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/N-2 and SO2/CO2 into sodium based aqueous solutions, with measurements of SO2 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/N-2 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/N-2); and region 3 where gas absorption reduced rapidly with pH, at pH values below 4.22 (4.82 for SO2/N-2). 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 3000 ppm, 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. (C) 2013 Elsevier Ltd. All rights reserved.