To address growing environmental concerns, the Ministry of Environment, Forest and Climate Change (MoEF&CC) mandated the installation of flue gas desulfurisation (FGD) systems in coal-based thermal power plants (TPPs) under the Environment (Protection) Amendment Rules, 2015. FGD systems are intended to curb sulphur dioxide (SO2) emissions, which are a significant contributor to air pollution. The Central Electricity Authority (CEA), under the Ministry of Power, partnered with IIT Delhi to conduct a comprehensive, measurement-based survey of ambient SO2 concentrations across Indian cities.
The study, conducted in three phases between February 2023 and February 2024, categorised cities into three groups – Category 1 (those without TPPs), Category 2 (those with TPPs lacking FGDs) and Category 3 (those with TPPs equipped with FGDs). The aim was to assess the impact of proximity to TPPs and the presence of FGDs on local air quality. Findings indicated that all surveyed cities remained within the National Ambient Air Quality Standards (NAAQS) for SO2 (80 µg/m³ over 24 hours). Category 2 and 3 cities, despite proximity to TPPs, also showed compliance but experienced higher mean SO2 levels compared to Category 1 cities. The study also evaluated the contribution of TPP emissions to sulfate aerosols and particulate matter (PM2.5 and PM10), finding that while TPPs do contribute, their impact on overall PM levels is modest.
Power Line presents an overview of the key findings of the report…
Major findings of the study
The study revealed that the daily mean ambient air SO2 concentrations in Category 1 cities ranged from 2.48-9.26 µg/m³.
Within this category, New Delhi recorded the highest maximum instantaneous SO2 concentration of 29.37 µg/m³ during Phase I (pre-monsoon season), while its lowest minimum instantaneous value was 0.83 µg/m³ during the same phase. In Category 2 cities, the daily mean ambient air SO2 concentrations ranged from 3.12-11.23 µg/m³. Among these cities, Kota, Rajasthan, recorded the highest maximum instantaneous SO2 concentration at 48.56 µg/m³ under normal conditions. This value spiked to 68.09 µg/m³ during festival days in Phase III (winter season). The lowest instantaneous concentration in this category was 0.75 µg/m³, observed in Kota during Phase II (summer monsoon season). For Category 3 cities, the daily mean SO2 concentrations ranged from 4.24-12.51 µg/m³. Gautam Buddha Nagar, Uttar Pradesh, registered the highest maximum instantaneous SO2 concentration at 36.41 µg/m³ under normal conditions, with a peak of 70.14 µg/m³ during festival days in Phase III. The lowest minimum concentration in this category was 0.54 µg/m³, also recorded in Gautam Buddha Nagar during Phase III.
Further, ambient air SO2 levels across all city categories and monitoring phases remained well within the NAAQS limit of 80 µg/m³, which represents the 24-hour average threshold established by the MoEF&CC. Even the maximum instantaneous values did not exceed this standard. The survey also highlighted spatial variability in SO2 levels, with measurement sites located downwind of TPPs exhibiting higher concentrations compared to upwind sites. In both Category 2 and 3 cities, SO2 concentrations were found to decrease with greater distance from the TPPs, indicating a clear impact of proximity to power plants on ambient air quality.
Moreover, the study assessed the effectiveness of FGD systems installed in Category 3 cities. No substantial or systematic differences in ambient SO2 levels were observed between Category 2 and 3 cities, casting doubt on the real-world efficacy of FGDs. In particular, the operation of FGD units in Gautam Buddha Nagar and Khargone, Madhya Pradesh – two cities selected for focused monitoring – did not result in noticeable reductions in SO2 concentrations. Furthermore, the daily variability in SO2 levels in these locations did not correspond with the operational status or the number of active FGD units, raising questions about the practical impact of these systems in reducing ambient SO2 pollution.
The study further assessed the impact of TPP-related SO2 emissions on particulate matter (PM2.5 and PM10) through the formation of sulfate aerosols. It found that the contribution to PM2.5 and PM10 was relatively minor, averaging between 0.96-5.21 per cent for PM2.5 and 0.57-3.67 per cent for PM10, with the highest impacts observed in Category 2 cities. Installing FGDs with an SO2 removal efficiency of 87.5 per cent across all TPPs would only reduce PM2.5 and PM10 levels by about 5 per cent, indicating a marginal improvement in overall air quality.
However, the implementation of FGDs comes with significant environmental and economic trade-offs. While they can reduce SO2 emissions by approximately 4.88 million tonnes (mt) annually, the associated increase in CO2 emissions is estimated at 14.4 mt per year due to additional energy use, mining, transport and chemical processes. Moreover, each molecule of SO2 captured generates a molecule of CO2. The full environmental cost also includes increased water consumption and secondary pollution. Given SO2’s short atmospheric lifetime (a few days to two weeks) versus CO2’s century-long persistence, this trade-off raises serious climate concerns.
India’s risk of acid rain from SO2 emissions is largely mitigated by the presence of alkaline substances such as ammonia from agriculture and minerals in dust. Thus, acid rain is a limited issue, mainly in the southern peninsular region. Coastal areas naturally benefit from the alkalinity of sea breezes and seawater droplets, which neutralise sulfuric acid.
The study underscores the need for a comprehensive life-cycle assessment (LCA) of FGD implementation in India. It also highlights the lack of credible, holistic studies that assess the long-term environmental, economic and climatic impacts of FGDs. The total capital cost of installing FGDs for India’s 211,860 MW of coal-based power capacity is estimated at Rs 2,542.32 billion, excluding operation and maintenance (O&M) costs. Without a clear understanding of these trade-offs, such investments may not yield meaningful environmental benefits and could inadvertently worsen climate outcomes.
Recommendations and way forward
Considering that the current ambient air SO2 levels across all surveyed cities remain well within the NAAQS, and recognising that the complete elimination of SO2 emissions from TPPs would only marginally improve PM air quality in these areas, the urgency for widespread FGD installation across the country necessitates reassessment. Moreover, existing FGD technologies are known to increase CO2 emissions, elevate water consumption and contribute to secondary pollution due to the mining and transport of limestone or gypsum. In light of these facts, it may be prudent to temporarily halt FGD installations in TPPs where the process has not yet commenced and to revisit this decision after a thorough review.
FGD projects currently under implementation may be allowed to proceed and operate for a trial period of one year. Following this, a comprehensive review should be conducted to evaluate the impact of FGD operations on ambient air quality and regional meteorological changes. This review will inform any necessary adjustments or further action.
A follow-up study is also essential to carry out a comprehensive LCA and life-cycle cost (LCC) analysis of FGD technologies in coal-based TPPs across India. This study should include a comparison of various available FGD technologies, their O&M costs and the effect of FGD implementation on the current cost of electricity generation in the country.
In addition, a detailed study should be undertaken to assess the health benefits of reducing sulfate aerosols, particularly in terms of mitigating respiratory diseases, against the potential negative health impacts arising from heat stress. These effects may result from the loss of regional cooling provided by sulfate aerosols, alongside the increase in CO2 emissions. This analysis should consider broader climate implications, such as intensified global warming and elevated heat stress, which may adversely affect human health, agriculture and the balance of electricity generation and demand.
It is also recommended to initiate a wider public and policy debate among stakeholders after the LCC study is complete. This discussion should critically examine whether investing heavily in FGD for CO2-intensive, non-sustainable energy sources is wise, or whether those funds would be better directed toward rapidly expanding renewable energy options such as solar and wind power.
Furthermore, India should begin a gradual phase-out of older and less efficient coal-based TPPs. This transition must be guided by principles of sustainability and environmental responsibility, without compromising national energy security. The deployment of FGD systems and carbon capture technologies should not serve as justification for prolonging the reliance on outdated, carbon-intensive energy infrastructure. Instead, priority should be given to shifting towards cleaner, renewable energy alternatives that offer long-term benefits for the environment, public health and national energy resilience. Ultimately, it is crucial to formulate balanced strategies that effectively address both air quality improvement and climate change mitigation objectives.
Conclusion
IIT Delhi’s study on ambient SO2 levels near TPPs highlights the complexity of balancing air quality improvements with broader environmental and economic considerations. While FGDs can significantly reduce SO2 emissions, their marginal impact on PM2.5 and PM10 levels, coupled with increased CO2 emissions, water use and costs, raises important questions about their overall efficacy. The findings suggest that a one-size-fits-all approach to FGD implementation may not be suitable for the country’s diverse power sector landscape. Instead, a more strategic path forward involves reassessing ongoing FGD installations, conducting a comprehensive life-cycle assessment of emission control technologies, shifting focus toward retiring ageing coal plants and accelerating investment in renewable energy. This approach would align India’s air quality goals with its climate commitments and support a more sustainable and economically viable energy future.
