Denitration device, heat recovery steam generator having the same, gas turbine combined cycle power plant and method of denitration

A denitration method for a denitration device comprising:

• a duct that leads and distributes exhaust gas from a turbine of a gas turbine having a compressor and the turbine;
• an ammonia injection grid that sprays, into the duct, an ammonia mixed gas in which ammonia gas and dilution air are mixed with each other;
• a denitration catalyst that is installed on a downstream side of flow of the exhaust gas of the ammonia injection grid in the duct, and performs a denitration reaction between the exhaust gas and the ammonia mixed gas;
• and
• an air bleeding line that is connected to a low compression portion of the compressor and supplies a low compression air bled of the compressor into the ammonia injection grid as the dilution air,
• wherein the air bleeding line includes a damper configured to adjust a flow rate of the low compression air bled of the compressor,
• the damper has a constant opening degree fixed during operation of the gas turbine regardless of an operating load of the gas turbine, and
• the denitration method comprising adjusting a concentration of the ammonia mixed gas by a flow rate of the ammonia gas.