An ammonia engine works by using ammonia (NH3) as the primary fuel in a combustion process to generate power.
At its core, an ammonia engine is a type of internal combustion engine (ICE) or external combustion engine (like a gas turbine) adapted to burn ammonia. Similar to engines running on gasoline, diesel, or natural gas, the engine converts chemical energy stored in the fuel into mechanical energy.
The Combustion Process
Burning ammonia releases energy. The primary chemical reaction for complete combustion is:
4 NH₃ + 3 O₂ → 2 N₂ + 6 H₂O + Energy
Ideally, this process yields only nitrogen gas (N₂) and water (H₂O), making it potentially carbon-free.
However, pure ammonia is challenging to burn efficiently in many engine designs. It has a high ignition temperature, a slow flame speed compared to conventional fuels, and a narrow flammability range. These characteristics make it difficult to achieve stable and complete combustion, especially across varying engine loads and speeds.
Enhancing Ammonia Combustion with Hydrogen
To overcome these challenges and ensure reliable operation, modifications and strategies are employed. A key method is blending ammonia with other fuels or additives that improve its combustion properties.
- Combustion Promotion: As highlighted in the reference, ammonia is typically mixed with a small fraction of hydrogen (H2), which serves as combustion promoter to achieve stable combustion over a large range of engine operation. Even small amounts of hydrogen (often just a few percent by volume) significantly improve ammonia's combustion characteristics.
- Why Hydrogen Helps: Hydrogen has a very fast flame speed, low ignition energy, and wide flammability limits. When added to ammonia, it acts like a "spark plug" for the flame, helping the mixture ignite more easily and burn faster and more completely. This stabilizes the combustion process across different engine operating conditions, preventing misfires and ensuring consistent power output.
Engine Types and Adaptations
Ammonia can be used in various engine architectures:
- Spark Ignition (SI) Engines: Similar to gasoline engines, requiring a spark plug to ignite the fuel-air mixture. Modifications are needed for the fuel injection system and combustion chamber design to handle ammonia's properties and the blend.
- Compression Ignition (CI) Engines: Similar to diesel engines, where fuel is injected into hot compressed air, causing auto-ignition. Ammonia's high auto-ignition temperature makes pure ammonia challenging for this type, often requiring a pilot fuel (like diesel or hydrogen) or significant modifications.
- Gas Turbines: Used in larger power generation or propulsion systems. Ammonia-hydrogen blends or pure ammonia can be injected into the combustor, where they mix with compressed air and burn continuously.
The Engine Cycle (Simplified)
Regardless of the specific type, the general principle involves:
- Intake: A mixture of ammonia (or ammonia-hydrogen blend) and air enters the combustion chamber.
- Compression: The piston (in reciprocating engines) or compressor (in gas turbines) compresses the mixture, increasing its temperature and pressure.
- Combustion/Expansion: The fuel ignites (via spark or compression), burns rapidly, and creates high-pressure gases that push the piston down (generating power) or expand through a turbine.
- Exhaust: The burnt gases are expelled from the engine cylinder or turbine.
Challenges Beyond Combustion
While hydrogen blending helps with combustion stability, ammonia engines still face other considerations:
- NOx Emissions: Ammonia combustion can produce nitrogen oxides (NOx), harmful pollutants. Managing and reducing NOx emissions through optimized combustion or after-treatment systems is crucial.
- Toxicity and Corrosiveness: Ammonia is toxic and can be corrosive to certain materials, requiring careful handling and appropriate material selection in fuel systems and engines.
- Fuel Storage: Storing ammonia requires pressurized or refrigerated tanks.
In summary, an ammonia engine generates power by combusting ammonia fuel, often enhanced by blending with hydrogen to ensure stable and efficient burning across various operating conditions, despite the inherent combustion challenges of pure ammonia.
