Oxygen enhanced low nox combustion

Abstract

Fuel such as coal is combusted in a staged combustion device in a method comprising feeding into a first combustion stage of said furnace said fuel and gaseous oxidant containing more than 21 vol. % oxygen, and preferably 21.8 to 29 vol. % oxygen, at a stoichiometric ratio below that which, if the stage were operated with air as the only oxidant, would produce the same amount of NOx, and combusting said fuel with said gaseous oxidant in said combustion stage to produce combustion products and unburned fuel.

What is claimed is:  
     
         1 . A method for combusting hydrocarbon fuel, comprising: 
 feeding into a first combustion stage said fuel and gaseous oxidant containing more than 21 vol. % oxygen, at a stoichiometric ratio below that which, if the stage were operated with air as the only oxidant, would produce the same amount of NOx, and combusting said fuel with said gaseous oxidant in said combustion stage to produce combustion products and unburned fuel.    
     
     
         2 . A method according to  claim 1  wherein the average oxygen concentration of the oxidant fed to the first combustion stage is 21.8 vol. % to 29 vol. % oxygen.  
     
     
         3 . A method according to  claim 1  further comprising heating the oxidant before it is fed to said first combustion stage.  
     
     
         4 . A method according to  claim 1  further comprising combusting said unburned fuel in a second combustion stage with additional gaseous oxidant comprised such that the average oxygen content of the oxidant fed to said first and second stages is in the range of 20.9-27.4 vol. % oxygen while removing sufficient heat from the combustion products and unburned fuel from the first stage to reach a temperature low enough to minimize additional formation of NOx in said combustion in said second stage.  
     
     
         5 . A method according to  claim 1  wherein the stoichiometric ratio in said first stage is below that which, if the stage were operated with air as the only oxidant, would produce the same amount of NOx, but is at least the lower stoichiometric ratio at which the amount of NOx formed by combustion of said fuel with said oxidant under otherwise identical conditions is said same amount.  
     
     
         6 . A method according to  claim 1  wherein said fuel is coal.  
     
     
         7 . A method according to  claim 6  wherein the average oxygen concentration of the oxidant fed to the first combustion stage is 21.8 vol. % to 29 vol. % oxygen.  
     
     
         8 . A method according to  claim 6  further comprising heating the oxidant before it is fed to said first combustion stage.  
     
     
         9 . A method according to  claim 6  further comprising combusting said unburned fuel in a second combustion stage with additional gaseous oxidant comprised such that the average oxygen content of the oxidant fed to said first and second stages is in the range of 20.9-27.4 vol. % oxygen while removing sufficient heat from the combustion products and unburned fuel from the first stage to reach a temperature low enough to minimize additional formation of NOx in said combustion in said second stage.  
     
     
         10 . A method according to  claim 6  wherein the stoichiometric ratio in said first stage is below that which, if the stage were operated with air as the only oxidant, would produce the same amount of NOx, but is at least the lower stoichiometric ratio at which the amount of NOx formed by combustion of said fuel with said oxidant under otherwise identical conditions is said same amount.  
     
     
         11 . A method for operating a furnace in which a hydrocarbon fuel is combusted, so as to reduce the amount of NOx formed by the furnace compared to the amount of NOx formed by combustion of said fuel in said furnace with air as the only oxidant, comprising: 
 feeding into a first combustion stage of said furnace said fuel and gaseous oxidant containing more than 21 vol. % oxygen, at a stoichiometric ratio below that which, if the stage were operated with air as the only oxidant, would produce the same amount of NOx, and combusting said fuel with said gaseous oxidant in said combustion stage to produce combustion products and unburned fuel.    
     
     
         12 . A method according to  claim 11  wherein the average oxygen concentration of the oxidant fed to the first combustion stage is 21.8 vol. % to 29 vol. % oxygen.  
     
     
         13 . A method according to  claim 11  further comprising heating the oxidant before it is fed to said first combustion stage.  
     
     
         14 . A method according to  claim 11  further comprising combusting said unburned fuel in a second combustion stage with additional gaseous oxidant comprised such that the average oxygen content of the oxidant fed to said first and second stages is in the range of 20.9-27.4 vol. % oxygen while removing sufficient heat from the combustion products and unburned fuel from the first stage to reach a temperature low enough to minimize additional formation of NOx in said combustion in said second stage.  
     
     
         15 . A method according to  claim 11  wherein the stoichiometric ratio in said first stage is below that which, if the stage were operated with air as the only oxidant, would produce the same amount of NOx, but is at least the lower stoichiometric ratio at which the amount of NOx formed by combustion of said fuel with said oxidant under otherwise identical conditions is said same amount.  
     
     
         16 . A method according to  claim 11  wherein said fuel is coal.  
     
     
         17 . A method according to  claim 16  wherein the average oxygen concentration of the oxidant fed to the first combustion stage is 21.8 vol. % to 29 vol. % oxygen.  
     
     
         18 . A method according to  claim 16  further comprising heating the oxidant before it is fed to said first combustion stage.  
     
     
         19 . A method according to  claim 16  further comprising combusting said unburned fuel in a second combustion stage with additional gaseous oxidant comprised such that the average oxygen content of the oxidant fed to said first and second stages is in the range of 20.9-27.4 vol. % oxygen while removing sufficient heat from the combustion products and unburned fuel from the first stage to reach a temperature low enough to minimize additional formation of NOx in said combustion in said second stage.  
     
     
         20 . A method according to  claim 16  wherein the stoichiometric ratio in said first stage is below that which, if the stage were operated with air as the only oxidant, would produce the same amount of NOx, but is at least the lower stoichiometric ratio at which the amount of NOx formed by combustion of said fuel with said oxidant under otherwise identical conditions is said same amount.