What are flue gas desulphurisation units ?

Context

• On June 4, The Hindu reported that a committee of experts, chaired by Principal Scientific Advisor (PSA) Ajay Sood, has recommended that India do away with a decade long policy of mandating Flue Gas Desulphurisation (FGD) units in all coal-fired thermal power plants (TPPs)

Flue Gas Desulphurisation

• Flue gas is emitted as a byproduct of combustion of fossil fuels.

• It mainly contains pollutants such as carbon dioxide (CO2), sulphur dioxide (SO2), nitrogen oxides, particulate matter, etc.

• FGD units specifically target the SO2 emissions in flue gas.

• SO2 is an acidic gas, and is usually treated with a basic compound in the FGD unit to neutralise the pollutant.

• There are three common types of FGD systems around the world — dry sorbent injection, wet limestone treatment, and using sea water to remove SO2.

• The dry sorbent injection method involves adding a powdered sorbent like limestone to the flue gas, where it reacts with SO2. The resultant compound can be removed by using an electrostatic precipitator

• The wet limestone treatment method also uses limestone to remove SO2, but instead of using it in a powdered form, it uses a limestone slurry.

• Passing SO2 through this slurry results in the formation of gypsum, which is a stable compound and has wide applications in industries like construction. This is the commonly used technology, and has very high efficiency.

• Sea water treatment is used in plants located near coastal areas. Sea water first absorbs SO2 from flue gas, and then the water is treated to make it suitable to be discharged back into the sea

Issue with FGD

• SO2 is one of the major greenhouse gases that cause global warming, and can cause respiratory problems in humans.

• In 2015, the Union Environment Ministry issued a policy that mandated all 537 coal-fired TPPs in India to install FGD units to reduce SO2 emissions.

• It takes around two years to install an FGD unit.

• Installing FGD units is a costly affair. According to the Central Electricity Authority, FGD costs approximately ₹1.2 crore per MW to install.

• As of April 2025, India’s installed coal capacity stood at 2,19,338 MW, which is more than 46% of the country’s total electricity installed capacity. This is expected to rise in the coming years. In his statement at a June 10 press conference, Union Power Minister Manohar Lal Khattar said, “About 97,000 MW of power will be added, and implementing FGD means an additional expense of ₹97,000 crore.