Changes in the infrared reflection spectrum of a thin film of silica-like resinous material sandwiched between metal electrodes can be induced by applying an electric potential to a top electrode which is semitransparent. A control unit communicates with the beam property monitor and guiding unit and determines deviations of actual properties from desired properties of the beam and transmits control signals for adapting the beam properties to the guiding unit, using said deviations. Heat is recovered and process streams are heated by separately heating portions of the oxygen-containing stream and the boiler feed water stream with a retentate stream produced from the oxygen separation and a flue gas stream generated from the combustion.