Unmixed combustion: An alternative to fire

A new form of combustion has been studied with properties that are greatly different from combustion via fire. Called unmixed combustion, it occurs when fuel and air alternately pass over a catalyst that undergoes oxidation and reduction, storing oxygen from the air and delivering it to the fuel. Examples of catalysts include finely divided Cu/CuO or FeO/Fe2O3 supported on gamma-alumina. The full heat of combustion of the fuel is released, the fuel is converted to CO2 and water, and the air is depleted of oxygen, all without any need for the fuel and air to mix. Thus unmixed combustion is an alternative to fire, another way of using fuel and air to generate heat. The properties and characteristics of unmixed combustion are different from those of combustion by fire in a number of ways, some obvious, some subtle, suggesting a number of applications where conventional combustion is not or cannot be used. One example are situations in which it is difficult to provide complete mixing but in which complete combustion is required; e.g., rotary kiln incinerators. These incinerators have a failure mode called "puffing" to which unmixed combustion may be relevant. In the area of pollution control unmixed combustion is capable of burning natural gas and pyridine with zero NOx production; of burning sulfur-containing fuels in a manner that facilitates subsequent removal of the SO2; and of burning coal in a manner that directly provides sequestration ready CO2. Greatly enhanced delivery of heat to surfaces, rapidly supplying heat for cold starting engines, and direct generation of dry inert gases are also possible. Unmixed combustion also allows the delivery of heat uniformly throughout a volume. This method of heat delivery is potentially useful for supplying heat to endothermic reactions carried out in packed beds of catalyst. Experimental evaluation of this technique for steam reforming shows that while conventional steam reforming is a strongly endothermic reaction with an unfavorable equilibrium, the use of unmixed combustion allows a redefinition of the system's thermodynamics, making the reaction weakly exothermic with a more favorable equilibrium. (C) 2000 by The Combustion Institute.