Biocoke: Role of surface energy in binding char with coal during coking and the effect of char on coke pore size

The incorporation of biomass-derived char into coking coal is attracting increasing interest as a decarbonisation strategy; however, its influence on coal behaviour during carbonisation and the resulting coke structure remains insufficiently understood. This study investigates the role of surface energy in coal–char interactions during pyrolysis and examines the effect of char addition on the pore structure of coke. The rheological behaviour of two Australian bituminous coals of differing rank, a medium-volatile coal (MVC) and a high-volatile coal (HVC), with vitrinite contents of 73.6 % and 77.2 %, respectively, was analysed to map their viscoelastic pathways and understand bubble evolution during pyrolysis. MVC exhibited a relatively high viscosity (>10 5 Pa·s) within its liquid-like state, enabling stable bubble growth before coalescence. In contrast, HVC exhibited a lower viscosity (3 × 10 4 Pa·s), resulting in unstable bubbles that collapsed shortly after nucleation. Surface energy measurements obtained using the film flotation method revealed that MVC possessed a surface energy of 57 mJ m⁻ 2 during the bubble growth stage, whereas HVC exhibited a substantially lower value of 41 mJ m⁻ 2 . Notably, bagasse char produced at 800 °C exhibited a surface energy of 57 mJ m⁻ 2 , matching that of MVC. Optical microscopy confirmed that approximately 95 % of this char was incorporated into MVC, compared with only 64 % incorporation in HVC. These observations suggest that surface energy matching promotes wetting and interfacial bonding between coal and char during coking. Micro-CT analysis and pore size characterisation further demonstrated that char addition substantially reduced the pore size distribution and overall porosity of both coals, resulting in denser coke structures. These findings demonstrate that surface energy compatibility and coal viscoelastic behaviour play important roles in coal–char interactions, pore development, and ultimately coke strength, thereby advancing the substitution of fossil coal with renewable carbon in ironmaking applications.