How Produce CNG from Biogas: Methods and Application

 

What is Biogas vs Bio-CNG?

• Producing CNG from biogas is basically upgrading raw biogas into Bio-CNG (Compressed Biomethane, CBM) by removing impurities and compressing methane to high pressure. Here’s a clear, engineering-focused explanation.
• Producing CNG from biogas (commonly called Bio-CNG or Compressed Biomethane – CBM) is an important clean-energy solution, especially in India where agricultural and organic waste is abundant. Below is a structured, engineering-focused guide covering methods and applications.

• Biogas composition:
  • Methane (CH₄): 50–65%
  • CO₂: 35–45%
  • Traces: H₂S, moisture, siloxanes
• CNG / Bio-CNG requirement:
  • Methane ≥ 90–95%
  • High calorific value
  • Compressed at 200–250 bar

So, the goal is purification + compression

 Step-by-Step Process to Convert Biogas to CNG

 Step 1: Biogas Production

• Source: cow dung, food waste, agricultural waste, sewage
• Process: Anaerobic digestion
• Output: Raw biogas

 Step 2: H₂S Removal (Desulfurization)

• H₂S is corrosive and toxic
• Methods:
  • Iron oxide (iron sponge)
  • Activated carbon
  • Biological scrubbers

 Step 3: CO₂ Removal (Upgrading)

This is the most critical step.

Common technologies:

Method	Working Principle	Suitability
Water Scrubbing	CO₂ dissolves in water	Small–medium plants
PSA (Pressure Swing Adsorption)	CO₂ adsorbed on media	Medium plants
Membrane Separation	Selective gas separation	Modern plants
Amine Scrubbing	Chemical absorption	Large plants

Output: Biomethane (~95% CH₄)

 Step 4: Moisture Removal (Drying)

• Methods:
  • Chilling + condensation
  • Silica gel dryers

Required to avoid pipeline corrosion & freezing

 Step 5: Compression

• Compress gas to 200–250 bar
• Equipment:
  • Multi-stage compressor
  • Cascade cylinders

Final product = Bio-CNG

 3. Basic Process Flow Diagram

Biogas Plant → H₂S Removal → CO₂ Removal → Dryer → Compressor → Storage (CNG cylinders)

 4. Key Equipment Required

• Digester (biogas plant)
• Gas scrubber (H₂S removal)
• CO₂ removal unit (PSA / water scrubber)
• Gas dryer
• High-pressure compressor
• Storage cylinders (cascade system)

---

🔷 5. Typical Output & Efficiency

Parameter	Value
Methane purity	90–98%
Energy content	~50 MJ/kg
Conversion efficiency	85–95%
Compression pressure	200–250 bar

 6. Applications of Bio-CNG

• Vehicle fuel (cars, buses, tractors)
• PNG replacement
• Industrial burners (you may find this relevant given your burner interest)
• Power generation

 7. Indian Context (Important)

• Government scheme: SATAT (Sustainable Alternative Towards Affordable Transportation)
• Oil companies buy Bio-CNG
• High demand for:
  • Dairy waste plants
  • Municipal waste plants

 8. Practical Challenges

• High initial cost of upgrading plant
• CO₂ removal efficiency
• Compressor maintenance
• Feedstock consistency

 9. Small vs Large Plant Insight

Plant Type	Capacity	Technology
Small (farm level)	10–100 m³/day	Water scrubber
Medium	100–1000 m³/day	PSA
Large (commercial)	>1000 m³/day	Membrane / Amine

 10. Simple Insight (Engineering View)

👉 Raw biogas is low-grade fuel
👉 Bio-CNG is pipeline/vehicle-grade fuel

The entire process is about:

• Removing CO₂ (increase calorific value)
• Removing impurities (protect equipment)
• Compressing gas (store & transport)

---

If you want, I can go deeper into:

• Design of biogas upgrading system (PSA/membrane)
• Cost estimation for India (small/medium plant)
• CFD scope in gas cleaning and burner design using Bio-CNG