AP-TPR investigation of the effect of pyrite removal on the sulfur characterization of different rank coals

Fuel Processing Technology 76 (2002) 121 – 138 www.elsevier.com/locate/fuproc

AP-TPR investigation of the effect of pyrite removal on the sulfur characterization of different rank coals Piotr Rutkowski a, Steven Mullens b, Jan Yperman b, Graz˙yna Gryglewicz a,* a

Institute of Chemistry and Technology of Petroleum and Coal, Wroclaw University of Technology, ul. Gdan´ska 7/9, 50-344 Wroclaw, Poland b Laboratory of Applied Chemistry, IMO, Limburgs Universitair Centrum, Universiteitslaan, B-3590 Diepenbeek, Belgium Received 9 July 2001; received in revised form 18 January 2002; accepted 11 March 2002

Abstract The effect of pyrite removal on the determination of organic sulfur functionalities of different rank coals by atmospheric pressure-temperature-programmed reduction (AP-TPR) method was studied. The nitric acid treatment and sink – float separation were performed to reduce the content of pyritic sulfur in the samples. Microscopic examination of raw coals was performed to determine the mode of pyrite occurrence and to estimate the possibility of its removal using high-density liquids, i.e. ZnCl2 and CHBr3/CCl4 mixture. The AP-TPR investigation showed a different impact of nitric acid treatment and sink – float separation on the distribution of organic sulfur functionalities in coal. The evolution of sulfur dioxide observed by AP-TPR-mass spectrometry (MS) confirmed the formation of oxidized sulfur compounds during nitric acid treatment. Organic sulfonic groups are nearly totally decomposed to SO2 without reduction of the latter to H2S under AP-TPR conditions. Some sulfur species remained in the TPR residue which was proven by AP-TPO-MS analysis through SO2 monitoring. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Coal; Pyrite removal; Reductive pyrolytic treatment; Organic sulfur functionalities

1. Introduction Sulfur in coal is known to occur in two different forms, as inorganic and organic sulfur. Among the inorganic sulfur, the most common species is pyrite. The removal of sulfur *

Corresponding author. Tel.: +48-7132-06398; fax: +48-7132-21580. E-mail address: [email protected] (G. Gryglewicz).

0378-3820/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 ( 0 2 ) 0 0 0 1 9 - X

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from coal is necessary due to ecological aspects, i.e. reduction of SO2 emission during coal combustion. Nevertheless, it requires prior knowledge of the distribution and the type of inorganic and organic sulfur forms. Generally, inorganic sulfur may be relatively easily removed from coal by physical processes, while organic sulfur removal may be achieved only by chemical or microbiological desulfurization methods [1,2]. An understanding of sulfur behavior in coal during heat treatment processes has received a lot of attention [3– 6]. This is very important for the rational utilization of coal and, consequently, for environmental protection. Pyritic sulfur and its chemical transformations during thermal processes are quite well known [7– 9], while the knowledge about organic sulfur is still relatively scant [10 –12]. Nevertheless, there are many reports on the study of organic sulfur functionalities in coals and coal-derived materials using new different instrumental methods such as XPS, XANES, Py-GC-MS [13 –16]. Atmospheric pressure-temperature-programmed reduction (AP-TPR) has been successfully used for determining organic and inorganic sulfur functionalities in different solid materials [17]. The AP-TPR method is based upon the fact that specific sulfur functional groups are hydrogenated/reduced at specific temperatures. A general limitation in the study on organic sulfur functionalities by AP-TPR method is the overlapping of H2S evolution derived from the hydrogenation/reduction of organic sulfur groups with this one which is attributed to inorganic compounds, more specific pyrite or organic oxidized sulfur forms. In the last few years, several improvements in AP-TPR setup and pre-run sample preparation were done [18]. However, the technique still faces some drawbacks connected with the secondary reactions occurring during the test, monitoring the organic sulfur compounds, which are not stable in the conditions of AP-TPR experiment. Also, an incomplete hydrogenation/reduction of some sulfur groups, due to their inaccessibility or resistance towards the hydrogenation/reduction conditions, sometimes leads to a poor sulfur recovery. In the work focused on the organic sulfur compounds in coal, the pyrite plays a role of ballast. There are some methods for pyrite removing by chemicals, i.e. using nitric acid [19,20] and LiAlH4 [20,21]. However, in this case, some changes in organic part of coal to a less or more extent can take place. Previous works [20,22] showed that chemical treatment of coal with nitric acid causes not only pyritic, but also partial organic sulfur removal. It was demonstrated that even mild oxidizing conditions destroy some organic sulfur species, i.e. dialkyl and alkyl – aryl sulfides. Moreover, it was reported that increasing oxidation of coal results in decreasing sulfur recovery in an AP-TPR experiment. Undoubtedly, the use of physical desulfurization methods does not cause any change in coal structure. However, the degree of pyrite removal by physical method cannot be achieved as high as for chemical methods and it depends on the morphology of pyrite, its distribution and degree of association with organic matrix. In order to solve the problem of low sulfur recovery for some samples during AP-TPR analysis, a reliable method is required to check if all sulfur compounds present in the sample was hydrogenated/reduced to H2S. For this purpose, we have extended the potentiometric detection system, which allows only H2S monitoring, by mass spectrometry [18,23] to examine other volatile sulfur species, i.e. SO2/SO, simple thiols and volatile organic fragments, evolved during this hydrogenative/reductive pyrolytic analysis. A lower sulfur recovery may be also arisen by condensed thiophenes preserved in the AP-

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TPR char and by small amounts of volatile organic sulfur compounds, mainly in the form of thiophenes, passed into the tar. To check if any sulfur remained in the residue after APTPR experiment, the temperature-programmed oxidation (AP-TPO-MS) analysis was performed using the setup equipped with mass spectrometer [24]. The aim of this work was to determine the possibility of organic sulfur investigation using AP-TPR method for high pyritic sulfur coals, which have prior been subjected to sink – float centrifugal procedure to remove pyrite. The efficiency of the physical method used in this work for pyrite removing was estimated and compared with the nitric acid treatment.

2. Experimental 2.1. Samples In this research, three coals of different rank and high sulfur content were used. The coals came from Polish mines: Be»chato´w (lignite A), Siersza (subA) and 1 Maja (mvb). The lignite was taken from unexplored deposits as being rich in organic sulfur according to the geological documentation. The proximate, ultimate and sulfur forms analyses of the coals are listed in Table 1. 2.2. Elemental analysis and sulfur forms A Perkin-Elmer 2400 CHN elemental analyzer was used for CHN determinations. The total sulfur was determined using a Leco SC-132 sulfur analyzer. The oxygen content was calculated from the difference. Pyritic, sulfatic and organic sulfur contents were deterTable 1 Characteristic of raw coals Be»chato´w

Siersza

1 Maja

Proximate analysis (wt.%) Moisture Ash (db) Volatile matter (daf)

29.4 23.9 54.5

16.9 13.2 38.3

1.1 9.5 29.8

Ultimate analysis (wt.%) C (daf) H (daf) N (daf) S (db) Odiff (daf)

62.6 6.7 1.3 8.5 20.9

79.4 5.4 1.3 1.75 12.7

87.5 4.7 1.8 4.3 4.3

Sulfur forms (wt.%, db) Organic Sulfatic Pyritic Elemental

3.74 0.59 4.08 0.09

1.18 0.03 0.52 0.02

1.55 0.27 2.36 0.12

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mined according to Polish Standards which are very similar to ISO standards. Elemental sulfur was extracted from the coal sample with cyclohexane in the Soxhlet apparatus for 6 h and analyzed by GC-MS [25]. 2.3. Microscopic examinations Optical microscopy was used to characterize the mode of occurrence of pyrite in the original coals and to estimate the efficiency of its removal by sink – float centrifugation technique. The analysis was performed on a polished section for oil immersion microscopy. 2.4. HNO3 treatment In each experiment, 10 g of coal (< 0.2 mm) was added to 100 ml of the 17% nitric acid solution and stirred at 60 jC for 1 h. After nitric acid treatment, the sample was filtered and washed with distilled water to remove remaining HNO3, thereafter the sample was dried overnight in a vacuum oven. 2.5. Sink –float centrifugation Sink – float separations were run at a specific gravity of 1.8 with aqueous ZnCl2 solution and CHBr3/CCl4 mixture on the coal fraction below 0.063 mm. The centrifugation operation was carried out at 5500 rpm using JANETZKI-T23 centrifuge. The float was separated and dried until constant weight when CHBr3/CCl4 mixture was used. For ZnCl2 solution, the float was washed with distilled water until chloride ions were completely removed. Finally, the sample was dried in a vacuum oven. 2.6. Atmospheric pressure-temperature-programmed reduction (AP-TPR) Detailed information about the apparatus and the procedure for the AP-TPR technique can be found elsewhere [17]. For each AP-TPR experiment f40 mg of finely ground coal sample (d < 88 Am) and 30– 40 mg of fumed silica (p.a.) are placed in the reactor and subjected under a 100 ml/min flow of pure hydrogen as a reducing gas. The H2S formed is potentiometrically detected as S2 . The sulfur recovery is expressed as the ratio of H2S evolving during AP-TPR experiment to the total sulfur content of an analysed sample. Characteristic temperature regions [17] in which different sulfur functionalities are hydrogenated/reduced are presented in Table 2. All obtained profiles are normalized, i.e. AP-TPR data are divided by the experimental sulfur recovery and multiplied by the total sulfur content of the sample. In this way, kinetograms are independent of experimental sulfur recoveries and quantitative comparison between different AP-TPR results is possible. Prior to AP-TPR analysis, each coal sample was ground to less than 0.2 mm, and demineralized with 20 wt.% HCl for 5 h at 60 jC to avoid the H2S capturing by calcium minerals which implies inaccurate sulfur characterization. Demineralization process was carried out in inert atmosphere using a reaction flask, equipped with cooling and stirring

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Table 2 AP-TPR temperature region of different sulfur functionalities [17] Temperature (jC)

Sulfur functional group

180 – 390 200 – 250 400 – 450 480 – 580 440 – 560 480 – 590 500 – 630 z600 740 – 850 z800

Thiols Polysulfides Disulfides Dialkyl sulfides Alkyl – aryl sulfides Pyrite Diaryl sulfides Thiophenes Troilite Sulfates

systems. After the treatment, the product was filtered, washed with deionized water until it was free of chloride ions. Finally, the product was dried at 60 jC in a vacuum oven. The coupling of the AP-TPR reactor with the mass spectrometer (FISONS-VG Thermolab MS) through the capillary heated at 135 jC permits to follow H2S as well as other volatile sulfur specific compounds and volatile organic fragments which can evolve during an AP-TPR experiment. The mass spectrometer, equipped with quadrupole analyzer, was set at an ionizing voltage of 70 eV with the mass range m/z 15 –150. To check if any sulfur is left in the AP-TPR residue (char and tar together), atmospheric pressure-temperature-programmed oxidation-mass spectrometry (AP-TPO-MS) was applied. The reactor was the same as in AP-TPR analysis, but hydrogen was replaced by oxygen. The flow rate of the gas was set at 100 ml/min. The SO2 evolution, besides others, was monitored heating the reactor at 20 jC/min from 25 up to 1200 jC.

3. Results and discussion 3.1. Sulfur forms analysis The total sulfur content in the original coals ranges between 1.7 and 8.5 wt.%, Table 1. Pyritic sulfur content is as high as 0.52 wt.% for Siersza coal, 2.36 wt.% for 1 Maja coal and 4.08 wt.% for Be»chato´w lignite. The contribution of pyritic sulfur to total sulfur is the highest in 1 Maja coal. The studied coals are characterized by high organic sulfur content, 1.18 –3.74 wt.%. The sulfate sulfur content of the coals ranges between 0.03 and 0.59 wt.%. All samples contain elemental sulfur in the range of 0.02 –0.12 wt.%. The presence of sulfates and elemental sulfur is due to air oxidation, which could take place during samples storage [26]. 3.2. Microscopic examinations of pyrite in original coals Optical microscopy allows specifying pyrite forms in coals and evaluating the efficiency of its removal. It was assumed that, in some cases, sink – float centrifugation technique

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would be effective enough to get samples containing such a small amount of pyrite that it would not affect the organic sulfur fuctionalities distribution monitored by AP-TPR. Microscopic observations show that the examined coals differ in mode of occurrence and size of pyrite (Fig. 1). For Be»chato´w lignite (Fig. 1a), different pyrite forms are found such as massive, framboidal and euhedral pyrite. Massive pyrite occurs mostly as loose grains. A large amount of framboidal pyrite is embedded within the coal particles and located at the edge of the coal particles. Most of euhedral pyrites are evenly disseminated throughout coal grains. The above observations suggest that upon finer grinding of the lignite sample, liberated and partially bound pyrite particles could be removed by sink – float separation. However, it is also expected that most of microsized pyrite would remain in the float. In Siersza coal (Fig. 1b), most of the pyrite is completely liberated. Microsized and framboidal pyrite has a low contribution and is associated with organic matrix and minerals, mostly clays. Pyrite predominates as liberated irregular coarse particles. It seems that most of pyrite would be removed if sample was ground finely. For 1 Maja coal (Fig. 1c), coarse and fine pyrite particles are observed. The coarse particles occur mainly in dendritic form as was revealed by SEM examination [27]. Other pyrite forms observed in this coal include framboidal and euhedral pyrite. Framboidal pyrite ranges between 5 and 10 Am. Mostly, it is liberated; however, some particles are embedded within the coal particles. Fine individual particles of pyrite (< 5 Am) are, in most cases, completely bound and strongly distributed in the coal matrix. Presumably, most of liberated pyrite would be removed by sink – float separation. 3.3. AP-TPR of raw coals The H2S evolution of the untreated coals is shown in Figs. 2 – 4 (profile ‘a’). Because the contribution of pyritic sulfur in the coals is significant, i.e. 54.9% of total sulfur in 1 Maja coal, 48.0% in Be»chato´w lignite and 29.7% in Siersza coal, the analysis of these profiles is complicated. As can be seen, for Be»chato´w lignite (Fig. 2a) and 1 Maja coal (Fig. 4a), the AP-TPR profiles have two dominant, well-separated peaks with maxima at 400– 450 and 780 –850 jC. These distinct signals are attributed to reduction of pyrite to troilite and subsequent reduction of troilite to iron. The small peak for 1 Maja coal and the shoulder for Be»chato´w lignite, both at around 250 jC, can correspond to the reduction of thiols and elemental sulfur. For the Be»chato´w lignite, the first main signal is much broader than the first peak in the 1 Maja coal, this refers to the presence of a greater amount of alkyl sulfides and mixed alkyl – aryl sulfides present in Be»chato´w lignite. For both coals, the contribution of organic sulfur to total sulfur is close and the sulfur recovery is almost the same. This means that 1 Maja coal, therefore, contains more aryl sulfides and thiophenic sulfur than Be»chato´w lignite. For 1 Maja coal, the small peak plateau around 500 jC refers to the presence of mixed alkyl – aryl sulfides. The fact that the second peak in the 1 Maja spectra is found at a lower temperature (750 jC) than for Be»chato´w (850 jC) can be explained by the fact that pyrite particles in 1 Maja coal are finer and better accessible to hydrogen than the ones found for Be»chato´w. For Siersza coal, the AP-TPR profile (Fig. 3a) is characterized by three intensive signals. The first one with maximum at 470 jC is attributed to the reduction of pyrite and

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Fig. 1. Optical micrographs of the pyrite forms in coals: (a) Be»chato´w lignite; (b) Siersza coal; (c) 1 Maja coal.

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Fig. 2. AP-TPR kinetograms of Belchato´w lignite: (a) raw coal; (b) HNO3-treated; (c) CHBr3/CCl4-treated; (d) ZnCl2-treated.

Fig. 3. AP-TPR kinetograms of Siersza coal: (a) raw coal; (b) HNO3-treated; (c) CHBr3/CCl4-treated; (d) ZnCl2treated.

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the two others with maxima at 680 and 760 jC may correspond to the hydrogenation of diaryl sulfides and thiophenes and the reduction of troilite to iron (Table 2). The asymmetric profile of the first main signal suggests the presence of dialkyl and alkyl – aryl sulfides. The first peak for the Siersza coal is also broader than the one for 1 Maja and, as in the case of Be»chato´w, this refers to the presence of a greater amount of alkyl sulfides and alkyl –aryl sulfides. Here, no thiols seem to be present. The evidence for that is a lack of signal below 300 jC, even when the AP-TPR experiment is performed with the socalled reducing solvent mixture [28]. This mixture is a much better hydrogenation medium for the formation of H2S in the lower temperature region of an AP-TPR experiment. For the raw coals, the temperature maximum of pyrite reduction into troilite is not always the same and varies between 420 and 460 jC. This can be explained by the difference in the morphology and particle size of pyrite present in these coals [29]. However, a much higher difference is observed in the case of the temperature range for the troilite reduction. The maximum of the reduction peak is found at 760 jC for Siersza coal and at 750 jC for 1 Maja coal and, finally, at 860 jC for Be»chato´w lignite. In a previous study [29] performed on coal-derived pyrite, it was demonstrated that the maximum rate of troilite reduction exhibits a tendency towards higher temperature values with increasing sample mass for analysis. This finding can be confirmed by the results obtained in this work. The maximum temperature reduction of troilite to iron varies between the coals in the same way as the total sulfur content, i.e. 1 Maja
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Table 3 Sulfur forms of treated coals (wt.%, dry basis) Sample

Total

Pyritic

Organic

Sulfate

Pyritic sulfur removal

Be»chato´w – HNO3 Be»chato´w – CHBr3/CCl4 Be»chato´w – ZnCl2 Siersza – HNO3 Siersza – CHBr3/CCl4 Siersza – ZnCl2 1 Maja – HNO3 1 Maja – CHBr3/CCl4 1 Maja – ZnCl2

2.34 4.91 4.21 0.99 1.20 1.34 1.35 2.08 1.98

0.0 1.84 1.62 0.0 0.0 0.06 0.0 0.69 0.54

2.34 3.07 2.57 0.99 1.20 1.26 1.35 1.39 1.42

0.0 0.0 0.02 0.0 0.0 0.02 0.0 0.0 0.02

100 54.9 60.3 100 100 88.5 100 70.3 78.8

organic sulfur is undesirable in the view of gaining knowledge about the distribution of organic sulfur functionalities of initial coal. In Figs. 2 – 4, the AP-TPR kinetograms of coal samples treated with nitric acid are shown (profile ‘b’). The comparison of profiles ‘a’ and ‘b’ confirms that not only inorganic sulfur, but also some part of organic sulfur compounds has been removed. The AP-TPR sulfur recovery for HNO3-treated coals is much lower than that of respective raw coals and increases in the order: Be»chato´w < Siersza < 1 Maja (Table 4). This suggests that the extent of oxidation is the highest for the lignite giving the oxidized sulfur compounds which are not reducible to H2S under AP-TPR conditions. In previous works [23,24], it was reported that oxidized sulfur groups are not quantitatively detected in our AP-TPR setup. 3.6. Sink –float separation 3.6.1. Sulfur forms analysis The results of the wet sulfur analysis of the floats obtained on sink –float separation using CHBr3/CCl4 mixture and ZnCl2 aqueous solution are presented in Table 3. Using CHBr3/CCl4 mixture, all sulfates are removed from the coals studied. Traces of sulfates are left after separation with ZnCl2. In the case of pyrite, Siersza coal is the only one free of pyrite. When CHBr3/CCl4 mixture was used, both Be»chato´w lignite and 1 Maja coal still contain pyritic sulfur, 1.84 and 0.69 wt.%, respectively, which corresponds to 54.9% and 70.3% of pyrite removal. The sink –float separation using ZnCl2 causes a decrease in the pyritic sulfur content to 1.62 wt.% for Be»chato´w lignite and 0.54 wt.% for 1 Maja coal. However, even if a significant part of the pyrite was removed, its reduction during APTable 4 Sulfur recovery of AP-TPR analysis, % Sample

Be»chato´w

Siersza

1 Maja

Raw coal HNO3-treated CHBr3/CCl4-treated ZnCl2-treated

93 43 82 76

90 61 81 81

91 72 76 71

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TPR would still have an essential influence on the H2S profile. It was also noticed that the ratio of organic sulfur to pyritic sulfur is much higher in the floats than in respective raw coals. A marked loss of organic sulfur content in Be»chato´w lignite during separation procedure using heavy liquids has been found. For CH3Br/CCl4 mixture, some organic sulfur compounds could be extracted from the lignite. When ZnCl2 solution is used, the formation of strong complexes of ZnCl2 with alkyl and aryl thiols present in the raw lignite could play an essential role [33]. Such a loss of organic sulfur is not observed for higher rank coals, i.e. Siersza coal and 1 Maja coal, which do not contain thiols. Taking into account the level of pyritic sulfur content in Be»chato´w lignite and 1 Maja coal (Table 3), the problem we had faced in organic sulfur determination by AP-TPR for raw coals is to be expected in the case of their floats as well. To get more effective pyrite removal by sink – float separation, the optimization of parameters such as specific gravity of liquids and grain size of coal sample will be done in a near future. 3.6.2. Microscopic examination As expected, sink – float centrifugation technique allowed removing most of liberated pyrite from coals. However, very fine pyrite particles associated with a vitrinic material are still present in the float. Microscopic observations showed that Be»chato´w lignite contains a significant amount of fine pyrite evenly dispersed in the coal matrix. Most of the massive pyrite was removed from this coal. For Siersza coal, only a few fine particles of euhedral and framboidal pyrite are observed in the float. In the case of 1 Maja, a relatively high amount of framboidal and microsized pyrite is still present. Some small loose grains of pyrite are also found. 3.6.3. AP-TPR results Figs. 2 –4 show the AP-TPR kinetograms of samples after sink – float centrifugation method using CHBr3/CCl4 mixture (profile ‘c’) and ZnCl2 solution (profile ‘d’). The sulfur recovery for enriched samples is lower in comparison with raw coals, but higher than the HNO3-treated ones (Table 4). For Be»chato´w lignite, in spite of a marked removal of pyrite to more than 50%, there are still two well-separated peaks at 400 –450 and 690 –750 jC corresponding to pyrite and troilite reduction (Fig. 2c and d), which makes the kinetogram to be difficult for organic sulfur functionalities determination. We can only estimate what kind of organic sulfur functionalities have been oxidized by nitric acid. Comparing profiles 2b with 2c or 2d, it can be concluded that these ones, which give the signals in the temperature range of 330– 630 jC, are assigned to dialkyl, alkyl –aryl and diaryl sulfides. Nearly complete removal of pyrite from Siersza coal with CHBr3/CCl4 mixture permits to observe the presence of organic sulfides which are hydrogenated in the temperature range of 400– 600 jC (Fig. 3c). The intensive peak at 465 jC observed for the raw coal (Fig. 3a), which is attributed to the pyrite reduction, has been replaced by two less intensive signals (Fig. 3c). The first signal at 440 jC corresponds to dialkyl sulfides and the second one at 495 jC is caused by the hydrogenation of alkyl –aryl sulfides. Two broad peaks at 650 and 760 jC can be explained by hydrogenation of diaryl sulfides and thiophenes, respectively. Fig. 4 shows the AP-TPR profiles of 1 Maja coal after sink – float separation with CHBr3/CCl4 mixture (profile ‘c’) and with ZnCl2 (profile ‘d’). Both kinetograms are again

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Fig. 4. AP-TPR kinetograms of 1 Maja coal: (a) raw coal; (b) HNO3-treated; (c) CHBr3/CCl4-treated; (d) ZnCl2treated.

very alike. Although there is some amount of pyrite in the sample as proven by wet analysis and microscopic examination, it is possible to observe organic sulfides (<600 jC) and thiophenes ( > 600 jC) on both kinetograms. The shoulder at 415 jC indicates the presence of pyrite, while the peak at 490 jC may be assigned to alkyl – aryl sulfides hydrogenation. The signals above 600 jC can be attributed in the same way as in Siersza sample, the first peak at 640 jC for 1 Maja – CHBr3/CCl4 (Fig. 4c) and at 680 jC for 1 Maja – ZnCl2 (Fig. 4d) corresponds to diaryl sulfides, and the second one at 720 jC (Fig. 4c) and at 740 jC (Fig. 4d) can be assigned to thiophenes and troilite. 3.6.4. AP-TPR-MS results Coupling AP-TPR reactor with mass spectrometer is particularly promising when we expect the evolution of not only H2S, but also other volatile sulfur-containing compounds. This is the case for oxidized coals when oxidized sulfur functionalities are expected. The H2S and SO2 profiles obtained for coals treated with nitric acid are presented for Be»chato´w in Fig. 5, as an example. Signals characteristic for hydrogen sulfide (m/z 33 and 34) give similar profiles (‘a’) to these obtained in AP-TPR analyses with potentiometric detection of S2 . The signals of m/z 48 (SO+ ) and 64 (SO2+ ) ions exhibit the same course and are presented as ‘b’ profile. The SO2 kinetogram consisted of one large peak at a maximum in the range of 200– 300 jC and a broad shoulder around 600 jC. The latter can be assigned to sulfoxides and sulfones [23]. The SO2 evolution observed in a lower temperature range is considered to be connected with the decomposition of organic sulfonic groups which could have been formed on nitric acid oxidation of coals.

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Fig. 5. AP-TPR-MS analysis. H2S (a) and SO2 (b) profiles of HNO3-treated Be»chato´w lignite.

To check this assumption, an AP-TPR-MS measurement of two model compounds, i.e. dodecyl-benzenesulfonic acid (DBS) and styrene-divinylbenzoic cationite containing – sulfonic groups (Wofatit KPS), was performed. The AP-TPR-MS mass spectrum of DBS (profile ‘a’) and the cationite (profile ‘b’) is shown in Fig. 6. For both samples, very intensive signals of the m/z 48 and 64 ions are observed. The AP-TPR-MS analysis shows that DBS is decomposed in two steps with a temperature maximum at 200 and 470 jC. The first peak is due to the decomposition of a part of sulfonic groups, while the second one may be attributed to the decomposition of disulfone and/or polysulfones formed on DBS heat treatment in the TPR reactor according to the reaction [34]:

For the cationite, one SO2 peak with a maximum at 340 jC is found in the TPR-MS spectra. The cationite belongs to high molecular-weight materials showing a relatively high thermal stability. DBS is not so stable under AP-TPR conditions. The latter starts to loose –SO3H groups around 120 jC and simultaneously undergoes secondary reactions leading to the formation of sulfones. Thus, for sulfonic groups containing model compounds, a temperature maximum of – SO3H decomposition is found to be in a wide interval between 200 and 340 jC, depending on its chemical structure. The SO2 evolved from decomposition of – SO3H groups is reduced to H2S to a small extent without a distinct maximum (profile ‘c’). This means that the sulfonic groups, if present in coal, have a very low contribution to the H2S profile resulting in a lower AP-TPR sulfur recovery

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Fig. 6. AP-TPR-MS analysis. SO2 profiles of DBS (a) and Wofatit KPS (b). H2S profile of Wofatit KPS (c).

expressed as H2S. The above observation can explain much lower sulfur recovery observed for all coals treated with HNO3, especially in the lower temperature range. For raw coals and the floats obtained using ZnCl2, very small amount of SO2 is evolved during AP-TPR experiment. 3.6.5. AP-TPO-MS results To check if there was any sulfur present in the residue after AP-TPR analysis, AP-TPOMS evolution of SO+ and SO2+ ions was investigated. Low sulfur recovery, in some cases, suggests that a part of sulfur might remain in the char and in the tar. Figs. 7– 9 show for the studied coals, the SO2 profiles for the residue (char + tar) left after AP-TPR experiments. The SO2 evolution profile is characterized by two maxima, one at around 600 jC and the second one at 1000 jC. The low-temperature evolution can be attributed to oxidation of organic sulfur groups. The high temperature one is the result of oxidation of inorganic sulfates and sulfides [24]. 3.6.5.1. Untreated coals. Because of very high sulfur recovery obtained during AP-TPR analyses, higher than 90%, we did not expect an intensive signal of SO2 due to the oxidation of AP-TPR residue of original coals. The AP-TPO-MS results presented in Fig. 7 indicate that there is some organic sulfur present in the char and tar. These compounds seem to be different from coal to coal, resulting in different profile. At the moment, we do not have enough information to assign these signals to different sulfur functionalities. The signal could be a result of oxidation of original present sulfur groups, but also sulfur

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Fig. 7. AP-TPO-MS analysis. SO2 profiles of residue of original coals: (a) Be»chato´w lignite; (b) Siersza coal; (c) 1 Maja coal.

groups formed during AP-TPR experiment. Nevertheless, it deals only with less than 10% sulfur original present in the coal. 3.6.5.2. Samples after sink – float centrifugation with ZnCl2. In Fig. 8, the AP-TPO-MS results of TPR residues of coals enriched using ZnCl2 solution are shown. It can be seen that after TPR analysis not all sulfur has been recovered. All residues contain significant amounts of inorganic sulfur which is manifested by the SO2 evolution at >1000 jC. As far as organic sulfur is concerned, the results are very similar to these obtained for original coals. A distinct signal present at higher temperature is due to oxidation of zinc sulfide [24], possibly formed during TPR analysis from evolving H2S and ZnCl2 traces remained in the sample after sink – float separation. Peaks at >1000 jC are much higher and wider than those at 600 jC, which suggests that there is more inorganic than organic sulfur in these residues. 3.6.5.3. Samples after nitric acid treatment. The results of AP-TPO-MS experiment on AP-TPR residues of HNO3-treated samples are presented in Fig. 9. A clear difference in the SO2 signal of the studied coals can be seen. A very strange is the low oxidation temperature signal at 290 jC for Be»chato´w lignite which is difficult to interpret at this stage of research. Presumably, this signal corresponds to organic functionalities different from thiophenes but also not reducable under TPR conditions; however, more reactive towards oxygen. It is worth emphasizing that, among the studied coals Be»chato´w lignite,

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Fig. 8. AP-TPO-MS analysis. SO2 profiles of residue of coals sink floated with ZnCl2: (a) Be»chato´w lignite; (b) Siersza coal; (c) 1 Maja coal.

Fig. 9. AP-TPO-MS analysis. SO2 profiles of residue of HNO3-treated coals: (a) Be»chato´w lignite; (b) Siersza coal; (c) 1 Maja coal.

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gives the lowest sulfur recovery. A clear difference can be also seen in the temperature range of 500– 700 jC (Fig. 9). We may say that, for the residue, the more mature coals the higher temperature of SO2 evolution is observed. In other words, as coal rank increases, the reactivity of organic sulfur left after TPR experiment to oxygen decreases. This can be connected with the reactivity of the whole residue substance. Remarkable is also the presence of inorganic sulfur for 1 Maja coal and not for the others, which is manifested by a signal around 950 jC. All the results mean that there is some difference in the chemical structure of sulfur compounds present in TPR residues.

4. Conclusions For high pyritic sulfur coals, the interpretation of the AP-TPR profiles in view of organic sulfur characterization without pyrite removal is very difficult or even impossible. Although the nitric acid is very effective in pyrite removal, it is too drastic because also oxidation of organic sulfur compounds and changes in the coal structure occur. As a result, classical AP-TPR sulfur recovery will be very low due to the ineffective reducing circumstances of the AP-TPR experiment. The AP-TPR-MS study clearly showed that for nitric acid-treated sample, a part of organic sulfur evolves as SO2 referring to the presence of oxidized sulfur forms. It was found that for organic sulfonic groups, nearly the whole sulfur is evolved as SO2. The sink – float centrifugation used in this work appears to be effective in pyrite removing. The degree of pyrite removal ranges between 54.9% and 100% and is related to the mode of pyrite occurrence, i.e. morphology, size and distribution in coal matrix. For some coals, it is possible to remove the pyrite to such an extent that a better insight into organic sulfur functionalities has been achieved, especially in view of organic sulfides. The AP-TPO-MS measurements of residue from TPR analysis indicate the presence of sulfur in char and tar, which means that not all sulfur is reduced to H2S in AP-TPR experiment. It was found that SO2 evolves at different temperatures for different coals. As coal rank increases, TPR residue contains sulfur compounds which are less reactive towards oxygen.

Acknowledgements The authors thank Mr. G. Reggers, Mr. J. Kaelen and Mr. K. Van Vinckenroye for their assistance with the AP-TPR experiments. This study was partly supported by the Polish State Committee for Scientific Research (KBN), project no. 3 T09B 01517.

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