The principle of Selective Catalytic Reduction (SCR) is that under the action of a catalyst, the reducing agent NH3 selectively reduces NO and NO2 to N2 at high temperatures in engine exhaust, without almost any oxidation reaction between NH3 and O2, thereby improving the selectivity of N2 and reducing NH3 consumption.

Components of SCR system:

As shown in the figure, the main components of the SCR system are the urea tank, mixing pipe, reaction chamber, catalyst, and control system. The control system includes the main control cabinet, metering cabinet, urea pump, booster pump, and injection device.

Working principle and process of SCR system

The entire system is installed in the engine exhaust pipe, preferably as close as possible to the position of the engine exhaust pipe, as the entire reaction requires a certain temperature to improve reaction efficiency. The control system is a closed-loop system that collects various signals to adjust the urea injection amount according to changes in nitrogen oxides in exhaust emissions. This can not only fully reduce nitrogen oxides in the exhaust gas, but also prevent excess ammonia solution from escaping.

When the engine starts, the main control cabinet will receive a start signal and send a signal to start the injection control cabinet, air compressor, and urea pump.

When the engine is first started, urea will be sprayed through the nozzle according to a pre-set curve. After entering the mixing pipe, it will undergo a series of synchronous reactions such as atomization, evaporation, hydrolysis, etc. under a certain temperature to form an ammonia solution. At the same time, the engine exhaust also enters the mixing pipe and is fully mixed with the injected urea.

A detection device will be installed at the end of the mixing pipe to detect the NOx content in the exhaust gas, which will then be transmitted to the main control cabinet.

After being fully mixed with the reducing agent NH3, the exhaust gas continues to enter the SCR reaction chamber. Under the catalytic action of the catalyst, NOx in the exhaust gas will react with NH3 in a very short time, and NOx will be reduced to N2 while generating some H2O, which will be discharged together.

To prevent the incomplete reaction of NH3 in the reaction chamber from being discharged into the atmosphere and causing secondary pollution, a catalyst for ammonia escape can be added to the reaction chamber to further fully react ammonia and control ammonia escape to a very small amount.

A NOx detection device will also be installed at the end of the reaction chamber, allowing the main control cabinet to adjust the urea injection dose in a timely manner by comparing the NOx content in the inlet and outlet exhaust gases, ensuring that NOx can be fully reduced.