CO₂ quality control by scrubbing in oxy-fuel combustion prior to compression: relating pH to the liquid composition from absorption of SO₂ into sodium based solutions to identify an operational pH window

Oxy-fuel combustion is an emerging technology to mitigate CO₂ emissions from power plants. Compared with other CO₂ capture technologies, gas impurities in oxy-fuel flue gas are highly concentrated, among which SO₂ is of concern. Sodium based quench units have been used in oxyfuel 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. Dynamic transient experiments were therefore conducted in a semi-batch well stirred reactor (WSR) to study the mechanisms of SO₂ absorption from gas mixtures of both SO₂/N₂ and SO₂/CO₂ into sodium based aqueous solutions, with measurements of SO absorption rate and decreasing liquid pH during the experiments. The liquids were analysed by ion chromatography (IC) and acid titration with equilibrium calculations to estimate sulfur and carbon species formed in the liquid. The dynamic absorption results for SO₂/CO₂ show three pH regions of absorption behaviour as pH reduced during the experiments, namely, region 1 with a constant gas absorption rate at pH values above 8.32 (this being 9.65 for the SO₂/N₂ experiments); region 2 where the gas absorption rate reduced at pH values from 8.32 to 4.22 (from 9.65 to 4.82 for SO₂/N₂); and region 3 where gas absorption reduced rapidly with pH, at pH values below 4.22 (4.82 for SO₂/N₂). From liquid analyses and thermodynamic calculations it is concluded that region 1 is associated with the consumption of OH¯ to give SO₃²¯, region 2 with the consumption of HCO₃¯/SO₃²¯ and the formation of HSO₃¯, and region 3 with the consumption of H₂O and the accumulation of HSO₃¯ and SO₂. The operational pH of the sodium based quench units is recommended to be in region 2, where a high absorption rate of SO₂ and low sodium loss are expected. An operational pH window is thereby defined in terms of upper and lower limits. The operational pH window is found to be related to the concentration of sodium solutions when an inlet concentration of SO₂ is proximately 3000ppm, and the window narrows at high concentrations of sodium solutions. The operational window is secondarily related with the concentrations of SO₂ and narrows at low concentrations of SO₂