Environmentally Acceptable Treatment Options for FGD Wastewater

Transcription

1 Environmentally Acceptable Treatment Options for FGD Wastewater Kumar Sinha Colleen Layman Colleen Chapman Bechtel Power Corporation, Frederick, MD ELECTRIC POWER Keywords: FGD wastewater, coal-fired power plants, metals removal, trace metals, Hg, Se, As, treatment methods, BAT, sulfide/ hydroxide/carbonate precipitation, ZLD, dewatering, co-disposal of sludge, pk sp values Abstract: With the increased number of FGD systems being installed in the US and around the world, wastewater purged from the FGD system is a concern for many power plant operators due to the regulatory attention being given to this wastewater discharge and the treatment required to meet the discharge permit. The targeted constituents that should be removed from FGD wastewater are mercury, selenium, and arsenic. Due to the high chloride, sulfates, TSS, and COD content of this wastewater stream, its recycle for plant reuse, even after removal of metals and TSS, is extremely difficult but not impossible. If recycle is not feasible, the only options available are discharge under regulatory purview and predicated conditions or zero discharge which is extremely expensive. The technical paper will discuss treatment options and case histories where FGD wastewater treatment systems are being designed for discharge, recycle where possible, and/or zero discharge. INTRODUCTION Reagents and sorbents used in Flue Gas Desulphurization (FGD) Systems remove mercury, selenium, chromium, cadmium, arsenic and other trace metals from the boiler flue gases. Whereas the predominant process is control of SO 2 emissions, the FGD purge stream containing these metals is also receiving close scrutiny by the regulatory agencies. With the dramatic increase in the efficiency of the SO 2 removal methods assisted by the use of specialty chemicals with reagents, there has been enhanced improvement in reducing mercury and other trace metal emissions from power plant stacks. The FGD purge stream (wastewater or blowdown) contains both dissolved and undissolved contaminants. The common dissolved constituents present in the purge streams are chlorides, sulfates, nitrates, fluorides, calcium, magnesium, sodium, and contaminant metals that are entrapped during the FGD process as suspended, finer particles of solids and later dissolved in the reagent slurry. The un-dissolved contaminants consist of both suspended particles of ash as well as metal oxides that are captured by the FGD slurry but do not dissolve in the FGD slurry water With older power plants being retrofitted with wet FGD systems using lime and limestone slurries, the FGD purge streams add constraints on the already over-worked wastewater treatment and disposal systems. Nonetheless, the FGD purge streams must still be disposed of in an environmentally acceptable manner or recycled where feasible. If there is no spare capacity in the existing wastewater clarification / softening system originally installed with the power plant infrastructure, a separate treatment system with individual steps specifically designed for removing trace metals from the FGD purge streams is required. A FGD wastewater treatment plant would also be required for grass roots power plants where wet FGD systems are incorporated. However, incorporating a design with the balance of the power plant wastewater treatment systems would be much easier. BACKGROUND The treatment of FGD wastewater has always been a challenge with the presence of metals such as Hg, Se, and As in the purge stream., The use of organic acids in the FGD process as a ph buffer has further impacted the issue of FGD wastewater disposal. The organic acids used in the FGD process are formic acid and dibasic acid (DBA) compound. DBA Page 1

2 consists of a blend of glutaric, succinic, and adipic acids. The use of these organic acids in the absorber vessel may add process treatment steps in the form of biological processes to remove the organics, nutrients and selenium. Selenium converts from selenites to selenates when organic acids are added and requires a reducing bacteria for conversion back to the selenite form for the removal of selenium. Utilities are convinced that use of DBA is saving them millions of dollars in annual O&M costs by realizing that use of DBA increases reactivity of wet limestone and improves SO 2 removal efficiency. Field studies have shown that the rate at which the limestone slurry dissolves in the reactor increases when DBA is added to the slurry resulting in lower reagent use. Therefore, the treatment and disposal options for FGD wastewater cannot ignore the increased cost of biological treatment associated with DBA to meet selenium and organics concentrations in the FGD discharge stream. Some utilities are also considering injection of organosulfides such as TMT 15 directly into the FGD absorber vessel to minimize mercury reemission in the treated flue gases. Although desirable from a wastewater treatment perspective, the organosulfides add complexity in the overall control of metals precipitated in the clarification process due to the other chain reactions caused by these organosulfides in wastewater when compared to inorganic sodium sulfide. The FGD wastewater treatment processes installed so far with FGD retrofitted systems have been regulated for mercury, selenium, arsenic, cadmium, nickel, lead, selenium, zinc, and total chromium in addition to the conventional pollutants such as TSS, phosphates, iron, copper, and manganese. Wastewater ph is also controlled to preclude excessively acidic or basic wastewater from being discharged. Control of ph is also critical for the FGD wastewater treatment process as metals preferentially precipitate as hydroxides, carbonates, and sulfides. Table 1 (attached at the end of the text) provides the well published pk sp (a.k.a. Ksp) values for heavy metals and other trace metals that can be precipitated in their respective carbonate, sulfide, and hydroxide forms. Although no pk sp values have been published for selenium, the common notion is that inorganic selenium (as Se IV) can co-precipitate with other metals when ferrous compounds are added as coagulants. Bechtel has conducted many studies on licensing approaches to these FGD wastewater treatment plants. Some of these are additions due to FGD system retrofits and some are grass roots facilities. Most of these studies are conceptual but some are site specific due to the types of coal burnt and the FGD process being considered for installation. For grass roots facilities, the FGD wastewater treatment system can be integrated with the balance of the water management systems or could be a stand alone system with the sole purpose of discharge as a controlled and regulated stream. No matter what the environmental licensing scenario is, the treatment, monitoring, and control of FGD wastewater is a challenge. Although EPA s best available technologies (BATs) exist for the control of individual metal concentrations in drinking water streams, it is difficult to establish BATs that deal with the precipitation of a multitude of metals present in wastewater solutions that are contaminated with organics and emanate from the different FGD treatment processes. INSTALLING FGD WASTEWATER TREATMENT SYSTEMS Most of the newly installed FGD processes and retrofits use limestone slurry with the following constraints: Organic acids such as DBA and formic acid are added to the absorber to lower limestone consumption and improve SO 2 removal Organic sulfide products with trade names such as TMT 15 or Nalmet are being added to the absorber for enhanced mercury removal and to precipitate mercury before re-emission reactions occur in wet scrubbers Whereas the use of TMT 15, which is an active organosulfide, augments removal of mercury in the FGD blowdown treatment system also and is therefore desirable, the use of DBA requires additional steps to the FGD blowdown treatment process for selenium and organics removal. These additional steps are mostly biological waste treatment steps that need monitoring and control of aerobic and anaerobic bacteria for meeting low selenium and organics limits required for discharge. Although the physical, chemical and biological methods are acceptable environmentally, the challenges in producing a consistent FGD effluent for discharge are plenty. Some of these challenges are in controlling the proper ph in the reaction tanks Page 2

3 to allow the desired precipitation of TSS, COD, BOD, and the controlled metals. The most ideal situation from a process view-point would be to use the physical/ chemical method and precipitate as many metals to the lowest limits practical and reuse or discharge the FGD treated wastewater. A more complicated situation would be adding aerobic and anaerobic biological treatment to the physical / chemical methods for removal of selenium that will be required if organic acids are used. The most complicated situation would be to design a zero liquid discharge system using evaporators, crystallizers, and salt dewatering equipment with the pretreatment require to just make the evaporator/ crystallizer more adaptable for treating wastewater containing metals and organics. LICENSING APPROACH From a licensing view-point, the following scenarios are presented with their acceptability in no particular order and being subject to review by the regulating authorities: Eliminate FGD blowdown treatment effluent discharge by using Zero Liquid Discharge (ZLD) equipment with upstream pretreatment as required. Treat FGD wastewater with the power plant combined wastewater using just the physical/ chemical methods and coprecipitate the metals as hydroxides using alum or ferric salts to meet the established combined liquid discharge limits for the power plant liquid effluents Treat FGD wastewater separately using physical/ chemical methods and precipitate the metals as hydroxides and carbonates to meet established discharge limits for FGD wastewater Treat FGD wastewater separately using physical/ chemical methods and precipitate the metals as hydroxides and sulfides to meet established discharge limits for FGD wastewater, particularly mercury Treat FGD wastewater separately by precipitating the metals present but recycle the treated effluent within the plant for uses such as ash conditioning, cooling tower makeup, dust suppression, etc. Another option that exists for retrofits is treating the wastewater with conventional clarification/ softening methods with the combined power plant wastewater and discharging it to an existing ash pond. Although the pk sp values listed in Table 1 show that the metals could potentially be precipitated out to very low limits based on solubility of the metals, as metal carbonates, metal sulfides, and/or metal hydroxides, there is no guarantee that these limits can be achieved in organically complexed wastewater without bench scale tests or pilot tests. The clarification/ softening technologies are well established with reagents providing the hydroxide, carbonate, and sulfide ions to cause precipitation to very low levels but not to the theoretical levels calculated from the pk sp values. The difficulties are in disengaging the organically complexed metals to the purer forms for precipitation. A NEW LICENSING APPROACH FOR FGD WASTEWATER Bechtel has conceptualized plant water balances incorporating the treated FGD wastewater effluents with cooling tower makeup and characterized both the circulating water chemistry and the cooling tower drifts. This approach is much simpler from an environmental standpoint and has not been presented as treatment option so far. To pursue this licensing approach, the water balances for these plants and the waste characterizations must be presented with the application for permit to construct the FGD wastewater treatment facility and discharge the treated water. The impact of chlorides and trace metal concentrations in FGD wastewater effluent streams are incorporated in both the cooling tower drifts and recirculating water to capture the increase in materials cost if any and adverse environmental effects due to cooling tower drifts. The following case histories are presented to summarize this approach and the impact of injecting the treated FGD wastewater on cooling tower recirculating water chemistry. Case Study #1 Two 800 MW pulverized coal-fired supercritical units to be located in Illinois. FGD wastewater purge stream of approximately 290 gpm (145 gpm per unit) will be treated using alkaline sulfide precipitation process and recycled for usage in the plant for ash conditioning and potentially cooling tower makeup. After ash conditioning, 100 gpm of treated FGD purge would be available for cooling tower makeup. Normal cooling tower makeup is river water pretreated by clarification and cold lime softening. Chlorides in the FGD system are limited to 8,000 mg/l. Page 3

4 Cooling tower recirculating chemistry based on 50 gpm of treated FGD purge wastewater recycled to each cooling tower and 7 COC in the cooling tower: TDS 4965 mg/l Cl 705 mg/l As 0.08 mg/l Cu 0.16 mg/l Pb 0.04 mg/l Hg mg/l Se 0.36 mg/l Zn 0.51 mg/l Condenser tube materials and other system metallurgy will be 316 SS based on the anticipated cooling tower chemistry. Tube material selection in this case will not be impacted by the addition of the recycled FGD purge water to the cooling tower basin. Without the addition of the recycled wastewater to the system, the chloride content of the circulating water would be approximately 329 mg/l. Case Study #2 One 900 MW pulverized coal-fired supercritical unit on a yet to be permitted site in the eastern US. FGD wastewater purge stream of approximately 125 gpm will be treated using alkaline sulfide precipitation process. The wastewater will either be discharged off-site to a nearby stream or will be recycled as cooling tower makeup water depending on the requirements of the plant NPDES permit. There are no other potential uses for FGD wastewater recycle at this facility. Normal cooling tower makeup is mine tunnel drainage water pretreated by clarification and cold lime softening. Chlorides in the FGD system are limited to 15,000 mg/l. Cooling tower recirculating chemistry based on 125 gpm of treated FGD purge wastewater recycled to the cooling tower and 7 COC in the cooling tower: TDS 9230 mg/l Cl 1614 mg/l As 0.03 mg/l Cu 0.20 mg/l Pb 0.07 mg/l Hg mg/l Se 0.04 mg/l Zn 3.01 mg/l Condenser tube materials and other system metallurgy will be 317 SS based on the anticipated cooling tower chemistry. Tube material selection in this case will be impacted by the addition of the recycled FGD purge water to the cooling tower basin. Without the addition of the recycled wastewater to the system, the chloride content of the circulating water would be less than 150 mg/l, allowing the usage of 304 or 316 SS materials. TRADITIONAL LICENSING APPROACH FOR FGD WASTEWATER The concentration of metals that are removed from the FGD purge water and the treated FGD effluent are controlled by the following equilibrium reactions for bivalent metals: K Sp OH = [M + ] [OH - ] K Sp CO 3 = [M + ] [CO 3 - ] K Sp S = [M + ] [S - ] The above equations are used to predict the precipitation of metals at the controlled ph and to calculate the concentrations of metals in the resulting effluent stream. K sp or pk sp is the solubility constant of the metal either as the hydroxide, carbonate, or sulfide, M + is the metal ion concentration, and OH -, CO3 2-, and S - are the hydroxyl, carbonate, and sulfide ion concentrations. Hydroxide and Sulfide Precipitation The combination of these two precipitation processes is also referred to as the Alkaline Sulfide Process. Bechtel is completing construction on its Elm Road Plant where this traditional approach has been taken for designing the FGD wastewater treatment system. The process consists of two stages of precipitation with intermediate reaction tanks where reagents are added to precipitate metals as hydroxides and sulfides. Figure 1 provides a simplified flow diagram of the process showing the main chemical precipitation steps. The addition of the metal sulfide precipitation step using sodium sulfide is required at this facility to meet the stringent Hg, Se, As, and other trace metal requirements that will be implemented when the plant is operational. Table 2 shown below lists the concentrations of metals that are controlled for discharge of the treated FGD wastewater stream. Table 2 Elm Road FGD Treatment System Effluent Metals Guarantee Requirements Page 4

5 Total Aluminum, mg/l <5.0 Total Antimony, mg/l <0.5 Total Arsenic, mg/l <0.3 Total Barium, mg/l <5.0 Total Beryllium, mg/l <0.2 Total Cadmium, mg/l < 0.5 Total Chromium, mg/l <0.5 Total Cobalt, mg/l <1.0 Total Copper, mg/l < 1.0 Total Iron, mg/l <0.5 Total Lead, mg/l < 0.7 Total Manganese, mg/l <3.0 Total Mercury, mg/l < Total Molybdenum, mg/l <0.5 Total Nickel, mg/l < 2.0 Total Selenium, mg/l <0.3 Total Thallium, mg/l <0.5 Total Tin, mg/l <0.5 Total Zinc, mg/l < 4.0 Although this licensing approach is considered traditional, the biggest challenge is characterizing the wastewater stream purged form the FGD system (using the various blends of coals used at the power plant) that is the basis for design of the FGD wastewater treatment plants. With the exception of chlorides and TDS, the treated FGD wastewater effluent after precipitation of metals, is frequently of better quality than the plant makeup water used in the FGD system. Table 2 (above) lists the FGD effluent concentrations for the Elm Road plant that require compliance and are included in the plant NPDES permit. The treatment equipment selected to precipitate the controlled metals guarantees 100% compliance. Bechtel is also designing a similar alkaline sulfide precipitation process for First Energy s Sammis Power Plant. Further discussion of this will be the subject of a future paper at this or some other national or international conference. The process steps for this system are common with the exception of allowing suppliers to use either inorganic sulfides or organic sulfide products such as TMT 15. The alkaline sulfide process is in operation at the following power plants to treat the FGD wastewater: Homer City Generating Station Unit 3, Pennsylvania (1977) Pennsylvania Electric Company, Conemaugh Power Plant, Pennsylvania (1994) B.L. England Generating Station, Atlantic City, New Jersey (1994) Tampa Electric Company Big Bend Station, Tampa, Florida (1985) City of Lakeland, C. D. McIntosh Power Plant, Lakeland, Florida (1982) Northern Indiana Public Service Co, Bailey Power Station, Indiana (1992) The following other power plants that are currently in various stages of the design phase will also incorporate an alkaline sulfide process for treating FGD wastewater: Pleasant Prairie Power Plant, Wisconsin Electric Power Co Marshall Steam Station, Duke Power Co, North Carolina AEP Mountaineer Power Plant, New Haven, West Virginia There are many FGD wastewater treatment systems utilizing the alkaline sulfide process that are operating in Germany. Most of these were installed after 1986 when the West German Government mandated the utilities to control the hazardous constituents in their wastewater discharge. In other parts of Europe including the four KEMA coal-fired plants in Holland, FGD wastewater treatment systems are installed and working well to control the hazardous pollutants in FGD wastewater. Hydroxide and Carbonate Precipitation These types of treatment processes utilizing the carbonate and hydroxide precipitation methods for the metals have been installed at the following power plants to treat the FGD wastewater: Salt River Project, Coronado Generating Station, Arizona ( ) Salt River Project, Navajo Generating Station, Arizona (1999) Arizona Public Service Co, Four Corners Units 1-5, New Mexico ( ) Orlando Utilities Commission, Stanton Energy, Florida (1987) Florida Power & Light and Jacksonville Electric Authority, St. Johns River Power Park, Jacksonville, Florida ( ) Seminole Electric Cooperative, Seminole Power Plant Units 1 and 2, Palatka, Florida (1984) Southern Indiana Gas & Electric, AB Brown Power Plants, Units 1 and 2 (1979 and 1986) Page 5

6 Cincinnati Gas & Electric, W. H. Zimmer Power Plant, Ohio (1991) Monongahela Power Co, Pleasants 1 and 2 Power Plants, West Virginia ( ) In this treatment option, the FGD blowdown stream is treated as a low volume wastewater stream and is allowed to be mixed with other power plant low volume wastewater streams prior to final discharge. This option consists of treatment of the FGD blowdown stream in conventional clarifiers at controlled ph with injection of lime, soda ash, and coagulants such as ferric sulfate, ferric chloride, and alum, following EPA s Best Available Technology (BAT). Two steps of clarification are employed with different ph values maintained in each clarifier to favor the precipitation of metals as hydroxides or as carbonates. This treatment option will reduce suspended solids, total organics, and precipitate the metals as hydroxides and/or carbonates. Whereas this method is very effective for the removal of iron, manganese, copper, zinc, nickel, aluminum, cadmium, arsenic, chromium, and lead, it does not guarantee their removal to the low ppb levels that the EPA is now enforcing in many states for FGD blowdown streams. This treatment option will not remove mercury to the low ppb levels that are likely to be mandated by the permitting authorities. However, this is still a feasible option and for many metals, this is the established BAT (i.e., for the removal of zinc, nickel, cadmium, and lead) using solubility constants that are well-documented. Hg removal should be monitored in the hydroxide carbonate precipitation process and if the concentrations of Hg are low enough in the FGD effluent, a recycle should be initiated after characterizing metal concentrations in the combined plant wastewater effluent. Biological Treatment Biological treatment may be required if the organic acids in the FGD process dissolve the metals present in the flue gases and produce BOD. BOD is removed by aerobic treatment. However, if selenium needs to be removed, an anaerobic biological system is also required for the removal of selenates (the common form of selenium present in the FGD wastewater stream). The selenates are reduced by denitrification and sulfate reducing bacteria to selenium which can be coagulated for removal in a clarifier. The process utilizes the oxygen associated with selenates and sulfates for respiration that is effective for this selenate to selenium reduction. ZERO LIQUID DISCHARGE This option consists of concentrating the FGD wastewater further in evaporators (brine concentrators) and crystallizers and recycling the product water streams for the plant s reuse. The remaining solid wastes (sludge formed and crystallized solids) are disposed off-site at a suitable landfill. This option would be expensive and energy and operator/ maintenance intensive. The evaporator system would require a power supply from the house electrical load or a dedicated steam flow extracted from an auxiliary boiler or the steam cycle. This technology is not new but its application to treat FGD wastewater would be mainly for licensing scenarios where discharge levels for metals cannot be met by sulfide precipitation methods. Special metallurgical requirements need to be specified to deal with the highly corrosive nature of the concentrated FGD wastewater. For example, a brine concentrator will be used at a power plant in the city of Springfield, Illinois for treating the FGD system wastewater. CONCLUSIONS In conclusion, the authors have presented some diverse licensing scenarios. These include coagulation in the form of common hydroxides, carbonates, and sulfides. Considerations have also been given to adding a biological treatment system for removing BOD and selenium driven by the use of organic acids in the FGD process. Finally, the use of zero liquid discharge (ZLD) systems are recommended where required. The main technical argument this paper presents is that perhaps a more simple treatment option exists and this may be sufficient for treating FGD wastewater streams. This is feasible only when the power plant can demonstrate that heavy metals such as Hg, Se, and As do not exceed the site NPDES permit limits when applied to the combined wastewater stream after recycling. The case histories presented in this paper demonstrate that the simpler licensing option is definitely feasible with recycle and control of other large volume wastewater streams produced at the plant. For plants with cooling towers, the partially treated FGD wastewater stream can be used as cooling tower makeup without impacting the controlled chemistry parameters in the combined discharge. This scenario should be first presented to the licensing authority before exploring the added Page 6

7 sulfide precipitation process or the complex biological and zero discharge processes. ACKNOWLEDGEMENTS The authors wish to acknowledge the contributions of their colleague, Michael Chuk for researching the solubility (pk sp values) presented in this paper and performing calculation on concentration limits for these metals that can be precipitated using these three different (OH, S, CO 3 ) precipitation methods. Page 7

8 Table 1 pk sp Values for Precipitated Metals Solubility Solubility Solubility Carbonate Sulfide Hydroxide Metal Symbol Product Product Product CO 3 S OH K sp X x -CO 3 K sp X x -S K sp X x -OH Mercury Hg HgCO x Hg 2 S 1.0 x Hg(OH) x Cadmium Cd CdCO x CdS 8.0 x Cd(OH) x Arsenic R4 As - As 2 S x As 2 S 2 Note As 2 S x10-37 Note 7 - Selenium R3 Se SeS Note 5 - Zinc Copper Iron Manganese Zn Cu Fe Mn ZnCO x ά-zns 1.6 x Zn(OH) x β-zns 2.5 x CuCO x Cu 2 S 2.5 x CuOH 1.0 x CuS 6.3 x Cu(OH) x FeCO x FeS 6.3 x Fe(OH) x Fe(OH) x MnCO x MnS (a) 2.5 x Mn(OH) x MnS (c) 2.5 x Magnesium 4,R2 Mg MgCO x MgSO x Mg(OH) x Cobalt Nickel Co Ni CoCO x ά-cus 4.0 x Co(OH) x β-cus 2.0 x Co(OH) x NiCO x ά-nis 3.2 x Ni(OH) x β-nis 1.0 x γ-nis 2.0 x Silver Ag Ag 2 CO x Ag 2 S 6.3 x AgOH 2.0 x Barium 3 Ba BaCO x BaSO x Ba(OH) 2 8H 2 O 2.55 x Lead Pb PbCO PbS 8.0 x Pb(OH) x Chromium Cr - - Cr(OH) x Cr - - Cr(OH) x Notes: 1. (a) amorphous 2. (c) crystalline 3. Barium does not readily form a sulfide compound Barium typically form Barium Sulfate. K sp for Barium Sulfate is shown. 4. Magnesium sulfide is decomposed by water. Ksp for Magnesium Sulfite is shown. 5. i- insoluble in water 6. Arsenic Disulfide is insoluble in water below 50 C, above 50 C Arsenic Disulfide decomposes. 7. Calculated based on solubility provided in Reference 4 References: 1. Lange's Handbook of Chemistry (15 th Edition); Editor: Dean, J.A; 1999 McGraw Hill 2. International Critical Tables of Numerical Data, Physics, Chemistry and Technology (1st Electronic Edition); Washburn, E.W. ( ;2003); K Knovel. 3. Handbook of Chemistry and Physics (51st Edition); Editor: West, Robert C.; 1970 The Chemical Rubber Co. 4. Chemical Engineer's Handbook; Perry & Chilton; 1973 McGraw Hill 5. Wastewater Engineering Treatment and Reuse; Metcalf & Eddy; 2003 McGraw Hill Page 8

9 Figure 1 Page 9