A review of fractional distillation to improve hydrothermal liquefaction biocrude characteristics; future outlook and prospects

Hydrothermal liquefaction (HTL) is an efficient thermochemical method for biomass conversion into biocrude which could be operated with a wide range of feedstock resources. However, HTL biocrude characteristics including viscosity, density, heating value, composition and stability are not comparable with conventional products. The current focus for upgrading mainly relates to catalytic and hydrogenation processes; however, physical processes are cheaper and more reliable. Fractional distillation has potential as a cost-efficient physical technique for biocrude upgrading or even co-processing with crude oil in a refinery. This review summarises and discusses changes in physical and chemical properties of biocrude using fractional distillation. Distillation reduces the oxygen content of biocrude for heavy fractions more than 53% on average. It also decreases the sulphur and nitrogen content of biocrude up to 5-44%. The potential role of distillation in preparing fuel suitable for diesel engines is investigated. The challenges and technical limitations in HTL biocrude application in industry are also discussed alongside with possible solutions and future research potential which addresses these challenges.