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Show Table 1, adapted from Bayles' article, shows how the mass and free-fall time of a coal particle change with particle size: As coal particles are ground to finer sizes, the mass of the particles rapidly diminishes and the falling time rapidly increases. The falling time is a good measure of the tendency of a particle to follow a moving gas stream. Thus, the smaller molten ash particle would have a greater tendency to flow with the gases than to stick to the surfaces because the smaller ash particle would have a lower terminal velocity. Likewise, smaller coal particles would stay in suspension better than large coal particles, which would tend to fall to the floor of the boiler unburned and become part of the ash or slag. Bayles shows very few data on the combustion of the small coal particles in a boiler, and no conclusions can be made with any certainty from his article. Although particles in pulverized fuel flames would not burn in exactly the same manner as individual particles, the smaller the particles are, the better the "fuel-air mixing" and the faster and more complete the combustion should be. 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 (76v diameter) showed that a considerably reduced combustion chamber volume is required for complete combustion when smaller particles are fired. The burners used produced a flame of 2 to 6 meters in length, which indicates that mixing was not intense. However, the flames shortened and moved closer to the burner as the coal was ground finer. This would not be the type of coal combustion most desirable in a oil-fired boiler where high combustion intensity is needed. Experiments with bituminous and subbituminous coal, however, are lacking, as are data with grinds much finer than 200 mesh. Although very fine grinding of coal can reduce flame lengths, increase heat-release rates in the flame, and reduce the combustion volume necessary for a given thermal input, some preliminary research was needed to determine how the flame changes as a function of coal grind before gathering fundamental data and attempting to convert existing oil and natural gas fired boilers and furnaces to pulverized coal. Milling a bituminous coal to below 37y was the first step of this project. A pulverized coal feeding system, a boiler burner, and the pulverized coal nozzles were installed in the existing furnace. The combustion system was modified to produce the shortest flame with the characteristics most like those of oil or even natural gas flame The burner nozzle and its position in the burner were changed to create the most intens 16-5 |