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Show For the fine-ground coal flame, the unburned carbon in the fly ash was much lower (3.5%, 8.2%), 10.9%) in the furnace interior with the 1-1/4-in. nozzle than the pulverized coal in the flue with the same nozzle. The greatest reduction of unburned carbon was with the fine-ground coal and the gun-type nozzle, which showed only 1.4% unburned carbon at 35 cm from the burner and 1.14% at 90 cm on the furnace axis. Flue-Gas Analysis Analysis of the flue gas at the entrance of the flue at 20% excess air using the 1- 1/4 inch stainless-steel pipe nozzle with 10% primary air showed 15% CO-,, 100 ppm CO, 3.5% 02, 760 ppm NO, and 820 ppm N O x on a dry basis in the flue for the fine-ground coal. The nitrogen in the flue was evaluated by subtraction of other gases and by analysis of fuel and flue oxygen, which agreed at 81.5% on a dry basis. The percent conversion of fuel nitrogen to nitrogen oxides for the 1-1/4 inch stainless-steel pipe nozzle with 10% primary air and 13% primary air are shown in Table 9. Slagging Slag and ash were a constant annoyance when coal was fired. Slag was constantly building up in the hot flue duct, thus reducing the opening to the flue. This slag was periodically removed while the furnace was still hot. At this temperature [ 900°C (1600°F)], the slag had the viscosity of a heavy gum. Powdered ash coated the surfaces of the water-cooled stainless-steel pipes, which could have reduced the heat transferred to these pipes and increased the exit gas temperatures. After the furnace cooled, the refractory surfaces were covered with a shiny blueish-black coating, which was very hard to remove. In the flue duct after cooling, the slag was a gray, brittle matter built up to 5 to 15 cm on the surfaces. This slag was easily broken away from the flue. The molten ash in the flue gases made the use of a suction pyrometer very difficult because of blockage of the ceramic radiation shield with ash. Figure 14 shows the buildup of ash on the water-cooled tube in the furnace while firing fine-ground coal. This powdered ash was easily removed with a light brushing or with the use of compressed air while firing. After cleaning, the water-cooled tube appeared to be unaffected by the ash except for a slight darkening of the surface. No marring, pitting, or any other damage was observed on the tube surfaces. 16-32 |