Research on the synergy of co‐pyrolysis of oily sludge and walnut shell: effects of blending ratio on the quality of bio‐oil and bio‐gas

Abstract BACKGROUND Oily sludge (OS) is a hazardous petroleum waste whose pyrolysis often suffers from high char yield and heavy (C 21+ ) fractions, while co‐pyrolysis with biomass such as walnut shell (WS) can potentially upgrade product quality by providing hydrogen and a porous carbonaceous catalytic scaffold. RESULTS Co‐pyrolysis of oily sludge (OS) and walnut shell (WS) was investigated in a fixed‐bed reactor to assess synergistic effects of temperature (300–700 °C) and blend ratios (OS: WS = 2:1, 1:1, and 1:2). The results demonstrated that co‐pyrolysis significantly reduces char formation, with the 1:1 ratio at 600 °C yielding only 44.76% char, compared to 70.06% for OS pyrolysis at 700 °C. Additionally, the heavy oil fraction (C 21+ ) was notably reduced, with lighter compounds such as phenols and ketones enriching the bio‐oil. The heavy oil content decreased from 43.17% in OS pyrolysis oil to 20.24% in co‐pyrolysis oil. Synergistic gas production was observed, as indicated by a Synergy Index of 0.22, with CH 4 yield increasing to 25.76% at the 1:1 ratio. H 2 and CO yields also increased with higher WS content (up to 8.92% and 12.24%, respectively), while CO 2 production decreased to 10.23% at the 1:2 ratio. These enhancements were attributed to WS acting as both a hydrogen donor and catalytic scaffold, where alkali metals (K, Ca) promoted the cracking of heavy components via dehydrogenation and dealkylation. CONCLUSIONS The optimal conditions, achieved at 600 °C with a 1:1 OS: WS ratio, minimized char formation while maximizing oil stability and gas calorific value, offering an effective strategy for waste valorization. © 2026 Society of Chemical Industry (SCI).