A cement plant fires clinker kiln at 1,450 C on coal, petcoke or RDF. Coal mill, clinker cooler and by-pass dust collectors hold combustible dust. NFPA 664 (originally for wood products but used as reference) with NFPA 652 DHA and NFPA 68 explosion panels are mandatory.
Risk Points
- Coal mill: 70-90 C internal - self-ignition risk
- Coal bin: 20 t stock - CO monitoring
- Clinker cooler dust filter: 450 C dust buildup
- By-pass dust silo: alkali salts + dust-laden air
- RDF bunker: mixed plastics/textile alternative fuel
Coal Mill Dust Properties
- Kst (coal): 50-150 bar m/s (St1)
- Pmax: 6-8 bar
- MEC: 60 g/m3
- MIE: 50-100 mJ
- Petcoke higher: Kst 200 bar m/s, MEC 30 g/m3
NFPA 68 Venting
Coal mill 50 m3:
- Av = Kst × V^0.75 / (Pred × 1000)
- Av = 150 × 50^0.75 / (0.5 × 1000) = 5.6 m2
- Panel must vent outdoors - no personnel
- Vent duct behind panel increases Av (factor 1.5-2)
CO Monitoring and Inerting
- Coal bin CO: 50 ppm alarm, 200 ppm N2 inert
- N2 inerting: maintain O2 <8 percent
- CO2 inerting: alternative, lower cost
- Temperature: bunker thermocouples
- Bearing monitoring: coal mill bearing temp >70 C alarm
Coal mill venting with MEP Calc
MEP Calc sizes NFPA 68 venting Av, N2 inerting flow and CO monitoring setpoints for coal mills.
Download MEP Calc on the App Store
Sources & Further Reading
NFPA 652, NFPA 68, NFPA 69, NFPA 664, CEMBUREAU Technical Report, ACerS. NFPA 664 Wood Processing standard.