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Show including detailed nitrogen chemistry, and allows the primary coal gas combustion processes as well as the reburning process to be simulated. 3.2 Results for coal gas mixture fraction fields Sample results from these LIM simulations can be found at the NGB Technologies web site www.ngbtech.com, including downloadable color files of instantaneous coal gas mixture fraction and molecular mixing rate fields, as well as a 9 M B movie showing the time-dependent evolution of the LIM surfaces within the furnace. Owing to the time-varying nature of the flow in the furnace, these provide essential insights into the in-furnace flow, mixing, and combustion processes, and should be referred to to augment the discussion given below. Figure 7 shows typical results obtained for the instantaneous coal gas mixture fraction field within the furnace. Colors in the furnace interior show the coal gas mixture fraction values, with black denoting zero and colors increasing from red to yellow denoting increasingly higher mixture fractions. Note the flows issuing from the coal pipes and the SOFA ducts, and the resulting pattern of coal gas mixture fractions within the furnace. Evidence of large-scale vortical motions (eddies) can be seen at the coal gas interface in the near-burner zone and throughout the furnace. The movie noted above provides direct time-dependent visualization of these large-scale motions within the furnace flow, mixing, and combustion process. Representative results for the coal gas mixture fraction fields C,(x,t) and corresponding scalar dissipation rate fields V£ V^(x,r)are shown at three separate instants of time in Figs. 8 and 9. indicating the range of variation found in these fields. While these fields can be seen to vary widely with time, there is a basic spatial pattern that results in the mixing and combustion processes within the furnace. Of particular relevance to the reburning performance, the SOFA system is seen to create a region of high mixture fraction values (evidenced by red and yellow shades) near the front wall. extending downward beyond the 5th-floor elevation at which the reburn gas injectors are located. An accompanying region of high oxygen concentrations, seen by dark red and black shades, forms behind this coal gas rich region as a consequence of the SOFA system, and extends toward the furnace back wall. At the 5th-floor elevation. the mixing layer separating the fuel-rich zone near the furnace front wall and the oxygen-rich zone extending to the back wall is predicted to begin at a distance of about 8 feet from the front wall. This is in good agreement with results obtained from ESA's furnace probing measurements at this elevation. As will be seen below, this oxygen mixing layer seen in the simulation results appears to control the rebum system performance in this facility. 3.4 Results for natural gas injection and reburning When natural gas is injected through the front wall, the resulting gas jets can penetrate through the mixing layer and into the oxygen-rich region extending toward the furnace back wall. This can be seen in the LIM simulation results in Figs. 10 and 1 1 when gas is injected at the 5th-floor elevation. Typical resulting instantaneous gas mixture fraction fields are shown in Fig. 10. and the associated molecular mixing rate fields in Fig. 11. The resulting jets can be seen to penetrate far across the furnace. as was intended for the original gas injection system design. |