FQA
1. Analysis of the Working Principle of Double Membrane Gas Holders
The double membrane gas holder is typically designed in a 3/4 sphere or 1/2 sphere shape and is fixed onto a concrete foundation or the top of an anaerobic digester tank by clamping plates.
The membrane material is mainly made of PVDF (Polyvinylidene Fluoride), which offers excellent performance in:
- Corrosion resistance
- Anti-aging
- Resistance to microbial degradation
- UV resistance
The material also achieves a B1 fire protection rating, and its tensile strength exceeds 4000N/5cm.
The inner membrane of the gas holder is specially designed for corrosion resistance. Depending on the origin and quality of the membrane material, the service life can reach up to 15 years.
Application scope:
Suitable for storing various neutral gases such as:
- Biogas
- Air
- Carbon dioxide
- Oxygen
- Biomass gas / straw gas, etc.
2. Material Selection for Double Membrane Gas Holders
The membrane material used for double membrane gas holders should be a corrosion-resistant composite material specially designed for environmental protection applications.
The material is mainly composed of:
- High-strength tensile fibers
- Gas-tight anti-corrosion coating
- Surface protective coating
It provides excellent performance in:
- Corrosion resistance
- Anti-aging
- Resistance to microorganisms
- UV resistance
The fire protection level should meet at least Class B standards.
Main Reasons Why the Material Turns Red or Yellow
1. Unsuitable Materials Were Used for Biogas Applications
The membrane material is generally made of high-strength polyester fiber composite material, which has many applications such as:
- Architectural membrane structures
- Truck tarpaulins
- Advertising fabrics, etc.
However, only specially treated anti-corrosion materials are suitable for biogas environments.
To achieve proper corrosion resistance, expensive specialized materials must be used, which increases the overall cost.
Some low-quality manufacturers use non-corrosion-resistant materials in order to maximize profits.
This situation is similar to the difference between carbon steel and stainless steel — the key issue is the material quality rather than the country of origin. Even some imported materials may have the same problem.
2. The Outer Membrane Was Corroded Due to Excessive Biogas Exposure
This type of material is usually used as the outer membrane.
In theory, the outer membrane should not come into contact with biogas. However, in actual operation, this is not always the case.
If the manufacturer’s production process is not sufficiently precise, or installation problems occur, biogas concentration inside the outer membrane space may exceed acceptable levels.
Once this happens, the outer membrane may begin to corrode, eventually causing discoloration such as yellowing or reddening.
3. How Are Anti-Freezing and Anti-Leakage Measures Applied in Double Membrane Gas Holders?
A double membrane gas holder adopts a double-layer hollow structure with an independent air space between the two membranes. This air layer provides excellent thermal insulation performance and greatly reduces condensation and water dripping inside the system.
The structure not only improves light transmission and reduces internal humidity and related corrosion risks, but also lowers thermal conductivity and increases thermal resistance. As a result, it provides both heat insulation and heat retention capabilities while helping stabilize internal temperature conditions.
The membrane material is formulated with anti-aging additives, giving it a long service life. Reinforcement structures are integrated inside the system, allowing it to withstand strong winds and snow loads.
Since double membrane gas holders are exposed to various outdoor climate conditions for long periods, several important issues must be considered in outdoor applications.
1. Anti-Freezing Measures
This issue mainly concerns cold northern regions.
Unlike traditional water-sealed gas holders, double membrane gas holders do not contain water inside the structure, so freezing problems are greatly reduced.
As a result, there is generally no need to worry about ice formation during winter operation.
2. Resistance to External Environmental Effects
The gas holder is continuously exposed to external environmental factors such as:
- Wind
- Sunlight and UV radiation
- Snow loads
- Rain and moisture
These conditions mainly test the performance and durability of the membrane materials.
The inner membrane, outer membrane, and bottom membrane should all use corrosion-resistant materials. Only membranes with strong anti-corrosion and acid-resistant properties can ensure long-term stable operation of the gas holder.
3. Gas Tightness and Anti-Leakage Performance
Since the membrane structure is formed by welding multiple membrane panels together, the quality of the welding seams and clamping system is extremely important.
Proper welding and sealing processes ensure that there are no leakage gaps between membrane sections, maintaining excellent gas tightness throughout the system.
The inner membrane and bottom membrane of the double membrane gas holder are usually made of double-sided PVC/PVDF special membrane materials designed specifically for biogas applications.
These materials offer:
- Aging resistance
- Wear resistance
- Folding resistance
- Methane permeation resistance
The inner membrane and bottom membrane form a variable-capacity airtight chamber for biogas storage. Meanwhile, a blower continuously supplies air between the outer membrane and inner membrane to maintain constant storage pressure.
Installation is both time-saving and labor-saving.
Because the system is lightweight and compact, most membrane fabrication is completed in the factory. On-site work mainly involves connecting a small number of joints and auxiliary equipment.
In most cases, on-site installation can be completed within 1–2 days.
Since there is no water inside the structure, freezing problems in cold regions are generally not a concern.
4. What Size of Biogas Storage Tank Should I Choose?
The storage capacity of a biogas holder is mainly designed to handle fluctuations in biogas production and gas consumption. It should also take temperature changes and possible equipment shutdowns into consideration.
The appropriate storage volume should be determined based on factors such as:
- Overall biogas plant design
- Type and composition of feedstock
- Project operation and management methods
The main goal is to ensure that downstream gas-consuming equipment can continue operating steadily at high capacity without being affected by fluctuations in biogas production.
5.How Is Storage Capacity Determined for Base Load Operation?
Under base load operating conditions, the main functions of the gas holder are:
- Buffering excess biogas production
- Providing reserve gas when production temporarily decreases
In most cases, the storage volume is designed based on approximately 3–4 hours of biogas production capacity to ensure continuous and stable operation of downstream equipment.
6.What Should Be Considered Under Peak Load Conditions?
Under peak load conditions, the biogas holder must balance:
- Continuous and stable biogas production
- Intermittent high gas consumption demand
This means the storage system should be capable of:
- Storing gas generated during low-demand periods for longer durations
- Releasing large amounts of gas quickly during short-term peak demand periods
Therefore, projects with peak load requirements usually require larger biogas storage capacities.
7.What Operating Pressure Should Be Selected for a Gas Holder?
The common operating pressure range for gas holders is usually:
0–50 mbar
The specific operating pressure should be determined based on factors such as:
- Process design
- Pipeline dimensions
- Safety devices
- Condensate drainage systems
- Flare systems
- Biogas upgrading equipment
- Pressure boosting equipment
The operating pressure will also directly affect the sizing and equipment selection of the entire gas handling system.
8.What External Loads Must the Outer Membrane of a Double Membrane Gas Holder Withstand?
Wind Load
This depends on the installation location of the gas holder and local wind conditions.
Temperature and Solar Radiation
Different climate conditions and sunlight exposure levels may affect the performance of the membrane material.
Snow Load
Snow load depends on local snowfall conditions and system operating conditions.
During normal operation, the waste heat generated by the digester can usually help remove snow and prevent ice accumulation on the membrane surface.
8.Up to What Diameter Can Gas Holders Be Built?
Membrane gas holders and membrane roof structures can be designed in very large dimensions and are generally not significantly limited by tank diameter.
In addition, most gas holder suppliers can reserve customized openings according to project requirements.
9.What Costs Should Be Considered When Selecting a Biogas Storage System?
When choosing a biogas storage system, it is important to consider not only:
- Initial investment cost
- Daily operating cost
but also:
- Future replacement cost
- Downtime losses
- Opportunity costs
Opportunity costs mainly refer to hidden losses caused by system mismatch, such as:
- Reduced power generation efficiency
- Lower heat utilization efficiency
- Increased feedstock consumption
In many cases, if the CHP (Combined Heat and Power) system cannot operate at full capacity, the accumulated losses over several years may even exceed the original equipment investment.
10.What Flexible Membrane Materials Are Commonly Used for Biogas Holders?
Biogas holders are typically manufactured using high-quality flexible membrane materials that must provide:
- Excellent durability
- Strong chemical corrosion resistance
- Superior gas tightness
Common membrane materials include:
- PVC (Polyvinyl Chloride)
- EVA (Ethylene Vinyl Acetate)
- Polyurethane (PU)
In addition, membrane materials are usually enhanced with:
- UV stabilizers
- Anti-aging additives
- Special protective coatings
to improve service life and operational performance under different environmental conditions.
11.What is biogas?
Biogas is a by-product generated during the decomposition of any kind of organic matter by means of anaerobic bacteria. Biogas is mostly of 50-75% methane, 25-40% CO2 and other traces of moisture, H2S, Nitrogen, Oxygen & Hydrogen. Biogas is a clean and renewable energy source which substitutes LPG and CNG for cooking, steam generation, transportation fuel, electricity generation, etc., At room pressure and temperature biogas is in gaseous form, not liquid like LPG (propane).
12.What type of waste produces biogas?
Biogas can be produced by the decomposition of any kind of organic waste. Few examples are as mentioned below. 1. Kitchen Waste like food waste, Fruit and Vegetable waste, etc., 2. Farm manure like Cow dung, Poultry litter, Horse dung, Pig manure, etc. 3. Industrial Waste like Food processing industrial waste, Meat packing waste, Fish packaging industry waste, Dairy industry waste, Tapioca industrial waste, Sugar factory wastes like Press Mud, Bagasse wash water & Molasses, Brewery & Distillery waste, etc. 4. Agricultural Waste like Green Grass, Corn Waste, Harvest residues, Whole crop fodder, etc.,
13.What is a Bio-digester?
A Bio-digester is a sealed tank or container specifically designed to provide anaerobic conditions, allowing organic matter to decompose and produce biogas.
14.How does a digester work?
Organic waste is put into a sealed tank known as a digester or bioreactor.
In an anaerobic environment, anaerobic bacteria decompose and consume organic matter, producing biogas during their reproduction process.
15.How much energy is contained in biogas?
Each cubic meter (m³) of biogas contains approximately the same amount of thermal energy as 6 kWh (kilowatt-hours).
However, when biogas is converted into electrical energy through a biogas generator, it typically produces only about 2 kWh of usable electricity, with the remaining energy being converted into heat. This heat can also be utilized for purposes such as heating.
2 kWh of electric energy is sufficient for:
Let a 100W bulb work continuously for 20 hours;
Or let a 2000W hair dryer operate for 1 hour.
16.How much biogas can I get out of my kitchen waste?
The amount of biogas you can extract from your organic waste depends on the quantity of waste. Around 6-10 kg of food & vegetable waste can generate 1 m3 of biogas.
17.What happens to waste after digestion?
Although many people believe that the amount of waste entering the digestion tank is nearly the same as the amount discharged, in reality, the quality of the waste has been significantly improved, for example:
Less odor, better fertilizer efficiency, reduced organic load, and less environmental pollution.
The substance discharged from the digestion tank (also known as biogas slurry or Bio-slurry) can undergo solid-liquid separation:
The solid part can be further composted; the liquid part can be used as liquid fertilizer or discharged after further treatment.
In addition, biogas slurry has various applications, such as:
Aquaculture serves as a feed additive for pigs, fish, and poultry.
18.How much biogas can I get out of my cow’s dung?
1 m3 of biogas can be generated from 20-25 kg of cow dung.
19.How long will 1m3 biogas plant burns equivalent to LPG?
1 m3 of biogas is equivalent to 0.5 kg of LPG which will continuously burn in a single biogas burner for around 2 hrs. 1m3 biogas plant replaces 1 domestic LPG cylinder per month.
20.How can the compost be used effectively?
Waste coming out of the digester can be directly used as organic fertilizer (after mixing with water in the ratio of 1:10) in Gardens & Farms.
21.What happens if I am not using the generated biogas as I am out of station for a long period?
The generated biogas will be stored in the gas holder fitted along with the bio digester. As you are outstation you will not feed it & so, no more excess gas will be produced.
22.What will happen, if I am feeding the digester continuously without using the Gas?
The gas generated will get stored in the Gas holder and the excess gas generated will escape out through the water jacket portion of the biogas plant easily.
23.Is there any chance of Explosion, if the Biogas leaks?
Not at all. The Biogas is highly safer than LPG. Density of LPG is heavier than air. When LPG is leaked, it flows for long distances along the ground (Downward direction), and can collect in drains, gullies and cellars which causes explosion. Unlike LPG, the density of biogas is lighter than air. When Leaked, biogas moves in the upward direction in air and easily mixes in the atmosphere.
24.Where can we place the biogas plant?
Biogas Plant can be placed anywhere.Partially below the ground level.Above the ground level.In terrace It is recommended to keep the biogas plant in open place exposed to sunlight.