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.
The other common reason for purging wastewater is the buildup of chlorides, which are typically limited to 12,000 mg/l, based on the metallurgy used in the scrubbers. With corrosion-resistant metallurgy, chlorides up to 35,000 mg/l can be handled, thus reducing wastewater production.
There is no consistency in the definition of wastewater produced by an FGD system. Some designate hydroclone overflow as wastewater, whereas others that use a thickener or settling (gypsum stacking) pond designate effluent from these to be wastewater. Some may employ primary and secondary hydroclones to maximize the capture of solids before gypsum dewatering. Plants that produce wallboard-quality gypsum will wash the gypsum and reject more solids to wastewater. Therefore, FGD wastewater may contain as much as 7% suspended solids (if primary hydroclone overflow is used) or as little as 30 ppm of suspended solids (when using thickener or stacking pond overflow). Moreover, because it is common for a plant to change coal and limestone suppliers, the wastewater constituents will change over time during operation of the FGD system. Complicating matters further, the plant’s wastewater treatment system must be flexible enough to handle these varying inputs yet produce a treated stream that meets the plant’s wastewater discharge permit requirements.
FGD treatment systems are usually designed before any wastewater is available for testing. Therefore, much effort is expended in obtaining accurate limestone and coal analyses and estimating how much heavy metals will end up in the wastewater. From a treatment standpoint, soluble concentrations are of most interest, as particulate metals can be easily removed by simple solids removal processes.
Developing Design Guidelines
There is a growing body of data on the characteristics of FGD wastewaters taken from different points in the wet FGD system. CH2M HILL contacted the Electric Power Research Institute (EPRI), FGD wastewater treatment equipment vendors, and scrubber manufacturers for typical FGD wastewater composition for seven different FGD systems (Table 1).
Table 1. Typical composition of FGD wastewater. The major chemical constituents found in samples taken from seven wet flue gas desulfurization (FGD) systems are summarized in this table. Suspended solids concentration varies widely, and treating for its removal dominates the treatment process selection options. Other constituents are shown as soluble, because once the solids are handled, the soluble constituents determine treatment requirements. The water is supersaturated in calcium sulfate and is hot, mildly acidic, and corrosive. Source: CH2M HILL
The wastewater data summary illustrates the variety and complexity of wastewater constituents across the industry. Four of the samples were of wastewater that had undergone settling before the collection point, so these were not analyzed for total constituents. For the other four sites, total and soluble (filtered) analyses were performed. Pond and thickener effluents were not included, as these are considered to be wastewater treatment systems in their own right, and they would have skewed the data to the low side.
In a review of metals data from these plants (Figure 3), soluble iron ranged from about 0.1 to 1 ppm, with this representing about 1% of the total iron in the sample. Boron was the metal with the highest concentration, and all of the boron was soluble. Two metals of particular interest are mercury and selenium. Mercury was mostly in the form of particulates, with soluble concentrations ranging from about 0.1 to 10 ppb. Selenium was mostly soluble and ranged from 0.1 to 1.0 ppm.
3. Typical metal concentrations in FGD wastewater. Generally, FGD treatment systems are designed before any wastewater is available for testing. From a treatment standpoint, information about soluble concentrations is what’s most needed, as particulate metals will come out in solids removal processes. Blue lines represent the range and median concentrations of soluble metals. Red lines represent the soluble fraction of the total metals in the sample (soluble plus particulate). Source: CH2M HILL
Some general conclusions can be drawn from this brief study of FGD wastewater characteristics. The most obvious conclusion is that there is no one-size-fits-all FDG wastewater treatment facility. In general, our analysis of these samples also found that:
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There are high concentrations of total dissolved solids, principally consisting of chloride, sulfate, bicarbonate, calcium, sodium, potassium, and magnesium.
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There is a high total suspended solids (TSS) level, due to gypsum solids, fly ash, and limestone inert material.
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There is high buffering, due to bicarbonates and dissolved carbonic acid.
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They are supersaturated with calcium sulfate.
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The temperature was as high as 140F.
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There is a potentially high organic concentration from dibasic acid addition.
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They contain ammonia, from the ammonia slip for electrostatic precipitator conditioning and NOx control, and nitrates (as a result of incomplete selective catalytic reduction).
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They contain miscellaneous regulated heavy metals and trace constituents (including arsenic, mercury, selenium, and boron), which vary by coal type; these metals seem to be concentrated in fine solids.
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Hydroclones separate FGD purge water into an underflow stream that is predominately gypsum solids and an overflow stream that consists of immature gypsum crystals, inerts from limestone, and a concentration of metal solids higher than that in the underflow.
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Metals vary: They may be present in soluble form (that is, able to pass through a 0.45-µm filter) or particulate form.
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Selenium is present mostly in soluble form at concentrations up to several parts per million; it is being regulated to parts-per-billion levels.
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High-soluble selenium in newer forced-oxidation scrubbers appears to be due to the conversion of selenium into the oxidized selenate form, which is difficult to treat.
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Mercury tends to be present predominately in particulate form, but its soluble fraction is in the low parts-per-billion range; it is starting to be regulated to parts-per-thousand levels.
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