Maximizing conversion of nitrogen oxides in the treatment of combustion

Abstract

A three-catalyst process for maximizing the conversion of nitrogen oxides during the treatment of combustion exhaust gases is provided. This process comprises in sequence contacting combustion exhaust gas in a first zone with a nitrogen-oxide-reduction catalyst to form a nitrogen-oxide-reduced gas; introducing a first stream of secondary air into said nitrogen-oxide-reduced gas to form a first air-gas mixture; contacting said first air-gas mixture in a second zone with a catalyst for decreasing ammonia to form a gas containing decreased amounts of ammonia; introducing a second stream of secondary air into said gas containing decreased amounts of ammonia to form a second air-gas mixture; and contacting said second air-gas mixture in a third zone with an oxidation catalyst for the oxidation of carbon monoxide and hydrocarbons to form purified exhaust gas. The first stream of secondary air provides about 3 to about 8 volume percent air, based on the volume of primary air being employed in the combustion, while the second stream of secondary air is sufficient, when added to the oxygen remaining in the gas containing decreased amounts of ammonia, to provide substantially complete oxidation of the carbon monoxide and hydrocarbons in the gas containing decreased amounts of ammonia. The process may be employed to treat automobile exhaust gases.