Combustion of woodfuel to produce heat and electricity

Interior of a bark burning furnace, a component part of a wet burning systemThe combustion of woodfuel is a complex process involving several main stages and resulting in by-products, some of which have the potential to be very useful.

Developing the technology

There is a wide range of possible technologies for generating heat and electricity from woodfuel. Three principal methods are currently being developed:

  • Pyrolysis
    During pyrolysis, wood is heated in the complete absence of oxygen.
  • Gasification
    When the supply of air is restricted as wood is heated, combustible gases are given off. These gases can be ‘cleaned’ and used to drive an engine; the engine in turn can be used to generate electricity.
  • Combustion
    During combustion, wood is completely oxidised and the resulting hot gases are used to raise steam, which drives a steam turbine to produce electricity.

Wood ash

Two types of ash are formed from woodfuel combustion: ‘bottom ash’ which collects in the combustion unit (the residue that is left in the combustion chamber) and ash collected from the flue gases in the larger plant.

Suitably processed ash has the potential to form a valuable by-product as a low nitrate fertiliser for use in areas high in nitrogen.


There are two forms of emission from woodfuel: leachates and atmospheric emissions. Leachates can be produced by stored branchwood and residues and can enter groundwater or watercourses as rain runs over the ground or through the fuel, picking up contaminants. Atmospheric emissions are released following burning. They include particulates, carbon dioxide and carbon monoxide as well as organic compounds, nitrogen oxides and water vapour.


What's of interest

This is a selection of the respective information in the Woodfuel Information Pack. The full pack comprises fact sheets giving more extensive information.