How Does a Double Membrane Gas Holder Work?

The inner membrane forms the variable gas-storage chamber. Biogas enters the chamber through the gas inlet. As more gas enters than leaves, the membrane rises and the stored volume increases. When downstream equipment consumes more gas than the plant produces, the membrane moves downward and the stored volume decreases.

Because raw biogas may contain methane, carbon dioxide, water vapor, hydrogen sulfide and other trace compounds, membrane selection needs to consider gas tightness, chemical resistance, tensile performance, temperature range and long-term durability rather than mechanical strength alone.

The outer membrane provides the visible shape of the gas holder. It is not normally the primary variable-volume gas-storage chamber. Instead, a controlled air space is maintained between the outer and inner membranes. The outer membrane forms a stable enclosure and protects the internal membrane system from outdoor environmental exposure.

For outdoor projects, the structural and membrane design must also account for project-specific environmental conditions such as wind, temperature, snow where applicable, anchoring conditions and local design requirements.

The support-air blower supplies air into the space between the two membranes. This air performs two important functions: it keeps the outer membrane inflated, and it contributes to the pressure environment that allows the gas chamber to operate at the specified low pressure.

The blower should therefore not be understood simply as a fan that makes the dome look round. It is part of the gas holder pressure-control system. Depending on the project design, blower operation can be coordinated with pressure transmitters, control valves, redundant blower arrangements and PLC logic.

Because the storage chamber changes shape, the system needs to know approximately how much usable gas remains. Level or position measurement can be used to estimate the filling condition of the inner membrane and transmit this information to the plant control system.

• High gas level: increase gas utilization, prepare flare operation, or trigger a high-level alarm according to the process philosophy.
• Low gas level: reduce downstream gas demand or protect gas-consuming equipment from insufficient gas supply.

A common misunderstanding is that the gas holder works primarily because the weight of the membrane presses down on the gas. That is not the main operating principle of a modern double membrane gas holder.

The actual pressure relationship is governed by the gas chamber, support-air chamber, membrane geometry, blower control, pressure-control devices and the hydraulic or mechanical characteristics of the connected gas system.

As an example, MOTET currently lists project-dependent working-pressure ranges of approximately 1,000–5,000 Pa for floor-standing systems and approximately 800–3,000 Pa for tank-top configurations. These values should be treated as product design ranges rather than universal values for every double-membrane system.

These systems therefore operate at relatively low pressure compared with compressed-gas storage vessels. This distinction matters when engineers specify piping, blowers, pressure protection, flare systems and downstream equipment.

A gas holder is not an isolated piece of equipment. It may be connected to anaerobic digesters, desulfurization systems, gas dryers, CHP engines, boilers, biogas upgrading units, compressors and flares. The pressure design of the gas holder therefore needs to be coordinated with the pressure limits and flow requirements of the entire gas train.

Insufficient pressure may cause unstable downstream operation, while excessive pressure can create unnecessary loads and increase the risk of abnormal operation. Gas-holder pressure design should therefore be considered together with the complete process rather than selected independently.

A tank-top double membrane gas holder is installed directly above a digester or another suitable tank structure. This configuration can reduce separate plot requirements and shorten the gas route between digestion and storage. The tank structure and anchoring system must be checked to ensure that all membrane, pressure, wind and other project loads can be transferred safely.

MOTET currently lists an effective-volume range of approximately 300–150,000 m³ for floor-standing configurations and a typical range of approximately 500–12,000 m³ for tank-top configurations. Actual selection should always be based on the specific project requirements and structural conditions.

Gas-holder sizing should not be based only on the plant daily biogas production. A more useful starting point is the cumulative difference between gas production and gas consumption over the design operating cycle:

In practical engineering terms, the designer should calculate how much surplus gas accumulates during periods when production exceeds demand, and how much stored gas is needed when demand exceeds production.

The required design volume should then consider additional factors such as operating reserve, usable versus geometric volume, process fluctuations, flare philosophy, maintenance scenarios, future expansion and project-specific safety margins. Useful sizing inputs for an EPC project include:

• Average biogas production, Nm³/h
• Maximum and minimum biogas production, Nm³/h
• Gas consumption profile versus time
• Required buffering time, h
• Required operating pressure, Pa
• Gas composition, including CH₄, CO₂, H₂S and moisture
• Site temperature and relevant environmental conditions
• Installation configuration: floor-standing or tank-top

This approach produces a more defensible selection than applying a simple fixed percentage of daily gas production without reviewing the actual operating profile.