Role of heating rate and atmosphere in pore structure evolution and reactivity of mineral-free biomass-derived chars

The structural evolution and oxidation reactivity of partially converted mineral-free biomass-derived chars were investigated under both low and high heating rate conditions in O 2 and CO 2 atmospheres. Applying two-dimensional non-local density functional theory (2D-NLDFT) to CO 2 and N 2 physisorption data allowed for a detailed analysis of individual pore regimes as defined by IUPAC. Low heating rate experiments revealed that char conversion proceeds via kinetically controlled ultramicropore growth, followed by growth from ultramicropores to micropores and mesopores. Under kinetically controlled char conversion in O 2 atmosphere, char conversion contributes to pronounced pore growth to overall surface areas three times as high as for the unconverted chars, dominated by ultramicropore formation. For gasification, both the generation of ultramicropores and their subsequent evolution into larger pores strongly dominate pore development, with an overall surface area increase by a factor of five. High heating rate treatments induced simultaneous pyrolysis and char conversion, resulting in the development of ultramicropores through devolatilization and reactions. These interdependent processes altered the distribution of reactive sites and reversed the relative reactivity trends observed under low heating rates. Structure/reactivity correlations demonstrate that oxidation reactivity is governed not only by total surface area but primarily by the distribution of pore regimes and reactive carbon sites. These findings highlight the critical role of heating rate and reaction atmosphere in determining char microstructure and performance.