Simulation of the HiPowAR power generation system for steam-nitrogen expansion after ammonia oxidation in a high-pressure oxygen membrane reactor

The EU project HiPowAR studies a novel power generation system based on ammonia flameless oxidation with pure oxygen in a high-pressure membrane reactor and expansion of the resulting high-temperature H2O-N2 stream. The system combines the advantages of high temperature at expander inlet, typical of gas turbines, and small compression demand, typical of steam cycles. Water is injected into the reactor to control the very high adiabatic temperature, at the limited energy expenditure of liquid pumping. This work assesses the performance potential of the HiPowAR system under different design conditions, through simulations with a model developed in Aspen Plus®. The system shows a high efficiency (up to 55%) when operating at high temperature (e.g., 1350°C at expander inlet); hence, O2 membranes capable of working at very high temperature are required. The cycle features an optimal sub-atmospheric expansion pressure (in the range 0.1-0.2 bar), which requires the re-pressurization of the off-gas (steam-saturated nitrogen). The system also produces liquid water as a net output. A reduction of the expander inlet temperature to values acceptable by typical steam cycles (600°C) significantly limits the efficiency, despite allowing to demonstrate the process using conventional steam expanders.