The carbon-free fire tube boiler is a boiler for heating water by combustion of hydrocarbon fuels with oxygen. The boiler includes a housing defining first and second heat transfer stages. At least one oxygen transport reactor is received within the first heat transfer stage. The oxygen transport reactor includes an outer wall and an inner cylindrical ion transport membrane. The membrane receives pressurized air and separates gaseous oxygen therefrom, transporting the oxygen into an annular region between the membrane and the outer wall. The gaseous hydrocarbon fuel is delivered into the annular region for combustion, producing gaseous carbon dioxide and water vapor. A water reservoir is defined within the first and second stages, with the at least one oxygen transport reactor and at least one transfer tube passing therethrough. The carbon dioxide and the water vapor are delivered through the at least one transfer tube for heating water contained therein.