Catalysis of Domestic Biomass Combustion
The fumes resulting from the incomplete combustion of biomass 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 biomass 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 (‘lights off’ in the jargon) 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 biomass stoves are often referred to as ‘catalytic combustors’.
Designing an efficient catalytic 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 biomass 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. The subject is quite complicated but Whitebeam offers design guidelines and advice to assist stove manufacturers which are unfamiliar with this technology.