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Modes of occurrence and thermal stability of mercury in different samples from Guandi coal preparation plant
Fuel 200 (2017) 22–30
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Full Length Article
Modes of occurrence and thermal stability of mercury in different samples from Guandi coal preparation plant Libing Gao a,b, Yiping Wang a, Qunwu Huang a,⇑, Shaoqing Guo b,⇑ a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China School of Environment and Safety, Taiyuan University of Science and Technology, Taiyuan 030024, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The Hg in samples was studied by
TPD-AFS technique coupled with acid leaching. The Hg is enriched in gangue and slime and decreased in cleaned coal and middlings. The Hg has close relation with ashforming mineral in the samples. The distributions of modes of occurrence of Hg in all samples are different. The thermal stability of HCl-soluble and pyrite-bound Hg is similar in all samples.
a r t i c l e
i n f o
Article history: Received 12 November 2016 Received in revised form 11 February 2017 Accepted 15 March 2017
Keywords: Coal preparation Mercury Modes of occurrence Thermal stability
a b s t r a c t The raw coal, cleaned coal, middlings, gangue and slime samples were collected from Guandi coal preparation plant, China. The modes of occurrence and thermal stability of mercury in the samples were characterized by the temperature programmed decomposition-atomic fluorescence spectroscopy (TPD-AFS) technique coupled with acid leaching. The results show that the Hg contents of cleaned coal and middlings are lower than that in raw coal while the Hg content of raw coal is less than that of gangue or slime. HCl-soluble Hg, organic-bound Hg, pyrite-bound Hg and silicate-bound Hg exist in all the samples. However, the distributions of modes of occurrence of Hg in all samples are different. The contents of HCl-soluble Hg and pyrite-bound Hg are in the order of gangue > slime > raw coal > middlings > cleaned coal. The silicate-bound Hg contents are the least and in the order of middlings > gangue > slime > raw coal > cleaned coal. The organic-bound Hg proportion has significant correlation with the organic material content of the samples. For HCl-soluble Hg and pyrite-bound Hg, the thermal stability is similar in all coal samples. The thermal stability of the silicate-bound Hg is highest among all the modes of occurrence of Hg and shows a certain dependency upon mineral in samples. The organic-bound Hg is complex and its thermal stability is different with different samples. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Huang), [email protected] (S. Guo). http://dx.doi.org/10.1016/j.fuel.2017.03.045 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
Mercury (Hg) has attracted increasing attention because it is highly toxic to both humans and the ecosystem. Coal combustion is a main source of Hg emission to the environment [1]. In the past few decades, considerable efforts have been made to understand
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Hg emissions and to develop Hg emission control technology. The coal preparation has been proved to be an economical and effective method of reducing Hg prior to coal combustion [2–8].Therefore, the raw coal from coal mines is usually processed to produce cleaned coal in coal preparation plant. Meanwhile, some byproducts are also produced, such as gangue, middlings and slime. Generally, Hg in coal can be removed through coal preparation process and the Hg content of cleaned coal is significantly decreased compared with raw coal [9–11]. However, accompanied by cleaned coal producing, large amounts of Hg are inevitably enriched in gangue, middlings and slime [12,13]. Recently, in order to reduce the disposal cost and bring economic benefits, gangue, middlings and slime have been extensively utilized as raw fuel in low calorific value coal-burning power plants in China [14,15]. The low calorific value coal-burning power plants are usually located near the coal mines. According to Annual Report on Comprehensive Utilization of Resources of China (2014), the installed capacity of low calorific value coal-burning power plants was up to 30 million kW in 2013 in China. In those power plants, gangue, middlings, slime or their blends are often utilized as fuel materials. As a result, the combustion of the low calorific value coals (gangue, middlings and slime) has become a new anthropogenic discharge source of Hg [14–17]. The low calorific value coals are separated from raw coal during coal preparation process, and they have some similar characters with the raw coal. But they have different contents of mineral, organic material and Hg [12,13,17]. Over the past decades, extensive studies have been conducted on the thermal stability of Hg as well as the modes of occurrence of Hg in coal [18–25]. However, there are very few reports on the low calorific value coals except several reports about gangue [16,17]. An understanding of the modes of occurrence of Hg and their thermal stability in low calorific value coals is crucial to predict Hg speciation and behavior during low calorific value coal combustion. The above work can provides insight into the developing of advanced Hg control technologies. In the present paper, the samples of raw coal, cleaned coal, middlings, gangue and slime from Guandi coal preparation plant were studied. The modes of occurrence of Hg and their thermal stability of all samples were characterized by the temperature programmed decomposition-atomic fluorescence spectroscopy (TPD-AFS) technique coupled with chemical leaching.
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2.2. Thermal decomposition experiments A quartz boat with 0.25 g sample was put in a fixed bed quartz tube reactor. Under the flow of N2 (300 cm3min 1), the sample was heated from room temperature to 1200 °C at the heating rate of 20 °Cmin 1. The produced gas during thermal treatment was directly swept to the AFS analyzer via the N2 flow. The dynamic release behavior of Hg0 from the thermal decomposition of samples was recorded continually. The details about the TPD AFS technique can be found in our previous literature [26]. 2.3. Chemical leaching The chemical leaching method was described by Guo et al. [20]. The detailed procedures are as follows: 5 g sample was treated with 50 ml HCl solution (5 M), then shaken for 2 h at 60 °C, followed by centrifugation at 4000 rmin 1 for 20 min and filtered. The residue was dried at 60 °C. HCl solution helped to dissolve sulfates, carbonates, oxides and phosphates. So the residue was free of HCl-soluble Hg. The obtained residue was then treated with HF (40%) and then shaken for 2 h at 60 °C. The rest of the steps were the same as above. HF can dissolve silicates and aluminosilicates. The residue was free of HCl-soluble Hg and silicate-bound Hg. After this, the residue was treated with dilute HNO3 (2 M) and then shaken for 2 h at 45 °C. The rest of the steps were the same as above. Dilute HNO3 can dissolve pyritic minerals and the residue was free of HCl-soluble Hg, silicate-bound Hg and pyrite-bound Hg. As a result, the remaining Hg in the residue only contains organicbound Hg. 2.4. Sample digestion and Hg analysis The samples were digested in microwave digestion system. On the basis of the method described by literature [17], the sample digestion was performed in the mixture of 6 ml of HNO3, 2 ml of HCl and 2 ml of HF. The liquids obtained from the digestions were solid-free and therefore suitable to for Hg analysis using AFS. The detection limit of AFS for Hg in solution is 0.01 ngmL 1 and the analysis uncertainty of obtained Hg content is less than 3%. 3. Results and discussion
2. Materials and methods
3.1. The Hg contents of different samples
2.1. Plant description and samples
The Hg contents of all samples are shown in Fig. 1. It can be seen that the Hg content of cleaned coal is much less than that in raw coal, proving that the coal preparation process is an effective way to remove the Hg in coal. The Hg content of gangue is highest among all the samples, suggesting that Hg is associated with minerals, which is in agreement with the reports [6,12]. The Hg content of middlings is less than that in raw coal, while the Hg content of raw coal is less than that of slime. This is consistent with the laboratory experiment results reported by Wang et al. [13]. In general, the Hg contents of the samples are in the order of gangue > coal slime > raw coal > middlings > cleaned coal. It is interesting that the Hg contents have a positive relation with the ash contents (see Fig. 1), i.e. the more ash contents, the more Hg contents. The regression coefficient for the relationship between Hg contents and the ash contents was 0.966, which suggests that the Hg has close relation with ash-forming mineral in the samples. This result is greatly in agreement with the previous findings [27]. Since the Hg contents of the samples are in the order of gangue > middlings > cleaned coal and the density of the samples shows the same tendency, it is concluded that the Hg content increases with the increased density of the samples from GD coal
The raw coal, cleaned coal, middlings, gangue and slime samples were collected from the Guandi (GD) coal preparation plant in Shanxi province, China. The main device in this plant is dense medium cyclone, which has been widely used to upgrade run-of-mine coal in the modern coal preparation industry. The raw coal (bituminous coal) was firstly classified into the coarse coal (>0.5 mm) and the fine coal (<0.5 mm) after being crushed, then the fine coal were sent to produce the slime by flotation technology. The mass yield of slime is about 5%. The coarse coal were classified into the cleaned coal (<1.4 gcm 3), middlings (1.4 –1.8 gcm 3) and gangue (>1.8 gcm 3) by three-product dense medium cyclone. The mass yields of the cleaned coal, middlings and gangue are about 60%, 10 –20% and 5 –15%, respectively. About 60 kg samples were collected for each sample according to the Chinese national standard, GB475-2008. The samples were firstly crushed to about 2 mm by Jaw crusher and mixed thoroughly, then pulverized to finer than 200 mesh screen and air-dried prior to analysis. Proximate and ultimate analyses of the samples are listed in Table 1.
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Table 1 Proximate and Ultimate Analyses of feed coals (wt%). Samples
raw coal cleaned coal middlings gangue slime
Proximate analysis, ad
Ultimate analysis, ad
Qnet,ad (MJ/kg)
Hg(ng/g)
24.66 30.08 25.83 5.68 21.23
239.33 130.25 145.50 654.00 314.25
a
M
A
V
C
H
N
S
O
0.62 0.61 0.57 0.57 0.58
23.08 13.08 22.67 72.12 35.30
12.86 11.83 12.28 10.29 11.84
67.66 78.08 68.12 17.96 55.46
3.32 3.53 3.23 1.56 2.86
0.98 1.08 0.94 0.28 0.84
1.10 1.15 0.94 1.87 1.37
3.24 2.47 3.53 5.64 3.59
ad: air dried basis; M : Moisture;A : ash; V: volatile matter; Qnet,ad: Net calorific value; a: by difference.
Fig. 1. Relationship between Hg and ash contents of all coal samples.
preparation plants. It is generally in agreement with the Hg distribution of coal samples obtained by density fractionation reported in literatures [27,28]. 3.2. Distribution of modes of occurrence of Hg in different samples The modes of occurrence of Hg in different samples are obtained on the basis of the chemical leaching method [20]. The Hg dissolved in HCl, HF and HNO3 was denoted as HCl-soluble Hg, silicate-bound Hg and pyrite-bound Hg, respectively. After the HNO3 extraction procedure, the Hg content of the residue was analyzed and named as organic-bound Hg. The results are listed in Table 2. As shown in Table 2, all the samples contain HCl-soluble Hg, silicate-bound Hg, pyrite–bound Hg and organic-bound Hg. However, the distributions of modes of occurrence of Hg in all samples are different. Overall, the pyrite-bound Hg is the dominant form of Hg for all samples, which accounts for 49.29% for cleaned coal and 64.07% for gangue. This result is similar to other reports about samples from coal preparation plant [29]. It also shows that the
second most abundant form of Hg is the organic-bound Hg, in the range from 26.30% of gangue to 39.49% of cleaned coal. This result is consistent with previous literates [20,28]. It was reported that the Hg in coal is most likely to be occurred as pyrite-bound Hg, followed by organic-bound Hg [20,28]. Generally, the cleaned coal consists of large amount of organic material and small portion of fine-grained minerals encapsulated in organic coaly constituent [6]. The middlings contains the relatively larger density fractions with many fine-grained minerals closely associated with the organic constituents [13]. The gangue mainly contains coarse-grained minerals with a greater density and the Hg in the gangue is possibly associated with coarsegrained minerals [12]. The slime is the physically separable fine fractions and mineral fragments falling from the coal fractures [12]. The contents of HCl-soluble Hg and pyrite-bound Hg are in the order of gangue > slime > raw coal > middlings > cleaned coal, which have close relation with the ash contents of the samples (Table 1). The HCl-soluble Hg and pyrite-bound Hg are mainly enriched in gangue and slime, depleted in cleaned coal and middlings. This suggests that HCl-soluble Hg and pyrite-bound Hg are mostly bound or present in large-grained minerals or mineral fragments, which can be effectively removed during the coal cleaning process [3,6]. However, the Hg-bearing fine-grained grains in cleaned coal and middlings could not be ignored. The contents of silicate-bound Hg are the least and in the order of middlings > gangue > slime > raw coal > cleaned coal. The contents of the organic-bound Hg are in the order of gangue > slime > raw coal > middlings > cleaned coal. However, the proportion of organic-bound Hg in samples is in the opposite order. For example, the cleaned coal has the largest proportion (about 39.49%) of organic-bound Hg while the gangue has the least proportion (about 26.30%) of organic-bound Hg. It implies that the proportion of organic-bound Hg has significant correlation with the organic material content of samples (Table 1).
3.3. Thermal stability of Hg in different samples The thermal stability of Hg in different samples from GD coal preparation plant is identified by TPD-AFS coupled with acid leaching and the results are given in the following sections.
Table 2 Distribution of modes of occurrence of Hg in samples. Modes of occurrence of Hg
Raw coal
Cleaned coal
Middlings
Gangue
Slime
HCl-soluble
ng/g wt%
14.23 5.95
10.67 8.19
11.80 8.11
59.10 9.51
22.50 7.16
Silicate-bound
ng/g wt%
7.22 3.02
6.80 5.22
10.07 6.92
9.71 1.56
8.42 2.68
Pyrite-bound
ng/g wt%
127.61 53.32
64.20 49.29
78.05 53.64
397.98 64.07
163.45 52.01
Organic-bound
ng/g wt%
84.05 35.12
51.44 39.49
53.91 37.05
163.44 26.30
99.62 31.70
Recovery
wt%
97.40
102.20
105.73
101.45
93.55
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3.3.1. Raw coal The dynamic Hg release profile of raw coal as well as the release profiles of different modes of occurrence of Hg (HCl-soluble Hg, pyrite-bound Hg, silicate-bound Hg and organic-bound Hg) is shown in Fig. 2. HCl-soluble Hg is identified by subtracting the profile of HCl-leached coal from that of raw coal. Pyrite-bound Hg is identified by subtracting the profile of HCl-HF-HNO3-leached coal from that of HCl-HF-leached coal. Silicate-bound Hg is identified by subtracting the TPD AFS profile of HCl-HF-leached coal from that of HCl-leached coal. Organic-bound Hg is identified by the profile of HCl-HF-HNO3-leached coal [20]. Fig. 2a shows three peaks at different temperature range for raw coal. There is a broad and overlapped peak (denoted as peak A) at 150–450 °C, implying a diversity of modes of occurrence of Hg
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existing in the coal sample [30–33]. Also, there is another broad peak (denoted as peak B) at 450–950 °C with a spike at approximately 580 °C, which can ascribe to the release of pyrite-bound Hg [17,20,28]. Meanwhile, a minor peak (denoted as peak C) at temperature > 900 °C is possibly from silicate-bound Hg [17,20]. The HCl-soluble Hg (Fig. 2b) is released at the temperature range of 150–300 °C with a spike at approximately 240 °C. This is in agreement with the literatures and might be attributed to HgCl2-, Hg2Cl2- or HgBr2-type Hg [20,30]. The pyrite-bound Hg (Fig. 2c) releases at about 350–950 °C with a small peak at 400 °C and a spike peak at 580 °C, which agrees with other reports [20,28]. Compared with the profile of raw coal in Fig. 2a, it suggests that the small peak is located in the peak A and the spike peak belongs to the peak B. The silicate-bound Hg (Fig. 2d) releases at
Fig. 2. The dynamic Hg release profiles of raw coal (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
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about 950–1150 °C, which is consistent with the reports [17,20]. According to the comparison between Fig. 2d and Fig. 2a, it is reasonable to be concluded that the peak C (Fig. 2a) is from the release of silicate-bound Hg. The Fig. 2e shows that the organic-bound Hg releases at about 180 °C with a spike at approximately 290 °C. Moreover, there exists a small peak at about 580 °C. Compared with Fig. 2a and c, this small peak should be from the release of pyrite-bound Hg. This indicates that some pyrite-bound Hg is encapsulated in the organic material, thereby pyrite-bound Hg still remained in the organic-bound material after HCl, HF and HNO3 extraction procedure [6]. 3.3.2. Cleaned coal The dynamic Hg release profiles of cleaned coal and its different modes of occurrence of Hg are shown in Fig. 3. There are two peaks
in Fig. 3a and the Hg release behavior at temperature < 900 °C is similar to that in raw coal. Unlike raw coal, a minor peak is absent at temperature > 900 °C, which implies that the silicate-bound Hg in cleaned coal is thermal stable and difficult to release. For HClsoluble Hg (Fig. 3b) and pyrite-bound Hg (Fig. 3c), the releasing temperature ranges and peak positons are similar to that of raw coal. However, the peak intensity is obviously decreased, which is ascribed to the lower contents of HCl-soluble Hg and pyritebound Hg in cleaned coal. Although there is no silicate-bound Hg release from cleaned coal according to the profile of Fig. 3a, Fig. 3d shows some silicate-bound Hg released at 950–1100 °C. In fact, not only in cleaned coal, but also in other samples (raw coal, middlings, gangue and slime), more silicate-bound Hg is released after HCl-leaching process. This result indicates that silicatebound Hg is not completely released out during thermal decompo-
Fig. 3. The dynamic Hg release profiles of cleaned coal (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
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sition because of its higher thermal stability. Actually, HCl can dissolve some minerals in coal during HCl leaching [34]. With the removal of HCl-soluble mineral, a portion of silicate-bound Hg surrounded by HCl-soluble mineral can be released more easily. Compared with Figs. 3e and 2e, it can be identified that more Hg releases at about 580 °C, implying more pyrite-bound Hg encapsulated by the organic material in cleaned coal. Also, a minor peak can be seen at about 1000 °C, which should be from silicatebound Hg encapsulated by the organic material in cleaned coal. This result indicates that some pyrite mineral and silicate mineral cannot be leached out during acid extraction procedure because they are closely surrounded by the higher content of organic material in cleaned coal.
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3.3.3. Middlings Fig. 4a shows the result of the dynamic Hg release profiles of middlings and its different modes of occurrence of Hg. It shows similar peak positons and temperature range with that of raw coal. The HCl-soluble Hg (Fig. 4b), pyrite-bound Hg (Fig. 4c), silicatebound Hg (Fig. 4d) and organic-bound Hg (Fig. 4e) also show similar releasing temperature ranges and peak positons with that of raw coal. It might be caused by the fact that the difference in the composition between the middlings and the raw coal are minor (Table 1). Despite of the similar releasing temperature ranges and peak positons with raw coal, the different peak intensity can be observed. Also, the peak intensity of HCl-soluble Hg, pyritebound Hg and organic-bound Hg are all lower than that from raw coal, which is proportional to the their contents of middlings.
Fig. 4. The dynamic Hg release profiles of middlings (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
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Fig. 5. The dynamic Hg release profiles of gangue (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
However, the peak intensity of silicate-bound Hg is higher than that in raw coal, indicating a more amount of silicate-bound Hg in middlings. 3.3.4. Gangue The results of gangue and its corresponding modes of occurrence of Hg are shown in Fig. 5. The similar Hg releasing temperature ranges and peak positons with that of raw coal is also found in Fig. 5a. Meanwhile, HCl-soluble Hg (Fig. 5b) and pyrite-bound Hg (Fig. 5c) also present similar Hg peak positons with that in raw coal. Nevertheless, the peak intensity of HCl-soluble Hg and pyrite-bound Hg are much higher than that of raw coal, cleaned coal and middlings, which coincides with the fact that the contents of HCl-soluble Hg and pyrite-bound Hg in gangue are highest among all the samples. Also, the peak intensity of silicate-bound
Hg is higher than that of raw coal shown in Fig. 2d, which is ascribable to its higher content of silicate-bound Hg in gangue than raw coal. Note that there is no Hg release peak at temperature > 450 °C in Fig. 5e, while the Hg release > 450 °C can be observed for Fig. 2e, Figs. 3e and 4e. It indicates that the pyrite-bound Hg and silicatebound Hg in gangue are completely soluble during HNO3 extraction procedure, which may due to the higher mineral constituents and less organic material matter in gangue (Table 1). Meanwhile, there exists a minor peak at about 200 °C in Fig. 5e, which indicates that the forms of organic-bound Hg are different with other samples. 3.3.5. Slime The results of slime and its corresponding modes of occurrence of Hg are shown in Fig. 6. It is interesting that Fig. 6a shows a broad
L. Gao et al. / Fuel 200 (2017) 22–30
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Fig. 6. The dynamic Hg release profiles of slime (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
peak with three sub-peaks at 150–450 °C. Based on the discussion above, the broad peak should contain HCl-soluble Hg, pyrite-bound Hg and organic-bound Hg. It might be caused by the mixing characteristics in the original composition of slime since the slime is the physically separable fine fractions and mineral fragments falling from the coal fractures. The HCl-soluble Hg (Fig. 6b), pyrite-bound Hg (Fig. 6c) and silicate-bound Hg (Fig. 6d) also show similar behavior with raw coal. However, their peak intensity are all higher than that of raw coal, which is due to the higher contents of HCl-soluble Hg, pyrite-bound Hg and silicate-bound Hg of slime. Similar with Fig. 5e, there is also no Hg release peak at temperature > 450 °C in Fig. 6e, which suggests that the pyrite-bound Hg and silicate-bound Hg in slime are easier to leach out by chemical leaching.
In summary, HCl-soluble Hg as well as pyrite-bound Hg for all samples shows similar releasing temperature ranges and peak positons. It indicates that the thermal stability of HCl-soluble Hg in all samples is all similar. Also, the pyrite-bound Hg in all samples presents similar thermal stability. This further suggests that the thermal stability of HCl-soluble Hg and pyrite-bound Hg have no significant correlation with the density or particle sizes of the samples because all samples are separated by density or particle sizes during coal preparation process. For silicate-bound Hg in all samples, it cannot completely release out because of its higher thermal stability. After chemical leaching, silicate-bound Hg in all samples are easier to release out, implying that the thermal stability of silicate-bound Hg shows a certain dependency upon mineral in samples. For organic-bound Hg, the releasing character and tem-
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perature range are different with different samples, indicating that the organic-bound Hg is complex in the samples. 4. Conclusions The raw coal, cleaned coal, middlings, gangue and slime samples from GD coal preparation plant were studied to identify the modes of occurrence of Hg and their thermal stability. The main findings can be summarized as follows: 1. The Hg is mainly enriched in gangue and slime and partially removed in cleaned coal and middlings during coal preparation process. 2. All samples contain HCl-soluble Hg, organic-bound Hg, pyritebound Hg and silicate-bound Hg with different contents. The pyrite-bound Hg is the dominant form of Hg for all coal samples. The content of silicate-bound Hg is the least among the four modes of occurrence of Hg. HCl-soluble Hg and pyritebound Hg mainly enriched in gangue and slime. The organicbound Hg proportion has significant correlation with the organic material content. 3. The thermal stability of HCl-soluble Hg as well as pyrite-bound Hg for all samples is similar. Silicate-bound Hg cannot completely release out because of its higher thermal stability and its thermal stability shows some relation with the mineral in samples. The release behavior of organic-bound Hg is different with different samples.
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Full Length Article
Modes of occurrence and thermal stability of mercury in different samples from Guandi coal preparation plant Libing Gao a,b, Yiping Wang a, Qunwu Huang a,⇑, Shaoqing Guo b,⇑ a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China School of Environment and Safety, Taiyuan University of Science and Technology, Taiyuan 030024, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The Hg in samples was studied by
TPD-AFS technique coupled with acid leaching. The Hg is enriched in gangue and slime and decreased in cleaned coal and middlings. The Hg has close relation with ashforming mineral in the samples. The distributions of modes of occurrence of Hg in all samples are different. The thermal stability of HCl-soluble and pyrite-bound Hg is similar in all samples.
a r t i c l e
i n f o
Article history: Received 12 November 2016 Received in revised form 11 February 2017 Accepted 15 March 2017
Keywords: Coal preparation Mercury Modes of occurrence Thermal stability
a b s t r a c t The raw coal, cleaned coal, middlings, gangue and slime samples were collected from Guandi coal preparation plant, China. The modes of occurrence and thermal stability of mercury in the samples were characterized by the temperature programmed decomposition-atomic fluorescence spectroscopy (TPD-AFS) technique coupled with acid leaching. The results show that the Hg contents of cleaned coal and middlings are lower than that in raw coal while the Hg content of raw coal is less than that of gangue or slime. HCl-soluble Hg, organic-bound Hg, pyrite-bound Hg and silicate-bound Hg exist in all the samples. However, the distributions of modes of occurrence of Hg in all samples are different. The contents of HCl-soluble Hg and pyrite-bound Hg are in the order of gangue > slime > raw coal > middlings > cleaned coal. The silicate-bound Hg contents are the least and in the order of middlings > gangue > slime > raw coal > cleaned coal. The organic-bound Hg proportion has significant correlation with the organic material content of the samples. For HCl-soluble Hg and pyrite-bound Hg, the thermal stability is similar in all coal samples. The thermal stability of the silicate-bound Hg is highest among all the modes of occurrence of Hg and shows a certain dependency upon mineral in samples. The organic-bound Hg is complex and its thermal stability is different with different samples. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Huang), [email protected] (S. Guo). http://dx.doi.org/10.1016/j.fuel.2017.03.045 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.
Mercury (Hg) has attracted increasing attention because it is highly toxic to both humans and the ecosystem. Coal combustion is a main source of Hg emission to the environment [1]. In the past few decades, considerable efforts have been made to understand
L. Gao et al. / Fuel 200 (2017) 22–30
Hg emissions and to develop Hg emission control technology. The coal preparation has been proved to be an economical and effective method of reducing Hg prior to coal combustion [2–8].Therefore, the raw coal from coal mines is usually processed to produce cleaned coal in coal preparation plant. Meanwhile, some byproducts are also produced, such as gangue, middlings and slime. Generally, Hg in coal can be removed through coal preparation process and the Hg content of cleaned coal is significantly decreased compared with raw coal [9–11]. However, accompanied by cleaned coal producing, large amounts of Hg are inevitably enriched in gangue, middlings and slime [12,13]. Recently, in order to reduce the disposal cost and bring economic benefits, gangue, middlings and slime have been extensively utilized as raw fuel in low calorific value coal-burning power plants in China [14,15]. The low calorific value coal-burning power plants are usually located near the coal mines. According to Annual Report on Comprehensive Utilization of Resources of China (2014), the installed capacity of low calorific value coal-burning power plants was up to 30 million kW in 2013 in China. In those power plants, gangue, middlings, slime or their blends are often utilized as fuel materials. As a result, the combustion of the low calorific value coals (gangue, middlings and slime) has become a new anthropogenic discharge source of Hg [14–17]. The low calorific value coals are separated from raw coal during coal preparation process, and they have some similar characters with the raw coal. But they have different contents of mineral, organic material and Hg [12,13,17]. Over the past decades, extensive studies have been conducted on the thermal stability of Hg as well as the modes of occurrence of Hg in coal [18–25]. However, there are very few reports on the low calorific value coals except several reports about gangue [16,17]. An understanding of the modes of occurrence of Hg and their thermal stability in low calorific value coals is crucial to predict Hg speciation and behavior during low calorific value coal combustion. The above work can provides insight into the developing of advanced Hg control technologies. In the present paper, the samples of raw coal, cleaned coal, middlings, gangue and slime from Guandi coal preparation plant were studied. The modes of occurrence of Hg and their thermal stability of all samples were characterized by the temperature programmed decomposition-atomic fluorescence spectroscopy (TPD-AFS) technique coupled with chemical leaching.
23
2.2. Thermal decomposition experiments A quartz boat with 0.25 g sample was put in a fixed bed quartz tube reactor. Under the flow of N2 (300 cm3min 1), the sample was heated from room temperature to 1200 °C at the heating rate of 20 °Cmin 1. The produced gas during thermal treatment was directly swept to the AFS analyzer via the N2 flow. The dynamic release behavior of Hg0 from the thermal decomposition of samples was recorded continually. The details about the TPD AFS technique can be found in our previous literature [26]. 2.3. Chemical leaching The chemical leaching method was described by Guo et al. [20]. The detailed procedures are as follows: 5 g sample was treated with 50 ml HCl solution (5 M), then shaken for 2 h at 60 °C, followed by centrifugation at 4000 rmin 1 for 20 min and filtered. The residue was dried at 60 °C. HCl solution helped to dissolve sulfates, carbonates, oxides and phosphates. So the residue was free of HCl-soluble Hg. The obtained residue was then treated with HF (40%) and then shaken for 2 h at 60 °C. The rest of the steps were the same as above. HF can dissolve silicates and aluminosilicates. The residue was free of HCl-soluble Hg and silicate-bound Hg. After this, the residue was treated with dilute HNO3 (2 M) and then shaken for 2 h at 45 °C. The rest of the steps were the same as above. Dilute HNO3 can dissolve pyritic minerals and the residue was free of HCl-soluble Hg, silicate-bound Hg and pyrite-bound Hg. As a result, the remaining Hg in the residue only contains organicbound Hg. 2.4. Sample digestion and Hg analysis The samples were digested in microwave digestion system. On the basis of the method described by literature [17], the sample digestion was performed in the mixture of 6 ml of HNO3, 2 ml of HCl and 2 ml of HF. The liquids obtained from the digestions were solid-free and therefore suitable to for Hg analysis using AFS. The detection limit of AFS for Hg in solution is 0.01 ngmL 1 and the analysis uncertainty of obtained Hg content is less than 3%. 3. Results and discussion
2. Materials and methods
3.1. The Hg contents of different samples
2.1. Plant description and samples
The Hg contents of all samples are shown in Fig. 1. It can be seen that the Hg content of cleaned coal is much less than that in raw coal, proving that the coal preparation process is an effective way to remove the Hg in coal. The Hg content of gangue is highest among all the samples, suggesting that Hg is associated with minerals, which is in agreement with the reports [6,12]. The Hg content of middlings is less than that in raw coal, while the Hg content of raw coal is less than that of slime. This is consistent with the laboratory experiment results reported by Wang et al. [13]. In general, the Hg contents of the samples are in the order of gangue > coal slime > raw coal > middlings > cleaned coal. It is interesting that the Hg contents have a positive relation with the ash contents (see Fig. 1), i.e. the more ash contents, the more Hg contents. The regression coefficient for the relationship between Hg contents and the ash contents was 0.966, which suggests that the Hg has close relation with ash-forming mineral in the samples. This result is greatly in agreement with the previous findings [27]. Since the Hg contents of the samples are in the order of gangue > middlings > cleaned coal and the density of the samples shows the same tendency, it is concluded that the Hg content increases with the increased density of the samples from GD coal
The raw coal, cleaned coal, middlings, gangue and slime samples were collected from the Guandi (GD) coal preparation plant in Shanxi province, China. The main device in this plant is dense medium cyclone, which has been widely used to upgrade run-of-mine coal in the modern coal preparation industry. The raw coal (bituminous coal) was firstly classified into the coarse coal (>0.5 mm) and the fine coal (<0.5 mm) after being crushed, then the fine coal were sent to produce the slime by flotation technology. The mass yield of slime is about 5%. The coarse coal were classified into the cleaned coal (<1.4 gcm 3), middlings (1.4 –1.8 gcm 3) and gangue (>1.8 gcm 3) by three-product dense medium cyclone. The mass yields of the cleaned coal, middlings and gangue are about 60%, 10 –20% and 5 –15%, respectively. About 60 kg samples were collected for each sample according to the Chinese national standard, GB475-2008. The samples were firstly crushed to about 2 mm by Jaw crusher and mixed thoroughly, then pulverized to finer than 200 mesh screen and air-dried prior to analysis. Proximate and ultimate analyses of the samples are listed in Table 1.
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L. Gao et al. / Fuel 200 (2017) 22–30
Table 1 Proximate and Ultimate Analyses of feed coals (wt%). Samples
raw coal cleaned coal middlings gangue slime
Proximate analysis, ad
Ultimate analysis, ad
Qnet,ad (MJ/kg)
Hg(ng/g)
24.66 30.08 25.83 5.68 21.23
239.33 130.25 145.50 654.00 314.25
a
M
A
V
C
H
N
S
O
0.62 0.61 0.57 0.57 0.58
23.08 13.08 22.67 72.12 35.30
12.86 11.83 12.28 10.29 11.84
67.66 78.08 68.12 17.96 55.46
3.32 3.53 3.23 1.56 2.86
0.98 1.08 0.94 0.28 0.84
1.10 1.15 0.94 1.87 1.37
3.24 2.47 3.53 5.64 3.59
ad: air dried basis; M : Moisture;A : ash; V: volatile matter; Qnet,ad: Net calorific value; a: by difference.
Fig. 1. Relationship between Hg and ash contents of all coal samples.
preparation plants. It is generally in agreement with the Hg distribution of coal samples obtained by density fractionation reported in literatures [27,28]. 3.2. Distribution of modes of occurrence of Hg in different samples The modes of occurrence of Hg in different samples are obtained on the basis of the chemical leaching method [20]. The Hg dissolved in HCl, HF and HNO3 was denoted as HCl-soluble Hg, silicate-bound Hg and pyrite-bound Hg, respectively. After the HNO3 extraction procedure, the Hg content of the residue was analyzed and named as organic-bound Hg. The results are listed in Table 2. As shown in Table 2, all the samples contain HCl-soluble Hg, silicate-bound Hg, pyrite–bound Hg and organic-bound Hg. However, the distributions of modes of occurrence of Hg in all samples are different. Overall, the pyrite-bound Hg is the dominant form of Hg for all samples, which accounts for 49.29% for cleaned coal and 64.07% for gangue. This result is similar to other reports about samples from coal preparation plant [29]. It also shows that the
second most abundant form of Hg is the organic-bound Hg, in the range from 26.30% of gangue to 39.49% of cleaned coal. This result is consistent with previous literates [20,28]. It was reported that the Hg in coal is most likely to be occurred as pyrite-bound Hg, followed by organic-bound Hg [20,28]. Generally, the cleaned coal consists of large amount of organic material and small portion of fine-grained minerals encapsulated in organic coaly constituent [6]. The middlings contains the relatively larger density fractions with many fine-grained minerals closely associated with the organic constituents [13]. The gangue mainly contains coarse-grained minerals with a greater density and the Hg in the gangue is possibly associated with coarsegrained minerals [12]. The slime is the physically separable fine fractions and mineral fragments falling from the coal fractures [12]. The contents of HCl-soluble Hg and pyrite-bound Hg are in the order of gangue > slime > raw coal > middlings > cleaned coal, which have close relation with the ash contents of the samples (Table 1). The HCl-soluble Hg and pyrite-bound Hg are mainly enriched in gangue and slime, depleted in cleaned coal and middlings. This suggests that HCl-soluble Hg and pyrite-bound Hg are mostly bound or present in large-grained minerals or mineral fragments, which can be effectively removed during the coal cleaning process [3,6]. However, the Hg-bearing fine-grained grains in cleaned coal and middlings could not be ignored. The contents of silicate-bound Hg are the least and in the order of middlings > gangue > slime > raw coal > cleaned coal. The contents of the organic-bound Hg are in the order of gangue > slime > raw coal > middlings > cleaned coal. However, the proportion of organic-bound Hg in samples is in the opposite order. For example, the cleaned coal has the largest proportion (about 39.49%) of organic-bound Hg while the gangue has the least proportion (about 26.30%) of organic-bound Hg. It implies that the proportion of organic-bound Hg has significant correlation with the organic material content of samples (Table 1).
3.3. Thermal stability of Hg in different samples The thermal stability of Hg in different samples from GD coal preparation plant is identified by TPD-AFS coupled with acid leaching and the results are given in the following sections.
Table 2 Distribution of modes of occurrence of Hg in samples. Modes of occurrence of Hg
Raw coal
Cleaned coal
Middlings
Gangue
Slime
HCl-soluble
ng/g wt%
14.23 5.95
10.67 8.19
11.80 8.11
59.10 9.51
22.50 7.16
Silicate-bound
ng/g wt%
7.22 3.02
6.80 5.22
10.07 6.92
9.71 1.56
8.42 2.68
Pyrite-bound
ng/g wt%
127.61 53.32
64.20 49.29
78.05 53.64
397.98 64.07
163.45 52.01
Organic-bound
ng/g wt%
84.05 35.12
51.44 39.49
53.91 37.05
163.44 26.30
99.62 31.70
Recovery
wt%
97.40
102.20
105.73
101.45
93.55
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3.3.1. Raw coal The dynamic Hg release profile of raw coal as well as the release profiles of different modes of occurrence of Hg (HCl-soluble Hg, pyrite-bound Hg, silicate-bound Hg and organic-bound Hg) is shown in Fig. 2. HCl-soluble Hg is identified by subtracting the profile of HCl-leached coal from that of raw coal. Pyrite-bound Hg is identified by subtracting the profile of HCl-HF-HNO3-leached coal from that of HCl-HF-leached coal. Silicate-bound Hg is identified by subtracting the TPD AFS profile of HCl-HF-leached coal from that of HCl-leached coal. Organic-bound Hg is identified by the profile of HCl-HF-HNO3-leached coal [20]. Fig. 2a shows three peaks at different temperature range for raw coal. There is a broad and overlapped peak (denoted as peak A) at 150–450 °C, implying a diversity of modes of occurrence of Hg
25
existing in the coal sample [30–33]. Also, there is another broad peak (denoted as peak B) at 450–950 °C with a spike at approximately 580 °C, which can ascribe to the release of pyrite-bound Hg [17,20,28]. Meanwhile, a minor peak (denoted as peak C) at temperature > 900 °C is possibly from silicate-bound Hg [17,20]. The HCl-soluble Hg (Fig. 2b) is released at the temperature range of 150–300 °C with a spike at approximately 240 °C. This is in agreement with the literatures and might be attributed to HgCl2-, Hg2Cl2- or HgBr2-type Hg [20,30]. The pyrite-bound Hg (Fig. 2c) releases at about 350–950 °C with a small peak at 400 °C and a spike peak at 580 °C, which agrees with other reports [20,28]. Compared with the profile of raw coal in Fig. 2a, it suggests that the small peak is located in the peak A and the spike peak belongs to the peak B. The silicate-bound Hg (Fig. 2d) releases at
Fig. 2. The dynamic Hg release profiles of raw coal (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
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L. Gao et al. / Fuel 200 (2017) 22–30
about 950–1150 °C, which is consistent with the reports [17,20]. According to the comparison between Fig. 2d and Fig. 2a, it is reasonable to be concluded that the peak C (Fig. 2a) is from the release of silicate-bound Hg. The Fig. 2e shows that the organic-bound Hg releases at about 180 °C with a spike at approximately 290 °C. Moreover, there exists a small peak at about 580 °C. Compared with Fig. 2a and c, this small peak should be from the release of pyrite-bound Hg. This indicates that some pyrite-bound Hg is encapsulated in the organic material, thereby pyrite-bound Hg still remained in the organic-bound material after HCl, HF and HNO3 extraction procedure [6]. 3.3.2. Cleaned coal The dynamic Hg release profiles of cleaned coal and its different modes of occurrence of Hg are shown in Fig. 3. There are two peaks
in Fig. 3a and the Hg release behavior at temperature < 900 °C is similar to that in raw coal. Unlike raw coal, a minor peak is absent at temperature > 900 °C, which implies that the silicate-bound Hg in cleaned coal is thermal stable and difficult to release. For HClsoluble Hg (Fig. 3b) and pyrite-bound Hg (Fig. 3c), the releasing temperature ranges and peak positons are similar to that of raw coal. However, the peak intensity is obviously decreased, which is ascribed to the lower contents of HCl-soluble Hg and pyritebound Hg in cleaned coal. Although there is no silicate-bound Hg release from cleaned coal according to the profile of Fig. 3a, Fig. 3d shows some silicate-bound Hg released at 950–1100 °C. In fact, not only in cleaned coal, but also in other samples (raw coal, middlings, gangue and slime), more silicate-bound Hg is released after HCl-leaching process. This result indicates that silicatebound Hg is not completely released out during thermal decompo-
Fig. 3. The dynamic Hg release profiles of cleaned coal (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
L. Gao et al. / Fuel 200 (2017) 22–30
sition because of its higher thermal stability. Actually, HCl can dissolve some minerals in coal during HCl leaching [34]. With the removal of HCl-soluble mineral, a portion of silicate-bound Hg surrounded by HCl-soluble mineral can be released more easily. Compared with Figs. 3e and 2e, it can be identified that more Hg releases at about 580 °C, implying more pyrite-bound Hg encapsulated by the organic material in cleaned coal. Also, a minor peak can be seen at about 1000 °C, which should be from silicatebound Hg encapsulated by the organic material in cleaned coal. This result indicates that some pyrite mineral and silicate mineral cannot be leached out during acid extraction procedure because they are closely surrounded by the higher content of organic material in cleaned coal.
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3.3.3. Middlings Fig. 4a shows the result of the dynamic Hg release profiles of middlings and its different modes of occurrence of Hg. It shows similar peak positons and temperature range with that of raw coal. The HCl-soluble Hg (Fig. 4b), pyrite-bound Hg (Fig. 4c), silicatebound Hg (Fig. 4d) and organic-bound Hg (Fig. 4e) also show similar releasing temperature ranges and peak positons with that of raw coal. It might be caused by the fact that the difference in the composition between the middlings and the raw coal are minor (Table 1). Despite of the similar releasing temperature ranges and peak positons with raw coal, the different peak intensity can be observed. Also, the peak intensity of HCl-soluble Hg, pyritebound Hg and organic-bound Hg are all lower than that from raw coal, which is proportional to the their contents of middlings.
Fig. 4. The dynamic Hg release profiles of middlings (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
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L. Gao et al. / Fuel 200 (2017) 22–30
Fig. 5. The dynamic Hg release profiles of gangue (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
However, the peak intensity of silicate-bound Hg is higher than that in raw coal, indicating a more amount of silicate-bound Hg in middlings. 3.3.4. Gangue The results of gangue and its corresponding modes of occurrence of Hg are shown in Fig. 5. The similar Hg releasing temperature ranges and peak positons with that of raw coal is also found in Fig. 5a. Meanwhile, HCl-soluble Hg (Fig. 5b) and pyrite-bound Hg (Fig. 5c) also present similar Hg peak positons with that in raw coal. Nevertheless, the peak intensity of HCl-soluble Hg and pyrite-bound Hg are much higher than that of raw coal, cleaned coal and middlings, which coincides with the fact that the contents of HCl-soluble Hg and pyrite-bound Hg in gangue are highest among all the samples. Also, the peak intensity of silicate-bound
Hg is higher than that of raw coal shown in Fig. 2d, which is ascribable to its higher content of silicate-bound Hg in gangue than raw coal. Note that there is no Hg release peak at temperature > 450 °C in Fig. 5e, while the Hg release > 450 °C can be observed for Fig. 2e, Figs. 3e and 4e. It indicates that the pyrite-bound Hg and silicatebound Hg in gangue are completely soluble during HNO3 extraction procedure, which may due to the higher mineral constituents and less organic material matter in gangue (Table 1). Meanwhile, there exists a minor peak at about 200 °C in Fig. 5e, which indicates that the forms of organic-bound Hg are different with other samples. 3.3.5. Slime The results of slime and its corresponding modes of occurrence of Hg are shown in Fig. 6. It is interesting that Fig. 6a shows a broad
L. Gao et al. / Fuel 200 (2017) 22–30
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Fig. 6. The dynamic Hg release profiles of slime (a), HCl-soluble Hg (b), pyrite-bound Hg (c), silicate-bound Hg (d) and organic-bound Hg (e).
peak with three sub-peaks at 150–450 °C. Based on the discussion above, the broad peak should contain HCl-soluble Hg, pyrite-bound Hg and organic-bound Hg. It might be caused by the mixing characteristics in the original composition of slime since the slime is the physically separable fine fractions and mineral fragments falling from the coal fractures. The HCl-soluble Hg (Fig. 6b), pyrite-bound Hg (Fig. 6c) and silicate-bound Hg (Fig. 6d) also show similar behavior with raw coal. However, their peak intensity are all higher than that of raw coal, which is due to the higher contents of HCl-soluble Hg, pyrite-bound Hg and silicate-bound Hg of slime. Similar with Fig. 5e, there is also no Hg release peak at temperature > 450 °C in Fig. 6e, which suggests that the pyrite-bound Hg and silicate-bound Hg in slime are easier to leach out by chemical leaching.
In summary, HCl-soluble Hg as well as pyrite-bound Hg for all samples shows similar releasing temperature ranges and peak positons. It indicates that the thermal stability of HCl-soluble Hg in all samples is all similar. Also, the pyrite-bound Hg in all samples presents similar thermal stability. This further suggests that the thermal stability of HCl-soluble Hg and pyrite-bound Hg have no significant correlation with the density or particle sizes of the samples because all samples are separated by density or particle sizes during coal preparation process. For silicate-bound Hg in all samples, it cannot completely release out because of its higher thermal stability. After chemical leaching, silicate-bound Hg in all samples are easier to release out, implying that the thermal stability of silicate-bound Hg shows a certain dependency upon mineral in samples. For organic-bound Hg, the releasing character and tem-
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L. Gao et al. / Fuel 200 (2017) 22–30
perature range are different with different samples, indicating that the organic-bound Hg is complex in the samples. 4. Conclusions The raw coal, cleaned coal, middlings, gangue and slime samples from GD coal preparation plant were studied to identify the modes of occurrence of Hg and their thermal stability. The main findings can be summarized as follows: 1. The Hg is mainly enriched in gangue and slime and partially removed in cleaned coal and middlings during coal preparation process. 2. All samples contain HCl-soluble Hg, organic-bound Hg, pyritebound Hg and silicate-bound Hg with different contents. The pyrite-bound Hg is the dominant form of Hg for all coal samples. The content of silicate-bound Hg is the least among the four modes of occurrence of Hg. HCl-soluble Hg and pyritebound Hg mainly enriched in gangue and slime. The organicbound Hg proportion has significant correlation with the organic material content. 3. The thermal stability of HCl-soluble Hg as well as pyrite-bound Hg for all samples is similar. Silicate-bound Hg cannot completely release out because of its higher thermal stability and its thermal stability shows some relation with the mineral in samples. The release behavior of organic-bound Hg is different with different samples.
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