Biogas Holder Design is remarkably similar for most of the common type of commercial AD plants, so here we devote our discussion of anaerobic digestion plant gas storage to gas holders for circular digester tanks and circular gas holders. Most of those are mesophilic type single stage continuously stirred biogas reactors (CSRs).
Biogas holders designed for WasteWater Treament Plant (sewage works) sludge digestion units, usually differ from the designs discussed here. Similarly, dry anaerobic digestion plants may also have gas storage, but again, those biogas holder designs are not part of this article.
Although biogas holders look large, they are not provided for long-term storage. For long-term storage at any AD facility the biogas holders would need to be too large to provide long-term storage, or if smaller too many would be needed to be economically justifiable.
To look at this in another way, [in this biogas holder design], if an equipment failure occurs (even where a gas holder exists) it is not possible to create a biogas holder design which can store the biogas for more than a few hours. [The volume needed would be too large.] For that reason all biogas plants should also be provided with an emergency flare so that the excess biogas can at least be vented as carbon dioxide (CO2) and not methane (CH4) which is a much more climate change damaging gas than CO2.
Correcting a Common Misconception Necessary Before Starting Any Biogas Holder Design Article
To all normal purpose it is a mistake to assume (although many do) that masonry, mortar and concrete are gas-tight. They are not. The use of mortar additives to improve gas tight properties is not sufficient. Gas-tightness can only be achieved through a combination of good, careful workmanship and special coatings.
Special coatings can be applied to masonry and plaster that is strong and free of cracks, and these may be gas tight when constructed. However, masonry and plaster are stiff material which will eventually crack at some location. A coating which has been applied to such surfaces would have to be very special not to also crack. Once cracks have developed, many experts say that not even the best seal coating can render cracks permanently gas-tight thereafter.
Types of Gasholders for Biogas Plants – Floating Drum and Stand-alone and Fixed Dome
Depending on the need and the capacity, different types of biogas plants have different gasholders. Major types of the gasholders are summarised as follows:
Most floating-drum gas-holders are made of 2-4 mm thick sheet steel, with the sides made of thicker material than the top in order to compensate for the higher degree of corrosive attack. Structural stability is provided by L-bar bracing that also serves to break up surface scum when the drum is rotated. A guide frame stabilises the gas drum and prevents it from tilting and rubbing against the masonry.
The two equally suitable and most frequently used types are:
- an internal rod & pipe guide with a fixed (concrete-embedded) cross pole (an advantageous configuration in connection with an internal gas outlet);
- external guide frame supported on three wooden or steel legs.
For either design, substantial force can be necessary to rotate the drum, especially if it is stuck in a heavy layer of floating scum. All grades of steel normally used for gas-holders are susceptible to moisture-induced rusting both in and outside. Consequently, a long service life requires proper surface protection with a corrosion protection paint.
Materials regarded as suitable alternatives to standard grades of steel are galvanised sheet metal, plastics (glass-fibre reinforced plastic (GRP), plastic sheeting) and ferro-cement with a gas-tight lining.
A fixed-dome gas-holder can be either the upper part of a hemispherical digester (CAMARTEC design) or a conical top of a cylindrical digester (e.g. Chinese fixed-dome plant). In a fixed-dome plant the gas collecting in the upper part of the dome displaces a corresponding volume of digested slurry.
Types of Gasholders for Biogas Plants – Plastic Membrane Balloon or Bag Type
Gas-holders made of plastic membrane sheets welded together serve as integrated gas-holders, and in stand-alone installations on their own ground slabs.
For plastic (sheet) gas-holders, the structural details are of less immediate interest than the question of which materials can be used.
Differentiation is made between:
- low-pressure, wet and dry gas-holders (10-50 mbar). Basically, these gas-holders are identical to integrated and/or plastic (sheet) gas-holders. Separate gas-holders cost more and are only worthwhile in case of substantial distances (at least 50-100 m) or to allow repair of a leaky fixed-dome plant. This type of separate gas-holder is also used to buffer extreme differences between gas-production and gas-use patterns.
- medium or high-pressure gas-holders (8-10 bar / 200 bar).
Plastic membrane biogas holder design is a proven technique which can work very well, and that has been proven by it's choice in so many anaerobic digestion facility designs. via Gasholders for Biogas Plants – energypedia.info
Now let us look at the manufacturers and suppliers which Design and Build digester gas holders, and what they have to say about their gas holding systems:
Dystor® Double Membrane Digester Gas Holder Systems
The Dystor® biogas holder design uses a dome-shaped, engineered membrane system to store biogas, and prevent odours.
The Dystor system’s outer membrane remains inflated in a fixed position. The inner membrane moves freely as it stores or releases biogas generated from the anaerobic digestion process. This system can hold up to six times as much gas as conventional steel digester covers, maximising the energy benefits of the process and providing cost savings.
DMG – Biogas Storage – Double Membrane Biogas Holder by Bilgeri EnvironTec GmbH
Each store is individually calculated in terms of the loads (internal pressure, wind and snow loads). As a result, in accordance with good biogas holder design methods, membranes are selected with high tensile strength.
The PVC-coated membranes are hardly combustible according to DIN 4102 B1, fungicidal treatment and equipped with a UV protection. To protect the membranes from the substances contained in the biogas (CH4, CO2, H2S, etc.), the interior and bottom membranes have a special coating.
In [this biogas holder design,] the supporting air blower provides the necessary pressure of the gas reservoir to absorb the external forces such as snow and wind load and to generate the form on the gas system. A separate air hose connects the fan to the memory.
In this biogas holder design, the check valve prevents air leakage in case of failure of the blower. For safety, a pressure control valve is installed in the air system.
This valve controls the operating pressure and closes automatically at a minimum required pressure (static stability, failure of the fan) in the outer membrane. By this device, the membrane remains stable at existing gas production. A [digester substrate level] level measurement sensor enables an optimum utilisation of the storage volume, and control of the slave devices (e.g. flare, motor, burner, etc.). via DMG – Double Membrane Holder
Tecon Double Membrane Biogas Storage
Custom-Made Biogas Storage For Wastewater Treatment Plants and Energy Recovery Facilities — The TECON gasholder may be mounted on the ground typically in the form of a ¾ sphere, or mounted on the top of an anaerobic digester or tank functioning as a durable tank cover and storage vessel.
The double membrane gasholder consists of an outer and inner membrane.
The outer membrane is the protection layer and is always under a pressure to withstand wind and snow loads, no matter climate extremes.
The inner membrane expands and contracts beneath the outer membrane according to gas generation and usage.
If the gasholder is ground-mounted on a concrete foundation, there is a third, bottom membrane which seals the gas storage to the foundation.
Every gas holder is designed in conformance with the strictest European and U.S. standards (e.g. materials of construction, structural design, storage capacity, snow load, wind load, operating pressure fire resistance).
Also all membrane gasholders are custom designed and manufactured in accordance with the specific project requirements for size, configuration (e.g. dome, rectangular, square, donut, ¾ sphere) and strength using high-quality PVC coated polyester fabrics. Product design and manufacturing principles place a priority on system safety, reliability and control simplicity. via TECON – Biogas Storage System – Treatment Solutions …
Biogas Storage Tanks from Sattler BioGas
Sattler Double Membrane Gas Storage Tank Design Principles
The SATTLER double membrane gas storage tank consists of an external membrane which forms the outer shape of the tank, as well as an internal membrane and a bottom membrane which make up the actual gas space.
A permanently running support air blower provides air to the space between inner and outer membrane, and thus keeps the gas pressure up at a constant level – irrespective of gas supply and gas withdrawal.
The pressurised air keeps the outer membrane in shape to withstand external wind and snow loads.
It exerts a constant pressure on the inner membrane and thus pushes gas at constant volume and pressure into the outlet pipe. The gas charge and discharge pipes are cast into the concrete foundations that are usually supplied by the customer. All three membranes are then clamped to the foundation by means of an anchor ring.
The safety valve prevents the gas holder from over-pressure on the gas-side. To maintain a constant pressure within the gas holder, a pressure control valve is mounted on the air-side. Filling levels are measured by means of ultrasonic and wire length measurement systems.
As the leading supplier of such biogas holder storage systems, we keep the value chain as long as possible in our own company. Thus their own weaving and coating units [are stated by them to] ensure outstanding quality for the supply of the base material, PVC-coated polyester fabrics.
Picking the perfect storage tank for every challenge
WHAT VOLUME OF BIOGAS STORAGE SHOULD I CHOOSE?
Storage tank volumes (biogas holder vessel volumes) are designed to compensate for fluctuations in biogas production and consumption, and for volume changes due to varying temperatures and for interruptions to biogas consumption.
With these parameters the optimal dimension can be determined. The ideal storage volume varies according to the plant design, the substrate mixture, and the management of the plant.
BASE LOAD OPERATION
The goal is to enable consumers to operate at high capacity. Consumers’ performance should neither be influenced nor determined by fluctuating gas production, but should constantly operate at full load. Flaring excess gas should be avoided. Therefore excess gas production is buffer stored and lack of gas production is replaced by the stored gas. Conventional storage tanks hold volumes corresponding to a gas production of three to four hours.
The storage tank serves to balance out the continuous gas production and the discontinuous consumption. Low gas production will be stored over a longer period of time, followed by a shorter and higher-powered consumption period. That is why bigger storage tanks are required.
WHICH PRESSURE DO I CHOOSE?
The operating pressure is predetermined by the plant design and the dimensioning of the various components. We are talking about low-pressure systems, starting from zero pressure to 50 mbar. The operating pressure influences the dimensioning of the entire gas line. Among them are: gas ducts, safety devices for protection against over-pressure, condensate traps, flares, devices for gas purification, and gas pressure intensifiers.
WHICH EXTERNAL LOADS DOES A STORAGE TANK HAVE TO WITHSTAND?
Depending on its location, wind load, temperature and sun radiation all have an impact on the gas holder. The impact of snow loads depends both on the location and operating conditions since waste heat from the digester results in de-icing in regular operation. When not in operation, the digester does not produce any waste heat. In this case the full snow load must be taken into account.
UP TO WHICH TANK DIAMETER DO WE BUILD GAS STORAGE TANKS?
Textile gas storage tanks and coverings can be built to very large dimensions. The tank diameter basically does not pose any limitation as to the choice of a suitable gas storage tank.
WHICH KIND OF AGITATOR TECHNOLOGY CAN BE USED?
There is a suitable biogas holder based storage system for each type of stirring device. Where submersible mixers are installed, these will usually be accessible via service manholes or service shafts in the membrane. It is not [in normal circumstances] necessary to remove membranes for service purposes.
WHICH COSTS DO I HAVE TO TAKE INTO ACCOUNT?
In terms of economic evaluation of a storage system, not only the initial investment and the running costs play a major role, but also the replacement investment and opportunity costs need to be considered. Opportunity costs arise from lesser yields in power and heat generation and from a higher use of substrate in less suitable storage systems.
The highest costs arise when the CHP (combined heat & power unit, including the biogas generator) is not able to operate at full capacity. These opportunity costs can exceed the initial costs of the biogas storage tank after only a few years of operation.
More About Sattler/Ceno DMGS TM
Both membranes are clamped to the crown of the storage tank or anchored to the external wall of steel or concrete tanks.
A supporting structure prevents the inner membrane from immersing into the substrate and being damaged by the stirring device. The safety valves protect the gas holders against over- or low gas pressure. The pressure regulation is carried out by an airside pressure control valve.
Filling levels are measured by means of hydraulic and wire length measurement systems. Where plants are designed with more than one storage tank, you can combine Sattler double membrane gas storage tanks and single membrane gastight covers.
The intention is that variation in the combination of storage tanks and covers, and the numbers sold, should offer an ideal storage volume, at lowest total costs.
JDV Double Membrane Biogas Holder
The unique design allows for variable biogas storage within the inner membrane at constant pressure during gas production and utilisation , while the air inflated outer membrane provides gas pressure and protection. The outer membrane is constructed of a high tech cross woven fabric, coated with PVC and Ultraviolet (UV) ray protection, which has proven to endure the weather elements.
An ultra sonic sensor is positioned at the centre of the sphere to monitor the volume of gas present. This gives the operators full control of optimising the utilisation of biogas to feed generators and/or heating systems.
Storing digester biogas and eliminating or reducing ‘flaming' from the digester(s), can help your facility reduce or completely eliminate the need for electric grid power when generators and/or hot water boilers are incorporated into the system/facility design.
DuoSphere Double Membrane Gas Holder for Biogas Storage
Over the years, WesTech has supplied customers with anaerobic digestion equipment that has added value to their plant operations. WesTech provides the DuoSphere™ Dual Membrane Gasholder biogas holder design.
The DuoSphere™ offers innovative designs for digester gas storage and alternatives for conventional digester covers.
By storing biogas, the plants are able to have a constant feed of gas to their generator.
DuoSphere™ Gas Holders
Slab-mounted dual membranes provide convenient digester gas storage for your plant operation needs.
- Simple gas storage capability
- Mounts to flat concrete slab
- Constant pressure regulation
- Inner membrane access
- Custom flow and sizes
Tank-mounted digester covers provide an innovative and cost-effective solution for new and replacement covers, in this biogas holder design.
- Lower capital and installation costs
- Absolute odor containment
- New or existing digesters
- Variable liquid level
Biogas Holder Design Features
- Critical inner gas holding membrane is independent of the outer membrane
- Membranes are manufactured from the highest quality materials available
- Access is available to inspect the inner membrane at all times
- Airflow between the membranes is accurately controlled
- Gas detection equipment monitors the air between the membranes
- Spark-resistant and explosion-proof pressurization fans
- Complete service from design to fabrication to installation and commissioning
- Fast installation (two to five days)
- Gasholder volumes from 700 ft3 to 176,000 ft3 (20m3 to 5000m3)
- Volume is measured ultrasonically. via DuoSphere Holder
BIODOME® Double Membrane Gas Holder
This was one of the first designs installed in the UK. Other AD plants now also have digester tanks incorporating the now industry-leading BIODOME® Double Membrane Gas Holder roof biogas holder design to store the biogas produced during the treatment of their food waste, ready for conversion to renewable electricity. via ANAEROBIC DIGESTION « KIRK GROUP BLOG
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