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Dynamics of trace elements release in a coal pyrolysis process☆
Fuel 82 (2003) 1281–1290 www.fuelfirst.com
Dynamics of trace elements release in a coal pyrolysis processq Elwira Zajusz-Zubek*, Jan Konieczyn´ski Faculty of Energy and Environmental Engineering, Silesian University of Technology, ul. Akademicka 2, Gliwice 44-101, Poland Accepted 1 July 2001; available online 27 February 2003
Abstract Samples of coal and solid carbonization product obtained at four temperatures: 400, 600, 850 and 1000 8C were tested on account of the contents of trace elements. The following hazardous trace elements were considered: arsenic, beryl, cadmium, manganese, nickel, lead, mercury and selenium. The release curves for the elements tested were determined. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Trace elements; Coal pyrolysis; Air pollution
1. Introduction About 10% of coal produced in the world are processed into coke. Global production power of the coke industry is evaluated as 250– 270 million Mg of coke per year. There is a good balance between the coke supply and demand. However, in the nearest future, coke deficiency may be expected in some regions due to strict observation of the environmental protection regulations. Although coke, being smokeless, is considered a clean fuel, its production is accompanied by the air, water and air pollution. In particular the air cleanness is at risk in result of emission of dusts and gases including sulphur oxides, carbon monoxide and nitric oxide as well as numerous organic aromatic polynuclear hydrocarbons. Emission of the matters polluting the air is a result of lack of full airtight sealing during the basic stage of coke production, i.e. thermal decomposition (pyrolysis) of the coal. This process leads to significant changes in the coal structure and chemical constitution and to formation of solid, gaseous and liquid products (Table 1). In spite of the extensive knowledge of the effect of coal coking on the environment, little attention has been paid so far to the environment pollution with trace elements. On the other hand, one may expect they will be found among the pollutants penetrating to the environment if they occur in the coal subjected to the coking process. * Corresponding author. Tel./fax: þ 48-32-237-12-90. E-mail address: [email protected] (E. Zajusz-Zubek). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
Coal, besides its main components, includes a lot of elements of low concentrations of the order of 0.01 or 0.001 wt%. They are trace elements connected with organic coal matter or mineral matter comprised in coal. Their accumulation in coal is related to the origin of the coal beds and coal metamorphism. The contents of trace elements usually decrease when the rank of coal increases. The most often mentioned trace elements are: As, Ag, B, Ba, Be, Cd, Cl, Co, Cr, Cu, F, Ga, Ge, Hf, Hg, Li, Mn, Mo, Ni, Pb, Ra, Rb, Sb, Sn, Sr, Se, V and Zn. Concentrations of the trace elements are different, depending on the coal bed and basin. Table 2 shows the range of the average concentrations of some trace elements found in most types of coal produced in the world. Table 3 shows the adequate range for coal ashes from 20 mines situated in the Upper Silesia (Southern Poland) coal basin. This occurrence of the trace elements in the organic (lighter) and mineral (heavier) matter constituting the coal makes it possible to remove a part of the trace elements, connected with the mineral matter, from the coal. When testing American coals, [5] found organically associated Ge, Be, Ga, V, Cr, Co, Pb and Zn. In Poland, De˛bska-Bes, Menzla et al. [6,7] tested the way in which some trace elements were connected with organic and mineral coal matter in coking coals. The samples were separated in heavy liquids. Then, coal fractions of different specific gravity were extracted by means of the adequate solvents. On the basis of the acquired results it was found that most elements were connected with mineral coal matter, i.e. coal fraction of the density lower than 1.35 g/cm3. In this fraction significant increase in Co, Sr,
0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00031-0
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Table 1 Chemical composition of coking coal and coke, (%) [1,2]
Coking coal Coke a
Cdaf
Hdaf
Odaf a
Ndaf
Sdaf
88.2 97.5
4.8 0.4
4.5 0.5
1.8 0.7
0.7 0.9
From the difference.
Ni, V and Ga concentration was observed. On the other hand, Pb and Zn occurrence in fractions of the density higher than 1.45 g/cm3 proves the connection with mineral matter. When applying coal cleaning processes, based on the difference in density, the contents of Mn, Co, As, Hg, Zn, Cd and Pb may be reduced. Removal of pyrite from coal results in substantial decrease in Pb, Cd and As contents. It should be pointed out that the charge coal in a coking plant is most often dressed. Behaviour of trace elements in a combustion process reveals their different ability to form the gaseous phase. Emission of trace elements from coal furnaces takes place both in gaseous phase (e.g. Hg) and in solid phase. Their presence in dust particles is a result of their origin from mineral matter in coal as well as of condensation and adsorption from gaseous phase during the waste gas transport from the source to the emitter. Only a few researchers have so far studied the behaviour of trace elements found in coal during coal pyrolysis [8]. The literature available includes papers by Eisenhut [9] who tested the risk for the air, caused by heavy metals emitted during coal carbonization in Germany. There are also Polish papers, referring to the concentration range of some trace elements in coal and coke [6,7], and papers referring to the concentration level of some trace elements in the air, earth and water in coking plant surroundings [10 – 13]. Increased pollution of the environment with these trace elements may be caused by their release during coal pyrolysis and by emission, including volatile matter emitted from leaks in
Table 2 Ranges (ppm) of the average concentrations of some elements found in most types of coal produced in the world [3] Element
Range
Element
Range
As B Ba Be Cd Cl Co Cr Cu F Hg Mn
0.5–80 5–400 20–1000 0.1–15 0.1–3 50–2000 0.5–30 0.5–60 0.5–50 20–500 0.02–1 5–300
Mo Ni P Pb Sb Se Sn Th Tl U V Zn
0.1–10 0.5–50 10–3000 2–80 0.05–10 0.2–10 1–10 0.5–10 ,0.2–1 0.5–10 2–100 5–300
Table 3 Concentration intervals of trace elements found in coal ashes from the Upper Silesia [4] Element
Concentration (ppm)
Ag As Ba Be Cd Co Cr Cu Mn Mo Ni Pb V Zn Hga
0–2 0–82 504–3620.5 7–26 0–18 41–167 72–231.5 102–508.5 311.5–1186.5 0–7 82.5–359 59–248 249–675 148–714 0.1–0.23
a
In coal.
coke oven batteries when the coke chambers are charged and discharged and during coke oven battery firing. Thus, in order to get more information to better evaluate the coal coking hazards to the environment, it was decided to begin a research into the release of trace elements during coal pyrolysis.
2. Research concept and scope The aim of the research was to test the dynamics of the release of some selected trace elements from coal subjected to pyrolysis to the stream of volatile matter. Taking into account their toxic effect, the trace elements selected were As, Be, Cd, Mn, Ni, Pb, Hg and Se (they are all on the EPA list of hazardous air pollutants). The tests were carried out in a laboratory. The coal samples were carbonised in an electric muffle furnace in the same way as the way used when determining volatile matter. Dynamics of the trace elements release was measured by comparing their concentrations in a coal sample and in solid carbonization product samples obtained at carbonization temperatures 400, 600, 850 and 1000 8C, respectively. The temperature levels corresponded to the following stages, used in the coal coking theory: primary coal carbonization (400 8C), semi-coke formation (600 8C), secondary coal carbonization (850 8C) and coke formation (1000 8C). Samples of orthocoking coal (type 35.2) and of coal blend, consisting of two coal types and used in one of Polish coking plants, were tested. The dynamics of the trace element release was confirmed by their decreasing contents when the temperature of the process increased. It could be evaluated by plotting the release curve, whose shape depends on that dynamics or by using a calculated comparative index.
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The following coal types were tested:
Table 4 Technical analysis of the coals tested Coal tested
Coal blend used as charge in the coking plant Orthocoking coal (floatation concentrate) type 35.2
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Contents (%) Moisture
Ash
Volatile matter
0.2
7.7
24.0
0.3
7.64
21.9
3. The test method 3.1. Sampling The coal sampling, preparing and milling to get grains smaller than 0.2 mm as well as moisture, ashes and volatile matter determining were done according to the Polish standards.
† Coal blend used in a coking plant consisting of gascoking coal type 34.1 (70%) and orthocoking coal type 35.1 (30%). † Orthocoking coal (floatation concentrate) type 35.2. The coal characteristics are presented in Table 4. 3.2. Tests of release dynamics of the trace elements selected The release dynamics of the trace elements selected was tested in the following way. Weighed samples, the mass of which was 1.0000 g, were separated from the analytical samples of the coals tested. Crucibles with the weighed samples of coal (three weighed samples from each analytical sample), covered with lids, were placed on the preheated stools in the muffle furnace heated to the temperature of 400 8C. Then they were heated without any air supply, in the way similar to the way used when determining the contents of volatile matter, for 15 min. Carbonization of the coal samples at the temperature of 600, 850 and 1000 8C was carried out in the same way by placing
Table 5 Concentration of the trace elements selected in the coal blend and solid carbonization product obtained at the relevant temperatures No
Carbonization temperature (8C)
Solid carbonization product (%)
Concentration of the elements selected (ppm)
Contents of the elements selected in the solid carbonization product compared with the primary coal (%)
Pb
Cd
Mn
Ni
Pb
Cd
Mn
Ni
46.16
3.51
76.67
19.59
1
Primary coal
2 3 4 5 6 7
400
97.91 97.22 98.45 97.73 97.00 98.03
14.85 15.53 15.50 15.03 14.94 15.39
1.00 0.95 0.90 1.10 0.92 0.90
67.16 70.13 73.91 71.30 71.33 73.98
19.00 18.50 18.85 18.90 18.30 18.00
31.50 32.71 33.06 31.82 31.39 32.69
27.89 26.31 25.24 30.63 25.42 25.14
85.77 88.93 94.90 90.88 90.24 94.59
94.96 91.81 94.73 94.29 90.61 90.08
8 9 10 11 12 13
600
79.08 79.06 80.59 80.62 79.68 79.48
15.45 15.17 14.85 14.55 15.00 15.57
0.71 0.73 0.77 0.75 0.80 0.90
84.54 86.29 84.65 84.50 86.09 81.15
21.45 21.25 21.50 20.36 22.10 20.65
26.47 25.98 25.93 25.41 25.89 26.81
16.00 16.44 17.68 17.23 18.16 20.38
87.20 88.98 88.98 88.85 89.47 84.12
86.59 85.76 88.44
14 15 16 17 18 19
850
72.35 72.76 72.73 72.70 73.01 72.76
14.33 14.50 14.56 13.30 14.10 14.62
0.60 0.71 0.62 0.80 0.65 0.56
85.31 83.53 79.80 83.50 83.64 85.38
23.22 23.90 23.80 22.76 23.95 23.50
22.46 22.86 22.94 20.95 22.30 23.05
12.37 14.72 12.85 16.57 13.52 11.61
80.51 79.27 75.70 79.17 79.64 81.03
85.76
20 21 22 23 24 25
1000
71.08 71.09 71.00 70.56 70.62 70.77
9.00 9.14 9.08 10.05 8.37 9.14
0.52 0.45 0.59 0.48 0.55 0.51
78.46 75.89 76.79 76.03 76.70 76.78
23.90 24.00 24.50 23.85 22.69 23.84
13.86 14.08 13.97 15.36 12.81 14.01
10.53 9.11 11.93 9.65 11.07 10.28
72.74 70.37 71.11 69.97 70.65 70.87
86.72 87.10
89.88
88.36 84.46 87.29
85.91 81.80 86.12
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the samples into the muffle furnace heated to the relevant temperature. 3.3. Mineralization In order to minimise the loss of especially volatile elements: As, Be, Hg and Se, the coal and solid carbonization product samples were mineralised by combustion in a bomb calorimeter at the oxygen pressure of about 2.5 MPa, 10 cm3 of spectrally pure nitric acid (1:9) and a 1 g weighed sample, mixed with 0.5 g spectrally pure benzene carboxylic acid on a filter free from heavy metals, were introduced inside the bomb. In order to determine Cd, Mn, Ni and Pb wet mineralization was applied with a mixture of concentrated spectrally pure nitric acid and perchloric acid at the volume ratio 1:1 at the atmospheric pressure and at the temperature about 200 8C. The weighed sample mass was 0.2 g. 3.4. Determination As, Hg and Se in the solutions obtained in the way described earlier were determined by an absorption method employing the technique of ASA –VAP hydride generation on a spectrophotometer AAS Philips PU-9100x. Be content was determined by the ASA method with the use of flame atomisation in an acetylene-nitrous acid flame and verified by ICP method. The remaining trace elements, i.e. Pb, Cd, Mn and Ni were determined by ASA method applying the flame variant with deuterium background correction. In order to determine these elements, typical parameters were used, recommended by the producer of ASA Carl Zeiss Jena
spectrophotometer type AAS-3 as optimum for particular elements. The results obtained are shown in Tables 5– 8. Figs. 1 –8 show the plots of the relevant element release in the coal tested.
4. Experimental For experimental details see Tables 5 – 8 and Figs. 1 – 9.
5. Discussion of the results In the two coal types tested, in which the content of ash was similar, there were not any essential differences in the concentrations of the trace elements investigated. The highest concentrations, higher than 40 ppm, were found for mercury, lead, manganese, about 20 ppm for nickel, approximately a few ppm for arsenic and beryl, the lowest, lower than 1 ppm—for selenium. Satisfactory repeatability of lead, cadmium, manganese, nickel and mercury determination was acquired. Hence, it was possible to plot the curves of their release in the whole range of the carbonization temperatures tested. Owing to the worse repeatability of selenium and beryl determination their release curves were not plotted and the final result was illustrated only for the temperature of 1000 8C. When comparing the release runs obtained for the elements tested, significant differences in the release rate may be noticed. The release was investigated by watching the mass of the particular element contained in the coal subjected to carbonization. The content of the element, determined by an analytical method, was assumed to be 100.
Table 6 Concentration of the trace elements selected in the coal blend and solid carbonization product obtained at the relevant temperatures No
Carbonization temperature (8C)
Solid carbonization product (%)
Concentration of the elements selected (ppm)
Contents of the elements selected in the solid carbonization product compared with the primary coal (%)
Hg
As
Se
Be
Hg
As
61.22
5.00
0.74
6.29
1
Primary coal
2 3 4
400
97.03 97.94 97.23
34.21 28.56 30.02
3.42 4.99 5.15
0.68 0.86 0.86
6.29 5.42 4.86
54.22 45.69 47.68
66.32 97.68 100.08
5 6 7
600
80.22 79.22 79.26
15.53 16.99 10.92
4.73 3.94 4.37
0.95 0.68 0.82
5.27 5.03 5.60
20.35 21.98 14.14
75.84
8 9 10
850
72.76 72.31 73.40
12.08 14.46 10.89
4.62 5.42 4.94
0.57 0.43 0.58
5.54 6.94 3.20
14.36 17.08
67.19
11 12 13
1000
70.53 70.53 70.38
13.34 14.13 13.31
5.56 3.87 5.55
0.74 0.67 0.89
5.33 5.35 6.06
Se
Be
70.85 64.15
59.79 60.02
69.22
72.47 78.37 54.55 78.07
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Table 7 Concentration of the trace elements selected in the orthocoking coal and solid carbonization product obtained at the relevant temperatures No
Carbonization temperature (8C)
Solid carbonization product (%)
Concentration of the elements selected (ppm)
Contents of the elements selected in the solid carbonization product compared with the primary coal (%)
Pb
Cd
Mn
Ni
73.54
1.38
70.14
25.33
Pb
Cd
Mn
Ni
1
Primary coal
2 3 4 5 6 7
400
98.29 98.68 98.75 98.10 98.27 97.91
56.95 56.90 53.60 54.55 62.72 56.97
0.52 0.59 0.52 0.55 0.55 0.54
61.56 65.74 62.65 61.92 62.64 61.39
20.83 19.76 22.05 18.08 20.18 20.10
76.12 76.35 71.97 72.77 83.81 75.85
37.04 42.19 37.21 39.10 39.16 38.31
86.27 92.49 88.20 86.60 87.76 85.70
80.83 76.98 85.96 70.02 78.29 77.69
8 9 10 11 12 13
600
80.77 80.75 80.79 82.24 83.19 81.66
46.22 42.11 48.84 44.61 46.20 49.33
0.40 0.39 0.45 0.39 0.45 0.41
65.47 65.71 65.78 65.70 65.88 65.69
22.15 22.10 20.99 22.93 23.16 21.52
50.76 46.24 53.66 49.89 52.26 54.77
23.41 22.82 26.34 23.24 27.13 24.26
75.39 75.65 75.77 77.03 78.14 76.48
70.63 70.45 66.95 74.45 76.07 69.37
14 15 16 17 18 19
850
74.65 74.84 74.67 74.54 74.93 74.68
45.42 48.83 51.70 48.80 49.37 48.82
0.30 0.40 0.33 0.20 0.40 0.35
67.22 65.82 67.61 68.51 68.91 67.60
23.49 23.47 23.50 23.48 22.93 23.09
46.11 49.70 52.50 49.46 50.30 49.58
16.23 21.69 17.86 10.80 21.72 18.94
71.54 70.24 71.98 72.81 73.61 71.98
69.23 69.35 69.28 69.10 67.83 68.08
20 21 22 23 24 25
1000
72.86 73.09 72.78 71.30 72.99 72.77
23.94 25.15 24.79 26.09 30.50 26.10
0.05 0.05 0.05 0.05 0.05 0.05
62.71 57.20 60.51 64.81 61.30 61.31
21.10 21.40 21.18 20.33 22.05 20.21
23.72 25.00 24.53 25.30 30.27 25.83
2.64 2.65 2.64 2.58 2.64 2.64
65.14 59.60 62.78 65.88 63.79 63.61
60.70 61.75 60.85 57.23 63.53 58.06
Table 8 Concentration of the trace elements selected in the orthocoking coal and solid carbonization product obtained at the relevant temperatures No
Carbonization temperature (8C)
Solid carbonization product (%)
Concentration of the elements selected (ppm)
Contents of the elements selected in the solid carbonization product compared with the primary coal (%)
Hg
As
Se
Be
97.08
8.04
0.29
7.66
Hg
As
1
Primary coal
2 3 4
400
98.27 98.05 97.96
16.59 7.76 8.67
7.97 6.79 6.67
0.13 0.19 0.20
9.29 6.99 9.34
16.79 7.84 8.75
97.41 82.80 81.27
5 6 7
600
82.31 82.08 81.03
7.68 6.27 6.98
8.37 7.67 6.63
0.14 0.21 0.21
8.37 9.76 7.11
6.51 5.30 5.83
85.68 78.31 66.82
8 9 10
850
74.64 74.98 75.36
6.41 4.84 5.43
7.48 6.91 6.65
0.28 0.28 0.20
7.69 7.46 8.55
4.93 3.74 4.22
69.44 64.44
11 12 13
1000
72.97 72.57 72.42
7.90 5.86 6.49
8.62 8.06 7.21
0.22 0.15 0.22
10.78 7.62 8.66
5.94 4.38 4.84
Se
Be
56.00 55.58
72.16 81.84
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Fig. 1. Pb release in the coals tested.
In result of the release the content of the element tested decreased in the solid carbonization product samples when the carbonization temperature increased. The method applied consisted in determination of the elements tested in the solid carbonization product samples. The volatile matter was not analysed. The percentage of the element mass, which remained in the solid carbonization product sample, was calculated. It was assumed that the release
degree of the element tested during the carbonization was determined by means of the remaining amount of the element in the solid carbonization product. To this purpose the content of the element in the primary coal sample was calculated on the basis of the analysis results. Then the content of the element tested was referred to the solid carbonization product and expressed in per cent. These values are shown in Tables 5– 8. The rank of coal, defining
Fig. 2. Cd release in the coals tested.
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1287
Fig. 3. Mn release in the coals tested.
the part of the trace element released during carbonization at the particular temperature interval of the process, was calculated by subtracting these values from 100. When the release measure applied was the final result (at the temperature of 1000 8C) the highest volatility was found for cadmium—more than 90%, then for mercury—85% and lead—about 80%. The mean volatility was found for selenium—of the order of 40% and the lowest for nickel, manganese, arsenic and beryl—about 30%. Some differences between the floatation concentrate and the coal blend were also observed. They may be explained by differences
likely to be found in the share of organic and mineral coal matter in the origin of the trace elements. Besides the final degree of release, the dynamics of the process was also of interest. Mercury, cadmium and lead were released to great extent already at the temperature of heating equal to 400 8C. In the coals tested almost complete release of Hg, Cd and Pb was observed when heating to the temperature of 400 8C was applied. In the case of orthocoking coal, 90% Hg release occurred already at 400 8C, reaching 95% release rate at 600 8C. In the coal blend 50% Hg release and 80% Hg release were observed at 400 and 600 8C, respectively.
Fig. 4. Ni release in the coals tested.
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Fig. 5. Hg release in the coals tested.
Fig. 6. As release in the coals tested.
Fig. 7. Se content (%) in the solid carbonization product obtained at the temperature of 1000 8C compared with the primary coal.
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Fig. 8. Be content (%) in the solid carbonization product obtained at the temperature of 1000 8C compared with the primary coal.
Dynamics of Hg release was similar to mercury vaporization during coal combustion process, described in the literature. Selenium release reached 45% in the orthocoking coal and 30% in the coal blend. Cadmium release was very similar for both the coal types, dynamic at the first interval of the heating temperature (60 – 70%) and moderately increasing to more than 90% at the temperature of 1000 8C. At similar levels of final lead release rates, about 70– 85%, the release dynamics was different for the two coal types: it was significantly lower in the case of orthocoking coal. This referred
particularly to the first temperature interval. At 400 8C, 70% release from the coal blend and only 25% release from the orthocoking coal were found. The solid carbonization products obtained at 600 and 850 8C were also different The observed lead release from the carbonised orthocoking coal was lower by more than 20%. The plots of dynamics of arsenic and manganese release are regular taking the form of straight lines. The difference between the two solid carbonization products is very small (arsenic) and small (manganese). The arsenic release reaches 12% at 400 8C, 25% at 600 8C and 30% at 850 8C for the coals tested. In the case
Fig. 9. Pb release in the coal blend.
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1290 Table 9 Volatility indices for the trace elements tested Element
Hg Cd Pb As Mn Ni
Volatility indices Coal blend P ( £ 104)
Orthocoking coal (floatation concentrate) type 35.2 ( £ 104)
6.282 6.909 6.364 2.037 1.494 0.996
7.776 6.604 4.141 2.021 2.106 2.536
of manganese the release reaches 10% at 400 8C for both the coals tested. At 600 8C it reaches 12% from the coal blend and 25% from the orthocoking coal. At 850 8C 20% Mn release rate from the coal blend and 30% Mn release rate from the orthocoking coal were observed. An apparently marked temperature interval at which the highest nickel release rate could be observed was not found. The only observation made was that the final Ni release rate reached 40% for the orthocoking coal and only 15% for the coal blend. At 400 8C only 7% release was observed in the case of the coal blend and 20% release in the case of the orthocoking coal. At 600 8C the release was 12% Ni and 30% Ni for the coal blend and orthocoking coal, respectively. At 850 8C 14% Ni and 31% Ni were released from the coal blend and orthocoking coal, respectively. Beryl is an element of small volatility. Its release reaches 20% in the case of the orthocoking coal and 40% in the coal blend. Assuming that the surface below the curve in the release plot is the measure of the volatility dynamics of the trace elements tested (the highest volatility, the largest the surface), one may estimate the trace elements by a dimensionless index defined by the surface area P (an example is shown in Fig. 9). The indices calculated in this way are presented in Table 9. In this way it was found that the most volatile elements are Hg, Cd and Pb, whereas As, Mn and Ni volatility is significantly lower in the coals tested. The release of the elements tested, occurring with different dynamics and to different extents, always means
their passing together with the volatile matter to the carbonization products. The difference in the release rate reveals the way in which the trace elements tested are bound with the coal structural elements. However, besides such the qualitative estimation, the tests did not make any conclusion regarding the way, in which they are bound, possible. It requires application of another test method.
6. Conclusions (1) During coal carbonization some parts of the trace elements contained in the coal are released. (2) The release rate is the highest for cadmium, mercury and lead, mean for selenium and the lowest for nickel manganese, arsenic and beryl. (3) The release dynamics is different; it is also the highest for cadmium, mercury and lead. (4) Turning of the great part of the trace elements into gaseous products during carbonization proves that the limited airtight sealing of the coal coking process as well as coke-oven gas combustion contribute to the pollution of the environment with trace elements. (5) Actual hazards mentioned in (4) refer to mercury, cadmium and lead when, in addition, the toxicity of these metals is taken into account.
References [1] Jasien´ko S. (red.), Chemia i fizyka we˛gla, Wyd. Polit. Wrocl⁄awskiej, Wrocl⁄aw; 1995. [2] Zielin´ski H. (red.), Koksownictwo, Wyd. ‘S´la˛sk’, Katowice; 1986. [3] Swaine DJ, Goodarzi F. Environmental aspects of trace elements in coal. Netherlands: Kluwer; 1995. [4] Staisz J, Pason´-Konieczyn´ska A, Konieczyn´ski J. Archiwum Ochrony S´rodowiska 2000;26:7. [5] Roy LC, Chakrovarty DK. Chem Econ Engng Rev 1984;16:23. [6] De˛bska-Bes M, Menzla H. Koks, Smol⁄a, Gaz 1989;34:51. [7] Menzla H, De˛bska-Bes M. Koks, Smol⁄a, Gaz 1990;35:40. [8] Yakovleva TP, Dolzhanskaya YuB. Koks i Khimiya 1995;(5):31. [9] Eisenhut W, Klein F. ICP Inf Newslett 1991;17:164. [10] Kapal⁄a J. Koks, Smol⁄a, Gaz 1989;34:139. [11] Konieczyn´ski J, Z˙elin´ski J. Ochrona Powietrza 1991;25:78. [12] Zajusz-Zubek E, Go´rka P, Kwapulin´ski J. Substancje Toksyczne w S´rodowisku 1995;4–5:13. [13] Zajusz-Zubek E. Praca doktorska, Pol. S´l., Gliwice; 1999.
Dynamics of trace elements release in a coal pyrolysis processq Elwira Zajusz-Zubek*, Jan Konieczyn´ski Faculty of Energy and Environmental Engineering, Silesian University of Technology, ul. Akademicka 2, Gliwice 44-101, Poland Accepted 1 July 2001; available online 27 February 2003
Abstract Samples of coal and solid carbonization product obtained at four temperatures: 400, 600, 850 and 1000 8C were tested on account of the contents of trace elements. The following hazardous trace elements were considered: arsenic, beryl, cadmium, manganese, nickel, lead, mercury and selenium. The release curves for the elements tested were determined. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Trace elements; Coal pyrolysis; Air pollution
1. Introduction About 10% of coal produced in the world are processed into coke. Global production power of the coke industry is evaluated as 250– 270 million Mg of coke per year. There is a good balance between the coke supply and demand. However, in the nearest future, coke deficiency may be expected in some regions due to strict observation of the environmental protection regulations. Although coke, being smokeless, is considered a clean fuel, its production is accompanied by the air, water and air pollution. In particular the air cleanness is at risk in result of emission of dusts and gases including sulphur oxides, carbon monoxide and nitric oxide as well as numerous organic aromatic polynuclear hydrocarbons. Emission of the matters polluting the air is a result of lack of full airtight sealing during the basic stage of coke production, i.e. thermal decomposition (pyrolysis) of the coal. This process leads to significant changes in the coal structure and chemical constitution and to formation of solid, gaseous and liquid products (Table 1). In spite of the extensive knowledge of the effect of coal coking on the environment, little attention has been paid so far to the environment pollution with trace elements. On the other hand, one may expect they will be found among the pollutants penetrating to the environment if they occur in the coal subjected to the coking process. * Corresponding author. Tel./fax: þ 48-32-237-12-90. E-mail address: [email protected] (E. Zajusz-Zubek). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
Coal, besides its main components, includes a lot of elements of low concentrations of the order of 0.01 or 0.001 wt%. They are trace elements connected with organic coal matter or mineral matter comprised in coal. Their accumulation in coal is related to the origin of the coal beds and coal metamorphism. The contents of trace elements usually decrease when the rank of coal increases. The most often mentioned trace elements are: As, Ag, B, Ba, Be, Cd, Cl, Co, Cr, Cu, F, Ga, Ge, Hf, Hg, Li, Mn, Mo, Ni, Pb, Ra, Rb, Sb, Sn, Sr, Se, V and Zn. Concentrations of the trace elements are different, depending on the coal bed and basin. Table 2 shows the range of the average concentrations of some trace elements found in most types of coal produced in the world. Table 3 shows the adequate range for coal ashes from 20 mines situated in the Upper Silesia (Southern Poland) coal basin. This occurrence of the trace elements in the organic (lighter) and mineral (heavier) matter constituting the coal makes it possible to remove a part of the trace elements, connected with the mineral matter, from the coal. When testing American coals, [5] found organically associated Ge, Be, Ga, V, Cr, Co, Pb and Zn. In Poland, De˛bska-Bes, Menzla et al. [6,7] tested the way in which some trace elements were connected with organic and mineral coal matter in coking coals. The samples were separated in heavy liquids. Then, coal fractions of different specific gravity were extracted by means of the adequate solvents. On the basis of the acquired results it was found that most elements were connected with mineral coal matter, i.e. coal fraction of the density lower than 1.35 g/cm3. In this fraction significant increase in Co, Sr,
0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00031-0
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Table 1 Chemical composition of coking coal and coke, (%) [1,2]
Coking coal Coke a
Cdaf
Hdaf
Odaf a
Ndaf
Sdaf
88.2 97.5
4.8 0.4
4.5 0.5
1.8 0.7
0.7 0.9
From the difference.
Ni, V and Ga concentration was observed. On the other hand, Pb and Zn occurrence in fractions of the density higher than 1.45 g/cm3 proves the connection with mineral matter. When applying coal cleaning processes, based on the difference in density, the contents of Mn, Co, As, Hg, Zn, Cd and Pb may be reduced. Removal of pyrite from coal results in substantial decrease in Pb, Cd and As contents. It should be pointed out that the charge coal in a coking plant is most often dressed. Behaviour of trace elements in a combustion process reveals their different ability to form the gaseous phase. Emission of trace elements from coal furnaces takes place both in gaseous phase (e.g. Hg) and in solid phase. Their presence in dust particles is a result of their origin from mineral matter in coal as well as of condensation and adsorption from gaseous phase during the waste gas transport from the source to the emitter. Only a few researchers have so far studied the behaviour of trace elements found in coal during coal pyrolysis [8]. The literature available includes papers by Eisenhut [9] who tested the risk for the air, caused by heavy metals emitted during coal carbonization in Germany. There are also Polish papers, referring to the concentration range of some trace elements in coal and coke [6,7], and papers referring to the concentration level of some trace elements in the air, earth and water in coking plant surroundings [10 – 13]. Increased pollution of the environment with these trace elements may be caused by their release during coal pyrolysis and by emission, including volatile matter emitted from leaks in
Table 2 Ranges (ppm) of the average concentrations of some elements found in most types of coal produced in the world [3] Element
Range
Element
Range
As B Ba Be Cd Cl Co Cr Cu F Hg Mn
0.5–80 5–400 20–1000 0.1–15 0.1–3 50–2000 0.5–30 0.5–60 0.5–50 20–500 0.02–1 5–300
Mo Ni P Pb Sb Se Sn Th Tl U V Zn
0.1–10 0.5–50 10–3000 2–80 0.05–10 0.2–10 1–10 0.5–10 ,0.2–1 0.5–10 2–100 5–300
Table 3 Concentration intervals of trace elements found in coal ashes from the Upper Silesia [4] Element
Concentration (ppm)
Ag As Ba Be Cd Co Cr Cu Mn Mo Ni Pb V Zn Hga
0–2 0–82 504–3620.5 7–26 0–18 41–167 72–231.5 102–508.5 311.5–1186.5 0–7 82.5–359 59–248 249–675 148–714 0.1–0.23
a
In coal.
coke oven batteries when the coke chambers are charged and discharged and during coke oven battery firing. Thus, in order to get more information to better evaluate the coal coking hazards to the environment, it was decided to begin a research into the release of trace elements during coal pyrolysis.
2. Research concept and scope The aim of the research was to test the dynamics of the release of some selected trace elements from coal subjected to pyrolysis to the stream of volatile matter. Taking into account their toxic effect, the trace elements selected were As, Be, Cd, Mn, Ni, Pb, Hg and Se (they are all on the EPA list of hazardous air pollutants). The tests were carried out in a laboratory. The coal samples were carbonised in an electric muffle furnace in the same way as the way used when determining volatile matter. Dynamics of the trace elements release was measured by comparing their concentrations in a coal sample and in solid carbonization product samples obtained at carbonization temperatures 400, 600, 850 and 1000 8C, respectively. The temperature levels corresponded to the following stages, used in the coal coking theory: primary coal carbonization (400 8C), semi-coke formation (600 8C), secondary coal carbonization (850 8C) and coke formation (1000 8C). Samples of orthocoking coal (type 35.2) and of coal blend, consisting of two coal types and used in one of Polish coking plants, were tested. The dynamics of the trace element release was confirmed by their decreasing contents when the temperature of the process increased. It could be evaluated by plotting the release curve, whose shape depends on that dynamics or by using a calculated comparative index.
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The following coal types were tested:
Table 4 Technical analysis of the coals tested Coal tested
Coal blend used as charge in the coking plant Orthocoking coal (floatation concentrate) type 35.2
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Contents (%) Moisture
Ash
Volatile matter
0.2
7.7
24.0
0.3
7.64
21.9
3. The test method 3.1. Sampling The coal sampling, preparing and milling to get grains smaller than 0.2 mm as well as moisture, ashes and volatile matter determining were done according to the Polish standards.
† Coal blend used in a coking plant consisting of gascoking coal type 34.1 (70%) and orthocoking coal type 35.1 (30%). † Orthocoking coal (floatation concentrate) type 35.2. The coal characteristics are presented in Table 4. 3.2. Tests of release dynamics of the trace elements selected The release dynamics of the trace elements selected was tested in the following way. Weighed samples, the mass of which was 1.0000 g, were separated from the analytical samples of the coals tested. Crucibles with the weighed samples of coal (three weighed samples from each analytical sample), covered with lids, were placed on the preheated stools in the muffle furnace heated to the temperature of 400 8C. Then they were heated without any air supply, in the way similar to the way used when determining the contents of volatile matter, for 15 min. Carbonization of the coal samples at the temperature of 600, 850 and 1000 8C was carried out in the same way by placing
Table 5 Concentration of the trace elements selected in the coal blend and solid carbonization product obtained at the relevant temperatures No
Carbonization temperature (8C)
Solid carbonization product (%)
Concentration of the elements selected (ppm)
Contents of the elements selected in the solid carbonization product compared with the primary coal (%)
Pb
Cd
Mn
Ni
Pb
Cd
Mn
Ni
46.16
3.51
76.67
19.59
1
Primary coal
2 3 4 5 6 7
400
97.91 97.22 98.45 97.73 97.00 98.03
14.85 15.53 15.50 15.03 14.94 15.39
1.00 0.95 0.90 1.10 0.92 0.90
67.16 70.13 73.91 71.30 71.33 73.98
19.00 18.50 18.85 18.90 18.30 18.00
31.50 32.71 33.06 31.82 31.39 32.69
27.89 26.31 25.24 30.63 25.42 25.14
85.77 88.93 94.90 90.88 90.24 94.59
94.96 91.81 94.73 94.29 90.61 90.08
8 9 10 11 12 13
600
79.08 79.06 80.59 80.62 79.68 79.48
15.45 15.17 14.85 14.55 15.00 15.57
0.71 0.73 0.77 0.75 0.80 0.90
84.54 86.29 84.65 84.50 86.09 81.15
21.45 21.25 21.50 20.36 22.10 20.65
26.47 25.98 25.93 25.41 25.89 26.81
16.00 16.44 17.68 17.23 18.16 20.38
87.20 88.98 88.98 88.85 89.47 84.12
86.59 85.76 88.44
14 15 16 17 18 19
850
72.35 72.76 72.73 72.70 73.01 72.76
14.33 14.50 14.56 13.30 14.10 14.62
0.60 0.71 0.62 0.80 0.65 0.56
85.31 83.53 79.80 83.50 83.64 85.38
23.22 23.90 23.80 22.76 23.95 23.50
22.46 22.86 22.94 20.95 22.30 23.05
12.37 14.72 12.85 16.57 13.52 11.61
80.51 79.27 75.70 79.17 79.64 81.03
85.76
20 21 22 23 24 25
1000
71.08 71.09 71.00 70.56 70.62 70.77
9.00 9.14 9.08 10.05 8.37 9.14
0.52 0.45 0.59 0.48 0.55 0.51
78.46 75.89 76.79 76.03 76.70 76.78
23.90 24.00 24.50 23.85 22.69 23.84
13.86 14.08 13.97 15.36 12.81 14.01
10.53 9.11 11.93 9.65 11.07 10.28
72.74 70.37 71.11 69.97 70.65 70.87
86.72 87.10
89.88
88.36 84.46 87.29
85.91 81.80 86.12
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the samples into the muffle furnace heated to the relevant temperature. 3.3. Mineralization In order to minimise the loss of especially volatile elements: As, Be, Hg and Se, the coal and solid carbonization product samples were mineralised by combustion in a bomb calorimeter at the oxygen pressure of about 2.5 MPa, 10 cm3 of spectrally pure nitric acid (1:9) and a 1 g weighed sample, mixed with 0.5 g spectrally pure benzene carboxylic acid on a filter free from heavy metals, were introduced inside the bomb. In order to determine Cd, Mn, Ni and Pb wet mineralization was applied with a mixture of concentrated spectrally pure nitric acid and perchloric acid at the volume ratio 1:1 at the atmospheric pressure and at the temperature about 200 8C. The weighed sample mass was 0.2 g. 3.4. Determination As, Hg and Se in the solutions obtained in the way described earlier were determined by an absorption method employing the technique of ASA –VAP hydride generation on a spectrophotometer AAS Philips PU-9100x. Be content was determined by the ASA method with the use of flame atomisation in an acetylene-nitrous acid flame and verified by ICP method. The remaining trace elements, i.e. Pb, Cd, Mn and Ni were determined by ASA method applying the flame variant with deuterium background correction. In order to determine these elements, typical parameters were used, recommended by the producer of ASA Carl Zeiss Jena
spectrophotometer type AAS-3 as optimum for particular elements. The results obtained are shown in Tables 5– 8. Figs. 1 –8 show the plots of the relevant element release in the coal tested.
4. Experimental For experimental details see Tables 5 – 8 and Figs. 1 – 9.
5. Discussion of the results In the two coal types tested, in which the content of ash was similar, there were not any essential differences in the concentrations of the trace elements investigated. The highest concentrations, higher than 40 ppm, were found for mercury, lead, manganese, about 20 ppm for nickel, approximately a few ppm for arsenic and beryl, the lowest, lower than 1 ppm—for selenium. Satisfactory repeatability of lead, cadmium, manganese, nickel and mercury determination was acquired. Hence, it was possible to plot the curves of their release in the whole range of the carbonization temperatures tested. Owing to the worse repeatability of selenium and beryl determination their release curves were not plotted and the final result was illustrated only for the temperature of 1000 8C. When comparing the release runs obtained for the elements tested, significant differences in the release rate may be noticed. The release was investigated by watching the mass of the particular element contained in the coal subjected to carbonization. The content of the element, determined by an analytical method, was assumed to be 100.
Table 6 Concentration of the trace elements selected in the coal blend and solid carbonization product obtained at the relevant temperatures No
Carbonization temperature (8C)
Solid carbonization product (%)
Concentration of the elements selected (ppm)
Contents of the elements selected in the solid carbonization product compared with the primary coal (%)
Hg
As
Se
Be
Hg
As
61.22
5.00
0.74
6.29
1
Primary coal
2 3 4
400
97.03 97.94 97.23
34.21 28.56 30.02
3.42 4.99 5.15
0.68 0.86 0.86
6.29 5.42 4.86
54.22 45.69 47.68
66.32 97.68 100.08
5 6 7
600
80.22 79.22 79.26
15.53 16.99 10.92
4.73 3.94 4.37
0.95 0.68 0.82
5.27 5.03 5.60
20.35 21.98 14.14
75.84
8 9 10
850
72.76 72.31 73.40
12.08 14.46 10.89
4.62 5.42 4.94
0.57 0.43 0.58
5.54 6.94 3.20
14.36 17.08
67.19
11 12 13
1000
70.53 70.53 70.38
13.34 14.13 13.31
5.56 3.87 5.55
0.74 0.67 0.89
5.33 5.35 6.06
Se
Be
70.85 64.15
59.79 60.02
69.22
72.47 78.37 54.55 78.07
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Table 7 Concentration of the trace elements selected in the orthocoking coal and solid carbonization product obtained at the relevant temperatures No
Carbonization temperature (8C)
Solid carbonization product (%)
Concentration of the elements selected (ppm)
Contents of the elements selected in the solid carbonization product compared with the primary coal (%)
Pb
Cd
Mn
Ni
73.54
1.38
70.14
25.33
Pb
Cd
Mn
Ni
1
Primary coal
2 3 4 5 6 7
400
98.29 98.68 98.75 98.10 98.27 97.91
56.95 56.90 53.60 54.55 62.72 56.97
0.52 0.59 0.52 0.55 0.55 0.54
61.56 65.74 62.65 61.92 62.64 61.39
20.83 19.76 22.05 18.08 20.18 20.10
76.12 76.35 71.97 72.77 83.81 75.85
37.04 42.19 37.21 39.10 39.16 38.31
86.27 92.49 88.20 86.60 87.76 85.70
80.83 76.98 85.96 70.02 78.29 77.69
8 9 10 11 12 13
600
80.77 80.75 80.79 82.24 83.19 81.66
46.22 42.11 48.84 44.61 46.20 49.33
0.40 0.39 0.45 0.39 0.45 0.41
65.47 65.71 65.78 65.70 65.88 65.69
22.15 22.10 20.99 22.93 23.16 21.52
50.76 46.24 53.66 49.89 52.26 54.77
23.41 22.82 26.34 23.24 27.13 24.26
75.39 75.65 75.77 77.03 78.14 76.48
70.63 70.45 66.95 74.45 76.07 69.37
14 15 16 17 18 19
850
74.65 74.84 74.67 74.54 74.93 74.68
45.42 48.83 51.70 48.80 49.37 48.82
0.30 0.40 0.33 0.20 0.40 0.35
67.22 65.82 67.61 68.51 68.91 67.60
23.49 23.47 23.50 23.48 22.93 23.09
46.11 49.70 52.50 49.46 50.30 49.58
16.23 21.69 17.86 10.80 21.72 18.94
71.54 70.24 71.98 72.81 73.61 71.98
69.23 69.35 69.28 69.10 67.83 68.08
20 21 22 23 24 25
1000
72.86 73.09 72.78 71.30 72.99 72.77
23.94 25.15 24.79 26.09 30.50 26.10
0.05 0.05 0.05 0.05 0.05 0.05
62.71 57.20 60.51 64.81 61.30 61.31
21.10 21.40 21.18 20.33 22.05 20.21
23.72 25.00 24.53 25.30 30.27 25.83
2.64 2.65 2.64 2.58 2.64 2.64
65.14 59.60 62.78 65.88 63.79 63.61
60.70 61.75 60.85 57.23 63.53 58.06
Table 8 Concentration of the trace elements selected in the orthocoking coal and solid carbonization product obtained at the relevant temperatures No
Carbonization temperature (8C)
Solid carbonization product (%)
Concentration of the elements selected (ppm)
Contents of the elements selected in the solid carbonization product compared with the primary coal (%)
Hg
As
Se
Be
97.08
8.04
0.29
7.66
Hg
As
1
Primary coal
2 3 4
400
98.27 98.05 97.96
16.59 7.76 8.67
7.97 6.79 6.67
0.13 0.19 0.20
9.29 6.99 9.34
16.79 7.84 8.75
97.41 82.80 81.27
5 6 7
600
82.31 82.08 81.03
7.68 6.27 6.98
8.37 7.67 6.63
0.14 0.21 0.21
8.37 9.76 7.11
6.51 5.30 5.83
85.68 78.31 66.82
8 9 10
850
74.64 74.98 75.36
6.41 4.84 5.43
7.48 6.91 6.65
0.28 0.28 0.20
7.69 7.46 8.55
4.93 3.74 4.22
69.44 64.44
11 12 13
1000
72.97 72.57 72.42
7.90 5.86 6.49
8.62 8.06 7.21
0.22 0.15 0.22
10.78 7.62 8.66
5.94 4.38 4.84
Se
Be
56.00 55.58
72.16 81.84
1286
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Fig. 1. Pb release in the coals tested.
In result of the release the content of the element tested decreased in the solid carbonization product samples when the carbonization temperature increased. The method applied consisted in determination of the elements tested in the solid carbonization product samples. The volatile matter was not analysed. The percentage of the element mass, which remained in the solid carbonization product sample, was calculated. It was assumed that the release
degree of the element tested during the carbonization was determined by means of the remaining amount of the element in the solid carbonization product. To this purpose the content of the element in the primary coal sample was calculated on the basis of the analysis results. Then the content of the element tested was referred to the solid carbonization product and expressed in per cent. These values are shown in Tables 5– 8. The rank of coal, defining
Fig. 2. Cd release in the coals tested.
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1287
Fig. 3. Mn release in the coals tested.
the part of the trace element released during carbonization at the particular temperature interval of the process, was calculated by subtracting these values from 100. When the release measure applied was the final result (at the temperature of 1000 8C) the highest volatility was found for cadmium—more than 90%, then for mercury—85% and lead—about 80%. The mean volatility was found for selenium—of the order of 40% and the lowest for nickel, manganese, arsenic and beryl—about 30%. Some differences between the floatation concentrate and the coal blend were also observed. They may be explained by differences
likely to be found in the share of organic and mineral coal matter in the origin of the trace elements. Besides the final degree of release, the dynamics of the process was also of interest. Mercury, cadmium and lead were released to great extent already at the temperature of heating equal to 400 8C. In the coals tested almost complete release of Hg, Cd and Pb was observed when heating to the temperature of 400 8C was applied. In the case of orthocoking coal, 90% Hg release occurred already at 400 8C, reaching 95% release rate at 600 8C. In the coal blend 50% Hg release and 80% Hg release were observed at 400 and 600 8C, respectively.
Fig. 4. Ni release in the coals tested.
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Fig. 5. Hg release in the coals tested.
Fig. 6. As release in the coals tested.
Fig. 7. Se content (%) in the solid carbonization product obtained at the temperature of 1000 8C compared with the primary coal.
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Fig. 8. Be content (%) in the solid carbonization product obtained at the temperature of 1000 8C compared with the primary coal.
Dynamics of Hg release was similar to mercury vaporization during coal combustion process, described in the literature. Selenium release reached 45% in the orthocoking coal and 30% in the coal blend. Cadmium release was very similar for both the coal types, dynamic at the first interval of the heating temperature (60 – 70%) and moderately increasing to more than 90% at the temperature of 1000 8C. At similar levels of final lead release rates, about 70– 85%, the release dynamics was different for the two coal types: it was significantly lower in the case of orthocoking coal. This referred
particularly to the first temperature interval. At 400 8C, 70% release from the coal blend and only 25% release from the orthocoking coal were found. The solid carbonization products obtained at 600 and 850 8C were also different The observed lead release from the carbonised orthocoking coal was lower by more than 20%. The plots of dynamics of arsenic and manganese release are regular taking the form of straight lines. The difference between the two solid carbonization products is very small (arsenic) and small (manganese). The arsenic release reaches 12% at 400 8C, 25% at 600 8C and 30% at 850 8C for the coals tested. In the case
Fig. 9. Pb release in the coal blend.
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1290 Table 9 Volatility indices for the trace elements tested Element
Hg Cd Pb As Mn Ni
Volatility indices Coal blend P ( £ 104)
Orthocoking coal (floatation concentrate) type 35.2 ( £ 104)
6.282 6.909 6.364 2.037 1.494 0.996
7.776 6.604 4.141 2.021 2.106 2.536
of manganese the release reaches 10% at 400 8C for both the coals tested. At 600 8C it reaches 12% from the coal blend and 25% from the orthocoking coal. At 850 8C 20% Mn release rate from the coal blend and 30% Mn release rate from the orthocoking coal were observed. An apparently marked temperature interval at which the highest nickel release rate could be observed was not found. The only observation made was that the final Ni release rate reached 40% for the orthocoking coal and only 15% for the coal blend. At 400 8C only 7% release was observed in the case of the coal blend and 20% release in the case of the orthocoking coal. At 600 8C the release was 12% Ni and 30% Ni for the coal blend and orthocoking coal, respectively. At 850 8C 14% Ni and 31% Ni were released from the coal blend and orthocoking coal, respectively. Beryl is an element of small volatility. Its release reaches 20% in the case of the orthocoking coal and 40% in the coal blend. Assuming that the surface below the curve in the release plot is the measure of the volatility dynamics of the trace elements tested (the highest volatility, the largest the surface), one may estimate the trace elements by a dimensionless index defined by the surface area P (an example is shown in Fig. 9). The indices calculated in this way are presented in Table 9. In this way it was found that the most volatile elements are Hg, Cd and Pb, whereas As, Mn and Ni volatility is significantly lower in the coals tested. The release of the elements tested, occurring with different dynamics and to different extents, always means
their passing together with the volatile matter to the carbonization products. The difference in the release rate reveals the way in which the trace elements tested are bound with the coal structural elements. However, besides such the qualitative estimation, the tests did not make any conclusion regarding the way, in which they are bound, possible. It requires application of another test method.
6. Conclusions (1) During coal carbonization some parts of the trace elements contained in the coal are released. (2) The release rate is the highest for cadmium, mercury and lead, mean for selenium and the lowest for nickel manganese, arsenic and beryl. (3) The release dynamics is different; it is also the highest for cadmium, mercury and lead. (4) Turning of the great part of the trace elements into gaseous products during carbonization proves that the limited airtight sealing of the coal coking process as well as coke-oven gas combustion contribute to the pollution of the environment with trace elements. (5) Actual hazards mentioned in (4) refer to mercury, cadmium and lead when, in addition, the toxicity of these metals is taken into account.
References [1] Jasien´ko S. (red.), Chemia i fizyka we˛gla, Wyd. Polit. Wrocl⁄awskiej, Wrocl⁄aw; 1995. [2] Zielin´ski H. (red.), Koksownictwo, Wyd. ‘S´la˛sk’, Katowice; 1986. [3] Swaine DJ, Goodarzi F. Environmental aspects of trace elements in coal. Netherlands: Kluwer; 1995. [4] Staisz J, Pason´-Konieczyn´ska A, Konieczyn´ski J. Archiwum Ochrony S´rodowiska 2000;26:7. [5] Roy LC, Chakrovarty DK. Chem Econ Engng Rev 1984;16:23. [6] De˛bska-Bes M, Menzla H. Koks, Smol⁄a, Gaz 1989;34:51. [7] Menzla H, De˛bska-Bes M. Koks, Smol⁄a, Gaz 1990;35:40. [8] Yakovleva TP, Dolzhanskaya YuB. Koks i Khimiya 1995;(5):31. [9] Eisenhut W, Klein F. ICP Inf Newslett 1991;17:164. [10] Kapal⁄a J. Koks, Smol⁄a, Gaz 1989;34:139. [11] Konieczyn´ski J, Z˙elin´ski J. Ochrona Powietrza 1991;25:78. [12] Zajusz-Zubek E, Go´rka P, Kwapulin´ski J. Substancje Toksyczne w S´rodowisku 1995;4–5:13. [13] Zajusz-Zubek E. Praca doktorska, Pol. S´l., Gliwice; 1999.