Tuning sorbent properties to reduce the cost of direct air capture

Abstract

The question has shifted from whether solid sorbents can work in direct air capture (DAC) technologies to which solid sorbents are more economically advantaged. Determining this is challenging due to the influence of many different yet interconnected sorbent properties on the cost of CO₂ capture. Existing DAC economic models oversimplify sorbent stability by treating it as a simple replacement rate, neglecting crucial factors such as capacity loss rate and form. To address this challenge, we have developed an economic model that accounts for sorbent degradation in DAC processes. By factoring in capacity loss over time, our model provides a more accurate estimate of the cost associated with DAC and highlights the optimum time for sorbent replacement. We then identified sorbent characteristics and process features that minimize both the carbon footprint and the cost of captured CO₂. To further investigate the interplay of sorbent properties and DAC cost, we constructed a series of alkyl- and epoxy-functionalized polyamine sorbents. The sorbents’ CO₂ uptake, heat of adsorption and capacity fade were adjusted via a one-step modification, varying the proportions of primary, secondary, and tertiary amines. We then integrated the experimentally-measured parameters, including the form of degradation, into our economic model to probe which combination of sorbent properties results in the lowest cost of DAC for a fixed operating condition. The results provide guidelines and priorities for sorbent performance metrics that will yield the most cost-effective DAC technologies.