- Title
- Gas quality control in oxy-fuel technology for carbon capture and storage: scrubbing of CO₂ prior to compression
- Creator
- Liu, Dunyu
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2014
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Oxy-fuel combustion is an emerging technology intended to mitigate CO₂ emissions from power plants. Compared with other CO₂ capture technologies, non-CO₂ components in Oxy-fuel flue gas are highly concentrated, among which SO₂ is of concern. Sodium based quench units have been used in Oxy-fuel projects to directly cool the gas prior to compression and to also remove SO₂. However, the high concentration of CO₂ in the flue gas can interfere with the capture of SO₂. This study considers the mechanisms by which SO₂ in CO₂ is absorbed by sodium containing liquids, using laboratory experimentation and experiments at the Callide Oxy-fuel Project. Conditions for the operation of Oxy-fuel scrubbers are recommended, including operational pH levels based on both the SO₂ absorption rate and the effective use of Na⁺ in liquid. In the laboratory evaluation, dynamic experiments were designed to elucidate the reaction mechanisms of SO₂ absorption from gas mixtures of SO₂/N₂, SO₂/CO₂, SO₂/N₂/O₂ and SO₂/CO₂/O₂ when introduced into solutions of NaOH, Na₂CO₃ and NaHCO₃ with known initial concentrations and pH. Correspondingly, the steady state experiments were designed to understand the absorption rate of SO₂ from gas mixtures of SO₂/CO₂ when introduced into sodium solutions with known mixtures of NaHCO₃ and NaHSO₃. The SO₂ concentration of the exhaust gas and the changes in the pH of the liquid were obtained during experiments in both dynamic and steady state processes. The changes in both parameters were recorded during experiments for dynamic processes; whereas both parameters were controlled to reach target values during experiments for steady state processes. Additionally, liquid samples were obtained on a regular basis for the analysis of HCO₃-, S (IV) and S (VI) in both dynamic and steady state processes. The absorption rate of SO₂ in dynamic experiments was found to be reduced in CO₂ in the gas phase controlled region primarily due to the reduced diffusivity of SO₂ in CO₂ and reduced kinematic viscosity of CO₂ compared to N₂. The dynamic absorption results for gas mixtures of SO₂/CO₂ demonstrated three pH regions of absorption rate behaviour and the absorption rate of SO₂ was correlated with speciation in these regions, as pH decreased during the experiments. The steady state experiments of SO₂ absorption into mixtures of NaHCO₃ and NaHSO₃ investigated solution chemistry at various pH values, the significance of solution pH and the concentration of SO₂ on the absorption rate of SO₂. The absorption rates of SO₂ obtained in steady state experiments were predicted by the model based on the instantaneous reaction assumption. The model generally overestimates the absorption rates of SO₂ at pH values below pH 6 indicating a kinetic limitation of SO₂ and water reaction at low pH values. Experiments at the Callide Oxy-fuel Project yielded both gas and liquid analyses. Gas analysis for SO₂, CO₂, O₂, CO, NO and NO₂ were obtained at three positions: before, at the intermediate location of, and after the atmospheric scrubbing system. Liquid analysis for dissolved CO₂, HCO₃-, S (IV) and S (VI) was obtained from two columns. From the gas analysis, it could be observed that the initial Quencher column captured most of the SO₂ (97%) and the subsequent low pressure (LP) scrubber captured a limited amount of the SO₂ (1%). Thus, from the liquid analysis, the amount of total sulfur measured in the liquid discharge from the Quencher is therefore 100 times higher than the total sulfur measured in the liquid obtained from the LP scrubber. This work has implications for the absorption of SO₂ into sodium solutions in a spray tower. Simulations on the absorption rate of SO₂ into droplets were conducted based on the instantaneous reaction model and demonstrated the impacts of pH, SO₂ concentration, droplet size, droplet position and gas phase CO₂ on the absorption rate of SO₂. More importantly, simulations revealed the three pH regions for droplets in a spray tower. In region 1, the absorption rate of SO₂ is the highest; however, a large amount of CO₂ is absorbed instead of SO₂. In region 2, the absorption rate of SO₂ is moderate and Na⁺ is effectively utilised. In region 3, the absorption rate of SO₂ is low and dissolved SO₂ is not fixed. The operational pH of the sodium based quench unit is recommended to be in region 2, where a high absorption rate of SO₂ and low sodium losses are expected. The operational window is primarily related to the concentration of sodium solutions and the window narrows at high sodium solution concentrations (refer to dynamic experiments). The operational window is secondarily related to the concentration of SO₂ (refer to dynamic experiments). This operational pH region is also related to droplet position and droplet size (refer to droplet simulations). The operational pH region 2 can be further divided into three sub regions. Three sub regions are defined as follows. Region 2-1 is the pH region where the absorption rates of SO₂ at intermediate to high concentrations from 600ppm to 1500ppm are moderate; region 2-2 is the pH region where the absorption rates of SO₂ at all concentrations are moderate; region 2-3 is the pH region of minimal Na⁺ waste for CO₂ capture. In the region 2-1, the absorption rate is moderate, but there is a large amount of Na⁺ consumed for CO₂ capture. In the region 2-2, there is moderate consumption of Na⁺ for CO₂ capture. In the region 2-3, Na⁺ consumed for CO₂ capture is minimized, but the effective ratio of Na⁺ is still not 100%. The operational pH region graph can be used to optimise the operation of a spray tower. This work also has implications for the disposal of waste liquids and use of reagents. The disposal of waste liquids should take into consideration the presence of HCO₃- in liquids and the effect on the final pH for extended exposure of liquid solutions to air. In the presence of HCO₃-, the final pH reached will be pH around 8 and in the absence of HCO₃-, the final pH will reach around 4. The cost of reagents should also be considered. Using soda ash instead of caustic soda and sodium bicarbonate could reduce costs by 58-79%.
- Subject
- oxy-fuel; CCS; SO₂ absorption; scrubber; pH; sodium solution
- Identifier
- http://hdl.handle.net/1959.13/1058860
- Identifier
- uon:16483
- Rights
- Copyright 2014 Dunyu Liu
- Language
- eng
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