Climate and air quality impact of using ammonia as an alternative shipping fuel

Abstract

As carbon-free fuel, ammonia has been proposed as an alternative fuel to facilitate maritime decarbonization. Deployment of ammonia-powered ships is proposed as soon as 2024. However, NO _x , NH _3 and N _2 O from ammonia combustion could impact air quality and climate. In this study, we assess whether and under what conditions switching to ammonia fuel might affect climate and air quality. We use a bottom–up approach combining ammonia engine experiment results and ship track data to estimate global tailpipe NO _x , NH _3 and N _2 O emissions from ammonia-powered ships with two possible engine technologies (NH _3 –H _2 (high NO _x , low NH _3 emissions) vs pure NH _3 (low NO _x , very high NH _3 emissions) combustion) under three emission regulation scenarios (with corresponding assumptions in emission control technologies), and simulate their air quality impacts using GEOS–Chem high performance global chemical transport model. We find that the tailpipe N _2 O emissions from ammonia-powered ships have climate impacts equivalent to 5.8% of current shipping CO _2 emissions. Globally, switching to NH _3 –H _2 engines avoids 16 900 mortalities from PM _2.5 and 16 200 mortalities from O _3 annually, while the unburnt NH _3 emissions (82.0 Tg NH _3 yr ^−1 ) from pure NH _3 engines could lead to 668 100 additional mortalities from PM _2.5 annually under current legislation. Requiring NH _3 scrubbing within current emission control areas leads to smaller improvements in PM _2.5 -related mortalities (22 100 avoided mortalities for NH _3 –H _2 and 623 900 additional mortalities for pure NH _3 annually), while extending both Tier III NO _x standard and NH _3 scrubbing requirements globally leads to larger improvement in PM _2.5 -related mortalities associated with a switch to ammonia-powered ships (66 500 avoided mortalities for NH _3 –H _2 and 1200 additional mortalities for pure NH _3 annually). Our findings suggest that while switching to ammonia fuel would reduce tailpipe greenhouse gas emissions from shipping, stringent ammonia emission control is required to mitigate the potential adverse effects on air quality.

Keywords

• shipping
• decarbonization
• air quality
• PM2.5
• ozone
• ammonia energy