Study of Direct Air Capture (DAC) Using a KOH/K2CO3 Absorbing Solution for CO2 Capture

Direct Air Capture (DAC) consists of sequestrating CO2 from ambient air using absorption or adsorption mechanisms. Within the former, alkaline solutions, such as KOH, are generally used as the absorbing agent due to the high reached pH values favouring CO2 absorption. However, these solutions' alkalinity induces high corrosivity and harmful properties, making them challenging to handle. The KOH/K2CO3 mixture solution is studied in this paper and proposed as an alternative to the more generally used strong base solution. This mixture solution aims at taking advantage of the low toxicity and corrosivity of K2CO3 solutions while still including a fraction of KOH. It guarantees sufficiently high alkalinity and counteracts K2CO3 poor mass transfer performance. The purpose of this paper is to have first estimations of the potential of KOH/K2CO3 solutions as the absorbing agent for carbon capture in ambient air by taking only equilibrium considerations and assuming an ideal aqueous solution. Within this framework, CO2 removal efficiencies are calculated for various pH values and phase ratios, and then the results are compared with literature data. This comparison highlights the importance of kinetics limitations for CO2 absorption in aqueous solutions. A significant part of this work is focused on the potential precipitation issues that may occur during CO2 absorption. These issues’ likelihood is compared for the case of a KOH/K2CO3 and a NaOH/K2CO3 solution. It turns out that precipitation is only encountered in the latter situation when extreme concentration conditions are encountered. In further work, the results should be refined by including the absorption kinetics and appropriately modelling the aqueous phase, accounting for the existing interactions. These factors may influence the conditions under which precipitation occurs. Nevertheless, the present study gives the necessary thermodynamics fundamental knowledge for addressing the challenges to be encountered in KOH/K2CO3 solutions.