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<p>This study investigates the effects of acid deashing on the hydrothermal carbonization (HTC) of sewage sludge (SS) and the resulting fuel characteristics of the produced hydrochar. SS was pre-treated using a mixed acid solution containing 5&#xa0;M hydrochloric acid (HCl) and 5&#xa0;M hydrofluoric acid (HF), resulting in significant ash removal (65.93%) and consequently improved HTC solid yield—from 72.56 to 89.50%. The reduction in inorganic content was also accompanied by an increase in total organic carbon (TOC) in the liquid phase (from 24.10 to 30.60&#xa0;g/L), suggesting that inorganic removal from the SS inhibits the pathway toward gas production under hydrothermal conditions. The hydrochar produced from the deashed sludge (DA-HC) exhibited markedly higher carbon content and enhanced physicochemical properties compared to the hydrochar derived from the untreated sewage sludge (SS-HC). Structural analysis revealed increased surface area and porosity post-deashing, which contributed to better combustion behavior. The higher heating value (HHV) of DA-HC of 22.25&#xa0;MJ/kg, compared to 17.03&#xa0;MJ/kg of SS-HC, is comparable to medium–high calorific coal. The thermogravimetric analysis further revealed enhanced ignition and burnout characteristics and thermal reactivity, while kinetic modeling via Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods confirmed reduced activation energies for DA-HC, confirming its enhanced combustion performance.</p> Graphical Abstract <p></p>

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Effects of acid deashing on the hydrothermal carbonization of sewage sludge and the properties and combustion kinetics of produced hydrochar

  • 期刊论文

  • 发表日期：

    2026年8月1日

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  • Siti Zaharah Roslan,
  • Muzakkir Mohammad Zainol,
  • Xiangyi Long,
  • Syed Shatir A. Syed-Hassan

摘要

This study investigates the effects of acid deashing on the hydrothermal carbonization (HTC) of sewage sludge (SS) and the resulting fuel characteristics of the produced hydrochar. SS was pre-treated using a mixed acid solution containing 5 M hydrochloric acid (HCl) and 5 M hydrofluoric acid (HF), resulting in significant ash removal (65.93%) and consequently improved HTC solid yield—from 72.56 to 89.50%. The reduction in inorganic content was also accompanied by an increase in total organic carbon (TOC) in the liquid phase (from 24.10 to 30.60 g/L), suggesting that inorganic removal from the SS inhibits the pathway toward gas production under hydrothermal conditions. The hydrochar produced from the deashed sludge (DA-HC) exhibited markedly higher carbon content and enhanced physicochemical properties compared to the hydrochar derived from the untreated sewage sludge (SS-HC). Structural analysis revealed increased surface area and porosity post-deashing, which contributed to better combustion behavior. The higher heating value (HHV) of DA-HC of 22.25 MJ/kg, compared to 17.03 MJ/kg of SS-HC, is comparable to medium–high calorific coal. The thermogravimetric analysis further revealed enhanced ignition and burnout characteristics and thermal reactivity, while kinetic modeling via Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods confirmed reduced activation energies for DA-HC, confirming its enhanced combustion performance.

Graphical Abstract