OCR Text |
Show mu1tic10ne and fan will be installed. The fan will produce the high pressure necessary to carry the natural gas across the furnace. The overfire air will be obtained from the existing windbox supply ducts. An overfire air booster fan will be required. INITIAL GAS REBURNING DATA GR testing on the 71 MW tangentially fired boiler is now in progress. The test i ng has focused on eva 1 uat i on of performance over a range of operat i ng conditi~~s leading to definition of optimum conditions for the long term tests19 , • The Sorbent Injection portion of the tests will begin in Spring 1991. Figure 5 shows some preliminary GR data from these tests. The unit was operating at near full load. The test involved injecting varying amounts of reburning fuel while the overall excess air was maintained constant. NOx emissions are plotted as a function of reburning zone stoichiometry. The data with highest reburning zone stoichiometry (115 to 125 percent theoretical air) is baseline data with no GR. However, there is some staging due to cooling air through the overfire air ports. Brief tests where the cooling air was shut off resulted in NOx emissions 9 to 10 percent higher, in the range of 550 ppm. As the natural gas inject i on was increased, the reburn stoi ch i ometry decreased toward the design point of 90 percent theoretical air and NOx emissions decreased to about 125 ppm. This corresponds to 77 percent reduction from the baseline. The effect of overall excess air is shown in Figure 6 for three operating conditions: baseline, staging with overfire air ports, and GR. The effect of excess air is similar for all three conditions with a nominal slope of 8 ppm per percent excess air. Staging via overfire air reduces NOx emissions by 75 ppm compared to baseline. The minimum NOx is limited by the need to operate the coal burners with sufficient excess air to achieve carbon burnout. At similar overall excess air, GR reduces NO emissions an additional 150 ppm. With GR, the excess air may be decreased further, since carbon burnout is achieved by operating the coal burners under excess air conditions. This reduces NOx emissions to the range of 125 to 150 ppm. The analysis conducted as part of design showed that a flue gas carrier was required for optimum performance in this boiler. Figure 7 shows confirming test results. Increasing flue gas recirculation improves mixing and reduces NOx by about 50 ppm per percent flue gas recirculation. As mentioned above, the design of the reburning injectors is site specific. It may be possible to achieve adequate mixing without flue gas recirculation in some units reducing the cost of the GR system. The field evaluations are providing data to validate the GR design methodology. This will allow future GR installations to be designed with confidence to meet NO guarantees. Figure 8 compares predictions from EER's desi gn methodology with initial test data for comparable operating conditions. The greement is good. Add it i ona 1 compari sons wi 11 be made as new data are acquired and the design methodology will be refined accordingly. It must be emphasized that these data are from initial short term tests and may not be representative of emission reduction which can be achieved in long 8 |