Emissions of Sulfur Trioxide from Coal-Fired Power Plants

Abstract

Emissions of sulfur trioxide (SO 3 ) are a key component of plume opacity and acid deposition. Consequently, these emissions need to be low enough to not cause opacity violations and acid deposition. Generally, a small fraction of sulfur (S) in coal is converted to SO 3 in coal-fired combustion devices such as electric utility boilers. The emissions of SO 3 from such a boiler depend on coal S content, combustion conditions, flue gas characteristics, and air pollution devices being used. It is well known that the catalyst used in the selective catalytic reduction (SCR) technology for nitrogen oxides control oxidizes a small fraction of sulfur dioxide in the flue gas to SO 3 . The extent of this oxidation depends on the catalyst formulation and SCR operating conditions. Gas-phase SO 3 and sulfuric acid, on being quenched in plant equipment (e.g., air preheater and wet scrubber), result in fine acidic mist, which can cause increased plume opacity and undesirable emissions. Recently, such effects have been observed at plants firing high-S coal and equipped with SCR systems and wet scrubbers. This paper investigates the factors that affect acidic mist production in coal-fired electric utility boilers and discusses approaches for mitigating emission of this mist. INTRODUCTIONAs understanding of the adverse effects of air pollution has grown, so also has the complexity of coal-fired power plant design and operation, especially with regard to air pollution control systems. Control of air pollutant emissions is not only a legal requirement but also is becoming a financial necessity, as salability of effluents and trading of emissions increase the direct monetary value of emissions control. The days when one must only consider the nuisance value of fly ash are long past. 1 As plant complexity has increased, so have unexpected consequences of changing segments of the total chemical process that occurs between fuel preparation and ultimate emissions. One of the more discernible adverse consequences is the formation and emission of sulfur trioxide (SO 3 )/sulfuric acid (H 2 SO 4 ), as highlighted by the recent and well-publicized experiences of a power plant in Ohio. 2 Although not directly subject to emission limits, SO 3 is important to consider during the design and operation of coal-fired utility boilers for a number of environmental and plant performance reasons.The formation of SO 3 will occur during the combustion of sulfur (S)-bearing fuels such as coal and heavy fuel oils. Virtually all of the SO 3 converts to H 2 SO 4 as flue gas is cooled in the air preheater (APH). Relatively high concentrations of SO 3 /H 2 SO 4 in the boiler, stack, or plume can cause adverse impacts to plant equipment and to the environment. Impacts on plant equipment can include corrosion, fouling, and plugging and may require additional hardware or changes in operation to minimize SO 3 / H 2 SO 4 concentrations and the resulting adverse impacts.

Reactions of SO3 with the O/H Radical Pool under Combustion Conditions

“…Another important reaction for the SO 3 /SO 2 ratio is the pressure-dependent step, Under the excess oxygen conditions typical of the post-flame region in a combustion system it is believed that the SO 3 /SO 2 ratio is mainly controlled by the sequence (4), (2), 2,7-10 whereas reactions 1 and 3 presumably are less important. Although the SO 3 formation step (4) is characterized over a wide temperature and pressure range, 11,12 there is a significant uncertainty in the rate constants for the consumption reactions (1)- (3). There are no reported measurements for the SO 3 + H reaction; kinetic models for sulfur chemistry rely on estimated values for k 1 13-15 or adopt a QRRK estimate 7 for the reverse reaction SO 2 + OH.…”

“…There are no reported measurements for the SO 3 + H reaction; kinetic models for sulfur chemistry rely on estimated values for k 1 13-15 or adopt a QRRK estimate 7 for the reverse reaction SO 2 + OH. 10,16,17 The rate constant for the reaction between SO 3 and O (2) has not been measured directly and reported data vary by several orders of magnitude. Indirect determinations have been reported on the basis of results from laminar premixed flames [18][19][20][21][22] and, for the reverse step, from reactor experiments.…”

“…Sulfur trioxide in combustion systems causes formation of sulfuric acid, which will condense and corrode metal surfaces at temperatures below the dew point. 2,3 In addition, it may contribute to aerosols and particulate emissions. [4][5][6] A better understanding of the mechanisms for SO 3 formation and destruction is a necessary step in the effort to minimize these problems.…”

“…A number of reactions may conceivably affect the SO 3 concentration. Reactions of particular interest are those of SO 3 with H, O, and OH, Depending on the reaction conditions, these steps may serve to either consume SO 3 (forward direction) or form SO 3 (reverse direction). Another important reaction for the SO 3 /SO 2 ratio is the pressure-dependent step, Under the excess oxygen conditions typical of the post-flame region in a combustion system it is believed that the SO 3 /SO 2 ratio is mainly controlled by the sequence (4), (2), 2,7-10 whereas reactions 1 and 3 presumably are less important.…”

“…1 The combustion of fuel-bound sulfur results in formation of oxides of sulfur, mainly SO 2 and SO 3 . Sulfur trioxide in combustion systems causes formation of sulfuric acid, which will condense and corrode metal surfaces at temperatures below the dew point.…”