ARTICLE pubs.acs.org/EF
Analysis of Trace Elements in Flue Gas Desulfurization Water in the Coal Combustion System and the Removal of Boron and Mercury from the Water Akira Ohki,* Kenta Yamada, Takuya Furuzono, Tsunenori Nakajima, and Hirokazu Takanashi Department of Chemistry, Biotechnology, and Chemical Engineering, Graduate School of Science and Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan ABSTRACT: A detailed analysis was performed on the slurry supernatant water obtained from a wet flue gas desulfurization (FGD) system in a pilot plant of a coal combustion facility using five different coals. The determination of trace elements in those samples was performed by use of inductively coupled plasma atomic emission spectrometry (ICP AES), inductively coupled plasma mass spectrometry (ICP MS), and other methods, while the analysis of major cations and anions was conducted by ICP AES and ion chromatography, respectively. The levels of B, Se, and Hg in the FGD water were sometimes high (44 62 mg/L for B, 0.24 0.37 mg/L for Se, and 0.11 0.23 mg/L for Hg), and these values exceeded the Japanese national effluent standards for terrestrial water (10, 0.1, and 0.005 mg/L for B, Se, and Hg, respectively). Simulated FGD water was prepared as a result of the analysis mentioned above, and the removal of those hazardous elements was attempted using various adsorbents, including N-methylglucamine resins (DIAION CRB02 and CRB05), a N-methylglucamine fiber (Chelest fiber GRY-L), an iminodiacetic acid resin (DIAION CR11), a polyamine resin (DIAION CR20), an anion-exchange resin (DOWEX 1X8), activated alumina, and activated carbon. For the simultaneous removal of B and Hg from the simulated FGD water, which contained 60 mg/L of B and 0.11 mg/L of Hg, CRB02 was the most effective and the percent removal values for B and Hg were 88 and 97%, respectively. The resulting water met the effluent standards.
1. INTRODUCTION Coal contains various kinds of elements, including hazardous elements, and these elements are released from coal when coal is combusted.1 8 Modern coal-fired boilers are usually equipped with an electrostatic precipitator (ESP) and a wet flue gas desulfurization (FGD) system. Thus, the trace elements are distributed to ashes (bottom and fly ashes), FGD residue (gypsum) and supernatant water, and air in the process of the coal combustion system. Several studies have been performed about the presence of hazardous trace elements in FGD water in coal combustion systems. Cheng et al. investigated the partitioning of several hazardous elements in a coal combustion system with selective catalytic reduction (SCR), ESP, and FGD and described the concentrations of B, Hg, and Se in the FGD water.9 Similar studies have been performed for coal combustion systems with FGD.10,11 For those studies, only one kind of coal was used in each case, and thus, there has been no evident information about the relationship between the element concentration in coal and that in FGD water. Because FGD water may contain several hazardous trace elements, the removal of those elements is important before the water is discharged into the environment. To develop effective removal technology, a detailed characterization of FGD water is needed. The detailed analysis should include the analysis of trace elements as well as that of major cationic and anionic species. Such information is required when a fully simulated FGD water is prepared for the development of removal technology. Although B is an important micronutrient for plants, it is harmful and lethal in excess doses.12 For humans and animals, long-term excessive ingestion may cause a failure in the function of circulatory and alimentary systems. It has been reported that, when various B-contaminated waters are treated, the adsorption method is an effective technology to be adapted for drinking and effluent waters, in r 2011 American Chemical Society
comparison to the conventional method, such as coagulation.13 16 Kabay et al. reported the removal of B from geothermal wastewaters13,14 and seawater15 by use of N-methylglucamine resins. Simonnot et al. studied the removal of B from drinking water by the same type of resin.16 Also, N-methylglucamine resins were examined for the B removal from various aqueous media.17 19 An anionexchange resin,20 activated carbon,21,22 and activated alumina23,24 were used for the removal of B. However, the examination of adsorbent for the removal of B from FGD water has not been systematically conducted. Especially, simultaneous removal of B and other hazardous elements has not been investigated. In this study, we first examined the detailed features of FGD water, which had been obtained from a pilot plant of a coal combustion facility with SCR, ESP, and FGD systems using five different coals. The relationship between the element concentrations in coal and those in the FGD water was studied. As a result of the analysis of these real FGD water samples, simulated FGD water was prepared. The simultaneous removal of B and Hg from the simulated water using various adsorbents was investigated.
2. EXPERIMENTAL SECTION 2.1. Samples and Reagents. FGD water samples (supernatant of the FGD slurry) were obtained from a pilot plant of a coal combustion facility in Babcock Hitachi K.K. (Kure, Japan). A schematic description of the plant is shown in Figure 1, and the sampling point is the outlet of the circulation pump in the FGD system. The plant has SCR, ESP, and FGD equipment. The combustor is a vertical furnace with a burner Received: March 11, 2011 Revised: July 19, 2011 Published: July 19, 2011 3568
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Figure 1. Schematic description of the plant and sampling point: SCR, selective catalytic reduction; GGH, gas gas heat exchanger; ESP, electrostatic precipitator; and FGD, flue gas desulfurization.
Table 1. Proximate and Ultimate Analyses of Coals proximate analysis (wt %, as received)
ultimate analysis (wt %, dry)
moisture
ash
volatile matter
fixed carbon
C
H
N
S
O
A1 (U.S.A.) A2 (U.S.A.)
9.2 4.0
8.5 27.3
36.7 31.4
45.6 37.4
71.8 54.5
4.7 3.9
1.3 0.9
2.7 3.6
10.2 8.9
C1 (Canada)
3.7
13.5
35.0
47.9
68.6
4.8
0.8
0.4
11.5
C2 (Canada)
5.6
11.6
33.9
48.9
68.7
4.4
0.8
0.3
13.8
Ch (China)
2.0
46.9
18.6
32.4
40.9
2.3
0.6
0.8
8.0
coal (country)
installed on its top. The coal feed rate was 100 120 kg/h depending upon the kind of coal, while the amount of flue gas was 1000 1200 m3 N h 1. Five coals [two American coals (A1 and A2), two Canadian coals (C1 and C2), and one Chinese coal (Ch)] were subjected to the pilot plant, and one to three FGD water samples for each coal were obtained. Table 1 presents the proximate and ultimate analyses of these coals. The sampling of FGD water was performed 30 36 h after the combustion run was started. The FGD water sample was sent to our university every time the combustion run was finished, and the analysis was carried out as soon as possible. Nitric acid (HNO3, 60%) and hydrogen peroxide (H2O2, 30%) were of ultrapure grade from Kanto Chemical Co, Inc. and Wako Pure Chemical Industries, Ltd., respectively. A multi-element standard solution (Merck) and a single-element standard solution (Wako) for Hg were used. Other chemicals used were of analytical-grade quality. Ultrapure water was prepared by Purelab Ultra Ionic (Organo Co., Ltd.) and used throughout the experiments. Four DIAION resins, including CRB02 (N-methylglucamine resin), CRB05 (N-methylglucamine resin), CR11 (iminodiacetic acid resin), and CR20 (polyamine resin), were obtained from Mitsubishi Chemical Corp. Also, a Nmethylglucamine fiber (Chelest Fiber GRY-L, Chelest Corp., Osaka, Japan), an anion-exchange resin (DOWEX 1X8, Dow Chemical Co.), an activated alumina (KHD-46, Sumitomo Chemical Co., Ltd.), and an activated carbon (Filtrasorb 400, Calgon Carbon Corp.) were examined. 2.2. Determination of Trace Elements in Coal. The coal was digested in a similar manner described in our previous papers.5,7 Powdered coal (ca. 0.25 g) was precisely weighed and digested with an acid mixture containing HNO3 and H2O2 using a microwave processor (Milestone ETHOS 1). Detailed conditions about the microwave processing were described in the literature.7 After cooling and filtration, the filtrate was diluted to a fixed volume and analyzed by inductively coupled plasma mass spectrometry (ICP MS, Agilent 7500cx) with a dilution factor of 1300 2500. To analyze Hg in coal, powdered coal was directly measured by heat vaporization atomic absorption spectrometry (HVAAS, Nippon Instruments MA-2000).
2.3. Analysis of FGD Water. After filtration, the FGD water was analyzed for major and trace elements. For B, Ca, and Mg, inductively coupled plasma atomic emission spectrometry (ICP AES, PerkinElmer Optima 3100RL) was used, while hydride generation atomic absorption spectrometry (HGAAS, Thermo Scientific AA spectrometer S2) and cold vapor atomic absorption spectrometry (CVAAS, Nippon Instruments RA-3110A) were used for Se and Hg, respectively. The rest of the elements were measured by ICP MS. The pH value of the water samples was measured with a pH meter (TOA HM-35 V), while the determination of anion was performed by ion chromatography (JASCO PU-2080 Plus with a Dionex AS4A-SC column and a suppressor). The fractional determination of SeVI and SeIV in FGD water was performed by the conventional with and without pre-reduction procedure with HGAAS as follows. Portions of FGD water (5 mL) and HCl (37%, 5 mL) were mixed and heated at 80 90 °C for 20 min, and the resulting solution was diluted to a fixed volume. The HGAAS measurement was performed for the solution to determine the total Se concentration. The SeIV concentration in the FGD water was measured without the prereduction procedure. The SeVI concentration was calculated from the difference between the concentration of total Se and that of SeIV. 2.4. Preparation of Simulated FGD Water. After the analysis of FGD water samples, simulated wastewater was prepared as follows. About 5000 mg of CaCl2, 600 mg of NaCl, 700 mg of MgSO4, and 700 mg of Ca(NO3)2 3 4H2O were added and dissolved in 1 L of ultrapure water. Moreover, appropriate amounts of Na2B4O7 3 10H2O, Na2SeO4, and HgCl2 were added in the resulting solution to prepare the simulated FGD water. The pH of the simulated water was adjusted to 6.0 6.5 by the addition of HCl solution. 2.5. Removal of B and Other Hazardous Elements. An appropriate amount of adsorbent was added to 40 mL of the simulated FGD water and shaken for 30 min in a stoppered centrifuge tube at room temperature (23 25 °C). After filtration to separate the adsorbent, the concentrations of B, Hg, and Se in the filtrate were measured by ICP AES, CVAAS, and HGAAS, respectively. The percentage of element removal was calculated from the difference in the element 3569
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concentrations in the simulated FGD water before and after the adsorption procedure.
3. RESULTS AND DISCUSSION 3.1. Trace Elements in Coal. Trace elements in coal were determined by microwave-assisted acid digestion followed by ICP MS analysis, except for Hg, where HVAAS was used. The results are presented in Table 2. It has been reported that the concentration of B in coal greatly varies depending upon the kind of coal.1 The B concentrations in American coals are considerably higher than those in other coals. The concentration of Hg decreased in the order: coal Ch > American coals > Canadian coals, while Canadian coals had relatively low concentrations of Se. For other heavy metals, such as Cr, Cu, Mn, and Pb, coal Ch had fairly high concentrations, probably because the ash content is remarkably high (Table 1). 3.2. Analysis of FGD Water. From the pilot plant of coal combustion facility mentioned above, several FGD water samples were obtained. For some coals, multiple combustion runs under identical conditions, except for the ESP temperature, were performed. Consequently, 12 samples of FGD waters
Table 2. Concentrations of Trace Elements in Coals concentrations of trace elements in coals (μg/g) element As B Co Cr Cu Hg
coal A1 2.5
coal A2 3.3
220
2.3
250
3.7 8.2 0.13
3.1 7.7
74
11 0.09
25 0.03
8.9 0.02
32 0.29
90
53
13
31
26
Pb
17 12
47
20
62
Zn
4.1
62
25
Ni
2.0
coal Ch
40
Mn
Se
coal C2 3.5
72
5.0
20
coal C1
40
220
5.5
39 25
8.7
9.0
7.3
2.8
0.6
0.8
89
7.2
22
2.6
7.0
25
(3 from coal A1, 1 from coal A2, 3 from coal C1, 2 from coal C2, and 3 from coal Ch) were analyzed. The determination of trace elements in those samples was performed by use of ICP AES, HGAAS, CVAAS, and ICP MS. The results are indicated in Table 3. The Japanese national effluent standards (terrestrial water) of B, Se, and Hg are 10, 0.1, and 0.005 mg/L, respectively. Therefore, it is found that the levels of those three elements in the FGD water samples sometimes exceed the effluent standards. For other hazardous elements, such as As and Pb, the concentrations in the FGD water samples were below the effluent standards. The concentrations of major cationic and anionic species in the FGD water samples were measured by ICP AES and ion chromatography, respectively. The molar concentrations of cations and anions are presented in Table 4. For all of the FGD water samples, Ca2+ and Mg2+ were the predominant cationic species, which contribute to the ionic strength in the samples, while Cl and SO42 were major anionic species; the gram equivalent of cationic species was about the same as that of anionic species for all of the FGD water samples. A small amount of carbonate (0.5 1.0 mM) was observed in the FGD water samples. The pH values in the FGD water samples were measured, and the variation in the pH values is small (5.8 6.2) (Table 4). The fractional determination of SeVI and SeIV in the FGD water samples was carried out by the conventional method using HGAAS. For all of the FGD water samples, the majority of Se species was SeVI (Table 3), which has been described elsewhere.25,26 3.3. Trace Element Concentrations in FGD Water and Those in Coal. A comparison of trace element concentrations in FGD water and in source coal was conducted. In Figure 2, the concentrations of B in the FGD water samples are plotted against those in the source coals. It is apparent that a high concentration of B in FGD water is obtained when the source coal has a high B concentration. As shown in Figure 3, when the source coal with a high Se concentration was used, the resulting FGD water also contained a high level of Se. However, coal Ch gave quite low Se concentrations in the FGD water, even though the concentration of Se in the coal was high. This is because the ash content of the coal is extremely high (47%) and most Se released, when coal is burned, may be adsorbed on fly ash.
Table 3. Concentrations of Trace Elements in FGD Water Samples concentrations of trace elements in FGD water samples (mg/L)a coal A1
a
element
run 1
As B
ND 51
Co
ND
Cr
ND
b
run 2
c
ND 45
coal A2 run 3
c
run 1
c
coal C1 b
run 1
run 2
b
coal C2 run 3
c
run 1
b
run 1
run 2d
run 3c
ND 15
ND 9.5
ND 19
ND 44
ND 1.4
ND 4.9
ND 3.6
ND
ND
ND
0.03
0.01
0.01
ND
ND
ND
ND
ND
0.06
0.12
0.07
0.02
0.05
0.10
0.13
0.01
0.02
0.29
ND
ND
ND
0.08
0.07
0.01
0.01
0.01
ND
ND
ND
ND
ND
0.006
ND
0.23
Cu
ND
ND
ND
Hg
0.060
0.002
0.048
ND 0.11
ND 1.9
b
ND 62
0.84
ND 4.5
run 2
coal Ch c
Mn
ND
ND
ND
ND
0.25
0.55
0.31
0.35
0.10
0.32
0.13
ND
Ni
1.8
2.1
2.3
5.4
0.18
0.83
0.70
0.60
0.56
1.7
1.2
2.6
Pb SeVI
ND 0.24
ND 0.27
ND 0.32
ND 0.37
ND ND
ND 0.004
ND ND
ND 0.003
ND 0.002
ND 0.024
ND 0.019
ND 0.079
SeIV
ND
0.001
0.001
ND
ND
ND
ND
ND
0.001
0.001
0.001
ND
Zn
0.36
0.32
0.32
ND
ND
ND
ND
ND
ND
0.15
0.13
0.22
ND = not detected. b ESP temperature was 90 °C. c ESP temperature was 160 °C. d ESP temperature was 130 °C. 3570
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Table 4. Molar Concentrations of Major Ionic Species and pH in FGD Water Samples molar concentrations of ionic species in FGD water samples (mM)a coal A1
a
ionic species
run 1b
run 2c
Ca2+
24
30
coal A2 run 3c 30
67
11
12
Mg2+
9.5
7.3
Na+
2.2
2.0
2.2
K+
0.17
0.19
0.18
Cl
48
SO42
11
61 9.9
run 1c
71 10
coal C1
coal C2
coal Ch
run 1b
run 2b
run 3c
run 1b
run 2c
run 1b
run 2d
run 3c
36
51
52
32
32
40
38
43
0.49
0.26
0.18
1.5
0.63
3.4
2.0
5.1
7.1
2.4
2.9
3.1
3.0
2.9
2.8
2.6
2.8
0.12
0.01
0.01
0.01
0.01
0.01
0.18
0.15
65
69
90
91
69
47
76
72
0.20 95
2.9
8.4
7.5
7.2
7.5
4.3
7.4
8.0
8.2
NO3
0.24
0.22
0.34
5.4
ND
ND
ND
ND
ND
ND
ND
ND
pH
5.9
6.0
6.0
5.9
5.9
6.1
6.1
6.2
6.0
6.0
5.8
6.1
ND = not detected. b ESP temperature was 90 °C. c ESP temperature was 160 °C. d ESP temperature was 130 °C.
Figure 2. Plot of B concentrations in FGD water samples against those in coals. ESP temperatures were 90 °C (open), 130 °C (half filled), and 160 °C (filled).
Figure 3. Plot of Se concentrations in FGD water samples against those in coals. ESP temperatures were 90 °C (open), 130 °C (half filled), and 160 °C (filled).
Because the amount of gypsum produced in FGD systems is dependent upon the sulfur content in coal and some trace elements may distribute into the gypsum, the content ratio of B/S or Se/S may affect the trace element concentrations in FGD water. However, the number of coals examined in this study was limited, and a clear relationship was not observed. A portion of the trace elements is adsorbed on fly ash in ESP, and thus, the amount of trace elements that goes into FGD may decrease when a coal with a high content of ash is used. For coals A1 and A2, which are quite different in terms of their ash contents (Table 1), the B concentrations in the resulting FGD water samples are similar to each other (Figure 2), probably because the capture of B on fly ash in ESP is not large. For Hg, the deviation in data, when the same coal was tested, was considerably large. The slurry in the FGD scrubber circulates within the system. During this cycle, the concentration of Hg in the FGD water may vary. Also, some deviation in the concentrations of B and Se in the FGD water samples was observed (Figures 2 and 3), probably because of the same reason. 3.4. Removal of Hazardous Trace Elements from FGD Water. The removal of hazardous trace elements from simulated FGD water was attempted by use of various adsorbents. The amount of real FGD water for a single sampling procedure was limited, and the component of FGD water somewhat varied when multiple runs under similar conditions were performed, as mentioned above. Therefore, simulated FGD water was used to evaluate the ability of various adsorbents under constant component conditions. The simulated FGD water was prepared as described in the Experimental Section and contained 60 mg/L B, 0.11 mg/L Hg, and 0.50 mg/L Se, while 48 mM Ca2+, 6.0 mM Mg2+, 10 mM Na+, 100 mM Cl , 6.0 mM SO42 , and 6.0 mM NO3 were present as major ionic species. The concentrations of FGD water components, trace elements (B, Se, and Hg) and major ionic species, were set at relatively high values, as a result of the analysis of real FGD water samples mentioned above (Tables 3 and 4). Because the Se species in the FGD water samples analyzed was exclusively SeVI (Table 3), the simulated water contained only SeVI for Se species. Eight adsorbents were tested for the removal of hazardous elements, B, Se, and Hg, from the simulated FGD water. The results are listed in Table 5. For B, a N-methylglucamine fiber (GRY-L) showed the highest removal (95%), while N-methylglucamine resins (CRB02 and CRB05) were also effective (82 88% removal). By use of these adsorbents, the resulting 3571
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Table 5. Removal of Hazardous Elements from Simulated FGD Water by Various Adsorbentsa percent removal of element (%) adsorbent
B
Hg
Se
CRB02
88
97
0
CRB05
82
94
0
GRY-L CR11
95 0
86 100
0 3
CR20
0
84
0
1X8
0
100
48
activated alumina
14
28
18
activated carbon
12
99
3
a
The added amount of adsorbent was 15 g/L, and other conditions were described in the text. Figure 5. Effect of the concentration of adsorbent added upon the removal of Hg (simulated FGD water).
Table 6. Removal of B and Hg by CRB02 under Various Conditions (Simulated FGD Water)
shaking time (h) a
a
Figure 4. Effect of the concentration of adsorbent added upon the removal of B (simulated FGD water).
concentration of coexisting ionic
percent removal of
species (mM)
element (%)
SO42
S2O82
B
Hg
0.5
6.0
0
88
97
12 0.5
6.0 18
0 0
96 86
97 97
0.5
6.0
1.5
87
97
The conditions were the same as in Table 5.
Table 7. Removal of B and Hg by CRB02 from Real FGD Water Samples percent removal of element (%)
water can meet the Japanese national effluent standard for B. Activated alumina and activated carbon were not very effective (12 14% removal), and the rest of the adsorbents scarcely gave the B removal. All of the adsorbents, except for activated alumina, had high abilities in the removal of Hg from the simulated water. Considering the effluent standards for both B and Hg, the resulting FGD water meets the standards, only when CRB02 is used as an adsorbent. Although N-methylglucamine resins have been known as a good adsorbent for B, it was reported that the resins can also adsorb Hg.27 However, the simultaneous removal of B and Hg using the resins has not yet been reported. For Se, an anion-exchange resin 1X8 was moderately effective (48% removal), whereas the rest of the adsorbents were not effective. It has been reported that SeVI (SeO42 ) is hard to remove from aqueous media, especially when an excessive amount of SO42 is present, because SeO42 and SO42 has similar chemical structures.26 Therefore, even an anion-exchange resin did not work well, and increasing the added amount of 1X8 did not improve the removal efficiency. In our previous paper, we tried the removal of SeVI from a FGD water by use of photocatalytic reduction.28 Figures 4 and 5 show the effect of the concentration of adsorbent added (CRB02 or GRY-L) upon the simultaneous removal of B and Hg, respectively, from the simulated FGD water. For both
a
FGD watera
B
Hg
coal A1, run 1
87
96
coal A1, run 3
92
92
coal C1, run 2 coal Ch, run 1
>95 >95
b >90
See Table 3. b The FGD water sample initially contained almost no Hg.
adsorbents, when the added amount was raised, the removal of B as well as that of Hg increased. It is evident that the removal of these elements is caused by the adsorption of the elements. It is found that GRY-L is somewhat superior to CRB02 for the removal of B, whereas the former is inferior to the latter for the removal of Hg. In Table 6, the simultaneous removal of B and Hg from the simulated FGD water was attempted by use of CRB02 under various conditions. When the shaking time was lengthened, the B removal was fairly promoted. The increase in the SO42 concentration in the simulated FGD water did not affect the removal of B and Hg. It was reported that FGD water in practical coal combustion systems sometimes contains S2O82 .25 The addition of S2O82 into the water did not affect the simultaneous removal of B and Hg. The removal of B and Hg by CRB02 from the real FGD water samples was attempted. As shown in Table 7, the removal of B 3572
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Energy & Fuels and Hg was effectively carried out. It is proven that the adsorption system can be favorably applied to real FGD water. This batch adsorption system should be extended to a practical column system in future work.
4. CONCLUSION Various FGD water samples obtained from a pilot plant of a coal combustion facility were analyzed for trace elements as well as major cationic and anionic species. For B and Se, some coals gave high concentrations of these elements (44 62 mg/L for B and 0.24 0.37 mg/L for Se) in the FGD water samples. For Hg, when similar runs were performed, the deviation in data was considerably large. Consequently, the levels of B, Se, and Hg in the FGD water samples sometimes exceeded the Japanese national effluent standards. Thus, it is found that, before FGD water is discharged, appropriate treatment is needed. The removal of B and Hg from simulated FGD water was attempted with various adsorbents. For the removal of B, a Nmethylglucamine fiber, GLY-L, was the most effective. For the simultaneous removal of B and Hg, a N-methylglucamine resin, CRB02, was the most preferable and the resulting water met the effluent standards. For the removal of Se from FGD water, the adsorption method did not favorably work and some other methods, such as chemical reduction,29 photocatalytic reduction,28 and biological reduction,30 in which SeVI is reduced to insoluble Se0, are needed. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: 81-99-285-8335. Fax: 81-99-285-8339. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was carried out as a part of the Energy Innovation Program: Strategic Technical Platform for Clean Coal Technology (STEP-CCT), which has been supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. We thank Dr. Hirofumi Kikkawa and Mr. Noriyuki Imada of Babcock Hitachi K.K. for providing samples and helpful discussions.
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