The air for dilution can be stably supplied to the ammonia injection device, and the exhaust gas discharged from the gas turbine is suppressed from flowing into the ammonia injection device, and the generation and deposition of ammonium carbonate in the ammonia injection device are prevented. It aims to prevent blockage. A duct 22 for guiding and circulating exhaust gas from a turbine 52 of a gas turbine 50 having a compressor 52 and a turbine 56, and a mixed gas of ammonia gas and air for dilution are placed in the duct 22. An ammonia injection device 24 to be sprayed and a denitration catalyst 26 installed in the exhaust gas flow downstream of the ammonia injection device 24 in the duct 22 are provided, and the storage of the compressor 52 of the gas turbine 50 is provided. A bleeding line 76 is provided connected to the compression unit to supply air extracted from the compressor 52 as dilution air to the ammonia injection device 24.
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