The number of vehicles on the roads seems to be keeping pace with the country's rapidly growing population. It is estimated that there are over 40 million vehicles on the roads, and this is growing at an average annual rate of 10 per cent. This growing automobile population spells doom as far as clean air is concerned -- atmospheric pollution is a growing menace, especially in the urban areas where traffic density is high. Auto pollution is being tackled all over the world, as in India, by enforcing strict emission control norms from the manufacturing stage onwards. In India, the government has amended the Central Motor Vehicle Rules to check emission of smoke from vehicles and the states have the necessary powers to carry out effectively and implement the rules.

One of the parts responsible for controlling (or causing) auto pollution is the ‘silencer’ or auto-exhaust muffler. These mufflers fall in the category of items reserved for the small-scale industry, so these units are now under scrutiny. In order to minimise auto-exhaust pollution, the muffler manufacturers have been advised to incorporate 'catalytic converters' in their design. Manufacturing mufflers fitted with catalytic converters could prove a growth area for SMEs, as people and governments are increasingly becoming aware of the dangers of auto pollution.

Catalytic cracking of gases:

In general, a catalyst is a substance that alters, initiates, accelerates or retards the rate of chemical reaction. Certain types of catalysts can act on pollutant gases like carbon monoxide to render them harmless by converting them into carbon di oxide. The following catalysts have been found effective in neutralising exhaust gases:

•  Oxides of manganese, iron, chromium, copper or cobalt
•  Copper chromite
•  Platinum, palladium, rhodium

These catalysts are composite materials, usually in the form of cylindrical or spherical granules or pellets, built up from a porous refractory material like alumina. They are called 'heterogeneous' catalysts since the catalyst is in a different phase than that of the reactants.

A catalytic converter is a device which when fitted to a vehicle, brings down the level of pollutant gases in exhaust emissions.  The level of the most toxic gas, carbon monoxide, is reduced up to 90 per cent. Given below are the outlines on the design and operation of the catalytic devices used in auto-exhausts.

There are basically three types of catalytic converters:

• Flat bed catalytic device
• Radial flow converter
• Honeycomb reactor

Flat bed catalytic device:

The catalyst is packed in the muffler in the form of a solid cylindrical bed. Exhaust gases flow through the downstream end of the catalyst bed, focussing the heat load of hydrocarbon/carbon monoxide combustion. The bed thickness, catalyst granule dimension and the perforations on the sheet steel supporting the catalyst bed influence pressure drop through the catalysis bed. The location of the catalytic device in terms of the distance from exhaust manifold is critical.

Radial flow converter:

The catalyst is packed around the central tube so that the catalyst bed assumes the form of a hollow cylinder.  The exhaust outlet is placed above this tube.

Honeycomb reactor:

Auto catalysts based upon the platinum group metals are the latest gadgets adopted by the world's motor industry. A platinum palladium oxidation catalyst supported on a ceramic or metal honeycomb support is used. Recently, a three-way catalyst of platinum, palladium and rhodium was found to be a satisfactory solution to the control of all three noxious exhaust emissions under these conditions.

Air-fuel ratio:

Several factors influence the emissions from an engine, but the main culprit is the air-fuel ratio. At low air-fuel ratios, when fuel is in excess, combustion is poor and emissions of hydrocarbons and carbon monoxide are high. As the fuel content reaches a point (Stoichiometric region) at which there is exactly the right amount of oxygen from the air to burn the fuel, the hydrocarbon and carbon monoxide emissions fall, but nitrogen oxide emissions rise.  In practice, most conventional engines tend to be tuned to operate in the Stoichiometric region.  The three-way auto catalyst is used to control nitrogen oxide as well as carbon monoxide and hydrocarbon emissions under these conditions.

Maintenance of catalytic devices

SMEs manufacturing catalytic converters must also bear in mind that this equipment can become ineffective due to the following two reasons:

1. Catalytic poisoning: Heterogeneous catalysts are very sensitive to the presence of traces of foreign substances, which render them ineffective.  Any substance that reduces or completely destroys the activity of the catalyst is called 'poison' and the phenomenon is known as 'catalytic poisoning'.  Lead in petrol can be carried away along with the exhaust gases and cause catalytic poisoning (permanent poisoning). The catalyst has to be discarded and replaced in this case.

2. Carbon deposition/sooting: After considerable service, or at a poor air-fuel ratio, the pores in the catalyst can be choked or blocked by fine particles of carbon (temporary poisoning).  When this occurs, the catalyst cartridge can be removed and the soot burnt off in order to regenerate or reactivate the catalyst.

For efficient functioning of a catalytic device, the following precautions are necessary:

• Maintain an optimum air-fuel ratio in the engine
• Use unleaded petrol
• Reactivate the catalyst as and when necessary
• Avoid excess peak combustion temperature.

Peak combustion temperature is the optimum temperature at which the efficiency of the catalyst is the maximum.

With the growing green movement throughout the world, pollution control norms are bound to get tougher. So any industry in the business of reducing pollution – especially in the auto industry – is bound to see high growth. Catalytic converters are sure to be a high growth business for SMEs in the coming years.