Figure 7. Reactant mole fraction profiles in the streamwise direction of
the pre-mixed homogeneous-heterogeneous hybrid system.
The inner wall temperature profiles in the streamwise direction are
presented in Figure 8 for the pre-mixed homogeneous-heterogeneous hybrid
system. The higher the temperature of the combustion gas entering the
turbine, the higher the efficiency of the turbine. However, absent
perfect mixing of fuel and air, there will be islands of high fuel
concentration where the temperature is high enough to form nitrogen
oxides [77, 78]. Catalytic combustion offers the possibility of
reducing the nitrogen oxides concentration [79, 80]. Temperature
control in a catalytic combustor is a serious problem [81, 82]. 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 [83, 84]. 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 [85, 86]. The gas that results is lowered to a
predetermined turbine inlet temperature by the addition of cooling air
and diluent air, then injected through a turbine nozzle into a gas
turbine [87, 88]. The catalyst in the catalytically supported
thermal combustion generally operates at a temperature approximating the
theoretical adiabatic flame temperature of the fuel-air admixture
charged to the combustion zone. The entire catalyst may not be at these
temperatures, but preferably a major portion, or essentially all, of the
catalyst surface is at such operating temperatures. The temperature of
the catalyst zone is controlled by adjusting the composition and initial
temperature of the fuel-air admixture as well as the uniformity of the
mixture. Relatively higher energy fuels can be admixed with larger
amounts of air in order to maintain the desired temperature in a
combustion zone. At the higher end of the temperature range, shorter
residence times of the gas in the combustion zone appear to be desirable
in order to lessen the chance of forming nitrogen oxides. The residence
time is governed largely by temperature, pressure and space throughput,
and generally is measured in milliseconds. Part of the catalytically
supported thermal combustion of the fuel-air mixture is conducted
upstream of the turbine by combustion of the mixture while passing
through an insufficient amount of catalyst to effect complete combustion
of the fuel prior to passing through the turbine wheel or expansion
zone. The partially combusted effluent from the catalyst, with or
without substantial intermediate but incomplete further combustion, is
introduced into a gas expansion zone of the turbine so that the
partially combusted effluent is further thermally combusted while
undergoing expansion. The further thermal combustion converts remaining
combustible fuel components to carbon dioxide and water. The addition of
heat to the system provided by combustion in the turbine gas expansion
area counteracts the cooling in the system concomitant with the gas
expansion. The turbine is thus operated under conditions which may be
referred to as continuous reheating. The operating efficiency and work
output of the turbine are thereby enhanced according to known reheating
principles. Also, the reheating effect can be obtained without the
necessity of employing plural turbine stages and separate reheating
equipment. The present design thus provides benefits even in turbines
with a single impeller wheel or single stage; however, the present
design can also be used where the combustion is accomplished in the gas
expansion zones of a plurality of turbine wheels or stages. In the
latter type systems, advantageous efficiencies can be obtained. Partial
combustion is conducted upstream of the turbine by catalytically
supported thermal combustion. A lesser amount of catalyst is required
for complete combustion of the fuel is used in the catalyst zone.
Accordingly, and since at least a portion of the combustion is conducted
in a downstream turbine gas expansion area, the system responds quickly
to desired changes in operation, and yet the combustion can still
produce an effluent from the work-performing zone having a relatively
small amount of nitrogen oxides. As a consequence, the operation can be
highly advantageous in turbines used for propelling automotive vehicles.
A temperature lowering effect from expansion of the gases within the
turbine expansion zone normally occurs in turbines, and this is
counteracted by providing at least some thermal combustion of fuel
within the turbine gas expansion zone. Thus, for a turbine which
operates at a given temperature, more fuel can be burned in the method
as compared with conventional operations, without producing excessive
temperatures in the turbine. The catalytic metal may be in a combined
form, such as an oxide, rather than being solely in the elemental state,
and preferably the catalytic metal compound is carried by a less
catalytically-active, or even an essentially inert, support which may
be, for instance, ceramic in nature. In these catalysts, the more
catalytically-active metal component is often a minor amount of the
catalyst, while the support is the major portion.