At restaurants, people often order food based on how hungry they feel. The eyes are often larger than the stomach, so to get the biggest bang per buck, leftovers are boxed up and brought home for later consumption. While that takeout box serves as the buffer between overeating and a loss on already purchased food, the same concept applies to biogas storage options, which help balance the amount of gas generated versus used. “In the biogas plant industry, if you’re trying to generate electricity, you often need to equalize the biogas flow to the generating system,” says Darin Evans, vice president of product management with Geomembrane Technologies Inc. “They need to keep the engines running at a constant speed, so if there are variances in the digestion and creation of biogas, they need a way to attenuate the flow because the generator wants a steady, constant feed of biogas.” Biogas is mainly stored for later on-site usage, or stored before or after transport to off-site distribution points or systems. Still, in practice, biogas is used as it is produced, making the need for biogas storage—like leftovers—usually temporary, at times when production exceeds consumption or during maintenance of digester equipment. “Long-term biogas storage from a biogas plant is just not realistic,” says Pat Howell, global business development director of bioenergy with CST Industries Inc. “You want to move that gas as fast as you can, because it will go bad. You also want to move it to receive the revenues. It’s really, produce it, move it on and get paid for what you produce.” The added storage component at biogas plants can save producers, in the case of operational flux. “Gas storage tanks are designed to compensate for fluctuations in production and consumption—for volume changes due to varying temperatures and, for example, stagnating consumption,” says Patrick Johnson, business development manager for Sattler AG. “For example, if the gas motor was not pulling as much gas as it would be under full power, then you need to store the extra gas that is being produced, rather than flaring it off. All of this money would just be burned up and turned into CO2 in the air, if the gas was flared. Whereas, when you have a variable-volume gasholder, you are able to store this extra gas and use it in the future when there is not as much being produced.” Capacity to compensate for generation inconsistencies can make a significant contribution to the efficiency and safety of an anaerobic digestion (AD) plant’s operation. Biogas storage options—whether integrated onto the digester itself or as a standalone unit—are making their way onto biogas project blueprints in North America. Suppliers of equipment, such as biogas holders, roofs and covers, made for biogas storage or to compensate for some storage, are working with project developers across the globe to help manage production and consumption variability. Dually Loaded Ideal gas storage volume varies based on each biogas installation design, substrate mixture and plant management. AD storage equipment design was transformed in the early 1980s with the invention of the dual-membrane concept. Austrian-based Sattler AG began developing the double-membrane gas storage (DMGS) tank in the form of a three-quarter sphere over 30 years ago. Sattler teamed up with its sister company Ceno Tec, a textile structure builder, to provide storage for biogas, substrates and fermentation residue. “Over the years, we developed the concept from the three-quarter sphere type of layout into several different types of products,” Johnson says. The traditional stand-alone, external biogas storage system Sattler provides is the DMGS tank. The general concept the company invented over 30 years ago is still the design standard used today, according to Johnson. “It has an outer membrane, which is basically the protection against the elements,” he says. “Inside this external air-inflated shell, there is an inner membrane: and a bottom membrane. These two components are combined to form the gas space of the gasholder.” Johnson adds that the same concept is carried on to the tank-mounted option with the exception of the bottom membrane. “Tank-mounted units allow the gas to come off the substrate directly, and to be collected by the inner membrane on top of the tank itself,” he says. Each of the membranes are anchored to the crown or to the outer wall of the steel or concrete tank using clamping rails. Both storage system configurations are air-supported. There is a supporting air blower that is constantly running, which keeps the outer membrane in shape to allow it to be resistant to wind and snow loading,” he says. At the same time, the supporting air blower keeps the gas contained in the inner membrane under constant pressure, called the operating pressure, which serves to recirculate the biogas stored in the plant. GTI, another company in the biogas storage space, is now an exclusive supplier of dual-membrane systems provided by VSO Biogas Technologies of France. The company supplies floating gas-collection covers to recover biogas from lagoons and tanks, and inflated cover systems, both single- and dual-membrane. If storage is the goal, dual-membrane applications work best. “The floating style covers are used over areas too large for dual-membrane systems,” Evans says. “They lay on the water surface and operate under a negative pressure. Therefore, there is no storage. Inflated single-membrane covers rely on stored biogas to remain stable, which means the biogas is not available to attenuate flows like the dual-membrane systems.” The dual membrane cover’s initial purpose is to regulate the pressure of the digester, Evans adds, and it also has the ability to store some volume of biogas based on a customer’s needs. CST is a third storage solution company that supplies aluminum domes, coated-steel roofs, membrane covers, storage tanks and more. Howell says in the past few years, the company has keyed in on large projects, like one with Harvest Power near Disney World and another at the University of California Davis. “That’s where our business has been, and that has really been driven by the market, not by our choice,” he says. Tank-mounted roofs and covers, and standalone biogas holders and tanks available for storage at small- and large-scale plants, can vary in how long they store gas. Sattler has received inquiries requiring stored gas for eight to 10 hours. “I think that is mainly because plant operators need to have a certain amount of time for maintenance, for example, on a boiler or CHP combined-heat-and-power to change oil and things like this,” Johnson says. He adds that Sattler has seen some plant designs requiring storage for the whole weekend because no gas-consuming equipment will be running. Johnson says some producers will also want to store gas during low feed-in tariff (FIT) times and supply electricity or upgraded renewable natural gas to the grid during peak-FIT times. Evans considers the ability to accumulate a volume of biogas for  burning at times of peak energy demand a storage advantage, but not all plants benefit from peak energy generation; it depends on the rate agreement with the power utility. Storage also provides an assurance in meeting offtake agreements with electricity providers. Howell says long-term biogas storage, maybe four or five days at the max, may be needed.  “Long-term is more of an insurance policy, in case they are down and need to do some repairs,” he says. State of Storage Whereas dual-membrane designs are inflated, offering a variable amount of gas storage, single-layer biogas roofs have only a static volume of gas available inside, Johnson says. Operational pressure is available for dual-membrane roofs because an air-supported system is added. Gas bags have no operational pressure inside, and although they can have variable volume, they will most likely need to be emptied via gas-suction pumps. Johnson adds that single-layer gas roofs and gas bags can serve as a buffer or means to collect gas, but they  do not provide operational pressure. The operational pressure of the gas-holder can be adjusted to meet wind and snow-loading requirements, according to Johnson. “Let’s just say, for example, there is a higher wind load requirement for a project that is on an island, then you need to have higher pressures to operate at to resist the wind loading,” he says. Sattler operates at pressures up to 50 millibar (mbar). CST’s roof and membrane options range from 10 mbar for tank-mounted, dual-membrane foil covers to 60 mbar for its pressure dome and external-supported-fixed  roof. “Whether you’re doing storage on top of a tank or on a ground-mounted storage system, whether it’s a dual membrane or a tank just storing methane, the most economical design for anybody’s tank or storage system will always be the lower-pressure range just from a structural-design standpoint,” Howell says. Biogas storage systems can store biogas in its raw state. Another way to store biogas, especially in larger volumes at higher pressures, is to treat the gas by cleaning and compressing it. Low-pressure systems, usually below 2 pounds per square inch (psi), can store raw biogas, and tend to be the least expensive and easiest to use for intermediate storage and later on-site applications. Medium- (2 to 200 psi) and high-pressure storage systems, usually stored as compressed biomethane (CBM) or liquefied biomethane (LBM) (2,000 to 5,000 psi) require some treatment of the biogas, making them costly and high-maintenance options for noncommercial use. In some instances, to prevent corrosion of the tank components and to ensure safe operation, the biogas must first be cleaned by removing hydrogen sulfide (H2S) and then slightly compressed prior to storage. “Storage is problematic unless you get all of the water and all of the H2S and, if possible, all of the CO2 out of the biogas before you compress it,” says Bernard Sheff, chairman of the board of the American Biogas Council. Biogas that has been upgraded to biomethane by removing the H2S, moisture and CO2 can be used as transportation fuel. This fuel typically exceeds immediate on-site demand, so it must be stored, in most cases, as CBM or LBM. LBM can be transported relatively easily and dispensed to either liquefied natural gas or compressed natural gas vehicles. Biomethane can be stored as CBM to save space, stored in steel cylinders. In the case of the stand-alone, foundation-mounted units, the removal of H2S is typically done before the gasholder, Johnson says. “We have built into the design of our tank-mounted units some supporting belts,” he says. “On top of these belts, we call it a sulphur net that can be installed. What happens is that hydrogen-sulfide-consuming bacteria grow in colonies. Basically, it looks like a fish net; these hydrogen-sulfide-consuming bacteria grow there on the net, and they substantially lower the amount of H2S in the biogas.” While Howell believes the H2S level is something to carefully monitor, he says storing raw biogas is usually not an issue. “Whether on top of a tank or on the ground, you design the coatings of the materials that you build the storage structure out of to handle that H2S level or that methane gas mix level,” he says. Secret is in the Sauce Nearly 75 percent of digesters built worldwide are sectional-bolted tanks either in an epoxy, stainless steel or glass coating, Howell says. He attributes this to speed of construction. To Howell, one of the most important details for a tank manufacturer or a membrane or gas holder supplier is selecting the right coating. “What I have seen in biogas plants that have had tank or membrane failures, is really attributed to whoever sold it to them originally didn’t select the right material, the right coating, to give them the longevity that they should get out of that piece of equipment,” Howell says. CST offers glass, stainless steel, epoxy, aluminum and PVC-coated polyester coating materials for its biogas roofs. The company has manufacturing plants in Illinois and Kansas that together create the panels in all options.  Sattler’s membranes are produced in-house. “We start by weaving the individual threads into a foundation fabric and determine the best type of coating to put on the fabric-base mat, and then apply the various specialized layers of coatings, so it has the qualities that we need in order to resist biogas effects and have good UV protection/resistance, flexibility and things like this,” Johnson says. “The secret is in the sauce with all of the components that make up the membrane.” If the right coating is chosen as a storage solution, suppliers agree minimal maintenance is needed and longevity will remain. “The longest one we’ve had in service is 23 years on the original membranes itself,” Johnson says. “Different gases have different effects on gasholders, so some aggressive gases can cause a shortened life span and some very inert gases can cause a very long life span on the gas holder.” GTI, CST, Sattler and others team up with project developers to seek the best biogas storage solution for the unique array of AD installations. “Our standard for going into the biogas market is to build a relationship with the process companies, the engineers who are actually developing the project and the customer, and help them size and choose tank roofs, whether they want a dual membrane for biogas storage or a fixed cover to put mixers in,” Howell says. “We want to help them find the most economical tank design that will give them the longevity they want out of their biogas plant.” Evans believes that, ultimately, it is better to not rely on storage and try to size the generating system to match the amount of gas produced. “That is really the first step, in my mind, for any developer, to try to match the two systems—the amount you are going to generate versus the amount you are going to consume—and that these products are just meant to make the operation of the system a bit smoother.” In the end, equipment used for biogas storage is in place to improve a plant’s efficiency, and the ultimate goal for equipment providers is to help the customer make sure their baseload is covered. Johnson says, “It’s nice to have a gas buffer in place to ensure that a constant supply of gas is available to run the engine at a constant speed all of the time, while simultaneously having the capacity to store extra gas instead of flaring it.” Author: Katie Fletcher Associate Editor, Biomass Magazine 701-738-4920 [email protected]