EXHAUST GAS TREATMENT SYSTEM AND TREATMENT METHOD

The present invention provides a flue gas treatment system and a flue gas treatment method that enable the stable long-term operation of a plant by reducing NO x in a combustion flue gas and reducing the concentration of SO 3 more compared with that available conventionally. The flue gas treatment method of removing NO x and SO 3 in the gas that includes NO x and SO 3 includes a denitration and SO 3 reduction step of denitrating the gas and reducing SO 3 into SO 2 , in which NH 3 as a first additive and a second additive including one or more selected from the group consisting of an olefinic hydrocarbon expressed by a general formula: C n H 2n (n is an integer of 2 to 4) and a paraffinic hydrocarbon expressed by a general formula: C m H 2m+2 (m is an integer of 2 to 4) are added to the gas before bringing the gas into contact with a catalyst.