Nitric acid

Nitrogen oxides are major pollutants in the atmosphere, being a precursor to acid rain, photochemical smog, and ozone accumulation. The oxides are mainly nitric oxide (NOx) and nitrogen dioxide (NO₂) both of which are corrosive and hazardous to health. With the use of catalytic converters, the focus of controlling of NOx emissions has become economically impractical. Consequently, the stationary sources of NOx emissions are now being subjected to more stringent standards in many countries. The sources include nitric acid manufacturing plants, manufacturers of nitrated materials such as fertilizer and explosives, and industrial manufacturers, metallurgical processors, glass manufacturers, cement kilns and power generator where high processing temperatures are used. Because of the environmental concerns posed by air pollution, it is important to design and implement scrubber system for controlling NOx emissions.

Nitrogen Oxide Sources

• Combustion of fuels: At high temperatures, oxygen and nitrogen present in air combine to form nitrogen oxides. Typical flue gas samples contain 100-1500 ppm of nitrogen oxides.
• Nitric acid manufacture: Waste gases which cannot be economically recovered in the final absorber usually contain 2-3% nitrogen oxides based on weight of acid produced.
• Metal finishing operations: Many metal surface treatment operations which use nitrates, nitrites, or nitric acid which evolve nitrogen oxides.
• Chemical processes: Many processes where nitric acid, nitrates, or nitrites are used as reagents evolve nitrogen oxides. Examples include manufacture of explosives, plastics, dyes, etc.
• High temperature processes: Processes where materials are made at high temperatures such as glass manufacturing, electric furnaces, and cement kilns evolve nitrogen oxides.
• “Breathing” of the tank during changes in ambient temperatures, resulting in very low rates of emissions.
• Tank filling that displaces gaseous volumes equal to the volumes flowing into the storage tank. 

Mechanism

The process entails injecting H₂O₂ into the absorber feed water to provide a dilute solution containing 0.5 – 1 wt.% H₂O₂. This solution is passed through the absorber where NOx constituents are transferred from the vapor into the solution and oxidized to high-grade HNO₃. The oxidation reactions are rapid at moderate temperatures (30-80 degrees C), with about 1.7 and 0.4 lbs H2O2 required per lb of NO and NO2, respectively.