Optimizing Ptx Carbon Capture: Rapid KOH Reaction Dynamics
Authors: Lund Biel-Nielsen Tessa , Neerup Randi , Vinjarapu Sai Hema Bhavya , Bonde Jacob Lindnar , Villadsen Sebastian Nis Bay , Fosbøl Philip Loldrup 
Published: December 2024

Introduction:

Unprecedented increase in atmospheric CO2 levels calls for efficient, sustainable, and cost-effective technologies for CO2 removal. Advancing carbon capture technologies depends significantly on the innovation of effective absorbents. In the development of new solvents, the following criteria should be prioritized: 1) economic viability, higher CO2 uptake, fast kinetics, reduced degradation rates, energy-efficient solvent regeneration, environmental effects, and scalability. Hydroxide solvents meet most of these criteria. They are cheap, highly available in the industry market, scalable, and environmentally friendly. In addition, they are applicable in innovative CO2 capture set-ups, specifically in electro-scrubbers. This provides lower regeneration costs and increases operational flexibility. Despite their advantages, hydroxides have so far been underutilized in carbon capture for two reasons: 1) energy concerns in thermal capture set-ups (500-800 kJ/mol CO2 required for solvent regeneration and 2) a perceived slow CO2 absorption rate in alkaline solutions. However, the reaction dynamics of hydroxides in electrochemical capture technologies are not yet fully understood and may present new opportunities. In fact, the whole assumption of kinetic limitations may be mistaken thereby hindering hydroxide-based PtX systems to gain traction.

Methodology

The primary objective of this study is to determine the absorption mechanisms between CO2 and hydroxide-based solvents in an electrochemical framework. To do so, the kinetic rate of absorption of carbon dioxide by potassium hydroxide has been measured experimentally using a wetted wall column set-up. Kinetics have been determined at varying operating conditions including temperatures of 293-333 K and ionic strengths of 5-25 w/w%. Both mass transfer constants as well as the second order rate constant (k2) have been determined. The traditional capture solvent monoethanolamine (MEA) has been used as a benchmark for evaluating the effectiveness of the hydroxides.

Subsequently, the hydroxide solvent has been tested in a pilot-scale PtX carbon capture set-up. The cell used operates on a pH-swing concept, where the hydroxide solvent is readily reduced and oxidized through electrical inputs. The process depends on the speciation equilibrium allowing CO2 capture at high pH (as HCO3
- and CO3
2-) and CO2 release at low pH.

The specific anode half-cell reaction:

This reaction locally lowers the pH, which pushes the HCO3
-/CO2 equilibrium to the right, resulting in evaporation of CO2 from the solvent:

The cathode half-cell reaction:

CO2 and O2 will be desorbed in a ratio of 4:1 at the anode, whereas high-purity hydrogen is released from the cathode.

Results

In this study new kinetic models for determining the reaction constant, k2, in KOH solutions have been developed. These models account for both variations in temperature and ionic strength. Experimental results emphasize superior reactivity of potassium hydroxide (KOH) in comparison to MEA. In an electrochemical setting, the enhanced kinetic properties can be successfully utilized, offering efficient CO2 absorption, desorption, and the added benefit of hydrogen production as a byproduct. The production of CO2, O2, and H2 is largely governed by the choice of electrodes and membrane configurations in the electrochemical cell. The influence of these parameters on the system has also been investigated when operating with a hydroxide solvent.

This research offers new insight into the reaction mechanisms between CO2 and hydroxide. It clarifies why the application of hydroxides is more hindered by energy-related limitations in a thermal system compared to an electrochemical one. In addition, extensive electrolyte modelling has also proved that the absorption process with KOH is limited by a scarcity of OH- ions rather than by kinetic factors. At a loading of 0.5 mol CO2/mol K essentially all OH- has been depleted (< 3% of the original OH- content remains).

Scientific Innovation

The adoption of hydroxide solvents opens the door to creating a new generation of electro-scrubbers seamlessly integrating into future energy infrastructures. By the technology presented in this study, CO2 capture, and conversion becomes closely linked to alternative energy sources and Power-to-X/e-fuel applications including hydrogen production. This proposed coupling of sectors can bring down the costs of implementing carbon capture in small-scale industries – a segment which in the future will certainly need cost-effective emission mitigation strategies. The produced hydrogen can be directly employed in, e.g., fuel cells for marine industries, or directed towards various PtX applications, encompassing renewable energy storage, fuel production, and chemical synthesis. Once separated from O2, the CO2 produced can be used in the food and beverage industry, in green houses, or in generation of renewable fuels.