Bio-oil production from hydrothermal liquefaction of high-protein high-ash microalgae including wild Cyanobacteria sp. and cultivated Bacillariophyta sp.

Decomposition behavior of two kinds of high-protein high-ash microalgae, including wild Cyanobacteria sp. and cultivated Bacillariophyta sp., was examined via hydrothermal liquefaction (HTL). Physicochemical properties of bio-oil were extensively investigated through various techniques. Special attention was paid on transfer behavior of elements (i.e. C and N) during HTL process. A maximum bio-oil yield of 21.10 wt% (Cyanobacteria sp.) and 18.21 wt% (Bacillariophyta sp.) on dry basis was achieved at 325 °C for 45-60 min, accompanying with the maximum (51.38% and 48.76%) of energy recovered in bio-oil. Although high HHV (33.87-36.51 MJ kg−1) and high H/C ratio (1.37-1.62) were estimated for bio-oil products, high N/C ratio (0.06-0.09) and nitrogen content (5.31-7.50%) negatively affected its quality. GC-MS analysis combining with FTIR and NMR detection revealed that components and functional structure of bio-oils were greatly distinguished from other algae bio-oil. TG analysis showed approximate 72.32-75.79% of bio-oils was distilled in the range of 200-550 °C. Large fraction of carbon and small fraction of nitrogen were recovered in bio-oils, together with high portion of nitrogen was enriched into aqueous liquid. This study demonstrated the feasibility of applying HTL to produce bio-oil from high-protein high-ash microalgae, and the findings on bio-oil properties and transfer behavior of carbon and nitrogen supplied useful information for downstream utilization.