Figure 2. Temperature contour plots in the pre-mixed
homogeneous-heterogeneous hybrid system with combustion of small alkanes
on noble metal surfaces.
The front wall edge temperature profiles in the transverse direction are
presented in Figure 3 for the pre-mixed homogeneous-heterogeneous hybrid
system. Intake air in the pre-mixed homogeneous-heterogeneous hybrid
system is compressed by a compressor turbine, and the compressed air
then may be apportioned into at least two parts, one part of which is
intimately admixed with a carbonaceous fuel and introduced into a
combustion zone to be thermally combusted, preferably by homogeneous
thermal combustion and a second part of which is combined with the
effluent from such combustion. The relative amount of air in each part
or portion is adjustable and proportionately interdependent. Attempts
have been made to control the fuel-to-air ratio in conventional
combustors [57, 58]. Such attempts, however, have not satisfactorily
dealt with the problem of sustained low emission combustion which is
responsive to variations in load on the engine and other operating
conditions [59, 60]. In fixed geometry conventional combustors which
burn fuel in air at approximately the stoichiometric ratio, the hole
pattern of the combustor liner is ordinarily designed for best operation
of the primary or combustion zone, at nearly full load. With such
apparatus, the overall fuel-air ratio decreases at light load or at idle
resulting in a leaner mixture in the combustion zone, which can lead to
reduced combustion efficiency and increased exhaust emissions. This
problem can be overcome by using variable combustor geometry to operate
the primary or combustion zone at a constant fuel-air ratio, namely
close to stoichiometric at all turbine operating conditions. Although
this does solve the hydrocarbon and carbon monoxide problem, it does not
even address the nitrogen oxides problem. Formation of nitrogen oxides
occurs at relatively high temperatures, which inevitably are reached in
adiabatic combustion systems with near-stoichiometric fuel-air ratios.
Consequently, simply changing the combustor geometry to maintain
near-stoichiometric ratios will not avoid nitrogen oxides formation.
Further, at a fixed fuel-air ratio the combustion temperature will vary
accordingly to variations in the temperature of the air at the inlet to
the combustor so that combustion temperature is not fixed. Flammable
mixtures of most fuels, for complete combustion, normally burn at
relatively high temperatures, namely above about 1800 °C, which
inherently results in the formation of substantial amounts of nitrogen
oxides. In the case of conventional gas turbine thermal combustors,
formation of nitrogen oxides can be reduced by limiting the residence
time of the combustion products in the combustion zone. However, due to
the large quantities of gases being handled, undesirable amounts of
nitrogen oxides are produced. Many conventional combustors, by injecting
the fuel into the combustor in droplet form and separately from the air
used for combustion present serious drawbacks to low pollution
operation. Such a system substantially precludes very lean sustained
combustion. Consequently, the combustion temperature of the droplet
boundaries in the pre-mixed homogeneous-heterogeneous hybrid system will
frequently be approximately the theoretical adiabatic flame temperature
of a stoichiometric mixture of the fuel and air; this temperature will
be substantially over 1800 °C, and typically in excess of 2200 °C.
Therefore, even though the overall temperature in the combustor may be
quite low and not high enough to form nitrogen oxides, the temperature
near the droplet surface is typically in excess of that required to form
nitrogen oxides. Consequently, nitrogen oxides forms and is present in
the combustor effluent.