Pellets Under the Dome

Looking at the aerial image of the Drax Power station in the UK, the four white pellet storage domes resemble a giant clutch of eggs. If it had been built prior to 2009 it might have been a source of inspiration for Stephen King’s “Under the Dome” sci-fi novel and subsequent TV series. Each of the four storage domes is enormous, measuring just over 50 m in height and 63 m in diameter and capable of holding 80 000 tonnes of pellets. In total the domes could store a massive 320 000 tonnes of wood pellets, equivalent to the annual production of a reasonably sized pellet plant.

The first-of-its-kind biomass storage project in the UK and a new storage solution for Drax, who had previously chosen flat storage and silos, it was also the largest project ever for Dome Technology. While the size and scale of the Drax power station storage project were unprecedented it was not the first wood pellet dome project.

– We have several biomass projects in the US, such as the Savannah Bulk Terminal in Georgia for Georgia Biomass; the Enviva Biomass Terminal in Chesapeake, Virginia and Enviva in Wilmington, North Carolina and for Quebec Stevedoring (QSL) in Canada. The domes were more economical and have a smaller footprint than other technologies and a quicker build process, recounted Lane Roberts, Sales Manager, Dome Technology.

– We have completed over 270 industrial projects worldwide. These are found at power plants like Drax, but also mines, heavy industry and terminals for storing dry-bulk products – for instance cement, potash, coke or agri-bulk like corn, soybeans, sugar, canola and wood pellets, said Roberts.

Inherent strength provides options

The thin-shell structure consists of several layers and whilst the domes may appear alike, no two projects are the same since each is tailored to the indivudual project. A bulk-storage project usually begins with assessing the material to be stored along with site constraints and operational requirements of the client factored into the initial design and engineering of foundations, tunnels, domes and operational systems. Precast concrete tunnels can speed up project delivery and, paired with various dome shapes and floor configurations, accommodate efficient underground mechanical reclaim systems.

– The inherent strength of the dome allows for diverse foundation and product reclaim options. The concrete ring beam serves as both footing for structure and the belt keeping the structure in tension, explained Roberts. In locations where soil conditions would require deep foundations beneath traditional structures, intermediate options can often suffice with a dome.

– The Savannah Bulk Terminal project is a good example. Here the operators, Peeples Industries, were originally considering a conventional A-frame storage solution for 50 000 tonnes of pellets. This would have required a deep pile foundation on account of the soggy Savannah soil, said Roberts, adding that the cost of piling alone would have been equal to about half the cost of one of the two 25 000 tonne capacity domes that were built.

Novel construction process

The feature that gives the dome its initial shape is the innovative PVC air-form that acts as the air-supported structure for applying additional layers of shotcrete. Upon completion, it remains in place as the outer waterproof membrane. Once attached to the ring beam and inflated, the air-form creates a protected environment, allowing construction to continue inside the dome in almost any weather conditions. The continuous single-ply PVC membrane is coated on both sides with a mould-resistant UV-protective resin that provides long-term protection from sunlight and microbial degradation.

– The air-form is one of few components not sourced locally. We fabricate it at our facility in east Idaho to match precise design requirements, revealed Roberts.  Multiple layers of polyurethane foam are then applied to the inflated air-form. With fasteners inserted and the foam hardened, this foam serves a structural role in supporting the first layers of steel matting and concrete. The application of multiple layers of steel-reinforced concrete is the final step in the dome construction process before project-specific features are included.

Cut outs in the dome accommodate gravity feeding, explosion venting, front-loader access, conveyor access, and other access needs. Head-houses are constructed atop the dome, along with other steel structures. Pneumatic floors, mechanical screws, other reclaim systems, climate systems, and specialized hardware are installed within a dome.

– Upon completion, the air-form remains in placeas the outer waterproof membrane, the foam acts as a thermal insulator protecting both the concrete dome structure and the product stored inside, and the reinforced concrete is what gives our domes their brute strength, said Roberts.

Climate control

Brute strength is not an overstatement as Roberts revealed that for the Enviva Biomass project in Chesapeake, the domes were required to be able to withstand hurricanes with 320 km/h (200+ mph) winds. Nonetheless, all factors being equal to other storage options there are essentially two major characteristics that would seem to give Dome Technology a specific advantage for large-volume wood pellet storage.

– Our domes are waterproof, insulated and almost completely airtight. As a result temperature fluctuations inside the dome are tempered, the formation of condensation is avoided and the internal atmosphere can be manipulated to mitigate off-gassing and self-combustion; for instance nitrogen can be used. Incidents with self-heating and combustion happen far less frequently compared to other storage forms, explained Lane Roberts.

Domes that store wood pellets are built with an explosion panel over the vent at the apex; this panel seals out moisture but is rated to release in an explosion event or when air pressure is excessive.

– When the explosion panel is loosened, the shape of the structure channels energy through the panel, meaning less chance of your dome being compromised should an explosion occur, said Roberts.

The unique “tops” for the Drax Power station domes were specially designed. Calculated to meet ATEX and DSEAR requirements the weather-proof design incorporates a 90-foot opening at the apex of each dome to accommodate panels, which relieve and dissipate pressure should it arise. A stiffening structure was used to spread the load of the head house, which sits between the relief panels, evenly down into the dome shell.

Drax Port Allen project

In the US, Drax has two 450 000 tonne per annum pellet plants in operation, Amite BioEnergy in Gloster, Mississippi and Morehouse BioEnergy in Bastrop, Louisiana. The company has also recently completed its port facility in Baton Rouge, Louisiana. Located at the Port of Baton Rouge on the east bank of the Mississippi River, the Drax Port Allen facility acts as a hub for wood pellets from its US Gulf plants and third-party suppliers, providing both storage and loading terminal prior to shipment. The new facility forms part of Drax’s US$350 million investment in its biomass operations in Louisiana and Mississippi.

Delivered by road and rail

Operating within the Port of Baton Rouge secure area, the port-side facility features a ship loader, open plan control building with a panoramic view of the terminal, and two 40 000 tonne capacity domes for storing pellets.

– Like in Savannah, a location near the Gulf of Mexico means soggy soil and therefore potentially expensive foundation work. Furthermore, in this case, building a facility within 1 500 feet (457 m) of a levee meant getting design approval from the Army Corps of Engineers, said Roberts.

The port facility primarily handles pellets from Drax’s two pellet plants. From Morehouse, which is located omre than 380 km (238 miles) from the port the pellets are railed using railcar train sets. Each railcar can load 90 tonnes. Each train set has 80 railcars and is about 1.6 km (1 mile) long end-to-end and can carry about 7 200 tonnes of pellets. The facility is designed to unload six to seven railcars, or 540–630 tonnes of pellets, per hour.

From Gloster the pellets are trucked and to ensure drivers can unload quickly and safely at the port, a sophisticated automated system has been designed to handle and unload trucks at the touch of a button. The fully automated system discharges each pellet load in less than three minutes, keeping drivers a safe distance from the conveyors and chutes, with no need to climb on or under their trucks to unload.

Ship loader from Bruks

The domes are connected to rail- and truckunloading pits via transport towers and conveyors. An automated mechanical screw system is used for reclaim from the domes to minimize dust. Downstream of the domes is the ship loader, which is located on a dock that dates back to the mid-1950s. Without the benefit of a new ‘greenfield’ site, plans had to be adapted to accommodate constraints presented by the existing dock. Major challenges included building the ship loader and anchor points for the rails onto the existing dock as well as providing new mooring points for larger vessels. The moorings have to be designed with sufficient strength to hold vessels on berth even at times of the high water flows, which are typical on the Mississippi River.

The shiploader was supplied by BRUKS Rockwood, a global leader in mechanical engineering and equipment supply for bulk-storage facilities. The scope of supply included the complete traveling, luffing and slewing ship loader. It is designed to transport and load pellets at a rate of 1 200 tonnes per hour and fully load a Panamax vessel with 65 000 tonnes in three days.

In April 2015 the first boat, a ‘Handy’ sized vessel, set sail on the Mississippi River from Drax’s new port facility. It carried 25 000 tonnes of the first wood pellets produced by the Gloster plant testing the supply chain, from pellet plant to port and port to vessel, in preparation for the first Panamax-sized vessel.

According to Lane Roberts, BRUKS is an important vendor for Dome Technology projects in the biofuel realm with multiple projects utilizing BRUKS equipment in Dome’s project portfolio, the most common being equipment used to deliver material to or from the dome, especially in the form of ship loaders and unloaders.

– For these projects, BRUKS completes the engineering for their systems and that requires a thorough understanding of the machinery and the bulk product in question, ended Lane Roberts.

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