Biogas plants ferment agricultural waste, manure and sewage sludge. The biogas is 55-70 percent CH4, 30-45 percent CO2 and 100-3000 ppm H2S. NFPA 820 classifies wastewater and biogas plants and labels the digester zone Class I Div 1.
Biogas Chemistry
- CH4: 55-70 percent, LEL 5 percent
- CO2: 30-45 percent
- H2S: 100-3000 ppm (very toxic)
- Humidity: 100 percent RH
- Calorific: 20-25 MJ/m3
NFPA 820 Classification
- Inside digester: Class I Div 1 Zone 0
- Digester outside 3 m: Class I Div 2 Zone 2
- Gas holder: Div 1 Zone 1
- Holding/buffer room: Div 2 Zone 2
- Flare stack: 3 m hot zone
NFPA 820 Ventilation
- Digester room: 12 ACH
- Pump room: 6 ACH and Zone 2
- CHP container: 30 ACH
- CH4 detection: 20 percent LEL alarm
- H2S detection: 10 ppm alarm, 20 ppm evacuate
CHP and Flare
- CHP engine: 500 kW-5 MW typical
- Gas treatment: Fe2O3 H2S scrubbing
- Flare: Emergency 1200 °C
- Scrubber: Biogas upgrade to RNG
- Grid injection: 97+ percent CH4 biomethane
Turkish Biogas Examples
- Large-scale plant: 18 MW biogas
- Municipal plants: Municipal waste
- Sugar industry: sugar-beet pulp fermentation
- Poultry sector: Chicken manure
- EPDK: 400+ licensed biogas plants
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MEP Calc sizes biogas digester-room 12 ACH, CH4 5 percent LEL alarm, H2S detection, Zone 1 electrical and NFPA 820 zoning.
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Sources & Further Reading
NFPA 820, NFPA 30, EPA 40 CFR 503, ATEX 137, EPDK regulation. NFPA 820 Wastewater Treatment.