Abstract: A comburant gas supply system is provided for a combustion boiler/turbine of a thermal power plant, a combustion boiler/turbine system and a thermal power plant. The gas supply system has an air separation module to separate and output an oxygen rich gas from an input air supply; a comburant gas storage module fluidly connected to the output of the air separation module for storage in liquid state of separated oxygen rich gas; a comburant gas supply module to supply the oxygen rich gas to the combustion boiler selectively from the air separation system and/or the comburant gas storage system. The air separation module has an oxygen rich gas output capacity determined from a demand rating for the combustion boiler/turbine adjusted with reference to a load factor across a predetermined operating period and/or the ASU size is increased to provide longer term energy storage capacity.