(501c) Metal Halide Absorbents for the Production of Green Ammonia

Ammonia is the second most produced chemical by mass (200 Mt yr^-1) due to its central role in the production of fertilisers which enable food production for 50% of the global population. This global demand is projected to triple in the next 25 years due to the increasingly recognised potential of green ammonia in storing renewable energy in its carbon-free chemical bonds, effectively serving as a renewable fuel replacement for fossil fuels. However, the realisation of the so-called ‘green ammonia economy’ requires innovations to convert the conventional, emissions-intensive (2.4 t_CO2/t_NH3), continuous Haber-Bosch process into one that is driven by renewable energy.^1,2 One of the main challenges in achieving this is related to the intermittent and distributed nature of renewable energy which necessitates the need for small-scale, flexible ammonia production processes that can effectively ramp production capacity.^3,4 In this context, absorption-enhanced ammonia synthesis opens the door to mild pressure operations at 30 bar rather than traditional Haber-Bosch conditions of 200 bar, thereby enabling the process to ramp up and down quickly and safely. ^5,6

Whilst the thermodynamics and capacities of many absorbent materials are well documented, reliable kinetic data has remained elusive due to the transient nature of the reaction which complicates kinetic measurements. Furthermore, metal halide absorbents experience agglomeration under cyclic conditions as well as decomposition during pre-treatment, both of which have critical implications on the absorption kinetics and capacities of these materials. In this work, we present a robust way of measuring the kinetics of absorption and desorption of ammonia in metal halides. We have developed a novel experimental protocol to measure reaction kinetics in a variety of absorbent materials in flow under isothermal and isobaric conditions. These new capabilities provide insights into the agglomeration of metal halides under reaction conditions and how losses in capacity can be mitigated through material design.

The findings from these studies will have a profound impact on the design of absorption-enhanced processes to accelerate the deployment of these materials not only for green ammonia production but also in the refrigeration and energy storage industries where these metal halide absorbents are seeing increased usage and prevalence.

References

[1] IRENA. Innovation Outlook: Renewable Ammonia. 2022.

[2] David, William I. F., et al. “2023 Roadmap on Ammonia as a Carbon-Free Fuel.” JPhys Energy, vol. 6, no. 2, IOP Publishing, Nov. 2023, https://doi.org/10.1088/2515-7655/ad0a3a.

[3] Smith, Collin, and Laura Torrente-Murciano. “The Importance of Dynamic Operation and Renewable Energy Source on the Economic Feasibility of Green Ammonia.” Joule, vol. 8, no. 1, Jan. 2024, pp. 157–74, https://doi.org/10.1016/j.joule.2023.12.002.

[4] Torrente‐Murciano, Laura, and Collin Smith. “Process Challenges of Green Ammonia Production.” Nature Synthesis, vol. 2, no. 7, Nature Portfolio, June 2023, pp. 587–88, https://doi.org/10.1038/s44160-023-00339-x.

[5] Smith, Collin, and Laura Torrente‐Murciano. “Exceeding Single‐Pass Equilibrium with Integrated Absorption Separation for Ammonia Synthesis Using Renewable Energy—Redefining the Haber‐Bosch Loop.” Advanced Energy Materials, vol. 11, no. 13, Feb. 2021, p. 2003845, https://doi.org/10.1002/aenm.202003845

[6] Smith, Collin, et al. “Current and Future Role of Haber–Bosch Ammonia in a Carbon-Free Energy Landscape.” Energy & Environmental Science, vol. 13, no. 2, Feb. 2020, pp. 331–44, https://doi.org/10.1039/C9EE02873K.