Thermochemical production of bio-oil: a review of downstream processing technologies for bio-oil upgrading, production of hydrogen and high value-added products

Bio-oil produced from biomass pyrolysis and hydrothermal liquefaction is considered as the most sustainable alternative for depleting fossil fuels. However, the poor bio-oil properties, such as high viscosity, presence of solid particles, low calorific value and high instability are restricting its use as a drop-in fuel. The bio-oil properties can be significantly improved using different methods, such as catalytic upgrading, biomass pre-treatment and downstream bio-oil upgrading. This article focusses on the widely used methods for downstream bio-oil upgrading, such as hydrotreatment, solvent addition, emulsification, microfiltration and electrocatalytic hydrogenation. The bio-oil upgrading using non-polar solvents or preparing emulsions using surfactants have shown a significant increase in the calorific values and a considerable decrease in viscosity of the bio-oil. On the other hand, filtration of the bio-oil using membranes can remove the char particles and alkali and alkali earth metals from the bio-oil, consequently, leading to higher stability of the bio-oil. Electrocatalytic hydrogenation of the bio-oil has shown promising results to increase the content of hydrocarbons and increased pH by removing the carbonyl group-containing compounds from the bio-oil. The bio-oil can also be upgraded to other clean fuels, such as H ₂ using steam reforming approach, has been critically reviewed. Basic principles of the processes and effects of different parameters on bio-oil upgrading are thoroughly discussed. In addition, techno-economic analysis, policy analysis, challenges and future recommendations related to downstream processes are provided in the article. Overall, this review article provides critical information about downstream bio-oil upgrading and production of other high value-added fuels.