Valorizing Every Carbon Atom: A Cascade Bioprocess for Advanced Biofuels from Corn-Stover-Derived Lignocellulose

Conventional biorefining of lignocellulosic biomass, such as corn stover (CS), is hampered by poor carbon efficiency, as nearly half of the substrate carbon is lost as CO₂ during ethanol fermentation. This study presents a holistically integrated cascade process designed to capture and valorize all major carbon streams. The system synergistically couples three stages: (1) high-productivity ethanol fermentation (1.68 g/L/h) using the engineered yeast <i>Saccharomyces cerevisiae</i> CE10; (2) anaerobic digestion of the resulting stillage, which efficiently converted residual organics into methane (171 L/kg COD) with ca. 80% COD removal; and (3) cultivation of the cyanobacterium <i>Desertifilum tharense</i> BERC03 using the nutrient-rich digestate and captured fermentation CO₂. This integrated approach boosted the carbon utilization from a baseline of 48% to 62%. A comprehensive techno-economic analysis of an industrial-scale (2000 t/d) facility projected a Minimum Ethanol Selling Price (MESP) of $2.44 per gallon, a value approaching current market competitiveness. The analysis identified the feedstock (30%) and cellulase (17%) as the primary cost drivers. These findings demonstrate a validated biorefinery model that significantly enhances carbon recovery and outlines a viable pathway for the coproduction of multiple biofuels from lignocellulosic resources.