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Short Communication
Transformations of sulphur compounds in high-sulphur coals during reduction in the potassium/liquid ammonia systemq M. Kozłowskia,*, H. Wachowskaa, J. Ypermanb a Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, Poznan´ 60-780, Poland Laboratory of Applied Chemistry, IMO, Limburgs Universitair Centrum, Diepenbeek B-3590, Belgium.
b
Received 5 April 2002; accepted 17 December 2002; available online 30 January 2003
Abstract The two types of high-sulphur coals Mequinenza and Illinois No. 6, in the initial form and subjected to potassium/liquid ammonia reduction, were analysed by atmospheric pressure –temperature programmed reduction (AP – TPR) method. It has been shown that preliminary demineralisation was beneficial for AP– TPR measurements because of the removal of calcium compounds. The reduction of sulphides and disulphides in the potassium/liquid ammonia system was found to lead to formation of aromatic and aliphatic thiols. The presence of the latter is better manifested in the AP – TPR kinetograms when the measurements are performed in the presence of a special reducing mixture. It has been shown that the coal reduction in the potassium/liquid ammonia system apart from transformations of nonthiophene sulphur groups also leads to breaking up of the C– S bonds in some thiophene systems. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Coal; Reduction; Sulphur; Atmospheric pressure–temperature programmed reduction method
1. Introduction All types of coal contain sulphur, whose concentration may vary from trace amounts to several percent. Usually it occurs in the dominant form of inorganic sulphur (mainly pyrite), however, some coals, like the Croatian Rasˇa [1], contain significant amounts of organic sulphur. Very often utilisation of high-sulphur content coals has negative effects on the natural environment [2]. Rational and safe utilisation of coal requires first of all a full recognition of the amount and types of sulphur content in them. Determinations of the inorganic sulphur compounds are relatively simple, whereas analysis of the organic sulphur has been still a challenge. Interesting information on the inorganic and organic sulphur compounds occurring in coal has been obtained from the studies using methods such as XANES [3], XPS [4], CAPTO [5], HP – TPR [6] and AP – TPR [7]. In our earlier work [8] on analysis of inorganic sulphur forms by atmospheric pressure –temperature programmed reduction (AP –TPR), we have shown a good agreement between the AP – TPR results and those obtained by * Corresponding author. E-mail address: [email protected] (M. Kozłowski). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
the classical chemical analysis. The AP –TPR analysis of organic sulphur proved to be more complex because some of its forms give signals in the same temperature range [9, 10]. To avoid this problem in our next study [11], the AP – TPR analysis was preceded by the coal reduction in the potassium/liquid ammonia system. In this process, sulphides and disulphides are converted into corresponding thiols characterised by a low temperature of hydrogenation and thus being easy to be identified in AP – TPR kinetogram. Unfortunately, the presence of relatively large amounts of calcium compounds in the samples studied initiated side reactions in which some of the H2S released was captured in the form of CaS, which has significantly disturbed interpretation of the results. For this reason, in this study, the coal samples were preliminary demineralised by the method of Radmacher and Mohrhauer [12]. This process does not have a significant influence on the course of the coal reduction in the potassium/liquid ammonia system [13], and removes the disturbing mineral compounds. As the changes in the coal structure increase with a repetition of the process [14], the measurements were performed for once and twice reduced coals. The study is a continuation of our earlier work on AP – TPR analysis of reduced coals, and our main aim was to
0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00005-X
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M. Kozłowski et al. / Fuel 82 (2003) 1149–1153
analyse the transformations of organic sulphur occurring in the process of reduction in the potassium/liquid ammonia system.
pyrogallol [20]) to the sample. Identification of particular sulphur compounds was performed on the basis of a study of model compounds [9,10]. The AP – TPR kinetograms are shown in Figs. 1 – 4.
2. Experimental 3. Results and discussion 2.1. The samples The high-sulphur coal samples to be studied were the Spanish coal Mequinenza and American-Illinois No. 6. The coal samples were ground and sieved to grain size # 0.06 mm, and subjected to demineralisation by the method of Radmacher and Mohrhauer [12]. The raw and demineralised coal samples are characterised in Table 1. The reduction in the potassium/liquid ammonia system was performed according to the procedure given elsewhere [8], using 14 g of metallic potassium dissolved in 300 cm3 of liquid ammonia per 12 g of carefully dried coal. The protonating agent was ethyl alcohol. After acidification with 5% HCl and careful washing with warm distilled water, the product was dried at 60 8C to constant weight in vacuo. The process of reduction was carried out twice. 2.2. Analytical procedures The total, sulphate, pyrite and elemental sulphur were determined by the classical chemical methods according to the standards [15 –18]. The content of organic sulphur was determined as the difference between that of the total and inorganic sulphur. The results are given in Table 2. No elemental sulphur was detected in the samples studied. The AP –TPR measurements were carried out according to the procedure given in Ref. [19]. Portions of 40 mg of the samples studied were mixed with 60 mg of fumed silica and placed in a quartz reactor heated from room temperature to 1000 8C at a linear temperature increase at the rate of 5 8C/ min. Hydrogen gas was blown through the reactor at the rate of 50 cm3/min. The formed H2S was dissolved in an antioxidising pH-buffered solution and determined potentiometrically as S22. Some AP – TPR measurements were conducted with the addition of a special reducing mixture (9,10-dihydroanthracene, phenanthrene, resorcinol and
Table 1 presents results of proximate and elemental analysis of the raw and demineralised coals, whereas their full analytical and spectral characterisation (and the characterisation of the products of their reduction) is given in Ref. [21]. Demineralisation of the two types of coal by HCl and HF leads to a great decrease in the content of ash, however, the content of volatile components decreases only a little. Analysis of the data from Table 2 shows that demineralisation of the two types of coal does not remove pyrite from them, because this mineral is insoluble in HCl and HF [22]. The content of sulphate sulphur considerably decreases after demineralisation. The reduction of the demineralised coal samples in the potassium/liquid ammonia system leads to a small loss of pyrite and total removal of sulphates. With repetition of the process, the content of organic sulphur decreases as a result of breaking up some C – S bonds, which sometimes leads to elimination of sulphur from coal [23]. The main form of sulphur occurring in all samples studied is organic one. It has been found [24] that the values of sulphur recovery obtained on AP – TPR measurements depend on three factors: (1) the content of calcium compounds binding H2S appearing on measurements, (2) the presence of sulphur in the from of particularly stable thiophene groups not undergoing hydrogenation to H2S and (3) the formation of volatile sulphur compounds other than H2S, potentiometrically undetected. The sulphur recovery values are high (Table 3), which means that almost all sulphur content is recorded by AP – TPR. The values of sulphur recovery increase after demineralisation as a result of removal of calcium compounds. As the increase is greater for Mequinenza coal, this coal can be expected to contain more calcium than Illinois No. 6. This supposition was confirmed by the analytical results specifying the content of
Table 1 Proximate and elemental analyses of raw and demineralised coals (wt%) Coal
Moisture
Ash (dry coal)
VMdaf
Cdaf
Hdaf
Ndaf
Sdaf org
Odaf a
Mequinenza coal Raw Demineralised
4.9 0.0
21.9 1.3
51.0 48.1
62.3 64.8
5.5 5.3
0.6 0.8
9.8 10.4
21.8 18.7
Illinois No. 6 coal Raw Demineralised
6.7 0.0
10.7 0.6
40.4 38.5
71.9 74.1
4.5 5.2
1.3 1.4
3.0 3.0
19.3 16.3
a
By difference.
M. Kozłowski et al. / Fuel 82 (2003) 1149–1153
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Table 2 Content of different forms of sulphur in raw, demineralised and modified coals (wt%, dry coal) Coal
Sulphur Total
Pyritic
Sulphate
Organic
Mequinenza coal Raw Demineralised Reduced 1 £ Reduced 2 £
9.67 11.60 10.96 9.74
1.22 1.29 1.24 1.23
0.81 0.03 0.00 0.00
7.64 10.28 9.72 8.51
Illinois No. 6 coal Raw Demineralised Reduced 1 £ Reduced 2 £
3.91 3.51 2.92 2.55
0.35 0.46 0.36 0.40
0.92 0.03 0.00 0.00
2.64 3.02 2.56 2.15
2.89% of calcium (as CaO) in Mequinenza and 0.52% in Illinois No. 6 [11]. After the reduction in the potassium/ liquid ammonia system, the values of sulphur recovery decrease. This observation is interpreted as a probable result of fragmentation of the coal structure and conversion of some sulphur compounds into others, which do not undergo hydrogenation to H2S on AP –TPR measurements (e.g. volatile thiols, which evaporate before they could be hydrogenated). The AP –TPR analysis was also made with the addition of a special reducing mixture [20], whose presence decreases the temperature of hydrogenation of certain sulphur compounds (mainly thiols) [9]. The influence of the mixture on the sulphur recovery depends on the character of the coal studied. For the raw coals reduced in the potassium/liquid ammonia system, the values of the sulphur recovery increased in the presence of the reducing mixture [11], but for the coals oxidised with different
Fig. 1. AP–TPR kinetograms of Mequinenza coal: (—) raw coal; (- - -) demineralised coal; (-·-·-) demineralised coal once reduced; (· · ·) demineralised coal twice reduced.
Fig. 2. AP –TPR kinetograms of Mequinenza coal with reducing mixture: (—) raw coal; (- - -) demineralised coal; (-·-·-) demineralised coal once reduced; (· · ·) demineralised coal twice reduced.
agents, the influence of this mixture proved rather disadvantageous [24]. As follows from the data in Table 3, for the samples studied, the addition of the reducing mixture proved to be advantageous. Similarly as for the samples without the mixture, the sulphur recovery increases after demineralisation and gradually decreases with repetition of the process of reduction in the potassium/liquid ammonia system. The majority of the AP –TPR kinetograms presented in Figs. 1 – 4 reveal two broad peaks. The first, with the maximum in the range of 370– 450 8C, on the basis of the results for model compounds, can be assigned to superposition of signals from different forms of non-thiophene
Fig. 3. AP–TPR kinetograms of Illinois No. 6 coal: (—) raw coal; (- - -) demineralised coal; (-·-·-) demineralised coal once reduced; (· · ·) demineralised coal twice reduced.
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Fig. 4. AP–TPR kinetograms of Illinois No. 6 coal with reducing mixture: (—) raw coal; (- - -) demineralised coal; (-·-·-) demineralised coal once reduced; (· · ·) demineralised coal twice reduced.
sulphur, while the broad peak with the maximum at about 700 8C can be assigned to thiophene sulphur [9,10]. The shapes of AP –TPR kinetograms of the raw and demineralised Mequinenza samples (Fig. 1) are similar up to , 550 8C (except for the small widening in the lower temperature range of the demineralised sample, due to the removal of calcium compounds). Above this temperature, the intensity of the signal in the kinetogram of the raw sample decreases compared with the demineralised sample, which is again explained as a result of the presence of calcium ions binding the H2S produced during the analysis. In the kinetograms of the demineralised samples, so after removal of disturbing calcium compounds, a peak with a maximum at , 680 8C assigned to thiophene sulphur clearly appears (in contrast to that for the raw sample). The reduction of the demineralised coal in the potassium/liquid ammonia system leads to a decrease in the intensity of the peaks assigned both to thiophene and nonthiophene sulphur. This decrease is a consequence of elimination of sulphur (Table 2) by the process of breaking Table 3 Sulphur recovery by AP–TPR method (%) Coal
Sulphur recovery
Sulphur recovery (with mix)
Mequinenza coal Raw Demineralised Reduced 1 £ Reduced 2 £
73 88 82 82
84 91 87 82
Illinois No. 6 coal Raw Demineralised Reduced 1 £ Reduced 2 £
93 100 88 77
98 100 89 77
up of some C –S bonds, by potassium in liquid ammonia, and a decrease of the sulphur recovery value. Moreover, after the reduction, the peak localised initially at 410 8C is shifted towards lower temperatures, which is interpreted as a result of conversion of sulphides and disulphides in thiol groups. As the temperature of hydrogenation of the thiol groups on AP – TPR measurements is lower than that of the sulphide and disulphide groups from which they originated [9,10], the maximum of the peak being a superposition of the signals from all these groups is shifted towards lower temperatures. The value of the shift increased after repetition of the process of reduction in the potassium/ liquid ammonia system, which proves that a single process of reduction is insufficient to convert all sulphides and disulphides into thiols. It was also noted that as a result of twice-repeated reduction, the intensity of the signal assigned to thiophene sulphur decreases, but the intensity of that assigned to non-thiophene sulphur does not change. This observation proves the breaking up of some C –S bonds in the thiophene structures by potassium in liquid ammonia. According to the results of the study of model compounds, sometimes the presence of the reducing mixture during AP –TPR analysis causes a disturbance in the kinetograms obtained [9]. A comparison of kinetograms of the demineralised sample (Fig. 1) and the same sample with addition of the reducing mixture (Fig. 2) proves, however, that for the samples studied, the above phenomenon practically does not occur. The kinetograms taken for the potassium/liquid ammonia reduced samples with the addition of the reducing mixture in the AP –TPR experiment reveal the appearance of a new peak at about 230 8C (Fig. 2). On the basis of the study of model compounds, this peak can be assigned to non-volatile thiols formed as a product of reduction of sulphides and disulphides in the potassium/liquid ammonia system. In the presence of the additional reducing mixture, thiols undergo hydrogenation on AP – TPR analysis at a temperature lower than without this additional mixture [9], and therefore, they were manifested as a separate peak, while in the kinetograms in Fig. 1, the peak corresponding to them was a component of a broad signal with a maximum at , 400 8C. Despite the appearance of a peak at , 230 8C, a small shift of the peak assigned to non-thiophene sulphur is still observed in the kinetograms of the reduced samples. This fact suggests that different kinds of thiols are obtained as a result of coal reduction in the potassium/liquid ammonia system. Most probably the signal at 230 8C can be assigned to aliphatic thiols, whereas the one being a component of the broad peak at , 400 8C to more thermally stable aromatic thiols. This interpretation suggests the presence of different kinds of sulphides and disulphides in the coal sample studied (e.g. aromatic – aromatic, aromatic – aliphatic, aliphatic – aliphatic). As the signal at , 230 8C has almost the same intensity in the kinetograms taken after the first and the second process of reduction, we can conclude that a single reduction is sufficient for breaking up all C –S sulphide and
M. Kozłowski et al. / Fuel 82 (2003) 1149–1153
disulphide bonds, leading to formation of aliphatic thiols. The absence of the peak at about 230 8C in the kinetogram of the initial Mequinenza coal proves that this coal practically does not contain non-volatile aliphatic thiols. The results of AP – TPR analysis for Illinois No. 6 coal, without the additional reducing mixture, are shown in Fig. 3. In contrast to the results for Mequinenza coal (Fig. 1), the shapes of the kinetograms taken for the raw and demineralised samples are very close. It is a consequence of the earlier discussed differences in the content of calcium compounds in the two types of coal. A comparison of the kinetograms of the Mequinenza coal and Illinois No. 6 shows that the content of non-thiophene sulphur in the former is greater. Reduction of the demineralised Illinois No. 6 coal in the potassium/liquid ammonia system leads to a shift of the maximum of the peak, assigned to nonthiophene sulphur, towards lower temperatures. Similarly as for Mequinenza, the value of the shift increases with the repetition of the reduction process. This observation is the evidence of the thiol group formation accompanied by the disappearance of sulphides and disulphides. The reduction leads to a gradual decrease of intensity of all signals as a consequence of a decreasing value of sulphur recovery and elimination of sulphur via breaking up of the C – S bonds in thiophene and non-thiophene systems. The results of AP – TPR analysis of Illinois No. 6 coal in the presence of the additional reducing mixture (Fig. 4) clearly indicate that the initial coal contains certain amount of aliphatic thiols (the peak at , 210 8C which is in contrast to the Mequinenza coal). Their amount increases as a result of reduction in the potassium/liquid ammonia system, and, similarly as for Mequinenza coal, once performed process is sufficient to break up all sulphides and disulphides from which aliphatic thiols are formed. The proceeding reduction causes the appearance of new aromatic thiols, which is manifested by a gradual shift of the maximum of the peak assigned to non-thiophene sulphur towards lower temperatures.
4. Conclusions On the basis of a comparison of the results obtained in this work with those obtained for raw coals [11], we can conclude that the preliminary demineralisation of coals leads to better results of AP – TPR analysis. This beneficial effect is mainly due to a removal of the calcium compounds binding H2S formed during AP – TPR measurements, which causes deformation of the kinetograms obtained and makes their interpretation difficult. The preliminary reduction of coals in the potassium/liquid ammonia system prior to AP – TPR analysis permits getting the information about the sulphur compounds, which would be impossible in measurements of non-modified samples. It has been shown that both Mequinenza and Illinois No. 6 coals contain different types of sulphides and disulphides (e.g. aromatic – aromatic,
1153
aromatic – aliphatic and aliphatic – aliphatic). The once performed reduction of the coals in the potassium/liquid ammonia system of the two types is sufficient for breaking up all sulphides and disulphides from which aliphatic thiols are formed. The presence of the thiols is much better manifested on the AP – TPR kinetogram if the measurements are performed in the presence of a special reducing mixture. The presence of this reducing mixture is also beneficial because of the increase of the sulphur recovery values. It has been established that apart from conversions in the nonthiophene sulphur groups, the coal reduction in the potassium/liquid ammonia system also causes cleavage of the C –S bonds in some thiophene groups.
Acknowledgements The study was carried out within the KBN project no. 3 T09A 062 17.
References [1] Sinninghe Damste´ JS, White CM, Green JB, de Leeuw JW. Energy Fuels 1999;13:728. [2] Chadwick MJ, Highton NH, Lindman N, editors. Environmental impacts of coal mining and utilization. Oxford: Pergamon Press; 1987. [3] Brown JR, Kasrai M, Bancroft GM, Tan KH, Chen JM. Fuel 1992;71: 649. [4] Gorbaty ML, Kelemen SR, George GN, Kwiatek PJ. Fuel 1992;71: 1255. [5] LaCount RB, Anderson RR, Friedman S, Blaustein BD. Fuel 1987;66: 909. [6] Mitchell SC, Snape CE, Garcia R, Ismail K, Bartle K. Fuel 1994;73: 1159. [7] Attar A. In: Karr C Jr, editor. Analytical methods for coal and coal products, vol. III. New York: Academic Press; 1979. p. 585. [8] Kozłowski M, Maes II, Wachowska H, Yperman J, Franco DV, Mullens J, Van Poucke LC. Fuel 1999;78:769. [9] Ismail K, Mitchell SC, Brown SD, Snape CE, Buchanan AC, Britt PF, Franco DV, Maes II, Yperman J. Energy Fuels 1995;9:707. [10] Maes II, Yperman J, Van den Rul H, Franco DV, Mullens J, Van Poucke LC, Gryglewicz G, Wilk P. Energy Fuels 1995;9:950. [11] Kozłowski M, Wachowska H, Yperman J, Franco DV, Mullens J, Van Poucke LC. Energy Fuels 1998;12:1142. [12] Radmacher W, Mohrhauer P. Brennstoff-Chemie 1956;37:353. [13] Wachowska H, Kozłowski M. Fuel 1995;74:1319. [14] Wachowska H, Kozłowski M. Fuel 1995;74:1398. [15] Polish Standard PN-76/G-04514.02. [16] Polish Standard PN-77/G-04514.11. [17] Polish Standard PN-87/G-04514.09. [18] Polish Standard PN-80/G-04514.15. [19] Majchrowicz BB, Yperman J, Mullens J, Van Poucke LC. Anal Chem 1991;63:760. [20] Yperman J, Franco D, Mullens J, Van Poucke LC, Gryglewicz G, Jasien´ko S. Fuel 1995;74:1261. [21] Kozłowski M, Wachowska H. Fuel 1999;78:1875. [22] Wachowska H, Kozłowski M, Pietrowski M. Karbo-EnergochemiaEkologia 1996;41:199. [23] Ignasiak BS, Fryer JF, Jadernik P. Fuel 1978;57:578. [24] Pietrzak R, Wachowska H, Yperman J. Fuel 2002; 81: 2397.
Short Communication
Transformations of sulphur compounds in high-sulphur coals during reduction in the potassium/liquid ammonia systemq M. Kozłowskia,*, H. Wachowskaa, J. Ypermanb a Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, Poznan´ 60-780, Poland Laboratory of Applied Chemistry, IMO, Limburgs Universitair Centrum, Diepenbeek B-3590, Belgium.
b
Received 5 April 2002; accepted 17 December 2002; available online 30 January 2003
Abstract The two types of high-sulphur coals Mequinenza and Illinois No. 6, in the initial form and subjected to potassium/liquid ammonia reduction, were analysed by atmospheric pressure –temperature programmed reduction (AP – TPR) method. It has been shown that preliminary demineralisation was beneficial for AP– TPR measurements because of the removal of calcium compounds. The reduction of sulphides and disulphides in the potassium/liquid ammonia system was found to lead to formation of aromatic and aliphatic thiols. The presence of the latter is better manifested in the AP – TPR kinetograms when the measurements are performed in the presence of a special reducing mixture. It has been shown that the coal reduction in the potassium/liquid ammonia system apart from transformations of nonthiophene sulphur groups also leads to breaking up of the C– S bonds in some thiophene systems. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Coal; Reduction; Sulphur; Atmospheric pressure–temperature programmed reduction method
1. Introduction All types of coal contain sulphur, whose concentration may vary from trace amounts to several percent. Usually it occurs in the dominant form of inorganic sulphur (mainly pyrite), however, some coals, like the Croatian Rasˇa [1], contain significant amounts of organic sulphur. Very often utilisation of high-sulphur content coals has negative effects on the natural environment [2]. Rational and safe utilisation of coal requires first of all a full recognition of the amount and types of sulphur content in them. Determinations of the inorganic sulphur compounds are relatively simple, whereas analysis of the organic sulphur has been still a challenge. Interesting information on the inorganic and organic sulphur compounds occurring in coal has been obtained from the studies using methods such as XANES [3], XPS [4], CAPTO [5], HP – TPR [6] and AP – TPR [7]. In our earlier work [8] on analysis of inorganic sulphur forms by atmospheric pressure –temperature programmed reduction (AP –TPR), we have shown a good agreement between the AP – TPR results and those obtained by * Corresponding author. E-mail address: [email protected] (M. Kozłowski). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com
the classical chemical analysis. The AP –TPR analysis of organic sulphur proved to be more complex because some of its forms give signals in the same temperature range [9, 10]. To avoid this problem in our next study [11], the AP – TPR analysis was preceded by the coal reduction in the potassium/liquid ammonia system. In this process, sulphides and disulphides are converted into corresponding thiols characterised by a low temperature of hydrogenation and thus being easy to be identified in AP – TPR kinetogram. Unfortunately, the presence of relatively large amounts of calcium compounds in the samples studied initiated side reactions in which some of the H2S released was captured in the form of CaS, which has significantly disturbed interpretation of the results. For this reason, in this study, the coal samples were preliminary demineralised by the method of Radmacher and Mohrhauer [12]. This process does not have a significant influence on the course of the coal reduction in the potassium/liquid ammonia system [13], and removes the disturbing mineral compounds. As the changes in the coal structure increase with a repetition of the process [14], the measurements were performed for once and twice reduced coals. The study is a continuation of our earlier work on AP – TPR analysis of reduced coals, and our main aim was to
0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00005-X
1150
M. Kozłowski et al. / Fuel 82 (2003) 1149–1153
analyse the transformations of organic sulphur occurring in the process of reduction in the potassium/liquid ammonia system.
pyrogallol [20]) to the sample. Identification of particular sulphur compounds was performed on the basis of a study of model compounds [9,10]. The AP – TPR kinetograms are shown in Figs. 1 – 4.
2. Experimental 3. Results and discussion 2.1. The samples The high-sulphur coal samples to be studied were the Spanish coal Mequinenza and American-Illinois No. 6. The coal samples were ground and sieved to grain size # 0.06 mm, and subjected to demineralisation by the method of Radmacher and Mohrhauer [12]. The raw and demineralised coal samples are characterised in Table 1. The reduction in the potassium/liquid ammonia system was performed according to the procedure given elsewhere [8], using 14 g of metallic potassium dissolved in 300 cm3 of liquid ammonia per 12 g of carefully dried coal. The protonating agent was ethyl alcohol. After acidification with 5% HCl and careful washing with warm distilled water, the product was dried at 60 8C to constant weight in vacuo. The process of reduction was carried out twice. 2.2. Analytical procedures The total, sulphate, pyrite and elemental sulphur were determined by the classical chemical methods according to the standards [15 –18]. The content of organic sulphur was determined as the difference between that of the total and inorganic sulphur. The results are given in Table 2. No elemental sulphur was detected in the samples studied. The AP –TPR measurements were carried out according to the procedure given in Ref. [19]. Portions of 40 mg of the samples studied were mixed with 60 mg of fumed silica and placed in a quartz reactor heated from room temperature to 1000 8C at a linear temperature increase at the rate of 5 8C/ min. Hydrogen gas was blown through the reactor at the rate of 50 cm3/min. The formed H2S was dissolved in an antioxidising pH-buffered solution and determined potentiometrically as S22. Some AP – TPR measurements were conducted with the addition of a special reducing mixture (9,10-dihydroanthracene, phenanthrene, resorcinol and
Table 1 presents results of proximate and elemental analysis of the raw and demineralised coals, whereas their full analytical and spectral characterisation (and the characterisation of the products of their reduction) is given in Ref. [21]. Demineralisation of the two types of coal by HCl and HF leads to a great decrease in the content of ash, however, the content of volatile components decreases only a little. Analysis of the data from Table 2 shows that demineralisation of the two types of coal does not remove pyrite from them, because this mineral is insoluble in HCl and HF [22]. The content of sulphate sulphur considerably decreases after demineralisation. The reduction of the demineralised coal samples in the potassium/liquid ammonia system leads to a small loss of pyrite and total removal of sulphates. With repetition of the process, the content of organic sulphur decreases as a result of breaking up some C – S bonds, which sometimes leads to elimination of sulphur from coal [23]. The main form of sulphur occurring in all samples studied is organic one. It has been found [24] that the values of sulphur recovery obtained on AP – TPR measurements depend on three factors: (1) the content of calcium compounds binding H2S appearing on measurements, (2) the presence of sulphur in the from of particularly stable thiophene groups not undergoing hydrogenation to H2S and (3) the formation of volatile sulphur compounds other than H2S, potentiometrically undetected. The sulphur recovery values are high (Table 3), which means that almost all sulphur content is recorded by AP – TPR. The values of sulphur recovery increase after demineralisation as a result of removal of calcium compounds. As the increase is greater for Mequinenza coal, this coal can be expected to contain more calcium than Illinois No. 6. This supposition was confirmed by the analytical results specifying the content of
Table 1 Proximate and elemental analyses of raw and demineralised coals (wt%) Coal
Moisture
Ash (dry coal)
VMdaf
Cdaf
Hdaf
Ndaf
Sdaf org
Odaf a
Mequinenza coal Raw Demineralised
4.9 0.0
21.9 1.3
51.0 48.1
62.3 64.8
5.5 5.3
0.6 0.8
9.8 10.4
21.8 18.7
Illinois No. 6 coal Raw Demineralised
6.7 0.0
10.7 0.6
40.4 38.5
71.9 74.1
4.5 5.2
1.3 1.4
3.0 3.0
19.3 16.3
a
By difference.
M. Kozłowski et al. / Fuel 82 (2003) 1149–1153
1151
Table 2 Content of different forms of sulphur in raw, demineralised and modified coals (wt%, dry coal) Coal
Sulphur Total
Pyritic
Sulphate
Organic
Mequinenza coal Raw Demineralised Reduced 1 £ Reduced 2 £
9.67 11.60 10.96 9.74
1.22 1.29 1.24 1.23
0.81 0.03 0.00 0.00
7.64 10.28 9.72 8.51
Illinois No. 6 coal Raw Demineralised Reduced 1 £ Reduced 2 £
3.91 3.51 2.92 2.55
0.35 0.46 0.36 0.40
0.92 0.03 0.00 0.00
2.64 3.02 2.56 2.15
2.89% of calcium (as CaO) in Mequinenza and 0.52% in Illinois No. 6 [11]. After the reduction in the potassium/ liquid ammonia system, the values of sulphur recovery decrease. This observation is interpreted as a probable result of fragmentation of the coal structure and conversion of some sulphur compounds into others, which do not undergo hydrogenation to H2S on AP –TPR measurements (e.g. volatile thiols, which evaporate before they could be hydrogenated). The AP –TPR analysis was also made with the addition of a special reducing mixture [20], whose presence decreases the temperature of hydrogenation of certain sulphur compounds (mainly thiols) [9]. The influence of the mixture on the sulphur recovery depends on the character of the coal studied. For the raw coals reduced in the potassium/liquid ammonia system, the values of the sulphur recovery increased in the presence of the reducing mixture [11], but for the coals oxidised with different
Fig. 1. AP–TPR kinetograms of Mequinenza coal: (—) raw coal; (- - -) demineralised coal; (-·-·-) demineralised coal once reduced; (· · ·) demineralised coal twice reduced.
Fig. 2. AP –TPR kinetograms of Mequinenza coal with reducing mixture: (—) raw coal; (- - -) demineralised coal; (-·-·-) demineralised coal once reduced; (· · ·) demineralised coal twice reduced.
agents, the influence of this mixture proved rather disadvantageous [24]. As follows from the data in Table 3, for the samples studied, the addition of the reducing mixture proved to be advantageous. Similarly as for the samples without the mixture, the sulphur recovery increases after demineralisation and gradually decreases with repetition of the process of reduction in the potassium/liquid ammonia system. The majority of the AP –TPR kinetograms presented in Figs. 1 – 4 reveal two broad peaks. The first, with the maximum in the range of 370– 450 8C, on the basis of the results for model compounds, can be assigned to superposition of signals from different forms of non-thiophene
Fig. 3. AP–TPR kinetograms of Illinois No. 6 coal: (—) raw coal; (- - -) demineralised coal; (-·-·-) demineralised coal once reduced; (· · ·) demineralised coal twice reduced.
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Fig. 4. AP–TPR kinetograms of Illinois No. 6 coal with reducing mixture: (—) raw coal; (- - -) demineralised coal; (-·-·-) demineralised coal once reduced; (· · ·) demineralised coal twice reduced.
sulphur, while the broad peak with the maximum at about 700 8C can be assigned to thiophene sulphur [9,10]. The shapes of AP –TPR kinetograms of the raw and demineralised Mequinenza samples (Fig. 1) are similar up to , 550 8C (except for the small widening in the lower temperature range of the demineralised sample, due to the removal of calcium compounds). Above this temperature, the intensity of the signal in the kinetogram of the raw sample decreases compared with the demineralised sample, which is again explained as a result of the presence of calcium ions binding the H2S produced during the analysis. In the kinetograms of the demineralised samples, so after removal of disturbing calcium compounds, a peak with a maximum at , 680 8C assigned to thiophene sulphur clearly appears (in contrast to that for the raw sample). The reduction of the demineralised coal in the potassium/liquid ammonia system leads to a decrease in the intensity of the peaks assigned both to thiophene and nonthiophene sulphur. This decrease is a consequence of elimination of sulphur (Table 2) by the process of breaking Table 3 Sulphur recovery by AP–TPR method (%) Coal
Sulphur recovery
Sulphur recovery (with mix)
Mequinenza coal Raw Demineralised Reduced 1 £ Reduced 2 £
73 88 82 82
84 91 87 82
Illinois No. 6 coal Raw Demineralised Reduced 1 £ Reduced 2 £
93 100 88 77
98 100 89 77
up of some C –S bonds, by potassium in liquid ammonia, and a decrease of the sulphur recovery value. Moreover, after the reduction, the peak localised initially at 410 8C is shifted towards lower temperatures, which is interpreted as a result of conversion of sulphides and disulphides in thiol groups. As the temperature of hydrogenation of the thiol groups on AP – TPR measurements is lower than that of the sulphide and disulphide groups from which they originated [9,10], the maximum of the peak being a superposition of the signals from all these groups is shifted towards lower temperatures. The value of the shift increased after repetition of the process of reduction in the potassium/ liquid ammonia system, which proves that a single process of reduction is insufficient to convert all sulphides and disulphides into thiols. It was also noted that as a result of twice-repeated reduction, the intensity of the signal assigned to thiophene sulphur decreases, but the intensity of that assigned to non-thiophene sulphur does not change. This observation proves the breaking up of some C –S bonds in the thiophene structures by potassium in liquid ammonia. According to the results of the study of model compounds, sometimes the presence of the reducing mixture during AP –TPR analysis causes a disturbance in the kinetograms obtained [9]. A comparison of kinetograms of the demineralised sample (Fig. 1) and the same sample with addition of the reducing mixture (Fig. 2) proves, however, that for the samples studied, the above phenomenon practically does not occur. The kinetograms taken for the potassium/liquid ammonia reduced samples with the addition of the reducing mixture in the AP –TPR experiment reveal the appearance of a new peak at about 230 8C (Fig. 2). On the basis of the study of model compounds, this peak can be assigned to non-volatile thiols formed as a product of reduction of sulphides and disulphides in the potassium/liquid ammonia system. In the presence of the additional reducing mixture, thiols undergo hydrogenation on AP – TPR analysis at a temperature lower than without this additional mixture [9], and therefore, they were manifested as a separate peak, while in the kinetograms in Fig. 1, the peak corresponding to them was a component of a broad signal with a maximum at , 400 8C. Despite the appearance of a peak at , 230 8C, a small shift of the peak assigned to non-thiophene sulphur is still observed in the kinetograms of the reduced samples. This fact suggests that different kinds of thiols are obtained as a result of coal reduction in the potassium/liquid ammonia system. Most probably the signal at 230 8C can be assigned to aliphatic thiols, whereas the one being a component of the broad peak at , 400 8C to more thermally stable aromatic thiols. This interpretation suggests the presence of different kinds of sulphides and disulphides in the coal sample studied (e.g. aromatic – aromatic, aromatic – aliphatic, aliphatic – aliphatic). As the signal at , 230 8C has almost the same intensity in the kinetograms taken after the first and the second process of reduction, we can conclude that a single reduction is sufficient for breaking up all C –S sulphide and
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disulphide bonds, leading to formation of aliphatic thiols. The absence of the peak at about 230 8C in the kinetogram of the initial Mequinenza coal proves that this coal practically does not contain non-volatile aliphatic thiols. The results of AP – TPR analysis for Illinois No. 6 coal, without the additional reducing mixture, are shown in Fig. 3. In contrast to the results for Mequinenza coal (Fig. 1), the shapes of the kinetograms taken for the raw and demineralised samples are very close. It is a consequence of the earlier discussed differences in the content of calcium compounds in the two types of coal. A comparison of the kinetograms of the Mequinenza coal and Illinois No. 6 shows that the content of non-thiophene sulphur in the former is greater. Reduction of the demineralised Illinois No. 6 coal in the potassium/liquid ammonia system leads to a shift of the maximum of the peak, assigned to nonthiophene sulphur, towards lower temperatures. Similarly as for Mequinenza, the value of the shift increases with the repetition of the reduction process. This observation is the evidence of the thiol group formation accompanied by the disappearance of sulphides and disulphides. The reduction leads to a gradual decrease of intensity of all signals as a consequence of a decreasing value of sulphur recovery and elimination of sulphur via breaking up of the C – S bonds in thiophene and non-thiophene systems. The results of AP – TPR analysis of Illinois No. 6 coal in the presence of the additional reducing mixture (Fig. 4) clearly indicate that the initial coal contains certain amount of aliphatic thiols (the peak at , 210 8C which is in contrast to the Mequinenza coal). Their amount increases as a result of reduction in the potassium/liquid ammonia system, and, similarly as for Mequinenza coal, once performed process is sufficient to break up all sulphides and disulphides from which aliphatic thiols are formed. The proceeding reduction causes the appearance of new aromatic thiols, which is manifested by a gradual shift of the maximum of the peak assigned to non-thiophene sulphur towards lower temperatures.
4. Conclusions On the basis of a comparison of the results obtained in this work with those obtained for raw coals [11], we can conclude that the preliminary demineralisation of coals leads to better results of AP – TPR analysis. This beneficial effect is mainly due to a removal of the calcium compounds binding H2S formed during AP – TPR measurements, which causes deformation of the kinetograms obtained and makes their interpretation difficult. The preliminary reduction of coals in the potassium/liquid ammonia system prior to AP – TPR analysis permits getting the information about the sulphur compounds, which would be impossible in measurements of non-modified samples. It has been shown that both Mequinenza and Illinois No. 6 coals contain different types of sulphides and disulphides (e.g. aromatic – aromatic,
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aromatic – aliphatic and aliphatic – aliphatic). The once performed reduction of the coals in the potassium/liquid ammonia system of the two types is sufficient for breaking up all sulphides and disulphides from which aliphatic thiols are formed. The presence of the thiols is much better manifested on the AP – TPR kinetogram if the measurements are performed in the presence of a special reducing mixture. The presence of this reducing mixture is also beneficial because of the increase of the sulphur recovery values. It has been established that apart from conversions in the nonthiophene sulphur groups, the coal reduction in the potassium/liquid ammonia system also causes cleavage of the C –S bonds in some thiophene groups.
Acknowledgements The study was carried out within the KBN project no. 3 T09A 062 17.
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