In some coal-fired boilers, corrosion rates can be dominated by by the presence of reducing gases (CO and H2S) or by the deposition of ash or char containing partially reacted material (sulfur and carbon). Accordingly, REI has modeled the evolution of sulfur species in the combustion zone.

The evolution of SO3 in the backpass of a coal-fired boiler impacts both operation (air heater fouling, low-temperature corrosion) and emissions (visible plumes). REI uses models for SO2 oxidation to predict production of SO3. Detailed CFD modeling of injection of calcium and magnesium for SO3 control in the backpass has also been carried out.

The impact of SO3 on mercury control has also been studied using REI’s mercury process model, MerSim.  SO3 is known to interfere with mercury capture by activated carbon (or unburned carbon in fly ash). SO3 condenses out of the gas phase as H2SO4 when temperatures drop below the H2SO4 dew point. Most power plants operate at flue gas temperatures above the dew point to avoid corrosion. However, many particle surfaces or equipment surfaces may be below the dew point. H2SO4 condenses on the surfaces and on unburned carbon, removing sites for mercury oxidation and absorption.

Sulfur emissions have also been studied in other applications, such as copper smelting.