Catalytic converters are widely used in car all over the world.
It is cylindrical canister placed between exhaust manifold and silencer. It contains plastics pallets coated with the catalyst.
Catalytic converter is designed for the oxidation of pollutant gases escaping after primary combustion in the engine, within the exhaust system. The simplest type of catalytic converter.
The temperature required for bulk gas oxidation and reduction of hydrocarbon gases (HC), carbon monoxide (CO) and oxides of nitrogen (NO) is about 600 to 700°C. The temperature of exhaust gases in exhaust system is lower. The catalytic converter oxidizes and reduces all the three pollutant gases at lower temperature because of catalytic chemical reaction. A catalytic converter becomes effective in the temperature between 250 to 300°C.
1. TWO WAY CATALYTIC CONVERTER
A converter is filled with a monolithic substrate coated with small amount of platinum and palladium.
Through catalytic action, a chemical change converts carbon mono oxide (CO) and hydrocarbons (CH) into carbon dioxide and water. Such a converter is called ‘two way catalytic converter’.
THREE WAY CATALYTIC CONVERTER
Three way catalytic converter is installed on cars to check pollution. Such a converter uses thin coating of platinum, palladium and rhodium over a support metal (generally alumina) and acts on all three major constituents of exhaust gas pollution i.e. hydrocarbons, carbon monoxide and oxides of nitrogen, oxidizing these to water, carbon dioxide and free hydrogen and nitrogen respectively.
Three way catalytic converter operates in two stages.
The first converter stage uses rhodium to reduce the NO, in the exhaust into nitrogen and охygen.
In the second stage converter platinum or palladium acts as oxidation catalyst to change HC and CO into harmless water and CO2.
For supplying the oxygen required in the second stage air is fed into the exhaust after the first stage. The catalyst allows the oxidation of the exhaust gas at a much lower temperature than in the combustion chamber.
Reactions within catalyst produce additional heat that reaches temperature of 900°C,
which is required for the catalytic converter to operate at complete efficiency. To safeguard from this high temperature, the catalytic converter is made of stainless steel and special heat shields are also used.
No regular maintenance is required for catalytic converter.
Catalytic converter may be replaced at about 80,000 km or more.
The engine emissions HC, CO and NO, are removed from the exhaust gas by oxidation.
LIMITATIONS OF CATALYTIC CONVERTER
- Since lead destroys catalytic activity, the engine cannot use leaded petrol.
- Exhaust systems are hotter than normal as a result of exothermic reaction in catalyst bed.
- The emission of SO3 increases if fuel contains sulphur.
- The use of equipment adds to the cost.
- Periodic replacement of air filter of induction system is required.