How Operation of a WFGD System Can Impact the Wastewater Quality Generated

The Environmental Protection Agency (EPA) updated the Effluent Limitation Guidelines (ELG) standards applicable to Steam Electric Generating Units on January 4, 2016.

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By Bryan Hansen and Corey Kipp

The Environmental Protection Agency (EPA) updated the Effluent Limitation Guidelines (ELG) standards applicable to Steam Electric Generating Units on January 4, 2016. The final ELGs establish new or additional effluent limitations for affected plants within the steam electric industry. Direct dischargers of wastewater streams (i.e., those discharging directly to surface waters) must comply with effluent limitations in their National Pollutant Discharge Elimination System (NPDES) permits. It is through the NPDES program that compliance with the proposed ELG limitations will be enforced. The final rule requires compliance as soon as possible, beginning November 1, 2018, but no later than December 31, 2023. Recent EPA actions may revise or delay implementation of the ELG rule.

The ELG requirements apply to discharges of wastewater associated with the following processes and byproducts:

  • Flue gas desulfurization (FGD) wastewater
  • Fly ash transport water
  • Bottom ash transport water
  • Combustion residuals leachate
  • Gasification of fuels such as coal and petroleum coke
  • Flue gas mercury control (FGMC) wastewater

Limits for pH, total suspended solids (TSS), and oil/grease are set at the same levels as previously established by the ELGs. New limits for arsenic, mercury, nitrate/nitrite, and selenium have been introduced with the revised ELGs. Table 1 presents the final ELG limits for existing discharges of FGD wastewater.

The only anti-circumvention provision the EPA included in the final ELGs is in regard to streams that have a zero-discharge provision. These streams may not be mixed with any other stream that results in an eventual discharge. The only exception to this anti-circumvention provision is the use of fly ash or bottom ash transport water as FGD makeup water.

While the anti-circumvention provisions do not apply to other waste streams, the ELGs make clear that when any two streams are mixed, the resulting discharge limits should be prorated to account for any mixing dilution effect within the streams. Mixing is allowed, but the eventual discharge limit will be reduced to ensure the resulting discharge will contain the same amount of contaminants as if the mixing had not occurred.

Controllable FGD System Parameters

Control of FGD chemistry and operating parameters including chloride levels, pH, oxidation, and ORP in wet FGD (WFGD) systems can impact both the quality and quantity of wastewater generated. Table 2 shows parameters that can be used to control FGD systems.

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Chloride Control

The most effective way to reduce the cost of wastewater treatment is to decrease the FGD system blowdown rate. Doing so increases the chloride concentration in the absorber slurry, which is limited by the existing materials of construction. FGD systems should be operated at the highest allowable chloride concentration for the existing materials of construction. Owners should evaluate the cost of upgrading materials of construction to operate at higher chloride concentrations and produce less FGD blowdown versus the cost of wastewater treatment systems.

One advantage to increasing chloride concentration is that mercury re-emission rates decrease as the concentration of chlorides in the WFGD liquor increases. This is due to increased formation of mercuric-chloride compounds. Mercuric-chloride compounds are made of oxidized mercury, and increase the likelihood of mercury being captured in the WFGD liquids.1

Upgrading materials of construction to operate at higher chlorides may have undesirable effects. One consequence is the possibility that elevated chloride levels in the FGD system make the sale of gypsum difficult. Salable-grade gypsum must typically contain less than 100 ppm of chlorides, normally achieved by washing the gypsum with fresh water. At chloride concentrations of greater than 20,000 ppm, gypsum becomes difficult to wash to an acceptable chloride concentration for sale. Losing the ability to sell gypsum at elevated chloride concentrations must be weighed against the reduction in the cost of any wastewater treatment system. A side benefit of not selling gypsum is that some FGD wastewater is entrained with the gypsum resulting in less wastewater to treat overall.

Switching to a fuel with a lower chloride concentration could also be considered. Lower chlorides in the fuel result in less FGD blowdown required to maintain the same chloride concentration. Less FGD blowdown reduces the size of the wastewater treatment system required.

Operating at higher chloride concentrations in the FGD system also results in higher concentrations of other constituents, which in turn could make it harder to treat and remove in downstream wastewater treatment systems. Biological wastewater treatment processes are particularly suspect to elevated chloride concentrations.

pH Control

The pH of absorber liquids can most readily be controlled by reagent addition to the vessel. Changing the pH set point will affect the quantity of reagent added to the vessel. As more reagent is added, the pH will rise. An increase in pH within the absorber will affect many measurements such as SO2 removal, reagent utilization, and the possibility of increased scaling. As shown in Figure 1, the ideal operating pH range is limited. Reagent utilization decreases with increasing pH; however, SO2 removal rate increases with increasing pH. Decreasing pH will have the opposite effects on each of these factors.

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For a majority of WFGD systems, the ideal operating range for pH is between a value of 5.2 and 6.2. Operation at a pH of greater than 6.2 will likely result in significant scaling as the equilibrium dissolution of calcium sulfite occurs at a pH of 6.3, forming scaling solids. Operation at a pH of less than 5.2 will likely decrease SO2 removal rate due to inadequate reagent available for reactions.

Oxidation Control

Forced oxidation, natural oxidation, and inhibited oxidation operating modes are available for WFGD systems. The desired operating value for oxidation within an absorber is greater than 95 percent (forced oxidation) or less than 10 percent (inhibited oxidation). Any oxidation percentages between 10 percent and 95 percent are likely to cause scaling within the absorber and are not desirable.

Forced oxidation requires the addition of oxygen into the absorber reactor vessel to increase oxidation, increasing calcium sulfate (gypsum) production. Inhibited oxidation is the injection of oxygen scavengers (e.g., sodium thiosulfate) to prevent the oxidation of sulfite to sulfate within the WFGD system. The primary factor in the decision to operate with forced oxidation is the ability to sell gypsum. Forced oxidation is required to produce a salable-grade gypsum, resulting in increased profits for the plant. Forced oxidation systems typically have higher capital costs due to the need for blowers or air compressors to increase oxygen content within the absorber. Forced oxidation reduces the chances of the mercury re-emissions phenomenon.

Inhibited oxidation typically requires blending of fly-ash and lime with the calcium sulfite by-product from the WFGD system to be landfilled. Inhibited oxidation has the potential to eliminate FGD blowdown at the cost of losing potential fly-ash and gypsum sales.

ORP Control

Oxidation-reduction potential (ORP) in an absorber can be indicative of many operating parameters within the WFGD unit. ORP measures an aqueous system’s capacity to transfer electrons from chemical reactions.2 The primary means of altering ORP include changing the pH set point, changing the liquid to gas feed ratio (L:G), changing the sulfur dioxide to oxygen ratio (SO2:O2) in the flue gas, changing the slurry level in the reaction tank, or using chemical additives.

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Increasing the pH value will decrease ORP, while lowering the pH will raise ORP. Methods of changing the L:G ratio in a WFGD system include adding more spray levels, changing the number of operational recycle pumps, and adding gas/liquid contact devices such as wall rings or perforated trays. Lower L:G ratios correspond to a lower ORP value. The SO2:O2 ratio of an absorber can be most readily impacted by changing the excess oxygen percentage in the boiler, or by switching to a fuel source that has a different sulfur content. Achieving a higher SO2:O2 ratio can lower ORP. Decreasing the oxygen content in the boiler may risk incomplete combustion of fuel. Lowering the slurry level in the absorber reaction tank for a WFGD system will decrease ORP by decreasing oxygen transport rate into the slurry. Improving particulate collection systems upstream of the WFGD unit can also decrease ORP, as fly ash that enters the WFGD unit can increase ORP.

The ORP set point affects selenium and mercury within a WFGD unit. At ORP values greater than 250 mV, selenium will tend to be in the selenate form, which is undesirable, while mercury will remain oxidized in the FGD liquor, which is desirable. At ORP values less than 250 mV, selenium will tend to be in the selenite form, which is desirable, while mercury will reduce back into a particulate form and re-emit into the flue gas, which is undesirable. Plant operational experience and testing data have shown that the desirable operating range for ORP in WFGD systems is between 150 and 250 mV. This attempts to minimize selenate formation and mercury re-emissions.

Conclusion

Compliance with ELGs will require the treatment of WFGD wastewater for removal of arsenic, mercury, chlorides, selenium, and nitrates/nitrites. Having a solid understanding of WFGD chemistry and operating factors that affect the wastewater quality as well as SO2 removal rate is important to finding the best solution for overall compliance and reduced wastewater treatment costs. Chloride, pH, oxidation, and ORP control are operating parameters that can be readily adjusted to control both the quality and quantity of FGD wastewater generated.

References

1. Blythe, G., et al. "Investigation of Mercury Control by Wet FGD Systems." Paper Presented at the Symposium on Air Quality VIII, Arlington, VA, October 2011.

2. AECOM. "Effects of MATS Control and Variable Unit Load on ORP and Trace Metals in FGD Wastewater." APC-Wastewater Round Table & Expo Presentation, Dearborn, MI, July 2016.


About the Authors: Bryan Hansen works at Burns & McDonnell in the Energy Division. Hansen has 25 years of experience working on projects and studies involving industrial water treatment and air pollution control systems. Corey Kipp works at Burns & McDonnell in the Energy Division. Kipp works on projects and studies involving industrial water treatment and air pollution control systems.

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