Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Chemical Reaction Mechanisms Assessment for Simulation of Methane Combustion in Domestic Gas Cooking Burners Saúl Laguillo,† Jose ́ Salvador Ochoa,‡ and Alfredo Ortiz*,† †
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Department of Chemical and Biomolecular Engineering, University of Cantabria, Avenida Los Castros 46, 39005, Santander, Cantabria, Spain ‡ Gas Competence Center, Product Division Cooking, BSH Home Appliances Group, Avenida Eduardo García del Rio 30, 39011, Santander, Cantabria, Spain S Supporting Information *
ABSTRACT: The use of detailed chemical kinetics combined with complex three-dimensional computer aided design (3-D CAD) geometries makes it practically unaffordable to simulate combustion in real configurations such as domestic gas cooking burners. Global or reduced reaction mechanisms are alternatives to alleviate this high computational load, but they are generally formulated on certain operating ranges or assumptions. In this work, the performance of chemical reaction mechanisms to simulate methane combustion is analyzed comparing with available experimental measurements and detailed chemistry. Examined geometrical configurations are one-dimensional (1-D) premixed flame, two-dimensional (2-D) partially premixed flame, and 3-D domestic gas cooking burner. Laminar flame speed and variables of interest such as temperature, major species, and heat exchange are the targets for performance comparison. Results show that global mechanisms could be used in certain cases for getting overall temperature fields and thermal balances, but never to accurately predict laminar flame speed and pollutant emissions. Skeletal models predominantly present good behavior in all the analyzed cases, although some of them show slight deviations. Mechanisms are ranked based on their statistical evaluation of agreement with reference data. In general, results state that the more species and reactions included, the better accuracy obtained but the higher computational time required. Most skeletal mechanisms are good alternatives to detailed chemistry; therefore, the final choice will be mainly influenced by the availability of computational resources and the required target of the computational fluid dynamics (CFD) analysis.
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INTRODUCTION Natural gas (NG) is a resource consisting mainly of methane (>90% volume dry) and small traces of heavier hydrocarbons and gases such as carbon dioxide or nitrogen.1 NG is a very important energy source in the world. For instance, 37.1% of the final energy consumption in the residential sector of Europe is covered by NG,2 while in the United States about half of the homes use NG mainly to heat buildings and water, dry clothes, and cook.3 In any of the above uses, combustion is the mechanism to convert the fuel chemical energy into heat, and this resource will unanimously continue playing an important role in the energy mix for a long time. In that context, designing efficient gas burners with low release of pollutant emissions and greenhouse gases such as carbon monoxide (CO) and carbon dioxide (CO2) is essential. Numerical simulation is becoming an increasingly important tool in the design of new, safer, and more efficient gas cooking burners. Computational fluid dynamics (CFD) techniques are increasingly being employed to solve the combustion process taking place in these devices. An accurate modeling of the reactive flow in a gas cooking burner comprises complex threedimensional computer aided design (3-D CAD) geometries which need to be discretized into several millions of cells. The chemical reaction process is taken into account through a set of chemical reactions and species, including their respective thermokinetic parameters, typically known as kinetic or chemical reaction mechanisms. Depending on the fuel © XXXX American Chemical Society
composition, these chemistry databases can comprise thousands of reactions involving hundreds of chemical species. This often makes unaffordable the cost of simulating real configurations with full chemistry, at least when high performance hardware is not available. In the case of methane combustion, a widely studied hydrocarbon, the reference reaction mechanism GRI-Mech 3.0 contains tens of species and hundreds of reactions.4 Given the strong impact of the detailed mechanism size in the modeling of a gas burner in both accuracy of results and computational time, it is a common practice to use global or reduced kinetics to represent the combustion reaction. However, the reduction process or the definition of chemical kinetics parameters are usually set up to fulfill specific targets under a narrow range of operating conditions. Specifically concerning the analysis of domestic gas burners, country regulations such as European Standards5,6 impose a restrictive threshold value for CO emissions in addition to a minimum value of thermal efficiency and flame stability requirements in all the operational range. The correct CFD prediction of these phenomena highly depends on the quality of chemical kinetics description as the combustion develops under a laminar flow regime. To the authors’ knowledge, there Received: May 20, 2019 Revised: August 10, 2019 Published: August 12, 2019 A
DOI: 10.1021/acs.energyfuels.9b01598 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
where Y stands for the mass fraction of the mixture components. There exist two previous releases which are still suitable to be used, namely, GRI-Mech 1.2 and GRI-Mech 2.11. These three versions, along with the Leeds mechanism,22 fill out the detailed mechanisms group in this study. Table 1 classifies all the evaluated options, including their numbers of species and reactions.
are very few published studies considering the numerical simulation of methane combustion in a domestic gas cooking burner, and global mechanisms are employed in all of them. The two-step mechanism developed by Westbrook and Dryer7 is used by Gattei8 in the prediction of occurrence of the flame lift instability, relying on the CO value as a marker of the starting reaction. Another two-step mechanism for natural gas combustion developed by Bibrzycki et al.9 is employed by Boggavarapu et al.,10 who do not compare experimental results with numerical predictions but only use them as a guidance for trends in order to improve the performance of the burner. There are some studies involving different type of methane− air burners, but either they use a similar approach for chemical kinetics11 or the combustion is developed under turbulent regime, and therefore the turbulence−chemistry interactions determine the approach to be used.12,13 Some recent studies consider precalculating and storing the chemistry by means of the flamelet-generated-manifold (FGM) approach in domestic gas burners14 as an alternative to alleviate the computational load. However, although the precalculation is carried out using simple one-dimensional flames, a reaction mechanism is needed to calculate every thermochemical state to be expected in a real configuration. In this study, the performance of reaction mechanisms developed to describe methane combustion is analyzed under different configurations, focused in CFD simulations of domestic gas cooking burners. The final goal is to determine the best mechanisms to surrogate detailed chemistry, taking into account both prediction of parameters of interest such as pollutant emissions and thermal efficiency, and their computational load. Assessment performance is also evaluated through a comparison with experimental data when available. The evaluation is done first resorting to classical onedimensional (1-D) free flames, where mechanisms are evaluated comparing laminar flame speed (LFS) values. This parameter is usually considered a fundamental reference for the mechanism behavior, because it outlines the flame shape, and therefore influences the temperature field and chemical species production and consumption. Then, a two-dimensional (2-D) axisymmetric partially premixed laminar flame is employed to analyze temperature and major species profiles. Given its simplicity and the availability of experimental data, this configuration is a great candidate to be used as a reference case to validate numerical approaches. Finally, a realistic 3-D (slice) gas cooking burner configuration is used and parameters related to certification standards (thermal efficiency and CO emissions) are assessed.
Table 1. Classification of the Evaluated Chemical Reaction Mechanisms Suitable for Methane−Air Combustion type global
skeletal
detailed
no. reactions
ref
5 6 7 7 16 17 21 24 30 32 37 49 53
1 2 4 5 35 73 84 104 184 177 351 279 325
7 7 15 16 17 18, 19 20 20 18, 21 23 22 24 4
Independently generated. bObtained from GRI-Mech 1.2. cObtained from GRI-Mech 3.0.
The far side of possibilities is represented by global reaction mechanisms, which generally reproduce essential properties of flames such as laminar flame speed, major species, or temperature under certain mixing or environmental conditions. Their deduction process is based on general optimization or empirical methods. The simplest mechanisms for methane−air combustion, described by one and two reaction steps (hereinafter 1Step and 2Steps, respectively), are included in this group. These mechanisms were originally developed by Westbrook and Dryer7 and are the options normally included by default in commercial CFD codes for chemical kinetics problems. The four-step mechanism developed by Jones and Lindstedt15 (JL) and its five-step subsequent modification by Andersen et al.16 (JL-mod) are also classified as global mechanisms. The spectrum of the analyzed mechanisms in this study is completed by the skeletal ones, which are those obtained by neglecting irrelevant species and reactions from a detailed mechanism in order to produce a reduced-size one. This type of reduction is based on either comprehensive consideration or a particular application. It can be achieved by several methods, e.g., Jacobian analysis,25 detailed reduction,26 directed relation graph27 (DRG), DRG with error propagation28 (DRGEP), and DRG-aided sensitivity analysis29 (DRGASA). Skeletal mechanisms in this work can be further classified depending on their origin. First is the mechanism presented by Smooke and Giovangigli17 (Smooke), which is directly proposed as a skeletal methane−air reaction mechanism. Second are mechanisms developed by Lu et al.18,19 (Lu-sk17) and Kazakov and Frenklach20 (DRM19 and DRM22), that were originally obtained from GRI-Mech 1.2. Finally is another mechanism by Lu et al.18,21 (Lu-sk30), which is based on GRI-Mech 3.0. Besides the Leeds and GRI-Mech detailed mechanisms considered in this paper, there exist in the literature some other
CHEMICAL KINETICS OF NATURAL GAS COMBUSTION For the analysis of NG combustion, GRI-Mech 3.0 is widely used as the reference reaction mechanism. It is an optimized set of 325 elementary chemical reactions, associated rate coefficient expressions, and thermochemical parameters for 53 species, including NOx formation and reburn chemistry. It was originally designed over the ranges 1000−2500 K, 1−1000 kPa, and Φ from 0.1 to 5 for premixed systems,4 where Φ is the equivalence ratio that relates stoichiometric and local mixture conditions. It can be defined as (Yair /Yfuel)stoi (Yair /Yfuel)
no. species
a
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Φ=
mechanism 1Step 2Steps JL JL-mod Smookea Lu-sk17b DRM19b DRM22b Lu-sk30c GRI-Mech 1.2 Leeds GRI-Mech 2.11 GRI-Mech 3.0
(1) B
DOI: 10.1021/acs.energyfuels.9b01598 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
conditions, so the process is entirely governed by chemical kinetics and no turbulence−chemistry interaction is needed. However, the mixing between fuel and primary air in the 3-D gas burner configuration is developed under a turbulent environment; thus, a turbulence model needs to be enabled in these simulations. The mixture is considered to be a multicomponent ideal gas where density depends only on temperature and composition. Kinetic theory is invoked for transport properties such as viscosity, thermal conductivity, and mass diffusivity. A complete overview of the transport equations for each specific case can be found in ref 40 for the 1-D configuration and in ref 41 for CFD calculations. One-Dimensional Laminar Premixed Flame. Laminar flame speed (LFS) is commonly used to characterize the combustion of various fuel−oxidizer combinations and to determine mixture flammability limits. Burner-stabilized onedimensional laminar premixed flames are often employed to study chemical kinetics in a combustion environment and obtain LFS values. Such flames can be highly stable, facilitating detailed measurements of temperature and species profiles. LFS experiments using a methane−air flame at atmospheric pressure and an unburnt gas temperature of 298 K were reported by Taylor42 and Vagelopoulos et al.,43 varying the equivalence ratio between 0.5 and 1.6. These studies are selected for comparison purposes. LFS computational values are determined using a 1-D premixed laminar flame model embedded in the software ANSYS Chemkin-Pro,40 solving the set of governing equations that describe the flame dynamics using an implicit finite difference scheme with a combination of time-dependent and steady-state methods. Temperature, pressure, and equivalence ratio range values are set as in the experiments. Additionally, thermal diffusion (Soret effect) is enabled and multicomponent diffusion transport is chosen for species calculation. The used 1-D domain allows a discretization up to 500 grid points, and the length of the domain is set to 22 mm. Two-Dimensional Laminar Partially Premixed Flame. A simplified, 2-D axisymmetric laminar partially premixed flame (frequently known as a “Yale flame”) is chosen to assess the performance of the mechanisms in terms of temperature and major species prediction. The use of this flame as a validation case is motivated by its simplicity, the availability of experimental measurements, and the fact that it is representative of the processes happening in a real gas cooking burner. The experimental setup was initially described by Santoro et al.44 and McEnally and Pfefferle.45 This flame has been widely studied and reported in the literature.45−54 Its configuration is similar to that of a Bunsen burner: the fuel enters a cylindrical combustion chamber through an axial duct, surrounded by a coaxial coflow of air (Figure 1). The fuel stream consists of a partially premixed mixture of methane and oxygen-enriched air. The flame presents two distinct combustion zones. The first one is located near the nozzle, where the chemistry of the rich mixture controls the reaction as a premixed combustion. Once the primary oxygen is depleted, the second zone is formed, where the diffusion of air from coaxial flow controls the combustion progress. The experiments reported by McEnally et al.53 were carried out with several equivalence ratios for the fuel stream, ranging between Φ = 2.5 and ∞ (the latter corresponding to a purely diffusion flame). Major species (CH4, O2, CO2) concentration
possibilities which include C1−C4 hydrocarbon chemistry (e.g., AramcoMech,30 USC Mech II,31 UCSD mechanism32). All these detailed options are not normally used in CFD simulations without a prior reduction process. Furthermore, other strategies utilize a combination of skeletal reaction mechanisms and the quasi-steady state (QSS) approximation for intermediate species with short time scales that reach chemical equilibrium.33 Thus, the integration of transport equations is replaced by solving simple algebraic expressions.18,19,21,34−38 However, this kind of approach does not fully comply with the well-known Arrhenius formulation. These mechanisms are not directly readable and functional in commercial CFD solvers, and their implementation requires an adaptation through external programmed subroutines. The performance of the mechanisms presented previously is analyzed in terms of laminar flame speed and variables of interest such as temperature, major species, and heat fluxes, among others. Table 2 shows the geometrical configurations that are used to assess this performance. Table 2. Relationship between the Assessment Parameter and the Geometrical Configuration Used To Obtain It studied parameter laminar flame speed temperature and major species CO emissions and thermal efficiency
configuration utilized 1-D laminar premixed flame CFD: 2-D laminar partially premixed flame CFD: 3-D gas cooking burner
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DESCRIPTION OF THE GEOMETRICAL CONFIGURATIONS Initially, a one-dimensional flame model is used to obtain laminar flame speed profiles. Subsequently, CFD is utilized to describe two-dimensional and three-dimensional configurations to compare variables of interest such as temperature, major species, or heat exchange. In all the former cases, conservation equations of mass, momentum, energy, and species need to be solved. The final formulation of the equations naturally depends on the dimensionality of each type of configuration. The solver code addresses the specific problem in steady-state formulation. Likewise, net chemical production rates of each species are taken into account adding the specification of gas-phase reaction, thermodynamic, and transport properties data contained by means of special formatted files (Chemkin format) to the solver. These files are directly read by the software. The approach to numerically describe the combustion process that takes place in any configuration depends on the flow regime, being laminar or turbulent. The maximum Reynolds number for each studied configuration is presented in Table 3. Assuming that the flow is laminar when Re < 2000,39 in all cases combustion is developed under laminar Table 3. Maximum Reynolds Number and Flow Regime in Analyzed Configurations configuration
zone
Vmax (m/s)
Remax
regime
1-D 2-D 3-D
− − mixing combustion
