Biomass-Based Biofuels: Technological Innovations, Sustainability Metrics, and Policy Pathways for a Low-Carbon...

Biomass-derived biofuels are central to reducing greenhouse gas (GHG) emissions and dependence on fossil fuels, yet large-scale deployment faces technical, economic, and environmental barriers. This review synthesizes advances in feedstock utilization, pretreatment methods, conversion pathways, hybrid systems, and policy frameworks shaping the biofuel landscape. First-generation crops such as corn and sugarcane yield 4000-7000 L/ha ethanol with limited pretreatment but compete with food supplies and consume 500-1000 L water per liter of ethanol. Second-generation residues (e.g., corn stover, switchgrass) achieve 280-300 L/ton ethanol and cut GHG emissions by 70%-90% (50-70 g CO₂/MJ vs. 120-150 g CO₂/MJ for fossil fuels). Third-generation microalgae produce 200-300 L/ton biocrude, though energy-intensive dewatering (10-15 MJ/kg) restricts feasibility. Pretreatment options-including physical (60%-70% sugar yield), chemical (90-95%), biological (50%-60%), and integrated systems (85%-95%)-enhance accessibility but remain costly ($0.05-10/kg). Conversion routes such as pyrolysis (70%-75% bio-oil), hydrothermal liquefaction (80%-85% efficiency), fermentation (280-300 L/ton ethanol), and anaerobic digestion (300-400 m³/ton biogas) offer versatile outputs, though bottlenecks like pentose fermentation losses persist. Integrated biorefineries and emerging platforms, including catalytic upgrading (80%-90% hydrocarbons) and bioelectrochemical systems (0.1-0.3 m³/m³/day H₂), improve yields by 30%-50% but demand high capital costs ($100-200 million/plant). Policy interventions-such as renewable fuel standards (e.g., US RFS2) and carbon pricing ($50-100/ton CO₂)-reduce costs by 10%-20% and boost production by 15%-25%, albeit with compliance challenges. Future priorities include cost-effective pretreatment, scalable biorefineries, durable catalysts, and AI-driven life cycle assessments to enable GHG reductions of 90-100 g CO₂/MJ, positioning biofuels as a cornerstone of sustainable, low-carbon energy systems.