Corrosion in CO2 systems with impurities creating strong acids

There are several proposed specifications for CO2 transport regarding how much impurities that can be allowed in the CO2 stream. Many of these specifications are based on health, safety and environment (HSE) considerations in case of accidental spill, and only limited focus has been on the pipeline integrity. Previous work has demonstrated that many of the impurities that are expected to be present in CO2 captured from flue gasses may react and form corrosive species. The present paper studied impurity reactions and corrosion under simulated transport conditions (25 °C and 10 MPa of CO2). An experiment was carried out in a transparent autoclave which allowed for in-situ visual observation. Chemical reactions between the impurities were observed even at very low concentrations (<100 ppmv). These reactions contributed to the production of nitric and sulfuric acid together with formation of elemental sulphur. Corrosion was observed on coupons of carbon steel, but not on stainless steels. The corrosion rate of carbon steel was low, but the amount of acids and solids (corrosion products) produced cannot be accepted from a pipeline integrity perspective. Further experimental studies are needed to determine specific limits for impurity concentrations in captured CO2 for transport. INTRODUCTION The carbon dioxide (CO2) concentration in the atmosphere has increased over the last century, mainly due to human activities such as combustion of fossil fuels. Since CO2 contributes to global warming, there is an international agreement to reduce emission of CO2 to the atmosphere.1 One method to reduce emission is CCS (Carbon Capture and Storage). In many cases there will be a significant distance between the CO2 capture site and the storage site and the CO2 will be transported in pipelines, by ships or a combination of both. Pipeline transport is regarded as the most cost-effective alternative for large volumes, but for temporary and small storage sites ships might be the preferred method of transportation for offshore storage.2 USA has routinely transported CO2 from naturally occurring sources in on-shore pipelines for more than 40 years. No serious corrosion problems have been reported, but for captured anthropogenic CO2 the conditions might be different due to the presence of small amounts of other compounds, such as SOx, NOx, O2, H2S, CO. These compounds are referred to as impurities in the present work since CO2 is the main product of the capturing process and anything else would only be present because further cleaning/purification of CO2 is not economically feasible. Although valuable data can be acquired from the existing CO2 transport network, it cannot say much about the effect of the additional impurities and if they may affect the integrity of a pipeline or a ship. There are numerous ways these impurities can react, and many reaction products can potentially form.3 If all these reactions occurred only in the capture plant it would probably not be a large problem in practice since the reaction products could be removed before the CO2 leaves the plant. However, it is expected that a future CCS network will be more complex, with several capturing plants connected through a joint CO2 transport system. This is proposed in a feasibility study from the Norwegian Ministry of Petroleum and Energy, where the goal is to handle 1.3 million tons/year of CO2 from three different sources and store it in the Smedaheia formation.2 In cases like this it will be important to avoid cross-impurity reactions that form corrosive species, for example if CO2 streams with different impurities are mixed. Several papers address the impacts and acceptable concentrations of impurities in the CO2 stream, and several CO2 specifications have been suggested.4 These specifications are often based on literature studies (see Table 1) and the suggested maximum limits for SOx, NOx, and H2S are commonly based on HSE considerations in case of accidental release of the CO2, while pipeline integrity threats like corrosion or formation of solids were not considered. Thus, there is a lack of experimental data to verify that these limits are safe from a pipeline or ship transport point of view. Only a limited number of projects have performed experiments with multiple impurities with continuous replenishment,5-7 and the reported corrosion rates from those experiments were lower than 0.1 mm/y. Several experiments were carried out in closed systems (autoclaves) with batch injection6-16 of the impurities, usually without replenishment.6-15, 17-20 The corrosion rates reported are in the range from 0.005 to 7 mm/y, the water saturated experiments tend to report higher corrosion than the under-saturated or fully dissolved water experiments. Since the impurities (except for water) typically were present at a very low concentration (~50 – 200 ppmv) they may have been consumed rapidly if corrosion occurred. This is an issue which can possibly explain why some authors report higher corrosion rates in short duration tests as compared to long term experiments. If the impurities are consumed by corrosion or chemical reactions during the initial period of exposure, essentially all corrosion will take place in the initial phase after which corrosion might slow down or stop. This will give errors when the mass loss is divided by the full exposure time. The corrosion mechanism depends on which impurity is present in the experiments but most of the suggested mechanisms16, 19, 21-23 in the literature follows an absorption of impurity in an existing water film on the surface to create acids like sulfuric/sulfurous, and/or nitric/nitrous acid. The corrosion products often found are FeSO3, FeCO3, Fe2O3, and FeOOH12, 16, 19. Some has also reported FeSO4 5, 21, 24 and Fe(NO3)3, the latter compound is not common since it is regarded as unstable and instead will usually react to Fe2O3. In water-saturated experiments it is likely that a large or thick water film will form, and all the suggested mechanism could be valid. For dissolved water or undersaturated conditions with water concentration as low as 100 ppmv this would not be straight forward, since the water film would consist of only a few monolayers of water25. An experimental setup which allows the impurities to be continuously replenished and analyzed during the experiments was used in the present work. This removes uncertainties related to consumption and depletion of impurities due to reactions or corrosion. The present paper discusses the result of an experiment that was performed in a transparent autoclave with 10 MPa CO2 containing multiple impurities. The objective was to compare the results with previously published data,5 to identify if any reactions take place in what is regarded as a safe CO2 specification and to determine if these reactions may cause corrosion. EXPERIMENTAL PROCEDURE The experiment was carried out in an in-house built autoclave with transparent end-plates. The body material was SS316L, with lime soda glass and poly carbonate as the see-through material (Figure 1). The total volume of the autoclave was 330 ml and the maximum design pressure was 29 MPa. The autoclave had separate injection lines for the different impurities (Figure 2), a measure which prevented the impurities from reacting before entering the autoclave. All lines where of 1/16” tubing to minimize the volume and hence the lag time in the injection and sampling system. The water was pre-dissolved in the CO2 (at 10 MPa) before entering the autoclave, meaning that no liquid water was injected in the autoclave at any time. An in-house built moisture generator was used for this purpose. It was an autoclave filled with water. CO2 was injected at the bottom and moist CO2 taken out from the top. The water saturated CO2 was then mixed with dry CO2 at a given ratio to create the target level of dissolved water. The retention time through the water moisturizer was about 20 hours. All other impurities were pre-mixed with CO2 in separate high-pressure precision piston pumps operating at 10 MPa. The impurity stock solutions had concentrations from around 1000 to 3000 ppmv. The injection rates of the piston pumps were adjusted according to the total CO2 injection rate to get the target impurity concentration in the autoclave. Some minor impurity fluctuations were observed due to daynight temperature variations. The exhaust CO2 from the autoclave was depressurized to 0.2 MPa over a heated gas regulator and the flow rate was controlled by a mass flow controller at the low-pressure side. The impurity content of the exhaust CO2 was analyzed using OFCEAS (Optical Feedback Cavity Enhanced Absorption Spectroscopy) for H2O, H2S and O2. A NDIR/UV photometer was used to measure NO, NO2 and SO2, and a zirconium oxide sensor was used as a second O2 analyzer. Exhaust CO2 (with impurities) was finally ventilated to a safe area with scrubbers. Gas qualities used in the experiment are listed in Table 2. Three metal specimens (corrosion coupons) where prepared by grinding up to P1000 paper, rinsing in isopropanol and then cleaning in ultrasonic bath with acetone for 10 minutes, followed by 10 minutes in isopropanol before drying. Three different metals were exposed (Table 3); N10276 (nickel-alloy), S355MC (carbon steel), and S32205 (duplex stainless steel). The coupons were mounted in a holder made from polyether ether ketone (PEEK), as shown in Figure 2, the coupons size was 9.5x9.5 mm. The back side of the carbon steel coupon was covered with mill scale consisting of 5 wt.% wüstite and 95 wt.% magnetite. When the coupon holder was in place, the autoclave was closed and flushed for several hours with dry low-pressure CO2 to remove water and oxygen. Then the autoclave was filled with dry CO2 to a pressure of 10 MPa. Injection of dry CO2 continued until the measured water concentration was below 5 ppmv. The CO2 (with impurities) flow rate was 80 g/h. This flow rate di