Here,　a　large-scale　(~40000　atoms)　atomistic　coal　char　representation　(with　a　measure　of　control　over　structural　and　physical　properties)　was　constructed　using　in-house　scripts　Fringe3D　and　Vol3D.　A　simplified　inexpensive　simulation　approach　proposed　in　our　earlier　work　was　employed　to　compare　the　char　combustion　reactivity　and　char　micro-structure　evolution　under　O2/CO2conditions.　Simulation　results　indicate　that　compared　with　air　combustion,　char　combustion　in　O2/CO2atmosphere　is　obviously　prolonged.　The　present　simulation　process　captures　the　char　reactivity　disparities　between　different　combustion　conditions.　As　char　is　combusted,　lower　reactive　carbon　decreases　while　higher　reactive　carbon　first　increases　than　decreases.　In　addition,　accessible　surface　area　of　char　has　a　close　relevance　with　its　combustion　reactivity.　©　2016,　Science　Press.　All　right　reserved.