Biogas and Landfill CHP Gensets: Fuel Gas Treatment, Methane Number and ISO 8528

Engineering guide for biogas, landfill and sewage gas cogeneration gensets covering fuel gas conditioning, Methane Number (MN) knock limits, thermal recovery, a

1. Gas Quality Challenges in Biogenic Energy Conversion

Reciprocating gas engine generator sets operating on anaerobic digester biogas, sewage gas or landfill gas face severe fuel quality variations compared to pipeline natural gas. High moisture (100% relative humidity at digester outlet), hydrogen sulfide (H2S up to 5000 ppm), and volatile organic silicon compounds (siloxanes) cause rapid lubricating oil degradation, abrasive silica (SiO2) cylinder deposits, and turbocharger corrosion.

2. Methane Number, Heating Value & Engine Derating

Gas CharacteristicNatural Gas (Pipeline)Agricultural BiogasLandfill / Sewage Gas
Methane Content (CH4)90 - 98 vol%50 - 65 vol%38 - 52 vol%
Lower Heating Value (LHV)36 - 38 MJ/Nm318 - 23 MJ/Nm314 - 18 MJ/Nm3
Methane Number (MN)70 - 85> 100 (CO2 knock inhibitor)> 110
Pre-Treatment RequiredBasic filtrationDehumidification + Biological H2SDeep chilling (+4°C) + Carbon bed
Duty Rating (ISO 8528-1)Baseload / PeakingContinuous Operating Power (COP)Continuous Operating Power (COP)

The electrical power output from gas flow is calculated by:

Pe = ηe · V̇gas · LHVgas [kW]

Where:

  • ηe is the electrical efficiency of the lean-burn gas engine (40% - 44% for modern turbocharged units).
  • V̇gas is the normalized volumetric gas feed rate in Nm3/h.
  • LHVgas = (%CH4) / (100) · 35.8 MJ/Nm3.

3. Combined Heat and Power (CHP) Thermal Integration

In biogas cogeneration installations, overall energy efficiency exceeds 88% by recovering two primary heat streams:

1. High-Temperature Circuit: Exhaust gas heat exchanger produces hot water (90/70°C) or low-pressure steam (180°C) recovering ≈ 40% of fuel thermal energy.

2. Low-Temperature Circuit: Engine jacket water, lubricating oil plate cooler and first-stage mixture intercooler deliver warm water (60/45°C) for digester tank heating.

4. Specification Requirements for Gas Engine Procurement

  • Mandate gas dehumidification chiller (+4°C dew point) and activated carbon filter upstream of engine throttle.
  • Require automated air-fuel ratio (A/F) control with fast oxygen sensor (λ = 1.6 - 1.8 lean-burn combustion) to maintain NOx < 250 mg/Nm3 (TA-Luft 2021).
  • Verify alternator insulation class H with Class F temperature rise (105 K) for 24/7 continuous baseload operation.
  • Specify anti-corrosive stainless-steel piping and nickel-plated gas mixer for corrosive landfill gas duty.

*Reference: ISO 8528-1:2018; ISO 3046-1; DVGW Code of Practice G 262; TA-Luft 2021.*