Figure 5. Propane mole fraction profiles along the fluid centerline of
the gas turbine combustor under different fluid velocity conditions.
The homogeneous reaction rate profiles along the fluid centerline of the
gas turbine combustor are presented in Figure 6 under different fluid
velocity conditions. In a gas turbine engine, inlet air is continuously
compressed, mixed with fuel in an inflammable proportion, and then
contacted with an ignition source to ignite the mixture which will then
continue to burn [71, 72]. The heat energy thus released then flows
in the combustion gases to a turbine where it is converted to rotary
energy for driving equipment such as an electrical generator [73,
74]. The channels through the unitary body or skeletal structure can
be of any shape and size consistent with the desired superficial surface
and should be large enough to permit relatively free passage of the gas
mixture. The channels may be parallel, or generally parallel, and extend
through the support from one side to an opposite side, such channels
being separated from one another by preferably thin walls. The channels
may also be multi-directional and may even communicate with one or more
adjacent channels. The channel inlet openings can be distributed across
essentially the entire face or cross-section of the support subject to
initial contact with the gas to be oxidized. The unitary, skeletal
structure support type of oxidation catalyst can be characterized by
having a plurality of flow channels or paths extending therethrough in
the general direction of gas flow. The flow channels need not pass
straight through the catalyst structure and may contain flow diverters
or spoilers. The skeletal structure support is preferably constructed of
a substantially chemically inert, rigid, solid material capable of
maintaining its shape and strength at high temperatures. The support may
have a low thermal coefficient of expansion and low thermal
conductivity. Often, the skeletal support is porous but its surface may
be relatively non-porous, and it may be desirable to roughen its surface
so that it holds the catalyst coating better, especially if the support
is relatively non-porous. The support may be metallic or ceramic in
nature or a combination thereof. The combustion gases are exhausted to
atmosphere after giving up some of their remaining heat to the incoming
air provided from the compressor. Operating the combustion process in a
very lean condition is one of the simplest ways of achieving lower
temperatures and hence lower nitrogen oxides emissions. Very lean
ignition and combustion, however, inevitably result in incomplete
combustion and the attendant emissions which result therefrom. In
addition, combustion processes cannot be sustained at these extremely
lean operating conditions. In a catalytic combustor, fuel is burned at
relatively low temperatures. While emissions can be reduced by
combustion at these temperatures, the utilization of catalytic
combustion is limited by the amount of catalytic surface required to
achieve the desired reaction and the attendant undesirable pressure drop
across the catalytic surface. Also, the time to bring the catalytic
combustor up to operating temperature continues to be of concern. In
conventional thermal combustion systems, a fuel and air in flammable
proportions are contacted with an ignition source to ignite the mixture
which will then continue to burn. Flammable mixtures of most fuels are
normally burned at relatively high temperatures, which inherently
results in the formation of substantial emissions of nitrogen oxides. In
the case of gas turbine combustors, the formation of nitrogen oxides can
be decreased by limiting the residence time of the combustion products
in the combustion zone. However, even under these circumstances,
undesirable quantities of nitrogen oxides are nevertheless produced. In
combustion systems utilizing a catalyst, there is little or no nitrogen
oxides formed in a system which burns the fuel at relatively low
temperatures. Such combustion heretofore has been generally regarded as
having limited practicality in providing a source of power as a
consequence of the need to employ amounts of catalyst so large as to
make a system unduly large and cumbersome. Consequently, combustion
utilizing a catalyst has been limited generally to such operations as
treating tail gas streams of nitric acid plants. The catalytically
supported thermal combustion involves essentially adiabatic combustion
of a mixture of fuel and air or fuel, air, and inert gases in the
presence of a solid oxidation catalyst operating at a temperature
substantially above the instantaneous auto-ignition temperature of the
mixture, but below a temperature which would result in any substantial
formation of oxides of nitrogen under the conditions existing in the
catalyst.