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Show Each burner was optimized for minimum N 0 X and LOI at full load (100 MBtu/hr) and 1 7% excess air (1.17 stoichiometric ratio) by systematic variation of swirl vane angles and/or swirling device position in the coal nozzle. Other major experimental variables included excess air and thermal load. Unburned carbon loss (UBCL) was calculated from the LOI measurement and fuel analysis. U B C L is a measure of the unutilized fuel and combustion efficiency. All burner hardware were evaluated initially with the high volatile Mahoning coal. Full load performance characterization of the burners at different excess air levels is shown in Figure 5. As expected, raising the excess air converted more fuel-N to N O x and decreased the C O formation and LOI. Differences in N O x , C O , and LOI levels of the two standard D R B - X C L® burners are attributable to the differences in their velocities which in turn affects the fuel/air mixing characteristics. Consistent with the previous results of the full-diameter burner [5,7], certain coal nozzle swirlers performed better when installed in combination with a flame cone. With the flame cone alone, N O x emissions were about 2 7 % lower than the corresponding standard burner results. Visual observations indicated that the plug-in velocity burner flames were shorter and wider than those from the full-diameter burner. Part load operation at a fixed burner stoichiometry generated lower N O x and higher LOI than the full load results due to the reduced mixing and cooler furnace environment. Part load conditions with typically higher excess air levels resulted in equal or lower N O x and LOI than values measured during full load operation. Combined use of the flame cone with a small swirler provided the best overall plug-in burner performance. With the swirler, near-burner fuel/air mixing was enhanced sufficiently to promote fuel dispersion and carbon burnout while maintaining low N O x emissions. Further design improvement to the swirler-equipped plug-in burner involved recessing the flame stabilizing cone just inside the burner. At 1 7 % excess combustion air, average N O x , C O , and LOI values for the swirler-equipped burner were 249 P P M V (0.34 lb N02/MBtu), 11 P P M V , and 1.90%, respectively. By comparison, N O x , C O , and LOI levels for the standard plug-in D R B - X C L ® burner were 326 P P M V (0.45 lb NOz/MBtu), 23 P P M V , and 4.79%. Table 3 summarizes the test results from firing the high volatile bituminous Mahoning coal with different burner hardware under normal operating conditions (100 MBtu/hr and 1 7 % excess air). Under these conditions, the burner pressure drop was close to 3 inches of water for the standard full-diameter burner and about 6 inches of water for the standard plug-in burner. For the other plug-in configurations (with or without the swirler) where the air separation vane was removed, the burner pressure drop was around 5 inches of water. Numerical modeling is being applied presently to redesign the secondary air swirl vanes and further reduce the plug-in burner pressure drop. 7 |
