Insights into the nanopore characteristics and synergistic effects of coal, biomass, and co-pyrolysis chars using very small-angle neutron scattering

The nanopore evolution in chars derived from coal (XBD), rice straw (RS), and their blend (RS/XBD) was systematically investigated using very small-angle neutron scattering (VSANS) at 30–600 °C. VSANS can achieve a broader characterization scale (1–1000 nm) for nanopore structures, far surpassing the 1–100 nm limit of SANS. Results reveal that the average nanopore diameters of all chars expand continuously, delineated into three distinct stages: rapid expansion (30–200 °C) driven by moisture and light volatile release, decelerated growth (200–400 °C) due to structural realignments, and a coal-dominated expansion phase (400–600 °C). Specifically, the average pore diameter of the RS/XBD blend increased from 79 nm to 110 nm. While all chars exhibit sustained pore expansion upon heating, the co-pyrolysis char uniquely demonstrates an anomalous non-monotonic evolution at elevated temperatures, characterized by a regression in peak pore size from 97 nm to 89 nm between 500 and 600 °C. Fractal analysis reveals that pure coal undergoes a critical transition from an intricate mass fractal network (Dm ≈ 2.91) to a defined surface fractal structure (Ds ≈ 2.40) above 500 °C. Concurrently, co-pyrolysis effectively mitigates the severe surface roughening (Ds = 2.96) observed in pure biomass char. Crucially, a cross-scale synergistic decoupling was identified: volatile-char interactions inhibit macroscopic mass loss (negative synergy) while simultaneously driving nanoscale pore dilation (positive synergy) via elevated internal gas pressure. These findings provide direct quantitative evidence of the structural regulation mechanisms during co-pyrolysis, offering a robust framework for tailoring functional carbon materials using VSANS.