Valorization of the microalgae fixing CO2 from flue gas by co-hydrothermal liquefaction with high-protein microalgae: Denitrogenation of bio-oil by ash and high energy recovery

Microalgae have significant potential for carbon sequestration due to their high photosynthesis efficiency. In this study, co-hydrothermal liquefaction (co-HTL) of these high-ash microalgae (Scenedesmus) and the high-protein microalgae (Spirulina) was carried out to obtain the high quality and high energy recovery bio-oil. The results show that co-HTL facilitates the conversion of organic matter in microalgae to bio-oil and gas products. Compared with individual Spirulina HTL, the contents of carbon and hydrogen in bio-oil derived from 7:3 (Spirulina to Scenedesmus mass ratio, BO-7:3) co-HTL increase to 74.09% and 8.83%, and the nitrogen content is significantly reduced to 6.67%. Besides, BO-7:3 has a maximum energy recovery of 94.64%. The relative contents of nitrogen-heterocyclic compounds and amides/amines in bio-oil derived from co-HTL decrease, and that of hydrocarbons increase. The actual value of ammonium ion conversion of nitrogen in microalgae is greater than the theoretical value, indicating that ash can effectively promote the conversion of nitrogen in microalgae to the aqueous product. The DTG curve of BO-7:3 has obvious peaks in low-temperature oxidation stage and high -temperature oxidation stage. Besides, co-HTL can effectively reduce the release of nitrogen-containing gas pol-lutants, which is beneficial to the application of microalgae-derived bio-oil.