Catalytic Cracking of Pyrolytic Vapors of Low-Rank Coal over

Low-grade natural iron ore, limonite, is proposed as an inexpensive catalyst to increase the light liquid production from coal pyrolysis. Experiments ...
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Catalytic Cracking of Pyrolytic Vapors of Low-Rank Coal over Limonite Ore Lu He,†,‡ Songgeng Li,*,†,‡ and Weigang Lin† †

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: Low-grade natural iron ore, limonite, is proposed as an inexpensive catalyst to increase the light liquid production from coal pyrolysis. Experiments were conducted at a pyrolyzer combined with gas chromatography/mass spectrometry (Py− GC/MS). It is found that limonite favors the formation of light aromatic hydrocarbons, while the aliphatics and oxygenated compounds in tar are significantly reduced. Tests with model compounds (C19 alkane and o-cresol) indicate that the increased light aromatics could result from the conversions of either oxygenated compounds or aliphatics promoted by limonite. Gaseous products are determined with online gas chromatography (GC). A strong correlation between COx and light aromatics is revealed in the catalytic pyrolysis process, indicating that COx could be seen as an index of light aromatic formation. There is a maximum value for the yield of liquids when the temperature is increased, which is quite different from the tests without limonite. It is postulated that pyrolytic water is decreased with the temperature because the total tar yield exhibits an increasing trend.

1. INTRODUCTION Low-rank coals, such as lignite and sub-bituminous coal, constitute over 50% of total coal reserves worldwide.1 These low-rank coals generally have low calorific values and high moisture and oxygen contents, which significantly impact their utilization, including low thermal efficiency for direct combustion, high cost in transportation, and the risk of spontaneous combustion in storage.2 However, low-rank coals contain high volatile matter. These volatiles can be readily released through pyrolysis at moderate temperatures and further converted into liquid fuels, valuable chemicals, and gaseous products. Thus, various processes for the production of liquid fuels and gaseous products based on pyrolysis of lowrank coals has been developed, especially in China because there is a lack of petroleum and gas. A challenge of the process industrialization is how to minimize the heavy component production in tar and increase the selectivity to light liquids. The heavy components would not only result in the difficulty in further processing the tar but also cause serious operation problem in downstream equipment of the process, such as pipe blocking.3 Thermal cracking seems to be the simplest way to reduce the heavy components in tar. However, the selectivity to light liquids is insufficient. Catalytic cracking is considered as a potential way to convert heavy compounds into light liquids with high selectivity.4 Iron is very active in the catalytic chemistry.5 Iron-based catalysts have widely been used in coal liquefaction and gasification.6−12 Fe/SiO2 and Fe/activated carbon exhibit good selectivity for the hydrodeoxygenation of lignin vapors to benzene, toluene, and xylene (BTX).13 Some researchers attempt to use iron-containing red mud as a catalyst for deoxygenation of biomass pyrolytic vapors.14,15 In the study of hydrocracking of crude vacuum residue, it is found that limonite favors the formation of light liquid hydrocarbons.16 © 2016 American Chemical Society

More recently, it has been reported that there is a correlation between Fe and the aromatic hydrocarbon yield in pyrolysis of biomass over HZSM-5.17 These studies suggest that iron could be effective in the production of light liquids from coal pyrolytic vapors. It is known that coal volatiles are a complex mixture of various compounds with different structures. Catalysts are easily deactivated in this environment as a result of coke formation.18 Research indicates that regeneration by burning the coke does not seem to completely restore their performance.19 The use of natural iron ore as a catalyst might not necessarily be regenerated and reactivated because the deactivated iron ore can be recycled as the feedstock of iron- and steel-making industries.20−22 Moreover, iron ores are easily available and cheap. In view of the above-mentioned fact, natural iron ore is chosen for investigation in this work. In this work, the catalytic behaviors of natural limonite ore on coal pyrolytic vapors are studied using a pyrolyzer combined with gas chromatography/mass spectrometry (Py−GC/MS). Non-condensable gaseous products generated during pyrolysis are simultaneously measured on line with gas chromatography (GC). Effects of limonite ore on the yields of coal pyrolytic products and their compositions at different temperatures are discussed in an attempt to reveal the intercorrelations of their variations. A selection of relevant model compounds are pyrolyzed to elucidate the possible reaction pathway acting by limonite. Received: May 17, 2016 Revised: August 20, 2016 Published: August 22, 2016 6984

DOI: 10.1021/acs.energyfuels.6b01182 Energy Fuels 2016, 30, 6984−6990

Article

Energy & Fuels

2. EXPERIMENTAL SECTION

Table 3. BET Surface Area, Pore Size, and Pore Volume of Calcined Limonite

2.1. Coal Sample and Catalyst. A lignite from Baori, Inner Mongolia, was used for the pyrolysis test throughout this study. The coal samples were ground and sieved to a size fraction below 75 μm and dried at 105 °C for 4 h prior to experimentation. The proximate and ultimate analyses of the coal are given in Table 1. It is noteworthy that oxygen contained in the coal is as high as 20.1%.

a

ultimate analysis (wt %, dry)

V

A

FC

C

H

N

S

Oa

44.0

12.0

44.0

62.6

4.3

0.8

0.3

20.1

pore volume (cm3 g−1)

pore diameter (nm)

65

0.050

4

grammed as follows: begin at 45 °C, with holding for 5 min, then increase to 260 °C by 15 °C/min, with holding for 40 min, and further increase to 280 °C by 30 °C/min, with holding for 5 min. The yields of gases are quantified with TCD responses. An external standard method was used for calibrations of the TCD responses with known concentrations of the target gases. For each run, a coal sample of 1.0 ± 0.03 mg and a limonite ore of 1.0 ± 0.03 mg are separately loaded in a small quartz tube (2.5 mm in outside diameter × 25 mm in length). Quartz wool is placed in between and at both ends of the samples to prevent the particles from moving by a purging gas. The samples are then heated to a desired temperature at a rate of 20 °C/ms under an argon atmosphere, holding for 25 s at a set temperature (500, 600, 700, and 800 °C). All of the experiments were conducted 3 times. The relative errors are less than ±5%, indicating good repeatability. 2.3. Determination of Yields of Pyrolysis Products. The yield of volatiles can be obtained through the difference of the weight of the coal sample before and after pyrolysis. A high precision balance with an accuracy of 0.001 mg was used to weigh the sample before and after the experimentation. The volatile yield is calculated as

Table 1. Proximate and Ultimate Analyses of the Coal Sample proximate analysis (wt %, dry)

SBET (m2 g−1)

By difference.

Natural limonite ore, provided by the Research Academy of Shandong Metallurgy, was employed as a catalyst. The limonite ore is a brown powder with an average diameter of 34 μm, composed mainly of goethite (FeOOH) with a small amount of a soft, gray talc-like mineral dispersed heterogeneously. Chemical analysis of the asreceived limonite sample is presented in Table 2. The iron contained in the ore is 40.24%. Prior to experimentation, the iron ore sample was calcined at 450 °C for 1 h in a muffle furnace to remove combined water. The calcined ore sample was kept in a sealed container to prevent moisture readsorption. Goethite becomes hematite after calcination. The Brunauer−Emmett−Teller (BET) surface area, average pore size, and pore volume of the calcined limonite are given in Table 3. It is seen that it has a large surface area and nanoporous structure, which could be the result of evaporation of the combined water during the calcination. 2.2. Apparatus. Pyrolysis was carried out at a platinum filament pyrolyzer (CDS 5200, CDS Analytical, Oxford, PA). The pyrolyzer can be operated in two modes: Py mode and trap mode. A detailed description of the operation modes can be found in the literature.23 The trap mode was employed throughout this study. In the trap mode, the released volatiles first go to an adsorption trap, which is kept at 40 °C. The escaped incondensable gases were directed into a GC instrument for analysis. The adsorbed volatiles by the trap are desorbed at an elevated temperature (300 °C) with argon as a purging gas, analyzed with a GC/MS analyzer (TRACE GC, ISQ MS, Thermo Scientific Co.). This kind of setup can simultaneously measure the incondensable and condensable gases from coal pyrolysis on line, which helps to reveal the possible reaction pathways of the pyrolysis vapors over limonite. The chromatographic separations are performed using a WAX column (30 m × 0.25 mm × 0.25 μm). The oven for the GC column is programmed from 40 °C (holding for 3 min) to 100 °C (holding for 1 min) at a heating rate of 6 °C/min, then to 180 °C (holding for 2 min) at a heating rate of 6 °C/min, further to 240 °C (holding for 5 min) at a heating rate of 6 °C/min, and finally to 280 °C (holding for 1 min) at a heating rate of 6 °C/min. The injector temperature is at 300 °C with a split ratio of 115:1 and a helium flow rate of 1.2 mL/ min. The transfer line temperature between GC and MS is kept at 280 °C. The mass spectrometer is operated in electron ionization (EI) mode at 70 eV, and the ion source temperature is 250 °C. The m/z range is set from 2 to 500. The peaks can be identified by matching with data in the National Institute of Standards and Technology library. GC is equipped with a Carboxen 1000 column and a thermal conductivity detector (TCD). The column temperature is pro-

Yvolatile =

M 0 − M1 × 100% M0

in which M0 is the mass of the sample before pyrolysis and M1 is the mass of the sample after pyrolysis. The char yield is obtained as

Ychar = 1 − Yvolatile The total yield of gaseous products is described as n

Ygas =

∑ Ygas,j j=1

where Ygas,j is the yield of component j in the gaseous products, determined by GC via an external standard method. The liquid yield is obtained as

Yliquid = 1 − Ygas − Ychar which includes tar and pyrolytic water. In general, the chromatographic peak area of a compound is considered linear with its quantity. For each product, the peak area obtained under different reaction conditions can be compared to reveal the changing of its yield in consideration of the fact that the nearly exactly same amount of the sample was used for each test in this work.24 Thus, the peak area is taken in this work as an indication of the variations of tar and its composition yields under different pyrolysis conditions.

3. RESULTS AND DISCUSSION 3.1. Pyrolysis Product Distribution. Figure 1 presents the variation of the amount of tar before and after catalysis with the temperature. It can be seen that the tar with and without catalysis show a monotonic increase with the temperature. The difference is that the tar yield is significantly reduced in the presence of limonite. Correspondingly, the yield of gaseous products is increased after catalysis as a result of tar cracking

Table 2. Composition of Limonite component

Fetotal

SiO2

Al2O3

CaO

MgO

P

S

combined water

wt %

40.24

8.40

0.65

11.95

1.17

0.041

0.087

4.84

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of the apparatus limitation. However, on a basis of mass balance, it can be postulated that the dramatic decrease in the yield of liquids for the case with catalysis could result from the reduction of pyrolytic water because the tar keeps an increase with the temperature, as indicated in Figure 1. Further experimentation is certainly necessary to confirm this fact. 3.2. Tar Compositions. Light aromatics, such as benzene, toluene, ethylbenzene, xylene, and naphthalene (BTEXN), are important industrial chemicals, widely used as fuel additives, solvents, and the feedstock for production of pesticides, plastic products, and synthetic fibers. Figure 4 presents BTEXN

Figure 1. Variation of the amount of tar before and after the catalysis of limonite with the temperature.

over limonite, as shown in Figure 2. It is interesting to note that the liquid yields before and after catalysis appear to follow a

Figure 4. BTEXN production before and after the catalysis of limonite at different temperatures.

production before and after the catalysis at different temperatures. The amount of BTEXN released from coal pyrolysis increases with an increase in the temperature. The trend remains similar after the pyrolytic vapors pass through the catalyst. However, the increase is relatively faster in comparison to the test without limonite. BTEXN production is significantly enhanced as a result of the presence of limonite. The amount of BTEXN increases by 38−58%, although the total tar yield is reduced. It is shown that the increment increases with the temperature and reaches a maximum at 700 °C, indicating that limonite has the highest selectivity to BTEXN at this temperature. Further looking at the changes of individual components of BTEXN, it is found that the increment of benzene is the most significant, indicating that limonite has a relatively higher selectivity to benzene. In contrast to BTEXN, aliphatics, phenols, and oxygencontaining compounds are dramatically reduced in the presence of limonite (shown in Figure 5), which indicates that limonite has strong decomposition for C−C, C−H, and C−O. The presence of limonite seems no obvious effect on the trends of aliphatic hydrocarbons, phenols, and other oxygen-containing compounds with the temperature. These oxygenates before and after catalysis exhibit similar increasing trends within the examined temperature range. They significantly increase with an increase in the temperature at the relatively low temperature range (500−600 °C). With a further increase in the temperature, they appear to level off. Table 4 lists the major oxygen-containing compounds before and after the catalysis of limonite. It is seen that phenol and cresol are dominant compounds in the oxygenates. These oxygenates are significantly reduced as a result of the presence of limonite. The significant reduction in the oxygenates after catalysis can be explained by the fact that iron can facilitate oxygen-transfer

Figure 2. Yields of gas with and without limonite at different temperatures.

Figure 3. Yields of liquid with and without limonite at different temperatures.

different trend with the temperature (shown in Figure 3). The yield of liquids without catalysis monotonically increases with the increasing temperature. After the catalysis of limonite, the yield of liquids increases with the temperature to a maximum value and then dramatically decreases. It is known that the liquid obtained from coal pyrolysis is composed of tar and pyrolytic water.25,26 The water was also detected by GC in this work, although quantification of the water is difficult as a result 6986

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Figure 5. Aliphatics, phenols, and other oxygen-containing compounds before and after the catalysis of limonite.

Figure 6. Yields of carbon oxides before and after catalysis at different temperatures.

Table 4. Major Oxygen-Containing Compounds with and without the Presence of Limonite

a

no catalyst (peak area, ×108)

limonite (peak area, ×108)

oxygen-containing compounds

500 °C

600 °C

500 °C

600 °C

phenol cresol xylenol (ethylphenol) napthol benzenediol alkyl phenol alkoxy phenol acetic acid n-hexadecanoic acid dodecyl propyl carbonate hexadecen-1-ol, trans-9octadecanoic acid 1-dodecanol, 2-octyl-, acetate 2-cyclopenten-1-one, 3-methylbenzofuran benzofuran,2,3-dihydro-

23.9 20.3 6.8 5.4 3.9 5.5 13.4 9.8 8.6 2.0 3.3 2.7 2.1 2.2 1.5 3.5

53.9 47.1 19.6 14.3 7.6 10.6 13.1 12.3 7.3 4.8 5.3 1.1 2.8 3.2 3.1 5.8

9.7 5.4 a a a a a 0.5 2.3 1.9 1.3 0.7 0.4 0.6 1.3 1.6

32.4 17.8 4.3 1.7 a 1.1 a 1.4 5.0 4.1 3.2 0.6 2.6 0.7 2.7 2.0

limonite. It is reasonable that more hydrogen is generated after passing through the catalyst, as shown in Figure 7. There is almost no change in the yield of methane by comparison of before and after catalysis, as shown in Figure 8.

Figure 7. Yields of hydrogen before and after catalysis of limonite.

Below the detection limit.

redox reactions, owing to its stability at both Fe2+ and Fe3+ oxidation states. 3.3. Gaseous Products. CO, CO2, H2, and CH4 are main gaseous products. There are very few C2 and C3 (only detected at 800 °C; thus, they are not given here). The yields of CO2 and CO before and after the catalysis of limonite are shown in Figure 6. It is found that the yield of CO is dramatically reduced after catalysis, while CO2 is significantly augmented. The significant reduction in CO after catalysis is a bit out of expectation in consideration of the reduction of oxygenated compounds in tar as a result of the presence of limonite. It is known that oxygen in the oxygenated compounds is released in the form of water, CO, and CO2. Despite the reduction in the yield of CO, the total COx (CO2 and CO) is significantly augmented after the catalysis, which consolidates the fact that more oxygenated compounds are converted in the presence of

Figure 8. Yields of CH4 before and after catalysis of limonite. 6987

DOI: 10.1021/acs.energyfuels.6b01182 Energy Fuels 2016, 30, 6984−6990

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Energy & Fuels 3.4. Possible Reaction Pathways. In consideration of the significant reduction in oxygen-containing compounds and aliphatics and the increase in BTEXN after the catalysis of limonite, one may speculate that the increase of BTEXN could result from either aliphatic or oxygenate conversion. To confirm this point, C19 alkane and o-cresol chosen as model compounds were pyrolyzed at a temperature of 800 °C with and without the presence of limonite (shown in Figures 9 and

Figure 11. Correlation of COx with the production of BTEXN in the presence of limonite.

as a whole (catalytic derived plus thermal derived). It is found that there is exactly a linear relation between them. This may indicate that BTEXN formation is strongly connected with deoxygenation in the catalytic coal pyrolysis. The yield of COx can be used as an index of BTEXN formation for the pyrolysis of coal over limonite. To further distinguish the catalytic effect from the thermal effect, catalytic-derived COx (defined as the difference of the yield of COx with and without the presence of limonite) was plotted against catalytic-derived BTEXN (defined as the difference of BTEXN with and without limonite), which is shown in Figure 12. It is seen that the

Figure 9. Chromatogram of pyrolysis products of o-cresol at 800 °C.

Figure 10. Chromatogram of pyrolysis products of C19 alkane at 800 °C.

10, respectively). It can be seen that benzene and toluene as major products from the pyrolysis of o-cresol are identified. Their amounts are considerably increased in the presence of limonite. Research has shown that phenols are easily absorbed on the iron oxide surface to form surface phenoxy species, which would undergo several reactions, such as breaking of C− C and C−O, to generate more benzene and toluene.27 In the pyrolysates of C19 alkane, few aromatics are found without limonite. In contrast, benzene, toluene, and naphthalene are significantly enhanced when limonite is present. It has been proven that iron is very active for the deoxygenation of pyrolysis vapors.28−30 Oxygen is removed from oxygenates in the form of CO, CO2, and water during deoxygenation reactions. There should be some correlation between the yields of COx and BTEXN if the conversion from oxygenates is dominant in BTEXN production. It is known that aromatics formed during the catalytic process can arise from either of the two mechanisms. One is direct thermal decomposition of coal and coal pyrolytic vapors. The other is catalytic decomposition of pyrolytic vapors. Likewise, carbon oxides can also be categorized into thermal and catalytic origin. Figure 11 presents the plot of the yields of COx versus BTEXN

Figure 12. Correlation of catalytic-derived COx with catalytic-derived BTEXN.

correlation coefficient is 0.72, which is not as high as the correlation as a whole. One possible explanation for the relatively lower coefficient is that there exists other conversion routes (such as cyclization of aliphatics mentioned above) promoted by limonite besides the deoxygenation.

4. CONCLUSION Major conclusions can be summarized as follows: (1) The gas yield is increased at the expense of the tar in the presence of limonite. The total yield of liquids, including tar and pyrolytic water, exhibits a peak value at a temperature of 600 °C. It is postulated that the pyrolytic water is reduced with the increasing temperature as a result of the presence of limonite in consideration of the fact that the tar yield shows a monotonic increasing trend with the temperature. (2) Limonite significantly promotes the formation of light aromatic hydrocarbons. BTEXN increases by 38−58% within the examined temper6988

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ature range. Oxygenates and aliphatics in liquids are significantly reduced. Total COx in the gaseous products is increased after passing through limonite, although CO is decreased. The increase in total COx could be attributed to the decrease of oxygenated compounds in liquids, which indicates that limonite has strong deoxygenation ability for coal pyrolytic vapors. There is no obvious change in the yield of methane. A significant increase in H2 is observed after the catalysis of limonite. (3) A strong correlation between BTEXN and total COx (catatlytic derived plus thermal derived) is found. Deoxygenation plays a crucial role in the formation of BTEXN. Increased BTEXN can be attributed to the conversions of oxygenated compounds and aliphatics promoted by limonite.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-10-82544815. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Joint Program of the Natural Science Foundation of China and the Shenhua Group Corporation Limited (51174284), the National Key Technology R&D Program of Ministry of Science and Technology of China (2013BAC12B02), the Chinese Academy of Sciences (CAS)/State Administration of Foreign Expert Affairs (SAFEA) International Partnership Program for Creative Research Teams, and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDA07010200). The authors thank Dr. Lufei Jia from Natural Resources Canada’s CanmetENERGY for his contribution to this work.

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DOI: 10.1021/acs.energyfuels.6b01182 Energy Fuels 2016, 30, 6984−6990