Figure 6. Homogeneous reaction rate profiles along the fluid centerline of the gas turbine combustor under different fluid velocity conditions.
The pressure contour plots in the gas turbine combustor are illustrated in Figure 7 for the reduction of nitrogen oxides emissions by heterogeneous catalysis. At relatively low temperatures, the catalytic reaction rate increases exponentially with temperature. As the temperature is raised further, the reaction rate enters a transition zone in which the rate at which the fuel and oxygen are being transferred to the catalytic surface begins to limit further increases in the reaction rate. As the temperature is raised still further, the reaction rate enters a so-called mass transfer limited zone in which the reactants cannot be transferred to the catalytic surface fast enough to keep up with the catalytic surface reaction and the reaction rate levels off regardless of further temperature increases. In the mass transfer limited zone, the reaction rate cannot be increased by increasing the activity of the catalyst because catalytic activity is not determinative of the reaction rate. The only apparent way to increase the reaction rate in a mass transfer limited reaction is to increase mass transfer. However, this typically requires an increase in the pressure drop across the catalyst and consequently a substantial loss of energy. Sufficient pressure drop may not even be available to provide the desired reaction rate. Of course, more mass transfer can be affected, and hence more energy can always be produced by increasing the amount of catalyst surface. However, this results in catalyst configurations of such size and complexity that the cost is prohibitive and the body of the catalyst is unwieldy. For example, in the case of gas turbine engines, the catalytic reactor might very well be larger than the engine itself. In general, conventional adiabatic, thermal combustion systems operate at such high temperatures in the combustion zone that undesirable nitrogen oxides, The conventional gas turbine combustor, as used in a gas turbine power generating system, requires a mixture of fuel and air which is ignited and combusted uniformly. Generally, the fuel injected from a fuel nozzle into the inner tube of the combustor is mixed with air for combustion, fed under pressure from the air duct, ignited by a spark plug and combusted. The gas that results is lowered to a predetermined turbine inlet temperature by the addition of cooling air and dilutant air, then injected through a turbine nozzle into a gas turbine. Catalytic combustion systems, though, are capable of achieving ultra-low emissions [75, 76]. However, catalytic combustion systems are not able to offer the accuracy and controllability of the air staging system over a wide range of power levels, fuel properties and ambient operating conditions [77, 78]. The system may be operated in different manners to allow for low and high-power operation, as well as according to a controlled schedule that may be programmed. Under low power operation oxidation does not occur in the catalyst section. However, the mixing of the fuel and air in the fuel preparation and mixing section is enhanced by the presence of the catalyst. As the engine power level increases the compressor outlet air temperature will become high enough to activate the catalyst, and partial oxidation reactions will occur.