Economizer bypass with ammonia injection

Abstract

A NOx removal system for a furnace or boiler with an economizer has two separate ammonia injection grids (AIG) upstream of an SCR. A primary AIG is positioned adjacent the flue gas outlet of the economizer and upstream of an economizer bypass outlet. A secondary AIG is located within an economizer flue gas bypass. The separate AIGs permit a reduction in the overall length of the flue between the economizer and SCR, thereby reducing the NOx removal system footprint, cost and quantity of materials.

We claim:  
     
         1 . A NOx removal system for a furnace or boiler having an economizer with a flue gas bypass duct for bypassing flue gas around the economizer, the bypass duct extending between a bypass duct inlet located upstream of the economizer and a bypass duct outlet downstream of an economizer flue gas outlet, the bypass duct outlet being upstream of a selective catalytic reduction reactor, the system comprising: 
 a primary ammonia injection grid positioned between the economizer flue gas outlet and the bypass duct outlet; and    a secondary ammonia injection grid positioned within the flue gas bypass duct.    
     
     
         2 . A system according to  claim 1 , further comprising ammonia control means for controlling the amount of an ammonia supply injected through each of the primary and secondary ammonia injection grids.  
     
     
         3 . A system according to  claim 2 , further comprising first dampers adjacent the economizer outlet and second dampers adjacent the bypass duct inlet.  
     
     
         4 . A system according to  claim 3 , wherein the primary ammonia injection grid is positioned downstream of the first dampers.  
     
     
         5 . A system according to  claim 3 , wherein the secondary ammonia injection grid is positioned downstream of the second dampers.  
     
     
         6 . A system according to  claim 3 , wherein the first and second dampers are adjustable to direct from 0-100% of the flue gases through the bypass duct.  
     
     
         7 . A system according to  claim 6 , wherein the ammonia control means is adjustable to direct from 0-100% of the ammonia through the secondary ammonia injection grid.  
     
     
         8 . A system according to  claim 1 , wherein the secondary ammonia injection grid comprises at least one injection pipe.  
     
     
         9 . A system according to  claim 1 , wherein the secondary ammonia injection grid comprises a plurality of injection pipes each having a plurality of injection holes.  
     
     
         10 . An ammonia injection system for introducing ammonia to a flue gas stream of a furnace or boiler having an economizer and an economizer flue gas bypass upstream of a selective catalytic reduction reactor, the system comprising: 
 a primary ammonia injection grid positioned between a flue gas outlet of the economizer and a bypass outlet;    a secondary ammonia injection grid located in the economizer flue gas bypass; and    control means for controlling a percentage of an ammonia supply delivered to each of the primary and secondary ammonia injection grids.    
     
     
         11 . A system according to  claim 10 , wherein the control means is adjustable to deliver from 0-100% of the ammonia supply through the secondary ammonia injection grid.  
     
     
         12 . A system according to  claim 10 , wherein the secondary ammonia injection grid comprises at least one injection pipe.  
     
     
         13 . A system according to  claim 10 , wherein the secondary ammonia injection grid comprises a plurality of injection pipes each having a plurality of injection holes.