Biofuels

Biomass is normally derived from animal and plant wastes. It can be used to generate electricity and or heat in a variety of ways. The main advantage with biomass fuels is that they are effectively a waste product and will not contribute to depletion of the world's natural resources.

 

Currently, agricultural wastes such as straw and other crop residues are used for energy production, although it is becoming common to grow crops such as willow and oil seed rape specifically for energy production.

 

A number of processes exist to extract or recover energy from biomass and this will depend very much on the type of raw fuel.

 

Dry fuels

The most straightforward way to recover energy from dry biomass fuels is by simple combustion to provide heating or hot water. Typical applications range from simple log fires and stoves, to sophisticated wood or straw fuelled boiler systems.

 

Combined Heat and Power (CHP) plants are fast becoming a commercially attractive means of turning biomass into either heat or electricity or a combination of both. A number of plants of this kind are in operation throughout the UK .

 

The most common raw fuel has to date been forest residues, wood wastes and the agricultural wastes discussed above. However recently a trend is occurring where 'energy crops' are grown specifically for the purpose of providing raw fuel for CHP plants. These 'energy crops' including willow, poplar and although not grown in the UK may be grown specifically for use as a fuel, and can provide long-term secure resources.

 

In addition biomass fuels are being increasingly used with advanced conversion technologies, such as gasification systems, offering superior efficiencies compared with conventional power generation. Gasification itself is a thermo-chemical process in which biomass is heated with little or no oxygen to produce a low-energy gas. The actual composition of the gas will depend on the nature of the gasification process and raw fuel source used. This gas can then be used as a raw fuel for a gas turbine or a combustion engine to generate electricity.

 

Wet fuels

Cattle, poultry and pigs all produce slurry which can be used to produce biogas. By fermenting the slurry in an anaerobic digester biogas is produced that mainly methane and carbon dioxide. This gas can then be used in gas engines to generate electricity or in boilers to produce heat. Between 40-60% of the organic matter present in the slurry is converted into biogas. The remainder provides a stabilised residue that can be used as a soil conditioner.

 

Biogas is produced when organic matter decomposes in the absence of oxygen and in addition to being produced by an anaerobic digester a landfill site will also create a similar reaction and provide landfill gas.

 

Landfill gas

Landfill gas is a mixture comprising mainly methane and carbon dioxide, formed when biodegradable wastes break down within a landfill as a result of anaerobic microbiological action. The biogas can be collected by drilling wells into the waste and extracting it as it is formed. It can then be used in an engine or turbine for power generation. Landfill sites can generate commercial quantities of landfill gas for up to 30 years after wastes have been deposited. Recovering this gas and using it as a fuel not only ensures the continued safety of the site after landfilling has finished, but also provides a significant long term income from power and/or heat sales.

 

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