To reduce global warming and other environmental burden, there is a social demand for reduction of carbon dioxide (CO2) emissions. CO2 is produced, for example, when fossil fuels, such as coal, petroleum, and natural gas, and the like are burned. Thus, boilers which burn fossil fuels are expected to reduce CO2 emissions produced as a result of the combustion.
Thus, Patent Literature 1 proposes to burn fuel in a so-called oxygen combustion mode, increasing CO2 concentration in combustion flue gas, and thereby separate and recover CO2 from the combustion flue gas, where the oxygen combustion mode involves separating air into gas mainly composed of oxygen and gas mainly composed of nitrogen and burning fuel using combustion gas made of a mixture of separated gas rich in oxygen (hereinafter referred to as oxygen-rich gas) and combustion flue gas. Also, Patent Literature 1 proposes to burn the fuel with air during boiler start-up when combustion flue gas is in short supply and burn the fuel by switching from air to mixed gas of oxygen-rich gas and combustion flue gas upon completion of the start-up.
On the other hand, Patent Literature 2 proposes to increase oxygen concentration in the combustion gas in the oxygen combustion mode, in which the combustion gas contains a lot of CO2 with high specific heat, increasing a heating-up period of the fuel ejected from a fuel nozzle and resulting in an unstable flame.