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 pretreatment 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 H2 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.