OCR Text |
Show 1% O2 operation. NO emissions were found to incfs1se slightly with increasing excess ai~ as predicted by Zelodovitch . NO emissions were strongly reduced by increasing either OFA or FGR (Figures X 7 and 8 respectively). Further significant NO emissions reductions were achieved by combining OFA and FGR (Figure 9). x The best NO emissions obtained, at a commercially acceptable operating condition, ~as 25 ppm (corrected to 3% O 2 ), Operating conditions were 1% excess O 2 with 30% total OFA with 20% FGR. Overfire air (OFA) parameters such as velocity, injection angle, and elevation affected NO emissions only slightly. However, CO emissions were unacceptable at ~ertain OFA angles and velocities. Furnace outlet gas temperature was linearly reduced with increased FGR flow. It should be noted that these tests simulated a multi-fuel furnace and, therefore, during gas and oil firing the NO emissions and furnace outlet gas temperatures were relatively low as com~ared with coal firing. The effect of operating variables and the NO emissions achieved are expected to be different with oil or gas-only uni~s and will be evaluated in future tests. Oil Firing Results Oil firing results are shown in Figures 10, 11, and 12. Baseline NO emissions were 140 ppm (corrected to 3% 02) at a nominal operating e~cess O 2 of 1.5%. NO emissions increased strongly with increasing O 2 as seen in Figure 10. this increase is greater than that predicted by Zeldovitch , which describes thermal fixation of atmospheric nitrogen (thermal NO formation). During oil or coal firing, the fuel bound nitrogen is also being oxidized (fuel NO formation). Fuel NO formation is sensitive to O 2 levels, but not temp~rature. With these fUels, controlling the availability of oxygen in the early stages of combustion is required to control overall NO emissions. x For this reason, NO emissions were reduced substantially with OFA (Figure 10). However, becaUse of the "fuel NO " formation mechanism, NO emissions were found to be only modera~ely reduced by flue gas x recirculation (Figure 11). During operation with large quantities of OFA (~30%), NO emissions were unaffected by FGR (Figure 12). x The best NO emissions obtained at a commercially acceptable operating condition w~s by operation at 30% OFA with no FGR at 1.5% excess O . This represented a 44% reduction from baseline NO levels with acceptable co x emissions . APPLICATION OF REBURNING TECHNIQUES Tests to date in the laboratory and field have shown ~hat CCTFS can meet the targeted NO emissions control level of 400 mg/Nm @ 6% O 2 (240 ppm @ 3% O 2 ) for coalXfiring. While further work is ongoing to demonstrate CCTFS performance for a range of fuels and boiler operation, it is generally felt that tg~lication of reburning techniques will yield even lower NO emissions . Thus studies are in progress on the application of rebur~ing to ENEL tangentially fired boilers . This experimental study is evaluating oil and gas reburn fuels applied to gas, oil, and coal fired 5 |