Enhancing the conversion of waste motor oil into diesel-like fuels using mineral-impregnated biochar catalysts

This study shows how the presence of metal-impregnated biochar influences the kinetics of the catalytic conversion of waste motor oil (WMO) into a diesel-like fuel (DLF). First, the initial metal concentration in the wet impregnation process was optimized, showing that high metal concentrations tend to clog the biochar pores, reducing the reaction’s conversion and speed. Zinc and calcium showed higher kinetic constants at the optimized conditions than nickel, while the yield- and selectivity-to-liquid products were unaffected. Then, a reduction method was developed for impregnating metal particles of calcium oxide to enhance the metal dispersion at the biochar surface. This material showed the best performance in the reaction due to the presence of small particles instead of metal agglomerates. An increase of about 280% in the kinetic constant at 420 °C and a much lower activation energy (246 kJ mol⁻¹) compared to the thermal cracking (308 kJ mol⁻¹) was obtained. Finally, the DLF from the best catalytic system was analyzed. Interestingly, the fuel showed similar rheological properties to commercial diesel with a similar hydrocarbon distribution but with the presence of smaller hydrocarbon chains.