Analysis of Tars Produced in Pyrolysis of Four Coals under Various

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Analysis of Tars Produced in Pyrolysis of Four Coals under Various Conditions in a Viewpoint of Radicals Wenjing He,† Zhenyu Liu,*,† Qingya Liu,† Muxin Liu,‡ Xiaojin Guo,† Lei Shi,† Junfei Wu,† Xiaofen Guo,§ and Donghui Ci§ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People’s Republic of China § Coal Upgrading Center, National Institute of Clean-and-Low-Carbon Energy, Beijing 102209, People’s Republic of China ABSTRACT: Many studies tried to correlate coal pyrolysis conditions with reactivity of tars produced, but few analyzed the radical concentration of tars. This work studies pyrolysis of four coals, with carbon contents in a range of 74−82 wt %, in a fixedbed tubular reactor at three heating rates and three N2-purging rates. The changes in the radical concentration of tars under various conditions, such as in solvents (hexane or tetrahydrofuran) or at high temperatures (300, 350, 400, and 450 °C), are analyzed. The effect of pyrolysis conditions on the radical concentration of tars is ascribed to the temperature difference between the gas phase and the coal in the reactor, which results in further cracking of tars. The difference in the radical concentration in tars obtained from pyrolysis of different coals is elaborated in a viewpoint of the coal structure and pyrolysis mechanism.

1. INTRODUCTION Pyrolysis is an effective and simple way to produce coke, tar, and coke oven gas from coals. Coal pyrolysis technologies can be categorized into slow pyrolysis and fast pyrolysis according to the heating rate to the coals. A typical case of the slow pyrolysis technologies is coking, which has long been practiced at industrial scale and has been the backbone of the iron and steel industry. The tar yield of coking is low, usually less than 3.5 wt %, and the coking time is long, usually about 20 h.1 The fast coal pyrolysis technologies produce more tars, 7 wt % or higher, in a short time, a few minutes,2 for example, but the coke produced is poor in quality and cannot be used in the iron and steel industry. Because of the shortage of the petroleum supply that has occurred frequently in recent decades, fast coal pyrolysis technologies have been studied worldwide aiming toward the production of tars. However, the attempts for commercial application of fast pyrolysis technologies have not been successful, because the tars obtained are usually high in particulate and pitch contents and poor in stability.3,4 Furthermore, system plugging often occurs because of solid fouling. It is, therefore, necessary to make a deeper understanding on the pyrolysis reaction, especially on the relationship between the pyrolysis conditions and the tar quality and reactivity. It is commonly recognized that coal pyrolysis follows a twostep radical mechanism. The first step is thermal cracking of weak covalent bonds in coal to form volatile radical fragments. The second step is reaction of the volatile radical fragments (including coupling and condensation) to produce pyrolysis products, such as gas, tar, and char.1,5−7 Because the radical fragments and their reaction products may crack further,8 the two-step reactions take place successively and concurrently, which makes it difficult to distinguish them and study each step experimentally.9 Despite this complexity, it is certain that the main factors that influence © 2015 American Chemical Society

the quantity and quality of the tars produced in coal pyrolysis are two: (1) the temperature at which the weak covalent bonds in coal crack and (2) the reaction temperature and time of the volatile radical fragments and their products. Studies have shown that the cracking of the covalent bond in coal is mainly temperature-dependent and not significantly influenced by the heating rate.10,11 Little work, however, can be found in the literature on reaction of the volatiles under conditions of large reactors. Our recent work showed that the temperature of volatiles may be a few degrees Celsius higher than that of coals in a slow heating laboratory fixed-bed reactor but may be as high as 300 °C higher than that of coals in a fast heating fluidized-bed reactor or reactors using solid heat carriers.1 This indicates that the major difference between a slow pyrolysis technology and a fast pyrolysis technology is the reaction temperature and time of the volatiles. Huge amounts of work have been carried out to study pyrolysis of coals, but only a few attempted to quantify the behavior of radicals during the pyrolysis. The available information shows that the radical population changes regularly in the course of pyrolysis12−14 and that the radicals detectable are mainly stable radicals.15 Coal tars contain radicals with concentrations ranging from 1016 to 1017 spins g−1,16−18 and these radicals are mainly distributed in heavy fractions of tars.19 The tars are reactive and easy to age, and the aging processes are accompanied by changes in the radical population.17,18,20−22 However, the relationship between the radical concentration of tars and the pyrolysis conditions has not been reported, which is important for understanding the reactions of volatiles. To understand the relationship, this work studies pyrolysis of four Received: March 20, 2015 Revised: May 10, 2015 Published: May 11, 2015 3658

DOI: 10.1021/acs.energyfuels.5b00594 Energy Fuels 2015, 29, 3658−3663

Article

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

a

ultimate analysis (wt %, daf)

coal

Mad

Aad

Vad

C

H

Ob

N

S

HLBE BLT BET DLT

30.99 13.04 5.13 4.89

9.10 12.34 14.77 6.11

29.94 30.55 31.11 28.97

73.93 79.76 80.44 82.02

5.12 4.85 4.77 4.58

19.43 13.92 13.57 11.90

1.14 1.06 1.02 0.92

0.38 0.40 0.20 0.59

ad, air-dried basis; daf, dry and ash-free basis. bBy difference. before the coal sample was placed in the center of its isothermal zone; and (3) the coal sample was placed in the center of the isothermal zone of the furnace initially and then heated at a rate of 5 °C min−1. The changes in the temperature of the coal samples with time under these heating mechanisms are shown in Figure 2, which yield average heating

coals, with carbon contents in a range of 74−82 wt %, at three heating rates and three N2-purging rates, corresponding to three residence times of volatiles. The radical concentration and radical distribution in the as-obtained tars and the tars mixed with solvents or experiencing additional high temperatures, 300−450 °C, are measured and analyzed.

2. EXPERIMENTAL SECTION 2.1. Materials. The four coals studied are Hulunber coal (HLBE), Bulianta coal (BLT), Buertai coal (BET), and Daliuta coal (DLT), which were ground to sizes of 120−140 mesh and dried at 383 K in vacuum for 4 h. The proximate and ultimate analyses of the coals are shown in Table 1. 2.2. Fixed-Bed Pyrolysis Experiments. The pyrolysis experiments were carried out in a fixed-bed tubular reactor shown in Figure 1. The

Figure 2. Changes in the temperature of the coal sample with time under three heating mechanisms (Tf stands for the temperature of the furnace).

rates of about 210, 80, and 5 °C min−1, respectively, at the main pyrolysis stage (350−540 °C). The N2 purging rate was 200 mL min−1, corresponding to a linear gas velocity of 9.7 × 10−3 m s−1 under standard temperature and pressure. 2.2.2. Pyrolysis with Different Residence Times. The experiments were carried out at an average pyrolysis heating rate of 80 °C min−1 and three N2 purging rates of 20, 100, and 200 mL min−1, corresponding to the linear velocities of 9.7 × 10−4, 4.9 × 10−3, and 9.7 × 10−3 m s−1 under the standard temperature and pressure, respectively. Because the volatiles generated in the main pyrolysis stage were greater than 120 mL min−1, their average residence time in the isothermal zone of the reactor were about 2.4, 1.5, and 1.1 s, respectively. 2.3. Radical Concentration and Reactivity Tests of the Tars. 2.3.1. Measurement of the Radicals and Their Distribution in the Tars. The radical concentrations of tars were measured by ESR (JES-FA 200, JEOL, Ltd.). The tar sample sealed in the capillary was loaded into the ESR sample tube and measured directly at room temperature. The results were calibrated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) of known radical concentrations. The capillary showed no influence on the ESR measurement. The water in tars was found to have little effect on the ESR measurements because of its low content, less than 2.85 wt %. Pre-experiments showed that, for a tar sample of 5 mg, a water content of 20 wt % decreases the ESR reading by about 8%. Previous studies suggested that the radicals detected in coal tar are not able to react with each other because of steric hindrance or low mobility. These radicals are likely large in size and distribute in all of the tar fractions, such as oil, asphaltene, pre-asphaltene, and fine coke particulates suspended in the tars.19 To understand this distribution, the tars were mixed with different solvents and the changes in the radical concentration of tars were recorded over time. The procedure is (1) introduction of 60 μL of hexane or tetrahydrofuran (THF) into the

Figure 1. Schematic diagram of the pyrolysis apparatus: (a) before and after the pyrolysis experiment and (b) during the pyrolysis. isothermal zone of the furnace is 10 cm. The quartz reactor is 2 cm in inner diameter and holds 7 g of coal. Two thermocouples, 1 and 2 in Figure 1, were used to monitor the temperatures of the reactor and the coal sample, respectively. A third thermocouple, 3 in Figure 4, was used to measure the gas-phase temperature inside the reactor near the wall. The pyrolysis was operated in two modes. In one mode, the coal sample was placed outside the furnace downstream initially (Figure 1a). When the temperature of the furnace reached the set value, the reactor was pushed into the furnace to have the coal sample be located at the center of the isothermal zone (Figure 1b). In another mode, the coal sample was initially located in the center of the isothermal zone, as shown in Figure 1b, and was then heated at a constant rate. The pyrolysis experiments were terminated at a coal temperature of 540 °C (±5 °C) by removing the reactor from the furnace while keeping the N2 purging. The tar condensed at the cold end of the reactor was sampled with capillaries with a diameter of 1 mm, weighed and sealed in glass tubes (30 × 2 mm inner diameter), and finally stored at the liquid nitrogen temperature for the electron spin resonance (ESR) test. The average sample size is about 5 mg. The water contents of the tars were found to be less than 2.85 wt %, which were determined by a multipurpose water titrator (ZDJ-400S, Xianquweifeng Co., Ltd.) employing the Karl Fisher method. The overall error in sampling and radical concentration test of the tars is no more than 15%. 2.2.1. Pyrolysis at Different Heating Rates. The three heating rates were accomplished by using the following heating mechanisms: (1) the furnace was preheated to 800 °C before the coal sample was placed in the center of its isothermal zone; (2) the furnace was heated to 600 °C 3659

DOI: 10.1021/acs.energyfuels.5b00594 Energy Fuels 2015, 29, 3658−3663

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Energy & Fuels

produced at heating rates of 5, 80, and 210 °C min−1 are 5.9 × 1016, 12.5 × 1016, and 23.4 × 1016 spins g−1, respectively. The prevailing explanation of the phenomenon may be that, in comparison to a low heating rate, a high heating rate accelerates the rate of volatile generation from coal and results in a high concentration of volatiles in a shorter time, which alters the reactions of the volatiles.23 This “concentration effect”, however, provides no information on the changes in the radical concentration of tars observed here. A possible and simple explanation of the phenomenon is that a higher heating rate results in a larger temperature gradient in the reactor in the radial direction, which makes the temperature difference between the gas phase and the coal greater and, consequently, intensifies cracking of volatiles in the gas phase, resulting in more unpaired electrons been confined in the macromolecules formed from condensation of radicals. The above explanation is supported by Figure 4, where the temperature of the gas phase is plotted against the temperature of

capillary loaded with 5 mg of tars under a nitrogen atmosphere and (2) measurement of radical concentrations of the tars hourly in 4 h. 2.3.2. Radical Concentration of the Tars Maintained at Various Temperatures. It is commonly recognized that the volatiles that condensed into tar are mainly generated in the coal temperatures of 300−600 °C. Most of the large pyrolysis reactors, however, are operated at temperatures higher than this range. These suggest that the volatiles, as soon as they are generated from the coal surface, will enter a hightemperature environment and react further. To study the reaction of volatiles, the tar samples obtained from the above pyrolysis experiments, which had experienced different temperature increases at different pyrolysis conditions, were heated in a heating jacket maintained at 300, 350, 400, and 450 °C. The radical concentrations of the samples were tested at room temperature hourly during a 4 h period.

3. RESULTS AND DISSUSSION 3.1. Effect of the Heating Rate on the Radical Concentration of the Tars. Figure 3 shows the radical

Figure 3. Radical concentration of the tars collected in pyrolysis at three heating rates.

concentration of the tars collected in pyrolysis of the four coals at three heating rates and a N2-purging rate of 200 mL min−1. Each point is an average of four tar samples collected at the same time in an experiment, with an average error of less than 15%. It is interesting to see that the radical concentration of tars increases with an increase in the carbon content of the coals. This phenomenon should not be accidental and may be ascribed to the difference in the structure of the coals based on the following facts and/or inferences: (1) coal consists of aromatic unities interconnected by weak covalent bonds, and in general, the aromatic unities in a higher rank coal (high in C %) are larger in size than those in a lower rank coal; (2) an aromatic unity in a higher rank coal may interconnect with more aromatic unities than that in a lower rank coal; (3) the free-radical fragments generated in pyrolysis are usually centered on aromatic unity(s),23 and the generation of a free-radical fragment from a higher rank coal requires cleavage of more interconnecting covalent bonds than that from a lower rank coal. These suggest that a free-radical fragment generated from a higher rank coal may contain more unpaired electrons than that generated from a lower rank coal, and therefore, coupling of all of the unpaired electrons in a free-radical fragment from a higher rank coal is more difficult (with more steric hindrance) than that from a lower rank coal. This results in more unpaired electrons being confined in the tars produced from a higher rank coal than that produced from a lower rank coal. Figure 3 also shows that the heating rate has an obvious influence on the radical concentration of tars: the higher the heating rate, the higher the radical concentration of tars. Taking BLT coal as an example, the radical concentrations of the tars

Figure 4. Temperatures of the gas phase and coal sample in the same axial position.

coal. Clearly, the temperatures of the gas phase are always higher than that of the coal, and the temperature difference between the gas phase and coal (ΔT) increases with an increase in the heating rate. This indicates that the tar sample collected in pyrolysis with a high heating rate had reacted under a higher gas-phase temperature than that collected in pyrolysis with a low heating rate. To quantify the effect of the gas-phase temperature on the radical concentration of the tars, ΔT for volatiles generated at a coal temperature of 450 °C was chosen as the representative temperature increase in volatiles at each heating rate, because the generation of volatiles for these coals peaks at temperatures around 450 °C. Figure 5 shows that the radical concentration of tars is proportional to ΔT; i.e., the higher the ΔT, the higher the radical concentration of tars. This agrees with the earlier discussion that cracking of volatiles results in more condensation of free-radical fragments, which leads to the formation of more heavy components that contain more confined radicals.16 It is noted that the heavy components of tars that contain confined radicals include fine coke particulates suspended in the 3660

DOI: 10.1021/acs.energyfuels.5b00594 Energy Fuels 2015, 29, 3658−3663

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Energy & Fuels

amounts of radicals that disappeared in hexane (in the oil fraction) and in THF (in the pitch fraction, excluding those that disappeared in hexane) and those that survived in THF (assuming in coke). It can be seen that the tars from the coals show the same trend; i.e., the radical concentrations of tars, the radicals that disappeared in hexane and in THF, and the radicals that remained in THF all increase with an increasing pyrolysis heating rate. In other words, the tars obtained in pyrolysis at a high heating rate are likely poor in quality because they contain more radicals and a high percentage of them are inaccessible by other radicals, even in THF. These behaviors can again be ascribed to cracking of the volatiles that occurs more significantly in pyrolysis at a high heating rate. The effect of the temperature on the cracking of the volatiles has been demonstrated recently by us using tars obtained from pyrolysis of two kinds of biomass16 and the four coals used in this work.18 The tars were heated to various temperatures, and their radical concentration and THF-insoluble matter content were measured periodically. It was found that the tars start to crack at temperatures higher than 300 °C to generate THF-insoluble matter that contains high concentrations of radicals, even to a level of 1019 spins g−1; the reactivity of tars can be related to the properties of biomass and coals. The behaviors shown in Figures 3, 5, and 6 of this work suggest that the cracking behavior of the tars obtained from pyrolysis at different heating rates may also behave differently if they are heated to temperatures of 300 °C and higher. This is therefore studied, and the results are shown in Figure 7 for BET tars, as an example. It can be seen that the tars obtained from pyrolysis of the coal under different heating rates do behave differently. In general, the tars obtained at a high pyrolysis heating rate, 210 °C min−1, for example, contain more radicals initially and form more radicals at all four temperatures, 300, 350, 400, and 450 °C, than the tars obtained at a low pyrolysis heating rate, 5 °C min−1, for example. This indicates again that the quality and stability of tars produced from pyrolysis at a high heating rate are poorer than that produced from pyrolysis at a low heating rate. A major difference in these tars could be the initial coke and radical contents because coking follows an autocatalytic and radical mechanism.24 3.2. Effect of the Carrier Gas Flow Rate on the Radical Concentration of Tars. The correlation presented above between the pyrolysis heating rate and increases in the temperature and reactivity of volatiles suggests that the residence time of volatiles in the reactor is also a factor affecting the properties of tars. Figure 8 shows the radical concentration of tars obtained from pyrolysis of four coals at a heating rate of 80 °C min−1 under different N2-purging rates. It can be seen that the radical concentration of tars from pyrolysis at a low N2-purging rate (corresponding to a long residence time of volatiles in the reactor) is high, and an increase in the N2-purging rate (corresponding to a decrease in the residence time of volatiles in the reactor) results in a decrease in the radical concentration of tars. This behavior agrees with the observations made above; i.e., cracking of volatiles leads to the formation of radical-containing matter in tars. The differences in the tars obtained from pyrolysis under different N2-purging rates were also studied by mixing the tars with hexane and THF as well as heating the tars to temperatures of 300 °C and higher. The differences in the radical concentration of the tars, however, are within experimental error; therefore, the results are not reported here. It should be noted that the changes in the radical concentration of tars with changes in the N2-purging rate in pyrolysis shown in Figure 8 include also changes in the

Figure 5. Relationship between the radical concentration of tars and ΔT (temperature difference between the gas phase and coal sample) for volatiles generated at a coal temperature of 450 °C.

tars,4,23 pitch (asphaltene and pre-asphaltene), and large oil molecules that are poor in mobility. To estimate radical distribution in each of the fractions of tars, the tars were mixed with hexane or THF to have their oil fraction or the oil + pitch fraction dissolved, respectively, to increase their mobility. The results for BET tars are shown in Figure 6. It can be seen in the

Figure 6. Changes in the radical concentration of BET tars mixed with hexane and THF.

figure that, when a tar is mixed with a solvent, its radical concentration decreases to a stable level over time. This indicates that the tars contain radicals that are confined or buried in macromolecules, and some of these radicals are reactive when their mobility is increased. The small decreases in the radical concentration in hexane indicate that the oil fraction contains small amounts of reactive radicals, especially the oil obtained at a low pyrolysis heating rate. The large decreases in the radical concentration in THF indicate that the pitch fraction contains relatively large amounts of reactive radicals, especially the pitch obtained at a low pyrolysis heating rate. It is also clear that the tars obtained at a higher pyrolysis heating rate contain more unreactive radicals, which agrees with the observation that a higher ΔT between the gas phase and the coal results in more cracking of volatiles that generates more coke particulates. Table 2 shows radical concentrations of the tars obtained from pyrolysis of four coals at the three pyrolysis heating rates and the 3661

DOI: 10.1021/acs.energyfuels.5b00594 Energy Fuels 2015, 29, 3658−3663

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Energy & Fuels Table 2. Distribution of Radicals in Various Tar Fractions radicals (×1016 spins g−1 of tar) coal HLBE

BLT

BET

DLT

a

−1

pyrolysis heating rate (°C min )

16

−1

radicals in tars (×10 spins g )

210 80 5 210 80 5 210 80 5 210 80 5

disappeared in hexane a

14.4 5.4 1.8 23.4 12.5 6 24.1 10.8 6.1 25.5 16.4 9.2

1.2 (8.1) 1.7 (31.3) 0.1 (8.0) 8.8 (37.5) 4.3 (34.0) 0.9 (15.4) 2.9 (11.9) 2.5 (23.0) 0.5 (7.6) 2.4 (9.6) 1.2 (7.4) 1.0 (10.8)

disappeared only in THF

retained in THF

4.6 (31.8) 2.8 (51.4) 1.7 (92.0) 10.0 (42.8) 5.9 (46.9) 4.8 (79.6) 4.7 (19.7) 5.0 (47.8) 4.5 (73.6) 12.5 (49.1) 11.1 (67.7) 6.2 (67.4)

8.7 (60.1) 0.9 (17.3) 0 (0) 4.6 (19.7) 2.4 (19.1) 0.3 (15.0) 16.5 (68.4) 3.2 (29.2) 1.1 (18.8) 10.5 (41.3) 4.1 (24.9) 2.0 (21.8)

The percentage of radicals in tars is in parentheses.

Figure 7. Changes in the radical concentration of BET tars maintained at high temperatures.

reported by us earlier.18 As shown in Figure 9 for BET tars, the coke formation is mainly influenced by the changes in the residence time of volatiles. The temperature difference caused by the changes in the residence time of volatiles plays a much minor role on the coke formation in tars.

temperature of volatiles because the reactor temperature increases with time. It is estimated that, for the volatiles generated at a coal temperature of 450 °C, the maximum temperatures of the volatiles experienced are 543.6, 542.8, and 542.4 °C when the pyrolysis heating rate is 80 °C min−1 and the N2-purging rates are 20, 100, and 200 mL min−1, corresponding to the residence times of volatiles of 2.4, 1.5, and 1.1s, respectively. The effect of the small changes in the temperature of volatiles on the properties of tars is not observable in the experiments because of large experimental errors but can be estimated by the coking kinetics of tars from the same coals

4. CONCLUSION This work studies tars obtained from pyrolysis of four coals in a fixed-bed tubular reactor at different heating rates and different N2-purging rates. The observations and conclusions made are as follows: (1) The radical concentration of tars increases with an 3662

DOI: 10.1021/acs.energyfuels.5b00594 Energy Fuels 2015, 29, 3658−3663

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Energy & Fuels

the latter, which may be attributed to the high radical concentration and high coke content of the former. (5) An increase in the residence time of volatiles in the pyrolysis reactor promotes cracking of the volatiles and yields tars of higher radical concentration.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-64421073. Fax: +86-10-64421077. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Basic Research Program of China (2011CB201300), the Natural Science Foundation of China (21276019), and the Shenhua Group (ST930012SH05) for financial support.

Figure 8. Radical concentration of the tars collected in pyrolysis at different N2-purging rates and a heating rate of 80 °C min−1.

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REFERENCES

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Figure 9. Amounts of coke formed at different temperatures and different residence times of volatiles.

increase in the coal rank, i.e., the carbon content of coals. This is because the generation of a radical fragment from a higher rank coal requires cleavage of more interconnected covalent bonds, while the generation of a radical fragment from a lower rank coal requires cleavage of fewer interconnected covalent bonds. Consequently, a radical fragment generated from a higher rank coal contains more unpaired electrons than that generated from a lower rank coal. In comparison to coupling of radical fragments from a lower rank coal, coupling of radical fragments from a higher rank coal is more difficult to pair all of the electrons and results in confinement of more radicals in the product. (2) The radical concentration of tars increases with an increase in the heating rate of coals in pyrolysis. This is because the volatiles generated from coal experience a higher gas-phase temperature at a higher pyrolysis heating rate, which promotes cracking of volatiles and formation of more heavy tar fractions that are high in radical concentration. (3) Radicals in tars include those reactive but low in mobility because of the high viscosity of tars and those unreactive because the unpaired electrons are confined in macromolecules. An increase in the coal pyrolysis heating rate increases the amounts of both radicals in tars. (4) The tars obtained from coal pyrolysis at a high heating rate are more reactive than those obtained at a low heating rate. The former yields more radicals at high temperatures (300 °C or higher) than 3663

DOI: 10.1021/acs.energyfuels.5b00594 Energy Fuels 2015, 29, 3658−3663