Catalysis in exhaust treatment

Generally, there are three methods of oxidation of volatile organic compounds (VOC):

• In the flame, such as in a thermal afterburning system, at temperatures around 1,200 °C

• at a glowing surface, such as thermal-regenerative afterburning.  In this case, a bed of stone or ceramic material is heated to 800 °C red hot.  The molecules oxidize in the bed.

• By catalytic afterburning at a catalyst at temperatures around 350 °C.

There are clear advantages for the catalytic method in a system oxidizing VOC:

• Catalysis proceeds without a flame

• The energy consumption and the emission of CO2 are clearly lower.

• Due to the lower temperatures, catalytic afterburning proceeds as an autothermal process already at significantly lower concentrations than other forms of afterburning.

• There are no emissions of nitric oxides (NOx).

• Dioxin does not form.

The disadvantage of the catalytic method: Although modern catalysts are a lot more robust and versatile than say 15 years ago, a catalyst cannot be used just for everything.  In planning a catalytic afterburning process, a careful  look should be taken at the list of substances and potential emissions to make sure that no potential catalytic poisons are involved.  These would substantially reduce the life of the catalyst, in some instances to a few months or even days.

Like an adsorbing material, the catalyst can be blocked by incompatible substances. If it becomes too hot - which is possible due to reaction enthalpy if the input concentration is too high - the catalyst can sinter.  These points must be considered in planning the system equipment and the system control.