SCR systems: how NOx abatement works

What NOx are

NOx (nitrogen oxides) are chemical compounds made up mainly of nitric oxide (NO) and nitrogen dioxide (NO₂). They form in high-temperature combustion processes, such as those in diesel engines, boilers, gas turbines and many industrial furnaces.

These gases are among the main causes of air pollution and contribute to phenomena such as:

What an SCR system is

SCR (Selective Catalytic Reduction) is a technology designed to drastically reduce NOx emissions in exhaust and flue gases.

It is currently one of the most effective methods available and is used both on diesel engines, particularly in heavy-duty vehicles, and in industrial and power generation plants.

How an SCR system works

It works through a chemical reaction that, in the presence of a catalyst, converts NOx into harmless substances.

Main stages

The reaction is efficient within a well-defined temperature range that depends on the type of catalyst (roughly 200 to 450 °C). Reagent dosing must be precisely controlled: too much causes unreacted ammonia to be released at the stack (known as ammonia slip), too little reduces abatement efficiency. For this reason SCR systems are normally combined with continuous measurement of NOx and, where required, NH₃.

Advantages of SCR systems

Conclusion

SCR is a key solution for reducing the environmental impact of engines and combustion plants. Thanks to its effectiveness it has become a standard, making a significant contribution to reducing harmful emissions.