Atomistic Simulations of O ₂ , N ₂ , and CO ₂ Diffusion in Hydrochar─Quantifying the Effect of the Pore Size Regime

A rigorous yet flexible methodology was developed to generate realistic molecular char models that take experimental data on char geometry and elemental composition into account parametrically. Following this methodology, three upper-nanoscale models of an activated hydrochar derived from cellulose, with the composition C 17708 H 5036 O 5962 were constructed, with densities from 0.90 to 1.15 gcm –3, porosities ranging from 20% to 36%, and maximum pore sizes between 20 and 30 Å. These models were employed to investigate the mobility of the light gases O 2, N 2, and CO 2, which are typically present in oxy-fuel atmospheres confined within char nanopores and for which literature data are sparse and rather inconclusive. To achieve this, nonreactive molecular dynamics simulations were performed for several nanoseconds, using a modified formulation of the generic Universal Force Field. The studied combinations of char systems and gas molecules were investigated systematically with respect to gas molecule load and char pore size regime, encoded by the char density, porosity, and maximum pore diameter, to resolve the dependence of the diffusion dynamics of the confined gases on these parameters. Their interplay was quantified by computing self-diffusion coefficients for the confined gas species at 298.15 K, revealing absolute values in the 10 –8 m 2 s –1 range for O 2 and N 2 and in the 10 –9 m 2 s –1 range for CO 2 . The obtained values are comparable to the effective diffusion constants measured in previous experimental studies on related porous media, indicating that the models are sufficiently realistic within the chosen size regime. Furthermore, a linear decay of the gas diffusivities with the char density, linear growth with the char porosity, and superlinear growth with the maximum pore diameter were observed. Regression models fitted to the diffusion data may be incorporated into general diffusion equations, taking the pore size regime into account and supplementing future experimental studies on gas diffusion in char. The employed computational workflow can be used to characterize char and therein confined gases under experimentally inaccessible conditions.