Life cycle assessment of biomass-based ethylene production in Sweden - is gasification or fermentation the environmentally preferable route?

To reduce its environmental impact, the chemical industry is investigating the biomass-based production of chemicals such as ethylene, including fermentation and gasification conversion processes. However, a comprehensive study that compares the environmental impact of these biomass routes is missing. This study assesses and compares a wood gasification with a wood fermentation route to ethylene in Sweden, as well as compares it with the commercialized sugarcane and fossil oil alternatives. The study followed the methodology of life cycle assessment. A cradle-to-gate perspective for the production of 50,000 t ethylene/year was applied, and the following impact categories were investigated: global warming (GWP), acidification (ACP), photochemical ozone creation (POCP), and eutrophication (EP). Biomass acquisition including transport to the gasification plant was the major cause of the gasification route's GWP and POCP, suggesting improvements with regard to fuel source and machine efficiency. NOx emissions from the gasification process had the main share on the ACP and EP. The comparison of the gasification with a wood and a sugarcane fermentation route showed a lower impact for the gasification route. Among other things, this is caused by high emissions from transport and cultivation for the sugarcane route and high emissions from enzyme and ethanol production for the wood fermentation route. The results were less distinct for a comparison of the gasification with a fossil-based route. Fossil-based ethylene production was found to be preferable for the ACP and the EP, but less preferable for the GWP and POCP. However, it needs to be considered that the fossil route has been optimized for decades and is still ahead of the gasification and other biomass routes. The study shows that a gasification-based production of ethylene could outperform a fermentation-based one; however, further investigations are recommended, given the state of development of the investigated biomass routes. Moreover, based on the limited availability of biomass, further investigations into economical and ecological restrictions are of importance.