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Show Unmixed Combustion for Efficient Heat and American Flame Research Committee Mass Transfer in Chemical Processing Systems 1997 Fall International Symposium BACKGROUND In unmixed combustion fuel and air are alternately passed through a bed of a readily reduced metal oxide/readily oxidized metal-copper and its oxide provide a convenient example. The process can be conducted as simply as shown in Figure 1. In this figure a packed bed of copper or its oxide, supported on a high surface area ceramic is provided with sources of air and a gaseous fuel. A three-way valve serves to select whether fuel or air are admitted to the packed bed at any given moment. If the packed bed contains metallic copper and air is admitted the copper will be oxidized to C u O and the air vitiated. This process is very exothermic and the solid phase is effectively heated. Subsequently, if fuel is admitted to the packed bed the copper oxide will be reduced to the metallic state and the fuel oxidized. This process too is exothermic and further heats the packed bed. The action of the copper/copper oxide in this process meets the definition of a catalyst in that it "increases the rate of reaction toward equilibrium without being appreciably consumed in the process"2, and so m a y be called an unmixed combustion catalyst. However, unlike many heterogeneous catalysts, the unmixed combustion catalyst does not facilitate the interaction of two reactants on its surface, rather it serves to transfer mass between one step of the process and the other. Hence it m a y be called a mass transfer catalyst. One of the advantages of UMC is that, while some conventional combustion systems emit pollutants in the form of oxides of nitrogen, unmixed combustion is completely without nitrogen oxide production. T o demonstrate this aspect of unmixed combustion E E R built the prototype 2 k W natural gas combustor shown in Figure 2. Table 1 shows data from the operation of this combustor. Table 1. Unmixed combustion in a 2 kW prototype combustor Power, Watts Power Density, MW/m3 Fuel/Air Equivalence Ratio CO, ppm NOx, PP™ CH4, ppm CH20, ppm 340 .29 .422 4 0 ± 0.03 9-70 0 680 .59 .516 1.33 0 ± 0.03 55 - 450 0 680 .59 .289 1.1 0 ± 0.03 100- 1000 0 1360 1.17 .427 5.5 0 ± 0.03 200->1000 0 2040 1.76 .496 9 0 ± 0.03 >1000 0±0.1 Cycle time: air on 1 s, CH4 on 1 s Reactor temperature set point: 830 °C In this example the reactor was first heated up to 830 °C in an electric oven. Methane and air were then alternately supplied to the reactor using time intervals and flow rates corresponding to fuel/air equivalence ratios ranging from 0.289 to 0.516. A n interesting observation during this test Fairmont Hotel Chicago, Illinois September 21 -241997 Page 3 |