Figure 2. Temperature contour plots in the pre-mixed homogeneous-heterogeneous hybrid system with combustion of small alkanes on noble metal surfaces.
The front wall edge temperature profiles in the transverse direction are presented in Figure 3 for the pre-mixed homogeneous-heterogeneous hybrid system. Intake air in the pre-mixed homogeneous-heterogeneous hybrid system is compressed by a compressor turbine, and the compressed air then may be apportioned into at least two parts, one part of which is intimately admixed with a carbonaceous fuel and introduced into a combustion zone to be thermally combusted, preferably by homogeneous thermal combustion and a second part of which is combined with the effluent from such combustion. The relative amount of air in each part or portion is adjustable and proportionately interdependent. Attempts have been made to control the fuel-to-air ratio in conventional combustors [57, 58]. Such attempts, however, have not satisfactorily dealt with the problem of sustained low emission combustion which is responsive to variations in load on the engine and other operating conditions [59, 60]. In fixed geometry conventional combustors which burn fuel in air at approximately the stoichiometric ratio, the hole pattern of the combustor liner is ordinarily designed for best operation of the primary or combustion zone, at nearly full load. With such apparatus, the overall fuel-air ratio decreases at light load or at idle resulting in a leaner mixture in the combustion zone, which can lead to reduced combustion efficiency and increased exhaust emissions. This problem can be overcome by using variable combustor geometry to operate the primary or combustion zone at a constant fuel-air ratio, namely close to stoichiometric at all turbine operating conditions. Although this does solve the hydrocarbon and carbon monoxide problem, it does not even address the nitrogen oxides problem. Formation of nitrogen oxides occurs at relatively high temperatures, which inevitably are reached in adiabatic combustion systems with near-stoichiometric fuel-air ratios. Consequently, simply changing the combustor geometry to maintain near-stoichiometric ratios will not avoid nitrogen oxides formation. Further, at a fixed fuel-air ratio the combustion temperature will vary accordingly to variations in the temperature of the air at the inlet to the combustor so that combustion temperature is not fixed. Flammable mixtures of most fuels, for complete combustion, normally burn at relatively high temperatures, namely above about 1800 °C, which inherently results in the formation of substantial amounts of nitrogen oxides. In the case of conventional gas turbine thermal combustors, formation of nitrogen oxides can be reduced by limiting the residence time of the combustion products in the combustion zone. However, due to the large quantities of gases being handled, undesirable amounts of nitrogen oxides are produced. Many conventional combustors, by injecting the fuel into the combustor in droplet form and separately from the air used for combustion present serious drawbacks to low pollution operation. Such a system substantially precludes very lean sustained combustion. Consequently, the combustion temperature of the droplet boundaries in the pre-mixed homogeneous-heterogeneous hybrid system will frequently be approximately the theoretical adiabatic flame temperature of a stoichiometric mixture of the fuel and air; this temperature will be substantially over 1800 °C, and typically in excess of 2200 °C. Therefore, even though the overall temperature in the combustor may be quite low and not high enough to form nitrogen oxides, the temperature near the droplet surface is typically in excess of that required to form nitrogen oxides. Consequently, nitrogen oxides forms and is present in the combustor effluent.