The direct solid-solid reaction between coal char and iron-based oxygen carrier and its contribution to solid-fueled chemical looping combustion

•Direct reaction between coal char and iron-based OC particles investigated in the dynamic and static contact regimes.•Determines reaction kinetics and carbon conversion of the direct solid-solid reaction.•Operational temperature and the use of different OCs significantly affect the reaction kinetics.•In-situ gasification and solid-solid reaction in competition with each other in steam-gasified CLC.•Methods developed to determine the contribution of solid-solid reaction in CLC.