3. Results and discussion
The propane mole fraction contour plots in the gas turbine combustor are illustrated in Figure 3 for the reduction of nitrogen oxides emissions by heterogeneous catalysis. In order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimize the quantity of the oxides of nitrogen produced [59, 60]. The fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature and this requires premixing of the fuel and all the combustion air before combustion takes place [61, 62]. The present design provides a gas turbine engine combustion chamber comprising a primary combustion zone and a secondary combustion zone downstream of the primary combustion zone. In this way, the temperatures in each stage can be reduced. Suitable temperature ranges in each stage are from 900 K to 1500 K, depending on the particular catalyst and support. An advantage of operating in this temperature range is that it is below the fixation temperature of nitrogen and consequently the combusted gases are free of nitrogen oxides. Additionally, catalytic combustion results in lower un-combusted fuel content. A further advantage of catalytic combustion is that it is possible to operate with minimum of air for combustion, namely excess oxygen in the combusted gases can be reduced almost to zero. The catalyst may be supported on a monolith. The preferred characteristics of the metallic monolith having a catalyst deposited thereon are that is presents low resistance to the passage of gases by virtue of its possession of a high ratio of open area to blocked area and that it has a high surface to volume ratio. Preferably, the metallic monolith is formed from one or more metals selected from the group comprising ruthenium, rhodium, palladium, iridium, and platinum. However, base metals may be used or base metal alloys which also contain a platinum group metal component may be used. Oxygen is the required element to support combustion. It is possible to achieve essentially adiabatic combustion in the presence of a catalyst at a reaction rate many times greater than the mass transfer limited rate. In particular, if the operating temperature of the catalyst is increased substantially into the mass transfer limited region, the reaction rate again begins to increase rapidly with temperature. This is in apparent contradiction of the laws of mass transfer kinetics in catalytic reactions. The phenomenon may be explained by the fact that the temperature of the catalyst surface and the gas layer near the catalyst surface are above the instantaneous auto-ignition temperature of the mixture of fuel, air, and any inert gases and at a temperature at which thermal combustion occurs at a rate higher than the catalytic combustion rate. The fuel molecules entering this layer burn spontaneously without transport to the catalyst surface. As combustion progresses and the temperature increases, the layer in which thermal combustion occurs becomes deeper. Ultimately, substantially all of the gas in the catalytic region is raised to a temperature at which thermal combustion occurs in virtually the entire gas stream rather than just near the surface of the catalyst. Once this stage is reached within the catalyst, the thermal reaction appears to continue even without further contact of the gas with the catalyst.