Thermochemical simulations of hydrogen production from polypropylene plastic waste coupled with methanation of CO & CO₂ from steelmaking off-gases

Industrial decarbonization requires scalable pathways to recycle carbon-rich waste and produce low-emission fuels. Steelmaking emits substantial CO and CO₂ via off-gases, while plastic waste particularly polypropylene (PP) offers a hydrogen-rich feedstock. This work presents a thermodynamic simulation that employs Gibbs free energy minimization to optimize methane synthesis from steelmaking off-gases (CO and CO₂). The process is driven by hydrogen produced through polypropylene ((–C₃H₆–)ₙ, PP) pyrolysis, enabling the conversion of two industrial waste streams into synthetic methane (CH₄). Energy and exergy efficiencies were evaluated to assess the viability and performance of this integrated approach. PP pyrolysis at 650 °C and 1 bar was found to yield 7 mol h⁻¹ of H₂, achieving energy and exergy efficiencies of 65 % and 35 %, respectively. This H₂ was directly coupled to methanation of CO and CO₂ at 250 °C and 10 atm, yielding CH₄ with an 82 % selectivity and complete (100 %) conversion of both carbon sources. The methanation step displayed peak energy and exergy efficiencies near 78 %, while coke formation remained suppressed due to effective carbon reconversion at ≤ 300 °C. The synergy process enables enhanced thermodynamic performance and system integration, transforming waste plastics and metallurgical off-gases into clean, usable fuels. The combined pathway offers a circular, low-carbon solution for hydrogen and methane synthesis using industrial residues, supporting both energy transition goals and waste management.