Biochemical Conversion of Lignocellulosic Biomass in Biorefinery Systems

Lignocellulosic biomass is one of the most abundant renewable carbon resources available, currently used predominantly for energy generation through direct combustion, yet still underutilized as a feedstock for higher-value biochemical conversion. Its structural complexity and intrinsic recalcitrance continue to challenge efficient biological processing. Overcoming these barriers requires an integrated understanding of plant cell-wall architecture, pretreatment chemistry, enzymatic mechanisms, and process engineering. This review provides a clear and conceptually grounded synthesis of these elements, illustrating how they converge to enable the development of second-generation (2G) lignocellulosic biorefineries. This review examines the hierarchical organization of cellulose, hemicelluloses, and lignin; the principles and performance of modern pretreatment technologies; the synergistic action of cellulolytic systems, including lytic polysaccharide monooxygenases (LPMOs) and non-hydrolytic proteins such as swollenins; advances in C5/C6 sugar fermentation; and emerging strategies for lignin upgrading. In addition to a comprehensive analysis of the literature, representative industrial and experimental case studies reported in the literature are discussed to illustrate practical process behavior and design considerations. By integrating mechanistic insight with industrially relevant examples, this review highlights the technical feasibility, current maturity, and remaining challenges of lignocellulosic biorefineries, underscoring their strategic role in enabling a competitive, low-carbon bioeconomy.