CH4 decomposition over a wood char was investigated as an alternative green catalyst.to produce hydrogen from hydrocarbons. Pyrolytic carbon (pyrocarbon) deposition leads to apparent deactivation of the catalyst by pore-mouth plugging. The activity of the carbon bed and its available surface area are easily restored by H2O gasification. The used char with pyrocarbon deposition was even found to be more reactive to gasification than the fresh char used in our conditions. This finding was highlighted by: (i) determination of gasification reaction extents by steam, (ii) temperature-programmed oxidation (TPO) of the fresh, used and reactivated chars and (iii) TPO under differential scanning calorimetry of these chars and demineralised chars. High-resolution transmission electron microscope (HRTEM) analysis of the chars showed different multiscale organisation of the carbon materials (disordered and graphitic mesoporous nanostructures). The fast regeneration of the used char could be attributed to the catalytic effect of the minerals present in the char that are reduced under our conditions of CH4 conversion. The predominant oxidation of the pyrocarbon compared to the char during its regeneration is evidenced through differential annealing (at 1800 degrees C) followed by XRD analysis. The oxidation of pyrocarbon is faster than the oxidation of the weakly reactive mesoporous carbon in char as shown by the HRTEM analysis. Consequently, wood char is an effective, easy to regenerate and cheap catalyst for converting hydrocarbons (CH4 or tar) into syngas. (C) 2014 Elsevier B.V. All rights reserved.

In order to optimise hydrogen production from biomass gasification, catalytic conversion of methane contained in a surrogate biomass syngas (CH4 14%; CO 19%; CO2 14%; H2 16%; H2O 30%; N2 7%) is investigated over a fixed bed of porous wood char as a function of temperature (800–1000°C) and space time (1.6–6.2mingL−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 1nm 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 300m2/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.

The catalytic activity of a wood char towards CH4 decomposition in a pyrolysis gas was investigated in a fixed bed reactor for maximising hydrogen production from biomass gasification. Wood char is suggested to be the cheapest and greenest catalyst for CH4 conversion as it is directly produced in the pyrolysis facility. The conversion of methane reaches 70% for a contact time of 120 ms at 1000 °C. Because steam and CO2 are simultaneously present in the pyrolysis gas, the carbon catalyst is continuously regenerated. Hence the conversion of methane quickly stabilises. Such a phenomenon is shown to be possible through the oxidation of the char by CO2 and H2O at high temperature, which prevents the blocking of the mouth of pores by the concurrent pyrolytic carbon deposition. In the experimental conditions, oxygenated functional surface groups are continuously formed (by steam and CO2 oxidation) and thermally decomposed. The active sites for CH4 chemisorption and decomposition are suggested to be the unsaturated carbon atoms generated by the evolution of the oxygenated functions at high temperature.

The continuous self stirred tank reactor is a well known suitable device for studying the kinetics of high-temperature gas phase reactions. Temperature and composition are uniform at any point of the reactor, and for a given residence time, it is easy to calculate the values of kinetic constants from a very simple mathematical model. The aim of the present paper is to report the first experimental results obtained on the thermal cracking of vapours produced by the pyrolysis of biomass. The experiments are carried out between 836 and 1303K and under mean residence times ranging from 0.3 to 0.5 s. The calculated activation energy (59kJmol-1) and preexponential factor (1930s-1) are in good agreement with those obtained by other authors operating in more usual devices, but needing the solving of more sophisticated models.