Understanding Material Compatibility in CO₂ Capture Systems Using Molten Alkali Metal Borates

Abstract

Molten alkali metal borates have been proposed as energy-efficient sorbents for the low-cost capture of CO2 at high temperatures. The molten sorbents could help to mitigate global warming by capturing CO2 from industrial sources and preventing the release of CO2 into the atmosphere. However, these novel materials operate under harsh conditions, introducing challenges of which material compatibility is one of the most important. Other than platinum, where a less than 0.1% change in performance was observed over 1000 h of continuous use, few materials were found to be compatible with the molten salts. Common ceramics, steels, and superalloys were eliminated from consideration due to corrosive oxidation of the substrate and contamination of the melt resulting in chemical degradation and reduction in the sorbent’s working capacity. A high-purity nickel alloy, Nickel 200/201, with a protective oxide layer was found to perform optimally with regards to both corrosive degradation and chemical degradation. Modest corrosion rates on the order of 0.3–0.5 mm/year were estimated, and the sorbent capacity was found to drop by between a manageable 0.5 and 20% over 100 h. Various protective measures are proposed, and future work suggested, to ensure that material compatibility does not limit the potential of molten alkali metal borates to reduce CO2 emissions and contribute to a clean energy future.

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

“…Valuable lessons may be learned also from hightemperature CO 2 -conducting membranes, where the molten carbonate needs to be retained within the porous solid matrix, 359,682−685 or salt-based energy storage systems. 686 Recently, Hatton et al 687,688 started to look into practical issues related to the use of molten salt-based sorbents for CO 2 (and potentially other gaseous species), in particular the compatibility with other materials in which the molten salt is contained (e.g., a vessel or pipes). They found that ceramics, stainless steel, and many superalloys (e.g., Inconel 600) are not suitable in combination with the molten salts investigated (Na 0.75 B 0.25 O 0.75 and (Li 0.5 Na 0.5 ) 0.75 B 0.25 O 0.75 ) owing to their corrosive oxidation and their contamination of the molten sorbent that decreases the sorbent's cyclic CO 2 uptake capacity.…”

Capture and electrochemical conversion of CO2 in molten alkali metal borate–carbonate blends

“…Our group has advanced molten alkali metal borates such as lithium borate (Li 3 BO 3 ) and mixed lithium, sodium borates as reversible CO 2 sorbents due to their large and temperature-reversible sequestration capacity for CO 2 and acid gases, which generally exceeds the capacity of other molten salts. 11–19 Thermoreversible reactions of the alkali metal borate glasses with CO 2 are well-documented. 11–20 The chemisorption of CO 2 by alkali metal (M) borate M 3 BO 3 results in the reversible formation of carbonate (M 2 CO 3 ), metal oxide (M 2 O)–boron oxide (B 2 O 3 ) binary compounds, and metaborate (MBO 2 ) as follows.4M 3 BO 3(l) + 3CO 2(g) ↔ 3M 2 CO 3(l) + M 6 B 4 O 9(l) 4M 3 BO 3(l) + 5CO 2(g) ↔ 5M 2 CO 3(l) + M 2 B 4 O 7(l) M 3 BO 3(l) + CO 2(g) ↔ M 2 CO 3(l) + MBO 2(l) 2M 3 BO 3(l) + 3CO 2(g) ↔ 3M 2 CO 3(l) + B 2 O 3(l) (M = Li, Na, K, etc .…”

“…11–19 Thermoreversible reactions of the alkali metal borate glasses with CO 2 are well-documented. 11–20 The chemisorption of CO 2 by alkali metal (M) borate M 3 BO 3 results in the reversible formation of carbonate (M 2 CO 3 ), metal oxide (M 2 O)–boron oxide (B 2 O 3 ) binary compounds, and metaborate (MBO 2 ) as follows.4M 3 BO 3(l) + 3CO 2(g) ↔ 3M 2 CO 3(l) + M 6 B 4 O 9(l) 4M 3 BO 3(l) + 5CO 2(g) ↔ 5M 2 CO 3(l) + M 2 B 4 O 7(l) M 3 BO 3(l) + CO 2(g) ↔ M 2 CO 3(l) + MBO 2(l) 2M 3 BO 3(l) + 3CO 2(g) ↔ 3M 2 CO 3(l) + B 2 O 3(l) (M = Li, Na, K, etc . ).…”

Sorbents for the Capture of CO2 and Other Acid Gases: A Review

“…A number of advantages of certain salts for CO 2 capture, including high capacities, fast kinetics, and excellent stability, have been demonstrated with molten alkali metal borates. ,, These materials may be tuned for entirely liquid phase reactions or for the phase change precipitation of reaction products, introducing opportunities to handle captured SO x and NO x impurities . As high temperature liquid phase materials, these sorbents are uniquely positioned, although a number of uncertainties and challenges remain around their corrosive nature and their practical operation at scale. , …”

“…97,124,125 These materials may be tuned for entirely liquid phase reactions or for the phase change precipitation of reaction products, introducing opportunities to handle captured SO x and NO x impurities. 304 As high temperature liquid phase materials, these sorbents are uniquely positioned, although a number of uncertainties and challenges remain around their corrosive nature 305 and their practical operation at scale. 136,306 7.9.…”