Good material flow can reduce coal yard maintenance and operational problems
By Martin O’Neil, Parramatta Group
If coal dust collects under transfer chutes or in the transfer area, it’s cleaned up. When line components wear out or fail, they’re replaced. Should the amount of respirable dust in the air rise to levels prohibited by government regulations, steps are taken to remove it.
Many power plant managers view these and other regular maintenance activities simply as the cost of doing business. What they may fail to realize is that an often-overlooked - yet vital - component of material transfer systems could be the cause of their daily, weekly and monthly maintenance challenges.
Historically, conveyor system design focused on the system’s overall structural integrity, while ensuring a proper fit within the facility’s footprint. Material transfer points were often an afterthought with little design analysis or consideration for material flow characteristics. The solution for controlling flow was to use simple rock boxes, which are still common at many North American power generation facilities.
Airborne coal dust around forms a haze around a conventional coal chute installation
The good news for management is that a plant tour and thorough inspection of the conveyor system can reveal some obvious signs of underperforming transfer points. An audit of the old technology will help determine whether or not plant owners should look to an engineered transfer solution.
The following list does not suggest an order of importance. Rather, items are ordered starting from inside the chute walls, moving to liners, then to general build-up and outside spillage.
Reason No. 10: PRB Coal -Significant cost advantages exist in using Powder River Basin (PRB) coal to fuel a power plant’s boilers. As a result, many facilities have started using a blend of PRB and other coals. In some cases, facilities have switched to PRB coal completely.
Although cost-effective, using PRB coal creates challenges, especially for older transfer systems. PRB coal has a tendency to be “more powdery.” The coal degrades much easier and creates more dust than other coals. In addition, more PRB coal has to be burned to generate the heat required to fuel the boilers. If a power plant uses PRB coal, older transfer systems may be unable to keep up with demand or effectively handle PRB coal.
Reason No. 9: Throughput - Conveyors prone to material buildup, bridging and blocking need a reduced material flow to avoid these pitfalls. As a result, systems operate below optimal performance levels. The belt speed remains the same, but less material moves. Conversely, an engineered chute design can accommodate higher tonnages. This is especially important for facilities using PRB coal.
The key to an engineered transfer is in keeping the material moving throughout the system. At the same time, it’s important to introduce subtle changes in direction to affect the material velocity. This process requires less volumetric chute capacity or smaller components. The system can be built to match the size of existing chute components. As a result, flow rates and throughput will increase.
Reason No. 8: Material Build-Up - Regular material transfer system inspections will reveal a lot about whether the system is doing its job or simply creating more work for plant employees.
When a coal stream hits a stationary chute wall or abruptly changes speed or direction, forces are generated that degrade the materials. This causes significant dust build-up inside the chute. A properly engineered transfer will reduce material impact and energy changes during its transfer and eliminate this buildup. Plant personnel should open the transfer chute inspection doors on a regular basis to check the chute walls for dust buildup.
Reason No. 7: Wear Liners - Wear liner should be checked for signs of accelerated wear including scratches, dents or abrasions. Wear liners are available in a variety of materials and thickness depending on the material being transferred. For most coal applications, wear liners made of alumina ceramic tiles are recommended.
Ceramic lined transfer chutes are capable of withstanding up to 200 million tons of coal throughput, or more, without maintenance if they are part of a correctly installed transfer system. After 10 years, the amount of damaged tiles replaced in a traditional transfer system will eventually add up to a complete liner change.
Reason No. 6: Spillage - Outside the transfer chute, the challenges associated with an older transfer design are more visible. Past transfer chute design is based on containing materials rather than controlling the velocity of the material as it falls.
When material is dropped onto a conveyor belt, the velocity causes the coal to spread out. Fully enclosed skirting rubber is used to keep the material on the belt and allow it to change direction. But, the speed of the belt often differs from the speed of the falling material. The belt deflects the material between the rollers and the material finds a way to “leak out” under the skirting rubbers.
As a result, conveyor load points pose a maintenance problem due to spillage around the receiving area. Piles of material and dust on the floor create a potential slip and trip hazard for workers. Falling coal pieces from transfer points above the plant floor also can compromise safety. In addition, spillage can limit access for personnel to inspect equipment and perform necessary maintenance checks. If cleaning up spillage is part of a regular maintenance program, it’s likely the transfer chutes are the root of the problem.
Reason No. 5: Skirt System Wear - The traditional enclosed skirting system-used when attempting to contain spillage-is another component that will require periodic maintenance. The material’s abrasiveness causes skirt rubber to show significant wear. It should be inspected regularly.
An engineered transfer solution can control spillage so effectively that it makes a skirting system optional. A transfer system without skirt rubber can operate effectively for 5 to 10 years with no maintenance.
Reason No. 4: Dust - As with spillage, it’s easy to see where older transfer systems are underperforming. A haze of coal dust in the air and a coating of dust on mechanical conveyor components are good indicators, even before measurements are taken to determine the amount of respirable coal dust in the air.
A view from the top of a conventional coal chute. Photos courtesy of Parramatta Group.
Government organizations such as the Occupational Safety and Health Administration, the Mine Safety and Health Administration and the National Institute for Occupational Safety and Health are increasing their monitoring of dust levels at coal handling facilities. Many power plants are adopting standards that allow 2.0 milligrams or less of respirable dust per cubic meter of air.
Enclosed skirting systems, bag houses and dust collectors used to collect and process respirable dust are typically treating the symptom rather than eliminating the primary cause. Material accelerated or decelerated and directed through the use of low angles of impact and minimal chute contact results in little dust generation and material degradation.
Reason No. 3: Plugging - Chemicals such as sprays, foams and emulsions are another way power plants attempt to suppress coal dust.
However, using these chemicals has its drawbacks, aside from cost. For example, PRB coal can become sticky when sprayed with suppression chemicals. In addition, a coal with heavy moisture content, such as lignite, may cause transfer chute entry and exit points to become vulnerable to plugging.
Furthermore, it takes a lot of chemicals to prevent a severe dust problem. If a power plant is using an inordinate amount of dust suppression chemicals each month, or is experiencing emergency line stoppages due to plugging, it’s likely the transfer chutes are a contributing factor.
Large pieces of coal can accelerate belt wear. Courtesy of Parramatta Group.
Reason No. 2: Belt Wear - Nothing can bring a material handling operation to a halt like a damaged conveyor belt. When falling in an uncontrolled state, coal will cause significant belt wear. Evidence of belt wear will appear along the length and in the center of the belt in the load zone.
An engineered transfer solution reduces the material’s impact on the belt by using a low angle of incidence. This lays the material in the belt’s center at the same speed as the receiving conveyor and in the same direction.
Reason No. 1: Belt Tracking - Traditional transfer chute designs result in off-center loading because of their variable flow rates and coal consistency. Subsequently, belt tracking can be thrown off, creating additional maintenance issues. If the tracking is bad enough, it could trip the failsafe switch and abruptly stop the conveyor belt. It’s important to continually monitor belt tracking to avoid significant downtime.
From dust and spillage to conveyor component wear and belt tracking, each potentially poses a big challenge to a power generation facility’s material transfer system. Each affects productivity, throughput and the bottom line. With some creative thinking and a systemwide approach, each can be addressed.
Author: Martin O’Neil is business development manager for Parramatta Group. He has been involved with the sale, design and manufacture of conveyor applications since 1989 serving the iron ore, coal, steel, power generation, aggregate and surface mining markets.