Enhancing Green Ammonia Capture and Storage: Structure and Composition of Thermally Conductive MgCl₂–Al Absorbents for Reproducible Cyclic Performance

Metal halides such as MgCl2 have a high capacity for ammonia absorption and can be used in place of a condenser to efficiently separate ammonia from the reactor effluent in the Haber–Bosch process. Although these salts are effective at capturing and storing ammonia, they show reduced working capacity in their pure form with repeated cycling due to slowed uptake and release with particle agglomeration and heat transfer limitations. To stabilize the ammonia uptake performance, MgCl2 salt crystals were supported on aluminum fibers. Incorporating aluminum fibers into pure MgCl2 was found to reduce the sample charging during electron microscopy and increase the bed thermal conductivity. By using a 40 wt % MgCl2–Al absorbent mixture, cyclic ammonia working capacity was increased by a third over our previously reported absorbent (40 wt % MgCl2–SiO2) and the bed thermal conductivity increased by an order of magnitude. Higher performance stability using aluminum was attributed to two possible factors: the aluminum fibers guide recrystallization so as to maintain a crust of small crystals that adhered to the metal surface and the high thermal conductivity of the aluminum fibers can help suppress temperature overshoot or undershoot during ammonia uptake or release, respectively.