Pulverized fuel particles

The process of particle combustion depends on the physical and chemical nature of the solid as it heats and burns. Coal is a complex material of volatile and nonvolatile components which becomes increasingly porous during volatilization of low-boiling constituents in burning. The crucial practical questions for boiler design concern whether pulverized fuel combustion is controlled by oxidizer diffusion or by chemical kinetics. 

The particles to he removed may range in size from large molecules, measuring a few hundredths of a micrometer, to the coarse dusts arising from the attrition of catalysts or the fly ash from the combustion of pulverized fuels. 

The relative amount of coal mineral quartz surviving in the pulverized fuel flame depends on the particle size and temperature. 

During combustion of the mineral rich coal particles in the pulverized fuel flame, ash envelopes may be created which can take the form of censopheres as shown in Figure 7c and d. The gas bubble evolution leading to cenosphere formation (16,22) and fly ash usually contains between 0.1 and 2 per cent by weight of the lightweight ash. The mineral rich coal particles may leave the combustion ash residue also in the form of plerosphere (spheres-... 

Use of the Field tube for determining the performance of coals is limited by the short residence time of 0.3 seconds (2.1 m) and the inability to use particles larger than 0.14 mm. This limits the extent of particle size changes and the structure of the mineral substance relative to pulverized fuels or coarse-crushed brown coals used in utility boilers. This difference probably has an effect on the formation of deposits. 

Fuel preparation for a pc boiler occurs in a mill, where coal is typically reduced to 70% through a 200-mesh (JA-fim) screen. Figure 2 shows a particle-size distribution from a bowl-mill grinder typically used in power plants. A typical utility specification for particle size distribution of the coal is that 70% of the mass should pass through a 200-mesh screen, which is equivalent to 70% less than 74 /tm. Sometimes the top size is limited to about 120 /rm. The pulverized fuel is pneumatically transported to burner levels (commonly one level fed by each mill) with each level made up of a series of burners. 

Mengiig MP, Manickavasagam S, D Sa DA Determination of radiative properties of pulverized coal particles from experiments. Fuel 73(4) 613—625, 1994. 

In a fluidized-bed combustor, heat and mass transfer between the fluidizing gas and the solid particles are extremely efficient and fluidized-bed reactors are used for carrying out many chemical reactions on an industrial scale (e.g., catalytic cracking of hydrocarbons) (Speight, 2007). An additional advantage of fluidized-bed combustors is that they can use fairly coarse coal particles (ca. 0.04 in., ca. 1 mm diameter) and there is no need for much of the costly crushing equipment associated with the preparation of pulverized fuel. 

In the entrained-bed combustor, the feed coal must be introduced to the combustor as small. The particles (generally <200 mesh) are carried by the gas into the furnace and travel in a suspended state through a hot zone where they are consumed. After combustion, approximately 20% of the ash falls to the bottom of the furnace and is removed there. Combustion gases, which contain about 80% of the ash, pass out of the furnace and are treated to remove the remaining particulates (usually in an electrostatic precipitator) and sulfur compounds (usually in a stack gas scrubber). This type of combustion system (PCC) is in common use for large-scale utility boilers. Higher combustion rates can be attained in pulverized fuel combustors than in fixed-bed combustors. 

Pulverized fuel combustion swirl jets improve particle residence time in the combustion zone, and consequently improve combustion efficiency. Increasing the swirl number and thereby the jet angle moves... 

Milling of coal or coke produces a powder called pulverized fuel which contains particles of a wide range of sizes. As we saw in Chapter 3, the distance at which a particle in a particle-laden jet will travel in a combustion chamber plays a role in the damping of the jet s turbulent energy. Therefore theoretical analysis of combustion must take the particle size distribution of the fuel into account. Pulverized fuel fineness is therefore an important parameter in the modeling of coal combustion. An analytical expression of particle size distribution that has found a wide application for expressing the fineness of pulverized fuel is the Rosin-Rammler relation. The relationship is given by (Field et al., 1964)... 

As was shown in Figure 6.9, combustion begins with flaming pyrolysis, which is an endothermic reaction requiring initial heat to proceed when the fuel particle, whether liquid droplets or pulverized, reaches a certain threshold temperature. This is followed by the... 

Feed coarse particle sizes of crushed carbonaceous fuel compared to pulverized fuel-fired combustors, thus not requiring expensive, powerconsuming fuel pulverization. 

Specific gravity is direcdy related to the bulk density of waste fuels prepared ia a variety of ways. Solid oven-dry (OD) wood, for example, has a typical bulk density of 48.1 kg/m (301b/ft ). In coarse hogged form, eg, <1.9-cm minor dimension, this bulk density declines to about24kg/m (151b/ft ). In pulverized form, at a particle size <0.16 cm, this bulk density declines to 16—19 kg/m (10—12 Ib/ft ). Similar relationships hold for municipal waste, agricultural wastes, and related fuels. 

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. 

Sohd fuels are burned in a variety of systems, some of which are similar to those fired by Hquid fuels. In this article the most commonly burned soHd fuel, coal, is discussed. The main coal combustion technologies are fixed-bed, eg, stokers, for the largest particles pulverized-coal for the smallest particles and fluidized-bed for medium size particles (99,100) (see Coal). 

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