An SCR uses a catalyst and ammonia (NH₃) or a urea solution to convert NO_X into harmless nitrogen (N₂) and water vapor (H₂O). Below, this process is detailed for a urea solution: 

• The process begins with the intake of exhaust gases containing nitrogen oxides (NO_X) into the SCR reactor.
• A urea solution is added to the exhaust system before the SCR reactor.
• In the SCR reactor, the exhaust gas passes through a catalyst bed; this catalyst material accelerates the chemical reactions required for NO_X reduction.
• When the exhaust gas comes in contact with the catalyst bed and urea, a chemical reaction takes place. The nitrogen oxides (NO_X) in the exhaust react with urea (NH₂CONH₂) to form nitrogen (N₂) and water vapor (H₂O) according to the following reaction:

6NO_X + 4(NH₂CONH₂) → 5N₂ + 6H₂O + 2CO₂ + 2NH₃

• As a result of this reaction, the harmful NO_X compounds are converted into harmless nitrogen gas, water vapor, carbon dioxide and ammonia. This reduction in NO_X emissions helps reduce air pollution and complies with environmental regulations.
• The purified exhaust, which now contains significantly reduced NO_X emissions, leaves the SCR reactor and is emitted through the exhaust system.

Unlike SCR (Selective Catalytic Reduction), SNCR does not use a catalyst, but instead non-catalytic chemicals to reduce NOX emissions. Here's how an SNCR system works:

• The process begins by introducing exhaust gases, which contain harmful nitrogen oxides (NOX), into the SNCR system.
• A reagent, usually urea, is injected into the exhaust system. This is often done at a specific location where the temperature and concentration of NOX are optimal.
• The injected urea undergoes thermal decomposition at high temperatures in the exhaust gas. This process leads to the formation of ammonia and isocyanates:

(NH₂)2CO → 2NH₃ + CO₂

• The ammonia and isocyanates formed react with the nitrogen oxides (NO_X) in the exhaust to produce nitrogen (N₂) and water vapor (H₂O). The reaction thus reduces the amount of NO_X in the emissions.

SNCR systems are often simpler and less expensive than SCR systems, but they are less efficient at low temperatures and can lead to higher emissions of byproducts such as ammonia or isocyanates. Therefore, SNCR systems are usually used in industrial applications where exhaust gas temperatures are high enough for effective NO_X reduction.