Combustion characteristics and flame stability of catalytically supported thermal combustion gas turbine systems with low emissions of nitrogen oxides
Creators: name: Chen, Junjie
Publisher: name: Zenodo
Year: 2022
Subjects: Combustion characteristics; Flame stability; Fluid mechanics; Gas turbines; Ignition temperatures; Transport phenomena

description: <strong>Abstract: </strong>This study relates to the essential characteristics of catalytically supported thermal combustion in a micro-scale gas turbine system with a heat-recirculating structure. Numerical simulations were conducted to gain insights into system performance and to determine what changes can be made to improve its robustness and stability. Detailed kinetics were used for modeling the catalytically supported thermal combustion gas turbine system in computational fluid dynamics. Methods of applying a heat-recirculating structure to the channel walls were employed, which may be utilized with presently existing designs of micro-scale thermal combustion gas turbine systems. The factors affecting combustion stability were determined for the gas turbine system. The results indicated that catalytically-supported thermal combustion surmounts the mass transfer limitation. Catalytically-supported thermal combustion is achieved by contacting at least a portion of the carbonaceous fuel intimately admixed with air with a solid oxidation catalyst having an operating temperature substantially above the instantaneous auto-ignition temperature of the fuel-air admixture. The adiabatic flame temperature of fuel-air admixtures at any set of conditions is established by the ratio of fuel to air. The selection of the adiabatic flame temperature limits is governed largely by residence time and pressure. The adiabatic flame temperature employed can depend on such factors as the desired composition of the effluent and the overall design of the system. The residence time is governed largely by temperature, pressure and space throughput, and generally is measured in milliseconds. The total residence time in the combustion system should be sufficient to provide essentially complete combustion of the fuel, but not so long as to result in the formation of nitrogen oxides. In order to carry out the process, it is necessary to first mix fuel and air to prepare a mixture with the desired adiabatic flame temperature. <strong>Keywords: </strong>Combustion characteristics; Flame stability; Fluid mechanics; Gas turbines; Ignition temperatures; Transport phenomena <strong>Contributor</strong>: Junjie Chen, ORCID: 0000-0002-5022-6863, E-mail address: koncjj@gmail.com, Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China <strong>Declaration of competing interest</strong>: The author declares that there is no conflict of interest. <strong>1. Introduction</strong> 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 [1, 2]. In the case of gas turbine combustors, the formation of nitrogen oxides can be greatly reduced by limiting the residence time of the combustion products in the combustion zone [3, 4]. However, even under these circumstances undesirable quantities of nitrogen oxides are nevertheless produced. In addition, limiting such residence time makes it difficult to maintain stable combustion even after ignition. In purely catalytic combustion systems, there is little or no nitrogen oxides formed in a system which burns the fuel at relatively low temperatures. Catalytic combustion heretofore has been generally regarded as having limited practicality in providing a source of power as a consequence of the need to employ impractically large amounts of catalyst so as to make a system unduly large and cumbersome [5, 6]. Consequently, catalytic combustion has been limited generally to such operations as treating tail gas streams of nitric acid plants, where the catalytic reaction is employed to heat spent process air. For any given catalyst and set of reaction conditions, as the temperature in catalytic combustion is initially increased, the reaction rate is also increased. This rate of increase is exponential with temperature. As the temperature is raised further, the reaction rate then passes through a transition zone where the limiting parameters determining reaction rate shift from catalytic to mass transfer. When the catalytic rate increases to such an extent that the reactants cannot be transferred to the catalytic surface fast enough to keep up with the catalytic reaction rate, the reaction shifts to mass transfer control, and the catalytic reaction rate levels off regardless of further temperature increases. The reaction is mass transfer limited. In mass transfer controlled catalytic reactions, one cannot distinguish between a more active catalyst and a less active catalyst because the intrinsic catalyst activity is not determinative of the rate of reaction [7, 8]. Regardless of any increase in catalytic activity above that required for mass transfer control, a greater catalytic conversion rate cannot be achieved for the same set of conditions. Because of this limitation, in order to increase the conversion rate for any given system, it appears essential either to increase the amount of catalyst surface or to increase the rate of mass transfer of reactants to the surface [9, 10]. The former, for practical combustion systems, would require either a catalyst size of such magnitude as to be unwieldy or a catalyst configuration which results in increased specific pressure drop and which would require unwieldy geometry to hold a total pressure drop constant. For example, in the case of gas turbine engines, the catalytic reactor might very well be larger than the engine itself. On the other hand, increasing the rate of mass transfer of reactants to the catalytic surfaces would result in increased pressure drop and consequently a substantial loss of energy; sufficient pressure drop may not even be available to provide the desired rate of reaction [11, 12]. Quite obviously, these approaches, while theoretically possible, are quite impractical. This study relates to the essential characteristics of catalytically supported thermal combustion in a micro-scale system with a heat-recirculating structure. Numerical simulations were conducted to gain insights into system performance and to determine what changes can be made to improve its robustness and stability. Detailed kinetics were used for modeling the catalytically supported thermal combustion system in computational fluid dynamics. Methods of applying a heat-recirculating structure to the channel walls were employed, which may be utilized with presently existing designs of micro-scale thermal combustion systems. The factors affecting combustion stability were determined for the system. The objective of this study is to investigate the essential characteristics of catalytically supported thermal combustion in a micro-scale heat-recirculating system so as to gain a greater understanding of the mechanisms of flame stabilization. Particular focus is placed on determining essential factors for design considerations of the catalytically supported thermal combustion system with improved flame stability and combustion characteristics so that the system operates more efficiently. <strong>2. Methods</strong> The catalytically supported thermal combustion system comprises a concentric annular channel, wherein the concentric annular channel further comprises an inner annular channel and an outer annular channel. A platinum catalyst is deposited only upon the interior surface of the inner channel, and the wall of the outer channel is chemically inert and catalytically inactive. The reactant stream flows through the catalytically-coated inner channel and the product stream flows out of the outer non-catalytic channel. Fuel is present for combustion in both the catalytic and non-catalytic channels. The concentrically arranged annular channel is 5.0 mm in inner channel length, 5.6 mm in outer channel length, 0.8 mm in innermost diameter, 2.6 mm in outermost diameter, 0.1 mm in catalyst layer thickness, and 0.2 mm in wall thickness, unless otherwise stated. The system can have any dimension unless restricted by design requirements. All the walls have the same thickness. The spacing between the inner channel and the outer channel is 0.4 mm and remains constant. One of the potential problems associated with the system, as with all micro-scale combustion systems, continues to be combustion stability. The scale of the catalytically supported thermal combustion system is on the order of sub-millimeters, which is much smaller than the quenching distance of the combustible mixture in the absence of a catalyst. The quenching distance defines a critical dimension under which propagation of the methane flame is not possible. The quenching distance is approximately 2.5 mm, at which combustion cannot be sustained within the system in the absence of a catalyst. The system is modeled as two channels and a catalyst layer with dimensions described in detail above. These dimensions could be increased or decreased as desired for particular applications. This design analysis treats some of these dimensions as variables, and some as constrained by combustion stability considerations. In order for flame holding to occur, methane and air must be premixed and provided in an appropriate velocity region to reside. Methane and air are perfectly premixed with a temperature of 300 K before they enter the system prior to combustion. The strength of the mixture composition is typically expressed in terms of its equivalence ratio. Platinum is catalytically-active towards promoting the heterogeneous surface reaction. Complex modeling methods and algorithms are required for the system due not only to the complex geometry of the system but also the complex physicochemical processes involved. It is therefore essential to reduce the complexity of the model through use of certain simplifying assumptions. Steady-state analyses are performed, variations in system pressure and temperature are determined in accordance with the ideal gas law, and the system operates in the laminar flow regime due to the small Reynolds numbers. The maximum Reynolds number is less than 360 at the flow inlet and 960 when the velocity of the flow of the fluid is highest in the channels. The model is implemented in commercially available software FLUENT to obtain the solution of the problem. In the following detailed description, the conservation equations relevant to heat transfer and for laminar flow and chemical species in the system are presented. Detailed chemistry is included in the model. Detailed chemical mechanisms are playing an increasingly important role in developing chemical kinetics models for combustion. Detailed chemical mechanisms are incorporated into the reacting flow for the system. The homogeneous combustion is modeled with the detailed chemical mechanism for methane oxidation in CHEMKIN format. Additionally, detailed heterogeneous chemistry in SURFACE-CHEMKIN format is also included in the model. The rates of the elementary reactions involved in the combustion process are determined by Arrhenius kinetic expressions. Numerical simulations with the detailed chemical mechanism are typically computationally expensive. The detailed chemical mechanism is invariably stiff and therefore its numerical integration is computationally costly. A computational fluid dynamics analysis is carried out on the mesh. A mesh independence test is performed to assure independence of the solution to the problem. Velocity inlet boundary conditions are used to define the velocity properties of the flow at the inlet boundary of the fluid region. A uniform velocity profile is specified at the flow inlet. The temperature of the mixture is prespecified at the flow inlet, as described previously. The complex physicochemical processes involved in the system can require extensive computing resources. The computational fluid dynamics calculations may take days in order to arrive at a reasonably accurate solution, using fine grids of the system, due to the time-consuming nature of the model. Natural parameter continuation is performed by moving from one stationary solution to another. A critical point is denoted as the solution to the problem when a turning point is reached. Knowledge of critical parameters of flow velocity and heat loss coefficient gains a fundamental understanding of the essential factors affecting the stability of the combustion process. The critical parameters are useful as the design guides associated with the system. <strong>3. Results and discussion</strong> The contour plots of temperature are presented in Figure 1 for the catalytically supported thermal combustion system with a heat-recirculating structure. It is possible to achieve essentially adiabatic combustion in the presence of a catalyst at a reaction rate many times greater than the mass transfer limited rate. Catalytically-supported thermal combustion surmounts the mass transfer limitation. If the operating temperature of the catalyst is increased substantially into the mass transfer limited region, the reaction rate again begins to increase exponentially with temperature. This is an apparent contradiction of catalytic technology and the laws of mass transfer kinetics. The phenomena may be explained by the fact that the catalyst surface and the gas layer near the catalyst surface are above a temperature at which thermal combustion occurs at a rate higher than the catalytic rate, and the temperature of the catalyst surface is above the instantaneous autoignition temperature of the fuel-air admixture [13, 14]. The fuel molecules entering this layer spontaneously burn without transport to the catalyst surface. As combustion progresses, the layer becomes deeper. The total gas is ultimately raised to a temperature at which thermal reactions occur in the entire gas stream rather than only near the surface of the catalyst. Once this stage is reached within the catalyst, the thermal reactions continue even without further contact of the gas with the catalyst as the gas passes through the combustion zone. The term "instantaneous auto-ignition temperature" for a fuel-air admixture as used herein is defined to mean that the temperature at which the ignition lag of the fuel-air mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion. Figure 1. Contour plots of temperature for the catalytically supported thermal combustion system with a heat-recirculating structure. The contour plots of methane mole fraction are presented in Figure 2 for the catalytically supported thermal combustion system with a heat-recirculating structure. Catalytically-supported thermal combustion is achieved by contacting at least a portion of the carbonaceous fuel intimately admixed with air with a solid oxidation catalyst having an operating temperature substantially above the instantaneous auto-ignition temperature of the fuel-air admixture. At least a portion of the fuel is combusted under essentially adiabatic conditions. Combustion is characterized by the use of a fuel-air admixture having an adiabatic flame temperature substantially above the instantaneous auto-ignition temperature of the admixture but below a temperature that would result in any substantial formation of oxides of nitrogen [15, 16]. The adiabatic flame temperature is determined at catalyst inlet conditions. The resulting effluent is characterized by high thermal energy useful for generating power and by low amounts of atmospheric pollutants. Where desired, combustible fuel components, for example, un-combusted fuel or intermediate combustion products contained in the effluent from the catalytic zone, or fuel-air admixture which has not passed through a catalytic zone, may be combusted in a thermal zone following the catalytic zone, as explained hereinbelow in greater detail. Sustained catalytically-supported thermal combustion occurs at a substantially lower temperature than in conventional adiabatic thermal combustion and therefore it is possible to operate without formation of significant amounts of nitrogen oxides. Combustion is no longer limited by mass transfer as in the case of conventional catalytic combustion, and at the specified operating temperatures the reaction rate is substantially increased beyond the mass transfer limitation, for example, at least about 5 or 10 times greater than the mass transfer limited rate. Reaction rates of up to about 100 or more times the mass transfer limited rate may be attainable [17, 18]. Such high reaction rates permit high fuel space velocities which normally are not obtainable in catalytic reactions. There is, moreover, no necessity of maintaining fuel-to-air ratios in the flammable range, and consequently loss of combustion (flame-out) due to variations in the fuel-to-air ratio is not the problem it is in conventional combustors. Figure 2. Contour plots of methane mole fraction for the catalytically supported thermal combustion system with a heat-recirculating structure. The contour plots of flow velocity are presented in Figure 3 for the catalytically supported thermal combustion system with a heat-recirculating structure. The adiabatic flame temperature of fuel-air admixtures at any set of conditions (for example, initial temperature and, to a lesser extent, pressure) is established by the ratio of fuel to air. The admixtures utilized are generally within the inflammable range or are fuel-lean outside of the im-flammable range, but there may be instances of a fuel-air admixture having no clearly defined inflammable range but nevertheless having a theoretical adiabatic flame temperature within the operating conditions. The proportions of the fuel and air charged to the combustion zone are typically such that there is a stoichiometric excess of oxygen based on complete conversion of the fuel to carbon dioxide and water. Preferably, the free oxygen content is at least about 1.5 times the stoichiometric amount needed for complete combustion of the fuel. Although the design is described herein with particularity to air as the non-fuel component, it is well understood that oxygen is the required element to support proper combustion. Where desired, the oxygen content of non-fuel component can be varied and the term "air" is used herein to refer to the non-fuel components of the admixtures. The fuel-air admixture fed to the combustion zone may have as low as 10 percent free oxygen by volume or less, which may occur, for example, upon utilization as a source of oxygen of a waste stream wherein the portion of this oxygen has been reacted [19, 20]. In turbine operations, the weight ratio of air to fuel charged to the combustion system is often above about 30:1 and some turbines are designed for air-to-fuel ratios of up about 100 or 200 or more:1. The carbonaceous fuel, which when burned with a stoichiometric amount of air (atmospheric composition) at the combustor inlet temperature usually has an adiabatic flame temperature of at least about 2000 K, is combusted essentially adiabatically in the catalyst zone. Although the instantaneous auto-ignition temperature of a typical fuel may be below about 1400 K, stable adiabatic combustion of the fuel below about 2000 K is extremely difficult to achieve in practical primary combustion systems. Figure 3. Contour plots of flow velocity for the catalytically supported thermal combustion system with a heat-recirculating structure. The contour pl
description: Contributor: Junjie Chen, ORCID: 0000-0002-5022-6863, E-mail address: koncjj@gmail.com, Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China
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