Figure 7. Pressure contour plots in the gas turbine combustor designed
for the reduction of nitrogen oxides emissions by heterogeneous
catalysis.
The chemical enthalpy contour plots in the gas turbine combustor are
illustrated in Figure 8 for the reduction of nitrogen oxides emissions
by heterogeneous catalysis. Nitrogen oxides formation is
thermodynamically favored at high temperatures. Since the nitrogen
oxides formation reaction is so highly temperature dependent, decreasing
the peak combustion temperature can provide an effective means of
reducing nitrogen oxides emissions from gas turbine engines as can
limiting the residence time of the combustion products in the combustion
zone. The limits of the operating temperature are governed largely by
residence time and pressure. The instantaneous auto-ignition temperature
of the mixture is defined herein to mean the temperature at which the
ignition lag of the mixture entering the catalyst is negligible relative
to the residence time in the combustion zone of the mixture undergoing
combustion. Essentially adiabatic combustion means in this case that the
operating temperature of the catalyst does not differ by more than about
80 K, from the adiabatic flame temperature of the mixture due to heat
losses from the catalyst. Among the unique advantages of the
catalytically supported thermal combustion in the presence of a catalyst
is the fact that mixtures of fuel and air which are too fuel-lean for
ordinary thermal combustion can be burned efficiently. Since the
temperature of combustion for a given fuel at any set of conditions is
dependent largely on the proportions of fuel, of oxygen available for
combustion, and of inert gases in the mixture to be burned, it becomes
practical to burn mixtures which are characterized by much lower flame
temperatures. In particular, carbonaceous fuels can be burned very
efficiently and at thermal reaction rates at temperatures in the range
from about 1,200 K to about 2,000 K. At these temperatures, very little
if any nitrogen oxides are formed. In addition, because the
catalytically supported thermal combustion is stable over a wide range
of mixtures, it is possible to select or control reaction temperature
over a correspondingly wide range by selecting or controlling the
relative proportions of the gases in the mixture. The method may be
carried out in various ways, including heating the catalyst body by
electrical means, or by first thermally combusting a fuel and air
mixture and applying the heat produced to the catalyst body. Once a
catalyst temperature is reached at which the catalyst will function to
sustain mass transfer limited operation, the combustion of fuel in the
presence of the catalyst will bring it rapidly to the required operating
temperature. Once operating temperature is reached, the catalyst will
provide for sustained combustion of the fuel vapor. After the catalyst
body reaches a temperature at which it will sustain mass transfer
limited operation, the aforementioned application of heat to the
catalyst body is no longer necessary and an admixture of unburned fuel
and air is introduced into the system to establish the supported thermal
combustion to provide a motive fluid for a turbine or heat to a furnace.
No serious start-up problem normally is presented. The operation is
substantially continuous and it is necessary to start the system only at
infrequent intervals. Consequently, the substantial emissions of
atmospheric pollutants which tend to occur in start-ups are not serious
because of the small number of infrequent start-ups. While this
pollution may be tolerated in stationary operations which are normally
used continuously and for long periods of time, it cannot be tolerated
in the vehicular type of installation where start-ups are frequent, due
to intermittent operation [79, 80]. Also, the start-up must be rapid
in order to be as efficient as in the conventional present-day
automobile [81, 82]. The mixture of unburned fuel and air is not
introduced to the catalyst body until it has reached a temperature at
which it will sustain the desired rapid combustion. Such preferred
procedure minimizes pollutant emissions during start-up. Once combustion
in the zone containing the catalyst is achieved, the fuel-air admixture
is passed to the catalyst at a gas velocity, prior to or at the inlet to
the catalyst, in excess of the maximum flame propagating velocity. This
avoids flash-back that causes the formation of nitrogen oxides [83,
84]. Various means have been proposed in the past to prevent such
flash backs from traveling back to the gas supply source [85, 86].
Preferably, this velocity is maintained adjacent to the catalyst inlet
[87, 88]. Suitable linear gas velocities are usually above about 8
meters per second, but considerably higher velocities may be required
depending upon such factors as temperature, pressure, and composition.