Cm

Solids in gas, lb/ft3

FIGURE 5-15. Relationship between heat transfer and solids loading/bed density. (From Elliot, T. C., Ed., Standard Handbook of Powerplant Engineering, McGraw-Hill, New York, 1989. With permission.)

Circulating fluidized-beds include these major components: a refractory-lined combustor bottom section with fluidizing nozzles on the floor above the windbox; an upper combustor section, usually with waterwalls; a transition pipe, including a hot-solids separator and reentry down-comer; convective boiler section; and, in some designs, an external heat exchanger [14]. An external heat exchanger is a refractory-lined box containing an air distribution grid and an immersed tube bundle designed to cool material from the hot-solids separator and that is used to compensate for variations in the heat absorption rate caused by changes in fuel properties and load conditions. The solids separators are refractory-lined cyclones that are

Solids in gas, lb/ft3

FIGURE 5-15. Relationship between heat transfer and solids loading/bed density. (From Elliot, T. C., Ed., Standard Handbook of Powerplant Engineering, McGraw-Hill, New York, 1989. With permission.)

used to keep the solids circulating. The solids reinjection device, called an L-valve, J-valve, loop seal, Fluoseal, or a sealpot, depending on the manufacturer and configuration, provides a simple, nonmechanical hydraulic barometric seal against the combustor shell.

Suspension Firing

Pulverized coal-firing is the method of choice for large industrial boilers (e.g., >250,000 pounds of steam per hour) and coal-fired electric utility generators because pulverized coal-fired units can be constructed to very large sizes (i.e., up to ~1300 MW or ~9.5 million pounds of steam per hour), and, unlike stoker units where some designs have coal restrictions, they can accommodate virtually any coal with proper design provisions. The coal size distribution for pulverized coal-fired units is typically <2% by weight greater than 50 mesh (300 ^m) with 65 to 70% less than 200 mesh (74 ^m) for lignites and subbituminous coals and 80 to 85% less than 200 mesh for bituminous coals [19]. After the coal is pulverized, it is pneumatically transported to the burners using a portion of the combustion air, typically 10% of the total combustion air (the remaining combustion air is introduced at or near the burner), in a manner that permits stable ignition, effective control of flame shape and travel, and thorough and complete mixing of fuel and air.

Pulverized coal-fired units are typically classified into two types, depending on the furnace design for ash removal. In dry-bottom furnaces, the ash is removed from the system in dry form; in wet-bottom or slag-tap furnaces, the ash is removed in molten form. Dry-bottom furnaces are the more common of the two types and are now almost the only type sold in the United States. Dry-bottom furnaces are simpler to operate, more flexible with respect to fuel properties, and more reliable than slag-tap furnaces [2]. Dry-bottom furnaces are larger (hence, more costly) than wet-bottom furnaces since they must be sized to accommodate the ash where most of it (>80%) remains entrained in the flue gas and must be removed by par-ticulate control devices at the back end of the system. Slag-tap units were developed to reduce the amount of fine fly ash that had to be handled by producing a heavier, granular ash and retaining most of the ash (up to 80%) in the furnace.

Dry-Bottom Firing The most frequently used dry-bottom furnace and burner configurations are shown in Figure 5-16 [2]. These arrangements cover firing systems suitable for all ranks of coal and coal qualities, including high ash or moisture content, low heating value, low ash fusion temperature, and high potential for ash deposition. Dry-bottom furnaces are designed to remove the ash as a solid; therefore, the rate of heat transfer and temperature in the furnace must be controlled. The dry-bottom furnaces are designed such that the heat release rates are much lower than wet-bottom and cyclone furnaces, and this, coupled with maintaining the furnace exit gas temperature below the ash fusion temperature, results in larger furnace designs.

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