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

Abstract

● 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.

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

References

[1] Adar E, Ince M, Bilgili M S. (2020). Supercritical water gasification of sewage sludge by continuous flow tubular reactor: a pilot scale study. Chemical Engineering Journal, 391: 123499

[2] Bairq Z A S, Li R D, Li Y L, Gao H X, Sema T, Teng W C, Kumar S, Liang Z W. (2018). New advancement perspectives of chloride additives on enhanced heavy metals removal and phosphorus fixation during thermal processing of sewage sludge. Journal of Cleaner Production, 188: 185–194

[3] Bühler W, Dinjus E, Ederer H J, Kruse A, Mas C. (2002). Ionic reactions and pyrolysis of glycerol as competing reaction pathways in near- and supercritical water. Journal of Supercritical Fluids, 22(1): 37–53

[4] Chen G, Andries J, Luo Z, Spliethoff H. (2003). Biomass pyrolysis/gasification for product gas production: the overall investigation of parametric effects. Energy Conversion and Management, 44(11): 1875–1884

[5] Chen G, Yang X, Chen S, Dong Y, Cui L, Zhang Y, Wang P, Zhao X, Ma C. (2017). Transformation of heavy metals in lignite during supercritical water gasification. Applied Energy, 187: 272–280

[6] Chen Y, Yi L, Wei W, Jin H, Guo L. (2022). Hydrogen production by sewage sludge gasification in supercritical water with high heating rate batch reactor. Energy, 238: 121740

[7] Cui J, Wen X, Chen F, Yan M, Hantoko D. (2023). Migration of nitrogen and phosphorus during supercritical water gasification of sewage sludge: effect of alkaline additives. Fuel Processing Technology, 245: 107727

[8] Ding A, Zhang R R, Ngo H H, He X, Ma J, Nan J, Li G B. (2021). Life cycle assessment of sewage sludge treatment and disposal based on nutrient and energy recovery: a review. Science of the Total Environment, 769: 144451

[9] Fang Y R, Li S, Zhang Y, Xie G H. (2019). Spatio-temporal distribution of sewage sludge, its methane production potential, and a greenhouse gas emissions analysis. Journal of Cleaner Production, 238: 117895

[10] Gabarrón M, Zornoza R, Martínez-Martínez S, Muñoz V A, Faz A, Acosta J A. (2019). Effect of land use and soil properties in the feasibility of two sequential extraction procedures for metals fractionation. Chemosphere, 218: 266–272

[11] Hantoko D, Antoni E, Kanchanatip M, Yan Z, Weng Z, Gao Y. (2019). Assessment of sewage sludge gasification in supercritical water for H₂-rich syngas production. Process Safety and Environmental Protection, 131: 63–72

[12] Jia J, Bai J, Xiao R, Tian S, Wang D, Wang W, Zhang G, Cui H, Zhao Q. (2022). Fractionation, source, and ecological risk assessment of heavy metals in cropland soils across a 100-year reclamation chronosequence in an estuary, South China. Science of the Total Environment, 807: 151725

[13] Lee C S, Conradie A V, Lester E. (2021). Review of supercritical water gasification with lignocellulosic real biomass as the feedstocks: process parameters, biomass composition, catalyst development, reactor design and its challenges. Chemical Engineering Journal, 415: 128837

[14] Leng L, Leng S, Chen J, Yuan X, Li J, Li K, Wang Y, Zhou W. (2018). The migration and transformation behavior of heavy metals during co-liquefaction of municipal sewage sludge and lignocellulosic biomass. Bioresource Technology, 259: 156–163

[15] Li B, Ding S, Fan H, Ren Y. (2021). Experimental investigation into the effect of pyrolysis on chemical forms of heavy metals in sewage sludge biochar (SSB), with brief ecological risk assessment. Materials, 14(2): 447

[16] Li C X, Li J, Xie S Y, Zhang G Y, Pan L J, Wang R M, Wang G, Pan X F, Wang Y, Angelidaki I. (2022a). Enhancement of heavy metal immobilization in sewage sludge biochar by combining alkaline hydrothermal treatment and pyrolysis. Journal of Cleaner Production, 369: 133325

[17] Li D, Shan R, Jiang L, Gu J, Zhang Y, Yuan H, Chen Y. (2022b). A review on the migration and transformation of heavy metals in the process of sludge pyrolysis. Resources, Conservation and Recycling, 185: 106452

[18] Li H, Jin Y, Mahar R, Wang Z, Nie Y. (2008). Effects and model of alkaline waste activated sludge treatment. Bioresource Technology, 99(11): 5140–5144

[19] Li L, Cao W, Wang G, Peng P, Liu S, Jin H, Wei W, Guo L. (2022c). Experimental and kinetic study of heavy metals transformation in supercritical water gasification of oily sludge. Journal of Cleaner Production, 373: 133898

[20] Li L, Li X, Cao W. (2023). An experimental and thermodynamic equilibrium investigation of heavy metals transformation in supercritical water gasification of oily sludge. Journal of Environmental Management, 348: 119365

[21] Li Q, Zhong Z, Du H, Zheng X, Zhang B, Jin B. (2022d). Co-pyrolysis of sludge and kaolin/zeolite in a rotary kiln: analysis of stabilizing heavy metals. Frontiers of Environmental Science & Engineering, 16(7): 85

[22] Liang Y, Xu D, Feng P, Hao B, Guo Y, Wang S, Klemes J J. (2021). Municipal sewage sludge incineration and its air pollution control. Journal of Cleaner Production, 295: 126456

[23] Lin Y, Wang D, Wu S, Wang C. (2009). Alkali pretreatment enhances biogas production in the anaerobic digestion of pulp and paper sludge. Journal of Hazardous Materials, 170(1): 366–373

[24] Liu L, Huang L, Huang R, Lin H, Wang D. (2021a). Immobilization of heavy metals in biochar derived from co-pyrolysis of sewage sludge and calcium sulfate. Journal of Hazardous Materials, 403: 123648

[25] Liu M, Han X, Liu C Q, Guo L, Ding H, Lang Y. (2021b). Differences in the spectroscopic characteristics of wetland dissolved organic matter binding with Fe³⁺, Cu²⁺, Cd²⁺, Cr³⁺ and Zn²⁺. Science of the Total Environment, 800: 149476

[26] Lu Y F, Allen H E. (2002). Characterization of copper complexation with natural dissolved organic matter (DOM)-link to acidic moieties of DOM and competition by Ca and Mg. Water Research, 36(20): 5083–5101

[27] Muangrat R, Onwudili J A, Williams P T. (2010). Influence of alkali catalysts on the production of hydrogen-rich gas from the hydrothermal gasification of food processing waste. Applied Catalysis B: Environmental, 100(3−4): 440–449

[28] Nanda S, Dalai A K, Kozinski J A. (2016). Supercritical water gasification of timothy grass as an energy crop in the presence of alkali carbonate and hydroxide catalysts. Biomass and Bioenergy, 95: 378–387

[29] Okolie J A, Nanda S, Dalai A K, Kozinski J A. (2021). Techno-economic evaluation and sensitivity analysis of a conceptual design for supercritical water gasification of soybean straw to produce hydrogen. Bioresource Technology, 331: 125005

[30] Quan L M, Kamyab H, Yuzir A, Ashokkumar V, Hosseini S E, Balasubramanian B, Kirpichnikova I. (2022). Review of the application of gasification and combustion technology and waste-to-energy technologies in sewage sludge treatment. Fuel, 316: 123199

[31] Su W, Zhao M, Xing Y, Ma H, Liu P, Li X, Zhang H, Wu Y, Xia C. (2022). Supercritical water gasification of hyperaccumulators for hydrogen production and heavy metal immobilization with alkali metal catalysts. Environmental Research, 214: 114093

[32] Sun J, Feng H, Kou J, Jin H, Chen Y, Guo L. (2021). Experimental investigation on carbon microstructure for coal gasification in supercritical water. Fuel, 306: 121675

[33] Tang W, Sun L, Shu L, Wang C. (2020). Evaluating heavy metal contamination of riverine sediment cores in different land-use areas. Frontiers of Environmental Science & Engineering, 14(6): 104

[34] Wang C, Wu C, Hornung U, Zhu W, Dahmen N. (2021a). Suppression of tar and char formation in supercritical water gasification of sewage sludge by additive addition. Chemosphere, 262: 128412

[35] Wang Y, Tang Y, Li R, Guo X, Hurley J P, Finkelman R B. (2021b). Measurements of the leachability of potentially hazardous trace elements from solid coal gasification wastes in China. Science of the Total Environment, 759: 143463

[36] Wang Y, Zhu Y, Liu Z, Wang L, Xu D, Fang C, Wang S. (2019). Catalytic performances of Ni-based catalysts on supercritical water gasification of phenol solution and coal-gasification wastewater. International Journal of Hydrogen Energy, 44(7): 3470–3480

[37] Wei N, Xu D H, Hao B T, Guo S W, Guo Y, Wang S Z. (2021). Chemical reactions of organic compounds in supercritical water gasification and oxidation. Water Research, 190: 116634

[38] Xu D, Liu L, Wei N, Guo Y, Wang S, Wu Z, Duan P. (2019). Catalytic supercritical water gasification of aqueous phase directly derived from microalgae hydrothermal liquefaction. International Journal of Hydrogen Energy, 44(48): 26181–26192

[39] Yan M, Cui J, Li T, Feng H, Hantoko D, Kanchanatip E. (2023). Transformation and distribution of nitrogen and phosphorus in sewage sludge during supercritical water gasification. Fuel, 332: 125918

[40] Yan M, Feng H Y, Zheng R D, Yu C M, Hantoko D, Zhou Z H, Zhang Y, Kanchanatip E. (2021). Sulfur conversion and distribution during supercritical water gasification of sewage sludge. Journal of the Energy Institute, 95: 61–68

[41] Yang G, Zhang G, Wang H. (2015). Current state of sludge production, management, treatment and disposal in China. Water Research, 78: 60–73

[42] Yu J, Sun L, Xiang J, Hu S, Su S, Qiu J. (2012). Vaporization of heavy metals during thermal treatment of model solid waste in a fluidized bed incinerator. Chemosphere, 86(11): 1122–1126

[43] Zhou B, Wu W, Li X, Dai R, Wang Z. (2023). A state-of-the-art review on anaerobic digestion of sewage sludge based on microbial abundance: correlations among microbiota, performance and process parameters. Critical Reviews in Environmental Science and Technology, 54(15): 1093–1116

[44] Zhou W Z, Chen X G, Wang Y, Tuersun N, Ismail M, Cheng C, Li Z N, Song Q, Wang Y Q, Ma C Y. (2021). Anaerobic co-digestion of textile dyeing sludge: digestion efficiency and heavy metal stability. Science of the Total Environment, 801: 149722

RIGHTS & PERMISSIONS

Higher Education Press 2024