Nitrogen Oxides (2)

What are the effects of nitrogen oxides?

High concentrations of NO2 can produce an abnormally high accumulation of fluid in lung tissue. For exposures ranging from several minutes to one hour, a level of 50 – 100 ppm NO2 causes inflammation of lung tissue for a period of 6 – 8 weeks, after which time the subject normally recovers. Exposure of the subject to 150 – 200 ppm of NO2 causes bronchititis fibrosa obliterans , a conditions fatal within 3 – 5 weeks after exposure. Death generally results within 2 – 10 days after exposure to 500 ppm or more of NO2.

NO2 also causes extensive damage to plants through its secondary products such as peroxy acyl nitrate formed in smog. Exposure of plants to several parts per million of NO2 in the laboratory causes leaf spotting and break down of plant tissue. It also causes fading of dyes and inks used in some textiles. Much of the damage to materials caused by NOx, such as stress – corrosion cracking of electrical apparatus, comes from secondary nitrates and nitric acid.

How to control nitrogen oxides' emission?

It is possible to lower nitrogen oxides by carrying out the combustion in two stages, the first of which is rich in fuel and the second of which is rich in air. In this way the fuel is burned completely, but the temperature is never as high as it would be for a stoichiometric mixture. This two-stage approach is being incorporated in power plants; it has been tried in cars but with less success.

The other method that is done to reduce emissions is to remove the pollutant from the exhaust gases. In automobiles, this is done by the use of a threeway catalytic converter.

In order to deal with both NO and unburned gases the converter has two chambers in succession. In the reduction chamber, NO is reduced to N2 by hydrogen, which is generated at the surface of a rhodium catalyst by the action of water on unburned fuel molecules.

HC +H2O→H2 + CO
2NO+ 2H2 →N2 + 2H2O

In the oxidation chamber, air added, and the CO and unburned hydrocarbons are oxidised to CO2 and H2O at the surface of platinum/palladium catalyst.

2CO + O2 →2CO2
HC + 2O2 → CO2 + 2H2O