The flue gas dispersion from the stack of a power station was investigated prior to addition of a high efficiency heat recovery facility. Decrease of the flue gas temperature from 115°C to 40°C by heat recovery could influence the local humidity and thermal NOx level. It might also result in the formation of a white large plume due to the early saturation of vapor around the stack, and deteriorate dispersion of thermal NOx. Numerical simulation revealed that the area influenced by flue gas has been enlarged, particularly in winter. The volume of visible plume indicating RH 1.0 increased to 120 m high and 80 m wide for the flue gas at 40°C, while a smaller plume was formed that was 85 m high and 50 m wide for that at 115°C. The humid air of the flue gas extended nearly 160 m further along the ground. The distance for the maximum NO2 concentration on the ground increased by 80 m and 50 m for 1 m/s wind and 3 m/s, respectively. The area influenced at the ground level expanded more than 250 m at 1 m/s wind after heat recovery. In particular, lowering the temperature of flue gas may affect the local environment more significantly in unusual cases including temperature inversions.