Biogas Basics
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What is biogas?
Biogas typically refers to a gas produced by the anaerobic digestion of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste or any other biodegradable feedstock, under anaerobic conditions. Biogas is comprised primarily of methane and carbon dioxide. It also contains smaller amounts of hydrogen sulphide, nitrogen, hydrogen, methylmercaptans and oxygen[1].
Biogas originates from bacteria in the process of bio-degradation of organic material under anaerobic (without air) conditions. The natural generation of biogas is an important part of the biogeochemical carbon cycle. Methanogens (methane producing bacteria) are the last link in a chain of micro-organisms which degrade organic material and return the decomposition products to the environment. In this process biogas is generated, a source of renewable energy.
Biogas and the global carbon cycle
Each year some 590-880 million tons of methane are released worldwide into the atmosphere through microbial activity. About 90% of the emitted methane derives from biogenic sources, i.e. from the decomposition of biomass. The remainder is of fossil origin (e.g. petrochemical processes). In the northern hemisphere, the present tropospheric methane concentration amounts to about 1.65 ppm.
Biology of methanogenesis
Knowledge of the fundamental processes involved in methane fermentation is necessary for planning, building and operating biogas plants. Anaerobic fermentation involves the activities of three different bacterial communities. The process of biogas-production depends on various parameters. For example, changes in ambient temperature can have a negative effect on bacterial activity.
Substrate and material balance of biogas production
In principle, all organic materials can ferment or be digested. However, only homogenous and liquid substrates can be considered for simple biogas plants: faeces and urine from cattle, pigs and possibly from poultry and the wastewater from toilets. When the plant is filled, the excrement has to be diluted with about the same quantity of liquid, if possible, the urine should be used. Waste and wastewater from food-processing industries are only suitable for simple plants if they are homogenous and in liquid form. The maximum of gas-production from a given amount of raw material depends on the type of substrate.
Composition and properties of biogas
Biogas is a mixture of gases that is composed chiefly of:
- methane (CH4): 40-70 vol.%
- carbon dioxide (CO2): 30-60 vol.%
- other gases: 1-5 vol.%; including hydrogen (H2: 0-1 vol.%) and hydrogen sulfide (H2S: 0-3 vol.%)
Like those of any pure gas, the characteristic properties of biogas are pressure and temperature-dependent. They are also affected by the moisture content. The factors of main interest are:
- change in volume as a function of temperature and pressure,
- change in calorific value as a function of temperature, pressure and water-vapor content, and
- change in water-vapor content as a function of temperature and pressure.
The calorific value of biogas is about 6 kWh/m3 - this corresponds to about half a litre of diesel oil. The net calorific value depends on the efficiency of the burners or appliances. Methane is the valuable component under the aspect of using biogas as a fuel.
Utilization
The history of biogas utilization shows independent developments in various developing and industrialized countries. The European biogas-history and that of Germany in particular, as well as developments in Asian countries form the background of German efforts and programmes to promote biogas technology worldwide.
With biogas technology, waste (called slurry) is stored in specially constructed containers while being digested. There are a number of technologies used to accomplish this:
- Batch type digesters treat a large amount of material at once. They are used for large scale application
- Continuous flow units add and remove waste material on a daily or regular basis. They are best suited for small-scale domestic applications[1].
Normally, the biogas produced by a digester can be used as it is, just in the same way as any other combustible gas. But it is possible that a further treatment or conditioning is necessary, for example, to reduce the hydrogen-sulfide content in the gas. When biogas is mixed with air at a ratio of 1:20, a highly explosive gas forms. Leaking gas pipes in enclosed spaces constitute, therefore, a hazard. However, there have been no reports of dangerous explosions caused by biogas so far.
A first overview of the physical appearance of different types of biogas plants describes the three main types of simple biogas plants, namely:
- balloon plants,
- fixed-dome plants and
- floating-drum plants.
Digester temperature is an important factor in maintaining the bacteria necessary for digestion. This is one reason why fixed dome digesters tend to be more successful in areas with extreme temperature fluctuations. Gas production is dependent upon digester temperature, fermentation or retention time and the feedstock material.
Small scale farmers that keep zero-grazed animals are good candidates for installing household biogas units. However, a biogas unit will only yield good results if it is properly planned, constructed, operated and maintained. Regular supply of water is essential for operation of biogas plants.
Information required to design and install a biogas digester include:
- Size of institution, family and daily cooking (and lighting) requirements.
- Availability and amount of feedstock (water, number and type of cows, pigs etc)
- Materials available on site (bricks, etc) for construction of digester[1].
Applications of Biogas
Biogas can be used for direct combustion in cooking or lighting applications; and to power combustion engines for motive power or electricity generation. The technology is particularly valuable in agricultural, waste treatment or animal processing units where there is excess manure, farm waste or municipal waste. Typical applications in off grid settings in developing regions are cooking and lighting[1].
The Benefits of Biogas Technology
Well-functioning biogas systems can yield a whole range of benefits for their users, the society and the environment in general:
- production of energy (heat, light, electricity);
- transformation of organic waste into high quality fertilizer;
- reduction of volume of disposed waste products;
- improvement of hygienic conditions through reduction of pathogens, worm eggs and flies;
- encouragement of better sanitation;
- reduction of workload, mainly for women, in firewood collection and cooking.
- environmental advantages through protection of soil, water, air and woody vegetation;
- micro-economical benefits through energy and fertilizer substitution, additional income sources and increasing yields of animal husbandry and agriculture;
- macro-economical benefits through decentralized energy generation, import substitution and environmental protection
Thus, biogas technology can substancially contribute to conservation and development, if the concrete conditions are favorable. However, the required high investment capital and other limitations of biogas technology should be thoroughly considered.
The Costs of Biogas Technology
An obvious obstacle to the large-scale introduction of biogas technology is the fact that the poorer strata of rural populations often cannot afford the investment cost for a biogas plant. This is despite the fact that biogas systems have proven economically viable investments in many cases.
Efforts have to be made to reduce construction cost but also to develop credit and other financing systems. A larger numbers of biogas operators ensures that, apart from the private user, the society as a whole can benefit from biogas. Financial support from the government can be seen as an investment to reduce future costs, incurred through the importation of petrol products and inorganic fertilizers, through increasing costs for health and hygiene and through natural resource degradation.
Fuel and Fertilizer
In developing countries, there is a direct link between the problem of fertilization and progressive deforestation due to high demand for firewood. In many rural areas, most of the inhabitants are dependant on dung and organic residue as fuel for cooking and heating. Such is the case, for example, in the treeless regions of India (Ganges plains, central highlands), Nepal and other countries of Asia, as well as in the Andes Mountains of South America and wide expanses of the African Continent. According to data published by the FAO, some 78 million tons of cow dung and 39 million tons of phytogenic waste were burned in India alone in 1970. That amounts to approximately 35% of India's total noncommercial/nonconventional energy consumption.
The burning of dung and plant residue is a considerable waste of plant nutrients. Farmers in developing countries are in dire need of fertilizer for maintaining cropland productivity. Nonetheless, many small farmers continue to burn potentially valuable fertilizers, even though they cannot afford to buy chemical fertilizers. At the same time, the amount of technically available nitrogen, pottasium and phosphorous in the form of organic materials is around eight times as high as the quantity of chemical fertilizers actually consumed in developing countries. Especially for small farmers, biogas technology is a suitable tool for making maximum use of scarce resources: After extraction of the energy content of dung and other organic waste material, the resulting sludge is still a good fertilizer, supporting general soil quality as well as higher crop yields.
Public and Political Awareness
Popularization of biogas technology has to go hand in hand with the actual construction of plants in the field. Without the public awareness of biogas technology, its benefits and pitfalls, there will be no sufficient basis to disseminate biogas technology at grassroots level. At the same time, awareness within the government is essential. Since impacts and aspects of biogas technology concern so many different governmental institutions (e.g. agriculture, environment, energy, economics), it is necessary to identify and include all responsible government departments in the dissemination and awareness-raising process.
Links to further reading
Wikipedia: Biogas [1]
Biogas & IBBK Bioenergie [2]
References
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