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Show BURNER DEVELOPMENT The bulk of the development work involved optimizing the fuel staging method for minimum NOx combined with stable operation with flue gas recirculation at ~II firing rates from maximum to minimum. This work involved determining the optimum ratio of primary to secondary fuel, primary and secondary fuel injection pattern, number and location of primary and secondary fuel injectors and air velocity and flow pattern. After determining the optimum design configuration while firing primarily natural gas, the burner was tested with a variety of other fuels and further optimization was done. TEST RESULTS Figure 3· shows some of the data collected for natural gas firing. The lower curve shows the variation of NOx with flue gas recirculation for ambient combustion air. The data shows that the NOx level was about 27 PPM without FGR, which is well below the 40 PPM limit of rule 1146 and very near the limit of 25 PPM (0.03 Ib of NOx per million Btu) mandated by Rule 1109. By introducing flue gas recirculation this low NOx level was further reduced. With 15% FGR the level was less than half. The upper curve shows the behavior with 500°F combustion air. Without FGR the NOx was nearly double that seen with ambient air. However, with less than 5% FGR, the Rule 1146 level is met and with 15% FGR the NOx level was below the 0.03 Ib per million Btu level mandated by Rule 1109. Figure 4 compares the ambient air natural gas NOx data with data for a mixture of 5O%{volume) hydrogen and 50% natural gas. Note that the emissions for this fuel gas mixture is higher than that for natural gas. Note also that the Rule 1109 emission limit, which is mandated in terms of NOx production rate, is also slightly higher (27 PPM versus 25 PPM) for the fuel mix when expressed in terms of concentration. The Rule 1146 level of 40 PPM for this fuel can be met with the burner alone and, as shown, the Rule 1109 limit can be easily met for these operating conditions by the addition of a small amount of FGR. Figure 5 compares the natural gas data with data for a mixture of 30% propane, 40% hydrogen and 30% natural gas for ambient and 500°F combustion air. Once again the NOx for the fuel gas mixture is higher than that for natural gas, but the required emission limits are easily reached. With ambient air the Rule 1146 level of 40 PPM was met without FGR. For the preheated air case FGR was required to meet both rules. With 500°F air Rule 1109 level required the addition of nearly 20% flue gas recirculation. The data show FGR levels up to 35%. This high FGR rate is possible because the hydrogen in the fuel aids in stabilizing the flame. 6 |
