Hydrothermal carbonization of poplar sawdust: Effect of deep eutectic solvents on hydrochar's physicochemical properties

In order to enhance carbonization, deep eutectic solvents (DES) as a green solvents has been utilized to remove partial lignin in poplar sawdust (PS) before hydrothermal carbonization (HTC). Results showed that DES pretreatment has a notable role in increasing degree of carbonization for hydrochar. The key role was that after DES pretreatment, more cracks can be found on the surface of PS, leading to a higher temperature on the interior of crack to promote cellulose hydrolysis and dehydration and enhance contact between aqueous phase with PS. After DES pretreatment, the cellulose and lignin still could be found in hydrochar. The crack formation on the surface of PS changed the HTC style for PS and accelerated the hydrolysis and carbonization. DES pretreatment was an effective method to increase the degree of carbonization for hydrochar during HTC. It would pave a new way to utilization of hydrochar for its high degree of carbonization.