OCR Text |
Show The effect of firing rate on NOx in the test furnace was relatively small. The deviation of data is primarily due to different burner adjustments. The size of the visual flame body and CO emissions measured with a traversing probe at different distances from the furnace front were also a strong function of the overall excess air. At 3% 02 in the flue gas for example the combustion was practically complete within the volume corresponding to the combustion intensity about 75-80 kBtu/ft3. When testing modifications of the burner designed to higher flame intensity (smaller flame diameter) NOx emissions were inevitably higher. To be able to apply QLN burners to short boilers a version of QLN, called QLN/5 (where "5" stands for "swirl") was developed. The flame was shortened and widened without any penalties on the NOx emission side. Interesting results were achieved when operating with preheated air .. The initial data showed that the function of percentage of NOx increase with air temperature was practically identical to a conventional burner without staged combustion. However, with an adjustment was made to the fuel distribution, the rate of NOx increase was much slower, Fig. 3. This means that optimum settings of the fuel distribution is a function of air temperature. The burner was also tested when operating with the flue gas bulk mixed with the combustion air and with the flue gas introduced into the furnace through the ports surrounding the secondary gas spuds separately from the combustion air. In the case of bulk mixed flue gas, the distribution of fuel had to be shifted to the primary gas and the effectiveness of flue gas on a percentage basis was slightly less, if compared with FGR brought selectively, Fig. 4. With either type of FGR NOx emissions could be reduced to 10-15 ppm with 15-25% FGR until the flame started showing signs of instability by increased fluctuations of pressure in the fumace. With higher FGR rates the stability range of the burner became more narrow with respect to the excess air. In order to maintain NOx below the 15 ppm level, both combustion air and FGR had to be very tightly controlled, and that was beyond the capabilities of most of the available control systems. Deeper NOx reduction to 7-9 ppm was possible when firing a modified QLN burner with the induced FGR without the flow of gas through the secondary spuds, but again very tight controls were required. This level of NOx reduction with ambient combustion air was possible with up to 40% of FGR. 4 |