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Show Nozzle Variation and Flame Characteristics Four different nozzles were used to measure the flame characteristics of fine-ground coal combustion and two for pulverized coal. Table 7 shows the variations of nozzles, percent of primary air, combustion air temperatures, oxygen and carbon monoxide in the flue gas, and the resulting flame length and flame stability. A 1-inch stainless-steel pipe nozzle was used in the first sets of experiments. This nozzle gave a fuel injection velocity of 30 m/s (100 ft/s) for the natural gas and the air/coal mixture with the primary air 10% of the total air flow. The optimum placement of the nozzle in the burner throat was determined by moving the nozzle along the axis and observing the flame length and shape. For both natural gas and coal, the flame appeared shortest when the end of the pipe was 15 cm (6 in.) back from the furnace end of the burner throat. The flame produced by the 1-inch stainless-steel pipe nozzle was visually 45 cm (14 in.) long when using 230°C (450°F) combustion air with fine-ground coal. Initially, the flame appeared very stable, but it became very irregular after 4 hours. After shutting down the furnace, slag was found in the burner throat between the nozzle and the burner block. This slag blocked about one-third of the throat opening. The nozzle was changed to a 1-1/4 inch stainless-steel pipe 3.5 cm (1.38 in.) ID , and the fine-ground coal was fired with 10% and 13% primary air using ambient combustion air. Both flames appeared stable with a flame shaft and length similar to the previous flame but with no apparent slag formations on the burner block. The change from preheated (230°C) combustion air to ambient air had no visual effect on the flame. Next, a gun-type fuel nozzle consisting of a 1-1/2 inch stainless-steel pipe with a hemispherical cap that has six ports symetrically arranged at 45 degrees from the axis was used. Initially, the ports had a 1.19-cm (15/32-in.) diameter and the tip of the nozzle was flush with the end of the burner block throat. Coal was fired with 8% primary air with 160°C (320°F) combustion air and with 10% primary air with ambient combustion air. The flame shape appeared as six individual flames jetting out of the burner block with lengths of 40-cm (16 in.). (See Figure 11.) These flames had periodic puffing of coal but were otherwise stable, and there was no problem with slagging on the block. The puffing was caused by the feeding system conveying the coal to the nozzle at the pressure and flow needed, which was greater than the nozzle ports could accommodate. 16-26 |