Design Basis for SCR System

Customize the design based on the original project design parameters and target emission standards. The system includes: SCR reactor, mixing unit, urea storage unit, urea replenishment unit, gas supply unit, metering injection unit, PLC control unit, and maintenance bypass (optional), etc.

Model: BST-400DG-SCR/WSK medium temperature catalyst, suitable for engine exhaust temperatures of 250-420 degrees.

The concentration limit in the second level standard of the second period of the Chinese diesel generator set emission standard "Air Pollutant Emission Limits" (DB44/27-2001)

Maximum allowable emission concentration of pollutants (mg/m3)

NOX nitrogen oxides ≤ 120mg/Nm3

1. Working principle of SCR

SCR stands for Selective Catalytic Reduction. Its working principle is to inject a reducing agent into the exhaust pipe, so that the nitrogen oxides in the exhaust react with the reducing agent under the action of the catalyst and are reduced into nitrogen and water. The reducing agent currently used in SCR systems is urea.

Urea aqueous solution decomposes into NH3 and CO2 at high temperatures:

NH2CONH2+ H2O → 2NH3 +CO2

NH3 reacts with NO and NO2 in the exhaust to produce nitrogen and water:

NO+NO2+2NH3 → 2N2+3H2O

4NO+O2+4NH3 → 4N2+6H2O

2NO2+O2+4NH3 → 3N2+6H2O

To prevent ammonia leakage during the entire use of the SCR system, the SCR catalyst may be equipped with an ammonia oxidation layer, in which NH3 reacts with O2 to generate nitrogen gas and water. The reaction equation is as follows:

4NH3+3O2 → 2N2+6H2O 

2. Design process flow description and system architecture diagram

SCR denitrification technology mainly consists of SCR reaction chamber, catalyst, mixing pipe, SCR control system, injection cabinet (urea pump, urea tank, urea solution sensor, urea injection pipe, urea return pipe), urea nozzle, NOX sensor, temperature sensor, pressure sensor, flow meter, urea storage tank (1 ton) and other parts. Urea is stored in a urea tank in the form of a calculated concentration (32.5%) solution, and is sent to the spray gun in the urea injection tank under the action of the compressor and urea pump. Then, it is thoroughly mixed with flue gas and air and enters the SCR reaction chamber. When the flue gas passes through the catalytic layer of the SCR reaction chamber, selective catalytic reduction reaction occurs.

(1) The SCR nitrogen oxide catalytic reduction device process used is arranged in a medium temperature range. The catalyst can operate normally and stably in the range of 320-400 degrees Celsius at higher temperatures, and urea can be completely decomposed in this temperature range, achieving the best use effect with a denitrification efficiency of 80% -95%.

(2) The catalyst arranged in the medium temperature range is suitable for engine exhaust temperatures of 250-420 degrees Celsius, with the optimal catalytic temperature of 320-400 degrees Celsius. If the temperature exceeds 420 degrees Celsius, the bypass will be opened and discharged from the bypass to protect the catalyst from burning out.