HC mitigation to reduce NOx spike

Abstract

Systems and methods are disclosed for ameliorating NOx slip from a lean NOx trap by reducing the amount of hydrocarbons reaching the lean NOx trap during the early stages of, or in a period immediately preceding, a rich regeneration. In one embodiment, a hydrocarbon absorber is configured downstream from a fuel reformer, but upstream from the lean NOx trap, in order to reduce the quantity of hydrocarbons that reach the lean NOx trap during lean reformer warm-up and rich regeneration phases. In another embodiment, the fueling rate to a fuel reformer configured in an exhaust line upstream from the lean NOx trap is limited to reduce NOx slip.

1 . A power generation system, comprising:
 a diesel engine operative to produce exhaust;   an exhaust system configured to receive the exhaust from the diesel engine;   a lean NOx trap configured within an exhaust line of the exhaust system;   a hydrocarbon absorber configured within the exhaust system upstream from the lean NOx trap;   a fuel reformer configured to supply syn gas for regenerating the lean NOx trap;   wherein the exhaust system configuration requires the syn gas to pass through the hydrocarbon absorber in order to reach the lean NOx trap.   
   
   
       2 . The power generation system of  claim 1 , wherein the fuel reformer is configured to receive most of the exhaust from the engine. 
   
   
       3 . The power generation system of  claim 1 , wherein the exhaust system does not contain any valves other than exhaust gas recirculation valves. 
   
   
       4 . The power generation system of  claim 1 , wherein the hydrocarbon absorber is functional to absorb at least about 25% of the hydrocarbons with more than three hydrocarbons in the exhaust passing through the hydrocarbon absorber during a regeneration of the lean NOx trap while at a temperature of 275° C. 
   
   
       5 . The power generation system of  claim 1 , wherein the hydrocarbon absorber is functional to adsorb hydrocarbons at 275° C. and release most of the stored hydrocarbons at 450° C. 
   
   
       6 . The power generation system of  claim 1 , wherein the hydrocarbon absorber comprises an effective amount of a zeolite for adsorbing hydrocarbons from the exhaust. 
   
   
       7 . The power generation system of  claim 2 , wherein the reformer is of a type that is not effective for reforming diesel fuel contained in the exhaust at 350° C., but is effective for reforming diesel fuel contained in the exhaust at 600° C. 
   
   
       8 . The power generation system of  claim 1 , further comprising an ammonia-selective catalytic reduction catalyst configured in the exhaust system downstream from the lean NOx trap. 
   
   
       9 . The power generation system of  claim 1 , wherein the hydrocarbon absorber has a significantly greater thermal mass than the fuel reformer. 
   
   
       10 . A vehicle comprising the power generation system of  claim 1 . 
   
   
       11 . A method of operating a power generation system, comprising:
 operating a diesel engine to produce an exhaust comprising NOx;   passing at least a portion of the exhaust from the engine to a lean NOx trap through a fuel reformer;   using the lean NOx trap, adsorbing NOx from the exhaust under lean conditions;   preparing to regenerate the lean NOx trap by injecting diesel fuel into the exhaust upstream from the fuel reformer, whereby the injected fuel combusts in the fuel reformer under lean conditions to heat the fuel reformer over a period;   as the fuel reformer heats, trapping hydrocarbons slipping past the fuel reformer in a hydrocarbon absorber downstream from the fuel reformer, but upstream from the lean NOx trap;   regenerating the lean NOx trap by increasing the fuel injection rate to form syn gas under rich conditions within the fuel reformer.   
   
   
       12 . The method of  claim 11 , wherein the fuel reformer is heated to at least about 500° C. under lean conditions. 
   
   
       13 . The method of  claim 12 , wherein over the course of the lean NOx trap regeneration, the fuel reformer is heated by at least 200° C., but the lean NOx trap is heated by less than about 100° C. 
   
   
       14 . The method of  claim 12 , wherein the hydrocarbon absorber is eventually heated to a temperature at which it releases the hydrocarbons it adsorbed during the period of heating the fuel reformer under lean conditions. 
   
   
       15 . The method of  claim 11 , further comprising oxidizing the adsorbed hydrocarbons within the hydrocarbon absorber under lean conditions. 
   
   
       16 . The method of  claim 11 , wherein at least about 5% of the hydrocarbon injected to heat the fuel reformer under lean conditions slips past the fuel reformer without being oxidized. 
   
   
       17 . The method of  claim 11 , wherein the hydrocarbon absorber functions to substantially reduce the amount of NOx released from the lean NOx trap without being reduced. 
   
   
       18 . The method of  claim 11 , wherein the hydrocarbon absorber functions to substantially reduce hydrocarbon poisoning of an ammonia selective catalytic reduction catalyst configured downstream from the lean NOx trap. 
   
   
       19 . A method of operating a power generation system, comprising:
 operating a diesel engine to produce an exhaust comprising NOx;   passing the exhaust from the engine to a lean NOx trap through a fuel reformer;   adsorbing NOx from the exhaust under lean conditions using the lean NOx trap;   regenerating the lean NOx trap by injecting diesel fuel into the exhaust upstream from the fuel reformer to create rich conditions within the fuel reformer, whereby a portion of the injected diesel fuel is reformed into syn gas that regenerates the lean NOx trap; and   while regenerating the lean NOx trap, adsorbing hydrocarbons from the exhaust downstream from the fuel reformer but upstream from the lean NOx trap in a hydrocarbon absorber.   
   
   
       20 . The method of  claim 19 , wherein at least about 5% of the hydrocarbon injected to create rich conditions within the fuel reformer slips past the fuel reformer without being completely oxidized or converted to syn gas. 
   
   
       21 . The method of  claim 19 , wherein the hydrocarbon absorber functions to substantially reduce the amount of NOx released from the lean NOx trap without being reduced during the regeneration of the lean NOx trap. 
   
   
       22 . The method of  claim 19 , wherein the hydrocarbon absorber functions to reduce the amount of NOx released from the lean NOx trap without being reduced to nitrogen or ammonia to below 10% of the amount of NOx removed from the lean NOx trap over the course of the regeneration. 
   
   
       23 . The method of  claim 19 , wherein the hydrocarbons adsorbed by the hydrocarbon absorber are desorbed as a result of the hydrocarbon absorber becoming heated during the later part of the lean NOx trap regeneration. 
   
   
       24 . The method of  claim 19 , further comprising oxidizing the adsorbed hydrocarbons within the hydrocarbon absorber under lean conditions. 
   
   
       25 . The method of  claim 19 , wherein the hydrocarbon absorber functions to substantially reduce hydrocarbon poisoning of an ammonia selective catalytic reduction catalyst configured downstream from the lean NOx trap.