Figure 5. Velocity contour plots in the pre-mixed
homogeneous-heterogeneous hybrid system with combustion of small alkanes
on noble metal surfaces.
The reactant mole fraction contour plots in the pre-mixed
homogeneous-heterogeneous hybrid system are illustrated in Figure 6 with
combustion of small alkanes on noble metal surfaces. The use of
catalytic processes for combustion or oxidation is a well-known method
for potentially reducing levels of nitrogen oxides emissions from gas
turbine engine systems [69, 70]. There are various processes for
converting the chemical energy in a fuel to heat energy in the products
of the conversion [71, 72]. The primary processes are gas phase
combustion, catalytic combustion, and catalytic oxidation. There are
also combinations of these processes, such as processes having a first
stage of catalytic oxidation followed by a gas phase combustion process.
In catalytic oxidation, an air-fuel mixture is oxidized in the presence
of a catalyst. In all catalytic processes the catalyst allows the
temperature at which oxidation takes place to be reduced relative to
non-catalytic combustion temperatures. Lower oxidation temperature leads
to reduced nitrogen oxides production. In catalytic oxidation, all
reactions take place on the catalytic surface. There are no local high
temperatures and therefore the lowest possible potential for nitrogen
oxides to be formed. In either catalytic combustion or catalytically
stabilized combustion, some part of the reaction takes place in the gas
phase, which increases local temperatures and leads to higher potential
for nitrogen oxides being formed. Such low levels in general cannot be
achieved with conventional non-catalytic combustors, catalytic
combustion, or catalytically stabilized combustion. The term catalytic
combustor is used to refer to any combustor utilizing catalysis,
preferably one utilizing catalytic oxidation. The catalyst employed in a
catalytic combustor tends to operate best under certain temperature
conditions. In particular, there is typically a minimum temperature
below which a given catalyst will not function. For instance, palladium
catalyst requires a combustor inlet temperature for the air-fuel mixture
higher than 500 °C when natural gas is the fuel. In addition, catalytic
oxidation has the disadvantage that the physical reaction surface which
must be supplied for complete oxidation of the hydrocarbon fuel
increases exponentially with decreasing combustor inlet temperatures,
which greatly increases the cost of the combustor and complicates the
overall design. The need for a relatively high combustor inlet
temperature is one of the chief reasons why catalytic combustion in
general, and catalytic oxidation in particular, has not achieved
widespread use in gas turbine engine systems. More specifically, such
high combustor inlet temperatures generally cannot be achieved in gas
turbines operating with compressor pressure ratios less than about 40
unless a recuperated cycle is employed. In a recuperated cycle, the
air-fuel mixture is pre-heated, prior to combustion, by heat exchange
with the turbine exhaust gases. Recuperation thus can help achieve the
needed combustor inlet temperature for proper catalyst operation, at
least under some conditions. However, there are often other operating
conditions that will be encountered at which the minimum required
combustor inlet temperature still cannot be achieved even with
recuperation. For instance, when recuperation is applied in small gas
turbines, material temperature limitations in the recuperator can limit
the maximum air or air-fuel mixture temperature.