the clean fuel that warms the sea

Problem statement

Ammonia is a leading candidate zero-carbon fuel for shipping, but its combustion in marine engines produces nitrous oxide (N2O) — a greenhouse gas with ~273× the warming potential of CO2 over 100 years. At partial engine loads (which dominate actual ship operations during maneuvering and slow steaming), N2O emissions are worst and poorly characterized. No after-treatment catalyst simultaneously addresses NOx, N2O, and unburned ammonia slip across the full operating range.

Why this matters

International shipping accounts for roughly 3% of global CO2 emissions (~2.7% of global energy-related CO2; Wong et al. 2024). Ammonia is among the most viable zero-carbon fuels for deep-sea vessels because of its energy density and existing global handling infrastructure. But the climate case depends entirely on holding N2O slip near zero: modeling of a full ammonia-adoption scenario finds that tailpipe N2O from ammonia-powered ships would have climate impacts equivalent to 5.8% of current shipping CO2 emissions even at expected slip rates (Wong et al. 2024), and uncontrolled slip at the worst operating points erodes the benefit further — every gram of N2O carries the warming of ~273 grams of CO2.

What’s been tried and why it hasn’t worked

Engine manufacturers (MAN Energy Solutions, WinGD) are developing two-stroke ammonia engines, with WinGD reporting N2O below 3 ppm and ammonia slip below 10 ppm in testing, and MAN reporting N2O "typically well below 5 ppm" (Rouwenhorst 2025). Selective catalytic reduction (SCR) is a mature NOx control for diesel engines. Dual-fuel engine designs co-fire ammonia with pilot fuels. However, the strongest results are at high load: MAN's own figures are under 1% CO2-equivalent (versus a fuel-oil engine) at full load but under 2% across the operating envelope, and systematic N2O data at the partial loads that dominate maneuvering and slow steaming remain sparse. SCR catalysts designed for NOx do not reliably decompose N2O. Engine-out emissions of unburned NH3 and N2O vary dramatically with ammonia/pilot fuel ratio and injection timing (Niki 2023), making single-catalyst solutions inadequate. No standardized emissions testing protocol exists for ammonia engines across real operating profiles.

What would unlock progress

After-treatment catalysts effective against N2O across partial to full load conditions. Combustion chamber geometries that minimize N2O formation at low loads. Standardized emissions testing protocols for ammonia marine engines across real operating profiles including maneuvering and slow steaming.

Entry points for student teams

A team could characterize N2O formation mechanisms in ammonia combustion at varying equivalence ratios and temperatures using a bench-scale combustor, mapping the conditions that maximize N2O production. A catalysis-focused team could screen candidate materials for simultaneous N2O decomposition and unburned ammonia oxidation. Combustion science, catalysis, and marine engineering skills apply.