Nick Koppelaar, Plastruct-Polyzone group, Canada, explains how the application of a polymer lining can boost the flow of coal through transfer points.
When looking to boost the flow of coal through transfer points, operators and frontline staff at handling systems must understand four key facts:
- Fact 1: A smooth flowing coal handling system is usually an oxymoron.
- Fact 2: This has remained unchanged for decades and becomes worse as coal quality changes.
- Fact 3: Most handling systems insufficiently deal with the resistive nature of coal flowability.
- Fact 4: There is no one, single magic answer, but a series of smaller victories.
Why coal gets stuck
Moisture – both inherent and absorbed – is the key reason for poor coal flows. The higher the coal moisture level above the 5% level, the more issues associated with the product there are, and the more inefficient the coal handling system becomes. An important victory, therefore, is reducing moisture or – just as importantly – preventing additional moisture being absorbed. Reviewing coal content may provide solutions. Higher clay content not only retains higher moisture levels, it also increases stickiness. Many facilities have also turned to low sulfur sub-bituminous coal due to availability, but this creates a new range of parameters that most handling systems are not capable of dealing with.
Despite this, coal handling workers must ensure the coal continues to flow within the existing infrastructure. Maximising that existing system(s) is thus practically the only option available. A system evaluation is an absolute necessity. It is often best practice to break the evaluation down into sections and have it completed by frontline staff. Room should be left on the forms for notes. The evaluation should include production loss feedback, which will highlight the true cost in lost labour production. Management must be prepared to read notes of exasperation, as employees deal with the frustration of poor coal flow on a daily basis.
Once armed with this report, it will provide insight into lost production. Additionally the risk of stagnant coal that has built up in a system must be factored into the equation, as build-up of heat and spontaneous combustion can the result. This cost often only truly becomes realised after the damage is done.
It is also important not to overlook the obvious. For example, consider a typical coal handling structure: these are steel with a small portion made from concrete. Most chutes are also steel. While steel is not susceptible to moisture equilibrium, it does absorb moisture. With a coal at a moisture level of 5.5%, a significant percentage of coal will successfully adhere to the structure. As temperatures decrease in colder climates, the phenomena exponentially increases. Quite simply, if there were a way to reduce this fatal attraction between coal and steel, operators would solve a significant portion of their coal handling woes.
A fatal attraction
If the attraction between coal and steel is mostly due to moisture absorption, then it follows that the removal of this moisture, either in the coal or in the steel, will significantly help – but this is often impractical or too costly. Another problem can also be created by trying to do so: namely an increase in coal dust. To remove the moisture absorption of steel is also not practical or even possible.
Using a polymer material as a solution requires that the polymer has certain physical characteristics:
- Water absorption of zero.
- Low co-efficient of friction.
- Adequate impact strength.
- A tensile strength that allows the co-operation of a joint effort with the steel structure.
This is a tall order, as polymers that exhibit one characteristic may be poor in another. Combine this with the fact that handling bulk materials requires extreme resistance to sliding abrasion, then nearly all polymers fail to meet the requirements.
Fortunately for coal handling operators, in the days of the polymer development boom, a product was developed that does meet the criteria. Tivar88 was developed specifically to have zero water absorption, low co-efficient of friction and high impact strength. Notched izod strength (how quickly the material will tear or crack when scratched or cut) is extremely good with a no-break ASTM result. The weakest parameter of this material is the thermal expansion properties, which differ significantly from that of steel.
It is possible to use this material together with the coal handling structure to boost coal flow. To do so requires operators to go back to their system evaluation. They must look at the hot spots noted by staff. Typically, these will be the discharge areas or areas with unseemly geometry. Once these hot spots have been noted, each one must be expanded upstream to include a slightly larger area. This will be the area that requires the liner.
The following brief case study shows the improvement of coal flow in an existing coal handling system used at a steelmaking plant.
- The coal is stored outdoors in piles.
- It is loaded into ground hoppers by belly loaders.
- The ground hoppers discharge onto a vibratory chute, which discharges onto belt conveyor.
- The conveyor, through a series of transfer points, delivers coal to mixing plant.
- A rotary chute redirects the coal into the appropriate hopper.
- After the mixing process, the coal blend is then transferred to its final destination.
A system evaluation clearly identified the hot spots. Operators were able to pinpoint the problem areas, as well as provide a summary of labour time required to deal with the poor flow. A time study was also completed to provide a thorough understanding of the problems with the flow through the system, while also providing a measure of the progress to fix these issues.
Engineers produced drawings for Tivar88 liner panels to be fitted into the hot spots, allowing for thermal expansion variance from the steel chutes. Installation was done in accordance to polymer standards. All leading flow edges of the Tivar88 were protected by a polymer protection profile. In some cases, the parts installed were very small, targeted directly at the hotspot, while other areas were enlarged towards the flow to create mass flow.
The results were dramatic. Ground hopper discharge through to initial conveyor contact was 30% faster with no hang-up points. Coal flowed smoothly at transfer chutes (a side benefit here was minimal coal build-up on pulleys and bearings). A rotary chute, which previously clogged due to a low wall angle, required no labour to keep coal flowing. The operators were able to turn the vibrators off on the mixing hopper (these vibrators had previously created structural damage to the hopper, which in turn required a costly fix).
Inspectors of coal handling systems often report instances of sledge hammer beat marks on discharge chutes and hoppers. In addition to this, pokers are often seen to help operators initiate flow. As expected in cold, moist climates, these efforts take on a feverish pitch. The frustration level of operators and frontline staff alike is apparent as lost production time and poor output numbers continually haunt them. Once the coal flow has been improved, a significant weight is lifted from these workers’ shoulders. For coal handling operators, there are not many professional joys above seeing coal flowing continuously through their system.
Note: This article first appeared in the July 2014 issue of World Coal.
Written by Nick Koppelaar. Edited by Jonathan Rowland
Read the article online at: https://www.worldcoal.com/handling/14082014/world-coal-how-the-application-of-a-polymer-lining-can-boost-the-flow-of-coal-through-transfer-points-coal1210/