A study conducted by the Centre for Atmospheric Sciences at the Indian Institute of Technology (IIT) Delhi surveyed ambient atmospheric Sulfur Dioxide (SO2) concentrations in various categories of cities across India, based on their proximity to coal-based Thermal Power Plants (TPPs). The report, dated May 31, 2024, was prepared for the Central Electricity Authority (CEA) under the direction of the Ministry of Power (MoP) to assess the impact of TPPs on air quality following the implementation of emission control regulations. This report was published on May 2, 2025.
The study, spanning from February 2023 to February 2024, categorized cities into three groups: those without coal-based TPPs (Category-1), those with TPPs without Flue Gas Desulfurization (FGD) units (Category-2), and those with TPPs equipped with FGD units (Category-3). The survey involved three phases of measurements in selected cities within each category.
Key Findings of the Survey
The study found that all surveyed locations, across all categories, registered ambient SO2 concentrations within the National Ambient Air Quality Standards (NAAQS) limit of 80 μg/m³ (based on 24-hour average measurements).
Daily mean SO2 concentrations varied across city categories:
Category-1 (No TPPs): 2.48 ± 0.49 μg/m³ to 9.26 ± 3.28 μg/m³ (e.g., New Delhi, Hyderabad, Lucknow).
Category-2 (TPPs without FGD): 3.12 ± 0.81 μg/m³ to 6.48 ± 3.08 μg/m³ (e.g., Kolkata, Nagpur, Kota, Shahdol).
Category-3 (TPPs with FGD): 4.24 ± 0.31 μg/m³ to 12.51 ± 2.82 μg/m³ (e.g., Ghaziabad, Khargone, Gautam Buddha Nagar).
The study assessed the contribution of SO2 emissions from coal-based TPPs to ambient SO2 levels:
Category-1 cities showed a relatively low contribution from the energy sector (11.44% to 16.53%).
Category-2 cities exhibited more significant contributions (27.99% to 38.57%).
Category-3 cities showed intermediate contributions (17.37% to 22.54%) with seasonal variations.
The contribution of sulfate aerosols (formed from SO2) from TPPs to ambient PM2.5 and PM10 concentrations was found to be relatively minor across all cities and seasons, ranging from 0.57% to 5.21%. The highest contributions were observed in Category-2 cities.
The study estimates that installing FGD systems with 87.5% SO2 removal efficiency across all Indian coal-based TPPs would only reduce PM2.5 concentrations by up to 4.56% and PM10 concentrations by up to 3.21%. The researchers suggest that the impact on overall PM air quality would be marginal (around 5%).
The report discusses the likelihood of acid rain in India, noting that alkaline substances like ammonia from agriculture and carbonate minerals in dust generally neutralize sulfuric acid, mitigating widespread acid rain. However, the southern peninsular region could be more susceptible due to lower prevalence of these alkaline substances. Coastal areas benefit from sea breeze and the alkalinity of seawater.
The study highlights the lack of comprehensive life-cycle assessments (LCA) for FGD implementation in the Indian context. It points out that while FGDs reduce SO2, they can increase CO2 emissions through various processes and also lead to increased water consumption and secondary pollution. The report estimates that while FGD installation could reduce annual SO2 emissions by 4.88 million tonnes, it could immediately increase CO2 emissions by 14.4 million tonnes annually. Considering broader LCA, the increase in equivalent CO2 emissions and water consumption could be even more significant.
Based on the findings of low ambient SO2 levels within NAAQS and the potentially marginal improvement in PM air quality with significant increases in CO2 emissions and water usage, the IIT Delhi report recommends a reassessment of the urgency of FGD installation, particularly for TPPs that have not yet begun the process.
The report makes the following recommendations:
- Considering halting FGD installation in TPPs where work has not commenced and revisiting the decision after a comprehensive LCA study.
- Conducting a thorough follow-up study evaluating the life-cycle costs and benefits of FGD implementation across all Indian coal-based TPPs, including impacts on electricity prices.
- Initiating a broader stakeholder debate to weigh the investment in FGDs against the expansion of renewable energy sources.
- Gradually phasing out older and less efficient coal-based TPPs and transitioning to a sustainable energy future.
The IIT Delhi study provides insights into the current ambient SO2 levels near thermal power plants in India and raises questions about the overall environmental and economic implications of widespread FGD adoption. The findings suggest a need for a balanced approach that considers both air quality improvement and climate change goals, while also ensuring energy security.
