| OCR Text |
Show 6 reduces the NOx emissions from 240 to 113 - 118 ppm at 3 % 02 in the waste gas, at 100 and 70 % of full load, a 50% reduction. The lower figure at full load can in part be explained by the lower stoichiometry. It was accompanied by a higher CO concentration in the gas at level 6, bullnose, and higher carbon concentration in the flyash. The difference however, falls within the experimental error, since the coal consumption could not be measured, but had to be calculated from the thermal load of the boiler.The available volumes allowed longer residence times both in the primary zone, 2500 - 3600 ms, and the reburning zone, 700 - 1000 ms, than reported by other authors, 300 - 400 ms. /Chen 1986/ Fig 4 compares mean values of NOx and temperatures in the baseline case (only OFA) and rebuming (+OFA and reburning). The funktion of the overfire air is to reduce the available oxygen in the beginning of the flame, and its influence on NOs formation is the same in both baseline and reburning trials. The difference in NOs, 70 ppm in the UFI level, can be attributed partly to experimental error, partly to the lower stoichiometric ratio in the primary zone in the reburning case. In the baseline case the overfire air brought the air excess up to 20 %, while in the reburning case the corresponding figure was 12 to 13 %. The temperature in the lower end of the main combustion zone is lower, which according to (m) 26 24 22 20 18 16 14 12 0 Fig. 4 (m) 26 24 22 20 18 16 OFA burners 14 1 12 100 200 300 400 800 1000 1200 1400 NOx (ppm) temperature °c Comparison of NOx and temperature at different levels, between baseline and reburning at full load /Takahashi 1981/calls for lower stoichiometric ratio.The reburning fuel in fig 4 reduces the NOs to 113 ppm, and it should be noted that the difference in temperature between the two cases remains throughout the rebuming and burnout zones. Influence of various Factors Stoichiometry. reburnin& fuel rate. Figs 5 and 6 show that the reduction of NOx is inversely proportional to the stoichiometry in primary and reburning zones. The latter is dependent on the amount of reburning fuel. |
