The main sources of sulfur compounds in air are anthropogenic, with sulfur dioxide (SO2) the predominant form.
Emissions of sulfur oxides are mainly generated during combustion as the sulfur contained in the fuels is oxidized; therefore, sulfur oxides emissions are almost exclusively dependent on the sulfur content of the fuel and not on boiler size, burner design, or fuel grade.
In combustion systems, approximately 95% of the sulfur present in the fuel is converted to sulfur dioxide (SO2), between 1 to 5% is further oxidized to sulfur trioxide (SO3), and 1 to 3% is emitted as sulfate particulates.
As SO2 is a colorless, corrosive gas, it has a very harmful effect on plants, animals and humans and even on the physical environment. In air it can be further oxidized to SO3, which reacts with water vapor to form sulfuric acid (H2SO4), one major component of acid rain.
Also, the sulfate anion (SO4=) can be inhaled by humans into the lungs where it is very detrimental. In addition, sulfate particles contribute to impairment of visibility and affect the Earth's albedo, or global radiation balance, which in turn, has an effect on climate [46–48].