Toward Efficient Biofuel Production: A Review of Online Upgrading Methods for Biomass Pyrolysis

The reduction in fossil fuel reserves has driven the scientific community and industrial stakeholders to seek alternative renewable energy sources. Additionally, the detrimental environmental effects of fossil fuel extraction and combustion have further emphasized the need for these alternatives. One such source is bio-oil, which can be synthesized from biomass through fast pyrolysis. However, bio-oil produced via fast pyrolysis exhibits several significant drawbacks, including poor quality due to water and oxygenated compounds, high viscosity, storage instability, low heating value, and high acidity, resulting in corrosiveness. To address these limitations and enhance the quality of bio-oil, various methods and innovative techniques have been proposed. This review provides a comprehensive analysis of recent advancements in biomass fast pyrolysis and bio-oil upgrading. It explores various traditional physical methods and novel chemical processes, including innovative techniques, such as plasma reactors, microwave-assisted methods, and pyrolysis under supercritical conditions. These approaches are critically evaluated for their effectiveness in enhancing bio-oil performance. Additionally, a novel perspective is proposed from the electrochemical catalysis point of view. It highlights the capabilities of solid oxide electrolysis cells, which can perform electrochemical hydrogenation and thermochemical hydrodeoxygenation reactions simultaneously under moderate temperature conditions. When integrated with fast pyrolysis, these processes enable the online electrochemical upgrading of biomass pyrolysis vapors.