(57c) Corrosion Analysis of RTI’s Non-Aqueous Solvent for Carbon Capture

RTI International is developing a novel non-aqueous solvent (eCO₂Sol) based process for post-combustion CO₂ capture from coal-fired power plant flue gas. The process has the potential of substantially reducing the thermal energy required for solvent regeneration compared to the NETL baseline Case 12. RTI has already shown the reduction of thermal energy using NAS in a Bench-scale Gas Absorption System (BsGAS) at RTI with simulated flue gas and a pilot-scale unit at SINTEF’s Tiller facility in Trondheim, Norway with coal-derived flue gas. Further NAS testing at the National Carbon Capture Center (NCCC) was used to investigate solvent degradation, corrosion, and emissions using the Slipstream Solvent Test Unit (SSTU) with long-term exposure to coal-derived flue gas from Southern Company’s Plant Gaston in Wilsonville, Alabama to reduce the scale-up risks for this technology. Testing was conducted during May-August 2018, with 580 hours of time on stream with coal-derived flue gas from the power plant. The post-combustion test facilities at NCCC include the 0.5 MWe Pilot-Scale Solvent Test Unit (PSTU), the 50 KW Slipstream Solvent Test Unit (SSTU), and other customer supplied test units.

The capital cost of CO₂ capture plants is one of the largest drivers in the cost of CO₂ captured. As a result, efforts to reduce the capital costs are one of the most effective ways to bring down the cost of a technology. Most capture systems are currently built with stainless steel due to the corrosive nature of MEA and other aqueous solvents, but cheaper alternatives including carbon steel or plastic liners may be suitable for solvents like eCO₂Sol. This paper will focus on the corrosion analysis of the eCO₂Sol completed during testing at NCCC.

Corrosion coupons were inserted in the piping downstream of the absorber and reboiler that each included carbon steel and stainless steel as well as polypropylene at the absorber outlet. The coupons were removed from the system at the end of the testing and were analyzed for weight change and any surface defects. It was observed that the eCO₂Sol (0.015 mm/yr) exhibited extremely lower corrosion rates, ~100x less than aqueous solvents (MEA – 50 mm/yr, Gunasekaran et al., 2013) for carbon steel due to the lower water content of the solvent. The corrosion rate can be correlated with the conductivity of the solvent. The corrosion rate for stainless steel was about one order of magnitude lower than for the carbon steel. Comparisons with other values reported in literature showed power law correlations fit well between the conductivity of solvents and the corrosion rate.

Samples of the solvent were routinely taken at various points in the system during steady state operation to analyze the system operation and detect any changes in the solvent. Samples were analyzed for amine, water, and CO₂ content from both the absorber and desorber side of the system. In addition, the concentrations of metals in the samples were analyzed using inductively coupled plasma mass spectrometry (ICPMS) for metal content. Chromium (Cr), iron (Fe), nickel (Ni), manganese (Mn), and cobalt (Co) metals are present in the SSTU materials of construction and are not present in the flue gas in significant quantities. The presence of these metal ions shows that the occurrence of corrosion in the process equipment. Fe, Cr, and Ni ions were significantly lower than MEA in the tests. The major metal components present in MEA are two orders of magnitude higher than the metals measured in eCO₂Sol. For instance, a maximum of ~100 ppm Fe was seen in the MEA campaign, whereas ~1 ppm Fe was observed in the eCO₂Sol campaign.

Fischer et al. (2017) suggested that a linear correlation exists between the rate of corrosion and the rate of formate ion production. The corrosion rate increases with an increase in formate ion concentration. The measured corrosion rates of eCO₂Sol and others from literature were compared and a power law correlation was fit to the data of multiple solvents.

Long-term corrosion studies are also underway for the eCO₂Sol and will be compared to the data collected at NCCC. The low corrosion rate of the eCO₂Sol indicates that CO₂ capture plants could use low-cost materials of construction, lowering the capital cost significantly and reducing the cost of CO₂ capture. A technoeconomic analysis will also be included to show the reduction in cost of capture from the use of cheaper materials.