By Rajesh Mendiratta, Christopher Humphrey, and Bob Klare
(Illustration by Andrew Berridge)
A great deal of effort has been put into developing technologies designed to reduce acid rain, pollution and toxins (NOx and SO2), and global warming. While the new technologies are a huge step in air quality controls, their development is just the tip of the entire process. Whether or not these technologies can be retrofitted into an existing plant or designed into a new plant depends on the economics of the process.
Sulfur dioxide control technologies attempt to reduce SO2 emissions through technologies utilizing conventional limestone and gypsum in flue gas desulfurization (FGD) systems. The integration of the material-handling systems for FGD projects with the existing coal-handling plant is the key to success. The material-handling design criteria of these plants often vary from the existing coal-handling systems. Various factors need to be considered and evaluated before and during the design of new material-handling systems for limestone and gypsum FGD systems.
Many power plant designs incorporate a redundant equipment philosophy to avoid plant breakdowns. However, such a philosophy may not apply to the new limestone- and gypsum-handling systems. There can be large economic advantages to single conveyors over dual conveyors. Dual conveyor systems inherently result in substantial capital investments for equipment, structure, foundations, and electrical power distribution systems.
An alternative solution to redundancy may be achieved through the use of dual conveyor drives as opposed to dual conveyors, since the drive is the most likely component to be out of service. In contrast, idlers, belts, and pulley assemblies have low failure rates. Costs for initial equipment, structural steel, and installation are lower, and increased reliability of systems may often be realized.
There are many specific equipment considerations when incorporating FGD systems. Different material-handling characteristics of limestone and gypsum effect the system equipment in several specific ways and require consideration when choosing the angles for chutework, wear liner material, idler rolls, and more.
When comparing chutework for different systems, the valley angles depend on the specific bulk density, lump sizes, and drop distance of the material. Limestone systems have a minimum valley angle similar to that of coal systems—around 60º. However, gypsum systems require steeper valley angles of 70º or more to prevent material buildup and plugging. Steeper valley angles in hoppers and chutework require the equipment to occupy more vertical space. It is important to account for the extra vertical space early in the design and layout of the system because adding vertical space after a structure or system is designed is the most difficult and costly of all modifications.
(Illustration by Andrew Berridge)
Equipment for each system is specific to the chosen material. This can be seen when choosing wear liner material. The material required for limestone and gypsum systems may be less expensive than what is needed for coal systems due to the fact that the liner requirements differ for handling these materials. Abrasion-resistant liners with a Brinell hardness of 400 are adequate for limestone systems, whereas urethane or UHMW liners are often used for gypsum since they are less abrasive, even though they can be more chemically aggressive with their chloride concentration levels. A benefit of using urethane or UHMW liners is lower initial and replacement costs. Another factor that affects liner costs is the selected liner thickness. It may be selected from ¼- to ½-in. depending on the desire to delay replacement of the liners versus the higher initial capital cost.
Just as important as wear liner material is the conveyor idler rolls. Care should be given to the selection of the CEMA rating for the idler rolls, to avoid overdesign and the associated capital cost. Normally the system throughput capacity requirement for limestone and gypsum is less than coal-handling systems, which results in lighter-duty service for limestone and gypsum systems. Polyurethane-coated return idlers, which are valuable for gypsum conveyors, are not necessary for limestone conveyors—conventional steel roll idlers offer more than adequate service life.
The belt specifications also change depending on the material to be conveyed, especially when determining the top cover requirements based on that material. Since limestone, gypsum, and coal have different material characteristics, the impact zone needs to be designed differently based on bulk density, lump sizes, and material drop. Another important specification is the belt cleaner arrangement for different materials. Gypsum systems require well-designed belt cleaner arrangements due to the inherent stickiness of the material.
Material characteristics like these should be addressed in the initial design, as retrofits for adding or modifying belt cleaners are always difficult and sometimes almost impossible.
Along with the material equipment specifications, the proper classification of hazardous areas can also affect the project. Different materials and equipment depend on the classification grade, and proper classifications can save unneeded expenses and time. For example, specifying explosionproof motors for the drive of an outdoor limestone conveyor is unnecessary because of the different hazardous area classification. By using an explosionproof motor, the associated power distribution system is also required to include explosionproof connections resulting in unwarranted higher capital expenditures, both in materials and installation labor.
Lastly, the proper selection of high-speed coupling and coordinating motors can save 2—10% of the drive component costs. The selection of the motor and high-speed coupling should be selected based on the acceleration time taken by the conveyor to overcome the mass moment of inertia (GD2 value of the system). Such preliminary calculations allow proper equipment selection and also help optimize the drive components.
After taking all the equipment decisions into consideration, the final component of an FGD system is ensuring its protection and longevity. While this may be overlooked to save capital costs in the beginning, the investment pays off in the long run. Depending upon the environment, economic evaluation for the protective coating system versus galvanizing should be performed for the structural components. In some cases, galvanizing may be the more cost-effective selection.
There are many ways to develop your material-handling systems; it is worth the time to investigate all options to ensure that the system is both economical and reliable. Proper planning and equipment selection save time and money, while also improving throughput and equipment optimization.
Christopher Humphrey is vice president and general manager of the Salt Lake City office of River Consulting. Bob Klare and Rajesh Mendiratta are program managers for River Consulting.