SELECTIVE CATALYTIC REDUCTION OF NOx ENABLED BY SIDESTREAM UREA DECOMPOSITION

Abstract

A preferred apparatus arrangement utilizes the enthalpy of the flue gas, which can be supplemented if need be, to convert urea ( 30 ) into ammonia for SCR. Urea ( 30 ), which decomposes at temperatures above 140 .degree. C., is injected ( 32 ) into a flue gas stream split off ( 28 ) after a heat exchanger ( 22 ), such as a primary superheater or an economizer. Ideally, the side stream would gasify the urea without need for further heating; but, when heat is required it is far less than would be needed to heat either the entire effluent ( 23 ) or the urea ( 30 ). This side stream, typically less than 3% of the flue gas, provides the required temperature and residence time for complete decomposition of urea ( 30 ). A cyclonic separator can be used to remove particulates and completely mix the reagent and flue gas. This stream can then be directed to an injection grid ( 37 ) ahead of SCR using a blower ( 36 ). The mixing with the flue gas is facilitated due to an order of magnitude higher mass of side stream compared to that injected through the AIG in a traditional ammonia-SCR process.

1 . A process for reducing the concentration of nitrogen oxides in a stream of combustion gases, comprising: providing a side stream of gases at a temperature of at least 140 C; introducing an aqueous solution of urea into said side stream under conditions effective to gasify said aqueous urea; introducing said side stream of gases containing the gases resulting from the gasification of the urea into a primary stream of NOx-containing gases of greater volume than the side stream to create a combined gas stream; and passing the combined gas stream through a NOx-reducing catalyst under conditions effective to reduce the concentration of NOX in the combined gas stream. 
     
     
         2 . A process according to  claim 1 , wherein the side stream comprises combustion gases separated from a combustion gas stream to produce said side stream and said primary stream. 
     
     
         3 . A process according to  claim 1 , wherein the side stream comprises outside air. 
     
     
         4 . A process according to  claim 1 , wherein the side stream comprises gases withdrawn from said combined gas stream following their passage through said NO x  reducing catalyst. 
     
     
         5 . A process according to  claim 1 , wherein said side steam of gases is heated to a temperature of at least 200 C prior to introducing the aqueous solution of urea. 
     
     
         6 . A process according to  claim 1 , wherein the urea solution is introduced at a rate relative to the NOX concentration in said combined stream prior to passage through said NOx-reducing catalyst effective to provide an NSR of from 0.1 to 2.0. 
     
     
         7 . A process according to  claim 1 , wherein the aqueous urea has a concentration of from 5 to 70%. 
     
     
         8 . A process according to  claim 1 , wherein the side stream is heated by the use of steam to facilitate gasification of the urea. 
     
     
         9 . A process according to  claim 1 , wherein the side stream is passed through a mixing device prior to introducing said side stream of gases containing the gases resulting from the gasification of the urea into said primary stream of NOX-containing gases to create said combined gas stream. 
     
     
         10 . A process according to  claim 1 , wherein urea is introduced into the side stream following passage of the gases therein through particulate reduction means. 
     
     
         11 . A process according to  claim 1 , wherein the urea is a solid reagent. 
     
     
         12 . A process according to  claim 1 , wherein said side steam of gases is heated to a temperature of at least 200 C prior to introducing the aqueous solution of urea having a concentration of from 5 to 70% at a rate relative to the NOX concentration in said combined stream prior to passage through said NOx-reducing catalyst effective to provide an NSR of from 0.1 to 2.0, and the side stream is passed through a mixing device prior to introducing said side stream of gases containing the gases resulting from the gasification of the urea into said primary stream of NOx-containing gases to create said combined gas stream. 
     
     
         13 . A process according to  claim 1 , wherein said side steam of gases comprises less than 10% of the volume of the combined gas stream under standard conditions. 
     
     
         14 . A process according to  claim 1 , comprising: providing a side stream of gases at a temperature of at least 200 C, said side stream comprising combustion gases separated from a combustion gas stream to produce said side stream and a primary stream, wherein said side stream of gases comprises less than 10% of the volume of the combustion gases under standard conditions; introducing an aqueous solution of urea into said side stream under conditions effective to gasify said aqueous urea, said urea having a concentration of from 5 to 70% and is introduced at a rate relative to the NOX concentration in said combined stream prior to passage through said NOx-reducing catalyst effective to provide an NSR of from 0.1 to 2.0; introducing said side stream of gases containing the gases resulting from the gasification of the urea into said primary stream of NOx-containing gases of greater volume than the side stream to create a combined gas stream; and passing the combined gas stream through a NOx-reducing catalyst under conditions effective to reduce the concentration of NOx in the combined gas stream. 
     
     
         15 . A process according to  claim 1 , comprising: providing a side stream of gases at a temperature of at least 200° C., said side stream comprising combustion gases separated from a combustion gas stream to produce said side stream and a primary stream, wherein said side stream of gases comprises less than 10% of the volume of the combustion gases under standard conditions; introducing an aqueous solution of urea into said side stream under conditions effective to gasify said aqueous urea, said urea having a concentration of from 5 to 70% and is introduced at a rate relative to the NOx concentration in said combined stream prior to passage through said NOx-reducing catalyst effective to provide an NSR of from 0.1 to 2.0; introducing said side stream of gases containing the gases resulting from the gasification of the urea into said primary stream of NOx-containing gases of greater volume than the side stream to create a combined gas stream; and passing the combined gas stream through a NOx-reducing catalyst under conditions effective to reduce the concentration of NO, in the combined gas stream; wherein said combustion gases comprised in said side stream are separated from said combined gas stream following passage through the NOx-reducing catalyst. 
     
     
         16 . A process according to  claim 1 , comprising: providing a side stream of gases at a temperature of at least 200° C., wherein said side stream of gases comprises less than 10% of the volume of the combustion gases under standard conditions and are supplied from a source external of the combustion gases; introducing an aqueous solution of urea into said side stream under conditions effective to gasify said aqueous urea, said urea having a concentration of from 15 to 70% and is introduced at a rate relative to the NOx concentration in said combined stream prior to passage through said NOx-reducing catalyst effective to provide an NSR of from 0.1 to 2.0; introducing said side stream of gases containing the gases resulting from the gasification of the urea into said primary stream of NOx-containing gases of greater volume than the side stream to create a combined gas stream; and passing the combined gas stream through a NOx-reducing catalyst under conditions effective to reduce the concentration of NOx in the combined gas stream. 
     
     
         17 . An apparatus for reducing the concentration of nitrogen oxides in a stream of combustion gases, comprising: conduit means for transporting a side stream of gases at a temperature of at least 140 C; means for introducing an aqueous solution of NOx-reducing agent into said side stream under conditions effective to gasify said aqueous NOx-reducing agent; means for introducing said side stream of gases containing the gases resulting from the gasification of the NOx-reducing agent into a primary stream of NOx-containing gases of greater volume than the side stream to create a combined gas stream; and means for passing the combined gas stream through a NOx-reducing catalyst under conditions effective to reduce the concentration of NO, in the combined gas stream. 
     
     
         18 . An apparatus according to  claim 17 , wherein means are provided for separating the side stream from a combustion gas stream to produce said side stream and said primary stream. 
     
     
         19 . An apparatus according to  claim 17 , wherein means are provided for introducing outside air as the side stream. 
     
     
         20 . An apparatus according to  claim 17 , wherein means are provided for withdrawing gases from said combined gas stream following their passage through said NOx-reducing catalyst to form said side stream. 
     
     
         21 . (canceled) 
     
     
         22 . (canceled) 
     
     
         23 . (canceled) 
     
     
         24 . (canceled) 
     
     
         25 . (canceled) 
     
     
         26 . (canceled) 
     
     
         27 . (canceled)