Chemical looping based Low-pressure ammonia synthesis
Hangzuo Guo, Alexander R.P. Harrison, Mingchen Gao, Xusheng Zhang, Qicheng Chen, Zhanfeng Cui, Binjian Nie
Abstract
• Ammonia can be synthesized by chemical looping using nitride or imide N-carriers. • Imides show faster kinetics but are sensitive to gaseous impurities. • Thermodynamics and process design limit maximum system efficiency. • Doping with transition metals and structural modification improve reaction kinetics. Ammonia (NH 3 ) is a promising alternative to fossil fuels due to its ability to store and release hydrogen without carbon emissions, with high energy density. The traditional Haber-Bosch (H-B) process for ammonia synthesis is energy-intensive, requiring extreme pressure conditions (10–20 MPa). Chemical Looping-based Ammonia Synthesis (CLAS) offers a sustainable alternative, via a cyclic process where hydrogen gas reacts with a nitrogen containing solid (termed a ‘nitrogen carrier’) at near ambient pressure to form ammonia. The nitrogen carrier is regenerated in N 2 gas in a separate step. However, practical implementation of CLAS faces challenges, primarily in the development of effective nitrogen carriers. This review analyses nitrogen carrier materials for the CLAS process, focusing on material screening and fabrication techniques, as well as strategies to reduce energy consumption and increase ammonia production at reactor and system level. The latest advancements in transition metal nitrides and alkali/alkaline earth metal imides are highlighted, as well as external field-assisted technologies to apply the CLAS process at industrial scale.