SO3 Issues for Coal-Fired Plant
AUTHOR: Rohan Fernando
DATE: September 2003
During the combustion of coal in a furnace most of the sulphur in the fuel is oxidised to SO2 but a small and variable percentage is further oxidised to SO3. The SO3 is produced by two distinct mechanisms – one homogeneous and the other heterogeneous. The homogeneous reactions involve the oxidation of SO2 by gas phase species. The heterogeneous process involves the oxidation of SO2 by surface catalysts. Higher concentrations of sulphur trioxide are present in oil-fired plant than in coal-fired plant as residual fuel oil generally contains higher levels of sulphur and much higher levels of transition metals such as vanadium than coal. Moreover most coals contain much higher amounts of ash which can adsorb any SO3 that is formed. Hence typical SO3 concentrations from coal-fired plant are less than 5 ppm whereas for oil-fired plant, SO3 levels are typically 15–50 ppm.
Once formed it readily reacts with water vapour to produce sulphuric acid. Sulphur trioxide plays an important role in several aspects of plant operation. In most respects its presence is problematic but it does have one useful role. Some sulphur trioxide is required for adequate operation of ESPs and if sufficient SO3 is not already present, it may be necessary to inject some. The corrosion potential of SO3 means that flue gas back end temperatures must be kept sufficiently high to prevent condensation. Though there are several techniques to measure SO3, none is straightforward and all require considerable care and attention to detail. The emission of high concentrations of SO3 can cause problems with plume visibility. This is particularly likely if SCR is installed on units firing high-S coal. It is possible to control SO3 either by alkali injection or utilising wet ESPs.