Sulphur Dioxide and Coal Power Plants

Why in news?

Background

Sulphur dioxide is a colourless gas with a sharp, irritating smell, and its chemical formula is SO₂.

Burning sulphur-containing coal or oil releases the gas, and metal smelting and some industrial processes also produce it.

Volcanoes are an important natural source, and human exposure is often highest near power plants, industries and busy ports.

Sulphur dioxide is a primary pollutant because sources release it directly, and it can later form damaging secondary pollutants.

How does it create fine-particle pollution?

1. Coal combustion releases sulphur dioxide into the atmosphere.
2. Atmospheric reactions oxidise some gas into sulphur compounds.
3. These compounds react further with water and ammonia.
4. Small sulphate particles then form in the air.
5. Wind can transport these particles over long distances.

Particles measuring 2.5 micrometres or less are called particulate matter 2.5. They are much smaller than a human hair.

Sulphate is an important component of this fine pollution, and emissions outside a city can therefore affect urban air quality.

How does sulphur dioxide affect health?

The gas irritates the nose, throat and lungs, and people with asthma can react after short exposure.

Children, older adults and outdoor workers face greater risk, and exercise increases the amount of polluted air entering the lungs.

Fine sulphate particles can reach deep lung tissue, and long exposure is linked with respiratory and cardiovascular disease.

Atmospheric sulphur compounds also contribute to acid rain, and acid deposition can damage plants, soils, lakes and buildings.

What is flue gas desulphurisation?

Flue gas is the exhaust leaving a boiler or furnace. Desulphurisation removes sulphur dioxide before that exhaust reaches the air.

Many plants spray a limestone or lime mixture through the gas. The material reacts with sulphur dioxide and captures it.

The process can produce gypsum as a usable by-product, and well-operated systems can remove up to about 95 per cent.

Installation alone is not sufficient, and plants must operate the equipment continuously and dispose of residues safely.

How did India’s rules change?

1. India introduced tighter thermal-power emission standards in December 2015.
2. Coal plants initially received a 2017 compliance deadline.
3. Authorities later extended deadlines several times.
4. A 2021 framework divided plants into three location categories.
5. Category A covered plants near the most polluted cities.
6. Category B covered plants near certain sensitive areas.
7. Category C covered the remaining plants.
8. Rules issued in July 2025 relaxed requirements for many plants.

Category A plants must meet the revised requirement by 2027, and Category B plants receive case-by-case assessment.

Category C plants are exempt from mandatory flue gas desulphurisation. The change covered about 78 per cent of India’s coal plants.

Who conducted the new analysis?

The Centre for Research on Energy and Clean Air conducted it. This independent research organisation studies air pollution and energy systems.

The team examined twelve coal plants within 300 kilometres of Delhi, and these plants contained thirty-seven generating units.

Public emissions data were available for only twenty-five units, and researchers estimated annual emissions using available operating and fuel information.

What were the main findings?

• The twenty-five assessed units emitted an estimated 154 kilotonnes during 2025.
• Category C units produced about 125 kilotonnes.
• This represented nearly 81 per cent of estimated emissions.
• Category A units produced the remaining 29 kilotonnes.
• Units without desulphurisation produced about 90 per cent.
• Twenty units exceeded applicable emission standards.
• Five units complied with those standards.
• Twelve units lacked sufficient public data for assessment.

A kilotonne equals one thousand tonnes, and the figures represent estimated yearly emissions, not pollution concentrations at one Delhi monitor.

The analysis estimated 1,775 and 2,154 tonnes for two controlled units. Comparable uncontrolled units produced much larger estimates.

The Rajpura units had estimates of 20,851 and 22,690 tonnes. They were the study’s largest sources.

Why does distance-based exemption face criticism?

The 2025 framework relies strongly on distance from selected cities or sensitive locations. Air pollution does not stop at those boundaries.

Tall chimneys release pollution high into moving air, and winds can carry gases and particles across several states.

The study therefore treats Delhi and surrounding states as one airshed. An airshed is a region sharing atmospheric pollution movement.

Delhi’s wider region is called the National Capital Region, and sources outside its formal boundary can still affect its air.

What are the study’s limitations?

The analysis used publicly available data and estimates, but it did not continuously measure every chimney during 2025.

Data covered only twenty-five of thirty-seven units, and missing information reduces certainty about the complete regional total.

Weather determines how much pollution reaches Delhi at any time, and annual emissions do not show daily exposure by themselves.

The report also discusses a possible El Niño weather pattern, and that future scenario is a projection, not a certainty.

What policy steps follow from the findings?

• Large emitters should install and operate effective controls.
• Real-time monitoring data should become publicly accessible.
• Regulators should audit equipment operation, not only installation.
• Airshed modelling should guide location-based requirements.
• Older inefficient units need clear retirement plans.
• Renewable power can reduce dependence on coal combustion.

Conclusion

The analysis shows that distant exemptions can carry regional costs. Transparent monitoring and working controls remain essential for cleaner air.