Coal, the primary U.S. resource for producing electricity, generates over half the nation’s electricity. Balancing a dependence on foreign oil, coal offers over 274 billion tons of untapped supplies - enough to energize America for 250 years. According to The Energy Blog, The National Coal Council (NCC) released a study recommending maximized usage of coal to generate electricity over the next 20 years. Coal combustion in power generation facilities results in energy release, solid waste produced as bottom ash and fly ash, and flue gas emissions to the atmosphere. Suspended particulate matter, or fly ash in flue gas, is controlled through electrostatic precipitators (ESPs) and/or baghouses. ESP performance may be enhanced by adding ammonia to the flue gas.
Exhibit 1. Typical flue gas desulfurization (FGD) unit - Spray tower absorber. (SOURCE: Alstom Power, Environmental Control Systems)
The primary gas emissions are criteria pollutants (e.g., sulfur dioxides [SOx], nitrogen dioxides [NOx], particulate matter, carbon monoxide and others). About two thirds of all SOx and a quarter of all NOx in our atmosphere come from electric power generation, achieved by burning coal and other fuels. SOx causes acid rain, and NOx creates atmospheric ozone. Secondary emissions, such as mercury, arsenic, selenium, and boron, are indigenous to the type of coal being combusted. Clean Air Act Amendments regulate many of these pollutants, and require stringent controls of SOx and NOx emissions.
NOx compliance requires use of controls, such as: selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), and low NOx burners. Both SCR and SNCR cause ammonia slip, which allows unreacted ammonia to slip past the NOx control device for subsequent removal in the electrostatic precipitator or scrubber. If the water is discharged to an ash pond, this ammonia may oxidize to nitrite and nitrate.
Exhibit 2. Wet FGD scrubber (SOURCE: The Babcock & Wilcox Co.)
SOx compliance mandates use of flue gas desulfurization (FGD). The three methods used in the U.S. to desulfurize flue gas include wet scrubbing (85%), dry scrubbing (12%), and dry sorbant injection (3%). Wet scrubbers (see Exhibit 2) achieve SOx removal efficiency of greater than 90%, compared to dry scrubbers at 80%.
Wet Scrubbing FGD
This article focuses on the wet scrubbing methods used to desulfurize flue gas in coal-fired power plants. Three main scrubbers used for wet scrubbing - venturi, packed and spray scrubbers - are characterized by the following treatment steps:
- Scrubbing agent injection-alkaline agents with calcium - typically limestone (i.e., calcium carbonate) and/or lime and caustic soda
- Scrubber blowdown solids separation
- Calcium solids dewatering
- Wastewater treatment
Exhibit 3. Simplified FGD system
Exhibit 3 depicts a typical wet scrubber FGD system. The SOx removal efficiency can be enhanced by adding dibasic acids (DBA), e.g., succinic, glutaric and adipic acids, to remove nearly 99% of SOx. Nitrates and ammonia nitrogen can be expected in wet scrubber FGD where ammonia is present, either for ESP conditioning or NOx control. Excess air is injected into the wet scrubber to oxidize sulfites to sulfate for production of gypsum (i.e., calcium sulfate) for reuse in wall board production.
FGD Wastewater Treatment
Wastewater from the wet scrubbing FGD process must itself be treated before being released to surface water. National Pollutant Discharge Elimination System (NPDES) requirements for wastewater discharges from FGD processes vary depending on receiving water quality and quantity. Typical NPDES parameters include BOD, TSS, heavy metals, selenium, arsenic, boron, temperature, pH, and TDS. FGD wastewater typically is combined with other water discharges from the power plant (i.e., wet fly ash handling, cooling water, steam condensate, etc.), thereby further defining exact wastewater treatment requirements.
FGD wastewater poses a challenge to treat because of the following unique characteristics:
- High concentrations of TDS and TSS
- High buffering capacity
- Supersaturation in sulfates
- High temperature
- Potentially high organic concentration from DBA addition
- Ammonia, from the ammonia slip for ESP conditioning and NOx control; and potentially nitrites and nitrates (if they’re produced by the SCR or by the ammonia treatment)
- Miscellaneous regulated heavy metals and trace constituents (i.e., arsenic, mercury, selenium, boron, etc.) present that vary by coal type.
Due to the highly complex nature of FGD wastewater, several stages are required to treat wastewater blowdown from the FGD scrubber to meet NPDES requirements for surface water discharge. This treatment typically consists of the following steps:
- Calcium sulfate desaturization
- Primary solids removal (see Exhibit 4)
- Trace metals precipitation
- Secondary solids removal
- Polishing (filtration)
Exhibit 4. Clarifier under construction
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Exhibit 5 depicts a wastewater treatment scheme developed by CH2M Hill to treat FGD wastewater and remove heavy metals. In this particular plant, it was decided to desaturate the wastewater of sulfates and remove the bulk of the insoluble suspended solids prior to tertiary treatment of heavy metals and arsenic using a chemical/physical treatment process. Additional treatment could be provided (i.e., anoxic biological treatment) for selenium, nitrates and organics. This system didn’t require that level of treatment to meet NPDES permit requirements.
Exhibit 5. FGD wastewater treatment plant
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Tertiary treatment is target-pollutant specific. Metals can be treated through chemical, physical and biological means. Iron co-precipitation removes cationic metals and arsenic. Adding organosulfide precipitants lowers levels of cationic metals such as mercury. Selenium, particularly the oxidized form (selenate, the principal form of selenium in forced oxidation scrubbers), can be treated through biological processes (i.e., anoxic biological treatment processes and/or constructed wetlands). Organics can be treated through the same processes if properly designed. Careful consideration of chemical and biochemical stoichiometry of the pollutants, as well as temperature, pH, and TDS is required in the process design.
CH2M Hill completed conceptual evaluations, treatability test programs, preliminary engineering designs, and detailed designs to treat FGD wastewater at several different coal-fired power plants. While completing this and other ongoing FGD wastewater treatment work, it identified several key issues that should be considered in designing infrastructure for FGD wastewater:
- Most flue gas constituents will end up in the wet scrubber water.
- Wastewater characterization is important, given variations in add-on air emission controls (i.e., ESP, baghouse, SNCR, FGD, DBA, etc.), and coal and limestone types that can influence variability of flow and pollutant parameters.
- Treatability testing (i.e., bench and pilot scale) is critical to define wastewater treatment system performance, especially when tertiary treatment is required to polish problematic pollutants (e.g., selenium, arsenic, mercury, BOD, etc.).
- Calcium sulfate scaling potential is high and must be considered in the design.
- Residuals management strategies must be carefully considered, given the volume and mass of solids produced.
Exhibit 6. Calcium sulfate scaling in clarifier overflow weir
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Many unanswered questions persist involving the application of treatment technologies to FGD wastewater, especially tertiary treatment of problematic constituents. In fact, many of the primary and tertiary wastewater treatment technologies being constructed by the power industry today haven’t been demonstrated full-scale on actual FGD wastewater. CH2M Hill believes the power industry should work jointly to address these questions and share lessons learned.
About the Authors: Dr. Thomas E. Higgins is a vice president and principal technologist with CH2M Hill. He has served as senior process engineer for alternatives evaluations, detailed design, construction, and startup of numerous FGD wastewater treatment plants. Higgins holds a doctorate in environmental engineering and a bachelor’s degree in civil engineering from the University of Notre Dame.
A. Thomas Sandy is a principal technologist and director of CH2M Hill Global Water and Process Practice. With broad experience in management and design of residuals/solid waste treatment processes, he’s accomplished in biological control and chemical/physical wastewater treatment, including for FGD systems. Sandy holds degrees in engineering and chemistry from West Virginia University.
Both are registered professional engineers.