The Ultimate Guide to Biogas Purification Methods: From Raw Gas to Clean Energy

Biogas purification methods play a substantial role as renewable energy sources become more prominent. European biogas production alone creates 63.3 TWh of electricity and powers 14.6 million European households each year. Raw biogas can’t be used directly as a clean energy source because it contains about 60% methane mixed with 40% carbon dioxide and other contaminants.

Raw biogas needs proper upgrading to realize its full potential. Biogas upgrading and purification technologies remove carbon dioxide and other impurities to substantially improve fuel efficiency. The methane enrichment process is a vital step that transforms biogas from a simple energy source into versatile fuel. This clean fuel works well for cooking, lighting, transportation, and power generation. The production process also serves two purposes: it recycles organic waste into valuable fertilizer and creates renewable energy.

Let’s get into the most effective biogas purification systems available today. You’ll learn about various purification methods and find the right biogas cleaning solution that meets your needs.

Understanding Raw Biogas and Its Impurities

Raw biogas is a mix of gasses that forms when organic materials break down without oxygen. Let’s take a closer look at what makes up this gas and what’s in it before we learn about cleaning it up.

What is raw biogas made of?

We see two main gasses in raw biogas: methane (CH₄) and carbon dioxide (CO₂). The mix changes based on what organic material you use, how you run things, and your digester setup. Raw biogas usually contains:

Methane: 50-75% by volume
Carbon dioxide: 25-50% by volume
Water vapor: 1-10%
Nitrogen: 0-15%
Oxygen: 0-3%
Hydrogen: 0-1%
Trace compounds: including hydrogen sulfide, ammonia, and siloxanes

The mix changes a lot depending on what you feed it. To cite an instance, biogas from agricultural waste usually has 60-75% methane, while household waste gives you 50-60% methane. The temperature, pressure, digester pH, and how long you keep the material also affect the final gas mix.

Common contaminants in biogas

Raw biogas has several unwanted substances that make it hard to use. Hydrogen sulfide (H₂S) is one of the biggest headaches, showing up in amounts up to 10,000 ppmv. Agricultural waste digesters make biogas with very high H₂S levels, sometimes reaching thousands of parts per million.

Siloxanes are another big problem, especially in biogas from wastewater treatment plants, where they average about 2.55 ppm. These come from products with silicon, like detergents, shampoos, and cosmetics.

Other unwanted substances include:

Ammonia (NH₃): Usually below 100 ppmv, coming from protein breakdown
Volatile organic compounds (VOCs): Usually 0-200 mg/m³, more common in landfill gas
Halocarbons: Found mostly in landfill gas at about 6 ppm
Water vapor: Makes up to 10% of the gas mix

Each source of biogas creates its own mix of contaminants. Landfill gas has more aromatic compounds (averaging 109 ppm), while wastewater treatment biogas has more siloxanes.

Why purification is essential for energy use

The impurities in raw biogas limit what you can do with it and cause many problems. Carbon dioxide waters down the methane content, which makes the gas less valuable and harder to transport and compress.

H₂S is the most urgent problem because it corrodes everything it touches. When it mixes with water vapor, it makes sulfuric acid that eats away at engines, pipes, and storage tanks. When it burns, H₂S also makes toxic sulfur dioxide.

Siloxanes create another vital issue. They turn into silicon dioxide deposits when burned, which wear down engine parts and cause overheating and breakdowns. These deposits build up to several millimeters thick and need chemical or mechanical cleaning.

Simple heating needs just basic cleanup. But other uses need more thorough purification:

Pipeline injection: Needs CO₂ below 3%, water under 112 mg/m³, and H₂S less than 4 ppm
Vehicle fuel (compressed): Must have methane above 97%
Liquefied biomethane: Needs very low levels of contaminants—under 25 ppm CO₂, 4 ppm H₂S, and 1 ppm water—to stop ice formation and corrosion

Clean biogas becomes a valuable, clean energy source that works well in many different industries when you purify it properly.

Key Requirements for Biogas Purification

Proper standards are the life-blood of biogas purification in the industry. Biogas producers struggle with varying requirements that drive up costs and slow down projects when guidelines don’t exist. Let’s get into the key standards that make biogas upgrading and purification work well.

Pipeline and vehicle fuel standards

RNG producers face their biggest problem with different gas quality rules from various utilities. The American Biogas Council created standard RNG purity guidelines based on data from dozens of US facilities. Some of these plants have run for over twenty years. These guidelines match typical natural gas requirements and create unity in the industry.

Purified biogas headed for pipelines needs less than 3% carbon dioxide, water content under 112 mg/m³, and hydrogen sulfide below 4 ppm. RNG in natural gas pipelines usually contains 96-98% methane. Strict limits also apply to oxygen (less than 0.4 mol%), total inerts (less than 5 mol%), and siloxanes (less than 1 ppm(v)).

Fresh Kills landfill in Staten Island, NY became the first site to put RNG into a natural gas pipeline in 1982. Standards have grown over the last several years to make sure biogas-based fuels work with current infrastructure.

Methane enrichment targets

The amount of methane directly sets the energy value of purified biogas. Each application needs specific methane levels to work right. Higher methane means more energy—biogas with 70% methane gives about 21.5 MJ/Nm³, while pure methane produces 35.8 MJ/Nm³.

Different uses need different purity levels:

CNG vehicle applications: at least 97% methane content
Liquefied biomethane: very strict limits of less than 25 ppm CO₂, less than 4 ppm H₂S, and less than 1 ppm H₂O to stop dry ice formation and corrosion
Simple grid injection: usually 90-95% methane, based on local rules

Purification methods show different success rates. Water scrubbing systems reach 96.1% methane purity, pressure swing adsorption hits 95.8%, chemical absorption gets to 94.6%, membrane separation achieves 90.3%, and cryogenic processes deliver about 88% methane purity.

Environmental and safety considerations

Biogas purification must handle both environmental and safety issues well. Biogas technology helps prevent greenhouse gasses from escaping organic material storage. Yet methane leaks remain an issue, with CHP units possibly releasing 1-2% of total methane made.

A case from Germany in 2015 shows why proper management matters. The whole ordeal involved 350,000 liters of slurry leaking into nearby waters, killing six tons of fish.

These practices help reduce environmental damage and safety risks:

Gas-tight construction stops unwanted emissions. Automatic flare systems must be in place, especially during CHP unit maintenance when biogas production continues. Workers need to follow manufacturer safety rules carefully when using iron compounds or activated carbon for desulfurization because these materials can heat up during regeneration.

Air for desulfurization needs careful control. It should not exceed 6% of the biogas volume made in the same period to avoid explosive mixtures. Non-return valves close to the gas room stop any backflow.

Physical and Chemical Purification Methods

The path from raw biogas to clean energy mostly depends on how well we can separate its components. Several tested physical and chemical methods can purify biogas. Each method works best for specific uses and different types of contaminants.

Water scrubbing: how it works and when to use it

Water scrubbing works on a basic principle—CO₂ and H₂S dissolve in water more easily than methane. The process starts when pressurized biogas (usually 8-10 bars) enters the bottom of a packed column. Water flows down from the top to create a counter-current absorption system. CO₂ dissolves into the water, which lets methane rise to the top much purer than before.

High Pressure Water Scrubbing (HPWS) leads the way in biogas upgrading because it’s simple and works well. The system can reach methane purity levels around 94% when run properly. The gas-to-liquid ratio usually stays around 5 to balance good results with running costs.

Water scrubbing systems come with clear benefits:

They remove both CO₂ and H₂S at once
They cost less to build and run
They’re easy to operate with few chemicals needed
You can reuse water through vacuum desorption

Pressure swing adsorption (PSA)

PSA works by picking out specific molecules with special porous materials that catch CO₂. Charles Skarstrom first created it in 1960, and today’s PSA systems work great for biogas applications.

PSA works in four main steps:

Pressure build-up
Adsorption (CO₂ sticks to the material)
Depressurization
Regeneration of the adsorption media

PSA usually uses carbon molecular sieves or zeolites to absorb gasses. These systems run at room temperature and can make biomethane that’s more than 96% pure. They can recover up to 98% of methane from the original biogas. PSA has become more popular as new technology has made it more budget-friendly by using less energy.

Amine absorption and solvent-based systems

Amine absorption uses chemical reactions instead of physical properties like water scrubbing. This method uses amine solutions (like monoethanolamine or MEA) that form chemical bonds with CO₂. The solution gets heated to 80-150°C to release the CO₂ and become ready for reuse.

Amine systems stand out because they can recover all the methane from biogas. The output gas can be amazingly pure—often more than 99% methane.

But traditional amine systems need lots of energy for regeneration. New advances include using protic solvents instead of water, which cuts energy needs from 3000 kJ to 1200 kJ per mole of CO₂. Modern systems also mix solvents like ethylene glycol and propanol to stop solid buildup while keeping strong absorption power.

Cryogenic separation for high-purity methane

Cryogenic separation ranks among the most advanced ways to purify biogas. It uses temperature differences to separate gasses by turning them into liquids. By cooling biogas to very low temperatures, this method creates two valuable products: very pure biomethane and food-grade liquid CO₂.

Cryogenic systems can make very pure gas (95-99% methane) but need lots of energy for compression and cooling. Still, this method offers unique advantages:

Makes liquid CO₂ that’s up to 98% pure
Loses very little methane (less than 1%)
Doesn’t need extra steps to compress CO₂ for storage or moving

Green Gas Inc. creates custom biogas purification systems based on these technologies. They match the right system to what each project needs and how it runs.

Biological and Advanced Purification Technologies

Advanced biological and membrane technologies lead the way in biogas upgrading solutions, going beyond traditional purification methods.

Biological methanation using hydrogenotrophic microbes

Hydrogenotrophic methanogens are specialized microorganisms that convert carbon dioxide and hydrogen into methane through biological methanation. The process produces biomethane with over 96% purity. The reaction follows a simple formula: CO₂ + 4H₂ → CH₄ + 2H₂O, releasing 131 kJ of heat.

The process can be implemented through two main approaches: in-situ (within the digester) and ex-situ (in separate reactors). Ex-situ methods have become more popular because they provide better control and don’t interfere with anaerobic digestion. Research shows that methanogenic Archaea populations can double during discontinuous H₂-feeding, which makes the system more resilient.

Membrane separation systems

Selective polymer barriers in membrane technology separate methane from carbon dioxide. The process works through pressure difference, where high-pressure biogas meets the membrane surface and CO₂ passes through much faster than CH₄. The system offers several advantages:

Energy efficiency (approximately 0.2 kWh/Nm³ of raw biogas)
No need for additional chemicals or operating media
Very high methane purity achievement (>99.8%)
Complete CH₄ recovery potential

Hollow fiber membranes now represent the most advanced option, with thousands grouped into modules for optimal separation.

Hybrid systems combining multiple methods

Multiple purification technologies work together in hybrid configurations. Membrane-PSA systems stand out as they achieve biomethane purity above 96% and methane recovery over 99%, while producing CO₂ with purity greater than 95% and recovery above 93%. These combined systems reduce upgrading costs by about 15% compared to single technology solutions.

Bio-adsorbents and activated carbon

Activated carbon is vital for biogas purification as it removes hydrogen sulfide, siloxanes, and volatile organic compounds. Specially impregnated activated carbon captures up to 60% of its weight in contaminants due to its high adsorption capacity. Activated carbon made from biomass waste shows great results for CO₂ removal, with some materials reaching CO₂ adsorption capacities of 9.2 mmol/g.

How to Choose the Right Biogas Purification System

Biogas purification technology selection needs thorough analysis of multiple factors. Your choice shouldn’t just focus on getting the “cheapest” solution. The right technology must match your specific utilization requirements.

Factors to consider: cost, scale, and feedstock

Your organic feedstock choice affects biogas composition and shapes purification needs. Poultry waste generates methane concentrations ranging from 40% to 70%. The feedstock you use determines which contaminants your system needs to handle.

Scale plays a vital role—small operations need compact, modular systems. Larger facilities work better with more extensive, integrated solutions. Large biogas plants show better economics despite higher upfront costs. A small-scale plant (250 Nm³/hour) ranges between $4,400-$5,950/Nm³/hour. Large-scale operations (over 2,000 Nm³/hour) cost about $1,950/Nm³/hour.

Comparing efficiency and methane recovery

Different purification technologies show varying efficiency levels:

Water scrubbing: 96.1% methane purity with 98% recovery rate
Pressure swing adsorption: 95.8% purity
Chemical absorption: 94.6% purity with potential for 99.5% recovery
Membrane separation: 90.3% purity, though advanced systems can reach 99.5% purity

Note that economically viable operations need more than 90% purity and recovery. Membrane technology gives a good balance of energy efficiency (0.12-0.21 kWh/Nm³) and performance for most applications.

Why Green Gas Inc. is a trusted solution provider

Green Gas Inc. has become a reliable partner for biogas purification solutions in this complex field. They analyze each site’s unique needs instead of using generic solutions. This approach recognizes how local conditions and specific needs shape the best purification strategy. Their expertise helps clients find the perfect balance between purity, recovery, and energy use—a sweet spot that boosts profitability while meeting application needs.

Conclusion

Raw biogas transforms into a valuable renewable energy resource through purification. This piece explored how various contaminants affect biogas quality by a lot. The trip from raw biogas to clean energy needs careful selection of appropriate technologies based on project requirements.

Water scrubbing, pressure swing adsorption, amine absorption, cryogenic separation, biological methanation, and membrane systems each offer unique advantages based on your situation. The best approach matches technology capabilities with your feedstock, scale requirements, and end-use applications.

Methane enrichment targets change based on applications, from simple heating to pipeline injection and vehicle fuels. Your purification system must line up with these purity requirements while balancing operational costs and efficiency. The right system delivers optimal methane recovery rates without wasting energy or needing excessive maintenance.

Green Gas Inc. proves to be a reliable partner for biogas purification needs. They offer customized solutions instead of one-size-fits-all approaches. Their expertise helps pick the most suitable technology for your circumstances, which ensures high biogas quality while maximizing return on investment. On top of that, their site-specific analysis makes sure all local factors and requirements get proper attention when designing your purification system.

Clean energy’s future depends on knowing how to turn organic waste into high-quality biomethane efficiently. Biogas purification technologies produce renewable energy and help manage waste streams effectively, creating two environmental benefits. These technologies keep improving, but the simple principles and considerations in this piece will stay relevant for years.

Note that success depends on picking the right purification approach for your needs. Working with experienced providers like Green Gas Inc. helps you guide through technical considerations with confidence and turn your biogas resource into clean, valuable energy.

FAQs

Q1. What are the main methods for purifying biogas?

The primary biogas purification methods include water scrubbing, pressure swing adsorption, amine absorption, cryogenic separation, biological methanation, and membrane systems. Each method has unique advantages and is suitable for different scenarios depending on factors like feedstock, scale, and end-use requirements.

Q2. Why is biogas purification necessary?

Biogas purification is essential because raw biogas contains impurities like carbon dioxide, hydrogen sulfide, and siloxanes that can reduce its energy value and damage equipment. Purification increases methane concentration, making the gas suitable for various applications such as pipeline injection, vehicle fuel, and power generation.

Q3. What level of methane purity is required for different biogas applications?

Methane purity requirements vary by application. For basic grid injection, 90-95% methane content is typically needed. Compressed natural gas (CNG) for vehicles requires at least 97% methane, while liquefied biomethane demands extremely high purity with less than 25 ppm CO2 and 4 ppm H2S.

Q4. How does water scrubbing purify biogas?

Water scrubbing works by exploiting the higher solubility of CO2 and H2S in water compared to methane. Pressurized biogas is passed through a column where it contacts water flowing in the opposite direction. The CO2 and H2S dissolve in the water, leaving behind purified methane with typical purity levels around 94%.

Q5. What factors should be considered when choosing a biogas purification system?

Key factors to consider include the type of organic feedstock, which affects biogas composition; the scale of operation, as larger plants generally have better economics; the desired methane purity and recovery rate; energy efficiency; and specific end-use requirements. It’s crucial to conduct a site-specific analysis to determine the most suitable purification technology for your particular circumstances.