From Hydrothermal Treatment to Pyrolysis: Kinetics, Thermodynamics, Evolved Gas Behavior, and Reaction Mechanism During Coupled Thermal Conversion of Chicken Manure

Chicken manure represents a major agricultural waste stream with significant environmental risks if left unmanaged, yet its hydrothermally treated hydrochar (HCCM) holds promise as a renewable solid fuel. This study investigates the pyrolysis behavior, kinetics, thermodynamics, and gas evolution mechanism of HCCM using thermogravimetric analysis coupled with FTIR and mass spectrometry (TG-FTIR/TG-MS) at five heating rates (5–25 °C/min). TG-DTG results revealed a characteristic three-stage decomposition, with the main devolatilization occurring between 180 and 400 °C and a stable char residue of 22–27% remaining at 800 °C; DTG peaks shifted to higher temperatures and intensified with increasing heating rate, confirming a kinetically controlled process. Isoconversional analysis using the Flynn–Wall–Ozawa (FWO), Kissinger–Akahira–Sunose (KAS), and Starink (STK) methods; R2 > 0.98 showed the apparent activation energy rising sharply from ~150 to over 320 kJ/mol with conversion, while consistently positive Gibbs free energy (130–210 kJ/mol) confirmed an endothermic, non-spontaneous reaction pathway. TG-FTIR and TG-MS identified CO2, H2O, carbonyl/amide fragments, and light hydrocarbons as dominant volatiles, with delayed H2 release reflecting secondary aromatization and char condensation. These findings establish a mechanistic framework linking dehydration, decarboxylation, and aromatization reactions, supporting HCCM as a viable feedstock for sustainable energy recovery within a circular waste valorization strategy.