Effect of micropores diffusion on kinetics of CH 4 decomposition over a wood-derived carbon catalyst

Abstract

In order to optimise hydrogen production from biomass gasification, catalytic conversion of methane contained in a surrogate biomass syngas (CH 4 14%; CO 19%; CO 2 14%; H 2 16%; H 2O 30%; N 2 7%) is investigated over a fixed bed of porous wood char as a function of temperature (800–1000 °C) and space time (1.6–6.2 min g L −1). Determination of Thiele modulus evidences a change of kinetic regime from chemically- to diffusion-controlled when the temperature increases; this finding is particularly relevant when porous chars having an average pore width of 1 nm are used as catalysts. Mass diffusion transfers are accounted for by a model introducing an internal effectiveness factor. Knudsen diffusion in micropores is shown to limit the conversion rate of methane per unit mass of catalyst, and explains why such a rate is not proportional to the BET surface area, especially when the latter is higher than typically 300 m 2/g. It is concluded that diffusion limitations in micropores should be taken into account, otherwise underestimated activation energy and intrinsic kinetic constant are obtained in some experimental conditions.