The combustion of coal in power generation facilities produces solid waste, such as bottom and fly ash, and flue gas that is emitted to the atmosphere. Many plants are required to remove SOx emissions from the flue gas using flue gas desulfurization (FGD) systems. The three leading FGD technologies used in the U.S. are wet scrubbing (85% of the installations), dry scrubbing (12%), and dry sorbent injection (3%). Wet scrubbers typically remove more than 90% of the SOx, compared to dry scrubbers, which remove 80%. This article presents state-of-the-art technologies for treating the wastewater that is generated by wet FGD systems.
Wet FGD Basics
Wet FGD technologies have in common a slurry reactor section and a solids dewatering section. Various types of absorbers have been used, including packed and tray towers, venturi scrubbers, and spray scrubbers in the reactor section. The absorbers neutralize the acidic gasses with an alkaline slurry of lime, sodium hydroxide, or limestone. For a number of economic reasons, newer scrubbers tend to use limestone slurry.
When limestone reacts with SOx in the reducing conditions of the absorber, SO 2 (the major component of SOx) is converted into sulfite, and a slurry rich in calcium sulfite is produced. Earlier FGD systems (referred to as natural oxidation or inhibited oxidation systems) produced a calcium sulfite by-product. Newer FGD systems employ an oxidation reactor in which the calcium sulfite slurry is converted to calcium sulfate (gypsum); these are referred to as limestone forced oxidation (LSFO) FGD systems.
Typical modern LSFO FGD systems use either a spray tower absorber with an integral oxidation reactor in the base (Figure 1) or a jet bubbler system. In each the gas is absorbed in a limestone slurry under anoxic conditions; the slurry then passes to an aerobic reactor or reaction zone, where sulfite is converted to sulfate, and gypsum precipitates. Hydraulic detention time in the oxidation reactor is about 20 minutes.
1. Spray column limestone forced oxidation (LSFO) FGD system. In an LSFO scrubber slurry passes to a reactor, where air is added to force oxidation of sulfite to sulfate. This oxidation appears to convert selenite to selenate, resulting in later treatment difficulties. Source: CH2M HILL
These systems typically operate with suspended solids of 14% to 18%. Suspended solids consist of fine and coarse gypsum solids, fly ash, and inert material introduced with the limestone. When the solids reach an upper limit, slurry is purged. Most LSFO FGD systems use mechanical solids separation and dewatering systems to separate gypsum and other solids from the purge water (Figure 2).
2. FGD purge gypsum dewatering system. In a typical gypsum dewatering system particles in the purge are classified, or separated, into coarse and fine fractions. Fine particles are separated in the overflow from the hydroclone to produce an underflow that consists mostly of large gypsum crystals (for potential sale) that can be dewatered to a low moisture content with a vacuum belt dewatering system. Source: CH2M HILL
Some FGD systems use gravity thickeners or settling ponds for solids classification and dewatering, and some use centrifuges or rotary vacuum drum dewatering systems, but most new systems use hydroclones and vacuum belts. Some may use two hydroclones in series to increase solids removal in the dewatering system. A portion of the hydroclone overflow may be returned to the FGD system to reduce wastewater flow.
Purging may also be initiated when there is a buildup of chlorides in the FGD slurry, necessitated by limits imposed by the corrosion resistance of the FGD system’s construction materials.
FGD Wastewater Characteristics
Many variables affect FGD wastewater composition, such as coal and limestone composition, type of scrubber, and the gypsum-dewatering system used. Coal contributes acidic gases — such as chlorides, fluorides, and sulfate — as well as volatile metals, including arsenic, mercury, selenium, boron, cadmium, and zinc. The limestone contributes iron and aluminum (from clay minerals) to the FGD wastewater. Limestone is typically pulverized in a wet ball mill, and the erosion and corrosion of the balls contribute iron to the limestone slurry. Clays tend to contribute the inert fines, which is one of the reasons that wastewater is purged from the scrubber.
From: Thomas E. Higgins, PhD, PE; A. Thomas Sandy, PE; and Silas W. Givens, PE.
Email: caroline@rbsic-sisic.com
Post time: Aug-04-2018