NSF-BSF: Plasma Reformed Ammonia as a Carbon Free Fuel: Study of Nanosecond-Pulsed Discharge Kinetics and Combustion Enhancement

Renewable energy systems (e.g., wind, hydro, solar) are primed to play the dominant role in supplying the world’s power, but before independence from fossil fuels can be achieved there must be cost-effective ways to store the renewable energy and utilize it in hard to decarbonize transportation sectors. Ammonia (NH3), which can be synthesized using renewable energy and can easily be stored/transported as a liquid, has garnered significant interest as a carbon-free low-cost chemical energy storage medium and potential fuel for heavy duty freight/marine applications.

While the attention being paid to ammonia as an energy storage solution and fuel is warranted, its low reactivity and propensity to produce nitric oxide emissions during combustion represent critical impediments to widescale adoption. Through the joint NSF-BSF program, researchers at Colorado State University and Technion - Israel Institute of Technology are partnering to progress the scientific understanding needed to develop plasma assisted ammonia reforming strategies to enable the use of ammonia in state-of-the-art combustion devices.

Key outcomes from the proposed effort include a validated ammonia-plasma chemistry model and a validated combustion simulation strategy for the plasma-reformed ammonia mixtures at engine relevant conditions. The research output will benefit both the energy and transportation sectors – industries which together account for 73% of greenhouse gas emissions worldwide – by supporting the development of high-fidelity models required to shorten the design cycles of next generation carbon-free energy conversion devices.

This unique funding opportunity from NSF/BSF will help foster international collaboration leveraging the unique expertise in plasma reformation at Technion and combustion and laser diagnostics at Colorado State University and will support high impact educational and diversity building activities across borders. Findings will be shared in publications, at conferences, and will be integrated into joint undergraduate and graduate coursework at the collaborating institutions. The research will be regularly highlighted in K-12 outreach events.

Preliminary plasma- and combustion-kinetic modeling by the proposal team has suggested that the ignitability and flame speed of NRP plasma reformate blends could be comparable to those of conventional fuels across a wide range of operating conditions. Detailed kinetics within the plasma reactor have been examined, including identifying the key role of the NH2 radical for reduction of NO combustion emissions.

This work aims to expound upon these modeling results via experimental validation to build a foundation for future system-based development and optimization efforts. The main elements of the work plan are: (1) reformate species measurements within a newly constructed NRP plasma reactor to validate and refine existing NH3 plasma kinetic models, (2) NH2 measurements within the NRP by cavity ring-down spectroscopy (CRDS) to elucidate NH2’s role in plasma kinetics and NOx reduction, and (3) ignition delay and flame speed measurements of the resulting NH3 reformate blends at relevant conditions using a laser-ignited rapid compression machine.

These efforts will address gaps in the current literature including: (1) the detailed study of the relative impact of plasma pulse frequency, amplitude, and reactor pressure/temperature on NH3 and NH3/air reformation efficiency in an NRP reactor, constituting a validation dataset for kinetic modeling efforts; (2) the first direct measurement of NH2 radical formation in ns-pulsed systems for quantitative comparison with kinetic models; and (3) the expansion of ignition delay and flame speed measurements of NH3/H2/O2/N2 reformate blends into temperature/pressure regimes that are sparse in current literature, with trace NO/NO2, varying equivalence ratio, and induced turbulence, providing an invaluable dataset to support NH3 combustion chemical mechanism and multi-dimensional simulation development. The improved understanding of NRP plasma reactors and the combustion characteristics of their reformate gases will set the foundation for the development and optimization of a variety of ammonia-to-power conversion systems including internal combustion engines and gas turbines, which could otherwise only progress through cumbersome empirical iteration.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.