Providing biogas holders for anaerobic digestion plants is a growing business sector. Every biogas plant has a gas holder in some form. In fact you can very often spot an Anaerobic Digestion Plant by the presence of its spherical dome, or cylinder and these can at times be very striking architectural features.
But how much do most people know about biogas holders? I would wager that even those involved in biogas companies often don’t realise what technology sits beneath those futuristic shapes, hence, the name of this article to emphasise that the best of those domes are packaged with ingenious devices to carry out their hidden function.
So, what is their hidden function?
Watch the video below and scroll down and read the article that follows and you will discover the hidden function of biogas holders.
That main function of biogas holders is to control the gas, both upstream and downstream, by applying an adequate driving force (pressure) to carry the biogas around the system at the required pressure and flow rate.
It is also, crucially, the device that provides the signals to equipment, to turn them on or off, and to raise signals or alarms. In this sense, the best biogas holders are more of a process instrument than a storage tank. That is because the biogas holder is the controlling system which governs the satisfactory operation of engines, boilers and/ or waste gas burners.
Traditionally, the materials used to construct gas storage vessels, or biogas holders, have been steel and concrete. Those have for many years been the two main types of gas holder which used to be seen on sewage treatment works AD Plants. The main types in those days were:
There are other types which were sometimes used, such as dry-seal gas holders, with a membrane giving a seal between an upper biogas holding tank and a lower tank structure. But these were usually found only in the biggest sewage works.
As biogas technology has developed it has become more essential that gas storage is reliable, and low maintenance requirements are even more of an issue. The economic value and reliability of biogas holders is much more important now, as the value of the biogas itself rises.
Most of all has been the realisation that despite the effectiveness, in many applications, of anti-corrosion coating technology, one of the big disadvantages of using steel vessels for holding biogas is corrosion. Raw biogas is a unfortunately, a corrosive gas, containing varying amounts of hydrogen sulphide (H2S).
When the H2S in the biogas chemically combines with the water in the saturated gas, it produces weak sulphuric acid, which is particularly aggressive to steel. This can be overcome by coating the surfaces of the steel that are in contact with the gas with, for example, epoxy resin paints, which can be very effective. But, this is achieved at a high cost, and the results are never certain. Coatings do eventually become chipped, or become detached from the steel.
The modern double membrane biogas holder was developed in the s, using fabrics that were developed in the architectural industry for canopies and other innovative roofs. The breakthrough came when it was realised that different fabrics could be combined in one membrane, and the development of dense PVC coatings also helped.
The development of dense PVC brought the solution to the inconvenient feature of fabrics, which was that they had previously been too gas-permeable. Now combining materials produces membranes which are both effective barriers to the permeation of gas, and also very strong and long-lasting, with very low maintenance costs.
These materials are polyester-based fabrics, with very high tensile strengths, coated with specialised PVC materials.
The interesting feature of a double-membrane biogas holder, such as the designs offered by gas holder manufacturer Vergas, is that the outer membrane, which is inflated by air, acts as the solid support structure.
It is the outer membrane which resists wind and snow loads, and a separate inner membrane, which actually provides the seal and support for the biogas inside.
So, in effect the structure comprises a gas-holding membrane within an air-supported outer shell. The materials are resistant to the acidic and unscrubbed gas; and are actually less expensive than steel. They are quick to manufacture and install and, more as maintenance-free as any structure can be.
The structure actually is a live structure as well though. Small fans produce an air pressure to keep the structure inflated to resist all climatic conditions, and that same air pressure is the driving force on the inner membrane, providing the motive force to push gas round the biogas system.
The operator of the plant can also make safe entry within the gas holders provided by Vergas. The company reports that it has made a point of giving the operator an easier life. Its range of gas holders provides a full sized man-access through the outer membrane into the space between the two membranes.
This means that suitably trained operators can inspect the gas holder without taking it out of service.
In addition, the availability of telemetry signals from an ultrasonic level detection system that detects the pressure level of the inner membrane, means that this modern biogas holder design gives the operator a system that will give trouble free service for very many years. The system includes a pressure level stabilisation system to control the air pump system to keep the biogas pressure inside constant while changing the air volume in the outer membrane area, and ensuring that the cover on top of the digester, or around the free-standing type, is always kept taught.
Vergas has reported that it has supplied biogas holders to many UK and overseas projects, including those for the Anglian Water Biosolids Programme at Cotton Valley, Kings Lynn, and Great Billing.
KIRK ENVIRONMENTAL, part of KIRK GROUP, was awarded a contract back in for one of the first commercial biogas plants in the UK which was the start of a 6 year relationship with the customer and two extensions to the initial plant.
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Cannington Cold Stores is a family business based in Somerset, UK, which began storing vegetables for local companies. The business then expanded into providing washed, peeled, prepared and packaged products to UK’s leading supermarkets and today they can offer chilled, frozen and ambient temperature storage facilities with a capacity in excess of 10,000 pallets.
In early the business were facing increasing power utility bills and decided to carry out some exploratory research into creating renewable energy from food waste. Cannington Bio-Energy was established to fulfil their corporate responsibility by producing an affordable renewable energy solution for their cold storage plant through the use of Anaerobic Digestion and waste management.
KIRK ENVIRONEMNTAL were initially approached to design, supply and construct two Anaerobic Digestion Tanks and two Pasteurisation Tanks. This was one of the first plants in the UK to also have the digester tanks incorporating with the now industry-leading BIODOME® Double Membrane Gas Holder roof to store the biogas produced during the treatment of the food waste ready for conversion to renewable electricity.
The initial project was a great success, with enough energy being produced to not only power the cold storage plant but also to create extra revenue for the customer by being able to release the surplus energy back into the National Grid. This resulted in an extension to the plant in with a further Digester Tank and Pasteurisation Tank being added by KIRK. Now in KIRK are currently working on a second extension to the site which will see the addition of 3 more Digester Tanks and four Pasteurisers, all is on track for completion in summer bringing the total digestion volume on site to an incredible 18,000m³ with a generating capacity in excess of 3.3 MW
Typically food waste, when disposed of to landfill, can generate large quantities of methane gas, contributing to global warming as greenhouse gas. By treating the food waste in the digesters and capturing the biogas in the BIODOME® roofs, Cannington Bio-Energy are utilising the food waste to create renewable energy and doing their part in helping protect the environment. They process a wide range of different wood waste including liquids, plant material and animal by-product, also taking that extra step by separating packaging from the waste and recycling as much as possible. After their successful experience with anaerobic digestion and recycling, Cannington Bio-Energy now also work closely with local authorities, contractors and businesses providing them with valuable advice on how to process and treat their waste.
“We have used Kirk for all three phases of the development of our plant,” explains Tim Roe from Cannington Cold Stores.
“Ever since our first contact with them they have been both professional and flexible to their approach for providing the required tanks. The entire tank procurement and erection process has been relatively straight forward and hassle free.”
Each Digester Tank was constructed from Glass-Fused-to-Steel panels, a well-established and proven tank finish which leads the market with quick installation times, low maintenance and long project asset life. The high temperature fusion of glass to steel fired at 850°C results in an inert, durable finish. Applied to both interior and exterior, this technology is able to withstand the rigorous of a construction site and provide many years of trouble free service in harsh environments.
The award-winning BIODOME® Double Membrane gas holder is a state-of the-art construction with utilises the highest strength membrane available in the global market making it suitable for a wide range of working gas pressures, storage volumes and gas production rates. Produced in the UK with the latest machinery and production techniques enables KIRK to deliver better production time scales and quality controls, always providing the best possible service to customers in the Wastewater and Bioenergy markets throughout the world.
By installing the gas holder on top of the digester rather than having it as a stand-alone unit provided Cannington Cold Stores with some major benefits never before experienced in the UK. A ground-mounted stand-alone gas holder would require a concrete base to be built on and additional pipework to connect it to the digester tank which would have needed to be constructed with a steel roof. All of these extra requirements would have had a significant impact on the capital cost of the project. A top-mounted gas holder also results in further reductions to a project foot-prints and a lower maintenance regime. The outer membrane of the gas holder is constantly inflated by utilising electrically operated blowers. When gas production exceeds the rate of use, the internal membrane rises and vice versa, when the gas usage exceeds the rate of production, the internal membrane falls, with the air pressure inside the air compartment maintaining the system pressure.
“We are delighted to have involved in the development of the Cannington Bio-Energy facility over the last few years”, comments Andrew Peace, Managing Director at KIRK GROUP.
“Our team has worked closely with the customer to understand their plans and technical requirements and to ensure that the items of the plant provided by KIRK were delivered on time and within budget. Hopefully we will continue our involvement for many years to come and support Cannington Cold Stores through any future stages of development and upgrading”.
In BIODOME® won the Environmental Technology Business of the Year in the Red Rose Awards, recognising the portfolio of completed projects and the amazing statistics of carbon footprints reduction and green energy generation at a large number of plants across the UK. Joining this award was also the title of Environmental Innovation at the Lancashire Business Environment Awards, highlighting new technologies and services that other organisations would benefit from. The product needed to be tried and tested to enter the awards with quantified benefits to business practices, environmental impact and sustainability. In the BIODOME® product saw some innovative advancements in product specifications and greater safety control levels; strategic partnerships developments and considerable investments in research & development resulted in a change to the equipment offered to both gas pressure relief and air envelope regulation. These improvements, together with the manufacture of the product based in the UK, places the product as the industry-leader with proven results and advantages.
“It is great a great honour to be recognised in our local community for the work we do”, comments Bill Leach, CEO of KIRK GROUP.
“We are always reviewing our ever growing product range, always providing excellence in liquid and biogas storage solutions. Our knowledge, expertise, product quality and customer service are the foundations to the great success at the Cannington Cold Stores Anaerobic Digestion Plant.”
KIRK always aims to deliver industry leading customer service and product quality with every project: we plan, organise, direct and control all resources required to deliver the work on time, working closely with the client and other key suppliers to form a project team that will ensure all interface issues are managed throughout project implementation, thus providing a smooth transition through the varying scope of work. KIRK has invested considerable amount of time and resources into systems and procedures that allow the highest certification and accreditations in the industry to be gained and maintained. These procedures and certifications ensure that our management systems and project performance will satisfy the expectations of any client.
Taking a leading stand at this year’s UK AD & Biogas Exhibition at the NEC Birmingham, KIRK ENVIRONMENTAL is currently the only UK Company in a position to offer an unbiased opinion on the subject of tank selection thanks to an industry-leading product portfolio which includes: glass-fused-to-steel tanks, epoxy coated steel tanks, poured in situ concrete tanks, pre-cast concrete tanks, biogas storage and in-house manufactured steel fabrication.
Anaerobic Digestion Tanks are the most noticeable part of any Anaerobic Digestion plant and in most cases this is where all the action takes place, getting the right base material, coating type, size and design of the tank is a vital part of any AD project.
Tanks come in a variety of sizes which will vary depending on the expected throughput of the proposed plant and the material (feedstock) it is treating which can include industrial by-products, food waste, slurry and purpose-grown crops such as maize; the process technology provider will determine the preferred dimensions of the tank to achieve the best process conditions but then the type of tank needs to be selected.
The main basic materials for tanks found in the UK market are either concrete or steel; however after that initial material selection there are then a number of additional options which all have their place in the market but need careful consideration to achieve the right balance of being suitable for duty/application, initial capital cost, ongoing maintenance costs and any allowance for how your plant might be operated in the future. With concrete tanks you have the option to construct on site or to precast and with steel you have the options of glass-fused-to-steel, epoxy coated steel, stainless steel or a combination (hybrid) tank to suit different operational conditions in the liquid and gas phases of the digester.
So, which is best? There is no clear-cut right or wrong answer as there are so many variables that can influence selection criteria to each AD plant. This is why UK based KIRK ENVIRONMENTAL have put together a portfolio of products that covers all of the above options and allows KIRK to offer a neutral and unbiased opinion on the subject of tank selection. ‘When it comes to tank selection KIRK are in a position where we can sit with any client and offer independent, impartial advice on product selection for their AD Plant. Very often we find that a combination of all the above tank material types gives the client the best commercial and asset life advantages’ says Andrew Peace, Managing Director of KIRK GROUP. ‘We see plants being built with Coated Steel Digestion Tanks & Stainless Steel Roofs, Concrete Digestate Tanks and BIODOME Dual Membrane Gas Holders, all of which are an essential part of any AD plant but not necessary all suited to one specific material type’
Another key part to consider on the tank is the roof. Again there are many influencing factors here and many different options including Flexible PVC Membranes as an additional option to the steel and concrete options available for tanks. Factors such as pressure, mixing types, access and the kind of waste being digested have a major influence on roof selection as this top part of the tank has the most aggressive operating environment containing the biogas which can have a very corrosive composition. Andrew Peace comments ‘the selection criteria for roofs is much more prescriptive than with tanks as there are limiting factors with some roofs that automatically lead you to a certain material type, however there are some areas where many different roof types are suitable and it is important to understand the merits of each system before making your selection’
KIRK ENVIRONMENTAL provides liquid and biogas storage solutions with all ancillary equipment as a specialist design and build contractor for the water, waste and bioenergy sector.
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