Fractional valorization of bio-oil distillation residue: Strategically perfecting the pivotal step of biomass refinery system

With the gradual maturation of biomass refinery system, the eco-friendly disposal of bio-oil distillation residue (DR) is still required to be explored towards ecological protection and contaminant management. Here, we proposed a fractional process for valorizing DR through coupling of torrefaction and KOH impregnation (KI), with the emphasis on thermolysis behaviors, kinetic responses, gaseous emissions, product distribution and pyrolytic mechanism. The results indicated low-temperature torrefaction (LTT) promoted the enhancement of pyrolysis rate and accelerated the emissions of light compounds. The elevated temperatures for high-temperature torrefaction (HTT) weakened the pyrolysis rate, with the diminishment of 46.02–59.46%, while the subsequent KI process facilitated the pyrolysis rate of HTT-derived DR. The kinetic responses illustrated the activation energies with the enhancement of 21.93%–30.12% increased as pretreatment temperatures increased, ranging from 112.97 to 159.77 kJ/mol. Light torrefaction promoted the emissions of C=C, O−H, and C−H, while the phenols and hydrocarbons among pyrolysates were the most susceptible to the sequential temperature-dependency responses. The fractional valorization process was more conducive to producing hydrocarbons, ketones, and furans, unfortunately with reductions in phenolic contents, which might be attributed to the hydrogenated DR and decreased ether bonds after torrefying. In addition, cooperating of LTT and subsequent KI was inclined to destroy microcrystalline structure and carbonaceous skeleton for the sake of promoting reaction rate, whereas HTT-derived DR endowed a stable carbon skeleton, which was against the generation of pyrolysates and enhancement of pyrolytic rate. Generally, the fractional pretreatment was favorable to the acceleration of reaction rates and directional product distribution. Our findings can provide a feasible strategy for efficient disposal of DS towards cleaner production and energy recovery, and laid a puissant foundation for the future large-scale downstream processing of DR and perfection of biomass refinery system towards waste recycling and contaminant control.