Microwave-assisted wet torrefaction of microalgae under various acids for coproduction of biochar and sugar

Wet torrefaction is a promising method to converts the high moisture microalgae to biofuel. In this study, the microalgae were torrefied in water or dilute acid solutions with the aid of microwaves irradiation. The reaction temperature and heating time were fixed at 160 °C and 10 min. The effects of sulfuric, phosphorus, and succinic acids on two different microalgae species (Chlorella vulgaris ESP-31 and FSP-E) were investigated. As a result, the disruption of the microalga FSP-E (high protein) was not notable in the acidic solution. The higher heating value of wet torrefied microalga ESP-31 (high carbohydrate) in succinic acid was enhanced up to 40% with at least 45% of energy yield. The use of 0.1 M of phosphorus acid in wet torrefaction produced the highest ash content of 1.61 and 11.60 wt% for microalgae ESP-31 and FSP-E, respectively. Thermogravimetric analysis revealed that the carbohydrate content of microalga ESP-31 has the highest degradation in sulfuric acid solution. In contrast, crystalline cellulose in the microalgae is hardly affected by the wet torrefaction process in low-temperature acidic solution. In terms of liquid product, a maximum glucose extraction of 35.39 g/L occurred with the use of 0.1 M of sulfuric acid. In conclusion, biochar produced using organic acid is desirable as solid fuel, whereas the use of sulfuric acid is more suitable to produce sugar for bioethanol production.