Method and improved furnance for reducing emissions of nitrogen oxides

Abstract

A method and improved furnace for reducing nitrogen oxide emissions from a furnace having a plurality of primary fuel injectors and a plurality of spaced apart over-fire air injectors positioned above the primary fuel injectors are disclosed. Injection of over-fire air produces zones of cooler combustion gasses containing over-fire air that separate zones of hot combustion gasses containing nitrogen oxides. Reburn fuel injectors inject a reburn fuel into the zones of hot combustion making the effluent combustion gases containing nitrogen oxides partially or totally fuel-rich in order to further reduce nitric oxide.

We claim: 
     
         1 . An improved furnace of the type having a plurality of primary fuel injectors on a wall of the furnace and a plurality of spaced apart over-fire air injectors on the wall of the furnace and positioned above the primary fuel injectors such that centerlines through each a pair of adjacent over-fire air injectors define a region on the wall of the furnace that extends upward and downward on the wall of the furnace between that pair of adjacent over-fire air injectors, wherein the improvement comprises a plurality of reburn fuel injectors, each reburn fuel injector located within one of the regions. 
     
     
         2 . The improved furnace of  claim 1  wherein the reburn fuel injectors are slot injectors. 
     
     
         3 . The improved furnace of  claim 1  wherein the reburn fuel injectors are capable of being tilted to direct the flow of reburn fuel in a selected upward direction or a selected downward direction. 
     
     
         4 . The improved furnace of  claim 1  wherein the reburn fuel injectors are capable of being turned left or turned right to direct the reburn fuel in one of several selected directions. 
     
     
         5 . The improved furnace of  claim 1  wherein the furnace is comprised of a series of substantially parallel boiler tubes and webbing extending between each pair of adjacent boiler tubes and the reburn fuel injectors are in the webbing. 
     
     
         6 . The improved furnace of  claim 1  wherein the reburn fuel injectors are positioned above the over-fire air injectors. 
     
     
         7 . The improved furnace of  claim 6  wherein the reburn fuel injectors are positioned about fourteen feet above the over-fire air injectors. 
     
     
         8 . The improved furnace of  claim 1  wherein the furnace is a face-fired furnace, an opposed-fired furnace, a tangentially-fired furnace, a turbo-fired furnace or grate-fired furnace. 
     
     
         9 . The improved furnace of  claim 1  wherein the reburn fuel injectors have an orifice that is round, square or oblong. 
     
     
         10 . The improved furnace of  claim 1  wherein the injectors penetrate the boiler through existing openings. 
     
     
         11 . The improved furnace of  claim 10  wherein the furnace has a primary burner air duct and at least one of the existing openings are the primary burner air duct. 
     
     
         12 . A method for reducing nitrogen oxide emissions from a furnace having a plurality of primary fuel injectors on a wall of the furnace and a plurality of spaced apart over-fire air injectors on the wall of the furnace and positioned above the primary fuel injectors wherein during operation of the furnace combustion gasses will flow from the ignition zone adjacent the fuel injectors toward and past the over-fire air injectors and the injection of over-fire air creates zones of hot combustion gases containing nitrogen oxides, the zones separated by zones of cooler combustion gases containing over-fire air, the method comprising injecting a reburn fuel into at least one of the zones of hot combustion gases. 
     
     
         13 . The method of  claim 12  wherein the reburn fuel is injected at a downward or upward angle into the at least one of the zones of hot combustion gasses. 
     
     
         14 . The method of  claim 12  wherein the reburn fuel is a high hydrogen content gas. 
     
     
         15 . The method of  claim 14  wherein the high hydrogen content gas is natural gas or coal syn-gas. 
     
     
         16 . The method of  claim 12  also comprising identifying the zones of hot combustion gases. 
     
     
         17 . The method of  claim 16  wherein the zones of hot combustion gases are identified by observation or by computer modeling. 
     
     
         18 . The method of  claim 17  wherein the computer modeling includes flow modeling of high velocity injectors designed to form nitric oxide reburn zones. 
     
     
         19 . The method of  claim 12  also comprising injecting reburn fuel at velocities sufficient to form fuel-rich reburn zones within at least one of the zones of hot combustion gases.