Biomethane from Short Rotation Forestry and Microalgal Open Ponds: System Modeling and Life Cycle Assessment

•CO2 injection is the main source of impact of biomethane production from microalgae.•Gasification of poplar chips could be optimized by increasing heat recovery methods.•Absorption by triethylene glycol and MEA provides the best removal of CO2 and water.•Integration of microalgae cultivation in wastewater plants reduces fertilizers demand.•Re-injection of CO2 emitted during biogas upgrading is recommended. Gasification of Short Rotation Forestry (SRF) poplar wood chips and anaerobic digestion of the microalga Chlorella vulgaris have been analyzed as alternative supply chains for the production of biomethane. Life Cycle Assessment (LCA) was performed from the biomass cultivation to the upgrading stages. Process simulation of gasification and upgrading was carried out, environmental impacts of the entire supply chains have been estimated and discussed. The highest CO2 removal has been reached by absorption on monoethanolamine. Electricity requirements heavily affect the SRF chain, while productions of carbon dioxide and fertilizers are the main sources of impact of the microalgae cultivation. The recycle of non-absorbed fertilizers, as well as integration of microalgae digestion in wastewater plants, are recommended. Capture and re-injection of the CO2 lost during the upgrading stages would result, simultaneously, in an 8.53% reduction of the atmospheric emission, and in a minor demand to promote algal growth.