Method for reducing nitrogen oxide emissions in oxyfuel combustion

Abstract

A method for reducing nitrogen oxide emissions in oxyfuel combustion, which method comprises supplying in a furnace ( 11 ) of a circulating fluidised bed boiler ( 10 ) at least one primary gas flow ( 15 ) and at least one secondary gas flow ( 16 ), which both have been produced by mixing oxygen and circulated flue gas together. The oxygen content of the primary gas ( 15 ) is adjusted such that a reducing zone (I) is formed at the bottom of the furnace, in which zone nitrogen oxides are reduced to nitrogen. The oxygen content of the secondary gas is adjusted such that above the reducing zone (I) is formed an oxidising zone (II), in which zone combustion is completed.

1 . A method for reducing nitrogen oxide emissions in oxyfuel combustion, the method comprising supplying in a furnace of a circulating fluidised bed boiler at least one primary gas flow and at least one secondary gas flow, which primary gas and secondary gas have been produced by mixing oxygen and circulated flue gas together, characterised by adjusting the oxygen content of the primary gas such that at the bottom of the furnace is formed a reducing zone (I), and adjusting the oxygen content of the secondary gas such that above the reducing zone (I) is formed an oxidising zone (II), wherein the oxygen contents of the primary gas and the secondary gas are adjusted by changing the ratio of oxygen to circulated flue gas in said gas flows. 
     
     
         2 . A method according to  claim 1 , characterised by supplying in the furnace two or more secondary gas flows at two or more different heights. 
     
     
         3 . A method according to  claim 2 , characterised by adjusting the oxygen contents of the secondary gas flows supplied at different heights different from each other. 
     
     
         4 . A method according to any one of  claims 1 - 3 , characterised by mixing oxygen and circulated flue gas together with mixing means in connection with the furnace just before supplying the primary gas or the secondary gas to the furnace. 
     
     
         5 . A method according to  claim 1 , characterised by mixing oxygen and circulated flue gas together with mixing means just before supplying the primary gas flow or the secondary gas flow into connection with the furnace. 
     
     
         6 . A method according to  claim 1 , characterised by adjusting the temperature of the furnace by varying the oxygen content of the primary gas and/or the secondary gas.