By Rebecca Long Pyper for Dome Technology
With Japan and South Korea joining the list of countries expanding their reliance on biomass, the demand for wood-pellet storage is growing too, and storage concerns are consistent worldwide.
William Strauss with independent consultancy group for the wood-pellet sector FutureMetrics identifies the main concerns as safety, the ability to deal with fire, and efficiency considerations like the ability to fill and empty a storage structure with minimum damage to pellets, cost versus volume and maximum storage with a minimal footprint.
Multiple storage options provide the necessary square footage “but the particular advantage (with Dome Technology) is what people from the outside don’t see, and that’s all the internal stuff — the way the floors are shaped, the conveying systems and everything that’s set up to make sure the pellets drop into the domes in a gentle way,” said Gordon Murray, Wood Pellet Association of Canada executive director.
Companies that choose a DomeSilo™ from Dome Technology receive a storage solution with these features and more.
Safety & fire prevention
When Drax selected Dome Technology to build four wood-pellet domes that store 80,000 metric tons apiece — Europe’s largest decarbonization project and Dome Technology’s largest wood-pellet project to date — a custom dome “top” was designed to meet company specifications on relieving pressure should it arise. So it was important that these tops be completely weatherproof, as biomass storage demands, and “to be as close to gas-tight as possible — we don’t want oxygen being drawn in, nor do we want our inert gas atmosphere within the dome escaping,” Drax’s strategic project engineering manager Jason Shipstone said.
The insulated nature of the shell, composed of polyurethane foam insulation and reinforced shotcrete, moderates externally generated temperature fluctuations, and the more constant an environment, the more stable pellets will be.
The PVC airform covering the entire dome structure provides waterproofing, an important feature since moisture-content changes will produce heat, with either the drying effect or the hydrating effects resulting in self-heating. The dome’s insulated nature also reduces heating and cooling of the walls and air inside, preventing condensation from forming and introducing moisture to the product.
The dome is airtight except wherever penetrations are made, aiding in the containment of inert gases pumped inside to deter fires and to minimize available oxygen that feeds fires. Domes are engineered with systems to monitor temperature, humidity and off gasses. Spark detection recognizes sparks when product is loaded into the dome, and monitor conveyors analyze idlers on the belt to anticipate fire. Dust collection also comes standard with a dome.
Round explosion panels
Until now, square and rectangular explosion venting has been the norm in storing products prone to deflagration, but Dome Technology’s team has pioneered a round hybrid model that was installed on bulk-storage domes for two biomass companies in 2016.
“No matter what system, you’re creating a weak spot with panels, whether it’s a pre-manufactured rectangular panel or a metal cladding piece. This is a round panel, which in a dome is nice because you don’t get sharp corners for stress concentrators,” said engineer for Dome Technology Adam Aagard.
The proprietary explosion vents are comprised of a metal ring fastened directly to the dome with explosion-venting relief screws; these screws remain secure through dead, live and wind loads but will release should interior pressure reach critical levels. To the metal ring is fastened another ring, this one attached to a geodesic steel lattice covered with fabric. The lattice helps the fabric hold its shape, and the fabric acts as a waterproofing shield.
When an explosion occurs, the fabric accepts the load and transfers it uniformly around the ring’s circumference. “Because it’s circular we can predict the load going to each of the fasteners really well,” said Jason South, Dome Technology Vice President of Engineering, Research and Development. “If it were rectangular, the pressure going to each fastener could be different,” and more difficult to estimate.
In addition, the shape of the structure channels energy through the openings, reducing the chance of the dome shell being compromised should an explosion occur. The truss-free structure discourages dust build-up, and the double curvature has proven in real-world examples that a dome is structurally stable under extreme fire and heat conditions.
Product protection: The filling tube
Wood-pellet companies working with Dome Technology have the option of installing a filling tube as an innovative way to load pellets with reduced dust and better product protection.
The filling tube is based on a concept common in coal and ore storage. A belt conveyor moves product into the top of the dome, where pellets enter the filling tube. Openings are incrementally located along the tube’s shaft, and product rolls through the openings and into the pile, resulting in smoother, more even pile creation. Dome Technology sales manager Lane Roberts said he is fielding more calls than ever about installing filling tubes.
According to engineer for Dome Technology Doug Weber, the difference in the pile is obvious; pellets loaded with a filling tube look cleaner and are stacked more uniformly. This mode of filling also reduces the likelihood and/or severity of deflagration. This is possible since the airborne dust concentration is reduced. Most dust is confined within the tube; as a result, the filling tube acts as a crucial part of a dome’s deflagration-mitigation plan.
“To have an explosion due to dust particles, you have to have a certain concentration in the air, and we can drive this concentration below that level,” Weber said. “This is really mitigating the risk of deflagration.”
The tube also protects product integrity, minimizing fines generated during the drawdown process by reducing the pressure head on the product flow line. Although there are no guarantees regarding the reduction of fines, “based on experience, it reduces the fines considerably,” Aagard said.
Four filling tubes have been installed in four different wood-pellet domes, two for Savannah Bulk Terminal in Georgia, USA, and two for QSL in Quebec, Canada. Customers with a filling tube report lower percentages of fines during ship loading than previously experienced.
According to Peeples Industries project manager Brad Orwig, the filling tubes in the Savannah domes have worked very well for the company since installation four years ago. “The pack factor has been reduced or completely eliminated. The stratified dust blanket has been reduced or eliminated, allowing a reduction of degradation during storage as well as improving the breathability of the pile,” he said, adding that on the discharge side, the shear within the pile is reduced.
An existing structure can be retrofitted to accommodate a filling tube, but Aagard said the less-expensive option is to have the foundation planned and constructed with the filling tube in mind.