Toor, M., Department of Environmental Science and Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana  125 001, India; Kumar, S.S., Department of Environmental Science and Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana  125 001, India;  Bishnoi, N.R., Department of Environmental Science and Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana  125 001, India; Mathimani, T., Department of Energy and Environment, National Institute of Technology, Tiruchirappalli, Tamil Nadu  620 015, India; Rajendran, K., Department of Environmental Science, SRM University-AP, Amaravati, Andhra Pradesh  522502, India; Pugazhendhi, A., Innovative Green Product Synthesis and Renewable Environment Development Research Group, Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Viet Nam

Lignocellulosic ethanol has been proposed as a green alternative to fossil fuels for many decades. However, commercialization of lignocellulosic ethanol faces major hurdles including pretreatment, efficient sugar release and fermentation. Several processes were developed to overcome these challenges e.g. simultaneous saccharification and fermentation (SSF). This review highlights the various ethanol production processes with their advantages and shortcomings. Recent technologies such as singlepot biorefineries, combined bioprocessing, and bioenergy systems with carbon capture are promising. However, these technologies have a lower technology readiness level (TRL), implying that additional efforts are necessary before being evaluated for commercial availability. Solving energy needs is not only a technological solution and interlinkage of various factors needs to be assessed beyond technology development. © 2019 Elsevier Ltd