OCR Text |
Show No. 6 fuel oil temperature for 150 SSU viscosity is about 210°F. Satisfactory atomization of COM fuels can be obtained at fuel viscosities higher than 150 SSU. For instance, 30, 40, and 50 percent COM were fired at 230°, 230°, and 265°F, respectively, with the respective viscosities being about 340, 600, and 1500 SSU. The firing rate of the boiler was controlled by adjusting the flow rate of COM by means of a variable-speed-drive Moyno pump. During all COM combustion tests, the 700-hp boiler was able to operate at full capacity; no derating of the boiler was experienced. Effect of Coal Concentration The results of the coal concentration tests indicated that the carbon-conversion efficiencies were all above 97 percent. In full load tests, the carbon-conversion efficiency decreased slightly with increasing coal concentration in the COM and with decreasing excess air level. The boiler efficiencies, based on the ASME heat-loss method, for COM containing 30, 40, and 50 percent coal were about the same as those when firing No. 6 fuel oil within the accuracy of the boiler efficiency determination. Boiler efficiencies when firing No. 6 fuel oil at full-load operation were 82.3 and 82.6 percent at excess-oxygen levels of 3 and 2.4 percent, respectively. The boiler efficiencies ranged from 81.2 to 82.7 percent when firing COM in the range of 2.5 to 3.5 percent excess oxygen. Effect of Coal Particle Size Consist The carbon conversion efficiency depends on the coal particle size consist in the COM fuel. The average values of carbon conversion efficiency (ignoring the slight effect of excess air level) at full load ranged from 97.5 percent when using coarse coal (60 percent minus 200 mesh) to 99.3 percent when using fine coal (92 percent minus 325 mesh), in 40 percent COM. 14-4 |