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Show INTRODUCTION The Institute of Gas Technology (IGT) has investigated the effect of coal particle size on the flame characteristics of pulverized coal to determine if fine grinding of coal will allow replacement of oil by coal in some industrial applications. Direct combustion of pulverized coal, especially for converting an oil-fired unit to coal firing, is becoming increasingly attractive for many industrial boilers and even some furnaces. Currently, pulverized coal-fired boilers use a standard grind of 7 0 % through 200 mesh (74y) sieve, which has an average particle diameter of 25y. In this study, coal was fine ground to 99.9% less than 400 mesh (37y), directly combusted, and the resultant flame analyzed and compared with both a natural gas flame and a standard-ground pulverized coal flame. Standard-ground pulverized coal is burned with lower heat-release densities than oil because it has slower burnout times than oil droplets and the furnace or boiler exit gas temperature must be below the ash fusion temperature to prevent slagging of the convective surfaces or flue passages. Therefore, for a coal-fired furnace or boiler to have the same capacity as an oil-fired unit, the combustion chamber must be 3 to 4 times larger, which makes conversion from oil to direct pulverized coal firing impractical. The function of the size of the coal particle in combustors has been reported in terms of the rate of burning time (burnout time) for various particle diameters and the relation of particle size to particle suspension and fouling tendency. The burnout times of coal particles are dependent on particle size, and evidence suggests that very fine grinding of coal would allow for much higher heat-release rates than are currently employed in pulverized coal firing. Essenhigh reports that above about 10 microns, the burning times of single particles are proportional to the squares of their diameter, while below 10 microns the burning times are proportional to the diameter to the 1.4 power. (See Figure 1.) In a 1957 article, A. L. BaylesJ points out the potential benefits of reducing the average particle size of combustion coal. The available reactive area for combustion increases with the square of the particle diameter. Decreasing particle size also decreases the amount of surrounding air required for combustion of the particle, thus helping to increase the combustion intensity, and decreases the size of the resulting ash particle, which may reduce the amount of slag settling on the boiler tubes or flue passages. Increased combustion intensity and reduced slag settling both contribute to higher heat transfer from the combustion of the smaller coal particles. 16-3 |