Figure 1. Enthalpy contour plots in the pre-mixed homogeneous-heterogeneous hybrid system with combustion of small alkanes on noble metal surfaces.
The temperature contour plots in the pre-mixed homogeneous-heterogeneous hybrid system are illustrated in Figure 2 with combustion of small alkanes on noble metal surfaces. The products of combustion of fossil fuels include carbon dioxide, carbon monoxide, unburnt hydrocarbons and nitrogen oxides [53, 54]. Various control schemes and hardware configurations have been used to control the concentration of such emissions while at the same time providing fuel-efficient and stable engine operation [55, 56]. Regulatory changes continue to reduce the allowable level of emissions from electric power generating plants utilizing gas turbine engines. To achieve low levels of emissions, it is necessary to establish and to maintain very lean combustion conditions. Lean combustion is known to be less stable than rich combustion, and lean-burn combustors are more prone to damaging pressure pulsations generated within the combustor. Precise tuning of the combustion process is needed to establish a balance between stable combustion and low emissions. A precisely tuned engine may be susceptible to drift over time, with a resulting increase in emissions or an increase in the level of combustion instability. One known approach to controlling the emissions from a gas turbine power plant is to run the combustor at a relatively rich setting, thereby ensuring stable combustion while generating excessive amounts of undesirable emissions. The exhaust gas is then cleaned to regulatory limits by passing it through a combustion catalyst installed downstream of the combustor in the turbine exhaust system. Alternatively, a catalyst may be used to achieve a majority of the fuel combustion, with only a final portion of the combustion being accomplished in a flame combustor located downstream of the primary catalyst. Catalyst systems are very expensive and are often used as a last resort in especially rigorous regulatory situations. The generation of nitrogen oxides emissions is directly related to the peak flame temperature in the combustor. The peak flame temperature in a gas turbine combustor can be controlled by injecting water into the combustor. The cost of the demineralized water used for water injection can be significant, particularly in areas where the supply of fresh water is limited. Accordingly, it is beneficial to limit the use of injected water to the extent possible. A radial flow catalyst element can be integrated into an aerodynamically stabilized burner to provide a catalytically reacted fuel-air mixture for enhanced flame stabilization with catalyst temperature maintained by recirculation of hot combustion gases at a temperature high enough even for combustion of methane at ambient combustor inlet air temperatures yet at a temperature well below the adiabatic combustion temperature thus allowing burner outlet temperatures high enough for modern gas turbines. An aerodynamically stabilized combustor or burner is one wherein gas phase combustion is stabilized by aerodynamic recirculation of hot combustion products such as induced by a swirler, a bluff body, opposed flow jets, or a flow dump. Preferred are swirlers. In operation of a burner, a fuel-air mixture is passed into contact with a catalytic element for reaction thereon. The resulting reacted admixture is then admixed with the fresh fuel and air passing into the combustor thus enhancing reactivity and enabling stable combustion even with very lean fuel-air admixtures. Light-off of burners may be achieved using any conventional ignition means such as spark plugs, glow plugs, laser beams, or microwave energy. Advantageously, for ignition the catalytic element is heated electrically to a temperature high enough for fuel ignition followed by introduction of fuel and air. This not only achieves ignition but assures that the catalyst is at an effective temperature to stabilize lean combustion in the burner from the start of combustion.