The 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:
During pyrolysis, wood is heated in the complete absence of oxygen.
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.
During combustion, wood is completely oxidised and the resulting hot gases are used to raise steam, which drives a steam turbine to produce electricity.
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.