| OCR Text |
Show Figure 8 shows the mean stoichiometry and percentage of mass flow at reducing conditions predicted for each elevation in the SBS. The mean stoichiometty is 1.1,0.9, 1.15 for the cyclone, the reburn zone, and the burnout zone, respectively. In the reburn zone, the amount of flow with SR<I increases with elevation and approximately 80% of the flow achieves reducing conditions. In the burnout zone, the amount of flow with SR<I decreases with elevation. Mixing in the burnout zone is complete, with all of the gases and particles achieving the oxidizing condition before leaving the furnace. The numerical flow predictions quantify the SBS reburning system mixing perfonnance that achieves over 50% NOx reduction. Based on these results, a methodology was developed for scale-up to the lOO-MWe boiler using the pilot-scale data, and physical and numerical modeling results. SCALE-UP METHODOLOGY Comparison of the baseline conditions of the SBS and Nelson Dewey Station shows that the pilot-scale facility sufficiently simulates the full-scale conditions. Since the demonstration coal was tested in the SBS, the effect of coal properties is eliminated. The temperature profiles in the SBS and Nelson Dewey were generally in agreement; above 3000·F at cyclone throat and approximately 2200·F at the furnace exit with an average furnace residence time of approximately 1.4 seconds. The baseline NOx level was higher for the SBS (1025 ppm) than for Nelson Dewey (662 ppm). The only apparent rationale for this difference is that I) coal moisture content during the Nelson Dewey baseline tests was substantially higher than 14 12 10 g c: ~ 8 CIS > Q) w 6 4 2 ~85 ! ; r ! 1.00 the baseline SBS tests (16.74% versus 3.79%) and 2) required inherent SBS design features due to surface-to-volume differences, e.g., higher secondary air temperature and smaller coal particle sizes as compared to full-scale operation. Although this difference in the baseline NOx concentrations is not completely understood, it is not defeating since the NOx reduction effectiveness of reburning is not strongly sensitive to NOx levels entering the reburn zone in the 500 to 1000 ppm range (3). In addition, flexibility at the demonstration site will provide the capability to allow a higher percentage of coal to be switched to the reburning burners if required. The carbon content of the fly ash was lower in the SBS than the Nelson Dewey station during the baseline conditions, presumably due to finer coal particles in the SBS cyclone. However, the combustion efficiency of reburning coal (and, therefore, the impact on combustible loses) obtained in the SBS will be similar to that for full-scale since the reburn coal particle size distribution and the thennal and chemical environments of the two boilers are similar. It is in our best judgement that the Nelson Dewey's reburning system perfonnance will be close to the perfonnance of the SBS if the mixing in the reburn and burn-out zones of the two boilers are similar. This will be discussed below. In-furnace flow measurement and physical flow modeling were used to benchmark the numerical flow model for the SBS and Nelson Dewey unit Numerical models are based on a fundamental description of turbulent flow processes which are the same regardless of scale. Once validated with pilot-scale or physical flow modeling results, the numerical flow model can be used for quantitative evaluation and scale-up of the reburning process from the 6-million Btu/hr pilot-scale facility to the commercialscale boiler. 14 12 -- 10 -c: .C2ii 8 ~ W \ 6 4 2 1.2 00 20 40 60 80 100 Mean Stoichiometric. Ratio % Mass Row with SR < 1 Rgure 8. Predicted Mixing PerformalJCl for the SBS With Coal Rebumlng 7 |
