Sulphur Dioxide (SO2)

Sulphur dioxide (SO₂) is a colourless, pungent, acidic gas formed when sulphur-bearing fuels and materials are burned or when sulphide ores are smelted. It is one of the twelve criteria pollutants in India's NAAQS, with a 24-hour ambient limit of 80 µg/m³ and an annual limit of 50 µg/m³ (20 µg/m³ in ecologically sensitive areas). At the stack, SO₂ from combustion sources is controlled by source-specific emission standards and, for large sources, by mandatory flue-gas desulphurisation.

SO₂ harms in three ways. It is a respiratory irritant that aggravates asthma and bronchitis even at low concentrations. In the atmosphere it oxidises to sulphuric acid, the main driver of acid rain that acidifies soil and water and corrodes structures. And it converts to sulphate aerosols that form secondary PM2.5, linking SO₂ control directly to particulate pollution control.

In recycling and allied operations, SO₂ arises wherever sulphur-containing feedstock or fuel is burned: tyre pyrolysis and tyre/rubber processing (rubber is vulcanised with sulphur, so the char and gas streams carry it), DG sets and boilers run on high-sulphur diesel or furnace oil, and any co-processing of sulphur-bearing waste. Tyre-derived char and pyro-oil are notably sulphur-rich, making SO₂ a defining emission concern for the tyre pyrolysis sector.

Control follows the sulphur in the fuel. Use low-sulphur fuels (BS-VI diesel at 10 ppm sulphur instead of furnace oil), install wet or dry scrubbing (lime/limestone or caustic) on stacks where the load is high, and for tyre pyrolysis, desulphurise the pyro-oil and manage the char so downstream burning does not release SO₂. Stack SO₂ is measured during compliance testing and, above a capacity threshold, tracked live by OCEMS reporting to the SPCB.