The Environment Ministry on July 11 exempted the majority of India’s coal-based thermal plants from installing systems that are designed to remove sulphur dioxide (SO2) emissions, a key contributor to air pollution. Known as flue gas desulphurisation (FGD) devices, these systems cut SO2 from flue gas, which is a residue from thermal plants.
Of the remaining plants, those around Delhi-NCR have to comply with a December 2027 deadline, and the rest will be asked to fit the devices on a case-to-case basis. Originally, these plants had to install FGD devices by 2017. However, over the years, they have received several extensions to comply.
What is the reason behind the Centre’s move? What do the revised rules exactly say? And why have some experts criticised the decision?
But first, why are SO2 emissions from thermal plants an issue?
Once released into the atmosphere, SO2 usually reacts with ammonia (NH3) to produce ammonium sulfate, which is responsible for roughly one-third of India’s fine particulate matter (PM2.5) pollution, according to recent research by the Centre for Research on Energy and Clean Air (CREA). PM2.5 is injurious to human health as it can lead to chronic diseases such as asthma, heart attack, bronchitis, and other respiratory problems.
Direct exposure to high levels of SO2 can irritate the eyes, throat, and lungs. Long-term exposure can result in increased risk of heart attacks, strokes, and premature death, according to CREA.
SO2 also affects the environment as it contributes to the formation of acid rain, which can damage ecosystems.
One of the biggest sources of SO2 emissions in India is the power sector, especially the coal-based thermal plants. The CREA research found the SO2 levels were higher in 2023 compared to 2019, with a notable increase in regions dominated by coal-based thermal plants. This highlighted “the influence of power plant emissions on air quality”, the research said.
What has the government done to address the issue over the years?
In 2015, the Environment Ministry notified the first-ever emission norms for control of SO2 and other harmful substances from coal-based thermal plants. These were required to install FGD devices by December 2017.
However, this did not happen as the deadline was extended four times at the request of thermal plants. They argued that the installation of FGD devices was costly, and could cause several issues such as disruption of power supply due to shutdown for installation, and an increase in electricity bills.
To address these concerns, the Centre amended the emission norms in 2021. It put 596 coal-based thermal plant units (one thermal plant can have multiple units) in three categories.
Category A: Plants located within a 10 km radius of the National Capital Region (NCR) or cities having a million-plus population. Their deadline for compliance was 2022.
Category B: Plants located within a 10 km radius of critically polluted areas or non-attainment cities. Their deadline was 2023.
Category C: Remaining plants whose deadline was 2024.
Note that nearly 78% of the plants were put in Category C. About 11% were in Category A, and the rest were in Category B.
The latest change in the norms has exempted Category C plants from installing FGD devices. Category A plants, which are in operation and under construction, have to comply by the end of 2027. The Centre will decide compliance for Category B plants on a case-by-case basis.
The Environment Ministry has also said that plants which are supposed to retire before December 2030 will not be required to meet the SO2 emission norms, provided they submit an undertaking for exemption.
What is behind the latest change in norms?
According to the Centre, the change in the norms is based on three studies, which suggest FGD devices are not necessary for coal-based thermal plants. These studies have been carried out by the Indian Institute of Technology–Delhi, the National Institute of Advanced Sciences (NIAS), and the National Environmental Engineering Research Institute (NEERI).
The studies have argued against the installation of FGD devices, saying that SO2 levels around the plants are well within the norms prescribed under the National Ambient Air Quality Standards. They have also suggested that the government needs to focus on curbing not just SO2 emissions but rather the overall particulate matter pollution around the plants.
The IIT-D and NIAS studies have said that while reducing SO2 emissions, the operation of FGD devices has increased carbon dioxide and PM pollution.
All three studies were commissioned by the government or its agencies. For example, the NEERI research was commissioned by NITI Aayog.
Why have experts criticised the change in norms?
Experts have said that the claim made by the studies that SO2 levels around plants are low is misleading. For instance, CREA in a statement said, “The air quality monitoring stations (CAAQMS) don’t capture the real impact of power plant pollution because they don’t track whether emissions drift upwind or downwind, and they certainly don’t account for chemical reactions that convert SO₂ into other pollutants like PM2.5.”
A 2021 analysis by the Centre for Science and Environment (CSE) also contested this claim. It said that plumes from power stations travel long distances of 300 km, and during this time, they get converted to secondary pollutants. The analysis also found that the atmospheric lifetime of SO2 was about 10 days.
Durgapur Thermal Power station. (Photo: Wikimedia Commons) 
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