Influence of inorganic content on hydrochar quality from high-ash agar extraction waste via hydrothermal carbonization

Andi Firdaus Sudarma (Division of Biosystem Engineering, Faculty of Engineering and Technology, IPB University)
Edy Hartulistiyoso (Division of Biosystem Engineering, Faculty of Engineering and Technology, IPB University)
Yohanes Aris Purwanto (Division of Biosystem Engineering, Faculty of Engineering and Technology, IPB University)
Leopold Oscar Nelwan (Division of Biosystem Engineering, Faculty of Engineering and Technology, IPB University)
Obie Farobie (Division of Biosystem Engineering, Faculty of Engineering and Technology, IPB University)
Nono Darsono (Research Center for Fuel Technology, National Research and Innovation Agency (BRIN))
Khuzaimah Arifin (Research Center for Energy Materials, National Research and Innovation Agency (BRIN))
Maharani Dewi Solikhah (Research Center for Fuel Technology, National Research and Innovation Agency (BRIN))
Tri Yulni (Research Center for Equipment Manufacturing Technology, National Research and Innovation Agency (BRIN))
Imam Hidayat (Department of Mechanical Engineering, Faculty of Engineering, Universitas Mercu Buana)
Muhamad Fitri (Department of Mechanical Engineering, Faculty of Engineering, Universitas Mercu Buana)
Edy Herianto Majlan (Fuel Cell Institute, Universiti Kebangsaan Malaysia)

Sinergi
Vol. 30 No. 3 (2026)

Agar solid waste (ASW) has potential as a feedstock for hydrochar production because of its organic content; however, its high inorganic fraction, predominantly silica, limits its application. Furthermore, studies specifically investigating the hydrothermal carbonization (HTC) of ASW remain limited, given its high ash content and low energy value. This study investigated the HTC of ASW at varying temperatures (180–220 °C) and reaction times (30–90 minutes), with particular emphasis on how two inorganic components (celite versus perlite) affect hydrochar yield, composition, and energy properties. The resulting hydrochars were characterized using CHNSO, proximate, SEM, and FTIR analysis. The results revealed high ash contents reaching 77.7–92.08%, corresponding to HHV of 2.3–3.4 MJ kg⁻¹, while FTIR spectra confirmed strong silica presence through prominent Si–O–Si absorption bands. Therefore, the study provides valuable insights into silica-rich biomass behavior during HTC. The study also evaluated the relationship between HHV and ash content and analyzed the kinetic model. The results demonstrate the significant influence of inorganic components on carbonization pathways, highlighting opportunities to enhance hydrochar quality through pretreatment strategies such as acid/alkaline leaching or mineral separation.