After the ammonia decomposer vaporizes liquid ammonia, it is decomposed into a mixture of 75% H₂ and 25% N₂ (2NH₃ → N₂ + 3H₂) in a nickel-based catalyst at a temperature above about 900 °C. This gas has strong reducing properties and relatively low cost. It is widely used in bright annealing, brazing, powder metallurgy sintering of stainless steel and silicon steel, and as a diluent gas to adjust the nitriding potential in nitriding. The key quality indicators are residual ammonia and dew point, and the key safety point is the explosion-proof management of hydrogen.
- Reaction: 2NH₃ → N₂ + 3H₂, product 75% H₂ + 25% N₂
- Conditions: high temperature + nickel-based catalyst
- Usage: bright annealing, brazing, sintering, nitriding to dilute Kn
- Quality indicators: residual NH₃, dew point
- Safety: Hydrogen is flammable and explosive and requires leakage detection and interlocking
Principle
Liquid ammonia turns into gaseous ammonia through the vaporizer, enters the reaction tank heated to high temperature, and decomposes under the action of a nickel-based catalyst:
2NH₃ → N₂ + 3H₂
After complete decomposition, the gas composition is 75% H₂, 25% N₂ (volume ratio). The reaction is endothermic and requires external heating to maintain temperature.
Main components
- Liquid ammonia vaporization system: Convert liquid ammonia into gaseous ammonia with stable pressure;
- Reaction tank and catalyst: Complete the decomposition reaction;
- heating system: Mainly electric heating;
- cooler: Cooling product gas;
- Purification device (optional): The dryer removes moisture, lowers the dew point, and removes residual ammonia if necessary;
- Control and security: Temperature control, pressure control, hydrogen leak alarm.
Quality index
| Index | Significance |
|---|---|
| residual ammonia | It increases when the reaction temperature is insufficient or the catalyst activity decreases; it is detrimental to bright annealing. |
| Dew point | High moisture will cause the surface of stainless steel and other materials to oxidize and darken. |
| Traffic stability | Affects the atmosphere in the downstream furnace |
Application
1. Bright annealing Stainless steel, silicon steel, copper alloy, etc. are annealed in a highly reducing hydrogen and nitrogen atmosphere, and the surface remains bright.
2. Brazing and sintering Provides a reducing atmosphere to remove surface oxides and improve wetting and bonding.
3. Dilution and Kn adjustment in nitriding In nitriding, passing pre-decomposed ammonia (hydrogen-nitrogen mixture) into the furnace can reduce the nitriding potential without changing the temperature, achieving fine control of Kn, which is especially useful when low Kn or no compound layer nitriding is required.
Safety tips
- Liquid ammonia is a hazardous chemical, and its storage and use must comply with relevant safety regulations;
- The product contains 75% hydrogen, which is flammable and explosive. The pipelines need to be leak-proof, and the workshop needs hydrogen alarms and ventilation;
- Before entering the furnace, it is necessary to confirm that the furnace temperature reaches a safe temperature or use nitrogen purge;
- Replace the pipeline with nitrogen when shutting down.
Frequently asked questions
What are the advantages of dissociated ammonia over pure hydrogen?
The cost is relatively low and the supply is stable; it contains 25% nitrogen and has a slightly lower flammability risk than pure hydrogen. However, it still needs to be strictly managed according to hydrogen safety regulations.
How to deal with high residual ammonia?
Check whether the reaction temperature, catalyst activity and flow rate exceed the design capacity; replace the catalyst or reduce the load if necessary.
Can the nitriding furnace directly use the gas from the ammonia decomposer to adjust Kn?
Can. This is one of the common Kn adjustment methods. It needs to cooperate with the control system to automatically adjust the ratio of ammonia gas and decomposition gas according to the Kn setting value.