Methane combustion mechanism

Zsely, I.G., Turmyi, T. Investigation and reduction of two methane combustion mechanisms. Arch. Combust. 21, 173-177 (2001)... 

Using a temperature-programmed surface reaction (TPSR) technique, Li et al. (154) showed that this complete oxidation of methane took place on the NiO catalyst during the CHfOi reaction. Weng et al. (145) used in situ microprobe Raman and in situ time-resolved IR spectroscopies to obtain a relationship between the state of the catalyst and the reaction mechanism. These authors showed that RuC>2 in the Ru/SiC>2 catalyst formed easily at 873 K in the presence of a CH4/02/Ar (2/1/45, molar) mixture and that the dominant pathway to synthesis gas was by the sequence of total oxidation of CH4 followed by reforming of the unconverted CH4 by C02 and H20. Thus, these results indicate that the oxidation of methane takes place principally by the combustion mechanism on the oxidized form of this catalyst. 

The approach is to start with analysis of the smallest of the hydrocarbon molecules, methane. It is interesting that the combustion mechanism of methane was for a long period of time the least understood. In recent years, however, there have been many studies of methane, so that to a large degree its specific oxidation mechanisms are known over various ranges of temperatures. Now among the best understood, these mechanisms will be detailed later in this chapter. 

In terms of temperature regions, low-temperature combustion occurs over the range 298-550 K, whereas high-temperature combustion mechanisms dominate at temperatures over 1000 K. Intermediate temperatures, from 550 to 700 K, demonstrate an unusual phenomenon called the negative temperature coefficient (NTQ, which is observed for methane and larger hydrocarbon fuels. As shown in Fig. 3, when the correct alkylperoxy radical chemistry is included in a fuel s combustion mechanism, a NTC range exists (Fig. 3, plot C) where an increase in temperature causes a decrease... 

The simplest hydrocarbon, methane, has posed a wealth of challenges to experimentalists and theoreticians seeking to discern its combustion mechanism. Methane s reactions have been explored in a wide variety of contexts over the past several decades. We have discussed these briefly the interested reader is referred to the reviews cited in our previous discussion for further details. Due to the scope of this review, we are primarily interested in these reactions insofar as they provide useful benchmarks for the reactions of larger alkylperoxy (RO2 ) and alkoxy (RO ) systems. With respect to the reactive intermediates present in methane combustion and their implications for larger systems, Lightfoot has published a review on the atmospheric role of these species, while Wallington et al. have provided multiple overviews of gas-phase peroxy radical chemistry. Lesclaux has provided multiple reviews of developments in peroxy radical chemistry. Batt published a review of the gas-phase decomposition reactions available to the alkoxy radicals. ... 

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. 

The photograph is included to make two points. First, the particle paths show qualitatively that the flow follows the anticipated streamlines. Even for the relatively small dimensions, the edge effects that could interrupt similarity behavior at the outflow appear to be minor. Second, and more striking, is the fact that the flame zone is extremely flat. Here is a situation that includes a considerable amount of chemistry (methane combustion) and complex heat and mass transfer. The fact that the flame zone shows no radial dependence is is convincing evidence that the fluid mechanical similarity is indeed valid. 

TM. Sloane. Ignition and Flame Progation Modeling with an Improved Methane Oxidation Mechanism. Combust. Sci. Techn., 63 287-313,1989. 

G.P. Smith, DM. Golden, M. Frenklach, N.W. Moriarty, B. Eiteneer, M. Golden-berg, C.T. Bowman, R.K. Hanson, S. Song, W.C. Gardiner, V. Lissianski, and Z. Qin. GRI-Mech—An Optimized Detailed Chemical Reaction Mechanism for Methane Combustion. Technical Report http //www.me.berkeley.edu/gri-mech, Gas Research Institute, 1999. 

While Table 8 includes reactions for the formation of thermal NO, it does not include those for prompt NO. Mechanisms and reaction rate data for prompt NO formation and various methods for the reduction of NO have been described by Millerand Bowman (Prog. Energy Combust. 5ci., 15,287,1989). The GRIMECH reaction set (http //www.me.berkeley.edu/grijnech) is an example of a high temperature methane oxidation mechanism that includes both thermal and prompt NO production. 

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. 

P. Glarborg, N.J. Lilleheie, S. Byggstoyl, B.F. Magnussen, P. Kilpinen and M. Hupa, A Reduced Mechanism for Nitrogen Chemistry in Methane Combustion, 24th Symp. (Int.) Comb. (1992) pp. 889-898. 

The main advance of the chosen model is the possibility of handling detailed reaction mechanisms for investigating the gas phase reactions. There are several reaction mechanisms available for natural gas (methane) combustion including nitrogen chemistry. The mechanism selected for the present model is the GRl-Mechanism V2.11 (49 species, 279 primary reactions). For modeling the gas phase reactions of the furnace described model, the CHEMKIN II software package was used. The generation of the input and output data of the different processes is accomplished with separate input routines. 

As discussed above, LES/FMDF can be implemented with (1) nonequilibrium and (2) near-equilibrium combustion models. The former uses a direct ODF solver for the chemistry and can handle finite-rate chemistry effects. In the latter, a flamelet library is coupled with the LFS/FMDF solver in which transport of the mixture fraction is considered. The latter approach has the advantage it is computationally much less intensive and can be conducted with very complex chemical kinetics models. Below, some of the results recently obtained via Fq. (4.2) are presented. The flamelet library is generated with the full methane oxidation mechanism of the Gas Research Institute (GRI) [6] accounting for 53 species and about 300 elementary reactions. 

The methane combustion at high temperatures over metallic oxides can be described by a redox mechanism. The rate of oxygen incorporation to the lattice is much faster than the oxygen consumption hence the rate is zero-order with respect to the oxygen partial pressure ... 

---