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Show INTRODUCTION Currently, boilers designed for pulverized coal firing use a standard grind of 70% through a 200 mesh (74 ym) sieve, which has an average particle diameter of 55 ynr ^ . In this study, coal was ground to three finer particle size ranges, and the resultant flames were analyzed and compared for combustion characteristics and slagging tendencies. Burnout times of coal particles are dependent on particle size, and evidence suggests that very fine grinding of coal would allow much higher heat-release rates than are currently employed in pulverized coal firing. R. H. Essenhigh'^) reports that above about 10 microns, the burning times of single particles are proportional to the squares of their diameter; below 10 microns, the burning times are proportional to the diameter to the 1.4 power. A. L. Bayles^^ points out the potential benefits of reducing the average particle size of coal. The available surface 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. Bayles shows very little data on the combustion of the small coal particles in a boiler, and no conclusions can be made with any degree of certainty from his article. J. M. Beer' ' studied the effect of fineness of grind on the combustion of low-volatile pulverized coal (anthracite) at the International Flame Research Foundation facilities. The experiments with coal particles from 20% to 100% less than 200 mesh (74 ym diameter) showed that a considerably reduced combustion chamber volume is required for complete combustion when smaller particles are fired. The burners produced a flame from 2 to 6 meters in length, which indicates that mixing was not intense. This would not be the type of coal combustion most desirable in an oil-fired boiler where high combustion intensity is 3 |