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Show co~g t~?,~gh. the center damper and that air flow was restricted at the furnace walls. P~sl~lon. 2 Indicates that the dampers were all at the same position, indicating equal air dlstnbutlon under the grate. Position "3" indicates that air flow was restricted at the center of the furnace and most of the air was coming through at the walls. The data show that as the undergrate air in the center of the furnace is reduced the NO • • , x concentration IS reduced from about 85% to 65% of the maximum value and that CO concentration increases from about 5% to about 15% of the maximum value. A typical ?utput of the computational fluid dynamic (CFD) model (developed by B& W) of the boller furnace at Richmond utilizing METHANE de-NOX reburn is shown in Figure 4. Calculated average NOx concentrations as a function of elevation are shown which indicate that total NOx concentration decreases rapidly from about 425 ppm to about 150 ppm, at 3% oxygen, about a 65% reduction. The model indicates that most of the NOx is from the fuel bound and only about 20% is thermal NOx. In the figure, an elevation of 118 feet corresponds to the grate level. RETROFIT TESTING During the spring and summer of 1996, a METHANE de-NOX reburn system was installed on the stoker boiler and the unit was prepared for continuous operation. The reburn system consists of a natural gas supply system, flue gas recirculation system(FGR) and also includes a modification of one row of overfire air nozzles without any retrofit of the boiler waterwall tubes. Retrofit testing was conducted at the Richmond site on July and August 1996. The SNCR system was shut off per a prior agreement between Cogentrix and the Virginia Department of Environmental Quality, as it was for baseline testing. The major goals of the retrofit tests were to: validate and deploy the METHANE de-NOX reb urn process for a coalfired stoker boiler, and determine the operating parameters for the reburn system and retrofitted boiler for continuous operation. The variable parameters of the retrofit tests were: combustion zone stoichiometry, total heat input, natural gas input and distribution, FGR input and distribution, total excess air, and undergrateloverfire air split. Nineteen retrofit tests were conducted according to the test plan and matrix. F our loads were tested, 40, 60, 80, and 100%; natural gas input was varied from 5 to 25% with different distribution between the injection levels. The FGR flow was in the range of 10 to 35% and also with different distribution. The total excess air was varied with O2 concentration in the flue gas at the furnace exit from 2 to 5%, and undergrate/overfire air ratio was in the range of 15 to 32% at different boiler loads. The retrofit test measurements included the same volume of parameters as for baseline test, but in addition flue gas composition and temperature were measured at the boiler exit. The major retrofit test result is that the METHANE de-NOX reburn process was able to reduce NOx up to 70% and allowed operation of the coal-fired stoker boiler in compliance with the Virginia Department of Environmental Quality NOx and CO regulations without any urea injection. 6 |