Method for purification of an exhaust gas from a diesel engine

Abstract

A method for removing impurities in exhaust gas from a diesel engine, where the impurities comprise nitrogen oxides, carbon monoxide, particulate matter and incompletely combusted hydrocarbons. The method comprises the steps of injection of a reductant comprising urea or ammonia into the exhaust gas from the engine, selective catalytic reduction of the nitrogen oxides in the exhaust gas by the reductant, and intermittent injection of a hydrocarbon into this effluent. The succeeding steps are oxidation of carbon monoxide, particulate matter, incompletely combusted hydrocarbons and injected hydrocarbon to carbon dioxide and water, and in selectively oxidising possible excess of reductant to free nitrogen, and finally filtration of the effluent by passing the gas through a catalysed filter. The remaining particulate matter is retained in the filter, and the carbon monoxide, particulate matter and hydrocarbons are oxidixed to carbon dioxide and water, and the reductant is selectively oxidixed to nitrogen, creating a purified exhaust gas.

1 . A method for removing impurities in exhaust gas from a diesel engine, where the impurities comprise nitrogen oxides, carbon monoxide, particulate matter and incompletely combusted hydrocarbons,
 which method comprises the steps of   (a) injection in excess compared to stoichiometric ratio of a reductant comprising urea or ammonia into the exhaust gas from the engine;   (b) reduction of the nitrogen oxides in the exhaust gas by the reductant in the presence of a catalyst active in selective reduction of nitrogen oxides to nitrogen;   (c) intermittent injection of a hydrocarbon into the effluent from step (b);   (d) oxidation of carbon monoxide, particulate matter, incompletely combusted hydrocarbons and injected hydrocarbon in the presence of a catalyst active in oxidising carbon monoxide, particulate matter and hydrocarbons, to carbon dioxide and water, and in selectively oxidising excess of reductant to free nitrogen;   (e) filtration of the effluent from step (d) by passing the gas through a catalysed filter, wherein the remaining particulate matter is retained in the filter, and wherein the catalyst is active in oxidising carbon monoxide, particulate matter and hydrocarbons to carbon dioxide and water, and in selective oxidising reductant to nitrogen, creating a purified exhaust gas; and   (f) withdrawal of the purified exhaust gas.   
   
   
       2 . A method according to  claim 1 , further comprising the step of a pre-oxidation of carbon monoxide, particulate matter, nitrogen oxides and incompletely combusted hydrocarbons in the exhaust gas from the engine in the presence of a catalyst active in oxidising the carbon monoxide, nitrogen oxides, hydrocarbons, and particulate matter to carbon dioxide, nitrogen dioxide and water prior to the step (a). 
   
   
       3 . A method according to  claim 1 , wherein the injected hydrocarbon is diesel fuel. 
   
   
       4 . A method according to  claim 1 , wherein the catalyst for selective reduction is a zeolite or an ion exchanged zeolite on a cordierite catalyst support, or one or more base metal oxides catalyst on a catalyst support of one or more metal oxides, the catalyst having the form of a monolite, a foam or a metal mesh. 
   
   
       5 . A method according to  claim 1 , wherein the oxidation catalyst is one or more precious metals or one or more base metals on a catalyst support of a zeolite or a metal oxide, the catalyst having the form of a monolite, a foam or a metal mesh. 
   
   
       6 . A method according to  claim 1 , wherein the catalyst coated on the filter is a precious metal, but not including platinum, and on a catalyst support of a metal oxide. 
   
   
       7 . A method according to  claim 2 , wherein the preoxidation catalyst is one or more precious metals or one or more base metals on a catalyst support of a zeolite or of an oxide of one or more metals, the catalyst having the form of a monolite, a foam or a metal mesh. 
   
   
       8 . A method according to  claim 1 , wherein the catalyst base metal is one or more of vanadium, tungsten, cerium and manganese, the support metal oxide is titania, alumina and/or ceria and the ion exchanged zeolite is Cu/Fe exchanged β zeolite or ZSM-5 zeolite. 
   
   
       9 . A method according to  claim 1 , wherein the support for the oxidation catalyst is a zeolite, titania, alumina, ceria or zirconia, and wherein the catalyst precious metal is platinum, palladium and/or rhodium and the catalyst base metal is manganese, copper, cobalt and/or chromium. 
   
   
       10 . A method according to  claim 1 , wherein the support for the catalyst is titania, alumina, ceria or zirconia, and the catalyst is palladium. 
   
   
       11 . A method according to  claim 2 , wherein the catalyst support is one or more oxides of aluminium, cerium, zirconium and/or titanium, and the catalyst precious metal is platinum and/or palladium and the catalyst base metal is manganese, copper, cobalt and/or chromium. 
   
   
       12 . A method according to  claim 1 , wherein the catalyst is vanadium/tungsten oxide on a titania support, cerium/tungsten oxide on a titania support or manganese oxide on a titania support. 
   
   
       13 . A method according to  claim 1 , wherein the catalyst is platinum on a titania support or platinum/palladium on a titania support. 
   
   
       14 . A method according to  claim 1 , wherein the catalyst is palladium on a titania support. 
   
   
       15 . A method according to  claim 2 , wherein the catalyst is platinum/palladium on an aluminium/cerium oxide support.