Figure 3. Propane mole fraction contour plots in the gas turbine combustor designed for the reduction of nitrogen oxides emissions by heterogeneous catalysis.
The temperature contour plots in the gas turbine combustor are illustrated in Figure 4 for the reduction of nitrogen oxides emissions by heterogeneous catalysis. While gas turbine engines employing purely thermal combustion have been used extensively as prime movers, especially in aircraft and stationary power plants, they have not been found to be commercially attractive for propelling land vehicles, such as trucks, buses and passenger cars [63, 64]. One reason for this is the inherent disadvantages of systems based purely on thermal combustion or conventional catalytic combustion [65, 66]. Catalysts are available for promoting catalytic combustion. These catalysts are useful in eliminating nitrogen oxides from gases which also contain some oxygen. In general, reaction of oxygen with combustible hydrocarbons or hydrogen provides sufficient heat to raise the temperature of the gas mixture so that nitrogen oxides present will decompose when the oxygen present is depleted by combustion. The reactions are all exothermic and provide considerable heat. Reactions between oxygen and hydrocarbon gases or between oxygen and hydrogen initiate at different temperatures. Plant efficiencies are markedly reduced and pollution limits for nitrogen oxides exceeded when catalysts do not function. There is sufficient heat in gases leaving the catalytic combustion catalyst to reform a mixture of natural gas or other hydrocarbon and steam. Use of this heat via indirect heat exchange for reforming converts the incoming natural gas or hydrocarbon into a stream of gas containing a high percentage of hydrogen. This hydrogen is more than sufficient to initiate the catalytic combustion and to raise temperatures in the catalyst to a point where the remaining combustibles in the gas stream will also react with oxygen, reducing the oxygen content and further raising the temperature so that nitrogen oxide will decompose. According to the catalytically-supported, thermal combustion method, carbonaceous fuels can be combusted very efficiently at temperatures between about 1,200 K and about 2,000 K, for example, without the formation of substantial amounts of carbon monoxide or nitrogen oxides by a process designated catalytically-supported, thermal combustion. In conventional thermal combustion of carbonaceous fuels, a flammable mixture of fuel and air or fuel, air, and inert gases is contacted with an ignition source to ignite the mixture. Once ignited, the mixture continues to burn without further support from the ignition source. Flammable mixtures of carbonaceous fuels normally burn at relatively high temperatures. At these temperatures, substantial amounts of nitrogen oxides inevitably form if nitrogen is present, as is always the case when air is the source of oxygen for the combustion reaction. Mixtures of fuel and air or fuel, air, and inert gases which would theoretically burn at temperatures below about 2,080 K are too fuel-lean to support a stable flame and therefore cannot be satisfactorily burned in a conventional thermal combustion system.