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Thermal behaviour of sulphur forms in spanish low-rank coals.
Coal Science J.A. Pajares and J.M.D. Tasc6n (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1665
Thermal behaviour of sulphur forms in Spanish low-rank coals. Ana J. Bonet, Jos~ V. Ibarra and Rafael Moliner Instituto de Carboqufmica, C.S.I.C., P.O. Box 589, 50080-Zaragoza, Spain
1. INTRODUCTION The development of new tactics for the removal of the sulphur compounds from coal depends, in part, upon a knowledge of their chemical constitution. It is generally assumed that the sulphur in coal is distributed among organic sulphur, sulphatic sulphur and pyritic sulphur compounds. The organic sulphur value of coal is derived as the difference between the total sulphur content of coal and the amount of pyritic plus sulphatic sulphur. Elemental sulphur, which would be accounted for as "organic sulphur", has been mentioned only in a few references in the literature [1]. In this paper, the thermal behaviour of the different sulphur forms, including elemental sulphur, of a series of Spanish-low rank coals has been investigated, by means of temperature programmed degradation (TPD) and X-ray based spectroscopic techniques. The thermal evolution of volatile sulphur compounds was followed by sulphide electrode and FTIR spectroscopy and the speciation of organic sulphur forms by XPS. The influence of coal rank, pyrite content and operating conditions on the thermal stabilities of sulphur structures were also established.
2. EXPERIMENTAL SECTION A series of low rank coals with high sulphur contents were studied. They were selected from the Teruel and Mequinenza basins on the basis of their different sulphur form content (mainly, pyritic and organic). The samples were ground to less than 200 /~m. The main characteristics of some of the selected coals are given in Table 1. Analyses of sulphur forms were carded out according to ASTM method D-2492. Mequinenza coal (M6) was chosen for its high organic sulphur content. The AA6-F, EC4-F and AA5-F samples are freshly mined while the AA6, EC10 and AA5 coal samples have been stored under laboratory conditions for up to five years since collection and contain high amounts of sulphatic sulphur. Elemental sulphur (S~ was determined by extracting the coal samples (20 g) in a Soxhlet with tetrachloroethylene (PCE) (200 ml) at its boiling point (120~ for up to six hours. The SO contents in PCE extracts were determined using Gas Chromatography. The sulphur quantization was carried out using a Varian 3400; split-splitless injector running in splitless way and FID-FPD detector. A column of 50m x 0.25mm i.d. covered with a 0.4 ~m film of CP-sil-5 CB chemically bonded phase was used. Detector and injector temperature: 300~
1666 initial temperature of 120~ (2 min); heating rate of 40~
up to 240~
Table 1. Main characteristics of coal samples. AA6
AA6-F
AA5
AA5-F
EC-8
EC-10
EC4-F
M6
Oxidation state
Wc
Fc
W
F
W
W
F
W
% C"
65
71.7
69.6
73.7
79.1
74.5
79.2
69.6
7.40
6.44
5.28
3.19
6.68
2.94
9.83
7.48 a) daf b) air dried basis c) W:weathered; F:fresh
Pyrolysis runs were carded out in a swept fixed bed reactor (30 cm length, 2 cm i.d.) with the coal samples placed into a quartz tube and heated up to 850~ at a heating rate of 5~ min -~. The tar produced was trapped at 0~ and the evolved gas was swept with N2 to the sulphur detector system. Sulphide electrode (SE) and infrared spectroscopy (FTIR) were used as sulphur detectors, both coupled on-line with the reactor system. More experimental details have been previously described [2]. Coal samples with different pyrite contents were obtained by separation into density fractions (d= 1.4-2.8 g cm -3) with defined limits by float-sink using mixtures of liquids of different density (bromoform, xylene and perchloroethylene). Morphological studies and the determination of organic sulphur in the coal or char matrix were conducted with a scanning electron microscope (SEM) coupled to a Si-Li detector and a processor for energy-dispersive X-ray analysis (EDX).
3. RESULTS AND DISCUSSION 3.1. Elemental sulphur.
-I
From the sulphur forms present in coal, only elemental sulphur was shown to be extracted by PCE [3]. Table 1 summarizes the results for PCE extractions of samples taken from seven different coals as a function of weathering evaluated by the sulphatic sulphur content.
8
0 r-
C~.4
2
o
oi~
o12
o13
o14
o,5
% elemental sulphur
Figure 1. Sulphatic sulphur v.s. elemental sulphur content.
AA6-F, EC4-F and AA5-F coal samples produced only trace amounts of S~ PCE extracts of weathered coals always contained SO. The linear correlation between the elemental sulphur and the sulphate content of coal (Figure 1) suggests that both sulphur forms are the result of weathering reactions in coal. The absence of SOin PCE extracts of M6
1667 coal, which has been exposed to an air oxidation process but does not contain pyritic sulphur, can be explained on the basis that the pyrite oxidation is the source of both SOand sulphate in coal, as appears in literature.
3.2. Changes in sulphur forms during coal pyrolysis. The study of the evolution of sulphur forms in chars obtained at different temperatures shows how the sulphate content decrease markedly over 400~ in chars from weathered coals. Other species except iron sulphates (mainly calcium sulphates) remain in chars. Pyrite seems to be stable up to 450~ but it is entirely decomposed over 550~ Sulphides begin to appear coinciding with the pyrite decomposition to pyrrhotite and they increase with temperature of pyrolysis. At temperatures as low as 600~ a high percentage of the organic sulphur has already been removed between 30-55 % for Teruel coals and 75 % for Mequinenza coal. This removal is greatly influenced by the weathering and pyrite content of the samples. For the studied coals, total sulphur removal between 35 and 65 % has been achieved by low temperature pyrolysis (600~ 3.3. Evolution of sulphur compounds during pyrolysis. The H2S evolved from the pyrolysis of Teruel coals (Figure 2a) shows the presence of two peaks related to the decomposition of organic sulphur in thermally labile structures (250450~ and to the decomposition of pyrite into pyrrhotite (500-650~ respectively. :....
--~20 ---802
8 :
~6
"..
:
"-,
r
Io4
r
2~
40
800
Temperature(~ Figure 2a. H2S evolution with pyrolysis temperature for EC8 coal.
0 200
,
300
,oo s;o o;o Ternperature(~
r;o
Figure 2b. Evolution of oxygenated sulphur forms for AA6 coal.
Evidence of the capture by the coal matrix of sulphur from pyrite decomposition as well as the influence of the coal organic matter on the decomposition of pyrite was obtained by SEMEDX and XRD using float-sink coal fractions with different organic matter and pyrite contents. These findings demonstrate that desulphurization by pyrolysis has to be considered individually for each coal, depending on its rank, degree of weathering and distribution of sulphur forms, since the interactions observed greatly limit the efficiency of desulphurization. The evolution of oxygenated sulphur compounds (COS and SO2) during pyrolysis was followed by FTIR spectroscopy (Figure 2b). The evolution of COS shows a similar trend to that of H2S. Therefore, the study of this evolution can be used to follow the decomposition
1668 of sulphur forms of coal during pyrolysis. The evolution of SO2 seems to be related to the decomposition of sulphates coming from the weathering of coal. 3.4. Speciation of organic sulphur forms in chars. Determination of the distribution of organic sulphur forms in these coals and their evolution during low temperature pyrolysis was carried out by X-Ray Photoelectron Spectroscopy (XPS). For the correct assignation of S 2p binding energies (BE) to different sulphur forms, model compounds, including pyrite concentrates and coal organic matter fractions, were used [4].
In Mequinenza coal, a noticeable contribution of sulphidic sulphur was observed. This unstable sulphur form vanishes simply by heating up to moderate temperatures (400~ Starting from 400~ thiophenic sulphur is the major organic sulphur form and little changes are produced with increasing temperature of pyrolysis. Sulfoxide structures have also been detected in proportions close to 20%. They remain stable at the assayed temperatures. Coals from Teruel show the presence of oxided sulphur (S6§ that can be attributed to sulphonic structures but also iron sulphates from the pyrite oxidation. This last assignation is supported by the marked decrease observed for the $6+/$2+ ratio in the temperature range of sulphate decomposition. Above 600~ when pyrite has already decomposed, thiophenic sulphur is the major organic sulphur form in char (over 80 %) though sulphone structures have also been detected. The marked increase of the sulphur to carbon ratio observed for high temperature chars in relation to starting coals and low temperature chars suggests that the organic sulphur could be incorporated to char from pyrite decomposition mainly as thiophenic structures.
Acknowledgements. The authors wish to thank the ECSC (Project 7220-EC/756), CICYT and DPT for financial support. A.J.B. also thanks CONAI of the DGA for a grant.
REFERENCES 1. Duran J.E., Mahasay S.R., Stock L.M., Fuel 65, 1167-1168, 1986 2. Ibarra J.V., Bonet A.J., Moliner R., Fuel 73,933-939, 1994 3. Huggins F.E., Vaidya S.V., Shah N., Huffman G.P., Fuel Proc. Technology, 35, 233257, 1993 4. Fierro J.L.G., Palacios J.M., Moliner R., Ibarra J.V., 7th International Conference on Coal Science, (K.H. Michaelian Ed.), Banff, Canada, IEA, Vol. I, 489-492, 1993
1665
Thermal behaviour of sulphur forms in Spanish low-rank coals. Ana J. Bonet, Jos~ V. Ibarra and Rafael Moliner Instituto de Carboqufmica, C.S.I.C., P.O. Box 589, 50080-Zaragoza, Spain
1. INTRODUCTION The development of new tactics for the removal of the sulphur compounds from coal depends, in part, upon a knowledge of their chemical constitution. It is generally assumed that the sulphur in coal is distributed among organic sulphur, sulphatic sulphur and pyritic sulphur compounds. The organic sulphur value of coal is derived as the difference between the total sulphur content of coal and the amount of pyritic plus sulphatic sulphur. Elemental sulphur, which would be accounted for as "organic sulphur", has been mentioned only in a few references in the literature [1]. In this paper, the thermal behaviour of the different sulphur forms, including elemental sulphur, of a series of Spanish-low rank coals has been investigated, by means of temperature programmed degradation (TPD) and X-ray based spectroscopic techniques. The thermal evolution of volatile sulphur compounds was followed by sulphide electrode and FTIR spectroscopy and the speciation of organic sulphur forms by XPS. The influence of coal rank, pyrite content and operating conditions on the thermal stabilities of sulphur structures were also established.
2. EXPERIMENTAL SECTION A series of low rank coals with high sulphur contents were studied. They were selected from the Teruel and Mequinenza basins on the basis of their different sulphur form content (mainly, pyritic and organic). The samples were ground to less than 200 /~m. The main characteristics of some of the selected coals are given in Table 1. Analyses of sulphur forms were carded out according to ASTM method D-2492. Mequinenza coal (M6) was chosen for its high organic sulphur content. The AA6-F, EC4-F and AA5-F samples are freshly mined while the AA6, EC10 and AA5 coal samples have been stored under laboratory conditions for up to five years since collection and contain high amounts of sulphatic sulphur. Elemental sulphur (S~ was determined by extracting the coal samples (20 g) in a Soxhlet with tetrachloroethylene (PCE) (200 ml) at its boiling point (120~ for up to six hours. The SO contents in PCE extracts were determined using Gas Chromatography. The sulphur quantization was carried out using a Varian 3400; split-splitless injector running in splitless way and FID-FPD detector. A column of 50m x 0.25mm i.d. covered with a 0.4 ~m film of CP-sil-5 CB chemically bonded phase was used. Detector and injector temperature: 300~
1666 initial temperature of 120~ (2 min); heating rate of 40~
up to 240~
Table 1. Main characteristics of coal samples. AA6
AA6-F
AA5
AA5-F
EC-8
EC-10
EC4-F
M6
Oxidation state
Wc
Fc
W
F
W
W
F
W
% C"
65
71.7
69.6
73.7
79.1
74.5
79.2
69.6
7.40
6.44
5.28
3.19
6.68
2.94
9.83
7.48 a) daf b) air dried basis c) W:weathered; F:fresh
Pyrolysis runs were carded out in a swept fixed bed reactor (30 cm length, 2 cm i.d.) with the coal samples placed into a quartz tube and heated up to 850~ at a heating rate of 5~ min -~. The tar produced was trapped at 0~ and the evolved gas was swept with N2 to the sulphur detector system. Sulphide electrode (SE) and infrared spectroscopy (FTIR) were used as sulphur detectors, both coupled on-line with the reactor system. More experimental details have been previously described [2]. Coal samples with different pyrite contents were obtained by separation into density fractions (d= 1.4-2.8 g cm -3) with defined limits by float-sink using mixtures of liquids of different density (bromoform, xylene and perchloroethylene). Morphological studies and the determination of organic sulphur in the coal or char matrix were conducted with a scanning electron microscope (SEM) coupled to a Si-Li detector and a processor for energy-dispersive X-ray analysis (EDX).
3. RESULTS AND DISCUSSION 3.1. Elemental sulphur.
-I
From the sulphur forms present in coal, only elemental sulphur was shown to be extracted by PCE [3]. Table 1 summarizes the results for PCE extractions of samples taken from seven different coals as a function of weathering evaluated by the sulphatic sulphur content.
8
0 r-
C~.4
2
o
oi~
o12
o13
o14
o,5
% elemental sulphur
Figure 1. Sulphatic sulphur v.s. elemental sulphur content.
AA6-F, EC4-F and AA5-F coal samples produced only trace amounts of S~ PCE extracts of weathered coals always contained SO. The linear correlation between the elemental sulphur and the sulphate content of coal (Figure 1) suggests that both sulphur forms are the result of weathering reactions in coal. The absence of SOin PCE extracts of M6
1667 coal, which has been exposed to an air oxidation process but does not contain pyritic sulphur, can be explained on the basis that the pyrite oxidation is the source of both SOand sulphate in coal, as appears in literature.
3.2. Changes in sulphur forms during coal pyrolysis. The study of the evolution of sulphur forms in chars obtained at different temperatures shows how the sulphate content decrease markedly over 400~ in chars from weathered coals. Other species except iron sulphates (mainly calcium sulphates) remain in chars. Pyrite seems to be stable up to 450~ but it is entirely decomposed over 550~ Sulphides begin to appear coinciding with the pyrite decomposition to pyrrhotite and they increase with temperature of pyrolysis. At temperatures as low as 600~ a high percentage of the organic sulphur has already been removed between 30-55 % for Teruel coals and 75 % for Mequinenza coal. This removal is greatly influenced by the weathering and pyrite content of the samples. For the studied coals, total sulphur removal between 35 and 65 % has been achieved by low temperature pyrolysis (600~ 3.3. Evolution of sulphur compounds during pyrolysis. The H2S evolved from the pyrolysis of Teruel coals (Figure 2a) shows the presence of two peaks related to the decomposition of organic sulphur in thermally labile structures (250450~ and to the decomposition of pyrite into pyrrhotite (500-650~ respectively. :....
--~20 ---802
8 :
~6
"..
:
"-,
r
Io4
r
2~
40
800
Temperature(~ Figure 2a. H2S evolution with pyrolysis temperature for EC8 coal.
0 200
,
300
,oo s;o o;o Ternperature(~
r;o
Figure 2b. Evolution of oxygenated sulphur forms for AA6 coal.
Evidence of the capture by the coal matrix of sulphur from pyrite decomposition as well as the influence of the coal organic matter on the decomposition of pyrite was obtained by SEMEDX and XRD using float-sink coal fractions with different organic matter and pyrite contents. These findings demonstrate that desulphurization by pyrolysis has to be considered individually for each coal, depending on its rank, degree of weathering and distribution of sulphur forms, since the interactions observed greatly limit the efficiency of desulphurization. The evolution of oxygenated sulphur compounds (COS and SO2) during pyrolysis was followed by FTIR spectroscopy (Figure 2b). The evolution of COS shows a similar trend to that of H2S. Therefore, the study of this evolution can be used to follow the decomposition
1668 of sulphur forms of coal during pyrolysis. The evolution of SO2 seems to be related to the decomposition of sulphates coming from the weathering of coal. 3.4. Speciation of organic sulphur forms in chars. Determination of the distribution of organic sulphur forms in these coals and their evolution during low temperature pyrolysis was carried out by X-Ray Photoelectron Spectroscopy (XPS). For the correct assignation of S 2p binding energies (BE) to different sulphur forms, model compounds, including pyrite concentrates and coal organic matter fractions, were used [4].
In Mequinenza coal, a noticeable contribution of sulphidic sulphur was observed. This unstable sulphur form vanishes simply by heating up to moderate temperatures (400~ Starting from 400~ thiophenic sulphur is the major organic sulphur form and little changes are produced with increasing temperature of pyrolysis. Sulfoxide structures have also been detected in proportions close to 20%. They remain stable at the assayed temperatures. Coals from Teruel show the presence of oxided sulphur (S6§ that can be attributed to sulphonic structures but also iron sulphates from the pyrite oxidation. This last assignation is supported by the marked decrease observed for the $6+/$2+ ratio in the temperature range of sulphate decomposition. Above 600~ when pyrite has already decomposed, thiophenic sulphur is the major organic sulphur form in char (over 80 %) though sulphone structures have also been detected. The marked increase of the sulphur to carbon ratio observed for high temperature chars in relation to starting coals and low temperature chars suggests that the organic sulphur could be incorporated to char from pyrite decomposition mainly as thiophenic structures.
Acknowledgements. The authors wish to thank the ECSC (Project 7220-EC/756), CICYT and DPT for financial support. A.J.B. also thanks CONAI of the DGA for a grant.
REFERENCES 1. Duran J.E., Mahasay S.R., Stock L.M., Fuel 65, 1167-1168, 1986 2. Ibarra J.V., Bonet A.J., Moliner R., Fuel 73,933-939, 1994 3. Huggins F.E., Vaidya S.V., Shah N., Huffman G.P., Fuel Proc. Technology, 35, 233257, 1993 4. Fierro J.L.G., Palacios J.M., Moliner R., Ibarra J.V., 7th International Conference on Coal Science, (K.H. Michaelian Ed.), Banff, Canada, IEA, Vol. I, 489-492, 1993