Volatility and mobility of some trace elements in coal from Shizuishan Power Plant

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 39, Issue 5, May 2011 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2011, 39(5), 328332

RESEARCH PAPER

Volatility and mobility of some trace elements in coal from Shizuishan Power Plant SONG Dang-yu1,2,*, MA Yin-juan1, QIN Yong3, WANG Wen-feng3, ZHENG Chu-guang4 1

Institute of Resources and Environment, Henan Polytechnic University, Jiaozuo 454000, China

2

Henan Provincial Key Laboratory of Organism Traces and Mineralizing Procedure, Henan Polytechnic University, Jiaozuo 454000, China

3

College of Mineral Resources and Geoscience, China University of Mining and Technology, Xuzhou 221008, China

4

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China

Abstract: The volatility and mobility of eleven trace elements (Hg, As, Se, Pb, Cr, Cd, Mo, Ni, Co, U and Th) in coal from Shizuishan Power Plant were investigated. Column leaching tests on bottom ash and fly ash by sulfuric acid for 60 h were conducted. The contents of trace elements in input coal, bottom ash, fly ash and leaching solutions were determined by inductively coupled plasma mass spectrometry (ICP-MS) and atomic fluorescence spectroscopy (AFS). The volatility of trace elements during coal combustion was calculated based on the trace elements concentration in coal, ash and the ash yield of the raw coal. The results demonstrate that over 50% of As, Pb, and Hg volatilize to the atmosphere. Leaching test results indicate that the maximum leaching proportion of As, Cd, Ni, and Mo is 1.8%–6.2%. Based on the volatility and leaching characteristics, the volatilization and migration model of trace elements in the process of combustion and leaching was established. The results show that volatility of trace elements is the main pollution sources on environment in Shizuishan Power Plant. The concentration of Pb, Cd, Mo, Co, Ni, and Cr in leachate is extremely high and they are potentially harmful elements to the environment. Keywords: coal; trace elements; volatility; leaching test

Coal is the uppermost energy in China with output of 3.05 Gt in 2009. Majority of coals were combusted directly by power plant for electric power. Over the past decades, serious environmental concerns have been posed pertaining to the emissions of pollutants from coal combustion plants, which contain potentially hazardous trace elements. During coal combusting in power plant a series of physicochemical transformations take place, bringing about dispersion of potentially harmful trace elements into the environment. The fate of trace elements during combustion is of environmental concern due to either atmospheric emission or potential mobility in the aqueous environment because elements enrich in combustion residues[1]. Many research works have been done on volatility of trace elements in coal combustion and leaching feature of trace elements in different solutions[2–7]. Clarke has provided a valuable summary of behavior of trace elements in coals and combustion[8]. Seferinoglu reports the leaching mobility characteristics of environmentally hazardous trace elements in coal and ash by direct acid leaching[9]. But few researches have

been conducted on the release model of trace elements in entire utilization process that includes combustion and leaching. The purpose of the present work is to evaluate the distributing proportions of some potentially harmful trace elements in coal combustion and ash leaching based on coal and ash samples collected from Shizuishan Power Plant. The aim of the valuation is to assess the environmental impact caused by harmful trace elements in coal combustion and ash utilization.

1 1.1

Experimental Sample preparation

In the present study, input coal, fly ash and bottom ash samples were collected from Shizuishan Power Plant. The bulk coal, fly ash and bottom ash were collected from conveyer belt, ESP, and bottom ash tank, respectively. The coal samples were dried in an oven at 40°C for 16 h. Some samples were ground to pass 100-mesh for proximate analysis, trace elements and

Received: 26-Oct-2010; Revised: 30-Jan-2011 * Corresponding author. Tel: +86-319-3986265; Fax: +86-319-3987962; E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (40772093) and in part by the Program for Science˂Technology Innovation Talents in Universities of Henan Province (2010HASTIT007). Copyright ” 2011, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

SONG Dang-yu et al. / Journal of Fuel Chemistry and Technology, 2011, 39(5): 328332

sulfur determination, while others were ground to 40–100 mesh for leaching experiment.

was filled to the volume mark with water after the solution reached room temperature.

1.2

1.2.2

Analytical procedures

Column leaching tests were conducted on fly ash and bottom ash to study the leaching characteristics of As, Se, Pb, Hg, Cd, Cr, Co, Ni, Th, U, and Mo. It is well known that the maximum equilibration time of some elements could be several years in natural condition, and it is very difficult to simulate leaching condition with extremely long time. Thus, sulfuric acid (pH=2), of which the acidity is much stronger than acid rain, was used as leaching solution and leaching time intervals were 22, 23, 24 and 25 h, respectively. All test utensils and glasses were soaked in a 14% HNO3 solution for 24 h and rinsed using distilled water prior to the leaching experiments. 20 g of fly ash and bottom ash were transferred into fixed glass column (10 mm in internal diameter and 70 cm in length), respectively. A small amount of glass fibre was also packed at the bottom of the glass column to prevent fine particles loss during leaching. The leaching solutions were controlled to flow into the columns at room temperature and kept the liquid above the sample maintaining about 5 cm high. The outflow velocity of resulting leachates was controlled at 3.0 mL/h by governing valve. The resulting leachates were sampled after 4 h, 12 h, 28 h and 60 h, and put into 25 mL, 25 mL and 50 mL volumetric tubes and 100 mL capacity bottle for element analysis, respectively. The concentrations of elements in the resulting leachates and leaching solutions (blank value) were determined by AFS for As, Hg, and Se, and by ICP-MS for Cd, Co, Cr, Mo, Ni, Pb, Th, and U. Analytical errors were estimated less than 10% for most of the elements (about 10% for Cd and Mo). The analysis results were obtained by calculating the differences between the blank value and the concentration value for the leachate (Table 3).

The proximate analysis, sulfur content and maceral composition are given in Table 1. The proximate analysis and maceral composition were conducted according to GB/T 212-2001 and GB/T 8899-1998. The total sulfur and forms of sulfur were determined using chemical method following GB/T 214-1996 and GB/T 215-2003, respectively. 1.2.1

Leaching tests

Sample digestion

The concentration of trace elements in input coal and ash was determined by inductively coupled plasma mass spectrometry (ICP-MS) and atomic fluorescence spectroscopy (AFS) (Table 2). Cd, Se, Cr, Co, Ni, Mo, U, and Th were determined by ICP-MS. As, Pb, and Hg were determined by AFS. Samples were digested using a mixture of acids (HNO3, HF, and HClO4). 0.1000r0.0001 g of sample was weighed and transferred into a screw-capped Teflon flask, to which 1 mL HNO3 (1:1) and 3 mL HF were added. Then the flasks were fitted into a microwave oven and heated for 1 min at 1000 W. After the flask was cooled to room temperature, it was heated in a metallic block at 160°C for 48 h. After it cooled to room temperature, the flask was opened and returned to the hot metallic block, and the mixture within was evaporated to complete dryness. 1 mL HClO4 was added, and then the flask was heated until the white fog disappeared. The flask was removed from the hot plate and allowed to cool for 5 min, and 3.5 mL of HNO3 was added. The flask was returned to the hot plate and heated at 160°C for 12 h. After cooled, the flask was opened and the solution was transferred to a 50 mL linear polyethylene volumetric flask. This flask

Table 1 Proximate analysis, sulfur contents and maceral composition of input coal

w /%

Maceral composition

Mad

Ad

Vdaf

St

So

Sp

Ss

1.14

39.32

30.62

1.50

0.52

0.75

0.23

vitrinite

inertinite

exinite

mineral matter

60.7

34.0

1.1

4.2

Table 2 Contents of trace elements in coal, fly ash, and bottom ash (10–6) Samples

w /10–6 As

Pb

Hg

Cd

Se

Cr

Co

Ni

Mo

U

Th

Input coal

5.35

24.57

0.290

0.092

5.05

17.24

4.27

7.87

3.16

5.24

19.23

Bottom ash

1.59

2.38

0.015

0.045

0.24

52.74

11.15

30.00

9.96

12.50

36.38

Fly ash

5.67

36.62

0.100

0.137

3.60

38.11

10.10

25.29

7.56

11.52

30.90

Chinese coal[10]

5.0

13.0

0.15

0.2

2.0

12.0

7.0

14.0

4.0

3.0

6.0

SONG Dang-yu et al. / Journal of Fuel Chemistry and Technology, 2011, 39(5): 328332 Table 3 Concentrations of trace elements in different leachates (ng·mL–1) Trace

Ash

elements

type

0–4

4–12

12–28

28–60

As

bottom ash

0.63

0.94

31.75

21.11

fly ash

9.83

11.67

21.63

42.91

Se

bottom ash

0.25

0.06

0.25

0.31

fly ash

1.89

0.46

0.31

0.6

bottom ash

34.01

1.53

5.25

10.12

fly ash

8.39

–

–

–

bottom ash

0.005

0.004

0.005

0.015

fly ash

0.007

0.002

0.007

0.025

Cd

bottom ash

1.30

0.57

0.76

0.82

fly ash

0.40

0.10

0.09

0.30

Cr

bottom ash

13.26

0.29

7.13

77.01

fly ash

2.86

3.51

10.47

17.15

Co

bottom ash

47.89

34.49

94.32

152.94

fly ash

3.91

0.88

2.10

38.32

bottom ash

183.95

165.45

330.03

487.11

fly ash

22.52

9.97

20.17

127.46

Pb

Hg

Ni

Mo

Leaching time / h

bottom ash

0.83

–

0.03

2.12

fly ash

122.29

121.12

89.51

38.78

U

bottom ash

3.73

0.76

2.61

35.64

fly ash

3.71

0.12

6.97

19.55

Th

bottom ash

0.69

0.02

0.24

0.09

fly ash

0.09

–

0.02

0.01

2 2.1

Results and discussion Concentration of trace elements in coal and coal ash

In comparison with the average contents of trace elements in Chinese coals, the contents of Pb, Hg, Se and Th in coal from Shizuishan Power Plant are much higher. Their ratios of element content in Shizuishan coal to the average value in Chinese coals change from 1.9 to 3.2. The content of others is similar or much less than the average values in Chinese coals. For non-volatile elements such as Cr, Co, Ni, Mo, U, and Th[11], their contents in ash are higher than those in coal, indicating they enrich in ash, especially in bottom ash. Most part of volatile elements (Hg, As, and Se) and part of semi-volatile elements (Pb)[11] escaped into atmosphere in combustion, resulting in the lower concentration in ash. Because of the absorption in cooling processes, the contents of elements are higher in fly ash than those in bottom ash. 2.2

Volatility of trace elements in combustion

Mass balances were used to study the volatility of trace elements in combustion by pulverized fuel boiler. Based on the ash yield of the input coal, the contents of trace elements in

bottom ash and fly ash, and the weight proportion of bottom ash to fly ash about 20/80 at Shizuishan Power Plant, the volatility of trace elements can be calculated by the following equation: Ve

(1 

0.2w(A) u w(ebottom ash )  0.8w(A) u w(efly ash ) 100 u w(ecoal )

) u 100%

(1)

where Ve is the volatility of an element, %, w(A) is the ash yield of the input coal, %, w(ebottom ash) is the content of an element in bottom ash, 10–6, w(efly ash) is the content of an element in fly ash, 10–6, w(ecoal) is the content of an element in the input coal, 10–6. The results are shown in Table 4. Volatility of the trace element reflects the release characteristics in combustion. The volatility of Hg, Se, As, Pb, and Cd changes from 49.31% to 88.75%, whereas that of Cr, Co, Ni, Mo, and U is much lower, being less than 12.5%. There are some mistakes with Ni and Mo since their volatility is negative. This indicates that there are some disagreements between input and output compositions. There is also abnormal for Th, of which the volatility is as high as 34.58%. The possible reason is that the distribution of trace elements in coal is heterogenous[12,13], and the coal samples which convert to ash have different trace elements content with the coal sample for trace elements determination. Although the volatility deduced by this sample collection method cannot express the volatile portion accurately, it also reflects the migration characteristics of trace elements in combustion. 2.3

Mobility of trace elements in leaching

Leaching is a method to remove soluble components from a solid matrix. Column leaching tests are good methods to simulate the flow of percolating groundwater through a porous bed of granular material[14,15]. However, it is also one of the most appropriate methods to estimate the environmental consequences of the coal ash disposal. It is not scientific to evaluate the moving characteristics in leaching just using trace elements concentration in leachate because of great difference of trace element content in ash. Thus, the leaching portion, which is the mass proportion of the elements in leachant to the elements in solid matrix, was used. The leaching portion can be deduced by the following equation: 4

¦a

x , i u v u ti Lx˙ i 1 1000 u Ax u M

(2)

where Lx is the leaching proportion of element x, %, ax,i is the concentration of element x in the leachate of time interval i, ng/mL, v is the leachant flow rate, it is 3 mL/h in the experiment, ti is leaching time interval (t1=4 h, t2=8 h, t3=16 h, t4=32 h in experiment), Ax is the content of element x in bottom ash or fly ash, 10–6, M is the mass of bottom ash or fly ash, g.

SONG Dang-yu et al. / Journal of Fuel Chemistry and Technology, 2011, 39(5): 328332 Table 4 Volatility of trace elements in Shizuishan Power Plant (%) Trace elements Volatility /%

As

Pb

Hg

Cd

Se

Cr

Co

Ni

Mo

U

Th

64.33

52.34

88.75

49.31

77.20

6.41

5.06

-31.06

0.00

12.09

34.58

Table 5 Mobility of different elements in bottom ash and fly ash in leaching experiment (%) Trace elements

As

Pb

Hg

Cd

Se

Cr

Co

Ni

Mo

U

Th

Leaching proportion of trace elements in bottom ash

11.26 3.51

0.61 16.08 0.96

0.75

9.24 11.46 1.08

1.44

0.39

Leaching proportion of trace elements in fly ash

4.90

0.14

0.30

1.90

0.98

0.04

0.01

1.45

0.15

2.71

8.20

evaluation. Fig. 1 shows the distribution model of eleven trace elements in coal combustion and ash leaching process. The volatility of Hg, As, Pb, and Se exceeds 50%, so flue gas is the main path causing environmental pollution. More than 1.0% of As, Cd, Co, Ni, and Mo was released into the liquid system, which can possibly cause the underground pollution. Most part of Cr, Co, Ni, U, and Th keeps in the ash, which is stable and harmless. 2.5 Fig. 1 Release models of trace element in combustion and leaching

The results are shown in Table 5. The trace elements were divided into two kinds according to their leaching proportion from ash: leachable and non-leachable. As, Pb, Cd, Co, Ni, Mo, and U belong to leachable elements and their maximal leaching proportion is 1.0%–16.1%. Hg, Se, Cr, and Th belong to non-leachable elements with their maximal leaching proportion less than 1.0%. For the same element, there are also some differences in leaching proportion between bottom ash and fly ash. Except for Mo, the leaching proportion of other elements in bottom ash is much higher than that in fly ash. For example, the leaching portion of Hg in bottom ash is 351 times greater than that in fly ash. The differences of the trace element occurrence in fly ash and bottom ash result in the different leaching proportions. Due to the leaching time is very short compared with the natural condition, most of the trace elements did not reach the leaching balance although strong acidic leachant solution was used. The concentration of the most trace elements in the last interval leachate is maximal. 2.4

Distribution of trace elements

High initial content of trace elements in input coal is the most important factor deducing environmental problems[16]. Volatility of trace elements in coal in combustion and mobility of trace elements in ash by leaching are the main paths of pollution. So it is necessary to study the release model of trace elements in entire utilization process in environmental

Environmental issue of trace element

Although the distribution model of trace elements can express the enrichment, volatility, and release proportion, it cannot be used to evaluate the environmental effect directly. Based on the national environmental quality standard of soil and underground water of China, the environmental effect of trace elements was evaluated (Table 6). 1) The maximal contents of Hg, As, and Pb in solid samples (coal or ash) are a little higher than their content levels of soil of the first level, but much less than those of the third level. 2) The maximal concentrations of Pb, Cd, Mo, Co, Ni, and Cr in leachate exceed their environmental quality standard of underground water of the third level.

3

Conclusions

1. The contents of most trace elements in coals from Shizuishan Power Plant were found to be less than the average levels in Chinese coals. The volatility of Hg, Se, As, and Pb exceeds 50%, especially Hg up to 88.75% in combustion. The flue gas is the main path causing environmental problems. 2. The highest releasing portion of Hg, Mo, Ni, Co, As, and Cd exceeds 1.3% in strong acidic solution during 60 h, and most elements do not reach the leaching balance, indicating leaching release is their possible pollutant path. 3. The maximal contents of Hg, As, and Pb in solid samples (coal or ash) are a little higher than their content level of soil of the first level, but much less than the third level. The maximal concentrations of Pb, Cd, Mo, Co, Ni, and Cr in leachate exceed the environmental quality standard of underground water of the third level.

SONG Dang-yu et al. / Journal of Fuel Chemistry and Technology, 2011, 39(5): 328332 Table 6 Evaluation parameters of trace elements in environmental evaluation Evaluation parameters Maximum contents of trace elements in solid sample /10–6 National environmental quality standard of soil-the first level /10–6 Maximum contents of trace elements in solution /10–9 Environment quality standard of underground water-the third level /10–9

Hg

As

Pb

Cd

Se

Mo

Co

Ni

Th

Cr

U

0.29

5.67

36.62

0.14

5.05

9.96

11.15

30.0

31.1

52.73

36.38

0.15

15

35

0.3

–

–

–

40

–

90

–

0.025

42.91

86.32

13.54

4.25

0.69

77.0

114.46

1.0

50

50

10

10

–

50

–

Acknowledgments

511.40 929.56 487.1 100

50

50

[7] Bin-Shafique S, Benson C H, Edil T B, Hwang K. Leachate concentrations from water leach and column leach tests on fly

Thanks were given to Dr. Yinghui Liu for reading an earlier version of the manuscript and providing thoughtful comments and constructive suggestions. The authors wish to acknowledge Jinshuang Yin for his help in elements concentration determination. Special thanks also to Wenshun You and Yuelong Ma for their supports in coal and ash sampling.

[10] Tang X Y, Huang W H. Trace elements in coals of China.

References

[11] Meji R. The distribution of trace elements during the combustion

ash-stabilized soils. Environ Eng Sci, 2006, 23(1): 53–67. [8] Clarke L B. The fate of trace elements during coal combustion and gasification: an overview. Fuel, 1993, 72(6): 731–736. [9] Seferinoglu M, Paul M, Sandstrom A, Koker A, Toprak S, Paul J. Acid leaching of coal and coal-ashes. Fuel, 2003, 82(14): 1721–1734. Beijing: The Commercial Press, 2004: 6–11. (in Chinese). of coal. Swaine D, Goodarzi F. Environmental Aspect of Trace

[1] Spears D A, Martinez-Tarrazona M R. Trace elements in combustion residues from a UK power station. Fuel, 2004, 83(17/18): 2265–2270. [2] Querol X, Fernandez-Turiel J L, Lopez-Soler A. Trace elements in coal and their behaviour during combustion in a large power station. Fuel, 1995, 74(3): 331–343. [3] Huang,Y Jin B, Zhong Z, Xiao R, Tang Z, Ren H. Trace elements (Mn, Cr, Pb, Se, Zn, Cd and Hg) in emissions from a pulverized coal boiler. Fuel Processing Technology, 2004, 86(1): 23–32. [4] Guo X, Zheng C-G, Xu M-H. Characterization of arsenic emissions from a coal-fired power plant. Energy Fuels, 2004, 18(6): 1822–1826 [5] Kim A G, Peter Hesbach. Comparison of fly ash leaching methods. Fuel, 2009, 88(5): 926–937.

Elements in Coal. Dordrecht: Kluwer Academic Publishers, 1995: 111–127. [12] Song D, Qin Y, Zhang J, Wang W, Zheng C. Concentrati on and distribution of trace elements in some coals from Northern China. International Journal of Coal Geology, 2007, 69(3): 179–191. [13] SONG D, WANG M, ZHANG J, ZHENG C. Contents and Occurrence of Cadmium in the Coals from Guizhou Province, China. Ann NY Acad Sci, 2007, 1140(1): 274–281. [14] Wang W, Qin Y, Song D, Wang K. Column leaching of coal and its combustion residues, Shizuishan, China. Int J Coal Geol, 2008, 75(2): 81–87. [15] Hassett D J, Pflughoeft-Hassett D F, Heebink L V. Leaching of CCBs: observations from over 25 years of research. Fuel, 2005, 84(11): 1378–1383. [16] Miller S F, Ronald T W, Bruce G. M, Alan W S. Trace element

[6] Thorneloe S A, Kosson D, Helms G, And Garrabrants A C.

emissions when firing pulverized coal in a pilot-scale

Improved leaching test methods for environmental assessment of

combustion facility. 23rd International Technical Conference on

coal ash and recycled materials used in construction.

Coal Utilization & Fuel Systems Clearwater, US, Florida March

Proceedings Sardinia 2009, Twelfth International Waste

9–13, 1998.

Management and Landfill Symposium. Italy: CISA Publisher, 2009: 1–11.