April 23, 2024

Car urea and SCR, have to tell the story

Since its introduction in 1892, diesel engines have been widely used in various power plants, ships, and vehicles due to their good dynamic performance, economy, and durability. Since the late 1980s, more and more diesel engines have also been used on cars. Of the 1.4- to 2.0-liter displacement cars produced in Germany, 61% are diesel cars, and the proportion of French diesel engine cars is as high as 88%. From a worldwide perspective, the dieselization of automobiles has become an irreversible trend.

Urea for Cars

Compared with the equivalent power gasoline engine, particulate matter (PM) and nitrogen oxides (NOx) are the two most important pollutants in emissions. From the current technical approach to reduce vehicle exhaust emissions, in order to meet the national IV emission standards, measures are generally no longer taken from the engine's own structural aspects. Usually exhaust post-treatment is used to reduce pollutant emissions, while urea As a selective catalytic reduction (SCR) is the most practical method, it can reduce the NOx in the engine exhaust by more than 50%.

What is SCR?

SCR is an abbreviation for Selective Catalytic Reduction, a selective catalytic reduction converter. SCR is used to reduce the NOx (nitrogen oxides) in the engine exhaust, so that the vehicle meets higher emission regulations, such as Euro IV and National IV emission requirements. The principle of SCR is to spray urea aqueous solution (Adblue) in the tail gas containing NOx to reduce NOx into N2 (nitrogen) and H2O (water).

Since NH3 can selectively react with NOx instead of being oxidized by oxygen, the reaction is said to be selective: trucks or cars with SCR systems use aqueous urea, and if not added, OBD will alarm, and for more than 2 hours Addition, OBD will limit the engine's output power, vehicle acceleration is not effective, and for a longer time, OBD will lock the engine.

The SCR is installed in an exhaust system of a diesel vehicle and catalytically reduces NOx in the exhaust gas of the diesel engine to N2 and O2. The urea solution is sucked from the urea tank through a heated pipe, and then the compressed urea solution is evenly blown into the exhaust gas. The urea solution is vaporized and releases the ammonia gas, which is then used in the catalyst of the SCR system. NOx accelerates conversion to pure nitrogen and water.

Urea pyrolysis principle:

Urea is hydrolyzed to ammonia and water.

(NO2) 2CO→HNCO+2NH3

Hydrolysis: HNCO+H2O→CO2+2NH3

Synthesis: (NH2) 2CO+H2O→CO2+2NH3

NOx conversion: 4NO+4NH3+O2→CO2+2NH3

6NO2+8NH3→7N2+12H2O

The catalytic reduction of ammonia and NOx produces N2 (nitrogen) and H2O (water) that are harmless to the human body.

The injected amount of urea must be matched with the NOx concentration, and it must not exceed a certain dose while ensuring the reduction of NOx. Insufficient injection of urea will not reach the proper treatment level. Excessive injection of urea will exhaust excess ammonia into the atmosphere, causing new pollution. Therefore, it is necessary to have a highly sensitive NOx concentration sensor and a corresponding high-precision urea injection device.

In order to select the best method for removing NOx, it is necessary to study the exhaust emissions of diesel engines, especially the temperature range and concentration of NOx emissions. The test data shows that about 80% of NOx is emitted at the maximum load of the diesel engine and the corresponding temperature range is 350 to 550°C. Since the exhaust gas passes through the exhaust pipe to a certain degree, the operating temperature of the catalytic converter can be limited to 250 to 500°C.

The operating efficiency of the SCR depends on the temperature of the gas. If it is operated in the temperature range of 200-500°C, the efficiency is 85%. The actual vehicle operating conditions can meet this requirement. The conversion rate required by the Euro IV standard is 50%, and the Euro V standard requires 70%, so the SCR system can fully meet the requirements.

China is actively implementing more stringent emission regulations, in particular the development of emission standards for medium and heavy-duty diesel vehicles. The implementation steps are: April 1, 2016 Implementation of the Five-State Emission Regulations in 11 Eastern Provinces and Cities, January 1, 2018 Japan implements the national five emission regulations.

To meet the national five emission standards, urea-SCR catalytic purification technology is one of the most effective technologies for diesel exhaust pollution control. However, there are many problems to make this technology available in our country. The first is the technical problem. It is necessary to improve the combustion technology in the engine and add post-processing devices. In addition, it is a cost issue. SCR technology is more complex than DPF technology and the cost is relatively high. The engine is basically a high pressure common rail technology. The cost of this technology is about 50% of the engine cost. For engine manufacturers, A lot of pressure. In addition, there are problems with urea filling stations, which require national planning and strong government support. The Kelansu Chain Refueling Station effectively solves the problem of the five cars in the four countries of the country. It is crucial to promote it within the domestic scope.


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