Studying the effect of dynamic operation conditions on green ammonia production synthesis loop
Raj Kishore Patel, Amin Soleimani Mehr, Jimena Incer Valverde, Günter Scheffknecht, Jörg Maier, Reihaneh Zohourian
Abstract
The global challenges meeting hydrogen demands due to limited renewable resources urge the need for low-cost imports. Green ammonia, promising for its existing infrastructure, encounters inflexibility challenges in large-scale production with renewables. This study delves into ammonia synthesis loop flexibility amid renewable intermittencies. Utilizing an Aspen Plus® model of 1223 tonnes per day ammonia production capacity, the transient behavior under varied feed flow was investigated with Aspen Dynamics™ simulations. The findings indicated that effective strategies enabled managing a minimum load of 10 % or lower under stoichiometric conditions, constrained by the electrolysis system's lower load. The study also concluded that the ammonia synthesis unit's 20 %/hr feed flow ramp rate is restricted by the thermal cycling of the reactor catalyst; the consequences of fast ramp-up and ramp-down of the operational parameters such as feed flow or stoichiometric ratio are the primary limits in green ammonia production or dynamic operation of the plant. • Comprehensive Ammonia Synthesis Model : Developed a detailed ammonia synthesis model, including a catalyst model, based on a steady-state framework to investigate unit flexibility • Dynamic Simulations for Enlightening Flexibility Analysis : Transformed the steady-state model into dynamic flow-driven models using Aspen Dynamics™ to analyze transient behavior • Data Validation : Validated steady-state model data against academic literature and industrial sources before conducting dynamic simulations. • Dynamic Feed Simulation : Employed Aspen Dynamics™ simulations to evaluate the effects of dynamic feed conditions on process stability and operational flexibility. • Minimum Load Factor (turn-down ratio) : Demonstrated that the ammonia synthesis unit could achieve a minimum load factor of 12 % while maintaining stability and efficiency. • Practical Implications of Ramp Rate Constraints : Identified that the heating and cooling rates of the catalyst bed limited the maximum ramp rate of 20 % per hour. • Challenges with Purging and H 2 Ratio : We found that adjusting the H2 feed ratio and purging was impractical due to low inert content and the risk of hydrogen loss.