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Effects of chromium ion on sulfur removal during pyrolysis and hydropyrolysis of coal
Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
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Effects of chromium ion on sulfur removal during pyrolysis and hydropyrolysis of coal Jiqing Huang a,b , Zongqing Bai a,∗ , Zhenxing Guo a , Wen Li a , Jin Bai a a b
State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 17 February 2012 Accepted 14 April 2012 Available online 10 May 2012 Keywords: Chromium (III) ions Sulfur removal Pyrolysis Hydropyrolysis
a b s t r a c t The effects of impregnated Cr3+ on sulfur removal during pyrolysis and hydropyrolysis of coal were investigated by loading CrCl3 into raw, demineralized and pyrite removed coal, respectively. The results indicate that Cr has no effect on the removal of pyrite. Cr affects the removal of total sulfur by forming Cr7 S8 and affecting the removal of organic sulfur. Cr acts as the sulfur removing agent by promoting the decomposition of the unstable organic sulfur at low temperature. However, it behaves to be sulfur fixing agent between 400 and 700 ◦ C so as to inhibit the evolution of H2 S, even in hydropyrolysis. With the increase of temperature from 700 to 1050 ◦ C, a certain ratio of Cr7 S8 is converted into organic sulfur during pyrolysis; however, almost all the Cr7 S8 is reduced into Cr at 1050 ◦ C during hydropyrolysis. And Cr significantly promotes the removal of organic sulfur at high temperature within reducing atmosphere. The XPS results indicate that the sulfur is enriched on coke surface by Cr, which is attributable to the formation of Cr7 S8 as well as the transfer of organic sulfur from bulk to surface during pyrolysis and hydropyrolysis. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a major pollution source, sulfur in coal inhibits the effective and extensive utilization of coal [1,2]. Pyrolysis and hydropyrolysis are not only an initial and important intermediate stage in coal gasification, combustion and liquefaction, but also simple and effective approaches towards clean conversion of coal [3–8]. In these processes, both inorganic and organic sulfur can be removed effectively, especially in hydropyrolysis. Hydrogen sulfide (H2 S) is the dominant gaseous sulfur product during pyrolysis and hydropyrolysis, which can be easily recovered in the form of sulfur [5,6]. As an important part of coal, minerals have complex effects on the sulfur transformation during pyrolysis and hydropyrolysis. During coal conversion process, acid minerals in coal promote the decomposition of organic sulfur, while alkaline minerals reduce the sulfur removal by forming metal sulfide [2]. Karaca pointed out that clay in coal catalyzed the conversion of unstable organic sulfur into stable one, while Fe, Ca, Mg in coal had catalytic effect on the organic sulfur removal [9]. Liu et al. [10] reported that inherent mineral matters in coal had little effect on the decomposition of pyrite. Chen et al. [11] confirmed that demineralization can enhance the
∗ Corresponding author. Tel.: +86 351 4048967; fax: +86 351 4050320. E-mail address: [email protected] (Z. Bai). 0165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2012.04.011
removal of sulfur, so as to introduce more of which into the tar and gases during pyrolysis and hydropyrolysis. Additive minerals also have remarkable effects on the sulfur removal of coal during pyrolysis and hydropyrolysis. NaCl/KCl and KOH/NaOH were proved to have apparent sulfur fixing effects at low temperature range, but have little capacity of sulfur retention at high temperature due to the volatilization of Na and K during pyrolysis above 700 ◦ C [12,13]. The extent of desulfurization and formation of H2 S was reduced by alkaline inorganic additives [14]. The additives (H3 BO3 , MgCl2 and AlCl3 ) assisted by surfactant promoted the sulfur removal during simulated coking process of coal [15]. In hydropyrolysis at 150 bars, levels of desulfurization (excluding sulfate sulfur) for lignite were improved to over 90% by employing sulfide molybdenum as the catalyst [16]. Other reports also demonstrated that the removal of organic sulfur can be reinforced by iron through forming active complex with sulfur, so as to catalyze the break of C S bond [17]. However, much attention have been paid to the effects of alkali, alkaline earth metal and iron on sulfur transformation, while little work have been done on the additive effects of other transition metals on sulfur removal of coal during pyrolysis and hydroypyrolysis. As a typical transition metal, Cr is usually used for oil desulfurization and shows good catalytic effect for that [18]. And as a common trace element in coal, Cr is stable and weak-volatilized, thus most of which retains in coke after pyrolysis at 1000 ◦ C [19,20]. In addition, iron and steel acid washing waste liquid contains abundant Cr3+ ,
J. Huang et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
which is a potential desulfurizer. Therefore, it is of great importance to clarify the role of Cr on the sulfur removal during pyrolysis and hydropyrolysis of coal. In present paper, the effects of Cr on sulfur removal of coal have been studied systematically. In order to eliminate the influence of inherent minerals, the coals were leached by acid firstly. The pre-treated coal was further impregnated by Cr, so as to investigate the comprehensive effects of which towards the transformation of sulfur in both solid and gas products. 2. Experimental 2.1. Coal sample
25
Total sulfur removal (wt%)
144
LS Cr-LS Dem Cr-Dem
20
15
10
A Chinese high-sulfur coal, Lingshi (LS) bituminous coal was used in this work and it was ground and sieved to 150–250 m for experiment. Preparation of the demineralized sample was carried out as follows: raw coal was washed sequentially by 5 M HCl, concentrated HF and concentrated HCl in a plastic beaker at 60 ◦ C. Then, the coal was filtered and washed with hot distilled water until no chloride ions were detected and this acid-treated coal was denoted as Dem coal. According to this method, most carbonate, oxide and sulfate were removed, but pyrite and organic sulfur are largely unaffected [11]. In order to remove pyrite from the demineralized sample, Dem coal was treated again according to ASTM D249280. Briefly speaking, the Dem coal was dipped in HNO3 solution (17 wt%) and stirred at 60 ◦ C for 45 min, then filtered and washed until no ferric ions were detected. The sample obtained is denoted as Demp coal. The Cr-loaded coal was prepared by impregnation. Typically, the coal was immersed into the solution of chromic chloride and the weight ratio of coal to solution was controlled at 2, and the mixture obtained was agitated for 240 min at 80 ◦ C and then dried for 8 h at 80 ◦ C in vacuum drier. By controlling the concentration of the Cr3+ solution, the amount of Cr3+ accounting for 2 wt% of dry ashfree basis coal was obtained. And the Cr-loaded samples of LS raw coal, Dem and Demp were denoted as Cr-LS, Cr-Dem and Cr-Demp, respectively. The properties of all samples are listed in Table 1.
2.4. Calculation methods
2.2. Pyrolysis experiments
3.1. Effects of Cr on total sulfur removal
Pyrolysis experiments were conducted in a quartz-tube fixed bed reactor under N2 or H2 , and the flow rate of the carrier gas was 200 mL/min. About 6 g sample was loaded into the constant temperature zone of the reactor at room temperature and swept by N2 for 20 min, then heated at 5 ◦ C/min up to 400, 500, 600, 700, 900 and 1050 ◦ C, respectively. The solid was collected from the reactor after the experiment, when the sample reached the room temperature with shield gas (Ar).
As shown in Table 1, after demineralization and pyrite removal by acid washing, the ash contents of both Dem and Demp coal are significantly decreased with the increase of volatile matters. Demineralization exhibit little effect on the content of carbon, hydrogen, oxygen, nitrogen and organic sulfur of Dem coal. However, due to the oxidization effect by HNO3 , the content of oxygen is increased with a simultaneous decrease of organic sulfur. Though the ash and volatile matters of the Cr-loaded coal are increased due to the impregnation of CrCl3 , no evidence was shown for the transformation of sulfur form. As the content of sulfate sulfur in LS raw coal is as low as 0.09 wt%, while no sulfate sulfur is detected in Dem coal, only the effects of Cr on the removal of total sulfur, pyrite sulfur and organic sulfur are considered in this work. Removal of total sulfur from LS raw, Dem, Cr-LS and Cr-Dem coal as a function of temperature during pyrolysis is shown in Fig. 1. The total sulfur removal increases significantly with the increase of temperature for four samples, however, the trend become steady over 700 ◦ C. The total sulfur removal of raw coal is lower than that of Dem coal, for sulfur species is fixed by the inherent mineral matters in the raw coal during pyrolysis. The reaction between minerals and sulfur containing gases (mainly H2 S) is one of the major reasons for retention of sulfur in coke [2,21], the demineralization with acid washing weakens those sulfur fixing reactions and promotes the sulfur removal and the H2 S evolution (Fig. 2). As indicated in Fig. 1, the total sulfur removal of Cr-LS and Cr-Dem coal is lower than that of LS raw and Dem coal, respectively. Comparing with LS and
2.3. Analysis and characterization method The amount of the total sulfur in coal and coke was determined by Coulomb method and pyrite sulfur content was determined by ASTM D2492-80. The organic sulfur content in coal or coke was analyzed as follows: coal or coke sample was washed by 5 M HCl and 2 M HNO3 according to ASTM method, and then the sulfur content in coal or coke was analyzed by Coulomb method directly, which could be considered as the amount of the organic sulfur. Xray diffraction analysis of coke was performed on a D/Max-2400 (Rigaku, Japan) diffractometer using Cu K␣ radiation with a wave length of 0.154 nm. The evolved gas during pyrolysis was collected by a syringe at an interval of 50 ◦ C and analyzed by gas chromatograph coupled with flame photometry detectors (GC-FPD). The XPS measurements of the coal or coke were carried out in the analysis chamber of the electron spectrometer ESCALAB 250 (Thermo Scientific, USA) at a base pressure of 1 × 10−8 Pa. The spectra were excited
5
400
600
800
1000
o
Temperature / c Fig. 1. Total sulfur removal of four samples during pyrolysis.
with an Al-Ka radiation (h = 1486.6 eV). The C1s line had a binding energy of 284.8 ± 0.1 eV and no charge effects were observed. The acquisition time for S2p spectra was over 3 h at 150 W X-ray power (20 mA/10 kV).
The sulfur removal during pyrolysis or hydropyrolysis was calculated as the following expression:
Sulfur removal (wt%) = 1 −
Scoke × Wcoke Scoal × Wcoal
× 100
(1)
where Scoke and Scoal is content of a specific form of sulfur in the coke and coal (dry basis), Wcoke and Wcoal is the mass of the coke and coal, respectively. 3. Results and discussion
J. Huang et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
145
Table 1 Proximate and ultimate analysis of raw and Cr-loaded coal samples (wt%). Coal
Proximate analysis (d)
LS Dem Demp Cr-LS Cr-Dem Cr-Demp
Ultimate analysis (daf)
Sulfur forms analysis (d)
FC*
A
V
C
H
O*
N
St
Ss
Sp
So
66.13 75.25 70.95 62.73 71.5 67.85
12.21 1.74 0.46 14.62 4.29 2.98
21.66 23.01 28.59 22.65 24.21 29.17
83.74 83.53 82.77 83.71 83.52 82.77
4.33 4.25 4.12 4.32 4.25 4.11
8.22 8.67 10.74 8.33 8.71 10.75
1.12 1.12 1.11 1.10 1.11 1.11
2.59 2.43 1.26 2.54 2.41 1.26
0.09 – – 0.09 – –
0.92 0.98 – 0.91 0.95 –
1.25 1.40 1.23 1.24 1.36 1.23
FC: fixed carbon, A: ash, V: volatile matters; d: dry basis; daf: dry ash-free basis. *: by difference; –: not detected.
Dem coal, the total sulfur removal of Cr-LS and Cr-Dem at 1050 ◦ C is decreased by 10 wt% and 14 wt%, respectively. As a sulfur fixing agent, Cr will inhibit the total sulfur removal of Cr-LS and Cr-Dem coal during pyrolysis of coal in inert gas. Fig. 2 demonstrates the evolution profiles of H2 S for the four samples during pyrolysis. The evolution of H2 S from the raw coal and Dem coal is attributable to the decomposition of both FeS2 and organic sulfur. Comparing with the raw coal, the amount of H2 S released from the Dem coal is remarkably higher. Besides, the evolution peak temperature of H2 S for Dem coal appears at 600 ◦ C, which is 50 ◦ C lower than that of LS raw coal. This is in constant with the results of previous study [11]. The evolution of H2 S from Cr-LS and Cr-Dem below 400 ◦ C is higher than that of LS raw and Dem coal, respectively. The removal of total sulfur is mainly due to the decomposition of unstable organic sulfur, as pyrite always remains stable below 400 ◦ C [1,10]. Therefore, it is believed that the decomposition of unstable sulfur is promoted by Cr at low temperature. This is in accordance to the other reports [9,17], which also confirmed that transition metals facilitate the decomposition of unstable organic sulfur as well as reduce the evolution temperature of the sulfur containing gases. The H2 S evolution of Cr-loaded coals from 400 ◦ C to 800 ◦ C is much lower than that of non-Cr loaded coal, as H2 S is trapped by Cr in the form of Cr7 S8 (Fig. 3, in case of N2 ). The H2 S evolution of Cr-loaded coal above 850 ◦ C is increased slightly due to the decomposition of the as-formed Cr7 S8 . As shown in Fig. 3, the diffraction peaks of both elemental Cr and Cr7 S8 are appeared in the coke after 1050 ◦ C pyrolysis, indicating that only part of the Cr7 S8 was decomposed under N2 which is unfavorable for the decomposition of Cr7 S8 . The total sulfur removal of four samples during hydropyrolysis is shown in Fig. 4. As expected, the total sulfur removal increases with the increase of temperature during hydropyrolysis, which are
a -- Cr2O3; a
a
a
a
a
b
b O
700 C, H2
c
a
a
c
fb
a
a
b
c c
f
O
1050 C, N2 a O
1050 C, H2 30
35
40
45
50
55
60
65
Fig. 3. XRD patterns of the Cr-Demp coal during pyrolysis and hydropyrolysis.
higher than that during pyrolysis. Due to the elimination of the sulfur fixing effects of mineral matters by acid washing, the total sulfur removal of the Dem coal are higher than those of LS raw coal at all temperatures investigated. Comparing with LS raw and Dem coal, the total sulfur removal of Cr-loaded coals at 400–700 ◦ C is relatively lower, which can also be attributed to the formation of Cr7 S8 (Fig. 3). It is interesting to find that the produced H2 S can be trapped by Cr in the form of Cr7 S8 even in the H2 atmosphere, and only partial Cr7 S8 is reduced to Cr at 700 ◦ C. Cr accelerates the removal of total sulfur and the H2 S formation of Cr-loaded coals during hydropyrolysis. At 1050 ◦ C, the total sulfur removal of Cr-LS and CrDem increase by 13.5 wt% and 12.4 wt%, respectively, compared to
Total sulfur removal (wt%)
-6
H2S concentration (10 /g, daf)
100
b a
a
b
O
LS Cr-LS Dem Cr-Dem
60
200
a
f -- Fe
700 C, N2
300
LS Cr-LS Dem Cr-Dem
b -- Cr7S8; c -- Cr;
40
20
0 200
400
600
800
1000
Temperature ( oc) Fig. 2. H2 S evolution profiles of four samples during pyrolysis.
400
600
800
1000
o
Temperature ( c) Fig. 4. Total sulfur removal of four samples during hydropyrolysis.
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LS Cr-LS Dem Cr-Dem
100
Pyrite sulfur removal (wt%)
-6
H2S concentration (10 /g, daf)
600
400
200
Dem 80
Cr-Dem
60
LS
40
Cr-LS
20
LS Cr-LS Dem Cr-Dem
0
0 200
400
600
o
800
400
1000
500
600
700
o
Temperature ( c)
Temperature ( c)
Fig. 5. Evolution profiles of H2 S of samples during hydropyrolysis.
Fig. 6. Pyrite sulfur removal of four samples during pyrolysis.
that of LS raw and Dem coals. In a word, the effect of Cr on total sulfur removal below 700 ◦ C during hydropyrolysis is similar with that during pyrolysis; however, the effect of Cr on total sulfur removal above 700 ◦ C during hydropyrolysis is opposite to that in pyrolysis. Fig. 5 shows the H2 S evolution profiles of four samples during hydropyrolysis. The H2 S evolution peaks of LS raw coal and Dem coal appear at about 500 ◦ C, which are attributed to the decomposition of FeS2 and the unstable organic sulfur. The peaks at about 950 ◦ C are due to the decomposition of FeS and the stable organic sulfur. The effects of Cr on H2 S evolution below 650 ◦ C during hydropyrolysis are similar to those in pyrolysis. Briefly, comparing with the LS raw and Dem coal, the H2 S evolution of Cr-loaded coals is enhanced by the addition of Cr below 250 ◦ C, which is inhibited at 250–650 ◦ C. However, H2 S evolution of the Cr-loaded samples increases sharply above 600 ◦ C, which is much higher than that of LS and Dem coals. The increase may be attributed to two aspects: (i) the decomposition of FeS or Cr7 S8 into H2 S and element Fe or Cr (Fig. 3, in case of H2 ), respectively; (ii) the catalytic decomposition of stable organic sulfur by element Fe and Cr as below:
attributed to the enhancement of mass and/or heat transfer of coal by acid washing [9]. Little difference in the removal of the pyrite sulfur is observed between Dem and Cr-Dem coal, suggesting that Cr has little effect on pyrite decomposition. As the effect of inherent mineral and mass and/or heat transfer on pyrite sulfur removal for Cr-Dem coal is similar to that for Dem, the difference in pyrite removal between both two samples is mainly due to the addition of Cr. The pyrite sulfur removal of Cr-LS coal is slightly higher than that of raw coal, however, much lower than that of Dem coal. For Cr-LS raw coal, Cr benefits the removal of pyrite sulfur by promoting the coal decomposition. It is believed that the impregnation of metal ions can change the structure of coal and promote the pyrolysis of coal [23]. However, the change in the structure of coal caused by added Cr is weaker than that by demineralization.
2R C S + 3H2 → 2H2 S ↑ + 2R CH
(2)
FeS + H2 → H2 S ↑ + Fe
(3)
Cr7 S8 + 8H2 → 8H2 S ↑ + 7Cr
(4)
3.3. Effects of Cr on organic sulfur removal The effects of Cr on organic sulfur removal of Demp coal during pyrolysis and hydropyrolysis are shown in Fig. 8. The results indicate that H2 is favorable for the removal of organic sulfur comparing with N2 . During pyrolysis, the organic sulfur removal of Demp always increases with temperature. With the increase of temperature, the organic sulfur removal from Cr-Demp coal increases significantly and peaks at 600 ◦ C, afterwards the value is decreased.
Strong diffraction peaks of elemental Cr with very weak Cr7 S8 peaks are shown for coke after 1050 ◦ C hydropyrolysis (Fig. 3), which indicates that almost all the Cr7 S8 has been reduced into Cr.
Removal of pyrite sulfur during pyrolysis and hydropyrolysis is shown in Figs. 6 and 7, respectively. Pyrite in raw coal remains stable up to 400 ◦ C under N2 , and then decomposes quickly to generate FeS [22]. In the case of hydropyrolysis, the pyrite removal at 500 ◦ C is much higher than that of pyrolysis, which implies that the decomposing speed of pyrite in H2 is much faster than that in N2 . At 700 ◦ C, nearly 100% pyrite is removed under either atmosphere. The pyrite removal of Dem coal is higher than that of raw coal at the same temperature below 700 ◦ C in both atmospheres, it is indicated that demineralization promotes the removal of pyrite sulfur. As described above, the inherent minerals have little effects on the removal of pyrite sulfur during coal pyrolysis [10], thus the promotion of pyrite removal from Dem coal by demineralization is mainly
Pyrite sulfur removal (wt%)
3.2. Effects of Cr on pyrite sulfur removal
100
Cr-Dem
Dem
80
LS Cr-LS 60
40
LS Cr-LS Dem Cr-Dem
20
400
500
600 o
Temperature ( c) Fig. 7. Pyrite sulfur removal of four samples during hydropyrolysis.
700
J. Huang et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
147
Table 2 Sulfur forms analysis of coal/coke surface by XPS (wt%).
N2
H2
Sample
Demp
Temperature, ◦ C
80
400
700
1050
Cr-Demp 80
400
700
1050
St Ssulfide So St Ssulfide So
1.43 – 1.43 1.43 – 1.43
1.19 – 1.19 1.18 – 1.18
1.17 – 1.17 0.86 – 0.86
1.36 – 1.36 0.6 – 0.6
1.46 – 1.46 1.46 – 1.46
1.76 0.23 1.53 0.99 0.06 0.93
2.58 0.83 1.75 3.00 1.36 1.74
2.38 0.67 1.71 0.39 – 0.39
Comparing with Demp coal, the organic sulfur removal of Cr-Demp coal is higher below 600 ◦ C, however and is lower above 700 ◦ C. On the one hand, as a typical transition metal, Cr can promote the decomposition of organic sulfur removal during pyrolysis and hydropyrolysis. On the other hand, Cr suppresses the formation of new organic sulfur by reacting with H2 S. Zhou et al. [24] believed that additive mineral matters reacted with H2 S, so as to suppress the reaction of which with coal matrix to form stable sulfur compounds. That’s why the organic sulfur removal of Cr-Demp coal is increased below 600 ◦ C during pyrolysis. Unfortunately, Cr will also react with H2 S to give Cr7 S8 (Fig. 3), which will be further converted into new stable organic sulfur. These newly introduced species is even more difficult to be removed than the inherent organic sulfur. The above phenomenon has also been observed by Cernic-Simic [25], and thus can explain the decrease of the organic sulfur removal of Cr-Demp coal after 600 ◦ C. During hydropyrolysis, with the increase of temperature, the organic sulfur removal always increases and is enhanced by the addition of Cr. Comparing with Demp coal, the higher organic sulfur St , wt% =
40
Demp-N2
Organic sulfur removal (wt%)
Cr-Demp-N2 Demp-H2 Cr-Demp-H2
20
10
0
400
600
3.4. XPS study on sulfur removal in coal surface X-ray photoelectron spectroscopy (XPS) is employed to analyze the different sulfur components on the coal surface quantitatively. The signal of a sulfur single species was composed by two peaks representing 2p3/2 and 2p1/2 components having a 2:1 relative intensity and separated in energy by 1.2 eV. The limitation of this technique is that it determines 2p signals from sulfur atoms situated only on sample surface. In this work, XPS spectra were interpreted using a curve resolution method. The peak of S2p is synthesized by mixed Gaussian and Lorentzian line shapes for each sulfur species, and the assignments of the sulfur forms were based on reference data [26,27]. The elemental composition, total sulfur (St ) and sulfur forms on the surface of coal/coke are calculated as follows: C, at% + O, at% + S, at% + N, at% + Cr, at% + Cl, at% = 100 at%
Ms × S, at% Mc × C, at% + Ms × S, at% + Mo × O, at% + Mn × N, at% + MCr × Cr, at% + MCl × Cl, at%
removal of Cr-Demp coals below 600 ◦ C is due to the promotion to organic sulfur decomposition as well as the inhibition to the formation of new organic sulfur by Cr. Moreover, the organic sulfur within Cr-Demp coal is prevented from being removed at 600–700 ◦ C, due to the introduction of stable organic sulfur converted from Cr7 S8 under H2 at 700 ◦ C. Cr7 S8 is reduced by H2 above 700 ◦ C and thus inhibits the conversion from inorganic sulfur to organic sulfur. Furthermore, hydrodesulfurization of the organic sulfur is accelerated by the catalytic effect of elemental Cr in H2 atmosphere at high
30
temperature, so the organic sulfur removal of Cr-Demp coal is continuously increased.
800
1000
o
Temperature ( c) Fig. 8. Organic sulfur removal of Demp and Cr-Demp coals during pyrolysis and hydropyrolysis.
Ssulfur forms , wt% =
A
sulfur forms
Atotal sulfur
× St , wt%
(5) (6) (7)
whereas M is atomic mass of the each element, at% is the atomic percentage of the each element. A is the integrated peak area of the XPS spectra of sulfur. As discussed above, Cr has no effect on the removal of pyrite sulfur. In order to figure out the effects of Cr on the sulfur removal accurately, Demp coal, which has been pre-treated by removing other minerals and FeS2 , was selected for XPS analysis. The relative contents of various sulfur forms on the sample surface by XPS are summarized in Table 2. With the increase of temperature during pyrolysis, the total sulfur content on Demp coal surface decreases initially to a minimum value of 700 ◦ C, followed by a subsequent increase. It is known that almost all the desulfurization reactions occur below 700 ◦ C (Section 3.1), the increase of total sulfur content on Demp coke surface after 700 ◦ C can be attributed to the transfer of sulfur from bulk to surface. As reported by Liu et al. [28], this is believed to be an important reason for a higher S/C ratio on surface than in the bulk. Conversely, with the increase of temperature during pyrolysis, the contents of total sulfur, sulfide and organic sulfur on the surface of Cr-Demp coke increases with a maximum value at 700 ◦ C, followed by a slight increase. This suggests that the sulfur is enriched on the coke surface by Cr, especially at 700 ◦ C. Therefore, the increase in total sulfur on the surface of Cr-Demp coke is mainly due to the formation of Cr7 S8 , as well as the transfer of sulfur. During hydropyrolysis, with the increase of temperature, the total sulfur on Demp coke surface decreases continuously. However, the total sulfur, sulfide and organic sulfur on the surface of Cr-Demp coke decrease initially followed by an increase to reach a peak at 700 ◦ C, and then the value drops again. Although small amount of Cr7 S8 is formed at 400 ◦ C, the total sulfur and organic
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J. Huang et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
sulfur on the surface of Cr-Demp coke are lower than that of Demp coke, as the decomposition of organic sulfur is promoted by Cr at low temperature. At 700 ◦ C, due to the formation of considerable amount of Cr7 S8 (1.36 wt%) and the increase of the organic sulfur, the total sulfur in Cr-Demp coke surface is higher than that of Demp coke. At 1050 ◦ C, because of the reduction of Cr7 S8 and organic sulfur by H2 and the catalytic hydrodesulfurization of organic sulfur by Cr, the total sulfur content in Cr-Demp coke surface declines sharply and is lower than that on Demp coke surface. The above results are in consistent with the results as shown in Fig. 4. It is interesting to find that, at 700 ◦ C the contents of Cr7 S8 and organic sulfur on Cr-Demp coke surface during hydropyrolysis are higher than that during pyrolysis. It is well known that comparing with N2 , the decomposition of organic sulfur is promoted by H2 to generate abundant H2 S. Therefore, during hydropyrolysis of CrDemp coal, more Cr7 S8 and new organic sulfur are formed when H2 S escapes from the bulk to surface of coal. 4. Conclusions Cr has no effect on the decomposition of pyrite. At low temperature, Cr enhances the total sulfur removal by promoting the decomposition of the unstable organic sulfur during pyrolysis and hydropyrolysis. However, the total sulfur removal and the H2 S evolution of Cr-loaded coal decreases significantly at 400–700 ◦ C even under H2 , as Cr reacts with H2 S to give Cr7 S8 to retain in coke. Above 700 ◦ C, the total sulfur removal from all coals changes little during pyrolysis. But more organic sulfur is retained Cr-loaded coals comparing with those without Cr, due to the conversion from Cr7 S8 to organic sulfur. During hydropyrolysis, Cr7 S8 is reduced to element Cr which promotes the further decomposition of organic sulfur under high temperature and reducing atmospheres, so both the total sulfur removal and H2 S evolution are improved by Crloading. The XPS results indicate that the sulfur is enriched on the coke surface by forming Cr7 S8 and sulfur transferring during pyrolysis and hydropyrolysis. This sulfur enrichment on the surface of Cr-Demp by Cr is more apparent for hydropyrolysis rather than pyrolysis. The results from chemical method are further correlated with the XPS result, so as to provide insightful science for the sulfur removal from coal, and thus benefit the industrial application of high-sulfur coking. Acknowledgement The authors thank National Basic Research Program of China (2011CB201401) for financial support. References [1] W.H. Calkins, The chemical forms of sulfur in coal: a review, Fuel 73 (1994) 475–484.
[2] A. Attar, Chemistry, thermodynamics and kinetics of reactions of sulphur in coal–gas reactions: a review, Fuel 57 (1978) 201–212. [3] J. Yperman, D. Franco, J. Mullens, L.C. Van Poucke, G. Gryglewicz, S. Jasienko, Determination of sulfur groups in pyrolysed low-rank coal by atmosphericpressure TPR, Fuel 74 (1995) 1261–1266. [4] Y.Y. Shi, S.Y. Li, H.Q. Hu, Studies on pyrolysis characteristic of lignite and properties of its pyrolysates, J. Anal. Appl. Pyrol. 95 (2012) 75–78. [5] P. Arendt, K.H. Van Heek, Comparative investigations of coal pyrolysis under inert gas and H2 at low and high heating rates and pressures up to 10 MPa, Fuel 60 (1981) 779–787. [6] H.K. Chen, B.Q. Li, J.L. Yang, B.J. Zhang, Transformation of sulfur during pyrolysis and hydropyrolysis of coal, Fuel 77 (1998) 487–493. [7] B. Saina, P.C. Saikiaa, B.P. Baruaha, C.S. Bordoloia, B. Mazumdera, Desulphurization of Assam coal by chlorinolysis and hydrothermal techniques, Fuel 70 (1991) 753–755. [8] J.X. Fei, J. Zhang, F.C. Wang, J. Wang, Synergistic effects on co-pyrolysis of lignite and high-sulfur swelling coal, J. Anal. Appl. Pyrol. 95 (2012) 61–67. [9] K. Semra, Desulfurization of a Turkish lignite at various gas atmospheres by pyrolysis: effect of mineral matter, Fuel 82 (2003) 1509–1516. [10] Q.R. Liu, H.Q. Hu, Q. Zhou, S.W. Zhu, G.H. Chen, Effect of mineral on sulfur behaviour during coal pressurizing pyrolysis, Fuel Process. Technol. 85 (2004) 863–871. [11] H.K. Chen, B.Q. Li, B.J. Zhang, Effects of mineral matter on products and sulfur distributions in hydropyrolysis, Fuel 78 (1999) 713–719. [12] Q.R. Liu, H.Q. Hu, S.W. Zhu, Q. Zhou, W.Y. Li, X.Y. Wei, K.C. Xie, Desulfurization of coal by pyrolysis and hydropyrolysis with addition of KOH/NaOH, Energy Fuels 19 (2005) 1673–1678. [13] D.Z. Lv, M.H. Xu, H. Yao, X.W. Liu, W. Jiang, H.L. Cao, Zh.H. Zhan, Effects of indigenous and added minerals on transformation of organic and inorganic sulfur in density separated coal fractions during CO2 -pyrolysis, Energy Fuels 24 (2010) 123–130. [14] M.R. Khan, Prediction of sulfur distribution in products during low temperature coal pyrolysis and gasification, Fuel 68 (1989) 1439–1449. [15] X.Y. Zhong, Study of the Effects of Additives on Desulfurization Properties of Coking Coal in Coking [D], University of Science and Technology LiaoNing, 2008. [16] R. Garcia, S.R. Mcinelo, Pyrolytic desulfurizatuion of some high-sulfur coals, Energy Fuels 5 (1991) 582–586. [17] S. Matsumoto, P.L. Walker Jr., Coke gasification in steam at 1123 K catalyzed by K, Na, Ca and Fe—effect of H2 , H2 S and COS, Carbon 24 (1986) 277–283. [18] S. Kumar, V.C. Srivastava, R.P. Badoni, Oxidative desulfurization by chromium promoted sulfated zirconia, J. Anal. Appl. Pyrol. 93 (2012) 18–25. [19] H.L. Lv, H.K. Chen, W. Li, B.Q. Li, Occurrence and volatilization behavior of Pb, Cd, Cr in Yima coal during fluidized bed pyrolysis, Fuel 83 (2004) 39–45. [20] R.X. Guo, J.L. Yang, D.Y. Liu, Z.Y. Liu, The fate of As, Pb, Cd, Cr and Mn in a coal during pyrolysis, J. Anal. Appl. Pyrol. 70 (2003) 555–562. [21] M.P. Skhonde, C.A. Strydom, J.R. Bunt, H.H. Schobert, Sulphur capturing by an inertinite rich high ash bituminous coal during conversion in a pilot packed bed reactor, J. Anal. Appl. Pyrol. 91 (2011) 205–209. [22] J.D. Yan, L. Xu, J.L. Yang, A study on the thermal decomposition of coal-derived pyrite, J. Anal. Appl. Pyrol. 82 (2008) 229–234. [23] V.N. Shevoplyas, V.I. Saranchuk, The impregnation effect on low and middle rank coals structure reorganization and their behavior during pyrolysis, Fuel 79 (2000) 557–565. [24] S.F. Zhou, J.L. Yang, Z.Y. Liu, J.D. Yan, B.Q. Li, Effect of temperature on the reaction of H2 S with a coke, Fuel Process. Technol. 92 (2011) 1671–1674. [25] S. Cernic-Simic, A study of factors that influence the behavior of coal sulfur during carbonization, Fuel 41 (1962) 141–151. [26] S.R. Kelemen, G.N. George, M.L. Gorbaty, Direct determination and quantification of sulphur forms in heavy petroleum and coals: 1. The X-ray photoelectron spectroscopy (XPS) approach, Fuel 69 (1990) 939–944. [27] S.P. Marinov, G. Tyuliev, M. Stefanova, R. Carleer, J. Yperman, Low rank coals sulphur functionality study by AP-TPR/TPO coupled with MS and potentiometric detection and by XPS, Fuel Process. Technol. 85 (2004) 267–277. [28] F.R. Liu, W. Li, H.K. Chen, B.Q. Li, Uneven distribution of sulfurs and their transformation during coal pyrolysis, Fuel 86 (2007) 360–366.
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Effects of chromium ion on sulfur removal during pyrolysis and hydropyrolysis of coal Jiqing Huang a,b , Zongqing Bai a,∗ , Zhenxing Guo a , Wen Li a , Jin Bai a a b
State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 17 February 2012 Accepted 14 April 2012 Available online 10 May 2012 Keywords: Chromium (III) ions Sulfur removal Pyrolysis Hydropyrolysis
a b s t r a c t The effects of impregnated Cr3+ on sulfur removal during pyrolysis and hydropyrolysis of coal were investigated by loading CrCl3 into raw, demineralized and pyrite removed coal, respectively. The results indicate that Cr has no effect on the removal of pyrite. Cr affects the removal of total sulfur by forming Cr7 S8 and affecting the removal of organic sulfur. Cr acts as the sulfur removing agent by promoting the decomposition of the unstable organic sulfur at low temperature. However, it behaves to be sulfur fixing agent between 400 and 700 ◦ C so as to inhibit the evolution of H2 S, even in hydropyrolysis. With the increase of temperature from 700 to 1050 ◦ C, a certain ratio of Cr7 S8 is converted into organic sulfur during pyrolysis; however, almost all the Cr7 S8 is reduced into Cr at 1050 ◦ C during hydropyrolysis. And Cr significantly promotes the removal of organic sulfur at high temperature within reducing atmosphere. The XPS results indicate that the sulfur is enriched on coke surface by Cr, which is attributable to the formation of Cr7 S8 as well as the transfer of organic sulfur from bulk to surface during pyrolysis and hydropyrolysis. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a major pollution source, sulfur in coal inhibits the effective and extensive utilization of coal [1,2]. Pyrolysis and hydropyrolysis are not only an initial and important intermediate stage in coal gasification, combustion and liquefaction, but also simple and effective approaches towards clean conversion of coal [3–8]. In these processes, both inorganic and organic sulfur can be removed effectively, especially in hydropyrolysis. Hydrogen sulfide (H2 S) is the dominant gaseous sulfur product during pyrolysis and hydropyrolysis, which can be easily recovered in the form of sulfur [5,6]. As an important part of coal, minerals have complex effects on the sulfur transformation during pyrolysis and hydropyrolysis. During coal conversion process, acid minerals in coal promote the decomposition of organic sulfur, while alkaline minerals reduce the sulfur removal by forming metal sulfide [2]. Karaca pointed out that clay in coal catalyzed the conversion of unstable organic sulfur into stable one, while Fe, Ca, Mg in coal had catalytic effect on the organic sulfur removal [9]. Liu et al. [10] reported that inherent mineral matters in coal had little effect on the decomposition of pyrite. Chen et al. [11] confirmed that demineralization can enhance the
∗ Corresponding author. Tel.: +86 351 4048967; fax: +86 351 4050320. E-mail address: [email protected] (Z. Bai). 0165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2012.04.011
removal of sulfur, so as to introduce more of which into the tar and gases during pyrolysis and hydropyrolysis. Additive minerals also have remarkable effects on the sulfur removal of coal during pyrolysis and hydropyrolysis. NaCl/KCl and KOH/NaOH were proved to have apparent sulfur fixing effects at low temperature range, but have little capacity of sulfur retention at high temperature due to the volatilization of Na and K during pyrolysis above 700 ◦ C [12,13]. The extent of desulfurization and formation of H2 S was reduced by alkaline inorganic additives [14]. The additives (H3 BO3 , MgCl2 and AlCl3 ) assisted by surfactant promoted the sulfur removal during simulated coking process of coal [15]. In hydropyrolysis at 150 bars, levels of desulfurization (excluding sulfate sulfur) for lignite were improved to over 90% by employing sulfide molybdenum as the catalyst [16]. Other reports also demonstrated that the removal of organic sulfur can be reinforced by iron through forming active complex with sulfur, so as to catalyze the break of C S bond [17]. However, much attention have been paid to the effects of alkali, alkaline earth metal and iron on sulfur transformation, while little work have been done on the additive effects of other transition metals on sulfur removal of coal during pyrolysis and hydroypyrolysis. As a typical transition metal, Cr is usually used for oil desulfurization and shows good catalytic effect for that [18]. And as a common trace element in coal, Cr is stable and weak-volatilized, thus most of which retains in coke after pyrolysis at 1000 ◦ C [19,20]. In addition, iron and steel acid washing waste liquid contains abundant Cr3+ ,
J. Huang et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
which is a potential desulfurizer. Therefore, it is of great importance to clarify the role of Cr on the sulfur removal during pyrolysis and hydropyrolysis of coal. In present paper, the effects of Cr on sulfur removal of coal have been studied systematically. In order to eliminate the influence of inherent minerals, the coals were leached by acid firstly. The pre-treated coal was further impregnated by Cr, so as to investigate the comprehensive effects of which towards the transformation of sulfur in both solid and gas products. 2. Experimental 2.1. Coal sample
25
Total sulfur removal (wt%)
144
LS Cr-LS Dem Cr-Dem
20
15
10
A Chinese high-sulfur coal, Lingshi (LS) bituminous coal was used in this work and it was ground and sieved to 150–250 m for experiment. Preparation of the demineralized sample was carried out as follows: raw coal was washed sequentially by 5 M HCl, concentrated HF and concentrated HCl in a plastic beaker at 60 ◦ C. Then, the coal was filtered and washed with hot distilled water until no chloride ions were detected and this acid-treated coal was denoted as Dem coal. According to this method, most carbonate, oxide and sulfate were removed, but pyrite and organic sulfur are largely unaffected [11]. In order to remove pyrite from the demineralized sample, Dem coal was treated again according to ASTM D249280. Briefly speaking, the Dem coal was dipped in HNO3 solution (17 wt%) and stirred at 60 ◦ C for 45 min, then filtered and washed until no ferric ions were detected. The sample obtained is denoted as Demp coal. The Cr-loaded coal was prepared by impregnation. Typically, the coal was immersed into the solution of chromic chloride and the weight ratio of coal to solution was controlled at 2, and the mixture obtained was agitated for 240 min at 80 ◦ C and then dried for 8 h at 80 ◦ C in vacuum drier. By controlling the concentration of the Cr3+ solution, the amount of Cr3+ accounting for 2 wt% of dry ashfree basis coal was obtained. And the Cr-loaded samples of LS raw coal, Dem and Demp were denoted as Cr-LS, Cr-Dem and Cr-Demp, respectively. The properties of all samples are listed in Table 1.
2.4. Calculation methods
2.2. Pyrolysis experiments
3.1. Effects of Cr on total sulfur removal
Pyrolysis experiments were conducted in a quartz-tube fixed bed reactor under N2 or H2 , and the flow rate of the carrier gas was 200 mL/min. About 6 g sample was loaded into the constant temperature zone of the reactor at room temperature and swept by N2 for 20 min, then heated at 5 ◦ C/min up to 400, 500, 600, 700, 900 and 1050 ◦ C, respectively. The solid was collected from the reactor after the experiment, when the sample reached the room temperature with shield gas (Ar).
As shown in Table 1, after demineralization and pyrite removal by acid washing, the ash contents of both Dem and Demp coal are significantly decreased with the increase of volatile matters. Demineralization exhibit little effect on the content of carbon, hydrogen, oxygen, nitrogen and organic sulfur of Dem coal. However, due to the oxidization effect by HNO3 , the content of oxygen is increased with a simultaneous decrease of organic sulfur. Though the ash and volatile matters of the Cr-loaded coal are increased due to the impregnation of CrCl3 , no evidence was shown for the transformation of sulfur form. As the content of sulfate sulfur in LS raw coal is as low as 0.09 wt%, while no sulfate sulfur is detected in Dem coal, only the effects of Cr on the removal of total sulfur, pyrite sulfur and organic sulfur are considered in this work. Removal of total sulfur from LS raw, Dem, Cr-LS and Cr-Dem coal as a function of temperature during pyrolysis is shown in Fig. 1. The total sulfur removal increases significantly with the increase of temperature for four samples, however, the trend become steady over 700 ◦ C. The total sulfur removal of raw coal is lower than that of Dem coal, for sulfur species is fixed by the inherent mineral matters in the raw coal during pyrolysis. The reaction between minerals and sulfur containing gases (mainly H2 S) is one of the major reasons for retention of sulfur in coke [2,21], the demineralization with acid washing weakens those sulfur fixing reactions and promotes the sulfur removal and the H2 S evolution (Fig. 2). As indicated in Fig. 1, the total sulfur removal of Cr-LS and Cr-Dem coal is lower than that of LS raw and Dem coal, respectively. Comparing with LS and
2.3. Analysis and characterization method The amount of the total sulfur in coal and coke was determined by Coulomb method and pyrite sulfur content was determined by ASTM D2492-80. The organic sulfur content in coal or coke was analyzed as follows: coal or coke sample was washed by 5 M HCl and 2 M HNO3 according to ASTM method, and then the sulfur content in coal or coke was analyzed by Coulomb method directly, which could be considered as the amount of the organic sulfur. Xray diffraction analysis of coke was performed on a D/Max-2400 (Rigaku, Japan) diffractometer using Cu K␣ radiation with a wave length of 0.154 nm. The evolved gas during pyrolysis was collected by a syringe at an interval of 50 ◦ C and analyzed by gas chromatograph coupled with flame photometry detectors (GC-FPD). The XPS measurements of the coal or coke were carried out in the analysis chamber of the electron spectrometer ESCALAB 250 (Thermo Scientific, USA) at a base pressure of 1 × 10−8 Pa. The spectra were excited
5
400
600
800
1000
o
Temperature / c Fig. 1. Total sulfur removal of four samples during pyrolysis.
with an Al-Ka radiation (h = 1486.6 eV). The C1s line had a binding energy of 284.8 ± 0.1 eV and no charge effects were observed. The acquisition time for S2p spectra was over 3 h at 150 W X-ray power (20 mA/10 kV).
The sulfur removal during pyrolysis or hydropyrolysis was calculated as the following expression:
Sulfur removal (wt%) = 1 −
Scoke × Wcoke Scoal × Wcoal
× 100
(1)
where Scoke and Scoal is content of a specific form of sulfur in the coke and coal (dry basis), Wcoke and Wcoal is the mass of the coke and coal, respectively. 3. Results and discussion
J. Huang et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
145
Table 1 Proximate and ultimate analysis of raw and Cr-loaded coal samples (wt%). Coal
Proximate analysis (d)
LS Dem Demp Cr-LS Cr-Dem Cr-Demp
Ultimate analysis (daf)
Sulfur forms analysis (d)
FC*
A
V
C
H
O*
N
St
Ss
Sp
So
66.13 75.25 70.95 62.73 71.5 67.85
12.21 1.74 0.46 14.62 4.29 2.98
21.66 23.01 28.59 22.65 24.21 29.17
83.74 83.53 82.77 83.71 83.52 82.77
4.33 4.25 4.12 4.32 4.25 4.11
8.22 8.67 10.74 8.33 8.71 10.75
1.12 1.12 1.11 1.10 1.11 1.11
2.59 2.43 1.26 2.54 2.41 1.26
0.09 – – 0.09 – –
0.92 0.98 – 0.91 0.95 –
1.25 1.40 1.23 1.24 1.36 1.23
FC: fixed carbon, A: ash, V: volatile matters; d: dry basis; daf: dry ash-free basis. *: by difference; –: not detected.
Dem coal, the total sulfur removal of Cr-LS and Cr-Dem at 1050 ◦ C is decreased by 10 wt% and 14 wt%, respectively. As a sulfur fixing agent, Cr will inhibit the total sulfur removal of Cr-LS and Cr-Dem coal during pyrolysis of coal in inert gas. Fig. 2 demonstrates the evolution profiles of H2 S for the four samples during pyrolysis. The evolution of H2 S from the raw coal and Dem coal is attributable to the decomposition of both FeS2 and organic sulfur. Comparing with the raw coal, the amount of H2 S released from the Dem coal is remarkably higher. Besides, the evolution peak temperature of H2 S for Dem coal appears at 600 ◦ C, which is 50 ◦ C lower than that of LS raw coal. This is in constant with the results of previous study [11]. The evolution of H2 S from Cr-LS and Cr-Dem below 400 ◦ C is higher than that of LS raw and Dem coal, respectively. The removal of total sulfur is mainly due to the decomposition of unstable organic sulfur, as pyrite always remains stable below 400 ◦ C [1,10]. Therefore, it is believed that the decomposition of unstable sulfur is promoted by Cr at low temperature. This is in accordance to the other reports [9,17], which also confirmed that transition metals facilitate the decomposition of unstable organic sulfur as well as reduce the evolution temperature of the sulfur containing gases. The H2 S evolution of Cr-loaded coals from 400 ◦ C to 800 ◦ C is much lower than that of non-Cr loaded coal, as H2 S is trapped by Cr in the form of Cr7 S8 (Fig. 3, in case of N2 ). The H2 S evolution of Cr-loaded coal above 850 ◦ C is increased slightly due to the decomposition of the as-formed Cr7 S8 . As shown in Fig. 3, the diffraction peaks of both elemental Cr and Cr7 S8 are appeared in the coke after 1050 ◦ C pyrolysis, indicating that only part of the Cr7 S8 was decomposed under N2 which is unfavorable for the decomposition of Cr7 S8 . The total sulfur removal of four samples during hydropyrolysis is shown in Fig. 4. As expected, the total sulfur removal increases with the increase of temperature during hydropyrolysis, which are
a -- Cr2O3; a
a
a
a
a
b
b O
700 C, H2
c
a
a
c
fb
a
a
b
c c
f
O
1050 C, N2 a O
1050 C, H2 30
35
40
45
50
55
60
65
Fig. 3. XRD patterns of the Cr-Demp coal during pyrolysis and hydropyrolysis.
higher than that during pyrolysis. Due to the elimination of the sulfur fixing effects of mineral matters by acid washing, the total sulfur removal of the Dem coal are higher than those of LS raw coal at all temperatures investigated. Comparing with LS raw and Dem coal, the total sulfur removal of Cr-loaded coals at 400–700 ◦ C is relatively lower, which can also be attributed to the formation of Cr7 S8 (Fig. 3). It is interesting to find that the produced H2 S can be trapped by Cr in the form of Cr7 S8 even in the H2 atmosphere, and only partial Cr7 S8 is reduced to Cr at 700 ◦ C. Cr accelerates the removal of total sulfur and the H2 S formation of Cr-loaded coals during hydropyrolysis. At 1050 ◦ C, the total sulfur removal of Cr-LS and CrDem increase by 13.5 wt% and 12.4 wt%, respectively, compared to
Total sulfur removal (wt%)
-6
H2S concentration (10 /g, daf)
100
b a
a
b
O
LS Cr-LS Dem Cr-Dem
60
200
a
f -- Fe
700 C, N2
300
LS Cr-LS Dem Cr-Dem
b -- Cr7S8; c -- Cr;
40
20
0 200
400
600
800
1000
Temperature ( oc) Fig. 2. H2 S evolution profiles of four samples during pyrolysis.
400
600
800
1000
o
Temperature ( c) Fig. 4. Total sulfur removal of four samples during hydropyrolysis.
146
J. Huang et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
LS Cr-LS Dem Cr-Dem
100
Pyrite sulfur removal (wt%)
-6
H2S concentration (10 /g, daf)
600
400
200
Dem 80
Cr-Dem
60
LS
40
Cr-LS
20
LS Cr-LS Dem Cr-Dem
0
0 200
400
600
o
800
400
1000
500
600
700
o
Temperature ( c)
Temperature ( c)
Fig. 5. Evolution profiles of H2 S of samples during hydropyrolysis.
Fig. 6. Pyrite sulfur removal of four samples during pyrolysis.
that of LS raw and Dem coals. In a word, the effect of Cr on total sulfur removal below 700 ◦ C during hydropyrolysis is similar with that during pyrolysis; however, the effect of Cr on total sulfur removal above 700 ◦ C during hydropyrolysis is opposite to that in pyrolysis. Fig. 5 shows the H2 S evolution profiles of four samples during hydropyrolysis. The H2 S evolution peaks of LS raw coal and Dem coal appear at about 500 ◦ C, which are attributed to the decomposition of FeS2 and the unstable organic sulfur. The peaks at about 950 ◦ C are due to the decomposition of FeS and the stable organic sulfur. The effects of Cr on H2 S evolution below 650 ◦ C during hydropyrolysis are similar to those in pyrolysis. Briefly, comparing with the LS raw and Dem coal, the H2 S evolution of Cr-loaded coals is enhanced by the addition of Cr below 250 ◦ C, which is inhibited at 250–650 ◦ C. However, H2 S evolution of the Cr-loaded samples increases sharply above 600 ◦ C, which is much higher than that of LS and Dem coals. The increase may be attributed to two aspects: (i) the decomposition of FeS or Cr7 S8 into H2 S and element Fe or Cr (Fig. 3, in case of H2 ), respectively; (ii) the catalytic decomposition of stable organic sulfur by element Fe and Cr as below:
attributed to the enhancement of mass and/or heat transfer of coal by acid washing [9]. Little difference in the removal of the pyrite sulfur is observed between Dem and Cr-Dem coal, suggesting that Cr has little effect on pyrite decomposition. As the effect of inherent mineral and mass and/or heat transfer on pyrite sulfur removal for Cr-Dem coal is similar to that for Dem, the difference in pyrite removal between both two samples is mainly due to the addition of Cr. The pyrite sulfur removal of Cr-LS coal is slightly higher than that of raw coal, however, much lower than that of Dem coal. For Cr-LS raw coal, Cr benefits the removal of pyrite sulfur by promoting the coal decomposition. It is believed that the impregnation of metal ions can change the structure of coal and promote the pyrolysis of coal [23]. However, the change in the structure of coal caused by added Cr is weaker than that by demineralization.
2R C S + 3H2 → 2H2 S ↑ + 2R CH
(2)
FeS + H2 → H2 S ↑ + Fe
(3)
Cr7 S8 + 8H2 → 8H2 S ↑ + 7Cr
(4)
3.3. Effects of Cr on organic sulfur removal The effects of Cr on organic sulfur removal of Demp coal during pyrolysis and hydropyrolysis are shown in Fig. 8. The results indicate that H2 is favorable for the removal of organic sulfur comparing with N2 . During pyrolysis, the organic sulfur removal of Demp always increases with temperature. With the increase of temperature, the organic sulfur removal from Cr-Demp coal increases significantly and peaks at 600 ◦ C, afterwards the value is decreased.
Strong diffraction peaks of elemental Cr with very weak Cr7 S8 peaks are shown for coke after 1050 ◦ C hydropyrolysis (Fig. 3), which indicates that almost all the Cr7 S8 has been reduced into Cr.
Removal of pyrite sulfur during pyrolysis and hydropyrolysis is shown in Figs. 6 and 7, respectively. Pyrite in raw coal remains stable up to 400 ◦ C under N2 , and then decomposes quickly to generate FeS [22]. In the case of hydropyrolysis, the pyrite removal at 500 ◦ C is much higher than that of pyrolysis, which implies that the decomposing speed of pyrite in H2 is much faster than that in N2 . At 700 ◦ C, nearly 100% pyrite is removed under either atmosphere. The pyrite removal of Dem coal is higher than that of raw coal at the same temperature below 700 ◦ C in both atmospheres, it is indicated that demineralization promotes the removal of pyrite sulfur. As described above, the inherent minerals have little effects on the removal of pyrite sulfur during coal pyrolysis [10], thus the promotion of pyrite removal from Dem coal by demineralization is mainly
Pyrite sulfur removal (wt%)
3.2. Effects of Cr on pyrite sulfur removal
100
Cr-Dem
Dem
80
LS Cr-LS 60
40
LS Cr-LS Dem Cr-Dem
20
400
500
600 o
Temperature ( c) Fig. 7. Pyrite sulfur removal of four samples during hydropyrolysis.
700
J. Huang et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 143–148
147
Table 2 Sulfur forms analysis of coal/coke surface by XPS (wt%).
N2
H2
Sample
Demp
Temperature, ◦ C
80
400
700
1050
Cr-Demp 80
400
700
1050
St Ssulfide So St Ssulfide So
1.43 – 1.43 1.43 – 1.43
1.19 – 1.19 1.18 – 1.18
1.17 – 1.17 0.86 – 0.86
1.36 – 1.36 0.6 – 0.6
1.46 – 1.46 1.46 – 1.46
1.76 0.23 1.53 0.99 0.06 0.93
2.58 0.83 1.75 3.00 1.36 1.74
2.38 0.67 1.71 0.39 – 0.39
Comparing with Demp coal, the organic sulfur removal of Cr-Demp coal is higher below 600 ◦ C, however and is lower above 700 ◦ C. On the one hand, as a typical transition metal, Cr can promote the decomposition of organic sulfur removal during pyrolysis and hydropyrolysis. On the other hand, Cr suppresses the formation of new organic sulfur by reacting with H2 S. Zhou et al. [24] believed that additive mineral matters reacted with H2 S, so as to suppress the reaction of which with coal matrix to form stable sulfur compounds. That’s why the organic sulfur removal of Cr-Demp coal is increased below 600 ◦ C during pyrolysis. Unfortunately, Cr will also react with H2 S to give Cr7 S8 (Fig. 3), which will be further converted into new stable organic sulfur. These newly introduced species is even more difficult to be removed than the inherent organic sulfur. The above phenomenon has also been observed by Cernic-Simic [25], and thus can explain the decrease of the organic sulfur removal of Cr-Demp coal after 600 ◦ C. During hydropyrolysis, with the increase of temperature, the organic sulfur removal always increases and is enhanced by the addition of Cr. Comparing with Demp coal, the higher organic sulfur St , wt% =
40
Demp-N2
Organic sulfur removal (wt%)
Cr-Demp-N2 Demp-H2 Cr-Demp-H2
20
10
0
400
600
3.4. XPS study on sulfur removal in coal surface X-ray photoelectron spectroscopy (XPS) is employed to analyze the different sulfur components on the coal surface quantitatively. The signal of a sulfur single species was composed by two peaks representing 2p3/2 and 2p1/2 components having a 2:1 relative intensity and separated in energy by 1.2 eV. The limitation of this technique is that it determines 2p signals from sulfur atoms situated only on sample surface. In this work, XPS spectra were interpreted using a curve resolution method. The peak of S2p is synthesized by mixed Gaussian and Lorentzian line shapes for each sulfur species, and the assignments of the sulfur forms were based on reference data [26,27]. The elemental composition, total sulfur (St ) and sulfur forms on the surface of coal/coke are calculated as follows: C, at% + O, at% + S, at% + N, at% + Cr, at% + Cl, at% = 100 at%
Ms × S, at% Mc × C, at% + Ms × S, at% + Mo × O, at% + Mn × N, at% + MCr × Cr, at% + MCl × Cl, at%
removal of Cr-Demp coals below 600 ◦ C is due to the promotion to organic sulfur decomposition as well as the inhibition to the formation of new organic sulfur by Cr. Moreover, the organic sulfur within Cr-Demp coal is prevented from being removed at 600–700 ◦ C, due to the introduction of stable organic sulfur converted from Cr7 S8 under H2 at 700 ◦ C. Cr7 S8 is reduced by H2 above 700 ◦ C and thus inhibits the conversion from inorganic sulfur to organic sulfur. Furthermore, hydrodesulfurization of the organic sulfur is accelerated by the catalytic effect of elemental Cr in H2 atmosphere at high
30
temperature, so the organic sulfur removal of Cr-Demp coal is continuously increased.
800
1000
o
Temperature ( c) Fig. 8. Organic sulfur removal of Demp and Cr-Demp coals during pyrolysis and hydropyrolysis.
Ssulfur forms , wt% =
A
sulfur forms
Atotal sulfur
× St , wt%
(5) (6) (7)
whereas M is atomic mass of the each element, at% is the atomic percentage of the each element. A is the integrated peak area of the XPS spectra of sulfur. As discussed above, Cr has no effect on the removal of pyrite sulfur. In order to figure out the effects of Cr on the sulfur removal accurately, Demp coal, which has been pre-treated by removing other minerals and FeS2 , was selected for XPS analysis. The relative contents of various sulfur forms on the sample surface by XPS are summarized in Table 2. With the increase of temperature during pyrolysis, the total sulfur content on Demp coal surface decreases initially to a minimum value of 700 ◦ C, followed by a subsequent increase. It is known that almost all the desulfurization reactions occur below 700 ◦ C (Section 3.1), the increase of total sulfur content on Demp coke surface after 700 ◦ C can be attributed to the transfer of sulfur from bulk to surface. As reported by Liu et al. [28], this is believed to be an important reason for a higher S/C ratio on surface than in the bulk. Conversely, with the increase of temperature during pyrolysis, the contents of total sulfur, sulfide and organic sulfur on the surface of Cr-Demp coke increases with a maximum value at 700 ◦ C, followed by a slight increase. This suggests that the sulfur is enriched on the coke surface by Cr, especially at 700 ◦ C. Therefore, the increase in total sulfur on the surface of Cr-Demp coke is mainly due to the formation of Cr7 S8 , as well as the transfer of sulfur. During hydropyrolysis, with the increase of temperature, the total sulfur on Demp coke surface decreases continuously. However, the total sulfur, sulfide and organic sulfur on the surface of Cr-Demp coke decrease initially followed by an increase to reach a peak at 700 ◦ C, and then the value drops again. Although small amount of Cr7 S8 is formed at 400 ◦ C, the total sulfur and organic
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sulfur on the surface of Cr-Demp coke are lower than that of Demp coke, as the decomposition of organic sulfur is promoted by Cr at low temperature. At 700 ◦ C, due to the formation of considerable amount of Cr7 S8 (1.36 wt%) and the increase of the organic sulfur, the total sulfur in Cr-Demp coke surface is higher than that of Demp coke. At 1050 ◦ C, because of the reduction of Cr7 S8 and organic sulfur by H2 and the catalytic hydrodesulfurization of organic sulfur by Cr, the total sulfur content in Cr-Demp coke surface declines sharply and is lower than that on Demp coke surface. The above results are in consistent with the results as shown in Fig. 4. It is interesting to find that, at 700 ◦ C the contents of Cr7 S8 and organic sulfur on Cr-Demp coke surface during hydropyrolysis are higher than that during pyrolysis. It is well known that comparing with N2 , the decomposition of organic sulfur is promoted by H2 to generate abundant H2 S. Therefore, during hydropyrolysis of CrDemp coal, more Cr7 S8 and new organic sulfur are formed when H2 S escapes from the bulk to surface of coal. 4. Conclusions Cr has no effect on the decomposition of pyrite. At low temperature, Cr enhances the total sulfur removal by promoting the decomposition of the unstable organic sulfur during pyrolysis and hydropyrolysis. However, the total sulfur removal and the H2 S evolution of Cr-loaded coal decreases significantly at 400–700 ◦ C even under H2 , as Cr reacts with H2 S to give Cr7 S8 to retain in coke. Above 700 ◦ C, the total sulfur removal from all coals changes little during pyrolysis. But more organic sulfur is retained Cr-loaded coals comparing with those without Cr, due to the conversion from Cr7 S8 to organic sulfur. During hydropyrolysis, Cr7 S8 is reduced to element Cr which promotes the further decomposition of organic sulfur under high temperature and reducing atmospheres, so both the total sulfur removal and H2 S evolution are improved by Crloading. The XPS results indicate that the sulfur is enriched on the coke surface by forming Cr7 S8 and sulfur transferring during pyrolysis and hydropyrolysis. This sulfur enrichment on the surface of Cr-Demp by Cr is more apparent for hydropyrolysis rather than pyrolysis. The results from chemical method are further correlated with the XPS result, so as to provide insightful science for the sulfur removal from coal, and thus benefit the industrial application of high-sulfur coking. Acknowledgement The authors thank National Basic Research Program of China (2011CB201401) for financial support. References [1] W.H. Calkins, The chemical forms of sulfur in coal: a review, Fuel 73 (1994) 475–484.
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