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Biomethanation

Norman L. Brown and Prakasam B.S. Tata

 

Overview

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.

Analysis

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

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.

Production Technology

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.