As limits on SO
2 emissions become tighter, many coal-fired utility power plants are adding new flue gas desulfurization (FGD) systems or upgrading existing ones. The majority of these systems employ wet FGD technologies. Many plants with wet FGD systems use reheated flue gas to dry the saturated gases exiting the system's absorber before they are sent to the stack.
However, due to the increasingly high cost of energy and/or the need to decrease total plant SO2 emissions, flue gas reheating typically is no longer used on new plants and is being eliminated at plants already in service. Without reheat, saturated flue gases exiting the absorber enter the stack directly. Along with droplets that are carried over from the mist eliminator, water vapor condensing from the saturated flue gases create a liquid film that covers the walls of the absorber outlet ducting and stack liner. This film must be collected and drained from the system by a process called wet stack operation. If the ductwork, stack liner, and liquid collection system are not properly designed, unacceptable amounts of liquid droplets may be discharged from the top of the stack.
Trouble-free operation of an FGD unit with wet ducts and stacks requires investigation of several potential problem areas related to the handling and discharge of wet flue gas. This must be done during the design of the stack and duct system. This article outlines the important design aspects that must be addressed if liquid discharge from a new or retrofitted wet stack installation is to be minimized.
Wetter is better . . . but
Although wet FGD systems have been used for decades, most operated with dry stacks. In the late 1970s, some utilities began using wet ducts and stacks to reduce their operating costs, but in many cases the (unforeseen) result of this switch was unacceptably high levels of stack liquid discharge (SLD). To improve understanding of the physical processes taking place in wet FGD systems, in the 1980s, EPRI sponsored a number of programs to determine the key variables contributing to liquid re-entrainment. The results of one of these studies were summarized in EPRI Report No. CS-2520, "Entrainment in Wet Stacks."
By the late 1990s many utility plants were operating with wet duct/stack systems—most equipped with liquid collectors—and a sufficient body of experience had been developed for EPRI to sponsor another program to develop practical guidelines for wet stack design and operation. The results of this study were summarized in EPRI Report No. TR-107099, "Wet Stacks Design Guide."
As the two reports detailed, the development of an effective wet stack installation requires a thorough fluid dynamic design of the wet duct/stack system and of the liquid collectors and drains. The design process, usually performed by a flow modeling laboratory with experience in this area, typically consists of five distinct phases:
- Phase 1—Initial review of the proposed system design
- Phase 2—Condensation calculations
- Phase 3—Design and development of the liquid collection system
- Phase 4—Study of the plume downwash
- Phase 5—Field installation and operational inspections
Using the results of the first four phases, utility engineers can finalize the design of the wet FGD system and write the specifications for supply and construction bids. The results of the fifth phase are used to support the installation of the liquid collection system.
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