What Is a Gas Holder?
A gas holder stores the biogas produced by a biodigester at low pressure. Storage types, the double-membrane design and its role in a biogas plant.
A gas holder is the vessel that stores the biogas produced by a biodigester, keeping it at low pressure until it is consumed or processed. It works as a buffer between gas production (continuous, driven by the reactor's biology) and consumption (variable, depending on the engine, boiler or upgrading system). Without a gas holder, any mismatch between what the biodigester produces and what the plant uses would be lost or flared off.
The most widespread type in industrial plants today is the double-membrane gas holder, a dome formed by two membranes: an inner one that contains the biogas and an outer one that gives it shape and protects it.
How the double-membrane gas holder works
The system has three elements: the outer membrane (protection against wind, rain and sun), the inner membrane (which stores the gas and inflates or deflates with volume) and an air blower that keeps the space between them pressurized. That air cushion presses on the inner membrane and delivers biogas to consumption at a stable pressure, typically in the order of 2 to 30 mbar (low pressure).
As the biodigester produces, the inner membrane fills and rises; when the plant consumes, it drops. Level sensors monitor the stored volume and command the safety flare if the holder fills above the setpoint.
Types of biogas storage
| Type | Typical pressure | Common use |
|---|---|---|
| Digester-integrated dome | 2–5 mbar | Digester with membrane roof; storage on the reactor itself |
| External double-membrane holder | 2–30 mbar | Standalone storage; larger buffer volume |
| Medium/high-pressure storage | > 100 mbar up to bar | Requires compression; plants with upgrading or transport |
Why it matters
The gas holder defines the stability and safety of the whole biogas plant. A well-sized buffer absorbs production peaks, feeds the CHP engine at constant pressure and reduces flared gas (a loss of energy and value). As a guide, it is usually sized to store several hours of production, so the plant keeps delivering gas even amid reactor variations or short consumption downtime.
Key functions of the gas holder
- Decouple biogas production and consumption (buffer effect).
- Deliver gas at stable pressure to the engine, boiler or upgrading.
- Reduce losses from flaring.
- Add safety with over/under-pressure valves and level control.
Where it fits in Tech Tank projects
In a turnkey plant, the gas holder is not an accessory: it is part of the system's core. Tech Tank integrates gas storage within its biogas and biodigesters line, coordinated with the reactor and the safety systems. To dig deeper into the specific storage solution, review the double-membrane gas holder for biogas storage, and to understand the whole set-up, what an industrial biodigester is.
Correct gas-holder sizing depends on the substrate, the hourly production and the consumption profile. For a design tailored to your project, get in touch and we will arrange a technical assessment.
Frequently Asked Questions
At what pressure does a biogas gas holder operate?
Most operate at low pressure, in the order of 2 to 30 mbar. Transport or grid injection requires additional compression to much higher pressures, which is a separate system from the gas holder.
How much biogas does a gas holder store?
It depends on the design, but it is usually sized for several hours of the biodigester's production. The goal is to buffer peaks and sustain consumption, not to store days of gas.
Is the double-membrane gas holder safe?
Yes, when installed with its devices: over- and under-pressure valves, level control, gas detection and a safety flare. The pressurized air between membranes also acts as insulation and mechanical protection.
Can biogas be stored on the digester itself?
Yes. Many digesters have a membrane roof that works as an integrated gas holder. An external holder is used when a larger buffer volume is needed or to separate storage from reaction.
What happens if the gas holder fills up?
Level sensors trigger the safety flare, which burns off the excess in a controlled way to prevent overpressure. It is an energy loss, which is why good sizing minimizes it.