Figure 6. Reactant mole fraction contour plots in the pre-mixed
homogeneous-heterogeneous hybrid system with combustion of small alkanes
on noble metal surfaces.
The reactant mole fraction profiles in the streamwise direction are
presented in Figure 7 for the pre-mixed homogeneous-heterogeneous hybrid
system. The ratio of fuel to air in the admixture introduced into the
combustion zone is ultimately determined by the desired operating
temperature of the combustion zone. The operating temperature is
determined by the theoretical adiabatic flame temperature of the
fuel-air admixture passed to the combustor and thus is dependent on the
initial temperature of the air as well as the amount of fuel contained
therein. In the operation of any engine various changes in load occur.
For example, increases or decreases in output speed of a turbine may be
required thus changing the amount of fuel needed. Further, even at
constant speed, load changes are encountered. In certain turbine
applications, for example, automobiles, increases or decreases in
turbine power often result in an increase or decrease in compressor
speed and respectively an increase or decrease in the compression and
temperature of the air supplied to the system by the compressor [73,
74]. The theoretical adiabatic flame temperature of an admixture
having a given fuel-to-air volume ratio will vary directly with the
temperature of the air in the admixture [75, 76]. Thus, for example,
as power requirements of the turbine are increased, a number of
interrelated control changes must take place. The air charged to the
system will be at a higher temperature which, unless compensated for,
would increase the adiabatic flame temperature of a given fuel-air
mixture. Further, a power increase implies a fuel flow increase;
therefore, the portion of the total compressed air which is directed to
the combustor must increase to maintain the theoretical adiabatic flame
temperature approximately constant. The portion of the total compressed
air directed to the bypass will be decreased to allow the temperature of
the combined effluent gas-additional air admixture to increase. The
relative amounts of each air stream, that is, the air introduced to the
combustor and the air directed to the bypass are varied in an inverse
manner. These interdependent changes maintain the combustor at an
approximately constant temperature by controlling the fuel-air admixture
introduced into the combustor to have an approximately constant
theoretical adiabatic flame temperature. The combustion effluent is
preferably combined with additional, cooler air to quench the effluent
and provide a motive fluid for the gas turbine at a desired temperature.
The additional air can be at any convenient temperature, for instance,
at about the temperature at which the air leaves the compressor, or it
can be at a somewhat lower or higher temperature achieved by, for
example, indirect heat exchange with the combustion zone or with the
exhaust gases from the turbine. Conveniently, the additional air is at a
temperature between about 40 and 1100 °C, and preferably about 260 to
800 °C. Thus, the greater the amount of cooler additional air combined
with the combustion effluent, the lower the temperature of the combined
gases and, hence, the less the power obtainable from the combined gases
when used as a motive fluid in a turbine system. Similarly, if the
amount of additional air employed is reduced, the power output from the
turbine is increased. The temperature of the combined gases passed to
the turbine inlet is generally about 400 °C to 1500 °C, and preferably
for increased turbine efficiency, about 600 °C to 1500 °C. Another means
for increasing the energy of the combined combustor effluent-bypass air
in response to increased amounts of fuel is to increase proportionately
both the amount of air to the combustion zone and the amount of
additional air to be combined with the combustion effluent. The solid
catalyst can have various forms and compositions and can be the types
used to oxidize fuels in the presence of molecular oxygen. The catalyst
can be in the form of relatively small, solid particles of various sizes
and shapes, often in sizes below about one inch in the largest
dimension, with a plurality of such particles being arranged together to
form one or more catalyst masses or beds in the combustion zone. The
catalyst is preferably of larger form and has a skeletal structure with
gas flow paths therethrough. The catalyst generally has one or more
metal containing components which are catalytically active towards
promoting the desired oxidation reactions, and in view of the rather
high temperatures at which the catalyst operates, materials normally
considered to be relatively inactive or insufficiently active, to
promote adequately the oxidation of the fuel, may be suitable.