J Therm Anal Calorim (2015) 119:1611–1618 DOI 10.1007/s10973-015-4403-9
Partitioning effect of mercury content and speciation in gypsum slurry as a function of time Zifeng Sui • Yongsheng Zhang • Wenhan Li • William Orndorff • Yan Cao • Wei-Ping Pan
Received: 2 September 2014 / Accepted: 4 January 2015 / Published online: 10 February 2015 Ó Akade´miai Kiado´, Budapest, Hungary 2015
Abstract For gypsum slurry samples taken from wet flue gas desulfurization (WFGD) systems, a mercury partitioning effect was observed between the solid and liquid components. This partitioning effect, which appears to be pseudo-second-order, is a function of time which results in an increase in mercury content for the solid portion of the gypsum slurry as time increases. Moreover, the mercury content for the solid portion of the slurry reaches a plateau after 24 h. Thus, the sampling method for the gypsum slurry is an important factor in determining an accurate mercury mass balance in the WFGD systems. Temperature-controlled decomposition techniques were employed to acquire an understanding of the mercury species in WFGD gypsum. It was determined that the primary mercury species in WFGD gypsum was mercuric sulfide (HgS). Moreover, the content of HgS as well as mercuric oxide (HgO) in the solid portion of the slurry was susceptible to the partitioning effect and steadily increased with separation time. Other mercury compounds present were mercury sulfate and mercury chloride; however, these mercury species exhibited less change with separation time. Additionally, a comparison of the mercury present in WFGD gypsum from five different coal-fired power plants using thermal decomposition techniques showed that the distribution of mercury species was not the same; however, HgS was primary and
Z. Sui Y. Zhang (&) W. Li Y. Cao W.-P. Pan Key Laboratory of Condition Monitoring and Control for Power Plant Equipment, Ministry of Education, North China Electric Power University, Beijing 102206, China e-mail:
[email protected] W. Orndorff Y. Cao W.-P. Pan Institute of Combustion Science and Environmental Technology, Western Kentucky University, 1906 College Heights Blvd, Bowling Green, KY 42101, USA
mercury chloride compounds or HgO secondary. HgO varied between 15 and 25 % of the total mercury in gypsum. No clear correlation between mercury speciation and sulfur and chlorine content in the coal was determined. Thermogravimetric analysis was employed to compare the effects of settling time with gypsum quality. Data show that longer settling times results in lower quality gypsum. Keywords Gypsum Separation time Temperatureprogrammed decomposition Mercury compounds
Introduction In 2011, the total amount of electricity produced in China was 4.7 trillion kWh from which 82 % was produced by coalfired power plants [1]. Coal consumption for this power/ energy output was 1.8 billion tons [2]. Coal utilization has become the largest anthropogenic mercury pollution source. In China, 70 % of the mercury pollution is generated by coal consumption, and around 34 % of atmospheric mercury emissions are from coal-fired power plants [3]. The coalfired power plant air emission standards passed in 2011 dictate the regulation of mercury and its compounds, which should be less than 0.03 mg m-3. These regulations are set to go into effect by January 1, 2015. Thus, the research on how to control mercury and its compounds emitted from coalfired power plant has drawn much attention in China. There are three dominate forms of mercury in flue gas from coal-fired power plants, gaseous elemental mercury (Hg0), oxidized mercury (Hg2?) and particulate-bound mercury (Hgp). In general, Hg0 accounts for the majority of the flux in the stack, due to its insolubility and low reactivity. [4, 5] Hg2? dissolves readily in water and hence is easily captured by the liquid slurry in a wet flue gas desulfurization
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(WFGD). The accurate analysis of the mercury content and speciation in gypsum slurry is essential to calculate the mercury mass balance in coal-fired power plants. In a WFGD system, calcium carbonate (limestone) is mixed with water and sprayed into the flue gas stream. Under the Chinese operating protocol, this mixture is continuously recycled with the rate of fresh limestone added to the reservoir based on the sulfur dioxide emission from the flue stack. The removed (and newly formed) gypsum slurry is transferred to a dewatering process immediately after the slurry is out of the absorber. This process often uses one or two hydroclones. The hydroclones separate the gypsum solids from the water using centrifuges. The underflow from the first set of hydroclones contains the gypsum solids and is transferred to vacuum dewatering belt. The vacuum dewatering belt then removes additional water from the gypsum, drying the gypsum to its desired moisture content. However, the central FGD slurry dewatering device is shared by multiple units in many power plants. Thus, the gypsum collected in the vacuum dewatering belt does not represent the real mercury content within a specific unit. Hence, the gypsum slurry is collected from the exit of the each absorber and the water is separated to obtain the desired gypsum sample. It will be shown, in this paper, that the gypsum slurry settling time (time taken to separate the solid gypsum from the majority of the water) influences the mercury content and speciation. The correct settling time needs to be determined in order to calculate the correct mercury mass balance. Temperature-programmed decomposition methods have been employed to identify the mercury compounds in solid samples, such as coal, fly ash, gypsum, contaminated soil, sediments and iron-based sorbents [6–11]. The results from these tests show that the Hg species can be semiquantitatively determined by the characteristic release (thermal decomposition) temperatures and the corresponding area under the curve. Liu et al. [12] conducted a similar study of gypsum and determined the mercury speciation. The reference samples in Liu et al. [12] study were prepared from solid mercury standard compounds mixed with gypsum, which had previously been heated to 600 °C for 4 h to remove the inherent mercury in the gypsum. Considering that moisture loss of gypsum may affect the mercury compounds decomposition curves, the reference samples in this study were prepared by injecting solutions of mercury compounds into heat-purified gypsum.
Experimental Slurry samples The samples for settling time study were collected in the morning and afternoon in December, 2013. The power
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plant was a 300 MW unit, utilizing bituminous coal, equipped with selective catalyst reduction (SCR), electrostatic precipitation (ESP) and wet flue gas desulfurization units (WFGD). The gypsum slurry was collected into 500-mL Teflon bottles following EPRI protocol (EPRI method H1, I2) [13]. The settling time (time taken to allow solids and liquid to naturally separate) was 2 min, 10 min, 30 min, 1 h, 2 h, 1 day, 2 days and 7 days. The gypsum slurry sample was then separated using a HONGKE tabletop low-speed centrifuge operating at 3,000 rpm for 30 min. The solid portion (and now considered the gypsum) was further air-dried at 35 °C for 12 h. The dried gypsum was then ground using mortar and pestle to \200 microns. To provide better results for the mercury speciation component of the study, the reference material (gypsum) was prepared by heating the sample at 600 °C for 4 h to remove the inherent mercury [12]. The heat-treated gypsum was then analyzed to identify any remaining mercury compounds. The heat-treated reference material was then mixed with 50–100 lL of 400 ppm mercury solution such as mercuric chloride (HgCl2), mercurous chloride (Hg2 Cl2), mercuric sulfate (HgSO4), mercurous sulfate (Hg2 SO4), mercuric sulfide (HgS) and mercuric oxide (HgO) to establish the characteristic release curves. These substances then served as the mercury reference materials. The heattreated gypsum was also mixed with solid mercury compounds to determine whether there was any difference in the mercury release curves between the solution and solid prepared standard samples. It was determined that there was no significant difference between the solid versus solution prepared mercury reference materials except the mercurous compounds. It is assumed that this phenomenon was due to the Hg? in solution, which is kinetically favored to shift to Hg2? and Hg0. Gypsum samples Four of the gypsum samples (separation times of 2 min, 10 min, 2 days and 7 days) that were collected at the outlet of the absorber (before the vacuum dewatering belt) from power plant #1 (PP#1) were selected for thermogravimetric (TG) analysis and comparison. Additionally, the 1 hour settling time gypsum slurry sample (GYP#1) from PP#1 was chosen for comparison with the other four WFGD gypsum samples (GYP#2–GYP#5) which were collected from four different coal-fired power plants in China. The gypsum from the other four power plants is collected at the end of the vacuum dewatering belt, dried at 35 °C for 12 h, and ground using mortar and pestle to \200 microns. The coal utilized at all power plants was bituminous coal, which was equipped with ESP and WFGD for removing particulate matter and SO2. The coal and limestone
Partitioning effect of mercury content and speciation
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elemental analysis, as well as mercury analysis for gypsum, is shown in Table 1. Apparatus and method As shown in Fig. 1, the test apparatus consists of an air source, a quartz tube equipped with a temperature-controlled furnace, a high-temperature furnace and an Ohio Lumex RA-915? Mercury Analyzer. The detection limit of real-time mercury gas content is approximately 2 ng m-3. A quartz crucible was used to load the gypsum samples into the quartz tube under a heating rate of 10 °C min-1. The thermally decomposed and now gaseous mercury compounds were subsequently carried by air flow into a high-temperature furnace, where it is assumed that all of the mercury is converted into Hg0. This gas stream was then directed into an Ohio Lumex RA915? instrument to measure Hg0 as a function of temperature. This mercury signal versus temperature established the characteristic thermally decomposed curve for each mercury species and corresponding mercury reference materials. Moreover, the mercury content of each species was calculated by integrating the area under the curve. The thermal release curves from the unknown samples were then compared to the mercury standards.
Results and discussion Slurry samples The effect of separation time on Hg content As shown in Fig. 2, the Hg content in gypsum increases logarithmically for both the morning and afternoon samples. After 24 h, the mercury content does not change significantly. Additionally, there were significant differences in the mercury content between the morning and afternoon samples. This may be due to different operating parameters of the WFGD system. However, both sets of samples have the same trend.
An investigation of the uptake rate of mercury in the gypsum was conducted using the pseudo-first-order reaction model (Eq. 1) and pseudo-second-order reaction model (Eq. 2). dqt ¼ k1 ðqe qt Þ dt dqt ¼ k2 ðqe qt Þ2 dt
ð1Þ ð2Þ
where t is sampling time (h), qe is adsorbed Hg content (lg g-1) in gypsum at equilibrium, and qt is the adsorbed Hg content (lg g-1) in gypsum at time t. k1 and k2 are the pseudo-first-order and pseudo-second-order reaction constants (h-1 and g lg-1 h-1), respectively. By integrating Eqs. (1) and (2) over time t with the initial condition as qt = 0 at t = 0, the following linear equations can be obtained for the first-order and the pseudo-second-order models, respectively. lnðqe qt Þ ¼ ln qe k1 t
ð3Þ
t 1 1 ¼ tþ qt qe k2 q2e
ð4Þ
Equations (3) and (4) can then be plotted as ln(qe-qt) versus t and t/qt versus t, respectively. A linear dependency would mean a good fit between experimental results and model calculations. The equations given earlier were employed to produce the pseudo-first-order curve (Fig. 3) and the pseudo-second-order curve (Fig. 4). From these graphs, it is readily apparent that the mercury partitioning process in gypsum slurry is more in line with the pseudosecond-order curve. Gypsum samples TG curves of gypsum samples settled at varying times The gypsum samples that were collected at the outlet of absorber (before the vacuum dewatering belt) from power plant #1 (PP#1) at varying separation times (2 min, 10 min, 2 days and 7 days) were subsequently separated by
Table 1 Elemental analysis for the coal, limestone, and gypsum Coal
Limestone
C
H
N
S
O
PP#1
69.60
4.22
0.97
0.60
11.11
PP#2
64.72
3.98
0.90
0.53
PP#3
57.14
4.44
0.98
0.37
PP#4
68.09
4.24
0.93
PP#5
69.35
4.17
0.85
Cl/ppm
Gypsum
Hg/ppb
S/%
Cl/%
566
43
0.14
0.03
13.07
394
41
0.10
21.25
267
20
0.09
0.42
10.61
1,717
10
1.07
10.95
288
74
Hg/ppb
Hg/ppb
3.6
585
0.14
8.6
326
0.03
11.0
788
0.07
0.01
3.9
399
0.12
0.02
3.6
843
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High temperature furnace Lumex
air
RA915 +
quartz tube
sample
quartz crucible
1,800
16
1,600
14
1,400
12
1,000 800
y = 0.0009x + 0.0992 R² = 0.9963
6 4 2
400
0 Gypsum a.m.
200
Gypsum p.m. 0 0
2,000
4,000
6,000
8,000
10,000
Separation time/min
Fig. 2 Relationship between separation time and Hg content in gypsum 3.5
Gypsum a.m. Gypsum p.m.
3 2.5
ln(qe–qt)
8
600
2
y = –0.0003x + 2.9113 R² = 0.7362
1.5 1
y = –0.0009x + 2.829 R² = 0.9897
0.5 0
y = 0.0013x + 0.0929 R² = 0.9996
10
1,200
t/qt
Hg concentration/ng g–1
Fig. 1 Schematic diagram of the text apparatus
0
1,000
2,000
3,000
4,000
Time/min Fig. 3 Pseudo-first-order curve of gypsum samples
centrifuge and analyzed in a TAI Q500 TG analyzer. The samples were heated at 10 °C min-1 in air from 30 to 950 °C. The first thermal decomposition stage is due to the loss of the crystal water (CaSO42H2O). The second thermal decomposition stage is an overlapping reaction between 450 and 750 °C. The first shoulder decomposition is due the decomposition of Ca(OH)2, and the second larger peak is due to the decomposition of un-reacted CaCO3. Comparing the TG curves, it can be found that with separation time increasing, the mass loss of water reduces from 17.43 to 15.47 %. It may be difficult to separate the second overlapping peak to determine the amount of Ca(OH)2 and calcium carbonate in the mixture samples Fig. 5.
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Gypsum a.m. Gypsum p.m. 0
2,000
4,000
6,000
8,000
10,000
12,000
Time/min Fig. 4 Pseudo-second-order curve of gypsum samples
It was observed that the longer settling time (2-day and 7-day) samples contain less CaSO42H2O in the final products. The shorter settling time samples contain approximately 84 % CaSO42H2O and the longer settling time contain 76 %, which is based on the loss of water. The total amount of mass loss in the overlapping peaks also shows that the short settling time samples have less mass loss (around 5.11 %) as compared to that of longer settling time which is around 6.18 %. This confirms that the final product of the longer settling time samples contains less CaSO42H2O and more unused CaCO3 and Ca(OH)2. Hence, longer settling time produces poorer quality of gypsum. In power plant #1 (PP#1), the process time between outlet of the absorber and the vacuum dewatering belt is around 5 minutes. The Hg content of gypsum collected at the end of vacuum dewatering belt in the morning and afternoon was 563 ± 62 and 551 ± 74 ng g-1, respectively. These mercury contents are very similar to the results obtained from gypsum separated from slurry (the slurry was collected in the outlet of the absorber) with a 1 h settling time. It was observed that 1/2 to 2/3 of the mercury was not adsorbed by the gypsum. The majority of the dewatered portion (gypsum) will be recirculated to the absorber, and the water portion will go into the wastewater treatment system. The mercury in the wastewater will be captured by organic sulfur compounds such as TMT-15 to form the solid waste. The capture rate of sulfur organic compounds and subsequent formation/stability of mercury compounds is underway in another study.
Partitioning effect of mercury content and speciation
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Gypsum 2 min Gypsum 10 min Gypsum 2 days Gypsum 7 days
100 95
15.47 %
25
Mass/%
17.43 % 85
15
80
6.18 %
5.11 %
10
75
Deriv mass/% min–1
20 90
5 70 0
65 0
200
400
600
800
1,000
Temperature/°C Fig. 5 TG curves of four gypsum samples under different separation times
Mercury compounds in the gypsum
The Hg content of gypsums that was collected at the outlet absorber with different settling times and the gypsum sample collected at the end of the vacuum dewatering belt in power plant #1 were utilized to calculate the mercury mass balance (shown in Fig. 6). The acceptable mass balance closures are between 70 and 130 % [14]. When settling times over 1 h were used, the mercury captured by the FGD unit resulted in erroneously high capture efficiencies, and conversely, when settling times were less than 1 h, the efficiencies were low. Thus, the gypsum separated from the slurry at 1 h should be used to calculate mercury mass balance. 200.0 %
Study of thermal release curves from mercury reference material The thermal decomposition curves obtained from the mercury reference material, including HgCl2, Hg2Cl2, HgSO4, Hg2SO4, HgS and HgO, are shown in Fig. 7. The signals have been normalized (ratio of the Hg signal to the maximum Hg signal) for ease of comparison. The peak temperature of each curve represents the thermal dissociation temperature for each mercury reference material and is
Hg removal efficiency of FGD
180.0 %
Hg in flue gas
160.0 %
Hg removal efficiency of ESP
140.0 % 120.0 % 100.0 % 80.0 % 60.0 % 40.0 % 20.0 %
1
2
7 day
2 day
1 day
2h
1h
30 min
10 min
2 min
7 day
2 day
1 day
2h
1h
30 min
10 min
2 min
0.0 %
3
Fig. 6 Calculative efficiency of power plants to take off the mercury. 1 Gypsum collected from vacuum dewatering belt; 2 Gypsum a.m.; 3 Gypsum p.m
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HgSO4
HgCl2 Hgs Hg2SO4
1
Signal intensity/a.u.
350
0.8
Treated Hg signal
Gypsum Hg2Cl2 + HgCl2 HgS Hg2SO4 HgO HgSO4
400
HgO
Hg2Cl2 0.6
0.4
300 250 200 150 100 50
0.2
0 –50
0 0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
Temperature/°C
Temperature/°C
Fig. 7 Thermal release profile of reference mercury compounds
Fig. 8 Thermal decomposition profiles of Hg compounds in gypsum from #1 power plant
provided in Table 2. It can be shown that each Hg compound is indicated by one or two specific temperatures and each species has a different peak range. Each mercury reference material was repeated at least three times. The decomposition temperatures of Hg2Cl2, HgCl2 and HgS showed little differences compared with the study of Liu and others which also used a gypsum matrix [12]. The decomposition temperatures of HgO and HgSO4 were different from those obtained by Lopez-Anton and others in which fly ash and quartz were used [7]. It is assumed that the matrix material, the Hg reference solution and the carrier gas may cause the slight differences on the peak maximum temperatures. Several peaks (359 and 560 °C) were observed for Hg2SO4.
(dashed curve) represent the combination of Hg2Cl2 and HgCl2. Additionally, this gypsum sample also contains HgO, HgSO4 and Hg2SO4. The HgS and HgO contents in the solid portion of the gypsum slurry steadily increased with settling time, while the other mercury compounds such as HgSO4, Hg2SO4, Hg2Cl2 and HgCl2 demonstrated relatively small changes.
The relationship between separation time and the Hg compounds in gypsum The thermal decomposition curves of Hg compounds in the FGD gypsum are shown in Fig. 8. After resolving overlapping peaks by using PeakFit v4.12, different mercury species can be identified. The main Hg compound in 1# power plant gypsum is HgS, with more than 50 % of the total amount of mercury (Fig. 9). The first two peaks
Study of Hg compounds of five power plant gypsums In fact, as there are differences for Hg content and Hg compounds during different separation time, it is interesting to know the information about Hg content and Hg compounds for different power plants during same separation time. Here, five gypsum samples were used for Hg compounds study, in which all of them were separated from slurry at 1 h. The thermal decomposition curves obtained from the Hg samples collected in five power plants are shown in Fig. 10. Each sample was repeated at least three times until three similar thermal decomposition curves were obtained to guarantee the reliability of the analysis of Hg species. Utilizing the peak area under these curves, the ratio of each Hg compound to the total Hg is shown in Table 3.
Table 2 Thermal decomposition temperatures for reference materials and comparison with literature Hg compounds
Maximum peak temperature T/°C
Standard deviation/°C
Hg2Cl2
138,228
4,5
HgCl2 HgS
249 317
8 8
Peak range/°C
Test replicates
Peak T in literature/°C
30–370
4
148,240 [12]
50–600 240–400
3 4
212 [12] 350 [12]
Hg2SO4
359,560
19,4
240–640
3
HgO
422
12
220–520
4
505 [7]
HgSO4
575
6
200–620
4
540 [7]
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45,000 10 min
40,000
30 min
Peak area
35,000
1h
30,000
2h
25,000
1d
20,000 15,000 10,000 5,000 0 Hg2Cl2 + HgCl2 HgSO4
HgO
HgS
Hg2SO4
(a) 50,000 10 min
45,000
30 min
40,000
1h
Peak area
35,000
2h
30,000
1d
25,000 20,000 15,000 10,000 5,000 0 Hg2Cl2 + HgCl2 HgSO4
HgO
HgS
Hg2SO4
(b) Fig. 9 Peak areas of Hg compounds in gypsum on different separation times. a Gypsum a.m., b Gypsum p.m.
does have the highest sulfur content (1.07 %) and the highest mercury content (74 ppb). Since the feeding rate of limestone into the WFGD system is based on sulfur dioxide emissions from the stack, this result may be a manifestation of the higher limestone feed rate. PP#5 has the highest mercury content and the PP#3 has the second highest mercury content of the five gypsum samples. Both gypsum samples do show higher amounts of mercury chloride compounds content. There is no clear relationship between sulfur (S) and chlorine (Cl) content in the coal and the mercury content in the gypsum. The HgO content did not vary significantly in the gypsum samples, accounting for 15–25 % of the total. This result may be due to the fact that all five samples were from FGD forced oxidation units. In a forced oxidation unit, excess air is used in the absorber which possibly resulted in the similar amount of HgO for all five samples. The main reaction steps for SO2 to be removed from the flue gas in a forced oxidation FGD process can be written as [15]: SO2ðgÞ þ H2 O ! SO2ðgÞ H2 OðaqÞ
ð5Þ
þ SO2ðgÞ H2 OðaqÞ þ H2 OðlÞ ! HSO 3 þH
ð6Þ
2 þ HSO 3ðaqÞ þ H2 O ! SO3 þ H
ð7Þ
2 CaCO3 þ 0:5H2 O þ SO2 3 ! CaSO3 0:5H2 O þ CO3
ð8Þ 2 2SO2 3 þ O2 ! 2SO4
PP#1 PP#2 PP#3 PP#4 PP#5
Signal intensity/a.u.
600 500 400
ð9Þ
2 CaCO3 þ 2H2 O þ SO2 4 ! CaSO4 2H2 O þ CO3
ð10Þ
Additionally, a more stable form of mercury, such as mercuric disulfite complex, might also form (Eq. 11) [16]. þ2 2SO2 ! HgðSO3 Þ2 3 þ Hg 2
ð11Þ
300 200 100 0 0
100
200
300
400
500
600
700
Temperature/°C
Fig. 10 Thermal release of mercury from gypsum (the five power plants)
As exhibited in Fig. 10, all of the gypsum samples show very similar decomposition curves, except the PP#5. Nevertheless, the main Hg compound is HgS for all five coal-fired power plant samples. However, the mercury chloride compounds peak is a stand out for PP#5. This result is somewhat surprising since this coal has one of the lowest chlorine contents (288 ppm); however, this coal
With higher loads of sulfur, disproportionation of sulfite might also occur (Eq. 12). The reduced sulfur ions may react with mercury and form mercuric sulfide (Eq. 13), which is highly insoluble and could easily be removed from the liquid phase, as a solid, as suggested by Blythe et al. [16] and Van Loon et al. [17]. This phenomenon may lead to HgS becoming the main Hg compound. 2 2 4SO2 3 $ 3SO4 þ S
ð12Þ
Hgþ2 þ S2 $ HgSðsÞ
ð13Þ
Mercury reactions are influenced by temperature, ionic strength, initial reactant concentrations, pH, chloride, thiosulfate and other complexing agents. [16] Thus, the Hg reactions in gypsum slurry are complicated, and the composition of Hg compounds in it is likewise difficult. Given the very low mercury compound content in the gypsum, it is nearly impossible to determine mercury speciation using
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Table 3 Mercury speciation of gypsum from each power plant PP#1
PP#2
PP#3
PP#4
PP#5
HgSO4
0.08
HgO
0.15
0.25
0.18
0.19
0.19
HgS
0.59
0.62
0.53
0.59
0.41
0.13
0.29
0.08
Hg2SO4
0.03
Mercury chloride compounds (Hg2Cl2 ? HgCl2)
0.15
XRD. Thus, a better understanding of mercury compounds in gypsum is needed.
Conclusions Gypsum slurry samples taken from WFGD systems demonstrate a mercury partitioning effect between the solid and liquid components. This partitioning effect appears to be pseudo-second-order with mercury increases in the solid component. Results from this study suggest that the collected slurry sample from the outlet of the absorber should be centrifuged within 1 h in order to accurately calculate the mercury material balance for the power plant. Thermogravimetric (TG) data also show that gypsum quality decreases with settling time. The major mercury species present in gypsum is HgS (40–60 %), with mercury chloride compounds (Hg2Cl2 and HgCl2) (10–40 %) and HgO (15–25 %) serving secondary roles. There was no clear relationship between sulfur and chlorine content in the coal and mercury speciation in the gypsum. Thus, the formation and quantity of HgS and mercury chloride compounds are based on the concentration of sulfur and chloride in the slurry and other conditions such as temperature, pH value of slurry and other, as yet, unidentified factors. Acknowledgements Financial support from the National High Technology Research and Development Program of China (No. 2013AA065404), 111 Project (B12034) and Fundamental Research Funds for the Central Universities (13ZD04) is gratefully acknowledged.
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