Conceptual CFD design of a molten salt-heated ammonia cracking reactor for efficient hydrogen production

Abstract

As hydrogen demand increases in the transition toward a carbon-neutral society, solutions for hydrogen storage and transportation are necessary. Among the various options, ammonia is a promising hydrogen carrier due to its high volumetric density, and ammonia cracking process is indispensable to regenerate hydrogen. However, because ammonia cracking is energy-intensive, the development of large-scale ammonia cracking reactors with energy efficient heat management system is essential to ensure reliable hydrogen supply to satisfy hydrogen demand. In this work, a numerical model of shell-and-tube ammonia cracking reactor suitable for both centralized and distributed type plants is developed, employing molten salt as the heat transfer fluid to achieve uniform and efficient heat supply. When the ammonia temperature is sufficiently raised, conversion exceeding 75% and 83.1% of energy efficiency are achieved, while the molten salt exhibited only 22.6% temperature drop compared with the corresponding gas flow temperature rise. The reactor performances for various tube lengths or diameters yield conversion rates from 65 to 88%, indicating that a molten salt-heated shell-and-tube ammonia cracker can be designed as a compact reactor. Furthermore, sensitivity analyses of key parameters, including inlet temperatures, pressure, and space velocity, are found to have strong influence on temperature profiles and reaction progress, and the small multi-tubular model remains the performance well similar for the singular tube system. This study demonstrates for the first time via a CFD conceptual design of ammonia cracker, that molten salt heating is viable for maintaining uniform temperature gradients (<40 degrees C) and achieve >65% NH(3 )conversion rate in compact ammonia cracking reactor under industrial-grade gas hourly sapce velocity (GHSV), providing a viable design pathway for demonstration of large-scale ammonia crackers and contributing to the realization of an integrated hydrogen value chain.