Recently, biomass has shown its viability as an alternative to fossil fuels. Due to the growing trend in greenhouse gas emissions generated by the continual burning of fossil fuel products, it will be advantageous for humanity to seek a more sustainable and renewable source of energy. Due to its availability, biomass has a promising approach as a feedstock for bioconversion processes that produce energy, fuels, and other chemicals. The carbon dioxide generated by burning biomass has no influence on atmospheric carbon dioxide since it is derived from a renewable source. Despite these benefits, its adoption in bioconversion and biorefinery processes has traditionally been hindered by its recalcitrant nature, as indicated by its intrinsic characteristics. Prior to any conversion process, biomass must be pretreated to enhance product recovery. To satisfy the rising need for renewable and sustainable energy sources, the present conversion efficiency must be improved and the biorefinery concept must transition from using just one biomass component (cellulose) to utilizing the complete biomass component. This study examines numerous pretreatment procedures used prior to any conversion process, the challenges faced, and the future of biomass pretreatment technologies. Physical, hydrothermal, chemical, oxidation, biological, and hybrid pretreatment techniques are evaluated. The review indicates that the ideal approach to biomass pretreatment must be able to deal with the recalcitrant nature of biomass, enhance the crystallinity of cellulose, and provide the greatest recovery of biofuels, bio-char, sugars, and other industrially relevant bioproducts. The data offered in this study will equip readers with the knowledge necessary to effectively identify solutions to pretreatment problems and energy generation from pretreated biomass.
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