Figure 5. Velocity contour plots in the pre-mixed homogeneous-heterogeneous hybrid system with combustion of small alkanes on noble metal surfaces.
The reactant mole fraction contour plots in the pre-mixed homogeneous-heterogeneous hybrid system are illustrated in Figure 6 with combustion of small alkanes on noble metal surfaces. The use of catalytic processes for combustion or oxidation is a well-known method for potentially reducing levels of nitrogen oxides emissions from gas turbine engine systems [69, 70]. There are various processes for converting the chemical energy in a fuel to heat energy in the products of the conversion [71, 72]. The primary processes are gas phase combustion, catalytic combustion, and catalytic oxidation. There are also combinations of these processes, such as processes having a first stage of catalytic oxidation followed by a gas phase combustion process. In catalytic oxidation, an air-fuel mixture is oxidized in the presence of a catalyst. In all catalytic processes the catalyst allows the temperature at which oxidation takes place to be reduced relative to non-catalytic combustion temperatures. Lower oxidation temperature leads to reduced nitrogen oxides production. In catalytic oxidation, all reactions take place on the catalytic surface. There are no local high temperatures and therefore the lowest possible potential for nitrogen oxides to be formed. In either catalytic combustion or catalytically stabilized combustion, some part of the reaction takes place in the gas phase, which increases local temperatures and leads to higher potential for nitrogen oxides being formed. Such low levels in general cannot be achieved with conventional non-catalytic combustors, catalytic combustion, or catalytically stabilized combustion. The term catalytic combustor is used to refer to any combustor utilizing catalysis, preferably one utilizing catalytic oxidation. The catalyst employed in a catalytic combustor tends to operate best under certain temperature conditions. In particular, there is typically a minimum temperature below which a given catalyst will not function. For instance, palladium catalyst requires a combustor inlet temperature for the air-fuel mixture higher than 500 °C when natural gas is the fuel. In addition, catalytic oxidation has the disadvantage that the physical reaction surface which must be supplied for complete oxidation of the hydrocarbon fuel increases exponentially with decreasing combustor inlet temperatures, which greatly increases the cost of the combustor and complicates the overall design. The need for a relatively high combustor inlet temperature is one of the chief reasons why catalytic combustion in general, and catalytic oxidation in particular, has not achieved widespread use in gas turbine engine systems. More specifically, such high combustor inlet temperatures generally cannot be achieved in gas turbines operating with compressor pressure ratios less than about 40 unless a recuperated cycle is employed. In a recuperated cycle, the air-fuel mixture is pre-heated, prior to combustion, by heat exchange with the turbine exhaust gases. Recuperation thus can help achieve the needed combustor inlet temperature for proper catalyst operation, at least under some conditions. However, there are often other operating conditions that will be encountered at which the minimum required combustor inlet temperature still cannot be achieved even with recuperation. For instance, when recuperation is applied in small gas turbines, material temperature limitations in the recuperator can limit the maximum air or air-fuel mixture temperature.