Article pubs.acs.org/est
Mercury Vapor Pressure of Flue Gas Desulfurization Scrubber Suspensions: Effects of pH Level, Gypsum, and Iron Jan Schuetze,* Daniel Kunth, Sven Weissbach, and Heinz Koeser* Chair of Environmental Technology, Otto-von-Guericke-University Magdeburg, c/o Centre of Engineering Sciences, Martin-Luther-University Halle-Wittenberg, 06099 Halle (Saale), Germany S Supporting Information *
ABSTRACT: Calcium-based scrubbers designed to absorb HCl and SO2 from flue gases can also remove oxidized mercury. Dissolved mercury halides may have an appreciable partial vapor pressure. Chemical reduction of the dissolved mercury may increase the Hg emission, thereby limiting the coremoval of mercury in the wet scrubbing process. In this paper we evaluate the effects of the pH level, different gypsum qualities, and iron in flue gas desulfurization (FGD) scrubber suspensions. The impact of these parameters on mercury vapor pressure was studied under controlled laboratory conditions in model scrubber suspensions. A major influence is exerted by pH values above 7, considerably amplifying the mercury concentration in the vapor phase above the FGD scrubber suspension. Gypsum also increases the mercury re-emission. Fe(III) decreases and Fe(II) increases the vapor pressure significantly. The consequences of the findings for a reliable coremoval of mercury in FGD scrubbers are discussed. It is shown that there is an increased risk of poor mercury capture in lime-based FGD scrubbers in comparison to limestone FGD scrubbers.
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INTRODUCTION The adsorbing slurry of flue gas desulfurization (FGD) scrubbers removes mainly acidic components, such as sulfur dioxide and hydrogen chloride, from flue gases by means of the alkaline sorbents lime and limestone. The sorbents are added to the aerated recycle tank/sump or the recirculation pumps of the absorber. Generally, the absorbing suspension is contacted in the form of fine droplets with the flue gas in spray scrubbers to remove the acidic gas components. The liquid of the scrubber suspension has a retention time of up to four weeks in the absorber system, depending on the chlorine and sulfur content of the fuel input. Gaseous oxidized mercury (Hgoxg), as a water-soluble minor combustion product, is also absorbed in the sprayer zone of FGD in the washing suspension. Mercury capture in FGD scrubbers is quoted with 30−85% of the total gaseous mercury (Hgtotalg).1−3 The capture of the oxidized mercury Hgoxg is nowadays regarded as an environmentally important cobenefit of wet FGD scrubbers.2,4−6 A removal of the Hgoxg species of up to 98% can be achieved in wet FGD. In comparison, the cocapture of mercury seems to be smaller in power plants with the spray dryer absorber and fabric filter technique, with a Hg removal of 90%.8 The pH level and the gypsum content are the two foremost operating parameters of a wet FGD scrubber. Consequently, their effect on the mercury vapor pressure of the scrubber suspension is of vital importance for the coremoval of mercury. The normal operating pH level range in FGD scrubbers is between 4 and 7.9 Lower pH values increase the dissolution rate of the limestone but are detrimental to the solubility of the © 2012 American Chemical Society
SO2. A pH value of 4 will be encountered in the prescrubber of two-stage scrubbers. A higher pH value of 6.5 is more likely in lime scrubbers. Absorbed acidic gases in the sprayer zone lower the pH level in the suspension by half a pH unit.9 Later, FGD effluent wastewater passes heavy metal precipitation, where lime addition causes pH values of 7−9 to precipitate and remove the dissolved heavy metals.10 Dissolved oxidized mercury (Hgoxaq) may have a fairly high vapor pressure in water as long as no complex ions, such as HgX3− and HgX42− (X = Cl−, Br−, I−), are formed.11,12 HgI2, in particular, is highly volatile. Sulfur(IV) compounds are major drivers for the chemical reduction of dissolved oxidized mercury to elemental mercury, resulting in the re-emission of mercury from the wet FGD absorber.13−15 If these two reemission mechanisms are not controlled, the coremoval of mercury in FGD scrubbers may be severely jeopardized. With the exception of halogenides12 and sulfite,11,16−18 not much information is available on the effect of other major FGD scrubber constituents on the coremoval of mercury. In the present paper, the results of a controlled laboratory investigation on the effects of a range of FGD liquor constituents and operating parameters on the mercury vapor pressure are presented. The effect of the pH value and gypsum content in the scrubber suspension are discussed. Inoue19 and Kim20 describe that iron(III) precipitation will remove mercury, but not much is known about the effect of the Received: Revised: Accepted: Published: 3008
October 17, 2011 February 6, 2012 February 11, 2012 February 12, 2012 dx.doi.org/10.1021/es203605h | Environ. Sci. Technol. 2012, 46, 3008−3013
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mercury vapor phase and the effect of iron in the divalent state under FGD scrubber conditions. With a better understanding of the influence of these constituents and parameters, such as the pH level, on the mercury vapor pressure and, therefore, the mercury removal from the gas phase by scrubber liquors, it should be possible to optimize FGD scrubbers concerning the coremoval of mercury.
Table 1. Composition of the Model Suspension (RSL) and Information on the FGD Gypsum
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EXPERIMENTAL PROCEDURE The experiments were conducted on a laboratory-scale bench reactor simulating the oxidation zone/sump with two model suspensions. Figure 1 gives a sketch of the apparatus used for measuring the vapor pressure of FGD suspensions dynamically.
substance
RSL1
RSL2
Cl− as CaCl2 [g/L] Br− as CaBr2 [mg/L] NO3− as NaNO3 [mg/L] CaCO3 [g/L] Hg(II) as HgCl2 [mg/L] technical gypsum [g/L] gypsum from power plant/coal fired
4.5 70
16 300
Hg [μg/kg]
A/lignite C/bituminous coal D/bituminous coal p.A./pure (synthetic)
100 610 530 3
500 3 10 150 humidity [wt %] 10.2 6 8 0.04
concentration of the mercury in the vapor phase in contact with the model suspension could be measured more readily. It took approximately 45 min to bring 300 mL of the FGD model suspension to the standard measuring temperature of 60 °C in an oven with air circulation. This temperature was chosen to represent an average between the operation temperatures of FGD scrubbers in bituminous coal (∼50 °C) and lignite (∼70 °C) fired power plants. The duration of a vapor pressure experiment varied between 2 and 6 h subject to the number of test variables and the time needed to achieve stable Hgg vapor concentrations. The suspension was not replenished during the experiment. However, the Hgaq concentration in the suspension generally did not decrease much, and only cases with a high Hgg concentration (>500 μg/m3) over hours resulted in a decrease of 5−10%. The Hgtotalg concentraton in the effluent carrier gas was measured continuously with the Durag/Verewa EPM/731 series Hg monitor after scrubbing with a NaBH4 solution (∼20 mL/h of reduction solution of 5 g/L NaBH4 and 6 g/L NaOH) and a HCl solution (∼20 mL/h of 1 M HCl). A second Hg monitor from Sick-Maihack of the MERCEM series with a 1 M NaCl solution prescrubbing to remove the oxidized mercury was used to measure the Hgelg concentration. Furthermore, there was an online pH probe (WTW GL538 monitor with the probe WTW Sentix41) immersed in the model FGD suspension. The pH level of the scrubber suspension was adjusted by adding H2SO4 or NaOH. The ORP of the model suspension was measured selectively with the sensor EMC from Sensortechnik Meinsberg. The Hgaq concentration in the model suspension was measured with the Lumex RA915 Hg monitor equipped with the liquid analytical kit Lumex RP91 operated with a SnCl2 reduction solution (25 g/L SnCl2 stabilized with 1.2 M HCl). The samples from the model suspension were centrifuged at 4000 rpm for 10 min before measurement of the dissolved Hgaq. As a main difference from technical FGD scrubber suspensions, the model suspensions include no reducing matter. Generally, the reducing milieu is situated in the sprayer zone of the FGD and can cause critical Hgelg re-emission.13−15 The reduction stability was evaluated by adding sodium disulfite to the model suspension and measuring the resulting effect on the mercury concentration in the gas phase. A molar ratio SO32−:Hg2+ of 100:1 (475 mg/L Na2S2O5) reduces Hgaq, which is not stable bound to Hgel. A marked concentration of Hgelg appears temporarily in the gas phase of the suspension until the sulfite is oxidized. Smaller SO32−:Hg2+ ratios such as
Figure 1. Schematic of the apparatus for measuring the Hg vapor pressure over FGD suspensions.
The test setup consists of three main parts: the gas generation, the reactor, and the analytical equipment. The synthetic gas is generated by mixing air and nitrogen using mass flow controllers (MFCs). This gas is passed into the reactor, which is contained in an oven with the temperature kept at 60 °C. The reactor consists of three impingers (nos. 1−3) and a heat exchanger, and a magnetic stirrer is used for impinger 3. The gas enters the reactor through the heat exchanger, where it is heated. It passes impingers 1 and 2, which are filled with deionized water, to be humidified. The scrubber suspensions to be investigated are contained in impinger 3. Here, the pH value, temperature, oxidation reduction potential (ORP), and dissolved mercury (Hgaq) concentration are measured. Elemental (Hgelg) and total (Hgtotalg) gas-phase mercury concentrations are measured using continuous mercury monitors in the gas downstream of the reactor. For the tests, 300 mL of an FGD model suspension (RSL, reaction standard liquid), with a composition listed in Table 1, was introduced into the scrubber vessel. The composition of the model suspensions could be kept constant over the course of the investigation. Technical FGD suspensions show chemical aging processes, which may considerably affect the mercury vapor pressure and its solubility, as recognized in the initial investigations (see Figure S1, Supporting Information). Most of the chemicals used in the course of the experiment were of p.a. laboratory quality with a 98% and higher specified purity. The concentration of the mercury in the model scrubber suspension of 10 mg/L was set more than 10-fold higher than usual in technical FGD suspensions. Consequently, the mercury vapor pressure of the model suspension was also higher. In this way, the effects of the constituents added to the 3009
dx.doi.org/10.1021/es203605h | Environ. Sci. Technol. 2012, 46, 3008−3013
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decreased slowly. The Hg was mainly emitted as elemental species. Detailed investigations followed to identify the limiting parameter. It is known that the chloride concentration is important for the vapor pressure exerted by Hgaq.12 In Figure 3,
1:1 led mostly to a temporary decrease in mercury gas concentration by complexing the Hgaq.18 The mercury concentration measured in the gas phase of the suspension cannot be treated as true equilibrium data in a physical chemical sense. Chosen conditions with contact time between the gas and suspension phases in the experimental setup were not sufficient to achieve the equilibrium (see Figure S2, Supporting Information, for mercury concentration−carrier gas flow rate dependency). The mass transfer area of 100−400 m2/(L h) (at a contact time of 50 μg/m3 existed in the vapor phase near the liquor surface of these vessels. Some experiments were conducted to avoid these emissions. It is suggested that the usual order of water treatment steps should be changed to avoid these pH-driven Hg re-emissions. Instead of lime followed by sulfidic precipitation, the sequence should be reversed. First, the Hgaq has to be stabilized by the formation of insoluble Hg−sulfur compounds, whereupon higher pH levels will have no effect on the Hg (see Figure S3, Supporting Information). Some power plants of the E.On Co. in Germany already use the changed order of sequence in their FGD wastewater treatment plants.
the vapor phase, divalent iron had only small effects under acidic pH level conditions. Fe(II) addition caused a marked decrease of the ORP EH by about 200−250 mV; see Table 3. This is in line with the Table 3. Effect of Iron(II) and Iron(III) on the Dissolved Hg(II) Concentration and the ORP of a Model FGD Suspension (RSL1, 150 mg/L Gypsum C, 10 mg/L Hg(II)aq, pH 3.6−6, 60 °C) 500 mg/L Fe(II) step (1) before adding Fe (2) after adding Fe
(3) after adding Fe and 475 mg/L Na2S2O5
ORP and Hgaq ratio EH [mV] Hgaq(2)/ Hgaq(1) EH [mV] Hgaq(3)/ Hgaq(1) EH [mV]
500 mg/L Fe(III)
pH 3.6
pH 6
pH 4
pH 6
735 0.93
331 0.01
434 0.97
478 1.02
470 0.83
158 0.00
866 0.95
478 1.01
538
155
728
485
reducing properties of this iron species. Fe(III) at pH 4 increased the redox potential by more than 400 mV. At the same time, the elemental Hgelg concentration of the FGD suspension was lowered; compare with Table 2. The change of the Hgaq in the suspension during the investigation is also listed in Table 3. After the addition of Fe(II) at a pH value of 6, nearly all the Hgaq was removed from the FGD liquor. This may be caused by the soluble Hg(II) being reduced to less soluble Hg(I) compounds and to a certain degree to elemental Hg (compare eqs 6 and 7). 2Hg(OH)2 + 2Fe(OH)2 → Hg 2(OH)2 + 2Fe(OH)3 (6)
Hg(OH)2 + 2Fe(OH)2 → Hg + 2Fe(OH)3
(7)
The Hg mass balance of this experiment showed that nearly 60% was found in the fine particle fraction and 25% emitted. The solubility of Hg(I) species (e.g., 3.45 mg/L Hg2Cl226 or 0.785 mg/L Hg2Br227 at 25 °C) and elemental mercury (0.0562 mg/L Hgel at 25 °C28) is approximately 10−4-fold lower than that of Hg(II) species. In another experiment, after a dosage of 50 mg/L Fe(II), the pH level was raised stepwise from 3 to 6 over a period of 2 h under continuous aeration of the FGD suspension. In this case, the vapor Hgelg concentration did not increase and the dissolved Hgaq concentration declined only marginally. Under these conditions, the Fe(II) was probably oxidized before it could develop its reductive effect at the higher pH level and lower redox potential. Adding Fe(III) did not directly affect the dissolved mercury at the pH levels tested; compare with Table 3. It can be assumed from these results that incoming iron (by fly ash or limestone) will be oxidized under normal aerated FGD conditions. Conditions, such as insufficient aeration, that may produce Fe in the bivalent oxidation state should be avoided. If Fe(II) exists, the results in Table 3 identify Fe(II) as one ingredient of the FGD liquor that may transfer the dissolved Hg into the particle-bound form. It is easier to separate dissolved than particle-bound mercury from FGD gypsum. Measurement of 40 suspension samples from 3 FGDs with forced air oxidation and 1 with natural oxidation showed that an average of 85% (±15% standard deviation) of the particle-bound Hg could be
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ASSOCIATED CONTENT
S Supporting Information *
Information on the stability of technical FGD scrubber suspensions, equilibrium in the test apparatus, prevention of Hg re-emission during FGD effluent wastewater treatment, and the precision of the vapor pressure measurement. This material is available free of charge via the Internet at http://pubs.acs. org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] and heinz.koeser@iw. uni-halle.de. Phone: ++49-3461-462702. Fax: ++49-3461462710. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support from the VGB Research Foundation/Essen and for helpful discussions with Frans van Dijen, Heinz Gutberlet, Bernhard Pieper, Harald Thorwarth, Bernhard Vosteen, and Hermann Winkler. 3012
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