Hybrid system comprising HC-SCR, NOx-trapping, and NH3-SCR for exhaust emission reduction

Abstract

An exhaust aftertreatment system is provided with a first SCR catalyst, a NOx adsorber-catalyst, and an ammonia-SCR catalyst. The first catalyst is generally a hydrocarbon-SCR catalyst, but can be a carbon monoxide-SCR catalyst or a hydrogen-SCR catalyst. The first catalyst is functional to reduce NOx in lean exhaust using the corresponding reductant. The NOx adsorbant-catalyst is functional to adsorb NOx and to produce ammonia during regeneration. The ammonia SCR catalyst is configured to adsorb ammonia so produced and is functional to subsequently use that ammonia to reduce NOx in lean exhaust. The first SCR catalyst is useful to reduce the frequency with which the NOx adsorber-catalyst needs to be regenerated, and can thereby extends the life of that catalyst. In one embodiment, reductant for the first SCR catalyst is stored during regeneration of the NOx adsorber-catalyst and is used to convert additional NOx in a subsequent lean phase.

1 . A power generation system comprising an exhaust aftertreatment system, comprising: 
 an effective amount of a first SCR catalyst selected from the group consisting of hydrocarbon-SCR catalysts, carbon monoxide-SCR catalysts, and hydrogen-SCR catalysts;    an effective amount of a NOx adsorbant-catalyst; and    an effective amount of an ammonia-SCR catalyst;    wherein the first SCR catalyst is functional to reduce NOx in lean exhaust using a reductant selected from the group consisting of hydrocarbons, carbon monoxide, and hydrogen;    the NOx adsorbant-catalyst is functional to adsorb NOx from lean exhaust and to produce ammonia during regeneration; and    the SCR catalyst is configured to adsorb ammonia so produced and is function to use that ammonia to reduce NOx in exhaust.    
   
   
       2 . The system of  claim 1 , wherein the first SCR catalyst is configured upstream of the ammonia-SCR catalyst  
   
   
       3 . The system of  claim 1 , wherein the first SCR catalyst is a hydrocarbon-SCR catalyst  
   
   
       4 . The system of  claim 1 , further comprising an in-line reformer configured upstream of the first SCR catalyst, the NOx-adsorber catalyst, and the ammonia-SCR catalyst.  
   
   
       5 . The system of  claim 1 , further comprising a in-line reformer configured downstream of the first SCR catalyst, but upstream of the NOx-adsorber catalyst, and the ammonia-SCR catalyst.  
   
   
       6 . The system of  claim 5 , wherein the power generation system is configured to inject diesel fuel into the exhaust before the first SCR catalyst, which is a hydrocarbon-SCR catalyst.  
   
   
       7 . The system of  claim 6 , wherein the hydrocarbon-SCR catalyst is adapted to store hydrocarbon and effectively use it to reduce NOx after diesel fuel injection into the exhaust ceases.  
   
   
       8 . The system of  claim 1 , wherein the first SCR catalyst is functional to adsorb the reductant during regeneration of the NOx adsorber-catalyst and is functional to subsequently use that reductant to reduce NOx in lean exhaust.  
   
   
       9 . The system of  claim 8 , wherein the reductant is produced by the reformer.  
   
   
       10 . The system of  claim 1 , wherein the first SCR catalyst is combined with the NOx-adsorber catalyst.  
   
   
       11 . The system of  claim 1 , wherein the ammonia-SCR catalyst is combined with the NOx-adsorber catalyst.  
   
   
       12 . A method of treating NOx-containing lean exhaust, comprising: 
 in a first phase: 
 contacting the exhaust with a first SCR catalyst selected from the group consisting of hydrocarbon-SCR catalysts, and carbon monoxide-SCR catalysts to reduce a portion of the NOx by reactions with hydrocarbons or CO contained in the exhaust;  
 contacting the exhaust with a NOx adsorber-catalyst to remove another portion of the NOx from the exhaust by adsorption; and  
 contacting the exhaust with an ammonia-SCR catalyst to reduce a further portion of the NOx by reactions with stored ammonia;  
   and in a second phase:    enriching the exhaust to reduce NOx stored in the NOx adsorber-catalyst and in the process produce ammonia that becomes stored in the SCR catalyst.    
   
   
       13 . The method of  claim 12 , wherein the ammonia-SCR catalyst is downstream of the first SCR catalyst.  
   
   
       14 . The method of  claim 12 , wherein: 
 the exhaust is enriched in the second phase by injecting diesel fuel; and    the enriched exhaust is processed through a reformer upstream of the first SCR catalyst.    
   
   
       15 . The method of  claim 14 , wherein the diesel fuel is injected at a rate in excess of a stoichiometric rate for producing reformate, whereby insufficient oxygen is available in the reformer to completely reform all the injected diesel fuel.  
   
   
       16 . The method of  claim 12 , wherein: 
 the exhaust is enriched in the second phase by injecting diesel fuel upstream of the first SCR catalyst; and    the enriched exhaust is processed through a reformer downstream of the first SCR catalyst.    
   
   
       17 . The method of  claim 12 , wherein at least about 15% of the NOx is reduced by the first SCR catalyst.  
   
   
       18 . The method of  claim 12 , wherein at least about 30% of the NOx is reduced by the first SCR catalyst.  
   
   
       19 . A method of treating NOx-containing lean exhaust, comprising: 
 in a first phase: 
 contacting the exhaust with a first SCR catalyst selected from the group consisting of hydrocarbon-SCR catalysts, carbon monoxide-SCR catalysts, and hydrogen-SCR catalysts to reduce a portion of the NOx by reactions with a stored first reductant selected from the group consisting of hydrocarbons, carbon monoxide, and hydrogen;  
 contacting the exhaust with a NOx adsorber-catalyst to remove another portion of the NOx from the exhaust by adsorption; and  
 contacting the exhaust with an ammonia-SCR catalyst to reduce a further portion of the NOx by reactions with stored ammonia;  
   and in a second phase: 
 making the exhaust rich;  
 storing the first reductant in the first SCR catalyst; and  
 reducing NOx stored in the NOx adsorber-catalyst and in the process producing ammonia that becomes stored by the SCR-catalyst.  
   
   
   
       20 . The method of  claim 19 , wherein making the exhaust rich comprises injecting diesel fuel into the exhaust and processing the diesel fuel through a reformer to produce reformate while removing excess oxygen from the exhaust.  
   
   
       21 . The method of  claim 20 , wherein the first SCR catalyst is a hydrocarbon-SCR catalyst and the first reductant is hydrocarbon.  
   
   
       22 . The method of  claim 21 , wherein the hydrocarbon-SCR catalyst is configured upstream of the reformer.  
   
   
       23 . The method of  claim 20 , wherein the first reductant is produced by the reformer.  
   
   
       24 . The method of  claim 23 , wherein the diesel fuel is injected at a rate in excess of a stoichiometric rate for producing reformate, whereby insufficient oxygen is available in the reformer to completely reform all the injected diesel fuel.