Catalyst to improve low temperature deNOx activity in a reformer-LNT exhaust aftertreatment system

Abstract

A low temperature lean NO x catalyst is configured in a diesel exhaust treatment system upstream of a fuel reformer-LNT system to provide low temperature performance. During system warm-up and anytime the LNT falls below an effective operating temperature, the exhaust aftertreatment system is provided with hydrocarbons at a rate proportional to the exhaust NO x flow rate, whereby NO x is reduced over the lean NO x catalyst. Optionally, the lean NO x catalyst is also adapted for use warming up the fuel reformer and or catalyzing NO to NO 2 conversion. These concepts allow the reformer and LNT to be designed for high temperature performance and durability without also having to be adapted for NO x mitigation at the exhaust system's lowest temperatures.

1 . A power generation system, comprising:
 a diesel engine operative to produce exhaust containing NO x ; and   an exhaust aftertreatment system configured to treat the exhaust with a hydrocarbon SCR catalyst, a fuel reformer, and a LNT configured in that order within an exhaust line;   wherein the hydrocarbon SCR catalyst is functional to catalyze selective catalytic reduction of NO x  with hydrocarbons in a lean environment at 200° C.   the fuel reformer is functional to produce reformate when the fuel reformer is sufficiently warm, the exhaust is rich, and the exhaust contains diesel fuel; and   the LNT is adapted to store NO x  when the exhaust from the fuel reformer is lean and to reduce stored NO x  and regenerate when the exhaust from the fuel reformer is rich and contains reformate.   
   
   
       2 . The power generation system of  claim 1 , further comprising a fuel injector configured to inject fuel into the exhaust line upstream of the hydrocarbon SCR catalyst. 
   
   
       3 . The power generation system of  claim 2 , further comprising a second fuel injector configured to inject fuel into the exhaust line downstream of the hydrocarbon SCR catalyst, but upstream of the fuel reformer. 
   
   
       4 . The power generation system of  claim 1 , further comprising a fuel injector configured to inject fuel into the exhaust line downstream of the hydrocarbon SCR catalyst, but upstream of the fuel reformer. 
   
   
       5 . The power generation system of  claim 2 , further comprising:
 a controller configured to selectively inject fuel through the fuel injector while the exhaust aftertreatment system is warming up.   
   
   
       6 . The power generation system of  claim 2 , further comprising:
 a controller configured to selectively inject fuel through the fuel injector in order to induce combustion within the hydrocarbon SCR catalyst in order to warm the fuel reformer as needed prior to initiating a rich regeneration phase.   
   
   
       7 . The power generation system of  claim 1 , wherein the fuel reformer is adapted to produce reformate by steam reforming reactions. 
   
   
       8 . The power generation system of  claim 1 , further comprising:
 an ammonia SCR reactor configured in the exhaust line to receive at least a portion of the exhaust passing through the LNT;   wherein the ammonia SCR reactor is adapted to store ammonia produced by the LNT during regeneration and to reduce NO x  contained in the exhaust by catalyzing reactions between the NO x  and stored ammonia when the exhaust is lean.   
   
   
       9 . The power generation system of  claim 1 , wherein the hydrocarbon SCR catalyst is a PGM catalyst also functional to catalyze oxidation of NO to NO 2  at 200° C. 
   
   
       10 . The power generation system  claim 1 , wherein the hydrocarbon SCR catalyst comprises an effective amount of platinum. 
   
   
       11 . A method of operating a power generation system, comprising:
 starting a diesel engine;   operating the diesel engine to produce an exhaust containing NO x ;   channeling the exhaust through an exhaust aftertreatment system;   while the exhaust system is warming, dosing the exhaust with diesel fuel at a rate proportional to the NO x  flow rate in the exhaust, whereby a substantial portion of the NO x  is reduced over a lean NO x  catalyst contained in the exhaust aftertreatment system; and   in response to the exhaust system warming, discontinuing the diesel dosing to the exhaust system.   
   
   
       12 . The method of  claim 11 , further comprising:
 within the exhaust system, passing the exhaust first through the lean NO x  catalyst, then through a fuel reformer, and then through a LNT.   
   
   
       13 . The method of  claim 12 , further comprising:
 generating a control signal to regenerate the LNT;   in response to the control signal, dosing the exhaust aftertreatment with diesel fuel and combusting the dosed diesel fuel over the lean NO x  catalyst in order to heat the reformer; and   subsequently providing additional diesel dosing to make the exhaust rich, whereby the reformer produces reformate and regenerates the LNT.   
   
   
       14 . The method of  claim 13 , wherein providing additional diesel dosing comprises injecting diesel fuel into the exhaust aftertreatment system downstream of the lean NO x  catalyst, but upstream of the fuel reformer. 
   
   
       15 . The method of  claim 13 , wherein the fuel reformer functions to produce reformate by steam reforming reactions during LNT regeneration. 
   
   
       16 . The method of  claim 12 , wherein the lean NO x  catalyst is effective for oxidizing NO to NO 2  at 200° C. 
   
   
       17 . The method of  claim 12 , wherein the lean NO x  catalyst comprises an effective amount of platinum. 
   
   
       18 . The method of  claim 12 , further comprising passing the exhaust over an ammonia SCR catalyst after passing the exhaust through the LNT. 
   
   
       19 . The method of  claim 11 , further comprising:
 in response to the LNT falling below a critical temperature, reinitiating diesel dosing to the exhaust system at a rate proportional to the NO x  flow rate in the exhaust.