The chemical reactions which occur during the oxidation of pollutants are usually ‘exothermic’, which means that they give off heat.
In most applications, the goal is to minimise the emissions of pollutants and the heat produced is an unwanted by-product.
However, sometimes the heat is the priority and can be put to good use, for example to raise the temperature of exhaust gas so that it can help in the regeneration of a diesel particulate filter. Another situation is where a hot catalytic converter is used to generate infra-red light.
The temperature of a catalytic converter derives from two sources:
- the first is the temperature of the exhaust gas passing through it,
- and the second is the chemical energy which is released when pollutants or fuel are oxidised.
In some cases, this second component can be responsible for an increase in temperature of several hundred degrees centigrade.
One useful application of this phenomenon is the ‘fuel-added catalyst’ whereby a diesel engine is deliberately run with a rich mixture and a catalytic converter to create heat in the exhaust gas upstream of a diesel particulate filter.
The excess fuel required may be delivered via the standard fuel injectors, if the injection system is the ‘common-rail’ type, or alternatively an additional injector with its own pump can be installed into the exhaust system for this purpose.
Where an exothermic catalytic converter is installed on a spark ignition engine it is also possible to obtain very high surface temperatures, in excess of 1000 centigrade, with the key requirements being that the exhaust gas contains some unburnt fuel and enough oxygen to burn it.