HIGH EFFICIENCY NOx REDUCTION SYSTEM AND METHOD

Abstract

An exhaust gas stream aftertreatment system and method achieves high efficiency NOx reduction in exhaust gas emissions by arranging a selective catalytic reduction (SCR) catalyst element upstream from a NOx adsorber catalyst. In an exemplary embodiment, a reductant is introduced into the exhaust gas stream, exposing the exhaust gas stream containing the reductant to a selective catalytic reduction (SCR) catalyst element, and exposing the exhaust gas stream that was exposed to the SCR catalyst element to a NOx adsorber catalyst element. The NOx adsorber catalyst element can be regenerated by temporarily reducing an oxygen concentration in an exhaust gas stream to reduce the λ of the exhaust gas stream. While the oxygen concentration is reduced, reductant is introduced into the exhaust gas stream at a rate that achieves a fuel-rich condition, and the fuel rich exhaust stream is exposed to the NOx adsorber catalyst element.

What is claimed is: 
     
         1 . A method of reducing NOx in an exhaust gas stream emitted from an internal combustion engine, comprising:
 introducing a reductant into the exhaust gas stream;   exposing the exhaust gas stream containing the reductant to a selective catalytic reduction (SCR) catalyst element; and   exposing the exhaust gas stream that was exposed to the SCR catalyst element to a NOx adsorber catalyst element.   
     
     
         2 . The method of  claim 1 , further comprising intermittently regenerating the NOx adsorber catalyst by temporarily reducing the oxygen mass flux in the exhaust gas stream and increasing a rate that the reductant is introduced into the exhaust stream while the oxygen mass flux is reduced to create a net reducing condition in the NOx adsorber catalyst element. 
     
     
         3 . The method of  claim 2 , wherein temporarily reducing the oxygen mass flux in the exhaust gas stream comprises choking the air intake of the engine. 
     
     
         4 . The method of  claim 2 , wherein the intermittent regeneration of the NOx adsorber catalyst is performed periodically. 
     
     
         5 . The method of  claim 2 , further comprising sensing an amount of NOx in the exhaust stream downstream from the NOx adsorber catalyst, and triggering the intermittent regeneration when the sensed amount indicates the adsorber catalyst is at least one of full and approaching a saturation point. 
     
     
         6 . The method of  claim 2 , further comprising sensing an amount of reductant in the exhaust stream downstream from the NOx adsorber catalyst, and triggering the intermittently regeneration when the sensed amount of reductant indicates the adsorber catalyst is at least one of full and approaching a saturation point. 
     
     
         7 . The method of  claim 1 , wherein the reductant is ammonia or urea. 
     
     
         8 . The method of  claim 2 , wherein an intermittent regeneration further comprises reversing the order in which the exhaust gas stream is exposed to the SCR catalyst element and the NOx adsorber catalyst element. 
     
     
         9 . A NOx reduction system comprising a gas flow circuit for treating an exhaust gas stream emitted from the outlet of an internal combustion engine, said gas flow circuit comprising:
 a reductant introduction port;   a selective catalytic reduction (SCR) catalyst element positioned downstream from the reductant introduction port; and   a NOx adsorber catalyst element positioned downstream from the SCR catalyst element.   
     
     
         10 . The system of  claim 9 , further comprising a controller that monitors whether an intermittent regeneration triggering event occurs, and initiates regeneration of the NOx adsorber catalyst element after sensing a triggering event. 
     
     
         11 . The system of  claim 10 , wherein regeneration of the NOx adsorber catalyst element includes temporarily reducing the oxygen mass flux in the exhaust gas stream exiting the internal combustion engine and increasing a rate of injection of a reductant injected from the reductant injection port while the oxygen mass flux is reduced. 
     
     
         12 . The system of  claim 10 , wherein said intermittent regeneration triggering event comprises monitoring at least one sensor positioned downstream from the NOx adsorber catalyst and initiating the regeneration of the NOx adsorber catalyst when a sensed amount indicates the adsorber catalyst is at least one of full and approaching a saturation point. 
     
     
         13 . The system of  claim 12 , wherein the sensor is a NOx sensor. 
     
     
         14 . The system of  claim 12 , wherein the sensor is an ammonia sensor. 
     
     
         15 . The system of  claim 10 , further comprising a valve that can be switched from a first state in which the SCR catalyst element is connected upstream from the NOx adsorber catalyst, and a second state in which the NOx adsorber catalyst is connected upstream from the SCR catalyst element. 
     
     
         16 . The system of  claim 15 , wherein the controller switches the valve to the second state after determining that a triggering event has occurred. 
     
     
         17 . A method of regenerating a NOx adsorber catalyst element in an exhaust aftertreatment system including a selective catalytic reduction (SCR) catalyst element positioned upstream from the NOx adsorber catalyst element, comprising:
 temporarily reducing an oxygen concentration in an exhaust gas stream to reduce the λ of the exhaust gas stream;   while the oxygen concentration is reduced, introducing a reductant into the exhaust gas stream at a rate that achieves a fuel-rich condition; and   exposing the fuel-rich exhaust gas stream to the NOx adsorber catalyst.   
     
     
         18 . The method of  claim 17 , further comprising sensing an amount of NOx in the exhaust gas stream downstream from the NOx adsorber catalyst, and triggering regeneration when the sensed amount indicates the adsorber catalyst is at least one of full and approaching a saturation point. 
     
     
         19 . The method of  claim 17 , further comprising sensing an amount of reductant in the exhaust stream downstream from the NOx adsorber catalyst, and triggering regeneration when the sensed amount of reductant indicates the adsorber catalyst is at least one of full and approaching a saturation point. 
     
     
         20 . The method of  claim 17 , wherein regeneration of the NOx adsorber catalyst is performed periodically.