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Show 3 A 1-0 model is being used on a day-to-day basis for predicting unburned carbon in pulverized coalfired industrial and utility boilers. The model is a 1-0 description of the complex processes of flow, combustion, and heat transfer that is based on extensive 3-D modeling experience (Fiveland and Jamaluddin, 1992). This predictive tool, which runs on a personal computer, provides design engineers with reliable, parametric capabilities for evaluating carbon conversion for wide variations in coal qualities and operating conditions. Since the model contains fundamental descriptions of devolatilization, char oxidation, turbulence, and gas-phase combustion, applications can include unstaged and staged combustion systems with conventional or low NOx burners. This 1-0 model is used in conjunction with the 3-D models and field experience to develop and evaluate system designs. TECHNICAL APPROACH A straightforward approach is used for modeling staged systems. The furnace flow and combustion with the conventional system are characterized to provide a baseline. The model predictions are compared with operation'al and in-furnace probing data, if it exists. Then the operation with the staged system is modeled. The results are compared with the baseline results and analyzed to evaluate system performance. Several aspects of the results are examined including: flow patterns, chemical species concentrations, mixing effectiveness, combustion efficiency, furnace heat flux and absorption patterns, and gas temperatures including the furnace exit gas temperature (FEGT). Alternative designs are considered if variations in one or more of these aspects exceed acceptable tolerances. The results are used to identify possible improvements that are subsequently evaluated. The performance of the final system design can then be parametrically evaluated over a range of conditions. The predicted flow distributions and fuel and oxidant concentrations are used to determine the effectiveness of the mixing by an air staged system. The local stoichiometry is calculated from the fuel and air present in each control volume in the model. The deviation of the stoichiometry values from the perfectly mixed stoichiometry (os) on any plane can be calculated. This deviation is massflow weighted to account for variations in the flow over the plane. The percentage of the mass flow with a stoichiometry less than a specified value, typically 1.0, (us) can also be calculated. Changes in the air staging system to improve the mixing are indicated by decreases in Os and us. The computation time required to obtain combustion predictions is significantly longer than the time required to obtain flow solutions only. Although flow modeling does not provide a complete description of the system performance, it can be used to evaluate candidate system designs and enhancements more rapidly. A scalar transport equation is solved in conjunction with the mass, momentum, and turbulence equations. The transported scalar is the fuel mixture fraction (f) and is based on values specified at the burner and air port inlets. The local values of f are converted to stoichiometry, allowing the calculation of 8s and us. Combustion modeling can be used to evaluate selected systems in greater detail. |