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The mercury release during thermal treatment of two coal gangues and two coal slimes under N2 and air Xiangrui Niu, Shaoqing Guo, Libing Gao, Yanzhi Cao, and Xian-Xian Wei Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 20, 2017
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The mercury release during thermal treatment of two coal gangues and two coal slimes under N2 and air Xiangrui Niu, Shaoqing Guo*, Libing Gao, Yanzhi Cao, Xian-Xian Wei School of Environment and Safety, Taiyuan University of Science and Technology, Taiyuan,Shanxi 030024, P. R. China Keywords: Coal gangue; Coal slime; Mercury; Thermal treatment.
Abstract The mercury release from coal gangue and coal slime has attracted wide interest due to the utilization of coal gangue and coal slime in some power plants. In order to develop the Hg control technology, it is necessary to gain the information of the Hg release during thermal treatment of coal gangue and coal slime. This paper presents a study about Hg release during thermal treatment of two coal gangues and coal slimes under air and N2 atmosphere through the on-line atomic fluorescence spectroscopy aided with the Ontario Hydro Method. The result shows that the release profiles of Hg for the two coal gangues and two coal slimes are similar under the same atmosphere, respectively. Most of Hg released below 650oC under N2 atmosphere and that released below 500-550oC under air for all the samples. The release ratio of Hg2+ is much lower than that of Hg0 for each sample. The release ratio of Hg0 released under N2 is more than that under air. Owing to the high volatility of Hg, the release ratio of total Hg is much higher than the corresponding volatile yield in any case investigated in this study.
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Meanwhile, the Hg0 release profiles for the samples show two or three peaks under N2 atmosphere, suggesting different modes of occurrence of mercury presented in the coal gangues and coal slimes.
1. Introduction Coal gangue and coal slime are the major solid waste material generated during the coalwashing procedures. In recent years, due to the fast growth of coal washing industry in China, large amount of coal gangue and slime have produced each year, resulting in environmental pollution and energy loss.1,2 Therefore, the utilization of coal gangue and coal slime has drawn more and more attention. Currently, coal gangue and coal slime are served as fuel for power generation in order to effectively use their calorific valve and reduce their quantities, which becomes an important approach for utilization of them.3-5 However, the mercury in coal gangue and coal slime can emit into atmosphere during the combustion process since mercury is an easily volatile element, leading to some serious environmental problems.1,2 Mercury is a toxic trace element, which can cause some negative effects on the environment and all living beings.6-8 Generally, mercury emits into the environment both from natural source and anthropogenic source.9 It is estimated that the global emission of mercury is about 20006000 tons per year because of the increasing of anthropogenic activities, especially coal-fired power plants.9,10 As a consequence, the reduction of mercury emissions from coal-fired power plants has been considered as an important priority for mercury control.11 Also, the mercury from coal gangue or coal slime-fired power plants should not be neglected and some effective control technologies are much needed.12
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In order to provide information for the development of more efficient control technology, it is of great importance to study the mercury release behavior from power plants.11 Thus, some investigations about the mercury emission from coal-fired power plants have been conducted.13,14 The studies about mercury release during thermal treatment of coal have been done as well.10,15 Consequently, considerable information focusing on the mercury release from coal has been obtained.15-20 It is generally accepted that the mercury can be transformed into three species with different characters during coal combustion, including particle-bound mercury (Hgp), elemental mercury (Hg0) and oxidized mercury (Hg2+). Hgp can be easily removed by particulate control devices.14,21 The compounds of Hg2+ are water soluble and most of them can be removed by wet flue gas desulphurisation units (FGD).16,17,22-24 Compared with Hg2+, Hg0 can stay longer in the atmosphere and migrate long distances from the source of emission as a global pollutant.9,18 So it is harder to be controlled than the other two species. Therefore, it is attracting more and more attention in the world.19,20,25 To control Hg emission, some advanced technologies will need to be developed to prevent Hg emission into atmosphere.26 Therefore, the comprehensive knowledge of Hg release is much needed. There are many reports about the Hg release behavior during the coal combustion processes.14,26-28 Overall, most of the previous works have been focused on the Hg release from thermal treatment of coal or coal-fired power plant. However, little attention has been paid on the Hg release from coal gangue and coal slime. Therefore, the aim of this study is to research the Hg release characteristic during thermal treatment of coal gangue and coal slime, which can provide basic information for Hg release control for power plants utilizing coal gangue or coal slime as a primary fuel. To better understand the effect of atmosphere on the Hg release behavior, an inert gas (N2) and an oxidized gas (air) were used respectively in this study.
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2. Experimental section 2.1 Experimental samples For the present studies, two coal gangues (marked as 1# coal gangue and 2# coal gangue) and two coal slimes (marked as 1 # coal slime and 2 # coal slime) were used, which were collected from two low calorific value coal-burning power plants in Shanxi province, China. 1# coal gangue and 1# coal slime come from one coal cleaning plant. 2# coal gangue and 2# coal slime come from another coal cleaning plant. The samples were sieved to under 200mesh (74µm) and then dried prior to use. The proximate and ultimate analyses of samples and the content of Hg in samples are shown in Table 1. Table 1. Proximate and ultimate analyses of the samples, wt (%)
Samples
1#
Hg contentb
Ultimate analysis， daf
Proximate analysis, ad VM
A
6331.25
15.77
66.45
916.85
17.73
1345.35
289.75
FC
C
S
Oa
H
N
17.39 54.70
4.30
0.84
26.23 13.93
70.12
11.40 56.85
6.70
0.82
7.21
15.59
66.53
17.48 54.67
4.14
0.82
24.79 15.57
13.97
69.58
15.73 73.41
5.99
1.34
0.78
coal gangue 2#
28.42
coal gangue 1# coal slime 2#
18.48
coal slime ad: air dry basis; daf: dry ash-free basis; VM: volatile matter; FC: fixed-carbon; A: ash; a: by difference; b: ng/g. 2.2 Experiment of thermal treatment
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One quartz tubes with a diameter of 20 mm in a furnace was used as the reactor, which was directly connected to an on-line atomic fluorescence spectroscopy (AFS) or several absorbing bottles according to the Ontario Hydro method to determine the Hg0 and Hg2+ from the reactor.16,27,29 Nitrogen or air was used as the carrier gas with the flow rate of 0.3L/min. 0.5g coal gangue or coal slime samples were placed in the quartz boat located in the center of the quartz tube. Before the experiment was done, the leakage test was conducted to avoid gas leakage occurred. The temperature of sample controlled by a temperature controller was increased from the ambient temperature to 1200oC with a heating rate of 20oC/min and recorded by a computer. During the thermal treatment of the samples, the produced gas in the reactor was swept to the detector of the on-line AFS or the absorbing bottles by the carrier gas used. The intensity of Hg0 determined by the on-line AFS was recorded by a computer. The Hg0 and Hg2+ in the absorbing bottles at different temperature were analyzed following the Ontario Hydro method. At the final temperature of 1200oC, the experiment was finished and the sample in the quartz boat was moved to the cold side of the reactor to be cooled down with N2 flow. The sample after the experiment was weighed and analyzed for Hg content. Each experiment was done at least 3 times to ensure the reproduction of the result. The mercury in the samples was determined by an atomic fluorescence spectroscopy (AFS). The detail was presented in our previous paper.30 To clearly show the result of the mercury release, HgRR, Hg0 RR and Hg2+RR were used to illustrate the quantity of total Hg, Hg0 and Hg2+ released from thermal treatment of samples and defined as the percentage of total Hg, Hg0 and Hg2+ released to the total gaseous mercury in coal gangue or coal slime. The volatile yield (VY) stands for the percentage of volatile matters released and is defined as the ratio of the mass of the volatile matters to the mass of the raw
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samples. The mass of the volatile matters was calculated by the difference of the mass between the raw samples and the samples after the process of thermal treatment at different temperature. 3. Results and discussion 3.1 Mercury content in samples The mercury content in the coal gangues and coal slimes are listed in Table 1, which shows that the mercury contents of four samples vary from 289.75 ng/g to 6331.25 ng/g, which are generally higher than the mercury contents of coals.15,31-33 For example, the mercury content in six coals ranges from 94-432 ng/g was reported.29 Among them, 1# coal gangue has the highest mercury content while 2# coal slime has the lowest mercury content. Also, it can be found that the mercury contents in 1# and 2# coal gangues are higher than 1# and 2# coal slime,respectively. Note that 1# samples (coal gangue and coal slime) are from one same coal cleaning plant. 2# samples (coal gangue and coal slime) are from another coal cleaning plant. This result indicates that the mercury in coal gangue is generally higher than that in coal slime during coal cleaning process, which well agrees with other reports.34 Additionally, it can be observed a good correlation between the Hg content and the sulfur content in samples, as reported about coal by other literature.35 3.2 The mercury release from coal gangue 3.2.1 The mercury release from coal gangue under N2 The Hg0 and Hg2+ release profiles with the increase of temperature for two coal gangues under N2 atmosphere are shown in Fig. 1. Overall, the Hg0 and Hg2+ release from these two coal gangues is mainly at 200-600oC. Meanwhile, the release ratio of Hg2+ is much lower than that of
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Hg0. Generally, it is in good agreement with the Hg release from coal.26-28 It is reported that most of Hg released below 600oC and Hg0 is the dominant form of mercury released during thermal treatment of coals.33 The similar Hg release temperature range of coal gangue with that of coal indicated that coal gangue has some similarity with coal, such as they both contain organic matter and inorganic matter.30 Therefore, the similar modes of occurrence of Hg in them may be exist, resulting in similar release temperature range. It can be seen that there is a great difference in Hg0 release profiles between these two kinds of coal gangues. However, the similarity about Hg release for the two samples still can be observed. For example, regardless of the kind of coal gangue, the temperature range of Hg0 and Hg2+ release is nearly consistent. They start to release at about 150oC-200oC for two samples, which well agrees with the result of coals.30,33 Also, most of the Hg release out below 800oC during the thermal treatment of samples, which is also consistent with the report about coal.33 Overall, there are some similarities on Hg release between coal gangues and coal, which is possibly due to the same circumstance they exist in coal mine.36 It is reported that the Hg0 release under inert gas can reflect the thermal stability of modes of occurrence of Hg in samples.30,33 Fig. 1 shows that there are two main Hg0 peaks located at similar temperature range for both coal gangues. One peak ranges about 200-500oC with a broad profile and the other ranges about 500-650oC with a narrow profile. The peak of 200-500oC possibly origins from the release of organic-bound Hg, sulfide-bound Hg and carbonate-bound Hg in coal gangues since it was found that the organic-bound Hg, sulfide-bound Hg and carbonate-bound Hg in coal gangue all release out at about 200-500oC.30,33 Generally, some thermal decomposition reactions involving Hg compounds in coal gangues can be occurred, such as HgCl2= Hg0 + Cl2 or HgS (s) = Hg (g)+1/2S2 (g) etc., leading to some Hg0 release.11,37 The
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peaks of 500-650oC is possibly caused by pyrite-bound Hg since several studies presented that pyrite-bound Hg release out at this temperature range.38
1# coal gangue
0.4
0.6
0
Hg 2+ Hg
0
2# coal gangue
Hg 2+ Hg
0.3
HgRR
0.4
HgRR
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0.2
0.2 0.1
0.0
0.0 0
200
400
600
800
1000
1200
0
200
400
600
800
1000
1200
Temperature
Temperature
Fig. 1 Hg release profiles of two coal gangues under N2 Note that for 1# coal gangue, the first Hg0 peak is much higher than the second one while the first Hg0 peak is much lower than the second one for 2# coal gangue, indicating a different distribution of modes of occurrence of mercury between two coal gangues. The difference between the two coal gangues was possibly ascribed to the different type of coal gangues. Actually, these two coal gangues come from different coal cleaning plants in different coal mines. Meanwhile, it should be noted that there was still a minor Hg0 peak at the temperature higher than 1000oC for 2# coal gangue. However, it is absent for 1# coal gangue. Generally, the silicate-bound Hg releases at higher temperature according to the previous report.33 So, the minor Hg0 peak for 2# coal gangue might be the release of silicate-bound Hg in 2# coal gangue. The absence of the peak for 1# coal gangue indicates that the silicate-bound Hg in 1#coal gangue is possibly more thermal stable than that for 2# coal gangue, resulting in no mercury peak at higher temperature for 1# coal gangue. 3.2.2 The mercury release from coal gangue under air
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0.6
1# coal gangue
0.6
2# coal gangue
0
Hg 2+ Hg
0
Hg 2+ Hg
0.4
0.4
HgRR
HgRR
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0.2
0.2
0.0
0.0 0
200
400
600
800
Temperature
1000
1200
0
200
400
600
800
1000
1200
Temperature
Fig. 2 Hg release profiles of two coal gangues under air The Hg0 and Hg2+ release profiles for two coal gangues under air atmosphere are shown in Fig.2. It also presents a similar temperature range of the Hg release for both coal gangues, which is like that under N2. However, some difference on the Hg release behavior can be still found between N2 and air. Firstly, the Hg0RR under air is less than that under N2 for both coal gangues. Then, the temperature range of Hg0 release under air, which is mainly at 300-500oC, is narrower than that under N2. It indicates that the amount of Hg0 released is less than that under N2, which agrees with the result of the Ontario Hydro method listed in Table 2. Also, it is interesting to found that the Hg0 release profiles for two coal gangue is similar to that of coal under air.30 This phenomena further indicates that coal gangue and coal possibly have the similar modes of occurrence of Hg. It is reported that most of Hg released from coal or coal gangue is in the form of Hg0 under N2, which is well consistent with the result of this study.15,39 Meanwhile, it can be observed that the Hg2+RR under air is higher than that under N2, especially for 1# coal gangues. Actually, the air is different with N2 because of the existence of oxygen. It can react with Hg0 to produce oxidized Hg at suitable condition, following the reaction equation of Hg + O2 = HgO.41 Therefore, the reaction between Hg0 and oxygen in air result in the decrease of Hg0 and the increase of Hg2+
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released under air.41 Also, according to the thermodynamics models about Hg release from coal, Hg0 was generally more volatile under the reducing or inert atmosphere than in oxidizing conditions.39 Therefore, the amount of Hg0 released under N2 is higher than that under air. As to the narrower profile of Hg0 mainly at 300-500oC under air, it is also ascribed to the reaction of Hg0 with oxygen. According to Wang et al. report, the reaction would move to the right side below 300oC, leading to a significant decreasing Hg0 released below 300oC under air.41 Then, it would change direction (move to the left side) when the temperature is higher than 300oC, resulting in a slight decrease of Hg0.41 Meanwhile, it should be pointed out oxygen can also reacted with the coal gangue matrix besides the reaction with Hg0. The reaction can destroy the matrix of coal gangue, thus the mercury can be easily release at relatively lower temperature range compared with N2. 3.2.3 The comparison of volatile yield and the total Hg release ratio for two coal gangues
100
HgRR
1# coal gangue
100
2# coal gangue
HgRR
80
VY or HgRR
80
VY or HgRR
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60 40
VY 20
60 40
VY 20
0
0 0
200
400
600
800
Temperature
1000
1200
0
200
400
600
800
1000
1200
Temperature
Fig.3(a) The VY and the HgRR for two coal gangues under N2 Fig.3(a) shows the volatile yield (VY) and the total Hg release ratio (HgRR) for two coal gangues under N2 atmosphere. As shown in Fig.3(a), the behavior of the VY is similar for two coal gangues. They both increase with temperature and mostly keep constant of about 20% at >800oC. This implies that the volatile matter released for these coal gangues is nearly no relation
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with coal gangue type. Also, the HgRR shows the similar tendency with VY, increasing with temperature and keeping constant at higher temperature. The reason is possibly that the Hg released is actually part of volatile matter during thermal treatment of coal gangues. Nevertheless, it can be seen that, similar to coal, the HgRR is much higher than its corresponding VY for both coal gangues, which indicates that mercury releases more easily than coal gangue matrix, further verifying the volatile character of mercury.26,39 It is interesting to find that the VY and the HgRR are both increase rapidly at 400-600oC, which implies that the coal gangue has undergone some complex reactions such as thermal decomposition of organic matter or mineral at 400-600oC during thermal treatment process, leading to more volatile matter including Hg released.40
100
HgRR
1#coal gangue
100
2# coal gangue
HgRR
80
80
VY or HgRR
VVY or HgRR
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60
VY
40
60 40
VY
20
20
0
0 0
200
400
600
800
Temperature
1000
1200
0
200
400
600
800
1000
1200
Temperature
Fig.3(b) The VY and the HgRR for two coal gangues under air Fig.3(b) shows the VY and the HgRR for two coal gangues under air atmosphere. As shown in Fig. 3(b), the HgRR of 1#coal gangue is analogical for that of 2# coal gangue, both increasing rapidly 600oC. Simultaneously, the VY of two coal gangues presents the similar tendency with HgRR. However, the VY is also significantly less than the corresponding HgRR. Additionally, the VY increases rapidly
