Bioethanol Revolution: Integrating Feedstock Innovation with Energy-Efficient Processes under Life Cycle Assessment

The global energy system remains heavily dependent on fossil fuels, increasing the need for renewable liquid alternatives. Bioethanol is a major gasoline substitute that can reduce petroleum use in transport by 5–27% under typical blending mandates (E10–E27) and up to 100% on an energy basis in neat ethanol applications (E100). However, first-generation production from food crops raises economic and food–fuel concerns. This review synthesizes recent advances in second- and third-generation bioethanol from lignocellulosic biomass, agricultural and industrial residues, and microalgae, with emphasis on process intensification and sustainability performance. A comparative life-cycle assessment (LCA) and techno-economic analysis evaluate pathways based on net energy ratio, life-cycle GHG emissions, and production cost. Lignocellulosic ethanol can achieve 40–91% GHG reductions relative to gasoline, though costs remain variable (∼0. 21–1. 25/L). Mature sugar/starch routes show competitive costs (∼0. 34–0. 47/L) with significant carbon benefits when land-use change is minimized. Waste/residue pathways emerge as the most robust near-term option, while microalgae-based ethanol remains limited by energy and cost constraints (NER < 1; ∼0. 76–0. 91/L). Overall, this review identifies the most viable deployment routes and key research priorities for advancing low-carbon bioethanol systems. • Integrates novel feedstocks with modern bioethanol conversion technologies. • Reviews enzymatic, thermochemical, and microbial innovations for 2G ethanol. • Compares SSF, SSCF, and CBP processes for cost-effective bioethanol yield. • Includes life cycle assessment of various bioethanol production pathways. • Identifies scalable and sustainable strategies for future bioethanol deployment.