Biogas to syngas by microwave-assisted dry reforming in the presence of char

The aim of this paper is to study the reforming of CH₄ with CO₂ using as a catalyst a rich potassium char obtained from biomass pyrolysis. The reaction was carried out at 800 °C by means of two different methods of heating, microwave and electrical heating (MWH and EH, respectively). In addition, the individual reactions proposed for the dry reforming of methane, that is, (i) the decomposition of CH₄ to form hydrogen and carbon and (ii) the dissociative adsorption of CO₂ followed by reduction to give CO, were also studied with both heating methods at the same temperature. The results showed that MWH produces hot spots (microplasmas located inside the catalyst bed) that favor heterogeneous catalytic reactions. Thus, it was found that the conversion of CH₄ and CO₂ in the individual reactions was greater for MWH than for EH. An examination of the CH₄ decomposition reaction proved the formation of coke deposits, which reduced catalytic activity and CH₄ conversion. When a CH₄/CO₂ mixture was used, this problem was minimized since the CO₂ partly removed the carbon deposits formed, thereby prolonging the activity of the catalyst. This gasification reaction, catalyzed by the high level of K contained in the char, provides an "in situ" route for catalyst regeneration. The results indicated that the presence of CO₂ increased the conversion of CH₄ to H₂, the values being higher in MWH than in EH. Both heating methods produced an outlet gas composed mainly of syngas (CO + H₂) and practically free of CO₂ and CH₄ (especially in the case of MWH). In addition, the study of the exhausted catalysts by scanning electron microscopy revealed the presence of significant amounts of carbon nanofibers on the char surface, but only in the case of MWH, these nanofibers being more abundant in the dry reforming reaction than in the single CH₄ decomposition.