation stage include gaseous carbon dioxide that is used in the beverage industry and excess yeast that can be recycled or sold. Stillage-the residue remaining after distillation-has important uses in animal feedstocks, fertilizers, and biomethanation. The proper disposal of stillage represents the biggest environmental problem associated with ethanol production. Other environmental problems include personal hazards and air pollution from the use of alcohol fuels. The data do not clearly show whether gasoline or ethanol produces higher emissions of polluting chemicals.
The establishment of an industry to produce ethanol is often justified on the premise of national self-sufficiency and the creation of new jobs. Smaller units may be more relevant to poorer countries because they make better use of unskilled labour. Nonetheless, even small units require large investments by developing world standards, and it is unlikely that they can be built by individual farmers. Community distilleries may require investment and market guarantees to function. LDCs have experienced difficulties in the establishment of large, successful facilities.
Ethanol production is never economical in purely monetary terms because of the high cost of the feedstock, which can be used as a food source or an export. Therefore, government subsidization is necessary if ethanol is to compete with gasoline.
Ethanol Production from Cellulose
Pretreatment is necessary to make the cellulose component more suitable for enzyme hydrolysis. Mechanical reduction of size is usually a preliminary pretreatment step, and it is generally followed by either acid pretreatment or enzymatic delignification. Delignification is preferred because it does not decompose the sugars. Alternative pretreatment methods are solvent extraction and steam explosion.
After pretreatment, hydrolysis is undertaken with acids, alkalis, or enzymes. Because glucose inhibits the activity of most cellulase enzymes, it should be removed from the reaction medium. However, some microorganisms or enzyme systems can simultaneously hydroIyze cellulose and ferment glucose to ethanol, which eliminates the difficulties associated with glucose.
The by-products of cellulose-derived ethanol also influence the cost of production. If hemicellulose sugars are to be exploited, microorganisms must be found to convert them to liquid fuels. Hemicellulose can also be used as the source of other industrial chemicals (for example, furfural). Lignin, the other major byproduct, can be used as a source of phenols and benzene and as a source of fuel. Despite the value of these by-products, ethanol derived from cellulose is not competitive with alcohol derived from sugar or starch.
Biogas derived from anaerobic digestion is more efficient as a cooking fuel than kerosene or solid biomass, and it can be substituted for kerosene as a heat and light source. Anaerobic digestion also produces solid sludge, which can be used as fertilizer or as animal and fish feed. All types of biomass can be converted, at least partially, by anaerobic digestion. Recent research on feedstocks has included crop wastes, agro-industry by-products and wastes, human waste, domestic waste and refuse, industrial effluents, aquatic vegetation, and terrestrial crops. Digestion kinetics can be improved by improving and isolating new anaerobic microorganisms, increasing microorganism concentration by recycling digester effluent or enriching the level of microorganisms with external cultures, and increasing the feedstock concentration.
There are many types of digesters (vertical, cylindrical, fixed-dome, plastic, and simple ones made from oil drums). The choice of digester depends on budget, local conditions, and know-how. Temperature control is an important aspect of digester operation, and subterranean digesters are better insulated against climatic variations. In the developing world, expensive individual digesters will not benefit the poor unless there are government subsidies. Community digesters can be economically feasible but have seldom been successful.
Vegetable Oil and Hydrocarbon Fuels from Biomass
Plants that photosynthetically reduce carbon to hydrocarbons are potential sources of liquid fuels for petroleum replacement. Euphorbia lathyris, which is widespread on marginal lands throughout the world, was targeted as a likely candidate for exploitation, and many studies on its feasibility as a hydrocarbon producer have been undertaken. The results have been disappointing, but other plants may be more suitable. Systematic surveys of indigenous species have been carried out worldwide. The concept of the multipurpose crop that could supply both fuel and raw materials spawned the analysis of over 1000 wild North American plants to determine their whole-plant oil, hydrocarbon, phenolic, and bagasse content.
Vegetable oils may be economically preferable to other biomass-derived fuels because the oils can be easily extracted from the plant parts. Seeds must be cleaned, dried, and dehusked before they are placed in the expeller. Vegetable oils can be used as an additive to diesel fuel, admixed with gasoline, or cracked into high-grade gasoline. Because of their lower volatility, they are being developed for use in compression-ignition engines. For this purpose, their viscosity must be reduced either by blending with fatty acids or by esterification to produce methyl or ethyl esters, which are more volatile than the parent oils. Indigenous plants have also been subjected to worldwide screening for their seed-oil content.