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Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS
Journal of the Energy Institute xxx (xxxx) xxx
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Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS Yanan Zhu a, Wu Wen a, Yamin Li a, Lilin Lu b, Jiuzhong Yang a, **, Minggao Xu a, Yang Pan a, * a
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui, 230029, People's Republic of China Hubei Province Key Laboratory of Coal Conversion and New Carbon Materials, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, People's Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2018 Received in revised form 25 January 2019 Accepted 28 January 2019 Available online xxx
The pyrolysis behaviors of Huainan coal (HN coal) with three particle sizes, i.e., <40 mm (HN-S), 224 e500 mm (HN-M) and 1600e2000 mm (HN-L), were investigated by thermogravimetry (TG) and online pyrolysis photoionization time-of-flight mass spectrometry (Py-PI-TOF MS) using a krypton discharge lamp (10.6 eV) and synchrotron radiation vacuum ultraviolet light (SVUV) (14.2 eV and 15.5 eV) as the ionization sources. The fragment-free mass spectra of coal pyrolysis products were obtained at 600 C in real time. The released organic volatiles were characterized as alkenes, phenols and aromatics, and their evolved profiles were measured as a function of time in fixed temperature mode and as a function of temperature in programmed-temperature mode. A higher weight loss ratio of the TG curves was obtained when the particle size was increased. The intensities of the individual volatiles in the mass spectra were all enhanced following the increase in the particle size of HN coal. With regard to the time-evolved profiles, by increasing the particle size, the appearance times of volatiles were delayed. Tricyclic aromatics showed multireleasing processes in both time-evolved and temperature-programmed profiles. The release of the guest molecules lagged within the higher temperature region as the coal particle size increased. To explain the evolution trends of most organic volatiles, a mechanism combining intraparticle and interparticle mechanisms is presented. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Coal pyrolysis Particle size Online Photoionization mass spectrometry
1. Introduction The enormous consumption of coal over the past few decades has caused serious local and global environmental impacts such as acidic gas emissions, global warming and air pollution [1]. However, coal will continue to be a major energy source in the foreseeable future, especially for China. Thus, cleaner and higher efficiency coal conversion technologies are critically needed. Coal pyrolysis has attracted much attention because it represents the primary process to produce valuable products such as liquid fuels, fuel gases and various additional chemicals. Furthermore, pyrolysis is the initial step and exerts a significant influence on the thermochemical conversion processes of coal, such as carbonization, liquefaction, combustion and gasification. Pyrolysis reactions can provide basic information concerning thermal decomposition of individual functional groups and the coal molecular networks [2]. The compositions and release of the pyrolytic products are important for understanding the pyrolysis mechanisms and pollution control strategies. Many factors affect the coal pyrolysis process, including the type of coal [3e6], particle size [1,3,4,7e10], temperature [3,11], pressure [3,8], heating rate [8,9], and residence time [11], among which the influence of particle size is important because mass diffusion and caloric delivery inside coal particles during devolatilization might affect the pyrolysis process. The knowledge of particle size effects on coal pyrolysis product evolution and distribution remains in a state with some disagreements, although many efforts have been attempted. Tian et al. studied the effect of particle size on the pyrolysis characteristics of a bituminous coal, finding that larger particle size indicated higher pyrolysis reactivity [1], and a “metaplast” mechanism was proposed [1,12]. For a larger particle size of Fugu sub-bituminous and Liulin * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yang), [email protected] (Y. Pan). https://doi.org/10.1016/j.joei.2019.01.016 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Nomenclature: HN coal Huainan coal HN-S Huainan coal with <40 mm particle size HN-M Huainan coal with 224e500 mm particle size HN-L Huainan coal with 1600e2000 mm particle size Py-PI-TOF MS pyrolysis photoionization time-of-flight mass spectrometry TG thermogravimetry DTG differential thermogravimetry FTIR Fourier transform infrared spectrometry MS mass spectrometry MW molecular weight GC gas chromatography EI electron ionization TOF MS time-of-flight mass spectrometer SCCM standard cubic centimeter per minute VUV vacuum ultraviolet SVUV synchrotron vacuum ultraviolet light IE ionization energy
bituminous coals, Gong et al. found that the contents of aliphatics were remarkably reduced, while the contents of aromatics were distinctly augmented, which could be attributed to the intraparticle secondary reactions [3]. Based on thermogravimetry/differential thermogravimetry (TG/DTG) analysis and Coats-Redfern kinetic modeling, Duz et al. concluded that activation energy (Ea) increased as the particle size decreased [13]. However, Jayaraman et al. found that activation energy and pre-exponential factor (A) values increased with the increase of particle sizes and that the yields of CO2 and CO were decreased with the larger particle sizes of coal samples [9]. In another work, Liu et al. also suggested that the amounts of CO decreased with the increase of coal particle sizes [14]. Morris and Pather reported that the particle size was dominant in determining the yields of CH4 and CO2 [15,16]. Furthermore, large numbers of investigations were conducted on mass and heat transfer during pyrolysis of different coal particles [7,17e20]. Due to the impedance of internal heat conduction, the internal temperature distribution of large coal particles markedly differs, and so the pyrolysis within the center occurs much later than that on the particle surface [17,18]. This effect also implied that the path length for pore diffusion of the volatiles was reduced as the particle size decreased, thereby reducing the occurrence of secondary reactions within the pores and increasing the volatile matter [19,20]. However, Hanson et al. revealed that the pyrolysis was relatively insensitive to the coal particle size [7]. To understand the particle size effect during coal pyrolysis, aside from the yield of products, the release profiles of individual products should be studied. Several techniques have been utilized to study the pyrolysis of coal and to analyze the particle size effect. TG is widely used in thermal analysis, which could provide information regarding weight loss rate, Ea and A [21,22]. However, qualitative analysis cannot be realized by TG unless combined with Fourier transform infrared spectrometry (FTIR) [1,23,24] and/or gas chromatography-mass spectrometry (GC-MS) [18,25]. Nevertheless, the overlap between the adjacent absorption peaks of FTIR is strong, and only a few products with low molecular weight (MW) can be distinguished. GC-MS is time-consuming and is lacking information regarding nascent products. Pyrolysis photoionization time-of-flight mass spectrometry (Py-PI-TOF MS) using vacuum ultraviolet (VUV) light as the “soft” ionization source and a timeof-flight mass spectrometer (TOF MS) as the mass analyzer has been applied to analyze characteristics of coal pyrolysis [26,27]. Thanks to near-threshold photoionization, fragment-free mass spectra can be obtained and release profiles of individual products as a function of time/temperature can be shown in real time. In this work, the pyrolysis behaviors of Huainan (HN) coal, a bituminous coal obtained from Huainan coal mine (one of the five largest coal mines in China), were studied with thermogravimetry (TG) and Py-PI-TOF MS. Mass spectra of CH4, CO2, CO, H2O, H2 and light organic volatiles, and the time/temperature-evolved profiles of these pyrolysates were obtained in real time. The various pyrolysis characteristics of coal with different particle sizes were observed and used to elucidate the possible reaction and product-releasing mechanisms. 2. Experiments 2.1. Materials and chemicals The coal sample, designated as HN or HN coal, was obtained from Huainan coal mine (Huainan, China). The coal was dried at 105 C in a vacuum drying oven for 24 h. Then, the coal was ground and screened to three particle sizes, i.e., <40 mm, 224e500 mm and 1600e2000 mm, which were referred to as HN-S, HN-M, and HN-L, respectively. N2 (99.999%), He (99.999%) and C2H4 (99.95%) were obtained from Nanjing Special Gas Factory Co., Ltd. (Nanjing, China). 2.2. Proximate and ultimate analysis The proximate analysis was implemented based on the testing standards in China (GB/T212-2008). Ultimate analysis was completed with a Vario EL-II CHNS elemental analyzer (Elementar Analysensysteme Gmbh, Hanau, Germany). The weight percentage of oxygen in the coal sample was calculated by the difference. The results are shown in Table 1. Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Table 1 Proximate and ultimate analysis of HN coal employed in this study. Proximate Analysis (wt%, dry)
Ultimate Analysis (wt%, dry)
Sample
A
V
FCa
C
H
N
St
Oa
C/H
HN-S HN-M HN-L
8.69 8.09 4.26
30.39 30.81 29.85
60.92 61.10 65.89
75.185 80.785 81.050
4.930 5.460 5.450
1.440 1.575 1.675
0.924 0.570 0.486
17.521 11.610 11.339
15.251 14.796 14.872
t: total. A: ash. V: volatiles. FC: fixed carbon. a Estimated by difference.
2.3. TG analysis The weight loss of coal was analyzed by TG (Pyris 1 TGA/Frontier, Perkin Elmer, USA). Approximately 7 mg of each particle size sample was continuously weighed in the TG system while heated from room temperature up to 800 C at a 50 C/min heating rate. The TG system was purged under a helium atmosphere with a flow rate of 50 standard cubic centimeters per minute (SCCM). 2.4. Online Py-PI TOF MS analysis The detailed introduction to the online Py-PI-TOF MS setup can be found in previous research works [26,28,29]. Therefore, only a brief introduction of experimental conditions is provided here. The Py-PI-TOF MS setup consists of a tubular furnace, a transfer line, and a PI-TOF MS (see Fig. S1 in the Supplementary Material). The temperature of the furnace was controlled by a temperature controller (SKY Technology Development Co., Ltd., China). A K-type thermocouple was used to measure and report the temperature of the furnace. The other K-type thermocouple sealed in the quartz sample boat just above the coal sample was used to measure the temperature of coal in real time. The flow rate of the carrier gas (nitrogen) was set at 200 SCCM. When the temperature reached the set value, 20 mg coal samples with different sizes were placed in the quartz sample boat and introduced into the middle position of the furnace. The pyrolysis products were sampled through a deactivated fused-silica capillary (I.D. 250 mm) inside the heated transfer line (250 C) to reach the ionization chamber (0.75 Pa) mounted on the TOF MS, where the products were ionized by the vacuum ultraviolet light emitted from a Kr lamp with a photon energy of 10.6 eV (PKS106, Heraeus, Ltd., Germany). To realize the ionization of products with ionization energy (IE) higher than 10.6 eV, tunable synchrotron radiation light from the Combustion Beamline (BL03U) of the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China was utilized as the light source. The signals of CH4 (IE ¼ 12.6 eV), CO2 (IE ¼ 13.8 eV), CO (IE ¼ 14.0 eV), and H2O (IE ¼ 12.6 eV) were obtained at the photon energy of 14.2 eV, and H2 (IE ¼ 15.4 eV) was obtained at the photon energy of 15.5 eV. To remove the fine particles from the product gas stream, a glass fiber filter with a pore size of 1.2 mm was placed between the transfer line and the furnace. The formed ions were analyzed by a TOF MS. The ion signal was amplified with a preamplifier (VT120C, ORTEC, USA) and recorded by a multiscaler (P7888, FAST Comtec, Germany). The total acquisition time of the fixed-temperature mass spectrum was 250 s at 600 C (the temperature distribution of the reactor at 600 C is shown in Fig. S2). The time-evolved profiles of pyrolysis products were recorded continuously for 250 s at 5 s intervals, with the background noise subtracted. The temperature-programmed pyrolysis process was measured from 50 to 800 C at a heating rate of 50 C/ min (temperature profiles of HN-S, HN-M and HN-L can be found in Fig. S3). Before measurement, the coal sample was preheated at 50 C for 10 min. The temperature-programmed profiles of volatiles were recorded continuously at 10 s intervals. Mass spectra, time-evolved profiles and temperature-programmed profiles were repeated three times. To avoid the effect of light attenuation, the intensity of C2H4 (99.95%) was used to calibrate the intensities of volatiles. 3. Results and discussion 3.1. Proximate and ultimate analysis Table 1 shows the proximate and ultimate analysis of HN coal. For the proximate analysis, only a slight reduction in the yields of volatiles following a nearly 50-fold increase of particle diameter from 40 mm to 2000 mm was found. The ash content was found to be reduced with the increase of coal particle size. After grinding and sieving, the variation in ash content can be attributed to the higher mineral matter content in the smaller particles of the coal samples [4,7,30]. In this work, the ash content of HN-L (ash, 4.26%) is less than that of HN-M (ash, 8.09%) and HN-S (ash, 8.69%). Even so, the ash content is less than 10%, which shows that the HN coal used in this work is low ash coal. Morris studied the effect of particle size on the evolution rate of volatiles from low ash coal and declared that mineral materials exerted almost no effect on the coal pyrolysis characteristics [31]. For the ultimate analysis, the oxygen content and sulfur content were increased with the decrease of particle size. This effect was mainly caused by the enrichment of oxygen-containing and sulfur-containing minerals in smaller coal particles after grinding and sieving [4,7,13,32]. In addition, considering the mass balance, the carbon content would be reduced. The values of the atomic ratio of C/H are approximately identical for the three size fractions of HN coal, indicating no significant difference in maceral distribution among the three size fractions [4]. 3.2. Thermal behavior of HN coals The TG and DTG curves of HN coal samples as a function of reaction temperature are illustrated in Fig. 1(a) and Fig. 1(b), respectively. In general, the first stage is from 50 to 350 C. Weight loss in the temperature range 50e200 C is assigned to the vaporization of physically adsorbed moisture, which is not obvious because of the dried basis. DTG peaks of HN-S, HN-M and HN-L (from 200 to 350 C) with no Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 1. (a) TG curves and (b) DTG curves of HN coal with different particle sizes at 50 C/min heating rate.
appreciable weight loss were likely due to the evolution of guest molecules [33] such as aromatics, which could also be verified in the aromatic evolution profiles in section 3.4.3. The second stage, from approximately 350 to 600 C, is ascribed to the dominant pyrolysis process, which produces abundant organic compounds and inorganic gases such as CO2, CO, H2, H2O. As shown in Fig. 1(a), the weight losses of HN-S, HN-M and HN-L are 23.00%, 25.04% and 28.78%, respectively. This reveals that the larger particle size leads to the higher conversion ratio of coal. In addition, Fig. 1(b) shows that higher maximum weight loss of the second stage is obtained when the particle size is increased, indicating that HN coal with a larger particle size produces higher yields of pyrolysis volatiles.
3.3. Volatile release analysis by online Py-PI-TOF MS 3.3.1. Fixed-temperature mass spectra Due to the “soft” ionization characteristics of online Py-PI-TOF MS, the fragment-free mass spectra of coal pyrolysis products could be obtained. As shown in Fig. 2(a), the pyrolysates of HN-S are mainly separated into several categories, i.e., alkanes (m/z 58, 72), alkenes (m/z 28, 42, 56, 70 and 84), dienes or alkynes (m/z 40, 54, 68, 82), aromatics (m/z 78, 92, 104, 106, 120, 128, 142, 156, 170), and phenols (m/z 94, 108, 122). Small amounts of nitrogen and sulfur-containing compounds, such as NH3 (m/z 17) and H2S (m/z 34), could also be detected. Fig. 2(b) demonstrates the mass spectrum obtained at the photon energy of 14.2 eV, where CH4, CO2, CO, and H2O are ionized and observed. H2 can be found at 15.5 eV (not shown). On the basis of previous studies [27,34], the identification of the main pyrolysis products of coal were performed and listed in Table 2. Coal particle size is a critical factor for the yields of volatiles. Fig. 3 shows the integral intensities of several selected pyrolytic products at 600 C, where the amounts of CH4, CO2, CO, H2O, H2, alkenes (m/z 28, 42, 56, 70 and 84), phenols (m/z 94, 108 and 122), aromatics (m/z 78, 92, 106, 120, 128, 142, 156, 178, 192 and 206), H2S, and NH3 are all increased following the increase of particle sizes of HN coal. 3.3.2. Time-evolved profiles of volatiles By means of the online sampling capability of Py-PI-TOF MS, the intensities of pyrolysis products with the lapse of time were obtained during the pyrolysis process. Fig. 4 exhibits the time-evolved profiles of six characteristic pyrolysis products. As shown in Fig. 4(aec), the profiles of relatively low MW molecules, such as propene (m/z 42), toluene (m/z 92), and cresol (m/z 108), all present one peak at approximately 56 s (for HN-S), indicating that these products are only produced by the pyrolysis of coal. However, double peaks or shoulder Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 2. Mass spectra of HN-S coal at 600 C (a) at the photon energy of 10.6 eV and (b) photon energy of 14.2 eV. Table 2 Mass assignments of the primary pyrolytic products of the HN coal samples. Mass (m/z)
Name or Type
Formula
Mass (m/z)
Name or Type
Formula
2 16 17 18 28
hydrogen methane ammonia water ethylene carbon monoxide hydrogen sulfide propyne allene propene carbon dioxide vinylacetylene 1,3-butadiene butene butane 1,3-cyclopentadiene cyclopentene 1,3-pentadiene,(E)furan pentene pentane benzene 1-methyl-1,3-cyclopentadiene hexadiene 2-methyl-furan hexane
H2 CH4 NH3 H2O C2H4 CO H2S C3H4 C3H4 C3H6 CO2 C4H4 C4H6 C4H8 C4H10 C5H6 C5H8 C5H8 C4H4O C5H10 C5H12 C6H6 C6H8 C6H10 C5H6O C6H12
92 94 102 104 106 108 110 116
toluene phenol phenylacetylene styrene C2 alkyl benzenes cresol thiophenol C3 alkynyl benzenes indene C3 alkenyl benzenes indane C3 alkyl benzenes C2 alkyl phenols C3 alkyl phenols methylcatechol guaiacol naphthalene C1 alkyl naphthalenes C2 alkyl naphthalenes C3 alkyl naphthalenes phenanthrene anthracene C1 alkyl phenanthrenes C1 alkyl anthracenes C2 alkyl phenanthrenes C2 alkyl anthracenes
C7H8 C6H6O C8H6 C8H8 C8H10 C7H8O C6H6S C9H8 C9H8 C9H10 C9H10 C9H12 C8H10O C9H12O C7H8O2 C7H8O2 C10H8 C11H10 C12H12 C13H14 C14H10 C14H10 C15H12 C15H12 C16H14 C16H14
34 40 42 44 52 54 56 58 66 68
70 72 78 80 82 82 84
118 120 122 136 124 128 142 156 170 178 192 206
peaks are found in the profiles of PAHs (Fig. 4(def)). For example, the profile of phenanthrene/anthracene (m/z 178) of HN-S exhibits two peaks at 25 and 56 s. This phenomenon can be explained by the occurrences of two processes, i.e., guest molecules releasing and subsequent coal pyrolysis. Moreover, heat transfer will greatly affect the releasing rates of volatiles. As shown in Fig. 4, the appearance times of all of the pyrolytic products are delayed for several seconds as the coal particle size increases, especially for PAHs. It is worth noting that the first guest release Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 3. Peak areas of main pyrolysis products of HN-S, HN-M and HN-L coal at 600 C: (a) CH4, CO2, CO, H2O (obtained at 14.2 eV) and H2 (obtained at 15.5 eV); (b) main organic products, NH3 and H2S (obtained at 10.6 eV).
process is considerably delayed, making the double peaks undistinguishable (Fig. 4(def)). Hanson et al. also concluded that the start of gas evolution generally showed a rising trend with particle size using a spouted bed reactor combined with a quadrupole mass spectrometer [7]. 3.3.3. Temperature-programmed profiles of volatiles The temperature-programmed profiles acquired from Py-PI-TOF MS could demonstrate the dynamic temperature-varying tendency of individual products, which is important to better understand the conversion and releasing mechanisms. The temperature-programmed profiles of some alkenes and phenols of HN-S, HN-M and HN-L coal at 50 C/min heating rate are shown in Fig. 5. The main pyrolysis region ranged from 300 to 600 C. From these profiles, it is obvious that the peak areas increase as the coal particle size increases, which is consistent with the TG/DTG curves. The trends of temperature-programmed profiles of benzene derivatives are similar to the temperature profiles of alkenes and phenols, which show a single peak in the temperature range of 300e600 C. In addition, it can be found that these evolution profiles typically consist of a peak with shoulder or multiple peaks, especially with respect to the tricyclic compounds. The peaks at relatively low temperatures (150e350 C for HN-S, 200e400 C for HN-M as well as HN-L) arise from the evaporation of unattached guest molecules (Fig. 6(gei)) [35]. The higher temperature peak ranging from 400 to 600 C is due to the release of coal fragments by bond breaking, polymerization reactions of light pyrolysis products, and the secondary pyrolysis of tar. For tricyclic pyrolysates in Fig. 6(gei), the first peak at relatively low temperature moves to higher temperature when the particle size of coal is enlarged from HN-S (<40 mm) to HN-M (224e500 mm). The HN-L coal with larger size of 1600e2000 mm exhibits nearly the same temperature-programmed profile as HN-M coal. This lag effect of the lowtemperature peak is primarily attributed to the fact that the heat transfer for larger coal particles from the external atmosphere to the internal space is more difficult. 3.4. The volatile product releasing mechanism Three stages in the evolution of volatiles have been reported in the slow pyrolysis of coal [15]. The first stage is the production of primary volatiles by thermal decomposition at the solid-gas interface and in the inner region of coal particles. In the second stage, secondary Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 4. Time-evolved profiles of typical organic species at different particle sizes at 600 C: (a) m/z 42 propene; (b) m/z 92 toluene; (c) m/z 108 cresol; (d) m/z 178 phenanthrene/ anthracene; (e) m/z 192 C1 alkyl phenanthrenes/anthracenes; (f) m/z 206 C2 alkyl phenanthrenes/anthracenes.
Fig. 5. Temperature-programmed profiles of main alkene and phenol products of HN coal with different particle sizes at 50 C/min: (a) m/z 42 propene; (b) m/z 56 butene; (c) m/z 70 pentene; (d) m/z 94 phenol; (e) m/z 108 cresol; (f) C2 alkyl phenols.
volatiles are produced by cracking or polymerization of the primary volatiles during diffusion through the intraparticle area to the surface of the particle. During the third stage, the freshly evolved volatiles undergo chemical changes after releasing from the surface of the particle, which occurs within the voids between the particles [15,36]. In this work, the first and second stages are considered as the intraparticle process, and the third stage is ascribed to the interparticle process. With regard to the intraparticle process (see the single particle evolution in Fig. 7), larger coal particles have a relatively low heating rate compared to small coal particles. Therefore, the formation rates of volatiles are decreased due to slower heat transfer, and the appearance times of volatiles are delayed (see Fig. 4). Some literature publications have developed models and showed that heat transfer is the rate-
Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 6. Temperature-programmed profiles of main aromatic products of HN coal with different particle sizes at 50 C/min: (a) m/z 78 benzene; (b) m/z 92 toluene; (c) m/z 106 C2 alkyl benzenes; (d) m/z 128 naphthalene; (e) m/z 142 C1 alkyl naphthalenes; (f) m/z 156 C2 alkyl naphthalenes; (g) m/z 178 phenanthrene/anthracene; (h) m/z 192 C1 alkyl phenanthrenes/anthracenes; (i) m/z 206 C2 alkyl phenanthrenes/anthracenes.
Fig. 7. Volatile evolving processes during pyrolysis of large and small coal particles.
controlling factor within coal particles, especially for the large coal particles during coal pyrolysis [37e39]. In addition, larger particles require longer transport lengths to release the newly formed volatiles. Hence, the opportunity for intraparticle secondary reactions would be increased [3,14]. As a result, coal with larger particle sizes undergoes more severe pyrolysis reactions and exhibits higher yields of secondary volatiles. Compared to the intraparticle process, the interparticle process is more complex. The neighboring particles have approximately the same temperature, so interparticle heat transfer can be ignored [40]. As shown in Fig. 7, during coal pyrolysis, “metaplast” would be formed and result in coal particle softening and intergranular swelling, leading to the coalescence of the coal particles [12,41e43]. It was reported that metaplast was an intermediate during a series of consecutive reactions: coal/ metaplast/ semicoke / coke [41]. Metaplast was also deemed to be the mixture of partially depolymerized coal and the fragments formed during coal pyrolysis [12,43]. Since metaplast was formed, coal particle morphology would be drastically changed and the interparticle space would be reduced. Consequently, the escape of volatiles from the particle surface to the external atmosphere would be hindered [1]. The larger particles exhibit lower swelling ratio [44] and increased interparticle space, which is beneficial to the free escape of the volatiles evolved from the coal particles and the release of more volatiles. In contrast, smaller particles display reduced interparticle space, cracking and polymerization among the fresh volatiles, and coal particles are more severe, which would affect the yield of volatiles. Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Based on the above results, both intraparticle and interparticle processes play important roles in coal pyrolysis. The intraprocess plays a key role in the appearance time and the yield of volatiles. For large particle size coal, heat transfer is impeded, resulting in the delayed evolution of volatiles. Furthermore, more intense secondary reactions lead to enhanced yields of volatiles. In contrast, the interprocess contributes to the yield of volatiles, but it is not as severe as the intraprocess. 4. Conclusions The pyrolysis products of HN coal with different particle sizes were investigated with TG and online Py-PI-TOF MS. TG curves show that larger particle size coal exhibited increased weight loss. Mass spectra at fixed temperature show that the yields of CH4, CO2, CO, H2O, H2, light alkenes, phenols and aromatics increase with the enlargement of particle size. The appearance times of pyrolysis products are delayed following the increase of particle size, which can be accounted for by the obstruction of heat transfer. The time-evolved profiles of PAHs show multireleasing processes, which verifies the existence of a guest molecule releasing process. The release of the guest molecules lagged within the higher temperature region as the coal particle size increased. These findings reveal that particle size is a predominant factor for product yield and release processes. The volatile product release mechanism was explained by the intraparticle and interparticle processes. Conflicts of interest The authors declare no competing financial interest. Acknowledgments This work was supported by grants from the National Key Research and Development Program of China (2017YFA0402800), the Natural Science Foundation of China (No. 91545120, 91845203, and 51706217), the Chinese Universities Scientific Fund, Anhui Provincial Natural Science Foundation (1708085ME103), the Major/Innovative Program of the Development Foundation of Hefei Center for Physical Science and Technology (2016FXCX008), and the Users with Excellence Project of Hefei Science Center CAS (2018HSC-UE001). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.joei.2019.01.016. References [1] B. Tian, Y.Y. Qiao, Y.Y. Tian, Q. Liu, Investigation on the effect of particle size and heating rate on pyrolysis characteristics of a bituminous coal by TGeFTIR, J. Anal. Appl. Pyrol. (121) (2016) 376e386. [2] Y. Fei, L. Giroux, M. Marshall, W. Roy Jackson, J.A. MacPhee, J.P. Charland, A.L. 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Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS Yanan Zhu a, Wu Wen a, Yamin Li a, Lilin Lu b, Jiuzhong Yang a, **, Minggao Xu a, Yang Pan a, * a
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui, 230029, People's Republic of China Hubei Province Key Laboratory of Coal Conversion and New Carbon Materials, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, People's Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2018 Received in revised form 25 January 2019 Accepted 28 January 2019 Available online xxx
The pyrolysis behaviors of Huainan coal (HN coal) with three particle sizes, i.e., <40 mm (HN-S), 224 e500 mm (HN-M) and 1600e2000 mm (HN-L), were investigated by thermogravimetry (TG) and online pyrolysis photoionization time-of-flight mass spectrometry (Py-PI-TOF MS) using a krypton discharge lamp (10.6 eV) and synchrotron radiation vacuum ultraviolet light (SVUV) (14.2 eV and 15.5 eV) as the ionization sources. The fragment-free mass spectra of coal pyrolysis products were obtained at 600 C in real time. The released organic volatiles were characterized as alkenes, phenols and aromatics, and their evolved profiles were measured as a function of time in fixed temperature mode and as a function of temperature in programmed-temperature mode. A higher weight loss ratio of the TG curves was obtained when the particle size was increased. The intensities of the individual volatiles in the mass spectra were all enhanced following the increase in the particle size of HN coal. With regard to the time-evolved profiles, by increasing the particle size, the appearance times of volatiles were delayed. Tricyclic aromatics showed multireleasing processes in both time-evolved and temperature-programmed profiles. The release of the guest molecules lagged within the higher temperature region as the coal particle size increased. To explain the evolution trends of most organic volatiles, a mechanism combining intraparticle and interparticle mechanisms is presented. © 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Coal pyrolysis Particle size Online Photoionization mass spectrometry
1. Introduction The enormous consumption of coal over the past few decades has caused serious local and global environmental impacts such as acidic gas emissions, global warming and air pollution [1]. However, coal will continue to be a major energy source in the foreseeable future, especially for China. Thus, cleaner and higher efficiency coal conversion technologies are critically needed. Coal pyrolysis has attracted much attention because it represents the primary process to produce valuable products such as liquid fuels, fuel gases and various additional chemicals. Furthermore, pyrolysis is the initial step and exerts a significant influence on the thermochemical conversion processes of coal, such as carbonization, liquefaction, combustion and gasification. Pyrolysis reactions can provide basic information concerning thermal decomposition of individual functional groups and the coal molecular networks [2]. The compositions and release of the pyrolytic products are important for understanding the pyrolysis mechanisms and pollution control strategies. Many factors affect the coal pyrolysis process, including the type of coal [3e6], particle size [1,3,4,7e10], temperature [3,11], pressure [3,8], heating rate [8,9], and residence time [11], among which the influence of particle size is important because mass diffusion and caloric delivery inside coal particles during devolatilization might affect the pyrolysis process. The knowledge of particle size effects on coal pyrolysis product evolution and distribution remains in a state with some disagreements, although many efforts have been attempted. Tian et al. studied the effect of particle size on the pyrolysis characteristics of a bituminous coal, finding that larger particle size indicated higher pyrolysis reactivity [1], and a “metaplast” mechanism was proposed [1,12]. For a larger particle size of Fugu sub-bituminous and Liulin * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yang), [email protected] (Y. Pan). https://doi.org/10.1016/j.joei.2019.01.016 1743-9671/© 2019 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Nomenclature: HN coal Huainan coal HN-S Huainan coal with <40 mm particle size HN-M Huainan coal with 224e500 mm particle size HN-L Huainan coal with 1600e2000 mm particle size Py-PI-TOF MS pyrolysis photoionization time-of-flight mass spectrometry TG thermogravimetry DTG differential thermogravimetry FTIR Fourier transform infrared spectrometry MS mass spectrometry MW molecular weight GC gas chromatography EI electron ionization TOF MS time-of-flight mass spectrometer SCCM standard cubic centimeter per minute VUV vacuum ultraviolet SVUV synchrotron vacuum ultraviolet light IE ionization energy
bituminous coals, Gong et al. found that the contents of aliphatics were remarkably reduced, while the contents of aromatics were distinctly augmented, which could be attributed to the intraparticle secondary reactions [3]. Based on thermogravimetry/differential thermogravimetry (TG/DTG) analysis and Coats-Redfern kinetic modeling, Duz et al. concluded that activation energy (Ea) increased as the particle size decreased [13]. However, Jayaraman et al. found that activation energy and pre-exponential factor (A) values increased with the increase of particle sizes and that the yields of CO2 and CO were decreased with the larger particle sizes of coal samples [9]. In another work, Liu et al. also suggested that the amounts of CO decreased with the increase of coal particle sizes [14]. Morris and Pather reported that the particle size was dominant in determining the yields of CH4 and CO2 [15,16]. Furthermore, large numbers of investigations were conducted on mass and heat transfer during pyrolysis of different coal particles [7,17e20]. Due to the impedance of internal heat conduction, the internal temperature distribution of large coal particles markedly differs, and so the pyrolysis within the center occurs much later than that on the particle surface [17,18]. This effect also implied that the path length for pore diffusion of the volatiles was reduced as the particle size decreased, thereby reducing the occurrence of secondary reactions within the pores and increasing the volatile matter [19,20]. However, Hanson et al. revealed that the pyrolysis was relatively insensitive to the coal particle size [7]. To understand the particle size effect during coal pyrolysis, aside from the yield of products, the release profiles of individual products should be studied. Several techniques have been utilized to study the pyrolysis of coal and to analyze the particle size effect. TG is widely used in thermal analysis, which could provide information regarding weight loss rate, Ea and A [21,22]. However, qualitative analysis cannot be realized by TG unless combined with Fourier transform infrared spectrometry (FTIR) [1,23,24] and/or gas chromatography-mass spectrometry (GC-MS) [18,25]. Nevertheless, the overlap between the adjacent absorption peaks of FTIR is strong, and only a few products with low molecular weight (MW) can be distinguished. GC-MS is time-consuming and is lacking information regarding nascent products. Pyrolysis photoionization time-of-flight mass spectrometry (Py-PI-TOF MS) using vacuum ultraviolet (VUV) light as the “soft” ionization source and a timeof-flight mass spectrometer (TOF MS) as the mass analyzer has been applied to analyze characteristics of coal pyrolysis [26,27]. Thanks to near-threshold photoionization, fragment-free mass spectra can be obtained and release profiles of individual products as a function of time/temperature can be shown in real time. In this work, the pyrolysis behaviors of Huainan (HN) coal, a bituminous coal obtained from Huainan coal mine (one of the five largest coal mines in China), were studied with thermogravimetry (TG) and Py-PI-TOF MS. Mass spectra of CH4, CO2, CO, H2O, H2 and light organic volatiles, and the time/temperature-evolved profiles of these pyrolysates were obtained in real time. The various pyrolysis characteristics of coal with different particle sizes were observed and used to elucidate the possible reaction and product-releasing mechanisms. 2. Experiments 2.1. Materials and chemicals The coal sample, designated as HN or HN coal, was obtained from Huainan coal mine (Huainan, China). The coal was dried at 105 C in a vacuum drying oven for 24 h. Then, the coal was ground and screened to three particle sizes, i.e., <40 mm, 224e500 mm and 1600e2000 mm, which were referred to as HN-S, HN-M, and HN-L, respectively. N2 (99.999%), He (99.999%) and C2H4 (99.95%) were obtained from Nanjing Special Gas Factory Co., Ltd. (Nanjing, China). 2.2. Proximate and ultimate analysis The proximate analysis was implemented based on the testing standards in China (GB/T212-2008). Ultimate analysis was completed with a Vario EL-II CHNS elemental analyzer (Elementar Analysensysteme Gmbh, Hanau, Germany). The weight percentage of oxygen in the coal sample was calculated by the difference. The results are shown in Table 1. Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Table 1 Proximate and ultimate analysis of HN coal employed in this study. Proximate Analysis (wt%, dry)
Ultimate Analysis (wt%, dry)
Sample
A
V
FCa
C
H
N
St
Oa
C/H
HN-S HN-M HN-L
8.69 8.09 4.26
30.39 30.81 29.85
60.92 61.10 65.89
75.185 80.785 81.050
4.930 5.460 5.450
1.440 1.575 1.675
0.924 0.570 0.486
17.521 11.610 11.339
15.251 14.796 14.872
t: total. A: ash. V: volatiles. FC: fixed carbon. a Estimated by difference.
2.3. TG analysis The weight loss of coal was analyzed by TG (Pyris 1 TGA/Frontier, Perkin Elmer, USA). Approximately 7 mg of each particle size sample was continuously weighed in the TG system while heated from room temperature up to 800 C at a 50 C/min heating rate. The TG system was purged under a helium atmosphere with a flow rate of 50 standard cubic centimeters per minute (SCCM). 2.4. Online Py-PI TOF MS analysis The detailed introduction to the online Py-PI-TOF MS setup can be found in previous research works [26,28,29]. Therefore, only a brief introduction of experimental conditions is provided here. The Py-PI-TOF MS setup consists of a tubular furnace, a transfer line, and a PI-TOF MS (see Fig. S1 in the Supplementary Material). The temperature of the furnace was controlled by a temperature controller (SKY Technology Development Co., Ltd., China). A K-type thermocouple was used to measure and report the temperature of the furnace. The other K-type thermocouple sealed in the quartz sample boat just above the coal sample was used to measure the temperature of coal in real time. The flow rate of the carrier gas (nitrogen) was set at 200 SCCM. When the temperature reached the set value, 20 mg coal samples with different sizes were placed in the quartz sample boat and introduced into the middle position of the furnace. The pyrolysis products were sampled through a deactivated fused-silica capillary (I.D. 250 mm) inside the heated transfer line (250 C) to reach the ionization chamber (0.75 Pa) mounted on the TOF MS, where the products were ionized by the vacuum ultraviolet light emitted from a Kr lamp with a photon energy of 10.6 eV (PKS106, Heraeus, Ltd., Germany). To realize the ionization of products with ionization energy (IE) higher than 10.6 eV, tunable synchrotron radiation light from the Combustion Beamline (BL03U) of the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China was utilized as the light source. The signals of CH4 (IE ¼ 12.6 eV), CO2 (IE ¼ 13.8 eV), CO (IE ¼ 14.0 eV), and H2O (IE ¼ 12.6 eV) were obtained at the photon energy of 14.2 eV, and H2 (IE ¼ 15.4 eV) was obtained at the photon energy of 15.5 eV. To remove the fine particles from the product gas stream, a glass fiber filter with a pore size of 1.2 mm was placed between the transfer line and the furnace. The formed ions were analyzed by a TOF MS. The ion signal was amplified with a preamplifier (VT120C, ORTEC, USA) and recorded by a multiscaler (P7888, FAST Comtec, Germany). The total acquisition time of the fixed-temperature mass spectrum was 250 s at 600 C (the temperature distribution of the reactor at 600 C is shown in Fig. S2). The time-evolved profiles of pyrolysis products were recorded continuously for 250 s at 5 s intervals, with the background noise subtracted. The temperature-programmed pyrolysis process was measured from 50 to 800 C at a heating rate of 50 C/ min (temperature profiles of HN-S, HN-M and HN-L can be found in Fig. S3). Before measurement, the coal sample was preheated at 50 C for 10 min. The temperature-programmed profiles of volatiles were recorded continuously at 10 s intervals. Mass spectra, time-evolved profiles and temperature-programmed profiles were repeated three times. To avoid the effect of light attenuation, the intensity of C2H4 (99.95%) was used to calibrate the intensities of volatiles. 3. Results and discussion 3.1. Proximate and ultimate analysis Table 1 shows the proximate and ultimate analysis of HN coal. For the proximate analysis, only a slight reduction in the yields of volatiles following a nearly 50-fold increase of particle diameter from 40 mm to 2000 mm was found. The ash content was found to be reduced with the increase of coal particle size. After grinding and sieving, the variation in ash content can be attributed to the higher mineral matter content in the smaller particles of the coal samples [4,7,30]. In this work, the ash content of HN-L (ash, 4.26%) is less than that of HN-M (ash, 8.09%) and HN-S (ash, 8.69%). Even so, the ash content is less than 10%, which shows that the HN coal used in this work is low ash coal. Morris studied the effect of particle size on the evolution rate of volatiles from low ash coal and declared that mineral materials exerted almost no effect on the coal pyrolysis characteristics [31]. For the ultimate analysis, the oxygen content and sulfur content were increased with the decrease of particle size. This effect was mainly caused by the enrichment of oxygen-containing and sulfur-containing minerals in smaller coal particles after grinding and sieving [4,7,13,32]. In addition, considering the mass balance, the carbon content would be reduced. The values of the atomic ratio of C/H are approximately identical for the three size fractions of HN coal, indicating no significant difference in maceral distribution among the three size fractions [4]. 3.2. Thermal behavior of HN coals The TG and DTG curves of HN coal samples as a function of reaction temperature are illustrated in Fig. 1(a) and Fig. 1(b), respectively. In general, the first stage is from 50 to 350 C. Weight loss in the temperature range 50e200 C is assigned to the vaporization of physically adsorbed moisture, which is not obvious because of the dried basis. DTG peaks of HN-S, HN-M and HN-L (from 200 to 350 C) with no Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 1. (a) TG curves and (b) DTG curves of HN coal with different particle sizes at 50 C/min heating rate.
appreciable weight loss were likely due to the evolution of guest molecules [33] such as aromatics, which could also be verified in the aromatic evolution profiles in section 3.4.3. The second stage, from approximately 350 to 600 C, is ascribed to the dominant pyrolysis process, which produces abundant organic compounds and inorganic gases such as CO2, CO, H2, H2O. As shown in Fig. 1(a), the weight losses of HN-S, HN-M and HN-L are 23.00%, 25.04% and 28.78%, respectively. This reveals that the larger particle size leads to the higher conversion ratio of coal. In addition, Fig. 1(b) shows that higher maximum weight loss of the second stage is obtained when the particle size is increased, indicating that HN coal with a larger particle size produces higher yields of pyrolysis volatiles.
3.3. Volatile release analysis by online Py-PI-TOF MS 3.3.1. Fixed-temperature mass spectra Due to the “soft” ionization characteristics of online Py-PI-TOF MS, the fragment-free mass spectra of coal pyrolysis products could be obtained. As shown in Fig. 2(a), the pyrolysates of HN-S are mainly separated into several categories, i.e., alkanes (m/z 58, 72), alkenes (m/z 28, 42, 56, 70 and 84), dienes or alkynes (m/z 40, 54, 68, 82), aromatics (m/z 78, 92, 104, 106, 120, 128, 142, 156, 170), and phenols (m/z 94, 108, 122). Small amounts of nitrogen and sulfur-containing compounds, such as NH3 (m/z 17) and H2S (m/z 34), could also be detected. Fig. 2(b) demonstrates the mass spectrum obtained at the photon energy of 14.2 eV, where CH4, CO2, CO, and H2O are ionized and observed. H2 can be found at 15.5 eV (not shown). On the basis of previous studies [27,34], the identification of the main pyrolysis products of coal were performed and listed in Table 2. Coal particle size is a critical factor for the yields of volatiles. Fig. 3 shows the integral intensities of several selected pyrolytic products at 600 C, where the amounts of CH4, CO2, CO, H2O, H2, alkenes (m/z 28, 42, 56, 70 and 84), phenols (m/z 94, 108 and 122), aromatics (m/z 78, 92, 106, 120, 128, 142, 156, 178, 192 and 206), H2S, and NH3 are all increased following the increase of particle sizes of HN coal. 3.3.2. Time-evolved profiles of volatiles By means of the online sampling capability of Py-PI-TOF MS, the intensities of pyrolysis products with the lapse of time were obtained during the pyrolysis process. Fig. 4 exhibits the time-evolved profiles of six characteristic pyrolysis products. As shown in Fig. 4(aec), the profiles of relatively low MW molecules, such as propene (m/z 42), toluene (m/z 92), and cresol (m/z 108), all present one peak at approximately 56 s (for HN-S), indicating that these products are only produced by the pyrolysis of coal. However, double peaks or shoulder Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 2. Mass spectra of HN-S coal at 600 C (a) at the photon energy of 10.6 eV and (b) photon energy of 14.2 eV. Table 2 Mass assignments of the primary pyrolytic products of the HN coal samples. Mass (m/z)
Name or Type
Formula
Mass (m/z)
Name or Type
Formula
2 16 17 18 28
hydrogen methane ammonia water ethylene carbon monoxide hydrogen sulfide propyne allene propene carbon dioxide vinylacetylene 1,3-butadiene butene butane 1,3-cyclopentadiene cyclopentene 1,3-pentadiene,(E)furan pentene pentane benzene 1-methyl-1,3-cyclopentadiene hexadiene 2-methyl-furan hexane
H2 CH4 NH3 H2O C2H4 CO H2S C3H4 C3H4 C3H6 CO2 C4H4 C4H6 C4H8 C4H10 C5H6 C5H8 C5H8 C4H4O C5H10 C5H12 C6H6 C6H8 C6H10 C5H6O C6H12
92 94 102 104 106 108 110 116
toluene phenol phenylacetylene styrene C2 alkyl benzenes cresol thiophenol C3 alkynyl benzenes indene C3 alkenyl benzenes indane C3 alkyl benzenes C2 alkyl phenols C3 alkyl phenols methylcatechol guaiacol naphthalene C1 alkyl naphthalenes C2 alkyl naphthalenes C3 alkyl naphthalenes phenanthrene anthracene C1 alkyl phenanthrenes C1 alkyl anthracenes C2 alkyl phenanthrenes C2 alkyl anthracenes
C7H8 C6H6O C8H6 C8H8 C8H10 C7H8O C6H6S C9H8 C9H8 C9H10 C9H10 C9H12 C8H10O C9H12O C7H8O2 C7H8O2 C10H8 C11H10 C12H12 C13H14 C14H10 C14H10 C15H12 C15H12 C16H14 C16H14
34 40 42 44 52 54 56 58 66 68
70 72 78 80 82 82 84
118 120 122 136 124 128 142 156 170 178 192 206
peaks are found in the profiles of PAHs (Fig. 4(def)). For example, the profile of phenanthrene/anthracene (m/z 178) of HN-S exhibits two peaks at 25 and 56 s. This phenomenon can be explained by the occurrences of two processes, i.e., guest molecules releasing and subsequent coal pyrolysis. Moreover, heat transfer will greatly affect the releasing rates of volatiles. As shown in Fig. 4, the appearance times of all of the pyrolytic products are delayed for several seconds as the coal particle size increases, especially for PAHs. It is worth noting that the first guest release Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 3. Peak areas of main pyrolysis products of HN-S, HN-M and HN-L coal at 600 C: (a) CH4, CO2, CO, H2O (obtained at 14.2 eV) and H2 (obtained at 15.5 eV); (b) main organic products, NH3 and H2S (obtained at 10.6 eV).
process is considerably delayed, making the double peaks undistinguishable (Fig. 4(def)). Hanson et al. also concluded that the start of gas evolution generally showed a rising trend with particle size using a spouted bed reactor combined with a quadrupole mass spectrometer [7]. 3.3.3. Temperature-programmed profiles of volatiles The temperature-programmed profiles acquired from Py-PI-TOF MS could demonstrate the dynamic temperature-varying tendency of individual products, which is important to better understand the conversion and releasing mechanisms. The temperature-programmed profiles of some alkenes and phenols of HN-S, HN-M and HN-L coal at 50 C/min heating rate are shown in Fig. 5. The main pyrolysis region ranged from 300 to 600 C. From these profiles, it is obvious that the peak areas increase as the coal particle size increases, which is consistent with the TG/DTG curves. The trends of temperature-programmed profiles of benzene derivatives are similar to the temperature profiles of alkenes and phenols, which show a single peak in the temperature range of 300e600 C. In addition, it can be found that these evolution profiles typically consist of a peak with shoulder or multiple peaks, especially with respect to the tricyclic compounds. The peaks at relatively low temperatures (150e350 C for HN-S, 200e400 C for HN-M as well as HN-L) arise from the evaporation of unattached guest molecules (Fig. 6(gei)) [35]. The higher temperature peak ranging from 400 to 600 C is due to the release of coal fragments by bond breaking, polymerization reactions of light pyrolysis products, and the secondary pyrolysis of tar. For tricyclic pyrolysates in Fig. 6(gei), the first peak at relatively low temperature moves to higher temperature when the particle size of coal is enlarged from HN-S (<40 mm) to HN-M (224e500 mm). The HN-L coal with larger size of 1600e2000 mm exhibits nearly the same temperature-programmed profile as HN-M coal. This lag effect of the lowtemperature peak is primarily attributed to the fact that the heat transfer for larger coal particles from the external atmosphere to the internal space is more difficult. 3.4. The volatile product releasing mechanism Three stages in the evolution of volatiles have been reported in the slow pyrolysis of coal [15]. The first stage is the production of primary volatiles by thermal decomposition at the solid-gas interface and in the inner region of coal particles. In the second stage, secondary Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 4. Time-evolved profiles of typical organic species at different particle sizes at 600 C: (a) m/z 42 propene; (b) m/z 92 toluene; (c) m/z 108 cresol; (d) m/z 178 phenanthrene/ anthracene; (e) m/z 192 C1 alkyl phenanthrenes/anthracenes; (f) m/z 206 C2 alkyl phenanthrenes/anthracenes.
Fig. 5. Temperature-programmed profiles of main alkene and phenol products of HN coal with different particle sizes at 50 C/min: (a) m/z 42 propene; (b) m/z 56 butene; (c) m/z 70 pentene; (d) m/z 94 phenol; (e) m/z 108 cresol; (f) C2 alkyl phenols.
volatiles are produced by cracking or polymerization of the primary volatiles during diffusion through the intraparticle area to the surface of the particle. During the third stage, the freshly evolved volatiles undergo chemical changes after releasing from the surface of the particle, which occurs within the voids between the particles [15,36]. In this work, the first and second stages are considered as the intraparticle process, and the third stage is ascribed to the interparticle process. With regard to the intraparticle process (see the single particle evolution in Fig. 7), larger coal particles have a relatively low heating rate compared to small coal particles. Therefore, the formation rates of volatiles are decreased due to slower heat transfer, and the appearance times of volatiles are delayed (see Fig. 4). Some literature publications have developed models and showed that heat transfer is the rate-
Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Fig. 6. Temperature-programmed profiles of main aromatic products of HN coal with different particle sizes at 50 C/min: (a) m/z 78 benzene; (b) m/z 92 toluene; (c) m/z 106 C2 alkyl benzenes; (d) m/z 128 naphthalene; (e) m/z 142 C1 alkyl naphthalenes; (f) m/z 156 C2 alkyl naphthalenes; (g) m/z 178 phenanthrene/anthracene; (h) m/z 192 C1 alkyl phenanthrenes/anthracenes; (i) m/z 206 C2 alkyl phenanthrenes/anthracenes.
Fig. 7. Volatile evolving processes during pyrolysis of large and small coal particles.
controlling factor within coal particles, especially for the large coal particles during coal pyrolysis [37e39]. In addition, larger particles require longer transport lengths to release the newly formed volatiles. Hence, the opportunity for intraparticle secondary reactions would be increased [3,14]. As a result, coal with larger particle sizes undergoes more severe pyrolysis reactions and exhibits higher yields of secondary volatiles. Compared to the intraparticle process, the interparticle process is more complex. The neighboring particles have approximately the same temperature, so interparticle heat transfer can be ignored [40]. As shown in Fig. 7, during coal pyrolysis, “metaplast” would be formed and result in coal particle softening and intergranular swelling, leading to the coalescence of the coal particles [12,41e43]. It was reported that metaplast was an intermediate during a series of consecutive reactions: coal/ metaplast/ semicoke / coke [41]. Metaplast was also deemed to be the mixture of partially depolymerized coal and the fragments formed during coal pyrolysis [12,43]. Since metaplast was formed, coal particle morphology would be drastically changed and the interparticle space would be reduced. Consequently, the escape of volatiles from the particle surface to the external atmosphere would be hindered [1]. The larger particles exhibit lower swelling ratio [44] and increased interparticle space, which is beneficial to the free escape of the volatiles evolved from the coal particles and the release of more volatiles. In contrast, smaller particles display reduced interparticle space, cracking and polymerization among the fresh volatiles, and coal particles are more severe, which would affect the yield of volatiles. Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016
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Based on the above results, both intraparticle and interparticle processes play important roles in coal pyrolysis. The intraprocess plays a key role in the appearance time and the yield of volatiles. For large particle size coal, heat transfer is impeded, resulting in the delayed evolution of volatiles. Furthermore, more intense secondary reactions lead to enhanced yields of volatiles. In contrast, the interprocess contributes to the yield of volatiles, but it is not as severe as the intraprocess. 4. Conclusions The pyrolysis products of HN coal with different particle sizes were investigated with TG and online Py-PI-TOF MS. TG curves show that larger particle size coal exhibited increased weight loss. Mass spectra at fixed temperature show that the yields of CH4, CO2, CO, H2O, H2, light alkenes, phenols and aromatics increase with the enlargement of particle size. The appearance times of pyrolysis products are delayed following the increase of particle size, which can be accounted for by the obstruction of heat transfer. The time-evolved profiles of PAHs show multireleasing processes, which verifies the existence of a guest molecule releasing process. The release of the guest molecules lagged within the higher temperature region as the coal particle size increased. These findings reveal that particle size is a predominant factor for product yield and release processes. The volatile product release mechanism was explained by the intraparticle and interparticle processes. Conflicts of interest The authors declare no competing financial interest. 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Please cite this article as: Y. Zhu et al., Pyrolysis study of Huainan coal with different particle sizes using TG analysis and online Py-PI-TOF MS, Journal of the Energy Institute, https://doi.org/10.1016/j.joei.2019.01.016