OCR Text |
Show Figure 5 shows the calculated gas temperature history for a case of slower mixing of primary and secondary flows with the recirculation zone products. The rapid mixing result is included for comparison. There is again predicted depletion of oxidant at about 10 ms, followed by a more gradual, mixing limited temperature rise at 20 ms. The subsequent dilution driven temperature decrease is less rapid than in the rapid mixing case. The relationship of gas temperature to coal volatile yield is shown in Figure 6. For these calculations, a single first order Arrhenius rate, describing coal decomposition to assigned volatile and char yields is used, with temperatures of the size distributed particles determined by integration of their thermal histories. The higher volatile yield curve, a= 0.80, behaves similarly to the a = 0.50 case, but with local stoichiometry effects more strongly emphasized. Gas temperature is higher at times longer than 40ms, indicating more nearly complete combustion. An initial temperature for the recirculation zone flow contribution, as well as burner input and geometry information, is used to generate this time history. Figure 7 shows a comparison of predicted combustion product temperature with the assumed recirculation gas temperature, for an assigned coal residence time consistent with the burner scale and flow field. The line BC represents the trajectory of possible stable solutions. Curve ASU shows the result of inputing various assumed recirculation gas temperatures into the model. It can be shown that point S represents a stable flame solution, for a coal with 50 percent (mass) available as volatile matter. The curve labeled Qt = 0.20, indicating a 20 percent volatile fuel, does not give a crossing of the stability line, and hence is predicted to be unusable in these particular burner conditions. A more complete dependence on available coal volatiles is shown in Figure 8, which describes coal in terms of its high heating rate, high yield volatile contents, rather than the commonly used ASTM proximate -12- ^7AVCO EVERETT |