Figure 1. Enthalpy contour plots in the pre-mixed
homogeneous-heterogeneous hybrid system with combustion of small alkanes
on noble metal surfaces.
The temperature contour plots in the pre-mixed homogeneous-heterogeneous
hybrid system are illustrated in Figure 2 with combustion of small
alkanes on noble metal surfaces. The products of combustion of fossil
fuels include carbon dioxide, carbon monoxide, unburnt hydrocarbons and
nitrogen oxides [53, 54]. Various control schemes and hardware
configurations have been used to control the concentration of such
emissions while at the same time providing fuel-efficient and stable
engine operation [55, 56]. Regulatory changes continue to reduce the
allowable level of emissions from electric power generating plants
utilizing gas turbine engines. To achieve low levels of emissions, it is
necessary to establish and to maintain very lean combustion conditions.
Lean combustion is known to be less stable than rich combustion, and
lean-burn combustors are more prone to damaging pressure pulsations
generated within the combustor. Precise tuning of the combustion process
is needed to establish a balance between stable combustion and low
emissions. A precisely tuned engine may be susceptible to drift over
time, with a resulting increase in emissions or an increase in the level
of combustion instability. One known approach to controlling the
emissions from a gas turbine power plant is to run the combustor at a
relatively rich setting, thereby ensuring stable combustion while
generating excessive amounts of undesirable emissions. The exhaust gas
is then cleaned to regulatory limits by passing it through a combustion
catalyst installed downstream of the combustor in the turbine exhaust
system. Alternatively, a catalyst may be used to achieve a majority of
the fuel combustion, with only a final portion of the combustion being
accomplished in a flame combustor located downstream of the primary
catalyst. Catalyst systems are very expensive and are often used as a
last resort in especially rigorous regulatory situations. The generation
of nitrogen oxides emissions is directly related to the peak flame
temperature in the combustor. The peak flame temperature in a gas
turbine combustor can be controlled by injecting water into the
combustor. The cost of the demineralized water used for water injection
can be significant, particularly in areas where the supply of fresh
water is limited. Accordingly, it is beneficial to limit the use of
injected water to the extent possible. A radial flow catalyst element
can be integrated into an aerodynamically stabilized burner to provide a
catalytically reacted fuel-air mixture for enhanced flame stabilization
with catalyst temperature maintained by recirculation of hot combustion
gases at a temperature high enough even for combustion of methane at
ambient combustor inlet air temperatures yet at a temperature well below
the adiabatic combustion temperature thus allowing burner outlet
temperatures high enough for modern gas turbines. An aerodynamically
stabilized combustor or burner is one wherein gas phase combustion is
stabilized by aerodynamic recirculation of hot combustion products such
as induced by a swirler, a bluff body, opposed flow jets, or a flow
dump. Preferred are swirlers. In operation of a burner, a fuel-air
mixture is passed into contact with a catalytic element for reaction
thereon. The resulting reacted admixture is then admixed with the fresh
fuel and air passing into the combustor thus enhancing reactivity and
enabling stable combustion even with very lean fuel-air admixtures.
Light-off of burners may be achieved using any conventional ignition
means such as spark plugs, glow plugs, laser beams, or microwave energy.
Advantageously, for ignition the catalytic element is heated
electrically to a temperature high enough for fuel ignition followed by
introduction of fuel and air. This not only achieves ignition but
assures that the catalyst is at an effective temperature to stabilize
lean combustion in the burner from the start of combustion.