Boilers, Emissions, Policy & Regulations, Water Treatment

Salt Dryers for TRUE Zero Liquid Discharge from WFGD Wastewater

Issue 6 and Volume 120.

By Steven Feeney and Michael Klidas

In November of 2015, the Federal Register published the final rule for the Effluent Limitation Guidelines (ELG) and Standards for Steam Electric Power Generators in the US. These new standards set numeric limits on mercury, arsenic, selenium and nitrate/nitrite as N in the discharge of flue gas desulfurization (FGD) wastewater.

While there are other streams listed in the standard, with their own set of limitations, the FGD wastewater of existing coal-fired power plants not only requires separate treatment, but is unique in its characteristics.

When evaluating treatment options for wet FGD wastewater from existing coal plants, we generally expect utilities will consider one of two paths for meeting the ELG limits, should they choose to keep the units on-line – either the treat and discharge path, where the plant must meet the assigned limits for mercury, arsenic, selenium and nitrates/nitrites as N, or a voluntary incentive program using evaporation technology.

The limits in Table 1 were determined by the EPA following a survey of existing utility wet FGD wastewater treatment technologies. The ELG mercury and arsenic limits were derived from operating data across several chemical precipitation systems, as these systems typically remove mercury and arsenic well. However, because chemical precipitation systems typically have low removal rates of selenium and nitrates/nitrites, these limits were based on operating data from several biological systems. The ELG limits are therefore based on a wet FGD wastewater system which would include chemical precipitation followed by biological treatment.

Yet, the variability of WFGD wastewater has made performance of the biological treatment systems, at times, uneven. For those utilities which have operated biological systems, many harbor some level of concern regarding the long-term consistent performance of biological systems when faced with uncertain WFGD discharge conditions.

Not only are there variations between plant sites based on the coal burned and wet FGD chemistry deployed, but daily load fluctuations can also lead to substantial WFGD wastewater changes as well. These changes are seen not just in anions, cations, and dissolved compounds, but also in oxidation reduction potential (ORP), which has been known to cause some difficulties in effectively removing certain selenium species. For this and other reasons, there are currently a not insignificant number of utilities that are considering the installation of pilot-scale biological systems at their site to see how the expected daily, weekly and monthly variabilities in WFGD chemistry affect the performance of these systems.

The second path for plant operators to consider is a voluntary incentive program using evaporation technology.

The ELG’s voluntary incentives program provides the certainty of more time (until December 2023) for plants to implement a technology capable of achieving more strict limitations on mercury, arsenic, selenium and total dissolved solids (TDS) based on evaporation technology. Since the ultimate goal of the Clean Water Act (CWA) is the elimination of pollutants into the nation’s waters, this program encourages such an approach. The EPA expects this choice to be made prior to issuance of a plant’s next National Pollutant Discharge Elimination System (NPDES) permit.

Yet, the ELG text considers evaporation from the standpoint of more traditional evaporation techniques such as evaporators, condensers, and brine coolers, and not the use of the latent heat of the boiler flue gas to evaporate the entire wet FGD waste stream, which is the function of a salt dryer. These voluntary incentives program limitations may be met with more traditional evaporation technology, yet the salt dryer results in the wastewater stream being entirely evaporated into the flue gas, leaving behind only FGD solids and previously dissolved salts such as calcium and magnesium chloride. These solids are collected in a downstream particulate collection device PCD..

Babcock & Wilcox investigated of the use of an evaporative technology that could build upon our expertise in the area of dry FGD, evaluating whether a spray dryer or circulating dry scrubber is more effective for drying wet FGD wastewater.

B&W’s research indicated the advantages of the spray dryer (aka salt dryer, when used in a wet FGD application) far outweighed those of the circulating fluid bed salt dryer. The circulating fluid bed salt dryer often required adding insoluble material, which increased cost; the circulating fluid bed salt dryer required a large recirculating load of hygroscopic salts and experienced elevated-pressure pumping issues as a result of the relatively high suspended solids in wet FGD wastewater. Additionally, there is an elevated risk of corrosion resulting from contact between damp particles high in chlorides with the circulating fluid bed salt dryer vessel carbon steel walls. We also looked at the waste-to-energy applications, which have very high chloride fuel and high spray dryer outlet temperatures similar to a salt dryer. These applications also favor spray drying for SO2 control. All these evaluation factors led us to choose the spray dryer system for wet FGD wastewater evaporation. The flue gas temperature exiting the selective catalytic reduction (SCR) system is generally anywhere from 650F to 725F, and it is a portion of this gas that needs to be bypassed around the air heater, and taken to the salt dryer. The salt dryer outlet temperature will generally be around 300F.

Depending on the amount of water to be evaporated and the size of the boiler, anywhere from 6 percent to 12 percent of the flue gas generated in the boiler will be used to evaporate the wet FGD wastewater. For instance, a 900MW unit burning low-chloride coal such as Powder River Basin coal with its typically low chlorine content, and a WFGD system capable of higher chloride loadings, may only necessitate blowdown of 30gpm – 50 gpm, which would require 2 percent to 3 percent of the flue gas to be bypassed. However, if this same unit was burning eastern fuel with high chloride levels, blowdown of 170gpm – 200 gpm or more may be necessary, requiring 10 percent to 12 percent flue gas bypass. An operator would want to limit this to no more than 12 percent in most circumstances.

A popular misunderstanding is that this level of bypass around the air heater leads to a significant impact on efficiency. However, we normally expect a 12 percent bypass to impact boiler efficiency by approximately 0.3 percent.

The solids/salt combination that exits the salt dryer at 300F may well be handled in the existing PCD. But if not, a separate fabric filter is needed if the additional solids loading to the PCD can’t be tolerated for some reason. This also would require a booster fan and possibly solids handling equipment. Another advantage of including a separate fabric filter is that flyash collected in the existing PCD will continue to be sellable, since it will not be contaminated with high-chloride salts from the WFGD. This may warrant the installation of a separate fabric filter even if the existing PCD can tolerate the increased solids flow.

Some plant operators use a wet FGD system with a limestone forced-oxidation system making a gypsum by-product. Increasingly, these customers are having difficulty finding a suitable market outlet for gypsum. Those who already process their wet FGD wastewater with chemical treatment systems may feel adding a biological system to meet regulatory requirements for selenium and nitrates/nitrites as N may be sufficient. While there seems to be a wide variation in what the costs may be, evaluating a salt dryer versus a biological system is somewhat akin to evaluating apples versus oranges. For those plants which currently have no substantial water treatment via chemical treatment, the economics tend more favorably to the salt dryer.

For example, if an operator is evaluating the discharge and treatment of 100 gpm of wet FGD wastewater and estimates the all-in engineer-procure-construct cost for the biological unit is approximately $30 million. For a 100 gpm salt dryer, we would estimate the all-in cost to be$30 million, assuming space is available for the vessel, flues are not overly distant, and sufficient time for installation is allotted. This price assumes a small add-on fabric filter, booster fan and solids handling. But let’s assume we missed the mark and the real salt dryer cost is $40 million. How does this stack up versus $30 million for an add-on biological treatment system?

When a biological system is added to an existing chemical precipitation system, all the equipment in the wastewater treatment building still needs to remain in operation. The wet FGD dewatering system still remains in operation, including hydroclones, holding tanks, agitators, pumps, piping and belt filters. Chemical usage rates remain roughly the same, with the exception of the added biological system.

B&W evaluated the entire U.S. wet FGD installation landscape, and determined the likelihood that 34 plants would be well-suited to adopt a ZLD approach, as opposed to treat and discharge. Of this number, we estimated half would choose evaporation/crystallization technology, and the others would adopt a salt dryer. One significant difference between a salt dryer and evaporation/crystallization is the look of the installation. The evaporation/crystallization technology looks very much like a refinery environment, whereas the salt dryer looks very much like a power plant environment. Fewer operators will be needed to operate the salt dryer when compared to the evaporation/crystallization system.

A midwestern U.S. plant has recently decided to install a more traditional evaporation/crystallization system for treating about 650 gpm of wet FGD wastewater. That estimated all-in price for physical/chemical plus biological treatment was slightly less than $210 million. But what is more interesting, is the risk of non-compliance with the limits in the current permit at that time were seen as moderate, even with the $210 million capital spend. Further, the risk and likelihood of non-compliance with future regulations was determined to be low for the now-published ELG limits, but high, should water quality based effluent limits be a future requirement for the plant. So, for a $210 million capital spend for physical/biological treat and discharge, a future non-compliance concern remains.

We estimate a 650 gpm all-in price for a salt dryer system at $175 million to $200 million, assuming proper time for detailed up-front engineering in the development and detailed design phase of the project, reasonable construction schedule, and sufficient space for salt dryer vessel installation. Yet maybe more importantly, there is zero risk a plant’s future water discharge limits can’t be met, thanks to zero liquid discharge coming from the wet FGD unit.

While this approach to meeting the effluent limitation guidelines is not for everyone, increasingly it is seen as a possible low-cost option to meet the ELG limits for wet FGD systems, and to allay concerns of how the next NPDES permit will affect wet FGD discharge limits. Those limits for the voluntary incentive program are not in play, since there is no wet FGD water being discharged except into the flue gas; a true Zero-Liquid Discharge ZLD approach.

Although this technology has already been successful in treating wet FGD effluent from the waste industry, we also expect this approach of treating wastewater to expand beyond wet FGD systems to other industries once it begins to catch on in coal fired wet FGD applications.


Steven Feeney is product line manager and Michael Klidas is applications engineer at Babcock & Wilcox