A novel crystallizer has shown promise in lowering costs for direct-air capture (DAC) of CO2 emissions. In recent work published in Nature Chemical Engineering, a research team from the University of Toronto; www.utoronto.ca), led by mechanical and industrial engineering professor David Sinton, details a crystallizer unit designed to convert CO2 from air directly into solid potassium carbonate crystals. “The device continuously wicks a potassium hydroxide (KOH) solution along fine polypropylene capillaries, and as natural wind flows over the surface, the water evaporates and the solution becomes more concentrated. Once concentrated, the KOH reacts immediately with CO2 in the air, forming solid carbonate crystals directly on the surface,” explains Dongha Kim, lead author on the new paper.
This passive, evaporative carbonate crystallization accelerates the reaction to form potassium carbonate by leveraging a newly discovered capture mechanism of KOH at ultra-high concentrations. Other DAC solutions have looked at porous, honeycomb-style materials to increase surface area, and thus reaction rate, but Kim says the team’s work instead focused on “long strands of polypropylene fiber immersed in a solution of potassium hydroxide.” This configuration enables the system to achieve extremely high KOH concentrations, which supports much faster CO2 capture — with a reported six-fold increase in capture flux over traditional DAC systems.
Conventional DAC plants often rely on large, energy-intensive fans and operate with a liquid-phase capture mechanism that requires additional solidification and filtration steps. In the new crystallizer, CO2 is solidified instantly, reducing process complexity and thereby lowering costs. “There are many challenges to removing carbon dioxide from the atmosphere, but none greater than cost,” emphasizes Sinton.
The technology has so far been demonstrated at the laboratory scale, with stable CO2 capture performance over a month of operation. The team foresees a modular and fully automated crystallizer design that can easily be scaled. “Because the crystallizer unit is structurally simple, we expect rapid and low-cost expansion through direct replication of these modules. Our next step is to deploy multiple crystallizer modules in parallel and expand the system to areas on the order of tens to hundreds of square meters. We also plan to integrate these crystallizer units with a large-scale electrochemical KOH regeneration system, enabling continuous CO2 capture and sorbent regeneration within a unified process,” says Kim.
Since the sorbent can be continuously recycled, the crystallization process does not create any notable waste streams. The high-purity CO2 generated in the process can be sequestered underground, or used in downstream chemical processes.