Figure 7. Reactant mole fraction profiles in the streamwise direction of the pre-mixed homogeneous-heterogeneous hybrid system.
The inner wall temperature profiles in the streamwise direction are presented in Figure 8 for the pre-mixed homogeneous-heterogeneous hybrid system. The higher the temperature of the combustion gas entering the turbine, the higher the efficiency of the turbine. However, absent perfect mixing of fuel and air, there will be islands of high fuel concentration where the temperature is high enough to form nitrogen oxides [77, 78]. Catalytic combustion offers the possibility of reducing the nitrogen oxides concentration [79, 80]. Temperature control in a catalytic combustor is a serious problem [81, 82]. The conventional gas turbine combustor, as used in a gas turbine power generating system, requires a mixture of fuel and air which is ignited and combusted uniformly [83, 84]. Generally, the fuel injected from a fuel nozzle into the inner tube of the combustor is mixed with air for combustion, fed under pressure from the air duct, ignited by a spark plug and combusted [85, 86]. The gas that results is lowered to a predetermined turbine inlet temperature by the addition of cooling air and diluent air, then injected through a turbine nozzle into a gas turbine [87, 88]. The catalyst in the catalytically supported thermal combustion generally operates at a temperature approximating the theoretical adiabatic flame temperature of the fuel-air admixture charged to the combustion zone. The entire catalyst may not be at these temperatures, but preferably a major portion, or essentially all, of the catalyst surface is at such operating temperatures. The temperature of the catalyst zone is controlled by adjusting the composition and initial temperature of the fuel-air admixture as well as the uniformity of the mixture. Relatively higher energy fuels can be admixed with larger amounts of air in order to maintain the desired temperature in a combustion zone. At the higher end of the temperature range, shorter residence times of the gas in the combustion zone appear to be desirable in order to lessen the chance of forming nitrogen oxides. The residence time is governed largely by temperature, pressure and space throughput, and generally is measured in milliseconds. Part of the catalytically supported thermal combustion of the fuel-air mixture is conducted upstream of the turbine by combustion of the mixture while passing through an insufficient amount of catalyst to effect complete combustion of the fuel prior to passing through the turbine wheel or expansion zone. The partially combusted effluent from the catalyst, with or without substantial intermediate but incomplete further combustion, is introduced into a gas expansion zone of the turbine so that the partially combusted effluent is further thermally combusted while undergoing expansion. The further thermal combustion converts remaining combustible fuel components to carbon dioxide and water. The addition of heat to the system provided by combustion in the turbine gas expansion area counteracts the cooling in the system concomitant with the gas expansion. The turbine is thus operated under conditions which may be referred to as continuous reheating. The operating efficiency and work output of the turbine are thereby enhanced according to known reheating principles. Also, the reheating effect can be obtained without the necessity of employing plural turbine stages and separate reheating equipment. The present design thus provides benefits even in turbines with a single impeller wheel or single stage; however, the present design can also be used where the combustion is accomplished in the gas expansion zones of a plurality of turbine wheels or stages. In the latter type systems, advantageous efficiencies can be obtained. Partial combustion is conducted upstream of the turbine by catalytically supported thermal combustion. A lesser amount of catalyst is required for complete combustion of the fuel is used in the catalyst zone. Accordingly, and since at least a portion of the combustion is conducted in a downstream turbine gas expansion area, the system responds quickly to desired changes in operation, and yet the combustion can still produce an effluent from the work-performing zone having a relatively small amount of nitrogen oxides. As a consequence, the operation can be highly advantageous in turbines used for propelling automotive vehicles. A temperature lowering effect from expansion of the gases within the turbine expansion zone normally occurs in turbines, and this is counteracted by providing at least some thermal combustion of fuel within the turbine gas expansion zone. Thus, for a turbine which operates at a given temperature, more fuel can be burned in the method as compared with conventional operations, without producing excessive temperatures in the turbine. The catalytic metal may be in a combined form, such as an oxide, rather than being solely in the elemental state, and preferably the catalytic metal compound is carried by a less catalytically-active, or even an essentially inert, support which may be, for instance, ceramic in nature. In these catalysts, the more catalytically-active metal component is often a minor amount of the catalyst, while the support is the major portion.