| OCR Text |
Show note from Fig. 7 that quite high jet velocities (Uj > 150 mls) can be achieved without blowing the flame out. For a methane/air diffusion flame, the blowout Mach n~ber for the sam~25 injector diameter (do = 1.59 mm) under ambient conditions (Ts - 300 K) IS about 0.06 (Ujmls) [18]. Hence, combustion with a high-temperature oxidant significantly enhances flame stability. Recall that the flame lift-off height correlation developed by Kalghatgi [14] (see Eq. 2) predicts a linear relationship between h and the jet velocity, Uo. This trend is not strictly evident in the data shown in Fig. 7. For the higher temperature case, Ts= 1366 K, the flame lift-off heights were very difficult to ascertain for oxygen concentrations below 7 %; hence, no measurements were made. Similarly, for the lower surrounding-gas temperature case, it was not possible to visualize the flame lift-off height for oxygen concentrations below 12 %. Flame lift-off measurements for the aforementioned cases were hampered due to a lack of contrast between the flame luminosity and the background, as well as flame stability issues. The lift-off height measurements could only be made up to nondimensional heights, h/do' of about 80 due to limitations in visual access. Thus, no implications regarding the flame blowout limits should be inferred from Fig. 7. As noted by Dahm and Dibble [19], even small coflow velocities significantly reduce the blowout velocity limits, thus the co flow velocity (-0.35 mls) of the hot surrounding gas may have had an influence on the lift-off height measurements made in this study, and is an issue which needs further analysis. Co-Firing Burner Arrangements The goal of the DOC program is to develop a burner system which yields NOx levels in the single-digit ppm level range for conditions typical of commercial furnaces. Prior to development of a pilot-scale or commercial-scale burner system, the DOC burner concept was evaluated in a laboratory-scale furnace (see Fig. 4). The pollutant formation characteristics (e.g., NOx, CO) of the DOC system were catalogued as a function of various parameters including the furnace wall temperature, the furnace nitrogen content and the burner arrangement (geometric). Since the in-furnace mixing of fuel, oxidant and combustion products is critical, a variety of burner arrangements were studied to ascertain which arrangements (and which general mixing patterns) would lead to low NOr The types of burner arrangements can be broadly classified into t~o categories; arrangements in which the fuel and oxidant were both injected from the front face of the furnace (co-frring) and those arrangements in which the fuel and oxidant were injected from opposite ends of the furnace (opposed-frring). For the most part, natural gas and oxygen were used as reactants; however, air was used as the oxidant for a limited number of runs. The NOx emissions for eight DOC arrangements in which both the fuel and oxidant were positioned on the same face (front) of the furnace are presented frrst. For the eight different burner arrangements, the furnace wall temperature was kept constant at 1370 K (2oo7°F), the oxygen concentration as measured in the flue was typically between 2-3% (vol. wet) and the frring rate was 185 kW ( - 0.6 MMBtu/h). The NOx power emission index is plotted as a function of furnace nitrogen content a d burner arrangement (co-frring cases) in Fig. 8. For furnace nitrogen concentrations below n 11 |
