Session: 08-02: Solar Chemistry: Thermochemical Fuel Production II

Paper Number: 138330

138330 - Solar-Driven Thermochemical Production of Green Ammonia via a Strontium-Based Cycle 

Abstract: 

The current conventional production of ammonia (NH₃) relies on the established Haber-Bosch synthesis involving a catalytic reaction between H₂ and N₂. The process takes place at temperatures around 400 °C and pressures around 300 bar. Despite the severe operating conditions, single-pass reaction extents do not exceed 25 %. The required high pressures and recycle flows of unreacted H₂ and N₂ impose complex equipment and, in turn, substantial costs, favouring large-scale centralized plants. Furthermore, as H₂ and N₂ are industrially obtained by means of energy-intensive processes using fossil fuels and additional pump work is required, the worldwide ammonia production is responsible for about 1.2 % of the anthropogenic global greenhouse gas emissions. Alternatively, two-step thermochemical cycles based on metal nitrides stand as a promising pathway to green ammonia production because they can substantially mitigate or even eliminate the concomitant CO₂ emissions [2]-[5]. In such cycles, concentrated solar energy is used as the source of the high-temperature process heat required to effect the endothermic reaction steps. The advantage of these cycles over the Haber-Bosch process is the decoupling of the high-temperature step to break the strong N₂ triple bond from the ammonia synthesis step, and the lower temperature and pressure requirement. This could make it economically feasible to commission smaller scale and distributed plants.

The investigation of the Strontium-based cycle is revealed a possible cycle based on the following net reactions:

Nitridation: 2 SrH₂ + N₂ → Sr₂HN + NH₃
Hydrogenation: Sr₂HN + 3 H₂ → 2 SrH₂ + NH₃

The nitridation proceeds endothermically (dH°_298K = 21.46 kJ/mol) at 443 °C and 1 bar while the hydrogenation reaction proceeds exothermically (dH°_298K = -113.25 kJ/mol) at 218 °C and 1 bar. Thermogravimetric (TGA) runs demonstrated the generation of ammonia during both the hydrogenation and nitridation step. The same material in a powder form was also tested in a pressurized packed-bed reactor. The gas composition at the outlet of the reactor was monitored on-line with a NH₃ analyzer, a mass spectrometer, and a volumetric flow meter. Solid reactants and products were characterized by X-ray powder diffraction and scanning electron microscopy. The effect of pressure on the reaction extent was investigated at up to 50 bar. For each step, an accurate mass balance was performed from which the selectivity of reactions was determined. Reaction stability with respect to sintering and deactivation of solids was investigated in multiple consecutive cycles.

References
[1]          C. Smith et al., Energy Environ. Sci. 13, 331–344, 2020.
[2]          M. E. Gálvez et al., Ind. Eng. Chem. Res.  46, 2042–2046, 2007.
[3]          R. Michalsky et al., Chem. Sci. 6, 3965–3974, 2015.
[4]          A. Jain et al., Int. J. Hydrogen Energy 42, 24897–24903, 2017.
[5]          N. P. Nguyen et al., Adv. Energy Mater., 1–10, 2023.

Presenting Author: Daniel Notter ETH Zurich

Presenting Author Biography: Daniel Notter acquired a Master of Science degree in Mechanical Engineering from ETH Zürich. After a short stint as a Research Assistant working on thermochemical energy storage, he started his Doctoral Studies in 2021 under the supervision of Prof. Aldo Steinfeld. His research and talk are centered around the sustainable production of ammonia by means of solar-driven thermochemical cycles.

Authors:

Daniel Notter ETH Zurich
María-Elena Gálvez Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza
Aldo Steinfeld ETH Zurich