7.1 SOx Control in Flue Gas

 

SO_x emission control is most essential requirement in today scenario because High concentrations of sulfur dioxide (SO₂) can result in breathing problems with asthmatic children and adults who are active outdoors.

Apart from health hazard it is very harmful for environment because Sulfur dioxide and nitrogen oxides are the major precursors of acid rain, which has acidified soils, lakes and streams, accelerated corrosion of buildings and monuments, and reduced visibility.

 

As much as 75% of total SO₂ ejected into atmosphere from Thermal power plant. There are several methods available for cleaning up flue gases from Sox.

-Dry Scrubbing – Absorption on lime/MgO/CaCO₃ bed.

- Wet Scrubbing – Spraying lime slurries/ NaOH/Caustic soda solution into the flue gas stream.

- Absorption of SO_x on coke or activated charcoal bed.

- Catalytic oxidation of SO₂ to SO₃ and subsequent conversion as well as removal of latter as H₂SO₄.

- SOx control during combustion of coal by adding Lime Stone.

In this article, we shall discus on SO_x control by lime stone.

Calcinations Process

The process of heating a substance to a high temperature but below the melting or fusing point, causing loss of moisture, reduction or oxidation, and dissociation into simpler sub- stances. The term was originally applied to the method of driving off carbon dioxide from limestone to obtain lime (calcium oxide). Calcinations are also used to extract metals from ores.

 

Desulfurization Process

It the process of removing sulfur from flue gas by using wet process, Semi-dry process or Dry process. The reaction between the SO₂ and the alkali can take place either in bulk solution (‘wet’ FGD processes) or at the wetted surface of the solid alkali (‘dry’ and ‘semi-dry’ FGD processes).

 

Limestone parameter:

Parameters of Limestone	Quantity in %
CaCO₃	82
MgCO₃	2.14
FeO₃	3.72
SiO₂	9
Al₂O₃	2.86

 

The reaction of Sulfur with Lime stone for desulfurization is occurs after the calcinations process as:

           Calcinations reaction @ 600 °C to 750°C temperature

                             Heat

CaCO₃          ---------------->                CaO   + CO₂ (g) ↑

          100 g                                         56 g    +         44 g

 

Desulfurization reaction @ 700 °C to 850 °C temperature

CaO  +	SO₂    + ½ O₂	Heat	CaSO₄
56 g    +	64 g     + 16 g	--------------->	136 g
56/64 g	64/64 g    16/64 g		136/64 g

 

From above reaction we can get how many grams of CaO & O₂ are required to desulfurization of sulfur.

 

We get 0.875gms of CaO & 0.25gms of O₂ required to desulfurization of 1gm of sulfur Dioxide (SO₂) and produce 2.125gms of CaSO₄.

 

Conclusion:

 From the above equation, we found that if we are using higher sulfur content fuel, i.e. Coal then we requires mixing the higher amount of Lime Stone to reduce the SOx. It depends upon the requirement for desulfurization. If we are mixing higher amount of Limestone then we get high amount of Calcium Oxides (CaO). It will help to desulfurize higher Sulfur or Sulfur dioxides (SO₂).

So it is most important to know the amount of Calcium Oxides (CaO) present in the lime stone that we are using. Lower the Calcium Oxides (CaO) will increases the Lime stone consumption.