Syngas production and heavy metal dynamics during supercritical water gasification of sewage sludge

Highlights

● The highest syngas yield of 10.9 mol/kg with 44.7% H₂ concentration was achieved.

● The distribution of heavy metals with/without alkaline additives was investigated.

● Risk Assessment Code of heavy metals after SCWG reduced to less than 1%.

● Possible reaction pathways of heavy metals during SCWG of sludge were proposed.

Abstract

The rising production of sewage sludge, characterized by high organic content and excessive heavy metals, necessitates an effective treatment method. This study investigated the production of syngas and the migration and transformation behavior of heavy metals such as Zn, Ni, Cr, Cu, and As during supercritical water gasification (SCWG) of sewage sludge. The experiments were conducted without or with alkaline additives at temperatures between 380 to 420 °C and retention time from 15 to 60 min. The results revealed that the highest syngas yield reached 10.9 mol/kg with an H₂ concentration of 44.7% at 420 °C and 60 min. In this process, heavy metals were effectively immobilized and converted into a more stable form, whereas higher temperatures and longer retention time enhanced this effect. The introduction of alkaline additives (NaOH, KOH, Ca(OH)₂, Na₂CO₃, and K₂CO₃) led to the redistribution of heavy metals, further promoting the stabilization of Zn, Cr, and Cu. An environmental risk assessment showed that SCWG could significantly lower the risk associated with heavy metals to a low or negligible level.

Graphical abstract

Keywords

Sludge / Heavy metal / Immobilization / Supercritical water gasification / Environmental risk assessment

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Credit Author Statement

Mi Yan: Conceptualization, Writing – review and editing, Methodology. Shuai Liu: Investigation, Formal analysis, Writing – original draft. Haihua Zhang: Resources. Rendong Zheng: Resources. Jintao Cui: Investigation, Formal analysis. Dan Wang: Investigation, Data curation. Dicka Ar Rahim: Validation, Data curation. Ekkachai Kanchanatip: Methodology, Writing – review and editing, Data curation, Supervision.

Acknowledgements

The authors greatly appreciate the financial support from the National Natural Science Foundation of China (No. 52250410339) and the “Pioneer” and “Leading Goose” R&D Program of Zhejiang (China) (No. 2024C03129).

Conflict of Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

RIGHTS & PERMISSIONS

2024 Higher Education Press 2024