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.