Sulfur dioxide is released by volcanoes and by various industrial processes such as from the combustions of fuels (oils and coal) in power plant or from fractional distillation process of crude oils. Generally, any fuels that contain sulfur compounds (e.g. Petroleum and coal) generate SO2 in their combustions unless the sulfur compounds are removed before it contacts with oxygen. However, the process of removal of sulfur compounds (e.g. hydrodesulphurization) from fuels is difficult and expensive and is not going to be discussed.
When sulfur dioxide dissolves in water droplets in clouds, it will form H2SO4, and thus it makes the rain more acidic than normal. This generates a phenomenon called acid rain. The impacts of acid rain are crucial to the environment; hence, the emission of SO2 from these fuels is regulated by federal laws in many countries.
There have been many methods developed to remove SO2 from the waste gas (flue gas). The most effective way is by microwave and beam processing which gives efficiency almost 100%. However, it’s never been easy and cheap to install the system. Therefore, it is not commercially applied in industries. In turn, a continuous Flue Gas Desulfurization (FGD) system is the most commercially-used method in many industries by using absorbance technology. Moreover, the installation and maintenance fees of FGD system are reasonably affordable.
The main contributor of SO2 is from power plants of coal or oils. However, for simplicity of making this report SO2 is injected from fractional distillation tower of crude oil. FGD system today has several process types:
a) Wet scrubbing
b) Dry scrubbing
c) Wet sulfuric acid method
d) Spray-dry
e) SNOX FGD
Regardless of the process type, the degree of SO2 removal attempted is a major consideration and wet scrubbing was found to give the highest efficiency of more than 95% removal of SO2. Wet scrubbing system itself uses various sorbents such as limestone (CaCO3), calcium oxide (CaO), lime slurry [Ca(OH)2)], magnesium hydroxide [Mg(OH)2], etc. To partially offset the cost of FGD-wet scrubbing installation, application of CaCO3 sorbent eventually produces a marketable by product (gypsum) when it is contacted with water. Commercially, this technique is known as forced oxidation and the overall reaction of the FGD system is:
CaCO3 (solid) + 1/2 O2 (gas) + 2H2O (liquid) + SO2 (gas) → CaSO4.2H2O (solid) + CO2 (gas)
A process diagram of FGD – wet scrubbing system is shown below:
According to US Energy Information Administration data, 27,973,918 barrels of sulfur was contained in a production of crude oils in 2010. This value is equal to 4,441,139,222 Liters of sulfur per year 2010.
By assuming an ideal system, if 1,174,904,556 gallons of sulfur comes into a distillation tower of crude oil, the same amount of SO2 will come out. Or by stoichiometry:
S +O2 → SO2
The amount of SO2 formed then will be going to the FGD-system as the inlet amount.
By having the stoichiometry of the overall reaction in the FGD system and knowing the fractional conversion of the process, we are able to determine how much gypsum is produced and how much SO2 is released every year from the system. Some calculations we have made:
4,441,139,222 Liters = 4,441,139.3 m3
m= 4,441,139.3 m3 x (2.6288 kg/mol) = 11,674,867 kg
11,674,867 kg (1000g/kg) (1mol/64g) = 182,419,797 mol of SO2 comes into the FGD system.
From stoichiometry, we can find directly that:
182,419,797 mols of CaCO3; 91,209,899 mols of O2; and 364,839,594 mols of H2O come into the batch.
However, we have constrained to take 96% efficiency of FGD wet scrubbing system (96% fractional conversion is taken), and by using atomic balances we are able to find how many moles of SO2, CO2, and gypsum yielded, and how many mols of CaCO3, O2, and H2O needed.
|
Compounds |
Mols per year |
| Oxygen (O2) | 87,561,503 mols needed |
| Limestone (CaCO3) | 182,419,797 mols needed |
| Water (H2O) | 350,246,010 mols needed |
| Carbon dioxide (CO2) | 175,123,005 mols released |
| Gypsum (CaSO4.2H2O) | 175,123,005 mols produced |
| Sulfur dioxide (SO2) | 7,296,722 mols released |
Since SO2 is an acid gas, the typical water used to help to absorb SO2 is sea water (alkaline).
The amount of gypsum produced as the byproduct of the reaction in turns can be sold in markets to produce valuable products such as plaster or plaster wallboard. The quality of gypsum produced is however affecting the selling price. Therefore, subsequent reduction-oxidation reactions for gypsum are sometimes applied in many industries to attain a high quality of gypsum.
Work Cited
Slack, A. V. (1971). Sulfur dioxide removal from waste gas. Noyes Data corporation.
(1991). Desulphurisation 2 technologies and strategies for reducing sulphur emissions. (symposium ed., Series no 123). Rugby, UK: Institution of Chemical Engineers (Great Britain).
