Methane combustion, kinetics

Y.-E. Han, L. Chen, K. Ramesh, E. Widjaja, S. ChUukoti, l.K. Suijami and J. Chen, Kinetic and spectroscopic smdy of methane combustion over a-Mn203 nanocrystal catalysts, J. Catal., 253, 261-268 (2008). 

Notably, the Gas Research Institute s mechanism (GRI-MECH) for methane combustion is well-established, drawing on research from several groups over several decades to define and calibrate kinetic and thermodynamic data for each elementary reaction step. Additional mechanisms" for methane oxidation are also available and updated periodically to include the most recent data. 

TJ. Mitchell, S.W. Benson, and S.B. Karra. Kinetic Model for Formation of Aromatics in the High Temperature Chlorination of Methane. Combust. Sci. Techn., 107 223-260,1995. 

The kinetics and mechanism of methane combustion have been the subject of many investigations, e.g.. Refs. 43-47, because of the importance of natural gas as a potential fuel for catalytic combustors. Under conditions expected in catalytic combustors, i.e., excess oxygen, a first order in methane is generally observed [48], whercas a variety of orders has been observed for other hydrocarbons [13]. The actual mechanism appears to be quite complex and depends on the fuel used. For instance, inhibiting effects are observed for the products carbon dioxide and water in methane combustion over supported palladium catalysts [49,50]. The inhibition of methane adsorption and the formation of a surface palladium hydroxide were proposed to explain the observation. 

Many models use only simple kinetics to describe the gas-phase reactions without taking radical transfer between the two phases into account. This may be an oversimplification, since radical transfer may strongly influence the results of the calculations. Moreover, homogeneous combustion kinetics, even without the presence of a catalyst, are extremely complex, with already over 300 known primary reactions for methane. 

Drake, M. C., Kinetics of Nitric Oxide Formation in Laminar and Turbulent Methane Combustion, Report no. GRI-85/0271, Gas Research Institute, Chicago, 1985. 

N. Bahlawane, Kinetics of methane combustion over CVD-made cobalt oxide catalysts Appl. Catal. B-Environ., 2006, in press. 

Arai et al. (1986) determined the kinetic parameters for methane combustion over Lao.6Sro 4MnC>3 prepared by decomposition of metal acetates and/or nitrates calcined... 

Kinetic parameters for methane combustion over La Sr MnO calcined at different temperatures ... 

The deactivation of bulk iron oxide during methane combustion has been studied. The observed deactivation behaviour has been explained as the result of two simultaneous deactivation mechanisms. In the initial phase of reaction both are in operation and the activity drops rapidly as a consequence of both catalyst sintering and of the depletion of lattice oxygen in the outer layers, due to a partial reduction of the catalytic surface. In later stages, catalyst deactivation is almost exclusively due to sintering imder reaction conditions. A kinetic model of deactivation is presented, together with the physicochemical characterization of fresh and partially deactivated catalysts. 

Kinetic study of methane combustion over Lao.9Ceo.iCo03... 

The kinetics of methane combustion over a perovskite catalyst (Lao.9Ceo.iCo03) has been studied in Micro-Berty and fixed bed reactors. Discrimination among twenty-three rival kinetic models from Eley-Rideal, LHHW and Mars-van Krevelen (MVK) types has been achieved by means of (a) the initial rate method as well as by (b) integral kinetic data analysis. Two MVK type models could be retained as a result of the two studies, with a steady-state assumption implying the equality of the rate of three elementary steps. 

Keywords methane combustion, perovskite, kinetics, model discrimination, MVK mechanism... 

The goal of the present study is to elucidate the base mechanism of methane combustion over Lao.9Ceo. Co03 perovskite. The influence of reactants, oxygen and methane was taken into account while Ae effect of the reaction products was neglected. Discrimination among 23 plausible kinetic models has been realized by means of two studies, viz. 

The estimated activation energy is in reasonable agreement with the values reported in the literature for methane combustion over Pd/y-AbOa (see Table 3). Particularly, the activation energy obtained in the present work is lower than that reported by Kolaczkowski et al. (19%) which is the unique kinetic expression found for a monoUth reactor. However, the reaction rates calculated using the kinetic parameters showed in Table 2, for different methane concentrations and temperatures, are lower than those estimated through the expression given by Kolaczkowski et al. (1996). The observed differences can not be explained because the catalyst characterization is not given by the authors. 

McCarty [125] used an annular reactor to evaluate kinetics of methane combustion over PdO-supported catalysts. The design of the apparatus had a small gap between cylinders (0.1-0.3 mm) and a thin coating (10 pm). Using high flow rates and dilute methane and oxygen in helium, the author claims to have measured the intrinsic rate of methane oxidation up to 900°C, without contributions from gas-phase reactions. Groppi et al. 

The kinetics of methane combustion on ceramic perovskites can be almost always described by the standard equation corresponding to bimolecular Rideal-Eley mechanism. To our knowledge, the full-term bimolecular Langmuir-Hinshelwood model has not been observed dimng methane oxidation. In the RE case, oxygen molecules are adsorbed in dissociative form on the surface metal ions while methane reacts with them from the gaseous phase or from a very weakly adsorbed state, the distinction been elusive. The equation describing those results reads... 

The above treatment is illustrated in a typical example referred to the kinetics of methane combustion over three mixed perovskites Lao.yCeo.sFeOs, Lao.7Sro.3-FeOs, and Lao.7Sro.iCeo.2Fe03. Details about these experiments are provided in Ref. [31]. The main experiment results are shown in Figure 16.6 and the obtained parameters are collected in Table 16.1. 

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