Measurement techniques for sulfur trioxide concentration in coal-fired flue gas: a review

Under the extensive implementation of ultra-low emission facilities, sulfur trioxide (SO₃) has received increasing attention. This article reviews the measurement techniques for SO₃ in flue gas, which include controlled condensation method (CCM), isopropanol absorption method (IPA), salt method, tunable diode laser absorption spectroscopy (TDLAS), ultraviolet absorption spectroscopy (UVs), and Fourier transform infrared spectroscopy (FTIR). The first three methods are chemical methods, which focus on the extraction of SO₃ from flue gas. With highly reactive nature and relatively low concentrations, which are about 5 mg/m³ even lower, achieving high-fidelity flue gas sampling and non-destructive extraction of SO₃ is the key to SO₃ measurement. The latter three methods belong to spectroscopic methods, which focus on the principle, system composition, and influencing factor analysis. With real-time response and 1-ppm detection limit, attention is attracted to spectroscopic methods on online measurement. This article comprehensively introduces the measurement techniques for SO₃ concentration in flue gas and presents conclusions so as to enable researchers to decide the direction of further investigation.