Thermal management of hybrid LNT/SCR aftertreatment during desulfation

Abstract

In an exhaust aftertreatment system comprising a NOx adsorber-catalyst followed by an SCR catalyst, means are provided for preventing the SCR catalyst from becoming heated to near the same peak temperatures as the NOx adsorber-catalyst during desulfation. In one embodiment, the means is a thermal mass between the NOx adsorber-catalyst and the SCR catalyst. In another embodiment, the means is a valve configured to selectively divert exhaust leaving the NOx adsorber-catalyst from the SCR catalyst. In a method of the invention, the NOx adsorber-catalyst temperature is cycled during desulfation. The peaks of the cycles are within an appropriate temperature range for desulfating the NOx adsorber-catalyst, but the average temperature is below the temperature range at which the SCR catalyst is damaged. The temperature peaks are damped as they travel from the NOx adsorber-catalyst to the SCR, whereby the SCR experiences much lower peak temperatures than the NOx adsorber-catalyst.

1 . An exhaust aftertreatment system, comprising: 
 a NOx adsorber-catalyst configured to treat an exhaust flow;    an ammonia SCR catalyst configured to adsorb ammonia released by the NOx adsorber-catalyst during denitration and to subsequently use the ammonia to reduce NOx in the exhaust; and    a thermal mass positioned between the NOx adsorber-catalyst and the ammonia SCR catalyst, the thermal mass being neither a particulate filter, an oxidation catalyst, an ammonia SCR catalyst, or a NOx adsorber-catalyst;    wherein the thermal mass is configured to exchange heat with the exhaust as it passes from the NOx adsorber-catalyst to the ammonia SCR catalyst; and    the thermal mass is functional to adsorb heat from the exhaust, store the adsorbed heat, and thereby prevent the ammonia SCR catalyst from becoming heated to near the same peak temperatures as the NOx adsorber-catalyst during desulfation of the NOx adsorber-catalyst.    
   
   
       2 . The exhaust aftertreatment system of  claim 1 , wherein the ammonia SCR catalyst, the thermal mass, and the NOx adsorber-catalyst are all contained in a single housing.  
   
   
       3 . The exhaust aftertreatment system of  claim 1 , further comprising a controller configured to cycle the exhaust gas temperature during desulfation of the NOx adsorber-catalyst.  
   
   
       4 . The exhaust aftertreatment system of  claim 1 , wherein the thermal mass is effective to make the peak exhaust gas temperatures entering the ammonia SCR catalyst at least about 100° C. less than the peak exhaust gas temperatures leaving the NOx adsorber-catalyst during desulfation.  
   
   
       5 . The exhaust aftertreatment system of  claim 1 , further comprising a particulate filter configured to remove particulates from the exhaust either upstream of the NOx adsorber-catalyst or downstream of the SCR catalyst.  
   
   
       6 . An exhaust aftertreatment system, comprising: 
 a NOx adsorber-catalyst configured to treat an exhaust flow;    an ammonia SCR catalyst configured to adsorb ammonia released by the NOx adsorber-catalyst during denitration and to subsequently use the ammonia to reduce NOx in the exhaust; and    a device, which is neither a particulate filter, an oxidation catalyst, an ammonia SCR catalyst, or a NOx adsorber-catalyst, positioned between the NOx adsorber-catalyst and the SCR catalyst;    wherein the device provides a thermal mass functional to substantially dampen temperature pulses transmitted from the NOx adsorber-catalyst to the SCR catalyst through the exhaust, whereby the temperature pulses are attenuated and have substantially lower peaks.    
   
   
       7 . The exhaust aftertreatment system of  claim 6 , wherein the ammonia SCR catalyst, the device, and the NOx adsorber-catalyst are all contained in a single housing.  
   
   
       8 . The exhaust aftertreatment system of  claim 6 , further comprising a controller configured to cycle the exhaust gas temperature during desulfation of the NOx adsorber-catalyst.  
   
   
       9 . The exhaust aftertreatment system of  claim 6 , wherein the device is effective to make the peak exhaust gas temperatures entering the ammonia SCR catalyst at least about 100° C. less than the peak exhaust gas temperatures leaving the NOx adsorber-catalyst during desulfation.  
   
   
       10 . The exhaust aftertreatment system of  claim 6 , further comprising a particulate filter configured to remove particulates from the exhaust either upstream of the NOx adsorber-catalyst or downstream of the ammonia SCR catalyst.  
   
   
       11 . A method of desulfating an NOx adsorber-catalyst in an exhaust aftertreatment system comprising an NOx adsorber-catalyst followed by an ammonia SCR catalyst, comprising: 
 cycling the temperature of the NOx adsorber-catalyst;    creating a reducing environment in the NOx adsorber-catalyst during portions of the cycles where the NOx adsorber-catalyst is at temperatures effective for desulfation;    wherein the cycles have peak temperatures in a range effective for desulfation and an average temperature that is at least 100° C. lower.    
   
   
       12 . The method of  claim 11 , further comprising positioning a device between the NOx adsorber-catalyst and the SCR catalyst that effectively dampens the temperature cycles.  
   
   
       13 . The method of  claim 11 , wherein the device effectively dampens exhaust temperature fluctuations without substantially reducing an average exhaust temperature by adsorbing and releasing heat.  
   
   
       14 . The method of  claim 11 , further comprising: 
 protecting the SCR catalyst from the peak temperatures using a device between the NOx adsorber-catalyst and the SCR catalyst that provides the protection by virtue of it thermal mass.    
   
   
       15 . The method of  claim 14 , wherein the device is neither a particulate filter, an oxidation catalyst, an ammonia SCR catalyst, or a NOx adsorber-catalyst.  
   
   
       16 . The method of  claim 14 , further comprising passing the exhaust through a diesel particulate filter either before the NOx adsorber-catalyst or after the ammonia SCR catalyst.