Combustion characteristics of fine-ground coal

The effect of coal particle size on pulverized coal flame and slagging/fouling characteristics for boiler application are determined. The replacement of high-cost liquid fuel with coal in utility and industrial boilers is the final goal of this project. An on-line milling and feeding system was constructed. The research furnace was equipped with a boiler type burner, water cooling radiative tubes along the combustion chamber walls, and gas cooling convective tubes in the flue. Most of the combustion trials were conducted at the high furnace exit gas temperatures typical of boilers designed for natural gas- and oil-firing. The coal fired in the tests was a highly volatile bituminous coal from Pittsburgh No. 8 seam. The combustion characteristics, slagging and fouling tendencies of three different coal particle sizes were studied, namely: 99.9% below 15.8 ..mu..m (average particle size 6.6 ..mu..m), 99.9% below 55 ..mu..m (average particle size 18.1 ..mu..m), and 99.9% below 148.0 ..mu..m (average particle size 41 ..mu..m). It was found that the flame length was reduced by about 60% from the largest to smallest particle size. The combustion intensity achieved for the finest grind of coal was more than twice that with the coarsest coal grind. The radiation heat fluxmore » and the overall amount of heat absorbed by furnace load was considerably higher for fine-ground coals compared to the coarser coals. The slag and ash deposition rate on the convective tube passes was reduced by about 80% with the finest coal compared to the coarsest coal. Nitrogen oxide emissions did not change with coal particle size. The finest coal (99.9% less than 15.8 ..mu..m) tested caused some feeding and transport difficulties; therefore, the feeding and transport systems for very fine coal should be further improved.« less