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Oxidative desulphurisation of sulphur-rich coal
Fuel 83 (2004) 1117–1122 www.fuelfirst.com
Oxidative desulphurisation of sulphur-rich coal S.V. Pysh’yeva, V.I. Gayvanovycha,1, A. Pattek-Janczykb,*, J. Stanekc,2 a
Institute of Chemistry and Chemical Technology, National University, Lvivska Polytecnika, St. Bandery 12, 79013 Lviv, Ukraine b Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krako´w, Poland c M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krako´w, Poland Accepted 12 December 2003; available online 8 January 2004
Abstract Process of oxidative desulphurisation of sulphur-rich containing Ukrainian coal was carried out in the temperature range of 623– 723 K by air/steam mixture in the fluidised bed reactor. Mo¨ssbauer spectroscopy was applied to study the reaction products in form of iron compounds after every step of thermal treatment. The pyritic sulphur that constitutes most of total sulphur oxidises selectively at these conditions. At lower temperature (623 K) the ferrous sulphates and pyrite oxidised mainly to the ferric sulphate. At 673 K and higher temperatures a-Fe2O3 and SO2 were basic products of FeS2 oxidation. The transformation of FeS2 into pyrrhotite as the first stage of desulphurisation was observed at 698 K independently of the partial pressure of oxygen. The iron monosulphide (FeS) was not found at any stage of desulphurisation suggesting the pyrrhotite Fe12xS oxidation before the sequential FeS2 ! Fe12xS ! FeS transformation. q 2004 Elsevier Ltd. All rights reserved. Keywords: Coal desulphurisation; Pyrites; Mo¨ssbauer spectroscopy; Pyrrhotite
The content of total sulphur in coal is usually not higher than 1 –1.5 wt%. However, there are a lot of coals belonging to sulphur-rich ones [1 –3]. In particular, some Ukrainian coals contain 8– 10 wt% of sulphur [1,4,5]. Therefore power stations which use such a coal must be equipped with the installations for removal of SO2 from effluent gases. Due to the high cost of these installations the industry is forced to use low-sulphur coal. Thus, the search for the optimal methods of coal desulphurisation is a crucial problem from economical and ecological point of view. In addition, recovery of sulphur from the effluent gases which content a high concentration of SO2 might be a source of this raw material. The present paper is the continuation of the earlier studies on the oxidative desulphurisation [1,2,4,6], influenced by temperature, time, expenditure and kind of air/ steam mixture as well as by dimension of coal grains. * Corresponding author. Tel.: þ 48-12-6336377/2246; fax: þ 48-126340515. E-mail addresses: [email protected], [email protected] (A. Pattek-Janczyk); [email protected] (V.I. Gayvanovych); jstanek@ theta.uoks.uj.edu.pl (J. Stanek). 1 Tel.: þ380-32-398-166; fax: þ 380-32-398-166. 2 Tel.: þ48-12-6336377x5537; fax: þ 48-12-6337086. 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2003.12.008
The optimal factors of the process were found and the model of the pyrite oxidising in the temperature range of 673– 698 K, partly through the FeS2 decomposition to S and FeS (or Fe12xS) was derived. In this paper, we discuss some aspects of the sulphur removal using the selective oxidation by air/steam mixture in low temperature range (623 – 723 K). Because the main part of sulphur in coal is bounded to iron we have applied the 57Fe Mo¨ssbauer spectroscopy (MS) to follow the changes of the chemical states of these compounds after various coal treatment. MS based on the recording of the resonant absorption of the g-ray radiation, is a powerful experimental technique for identifying and determining the amounts of various iron compounds. This technique is especially useful in case of complex systems for example, containing poorly crystallised small particles or nonstoichiometric precipitates. MS method was used in the past by many authors to characterise various iron-bearing compounds occurring in coals [7 – 10] as well as to observe the reactions that take place during the coal treatment [11 –13]. The purpose of this work was to study the conversion of iron-content compounds to FeS and further to iron oxides via pyrrhotite at different oxidation conditions.
1118
S.V. Pysh’yev et al. / Fuel 83 (2004) 1117–1122
The coal with low coalification index, denoted as G12, from the Lvov– Volyn region was chosen for the study. Its characteristics are given in Table 1. The samples were prepared from the sieve fraction of 0.1– 0.25 mm of the grounded material. The technical analysis showed that this material belongs to the high-sulphur coals with the total sulphur content of , 8 wt%. Approximately 90% of the total sulphur appears in the pyritic form. The desulphurisation procedure was performed in the fluidised bed reactor in the temperature range of 623 – 723 K, increasing the temperature in steps of 25 K, in the air/steam atmosphere. The steam content in the gas mixture was 30 vol.%. In addition, one sample (F) was studied in more wet atmosphere (70 vol.% of steam) at 698 K. The characteristics of the coal after final desulphurisation is given in Table 2. The 57Fe Mo¨ssbauer absorption spectra were recorded in the transmission geometry at room temperature. The data were numerically analysed by means of a least-squares procedure assuming Lorentzian shape of the absorption lines. The fits supplied the main parameters of the hyperfine interactions of the 57Fe nuclei: isomer shift, proportional to the electron density at the iron nuclei, quadrupole splitting related to the electric field gradient and Zeeman splitting proportional to the internal magnetic fields. These parameters compared to the parameters known from the literature data, were the basis for the identification of iron compounds in the samples. Moreover, the spectral area of every component was used for the determination of the relative amount of each iron compound (assuming equal recoil free fractions f for all phases).
scale (^ 4.0 mm/s) in order to determine precisely the parameters of doublets and to perform the identification of sulphur bounded in inorganic compounds present in the coal. Such a procedure was possible because no magnetic iron phases were observed. Samples C to F contained magnetic phases and their spectra had to be recorded in a high velocity scale (^ 10.0 mm/s). From the visual inspection of the spectra it is obvious, that the specimens contain a variety of iron compounds. Thus, the numerical deconvolution of the spectra was complicated. In the first attempt all spectra were fitted within the following scheme. We assumed one doublet for low spin iron (II), assigned to pyrite, FeS2, one or two doublets for high spin iron (II) in FeSO4·n H2O with different n; one doublet for paramagnetic iron(III) in high spin state related to Fe2(SO4)3 and/or to small particles of Fe2O3. In addition, one or two sextets for magnetically ordered Fe2O3 with different grain sizes were also taken into account. However, this model of fitting was not satisfactory for sample E, where additional magnetic phase was clearly observed with internal magnetic field ranging between 22 and 30 T (see Fig. 1E). This fraction was assigned to pyrrhotite, deficient in iron monosulphide FeS, with the general formula Fe12xS (typically 0.06 , x , 0.12). Small amount of pyrrhotite and its complicated spectrum (composed of three sextets) forced us to use in numerical analysis the known from the literature [7,14,15] Mo¨ssbauer pyrrhotite parameters (Section 3) as constrained ones. Such procedure, which results in the improvement of calculations, was applied first to the spectrum of sample E and next also to samples C and F. Final Mo¨ssbauer parameters of the phases identified in spectra are given in Table 3.
2. Results
3. Discussion
The Mo¨ssbauer spectra of the G12 coal before treatment (sample A) and after desulphurisation in different conditions (samples B, C, D, E and F) are shown in Fig. 1. In the case of samples A and B the spectra were recorded in low velocity
3.1. Untreated sample
1. Experimental
In the studied coal the main part of sulphur occurs as a pyritic sulphur (Table 1). In principle, iron disulphide may
Table 1 Characteristics of the coal Technical analysis
Content of sulphur (wt %)
Ash, A (wt %)
Moisture,W (wt %)
Volatiles,V (wt %)
Total, St
Pyritic, Sp
Sulphate, SSO4
Organic, So
16.60
1.21
33.62
7.85
7.11
0.23
0.51
Table 2 Characteristics of the coal after desulphurisation Technical analysis
Content of sulphur (wt %)
Ash, A (wt %)
Moisture,W (wt %)
Volatiles,V (wt %)
Total, St
Pyritic, Sp
Sulphate, SSO4
Organic, So
25.46
2.09
18.42
1.88
0.68
0.42
0.78
S.V. Pysh’yev et al. / Fuel 83 (2004) 1117–1122
1119
Fig. 1. Room temperature Mo¨ssbauer spectra of G 12 coal. (A) Before treatment; (B–E) after desulphurisation at 623, 673, 698, 723 K, respectively, with 30 vol.% of steam; (F) after desulphurisation at 698 K with 70 vol.% of steam.
appear in cubic structure of pyrite and rhombic structure of marcasite, depending on the geological origin of the coal. However, these two forms have similar chemical reactivity and the name of more frequent pyrite is often
used for iron disulphide FeS 2. The Mo¨ssbauer parameters of both forms are similar as well (for pyrite isomer shift IS ¼ 0:28 2 0:33 mm/s and quadrupole splitting QS ¼ 0:58 2 0:63 mm/s and for marcasite
22.6–29.8 2.1(6)
26(1), 2.1(3) 40(1), 0.66 3.2(8) 63(1), 0.66 15.9(1) 8.4 (7), 0.66 3.4(6)
22.6–29.8 2.5(5)
IS ¼ 0:26 2 0:29 mm/s and QS ¼ 0:50 2 0:53 mm/s [7, 15]). Thus, we did not attempt to distinguish them from Mo¨ssbauer spectra. Within this simplification the Mo¨ssbauer data (Table 3) confirm that in studied coal about 86% of iron exists in pyrite and the rest in form of ferrous sulphates. The certain amount of iron sulphates is usually present in the coal as a result of pyrite oxidising probably in the period between collection of the sample and its final studies and may be described by the reaction: FeS2 þ 3O2 þ nH2 O ! FeSO4 ·nH2 O þ SO2 :
c
IS, isomer shift versus room temperature a-Fe. QS, quadrupole splitting. A, relative contribution to the total spectrum.
698 F
a
723 E
3.2. Treated samples
b
698 D
0.20(2), 0.12(6) 0.20(2), 0.12(4) 0.20(2), 0.14(2) 0.22(2), 0.22(8) 0.30(2) 0.60(2) 85.9(3) 1.27(2) 3.08(4) 7(1) 1.32(2) 2.62(2) 7(1) 0.30(2) 0.60(2) 82.5(7) 1.27(2) 3.18(4) 3.4(9) 1.33(2) 2.60(2) 5.2(9) 0.39(2) 1.08(2) 8.9(7) 0.31(2) 0.62(2) 54.9(5) 1.3(1) 2.7(2) 2.0(3) 0.38(2) 1.11(2) 14.8(5) 0.38(2), 0.39(2) 0.31(2) 0.61(2) 44.6(4) 0.39(2) 1.09(2) 9.7(4) 0.38(2), 0.30(2) 0.31(2) 0.64(2) 13.3(3) 0.39(2) 1.19(2) 2.1(3) 0.38(2), 0.35(2) 0.30(2) 0.61(2) 61.1(6) 1.32(2) 2.56(2) 6.3(3) 0.38(2) 1.08(2) 18.8(6) 0.38(2), 0.41(4) RT 623 673 A B C
ð1Þ
The detailed numerical analysis of the sample A showed that in the subspectrum assigned to iron sulphates at least two slightly different forms of sulphates which differ in the number of hydrous water molecules might be distinguished: FeSO4·7H2O and FeSO4·H2O. Indeed, melanterite FeSO4·7H2O, szomolnokite FeSO4·H2O and coquimbite Fe2(SO4)3·9H2O are the most commonly observed iron sulphate minerals occurring in various kinds of coals [17].
52.17(2), 49.4(2) 51.70(2), 49.3(2) 51.80(2) 49.2(1) 50.76(7), 48.0(3)
IS (mm/s) H (T) QS (mm/s) Temp. (K)
QSb Ac ISa (mm/s) (mm/s) (%)
IS QS A (mm/s) (mm/s) (%)
IS QS A (mm/s) (mm/s) (%)
IS QS A (mm/s) (mm/s) (%)
IS (mm/s)
H (T)
A (%)
Fe1-xS a-Fe2O3 Fe3þ species FeSO4·H2O FeSO4·7H2O Sample Species FeS2
Table 3 Mo¨ssbauer data of the coal samples
22.6–29.8 5.3(4)
S.V. Pysh’yev et al. / Fuel 83 (2004) 1117–1122 A (%)
1120
In the studied desulphurisation process after the first heating at 623 K only tiny changes in the spectrum were observed (Table 3). The amount of pyrite (precisely the amount of iron bounded in pyrite) decreases slightly from 86 to 82% and the content of ferrous sulphates diminishes to the half of starting amount, from 14 to 8%. Additional doublet with parameters indicating oxidation to the Fe3þ was attributed to the new appearing phase. This new paramagnetic doublet is probably a superposition of two doublets. One of them may be assigned to the iron(III) sulphate formed on the surface of FeS2 and FeSO4 grains. Anhydrous iron(III) sulphate gives at room temperature the paramagnetic spectrum with an isomer shift of 0.39 mm/s and a quadrupole splitting of 0.60 mm/s [16], which is significantly less than the values reported in present studies. Larger value of quadrupole splitting which indicates a deviation from the symmetrical structure of Fe3þ species may be understood if the hydrated or surface forms are taken into account [8]. The second doublet could be attributed to small particles of hematite and/or related compounds such as oxyhydroxides like FeOOH. At higher temperature during thermal treatment (673 K) the amount of a-Fe2O3 in form of big grain increases; the last is to be seen by magnetic interactions. For bulk hematite the accepted value of the hyperfine magnetic field is 51.5 –51.8 T at room temperature. This value is reduced for small grains; for example for the particles of 158 nm in diameter the field is 49.6 T and for particles with 180 nm of grain size the field is 50.3 T, as it was found in the literature for supported Fe2O3/SiO2 [22]. We also observe such reduction in the magnitude of magnetic field, that is an evidence of the distribution of hematite grain sizes in the studied samples.
S.V. Pysh’yev et al. / Fuel 83 (2004) 1117–1122
In excess of oxygen the reaction predominantly proceeds according to the equations: 4FeS2 þ 11O2 ! 2Fe2 O3 þ 8SO2
ð2Þ
and 2FeS2 þ 7O2 ! Fe2 ðSO4 Þ3 þ SO2 :
ð3Þ
Sulphate formation is more favourable at the temperature below 673 K [21]. Indeed, at that temperature we observed the formation of some Fe3þ species and Fe2O3 from FeS2. Thus, it may be concluded that this reaction takes place predominantly at surface layers of the coal grains. At temperatures of 698 and 723 K the scenario of desulphurisation changes qualitatively. After such treatment the spectrum consists mainly of magnetic components (Fig. 1D and E). Moreover, at 723 K, besides hematite, a new magnetic phase with definitely lower hyperfine field in the range of 22– 30 T may be clearly observed. At room temperature, the stable phases of iron monosulphide are stoichiometric FeS (troilit), hexagonal pyrrhotite Fe0.90 – 0.94S and monoclinic pyrrhotite Fe7S8 which composition Fe7S8 (or Fe0.875S) may be derived from FeS by subtraction of one iron atom per eight FeS units. Stoichiometric troilit is antiferromagnetic with the magnetic field of H ¼ 31 T [7,16] and isomer shift of IS ¼ 0:66 – 0:76 mm/s. Both pyrrhotite forms represent the defect structures with different vacancies distributions [11, 14 –16]. The phase Fe7S8 is ferromagnetic and the room temperature Mo¨ssbauer spectrum shows three well resolved sextets with hyperfine fields in the ranges of 29.7 –30.0, 25.2 –25.5 and 22.1 – 22.8 T corresponding to vacancies distribution. All sextets have the isomer shift values between 0.65 and 0.70 mm/s [14 –16]. Hexagonal pyrrhotites are analysed in the literature [11, 14] within the model of three inequivalent iron sites with different hyperfine fields in the range of 30.0 –30.3, 27.8 – 28.0 and 25.6 –26.0 T. Thus a mixture of monoclinic and hexagonal pyrrhotites exhibits a spectrum that is a superposition of the spectra characteristic for both components and effectively consists of four Zeeman sextets (30, 28, 25 and 22 T). Pyrite decomposition into iron sulphide and sulphur going through the nonstoichiometric phases takes place in an inert atmosphere according to the reactions [18 – 20]: FeS2 ! FeS22x ! FeS þ 1=nSn : where 0 , x # 1
ð4Þ
In the previous studies [2,4] it has been foreseen that the pyrite oxidation in the temperature range 673– 698 K is partly proceeded by the decomposition of FeS2 to S and FeS (or Fe12xS). To elucidate the process of pyrrhotite formation sample F was desulphurised in the more wet atmosphere. It is worth to note that in case of the sample F the amount of pyrrhotite found from the Mo¨ssbauer spectrum analysis was similar to the amount determined
1121
for sample D desulphurised at the same temperature. In the sample E (reaction temperature 723 K) the higher content of pyrrhotite was found (Table 3). These observations confirmed that the process of pyrrhotite formation depends only on the temperature whereas the partial pressure of oxygen is irrelevant. Thus, the decomposition of pyrite to pyrrhotite appearing at 698 K results in the previously observed increase in the SO2 content in desulphurisation gases at the temperature range 673 – 698 K [4]. Moreover, because FeS2 decomposes to FeS in inert atmosphere, it may be concluded that in the studied samples this process appeared in bulk of the coal grains. We did not observe the formation of FeS, but it is interesting to note that Mo¨ssbauer data confirm the presence of small amount of Fe12xS (2 –5%) found earlier by chemical analysis as being equal to 0.03– 0.05 wt% [2]. It may suggest that the process of coal desulphurisation takes place in the kinetic regime because the pyrrhotite oxidation is far faster than the troilite (FeS) formation.
4. Summary Observing by 57Fe Mo¨ssbauer spectroscopy the reactions of the sulphur –iron compounds we were able to monitor and describe at microscopic level the process of oxidative desulphurisation. Hence the distinction of the surface oxidation of pyrite which involves the following species: FeSO4·n H2O (in virgin sample), next Fe2(SO4)3, small grains of Fe2O3, bulk Fe2O3 and finally the solid state conversion of FeS2 to Fe12xS followed by the oxidation to Fe2O3 by diffusing oxygen was possible. In particular, at temperatures lower than 673 K pyrite is oxidised to sulphates. Close the temperature of 698 K and above the desulphurisation is intensified due to two parallel processes: pyrite oxidation to iron(III) oxide and sulphur dioxide as well as pyrite decomposition into sulphur and pyrrhotite. The last being very reactive accelerates the whole process of desulphurisation, that was observed as higher amount of SO2 in gases when process is carried out at 673 K and higher [2,4]. Thus the process of oxidative desulphurisation by means of the proposed method should be carried out at the temperatures 698– 723 K. The further increasing of temperature favours the oxidation of the organic parts of coal and results in decreasing of the SO2 content in gases.
References [1] Gayvanovych VI, Surygala J. Karbo 1999;44:371. in Polish. [2] Pysh’yev SV, Gayvanovych VI, Surygala J. Karbo 2001;46:1. in Polish.
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[3] Kucowski J, Laudyn D, Przekwas M. Energetyka a ochrona s´rodowiska. Warszawa: WNT; 1997. [4] Pysh’yev SV. Oksydaciyna desulfuryzaciya vysokosirkovogo nyzkometamorfizovanogo vugilla: Dys. kand. tehn. Nauk: 05.17.07.—Lviv, 1999.—149 ? [5] Lvovsko-volynskyj kamennougolnyj bassieyn. Gieologiczieskiy oczierk/Struyev M.I., Isakov V.I. i dr.—Kiyev, Nauk. Dumka, 1984. 272 ? [6] Pysh’yev SV, Gayvanovych VI, Surygala J. Karbo 2001;46:7– 8. in Polish. [7] Huffman GP, Huggins FE. Fuel 1978;57:592. [8] Montano PA. Fuel 1977;56:397. [9] Smith GV, Liu J-H, Saporoschenko M, Shiley R. Fuel 1978;57:41. [10] Bommannavar AS, Montano PA. Fuel 1982;61:523. [11] Montano PA, Granoff B. Fuel 1980;59:214. [12] Wang L, Cui Z, Liu S. Fuel 1992;71:755. [13] Jacobs IC, Federighi C, McKee DW, Patchen HJ. J Appl Phys 1982; 53(11):8326.
[14] Gupta VP, Singh AK, Chandra K, Jaireth SK. Phys Stat Sol (a) 1984; 81:281. [15] Jeandey C, Oddou JL, Mattei JL, Fillion G. Solid State Commun 1991;78(3):195. [16] Greenwood NN, Gibb TC. Mo¨ssbauer spectroscopy. London: Chapman and Hall; 1971. [17] Casagrande DJ. Sulphur in peat and coal. In: Scott AC, editor. Coal and coal-bearing strata: recent advances. Blackwell Scientific Oxford; 1987. p. 87–105. [18] Jasien´ko S, editor. Chemia i fizyka wegla. Wroclaw, Poland: Oficyna Wydawnicza Politechniki Wroclawskiej; 1995. [19] Attar A. Fuel 1978;57:201. [20] Rao VUS. Energy Fuels 1994;8:44. [21] Stephenson MD, Rostam-Abadi M, Johnson LA, Kruse CW. In: Attia YA, editor. Processing and utilization of high sulfur coals, coal science and technology, vol. 9. Amsterdam: Elsevier; 1985. p. 353 –72. [22] Chen JP, Hausladen MC, Yang RT. J Catal 1995;151:135.
Oxidative desulphurisation of sulphur-rich coal S.V. Pysh’yeva, V.I. Gayvanovycha,1, A. Pattek-Janczykb,*, J. Stanekc,2 a
Institute of Chemistry and Chemical Technology, National University, Lvivska Polytecnika, St. Bandery 12, 79013 Lviv, Ukraine b Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krako´w, Poland c M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krako´w, Poland Accepted 12 December 2003; available online 8 January 2004
Abstract Process of oxidative desulphurisation of sulphur-rich containing Ukrainian coal was carried out in the temperature range of 623– 723 K by air/steam mixture in the fluidised bed reactor. Mo¨ssbauer spectroscopy was applied to study the reaction products in form of iron compounds after every step of thermal treatment. The pyritic sulphur that constitutes most of total sulphur oxidises selectively at these conditions. At lower temperature (623 K) the ferrous sulphates and pyrite oxidised mainly to the ferric sulphate. At 673 K and higher temperatures a-Fe2O3 and SO2 were basic products of FeS2 oxidation. The transformation of FeS2 into pyrrhotite as the first stage of desulphurisation was observed at 698 K independently of the partial pressure of oxygen. The iron monosulphide (FeS) was not found at any stage of desulphurisation suggesting the pyrrhotite Fe12xS oxidation before the sequential FeS2 ! Fe12xS ! FeS transformation. q 2004 Elsevier Ltd. All rights reserved. Keywords: Coal desulphurisation; Pyrites; Mo¨ssbauer spectroscopy; Pyrrhotite
The content of total sulphur in coal is usually not higher than 1 –1.5 wt%. However, there are a lot of coals belonging to sulphur-rich ones [1 –3]. In particular, some Ukrainian coals contain 8– 10 wt% of sulphur [1,4,5]. Therefore power stations which use such a coal must be equipped with the installations for removal of SO2 from effluent gases. Due to the high cost of these installations the industry is forced to use low-sulphur coal. Thus, the search for the optimal methods of coal desulphurisation is a crucial problem from economical and ecological point of view. In addition, recovery of sulphur from the effluent gases which content a high concentration of SO2 might be a source of this raw material. The present paper is the continuation of the earlier studies on the oxidative desulphurisation [1,2,4,6], influenced by temperature, time, expenditure and kind of air/ steam mixture as well as by dimension of coal grains. * Corresponding author. Tel.: þ 48-12-6336377/2246; fax: þ 48-126340515. E-mail addresses: [email protected], [email protected] (A. Pattek-Janczyk); [email protected] (V.I. Gayvanovych); jstanek@ theta.uoks.uj.edu.pl (J. Stanek). 1 Tel.: þ380-32-398-166; fax: þ 380-32-398-166. 2 Tel.: þ48-12-6336377x5537; fax: þ 48-12-6337086. 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2003.12.008
The optimal factors of the process were found and the model of the pyrite oxidising in the temperature range of 673– 698 K, partly through the FeS2 decomposition to S and FeS (or Fe12xS) was derived. In this paper, we discuss some aspects of the sulphur removal using the selective oxidation by air/steam mixture in low temperature range (623 – 723 K). Because the main part of sulphur in coal is bounded to iron we have applied the 57Fe Mo¨ssbauer spectroscopy (MS) to follow the changes of the chemical states of these compounds after various coal treatment. MS based on the recording of the resonant absorption of the g-ray radiation, is a powerful experimental technique for identifying and determining the amounts of various iron compounds. This technique is especially useful in case of complex systems for example, containing poorly crystallised small particles or nonstoichiometric precipitates. MS method was used in the past by many authors to characterise various iron-bearing compounds occurring in coals [7 – 10] as well as to observe the reactions that take place during the coal treatment [11 –13]. The purpose of this work was to study the conversion of iron-content compounds to FeS and further to iron oxides via pyrrhotite at different oxidation conditions.
1118
S.V. Pysh’yev et al. / Fuel 83 (2004) 1117–1122
The coal with low coalification index, denoted as G12, from the Lvov– Volyn region was chosen for the study. Its characteristics are given in Table 1. The samples were prepared from the sieve fraction of 0.1– 0.25 mm of the grounded material. The technical analysis showed that this material belongs to the high-sulphur coals with the total sulphur content of , 8 wt%. Approximately 90% of the total sulphur appears in the pyritic form. The desulphurisation procedure was performed in the fluidised bed reactor in the temperature range of 623 – 723 K, increasing the temperature in steps of 25 K, in the air/steam atmosphere. The steam content in the gas mixture was 30 vol.%. In addition, one sample (F) was studied in more wet atmosphere (70 vol.% of steam) at 698 K. The characteristics of the coal after final desulphurisation is given in Table 2. The 57Fe Mo¨ssbauer absorption spectra were recorded in the transmission geometry at room temperature. The data were numerically analysed by means of a least-squares procedure assuming Lorentzian shape of the absorption lines. The fits supplied the main parameters of the hyperfine interactions of the 57Fe nuclei: isomer shift, proportional to the electron density at the iron nuclei, quadrupole splitting related to the electric field gradient and Zeeman splitting proportional to the internal magnetic fields. These parameters compared to the parameters known from the literature data, were the basis for the identification of iron compounds in the samples. Moreover, the spectral area of every component was used for the determination of the relative amount of each iron compound (assuming equal recoil free fractions f for all phases).
scale (^ 4.0 mm/s) in order to determine precisely the parameters of doublets and to perform the identification of sulphur bounded in inorganic compounds present in the coal. Such a procedure was possible because no magnetic iron phases were observed. Samples C to F contained magnetic phases and their spectra had to be recorded in a high velocity scale (^ 10.0 mm/s). From the visual inspection of the spectra it is obvious, that the specimens contain a variety of iron compounds. Thus, the numerical deconvolution of the spectra was complicated. In the first attempt all spectra were fitted within the following scheme. We assumed one doublet for low spin iron (II), assigned to pyrite, FeS2, one or two doublets for high spin iron (II) in FeSO4·n H2O with different n; one doublet for paramagnetic iron(III) in high spin state related to Fe2(SO4)3 and/or to small particles of Fe2O3. In addition, one or two sextets for magnetically ordered Fe2O3 with different grain sizes were also taken into account. However, this model of fitting was not satisfactory for sample E, where additional magnetic phase was clearly observed with internal magnetic field ranging between 22 and 30 T (see Fig. 1E). This fraction was assigned to pyrrhotite, deficient in iron monosulphide FeS, with the general formula Fe12xS (typically 0.06 , x , 0.12). Small amount of pyrrhotite and its complicated spectrum (composed of three sextets) forced us to use in numerical analysis the known from the literature [7,14,15] Mo¨ssbauer pyrrhotite parameters (Section 3) as constrained ones. Such procedure, which results in the improvement of calculations, was applied first to the spectrum of sample E and next also to samples C and F. Final Mo¨ssbauer parameters of the phases identified in spectra are given in Table 3.
2. Results
3. Discussion
The Mo¨ssbauer spectra of the G12 coal before treatment (sample A) and after desulphurisation in different conditions (samples B, C, D, E and F) are shown in Fig. 1. In the case of samples A and B the spectra were recorded in low velocity
3.1. Untreated sample
1. Experimental
In the studied coal the main part of sulphur occurs as a pyritic sulphur (Table 1). In principle, iron disulphide may
Table 1 Characteristics of the coal Technical analysis
Content of sulphur (wt %)
Ash, A (wt %)
Moisture,W (wt %)
Volatiles,V (wt %)
Total, St
Pyritic, Sp
Sulphate, SSO4
Organic, So
16.60
1.21
33.62
7.85
7.11
0.23
0.51
Table 2 Characteristics of the coal after desulphurisation Technical analysis
Content of sulphur (wt %)
Ash, A (wt %)
Moisture,W (wt %)
Volatiles,V (wt %)
Total, St
Pyritic, Sp
Sulphate, SSO4
Organic, So
25.46
2.09
18.42
1.88
0.68
0.42
0.78
S.V. Pysh’yev et al. / Fuel 83 (2004) 1117–1122
1119
Fig. 1. Room temperature Mo¨ssbauer spectra of G 12 coal. (A) Before treatment; (B–E) after desulphurisation at 623, 673, 698, 723 K, respectively, with 30 vol.% of steam; (F) after desulphurisation at 698 K with 70 vol.% of steam.
appear in cubic structure of pyrite and rhombic structure of marcasite, depending on the geological origin of the coal. However, these two forms have similar chemical reactivity and the name of more frequent pyrite is often
used for iron disulphide FeS 2. The Mo¨ssbauer parameters of both forms are similar as well (for pyrite isomer shift IS ¼ 0:28 2 0:33 mm/s and quadrupole splitting QS ¼ 0:58 2 0:63 mm/s and for marcasite
22.6–29.8 2.1(6)
26(1), 2.1(3) 40(1), 0.66 3.2(8) 63(1), 0.66 15.9(1) 8.4 (7), 0.66 3.4(6)
22.6–29.8 2.5(5)
IS ¼ 0:26 2 0:29 mm/s and QS ¼ 0:50 2 0:53 mm/s [7, 15]). Thus, we did not attempt to distinguish them from Mo¨ssbauer spectra. Within this simplification the Mo¨ssbauer data (Table 3) confirm that in studied coal about 86% of iron exists in pyrite and the rest in form of ferrous sulphates. The certain amount of iron sulphates is usually present in the coal as a result of pyrite oxidising probably in the period between collection of the sample and its final studies and may be described by the reaction: FeS2 þ 3O2 þ nH2 O ! FeSO4 ·nH2 O þ SO2 :
c
IS, isomer shift versus room temperature a-Fe. QS, quadrupole splitting. A, relative contribution to the total spectrum.
698 F
a
723 E
3.2. Treated samples
b
698 D
0.20(2), 0.12(6) 0.20(2), 0.12(4) 0.20(2), 0.14(2) 0.22(2), 0.22(8) 0.30(2) 0.60(2) 85.9(3) 1.27(2) 3.08(4) 7(1) 1.32(2) 2.62(2) 7(1) 0.30(2) 0.60(2) 82.5(7) 1.27(2) 3.18(4) 3.4(9) 1.33(2) 2.60(2) 5.2(9) 0.39(2) 1.08(2) 8.9(7) 0.31(2) 0.62(2) 54.9(5) 1.3(1) 2.7(2) 2.0(3) 0.38(2) 1.11(2) 14.8(5) 0.38(2), 0.39(2) 0.31(2) 0.61(2) 44.6(4) 0.39(2) 1.09(2) 9.7(4) 0.38(2), 0.30(2) 0.31(2) 0.64(2) 13.3(3) 0.39(2) 1.19(2) 2.1(3) 0.38(2), 0.35(2) 0.30(2) 0.61(2) 61.1(6) 1.32(2) 2.56(2) 6.3(3) 0.38(2) 1.08(2) 18.8(6) 0.38(2), 0.41(4) RT 623 673 A B C
ð1Þ
The detailed numerical analysis of the sample A showed that in the subspectrum assigned to iron sulphates at least two slightly different forms of sulphates which differ in the number of hydrous water molecules might be distinguished: FeSO4·7H2O and FeSO4·H2O. Indeed, melanterite FeSO4·7H2O, szomolnokite FeSO4·H2O and coquimbite Fe2(SO4)3·9H2O are the most commonly observed iron sulphate minerals occurring in various kinds of coals [17].
52.17(2), 49.4(2) 51.70(2), 49.3(2) 51.80(2) 49.2(1) 50.76(7), 48.0(3)
IS (mm/s) H (T) QS (mm/s) Temp. (K)
QSb Ac ISa (mm/s) (mm/s) (%)
IS QS A (mm/s) (mm/s) (%)
IS QS A (mm/s) (mm/s) (%)
IS QS A (mm/s) (mm/s) (%)
IS (mm/s)
H (T)
A (%)
Fe1-xS a-Fe2O3 Fe3þ species FeSO4·H2O FeSO4·7H2O Sample Species FeS2
Table 3 Mo¨ssbauer data of the coal samples
22.6–29.8 5.3(4)
S.V. Pysh’yev et al. / Fuel 83 (2004) 1117–1122 A (%)
1120
In the studied desulphurisation process after the first heating at 623 K only tiny changes in the spectrum were observed (Table 3). The amount of pyrite (precisely the amount of iron bounded in pyrite) decreases slightly from 86 to 82% and the content of ferrous sulphates diminishes to the half of starting amount, from 14 to 8%. Additional doublet with parameters indicating oxidation to the Fe3þ was attributed to the new appearing phase. This new paramagnetic doublet is probably a superposition of two doublets. One of them may be assigned to the iron(III) sulphate formed on the surface of FeS2 and FeSO4 grains. Anhydrous iron(III) sulphate gives at room temperature the paramagnetic spectrum with an isomer shift of 0.39 mm/s and a quadrupole splitting of 0.60 mm/s [16], which is significantly less than the values reported in present studies. Larger value of quadrupole splitting which indicates a deviation from the symmetrical structure of Fe3þ species may be understood if the hydrated or surface forms are taken into account [8]. The second doublet could be attributed to small particles of hematite and/or related compounds such as oxyhydroxides like FeOOH. At higher temperature during thermal treatment (673 K) the amount of a-Fe2O3 in form of big grain increases; the last is to be seen by magnetic interactions. For bulk hematite the accepted value of the hyperfine magnetic field is 51.5 –51.8 T at room temperature. This value is reduced for small grains; for example for the particles of 158 nm in diameter the field is 49.6 T and for particles with 180 nm of grain size the field is 50.3 T, as it was found in the literature for supported Fe2O3/SiO2 [22]. We also observe such reduction in the magnitude of magnetic field, that is an evidence of the distribution of hematite grain sizes in the studied samples.
S.V. Pysh’yev et al. / Fuel 83 (2004) 1117–1122
In excess of oxygen the reaction predominantly proceeds according to the equations: 4FeS2 þ 11O2 ! 2Fe2 O3 þ 8SO2
ð2Þ
and 2FeS2 þ 7O2 ! Fe2 ðSO4 Þ3 þ SO2 :
ð3Þ
Sulphate formation is more favourable at the temperature below 673 K [21]. Indeed, at that temperature we observed the formation of some Fe3þ species and Fe2O3 from FeS2. Thus, it may be concluded that this reaction takes place predominantly at surface layers of the coal grains. At temperatures of 698 and 723 K the scenario of desulphurisation changes qualitatively. After such treatment the spectrum consists mainly of magnetic components (Fig. 1D and E). Moreover, at 723 K, besides hematite, a new magnetic phase with definitely lower hyperfine field in the range of 22– 30 T may be clearly observed. At room temperature, the stable phases of iron monosulphide are stoichiometric FeS (troilit), hexagonal pyrrhotite Fe0.90 – 0.94S and monoclinic pyrrhotite Fe7S8 which composition Fe7S8 (or Fe0.875S) may be derived from FeS by subtraction of one iron atom per eight FeS units. Stoichiometric troilit is antiferromagnetic with the magnetic field of H ¼ 31 T [7,16] and isomer shift of IS ¼ 0:66 – 0:76 mm/s. Both pyrrhotite forms represent the defect structures with different vacancies distributions [11, 14 –16]. The phase Fe7S8 is ferromagnetic and the room temperature Mo¨ssbauer spectrum shows three well resolved sextets with hyperfine fields in the ranges of 29.7 –30.0, 25.2 –25.5 and 22.1 – 22.8 T corresponding to vacancies distribution. All sextets have the isomer shift values between 0.65 and 0.70 mm/s [14 –16]. Hexagonal pyrrhotites are analysed in the literature [11, 14] within the model of three inequivalent iron sites with different hyperfine fields in the range of 30.0 –30.3, 27.8 – 28.0 and 25.6 –26.0 T. Thus a mixture of monoclinic and hexagonal pyrrhotites exhibits a spectrum that is a superposition of the spectra characteristic for both components and effectively consists of four Zeeman sextets (30, 28, 25 and 22 T). Pyrite decomposition into iron sulphide and sulphur going through the nonstoichiometric phases takes place in an inert atmosphere according to the reactions [18 – 20]: FeS2 ! FeS22x ! FeS þ 1=nSn : where 0 , x # 1
ð4Þ
In the previous studies [2,4] it has been foreseen that the pyrite oxidation in the temperature range 673– 698 K is partly proceeded by the decomposition of FeS2 to S and FeS (or Fe12xS). To elucidate the process of pyrrhotite formation sample F was desulphurised in the more wet atmosphere. It is worth to note that in case of the sample F the amount of pyrrhotite found from the Mo¨ssbauer spectrum analysis was similar to the amount determined
1121
for sample D desulphurised at the same temperature. In the sample E (reaction temperature 723 K) the higher content of pyrrhotite was found (Table 3). These observations confirmed that the process of pyrrhotite formation depends only on the temperature whereas the partial pressure of oxygen is irrelevant. Thus, the decomposition of pyrite to pyrrhotite appearing at 698 K results in the previously observed increase in the SO2 content in desulphurisation gases at the temperature range 673 – 698 K [4]. Moreover, because FeS2 decomposes to FeS in inert atmosphere, it may be concluded that in the studied samples this process appeared in bulk of the coal grains. We did not observe the formation of FeS, but it is interesting to note that Mo¨ssbauer data confirm the presence of small amount of Fe12xS (2 –5%) found earlier by chemical analysis as being equal to 0.03– 0.05 wt% [2]. It may suggest that the process of coal desulphurisation takes place in the kinetic regime because the pyrrhotite oxidation is far faster than the troilite (FeS) formation.
4. Summary Observing by 57Fe Mo¨ssbauer spectroscopy the reactions of the sulphur –iron compounds we were able to monitor and describe at microscopic level the process of oxidative desulphurisation. Hence the distinction of the surface oxidation of pyrite which involves the following species: FeSO4·n H2O (in virgin sample), next Fe2(SO4)3, small grains of Fe2O3, bulk Fe2O3 and finally the solid state conversion of FeS2 to Fe12xS followed by the oxidation to Fe2O3 by diffusing oxygen was possible. In particular, at temperatures lower than 673 K pyrite is oxidised to sulphates. Close the temperature of 698 K and above the desulphurisation is intensified due to two parallel processes: pyrite oxidation to iron(III) oxide and sulphur dioxide as well as pyrite decomposition into sulphur and pyrrhotite. The last being very reactive accelerates the whole process of desulphurisation, that was observed as higher amount of SO2 in gases when process is carried out at 673 K and higher [2,4]. Thus the process of oxidative desulphurisation by means of the proposed method should be carried out at the temperatures 698– 723 K. The further increasing of temperature favours the oxidation of the organic parts of coal and results in decreasing of the SO2 content in gases.
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