Biogases - Alternative energy sources

The term "biogases" refers to gases created by the anaerobic fermentation of biological materials. Their main constituents are methane and carbon dioxide.
Considerable quantities of biogases are produced by :
• anaerobic fermentation of agricultural and organic waste (biogas),
• sludge digestion in the tanks of sewage treatment plants (sewage gas),
• organic residues in garbage tips (landfill gas).

Biogas



Disposal and treatment of biological waste represent a major challenge for the waste industry. For a wide range of organic substances from agriculture, foodstuff or feed industries, anaerobic fermentation is a superior alternative to composting.
Biogas - a mixture of methane and carbon dioxide - is created during anaerobic fermentation and serves as a high-energy fuel that can be used as a substitute for fossil fuels. Biogas-fueled gas engines improve waste management while maximizing the use of an economical energy supply.
Biogas results from anaerobic fermentation of organic materials. As a metabolic product of the participating methane bacteria, the prerequisites for its production are a lack of oxygen, a pH value from 6.5 to 7.5 and a constant temperature of 15 to 25°C (psychrophile), 25 to 45°C (mesophile) or 45 to 55°C (thermophile). The fermentation period is approximately 10 days for thermophiles, 25 to 30 days for mesophiles and 90 to 120 days for psychrophile bacteria. The fermentation systems of today operate largely within the mesophile temperature range.

Sewage Gas



The anaerobic digestion of sewage sludge involves fermenting the sludge in tanks at a temperature of 90 to 93 °F (32 to 34 °C) for about 25 days. The thermal energy generated by a combined heat and power (CHP) unit preheats the sludge and keeps the temperature of the digestion tank constant.
The resulting biogas normally consists of 50 to 60 percent methane, 30 to 40 percent carbon dioxide and small quantities of residues. The gas is compressed and purified if it contains larger amounts of contaminants, and stored temporarily in a gasometer from which it is fed to a CHP unit at constant pressure. A gas engine transforms the energy stored in the biogas into mechanical and thermal energy. It also powers a synchronous generator, which in turn generates electrical energy for the operation of the sewage treatment plant.

Landfill Gas - Producing Usable Energy from Garbage



Landfill gas is created during the decomposition of organic substances and consists of methane (CH4), carbon dioxide (CO2) and nitrogen (N2). Uncontrolled venting of landfill gas hampers or prevents rapid, scheduled recultivation of a landfill site. To prevent this and to avoid offensive smells, smouldering fires or the migration of gas, the gas must be continuously extracted under controlled conditions. With a calorific value of about 5 kWh/m3N, landfill gas constitutes a high-value fuel for gas engines that can be effectively used for power generation.

Landfill gas is an alternative form of fuel to fossil fuels in the production of electricity. With landfill sites all over the UK, there is a large resource available.
Landfill Gas (LFG) is the product of the degradation of biodegradable waste and it is typically made up of about 50% methane (although this can reach about 65%), with the remainder mainly carbon dioxide plus small amounts of other gases. Both methane and carbon dioxide are ‘greenhouse gases’ contributing to global warming, so their use in this case as fuel reduces their release into the atmosphere. LFG is also a danger to the local environment causing problems for local vegetation, potential fires and explosions and even asphyxiation if released into a building.



Under optimum conditions one tonne of biodegradable waste can produce between 200 and 400 cubic metres of landfill gas. Due to the use of greenhouse gases and the fact that this energy source displaces the use of fossil fuels, landfill gas is deemed to be a ‘green energy’. The landfill gas is extracted from the site and burnt in order to produce electricity. Currently, 48% of Great Britain’s renewable gas and electricity production comes from landfill gas.
Landfill gas can also be used for direct firing (e.g. in brick kilns or for producing steam in boilers) or for direct heating (e.g. in horticultural greenhouses). Again, this reduces the amount of gas released into the atmosphere and can provide an income stream once the landfill site is full.

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