Hydrothermal carbonization coupling with liquid dimethyl ether extraction pretreatment of sewage sludge: Hydrochar performance improvement and low-nitrogen biocrude production.

ABSTRACT

Hydrothermal carbonization of sewage sludge converts waste into hydrochar; however, the complex organic composition of feedstock limits the product value. A novel process that combines liquid dimethyl ether extraction and hydrothermal carbonization (DE-HTC) was proposed for improving the product value by simultaneously producing biocrude/hydrochar and improving feedstock suitability for thermochemical conversion. Biocrude and hydrochar with a product yield of 2.62% and 55.83% were produced via DE-HTC, respectively. The hydrochar yield increased by 12.65%-29.90% compared to traditional single-step hydrothermal carbonization. The hydrochar energy densification was decreased by 1.16%-10.28%, while hydrochar's energy yield increased by 47%-66%, and it had a more prominent porous structure. By avoiding the decomposition of proteins during thermochemical conversion, the nitrogen content of the biocrude obtained via DE-HTC was only 0.38%. The biocrude was also further qualitatively analyzed. This study provides insights into the efficacy of a novel hydrothermal method with distinct product value advantages over direct hydrothermal carbonization.