Method of extending the useful life of an aged SCR catalyst bed in an exhaust system of a stationary source of NOx

A method of extending the useful life of an aged selective catalytic reduction (SCR) catalyst bed, which catalyses the conversion of oxides of nitrogen (NOx) to dinitrogen (N2) in the presence of a nitrogenous reductant, in the exhaust gas after treatment system of a stationary source of NOxso that the exhaust gas emitted to atmosphere from the system continues to meet proscribed limits for both NOxand ammonia emissions, which method comprising the step of retrofitting an additional honeycomb substrate monolith or a plate-type substrate comprising a catalyst (ASC) for converting ammonia in exhaust gas also containing oxygen to nitrogen and water downstream of the aged SCR catalyst bed, wherein the kNOxof the honeycomb substrate monolith comprising the catalyst for converting ammonia in exhaust gas also containing oxygen to nitrogen and water is greater than or equal to 80 m/hr between 300 and 400°C inclusive, wherein kNOx of a sample of the catalyst, which has been aged at 450°C in 10% H20 (as steam) in air for 48 hours, is determined by a SCR activity test in a laboratory scale reactor using a gas composition of 50 ppm CO, 30 ppm NO, 36 ppm NH3, 15% 02, 8% water, 3% C02, balance N2.