Synergistic and competitive mechanisms among multicomponent fractions during supercritical water oxidation of oily sludge: A ReaxFF MD study

• Structure and intermediate reactivity jointly control carbon conversion. • n-Hexadecane and resins ignite radical chains, boosting oxidation. • Methylnaphthalene-asphaltene interactions accelerate aromatic ring opening. • Multicomponent synergy intensifies radical competition among fragments. Oily sludge can be efficiently degraded under supercritical water oxidation conditions, but the component-level interactions among its petroleum fractions remain unclear. Using ReaxFF molecular dynamics, synergy among oil-phase fractions—n-hexadecane (H), methylnaphthalene (M), resin (R), and asphaltene (A)—was interrogated across 15 systems (single, binary, ternary, quaternary). The synergistic reaction mechanisms among the components were elucidated from multiple perspectives, including product distribution, reaction pathway tracing, radical chemistry, and ring-opening processes. Single-component Organic carbon conversion rate rank R (79.58%) > A (78.93%) > H (71.56%) ≈ M (71.36%), reflecting a trade-off between parent stability and fragment reactivity. All binary mixtures show positive overall synergy; M-A is strongest (+15.16%) via aromatic-core destabilization, whereas H-A is weakest (+5.65%). Two drivers explain binary synergy: (i) radical ignition by H/R and (ii) aromatic-core interaction in M-A. In multicomponent systems, the detrimental effects of radical competition are amplified. Owing to the lowest reactivity of its fragments, n-hexadecane becomes the “sacrificial” component in the reaction (Δ H = −6.26%, Δ M = +6.90%, Δ R = +3.40%, Δ A = +8.60%). The conclusions extend the theoretical understanding of supercritical water oxidation of complex multicomponent systems.