Life cycle assessment of lignocellulosic bioethanol: Environmental impacts and energy balance

A high number of life cycle assessment of the production of lignocellulosic bioethanol have been published to evaluate or to demonstrate its environmental benefits, in most of the cases the comparison of results and conclusions is difficult due the different methodological approaches of the analysts. The aim of this review is to synthesize and to analyze the available and updated information concerning to life cycle assessment (LCA) of lignocellulosic bioethanol and to compare its environmental impacts with conventional fossil fuels and the first-generation bioethanol. This review analyzes more than one hundred case studies reported in the last decade, whose main focus were the energy balance and the green house gas (GHG) emissions, considering the methods of analysis, assumptions and the most used impact categories. The studies showed a clear reduction in GHG emissions and ozone layer depletion, while results in other impact categories, such as acidification, eutrophication, human health and photochemical smog showed to be positively or negatively affected. The LCA results of the bioethanol production are highly influenced by the bioethanol proportion in the gasoline–bioethanol blend and by the source of raw material: GHG emissions reduction is less than 10% for E10 blend and higher than 40% for E85 and upper blends. When comparing raw materials for E100 blend, the highest GHG reduction per distance traveled were obtained for agricultural residues, with reductions between 82 and 91%. In the case of switchgrass and wood, the reduction values were between 53–93% and 50–62%, respectively. Most of the reviewed studies found that lignocellulosic bioethanol production is energetically sustainable, being this result dependent on the possibility of using by-products as fuel in a cogeneration system. It has been shown that lignocellulosic bioethanol have lower impacts in most of the categories and a positive energy balance compared with first generation bioethanol and gasoline.