An early design of oxygen transport membranes (OTM) developed and fabricated by Praxair have been tested at 850-900 degrees C in an experimental coal gasifier using a blend of Illinois and Powder River Basin (PRB) coals. Oxygen flux was measured and found to be stable over approximately 80 h of gasifier operation. Study of interactions between the OTM and coal ash and gas phase impurities indicated that the OTM components remain chemically and structurally stable against coal ash and do not show any indication of solid or liquid (slagging) compound formation. The structure of the active fuel oxidation layer in the OTM exposed to the coal gas also remained stable and no interactions between the consecutive layers (porous support/fuel oxidation layer/gas separation layer/oxygen incorporation layer) were identified in the tested OTM system. (C) 2015 Elsevier Ltd. All rights reserved.