Figure 4. Temperature contour plots in the gas turbine combustor designed for the reduction of nitrogen oxides emissions by heterogeneous catalysis.
The propane mole fraction profiles along the fluid centerline of the gas turbine combustor are presented in Figure 5 under different fluid velocity conditions. In conventional catalytic combustion, the fuel is burned at relatively low temperatures [67, 68]. However, catalytic combustion is regarded as having limited value as a source of thermal energy [69, 70]. In the first place, conventional catalytic combustion proceeds relatively slowly so that impractically large amounts of catalyst would be required to produce enough combustion effluent gases to drive a turbine or to consume the large amounts of fuel required in most large furnace applications. In the second place, the reaction temperatures normally associated with conventional catalytic combustion are too low for efficient transfer of heat for many purposes. Typically, catalytic combustion is also relatively inefficient, so that significant amounts of fuel are incompletely combusted or left un-combusted unless low space velocities in the catalyst are employed. Catalytic combustion is a chemical process whereby a combustible species, in the gas phase, is reacted over a solid catalyst to completely oxidize the target molecules. For molecules containing only carbon and hydrogen or molecules containing carbon, hydrogen, and oxygen, the products of catalytic combustion are solely carbon dioxide and water. Catalytic combustion is frequently, but not exclusively, employed in applications where the concentration of the target species is below its lower ignition limit. When combustible gases are below their lower ignition limit then the mixture will not ignite when exposed to an ignition source. At such dilute concentrations, it is more efficient to use catalysts to react, convert, or combust, the target compounds because a catalyst can facilitate complete oxidation of the target species at a temperature significantly lower than the auto-ignition temperature of the molecule. It is generally desirable to develop catalysts that are capable of facilitating catalytic combustion at the lowest possible temperature in order to reduce the energy costs associated with operating a catalytic combustion system. Complete catalytic combustion of a target species can only occur when oxygen gas is found in molar stoichiometric excess; a condition which is easily met when the target species is present in trace quantity in air. Even in a reduced-oxygen environment, provided more moles of oxygen gas are present compared to moles of carbon atoms to be combusted then complete combustion of the target species can be realized. The required residence time of the gases in the space between the catalyst and the inlet of the turbine expansion zone is a function of the temperature of the gases exiting the catalyst. In any event, the gas residence time between the exit of the upstream oxidation zone and the inlet of the turbine gas expansion zone may be minimal and is such that at least a significant amount of the combustion takes place in the turbine gas expansion zone. If desirable, this residence time may be so small that at least a major portion of the total combustion occurring subsequent to the upstream catalyst zone is in the turbine expansion zone. The fuel is combusted in contact with free or molecular oxygen and free or molecular nitrogen. The fuel may occur or be obtained in admixture with components which are essentially inert in the oxidation system. The fuel has a relatively high energy content and is of a nature which permits the preparation of the oxidation feed streams.