Nitrogen oxides (NOx) is a highly toxic yellow smoke that is released into the atmosphere through a chimney without treatment and forms a yellow (red) yellow smog. It is very difficult to control NOx.
The emission limit for nitrogen oxides specified in GB 21900–2008 is 200 mg/m3. In order to prevent the adverse effects of nitrogen oxides, the workshop must have a strong exhaust system and a reliable purification system.
The general treatment process is: three-stage reduction adsorption-secondary reduction adsorption-activated carbon adsorption-chimney discharge.
The NOx is reduced to N2 by a reducing lye adsorption method, and the volatilized nitric acid is neutralized.
Main reaction: 10NO2+4Na2S=4NaNO3+4NaNO2+4S+N2↑.
Side reaction: 4NO2+2Na2S=Na2S2O3+NaNO3+NaNO2+N2↑.
It can also be treated with 10% weakly acidic urea: NO+NO2+(NH2)2CO→2N2↑+CO2↑+2H2O, the absorption rate is over 90%, no secondary pollution, but the cost is high.