Introduction
Compressed Bio-Gas (CBG), commonly referred to as Bio-CNG, is rapidly gaining prominence as a clean, renewable, and indigenous alternative to fossil-based compressed natural gas. Derived from organic waste through anaerobic digestion, CBG aligns strongly with India’s objectives of energy security, waste management, reduction of greenhouse gas emissions, and achievement of Net Zero targets. With rising urbanization, increasing organic waste generation, and growing fuel demand, CBG offers a practical solution that simultaneously addresses environmental, economic, and social challenges.
Raw biogas produced in digesters typically contains only 55–65% methane, while the remaining portion consists of carbon dioxide (CO₂), hydrogen sulfide (H₂S), moisture, ammonia, nitrogen, oxygen, siloxanes, and trace volatile organic compounds. In its untreated form, biogas cannot be directly used as a transport fuel or injected into city gas distribution (CGD) pipelines due to low calorific value, corrosive nature, and safety concerns. Therefore, purification and upgradation are the most critical stages in the CBG value chain.
CBG purification refers to the removal of harmful and undesirable impurities such as H₂S, moisture, siloxanes, and particulates, whereas upgradation focuses on increasing methane concentration by separating CO₂ and other inert gases. Together, these processes convert low-grade biogas into high-quality, grid-compatible, and automotive-grade fuel that meets the stringent specifications of Oil Marketing Companies (OMCs) and Bureau of Indian Standards (BIS).
In recent years, significant technological advancements have taken place in CBG purification and upgradation. Traditional methods have evolved into highly efficient, automated, and digitally monitored systems capable of delivering high methane recovery with minimal losses. Innovations in adsorbent materials, membrane science, biological methanation, cryogenic separation, and hybrid process integration have transformed the technical and commercial feasibility of CBG projects.
As India accelerates the implementation of initiatives such as the SATAT scheme, GOBARdhan mission, and CGD expansion, the role of advanced purification and upgradation technologies becomes even more crucial. Selecting the right technology not only determines gas quality but also influences plant economics, operational stability, scalability, and long-term sustainability. This blog provides an in-depth technical and commercial overview of the latest technologies shaping the future of CBG purification and upgradation.
Why Purification & Upgradation Is Critical?
Purification and upgradation are the most crucial stages in the entire CBG production chain, as they directly determine gas quality, safety, market acceptability, and project profitability. Raw biogas produced from anaerobic digesters is unsuitable for direct use due to its low methane concentration and presence of corrosive, toxic, and inert components. Without proper purification and upgradation , the biogas cannot meet regulatory standards nor be safely compressed, transported, or utilized.
One of the primary objectives of upgradation is to increase the methane concentration from 55–65% in raw biogas to more than 90–95%, which significantly enhances the calorific value of the gas. Higher methane content ensures that CBG delivers energy performance comparable to conventional CNG, making it suitable for automotive fuel applications and pipeline injection into CGD networks.
Purification focuses on the removal of harmful impurities such as hydrogen sulfide (H₂S), moisture, ammonia, and siloxanes. H₂S is highly corrosive and toxic; even small concentrations can cause severe damage to compressors, storage cylinders, pipelines, and vehicle engines. Moisture in biogas leads to corrosion, ice formation during compression, and reduced equipment life. Siloxanes, commonly present in MSW-based biogas, can form abrasive silica deposits during combustion, leading to engine failure and increased maintenance costs.
From an operational perspective, effective purification and upgradation improve plant reliability and uptime. Clean, upgraded gas reduces fouling, corrosion, and unplanned shutdowns, thereby lowering maintenance costs and extending equipment lifespan. It also minimizes methane slip and gas losses, directly increasing saleable CBG volume and overall plant efficiency.
Regulatory compliance is another critical factor. Oil Marketing Companies (OMCs) and CGD operators mandate strict gas quality specifications under BIS standards, including limits on H₂S, moisture, oxygen, nitrogen, and particulates. Plants failing to meet these specifications risk rejection of gas, financial penalties, or termination of offtake agreements under schemes such as SATAT.
Environmental considerations further highlight the importance of advanced upgradation. Efficient systems reduce methane leakage—a potent greenhouse gas—and enable better utilization or conversion of CO₂. Emerging technologies such as biological methanation even allow conversion of CO₂ into additional methane, enhancing sustainability and reducing carbon footprint.
Major CBG Purification & Upgradation Technologies
CBG purification and upgradation technologies are designed to remove impurities and increase methane concentration to meet automotive and CGD standards. Selection of technology depends on plant capacity, feedstock type, raw biogas composition, availability of utilities, and economic considerations. Modern CBG plants generally use a multi-stage purification approach, combining pre-treatment systems with one or more upgradation technologies to achieve high methane purity, minimal losses, and stable operation.
CBG upgradation technologies can be classified into physical separation technologies, chemical absorption technologies, biological conversion technologies, and cryogenic separation technologies. Each category has its own operating principle, advantages, limitations, and suitability for specific plant sizes.
Before selecting any upgradation technology, it is important to understand the nature of raw biogas. Typical biogas composition varies with feedstock (press mud, cattle dung, MSW, Napier grass, sewage sludge), operating temperature, and digester design. Variations in CO₂ (30–45%), H₂S (500–5,000 ppm), moisture saturation, and siloxanes directly affect technology performance. Hence, modern CBG plants adopt customized, multi-stage purification trains rather than single-step solutions.
Upgradation technologies can broadly be classified into physical separation, chemical absorption, biological conversion, and cryogenic separation methods. Each has unique technical and economic implications.
1. Pressure Swing Adsorption (PSA)
PSA is one of the most established and widely used technologies in India.
Working Principle: Biogas is compressed and passed through adsorption vessels containing molecular sieves or activated carbon. CO₂, H₂S, moisture, and nitrogen are adsorbed under high pressure, while methane passes through. The adsorbent is regenerated by reducing pressure.
Latest Advancements:
- High-selectivity adsorbents
- Multi-bed PSA systems for continuous operation
- Automated valve sequencing
- Lower methane slip (<2%)
Advantages:
- Methane purity up to 96%
- No chemical usage
- Proven reliability
- Suitable for medium to large-scale plants
Limitations:
- Higher power consumption due to compression
- Performance sensitive to inlet gas composition
2. Membrane Separation Technology
Membrane-based upgradation is gaining popularity due to its compact design and modularity.
Working Principle: Selective membranes allow CO₂, H₂S, and water vapor to permeate faster than methane. Multiple membrane stages are used to achieve high methane purity.
Recent Innovations:
- Multi-stage membrane cascades
- Advanced polymer membranes with higher selectivity
- Hybrid membrane + PSA systems
Advantages:
- Compact and skid-mounted systems
- Lower energy consumption compared to PSA
- Easy scalability
- Fast start-up and shutdown
Challenges:
- Membrane fouling if pre-treatment is inadequate
- Slightly lower methane recovery without multi-stage design
3. Water Scrubbing (Advanced Wet Scrubbing)
Water scrubbing is a physical absorption process where CO₂ and H₂S are absorbed in water.
Technological Improvements:
- Closed-loop water circulation systems
- High-efficiency packed columns
- Reduced water consumption
Advantages:
- Simple and chemical-free process
- Suitable for high CO₂ biogas
- Moderate CAPEX
Limitations:
- Water requirement
- Lower methane recovery if not optimized
4. Chemical Scrubbing (Amine-Based Systems)
Chemical scrubbing uses solvents such as monoethanolamine (MEA) or methyldiethanolamine (MDEA) to selectively absorb CO₂.
Latest Developments:
- Improved solvent formulations
- Heat recovery and energy integration
- Lower solvent degradation
Advantages:
- Methane purity up to 99%
- Very low methane losses
- Suitable for large-scale plants
Limitations:
- High CAPEX and OPEX
- Skilled manpower required
- Chemical handling and disposal
5. Biological Upgradation Technologies
Biological upgradation is an emerging and environmentally friendly approach.
In-Situ Methanation: Hydrogen is injected directly into the digester, where microorganisms convert CO₂ into methane.
Ex-Situ Methanation: Biogas is treated in a separate bioreactor for CO₂ conversion.
Benefits:
- Converts CO₂ into additional methane
- Minimal chemical use
- Potentially carbon-negative process
Conclusion
Purification and upgradation technologies are the most decisive elements in the success of any CBG (Compressed Bio-Gas) project. While anaerobic digestion enables the production of biogas from organic waste, it is the purification and upgradation stage that converts this raw, low-grade gas into a high-quality, marketable, and regulation-compliant fuel. Without efficient removal of impurities and enhancement of methane concentration, biogas cannot meet the requirements of automotive fuel or City Gas Distribution (CGD) networks.
The continuous advancement of technologies such as Pressure Swing Adsorption (PSA), membrane separation, water scrubbing, chemical scrubbing, biological methanation, and cryogenic upgradation has significantly improved methane recovery, operational reliability, and energy efficiency. Modern systems are increasingly automated, digitally monitored, and designed to minimize methane losses, thereby improving both environmental performance and project profitability.
In India, where initiatives like SATAT, GOBARdhan Mission, and rapid CGD expansion are driving large-scale adoption of CBG, advanced purification and upgradation technologies play a critical role. Compliance with BIS and OMC quality standards is essential for securing long-term offtake agreements and ensuring stable revenue. Moreover, emerging trends such as integration with green hydrogen, carbon utilization, and AI-based plant optimization are making CBG plants future-ready and aligned with global decarbonization goals.