Studies of the Fate of Sulfur Trioxide in Coal-Fired Utility Boilers Based on Modified Selected Condensation Methods

The formation of sulfur trioxide (SO3) in coal-fired utility boilers can have negative effects on boiler performance and operation, such as fouling and corrosion of equipment, efficiency loss in the air preheater (APH), increase in stack opacity, and the formation of PM2.5. Sulfur trioxide can also compete with mercury when bonding with injected activated carbons. Tests in a lab-scale reactor confirmed there are major interferences between fly ash and SO3 during SO3 sampling. A modified SO3 procedure to maximize the elimination of measurement biases, based on the inertial-filter-sampling and the selective-condensation-collecting of SO3, was applied in SO3 tests in three full-scale utility boilers. For the two units burning bituminous coal, SO3 levels starting at 20 to 25 ppmv at the inlet to the selective catalytic reduction (SCR), increased slightly across the SCR, owing to catalytic conversion of SO2 to SO3, and then declined in other air pollutant control device (APCD) modules downstream to approximately 5 ppmv and 15 ppmv at the two sites, respectively. In the unit burning sub-bituminous coal, the much lower initial concentration of SO3 estimated to be approximately 1.5 ppmv at the inlet to the SCR was reduced to about 0.8 ppmv across the SCR and to about 0.3 ppmv at the exit of the wet flue gas desulfurization (WFGD). The SO3 removal efficiency across the WFGD scrubbers at the three sites was generally 35% or less. Reductions in SO3 across either the APH or the dry electrostatic precipitator (ESP) in units burning high-sulfur bituminous coal were attributed to operating temperatures being below the dew point of SO3.