Nitrogen oxide (NOx) is a kind of yellow smoke with great toxicity. It is discharged into the atmosphere through the chimney without treatment, forming striking brown (red) yellow smoke, commonly known as "Huanglong". NOx is the most difficult in many waste gas treatment, and is the culprit of atmospheric pollution.

The design and selection of suction hood, ventilation duct, waste gas purification tower and fan determine the normal operation and treatment effect of the whole ventilation and purification system.

Forced (centrifugal fan) mechanical ventilation is used to draw the NOx waste gas generated in the pickling tank into the bottom side of the waste gas purification tower through the capture exhaust hood and ventilation duct. The NOx waste gas is led to the bottom side of the tower and rises along the tower. The absorption liquid is evenly distributed in the packing layer or the swirl (inclined hole) plate and flows downward. Based on the theory of double membrane of gas and liquid mass transfer, the tower fully contacts the gas and liquid solvent for chemical reaction and is in a state of violent disturbance.

Mass transfer is an important and basic process in chemical reaction. In order to enhance the efficiency of gas-liquid mass transfer, the key is to select nozzles with high anti clogging, jet strength and fineness, and excellent packing or novel swirl (inclined hole) plate, which have the following advantages:

1) The specific surface area per unit volume is large, which increases the surface area of gas-liquid contact and has strong mass transfer capacity.

2) It can change the flow direction of gas, resulting in a continuous channel for gas-liquid exchange, and has great operational flexibility.

3) Corrosion resistance, long service life, strong anti pollution ability, can be used repeatedly.

NOx waste gas is different from SO2, HCl, H2 SO4 and other acid mist, and only one waste gas purification tower is required. According to the characteristics of complex composition, high concentration and difficult treatment of NOx waste gas, the system is designed with two stages, namely, two-stage waste gas purification (absorption) tower, to increase the mass transfer process between NOx and absorption liquid, and have sufficient reaction time to make NOx waste gas diffuse in the liquid phase, be absorbed, dissolved and purified.