Sahibi:
Prof.Dr. Abdurrahim Özgenoğlu
Yayın Kurulu :

Prof.Dr. İsmail Bircan
Prof.Dr. Oya Batum Menteşe
Uzman Nilüfer Ünal
Editör:  Nilüfer Ünal
             3 Ayda bir yayınlanır.

   COAL PREPARATION EFFECT ON THE REMOVAL OF  TRACE ELEMENTS

 

Gülhan Özbayoğlu                                                                                                                      

Prof.Dr., Dean of Engineering Faculty, Atilim University,Ankara-Turkey

Abstract: Several trace elements of environmental concern of Soma lignite are above the world averages. Float and sink tests prove that trace elements associated with mineral matter can be removed from coal by physical coal cleaning. Above 60% of Pb, Th, Mo and W could be rejected from Soma lignite at 1.90 sp.gr.

Key Words: Trace elements, Lignite, Coal preparation

1.  INTRODUCTION

The major use of coal in the world is for the generation of power by combustion on a large scale. Its share in electricity generation is 40%. However, domestic coal utilization for electricity generation in Turkey is around 20%

Coal is a complex heterogeneous mixture of organic and inorganic constituents. After combustion, up to 20% of the original mineral matter is found in the bottom ash and up to about 80 % in the fly ash. A small proportion, mostly fine particles of less than about 10 micron in diameter is emitted to the atmosphere with the stack gases. Trace elements in thermal coals are important as they are potentially hazardous to human health and ecosystems.

Around 25 trace elements are considered to be of environmental interest [1]. In Table 1, trace

elements of environmental concern classified by US National Research Council is given [2].

 Although, some of the trace elements are organically bound in coal, many trace elements are associated with mineral matter.  Trace elements in the bituminous coal were found mainly in the mineral matter fraction [3]. In the high sulfur coals of Illinois Basin, U.S.A., the excess amounts of S, Mo, B, and U were probably derived from sea water, because concentration of these elements in sea water are much higher than those in fresh water [4]. In Australian coals Hg, As, Ni, Pb, Cu and Zn are predominantly associated with sulphide minerals and organic matter [5].  Table 2 shows the results of an extensive study on the trace elements distributions in both coals and wastes [6].  

Table  SEQ Table \* ARABIC 1. Classification of the trace elements of environmental concern related to coal utilization [2].

Elements of greatest concern

As, B, Cd, Hg, Pb, Se

Elements of moderate concern

Cr, Cu, Ni, V, Zn

Elements of minor concern

Ba,Co, Ce, Li, Mn, Sb, Sr

Radioactive elements

Rn, Th, U

Elements of concern, but present in very small amount

Be, Sn, Te, Tl

 

 

Table  SEQ Table \* ARABIC 2. Trace elements distribution in coal

Affinity

Mineral group

Mineral Type

Trace Elements

Inorganic

Clays, feldspars

Kaolinite, Montmorillonite

Ba,Bi,Cr,Ce,Cu,Ga,Li,Ni,Pb,Sn,Sr,U,V,P,Rb,Th

 

Iron sulphides

Pyrite, Sphalerite

As,Cd,Co,Cu,Fe,Hg,Mo,Ni,Pb,Sb,Se,W,Tl,Zn,S

 

Carbonates

Calcite,Dolomite,Ankerite

Ca,Co,Mn —

 

Sulfates

---

Ba,Ca,Fe,S

 

 

Heavy minerals

Tourmaline

B

 

Organic

N,S,Be,B,Ge,V,W,Zr

(B in partial association with tourmaline in the heavy fraction, V with clays)

 

 

Most trace elements are released and redistributed into bottom ash, fly ash, fine fly ash and gaseous phase after combustion. Very few elements are in vapor state. The amount of trace elements emission from coal combustion is strongly dependent on the modes of occurrence in coal. Elements which are associated mostly with the coal organic and sulphide fractions tend to vaporize and then adsorb on the fine particles when the flue gas temperature drops. Trace elements detected in the flue gas of Australian coals were predominantly associated with sulphide minerals [5]. On the other hand, elements combined mainly with the excluded minerals remain in the ash. An option for controlling the released and distribution of these elements is to remove them before combustion. Coal cleaning, when applied to remove hazardous elements before utilization, is considered an economical and effective technique in minimizing environmental problems [7].   

The purpose of this study was to determine the content of trace elements in a Turkish lignite and to show the effect of coal cleaning on the removal of trace elements.

1.  PROCEDURE

Lignite sample was systematically collected over one week period from Soma Lignite Establishment in the western part of Turkey.

The representative sample was analyzed for proximate analysis as well as for trace elements. XRF and ICP-OES techniques were used in the determination of the trace elements in coal.

Screen analyses were conducted to determine the distribution of trace elements in + 50mm, 50x18mm, and -18mm size fractions. Float and sink tests were carried out on each screen fraction to determine the capability of cleaning processes for rejecting trace elements from the coals. ZnCl2 solution was used to generate sp.gr. between 1.3 and 1.9. The sinks obtained from float and sink tests of three size fractions were analyzed in terms of trace elements and the possibility of trace elements removal from the sink products were determined.

2.  RESULTS AND DISCUSSIONS

2.1. Chemical composition of the lignite sample

Representative sample of Soma lignite consisted of 40.65% ash, 39.97% volatile matter, 0.63% total

sulfur and 3098 kcal/kg low heating value on dry basis. Trace elements analysis of a representative sample is shown in Table 3.

Table  SEQ Table \* ARABIC 3. Trace contents of representative sample of Soma lignite

Trace Ele-ments

ppm

Trace Ele-ments

ppm

Trace Ele-ments

ppm

As

25.1

Co

4.19

Cd

2.00

Cr

29.2

Ni

14.21

Sn

12.93

Pb

8.7

Se

1.02

W

0.74

Th

5.7

Mo

0.59

Hg

0.79

U

7.6

V

79.3

Cu

13.3

The result of the trace element contents and distributions of screen fractions is indicated in Table 4.

1.1. Float and sink test of Soma lignite

Float and sink tests were performed on three screen fractions to determine the washability characteristics of Soma lignite and to see the contents and distributions of  trace elements in +1.90 sp.gr (sink) fractions  which is shown in Table 5. The total trace elements removal from the sink products of three screen fractions were combined and presented in Table 6.

As seen in Table 5, the trace elements contents of +1.90 sp.gr (sink) products of float and sink tests of three size fractions showed different behavior. Co, Cd, W, and Hg showed uniform contents in all size fractions; however, Cr, Ni and Sn contents decreased with the decrease in particle size. The rest of the trace elements contents increased with the decrease in particle size. The trace elements contents of all size fractions of +1.90 sp.gr. products of As, Cr, U and V are less than the feed levels, which means these trace elements are concentrated in coal, not in inorganic matter.

As seen in Table 6, physical coal cleaning could provide the removal of some trace elements by ash.  For Pb, Th, Mo, and W elements, the removals are above 60%. In other words, by cleaning of coal at 1.90 sp.gr., 60% of these elements are removed from the sink. Trace elements, indicating strong affinity to organic matter such as U and V were enriched in coal and could not be removed with ash.


 

Table  SEQ Table \* ARABIC 4. Trace elements distribution in screen fractions

Trace Elements

Screen Fractions

(+50 mm) Weight %:21.59

(-50+18)mm Weight %:26.71

(-18mm) Weight %:51.70

ppm

%Distr

ppm

%Distr

ppm

%Distr

As

17.6

13.93

24.0

23.51

33.0

62.56

Cr

16.7

11.33

41.5

24.21

38.9

64.47

Pb

6.5

14.59

8.3

23.05

11.6

62.36

Th

3.1

12.12

5.2

25.15

6.7

62.73

U

6.2

17.56

6.1

21.38

9.0

61.06

Co

3.0

15.46

3.0

19.13

5.3

65.41

Ni

7.6

11.55

12.6

23.69

17.8

64.77

Se

0.9

18.89

0.8

20.78

1.2

60.33

Mo

1.0

36.82

1.0

45.55

0.2

17.63

V

45.7

12.42

72.7

24.45

97.0

63.13

Cd

2.0

21.59

2.0

26.71

2.0

51.70

Sn

11.7

19.54

16.1

33.27

11.8

47.19

W

1.0

29.12

1.0

35.54

0.5

34.86

Hg

1.0

27.22

1.0

33.67

0.6

39.11

Cu

7.7

11.33

13.3

24.20

18.3

64.47

 

Table  SEQ Table \* ARABIC 5. Trace elements in +1.90 sp.gr.(sink) fractions of sink and float tests

Trace elements

 (+50 mm) +1.90 sp.gr

Weight %: 13.73

(-50+18)mm +1.90 sg.

Weight %: 13.60

(-18mm) +1.90 sp.gr

Weight %: 18.00

ppm

%Distr

ppm

%Distr

ppm

%Distr

As

12.91

6.50

10.26

5.12

15.79

10.42

Cr

22.20

8.76

13.50

5.28

13.70

7.09

Pb

13.84

19.76

12.26

17.34

15.42

28.86

Th

7.80

19.39

7.70

18.96

     9.10

29.66

U

1.16

2.09

1.30

2.32

2.80

6.61

Co

3.10

10.06

3.10

9.97

3.10

13.19

Ni

16.50

15.94

16.10

15.42

13.40

17.03

Se

1.30

17.36

1.10

14.55

1.10

19.25

Mo

1.00

23.41

0.40

9.28

1.00

30.70

V

37.00

6.39

33.70

5.77

42.80

9.70

Cd

2.00

13.73

2.00

13.60

2.00

18.00

Sn

14.10

14.98

15.80

16.65

12.10

16.89

W

1.03

19.07

1.03

18.89

1.03

25.00

Hg

1.00

17.31

1.00

17.15

1.00

22.69

Cu

13.90

13.00

13.50

12.51

13.70

16.80

 

       Table  SEQ Table \* ARABIC 6. Summary of trace elements removal at +1.90 sp. gr.fractions of float and sink  tests

Trace Elements

Removal in  +1.90 sink fraction,%

Trace Elements

Removal in +1.90 sink fraction,%

Trace Elements

Removal in +1.90 sink fraction,%

As

22.04

Co

33.22

Cd

45.33

Cr

21.13

Ni

48.39

Sn

48.52

Pb

65.96

Se

51.16

W

62.96

Th

68.01

Mo

63.39

Hg

57.15

U

11.02

V

21.86

Cu

42.31

1.  CONCLUSIONS

Several trace elements of environmental concern, namely, As, Cd, Cr, Cu, Th, U and V in Soma lignite are above the world averages while Ni and Pb concentrations are less than the world average . On the other hand, As, Co, Cr, Ni, Pb, Se,Th, U and V contents of this coal are below Turkish averages.

Float and sink tests proved that trace elements associated with mineral matter can be removed from coal by physical coal cleaning. Above 60% of Pb, Th, Mo and W could be rejected from Soma lignite at 1.90 sp.gr. One should be reminded that the success of rejection is related with the level of trace elements association with mineral matter and the degree of liberation.

ACKNOWLEDGEMENTS

The author would like to acknowledge Dr.Selahaddin Anaç, Director of Turkish Coal Enterprises, and the staff of Soma Coal Laboratories for conducting coal analysis.

2.  REFERENCES

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[3] Liu, G., Vassilev, S.V., Gao, L.,  Peng, Z.: Mineral and chemical composition and trace element contents in coals and coal ashes from Huaibei coal field, Energy Conversion and Management, Vol.46, Issue 13-14, pp 2001-2009, 2004.

 [4] Raask, E.: The mode of occurrence and concentration of trace elements in coal, Progress in

Energy and Combustion Science, 11, pp.97-118, 1985.

[5] Simento, N.: Analysing for trace elements, CRC for coal in sustainable development, http://www.ccsd.biz/products/traceelementsdb.cfm,2002.

[6] Querol, X., Fernandez-Turiel, J., Lopez-Soler, A.: Trace elements in coal and their behavior during combustion in a large power station, Fuel, 74(3): pp.331-343, 1995.

[7] Wang, W., Qin, Y., Wang,J., Li,J.: Partitioning of hazardous trace elements during coal preparation, Procedia Earth Planetary Science, 1, pp.838-844, 2009.

[8].   Tuncali, E., Ciftci, B., Yavuz, ,N., Toprak, S., Köker, A., Gencer, Z., Aycık, H., Şahin, N.: Chemical and technological properties of Turkish lignites, General  Directorate of Mineral Research and Exploration (M.T.A.), Turkey, ISBN: 6595, 2002.