Hydrothermal Carbonization of Lignin from Black Liquor to Enhance Biomethane Yield Through One-Pot Reaction

Abstract

Improper disposal of black liquor constitutes a significant factor contributing to environmental pollution within the pulp and paper industry. Black liquor, rich in organic matter, holds the potential for biogas production through anaerobic digestion (AD). However, the presence of lignin in black liquor poses a significant challenge to its anaerobic conversion process. In this study, we applied hydrothermal carbonization (HTC) to convert alkali lignin (AL) in black liquor into two different carbon materials, namely lignin hydrochar (LHC) and carbon quantum dots (CQDs). The characteristics of LHC and CQDs were characterized, and their impact on the AD process was evaluated. The results revealed that high concentrations of AL had a significant inhibitory impact on the methane production process. Nonetheless, the conversion of AL to LHC and CQDs, and subsequently addition of the two carbon materials into the AD system could mitigate the negative influence of AL, and obtained a significant biomethane enhancement (26.16% and 17.91%) at 3 g/L, respectively. Moreover, electrochemical tests and excitation emission matrix (EEM) revealed that CQDs have the ability to improve the conductivity and redox ability of AD systems, while LHC helps to enhance the redox capacity of the AD system and promote the production of humic acid-like organic compounds. Furthermore, microbial community and predicted functional gene analysis further showed that LHC and CQDs increased the abundance of microorganisms related to hydrolytic, acidifying, and direct interspecies electron transfer (DIET), and hence enhanced three methanogenic pathways.

Keywords: lignin, Anaerobic Digestion, carbon quantum dots, Lignin hydrochar, Direct interspecies electron transfer, Biogas