Method of and Arrangement for Feeding Fuel Into a Circulating Fluidized Bed Boiler

Abstract

A method of feeding at least one of light, fine and moist fuel into a furnace of a circulating fluidized bed boiler. Fuel is fed into the furnace through a fuel feed and the fuel is combusted in a turbulent, circulating fluidized bed. A fuel feed area is isolated from the turbulent circulating bed by arranging the fuel feed along at least one channel arranged in a wall of the furnace. Solids of the circulating bed material are introduced onto a first grid section at the bottom of the fuel feed area. The fuel and the solids are mixed and fluidized above the grid section to form a fuel-solids mixture that flows laterally onto a second grid section and a third grid section where the mixture is fluidized. The fluidized bed material is circulated both inside and outside of the furnace. The bed material is separated from the flue gases and returned to the furnace.

1 - 23 . (canceled) 
     
     
         24 . A method of feeding at least one of light, fine and moist fuel into a furance of a circulating fluidized bed boiler, the method comprising:
 feeding a flow of fuel into a furnace of a circulating fluidized bed boiler, through a fuel feed, the fuel comprising at least one of light, fine and moist fuel;   mixing the fuel with bed material in the furnace to form a turbulent circulating bed;   isolating a fuel feed area from the turbulent circulating bed by arranging the fuel feed along at least one channel arranged in a wall of the furnace;   introducing solids of the circulating bed material and fuel onto a first grid section at the bottom of the fuel feed area;   mixing and fluidizing the fuel and the solids above the first grid section to form a fuel-solids mixture;   allowing the fuel-solids mixture to flow substantially laterally onto a second grid section;   feeding the fuel-solids mixture substantially laterally along the second grid section to a third grid section where the mixture is efficiently fluidized, and the circulating fluidized bed is created;   combusting the fuel in the presence of the fluidized bed material in the furnace and forming flue gases;   circulating the fluidized bed material both inside the furnace, in an internal circulation, where bed material returns along walls of the furnace down to the bottom of the furnace, and outside the furnace, in an external circulation, at least via a solids separator arranged in flow communication with the furnace;   separating bed material from the flue gases in the separator;   removing the flue gases from the separator for further treatment; and   returning the separated bed material to the furnace.   
     
     
         25 . The method as recited in  claim 24 , further comprising isolating the fuel feed area from the turbulent circulating bed by introducing solids from one of (i) the internal circulation flowing down along the furnace walls, (ii) a discharge of solids from a fluidized bed heat exchange chamber, and (iii) the solids returning from the solids separator on top of the fuel feed, such that the solids form a curtain between the fuel and the turbulent circulating bed. 
     
     
         26 . The method as recited in  claim 24 , further comprising fluidizing and mixing the fuel and the solids above the first grid section by using an air velocity less than about 1.0 m/s. 
     
     
         27 . The method as recited in  claim 24 , further comprising fluidizing and mixing the fuel and the solids above the first grid section by using an air velocity of between about 0.1 and about 1.2 m/s. 
     
     
         28 . The method as recited in  claim 27 , further comprising assisting the movement of the fuel-solids mixture to the second grid section by one of (i) at least sloping the first grid section towards the second grid section and (ii) arranging directional air nozzles at the first grid section. 
     
     
         29 . The method as recited in  claim 24 , further comprising using one of directional nozzles and a step grid at the second grid section for spreading the fuel-solids mixture to the third grid section. 
     
     
         30 . The method as recited in  claim 29 , further comprising using an air velocity of between about 0.4 and about 3 m/s in spreading the fuel-solids mixture to the third grid section. 
     
     
         31 . The method as recited in  claim 29 , further comprising using an air velocity of less than about 2.5 m/s in spreading the fuel-solids mixture to the third grid section. 
     
     
         32 . The method as recited in  claim 29 , further comprising using an air velocity of less than about 2.0 m/s in spreading the fuel-solids mixture to the third grid section. 
     
     
         33 . The method as recited in  claim 24 , further comprising using flow guide arrangements for spreading the fuel-solids mixture from the second grid section to desired locations at the third grid section. 
     
     
         34 . An arrangement for feeding at least one of light, fine and moist fuel into a circulating fluidized bed boiler, the boiler comprising a furnace delimited at least by a grid at the bottom of the boiler, side walls, and a roof, a solids separator arranged in flow communication with an upper portion of the furnace, an air feed for providing the furnace with primary and secondary air, a return for returning the solids separated in the separator from the flue gases into the furnace, and a fuel feed for feeding a fuel flow into the furnace, the arrangement comprising:
 a fuel feed area isolated from the rest of the furnace by at least one channel arranged in a side wall of the furnace, for increasing the residence of time of the fuel in the furnace; and   a grid divided into three functionally different sections, a first grid section, a second grid section, and a third grid section.   
     
     
         35 . The arrangement as recited in  claim 34 , wherein the fuel feed area, being isolated from the rest of the furnace, is a portion of a substantially vertically oriented channel arranged in a side wall of the furnace, the channel having side walls, and a substantially vertical bottom wall comprising a fuel feed opening, the first grid section forming the bottom of the fuel feed area. 
     
     
         36 . The arrangement as recited in  claim 34 , wherein the channel is arranged in flow communication with one of (i) a fluidized bed heat exchange chamber, (ii) the internal circulation and (iii) the solids separator. 
     
     
         37 . The arrangement as recited in  claim 34 , wherein the fuel feed area is arranged in flow communication with the second grid section to provide flow conditions that are different from those of the first grid section. 
     
     
         38 . The arrangement as recited in  claim 34 , wherein at least one of the first grid section slopes towards the second grid section and is provided with directional air nozzles for assisting in transferring the fuel-solids mixture to the second grid section. 
     
     
         39 . The arrangement as recited in  claim 34 , wherein the second grid section is arranged in flow communication with the third grid section, the third grid section having flow conditions that are different from those of the first grid section and the second grid section. 
     
     
         40 . The arrangement as recited in  claim 34 , wherein the second grid section is provided with guide arrangements for spreading the fuel-solids mixture to desired locations at the third grid section. 
     
     
         41 . The arrangement as recited in  claim 40 , wherein the guide arrangements at the second grid section are one of (i) direction nozzles and (ii) a step grid for assisting in transferring the fuel-solids mixture to a desired location at the third grid section.