Energy & Fuels 2004, 18, 619-627
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An Augmented Reduced Mechanism for Methane Combustion T. Mendiara,* M. U. Alzueta, A. Millera, and R. Bilbao Department of Chemical and Environmental Engineering, Centro Polite´ cnico Superior, Marı´a de Luna, 3 (Torres Quevedo Building), 50018 Zaragoza, Spain Received May 6, 2003
An augmented reduced mechanism (ARM) for methane combustion has been developed from the detailed mechanism of Glarborg and co-workers (reported in 1998) that was formed by 447 elementary reversible reactions and 65 chemical species. First, an intermediate state of reduction, the skeletal mechanism, is reached by means of sensitivity analysis and reaction-rate analysis. After that, a systematic method for reducing mechanisms based on the steady-state hypothesis and the partial equilibrium assumption is used. The reduced mechanism is obtained via an algebraic procedure. This mechanism consists of 18 lumped reaction steps and 22 chemical species. Both the skeletal and reduced mechanisms are validated by comparison with the results predicted by the detailed mechanism for the main species in methane combustion (CH4, CO, and CO2) in a wide range of operating conditions and generally show good agreement.
Introduction Recently, there have been great advances in obtaining detailed kinetic mechanism schemes that are used to describe hydrocarbon combustion in a wide range of temperatures and operating conditions. These detailed mechanisms consist of an elevated number of elemental reactions (several hundreds) that reproduce, quite accurately, the real kinetics of the process. Nevertheless, the complete modeling of combustion systems needs to combine the kinetics with the main physical features of the system, such as mass and heat transfer, and fluidynamic aspects. The great number of elementary reactions and chemical species involved in the detailed mechanisms, together with the coupling of kinetic equations and physical features, indicate that the computational calculations for the complete simulation of the system are not adequate for the current means. One of the possibilities to cope with this problem is to obtain a simpler but representative kinetic scheme of the system. This would imply a reduction in the total number of chemical species and elementary reactions and would lead to a manageable computation. A reduced mechanism can achieve this goal. Apart from reducing the computational effort, the reduced mechanisms for hydrocarbons are powerful tools for the study of the structure of the combustion system. Moreover, the reduced mechanisms allow estimation of how the system responds to situations that are not possible to test experimentally. This is one of the most interesting advantages of numerical modeling. A few years ago, the reduced mechanisms consisted of four or five global steps that were capable of describing, to some extent, the chemistry of the combustion process. The first reduced mechanisms for methane * Author to whom correspondence should be addressed. E-mail address:
[email protected].
combustion were derived in different conditions by making steady-state and partial equilibrium approximations for some species. Initial attempts only considered the C1 chain to explain methane oxidation. That is the case of several studies for methane flames. The reduced four-step mechanism obtained by Peters and Kee1 provided the essential properties of methane-air diffusion flames. Bilger et al.2 obtained a four-step mechanism for methane-air combustion in non-premixed flames and also discussed a further reduction to a three-step or two-step mechanism. However, the description of rich methane flames was still insufficient. Therefore, the C2 chain for methane oxidation had to be included in the derivation of a reduced mechanism. An example is the four-step mechanism of Mauss and Peters.3 The accuracy improved but the great simplicity of these mechanisms was joined to a narrow applicability. This means that these reduced mechanisms were only able to reproduce the results for a given combustion system under very specific operating conditions. A detailed examination of the reduced mechanisms, especially those of methane combustion, shows that some of the crucial intermediate species, despite what was expected, are not in a steady state over a wide range of conditions. Because there are not too many of these species, it could be possible that the relaxation of the steady-state assumptions for them will lead to remarkable improvements in the descriptive capacity of the reduced mechanism. An augmented reduced mechanism (ARM), formed by 10 or 20 reactions, then could be a better approximation to real chemistry. This can be especially true for low-weighted alkanes, because of the fact that their chemical kinetics is better known. Some (1) Peters, N.; Kee, R. J. Combust. Flame 1987, 68, 17-29. (2) Bilger, R. W.; Stårner, S. H.; Kee, R. J. Combust. Flame 1990, 80, 135-149. (3) Mauss, F.; Peters, N. Lect. Notes Phys. 1993, 15, 58-75.
10.1021/ef030111u CCC: $27.50 © 2004 American Chemical Society Published on Web 04/07/2004
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Table 1. Operating Conditions Considered in the Modeling environment
λ
reaction time, tr (ms)
fuel-rich stoichiometric fuel-lean
0.5 1 5
0-500 0-500 0-500
initial concentrations
temperature, T (K)
pressure, P (atm)
CH4 (ppm)
H2O (%)
O2 (ppm)
N2 (%)
1100-1800 1100-1800 1100-1800
1 1 1
1000 1000 1000
2 2 2
1000 2000 10000
97.8 97.7 96.9
ARMs have been developed for methane oxidation, such as the reaction mechanism obtained by Sung et al.,4 which was developed from the GRIv1.2 mechanism of Frenklach et al.,5 which consisted of 12 lumped reaction steps and 16 chemical species. In this sense, the actual tendency to the ARMs is clear. In this context, the purpose of this work is to develop and supply an ARM for methane combustion, using a more-recently detailed mechanism (the GADM mechanism)6 as a starting point. We have chosen methane combustion because of its inherent importance, as well as its involvement in other processes (flames, engines, combustion chambers, etc.). Several detailed kinetic mechanisms for hydrocarbon combustion are available in the literature. These were validated under different operating conditions and used to predict different results (chemical species concentrations, ignition delay times, contour plots of the time to achieve a given temperature rise, ...), depending on the particular interest of the researchers. The GADM detailed model is one of the most-recent and most-used detailed mechanisms that are utilized in combustion applications, and it has been validated under determined conditions.7-9 As mentioned previously, some reduced mechanisms for CH4 combustion have been published in the literature; however, none of them were extracted from the GADM mechanism. In this sense, the development of an ARM from the GADM mechanism, to be used when a combination of the kinetics and physical features of the system is required, clearly is a item of interest; such a mechanism can be applied with CFD models in practical applications with a reasonable time of computation. The results of the ARM should be as similar as possible to those obtained with the GADM mechanism. In this work, the ARM is obtained using a systematic method that is based on the steady-state hypothesis, the partial equilibrium assumption, and the application of an algebraic procedure. This reduced mechanism is also validated, versus the detailed mechanism,6 for a wide range of operating conditions and considering principally kinetic aspects or those inherent in chemical reactions, i.e., the main species in methane combustion concentrations (CH4, CO, and CO2). The Skeletal Mechanism Development. In the present work, the detailed mechanism to be reduced is that which was proposed by Glarborg et al.6 It is a mechanism that was developed for the reburning process, and it includes the oxidation mechanisms of hydrocarbons, as well as the interactions between hydrocarbons and NO. It is formed by 447 reversible elementary reactions and 65 chemical species. It was initially validated for a temperature range of 800-1500 K and for fuels such as CH4, C2H2, C2H4, C2H6, and natural gas. The experimental data for the
validation of this detailed mechanism were basically extracted from the work of Alzueta et al.7 Moreover, more recently, it has also been used to simulate experimental results on reburning in both bench-scale and pilot-scale setups that cover an extended range of conditionssin particular, temperatures reaching as high as 1800 Kswith good results.8,9 In addition, the GADM mechanism, and modifications of it, have been used recently to model methane combustion in conditions that represent engine combustion or the evolution of unburned hydrocarbons in engine exhaust with success.10-12 The operating conditions that have been used in the present work are given in Table 1. Because our main interest consists of developing an ARM that is based on the Glarborg et al.6 mechanism, we have performed most of calculations in the temperature range of 8001500 K, which was the initial validation range of the detailed mechanism. However, as shown below, results obtained at higher temperatures seem also to be valid. Table 1 also shows that we have used very diluted conditions, with the purpose of assuming isothermal conditions for calculations. Because it is proposed that both skeletal and reduced mechanisms can be used in a wide range of operating conditions, fuel-rich, stoichiometric, and fuel-lean conditions are tested. The regime of operation is characterized by the air-excess ratio, λ. Taking into account the aforementioned considerations, prior to any other analysis, the first simplification done is the elimination of all the elementary reactions that describe the chemistry of nitrogen species from the detailed mechanism. The temperatures in this work are generally
