Investigation on bio-oil pyrolysis with Ni/Al2O3 blending: Influence of the blended catalyst on coke formation

Abstract

Bio-oil can be converted into syngas, chemicals, and carbon materials via thermochemical and/or catalytic conversion routes. However, coking remains a significant challenge during the heating of bio-oil. In this study, we investigated the influence of pre-blending Ni/Al2O3 and bio-oil in an in-situ catalytic pyrolysis process on the coking behaviours of bio-oil. Our results indicate that the presence of Ni/Al2O3 significantly reduces coke yield and increases gas yield (for example, by a 14.2 % reduction in coke and a 44.1 % increase in gas at 700 °C). This is attributed to the suppression of the cross-linking reactions responsible for coke formation through the pre-elimination of O-containing groups into CO and CO2. Additionally, pre-blending of Ni/Al2O3 is highly desirable for transforming or stabilising the highly reactive substances in the early pyrolysis stage of bio-oil, minimising their tendency to polymerise. Furthermore, the presence of Ni/Al2O3 facilitates the cracking of C–H bonds and self-reforming of bio-oil, increasing H2 production (which is increased by 42.3 % at 700 °C), thereby promoting the breakage of side chains of lignin-derived aromatics and the hydrogenation of unsaturated structures, suppressing the formation of small aromatic systems in coke (<6 rings). Nevertheless, enhanced formation of alkyl-aryl C–C bonds during aromatisation leads to more condensed coke structures.