1. Role in the Green Gas Value Chain
Anaerobic digestion and landfill gas recovery convert organic waste streams into renewable methane. AD is a controlled biological process using digesters, while LFG is generated naturally in landfills. Both routes produce methane‑rich gas requiring purification and upgrading to meet grid‑quality biomethane standards (CH₄ ≥96%, H₂S ≤6 mg/m³, dew point ≤–10°C).
Continuous analytics across the process chain support stable biological conversion, corrosion prevention, explosion safety, upgrading efficiency, and compliance with product gas specifications.
2. Process Overview
2.1 Anaerobic Digestion (AD)
Anaerobic digestion uses agricultural residues, manure, food waste, and municipal sludge as feedstock. After homogenization and pH adjustment, feedstock enters a digester where hydrolytic, acidogenic, acetogenic, and methanogenic microorganisms convert organic matter into raw biogas.
Raw biogas typically contains CH₄ (50–70%), CO₂ (30–50%), H₂S (100–10,000 ppm), NH₃ (<1500 ppm), H₂O, and trace H₂. Digestion operates under mesophilic (35–42°C) or thermophilic (50–60°C) conditions with pH maintained at 6.8–7.5.
Biogas is then desulfurized, decarbonized (PSA, membrane, or amine), dehydrated, and compressed to produce biomethane.
2.2 Landfill Gas Recovery (LFG)
Landfill gas forms through anaerobic degradation of organic matter in landfills. LFG typically contains CH₄ (35–55%), CO₂ (30–50%), O₂ (<3%), N₂, H₂S (50–500 ppm), siloxanes, and VOCs.
Gas is extracted under negative pressure through vertical or horizontal wells, followed by moisture removal, coarse filtration, desulfurization, siloxane removal, and upgrading via PSA or membrane separation.
Because LFG may contain air, continuous O₂ monitoring is required to maintain explosion safety (<1%).
3. Typical Gas Composition
3.1 Shared Components (AD & LFG)
CH₄: 35–70%
CO₂: 30–55%
H₂O: saturated at process temperature
3.2 AD‑Specific Components
H₂S: 100–10,000 ppm
NH₃: up to 1500 ppm
Trace H₂
3.3 LFG‑Specific Components
O₂: <3% (air intrusion)
N₂: variable
Siloxanes
VOCs
These differences define additional monitoring needs for O₂/N₂ in LFG and NH₃/H₂S in AD.
4. Key Measurement Points
Typical measurement locations across AD and LFG include:
Feedstock homogenization tank (pH)
Hydrolysis & acidogenesis tank (pH, ORP)
Digester headspace (CH₄, CO₂, H₂S, NH₃, O₂, H₂)
Landfill extraction wells (CH₄, CO₂)
Landfill main header (O₂, H₂S, H₂)
Desulfurization inlet/outlet (H₂S)
Cooling dehydrator (H₂O, dew point)
Upgrading unit inlet/outlet (CH₄, CO₂, O₂, N₂, H₂S, moisture)
Compressor inlet buffer tank (CH₄, moisture)
Flare inlet (CH₄)
5. Required Gas Analytics
5.1 Core Composition
CH₄
CO₂
H₂O (dew point)
5.2 Safety & Integrity (LFG)
O₂ (explosion prevention <1%)
N₂ (air intrusion indicator)
5.3 Contaminants
H₂S (corrosion, catalyst poisoning)
NH₃ (feedstock nitrogen indicator)
Siloxanes (engine abrasion)
VOCs (landfill-specific)
5.4 Biological Stability (AD)
pH
ORP
6. Analyzer Selection
6.1 Infrared Gas Analyzers
Used for CH₄, CO₂, CO.
Widely applied at digester outlets, landfill wells, upgrading units, and flare monitoring.
6.2 Ultraviolet Gas Analyzers
Used for H₂S.
Applied at digester outlets, desulfurization units, and landfill gas headers.
6.3 Laser Gas Analyzers
Used for NH₃ and HCl.
Provide high selectivity and fast response.
6.4 Oxygen Analyzers
Optical, electrochemical, or paramagnetic.
Critical for LFG air intrusion monitoring.
6.5 Hydrogen Analyzers
Used for AD trace H₂ and LFG H₂.
6.6 P₂O₅ Trace Moisture Analyzers
Used for trace moisture at compressor inlet/outlet and upgrading outlet.
6.7 Liquid‑Phase Analyzers
pH (solid‑state reference electrode)
ORP
Turbidity
All analyzer types correspond exactly to the measurement table in your PDF.
7. Integration with Biomethane Upgrading
AD and LFG gas must be upgraded to meet grid standards (CH₄ ≥96%, CO₂ ≤2.5%, O₂ ≤0.5%, H₂S ≤6 mg/m³, dew point ≤–10°C).
Analytics ensure:
Feed gas specification (CH₄, CO₂, H₂S, O₂, N₂, moisture)
PSA performance (CH₄ recovery, off‑gas CH₄ loss)
Membrane performance (permeate/retentate CH₄)
Amine absorption efficiency (CO₂ removal <0.5%)
Product gas quality (CH₄, CO₂, O₂, H₂S, moisture)
8. Engineering & Lifecycle Considerations
Key engineering aspects include:
Heated sampling lines and condensate management
Accessibility of analyzers in high‑contamination environments
Calibration intervals and validation methods
SCADA/DCS integration for alarms and trending
Lifecycle cost optimization (maintenance vs. downtime)
These considerations align with the PDF’s emphasis on O&M optimization and equipment protection.
9. Summary
Anaerobic digestion and landfill gas recovery share biological origins and overlapping gas composition.
A unified analytics strategy covering CH₄, CO₂, H₂S, NH₃, O₂, N₂, moisture, and pH/ORP ensures:
Stable biological conversion
Explosion safety
Corrosion and catalyst protection
Efficient upgrading
Compliance with biomethane specifications