Figure 3. Front wall edge temperature profiles in the transverse
direction of the pre-mixed homogeneous-heterogeneous hybrid system.
The fluid centerline temperature profiles in the streamwise direction
are presented in Figure 4 for the pre-mixed homogeneous-heterogeneous
hybrid system. In a conventional combustion system typical of industrial
and commercial burners, the combustion reaction is relatively
uncontrolled [61, 62]. That is, a flame can vary in conformation
such that its shape and location at any particular point in time is
unpredictable [63, 64]. This unpredictability, combined with high
peak temperatures encountered especially at the stoichiometric interface
in a diffusion flame can cause operational problems such as coking of
reaction tubes and uneven heating of steam tubes. Moreover, the length
of a conventional flame causes a relatively long residence time during
which combustion air is subject to high temperature. What is needed is a
technology for reducing pollutants released by combustion systems such
as industrial and commercial burners. What is also needed is a
technology that can improve flame control in such systems. Fuel and
combustion air are mixed to form a fuel-air mixture. The amount of
combustion air mixed with the fuel is sufficient to substantially fully
oxidize the fuel. Energy is transferred to the fuel-air mixture to
increase a temperature of the mixture prior to ingestion and oxidation
by the catalytic reactor so that the fuel-air mixture is above a minimum
operating temperature of the catalytic reactor when it is ingested. The
catalytic reactor oxidizes substantially all of the ingested fuel and
produces thermal energy. Exhaust gasses from the catalytic reactor
expand across a turbine. This causes the turbine to rotate and thereby
convert thermal energy to mechanical energy. Fuel may be injected into
the low-pressure inlet of a compressor, rather than downstream of the
compressor. This eliminates the need for complex or high-pressure fuel
delivery systems having fuel gas compression. A fuel mixer mixes the
fuel with combustion air to form the fuel-air mixture. The concentration
of any unburned fuel in the bleed air is reduced prior to being
exhausted to the environment. This is achieved by oxidizing any unburned
fuel with a catalyst. Gas-turbine engines and systems are capable of
sustained catalytic combustion over a wide range of operating power
levels. For sustained catalytic combustion, the operating temperature of
a gas-turbine engine should preferably remain within a limited band of
operating temperatures over a wide range of operating power levels. A
typical band of catalytic reactor operating temperatures is
approximately between 400 °C and 600 °C. However, for conventional
gas-turbine engines, operating temperature is a function of power level.
Thus, when conventional gas-turbine engines operate at part load, they
typically reduce their operating temperature while maintaining a
constant engine speed. This reduction of operating temperature causes
the operating temperature of the conventional gas-turbine engines to
fall outside the limited band of catalytic reactor operating
temperatures. Unlike known constant-speed gas-turbine engines,
gas-turbine engines adjust their engine speed with power level to
maintain a nearly constant operating temperature over a wide range of
operating power levels. In addition to adjusting engine speed with power
level to maintain a nearly constant operating temperature, gas-turbine
engines and systems have recuperators and pre-heaters that help maintain
the minimum operating temperature needed for sustained catalytic
combustion, even during periods of fuel interruption. There is no need
for complex or high-pressure fuel delivery systems having fuel gas
compression. In fact, in some cases, no fuel-metering valve is needed,
as controlling the speed of the gas-turbine engine can control the fuel
flow to the engine. During operation, fuel and air are mixed together to
form a fuel-air mixture having a desired fuel-air ratio. The pressure of
the fuel-air mixture is increased to a desired pressure by the
compressor. As an example, with conventional high-temperature materials
in the recuperator, the maximum safe operating temperature of the
recuperator may be about 600 °C, and hence an air-fuel mixture
temperature of about 500 to 550 °C is about the highest that can be
achieved. This temperature range is higher than the minimum catalyst
operating temperature for some types of catalysts and therefore the
catalytic combustor may operate properly at one particular operating
condition such as 100 percent load and standard-day ambient conditions.
At other operating conditions, however, such as part-load and cold
ambient conditions, the combustor inlet temperature may fall below the
minimum temperature. In order to be compatible with practical turbine
systems, it is desirable that the residence time of the combustion gases
in the catalytic and thermal oxidation zones is sufficient to give
essentially complete combustion of the fuel without the production of
objectional amounts of nitrogen oxides. Specific operations will
generally define the maximum flame velocity which is controlled by
various conditions of operations, such as amount of air and fuel
present, the type of fuel employed, temperature, and pressure. Suitable
gas velocities are above the actual maximum linear velocity for flame
propagation.