Coal covering challenges
Published by Stephanie Roker,
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 300 000 t of fuel in yards extending over 11 acres (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 an increased threat of spontaneous combustion, that could release uncontrolled smoke.
Despite these failings, the large footprint of a coal yard made it impractical to think of covering it. Industry, government and the public simply accepted an open coal yard as a consequence of using this type of fuel. Even though the status quo is changing, and now more and more companies are aware of the legal and environmental reasons to cover their stockpiles, Geometrica has been covering stockpiles and offering solutions since 1992.
Today, the company’s covers and domes can be designed in varying curvatures, shapes and sizes for complete flexibility. Environmentally conscious power plants and industrial solid fuel users are now covering their fuel stockpiles.
Coal stockpiles vary in both size and shape. These stockpiles range in size from 30 - 300 m. And, from small to large, there are typically four shapes: conical, ring, longitudinal and free-form. Each type presents unique challenges to covering them.
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: dust is released in large quantities when there is wind; and 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 it 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 plants and fertilizer plants.
Geometrica offers domes that hug the shape of the stockpile to control dust and minimise the amount of trapped air in the pile.
If the stacking equipment pivots 360° around a center point, it forms a ring stockpile. Large volumes of coal are stored 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. 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° 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 offers 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 is 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 50 - 80 m, and, for double piles, over 90 m.
Geometrica has designed and built longitudinal domes that span up to 11 0m and extend for hundreds of metres. When double piles require vault spans longer than 100 m, it is worth considering covering them with a free-form dome, as next described.
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 is even trademarked - Freedome®. This incredibly strong superstructure can span irregular shapes up to 300 m on uneven terrain without any interior columns.
Read the article online at: https://www.worldcoal.com/handling/12112018/coal-covering-challenges/
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