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Show ff~- Rebuming fuel portions provided the remaining 15 - 35% heat input In order to obtain various in-furnace reburning zone stoichiometries (0.85 - 0.95), the reburning burners were operated at substoichiometric conditions. The balance of air was then introduced through OF A ports located in the upper furnace. Under optimized test conditions, reburning burner stability was observed during each of the reburn fuel test phases. No indication of excessive CO levels (at the stack) or burner instability was observed during any of the optimum test conditions. ,. .!. ~ .. r.· The rebuming burners were flrst adjusted for optimum NOli emission levels via burner hardware. Changing the swirl component exiting the burner (via spin vanes in the outer zone) had an effect on resulting NOli levels. Reducing the amount of swirl within this system provided more reburning fuel penetration and improved NOli reduction capability. In addition, FGR could be introduced to the burner and an improvement in NOli reduction was also observed under this condition. Over 50% NOli reduction was achieved with natural gas, oil, and coal reburning at optimum conditions of natural gas, oil, and coal reburning burners. The optimum burner settings for each reburning fuel were detennined based upon NOli reduction capability, flame stability, and CO emission levels. NOli Emissions. A 40 - 75% NOli reduction (from the baseline NOll)evel) was achieved during reburning under various test conditions. These results are reported as overall reductions and consist of basically three components: • NOli reduction via lower heat input at the cyclone burner. • NOli reduction via substitution of main combustion zone coal input with oil or natural gas, thus reducing the total fuel nitrogen content to the furnace (oil and gas reburning tests only). NOli destruction via the reburning process. The following results are based upon the overall NOli reductions obtained. Reburning Zone Stoichiometries. Figure 5 shows that NOli emissions decreased with decreasing reburning zone stoichiometry for the three tested reburning fuels. The reburning burner throat stoichiometries were set at 0.4 for coal reburning and 0.2 for oil and natural gas reburning. Therefore, a greater portion of reburning fuel is required for coal reburning to achieve a given reburning zone stoichiometry. Varying the amount of natural gas and oil reburning fuels from 16 to 28% of total heat input changed the reburning zone stoichiometry from 0.95 to 0.85, respectively. To achieve the same reburning zone stoichiometry during coal reburning tests, 22 to 36% reburning coal had to be introduced to the furnace. Nitrogen-free natural gas provided the best NOli reduction . NOli concentrations ranged from 420 to 235 ppm while varying the reburning zone stoichiometry from 0.95 to 0.85 during gas rebuming operation. From the baseline NOli emission level of925 ppm, these NOli emission levels correspond to a 55 to 75% reduction. During No.6 fuel oil reburning tests, NOli reductions of 42 to 73% were achieved at reburning zone stoichiometries of 0.95 to 0.85. Pulverized coal reburning reduced the NOli levels 40 to 68% for the same range of reburning zone stoichiometry. For 50% NOli reduction from baseline conditions, 15% natural gas or 25% coal is required. Flue Gas Recirculation. Figure 6 shows that NOli emissions decreased with FGR rate to the rebuming burners. In these tests, cyclone and reburning burner stoichiometries and fuel portions were constant. Reburning fuel portions were 22% for natural gas or oil reburning and 28% for coal reburning. As explained before, the reburning burner throat was set at 0.2 for gas and oil reburning and 0.4 for coal reburning. These reburning fuel portions provided the reburning zone stoichiometry of 0.9. Addition of FGR helps to improve the mixing between furnace combustion gases and the reburning fuel. With coal reburning, NOli emissions were more sensitive to FGR than natural gas and oil reburning. This could be due to the presence of coal nitrogen in the reburning coal portions. Without FGR, some NOli is being formed through the volatile flame attached to the reburning burner. 10 |
