Unlocking high-pressure ammonia cracking
Accelerate catalyst screening and process optimisation with advanced high throughput techniques
Benjamin Mutz and Robert Baumgarten
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The 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 COX 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, demanding industrially applicable investigation, establishing long-term viability, and conducting accelerated ageing experiments.
Enhancing the NH₃ cracking process is necessary to achieve equilibrium NH₃ conversion, particularly when dealing with decentralised plant operations at lower temperatures. The purity of H₂ obtained from NH₃ is contingent upon its intended use; even NH₃ traces can act as a poison in applications such as fuel cells (Guo & Chen, 2017). The optimisation of NH₃ cracking, particularly at elevated pressures, is essential for downstream H₂ usage (such as syngas conversion) in industrial applications. While low-pressure conditions are thermodynamically favoured and commonly studied, industrial needs, such as efficient H₂ use, necessitate re-evaluating NH₃ cracking at higher pressures.
This article introduces the most advanced high throughput approach to accelerate catalyst screening and optimise the NH3 cracking process, thereby addressing the significant research gap in studies combining high temperature and pressure. A versatile 16-fold parallel fixed-bed reactor set-up operated in hte’s laboratories in Heidelberg was used, featuring high-temperature reactor technology designed for elevated pressures. Validation of the high throughput unit for the NH₃ cracking process was carried out using a Fe-based catalyst prepared in-house while screening a wide range of parameters. Subsequent customer projects aimed at ranking four different commercial catalysts and parameter screening under conditions relevant to industrial applications (temperature, pressure, and water concentration) also achieved successful outcomes.
High throughput technology
The most advanced 16-fold high throughput system designed for operation at high temperature combined with elevated pressures was used for this study. High-temperature metal alloy reactors were equipped with ceramic inlay tubes of 3-5 mm inner diameter to ensure the inert behaviour of the reactor material, which becomes relevant at >550°C. A similar test unit was validated and used for a reverse water-gas shift study published recently (Mutz, et al., 2022) and was further modified for processing NH₃.
Figure 1 shows a simplified scheme of the high throughput testing equipment. The feed components H₂, N₂, and Ar were dosed individually using mass flow controllers, whereas NH₃ and H2O were dosed as liquids using two separate high-precision syringe pumps. The feed mixture was equally distributed over the 16 parallel reactors. The catalysts were tested as particles in a size fraction of 125-160 µm and diluted in inert material of a similar size to minimise temperature gradients in the catalyst bed.
The reactor pressure was controlled downstream by feeding additional N₂. The reactor effluent gas was analysed using a multi-detector gas chromatograph (GC) configured in-house and equipped with different thermal conductivity detectors (TCDs) for permanent gas, NH₃, and H₂O quantification. hte process control software enabled fully automated and continuous operation and monitoring of the high throughput system. Data evaluation and charting were performed with the myhte software solution, which was specifically developed for processing large amounts of data produced by such a high throughput setup (Hauber & Sauer, 2022).
A flexible hte 16-fold parallel testing unit was upgraded to process NH₃ and subsequently validated using a Fe-based catalyst prepared in-house to further optimise the equipment and to produce a set of performance data while exploring a wide range of process parameters. The experiments were structured by performing a temperature ramp between 550-750°C in 50 K-steps, first at 40 barg followed by a second temperature ramp at 15 barg to investigate the influence of pressure. The temperature curves were generated at a gas hourly space velocity (GHSV) range of 11,000-100,000 h-1 concurrently, enabled by loading different catalyst bed lengths to the parallel reactors.
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