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.
Woodsmoke arises due to the fact that wood is difficult to burn completely, especially under the conditions present in a relatively simple stove. Part of the problem is that there is no such thing as ‘standard wood’: one species of tree burns differently from another and the moisture content and size of the logs also plays a part, meaning that wood burning stoves have to be designed so that they are not too fussy about what goes into them.
The result is that flue gases of wood burning stoves contain some substances which can be categorised as pollutants. These include gases, liquids and even solids. Some of the gases are responsible for the characteristic smell of burning wood, which is not always welcome, especially in congested cities. Liquid pollutants comprise tars and creosote which may condense on the internal walls of the chimney where they will gradually build up and constitute a fire hazard. Finally, solid particles may be carried some distance by the hot air currents in the flue gas but will eventually come to Earth as dust and soot.
So, the pollutants can be divided into three categories:
1. Those which arise from incomplete combustion
The principle pollutants are carbon monoxide, soot (i.e. carbon), hydrocarbons and wood tar compounds (creosote etc).
2. Those which arise from reactions between the components of the air
These comprise 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).
3. Inorganic components of the fuel
This category refers to ash.
Using a catalytic converter to reduce air pollution
The fumes resulting from the incomplete combustion of wood can be oxidised into carbon dioxide and water, as long as the temperature is right. The purpose of a catalytic converter is to lower this temperature so that the appropriate conditions can more easily be obtained. However, a range of different organic pollutants are present in the fumes, and each requires a different temperature threshold before it will oxidise. In addition, these pollutants may be in the gas, liquid or even solid phases, and most catalysts used for this application only promote the oxidation of pollutants in the gas phase.
Fortunately, the oxidation of organic pollutants releases heat (i.e. it is exothermic) so the catalytic converter gets going with those pollutants that have the lowest light-off temperature at about 225oC and then gets hotter. As the temperature increases it reaches the light-off temperature for additional pollutants, which may have switched to the gas phase, and these release more heat and so on in a virtuous circle. For this reason, a catalytic converter which is working well will often glow red-hot. Once the temperature reaches about 600oC, any organic matter (no matter which phase it is in) which has resisted catalytic oxidation so far will simply burn. This is why catalytic converters used in wood burning stoves are often referred to as ‘catalytic combustors’.
Designing an efficient catalytic wood burning, or biomass stove, is not simply a matter of starting with a standard model and inserting a catalytic converter in the flue. A well-designed catalytic stove uses the heat emitted from the catalytic converter as part of its output, rather than wasting it up the chimney, and also ensures that the catalytic converter runs at a high enough temperature so that it burns off any tar which lands on it.
There are many elements to consider so, if you are unfamiliar with the technology, the team at Whitebeam offer design guidelines and advice to assist stove manufacturers in making an informed choice on the best solution to reduce emissions from their product range.