Burning wood sounds simple, but it’s actually a surprisingly complicated process involving six discrete stages, during which the solid wood is gradually converted into flammable gases, which are then oxidised to produce heat. In an ideal combustion process the only emissions would be carbon dioxide and water, but in reality some pollutants are also produced.
These pollutants can be divided into three categories:
- Those which arise from incomplete combustion,
- Those which arise from reactions between the components of the air,
- Inorganic components of the fuel.
In the first category the principle pollutants are carbon monoxide, soot (i.e. carbon), hydrocarbons and wood tar compounds (creosote etc).
The second category comprises mainly oxides of nitrogen (NOx), which are generated both from the combustion air as well as the nitrogen content of the wood itself. If the NOx group is broken down further, the majority will be found to be nitric oxide (NO) with most of the remainder consisting of the toxic nitrogen dioxide (NO2).
The third category refers to ash.