Development, performance measurement, and modelling of a packed-bed, fire-tube heater
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Fire-tubes are good candidates for indirect heating of pyrolysis and gasification reactors because of their simplicity and low cost. Typical fire-tube designs use empty tubes filled with hot combustion gases. The open-tube design suffers from a low overall thermal efficiency, typically on the order of 30 to 70 percent. In this study, a new type of fire-tube was designed and evaluated. In the new design a multi-tube burner vas used to obtain a more symmetrical flame. A packed bed was used in the fire-tube to obtain a higher overall thermal efficiency. The packed bed not only confined the flame to the combustion zone, but also increased the radial thermal conductivity between the hot combustion gases and the fire-tube wall. The thermal performance of the new fire-tube design was determined by measuring temperature profiles along the tube wall, radiant heat flux profiles from the tube, and overall heat balances. The overall thermal efficiency was found to vary between 7 5 and 95 percent, depending on the gas flow rate, packing position, fuel-air ratio, and temperature of the surroundings. A fire-tube with the same burner design, but without packing, exhibited thermal efficiencies ranging between 40 and 75 percent. A mathematical model was developed to predict the fire-tube performance at pyrolysis and gasification reactor conditions. The model was tested by comparing computed and experimental temperature profiles for different operating conditions at much lower external temperatures than will be experienced in an actual application. The overall thermal efficiencies computed from the temperature profiles agreed with the experimental thermal efficiencies to within 3 percent.