Selective catalytic reduction of NOx in real exhaust gas of gas engines using unburned gas:Catalyst deactivation and advances toward long-term stability

摘要

Catalytic conversion of CH_4 and NOx in synthetic gas and real exhaust gas of a lean-burn gas engine was investigated.Rare earth promoted Pd-MOR catalyst proved to be very active in selective catalytic reduction of NOx by CH_4 under simulated tail gas conditions.Conversion of CH_4 with SCR was,however,incomplete.A dual bed configuration,in which an SCR and an oxidation catalyst are placed in series,could complete the conversion of CH_4.The high CH_4-SCR activity was also established in a real exhaust gas of gas engines: NOx,CH_4,higher hydrocarbons,CO and aldehydes were removed (in part) from the exhaust stream.However,the conversion of CH_4 at temperatures of 400 deg C and below was much lower than in the tests with synthetic gas: NOx was mostly converted in reactions with higher hydrocarbons (and CO,aldehydes),which are present in the exhaust gas mixture.Deactivation studies indicated that steam-dealumination and agglomeration of the active sites was significant at temperatures above 450 deg C.At 400 deg C,the catalyst showed initial deactivation in real exhaust gas during the first days on stream.At longer time on stream,the conversion persisted at a constant level.The initial decay of the activity,as indicated by a loss of NOx conversion by 15%,was caused by chemisorption of the sulphur species on the catalyst surface.The sulphur species originated from the odorant,which is added to the natural gas.TPR experiments indicated that temperatures above 500 deg C were necessary to remove (hydrogenate) these sulphur species.Calcium,sodium,potassium and phosphorus,as present in the lubricant,did not have a significant influence on the catalyst deactivation.