Method For Reducing Nitrogen Oxide Emissions of a Bubbling Fluidized Bed Boiler and an Air Distribution System of a Bubbling Fluidized Bed Boiler

Abstract

A method for reducing nitrogen oxide emissions of a bubbling fluidized bed boiler burning biofuel and an air distribution system for a bubbling fluidized bed boiler biofuel. A fluidized bed is arranged in a lower part of a furnace of the boiler. The bed is fluidized by fluidizing gas that includes primary air. Fuel is fed to the fluidized bed, which dries and pyrolizes into pyrolysis gas including volatile matter of the fuel. The gas rises upwards in the furnace and burns. Secondary air is supplied above the fluidized bed from secondary air nozzles, and tertiary air is supplied above the secondary air nozzles. A part of primary air is supplied in connection with fuel feeding such that the fuel is forced substantially on the surface of the fluidized bed, thus pyrolizing entirely, and at least a part of the pyrolysis gases formed in the pyrolysis is burned by primary air such that the air coefficient in relation to the volatile matter of fuel in the pyrolysis gases is in the substoichiometric area.

1 - 25 . (canceled) 
   
   
       26 . A method for reducing nitrogen oxide emissions of a bubbling fluidized bed boiler burning biofuel, the method comprising:
 supplying at least primary air to a fluidized bed arranged in a lower part of a furnace of the fluidized bed boiler for fluidizing bed material forming the fluidized bed in the furnace;   feeding fuel to the fluidized bed, which dries when coming into contact with hot bed material and pyrolizes into pyrolysis gas comprising volatile matter of fuel, which gas rises upwards in the furnace and burns there;   burning at least a part of the carbon residue from the pyrolysis in the fluidized bed with primary air;   supplying secondary air above the fluidized bed from secondary air nozzles;   supplying tertiary air above the secondary air nozzles; and   supplying a part of primary air to the furnace in connection with fuel feeding such that the fuel is forced substantially on the surface of the fluidized bed, and the fuel is pyrolyzed substantially entirely, and at least a part of the pyrolysis gases formed in the pyrolysis is burned in an atmosphere where the air coefficient in relation to the volatile matter of fuel in the pyrolysis gas is in the substoichiometric area.   
   
   
       27 . The method according to  claim 26 , wherein combustion air of volatile matter is supplied to the furnace in connection with fuel feeding in such a manner that at least a part of the pyrolysis gases are burnt before secondary air is supplied to the furnace. 
   
   
       28 . The method according to  claim 26 , wherein a part of the pyrolysis gases formed in the pyrolysis is burned by primary air supplied to the furnace. 
   
   
       29 . The method according to  claim 26 , wherein fuel is forced onto the surface of the fluidized bed by the momentum of the primary air supplied in connection with fuel feeding. 
   
   
       30 . The method according to  claim 26 , wherein a primary air zone, comprising a great deal of hydrocarbon radicals which reduce nitrogen oxides, is formed between the upper part of the fluidized bed and the secondary air nozzles. 
   
   
       31 . The method according to  claim 26 , wherein primary air is supplied to the furnace in connection with fuel feeding such that at least a part of the pyrolysis gases formed in the pyrolysis are burned in a manner such that an air coefficient in relation to volatile matter of fuel is 0.75 to −0.97. 
   
   
       32 . The method according to  claim 31 , wherein the air coefficient in relation to volatile matter of fuel is 0.90 to 0.95. 
   
   
       33 . The method according to  claim 31 , wherein primary air is supplied to the furnace in connection with fuel feeding such that at least a part of the pyrolysis gases formed in the pyrolysis are burned in a manner such that the total air coefficient is 0.5 to 0.8. 
   
   
       34 . The method according to  claim 33 , wherein the total air coefficient is 0.65. 
   
   
       35 . The method according to  claim 26 , wherein secondary air is supplied to the furnace from the secondary air nozzles such that a substoichiometric, nitrogen-oxides-reducing secondary air zone is formed between the secondary air nozzles and the tertiary air nozzles, in which zone the total air coefficient is 0.7 to 0.95. 
   
   
       36 . The method according to  claim 35 , wherein the total air coefficient is 0.85 to 0.9. 
   
   
       37 . The method according to  claim 26 , wherein the part of the primary air is supplied to the furnace together with the fuel or parallel with the fuel feeding. 
   
   
       38 . The method according to  claim 37 , wherein a part of primary air is supplied to the furnace around the fuel feeding opening. 
   
   
       39 . The method according to  claim 26 , wherein recirculating gas is supplied to the fluidized bed in order to fluidize the bed material forming the fluidized bed in the furnace. 
   
   
       40 . An air distribution system of a biofuel-burning bubbling fluidized bed boiler, comprising:
 nozzles arranged on a bottom of a furnace of the bubbling fluidized bed boiler for supplying at least primary air for fluidizing bed material forming the fluidized bed arranged in a lower part of the furnace;   fuel feeding means arranged on walls of the furnace for feeding fuel to the fluidized bed, which fuel dries when coming into contact with hot bed material and pyrolizes into pyrolysis gas comprising volatile matter of the fuel, which gas rises upwards in the furnace and bums there, wherein at least a part of the carbon residue from the pyrolysis is arranged to be burnt in the fluidized bed by means of primary air;   secondary air nozzles arranged on the walls of the furnace above the fluidized bed in order to supply secondary air into the furnace; and   tertiary air nozzles arranged on the walls of the furnace above the secondary air nozzles in order to supply tertiary air into the furnace,   wherein a part of the primary air is arranged to be supplied to the furnace in connection with fuel feeding such that the fuel is forced substantially on the surface of the fluidized bed, and the fuel is pyrolyzed substantially entirely, and at least a part of the pyrolysis gases formed in the pyrolysis is arranged to be burnt in an atmosphere where the air coefficient in relation to the volatile matter of fuel in the pyrolysis gas is in the substoichiometric area.   
   
   
       41 . The air distribution system according to  claim 40 , wherein combustion air of volatile matter is arranged to be supplied to the furnace in connection with fuel feeding in such a manner that at least a part of the pyrolysis gases are burnt before secondary air is supplied to the furnace. 
   
   
       42 . The air distribution system according to  claim 40 , wherein a part of the pyrolysis gases formed in the pyrolysis is arranged to be burnt by the primary air supplied to the furnace. 
   
   
       43 . The air distribution system according to  claim 40 , wherein the momentum of the primary air supplied in connection with fuel feeding forces the fuel onto the surface of the fluidized bed. 
   
   
       44 . The air distribution system according to  claim 40 , wherein a primary air zone, comprising a great deal of hydrocarbon radicals which reduce nitrogen oxides, is arranged to be formed between the upper part of the fluidized bed and the secondary air nozzles. 
   
   
       45 . The air distribution system according to  claim 40 , wherein at least a part of the pyrolysis gases formed in the pyrolysis is arranged to be burnt by means of the primary air supplied to the furnace in connection with fuel feeding in such a manner that the air coefficient in relation to volatile matter of fuel is 0.75 to 0.97. 
   
   
       46 . The air distribution system according to  claim 45 , wherein the air coefficient in relation to volatile matter of fuel is 0.90 to 0.95. 
   
   
       47 . The air distribution system according to  claim 45 , wherein at least a part of the pyrolysis gases formed in the pyrolysis is arranged to be burnt by means of the primary air supplied to the furnace in connection with fuel feeding in such a manner that the total air coefficient is 0.5 o 0.8. 
   
   
       48 . The air distribution system according to  claim 47 , wherein the total air coefficient is 0.65. 
   
   
       49 . The air distribution system according to  claim 40 , wherein secondary air is arranged to be supplied to the furnace from the secondary air nozzles in such a manner that a substoichiometric, nitrogen-oxides-reducing secondary air zone is formed between the secondary air nozzles and the tertiary air nozzles, in which zone the total air coefficient is 0.7 to 0.95. 
   
   
       50 . The air distribution system according to  claim 49 , wherein the total air coefficient in the zone is 0.85 to 0.9. 
   
   
       51 . The air distribution system according to  claim 40 , wherein the primary air supplied in connection with fuel feeding is arranged to be supplied to the furnace together with the fuel or parallel with the fuel feeding. 
   
   
       52 . The air distribution system according to  claim 51 , wherein the fuel feeding means comprises a fuel feeding opening and in its immediate vicinity at least one air supply opening, and wherein the primary air supplied in connection with fuel feeding is arranged to be supplied to the furnace around the fuel feeding opening parallel with the fuel fed from the fuel feeding opening. 
   
   
       53 . The air distribution system according to  claim 52 , wherein an air supply opening is arranged around the fuel feeding opening in order to supply the primary air supplied in connection with fuel feeding. 
   
   
       54 . The air distribution system according to  claim 40 , wherein the nozzles are arranged to supply recirculation gas to the fluidized bed in order to fluidize the bed material forming the fluidized bed in the furnace.