OCR Text |
Show 6 The dynamics of this process require that the fuel form a boundary layer between the combustion air and the inert furnace gases. FueUair mixing and combustion occurs at the shear layer between the fuel and the combustion air. Internal flue-gas recirculation results by mixing furnace gases and fuel at the outer shear layer. CFD models allowed exploring the potential of boundary layer staging. The models used a methane! carbon monoxide, two-step combustion process. Unknown thermal boundary profiles required setting model boundaries to averaged values. Defining the radial limit required using a weighted average technique to accommodate the variances caused by the 3x3 burner configuration. Since combustion must be completed in the furnace, the shortest possible furnace flame path length set the model axial limit. Thermal NOx values were trended using a probability density function (PDF) of temperature. Final NOx results used prompt NOx and PDFs of oxygen, and temperature. Model Results Model data indicated that boundary layer staging reduced NOx by 77% (Figure 4) when compared to the baseline model of the existing burner. However, flame length increased by 30 % which is still within acceptable limits of the furnace geometry. However, the model showed that increasing turbulence of the process through higher burner swirl number restored flame length to the baseline standard. The model also predicted less than 100 ppm carbon monoxide at the furnace outlet. Figures 5 through 9 compare baseline model results with the boundary layer staging model results. Raster plots for methane mass fraction, O2 mass fraction, CO mass fraction, temperature and NOx mass fraction are shown. -;)1 r:: o en !!l E w ~~ 1) U) ('0 I-oI-I -'ift (.68 Ib/mmBtu) (.17Ib/mmBtu) oLJILJIL Baseline Boundary Layer Staging MODEL NOx PREDICTIONS Figure 4 |