Flue Gas Desulfurization Slurry Research Articles

Determination of Sulfite Content in Wet FGD Slurry by No‐Acid‐Additive Iodimetry: Experimental and DFT Studies

AbstractIn this paper, experimental and density functional theory (DFT) studies on determining sulfite content in wet flue gas desulfurization (FGD) slurry by no‐acid‐additive iodimetry were carried out. Three different raw materials of slurry, filtrate, and filter residue were employed to react with iodine. The measurement accuracy of acid‐additive iodimetry and no‐acid‐additive iodimetry was compared by conducting the reactions in an acidic open system (AOS) and non‐acidic open system (NAOS). It was found that no‐acid‐additive iodimetry has a higher measurement accuracy compared to that of acid‐additive iodimetry, even though acid‐additive iodimetry could slightly accelerate the reactions. In no‐acid‐additive iodimetry, it was confirmed by DFT calculations that SO32−, HSO3−, and H2SO3 had different reaction mechanisms when reacting with iodine, and SO32− showed the lowest energy barrier of 5.78 kcal/mol. According to the relative abundance of SO32−, HSO3− and H2SO3, the dominant reaction pathway of sulfite reacting with iodine could be summarized as .

Comprehensive Effect of Oxidant Addition in an FGD Slurry on the Removal and Distribution of Selenium: A Field Study.

Flue gas desulfurization (FGD) scrubbers capture selenium in coal-fired power plants, leading to a high concentration of selenium in the slurry. This research proves that SO32- is preferentially oxidized compared to SeO32- by S2O82-. With the increase in the oxidation-reduction potential (ORP) caused by S2O82- addition, the conversion rate of SO32- increased and the size of gypsum grains grew from 31.2 to 34.6 μm. SeO32- migrates into gypsum grains during the growth of CaSO4·2H2O, leading to selenium fixation in gypsum. In a field study of a 350 MW unit, the ORP increased from 142 to 450 mV when Na2S2O8 was fed into the FGD slurry. With the addition of the oxidant, 65.1% of selenium in the liquid phase migrated into gypsum. The concentration of selenium in the leachate of gypsum after oxidant addition decreased by 68.0%. A 2.34% increase in the selenium removal rate was observed in the scrubber. This study focuses on the migration and conversion of selenium in an actual FGD slurry via a field test. The results found in the 350 MW unit are consistent with laboratory results. The change in ORP has been proven to be effective in adjusting the selenium distribution in the FGD slurry.

Selenium migration mechanism in wet FGD slurry: Experimental and DFT analysis

Selenium (Se) is one of the hazardous trace elements emitted from coal-fired power plants. The Se migration behavior in wet flue gas desulfurization (FGD) slurry is still unclear, and the species of Se in FGD gypsum remains controversial. In this research, the bubbling experiments using simulated slurry with/without gypsum crystallization process were conducted. The experimental results indicated that pure gypsum has poor capability to capture Se components, and only selenite could be trapped in gypsum during its crystal growth stage. Furthermore, the DFT calculation was conducted to provide the microscopic information of Se adsorption and substitution characteristics during gypsum crystallization process. The research findings of this study could help understand the mechanism of Se migration process in FGD slurry, and facilitate the development of effective Se emission control technologies in the future.

Case analysis and preventive measures of limestone-gypsum wet flue gas desulfurization slurry poisoning for 320MW unit

The slurry poisoning of the desulfurization system is a difficult problem in the current desulfurization operation. This paper analyzes the abnormality of the limestone-gypsum wet flue gas desulfurization slurry poisoning of a 320MW unit, and proposes a solution. Through the test method, the slurry composition and gypsum quality the analysis was carried out and effective measures were taken according to the actual situation to solve the slurry poisoning phenomenon and ensure the safe operation of the desulfurization system, which is of great significance to the flue gas emission standard of the desulfurization system of the power plant.

Selenium migration behaviors in wet flue gas desulfurization slurry and an in-situ treatment approach

Excessive discharge of Se-containing wastewater from coal-fired power plants is a disaster for aquatic life, and it must be strictly controlled. However, conventional coagulation-precipitation technology cannot efficiently remove selenium from wastewater, owing to its complex species and high solubility. In this study, an innovative approach was proposed to realize the in-situ removal of selenium in desulfurization slurry, instead of treating it in wastewater. Firstly, the migration behaviors of selenium in slurry was studied. Among various ions, the ferric ion could significantly convert liquid selenite into solid phase by the formation insoluble ferric selenite. Then, several oxidizing factors were tested and the S2O82− showed obvious inhibition effect. A multi-component removal agent containing Fe3+/Mn2+ or Fe3+/Fe2+ successfully removed selenite from slurry in the presence of S2O82−, and the efficiency was as high as 96.5%. Furthermore, the in-situ removal agent was tested using a field slurry obtained from a full-scale coal-fired power plant, and the selenium concentration could be reduced to 38.4 μg/L. It is promising that the application of this technology in full-scale power plants will achieve better performance because the formation of troublesome selenate can be avoided. No complex system or process is needed for this in-situ selenium removal approach, and it can easily combine with other available advanced deep removal technologies, to build the double defense walls for controlling selenium discharge.

Selective separation and recovery of fluoride ion from ammonia‐based flue gas desulfurization slurry using electrodialysis

AbstractBACKGROUNDThe ammonia‐based wet flue gas desulfurization (FGD) system generates fluoride‐rich slurry, the concentration of which reaches several thousand mg L−1. Therefore this slurry has to be treated to avoid damage to industrial equipment, human health and environment safety. This work aims at separating fluoride from high‐concentrated ammonia‐based wet FGD slurry and purifying the ammonium sulfate byproduct using electrodialysis (ED).RESULTSThe effects of experimental conditions such as applied potential, circular flow rate, pH and temperature were investigated for the ED process. The influence of competing ions such as Cl− and SO42− on the removal efficiency was also studied. The use of a homogeneous AMX membrane in the first stage of ED and an ACS monovalent anion selective membrane in the second stage of ED proved to be a feasible and cost‐effective strategy for F− removal. The total recovery rate of F− was 81.4%, while the content of NH4F increased from 0.35 to 40.70%. The power consumption and current efficiency were also studied to evaluate the suitability of ED for the defluoridation of FGD slurry.CONCLUSIONThe separation and recovery of F− using the designed ED is cost‐effective and efficient, and the process can be extended to other FGD techniques. © 2019 Society of Chemical Industry

Separation of fluoride and chloride ions from ammonia-based flue gas desulfurization slurry using a two-stage electrodialysis

This study reports the use of electrodialysis (ED) to separate fluoride (F−) and chloride (Cl−) ions from an ammonia-based flue gas desulfurization (FGD) slurry. The optimal operation voltage, circular flow rate, and pH were determined based on the ion migration efficiency and energy consumption. An applied voltage of 15 V and a circular flow rate of 200 L/h were determined to be the optimum conditions, while solutions with pH values in the range of 4–6 were beneficial for the migration of F−. The field slurry was also subjected to ED after being pretreated with microfiltration and ion exchange. The selected monovalent-anion-selective ACS membrane showed superior performance towards F− and Cl− separation and SO42− retention compared with a homogeneous AMX membrane. To further separate F− and Cl− from the multicomponent ammonia-based FGD slurry, experiments were carried out in two stages. The variation of separation efficiency and specific ion selectivity during the experiment were analyzed and the appropriate operation duration was obtained. As a result, the Cl−-containing solution reached a maximum purity of 98.6%, whereas the F−-containing solution reached a purity of 51.4%. The two-stage ED treatment process investigated herein can potentially be used to separate F− and Cl− and simultaneously purify ammonia-based FGD slurry.

Experiment study on mercury migration across wet flue gas desulfurization slurry under oxy-coal combustion atmosphere

Mercury in oxy-coal combustion flue gas may cause liquid metal embrittlement and material failure to aluminum (Al) heat exchangers, which needs to be removed with high efficiency before CO2 compression. Oxidized Hg (Hg2+) is soluble thus can be co-removed with SO2 in WFGD (Wet Flue Gas Desulfurization). However, the dissolved Hg2+ in slurry may also transform to the insoluble elemental mercury (Hg0) form and re-emitted, resulting a reduction of total Hg capture efficiency in WFGD. This paper experimentally examined the Hg migration across WFGD under high CO2 concentration atmosphere with simulated typical desulfurization slurry. Influences of solids (CaSO4, CaSO3) and anions (Cl−, NO3−, SO32−) in slurry are considered. According to the results, solids of CaSO3 with higher BET surface area are more favored for Hg retention than CaSO4. Slurry pH under high concentration of CO2 was a little bit lower than air combustion atmosphere. The decreased pH in turn hindered Hg retention on both solids while the gaseous Hg0 emission increased. For anions, 6mM NO3− and 5mM SO32− both led to large amount of Hg transformed from CaSO4 solids and liquid into the gas phase. The existence of 20mM Cl− can retain more Hg in the liquid, especially when NO3− and SO32− coexisted. However, this effect was quite pH sensitive. As to CaSO3 slurry, most of the Hg remained in slurry regardless the existence of other anions and atmosphere.

Method of flash evaporation and condensation – heat pump for deep cooling of coal-fired power plant flue gas: Latent heat and water recovery

Flue gas waste heat recovery and utilization is an efficient means to improve the energy efficiency of coal-fired power plants. At present, the surface corrosion and fouling problems of heat exchanger hinder the development of flue gas deep cooling. In this study, a novel flue gas deep cooling method that can reduce flue gas temperature below the dew point of vapor to recover latent heat and obtain clean water simultaneously is proposed to achieve improved energy efficiency. The heat transfer mode of this method is the direct contact mode, which takes the scrubber, e.g. the flue gas desulfurization (FGD) scrubber, as the deep cooling exchanger. The flash evaporation and condensation (FEC) device and heat pump (HP) are utilized to provide low-temperature medium, such as FGD slurry or water, for washing and deep cooling flue gas, to collect recovered water, and to absorb recovered waste heat. This method is called as the FEC–HP method. This paper elaborated on two optional models of the proposed method. The mechanism for recovering heat and water was also analyzed using the customized flue gas humidity chart, and the method to quantitate recovered heat and water, as well as the results of the case of a 300MW coal-fired generator set were provided. Net present value calculations showed that this method is profitable in the scenario of burning high-water-content coals. Several potential advantages of this method and suggestions for practical application were also discussed.

Process migration and transformation of mercury in simulated wet flue gas desulfurization slurry system

Wet flue gas desulfurization (WFGD) system has been increasingly recognized to be effective in Hg removal. But the generated secondary pollution due to Hg re-emission from desulfurization solutions or solid byproducts has received researchers’ attention. In this paper, the distribution of captured Hg2+(L) in the simulated WFGD slurry was studied as a function of various environmental factors. Experimental results indicated that the pH value had a strong effect on Hg2+ reduction but had negligible influence on the Hg2+ retention by the grained fraction of gypsum. Increasing in pH value resulted in higher Hg2+ reduction and over 46% of Hg2+ was found to be reduced into Hg0 at pH 5.0. Besides, increased slurry temperature also promoted the Hg2+ reduction and Hg0 re-emission as well as the Hg2+ retention efficiency by gypsum. Other factors including S (IV), Cl− and NO3− concentration was found to inhibit Hg2+ transformation into Hg0 but slightly promoted the Hg2+ retained in gypsum. Moreover, the addition of sodium sulfide (Na2S), 2,4,6-trimercaptotiazine, trisodium salt nonahydrate (TMT), and sodium dithiocarbamate (DTCR) prevented the Hg2+ reduction and precipitated out as insoluble HgS, Hg3(TMT)2 and Hg(DTCR)2 on gypsum. The findings presented in this study could provide theoretical basis for Hg removal in coal-fired power plants.

Experimental study on the absorption behaviors of gas phase bivalent mercury in Ca-based wet flue gas desulfurization slurry system

Gas phase oxidation and catalytic oxidation of element mercury (Hg 0) to bivalent mercury (Hg 2+) were proposed to improve the mercury removal efficiency in the wet flue gas desulfurization (WFGD) system. However, the re-emission of Hg 0, generated by the reduction of absorbed Hg 2+, would lead to a damping of the total mercury removal efficiency. In this paper, the absorption and reduction behaviors of bivalent mercury in the Ca-based WFGD slurry were evaluated in our purpose-built device. According to our experimental results, the slurry chemistry (such as CaSO 3 content, SO 4 2−, Cl − and pH value) had a strong influence on the reduction of absorbed bivalent mercury. And the inlet concentrations of SO 2 and O 2 contribute little to the mercury absorption. Within the typical pH value range of 4.5–5.5, about 70% of inlet bivalent mercury was converted to Hg 0. The re-emission of Hg would be greatly retarded with the increase of [SO 4 2−] due to the formation of HgSO 4 or Hg 3O 2SO 4. Moreover, it was found that Cl − would also inhibit the reduction of bivalent mercury through the ligands reactions between Cl − and Hg 2+.

Mercury in gypsum produced from flue gas desulfurization

Wet flue gas desulfurization (FGD) technologies used to remove sulfur dioxide (SO 2) from combustion gases can be effective in capturing oxidized mercury (Hg). Depending on the FGD process, a large portion of this Hg may be incorporated into the FGD slurry and its solid byproducts including synthetic gypsum, a material commonly used in the manufacturing of wallboard. The potential for atmospheric and groundwater releases of Hg arises during the manufacturing processes, during the preparation and use of the manufactured products, and eventually upon disposal of the wallboard or other products. In this paper, the fate and mobility of Hg in FGD products and process streams were investigated. Experimental approaches, including leaching studies and size separation techniques, were used to investigate products including FGD-outlet slurry and wallboard production line samples. Results of the experiments reported here indicate that, in a number of cases, Hg mobility is limited. Further, the agent responsible for the immobilization appears to be not the finer particles of gypsum itself but an iron-rich phase, such as iron coated clay minerals or iron oxide/hydroxide particles, probably introduced with the limestone used to form the SO 2-capture reagent.