OCR Text |
Show Table 2: Summary of the Experimental Conditions and Boiler Performance Data Test 1 Test 2 Firing Rate (kW) 4,400 4,400 Mass Flow Rate of Coal (kg/s) 0.17 0.17 Mass Flow Rate of Combustion Air (kg/s) 1.99 1.99 Excess Air (%) 20 20 Overall Swirl Number (No units) 2.1 1.9 Coal Combustion Efficiency (%) 98 97 Coal Seam Middle Kittanning Middle Kittanning 02% 3.0 3.0 CO Emissions (ppm) 90 180 S02 Emissions (ppm) 500 480 NOx Emissions (ppm) 700 380 According to Figure 3, the shape of the flame issued by the burner was relatively symmetrical. This flame symmetry was achieved by means of employing high values of swirl as characterized by the Overall Swirl Number of the flame (2.1). The symmetrical flame shape was maintained at different axial locations in the boiler, i.e. xID=8.75 and xID=14.25 (Figures 4 and 5). The measured temperature profiles, however, became moreuniform in going from xID=3.75 to xID=14.25 (Figures 3-5). The target of Test 1 was to maximize the combustion efficiency. This was achieved by maximizing the degree of swirl associated with all three air streams of the burner. It was speculated that at such high values of the Overall Swirl Number (2.1), a large internal recirculation zone (IRZ) was formed and large numbers of the micronized coal particles were entrained in the swirling combustion air. This would result in longer residence times of the particles with the accompanying high values of combustion efficiency (98-990/0). Measurements of gas temperature for Test 2 are depicted in Figures 6-8. The temperature measurements provided evidence for a temperature field different than that measured for Test 1. The wide scatter of the measured temperature values at xID=3.75 (Figure 6) implied that the mixing of combustion air and micronized coal achieved by the burner was also very different than that in Test 1. According to Figure 6, at xID=3.75, the highest value of the gas temperature measured was 1,616 K on the burnerlboiler centerline. The magnitude of the gas temperature dropped to 1,272 K at a radial distance ofr=-0.9 m. The objective of Test 2 was to reduce the NOx emissions while maintaining acceptable combustion efficiency. This was achieved by reducing the Overall Swirl Number of the flow from 2.1 to 1.9 at a firing rate of 4,400 kW. This reduction of the Overall Swirl Number resulted in a longer flame with temperature profiles different than those measured for Test 1. One explanation was that as a result of the reduction in the Overall Swirl Number, the intensity of the IRZ was reduced. This could allow for the micronized coal particles to penetrate the IRZ partially and devolatilize rapidly in the regions marked by the high temperatures (Figure 3). The rapid devolatilization of the coal in a high-temperature, fuel-rich, staged flame could then lead to the low values of NOx emissions associated with this flame. In conclusion, it can be stated that comparison of the measured temperature profiles at xID=3.75, 8.75 and 14.25 for Tests 1 and 2 (Figures 3-8) provides indirect evidence for the difference in the flow types associated with the two tests. |