Coal Storage Domes
Geometrica designs and builds geodesic domes for storing coal, petcoke and other combustible bulk materials. Coal is often transported long distances and stocked at various points between the mine and the user. Thus, coal piles come in many shapes, from the huge multiline longitudinal piles frequently found at ports, to ring blending beds at large powerplants, to simple conical or irregular piles common at industrial plants.
Coal Storage Domes, 125m diameter, 180,000t each, Taiwan
Combustibility
Although many of the same issues that apply to most other bulk materials are encountered when storing coal, combustibility makes it a special case and deserves careful treatment. The following comments apply also to other combustible materials such as woodchips, grains and sulfur.
Longitudinal coal storage dome, 60m x 200m, Tunisia
The National Fire Protection Association, in its publications NFPA 850 and 120, identifies the hazards associated with storage and handling of coal, and gives recommendations for protection against these hazards. NFPA recommends that storage structures be made of non-combustible materials, and that they are designed to minimize the surface area on which dust can settle, including the desirable installation of the cladding underneath a building’s structural elements.
The recommendation requires an explanation. First, coal is susceptible to spontaneous combustion due to heating during natural oxidation of new coal surfaces. Second, coal dust is highly combustible and an explosion hazard. If a coal dust cloud is generated inside an enclosed space, and an ignition source is present, an explosion can ensue. Dust clouds may generate wherever loose coal dust accumulates, such as on structural ledges, if there is a nearby impact or vibration due to wind, earthquake, or even maintenance operations.
The table above indicates that explosions occur with concurrence of several factors. But because of coal's propensity to heat spontaneously, ignition sources are almost impossible to eliminate in coal storage and handling, and any enclosed area where loose dust accumulates is at great risk. Further, even a small conflagration can result in a catastrophic “secondary” explosion if the small event releases a much larger dust cloud.
Additional Safety Recommendations
The standards also recommend several other items, including:
- Storage piles should be worked to prevent dead pockets of coal, a potential source of spontaneous heating. Storage of coal should be for as short a duration as possible. Coal piles should not be located above sources of heat such as, steam lines or manholes. Coal should be piled in layers and compacted to reduce air within the pile.
- The coal should be kept as dry as possible (sprinklers are not recommended).
- The piles should be constantly monitored for hot spots using temperature detection systems.
- When a hot spot occurs a management strategy should be in place to remove the ignited coal. Access should be provided for firefighting at sufficient intervals on the perimeter of an enclosure.
- The structure should have minimal surfaces for dust to accumulate inside the dome. For all surfaces where dust may accumulate, adequate dust removal systems should be provided.
- Ignition sources should be kept to an absolute minimum. Static electricity hazards should be minimized by the appropriate grounding of all equipment. All machinery and electrical equipment inside the dome should be approved for use in hazardous locations and provided with spark-proof motors.
- Provisions should be made to turn off all electrical circuits without the need for personnel to enter dust-producing locations.
- A sufficient ventilation system should be provided to reduce the chance of methane (and, in case of fire, smoke) build up and for explosion venting requirements. Ventilation should be provided at the apex to take advantage of the chimney effect. The structure should also be vented at the perimeter to reduce the damage to the structure due to blast overpressures.
122m petcoke storage domes with internal cladding, Florida, USA
Geometrica offers solutions to meet all the relevant recommendations. A dome protects the pile from rain and wind, which foster spontaneous combustion in open-air piles, and cause air and runoff pollution. Internal cladding prevents dust accumulation on the structure. A breakaway panel may provide for accidental overloading and ventilation at the base, and exhaust fans or ventilation openings insure against methane or smoke buildup.
Remarkable Long Span Applications
Geometrica has built the largest coal storage domes in the world, as well as domes for every coal pile shape (ring, conical, longitudinal and free-form). Typical coal dome sizes range from a 66m diameter dome over a 15m wall to store 35,000 t, to a 125m dome over an 18m wall for 180,000t. Capacity for domes on the ground varies with the shape of the pile. For instance, a 35,000t Freedome® required 3500 m2 of area, while 139,000t, 135m diameter piles have been covered with 144m diameter domes.
Dome features:
