Biomass pre-treatment techniques for the production of biofuels using thermal conversion methods-A review

This review aims to compile and discuss the pre-processing procedures utilized previously to biomass thermal conversion techniques, in order to promote the sustainable production of biofuels and other commodities from biomass. These will ultimately replace fossil-based alternatives under an environment-friendly framework, and contribute to the attainment of circular economy goals. Pre-processing methods account for a significantly improved efficiency, as a more homogeneous, dry, suitable and consistent feedstock is achieved, supporting cleaner and proficient conversion methods. This may prevent the complete depletion of non-renewable resources, alleviating the effects of their overexploitation. Findings concerning the main constrains when using biomass as a feedstock for thermal conversion reveal that size, format, moisture content and heterogeneity are the main encountered issues. Whilst mechanical processes, drying, torrefaction and pelletisation have shown enhanced results for physical aspects as well as moisture; hydrolysis, hydrothermal and microwave-based techniques are seen as the most utilized to solve problems related to morphology, degradability and digestibility. It was also found that the most significant thermal conversion techniques are torrefaction, hydrothermal processing, gasification, combustion, pyrolysis and plasma gasification. Relative to the major types of biomass applied for the production of biofuels, these are wood and woody biomass, followed by herbaceous and agricultural streams.