By Prankur Patel, Assistant Director, Central Electricity Authority
The installation of flue gas desulphurisation (FGD) systems across India’s coal-based thermal power plants (TPPs) has emerged as a controversial topic in recent years due to rising concerns over the efficacy of FGD in tackling air pollution. Nearly a decade after sulphur dioxide (SO2) emission standards were notified by the Ministry of Environment, Forest and Climate Change (MoEFCC), the sector continues to face significant challenges in implementation, and dilemma on whether there is a need for FGD systems.
Evolution of SO2 norms
The MoEFCC brought out a notification in 2015, which classified TPPs based on their commissioning dates and prescribed differentiated limits. Similarly, for SO2, pre-2017 plants under 500 MW were allowed the stack emission of 600 mg/Nm³, while larger units had to comply with a 200 mg/Nm³ cap. For post-2017 units, the SO2 ceiling was tightened to 100 mg/Nm³.
However, due to the diversity of TPPs, their location and designs, the Central Electricity Authority (CEA) deemed that a one-size-fits-all schedule for compliance is not practical. As a result, the MoEFCC issued a revised notification in March 2021, which introduced a location-based, category-wise implementation framework. Plants were divided into three categories: Category A, in which TPPs are located within 10 km of the National Capital Region (NCR) or cities with over 1 million population; Category B, with plants located within 10 km of critically polluted areas or non-attainment cities; and Category C, which includes all other locations not covered under Categories A or B. From this decision, it was clear that the ambient air quality of different category locations was different, and had location-specific requirements for pollution control equipment.
The corresponding compliance deadlines for meeting all norms were set as December 31, 2022 for Category A; December 31, 2023 for Category B; and December 31, 2024 for Category C.
Additionally, the government introduced a graded penalty framework for delayed implementation in the form of environmental compensation. TPPs that fail to meet emission compliance deadlines are subject to environmental compensation in the form of a per-unit penalty on electricity generated. For delays of up to 180 days beyond the prescribed timeline, the penalty is Re 0.20 per kWh. If the delay extends between 181 and 365 days, the penalty increases to Re 0.30 per kWh. For delays exceeding one year, the penalty is set at Re 0.40 per kWh.
The MoEFCC issued two subsequent revisions, one in September 2022 and another in December 2024. The latter extension pushed deadlines (only for SO2 compliance) to December 31, 2027 for Category A; December 31, 2028 for Category B; and December 31, 2029 for Category C. The last date for unit retirement to qualify for exemption from SO2 norms was also extended to December 31, 2030. The rationale behind these extensions was to prevent market manipulation due to limited vendor capability in the country; address unavoidable implementation challenges, such as extended installation timelines; and simultaneously gather sufficient on-ground data post FGD installation to assess its efficacy in reducing ambient SO2.
CEA’s analysis
The CEA had released two detailed reports highlighting the challenges in rolling out FGD systems across coal-based TPPs. The first report, titled “Plant Location-Specific Emission Standards”, suggested a phased, region-sensitive approach that actually prioritised polluted areas with higher populations. It called for avoiding uniform deadlines that may create unrealistic pressure on vendors and lead to equipment shortages. It also noted that blanket norms are not feasible and that emission standards must be based on the actual pollution levels of each location. The report also noted the need to complete installations over a 15-year horizon, promote domestic manufacturing and conduct cost-benefit analyses of SO2 reduction technologies. It cautioned that a tentative time schedule would create market scarcity and distort equipment pricing.
The second report, titled “Review Report on New SO2 Norms”, outlined key environmental trade-offs. While FGDs can potentially reduce SO2 emissions by 5.54 million tonnes (mt) annually (but are ineffective in lowering ambient SO2 significantly), they are expected to increase CO2 emissions by 14.4 mt, primarily due to higher auxiliary consumption, greater coal usage and the material intensity of limestone-based systems. Further, it increases water consumption rates, straining India’s water resources. Capital costs range between Rs 3.9 million and Rs 14 million per MW, depending on plant and site conditions, which would increase electricity tariffs significantly.
Survey of ambient atmospheric SO2: IIT Delhi Study 2024
Under an MoU with the CEA, IIT Delhi was tasked with conducting a study to assess ambient SO2 concentrations across various Indian cities and providing critical insights. The study categorised cities based on proximity to coal-based TPPs and found that the power sector’s contribution to ambient SO2 ranged from 2.48 to 12.51 µg/m³. Category 1 includes cities without any coal power plants within 10 km of their boundary, while Categories 2 and 3 include cities with TPPs within 10 km, distinguished by whether or not FGD systems are installed. While India’s National Ambient Air Quality Standard (NAAQS) for SO2 is 80 µg/m³ (daily average), these findings revealed that power plants are not dominant contributors to already low regional SO2 levels, which are well below the NAAQS. The total ambient SO2 levels in these areas vary from 2.48-9.26 µg/m³ in Category 1, 3.23-11.23 µg/m³ in Category 2, and 4.24-12.51 µg/m³ in Category 3. In terms of SO2, the contribution from the power sector ranges between 0.42 and 3.15 µg/m³ for Category 1 areas, 0.78 and 3.35 µg/m³ for Category 2, and 1.44 and 7.04 µg/m³ for Category 3. All these levels remain well below India’s NAAQS of 80 µg/m³ for SO2.
The study also examined the impact of SO2 emissions on PM 2.5 and PM 10 formation, particularly the sulphate fraction of aerosols. It had been cited by certain stakeholders that the sulphates formed through secondary reactions of SO2 contribute to fine PM, making it a public health concern. In this context, the study assessed the power sector’s contribution to sulphate particulate aerosols. Sulphate in PM 2.5 ranges from 0.32-3.23 µg/m³ in Category 1, 1.23 to 3.01 µg/m³ in Category 2, and 1.56 to 5.57 µg/m³ in Category 3. These figures are set against ambient PM2.5 levels of 34.59 to 92.26 µg/m³ 31.53 to 72.53 µg/m³ and 52.27 to 86.77 µg/m³, respectively. For PM10, sulphate contributions from the power sector range from 0.46-4.62 µg/m³ in Category 1, 1.91 to 2.76 µg/m³ in Category 2, and 2.23 to 4.64 µg/m³ in Category 3, compared to total ambient PM 10 levels of 52.57-157.43 µg/m³, 52.89-114.71 µg/m³ and 82.15-142.19 µg/m³, respectively. Hence, a key finding of the study was that TPP contributions to aerosols in PM are very low, and FGD systems are likely to have an insignificant impact on lowering emissions.
The implementation of FGD systems may lead to a significant reduction in SO2 emissions from the stacks of the coal-based TPPs, but their effect on ambient air SO2 levels remains insignificant. Moreover, the SO2 removal process is energy-intensive and results in increased CO2 emissions. This, along with reduced aerosol-induced solar radiation blocking, may actually accelerate long-term global warming impacts. The carbon trade-off from FGD implementation is evident when comparing the annual emissions impact. While FGD systems would lead to a yearly SO2 reduction of approximately 4.88 mt, they could simultaneously contribute to an estimated 14.4 mt of additional CO2 emissions due to the energy requirements of the process. While the atmospheric lifetime of SO2 is a few hours to 15 days, CO2 can persist in the atmosphere for about 100 years. This highlights the need for a balanced policy approach that weighs local air quality benefits against global climate impact.
The way forward
Given the challenges and trade-offs, the need of the hour is a calibrated, evidence-based strategy. The CEA has recommended shifting the focus from insignificant SO2 emissions from TPP stacks to significantly problematic PM emissions in power plants to tackle air pollution. There are various other studies conducted by scientific organisations such as the National Institute of Advanced Studies (NIAS), Bengaluru, and CSIR-National Environmental Engineering Research Institute (NEERI), Nagpur, which have given similar recommendations to the central government. Consequently, the overall stakeholder consensus lies in limiting FGD systems to areas with high pollution levels, and for low-impact zones or plants nearing retirement.
After continuous and rigorous deliberations, the Ministry of Power (MoP), CEA, MoEFCC and Central Pollution Control Board (CPCB) arrived at a consensus and submitted a set of recommendations to the central government. Taking these into account, the central government issued an amendment notification to the applicability of SO2 norms, as notified in the Gazette dated July 11, 2025. Following this notification, out of 600 units, 462 units under Category C have been exempted from the applicability of SO2 norms, which means they are not required to install FGD. Meanwhile, Category A units are required to comply with the SO2 emission standards by December 2027 and Category B units by December 2028. However, TPPs under Category B can apply for exemption, and their cases will be reviewed by the expert appraisal committee. In addition, TPPs retiring by December 2030 are also exempt from compliance, provided they give an intimation to the CPCB and CEA about the same.
The trajectory of emissions control in coal-based TPPs is guided by evidence-based policy reform. It began with independent study and survey reports from various institutions such as the CEA, IIT Delhi, CSIR-NEERI and NIAS, Bengaluru, which, among other things, evaluated the efficacy of FGD technology and its broader environmental implications. The findings were integrated into a multi-stakeholder dialogue with regulators and industry experts to assess implementation challenges. The discussions underscored the need for targeted reforms focused on timelines, regional prioritisation and technology choices. This led to the government’s amendment notification on July 11, 2025, balancing emission goals with prevailing environmental, economic and technical realities.
Ultimately, the goal is to align India’s emission control strategy with its twin objectives of environmental sustainability and energy affordability.
