Hydrothermal carbonization of lignin from black liquor enhances biomethane yield

Shuaishuai Ma, Hongliang Wang, Xiurong Ren, Ziyu Wang, Xin Gao, Chuanfei Bian, Luyao Wei, Shanshuai Chen, Zongjun Cui and Wanbin Zhu

Renewable and Sustainable Energy Reviews, 2025, vol. 211, issue C

Abstract: Improper disposal of black liquor significantly contributes to environmental pollution in the pulping industry. While rich in organic matter, black liquor's high lignin content complicates its anaerobic digestion (AD) for biogas production. In this study, hydrothermal carbonization was applied to convert alkali lignin (AL) in black liquor into two different carbon materials, namely lignin hydrochar (LHC) and carbon quantum dots (CQDs). These materials were characterized, and their impact on AD was evaluated. The results revealed that high concentrations of AL significantly inhibited methane production, but converting AL into LHC and CQDs and adding them to the AD system at 3 g/L mitigated these inhibitory effects, enhancing biomethane production by 26.16 % and 17.91 %, respectively. Both materials improved soluble chemical oxygen demand removal, increased coenzyme F420 activity, and boosted adenosine triphosphate concentration. Moreover, electrochemical tests and excitation emission matrix analyses revealed that CQDs enhanced the conductivity and redox capability of the AD systems, while LHC promoted redox capacity and the production of humic acid-like organic compounds. Further microbial community and predicted functional gene analyses indicated that LHC and CQDs increased the abundance of microorganisms related to hydrolytic, acidifying, and direct interspecies electron transfer, enhancing three methanogenic pathways. This comprehensive study provides a strategy to convert lignin into two additives for biomethane enhancement and offers new insights into efficient black liquor bioconversion.

Keywords: Lignin; Anaerobic digestion; Carbon quantum dots; Lignin hydrochar; Direct interspecies electron transfer; Biogas (search for similar items in EconPapers)
Date: 2025
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DOI: 10.1016/j.rser.2024.115265

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