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Show under the chimney, a feature known as "ears" in the industry. The ears allow flue gas from the bottom zone to join the rest of the flue and leave the furnace. Without the flue gas from the bottom zone, the average velocity in the tail end of the model furnace would be lower, providing more residence time for C O burnout when operating in the reburn mode. The fuel consists of 90% CH4, 5% C2H6 and 5% N2 by volume. The molecular weight is 17.3. Its gross heating value ( H H V ) is 1001 Btu/scf, and requires 15.76 lb. of air per lb. of gas. These fuel characteristics were used consistently in both the C F D model and the process model. A flame soot model has not been incorporated into the current modeling. Soot formation is a complex topic and currently only qualitative models are available for prediction. The accuracy of such a prediction depends critically on the flow and temperature fields, which in turn are intimate with mixing patterns. Since the 2 D model can not predict the 3 D mixing pattern in the furnace, even qualitative trends of soot may not be feasible for this geometrical approximation. The governing equations for the conservation of mass, momentum, energy and chemical species are solved with the F L U E N T software package (FLUENT User's Guide, 1996). It uses a control volume based finite difference scheme where nonlinear variations are included inside each control volume, similar to the concept of a shape function in a finite element scheme. This method is a variation of the original approach by Patankar (1980). This formulation ensures the balances of mass, momentum, energy and species locally (within each control volume) to achieve physically realistic results even on coarse grids. A grid of 131 by 112 computational nodes was used. Exploratory calculations showed that the results were independent of further grid refinement. To further assure the accuracy of the solution, a second order discretization scheme (Leonard, 1979) is used. The solution is allowed to iterate until the residuals are reduced by at least 5 orders of magnitude. More importantly, field variables are monitored to ensure they do not vary with further iterations, and the overall mass, species, and energy balances are satisfied. The overall error in the energy balance is believed to be on the order of 3 % of gross firing rate. CFD solutions are presented for two conditions, beginning with a baseline case representing design operation. Overall, about 5 5 % of the fuel energy ( H H V 140 MMBtu/hr) is absorbed by the steel to reach an average discharge temperature of 2120 °F. Water cooling amounts to 1 1 % loss, and refractory loss is 4 % . At an average exhaust temperature of 1725 °F, flue loss amounts to 2 7 % of total input. The flue temperature is in reasonable agreement with the recuperator design inlet temperature of 1600 °F. The average gas velocity at the nose is 9.8 ft/s, and the average gas temperature is 2166 °F. Axial furnace temperatures at baseline, Figure 7, show upper zone peak flame and bulk temperatures at around 3700 and 3200 °F, repectively. Temperatures are sufficiently high mid way along the top heat zone, about 2800 °F, to inject reburn fuel. The model also predicts primary combustion to be substantially complete at this location, e.g., CO<200 ppmv. The velocity profiles are shown in Figure 8. Note that the velocity at the point of reburn fuel and O F A injection is approximately 30 ft/s. Jet velocities are selected to penetrate at least 7 0 % through the bulk gas at these locations. The second case primarily evaluates the impact of increasing furnace production by 20% through oxygen enrichment. Oxygen enrichment is implemented by premixing it into the air supply duct for the top zone only. Furthermore, suppose 7 5 % (10.5 MMBtu/hr) of the reburn fuel energy is recovered inside the furnace. Iterative calculations predict the need for the top zone combustion air to be enriched to 5 0 % oxygen. The C F D model shows that the average steel discharge temperature is 2159 °F, and that the average exhaust flue temperature is 1640 °F, all within expectations. 7 |