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Draft PPS18: Renewable Energy
Annex 1 Biomass: Technology

B7. Energy generation based on biomass is technologically well advanced and widely utilised in many parts of the world.
B8. There are three main methods for converting dry biomass fuels into energy:
  • Direct combustion is used for heating water or to raise steam to drive a steam engine or turbine to generate electricity (steam cycle). Equipment ranges from very small wood stoves used for domestic heating to multi-megawatt plants for electricity production. The upper limit is restricted by local energy demand and availability of biomass rather than by combustion technology. Equipment design depends on the moisture content and particle size of the fuel;
  • Gasification is a technique in which the solid fuel undergoes incomplete combustion in a limited air supply to produce a combustible gas that can be burned in a boiler, or used as fuel for an engine or gas turbine. This technology is more applicable to multi megawatt plants, but smaller plants of under 5 MW are becoming more common; and
  • Pyrolysis involves heating in the absence of oxygen (rather like traditional charcoal production) to produce a combustible gas or liquid, which is used in a similar way to gas produced from gasification.
B9. Direct combustion is the most commonly used technology for ‘heat only’ plants, whilst both direct combustion and gasification are used for CHP and ‘electricity only’ plants. Pyrolysis is more commonly associated with the production of transport fuel, such as biodiesel. Combustion technology and generation of electricity using the steam cycle is an advanced, mature technology. Whilst becoming much more common, gasification and pyrolysis are much less mature technologies than direct combustion.
B10. The three technologies appear externally to be similar, and share much in common from a planning perspective. For a given capacity of plant, the size, extent and appearance of the development will be similar, similar amounts of fuel feedstock will be required, and emissions and other waste products will be similar, although pyrolysis and gasification plant may have a smaller footprint, as the process is more compact.
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