OCR Text |
Show CONCLUSIONS The flames from three fuels, fine-ground coal, standard pulverized coal, and natural gas were studied using a single air register boiler burner. The size of pulverized coal particles for a pulverized-coal-fired boiler is typically 70% to 80%) through 200 mesh (74p) sieve (25p average diameter). Seacoal (commerical brand name) was used as the standard pulverized coal, of which 76.4% (Table 2) passed through 200 mesh (74|J) sieve. The ultimate analyses for the Seacoal (Table 1) and the fine-ground coal (Table 3) are similar, so the particle size is the only major factor in the difference in the combustion of the two coals. The fine-ground coal was milled so that about 100% passed through 400 mesh (37y) sieve and had a 7p to 10u average diameter, which, therefore, increased the coal surface area by at least 4 times. The visual flame length of the fine-ground coal was from 75% to 85% of the pulverized coal flame length, which is a ratio of about four-to-one, similar to the surface area ratio for these two fuels. The visual flame length was verified to be a relatively good measure of complete combustion by measuring the C O concentration on the furnace axis on one flame (Figure 11) and the percentage of unburned carbon in particle samples for three flames (Figure 12). For the pulverized coal flame, over 10% of the carbon remained unburned entering the flue even when the visual flame ended 50 centimeters before the flue. The solid sampling for unburned carbon (Figure 12) on the furnace axis showed very good combustion of the fine-ground coal with the gun-type nozzle [99% combustion at 40 cm (end of the visual flame)] and poor combustion with the 1-1/4 inch stainless-steel pipe nozzle [ 95% combustion at 120 cm (3 times the visual flame length)]. All measurements were taken on the furnace axis and the solid sampling was not performed isokinetically so these measurements must be interpreted with care. The natural gas flame, as expected, had the shortest flame length (10 cm). The combustion intensities were evaluated visually at about 10 Kcal/hr-m3-atm (105 Btu/hr-ft3-atm) for the fine-ground coal, at 107 Kcal/hr-m3-atm (106 Btu/hr-ft3- atm) for natural gas, and at 105 Kcal/hr-m -atm (10 Btu/hr-ft -atm) for pulverized coal, which agree with other observed values in various combustors. Slag and ash buildup is always a potential problem in the combustion of coal and would be particularly difficult in retrofitting oil or natural gas boilers. 16-35 |