| Energy research in developing countries |
|Volume 8: Bioenergy|
J. Lamptey, M. Moo-Young, and H.F: Sullivan
This paper discusses processes for biomass bioconversion and thermochemical conversion. The production of ethanol from lignocellulosics is described in some detail, whereas butanol-acetone and hydrogen production are reviewed briefly. Thermochemical processes to convert lignocellulosics to energy (for example, methanol from synthetic wood gas) are described. Other renewable energy sources (solar, ocean, and wind energy) are discussed briefly.
Production of Ethanol-Ethanol has a number of commercial applications as an energy source and as an industrial solvent and chemical. Chemical synthesis and fermentation are the two major techniques used to produce ethanol. Chemical synthesis is the catalytic hydrolysis of ethylene derived from petroleum. For fermentation, which can be carried out through different conversion processes, the four categories of feedstocks are sugar-containing materials (which are the most expensive to obtain), starch (including corn, potatoes, and cassava), lignocellulose (which is the major renewable carbohydrate source in the world), and urban and industrial wastes. There are problems associated with the collection and conversion of each feedstock to produce a useable substrate.
The structural features of cellulosic biomass make pretreatment necessary. This process, which removes lignin and increases the surface area available to enzymes, promotes hydrolysis and enhances the rate and extent of cellulose saccharification. Pretreatment techniques are classified as physical, chemical, biological, or combined. One physical technique is milling, which reduces crystallinity and the degree of polymerization and increases bulk density. High power requirements make this technique costly. Chemical pretreatment involves the use of strong acids, bases, or agents that swell or dissolve the cellulose. These expensive chemicals are corrosive and toxic, which makes this technique commercially unattractive. Steam explosion, in which cellulose materials are saturated with water under high pressure and temperature, is another physical technique. When pressure is released, the wood fibres separate and the acetic acid that is released catalyzes the hydrolysis of the hemicellulose.
The three principle conversion processes for lignocellulose are acid hydrolysis, enzymatic hydrolysis, and direct microbial conversion. Acid hydrolysis, which is suitable for any cellulose feedstock (but is most often used with wood), can be carried out with dilute or concentrated acids. Because the use of concentrated acids is expensive, most development work involves dilute acids. An increase in acid concentration or reaction temperature increases the production of glucose.
Lignin, a by-product of acid hydrolysis, is likely only useful as fuel. Although acid hydrolysis has many advantages (for example, the ease of use of wood as a feedstock, the short reaction time, and the low cost of the acid catalyst), its limitations include lower potential yield and sugar concentration, the production of degradation products that are toxic to fermentation yeast, and severe operating conditions that produce corrosion.
Unlike acid hydrolysis of cellulosic materials, enzymatic hydrolysis is not yet used commercially. Because the enzymes are reaction specific, this process could potentially produce higher yields of sugars under mild operating conditions that do not require corrosion-resistant equipment. The major disadvantage is the requirement for an aseptic environment for enzyme production because hydrolysis is susceptible to microbial contamination.
Microbial conversion is also not used commercially, but it has theoretically the highest yields of all techniques. This process seeks to convert cellulose and produce alcohol in a single step using one or two microbial organisms. The conversion is not easily applied because no efficient cellulosic microbe that produces ethanol has been found. Microbes have a low tolerance for ethanol and tend to produce unwanted by-products.
Fermentation systems for ethanol production require substantial cost reductions. To minimize costs, a balance must be struck between three goals for ethanol fermentation (high substrate utilization, high ethanol productivity, and high ethanol concentration). Research is currently under way to increase the rate of fermentation and to decrease costs. Rapid fermentation can be achieved by maintaining a high yeast population and removing rate-inhibitory ethanol from the fermenter as quickly as it is formed. Researchers have used both adsorption and entrapment techniques in laboratory experiments to immobilize yeast and to increase the rate of fermentation. Industrial applications have not yet been developed. Although there seems to be a promising future for ethanol production, the cost of the glucose substrate remains the main bottleneck for fermentation.
Production of Butanol-Acetone and Hydrogen- Biomass feedstocks can also be used to produce both butanol-acetone mixtures and hydrogen. Fermentation of products containing starch and sugar can produce butanol-acetone mixtures. However, research in improved production technology is required to make the process commercially attractive.
Excluding gasification, hydrogen can be produced from biomass feedstocks by
· Photochemical methods (including a technique to simulate plant photosynthesis in vitro and a technique that uses water solutions of biomass components in the presence of catalysts),
· Biological methods (the use of anaerobic bacteria in dark reactions to ferment the biomass),
· Photobiological methods (the use of photosynthetic algae or bacteria under anaerobic conditions in the presence of light), and
· Biomass-water electrolysis (which involves anode depolarization with a water-soluble, oxidizable material such as glucose).
These processes have not been demonstrated on a large scale, and they may be made obsolete by the direct photocatalytic decomposition of water to hydrogen and oxygen.
Biomass feedstocks offer a number of advantages for thermochemical conversion processes (for example, high volatility and char reactivity). Drawbacks to this process include the decentralized nature of the resource and the high moisture content of the fuel. Pretreatment processes include cleaning, drying, and size reduction. Drying is crucial.
Thermochemical conversion processes include
· Direct combustion (traditional biomass includes wood and bagasse, but recent projects involve municipal solid waste, hog fuel, and pelleted wood fuels),
· Gasification to produce low- or intermediate-energy fuel gas or synthesis gas (gasification is considered a commercial or near-commercial technology and can, under certain conditions, be used as a substitute for fossil fuels),
· Direct liquefaction to produce heavy oils or, with upgrading, lighter boiling liquid products (liquefaction remains one of the least commercially attractive conversion processes because of its high capital and energy costs), and
· Pyrolysis to produce a mixture of pyrolysis oils, fuel gases, and char (rapid pyrolysis using rapid heating rates has emerged as a promising alternative for producing liquid fuels).
Production of Methanol -Although natural gas is the preferred carbon source to make synthesis gas for the production of methanol, it is possible to generate the synthesis gas from biomass. The two broad categories of wood conversion technologies for methanol production are oxygen gasifiers and pyrolytic gasifiers. The gasification of wood yields hydrogen, carbon monoxide, carbon dioxide, and other gases. The raw gas is purified and processed to extract a 2: I ratio of hydrogen - carbon monoxide. The synthesis gas is then compressed and passed through a methanol synthesis reactor. The major disadvantages of this process are the size of plant and the amount of raw material required.
Other Energy Sources
Other sources of renewable energy are solar energy, ocean energy, and wind energy. The three categories of direct solar energy are solar heating and cooling, solar thermal, and photovoltaics. The most promising application of solar energy may be in heating domestic water and buildings. Few industrial applications are currently demonstrated or operational. Research in solar photovoltaics is focusing on cheaper fabrication methods and materials for photovoltaic cells. Eventually, photovoltaics may be used for central power generation in conjunction with a conventional backup system. The diffusion of solar technology depends on cost, the solution of technical problems, and the rate of adoption of the technology.
Research is currently being undertaken in ocean energy to exploit the incident solar energy absorbed by surface waters and the energy available in ocean waves. Wind energy is another research area. In locations with strong winds and high fuel costs, wind turbines are already economic.
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.
Norman L. Brown and Prakasam B.S. Tata
This paper covers biomethanation at the household, community, and industrial levels. Biogas technology in the developing world is described from a historical perspective, and the properties, uses, substrates, and production technologies for biogas are discussed. The microbiology of biomethanation is explained. The biomethanation process and the factors that influence performance are described. The paper also presents lessons learned from experiences with biogas installations and makes recommendations for further research.
In China and India, biogas technology has been used for more than 50 years. Interest in biogas in China was revived in the 1970s, and biogas has been successfully used to provide cooking fuel and to conserve fertilizer and improve public health. This pioneering work stimulated interest in other countries (for example, Korea, the Philippines, and Taiwan). Although most of the activity has taken place in Asia, a number of African and Latin American countries are currently engaging in biomethanation projects.
Biogas is made up of methane and carbon dioxide in variable ratios (generally SO-70% methane and 30-50% carbon dioxide). The heating value of biogas is directly proportional to its methane content. Biogas can be used as a cooking fuel and in any gas-burning appliance that requires low-pressure gas (for example, lamps and refrigerators). If the removal of carbon dioxide is feasible and pressure containers are available, the remaining methane can be used as a transportation fuel. The residue from biomethanation has been used traditionally as a soil conditioner or fertilizer because the process produces chemical forms of minerals that are more soluble than the original forms. Because of evaporation problems, residue to be used as fertilizer should either be stored in a closed container or used immediately. Residue is also used as a feed supplement.
Any biomass can be considered as a potential source for biomethanation, but materials such as lignin, bark, and feathers, which are not easily degraded by microorganisms, are not desirable feedstocks unless they are pretreated. The most common feedstocks are crop residues, manure, and human excrement, although other feedstocks (for example, industrial wastes and marine and aquatic biomass) are also used. Generally, the feedstocks have competing uses as food, fuel, fibre, fodder, or fertilizer. These uses must be evaluated before investing in biomethanation. Information on the potential availability of crop residues, manure, and industrial wastes in developing countries is sparse, and this makes evaluation more difficult.
Biomethanation is accomplished by four interdependent groups of bacteria under anaerobic conditions. A group of hydrolytic and fermentative bacteria produces simpler organic compounds (for example, sugars, alcohols, fatty acids, hydrogen, and carbon dioxide). Acetogenic bacteria act on these products to produce methane and carbon dioxide. If the organic load received by the digester is excessive, too much hydrogen is formed and the acetogenic bacteria may be "washed out" before the methane bacteria have had a chance to use the excess hydrogen. This situation results in a "stuck reactor." The rate of digestion in biomethanation is related to the nature of the substrate, temperature, loading rate, and acidity.
The two broad categories of biomethanation digesters are
· Suspended-growth reactors, in which biological solids are suspended in the contents of the digester (these reactors can be batch or continuous), and
· Attached-growth reactors, in which the biological solids attach themselves to surfaces such as rock, plastic, or ceramic media (this technology is quite recent and not significantly disseminated in developing countries).
Systems to collect gas range from a simple plastic delivery tube for a family digester to the complex systems of large installations, which may include gas scrubbers and bottling equipment. In developing countries, small-scale systems usually consist of a gas holder, condensate trap, and flame arrester. Fixed-dome digesters also include a manometer and safety valve. Condensate traps remove the moisture carried by the gas stream, and they should be drained when they become full. Flame arresters prevent the flame of an appliance from travelling back through the pipe to the gas holder and causing an explosion. Manometers prevent damage caused by pressure buildup in fixed-dome digesters. Piping should be made of plastic or galvanized iron. In LDCs, biogas is not generally purified to enrich its methane content, but various purification methods (for example, scrubbing and physical and chemical absorption) are available to enhance the quality of the biogas. The bottling of biogas for use as a transportation fuel is not economically feasible in the developing world.
Collection, Storage, and Pretreatment of Substrates- The success of biomethanation depends on a steady supply of appropriate substrates. Collection and storage practices vary with location and depend on economic and sociocultural differences and on the nature of the substrate (solid, semisolid, or liquid). Dung is usually collected manually and transported to a storage pit near the digester.
Dry materials (for example, leaves and crop residues) can be transported to the digester either manually or by animal carts. Wet materials (for example, green leaves and aquatic weeds) can be shredded, dried, and stored. Prolonged storage of putrescible organic matter results in a loss of methane and valuable nutrients.
Pretreatment of substrates that are not easily biodegradable can be done using physical, chemical, and biological methods. Size reduction, steam explosion, and freeze explosion are all suitable pretreatment techniques for cellulosic materials. Heat treatment of biomass increases its digestibility and results in a higher methane content of the final gas. This process can use energy generated from the digester itself. Chemical pretreatment includes acid hydrolysis, alkaline hydrolysis, and the application of sulfur dioxide. Biological methods include fungi pretreatment to degrade lignin and enzymatic treatment of cellulose to promote saccharification.
Integrated Biomethanation Systems -Community-size integrated biomethanation systems have advantages over family-size units:
· Gas production is optimized by incorporating into the digester a good mixing and heating system that can be properly controlled by trained personnel.
· Resource recovery can be enhanced by the use of effluents as feed supplements.
· Pathogens can be controlled and the environment can be protected by the proper operation and maintenance of the system.
Biomethanation systems also have social and economic benefits:
· Diseases are reduced because of the use of clean fuel.
· Deforestation pressure is reduced.
· Electricity is made available to rural areas to improve living standards.
· Additional protein is made available because of the use of algae as a feed or feed supplement.
· Employment opportunities are produced.
Factors Influencing Performance -The rate and extent of biomethanation are affected by the nature of the substrate, the bacterial environment, and the design of the biomethanation system. A favourable bacterial environment depends primarily on the need for anaerobic conditions and acidity within a range of pH 6.7-7.6, temperature between 50° and 60°C, proper nutrients, and adequate mixing of the slurry.
Lesson. from Experience
It is difficult to learn from the experience of others in biomethanation because experience has largely been at a practical field level, where technological issues cannot be separated from social and economic influences. Rising concern for environmental, political, and social impact also makes traditional economic analysis, by which the technology has been evaluated in the past, inappropriate or at the least open to question. Benefit from the experience of others continues to be hampered by
· The lack of consistent technical, economic, and social criteria by which to monitor and evaluate installations, and
· The lack of consistent cost-benefit methods by which to evaluate the full social costs and benefits.
Suggestions for Further Research
Several areas that warrant further research are
· Efforts to reduce the cost of digesters,
· Investigations of the role of women in the dissemination of biogas technology,
· Controlled studies to determine whether there will be a net benefit if a portion of the biogas is used for heating and mixing,
· Experimentation with less common feedstocks,
· Testing of procedures for uniform reporting of basic data,
· Application of recently developed digester designs in LDCs,
· Comparison of the fertilizer value of slurries obtained from various feedstocks,
· The development of inexpensive and efficient biogas appliances,
· The impact of these systems on public health, and
· Evaluations of the socioeconomic factors that influence the success of various projects.
The Research Association for Petroleum Alternatives Development (RAPAD) has concentrated its efforts on ethanol derived from cellulose. This paper reviews the research accomplishments of RAPAD, describes efforts elsewhere in methane fermentation and microalgae technology, and discusses the utilization of alternative energy systems in Japan. Conversion technologies and research trends are also reviewed.
RAPAD has directed its research toward the production of ethanol from cellulose and concentrated on pretreatment, cellulase enzyme activity, saccharification, and concentration. The primary aim of pretreatment is to break down the cellulose by chemical or physical means to make it more susceptible to enzymatic hydrolysis. RAPAD results have demonstrated that a combination of chemical treatment and explosion can yield high rates of enzymatic conversion. RAPAD has also attempted to screen and develop strong cellulase-producing microorganisms to enhance enzymatic hydrolysis. Mutants of Trichoderma reesei have been selected for further research. In addition, four new cellulase producers, including moulds and bacteria, have also been identified for further research.
To improve enzyme saccharification, RAPAD seeks to produce higher yields of glucose syrup in a shorter time. A continuous saccharification system that uses a low substrate concentration to obtain a high ratio of glucose conversion has been developed.
The research objectives for the fermentation process are to improve the technology for immobilizing yeast cells and to develop an efficient and highly productive fermentation system. RAPAD has developed an alginate-entrapped pilot system to immobilize yeast cells. In operation, the system has worked well. Because of the importance of the fermenter to the efficiency of ethanol conversion, RAPAD has examined fermentation systems and selected a fixedbed, parallel-flow reactor that is used in conjunction with sheets of immobilized yeast cells. RAPAD was able to solve the problems of sludge adhesion to the yeast sheets and of contamination during fermentation.
Japanese research has shown that an immobilized methane-fermentation system improves the efficiency of methane fermentation by reducing the retention time from 16.9 d to 10.8 d and maintaining the same level of gas production. Conventional systems form the acid and gas in one batch, but the immobilized system ferments the methane in two phases (digestion or acid formation, and gasification or methane formation).
Microalgae are capable of direct synthesis of oil. These microalgae are produced commercially for high-value pharmaceuticals and health foods, but there is no technology developed for energy production from these organisms. The Japanese began to develop microalgae technology in 1980. Research has focused on increasing oil yields by species selection and improvement, but this process can be lengthy. Another approach is to increase cell yield by improving the culturing process. The four common technologies for large-scale outdoor production of microalgae are the open bubbling method, the closed circulation method, the open circulation system, and the open sewage-circulation system. Japan is designing its own large-scale outdoor cultivation systems and must choose among an open or closed system, a shallow or deep system, and a synthetic or waste-products medium.
Utilization of Technologies and Systems Analysis
The only significant biomass technology in commercial use in Japan is a program that pellets waste wood to produce briquettes for industrial purposes. The use of ethanol fuel has been studied, and tests have demonstrated that there is no difference between neat ethanol and gasoline.
A comparison of biochemical, thermochemical, and direct combustion technologies reveals that biochemical conversion for ethanol and methane production requires large quantities of raw materials that could be used at a lower cost for electricity generation or for direct combustion. Direct combustion is most feasible because the technology is simple and requires less raw material. Other systems require more research and development to become commercially feasible. The cost of the feedstock and the conversion technology are the major factors that affect the economics of these biomass systems.
Suggestions for Further Research
Areas for further research in Japan include
· The production of liquid fuel from cellulosic biomass,
· The development of systems for local community use,
· The development of systems for international use,
· Cellulosic pretreatment, the establishment of a strong cellulase-producing culture, and the development of an energy-saving process to recover ethanol, and
· The development of a compact pelletizer, a compact gasifier, and gas- or solid-fuel generators to lessen the dependence of agriculture on fossil fuels.
Kirk R. Smith and Jamuna Ramakrishna
This paper examines the health effects of indoor smoke produced by the burning of biomass in the households of developing countries. Although there are no specific studies comparing the health status of a population before and after the removal of smoke, enough information is available to draw some conclusions. The smoke emissions produced by the burning of biomass are described. Studies of the health impact of the smoke and its pollutants are reviewed. Recommendations are made for improving health conditions, assuming that a majority of the world's households will continue to burn biomass.
Air pollution can be measured from
· Fuel use, which provides a general indicator because pollutant release is roughly proportional to fuel use,
· Emissions monitoring, which is more accurate than measuring fuel use,
· Exposure monitoring, which takes into account concentration and duration and is usually the best and most practical measure for populations of any size, and
· Pollutant dose, which is the best indication but is relatively expensive to measure.
Smoke emissions are affected by fuel quality and the degree of incomplete combustion. When all of the organic matter in biomass is burned, essentially only carbon dioxide and water are emitted. The pollutants are not in the fuel; they are created during combustion. In small, simple domestic stoves, it is very difficult to achieve complete combustion.
The pollutants of concern in biomass smoke are suspended particulates, carbon monoxide, and hydrocarbons. The total suspended particulates (TSP) include inorganic carbon and a range of several hundred hydrocarbons, many of which can affect health. These toxic hydrocarbons, which exist in both gaseous and particulate form, include Benzo[a]pyrene (BaP) (the most studied of carcinogenic chemicals), acenaphthylene, and formaldehyde. Particulates from biomass combustion have toxicities that are similar to those of the particulates in the smoke from coal or diesel fuel.
A comparison of the emissions of common pollutants from several fuels indicates that small-scale biomass combustion produces higher emissions than fossil fuels. However, this does not prove that biomass combustion produces unacceptable pollution levels. Indoor concentrations are affected by such factors as fuelling rate, emission factor, ventilation rate, and room volume. Estimates of exposure include temporal and spatial factors. When air samplers are worn by cooks, a wide variation in exposures to common pollutants and BaP is observed. This is due to local weather and ventilation conditions. Generally, studies are inconclusive about the health impacts of exposure to indoor smoke because of the lack of data, inconsistent research techniques, the vast number of toxic pollutants to be studied, and the lack of comparative analysis with other pollutant exposures (for example, cigarette smoke). What seems clear is that a majority of the world's population in rural areas receives exposures and doses of major pollutants that exceed the levels experienced by their urban neighbours.
Research on the health impact of the burning of biomass shows a link between domestic smoke and poor health. Risks can be divided into noncancerous (respiratory abnormalities and acute respiratory infections) and cancerous risks.
Research in India, Nepal, and Papua New Guinea indicate that there is a link between respiratory abnormalities and symptoms and biomass smoke. More than 6 million children die each year from acute respiratory infections (ARI), and air pollution is a risk factor in ARI morbidity. In South Africa, a majority of respiratory problems in infants can be related to daily exposure to smoke. In Nepal, there is a positive correlation between ARI episodes and the amount of time that children spent near the fireplace. However, the suspected relationship between exposure to biomass smoke and cancer has not been established despite the large quantities of suspected carcinogens in some biomass smoke.
Studies of Specific Pollutants -Exposure to fossil-fuel combusion and cigarette smoke allows the study of two pollutants (carbon monoxide and particulates). Acute exposure to carbon monoxide can result in coma and death, and moderate short-term exposure can result in dizziness, headaches, and nausea. The long-term effects of lower exposures are not clear, but they are not likely to cause mortality although there is a growing link with impaired fetal development. Carbon monoxide may also increase the carcinogenic effects of other air pollutants. In addition, risk factors (for example, oxygen deprivation) increase sensitivity to exposure to carbon monoxide. In Guatemala, carbon monoxide exposure has a greater effect on people at higher elevations.
It is difficult to interpret the data on the impact of particulate exposure. It is easier to determine the health effects of acute exposure because there are more data than for chronic exposure. Acute exposures to fossil-fuel emissions show the joint effect of particulates and sulfur oxides (wood has a low concentration of sulfur oxides). Difficulties in controlling for compounding factors (for example, smoking and socioeconomic status) make many studies suspect. Nonetheless, there is reasonable certainty that acute exposures to the particulate pollution that is typical in rural kitchens of the developing world can increase both morbidity and mortality.
Polycyclic aromatic hydrocarbons (PAM) are organic compounds that are found in biomass smoke and are both mutagenic and carcinogenic. Although no link has been established between cancer and these compounds, there is much evidence of its carcinogenic potential.
Lessening the Impact of the Burning of Biomass
Despite the dangers of exposure to biomass smoke, hundreds of millions of households will continue to burn biomass. The use of cleaner fuels, improved stoves, and better ventilation could lessen the negative effects of the burning of biomass. Some changes in behaviour could also help reduce exposure.
Cleaner Fuels -Some species of wood burn more cleanly than others, and dry wood burns more efficiently. Upgrading the biomass fuel can result in more dramatic improvements. Charcoal manufacturing creates most of the emissions of particulates and carbon monoxide. Total household exposure is lower, but overall pollution is the same or higher. Dung can be upgraded by anaerobic digestion, which yields biogas and fertilizer. Biogas emissions are similar to the emissions from natural gas. Vegetable oils can also be upgraded, and in some remote locations, they are economical as fuels. Alcohol fuels, derived from the fermentation of biomass, are thought to be clean burning.
Improved Stoves and Better Ventilation - There are many designs for stoves that promote higher thermal efficiencies and lower emission factors; however, global dissemination of improved stoves has been frustratingly slow despite many isolated successes. Problems that arise in the field but have not been considered in the laboratory include
· Lack of flexibility in variety of fuels and choice of cooking methods,
· Limited capability to accommodate a variety of utensils,
· The large area occupied by the stove in homes where space is limited, and in some cases,
· Continued high exposure to smoke because the stoves were not installed correctly, maintained adequately, or operated properly.
The thermal efficiency of stoves can be improved by controlling the airflow to increase the residence time of the flue gases. The efficiency of single-pothole stoves has been improved by using a grate, a reflective surface, a water jacket, and insulation.
Despite a lack of empirical data, improved ventilation is probably the least expensive short-term way to reduce smoke exposure. Three ways to improve ventilation are minor changes in the ventilation of existing structures, relocation of the kitchen, and major redesign of the kitchen.
Behaviour Change. -Changes in behaviour can help reduce the impact of exposure to biomass smoke without new technologies. These changes include moving the stove outdoors or to a verandah and keeping pregnant women and young children out of the cooking area as much as possible.
Costs of Exposure Control
Generally, the three categories of policy tools that governments use to address pollution problems are
· Information (for example, government-sponsored R&D programs and rural education),
· The establishment and enforcement of ambient and emissions standards (generally, traditional regulatory approaches are ill-suited for domestic smoke pollution, but standards related to house and stove design may be relevant), and
· Economic tools that include severance taxes and subsidies (subsidies can be dangerous if poorly used because they create distortions and are expensive).
Cost-benefit analyses should include the costs of an increase in smoke exposure or the benefits of a decrease. The benefits of this type of pollution-control program might include decreases in medical care and in occupational disruption minus foregone benefits, such as the value of smoke as a form of thatch preservative. In the case of fuel savings, these benefits can be added to the benefits of reduced deforestation.
A rough analysis of the net present value (NPV) of the benefits of reduced exposure to domestic smoke selected as its benefit the reduction of chronic bronchitis in 10% of the population. Even without including the benefits of reduced ARI in children, decreased medical costs, decreased fuel costs, and increased household cleanliness, NPVs of more than 100 USD for most discount rates and degrees of impairment from exposure were found. This is more than enough to justify an improved cookstove program and investments in better ventilation and fuel.