Context: A high-powered committee of experts, led by Principal Scientific Advisor (PSA) has recommended that India should scrap its policy of mandating coal-fired thermal power plants (TPPs) to install Flue Gas Desulphurisation (FGD) units. FGD units are fitted in TPPs to cut harmful sulphur dioxide (SO2) emissions.
92% of India’s 600 TPPs have not yet installed FGD units. Instead, the committee recommends limiting FGD unit requirement to plants that use imported or high-sulphur (>0.5%) coal, as these contribute more significantly to SO₂ pollution.
Instead, the study recommends limiting this requirement to plants using imported or high-sulphur (>0.5%) coal, as these contribute more significantly to SO₂ pollution.
Relevance of the Topic: Prelims: Key facts related to Desulphurisation.
Rationale behind the Suggestions
- The rationale underlying the analysis is that 92% of the coal used in Indian plants has low sulphur content (0.3%-0.5%).
- SO2 levels in ambient air across the country are around 10-20 micrograms/cubic metre, well below India’s air quality norms of 80 micrograms/cubic metre.
- SO2 levels in cities near plants with operational FGD units do not differ significantly from those without these units.
- Particulate Matter samples in urban areas show low levels of elemental sulphur (max 8 micrograms/cubic metre) which is not a significant concern. Thus, FGD units may offer limited benefits in reducing PM pollution.
- Norms mandated by the Central Pollution Control Board that require stack heights (exhaust columns) in the thermal power plants be a minimum 220 metres, coupled with Indian climatic conditions, ensure that SO2 emissions do not threaten local air quality.
- A study by IIT-Delhi in 2024 found that acid rain, the most visible consequence of high SO2 emissions, was not a significant issue in India.
- Installing FGD in all coal plants would increase power consumption as well as freshwater consumption in the plants, resulting in an additional 69 million tonnes of CO2 (2025-30), while reducing SO2 emissions by 17 million tonnes.
- Unintended benefit of Sulphate Aerosols: When SO₂ is released into the atmosphere, it reacts with water vapour and other compounds to form sulphate aerosols. These aerosols reflect incoming solar radiation (shortwave radiation) back into space, which results in radiative cooling of the Earth's surface. This cooling effect masks or offsets part of the warming caused by greenhouse gases.
Flue Gas Desulphurisation
- Flue Gas Desulphurisation (FGD) is a clean technology system that separates the sulphur dioxide from the exhaust flue gas of coal-fired thermal power plants.
- FGD systems utilise various methods, including wet scrubbing with limestone slurry or dry scrubbing with a dry sorbent, to absorb SO2 from the flue gas.
How FGD Works?
- Flue Gas Collection: Flue gas, containing SO2 and other pollutants, is collected from the power plant's boiler or other combustion sources.
- SO2 Removal: The collected flue gas is then passed through an FGD system.
- In wet scrubbing, the gas is sprayed with a limestone slurry. The SO2 reacts with the limestone, forming a calcium sulfite or sulfate, which can be removed as a by-product or waste.
- In dry scrubbing, a dry sorbent, like lime or activated carbon, is introduced to the flue gas, where it absorbs the SO2.
- Waste Product Handling: The by-products or waste generated during FGD, such as gypsum or a dry waste product, are collected and either disposed of or utilized in other applications.
- Cleaned Flue Gas, now with reduced SO2 levels, is discharged into the atmosphere through the stack.