Flue gas desulfurization is an extremely important industrial process with respect to modern petrochemistry and modern petrochemical responsibility. Flue gas desulfurization is the process of removing sulfur dioxide and in some cases sulfur trioxide from high temperature boiler flue gas.
Naturally formed hydrocarbons such as natural gas in some areas have very high levels of sulfur bases compounds. Common among these are families of sulfur compounds known as sulfides and mercaptans. Gas and oil deposits that contain low levels of sulfur naturally are referred to as "sweet" while deposits containing high levels of sulfur are referred to as "sour". Sour hydrocarbons can be used in industrial processes such as boilers, but a specialized post-burning chemical scrubbing process must be in place to prevent toxic gases from escaping into the atmosphere.
The most common sulfur derivative formed in boiler burners is sulfur dioxide or SO2. This is formed from the combination of atmospheric oxygen and sulfides or mercaptans present in the hydrocarbon based fuels. Just as gas burns, so will the sulfides and mercaptans. The higher the level of sulfur in the fuel naturally, the higher the level of SO2 produced.
Sulfur dioxide itself falls into a class of sulfur oxides known as SOx gases. The "x" represents a multiple of oxygen and can be one, two or three. Sulfur oxides are acidic by nature so after particulate removal, the flue gas runs through an actual desulfurization process that in many cases is a chemical scrubber based on caustic chemistry. Caustic solution can trap sulfur oxides in the form of sulfates as well as NOx gases in the form of nitrates and carbon oxides in the form of carbonates.
Flue gas desulfurization is important because SOx gases, particularly SO3 can combine with moisture in the air to produce H2SO4, also known as sulfuric acid. This is what produces acid rain and it extremely detrimental to anything it comes in contact with such as paint, structural and protective materials and most importantly plant, soil and marine life.