Enrichment of phenolic products in walnut shell pyrolysis bio-oil by combining torrefaction pretreatment with fractional condensation.

Walnut shell pyrolysis liquefaction experiments for phenolic enrichment were conducted using torrefaction pretreatment and fractional condensation simultaneously. Bio-oil composition was comprehensively analyzed for elements, functional groups and componential contents. After high-temperature torrefaction, the proportion of lignin pyrolytic products in pyrolysis organic vapors showed significant increment. Fractional condensation further separated water, acetic acid and other small molecular compounds, and improved the phenolic content in the target bio-oil. Compared with the traditional pyrolysis liquefaction technology, when 363 K water bath was utilized to recover 300 °C torrefied walnut shell pyrolysis vapors, the C/O ratio in bio-oil increased from 0.5 to 1.6, and the proportions of H in aromatic ring and phenolic OH increased by 150% and 400%, respectively. The total content of detectable phenols was enriched from 2.7 wt% to 13.3 wt%. The optimized scheme on improving economic efficiency was provided for offsetting torrefaction energy consumption and low bio-oil yield. • Torrefaction pretreatment and fractional condensation were used to prepare bio-oil. • Variation in bio-oil composition under different preparation conditions was analyzed. • Phenolic enrichment mechanism by torrefaction and condensation was clarified. • Optimal temperatures of torrefaction and condensation were given to maximize phenols. • A revised scheme was provided for offsetting the disadvantages of torrefaction. [ABSTRACT FROM AUTHOR]