- Title
- CO₂ quality control through scrubbing in oxy-fuel combustion: an evaluation of operational pH impacts, and prediction of SO₂ absorption rate at steady state
- Creator
- Liu, Dunyu; Wall, Terry; Stanger, Rohan
- Relation
- International Journal of Greenhouse Gas Control Vol. 32, Issue January 2015, p. 37-46
- Publisher Link
- http://dx.doi.org/10.1016/j.ijggc.2014.10.019
- Publisher
- Elsevier
- Resource Type
- journal article
- Date
- 2015
- Description
- Oxy-fuel combustion is a promising CCS technology which is being demonstrated prior to commercialisation. While the flue gas in oxy-fuel combustion is concentrated in CO₂, it contains impurities such as SO₂. The elimination of SO₂ can provide a clean CO₂ stream ready for storage. This paper is to understand the absorption of SO₂ in scrubbing relevant to those used in oxy-fuel technology. Steady state experiments were conducted in a continuous well stirred reactor to understand the absorption rate of SO₂/CO₂ into a total concentration of 0.28M of mixtures of NaHSO₃ and NaHCO₃ simulating liquids formed by scrubbers using NaOH as the reagent at solution pH values from 4 to 7 with the exiting gas concentrations of SO₂ from 19ppm to 1500ppm and a constant CO₂ concentration of 70%. Online measurement included gas phase SO₂ and liquid pH, and offline measurement included CO₂ (aq), HCO₃⁻, S (IV), SO₃²⁻ and S (VI) after each experiment. Three aspects investigated were the impacts of pH on the solution chemistry, the significance of solution pH and the concentration of gas phase SO₂ on the absorption rate of SO₂.The total sulphur concentration in liquid was found to be related to the effectiveness of Na⁺. The effective ratio of Na⁺ can be defined as the total sulphur to Na⁺ ratio and this effectiveness ratio of Na⁺ is pH dependent. At pH<5, Na⁺ is 99% effective. It reduces dramatically from 99% at a pH 5 to less than 15% at pH above 7. With regard to carbon based species also absorbed, super saturation of CO₂ (aq) was observed at pH>5.5. The concentration of HCO₃⁻ increases dramatically above pH 6 and below this pH, the concentration of HCO₃⁻ is negligible. The absorption rate of SO₂ was found to increase with pH with some increase with the concentration of SO₂. The operational pH window for scrubbing may be defined by an upper limit pH where the absorption rate of SO₂ starts to decreases from the maximum absorption rate of SO₂ and the lower limit pH where the absorption rate of SO₂ reduces to half of the maximum absorption rate of SO₂. Both the upper limit and the lower limit decrease initially and stay stable with the concentration of SO₂. This decrease is caused by the reversible reaction of the hydrolysis of SO₂ and confirmed by equilibrium experiments of SO₂ and sodium solutions. Operation within region 2 (pH 5-6) is recommended, depending on the scrubber design. The operation exit pH of the produced liquid can be varied within the region. The absorption rates of SO₂ obtained in the steady state experiments were predicted by a model based on the instantaneous reaction assumption. This model generally overestimates the absorption rates of SO₂ at pH values below 6 indicating a kinetic limitation of SO₂ and water reaction at low pH values. The analysis on the controlling regions indicates that the gas side mass transfer resistance decreases with the concentration of SO₂. Liquid side resistance becomes more important at a lower pH and a higher concentration of SO₂.
- Subject
- CO₂ quality control; oxy-fuel; operational pH; SO₂ absorption
- Identifier
- http://hdl.handle.net/1959.13/1332876
- Identifier
- uon:26964
- Identifier
- ISSN:1750-5836
- Language
- eng
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