Using Industrial Alkaline Byproducts to Remove CO₂ Emissions

What is it?

Production of industrial goods and utilities such as cement, steel, batteries, and electricity from coal-fired power plants result in alkaline solid waste byproducts. “Alkaline” in this context means that the base-generating potential of alkaline cations such as Ca2+ and Mg2+ in the material exceeds the acid-generating potential of acidic anions such as SO42- or Cl-. This chemical imbalance can be neutralized by the introduction of CO₂, which is thereby stored stably in bicarbonate (HCO3-) or carbonate (CO32-) forms dissolved in water or as solid minerals. Such reactions are thermodynamically spontaneous, occur at moderate rates that can be accelerated through engineered processes, and may have co-benefits. These factors make industrial alkaline wastes promising as feedstocks that can store CO₂ and even make usable products.

How does it work?

CO₂ is an “acid gas”. That is, it is the anhydrous form of carbonic acid (CO₂ + H₂O = H₂CO3). Thus, when CO₂ dissolves in water, it produces a moderately acidic solution. This moderate acidity contributes to the refreshing taste of seltzer water. This is also the reason why CO₂ emissions lead to ocean acidification.

While industrial processes generate CO₂, many such processes also produce alkaline byproducts. One reason for this is that thermal processes often drive off volatiles such as CO₂ to produce oxides. Such is the case for calcination of limestone to produce calcium oxide, also known as quicklime (CaCO3 + heat = CaO + CO2). Another reason is that Earth’s average composition is highly alkaline. The most abundant mineral on Earth is olivine (Mg2SiO4), which is rich in the alkaline cation, Mg2+. Thus, mining and processing for valuable commodities such as nickel, cobalt, platinum, and diamonds, which are hosted in rocks that are compositionally similar to olivine, produce alkaline byproducts. Indeed, the weathering of natural silicate rocks is a dominant mechanism by which CO₂ concentrations in Earth’s atmosphere have been regulated over geologic time (Raymo and Ruddiman, 1992).

While neutralization of alkaline materials with CO₂ is spontaneous at Earth surface conditions, these processes occur at rates far slower than the pace at which humanity emits CO₂. The objective of the technologies summarized here is to speed up these processes. However, acceleration typically comes at an energetic cost. Thus, research in this area is geared toward optimizing the relationship between accelerating CO₂ uptake and minimizing energy input. 

There are two general approaches to using alkaline materials to remove CO₂ emissions. One is dissolution of the alkaline materials in ocean waters (or initially in soil pore waters), where they react with atmospheric CO₂ and are stored predominantly as dissolved HCO3- (Renforth and Henderson, 2017; Beerling et al., 2020). Another is the reaction of the materials with CO₂ to produce stable carbonates incorporating metals such as Ca, Mg, and Fe in their divalent forms (Hills et al., 2020). The CO₂ may derive from flue gas or directly from air and may be concentrated from these streams at an energetic cost, but often with transportation or reactivity benefits. Some of the myriad approaches to using alkaline materials to store CO₂ are depicted in Figure 1.

Figure 1. Flow chart of alkaline byproduct generation from industrial processes, methods to remove CO₂ emissions using alkaline materials, and beneficial reuse possibilities for carbonate mineral products.

Why is it important and what are the major barriers?

Scale is a critical topic for any CO₂ storage approach. Alkaline byproducts have the potential to achieve CO₂ storage on the scale of Gt yr-1 today, and generation of alkaline byproducts is projected to increase considerably over the coming century (Figure 2). The increased production stems from trends of population and economic growth as well as technological shifts including increased mining for battery metals in alkaline rock deposits. However, it is important to understand that the CO₂ emissions associated with the production of the higher-value commodities are often greater than the carbonation potential of the byproducts. Viewed in light of anthropogenic CO₂ emissions of 42.2 ± 3.3 Gt CO₂ in 2019 (Friedlingstein et al., 2020), it is evident that the reaction of CO₂ with alkaline byproducts will be a useful tool for climate change mitigation, but must be complemented by other approaches such as subsurface geologic CO₂ storage to meet the necessary scale of CO₂ management.

Figure 2. Forecast of CO₂ mineralization potential of alkaline materials to 2100 for a “middle-of-the-road” shared socio-economic pathway (SSP 2 of Riahi et al. (2017)) baseline scenario (no new climate policies beyond those in place today), after Renforth et al. (2019). The error bars represent the standard error for the range of pathways in the SSP (n = 5) together with uncertainties of material production and consumption, and chemistry.

Beyond scale, risks and co-benefits are important metrics by which to evaluate CO₂ storage solutions. One risk that is particularly pertinent to enhanced weathering and ocean alkalinization approaches is leaching of potentially toxic trace metals from the byproducts. Diligent screening and monitoring will be necessary to ensure such methods are implemented safely. These approaches will also need to empirically determine through robustly-designed demonstration projects how the theoretical CO₂ storage of the application of a given quantity of alkaline material relates to that actually observed in complex terrestrial and marine systems.

On the other hand, trace metals remediation and/or extraction can be potential co-benefits of technologies such as accelerated carbonation. This is because toxic metals can be mobile in alkaline waste stockpiles, and carbonation can help to solidify and immobilize these hazardous metals (Fernández Bertos et al., 2004; Hamilton et al., 2018). However, if the carbonated product contains materials that are hazardous or perceived as so, their value for beneficial reuse may be limited. In addition, carbonation may help add structural stability to tailings storage facilities (Vanderzee and Bradshaw, 2018). Further, alkaline byproducts may contain valuable trace metals. For example, approximately 30% of production of the metal germanium, which can be used in photovoltaic cells, is derived from leaching of coal fly ash (Bleiwas, 2012).

Challenges to deployment of CO₂-storing technologies that use alkaline industrial byproducts can be technical, economic, or informational. While the basic processes of carbonating many types of alkaline industrial byproducts are well documented, they are not always optimized at scale. In addition, technologies that exploit the synergies between CO₂ storage and extraction of valuable metals are fairly recent and generally at low technology readiness levels (Hamilton et al., 2021; Wang et al., 2021; Khan et al., 2021). Information access is a key challenge to overcome in scaling technologies that leverage alkaline materials for CO₂ storage. For example, while the locations of mining and industrial sites are reasonably well-catalogued by governments, the quantity of alkaline wastes stored onsite and their chemical compositions are seldom reported. Seeking those types of data and combining them with systems modeling including transportation logistics and life-cycle assessment are key areas of study.‍

What we do

Our activities in this field include experimental dissolution / carbonation of alkaline industrial wastes (Woodall et al., 2021) and techno-economic analyses of such processes. We also engage in market analysis of products of the carbonation process (Woodall et al., 2019). Geospatial analysis is an important part of our research in this area (Pilorgé et al., 2021). Ongoing work couples geospatial information with transportation cost modeling and life-cycle assessment.

Additional reading and references:

Beerling D. J., Kantzas E. P., Lomas M. R., Wade P., Eufrasio R. M., Renforth P., Sarkar B., Andrews M. G., James R. H., Pearce C. R., Mercure J.-F., Pollitt H., Holden P. B., Edwards N. R., Khanna M., Koh L., Quegan S., Pidgeon N. F., Janssens I. A., Hansen J. and Banwart S. A. (2020) Potential for large-scale CO2 removal via enhanced rock weathering with croplands. Nature 583, 242–248.

Bleiwas D. I. (2012) USGS Circular 1365: Byproduct Mineral Commodities Used for the Production of Photovoltaic Cells.

Fernández Bertos M., Simons S. J. R., Hills C. D. and Carey P. J. (2004) A review of accelerated carbonation technology in the treatment of cement-based materials and sequestration of CO2. J. Hazard. Mater. 112, 193–205.

Friedlingstein P., O’Sullivan M., Jones M. W., Andrew R. M., Hauck J., Olsen A., Peters G. P., Peters W., Pongratz J., Sitch S., Le Quéré C., Canadell J. G., Ciais P., Jackson R. B., Alin S., Aragão L. E. O. C., Arneth A., Arora V., Bates N. R., Becker M., Benoit-Cattin A., Bittig H. C., Bopp L., Bultan S., Chandra N., Chevallier F., Chini L. P., Evans W., Florentie L., Forster P. M., Gasser T., Gehlen M., Gilfillan D., Gkritzalis T., Gregor L., Gruber N., Harris I., Hartung K., Haverd V., Houghton R. A., Ilyina T., Jain A. K., Joetzjer E., Kadono K., Kato E., Kitidis V., Korsbakken J. I., Landschützer P., Lefèvre N., Lenton A., Lienert S., Liu Z., Lombardozzi D., Marland G., Metzl N., Munro D. R., Nabel J. E. M. S., Nakaoka S.-I., Niwa Y., O’Brien K., Ono T., Palmer P. I., Pierrot D., Poulter B., Resplandy L., Robertson E., Rödenbeck C., Schwinger J., Séférian R., Skjelvan I., Smith A. J. P., Sutton A. J., Tanhua T., Tans P. P., Tian H., Tilbrook B., van der Werf G., Vuichard N., Walker A. P., Wanninkhof R., Watson A. J., Willis D., Wiltshire A. J., Yuan W., Yue X. and Zaehle S. (2020) Global Carbon Budget 2020. Earth Syst. Sci. Data 12, 3269–3340.

Hamilton J. L., Wilson S. A., Morgan B., Turvey C. C., Paterson D. J., Jowitt S. M., McCutcheon J. and Southam G. (2018) Fate of transition metals during passive carbonation of ultramafic mine tailings via air capture with potential for metal resource recovery. Int. J. Greenh. Gas Control 71, 155–167.

Hamilton J. L., Wilson S. A., Turvey C. C., Morgan B., Tait A. W., McCutcheon J., Fallon S. J. and Southam G. (2021) Carbon accounting of mined landscapes, and deployment of a geochemical treatment system for enhanced weathering at Woodsreef Chrysotile Mine, NSW, Australia. J. Geochem. Explor. 220, 106655.

Hills C. D., Tripathi N. and Carey P. J. (2020) Mineralization Technology for Carbon Capture, Utilization, and Storage. Front. Energy Res. 8, 142.

Khan S., Wani O. B., Shoaib M., Forster J., Sodhi R. N., Boucher D. and Bobicki E. R. (2021) Mineral carbonation for serpentine mitigation in nickel processing: a step towards industrial carbon capture and storage. Faraday Discuss. 230, 172–186.

Pilorgé H., Kolosz B., Wu G. C. and Freeman J. (2021) Global Mapping of CDR Opportunities. In Carbon Dioxide Removal Primer (eds. J. Wilcox, B. Kolosz, and J. Freeman).

Raymo M. E. and Ruddiman W. F. (1992) Tectonic forcing of late Cenozoic climate. Nature 359, 117–122.

Renforth P. (2019) The negative emission potential of alkaline materials. Nat. Commun. 10, 1401.

Renforth P. and Henderson G. (2017) Assessing ocean alkalinity for carbon sequestration. Rev. Geophys. 55, 636–674.

Riahi K., van Vuuren D. P., Kriegler E., Edmonds J., O’Neill B. C., Fujimori S., Bauer N., Calvin K., Dellink R., Fricko O., Lutz W., Popp A., Cuaresma J. C., Kc S., Leimbach M., Jiang L., Kram T., Rao S., Emmerling J., Ebi K., Hasegawa T., Havlik P., Humpenöder F., Da Silva L. A., Smith S., Stehfest E., Bosetti V., Eom J., Gernaat D., Masui T., Rogelj J., Strefler J., Drouet L., Krey V., Luderer G., Harmsen M., Takahashi K., Baumstark L., Doelman J. C., Kainuma M., Klimont Z., Marangoni G., Lotze-Campen H., Obersteiner M., Tabeau A. and Tavoni M. (2017) The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: An overview. Glob. Environ. Change 42, 153–168.

Vanderzee S. and Bradshaw P. (2018) Targeting Highly Reactive Labile Magnesium in Ultramafic Tailings for Greenhouse-Gas Offsets and Potential Tailings Stabilization at the Baptiste Deposit, Central British Columbia (NTS 093K/13, 14).

Wang F., Dreisinger D., Jarvis M., Hitchins T. and Trytten L. (2021) CO2 mineralization and concurrent utilization for nickel conversion from nickel silicates to nickel sulfides. Chem. Eng. J. 406, 126761.

Woodall C. M., Lu X., Dipple G. and Wilcox J. (2021) Carbon Mineralization with North American PGM Mine Tailings—Characterization and Reactivity Analysis. Minerals 11, 844.

Woodall C. M., McQueen N., Pilorgé H. and Wilcox J. (2019) Utilization of mineral carbonation products: current state and potential. Greenh. Gases Sci. Technol. 9, 1096–1113.