Method of heating boiler process feed streams

A method of heating process streams fed to boiler utilizing an oxygen transport membrane unit, said method comprising:

• feeding process streams to the boiler that include a heated boiler feed water stream and a heated oxygen-containing stream;
• the heated boiler feed water stream being heated within the boiler through indirect heat exchange with a retentate stream and a flue gas stream to generate steam, the flue gas stream being produced by combustion of a fuel supported by oxygen separated from the heated oxygen-containing stream by the oxygen transport membrane device, thereby to also produce the retentate stream with a higher mass flow rate than the flue gas stream;
• indirectly transferring heat from the retentate stream to a first subsidiary oxygen-containing stream and thereafter, to a first subsidiary boiler feed water stream, thereby to produce a heated first subsidiary oxygen-containing stream and a first heated boiler feed water stream;
• indirectly exchanging further heat from the flue gas stream to a second subsidiary oxygen-containing stream and thereafter, to a second subsidiary boiler feed water stream, thereby to produce a second heated oxygen-containing stream and a second heated boiler feed water stream;
• combining the heated first subsidiary oxygen-containing stream and the heated second subsidiary oxygen-containing stream to form the heated oxygen-containing stream and combining the heated first boiler feed water stream and the heated second boiler feed water stream to form the heated boiler feed water stream;
• and
• minimizing heat exchange area required for the indirect heat exchange between the retentate stream, the flue gas stream and the oxygen-containing stream, the boiler feed water stream by providing the first oxygen-containing stream with a greater mass flow rate than that of the second oxygen-containing stream.