Kinetics of Methane Combustion

We shall discuss these approaches in detail and emphasize their complementary nature. 

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

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. 

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. 

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). 

Early studies in this field [35, 36] indicated that a high surface-to-volume ratio, which represents a hurdle for gas-phase combustion, is instead an advantage for catalytic combustion. In fact the small scale enhances considerably the rate of gas-solid mass transfer, which favors the kinetics of the combustion process and compensates for the short residence time. Also, as is well established for large-scale systems, the presence of a catalytic phase allows for stable combustion at significantly lower temperature than traditional homogeneous burners [55, 56]. This makes the design and operation of microcombustors more fiexible. Several recent studies have explored the potential of catalytic microcombustors using H2 [37, 38, 50], methane [37], propane [52,53,57] and mixtures of H2 with propane [57], butane [38,47,52] and dimethyl ether [52]. 

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. 

It is clear that the concept of formaldehyde as the relatively stable intermediate responsible for degenerate branching is a fundamental one in elaborating the kinetics of methane oxidation. Indeed the statement has been made (64) that the combustion of methane may be regarded as the formation and oxidation of formaldehyde. 

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. 

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

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. 

Bromly JH, Barnes FJ, Muris S, You X, Haynes BS. Kinetic and thermodjmamic sensitivity analysis of the NO-sensitised oxidation of methane. Combust Sci Technol 1996 115 259—96. 

Flame or Partial Combustion Processes. In the combustion or flame processes, the necessary energy is imparted to the feedstock by the partial combustion of the hydrocarbon feed (one-stage process), or by the combustion of residual gas, or any other suitable fuel, and subsequent injection of the cracking stock into the hot combustion gases (two-stage process). A detailed discussion of the kinetics for the pyrolysis of methane for the production of acetylene by partial oxidation, and some conclusions as to reaction mechanism have been given (12). 

Takahashi, F and Katta, V.R., Chemical kinetic structure of the reaction kernel of methane jet diffusion flames, Combust. Sci. Technol., 155, 243, 2000. 

CHEMClean and CHEMDiffs The Comparison of Detailed Chemical Kinetic Mechanisms Application to the Combustion of Methane, Rolland, S. and Simmie, J. M. Int. J. Chem. Kinet. 36(9), 467 471, (2004). These programs may be used with CHEMKIN to (1) clean up an input mechanism file and (2) to compare two clean mechanisms. Refer to the website http //www. nuigalway.ie/chem/c3/software.htm for more information. 

Erenklach, M., H. Wang, and J. J. Rabinowitz. 1992. Optimization and analysis of large chemical kinetic mechanisms using the solution mapping method — combustion of methane. Progress Energy Combustion Science 18 47-73. 

Another related problem is associated with over-oxidation of the substrate, in the extreme case resulting in complete combustion. In the case of methane oxidation by FeO", for example, the activation of methane occurs with about 10% of the gas-kinetic collision rate, whereas those of the putative oxygenation products CH3OH, CH2O, and HCOOH occur on every collision [60]. With regard to applied catalysis this would imply that the oxidation products are oxidized faster by about one order of magnitude compared to methane as the initial substrate. In the particular context of heterogeneous oxidation... 

There has been a great deal of research on the combustion of small hydrocarbons, including nitrogen-cycle chemistry leading to nitric-oxide formation and abatement [138]. There are a number of methane-air reaction mechanisms that have been developed and validated [274,276,278], the most popular one being GRI-Mech [366]. There is also active research on the kinetics of large hydrocarbon combustion [81,88,171,246,328-330,426]. 

Since the maximum attainable temperature is sought, we assume complete adiabatic (Q = 0) combustion. With the additional assumptions that the kinetic- and potential-energy changes are negligible and that there is no shaft work, the overall energy balance for the process reduces to AH = 0. For purposes of calculation of the final temperature, any convenient path between the initial and final states may be used. The path chosen is indicated in the diagram. With one mole of methane burned as the basis for all calculations,... 

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