Figure 3. Propane mole fraction contour plots in the gas turbine
combustor designed for the reduction of nitrogen oxides emissions by
heterogeneous catalysis.
The temperature contour plots in the gas turbine combustor are
illustrated in Figure 4 for the reduction of nitrogen oxides emissions
by heterogeneous catalysis. While gas turbine engines employing purely
thermal combustion have been used extensively as prime movers,
especially in aircraft and stationary power plants, they have not been
found to be commercially attractive for propelling land vehicles, such
as trucks, buses and passenger cars [63, 64]. One reason for this is
the inherent disadvantages of systems based purely on thermal combustion
or conventional catalytic combustion [65, 66]. Catalysts are
available for promoting catalytic combustion. These catalysts are useful
in eliminating nitrogen oxides from gases which also contain some
oxygen. In general, reaction of oxygen with combustible hydrocarbons or
hydrogen provides sufficient heat to raise the temperature of the gas
mixture so that nitrogen oxides present will decompose when the oxygen
present is depleted by combustion. The reactions are all exothermic and
provide considerable heat. Reactions between oxygen and hydrocarbon
gases or between oxygen and hydrogen initiate at different temperatures.
Plant efficiencies are markedly reduced and pollution limits for
nitrogen oxides exceeded when catalysts do not function. There is
sufficient heat in gases leaving the catalytic combustion catalyst to
reform a mixture of natural gas or other hydrocarbon and steam. Use of
this heat via indirect heat exchange for reforming converts the incoming
natural gas or hydrocarbon into a stream of gas containing a high
percentage of hydrogen. This hydrogen is more than sufficient to
initiate the catalytic combustion and to raise temperatures in the
catalyst to a point where the remaining combustibles in the gas stream
will also react with oxygen, reducing the oxygen content and further
raising the temperature so that nitrogen oxide will decompose. According
to the catalytically-supported, thermal combustion method, carbonaceous
fuels can be combusted very efficiently at temperatures between about
1,200 K and about 2,000 K, for example, without the formation of
substantial amounts of carbon monoxide or nitrogen oxides by a process
designated catalytically-supported, thermal combustion. In conventional
thermal combustion of carbonaceous fuels, a flammable mixture of fuel
and air or fuel, air, and inert gases is contacted with an ignition
source to ignite the mixture. Once ignited, the mixture continues to
burn without further support from the ignition source. Flammable
mixtures of carbonaceous fuels normally burn at relatively high
temperatures. At these temperatures, substantial amounts of nitrogen
oxides inevitably form if nitrogen is present, as is always the case
when air is the source of oxygen for the combustion reaction. Mixtures
of fuel and air or fuel, air, and inert gases which would theoretically
burn at temperatures below about 2,080 K are too fuel-lean to support a
stable flame and therefore cannot be satisfactorily burned in a
conventional thermal combustion system.