NH3 / ammonia issues in main engine combustion

Answer

As the IMO pushes toward Net-Zero, Ammonia is a leading alternative fuel candidate, but it presents severe combustion challenges:

Toxicity and Health Hazards

Ammonia is highly toxic to humans even at very low concentrations (IDLH - Immediately Dangerous to Life or Health is just 300 ppm). Any minor piping leak in the engine room can be fatal to the crew. This requires double-walled piping, extensive ventilation extraction systems, and highly sensitive NH3 leak detection networks.

Corrosivity and Material Selection

Ammonia induces Stress Corrosion Cracking (SCC) in conventional copper, zinc, brass, and high-strength steels. The entire fuel supply system (bunkering lines, valves, tanks, and internal engine components) must be constructed from specialized materials like low-carbon austenitic stainless steels or specific nickel alloys.

Combustion Inefficiency

Ammonia has a very low flame propagation speed, high auto-ignition temperature ( 651∘C), and a high lower explosive limit (LEL). To sustain stable combustion in an internal combustion engine, it requires a high compression ratio and a substantial amount of pilot fuel (MGO or biofuels) to initiate and sustain the ignition.

Environmental Trade-off (N2O and NOx Emissions)

While ammonia contains zero carbon, its combustion can generate high amounts of Nitrogen Oxides (NOx). More critically, incomplete or improper combustion can form Nitrous Oxide (N2O), a greenhouse gas that is approximately 298 times more potent than CO2. To counter this, advanced Selective Catalytic Reduction (SCR) systems and specialized exhaust gas after-treatment are mandatory.

Storage and Energy Density

Ammonia has a lower volumetric energy density compared to conventional VLSFO (requiring roughly 2.4 times more storage volume for the same energy content). It must be stored either completely refrigerated as a liquid at −33∘C at atmospheric pressure, or under a high pressure of approximately 10 bar at ambient temperature, which severely impacts cargo-carrying space and vessel stability design.

Calorific value

Lower Heating Value (LHV / Net Calorific Value): 18.6 MJ/kg (≈4,440 kcal/kg or 8,000 BTU/lb)

Higher Heating Value (HHV / Gross Calorific Value): 22.5 MJ/kg (≈5,370 kcal/kg or 9,690 BTU/lb)

Comparison with Marine & Conventional Fuels