Life cycle assessments of biofuel production from beach-cast seaweed by pyrolysis and hydrothermal liquefaction

Macroalgae blooms have been observed along the coastal zones of the Baltic Sea in recent decades, possibly as a result of global climate change. Excess algae biomass washed ashore the beaches reduces their attractiveness for recreational activities, produces greenhouse gases, and causes secondary pollution. To assess the most promising technology for processing excess beach-cast seaweed biomass into liquid biofuel, researchers conducted an inventory and a life cycle assessment analysis (LCA) of two thermochemical technologies: hydrothermal liquefaction (HTL) and pyrolysis. The resulting liquid fuels are expected to be used as heavy fuel oil (HFO) or fuel oil grade 6 in accordance with ASTM D396 in a mixture with HFO from fossil sources. The production of HFO from fossil sources was used as a basic comparison scenario. If one considers the possibility of replacing part of fossil hydrocarbons with synthetic fuels from seaweed biomass, the most climate-neutral would be Ulva sp. pyrolysis (GWP100 884.3 kg CO2-Eq per 1 Mg of fuel), but HTL would have GWP100 only for 9.6 % higher (969.6 kg CO₂-Eq per 1 Mg of fuel). Environmental and climatic impacts of pyrolysis and HTL are very sensitive to the type of electricity used, so shifting from a traditional electricity source to wind energy leads to GWP100 decreasing to a level of 838 and 628 kg CO2-Eq per 1 Mg of fuel for pyrolysis and HTL, respectively, and HTL becoming a technology with less environmental impact. In baseline scenario the ozone depletion potential for the two processes under consideration is almost equal (difference is only 2.4 %). HTL is more sustainable in comparison with pyrolysis in term of human toxicity (HTL potential is 1.6 times lower) and terrestrial acidification (HTL potential is 1.9 times lower).