The characteristics and evolution of nitrogen in bio-oil from microalgae pyrolysis in molten salt

Microalgae pyrolysis in molten salt is a promising way to produce high-quality bio-oil. This study provides deep insights into the characteristics of nitrogen-containing compounds (NCCs) during protein-rich spirulina platensis pyrolysis in a ternary eutectic carbonate (Li2CO3-Na2CO3-K2CO3) at different temperatures (450–600 °C). The results indicate that the molten carbonate significantly reduces the NCCs contents in bio-oil and heavy bio-oil by 25.89 % and 62.38 % respectively, which is due to the intensified removal of nitrogenous functional groups (CN, CN of the primary amide, and CN) in NCCs. The molten carbonate also intensifies the direct cleavage of amino acids and the conversion of amides/amines to form nitrile compounds but inhibits the dehydration of hexadecanamide to form hexadecanenitrile in bio-oil light components. Concerning the heavy components in bio-oil, molten carbonate intensifies the depolymerization of NCCs and plays an important role in intensifying the cyclane ring opening, double bond breaking, and cleavage of a benzene ring of NCCs. Finally, the possible pathways for nitrogen evolution in bio-oil are proposed, which can provide guidance for the high-quality bio-oil production by molten salt pyrolysis of algae.