Raw biogas typically contains 200–10,000 ppm of hydrogen sulfide (H₂S), depending on feedstock. Even at low concentrations, H₂S causes severe problems across the entire biogas utilization chain.
Without desulfurization
Consequences of untreated H₂S
Engine & compressorRapid corrosion
Combustion outputSO₂ emissions
Catalyst lifetimeSeverely reduced
Piping / heat exchangersSulfuric acid attack
Regulatory complianceEmissions violations
Worker safetyToxic hazard >100 ppm
After desulfurization
Benefits of H₂S removal
Equipment service life2–5× longer
Maintenance intervalsSignificantly extended
Combustion qualityClean, SO₂-free
Grid injection eligibilityAchievable (<1 ppm)
Carbon credit valueMaximized
Operational safetyCompliant
Process 1
Dry desulfurization
Dry desulfurization uses a solid adsorbent or chemisorption media — most commonly iron oxide (Fe₂O₃), zinc oxide (ZnO), or activated carbon — packed in a vessel. Biogas passes through the media bed and H₂S is captured through chemical reaction or adsorption.
Dry desulfurization — process flow
⚗️ Key chemical reactions — iron oxide media
Adsorption: 2 Fe₂O₃ + 6 H₂S → 2 Fe₂S₃ + 6 H₂O
H₂S is chemically bound to the iron oxide, forming iron sulfide. Water vapour is released.
Regeneration: 2 Fe₂S₃ + 3 O₂ → 2 Fe₂O₃ + 6 S⁰
Introducing air converts spent iron sulfide back to iron oxide, releasing elemental sulfur. The media can be regenerated multiple times before disposal.
Advantages and disadvantages
✅ Advantages
Simple installationNo wastewater generatedCompact footprintNo moving partsLow energy consumptionEasy to operateSuitable for remote sites
⚠️ Disadvantages
Media requires periodic replacementSaturates quickly at high H₂SSpent media disposal costSensitive to moisture and tarLess effective above 5,000 ppm
Process 2
Wet desulfurization
Wet desulfurization uses a liquid absorbent — typically sodium hydroxide (NaOH) solution, amine solutions, or water — to chemically absorb H₂S from the gas stream in a counter-current scrubber tower. The rich absorbent is then regenerated and recycled in a closed loop.
Wet desulfurization — process flow
⚗️ Key chemical reactions — NaOH scrubbing
Absorption: H₂S + 2 NaOH → Na₂S + 2 H₂O
H₂S is neutralized in alkaline solution. CO₂ is partially co-absorbed, which may need separate management.
Oxidation: 2 Na₂S + 2 H₂O + O₂ → 4 NaOH + 2 S⁰
In regenerative systems (e.g. Lo-Cat, SulfaTreat Liquid), the absorbent is oxidized, regenerating NaOH and producing elemental sulfur as a recoverable byproduct.
Advantages and disadvantages
✅ Advantages
Handles very high H₂S (>5,000 ppm)Ultra-low outlet (<1 ppm)Continuous operationAbsorbent can be regeneratedSulfur recovery possibleScalable to large flows
⚠️ Disadvantages
High capital costWastewater treatment requiredComplex operationChemical procurement ongoingLarger footprintHigher energy consumption
Technical comparison
Side-by-side comparison
The table below compares the two technologies across the most critical decision parameters for biogas plant operators.
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