Mitigation of Particulates and NOx in Engine Exhaust

Abstract

An emission treatment system and associated method for treating an exhaust stream containing nitrogen oxides and particulate matter are disclosed. One embodiment of the system comprises a flow-through oxidation catalyst, a reductant injector downstream from the oxidation catalyst, a particulate filter downstream from the reductant injector, an SCR catalyst downstream from the particulate filter and an ammonia oxidation catalyst downstream from the SCR catalyst. An embodiment of the method comprises passing the exhaust gas stream through the oxidation catalyst, injecting a reductant into the exhaust gas stream, passing the exhaust gas stream through the particulate filter, passing the exhaust gas stream through an SCR catalyst, and passing the exhaust gas stream through an ammonia oxidation catalyst.

1 . An emission treatment system for treating an exhaust stream containing nitrogen oxides and particulate matter comprising:
 a flow-through oxidation catalyst;   a reductant injector for introducing a reductant into the exhaust stream located downstream from the oxidation catalyst;   a particulate filter that does not contain an SCR catalyst for removing the particulate matter from the exhaust stream located downstream from the reductant injector; and   an SCR catalyst for promoting chemical reactions between the nitrogen oxides and the reductant located downstream from the particulate filter, the particulate filter being effective to disperse the reductant prior to the reductant contacting the SCR catalyst.   
   
   
       2 . The emission treatment system of  claim 1 , wherein the particulate filter does not contain any catalyst. 
   
   
       3 . The emission treatment system of  claim 1 , wherein the particulate filter and the SCR catalyst are closely coupled in the same canister. 
   
   
       4 . The system of  claim 1 , further comprising an ammonia oxidation catalyst located downstream from the SCR catalyst. 
   
   
       5 . The emission treatment system of  claim 1 , wherein the particulate filter is catalyzed. 
   
   
       6 . The emission treatment system of  claim 1 , wherein the reductant is urea and the particulate filter is configured so that substantially all of the urea that enters the particulate filter is converted to ammonia and isocyanic acid when it exits. 
   
   
       7 . The emission treatment system of  claim 1 , further comprising a hydrocarbon injector for introducing hydrocarbon compounds into the exhaust stream located upstream of the oxidation catalyst. 
   
   
       8 . The emission treatment system of  claim 7 , wherein the system further includes a metering system in fluid communication with the reductant injector. 
   
   
       9 . The emission treatment system of  claim 6 , wherein the reductant comprises urea and water. 
   
   
       10 . The emission treatment system of  claim 1 , wherein the SCR catalyst and the ammonia oxidation catalyst are located on the same substrate. 
   
   
       11 . The emission treatment system of  claim 5 , wherein the particulate filter has a first catalyst zone for creating NO 2  from the oxidation of NO. 
   
   
       12 . The emission treatment system of  claim 11 , wherein the particulate filter further includes a second catalyst zone for the oxidation of CO. 
   
   
       13 . The emission treatment system of  claim 12 , wherein the particulate filter further comprises a base metal oxide for burning soluble organic fraction of soot captured in the filter. 
   
   
       14 . The emission treatment system of  claim 13 , wherein the particulate filter further comprises a urea hydrolysis catalyst. 
   
   
       15 . The emission treatment system of  claim 1 , wherein the ammonia oxidation catalyst includes a catalyst for the treatment of CO and hydrocarbons. 
   
   
       16 . A method of treating an exhaust gas stream containing nitrogen oxides and particulate matter, comprising:
 passing the exhaust gas stream containing hydrocarbon compounds through an oxidation catalyst;   injecting a reductant into the exhaust gas stream downstream from the oxidation catalyst and upstream of a particulate filter that does not contain an SCR catalyst;   passing the exhaust gas stream through the particulate filter to cause the reductant to mix with the exhaust gas stream, the particulate filter being effective to disperse the reductant prior to the reductant contacting the SCR catalyst and;   passing the exhaust gas stream exiting the particulate filter through an SCR catalyst separated from the particulate filter.   
   
   
       17 . The method of  claim 16 , wherein the reductant comprises urea and the exhaust gas exiting the particulate filter contains substantially no urea. 
   
   
       18 . The method of  claim 17 , wherein the urea entering the exhaust gas filter is converted to ammonia and isocyanic acid prior to contacting the SCR filter. 
   
   
       19 . The method of  claim 16 , further comprising injecting hydrocarbon compounds into the exhaust gas stream upstream of the oxidation catalyst and passing the exhaust gas stream exiting the SCR catalyst through an ammonia oxidation catalyst. 
   
   
       20 . The method of treating an exhaust gas stream of  claim 16 , wherein passing the exhaust gas stream through the particulate filter comprises contacting the exhaust gas stream with a urea hydrolysis catalyst on the filter.