Reducing nitrogen content in bio-oil from hydrothermal liquefaction of microalgae by using activated carbon-pretreated aqueous phase as the solvent

Abstract

Hydrothermal liquefaction (HTL) is a promising thermochemical technology for converting wet biomass into liquid fuels, with aqueous phase (AP) as one main by-product. The strategy for recycling AP into the HTL process is extensively studied due to its potential to enhance bio-oil production. However, an increased concentration of nitrogen elements in the bio-oil has been reported in recycling cases. In the current work, the aqueous phase was first pretreated with activated carbon to solve this problem before recycling it back into the liquefaction system. Results showed that nitrogen content was decreased, and bio-oil quality was improved due to pre-adsorption. The optimal pretreatment conditions were 160 mg activated carbon and 40 min adsorption time. Although the bio-oil yield obtained from the recycling HTL with the pretreated solvent (35.75 wt%) was lower than the untreated case (39.77 wt%), the nitrogen content of the bio-oil was also reduced up to 39% simultaneously. Moreover, the bio-oil's higher-heating value (HHV) was increased slightly when using the pretreated solvent. And the content of n-hexadecanoic acid in the bio-oil was expanded due to the decline of N-based compounds. This study concluded that liquefaction of biomass with the activated carbon-pretreated AP solvent leading to high conversion efficiency and bio-oil quality would be more favorable from an economic point of view.