Method and system for the removal of noxious compounds from engine exhaust gas

Abstract

Method and system for the removal of noxious compounds from lean burning engines, the method comprising in series the steps of contacting the exhaust gas with a catalyst being active in oxidation of volatile organic compounds and carbon monoxide, passing the treated exhaust gas through a particulate filter catalysed with a first SCR catalyst, and passing the exhaust gas leaving the particulate filter through a second SCR catalyst, wherein ammonia is injected into the exhaust upstream of the catalysed particulate filter at a temperature below or at about 220° C. and wherein urea is injected into the exhaust gas between the first and the second SCR catalyst when the exhaust gas has reached a temperature of about 200° C.

1 . A method for the removal of noxious compounds from exhaust gas of a lean burning internal compression ignition engine comprising in series the steps of
 contacting the exhaust gas with a catalyst being active in oxidation of volatile organic compounds and carbon monoxide to carbon dioxide and water and nitrogen oxide to nitrogen dioxide;   passing the thus treated exhaust gas through a particulate filter being catalysed with a first SCR catalyst for selective reduction of nitrogen oxides; and   passing the exhaust gas leaving the filter through a second SCR catalyst for the selective reduction of nitrogen oxides, wherein ammonia reducing agent is injected into the exhaust gas upstream the catalysed particulate filter during a cold start phase of the engine when the gas has a temperature of below or at about 220° C., and wherein injection of ammonia is discontinued and urea as precursor for the ammonia reducing agent is injected into the gas between the first and second SCR catalyst when the gas has reached a temperature of about 200° C.   
     
     
         2 . The method of  claim 1 , wherein the ammonia reducing agent is injected into the exhaust gas prior to the contact with the oxidation catalyst. 
     
     
         3 . The method of  claim 1 , wherein the ammonia reducing agent is injected into the exhaust gas between the oxidation catalyst and the SCR catalyzed particulate filter. 
     
     
         4 . The method according to  claim 1 , wherein the ammonia reducing agent is released from an ammonia absorbent prior to injection into the exhaust gas. 
     
     
         5 . The method according to  claim 1 , wherein the exhaust gas is further passed through an ammonia oxidation catalyst for selective oxidation of ammonia downstream the second SCR catalyst. 
     
     
         6 . System for use in the method according to  claim 1  comprising within an engine exhaust gas channel connected to the engine, arranged in series
 an oxidation catalyst unit for the oxidation of volatile organic compounds and carbon monoxide to carbon dioxide and water and nitrogen oxide to nitrogen dioxide; 
 a particulate filter comprising a first catalyst for selective reduction of nitrogen oxides; 
 a second catalyst unit for the selective reduction of nitrogen oxides; 
 upstream the particulate filter, injection means for the injection of ammonia into the engine exhaust gas channel; and 
 between the particulate filter and the second catalyst for the selective reduction of nitrogen oxides, injection means for the injection of urea into the engine exhaust gas channel. 
 
     
     
         7 . The system of  claim 6 , wherein the injection means for injection of ammonia is arranged between the engine and the of the oxidation catalyst unit. 
     
     
         8 . The system of  claim 6 , wherein the injection means for injection of ammonia is connecter to a container holding a solid ammonia storage material. 
     
     
         9 . The system according to  claim 6 , wherein the oxidation catalyst unit and the particulate filter comprising a first catalyst for selective reduction of nitrogen oxides are arranged in close-coupled position. 
     
     
         10 . The system according to  claim 1  being further provided with a catalyst unit for the selective oxidation of ammonia to nitrogen downstream the second catalyst unit for the selective reduction of nitrogen oxides.