No abstract

This work presents a simulation activity on an innovative power generation cycle, developed within the EU Horizon 2020 project HiPowAR, using ammonia as fuel for an oxy-combustion process in a membrane reactor. The key advantages of the system are the low compression requirement, typical of steam cycles, and the large expander inlet temperature, typical of gas turbine cycles. The analysis explores the options of cooled or uncooled expander, either adopting a steam-cooled turbine made of conventional Ni-based alloys or using high temperature-resistant ceramic matrix composite (CMC) materials. The simulations show that, with a reactor outlet temperature of 1350°C, a cooled system could reach up to 48.2% efficiency, with limited additional advantages when further increasing the temperature. At the same temperature level, the uncooled system could instead achieve 52.5% efficiency, allowing also a substantial system simplification. However, since the expanded mixture contains nearly 90%mol steam, the use of CMC materials is made difficult by degradation issues and would require the development of suitable barrier coatings.

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.

Selection and peer-review under responsibility of the scientific committee of the 11th Int. Conf. on Applied Energy (ICAE2019).