Figure 6. Reactant mole fraction contour plots in the pre-mixed homogeneous-heterogeneous hybrid system with combustion of small alkanes on noble metal surfaces.
The reactant mole fraction profiles in the streamwise direction are presented in Figure 7 for the pre-mixed homogeneous-heterogeneous hybrid system. The ratio of fuel to air in the admixture introduced into the combustion zone is ultimately determined by the desired operating temperature of the combustion zone. The operating temperature is determined by the theoretical adiabatic flame temperature of the fuel-air admixture passed to the combustor and thus is dependent on the initial temperature of the air as well as the amount of fuel contained therein. In the operation of any engine various changes in load occur. For example, increases or decreases in output speed of a turbine may be required thus changing the amount of fuel needed. Further, even at constant speed, load changes are encountered. In certain turbine applications, for example, automobiles, increases or decreases in turbine power often result in an increase or decrease in compressor speed and respectively an increase or decrease in the compression and temperature of the air supplied to the system by the compressor [73, 74]. The theoretical adiabatic flame temperature of an admixture having a given fuel-to-air volume ratio will vary directly with the temperature of the air in the admixture [75, 76]. Thus, for example, as power requirements of the turbine are increased, a number of interrelated control changes must take place. The air charged to the system will be at a higher temperature which, unless compensated for, would increase the adiabatic flame temperature of a given fuel-air mixture. Further, a power increase implies a fuel flow increase; therefore, the portion of the total compressed air which is directed to the combustor must increase to maintain the theoretical adiabatic flame temperature approximately constant. The portion of the total compressed air directed to the bypass will be decreased to allow the temperature of the combined effluent gas-additional air admixture to increase. The relative amounts of each air stream, that is, the air introduced to the combustor and the air directed to the bypass are varied in an inverse manner. These interdependent changes maintain the combustor at an approximately constant temperature by controlling the fuel-air admixture introduced into the combustor to have an approximately constant theoretical adiabatic flame temperature. The combustion effluent is preferably combined with additional, cooler air to quench the effluent and provide a motive fluid for the gas turbine at a desired temperature. The additional air can be at any convenient temperature, for instance, at about the temperature at which the air leaves the compressor, or it can be at a somewhat lower or higher temperature achieved by, for example, indirect heat exchange with the combustion zone or with the exhaust gases from the turbine. Conveniently, the additional air is at a temperature between about 40 and 1100 °C, and preferably about 260 to 800 °C. Thus, the greater the amount of cooler additional air combined with the combustion effluent, the lower the temperature of the combined gases and, hence, the less the power obtainable from the combined gases when used as a motive fluid in a turbine system. Similarly, if the amount of additional air employed is reduced, the power output from the turbine is increased. The temperature of the combined gases passed to the turbine inlet is generally about 400 °C to 1500 °C, and preferably for increased turbine efficiency, about 600 °C to 1500 °C. Another means for increasing the energy of the combined combustor effluent-bypass air in response to increased amounts of fuel is to increase proportionately both the amount of air to the combustion zone and the amount of additional air to be combined with the combustion effluent. The solid catalyst can have various forms and compositions and can be the types used to oxidize fuels in the presence of molecular oxygen. The catalyst can be in the form of relatively small, solid particles of various sizes and shapes, often in sizes below about one inch in the largest dimension, with a plurality of such particles being arranged together to form one or more catalyst masses or beds in the combustion zone. The catalyst is preferably of larger form and has a skeletal structure with gas flow paths therethrough. The catalyst generally has one or more metal containing components which are catalytically active towards promoting the desired oxidation reactions, and in view of the rather high temperatures at which the catalyst operates, materials normally considered to be relatively inactive or insufficiently active, to promote adequately the oxidation of the fuel, may be suitable.