1. Introduction
All
flames can be classified either as premixed flames or as flames that
burn without premixing. A widely applied thermal theory, one of the
first flame propagation theories, implies that combustion proceeds
primarily at temperatures close to the maximum the flame can achieve
[1, 2]. A set of differential equations developed for thermal
conductivity and diffusion is reduced to one equation that yields the
burning velocity value [3, 4]. Further development of the theory has
been made, and computers now make most simplifications unnecessary [5,
6]. According to thermal theories, flame propagation is accounted for
by heat energy transport from the combustion zone to the unburned
mixture, which raises the temperature of the mixture [7, 8].
Diffusion theory assumes that thermodynamic equilibrium sets in behind
the flame front at a maximum temperature and that radicals produced in
this zone diffuse into the unburned mixture and ignite it [9, 10].
Both heat transport and diffusion of active particles must be considered
essential for ignition. Industrial flames, including those activated in
furnaces, belong to the type of turbulent diffusion [11, 12].
However, the theory of diffusion flames, is less advanced than that of
premixed flames. Since gas intermixing is mainly responsible for the
structure and the features of diffusion flames, the theory of diffusion
flames operates more in terms of thermodynamics and physics than
chemical reactions.
Combustion, with rare exceptions, is a complex chemical process
involving many steps that depend on the properties of the combustible
substance. It is initiated by external factors. The lean, pre-mixed
combustor is a combustor in which fuel is premixed with air prior to
combustion to form a largely homogeneous fuel lean admixture having an
adiabatic flame temperature less than about 1700 °C. This differs from a
diffusion flame combustor where the fuel is injected directly into the
combustion zone and mixed with air during combustion [13, 14]. As a
result, combustion is essentially at the stoichiometric fuel air ratio
with combustion flame front temperatures as high as 2200 °C. Unlike
diffusion flame combustors, lean, pre-mixed combustors avoid
stoichiometric combustion and are able to inherently achieve lower
nitrogen oxides emission levels. In both approaches the combustion
products are modified by dilution air to achieve the desired turbine
inlet temperature, however lower amounts are required in the premixed
system [15, 16]. To achieve single digit nitrogen oxides emission
levels in a lean, pre-mixed combustor requires operating at a flame
temperature of the fuel and air admixture no higher than about 1600 °C.
Unfortunately, as the flame temperature of a fuel and air admixture is
decreased to little more than 1500 °C, typically combustion becomes
unstable and high carbon monoxide emissions are generated [17, 18].
Thus, legal compliance requirements placed on both nitrogen oxides and
carbon monoxide make the operating window for a lean, premixed combustor
quite limited, even operating at rich enough conditions where nitrogen
oxides levels are as high as 20 milligrams per cubic meter. Accordingly,
various types of independently controlled pilots are employed in lean,
pre-mixed combustors to extend the stable operating window below 1500 °C
to minimize nitrogen oxides emissions [19, 20]. However, if the
pilot is a flame some nitrogen oxides are produced by it and often there
is little or no corresponding improvement in overall carbon monoxide
emissions [21, 22]. Consequently, there is a very small operating
window in which both nitrogen oxides and carbon dioxide emissions meet
environmental regulations.
Physical processes that transfer energy and mass by convection or
diffusion occur in gaseous combustion. In the absence of external
forces, the rate of component diffusion depends upon the concentration
of the constituents, pressure, and temperature changes, and on diffusion
coefficients, which are a measure of the speed of diffusion. The lean,
pre-mixed combustor is familiar with staging of combustion to achieve
low emissions over a wide engine operating range where lower turbine
inlet temperatures are required. However, staging has practical limits
both in terms of its ultimate ability to reduce emissions as well as the
level of complexity introduced into the design of the combustor system
[23, 24]. Even with this complexity, most lean, premixed combustors
cannot reliably achieve ever lower standards for carbon monoxide and
nitrogen oxides emissions, for example below 20 milligrams per cubic
meter. The lean, pre-mixed combustor is also familiar with the use of
catalysts to both improve combustion stability and reduce emissions in
combustors [25, 26]. A catalyst is applied to the inner surface of a
diffusion flame combustor for the purposes of flame stabilization. In
the event that the primary combustion zone is extinguished, a
re-ignition of the combustor can be achieved if the rich fuel-air
mixture can contact a sufficiently hot catalytic surface [27, 28].
The catalytic surface must be non-continuous so that the flame created
by the contact of the rich fuel and air mixture to it will leave the
liner wall and ignite the bulk combustor flow [29, 30]. The
discontinuity in the catalyst coating is identified in those regions
where film cooling of the combustor would be non-existent, the surfaces
prior to or directly over the film cooling air inlets [31, 32]. A
catalyst applied to a diffusion flame combustor should also tend to
reduce unburned hydrocarbons and carbon monoxide emissions [33, 34].
The combustor is completely film cooled, due to the temperature of
combustion, and that the flame, or reactants, contact the catalytic
surface [35, 36]. It is therefore necessary to allow achievement of
both lower nitrogen oxides emissions and lower carbon monoxide emissions
in lean, pre-mixed combustors. These reductions are possible in a lean,
premixed combustor both with and without open flame pilots [37, 38].
It is also therefore necessary to provide a means to operate at leaner
conditions if carbon monoxide emissions are the limiting factor in the
design, allowing lower firing temperatures and the associated
incremental reduction of nitrogen oxides.
In conventional thermal combustion, fuel and air in inflammable
proportions are contacted with an ignition source to ignite the mixture
which will then continue to burn. Flammable mixtures of most fuels are
normally burned at relatively high temperatures, which inherently
results in the formation of substantial emissions of nitrogen oxides. In
purely catalytic combustion systems, there is little or no nitrogen
oxides formed in a system which burns the fuel at relatively low
temperatures. The present study is focused primarily upon the combustion
characteristics of small alkanes on noble metal surfaces in pre-mixed
homogeneous-heterogeneous hybrid systems. The homogeneous-heterogeneous
combustion characteristics small alkanes on noble metal surfaces are
investigated to gain a greater understanding of the mechanisms of flame
stabilization and to gain new insights into how to design pre-mixed
combustors with improved stability and robustness. The essential factors
for design considerations are determined with improved combustion
characteristics and flame stability. The primary mechanisms responsible
for the loss of flame stability are discussed. The present study aims to
explore how to effectively operate catalytically supported thermal
combustion. Particular emphasis is placed upon the catalytic combustion
characteristics of small alkanes on noble metal surfaces in pre-mixed
homogeneous-heterogeneous hybrid systems.