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Mechanism of mercury vapor release from flue gas desulfurization gypsum
FUPROC-05179; No of Pages 6 Fuel Processing Technology xxx (2016) xxx–xxx
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Mechanism of mercury vapor release from flue gas desulfurization gypsum Zhen-Wu Zhu, Yu-Qun Zhuo ⁎, Ya-Ming Fan, Zhi-Peng Wang Key Laboratory for Thermal Science and Power Engineering, Ministry of Education, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 9 May 2016 Received in revised form 21 September 2016 Accepted 10 October 2016 Available online xxxx Keywords: Mercury vapor release FGD gypsum Temperature UV radiation Moisture content
a b s t r a c t Wet flue gas desulfurization (FGD) systems designed to remove sulfur dioxide can capture soluble oxidized mercury which then lead to mercury accumulation in FGD gypsum. In this study, the mercury species in three FGD gypsum samples were first determined using a temperature programmed decomposition method. The mercury species in the FGD gypsum was mainly HgS. Mercury vapor release to the atmosphere from FGD gypsum was then investigated. The effects of temperature, UV radiation and solid moisture content on mercury release were studied in controlled laboratory experiments. Detectable amounts of mercury were constantly emitted from three samples. The mercury release rates in the dark at room temperature were found to vary from 0.31% to 2.65% during the 180-day period tests. The temperature, UV radiation and solid moisture content all significantly enhanced the mercury release from the FGD gypsum with synergistic enhancements existing between the temperature/UV radiation and the solid moisture content. The promotion of oxidized mercury reduction and Hg0 desorption and diffusion might be the reasons for the enhancement of mercury release by these factors. Considering the temperature, UV radiation and solid moisture content fluctuations of FGD gypsum when exposed to natural environment, the mercury release from FGD gypsum disposed outdoor might be significant. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Mercury (Hg) is highly toxic and can be enriched through food chain which can lead to various diseases. Elemental mercury (Hg0) can enter into global atmospheric circulation and be transported over large distances before being deposited to soils and surface waters. This makes Hg emission to be a global environmental issue that is attracting more and more attention. Coal-fired power plants have been shown to be one of the largest anthropogenic point sources of atmospheric mercury [1,2]. China is the largest coal consumption country in the world and the total Hg emission from coal-fired power plants in China was about 132 tons in 2007 [3]. Wet flue gas desulfurization (FGD) systems, which are designed to control the release of sulfur dioxide, have been demonstrated to effectively capture soluble oxidized mercury (Hg2+) compounds [4,5]. The Hg captured is mainly retained in the FGD gypsum byproduct after dehydration [6,7]. Consequently, a considerable amount of Hg is bound to the FGD gypsum. This is especially true after halogen addition technology has been applied which will promote the oxidization of Hg0 in the flue gas [8]. Hao et al. [9] studied FGD gypsum samples from 70 coal-fired power plants in 20 provinces in China and the results showed that the Hg concentrations in FGD gypsum ranged from not detectable to 4330 ±620 μg/kg. Now FGD gypsum is not classified as hazardous ⁎ Corresponding author. E-mail address: [email protected] (Y.-Q. Zhuo).
waste in China and many other countries. There is no strict control on the transportation, reuse and storage of FGD gypsum. FGD gypsum has been regarded as an alternate of natural gypsum and been applied for several beneficial purposes both in China and many other countries. According to the American Coal Ash Association annual report, the production of FGD gypsum was about 24,400,000 tons in 2013 in American with 36% of the total production used in encapsulated applications such as cement, concrete and wallboard production, while 12.8% was used in non-encapsulated applications such as structural fills/embankments, agricultural, soil stabilization and mining reclamation [10]. And the remaining 52% was disposed as waste material in industrial landfills. About 52 million tons of FGD gypsum was produced in 2012 in China with 56% being used in beneficial applications such as wallboard production, agricultural soil reclamation and cement additives [9]. The remaining FGD gypsum would mainly end up in landfills or outdoor storage due to the lack of downstream industrial users. And this is especially common in less developed area of China. The Hg reactions in gypsum slurries in wet FGD systems are rather complicated with some of the absorbed Hg2+ in the slurry reduced by the sulfite ions and release as Hg0 [11,12]. Previous studies on soil have shown that Hg can cycle between adsorption and desorption among air-solid interfaces and background soil has been regarded as an important source to the global Hg cycle [13]. Hg released from the soil is mainly in form as Hg0 through complex physical, chemical and biological processes [14]. The organic matter in soil plays an important role in both abiotic and biotic processes that control the Hg release
http://dx.doi.org/10.1016/j.fuproc.2016.10.015 0378-3820/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
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Z.-W. Zhu et al. / Fuel Processing Technology xxx (2016) xxx–xxx
from soil [15]. Temperature, UV radiation and soil moisture content have also been found to be important parameters that influence Hg release from soil [16–19]. Concentrations of Hg may be higher in FGD gypsum compared to natural soil. Similarly, Hg retained in the FGD gypsum may also have the potential to release as Hg0. Some research works have revealed the volatilization of Hg0 from FGD gypsum [20–22]. However, the mechanisms of the Hg vapor release from FGD gypsum remain unclear. Unlike soil, Hg in FGD gypsum is mainly in the form as inorganic mercury species and FGD gypsum nearly contains no organic matters which have high affinity with mercury compounds and participate in the reactions related to Hg release from soil. The different material chemistry and Hg species in FGD gypsum suggest that the Hg vapor release from FGD gypsum may be different from that in soil. FGD gypsum would be in contact with the natural environment in the non-encapsulated applications such as agricultural practice and soil stabilization. The FGD gypsum applied for encapsulated applications such as wallboard and cement may also be affected by the changes of temperature and humidity of natural environment. And the large amounts of FGD gypsum that ended up in landfills or outdoor storage without any beneficial applications would also be exposed to outdoor environment. Because of current management scenarios of FGD gypsum, a large proportion of FGD gypsum would have the opportunity to be exposed to natural environment. The temperature, UV radiation and solid moisture content might also affect the Hg release from FGD gypsum as that occurs in soil. Therefore, it is necessary to study the influence of these natural environmental conditions on Hg release from FGD gypsum. Considering the huge amounts of FGD gypsum generated every year in China and many other countries, the Hg release from FGD gypsum may not be negligible. The emphasis of this study was to investigate the Hg vapor release characteristics from FGD gypsum. The effects of temperature, UV radiation and solid moisture content on the Hg release were studied in laboratory experiments. The Hg speciation in FGD gypsum was analyzed to evaluate the Hg release mechanism with the aim to comprehensively address the physicochemical processes controlling the Hg release from FGD gypsum. 2. Materials and methods 2.1. Sampling and characterization Samples of FGD gypsum labeled as CQ, LYG and EZ were obtained from three different coal-fired power plants in China. The samples were collected from fresh FGD gypsum and stored in sealed glass bottles at 4 °C until testing. The moisture contents of CQ, LYG and EZ samples were 13.68%, 8.95% and 25.55%, respectively. As listed in Table 1, the coal types of the three coal-fired power plants included bituminous, lignite and anthracite, representing all the major coal types burned in Chinese power plants. All the tested power plants were equipped with selective catalytic reduction (SCR), electrostatic precipitator (ESP) and wet FGD system to control the emissions of NOx, particulate matter and SO2. The Hg contents in the samples were determined using a Lumex RA915M + PYRO-915 (Lumex, Russia) which can achieve direct measurement of Hg in solid matrix. The contents of the other elements were determined by inductively coupled plasma mass spectrometry (ICP-MS, ThermoFisher X series II, German) after microwave digestion. The whole procedure followed US EPA Method 3052. X-ray diffraction
(XRD, D/Max-2500 pc, Rigaku Inc., Japan) analysis of air-dried samples was used to identify the basic crystalline phase. The structure and morphology of each sample was also examined by scanning electron microscopy (SEM, S-5500, Hitachi, Japan). 2.2. Mercury release measurement The Hg release from FGD gypsum was measured in the system as shown in Fig. 1. 40 g of FGD gypsum sample (as received) was placed in a clean quartz glass chamber of 1000 ml. Ambient air scrubbed of ambient mercury by activated carbon was slowly introduced into the chamber at the rate of l l/min. Any Hg release from the FGD gypsum was then carried out of the chamber and captured by the activated carbon trap at the exit. The Hg content in the activated carbon trap was then measured by Lumex 915M + PYRO-915 to determine the Hg release rate from FGD gypsum. The Hg release rate was the ratio of Hg release content to the total Hg content in the tested FGD gypsum samples. To investigate the impact of temperature and UV radiation, the Hg release experiments were conducted in parallel at three different conditions. The first batch of samples was placed in the dark without UV light at room temperature (maintained 25 ± 3 °C during the 180-day period of experiment). The second batch of samples was placed in light at room temperature to study the influence of UV light. The light was generated by a UV tube (340 nm, 40 W) which could mimic the ultraviolet region of solar radiation. The third batch of samples was placed in the dark in an oven with the temperature maintained at 50 °C to study the influence of temperature. The activated carbon traps were replaced and had their Hg contents measured in every 30 days, while the Hg release content was also recorded in every 2 days during the initial 10 days of the experiment. The release experiment lasted for 180 days to study the long-term release characteristics of Hg. The impact of solid moisture content was investigated by adding 5 ml deionized water to every sample homogeneously each time after the 180 days of continuous observation. The water additions were conducted three times. The experiments about the impact of solid moisture content lasted for 20 days and the Hg release content was recorded in every 2 days. The content of Hg release in the first 2 days before water addition was measured at the beginning. Then 5 ml water was added and the Hg release was measured along with the change of solid moisture content determined by weighing the chamber every 2 days in the following 8 days, so as to mimic the wetting and drying process of FGD gypsum in natural environment. The water was then added at the end of 10th day and 14th day in the experiments to confirm the effect of water addition and also to study the Hg release characteristic in the successive wetting events. All the experiments were conducted in triplicate to verify the data reliability with the mean values used. 3. Results and discussion 3.1. Characterization of FGD gypsum The mineralogy characterization of the FGD gypsum samples were studied by XRD analysis. As shown in Fig. 2, calcium sulfate dehydrate was the predominant crystalline phase in all the samples. This meant that most of the calcium sulfite (CaSO3) had been transformed to calcium sulfate (CaSO4) by the forced-oxidation process in the wet FGD system. There were also minor amounts of calcite and quartz in the
Table 1 General information of the tested power plants. Sample
Coal type
Air pollution control devices
Installed capacity (MW)
CQ LYG EZ
Lignite Bituminous Anthracite
SCR + ESP + wet FGD SCR + ESP + wet FGD SCR + ESP + wet FGD
300 1000 330
Fig. 1. Mercury release measurement system.
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
Z.-W. Zhu et al. / Fuel Processing Technology xxx (2016) xxx–xxx G G
G
Intensity
G
CQ
LYG EZ C=Calcite G=Gypsum Q=Quartz
G G
Q
0
10
20
G
30
GG Q
G
G
40 50 2θ (º)
C
60
C
C
70
80
90
Fig. 2. XRD patterns of the FGD gypsum samples.
samples, which might be impurities in the limestone or fly ash captured from the flue gas. However, no Hg species were detected by XRD since the Hg concentrations in the FGD gypsum samples were much lower than the detection limit of XRD. The particle morphology of the FGD gypsum samples were determined by SEM analysis. The results in Fig. 3 shown that the samples were mainly monoclinic crystalline and the EDX results showed that the main content was CaSO4. There were also some spheres in the LYG and EZ samples. The EDX results suggested that the spheres were mainly aluminosilicate and iron oxide, indicating that the spheres were the fly ash particles captured by the wet FGD system. The concentrations of major and trace elements in the samples (as received) were determined by ICP-MS. As listed in Table 2, Ca and S were the major elements in all the samples. There were also considerable amounts of Al, Fe and Si in the samples, which might come from the fly ash escaped from ESP. This was consistent with the result of SEM-EDX. As for the trace elements, the concentrations of As, Cr, Mn, Ni, Se and Pb were comparatively high. The Hg concentrations of the FGD gypsum samples were in the order of CQ (0.75 μg/g) b LYG (1.87 μg/g) b EZ (3.27 μg/g). Many factors could affect the Hg concentration in FGD gypsum, such as coal types, different air pollution control devices and different operating conditions. 3.2. Mercury speciation in FGD gypsum Based on the different thermal decomposition/desorption temperatures of Hg compounds, it is able to determine Hg species in solid matrix samples by temperature programmed decomposition (TPD) technique [23–25]. Previous studies have shown no organic mercury in the flue gas of coal combustion so the Hg compounds in FGD gypsum are expected to be HgCl2, HgO, HgSO4 and other inorganic species [26]. In our previous study, the thermal induced Hg release curves of pure mercury compounds (e.g. HgCl2, Hg2Cl2, black HgS, red HgS, HgO, HgSO4, and
3
Table 2 Elemental compositions of the FGD gypsum samples.
Al Ca Fe K Mg Na Sr Ti S Si As Be Cd Co Cr Hg Mn Ni Sb Se Pb
CQ Wt.%
LYG
0.492 ± 0.005 19.71 ± 0.11 0.423 ± 0.003 0.308 ± 0.006 0.306 ± 0.002 0.094 ± 0.001 0.078 ± 0.001 0.012 ± 0.001 15.18 ± 0.08 1.103 ± 0.006 mg/kg 2.72 ± 0.10 0.18 ± 0.02 0.08 ± 0.01 1.91 ± 0.03 9.88 ± 0.28 0.75 ± 0.03 79.01 ± 2.38 5.15 ± 0.18 0.015 ± 0.003 6.10 ± 0.18 1.36 ± 0.07
0.186 20.95 0.230 0.137 0.614 0.057 0.020 0.005 14.91 0.615
EZ ± ± ± ± ± ± ± ± ± ±
0.002 0.10 0.002 0.002 0.005 0.001 0.001 0.001 0.18 0.005
3.23 ± 0.17 0.13 ± 0.01 0.06 ± 0.01 0.91 ± 0.01 5.81 ± 0.50 1.87 ± 0.04 149.53 ± 0.91 2.43 ± 0.24 0.041 ± 0.002 8.17 ± 0.05 6.57 ± 0.21
0.295 17.42 0.173 0.069 0.391 0.035 0.022 0.006 12.92 0.627
± ± ± ± ± ± ± ± ± ±
0.004 0.18 0.002 0.001 0.006 0.001 0.001 0.001 0.10 0.007
5.04 ± 0.15 0.16 ± 0.01 0.45 ± 0.09 0.79 ± 0.01 11.02 ± 0.74 3.27 ± 0.09 83.97 ± 1.87 3.44 ± 0.06 0.174 ± 0.006 20.19 ± 0.16 4.47 ± 0.21
Hg2SO4) were obtained to identify the Hg compounds in FGD gypsum. The detailed speciation of Hg in FGD gypsum samples had been published elsewhere [27]. By comparing the thermal Hg release curves of the FGD gypsum samples with the pure mercury compounds, the Hg species in FGD gypsum samples could be determined. As listed in Table 3, HgS was the primary species in all the samples. This might be caused by the precipitation reaction of Hg2+ and S2− formed by the disproportionation of sulfite in the gypsum slurry [28]. 3.3. Mercury release from FGD gypsum 3.3.1. Mercury release in the dark at room temperature The Hg release from 40 g FGD gypsum sample in the dark at room temperature was recorded every 30 days in the continuous 180-day period. The temperature was maintained at about 25 °C. As listed in Table 4, the total Hg release content from CQ over the 180-day test was 2.356 ng/g FGD gypsum, while LYG and EZ was 9.021 and 86.50 ng/g FGD gypsum, respectively. The Hg release rate in the 180day test was 0.31%, 0.48% and 2.65% for CQ, LYG and EZ, respectively. Detectable amounts of Hg were constantly released from all the FGD gypsum samples. The Hg species in all the samples were mainly HgS with some HgCl2 and Hg2Cl2. No Hg0 was identified in the thermodesorption experiments in our previous study [27]. The Hg release could be due to the reduction of Hg2 + and the generation of Hg0 in FGD gypsum. Hg0 would then be desorbed from the gypsum particles and diffuse into the atmosphere. Gustin et al. [29] and Engle at al. [30] had found Hg release from both synthetic HgS in laboratory and HgS ores in situ. The release of Hg0 from HgS might occur because that HgS is thermodynamically stable under high temperatures and reducing
Fig. 3. SEM micrographs of the FGD gypsum samples.
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
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Table 3 Mercury species in the FGD gypsum samples.
Table 5 Mercury release content from FGD gypsum in the dark at 50 °C (ng/g FGD gypsum).
Sample
% Black HgS
% Red HgS
% Other
CQ LYG EZ
73.0 ± 0.5 88.5 ± 2.3 69.8 ± 3.3
9.0 ± 2.1 11.5 ± 2.3 24.0 ± 2.5
18.0 ± 2.5 HgCl2
CQ
6.2 ± 0.8 Hg2Cl2
conditions and might degrade to Hg0 at relatively low temperatures and oxidizing conditions [29]. The Hg release was in the order of CQ b LYG b EZ, which was the same order as the Hg concentrations in the FGD gypsum samples with much more Hg released from EZ than from CQ or LYG. The decomposition of Hg2Cl2 to form Hg0 in EZ might be one of the contributors. Although the total Hg release content increased with the Hg concentration in the FGD gypsum, there seemed to be no obvious functional relationship between them. This indicated that the Hg release from FGD gypsum was a rather complicated process. Besides the Hg concentration in FGD gypsum, many other intrinsic characteristics, such as Hg species, Hg bond modes, and mineralogy characterization of FGD gypsum might affect the Hg release process somehow. 3.3.2. Effect of temperature on mercury release As listed in Table 5, the total Hg release contents in 180-day period at 50 °C in the dark were 12.79, 125.3 and 352.3 ng/g FGD gypsum for CQ, LYG and EZ, respectively. And the Hg release rates of CQ, LYG and EZ were 1.71%, 6.7% and 10.77%. The elevated temperature significantly increased the Hg release from FGD gypsum which was 3.1–12.9 times greater than that at room temperature in the dark. Since the Hg in the FGD gypsum was in the form as Hg2+ and was unlikely to volatilize directly, the Hg release from FGD gypsum was most likely due to the reduction of Hg2+. The higher temperature would increase the reaction rate of Hg2 + reduction and promote the production of Hg0. Furthermore, the higher temperature would also promote the desorption and diffusion of Hg0 among FGD gypsum particles, which would eventually enhance the Hg0 mass transport across the FGD gypsum particle-air boundary and escape into the atmosphere. The experiments to simulate the wallboard production process in which FGD gypsum samples were heated to 160 °C from room temperature and then maintained at 160 °C for 1 h also showed a significant increase on the Hg release at elevated temperature. The release rates of Hg in the approximate 2 h duration experiment could reach up to 7.17%, 13.15% and 14.27% for CQ, LYG and EZ, respectively.
30 days 60 days 90 days 120 days 150 days 180 days Total
LYG
4.803 1.583 1.985 2.153 1.295 0.975 12.79
± 0.263 ± 0.085 ± 0.189 ± 0.185 ± 0.180 ± 0.141 ± 1.04
EZ
38.10 20.58 21.54 14.93 14.67 15.44 125.3
± 3.16 ± 1.37 ± 2.42 ± 0.10 ± 1.34 ± 1.06 ± 2.9
105.2 36.10 38.32 44.87 65.51 62.32 352.3
± 7.0 ± 2.54 ± 3.41 ± 0.71 ± 6.05 ± 7.36 ± 13.0
samples changed very little compared with those in the dark. The Hg release enhancement by UV radiation was independent of the solid temperature. Mercury is known to efficiently absorb light energy at the wave length of 315 nm. The absorption of photo energy by Hg0 atoms would reduce the apparent activation energy for Hg0 desorption [31]. This would facilitate the Hg0 release from FGD gypsum particles. Furthermore, the UV radiation also promotes the reduction of Hg2 + in FGD gypsum, which would then increase the available pool of Hg0 and its subsequent release from FGD gypsum. The main Hg species in all the FGD gypsum samples was HgS. Zhang [32] found that the decomposition of HgS occurred to release H2S and Hg2+ with incident light (λ N 300 nm), and Hg2+ could undergo in situ reductive deposition on HgS to form Hg0. Radepont et al. [33] found that red HgS would experience a multistep degradation process when exposed to UV radiation. When chloride existed, red HgS could be transformed to Hg2Cl2 (Eq. (1)) [33]. And Hg2Cl2 would decompose to HgCl2 and release Hg0 when exposed to UV radiation (Eq. (2)). This would also increase the release of Hg when the FGD samples were exposed to UV radiation. α−HgS↔γ−Hg3 S2 Cl2 ↔α−Hg3 S2 Cl2 ↔Hg2 Cl2
ð1Þ
Hg2 Cl2 →Hg0 þ HgCl2
ð2Þ
3.3.3. Effect of UV radiation on mercury release A series of samples were exposed to UV light (340 nm, 40 W) with the other conditions the same as for the samples in the dark at room temperature (about 25 °C). As listed in Table 6, the total Hg release contents in 180-day period were 7.224, 41.22 and 117.6 ng/g FGD gypsum for CQ, LYG and EZ, respectively. And the release rates of Hg for CQ, LYG and EZ were 0.96%, 2.2% and 3.6%, which were 0.4–3.6 times greater than that without UV radiation at the same conditions. Since the heat generation from the UV tube was negligible and the UV tube was placed about 5 cm from the glass chamber, the temperatures of FGD gypsum
3.3.4. Effect of water addition on mercury release The moisture content was found to affect the Hg release from soil [18,19]. As shown in Fig. 4, the Hg release in the following 2 days after first water addition was 4.3 to 118 times of that before water addition. The Hg release then decreased with the decreasing moisture content in the FGD gypsum. By monitoring the weights change of the FGD gypsum samples after water addition, it was found that the 5 ml water added to the gypsum had all evaporated out in 2 days after the addition. The moisture contents of the samples after 4 days in the record were about the same as that before water addition. However, the Hg release from the samples after 4 days was still greater than that before water addition. The Hg release returned to the level before water addition until 10th day in the record. The water addition at the end of 10th day and 14th day also significant promoted the Hg release from FGD gypsum and the extents of the promotion of these two water additions were almost the same as the first addition. This confirmed the promotion effect of solid moisture content. It also indicated that Hg release would be promoted with repeated wetting events, which means that
Table 4 Mercury release content from FGD gypsum in the dark at room temperature (25 °C) (ng/g FGD gypsum).
Table 6 Mercury release content from FGD gypsum in UV radiation at room temperature (25 °C) (ng/g FGD gypsum).
CQ 30 days 60 days 90 days 120 days 150 days 180 days Total
1.263 0.025 0.050 0.195 0.358 0.465 2.356
LYG ± ± ± ± ± ± ±
0.133 0.005 0.009 0.018 0.055 0.041 0.196
2.775 0.404 0.554 1.163 1.755 2.370 9.021
EZ ± ± ± ± ± ± ±
0.088 0.042 0.056 0.055 0.163 0.249 0.476
16.43 5.140 9.183 14.64 15.31 25.80 86.50
CQ ± 1.06 ± 0.700 ± 1.290 ± 0.03 ± 0.85 ± 1.18 ± 1.05
30 days 60 days 90 days 120 days 150 days 180 days Total
2.038 0.400 0.565 1.023 1.225 1.973 7.224
LYG ± ± ± ± ± ± ±
0.179 0.032 0.085 0.072 0.099 0.246 0.712
6.368 3.330 4.145 7.399 8.095 11.88 41.22
EZ ± 0.138 ± 0.244 ± 0.491 ± 0.550 ± 0.817 ± 0.85 ± 3.09
18.93 9.235 11.76 20.90 23.71 33.04 117.6
± 1.06 ± 0.375 ± 1.03 ± 2.26 ± 2.00 ± 0.45 ± 7.2
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
in the dark at 25 ºC UV radiation at 25 ºC in the dark at 50 ºC
3
2
1
watered
0 2
6
8
10 12 Days
14
16
18
20
in the dark at 25 ºC UV radiation at 25 ºC in the dark at 50 ºC
LYG
6 5
watered 4
watered
Hg release (ng/g FGD gypsum)
7
4
3
watered
2 1 0 2
4
6
8
10 12 Days
14
5
it could be available to be reduced and released from the surface. Studies have shown that the oxidative dissolution of HgS did exist and Hg(OH)2(aq) would be formed via Eq. (3) [34,35]. Hg(OH)2 could then be reduced to Hg0 which would also increase the Hg0 pool for release [36]. þ HgSðsÞ þ 2O2 ðaqÞ þ 2H2 O↔HgðOHÞ2 ðaqÞ þ SO2− 4 þ 2H
watered
CQ
watered
Hg release (ng/g FGD gypsum)
Z.-W. Zhu et al. / Fuel Processing Technology xxx (2016) xxx–xxx
16
18
20
ð3Þ
By comparing the Hg release from the same sample under different conditions after water addition in Fig. 4, it was obvious that the enhancements of Hg release from wet FGD gypsum with incident UV radiation or at elevated temperature were greater than that from the same samples in the dark or at room temperature. It indicated that there was a synergistic effect between solid moisture content and UV radiation/ temperature. Furthermore, the Hg release from the samples initially decreased and then increased with time during the 180-day period at room temperature as seen in Table 4 and Table 6. The experiments were conducted in Beijing of China from January to July. The room temperature in laboratory varied little and remained at approximately 25 °C in the entire 180-day period. It had been noticed that the air humidity increased from 16.9% to 75% during this period. The Hg release initially decreased as the original water evaporated in the FGD gypsum, which could be seen from Hg release in the initial 10 days of the 180-day period as shown in Fig. 5. Then the sample would absorb moisture from air with the increase of humidity and the Hg release increased accordingly. Although Hg release from FGD gypsum in the dark at room temperature was not very high, the temperature, UV radiation and solid moisture all could significantly increase the overall Hg release. This observation implied that, for the FGD gypsum stacked in open fields without any protection from sun radiation or rainfall, considerable amount of Hg it captured from flue gas in power plant could be eventually re-emitted into the atmosphere. 4. Conclusion
20
in the dark at 25 ºC UV radiation at 25 ºC in the dark at 50 ºC
EZ
18 16 14 12 10 8
watered
watered
watered
6
Wet FGD system can effectively capture oxidized mercury in flue gas which leads to Hg accumulation in FGD gypsum. The management scenarios of FGD gypsum give the opportunity for Hg to release from FGD gypsum to atmosphere. This may be non-negligible because of the huge amounts of FGD gypsum generated every year in the world. In this study, the measurement of Hg release from three FGD gypsum samples were conducted in controlled laboratory experiments. Detectable amounts of Hg were constantly emitted from all the samples. The Hg species in the FGD gypsum samples was mainly HgS with the Hg release due to the reduction of oxidized mercury in the samples. The Hg release
4
3.5
2 2
4
6
8
10 12 Days
14
16
18
20
Fig. 4. Hg release before and after water addition.
the Hg release from FGD gypsum storage outdoor would be enhanced by every rainfall in the environment. Water molecule is polar and could compete with Hg0 for binding sites on the surfaces of solid particles. Hg0 adsorbed on solid particles would be desorbed and become free Hg0 in the pores when water was added, so as to increase the Hg0 pool available for release. With water evaporation in the meantime, free Hg0 in the pores would be carried to the surface of solid by capillary effects and then released to the atmosphere in the end [18]. Moreover, soluble mercury compounds in the FGD gypsum samples such as HgCl2 would dissolve in water and be transported with the water evaporation to the particle surface where
Hg release (ng/g FGD gypsum)
Hg release (ng/g FGD gypsum)
22
in dark at 25 ºC UV radiation at 25 ºC
LYG
3.0 2.5 2.0 1.5 1.0 0.5 0.0 2
4
6 Days
8
10
Fig. 5. Mercury release in the initial 10 days of 180-day period, taking sample LYG as example.
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
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content from FGD gypsum in the dark at room temperature was quite low, with the Hg release rates in the 180-day period varying from 0.31% to 2.65% for the three samples. Temperature, UV radiation and solid moisture content all could enhance the Hg release from FGD gypsum. The Hg release rates at elevated temperature (50 °C) in 180-day period were 1.71% to 10.77% for the three samples. The Hg release rates with incident UV radiation were about 0.96% to 3.60% for the three samples during the 180-day period. The addition of 5 ml water after 180-day period could increase Hg release to 4.3 to 118 times of that without water addition. And the Hg release could be promoted with repeated wetting events. The enhancement of Hg release by these factors might be due to the increase of oxidized mercury reduction in FGD gypsum and desorption and diffusion of Hg0 among gypsum particles. The amount of Hg release in the dark and low temperature might not be very large for FGD gypsum stacked outdoors. However, when the FGD gypsum was exposed to solar radiation or experienced precipitation events, the UV radiation and temperature increase due to solar radiation and the water involved would significantly enhanced the Hg release. Considering the huge amounts of FGD gypsum generated every year in China and many other countries, more detailed research is required in future to fully understand its contribution to global Hg circulation. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51376109). References [1] N. Pirrone, G.J. Keeler, J.O. Nriagu, Regional differences in worldwide emissions of mercury to the atmosphere, Atmos. Environ. 30 (17) (1996) 2981–2987. [2] E.G. Pacyna, J.M. Pacyna, K. Sundseth, J. Munthe, K. Kindbom, S. Wilson, F. Steenhuisen, P. Maxson, Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and projections to 2020, Atmos. Environ. 44 (20) (2010) 2487–2499. [3] H.Z. Tian, Y. Wang, Z.G. Xue, Y.P. Qu, F.H. Chai, J.M. Hao, Atmospheric emissions estimation of Hg, As, and Se from coal-fired power plants in China, 2007, Sci. Total Environ. 409 (16) (2011) 3078–3081. [4] J.H. Pavlish, E.A. Sondreal, M.D. Mann, E.S. Olson, K.C. Galbreath, D.L. Laudal, S.A. Benson, Status review of mercury control options for coal-fired power plants, Fuel Process. Technol. 82 (2) (2003) 89–165. [5] L. Zhang, Y.Q. Zhuo, L. Chen, X.C. Xu, C.H. Chen, Mercury emissions from six coalfired power plants in China, Fuel Process. Technol. 89 (11) (2008) 1033–1040. [6] C.M. Cheng, P. Hack, P. Chu, Y.N. Chang, T.Y. Lin, C.S. Ko, P.H. Chiang, C.C. He, Y.M. Lai, W.P. Pan, Partitioning of mercury, arsenic, selenium, boron, and chloride in a fullscale coal combustion process equipped with selective catalytic reduction, electrostatic precipitation, and flue gas desulfurization systems, Energy Fuel 23 (10) (2009) 4805–4816. [7] Z.W. Zhu, Y.Q. Zhuo, Z.Y. An, W. Du, C.H. Chen, Trace element distribution during wet flue gas desulphurization system, Journal of Tsinghua University (Science and Technology) 3 (2013) 330–335 in Chinese. [8] Y. Cao, Z.Y. Gao, J.S. Zhu, Q.H. Wang, Y.J. Huang, C.C. Chiu, B. Parker, P. Chu, W.P. Pan, Impacts of halogen additions on mercury oxidation, in a slipstream selective catalyst reduction (SCR), reactor when burning sub-bituminous coal, Environ. Sci. Technol. 42 (1) (2007) 256–261. [9] Y. Hao, S.M. Wu, Y. Pan, Q. Li, J.Z. Zhou, Y.B. Xu, G.R. Qian, Characterization and leaching toxicities of mercury in flue gas desulfurization gypsum from coal-fired power plants in China, Fuel 177 (2016) 157–163. [10] N.H. Koralegedara, S.R. Al-Abed, M.K.J. Arambewela, D.D. Dionysiou, Impact of leaching conditions on constituents release from Flue Gas Desulfurization Gypsum (FGDG) and FGDG-soil mixture, J. Hazard. Mater. (2016) in press.
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Mechanism of mercury vapor release from flue gas desulfurization gypsum Zhen-Wu Zhu, Yu-Qun Zhuo ⁎, Ya-Ming Fan, Zhi-Peng Wang Key Laboratory for Thermal Science and Power Engineering, Ministry of Education, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 9 May 2016 Received in revised form 21 September 2016 Accepted 10 October 2016 Available online xxxx Keywords: Mercury vapor release FGD gypsum Temperature UV radiation Moisture content
a b s t r a c t Wet flue gas desulfurization (FGD) systems designed to remove sulfur dioxide can capture soluble oxidized mercury which then lead to mercury accumulation in FGD gypsum. In this study, the mercury species in three FGD gypsum samples were first determined using a temperature programmed decomposition method. The mercury species in the FGD gypsum was mainly HgS. Mercury vapor release to the atmosphere from FGD gypsum was then investigated. The effects of temperature, UV radiation and solid moisture content on mercury release were studied in controlled laboratory experiments. Detectable amounts of mercury were constantly emitted from three samples. The mercury release rates in the dark at room temperature were found to vary from 0.31% to 2.65% during the 180-day period tests. The temperature, UV radiation and solid moisture content all significantly enhanced the mercury release from the FGD gypsum with synergistic enhancements existing between the temperature/UV radiation and the solid moisture content. The promotion of oxidized mercury reduction and Hg0 desorption and diffusion might be the reasons for the enhancement of mercury release by these factors. Considering the temperature, UV radiation and solid moisture content fluctuations of FGD gypsum when exposed to natural environment, the mercury release from FGD gypsum disposed outdoor might be significant. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Mercury (Hg) is highly toxic and can be enriched through food chain which can lead to various diseases. Elemental mercury (Hg0) can enter into global atmospheric circulation and be transported over large distances before being deposited to soils and surface waters. This makes Hg emission to be a global environmental issue that is attracting more and more attention. Coal-fired power plants have been shown to be one of the largest anthropogenic point sources of atmospheric mercury [1,2]. China is the largest coal consumption country in the world and the total Hg emission from coal-fired power plants in China was about 132 tons in 2007 [3]. Wet flue gas desulfurization (FGD) systems, which are designed to control the release of sulfur dioxide, have been demonstrated to effectively capture soluble oxidized mercury (Hg2+) compounds [4,5]. The Hg captured is mainly retained in the FGD gypsum byproduct after dehydration [6,7]. Consequently, a considerable amount of Hg is bound to the FGD gypsum. This is especially true after halogen addition technology has been applied which will promote the oxidization of Hg0 in the flue gas [8]. Hao et al. [9] studied FGD gypsum samples from 70 coal-fired power plants in 20 provinces in China and the results showed that the Hg concentrations in FGD gypsum ranged from not detectable to 4330 ±620 μg/kg. Now FGD gypsum is not classified as hazardous ⁎ Corresponding author. E-mail address: [email protected] (Y.-Q. Zhuo).
waste in China and many other countries. There is no strict control on the transportation, reuse and storage of FGD gypsum. FGD gypsum has been regarded as an alternate of natural gypsum and been applied for several beneficial purposes both in China and many other countries. According to the American Coal Ash Association annual report, the production of FGD gypsum was about 24,400,000 tons in 2013 in American with 36% of the total production used in encapsulated applications such as cement, concrete and wallboard production, while 12.8% was used in non-encapsulated applications such as structural fills/embankments, agricultural, soil stabilization and mining reclamation [10]. And the remaining 52% was disposed as waste material in industrial landfills. About 52 million tons of FGD gypsum was produced in 2012 in China with 56% being used in beneficial applications such as wallboard production, agricultural soil reclamation and cement additives [9]. The remaining FGD gypsum would mainly end up in landfills or outdoor storage due to the lack of downstream industrial users. And this is especially common in less developed area of China. The Hg reactions in gypsum slurries in wet FGD systems are rather complicated with some of the absorbed Hg2+ in the slurry reduced by the sulfite ions and release as Hg0 [11,12]. Previous studies on soil have shown that Hg can cycle between adsorption and desorption among air-solid interfaces and background soil has been regarded as an important source to the global Hg cycle [13]. Hg released from the soil is mainly in form as Hg0 through complex physical, chemical and biological processes [14]. The organic matter in soil plays an important role in both abiotic and biotic processes that control the Hg release
http://dx.doi.org/10.1016/j.fuproc.2016.10.015 0378-3820/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
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from soil [15]. Temperature, UV radiation and soil moisture content have also been found to be important parameters that influence Hg release from soil [16–19]. Concentrations of Hg may be higher in FGD gypsum compared to natural soil. Similarly, Hg retained in the FGD gypsum may also have the potential to release as Hg0. Some research works have revealed the volatilization of Hg0 from FGD gypsum [20–22]. However, the mechanisms of the Hg vapor release from FGD gypsum remain unclear. Unlike soil, Hg in FGD gypsum is mainly in the form as inorganic mercury species and FGD gypsum nearly contains no organic matters which have high affinity with mercury compounds and participate in the reactions related to Hg release from soil. The different material chemistry and Hg species in FGD gypsum suggest that the Hg vapor release from FGD gypsum may be different from that in soil. FGD gypsum would be in contact with the natural environment in the non-encapsulated applications such as agricultural practice and soil stabilization. The FGD gypsum applied for encapsulated applications such as wallboard and cement may also be affected by the changes of temperature and humidity of natural environment. And the large amounts of FGD gypsum that ended up in landfills or outdoor storage without any beneficial applications would also be exposed to outdoor environment. Because of current management scenarios of FGD gypsum, a large proportion of FGD gypsum would have the opportunity to be exposed to natural environment. The temperature, UV radiation and solid moisture content might also affect the Hg release from FGD gypsum as that occurs in soil. Therefore, it is necessary to study the influence of these natural environmental conditions on Hg release from FGD gypsum. Considering the huge amounts of FGD gypsum generated every year in China and many other countries, the Hg release from FGD gypsum may not be negligible. The emphasis of this study was to investigate the Hg vapor release characteristics from FGD gypsum. The effects of temperature, UV radiation and solid moisture content on the Hg release were studied in laboratory experiments. The Hg speciation in FGD gypsum was analyzed to evaluate the Hg release mechanism with the aim to comprehensively address the physicochemical processes controlling the Hg release from FGD gypsum. 2. Materials and methods 2.1. Sampling and characterization Samples of FGD gypsum labeled as CQ, LYG and EZ were obtained from three different coal-fired power plants in China. The samples were collected from fresh FGD gypsum and stored in sealed glass bottles at 4 °C until testing. The moisture contents of CQ, LYG and EZ samples were 13.68%, 8.95% and 25.55%, respectively. As listed in Table 1, the coal types of the three coal-fired power plants included bituminous, lignite and anthracite, representing all the major coal types burned in Chinese power plants. All the tested power plants were equipped with selective catalytic reduction (SCR), electrostatic precipitator (ESP) and wet FGD system to control the emissions of NOx, particulate matter and SO2. The Hg contents in the samples were determined using a Lumex RA915M + PYRO-915 (Lumex, Russia) which can achieve direct measurement of Hg in solid matrix. The contents of the other elements were determined by inductively coupled plasma mass spectrometry (ICP-MS, ThermoFisher X series II, German) after microwave digestion. The whole procedure followed US EPA Method 3052. X-ray diffraction
(XRD, D/Max-2500 pc, Rigaku Inc., Japan) analysis of air-dried samples was used to identify the basic crystalline phase. The structure and morphology of each sample was also examined by scanning electron microscopy (SEM, S-5500, Hitachi, Japan). 2.2. Mercury release measurement The Hg release from FGD gypsum was measured in the system as shown in Fig. 1. 40 g of FGD gypsum sample (as received) was placed in a clean quartz glass chamber of 1000 ml. Ambient air scrubbed of ambient mercury by activated carbon was slowly introduced into the chamber at the rate of l l/min. Any Hg release from the FGD gypsum was then carried out of the chamber and captured by the activated carbon trap at the exit. The Hg content in the activated carbon trap was then measured by Lumex 915M + PYRO-915 to determine the Hg release rate from FGD gypsum. The Hg release rate was the ratio of Hg release content to the total Hg content in the tested FGD gypsum samples. To investigate the impact of temperature and UV radiation, the Hg release experiments were conducted in parallel at three different conditions. The first batch of samples was placed in the dark without UV light at room temperature (maintained 25 ± 3 °C during the 180-day period of experiment). The second batch of samples was placed in light at room temperature to study the influence of UV light. The light was generated by a UV tube (340 nm, 40 W) which could mimic the ultraviolet region of solar radiation. The third batch of samples was placed in the dark in an oven with the temperature maintained at 50 °C to study the influence of temperature. The activated carbon traps were replaced and had their Hg contents measured in every 30 days, while the Hg release content was also recorded in every 2 days during the initial 10 days of the experiment. The release experiment lasted for 180 days to study the long-term release characteristics of Hg. The impact of solid moisture content was investigated by adding 5 ml deionized water to every sample homogeneously each time after the 180 days of continuous observation. The water additions were conducted three times. The experiments about the impact of solid moisture content lasted for 20 days and the Hg release content was recorded in every 2 days. The content of Hg release in the first 2 days before water addition was measured at the beginning. Then 5 ml water was added and the Hg release was measured along with the change of solid moisture content determined by weighing the chamber every 2 days in the following 8 days, so as to mimic the wetting and drying process of FGD gypsum in natural environment. The water was then added at the end of 10th day and 14th day in the experiments to confirm the effect of water addition and also to study the Hg release characteristic in the successive wetting events. All the experiments were conducted in triplicate to verify the data reliability with the mean values used. 3. Results and discussion 3.1. Characterization of FGD gypsum The mineralogy characterization of the FGD gypsum samples were studied by XRD analysis. As shown in Fig. 2, calcium sulfate dehydrate was the predominant crystalline phase in all the samples. This meant that most of the calcium sulfite (CaSO3) had been transformed to calcium sulfate (CaSO4) by the forced-oxidation process in the wet FGD system. There were also minor amounts of calcite and quartz in the
Table 1 General information of the tested power plants. Sample
Coal type
Air pollution control devices
Installed capacity (MW)
CQ LYG EZ
Lignite Bituminous Anthracite
SCR + ESP + wet FGD SCR + ESP + wet FGD SCR + ESP + wet FGD
300 1000 330
Fig. 1. Mercury release measurement system.
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
Z.-W. Zhu et al. / Fuel Processing Technology xxx (2016) xxx–xxx G G
G
Intensity
G
CQ
LYG EZ C=Calcite G=Gypsum Q=Quartz
G G
Q
0
10
20
G
30
GG Q
G
G
40 50 2θ (º)
C
60
C
C
70
80
90
Fig. 2. XRD patterns of the FGD gypsum samples.
samples, which might be impurities in the limestone or fly ash captured from the flue gas. However, no Hg species were detected by XRD since the Hg concentrations in the FGD gypsum samples were much lower than the detection limit of XRD. The particle morphology of the FGD gypsum samples were determined by SEM analysis. The results in Fig. 3 shown that the samples were mainly monoclinic crystalline and the EDX results showed that the main content was CaSO4. There were also some spheres in the LYG and EZ samples. The EDX results suggested that the spheres were mainly aluminosilicate and iron oxide, indicating that the spheres were the fly ash particles captured by the wet FGD system. The concentrations of major and trace elements in the samples (as received) were determined by ICP-MS. As listed in Table 2, Ca and S were the major elements in all the samples. There were also considerable amounts of Al, Fe and Si in the samples, which might come from the fly ash escaped from ESP. This was consistent with the result of SEM-EDX. As for the trace elements, the concentrations of As, Cr, Mn, Ni, Se and Pb were comparatively high. The Hg concentrations of the FGD gypsum samples were in the order of CQ (0.75 μg/g) b LYG (1.87 μg/g) b EZ (3.27 μg/g). Many factors could affect the Hg concentration in FGD gypsum, such as coal types, different air pollution control devices and different operating conditions. 3.2. Mercury speciation in FGD gypsum Based on the different thermal decomposition/desorption temperatures of Hg compounds, it is able to determine Hg species in solid matrix samples by temperature programmed decomposition (TPD) technique [23–25]. Previous studies have shown no organic mercury in the flue gas of coal combustion so the Hg compounds in FGD gypsum are expected to be HgCl2, HgO, HgSO4 and other inorganic species [26]. In our previous study, the thermal induced Hg release curves of pure mercury compounds (e.g. HgCl2, Hg2Cl2, black HgS, red HgS, HgO, HgSO4, and
3
Table 2 Elemental compositions of the FGD gypsum samples.
Al Ca Fe K Mg Na Sr Ti S Si As Be Cd Co Cr Hg Mn Ni Sb Se Pb
CQ Wt.%
LYG
0.492 ± 0.005 19.71 ± 0.11 0.423 ± 0.003 0.308 ± 0.006 0.306 ± 0.002 0.094 ± 0.001 0.078 ± 0.001 0.012 ± 0.001 15.18 ± 0.08 1.103 ± 0.006 mg/kg 2.72 ± 0.10 0.18 ± 0.02 0.08 ± 0.01 1.91 ± 0.03 9.88 ± 0.28 0.75 ± 0.03 79.01 ± 2.38 5.15 ± 0.18 0.015 ± 0.003 6.10 ± 0.18 1.36 ± 0.07
0.186 20.95 0.230 0.137 0.614 0.057 0.020 0.005 14.91 0.615
EZ ± ± ± ± ± ± ± ± ± ±
0.002 0.10 0.002 0.002 0.005 0.001 0.001 0.001 0.18 0.005
3.23 ± 0.17 0.13 ± 0.01 0.06 ± 0.01 0.91 ± 0.01 5.81 ± 0.50 1.87 ± 0.04 149.53 ± 0.91 2.43 ± 0.24 0.041 ± 0.002 8.17 ± 0.05 6.57 ± 0.21
0.295 17.42 0.173 0.069 0.391 0.035 0.022 0.006 12.92 0.627
± ± ± ± ± ± ± ± ± ±
0.004 0.18 0.002 0.001 0.006 0.001 0.001 0.001 0.10 0.007
5.04 ± 0.15 0.16 ± 0.01 0.45 ± 0.09 0.79 ± 0.01 11.02 ± 0.74 3.27 ± 0.09 83.97 ± 1.87 3.44 ± 0.06 0.174 ± 0.006 20.19 ± 0.16 4.47 ± 0.21
Hg2SO4) were obtained to identify the Hg compounds in FGD gypsum. The detailed speciation of Hg in FGD gypsum samples had been published elsewhere [27]. By comparing the thermal Hg release curves of the FGD gypsum samples with the pure mercury compounds, the Hg species in FGD gypsum samples could be determined. As listed in Table 3, HgS was the primary species in all the samples. This might be caused by the precipitation reaction of Hg2+ and S2− formed by the disproportionation of sulfite in the gypsum slurry [28]. 3.3. Mercury release from FGD gypsum 3.3.1. Mercury release in the dark at room temperature The Hg release from 40 g FGD gypsum sample in the dark at room temperature was recorded every 30 days in the continuous 180-day period. The temperature was maintained at about 25 °C. As listed in Table 4, the total Hg release content from CQ over the 180-day test was 2.356 ng/g FGD gypsum, while LYG and EZ was 9.021 and 86.50 ng/g FGD gypsum, respectively. The Hg release rate in the 180day test was 0.31%, 0.48% and 2.65% for CQ, LYG and EZ, respectively. Detectable amounts of Hg were constantly released from all the FGD gypsum samples. The Hg species in all the samples were mainly HgS with some HgCl2 and Hg2Cl2. No Hg0 was identified in the thermodesorption experiments in our previous study [27]. The Hg release could be due to the reduction of Hg2 + and the generation of Hg0 in FGD gypsum. Hg0 would then be desorbed from the gypsum particles and diffuse into the atmosphere. Gustin et al. [29] and Engle at al. [30] had found Hg release from both synthetic HgS in laboratory and HgS ores in situ. The release of Hg0 from HgS might occur because that HgS is thermodynamically stable under high temperatures and reducing
Fig. 3. SEM micrographs of the FGD gypsum samples.
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
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Table 3 Mercury species in the FGD gypsum samples.
Table 5 Mercury release content from FGD gypsum in the dark at 50 °C (ng/g FGD gypsum).
Sample
% Black HgS
% Red HgS
% Other
CQ LYG EZ
73.0 ± 0.5 88.5 ± 2.3 69.8 ± 3.3
9.0 ± 2.1 11.5 ± 2.3 24.0 ± 2.5
18.0 ± 2.5 HgCl2
CQ
6.2 ± 0.8 Hg2Cl2
conditions and might degrade to Hg0 at relatively low temperatures and oxidizing conditions [29]. The Hg release was in the order of CQ b LYG b EZ, which was the same order as the Hg concentrations in the FGD gypsum samples with much more Hg released from EZ than from CQ or LYG. The decomposition of Hg2Cl2 to form Hg0 in EZ might be one of the contributors. Although the total Hg release content increased with the Hg concentration in the FGD gypsum, there seemed to be no obvious functional relationship between them. This indicated that the Hg release from FGD gypsum was a rather complicated process. Besides the Hg concentration in FGD gypsum, many other intrinsic characteristics, such as Hg species, Hg bond modes, and mineralogy characterization of FGD gypsum might affect the Hg release process somehow. 3.3.2. Effect of temperature on mercury release As listed in Table 5, the total Hg release contents in 180-day period at 50 °C in the dark were 12.79, 125.3 and 352.3 ng/g FGD gypsum for CQ, LYG and EZ, respectively. And the Hg release rates of CQ, LYG and EZ were 1.71%, 6.7% and 10.77%. The elevated temperature significantly increased the Hg release from FGD gypsum which was 3.1–12.9 times greater than that at room temperature in the dark. Since the Hg in the FGD gypsum was in the form as Hg2+ and was unlikely to volatilize directly, the Hg release from FGD gypsum was most likely due to the reduction of Hg2+. The higher temperature would increase the reaction rate of Hg2 + reduction and promote the production of Hg0. Furthermore, the higher temperature would also promote the desorption and diffusion of Hg0 among FGD gypsum particles, which would eventually enhance the Hg0 mass transport across the FGD gypsum particle-air boundary and escape into the atmosphere. The experiments to simulate the wallboard production process in which FGD gypsum samples were heated to 160 °C from room temperature and then maintained at 160 °C for 1 h also showed a significant increase on the Hg release at elevated temperature. The release rates of Hg in the approximate 2 h duration experiment could reach up to 7.17%, 13.15% and 14.27% for CQ, LYG and EZ, respectively.
30 days 60 days 90 days 120 days 150 days 180 days Total
LYG
4.803 1.583 1.985 2.153 1.295 0.975 12.79
± 0.263 ± 0.085 ± 0.189 ± 0.185 ± 0.180 ± 0.141 ± 1.04
EZ
38.10 20.58 21.54 14.93 14.67 15.44 125.3
± 3.16 ± 1.37 ± 2.42 ± 0.10 ± 1.34 ± 1.06 ± 2.9
105.2 36.10 38.32 44.87 65.51 62.32 352.3
± 7.0 ± 2.54 ± 3.41 ± 0.71 ± 6.05 ± 7.36 ± 13.0
samples changed very little compared with those in the dark. The Hg release enhancement by UV radiation was independent of the solid temperature. Mercury is known to efficiently absorb light energy at the wave length of 315 nm. The absorption of photo energy by Hg0 atoms would reduce the apparent activation energy for Hg0 desorption [31]. This would facilitate the Hg0 release from FGD gypsum particles. Furthermore, the UV radiation also promotes the reduction of Hg2 + in FGD gypsum, which would then increase the available pool of Hg0 and its subsequent release from FGD gypsum. The main Hg species in all the FGD gypsum samples was HgS. Zhang [32] found that the decomposition of HgS occurred to release H2S and Hg2+ with incident light (λ N 300 nm), and Hg2+ could undergo in situ reductive deposition on HgS to form Hg0. Radepont et al. [33] found that red HgS would experience a multistep degradation process when exposed to UV radiation. When chloride existed, red HgS could be transformed to Hg2Cl2 (Eq. (1)) [33]. And Hg2Cl2 would decompose to HgCl2 and release Hg0 when exposed to UV radiation (Eq. (2)). This would also increase the release of Hg when the FGD samples were exposed to UV radiation. α−HgS↔γ−Hg3 S2 Cl2 ↔α−Hg3 S2 Cl2 ↔Hg2 Cl2
ð1Þ
Hg2 Cl2 →Hg0 þ HgCl2
ð2Þ
3.3.3. Effect of UV radiation on mercury release A series of samples were exposed to UV light (340 nm, 40 W) with the other conditions the same as for the samples in the dark at room temperature (about 25 °C). As listed in Table 6, the total Hg release contents in 180-day period were 7.224, 41.22 and 117.6 ng/g FGD gypsum for CQ, LYG and EZ, respectively. And the release rates of Hg for CQ, LYG and EZ were 0.96%, 2.2% and 3.6%, which were 0.4–3.6 times greater than that without UV radiation at the same conditions. Since the heat generation from the UV tube was negligible and the UV tube was placed about 5 cm from the glass chamber, the temperatures of FGD gypsum
3.3.4. Effect of water addition on mercury release The moisture content was found to affect the Hg release from soil [18,19]. As shown in Fig. 4, the Hg release in the following 2 days after first water addition was 4.3 to 118 times of that before water addition. The Hg release then decreased with the decreasing moisture content in the FGD gypsum. By monitoring the weights change of the FGD gypsum samples after water addition, it was found that the 5 ml water added to the gypsum had all evaporated out in 2 days after the addition. The moisture contents of the samples after 4 days in the record were about the same as that before water addition. However, the Hg release from the samples after 4 days was still greater than that before water addition. The Hg release returned to the level before water addition until 10th day in the record. The water addition at the end of 10th day and 14th day also significant promoted the Hg release from FGD gypsum and the extents of the promotion of these two water additions were almost the same as the first addition. This confirmed the promotion effect of solid moisture content. It also indicated that Hg release would be promoted with repeated wetting events, which means that
Table 4 Mercury release content from FGD gypsum in the dark at room temperature (25 °C) (ng/g FGD gypsum).
Table 6 Mercury release content from FGD gypsum in UV radiation at room temperature (25 °C) (ng/g FGD gypsum).
CQ 30 days 60 days 90 days 120 days 150 days 180 days Total
1.263 0.025 0.050 0.195 0.358 0.465 2.356
LYG ± ± ± ± ± ± ±
0.133 0.005 0.009 0.018 0.055 0.041 0.196
2.775 0.404 0.554 1.163 1.755 2.370 9.021
EZ ± ± ± ± ± ± ±
0.088 0.042 0.056 0.055 0.163 0.249 0.476
16.43 5.140 9.183 14.64 15.31 25.80 86.50
CQ ± 1.06 ± 0.700 ± 1.290 ± 0.03 ± 0.85 ± 1.18 ± 1.05
30 days 60 days 90 days 120 days 150 days 180 days Total
2.038 0.400 0.565 1.023 1.225 1.973 7.224
LYG ± ± ± ± ± ± ±
0.179 0.032 0.085 0.072 0.099 0.246 0.712
6.368 3.330 4.145 7.399 8.095 11.88 41.22
EZ ± 0.138 ± 0.244 ± 0.491 ± 0.550 ± 0.817 ± 0.85 ± 3.09
18.93 9.235 11.76 20.90 23.71 33.04 117.6
± 1.06 ± 0.375 ± 1.03 ± 2.26 ± 2.00 ± 0.45 ± 7.2
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
in the dark at 25 ºC UV radiation at 25 ºC in the dark at 50 ºC
3
2
1
watered
0 2
6
8
10 12 Days
14
16
18
20
in the dark at 25 ºC UV radiation at 25 ºC in the dark at 50 ºC
LYG
6 5
watered 4
watered
Hg release (ng/g FGD gypsum)
7
4
3
watered
2 1 0 2
4
6
8
10 12 Days
14
5
it could be available to be reduced and released from the surface. Studies have shown that the oxidative dissolution of HgS did exist and Hg(OH)2(aq) would be formed via Eq. (3) [34,35]. Hg(OH)2 could then be reduced to Hg0 which would also increase the Hg0 pool for release [36]. þ HgSðsÞ þ 2O2 ðaqÞ þ 2H2 O↔HgðOHÞ2 ðaqÞ þ SO2− 4 þ 2H
watered
CQ
watered
Hg release (ng/g FGD gypsum)
Z.-W. Zhu et al. / Fuel Processing Technology xxx (2016) xxx–xxx
16
18
20
ð3Þ
By comparing the Hg release from the same sample under different conditions after water addition in Fig. 4, it was obvious that the enhancements of Hg release from wet FGD gypsum with incident UV radiation or at elevated temperature were greater than that from the same samples in the dark or at room temperature. It indicated that there was a synergistic effect between solid moisture content and UV radiation/ temperature. Furthermore, the Hg release from the samples initially decreased and then increased with time during the 180-day period at room temperature as seen in Table 4 and Table 6. The experiments were conducted in Beijing of China from January to July. The room temperature in laboratory varied little and remained at approximately 25 °C in the entire 180-day period. It had been noticed that the air humidity increased from 16.9% to 75% during this period. The Hg release initially decreased as the original water evaporated in the FGD gypsum, which could be seen from Hg release in the initial 10 days of the 180-day period as shown in Fig. 5. Then the sample would absorb moisture from air with the increase of humidity and the Hg release increased accordingly. Although Hg release from FGD gypsum in the dark at room temperature was not very high, the temperature, UV radiation and solid moisture all could significantly increase the overall Hg release. This observation implied that, for the FGD gypsum stacked in open fields without any protection from sun radiation or rainfall, considerable amount of Hg it captured from flue gas in power plant could be eventually re-emitted into the atmosphere. 4. Conclusion
20
in the dark at 25 ºC UV radiation at 25 ºC in the dark at 50 ºC
EZ
18 16 14 12 10 8
watered
watered
watered
6
Wet FGD system can effectively capture oxidized mercury in flue gas which leads to Hg accumulation in FGD gypsum. The management scenarios of FGD gypsum give the opportunity for Hg to release from FGD gypsum to atmosphere. This may be non-negligible because of the huge amounts of FGD gypsum generated every year in the world. In this study, the measurement of Hg release from three FGD gypsum samples were conducted in controlled laboratory experiments. Detectable amounts of Hg were constantly emitted from all the samples. The Hg species in the FGD gypsum samples was mainly HgS with the Hg release due to the reduction of oxidized mercury in the samples. The Hg release
4
3.5
2 2
4
6
8
10 12 Days
14
16
18
20
Fig. 4. Hg release before and after water addition.
the Hg release from FGD gypsum storage outdoor would be enhanced by every rainfall in the environment. Water molecule is polar and could compete with Hg0 for binding sites on the surfaces of solid particles. Hg0 adsorbed on solid particles would be desorbed and become free Hg0 in the pores when water was added, so as to increase the Hg0 pool available for release. With water evaporation in the meantime, free Hg0 in the pores would be carried to the surface of solid by capillary effects and then released to the atmosphere in the end [18]. Moreover, soluble mercury compounds in the FGD gypsum samples such as HgCl2 would dissolve in water and be transported with the water evaporation to the particle surface where
Hg release (ng/g FGD gypsum)
Hg release (ng/g FGD gypsum)
22
in dark at 25 ºC UV radiation at 25 ºC
LYG
3.0 2.5 2.0 1.5 1.0 0.5 0.0 2
4
6 Days
8
10
Fig. 5. Mercury release in the initial 10 days of 180-day period, taking sample LYG as example.
Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015
6
Z.-W. Zhu et al. / Fuel Processing Technology xxx (2016) xxx–xxx
content from FGD gypsum in the dark at room temperature was quite low, with the Hg release rates in the 180-day period varying from 0.31% to 2.65% for the three samples. Temperature, UV radiation and solid moisture content all could enhance the Hg release from FGD gypsum. The Hg release rates at elevated temperature (50 °C) in 180-day period were 1.71% to 10.77% for the three samples. The Hg release rates with incident UV radiation were about 0.96% to 3.60% for the three samples during the 180-day period. The addition of 5 ml water after 180-day period could increase Hg release to 4.3 to 118 times of that without water addition. And the Hg release could be promoted with repeated wetting events. The enhancement of Hg release by these factors might be due to the increase of oxidized mercury reduction in FGD gypsum and desorption and diffusion of Hg0 among gypsum particles. The amount of Hg release in the dark and low temperature might not be very large for FGD gypsum stacked outdoors. However, when the FGD gypsum was exposed to solar radiation or experienced precipitation events, the UV radiation and temperature increase due to solar radiation and the water involved would significantly enhanced the Hg release. Considering the huge amounts of FGD gypsum generated every year in China and many other countries, more detailed research is required in future to fully understand its contribution to global Hg circulation. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51376109). References [1] N. Pirrone, G.J. Keeler, J.O. Nriagu, Regional differences in worldwide emissions of mercury to the atmosphere, Atmos. Environ. 30 (17) (1996) 2981–2987. [2] E.G. Pacyna, J.M. Pacyna, K. Sundseth, J. Munthe, K. Kindbom, S. Wilson, F. Steenhuisen, P. Maxson, Global emission of mercury to the atmosphere from anthropogenic sources in 2005 and projections to 2020, Atmos. Environ. 44 (20) (2010) 2487–2499. [3] H.Z. Tian, Y. Wang, Z.G. Xue, Y.P. Qu, F.H. Chai, J.M. Hao, Atmospheric emissions estimation of Hg, As, and Se from coal-fired power plants in China, 2007, Sci. Total Environ. 409 (16) (2011) 3078–3081. [4] J.H. Pavlish, E.A. Sondreal, M.D. Mann, E.S. Olson, K.C. Galbreath, D.L. Laudal, S.A. Benson, Status review of mercury control options for coal-fired power plants, Fuel Process. Technol. 82 (2) (2003) 89–165. [5] L. Zhang, Y.Q. Zhuo, L. Chen, X.C. Xu, C.H. Chen, Mercury emissions from six coalfired power plants in China, Fuel Process. Technol. 89 (11) (2008) 1033–1040. [6] C.M. Cheng, P. Hack, P. Chu, Y.N. Chang, T.Y. Lin, C.S. Ko, P.H. Chiang, C.C. He, Y.M. Lai, W.P. Pan, Partitioning of mercury, arsenic, selenium, boron, and chloride in a fullscale coal combustion process equipped with selective catalytic reduction, electrostatic precipitation, and flue gas desulfurization systems, Energy Fuel 23 (10) (2009) 4805–4816. [7] Z.W. Zhu, Y.Q. Zhuo, Z.Y. An, W. Du, C.H. Chen, Trace element distribution during wet flue gas desulphurization system, Journal of Tsinghua University (Science and Technology) 3 (2013) 330–335 in Chinese. [8] Y. Cao, Z.Y. Gao, J.S. Zhu, Q.H. Wang, Y.J. Huang, C.C. Chiu, B. Parker, P. Chu, W.P. Pan, Impacts of halogen additions on mercury oxidation, in a slipstream selective catalyst reduction (SCR), reactor when burning sub-bituminous coal, Environ. Sci. Technol. 42 (1) (2007) 256–261. [9] Y. Hao, S.M. Wu, Y. Pan, Q. Li, J.Z. Zhou, Y.B. Xu, G.R. Qian, Characterization and leaching toxicities of mercury in flue gas desulfurization gypsum from coal-fired power plants in China, Fuel 177 (2016) 157–163. [10] N.H. Koralegedara, S.R. Al-Abed, M.K.J. Arambewela, D.D. Dionysiou, Impact of leaching conditions on constituents release from Flue Gas Desulfurization Gypsum (FGDG) and FGDG-soil mixture, J. Hazard. Mater. (2016) in press.
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Please cite this article as: Z.-W. Zhu, et al., Mechanism of mercury vapor release from flue gas desulfurization gypsum, Fuel Processing Technology (2016), http://dx.doi.org/10.1016/j.fuproc.2016.10.015