Particulate Control and Stack Height for Industrial Boilers
Solid-fuel boilers make dust, and the collector between boiler and chimney decides whether the stack meets its limit. The chimney itself then disperses what remains, and its minimum height is set by formula.
You'll learn
- Describe how each type of dust collector works
- Choose a collector for a fuel and a limit
- Apply the CPCB stack height formulas
- Recognize common collector failures
Before this: Stack Emission Norms and Consent
Where the dust comes from and where it goes
Coal and biomass contain ash. Some falls through the grate or drains from the bed as bottom ash; the rest leaves the furnace with the flue gas as fly ash. Between the boiler and the chimney, a collector removes as much of it as it can, and the ID fan draws the cleaned gas up the stack.
- Cyclone: gas spins, and dust is thrown to the wall and falls out. Simple, no moving parts, good on coarse dust.
- Multicyclone: many small cyclones in parallel. Smaller diameter spins harder and catches finer dust.
- Bag filter: gas passes through fabric bags; dust builds up on them and is pulsed off with compressed air. Catches fine dust very well.
- Electrostatic precipitator (ESP): dust is charged and drawn to collecting plates. Very low pressure drop at large gas volumes.
- Wet scrubber: dust is caught in water sprays. Also absorbs some acid gases, but makes a wet effluent to treat.
Choosing a collector
| Collector | Typical role | Watch out for |
|---|---|---|
| Cyclone / multicyclone | Pre-cleaner, or alone on small boilers with a lenient limit | Fine dust passes straight through |
| Bag filter | Meeting tight limits on biomass and coal | Sparks and hot gas burn bags; acid dew point blinds them |
| ESP | Larger boilers, large gas volumes | Performance depends on dust resistivity and gas conditions |
| Wet scrubber | Where gas is also sticky or acidic | Water treatment, corrosion, a wet plume |
With today’s limits of 150 and 100 mg/Nm³ for most fuels above 2 TPH, a cyclone alone rarely meets the standard; a multicyclone ahead of a bag filter or ESP is the usual answer, with the multicyclone taking the coarse load and sparks off the fine collector.
Stack height
CPCB guidelines set a minimum stack height from the emission rate, so that what leaves disperses before it reaches the ground. Two formulas apply; the stack must meet the greater of them and the minimum for the boiler’s size.
| Symbol | Meaning | Unit |
|---|---|---|
| Stack height above ground | m | |
| SO₂ emission rate | kg/h |
| Symbol | Meaning | Unit |
|---|---|---|
| Stack height above ground | m | |
| Particulate emission rate | t/h |
The minimum is 11 m for boilers below 2 TPH and 30 m for larger ones.
- 5,000 kg/h of steam at 10.54 kg/cm²(g), feedwater 80 °C; coal at 4,000 kcal/kg GCV, 82% efficiency; 0.5% sulfur, illustrative.
- Coal burnt: 5,000 × (2,781.8 − 335.8) ÷ (0.82 × 4,000 × 4.1868) = 891 kg/h.
- SO₂: every kilogram of sulfur becomes two of SO₂, so 891 × 0.005 × 2 = 8.9 kg/h.
- Height from SO₂:
- Minimum for a boiler of 2 TPH and above: 30 m.
Key takeaways
- Collectors: cyclone and multicyclone for coarse dust; bag filter and ESP for fine; scrubber for sticky or acidic gas.
- Above 2 TPH, meeting 150 or 100 mg/Nm³ usually takes a multicyclone plus a bag filter or ESP.
- (SO₂, kg/h) and (PM, t/h); take the larger, with a minimum of 11 m or 30 m.
- Protect bag filters from sparks and from running below the acid dew point.