Method for reducing nitrogen oxide emissions of a recovery boiler, and a recovery boiler

Abstract

A method for reducing nitrogen oxides of a recovery boiler, and a recovery boiler, where the furnace of the recovery boiler is supplied with primary air from primary air nozzles, black liquor from liquor nozzles, secondary air from secondary air nozzles above the primary air nozzles but still below the liquor nozzles, tertiary air from tertiary air nozzles above the liquor nozzles, and quaternary air from quaternary air nozzles above the tertiary air nozzles. Black liquor is supplied to the furnace from first liquor nozzles and second liquor nozzles, which liquor nozzles are arranged substantially on the same level with respect to the height of the furnace, and the droplet size of liquor fed from the second liquor nozzles is substantially smaller than the droplet size of liquor fed from the first liquor nozzles.

1 . A method for reducing nitrogen oxide emissions of a recovery boiler, in which method the furnace of the soda recover boiler is supplied with
 primary air from primary air nozzles to the lower part of the furnace,   black liquor from liquor nozzles,   secondary air from secondary air nozzles, which secondary air is supplied above the primary air nozzles but still underneath the liquor nozzles,   tertiary air from tertiary air nozzles, which tertiary air is supplied above the liquor nozzles,   quaternary air from quaternary air nozzles, which quaternary air is supplied above the tertiary air nozzles,   
       wherein black liquor is supplied to the furnace from first liquor nozzles and second liquor nozzles, which liquor nozzles are arranged substantially on the same level with respect to the height of the furnace, and that the droplet size of liquor fed from the second liquor nozzles is substantially smaller than the droplet size of liquor fed from the first liquor nozzles. 
     
     
         2 . The method according to  claim 1 , wherein the liquor fed from the first liquor nozzles and the pyrolysis gases formed thereof are burnt in a reducing zone and a combustion zone prevailing underneath the liquor nozzles. 
     
     
         3 . The method according to  claim 2 , wherein combustion air is fed into the reducing zone from the primary air nozzles so that substoichiometric, reducing conditions prevail in the reducing zone. 
     
     
         4 . The method according to  claim 2 , wherein combustion air is fed into the combustion zone from the secondary air nozzles so that excess air conditions prevail in the combustion zone. 
     
     
         5 . The method according to  claim 1 , wherein the liquor fed from the second liquor nozzles and the pyrolysis gases formed thereof are burnt in an afterburning zone and a burning-out zone prevailing above the second liquor nozzles. 
     
     
         6 . The method according to  claim 5 , wherein combustion air is fed into the afterburning zone from tertiary air nozzles so that substoichiometric, reducing conditions prevail in the afterburning zone. 
     
     
         7 . The method according to  claim 6 , wherein the temperature in the afterburning zone is about 950 to 1500° C., preferably about 1050 to 1400° C. 
     
     
         8 . The method according to  claim 5 , wherein combustion air is fed into the burning-out zone from the quaternary air nozzles so that excess air conditions prevail in the burning-out zone. 
     
     
         9 . The method according to  claim 8 , wherein the temperature in the burning-out zone is about 950 to 1200° C., preferably about 950 to 1050° C. 
     
     
         10 . The method according to  claim 1 , wherein black liquor is fed into the furnace from the first liquor nozzles and the second liquor nozzles which are arranged symmetrically with respect to the walls of the furnace. 
     
     
         11 . The method according to  claim 10 , wherein black liquor is fed into the furnace from the first liquor nozzles and the second liquor nozzles, the first liquor nozzles being arranged symmetrically on the walls of the furnace and the second liquor nozzles being arranged in the corners of the furnace. 
     
     
         12 . The method according to  claim 10 , wherein black liquor is fed into the furnace from the first liquor nozzles and the second liquor nozzles, the second liquor nozzles being arranged between the first liquor nozzles on the walls of the furnace. 
     
     
         13 . The method according to  claim 1 , wherein black liquor is fed into the furnace from the first liquor nozzles and the second liquor nozzles, the black liquor being supplied from the same liquor tank. 
     
     
         14 . A recovery boiler comprising
 a furnace,   primary air nozzles for feeding primary air to the lower part of the furnace,   liquor nozzles for feeding black liquor to the furnace,   secondary air nozzles for feeding secondary air into the furnace, the secondary air nozzles being placed above the primary air nozzles but still below the liquor nozzles with respect to the height of the furnace,   tertiary air nozzles for feeding tertiary air into the furnace, the tertiary air nozzles being placed above the liquor nozzles,   quaternary air nozzles for feeding quaternary air into the furnace, the quaternary air nozzles being placed above the tertiary air nozzles,   
       wherein the liquor nozzles consist of first liquor nozzles and second liquor nozzles, which liquor nozzles are arranged substantially on the same level with respect to the height of the furnace, and that the droplet size of liquor fed from the second liquor nozzles is substantially smaller than the droplet size of liquor fed from the first liquor nozzles. 
     
     
         15 . The recovery boiler according to  claim 14 , wherein below the liquor nozzles, the furnace is provided with a reducing zone and a combustion zone, in which zones the liquor fed from the first liquor nozzles and the pyrolysis gases formed thereof are arranged to be burnt. 
     
     
         16 . The recovery boiler according to  claim 15 , wherein the primary air nozzles are arranged to feed combustion air into the reducing zone. 
     
     
         17 . The recovery boiler according to  claim 15 , wherein the secondary air nozzles are arranged to feed combustion air into the combustion zone. 
     
     
         18 . The recovery boiler according to  claim 14 , wherein above the liquor nozzles, the furnace is provided with an afterburning zone and a burning-out zone, in which the liquor fed from the second liquor nozzles and the pyrolysis gases formed thereof are arranged to be burnt. 
     
     
         19 . The recovery boiler according to  claim 18 , wherein the tertiary air nozzles are arranged to feed combustion air into the afterburning zone. 
     
     
         20 . The recovery boiler according to  claim 19 , wherein the temperature in the afterburning zone is about 950 to 1500° C., preferably about 1050 to 1400° C. 
     
     
         21 . The recovery boiler according to  claim 18 , wherein the quaternary air nozzles are arranged to feed combustion air into the burning-out zone. 
     
     
         22 . The recovery boiler according to  claim 21 , wherein the temperature in the burning-out zone is about 950 to 1200° C., preferably about 950 to 1050° C. 
     
     
         23 . The recovery boiler according to  claim 14 , wherein the first and the second liquor nozzles are arranged symmetrically with respect to the walls of the furnace. 
     
     
         24 . The recovery boiler according to  claim 23 , wherein the first liquor nozzles are arranged symmetrically on the walls of the furnace and the second liquor nozzles are arranged in the corners of the furnace. 
     
     
         25 . The recovery boiler according to  claim 23 , wherein the second liquor nozzles are arranged between the first liquor nozzles on the walls of the furnace. 
     
     
         26 . The recovery boiler according to  claim 14 , wherein the black liquor fed from the first liquor nozzles and the second liquor nozzles into the furnace is supplied from the same liquor tank.