Emissions of SO₃ (or its hydrated form, H₂SO₄) have received increasing attention. Though much of this attention has focused on plume opacity, SO₃ also has significant negative impacts on plant performance and operations. Robert E. Moser, Codan Development LLC, writing in Power, related that these impacts include:

Burning any type of coal will produce some SO₃ in the boiler. It can vary from a few ppm to 40 ppm or more. The application of SCR for NO_x removal can also result in an increase in SO₃ produced. SO₃ exists in a vapor form in the high-temperature environment of the boiler. As the flue gas cools below 600° F to 700° F, and depending on the amount of moisture present, vaporous sulfuric acid is formed. By the time the average gas temperature falls to 280° F to 320° F downstream of the air heater, most of the SO₃ has assumed the form of vaporous sulfuric acid. As the flue gas is rapidly cooled in the FGD system, the vaporous sulfuric acid undergoes a shock condensation process that produces very fine aerosol particles. Note that the term “SO₃” invariably refers to varying proportions of vaporous SO₃ and vaporous sulfuric acid, and to sulfuric acid aerosol particles downstream of the wet FGD system.

The presence of SO₃ in flue gas creates a number of plant O&M issues that go far beyond “blue plumes,” although, the plumes are also problematic. The optical light-scattering effects of sulfuric acid aerosols produce a visual discoloration of the plume. The minimum concentration of SO₃ necessary for this phenomenon depends on atmospheric conditions (the amount of sunshine, temperature, relative humidity, wind speed) and stack specifics (stack diameter and exit gas velocity). However, it is generally accepted that if the SO₃ concentration is less than 5 ppm, there will be no visual discoloration effects.

When an SCR system is placed in service, the acid dew point can increase anywhere from 10° F to 20° F due to the increased SO₃ concentrations leaving the unit. If the air heater outlet temperature is not raised, the risk of acid corrosion increases. However, raising the outlet temperature reduces the unit heat rate. For example, raising air heater outlet temperature by 35° F would raise unit heat rate by 1 percent.

SO₃ and ammonia (NH₃) will react to form ammonium bisulfate (ABS) in the air heater if both substances are present when the gas in the heater cools to between 350° F and 420° F. The SO₃ must be present in molar concentration in excess of the molar concentration of NH₃. ABS is a sticky solid that can foul the air heater and increase the pressure drop. The driving force for this reaction becomes the amount of NH₃ slip from the SCR.

Depending on its concentration, SO₃ can also react with NH₃ under the catalytic conditions that exist in the SCR system at temperatures in the range of 530° F to 630° F. This is typically referred to as the SCR system’s minimum operating temperature (MOT). Operating at temperatures below the MOT can cause pluggage of the SCR catalyst.

The use of fabric filters at plants burning high-sulfur coals is usually avoided because SO₃ can foster corrosion of a filter’s metal components and adversely affect filter cake properties. However, with respect to mercury control with activated carbon injection, fabric filters are now being installed on a number of units and it is possible that significantly more will be installed in the near future. The greatest concern is the fact that as SO₃ collects on the filter cake, it becomes noticeably heavier and stickier, making the cake more difficult to remove, and increases pressure drop.

There is at least one positive impact that SO₃ has on plant operation. When SO₃ is absorbed onto flyash, the resistivity of the ash decreases, enhancing the ability of the ESP to remove it. Nevertheless, acid condensation on the ash causes corrosion of ESP components, raising the maintenance costs of ESP units.