Corrosion due to impurity reactions in dense-phase CO2: Thresholds definition through chemical equilibrium calculations and laboratory testing

Abstract

To reduce CO2 emissions by industry cost-effectively, hub projects for Carbon Capture and Storage (CCS) have been initiated that aim to capture CO2 from multiple emitters rather than a single source. Each CO2 stream from each emitter carries its own specific spectrum of impurities (H2O, SOx, H2S, NOx, O2 etc.). Upon mixing or a change in conditions, these impurities react and may separate out as solids or a corrosive polar phase containing reaction products such as H2SO4 and HNO3. To protect the CCS hub equipment, the operators set thresholds on the impurity concentrations which the emitter must achieve to join the hub project. This work discusses how test data and theoretical chemical equilibrium calculations inform the set limits. The equilibrium composition of the mixture is illustrated on diagrams that indicate different mechanisms of corrosion depending on the oxidative strength and the hydration level in the CO2 stream. Several corrosion mechanisms are investigated. Importantly, a correlation is confirmed between observed phase separation events and a calculated ‘equivalent concentration of sulfuric acid’, C acid. Threshold values for acid precipitation expressed as C acid were established for 100 bar CO2 at 25 °C and 4 °C for several combinations of impurities. The calculations and the test data confirm that the oxidizing strength is a major factor for the conversion of the impurities to acids. The kinetic factors that allow the reactions are identified: in the most important scenario, the process requires a radical chain initiator (NO2), an oxidant (O2) and a proton donor (e.g., H2S) to be simultaneously present in the mixture.