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The fate of sulfur during rapid pyrolysis of scrap tires
Chemosphere 97 (2014) 102–107
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Technical Note
The fate of sulfur during rapid pyrolysis of scrap tires Hongyun Hu, Yuan Fang, Huan Liu, Ren Yu, Guangqian Luo, Wenqiang Liu, Aijun Li, Hong Yao ⇑ State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
h i g h l i g h t s Sulfur species in pyrolysis char, tar and gases from scrap tires were investigated. Sulfur was mostly in the form of thermally unstable thiophenic in scrap tires. Besides H2S, CH3SH, COS and SO2 were formed during rapid pyrolysis of scrap tires. Interactions between volatiles and char promoted thiophenic and sulfones formation.
a r t i c l e
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Article history: Received 28 July 2013 Received in revised form 9 October 2013 Accepted 11 October 2013 Available online 13 November 2013 Keywords: Scrap tires Rapid pyrolysis Sulfur Speciation
a b s t r a c t The fate of sulfur during rapid pyrolysis of scrap tires at temperatures from 673 to 1073 K was investigated. Sulfur was predominant in the forms of thiophenic and inorganic sulfides in raw scrap tires. In the pyrolysis process, sulfur in organic forms was unstable and decomposed, leading to the sulfur release into tar and gases. At 673 and 773 K, a considerable amount of sulfur was distributed in tar. Temperature increasing from 773 to 973 K promoted tar decomposition and facilitated sulfur release into gases. At 1073 K, the interactions between volatiles and char stimulated the formation of high-molecular-weight sulfur-containing compounds. After pyrolysis, almost half of the total content of sulfur in raw scrap tires still remained in the char and was mostly in the form of sulfides. Moreover, at temperatures higher than 873 K, part of sulfur in the char was immobilized in the sulfates. In the pyrolysis gases, H2S was the main sulfur-containing gas. Increasing temperature stimulated the decomposition of organic polymers in scrap tires and more H2S was formed. Besides H2S, other sulfur-containing gases such as CH3SH, COS and SO2 were produced during the rapid pyrolysis of scrap tires. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Recently, a large number of scrap tires are continuously generated in China. The disposal and treatment of scrap tires have drawn much attention for both environmental concern and resources recovery (Llompart et al., 2013). The pyrolysis technology has been broadly applied for the utilization of scrap tires by using different types of reactors such as fluidized bed, spound bed, rotary kiln and fixed bed (Li et al., 2004; Kaminsky et al., 2009; López et al., 2010, 2011). Useful products such as gases, tar and char can be obtained through the pyrolysis process (Ucar et al., 2005; Mui et al., 2010; Dog˘an et al., 2012). However, sulfur is widely presented in tires and the release of the sulfur-containing gases during scrap tires pyrolysis could cause seriously environmental pollution. Moreover, sulfur remained in the solid products or transformed into pyrolysis tar would raise some safety and quality problems for the consequent use of these products (Rodriguez et al., 2001). As a result,
⇑ Corresponding author. Tel./fax: +86 27 87545526 (O). E-mail address: [email protected] (H. Yao). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.10.037
knowledge of sulfur transformation is essential for the utilization of scrap tires through pyrolysis process. So far, a few research findings have been focused on the fate of sulfur during the pyrolysis of scrap tires. Murena (2000) reported that sulfur compounds tended to release into gases at low temperatures. H2S was found as the main sulfur-containing component in gaseous products during flash pyrolysis of used tires by using a thermal plasma reactor (Tang and Huang, 2004). The initial temperature for the decomposition of scrap tires increased with the increasing of heating rate from 1 to 100 K min1 (Unapumnuk et al., 2006). At temperatures from 623 to 1123 K, most of sulfur was remained in the char rather than released into gases or condensed in the tar (Unapumnuk et al., 2008). In addition to the reaction conditions, the fate of sulfur during thermal conversions of coal was found to be significantly affected by the sulfur speciation and inorganic matters in the raw material (Liu et al., 2010; Zhang and Yani, 2011). Compared with coal, scrap tires are rich in carbon and hydrogen and mainly consist of organic compounds of high molecular weight. For the quality modification of rubber used for tires production, sulfur is usually used. According to previous studies, sulfur was mostly in the form of C–S bonds
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in scrap tires (Modrow et al., 2001; Quek and Balasubramanian, 2013). The investigation of the fate of organic sulfur during the pyrolysis of coal char indicated that SH radical was the key intermediate interacting with char to form secondary sulfur compounds in the pyrolysis products (Yan et al., 2005). The formation of the SH radical and its thermal behavior depended on the thermal stability of sulfur-containing organic compounds which might be particularly important during the pyrolysis of scrap tires due to the polymer characteristics. On the other hand, scrap tires were of high contents of Ca and Zn (Gieré et al., 2006; Alexandre-Franco et al., 2010). Compounds of these elements could enhance sulfur immobilization by forming thermal stable sulfates and/or sulfides (Hu et al., 2006; Ling et al., 2013). Nevertheless, no detailed information had been reported to further illuminate the mechanism of sulfur transformation during the pyrolysis of scrap tires. In this study, rapid pyrolysis of scrap tires was conducted at temperatures ranging from 673 to 1073 K. The pyrolysis product yields as well as the sulfur distribution in these products were observed. To forward the understanding of the mechanism of sulfur transformation, sulfur speciation in char, tar and pyrolysis gases were investigated by using a X-ray photoelectron spectrometer, GC–MS and a trace sulfur analyzer. The thermal behavior of inorganic materials was concerned for their effects on the sulfur immobilization. 2. Material and methods
conditions were similar to that in the literature (Chen et al., 2010). A fused-silica capillary HP-5 ms column (30 m 0.25 mm id) with 0.25 lm film thickness was used (J & W Scientific, Folsom, CA, USA). The column temperature program was firstly retained at 40 °C for 3 min and then heated at an increasing rate of 6 °C min1 to 300 °C, after which it was kept at 300 °C for 5 min. The sulfurcontaining gases in pyrolysis gases were measured by a HC-5 trace sulfur analyzer. All the experiments were repeated more than 3 times and the mass balance of sulfur was between 85% and 115%. Other gases like CH4, H2 and CO was determined by GC (Agilent 3000A micro-GC). In the process, N2 acted as the balance gas and the production of other types of gases were calculated on the basis of the determined concentrations and relative volume fraction in the gas. Furthermore, X-ray powder diffraction (XRD) was employed to provide detailed information about mineralogical characteristics of the char. And the sulfur speciation in scrap tires and char were investigated by using a VG Multilab 2000 X-ray photoelectron spectrometer (XPS). The relative concentrations of different sulfur forms were calculated according to the corresponding peak values fixed at 162.2 ± 0.6, 163.3 ± 0.4, 164.1 ± 0.2, 168.2 ± 0.2 and 170.0 ± 1.0 eV of binding energy for inorganic sulfides, aliphatic sulfur, thiophenic, sulfones, and sulfates, respectively (Kozłowski, 2004; Marinov et al., 2004; Ko et al., 2006; Liu et al., 2007, 2012).
3. Results and discussion
The scrap tires were sampled from used tires supplied by the nearby residents. The samples were shredded and sieved to the sizes of 0.5 to 1 mm. After drying to constant weight at 378 K, the proximate and ultimate analyses of the scrap tires were carried out by using a TGA2000 proximate analyzer and a Vario Microcube elemental analyzer, respectively. The results are shown in Table 1. The sample has a high content of carbon. The sulfur content in the sample is 2.05%. There is a high content of ash in the scrap tires and the chemical compositions of the ash were performed by X-ray fluorescence spectrometry (XRF). As shown in Table SM-1 in Supplementary material (SM), the major compositions of the ash are CaO, SO3, SiO2, MgO, ZnO, Al2O3 and Fe2O3. The rapid pyrolysis of scrap tires was conducted in a vertical furnace as shown in Fig. SM-1. At the beginning, 1 g of the sample was placed in the quartz basket which was fixed at the top of the reactor. Then the carrier gas N2 was input into the reactor at a certain flow rate (0.4 L min1) controlled by a mass flow controller. The reactor was heated by the electric furnace to the set temperatures ranging from 673 to 1073 K. When the furnace was stable at the set temperature, the quartz basket was put into the reaction zone rapidly. The pyrolysis gases were carried out by the N2 and were cooled down by the ice bath. The tar and gaseous products were separated and collected for the subsequent analysis. After pyrolysis, the char was pulled out and cooled down to room temperature under N2. Meanwhile, the tar were washed with dichloromethane and collected. The collected tar were dried at 313 K to remove the dichloromethane for subsequent analysis. The total content of sulfur in the tar was detected using a Vario Microcube elemental analyzer. The sulfur-containing compounds in the tar were identified by GC–MS (Agilent 7890 A/5975 C). The operating
3.1. Sulfur contents in scrap tires pyrolysis products The yields of the products from the pyrolysis of scrap tires are shown in Fig. 1a. Scrap tires mainly consists of organic polymers, the cleavage of which was strongly affected by the operating temperature (Leung et al., 2002). The results drawn from this work were similar to these in previous studies (Rodriguez et al., 2001; Lo9 pez et al., 2010). With the temperature increasing from 673 to 773 K, char yields underwent remarkable decrease. At temperatures high than 773 K, the char yields slightly decreased. In the present study, the tar yields (as well as sulfur contents in tar which were discussed in the next paragraph) were calculated as the difference between total and the sum of char yields and gas yields. With the decomposition of scrap tires, the production of gases and tar were facilitated. The further decomposition of char enhanced the tar production at 773 K. Compared with the gas and tar yields at 873 K, the gas yields increased notably while the tar yields decreased at 973 and 1073 K. Therefore, at these temperatures, the formation of small molecular gases was attributed to the cracking of high molecular-weight tar. Fig. 1b shows the sulfur distribution in the pyrolysis products of scrap tires. Most of sulfur was found in the char which was mainly due to the great thermal stability of sulfur-containing organic and/ or inorganic compounds formed during the pyrolysis process. However, a large amount of sulfur was transformed into gases. At 673 K, although the gas yields were quite low, a considerable amount of sulfur was found in the pyrolysis gases. The results indicated that some sulfur-containing compounds were unstable and easily decomposed at low temperatures. The decomposition of these compounds was facilitated at 773 K. As a result, the sulfur
Table 1 Properties of scrap tires. Proximate analysis (%)
*
Ultimate analysis (%)
Moisture
Volatile matter
Fixed carbon
Ash
C
H
O*
N
S
0.4
62.9
18.1
18.6
68.8
5.9
3.3
0.9
2.1
Calculated by difference.
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H. Hu et al. / Chemosphere 97 (2014) 102–107
Tar
(a)
Gas
Char
ð1Þ
Products yields (%)
As stated before, SH radical was confirmed as the key intermediate in sulfur transformation during thermal process of organic sulfur-containing compounds (Yan et al., 2005). With the increasing of pyrolysis temperature, the decomposition of sulfur-containing compounds was stimulated and more SH radicals were formed (Eq. (2)). The interactions between SH radicals and inorganic compounds promoted sulfur immobilization in the char by forming thermal stable sulfides (Eqs. (3) and (4)). As a result, more fraction of sulfur was found in the form of inorganic sulfides in the char with the temperature increasing from 673 to 973 K. However, the fraction of inorganic sulfides decreased in the char at 1073 K. To further understand the effects of metallic compounds on the sulfur transformation, the mineralogical characteristics of the char samples were investigated and the results are shown in Fig. 3. According to the XRD patterns, ZnS was the only sulfides detected in char samples collected at temperatures lower than 1073 K. Ca
80 60 40
S distribution in pyrolysis products (%)
20
(b)
R SH þ R0 SH ! R S R0 þ H2 S
100
0
100 80 60 40
Raw scrap tires 20
S2 0
S1 673
773
873
973
1073
Temperature/K Char 673K
Fig. 1. (a) Products yields during rapid pyrolysis of scrap tires, (b) sulfur distribution in pyrolysis products of scrap tires.
S2
S1 S3
Char 773K
Intensity (arbitray counts)
content in chars decreased while the sulfur contents in both tars and gases increased. With temperature increasing from 773 to 973 K, the contents of gas-containing sulfur increased continuously while the contents of tar-containing sulfur decreased. Meanwhile, the contents of char-containing sulfur slightly changed. So, it can be concluded that the increase of the sulfur contents in the gases was most likely attributed to the decomposition of sulfur-containing compounds in the tars. It was further confirmed by the results shown in Table SM-2. The sulfur contents in the tars decreased with temperature increasing from 673 to 973 K. Interestingly, the content of gas containing sulfur underwent an apparent decrease at 1073 K and more fraction of sulfur was found in the tar. It was supposed that the interactions between sulfur-containing gases and other volatiles and/or pyrolysis char enhanced sulfur enrichment in tar.
S1 S2 S3 Char 873K S1 S4
S2
Char 973K S1
3.2. Sulfur speciation in scrap tires char To investigate the mechanism of sulfur transformation during scrap tires pyrolysis, sulfur speciation in raw scrap tires and char were conducted by XPS. The spectra of S 2p are shown in Fig. 2 and the results are given in Table 2. In raw scrap tires, sulfur was in the form of thiophenic and inorganic sulfides. After pyrolysis, the fraction of sulfides in char increased while most of the organic sulfur-containing compounds were decomposed. These results further confirmed that the transformation of sulfur into gases and tar was attributed to the cracking of these organic sulfur-containing compounds. Moreover, the chemical forms of the sulfur-containing compounds changed during the pyrolysis process. As seen in Table 2, aliphatic sulfur was formed at 673 and 773 K (Eq. (1)). But these aliphatic compounds were unstable and were broken down at higher temperatures.
S2
S4 Char 1073K
S1 S2 S5 160
162
164
166
168
S4 170
172
Binding energy (eV) Fig. 2. XPS spectra of S 2p for raw scrap tires and pyrolysis char drawn at different temperatures from 673 to 1073 K (S1: inorganic sulfides, S2: aliphatic sulfur, S3: thiophenic, S4: sulfates, S5: sulfones).
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H. Hu et al. / Chemosphere 97 (2014) 102–107 Table 2 The sulfur forms of raw scrap tires and pyrolysis char drawn at temperatures ranged from 673 to 1073 K (wt%). Sulfur forms
Binding energy (eV)
Raw material
Inorganic sulfides Aliphatic sulfur Thiophenic Sulfones Sulfates
162.2 ± 0.6 163.3 ± 0.4 164.1 ± 0.2 168.2 ± 0.2 170.0 ± 1.0
30.1 – 69.9 – –
Char 673 K
773 K
873 K
973 K
1073 K
37.2 31.1 31.7 – –
67.3 6.0 26.7 – –
68.7 – 16.0 – 15.3
79.4 – 15.3 – 5.3
58.4 – 22.2 11.2 8.2
– Not determined.
pyrolysis gases indicated that a large amount of C–O groups was produced at 1073 K. The interactions between C–O groups and sulfur-containing active groups promoted the oxidation of organic sulfur compounds leading to the formation of high molecularweight sulfur-containing compounds. The formed products of high dew point remained in the char such as sulfones while the products of low dew point were released as volatiles and were finally condensed in tar. Hence, the fraction of sulfur content in the gases decreased while that in the tar increased at this temperature which was consistent with the results shown in Fig. 1b. The produced SH radical reacted with other volatile materials and facilitated the transformation of sulfur into tar in organic forms (Eq. (5)) (Calkins, 1994).
1. CaCO3 2. CaMg(CO3)2 1 354 11
3. SiO2 5 1
4. ZnS 5 Ca2Fe2S2O3
1 1
1073 K
Intensity (A. U.)
1 1 3 4 2 1 1 1 2 1 11 1
973 K
2 1 34 1
1 1 2 1 11 2
873 K
1 34 2 1
1 1 2 1 11 2
773 K
1 1 2 1 11
673 K
1 34
10
20
2
30
40
2
50
60
70
80
90
2θ (°) Fig. 3. XRD patterns of scrap tires pyrolysis char drawn at different temperatures from 673 to 1073 K.
and Mg were dominant in the form of carbonates and no obvious peaks of Ca/Mg sulfides were found. At 1073 K, part of Ca/Mg carbonates were decomposed and the formation of Ca2Fe2S2O3 was observed. The formation of inorganic sulfides was enhanced by increasing of operating temperature. However, at higher temperatures such as 1073 K, the deactivation of the reaction sites might take place due to the particle agglomeration and/or sintering, resulting in the suppress of the sulfides formation. On the other hand, according to the XPS results shown in Table 2, the formation of sulfates was found in the char samples collected at 873, 973 and 1073 K. The formation of sulfates could also enhance sulfur immobilization in the char.
R S ! R þ SH
ð2Þ
SH þ MO ! MS þ OH
ð3Þ
SH þ MCO3 ! MS þ OH þ CO2
ð4Þ
Another interesting phenomenon was that a considerable amount of thiophenic and sulfones was found in the char at 1073 K. It is due to the interactions between volatiles and char which stimulated the formation of thiophenic on the surface of char. And the effect was enhanced at high temperatures. During the investigation of sulfur behavior in coal pyrolysis process, similar phenomenon was found that the secondary reaction between volatiles and char strongly affected the sulfur transformation (Sugawara et al., 2003; Zhang et al., 2013). Additionally, as shown in Fig. SM-2, a certain amount of CO and CO2 was found in the pyrolysis gases at 1073 K. The production of CO and CO2 in
SH þ Volatiles ! Tars
ð5Þ
Table SM-3 shows the tentative GC/MS characterization of sulfur-containing compounds in scrap tires pyrolysis tar. Various types of sulfur-containing compounds were identified in the scrap tires pyrolysis tars collected at 673 and 773 K. In the tars collected at 973 and 1073 K, no sulfur-containing compounds were detected, whereas, a certain content of sulfur in these tars was determined by using a elemental analyzer (shown in Table SM-2). The results demonstrated that sulfur-containing compounds in these tars were of high dew points and were difficult to identify by using the GC– MS. Furthermore, these compounds were probably of high molecular-weight like the sulfur forms in the char collected at 1073 K which was in agreement with the discussion mentioned above. 3.3. Sulfur speciation in scrap tires pyrolysis gases Fig. 4 shows the sulfur-containing gases contents in pyrolysis gases of scrap tires. SO2, H2S, COS and CH3SH were found in the pyrolysis gases and H2S was the main sulfur-containing gas. The release of H2S and COS was of the same trend which was intensified with temperature increasing from 673 to 973 K while suppressed at 1073 K. In addition, the release of SO2 and CH3SH was of the same trend. The release of these gases was enhanced with the temperature increasing from 673 to 773 K while suppressed at higher temperatures. During the pyrolysis process, the decomposition of organic polymer in scrap tires led to the formation of H-containing radicals like CH3 and H (Zhang et al., 2013) which brought about sulfur release in the forms of CH3SH and H2S (Eqs. (6) and (7)). The decomposition of these organic polymers was enhanced with the increasing of temperature. Therefore, more CH3 and H were formed, resulting in the high contents of CH4 and H2 in the gases as shown in Fig. SM-2. The production of H enhanced the formation of H2S, contributing to the high concentration of H2S in gases. Meanwhile, CH3 was more likely combined with H than SH (Eq. (8)). Therefore, the formation of CH3SH was inhibited when more H was produced in the pyrolysis process at high temperatures.
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H. Hu et al. / Chemosphere 97 (2014) 102–107
5000 4000 3000 2000 1000
673 K 773 K 873 K 973 K 1073 K
160 120 80 40 0
Fig. 4. The distribution of sulfur-containing gases during rapid pyrolysis of scrap tires.
SH þ CH3 ! CH3 SH
ð6Þ
SH þ H ! H2 S
ð7Þ
H þ CH3 ! CH4
ð8Þ
The release of SO2 was mainly due to the oxidation of SH by oxygen-containing active radical such as OH (Eq. (9)) (Yan et al., 2005; Zhang and Yani, 2011). Miura et al. (2001) found that the heating rate was an important factor for the formation of SO2. In this research, heating of scrap tires was mainly attributed to radiative heat transfer and the heating rate was low in low-temperature pyrolysis process. As shown in Fig. 4, a large amount of SO2 released at 673 and 773 K. However, during high-temperature pyrolysis, the deactivation of the oxygen-containing functional groups was enhanced through a rapid decomposition. In addition, the interaction between SH and metal oxides was intensified at the oxygen-containing sites by forming sulfates (Eq. (10)). Therefore, less SO2 was formed during pyrolysis at high temperatures.
SH þ OH ! SO2 þ H2
SH þ MO þ OH ! MSO4 þ H2
ð9Þ ð10Þ
COS was formed by decomposition of the organic structures or second reactions of sulfur species (Chu et al., 2008; Zhang et al., 2013). As the temperature increased, the decomposition of the organic compounds of high molecular weight was reinforced and more C–O groups were formed, promoting the release of COS (Eq. (11)). However, a large amount of H2 was produced in high-temperature pyrolysis. The reaction between H2 and COS decreased COS content (Eq. (12)). Furthermore, the interaction between COS and active char or volatiles (Eqs. (13) and (14)) might also restrain the COS release by forming sulfur compounds of high molecular weight in the char.
SH þ C O ! COS þ H
ð11Þ
H2 þ COS ! CO þ H2 S
ð12Þ
COS þ Volatile ! Tars þ CO
ð13Þ
COS þ Char ! Chars þ CO
ð14Þ
4. Conclusions In the present study, the fate of sulfur during rapid pyrolysis of scrap tires by using a vertical furnace was investigated at
temperatures ranged from 673 to 1073 K. The sulfur speciation in the pyrolysis char, tar and gases were observed. In raw scrap tires, sulfur was predominant in the forms of thiophenic and inorganic sulfides. During pyrolysis, temperature played an important role in the sulfur transformation as well as sulfur speciation migration. Organic sulfur compounds were easily decomposed at low temperature 673 K and a large amount of sulfur was released into tar and gases. With temperature increasing from 673 to 773 K, more sulfur was transformed from char to tar and gases. At higher temperatures, the sulfur content in char slightly changed whereas the decomposition of tar contributed to sulfur release into gases. However, at 1073 K, the interactions between volatiles and char and/or among volatiles suppressed sulfur releasing into gases and stimulated the formation of high-molecular-weight sulfur-containing compounds in tar and char. After pyrolysis, almost half of the total content of sulfur in scrap tires remained in the char mainly in the form of sulfides. ZnS was found as the most important sulfide for the sulfur immobilization in the char. Furthermore, the formation of sulfates was observed in the chars at temperatures high than 873 K. In the pyrolysis gases, H2S was the main sulfur-containing gas and other sulfur-containing gases in the forms of CH3SH, COS and SO2 were found in small amounts. Temperature increasing stimulated the cracking of organic polymers in scrap tires and lead to more H2S formation. Acknowledgments This study was financially supported by National Natural Science Foundation of China (Grants, 51076053, 51161140330) and Key Project of Chinese National Programs for Fundamental Research and Development (973 program, 2011CB201505). The authors would also thank to the Analytical and Testing Center of Huazhong University of Science and Technology for the experimental measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2013.10.037. References Alexandre-Franco, M., Fernandez-Gonzlez, C., Alfaro-Domi9 nguez, M., Palacios, J.M., Go9 mez-Serrano, V., 2010. Devulcanization and demineralization of used tire rubber by thermal chemical methods: a study by X-ray diffraction. Energy Fuel. 24, 3401–3409. Calkins, W.H., 1994. The chemical forms of sulfur in coal: a review. Fuel 73, 475– 484. Chen, T.C., Shen, Y.H., Lee, W.J., Lin, C.C., Wan, M.W., 2010. The study of ultrasoundassisted oxidative desulfurization process applied to the utilization of pyrolysis oil from waste tires. J. Clean. Prod. 18, 1850–1858. Chu, X.J., Li, W., Li, B.Q., Chen, H.K., 2008. Sulfur transfers from pyrolysis and gasification of direct liquefaction residue of Shenhua coal. Fuel 87, 211–215. Dog˘an, O., Çelik, M.B., Özdalyan, B., 2012. The effect of tire derived fuel/diesel fuel blends utilization on diesel engine performance and emissions. Fuel 95, 340– 346. Gieré, R., Smith, K., Blackford, M., 2006. Chemical composition of fuels and emissions from a coal + tire combustion experiment in a power station. Fuel 85, 2278–2285. Hu, Y.Q., Watanabe, M., Aida, C., Horio, M., 2006. Capture of H2S by limestone under calcination conditions in a high-pressure fluidized-bed reactor. Chem. Eng. Sci. 61, 1854–1863. Kaminsky, W., Mennerich, C., Zhang, Z., 2009. Feedstock recycling of synthetic and natural rubber by pyrolysis in a fluidized bed. J. Anal. Appl. Pyrol. 85, 334–337. Ko, T.H., Chu, H., Tseng, J.J., 2006. Feasibility study on high-temperature sorption of hydrogen sulfide by natural soils. Chemosphere 64, 881–891. Kozłowski, M., 2004. XPS study of reductively and non-reductively modified coals. Fuel 83, 259–265. Leung, D.Y.C., Yin, X.L., Zhao, Z.L., Xu, B.Y., Chen, Y., 2002. Pyrolysis of tire powder: influence of operation variables on the composition and yields of gaseous product. Fuel Process. Technol. 79, 141–155.
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The fate of sulfur during rapid pyrolysis of scrap tires Hongyun Hu, Yuan Fang, Huan Liu, Ren Yu, Guangqian Luo, Wenqiang Liu, Aijun Li, Hong Yao ⇑ State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
h i g h l i g h t s Sulfur species in pyrolysis char, tar and gases from scrap tires were investigated. Sulfur was mostly in the form of thermally unstable thiophenic in scrap tires. Besides H2S, CH3SH, COS and SO2 were formed during rapid pyrolysis of scrap tires. Interactions between volatiles and char promoted thiophenic and sulfones formation.
a r t i c l e
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Article history: Received 28 July 2013 Received in revised form 9 October 2013 Accepted 11 October 2013 Available online 13 November 2013 Keywords: Scrap tires Rapid pyrolysis Sulfur Speciation
a b s t r a c t The fate of sulfur during rapid pyrolysis of scrap tires at temperatures from 673 to 1073 K was investigated. Sulfur was predominant in the forms of thiophenic and inorganic sulfides in raw scrap tires. In the pyrolysis process, sulfur in organic forms was unstable and decomposed, leading to the sulfur release into tar and gases. At 673 and 773 K, a considerable amount of sulfur was distributed in tar. Temperature increasing from 773 to 973 K promoted tar decomposition and facilitated sulfur release into gases. At 1073 K, the interactions between volatiles and char stimulated the formation of high-molecular-weight sulfur-containing compounds. After pyrolysis, almost half of the total content of sulfur in raw scrap tires still remained in the char and was mostly in the form of sulfides. Moreover, at temperatures higher than 873 K, part of sulfur in the char was immobilized in the sulfates. In the pyrolysis gases, H2S was the main sulfur-containing gas. Increasing temperature stimulated the decomposition of organic polymers in scrap tires and more H2S was formed. Besides H2S, other sulfur-containing gases such as CH3SH, COS and SO2 were produced during the rapid pyrolysis of scrap tires. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Recently, a large number of scrap tires are continuously generated in China. The disposal and treatment of scrap tires have drawn much attention for both environmental concern and resources recovery (Llompart et al., 2013). The pyrolysis technology has been broadly applied for the utilization of scrap tires by using different types of reactors such as fluidized bed, spound bed, rotary kiln and fixed bed (Li et al., 2004; Kaminsky et al., 2009; López et al., 2010, 2011). Useful products such as gases, tar and char can be obtained through the pyrolysis process (Ucar et al., 2005; Mui et al., 2010; Dog˘an et al., 2012). However, sulfur is widely presented in tires and the release of the sulfur-containing gases during scrap tires pyrolysis could cause seriously environmental pollution. Moreover, sulfur remained in the solid products or transformed into pyrolysis tar would raise some safety and quality problems for the consequent use of these products (Rodriguez et al., 2001). As a result,
⇑ Corresponding author. Tel./fax: +86 27 87545526 (O). E-mail address: [email protected] (H. Yao). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.10.037
knowledge of sulfur transformation is essential for the utilization of scrap tires through pyrolysis process. So far, a few research findings have been focused on the fate of sulfur during the pyrolysis of scrap tires. Murena (2000) reported that sulfur compounds tended to release into gases at low temperatures. H2S was found as the main sulfur-containing component in gaseous products during flash pyrolysis of used tires by using a thermal plasma reactor (Tang and Huang, 2004). The initial temperature for the decomposition of scrap tires increased with the increasing of heating rate from 1 to 100 K min1 (Unapumnuk et al., 2006). At temperatures from 623 to 1123 K, most of sulfur was remained in the char rather than released into gases or condensed in the tar (Unapumnuk et al., 2008). In addition to the reaction conditions, the fate of sulfur during thermal conversions of coal was found to be significantly affected by the sulfur speciation and inorganic matters in the raw material (Liu et al., 2010; Zhang and Yani, 2011). Compared with coal, scrap tires are rich in carbon and hydrogen and mainly consist of organic compounds of high molecular weight. For the quality modification of rubber used for tires production, sulfur is usually used. According to previous studies, sulfur was mostly in the form of C–S bonds
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in scrap tires (Modrow et al., 2001; Quek and Balasubramanian, 2013). The investigation of the fate of organic sulfur during the pyrolysis of coal char indicated that SH radical was the key intermediate interacting with char to form secondary sulfur compounds in the pyrolysis products (Yan et al., 2005). The formation of the SH radical and its thermal behavior depended on the thermal stability of sulfur-containing organic compounds which might be particularly important during the pyrolysis of scrap tires due to the polymer characteristics. On the other hand, scrap tires were of high contents of Ca and Zn (Gieré et al., 2006; Alexandre-Franco et al., 2010). Compounds of these elements could enhance sulfur immobilization by forming thermal stable sulfates and/or sulfides (Hu et al., 2006; Ling et al., 2013). Nevertheless, no detailed information had been reported to further illuminate the mechanism of sulfur transformation during the pyrolysis of scrap tires. In this study, rapid pyrolysis of scrap tires was conducted at temperatures ranging from 673 to 1073 K. The pyrolysis product yields as well as the sulfur distribution in these products were observed. To forward the understanding of the mechanism of sulfur transformation, sulfur speciation in char, tar and pyrolysis gases were investigated by using a X-ray photoelectron spectrometer, GC–MS and a trace sulfur analyzer. The thermal behavior of inorganic materials was concerned for their effects on the sulfur immobilization. 2. Material and methods
conditions were similar to that in the literature (Chen et al., 2010). A fused-silica capillary HP-5 ms column (30 m 0.25 mm id) with 0.25 lm film thickness was used (J & W Scientific, Folsom, CA, USA). The column temperature program was firstly retained at 40 °C for 3 min and then heated at an increasing rate of 6 °C min1 to 300 °C, after which it was kept at 300 °C for 5 min. The sulfurcontaining gases in pyrolysis gases were measured by a HC-5 trace sulfur analyzer. All the experiments were repeated more than 3 times and the mass balance of sulfur was between 85% and 115%. Other gases like CH4, H2 and CO was determined by GC (Agilent 3000A micro-GC). In the process, N2 acted as the balance gas and the production of other types of gases were calculated on the basis of the determined concentrations and relative volume fraction in the gas. Furthermore, X-ray powder diffraction (XRD) was employed to provide detailed information about mineralogical characteristics of the char. And the sulfur speciation in scrap tires and char were investigated by using a VG Multilab 2000 X-ray photoelectron spectrometer (XPS). The relative concentrations of different sulfur forms were calculated according to the corresponding peak values fixed at 162.2 ± 0.6, 163.3 ± 0.4, 164.1 ± 0.2, 168.2 ± 0.2 and 170.0 ± 1.0 eV of binding energy for inorganic sulfides, aliphatic sulfur, thiophenic, sulfones, and sulfates, respectively (Kozłowski, 2004; Marinov et al., 2004; Ko et al., 2006; Liu et al., 2007, 2012).
3. Results and discussion
The scrap tires were sampled from used tires supplied by the nearby residents. The samples were shredded and sieved to the sizes of 0.5 to 1 mm. After drying to constant weight at 378 K, the proximate and ultimate analyses of the scrap tires were carried out by using a TGA2000 proximate analyzer and a Vario Microcube elemental analyzer, respectively. The results are shown in Table 1. The sample has a high content of carbon. The sulfur content in the sample is 2.05%. There is a high content of ash in the scrap tires and the chemical compositions of the ash were performed by X-ray fluorescence spectrometry (XRF). As shown in Table SM-1 in Supplementary material (SM), the major compositions of the ash are CaO, SO3, SiO2, MgO, ZnO, Al2O3 and Fe2O3. The rapid pyrolysis of scrap tires was conducted in a vertical furnace as shown in Fig. SM-1. At the beginning, 1 g of the sample was placed in the quartz basket which was fixed at the top of the reactor. Then the carrier gas N2 was input into the reactor at a certain flow rate (0.4 L min1) controlled by a mass flow controller. The reactor was heated by the electric furnace to the set temperatures ranging from 673 to 1073 K. When the furnace was stable at the set temperature, the quartz basket was put into the reaction zone rapidly. The pyrolysis gases were carried out by the N2 and were cooled down by the ice bath. The tar and gaseous products were separated and collected for the subsequent analysis. After pyrolysis, the char was pulled out and cooled down to room temperature under N2. Meanwhile, the tar were washed with dichloromethane and collected. The collected tar were dried at 313 K to remove the dichloromethane for subsequent analysis. The total content of sulfur in the tar was detected using a Vario Microcube elemental analyzer. The sulfur-containing compounds in the tar were identified by GC–MS (Agilent 7890 A/5975 C). The operating
3.1. Sulfur contents in scrap tires pyrolysis products The yields of the products from the pyrolysis of scrap tires are shown in Fig. 1a. Scrap tires mainly consists of organic polymers, the cleavage of which was strongly affected by the operating temperature (Leung et al., 2002). The results drawn from this work were similar to these in previous studies (Rodriguez et al., 2001; Lo9 pez et al., 2010). With the temperature increasing from 673 to 773 K, char yields underwent remarkable decrease. At temperatures high than 773 K, the char yields slightly decreased. In the present study, the tar yields (as well as sulfur contents in tar which were discussed in the next paragraph) were calculated as the difference between total and the sum of char yields and gas yields. With the decomposition of scrap tires, the production of gases and tar were facilitated. The further decomposition of char enhanced the tar production at 773 K. Compared with the gas and tar yields at 873 K, the gas yields increased notably while the tar yields decreased at 973 and 1073 K. Therefore, at these temperatures, the formation of small molecular gases was attributed to the cracking of high molecular-weight tar. Fig. 1b shows the sulfur distribution in the pyrolysis products of scrap tires. Most of sulfur was found in the char which was mainly due to the great thermal stability of sulfur-containing organic and/ or inorganic compounds formed during the pyrolysis process. However, a large amount of sulfur was transformed into gases. At 673 K, although the gas yields were quite low, a considerable amount of sulfur was found in the pyrolysis gases. The results indicated that some sulfur-containing compounds were unstable and easily decomposed at low temperatures. The decomposition of these compounds was facilitated at 773 K. As a result, the sulfur
Table 1 Properties of scrap tires. Proximate analysis (%)
*
Ultimate analysis (%)
Moisture
Volatile matter
Fixed carbon
Ash
C
H
O*
N
S
0.4
62.9
18.1
18.6
68.8
5.9
3.3
0.9
2.1
Calculated by difference.
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H. Hu et al. / Chemosphere 97 (2014) 102–107
Tar
(a)
Gas
Char
ð1Þ
Products yields (%)
As stated before, SH radical was confirmed as the key intermediate in sulfur transformation during thermal process of organic sulfur-containing compounds (Yan et al., 2005). With the increasing of pyrolysis temperature, the decomposition of sulfur-containing compounds was stimulated and more SH radicals were formed (Eq. (2)). The interactions between SH radicals and inorganic compounds promoted sulfur immobilization in the char by forming thermal stable sulfides (Eqs. (3) and (4)). As a result, more fraction of sulfur was found in the form of inorganic sulfides in the char with the temperature increasing from 673 to 973 K. However, the fraction of inorganic sulfides decreased in the char at 1073 K. To further understand the effects of metallic compounds on the sulfur transformation, the mineralogical characteristics of the char samples were investigated and the results are shown in Fig. 3. According to the XRD patterns, ZnS was the only sulfides detected in char samples collected at temperatures lower than 1073 K. Ca
80 60 40
S distribution in pyrolysis products (%)
20
(b)
R SH þ R0 SH ! R S R0 þ H2 S
100
0
100 80 60 40
Raw scrap tires 20
S2 0
S1 673
773
873
973
1073
Temperature/K Char 673K
Fig. 1. (a) Products yields during rapid pyrolysis of scrap tires, (b) sulfur distribution in pyrolysis products of scrap tires.
S2
S1 S3
Char 773K
Intensity (arbitray counts)
content in chars decreased while the sulfur contents in both tars and gases increased. With temperature increasing from 773 to 973 K, the contents of gas-containing sulfur increased continuously while the contents of tar-containing sulfur decreased. Meanwhile, the contents of char-containing sulfur slightly changed. So, it can be concluded that the increase of the sulfur contents in the gases was most likely attributed to the decomposition of sulfur-containing compounds in the tars. It was further confirmed by the results shown in Table SM-2. The sulfur contents in the tars decreased with temperature increasing from 673 to 973 K. Interestingly, the content of gas containing sulfur underwent an apparent decrease at 1073 K and more fraction of sulfur was found in the tar. It was supposed that the interactions between sulfur-containing gases and other volatiles and/or pyrolysis char enhanced sulfur enrichment in tar.
S1 S2 S3 Char 873K S1 S4
S2
Char 973K S1
3.2. Sulfur speciation in scrap tires char To investigate the mechanism of sulfur transformation during scrap tires pyrolysis, sulfur speciation in raw scrap tires and char were conducted by XPS. The spectra of S 2p are shown in Fig. 2 and the results are given in Table 2. In raw scrap tires, sulfur was in the form of thiophenic and inorganic sulfides. After pyrolysis, the fraction of sulfides in char increased while most of the organic sulfur-containing compounds were decomposed. These results further confirmed that the transformation of sulfur into gases and tar was attributed to the cracking of these organic sulfur-containing compounds. Moreover, the chemical forms of the sulfur-containing compounds changed during the pyrolysis process. As seen in Table 2, aliphatic sulfur was formed at 673 and 773 K (Eq. (1)). But these aliphatic compounds were unstable and were broken down at higher temperatures.
S2
S4 Char 1073K
S1 S2 S5 160
162
164
166
168
S4 170
172
Binding energy (eV) Fig. 2. XPS spectra of S 2p for raw scrap tires and pyrolysis char drawn at different temperatures from 673 to 1073 K (S1: inorganic sulfides, S2: aliphatic sulfur, S3: thiophenic, S4: sulfates, S5: sulfones).
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H. Hu et al. / Chemosphere 97 (2014) 102–107 Table 2 The sulfur forms of raw scrap tires and pyrolysis char drawn at temperatures ranged from 673 to 1073 K (wt%). Sulfur forms
Binding energy (eV)
Raw material
Inorganic sulfides Aliphatic sulfur Thiophenic Sulfones Sulfates
162.2 ± 0.6 163.3 ± 0.4 164.1 ± 0.2 168.2 ± 0.2 170.0 ± 1.0
30.1 – 69.9 – –
Char 673 K
773 K
873 K
973 K
1073 K
37.2 31.1 31.7 – –
67.3 6.0 26.7 – –
68.7 – 16.0 – 15.3
79.4 – 15.3 – 5.3
58.4 – 22.2 11.2 8.2
– Not determined.
pyrolysis gases indicated that a large amount of C–O groups was produced at 1073 K. The interactions between C–O groups and sulfur-containing active groups promoted the oxidation of organic sulfur compounds leading to the formation of high molecularweight sulfur-containing compounds. The formed products of high dew point remained in the char such as sulfones while the products of low dew point were released as volatiles and were finally condensed in tar. Hence, the fraction of sulfur content in the gases decreased while that in the tar increased at this temperature which was consistent with the results shown in Fig. 1b. The produced SH radical reacted with other volatile materials and facilitated the transformation of sulfur into tar in organic forms (Eq. (5)) (Calkins, 1994).
1. CaCO3 2. CaMg(CO3)2 1 354 11
3. SiO2 5 1
4. ZnS 5 Ca2Fe2S2O3
1 1
1073 K
Intensity (A. U.)
1 1 3 4 2 1 1 1 2 1 11 1
973 K
2 1 34 1
1 1 2 1 11 2
873 K
1 34 2 1
1 1 2 1 11 2
773 K
1 1 2 1 11
673 K
1 34
10
20
2
30
40
2
50
60
70
80
90
2θ (°) Fig. 3. XRD patterns of scrap tires pyrolysis char drawn at different temperatures from 673 to 1073 K.
and Mg were dominant in the form of carbonates and no obvious peaks of Ca/Mg sulfides were found. At 1073 K, part of Ca/Mg carbonates were decomposed and the formation of Ca2Fe2S2O3 was observed. The formation of inorganic sulfides was enhanced by increasing of operating temperature. However, at higher temperatures such as 1073 K, the deactivation of the reaction sites might take place due to the particle agglomeration and/or sintering, resulting in the suppress of the sulfides formation. On the other hand, according to the XPS results shown in Table 2, the formation of sulfates was found in the char samples collected at 873, 973 and 1073 K. The formation of sulfates could also enhance sulfur immobilization in the char.
R S ! R þ SH
ð2Þ
SH þ MO ! MS þ OH
ð3Þ
SH þ MCO3 ! MS þ OH þ CO2
ð4Þ
Another interesting phenomenon was that a considerable amount of thiophenic and sulfones was found in the char at 1073 K. It is due to the interactions between volatiles and char which stimulated the formation of thiophenic on the surface of char. And the effect was enhanced at high temperatures. During the investigation of sulfur behavior in coal pyrolysis process, similar phenomenon was found that the secondary reaction between volatiles and char strongly affected the sulfur transformation (Sugawara et al., 2003; Zhang et al., 2013). Additionally, as shown in Fig. SM-2, a certain amount of CO and CO2 was found in the pyrolysis gases at 1073 K. The production of CO and CO2 in
SH þ Volatiles ! Tars
ð5Þ
Table SM-3 shows the tentative GC/MS characterization of sulfur-containing compounds in scrap tires pyrolysis tar. Various types of sulfur-containing compounds were identified in the scrap tires pyrolysis tars collected at 673 and 773 K. In the tars collected at 973 and 1073 K, no sulfur-containing compounds were detected, whereas, a certain content of sulfur in these tars was determined by using a elemental analyzer (shown in Table SM-2). The results demonstrated that sulfur-containing compounds in these tars were of high dew points and were difficult to identify by using the GC– MS. Furthermore, these compounds were probably of high molecular-weight like the sulfur forms in the char collected at 1073 K which was in agreement with the discussion mentioned above. 3.3. Sulfur speciation in scrap tires pyrolysis gases Fig. 4 shows the sulfur-containing gases contents in pyrolysis gases of scrap tires. SO2, H2S, COS and CH3SH were found in the pyrolysis gases and H2S was the main sulfur-containing gas. The release of H2S and COS was of the same trend which was intensified with temperature increasing from 673 to 973 K while suppressed at 1073 K. In addition, the release of SO2 and CH3SH was of the same trend. The release of these gases was enhanced with the temperature increasing from 673 to 773 K while suppressed at higher temperatures. During the pyrolysis process, the decomposition of organic polymer in scrap tires led to the formation of H-containing radicals like CH3 and H (Zhang et al., 2013) which brought about sulfur release in the forms of CH3SH and H2S (Eqs. (6) and (7)). The decomposition of these organic polymers was enhanced with the increasing of temperature. Therefore, more CH3 and H were formed, resulting in the high contents of CH4 and H2 in the gases as shown in Fig. SM-2. The production of H enhanced the formation of H2S, contributing to the high concentration of H2S in gases. Meanwhile, CH3 was more likely combined with H than SH (Eq. (8)). Therefore, the formation of CH3SH was inhibited when more H was produced in the pyrolysis process at high temperatures.
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5000 4000 3000 2000 1000
673 K 773 K 873 K 973 K 1073 K
160 120 80 40 0
Fig. 4. The distribution of sulfur-containing gases during rapid pyrolysis of scrap tires.
SH þ CH3 ! CH3 SH
ð6Þ
SH þ H ! H2 S
ð7Þ
H þ CH3 ! CH4
ð8Þ
The release of SO2 was mainly due to the oxidation of SH by oxygen-containing active radical such as OH (Eq. (9)) (Yan et al., 2005; Zhang and Yani, 2011). Miura et al. (2001) found that the heating rate was an important factor for the formation of SO2. In this research, heating of scrap tires was mainly attributed to radiative heat transfer and the heating rate was low in low-temperature pyrolysis process. As shown in Fig. 4, a large amount of SO2 released at 673 and 773 K. However, during high-temperature pyrolysis, the deactivation of the oxygen-containing functional groups was enhanced through a rapid decomposition. In addition, the interaction between SH and metal oxides was intensified at the oxygen-containing sites by forming sulfates (Eq. (10)). Therefore, less SO2 was formed during pyrolysis at high temperatures.
SH þ OH ! SO2 þ H2
SH þ MO þ OH ! MSO4 þ H2
ð9Þ ð10Þ
COS was formed by decomposition of the organic structures or second reactions of sulfur species (Chu et al., 2008; Zhang et al., 2013). As the temperature increased, the decomposition of the organic compounds of high molecular weight was reinforced and more C–O groups were formed, promoting the release of COS (Eq. (11)). However, a large amount of H2 was produced in high-temperature pyrolysis. The reaction between H2 and COS decreased COS content (Eq. (12)). Furthermore, the interaction between COS and active char or volatiles (Eqs. (13) and (14)) might also restrain the COS release by forming sulfur compounds of high molecular weight in the char.
SH þ C O ! COS þ H
ð11Þ
H2 þ COS ! CO þ H2 S
ð12Þ
COS þ Volatile ! Tars þ CO
ð13Þ
COS þ Char ! Chars þ CO
ð14Þ
4. Conclusions In the present study, the fate of sulfur during rapid pyrolysis of scrap tires by using a vertical furnace was investigated at
temperatures ranged from 673 to 1073 K. The sulfur speciation in the pyrolysis char, tar and gases were observed. In raw scrap tires, sulfur was predominant in the forms of thiophenic and inorganic sulfides. During pyrolysis, temperature played an important role in the sulfur transformation as well as sulfur speciation migration. Organic sulfur compounds were easily decomposed at low temperature 673 K and a large amount of sulfur was released into tar and gases. With temperature increasing from 673 to 773 K, more sulfur was transformed from char to tar and gases. At higher temperatures, the sulfur content in char slightly changed whereas the decomposition of tar contributed to sulfur release into gases. However, at 1073 K, the interactions between volatiles and char and/or among volatiles suppressed sulfur releasing into gases and stimulated the formation of high-molecular-weight sulfur-containing compounds in tar and char. After pyrolysis, almost half of the total content of sulfur in scrap tires remained in the char mainly in the form of sulfides. ZnS was found as the most important sulfide for the sulfur immobilization in the char. Furthermore, the formation of sulfates was observed in the chars at temperatures high than 873 K. In the pyrolysis gases, H2S was the main sulfur-containing gas and other sulfur-containing gases in the forms of CH3SH, COS and SO2 were found in small amounts. Temperature increasing stimulated the cracking of organic polymers in scrap tires and lead to more H2S formation. Acknowledgments This study was financially supported by National Natural Science Foundation of China (Grants, 51076053, 51161140330) and Key Project of Chinese National Programs for Fundamental Research and Development (973 program, 2011CB201505). The authors would also thank to the Analytical and Testing Center of Huazhong University of Science and Technology for the experimental measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2013.10.037. References Alexandre-Franco, M., Fernandez-Gonzlez, C., Alfaro-Domi9 nguez, M., Palacios, J.M., Go9 mez-Serrano, V., 2010. Devulcanization and demineralization of used tire rubber by thermal chemical methods: a study by X-ray diffraction. Energy Fuel. 24, 3401–3409. Calkins, W.H., 1994. The chemical forms of sulfur in coal: a review. Fuel 73, 475– 484. Chen, T.C., Shen, Y.H., Lee, W.J., Lin, C.C., Wan, M.W., 2010. The study of ultrasoundassisted oxidative desulfurization process applied to the utilization of pyrolysis oil from waste tires. J. Clean. Prod. 18, 1850–1858. Chu, X.J., Li, W., Li, B.Q., Chen, H.K., 2008. Sulfur transfers from pyrolysis and gasification of direct liquefaction residue of Shenhua coal. Fuel 87, 211–215. Dog˘an, O., Çelik, M.B., Özdalyan, B., 2012. The effect of tire derived fuel/diesel fuel blends utilization on diesel engine performance and emissions. Fuel 95, 340– 346. Gieré, R., Smith, K., Blackford, M., 2006. Chemical composition of fuels and emissions from a coal + tire combustion experiment in a power station. Fuel 85, 2278–2285. Hu, Y.Q., Watanabe, M., Aida, C., Horio, M., 2006. Capture of H2S by limestone under calcination conditions in a high-pressure fluidized-bed reactor. Chem. Eng. Sci. 61, 1854–1863. Kaminsky, W., Mennerich, C., Zhang, Z., 2009. Feedstock recycling of synthetic and natural rubber by pyrolysis in a fluidized bed. J. Anal. Appl. Pyrol. 85, 334–337. Ko, T.H., Chu, H., Tseng, J.J., 2006. Feasibility study on high-temperature sorption of hydrogen sulfide by natural soils. Chemosphere 64, 881–891. Kozłowski, M., 2004. XPS study of reductively and non-reductively modified coals. Fuel 83, 259–265. Leung, D.Y.C., Yin, X.L., Zhao, Z.L., Xu, B.Y., Chen, Y., 2002. Pyrolysis of tire powder: influence of operation variables on the composition and yields of gaseous product. Fuel Process. Technol. 79, 141–155.
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