OCR Text |
Show - 12 - The NOx concentration decreased from 800 ppm at the precombustor exit to approximately 380 ppm at the end of the rebum zone. At the end of the rebum zone, HeN and NH3 concentrations greater than 100 ppm were measured in the center of the forward flow out to 0.6 m horizontal position and trace amounts were detected at 0.4 m horizontal position. The N20 concentration was 1 ppm. The NOx emissions decreased from 410 ppm with air injected through Pipes "1,3" to 280 ppm with air injected through Pipes "3,4", Figure 1 O. The N20 flue gas emission was 2 ppm and was independent of tertiary air injection arrangement. Lower Aeff could account for the reduced NOx emissions observed with delayed tertiary air injection. When tertiary air recirculation into the rebum zone was considered, Aeff, calculated using Equation 2, decreased from 0.91 to 0.86 over the full range of tertiary air injection arrangements. This correlated with the general trend of NOx emissions as a function of Aeff for all flames, Figure 11, and suggested that reburn zone residence time above 0.3 s did not have a dominant influence on NOx reduction in these experiments. 3.4 Natural gas reburning Natural gas was used as the rebuming fuel in M8 at the same conditions as M7. The NOx concentration in the flue gas decreased from 290 ppm with tertiary air injected through Pipes "1,2" to 160 ppm with tertiary air injected through Pipes "3,4". The NOx in the flue was consistently 100-120 ppm less than in M7 with coal as the rebum fuel, Figure 10. The lower emission may have been due to better NOx reduction potential of natural gas or higher mixing of the reburn fuel with the primary products. It is suggested that the reduction potential of natural gas rebuming fuel was greater due to its high volatility and low N-species content and not due to improved mixing. If the mixing had been a significant effect, it would be expected that the difference between NOx emissions for natural gas and coal would become less as the tertiary air was delayed. However this was not the case since NOx emissions were consistently 100-120 ppm less than with coal rebuming. 1CXX> NOx emission (ppm) rebym f~1 ~ coal 800 r=:::J natural gas 600 400 r- - r- r- 200 f--- - o base 1:2 1:3 1: 4 2:3 2:4 3:4 tertiary air pipe injection location Figure 10: Effect of tertiary air injection location on NOx emissions |