Fossil-fuel power plant and fossil-fuel power plant operation method

A method of operating a thermal power plant comprising:

• an exhaust gas system configured to circulate an combustion gas, which is exhausted from a boiler, as an exhaust gas;
• a feed-water system configured to supply water to the boiler;
• a feed-water heater that is provided to the feed-water system and is configured to increase the temperature of feed-water by using extraction steam;
• an air system configured to supply air to the boiler;
• a main economizer, which is provided to a secondary side of feed-water from feed-water heater of the feed-water system and is configured to increase the temperature of feed-water by using residual heat of the combustion gas;
• a catalytic NOx removal equipment, which is provided to a secondary side of the exhaust gas from the main economizer of the exhaust gas system and the exhaust gas at a required temperature or higher is configured to be supplied to;
• a sub economizer, which is provided between the feed-water heater and the main economizer of the feed-water system and configured to increase a temperature of feed-water by using the exhaust gas on a secondary side of the NOx removal equipment;
• and
• an air preheater, which is provided to a secondary side of the exhaust gas from the sub economizer and is configured to increase a temperature of air of the air system by using the exhaust gas on the secondary side of the sub economizer,
• the method comprising the step of controlling a temperature of the exhaust gas on a secondary side of the air preheater of the exhaust gas system in such a way that the temperature is in a required temperature range.