Abstract: Controlled condensation method was adopted mostly for SO₃ sampling in the flue gas, because SO₃ is prone to condense and its capture rate is only about 80%. Based on the specifications of controlled condensation method in GB/T 21508-2008, the experiment of SO₃ capture was carried out. Through the experimental studies on the main factors affecting the capture rate of SO₃, such as the size and total length of serpentine coil, the extract flow as well as temperature of sampling, the best coil type and control parameters were obtained. The results show that, the heating temperature of the sampling gun should be at least 280℃. For serpentine coil with the inner diameter of 4 mm, 20 L/min is the optimal pumping flow rate, and the length of serpentine coil should be 240 mm. Under the optimal conditions, the SO₃ capture efficiency can reach 97% or higher, which will meet the needs of SO₃ field test. Finally by taking advantage of this sampling method, the SO₃ concentration in flue gas at each stage was measured and the SO₃ removal efficiency of various air pollutant control equipment from a 600 MW ultra low emission unit was calculated based on the measurement data.

Key words: coal-fired power plant, SO₃ testing, SO₃ sampling, controlled condensation method, sampling gun, coil, suction flow rate