Reduction of Sulphur Dioxide | Gaseous Control | Environmental Engineering

In this article we will discus about the removal and reduction of sulphur dioxide from flue gases.

Removal of SO₂ from Flue Gases:

The flue gases have high percentage of SO₂ gas, because the fuels have high content of sulphur. Coal having high contents of sulphur is most widely used as solid fuel in industries and thermal power stations. To some extent the percentage of SO₂, in the flue gases can be reduced by using low sulphur coals. But the coals available in the mines of India have more percentage of sulphur about 3.6%.

The sulphur can be removed by catalytic hydrogenation of coal suspended in tar at 100-250 atm at 450°C to achieve 75% desulphurization with the consumption of 20 kg of hydrogen per tonne of coal. The sulphur contents can also be removed from coal by destructive distillation or carbonisation of coal. Hydrogen sulphide formed in the above way can be burnt off through chimneys.

Crude oils have sulphur contents in varying proportions. The sulphur contents of the fuel oil can be reduced by caustic treatment. In this method hydrogen sulphide and other mercaptanes are formed, which also require their destruction at the chimneys heights. Hydro- desulphurisation method is commonly used for reducing sulphur contents in the crude oils.

Reduction of SO₂ in Flue Gases:

Various methods are used for reduction of SO₂ in flue gases which vary from industry to industry.

Following are some of the common methods employed for the reduction of SO₂, in flue gases:

(i) By liquid ammonia:

This method is commonly adopted in fertiliser industry where, the SO₂, containing gases are passed through ammonia solution, and ammonia, sulphate, is obtained and sold as by-product. The separation of SO₂ gases from tuet gases in contact with liquid phases are achieved by absorption towers (both packed and plate towers), venturi scrubbers and spray towers.

(ii) Cairox method:

In this method the clean gas containing SO₂, is mixed with KMnO₄ through spray, the oxidation takes place as follows:

2KMnO_A + 3SO₂ + 4KOH → 2 MnO₂ + 3K₂SO₄ + 2H₂O

The resultant slide obtained by the above process is discharged into sewer lines, where the manganese dioxide acts as oxidising and coagulating aid in the treatment of the sewage.

(iii) Lime stone-lime wash method:

Fig. 6.9 shows the limestone fluidized bed technique for removal of SO₂ In this method, the lime stone is injected into the bed of the plant. Now the gases containing SO₂ are allowed to pass through the prepared bed at 800°C-1000°C. In this process the SO₂ gas is separated up to 90% efficiency and form calcium sulphate. The carried over particles can be separated by other particulate matter separators.

In lime wash method, the gases containing SO₂ are allowed to pass through the lime water. The SO₂, gas is removed from the gases forming CaSO₄ solution which is thickened by the thickness, dried and disposed of as solid waste.

(iv) Molecular sieve absorption method:

From the industry producing 180 tonne per day H₂SO₄ plant using a resin at 40°C, very low sulphur dioxide levels of 15-25 ppm have been collected form the final tail gas. The SO₂, component from the flue gases is separated by the molecular sieve whereas it is then stripped iron the resin by dry air at 100°C and recycled back to the SO₃ converters, for the production of H₂SO₄.