Article pubs.acs.org/EF
Characterization of Coke Formed during Thermal Reaction of Tar Junfei Wu, Qingya Liu, Juantao Jiang, Zhengke Li, Lei Shi, Xinge Shi, Yuxin Yan, Xiaojie Cheng, and Zhenyu Liu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China S Supporting Information *
ABSTRACT: Coking of volatiles generated from coal in pyrolysis has been a focal issue in coal pyrolysis and upgrading of coal tar, but limited work can be found in the literature on evolution of coke in composition and structure under the pyrolysis conditions. This work characterizes the coke formed in reaction of a subbituminous coal tar at 300, 400, and 500 °C in 40 min in a semibatch system which allows natural evaporation of light fractions. The coke is categorized into two types, the one suspended in tetrahydrofuran (THF), coke-S, and the one deposited on the wall of tube reactor, coke-D. It is found that coke-D accounts for 70−85% of total coke. With increasing tar reaction temperature and time the quantity of coke increases from 1.0% to 16.3 wt % and the particle size of coke-S increases from a most probable size of approximately 0.1 to 700−800 μm. This change is accompanied by reduction in alkyl side chains and heteroatoms (O, N, and S), as well as the enrichment in the aromatic Car−Car bond, which lead to a decrease in H/C ratio from 0.9 to 0.6 and increase in aromaticity fa from 0.70 to 0.86. The carbon distribution in coke-S is similar to that in bituminous coals and is composed of 3−7 fused aromatic rings. The changes in coke-S also include increase in radical concentration and decreases in the radicals’ g value and line width, indicating continued pyrolysis and condensation of the coke due to the removal of oxygen atoms and side chains on the aromatic structure. When compared with coke-S, coke-D formed under the same conditions is more condensed as indicated by higher radical concentration and lower g value and line width. The morphological change in coke-D includes transformation of small irregular particles to spherical-like particles and to coke film that crack in 30 min at 300 °C or 10 min at 500 °C. Xu and Tomita8 studied volatiles reaction during pyrolysis of Liddell coal in a two-stage fixed-bed reactor, with the first stage for coal pyrolysis and the second stage for volatiles reaction. They found that when the second stage temperature was increased from 500 to 900 °C, while keeping the volatiles reaction time at 7 s, the coke content of tar increased from 1.2 to 4.9 wt % (based on daf coal), corresponding to 20−50 wt % tar conversion to coke. Our recent research on reaction of tar collected from pyrolysis of a subbituminous coal in a fixed-bed reactor showed 1−31 wt % tar conversions to coke in 5 min when the temperature was increased from 420 to 530 °C, and the coking behavior can be expressed by a zero-order or zeroorder plus autocatalytic kinetics.5 It has been reported that the volatiles or tar reaction follows the radical mechanism, involving dissociation of weak covalent bonds in tar to generate radical fragments and reaction of the radical fragments to form heavier and lighter products. During this course some of the radicals are confined or trapped in the heavy products such as coke and can be quantified by electron spin resonance (ESR).1,5 The coke is a generic name of largesize and carbon-rich matters, which are insoluble in some solvents such as tetrahydrofuran (THF). The structure of coke changes during the volatiles or tar reaction, but little work has been reported. Similar researches are in preparation of carbon materials from pitch, such as mesocarbon microbeads (MCMB) at 300−500 °C, which involves pyrolysis and polymerization of asphaltenes. The processes include growth of aromatic
1. INTRODUCTION Fast pyrolysis of low- and mid-rank coals at temperatures higher than 500 °C has been studied as an important route for the production of tar (including chemicals) and fuel gas, because it shortens the production time and presumably increases the tar yields. However, applications of fast coal pyrolysis technologies on a commercial scale frequently run into dusts/volatiles separation problems, which result in high dust content of tar, clogging of the product lines and downstream devices, and difficulties in tar upgrading.1,2 It was recently reported that the dusts are complex in composition because they originate from two sources, partially pyrolyzed coal fines3 and coke formed in reaction of volatiles at temperatures of the pyrolysis and high-temperature dust removal device.1,4,5 The coke formation in the reaction of coal volatiles cannot be fully avoided because it is not easy to cool down the volatiles as soon as they leave the coal surface. Even if it is technically achievable the volatiles would react easily, in the form of tar, during preheating of subsequent upgrading operations, leading to coke deposition in the preheater and/or deactivation of catalysts. It is therefore logical to allow the volatiles to react to a certain extent in the pyrolysis reactor to reduce the content of coke precursors and increase the light tar fraction in the volatile products, as seen in delayed coking in petroleum refining.6 Therefore, understanding the coking behavior in the volatiles reaction, including changes in coke structure and composition, is important for reactor design and process optimization of coal pyrolysis. Limited literatures can be found on coke formation in reaction of volatiles or tar produced from coal pyrolysis.1,5,7−9 © 2016 American Chemical Society
Received: October 19, 2016 Revised: December 5, 2016 Published: December 5, 2016 464
DOI: 10.1021/acs.energyfuels.6b02724 Energy Fuels 2017, 31, 464−472
Article
Energy & Fuels mc (A CalO /A t ) 12 m NCal−O = c (A Cal−O /A t ) 12 m NCar−O = o − NCalO − NCal−O 16 mc (A Car−H /A t ) NHar = 12 m NHal = H − NHar 1
structure,10,11 removal of heteroatoms (O, N and S) in the form of light fractions, and increase in viscosity.12 During processing of petroleum residues, the properties of coke deposited on hydrotreating catalysts also change with operating time and temperature. These changes can be characterized by modern instrumentations including Fourier transform infrared (FT-IR), 13 C cross-polarization magic-angle spinning (CP/MAS) NMR, thermogravimetry (TG), scanning electron microscopy (SEM), and elemental analysis.12−17 To understand the coke formation from volatiles generated in coal pyrolysis this work studies the properties of coke formed in a coal tar under the conditions of typical coal pyrolysis. The coke is characterized by its composition, morphology, structure, and reactivity.
NCalO =
2.1. Coal Pyrolysis and Tar Reaction. The subbituminous coal used contains 37.9 wt % volatile matters, 81.8 wt % C, and 4.3 wt % H, all on a dry-and-ash-free base, daf in short. Its proximate and ultimate analyses were reported elsewhere.5 The coal was ground and sieved to 20−40 mesh and subjected to pyrolysis in a vertical fixed-bed reactor coupled downstream with a U-tube at −13 °C for tar collection. Each pyrolysis experiment used approximately 10 g of coal and was under an Ar (≥99.999%) purging at a flow rate of 50 mL/min, corresponding to a volatiles’ residence time of 12.6 s. The pyrolysis was carried out initially at 110 °C for 1 h to remove moisture and then at a rate of 10 °C/min to 600 °C as reported in detail elsewhere.5 The tar collected in the U-tube was heated offline later to 300, 400, or 500 °C for up to 40 min to evaluate its reaction. The products of the tar reaction were extracted with 120 mL of THF, and the THF-insoluble matters were quantified in two categories. Those suspended in THF are termed coke-S while those deposited on the wall of U-tube are termed coke-D. 2.2. Coke Analysis. The particle sizes of coke-S were analyzed by a laser particle size analyzer (Mastersizer 2000) which is effective in a range of 0.02−2000 μm. The analysis was made to an aqueous solution containing a known dose of THF extract under stirring at 2000 rpm. The mass of coke-S was determined through filtering the THF extract through a hydrophobic filter membrane (PTFE) of 0.22 μm pore size, drying the filter cake at 40 °C under a vacuum for 6 h, and weighing the dried filter cake. The C, H, N, S, and O contents of coke-S were determined by a Vairo EL CUBE elemental analyzer with errors of less than 0.1%. FTIR spectra of coke-S were recorded on a Thermo Nicolet 6700 spectrometer using tablets compressed from well-mixed coke-S and KBr at a mass ratio of 1:200. Each spectrum ranges 4000−400 cm−1 and is an average of 32 scans with 4 cm−1 resolution. The coke-S samples were also analyzed using a solid-state CP/MAS 13 C NMR spectrometer (Bruker Avance 300). The analysis was carried out at 75.48 MHz with a contact time of 1 ms, a rotor-spinning rate of 12 kHz, and a pulse delay of 5 s. The spectra were analyzed by curvefitting using Origin. Equations 1−8 were used to quantify the aromaticity, aliphatic carbon, aromatic carbon, CalO bond, Cal−O bond, Car−O bond, aromatic hydrogen, and aliphatic hydrogen, termed fa, NCal, NCal, NCalO, NCal−O, NCar−O, NHar, and NHal, respectively. In the equations, ACar, ACal, ACalO, ACal−O, ACar−H, and At represent the integral area of aromatic carbons, aliphatic carbons, carbonyl carbons, aliphatic carbons bonded to oxygen, protonated aromatic carbons, and all carbons, respectively. The masses of C, O, and H that are contained in 1 g of coke-S were termed as mc, mO, and mH, respectively.
mc (A Cal /A t ) 12
(2)
NCar =
mc (A Car /A t ) 12
(3)
(6) (7) (8)
3. RESULTS AND DISCUSSION 3.1. Coke Formation in Reaction of Tars. The tar collected in the U-tube accounts 21.3 wt % of coal on a daf
Figure 1. Products distribution of tar subjected to temperatures of 300, 400, 500 °C.
basis. The tar contains approximately 8 wt % water and 0.2 wt % THF-insoluble matter; the latter includes fine particles entrained by the volatiles and coke formed in volatiles reaction in the pyrolysis reactor. Figure 1 shows products distribution of the tar subjected to a further heating to 300, 400, and 500 °C for 5, 15, 30, and 40 min, including a temperature ramping time of approximately 2 min before the tar reached the designated temperature. Repeated runs showed experimental errors of less than 2 wt %. It is can be seen that in 5 min the total yields of
(1)
NCal =
(5)
The radical concentration, line width, and g value of coke-S and some coke-D samples were measured by ESR (Bruker A200) using 1 mg of sample sealed in a glass tube under N2. The measurements were carried out at 25 °C with sweep time of 0.35 min, sweep width of 100 G, and a time constant of 0.04 s. The properties of coke-S were also analyzed with thermal gravimetric analysis (TG, Setaram-Setsys Evolution 1750) coupled online with a mass spectrometer (MS, Balzers Omnistar 200). Each analysis used 7.5 ± 0.2 mg of coke-S under an Ar (≥99.9993%) flow of 50 mL/min and followed a temperature program, from the room temperature to 110 °C at a rate of 10 °C/min and maintained at 110 °C for 30 min to remove the moisture, and from 110 to 600 °C at a rate of 10 °C/min and then maintained at 600 °C for 40 min. The releases of CH4, H2O, CO, CO2, SO2, and C7H8 were recorded by the MS with m/z of 16, 18, 28, 44, 64, and 91, respectively. The morphology of coke-D was analyzed by a scanning electron microscope (SEM, Hitachi S-4700) with a resolution of 2.1 nm for the secondary electron image. The samples were prepared in glass tubes of 2 mm in inner diameter under the same conditions as that in U-tube.
2. EXPERIMENTAL SECTION
fa = A Car /A t
(4)
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Figure 2. Size of particles suspended in THF.
coke-S and coke-D are low at all the temperatures, less than 1 wt %, but that of gas are large, approximately 29.0, 30.4, and 33.2 wt % at 300, 400, and 500 °C, respectively. The similar gas yields may be attributed mainly to evaporation of water and light tar fractions. The increases in reaction time to 15, 30, and 40 min raise the coke yield at increasing rates indicating an autocatalytic characteristic. At 300, 400, and 500 °C in 40 min the quantities of coke-D account for 70−85% the total coke. The gas yield, however, increases slowly with increasing reaction time in comparison to the coke yield, up to 44% in 40 min at 500 °C. It is noted that the coke formation in Figure 1 is significant at 300 °C, a temperature lower than that reported in the literature for batch experiments, 420 °C in a sealed reactor, for example.5 This indicates that the light fractions in tar restrain the condensation of heavy fraction in tar unless the temperature is high enough to cause condensation of the light fractions, such as at 500 °C. 3.2. Particle Size Distribution of Coke-S. Figure 2 shows the particle size distributions of coke-S in tar formed at various temperatures. Each sample was analyzed three times, and the average is shown with residual errors of less than 3%. It can be
Figure 3. FT-IR spectra of the coke-S. 300-15: coke-S formed at 300 °C in 15 min.
Figure 4. Solid-state 13C NMR spectrum of the coke-S formed at 500 °C in 30 min.
Table 1. Yields and Ultimate Analysis of Coke-S heating temp (°C)
heating time (min)
coke-S yield (wt %)
C (wt %)
H (wt %)
N (wt %)
S (wt %)
O (wt %)a
H/C (atomic)
300
15 30 40 15 30 40 15 30 40
0.45 1.11 4.08 0.59 1.54 4.67 0.88 2.92 4.97
76.0 79.6 80.4 80.9 81.9 83.0 80.9 82.4 82.7
5.8 5.1 4.9 4.2 4.3 4.0 4.1 4.1 4.0
2.4 1.9 1.5 1.8 1.5 1.5 1.8 1.5 1.4
0.4 0.2 0.1 0.2 0.2 0.1 0.2 0.1 0.1
15.4 13.2 13.1 12.9 12.1 11.4 13.0 11.9 11.8
0.92 0.77 0.73 0.62 0.63 0.58 0.61 0.59 0.58
400
500
a
By difference; the H/C of tar is 1.3. 466
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Energy & Fuels Table 2. Distribution of Carbon Functionalities in Coke-S Determined by 13C NMR
a
samplea
fa
Cal (mol/g)
Car (mol/g)
CalO (mol/g)
Cal−O (mol/g)
Car−O (mol/g)
Hal (mol/g)
Har (mol/g)
300-30 300-40 400-15 400-30 400-40 500-15 500-30 500-40
0.70 0.74 0.77 0.80 0.82 0.82 0.84 0.86
0.0175 0.0155 0.0142 0.0126 0.0111 0.0110 0.0101 0.0085
0.0443 0.0468 0.0507 0.0533 0.0558 0.0543 0.0565 0.0583
0.0015 0.0014 0.0012 0.0009 0.0009 0.0008 0.0007 0.0007
0.0046 0.0038 0.0040 0.0033 0.0033 0.0036 0.0033 0.0030
0.0033 0.0041 0.0039 0.0043 0.0041 0.0044 0.0045 0.0048
0.0366 0.0327 0.0235 0.0240 0.0200 0.0219 0.0214 0.0198
0.0144 0.0163 0.0185 0.0190 0.0200 0.0191 0.0196 0.0202
300-30: coke-S formed at 300 °C in 30 min.
Figure 5. Relation between content and radical concentration of cokeS.
seen in Figure 2a that the particle size of coke-S in the tar collected from the pyrolysis experiment is in a range of 0.1−1 μm with a peak at 0.16 μm. The coke-S particles grow at all the temperatures as evidenced by the decreasing peak in a range of 0.1−1 μm and the appearance of peaks of larger sizes. At 300 °C the most probable sizes of coke-S are 0.2, 4, 250, and 420 μm in 5, 15, 30, and 40 min of reaction, respectively (Figure 2b). The most probable sizes of coke-S increase further to 3, 250, 600, and 800 μm at 400 °C in 5, 15, 30, and 40 min, respectively (Figure 2c), and to greater than 400 μm at 500 °C for all the heating times, 700 μm in 40 min (Figure 2d), for example. These data indicate that the coke particles suspended in tar, and therefore in THF, agglomerate and/or grow due to radical reactions which generate carbon-rich radicals especially at higher temperatures. The maximum probable particle size of 700−800 μm may suggest that the coke-S of sizes bigger than 700−800 μm precipitates and deposits on the tube wall. 3.3. Composition of Coke-S. Table 1 shows the ultimate analyses of coke-S samples discussed in Figure 2. It should be noted that the C, H, N, S, and O contents of the initial tar are 79.2%, 8.9%, 0.8%, 0.1%, and 11.1%, respectively, and the cokeS data of 5 min at all the temperatures are not shown because the sample sizes are too small to be analyzed. It is clear that the tars’ C content increases while the H content decreases with an increase in temperature or time. The H/C molar ratio of coke-S formed is generally higher at a low temperature but lower at a high temperature, and decreases over time. The highest is 0.92 at 300 °C in 15 min, while the lowest is 0.58 at 400 and 500 °C in 40 min. This trend indicates the structure of coke-S condenses constantly, from approximately two-ring equivalent
Figure 6. (a) g value and (b) line width of radicals in coke-S.
to five-ring equivalent. This trend is similar to that reported for coking (THF-insoluble) of a petroleum-derived residue on a Mo/Al2O3 catalyst at 380 °C under a hydrogen pressure of 12 MPa, where the H/C ratio of coke decreased from 1.6 to 1.1 and then to 0.8 in 1, 120, and 6500 h, respectively.15,16 The contents of N and S in coke-S decrease markedly with increasing temperature and over time and reach constant values of 1.4−1.5 and 0.1 wt %, respectively. These show the quantities of N and S linked with the aromatic carbons because the N and S atoms linked with aliphatic carbons may have already dissociated under the conditions due to their low bond dissociation energies.13,16 It is interesting to note that the O content of coke-S is higher than that of the initial tar and decreases only slightly from 15.3 wt % at 300 °C in 15 min to 11.4−11.8 wt % at 400 and 500 °C 467
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averaged to 3−7 fused rings, which are slightly higher than that discussed earlier based on the overall H/C ratios and are similar to the coke deposited on hydrotreating catalysts during processing of petroleum residues.14,16 Table 2 also shows that the amounts of aliphatic carbon linked to O, i.e., CalO and Cal−O bonds, decrease but the amounts of aromatic carbon linked to O, i.e., Car−O bond, increase with increases in temperature and time. Furthermore, the amounts of Cal−O and Car−O are generally 3 times that of CalO. These data indicate that the coke-S samples are similar to that of bituminous coals.18 It can also be seen in Table 2 that the quantity of Hal decreases while that of Har increases with increasing temperature and time, and both become similar at 400 and 500 °C in 40 min, indicating gradual removal of aliphatic hydrocarbon moieties in coke-S during condensation especially at temperatures of 400 °C and higher. 3.4. ESR Properties of Coke-S. Since the coke formation can be characterized by changes in radicals concentration,5 Figure 5 correlates coke-S contents of tars and radical concentrations of the coke. The coke content and radical concentration of the initial tar are also shown. It is clear that the coke formation shows a two-stage behavior, increasing mass along with a fast increase in radical concentration in the early stage, and increasing mass with little change in radical concentration when the coke content of tar exceeds 1 wt %. Furthermore, a higher temperature leads to a faster increase and a higher maximum in radical concentration. This behavior indicates that coke-S formed in the early stage of reaction contains more aliphatic carbon moieties than that formed in the later stage. If the coke formation initiates from agglomeration of large THF-soluble matters, as reported,5,13,20 it should also be accompanied by more intensive self-condensation than that in the later stage, because the self-condensation involves release of lighter products. This also indicates that the nature of THFsoluble matters contributing to the coke formation varies over time, from rich in aliphatic carbon and heteroatoms initially to rich in aromatic carbon later on. This is understandable for the semibatch tar reaction of this work in which the dealkylation reactions occur not only in THF-insoluble matters but also in THF-soluble matters. This phenomenon may also indicate that the formation of coke on aromatic carbon-rich moieties is easier than that on aliphatic carbon-rich moieties as suggested by the autocatalytic coking mechanism.5 Clearly the structure of cokeS at different conditions differs; the one that experienced higher temperature for a longer time is more aromatic as shown by the characterizations presented above. In principle the g value determined in an ESR spectrum, the ratio of angular momentum and magnetic moment of unpaired electrons, tells the structure of the radicals.21,22 Furthermore, the line width of an ESR spectrum reflects the delocalization spaces of unpaired electrons which relates to the sizes of aromatic structure.23 The data in Figure 6a show that the g values of radicals in coke-S decrease with increases in temperature and time although the temperature effect is not significant between 400 and 500 °C; the trend is consistent with the changes in elemental composition of coke-S in Table 1. The decreases in g value in 5−40 min from 2.003 19 to 2.002 91 at 300 °C and that from 2.003 13 to 2.002 83 at 400 and 500 °C may indicate that the coke-S is mainly composed of fused aromatic rings (with g of 2.0025−2.0029 for π radicals21,24) and heteroatoms (with g of 2.0031 for N-containing radicals, 2.0080−2.0081 for S-containing radicals, and 2.0047−2.0038 for quinones radicals21) because the aliphatic π radicals are low
Figure 7. TG data of coke-S formed at 300, 400, 500 °C in 30 min. 300-30: coke-S formed at 300 °C in 30 min.
in 40 min. This may indicate that O plays an important role in coke formation and its bonding in coke structure is stable under the conditions used. This is similar to that reported by Doolan et al. in which the O in a tar is almost completely transferred into coke in 0.2 s at temperatures of 600−1100 °C.7 Figure 3 shows FT-IR spectra of the initial tar (Figure 3a) and coke-S formed under various heating conditions (Figure 3b−d). The peak intensities of the alkyl hydrogen stretching band (2975−2845 cm−1), oxygen-containing structures CO (1750−1650 cm−1) and aliphatic C−H bending band (1500− 1300 cm−1) are strong in tar but weak or absent in coke-S. The peak intensity of aromatic CC (1630−1500 cm−1) is higher in coke-S than that in tar, where the peak at 1595 cm−1 is typical for cokes.15 The differences in coke-S samples subjected to different heating conditions are not obvious, although the disappearance of peaks for alkyl side chain and aliphatic C−H is consistent with the decrease of H/C ratio shown in Table 1. The enhancement of the CC peak indicates polymerization of aromatic rings. These spectra changes of coke-S are similar to coke deposited on hydrotreating catalysts during processing of petroleum residues.15,17 Figure 4 shows 13C NMR spectrum of coke-S formed at 500 °C in 30 min; the spectra of other coke-S samples are shown in the Supporting Information. These spectra are deconvolved to fit eight subcurves based on chemical shifts, and the correlation coefficients, R2, are all greater than 0.99. The subcurves are ascribed to methyl (−CH3, 10−22 ppm), methylene (−CH2, 22−50 ppm), aliphatic carbon bonded to oxygen (Cal−O, 50− 90 ppm), protonated aromatic carbon (Car−H, 90−129 ppm), aromatic carbon in bridge heads (Car−Car, 129−137 ppm), alkyl-substituted aromatic carbon (Car−Cal/H, 137−148 ppm), aromatic carbon bonded to oxygen (Car−O, 148−164 ppm), and carboxyl and carbonyl carbons (−COOH/CO, 164−220 ppm). These carbon types are quantified in Table 2 for all the coke-S samples except that formed at 300 °C in 15 min due to small sample size. It can be seen that the aromaticity (fa) of coke-S increases with increasing temperature and time due to the decrease in aliphatic carbon (Cal) and the increase in aromatic carbon (Car). These changes are similar to that observed from a subbituminous coal to a bituminous coal18,19 and suggest that the aromatic structure in coke-S can be 468
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Figure 8. MS signals of coke-S formed at 300, 400, 500 °C in 30 min. 300-30: coke-S formed at 300 °C in 30 min.
in g value (2.0025−2.0026).21 The decreasing g value over time may be attributed to decrease in heteroatoms, especially O and N, and increase in aromatic carbon in coke-S. Figure 6b shows that the line width of radicals in coke-S decreases with increasing temperature and time, from 0.550, 0.529, and 0.504 mT in 5 min to 0.441, 0.405, and 0.398 mT in 40 min at 300, 400, and 500 °C, respectively. These changes reflect increases in aromaticity of coke-S during which the line width at 400 °C in 5 min corresponds to a structure with 2−4 fused aromatic rings.23 3.5. Thermal Properties of Coke-S. Figure 7 shows TG data of coke-S samples collected after 30 min of tar reaction at various temperatures. It can be seen in Figure 7a that the total mass losses of coke-S are 32.51%, 22.61%, and 18.08% for cokeS reacted at 300, 400, and 500 °C, respectively, indicating these coke samples are similar to mid- and high-rank bituminous coals in structure. The small and similar mass losses in the temperature range of 110−300 °C, around 2 wt %, indicates the coke-S samples contain limited water and carboxyl as evidenced by the H2O and CO signals shown in Figure 8, parts a and b,
respectively.25 This behavior agrees with the 13C NMR data discussed earlier, where the carboxyl and carbonyl carbons account only 1−2% of total carbon, corresponding to theoretical mass losses of 2−4% if these moieties decompose fully. The releases of small amounts of toluene at temperatures lower than 300 °C in Figure 8f suggest that coke-S contains single aromatic ring linked to aliphatic carbon that dissociates at low temperatures in the presence of unpaired electrons due to decreased bond dissociation energy, approximately 130 kJ/ mol.26,27 The major differences in TG of coke-S samples are in the temperature range of 300−600 °C. It is clear that the coke-S formed at a low temperature decomposes at a low temperature and yields a large mass loss while the coke-S formed at a high temperature decomposes at a high temperature and yields a small mass loss, for example, the differential thermogravimetry (DTG) of coke-S formed at 300, 400, and 500 °C peaks at 435, 520, and 580 °C, respectively. These indicate increased condensation in coke-S at a higher tar reaction temperature. Although the MS cannot detect heavy volatile products, the 469
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Figure 9. SEM of the coke-D formed at 300, 400, 500 °C. 300-1.5: coke-D formed at 300 °C in 1.5 min.
mol.16,26 The higher SO2 release from coke-S obtained from the tar reaction at 300 °C than that at higher temperatures indicates condensation of coke-S. The generation of CH4, C7H8, and C6H6 (not shown in the figure) indicates the presence of aliphatic linkages in coke-S as indicated by the 13C NMR analysis. 3.6. Radical Information and Morphology of Coke-D. As mentioned earlier, the quantities of coke deposited on the
gases detected such as H2O, CO, CO2, SO2, CH4, and C7H8 in Figure 8a−f indicate that the coke-S contains oxygen in the forms of ethers and quinones as discussed earlier. The double SO2 peaks at 320 and 550 °C indicate the presence of two types of S-containing bonds in coke-S, Cal−S and Car−S. The former is low in bond dissociation energy, about 200 kJ/mol or less than 150 kJ/mol in the presence of unpaired electrons.26 The latter is high in bond dissociation energy, about 300 kJ/ 470
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Energy & Fuels
Car bond. Consequently the H/C ratio decreases from 0.9 to 0.6 and fa increases from 0.70 to 0.86, corresponding to changes in aromatic structure from two to seven fused rings. The contents of N, S, and O in coke-S decrease from 2.4 to 1.5, 0.4 to 0.1, and 15.3 to 11.4, respectively. The radicals in coke-S increase in concentration, from a few micromoles per gram of coke-S to as high as 80 μmol/g of coke-S at 500 °C, and decrease in g value and line width, from 2.003 20 to 2.002 80 and from 0.55 to 0.40 mT, respectively. Coke-D is more condensed in aromatic structure than cokeS, as indicated by higher radical concentration and lower g value and line width. Coke-D contains initially irregular shaped coal fines of sizes around 0.1 μm, but large particles of smooth edge later on. Some particles merge to form coke film of increasing thickness due to deposition, cracking, and polymerization of heavy fractions in the tar. The coke film cracks into polygon in 30 min at 300 °C or in 10 min at 500 °C.
tube wall, coke-D, is more than that suspended in the tar, cokeS. However, it is not easy to perform detailed chemical analysis on this coke because it coats thinly on the tube wall and cannot be sampled in sufficient quantity. In this case the characterizations made to the coke are only radical information and morphology. It is found that the coke-D formed at 500 °C in 15 min contains 89 μmol/g of coke-D radicals, which is higher than that of the corresponding coke-S (74 μmol/g of coke-S). The g value and line width of the coke-D radicals are 2.002 89 and 0.42 mT, respectively, which are lower than those of the corresponding coke-S (2.002 96 and 0.44 mT, respectively). These indicate that coke-D is more condensed in aromatic structure than coke-S, which is understandable since the coke particles deposited on the tube wall are large in size as discussed earlier and the temperature of the tube wall is somewhat higher than that of the volatiles inside the tube, due to heat transfer from the outside to inside of the tube. Figure 9 shows that coke-D samples obtained from the tar reaction at 300 °C for 1.5 and 3 min, i.e., 300-1.5 and 300-3, appear to contain irregular particles of approximately 0.1 μm in size. These particles are likely to be coal fines or partially pyrolyzed coal fines entrained along with the volatiles generated from coal during pyrolysis because their rigid shape shows little surface tension effect. The size of these particles is at the lower end of the particles size distribution of coke-S shown in Figure 2. The coke-D 300-5, however, starts to show spherical particles approximately 0.25 μm in size, indicating their formation involving a molten stage, such as asphaltenes, which underwent polymerization and solidification on the tube wall over time. The round edge but imperfect spherical shape may be attributed to the high viscosity of the molten coke precursor and/or the presence of coal fines which hinder the symmetrical growth of spherical particles.12,28 The coke-D 300-10 and 30015 show coke film underneath and buried with densely distributed small particles in the former and large particles in the latter. Coke-D 300-30 shows large agglomerated particles adhered on to cracked thicker coke film. It can also be seen in the figure that the appearance of coke-D formed at 400 and 500 °C followed the same sequence as that at 300 °C but progressed at higher rates and with more obvious melting behavior. The cracking of the coke film occurred in 10 min at 500 °C which is much shorter than that at 300 °C. This continued evolving of coke formation on the tube wall simulates that actually occurring in the pyrolysis product lines or the preheaters for tar processing.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02724. 13 C NMR spectra of coke-S formed at 300, 400, and 500 °C in 40 min (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +86 0 10 64421073. ORCID
Qingya Liu: 0000-0003-0354-9026 Zhenyu Liu: 0000-0002-3525-273X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We thank the National Key Research and Development Program of China (2016YFB0600302) for financial support.
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4. CONCLUSIONS This work studies coke, defined as THF-insoluble matter, formed in the reaction of a tar at temperatures of 300, 400, and 500 °C in 40 min under semibatch conditions, allowing evaporation of light fractions. The tar was generated in pyrolysis of a subbituminous coal in a fixed-bed reactor. The coke is categorized in two types, the one suspended in tar and consequently in THF, coke-S, and the one deposited on the tube wall, coke-D. It is found that coke is formed at temperatures as low as 300 °C, and more coke is formed at a higher temperature and in a longer time. The quantities of coke-D are more than that of coke-S under the same conditions. With increasing temperature and time of the tar reaction, particles in coke-S increase from around 0.1 to 700−800 μm and undergo condensation leading to decreases in alkyl side chain and aliphatic Cal−H bond and increase in aromatic Car− 471
DOI: 10.1021/acs.energyfuels.6b02724 Energy Fuels 2017, 31, 464−472
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DOI: 10.1021/acs.energyfuels.6b02724 Energy Fuels 2017, 31, 464−472
