Thermal degradation behaviour and kinetic analysis of effluent treatment plant sludge towards sustainable bioenergy potential

Effluent treatment plant sludge (ETPS) is a significant byproduct of the pulp and paper industry and has been investigated to assess its thermal degradation behaviour, kinetic parameters, and bioenergy recovery potential. Physicochemical characterisation revealed lower moisture content (4.40 wt.%), a higher volatile content (67.79 wt.%), and a significant ash fraction (19.22 wt.%), rich in catalytically active alkali metals. Thermogravimetric analysis at 10–30 °C/min showed three distinct stages: dehydration (30–150 oC), active devolatilization (150–700 oC), and char stabilisation (>700 oC), with decomposition temperatures shifting higher at elevated heating rates due to kinetic effects. Furthermore, to capture the complex, multi-step reactions, eight iso-conversional kinetic models (KAS, OFW, FM, DAEM, STM, TM, VZM, and AVIC) were employed. The activation energy varied (139–350 kJ/mol) with conversion (0.1–0.8), confirming heterogeneous decomposition behaviour. The apparent average activation energies (185–225 kJ/mol) indicate moderate to high energy requirements influenced by inorganic interactions. Master plot analysis revealed a shift from diffusion-controlled mechanisms at early conversions to nucleation and interfacial reactions at higher conversions. Lastly, thermodynamic analysis indicated that ETPS pyrolysis is endothermic (ΔH > 0), non-spontaneous (ΔG>0), and disorder-promoting (ΔS>0), requiring continuous heat supply. These results demonstrate the technical feasibility of ETPS valorisation through pyrolysis while highlighting the need for process optimisation to overcome ash-related operational challenges.