Stack (chimney)

A stack is the vertical chimney or duct through which a facility's flue gas — from a boiler, dryer, genset, pyrolysis reactor or furnace — is released into the atmosphere after passing through pollution-control equipment. Its purpose is twofold: to vent the gas and, critically, to release it high enough that the plume disperses and dilutes before it reaches the ground or nearby receptors. The stack is the regulated point source at which emission concentration (mg/Nm³) is measured during compliance testing.

Every stack is fitted with a sampling port and platform at the height prescribed by CPCB guidelines (typically about 8 stack-diameters downstream and 2 diameters upstream of any disturbance) so that representative manual stack monitoring can be carried out. Larger or higher-polluting sources must also carry an Online Continuous Emission Monitoring System (OCEMS) mounted on the stack, transmitting live SO₂, NOₓ, PM and other readings directly to the SPCB and CPCB servers.

The stack works in series with the control train ahead of it — cyclone, baghouse, scrubber or ESP. The stack does not clean the gas; it disperses what the control equipment did not capture. Under-sizing control equipment and relying on a tall stack to "dilute" pollution is non-compliant: standards are concentration-based at the stack exit, so dilution by a tall stack does not lower the measured mg/Nm³.

For a recycler, the practical points are: build the stack to the height the SPCB prescribes for your capacity and pollutant load (see Stack Height), provide a safe, permanent monitoring platform and port, and keep the stack downstream of properly sized control equipment. A missing or inaccessible sampling port is a common reason boards reject consent renewal, because the unit cannot demonstrate compliance.