Simulated Wet Flue Gas Desulfurization Research Articles

Improvement of the reduction of condensable particulate matter in flue gas scrubbing process

Condensable particulate matter (CPM) is characterized by complex composition, non-negligible emission concentration, and fine or ultrafine in size after conversion to particles, which is difficult to remove. Current methods to control CPM are not fully developed and mainly focus on synergistic removal of CPM in existing air pollution control devices, such as CPM reduction through scrubbing processes in wet flue gas desulfurization (WFGD) systems. In this work, an experimental system including a simulated WFGD scrubber, also referred to as the primary scrubber (PS), and a secondary scrubber (SS) was built to explore measures to improve the CPM reduction performance during scrubbing. The operating parameters of the liquid-to-gas (L/G) ratio and the spray temperature in the two scrubbers were tuned in the experiments. The results indicated that CPM could be reduced in the PS by conversion to filterable particulate matter (FPM), and captured by the spray droplets through the effects of dissolution and condensation, but the reduction was not very efficient. In the SS, the reduction performance of CPM could be further improved due to increased dissolution of CPM caused by increased opportunities for gas-liquid contact, and increased condensation of CPM due to lower spray temperature. The FPM transformed from the CPM in the PS could also be reduced in the SS by the effects of diffusiophoresis and thermophoresis contributed by water vapor condensation. An increase in the L/G ratio could improve the CPM reduction.

Effects of metal ions on Hg0 re-emission under air and oxy-fuel combustion atmospheres in a WFGD system

The effects of metal ions, including Fe2+, Fe3+, Mn2+, and Cu2+, on Hg0 re-emission were investigated in a simulated wet flue gas desulfurization (WFGD) system under air and oxy-fuel combustion atmospheres. The results show that in the presence of metal ions, Hg0 re-emission is enhanced under a N2 atmosphere but inhibited under N2/9%O2, air, and oxy-fuel combustion atmospheres. In the inhibition cases, Hg2+ reduction and Hg0 re-emission were accelerated in the initial stages of the experiments. Fortunately, Fe2+, Fe3+, Mn2+, and Cu2+ have a positive impact on mercury stabilization in the liquid phase, and the precipitates of Fe2+/Fe3+ can also adsorb mercury in the solid phase, thus offsetting the acceleration of Hg0 re-emission in the initial stage. Therefore, Hg0 re-emission is suppressed by metal ions under N2/9%O2, air, and oxy-fuel combustion atmospheres. With an increase in CO2 concentration, the inhibition of Fe2+, Fe3+, and Cu2+ was intensified, whereas the suppression by Mn2+ was slightly weakened. Under an oxy-fuel combustion atmosphere, increasing O2 concentration would promote the inhibition of Mn2+ on Hg0 re-emission, but hardly affect the inhibitory effect of Cu2+.

The distribution and conversion of selenite and selenate with the bubbling of simulated flue gas in simulated WFGD slurry

Selenium is one of the hazardous trace elements emitted from coal-fired power plants. The distribution of selenium in Wet Flue Gas Desulfurization (WFGD) process is still unclear and even in controversial, impeding the development of selenium removal technologies. This research has found that the selenite in simulated slurry could be reduced by SO2 while selenate has not been affected. Characterization methods including X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to provide an evidence that the product of the reduction reaction is amorphous elemental selenium. Meanwhile, the influences of other gaseous components, pH, temperature and S2O82− in simulated slurry has also been considered in this research. It is found that with the increase of SO2 concentration in flue gas, the reduction of selenite increased and the reduction reaction is an exothermic reaction. Meanwhile, the oxidation effect of S2O82− competes with the reduction effect of SO2. This study introduced the influence of flue gas into the research of the conversion of selenium in FGD slurry and indicate the effect of flue gas on the potential emission treatment techniques of selenium in FGD slurry.

Mercury migration and re-emission in a simulated wet flue gas desulfurization system

Experiments were conducted in a simulated wet flue gas desulfurization (WFGD) system to investigate the effects of sulfite, slurry temperature, pH value and atmosphere on mercury migration and re-emission. The mechanisms were discussed based on the kinetics and chemical equilibrium. The results indicate that SO32− is a key factor for Hg0 re-emission in WFGD system. Increasing SO32− concentration at a relatively low level can promote Hg2+ in liquid phase reducing to gaseous Hg0, whereas Hg0 re-emission is significantly inhibited with excess SO32−. The reaction rate of mercury reduction is calculated to increase by approximately ten times when the slurry temperature rises from 40 to 60 °C. It reveals that a higher slurry temperature can accelerate mercury reduction. The proportion of mercury in liquid phase is detected to decline sharply with a drop in pH value. The mechanisms were further proposed that SO32− tends to be protonated and the stabilities of mercury–sulfite complexes are weakened at low pH value. Under both N2/O2 and O2/CO2 atmospheres, increasing O2 concentration can suppress Hg0 re-emission. However, the variation in CO2 concentrations has no impact on Hg0 re-emission. Compared with O2/CO2 atmosphere, there is more Hg0 re-emission under N2/O2 atmosphere due to the secondary release of Hg0.

Treatment of Wet FGD Wastewater by a Modified Chemical Precipitation Method Using a Solid Powder Reagent

This research focused on developing a modified chemical precipitation (MCP) method for treating wet flue gas desulfurization (FGD) waste water by adding a solid powder reagent directly. Simulated wet FGD wastewater was treated by MCP method in simulation experiments. Optimization experiments were carried out with the help of response surface methodology (RSM) and central composite design (CCD) to evaluate the effects and the interactions of experimental variables, including reagent dosage, temperature and pH value. The optimal reagent dosage, temperature and pH value were 3018.0 mg/L, 40.5 °C and 5.7, respectively. The RSM was demonstrated as an appropriate approach for the optimization of wet FGD wastewater treatment with the MCP method. A comparative study between the MCP method and the traditional chemical precipitation (TCP) method on raw wet FGD wastewater treatment was conducted. Results indicate that the MCP had less reagent dosage and variety than the TCP method had. Thus, the MCP method had a lower cost.

Mercury stability of byproducts from wet flue gas desulfurization devices

Mercury released from wet flue gas desulfurization (WFGD) devices can be inhibited by additives that can transfer mercury to its solid phase. However, the use of additives increases the mercury content in desulfurization gypsum, and the mercury is released again during the calcination process. In this study, NaHS, 2,4,6-trimercaptotiazine, trisodium salt nonahydrate (Na3(C3N3S3)·9H2O, or TMT) and sodium dithiocarbamate (DTCR) were used as additives to control mercury emissions in simulated WFGD devices. Then, different calcination methods were adopted to study mercury release behaviors from calcination process. The transformation of mercury compound species on gypsum was also investigated to explain its behavior during calcination. The result showed that the mercury content in the solid phase increased significantly in the presence of additives. Among three additives, simulated desulfurization slurry with TMT had the highest trapping efficiency over mercury. The mercury thermal stability in gypsum increased in the order of using DTCR, TMT and NaHS. Due to the migration of mercury to the solid phase and the stronger thermal stability of resultants, the residual content of mercury in gypsum with additives after calcination were higher than that without additives. In general, using TMT coupled with slow-speed calcination had the best effect on controlling mercury contamination from WFGD slurry and WFGD byproducts. Moreover, it was verified by temperature programmed decomposition experiments that, after using NaHS, TMT and DTCR, the mercury compound in the gypsum was mainly in the form of HgS, Hg3(TMT)2, Hg(DTCR)2, respectively, which explained the mercury release behaviors during calcination.

Performance of a pilot-scale wet electrostatic precipitator for the control of sulfuric acid mist.

The use of a wet electrostatic precipitator (WESP) is often regarded as a viable option to reduce sulfuric acid mist emitted from the wet flue gas desulfurization (WFGD) tower in coal-fired power plants. In this study, a pilot-scale wet electrostatic precipitator equipped with a wall-cooled collection electrode is investigated for the control of sulfuric acid mist from a simulated WFGD system. The results show that due to partial charging effect, the removal efficiency of sulfuric acid aerosol decreases when the aerosol size decreases to several tens of nanometers. Moreover, due to the plasma-induced effect, a large number of ultrafine sulfuric acid aerosols below 50nm formed at a voltage higher than 24kV inside the WESP. The percentages of submicron-sized aerosols significantly increase together with the voltage. To minimize the adverse plasma-induced effect, a WESP should be operated at a high gas velocity with an optimum high voltage. Even at a high flue gas velocity of 2.3ms(-1), the mass concentration and the total number concentration of uncaptured sulfuric acid aerosols at the WESP outlet are as low as ca. 0.6mgm(-3) and ca. 10(4) 1cm(-3) at 28kV, respectively. The corresponding removal efficiencies were respectively higher than 99.4 and 99.9% and are very similar to that at 1.1 and 1.6ms(-1). Moreover, the condensation-induced aerosol growth enhances the removal of sulfuric acid mist inside a WESP and enables a low emission concentration of ca. 0.65mgm(-3) with a corresponding removal efficiency superior to 99.4% even at a low voltage of 21kV, and of ca. 0.35mgm(-3) with a corresponding removal efficiency superior to 99.6% at a higher voltage level of 26kV.

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.

Study of mercury re-emission in a simulated WFGD solution containing thiocyanate and sulfide ions

In this paper, elemental mercury (Hg0) re-emissions from simulated wet flue gas desulfurization (WFGD) solutions were evaluated in the presence of thiocyanate ions (SCN−) or sulfide ions (S2− and HS−). Experimental results revealed that both thiocyanate ions and sulfide ions could inhibit the reduction of oxidized mercury to Hg0. The inhibition effect of SCN− was mainly attributed to the formation of HgSO3SCN− and HgSCNX−(X–2) (X=2–4). It was found that the mercury reduction process in the presence of sulfide ions followed different mechanism under acidic or alkaline condition, though the formation of stable HgS could be ascribed to the main inhibition reason. Under acidic condition, the bivalent mercury ions may not directly combine with sulfide ions to form HgS. This had led to a weaker inhibition effect under the acidic condition compared to that under alkaline condition.

Reduction of Selenate from Simulated Wet Flue Gas Desulfurization Wastewater Using Photocatalyst and Microorganism

The reduction of selenate [Se(VI)] from a simulated wet flue gas desulfurization (FGD) wastewater was attempted using photocatalyst and microorganisms. The photocatalyst reduction method consisted of TiO2 and HCOOH as a photocatalyst and a hole scavenger, respectively. When a simple 0.5 mg/L Se(VI) solution was used, the photocatalytic reduction gave an efficient removal of Se(VI). In contrast, the photocatalytic reduction could not remove Se(VI) from the simulated FGD wastewater, due to the excess amount of co-existing SO42- anion. Also, a biological reduction which used Se(VI)-reducing bacteria was investigated. When three kinds of bacteria (E. cancerogenus, P. stutzeri, and P. denitrificans) were used, Se(VI) in the simple 0.5 mg/L Se(VI) solution was effectively removed. However, for E. cancerogenus, the removal of Se(VI) was inhibited by the co-existing NO3- anion in the wastewater.

Study of Mercury Re-Emission from Simulated Wet Flue Gas Desulfurization Liquors

Hg2+captured by wet flue gas desulfurization (WFGD) systems can easily be reduced by reducing substances such as S(IV) (SO32-or HSO3-) and results in emissions of elemental mercury (Hg0). The re-emission of Hg0would lead to a damping of the total mercury removal efficiency by WFGD systems. The effects of the operating conditions, which included the pH, temperature, Cl-concentrations and oxygen concentrations, on Hg0re-emission from WFGD liquors was carried out. The experimental results indicated that the Hg0re-emission rate from WFGD liquors increased as the operational temperatures and pH values increased. The Hg0re-emission rates decreased as the O2concentration of flue gas and Cl-concentration of WFGD liquors increased. So the Hg0re-emission from WFGD system can be reduced or slowed by decreasing the temperature and pH, or by using forced oxidation. The present findings could be valuable for industrial application of characterizing and optimizing mercury control in WFGD systems.

Control of Hg<sup>0</sup> Re-Emission from Simulated Wet Flue Gas Desulfurization Liquors by Sodium Dithiocarbamate

Secondary atmospheric pollutions may result from wet flue gas desulfurization (WFGD) systems caused by the reduction of Hg2+to Hg0and lead to a damping of the co-benefit mercury removal efficiency by WFGD systems. The effects of the operating conditions, which included the pH, temperature, and oxygen concentrations, on Hg0re-emission inhibition efficiency by DTCR from WFGD liquors was carried out. The established data reflected an outstanding performance on the Hg0re-emission inhibition from the simulated WFGD liquors by adding DTCR. Our data suggested that a concentration of 0.0005% (v/v) was enough for Hg0re-emission inhibition in the simulated WFGD liquors. The results also indicated that the Hg0re-emission inhibition efficiency by adding DTCR decreased as the operational temperatures increased and the Hg0re-emission inhibition efficiency increased as pH values and the oxygen concentration in the flue gas increased. The present findings could be valuable for industrial application of DTCR as a precipitant for stabilizing dissolved Hg2+to prevent re-emission of Hg0in WFGD solutions.

Removal of Selenium (VI) from Simulated Wet Flue Gas Desulfurization Wastewater Using Photocatalytic Reduction

The removal of Se(VI) from a simulated wet flue gas desulfurization (FGD) wastewater was attempted by the use of a photocatalytic reduction system. Conventional method of removal, such as adsorption by ion-exchange resins, was not effective for Se(VI) removal from wastewater. The photocatalyst reduction system consisted of TiO2 and HCOOH as a photocatalyst and an electron hole scavenger, respectively. When the photo-irradiation was performed for a Se(VI) solution under the system, an efficient removal of Se(VI) was achieved, suggesting that the reduction of Se(VI) actually occurs via the photocatalytic system to produce Se(0) precipitate. The photocatalytic reduction system could be applied to Se(VI) removal from the simulated wastewater. However, excess amount of HCOOH was needed to achieve efficient Se(VI) removal from wastewater.

Hg 2+ reduction and re-emission from simulated wet flue gas desulfurization liquors

In this study, considering that Hg 2+ in wet flue gas desulfurization (FGD) systems can easily be reduced and then released into atmosphere, causing secondary pollution, the researches about Hg 2+ reduction and Hg 0 re-emission mechanism were carried out. The effects of several experimental parameters on the reduction were studied, including initial pH, temperature, and concentrations of Cl − and S(IV). Our experimental results indicated that Cl − had a restraining effect on the Hg 2+ reduction and Hg 0 re-emission, after 24 h reaction, only 20.5% of Hg 2+ was reduced with 100 mM Cl − in simulated desulfurization solution. Cl − can slow Hg 2+ reduction and Hg 0 re-emissions dramatically through changing reaction mechanism, with formation of new intermediate: ClHgSO 3 −, which can decompose to Hg 0, but much more slowly than Hg(SO 3) 2 2− or HgSO 3. Simulating the conditions of the practical application (initial pH 5, T = 50 °C, S(IV) = 5 mM, Cl − = 100 mM), we also found that Ca 2+, NO 3 −, F −, etc. all had obvious effects on reduction rates. Based on the material balance and characteristic of the reactants, the reduction emission mechanism of Hg 2+ has been established, providing theoretical basis for industrial application of mercury control in wet FGD systems.