Impurity reactions in dense phase CO₂ – corrosion and solid formation

Before a capture and treatment plant is built, it must be documented which CO₂ compositions can be transported in pipes and ships. There is little relevant field and lab data available and there is a need for reliable test equipment and comprehensive testing to determine what are acceptable levels for various impurities (H2O, H2S, NOx, SOx, CO, O2, glycol, amine) in the CO₂, especially when the streams from multiple point sources are collected in a network.

One of the biggest experimental challenges is to control the concentration of the impurities that are present in low concentrations (ppm level) and that can be consumed quickly due to. corrosion and reactions between the impurities. In this project, a test rig has been built that has a specially designed feeding system for continuous dosing of liquid (dense phase) CO₂ and impurities into a small reaction loop. CO₂ is taken from the feed system and the reaction loop, which goes to an analysis unit to determine differences in the composition and consumption of impurities.

The new test rig has been used to study and quantify under which conditions aqueous phases are formed that can cause corrosion and solid corrosion products and under which conditions elemental sulfur is formed that can join the CO₂ flow into the reservoir. For example, the experiments have shown that there is no significant corrosion when the content of water in dense phase CO₂ is below 1500 ppmv in the range 4 – 50 ° C and 50 – 150 bar, while corrosive acidic aqueous phases can be formed at lower water content when in in addition, SO2 and NO2 are present in the order of 50 ppm. The risk of corrosion increases further when oxygen is present. A set of CO₂ compositions with impurities has been identified where the formation of corrosive phases will be a problem. This will constitute a first step in defining an operating window for pipe transport of dense phase CO₂ with impurities.