Thermal decomposition kinetics of dairy manure hydrochars

Authors: MAINALI, KALIDAS - Oak Ridge Institute For Science And Education (ORISE); Sarker, Majher; Mullen, Charles; Sharma, Brajendra; Yadav, Madhav; Lew, Helen; GARCIA-PEREZ, MANUEL - Washington State University

Submitted to: Fuel
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2025
Publication Date: 4/7/2025
Citation: Mainali, K., Sarker, M.I., Mullen, C.A., Sharma, B.K., Yadav, M.P., Lew, H.N., Garcia-Perez, M. 2025. Thermal decomposition kinetics of dairy manure hydrochars. Fuel. 120: 1020088. https://doi.org/10.1016/j.joei.2025.102088.
DOI: https://doi.org/10.1016/j.joei.2025.102088

Interpretive Summary: Manure produced by concentrated animal feeding operation systems (CAFOs) such as dairy farms is an environmental concern. Utilization and disposal of manure for applications other than field fertilizer can help mitigate this pollution. Fuel characteristics can be increased when dairy manure is converted into solid products (hydrochars) through hydrothermal carbonization. The purpose of this study was to convert manure to hydrochars at various conditions. As compared to raw dairy manure (HHV=17.16 MJ/kg), the fuel properties of each produced hydrochar were significantly improved (up to 25.64 MJ/kg). A mathematical model was developed to determine the potential combustion efficiency of the hydrochars. This information will be useful to determine conditions that make the most useful solid fuel from dairy manure. This will be useful for those looking for solutions to manure pollution from dairy farms or looking to add value to this low-value waste.

Technical Abstract: Hydrothermal processes including hydrothermal carbonization (HTC) have emerged as a promising technology for the conversion of high-moisture feedstocks. A total of six hydrochars were generated by varying the reaction temperatures and residence times. The heating value of hydrochars produced using HTC (25.64 MJ/kg) has improved significantly from raw dairy manure (17.16 MJ/kg). The Friedman mathematical model was used to estimate the kinetic parameters for the thermal decomposition of the manure and its hydrochars (activation energy and the pre-exponential factor). Comparing three reaction atmospheres (CO2, N2, and air), lower activation energies were shown to be required in air environments for all designated hydrochars compared with the raw manure. Furthermore, Py-GC-MS studies revealed that light-oxygenated compounds were produced at 500 °C from the hydrochars upon flash pyrolysis. The acid treatment under autogenous pressure, significantly improved manure hydrochars physiochemical properties. The comprehensive theoretical and practical guidelines presented in this study for acid-promoted manure hydrochars indicate that dairy manure can be converted into a substitute energy source using the hydrothermal carbonization process. Additionally, a thorough comprehension of the kinetics of combustion, gasification, and pyrolysis is crucial for the design of industrial processes, feasibility assessments, and scale-up.