3. Results and discussion
The enthalpy contour plots in the pre-mixed homogeneous-heterogeneous
hybrid system are illustrated in Figure 1 with combustion of small
alkanes on noble metal surfaces. The term thermal combustion, as used
herein, is understood to mean a homogeneous combustion reaction which
proceeds without the aid of a catalyst. The combustion is conducted by
utilizing a selectively controlled fuel-to-air ratio to achieve an
approximately constant combustion temperature which is substantially
above the instantaneous auto-ignition temperature of the fuel-air
admixture but preferably is below a temperature that would result in the
substantial formation of oxides of nitrogen. Subsequently, additional,
selectively controlled air can be combined with the products or effluent
from the combustion of the fuel-air admixture. The combustion effluent
is characterized by high thermal energy and typically by low nitrogen
oxides content. The method and apparatus thus provide for highly
efficient turbine operation and quick response to changes in operation
of the system with relatively little atmospheric pollution. Broadly,
intake air is compressed and at least a portion of the compressed air is
intimately admixed with a carbonaceous fuel [49, 50]. The resulting
admixture is then passed to a temperature-controlled combustion or
primary zone where it is combusted above the instantaneous auto-ignition
temperature at an approximately constant temperature advantageously over
a period of turbine operation in which the amount of fuel charged to the
combustion zone is varied in response to power demand on the turbine
[51, 52]. The fuel-to-air volume ratio is adjusted, taking into
account the temperature of the gas entering the combustion zone, so that
the theoretical adiabatic flame temperature of the mixture remains about
constant over a wide range of fuel inputs. The effluent from the
combustion zone is combined in a secondary zone with at least a portion
and preferably most of the remaining compressed air charged to the
turbine system. The power obtained from the gas turbine can thus be
controlled without changing the temperature in the combustion zone by
adjusting the overall volume of fuel-air mixture to the combustion zone
and the volume of additional or bypass air or gas combined with the
combustion effluent. These adjustments regulate the temperature of the
gas entering the turbine and thus the power produced thereby. Since the
temperature of the combustor effluent-bypass air mixture is primarily
dependent on the temperature and relative amounts of combustor effluent
and bypass air, it is possible to obtain a quick response of the gas
turbine system to different power requirements by varying the amount of
additional or bypass air combined with the combustion effluent. Further,
even at low turbine inlet temperatures, low combustion temperatures can
be avoided. Thus, the combustor need not operate at low temperatures
which might impair performance with possible flame-out and also might
result in an effluent having a high content of carbon monoxide and
hydrocarbons. Similarly, avoidance of combustion temperatures
significantly in excess of about 1800 °C avoids the formation of
excessive amounts of nitrogen oxides during combustion.