Advancing carbon dioxide capture: investigation into the kinetics and efficiency of absorption in molten calcium oxide–chloride†

Abstract

One promising carbon capture technology is the absorption of carbon dioxide (CO₂) by molten salt, specifically the molten mixture of calcium oxide and chloride (CaO + CaCl₂, COC), as it solves some of the key issues with alternative methods, including thermal stability and capture efficiency. The kinetics of CO₂ absorption in a column of COC is examined by deriving a simple kinetic model and determining the kinetic constants under various conditions. The model emphasises the importance of the oxide anion (O²⁻) concentration and CO₂ partial pressure in driving the absorption rates. Applying this model to reported experimental data on CO₂ absorption with varying molten salt height, or CaO wt% in molten CaCl₂ produced values for the kinetic constants with high accuracy. The fastest rate of absorption, with a rate constant of 0.00313 L mol⁻¹ min⁻¹ was achieved at a 15 cm molten salt height. Conversely, the slowest rate, 0.00062 L mol⁻¹ min⁻¹, occurred at 20 wt% CaO in CaCl₂. Comparative analysis with conventional amine-based CO₂ capture systems reveals a slower absorption rate for the molten salt. Nonetheless, there are further elements which need to be explored to perform a full comparison with the amine system, for example the desorption kinetics or absorption capacity. This reinforces the need for further research into molten salt absorption kinetics to gain a more holistic understanding of this technology and enable an optimal process design for further assessment of the feasibility and scalability of molten salt-based CO₂ capture in current and future processes. Ultimately, this will promote the adoption of carbon capture technology, cultivating more sustainable practices in industry.