In this chapter four of the major gaseous air pollutants are considered, i.e., sulfur oxides (SOx), nitrogen oxides (NOx), ozone (O3), and carbon monoxide (CO).
Sulfur oxides include both sulfur dioxide (SO2) and sulfur trioxide (SO3), of which SO2 is more important as an air pollutant. Sulfur trioxide may be formed in the furnace by reaction between sulfur and O2, or SO2 and O2. Sulfur dioxide is probably the most dangerous of all gaseous pollutants on the basis of amounts emitted.
Sulfur oxide emission results from the combustion of sulfur-containing fossil fuels such as coal and oil. The sulfur content of coal ranges from 0.3 to 7% and the sulfur is in both organic and inorganic forms, while in oil the sulfur content ranges from 0.2 to 1.7% and its sulfur is in an organic form. The most important sulfur compound in coal is iron disulfide (FeS2) or pyrite. When heated at high temperatures, pyrite undergoes the following reactions:
FeS2 + 3 O2 s FeSO4 + SO2 4 FeS2 + 11 O2 s 2 Fe2O3 + 8 SO2
In the smelting process, sulfide ores of copper, lead, and zinc are oxidized (roasted) to convert a sulfide compound into an oxide. For example, zinc sulfide undergoes oxidation process in a smelter forming ZnO and SO2 as shown below:
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