A Comprehensive Review of Biomass Pretreatment Technologies for High-Quality Fuel Production: Emphasis on Washing, Torrefaction, and Combined Approaches

Biomass is an abundant renewable feedstock for combustion, co-firing, gasification, and pyrolysis, but in its raw form, it is a low-grade fuel. Low energy density, high moisture, and high contents of alkali and alkaline earth metals (AAEMs) and chlorine drive slagging, fouling, and corrosion in thermal equipment. Pretreatment is therefore required to meet the fuel-quality specifications. This review critically examines biomass pretreatment technologies, including physical, chemical, and biological, and then focuses on the two routes most relevant to solid-fuel upgrading, namely water/solvent washing (leaching) and torrefaction (thermochemical). Washing removes water-soluble and organically bound AAEMs, directly reducing slagging and fouling propensity, but it adds moisture, requires a drying step, and does not increase energy density. Torrefaction raises the higher heating value and carbon contents, lowers oxygen and moisture contents, and improves grindability and hydrophobicity, but the accompanying mass loss concentrates the residual AAEMs, so the treated fuel can retain or worsen its slagging tendency. Because the two methods correct complementary deficiencies, deashing versus deoxygenation and energy densification, this review evaluates the combined washing-torrefaction route as an integrated strategy. The combined route delivers what neither single method achieves alone: high AAEMs removal, increased higher heating value (HHV) and carbon content, reduced oxygen and moisture, and balanced bio-oil and biochar yields in subsequent pyrolysis. The review further compares the methods against the fuel specifications of each end-use, summarises the limited pilot and demonstration-scale evidence, and techno-economic questions that currently keep combined pretreatment at the laboratory stage.