Thinking outside the box
Published by Angharad Lock,
Digital Assistant Editor
As part of World Coal's Handling Week 2017, Harold A. Walker and Donna F. Walker, PICOR, USA, illustrate how modern coal ports can advance
Technological progress has made possible coal port facilities that are pollution free, visually inoffensive and can be built with low CAPEX and OPEX. To illustrate this modern coal port, the authors will assume the use of the self-loading and self-unloading coal transport vessels described in last month’s article 'A New Wave Of Technology'.1 It should be noted that most of the attributes of the port described below are available to ports that continue to load and unload bulk carriers through conventional deck hatches.
The capacity of a coal port is determined by market forces. One of those market forces is the cost and availability of the port. For someone contemplating the building of a coal port offering the above-mentioned advantages, the economy of its operation may warrant consideration of expandability.
While coal loading ports and coal receiving ports are similar, they are different. We will treat shiploading coal ports first.
Coal loading port
Coal receiving system
The ship-loading port considered will be designed to load 25 million tpy of coal into different capacity self-loading and self-unloading coal vessels. A port of this capacity can be serviced by one railroad and one 5000 tph railcar unloading system. The rail loop servicing the unloading system would be routed to avoid, as much as possible, populated areas. When in the vicinity of populated areas, the rail loop should be isolated as much as possible by earthen mounds planted with indigenous trees. Unloading 5000 tph of coal can be accomplished by bottom-dump or rotary-dump railcars. Either system would be enclosed by a dust hood from which sufficient air would be extracted to ensure no dust escapes. The air extracted would be filtered to separate coal dust. This would then be returned to the coal stream from clean air, which would be discharged vertically through a large diffuser to minimise the sound. The dust hood, as well as all other equipment and offices at the receiving station, would be contained in an industrial building designed to be visually appealing.
Coal hoppers would be located below the track and the top of the hoppers would be covered by the above-mentioned dust hood. Coal would be withdrawn from the hoppers by belt feeders at a rate to ensure each railcar dumps into an almost empty hopper. The hoppers will never be completely empty, eliminating the possibility of dust generation and coal spillage in the hopper vault.
Low-level detectors in the dump hoppers will slow or stop the reclaim belt feeders to prevent the dump hoppers from emptying. Feeder discharge will be controlled by PICOR trim gates and variable frequency drives (VFDs) to ensure the feeders withdraw from the dump hoppers equally and that the reclaim conveyor always operates at 100% of its rated capacity per unit of length (q).
A 1.8 m wide reclaim conveyor driven by a VFD with a CEMA capacity of 5500 tph will receive coal from the dump hopper belt feeders and transport it to the storage building where a tripper will direct the coal to a reversible traversing conveyor or onto the continuation of the reclaim conveyor. The traversing reversible conveyor is necessary to load the coal storage system. If coal is discharged back onto the reclaim conveyor, it will load directly from train to ship.
The reclaim conveyor will originate underground (below the dump hoppers) and will remain underground until it curves upward to enter the top of the storage building. The elevated reclaim conveyor will be totally enclosed in a properly ventilated sealed structure, preventing any particulate emissions.
As can be seen from Figure 2, an illustrated storage building, it is necessary that the loaded coal is distributed to maximise storage volume for this particular storage building design. The system shown can store a maximum of about 520 000 t of coal on ten discrete Walker Reclaimer surfaces. Each reclaim surface consists of a square conical surface with a reclaim port in the centre. Different coals can therefore be stored and reclaimed from different areas and, using the PICOR trim gate, blended with great accuracy.
To illustrate the versatility of this storage capacity, assume half of the storage capacity is used to store coal and the other half is left empty to ensure sufficient available capacity to receive trains:
- If rail service was interrupted, the port would be able to load five 50 000 t vessels before running out of coal.
- If ship travel was interrupted, it would be possible to receive 17 x 15 000 t trains before running out of storage space.
Whether that capacity is sufficient will be decided by the developer of the port. Increasing the capacity of the Walker Reclaimer is cost effective as there is no upper limit to the capacity of Walker Reclaimers.
Two reclaim conveyors with the same capacity as the feed system will be located under the Walker Reclaimer discharge openings. One of the conveyors will discharge to a transfer conveyor, which will discharge onto the other reclaim conveyor. Logic will be provided to ensure that the combined reclaim rate of both conveyors does not exceed the capacity of the reclaim conveyor receiving coal from the transfer conveyor. The reclaim conveyor extending past the transfer conveyor will remain underground until it must curve vertically to feed the receiving conveyor of the self-loading ship.
The reclaim conveyor that feeds the receiving conveyor on the self-loading ship will discharge into an enclosed, permanently installed hopper aboard the ship. A flexible, sealed enclosure will connect the reclaim conveyor enclosure with the ship’s receiving hopper enclosure, allowing motion between the shore and the ship. The reclaim conveyor will be designed to move vertically with the tides and draft of the ship being loaded.
If the average capacity of a ship being loaded is 50 000 t, it will be loaded in 10 hrs. An additional 20 000 tpd would then be loaded on average if the port is to achieve 25 million tpy. Ten hours would therefore be available for the loaded vessel to depart the loading dock and the next vessel to prepare to receive coal; it appears that one loading dock and one standby dock would suffice.
The entire system described herein would be computer controlled. Coal, when received, will be of a known quality and quantity and be stored in a known location. Scheduled coal deliveries will also be known, allowing a storage plan to be made.
Since the arrival of ships and the coal that is to be loaded into them should be known in advance, it therefore becomes a simple matter to develop a programme to load incoming ships and to store incoming coal. The computer recommendations should always be reviewed and approved by operations in case last minute changes occur.
Figure 1. 25 million tpy coal loading port.
To operate the entire port on a 24/7 basis will require one Port Manager, four Port Operators and five Console Operators. A shiploading supervisor will also be required but that person will be a member of the ship’s crew. Due to the interaction between delivery trains and port personnel, it may be advisable to also have three unloading technicians on call to assist train crews as necessary.
All parts of the system described require little maintenance. If bottom-dump railcars are used, the air handling and filter system will be the most maintenance intensive equipment in the coal receiving system. Belt conveyors, properly designed and installed, are very reliable and require little maintenance. With properly installed sensors, failure of bearings, reducers and motors can be predicted. A weekly visual check of the entire system by a motivated maintenance person should give advanced warning of any other impending conveyor problems. The storage building, since no mobile equipment will operate close to it, will require almost no maintenance.
Operation of the Walker Reclaimer is such that very little material moves on the support surface, thereby minimising wear. One system similar to the Walker Reclaimer, installed under a coal storage system using the same type of wear surface intended for the Walker Reclaimer, was in operation more than 20 yrs before the system liner was changed. Almost 5100 t of coal per square foot of liner was reclaimed with this system using the installed liner. The liner thickness, when measured after removal, was not distinguishable from the ‘as installed’ thickness. In other words, no wear was noted. This information is as reported by the mine personnel and not measured by the authors. The system shown contains over 400 000 ft2 of liner. If 5000 t of coal flow per square foot of liner were used as a replacement limit, the system shown would require the liner be changed after 80 yrs of service. Given this evidence, it is conservatively expected the system liner will not require replacement for at least 30 yrs.
The Walker Reclaimer will be activated by a number of electrically powered vibrators. The vibrators will be designed for long life and will only be used as required. The vibrators in the centre of each section will be operated the most and will be easily accessible for maintenance. To access vibrators outside the central vault will require emptying the section containing the vibrators, removing access panels, replacing the vibrators and reinstalling the access panels. Each vibrator that fails will be indicated to the operator.
When sufficient vibrators fail to affect performance, only then will it be necessary to replace them. It is expected the vibrators will have a mean time between failures of around 10 000 hrs. The facility will operate a total of 5000 hr/yr to ship 25 million t of coal. Since there are ten Walker Reclaimer sectors, each one will operate, on average, 500 hr/yr. Most of the vibrators will operate less than 300 hr/yr, resulting in an expected average vibrator life of 33 yrs.
It is expected the maintenance of the port may be low enough that consideration should be given to sub-letting port maintenance. The port should be landscaped and maintained in a presentable manner. While the landscape design may be included in the design contract for the port, a subcontractor should be retained to maintain the site’s appearance. It would seem that to be successful, it is important the port be a welcome contribution to the community ambiance.
One product the port will receive with coal is water. When in storage, water tends to drain to the lowest point. The Walker Reclaimer conical sections will tend to concentrate this drainage, requiring each section to have a small sump and sump pump. A small water clarification device will be necessary to clean this water. After cleaning, the clarified water could be used to irrigate the landscape. The reclaimed solids, depending on the quantity and quality, may be sent to a local landfill or mixed with the product shipped.
Figure 2. Coal storage system.
If compared to existing coal loading ports of the same capacity, it will be readily noticed this 25 million tpy coal loading port requires very little space, CAPEX, OPEX, and permit difficulty. As mentioned previously, if the total operating cost is attractive enough, demand may require expansion of the port’s capacity. The described system is almost totally modular. Another system could be built in parallel to the original without interfering with the port’s existing operation. The dock would have to be extended to add one loading dock and possibly one standby dock. To increase the port’s capacity to 75 million tpy would require only another module.
- WALKER, H. A. and WALKER, D. F., ‘A New Wave Of Technology’ World Coal Vol. 25 No. 6 (June 2016), pp. 48 – 52.
This is an excerpt from an article that was first published in World Coal July 2016. To register and receive your free trial of the magazine, click here.
Read the article online at: https://www.worldcoal.com/handling/28032017/thinking-outside-the-box/
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