Life Cycle Assessment of Biomethane from Offshore-Cultivated Seaweed

Algae are a promising source of industrial biomass for the future. In order to assess if aquacultured seaweed (macroalgae) could be considered an environmentally friendly source of biomass for bioenergy, life cycle assessments were performed for European countries, comparing methane as a biofuel from the anaerobic digestion (A) of whole seaweeds, (B) of alginate extraction residues, and (C) natural gas as a fossil fuel reference. These results clarify that the sources of electricity and energy used to heat the anaerobic digesters have an important impact. Recycling of materials and use of greenhouses at the nursery stage also allow environmental improvements for system (A). Ecodesign can make algal biomethane competitive in several categories compared to natural gas: a decrease of 21.9% and 54.2% in greenhouse gas (GHG) emissions and 58.6% and 68.7% in fossil depletion for systems (A) and (B), respectively, decrease in ozone depletion, and last but not least, improvement in the marine eutrophication index for system (A). For system (B), benefits are more arguable and dependent on the allocation. To conclude, seaweed could become competitive with terrestrial feedstock for biofuel production in the near future.