(562ap) Modeling Chemical Absorption of CO2 in Highly Alkaline Solutions for Direct Air Carbon Capture Applications

Carbon dioxide is known to be one of the gases causing current global climate change. In 2015, with the signature of the Paris Agreement, nations agreed to reduce global emissions annually by 3% in order to avoid a catastrophic scenario. However this hasn’t been achieved and the current path, which includes mainly treatment of point sources such as flue gas, is not enough to avoid a global temperature change greater than 2°C, rising urgency for new and innovative ways to mitigate new emissions and to reduce existing CO₂ levels in the air [1].

Although long term geological storage of CO₂ have been demonstrated as feasible, it remains unattractive from an economic perspective, as –besides applications involving enhanced oil recovery– the cost of capture and storage is significantly above the market price assigned to carbon pollution. Consequently, recent research efforts are aimed at complementing carbon capture and storage with utilization strategies that generate valuable products from CO₂ [2], [3].

The production of bioenergy and bioproducts using photosynthetic microbes is quite attractive as, theoretically, it only requires minimal energy inputs to achieve the conversion, given that most of the energy can be provided by sunlight. Furthermore, processes that rely on photosynthetic microbes can be deployed in marginal land and can capture CO₂ directly from the air.

Unfortunately, a major obstacle currently hindering the large-scale deployment of photobiological processes is their low volumetric productivity. This low productivity is partially caused by limited solubility of CO₂ in the cultivation media and very low mass transfer rates. Current approaches to overcome this obstacle rely on a continuous bubbling of CO₂ streams into photobioreactors or cultivation ponds, which is accompanied by excessive CO₂ losses and high-energy requirements.

Although several advances have been reported regarding methods suitable for capturing CO₂ directly from the air, such as metal-organic frameworks and cryogenic distillation, which obtain very high purities they are still very energy intensive. Chemical absorption, on the other hand, is a well-established technology that could be easily deployed in a sustainable way, especially if environmentally friendly and easy to regenerate solvents are used.

Here we report on the modelling of a chemical absorption process using a highly concentrated aqueous solution of sodium carbonate. The high concentration of the carbonate solution favors the mass transfer of carbon dioxide from atmospheric air into the solution, and the solution can be regenerated directly by photosynthetic microbes.

Several factors affect the suitability of direct air capture applications. First, mass transfer is limited due to the low concentration of CO₂ in the air (~400 ppm) as at these low concentrations the likelihood of interaction and therefore absorption, is reduced. Second, the stability of the nonreacting CO₂ in the liquid phase and the relatively high fugacity of the CO₂ in aqueous solutions introduce significant thermodynamic limitations [4]. Third, there are also kinetic limitations introduced by the complex reaction pathways of CO₂ and carbonate species in aqueous solutions. The kinetics of highly alkaline solutions have been studied [5] and correspond to a series of electrochemical reactions in which the absorption of CO₂ is enhanced.

To understand the absorption mechanism and the limitations of the proposed system, a mathematical model was developed in COMSOL Multiphysics, incorporating the limiting factors previously described. Two different models were developed in order to determine the controlling mechanism in the process: the falling film model, which describes a thin film of liquid falling and a bulk of gas around it, and the two film theory, which simulates the bulk of both the liquid and gas as well as the interface between them.

The two-film model was found to provide a better understanding of the reaction and its likelihood in the interface and the mass transfer of the system. From this model, it is clear that, due to the high alkalinity of the media, the hydroxide ions available for the reaction are in such abundance that their concentration at the interface enables an increased probability of interaction with the CO₂, which is present in lower concentration on air allowing chemical absorption to occur by the previously described mechanism.

References:

[1] NOAA, “Global Greenhouse Gas Reference Network,” 2018. [Online]. Available: https://www.esrl.noaa.gov/gmd/ccgg/trends/weekly.html.

[2] D. W. Keith, K. Heidel, and R. Cherry, “Capturing CO2 from the atmosphere: Rationale and Process Design Considerations,” Geo-Engineering Climate Change: Environmental necessity or Pandora’s box? pp. 107–126, 2010.

[3] H. Herzog, “Air Assessing the Feasibility of Capturing CO2 from the Air,” MIT Lab. Energy Environ., no. October, 2003.

[4] K. S. Lackner, “The thermodynamics of direct air capture of carbon dioxide,” Energy, vol. 50, no. 1, pp. 38–46, 2013.

[5] H. Knuutila, O. Juliussen, and H. F. Svendsen, “Kinetics of the reaction of carbon dioxide with aqueous sodium and potassium carbonate solutions,” Chem. Eng. Sci., vol. 65, no. 23, pp. 6077–6088, 2010.