Comparison between utilization methods of biogas from polylactic acid through life cycle analysis

Abstract

A life cycle analysis was conducted to estimate the life cycle CO₂ emission for each PLA recycling method, including a system in which polylactic acid (PLA), a biodegradable plastic, is hydrolyzed and then converted to biogas at a sewage treatment plant. The functional unit was defined as "provision and processing of 20 t-dry beverage cups for events," and it was assumed that the raw material for PLA was corn, which was made into PLA resin in the United States and then imported to Japan. As a result, the scenario in which PLA is treated by sewage digestion after dissolution and biogas is introduced into the urban gas pipeline resulted in the lowest GHG emissions. In addition, the urban gas substitution effect was larger than the electricity substitution effect, and the latter tended to be superior even when GHG emissions associated with gas purification were taken into account. The scenario in which CO₂ in biogas is converted to methane gas by methanation and then purified and introduced into the urban gas pipeline consumes a large amount of electricity to obtain the hydrogen needed for methanation by electrolysis, resulting in large GHG emissions.