It is responsible for causing acid rains, increasing acidity of agricultural soils rendering them infertile, in addition acid rain itself can cause plant damage and damage to buildings and statues ruining the beauty of a place. Apart from acid rain, exposure to sulfur dioxide itself can cause serious health problems in people. It can lead to the development of eye and skin irritation and long term exposure to higher concentrations of it can even cause skin cancers. Hence removal of this gas from the waste gases emitted by factories is highly essential to prevent the people and the environment from suffering from its dangerous consequences.
Flue gas desulfurization (also referred to as FGD) is a very beneficial way to remove sulfur dioxide gas from the effluent gases emitted from factories. There are various methods of FGD like spray towers, plate towers etc but the main purpose is cost effective and efficient removal of sulfur dioxide from the waste gases, no matter what relevant reagents are used. Background of Flue Gas Desulfurization Fossil fuels are the most commonly used fuels for combustion in the industries in order to produce energy required to run the industries. Fossil fuels include coal, oil, natural gas etc.
and contain considerable quantities of the element called sulfur, which upon combustion releases toxic gases like sulfur dioxide (SO2). Hence, even power plants which use coal for burning and producing energy to run turbines, emit destructive gases into the atmosphere. Industrial Revolution not only revolutionized our life styles but it also increased the chances of environmental pollution and health concerns. Huge amounts of waste were being emitted into the atmosphere without any acknowledgement of how harmful it could be to the human population and the whole of ecosystem.
The SO2 emissions problem did not receive much attention until 1929 (Flue Gas Desulfurization FGD Wikipedia) after which FGD plants were installed by various industries and factories till it was made essential by the governments of quite a few countries to use FGD plants by each and every flue gas emitting industry (or in other words it was made necessary for those industries which were using fuels for combustion). Different Methods of FGD and their Chemistry There are two basic systems used for flue gas desulfurization: nonregenerative and regenerative.
The difference between the two is that reagent used in regenerative systems is reused while in nonregenerative systems the reagent forms sludge and has to be disposed off. Though no sludge is produced in regenerative systems but because of the other benefits of nonregenerative systems they are used more extensively. Calcium oxide (CaO), sodium bi carbonate (Na2CaCO3), Sodium hydroxide (NaOH) and ammonia are the most commonly used reagents for the nonregenerative systems. Since sulfur dioxide (SO2) is an acidic gas, the main chemistry involved in removing it is the use of alkaline reactants.
Lime, limestone and sodium hydroxide (NaOH) remain as favorites in FGD scrubbers. However, magnesium hydroxide (Mg(OH) 2) is also favored in case of regenerative systems. The highest SO2 removal efficiencies (greater than 90%) are achieved by wet scrubbers and the lowest (less than 80%) by dry scrubbers. (Flue Gas Desulfurization Wikipedia) A few most commonly used reagents and their chemical reactions taking place in wet and dry scrubbers are described below: In case of calcium hydroxide (lime) as the reagent:
Lime scrubbing uses an alkaline slurry made by adding calcium oxide (CaO), approximately 90% pure to water. The slurry is sprayed in the absorber and reacts with sulfur dioxide (SO2) in the flue gas. (Lee p. 321) The following reaction takes place: Ca(OH)2 (solid) + SO2 (gas) > CaSO3 (solid) + H2O (liquid) (Flue Gas Desulfurization Wikipedia) In case of calcium carbonate (CaCO3 limestone) as the reagent: This process uses the same mechanism as that of calcium hydroxide (lime) scrubber. Calcium sulfite (CaSO3) and calcium sulfate (CaSO4) salts are formed in the reaction and are removed as sludge.
(Lee p. 322) Nevertheless in dry system where no water is used to mix calcium carbonate, carbon dioxide (CO2) is produced instead of calcium sulfate (CaSO4) as described below: CaCO3 (solid) + SO2 (gas) > CaSO3 (solid) + CO2 (gas) (Flue Gas Desulfurization Wikipedia) In case of sodium hydroxide (NaOH) as reagents: In comparison to lime it is more expensive however, it gives the benefit of solution formation as compared to slurry in case of lime. It produces a solution of sodium sulfite/bisulfite (depending on the pH), or sodium sulfate that must be disposed of.
(Flue Gas desulfurization Wikipedia) Its usage is mostly in smaller combustion units. In case of magnesium oxide (MgO) or hydroxide Mg(OH)2 : Magnesium oxide scrubbing is a regenerative FGD process to remove SO2 from combustion exhaust gas. Magnesium oxide slurry absorbs SO2 and forms magnesium sulfite. Magnesium sulfite solids are separated by centrifugation and dried to remove moisture. The procedure is calcined to produce magnesium oxide and concentrated SO2 gas for production of sulfuric acid or elemental sulfur. (Lee p. 322)
When magnesium hydroxide is used the following reaction takes place: Mg(OH)2 (solid) + SO2 (gas) > MgSO3 (solid) + H2O (liquid) (Flue Gas Desulfurization Wikipedia) Types of wet scrubbers used in FGD Approximately 85% of the flue gas desulfurization units installed in the US are wet scrubbers, 12% are spray dry systems and 3% are dry injection systems. (Flue Gas Desulfurization Wikipedia) Most common wet scrubbers used in FGD systems are mobile-bed scrubbers, venturi-rod scrubber, packed bed scrubbers, plate towers and spray towers.
Packed scrubbers efficiently remove sulfur dioxide by allowing it to get in good contact with the packed material present inside their towers. Mobile and packed bed scrubbers are quite similar except for the packing material in the mobile scrubbers floats in the gas stream. In case of spray towers droplets are produced by spray nozzles inside a tower. A venturi scrubber is a converging/diverging section of duct. (Flue Gas Desulfurization Wikipedia) The gas stream is accelerated at a high speed and the liquid stream is injected at the same time hence resulting in tiny droplets.
However, a lot of power is required for this process. Conclusion Removal of sulfur dioxide from waste gases is critical in saving the environment and the invention and extensive use of flue gas desulfurization techniques by many industries around the globe is a highly effective step towards protecting the environment. Flue gas desulfurization scrubbers have been applied to combustion units firing coal and oil that range in size from 5 MW to 1500 MW.
(Flue Gas Desulfurization Wikipedia) The cost of setting up, running and maintaining FGD systems ranges from around $200 to $4000 per ton. Though the power requirement and the cost of setting up and operating FGD systems is quite high, this process is very cost effective. Saving energy and money on the cost of increasing atmospheric pollution is an unacceptable choice.
Works Cited Lee, C. C. Environmental Engineering Dictionary. 2005. Rowman and Littlefield Publishers. Flue Gas Desulfurization. Wikipedia. 16 April, 2008.