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.
Keywords: Controlled condensation method; Fourier transform infrared spectroscopy; Isopropanol absorption method; SO3 concentration measurement; Salt method; Tunable diode laser absorption spectroscopy; Ultraviolet absorption spectroscopy.