Hydrothermal pretreatment of microalgae for production of pyrolytic bio-oil with a low nitrogen content

Microalgae can be converted to an energy-dense bio-oil via pyrolysis; however, the relatively high nitrogen content of this bio-oil presents a challenge for its direct use as fuels. Therefore, hydrothermal pretreatment was employed to reduce the N content in Nannochloropsis oculata feedstock by removing proteins without requiring significant energy inputs. The effects of reaction conditions on the yield and composition of pretreated algae were investigated by varying the temperature (150–225 °C) and reaction time (10–60 min). Compared with untreated algae, pretreated samples had higher carbon contents and enhanced heating values under all reaction conditions and 6–42% lower N contents at 200−225 °C for 30–60 min. The pyrolytic bio-oil from pretreated algae contained less N-containing compounds than that from untreated samples and the bio-oil contained mainly (44.9% GC–MS peak area) long-chain fatty acids (C14−C18) which can be more readily converted into hydrocarbon fuels in the presence of simple catalysts.

► Hydrothermal pretreatment was employed to reduce N content in algal feedstock. ► Pretreated algae had higher carbon contents and enhanced heating values. ► Pretreated algae had lower N contents at 200−225 °C for 30–60 min. ► More than 73% fatty acids were retained in the pretreated algae. ► Pyrolytic bio-oil of pretreated algae has less N-containing compounds.