Reducing NOx, SO2, and other air pollutants continues to be a challenge for the power generation industry. The technologies are well-understood, but the devil is always in the details, especially when a complex treatment system is retrofitted to an existing plant.
The most common method for reducing SO2 from plant emissions is the conventional lime- and limestone-based flue gas desulfurization (FGD) system. Material-handling systems for limestone and gypsum present specific challenges and opportunities that differ from those of coal-handling systems. This article looks at factors to consider before and during the design of a new material-handling system. The choices you make about these many variables will determine the cost and longevity of your system.
How Much Redundancy Is Needed?
The first consideration is how the material-handling system for a FGD project will be efficiently integrated with the existing coal-handling plant. We consider this one of the essential keys to a successful project. The material-handling design for an FGD project is often quite different from that of the plant’s familiar coal-handling system and requires a different set of design evaluation criteria.
For example, the redundant equipment philosophy frequently used in power plant design may not apply to new limestone- and gypsum-handling systems. There can be large economic advantages to using single conveyors instead of dual conveyors, including lower initial costs for equipment, structural steel, and installation. Increased reliability of systems can often be realized without using expensive redundant systems. That said, the choice of more or less redundancy must be evaluated in concert with site-specific project requirements and the project economics.
Dual conveyor systems result in substantial capital investments for equipment, structures, foundations, and electrical power distribution systems. For that reason, we suggest that redundancy be achieved through the use of dual conveyor drives rather than dual conveyors, because the drive is the component likely to fall out of service. Idlers, belting, and pulley assemblies, on the other hand, have low failure rates.
Different Requirements for Different Materials
The different material-handling characteristics of limestone and gypsum affect the system equipment in several specific ways. The valley angles for chutework (the steepness of walls in hoppers and chutes) depend on the specific bulk density, lump sizes, and drop distance of the material. Limestone systems have a minimum valley angle that’s similar to that of coal systems: around 60 degrees; however, gypsum systems require steeper valley angles of 70 degrees or more to prevent material build-up and plugging.
Steeper valley angles in hoppers and chutework also require the equipment to occupy more vertical space. Adding vertical space after a structure or system is designed is the most difficult and costly of all modifications, so ensure that this is accounted for early in the design and layout of the system.
Another difference is that the wear liner material required for limestone and gypsum systems may be less expensive than wear liner for coal systems. Current economic trends have made traditional liner materials more expensive, so a careful evaluation of the options should be made early in the project.
For limestone, abrasion-resistant liners with a Brinell hardness of 400 are adequate. Liner thickness may be selected from ¼" to ½", depending on whether you choose to delay replacement of the liners or enjoy a lower initial capital cost. Currently, stainless steel has a fabricated cost that’s three times the cost of abrasion-resistant steel plate, and it doesn’t have a comparable life-cycle for limestone applications, which makes it an unattractive choice. Gypsum can be more chemically aggressive than limestone or coal due to the concentration level of chlorides, but it’s less abrasive. Therefore, urethane or UHMW (ultra-high molecular weight) liners can often be used and will result in lower initial and replacement costs.
Conveyor Component Choices
Normally, the system throughput capacity requirement for limestone and gypsum is less than for coal, and you may be tempted to consider specifying a lighter-duty service for these systems. Accordingly, care should be given to selection of the Conveyor Equipment Manufacturers Association (CEMA) rating of idler rolls to avoid overdesigning the system—and associated capital costs.
For example, though you may typically require a CEMA E idler on your coal-handling conveyor belts, a CEMA D or even CEMA C idler would be appropriate for limestone or gypsum conveyors. The lower CEMA rating does not imply a lower-quality idler but one that is designed for a different service duty. Selecting the proper idler class will mean a lower initial capital cost without the sacrifice of a reasonable service life.
Polyurethane-coated return idlers are valuable for gypsum conveyors but are not necessary for limestone conveyors. For limestone systems, conventional steel roll idlers offer more than adequate service life. Utilizing polyurethane rolls in a limestone system is a poor use of capital.
Belt specifications also change depending on the material to be conveyed. This is especially true of top cover requirements. Because limestone, gypsum, and coal have different material characteristics, the belt’s impact zone is designed differently to accommodate different bulk density, lump sizes, and material drop.
Because of the inherit stickiness of gypsum, systems carrying it will also require well-designed belt cleaner components. This addition should be addressed in the initial design, as retrofits for adding or modifying belt cleaners are always difficult and sometimes almost impossible after the material-handling system is installed and operating.
More Choices Affecting Price and Performance
The proper classification of hazardous areas can also radically affect project design and cost. For example, specifying explosion-proof motors for the drive of an outdoor limestone conveyor is unnecessary. If you choose an explosion-proof motor, the power lines to the motor must also have explosion-proof connections, and so on. Overclassifying also drives up material and labor installation costs.
Structural systems play a large part in the overall capital cost of a material-handling system. The selection of a protective coating system is a large determinant in the cost of the structural steel. We recommend that the environment in which the system will be operating be carefully evaluated before specifying the protective coating. In some cases, galvanizing may be the most cost-effective choice.
Even the level of assembly of the system’s structural and mechanical components affects overall capital cost. The high cost of field labor means that shop installation of the carrying idlers is more economical. Shop installation of the return idlers is not recommended, however, as these idlers may come loose during shipping. Whether or not to use shop installation for the conveyor walkways remains a subject of debate. Though shop installation of the walkways will be more economical than field installation, freight costs for the conveyor trusses will increase because fewer trusses can be shipped in a single load.
Electrical control devices such as pull cord switches, belt misalignment switches, and zero-speed switches should not be shop-installed. These devices are at a high risk of damage during shipping. Motors and drives should be shop-assembled as a single unit on a common steel bed.
Use these recommendations as guidelines, and make sure you are satisfied with your choices. Your engineering firm should be able to explain the system and equipment choices it recommends to your satisfaction.
There are many ways to design and construct material-handling systems, and it is worth the time to investigate all your options to make the system both economical and reliable. After all, you will live the results for many years.
For More Information
A PDF of “Designing Material Handling Systems for FGD Projects,” originally presented at the 2008 ELECTRIC POWER Conference, is available for viewing. The presentation includes numerous photos and illustrations that augment the key points made in this article.
—Christopher Humphrey, PE (firstname.lastname@example.org) is vice president and general manager of the Salt Lake City office of River Consulting, a management, consulting and engineering firm. Bob Klare and Rajesh Mendiratta are program managers for River Consulting (www.riverconsulting.com).