Eighty-One Per Cent of Delhi's Coal Sulphur Comes From Plants Legally Excused
A 2025 notification exempted about 78% of India's coal units from installing desulphurisation equipment — new analysis maps what that means for the air over the capital
What happened
Delhi's air is usually discussed in terms of what happens inside Delhi — vehicles, construction dust, stubble burning. This analysis moves the frame outward and upward: a gas emitted three hundred kilometres away becomes particulate matter over the city days later. Learn the chemistry, because it explains why an exemption granted to power plants in other states is an air quality decision for the capital, and why the airshed rather than the municipality is the correct regulatory unit.
Who Still Has to Scrub Their Smoke
FGD Categories — July 2025 Notification
| Category | Requirement | Share of units |
|---|---|---|
| A | Must install FGD by 2027 | — |
| B | Case-by-case assessment | — |
| C | Exempted entirely | ~78% |
comes from exempted units
Source: Centre for Research on Energy and Clean Air; MoEFCC notification, July 2025
Sulphur dioxide (SO₂) is a colourless, heavy, pungent gas, soluble in water, forming sulphurous acid on dissolution.
●Its principal anthropogenic source is combustion of sulphur-bearing coal and oil, along with metal smelting; volcanic eruptions are the main natural source.
●Its importance for public health lies chiefly in secondary particulate formation: SO₂ oxidises in the atmosphere to sulphate aerosols that contribute to PM2.5, associated with respiratory and cardiovascular disease, stroke and premature death.
●Because the conversion takes hours to days, particles form far downwind of the emitting stack.
●Flue gas desulphurisation (FGD) is the control technology, typically wet limestone scrubbing, which removes SO₂ from exhaust gases.
●India first notified emission standards for thermal power plants covering SO₂, oxides of nitrogen, particulate matter, mercury and water use in 2015; compliance deadlines were repeatedly extended.
●A notification of July 2025 replaced universal application with three categories — Category A (compliance required by 2027), Category B (case-by-case assessment) and Category C, comprising roughly 78 per cent of India's coal-fired units, which are exempted.
●Analysis by the Centre for Research on Energy and Clean Air finds that nearly 81 per cent of estimated SO₂ emissions from coal plants within 300 km of Delhi-NCR now come from exempted units.
SO₂ matters less as a gas than as a precursor — the pollutant that harms health is the particulate it becomes, hundreds of kilometres from the chimney that emitted it.
◎ In Simple Words
When coal is burned in power stations it releases a gas called sulphur dioxide. Special equipment can strip most of that gas out of the smoke before it leaves the chimney. In 2015 India said all coal power stations must install this equipment. In July 2025 the government changed its mind and excused most of them — about three-quarters of all units. New research shows that these excused plants are responsible for roughly four-fifths of the sulphur pollution from coal power around Delhi. The gas matters because in the air it slowly turns into tiny particles, and those particles are what damage people's lungs and hearts.
Factual Pointers
Practice · 2 questions
With reference to sulphur dioxide (SO₂) as an air pollutant, consider the following statements:
1. It is a precursor to the formation of secondary particulate matter in the atmosphere.
2. Flue gas desulphurisation is the principal technology used to control its emission from coal-fired power plants.
3. Its health impact is confined to the immediate vicinity of the emitting source.
Which of the statements given above are correct?
Under the revised framework notified in July 2025 for thermal power plants in India, plants classified as 'Category C':
Mains Practice Questions
"The airshed, not the administrative boundary, is the only coherent unit for air quality regulation." Examine this proposition with reference to secondary particulate formation from coal-fired power. (250 words, GS3)
An environmental obligation dissolved through repeated extension damages more than the standard it concerned. Critically examine with reference to thermal power plant emission norms. (250 words, GS3)
Explain the difference between primary and secondary particulate matter and its implications for pollution control strategy. (150 words, GS3)
Frequently Asked
· People also askWhat did the CREA analysis find?
That nearly 81 per cent of estimated sulphur dioxide emissions from coal-fired power plants within a 300-kilometre radius of Delhi-NCR come from units exempted from installing flue gas desulphurisation equipment under the Union government's July 2025 notification.
GS3 · EnvironmentThe 300-kilometre radius matters because SO₂ converts to particulate matter over hours to days, so the pollution forms far downwind of the emitting stack rather than near it.
SOURCE Centre for Research on Energy and Clean Air; Down To Earth
What did the July 2025 notification change?
It replaced the universal requirement to install flue gas desulphurisation with a three-category framework: Category A plants must comply by 2027, Category B plants are assessed case by case, and Category C — roughly 78 per cent of India's coal-fired units — is exempted altogether.
Prelims · GS3This substantially reversed emission standards first notified in 2015, whose compliance deadlines had already been extended repeatedly.
SOURCE Ministry of Environment, Forest and Climate Change
Why is sulphur dioxide harmful if it is a gas, not a particle?
Because it becomes a particle. SO₂ oxidises in the atmosphere to sulphate aerosols that contribute directly to PM2.5 — the pollutant associated with respiratory disease, cardiovascular illness, stroke and premature death. It is a precursor rather than the final harmful agent.
GS3 · EnvironmentThe conversion takes hours to days, which is why the particulate forms hundreds of kilometres downwind and why near-source ambient SO₂ readings understate the health impact.
SOURCE Central Pollution Control Board
What is flue gas desulphurisation?
FGD is the control technology that removes sulphur dioxide from power plant exhaust gases, most commonly through wet limestone scrubbing. It is capital-intensive, requires plant shutdown for retrofitting, and raises the cost of generation — the practical reasons behind repeated compliance extensions.
Prelims · GS3The technology is well established: China mandated widespread FGD retrofitting in the 2000s and achieved substantial SO₂ reductions within about a decade, showing the constraint is cost-bearing rather than feasibility.
SOURCE Central Electricity Authority
Why can't Delhi fix its air quality on its own?
Because a substantial share of its PM2.5 forms from precursor gases emitted hundreds of kilometres away. Measures taken inside the city cannot address particulate created in the atmosphere from SO₂ released in other states, which is why the airshed rather than the municipality is the coherent regulatory unit.
GS3 · GovernanceThis is the reasoning behind the Commission for Air Quality Management in the National Capital Region and Adjoining Areas, created to act across state boundaries on an airshed-level problem.
SOURCE Commission for Air Quality Management
Is there a climate trade-off in removing sulphur?
Yes, a real one. Sulphate aerosols scatter incoming solar radiation and exert a cooling effect, so removing SO₂ produces a small warming signal alongside the health benefit.
GS3 · EnvironmentThe resolution is that the health case is immediate, local and large while the cooling effect is diffuse and transient — so the trade-off argues for simultaneous decarbonisation rather than for retaining sulphur emissions as a climate strategy.
SOURCE Atmospheric science literature