Formation of SO₃ in Flue Gas Under SNCR Conditions

Abstract

Experimental and numerical studies were conducted to investigate the formation of sulfur trioxide (SO³) during the selective non-catalytic reduction (SNCR) process. The effects of the inlet NH₃/NO ratio (R_AN), reaction temperature, inlet mole fractions of SO₂, O₂, CO and H₂O were assessed. The experiments were conducted using a perfectly stirred reactor (PSR) and SO₃ mole fraction was determined using the sulfur balance method. Corresponding numerical simulation was performed using detailed chemistry developed by Mueller and coworkers. Both experimental and numerical results revealed that the SO₃ formation was considerably affected by R_AN, SO₂ and O₂ mole fractions. The experimental results demonstrated that under typical SNCR conditions, 0.5 ~ 1.0% of SO₂ was converted into SO₃, and SO₃ mole fraction was 5–10 ppm. The SO₃ formation was noticeably enhanced by the addition of NH₃ when R_AN < 0.5. The conversion rate decreased as the initial SO₂ increased. A small amount of O₂ could promote the SO₃ formation remarkably, but this effect became much weaker as inlet O₂ mole fraction ≥1%. The numerical simulation indicated that the increase of the reaction temperature significantly promoted the SO₃ formation when the temperature was above 1173 K. A small amount of CO could significantly enhance the SO₃ formation. The H₂O addition could inhibit SO₃ formation. The detailed chemical kinetic analyses showed that the main reaction paths of the SO₃ formation were the oxidization reaction of SO₂ with O radical via SO₂ + O (+M) → SO₃ (+ M) (52.3) and the one of SO₂ with NO₂ via SO₂ + NO₂ → SO₃ + NO (52.4). The effect of the operational parameters, i.e., R_AN, reaction temperature, and SO₂, O₂, CO, H₂O mole fractions, could be well explained by the variation of the reaction rates of Eqs. 52.3) and (52.4).