Hydrochar stability: understanding the role of moisture, time and temperature in its physiochemical changes

Abstract

Limited information is available about potential physicochemical changes that can occur in hydrochar post-production, e.g. during drying and storage. Understanding these changes is crucial not just for shaping future research plans, but also for future practical applications. Here we studied the effect of moisture (69.2% and 2.4%) and three storage temperatures (− 18, 4, and 20 °C) over a year on selected organic and inorganic compounds in hydrochar produced from the Hydrothermal carbonization (HTC) of digested cow manure. Comparison of the control wet hydrochars (WHs) and dry hydrochars (DHs) showed changes in organic compound composition due to drying. Overall, the total amount of the selected organic compounds was notably greater in WH (15.2 g kg⁻¹ DM) compared to DH (11.8 g kg⁻¹ DM), with variations observed in individual compound concentrations. Drying, however, had no significant influence on the identified inorganic compounds. Storage caused significant changes in both WH and DH, particularly in organic compounds after 12 weeks. Sugars (2–sevenfold), acids (36–371%), and aromatics (58–120%) in stored samples at week 52 were significantly higher than their control values. Changes in the inorganic elements (e.g., Co, K, Mg, Mn, P, S, Sr, and Zn) occurred faster in WH, with significant differences starting from week 1 compared to their control values, while DH showed fewer changes. Based on these changes in both organic and inorganic content, we recommend the optimal storage conditions for future HTC studies to preserve hydrochar properties. Finally, we discussed potential applications for stored hydrochars, with DH showing greater stability, especially at − 18 °C, making it suitable for various applications.

Highlights

•	Drying of hydrochar and storage time affected concentrations of acids, aromatics, and sugars significantly
•	Changes were observed at all storage temperatures—dried hydrochar stored at − 18 °C exhibited higher stability.
•	Recommended storage conditions could be used for upcoming HTC research and hydrochar applications.

Keywords

Hydrochar / Stability / Organic compounds / Inorganic compounds / Storage / Application

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Funding

German Academic Exchange Service; Leibniz-Institut für Agrartechnik und Bio?konomie e.V. (ATB) (3461)