Valorization of agricultural waste for sustainable bioethanol production: A comprehensive review

Abstract

With increasing global energy demand and concerns regarding fuel emissions, bioethanol has emerged as a promising alternative energy source. Agricultural wastes such as rice husk, wheat straw, corn stover, sugarcane bagasse, millet husks, fruit and vegetable residues, and oilseed wastes serve as low-cost and abundant lignocellulosic feedstocks, addressing both energy needs and waste disposal challenges. Conventional (first-generation) bioethanol production based on sugar and starch crops is widely used; however, it presents limitations such as food versus fuel conflict, high feedstock cost, and environmental concerns, necessitating a shift toward waste-based bioethanol. This review focuses on the chemical composition of agricultural residues, including cellulose, hemicellulose, and lignin, which influence fermentable sugar production. Various pretreatment strategies, including physical, chemical, biological, and combined methods, are discussed to enhance biomass conversion efficiency. Fermentation approaches involving conventional and non-conventional microbial strains are also highlighted. Sustainability aspects such as reduced greenhouse gas emissions, waste valorization, and contribution to circular bioeconomy are emphasized. Additionally, bioethanol yield and cost considerations are discussed, with yields ranging from 220-310 L/ton based on dry lignocellulosic biomass under optimized processing conditions which may vary based on feedstock composition, pretreatment efficiency, enzymatic hydrolysis, and fermentation conditions. Despite challenges such as lignin recalcitrance and process costs, advancements in biorefineries and metabolic engineering offer significant potential for sustainable bioethanol production.