The background of SCR technology application

With the global shortage of fossil fuels and the increase in CO2 discharge pressure ,the diesel engines have been widely used as the advantage of  its low fuel consumption and high thermal efficiency , but the nitrogen oxygen (NOx) and particulates (PM) also cause serious harm to people’s living environment. Every country have introduced emission regulations one after another to greatly reduce the emissions of NOx from diesel vehicles.

At present, the domestic and foreign experts generally believe that through the combustion optimizing internal control measures such as it is difficult to meet the requirements of the five European six emission regulations, efficient exhaust after-treatment technology SCR for its high fuel economy, the nature of oil demand lower bai is recognized as one of the most potential technology, as a reductant in urea solution has mature technology, stable catalyst, NOx conversion efficiency higher advantages, so has been the most widely used.

The working principle of SCR technology

Fast SCR reaction

- When increasing the ratio of NO2 in NOx, the conversion efficiency of NH3-SCR to NOx under low temperature conditions can be improved.

4NH3+2NO+2NO2=4N2 +6H2O

-The reaction can be under the low temperature when the ratio of the concentration of NO and NO2 is 1 will be the best NOX catalytic conversion efficiency of the reaction under low temperature conditions of the reaction rate is 17 times of the standard SCR reaction, therefore, known as rapid SCR reaction, therefore, will be installed upstream of the SCR reactor pre oxidation device part into NO2 NO oxidation, can solve the low temperatures NH3 - SCR NOX into the problem of low efficiency.

The working principle of SCR system

Standard SCR reaction

-NOx emission of diesel engine accounts for more than 90% of NO. The main reactions of NH3-SCR catalytic reduction are as follows：

4NH3+4NO+O2=4N2+6H2O

-The reaction is called the standard SCR reaction. This reaction is the main reaction of NH3 catalytic reduction of NOx. It has a higher reaction efficiency at a temperature of 300-400°C, but at a lower temperature (250°C, such as diesel engine cold start) NOx’s conversion efficiency is low.

Therefore, it is necessary to find a method that can maintain a high NOx conversion efficiency when the exhaust temperature of diesel engine is low.