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Covering all dimensions

In certain circumstances, coal yards are under increasing pressure to go under cover, and there are various coverage systems available.

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Geometrica supplied this 126m coal storage dome to a customer in the Philippines
Geometrica supplied this 126m coal storage dome to a customer in the Philippines

Coal storage yards have traditionally been left in the open. Fuel stocks can extend over many acres and, in some cases, the stockpiles shift in shape as material is brought in and pushed around with heavy moving equipment. A typical power plant may stock 300,000t of fuel in yards extending over 4.5-ha.


But open-air fuel stocks produce pollution. Wind and rain carry fine particles into the atmosphere, as well as into harbours, streams and aquifers. Exposure to direct sunlight produces a threat of spontaneous combustion that could release uncontrolled smoke.

 

Dome manufacturer Geometrica says that, despite these challenges, the large footprint of a coal yard often makes it impractical to think of covering it. Industry, government and the public simply accept an open coal yard as a consequence of using this type of fuel.


But the status quo might be changing. More and more companies are aware of the legal and environmental reasons to cover their stockpiles.

 

Geometrica has been covering stockpiles and offering olutions since 1992. Today, the firm’s covers and domes can be designed in varying curvatures, shapes and sizes for flexibility. Environmentally conscious power plants and industrial solid fuel users are now covering their fuel stockpiles.


These stockpiles vary in both size and shape. They can range from 30m to 300m. And, from small to large, there are typically four shapes: conical, ring, longitudinal, and free-form. Each type presents unique challenges.

 

When material drops from a fixed conveyor onto the ground it creates a conical pile. The pile’s base is circular, and the slopes of the cone are given by the angle of repose of the material.

 

Conical stockpiles are usually not very big because of two issues with dropping coal from a fixed conveyor. First, dust is released in large quantities when there is wind, and, second, air is trapped in the
coal pile.


The first issue is easily solved with a cover. The second issue promotes spontaneous combustion of the coal, and is perhaps the primary reason to keep conical fuel piles small. Nevertheless, these types of piles are fairly common in industries that burn moderate amounts of coal, such as cement or fertiliser plants.

 

Geometrica offers domes that hug the shape of the stockpile to control dust and minimise the amount of trapped air in the pile.

A 300m long coal storage vault in Tunisia, supplied by Geometrica
A 300m long coal storage vault in Tunisia, supplied by Geometrica

Ring stockpiling

 

If the stacking equipment pivots 360 degrees around a centre point, it forms a ring stockpile. Large volumes of coal can be stored this way with a relatively small footprint. Ring stockpiles can rest on the ground, or be contained by a concrete wall.


In a ring stockpile, equipment clearances are very important, Geometrica points out. For a covered pile on the ground, it is often desirable to have moving equipment circulate around the pile inside its dome cover. To achieve this, the dome’s diameter increases and the springing angle needs to be close to 90 degrees so that there is enough clearance for these vehicles. Similarly, for piles contained with a wall, reclaiming equipment may demand a special profile for the dome cover.


Geometrica says it can provide domes with profiles that can achieve any clearance requirements, whether on a wall or on the ground.

 

Automated linear stacking and reclaiming equipment produces piles in the form of a triangular prism. These stockpiles can be very long, but their width and height are limited by the reach of the automated equipment. Some types of coal also require pile height restrictions to prevent crushing or the threat of spontaneous combustion.


Longitudinal coal piles are covered with vaults or longitudinal domes. In addition to the stockpile itself, longitudinal domes must cover conveyors and stacking/reclaiming equipment on either side of the stockpile.


Often, the stacking equipment is shared by two adjacent piles. These are called ‘double piles’. To cover them, a vault must clear a much wider span. Typical vault spans for single piles are between 50m and 80m, and, for double piles, over 90m.

 

Geometrica has designed and built longitudinal domes that span up to 110m and extend for hundreds of metres. When double piles require vault spans longer than 100m, it is worth considering covering them with a freeform dome.

 

Free-form

 

However, stockpiles are not always conveniently shaped and located on even terrain. Often, these free-form stockpiles cover a massive area and are continuously changing with moveable conveyors, truck drops and bulldozers. Many are also surrounded by pre-existing buildings, fence lines or equipment.

 

The limitations of a conventional building simply cannot accommodate covering these large spans on uneven terrain. With a dome, the entire stockpile can be covered. Since there aren’t any internal columns, maximum useable storage space is easily achieved.

 

Geometrica’s freestyle geodesic dome, trademarked Freedome, has a superstructure that can span irregular shapes up to 300m on uneven terrain without any interior columns.

A 70m Geometrica coal stockpile dome in Mexico
A 70m Geometrica coal stockpile dome in Mexico

Biomass specialist


Another supplier, Dome Technology, specialises in storage for biomass, particularly wood pellets. The company has now completed some 600 domes in various parts of the world.

 

The firm makes its business case on the basis that its domes can maximise storage capacity on any available footprint. This is especially important with prime port land, where users get less land for the same money as elsewhere.


Dome Technology’s DomeSilo is marketed as the model of choice for wood pellets. Although each project is unique, a few benefits are constant with every DomeSilo. Because of its geometry, it allows companies to store more product, the firm claims. The double curvature of each dome lends itself to the ability to build up, rather than out, and the curve provides strength at all points of the structure, even at the apex. The entire interior of a dome, then, can be used to store product.


Establishing the right foundation will always be a major concern in a port where low-strength soils are common. But Dome Technology specialises in exploring alternatives to expensive deep foundations.


Where the top 6ft to 8ft of ground is less-than-desirable material, it can be replaced with controlled structural fill. This model allows for some settlement, but the amount will be within tolerable parameters for a dome.


When the top 15ft to 50ft of soil does not provide the needed bearing strength, stone columns are an option. Crews use an auger to remove soil in a roughly 30- inch diameter hole down to a suitable, soil-bearing layer. Rock is placed in the hole and compacted, increasing the strength of the soil. With soft or clay-concentrated soils, a casing must be used to prevent it from collapsing in on itself.


The dome ring foundation, or ring beam, is a shallow foundation system that can be used at sites with acceptable soil conditions. This is an especially viable option at sites with soils consolidated from supporting heavy storage facilities in the past.


For areas where deeper foundations are required, other systems are available.

 

Various foundation systems can work for a dome and its smaller footprint, and a frontend engineering design study will determine which is best and provide budgetary numbers so companies can compare costs with other storage types. That being said, a dome might not require a deep foundation at all. Steel-reinforced concrete on its own tolerates some degree of settlement and movement compared to a large flat storage solution.

Dome Technology filling tubes have been installed in four different wood pellet domes
Dome Technology filling tubes have been installed in four different wood pellet domes

Explosion panels


Recent additions to Dome Technology’s features include 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 has been installed on domes since 2016.


No matter what system is used, panels create weak spots, whether it’s a pre-manufactured rectangular panel or a metal cladding piece. A round panel eliminates sharp corners where stress concentrates.

 

Dome Technology’s explosion vents are comprised of a circular geodesic steel lattice covered with the same PVC fabric used in the dome construction process. The panel is anchored to the dome with explosion-venting relief screws that remain secure during dead, live and wind loads. But in the event of a deflagration, the screws release the panel and allow for the release of the excessive internal pressure. The system is watertight and meets the required operational design loads.


If 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’s 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 removes dust build-up concerns, and the double curvature has proven in real-world examples that a dome is structurally stable under extreme fire and heat conditions.


Wood pellet companies also have the option of installing a filling tube as a 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.

 

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, and, as a result, the filling tube acts as a crucial part of a dome’s deflagration-mitigation plan.

 

The tube also protects product integrity, inimising 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, the company says.


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 shiploading 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 Dome Technology said the cheaper option is to have the foundation planned and constructed with the filling tube in mind.

 

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