Why concerns about SO₂ emissions, a major air pollutant, keep recurring

Indian Express

3 days ago

— Arunangshu Das
Recently, the government decided to scale back its 2015 mandate requiring coal-based thermal plants to install clean-air equipment – flue gas desulphurisation (FGD) devices – that remove sulphur dioxide (SO₂) emissions, a key contributor to air pollution. This move is primarily driven by the high cost of retrofitting older plants and the significant capital investment needed for new installations.
The government has reaffirmed its commitment to supplying low-cost electricity as a cornerstone of its economic and social development agenda — one that includes reducing regional disparities in electricity access and meeting the country's rising cooling demands. However, the rollback has raised concerns about the environmental implications of emissions, particularly with respect to sulphur dioxide (SO2), which is controlled by FGDs.
First, let's understand the types of coal used in power plants and how they contribute to pollution.
A significant portion of the world's electricity continues to be generated from coal. In India, coal accounts for more than 70 per cent of electricity generation. Although India has one of the lowest electricity tariffs in the world, its per capita electricity consumption remains low.
The theoretical maximum efficiency of a coal-fired power plant is 64 per cent, but even the most advanced plants globally achieve up to 45 per cent. In comparison, plants in India average about 35 per cent efficiency. However, efficiency has improved in recent years with the adoption of supercritical and ultra-supercritical technologies in 2010 and 2019, respectively. Since 2015, India has also introduced emission standards for existing coal plants, aligning with regulatory approaches in the EU and the US.
The dominant type of coal produced in India is 'sub bituminous', primarily of Gondwana origin, which has low sulphur and moisture content, which is beneficial in reducing emissions. However, 'sub bituminous' coal also has low carbon content and lower energy density. As a result, power plants burn more of this low carbon coal to produce the same amount of energy as higher-grade coals like anthracite.
In other words, due to low calorific value and high quartz content, coal-fired power plants in India consume more coal per kilowatt-hour of electricity generated, produce more carbon dioxide (CO2) per unit of electricity, and generate large volumes of ash.
Sulphur, a non-metal in Group 16 of the periodic table, forms acidic oxides, most notably sulphur dioxide (SO₂), which is a major air pollutant emitted mainly from coal-fired power plants. SO₂ contributes to acid smog, acid rain, and secondary aerosols.
In the US, SO₂ is listed as a criteria pollutant under the Clean Air Act and is regulated by the Environmental Protection Agency (EPA). In India, the Air (Prevention and Control of Pollution) Act, 1981, sets the annual average SO₂ limit of 50 µg/m³ for residential/industrial areas, and 20 µg/m³ for ecologically sensitive zones. The 24-hour average limit is 80 µg/m³ for both.
Besides coal, other significant sources of SO₂ include petroleum refining, copper smelting, and cement production. Coal contains 0.5–6 per cent sulphur, present as organic sulphur (bound to carbon) and inorganic sulphur (mainly iron pyrites, FeS₂). Notably, inorganic sulphur can be partially removed through washing and pulverising.
When oxidised, sulphur forms SO₂ and SO₃, which dissolve in water to produce sulphurous and sulphuric acids – the latter being one of the strongest known acids. SO₂ is also highly water-soluble and forms sulphate aerosols (0.2–0.9 µm in size), which reduce visibility and can penetrate deep into the lungs.
The conversion of SO₂ into sulphate particles takes days, with deposition mainly occurring through acid rain. SO₂ can adsorb onto airborne particles and travel hundreds of kilometres before settling. Studies in the US have linked coal power plants to severe acid rain events, such as those recorded near Mississippi in 2004.
The health impacts are worsened when SO₂ and sulphate particles act together, increasing mortality during air pollution episodes. Environmentally, SO₂ damages vegetation; acid rain leaches nutrients from soil, mobilises toxic aluminium that inhibits plant uptake and disrupts freshwater ecosystems.
One of the most catastrophic effects of acid deposition due to SO2 is perhaps the collapse of pH buffer capacity in freshwater bodies. This can cause massive death of fish populations because of gelation and deposition of aluminium salts in their gills, causing respiratory blockage.
Coal-fired power plants are operated using Rankine cycle, in which a working fluid (usually water) is alternately heated and condensed. Due to high levels of pollution from these plants, expensive pollution control measures have been in place since 1960. These systems can account for up to 40 per cent of the cost of building a new power plant and consume about 5 per cent of generated power, thereby reducing overall efficiency.
SO₂ emissions can be reduced through two broad approaches: pre-combustion control and post-combustion control.
Pre-combustion control
Pre-combustion techniques include fuel switching, fluidized bed combustion (FBC), and integrated gasification combined cycle (IGCC).
— Fuel switching: It involves using or blending low-sulphur coal, which can cut SO₂ emissions by 30–90 percent, but only temporarily.
— Coal washing: Using physical, chemical, or biological methods, it removes iron pyrites (FeS₂) due to its higher density. This can lower sulphur content by approximately 10 per cent, while reducing ash levels and improving fuel quality and boiler efficiency.
— Fluidized bed combustion: It uses crushed coal mixed with limestone in a fluidized bed; the lime reacts with SO₂ to form calcium sulfate. FBC can remove more than 90 per cent of sulphur, operates at lower temperatures (~800°C), thereby lowering NOₓ formation, and is less sensitive to coal quality.
— Integrated gasification combined cycle: It turns coal-water slurry to clean syngas, removing particulates, mercury, and sulphur. IGCC plants reach up to 45 per cent efficiency compared to approximately 40 per cent for conventional pulverized coal plants, and allow for CO₂ capture via deep injection.
Post-combustion control
Post-combustion control is mainly achieved through flue gas desulphurisation (FGD).
— In dry FGD systems, limestone (CaCO₃) slurry is injected into flue gas to form calcium sulphite/sulphate. Lime-based slurries work better but are costlier.
— In wet scrubbing, flue gas is bubbled through limestone slurry, producing gypsum as a by-product, which is used as a construction material. Scrubbers also consume large amounts of water and generate significant sludge as landfills with the consistency of toothpaste.
— Regenerative SO2 capture processes, like Wellman-Lord, generate economically important byproducts like sulphuric acid, and even elemental sulphur for industrial application.
— Biotechnological application of autotrophic sulphur bacteria in thermophilic conditions to produce economically attractive elemental sulphur is also another environmentally benign alternative for SO2 remediation.
Without any control measures in place, burning Indian coal can equate to SO₂ emission of 800 to 1600 mg/m3, which is higher than the 2015 standards set for SO₂ gas. In 2021, a report titled 'A pathway to reducing emissions from coal power in India', by the International Energy Agency for the Coal Industry Advisory Boards, recommended the implementation of some kind of SO₂ control measures. This is because SOx and NOx can bind to particulate matter and travel hundreds of kilometres from their source, posing serious health and environmental risks even in distant areas.
The installation cost of FGD is around 50 lakh/MW, though this can be partially offset by selling the gypsum generated during the desulphurisation process. More recent studies from India's premier institutions, however, have suggested that the overall impact of FGD technology is limited, as ambient SO₂ levels remain low even in the absence of FGD at most power plants, largely due to India's favourable geographic location and meteorological conditions.
Of the 206 GW capacity required to comply with the SO₂ norms, the centrally-owned plants, notably those of NTPC, already had a higher rate of implementation as about 68 per cent of capacity (35.4 GW). As of August 2024, out of 537 coal-based thermal power plants, 39 had completed installations, 238 had awarded contracts or had been under implementations, 139 units were in the process of tendering and 121 were at the initial stages of pre-tendering.
Globally, policy rollbacks on environmental regulations are becoming increasingly common. In 2017, the then Trump administration issued an executive order to reevaluate Clean Power Plant rules and eliminate other federal initiatives addressing climate change. However, these actions were subsequently settled by the Biden administration to reduce pollution from fossil fuel-fired power plants.
Nonetheless, with the return of the Trump administration for the second term, there is renewed momentum to reconsider these regulations made by the previous government, specifically relaxing the carbon emission rules and Mercury and Air Toxics Standards (MATS). According to some estimates, these rollbacks could result in up to seven times more CO₂ emissions than current levels.
Such rollbacks risk setting new precedence across the globe in an era of climate crises. It is hoped that science will judiciously tackle all concerns, which include not only human wellbeing but also planetary health in taking such crucial decisions.
Why has the government exempted 78 per cent of coal-based thermal plants from installing anti-pollution devices? Evaluate.
What is the dominant type of coal produced in India, and how does it produce more carbon dioxide (CO2) per unit of electricity, and generate large volumes of ash?
How does the formation of sulphate aerosols from SO₂ affect both air quality and human health?
How do pre-combustion and post-combustion SO₂ control strategies compare in terms of cost, efficiency, and environmental impact?
How effective has India's regulatory approach to SO₂ emissions been when compared to countries like the US or EU, particularly in enforcement and technological adaptation?
(Dr. Arunangshu Das is the Principal Project Scientist at the Centre for Atmospheric Sciences, Indian Institute of Technology, Delhi.)
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