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Show representative starting points in the model. Starting points are the location where flue gas temperature is at the upper ] mit of the process temperature window. Velocity and temperatur a a l. )ng the streamlines are converted to time-temperature history for the CKM analysis. The required number of streamlines is based on temperature and velocity variations in each unit. The CKM analysis determines the location where the chemical reaction needs to initiate on each streamline. A typical application will generate 9 to 16 streamlines. On each streamline, the chemical release location that gives maximum NO~ reduction and minimum by-product emissions is identified. This 1S completed by initiating chemical reaction at several locations along each streamline. The total number of runs can range between 45 and 80 or more. By connecting these optimum locations, a surface is generated where chemical reaction should start to obtain the optimum process performance. This surface then becomes the aim point for the injector model. The NOx reduction under ideal chemical distribution is a weighted average of the reductions on each streamline. Similarly, the ammonia concentration is a weighted average of all streamlines. The injector model provides injection strategies to distribute chemicals on the optimum surface without impingement on tube surfaces. Sprays of droplets are typically introduced from wall ports, although water-cooled multi-nozzle lances can distribute chemicals within cavities. Injector locations and droplet size and velocity distributions are varied to evaporate droplets and release chemicals near the target surface. Premature release of chemicals will decrease NOx reduction while droplet evaporation past the surface will increase ammonia slip. The overall expected NOx reduction, therefore, is the ideal reduction predicted by CKM adjusted by the inefficiencies due to incomplete chemical distribution. Used together, the CKM and CFD models provide a sound basis for predictions of expected performance. Example cases that utilized this partially integrated model are described below: Case 1. Coal Fired District Heating Boiler The NOxOUT Process was implemented on a stoker coal fired boiler rated at 100 tons/hr of steam. Three levels of injection were provided. Each level had injectors located on the four corners of the boiler. Starting from a baseline NOx of 400 ppm, the target was to achieve 75% reduction with less than 15 ppm ammonia slip. The boiler was modelled using the combination model. On Figure 4, the side sectional view at the center of the boiler shows CFD model results. Flue gas temperatures are displayed as contours from 600 °c to 1300 °c and flue gas velocities are shown as vectors. Nine flow streamlines on Figure 4 were used to generate temperatureresidence time history along these streamlines as shown on Figure 5. Although similar, temperatures varied as much as 100°C among |
