The performance of a carbon free fire-tube boiler utilizing two-pass oxygen transport reactors was numerically investigated. The influences of the oxygen transport reactors wall temperature on the reaction rate, oxygen permeation and heat flux were quantified. The performance of the reactors has been investigated at elevated temperature. It is observed that both heat transfer and combustion characteristics can be optimized at an elevated temperature of 1373 K. Increasing the mass fraction of methane in this reactor to 6% results in improvement of the heat transfer and combustion characteristics in the reactor. Further increase in CH4% did not lead to any significant improvement. The fuel flow rate variation did not have any significant impact on the reactor performance. It is indicated that the membrane temperature has significant effect on the reaction rates and oxygen flux in the upstream region in particular. Copyright (C) 2016 John Wiley & Sons, Ltd.