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Show An option suggested by ESA to address this coal-gas interaction was to inject gas at a higher furnace elevation, where the coal burnout is more advanced and thus lower coal-CO should be produced. A temporary gas header was therefore installed at the 722-ft elevation, and four additional gas injectors were introduced through existing observation door openings at this location. Modeling results such as those in Fig. lib indicated that only part of the gas could be injected at the 722-ft elevation before any reduction in coal-CO would be offset by increases in gas-CO. Results such as those in Fig. 11a indicated that gas injected at this higher boiler elevation would have relatively little effect on the NOx reduction performance of the system. Furthermore, analyses of the initial test results also showed that a disproportionate fraction of the C O was produced when gas was introduced through certain of the injectors. This is consistent with the conclusion that coal-gas interaction was responsible for the high CO levels in the initial tests. An additional option to reduce COemissions thus involved appropriately adjusting the orientations of the 714-ft injectors to mix the gas with the coal-rich core of the furnace more slowly. Accordingly, new injector tips installed at this elevation were angled between 30-and 60-deg. in the direction of the tangential swirl. These were designed to place gas along the outward edge of the fireball, allowing the gas to mix more slowly into the coal-rich core where coal burnout is taking place. 5.3 Final performance testing After these modifications to the gas injection system, a second set of performance tests were conducted at 140 MW, 90 M W and 65 M W boiler loads. Gas was injected at 714-ft elevation using the original straight injectors and the new angled injectors, as well as at 722-ft elevation using the four new injectors, and using combinations of the injectors at these two elevations. The resulting NOx and CO performance at full load are shown in Fig. 13, where these data are also compared with the initial test results. It can be seen in Fig. 13athat, as predicted by the modeling results in Fig. 11a the NOx reduction versus gas heat input was largely unaffected by these modifications to the injection system. Slightly higher NOx reduction was achieved when injecting gas through the 722-ft injectors, and this is also in agreement with the modeling predictions in Fig. 11a Average NOx emissions were 0.29 lb/MMBtu at 7 % gas heat input. However in contrast to the NOx emissions performance in Fig. 13a the CO emissions in Fig. 13b were significantly reduced when a portion of the gas was injected at the 722-ft elevation. The lowest COemissions, 251 ppm corrected to 3 % 02, were achieved when injecting 5 % of the gas through the angled injectors at the 714-ft elevation, and 2 % of the gas at the 722-ft elevation. When more gas was injected at the 722-ft elevation, COemissions rose, but there was a clear benefit obtained from injecting part of the gas at the higher elevation. This is consistent with the conclusion in §5.2 that the high CO in the initial tests was from incomplete coal combustion due to interference between the injected gas and the coal-rich core in the furnace. Also consistent with this, use of the combined 714-ft and 722-ft injectors returned LOI values to the original baseline level, and also reduced the stack gas opacity. Final performance test results at 90 MWand 65 MWare shown in Figs. 14 and 15. CO |