Removal of Hg0 from flue gases in wet FGD by catalytic oxidation with air – An experimental study

Fuel 89 (2010) 3167–3177

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Removal of Hg0 from flue gases in wet FGD by catalytic oxidation with air – An experimental study Andrej Stergaršek a, Milena Horvat a,*, Peter Frkal a, Jošt Stergaršek b a b

‘‘Jozˇef Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia Notranjski regijski park, Tabor 42, 1380 Cerknica, Slovenia

a r t i c l e

i n f o

Article history: Received 21 February 2010 Received in revised form 5 April 2010 Accepted 7 April 2010 Available online 20 April 2010 Keywords: Elemental mercury Removal Flue gas Oxidation FGD

a b s t r a c t About 46% of global mercury emissions are due to fossil fuel combustion for electrical and thermal energy production. Since more stringent emission standards are expected, important research efforts are being focused on the development of mercury removal technologies, mainly directed to two alternative approaches: (i) the enhancement of homogeneous oxidation in the flue gases of Hg0 to water soluble Hg2+ by the addition of chlorides or bromides to the boiler or; (ii) the adsorption of Hg2+ and Hg0 on impregnated activated carbon (AC). The latter may require the treatment of the entire gas volume of the thermal power plant and constantly consumes relatively large quantities of AC. A third option gaining more attention lately is based on the oxidation and retention of dissolved Hg0 in the wet flue gas desulphurisation (FGD) system. A series of chemical oxidants, such as halogens, hydrogen peroxide, sulphur and oxygen, are theoretically able to oxidize Hg0 in the wet FGD system. Most chemical oxidants when applied in the FGD, however, are non-selective and are largely consumed by SO2 absorbed from the flue gas. The less expensive oxidant, non-selective as well, is oxygen (as air) which is already 2 being dispersed into FGD absorbing suspension for the conversion of SO2 3 into SO4 . The experimental evidence of the present work showed that Hg0 present in the gaseous phase can be dissolved and oxidized to a high degree (70–90%) by air together with SO2 3 in wet FGD solutions. Transition metals such as Fe2+ and Mn2+ act as catalysts, chloride enhances the reaction, while some oxosulphur compounds, e.g. tetrathionate, inhibit the oxidation. A combination of several catalysts at a 1 and an adequate redox potential of the solution can concentration of sulphite (SO2 3 ) below 100 mg L assure reasonable mercury removal even in the presence of oxidation inhibiting compounds. The main competitive reactions that govern final Hg0 removal in the FGD are as follows: (1) oxidation of Hg0 together with SO2 with air, enhanced by catalysts; (2) removal of catalysts by precipitation in the form of Fe(OH)3 and eventually as MnO2 (to overcome this problem continuous addition of catalysts to the solution is required); (3) reduction of Fe3+ by tetrathionate to Fe2+ which (4) may reduce Hg2+ to Hg0 and probably (5) the complexation of Hg2+ by anions present which may play an important role in the mechanism by complexing the product(s) of the Hg0 oxidation reaction. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Mercury is present in most coals used in thermal power plants producing electrical energy. Due to the low stability of mercury compounds and the high volatility of Hg and some of its compounds at elevated temperature, almost all the Hg present in coal is mobilised and leaves the boiler in the flue gas. The fate of the element and its compounds in different devices after the boiler depends greatly on the coal composition and on processes used for fly ash separation from flue gas and removal techniques for SO2 and NOx [1]. The industrial sector using fossil fuel combustion for * Corresponding author. Tel.: +386 1 5885450; fax: +386 1 5884346. E-mail addresses: [email protected] (A. Stergaršek), [email protected] (M. Horvat), [email protected] (P. Frkal), [email protected] (J. Stergaršek). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.04.006

power and heating contributes approximately 890 ta1of Hg or 46% to the global emission balance, being about 1930 ta1 [2,3]. The most important contributors in this sector by country are the PR of China with 43%, the rest of Asia with 26%, Europe and North America with 9% each and all others with 13% [3]. It is a unanimous understanding that mercury is a very toxic element; further it is considered a global pollutant due to its high volatility. Consequently it is proposed that more stringent legally binding limitations of emissions [4] be introduced internationally in the framework of UNEP. In view of these expected restrictions on Hg emissions from thermal power plants, intensive research is being conducted in order to develop technological processes for the removal of Hg from flue gases with an efficiency of about 90%. Two major options are mainly being investigated to remove mercury from the flue gas: firstly, the enhanced oxidation of Hg0

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in flue gas to Hg2+ which is soluble in the aqueous solutions of wet flue gas desulphurization (FGD) installations and, secondly, the adsorption of Hg0 on impregnated activated carbon (AC) which is constantly dispersed in the gaseous phase. The first option, the oxidation of Hg0 in the gaseous phase, is based on the presence of halogenides, such as chloride and bromide, that are present in coal or added to the boiler where chlorine or bromine, Cl2 or Br2, formed with the involvement of oxygen according to the Deacon reaction, oxidize Hg0 to Hg2+ that is then dissolved and retained in the FGD solutions [1,5]. Halogenide addition is quite simple with respect to the equipment needed and the chemicals added are not very costly. The drawback of this option could be the increased emission of bromides to the atmosphere where bromides may participate in the chain reactions depleting ozone from the atmosphere. In the case of AC injection into the flue gas, AC can be removed in the fly ash separation devices or by a special filter that has to treat the entire flue gas emission of the boiler. This option is demanding with respect to investment and operating costs due to energy and AC consumption. If AC is filtered together with fly ash the later becomes less attractive as a construction material due to the elevated carbon and mercury content. The oxidation of Hg0 in flue gas by halogens and the adsorption of Hg0 on AC are technological alternatives which are already in the commercial demonstration stage. The third option for removal of Hg0 from flue gases that has attracted a number of researchers is still in the research stage. The principle of this approach is based on modification of the wet FGD chemistry in order to simultaneously remove SO2 and Hg0. The oxidation of Hg0 in the solution of wet FGD seems to be the most promising modification of FGD chemistry. The drawback of the use of chemical oxidants is that they are not selective and are largely consumed by SO2, rendering the process expensive due to the operational costs. The option here could be a two-stage reaction; in the first classical SO2 absorption and oxidation by air should occur, and in the second, mercury should be oxidized with lower oxidant consumption, but all the flue gas has to be treated twice. From the theoretical standpoint, a number of chemical oxidants are able to oxidize Hg0 under the chemical conditions at which wet FGD processes are performed, i.e. pH between 5 and 6, temperature usually between 40 and 65 °C, forced oxidation of SO2 by air sparged into the absorbing suspension tank, resulting in a concen1 tration of SO2 . Some of 3 ions usually between 10 and 1000 mg L the possible chemical reactions that could run spontaneously, as shown by negative Gibbs enthalpies, are listed below (data from [6–10]):

H2 SO3 ðaqÞ þ Hg0 þ O2 ! HgSO4 ðaqÞ þ H2 OðlÞ

DG0 ¼ 284:6 kJ=mol

ð1Þ

Hg0 ðaqÞ þ H2 O2 ðaqÞ ! HgðOHÞ2 ðundissÞ

DG0 ¼ 140:0 kJ=mol

ð2Þ

Hg0 ðaqÞ þ Cl2 ðgÞ ! HgCl2 ðaq; undissociatedÞ

DG0 ¼ 172:8 kJ=mol

ð3Þ



3Hg0 ðaqÞ þ ClO3 ðaqÞ þ 3H2 OðlÞ 

3Hg2þ ðaqÞ þ Cl ðaqÞ þ 6OH ;

DG0 ¼ þ138:1 kJ=mol ð4Þ



3Hg0 ðaqÞ þ ClO3 ðaqÞ þ 3H2 OðlÞ 

! 3HgðOHÞ2 ðaq; undissÞ þ Cl ðaqÞ

DG0 ¼ 235:8 kJ=mol ð5Þ

DG0 ¼ 46:4 kJ=mol

Hg0 ðaqÞ þ SðcÞ ! HgSðsÞ

ð6Þ



2Hg0 ðaqÞ þ ClO2 ðaqÞ þ 2H2 OðlÞ 

DG0 ¼ þ25:6 kJ=mol

$ 2Hg2þ ðaqÞ þ Cl ðaqÞ þ 4OH

ð7Þ



2Hg0 ðaqÞ þ ClO2 ðaqÞ þ 2H2 OðlÞ 

DG0 ¼ 223:7 kJ=mol ð8Þ

! 2HgðOHÞ2 ðaq; undissÞ þ Cl ðaqÞ



Hg0 ðaqÞ þ 2Mn3þ ðaqÞ þ 2Cl ðaqÞ

DG0 ¼ 201:5 kJ=mol

! HgCl2 ðaq; undissÞ þ 2Mn2þ ðaqÞ

ð9Þ 

Hg0 ðaqÞ þ 2Fe3þ ðaqÞ þ 2Cl ðaqÞ

DG0 ¼ 58:3 kJ=mol

$ HgCl2 ðaq; undissÞ þ 2Fe2þ ðaqÞ

ð10Þ 

2Hg0 ðaqÞ þ 0:5O2 ðgÞ þ H2 OðlÞ þ 2Cl ðaqÞ

DG0 ¼ 126:9 kJ=mol

! Hg2 Cl2 ðsÞ þ 2OH ðaqÞ

ð11Þ



Hg0 ðaqÞ þ 0:5O2 ðgÞ þ H2 OðlÞ þ 2Cl ðaqÞ

DG0 ¼ 89:3 kJ=mol



! HgCl2 ðaq; undissÞ þ 2OH ðaqÞ

ð12Þ 2+

The formation of complexes of Hg with anions present in the solution, e.g. Cl and OH, promotes the progress of the spontaneous reactions; see Eqs. (4) and (7) (non-spontaneous) and Eqs. (5) and (8) (spontaneous). A further observation is that the formation of Hg2+ and/or Hg22+ is possible (Eqs. (11) and (12)). This equilibrium should be reversible and greatly influenced by the solubility of the corresponding Hg(I) and Hg(II) compounds [10]. Simplified theoretical assumptions (not including kinetic considerations) and results found in industrial practice are not in agreement. The measurements of the removal potential of wet FGD in industrial installations show that Hg0 not only passes the FGD but also part of the dissolved Hg2+ is re-emitted. Consequently, the chemistry of Hg2+ reactions in the solution at wet FGD conditions was studied by a number of researchers, studying the reduction reactions of dissolved Hg2+ to Hg0 by SO2 3 and divalent ions of transition metals, like Fe2+, and its re-emission from the FGD solutions [1,11,12–15] and the oxidation reactions of   Hg0 by different oxidants, like ClO2 , ClO3 , H2 O2 , MnO 4 and others [16–18]. All investigations have considered the complexation reactions of oxidized mercury species with the anions present in the 2 FGD solutions. The anions studied included SO2 3 , SO4 , haloge2   nides, especially Cl , S2 O3 , OH and also some combinations of them. When studying the reduction of Hg2+ in the solution the authors generally did not control all chemical parameters that simulate the actual conditions in the FGD installations, e.g. the adequate oxygen supply to the make-up agitated tank at the absorbing unit and maintaining the proper level of the sulphite ions in the solution. Chang and Zhao [13] did not control the sulphite level, Wo et al. [12] used nitrogen to strip Hg0 out of the test solutions. When studying the oxidation reactions Diaz-Somoano et al. [17] report on the laboratory experiments where synthetic gas containing O2, H2O, SO2, N2 and Hg0 was bubbled through the solution and the mercury removal from the gas was monitored. CaO slurry was used for the neutralization instead of usual CaCO3. The redox potential was not measured. Up to 75% removal of Hg0 from the synthetic gas was achieved ant the correlation of the removal efficiency was observed with SO2 (though SO2 was not 3

A. Stergaršek et al. / Fuel 89 (2010) 3167–3177

measured or controlled) and pH and no correlation between removal efficiency and gas flow and HCl concentration in the gas. From the data published it seems that the specific load of the solution with the oxygen in air was not sufficient to show strong influence on the Hg0 removal. The correlation between oxygen supply and Hg0 removal efficiency was observed by Acuña-Caro et al. [18] that used similar laboratory bubbler stirred reactor in which the synthetic gas containing O2, SO2, HCl and Hg0 was introduced. At 15% of O2 in the gaseous phase used the removal rate of Hg0 was 22 to 28% at the concentration of Cl between 0 and 10 gL1. The authors did not measure the concentration of SO2 3 in the solution which is very important parameter as we will show later. They have determined the redox potential. From the data published it is not possible to evaluate the specific load of the slurry with oxygen. The authors showed significant influence of the chlorine species (Cl) on the oxidation degree of Hg0 and this effect is attributed to the formation of strong complexes of oxidized mercury with chloride ions. One should be aware that in the industrial FGD make-up tank at the absorber the oxidation air is introduced generally in the depth of 6 meters (because the Roots air blowers that are normally used can produce 0.7 bar pressure) and when one makes scaledown of the industrial tank it must be considered that in the agitated laboratory vessel the depth of some 10s of centimeter does not allow very efficient oxygen transfer from the gaseous to the liquid phase due to very short contact time and comparatively very low partial pressure of oxygen in the solution. To overcome this problem we used bubbler column with the solution depth of about 1 m. Beside this it should be mentioned that bubbling the synthetic flue gas through the solution in a gas–liquid contacting device simulates the absorber but one must consider that additional flow of air is sparged into the solution for the oxidation of sulphite. The same observation is given by Blythe et al. [15] reporting on the difficulties to obtain the target SO2 3 concentration in the solution, being 1 mM or less, in the bench scale apparatus with the slurry depth in inches as compared to the depth in the industrial installation being several feet. The problem was reportedly overcome with very intense oxidation and the conditions were established that closely represent the actual wet FGD conditions. The authors, however, limited themselves to the conclusion that under well oxidized conditions (forced oxidation by air and low sulphite concentration 1 mML1 or less) there is no re-emission of dissolved Hg2+ and they did not propose the oxidation of Hg0 by air as a potential process for the removal of Hg0 from the gaseous phase in wet FGD. One of the obstacles to the oxidation of Hg0 in the wet FGD scrubber is believed to be the poor or non-solubility of Hg0 in water [12,19]. However, studies of the solubility of elemental mercury in water [9,20,21] show that enough Hg0 is dissolved to allow chemical reactions to proceed. Recently, a case study implemented in a lignite fired boiler in Slovenia showed a significant removal of Hg0 in the wet FGD, where 76.2% of Hg0 present in the flue gas was retained in the solution along with 99.0% of Hg2+ and 98.1% of Hgp, respectively [22]. Based on this work an experimental laboratory project was initiated to study the influence of different parameters on oxidation Hg0 to Hg2+ by air in the wet FGD process. The results of these experiments are the subject of the present publication.

2. Experimental 2.1. Laboratory set-up Experimental laboratory apparatus was set-up for testing the oxidation of Hg0 in water solution by air, see Fig. 1. The installa-

3169

tion simulated the water column of the industrial FGD absorber tank in which the air for SO2 oxidation is introduced through dispersing perforations at a depth of 6 to 12 m of solution. The apparatus was composed from the following parts: (i) the thermostatic bubbler column with a diameter of 40 mm and 1.5 m high connected in a closed loop through a peristaltic pump (PP1) with (ii) an agitated reactor with a volume of 500 mL (AR1) where chemicals were added and pH, redox potential and temperature were measured and controlled, and (iii), a system for the preparation of a gaseous mixture spiked with Hg0 vapour. To perform the test, the column was filled with 1.8 L of solution and its recirculation over AR1 and PP1 started. The solution in the column was 1.0 m deep (no gas-hold-up included). The flow of a gaseous mixture of nitrogen and air was regulated by the valves V1, V2 and measured by rotameters R1 and R2, respectively. To this mixture a small stream of air saturated with Hg0 in the thermostatted agitated reactor AR2, containing 5 mL of liquid mercury, was dosed by means of the peristaltic pump PP2. The dilution ratio of the mercury saturated air stream and the N2/air mixture was from 300 to 1000. The flow rate of PP2 was regularly checked. The specific gas load of the solution in the bubbler column was kept at about 50 Lh1L1by means of the valves V4 and V8 and measured with the rotameter R4. By changing the ratio of air and nitrogen it was possible to vary the oxygen transfer rate to the liquid phase, while maintaining the hydrodynamic parameters, like the gas to liquid ratio and turbulence at constant levels. The gas phase was introduced at the bottom of the bubbler glass column through a glass frit. The gaseous phase left the bubbler column at the top saturated with moisture and was diluted by the stream of air regulated by the rotameter R3 and the valves V3, V5 and V6 in the ratio of about 1–5 prior to analysis in order to avoid condensation of moisture in the Lumex. The excess of inlet gas mixture (about 150 Lh1) was purged or led through the valve V4 to the Lumex after being diluted with air through rotameter R3 and valves V3 and V5. All tubes in contact with mercury vapour were made from Teflon, the only exception being the tubes in the peristaltic pumps which were made from silicon rubber, and the glass rotameters. To simulate the absorption of SO2 in the FGD, an aqueous solution of SO2 was constantly added to the AR1 and the pH kept constant by the automatic addition of a solution of Na2CO3 by another peristaltic pump (not shown on Fig. 1). Other chemicals were also added to the AR1 in a continuous or discontinuous way by a peristaltic pumps, burettes or pipettes (not shown on Fig. 1). The temperature, pH (glass probe, Metrohm) and redox potential (Pt/ saturated calomel reference electrode SENTEK) were measured with a Metrohm 826 pH/redox meter. All redox potential measurements were made with the Pt–saturated calomel electrode pair. The solution in the bubbler column used was either the real solution from the industrial FGD plant in some preliminary tests or, later, a synthetic solution with various compositions. The temperature was held by the thermostat at room level (about 21 °C) with fluctuations ±2 °C. The Hg0 content of the inlet and outlet gaseous mixture was measured continuously with a Lumex Zeeman Mercury Spectrometer, model RA-915+. Occasionally the Lumex measurements were verified by the absorption of elemental mercury vapour in liquid traps containing 50 mL of 1% KMnO4 in acid solution (0.5% H2SO4). Mercury in the liquid phase was then determined by a standard procedure based on reduction of mercury and its measurement by cold vapour atomic absorption spectrometry as described later [23]. Liquid samples were withdrawn through the valve V7, through which the system was also drained after each test and washed several times with HCl and MilliQ water in order to avoid contamination being carried over from test to test.

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To LUMEX or KMnO4 TRAPS

V6 V5

R4

BUBBLER COLUMN

CHEMICALS

PP1

pH E

R3

V4

PP2 R2

AIR IN

AR2

Hg(l)

R2

AR1

V1 V7

V8

V2 N2

AIR

DILUTION AIR

V3

LIQUID SAMPLES Fig. 1. Schematic view of experimental laboratory apparatus.

2.2. Analytical methods for mercury determination 2.2.1. Determination of elemental mercury in the gaseous phase This was measured by the Lumex Zeeman RA-915+Mercury Spectrometer. Simultaneous measurements of Hg0 were made at the outlet of the chamber using an RA-915+ Lumex Zeeman portable mercury spectrometer [24]. This system utilized atomic absorption in a column of chamber air which was constantly replenished by the pump maintaining an air flow of 12 L min1 through the system. By using Zeeman modulation techniques the system was particularly sensitive and real time measurements could be performed rather than relying on the customary off-line gold amalgamation technique for pre-concentration before atomic absorption spectrometric measurements. Spectra were collected and recorded by a computer as 10 s averages. The baseline correction time (period of time during which the level corresponding to a zero mercury vapour concentration in the analytical cell) was set to be 20 s and was measured every 5 min. The calibration and function of the instrument was checked prior to every measurement by measuring a known Hg vapour concentration in the test cell installed in the instrument. The relative deviation of the measured value of the Hg vapour concentration in the test cell from the

known value was within 5%. The calculated detection limits of the RA-915+ Zeeman Mercury Analyzer ranged between 0.5 and 1.0 ng m3. During the experiment, Hg concentrations were also repeatedly measured at the inlet and the outlet of the purging column, in order to check the efficiency of Hg removal from the incoming air. Occasionally the Lumex measurements of gaseous Hg were verified by selective trapping of mercury in 50 mL of 1% KMnO4 in acid solution (0.5% H2SO4). Mercury in the liquid phase was then determined by the procedure based on reduction of mercury and its measurement by cold vapour atomic absorption spectrometry [25]. 2.2.2. Dissolved elemental mercury in solution An aliquot of 100 mL of aqueous solution was transferred to a glass bubbler immediately after sampling. The carrier gas flow was 50–60 mL min1 and entered the glass bubbler through a glass frit sealed at the bottom of the bubbler. This guaranteed the most efficient purging of water samples, and allowed for quantitative removal of volatile Hg species in a short period of time (e.g. 8–10 min). Volatile mercury species were purged for 10 min and collected on a sampling gold-coated silica trap kept at room temperature with Hg-free argon (or nitrogen). In the measurement

A. Stergaršek et al. / Fuel 89 (2010) 3167–3177

step, the sampling gold trap was immediately transferred to a double amalgamation CV AFS analyzer system. The sampling gold trap was heated for 1 min (ramp heating to a maximum of 500 °C) and the mercury collected on this trap was released and purged with a flow of argon onto a permanent analytical gold trap kept at room temperature. After heating the analytical gold column, the trapped Hg was thermally desorbed (ramp heating to a maximum of 500 °C) into an argon stream that carried the released Hg vapour into the cell of a cold vapor atomic fluorescence spectrometer (Tekran 2500) for detection [23]. 2.2.3. Divalent mercury in aqueous solution Immediately after sampling, an aliquot of 50 mL of aqueous solution was transferred to a 300 mL reduction vessel filled with SnCl2 solution to convert ‘‘reducible” or ‘‘free” inorganic Hg2+ to Hg0. Reduced Hg0 was swept from the solution by aeration with N2 and concentrated on a gold trap. Mercury was then released from the gold trap by heating and measured on an LDC Milton Roy instrument by cold vapour atomic absorption spectrophotometry (CV AAS) [23]. Hg(II) in the solution was obtained by subtracting the value of dissolved elemental mercury obtained by the method described above. 2.2.4. Anions and cations in the solution Tetrathionate: the solution was pre-concentrated by cryogenic method and Raman spectra measured with ‘‘Renishaw Ramascope 2 1000”, exciting laser line He–Ne 633 nm. SO2 and S4 O2 4 , S2 O6 6 were determined with standard additions of anions after the procedure published elsewhere [26]. Gravimetric determination of sulphate (SO2 4 ) as barium sulphate: classical method was used [27]. Volumetric determination of sulphite (SO2 3 ): this determination was done by classical iodate/iodine method for the samples containing sulphite [27]. Complexometric determination of calcium or magnesium: magnesium and calcium were determined by direct complexometric titration with EDTA. Magnesium was determined in alkaline media (ammonia buffer of pH 10) using the indicator Methylthymol Blue and calcium in sodium hydroxide media using indicator Calcein [28]. Selenium and iodine was determined by X-ray fluorescence method [29]. Fluoride was determined with fluoride Ion Selective Electrode [30]. Bromide was determined by classical volumetric determination [27]. Chloride, nitrate and nitrite were determined by spectrophotometic determination [31]. Other cations like Na, K, Mn, etc. were determined by standard routine procedures. 2.3. Composition of the technical solution from the industrial FGD installation The composition of the FGD scrubber solution collected from the lignite fired power plant running in steady state operation, and filtered in a filter funnel by gravity, is presented in Table 1 [22]. Based on pH and E values of the solution the ionic species of the elements determined by the analysis were anticipated; the anticipated species have been used for the calculation of the balance of cations and anions. 2.4. Materials used The following chemicals were used (chemicals for analytical procedures are not included): MilliQ water, MgSO47H2O p.a.,

3171

Merck, NaCl, p.a., AppliChem, CaSO42H2O p.a., Kemika, Na2CO3, p.a., Merck, SO2(l), techn. grade, Messer, N2, pure, Messer, NaOH, p.a., Carlo Erba, K2S4O6, purrissimum for microbiology, Merck, Ca(OH)2, p.a., Merck – Alkaloid, MnSO4H2O, p.a., Riedel de Haen, FeSO47H20, p.a., Merck, NiSO47H20, p.a., Riedel de Haen, CrCl36H2O, purissimum, Merck, Na2MoO42H20, p.a., Merck, CoSO47H2O, p.a., Riedel de Haen, NH4VO3, p.a., Kemika, KMnO4, p.a., Merck, Hg(NO3)2, p.a., Merck. 3. Results and discussion 3.1. Work plan The basic parameter measured was the percentage of Hg0 removed from the gaseous mixture entering and passing the bubbler column and the influence of different parameters on this removal efficiency. The parameters included the constituents of and additives to the solution, the composition of the gaseous mixture and the operating conditions of the bubbler column, i.e. pH, redox potential, specific oxygen supply and temperature. A complete parametrical study including all components found in the FGD solution would require extensive and laborious testing, therefore priority parameters were chosen. The focus was on following the most promising groups of possibly influential compounds: transition metals likely to act as catalysts for the oxidation, anions like halogenides and oxosulphur compounds (e.g. thionates) that form strong complexes with mercury. The following groups of tests were performed:  Preliminary testing with the aim of repeating the high Hg0 removal efficiency measured in the industrial FGD installation, using the solution from the industrial FGD and the ability of Hg0 oxidation by air in the synthetic solution composed of major anions and cations, i.e. magnesium sulphate and sodium chloride under continuous SO2 addition as a water solution of SO2 (about 0.1 M)  Parametric testing of the effect of additions of Fe2+, Mn2+, Cu2+ and K2S4O6 with or without SO2 and Cl addition.  Effect of additional potential catalysts like Ni2+, V(V), Mo(VI), Co2+, as well as Fe2+ and Mn2+ on Hg0 removal efficiency  Influence of SO2 concentration and the influence of oxygen 3 (air) supply.  Reduction of dissolved Hg2+ by reducing agents like SO2 3 , tetrathionate, and transition metals in lower oxidation states. The results of each group of tests greatly influenced the programme of subsequent testing. In order to optimize the number of tests, some parameters were kept nearly constant or in a narrow range, e.g. pH (kept at a value around 5.6 which is very typical of a wet limestone FGD process), temperature, specific load of the bubbler column with gaseous mixture, concentrations of sulphate and chloride in the solution, inlet concentrations of Hg0 in the gaseous mixture. In each group of tests some of the previous tests were repeated to check the reproducibility. 3.2. Preliminary testing In preliminary testing Hg0 spiked air was bubbled through the column filled with water, with the solution and the slurry from the industrial FGD scrubber. In subsequent tests synthetic starting solutions were used and different chemical compounds added as non-continuous additions, except in the case of SO2. The temperature of the solution was kept at 20.5 °C (±2 °C). Hg0 concentrations were measured in the inlet gas until constant values were reached and then the measurement of the outlet gas started. Inlet concen-

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Table 1 Composition of industrial FGD scrubber solution. Analytical method

Cations (mmol L1 val1)

Anticipated species 3+

Anions (mmol L1 val1)

Al Ba Br Ca Cd Cl Co Cu Cr F Fe Hg I K Mg Mn Na Ni NO3 NO2 Pb S2O6

3.4 14.6 0.09 500 0.162 650 0.018 0.091 0.05 130 0.05 0.004 32.1 256 2940 24.7 595 1.51 370 1 0.08 1000

FAAS FAAS CM COMPLEXM FAAS CM ETAAS FAAS FAAS ISE FAAS CVAAS XRF FAAS COMPLEXM FAAS FAAS FAAS CM CM FAAS RAMAN

Al Ba2+ BrCa2+ Cd2+ Cl Co2+ Cu2+ Cr3+ F Fe2+ Hg2+ I K+ Mg2+ Mn2+ Na+ Ni2+ N03 NO2 Pb2+ S2 O2 6

12.5

S4O6

1000

RAMAN

S4 O2 6

8.92

SO4

12,500

GRAVI

260.1

SO3 Se

1 3.24

CM XRF

SO2 4 HSO3

Sr V

0.15 0.361

CM ETAAS

Zn Zr

1.61 0.04

FAAS FAAS

pH SUM

5.3

POTENTM

0.0011 25.0 0.0029 18.31 0.0006 0.0029 0.0029 6.84 0.0018 0.0000 0.2530 6.55 241.9 0.8992 25.9 0.0514 5.97 0.0217 0.0008

0.0125 0.0820

SeO2 3 Sr2+

0.0034

V10 O6 28 Zn2+

0.0493

0.0037

ZrO2 3 H+

0.0009 0.0000 301

Colorimetry Flame atomic absorption spectroscopy Gravimetry Potentiometry Raman spectroscopy mmol L1 divided by ion valence

Electrothermal AA spectroscopy Ion selective electrode X-ray fluorescence Cold vapour AAS Compleximetry

2000 15 mgL-1 Mn2+

400

1800 1600 1400

350 300

1200

250

1000

200

800 400 200 0

0.0

30 mgL-1 Mn2+

150

600

START SO2

trations were checked periodically and at the end of the test. Table 2 shows typical preliminary results as average values and the chemical conditions of the tests, while the time dependence of Hg0 removal and redox potential for two tests is graphically presented in Figs. 2 and 3. The data in Table 2 show that Hg0 can be removed to a high degree, about 80–90%, if the FGD solution is used or when SO2 is 3 added constantly to a synthetic solution composed of magnesium and calcium sulphate and sodium chloride. Control analyses of the solutions where high mercury removal efficiency was observed, showed that only a minor part of the total mercury, about 5–10%, was dissolved Hg0. The data in Figs. 2 and 3 show that Mn2+ and Fe2+ have very strong effects on Hg0 removal, which can be close to 100% immediately after their addition. However, this effect disappears after some time when the precipitation of yellow-reddish Fe(OH)3 or

313

ETAAS ISE XRF CVAAS COMPLEXM

Hg0 in outlet air. ngm-3

CM FAAS GRAVI POTENTM RAMAN mmol L1 val1: concentration in

0.3778 0.2126

E. mV

Concent (mg L1)

Element determined

0.5

Hg

100

E

50 0

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

time. h Fig. 2. Influence of Mn

2+

addition on Hg0 removal.

Table 2 Operating conditions and the results of preliminary testing. System

1 1 L ) SO2 3 (g h

1) SO2 4 (mg L

Cl (mg L1)

O2 (g h1 L1)

Hg0 inlet (ng m3)

Hg0 removal (%)

pH

E (mV)

Water FGD solution Synthetic solution Synthetic solution Synthetic solution

0 0 0.14 0.23 0.22

0 12,500 1000 10,000 10,000

0 625 88 880 0

8.6 7.8 11.0 11.5 8.8

5890 5151 2680 4950 5489

33 89 78 Fig. 2 Fig. 3

5.3 5.9 5.5 5.6

241 223 Fig. 2 Fig. 3

Notes Tap water See Table 1 for analysis Mn2+ added Fe2+ added

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A. Stergaršek et al. / Fuel 89 (2010) 3167–3177 400

6000 4.7 mgL-1 Fe2+

4.7 mgL-1 Fe2+

350 300

4000

250 200

3000

E. mV

Hg0 in outlet air. ngm -3

5000

150

2000 1000 0 3.0

Hg

100

E

50 0

4.0

5.0

6.0

7.0

8.0

some cases single, non-continuous additions of soluble compounds were used, including Fe2+, Mn2+, Hg2+ and potassium tetrathionate, K2S4O6. The aim of these tests was to screen the most influential compounds in respect to Hg0 removal by air oxidation. A series of parametric tests were performed in which the oxidation and removal efficiency of Hg0 from spiked air in the bubbler column was measured. In Table 3 basic experimental conditions are given, i.e. the composition of the synthetic solution and the rate of addition of reactants and catalysts as averages and the range of values. The results are presented in Figs. 4 and 5. The results in Fig. 4 again show that the presence of SO2 is 3 essential for the oxidation and removal of Hg0 from air (series 3)

time. h Fig. 3. Influence of Fe

2+

addition on Hg0 removal.

100 90 80

Hg 0 removal, %

in some cases, when the redox potential is high due to very good aeration, of dark brown MnO2 is observed. Evidently both catalysts are removed by precipitation reactions, shown schematically (data for DG0 from [10]):

DG0 ¼ 208:0 kJ=mol

Fe3þ ðaqÞ þ 3OH ðaqÞ ! FeðOHÞ3 ðaqÞ

60

2

50

3

40

4

30

5

20

6

10

ð13Þ 2+

70

0

0

3+

where, Fe is oxidized to very insoluble Fe at the pH normally encountered in the FGD process, i.e. 5–6, and

DG0 ¼ þ16:8 kJ=mol

400

500

600

ð14Þ

2+

2+

2+

90 80

0

on Hg removal

0

Cl , K2S4O6, Fe , Mn and Cu

100

Hg removal, %

3.3. Effect of efficiency



300

E, mV

known as autocatalytic oxidation of Mn2+ to insoluble MnO2 [9]. The reaction is nearly reversible (as shown by Gibbs free enthalpy close to 0), depending at a given pH value mainly on the redox potential. In the FGD solution the concentration of Mn2+ is quite high (25 mg L1), as shown in Table 1. SO2 3 ,

200

 2+ 2+ Fig. 4. Influence of SO2 and Cu2+ on Hg0 removal efficiency. 3 , Cl , K2S4O6, Fe , Mn Chemical conditions of single tests are given in Table 3.

Mn2þ ðaqÞ þ 0:5O2 ðgÞ þ H2 OðlÞ $ cMnO2 ðsÞ þ 2Hþ ðaqÞ

100

Based on the preliminary results further testing was performed using continuous addition of catalysts in order to counteract their loss by precipitation reactions. The main reactants and catalysts were added in continuous mode to a synthetic solution composed of high purity MilliQ water and the dissolved salts MgSO4, NaCl, CaSO4. Reactants were SO2 3 (water solution of SO2), oxygen (air) in air–nitrogen mixture and the catalysts were Fe2+ and Mn2+. In

70 60

1

50

7

40

8

30

9

20

10

10 0

0

100

200

300

400

500

600

E, mV  2+ 2+ Fig. 5. Influence of SO2 and Cu2+ on Hg0 removal efficiency. 3 , Cl , K2S4O6, Fe , Mn Chemical conditions of single tests are given in Table 3.

Table 3 Experimental conditions of parametric testing. Series in Figs. 4 and 5

1 SO2 4 (mg L ) (range)

Cl (mg L1) (range)

Fe2+ (mg h1 L1) (range)

Mn2+ (mg h1L1) (range)

K2S4O6 (mg L1) (range)

SO2 (g h1 L1) (range)

O2 (g h1 L1) (range)

1 2 3 4

0 0 0 0

0 0 0 0

0 0.32 (0.1–0.7) 0 0.41 (0.14–0.47)

4.3 5.1 (3.0–6.9) 3.6 4.0 (1.6–6.7)

3.0 (1.0–10.5)

0

0

0.33 (0.23–0.53)

5.3 (3.4–6.6)

6 7 8

10,000 10,000 10,000

0 5.20 (3.6–6.9) 1.64 (1.4–1.8)

5.34 (1.3–2.6) 6.20 (3.6–6.9) 0

0 0 500

0.28 (0.24–0.33) 0.48 (0.45–0.54) 0.37 (0.34–0.39)

4.26 (3.2–3.4) 4.3 (3.2–6.5) 3.5

9

8889 (5000– 10,000) 10,000

0 0 1000 4600 (1000– 5000) 5284 (1000– 9710) 0 5000 3000 (1000– 5000) 5000

0 1.3 (0.9–2.1) 0 0

5

10,000 10,000 10,000 8000 (5000– 10,000) 10,000

5.6 (2.3–9.4)

5.60 (2.3–9.4)

256 (83–378)

0.45 (0.42–0.51)

6.6 (6.3–6.8)

1000

6.25

5.84 (5.6–6.0) ADDED Cu2+:2.2 (2.2–2.3)

528

0.22 (0.21–0.24)

6.17

10

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A. Stergaršek et al. / Fuel 89 (2010) 3167–3177

and that Fe2+ is an effective catalyst (series 2), especially if chloride is present (series 5). Mn2+ seems to be less effective than Fe2+ if applied alone (series 6 vs. 2). Chloride enhances the removal efficiency, but chloride without SO2 3 is not effective (series 3 and 4). The results in Fig. 5 show that the presence of chloride, Mn2+ and Fe2+ makes the system very effective (series 7), while the addition of tetrathionate decreases the removal of Hg0 efficiency drastically, even in the presence of Fe2+ (series 8). Effective removal in the presence of thionate can be achieved by the application of a combination of more catalysts (series 9 and 10), Mn2+ and Fe2+ or Mn2+, Fe2+ and Cu 2+, respectively. An essential condition for good removal of Hg0 is a relatively high redox potential in the solution, Figs. 4 and 5. Series 1 in Fig. 5 represents the basic case where only 2 SO2 4 is present and no SO3 , chloride or catalysts are added, showing only moderate potential for the oxidation of Hg0 by oxygen alone. 3.4. Comparison of the catalytic effect of Fe2+, Mn2+, Ni2+, V(V), Mo(VI), Co2+ on Hg0 removal efficiency

dy operation with respect to pH (kept at a value of 5.6 ± 0.3), redox potential and outlet concentration of Hg0, the system was changed successively by potential catalyst addition, then potassium tetrathionate addition and finally by the addition of MnSO4 and in some cases FeSO4 solution. After each addition the system was run to steady state operation. The general operating conditions and the removal efficiency of Hg0 in % for different combinations of potential catalysts added and with or without tetrathionate addition, are given in Table 4. The results confirm that addition of tetrathionate always very significantly reduces the removal of Hg0. Mn, Ni, Co, Mo, V are not effective catalysts for the oxidation of Hg0 if tetrathionate is present. The Hg0 removal rate achieved is only from 2 to 40%. In the presence of tetrathionate only continuous addition of Fe 2+ to the system in combination with Mn2+ (added continuously or in separate increments) is effective, resulting in more than 90% Hg0 removal, provided that the redox potential is kept high with an appropriate oxygen (air) supply. 3.5. Influence of SO2 3 and oxygen supply

The effect of the addition of different cations, present beside Fe2+ and Mn2+ (the latter two proven to be effective catalysts as shown in the previous section), in the process solution in the FGD plant on the efficiency of Hg0 removal was studied in the next series of tests. Each test started with a synthetic solution of MgSO4, NaCl and CaSO4 in MilliQ water, and then air spiked with Hg0 was introduced until the outlet concentration of Hg0 was close to the inlet concentration. A solution of SO2 in MilliQ water was then added continuously, and the pH was kept constant by automatic addition of a diluted solution of Na2CO3. After certain time of stea-

In next series of tests the influence of two process parameters important for the desulphurization process, i.e. sulphite (SO2 3 ) concentration and oxygen (air) supply to the solution, on the removal rate of Hg0 was studied; basic parameters that were changed included the presence of Mn2+, Fe2+ and tetrathionate. In Table 5 operating conditions are summarised and the results are shown in Fig. 6. The data presented in Table 5 and on Fig. 6 confirm that the 0 presence of SO2 or SO2 3 is essential for good oxidation of Hg ; even

Table 4 Operating conditions and the results of screening tests for determination of the effect of potential catalysts and tetrathionate addition on Hg0 removal. Chemical added Unit Average Series 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Cl TT SO2 4 Conc. Conc. Conc. mg L1 7845 4251 449 Combination of conditions + + + + +  + +  + + + + + + + +  + + + + + + + +  + + + + + + + +  + + + + + + + + +

O2

SO2

Fe2+

Mn2+

Co (VI)

Mo (VI)

V (V)

Hg0

Add. g h1 L1 6.8

Add. g h1 L1 0.52

Add. mg h1 L1 2.36

Conc. mg L1 20.7

Add. Add. mg h1 L1 1.3 0.9

Ni2+

Add.

Add.

1,1

1.9

+ + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + +

+              +

+ +   +   +   +   + +

           + + + +

        + + +    

     + + +       

Inlet ng m3 12774 Hg0 removed% 90 95 96 19 24 79 34 40 92 3 2 99 28 32 95

   + +          

Legend: TT – potassium tetrathionate, K2S4O6; Conc. – concentration of chemical in column, Add. – continuous addition of O2, SO2, Fe2+ and other chemicals; Inlet – inlet concentration of Hg0 in the bubbler air; ‘‘+” – presence of a specific reagent; ‘‘‘‘ – absence of a specific reagent.

Table 5 2+ Operating conditions of tests to study of the influence of SO2 and Fe2+ on Hg0 removal. The Hg0 concentration in the inlet gas was 14704 ± 1488 ng m3. pH 3 , tetrathionate, Mn was kept constant at 5.6. Series on Fig. 6

1 SO2 4 (mg L )

Cl (mg L1)

1 SO2 3 (mg L )

K2S4O6 (mg L1)

Mn2+ (mg L1)

Fe2+ (mg h1 L1)

O2 (g h1 L1)

1 2 3 4 5 6

10876 8278 6705 6318 3103 4802

5438 4139 3352 3159 1552 2401

0 76.5 81.7 633.4 42.4 2158

0 0 0 969 774 736

0 0 6.6 19.4 9.9 14.7

0 0 0 0 0.68 1.65

4.3 4.7 3.5 2.3 6.7 6.7

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A. Stergaršek et al. / Fuel 89 (2010) 3167–3177 100

100 1 2 3 4 5 6

Hg 0 removal, %

80 70 60

y = 0,539x - 5,7763 2

R = 0,973

80

Hg 0 removal, %

90

50 40 30 20 10 0 0

60

40

20 50

100

150

200

250

300

350

400

E, mV

0 0

50

100

3.6. Reduction of dissolved Hg2+

100

200

90

180

80

160

70

140

60

120

50

100

40

80

30

60

E, mV

Hg 0 removal, %

The low solubility of Hg0 and in addition the reduction of predissolved Hg2+ in the solution of the absorption slurry of a wet

40

20 10

Hg removal,%

20

E, mV

200

250

2000

2500

Fig. 8. Hg0 removal vs. redox potential.

100 90 80

Hg 0 removal, %

if the redox potential is high (series 1), without SO32- the oxidation efficiency is poor. Further, the presence of tetrathionate drastically decreases the ability of the system to oxidize Hg0; the presence of Mn2+ catalyst alone cannot improve the oxidation (series 4) due to its inability to increase the redox potential high enough. Again, the presence of two catalysts together, Mn2+ and Fe2+, improves the effect greatly (series 5). However, even in the presence of both catalysts and a high redox potential, over 200 mV, the removal of Hg0 is not effective if the concentration of sulphite is relatively high (series 6). In some of the tests the influence of the specific oxygen (air) supply was studied. In this series of experiments no tetrathionate or Mn2+ or Fe2+ were added. Results are shown in Fig. 7. The results shown in Fig. 7 suggest that for efficient Hg0 removal by oxidation enough oxygen has to be supplied to the solution, i.e. at least 2 g h1 L1 of O2 for 80% Hg0 removal and 3 g h1 L1 of O2 for >90% of Hg0 removal, respectively. It is also evident that the redox potential and Hg0 removal rate are linearly dependant (if other experimental conditions are similar), as shown in Fig. 8. The results shown on Figs. 8 and 9 suggest that Hg0 can be effectively removed if we can provide efficient oxidation of sulphite, 2 SO2 3 to sulphate, SO4 , which can be done by the use of the synergistic effects of the catalysts Mn2+ and Fe2+ and good aeration of the solution. The measure of effective oxidation of sulphite and Hg0 is an appropriate level of the redox potential.

150

E, mV

Fig. 6. Influence of sulphite, tetrathionate, Mn2+ and Fe2+ on Hg0 removal.

70 60 50 40 30 20 10 0

0

500

1000

1500 -1

SO32- concentration, mgL

Fig. 9. Hg0 removal vs. SO2 3 (aq) concentration. The experimental conditions were as follows: Fe2+ addition 0–1.65 mg h1L1, Mn2+ concentration 0–25 mg L1, O2 1 1 supply 2.1–6.9 g h L , potassium tetrathionate concentration 0–1000 mg L1, 1 SO2 and Cl concentration 1500–5000 mg L1. 4 concentration 4250–10,000 mg L

FGD by SO2 was claimed to be the reason for the low removal efficiency of Hg0 or even its re-emission from the absorbing slurry, as mentioned before [12,19]. As we discussed in the Introduction and later, especially in the chapter 3.5, it is very important to keep SO2 3 concentration low enough to assure effective Hg0 oxidation by air as also demonstrated by Blyth and co-workers [15]. To demonstrate that under controlled oxidation conditions the prevailing reaction is the oxidation and not the reduction we performed a test in the bubbler column where Hg2+ was added to the solution, the gaseous phase (free of Hg0) was bubbled through the solution, SO2 was continuously added and successively portions of Fe2+ and later of tetrathionate and again Fe2+. The chemical parameters and the concentration of Hg0 produced by reduction were recorded in the outlet gaseous stream. Operating conditions are given in Table 6 and the time dependence of the most important parameters is graphically presented in Fig. 10. In Table 7 the meaning of symbols in Fig. 10 are detailed. When Hg2+ is added to the solution of sulphate and chloride, aerated with the air bubbled through the column, no reduction to Hg0 occurs. The same is true after the start of the addition of SO2 causing a drop of redox potential from about 260 to about

0

0 0

1

2

3

4

5

6

7

O 2, gh-1L-1 Fig. 7. The influence of oxygen supply to the solution on Hg0 removal. The Hg0 inlet concentration in the gaseous mixture was 14716 ± 1484 ng m3 and the SO2 supply 1 to the solution was 0.92 ± 0.23 g h1L1. SO2 4 concentration was 6180–9378 mg L and Cl concentration was 3090–4688 mg L1.

Table 6 Operating conditions of Hg2+ reduction test. Parameter

pH

Unit Average value

– 5.5

SO2 4 mg L1 10,000

Cl

O2 add.

SO2 add.

T sol.

mg L1 5000

g h1 L1 6.5

g h1 L1 0.55

°C 20.6

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A. Stergaršek et al. / Fuel 89 (2010) 3167–3177

Table 7 The meaning of symbols in Fig. 10. Symbol No. Time, min Additive Unit Addition % of Hg2+ reduced

1 Start 0  SO2 4 ; Cl g L1 10.0; 5.0 0

2 Hg2+ 14 Hg2+

3 SO2 23 SO2

4 Hg2+ 92 Hg2+

5 Fe-1 106 Fe2+

6 Fe-2 114 Fe2+

7 TT 119 K2S4O6

8 Fe-3 125 Fe2+

lg L1

g h1 L1 0.55 0

lg L1

mg L1 0.33 0.39

mg L1 0.33 0.22

mg L1 278 0

mg L1 0.22 0.51

1.0 0

1.0 0

12000

300

10000

250 1 START

200

E, mV

4 Hg2+

4000

150

3 SO2

6000

E, mV

8000

2 Hg2+

Hg0 concentration in outlet air, ngm-3

Legend to Table 7: ‘‘% of Hg reduced” – % of Hg2+ present in the solution that was reduced by Fe2+ addition to Hg0; ‘‘3 SO2” – time where SO2 addition started with the given rate.

100

2000

50

Hg

0

0

0 0

20

40

60

80

10 0

120

140

7 TT

8 Fe 2+ - 3

6 Fe 2+ - 2

5 Fe 2+ - 1

Time, min

Fig. 10. Course of the test of reduction of Hg2.

130 mV. The addition of Fe2+ in a molar ratio 334:1 with respect to Hg2+ present in the solution causes rapid reduction to Hg0 which stops in a few minutes; at the same time the redox potential increases significantly. During the emission of Hg0 from the solution caused by two additions of Fe2+, 0.22 and 0.39% of Hg2+ present in the solution were reduced and emitted. We attribute this course of the reduction to the instantaneous excess of Fe2+ which quickly disappears due to the oxidation of Fe2+ to Fe3+ by air (increase of redox potential) and the reduction of Hg2+ ceases. After the addition of potassium tetrathionate we did not observe any reduction of Hg2+ to Hg0. When Fe2+ is added to the solution of Hg2+ and tetrathionate, again reduction to Hg0 occurs (0.51% of Hg2+ present), but the decrease of the reduction rate is slower than in the absence of tetrathionate; this behaviour is attributed to the competitive reaction of the reduction of Fe3+ by tetrathionate, reported in the literature [10,11,32,33] and described in the following possible spontaneous chemical reaction [32]:

3 1 3þ S4 O2 þ 2 O2 þ 4 H 2 O 6 þ 3Fe 4 2 2þ þ þ 3Fe þ 9H DG0 ¼ 1231:9 kJ=mol ! 4SO2 4

ð15Þ

4. Conclusions The experimental results show that Hg0 can be dissolved, along with readily soluble Hg2+ species, in a synthetic solution similar in composition to the absorbing solution in a wet FGD installation and that Hg0 can be effectively oxidized by air sparged into the suspension for the oxidation of sulphite to sulphate at an appropriate rate. Transition metals like Mn and Fe act as catalysts, especially when combined, as is known from investigations of the oxidation of sulphite with oxygen in the atmosphere [34] and also in other

fields, e.g. in hydrometallurgy [35]. While Mn2+ is sufficiently soluble (order of magnitude of 10 g L1) at the pH range of FGD operation (being around 5.6), Fe2+ and Fe3+ are precipitated under these conditions. To sustain the oxidation of Hg0, the supply of Fe2+ has to be constantly replenished in the solution. Oxygen itself is not an effective oxidant and a certain amount of SO2 3 , of the order of magnitude of 10–100 mg L1, must be present for good oxidation of Hg0, a fact which is known (for acid media) from the literature [35]. Oxidation of SO2 with oxygen is a much investigated reaction and it is believed to be a chain reaction that can be initiated by metal ions having two valence states, like Fe2+/Fe3+, Mn2+/Mn3+, Co2+/ Co3+and Cu+/Cu2+ acting as electron transmitters and producing the sulphite radical, SO 3 . Further intermediate species include peroxomonosulphate, SO2 5 or hydroxo radicals, OH, which are strong oxidants [34]. Our experimental results, however, show that increased sulphite concentrations decrease Hg0 oxidation to very low levels, even if other components that enhance the oxidation of Hg0 to Hg2+ are present. Among anions chloride enhances the reaction. Some other constituents of FGD liquors, like tetrathionate, may significantly decrease the oxidation rate, presumably by the competitive reaction with Fe3+ which is reduced to Fe2+ form, indicated by the drop of redox potential in the presence of tetrathionate. Fe2+ can reduce Hg2+ to Hg0 [11]. An adequate supply of dispersed oxygen (air), i.e. >3 g h1 L1 O2, to the solution in our experimental set-up is essential for good oxidation and for a high redox potential that is proportional to the mercury removal efficiency. So, the degree of aeration required can easily be determined by measurement of the redox potential in the solution. Reduction of dissolved Hg2+ was observed neither with SO2, nor with tetrathionate, if the solution was aerated. It should be noted that realistic industrial solutions contain numerous anions that tend to form strong complexes with Hg2+, 2 2 2 e.g. Cl-, Br-, OH-, SO2 4 , SO3 , CO3 , S4 O6 , etc., probably playing an

A. Stergaršek et al. / Fuel 89 (2010) 3167–3177

important role in the relative inertness of Hg2+ against reducing agents, making the reaction product of the oxidation of Hg0, Hg2+ (and/or Hg22+), less available for chemical reaction. The fate of Hg2+ in solutions of the wet FGD will be an important field of further investigation. It seems that Hg0 oxidation with air in the wet FGD is viable if the following competitive reactions are controlled: 2 1. Oxidation of SO2 3 to SO4 by an adequate air supply to the solution and by catalytic support if inhibitors of the reaction are present. 2. The reduction reaction of Fe3+ to Fe2+ ions (later can directly reduce Hg2+ to Hg0) by sulphur compounds in lower oxidation states than 6+ (sulphite, tetrathionate) must be slowed down by the synergistic catalytic effect of Fe2+ and Mn2+ addition. 1 The SO2 by good aeration. 3 level must be kept below 100 mg L 3+ 3. Removal of a catalyst like Fe must be compensated for by continuous addition of the catalyst to the system.

The basic parameters that govern the oxidation of Hg0 in wet FGD chemistry by air were experimentally demonstrated, both those that enhance the reaction and those that inhibit it. Due to the great number of influential parameters, many of them have not been investigated in detail, e.g. the optimal quantity of catalysts, the influence of the temperature, the concentration of Hg0 in the inlet gas etc. This remains to be investigated further. Finally, the potential of the system described (patent applied for) for use in industrial FGD plants has to be verified by continuous pilot plant operation. Acknowledgment The work was supported by the Agency of Republic of the Slovenia for Research, Project L2-9023-0106-06. References [1] Pavlich JH, Sondreal EA, Mann MD, Olson ES, Galbreath KC, Laudal DL, et al. Status review of mercury control options for coal fired power plants. Fuel Process Technol 2003;82:89–165. [2] Sundseth K, Pacyna JM, Pacyna EG, Munthe J, Belhaj M, Astrom S. Economic benefits from decreased mercury emissions: projection for 2020. J Cleaner Prod 2009:1–9. [3] Pacyna EG, Pacyna JM, Sundseth K, Munthe J, Kindbom K, Wilson S, et al. Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and projections to 2020. Atmos Environ 2009. [4] Communication from the commission to the council and the European Parliament, community strategy concerning mercury. COM 2005;20 final. [5] Vosteen BB, Kanefke R, Koeser H. Bromine-enhanced mercury abatement from combustion flue gases – recent industrial application and laboratory research. VGB Power Technol 2006;86(3):70–5. [6] Hudson JL, et al. Flue gas desulphurisation. ACS Symposium Series 1982;188. [7] Brandon NP, Francis PA, Jeffrey J, Kelsall GH, Yin Q. Thermodynamics and electrochemical behaviour of Hg–S–Cl–H2O systems. J Electrochem Chem 2001;497:18–32. [8] Shock EL, Sassani DC, Willis M, Sverjensky DA. Inorganic species in geological fluids: correlations among standard molal thermodynamic properties of aqueous ions and hydroxide complexes. Geochim Cosmochim Acta 1997;61(5):907–50.

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