Flue Gas Desulfurization Wastewater Research Articles

Investigation on Friedel’s salt precipitation method for chloride removal from flue gas desulfurization wastewater

Flue gas desulfurization wastewater (FGDW) contained high concentration of chloride, which made it difficult to recycle or achieve zero liquid discharge (ZLD). Friedel’s salt precipitation (FSP) method could effectively dislodge chloride from FGDW and was expected to achieve ZLD of FGDW. Effects of reaction conditions, reagents dosage and types on chloride removal efficiency of FSP method were investigated systematically by single factor tests. Results showed that chloride removal by FSP method was fast and basically completed in 60 min. A higher stirring speed (≥90 rpm) would enhance the dispersion and dissolution of reagents, which was beneficial to improve the chloride removal efficiency. Moreover, the dosage and types of reagents had significant influence on chloride removal and should be selected according to actual need. A two-stage FSP method was applied to avoid the negative effect caused by the presence of sulfate, by which the sulfate and chloride removal efficiency reached 98% and 60%, respectively, in the real FGDW treatment. Through SEM and XRD analysis, precipitates formed in Stage I and II were proved to be CaSO4 and friedel’s salt, which belonged to Ca-Al-layered double hydroxides. The kinetics of FSP method removed chloride was attempted to study, and inferred the reaction as third order reaction. FSP method has enormous potential to be applied in industrial wastewater treatment.

Insights into chloride sorption and phase transformation of the synthesized Ca-Al bimetallic oxides in flue gas desulfurization wastewater

Sorbents enriched with Ca3Al2O6 (C3A) and Ca12Al14O33 (C12A7) were successfully synthesized by a solid-state sintering method for Cl− removal from the flue gas desulfurization (FGD) wastewater. C3A and C12Al7-rich sorbents prepared at 1200 and 1100 °C achieved the highest Cl− removal of 83.3% and 73.1%, and the former had a higher aluminum utilization rate of 45.2%. Compositional analyses revealed that sorbents synthesized using α-Al2O3 had higher atomic density, higher ratio of absorbed oxygen to lattice oxygen, and higher AlO45−/AlO69− ratio than those using Al2O3, resulting in enhanced reactivity and superior Cl− removal performance. Higher calcination temperature also improved the crystallinity of bimetallic oxides. The sorption isotherms and thermodynamic studies confirmed that the sorption of Cl− on Ca-Al oxides was spontaneous and exothermic, and the maximum sorption capacities were 243.7 mg/g on C3A and 204.1 mg/g on C12Al7, respectively. Kinetics and characterizations suggested that the hydration-induced mineral phase transformation led to structural reconstruction, and the sorption mechanism involved a synergistic action of chemisorption (as the primary force) and physisorption. These results suggested that Ca-Al-based sorbents are highly promising alternatives for application in the zero liquid discharge process of FGD wastewater.

Thermodynamic insights into selenium oxyanion removal from synthetic flue gas desulfurization wastewater with temperature-swing solvent extraction.

Temperature-swing solvent extraction (TSSE) is a cost-effective, simple, versatile, and industry-ready technology platform capable of desalinating hypersaline brines toward zero liquid discharge. In this work, we demonstrate the potential of TSSE in the effective removal of selenium oxyanions and traces of mercury with the coexistence of high contents of chloride and sulfate often encountered in flue gas desulfurization wastewater streams. We compare the rejection performance of the two common solvents broadly used for TSSE, decanoic acid (DA) and diisopropylamine (DPA), and correlate those with the solvent physicochemical properties (e.g., dielectric constant, polarity, molecular bulkiness, and hydrophobicity) and ionic properties (e.g., hydrated radii and H-bonding). The results show that TSSE can remove >99.5% of selenium oxyanions and 96%-99.6% of mercury traces coexisting with sulfate (at a sixfold Se concentration) and chloride (at a 400-fold Se concentration) in a synthetic wastewater stream. Compared to diisopropylamine, decanoic acid is more effective in rejecting ions for all cases, ranging from a simple binary system to more complex multicomponent systems with highly varied ionic concentrations. Furthermore, the H-bonding interaction with water and the hydrated radii of the oxyanions (i.e., selenate vs. selenite) along with the hindrance effects caused by the molecular bulkiness and hydrophobicity (or lipophilicity) of the solvents play important roles in the favorable rejection of TSSE. This study shows that TSSE might provide a technological solution with a high deionization potential for the industry in complying with the Environmental Protection Agency regulations for discharge streams from coal-fired power facilities.

Investigating the Corrosive Influence of Chloride Ions on Slag Recovery Machine Shells in Power Plants.

An effective strategy for achieving cost-effective and environmentally friendly desulfurization wastewater in coal-fired power plants involves the incorporation of desulfurization wastewater into the slag water system. The objective of this study was to analyze the corrosion behavior of Q235-A slag-picker shell material upon the introduction of FGD wastewater into the slag water system. The dynamic weight loss method, electrochemical testing method and microscopic phase characterization were employed to investigate the impact of varying chloride ion concentrations (ranging from 1000 mg/L to 30,000 mg/L) of flue gas desulfurization wastewater (FGD wastewater) on the corrosion of Q235-A slag-picker shell material. The test results indicate that as the concentration of chloride ions increases, the corrosion rate increases from 1.1487 mm/a to 1.5590 mm/a when the concentration is less than 10,000 mg/L. However, when the concentration exceeds 10,000 mg/L, the corrosion rate decreases from 1.559 mm/a to 1.0393 mm/a. The corrosion rate is above 1 mm/a at all concentrations. As the Cl- concentration, the quality of the corrosion product film initially increases and then decreases. The primary components of the corrosion product are α- FeOOH, γ-FeOOH, β-FeOOH, Fe3O4 and γ-Fe2O3.

Effect of external magnetic field on mass transport of reactive species in the reaction between nano zero valent iron and selenate anions in a simulated flue gas desulfurization wastewater

The mechanism involved in the reaction of regulated ions with nano Zero-Valent Iron (ZVI) in the presence of an applied weak magnetic field (WMF) was investigated. The WMF effect on mass transport of reacting ions and morphology and composition of the oxide layer in nano ZVI was elucidated using Electron Energy Loss Spectroscopy (EELS). The magnetic gradient force produced by ferromagnetic ZVI particles had a greater influence on the rate of selenate diffusion in solution than the Lorentz force due to the externally applied weak magnetic field. Moreover, the magnetic gradient force was suggested to suppress ZVI corrosion reactions. Despite changes in the iron oxide layer under the WMF influence, the rate of selenate reduction has not been significantly affected over short term lasting several hours. Most crucially, WMF had a negligible effect on the rate of mass transport of selenate anions, as compared to the dominant effect of forced convection.

Experimental study on coal-fired flue gas HCl removal by injecting adsorbent into flue duct

Abstract Adsorbent injection into flue ducts is an effective technology for controlling gaseous pollutant in coal-fired power plants. This study proposed a new technique of injecting dechlorinater into flue duct for HCl removal in order to realize the wet flue gas desulfurization (WFGD) wastewater sequestration and upgrade the gypsum quality, known as the source dechlorination method. Four alkaline-based adsorbents of CaO, Ca(OH)2 + 5 % NaOH, ethanol-modified CaO, and NaHCO3 were developed and investigated in a pilot scale 6 kW coal-fired circulating fluidized bed (CFB) combustion system for capturing flue gas HCl. The physical and chemical properties of the adsorbents were characterized to explore the reaction mechanisms affected by the adsorbent size and its distribution, active component loading, micro-structure, morphology, and crystal structure. The influences of the injection amount, resident time and flue gas temperature on the HCl removal efficiency were carried out, the dechlorination mechanism of the ethanol-modified CaO were discussed. The distribution of flue gas chlorine species across the air pollutant control devices (APCD) were obtained. This study provides basis for developing the technology of injecting dechlorinater into flue gas for HCl removal.

Effects of nanofiltration on desalination of flue gas desulfurization wastewater by electrodialysis: Treatment effect, fouling property and techno-economic analysis

This study seeks to compare the treatment effect and fouling property of the nanofiltration (NF) – electrodialysis (ED) hybrid system and single ED system, and launches a techno-economic analysis for the flue gas desulfurization wastewater treatment. The optimal operation conditions of ED and NF process were determined, and the fouling properties of NF membranes and ion exchange membranes (IEMs) were explored. Morphology and chemical structure analysis concluded that NF membranes are more susceptible to fouling than IEMs, ascribing to the discrepancy of driving force and fouling mechanisms. The economic impact of membrane fouling was calculated in detail, including costs of increased membrane resistance and pressure drop of feed channel, decreased water permeability, membrane replacement and cleaning-in-place (CIP). We have found that the major factor of increased operating cost resulting from fouling comes from the membrane replacement cost (≥66%), followed by additional energy cost (≥4.1%) and with only a minor contribution from the CIP costs (≥0.8%). Overall, the total operating cost for the NF-ED system was 35.9% more expensive than the single ED system with a total cost of 2.95 ¥/m3. The results can provide a useful reference for the application of membrane technology in water and wastewater treatment.

Recovery of magnesium from flue gas desulfurization wastewater using thermomorphic hydrophilicity amines

The concentrations of hardness ions in saline wastewater from flue gas desulfurization (FGD) are usually very high, which could result in severe membrane fouling in consequent membrane systems. Hence, we investigated the feasibility of recovering Mg2+ from saline wastewater collected from a coal-fired power plant through utilizing thermomorphic hydrophilicity amines. Triethylamine (TEA), diisopropylamine (DIPA) and N,N-dimethylisopropylamine (DMIPA) were chosen as thermomorphic hydrophilicity amines. The effects of temperature change on solubility of amines in aqueous phase were evaluated, and the influences of molar ratio of amine to Mg2+ were also investigated. Results show that the solubility of amines in aqueous phase decreased substantially when temperature was increased from 25 °C to 60 °C. Thermomorphic hydrophilicity amines could be regenerated by mild heating, and regenerated TEA could remove up to 95% of Mg2+ in the actual FGD wastewater. Meanwhile, three amines could effectively increase the pH value of the solutions, and the recovery of Mg2+ was more than 98% as Mg(OH)2 at an amine ratio molar ratio of 2.5. This study demonstrated the great potential of using thermoresponsive amines as alkali sources to recover Mg2+ and the synthesis of reactive MgO in FGD wastewater, providing a reference for the recovery of resources such as Mg2+.

Role of Refined Coal Use in Power Plant Effects on Downstream Drinking Water Quality

Use of refined coal at power plants has been incentivized in tax law due to perceived benefits in control of criteria air pollutants. Refined coal use at power plants increased by 20% from 99 million metric tons in 2016 to 119 million metric tons in 2020. Additives in the refining process (e.g., halogens) can be present in power plant wastewater, especially where wet flue gas desulfurization (FGD) is utilized. Discharge of wastewaters containing halogens can have unintended negative effects on water quality at downstream drinking water plants. The increases in refined coal consumption over the past decade, and incomplete information on the nature and quantity of chemical additives used in refining, have increased uncertainty of the potential effects on drinking water. Considering 2020, the total bromide load associated with discharge of wet FGD wastewater generated from use of refined coal is estimated to range from 470 thousand kg/day to 12,900 thousand kg/day. While tax credits expired in 2021, additive use may continue to manage air pollutants; use of additives is not reported. Additional information on halogen application rates is needed to improve estimates and enable assessment of the potential effects of this practice on downstream drinking water treatment facilities.

Treatment of flue gas desulfurization wastewater by a coupled precipitation-ultrafiltration process

Flue gas desulfurization (FGD) wastewater is characterized by low pH, high content of hardness ions and COD. In this work, chemical precipitation and ultrafiltration (UF) membrane separation were corporately coupled to treat the FGD wastewater on a lab scale to remove the hardness as well as some organic compounds for further advanced treatment. Firstly, influential factors for precipitation to deal with the simulated wastewater, including reaction time, stirring speed, and dosage, were investigated by batch tests, while CaO and Na2CO3 were used as the first and second precipitants. The batch results showed that the removal rates of Mg2+ and Ca2+ could be both higher than 98% under the optimal reaction condition. Then a polyvinylidene fluoride tubular UF membrane was used to remove the precipitates without adding flocculants. Thirdly, the coupled process with the optimal operation parameters was adopted to treat real FGD wastewater. Almost all the hardness ions and most COD could be removed, while rapid permeability decline was observed in the UF process due to the high COD, which could be recovered by a simple cleaning. In summary, when the mass ratios of [CaO]/[Mg2+] and [Na2CO3]/[Ca2+] were 2.5 and 2.7, the flow velocity was around 5 m/s, the removal rates of the hardness ions can be above 98% and the membrane flux can be recovered by 98% with a backwash with pure water and an HCl solution of pH = 2. This work proved that the coupled process is a technically feasible technology for treating FGD wastewater to realize the ZLD.

Aqueous Bromide Discharges from U.S. Coal-Fired Power Plants: Points of Origin, Concentration Ranges, and Effluent Treatment Costs

Bromide discharges from coal-fired power plants have received increased attention from regulatory bodies due to their contribution to the formation of disinfection byproducts (DBPs) in downstream drinking water treatment plants. This paper characterizes the relative contributions of bromide from coal feedstocks and bromine-based mercury control processes, estimates the distribution of bromide concentrations at 85 active coal-fired power plants across the United States (U.S.) with wet flue gas desulfurization units, and estimates the cost of bromide removal from wastewater discharge using year 2020 data. Bromide discharges are estimated at the plant level using a combination of the reported coal rank and composition combusted, estimates of bromide addition in mercury control techniques under multiple halogen addition scenarios, and the estimated flue gas desulfurization (FGD) wastewater flow rate. The median, simulated plant-level estimation of total FGD wastewater flow is 18.3 gallons/min at a bromide concentration of 319 mg/L, equivalent to ∼11.6 tonnes/year of bromide discharges to the environment. Next, we evaluated the expected cost of employing the best available technology (BAT) to control bromide discharges in FGD wastewater to prevent contributions to DBP formation. Treatment would need to remove more than 99.8% of bromide to reach the 0.2 mg/L voluntary incentive program (VIP) limit. The total cost of treatment depends on whether disposal is on- or off-site; the average costs for all plants combined come to an average of $110 million ($95.2/kgal) in 2021 U.S. dollars for on-site disposal, or $134 million ($115/kgal) for off-site disposal.

Effective removal of trace-level toxic metals from flue gas desulfurization wastewater using SiO2 supported hydrogel sorbent

Flue gas desulfurization (FGD) wastewater generated from coal-fired power plants contain potentially harmful heavy metal pollutants that pose a threat to public health and clean water. In this work, we present a water stable polyethylenimine-n,n’-methylenebisacrylamide (PEI-MBAA) functionalized SiO2 solid sorbent material (PMS-1.2/1/4) and investigate its metal adsorption kinetics, selectivity, regenerability, and space velocity. The kinetic studies of six of the toxic heavy metals (As, Cd, Cr, Pb, Se, and Hg) prepared with single elements in Milli-Q water showed the effect of chemical bonding and intraparticle mass transfer resistance on the sorption process. The selectivity studies demonstrated the significant adsorption efficiency toward trace-level heavy metals (Se, Cd, U, Al, etc.) from authentic industrial FGD wastewater. Through five consecutive adsorption−desorption cycles with the FGD (uptake)-citrate (release)-based buffer pair, the sorbent showed high heavy metal removal ability and good reusability. The maximum flow rate for the removal of Se from industrial FGD wastewater was determined to be as high as 8 bed volumes/minute of the sorbent bed. The results demonstrate the PMS-1.2/1.4 sorbent is a promising candidate for the removal of heavy metals from practical aqueous solutions.

Oxygen-vacancy enhanced CoO/CeO2 heterojunction for synchronous regulation of sulfur resourcing and selenium adsorption separation from flue gas desulfurization wastewater

A dual functional material of oxygen-vacancy-enriched cobalt-ceria bimetallic oxide (OV-CoO/CeO2) provides a new method for the simultaneous production of high-quality sulfate and separation of toxic Se from flue gas desulfurization wastewater.

Multiple functions of liquid ferrate in the pretreatment of flue gas desulfurization wastewater

Multiple functions of liquid ferrate in the pretreatment of flue gas desulfurization wastewater

Concentration effects of calcium ion on polyacrylamide fouling of ion-exchange membrane in electrodialysis treatment of flue gas desulfurization wastewater

Residual anion polyacrylamide (APAM) and fluctuating calcium ion (Ca2+) in flue gas desulfurization wastewater after pretreatment could result in ion-exchange membrane fouling during electrodialysis process. Concentration effects of Ca2+ on APAM fouling were systematically investigated and the mechanisms were illustrated. Results of desalination performance indicated that Ca2+ alleviated APAM fouling and the mitigating effect increased firstly and then declined, obtaining the mildest fouling phenomenon in the presence of 1.0 mM Ca2+. Zeta potential and particle radius of APAM in the feed solutions increased as Ca2+ changed the charge balance of APAM molecules and complexed with them. Morphology and chemical structure analysis concluded that APAM molecules were adsorbed on the membrane surface and the fouled layer changed with increasing Ca2+ concentration, from dense layer to reticular structure and then to sophisticated multilayer structure. Calculation of interfacial free energy based on Extended Derjaguin-Landau-Verwey-Overbeek theory confirmed that electrostatic interaction dominated the membrane fouling process. Concentration effects of Ca2+ were more remarkable on membrane-APAM attraction and APAM-APAM repulsion at 0.0–1.0 and 1.0–10.0 mM, respectively. The results contribute to establishing comprehensive understanding of the concentration effects of Ca2+ on polymer fouling of membranes and provide new insights into cleaning process optimization.

Mineralogy optimization for immobilization of potentially toxic elements and halides in Co-disposed flue gas desulfurization brines and bituminous coal fly ash

• Co-disposal of flue gas desulfurization wastewater brine and bituminous coal fly ash. • Zero liquid discharge through solidification/stabilization. • Contaminants encapsulation through mineralogy optimization. • Aluminate addition enhances the formation of Friedel's salt. • Gypsum addition enhances the formation of ettringite. The flue gas desulfurization (FGD) wastewater and coal fly ash (CFA) are waste products containing potentially toxic elements (PTEs) and halides of environmental concerns. A novel co-disposal strategy for these two wastes is a zero-liquid-discharge (ZLD) method which treats the concentrated FGD brine via a solidification/stabilization (S/S) process using CFA and alkaline additives. The objective of this study was to improve PTE and halide immobilization of this ZLD method by optimizing the mineralogy of the S/S solids in co-disposed FGD brines and bituminous CFA. The formation of Friedel's salt, which is critical in binding oxyanions and halides, was specifically evaluated. Various mixtures of FGD brines, CFA, and alkaline additives were systematically tested and the formed S/S solids were evaluated by long-term leaching tests and quantitative X-ray diffraction (QXRD) analyses. Addition of reactive aluminate (2.5 % by weight) and lime (10% by weight) yielded S/S solids with enhanced formation of Friedel's salt by up to 23%, which were able to retain the majority of selenate (>90%), arsenate (>99%), chromate (>99%) and chloride (>50%) from the brine after nine weeks of leaching tests. Comparatively, addition of gypsum promoted the formation of ettringite, which only enhanced the immobilization of borate, but not selenate and chloride. The hydraulic conductivity of the S/S solids was another important factor for contaminant leaching, because it could affect the stability of Friedel's salt under the leaching conditions and governed the immobilization of halides since the halide concentration from the brine exceeded the binding capacity of formed Friedel's salt.

High Performance of Commercial PAC on the Simultaneous Desulfurization and Denitrification of Wastewater From a Coal-Fired Heating Plant.

The flue gas desulfurization wastewater is highly saline and has too many refractory pollutants to be recycled during the desulfurization process of the coal-fired heating plant. Given that waste heat is abundant in coal-fired heating plants, a thermal treatment method was developed to simultaneously remove sulfates and nitrates from the wastewater, with the production of chemical-grade natroalunite and recycled water. The results showed that sulfates and nitrates were 50.3 and 10 g/L in the wastewater, respectively, and only 2.8% and 9.1% were removed after direct treatment at 270°C for 7 h; but these rates increased to 99.3% and 99.9%, respectively, with the addition of commercial poly aluminum chloride. Mass balance summarized that the treatment of 1 ton wastewater needed 0.1 ton PAC and produced 0.11 ton natroalunite and 0.92 ton recycle water. The removal of sulfates and nitrates was mainly done by the precipitation reaction of sulfates such as natroalunite and the redox reaction between nitrates and organics, respectively. Thermodynamic analysis demonstrated that the precipitate reaction occurred at 45°C and accelerated in the temperature range of 45–270°C, but became slow with the decrease of sulfate and Al concentrations in wastewater. Four other reagents were also used for wastewater treatment in comparison with PAC and showed the following order of performance: PAC > citrate calcium > limestone > subacetate aluminum > citrate ferric. This method provided a practical route to treat wastewater from flue gas desulfurization without generating secondary waste.

Numerical Simulation of Multi-Nozzle Droplet Evaporation Characteristics for Desulfurization Wastewater

Spraying flue gas desulfurization wastewater into flue ducts is an emerging technology that is receiving extensive attention in thermal power plants. In order to study the evaporative performance of wastewater-atomizing droplets under variable working conditions, a combined Euler–Lagrange model was developed to demonstrate the thermal behavior of FGD wastewater spray evaporation in flue gas. The effects of several control factors under various operating conditions were numerically determined and validated against experimental data. Due to the complicated parameters and various other conditions, a least-square support vector machine (LSSVM) model relying on numerical results was used to anticipate the evaporation rate of the droplets. We prove that the LSSVM model has high prediction accuracy for the evaporation rate at different cross-sections of flue under a different operating situation. The conclusion is that for the sake of improving the quality of evaporation, the spacing between two adjacent nozzles should be increased while increasing the flow rate. However, using a higher flue gas temperature, higher initial temperature and smaller diameter of droplets can shorten the time and distance of complete evaporation. In summary, this research analysis can be used effectively to determine the design of the FGD wastewater flue gas evaporative process in thermal power plants.

Designing triple-layer superhydrophobic/hydrophobic/hydrophilic nanofibrous membrane via electrohydrodynamic technique for enhanced anti-fouling and anti-wetting in wastewater treatment by membrane distillation

Developing high-performance membranes for membrane distillation (MD) to treat highly saline industrial wastewater is of great significance. In this work, a superhydrophobic/hydrophobic/hydrophilic triple-layer membrane combining an electrosprayed superhydrophobic top layer, an electrospun hydrophobic nanofibrous intermediate layer and a hydrophilic microporous membrane substrate was fabricated by using electrohydrodynamic techniques. The top superhydrophobic surface possesses a unique surface morphology composing of hydrophobic SiO2-polymer microbeads with nanoscaled protrusions and interconnected thin nanofibers, which contributed to the enhanced water flux for desalination in direct contact MD. By tuning the concentrations of hydrophobic SiO2 nanoparticles and polyvinylidene fluoride-co-hexafluoropropylene for electrospraying the top layer, the triple-layer membrane showed both enhanced anti-fouling and anti-wetting properties due to the reduced liquid-solid contact area and stable Cassie-Baxter state. The triple-layer membrane exhibited stable MD performances when using real seawater and industrial flue gas desulfurization wastewater as the feed solutions, while no obvious fouling and wetting being observed even at 60% water recovery. This study provides an effective approach for fabricating a high-performance triple-layer superhydrophobic/hydrophobic/hydrophilic membrane for potential practical MD applications for industrial wastewater treatment.

Black-TiO2 based photoelectrochemical oxidation of flue-gas desulfurization wastewater for effective reuse in flow-electrode CDI

This study proposes two integrated electrochemical methods for the reuse of flue-gas desulfurization wastewater (FGDW): photoelectrochemical oxidation (PECO) using a self-doped black TiO2 nanotube array (Black-TNA) catalyst and flow-electrode capacitive deionization (FCDI). The electrocatalytic activity and oxidative performance of the Black-TNA annealed at 600 °C were superior to those of the conventional Blue-TNA, as determined via Mott-Schottky and cyclic voltammetry plots. Furthermore, the removal of organic pollutants via PECO under sulfate-based conditions using the Black-TNA showed exceptional degradative results. Subsequently, FGDW was introduced to the FCDI system to evaluate its feasibility as a flow-electrode electrolytic solution in comparison with other synthetic electrolytes (NaCl, Na2SO4, and NaCl/Na2SO4 composite), and was confirmed for its feasibility. Thus, the integrated PECO-FCDI system was demonstrated to be an effective means of removing organic pollutants and salts without the use of additional chemicals.