Seasonal and Transient Effects of Atmospheric Humidity and Temperature on CO ₂ Capture Efficiency in Solid–Sorbent Packed Bed DAC Systems

Direct air capture (DAC) of CO 2 via solid adsorbents is a promising and scalable technology, but its implementation is limited by high energy demands driven by a low atmospheric CO 2 concentration and high desorption temperatures. Advancing this technology requires developing novel sorbents and process strategies that maximize the CO 2 adsorption capacity while enabling low-temperature regeneration. Competitive H 2 O adsorption is another major hurdle in CO 2 capture as H 2 O outcompetes CO 2 for active sites, reducing CO 2 adsorption capacity and increasing the energy penalty for regeneration due to the higher heat of H 2 O desorption. This work investigates how ambient humidity and temperature and their variability affect CO 2 adsorption on packed beds of zeolites 13X for DAC, with the aim of optimizing conditions for maximum CO 2 adsorption and helping to overcome the obstacles of low CO 2 concentration and high energy barriers for large-scale deployment. To this end, the dynamic behavior of CO 2 and H 2 O coadsorption in a DAC unit packed with zeolite 13X particles was studied via a two-dimensional, nonisothermal, two-scale heterogeneous dynamic model. Results indicate that while water competitively inhibits CO 2 adsorption, solid–sorbent DAC efficiency is optimized at lower temperatures and lower humidity, which enhances CO 2 adsorption while minimizing unnecessary energy-intensive water accumulation. A moderate and time-constant inlet relative humidity (around 30%) appears to be a potential optimal point for CO 2 capture (CO 2 reaches near-maximum adsorption capacity while minimizing unnecessary excess H 2 O accumulation) during summer night operation at 15 °C and moderate gas velocity. Higher gas velocities facilitate rapidly moving adsorption in packed beds, quickly maximizing CO 2 capture even at 50% humidity, but they result in rapid and uniform H 2 O accumulation that significantly increases regeneration energy. Conversely, in winter conditions, lower temperatures reduce the absolute H 2 O content in the air despite high relative humidity (e.g., 88%), slowing the rate of H 2 O adsorption throughout the second half of the packed bed, thus preventing premature saturation of the entire packed bed with H 2 O, allowing it to function effectively without immediate moisture overload.