Ammonia (NH3), a carbon-free chemical, has emerged as a promising hydrogen carrier and carbon-neutral fuel, playing a pivotal role in advancing green hydrogen trade and achieving a carbon-neutral society. This prominence is reinforced with its higher volumetric hydrogen content and energy density compared to liquid hydrogen, coupled with its well-established infrastructure for storage and transportation.
The ammonia cracking (decomposition) process is highly endothermic, requiring a minimum of 46 kJ/mol of thermal energy for the reaction, with additional thermal energy required for the phase transition from liquid to gas and subsequent heating of ammonia. The ammonia decomposition reaction is represented as follows:
NH3 (g) → 0.5N2 (g) + 1.5H2 (g) with ΔHo=46 kJ/mol
This highlights the critical importance of an effective thermal management strategy within the integrated process. To address this challenge, POSCO Holdings New Experience of Technology Hub (N.EX.T Hub) has undertaken research focusing on optimizing heat integration within the system, thereby maximizing the utilization of the highly efficient and innovative catalysts developed by POSCO Holdings and minimizing the reliance on external thermal energy inputs. The objective of the ammonia cracking process is to produce 1,000 Nm³/hr of hydrogen, utilizing an integrated approach to enhance thermal efficiency and sustainability.
[Methods]
An integrated ammonia cracking process was designed to produce hydrogen, with a focus on efficient thermal energy management through internal heat recovery. The process incorporates heat integration to supply the necessary thermal energy for both the ammonia decomposition reaction and the preheating of ammonia and air. The ammonia cracking process consists of an ammonia storage tank, heat recovery units, an ammonia cracker, and a hydrogen separation/purification system. Liquid ammonia supplied from the storage tank is vaporized and heated to the ammonia cracking reaction temperature. After the cracking reaction, the effluent is cooled using a series of heat integration units and then undergoes separation and purification processes to produce high-purity hydrogen. Additionally, the tail gas, composed of nitrogen, hydrogen, and ammonia, is combusted with air in the furnace to generate high-temperature flue gas. Waste heat generated within the process is utilized to supply the requisite heat for the integrated ammonia-based hydrogen production process, thereby enhancing the process efficiency of the system.
[Results and discussion]
The integrated ammonia cracking process with heat recovery was designed to enhance the process efficiency and sustainability of hydrogen production. Thermal energy was recovered from two primary sources: high-temperature ammonia cracking effluents and combustion flue gas of the tail gas. The recovered energy was utilized through direct heat exchange with the ammonia and air streams or by generating steam using Boiler Feed Water (BFW). This efficient thermal management ensured a continuous supply of the requisite heat for the ammonia cracking process. The process exhibited significant improvements in overall process efficiency, highlighting the effectiveness of the heat recovery strategy.
[Conclusions]
This study developed an integrated ammonia cracking process with heat recovery to enhance the process efficiency and sustainability of hydrogen production. By effectively recovering thermal energy from high-temperature ammonia cracking effluents and combustion flue gas of the tail gas, the process ensured a continuous supply of the requisite heat through direct heat exchange and steam generation. The process achieved significant improvements in overall process efficiency. These findings highlight the effectiveness of the heat recovery strategy in improving the efficiency and sustainability of hydrogen production, demonstrating its potential for large-scale implementation in achieving carbon-neutral energy solutions.
[Acknowledgments]
This research was supported by the National Research Council of Science & Technology(NST) grant by the Korea government (MSIT) (No. GTL24051-200), and the Project funded by the POSCO Holdings (Project; No. 2024H032).