Nitrogen oxides (NOx) are removed from exhaust gases primarily through a process that involves converting them into harmless nitrogen gas.
A key method for removing nitrogen oxides from exhaust gases is by reducing the nitrogen oxides to nitrogen with the use of a gaseous reductant, in contact with a catalyst and at elevated temperatures. This process, as described in the reference, typically involves contacting the NOx-containing gas in admixture with at least stoichiometric proportions of a gaseous reductant.
The NOx Reduction Process Explained
This method relies on a chemical reaction where NOx molecules (like NO and NO₂) are broken down. The process can be broken down into key components:
- Nitrogen Oxides (NOx): These are the target pollutants in the exhaust gas.
- Gaseous Reductant: A chemical substance introduced into the exhaust stream that reacts with NOx to remove oxygen, thus converting NOx into nitrogen gas (N₂) and water (H₂O). Common examples of gaseous reductants include ammonia (NH₃) or urea (which decomposes into ammonia).
- Catalyst: A material that speeds up the chemical reaction between the NOx and the reductant without being consumed itself. Catalysts used in this process often contain precious metals (like platinum, palladium, rhodium) or base metals (like vanadium, titanium).
- Elevated Temperatures: The reaction requires specific high temperatures to proceed effectively. The optimal temperature range depends on the specific catalyst and reductant used.
How the Reaction Works
The reductant selectively reacts with the NOx on the surface of the catalyst at the appropriate temperature. For instance, using ammonia as the reductant:
4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O
6NO₂ + 8NH₃ → 7N₂ + 12H₂O
The result is a significant reduction in the amount of harmful nitrogen oxides released into the atmosphere, replaced by nitrogen (the main component of air) and water vapor.
Key Components of the System
Here's a summary of the crucial elements involved in this method:
| Component | Role | Examples (Practical Insight) |
|---|---|---|
| Exhaust Gas | Contains Nitrogen Oxides (NOx) | Industrial flue gas, vehicle exhaust |
| Gaseous Reductant | Provides hydrogen/ammonia for the reduction reaction | Ammonia (NH₃), Urea (decomposes to NH₃), Hydrocarbons (less common) |
| Catalyst | Accelerates the reduction reaction | Vanadium-based (for stationary sources), Zeolite-based (for vehicles) |
| Elevated Temperatures | Required for catalyst activity and reaction speed | Typically several hundred degrees Celsius (°C) |
This type of NOx removal process is widely used in various applications, including:
- Power plants and industrial boilers
- Diesel engines (vehicles, ships, locomotives)
- Cement plants and glass furnaces
By implementing these catalyzed reduction processes at high temperatures, industries and vehicles significantly reduce their impact on air quality, mitigating the formation of smog and acid rain.
