OCR Text |
Show carbon as compared to Trial 1 at 470 kW (1.6 million Btu/hr) firing rate. For the fine-ground coal flames, the unburned carbon in the fly ash was much lower than in the coarser coal flames. At 140 cm (55 in.) from the burner on the furnace axis, unburned carbon was below 2% for fine-ground coals compared with more than 10% for the coarser coal. Hydrogen is consumed much faster in the coal combustion process and shows little dependence on coal particle size In the particle size ranges studied in this program (Figure 22). The amount of unburned hydrogen in the fly ash was very low at the end of the flames. Combustion Gas Temperature Figure 23 shows combustion gas temperatures measured with a suction pyrometer along the furnace axis. The results show maximum combustion gas temperatures near the burner wall for fine-ground coal flames (Trials 1 and 2) while for the coarser coal, the maximum combustion gas temperature was found at 200 to 250 cm (77 to 98 in.) from the burner wall (Trial 3). The temperature profiles in Trials 1 and 2 indicate an initial high temperature near the burner and a slow decline to the flue. The coarsest grind of coal in Trial 3 shows the gas temperature increasing until the middle of the furnace, which demonstrates less intensive combustion and a longer flame than the fine-ground coal flames. The gas temperature profile for Trial 1 shows the effect of the lower firing rate on combustion gas temperatures: they are much lower for the reduced firing rate. Radiation Heat Flux A Hy-Cal Radiometer with a 7° viewing angle was used to measure the radiation heat flux through the removable doors. The radiometer was water cooled and the interior was purged with dry nitrogen. The radiometer was positioned flush with the exterior wall and pointed across the furnace axis to the center of the opposite wall. The output voltage was read off a strip chart recorder and converted to heat flux by a calibration graph supplied by Hy-Cal Kngineering. A plot of the radiation heat flux along the length of the furnace for Trials 1, 2, and 3 is shown in Figure 24. The radiation heat flux does not vary much over the length of the furnace for each coal particle size. At similar 31 |