The Changing Pellet Storage Landscape

Like any combustible fuel, large-scale storage of wood pellets can be a dangerous proposition. Matters of dust, moisture, temperatures, deflagrations, density, structural integrity, location and public safety must all be thoroughly considered well before a single stake goes into the ground.

“The storage of large amounts of wood pellets is something that’s come on fairly recently,” says Eric Lapointe, director of engineering with Quebec Stevedoring Ltd., a terminal operator and stevedore present in 30 North American ports. “The industry is very young. We are more and more aware of the different risks associated with large-scale pellet storage—off-gassing, heating. And we are well aware of fires in the storage of pellets, both in Europe and in the southern U.S.”

Conventional storage vessels for grain and oilseeds—and more recently for wood pellets—are typically either concrete silos or corrugated steel bins. Chief Agri/Industrial Group has been around since 1961 and builds steel bins all over the world, mostly for grain storage, but more recently, for wood pellets, too. “We are currently using Chief storage silos at the Highland Pellet project in Pine Bluff, Arkansas,” says Jody Bruning, regional sales manager for pellet mill builder Astec Inc. Chief has also done a few projects for German Pellets, including building the silos in Port Arthur, Texas, that made headlines this year when pellets in one of the silos smoldered for months as extraction, extinguishing and debris cleanup efforts unfolded from April to July. The bin collapsed in June after on-site teams worked for a month to remove the contents. The incident remains under investigation, says Bradly Jorgenson, vice president of engineering at Chief.

When designing a corrugated steel bin, Jorgenson says knowing product density is critical. “Pellets weigh less per cubic foot than grain,” he says. The foundation must be capable of withstanding the vertical load. “We use external stiffeners that help carry vertical loads,” Jorgenson says. “As the product pushes up against the side wall of the silo, that helps push the force down.” Then there is horizontal load to consider. “As product goes into the bin, it creates hoop stress if the product is heavier than what the bin is designed for,” he says. The thickness of the steel on the bottom, where more force exerts pressure on the sidewalls, is also typically thicker than the bin’s sidewalls on top.

“Like grain, it’s important to keep wood pellets well-conditioned,” Jorgenson says. “If the aeration fans aren’t matched with your silo, you will not get the performance needed to keep the product properly conditioned.” He says if product starts to heat slightly, fans can aerate and bring it back into condition. “But wood pellets are very combustible,” Jorgenson says, “and feeding air after the temperature reaches a certain point is just feeding oxygen to it, so you have to be careful.”  

Domes
While conventional steel bins and concrete silos have served their purpose over the years and continue to dominate pellet storage, a relatively new design—domes and DomeSilos—is changing the landscape, both figuratively and literally.

“When we decided to build pellet storage in our terminal in the Port of Quebec, our client Rentech wanted us to load Post-Panamax vessels,” or very large ships unable to fit in Panama Canal’s locks, Lapointe says. “There aren’t too many places that can load ships this size. In order to load these, we needed fairly large storage.” He says the rule of thumb is to build port storage sized at one-and-a-half times the vessel load weight. The average Post-Panamax vessel can hold 50,000 metric tons, but QSL would need to be prepared to load up to 68,000 tons. “To store 75,000 tons, we would have needed to build eight steel bins right next to the port community,” Lapointe says. After visiting Enviva’s port storage in Chesapeake, Virginia, and Wilmington, North Carolina, and Georgia Biomass’ port storage in Savannah, Georgia, QSL contracted with Dome Technology and its partner company Engineering System Solutions (ES2) to design, engineer and construct two large domes, each capable of storing 37,500 tons of pellets.

“Dome Technology and ES2 have been working together as strategic partners for close to 20 years,” says Douglas Weber, a professional and structural engineer, and CEO of ES2. “ES2 designs the storages that Dome Technology constructs, and the two groups bring design concepts to a reality. Together, both companies have innovated to pioneer new uses for domes and construction techniques that allow a large variety of products to be stored and reclaimed from domes and DomeSilos.”

Dome Structure
Jason Miller, Dome Technology’s vice president of marketing, says the company, founded by Barry South, started building domes in 1975. “We’re the original steel-reinforced concrete dome builder,” Miller says. “Our method of building has been perfected over the years. Barry had the idea in the ‘70s that the best way to build these is through the inflation of a polyvinylchloride (PVC) membrane.” Miller likens the construction process to paper mache, but rather than inflating a balloon and coating the outside with paper soaked in a water-flour mixture, the membrane is inflated and intricate work is performed on the inside. A thin layer of polyurethane foam is first used to coat inside the PVC membrane, followed by placement of rebar and concrete. “When completed” Miller says, “you have an incredibly strong, steel-reinforced structure with a thin layer of polyurethane foam and a waterproof PVC membrane on the outside.”

Two reasons domes are safer than concrete silos or steel bins, according to Miller, are strength of design and construction, and their ability to keep temperatures stable. He adds that domes and DomeSilos can be engineered to allow for some limited ground settlement. “If a concrete silo leans, that’s a major problem,” Miller says. “But our domes have a larger footprint as compared to their height and, as a result, they can deal with some differential movement. Since the dome or DomeSilo is one continuous shell, without joints or seams, it has tremendous ability to transfer loads from one part of the structure to another. Consequently, less expensive foundation systems have been installed as compared to other storage types.”

Lapointe says one of QSL’s concerns in deciding which pellet storage design to choose for the Port of Quebec was temperature control. “We wanted to make sure there was no heating of the pellets,” he says. “We went with concrete domes to better control temperature.” Miller says the temperature inside the dome remains relatively stable, thanks to the concrete and layer of polyurethane foam. “When pellets are loaded, they can be warm,” he says, “while the ambient air temperature in Quebec can be quite cool. The dome composition allows the wood pellets to retain much more of their heat, leading to less condensation and product degradation.” 

With portside property at a premium, domes can also be advantageous because more product can be stored in a smaller footprint. “It’s important to maximize that land as best they can,” Miller says. Given QSL’s port terminal location in a community, Lapointe adds that the structures also had to be visually appealing. “Not just for the short term,” he says, “but for the long term. Concrete holds up nicer than steel silos over time.” Dome construction at QSL’s terminal in Port of Quebec began in fall 2013 and was completed by the end of 2015.

Dome Dust, Fire Safety
QSL could have chosen a flat, A-frame storage facility, “but because we are right here in Quebec City where a lot of tourists visit, we had to make sure there was no dust flying around—absolutely none, zero,” Lapointe says. “So all our conveyors are enclosed, the domes are sealed, and aeration inside makes sure the exhaust is filtered so there’s no dust flying out.”

Lapointe says instead of choosing a design like Drax’s domes in the U.K., which have large openings on top, QSL wanted a filling tube installed through the dome’s center that utilizes a dust-collection device. “It’s like a manifold, so when the pellets fall inside, it collects dust before it starts flying around inside the dome,” Lapointe says. This reduces the concentration of explosive dust inside the dome to less than 0.28 grams per cubic meter (g/m³) during filling, Lapointe says, adding that the limit is 70 g/m³. “We tested dust concentration in a worst-case scenario of dropping pellets in when the dome is completely empty,” Lapointe says. “During four full hours, we tested three different elevations inside the dome and maximum concentration never reached 0.28 g/m3. The system is very efficient.”

Weber says there are several opinions about what safety measures should be part of dome storage. “We believe that the greatest improvement to pellet storage is the central filling tube,” he says. “It has shown to decrease the amount of airborne dust to levels lower than that necessary for a self-igniting deflagration event. Also, it reduces fines generation during the reclaiming process, allowing operators to supply a better quality product to their customers.”

The domes at QSL’s terminals also feature smaller explosion vents in the unlikely event of deflagration from the very low concentrations of explosive dust. Explosion panels are used to allow pressure expansion without cracking the structure. “The size of the vent opening is critical,” Weber says. “The greater the open area, the more venting is possible and the less pressure build-up inside the storage.” But with the dust-collecting filling tube, Lapointe says the size of the vents needed were much smaller than would otherwise be required.

“We take pride in our innovation and safety,” Miller says. “Dome Technology, along with ES2, recently pioneered smaller, round explosion panels, which we built into the QSL domes as an optional feature.” He says it is harder to predict how rectangle explosion panels will react to a deflagration event. “But with round panels,” Miller says, “there’s a much more even hit, which would minimize damage to the structure if an event occurred.” Weber adds that a round opening has better performance characteristics structurally, since there are no corners, thereby eliminating stress concentrations.

Not only are round explosion vents safer, according to Miller, but Lapointe says QSL saved a lot of money coupling the newly designed explosion vents with the dust-collecting filling tube. “We currently have four 8-meter diameter openings,” Lapointe says. “According to the computational fluid dynamics model used, the size of a vent to keep the overpressure below 0.6 bars during a 500 g/m³ dust cloud deflagration would be one-third of the entire dome surface. The filling tube reduces the dust concentration and the volume of the cloud.”  

Each of the two domes at QSL’s terminal also contains 47 cable temperature probes. “We are monitoring the temperature all the time,” Lapointe says. “We’ve never reached a temperature higher than 48 degrees Celsius—even with pellets stored over six months.” 

Other features built into the structures include aeration and nitrogen pacification. “Aeration has been shown to help moderate the temperature within the DomeSilos, but its effectiveness is limited,” Weber says. The domes in Port of Quebec each feature two fans capable of moving 40,000 cubic feet per minute (cfm) of air, totaling 80,000 cfm per dome, Lapointe says. “They are equipped with variable speed drives to reduce the amount of air when it’s not needed, to save energy,” he adds. Because the domes are so big, separate vents are included with the ability to concentrate venting in certain areas if there’s a hot spot. “One thing we found out,” Lapointe says, “is aeration doesn’t have a short-term impact. It won’t produce a change in the pellet temperature overnight if it starts to heat—80,000 cfm is not enough to do that. So it’s not a drying or cooling ventilation system, just an aeration system. When used steadily long-term though, it helps keep temperatures regulated. It will slow down the heating vs. keeping the pellets cool.”

While QSL has never had to use it, its domes are also equipped with a 15,000-gallon nitrogen extinction system. The system features a top manifold to put a fire out on the surface of the pellets if needed, and the floor piping is divided into four quadrants with a manifold outside the dome to pinpoint nitrogen delivery inside. “The system has a capacity of 7,500 cubic meters per hour and is able to inert the dome head spaces within four hours,” Lapointe says.

To keep dust down in conveyance, QSL employed a French company to build enclosed tube conveyors. “The belt slides inside a tube,” Lapointe says. “There are no support idlers for the loaded belt, only the return belt.” One side is canvas to make it slide easier inside the tube, and the other is rubber. “With no idlers there’s less tension and the belt doesn’t need to be as thick,” he says. “Therefore, it needs less power.”

Lapointe says additional features at QSL’s port terminal include a gas analyzing system present in the head space of the domes to alert in case of combustion well before any smoke is visible. “Furthermore,” he adds, “to reduce off-gassing and heating related to the moisture content, we have a moisture analyzer on the rail car receiving conveyor. Any pellets above 10 percent moisture are automatically diverted to a reject bin before entering the domes.”    

Future Improvements
Weber says that over time, Dome Technology and ES2 have developed innovations for its structures to meet the industry’s needs while utilizing their material-handling experience. “We are using the best practices and have more experience storing wood pellets in domes than anyone else in the world,” Weber says. “We are using the latest technology and practices.” Miller says there’s a reason more and more pellet companies are going with Dome Technology. “They can store more pellets in a smaller footprint, potentially save money on foundation costs and are safer to use,” he says.

As far as what the future might hold for further improvements in storage technology, Miller says he’s excited to see what new developments may arise in reclaim systems. “They’re only getting better and better with time,” he says. “We have some customers that use front-end loaders and others that use automated systems that include reclaim screws and vibratory floors, so we’re excited to see innovations in  reclaim options. It’s important to keep people and workers outside the structures.”

Lapointe notes that the biggest challenge is not storage itself, but the mobile equipment working inside the domes to recover the remaining pellets. “Dust accumulates on hot components and becomes a fire hazard,” he says. “We have to regularly blow the dust accumulation. Manufacturers should address this.”

As time goes on, he adds, ocean transport vessels will continue to get larger. “With many weeks of traveling time, this will increase the risk of heating and off-gassing,” Lapointe says. “The shipping business will face the same challenges as large storage facilities.”

Ultimately, each storage facility faces different issues and is asked to manage different risks. “For example,” Lapointe says, “we were asked by the city to provide a smoke dispersion simulation in case of a surface fire inside a dome. To my knowledge this was not required elsewhere.” He would like to see a common risk analysis/assessment developed for all large storage facilities, and guidelines produced.

Last, Lapointe says that aerating large storage facilities can be energy-intensive and costly. “It would be good to have efficient algorithms to manage the use of fans based on inside and outside temperature, dew point, residence time and more,” he says.  

Dome designs for pellet storage have gradually leaned toward taller and more slender DomeSilos, Weber explains. “This is a result of needing a smaller footprint for the storage due to space utilization. In addition, a smaller footprint leads to economical savings, if aeration is desired.”

While Dome Technology has built many pellet storage domes over the past several years—with more builds on the docket—Miller says the biggest obstacle for his company is that the modern concrete dome, along with all of its benefits, is still often unknown to many people. “Even so, the pellet industry looks awfully bright for us,” Miller says.

Author: Ron Kotrba
Senior Editor, Biomass Magazine
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[email protected]