Energy efficiency and environmental performance of bioethanol production from sweet sorghum stem based on life cycle analysis

• Bioethanol based on sweet sorghum stem can produce positive net energy.
• Most negative environmental impacts were human toxicity and eutrophication.
• Main factors contributed to fossil energy consumption and emissions were analyzed.
• Vinasse treatment and inventory allocation exerted significant effects on results.
• Key points to better energy efficiency and environmental performance were discussed.

Life cycle analysis method was used to evaluate the energy efficiency and environmental performance of bioethanol production from sweet sorghum stem in China. The scope covers three units, including plant cultivation, feedstock transport, and bioethanol conversion. Results show that the net energy ratio was 1.56 and the net energy gain was 8.37 MJ/L. Human toxicity was identified as the most significant negative environmental impact, followed by eutrophication and acidification. Steam generation in the bioethanol conversion unit contributed 82.28% and 48.26% to total human toxicity and acidification potential, respectively. Fertilizers loss from farmland represented 67.23% of total eutrophication potential. The results were significantly affected by the inventory allocation methods, vinasse reusing approaches, and feedstock yields. Reusing vinasse as fuel for steam generation and better cultivation practice to control fertilizer loss could significantly contribute to enhance the energy efficiency and environmental performance of bioethanol production from sweet sorghum stem.