Decarbonisation Technology - February 2024 Issue

Unlocking high-pressure ammonia cracking

Accelerate catalyst screening and process optimisation with advanced high throughput techniques

Benjamin Mutz and Robert Baumgarten hte GmbH

T he increasing global demand for carbon- free energy sources positions hydrogen (H₂) as a frontrunner among the most promising energy solutions for the future. H₂ exhibits broad versatility and thus finds applications in diverse fields, such as fuel cells for electricity generation or as a reducing and hydrogenation agent in chemical processes. Furthermore, H₂ offers the enticing possibility of converting inevitable CO₂ emissions into valuable platform chemicals, thereby seamlessly reintegrating them into the respective lifecycle. However, the full realisation of hydrogen’s potential is affected by challenges related to its storage and transportation, limiting its overall efficiency. In this context, ammonia (NH₃) emerges as a highly promising candidate for overcoming these obstacles on a large scale. Notably, NH₃ features a considerable hydrogen storage capacity, positioning it as an ideal candidate for efficient, carbon-free energy storage and transportation over long distances (Klerke, et al., 2008) (Ristig, et al., 2022). Its widespread production capacity, coupled with scalability, further strengthens its candidacy in this role. NH₃ (green or blue) as a hydrogen carrier is well suited to being produced in regions abundant in renewable energy resources or locations where energy resources and natural gas are readily available, thus ensuring cost- effective feedstock. The straightforward liquefaction of NH₃ enables its seamless transportation to regions with substantial hydrogen demand, such as Japan (Aramco, 2023) or Europe (BP, 2023). The release of H₂ through NH₃ crackers enables direct utilisation

for various applications, as mentioned above. Notably, this process sidesteps the potential generation of CO X byproducts in reactors (for example, from steam methane reforming), highlighting its environmental and operational benefits. The decomposition of NH₃ is energy intensive, since high temperatures are required, especially when the cracker is operated at elevated pressures, which allows the favoured production and direct use of pressurised H₂ (Ristig, et al., 2022). The two probable models for distributing H₂, namely localised and centralised approaches, exhibit distinct technical demands influenced by both scale and H₂ purity considerations. Smaller, decentralised units are more economic, operating at lower temperatures and using highly active catalysts. However, the downside of functioning at these reduced temperatures becomes more pronounced, especially when combined with elevated pressures, as it leads to a less favourable equilibrium conversion. In contrast, larger centralised units designed for NH₃ cracking use base-metal catalysts and are operated at higher temperatures (Klerke, et al., 2008) (Ashcroft & Goddin, 2022). Next to the commonly used Ni, Ru, and Fe- based catalysts, various bimetallic materials are investigated for NH₃ cracking (Klerke, et al., 2008) (Lucentini, et al., 2021). Moreover, it is worth noting that the best NH₃ synthesis catalyst is not the best NH₃ cracking catalyst since different strengths in binding energy for nitrogen are required for the individual processes (Boisen, et al., 2005). Optimisation and fine-tuning of NH₃ cracking catalysts are essential, ensuring stability of the catalyst,