Tenth Australian Coal Preparation Conference

October 18, 2004

Precision Train Loading Systems

T Walker(1), M McPhan(2)

(1) PICOR, (2) Halley & Mellowes Pty Ltd

Abstract

A significant proportion of coal mined in Australia is transported to the user or to port facilities by rail. While reclaiming stored coal to be loaded into trains, the final product is often made by controlled blending of different qualities of coal. This paper examines the process of loading coal into wagons, defines different types of wagon loading systems and discusses the economics of different methods of reclaiming, blending, and loading coal into trains.

The loading systems discussed will include manual fiood loading systems capable of loading up to 11,000 tonnes per hour (t/h) and precision loading systems capable of loading, and providing a certified weight for, the precise quantity desired to be loaded into each wagon at up to 11 ,000t/h.

The operational characteristics of different systems will be described and the economics will be discussed with particular attention paid to capital cost, operating cost, product quality, and rail equipment utilisation. The ability to reduce transportation cost and add value to a coal production operation through the use of precision blending systems and precision loading systems will be demonstrated.

Introduction

The scope of this paper is necessarily limited to acquainting the reader with a very general description of volumetric train loading systems, precision train loading systems, the development of high capacity precision train loading systems, the development of blending systems as part of the train loading process, and the transfer of technology from high capacity train loading systems to an economical integrated precision train loading system.

Historically the usual method of loading coal into wagons was to load the wagon as the coal was produced. A preparation plant or mine usually included a rail yard for wagon storage. The wagons were loaded with a particular size of coal as it was produced and in effect served as the storage and transportation vehicles for coal.

Typically the rail company would deliver a string of empty wagons to a mine and collect the loaded wagons destined for delivery to customers. The string of wagons delivered to the mine would remain there until loaded, a time that could range from one to several days. The string of loaded wagons would then be weighed, and assembled into trains according to their destination. The amount of time the wagons spent idle was considerable and represented an opportunity for the rail companies to improve the economics of rail transport of coal.  If it were possible to cut the cycle time of a wagon, the increased utilisation of the rail company's as would potentially result in substantial cost savings.

As time passed, loading wagons was no longer tied to a plant's production as coal was increasingly stockpiled on the mine premises. Therefore a string of rail wagons was able to be shuttled back and forth from the port or customer's premises in an expeditious manner, thus requiring the automation and increased loading capacity of train loading systems. As rail wagons were more expeditiously loaded, the rail companies achieved an increased utilisation of rolling stock.


Train Loading Systems

Volumetric Train Loading

Coal is shipped in trainload quantities to maximise the utilisation of rail equipment, receiving facilities, and loading facilities. Loading and unloading trains expeditiously and expediting transit between destinations increases the efficiency of transporting coal by rail.

Many different types of train loading systems were constructed. Systems were built to allow trains to travel under open and enclosed storage piles to be loaded directly from the storage pile by means of a reclaim hopper or hoppers, a reclaim gate or gates, and a telescopic loading chute or chutes. Some mines elected to store coal in a silo or silos above the track and load train by means of a telescopic or swing-type loading chute as the train traveled under the silo. Other mines stored coal inside a storage silo or in uncovered storage stockpiles and reclaimed the coal to a train loading bin to load trains. Other methods of loading trains by means of mobile trippers, bifurcated chutes, and multiple loading points were built and operated at different sites with varying degrees of success.

One method of loading trains is to store enough coal to load the train, reclaim that coal from storage, and transport it to a train loading system. The train loading system typically consists of a bin of at least 200 tonnes capacity, a discharge gate, and a train loading chute (see figure 1 below). The train is moved under the loading chute by a locomotive while coal is "flood loaded" into the rail wagons. If a train were to be loaded at 3,000t/h, for example, the initial flow rate into the wagon would have to be greatly in excess of 3,000t/h to flood the front of the wagon.

While flood loading a train, after the level of coal in the wagon reaches the bottom of the loading chute, the chute fills with coal and the discharge rate of coal from the chute into the wagon is determined by the speed of the train. This makes it possible to "profile" the top of each wagon allowing the operator to increase the volumetric consistency of loading.

In order to not interrupt the loading operation it is necessary to keep a sufficient amount coal in the loading bin to flood the front of each wagon. By installing different types of level indicators on the loading bins, it is possible for the operator to control the train loading operation and maintain a relatively constant quantity of coal in the loading bin.

While train loading systems can efficiently load trains and support the economics of shipping coal by rail, there remains room for improvement. By volumetrically loading trains, some wagons are inevitably over loaded and some under loaded. Under loading results in decreased utilisation of the rail company's equipment and over loading results in increased maintenance cost. To discourage overloads, rail authorities may impose penalties on the shippers and often require them to unload the overloaded coal.

By under loading each wagon the shipper can minimise overload penalties. The rail company, however, would lose some shipping capacity of each train. Since most shipping contracts are on a weight shipped basis, the rail companies have a vested interest in encouraging shippers to load each wagon to its design capacity whilst preventing overloads.

It is not possible to load each wagon to design capacity using volumetric loading methods since coal density varies with changes in ash content, size, moisture, and other factors. To achieve the precision required, it is necessary to actually weigh the coal being loaded into each wagon.

Using track scales to monitor and control loading was tried without a great deal of success. While it is possible, given satisfactory track conditions, to obtain certified weights of railcars in motion, accurately weighing a railcar during the loading process has not been accomplished. By using a tare scale to weigh each railcar prior to loading and a gross scale after loading, a shipper can know the exact weight loaded into each railcar and correct overloads before they leave the loading site, but track scales alone do not allow control of the amount loaded into each railcar. It is therefore not possible to load trains at the required loading rates precisely by use of track scales alone.

Certified belt scales and a control system to divert coal from one railcar to the next can be used to achieve a degree of loading accuracy. Using this method, it is possible to obtain a certified weight for an entire trainload of coal but not for the amount loaded into each railcar. A system of this type could significantly improve the quantitative loading accuracy of individual railcars. The weight of coal loaded into each railcar cannot, however, be certified. For smaller operations desiring to eliminate overloading problems but not able to justify the expense of a certified precision loading system, this method represents a very possible economical option.

Batch Weighing Systems

By weighing coal to be loaded in batches before loading those batches into a wagon, it is possible to weigh the coal accurately and certify the weight to a static weighing accuracy 0.1%. Technology exists to transfer coal from a surge bin located above a weigh bin into the weigh bin rapidly enough to support train loading rates in excess of 3,000t/h and to control the amount of material transferred with sufficient accuracy to avoid overloading wagons or allowing wagons to be significantly underutilised.

The majority of batch weighing systems being constructed at present in the United States use a surge bin of 230 to 270 tonnes capacity positioned above a weigh bin of 90 to 120 tonnes capacity. Four bi-parting gates usually control the transfer of coal from the surge bin into the weigh bin. All four gates are opened initially to very rapidly transfer coal into the weigh bin and are progressively closed to meter coal into the weigh bin. After a batch has been weighed and the wagon is in position to be loaded, the operator opens the discharge gate to load the wagon. The discharge gate is sized to discharge coal into the wagon at a rate sufficient to flood the front of the in motion wagon. Gates of 1.2m to 1.5m square have most typically been used. By loading each wagon accurately, the shipper maximises the amount of coal in each wagon and thereby maximises the loading efficiency of each train. Since the weights loaded into each wagon are certified, the train can proceed directly from the loading point to its destination. Figure 2 shows atypical batch weighing system.

High Capacity Precision Loading System Development

Production rates of mines in the Powder River Basin in the United States require higher loading rates than are usual at other mines. Several of these mines were built with volumetric loading systems where by loading each railcar from more than one loading point, could load up to 11,000t/h. Several loading methods were adopted to achieve the desired loading rates. By using a 120 tonne weigh bin and charging the bin from more than one source, a 110 tonne wagon could be loaded in two 55 tonne batches, which are both controlled and certified. With this method rates of up to 8,000t/h were possible. Following this trend, it became necessary to retrofit high capacity precision loading systems to the originally designed volumetric loading systems.

BlackThunder Mine, one of the highest producing coal mines in the world, retrofitted two train loading silos that were originally built as volumetric loading systems with double bin precision loading systems in the 1980's. Each weigh bin capacity was limited, due to existing space constraints, to approximately 55 tonnes. This system was designed and installed by Ramsey Technology and achieved the desired loading rate of over 9,000t/h while loading each wagon accurately and providing a certified weight of the contents of each wagon.

In the early 1990's, Kennecott Energy's Cordero Rojo mine, another large producer in the Powder River Basin (figure 3 below), needed to convert their volumetric train loading silos to precision loading silos. Since each of their silos were equipped with two draw-downs, they wanted, as Black Thunder Mine had successfully done, to retrofit precision loading systems under their silos. The geometry of BlackThunder's silos and Cordero Rojo's silos are different. There is not enough vertical clearance between the top of rail and the underside of the existing silos at Cordero Rojo to accommodate the 55 tonne weigh bins. Since it was necessary to load 110 tonne wagons, this created a problem.

The solution to the problem, devised and patented by one of the authors, (Tony Walker, PICOR) was to install two 40 tonne capacity weigh bins under each silo and deposit two batches into each wagon from the first weigh bin and one batch into the same wagon from the second weigh bin. On first consideration, it seems this scheme should decrease the loading rate. To explain why it did not requires that some details concerning precision loading systems be reviewed.

To rapidly load rail wagons in this manner it is necessary that the front of each wagon be completely loaded, the entire wagon be loaded without voids, all coal loaded into each wagon be accurately weighed, and that the total amount of coal loaded be very close to the desired amount.

Since Cordero Rojo's loading system requires the use of two weigh bins, loading the front of each wagon at the required loading rate is not difficult. Partially loading the front of each wagon from the first bin reduces the volume required from the second bin to insure the front of each wagon is fully loaded.

Batching, weighing, and loading multiple batches from one weigh bin into a wagon moving at a speed sufficient to load 9,100t/h of coal is challenging. In order to load a train of 100 tonne capacity wagons that are 16.15m long overall at 9,100t/h requires the train to be moving at 0.41 m/s. If the inside length of the above mentioned wagon were 13.72m and the loading chute were 2.13m square, the available time to discharge two batches in one wagon would be approximately 23 seconds. It is therefore necessary, in the available 23 seconds, for the system to discharge the first batch into the wagon, close the discharge gate, obtain a tare weight of the weigh bin, open the weigh bin charging gate, transfer a quantity of coal into the weigh bin, close the weigh bin charging gate, obtain a gross weight of the weigh bin, open the discharge gate, and discharge the second batch into the same wagon.

A time motion study proved that there was enough time to accomplish all the above provided the second batch of coal loaded into the weigh bin be prepared utilising only one set point to close the weigh bin charging gate. To make an accurate batch requires the use of multiple set points to sequentially close the weigh bin charging gates as the coal in the weigh bin approaches the target capacity. Clearly, batching coal into a weigh bin accurately requires more time than batching without regard to accuracy.

Only the last batch of a multiple batch precision loading system affects the wagon loading accuracy. As long as the weights measured by the first bin are certified, subtracting the weight of the first two batches from the target weight of the wagon and batching precisely the resultant into the second weigh bin insures the amount loaded into each wagon closely approximates the desired amount.

High flow rates between the silo and weigh bin and the weigh bin and wagon must be achieved for this system to operate in the time allowed. By using 2.13m square weigh bin charging and discharge gates on the first weigh bin, flow rates of over 11 tonnes per second were achieved. The opening time and closing time of the gates were also critical. It was necessary to operate the gates at 0.762m per second.

The second weigh bin receives its target weight information after the second batch of the first weigh bin is weighed. This allows plenty of time to accurately batch the required amount of coal in the second weigh bin. Since the required flow rate into and out of the second weigh bin is lower than the first weigh bin and since the charging accuracy of the second weigh bin is critical, the second weigh bin charging and discharging gates are only 1.83m square. The second weigh bin charging gate is a four-blade gate that utilises two set points. At the first set point, three of the four blades are closed and at the second set point the last gate blade is closed. The discharge gate is a bi-parting gate. The gates on the second weigh bin also operate at 0.762m per second.

Impact of the opening and closing of the gates, if the hydraulic cylinder cushions are not properly set and maintained, can cause vibrations that delay the weighing operation. Each gate blade of a 2.13m double blade gate weighs approximately one tonne. Stopping a one tonne gate blade moving at 0.762m per second, if not cushioned, can result in significant forces. If properly cushioned, the forces are easily handled and normal gate operation is uneventful.

Impact forces of the coal charging a weigh bin at over twelve tonnes per second are also significant. In a conventional weigh bin, the flow rate is initially only slightly less than that through a 2.13m square charging gate. The load cells detect the impact load of coal transferring into a single batch weigh bin but since that impact load does not occur close to a critical set point, it ussually goes unnoticed. The impact load on the triple batch weigh bin, however, occurs at the single set point that closes the weigh bin charging gate and must be concidered. That impact load is shown in Figure 4 below.

Coal Market Requirements

Due to variations in coal quality requirements, it is often necessary to blend different qualities of coal to meet market specifications. While it is sometimes possible to blend coal going into storage to make the required product, if a producer has more than one contract specification which must be met, it is generally best to store different qualities of coal and blend to the market specification as the coal is being reclaimed and loaded. If coals are blended while being stored and the expected train fails to arrive in sequence, it would be necessary to prepare another blend to load the train. Therefore the ability to blend the specified product while loading greatly simplifies the logistics of loading abd shipping coal.

Precision Blending Systems

When the coal of different qualities are stored separately it is posible to reclaim different coals selectively thereby maximising the use of the most valuable coal while meeting the required specification.

If the quality of coal in each storage area is known, it is possible, using adjustable reclaim gates or feeders and monitoring the amount of coal reclaimed from each stockpile, to control the quality of the product to approximately that desired. By using the output of an on-line elemental analyser capable of providing real time analysis of coal being loaded to adjust relative reclaim rates, it is posible to adjust the blend precisely and accurately reclaim the desired quality.

Technology exists to control the reclaim rates of coal very accurately by several different means. Selection of the most cost effective reclaim method is dependant on the characteristics and quality of the coal to be reclaimed. Belt scales or weigh feeders can be used to monitor the exact feed rate from the reclaim point. Properly designed, a reclaim blending system can be controlled to within two percent qualitatively. If the quality of the coal were known perfectly in each stockpile, it would therefore be possible to blend to a target without fear of exceeding any quality specification. Since the exact composition of a stockpile varies throughout the stockpile, it is necessary to take that variation into account when determining the blend.

By installing an on line analyser on a reclaim conveyor, the output from the analyser is available to adjust the quantitative reclaim control thereby allowing the reclaim blending system to precisely meet the qualitative target. The reclaim blending system, properly designed, would be as qualitatively as the analyser. Since on line elemental analysers are improving in accuracy over the entire periodic table and currently suitable for monitoring certain elements for blending purposes, it is expected that in the near future they will be increasingly used to control precision blending systems.

Precision Loading Systems

Lessons learned through solving problems often to improvements in established methods of operating. Solving the problem of high capacity precision loading of coal into wagons in the restricted space allowed under Kennecott Energy's Cordero Rojo silos forced one of the authors to examine the operation of large, high speed transfer gates, the behavior of coal at high transfer rates for short periods of time, methods of deceasing the time required to batch and weigh a quantity of coal, and other considerations of loading coal into wagons at 5,500t/h or less, it became clear there was a very economical method of loading coal at 5,500t/h and below into wagons with the same quantitative accuracy as was currently being accomplished. That method was loading three batches of coal into each wagon from one weigh bin.   

In order to achieve a loading rate of 5,500t/h, the train must move at a relatively constant rate under the loading apparatus. As each wagon arrives under the loading chute, the amount of coal necessary to load the front of the wagon must be available and the weigh bin discharge gate must be adequate to provide a flow rate sufficient to load the front of the wagon before moves out of position. It is then necessary that the flow rate of coal from the weigh bin into the wagon be sufficient to fill the wagon without voids. Also, the quantity of coal loaded into each wagon must approximate the target weight and must be weighed in accordance with Australian regulations.

By selecting a precision loading system that uses three batches to load awagon, all of the above requirements are met.

Single Batch System

For our purposes we will assume the largest capacity wagon to be loaded will hold 110 tonnes. A single batch precision loading system must transfer the entire 110 tonnes of coal from the surge bin into the weigh bin and weigh the coal transferred in the time interval between the closing of the discharge gate while loading the preceding wagon and the arrival of the front of the wagon to be loaded under the loading chute. This normally presents no problem for the commercially available single batch systems operating at up to 5,500t/h. When the wagon arrives under the loading chute, the weigh bin discharge gate is opened and the front of the wagon is loaded. The operator then trims the top of the coal in the wagon to the desired level.

Double Batch System

Another method of precision loading wagons utilises two batches from a single weigh bin. With this system the first batch approximates half the capacity of the wagon, in this case, 55 tonnes. Batching and weighing that amount while the coupling traverses under the loading chute presents no problem. Also, loading the front of the wagon is straightforward. The second batch however, is approximately the same size as the first but must be transferred into the weigh bin accurately, weighed, and the discharge gate opened in the time required for coal from the first batch remaining in the loading chute after the weigh bin discharge gate is closed to be discharged into the wagon. As noted earlier, accurately transferring coal into a weigh bin requires multiple set points and consumes time. The time consumed transferring, weighing, and discharging the second batch limits the capacity of a two-batch precision loading system for wagons to between 3,500t/h and 4,500t/h.

Triple Batch System

A triple batch precision train loading system achieves its high throughput, loading accuracy, and certified weights for coal loaded into each wagon as follows:

* By using a weigh bin that holds approximately 45 tonnes, it is possible to make the first batch for each wagon large enough to fully load the front of the wagon and the loading chute.

* There is sufficient time, using a 1.83m square four blade weigh bin charging gate, to make the first batch large (45 tonnes) and accurate while the coupling moves under the loading chute.

* By using a 1.83m fast-acting, bi-parting discharge gate, the first batch discharges at over seven tonnes per second leaving approximately 8 tonnes of coal in the loading chute after the front of the wagon is filled, the weigh bin is empty, and the weigh bin discharge gate closes. Coal is then discharged from the chute as the train moves.

* The time available to make the second batch is the time required for the wagon to move far enough to unload the coal from the first batch remaining in the chute and move the loaded coal profile from under the loading chute. At a loading rate of 5,500t/h the train speed is approximately 0.8km/h and the loading rate into the wagon after the front of the wagon is flooded is approximately two tonnes per second. Approximately 12 seconds are therefore available to close the discharge gate, obtain a tare weight for the weigh bin, open the weigh bin charging gate, transfer an amount of coal, weigh the loaded weigh bin, and open the discharge gate. Since it is not necessary to make the second batch a precise quantity, that time is sufficient to transfer and weigh a relatively large batch.

* The third batch target weight is preset at approximately twenty-three tonnes and modified by the combined weight of the first two batches. The batch that determines the overall batching accuracy and loading rate is therefore small and relatively consistent.

By loading three batches per wagon, the required capacity of the weigh bin is reduced from 110 tonnes to 45 tonnes, the required capacity of the surge bin is reduced from approximately 275 tonnes to approximately 150 tons, the required feed conveyor can be significantly shortened and the required power to elevate coal to the top of the loading system is reduced. The number of mechanical components required to operate the system is reduced thereby reducing maintenance cost. Since the weight of the loading system as well as the capacity of the bins is reduced, the foundations cost is minimised.

Due to variability of train speeds, operating a loading system at relatively high loading rates with a small surge bin requires monitoring the amount of coal in the surge bin and modulating the reclaim rate to maintain that level relatively constant. A method of monitoring the surge bin level at the time the charging gate opens to load the first batch for each wagon has been developed. This method automatically adjusts the total reclaim rate to maintain the level of coal in the surge bin within acceptable limits while allowing significant train speed fluctuations. This is accomplished while maintaining automated qualitative blending system accuracy.

Conclusion

By using an automated reclaim system capable of blending two or more coals to make a qualitatively precise product the producer can maximise the value of their reserves and ensure customer satisfaction. By adding a precision loading system to the precision blending system, the producer can maximise the utilisation of the railroads resources making possible transportation cost savings for their customer. By incorporating an automatic surge bin level control and automatic loading system, the loading system operator can minimise the cost of operating the integrated precision train loading system.

In today's coal market, it is necessary for the coal producer to fully consider how coal reclaim and loading systems can add value to their operation. By providing a precision train loading system that is precise both quantitatively and qualitatively, it is possible for a mine to minimise its capital investment and operational costs while simultaneously maximising the value of its reserves.

Acknowledgements

The authors would like to acknowledge the continued support of the various industrial participants who contributed to this paper. In particular, the authors would like to recognise the management team at Halley &Mellowes Pty Ltd and the design team of PICOR, especially Douglas Wilson and Les Pearson.

References

Empey, ER, 2003, "A Major Step Forward for On-Line Coal Analysis", 20th International Coal Preparation Exhibition and Conference Proceedings and Index, Coal Prep 2003 April 29-May 1,2003, Lexington, KY U.S.A., p137-151.

Evans, MP, 2003, "A Major Step Forward for On-Line Coal Analysis", 20th International Coal Preparation Exhibition and Conference Proceedings and Index, Coal Prep 2003 April 29-May 1,2003, Lexington, KY U.S.A, p137-151.

Woodward, RC, 2003, "A Major Step Forward for On-Line Coal Analysis", 20th International Coal Preparation Exhibition and Conference Proceedings and Index, Coal Prep 2003 April 29-May 1,2003, Lexington, KY U.S.A, p137-151



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