Exhaust aftertreatment system

Abstract

An exhaust aftertreatment system connected to an internal combustion engine operative lean of stoichiometry includes first and second selective catalytic reactor devices. A preferred ratio of hydrocarbon:NOx to achieve a preferred concentration of ammonia immediately downstream of the first selective catalytic reactor device is determined. An ethanol-based reductant is dispensed upstream of the first selective catalytic reactor device.

1 . Method for operating an internal combustion engine operative lean of stoichiometry and providing an exhaust gas feedstream fluidly connected to an exhaust aftertreatment system including a first selective catalytic reactor device fluidly connected upstream to an ammonia-selective catalytic reactor device, comprising:
 monitoring operation of the internal combustion engine and the exhaust gas feedstream upstream of the first selective catalytic reactor device;   determining a preferred ratio of hydrocarbon:NOx for the exhaust gas feedstream upstream of the first selective catalytic reactor device effective to achieve a preferred concentration of ammonia immediately downstream of the first selective catalytic reactor device based upon the operation of the internal combustion engine and the exhaust gas feedstream upstream of the first selective catalytic reactor device; and   dispensing an ethanol-based reductant upstream of the first selective catalytic reactor device to achieve the preferred ratio of hydrocarbon:NOx in the exhaust gas feedstream upstream of the first selective catalytic reactor device.   
     
     
         2 . The method of  claim 1 , wherein the preferred concentration of ammonia corresponds to a preferred ratio of ammonia:NOx upstream of the ammonia-selective catalytic reactor device. 
     
     
         3 . The method of  claim 2 , wherein the preferred ratio of ammonia:NOx upstream of the ammonia-selective catalytic reactor device comprises substantially equal concentrations of NOx and ammonia. 
     
     
         4 . The method of  claim 1 , wherein monitoring operation of the internal combustion engine and the exhaust gas feedstream upstream of the first selective catalytic reactor device comprises determining a concentration of NOx, a temperature, and a mass flow of the exhaust gas feedstream upstream of the first selective catalytic reactor device, the method further comprising:
 determining a space velocity of the first selective catalytic reactor device; and   wherein dispensing an ethanol-based reductant upstream of the first selective catalytic reactor device comprises controlling a mass flow rate of the ethanol-based reductant upstream of the first selective catalytic reactor device based on the concentration of NOx and the temperature of the exhaust gas feedstream upstream of the first selective catalytic reactor device, and the space velocity of the first selective catalytic reactor device.   
     
     
         5 . The method of  claim 4 , wherein determining the preferred ratio of hydrocarbon:NOx for the exhaust gas feedstream upstream of the first selective catalytic reactor device comprises determining the preferred ratio of hydrocarbon:NOx for the exhaust gas feedstream upstream of the first selective catalytic reactor device effective to achieve substantially equal concentrations of NOx and ammonia upstream of the ammonia-selective catalytic reactor device corresponding to the concentration of NOx, the temperature, and the mass flow of the exhaust gas feedstream upstream of the first selective catalytic reactor device. 
     
     
         6 . Method for operating an internal combustion engine at a lean air/fuel ratio, comprising:
 fluidly connecting an exhaust aftertreatment system to an exhaust gas feedstream of the internal combustion engine, the exhaust aftertreatment system including a silver-alumina catalytic reactor device fluidly connected upstream to an ammonia-selective catalytic reactor device;   determining a concentration of NOx in the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device;   determining a preferred ratio of hydrocarbon:NOx for the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device effective to achieve substantially equal concentrations of NOx and ammonia in the exhaust gas feedstream immediately downstream of the silver-alumina catalytic reactor device; and   dispensing a reductant upstream of the silver-alumina catalytic reactor device to achieve the preferred ratio for hydrocarbon:NOx in the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device.   
     
     
         7 . The method of  claim 6 , wherein dispensing a reductant upstream of the silver-alumina catalytic reactor device comprises dispensing an ethanol-based reductant. 
     
     
         8 . The method of  claim 6 , further comprising:
 monitoring the internal combustion engine and the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device to determine the concentration of NOx in the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device, a temperature of the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device and a mass flow of the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device,   determining a space velocity of the silver-alumina catalytic reactor device corresponding to the mass flow of the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device; and   wherein dispensing a reductant upstream of the silver-alumina catalytic reactor device comprises controlling a mass flow rate of an ethanol-based reductant upstream of the silver-alumina catalytic reactor device based on the concentration of NOx and the temperature of the exhaust gas feedstream upstream of the silver-alumina catalytic reactor device, and the space velocity of the silver-alumina catalytic reactor device.   
     
     
         9 . Apparatus, comprising
 an internal combustion engine configured to operate lean of stoichiometry and provide an exhaust gas feedstream;   an exhaust aftertreatment system fluidly connected to the internal combustion engine to receive the exhaust gas feedstream, including an ethanol-selective catalytic reactor device fluidly connected upstream to an ammonia-selective catalytic reactor device and a reductant dispensing system configured to dispense an ethanol-based reductant upstream of the ethanol-selective catalytic reactor device;   a control system signally and operatively connected to the internal combustion engine and the exhaust aftertreatment system, the control system configured to monitor the internal combustion engine and the exhaust aftertreatment system and control the reductant dispensing system to dispense the ethanol-based reductant upstream of the ethanol-selective catalytic reactor device to achieve a preferred ratio of ammonia:NOx downstream of the ethanol-selective catalytic reactor device and upstream of the ammonia-selective catalytic reactor device.   
     
     
         10 . The apparatus of  claim 9 , wherein the ethanol-selective catalytic reactor device comprises a flow-through substrate coated with a silver-alumina catalyst. 
     
     
         11 . The apparatus of  claim 10 , wherein the silver-alumina catalyst comprises a 3 wt. % Ag 2 O catalyst supported on an alumina washcoat. 
     
     
         12 . The apparatus of  claim 9 , wherein the ammonia-selective catalytic reactor device comprises a flow-through substrate coated with a copper-zeolite catalyst. 
     
     
         13 . The apparatus of  claim 9 , further comprising the reductant dispensing system operative to dispense the ethanol reductant into the exhaust gas feedstream at a HC1:NOx ratio that achieves substantially equal concentrations of NOx and ammonia in the exhaust gas feedstream downstream of the ethanol-selective catalytic reactor device and upstream of the ammonia-selective catalytic reactor device.