3. Results and discussion
The propane mole fraction contour plots in the gas turbine combustor are
illustrated in Figure 3 for the reduction of nitrogen oxides emissions
by heterogeneous catalysis. In order to meet the emission level
requirements, for industrial low emission gas turbine engines, staged
combustion is required in order to minimize the quantity of the oxides
of nitrogen produced [59, 60]. The fundamental way to reduce
emissions of nitrogen oxides is to reduce the combustion reaction
temperature and this requires premixing of the fuel and all the
combustion air before combustion takes place [61, 62]. The present
design provides a gas turbine engine combustion chamber comprising a
primary combustion zone and a secondary combustion zone downstream of
the primary combustion zone. In this way, the temperatures in each stage
can be reduced. Suitable temperature ranges in each stage are from 900 K
to 1500 K, depending on the particular catalyst and support. An
advantage of operating in this temperature range is that it is below the
fixation temperature of nitrogen and consequently the combusted gases
are free of nitrogen oxides. Additionally, catalytic combustion results
in lower un-combusted fuel content. A further advantage of catalytic
combustion is that it is possible to operate with minimum of air for
combustion, namely excess oxygen in the combusted gases can be reduced
almost to zero. The catalyst may be supported on a monolith. The
preferred characteristics of the metallic monolith having a catalyst
deposited thereon are that is presents low resistance to the passage of
gases by virtue of its possession of a high ratio of open area to
blocked area and that it has a high surface to volume ratio. Preferably,
the metallic monolith is formed from one or more metals selected from
the group comprising ruthenium, rhodium, palladium, iridium, and
platinum. However, base metals may be used or base metal alloys which
also contain a platinum group metal component may be used. Oxygen is the
required element to support combustion. It is possible to achieve
essentially adiabatic combustion in the presence of a catalyst at a
reaction rate many times greater than the mass transfer limited rate. In
particular, if the operating temperature of the catalyst is increased
substantially into the mass transfer limited region, the reaction rate
again begins to increase rapidly with temperature. This is in apparent
contradiction of the laws of mass transfer kinetics in catalytic
reactions. The phenomenon may be explained by the fact that the
temperature of the catalyst surface and the gas layer near the catalyst
surface are above the instantaneous auto-ignition temperature of the
mixture of fuel, air, and any inert gases and at a temperature at which
thermal combustion occurs at a rate higher than the catalytic combustion
rate. The fuel molecules entering this layer burn spontaneously without
transport to the catalyst surface. As combustion progresses and the
temperature increases, the layer in which thermal combustion occurs
becomes deeper. Ultimately, substantially all of the gas in the
catalytic region is raised to a temperature at which thermal combustion
occurs in virtually the entire gas stream rather than just near the
surface of the catalyst. Once this stage is reached within the catalyst,
the thermal reaction appears to continue even without further contact of
the gas with the catalyst.