By Dan Navicky
Coal fired power plants are dusty places. The handling and transportation of coal creates dust. This causes health issues and can lead to explosions if not properly maintained. Most dust collection systems’ ductwork designs use straight horizontal ductwork and a high air velocity to keep the dust from falling out of the air stream and building up in the straight sections. An alternative to the high velocity design is the low velocity duct system which uses a saw-tooth design duct run.
This is a more expensive design to install but minimizes power and maintenance. A third option uses a point of use cyclone to capture the benefits of both high and low velocity systems. The cyclone can work in tandem with various types of dust collectors that may be present in the application.
In addition to trying to achieve accurate dust collection results, the release of the new NFPA 654 has put a spotlight on housekeeping around coal fired power plants. NFPA 654 is a National Fire Prevention Association standard for the prevention of fire and dust explosions from the manufacturing, processing and handling of combustible particulate solids. The standard applies to coal fired power plants because dust is no longer permitted to settle in a layer on the equipment. If the coal dust does produce a layer on the equipment then an air disturbance can cause the coal dust to be suspended in the facility’s air supply. Should that dust-laden air supply come in contact with a spark or fire source, a deflagration or explosion can occur. To keep the dust from layering on the equipment and facility rafters, improvements to dust collection systems and stricter protocols are required. The following will explain how you can stay in compliance with these new protocols.
A dust collection system consists of four parts 1) the collection hood 2) the ductwork 3) the dust collector and 4) the exhaust fan. The collection hood is used to pull the dust from the area that creates it. A properly designed hood minimizes the air required while maximizes the dust collected. The ductwork is used to transport the air from the collection hood to the dust collector. The dust collector is usually a baghouse, cartridge collector, cyclone, electrostatic precipitator, or a combination of these. The fan provides the motive force for the system and is usually located just before the exhaust stack. The following discusses the different types of ductwork designs for coal fired power plants and the advantages or disadvantages they offer.
High Velocity Ductwork System
The most common system for coal dust collection in power plants is high velocity air, 4000 FPM (feet per minute) or greater, in straight run ductwork. The high velocity airflow is meant to keep the coal dust from falling out of the airflow and collecting inside the ductwork. If the air velocity begins to drop lower than the 4000 FPM, coal dust can begin to accumulate inside the ductwork.
As this happens, the pressure drop in the ductwork begins to increase and less airflow is allowed through the system. This causes the pull at the hoods to fall, thereby letting more dust to remain at the collection point. Less dust collection in the hood will result in an increase in dust within the air throughout the facility. Additionally this will lead to dust producing a layer on the equipment in the plant. In order to comply with NFPA 654, additional measures will have to be taken to eliminate the dust from settling on the equipment and stay in compliance with the regulations.
The main advantage of the high velocity ductwork system is that it is less expensive than the alternatives and that it takes up the least amount of space. The higher velocity of the air allows smaller ductwork to be used, thereby decreasing the initial cost of the system. The straight horizontal runs provide the minimum amount of ductwork required when compared to the low velocity system. This again keeps initial costs lower and keeps the height required to just the height of the ductwork and supports.
A big disadvantage of the high velocity ductwork system is that there is a higher pressure drop for the system, and abrasive dust, such as coal, will cause greater wear on the equipment in the system. It is the higher velocity that causes greater collisions of the dust with the ductwork walls. Abrasive dust will begin to wear away the walls and cause holes to develop in the ductwork over time. Holes in the ductwork will allow air from undesired areas to enter the system, thereby decreasing airflow at the pickup points.
Another disadvantage to the high velocity air option is that it creates greater resistance to the airflow, which is the cause of the higher pressure drop in the system. A final disadvantage is that the higher pressure drop requires a larger fan, which requires more power. Therefore, the operating expense of the high velocity system is greater than the low velocity system.
Low Velocity Ductwork System
Another type of dust collection system ductwork design is the low velocity ductwork system. This system uses larger ductwork, so the air velocity is lower (1800 FPM). To keep the coal dust from falling out of the ductwork, the hoods are designed to capture only dust in the respiratory range (<10 micron). Larger dust particulate fall back to the process and isn’t collected. The low velocity system ductwork has a saw-tooth shape, where one leg of the duct is at a 45° angle from horizontal and then the next is 30° from horizontal. This allows any coal dust that would accumulate in the ducting to fall out to the bottom where it can be collected. However, the design will require more equipment and system modifications to pick up the larger dust not collected in order to comply with NFPA 654.
The main advantages of this style of system are that the overall operating costs are less expensive and the maintenance and the wear are less than what is found in high velocity systems. The low velocity ductwork system runs at an air velocity of 1800 FPM. This ductwork is larger than in a high velocity system. With the lower air speed, the resistance is lower, so the pressure drop of the system is less. The lower velocity also means that the dust abrasion on the walls will be less, thereby increasing duct life and decreasing the chances of holes developing in the ductwork.
There are a few major disadvantages to using low velocity ductwork system. First, the low velocity system uses larger ductwork than other dust collection systems. This means that the initial cost of the ductwork is greater. The saw-tooth design also has approximately 28% more straight runs along with additional turns than the high velocity system. The added ductwork will increase the cost of the system. The saw-tooth design also requires more vertical space than the other systems. Due to the angles in the saw-tooth, this can be as much as 3-5’ extra ductwork in the facility. If there is limited head space, this could force the ducting outside (requiring additional protection such as insulation, etc.). The final disadvantage is that the larger dust (<10 micron) isn’t captured in the hood and it falls back to the collection point. This can cause operational issues or clean-up issues near the pick-up point and may require additional cleaning. Explosive dust will require additional measures to collect the dust greater than 10 microns. With the new NFPA rules about dust layering, it can also be an OSHA violation to allow dust to collect on equipment.
Point of Use Cyclone System
The third type of dust collection system for coal fired power plants is the use of a cyclone as a point of use pre-filter. The cyclone will remove the majority of the coal dust from the airflow near the point of collection. With the majority of the dust removed (10 micron and larger), the air can then move to the primary dust collector at a middle velocity (2000 FPM). This lower velocity will provide a lower pressure drop than the high velocity system. And, since there is a higher velocity and most of the coal dust is removed, no saw-tooth ductwork design is required. This system design collects all dust sizes (not just <10 micron) with a low energy demand. No additional cleaning at the pick-up is required. The hybrid dust collection system requires less power than the high velocity ductwork design and provides greater dust removal than the low velocity ductwork design.
The ideal cyclone for the Point of Use hybrid system would be a cyclone less than 8ft3 that can be installed indoor per NFPA 654 without requiring additional expensive explosion prevention equipment. Airflows less than 2,000 CFM may be able to use a cyclone as a point of use dust collector per NFPA 654. As with all cyclones, the removal efficiency increases as the airflow and pressure drops increase. So deciding which cyclone to use as a point of use pre-filter is a trade-off of size vs removal efficiency and pressure drop. The following graph shows estimated removal efficiency of dust (SG of 1, particulate – spherical)
While cyclones can add extra cost, they require minimal maintenance. Using a cyclone as a point of use pre-filter will reduce on maintenance costs and loss of production time that would typically occur with the other ductwork systems. As with all dust collectors, a properly designed airlock should be provided so the greatest removal efficiency is achieved by the dust collector. Point of use dust collectors also have the advantage of being placed closely to where the dust is created. This allows for easy recycling or use of the dust. Coal dust that is collected can be fed into the fuel feeding system within the power plant.
Comparison of the Three Systems
The following chart shows a comparison of operating expenditure and power for a hypothetical system using the three different designs. As you can see, the Point of Use design provides performance similar to the low velocity system (Cyclone w/ dp of 1”) without the low velocity system issues of increased size and only capturing the smallest dust.
An Aerodyne GPC cyclone was used in the hybrid dust collection system in the comparison below.
Our Current System Delivering the Right Results?
Whether you are designing a new dust collection system or trying to troubleshoot your existing system, there are numerous things to consider for your coal dust application. The design of the ductwork plays a crucial part in the process and should be placed on the same level of priority as the hood, dust collector, and exhaust fans. A ductwork system for a coal fired plant that is not designed appropriately can lead to lower airflow at the hoods and a less efficient operation overall. Additionally, when designing or troubleshooting your system, always try and minimize the amount of ductwork being used. Reducing the amount of ductwork in a single system will help to decrease the resistance of the process or system. Using a Point of Use Cyclone as a pre-filter can help keep operating and maintenance costs down, while improving efficiency and removal levels. So if your dust collection system for your coal fired plant is not operating as you think it should or it is not achieving the results you want, then it might be time to take a look at the current design and rethink some of the components. Industrial cyclones used as pre-filters offer an ideal solution for coal fired plants. They can effectively capture dust of all sizes, reduce wear on the rest of the system, and reduce maintenance and energy costs.
Dan Navicky is the Sales Engineer at Aerodyne Environmental in Chagrin Falls, Ohio