High-set separated overfire air system for pulverized coal fired boilers

Abstract

A furnace ( 10 ) includes a series of lower compartments ( 18 ) extending in a vertical arrangement, with at least one of the lower compartments ( 18 ) having one or more fuel nozzles ( 20 ) which tangentially fires fuel into the combustion chamber at an offset from a diagonal (DD) passing through opposed corners of the combustion chamber. At least one of the lower compartments ( 18 ) has one or more air nozzles ( 22 ) which tangentially introduces air into the combustion chamber along an offset direction which is offset from the diagonal (DD) to the same side as the fuel firing offset direction. The tangentially fired air and the offset fired fuel create a swirling fireball (RB) in the combustion chamber. At least one overfire compartment ( 24 ) has at least one air nozzle ( 40 ) for injecting air in opposition to the swirling fireball (RB), along an opposition offset direction which is offset to the other side of the diagonal (DD), in a manner such that the injected air promotes the evolution of the swirling fireball (RB) into an upward flow.

1 . A pulverized coal-firing furnace comprising: 
 a combustion chamber having a boiler nose, a burner region disposed below the boiler nose, and four comers, each of the comers being substantially equidistant from each adjacent corner;    a series of lower compartments for introducing therethrough one of air, fuel, or air and fuel into the combustion chamber, the series of lower compartments extending in a vertical arrangement from a topmost one of the lower compartments to a bottommost one of the lower compartments;    at least one fuel nozzle for tangentially firing fuel from the series of lower compartments into the combustion chamber at an offset from a diagonal passing through one pair of opposed comers of the combustion chamber, the at least one fuel nozzle defining a topmost fuel injection compartment having a centerline at a height H tot  from the boiler nose;    at least one air nozzle for tangentially introducing air from the lower compartments into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction, the air fired tangentially from the lower compartments being in an amount less than the amount required for complete combustion with the fuel such that the offset fired fuel and air create a swirling fireball in the combustion chamber; and    a highset overfire position having a centerline and at least one overfire compartment having at least one air nozzle for injecting air from the at least one overfire compartment generally in opposition to the swirling fireball along an opposition offset direction which is offset to the other side of the diagonal in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow, the centerline of the highset overfire position being disposed at a height H sofa  from the centerline of the topmost fuel injection compartment,    wherein 0.5≦(H sofa /H tot )≦0.9.    
     
     
         2 . The furnace of  claim 1  wherein there are no overfire positions disposed between the highset overfire position and the topmost fuel injection compartment.  
     
     
         3 . A pulverized coal-firing furnace comprising: 
 a combustion chamber having a burner region, four corners, and four walls defining an axis extending vertically through a center of the burner region, each of the corners being substantially equidistant from each adjacent corner;    a series of lower compartments for introducing therethrough one of air, fuel, or air and fuel into the combustion chamber, the series of lower compartments extending in a vertical arrangement from a topmost one of the lower compartments to a bottommost one of the lower compartments;    at least one fuel nozzle for tangentially firing fuel from the series of lower compartments into the combustion chamber at an offset from a diagonal passing through one pair of opposed corners of the combustion chamber;    at least one air nozzle for tangentially introducing air from the lower compartments into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction, the air fired tangentially from the lower compartments being in an amount less than the amount required for complete combustion with the fuel such that the offset fired fuel and air create a swirling fireball in the combustion chamber;    at least one overfire compartment for injecting air into the combustion chamber, the at least one overfire compartment being disposed at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment; and    at least one high velocity air nozzle and at least one low velocity air nozzle for injecting air from the at least one overfire compartment generally in opposition to the swirling fireball along an opposition offset direction which is offset to the other side of the diagonal in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow, whereby high velocity air jets from the at least one high velocity air nozzle penetrate to the axis, preventing a fuel rich core at center of the boiler, and low velocity air jets from the at least one low velocity air nozzle sweep the furnace walls, preventing fuel rich pockets proximate to the furnace walls.    
     
     
         4 . The furnace of  claim 3  wherein the at least one overfire compartment includes a first overfire compartment and a second overfire compartment, the at least one high velocity air nozzle being disposed in the first overfire compartment and the at least one low velocity air nozzle being disposed in the second overfire compartment.  
     
     
         5 . The furnace of  claim 4  wherein the at least one high velocity air nozzle and the at least one low velocity air nozzle each have a free area, the free area of the at least one low velocity air nozzle being substantially equal to three times the free area of the at least one high velocity air nozzle.  
     
     
         6 . The furnace of  claim 5  wherein each of the overfire compartments has an air flow, the air flow of the second overfire compartment being substantially equal to the air flow of the first overfire compartment.  
     
     
         7 . The furnace of  claim 6  wherein the second overfire compartment includes a plurality of walls defining a flow passage and a damper disposed in the flow passage, the walls and damper defining a restricted passage for creating a pressure drop in the flow passage.  
     
     
         8 . The furnace of  claim 4  wherein each of the overfire compartments has an independent yaw control.  
     
     
         9 . A pulverized coal-firing furnace comprising: 
 a combustion chamber having a burner region, four corners, and four walls defining an axis extending vertically through a center of the burner region, each of the corners being substantially equidistant from each adjacent corner;    a series of lower compartments for introducing therethrough one of air, fuel, or air and fuel into the combustion chamber, the series of lower compartments extending in a vertical arrangement and including an upper first lower compartment, a lower second lower compartment, and a lowest third lower compartment, at least one of the upper first the lower second, and the lowest third lower compartments having at least one fuel nozzle for tangentially firing fuel into the combustion chamber at an offset from a diagonal passing through one pair of opposed corners of the combustion chamber, at least one of the upper first, the lower second, and the lowest third lower compartments having a plurality of air nozzles for tangentially introducing air from the lower compartments into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction, the air fired tangentially from the lower compartments being in an amount less than the amount required for complete combustion with the fuel such that the offset fired fuel and air create a swirling fireball in the combustion chamber;    at least one overfire compartment for injecting air into the combustion chamber, the at least one overfire compartment being disposed at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment; and    at least one air nozzle for injecting air from the at least one overfire compartment generally in opposition to the swirling fireball along an opposition offset direction which is offset to the other side of the diagonal in a manner such that the injected air promotes the evolution of the swirling fireball into an upward flow.    
     
     
         10 . The furnace of  claim 9  wherein each of the upper first, the lower second, and the lowest third lower compartments includes a damper.  
     
     
         11 . The furnace of  claim 9  wherein at least one of the upper first, the lower second, and the lowest third lower compartments has upper intermediate, and lower sub-compartments and, in the at least one of the upper first, the lower second, and the lowest third lower compartments having an upper, intermediate, and lower sub-compartment, the intermediate sub-compartment includes a damper and the upper and lower sub-compartments include a common damper.  
     
     
         12 . The furnace of  claim 9  wherein the second lower compartment includes a single tilt control.  
     
     
         13 . The furnace of  claim 11  wherein each of the sub-compartments includes a yaw control.  
     
     
         14 . The furnace of  claim 11  wherein a one of the sub-compartments has an offset from the diagonal which is different from the offset from the diagonal of the other sub-compartments.  
     
     
         15 . The furnace of  claim 11  wherein each of the sub-compartments has an offset from the diagonal which different from the offset from the diagonal of each other sub-compartment.  
     
     
         16 . The furnace of  claim 11  wherein one or two of the sub-compartments include at least one fuel nozzle for tangentially firing fuel into the combustion chamber at an offset from the diagonal.  
     
     
         17 . A pulverized coal-firing furnace comprising: 
 a combustion chamber having a burner region, four corners, and four walls defining an axis extending vertically through a center of the burner region, each of the corners being substantially equidistant from each adjacent corner;    a series of lower compartments for introducing therethrough one of air, fuel, or air and fuel into the combustion chamber, the series of lower compartments extending in a vertical arrangement from a topmost one of the lower compartments to a bottommost one of the lower compartments;    a plurality of fuel nozzles for firing fuel from the series of lower compartments into the combustion chamber, a lowest one of the lower compartments having at least one fuel nozzle defining a lowermost fuel injection compartment, each other of the lower compartments having at least one fuel nozzle defining an upper fuel injection compartment, the fuel nozzles of each upper fuel injection compartment tangentially firing fuel at an offset direction which is offset to one side of a diagonal passing through one pair of opposed corners of the combustion chamber;    at least one air nozzle for tangentially introducing air from the lower compartments into the combustion chamber along an offset direction which is offset from the diagonal to the same side as the fuel firing offset direction, the air fired tangentially from the lower compartments being in an amount less than the amount required for complete combustion with the fuel such that the offset fired fuel and air create a fireball rotating in a swirl direction in the combustion chamber;    at least one overfire compartment for injecting air into the combustion chamber, the at least one overfire compartment being disposed at a vertical distance from the topmost lower compartment which is greater than the vertical distance between any given one of the lower compartments and an adjacent lower compartment; and    at least one air nozzle for injecting air from the at least one overfire compartment generally in opposition to the rotating fireball along an opposition offset direction which is offset to the other side of the diagonal in a manner such that the injected air promotes the evolution of the rotating fireball into an upward flow;    wherein the at least one fuel nozzle of the lowermost fuel injection compartment fires fuel vertically downward and in the opposition offset direction.    
     
     
         18 . The furnace of  claim 17  wherein the at least one fuel nozzle of the lowermost fuel injection compartment fires fuel at 15 degrees from the diagonal in the opposition offset direction.  
     
     
         19 . The furnace of  claim 17  wherein the lowermost fuel injection compartment includes tilt and yaw controls.  
     
     
         20 . The furnace of  claim 19  wherein the lowermost fuel injection compartment is rotatable at ±15° tilt or at ±15° yaw.