The oxidation reactions which the catalytic converter promotes are what is known as ‘exothermic’ i.e. they release heat. This means that the temperature of the flue gas usually increases as it passes through the catalytic converter, often by more than 100oC. One benefit of this is that the reactions tend to be self-sustaining, once they have got started. Another benefit is that heavier molecules such as soot and tar may be burned by the higher temperature.
Catalytic converters can only oxidise toxic substances if there is oxygen present in the flue gas, and sometimes this is in short supply, especially in the case of charcoal ovens where the aim is often to restrict the air supply to the charcoal so that it does not burn too vigourously. If there is an oxygen-deficiency in the flue-gas, the catalytic converter will not work properly. The best way to overcome this is by using ‘secondary-air’, which means introducing fresh air into the flue upstream of the catalytic converter. This secondary air can either be delivered by a pump, or drawn in by using a feature such as a venturi.
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