(659g) Sustainable Thermochemical High-Purity Nitrogen Production Using Redox Oxides

In 2019, the global production of ammonia was 150 million metric tons of contained nitrogen, whereby 88 % of the ammonia was used for fertilizer production [1]. Therefore, the production of ammonia is fundamental to ensure the nutrition of the world population within the existing agriculture and diet. Due to the growing world population a further increase in demand for ammonia is expected.

Ammonia is synthesized via the Haber-Bosch process. The process itself has high energy consumption (32 MJ kg^-1) [2] and the required nitrogen is predominantly produced via energy intensive cryogenic air separation. An alternative method utilizes thermochemical cycles where a redox material is reduced at high temperature accompanied by a release of oxygen. At lower temperatures the material is oxidized and thus takes the oxygen from the incoming gas (see Fig. 1). An advantage of this method is the opportunity to use concentrated solar radiation (CSR) for the heat supply and therefore, providing a sustainable nitrogen production.

Perovskites are often used as redox material. They are described with the general formula AMO_3-δ, where A are alkali, alkaline earth or rare earth metal cations and the M site is occupied in most cases by transition metal cations. δ (δ = 0 – 0.5) indicates an oxygen non-stoichiometry. The partial reduction, which is described by Eq. 1, depends on the partial oxygen pressure p_O2 and Temperature T. During the reduction the perovskites maintain their crystal structure, only smaller distortions are observed. This enables fast redox kinetics (reactions are completed in less than one min) and highly reversible redox cycles with long lifetime expectancy [3].

AMO₃ ↔ AMO_3-δ + δ/2 O₂ (1)

For the Haber-Bosch process oxygen impurities of 3 ppm in N₂ can be tolerated [2]. Therefore, it is essential to produce high-purity N₂. A recent publication has proven that this is possible with thermochemical cycling using pre-purified air (1% O₂) and SrFeO_3-δ as redox material [5]. For the pre-purification of the air a pressure swing absorption devise (PSA) can be used. Economic analysis reveals that a combined solar-electrical driven PSA with a thermochemical system requires a lower thermal energy input than both solely solar-electrical driven PSA and solar-electrical cryogenic distillation (see Fig 2.).

Based on this analysis, the focus of the current work is the coupling of a PSA devise to a thermochemical reactor for the production of high-purity nitrogen. We present the concept design for this completely new approach, as well as an optimization of the process parameters based on thermochemical process simulations. Another field of research for improving the efficiency of thermochemical air separation is material science. The used perovskites determine the required temperatures and the achievable oxygen partial pressures. Doping of SrFeO_3-δ and CaMnO_3-δ leads to improved redox performance by either stabilizing the crystal structure and preventing phase changes, increasing the oxygen affinity or reducing the reduction and oxidation temperatures. We show the results of this material development and its positive effects on nitrogen production.

References

1. https://pubs.usgs.gov/periodicals/mcs2020/mcs2020.pdf, 7^th April 2020
2. J.R. Jennings, Catalytic ammonia synthesis: fundamentals and practice, Springer Science & Business Media, 2013
3. Vieten, J.; Bulfin, B.; Call, F.; Lange, M.; Schmücker, M.; Francke, A.; Roeb, M.; Sattler, C.; J. Mater. Chem. A 2016, 4 (35), 13652-13659
4. J. Vieten, D. Guban, B. Lachmann, B. Bulfin, D. Krauzner, 2019, patent no DE 10 2019 126 114.7
5. Bulfin, B.; Lapp, J.; Richter, S.; Gubàn, D.; Vieten, J.; Brendelberger, S.; Roeb, M.; Sattler, C.; Chem. Engineering Science, 2019, 203, 68-75