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Transformation and regulation of sulfur during pyrolysis of coal blend with high organic-sulfur fat coal
Fuel 249 (2019) 427–433
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Full Length Article
Transformation and regulation of sulfur during pyrolysis of coal blend with high organic-sulfur fat coal ⁎
T
⁎
Yanfeng Shena, Meijun Wanga,b, , Yongfeng Huc, Jiao Konga, Jiancheng Wanga, Liping Changa, , Weiren Baoa a
Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, PR China Shanxi Coking Coal Group Co. LTD, Taiyuan 030024, PR China c Canadian Light Source, Saskatoon, SK S7N 2V3, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: High organic-sulfur coal Sulfur K-edge XANES Sulfur transformation Sulfur regulation Interaction
Sulfur transformation and distribution during pyrolysis of an industrial coking coal blend with a high organicsulfur fat coal was studied in a fixed-bed reactor. Sulfur K-edge X-ray absorption spectroscopy was used to study the sulfur speciation in cokes. Two coals with high volatiles were selected to regulate the sulfur transformation behavior during pyrolysis and the interactions between volatile matters and nascent cokes were investigated. The addition of high organic-sulfur fat coal to coal blend resulted in the increase of sulfur content in coke, but the desulfurization rate also increased during coal blend pyrolysis. Higher amount of sulfur-containing radicals in volatile matters from high organic-sulfur fat coal promoted the interactions with nascent coke and resulted in the higher sulfur retention on the coke surface. Adding high volatile coal into coal blend affected the interactions between different coals and the sulfur transformation. The interactions between sulfur-containing radicals and nascent coke could be inhibited by volatile matters from high volatile coals. Further characterizations confirmed that the volatile matters from high volatile coal, which had a more overlapping temperature range to sulfur release in coal blend, leading to the lower sulfur content in coke, higher desulfurization and weaker S K-edge spectral intensity. The interactions between external volatile matters and nascent coke primarily occurred on the coke outer surface.
1. Introduction In the metallurgical industry, coke plays an important role and its strength is primarily dependent on the original coking coals. The distribution of coking coal reserves in China is quite disproportional and high-quality coking coals such as fat coal and prime coking coal only account for a small fraction. With the massive consumption of highquality coking coals in the past decades, these rare coal reserves have been depleted rapidly. On the other hand, high quality coke is required for the production of high standard iron and steel in the large-scale development of iron-making blast furnace, which is directly dependent on coking coals. In order to overcome the shortage of high quality coking coal reserves and meet the demand of metallurgical coke, and since much of the high-quality coking coal with low sulfur content has been exploited and consumed, it becomes increasingly necessary to investigate and utilize coking coal reserves with high sulfur content, which accounts for a high proportion in the coking coal reserves. These
high sulfur coking coals, such as the high organic-sulfur fat coal and prime coking coal, are mostly rich with organic sulfur. In China, based on the current price gap between high sulfur coking coal (Sd ＞ 1.50%) and low sulfur coking coal (Sd < 1.00%), for a coking plant with coke production of 3 million ton per year, replacing the low sulfur coking coal in coal blend with 1.00% of high sulfur coking coal (2.01% < Sd < 2.50%), the coking cost could be reduced by 5.61 million RMB. Therefore, investigating these relatively abundant high organic-sulfur coking coals for the coke-making industry will not only be a solution for the coke demand, but also improve the utilization of coking coal resources and reduce the coke-making cost. The sulfur retained in coke mostly comes from the raw coal during pyrolysis process. High sulfur content in coke is harmful for the metallurgical industry and resulting in the brittle fracture of iron and steel. So it is vital to remove the sulfur in coal during the pyrolysis process, especially when high sulfur coals are used. The inorganic sulfur in coal can be mostly removed during the physical pre-process of coke-making
⁎ Corresponding authors at: Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, PR China. E-mail addresses: [email protected] (M. Wang), [email protected] (L. Chang).
https://doi.org/10.1016/j.fuel.2019.03.066 Received 2 December 2018; Received in revised form 9 March 2019; Accepted 12 March 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Analytical data of coal samples. Sample
HVC1 HVC2 FC1 FC2 BC1 BC2
Proximate analysis/wt%
Ultimate analysis/wt%
Sulfur form/wt%, d *
G
Mad
Ad
Vdaf
Cdaf
Hdaf
Ndaf
Sd
O
Ss
Sp
So*
9.97 1.56 0.52 0.19 0.60 0.56
8.44 5.42 9.71 9.14 9.42 9.36
44.63 36.52 26.11 28.48 22.63 22.87
73.58 85.16 89.86 83.35 90.71 90.06
5.35 5.24 4.98 4.84 4.78 4.76
1.63 1.11 1.56 1.35 1.60 1.58
0.40 0.37 1.02 3.88 0.73 1.01
19.00 8.11 2.48 6.19 2.10 2.49
0.04 0.04 0.02 0.07 0.03 0.03
0.02 0.14 0.08 0.07 0.08 0.08
0.34 0.19 0.92 3.74 0.62 0.90
– 60.00 93.00 100.00 83.00 85.00
Note: ad is air dried basis; d is dried basis; daf is dry and ash free basis; Sp is pyritic sulfur; Ss is sulfate sulfur; So is organic sulfur. * By difference.
effect of external volatile matters on sulfur transformation during pyrolysis of coal blend with high organic-sulfur fat coal. Several experiments were also conducted to illuminate the interactions between volatile matters and cokes. The sulfur-containing gases released during pyrolysis were analyzed by mass spectrometer (MS). X-ray absorption near edge structure (XANES) at S K-edge was used to study the sulfur speciation in cokes [16–25]. The findings in this work could provide useful industrial guidance and application basis for the efficient utilization of high organic-sulfur coal in coking industry.
industry, while the organic sulfur is mostly associated with the coal matrix. Although some chemical processes can partially remove organic sulfur in coal, this will lead to the change of caking and coking property of coking coals [1–6]. Hence, for the utilization of high organic-sulfur coking coals, it is essential to study the transformation and regulation of organic sulfur during pyrolysis. Many researchers have studied the sulfur transformation behavior and desulfurization during coal pyrolysis. The influencing factors, such as coal property, pyrolysis conditions (temperature, pressure, and atmospheres) and mineral matters in coal, have been extensively investigated. Different coal properties play different roles in sulfur transformation, and sulfur form in coal is one of the important properties [7–13]. For organic sulfur in coal, the unstable aliphatic sulfur can decompose at lower temperature and the stable aromatic sulfur species such as thiophene won’t decompose even at quite high temperature. Yan et al. [14] have reported that SH radicals are critical intermediate products during organic sulfur transformation, and they come from the cleavages of Cal-S and Car-S bonds which undergo secondary reactions with the char to form various sulfur compounds. Physical pretreatment process is not very effective in removing high organic sulfur from coking coal, thus when high organic-sulfur coking coal is used in the coking process, the desulfurization rate will be quite limited. Therefore, beyond changing other pyrolysis conditions, regulating the sulfur transformation behavior during coal pyrolysis is the most feasible way to improve the coke quality. Based on previous studies, two main essential requirements are needed to realize this regulation. One is that there should be more decompositions of different sulfur forms in coal, the other is to have enough active hydrogen radicals as hydrogen donor to catalyze the cleavage of C–S/C–C bonds and bond with formed sulfur radicals [15–17]. Adding high organicsulfur coking coal in coal blend is likely to cause the insufficient amount of hydrogen-donor radicals (to catalyze the cleavage of some sulfur bonds and react with sulfur radicals) in the pyrolysis system, which will result in more inter-conversions of sulfur radicals and retention of sulfur in the coke, and finally, increasing the sulfur content in coke. So it is critical to promote the decomposition of organic sulfur and the coupling of hydrogen and sulfur radicals, in order to effectively utilize high organic-sulfur coking coal. Commonly, high volatile coal can generate extensive active hydrogen radicals during the pyrolysis process. Thus, it is feasible to regulate the sulfur transformation behavior during coal blend pyrolysis by combining high volatile coal with high organic-sulfur coking coal; and ultimately, promoting more sulfur release to the gas phase (which can be removed by the high efficient gas desulfurization process) and decreasing the sulfur content in coke to meet the coke quality criterion. In this work, an industrial coking coal blend was selected as the basic sample, a high organic-sulfur fat coal was used to replace the low sulfur fat coal in the coal blend and subjected to fixed-bed pyrolysis. The only difference between high sulfur fat coal and low sulfur fat coal will be the sulfur content, while other coal properties are similar. Two high volatile coals were added into the coal blend in order to study the
2. Experimental 2.1. Materials 11 coking coals from a well-performing coking plant in Shanxi Province was selected as the experimental samples. These 11 coals consist of two fat coals (FC), seven prime coking coals (CC), one 1/3 coking coal (1/3CC) and one lean coal (LC). A basic coal blend (denoted as BC1) was mixed by the 11 coals with a certain ratio. To study the influence of high sulfur and high volatile coals, a high sulfur fat coal (FC2) was selected from Shanxi province, one low sulfur fat coal of the original 11 coals, denoted as FC1, will be partially replaced by FC2 in coal blend; two high volatile coals, HVC1 (a long flame coal) and HVC2 (a gas coal), were selected from Xinjiang province and Shanxi province, respectively. All these were cleaned coals and were preserved in a brown container after ground and sieved into particle sizes ranging from 0.15 mm to 0.25 mm. The proximate and ultimate analyses, sulfur form analyses and caking index (G) of coal samples are listed in Table 1. 2.2. Pyrolysis apparatus and methods The pyrolysis experiments were conducted in a fixed-bed furnace with a quartz reactor. Fig. 1 shows the schematic diagram of the fixedbed apparatus used for coal pyrolysis in this study. The sintered quartz plates in the quartz reactor were all in the constant temperature zone of the furnace. The samples were heated from room temperature to 1000 °C at a heating rate of 10 °C/min under high-purity Ar (＞ 99.999%), and the total flow rate of carrier gas was 400 mL/min (by mass flowmeter with ± 0.2% repeatability). The volatile matters released during pyrolysis process firstly went through ice water bath and dry ice bath to condense tar and the gases were introduced to MS (Hiden QIC-20, the detection limit is 0.1 ppm) for further analysis. After reaching the final pyrolysis temperature, the quartz reactor was lifted out of the furnace under the protection of Ar and cooled to room temperature, then the cokes were collected and sealed for further analysis. Firstly, BC1 was pyrolyzed in the fixed-bed reactor, the total mass was 3 g ( ± 0.001 g). Then FC2 was added into BC1 by replacing the low sulfur fat coal (FC1, one of the 11 coals in the original coal blend (BC1)) with the same proportion, the replacing ratios were 3%, 5%, 10% and the resulting coal blend were denoted as 3%FC2-FC1, 5%FC2428
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Fig. 1. Schematic diagram of fixed-bed pyrolysis apparatus.
conditions were described elsewhere [16–18]. In this study, the samples were detected in the bulk sensitive fluorescence mode (FY) and the surface sensitive total electron yield (TEY), and the energy scale was calibrated using the Ar K-edge in the upstream ion chamber and a sulfate peak at 2481.6 eV. The accuracy of energies reported here is 0.1 eV. A series of model sulfur compounds were selected as the “references” to ascertain the sulfur forms in S K-edge spectra. Least squares method was chosen to fit the S K-edge spectra, which deconvoluted the experimental spectrum into several Gaussian peaks and one or two arctangent step functions [16–25]. Here the S K-edge spectra were fitted into 7 or 8 peaks representing FeS, FeS2, sulfide, thiophene, sulfoxide, sulfone, sulfonate and sulfate, based on the peak positions described in [16].
FC1 and 10%FC2-FC1, respectively. Secondly, 10%FC2-FC1 was selected as a coal blend with high sulfur fat coal (denoted as BC2), two high volatile coals (HVC1 and HVC2) were added into BC2 with ratios of 10%, 20% and 30%. The coal blend with HVC1 were denoted as 10% HVC1-BC2, 20%HVC1-BC2, 30%HVC1-BC2, and similarly indicated for coals added with HVC2. In these two series of experiments, coal blends, such as BC1, coal blend with FC2, coal blend with HVC1/HVC2, were placed on the upper plate. Thirdly, several experiments, in order to further study the influence of external volatile matters on sulfur transformation, were carried out in the fixed-bed reactor. In brief, HVC1 (HVC2) was placed on the lower sintered quartz plate and BC2 on the upper plate, the volatile matters released from HVC1 (HVC2) would flow with the carrier gas from bottom to top and react with BC2. Three ratios (1:2, 1:1, 2:1) of HVC1 (HVC2) to BC2 were selected.
3. Results and discussion
2.3. Total sulfur analysis
3.1. Sulfur transformation during pyrolysis of coal blend with FC2
The total sulfur content in coke was measured by HCS-140 High Frequency Infrared Carbon and Sulfur Analyzer (Shanghai Dekai Instrument Co. Ltd.), the detection error is 0.01%. The calculated sulfur content in coal blend coke was obtained by following equation:
S=
∑ Si × Yi × Pi ∑ Yi × Pi
It is necessary to first determine the sulfur forms in the two fat coals to investigate the sulfur transformation during pyrolysis of coal blend with high sulfur fat coal. The S K-edge spectra fitting results of FC1 and FC2 are listed in Table 2. The dominant sulfur species in the two coals are sulfide-S, thiophene-S and sulfoxide-S, with the five organic sulfur species accounting for about 87.07% for FC1 and 93.33% for FC2 in FY mode, 86.78% for FC1 and 93.61% for FC2 in TEY mode. We note that the TEY mode at the S K-edge is sensitive to the surface species, since only the surface electrons can escape and be detected, while FY mode is more bulk sensitive because of the high penetration of fluorescence (a few hundred nms). Compared to the sulfide-S, the thiophene-S in FY mode has a higher percentage than that in TEY mode, which indicates that the sulfur on coal surface is much easier to be removed than that in bulk coal. The above results show that the sulfur species in the two fat coals are also quite similar, the difference is mainly reflected in the total sulfur content (Table 1). The pyrolysis of BC1 and coal blend with FC2 are firstly carried out, the resulting sulfur content in coke and desulfurization rate are shown in Fig. 2. It can be seen that the sulfur content in coke increases with increasing ratio of FC2, because of the higher percentage of sulfur in FC2. Nevertheless, the desulfurization rate also increases with increasing FC2. This can be attributed to the increase of interactions
(1)
where S is the calculated sulfur content in coal blend coke (%); Si is the sulfur content in coke i (%); Yi is the coke yield of coal i (%); Pi is the proportion of coal i in coal blend (%). The desulfurization rate was obtained by following equation:
D = 100 −
Scoke × Y Scoal
(2)
where D is the desulfurization rate (%); Scoal is the sulfur content in coal (%); Scoke is the sulfur content in coke (%); Y is the coke yield (%). 2.4. Sulfur K-edge XANES Sulfur K-edge XANES analyses of coal and coke samples were performed at the Canadian Light Source using the Soft X-ray Micro characterization Beamline (SXRMB), and the detailed procedures and 429
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Table 2 Distribution of sulfur forms in total sulfur for FC1 and FC2 by S K-edge spectra. Mode
Sample
Percentage of sulfur form (%) FeS2
Sulfide
Thiophene
Sulfoxide
Sulfone
Sulfonate
Sulfate
FY
FC1 FC2
3.52 2.46
14.40 13.79
51.25 55.08
13.08 14.96
5.66 6.04
2.68 3.46
9.41 4.21
TEY
FC1 FC2
3.74 2.30
15.00 16.65
46.91 52.37
16.45 15.33
6.12 4.67
2.30 4.58
9.48 4.09
between different coals as a result of the increase in sulfur amount in coal blend. Fig. 3 shows the H2S and COS release curves during pyrolysis of coal blend with FC2. It can be seen that the release profiles of H2S and COS of different coal blend are similar, but the release amount increases with increasing ratio of FC2. On one hand, the sulfur content in FC2 is nearly four times in FC1, and with the exception of thiopheneS, there are still some unstable organic sulfur in FC2 (Table 2) which could be more easily decomposed during pyrolysis. Thus with the addition of FC2, the decomposition of unstable organic sulfur resulted in the increased amount of sulfur-containing gases. On the other hand, the interactions between high sulfur fat coal and other coals could also have some effects on the sulfur transformation after the addition of FC2. Fig. 4(a) shows the S K-edge spectra of coal blend cokes with FC2. It can be seen that there are clear differences between the spectra of coal blend cokes with FC2 and that of the BC1 coke. This could be due to the transformation and redistribution of sulfur at the surface of cokes during pyrolysis. When FC2, which has more organic sulfur, is added
Fig. 2. Sulfur content in coke and desulfurization rate during pyrolysis of coal blend with FC2.
Fig. 3. H2S and COS release curves during pyrolysis of coal blend with FC2.
Fig. 4. S K-edge spectra of coal blend cokes with FC2 (a), HVC1 or HVC2 (b). 430
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Table 3 Contents of sulfur forms in coal blend cokes (TEY mode). Sample
BC1 3%FC2-FC1 5%FC2-FC1 10%FC2-FC1 (BC2) 10%HVC1-BC2 20%HVC1-BC2 30%HVC1-BC2 10%HVC2-BC2 20%HVC2-BC2 30%HVC2-BC2
Total sulfur (%)
0.62 0.71 0.76 0.85 0.82 0.78 0.74 0.80 0.75 0.71
Content of sulfur form (%) FeS
Sulfide
Thiophene
Sulfoxide
Sulfone
Sulfate
0.03 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.02 0.00
0.09 0.15 0.18 0.23 0.23 0.24 0.25 0.25 0.26 0.29
0.17 0.21 0.23 0.29 0.26 0.22 0.19 0.24 0.19 0.13
0.17 0.18 0.19 0.18 0.15 0.14 0.10 0.18 0.15 0.14
0.02 0.02 0.04 0.04 0.03 0.03 0.05 0.04 0.05 0.03
0.13 0.14 0.10 0.08 0.12 0.11 0.11 0.06 0.08 0.12
into coal blend, the amount of sulfur-containing radicals in volatile matters increases, the interactions between volatile matters and nascent coke further lead to higher percentages of sulfide and thiophene on the surface of coal blend cokes than that of BC1 coke [17,26]. It is also noteworthy that the percentages of these two sulfur species increase with increasing ratio of FC2 (Table 3). The above results show that the sulfur content in coke increases inevitably while FC2 is added into coal blend, but the desulfurization rate also increases. Meanwhile, there is more sulfur transformation at coke surfaces due to the interactions between volatile matters and nascent coke. Therefore, it provides a feasible way to regulate the sulfur transformation behavior through inhibiting the interactions between sulfur-containing radicals and nascent coke on the coke outer surface during pyrolysis of coal blend with FC2. 3.2. Effect of external volatile matters on sulfur transformation 3.2.1. Selection of high volatile coals Adding high sulfur coal in coal blend could result in the increase of sulfur content in coke, so it is necessary to regulate the transformation of sulfur species to meet the coke quality requirement. As stated previously, two conditions must be met in order to achieve the directional regulation of sulfur transformation and ultimately decrease the sulfur content in coke: different sulfur species in coal should be more decomposed and enough active hydrogen radicals are donated to form SH radicals. Indeed, the volatile matters generated during coal pyrolysis process contain lots of active hydrogen-containing radicals, these radicals could induce the cleavage of sulfur-containing groups and react with formed sulfur radicals to release as sulfur-containing gases and further reduce the sulfur content in coke. Therefore, it should be feasible to add some coals with higher volatile content and high sulfur coal to the coal blend to tune the sulfur transformation behavior. A wide release temperature range of hydrogen-containing gases and low total sulfur content are desirable when selecting high volatile coals. Therefore, two high volatile coals (HVC1 and HVC2) were selected, both containing high volatile contents (higher than 36%, Table 1) and low sulfur contents (less than or equal to 0.40%, Table 1). Fig. 5 shows the release temperature ranges of CH4 and H2 for HVC1 and HVC2, and release temperature range of H2S for BC2. It can be seen that the release temperature ranges of CH4 and H2 are both wider than that of H2S, with the range of H2S for BC2 more similar to that of CH4, but rather different from that of H2. CH4 is mostly formed from the cleavage of aliphatic side chains and plenty of hydrogen-containing radicals will be generated during this process. These radicals must have some effects on the H2S release in this temperature range. While H2 is normally generated from the condensation of aromatic compounds at higher temperature, so when H2 starts to release, most of H2S will no longer be generated and the sulfur distribution in coke mainly undergoes interconversions [16,17]. In addition, once these hydrogen radicals are formed in the higher temperature range, it would be easier for
Fig. 5. Correlation between CH4/ H2 and H2S release curves.
hydrogen radicals to bond with each other to form H2, instead of bonding with other radicals. Thus, wider release temperature range of CH4 is more beneficial for regulating the sulfur transformation. 3.2.2. Sulfur transformation under the influence of high volatile coals The sulfur content in coke and desulfurization rate during pyrolysis of coal blend with HVC1 and HVC2, together with the calculated values of sulfur content in cokes, are shown in Fig. 6. It can be seen that the sulfur content in coke decreases with increasing ratio of high volatile coal. Since the sulfur content in these two high volatile coals is relatively low, the sulfur percentage in coal blend is expected to decrease with the increasing ratios of these coals. By comparing the sulfur content in coke between experimental and calculated values, the former value of sulfur content is clearly lower, and with increasing ratio of high volatile coal, the desulfurization rate also increases. This indicates that adding high volatile coals in coal blend not only causes additive effects, it also has an effect on the interactions between different coals. In addition, it can be noted that adding HVC2 could result a much more 431
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Fig. 6. Sulfur content in coke and desulfurization rate during pyrolysis of coal blend with HVC1 or HVC2.
reduced sulfur content in coke, which can be attributed to the more overlapping temperature range of the CH4 release with that of the H2S for BC2. The effect of high volatile coals in BC2 can also be studied by the S K-edge spectra of coal blend cokes. From Fig. 4(b), the normalized peak area of coal blend cokes with HVC1 and HVC2 is generally lower than that of BC2 coke, and the peak intensities of coal blend cokes with HVC2 are weaker than that of HVC1 added ones. Compared to the spectrum of BC2 coke, it is also clear that the percentage of thiophene S is reduced for HVC1 and HVC2, as indicated by the reduced peak intensities around 2475 and 2479 eV, which are part of thiophene features [16,20]. From the above results (Fig. 4(a)), more noticeable changes are also observed in the TEY spectra of coal blend cokes with the addition of high sulfur fat coal. So it is obvious that the interactions between sulfur-containing radicals (generated from high sulfur coal) and nascent coke are inhibited by adding high volatile coals. During pyrolysis of coal blend with high volatile coals, more hydrogen radicals will be generated from high volatile coals, by catalyzing the cleavage of some sulfur bonds and reacting with the formed sulfur radicals on coke surface. As a result, more sulfur species can be released as sulfur-containing gases and the sulfur content in coke will decrease. As for the differences between coal blend cokes with HVC1 and HVC2, it can be seen that the higher volatile content in high volatile coal is not better for the removal of sulfur in coke (Fig. 6). For the lower rank HVC1, the release of volatile matters and the generation of plenty of hydrogencontaining radicals occur at a lower temperature range. While the decomposition temperature of HVC2 is relatively higher (Fig. 5), the release temperature range for HVC2 to generate hydrogen radicals overlaps (better than HVC1) with the temperature range for BC2 to produce sulfur radicals. Thus, more active hydrogen radicals in coal blend with HVC2 can react with the generated sulfur radicals. For coal blend with HVC1, the untimely formed hydrogen radicals will be likely flushed out by carrier gas or reacted with other active radicals, resulting in the insufficient reactions between hydrogen radicals and sulfur radicals (Table 3). Comparing with the sulfur forms in BC2 coke, the sulfide content on coke surface increases with increasing ratio of high volatile coal, while the percentage of thiophene has an opposite trend (Fig. 4(b) and Table 3). This indicates that the volatile matters from high volatile coal participate in the interactions with cokes and regulate the sulfur transformation behavior, which inhibit the transfer of sulfur into thiophene-S, and promote the release of sulfur species as sulfurcontaining gases or retain as sulfide-S on coke surface. Consequently, the relatively stronger interactions between volatile matters in HVC2 and coke on the surface lead to the lower sulfur content in coke, higher desulfurization rate, and weaker S K-edge spectra peak intensity.
Fig. 7. S K-edge spectra of BC2 cokes.
3.3. Interactions between external volatile matters and BC2 coke To further investigate the effects of volatile matters in HVC1 and HVC2 on the sulfur forms of BC2 coke, we carried out more experiments in order to probe the influence of external volatile matters on BC coke. As stated in the experimental section, the samples of HVC1 or HVC2 and BC2 with mass ratios of 1:2, 1:1, 2:1 were selected and placed respectively on the lower and upper sintered quartz plates. Fig. 7 shows the S K-edge spectra of BC2 cokes obtained from these experiments. It can be seen that all the S K-edge spectra relative intensities (2473 eV) of cokes with external volatiles are obviously weaker than the BC2 coke; and similar to Fig. 4(b), the thiophene peak (2473 eV) is also weaker compared to that of the BC2 coke. This indicates that the interactions between volatile matters and BC2 coke mostly occur on the outer BC2 coke surface, or the degree of interactions in bulk coke is limited. This may be attributed to two reasons: one is that the volatile matters from HVC1 and HVC2 may pass through the BC2 coke with the carrier gas at relatively high speed and there is not enough time for interaction; the other possible reason is that the pyrolysis of HVC1, HVC2 and BC2 almost proceed simultaneously and when the volatile matters from the lower plate reach the upper plate, the volatile matters release from BC2 also diffuse from the inner side to outer side and inhibit the volatiles of HVC1 and HVC2 from permeating into the BC2 coke. In addition, since the BC2 is used as a metallurgical coke and has strong caking ability, the plastic mass, which has poor gas permeability, is formed during its pyrolysis process. Interactions between the plastic mass with other gases, liquids and solids will restrain the volatiles of HVC1 and HVC2 from interacting with inner BC2 coke matrix [27,28]. It can also be seen 432
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Appendix A. Supplementary data
from Fig. 7 that higher addition ratio of HVC2 in coal blend results in the weaker spectral intensity, which can be attributed to the more active volatile matters generated from HVC2 that can interact with more BC2 coke. In this study, relatively higher total sulfur content of high organicsulfur fat coal and its addition in coal blend are selected for more observable changes experimentally. The relatively high additive amount of high volatile coals is also chosen to be more beneficial for illuminating the interactions between volatiles and cokes. However, the addition of high organic-sulfur coal in coal blend should be limited, and a large amount of high volatile coal in coal blend may also lead to the weakening of coke strength. Therefore, based on the results from this study, in order to achieve the efficient utilization of high organic-sulfur coking coal, modification and optimization of the addition ratio of high organic-sulfur coking coal and high volatile coal in coal blend should be further conducted. In addition, the effects of coal particle sizes, mixing methods and other conditions on the regulation of sulfur transformation behavior should also be conducted to further investigate the interactions more profoundly.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.03.066. References [1] Calkins WH. The chemical forms of sulfur in coal: a review. Fuel 1994;73:475–84. [2] Hu HQ, Zhou Q, Zhu SW, Meyer B, Krzack S, Chen GH. Product distribution and sulfur behavior in coal pyrolysis. Fuel Process Technol 2004;85:849–61. [3] Cleyle PJ, Caley WF, Stewart L, Whiteway SG. Decomposition of pyrite and trapping of sulphur in a coal matrix during pyrolysis of coal. Fuel 1984;63:1579–82. [4] Mesroghli S, Yperman J, Jorjani E, Vandewijngaarden J, Reggers G, Carleer R, et al. Changes and removal of different sulfur forms after chemical desulfurization by peroxyacetic acid on microwave treated coals. Fuel 2015;154:59–70. [5] Li ZL, Sun TH, Jia JP. An extremely rapid, convenient and mild coal desulfurization new process: Sodium borohydride reduction. Fuel Process Technol 2010;91:1162–7. [6] Chou CL. Sulfur in coals: a review of geochemistry and origins. Int J Coal Geol 2012;100:1–13. [7] Chen HK, Li BQ, Yang JL, Zhang BJ. Transformation of sulfur during pyrolysis and hydropyrolysis of coal. Fuel 1998;77:487–93. [8] Chen HK, Li BQ, Zhang BJ. Effects of mineral matter on products and sulfur distributions in hydropyrolysis. Fuel 1999;78:713–9. [9] Gu Y, Yperman J, Vandewijngaarden J, Reggers G, Carleer R. Organic and inorganic sulphur compounds releases from high-pyrite coal pyrolysis in H2, N2 and CO2: Test case Chinese LZ coal. Fuel 2017;202:494–502. [10] Wang BF, Zhao SG, Huang YR, Zhang JJ. Effect of some natural minerals on transformation behavior of sulfur during pyrolysis of coal and biomass. J Anal Appl Pyrol 2014;105:284–94. [11] Mochizuki Y, Ono Y, Uebo K, Tsubouchi N. The fate of sulfur in coal during carbonization and its effect on coal fluidity. Int J Coal Geol 2013;120:50–6. [12] Liu QR, Hu HQ, Zhou Q, Zhu SW, Chen GH. Effect of mineral on sulfur behavior during pressurized coal pyrolysis. Fuel Process Technol 2004;85:863–71. [13] Ellis N, Masnadi MS, Roberts DG, Kochanek MA, Ilyushechkin AY. Mineral matter interactions during co-pyrolysis of coal and biomass and their impact on intrinsic char co-gasification reactivity. Chem Eng J 2015;279:402–8. [14] Yan JD, Yang JL, Liu ZY. SH radical: The key intermediate in sulfur transformation during thermal processing of coal. Environ Sci Technol 2005;39:5043–51. [15] Zhang YL, Wang MJ, Qin Z, Yang YL, Fu CH, Feng L, et al. Effect of the interactions between volatiles and char on sulfur transformation during brown coal upgrade by pyrolysis. Fuel 2013;103:915–22. [16] Wang MJ, Hu YF, Wang JC, Chang LP, Wang H. Transformation of sulfur during pyrolysis of inertinite-rich coals and correlation with their characteristics. J Anal Appl Pyrol 2013;104:585–92. [17] Wang MJ, Liu LJ, Wang JC, Chang LP, Wang H, Hu YF. Sulfur K-edge XANES study of sulfur transformation during pyrolysis of four coals with different ranks. Fuel Process Technol 2015;131:262–9. [18] Wang MJ, Jia TH, Wang JC, Hu YF, Liu FY, Wang H, et al. Changes of sulfur forms in coal after tetrachloroethylene extraction and theirs transformations during pyrolysis. Fuel 2016;186:726–33. [19] Vairavamurthy A. Using X-ray absorption to probe sulfur oxidation states in complex molecules. Spectrochim Acta Part A Mol Biomol Spectrosc 1998;54:2009–17. [20] Kasrai M, Brown JR, Bancroft GM, Yin Z, Tan KH. Sulphur characterization in coal from X-ray absorption near edge spectroscopy. Int J Coal Geol 1996;32:107–35. [21] Liu LJ, Liu HJ, Cui MQ, Hu YF, Wang J. Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: Transformations of sulfur. Fuel 2013;112:687–94. [22] Huffman GP, Shah N, Huggins FE, Stock LM, Chatterjee K, Kilbane JJ, et al. Sulfur speciation of desulfurized coals by XANES spectroscopy. Fuel 1995;74:549–55. [23] Prietzel J, Botzaki A, Tyufekchieva N, Brettholle M, Thieme J, Klysubun W. Sulfur speciation in soil by S K-Edge XANES spectroscopy: comparison of spectral deconvolution and linear combination fitting. Environ Sci Technol 2011;45:2878–86. [24] Taghiei MM, Huggins FE, Shah N, Huffman GP. In situ X-ray absorption fine structure spectroscopy investigation of sulfur functional groups in coal during pyrolysis and oxidation. Energy Fuels 1992;06:293–300. [25] Ravel B, Newville M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J Synchrotron Radiat 2005;12:537–41. [26] Liu FR, Li W, Chen HK, Li BQ. Uneven distribution of sulfurs and their transformation during coal pyrolysis. Fuel 2007;86:360–6. [27] Sakurovs R. Interactions between coking coals in blends. Fuel 2003;82:439–50. [28] Yu JL, Strezov V, Lucas J, Wall T. Swelling behaviour of individual coal particles in the single particle reactor. Fuel 2003;82:1977–87.
4. Conclusions Sulfur transformation and regulation during pyrolysis of an industrial coking coal blend with high organic-sulfur fat coal and high volatile coal were investigated. Main conclusions can be drawn as follows: The addition of high organic-sulfur fat coal to blend results in the increase of sulfur content in coke, but the desulfurization rate also increases during coal blend pyrolysis. Higher amount of sulfur-containing radicals in volatile matters from high sulfur fat coal promotes the interactions with nascent coke and results in the higher sulfur retention on the coke surface. With the addition of high volatile coals in coal blend, the generated hydrogen radicals inhibit the interactions between sulfur radicals (generated from high sulfur coal) and nascent coke, promote the release of sulfur species as sulfur-containing gases or retain as sulfide-S on coke surface. Further characterizations confirm that the volatile matters from high volatile coal, which have a more overlapping temperature range to sulfur release in coal blend, result in the lower sulfur content in coke, higher desulfurization and weaker S K-edge spectral intensity. The interactions between external volatile matters and nascent coke primarily occur on the coke outer surface. The results in this study indicate that it is feasible to regulate the sulfur transformation during pyrolysis of coal blend with high organic-sulfur coal, by selecting and blending high volatile coal with low sulfur content, wide and suitable release temperature range of CH4.
Acknowledgements The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (21878208, U1510111), China, Shanxi Coal Based Key Scientific and Technological Project (MJH2015-04), China and Research Project Supported by Shanxi Scholarship Council of China (2017-03), China, The S K-edge XANES measurement was carried out at the Canadian Light Source (CLS), Canada. We thank the CLS staff for the technical support.
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Full Length Article
Transformation and regulation of sulfur during pyrolysis of coal blend with high organic-sulfur fat coal ⁎
T
⁎
Yanfeng Shena, Meijun Wanga,b, , Yongfeng Huc, Jiao Konga, Jiancheng Wanga, Liping Changa, , Weiren Baoa a
Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, PR China Shanxi Coking Coal Group Co. LTD, Taiyuan 030024, PR China c Canadian Light Source, Saskatoon, SK S7N 2V3, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: High organic-sulfur coal Sulfur K-edge XANES Sulfur transformation Sulfur regulation Interaction
Sulfur transformation and distribution during pyrolysis of an industrial coking coal blend with a high organicsulfur fat coal was studied in a fixed-bed reactor. Sulfur K-edge X-ray absorption spectroscopy was used to study the sulfur speciation in cokes. Two coals with high volatiles were selected to regulate the sulfur transformation behavior during pyrolysis and the interactions between volatile matters and nascent cokes were investigated. The addition of high organic-sulfur fat coal to coal blend resulted in the increase of sulfur content in coke, but the desulfurization rate also increased during coal blend pyrolysis. Higher amount of sulfur-containing radicals in volatile matters from high organic-sulfur fat coal promoted the interactions with nascent coke and resulted in the higher sulfur retention on the coke surface. Adding high volatile coal into coal blend affected the interactions between different coals and the sulfur transformation. The interactions between sulfur-containing radicals and nascent coke could be inhibited by volatile matters from high volatile coals. Further characterizations confirmed that the volatile matters from high volatile coal, which had a more overlapping temperature range to sulfur release in coal blend, leading to the lower sulfur content in coke, higher desulfurization and weaker S K-edge spectral intensity. The interactions between external volatile matters and nascent coke primarily occurred on the coke outer surface.
1. Introduction In the metallurgical industry, coke plays an important role and its strength is primarily dependent on the original coking coals. The distribution of coking coal reserves in China is quite disproportional and high-quality coking coals such as fat coal and prime coking coal only account for a small fraction. With the massive consumption of highquality coking coals in the past decades, these rare coal reserves have been depleted rapidly. On the other hand, high quality coke is required for the production of high standard iron and steel in the large-scale development of iron-making blast furnace, which is directly dependent on coking coals. In order to overcome the shortage of high quality coking coal reserves and meet the demand of metallurgical coke, and since much of the high-quality coking coal with low sulfur content has been exploited and consumed, it becomes increasingly necessary to investigate and utilize coking coal reserves with high sulfur content, which accounts for a high proportion in the coking coal reserves. These
high sulfur coking coals, such as the high organic-sulfur fat coal and prime coking coal, are mostly rich with organic sulfur. In China, based on the current price gap between high sulfur coking coal (Sd ＞ 1.50%) and low sulfur coking coal (Sd < 1.00%), for a coking plant with coke production of 3 million ton per year, replacing the low sulfur coking coal in coal blend with 1.00% of high sulfur coking coal (2.01% < Sd < 2.50%), the coking cost could be reduced by 5.61 million RMB. Therefore, investigating these relatively abundant high organic-sulfur coking coals for the coke-making industry will not only be a solution for the coke demand, but also improve the utilization of coking coal resources and reduce the coke-making cost. The sulfur retained in coke mostly comes from the raw coal during pyrolysis process. High sulfur content in coke is harmful for the metallurgical industry and resulting in the brittle fracture of iron and steel. So it is vital to remove the sulfur in coal during the pyrolysis process, especially when high sulfur coals are used. The inorganic sulfur in coal can be mostly removed during the physical pre-process of coke-making
⁎ Corresponding authors at: Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, PR China. E-mail addresses: [email protected] (M. Wang), [email protected] (L. Chang).
https://doi.org/10.1016/j.fuel.2019.03.066 Received 2 December 2018; Received in revised form 9 March 2019; Accepted 12 March 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Analytical data of coal samples. Sample
HVC1 HVC2 FC1 FC2 BC1 BC2
Proximate analysis/wt%
Ultimate analysis/wt%
Sulfur form/wt%, d *
G
Mad
Ad
Vdaf
Cdaf
Hdaf
Ndaf
Sd
O
Ss
Sp
So*
9.97 1.56 0.52 0.19 0.60 0.56
8.44 5.42 9.71 9.14 9.42 9.36
44.63 36.52 26.11 28.48 22.63 22.87
73.58 85.16 89.86 83.35 90.71 90.06
5.35 5.24 4.98 4.84 4.78 4.76
1.63 1.11 1.56 1.35 1.60 1.58
0.40 0.37 1.02 3.88 0.73 1.01
19.00 8.11 2.48 6.19 2.10 2.49
0.04 0.04 0.02 0.07 0.03 0.03
0.02 0.14 0.08 0.07 0.08 0.08
0.34 0.19 0.92 3.74 0.62 0.90
– 60.00 93.00 100.00 83.00 85.00
Note: ad is air dried basis; d is dried basis; daf is dry and ash free basis; Sp is pyritic sulfur; Ss is sulfate sulfur; So is organic sulfur. * By difference.
effect of external volatile matters on sulfur transformation during pyrolysis of coal blend with high organic-sulfur fat coal. Several experiments were also conducted to illuminate the interactions between volatile matters and cokes. The sulfur-containing gases released during pyrolysis were analyzed by mass spectrometer (MS). X-ray absorption near edge structure (XANES) at S K-edge was used to study the sulfur speciation in cokes [16–25]. The findings in this work could provide useful industrial guidance and application basis for the efficient utilization of high organic-sulfur coal in coking industry.
industry, while the organic sulfur is mostly associated with the coal matrix. Although some chemical processes can partially remove organic sulfur in coal, this will lead to the change of caking and coking property of coking coals [1–6]. Hence, for the utilization of high organic-sulfur coking coals, it is essential to study the transformation and regulation of organic sulfur during pyrolysis. Many researchers have studied the sulfur transformation behavior and desulfurization during coal pyrolysis. The influencing factors, such as coal property, pyrolysis conditions (temperature, pressure, and atmospheres) and mineral matters in coal, have been extensively investigated. Different coal properties play different roles in sulfur transformation, and sulfur form in coal is one of the important properties [7–13]. For organic sulfur in coal, the unstable aliphatic sulfur can decompose at lower temperature and the stable aromatic sulfur species such as thiophene won’t decompose even at quite high temperature. Yan et al. [14] have reported that SH radicals are critical intermediate products during organic sulfur transformation, and they come from the cleavages of Cal-S and Car-S bonds which undergo secondary reactions with the char to form various sulfur compounds. Physical pretreatment process is not very effective in removing high organic sulfur from coking coal, thus when high organic-sulfur coking coal is used in the coking process, the desulfurization rate will be quite limited. Therefore, beyond changing other pyrolysis conditions, regulating the sulfur transformation behavior during coal pyrolysis is the most feasible way to improve the coke quality. Based on previous studies, two main essential requirements are needed to realize this regulation. One is that there should be more decompositions of different sulfur forms in coal, the other is to have enough active hydrogen radicals as hydrogen donor to catalyze the cleavage of C–S/C–C bonds and bond with formed sulfur radicals [15–17]. Adding high organicsulfur coking coal in coal blend is likely to cause the insufficient amount of hydrogen-donor radicals (to catalyze the cleavage of some sulfur bonds and react with sulfur radicals) in the pyrolysis system, which will result in more inter-conversions of sulfur radicals and retention of sulfur in the coke, and finally, increasing the sulfur content in coke. So it is critical to promote the decomposition of organic sulfur and the coupling of hydrogen and sulfur radicals, in order to effectively utilize high organic-sulfur coking coal. Commonly, high volatile coal can generate extensive active hydrogen radicals during the pyrolysis process. Thus, it is feasible to regulate the sulfur transformation behavior during coal blend pyrolysis by combining high volatile coal with high organic-sulfur coking coal; and ultimately, promoting more sulfur release to the gas phase (which can be removed by the high efficient gas desulfurization process) and decreasing the sulfur content in coke to meet the coke quality criterion. In this work, an industrial coking coal blend was selected as the basic sample, a high organic-sulfur fat coal was used to replace the low sulfur fat coal in the coal blend and subjected to fixed-bed pyrolysis. The only difference between high sulfur fat coal and low sulfur fat coal will be the sulfur content, while other coal properties are similar. Two high volatile coals were added into the coal blend in order to study the
2. Experimental 2.1. Materials 11 coking coals from a well-performing coking plant in Shanxi Province was selected as the experimental samples. These 11 coals consist of two fat coals (FC), seven prime coking coals (CC), one 1/3 coking coal (1/3CC) and one lean coal (LC). A basic coal blend (denoted as BC1) was mixed by the 11 coals with a certain ratio. To study the influence of high sulfur and high volatile coals, a high sulfur fat coal (FC2) was selected from Shanxi province, one low sulfur fat coal of the original 11 coals, denoted as FC1, will be partially replaced by FC2 in coal blend; two high volatile coals, HVC1 (a long flame coal) and HVC2 (a gas coal), were selected from Xinjiang province and Shanxi province, respectively. All these were cleaned coals and were preserved in a brown container after ground and sieved into particle sizes ranging from 0.15 mm to 0.25 mm. The proximate and ultimate analyses, sulfur form analyses and caking index (G) of coal samples are listed in Table 1. 2.2. Pyrolysis apparatus and methods The pyrolysis experiments were conducted in a fixed-bed furnace with a quartz reactor. Fig. 1 shows the schematic diagram of the fixedbed apparatus used for coal pyrolysis in this study. The sintered quartz plates in the quartz reactor were all in the constant temperature zone of the furnace. The samples were heated from room temperature to 1000 °C at a heating rate of 10 °C/min under high-purity Ar (＞ 99.999%), and the total flow rate of carrier gas was 400 mL/min (by mass flowmeter with ± 0.2% repeatability). The volatile matters released during pyrolysis process firstly went through ice water bath and dry ice bath to condense tar and the gases were introduced to MS (Hiden QIC-20, the detection limit is 0.1 ppm) for further analysis. After reaching the final pyrolysis temperature, the quartz reactor was lifted out of the furnace under the protection of Ar and cooled to room temperature, then the cokes were collected and sealed for further analysis. Firstly, BC1 was pyrolyzed in the fixed-bed reactor, the total mass was 3 g ( ± 0.001 g). Then FC2 was added into BC1 by replacing the low sulfur fat coal (FC1, one of the 11 coals in the original coal blend (BC1)) with the same proportion, the replacing ratios were 3%, 5%, 10% and the resulting coal blend were denoted as 3%FC2-FC1, 5%FC2428
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Fig. 1. Schematic diagram of fixed-bed pyrolysis apparatus.
conditions were described elsewhere [16–18]. In this study, the samples were detected in the bulk sensitive fluorescence mode (FY) and the surface sensitive total electron yield (TEY), and the energy scale was calibrated using the Ar K-edge in the upstream ion chamber and a sulfate peak at 2481.6 eV. The accuracy of energies reported here is 0.1 eV. A series of model sulfur compounds were selected as the “references” to ascertain the sulfur forms in S K-edge spectra. Least squares method was chosen to fit the S K-edge spectra, which deconvoluted the experimental spectrum into several Gaussian peaks and one or two arctangent step functions [16–25]. Here the S K-edge spectra were fitted into 7 or 8 peaks representing FeS, FeS2, sulfide, thiophene, sulfoxide, sulfone, sulfonate and sulfate, based on the peak positions described in [16].
FC1 and 10%FC2-FC1, respectively. Secondly, 10%FC2-FC1 was selected as a coal blend with high sulfur fat coal (denoted as BC2), two high volatile coals (HVC1 and HVC2) were added into BC2 with ratios of 10%, 20% and 30%. The coal blend with HVC1 were denoted as 10% HVC1-BC2, 20%HVC1-BC2, 30%HVC1-BC2, and similarly indicated for coals added with HVC2. In these two series of experiments, coal blends, such as BC1, coal blend with FC2, coal blend with HVC1/HVC2, were placed on the upper plate. Thirdly, several experiments, in order to further study the influence of external volatile matters on sulfur transformation, were carried out in the fixed-bed reactor. In brief, HVC1 (HVC2) was placed on the lower sintered quartz plate and BC2 on the upper plate, the volatile matters released from HVC1 (HVC2) would flow with the carrier gas from bottom to top and react with BC2. Three ratios (1:2, 1:1, 2:1) of HVC1 (HVC2) to BC2 were selected.
3. Results and discussion
2.3. Total sulfur analysis
3.1. Sulfur transformation during pyrolysis of coal blend with FC2
The total sulfur content in coke was measured by HCS-140 High Frequency Infrared Carbon and Sulfur Analyzer (Shanghai Dekai Instrument Co. Ltd.), the detection error is 0.01%. The calculated sulfur content in coal blend coke was obtained by following equation:
S=
∑ Si × Yi × Pi ∑ Yi × Pi
It is necessary to first determine the sulfur forms in the two fat coals to investigate the sulfur transformation during pyrolysis of coal blend with high sulfur fat coal. The S K-edge spectra fitting results of FC1 and FC2 are listed in Table 2. The dominant sulfur species in the two coals are sulfide-S, thiophene-S and sulfoxide-S, with the five organic sulfur species accounting for about 87.07% for FC1 and 93.33% for FC2 in FY mode, 86.78% for FC1 and 93.61% for FC2 in TEY mode. We note that the TEY mode at the S K-edge is sensitive to the surface species, since only the surface electrons can escape and be detected, while FY mode is more bulk sensitive because of the high penetration of fluorescence (a few hundred nms). Compared to the sulfide-S, the thiophene-S in FY mode has a higher percentage than that in TEY mode, which indicates that the sulfur on coal surface is much easier to be removed than that in bulk coal. The above results show that the sulfur species in the two fat coals are also quite similar, the difference is mainly reflected in the total sulfur content (Table 1). The pyrolysis of BC1 and coal blend with FC2 are firstly carried out, the resulting sulfur content in coke and desulfurization rate are shown in Fig. 2. It can be seen that the sulfur content in coke increases with increasing ratio of FC2, because of the higher percentage of sulfur in FC2. Nevertheless, the desulfurization rate also increases with increasing FC2. This can be attributed to the increase of interactions
(1)
where S is the calculated sulfur content in coal blend coke (%); Si is the sulfur content in coke i (%); Yi is the coke yield of coal i (%); Pi is the proportion of coal i in coal blend (%). The desulfurization rate was obtained by following equation:
D = 100 −
Scoke × Y Scoal
(2)
where D is the desulfurization rate (%); Scoal is the sulfur content in coal (%); Scoke is the sulfur content in coke (%); Y is the coke yield (%). 2.4. Sulfur K-edge XANES Sulfur K-edge XANES analyses of coal and coke samples were performed at the Canadian Light Source using the Soft X-ray Micro characterization Beamline (SXRMB), and the detailed procedures and 429
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Table 2 Distribution of sulfur forms in total sulfur for FC1 and FC2 by S K-edge spectra. Mode
Sample
Percentage of sulfur form (%) FeS2
Sulfide
Thiophene
Sulfoxide
Sulfone
Sulfonate
Sulfate
FY
FC1 FC2
3.52 2.46
14.40 13.79
51.25 55.08
13.08 14.96
5.66 6.04
2.68 3.46
9.41 4.21
TEY
FC1 FC2
3.74 2.30
15.00 16.65
46.91 52.37
16.45 15.33
6.12 4.67
2.30 4.58
9.48 4.09
between different coals as a result of the increase in sulfur amount in coal blend. Fig. 3 shows the H2S and COS release curves during pyrolysis of coal blend with FC2. It can be seen that the release profiles of H2S and COS of different coal blend are similar, but the release amount increases with increasing ratio of FC2. On one hand, the sulfur content in FC2 is nearly four times in FC1, and with the exception of thiopheneS, there are still some unstable organic sulfur in FC2 (Table 2) which could be more easily decomposed during pyrolysis. Thus with the addition of FC2, the decomposition of unstable organic sulfur resulted in the increased amount of sulfur-containing gases. On the other hand, the interactions between high sulfur fat coal and other coals could also have some effects on the sulfur transformation after the addition of FC2. Fig. 4(a) shows the S K-edge spectra of coal blend cokes with FC2. It can be seen that there are clear differences between the spectra of coal blend cokes with FC2 and that of the BC1 coke. This could be due to the transformation and redistribution of sulfur at the surface of cokes during pyrolysis. When FC2, which has more organic sulfur, is added
Fig. 2. Sulfur content in coke and desulfurization rate during pyrolysis of coal blend with FC2.
Fig. 3. H2S and COS release curves during pyrolysis of coal blend with FC2.
Fig. 4. S K-edge spectra of coal blend cokes with FC2 (a), HVC1 or HVC2 (b). 430
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Table 3 Contents of sulfur forms in coal blend cokes (TEY mode). Sample
BC1 3%FC2-FC1 5%FC2-FC1 10%FC2-FC1 (BC2) 10%HVC1-BC2 20%HVC1-BC2 30%HVC1-BC2 10%HVC2-BC2 20%HVC2-BC2 30%HVC2-BC2
Total sulfur (%)
0.62 0.71 0.76 0.85 0.82 0.78 0.74 0.80 0.75 0.71
Content of sulfur form (%) FeS
Sulfide
Thiophene
Sulfoxide
Sulfone
Sulfate
0.03 0.02 0.02 0.02 0.02 0.03 0.03 0.03 0.02 0.00
0.09 0.15 0.18 0.23 0.23 0.24 0.25 0.25 0.26 0.29
0.17 0.21 0.23 0.29 0.26 0.22 0.19 0.24 0.19 0.13
0.17 0.18 0.19 0.18 0.15 0.14 0.10 0.18 0.15 0.14
0.02 0.02 0.04 0.04 0.03 0.03 0.05 0.04 0.05 0.03
0.13 0.14 0.10 0.08 0.12 0.11 0.11 0.06 0.08 0.12
into coal blend, the amount of sulfur-containing radicals in volatile matters increases, the interactions between volatile matters and nascent coke further lead to higher percentages of sulfide and thiophene on the surface of coal blend cokes than that of BC1 coke [17,26]. It is also noteworthy that the percentages of these two sulfur species increase with increasing ratio of FC2 (Table 3). The above results show that the sulfur content in coke increases inevitably while FC2 is added into coal blend, but the desulfurization rate also increases. Meanwhile, there is more sulfur transformation at coke surfaces due to the interactions between volatile matters and nascent coke. Therefore, it provides a feasible way to regulate the sulfur transformation behavior through inhibiting the interactions between sulfur-containing radicals and nascent coke on the coke outer surface during pyrolysis of coal blend with FC2. 3.2. Effect of external volatile matters on sulfur transformation 3.2.1. Selection of high volatile coals Adding high sulfur coal in coal blend could result in the increase of sulfur content in coke, so it is necessary to regulate the transformation of sulfur species to meet the coke quality requirement. As stated previously, two conditions must be met in order to achieve the directional regulation of sulfur transformation and ultimately decrease the sulfur content in coke: different sulfur species in coal should be more decomposed and enough active hydrogen radicals are donated to form SH radicals. Indeed, the volatile matters generated during coal pyrolysis process contain lots of active hydrogen-containing radicals, these radicals could induce the cleavage of sulfur-containing groups and react with formed sulfur radicals to release as sulfur-containing gases and further reduce the sulfur content in coke. Therefore, it should be feasible to add some coals with higher volatile content and high sulfur coal to the coal blend to tune the sulfur transformation behavior. A wide release temperature range of hydrogen-containing gases and low total sulfur content are desirable when selecting high volatile coals. Therefore, two high volatile coals (HVC1 and HVC2) were selected, both containing high volatile contents (higher than 36%, Table 1) and low sulfur contents (less than or equal to 0.40%, Table 1). Fig. 5 shows the release temperature ranges of CH4 and H2 for HVC1 and HVC2, and release temperature range of H2S for BC2. It can be seen that the release temperature ranges of CH4 and H2 are both wider than that of H2S, with the range of H2S for BC2 more similar to that of CH4, but rather different from that of H2. CH4 is mostly formed from the cleavage of aliphatic side chains and plenty of hydrogen-containing radicals will be generated during this process. These radicals must have some effects on the H2S release in this temperature range. While H2 is normally generated from the condensation of aromatic compounds at higher temperature, so when H2 starts to release, most of H2S will no longer be generated and the sulfur distribution in coke mainly undergoes interconversions [16,17]. In addition, once these hydrogen radicals are formed in the higher temperature range, it would be easier for
Fig. 5. Correlation between CH4/ H2 and H2S release curves.
hydrogen radicals to bond with each other to form H2, instead of bonding with other radicals. Thus, wider release temperature range of CH4 is more beneficial for regulating the sulfur transformation. 3.2.2. Sulfur transformation under the influence of high volatile coals The sulfur content in coke and desulfurization rate during pyrolysis of coal blend with HVC1 and HVC2, together with the calculated values of sulfur content in cokes, are shown in Fig. 6. It can be seen that the sulfur content in coke decreases with increasing ratio of high volatile coal. Since the sulfur content in these two high volatile coals is relatively low, the sulfur percentage in coal blend is expected to decrease with the increasing ratios of these coals. By comparing the sulfur content in coke between experimental and calculated values, the former value of sulfur content is clearly lower, and with increasing ratio of high volatile coal, the desulfurization rate also increases. This indicates that adding high volatile coals in coal blend not only causes additive effects, it also has an effect on the interactions between different coals. In addition, it can be noted that adding HVC2 could result a much more 431
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Fig. 6. Sulfur content in coke and desulfurization rate during pyrolysis of coal blend with HVC1 or HVC2.
reduced sulfur content in coke, which can be attributed to the more overlapping temperature range of the CH4 release with that of the H2S for BC2. The effect of high volatile coals in BC2 can also be studied by the S K-edge spectra of coal blend cokes. From Fig. 4(b), the normalized peak area of coal blend cokes with HVC1 and HVC2 is generally lower than that of BC2 coke, and the peak intensities of coal blend cokes with HVC2 are weaker than that of HVC1 added ones. Compared to the spectrum of BC2 coke, it is also clear that the percentage of thiophene S is reduced for HVC1 and HVC2, as indicated by the reduced peak intensities around 2475 and 2479 eV, which are part of thiophene features [16,20]. From the above results (Fig. 4(a)), more noticeable changes are also observed in the TEY spectra of coal blend cokes with the addition of high sulfur fat coal. So it is obvious that the interactions between sulfur-containing radicals (generated from high sulfur coal) and nascent coke are inhibited by adding high volatile coals. During pyrolysis of coal blend with high volatile coals, more hydrogen radicals will be generated from high volatile coals, by catalyzing the cleavage of some sulfur bonds and reacting with the formed sulfur radicals on coke surface. As a result, more sulfur species can be released as sulfur-containing gases and the sulfur content in coke will decrease. As for the differences between coal blend cokes with HVC1 and HVC2, it can be seen that the higher volatile content in high volatile coal is not better for the removal of sulfur in coke (Fig. 6). For the lower rank HVC1, the release of volatile matters and the generation of plenty of hydrogencontaining radicals occur at a lower temperature range. While the decomposition temperature of HVC2 is relatively higher (Fig. 5), the release temperature range for HVC2 to generate hydrogen radicals overlaps (better than HVC1) with the temperature range for BC2 to produce sulfur radicals. Thus, more active hydrogen radicals in coal blend with HVC2 can react with the generated sulfur radicals. For coal blend with HVC1, the untimely formed hydrogen radicals will be likely flushed out by carrier gas or reacted with other active radicals, resulting in the insufficient reactions between hydrogen radicals and sulfur radicals (Table 3). Comparing with the sulfur forms in BC2 coke, the sulfide content on coke surface increases with increasing ratio of high volatile coal, while the percentage of thiophene has an opposite trend (Fig. 4(b) and Table 3). This indicates that the volatile matters from high volatile coal participate in the interactions with cokes and regulate the sulfur transformation behavior, which inhibit the transfer of sulfur into thiophene-S, and promote the release of sulfur species as sulfurcontaining gases or retain as sulfide-S on coke surface. Consequently, the relatively stronger interactions between volatile matters in HVC2 and coke on the surface lead to the lower sulfur content in coke, higher desulfurization rate, and weaker S K-edge spectra peak intensity.
Fig. 7. S K-edge spectra of BC2 cokes.
3.3. Interactions between external volatile matters and BC2 coke To further investigate the effects of volatile matters in HVC1 and HVC2 on the sulfur forms of BC2 coke, we carried out more experiments in order to probe the influence of external volatile matters on BC coke. As stated in the experimental section, the samples of HVC1 or HVC2 and BC2 with mass ratios of 1:2, 1:1, 2:1 were selected and placed respectively on the lower and upper sintered quartz plates. Fig. 7 shows the S K-edge spectra of BC2 cokes obtained from these experiments. It can be seen that all the S K-edge spectra relative intensities (2473 eV) of cokes with external volatiles are obviously weaker than the BC2 coke; and similar to Fig. 4(b), the thiophene peak (2473 eV) is also weaker compared to that of the BC2 coke. This indicates that the interactions between volatile matters and BC2 coke mostly occur on the outer BC2 coke surface, or the degree of interactions in bulk coke is limited. This may be attributed to two reasons: one is that the volatile matters from HVC1 and HVC2 may pass through the BC2 coke with the carrier gas at relatively high speed and there is not enough time for interaction; the other possible reason is that the pyrolysis of HVC1, HVC2 and BC2 almost proceed simultaneously and when the volatile matters from the lower plate reach the upper plate, the volatile matters release from BC2 also diffuse from the inner side to outer side and inhibit the volatiles of HVC1 and HVC2 from permeating into the BC2 coke. In addition, since the BC2 is used as a metallurgical coke and has strong caking ability, the plastic mass, which has poor gas permeability, is formed during its pyrolysis process. Interactions between the plastic mass with other gases, liquids and solids will restrain the volatiles of HVC1 and HVC2 from interacting with inner BC2 coke matrix [27,28]. It can also be seen 432
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Appendix A. Supplementary data
from Fig. 7 that higher addition ratio of HVC2 in coal blend results in the weaker spectral intensity, which can be attributed to the more active volatile matters generated from HVC2 that can interact with more BC2 coke. In this study, relatively higher total sulfur content of high organicsulfur fat coal and its addition in coal blend are selected for more observable changes experimentally. The relatively high additive amount of high volatile coals is also chosen to be more beneficial for illuminating the interactions between volatiles and cokes. However, the addition of high organic-sulfur coal in coal blend should be limited, and a large amount of high volatile coal in coal blend may also lead to the weakening of coke strength. Therefore, based on the results from this study, in order to achieve the efficient utilization of high organic-sulfur coking coal, modification and optimization of the addition ratio of high organic-sulfur coking coal and high volatile coal in coal blend should be further conducted. In addition, the effects of coal particle sizes, mixing methods and other conditions on the regulation of sulfur transformation behavior should also be conducted to further investigate the interactions more profoundly.
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4. Conclusions Sulfur transformation and regulation during pyrolysis of an industrial coking coal blend with high organic-sulfur fat coal and high volatile coal were investigated. Main conclusions can be drawn as follows: The addition of high organic-sulfur fat coal to blend results in the increase of sulfur content in coke, but the desulfurization rate also increases during coal blend pyrolysis. Higher amount of sulfur-containing radicals in volatile matters from high sulfur fat coal promotes the interactions with nascent coke and results in the higher sulfur retention on the coke surface. With the addition of high volatile coals in coal blend, the generated hydrogen radicals inhibit the interactions between sulfur radicals (generated from high sulfur coal) and nascent coke, promote the release of sulfur species as sulfur-containing gases or retain as sulfide-S on coke surface. Further characterizations confirm that the volatile matters from high volatile coal, which have a more overlapping temperature range to sulfur release in coal blend, result in the lower sulfur content in coke, higher desulfurization and weaker S K-edge spectral intensity. The interactions between external volatile matters and nascent coke primarily occur on the coke outer surface. The results in this study indicate that it is feasible to regulate the sulfur transformation during pyrolysis of coal blend with high organic-sulfur coal, by selecting and blending high volatile coal with low sulfur content, wide and suitable release temperature range of CH4.
Acknowledgements The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (21878208, U1510111), China, Shanxi Coal Based Key Scientific and Technological Project (MJH2015-04), China and Research Project Supported by Shanxi Scholarship Council of China (2017-03), China, The S K-edge XANES measurement was carried out at the Canadian Light Source (CLS), Canada. We thank the CLS staff for the technical support.
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