Decomposition kinetics and diffusion-controlled mechanisms of organic industrial waste: a non-isothermal TGA–iso-conversional study

Efficient valorization of organic industrial waste (OIW) requires reliable kinetic descriptors and mechanistic insight under realistic thermal programs. This work examines the pyrolysis behavior of an incinerable organic industrial waste reaction sludge from specialty chemical synthesis—using non-isothermal thermogravimetric analysis under N₂ (60 mL min−1) from 25 to 800 °C at heating rates of 20, 30, and 50 °C min−1. The feedstock shows high energy potential (HHV = 30.9 MJ kg−1; C = 62.9 wt%) and low moisture content (3.25 wt%). Differential thermogravimetry profiles reveal three stages: drying/light volatile release (≈30–200 °C), a dominant devolatilization zone (≈200–350 °C), and a slow high-temperature degradation region (≈350–800 °C). Maximum overall conversion (75.3%) and minimum residue (24.7%) occur at 30 °C min−1. Model-free iso-conversional analyses (Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Starink) describe the kinetics well, with FWO providing the most consistent interpretation. Average apparent activation energies in the main devolatilization zone range from 37.4 to 43.8 kJ mol−1 and decrease slightly with conversion. Negative apparent activation energies at high temperatures indicate overlapping secondary reactions and transport limitations. Criado master-plot analysis identifies diffusion-controlled mechanisms.