Here a new automated and inexpensive atomic simulation approach including sequential oxygen diffusion and simplified reaction was created to explore the char reactivity and combustion behavior during oxy-fuel combustion. Two large-scale (>40,000 carbon atoms) coal chars (higher- and lower-porosity chars) that differ primarily in pore volume and pore size distribution were evaluated against graphite with similar number of carbon atoms. The structural transformations, total atoms, reactive atoms, their location, conversion, rate (number of atoms reacting), size of the char, and apparent density were captured. Post analysis evaluated the total porosity, micro/mesoporosity volume, and atomic surface area. The higher porosity char burn-off was similar to 38% more rapid than the graphite (number of steps), similar to the lower porosity char (similar to 33% faster). The peak reactivity followed the maximum atomic surface area (with a delay of 5-10% conversion) than decreased as there were fewer reactive carbons. Porosity volume increases with conversion until 50-70% where the porosity declines. For both chars the microporosity contribution increased until 30-35% conversion then the pores either grow or coalesce into the mesoporous classification. The chars burning modes are initially changing density (pore growth accompany mass loss) until similar to 50% conversion when the mode became a combination of changing density and shrinking core. Graphite was less reactive and followed a shrinking core consistent with the simulation restrictions. This approach is likely to aid in the exploration of the influence of char structure and conversion conditions upon reactivity and char behavior in combustion and gasification. (C) 2016 Elsevier Ltd. All rights reserved.