The Biogas to Biomethane Process Explained: 4 Upgrading Technologies Compared

Raw biogas from a digester is a powerful starting point, but it’s not ready for every job. To unlock its full commercial and environmental value, it must be refined.

This refinement is the biogas to biomethane process. It’s a critical upgrade that transforms raw, variable gas into a consistent, high-purity renewable fuel.

This article explains how this cleaning and upgrading works. We’ll detail the main steps, compare the leading technologies used by international manufacturers, and look at where this high-value biomethane ends up.

If you’re evaluating the feasibility of a project, understanding this core process is the first step.

What is the Goal of Biogas Upgrading?

Raw biogas typically contains 50-65% methane (CH4), 35-50% carbon dioxide (CO2), water vapor, hydrogen sulfide (H2S), and other trace gases.

For simple on-site heat and power, this mixture can often be used directly. But for two key applications, the impurities, especially CO2, must be removed:

Injection into the Natural Gas Grid: Gas grids have strict specifications. The gas must be over 95% methane, often exceeding 98%, to match the quality of fossil natural gas.
Use as Vehicle Fuel (Bio-CNG/Bio-LNG): Vehicle engines and fuel stations require clean, high-calorific-value, and pressurized fuel. Removing CO2 increases the energy density and prevents corrosion.

The biogas to biomethane process strips out CO2 and contaminants, leaving nearly pure methane. This product is called renewable natural gas (RNG) or biomethane.

The Standard 5-Step Cleaning and Upgrading Process

While technologies differ, a complete upgrading system follows a logical sequence. Think of it as a production line for purifying gas.

Step 1: Pretreatment and Filtration
Raw biogas first passes through a scrubber or condensate trap to remove water vapor. It then goes through a filter, often filled with activated carbon, to adsorb hydrogen sulfide (H2S). H2S is highly corrosive and can damage equipment in later stages. This step protects the entire system.

Step 2: The Core Upgrading Technology
This is the heart of the biogas to biomethane process. Here, the bulk separation of methane from carbon dioxide happens. We will explore the four main technology types in the next section. The core objective is to produce a methane-rich stream (biomethane) and a separate CO2-rich off-gas.

Step 3: Gas Drying
After upgrading, the biomethane stream is still slightly moist. A final drying step ensures the dew point is low enough to prevent condensation in pipelines or during compression. This usually involves a desiccant dryer.

Step 4: Compression
For grid injection or use as vehicle fuel, the biomethane must be pressurized. A compressor raises the gas to the required pressure, which can be over 200 bar for CNG vehicle tanks or the specific pressure of the local gas grid.

Step 5: Odorization (For Grid Injection)
Natural gas is odorless. For safety, an odorant (like tetrahydrothiophene) is added to give it that distinctive "gas smell." This allows leaks to be easily detected. This final step completes the biogas upgrading process for grid-ready biomethane.

4 Leading Biogas Upgrading Technologies Compared

International biogas upgrading equipment manufacturers specialize in different core technologies. The choice depends on scale, energy costs, and desired purity.

1. Water Scrubbing (Pressure Water Wash)
This is a physical absorption process. Pressurized biogas is brought into contact with water in a column. CO2 is more soluble in water than methane, so it dissolves into the water. The cleaned methane exits the top.

Pros: Robust, relatively simple, handles fluctuations well.
Cons: Can require a lot of water (though closed-loop systems recycle it), uses energy for water pumps and gas compression.

2. Membrane Separation
This technology uses the difference in molecular size and solubility. High-pressure biogas flows across polymeric membranes. CO2 and water vapor permeate through the membrane walls faster than methane, separating the gases into two streams.

Pros: Compact, modular design. Low chemical consumption. Good for medium-scale projects.
Cons: Requires very thorough pretreatment to protect membranes. Multiple membrane stages are often needed for high purity.

3. Pressure Swing Adsorption (PSA)
PSA uses adsorbent materials (like zeolites or activated carbon) that trap CO2 molecules on their surface under pressure. The biogas flows through a vessel filled with this material. Once saturated, the vessel is depressurized to release the CO2, and it is ready for the next cycle. Plants use multiple vessels for continuous operation.

Pros: Produces very high-purity biomethane (>99%). Dry process, no chemicals.
Cons: Can have higher methane slippage (loss) if not tuned perfectly. The cyclic process requires complex valve systems.

4. Chemical Scrubbing (Amine Wash)
This is a chemical absorption process. The biogas is scrubbed with a liquid amine solution, which chemically bonds with and removes the CO2. The "rich" amine solution is then heated in a separate column to release the captured CO2, regenerating the "lean" amine for reuse.

Pros: Exceptionally high selectivity and purity. Proven technology from the natural gas industry. Efficient at large scale.
Cons: Higher capital cost. Requires thermal energy for solvent regeneration. Amines can degrade and require management.

Where Does the Biomethane Go? Key Applications

The end use of the biomethane often dictates the choice of biogas to biomethane process technology.

Gas Grid Injection: This is the most common pathway in many regions. The biomethane is injected into the local natural gas distribution network. Producers earn revenue through gas sales and green gas certificates. This application requires strict, continuous quality monitoring.

Vehicle Fuel (Bio-CNG/Bio-LNG): Upgraded biomethane is compressed (CNG) or liquefied (LNG) for use in buses, trucks, and waste collection vehicles. It offers a near-carbon-neutral transport fuel. Fuel-quality standards are equally stringent.

Industrial Direct Use: Some industries with high, steady thermal demands may pipe biomethane directly to their boilers or furnaces as a replacement for fossil gas, often under a private offtake agreement.

Costs and What Influences Them

The price tag for a biogas upgrading system is significant. A unit for a medium-sized plant can range from $1 million to $5 million or more, depending on capacity and technology.

Major cost drivers include:

System Capacity: Cost per normalized cubic meter of gas decreases with larger scale.
Technology Choice: Membrane systems may have lower CapEx but different efficiency. Amine scrubbers have high efficiency but higher initial and operational costs.
Site-Specific Engineering: Grid connection costs, required civil works, and integration with existing digester systems all add up.
Automation & Quality Control: Meeting grid specs requires advanced analyzers and control systems.

Operational costs are dominated by electricity consumption (for compressors and pumps) and, for some technologies, thermal energy for regeneration. Maintenance and periodic replacement of filters, adsorbents, or membranes are also factored in.

The Role of International Equipment Manufacturers

Building a reliable upgrading plant requires specialized expertise. The international biogas upgrading equipment manufacturing sector is composed of firms that provide core technology or complete turnkey plants.

Companies like Greenlane Renewables, DMT Environmental Technology (Carborex® MS), Bright Renewables, and EnviTec Biogas offer proven technologies. Some specialize in one method (e.g., membranes), while others provide multiple technology options.

When selecting a supplier, scrutinize their reference plants, especially those using a similar feedstock and producing for the same end-use. Ask for detailed data on methane recovery rates, purity guarantees, and specific energy consumption.

Frequently Asked Questions (FAQ)

Q1: How much methane is lost during the upgrading process?
A1: Methane loss, or "slip," is a critical performance metric. Modern, well-optimized systems aim for losses below 1-2%. The specific technology and its operation affect this. High methane slip hurts both project economics and the environmental benefit, so manufacturers strive to minimize it.

Q2: What happens to the separated CO2 from the biogas to biomethane process?
A2: This CO2 stream is usually vented to the atmosphere. However, since it is biogenic CO2 (from recently grown plants), it is part of the natural carbon cycle and doesn't add new carbon like fossil fuels. Innovatively, some projects are now exploring to capture and use this food-grade CO2 in greenhouses or for beverage carbonation.

Q3: Which upgrading technology is the "best"?
A3: There is no single best technology. The optimal choice depends on the project's scale, local energy costs, required methane purity, and feedstock consistency. A small farm project might choose a simpler, robust water scrubber, while a large industrial plant injecting into a high-pressure grid might opt for an amine or PSA system for its high purity.

Q4: How energy-intensive is the biogas upgrading process?
A4: It is a significant energy consumer, typically using 2-6% of the energy content of the raw biogas. Most of this energy goes to gas compression. The chosen technology impacts this: membrane systems often have lower thermal but higher electrical demand, while amine scrubbing requires heat. The goal is to maximize net energy output.

Q5: Can an upgrading system handle fluctuations in biogas composition or flow?
A5: Yes, but within design limits. All systems are engineered for a range of inputs. Sudden, large fluctuations can challenge the process control. High levels of contaminants like siloxanes (from certain waste feedstocks) can be particularly damaging and must be removed in pretreatment. Discuss your expected biogas variability with potential suppliers during design.