CATALYTIC INCINERATORS

Description

Catalytic incinerators are used to destroy gaseous pollutants such as VOCs. Catalysts increase the rate of chemical reactions without being changed by the reaction themselves. A catalytic incinerator permits the incineration reactions to occur more quickly and at reduced temperatures.

The best metal catalysts are platinum and palladium. However, less expensive oxides of copper, chromium, vanadium, nickel and cobalt can also be used. The catalyst is in the form of porous pellets, honeycombs or wire mesh designed to present a large surface area to the gas stream.

In a reversible-flow incinerator the airstream is preheated as it passes through a hot ceramic bed. When the gas reaches the catalytic surfaces, the organic compounds are oxidised and the gas temperature rises dramatically. The gas then passes through a second ceramic bed, which is cooler than the gas. Heat from the gas is absorbed by this bed and the cooled gas is vented to the atmosphere. The temperature of the second ceramic bed rises until it is no longer able to absorb heat from the gas. At this point a changeover valve operates and the direction of the gas flow is reversed and the process continues in the reverse direction.

Application

As with thermal oxidation, the aim of catalytic incineration is to convert toxic or ozone-forming organic pollutants into carbon dioxide and water.

Catalytic incinerators are used to remove VOCs such as solvents from printing and paint-spraying processes. There are similar applications in the food, tobacco, metal and chemical industries.

Catalytic incinerators are not suitable for emissions containing particulate or polymeric material as these may block the fine catalytic structure. Also, it should be noted that certain compounds, such as those containing chlorine or sulphur, are able to poison certain catalysts and prevent their operation.

Sizing

For a typical catalytic incinerator treating 15000 m3/hour of air contaminated with 03 g/m3 of VOCs, a 50 kW fan would be needed to pass the gas through the system, and a 150 kW heater would be required for supplementary heating.

The catalytic nature of operation results in reduced fuel costs, which may only be required for start up and can be as little as 40-60 % that required by a thermal oxidiser. The more efficient oxidation process also allows the equipment to be correspondingly lighter and more compact. However, catalytic incinerators have higher capital costs. Replacement catalyst may be required periodically.