Combustion, suspension-firing

Ptilverizers The pulverizer is the heart of any solid-fuel suspension-firing system. Air is used to dry the coal, transport it through the pulverizer, classify it, and transport it to the burner, where the transport air provides part of the air for combustion. The pulverizers themselves... 

Conceptually speaking, the technology for combustors is similar to that used for coal combustion (see Chapter 9) if solid biomass fuels are used. Typically used configurations for biomass combustion include [41] (1) pile-burned, (2) stoker-fired, (3) suspension-fired, and (4) fluidized-bed combustors. 

The suspension-fired boilers are similar to the pulverized coal firing technology and involve combusting the fuel in the form of small particles as they are fed into the boiler. A great deal of pretreatment is required of the fuel, which is a potential disadvantage. However, the higher boiler efficiency is an advantage. 

TABLE 6.2 Typical Moisture Contents and Heating Values of Waste Biomass for Combustion in Fluid-Bed, Grate, and Suspension Firing Units... 

A demonstration unit sponsored by the Environemtal Protection Agency (EPA) to produce RDF at St. Louis proved the basic feasibility of mechanical separation processes, transport and storage techniques, and combustion of fluff RDF to replace 5 to 27 percent of the pulverized coal used in suspension-fired utility boilers. However, the refinement of equipment components and the technical and economic optimization of the basic technology still require a great deal of work. 

Fluidized bed combustion of coal is used to reduce the emission of sulfur dioxide and NO into the enviromnent. In this technology the pulverized fuel is burnt in a fluidized bed combustor after being blended with limestone or dolomite, which act as soibents for SO2. The burning temperature is significantly lower than is common in the production of fly ashes in suspension-fired furnace chambers, at around 850°C. The process may take place at atmospheric pressure (atmospheric fluidized bed combustion, AFBC) or elevated pressure (pressurized fluidized bed combustion, PFBC). 

The first suspension-fired furnace in the United States was designed like the one shown for vertical firing in Fig 17.24. Pulverized coal (about 70% through a 200-mesh screen) is transported to the burner with primary air, the amount of this air being about 20 percent of that needed for complete combustion. The... 

The WP L cyclone boiler will bum I DE continuously with coal, as about 5% of its fuel mix, with htfle or no modification. By contrast, pulverized-coal boilers, which account for about 80% of the coal-fired capacity in the United States, probably caimot bum tire chips without significant modifications. In these boilers, which bum very fine coal particles in suspension, the heavy chips will fall from the area where best combustion occurs. 

CycUme Furnaces In cyclone firing (Fig. 24-l4d) the coal is not pulverized but is crushed to 4-mesh (4.76-mm) size and admitted tangentially with primary air to a horizontal cylindrical chamber, called a cydone furnace, which is connected peripherally to a boiler furnace. Secondary air also is admitted, so that most all of the coal bums within the chamber. The combustion gas then flows into the boiler furnace. In the cyclone furnace, finer coal particles burn in suspension and the... 

In a pulverized boiler, the coal is ground to the consistency of talcum powder in a mill, and then entrained in an air stream that is fed through the burners to the boiler combustion chamber.6 Firing, therefore, occurs in suspension. Pulverized boilers can be wet-bottom, which means that coals with low ash fusion temperatures are used, and molten ash is drained from the bottom of the furnace, or can be dry bottom, which means that coals with high ash fusion temperatures are used, and dry ash removal techniques can occur.6... 

Because most of the ash is removed as molten slag, addition of a bottom grate is not necessary.7 However, small TDF is required, because much of the combustion must occur in suspension.7 TDF that is too large to combust completely can get carried over into the boiler or dust collection system, and cause blockage problems.9 Therefore, particle size may inversely determine the amount of TDF that can be used in a cyclone boiler.11 Three cyclone-fired boilers at utilities have burned 1M x 1" TDF in test operation, one at the 2 percent, one at the 5 percent, and one at up to a 10 percent level.3,9,12 One pulp and paper mill plans the use of TDF in a cyclone-fired hog-fuel boiler.13... 

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