Mechanisms and Kinetics of Methane Thermal Conversion in a Syngas Anthony Dufour,* ,†,‡ Sylvie Valin, § Pierre Castelli, § Se ´bastien Thiery, § Guillaume Boissonnet, § Andre ´ Zoulalian, ⊥ and Pierre-Alexandre Glaude | Research & DeVelopment DiVision, Gaz de France, 361 aVenue du Pre ´sident Wilson, BP 33, F-93211 Saint-Denis-la-Plaine cedex, France, CEA, DEN, 17 rue des Martyrs, F-38054 Grenoble, France, Faculte ´ des Sciences & Techniques, LERMaB, Nancy-UniVersite ´, INRA, BP 239, F-54506 VandoeuVre le `s Nancy cedex, France, and DCPR, Nancy-UniVersite ´, CNRS, 1 rue GrandVille, B.P. 20451 F-54001 Nancy cedex, France In order to optimize H 2 and CO production from biomass gasification, the thermal decomposition of methane in a reconstituted syngas was investigated in a tubular reactor at 130 kPa, for a gas residence time of 2 s and as a function of temperature (1000-1400 °C), CH 4 (7, 14%), H 2 (16, 32%), and H 2 O (15, 25, 30%) initial mole fractions. H 2 showed an inhibiting effect on CH 4 conversion whereas H 2 O had few effects. Three detailed elementary mechanisms were used to predict the methane conversion rate and to identify the key reaction pathways. Flow rate analyses showed that carbon oxidation occurs mainly by addition of OH radicals on C 2 compounds. OH radicals are mainly produced by CO 2 (CO 2 + H ) CO + OH). The inhibiting role of H 2 on CH 4 conversion is explained by a competition between the OH radicals consumption channels (H 2 + OH ) H 2 O + H). The competition between thermal conversion of methane and reforming of unsaturated C 2 explains the soot formation. 1. Introduction Biomass gasification is a promising route of gaseous and liquid fuels production such as CH 4 ,H 2 , Fischer-Tropsch diesel, or methanol. Biomass gasification processes produce a syngas mainly composed of CO, H 2 ,H 2 O, CO 2 , CH 4 , and C 2 com- pounds and tar. CH 4 conversion into H 2 and CO has been identified as one of the main key points to optimize H 2 recovery and the overall energetic efficiency of the H 2 production from biomass gasification. 1,2 Methane is formed during biomass pyrolysis and tar cracking, and it roughly represents 30 mol % of the hydrogen element contained in the products of wood pyrolysis at 800 °C (including permanent gases, char, water, and tar). 3 The most studied methane up-grading technology is catalytic gas conditioning. Metal based catalysts and especially Ni-based catalysts have been extensively studied. 4-6 However, they are prone to deactivation due to coke accumulation on the catalytic surface and to sulfur poisoning, when used to up-grade a real biomass syngas. 7-9 Other alternative strategies for methane reforming could be the use of carbon-based catalysts such as wood char 10 or thermal decomposition (without any catalysts). Thermal decomposition of methane was reviewed by Khan et al., 11 Back and Back, 12 and Billaud et al. 13 Pyrolysis of pure CH 4 (diluted in an inert gas) was extensively studied in a wide range of conditions. 14-26 However, these works focused most of the time on C 2 H 2 production 17,18,20,21,23,25 or on carbon vapor deposition. 19,22 Numerous studies dealt with the effect of H 2 15,17,18,23-26 or H 2 O 17,24 addition on CH 4 thermal decomposition mainly in order to maximize the C 2 H 2 selectivity. The effect of CO 2 addition was studied, to our knowledge, exclusively in the presence of O 2 . 27,28 Moreover, numerous detailed mechanisms validated for an extensive range of CH 4 combustion conditions were developed. 29-35 The conversion of CH 4 in presence of H 2 , CO 2 ,H 2 O, and CO with a composition representative of a biomass pyrolysis gas has only been studied, to our knowledge, by Jo ¨nsson. 36 Jo ¨nsson investigated thermal reactions at temperatures lower than 1250 °C. Methane conversions lower than 60% were achieved, and no kinetic data were derived. To our knowledge, in spite of the numerous studies on CH 4 conversion, no work deals with the kinetic of CH 4 thermal decomposition under the conditions of a biomass pyrolysis gas, i.e. a syngas with a high content of H 2 ,H 2 O, CO 2 , and CO. This paper provides new kinetic data on the thermal decomposi- tion of CH 4 in such a syngas. The effects of temperature, CH 4 , H 2 O, and H 2 initial mole fractions on CH 4 conversion are analyzed. The main elementary reactions involved during the syngas thermal upgrading are described and discussed in order to determine the most efficient process conditions and to better understand the limitations, such as soot formation. 2. Materials and Method The experimental apparatus was formerly presented by Valin et al. 37 with a detailed description on the facility and reactor design. Only the salient points are summarized here. The syngas composition was controlled using mass flow rate regulators (Brooks Instrument) for each individual compound. Water was vaporized in a coiled tube before being mixed with permanent gases in a preheated zone. The nominal molar gas composition was the following: CH 4 7%, CO 19%, CO 2 14%, H 2 16%, H 2 O 25%, and N 2 /Ar 7%. When the partial pressure of a compound was modified, the partial pressures of other gases were kept constant by adjusting the nitrogen flow. The preheated gas was injected in an alumina tubular reactor (838 mm length and 70 mm diameter). The reactor was specifically designed to study the reforming of hydrocarbons such as methane and tar at high temperatures between 1000 and 1500 °C. 37 The absolute pressure in the reactor was approximately 130 kPa to make up for the pressure drops, the * To whom correspondence should be addressed. E-mail: anthony.dufour@yahoo.fr. † Gaz de France. ‡ Present adress: LSGC, Nancy-Universite ´, CNRS, 1 rue Grandville BP 451 54001 Nancy cedex, France. § CEA, DEN. ⊥ Faculte ´ des Sciences & Techniques, LERMaB, Nancy-Universite ´, INRA. | DCPR, Nancy-Universite ´, CNRS. Ind. Eng. Chem. Res. 2009, 48, 6564–6572 6564 10.1021/ie900343b CCC: $40.75  2009 American Chemical Society Published on Web 06/22/2009 Downloaded by INPL on July 10, 2009 Published on June 22, 2009 on http://pubs.acs.org | doi: 10.1021/ie900343b