Table of Contents

Scope for Bio-CNG (CBG) Production

A typical Biogas Purification and Upgradation System converts raw biogas (50–65% CH₄, 35–50% CO₂, H₂S, moisture, siloxanes, and trace gases) into Bio-CNG (Compressed Biogas) containing 95–98% methane, suitable for compression, bottling, vehicle fuel, or pipeline injection.

Instrumentation

A typical gas purification train includes:

Pressure transmitters (PT)
Temperature transmitters (TT)
Flow meters (FT)
Differential pressure transmitters (DPT)
H₂S analyzer
CH₄ analyzer
CO₂ analyzer
Moisture analyzer (optional)
Gas leak detectors
Pressure safety valves (PSV)
Emergency shutdown (ESD) valves
PLC and SCADA system

Documents

Document	Purpose
Process Flow Diagram (PFD)	Overall process flow
Piping & Instrumentation Diagram (P&ID)	Detailed piping and controls
General Arrangement (GA) Drawing	Equipment layout
Equipment Datasheets	Technical specifications
Piping Isometrics	Fabrication drawings
Instrument Index	Instrument list
Valve Schedule	Valve specifications
Cause & Effect Matrix	Safety interlocks
Electrical Single Line Diagram	Electrical distribution
PLC/SCADA Logic	Automation philosophy
Operation & Maintenance Manual	Plant operation procedures
As-Built Drawings	Final construction documentation

Sr. No.	Equipment	Function	Typical Outlet Condition
1	Biogas Blower	Supplies constant gas flow	20–100 mbar
2	Moisture Separator	Removes condensate and water droplets	Dry gas
3	Particulate Filter	Removes dust and solids	<5 μm particles
4	H₂S Scrubber	Removes hydrogen sulfide	<5–10 ppm H₂S
5	Gas Chiller	Condenses remaining moisture	Lower humidity
6	CO₂ Removal Unit	Increases methane concentration	CH₄: 95–98%
7	Fine Gas Filter	Removes adsorbent dust	Clean gas
8	Refrigeration Dryer	Reduces dew point	Approximately +3°C
9	Desiccant Dryer	Final moisture removal	Dew point ≤ −40°C
10	Methane Analyzer	Monitors CH₄ purity	Online measurement
11	Odorization Unit	Adds odorant for safety	Odorized gas
12	Buffer Vessel	Stabilizes pressure	4–10 bar
13	CNG Compressor	Compresses gas	200–250 bar
14	Cascade Storage	High-pressure storage	Ready for dispensing

Gas Composition Through the Process

Stage	CH₄ (%)	CO₂ (%)	H₂S (ppm)	Moisture
Raw Biogas	55–65	35–45	500–5,000	Saturated
After H₂S Scrubber	55–65	35–45	<5–10	Saturated
After CO₂ Removal	95–98	<2–3	<5	Moist
After Dryers	95–98	<2–3	<5	Dew point ≤ −40°C
After Compression	95–98	<2–3	<5	Dry, 200–250 bar

Common CO₂ Removal Technologies

Technology	Methane Purity	Advantages	Typical Applications
Water Scrubbing	96–98%	Low operating cost, no chemicals	Medium to large plants
VPSA	96–98%	Low energy, dry process	Medium to large plants
PSA	95–98%	Simple operation	Medium plants
Membrane Separation	96–99%	Compact, modular	Medium to large plants
Amine Scrubbing	98–99%	Very high purity	Large industrial plants

Typical Process Flow

Raw Biogas from Digester
        │
        ▼
Moisture Separator / Knock-out Pot
        │
        ▼
H2S Scrubbing Unit
        │
        ▼
CO2 Removal / VPSA Upgrading System
        │
        ▼
Gas Dryer (Refrigeration + Desiccant)
        │
        ▼
Odorization Unit
        │
        ▼
Buffer Storage Vessel
        │
        ▼
High Pressure CNG Compressor
        │
        ▼
Cascade Storage Cylinders
        │
        ▼
Dispensing / Cylinder Filling

Scope of Supply

Sr. No.	Equipment	Function	Typical Performance
1	H₂S Scrubbing Unit	Removes hydrogen sulfide	<5–10 ppm H₂S
2	CO₂ Scrubber Module	Removes carbon dioxide	CH₄ purity 95–98%
3	Vacuum Pressure Swing Adsorption (VPSA)	Final methane enrichment	CH₄ recovery >96%
4	Moisture Separator	Removes free water	Prevents corrosion
5	Refrigeration Dryer	Lowers dew point	Dew point about +3°C
6	Desiccant Dryer	Removes residual moisture	Dew point −40 to −70°C
7	Odorization Unit	Adds odorant for leak detection	Ethyl mercaptan/TBM
8	Surge Vessel	Dampens pressure fluctuations	Stable compressor suction
9	Buffer Storage Tank	Stores upgraded gas	Continuous compressor operation
10	Instrumentation	Pressure, flow, methane analyzer	Online monitoring

H₂S Scrubbing Unit

Hydrogen sulfide (H₂S) is one of the most critical impurities in raw biogas. It must be removed before biogas upgrading and compression because it is highly corrosive, toxic, and can damage downstream equipment such as CO₂ scrubbers, compressors, dryers, valves, pipelines, and CNG storage cylinders.

Why H₂S Removal is Necessary

Problem Caused by H₂S	Effect
Corrosion	Forms sulfuric acid in the presence of moisture, corroding carbon steel equipment and pipelines
Compressor Damage	Reduces compressor life and increases maintenance
Adsorbent Poisoning	Shortens the life of PSA/VPSA adsorbents and membranes
Environmental Impact	Produces SO₂ during combustion, contributing to air pollution
Safety	Toxic gas with a characteristic rotten egg odor
Product Gas Quality	Bio-CNG standards require very low H₂S levels

Removal of hydrogen sulfide before upgrading.

Typical H₂S Concentration

Feedstock	H₂S (ppm)
Cattle dung	200–2,000
Food waste	500–3,000
Press mud	1,000–5,000
Distillery spent wash	2,000–10,000
Sewage sludge	500–5,000

Technologies

Technology	H₂S Removal Efficiency	Application
Iron Oxide	99%	Small plants
Activated Carbon	99%	Final polishing
Biological Scrubber	High	Large plants
Caustic Scrubber	Very High	Industrial
Media Filters	High	CBG plants

Typical outlet H₂S:<5 ppm

H₂S Removal Technologies

1. Iron Oxide (Iron Sponge) Scrubber

Principle: H₂S reacts with iron oxide to form iron sulfide.

Reaction:

$F e_{2} O_{3} + 3 H_{2} S \to F e_{2} S_{3} + 3 H_{2} O$

Advantages

Low capital cost
Simple operation
High removal efficiency (>99%)
No chemical dosing
Suitable for small and medium Bio-CNG plants

Disadvantages

Media requires periodic replacement or regeneration
Not ideal for very high H₂S loading

Typical outlet H₂S: <5 ppm

H₂S Removal Technologies

Technology	CH₄ Purity	Advantages
Water Scrubber	96%	Low operating cost
Chemical Absorption	98–99%	High purity
PSA	96–98%	No chemicals
VPSA	97–98%	Lower power
Membrane	96–99%	Compact

Typical outlet gas:

Component	Value
CH₄	95–98%
CO₂	<2–3%
H₂S	<5 ppm

Activated Carbon Filter

Activated carbon, often impregnated with potassium iodide or metal oxides, adsorbs H₂S.

Advantages

Very high removal efficiency
Excellent polishing stage
Compact equipment

Disadvantages

Higher media cost
Frequent replacement with high H₂S concentrations

Typical outlet H₂S: <1 ppm

Biological Desulfurization

Sulfur-oxidizing bacteria convert H₂S into elemental sulfur.

Advantages

Very low operating cost
Environmentally friendly
Suitable for continuous operation and large plants

Disadvantages

Requires careful control of pH, temperature, and oxygen
Longer start-up period

Caustic (NaOH) Scrubber

H₂S reacts with sodium hydroxide solution.

Advantages

High removal efficiency
Suitable for high H₂S concentrations

Disadvantages

High chemical consumption
Spent solution treatment required

Water Scrubbing

Water absorbs a portion of H₂S along with CO₂.

Advantages

Removes CO₂ and part of H₂S simultaneously

Disadvantages

Usually insufficient as the only H₂S removal step for Bio-CNG specifications

Vacuum Pressure Swing Adsorption (VPSA)

Purpose

Final methane enrichment using adsorption.

Working Principle

Pressurization
Adsorption of CO₂
Depressurization
Vacuum regeneration
Re-pressurization

Advantages

High methane recovery
Low operating cost
No chemicals
Continuous operation
Automatic control

Typical Performance

Parameter	Value
Methane purity	96–98%
Methane recovery	96–99%
Pressure	4–8 bar
Vacuum	0.2–0.5 bar absolute

Moisture Removal System

Moisture must be removed before compression.

A. Moisture Separator

Removes:

Condensed water
Water droplets
Foam

Equipment:

Knock-out Drum
Cyclone Separator
Demister Pad

B. Refrigeration Dryer

Purpose:

Cool gas to condense moisture.
Typical outlet dew point:+3°C

C. Desiccant Dryer

Purpose:

Final moisture removal.

Adsorbents:

Molecular sieve
Activated alumina
Silica gel

Outlet dew point:

−40°C
−60°C
−70°C

5. Odorization Unit

Pure methane is odorless.

Odorant is added for safety.

Typical odorants:

Tertiary Butyl Mercaptan (TBM)
Ethyl Mercaptan

Features:

Metering pump
Storage tank
Injection nozzle
Level transmitter
PLC control

6. Surge Vessel and Buffer Storage Tank

Surge Vessel

Purpose:

Reduce pressure pulsation
Stabilize compressor inlet
Improve process stability

Typical pressure:

4–10 bar

Buffer Storage Tank

Purpose:

Temporary gas storage
Smooth process fluctuations
Ensure continuous compressor operation

Typical volume:

5–100 m³

Functional Testing, Leak Checks, and Trial Runs

Commissioning activities generally include:

Mechanical Completion

Equipment verification
Piping inspection
Valve verification
Instrument calibration

Leak Testing

Pneumatic pressure test
Soap solution leak test
Nitrogen pressure holding test
Flange inspection
Valve leakage inspection

Functional Testing

Instrument loops
PLC logic
Compressor interlocks
Emergency shutdown (ESD)
Pressure safety valves
Gas analyzers
Flow meters
Level transmitters

Performance Trial Run

Typical checks:

Methane purity
CO₂ concentration
H₂S concentration
Moisture content
Flow rate
Pressure stability
Compressor performance
Energy consumption

Submission of As-Built P&ID and GA Drawings

After successful commissioning, the contractor typically submits the final documentation.

A. As-Built P&ID

Includes:

Equipment tags
Valve numbering
Instrument numbering
Flow directions
Safety valves
Isolation valves
Control loops
Interlock philosophy
Utilities

B. General Arrangement (GA) Drawing

Includes:

Equipment layout
Foundation details
Pipe routing
Access platforms
Maintenance clearances
Cable trays
Structural supports
Drain lines

C. Gas Train Layout

Typical sequence:

Biogas Inlet
      │
Moisture Separator
      │
H2S Scrubber
      │
CO2 Scrubber / VPSA
      │
Dryer
      │
Gas Analyzer
      │
Buffer Vessel
      │
Compressor
      │
Cascade Storage
      │
Dispensing Unit

D. Purification Area Layout

Typical equipment:

H₂S Scrubber
CO₂ Removal Skids
VPSA Vessels
Blowers
Vacuum Pump
Air Compressor
Refrigeration Dryer
Desiccant Dryer
Buffer Vessel
Instrument Air Package
Analyzer Panel
Electrical Panels
PLC & SCADA Room

Typical Gas Quality Specification for Bio-CNG

Parameter	Typical Value
Methane (CH₄)	95–98%
Carbon Dioxide (CO₂)	<2–3%
Hydrogen Sulfide (H₂S)	<5–10 ppm
Oxygen (O₂)	<1%
Moisture Dew Point	≤ −40°C
Siloxanes	Trace / Nil
Particulates	Nil
Delivery Pressure (before compression)	4–10 bar
Compression Pressure	200–250 bar

Engineering Deliverables

For an EPC or detailed engineering package, the following documents are typically included:

Process Flow Diagram (PFD)
Process and Instrumentation Diagram (P&ID)
General Arrangement (GA) Drawings
Equipment Layout Drawings
Piping Layout and Isometrics
Instrument Hook-up Drawings
Cable Routing Drawings
Cause & Effect Matrix
Control Philosophy
Instrument Index
Equipment Datasheets
Valve Datasheets
Line List
Material Take-Off (MTO)
Commissioning Procedures
Functional Test Reports
Leak Test Reports
Performance Guarantee Test Report
Operation & Maintenance (O&M) Manual
As-Built Documentation

Conclusion

This configuration represents a standard industrial Bio-CNG purification and upgrading train suitable for agricultural waste, press mud, distillery spent wash, food waste, municipal organic waste, and other anaerobic digestion plants.
It can consistently produce vehicle-grade Bio-CNG meeting typical commercial methane purity requirements of 95–98% before high-pressure compression and storage.

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