OCR Text |
Show Selected retrofit test results are shown in Figures 5 to 9. The effect of primary combustion zone stoichiometry in the coal bed on the NOx in the center above the grate for two loads, 60 and 100%, is presented in Figure 5. The primary combustion air/coal ratio has a significant effect on NOx at the exit from the coal bed, and with load increase, the NOx level above the grate increases also. The NOx distribution versus furnace elevation at 100% load for different natural gas inputs is given in Figure 6. The reburning zone is located somewhere between the 120 and 130 feet elevations, and after the reburning zone the NOx concentration is nearly unchanged. The effect of natural gas input on NOx emissions for two loads is shown in Figure 7. When the natural gas portion in the total heat input is increased, NOx at the furnace exit decreases, and this effect is more pronounced at the higher load. The required FGR flow for different boiler loads is presented in Figure 8. At full load only 9% FGR is required for reburn process, and the portion of undergrate FGR is about 2%. As the boiler load decreases the percentage of undergrate FGR increases to replace the extra undergrate air used for regular grate operation. During all retrofit tests, the boiler operating data were continuously collected through the boiler's distributed control system. In Figure 9, two hours of boiler operational data showing steam flow, NOx and O2 in the stack at full load are shown. The data indicate that the NOx level is stable and in compliance over the time period shown. The boiler operating parameters for four loads, 40, 60, 80, and 100%, are shown in the Table 1. With about 8% natural gas injection, NOx was maintained in the range of 0.28-0.29 IblM:MBtu and CO emission was less than 0.07 IblMMBtu. The Virginia state regulations are: NOx < 0.32 IblMMBtu and CO < 0.20 IblMMBtu. The O2 at the furnace exit was about 3% at full load and slightly increased up to 4.5% at 40% load. These oxygen levels are much lower than boiler design oxygen concentrations, especially at partial load. As a result of this, stack losses were significantly decreased and boiler thennal efficiency increased by about 2%. After completion of retrofit testing, the METHANE de-NOX reburn system was put in continuous long-tenn operation, and now the boiler is operating in complian<?e_ with Virginia Department of Environmental Quality NOx and CO regulation without urea injection. CONCLUSIONS Based on retrofit test results, the 1t1ETHANE de-NOX reburn process is a..'1 attractive alternative to SNCR for emissions reduction in coal-fired stoker boilers and also for improvement of boiler operating perfonnance and thermal efficiency. At the present time, companies are allowed to take credit for operating a boiler below the permitted emission level and this presents another motivation to reduce NOx emissions using METHANE deNOX. 7 |