Air pollution control system

An air pollution control system, comprising:

• a denitration device configured to remove nitrogen oxide in flue gas from a boiler;
• a heat recovery unit including a heat transfer tube for recovering part of heat of the flue gas after denitration;
• a precipitator configured to remove soot and dust in the flue gas after heat recovery;
• a desulfurization device configured to remove sulfur oxide in the flue gas discharged from the precipitator;
• a reheater including a heat transfer tube for heating the flue gas discharged from the desulfurization device;
• a heat medium circulation pipe including a heat medium circulation pump configured to circulate a heat medium between the heat transfer tube of the heat recovery unit and the heat transfer tube of the reheater;
• a heat medium heater provided to the heat medium circulation pipe to heat the heat medium;
• a detection device configured to detect a state of ammonia at an outlet of the denitration device;
• a control device configured to control the heat medium heater based on a value of an ammonia concentration detected by the detection device;
• and
• a gas thermometer configured to measure a gas temperature of the flue gas at an outlet of the heat recovery unit, wherein
• the control device is configured to cause the heat medium heater to heat the heat medium when the ammonia concentration is equal to or higher than a certain value, and
• control operation of the heat medium heater such that the gas temperature of the flue gas is maintained under an upper limit value of operating temperature of the precipitator.