- Title
- CO₂ quality control through scrubbing in oxy-fuel combustion: simulations on the absorption rates of SO₂ into droplets to identify operational pH regions
- Creator
- Liu, Dunyu; Wall, Terry; Stanger, Rohan; Luo, Caimao
- Relation
- International Journal of Greenhouse Gas Control Vol. 37, Issue June 2015, p. 115-126
- Publisher Link
- http://dx.doi.org/10.1016/j.ijggc.2015.03.006
- Publisher
- Elsevier
- Resource Type
- journal article
- Date
- 2015
- Description
- Oxy-fuel combustion is a promising CCS technology which is being demonstrated prior to commercialization. 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. SO₂ is commonly washed by sodium based spray towers with high efficiency but CO₂ impacts are significant. A theoretical model was developed based on the two-film mass transfer model, and considering both the SO₂ and CO₂ reactions with sodium solutions. This model was firstly used to simulate the dynamic experimental results reported in our previous paper to confirm its applicability. With this model, simulations then were carried out on the absorption rate of SO₂ into droplets with three droplet sizes: 100µm, 500µm and 1000µm, one sodium concentration of 0.08M (back calculated from liquid analysis), a range of pH from 4 to 12.5 and a range of SO₂ concentrations from 19ppm to 1500ppm.Simulations focus on the impacts of droplet position, gas phase CO₂ and droplet size on the absorption rate of SO₂. These impacts are closely related with pH values. Taking a typical pH of 7 for example, the absorption rates of SO₂ for droplets close to nozzles which move relative to the gas are significantly higher than these below nozzles which are at the terminal velocities, and the differences between two positions are larger for higher concentrations of SO₂ with the concentrations range from 200ppm to 1500ppm. CO₂ has a negative impact on the absorption rate of SO₂ through reducing the gas phase mass transfer coefficient of SO₂ in the gas phase controlled region and also through generating more acidic conditions at the liquid phase interface. Reducing the droplet size from 500µm to 100µm has a more significant improvement on the absorption rate of SO₂ than from 1000µm to 500µm. Result: have implications in the controlling region and the operational liquid pH region. The controlling region is related to the droplet position. The droplets close to nozzles are located in the mixed controlled region or the gas film controlled region; and the droplets below nozzles can be located in three regions depending on the concentration of SO₂ and pH. A lower concentration of SO₂ and a higher pH are favourable for the absorption rates of SO₂ to be located in the gas phase controlled region. The operational pH of the exit liquid may be established based on two criteria: a reasonable absorption rate of SO₂ and a sodium reagent loss as NaHCO₃. The optimal operational region is then in the region 2 where the absorption rate of SO₂ is still high and reagent use is minimized. The region 2 can be further divided into four sub regions. In the region 2-1 and the region 2-2, the absorption rate is moderate, but there is too much Na⁺ wasted for CO₂ capture. In the region 2-3, there is a moderate amount of Na⁺ wasted for CO₂ capture. In the region 2-4, Na⁺ wasted for CO₂ capture is minimized, but the effective ratio of Na⁺ is still not the maximum.The necessary operational pH region graph can be used to guide the operation of a spray tower.
- Subject
- CO₂ quality control; oxy-fuel; droplets; operational pH; SO₂ absorption
- Identifier
- http://hdl.handle.net/1959.13/1332870
- Identifier
- uon:26963
- Identifier
- ISSN:1750-5836
- Language
- eng
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