3. Results and discussion
The enthalpy contour plots in the pre-mixed homogeneous-heterogeneous hybrid system are illustrated in Figure 1 with combustion of small alkanes on noble metal surfaces. The term thermal combustion, as used herein, is understood to mean a homogeneous combustion reaction which proceeds without the aid of a catalyst. The combustion is conducted by utilizing a selectively controlled fuel-to-air ratio to achieve an approximately constant combustion temperature which is substantially above the instantaneous auto-ignition temperature of the fuel-air admixture but preferably is below a temperature that would result in the substantial formation of oxides of nitrogen. Subsequently, additional, selectively controlled air can be combined with the products or effluent from the combustion of the fuel-air admixture. The combustion effluent is characterized by high thermal energy and typically by low nitrogen oxides content. The method and apparatus thus provide for highly efficient turbine operation and quick response to changes in operation of the system with relatively little atmospheric pollution. Broadly, intake air is compressed and at least a portion of the compressed air is intimately admixed with a carbonaceous fuel [49, 50]. The resulting admixture is then passed to a temperature-controlled combustion or primary zone where it is combusted above the instantaneous auto-ignition temperature at an approximately constant temperature advantageously over a period of turbine operation in which the amount of fuel charged to the combustion zone is varied in response to power demand on the turbine [51, 52]. The fuel-to-air volume ratio is adjusted, taking into account the temperature of the gas entering the combustion zone, so that the theoretical adiabatic flame temperature of the mixture remains about constant over a wide range of fuel inputs. The effluent from the combustion zone is combined in a secondary zone with at least a portion and preferably most of the remaining compressed air charged to the turbine system. The power obtained from the gas turbine can thus be controlled without changing the temperature in the combustion zone by adjusting the overall volume of fuel-air mixture to the combustion zone and the volume of additional or bypass air or gas combined with the combustion effluent. These adjustments regulate the temperature of the gas entering the turbine and thus the power produced thereby. Since the temperature of the combustor effluent-bypass air mixture is primarily dependent on the temperature and relative amounts of combustor effluent and bypass air, it is possible to obtain a quick response of the gas turbine system to different power requirements by varying the amount of additional or bypass air combined with the combustion effluent. Further, even at low turbine inlet temperatures, low combustion temperatures can be avoided. Thus, the combustor need not operate at low temperatures which might impair performance with possible flame-out and also might result in an effluent having a high content of carbon monoxide and hydrocarbons. Similarly, avoidance of combustion temperatures significantly in excess of about 1800 °C avoids the formation of excessive amounts of nitrogen oxides during combustion.