In conventional thermal power plants, air combustion systems equipped with an air combustion boiler adapted to burn fuel using air are in the mainstream. On the other hand, thermal power generation using an oxygen combustion system has been proposed as a method which makes it easy to recover carbon dioxide (CO2), which is one of causative substances of global warming and is emitted in the largest amount in industrial activities.
The oxygen combustion system supplies combustion gas to a boiler together with fossil fuel such as coal, burns the fossil fuel, and thereby discharges flue gas composed principally of CO2 and H2O, where the combustion gas is made by mixing part of the flue gas discharged from a boiler of an existing air combustion system with high-concentration oxygen (hereinafter referred to as oxygen for short, as appropriate) produced by an air separator. Consequently, CO2 can be separated easily from the flue gas by compressing and cooling the flue gas (see, for example, Patent Literatures 1 and 2).