Catalytic cracking of ethylbenzene as tar surrogate using pyrolysis chars from wastes

Abstract

This paper aims at studying the catalytic activity of waste-derived chars for the reforming of a tar compound (ethylbenzene), and to identify the relationships between the modification process, the physicochemical properties and their resulting catalytic behaviour. Two chars were produced by pyrolysis: (1) used wood pallets (UWP), and (2) a mixture of food waste (FW) and coagulation-flocculation sludge (CFS) from wastewater treatment plant. Two chemical-free modification processes were separately applied to the pyrolysis chars: a gas phase oxygenation at 280 °C, or a steam activation at 850 °C. At 650 °C, the ethylbenzene conversion due to thermal cracking was significantly increased by the catalytic activity of the chars (from 37.2 up to 85.8%). Ethylbenzene was decomposed into six molecules: hydrogen, carbon dioxide, ethylene, benzene, styrene, and toluene. Cracking, oxidative dehydrogenation, and hydrogenolysis reactions were involved in the decomposition mechanism of ethylbenzene. The catalytic efficiency of the char was also discussed based on the energy transferred from tar to syngas during tar cracking reactions. The characterization, performed with SEM, XRD, Raman, XRF, BET and TPD-μGC, evidenced that the presence of mineral species in the metallic form strongly increased the syngas production and quality by catalysing aromatic-ring opening reactions and Boudouard reaction. The oxidation of mineral species, occurring during the oxygenation process, decreased the char efficiency, while rising SBET increased the syngas production for UWP-based chars. This study demonstrated that waste-based chars were efficient catalysts to convert the lost energy contained in tar into useful syngas, thus increasing simultaneously the syngas yield and quality.