Nitrogen dioxide absorption into sulfite inhibited by thiosulfate




Suresh Babu, Athreya

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Emulsified sulfur was found to be the most suitable form of sulfur for in-situ thiosulfate production for sulfite inhibition with a maximum sulfur-to-thiosulfate-conversion of 50 % and t₅₀ of 6 hours. Increasing the ionic strength of the solution reduces the reaction rate between sulfur and sulfite. Reaction rate reduced by 3 when ionic strength of the solution was increased from 0.225 M to 2.95 M. The rate of reaction between sulfur and sulfite was found to be first-order in sulfur, half-order in sulfite, and zero-order in thiosulfate with a reaction rate constant of 5.48 x 10⁻³ mM [superscript -0.5] min⁻¹. Increasing the reaction temperature from 40 to 75 °C increased the interpreted reaction rate by a factor of 17. The activation energy of the reaction was found to be 74.2 kJ/mol, and this high value indicates that the reaction might be kinetically limited and not mass transfer limited. The reaction rate model predicts experimental bench-scale reaction rates with an absolute average deviation of 6.5%. In the pilot-scale prescrubber, at coal conditions, pH was observed to decrease as a function of time with 3 linear regions. These regions corresponded to CO₂ absorption to form carbonate, conversion of carbonate to bicarbonate, and CO₂ liberation from solution by reaction of bicarbonate with SO₂ respectively. The characteristic times of these linear regions corresponded to the rate of the reaction in each of these regions. Rate of oxidation of thiosulfate under 0-1 ppm NO₂ conditions was 0.13 gmol/hr which was half the rate at 0-5 ppm NO₂ conditions. Thiosulfate loss by tank bleed was found to be directly related to the amount of gas processed. Thiosulfate loss by bleed reduced from 60 gmol to 17.2 gmol when the flue gas flow rate reduced by 1/2.25 due to the lesser amount of water condensing in the prescrubber. A minimum of 25 mM sulfite was required to maintain NO₂ removal of 90% even under low NO₂ conditions. At NGCC conditions, thiosulfate degradation rates were 0.112 gmol/hr and 0.127 gmol/hr before and after thiosulfate addition respectively. Sulfite and thiosulfate in the prescrubber increased with SO₂ coming into the prescrubber. Thiosulfate and sulfite were correlated by a power-law relation just as in the coal condition


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