Organic transformation and kinetics in preheating and oxidation stages during supercritical water oxidation treatment of oily sludge

• Preheating initiates desorption and volatilization, while SCWO completes oxidation. • SCWO has a higher activation energy and faster reaction rates than preheating. • SCWO degrades esters, opens cyclic structures, and enhances nitrogen conversion. • Oxygen concentration significantly influences SCWO efficiency and reaction kinetics. The treatment of oily sludge presents significant environmental challenges due to its high organic pollutant content and the risk of soil and groundwater contamination. This study investigates the two-stage preheating-supercritical water oxidation (SCWO) process in treating oily sludge, focusing on the reaction mechanisms and organic component transfer across solid, liquid, and gas phases. During preheating, desorption, volatilization, and thermal decomposition of oil and solids drive organic migration; kinetic analysis reveals the limited reaction rates of preheating, leading to incomplete chemical oxygen demand (COD) removal and partial desorption at this stage. In the SCWO stage, rapid oil-solid desorption equilibrium is observed, with a reaction order of 2.11 ± 0.016 relative to sludge concentration and 0.82 ± 0.02 for oxygen concentration, underscoring oxidant effects in complex feedstock reactions. Preheating facilitates long-chain alkane cracking, ring-opening polymerization of cyclic siloxanes, and amine removal, while SCWO degrades macromolecular esters, opens cyclic structures, and enables nitrogen transfer. These insights improve process understanding, enhance treatment efficiency, and mitigate environmental impacts.