Recycled valorization of bio-oil distillation residue via CO2-induced co-pyrolysis with biomass

Bio-oil rectification is gradually employed as an industrialized platform to recover energy and commodity chemicals, but bio-oil distillation residue (DR) as an undesired by-product severely hampers the continuous operation of biomass refinery process. In the present work, a recycled processing of DR was proposed by copyrolysis with walnut shell (WS) under CO2-induced atmospheres. The insights into co-pyrolytic mechanism, kinetic responses, and product distribution were further examined. Meanwhile, the development prospects of condensed bio-oils were outlooked for engineering and commercial applications. The results indicated that the co-pyrolytic interaction significantly suppressed the high-temperature pyrolysis, endowing a deteriorative mass synergy index from -0.63%/min to -1.91%/min. The impacts of induced atmospheres and DR fractions on the evolutions of activation energies satisfied cubic polynomials, and the activation energy ranged from 121.54 kJ/mol to 153.69 kJ/mol. Additionally, the CO2-mixed atmospheres mainly changed the kinetic mechanism at the early stage of co-pyrolysis (X <50%), while the addition of DR induced a modified mechanism for the late stage (X >50%). The two-dimensional correlation spectroscopy further demonstrated a lower CO2 concentration modified the primary sequential temperature responses of pyrogenic gases in the order: ethers/carboxylic acids/ ketones < phenols < CO2 /ethers < alcohols, whereas pure CO2 induced the responses as below: CO2 < carboxylic acids/ketones/ethers. More importantly, co-processing of DR and WS facilitated the hydrodeoxygenation reaction of pyrolytic vapors and promoted the production of phenol-rich bio-oils, and the CO2-assisted pyrolysis was conducive to the scissions of aromatics and enhancement of hydrocarbon yield. Generally, the presence of DR and CO2 effectively accelerated the aromatics cracks and hydrogenation of co-pyrolytic vapors, which provided feasibilities for the abundances of product composition and recoveries of commodity chemicals. This work also offered a possible strategy for downstream disposal of DR, which further laid a puissant basis on perfection of biomass refinery system.