The demand for Compressed Biogas (CBG) is growing rapidly in India as industries, transport companies and energy users look for cleaner alternatives to conventional fossil fuels. However, setting up a successful CBG plant requires much more than installing a digester and gas upgrading system.
A properly designed CBG project requires detailed planning of feedstock, pretreatment, anaerobic digestion, biogas purification, CBG compression, water management, power consumption and digestate utilization.
This article explains the key technical aspects of a 5 TPD CBG plant based on paddy straw and Napier grass, including feedstock requirements, biogas generation, digester technology, gas upgrading, CBG production, FOM, LFOM, water requirements and major equipment.
What is a 5 TPD CBG Plant?
A 5 TPD CBG plant is designed to produce approximately 5 tonnes, or 5,000 kg, of compressed biogas per day.
It is important to understand that 5 TPD refers to the final CBG production and not to the quantity of raw material entering the plant.
The quantity of feedstock required depends on several factors, including:
- Feedstock type
- Moisture content
- Total solids
- Volatile solids
- Biodegradability
- Methane potential
- Pretreatment efficiency
- Digester efficiency
- Methane recovery
- Gas upgrading efficiency
Therefore, a detailed feedstock study and mass balance should always be completed before finalizing the capacity of a CBG plant.
Feedstock Requirement for a 5 TPD CBG Plant
A representative 5 TPD CBG plant based on paddy straw and Napier grass can be designed with the following feedstock:
Feedstock Quantity
Paddy Straw 30 TPD
Napier Grass 60 TPD
Total Feedstock 90 TPD
The design basis considered for this plant specifies paddy straw at approximately 85% solids and Napier grass at approximately 25% solids.
The difference in moisture content is extremely important.
For example, 60 tonnes of fresh Napier grass at 25% solids does not represent 60 tonnes of dry biomass. Therefore, CBG plant calculations should always consider both wet feedstock quantity and solids content.
Why Feedstock Pretreatment is Important
Paddy straw and Napier grass are fibrous biomass materials. Their structure can make them difficult to digest efficiently without suitable pretreatment.
A typical pretreatment system may include:
- Feedstock receiving
- Feed hopper
- Bale handling
- Chopping
- Shredding
- Size reduction
- Fibre opening
- Slurry preparation
- Controlled feeding
The objective of pretreatment is not simply to convert biomass into powder.
The objective is to improve:
- Particle size
- Fibre accessibility
- Pumpability
- Mixing
- Digestion efficiency
- Feeding consistency
For fibrous biomass, the correct pretreatment technology can have a significant impact on the overall performance of the CBG plant.
Slurry Preparation
After pretreatment, the biomass is mixed with water and recycled process liquid to produce a pumpable slurry.
In the representative plant design:
- Paddy straw + Napier grass feedstock = 90 TPD
- Recycle/process water = approximately 250 m³/day
- Total slurry = approximately 340 TPD
- Slurry solids = approximately 12%
The prepared slurry is then transferred to the anaerobic digesters.
This also demonstrates why water balance is an important part of CBG plant engineering.
CSTR Anaerobic Digester
The representative plant uses two Continuous Stirred Tank Reactors, or CSTR digesters.
The configuration includes:
2 × 8,000 m³ CSTR digesters
The digestion process operates under mesophilic conditions.
The design basis specifies:
- Operating temperature: approximately 36–40°C
- pH: around 7.2
- Hydraulic Retention Time: approximately 36 days
- Continuous mixing
Inside the digester, organic matter passes through the major biological stages of anaerobic digestion:
Hydrolysis
Complex organic materials are broken down into simpler compounds.
Acidogenesis
The simpler compounds are converted into organic acids and other intermediate products.
Methanogenesis
Methanogenic microorganisms convert suitable intermediate compounds into methane-rich biogas.
The resulting gas is called raw biogas.
Raw Biogas Production
The representative mass balance indicates raw biogas production of approximately:
11,900–13,200 Nm³/day
with methane concentration of approximately:
55–60% CH₄
This raw biogas cannot directly be considered CBG.
It must first undergo gas cleaning and upgrading to increase methane concentration and remove unwanted components.
H₂S Removal in a CBG Plant
Hydrogen sulphide, commonly known as H₂S, is an important impurity in raw biogas.
H₂S needs to be controlled because it can cause:
- Corrosion
- Equipment damage
- Maintenance problems
- Gas quality issues
- Safety concerns
The representative plant design includes an H₂S removal system using scrubbing and iron-based media.
The objective is to reduce H₂S to the required level before the gas enters the upgrading system.
CO₂ Removal and Biogas Upgrading
Raw biogas contains a significant quantity of carbon dioxide.
The purpose of the upgrading system is to remove CO₂ and other impurities while recovering as much methane as possible.
Different technologies can be used for biogas upgrading, including:
- Water Scrubbing
- Membrane Separation
- PSA/VPSA
- Amine Scrubbing
- Other specialized upgrading technologies
The representative project documentation considers water scrubbing and two-stage membrane technology as possible CO₂ removal options.
Water Scrubbing
CO₂ is absorbed into water under controlled operating conditions and subsequently separated from the process water.
Membrane Separation
Special membranes separate gases based on their permeability characteristics. CO₂ passes preferentially through the membrane while a larger portion of methane is retained.
The appropriate technology depends on:
- Raw biogas composition
- Required methane purity
- Methane recovery
- Plant capacity
- Power consumption
- Water availability
- CAPEX
- O&M requirements
Therefore, the upgrading technology should be selected according to the actual project conditions rather than using the same technology for every CBG plant.
Gas Drying
After upgrading, the gas must be dried before high-pressure compression.
Gas drying helps control moisture and prevents problems such as:
- Condensation
- Corrosion
- Compressor issues
- Gas quality problems
- Moisture accumulation in high-pressure systems
The representative design includes a desiccant-based gas drying system with a specified low dew point and moisture control.
Proper moisture monitoring is an important part of a professional CBG plant.
CBG Compression
After purification and drying, the upgraded biomethane is compressed to high pressure.
The representative plant includes a high-pressure reciprocating compressor with compression up to approximately 250 bar.
The compressed CBG can then be transferred to a cascade or filling system depending on the project's transportation and offtake arrangement.
The system can also include:
- Odorization
- Gas quality monitoring
- Online gas analyser
- PLC/SCADA controls
- High-pressure cascade
Expected CBG Production
The representative 5 TPD plant is designed for:
CBG Production: 5,000 kg/day
The target gas quality specified in the design includes approximately:
- Methane: ≥98%
- H₂S: <3 ppm
- CO₂: <2%
Actual gas quality requirements should always be checked against the applicable standards and the requirements of the intended CBG offtaker.
What Happens to the Digestate?
CBG production does not only produce gas.
Anaerobic digestion also produces digestate, which can be processed into organic manure products.
In the representative plant:
Digestate: approximately 325 m³/day at around 6% solids
The digestate is sent to a solid-liquid separation system.
The resulting streams are:
FOM – Fermented Organic Manure
Approximately:
20–25 TPD at 40% solids
LFOM – Liquid Fermented Organic Manure
Approximately:
50–55 m³/day at 1.8% solids
Therefore, a CBG plant can generate multiple value streams:
CBG + FOM + LFOM
This makes digestate management an important part of the overall project economics.
Major Equipment Required for a 5 TPD CBG Plant
A complete CBG plant can include the following major systems.
Feedstock Handling
- Feed hopper
- Belt conveyors
- Screw conveyors
- Chopper
- Shredder
- Grinder
- Feedstock handling equipment
Slurry Preparation
- Slurry preparation tank
- Mixers
- Transfer pumps
- Hydrolysis/feeding tank
Anaerobic Digestion
- CSTR digesters
- Central agitators
- Submersible mixers
- Heating system
- Insulation
- Temperature instrumentation
- pH monitoring
Biogas Handling
- Biogas holder
- Biogas blowers
- Gas pipelines
- Flare system
- Condensate management
Gas Cleaning and Upgrading
- H₂S removal system
- CO₂ removal/upgrading system
- Gas drying system
- Online gas analyser
CBG Compression
- High-pressure compressor
- Cascade
- Filling system
- Odorization system
- High-pressure piping and valves
Digestate Processing
- Screw press
- Pressure filter press
- FOM collection system
- LFOM collection system
- Storage tanks
- Transfer pumps
Water Requirement of a CBG Plant
Water requirement is often misunderstood when comparing different CBG plant quotations.
For the representative 5 TPD plant, the documented fresh water requirement for the bio-digester is:
5–7 m³/day
or:
5,000–7,000 litres/day
The plant also uses approximately:
250 m³/day of recycle water
for slurry preparation.
This recycle water should not be confused with fresh water consumption.
The biogas upgrading system has its own soft/RO water requirements depending on the selected technology and operating configuration.
Therefore, when evaluating a CBG plant, always ask the EPC company to clearly provide:
- Fresh water requirement
- Recycle water requirement
- Make-up water
- RO/soft water requirement
- Cooling water
- Utility water
Power Requirement
Power consumption is another major operating cost in a CBG plant.
The representative technical documentation provides an estimated consumption of:
Plant Section Estimated Consumption
Pretreatment + Bio-Digester 180
Extruder System 300
CBG Upgradation Plant 210
Miscellaneous 30
Total 720
These values are estimates and the documentation states that exact values are to be confirmed after detailed engineering.
For a proper feasibility study, power consumption should be clearly expressed in terms such as:
- Connected load
- Operating load
- kWh/day
- kWh/tonne feedstock
- kWh/kg CBG
This allows different CBG technologies to be compared properly.
Importance of Mass Balance in CBG Projects
One of the most important documents in a CBG project is the mass balance.
A simplified process flow for the plant is:
Paddy Straw + Napier Grass
↓
Feedstock Pretreatment
↓
Slurry Preparation
↓
CSTR Anaerobic Digestion
↓
Raw Biogas
↓
H₂S Removal
↓
CO₂ Removal / Biogas Upgrading
↓
Gas Drying
↓
Compression
↓
CBG
At the same time:
Digestate
↓
Solid-Liquid Separation
↓
FOM + LFOM
The representative plant design targets approximately 11,900–13,200 Nm³/day of raw biogas and 5,000 kg/day of CBG.
A detailed mass balance helps determine whether the proposed feedstock quantity is actually sufficient to achieve the guaranteed CBG production.
What Should Be Checked Before Buying a CBG Plant?
A CBG project should never be evaluated only on the basis of equipment price.
Before placing an order, the project developer should check:
Feedstock
- Daily availability
- Moisture
- TS
- VS
- C/N ratio
- BMP
- Seasonal availability
- Delivered cost
- Storage requirements
Digester
- Digester volume
- HRT
- OLR
- Operating temperature
- Mixing arrangement
- Heating requirement
Biogas
- Raw biogas quantity
- Methane percentage
- CO₂ percentage
- H₂S concentration
- Gas production guarantee
Upgrading
- Technology
- Methane recovery
- Methane slip
- Power consumption
- Gas quality
- Maintenance requirement
CBG
- Daily production
- Methane purity
- CO₂
- H₂S
- Moisture
- Compression pressure
Digestate
- Digestate quantity
- FOM quantity
- LFOM quantity
- Moisture
- Nutrient content
- Applicable specifications
Utilities
- Electricity
- Water
- Instrument air
- Heating
- Cooling
- DG backup
Commercial
- CAPEX
- GST
- Civil work
- Installation
- Commissioning
- Transportation
- Warranty
- Performance guarantee
- Exclusions
Performance Guarantee is Critical
A CBG EPC contract should clearly define what the EPC company guarantees.
The guarantee should ideally cover parameters such as:
- CBG production
- Methane concentration
- Methane recovery
- Raw biogas production
- Power consumption
- Water consumption
- FOM/LFOM production
- Feedstock conditions
In the representative project documentation, the performance guarantee run is specified as 48 cumulative hours after erection and commissioning, subject to availability of suitable feedstock, utilities and operating manpower.
The documented warranty provides 12 months for fabricated equipment from commissioning, while bought-out/OEM equipment is covered according to the respective OEM warranty terms.
Why Feedstock Should Come Before Plant Selection
One of the biggest mistakes in CBG projects is selecting the plant capacity first and trying to find feedstock later.
The correct approach is:
Feedstock Assessment
↓
Laboratory Analysis
↓
BMP / Methane Potential
↓
Mass Balance
↓
Plant Capacity
↓
Digester Design
↓
Gas Upgrading
↓
CBG Compression
↓
Financial Model
This approach significantly improves the reliability of the project design.
Biovardhan Energy Solutions – CBG Plant Engineering
Biovardhan Energy Solutions focuses on integrated CBG and Bio-CNG solutions for agricultural residues, animal waste and other organic feedstocks.
Our approach covers the complete CBG value chain:
PRE-CBG
Feedstock assessment → Pretreatment → Shredding → Chopping → Slurry Preparation
CBG
Anaerobic Digestion → Biogas Collection → H₂S Removal → CO₂ Removal → Gas Drying → Compression → CBG
POST-CBG
Digestate Separation → FOM → LFOM → Organic Manure Value Addition
The objective is not simply to supply machinery.
The objective is to develop a technically viable, commercially sustainable and feedstock-specific CBG project.
Conclusion
A CBG plant is a combination of biological processing, mechanical pretreatment, gas purification, gas compression and organic waste management.
The complete process can be summarized as:
Feedstock → Pretreatment → Slurry Preparation → Anaerobic Digestion → Raw Biogas → H₂S Removal → CO₂ Upgrading → Gas Drying → Compression → CBG
while the digestate is processed into:
FOM + LFOM
For a 5 TPD CBG project, the most important documents are the feedstock analysis, mass balance, process flow diagram, digester design, gas upgrading specification, water balance, power balance, equipment list, performance guarantee and commercial scope.
The right CBG plant is not necessarily the plant with the lowest price.
The right CBG plant is the one that is correctly designed for the available feedstock and can consistently deliver the promised gas production, gas quality and operating performance.
Planning a CBG Plant?
If you are planning a 3 TPD, 5 TPD, 10 TPD, 15 TPD or 20+ TPD CBG plant, Biovardhan Energy Solutions can help with feedstock evaluation, process design, plant configuration, technology selection and EPC planning.
Biovardhan Energy Solutions
CBG | Bio-CNG | Biogas | Waste-to-Energy | Organic Waste Management
Website: biovardhanenergy.com
Email: info@biovardhanenergy.com
Phone / WhatsApp: +91 92519 91977
Published 8 October 2026 by Biovardhan Energy Solutions Pvt. Ltd. · More articles