Challenges and innovation

The dome-building industry has faced a huge challenge over the past three years, in the form of low prices for most commodities. Falling demand and prices for coal in particular saw mining projects shelved and production capacity shut in.

Yet the vast array of products that are stored in domes – from cement, clinker and fertiliser, to sugar, soybeans and peanuts – means that diversification has been the key. In particular, the renewable energy boom has generated
a big rise in the uptake of biomass power generation, and biomass feedstock obviously has to be kept dry. Here, too, domes are the obvious solution.

Casting wide

The overwhelming sentiment in the industry is that new contracts have been secured by targeting a wider range of sectors, in order to compensate for the mining downturn.

“The mining and cement sectors have been challenging but, this has been offset by activity in the coal, power and architecture sectors,” said Francisco Castaño, CEO of Geometrica. “We are optimistic for the longterm and confident that any slowdowns are just part of the natural business cycle.” 

Lane Roberts, sales manager of US firm Dome Technology, said: “We’re sensing the cement market is coming back, and the coal market we hope may come back. Because the economy, in my opinion, has slowed down, Dome Technology has been pursuing other markets, such as grains and sugars and other agricultural products. We are also strengthening our presence in foreign markets to increase our company’s revenues.” Dome Technology, which is based in Idaho Falls (Idaho), has completed more than 550 domes for bulk storage, but also what it describes as “practical architectural facilities”, such as schools and churches.

Dome Technology’s recent orders include two domes for a wood pellet producer in North Carolina and two domes for Louis Dreyfus, one storing grain in Cahokia, Illinois, and the other storing oil seed in Saskatchewan, Canada. It has 
also provided a sugar dome for American Crystal Sugar Company in Montgomery, Illinois. 

FabricSpan

Dome Technology works closely with FabricSpan, which is also based in Idaho Falls. FabricSpan produces the membrane that Dome Technology uses on its domes, and which acts as airforms during construction and then later as weatherproofing.

Geometrica of Texas is currently working on two bulk storage projects for coal-fired power plants. The larger of the two is the Freedome at the Bocamina plant in Coronel on the coast of southern Chile, which it is developing for power
company Endesa. Geometrica describes it as one of its “most challenging and successful projects”, as it covers nearly 23,000 m2 but lacks any interior columns. The dome will house the coal feedstock, and its construction is part of a more general effort to improve the project’s technical and environmental performance. 

The smaller dome is being built at the 340 MW Therma Visayas Energy Project near Toledo City in Cebu Province in the Philippines. The circular coal storage dome will have a diameter of 120m and height of 40m, covering an area of about 11,500 m2, using a double-layer Vierendeel type geometry.

Tech advances

For the uninitiated, the process of building a dome is a lot more complicated than it may seem. Some domes are constructed by inflating a PVC dome, which is then covered with other layers of building materials, such as concrete. Carbon dioxide, oxygen, heat and pressure levels all have to be controlled. As well as the structures themselves, there are a wide range of systems within the structures, including chutes, augers, stacker reclaimers,  temperature control mechanisms, and fire-prevention and damping-down systems. Each one is different based on its size and the nature of the commodities to be stored. 

Materials handling systems have to be put in place in coal storage domes  to ensure that the first coal stored is also the first taken out, so that none is left for too long, as this increases the risk of explosion and fire. Coal is generally moved by full hopper systems or stacker reclaimers, particularly for more combustible types of coal. 

Completely automated systems are certainly becoming more popular, not least because of the savings in terms of manpower. In addition, more customers are opting for a single technology provider that can provide integrated, automated systems. Automation also allows problems to be identified and thus dealt with more quickly. 

FabricSpan uses airforms that are inflated before being sprayed inside with shotcrete that forms the dome’s shell when it hardens. The company said: “The airform stays permanently intact, acting as the exterior waterproofing membrane for the storage structure. When well maintained, the airform will last an average of 20 to 25 years and ensure proper protection from sun, wind, and rain.”

Cladding

Geometrica is keen to promote its approach of providing cladding on both the interior and exterior of domes  for added protection. This is possible because the weight on the company’s domes is kept down. Its structures are built with lightweight, strong, corrosion-resistant galvanised steel tubes that are connected via aluminium hubs. When all the tubes of a single hub are assembled, the joint is completed with washers and a single bolt, so no welding is required. 

Double cladding was included in Edo Cement’s storage dome in Nigeria, in order to prevent clinker from aggregating on the structural elements and creating extra loads. On a dome completed for JEA Northside in Florida, the addition of interior cladding minimised the surfaces on which dust could accumulate, and reduced the combustible atmosphere related to storing coal and coal dust. Similarly, the use of internal, fire-resistant plastic cladding protected Grupo
CICE’s dome in Venezuela from the stored urea. 

Kansas-based CST Industries is promoting its OptiDome, a flush batten aluminium geodesic dome design. Already available in the US and Canada, it began targeting other markets in October, although all manufacturing still takes place in Texas and Georgia. 

John Delaney, the vice president of CST Covers, commented: “Extensive testing across multiple applications, climates and geographical regions have been conducted on the OptiDome system to ensure a structurally sound and leak
tight cover with the new flush batten design. We’ve modified and optimised several components to create a premier standard design that is simple to install and allows for streamlined fabrication.” 

Explosion venting 

Dome Technology has developed explosion venting as an alternative to traditional vents. Dome Technology engineer Adam Aagard said: “No matter what system, you’re creating a weak spot with panels, whether it is 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.” 

The company’s vice president of engineering, research and development, Jason South, added: “Because it is circular we can predict the load going to each of the fasteners really well. If it were rectangular, the pressure going to each fastener could be different.” 

Another issue that must be considered is safety during the erection process, although CTS Covers considers this to be a given. CTS uses a central erection tower, permitting the assembly of the dome at grade and lifting it up as it ‘grows’ from the centre outward. Some other companies use cranes as they build from the wall inward, while others allow workers to climb the structure, although this is the most dangerous option. 

One industry insider, who asked to remain anonymous, does not think that there’s much new technology emerging, but he does cite ventilation as one area of innovation. “Clients are more concerned these days about ensuring that the inside is safe, and that means a lot of air movement,” he explained. 

Biomass growth 

Biomass has been one of the biggest growth areas in the industry in recent years. The renewable energy boom is transforming how the world produces electricity, but much of the new generating capacity is provided by intermittent 
technologies, such as solar photovoltaics (PV) and wind power, so biomass plays a key role in providing more flexible back-up and bulk load capacity. A great deal of international trade in wood pellets involves transport in Handymax ships, but larger vessels, up to Panamax size, are expected to be used as the level of trade increases. 

One of the biggest dome projects anywhere in the world over the past few years has been the construction of four giant domes at the Drax power plant at Selby in Yorkshire, UK. In line with government policy to shut down all coal-fired power generation by 2025, Drax had already converted two of its six coal-fired units to biomass, and completed the third one over the course of this year. Drax selected Dome Technology and Shepherd Construction to provide the storage domes. Shepherd is based just a few miles from the Drax plant, in York. 

Most of the biomass for Drax is imported from Latin America and the US, so more domes have been built at Baton Rouge to handle pellets before they are shipped out. Although US exports of wood pellets are increasing,  there is limited domestic demand because of the continued reliance on coal and access to cheap shale gas. 

The domes each have a storage capacity of 80,000t of compressed wood pellets. The pellets have to be kept dry and stored in sensitively calibrated environments to keep them in a safe and usable state. 

New approach

A spokesperson for Drax said: “At more than 50m tall, they’re the largest of their kind in the world – a new approach to construction had to be considered.” 

According to Drax, there were three key steps involved in the construction of each of the domes:

• Blowing up a giant balloon: the first stage was to prepare the foundation, which took the form of a massive concrete circular ring beam. A giant PVC airform dome was laid out over the ring beam and inflated using fans that are about the size of a telephone box, to form the outside of the dome.
• Insulating the inside: with the dome still air-inflated, a thin 15mm layer of polyurethane foam was sprayed on the inside, serving to both insulate the structure and provide purchase for the first layers of steel reinforcements.
• Completing the shell: once the first steel reinforcing grid was attached to the polyurethane, the concrete spraying process began. The dome wall was built up to a thickness of up to 350mm by adding further layers of steel reinforcement grid and reinforced concrete.

Hotting up

The pellets are stored inside the domes in vast quantities, creating high levels of pressure and a buildup of oxygen and heat. A spokesperson said: “The solution was a specially designed system that releases nitrogen into the dome. The gas forms non-flammable compounds with the oxygen, which keeps the inside of the dome stable.” 

An innovative solution to determine the volume of biomass in each dome was implemented at Drax by the two developers. Each dome is fitted with a sonar system that provides continuous feedback on how full the dome is. The sonar monitoring system provides level, profile and volume information, which is translated into a 3D image of the stored biomass. This allows the operators to view and monitor in real time the effects of their actions when filling and unloading the domes, so they can target specific areas, particularly when unloading and for fuel accounting purposes.

The pile temperature and gas levels are also measured, while a dome breather vent is fitted to the top of the dome to act as a vacuum breaker. It maintains a relatively even pressure by allowing air in during unloading and releasing
head space gasses while nitrogen is inserted. Finally, pressure levels are controlled by an aperture at the top of each dome called a slide gate, which is closed unless the dome is being filled to allow material to enter.

Outlook

Growth in new sectors is not only useful for generating additional income streams for dome developers. Solving challenges in one area can also help to generate in novations that can be applied more widely, even in established industries, where there are, at long last, signs of recovery.

Coking coal prices have more than doubled this year, and there have also been rises in the price of thermal coal. If sustained, the recovery should generate more demand for domes. In addition, greater concern over the health effects
of coal dust could boost demand for coal storage domes, including in emerging markets.