Oils and Phenols-and-Water-Free Tars Produced in Pyrolysis of 23

(1-9). It is well recognized that the tar is mainly produced in a temperature range of .... The U-tube with the liquid products was filled with 230 mL...
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Oils and Phenols-and-Water-Free Tars Produced in Pyrolysis of 23 Chinese Coals in Consecutive Temperature Ranges Lei Shi, Qingya Liu, Zhenyu Liu,* and Weize Wu State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: Coal pyrolysis has been widely studied, and the overall tar yield has been reported frequently in the literatures. It is recognized that the yield and composition of the tar vary with the coal rank and pyrolysis temperature, but detailed information is limited, especially for the oil fraction defined as the hexane-soluble matter. In this work, 23 Chinese coals of different rank with carbon content of 73.7−89.1% are pyrolized in a fixed-bed reactor, and the liquid products collected in the temperature ranges of 110−300, 300−400, 400−500, 500−600, and 600−800 °C are extracted by n-hexane. The yields of phenols-and-water-free tars and oils are estimated, and the oil fractions are analyzed by HPLC. It is found that the total yields of the phenols-and-water-free tar are generally greater than 10 wt % of the coals (dry and ash-free), and 30−60 wt % of the tars are soluble in n-hexane. Generally, about 50 wt % of the phenols-and-water-free tars and oils are produced in the temperature range of 400−500 °C. The HPLC analysis indicates that the oils from the coals with carbon contents of less than 80 wt % are dominated by aliphatic hydrocarbons, whereas those from the coals with carbon contents of greater than 80 wt % are dominated by one- and two-ring aromatics. Aliphatic hydrocarbons appear in all the oil fractions regardless of the pyrolysis temperature and coal rank, whereas the one-ring aromatics are found only in oils produced at 400−500 °C. sis.11,22−25 It can be categorized as paraffins, olefins and aromatics, or saturates, aromatics and polar compounds.22,23,26 Although numerous researchers have shown the yields of tar and oil as well as the oil composition, the information available is mainly for samples collected in broader temperature ranges, and a limited amount of work attempts to correlate the information with coal rank. This work studies the yields of tar and oil produced from pyrolysis of 23 Chinese coals in a fixedbed reactor at temperature ranges of 110−300, 300−400, 400− 500, 500−600, and 600−800 °C and analyzes the oil composition using HPLC. All the data are correlated with carbon content (C%) of the coals.

1. INTRODUCTION Coal is generally deficient in hydrogen, compared to petroleum, but contains some hydrogen-rich structures. Coal pyrolysis is recognized as a simple method for directly extracting the hydrogen-rich structure from coal to produce liquid fuels and chemicals. Abundant literature can be found on pyrolysis of coal under numerous operating conditions, which reported the yields of tar, gas, and char.1−9 It is well recognized that the tar is mainly produced in a temperature range of 300−600 °C, and its yields vary with pyrolysis apparatus and operating conditions. In flow systems, for example, the tar yields differ from 2 to 20 wt %.1,2,10−15 This difference, however, may also be attributed to the ambiguous definition of tar, as it often refers to all the materials condensed from the volatiles generated. It is often found that the tars contain not only oil, such as those defined frequently as the hexane-soluble matters,16,17 but also water, asphaltene, preasphaltene, and even organic and inorganic solid matters. These nonoil fractions may constitute 50−80 wt % of the tars.18−20 Most of the tars, therefore, are usually of high viscosity and turn into solids shortly at ambient conditions.18,19,21 This indicates a need to distinguish the yields of various tar fractions for better understanding and evaluation of pyrolysis techniques. There are many ways to define tar fractions. The method adopted frequently in laboratory researches is to dissolve a tar sequentially in hexane, toluene, and tetrahydrofuran (THF). This separates the tar into four fractions, such as oil (the hexane-soluble fraction), asphaltenes (the hexane-insoluble but toluene-soluble fraction), preasphaltenes (the toluene-insoluble but THF-soluble fraction) and residue (the THF-insoluble fraction). The oil is the most suitable fraction for further refining but is still very complex, as is indicated by many using high-performance liquid chromatography (HPLC) analy© 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Coal Pyrolysis Experiment. The 23 Chinese coals used in this work rank from lignite to bituminous coals with carbon contents (C%) of 73.7−89.1 wt %. The proximate analysis of the coals was determined following the Chinese National Standard GB/T 212−2008 (similar to ASTM D3172−2007, D3173−2008, D3174−2010, and D3175− 2007) as shown in Table 1. The Ultimate Analysis of the coals were measured following the Chinese National Standard GB/T 476−2001 (similar to ASTM D3176−2009, D3178− 1997, and D3179−2002) for carbon (C), hydrogen (H) and nitrogen (N), and GB/T 214−2007 (similar to ASTM D3177− 2007) for sulfur (S). All the ultimate analysis data are also shown in Table 1. All the coals were ground and sieved to 0.4−0.9 mm in size for the pyrolysis experiment. The pyrolysis was performed in a Received: June 29, 2013 Revised: August 31, 2013 Published: September 2, 2013 5816

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Table 1. Proximate and Ultimate Analyses of the Coalsa proximate analysis (wt %)

a

ultimate analysis (wt %, daf)

coal

Mad

Ad

Vdaf

C

H

N

O†

S

Sanji Gujiao unknown Tangshan Wuhaixi Xuanzhong Kailuan Mengxi Zhaogezhuang Shenmu Laoshidan Zhongliu Pingshuo Youyu Houan Yanzhou Yuling Bulianta Baorixile Huolinhe-1 Huolinhe-2 Chahaer Xiaolongtan

0.6 0.6 0.8 0.9 1.0 7.8 1.0 1.2 0.7 3.2 0.8 1.3 3.6 3.1 3.4 2.7 1.8 3.9 13.2 17.0 2.2 7.6 16.4

10.5 9.4 15.2 11.7 10.9 3.0 12.6 11.1 12.2 8.5 11.8 7.2 19.7 30.2 32.8 2.8 18.9 5.5 6.0 8.5 24.7 12.4 14.5

16.1 19.3 22.6 32.1 27.8 32.3 32.3 30.5 31.7 41.9 28.8 28.4 37.1 37.6 39.8 44.7 42.6 36.5 33.4 49.0 48.9 44.5 50.7

89.1 88.5 87.3 86.0 85.8 85.6 85.5 85.4 85.3 85.1 85.0 83.9 83.1 82.8 82.7 81.5 81.4 80.3 79.1 77.5 76.6 75.7 73.7

3.8 3.8 4.0 4.2 4.3 4.2 4.3 4.2 4.3 4.7 4.3 4.2 4.7 4.8 4.9 5.9 4.8 4.9 3.9 3.7 4.6 4.9 3.9

1.2 1.3 1.2 1.3 1.2 1.0 1.2 1.2 1.2 1.1 1.3 1.3 1.3 1.4 1.3 1.3 1.3 0.9 1.1 1.2 1.5 1.9 1.3

4.7 4.9 5.9 7.8 8.6 9.0 8.0 8.1 7.9 8.8 8.0 8.6 9.6 9.1 9.8 8.6 9.9 13.7 15.7 17.3 16.6 17.1 20.1

1.2 1.5 1.6 0.7 0.1 0.2 1.0 1.1 1.3 0.3 1.4 2.0 1.3 1.9 1.3 2.7 2.6 0.2 0.2 0.3 0.7 0.4 1.0

M = moisture; A = ash; V = volatile matter content; ad = air-dry basis; d = dry basis; daf = dry-and-ash-free basis; † = by difference.

vertical fixed-bed quartz reactor (Φ = 28 × 580 mm) coupled with a U-shaped quartz insert (U-tube, in short) as shown in Figure 1. The lower portion of the U-tube was placed in an

the U-tube, with and without the liquid. The amount of the liquid product is then divided by the amount of coal sample (daf) to form the yield of the liquid products, termed YLP, with a unit of g/g of coal (daf). After each experiment, including the hexane extraction of the liquid product described in the following, the used U-tube and the reactor were heated in an oven at about 500 °C for 2 h in air and then washed by deionized water after cooling down to room temperature. The yields of the total volatile matter generated from the coal samples in 110−800 °C were determined by a TGA (SETSYS Evolution 24, SETARAM) and denoted as Y110−800 in g/g of coal (daf). In each of the experiments, 30 mg coal was loaded into a quartz crucible and heated in a flow of Ar at 100 mL/min following a temperature program, moving from room temperature to 110 °C at a rate of 10 °C/min, maintaining at 110 °C for 30 min for the removal of moisture, and moving from 110 to 800 °C at a rate of 10 °C/min. 2.2. Analysis of the Liquid Products. The U-tube with the liquid products was filled with 230 mL n-hexane and soaked for 24 h. The n-hexane solution was then poured into a 250 mL volumetric flask and supplemented with n-hexane to 250 mL. The U-tube was then purged at room temperature with a flow of N2 of 500 mL/min for 10 min to remove residual n-hexane and water that was not dissolved in the n-hexane. The N2 purging operation was found to be sufficient to yield a constant mass of the U-tube with the hexane-insoluble materials (HIS, in short). The yield of HIS is thus obtained and denoted as YHIS in g/g of coal (daf). The n-hexane solution obtained from the extraction was analyzed by an HPLC (2695, Waters) with a photodiode array detector (PDAD, 221 nm), an NH2-bonded silica column (Φ 4.6 × 250 mm, Spherisorb NH2), and a mobile phase of nhexane (1 mL/min). The column temperature was 35 °C and the sample volume was 5 μL. Due to the strong adsorption of

Figure 1. Schematic diagram of the pyrolysis apparatus.

ice−water bath with 15 wt % NaCl to reach a temperature of −10 to −5 °C to condense the liquid products. A mixture of 3 g coal and 3 g quartz sand of the same size was loaded into the reactor and heated, in the absence of the U-tube, from room temperature to 110 °C at a rate of 10 °C/min under a N2 flow of 100 mL/min and maintained at 110 °C for 60 min to remove moisture in the coal. The mixture was then heated at the same rate to 800 °C with a fresh U-tube in each of the temperature intervals of 110−300, 300−400, 400−500, 500−600, and 600− 800 °C. The U-tube is connected to the reactor by a ground glass joint. The time needed for exchange of the U-tubes is usually 3 s. The volatile loss during the exchange of the U-tubes is estimated to be 0.5 wt % of the liquid collected, which takes 10 min or longer. The amount of the liquid collected in each temperature range was determined by the mass difference of 5817

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Figure 2. Retention time of 18 standards in an HPLC chromatogram and classification of the five group compositions.

phenols on the NH2-bonded silica column and insensitivity of the PDAD detector to water, the HPLC chromatograms show mainly the organic matters free of phenols and water. This fraction is termed oil in this work, as is frequently found in the literature. It is known that the oil is still very complex in composition, and identifying each of the compounds requires tremendous amounts of work. To fulfill the aim of this work (i.e., the major composition change in the oil versus the coal rank and pyrolysis temperature), the PDAD signals are categorized roughly into five groups, namely aliphatic, one-ring aromatics, two-ring aromatics, three-ring aromatics and four-ring aromatics. The mass of each of these groups is calibrated by an average response factor of a number of standards and the sum of the mass of the five groups’ yields of oil, which is then used to determine the yield of oil, denoted as YOil in g/g of coal (daf). Clearly, the procedure adopted above does not involve direct determination of the mass of the phenols and H2O generated in the pyrolysis. The yield of phenols + H2O, denoted as YPhenols+H2O in g/g of coal (daf), can only be estimated by eq 1, shown below. The term YOil+HIS, the sum of YOil and YHIS, is very close to but not the same as the term tar usually found in the literature because the tar contains also phenols and some water. This yield, therefore, is also termed phenols-and-waterfree tar in this work. The relation between the volatiles measured by TGA, Y110−800, and the products determined from the fixed bed pyrolysis can be estimated by eq 2, where YGas is the mass of the gaseous product of the pyrolysis in g/g of coal (daf). YLP − YOil + HIS = YPhenols + H2O

(1)

Y110 − 800 − YLP = YGas

(2)

Table 2. Retention Time and Response Factors of the Standards for Each Group ring no.

retention time (min)

average response factor (area counts/g)

0a 1

≤4.7 4.7−7.2

2.2 × 1012 5.3 × 1012

2

7.2−13.2

2.4 × 1013

3 ≥4

13.2−16.6 >16.6

3.0 × 1013 3.3 × 1013

a

standards in each group n-heptane, undecane benzene, toluene, cumene, tertbutybenzene, p-xylene, o-xylene, 1.3.5-trimethylbenzene, phenetole naphthalene, dimethylnaphthalene, acenaphthene, diphenylmethane, fluorene anthracene, phenanthrene pyrene

Aliphatic hydrocarbons.

therefore lumped into five groups as indicated in the figure. The response factors of these groups as determined by averaging the response factors of the standards in the same group are shown in Table 2. 3.2. Yields of the Pyrolysis Products of Coals of Different Rank. Because the mass of the pyrolysis products determined is far less than the mass of the U-tube, experimental errors in the yields of the pyrolysis products may be significant. The experimental errors therefore are analyzed taking Xiaolongtan coal as an example. The results of five experiments shown in Table 3 indicate that the relative standard deviation (RSD) of YLP and YHIS ranges from 3.5 to 26.6%, with the largest error for the smallest yield. For example, the RSD is 21.0% for YLP produced in 600−800 °C (0.006 g) and 26.6% for YHIS produced in 600−800 °C (0.001 g). Figure 3 shows Y110−800, YLP, YOil+HIS, and YOil that was obtained in the temperature range of 110−800 °C. As expected, Y110−800 decreases with an increase in the C% of the coals, from 0.5 g/g of coal (daf) for Xiaolongtan coal (C% = 73.7%) to 0.1 g/g of coal (daf) for Sanji coal (C% = 89.1%). The trends in YLP, YOil+HIS, and YOil are similar; they increase with an increase in the C% from 73 to 78%, fluctuate at high values in the C% range of 78−86%, and decrease with a further increase in the C %. YOil+HIS ranges from 0.025 to 0.175 g/g of coal (daf), with values higher than 0.100 g/g of coal (daf) for coals with a C% of 78−86%. YOil ranges from 0.015 to 0.080 g/g of coal (daf), with values higher than 0.050 g/g of coal (daf) for coals with a C% of 78−86%. As indicated in eq 1 the differences between YLP and YOil+HIS are the amounts of the phenols and H2O in the liquid products. It is clear that most of the liquid products contain significant

3. RESULTS AND DISCUSSION 3.1. Retention Time of the Group Composition in HPLC Analysis of the Oil. A schematic chromatograph of 18 standards representing the major compounds possibly in the oil is shown in Figure 2. It can be seen that the retention time of the aromatics increases with the order benzene (one-ring) < naphthalene (two-ring) < anthracene (three-ring) < pyrene (four-ring). A substituted aromatic ring usually has a longer retention time than an unsubstituted one. The effect of an alkyl substituent on the retention time is less significant than that of an aromatic substituent, and the effect of a polar substituent, such as phenyl or phenetole, is the largest. This retention time sequence agrees with the literature.23,24 These standards are 5818

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Table 3. Experimental Error in Yields of the Liquid Products and n-Hexane-Insoluble Mattersa mass determined in experiment (g/g of coal (daf)) yields

temp. range (°C)

1

2

3

4

5

AVb

RSDc(%)

YLP

110−300 300−400 400−500 500−600 600−800 110−300 300−400 400−500 500−600 600−800

0.021 0.050 0.033 0.006 0.005 0.005 0.020 0.011 0.002 0.001

0.018 0.049 0.028 0.005 0.005 0.004 0.018 0.010 0.001 0.001

0.017 0.053 0.032 0.005 0.007 0.003 0.020 0.010 0.001 0.002

0.018 0.052 0.031 0.005 0.005 0.003 0.021 0.010 0.001 0.001

0.018 0.051 0.033 0.006 0.006 0.003 0.022 0.010 0.001 0.001

0.018 0.051 0.032 0.006 0.006 0.004 0.020 0.010 0.001 0.001

7.7 3.5 6.9 12.0 21.0 25.0 8.1 5.1 15.5 26.6

YHIS

a

For Xiaolongtan coal. bAverage value. cRelative standard deviation.

Figure 3. Yields of pyrolysis products from coals of different rank.

Figure 5. Yields of phenols-and-water-free tars (Oil+HIS) and oils versus oxygen content of the coals.

amounts of phenols and H2O, especially for the liquid products derived from the coals with a C% of less than 86%. Figure 4 represents some of the data in Figure 3 to highlight the proportions of YOil+HIS in the total volatiles and YOil in YOil+HIS. It is worth noting again that YOil+HIS is the yield of phenols-and-water-free tar. It is clear that YOil+HIS/Y110−800 ranges from 10 to 55 wt % and is generally higher than 30 wt % for coals with a C% of 78−86%. YOil/YOil+HIS ranges from 30 to 60 wt % and is generally lower than 50 wt %. Because YOil+HIS is less than the tar yield, the values of YOil/YOil+HIS indicate that more than half of tars are not oil. It is interesting to note that the changes in the yields of the pyrolysis products for coals with a C% at 78% and 86% shown

in Figures 3 and 4 and discussed above agree with the coalification jumps observed in changes in reflectance of vitrinites reported in the literature27 where the first coalification jump was approximated at a C% of 77%, whereas the second coalification jump was approximated at a C% of 87%. The C% range of 77−87% covers the bituminous coals defined in ASTM classification.27 The large fluctuations in the yields of the pyrolysis products may suggest that the C% is not the only criteria of coal to correlate the pyrolysis behaviors. It is believed that the oxygen and hydrogen contents also determine the pyrolysis products.28,29 Figure 5 shows YOil+HIS and YOil versus the oxygen

Figure 4. Distribution of phenols-and-water-free tars (Oil+HIS) in the total volatiles (a) and oils in phenols-and-water-free tars (b). 5819

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content (O%) in the coals. YOil+HIS increases with an increase in O% from 4 to 10% and then declines with a further increase in O%. YOil shows a similar trend as YOil+HIS, indicating that the coals with 8−10% oxygen are better candidates for oil production via pyrolysis. Figure 6 shows YOil+HIS and YOil versus the hydrogen content (H%) in the coals. YOil+HIS generally increases with an increase in H% of the coal, as reported by many researchers.30,31 YOil also increases with an increase in the H% of the coal, but the increase is relatively small compared with that in YOil+HIS. 3.3. Behavior of Phenols-and-Water-Free Tars Obtained at Various Temperature Ranges. Figure 7 shows the yields of phenols-and-water-free tars (YOil+HIS) obtained at various pyrolysis temperature ranges. Clearly, YOil+HIS in the temperature ranges of 110−300 and 600−800 °C are relatively small, with total yields of less than 0.015 g/g of coal (daf), and do not vary much with the coal rank. This behavior is consistent with the common knowledge that limited tars are

Figure 6. Yields of phenols-and-water-free tars (Oil+HIS) and oils versus hydrogen content of the coals.

Figure 7. Yields of phenols-and-water-free tars (Oil+HIS) obtained at various pyrolysis temperature ranges. 5820

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Figure 8. Yield and group composition of the oils from pyrolysis of the coals at 110−800 °C.

These data agree with the general understanding that a higher rank coal contains more strong covalent bonds than a lower rank coal. 3.4. Composition of the oils obtained at various temperature ranges. Figure 8 shows composition of the total oils obtained at 110−800 °C. It can be seen that the oils obtained from coals with a C% of less than 80% are dominated by aliphatic hydrocarbons, whereas those from coals with a C% greater than 80% are dominated by one-ring and two-ring aromatics, especially by two-ring aromatics. The yields of threering and four-ring aromatics are much lower and observed only in oils from the coals with a C% of greater than 80%. Although these behaviors agree with the understanding that a lower rank coal contains more aliphatic side chains than a higher rank coal and the aromatic structure in a lower rank coal contains lesscondensed rings than a higher rank coal,11,32,33 it may also indicate that the compounds with three or more condensed aromatic rings are not very soluble in n-hexane. Figure 9 shows the yield and composition of oils obtained at different pyrolysis temperature ranges. The yields of the oils obtained in the temperature ranges 110−300 and 600−800 °C, which are dominated by aliphatic hydrocarbons, are very low, both less than 0.005 g/g of coal (daf). The yields of the oils obtained at 400−500 °C are the highest, about 0.040 g/g of coal (daf) for coals with a C% of 80−86%. This fraction consists mainly of one-ring and two-ring aromatics, aliphatic hydrocarbons, and small amounts of three-ring and four-ring aromatics, which agrees with the literature.23 The yields of the oils obtained at 300−400 and 500−600 °C are relatively low compared with that obtained at 400−500 °C, generally about 0.010 g/g of coal (daf). These fractions consist mainly of aliphatic hydrocarbons for coals with a C% of less than 80% and aliphatic hydrocarbons and two-ring aromatics for coals with a C% of greater than 80%. It is noted that while the aliphatic hydrocarbons can be found in oils obtained in all the temperature ranges, the one-ring aromatics are observed only in oils collected at 400−500 °C, whereas the two-ring aromatics are observed at 300−400 and 500−600 °C. This observation suggests that the linkage of onering aromatics with the coal matrix is limited mainly to a single covalent bond, which is likely to be the aliphatic carbon bonds (Cal−Cal) that break thermally in the temperature range of 400−500 °C;34,35 the linkages of two-ring aromatics with the coal matrix also involve covalent bonds between aliphatic carbon and heteroatoms, such as those between an aliphatic carbon and oxygen (Cal−O) or between an aliphatic carbon and aromatic carbon bond (Car−Cal).34,35

Figure 9. Yield and group composition of the oils from pyrolysis of the coals in various temperature ranges.

obtainable in these temperature ranges. The YOil+HIS collected at 400−500 °C are generally much larger than those collected in the other temperature ranges and account for greater than 50% of the total YOil+HIS. It should be noted that the peak of YOil+HIS shifts to coals of higher C% at a higher temperature range: about 79% at 300− 400 °C, 82−86% at 400−500 °C, and 85−86% at 500−600 °C. 5821

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(16) Uchino, H.; Yokoyama, S.; Satou, M.; Sanada, Y. Fuel 1985, 64, 842−848. (17) Katoh, T.; Ouchi, K. Fuel 1985, 64, 1260−1268. (18) Lang, E. W.; Lacey, J. C. Ind. Eng. Chem. 1960, 52, 137−140. (19) García, R.; Crespo, J. L.; Martin, S. C.; Snape, C. E.; Moinelo, S. R. Energy Fuels 2003, 17, 291−301. (20) Teo, K. C.; Watkinson, A. P. Fuel 1987, 66, 1123−1132. (21) Wolfesberger-Schwabl, U.; Aigner, I.; Hofbauer, H. Ind. Eng. Chem. Res. 2012, 51, 13001−13007. (22) Kahler, E. J.; Rowlands, D. C.; Brewer, J.; Powell, H.; Ellis, W. C. J. Chem. Eng. Data 1960, 5, 94−97. (23) Herod, A. A.; Ladner, W. R.; Stokes, B. J.; Berry, A. J.; Games, D. E.; Hohn, M. Fuel 1987, 66, 935−946. (24) Yu, L. E.; Hildemann, L. M. Energy Fuels 1998, 12, 450−456. (25) Zhang, C.; Zhang, X.; Yang, J.; Liu, Z. J. Chromatogr., A 2007, 1167, 171−177. (26) Wang, J.; Lu, X.; Yao, J.; Lin, W.; Cui, L. Ind. Eng. Chem. Res. 2005, 44, 463−470. (27) Elliott, M. A. Chemistry of Coal Utilization I; Wiley-Interscience: New York, 1981; pp 134. (28) Elliott, M. A. Chemistry of Coal Utilization I; Wiley-Interscience: New York, 1981; pp 473−478. (29) Sun, M.; Ma, X.; Yao, Q.; Wang, R.; Ma, Y.; Feng, G.; Shang, J.; Xu, L.; Yang, Y. Energy Fuels 2011, 25, 1140−1145. (30) Furlmsky, E.; Vancea, L.; Belanger, R. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 134−140. (31) Zhao, Y.; Hu, H.; Jin, L.; Wu, B.; Zhu, S. Energy Fuels 2009, 23, 870−875. (32) Snape, C. E.; Ladner, W. R.; Bartle, K. D. Fuel 1985, 64, 1394− 1400. (33) Jia, L.; Weng, J.; Wang, Y.; Sun, S.; Zhou, Z.; Qi, F. Energy Fuels 2013, 27, 694−701. (34) Shi, L.; Liu, Q.; Guo, X.; Wu, W.; Liu, Z. Fuel Process. Technol. 2013, 108, 125−132. (35) Stein, S. E. In New Approaches in Coal Chemistry; Blaustein, B. D., Bockrath, B. C., Friedman, S., Eds.; ACS Symposium Series 169; American Chemical Society: Washington, DC, 1981; pp 97−129.

4. CONCLUSIONS This work systematically studies the yields of oils and phenolsand-water-free tars and the group composition of the oils obtained from pyrolysis of 23 Chinese coals at temperature ranges 110−300, 300−400, 400−500, 500−600, and 600−800 °C. It is found that the yields of phenols-and-water-free tars are generally greater than 10 wt % of the coals (daf), with 30−60 wt % being in the oils. The phenols-and-water-free tars and oils produced in the temperature range of 400−500 °C generally account for 50% of the total tars and the total oils. Coals with a C% of 78−86 wt % and oxygen content of 8−10 wt % are suitable for pyrolysis to produce tars. Aliphatic hydrocarbons dominate the oils obtained from the coals with a C% less than 80 wt %, whereas aromatics, especially the one- and two-ring aromatics, dominate the oil obtained from coals with a C% greater than 80 wt %. Aliphatic hydrocarbons are found in all the oil fractions, regardless of the pyrolysis temperature and coal rank. The one-ring aromatics are found only in oils obtained at 400−500 °C, whereas the tworing aromatics are found in oils obtained at 300−600 °C, mainly at 400−500 °C. The main linkage between the one-ring aromatics with the coal matrix is likely limited to an aliphatic carbon−carbon bond.

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AUTHOR INFORMATION

Corresponding Author

*Z. Liu. E-mail: [email protected]. Phone: +86-10-64421073. Fax: +86-10-6442-1077. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from the National Basic Research Program of China (2011CB201306), the National Natural Science Foundation of China (21276019 and 20821004) are acknowledged.

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dx.doi.org/10.1021/ef401215h | Energy Fuels 2013, 27, 5816−5822