Process design and optimization of onsite hydrogen production from ammonia: Reactor design, energy saving and NOX control

Ammonia is an important commodity for both direct and indirect applications. It can be used directly as a carbon-free fuel source as well as for the purpose of renewable hydrogen storage and transport. In this study, hydrogen production from ammonia decomposition in a multi catalytic packed bed reactor with an intermediate heating system is reported using Aspen Plus V.12. The process simulation model results have been validated through experimental data for commercially available Ru/Al2O3 catalyst and Temkin-Phyzev reaction kinetics. The results exhibit that ammonia decomposition is a highly endothermic reaction, therefore requires significant heat energy. The decomposition parameters such as temperature and pressure are optimized for large-scale ammonia decomposition. For the ammonia combustion, thermal efficiency, fuel-saving, and product yield are analyzed and optimum values are found to be 59%, 22% and 77% for each parameter, respectively. During the pure ammonia combustion, NOX emissions are a major issue. To monitor the NOX emissions, parameters such as equivalence ratio, temperature and pressure are analyzed to witness the optimum operating conditions. To improve the flame quality such as laminar velocity, minimum ignition temperature, and adiabatic flame temperature of the ammonia combustion, waste hydrogen stream from the pressure swing adsorption (PSA) unit is blended with the fuel ammonia. The blending of hydrogen with fuel ammonia resulted in higher NOX emissions, which can be reduced by recirculation of 30% of the flue gas with air–fuel stream.