Formation, transformation, measurement, and control of SO3 in coal-fired power plants

The formation and emission of sulfur trioxide (SO3) in coal-fired power plants has received increasing attention due to its adverse effects on the operation of plant and environment. With the wide application of selective catalytic reduction (SCR) systems, the problem caused by SO3 has become severe, especially when high sulfur coal is burned. Emission regulations of SO3 for coal-fired power plants, which promote the development of SO3 measurement and control technologies, have been set in some countries and regions. In this paper, recent advances in the formation, transformation, measurement, and control mechanism and technologies of SO3 in coal-fired power plants were summarized. The formation mechanisms of SO3 in boiler and SCR systems and its form transition and corresponding effects on the performance of power plants were analyzed. Different SO3 test standards and methods were compared, and online SO3 monitor based on isopropanol absorption method were developed. Various SO3 control technologies, including simultaneous and specific removal technology, were summarized. Low-low temperature and wet electrostatic precipitators could remove up to 90% SO3/H2SO4 aerosol, and the removal efficiency of SO3 in wet flue gas desulfurization can be enhanced to 80% by absorber optimization and inlet flue gas temperature decrease. For power plants that burn high-sulfur coal, the injection of alkaline sorbent before the SCR or air preheater system can remove more than 90% of SO3 to decrease the formation of NH4HSO4 and the pressure drop in the air preheater. High SO3 removal technology and strategies for different scenarios were proposed to meet different SO3 emission requirements in the future.