Influence of Flue-Gas Components on Mercury Removal and

Article pubs.acs.org/EF

Influence of Flue-Gas Components on Mercury Removal and Retention in Dual-Loop Flue-Gas Desulfurization Silvio Farr,*,† Barna Heidel,† Melanie Hilber,‡ and Günter Scheffknecht† †

Institut für Feuerungs- und Kraftwerkstechnik (IFK, Institute of Combustion and Power Plant Technology), Universität Stuttgart, D-70176 Stuttgart, Germany ‡ EnBW Energie Baden-Württemberg AG, D-70567 Stuttgart, Germany ABSTRACT: Due to the system, the dual-loop wet limestone flue-gas desulfurization process is assumed to be well-suited for mercury retention. As the behavior of mercury in the dual-loop process has been insufficiently dealt with in literature so far, the influence of sulfur dioxide and halides on mercury removal and retention was investigated. The process conditions in the loops are highly relevant for the removal and retention of mercury. The circulating slurries there differ in pH-value and oxidation− reduction conditions as well as in halides and sulfite concentrations and solid compositions. This work presents investigations that were conducted at a laboratory-scale test rig treating the flue gas of a pulverized coal combustion facility. The share of the removal of mercury in both loops was investigated as well as the removal of oxidized mercury compounds, the mercury reemission, and the total mercury removal efficiency. Both loops showed comparable removal rates of oxidized mercury compounds. In the quencher, the removal and retention of mercury is assumed to be improved by the formation of stable mercury complexes with chloride. The decomposition of mercury complexes formed with sulfite and redox reactions are supposed to decrease mercury retention in the absorber. A higher sulfite concentration in the quencher induced by higher SO2 concentration in the flue gas can increase mercury removal by the enhanced formation of mercury complexes with sulfite. At the same time it can promote re-emissions due to a higher share of sulfite on total S(IV) at the resulting higher pH-value of the quencher. Furthermore, high S(IV) concentrations might result in an increased mercury adsorption on particles. Increased chloride concentrations in the absorber can lead to higher removal rates of sulfur dioxide and improved mercury retention as well as to relatively high shares of mercury in wastewater, whereas the latter constitutes the desired mercury pathway.

1. INTRODUCTION Mercury (Hg) and its compounds are toxic pollutants, whose emissions are a global problem due to their distribution and bioaccumulation. In the context of mercury removal in thermal power plants, the wet desulfurization process is of particular interest, because it is a favorable mercury sink for common air pollution control. Dual-loop flue-gas desulfurization (FGD) systems can absorb water-soluble components and thereby reduce mercury emissions of thermal power plants. In this study, the term “removal of mercury” is used for the description of the absorption process. In wet FGD, already absorbed mercury is re-emitted to a certain extent. The term “retention of mercury” is used for the description of the mercury reemissions: high mercury retention is equal to low mercury reemission. In summary, the total removal rate of mercury in FGDs is characterized both by the removal of mercury and by the retention of mercury. Theoretically, the dual-loop system should be suitable for mercury removal and retention equally well or even better than the standard single-loop process. In the first loop, there are favorable mercury removal and retention conditions such as high chloride concentrations of the slurry. The second loop only treats a lower mercury load due to the prior removal in the first loop. Although the technology is well-known and state of the art in full-scale application, there are rarely studies on the behavior of mercury in dual-loop systems; cf. refs 1−3. Therefore, the present study investigates the influence of sulfite and halide concentrations on mercury removal and © 2015 American Chemical Society

retention in a laboratory-scale dual-loop FGD. Thereby, the process conditions of the loops are highly relevant for the removal and retention of mercury. The circulating slurries there differ in pH-value and oxidation−reduction potential (ORP) as well as in halide and sulfite concentrations and solid composition. In order to optimize the mercury retention in the dual-loop process, the FGD parameters which play a key role regarding the system of a dual-loop FGD are identified. The investigations were conducted at a laboratory-scale dualloop FGD test rig treating the flue gas of a pulverized coal combustion facility. The share of the removal of mercury in both loops was investigated as well as the removal of oxidized mercury compounds, the mercury re-emission, and the total mercury removal efficiency. Mercury content on particles and in wastewater was also determined, whereas the latter constitutes the desired mercury pathway.

2. BEHAVIOR OF MERCURY IN THERMAL POWER PLANTS The investigated flue-gas cleaning system consists of a SCRDeNOx catalyst in high-dust configuration, an electrostatic precipitator (ESP), and a wet limestone FGD system. Important species with regard to the behavior of mercury in thermal power plants are elemental mercury (Hg0), oxidized Received: December 4, 2014 Revised: May 20, 2015 Published: May 26, 2015 4418

DOI: 10.1021/acs.energyfuels.5b00899 Energy Fuels 2015, 29, 4418−4427

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Energy & Fuels mercury compounds (Hg2+), and particle-bound mercury (HgP).4 Mercury and its compounds are released as Hg0 at a combustion temperature of more than 1000 °C.5 The species’ shares change in dependence on the conditions of the flue-gas cleaning system and the flue-gas composition during the cooling of the flue gas. Elemental mercury is oxidized by halogen compounds and SCR-DeNOx catalysts can promote mercury oxidation. In this context, the halide concentration in the flue gas influences the mercury oxidation potential of the SCR-DeNOx catalyst.6 Adsorbed mercury on fly ash (Hgp) can be removed by the electrostatic precipitator. Downstream from the ESP, the flue gas still contains gaseous elemental and oxidized mercury compounds such as HgCl2 and HgBr2. Elemental mercury is hardly soluble in water whereas mercury species such as HgBr2 and HgCl2 are very well water-soluble.7 Therefore, it can be assumed that a high share of HgCl2 is absorbed in the wet FGD,8,9 whereas almost all of the Hg0 remains in the gaseous phase. 2.1. Removal and Retention of Mercury in Wet FlueGas Desulfurization. Limestone wet FGD constitutes a gas scrubbing process, in which the flue gas passes a countercurrent scrubbing column. In the latter, an absorbent is sprayed by nozzles. Common absorbents for the operation of wet FGDs are slurries containing calcium carbonate in the form of limestone or calcium hydroxide. The water-soluble flue-gas components sulfur dioxide (SO2), halides such as hydrogen chloride (HCl) and hydrogen bromide (HBr), and oxidized mercury compounds (Hg2+) such as mercury dichloride (HgCl2) are absorbed in the FGD slurry. The dissociation products of sulfur dioxide are, e.g., a sulfite anion and two protons; see eq 2.1. This explains the influence of the pH-value on the SO2 mass transfer. For common pHvalues in wet FGD slurries (pH 4−6), sulfur dioxide is mainly apparent as bisulfite. At higher pH-values, the share of sulfite increases.

physically absorbed in the liquid phase. The physical dissolution can be characterized by Henry’s law. This implies the dependency of the concentration of dissolved mercury dichloride in liquid phase on the partial pressure of mercury dichloride in gaseous phase.18 HgCl2 hardly dissociates, as its molecular structure is characterized as being distinctly covalent.19 The formation of mercury complexes leads to a decrease of molecularly dissolved HgX2. The latter is transferred to dissociating mercury complexes, which are no longer in the gas−liquid balance (X = Cl, Br; see eq 2.4). This results in a decreased vapor pressure.17,20 HgX 2 + X− ↔ [HgX3]− ;

(2.4)

In suspensions containing chloride ions, HgCl2 reacts with chloride to give [HgCl3]− and further on to give [HgCl4]2−.14 If the concentration of chloride is higher than 0.2 mol/L, [HgCl4]2− constitutes the dominating complex.21 For chloride concentrations exceeding 1 mol/L, HgCl2 is complexed to [HgCl4]2− to a large extent.17 Besides chloride, compounds of sulfur with the oxidation number of +IV (S(IV)) are important ligands for the formation of mercury complexes. The concentration of S(IV) depends among others on the SO2 load and the aeration of the suspension. Higher concentrations of S(IV) in the slurry would promote the formation of [Hg[SO3]2]2− and by this inhibit oxidized mercury transformation to Hg0;22 cf. ref 23. Mercury complexes with sulfite gain improved stability at elevated pH-values,24 which is assumed to be caused by the higher share of SO32− on S(IV). The term “stability” describes the degree of the complexation, but not the reactivity of the complexes. According to Blythe et al.,25 mercury complexes with sulfite in a chloride-free slurry at a temperature of 55 °C and a pH-value below 5 only showed a stability of some minutes.1 In comparison to chlorides, bromides show a higher complexation degree at equal concentrations.17 So the stability of mercury complexes with bromide and iodide is increased; cf. ref 2. Multiligand complexes are suggested by Blythe et al.25 and Wo et al.26 They consist of Hg2+ with both chloride and sulfite anions as ligands; see eq 2.5 and eq 2.6, and cf. ref 26. [ClHgSO3]− decomposes much more slowly to Hg0 than [Hg(SO3)2]2− or [HgSO3].26

SO2 (aq) + 3H 2O ↔ H3O+ + HSO3− + H 2O ↔ 2H3O+ + SO32 −

(2.1)

2HSO3− + O2 (aq) + 2H 2O ↔ 2SO4 2 − + 2H3O+

(2.2)

CaCO3(s) + H+ ↔ Ca 2 + + HCO3−

(2.3)

[HgX3]− + X− ↔ [HgX]4 2 −

At common FGD pH-values, the dissociation of HCl and HBr is not dependent on the pH-value. Therefore, the removal rates of these halides are high. Chloride and bromide concentrations vary in fuels, and their corresponding concentrations in the FGD slurry are adjustable depending on the scrubber design, e.g., by the amount of water added to the process. Common chloride concentrations of 5−15 g/L in the slurry enhance the dissolution of limestone10 as well as the dissolution of SO2.11 The reaction of sulfite or bisulfite with calcium from the limestone dissolution (eq 2.3) and oxygen from the flue gas, but mostly from the additional aeration (eq 2.2), forms calcium sulfate dihydrate (gypsum). The latter is the product of the FGD process. Among others, the quality requirements for the commercial use of gypsum are its purity (calcium sulfate dihydrate content), a chloride content of less than 0.01%, and a calcium sulfite content of less than 0.5%.12 To achieve a high removal rate of SO2, wet scrubbers are commonly designed with enough residence time and liquid to gas ratio (L/G) to absorb up to 90% SO2 or even more.13 Relevant processes of the removal and retention of mercury in wet FGD are to be described.9,14−17 Mercury dichloride is

HgCl2 + SO32 − ↔ [Cl 2HgSO3]2 −

(2.5)

[HgSO3] + Cl− ↔ [ClHgSO3]−

(2.6)

The reduction of mercury compounds such as HgCl2 leads to a re-emission of elemental mercury.27 Redox reactions such as eq 2.7 can lead to re-emissions; cf. refs 22 and 28. In the reaction, two electrons are transferred from S(IV) to Hg0. Hg 2 + + SO32 − + 3H 2O ↔ Hg 0 + SO4 2 − + 2H3O+ (2.7)

A lower pH-value leads to a higher oxidation force by oxygen and a lower reduction force by sulfite; cf. ref 20. Increasing the pH-value of the slurry leads to a higher share of SO32− in total S(IV). The latter increases the reduction of mercury;28−30 cf. ref 26. With regard to the mercury content in the solid phase, it is assumed that the sorption of Hg on FGD gypsum occurs by physical and chemical adsorption.31 A connection between the sulfite concentration in the slurry and the mercury concen4419


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in the quencher leads to a higher vapor pressure. But the stabilizing of mercury in the slurry is favored by the enhanced formation of mercury complexes with Cl− at high chloride concentrations. In the absorber, the suspension is characterized by a high pH-value and, due to the previous removal in the quencher, low concentrations of Hg2+(aq) and chloride are expected. Furthermore, the S(IV) species in the absorber has a higher share of SO32−. Due to the passive aeration, there is a high sulfite concentration in the absorber resulting in a low ORP. The suspension is saturated, and calcium sulfite is precipitating. The availability of sulfite leads to the formation of mercury complexes with sulfite. Due to the higher pH-value, Schütze1 expects mercury re-emissions especially in the absorber. The adsorption on particles is more important, as there is a significant connection between sulfite concentration and mercury content on particles due to the formation of mercury disulfite;32 cf. ref 26. There are interactions between the two loops as the pHvalue of the quencher is set in dependence of the absorber pHvalue; cf. ref 3. The pH-value has a strong influence on the removal of SO2 as well as on the retention of mercury. Furthermore, additional S(IV) which is absorbed in the absorber could lead to the re-emission of mercury in the quencher by high S(IV) concentrations; cf. ref 25. The interactions of the removal rates are important, as the quencher removal rate defines the absorber raw gas concentration. Extremes of the pH-range in the absorber are associated with negative effects on Hg0 re-emissions and on a residual limestone content of the gypsum product.3 Even at lowest pH-values, significant re-emissions occurred, which indicates the relevance of the interactions between the loops in the dualloop system; cf. ref 3. Measurements in technical scale at a power plant show lower pH-values and higher halide concentrations in the quencher which means that the main removal of mercury could be expected here.1 As the absorber slurry is dosed into the quencher and the mercury share of the solids in the two washing circles was 50:50, Schütze1 concluded that there must be a significant removal of mercury in the absorber. Influencing factors on mercury retention in the dual-loop process have not been outlined in further literature so far. In order to optimize mercury retention, the parameters which play a key role regarding the interaction-influenced system of a dual-loop FGD have to be identified in the two loops. In the following, the influence of sulfite and halides concentrations on mercury retention is investigated with regard to the removal and retention of mercury in the FGD as well as with regard to the fate of mercury, as wastewater constitutes the desired mercury pathway.

tration in the gypsum by the formation of mercury disulfite was observed;32 cf. ref 26. With an increasing ORP, the mercury content of the gypsum decreased up to 60%;33 cf. ref 34. According to Chang et al.,27 disproportionation of sulfite might occur at a higher load of SO2. The sulfide anion may react with mercury and form the highly insoluble mercuric sulfide; cf. ref 25. 2.2. Mercury Behavior in Dual-Loop Flue-Gas Desulfurization. The idea of the dual-loop process is based on the compromise concerning the pH-value of the slurry in the single-loop FGD process, as the pH-value has a contrary influence on gypsum purity and on the removal of sulfur dioxide.34 In the dual-loop process, a low pH-value in the quencher can provide good conditions for gypsum quality; a higher pH-value in the absorber can promote the desulfurization process. The scheme of a dual-loop FGD process is shown in Figure 1. The flue gas first passes the quencher and then the

Figure 1. Scheme of the dual-loop FGD process.

absorber. Between the washing circles, there is a flow gap. Flue gas passes the flow gap from the bottom to the top, and the slurry is accumulated in the center of the absorber and then recirculated. The suspension in the quencher is aerated, and therefore, low concentrations of sulfite are expected. The quencher slurry is characterized by oxidative conditions at a low pH-value, which result in a high ORP. But the aeration can also lead to the stripping of mercury. High Hg2+(aq) and high chloride concentrations result, as mercury and halides removal comparable to acid prescrubbers2 is assumed. As exemplarily shown in VGB Guideline M 419,13 the chloride concentration in the quencher of a full-scale dual-loop FGD amounts to 30 g/ L and the corresponding concentration in the absorber to 2−3 g/L. According to Henry’s law, the high mercury concentration

3. EXPERIMENTAL SECTION Experimental studies in laboratory scale were conducted at a continuously wet FGD in single- and dual-loop configuration. The measurements of the states were performed under nearly steady-state conditions. This implies a constant ORP for several hours in the experiments. Consequently, for each state, the FGD was operated for about 8 h. 3.1. Setup of the Laboratory-Scale Firing System. The experiments were conducted with combustion flue gas at a laboratory furnace (cf. ref 35). The test setup disposed of a high-dust SCR alignment; therefore, the flue-gas first passes a SCR-DeNOx reactor, then an electrostatic precipitator, and finally a wet flue-gas desulfurization system. The FGD was made of glass, and it can be 4420


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Energy & Fuels operated in single- or dual-loop configuration. The oxygen concentration of the flue gas was controlled to 5 vol %. Due to the fuels used, the total mercury concentration upstream from the FGD was below 10 μg/m3. For a higher accuracy of the measurements, mercury was added to the combustion air prior to the burner in order to adjust the concentrations of mercury in the flue gas. Sulfur dioxide was added in order to simulate fuel with higher sulfur content. The flue-gas temperature prior to FGD was held at approximately 120 °C. The wet flue-gas desulfurization was operated continuously. Figure 2

absorber set the pH-value by the dosing of limestone slurry. The reference pH-value was set to pH 5.6. The quencher was fed by a continuous overflow dosing out of the absorber. The slurry temperature in the quencher and the single-loop sump was set to 50 °C. During the experiments, the L/G ratio was set to 11.4 L/m3, which is comparable to those of real FGDs. Figure 2 shows various gas sampling points at both loops in the system. The slurry for gypsum and wastewater treatment was continuously removed from the external sump in the quencher loop. A demister after the FGD minimizes the loss of water in the absorber. The solid content of the quencher slurry resulted in 8.5%. The experimental conditions and the flue-gas composition of the single- and dual-loop experiments are shown in Table 1. The total mercury concentrations prior to the FGD covered a certain range in order to increase the universality of the investigations. For this purpose, mercury was added in order to adjust the concentrations of mercury in the flue gas. In a first step, the removal and retention of mercury in a dual-loop FGD was investigated. In order to increase the universality of the investigations, the removal rates of three measurements were arithmetically averaged and error bars for the measurements were calculated (plus/minus standard deviation). The measurements represent stable operation states for both different fuels and flue-gas sulfur dioxide and mercury concentrations as well as a pH-value scope of pH 5.4− 5.6 and the respective ORP. The slurry conditions of the dual-loop and single-loop FGDs are shown in Table 2. The pH-values

Table 2. pH-Value, ORP, and Chloride Concentration of the Slurries during the Measurements dual-loop FGD pH-value ORP (mV) chloride (g/L) a

quencher

absorber

single-loop FGD

2.4−4.2 300−600 1.0 anda 1.6

5.4−5.6 (set) 102−180 0.05 anda 0.09

4.6−5.6 (set) 316−600 0.7−0.9

Two measurements.

in the absorber were set. The resulting pH-values in the quencher sump varied between 2.4 and 4.2. The chloride concentrations in the slurry in the absorber were a few times lower than in the quencher. The ORP was lower than in the quencher due to the lack of an additional aeration in the absorber. The pH-values in the single-loop FGD were set in a typical range, and the ORP was comparable to the one in the quencher of the dual-loop FGD. The chloride concentrations were lower than in the quencher of the dual-loop FGD. This is among others caused by the lower chloride concentration of the fuel in the single-loop experiments. The chlorine content of the fired coal was lower (0.12 g/(kg wf)) than the one of the mixture fuel (0.57 g/(kg wf)); cf. ref 35. In the applied test system, the chloride concentrations in the slurries were lower than in typical full-scale systems. The effect of sulfite concentration on mercury removal and fate in the dual-loop process was studied in an experiment, in which three states were compared (i, ii, and iii). The flue gas with a SO2 load of 1.4 g/h (650 mg/m3) originated from the combustion of a mixture fuel of coal and biomass. The first state (i) constituted the reference operation. The SO2 concentration in the raw gas was increased in the

Figure 2. Process scheme of the laboratory-scale dual-loop FGD. Reprinted with permission from 3. Copyright 2014 VGB PowerTech. shows the process scheme of the FGD in dual-loop setup. The flue-gas flow of 2.1 m3/h (STP, dry) first passes the quencher and then the absorber. The flue-gas contact time in the quencher resulted in 4 s; the one in the absorber in 6 s. The single-loop experiments were conducted by only using the quencher loop. In contrast to the dualloop experiments, the longer column of the absorber was installed at the first loop. The quencher loop conveyed in an external sump with oxidation air supply. There, the aeration of the slurry with a volume flow of 2 L of oxygen/min was conducted. The parameters ORP, temperature, and pH-value in the loops and in the sump were measured continuously. An automatic pH-value control in the

Table 1. Experimental Conditions and Flue-Gas Composition at the FGD Inlet

contact time (s) fuel SO2 (g/m3) flue-gas Hg total (μg/m3] share of Hg2+ (%)

dual-loop FGD

single-loop FGD

4 (quencher)/6 (absorber) mixture fuel of biomass and coal; coal with addition of HCl 0.65−0.90 42−100 >82

6 coal 0.93−0.99 16−52 >81

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Figure 3. Scheme of the mercury measurement setup. Reprinted with permission from ref 15. Copyright 2004 Michael Hocquel.

⎛ c(Hg 0 out) − c(Hg 0 in) ⎞ ⎟ × 100 RER/% = ⎜ 2+ 2+ ⎝ c(Hg in) − c(Hg out) ⎠

second state (3.8 g/h and 1591 mg/m3; ii). In the third state (3.3 g/h and 1579 mg/m3), the sump aeration was reduced (iii). The flue-gas mercury concentrations in these experiments were between 37 and 43 μg/m3, with a >73% share of oxidized mercury compounds (Hg2+). In order to identify the effect of halides concentration in the dualloop process, the dosed limestone slurry was replaced by a slurry containing limestone and FGD wastewater taken from the quencher. The investigations focused on the absorber loop, as hydrogen chloride is removed in the quencher of the dual-loop process to a large extent, resulting in low chloride concentrations in the absorber. The dosed slurry contains FGD wastewater and limestone. This led to the fact that wastewater flows into the absorber and a higher salinity and higher halide concentrations result. The wastewater recirculation was conducted in two steps, which means that the wastewater from the first recirculation experiment was used to feed the second experiment. 3.2. Analytical Methods. The gaseous phase was sampled in order to measure the mercury species Hg2+ and Hg0 at sampling points prior to and after the loops. The concentration of elemental mercury and total mercury (elemental and oxidized mercury) was measured separately with the same analyzer at each sampling point. The scheme of the measurement setup is shown in Figure 3. The online Hg analyzer (RA 915 AM, Ohio Lumex, St. Petersburg, Russia) used in the experiments works with the principle of atomic absorption (AAS) at 254 nm with Zeeman background correction. In addition to that, a speciation method for the mercury species with DOWEX, which selectively adsorbs Hg2+, was used; cf. ref 36. For this purpose, a glass pipe containing DOWEX was added in the gas sampling line. The concentrations for each measured sampling point and species had to be constant for minutes, and they were then arithmetically averaged. As the concentrations were not measured at the same time, additional experiments were conducted to ensure stable conditions in the FGD. Due to the sump air outlet flow, which was measured separately, the removal rate, RR(Hg0) (eq 3.2), was calculated including all relevant gaseous mass flows of Hg0. Because of the dependency of the rate on the Hg0 inlet, the rate underlies an acausal influence. Therefore, a rate in reference to the absorbed Hg2+ was developed, which describes its re-emission. The re-emission rate, RER, was calculated with a concentration c(Hg0), which is calculated from the mass flow; see eq 3.3. The total mercury removal, which is calculated in the removal rate RR(Hg) as shown in eq 3.4, describes the sum of mercury entering and exiting the FGD.

⎧ ⎛ c(Hg 2 + out) ⎞⎫ ⎟⎬ × 100 RR(Hg 2 +)/% = ⎨1 − ⎜ 2+ ⎝ c(Hg out) ⎠⎭ ⎩

(3.1)

⎧ ⎛ ṁ (Hg 0 out) ⎞⎫ ⎟⎬ × 100 RR(Hg 0)/% = ⎨1 − ⎜ 0 ⎝ ṁ (Hg in) ⎠⎭ ⎩

(3.2)

⎪

⎪

⎪

⎪

⎪

⎪

⎪

⎪

(3.3)

⎧ ⎛ c(Hg 0 out) + c(Hg 2 + out) ⎞⎫ ⎟⎬ × 100 RR(Hg)/% = ⎨1 − ⎜ 0 2+ ⎝ c(Hg in) + c(Hg in) ⎠⎭ ⎩ ⎪

⎪

⎪

⎪

(3.4) A gas analyzer was used for the measurement of SO2 with the principle of UV absorption; the oxygen content was measured paramagnetically at the same time. In order to ensure the comparability, the emissions were standardized to a 6 vol % oxygen content, dry. A continuous liquid analysis of the slurry in the quencher, the external sump, and the absorber was performed with electrodes for pH-value and platinum with silver chloride reference electrodes for ORP measurement. Gypsum samples were taken out of the process by continuously removing 200 mL of the quencher slurry. The latter was first filtered, then rinsed with 200 mL of deionized water, and finally dried overnight at 45 °C. The analysis of the product gypsum in order to measure the concentration of calcium sulfite semihydrate was done by iodometric titration according to the VGB Guideline M 701.37 The limestone content was measured by the addition of hydrogen chloride and gravimetry. Hereby, the content of calcium carbonate was calculated from the mass loss of CO2. This method was validated by thermogravimetric analysis; cf. ref 37. The mercury measurements of the slurries were conducted noncontinuously, as only a small slurry flow is produced in laboratory scale. Mercury was determined in wastewater and on particles. Liquid samples with a defined volume were added to a desorption reactor, where chemical reduction of Hg2+ by SnCl2 takes place. The emerging Hg0 was analyzed with cold vapor atomic absorption (CVAAS) in a mass flow controlled nitrogen flow. The chloride concentration in the liquid phase was determined by liquid chromatography of ions according to ISO 10304-1:2007.38 Solid samples were analyzed by temperature-programmed desorption. The sample was heated to 500 °C, and a defined gas flow was analyzed with CVAAS. The validation of the method was performed under total digestion and measurement of samples by CVAAS according to ISO 10304-1:2007.39

4. RESULTS AND DISCUSSION In the following, the removal and retention of mercury in the two loops of a dual-loop FGD with reference to a single-loop system are discussed. The studies focus on the effect of sulfite concentration and halides concentration on the removal and fate of mercury in the dual-loop process. 4.1. Removal and Retention of Mercury in Dual-Loop FGD. The removal rates of Hg2+ and Hg0 in the quencher and 4422


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Figure 4. Oxidized mercury compounds and elemental mercury removal in dual-loop FGD.

Figure 5. Mercury re-emission and total removal in dual-loop FGD.

to the high share of Hg2+. This results in a high negative percentage of more than −100 concerning the Hg0 removal rates and in respectively high error bars. The re-emission rate is defined in order to evaluate the reemission behavior of the process in dependence on the removed Hg2+. Figure 5 shows the corresponding mercury reemission rates and the total removal rates of mercury in a dualloop FGD with corresponding single-loop measurements. In the dual-loop process, the quencher shows a lower re-emission rate than the absorber due to the favorable conditions for forming mercury-chloride complexes, although the mercury load entering the quencher is higher. The re-emission rate of the absorber is measured slightly higher, as complexes with sulfite are assumed to only have a temporary stability under these conditions. Furthermore, a higher share of sulfite on S(IV) could promote mercury re-emission in the absorber by redox reactions. In total, the re-emission is measured higher in the investigated dual-loop process than in the single-loop process. In this context, the dual-loop has a higher mercury and sulfur dioxide removal rate due to the design of the FGD with a higher contact time. The higher removal rates result in a higher mercury and S(IV) content in the system, which leads to higher re-emissions of mercury in the investigated dual-loop system. The total mercury removal rate in the quencher is slightly higher than the one of the single-loop FGD. A lower pH-drop of the falling droplets in the quencher than in the single-loop FGD leads to more stable slurry conditions. A higher chloride concentration in the quencher tends to result from the separation of the absorption into two loops. In the dual loop, mercury is further removed in the absorber. The higher re-

in the absorber loop as well as in the single- and dual-loop FGDs are shown in Figure 4. The removal rate of Hg2+ in the quencher is measured comparably to the rate of the single-loop FGD, which amounts to 70%. The deviations result from the fact that the measurements represent stable operation states for both different fuels and flue-gas sulfur dioxide and mercury concentrations as well as for a pH-value scope of pH 5.4−5.6 and the respective ORP. The scale of the removal rate of Hg2+ in the quencher and the conditions in the slurry (chloride, ORP) are comparable to the single-loop system. A higher removal rate might result from the higher chloride content in the quencher slurry, which promotes the formation of charged mercury−chloride complexes. In the absorber, low chloride concentrations might lower the formation of charged mercury− chloride complexes. Due to the lack of aeration and high concentrations of S(IV) in the absorber, the formation of the [Hg[SO3]2]2− complex could accelerate the mass transfer of Hg2+ from the gaseous to the liquid phase. The removal rate of Hg2+ in the overall dual-loop FGD amounted to 90%. Figure 4 shows that the removal rates of Hg0 are negative due to the re-emission of already absorbed mercury in FGD. It is obvious that the re-emission takes place in the quencher to a relatively high share. This is caused by the higher mercury concentration in the slurry and the stripping effect of the aeration. The total re-emissions of the dual-loop FGD are higher than in the single-loop FGD experiments, which is caused by a higher removal rate of Hg2+ and sulfur dioxide in the dual-loop experiments. This leads to a higher absorbed mass flow of Hg2+ and S(IV) for the FGD. The removal rate of Hg0 depends on the flue-gas Hg0 content, which is very low due 4423


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Figure 6. Effect of SO2 load and oxidation air on the removal of SO2 in dual-loop FGD (three states with a 0.05% (i), 3.3% (ii), and 7.5% (iii) share of calcium sulfite in gypsum are compared).

Figure 7. Effect of SO2 load and oxidation air on mercury behavior and adsorption on particles. (Reference for Hg on particles is the first state (i) with a 0.05% share of calcium sulfite in gypsum.)

the solid phase amounted to 3.3%, which exceeds the limit for commercial use. As a second step (iii), the aeration is reduced in the depth of the dip pipe, which leads to a higher share of calcium sulfite of 7.4%. Figure 6 shows the effect of sulfur load and oxidation air on the removal of SO2 in dual-loop FGD. The SO2 concentration in the flue gas increases from 0.65 g/m3, STP, dry (i) to around 1.60 g/m3, STP, dry (ii). At the same time, the ORP in the quencher decreases due to the higher sulfur load and precipitation of calcium sulfite occurs. With less aeration (iii), the ORP further decreases, as less oxygen is absorbed in the slurry. The pH-value increases due to the slightly decreased oxidation of sulfite to sulfate. In general, the removal rate of sulfur dioxide decreases with increasing SO2 concentration in the raw gas. A higher dosing of fresh limestone suspension at a higher SO2 load results in increased pH-values of the slurry in the quencher loop (ii and iii). With less aeration (iii), the oxidation of bisulfite to sulfate is inhibited. Due to the pH-buffering effect of dissolved S(IV) species at high concentrations, increases of quencher pH-value and quencher SO2 removal rate are observed. In summary, at reduced aeration (iii) the total SO2 removal rate is higher than with full aeration (ii), but at the expense of gypsum purity. The effect of sulfur load and oxidation air on mercury behavior and adsorption on particles shows Figure 7. The Hg2+ removal rates and the re-emission rates in the quencher and in the overall dual-loop FGD are shown in the panel on the left side. As the removal rates of the quencher and the dual loop

emission in the absorber in comparison to the quencher is caused by redox reactions. In addition it is assumed that mercury sulfite complexes decompose which lowers the total mercury removal rate to 20%. The dual-loop FGD in these experiments removed 60% of the mercury in the flue gas in total. The deviations result from the fact that the measurements represent different stable operation states in order to increase the universality of the measurements. The highest deviations occur at the absorber due to a lower mercury flue-gas concentration there. The total mercury removal rates differ only to a minor degree. It can be concluded that in dual-loop FGD systems, the mercury load is assumed to be mainly removed in the quencher. Furthermore, the quencher is assumed to be a more favorable loop than the absorber due to the mercury stabilizing conditions in pH-value and chloride content. Thus, halides and sulfite concentrations seem to play a key role for mercury removal and retention. In order to achieve maximal mercury removal, both loops are supposed to be necessary. 4.2. Effect of Sulfite Concentration on the Removal and Fate of Mercury. The effect of sulfite concentration on mercury removal, retention, and fate in the dual-loop FGD was studied. Because the aeration performance of the FGD system is limited, increasing the SO2 load as a first step (ii) leads to a higher concentration of sulfite in the quencher due to a higher load of absorbed sulfur dioxide, which in turn leads to precipitation of calcium sulfite. The content of calcium sulfite in 4424


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Figure 8. Effect of chloride concentration on the removal of sulfur dioxide and mercury and on the adsorption of mercury on particles.

g/L. The same goes for bromide, which increases from 5 mg/L to more than 10 mg/L. The ORP in the absorber simultaneously increases from 180 to 217 mV. The same tendency was monitored during the process of adding sodium chloride to a FGD system.28 The removal rates of SO2 and Hg2+ increase as well, leading to a SO2 removal rate higher than 99% (reference 92%) and to a total mercury removal rate higher than 82% (reference 62%). The sulfur dioxide removal increases because of the increased SO2 dissolution at a higher salinity in the absorber and the enhanced dissolution of limestone. The pH-value in the quencher increases from pH 3.3 to pH 5.0 due to a higher SO2 removal rate in the absorber leading to a higher dosing of limestone slurry. Due to the recirculation there is also an enhanced dissolution of limestone in the quencher, which increases the pH-value. As a result of the increased concentration of halides such as chloride or bromide by wastewater recirculation, it is assumed that the formation of mercury complexes with halides is improved. In the absorber, multiligand complexes of sulfite and chloride are expected to be formed. The vapor pressure of mercury in the solution decreases which leads to a higher removal rate of oxidized mercury compounds and to increased mercury retention. Due to the higher removal rates of mercury and recirculated mercury, an increased liquid phase mercury concentration results. It is assumed that due to the increased formation of mercury complexes, mercury adsorption on particles remains unaffected or decreases. This results in comparable shares of mercury on particles (even lower than in the reference). It can be concluded that especially the dual-loop FGD process can benefit from increased halides concentrations in the absorber, as the formation of mercury complexes with for example chloride or multiligand complexes is favored. This results in lower mercury concentrations in clean gas and on particles.

differ only to a certain extent, the previously described dominating influence of the quencher in the dual-loop system is obvious. The oxizided mercury compounds removal rate decreases in the absorber at a higher sulfur load (ii), as a higher dosing of fresh limestone suspension results in lower halide concentrations. The chloride concentration, which is very low in the absorber in comparison to the quencher, decreases further on and, by this, the formation of mercury complexes with chloride and multiligand complexes is limited. The sulfite excess in ii and especially the lower aeration (iii) increases the Hg2+ removal rate in the quencher. Due to the lack of oxidation air and sulfite excess, mercury complexes with sulfur or multiligand complexes are supposed to be formed in the quencher. As the pH-value increases because of the lower oxidation of S(IV) at reduced aeration (iii), mercury sulfite complex formation is additionally preferred at higher shares of SO32− in the total amount of S(IV). The total re-emission of mercury in the FGD is dominated by the quencher due to the high S(IV) load in the quencher loop with high mercury content. Slightly increased S(IV) concentration in the quencher (ii) decreases mercury reemission, as mercury complexes with S(IV) or multiligand complexes are supposed to be formed. Reduced aeration (iii) leads to a considerable excess of S(IV) and an increase in pHvalue. Thus, the chemical reduction of Hg2+ by S(IV) and Hg0 re-emission is promoted. By reducing the aeration (iii), the removal rate of SO2 in the quencher increases resulting in a lower SO2 load in the absorber. A lower dosing of fresh limestone suspension leads to higher halide concentrations and a higher removal rate of Hg2+. The higher dissolved Hg2+ concentration in the absorber leads to increased re-emissions (iii). In this experiment, the total mercury removal decreases from 62% (i) to 56% (ii) and, with less aeration (iii), it ends up at 61%. The panel on the right side in Figure 7 shows that mercury in the slurry fulfils a relocation from liquid to solid phase up to 1200%, as the Hg adsorption on particles is favored and less mercury in the liquid phase is found. At the highest mercury load on particles, mercury is hardly found in the liquid phase. 4.3. Effect of Halides Concentration on the Removal and Fate of Mercury. The effects of halides concentration on the removal of sulfur dioxide and mercury and on the adsorption of mercury on particles are shown in Figure 8. The recirculation leads to an increase in the chloride concentration in the slurry in the quencher from 1.0 to 1.7

5. CONCLUSION The qualification of the dual-loop flue-gas desulfurization process on the removal of Hg2+ was shown in experiments with a laboratory wet FGD test rig. Due to the different conditions in the two loops, the shares of the mercury removal in each loop were investigated. The results show that both loops in dual-loop FGD systems seem to be relevant with regard to the total mercury removal. In the quencher, the removal and retention of mercury is supposed to be improved by the formation of mercury complexes with chloride. The rather 4425


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reduce mercury emissions). Emissionsminderung 2014, VDI Verlag: Dusseldorf, Germany, 2014; ISBN 978-3-18-092214-0. (3) Heidel, B.; Farr, S.; Scheffknecht, G.; Hilber, M. The Behaviour of Mercury in Dual-Loop Flue-Gas Desulphurization Systems. VGB Workshop “Flue Gas Cleaning 2014”, Marseille, France, May 21−22, 2014. (4) Scheffknecht, G.; Farr, S.; Heidel, B.; Schwämmle, T.; Brechtel, K. Mercury in Thermal Power Plants: Chemical Behavior and Possibilities for Separation. Chem. Ing. Tech. 2012, 84 (7), 1041−1051. (5) Li, Y.; Daukoru, M.; Suriyawong, A.; Biswas, P. Mercury Emissions Control in Coal Combustion Systems Using Potassium Iodide: Bench-Scale and Pilot-Scale Studies. Energy Fuels 2009, 23, 236−243. (6) Schwämmle, T.; Bertsche, F.; Hartung, A.; Brandenstein, J.; Heidel, B.; Scheffknecht, G. Influence of geometrical parameters of honeycomb commercial SCR-DeNOx catalysts on DeNOx-activity, mercury oxidation and SO2/SO3-conversion. Chem. Eng. J. (Amsterdam, Neth.) 2013, 222, 274−281. (7) Aylett, B. J. Comprehensive Inorganic Chemistry; Pergamon Press: Oxford, U.K., 1975. (8) Pavlish, J. H.; Sondreal, E. A.; Mann, M. D.; Olson, E. S.; Galbreath, K. C.; Laudal, D. L.; Benson, S. A. Status review of mercury control options for coal-fired power plants. Fuel Process. Technol. 2003, 82, 89−165. (9) Heidel, B.; Farr, S.; Brechtel, K.; Scheffknecht, G.; Thorwarth, H. Influencing factors on the emission of mercury from wet flue gas desulphurisation slurries. VGB PowerTech 2012, 3, 64−70. (10) VDI 3927 Bl. 1 draft: Waste gas cleaningRemoval of sulphur oxides, nitrogen oxides and halides from combustion flue gases; Beuth Verlag: Berlin, Germany, 2013. (11) Wedzicha, B. L.; Webb, P. P. Vapour pressure of SO2 above solutions of sulphur(IV) oxospecies: The effects of chloride ion and glycerol. Food Chem. 1996, 55 (4), 337−341. (12) Eurogypsum. FGD Gypsum Quality Criteria and Analysis Methods; Eurogypsum: Brussels, Belgium, 2012; http://www. eurogypsum.org (accessed Jan 13, 2015). (13) VGB PowerTech. M 419. Merkblatt für Bauart, Betrieb und Wartung von Rauchgasentschwefelungsanlagen (REA) (Translation: Guideline for design, operation and maintenance of flue-gas desulfurization systems (FGD)); VGB PowerTech Service GmbH: Essen, Germany, 2009. (14) Acuña Caro, C. Chemical Behavior of Mercury in Wet Flue Gas Desulphurisation Systems. Ph.D. Dissertation, University of Stuttgart, Germany, 2013. (15) Hocquel, M. The Behaviour and Fate of Mercury in Coal-Fired Power Plants with Downstream Air Pollution Control Devices. VDI Fortschritt-Berichte, Vol. 251; VDI-Verlag: Düsseldorf, Germany, 2004. (16) Riethmann, T. Untersuchungen zur Sorption von Quecksilber aus Verbrennungsabgasen und Nebenprodukten in Entschwefelungsanlagen (Translation: Investigations on the sorption of mercury from combustion gases and side products in desulfurization systems). Ph.D. Dissertation, University of Stuttgart, Germany, 2013. (17) Kanefke, R. Durch Quecksilberbromierung verbesserte Quecksilberabscheidung aus den Abgasen von Kohlekraftwerken und Abfallverbrennungsanlagen (Translation: Improved mercury removal from coal power plants and waste incineration flue-gases by mercury bromination). Ph.D. Dissertation, MLU Halle-Wittenberg, Germany, 2008. (18) Schütze, J.; Köser, H. Strategies for enhancing the co-removal of mercury in FGD scrubbers of power plantsOperating parameters and additives. VGB PowerTech 2012, 3, 71−77. (19) Griffiths, T. R.; Anderson, R. A. Structure of Mercury(II) Halides in Solution and Assignment of Their Resolved Electronic Spectra. Faraday Trans. 1979, 8/1684, 957−970. (20) Bittig, M. Zum Einfluss unterschiedlicher Liganden auf die Quecksilberabscheidung in absorptiven Abgasreinigungsstufen (Translation: On the influence of different ligands on the mercury removal in absorptive flue-gas cleaning systems). Ph.D. Dissertation, University of Duisburg Essen, Germany, 2010.

unstable mercury complexes formed with sulfite and redox reactions reduce mercury retention in the absorber. Flue-gas SO2 and halides load can affect the behavior of mercury and lead to an increased mercury adsorption on particles for high SO2 loads. A higher sulfite concentration in the quencher induced by a higher SO2 concentration in the flue gas can enhance mercury removal by forming mercury complexes with sulfite. At the same time, it can promote reemissions by redox reactions with a share of sulfite in total S(IV) at a higher pH-value in the quencher. Furthermore, by exceeding the limits of the FGD design, the gypsum product might not meet the quality criteria due to its high calcium sulfite content. Promoting the formation of charged mercury complexes with halides in the absorber at increasing halides concentrations by wastewater recirculation can lead to a higher mercury removal and retention of the system with relatively high shares of mercury in wastewater. The latter constitutes the desired mercury pathway. In the applied test system, the chloride concentrations in the slurries were lower than in typical full-scale systems. Therefore, it is assumed that the mercury re-emissions in this study are higher than in full-scale systems. Further studies are recommended in order to confirm the results for full-scale applications. Optimization should be performed by avoiding re-emissions both in the absorber and in the quencher. This involves the adaption of the operational pH-value, as it significantly influences the removal rates of sulfur dioxide and mercury reemission processes. The same goes for the aeration, as it affects the S(IV) concentration in the quencher and for the rate of wastewater recirculation with regard to the salinity of the absorber slurry, as the formation of mercury complexes with halides could be enhanced.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +46 72 8789830. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was kindly supported by EnBW Energie BadenWürttemberg AG. ABBREVIATIONS AAS = atomic absorption CV = cold vapor ESP = electrostatic-precipitator FGD = flue-gas desulfurization Hg = mercury IFK = Institute of Combustion and Power Plant Technology ORP = oxidation−reduction potential S(IV) = sulfur in oxidation state IV SCR = selective catalytic reduction

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REFERENCES

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