Hydrothermal liquefaction vs. fast/flash pyrolysis for biomass-to-biofuel conversion: new insights and comparative review of liquid biofuel yield, composition, and properties

Abstract

Hydrothermal liquefaction (HTL) and fast/flash pyrolysis are thermochemical processes (TPs) with proven potential to convert biomass into liquid biofuel, which can be comparable to crude oil. HTL is generally preferred for wet biomass, while fast/flash pyrolysis is more suitable for dried biomass, as moisture content plays a crucial role in determining the appropriate conversion method. Beyond moisture content, the biochemical and elemental composition of biomass significantly impacts the physical and chemical characteristics of the resulting liquid biofuels, often increasing the need for upgrading. This review provides a comprehensive comparison of HTL and fast/flash pyrolysis for converting five biomass types—lignocellulosic biomass, microalgae, macroalgae, municipal sludge, and food waste—into liquid biofuels, highlighting the impact of biomass composition on biofuel yield and quality. By linking biomass type, process severity, and liquid biofuel quantity, this study offers a structured framework for selecting the optimal conversion process and severity range to maximize biofuel yield in large-scale applications. Additionally, this review identifies various organic compounds and their concentrations in liquid biofuels produced through HTL and fast/flash pyrolysis from different biomass sources, serving as a valuable resource for developing novel multistage and selective upgrading processes.