Hydrothermal liquefaction of macroalgae Alaria esculenta for possible application as source of biofuels

Background : Among the different types of biomass, algae are becoming an interesting source of biofuels due to their higher photosynthetic efficiency and faster growing rates than terrestrial biomass. Currently, much attention is being devoted to microalgae, with less research efforts dedicated to macroalgae. The latter are known to cause disturbances in marine ecosystems as invasive species and as coastal waste at beach areas. Hydrothermal liquefaction is a potential future application to alleviate these problems while simultaneously producing biofuel from the algae that accumulate yearly in the coasts. Aim : This work aims to study the feasibility of macroalgae as feedstock for biofuel via hydrothermal liquefaction, with special focus on maximizing the biocrude oil yield through adjusting several process parameters (i.e. reaction temperature, content of marine salts in the reaction medium). An additional focus was directed to the recovery of nutrients in the aqueous phase for possible nutrient recycling. Methods : Microautoclaves of 10 mL were used to determine the highest biocrude yield at reaction temperatures varying from 330 to 370 °C, with an algae:water ratio of 1:10, a reaction time of 15 min, an initial atmosphere of 20 bar of nitrogen, and a heating rate of 20°C/min. For some experiments, the marine salts naturally present in the macroalgae were washed out to assess their influence in the overall process. HPLC and IC analyses were performed on the aqueous phase, as well as elemental analysis on the biomass feedstock, solid residue and biocrude oil. The gas phase was analysed by GC-WLD, while FTIR was used to characterize the feedstock, biocrude oil and the solid residue. The presence of mineral matter in several process phases was studied by means of ICP-AAS. Results : The experiments performed indicate that, below the critical temperature of water (374 °C), biocrude oil production tends to increase with the temperature, showing variations of around 30% in the yield. Results also indicate that nitrogen can be partially recovered in the aqueous phase, mainly in the form of NH4+, although this recovery appears to be strongly dependent from the temperature. Summary/conclusions : This work shows that a sound characterization of the organic, salt and water content is crucial for a proper assessment of this technology and its mass balances. To the best of our knowledge, no study has yet analysed Alaria esculenta for possible oil production. According to the results presented here, this species can effectively be converted to biocrude oil with yields among the highest currently reported in literature. This study has also shown the possibility of partially recovering nutrients (mainly N) in the water phase, which would have a significant impact on the economics of the process