Figure 7. Pressure contour plots in the gas turbine combustor designed for the reduction of nitrogen oxides emissions by heterogeneous catalysis.
The chemical enthalpy contour plots in the gas turbine combustor are illustrated in Figure 8 for the reduction of nitrogen oxides emissions by heterogeneous catalysis. Nitrogen oxides formation is thermodynamically favored at high temperatures. Since the nitrogen oxides formation reaction is so highly temperature dependent, decreasing the peak combustion temperature can provide an effective means of reducing nitrogen oxides emissions from gas turbine engines as can limiting the residence time of the combustion products in the combustion zone. The limits of the operating temperature are governed largely by residence time and pressure. The instantaneous auto-ignition temperature of the mixture is defined herein to mean the temperature at which the ignition lag of the mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion. Essentially adiabatic combustion means in this case that the operating temperature of the catalyst does not differ by more than about 80 K, from the adiabatic flame temperature of the mixture due to heat losses from the catalyst. Among the unique advantages of the catalytically supported thermal combustion in the presence of a catalyst is the fact that mixtures of fuel and air which are too fuel-lean for ordinary thermal combustion can be burned efficiently. Since the temperature of combustion for a given fuel at any set of conditions is dependent largely on the proportions of fuel, of oxygen available for combustion, and of inert gases in the mixture to be burned, it becomes practical to burn mixtures which are characterized by much lower flame temperatures. In particular, carbonaceous fuels can be burned very efficiently and at thermal reaction rates at temperatures in the range from about 1,200 K to about 2,000 K. At these temperatures, very little if any nitrogen oxides are formed. In addition, because the catalytically supported thermal combustion is stable over a wide range of mixtures, it is possible to select or control reaction temperature over a correspondingly wide range by selecting or controlling the relative proportions of the gases in the mixture. The method may be carried out in various ways, including heating the catalyst body by electrical means, or by first thermally combusting a fuel and air mixture and applying the heat produced to the catalyst body. Once a catalyst temperature is reached at which the catalyst will function to sustain mass transfer limited operation, the combustion of fuel in the presence of the catalyst will bring it rapidly to the required operating temperature. Once operating temperature is reached, the catalyst will provide for sustained combustion of the fuel vapor. After the catalyst body reaches a temperature at which it will sustain mass transfer limited operation, the aforementioned application of heat to the catalyst body is no longer necessary and an admixture of unburned fuel and air is introduced into the system to establish the supported thermal combustion to provide a motive fluid for a turbine or heat to a furnace. No serious start-up problem normally is presented. The operation is substantially continuous and it is necessary to start the system only at infrequent intervals. Consequently, the substantial emissions of atmospheric pollutants which tend to occur in start-ups are not serious because of the small number of infrequent start-ups. While this pollution may be tolerated in stationary operations which are normally used continuously and for long periods of time, it cannot be tolerated in the vehicular type of installation where start-ups are frequent, due to intermittent operation [79, 80]. Also, the start-up must be rapid in order to be as efficient as in the conventional present-day automobile [81, 82]. The mixture of unburned fuel and air is not introduced to the catalyst body until it has reached a temperature at which it will sustain the desired rapid combustion. Such preferred procedure minimizes pollutant emissions during start-up. Once combustion in the zone containing the catalyst is achieved, the fuel-air admixture is passed to the catalyst at a gas velocity, prior to or at the inlet to the catalyst, in excess of the maximum flame propagating velocity. This avoids flash-back that causes the formation of nitrogen oxides [83, 84]. Various means have been proposed in the past to prevent such flash backs from traveling back to the gas supply source [85, 86]. Preferably, this velocity is maintained adjacent to the catalyst inlet [87, 88]. Suitable linear gas velocities are usually above about 8 meters per second, but considerably higher velocities may be required depending upon such factors as temperature, pressure, and composition.