Corrosion of carbon steel in simulated CCS streams

Abstract

It is expected that Carbon capture and storage (CCS) may involve a large network with many sources, transportation systems and storage sites. Depending on factors like flue gas source, capturing technique and process parameters, the captured CO2 may contain various types and concentrations of other components. These components are expected to be present at low concentrations (in the ppm range) and are therefore commonly referred to as impurities. It is well known that some impurities may react and produce species that are corrosive to carbon steel. Long transport pipelines will be made of carbon steel due to cost considerations, and it is therefore essential to avoid formation of corrosive species. In the present work, corrosion of carbon steel was tested in a CO2 stream containing low levels of water and the impurities H2S, SO2, NO2 and O2 at a temperature of 25 °C and a pressure of 99 bar. The experiment was carried out in a custom-made autoclave with transparent windows that allowed for in-situ observation of any chemical reactions or corrosion processes. Each impurity, including water, was pre-mixed with CO2 at a high concentration (impurity stock solutions) and continuously injected through individual feed lines into the autoclave. This ensured that all impurities were injected fully dissolved in the CO2 phase and that no reactions could take place before the impurities were mixed inside the autoclave. The impurity injection rate was adjusted relative to the to the total CO2 rate, to get the desired impurity concentration in the autoclave. Exhaust CO2 was continuously removed from the autoclave and analysed for impurity content by laser spectroscopy. Comparison of the injected and exhaust impurity concentration made it possible to identify chemical reactions as concentration changes. The experiment demonstrated that there were no corrosion reactions when the impurities were at the target concentrations. However, chemical reactions between the impurities did take place. An episode with clogging of the vent line, which resulted in a temporarily increase of the impurity concentrations, resulted in formation of a separate liquid phase containing sulfuric and nitric acid formed. This also started corrosion of the carbon steel coupon.