Figure 4. Temperature contour plots in the gas turbine combustor
designed for the reduction of nitrogen oxides emissions by heterogeneous
catalysis.
The propane mole fraction profiles along the fluid centerline of the gas
turbine combustor are presented in Figure 5 under different fluid
velocity conditions. In conventional catalytic combustion, the fuel is
burned at relatively low temperatures [67, 68]. However, catalytic
combustion is regarded as having limited value as a source of thermal
energy [69, 70]. In the first place, conventional catalytic
combustion proceeds relatively slowly so that impractically large
amounts of catalyst would be required to produce enough combustion
effluent gases to drive a turbine or to consume the large amounts of
fuel required in most large furnace applications. In the second place,
the reaction temperatures normally associated with conventional
catalytic combustion are too low for efficient transfer of heat for many
purposes. Typically, catalytic combustion is also relatively
inefficient, so that significant amounts of fuel are incompletely
combusted or left un-combusted unless low space velocities in the
catalyst are employed. Catalytic combustion is a chemical process
whereby a combustible species, in the gas phase, is reacted over a solid
catalyst to completely oxidize the target molecules. For molecules
containing only carbon and hydrogen or molecules containing carbon,
hydrogen, and oxygen, the products of catalytic combustion are solely
carbon dioxide and water. Catalytic combustion is frequently, but not
exclusively, employed in applications where the concentration of the
target species is below its lower ignition limit. When combustible gases
are below their lower ignition limit then the mixture will not ignite
when exposed to an ignition source. At such dilute concentrations, it is
more efficient to use catalysts to react, convert, or combust, the
target compounds because a catalyst can facilitate complete oxidation of
the target species at a temperature significantly lower than the
auto-ignition temperature of the molecule. It is generally desirable to
develop catalysts that are capable of facilitating catalytic combustion
at the lowest possible temperature in order to reduce the energy costs
associated with operating a catalytic combustion system. Complete
catalytic combustion of a target species can only occur when oxygen gas
is found in molar stoichiometric excess; a condition which is easily met
when the target species is present in trace quantity in air. Even in a
reduced-oxygen environment, provided more moles of oxygen gas are
present compared to moles of carbon atoms to be combusted then complete
combustion of the target species can be realized. The required residence
time of the gases in the space between the catalyst and the inlet of the
turbine expansion zone is a function of the temperature of the gases
exiting the catalyst. In any event, the gas residence time between the
exit of the upstream oxidation zone and the inlet of the turbine gas
expansion zone may be minimal and is such that at least a significant
amount of the combustion takes place in the turbine gas expansion zone.
If desirable, this residence time may be so small that at least a major
portion of the total combustion occurring subsequent to the upstream
catalyst zone is in the turbine expansion zone. The fuel is combusted in
contact with free or molecular oxygen and free or molecular nitrogen.
The fuel may occur or be obtained in admixture with components which are
essentially inert in the oxidation system. The fuel has a relatively
high energy content and is of a nature which permits the preparation of
the oxidation feed streams.