Polymers and Chemical Composition of Hardwood and Softwood (Bark, Sapwood, and Heartwood) for Biofuel Production: A Comprehensive Review

Lignocellulosic biomass from hardwood and softwood species represents a highly abundant and renewable resource for biofuel and bio-based material production. This review provides a comprehensive analysis of the chemical composition and structural organization of the three major polymers—hemicellulose, cellulose, and lignin—across different wood fractions, including bark, sapwood, and heartwood. Typically, wood consists of a significant number of these components, approximately 20–35% hemicellulose, 40–50% cellulose, and 20–30% lignin. Significant variations exist between hardwood and softwood species, particularly in lignin composition and hemicellulose structure, which strongly influence biomass recalcitrance and conversion efficiency. Bark is rich in lignin (often 20–40%) and extractives, making it suitable for thermochemical processes, while sapwood exhibits higher carbohydrate accessibility, favoring biochemical conversion. Heartwood, enriched with extractives and condensed lignin, shows reduced reactivity but high potential for value-added chemicals. The review also evaluates extraction techniques and conversion pathways, highlighting the importance of fraction-specific processing strategies. Understanding these variations is essential for optimizing biorefinery performance and advancing sustainable biomass utilization.