Stack height (chimney height)

Stack height is the regulated minimum height of a chimney, set so that the emitted plume disperses high enough to keep ground-level pollutant concentrations within the National Ambient Air Quality Standards (NAAQS). The CPCB prescribes it either by a formula tied to the pollutant emission rate or, for many categories, by a fixed minimum linked to the process and capacity. For example, the well-known SO₂ rule sets stack height H = 14 × Q^0.3, where Q is the SO₂ emission rate in kg/hr; for DG sets the minimum is often "height of building + 0.2 × √(kVA capacity)".

The logic is dispersion physics: a taller release point gives the plume more vertical distance to dilute in the atmosphere before it descends to where people breathe. Stack height is therefore a tool for protecting ambient air quality, distinct from emission standards which control the concentration leaving the stack. A plant must satisfy both — the right concentration AND the right release height.

For combustion and thermal recycling sources, the SPCB quotes the required stack height directly in the Consent to Operate. Common figures: DG sets follow the building-plus-square-root formula; small boilers and furnaces have category minimums (often 30 m or a height tied to steam generation); incineration and pyrolysis stacks are frequently fixed at a minimum of 30 m or more regardless of formula, to manage dioxin and acid-gas dispersion.

The practical mistake is building a short stack to save on structural cost and then being unable to get consent, or being forced to extend it later. Calculate the required height during plant design from your worst-case emission rate or the applicable category minimum, build to it (with the sampling platform at the correct level), and document the calculation in the consent application. Retrofitting stack height after commissioning is expensive and disruptive.