Low temperature reforming of biogas over K-, Mg- and Ce-promoted Ni/Al2O3 catalysts for the production of hydrogen rich syngas: Understanding the plasma-catalytic synergy

Plasma-catalytic biogas reforming over Ni-X/Al2O3 catalyst (X = K, Mg and Ce) has been carried out in a coaxial dielectric barrier discharge (DBD) plasma reactor at 160 °C. Three different process modes: plasma-alone, catalysis-alone and plasma-catalysis have been investigated to get new insights into the synergistic effect resulted from the interaction of the plasma with the promoted Ni catalysts. Compared to the biogas reforming using either plasma-alone or catalysis-alone mode at the same temperature (160 °C), the combination of the plasma with the Ni-based catalysts exhibited a low temperature synergistic effect, as evidenced from the much higher reforming performance of the plasma-catalytic process compared to that of the sum of the individual processes (plasma-alone and catalysis-alone). The addition of promoters (K, Mg and Ce) into the Ni/Al2O3 catalyst enhanced the conversion of CH4, the yield of H2 and the energy efficiency of the plasma process. In this study, the behaviour of K, Mg and Ce promoters in the low temperature plasma-catalytic biogas reforming was clearly different from that in high temperature thermal catalytic process in terms of the conversion of CH4 and carbon deposition, which could be ascribed to the temperature-dependent character of the promotors. In the plasma-catalytic biogas reforming, the Ni-K/Al2O3 catalyst showed the best performance, enhancing the conversion of both CO2 and CH4, the yield of H2, CO and C2–C4 alkanes and the energy efficiency of the plasma process. The highest conversion of CO2 (22.8%) and CH4 (31.6%) was achieved by placing the K-promoted catalyst in the plasma reforming process. The Mg-promoted catalyst remarkably increased the H2/CO molar ratio in the gas products (up to 2.2) due to the decreased CO2 conversion. In addition, compared to the un-promoted Ni/Al2O3 catalyst, although the use of the promoted catalysts increased the carbon deposition on the surface of the spent catalysts by 22%–26%, the total amount of deposited carbon was still less than that reported in high temperature catalytic dry reforming processes. More than 80% of the increased carbonaceous species was in the form of reactive carbon species, which can be easily oxidized by CO2 and O atoms and maintain the stability of the catalysts during the reforming reaction.