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Impact of additives for enhanced sulfur dioxide removal on re-emissions of mercury in wet flue gas desulfurization
Applied Energy 114 (2014) 485–491
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Impact of additives for enhanced sulfur dioxide removal on re-emissions of mercury in wet flue gas desulfurization Barna Heidel a,⇑, Melanie Hilber b, Günter Scheffknecht a a b
Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Pfaffenwaldring 23, D-70569 Stuttgart, Germany EnBW Erneuerbare und Konventionelle Erzeugung AG, Schelmenwasenstraße 15, D-70567 Stuttgart, Germany
h i g h l i g h t s Mercury removal in wet FGD. Re-emission of mercury. Additives for enhanced SO2 removal in wet FGD. Reaction mechanisms of mercury re-emissions. Multi pollutant control by wet FGD.
a r t i c l e
i n f o
Article history: Received 17 July 2013 Received in revised form 25 September 2013 Accepted 28 September 2013
Keywords: Mercury FGD Organic acid Additive Re-emission Multi-pollutant control
a b s t r a c t The wet flue gas desulfurization process (FGD) in fossil fired power plants offers the advantage of simultaneously removing SO2 and other water soluble pollutants, such as certain oxidized mercury compounds (Hg2+). In order to maximize SO2 removal efficiency of installed FGD units, organic additives can be utilized. In the context of multi-pollutant control by wet FGD, the effect of formic and adipic acid on redox reactions of dissolved mercury compounds is investigated with a continuously operated lab-scale testrig. For sulfite (SO2 3 ) concentrations above a certain critical value, their potential as reducing agent leads to rapidly increasing formation and re-emission of elemental mercury (Hg0). Increasing chloride concentration and decreasing pH and slurry temperature have been identified as key factors for depressing Hg0 re-emissions. Both organic additives have a negative impact on Hg-retention and cause increased Hg0 reemissions in the wet FGD process, with formic acid being the significantly stronger reducing agent. Different pathways of Hg2+ reduction were identified by qualitative interpretation of the pH-dependence and by comparison of activation enthalpies and activation entropies. While the first mechanism proposed identifies SO2 3 as reducing agent and is therefore relevant for any FGD process, the second mechanism involves the formate anion, thus being exclusively relevant for FGDs utilizing formic acid as additive. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Sulfur (S) and Mercury (Hg) enter the combustion chamber as constituents of coal and secondary fuels [1]. The dominant gas phase compound of sulfur is sulfur dioxide (SO2). However, due to subsequent reactions with other flue gas constituents and in dependence of the configuration of the installed air pollution control devices (APCD), the formation of SO3 can be promoted, leading to cold-end corrosion phenomena. The most common technology for SOx-removal in Europe is the wet flue gas desulphurization (FGD) process with limestone, enabling SOx-removal rates of >95%. An overview about available SO2 control technologies and the corresponding abatement costs can be found in [2]. ⇑ Corresponding author. Tel.: +49 (0)711 685 68946; fax: +49 (0)711 685 63491. E-mail address: [email protected] (B. Heidel). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.09.059
The reactions of the trace element Hg in the flue gas and its interaction with other flue gas components are of complex nature. Due to the high temperatures in the combustion chamber, the thermodynamic equilibrium of Hg-reactions is shifted to its elemental form (Hg0). With decreasing temperature and affected by the flue gas composition, gaseous Hg0 undergoes oxidation to bivalent mercury (Hg2+), leading to the formation of HgCl2 in the presence of HCl, which is the dominant hydrogen halide in coal derived flue gas without additives. The heterogeneous oxidation of Hg0 is catalyzed by high-dust SCR DeNOx systems, resulting in higher shares of HgCl2. A certain fraction of mercury is bound to particulate matter (Hgp), due to sorption on the surface of ash particles. While elemental mercury is considered not to be affected by conventional flue gas cleaning systems, Hgp is removed by particle filters [3]. Due to the high solubility of HgCl2 in water, wet FGD offers the possible side benefit of removing HgCl2 from
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the flue gas efficiently. Therefore, wet FGD systems have considerable influence on mercury removal [4]. However, due to the utilization of high-sulfur coal blends, the efficiency of installed FGD units is often driven to its limits. A reasonable strategy for the optimization of the process with low investment costs is the addition of organic pH-buffer systems to the slurry. These additives lower the pH-drop caused by the absorption of SO2 in the slurry droplets, leading to an increased mass transfer during the whole contact time between slurry and flue gas and subsequently improved SO2 removal efficiency. For this purpose, carboxylic acids are often utilized, such as formic and adipic acid. It is known that under unfavorable FGD operational conditions and in presence of suitable reducing agents in the liquid phase, dissolved Hg2+ can be reduced to Hg0, which is released to the clean gas [5]. Besides compounds of sulfur with the oxidation number of +IV (S(IV)), (di-) carboxylic acids are known to be possible electron donors [6]. Thus, these organic compounds are able to act as reducing agents under specific conditions. Therefore, their presence in the liquid phase of the slurry could lead to the previously described chemical reduction of dissolved HgCl2, restricting the benefit of co-removal of mercury by the wet FGD system. In FGD slurries, there is a permanent interaction of complex formation and redox reactions of mercury compounds. Considering the large volume of the circulating slurry containing dissolved HgCl2, re-emissions of Hg0 are able to negatively impact the overall Hg removal efficiency of the FGD. Thus, factors leading to stable dissolved mercury compounds are essential for overall high Hg removal rates. Previous studies indicate that the optimization of operational parameters can lead to significantly lower Hg-emissions [7,8]. 2. Reactions of SO2 and Hg compounds in wet FGD There are several parameters determining the rate of SO2absorption in wet FGD slurries. The impact of numerous operating parameters can be found in [9]. However in this work, main focus is put on temperature and pH-value of the slurry. Considering Henry’s Law, there is an exponential dependence of the solubility of SO2 on temperature. Dissociation of SO2(aq) and relevant S(IV) compounds þ SO2ðaqÞ þ 3H2 O ¢ HSO3 þ H3 Oþ H2 O ¢ SO2 3 þ 2H3 O
ð1Þ
Dissociation product of SO2 is the (bi-) sulfite anion and (one) two proton(s), which explains the impact of pH on SO2 mass transfer. By raising pH, the equilibrium is shifted to the deprotonated sulfite side. pH-buffer effect of di-carboxylic acids
H2 A þ 2H2 O ¢ HA þ H2 O þ H3 Oþ ¢ A2 þ 2H3 Oþ
ð2Þ
In presence of the organic additives investigated in this study, both being weak organic acids and co-existing with their conjugated base in the solution, the drop of pH-value in the circulating slurry caused by the uptake of SO2 is reduced. The buffering capacity is directly linked to the operational slurry pH-value of the process. From the Henderson–Hasselbalch equation it can be derived, that the maximum buffering capacity is obtained for identical pH- and pKA-values of the carboxylic acid utilized. Being the negative logarithm of the acid constant, the pKA-value of suitable organic additives should be within the range of applicable operating pH-values in the FGD process. Oxidation of S(IV) 2 þ 2HSO2 3 þ O2 þ 2H2 O ¢ 2SO4 þ 2H3 O
ð3Þ
The oxygen content of the flue gas enables partial oxidation of S(IV)-compounds in the absorber section of the FGD. In forced oxidation mode, S(IV)-oxidation is enhanced by blowing an airflow
into the sump of the FGD. The overall reaction product of the described process is gypsum. Therefore, the pH of the slurry is controlled to a constant value by dosing fresh limestone slurry in order to replace the consumed CaCO3 and to neutralize the acidic slurry. Limestone has a low solubility in water; however the Ca2+ concentration in the slurry increases for decreasing pH-value. Hence, the influence of pH on the FGD-process is ambivalent: acidic slurries enhance limestone solubility on the one hand, but inhibit H2SO3 dissociation and overall reaction rate on the other. The previously described operational parameters and additional dissolved compounds have a strong impact on the fate of Hg compounds entering the scrubber. Due to the high solubility of HgCl2 in water and its low gaseous mass flow compared with SO2, FGD units are able to remove HgCl2 almost quantitatively. Because of its very low solubility and high volatility, the elemental form of gaseous mercury is usually not absorbed by the slurry in the scrubber. Former studies show rather an increase of the concentration of Hg0 at the outlet of wet FGD systems [5]. To clarify this phenomenon, it is necessary to identify possible reaction pathways leading to the reduction of previously absorbed Hg2+-compounds to Hg0. HgCl2 is a compound with covalent bonds; the linear shape molecule is hardly dissociated in aqueous solutions [10]. Dissolved HgCl2 has the affinity to form complexes in solution with suitable electron donor ligands. An approach in predicting the complex chemistry of mercury is the hard and soft acid bases theory (HSAB), which introduces two classes (and intermediates) of acids and bases. It states that complexes formed by combination of acceptors and donors of the same class show strong bindings and high stability [11]. Hg2+ is classified as a typically soft acid, thus forming stable complexes with soft bases. Regarding the composition of the aqueous phase in FGD slurries, possible complexing agents for 2 Hg2+ considered in this work are HSO and Cl. Both being 3 /SO3 classified as borderline bases between hard and soft, while SO2 4 , OH and CO2 are regarded as classic hard bases and therefore 3 unsuitable to combine with Hg2+ under typical FGD conditions. The formation of stable complexes is essential to permit the reduction of uncomplexed HgCl2 by electron donors. For increasing halide content (e.g. Cl) of the slurry, HgCl2 coexists with [HgCl3] and [HgCl4]2 complexes. According to the stability diagram in [12], [HgCl4]2 is the dominating complex when the Cl concentration is higher than 0.2 mol/l. This is in good agreement with calculations in [13]. It is stated, that increasing the activity of chloride 2+ ions greatly extends the stability of Hg2þ ions by the for2 and Hg mation of Hg2Cl2 and [HgCl4]2 complexes. These statements are also true when exchanging Cl by Br. The stability constants of compounds consisting of Hg2+ with Br-ligands are even higher than of those with Cl. Therefore, for power plants using both wet FGD and bromide based Hg control strategies, the impact of Br concentration of the slurry has to be considered [14]. Formation of Hg2+–Cl complexes in FGD slurries
ð2xÞ
HgCl2 þ ðx 2ÞCl ¢ ½HgClx
06x64
ð4Þ
Also, multi-ligand complexes consisting of Hg2+ as central ion and with several different ligands (e.g. Cl, Br and S(IV)-compounds) are suggested by several authors [15,16]. Mercury is a relatively noble element, the Hg2+/Hg0 half cell reaction is listed with a standard electrode potential of +0.85 V [10]. Therefore, redox reactions with participation of dissolved HgCl2 are likely to occur in FGD slurries. The equilibrium of dissociation of the covalent molecule is strongly dependent on pH and oxidation–reduction potential (ORP) [17]. However, a certain amount of dissociated Hg2+ can be expected in FGD slurries [18]. Reaction products of the chemical reduction of Hg2+ by S(IV) are sulfate and elemental mercury, which is emitted to the gas phase. Chemical reduction of Hg2+
B. Heidel et al. / Applied Energy 114 (2014) 485–491 2 þ 0 Hg2þ þ SO2 3 þ 3H2 O ¢ Hg þ SO4 þ 2H3 O
ð5Þ
Each of these reactions includes the transfer of two electrons from S(IV) to Hg2+, therefore a change in oxidation number of +2 to 0 2 (Hg2+/Hg0) and of +4 to +6 (SO2 3 /SO4 ). According to [19], potential intermediates of these reactions are formed by the transfer of a single electron, such as mercurous species (Hg+) and the sulfite radical + anion (SO 3 ). The disproportionation of Hg is an additional pathway of Hg0 generation. By the application of organic acids to the slurry, additional pathways of Hg2+ reduction are possible. Eq. (6) is an example for the mechanism of a redox-reaction of the reactants formic acid and Hg2+, visible by the change in oxidation number of the carbon atom of the carboxylic group from +II to +IV, while Hg2+ acts as an electron acceptor and is reduced to Hg0. The impact of pH-value on this mechanism is indicated by the appearance of the oxonium ion on the product side. Hg2+ reduction by formic acid
Hg2þ þ HCOOH þ 4H2 O ¢ Hg0 þ HCO3 þ 3H3 Oþ
ð6Þ
3. Experimental method Experimental investigations have been carried out at a lab-scale test facility. The functional principle of the wet FGD scrubber is shown in Fig. 1. The gas composition of the carrier gas is 15 Vol.% CO2, 3.5 Vol.% O2 and balance N2, which is typical for hard coal firing power plants. It is conditioned to adjustable concentrations of HCl, H2O, SO2, Hg0 and HgCl2. The simulated raw flue gas is heated to 150 °C and subjected to the circulating slurry in the absorber tower in a continuous countercurrent process. Gas phase concentrations are measured online. The slurry at the bottom of the absorber is directed to an external continuously stirred and electrically heated vessel, where the forced oxidation of dissolved S(IV) by aeration takes place. The gas phase concentration of Hg in the exhaust air of the external sump is also measured online. The content of dissolved chloride is determined by ion chromatography, while the aqueous concentration of mercury compounds is determined by atomic absorption spectroscopy. The pH-value and the ORP of the slurry are continuously measured in the external sump. The pH at this point is controlled to a constant level by dosing fresh limestone slurry. The level of the slurry inside the vessel is held constant by integration of a fixed overflow channel. Organic
Gas Analyzer
Air
Carrier gas
H2O HCl SO 2 Hg0 HgCl 2
3,5% O 2 15% CO2 balance N2
Absorber Column
CaCO3
External Sump
Fig. 1. Functional principle of the lab-scale FGD.
487
additive and chloride concentrations are adjusted by dissolution of sodium formate, adipic acid, KCl and NaCl. Due to laboratory scale and functional components capacities, the solids content is set to 3.5 wt.%. Absolute flow rates of gases and liquids are controlled by MFCs and peristaltic pumps. In reference settings, the scrubber slurry is controlled to pH 4.1 and heated to 40 °C. The liquid to gas ratio (L/G) is set to 28 l/m3. The aqueous concentration of Cl is adjusted to 3 g/l. Flue gas flow rate is 3 l/min, with a SO2 raw-gas concentration of 3000 mg/m3, the aeration flow is set to 2 l/min. The experimental setup allows observing the change in concentration of mercury and SO2 at the in- and outlet of the absorber on the one hand and revealing the chemical reduction of dissolved Hg2+-compounds and subsequent re-emission of Hg0 in the external sump on the other. At steady state conditions, all the described concentrations and mass flows are constant. In this paper, focus is put on Hg0 re-emission phenomena taking place in the aerated external sump. In a previous study [20], it was presented that Hg0 re-emissions occur simultaneously in both the absorber and the external sump section of the lab-scale FGD. The magnitudes of the re-emitted Hg0 mass-flows in absorber and sump were comparable. Thus, operating conditions with low Hg0 re-emissions in the sump were equivalent to simultaneous low re-emissions in the absorber and vice versa. Hg0 re-emissions in the absorber cannot be measured directly, but calculated by the difference in Hg concentration and speciation up- and downstream of the absorber, using multiple on-line Hg analyzers simultaneously. Initially, the oxidation air contains no Hg at all, thus the measured Hg0 concentration in the exhaust air of the sump is directly associated with Hg0 re-emissions. Therefore in contrast to absorber measurements, only one analyzer is required for the quantification of Hg0 re-emissions in the external sump, as presented in this work. For the common configuration of wet FGDs without external sump, this distinction cannot be made due to the immediate mixing of exhaust air and flue gas. For transitional operation conditions, the equilibrium of reactions involving mercury compounds is shifted and can result in an increase of the rate of reaction leading to Hg2+ reduction and subsequent Hg0 re-emission. Until reaching equilibrium, the total gaseous mass flow of Hg emitted by the system can be higher than the input of Hg. This implies that desorption of mercury from the slurry surpasses absorption and the aqueous concentration of mercury decreases. The external sump of the lab-scale FGD system is regarded as an ideal continuously stirred tank reactor, in order to use the empirical correlation for reaching steady state conditions. Hence, the minimal reaction time to achieve ideal constant concentrations at the outlet of the reactor at least five times the hydrodynamic residence time of the sump [21]. The consequence for the lab-scale FGD presented here, run at reference settings, is that ideal steady state is not reached before several days of constant operation. However, it has been observed that for the case of the labscale FGD, the time response of the ORP is a reliable indicator for distinguishing between dynamic and near steady state conditions. Therefore, stationary conditions presented in this work imply constant ORP for several hours, while the dynamic response of the FGD system is indicated by an unsteady trend of the ORP. Considering the common change in load and operating conditions of full-scale FGD units, these definitions ensure transferability of the conclusions from lab- to full-scale.
4. Results and discussion Overflow Liquid Analysis
4.1. Fundamental investigations on re-emissions of Hg0 The concentration of Hg in the exhaust air of the sump can be directly associated with Hg0 re-emissions, since reducing
B. Heidel et al. / Applied Energy 114 (2014) 485–491
Hg
15 10 5 0 0
1
2
3
4
5
Hg0 re-emission [µg/m³]
Hg0 re-emission [µg/m³]
20 15
30%
10
20%
5
10%
0
0% 30
Chloride concentration [g/l]
2SO SO 3 /S(IV)
Ratio SO32-/S(IV)
488
40 50 60 70 Temperature [°C]
80
Fig. 2. Impact of chloride concentration on Hg0 re-emission. Fig. 4. Hg0 re-emission and SO2 3 concentration as a function of temperature.
conditions in the slurry lead to the chemical reduction of previously absorbed HgCl2 and subsequent desorption of Hg0. In order to quantify the contribution of organic acids to Hg0 re-emissions, baseline experiments in absence of the additives were conducted under reference settings. Fig. 2 points out the importance of complex formation for the retention of Hg2+ in FGD-slurries. The experiment was conducted by continuously dosing KCl-solution to the slurry with initially low concentration of Cl at a temperature of 60 °C. The elevated temperature was chosen due to very low re-emissions for the reference setting of 40 °C. For increasing concentration of the ligand chloride, the ratio of uncomplexed Hg2+ decreases, which is equivalent to declining Hg0 re-emissions. The increase in chloride content leads to decreasing Hg0 formation. This observation is in-line with the theoretical share of uncomplexed Hg2+ in aqueous systems containing Hg2+ and Cl [13]. Further results of experimental investigations on the impact of the pH-value of the slurry are presented in Fig. 3. In the context of the scope of the work described in this paper, special emphasis has to be put on this parameter. It can be observed, that the concentration of Hg0 in the exhaust air is strongly dependent on the operational pH-value. A possible interpretation of this phenomenon is delivered by the thermodynamic equilibrium calculation of the ratio of SO2 3 /S(IV)total, visualized by the secondary axis. For pH > 5, both Hg0 re-emission rate and SO2 3 /S(IV)-ratio show similar behavior. This observation implies that the accelerated rate of Hg2+ reduction reaction is linked to the increased concentration of SO2 3 in the liquid phase (or the corresponding decreasing share 2 of HSO being the 3 ). This can be regarded as a reference to SO3 dominant reducing agent for the reduction of Hg2+. Detailed investigations on the rate of reaction were carried out by continuously increasing the temperature of the slurry. The
2-
100%
4
80%
3
60%
2
40%
1
20% 0%
0 4
5 6 pH-value
7
Fig. 3. Impact of pH-value on Hg0 re-emission.
Hg
Hg0 re-emission [µg/m³] ORP [mV]
5
ORP
SO2 SO 2
300
9000
250
7500
200
6000
150
4500
100
3000
50
1500
SO2 raw gas [mg/m³]
SO3 /S(IV)
Ratio SO32-/S(IV)
Hg0 re-emission [µg/m³]
Hg
results for reference settings with the aqueous concentration of Hg2+ = 100 lg/l and pH = 5.1 are illustrated by Fig. 4. The acceleration of the reaction rate for elevated temperatures is in accordance with the empirical Arrhenius equation. However, it may be worth stating that a secondary effect might contribute to the enhanced formation of Hg0. When calculating the acid constants of H2SO3 as functions of temperature, increased share of SO2 3 in connection with elevated Hg0 re-emissions can be observed by plotting the SO2 3 ratio for the actual temperature regime at constant pH-value of 5.1. Further experiments with focus on the impact of S(IV)concentration were conducted with the same slurry by successive increasing the SO2 concentration of the raw gas treated by the lab-scale FGD, presented in Fig. 5. Due to the higher driving force, mass transfer of SO2 is enhanced, resulting in a higher flux of S(IV)compounds into the liquid phase. For constant retention time in the sump at stationary L/G-ratio, this results in higher aqueous S(IV)-concentrations, detectable by the decrease of the ORP. The simultaneous sharp increase of Hg0 re-emissions and the drop of ORP after 25 min illustrated by Fig. 5 indicate the existence of a critical S(IV)-concentration in the context of Hg0 re-emissions. By applying a correlation method of the ORP and actually analyzed S(IV)-concentrations [22] on this data, this aspect is visualized more descriptively by the plot of five minute average values of re-emissions over S(IV)-concentrations by Fig. 6. It is obvious, that for S(IV)-concentrations higher than the critical value at the inflection point of the curve, the rate of reaction of the Hg2+ reduction is enhanced extensively and reaches a maximum. The exact values of both, critical concentration and height of the maximum are functions of the characteristics of the slurry (e.g. concentrations of Hg2+ and ligands, pH-value, redox-conditions, temperature).
0
0 0
20 40 Time [min]
60
Fig. 5. Impact of raw gas SO2 content on ORP and Hg0 re-emission.
489
Hg0 re-emission [µg/m³]
B. Heidel et al. / Applied Energy 114 (2014) 485–491
50
dependence of Hg2+ reduction on pH is qualitatively comparable for each case. Quantitative analysis reveal the reducing character of both formic and adipic acid, with formic acid being the much stronger reducing agent. It has to be stated, that due to the enhanced removal efficiency of SO2, FGDs utilizing such additives can be operated at reduced levels of pH. For the case of adipic acid, SO2 removal efficiency at pH = 4.3 (94%) is still superior to the reference case without additives at pH = 5.3 (88%), at only slightly elevated levels of Hg0 re-emissions. For the case of formic acid, there is no such appropriate compromise between SO2-removal and Hgretention, since Hg0-formation remains on a constant high level for pH < 4.5, indicating a differing reaction mechanism. Therefore, it can be concluded, that for all slurries investigated, one of the possible reaction pathways of Hg2+-reduction includes S(IV)-species (Eq. (5)). For the case of slurries containing formic acid, a second mechanism is proposed (Eq. (6)), where the electron donor is the formic acid itself, decomposing to CO2. For low pH-values, the Hg0-flux generated by the formate mechanism is the dominant one and not depending on S(IV)-concentration. The exponential dependence of Hg2+ reduction on temperature in the absence of additives was presented in Fig. 4, therefore subsequent experiments with slurries containing dissolved formic and adipic acid at identical molar concentrations (0.0174 mol/l) were conducted. The results presented in Fig. 8 show similar qualitative
Critical S(IV)
40 30 20 10 0 0
50 100 150 200 250 S(IV) [mmol/m³]
Fig. 6. Hg0 re-emission as a function of SO2 3 concentration.
4.2. Impact of organic additives on the re-emission of Hg0
Hg0 re-emission [µg/m³], ORP [mV]
In order to identify the impact of organic acids on the re-emission of Hg0, further investigations were conducted in presence of formic and adipic acid. The operating conditions of the test-rig and the chemical composition of synthetic flue gas and slurries, except of the addition of the actual organic acids were identical to the reference studies described in Section 4.1. Fig. 7 compares measured ORP profiles and the corresponding behavior of Hg in the lab-scale FGD for three different slurries at 40 °C with a chloride content of 3 g/l (reference set-up). The S(IV)-concentration of the slurries is considered to be comparable, since oxidation air flow rate (2 l/min), sump residence times and the individual removal rates of SO2 are similar (>80%). Slurry 1 is the reference slurry without organic acids. Slurry 2 contains 0.0174 mol/l adipic acid. For direct comparison, slurry 3 contains no adipic acid, but the same molar concentration of formic acid. The most important conclusion of the data presented is, that both organic acids affect the retention of dissolved HgCl2, promoting the re-emission of Hg0. Within the pH-range investigated, adipic acid enhances the formation of Hg0 approximately by one order of magnitude. In presence of formic acid, two orders of magnitude enhanced Hg0 re-emissions are detected, surpassing 500 lg/m3 (not presented due to scaling of Fig. 7) for pH higher than 4.9. These observations are in-line with the declining ORP-signals for increasing pH of the three slurries investigated, with high reemissions for low values of ORP. It can be concluded, that the
Hg: no additive Hg: Formic Acid
Hg0 re-emission [µg/m³]
500 400 300 200 100 0 40
50 60 70 Slurry temperature [°C]
Hg: no additive
Hg: Adipic Acid
Hg: Formic Acid
ORP: no additive
ORP: Adipic Acid
ORP: Formic Acid
250 200 150 100 50 0 4.2
4.4
80
Fig. 8. Impact of slurry temperature in presence of additives on re-emissions of Hg0.
300
4.0
Hg: Adipic Acid
4.6
4.8
5.0
5.2
5.4
pH-value Fig. 7. pH-dependence of Hg0 re-emissions and ORP of slurries containing organic acids.
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B. Heidel et al. / Applied Energy 114 (2014) 485–491
Table 1 Thermodynamic analysis of Hg2+-reduction. Slurry composition
Additive concentration [mol/l]
No organic acids Adipic acid Formic acid
0 0.0174 0.0174
Hg2þ ðaqÞ
ClðaqÞ
[lg/l]
[g/l]
DHà [kJ/ mol]
DSà [J/ mol/K]
90.8
3
99.2
51.8
104.0 63.5
3 3
97.2 108.0
37.8 22.6
tendencies for the slurries investigated. Quantitative comparison clearly states, that the magnitudes of Hg0 re-emissions are the highest in presence of formic acid, demonstrating the strong reducing character of this additive. For the case of adipic acid, the re-emission of Hg0 can be limited to comparably low levels, applying slurry temperatures below 50 °C. Results of the analytical analysis of the data in Fig. 8 using the Eyring equation [23] are summarized in Table 1. For the calculation, complex formation constants of the Hg2+/Cl system were used from [24], assuming that the complexes [HgCl3] and [HgCl4]2 are masked and thus not available as reactants of redox reactions. The obtained values for the activation enthalpies DHà are comparable for the slurry without additives and in presence of adipic acid. For the slurry containing formic acid, DHà differs considerably. This observation is even more evident when comparing the individual activation entropies DSà. The activation entropy of the reaction in presence of formic acid is at a significantly higher level than DSà of the other two reactions. For the purpose of identification of relevant reaction mechanisms involved, this is also a hint for an alternative, more rapid pathway of Hg0-formation in presence of formic acid. According to the transition state theory, high positive values of DSà indicate fast reactions, while a negative entropy is associated with slow kinetics [25]. This is in good agreement with the levels of Hg0 re-emissions observed in all of the experiments conducted. For the additive-free slurry, the importance of the availability of the ligand chloride for the formation of stable Hg2+-complexes was demonstrated by Fig. 2. In order to reveal the contribution of dissolved Cl to suppress Hg0 re-emissions in slurries containing the more promising additive adipic acid, additional experiments were conducted by dosing NaCl-solution to the liquid phase continuously. According to Fig. 9, increasing chloride content from 3 g/l to 16 g/l at a pH of 5.1 reduces the re-emission of Hg0 to less than 50% of the original value. The increase in chloride content can be visualized under these conditions by monitoring the corresponding ORP on the secondary axis. Overall, the stabilizing effect
ORP 250
15
225
10
200
5
175
ORP [mV]
Hg0 re-emission [µg/m³]
Hg 20
150
0 2
6
10
14
18
Chloride concentration [g/l] Fig. 9. Hg0 re-emissions for increasing chloride content in presence of adipic acid.
of Cl availability on re-emissions is obvious, whiles the actual Hg0-concentration in the exhaust air of the sump is always to some extent higher when the slurry contains dissolved adipic acid. 5. Conclusion In this study the chemical reduction and re-emission of mercury compounds from slurries of a lab-scale wet FGD system was investigated. Emphasis was put on the effect of formic and adipic acid, which are known additives for enhancing SO2 removal efficiency of full-scale FGDs. The obtained results allow an improved insight into the role of operational parameters and slurry composition on the stability of dissolved mercury compounds. The effect of S(IV)-concentration and -species on Hg chemistry was presented. SO2 3 -concentration was changed indirectly through the variation of absorbed SO2-mass flow and pH-value. For S(IV)-concentrations above a certain critical value, its potential as reducing agent leads to rapidly increasing formation and re-emission of Hg0. Increasing chloride concentration has been identified as an important factor for depressing Hg0 re-emissions for all conditions investigated. Lowering operational pH-value could always be correlated with decreasing Hg0 re-emissions. Both organic additives have a negative impact on Hg-retention in the wet FGD process, with formic acid being the significantly stronger reducing agent. Hg2+-complex decomposition and Hg0 re-emissions are exponentially depending on temperature for all slurries utilized. Two different pathways of Hg2+ reduction were identified by qualitative interpretation of the pH-dependence and by comparison of calculated activation enthalpies and activation entropies. While the first mechanism proposed involves SO2 3 as reducing agent and is therefore relevant for any FGD process, the second mechanism involves the formate anion, thus being exclusively relevant for FGDs utilizing formic acid as additive. However, due to greatly enhanced SO2-removal at low operating pH-values, the negative impact of adipic acid can be compensated by operating the FGD at lower pH, ensuring both increased SO2 removal efficiency and low levels of Hg0 re-emissions. Acknowledgement This work was kindly supported by EnBW Erneuerbare und Konventionelle Erzeugung AG. References [1] Pacyna, Elisabeth G, et al. Global anthropogenic mercury emission inventory for 2000. Atmos Environ 2006;40(22):4048–63. [2] Islas J, Grande G. Abatement costs of SO2-control options in the Mexican electric-power sector. Appl Energy 2008;85(2-3):80–94. [3] Wang Yun-jun et al. Comparison of mercury removal characteristic between fabric filter and electrostatic precipitators of coal-fired power plants. J Fuel Chem Technol 2008;36(1):23–9. [4] Ito Shigeo, Yokoyama Takahisa, Asakura Kazuo. Emissions of mercury and other trace elements from coal-fired power plants in Japan. Sci Total Environ 2006;368(1):397–402. [5] Chang, John CS, Behrooz Ghorishi S. Simulation and evaluation of elemental mercury concentration increase in flue gas across a wet scrubber. Environ Sci Technol 2003;37(24):5763–6. [6] Latscha, Hans Peter, Uli Kazmaier, Helmut Alfons Klein. Organische chemie chemie-basiswissen ii. London: Springer; 2008. [7] Wo Jingjing et al. Hg2+ reduction and re-emission from simulated wet flue gas desulfurization liquors. J Hazard Mater 2009;172:1106–10. [8] Wang Yunjun et al. Experimental study on the absorption behaviors of gas phase bivalent mercury in Ca-based wet flue gas desulfurization slurry system. J Hazard Mater 2010;183(1):902–7. [9] Sun Zhongwei et al. Experimental study on desulfurization efficiency and gas– liquid mass transfer in a new liquid-screen desulfurization system. Appl Energy 2010;87(5):1505–12. [10] Holleman, Arnold F., Egon Wiberg, and Nils Wiberg, eds. Lehrbuch der anorganischen Chemie. Walter de Gruyter, 1995. [11] Ho Tse-Lok. Hard soft acids bases (HSAB) principle and organic chemistry. Chem Rev 1975;75(1):1–20.
B. Heidel et al. / Applied Energy 114 (2014) 485–491 [12] Benes P, Havlik B. Speciation of mercury in natural waters. In: Nriagu J, editor. The biogeochemistry of mercury in the environment. Elsevier/North-Holland Biomedical Press; 1979. p. 175–202. [13] Brandon NP et al. Thermodynamics and electrochemical behaviour of Hg-S-ClH2O systems. J Electroanal Chem 2001;1–2(2):18–32. [14] Ochoa-González R, Díaz-Somoano M, Martínez-Tarazona MR. Effect of anion concentrations on Hg2+ reduction from simulated desulphurization aqueous solutions. Chem Eng J 2013;214:165–71. [15] Griffiths R, Trevor R, Anderson Richard A. The electronic spectra of the mixed mercury dihalides Part 2. J Chem Soc Dalton Trans 1980:209–15. [16] Blythe, Gary, David W. DeBerry, and Sara Pletcher. ‘‘Bench-scale Kinetics Study of Mercury Reactions in FGD Liquors’’. Final Report: DE-FC26-04NT42314 for the National Energy Technology Laboratory, Austin, TX (2008). [17] Becking, Baas LGM, Kaplan Ian R, Derek Moore. Limits of the natural environment in terms of pH and oxidation–reduction potentials. J Geol 1960:243–84. [18] Warneck P. Chemistry of clouds and precipitation. Chemistry of the natural atmosphere. New York: Academic Press; 1988. 734–421.
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[19] Loon Van, Lisa, Elizabeth Mader, Scott Susannah L. Reduction of the aqueous mercuric ion by sulfite: UV spectrum of HgSO3 and its intramolecular redox reaction. J Phys Chem A 2000;104(8):1621–6. [20] Heidel, Barna, et al. ‘‘Influencing factors on the emission of mercury from wet flue gas desulphurisation slurries’’. VGB powertech 3 (2012). [21] Baerns M, Hofmann H, Renken A. Chemische Reaktionstechnik Lehrbuch der Technischen Chemie Band 1. G. Thieme Verlag, Stuttgart; New York (1987): 289. [22] Heidel, Barna et al., Spectrophotometrical determination of sulphite concentration in slurries from wet flue gas desulphurization, VGB Chemistry in Power Plants 2012, Hamburg, (2012). [23] Eyring Henry. The activated complex in chemical reactions. J Chem Phys 1935;3:107. [24] Smith, R. M., A. E. Martell, and R. J. Motekaitis. ‘‘NIST standard reference database 46’’. NIST Critically Selected Stability Constants of Metal Complexes Database Ver 2 (2003). [25] Dickerson R, Richard E, Gray Harry B, Haight Gilbert P. Chemical principles. The Benjamin/Cummings Publishing Company, Inc.; 1979.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Impact of additives for enhanced sulfur dioxide removal on re-emissions of mercury in wet flue gas desulfurization Barna Heidel a,⇑, Melanie Hilber b, Günter Scheffknecht a a b
Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Pfaffenwaldring 23, D-70569 Stuttgart, Germany EnBW Erneuerbare und Konventionelle Erzeugung AG, Schelmenwasenstraße 15, D-70567 Stuttgart, Germany
h i g h l i g h t s Mercury removal in wet FGD. Re-emission of mercury. Additives for enhanced SO2 removal in wet FGD. Reaction mechanisms of mercury re-emissions. Multi pollutant control by wet FGD.
a r t i c l e
i n f o
Article history: Received 17 July 2013 Received in revised form 25 September 2013 Accepted 28 September 2013
Keywords: Mercury FGD Organic acid Additive Re-emission Multi-pollutant control
a b s t r a c t The wet flue gas desulfurization process (FGD) in fossil fired power plants offers the advantage of simultaneously removing SO2 and other water soluble pollutants, such as certain oxidized mercury compounds (Hg2+). In order to maximize SO2 removal efficiency of installed FGD units, organic additives can be utilized. In the context of multi-pollutant control by wet FGD, the effect of formic and adipic acid on redox reactions of dissolved mercury compounds is investigated with a continuously operated lab-scale testrig. For sulfite (SO2 3 ) concentrations above a certain critical value, their potential as reducing agent leads to rapidly increasing formation and re-emission of elemental mercury (Hg0). Increasing chloride concentration and decreasing pH and slurry temperature have been identified as key factors for depressing Hg0 re-emissions. Both organic additives have a negative impact on Hg-retention and cause increased Hg0 reemissions in the wet FGD process, with formic acid being the significantly stronger reducing agent. Different pathways of Hg2+ reduction were identified by qualitative interpretation of the pH-dependence and by comparison of activation enthalpies and activation entropies. While the first mechanism proposed identifies SO2 3 as reducing agent and is therefore relevant for any FGD process, the second mechanism involves the formate anion, thus being exclusively relevant for FGDs utilizing formic acid as additive. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Sulfur (S) and Mercury (Hg) enter the combustion chamber as constituents of coal and secondary fuels [1]. The dominant gas phase compound of sulfur is sulfur dioxide (SO2). However, due to subsequent reactions with other flue gas constituents and in dependence of the configuration of the installed air pollution control devices (APCD), the formation of SO3 can be promoted, leading to cold-end corrosion phenomena. The most common technology for SOx-removal in Europe is the wet flue gas desulphurization (FGD) process with limestone, enabling SOx-removal rates of >95%. An overview about available SO2 control technologies and the corresponding abatement costs can be found in [2]. ⇑ Corresponding author. Tel.: +49 (0)711 685 68946; fax: +49 (0)711 685 63491. E-mail address: [email protected] (B. Heidel). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.09.059
The reactions of the trace element Hg in the flue gas and its interaction with other flue gas components are of complex nature. Due to the high temperatures in the combustion chamber, the thermodynamic equilibrium of Hg-reactions is shifted to its elemental form (Hg0). With decreasing temperature and affected by the flue gas composition, gaseous Hg0 undergoes oxidation to bivalent mercury (Hg2+), leading to the formation of HgCl2 in the presence of HCl, which is the dominant hydrogen halide in coal derived flue gas without additives. The heterogeneous oxidation of Hg0 is catalyzed by high-dust SCR DeNOx systems, resulting in higher shares of HgCl2. A certain fraction of mercury is bound to particulate matter (Hgp), due to sorption on the surface of ash particles. While elemental mercury is considered not to be affected by conventional flue gas cleaning systems, Hgp is removed by particle filters [3]. Due to the high solubility of HgCl2 in water, wet FGD offers the possible side benefit of removing HgCl2 from
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the flue gas efficiently. Therefore, wet FGD systems have considerable influence on mercury removal [4]. However, due to the utilization of high-sulfur coal blends, the efficiency of installed FGD units is often driven to its limits. A reasonable strategy for the optimization of the process with low investment costs is the addition of organic pH-buffer systems to the slurry. These additives lower the pH-drop caused by the absorption of SO2 in the slurry droplets, leading to an increased mass transfer during the whole contact time between slurry and flue gas and subsequently improved SO2 removal efficiency. For this purpose, carboxylic acids are often utilized, such as formic and adipic acid. It is known that under unfavorable FGD operational conditions and in presence of suitable reducing agents in the liquid phase, dissolved Hg2+ can be reduced to Hg0, which is released to the clean gas [5]. Besides compounds of sulfur with the oxidation number of +IV (S(IV)), (di-) carboxylic acids are known to be possible electron donors [6]. Thus, these organic compounds are able to act as reducing agents under specific conditions. Therefore, their presence in the liquid phase of the slurry could lead to the previously described chemical reduction of dissolved HgCl2, restricting the benefit of co-removal of mercury by the wet FGD system. In FGD slurries, there is a permanent interaction of complex formation and redox reactions of mercury compounds. Considering the large volume of the circulating slurry containing dissolved HgCl2, re-emissions of Hg0 are able to negatively impact the overall Hg removal efficiency of the FGD. Thus, factors leading to stable dissolved mercury compounds are essential for overall high Hg removal rates. Previous studies indicate that the optimization of operational parameters can lead to significantly lower Hg-emissions [7,8]. 2. Reactions of SO2 and Hg compounds in wet FGD There are several parameters determining the rate of SO2absorption in wet FGD slurries. The impact of numerous operating parameters can be found in [9]. However in this work, main focus is put on temperature and pH-value of the slurry. Considering Henry’s Law, there is an exponential dependence of the solubility of SO2 on temperature. Dissociation of SO2(aq) and relevant S(IV) compounds þ SO2ðaqÞ þ 3H2 O ¢ HSO3 þ H3 Oþ H2 O ¢ SO2 3 þ 2H3 O
ð1Þ
Dissociation product of SO2 is the (bi-) sulfite anion and (one) two proton(s), which explains the impact of pH on SO2 mass transfer. By raising pH, the equilibrium is shifted to the deprotonated sulfite side. pH-buffer effect of di-carboxylic acids
H2 A þ 2H2 O ¢ HA þ H2 O þ H3 Oþ ¢ A2 þ 2H3 Oþ
ð2Þ
In presence of the organic additives investigated in this study, both being weak organic acids and co-existing with their conjugated base in the solution, the drop of pH-value in the circulating slurry caused by the uptake of SO2 is reduced. The buffering capacity is directly linked to the operational slurry pH-value of the process. From the Henderson–Hasselbalch equation it can be derived, that the maximum buffering capacity is obtained for identical pH- and pKA-values of the carboxylic acid utilized. Being the negative logarithm of the acid constant, the pKA-value of suitable organic additives should be within the range of applicable operating pH-values in the FGD process. Oxidation of S(IV) 2 þ 2HSO2 3 þ O2 þ 2H2 O ¢ 2SO4 þ 2H3 O
ð3Þ
The oxygen content of the flue gas enables partial oxidation of S(IV)-compounds in the absorber section of the FGD. In forced oxidation mode, S(IV)-oxidation is enhanced by blowing an airflow
into the sump of the FGD. The overall reaction product of the described process is gypsum. Therefore, the pH of the slurry is controlled to a constant value by dosing fresh limestone slurry in order to replace the consumed CaCO3 and to neutralize the acidic slurry. Limestone has a low solubility in water; however the Ca2+ concentration in the slurry increases for decreasing pH-value. Hence, the influence of pH on the FGD-process is ambivalent: acidic slurries enhance limestone solubility on the one hand, but inhibit H2SO3 dissociation and overall reaction rate on the other. The previously described operational parameters and additional dissolved compounds have a strong impact on the fate of Hg compounds entering the scrubber. Due to the high solubility of HgCl2 in water and its low gaseous mass flow compared with SO2, FGD units are able to remove HgCl2 almost quantitatively. Because of its very low solubility and high volatility, the elemental form of gaseous mercury is usually not absorbed by the slurry in the scrubber. Former studies show rather an increase of the concentration of Hg0 at the outlet of wet FGD systems [5]. To clarify this phenomenon, it is necessary to identify possible reaction pathways leading to the reduction of previously absorbed Hg2+-compounds to Hg0. HgCl2 is a compound with covalent bonds; the linear shape molecule is hardly dissociated in aqueous solutions [10]. Dissolved HgCl2 has the affinity to form complexes in solution with suitable electron donor ligands. An approach in predicting the complex chemistry of mercury is the hard and soft acid bases theory (HSAB), which introduces two classes (and intermediates) of acids and bases. It states that complexes formed by combination of acceptors and donors of the same class show strong bindings and high stability [11]. Hg2+ is classified as a typically soft acid, thus forming stable complexes with soft bases. Regarding the composition of the aqueous phase in FGD slurries, possible complexing agents for 2 Hg2+ considered in this work are HSO and Cl. Both being 3 /SO3 classified as borderline bases between hard and soft, while SO2 4 , OH and CO2 are regarded as classic hard bases and therefore 3 unsuitable to combine with Hg2+ under typical FGD conditions. The formation of stable complexes is essential to permit the reduction of uncomplexed HgCl2 by electron donors. For increasing halide content (e.g. Cl) of the slurry, HgCl2 coexists with [HgCl3] and [HgCl4]2 complexes. According to the stability diagram in [12], [HgCl4]2 is the dominating complex when the Cl concentration is higher than 0.2 mol/l. This is in good agreement with calculations in [13]. It is stated, that increasing the activity of chloride 2+ ions greatly extends the stability of Hg2þ ions by the for2 and Hg mation of Hg2Cl2 and [HgCl4]2 complexes. These statements are also true when exchanging Cl by Br. The stability constants of compounds consisting of Hg2+ with Br-ligands are even higher than of those with Cl. Therefore, for power plants using both wet FGD and bromide based Hg control strategies, the impact of Br concentration of the slurry has to be considered [14]. Formation of Hg2+–Cl complexes in FGD slurries
ð2xÞ
HgCl2 þ ðx 2ÞCl ¢ ½HgClx
06x64
ð4Þ
Also, multi-ligand complexes consisting of Hg2+ as central ion and with several different ligands (e.g. Cl, Br and S(IV)-compounds) are suggested by several authors [15,16]. Mercury is a relatively noble element, the Hg2+/Hg0 half cell reaction is listed with a standard electrode potential of +0.85 V [10]. Therefore, redox reactions with participation of dissolved HgCl2 are likely to occur in FGD slurries. The equilibrium of dissociation of the covalent molecule is strongly dependent on pH and oxidation–reduction potential (ORP) [17]. However, a certain amount of dissociated Hg2+ can be expected in FGD slurries [18]. Reaction products of the chemical reduction of Hg2+ by S(IV) are sulfate and elemental mercury, which is emitted to the gas phase. Chemical reduction of Hg2+
B. Heidel et al. / Applied Energy 114 (2014) 485–491 2 þ 0 Hg2þ þ SO2 3 þ 3H2 O ¢ Hg þ SO4 þ 2H3 O
ð5Þ
Each of these reactions includes the transfer of two electrons from S(IV) to Hg2+, therefore a change in oxidation number of +2 to 0 2 (Hg2+/Hg0) and of +4 to +6 (SO2 3 /SO4 ). According to [19], potential intermediates of these reactions are formed by the transfer of a single electron, such as mercurous species (Hg+) and the sulfite radical + anion (SO 3 ). The disproportionation of Hg is an additional pathway of Hg0 generation. By the application of organic acids to the slurry, additional pathways of Hg2+ reduction are possible. Eq. (6) is an example for the mechanism of a redox-reaction of the reactants formic acid and Hg2+, visible by the change in oxidation number of the carbon atom of the carboxylic group from +II to +IV, while Hg2+ acts as an electron acceptor and is reduced to Hg0. The impact of pH-value on this mechanism is indicated by the appearance of the oxonium ion on the product side. Hg2+ reduction by formic acid
Hg2þ þ HCOOH þ 4H2 O ¢ Hg0 þ HCO3 þ 3H3 Oþ
ð6Þ
3. Experimental method Experimental investigations have been carried out at a lab-scale test facility. The functional principle of the wet FGD scrubber is shown in Fig. 1. The gas composition of the carrier gas is 15 Vol.% CO2, 3.5 Vol.% O2 and balance N2, which is typical for hard coal firing power plants. It is conditioned to adjustable concentrations of HCl, H2O, SO2, Hg0 and HgCl2. The simulated raw flue gas is heated to 150 °C and subjected to the circulating slurry in the absorber tower in a continuous countercurrent process. Gas phase concentrations are measured online. The slurry at the bottom of the absorber is directed to an external continuously stirred and electrically heated vessel, where the forced oxidation of dissolved S(IV) by aeration takes place. The gas phase concentration of Hg in the exhaust air of the external sump is also measured online. The content of dissolved chloride is determined by ion chromatography, while the aqueous concentration of mercury compounds is determined by atomic absorption spectroscopy. The pH-value and the ORP of the slurry are continuously measured in the external sump. The pH at this point is controlled to a constant level by dosing fresh limestone slurry. The level of the slurry inside the vessel is held constant by integration of a fixed overflow channel. Organic
Gas Analyzer
Air
Carrier gas
H2O HCl SO 2 Hg0 HgCl 2
3,5% O 2 15% CO2 balance N2
Absorber Column
CaCO3
External Sump
Fig. 1. Functional principle of the lab-scale FGD.
487
additive and chloride concentrations are adjusted by dissolution of sodium formate, adipic acid, KCl and NaCl. Due to laboratory scale and functional components capacities, the solids content is set to 3.5 wt.%. Absolute flow rates of gases and liquids are controlled by MFCs and peristaltic pumps. In reference settings, the scrubber slurry is controlled to pH 4.1 and heated to 40 °C. The liquid to gas ratio (L/G) is set to 28 l/m3. The aqueous concentration of Cl is adjusted to 3 g/l. Flue gas flow rate is 3 l/min, with a SO2 raw-gas concentration of 3000 mg/m3, the aeration flow is set to 2 l/min. The experimental setup allows observing the change in concentration of mercury and SO2 at the in- and outlet of the absorber on the one hand and revealing the chemical reduction of dissolved Hg2+-compounds and subsequent re-emission of Hg0 in the external sump on the other. At steady state conditions, all the described concentrations and mass flows are constant. In this paper, focus is put on Hg0 re-emission phenomena taking place in the aerated external sump. In a previous study [20], it was presented that Hg0 re-emissions occur simultaneously in both the absorber and the external sump section of the lab-scale FGD. The magnitudes of the re-emitted Hg0 mass-flows in absorber and sump were comparable. Thus, operating conditions with low Hg0 re-emissions in the sump were equivalent to simultaneous low re-emissions in the absorber and vice versa. Hg0 re-emissions in the absorber cannot be measured directly, but calculated by the difference in Hg concentration and speciation up- and downstream of the absorber, using multiple on-line Hg analyzers simultaneously. Initially, the oxidation air contains no Hg at all, thus the measured Hg0 concentration in the exhaust air of the sump is directly associated with Hg0 re-emissions. Therefore in contrast to absorber measurements, only one analyzer is required for the quantification of Hg0 re-emissions in the external sump, as presented in this work. For the common configuration of wet FGDs without external sump, this distinction cannot be made due to the immediate mixing of exhaust air and flue gas. For transitional operation conditions, the equilibrium of reactions involving mercury compounds is shifted and can result in an increase of the rate of reaction leading to Hg2+ reduction and subsequent Hg0 re-emission. Until reaching equilibrium, the total gaseous mass flow of Hg emitted by the system can be higher than the input of Hg. This implies that desorption of mercury from the slurry surpasses absorption and the aqueous concentration of mercury decreases. The external sump of the lab-scale FGD system is regarded as an ideal continuously stirred tank reactor, in order to use the empirical correlation for reaching steady state conditions. Hence, the minimal reaction time to achieve ideal constant concentrations at the outlet of the reactor at least five times the hydrodynamic residence time of the sump [21]. The consequence for the lab-scale FGD presented here, run at reference settings, is that ideal steady state is not reached before several days of constant operation. However, it has been observed that for the case of the labscale FGD, the time response of the ORP is a reliable indicator for distinguishing between dynamic and near steady state conditions. Therefore, stationary conditions presented in this work imply constant ORP for several hours, while the dynamic response of the FGD system is indicated by an unsteady trend of the ORP. Considering the common change in load and operating conditions of full-scale FGD units, these definitions ensure transferability of the conclusions from lab- to full-scale.
4. Results and discussion Overflow Liquid Analysis
4.1. Fundamental investigations on re-emissions of Hg0 The concentration of Hg in the exhaust air of the sump can be directly associated with Hg0 re-emissions, since reducing
B. Heidel et al. / Applied Energy 114 (2014) 485–491
Hg
15 10 5 0 0
1
2
3
4
5
Hg0 re-emission [µg/m³]
Hg0 re-emission [µg/m³]
20 15
30%
10
20%
5
10%
0
0% 30
Chloride concentration [g/l]
2SO SO 3 /S(IV)
Ratio SO32-/S(IV)
488
40 50 60 70 Temperature [°C]
80
Fig. 2. Impact of chloride concentration on Hg0 re-emission. Fig. 4. Hg0 re-emission and SO2 3 concentration as a function of temperature.
conditions in the slurry lead to the chemical reduction of previously absorbed HgCl2 and subsequent desorption of Hg0. In order to quantify the contribution of organic acids to Hg0 re-emissions, baseline experiments in absence of the additives were conducted under reference settings. Fig. 2 points out the importance of complex formation for the retention of Hg2+ in FGD-slurries. The experiment was conducted by continuously dosing KCl-solution to the slurry with initially low concentration of Cl at a temperature of 60 °C. The elevated temperature was chosen due to very low re-emissions for the reference setting of 40 °C. For increasing concentration of the ligand chloride, the ratio of uncomplexed Hg2+ decreases, which is equivalent to declining Hg0 re-emissions. The increase in chloride content leads to decreasing Hg0 formation. This observation is in-line with the theoretical share of uncomplexed Hg2+ in aqueous systems containing Hg2+ and Cl [13]. Further results of experimental investigations on the impact of the pH-value of the slurry are presented in Fig. 3. In the context of the scope of the work described in this paper, special emphasis has to be put on this parameter. It can be observed, that the concentration of Hg0 in the exhaust air is strongly dependent on the operational pH-value. A possible interpretation of this phenomenon is delivered by the thermodynamic equilibrium calculation of the ratio of SO2 3 /S(IV)total, visualized by the secondary axis. For pH > 5, both Hg0 re-emission rate and SO2 3 /S(IV)-ratio show similar behavior. This observation implies that the accelerated rate of Hg2+ reduction reaction is linked to the increased concentration of SO2 3 in the liquid phase (or the corresponding decreasing share 2 of HSO being the 3 ). This can be regarded as a reference to SO3 dominant reducing agent for the reduction of Hg2+. Detailed investigations on the rate of reaction were carried out by continuously increasing the temperature of the slurry. The
2-
100%
4
80%
3
60%
2
40%
1
20% 0%
0 4
5 6 pH-value
7
Fig. 3. Impact of pH-value on Hg0 re-emission.
Hg
Hg0 re-emission [µg/m³] ORP [mV]
5
ORP
SO2 SO 2
300
9000
250
7500
200
6000
150
4500
100
3000
50
1500
SO2 raw gas [mg/m³]
SO3 /S(IV)
Ratio SO32-/S(IV)
Hg0 re-emission [µg/m³]
Hg
results for reference settings with the aqueous concentration of Hg2+ = 100 lg/l and pH = 5.1 are illustrated by Fig. 4. The acceleration of the reaction rate for elevated temperatures is in accordance with the empirical Arrhenius equation. However, it may be worth stating that a secondary effect might contribute to the enhanced formation of Hg0. When calculating the acid constants of H2SO3 as functions of temperature, increased share of SO2 3 in connection with elevated Hg0 re-emissions can be observed by plotting the SO2 3 ratio for the actual temperature regime at constant pH-value of 5.1. Further experiments with focus on the impact of S(IV)concentration were conducted with the same slurry by successive increasing the SO2 concentration of the raw gas treated by the lab-scale FGD, presented in Fig. 5. Due to the higher driving force, mass transfer of SO2 is enhanced, resulting in a higher flux of S(IV)compounds into the liquid phase. For constant retention time in the sump at stationary L/G-ratio, this results in higher aqueous S(IV)-concentrations, detectable by the decrease of the ORP. The simultaneous sharp increase of Hg0 re-emissions and the drop of ORP after 25 min illustrated by Fig. 5 indicate the existence of a critical S(IV)-concentration in the context of Hg0 re-emissions. By applying a correlation method of the ORP and actually analyzed S(IV)-concentrations [22] on this data, this aspect is visualized more descriptively by the plot of five minute average values of re-emissions over S(IV)-concentrations by Fig. 6. It is obvious, that for S(IV)-concentrations higher than the critical value at the inflection point of the curve, the rate of reaction of the Hg2+ reduction is enhanced extensively and reaches a maximum. The exact values of both, critical concentration and height of the maximum are functions of the characteristics of the slurry (e.g. concentrations of Hg2+ and ligands, pH-value, redox-conditions, temperature).
0
0 0
20 40 Time [min]
60
Fig. 5. Impact of raw gas SO2 content on ORP and Hg0 re-emission.
489
Hg0 re-emission [µg/m³]
B. Heidel et al. / Applied Energy 114 (2014) 485–491
50
dependence of Hg2+ reduction on pH is qualitatively comparable for each case. Quantitative analysis reveal the reducing character of both formic and adipic acid, with formic acid being the much stronger reducing agent. It has to be stated, that due to the enhanced removal efficiency of SO2, FGDs utilizing such additives can be operated at reduced levels of pH. For the case of adipic acid, SO2 removal efficiency at pH = 4.3 (94%) is still superior to the reference case without additives at pH = 5.3 (88%), at only slightly elevated levels of Hg0 re-emissions. For the case of formic acid, there is no such appropriate compromise between SO2-removal and Hgretention, since Hg0-formation remains on a constant high level for pH < 4.5, indicating a differing reaction mechanism. Therefore, it can be concluded, that for all slurries investigated, one of the possible reaction pathways of Hg2+-reduction includes S(IV)-species (Eq. (5)). For the case of slurries containing formic acid, a second mechanism is proposed (Eq. (6)), where the electron donor is the formic acid itself, decomposing to CO2. For low pH-values, the Hg0-flux generated by the formate mechanism is the dominant one and not depending on S(IV)-concentration. The exponential dependence of Hg2+ reduction on temperature in the absence of additives was presented in Fig. 4, therefore subsequent experiments with slurries containing dissolved formic and adipic acid at identical molar concentrations (0.0174 mol/l) were conducted. The results presented in Fig. 8 show similar qualitative
Critical S(IV)
40 30 20 10 0 0
50 100 150 200 250 S(IV) [mmol/m³]
Fig. 6. Hg0 re-emission as a function of SO2 3 concentration.
4.2. Impact of organic additives on the re-emission of Hg0
Hg0 re-emission [µg/m³], ORP [mV]
In order to identify the impact of organic acids on the re-emission of Hg0, further investigations were conducted in presence of formic and adipic acid. The operating conditions of the test-rig and the chemical composition of synthetic flue gas and slurries, except of the addition of the actual organic acids were identical to the reference studies described in Section 4.1. Fig. 7 compares measured ORP profiles and the corresponding behavior of Hg in the lab-scale FGD for three different slurries at 40 °C with a chloride content of 3 g/l (reference set-up). The S(IV)-concentration of the slurries is considered to be comparable, since oxidation air flow rate (2 l/min), sump residence times and the individual removal rates of SO2 are similar (>80%). Slurry 1 is the reference slurry without organic acids. Slurry 2 contains 0.0174 mol/l adipic acid. For direct comparison, slurry 3 contains no adipic acid, but the same molar concentration of formic acid. The most important conclusion of the data presented is, that both organic acids affect the retention of dissolved HgCl2, promoting the re-emission of Hg0. Within the pH-range investigated, adipic acid enhances the formation of Hg0 approximately by one order of magnitude. In presence of formic acid, two orders of magnitude enhanced Hg0 re-emissions are detected, surpassing 500 lg/m3 (not presented due to scaling of Fig. 7) for pH higher than 4.9. These observations are in-line with the declining ORP-signals for increasing pH of the three slurries investigated, with high reemissions for low values of ORP. It can be concluded, that the
Hg: no additive Hg: Formic Acid
Hg0 re-emission [µg/m³]
500 400 300 200 100 0 40
50 60 70 Slurry temperature [°C]
Hg: no additive
Hg: Adipic Acid
Hg: Formic Acid
ORP: no additive
ORP: Adipic Acid
ORP: Formic Acid
250 200 150 100 50 0 4.2
4.4
80
Fig. 8. Impact of slurry temperature in presence of additives on re-emissions of Hg0.
300
4.0
Hg: Adipic Acid
4.6
4.8
5.0
5.2
5.4
pH-value Fig. 7. pH-dependence of Hg0 re-emissions and ORP of slurries containing organic acids.
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B. Heidel et al. / Applied Energy 114 (2014) 485–491
Table 1 Thermodynamic analysis of Hg2+-reduction. Slurry composition
Additive concentration [mol/l]
No organic acids Adipic acid Formic acid
0 0.0174 0.0174
Hg2þ ðaqÞ
ClðaqÞ
[lg/l]
[g/l]
DHà [kJ/ mol]
DSà [J/ mol/K]
90.8
3
99.2
51.8
104.0 63.5
3 3
97.2 108.0
37.8 22.6
tendencies for the slurries investigated. Quantitative comparison clearly states, that the magnitudes of Hg0 re-emissions are the highest in presence of formic acid, demonstrating the strong reducing character of this additive. For the case of adipic acid, the re-emission of Hg0 can be limited to comparably low levels, applying slurry temperatures below 50 °C. Results of the analytical analysis of the data in Fig. 8 using the Eyring equation [23] are summarized in Table 1. For the calculation, complex formation constants of the Hg2+/Cl system were used from [24], assuming that the complexes [HgCl3] and [HgCl4]2 are masked and thus not available as reactants of redox reactions. The obtained values for the activation enthalpies DHà are comparable for the slurry without additives and in presence of adipic acid. For the slurry containing formic acid, DHà differs considerably. This observation is even more evident when comparing the individual activation entropies DSà. The activation entropy of the reaction in presence of formic acid is at a significantly higher level than DSà of the other two reactions. For the purpose of identification of relevant reaction mechanisms involved, this is also a hint for an alternative, more rapid pathway of Hg0-formation in presence of formic acid. According to the transition state theory, high positive values of DSà indicate fast reactions, while a negative entropy is associated with slow kinetics [25]. This is in good agreement with the levels of Hg0 re-emissions observed in all of the experiments conducted. For the additive-free slurry, the importance of the availability of the ligand chloride for the formation of stable Hg2+-complexes was demonstrated by Fig. 2. In order to reveal the contribution of dissolved Cl to suppress Hg0 re-emissions in slurries containing the more promising additive adipic acid, additional experiments were conducted by dosing NaCl-solution to the liquid phase continuously. According to Fig. 9, increasing chloride content from 3 g/l to 16 g/l at a pH of 5.1 reduces the re-emission of Hg0 to less than 50% of the original value. The increase in chloride content can be visualized under these conditions by monitoring the corresponding ORP on the secondary axis. Overall, the stabilizing effect
ORP 250
15
225
10
200
5
175
ORP [mV]
Hg0 re-emission [µg/m³]
Hg 20
150
0 2
6
10
14
18
Chloride concentration [g/l] Fig. 9. Hg0 re-emissions for increasing chloride content in presence of adipic acid.
of Cl availability on re-emissions is obvious, whiles the actual Hg0-concentration in the exhaust air of the sump is always to some extent higher when the slurry contains dissolved adipic acid. 5. Conclusion In this study the chemical reduction and re-emission of mercury compounds from slurries of a lab-scale wet FGD system was investigated. Emphasis was put on the effect of formic and adipic acid, which are known additives for enhancing SO2 removal efficiency of full-scale FGDs. The obtained results allow an improved insight into the role of operational parameters and slurry composition on the stability of dissolved mercury compounds. The effect of S(IV)-concentration and -species on Hg chemistry was presented. SO2 3 -concentration was changed indirectly through the variation of absorbed SO2-mass flow and pH-value. For S(IV)-concentrations above a certain critical value, its potential as reducing agent leads to rapidly increasing formation and re-emission of Hg0. Increasing chloride concentration has been identified as an important factor for depressing Hg0 re-emissions for all conditions investigated. Lowering operational pH-value could always be correlated with decreasing Hg0 re-emissions. Both organic additives have a negative impact on Hg-retention in the wet FGD process, with formic acid being the significantly stronger reducing agent. Hg2+-complex decomposition and Hg0 re-emissions are exponentially depending on temperature for all slurries utilized. Two different pathways of Hg2+ reduction were identified by qualitative interpretation of the pH-dependence and by comparison of calculated activation enthalpies and activation entropies. While the first mechanism proposed involves SO2 3 as reducing agent and is therefore relevant for any FGD process, the second mechanism involves the formate anion, thus being exclusively relevant for FGDs utilizing formic acid as additive. However, due to greatly enhanced SO2-removal at low operating pH-values, the negative impact of adipic acid can be compensated by operating the FGD at lower pH, ensuring both increased SO2 removal efficiency and low levels of Hg0 re-emissions. Acknowledgement This work was kindly supported by EnBW Erneuerbare und Konventionelle Erzeugung AG. References [1] Pacyna, Elisabeth G, et al. Global anthropogenic mercury emission inventory for 2000. Atmos Environ 2006;40(22):4048–63. [2] Islas J, Grande G. Abatement costs of SO2-control options in the Mexican electric-power sector. Appl Energy 2008;85(2-3):80–94. [3] Wang Yun-jun et al. Comparison of mercury removal characteristic between fabric filter and electrostatic precipitators of coal-fired power plants. J Fuel Chem Technol 2008;36(1):23–9. [4] Ito Shigeo, Yokoyama Takahisa, Asakura Kazuo. Emissions of mercury and other trace elements from coal-fired power plants in Japan. Sci Total Environ 2006;368(1):397–402. [5] Chang, John CS, Behrooz Ghorishi S. Simulation and evaluation of elemental mercury concentration increase in flue gas across a wet scrubber. Environ Sci Technol 2003;37(24):5763–6. [6] Latscha, Hans Peter, Uli Kazmaier, Helmut Alfons Klein. Organische chemie chemie-basiswissen ii. London: Springer; 2008. [7] Wo Jingjing et al. Hg2+ reduction and re-emission from simulated wet flue gas desulfurization liquors. J Hazard Mater 2009;172:1106–10. [8] Wang Yunjun et al. Experimental study on the absorption behaviors of gas phase bivalent mercury in Ca-based wet flue gas desulfurization slurry system. J Hazard Mater 2010;183(1):902–7. [9] Sun Zhongwei et al. Experimental study on desulfurization efficiency and gas– liquid mass transfer in a new liquid-screen desulfurization system. Appl Energy 2010;87(5):1505–12. [10] Holleman, Arnold F., Egon Wiberg, and Nils Wiberg, eds. Lehrbuch der anorganischen Chemie. Walter de Gruyter, 1995. [11] Ho Tse-Lok. Hard soft acids bases (HSAB) principle and organic chemistry. Chem Rev 1975;75(1):1–20.
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