Biogas Upgrading to Renewable Natural Gas via Biological Methods

Introduction

Biogas, produced through anaerobic digestion of organic waste, contains roughly 50–65% methane (CH₄) and 35–50% carbon dioxide (CO₂), along with traces of hydrogen sulfide (H₂S), water vapor, and siloxanes. While biogas can be used directly for heating or electricity, its low energy density and impurities limit broader applications.

To make it compatible with natural gas infrastructure, biogas is upgraded to Renewable Natural Gas (RNG)—also known as biomethane—by increasing its CH₄ content to >95%. Biological upgrading methods offer a cost-effective, low-energy, and environmentally friendly alternative to physical and chemical techniques, leveraging microbes to remove CO₂, H₂S, and other contaminants from raw biogas.

What Products Are Produced?

• Biomethane / Renewable Natural Gas (RNG) – Pipeline-quality methane (>95% CH₄)
• CO₂-depleted offgas – Can be reused in greenhouses or algae cultivation
• High-purity sulfur (from H₂S removal) – In biological desulfurization systems

Pathways and Production Methods

1. Biological Methanation (CO₂ + H₂ → CH₄)

• Hydrogenotrophic methanogens convert CO₂ and green H₂ into CH₄
• Occurs either in-situ (in the digester) or ex-situ (in separate reactor)
• Reaction: CO₂ + 4H₂ → CH₄ + 2H₂O

2. Biological Desulfurization

• Sulfur-oxidizing bacteria (SOBs) oxidize H₂S to elemental sulfur or sulfate
• Operates under aerobic conditions in trickling filters or bio-scrubbers

3. Microalgae-Based CO₂ Removal

• Algal photobioreactors consume CO₂ from biogas during photosynthesis
• Simultaneously produce oxygen and biomass for further use

4. Biocatalyst Membrane Systems (Emerging)

• Immobilized enzymes or microbial consortia selectively absorb CO₂
• Integrated into compact bioreactor modules

Catalysts and Key Tools Used

Microbial Agents:

• Methanothermobacter thermautotrophicus, Methanococcus maripaludis – Hydrogenotrophic methanogens
• Thiobacillus, Acidithiobacillus – H₂S oxidizers in bio-desulfurization
• Chlorella vulgaris, Scenedesmus obliquus – CO₂-scavenging microalgae

Reactor Types:

• Trickling biofilters – For H₂S removal
• Gas-lift loop bioreactors – For ex-situ methanation
• Photobioreactors – For microalgae-based CO₂ capture
• Membrane biofilm reactors (MBfR) – For emerging compact setups

Sensors and Controls:

• CH₄/CO₂ ratio monitors
• pH, redox potential, and H₂ dosing control systems

Case Study: Ex-Situ Biological Methanation at MicrobEnergy GmbH (Germany)

Highlights

• Used surplus renewable electricity to generate H₂
• Coupled with CO₂ from biogas to produce pipeline-grade methane
• Achieved >97% CH₄ in upgraded gas, suitable for grid injection

Timeline

• 2013 – Pilot facility initiated
• 2016 – Fully automated ex-situ biomethanation reactor deployed
• 2020 – Integrated into municipal biogas-to-grid pipeline system
• 2023 – Demonstrated continuous operation using solar-powered H₂

Global and Indian Startups Working in This Area

Global

• Electrochaea (Germany/USA) – Industrial-scale biological methanation
• MicrobEnergy (Viessmann Group, Germany) – Pioneers of ex-situ biogas upgrading
• Hitachi Zosen Inova (Switzerland) – Integrated biogas-to-RNG solutions
• Malmberg Water (Sweden) – Biological desulfurization modules

India

• Greenjoules (Pune) – Waste-to-biomethane solutions
• A2O Energy (Delhi NCR) – Upgrading municipal biogas via bio-desulfurization
• CSIR-IIP and IITs – R&D on bio-methanation and gas enrichment
• Bharat Biogas Energy – Decentralized biomethane units with biofilters

Market and Demand

The global RNG market was valued at USD 10.3 billion in 2023, projected to reach USD 33.1 billion by 2030, with a CAGR of ~18.1%. Biological upgrading methods are gaining attention for low-energy, modular deployments, especially in agriculture, wastewater treatment, and municipal waste sectors.

Major End-Use Segments:

• Transportation (CNG fleets, buses, tractors)
• Grid injection and domestic cooking gas
• Industrial boilers and heating
• Green hydrogen precursor via steam reforming
• Onsite power generation in rural areas

Key Growth Drivers

• Government mandates for RNG blending in India and EU
• Need to decarbonize the gas grid
• Growing urban food and organic waste streams
• Interest in Power-to-Gas (P2G) systems using surplus renewable electricity
• Low environmental impact compared to chemical upgrading

Challenges to Address

• Low solubility and mass transfer of H₂ in liquid-phase reactors
• Sensitivity of methanogens to oxygen, contaminants, and pH shifts
• Process scalability and hydrogen supply integration
• Variability in biogas quality and composition
• CAPEX for ex-situ reactor setup in small-scale plants

Progress Indicators

• 2010 – First lab-scale ex-situ methanation demonstrated
• 2015 – EU pilot programs integrate RNG into national grids
• 2018 – Indian initiatives for SATAT bio-CNG scheme launched
• 2021 – Biological desulfurization systems commercialized in India
• 2024 – Integrated RNG hubs with biological upgrading piloted across 3 Indian states

Biogas upgrading to RNG via biological methods is at TRL 8–9 globally (in commercial operation) and at TRL 6–7 in India, with advanced pilot demonstrations and state-supported rollouts.

Conclusion

Biological upgrading of biogas to RNG offers a clean, modular, and scalable pathway to transform waste into valuable energy while capturing and utilizing CO₂. With growing pressure to decarbonize energy and manage organic waste, this method aligns perfectly with the goals of a circular bioeconomy and net-zero targets.

India’s expanding SATAT program, rural digesters, and urban biogas systems are ideal candidates for deploying low-cost biological upgrading—turning methane-rich biogas into grid-ready, clean-burning RNG for transport, cooking, and industry.

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