Figure 3. Front wall edge temperature profiles in the transverse direction of the pre-mixed homogeneous-heterogeneous hybrid system.
The fluid centerline temperature profiles in the streamwise direction are presented in Figure 4 for the pre-mixed homogeneous-heterogeneous hybrid system. In a conventional combustion system typical of industrial and commercial burners, the combustion reaction is relatively uncontrolled [61, 62]. That is, a flame can vary in conformation such that its shape and location at any particular point in time is unpredictable [63, 64]. This unpredictability, combined with high peak temperatures encountered especially at the stoichiometric interface in a diffusion flame can cause operational problems such as coking of reaction tubes and uneven heating of steam tubes. Moreover, the length of a conventional flame causes a relatively long residence time during which combustion air is subject to high temperature. What is needed is a technology for reducing pollutants released by combustion systems such as industrial and commercial burners. What is also needed is a technology that can improve flame control in such systems. Fuel and combustion air are mixed to form a fuel-air mixture. The amount of combustion air mixed with the fuel is sufficient to substantially fully oxidize the fuel. Energy is transferred to the fuel-air mixture to increase a temperature of the mixture prior to ingestion and oxidation by the catalytic reactor so that the fuel-air mixture is above a minimum operating temperature of the catalytic reactor when it is ingested. The catalytic reactor oxidizes substantially all of the ingested fuel and produces thermal energy. Exhaust gasses from the catalytic reactor expand across a turbine. This causes the turbine to rotate and thereby convert thermal energy to mechanical energy. Fuel may be injected into the low-pressure inlet of a compressor, rather than downstream of the compressor. This eliminates the need for complex or high-pressure fuel delivery systems having fuel gas compression. A fuel mixer mixes the fuel with combustion air to form the fuel-air mixture. The concentration of any unburned fuel in the bleed air is reduced prior to being exhausted to the environment. This is achieved by oxidizing any unburned fuel with a catalyst. Gas-turbine engines and systems are capable of sustained catalytic combustion over a wide range of operating power levels. For sustained catalytic combustion, the operating temperature of a gas-turbine engine should preferably remain within a limited band of operating temperatures over a wide range of operating power levels. A typical band of catalytic reactor operating temperatures is approximately between 400 °C and 600 °C. However, for conventional gas-turbine engines, operating temperature is a function of power level. Thus, when conventional gas-turbine engines operate at part load, they typically reduce their operating temperature while maintaining a constant engine speed. This reduction of operating temperature causes the operating temperature of the conventional gas-turbine engines to fall outside the limited band of catalytic reactor operating temperatures. Unlike known constant-speed gas-turbine engines, gas-turbine engines adjust their engine speed with power level to maintain a nearly constant operating temperature over a wide range of operating power levels. In addition to adjusting engine speed with power level to maintain a nearly constant operating temperature, gas-turbine engines and systems have recuperators and pre-heaters that help maintain the minimum operating temperature needed for sustained catalytic combustion, even during periods of fuel interruption. There is no need for complex or high-pressure fuel delivery systems having fuel gas compression. In fact, in some cases, no fuel-metering valve is needed, as controlling the speed of the gas-turbine engine can control the fuel flow to the engine. During operation, fuel and air are mixed together to form a fuel-air mixture having a desired fuel-air ratio. The pressure of the fuel-air mixture is increased to a desired pressure by the compressor. As an example, with conventional high-temperature materials in the recuperator, the maximum safe operating temperature of the recuperator may be about 600 °C, and hence an air-fuel mixture temperature of about 500 to 550 °C is about the highest that can be achieved. This temperature range is higher than the minimum catalyst operating temperature for some types of catalysts and therefore the catalytic combustor may operate properly at one particular operating condition such as 100 percent load and standard-day ambient conditions. At other operating conditions, however, such as part-load and cold ambient conditions, the combustor inlet temperature may fall below the minimum temperature. In order to be compatible with practical turbine systems, it is desirable that the residence time of the combustion gases in the catalytic and thermal oxidation zones is sufficient to give essentially complete combustion of the fuel without the production of objectional amounts of nitrogen oxides. Specific operations will generally define the maximum flame velocity which is controlled by various conditions of operations, such as amount of air and fuel present, the type of fuel employed, temperature, and pressure. Suitable gas velocities are above the actual maximum linear velocity for flame propagation.