Upgrading gasification performance of bio-oil distillation sludge via co-pyrolysis with walnut shell: Correlating carbonaceous structure parameters and gasification indices

This investigation evaluates the synergistic valorization of bio-oil distillation sludge (DS) through integrated thermochemical processing with agricultural and forestry biomass. The research focuses on co-pyrolysis mechanisms between DS and walnut shell (WS) feedstocks, with particular emphasis on carbonaceous structure evolution and gasification reactivity enhancement. Advanced characterization techniques were employed to monitor structural transformations during thermal conversion processes. The results showed that increased DS loading ratios (50–75 wt%) progressively reduced the surface textural complexity of chars obtained from mixture of DS and WS, as evidenced by morphological analysis. Raman analysis on the char from mixture of DS and WS revealed intensified graphitic domain development proportional to increasing of DS blending ratios, suggesting its catalytic effects of graphitization on the produced char. Reactivity assessment through thermogravimetric analysis showed 58–158 % improvement in gasification performance indices compared to those of individual feedstocks. Kinetic evaluation on the thermogravimetric analysis identified the Random Pore Model (RPM) as optimal for describing co-pyrolysis char conversion dynamics (R²>0.9985), except for DSc which exhibited distinct decomposition behavior. Mineralogical analysis revealed that silicon-dominated DS ash mitigated sintering of ash derived from mixture of DS and WS, whereas WS-derived potassium and calcium species increased ash fusion tendencies. Multivariate analysis of carbon structure-gasification reactivity relationships revealed differential synergy manifestations across conversion stages of the mixtures of DS and WS. Strongest parametric correlations emerged with late-stage reactivity indices R 0.9 , suggesting enhanced interfacial interactions during advanced carbon conversion phases. These findings provide new insights into process optimization for sustainable management of refinery byproducts through tailored biomass blending strategies. • Co-pyrolysis technology enhanced the gasification characteristics of bio-oil distillation sludge (DS). • The carbonaceous structure of co-pyrolysis char unveiled alterations occurring. • Gasification indices provided evidence of synergy between DS and walnut shell (WS) during pyrolysis. • Mineral composition was conducted to assess the gasification deposition tendency. • Correlations between structure parameters and gasification indices were examined.