Biogas

The most straightforward exploitation of biogas involves the tapping of methane produced by decaying waste material in landfill sites. This has a considerable environmental benefit since it burns the methane which would otherwise add more intensively to the greenhouse problem. Gas is collected using a series of vertical collection wells connected to a blower which draws gas from the waste. Foreign matter is extracted and the gas then fed to a conventional engine which drives a generator. The engine would use 'lean-burn' technology to minimise emissions of nitrogen oxides and carbon monoxide (Power generation from landfill gas; Cuxton, UK, DTI Renewable Energy Case Study 2).

Anaerobic digestion uses wet waste products to produce energy in the form of methane-rich biogas. The process, which involves a fermentation stage, takes place in large heated tanks at either 30-35°C or 55°C during which 60 per cent of the organic material is converted to biogas. The liquids and solids which remain after digestion are used as fertilisers and soil conditioners.

The methane-rich gas is most effectively employed to fuel combined heat and power (CHP) schemes which is how the technology has been employed on an ambitious scale in Denmark. Here co-operative ventures receiving waste from all the farms in a viable collection area are combined with non-toxic industrial and food waste to fuel extensive CHP networks.

The next logical step is to employ the technology to exploit the energy potential of human sewage on a national scale, thus alleviating two problems simultaneously.

One of the most promising technologies to have developed in recent years involves the gasification of municipal waste. Non-solid waste is superheated to produce methane which then fuels a steam turbine. The process involves heat recovery so that there is a commercial net energy gain in the process. The unit price of electricity generated by this process can be offset by the avoided costs of landfill disposal together with the taxes this incurs.

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