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.