What are the key technical barriers to using ammonia as a clean shipping fuel?

What are the key technical barriers to using ammonia as a clean shipping fuel?
Ammonia's promise as a zero-carbon marine fuel faces two hard technical hurdles: poor ignitability and NOx emissions. Ammonia has high auto-ignition temperature and slow flame speed, requiring catalysts or pilot fuels to sustain combustion. During combustion, its nitrogen content readily forms NOx, while unburned ammonia can escape - both regulated pollutants. Recent progress includes catalytic combustion achieving stable operation from 200°C to 1100°C with low NOx, and Weichai Power's patent for a multi-stage SCR aftertreatment system specifically for ammonia engines. The shipping industry is hedging bets: methanol dual-fuel engines already meet IMO Tier III without SCR, giving methanol a near-term advantage. However, ammonia's higher volumetric energy density (12.7 MJ/L vs 8.5 MJ/L for liquid hydrogen) and existing global trade infrastructure make it the stronger long-term candidate. By 2050, ammonia demand as shipping fuel could add 100-200 million tonnes annually, requiring massive capacity expansion from the current 170 million tonne baseline.