This paper addresses the important issue of feeding oxygen to a fluidized bed gasifier in an efficient way, in cases of small to medium scale units (a few MWth), to obtain a syngas free of nitrogen and with relatively high calorific value, without the need to utilize a complex dual fluidized bed system. To this scope, the application to biomass conversion systems of ion transport membrane (ITM) technology for oxygen separation from air is studied by coupling an oxygen transfer model to a gasification model that considers thermodynamic and kinetic constraints. Numerical evaluations are performed of char partial combustion with oxygen permeated through the membrane, in the gasifier region close to the tubular ITM surface, as a means to provide the necessary input of heat to biomass gasification, a globally endothermic process. The results show that the membrane surface needed to provide the required oxygen flow to the gasifier is small enough to be arranged inside the fluidized bed volume, assuring feasibility of an autothermal process. The model is also helpful to optimize the location of the membrane module and evaluate different options. Experimental investigations are needed to check the resistance and durability of ITM materials in the gasifier environment. © 2014 Elsevier B.V.