Thermodynamic insights into the synergistic effects during Co-gasification of digested sewage sludge and LDPE for hydrogen-rich syngas

Global waste management challenges associated with digested sewage sludge (DSS) and plastic waste (PW) have intensified the search for sustainable waste-to-energy solutions. This study presents a thermodynamic assessment of hydrogen-rich syngas production from the co-gasification of DSS and low-density polyethylene (LDPE) using air/steam mixtures in Aspen Plus. A Gibbs free energy minimization model was developed to predict syngas composition, conversion efficiency, and reaction pathways across varying blend ratios and operating conditions. Results reveal that at low to moderate equivalence ratios (ER = 0.05–0.20), LDPE-rich blends (≥50 %) maximize hydrogen and light hydrocarbon yields while minimizing CO 2 formation, producing energy-dense syngas. Optimal process performance occurs at moderate ER values (0.25–0.35), where carbon conversion efficiency approaches completion and cold gas efficiency peaks, with sludge-rich blends favoring higher gasification temperatures and char oxidation. Specifically, ER values of 0.242, 0.285, and 0.3176 were identified as optimal values for achieving H 2 /CO = 1, while steam injection at SFR values of 0.625–1.043 enabled H 2 /CO = 2 across varying LDPE fractions. Controlled steam addition at moderate levels (SFR = 0.5–1) enhanced hydrogen yield, though excessive steam (≥2) reduced syngas quality and reactor efficiency by shifting equilibrium toward CO 2 and CH 4 and lowering gasifier temperature. Synergistic analysis confirms that antagonism dominates under oxygen-deficient conditions (ER ≤ 0.20), strong positive synergy emerges at moderate ER (0.25–0.35), and synergy diminishes at higher ER (≥0.40). Collectively, the findings demonstrate that precise air control, moderated steam injection, and optimized DSS–LDPE blending are essential for maximizing syngas quality, hydrogen production, and overall process efficiency. The identified operating windows provide practical guidelines for producing synthesis-grade syngas with tailored H 2 /CO ratios suitable for downstream applications such as Fischer–Tropsch fuel synthesis, methanol production, and hydrogen generation. Beyond improving gasification performance, the proposed co-gasification strategy offers a sustainable pathway for simultaneously valorizing municipal sewage sludge and plastic waste, thereby supporting resource recovery, circular economy objectives, and the development of low-carbon waste-to-energy systems.