Effect of Catalysts on the Pyrolysis of Turkish Zonguldak Bituminous

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Energy & Fuels 2000, 14, 820-827

Effect of Catalysts on the Pyrolysis of Turkish Zonguldak Bituminous Coal Nurs¸ en Altuntas¸ O ¨ ztas¸ † and Yuda Yu¨ru¨m*,‡ Department of Chemistry, Hacettepe University, 06532 Beytepe, Ankara Turkey, and Faculty of Engineering and Natural Sciences, Sabanci University, 81474 Tuzla, Istanbul, Turkey Received August 26, 1999

Raw coal, HCl-treated coal, and HCl-HF treated coal samples ZnCl2, NiCl2, CoCl2, CuCl2, and Fe2O3‚SO4 as catalysts were pyrolyzed under various temperature and time conditions in an inert atmosphere. The conversion of organic matter was calculated. The change in the aliphatic hydrogen and the degree of aromatic condensation by FTIR technique have been examined a function of temperature, time, and catalyst type. The volumetric swelling ratio of the chars obtained in different experimental conditions have been measured using the pyridine swelling technique, and the extent of cross-linking in the macromolecular network of chars was examined. It was found that the cross-linking increased with the catalysts.

Introduction Mineral matter and inorganic compounds change coal behavior during the gasification and pyrolysis processes. These compounds have catalytic effects and change swelling and coke forming characteristics of the coal. How these compounds act is difficult to describe since they may behave as chemical agents or catalysts.1-4 The effects of various additives, selected mainly for their acidic-basic characteristics, on coal pyrolysis were examined.5-9 Lewis acids, in particular zinc hallides, have been used as test catalysts for lowering the operating conditions and for increasing product selectivity. Iron, copper, nickel, and zinc chlorides are mainly used as catalysts for liquefaction, gasification, and pyrolysis of coals.10,11 The catalytic activity of Fe2O3 for the hydrocracking of a bituminous coal increases when a small amount of SO42- is included in the catalyst.12 The effect of nitrates of Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) on the gasification rate and the composition of the evolved gases during steam gasification of Spanish lignite coke have been studied.13 Among the metals studied Ni showed the highest carbon conversion, although at steady conditions Fe was the best catalyst. *Corresponding author. † Hacettepe University. ‡ Sabanci University. (1) Franklin, H. D.; Peters, W. A.; Cariello, F.; Howard, J. B. Ind. Eng. Chem. Process. Des. Dev. 1981, 10, 670. (2) Franklin, H. D.; Peters, W. A.; Howard, J. B. Fuel 1982, 61, 155. (3) Franklin, H. D.; Peters, W. A.; Howard, J. B. Fuel 1982, 61, 1213. (4) Bexley, K.; Green, D. P.; Thomas, M. K. Fuel 1986, 65, 47. (5) Bell, T.A.; Mobley, D. P. Fuel 1979, 58, 661. (6) Jolly, R.; Charcosset, H.; Boudou, J. P.; Guet, J. M. Fuel Proc. Technol. 1988, 20, 51. (7) Kandiyoti, R.; Lazaridis, J. I.; Dyrvold, B.; Weerasinghe, C. R. Fuel 1984, 63, 1583. (8) Ibarra, J. V.; Moliner, R.; Gavilab, M. P. Fuel 1991, 70, 408. (9) Ibarra, J. V.; Moliner, R.; Palacios J. M. Fuel 1991, 70, 727. (10) Song, C.; Nomura, M.; Miyake, M. Fuel 1986, 65, 922. (11) Nishiyama, Y. Fuel 1986, 65, 1404. (12) Tanabe, K.; Hattari, H.; Yamaguchi; Yokoyama, S.; Umematsu, J.; Sanada, Y.; Fuel 1982, 61, 389. (13) Lopez-Peinado, A.; Carrasco-Marin, F.; Rivera-Utrilla, J.; MorenoCastilla, C. Fuel 1992, 71, 105.

In this work the catalytic effects of ZnCl2, NiCl2, CoCl2, CuCl2, and Fe2O3‚SO4 during pyrolysis were examined. Changes in the conversion of organic material and cross-linking matter of the Zonguldak bituminous coal were investigated, and the char samples obtained were characterized by FTIR. Experimental Section Sample. The Zonguldak bituminous coal has a carbon content of 89 wt % (dmmf). The composition of the Zonguldak coal is presented in Table 1. The coal sample was ground under a nitrogen atmosphere to -65 mesh ASTM and stored under nitrogen. Coal sample was Soxhlet extracted with a tolueneethyl alcohol solvent couple (1:1) at its atmospheric boiling point to separate the resins of the coal and dried in a vacuum oven at 50 °C, for 24 h under a nitrogen atmosphere. This sample is referred to as raw coal throughout the text of the present work. Then, this sample was demineralized with HCl and HF by standard methods.14 A 2 L aliquot of 6 N HCl was added to 200 g of coal. The slurry was stirred for 24 h under a nitrogen atmosphere; then it was filtered and washed with distilled water until the filtrate became neutral. Consecutively, 1.6 L of aqueous (40%) HF was added to HCl-washed coal, and this mixture was stirred for ∼24 h under a nitrogen atmosphere. After filtering, the demineralized coal was washed with 1 L of distilled water and dried at 50 °C, for 24 h under vacuum. Pyrolysis Experiments. Pyrolysis experiments were made with a Mettler TA 3000 differential scanning calorimeter. Raw and HCl- and HCl/HF-washed Zonguldak bituminous coal samples (approximately 6 mg, particle size of 210 µm) and coal samples containing catalysts were placed in a standard aluminum crucible in the differential scanning calorimetry (DSC) cell to run the pyrolysis experiments. ZnCl2, NiCl2, CuCl2, CoCl2, and Fe2O3‚SO4 compounds were used as catalysts in pyrolysis experiments. The percentage of the catalysts added to raw and HCl- and HCl/HF-treated coal samples was 5, 10, and 20%. The powdered coal samples were mechanically mixed with the catalysts. All chemicals used as (14) Yu¨ru¨m, Y.; Kramer, R.; Levy, M. Thermochim. Acta 1985, 94, 285.

10.1021/ef9901847 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/22/2000

Pyrolysis of Turkish Zonguldak Bituminous Coal

Energy & Fuels, Vol. 14, No. 4, 2000 821

Table 1. Composition of the Zonguldak Bituminous Coal Sample Proximate Analysis (wt %) volatile matter 22.4 fixed carbon 23.2 ash 53.4 moisture 1.0 Elemental Analysis (wt %) C 89.0 H 5.0 S (total) 1.2 N 1.3 O (by difference) 3.5 Maceral Group Analysis (vol %) vitrinite 38.0 liptinite inertinite 6.0 minerals 56.0 carbonates 1 clay minerals 52 pyrite 3 a

86.0a 14.0a

Mineral-free.

catalysts were of reagent grade quality. Fe2O3‚SO4 was prepared according to the method given by Farcasiu and Smith.15 Ferric ammonium sulfate (25 g) and urea (50 g) were dissolved in 1000 mL of distilled water. The solution was heated at 95 °C for 2 h with constant rapid stirring. Precipitation of FeO(OH) particles takes place during the heating by reaction with ammonia formed from urea and water. After precipitation was complete, the precipitate was washed 4-5 times with hot (∼70 °C) water to remove impurity ions. The washed precipitate was filtered, dried at 110 °C for 24 h, and finally calcined at 500 °C in air for 3 h. All of the pyrolysis experiments were made in a constant flow rate of N2 gas (25 mL min-1) and with a constant heating rate of 50 K min-1. The pyrolysis reactions were investigated in the temperature range of 300-500 °C and between 0 and 60 min. Conversion of pyrolysis experiments was calculated with the following equation:

% conversion )

W 0 - W1 W0(1 - a)

× 100

where W0 is the weight of the coal (dry basis) at the beginning, W1 is the weight of the residue, and a is the amount of ash (dry basis). Solvent Swelling Experiments. The swelling behavior of the residues was studied by Liotta’s method.16 Approximately 100 mg of a coal sample was placed in a 6 mm o.d. tube and centrifuged for 5 min at 2500 rev/min. The height of the sample was measured as h1. Excess pyridine (∼1 mL) was added into the tube, the contents of the tube were mixed, the tube was centrifuged after 24 h, and the height of the sample in the tube (h2) was measured. The volumetric swelling ratio was calculated as Qv ) h2/h1. Qv values were normalized between 0 and 1 using an X parameter introduced by Solomon et al.:17

X)

Qv,coal - Qv,residue Qv,coal - 1

1 - X ) 1 corresponds to the raw coal, and 1 - X ) 0, to the maximum cross-linked residue. FTIR Analysis. Infrared spectra of both coal samples and coke samples with and without catalyst were obtained with a Mattson 1000 FTIR spectrometer. KBr pellets were prepared by grinding exactly 2.5 mg of dried coal, and a coke sample (15) Farcasiu, M.; Smith, C. Fuel Process. Technol. 1991, 29, 199. (16) Liotta, R.; Brons, G.; Isaacs, J. Fuel 1983, 62, 781. (17) Solomon, P. R.; Serio, M. A.; Desphande, G. V.; Kroo, E. Energy and Fuels 1990, 4, 42.

Figure 1. Changes in conversion of the organic matter of Zonguldak bituminous coal with pyrolysis temperature and percentage of ZnCl2: (a) raw coal sample; (b) HCl-treated coal sample; (c) HCl/HF-treated coal sample. with 200 mg KBr pellets, 12.9 mm in diameter and 0.4 mm thickness, was pressed in an evacuated die from a 60.0 mg mixture of KBr and sample and dried at 110 °C for 72 h under a nitrogen atmosphere to remove water. Spectra were obtained with 300 scans at a resolution of 2 cm-1. The peak integration of aliphatic C-H bands (2800-3000 cm-1) was done by using a software present in a Mattson 1000 FTIR spectrometer. Infrared spectra of the coal and coke samples for aromatic C-H out-of bending bands in the range 600-900 cm-1 were resolved by the least squares curve fitting technique described previously by Yu¨ru¨m and Altuntas¸ 18

Results and Discussion Pyrolysis Experiments. In the present work, the effect of pyrolysis temperature and the percentage of catalytic chemicals was investigated in the pyrolysis of raw, HCl-treated, and HCl/HF-treated coal samples. ZnCl2, NiCl2, CoCl2, CuCl2, and Fe2O3‚SO4 were used as catalysts. The experimental time in all of the pyrolysis experiments was 60 min. Figure 1 shows the change (18) Yu¨ru¨m, Y.; Altuntas¸ , N. Fuel Sci. Technol. Int. 1994, 12, 1115.

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Energy & Fuels, Vol. 14, No. 4, 2000

in organic matter conversion with pyrolysis temperature and the percentage of catalyst in the pyrolysis of raw, HCl-treated, and HCl/HF-treated coal samples using ZnCl2 as catalyst. Conversion of raw coal in the pyrolysis reactions without catalyst changed from 10 to 38% as the temperature was increased from 300 to 500 °C. Addition of ZnCl2 to the raw coal increased the conversion significantly. Conversion of experiments with 5% ZnCl2 changed between 10 and 55% as the temperature of pyrolysis was raised from 300 to 500 °C. But it should also be noted that in experiments at 300 °C, 350, and 400 °C the conversions measured were similar to those obtained in experiments with no catalyst. Conversion seemed to increase in experiments at 450 °C from 25 (no catalyst) to 38% and in experiments at 500 °C from 35 (no catalyst) to 55%. Same situation was also observed in experiments with 10% ZnCl2. Conversions with 10% catalyst were similar to those with no catalyst in experiments at 300, 350, and 400 °C, and an increase in conversion was observed in experiments at 450 and 500 °C, from 30 (no catalyst) to 40% and from 35 (no catalyst) to 60%, respectively. These results indicated that ZnCl2 added to the raw coal up to 10 wt % started to be active as a catalyst at temperatures 450 °C and above. When the catalyst percentage was raised to 20%, the conversion of pyrolysis increased to 20-80% within the temperature range of 300-500 °C. The increases in conversion were as follows: at 300 °C from 10 (no catalyst) to 20%, at 350 °C from 15 (no catalyst) to 28%, at 400 °C from 25 (no catalyst) to 40%, at 450 °C from 30 (no catalyst) to 60%, and at 500 °C from 35 (no catalyst) to 80%. It seemed that as the percentage of the ZnCl2 was increased to values higher than 10%, the temperature at which the catalyst started to be active was lowered to 300 °C. This might be due to the interaction between the mineral matter and the catalytic material added.19 The action of the catalytical material added can be explained by the anchorage of metal ions to the minerals by a reaction with surface -OH groups.20 Therefore the presence of minerals containing the surface -OH groups might be considered as the starting point for the synergetic catalytic activity of both the minerals and the chemicals added. Conversion values for HCl-treated and HCl/HFtreated coal samples within the same temperature limits were 7-25% and 8-35%, respectively. The leaching of the mineral matter first with HCl and then with HF caused substantial decreases in the conversion values. The reason for this behavior can be explained with the presence of mineral matter in the raw coal samples. Raw Zonguldak bituminous coal contained carbonate, clay, and pyrite minerals.21 Calcite minerals are known to catalyze degradation of phenols in the pyrolysis of coals;22 therefore, removal of these minerals might have changed the behavior of the HCl-treated coals when they were compared with the raw coals. Pyrolysis of HCl/HF-treated coal samples gave rise to lower conversions than those of raw coal samples. (19) Artok, L.; Malla, P. B.; Komerneni, S.; Schobert, H. H. Energy Fuels 1993, 7, 430. (20) Iwasawa, Y. Adv. Catal. 1987, 35, 187. (21) Altuntas¸ , Oztas, N. Yu¨ru¨m, Y. Fuel 2000, 79, 1221. (22) Cypress, R.; Furfari, S. Fuel 1982, 61, 447.

Altuntas¸ O ¨ ztas¸ and Yu¨ ru¨ m

Figure 2. Changes in conversion of the organic matter of Zonguldak bituminous coal with pyrolysis temperature and percentage of Fe2O3‚SO4: (a) raw coal sample; (b) HCl-treated coal sample; (c) HCl/HF-treated coal sample.

When ZnCl2 was added to HCl- and HCl/HF-treated coals, conversions observed were lower than those of the ZnCl2/raw coal mixture. This indicated that neither the mineral matter nor the added chemicals acted catalytically when they are by themselves. But catalysis seemed to be active when both the original minerals and ZnCl2 were present in the pyrolysis system. This indicated that there existed a synergetic effect between the minerals present in the coal and the chemicals added as catalysts. Changes in conversion in the pyrolysis of raw coal, HCl-treated coal, and HF-treated coal samples using Fe2O3‚SO4, CoCl2, CuCl2, and NiCl2 are presented in Figures 2-5, respectively. In all of these figures the results seemed to be parallel to those of experiments with ZnCl2. Comparison of conversion values in the pyrolysis of raw coal with the least amount of catalyst (5%) is presented in Table 2. The order of the efficiency of the catalytical chemicals is as follows: ZnCl2 > CoCl2 > NiCl2 > Fe2O3‚SO4 > CuCl2. Reduction potentials of the reactions M2+ + 2e- f M (M3+ + 3e- f M, in the case of iron salt) are in the following order:23,24 -0.76 V


Figure 3. Changes in conversion of the organic matter of Zonguldak bituminous coal with pyrolysis temperature and percentage of CoCl2: (a) raw coal sample; (b) HCl-treated coal sample; (c) HCl/HF-treated coal sample.

(ZnCl2) < -0.28 V (CoCl2) < -0.23 V (NiCl2) < -0.04 V (Fe2O3‚SO4) < 0.34 V (CuCl2). The decreasing order in the efficiency for the catalysts was exactly the same as the increasing order of the reduction potentials of the chemicals that were used. It seemed that the lower the reduction potential of the ionic species, the higher the conversion obtained in the pyrolysis and the higher the efficiency of the catalyst. The metallic salts attached to the surface structures can readily be reduced by hydrogen evolved during pyrolysis of the organics. When the metal is reduced to the zerovalent state, it migrates over the surface and forms aggregates of metal atoms, thereby giving the most common form of a supported metal catalyst.25 Therefore it might be considered that the active species of the chemicals was the metallic form of the cation which was created by the reductive action of the hydrogen evolved during pyrolysis reactions. (23) Ebbing, D. D. General Chemistry; Houghton Mifflin Co.: Boston, 1984, p 696. (24) Oxtoby, D. W.; Gillis, H. P.; Nachtrieb, N. H. Principles of Modern Chemistry, 4th ed.; Saunders College Publishing: New York, 1999; p A39. (25) Gates, B. C. Catalytic Chemistry; John Wiley and Sons: New York, 1992; p 338.


Figure 4. Changes in conversion of the organic matter of Zonguldak bituminous coal with pyrolysis temperature and percentage of CuCl2: (a) raw coal sample; (b) HCl-treated coal sample; (c) HCl/HF-treated coal sample.

Solvent Swelling Experiments. The volumetric swelling ratios of the pyrolysis chars obtained in different experimental conditions with and without a catalyst were measured using the pyridine swelling technique, and the extent of cross-linking in the macromolecular network of chars was examined with respect to the catalyst type and the percentage of the catalyst. The functional group dependence of crosslinking reactions in coal pyrolysis has been recently shown.8,9,26 The change in the volumetric swelling ratios of the coke samples obtained from the raw coal sample with the catalyst type used and the catalyst percentage (at 500 °C; time, 60 min) is presented in Figure 6. As it is seen in Figure 6, when the coal was pyrolyzed without any catalyst, an increase in cross-linking was observed, X ) 0 (raw coal) and X ) 0.70 (coke obtained in pyrolysis). Addition of 5% of NiCl2 or CoCl2 to the raw coal further increased the cross-links (decreased the swelling in pyridine) in the cokes produced, X ) 0.70 (coke without catalyst) and X ) 1 (cokes with catalyst). (26) Ibarra, J. V.; Moliner, R.; Bonet, A. J. Fuel 1994, 73, 918.

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Figure 6. Change of the volumetric swelling ratio of coke samples obtained from the raw bituminous coal sample versus the catalyst type and the percentage of catalyst (500 °C, t ) 60 min).

Figure 7. Change of the volumetric swelling ratio of coke samples obtained from the HCl-treated bituminous coal sample versus the catalyst type and the percentage of catalyst (500 °C, t ) 60 min).

Figure 5. Changes in conversion of the organic matter of Zonguldak bituminous coal with pyrolysis temperature and percentage of NiCl2: (a) raw coal sample; (b) HCl-treated coal sample; (c) HCl/HF-treated coal sample. Table 2. Comparison of the Conversion Values in the Pyrolysis of Raw Coal with the Chemicals Used as Catalysts catalyst, percentage

% conversion of raw coal

ZnCl2, 5% CoCl2, 5% NiCl2, 5% Fe2O3‚SO4, 5% CuCl2, 5%

56.5 43.5 42.8 41.6 40.2

The value of X was still 1 when the percentage of NiCl2 and CoCl2 was increased to 10. When the percentages of ZnCl2 added were 5 and 10, cross-linking increased to X ) 0.85 and X ) 0.90, respectively, from X)0.70 (coke without catalyst). X)0.92 was measured both for 5 and 10% of the Fe2O3‚SO4 used with the raw coal in pyrolysis reactions. CuCl2 up to 5% increased X to 0.87, and further increase of the percentage to 10% decreased X to 0.75. Since the minerals were removed by successive acid washings, the tendency to form cross-linking decreased. X increased from X ) 0 (raw coal) to X ) 0.70 (coke of raw coal) (Figure 6) and decreased to X)0.61 (coke of

Figure 8. Change of the volumetric swelling ratio of coke samples obtained from the HCl/HF-treated bituminous coal sample versus the catalyst type and the percentage of catalyst (500 °C, t ) 60 min).

HCl treated coal) (Figure 7) and X ) 0.56 (coke of HCl/ HF-treated coal) (Figure 8). Variation of the crosslinking in the cokes obtained with catalyst type and percentage, in HCl- and HCl/HF-treated coal samples, are presented in Figure 7 and Figure 8, respectively. As the percentage of the catalysts added to the HCland HCl/HF-treated coal samples was increased, more cross-linking developed in the pyrolysis cokes relative to those of the acid-treated coal samples. The highest degree of cross-linking in the cokes of HCl-treated coal (X ) 0.85) and HCl/HF-treated coal (X ) 0.93) was

Pyrolysis of Turkish Zonguldak Bituminous Coal Table 3. Changes of the Total Aliphatic Peak Area (2800-3000 cm-1) in the Coke Sample Obtained from HCl/HF-Treated Coal Sample with Temperature and Time pyrolysis temp, °C

pyrolysis time, min

peak area, abs‚cm-1

300

0 30 60 0 30 60 0 30 60

5.77 4.67 3.89 3.35 3.27 1.35 1.48 1.05 0.61

400 500

observed when 5% ZnCl2 was used as catalyst. It seemed that when catalysts added to the raw and HCl- and HCl/ HF-treated samples, the pyrolysis cokes contained more cross-linked structures. This was probably due to the synergetic effect of the native minerals present in the raw coal and the chemicals added as catalysts. Characterization of Chars by FTIR Spectroscopy. Aliphatic Hydrogen. Since most of the native minerals were separated in HCl/HF-treated coal samples, it was thought that it would be rational to observe the effect of the chemicals added as catalysts on the organic material of the coal by investigating the cokes produced from the pyrolysis reactions. Table 3 shows the variation in the areas of all of the aliphatic C-H stretching peaks in the region of 2800-3000 cm-1 in the FTIR spectra of the coke samples obtained from the pyrolysis of HCl/ HF-treated coal samples at 300, 400, and 500 °C for 0, 30, and 60 min (0 min means the time at which the furnace temperature reached 300, 400, or 500 °C at a heating rate of 50 °C/min). The total aliphatic C-H peak area in the unpyrolyzed HCl/HF-treated coal sample was 5.74. It seemed that, in experiments done at 300 °C and t ) 0 min, the aliphatic structures remained unchanged since the area of the total aliphatic C-H peak area stayed at 5.77. When the pyrolysis temperature was increased from 300 to 500 °C and the experimental time from 0 to 60 min, the total peak area of the aliphatic C-H bands decreased sharply. Practically most of the aliphatic structures were removed in experiments done at 500 °C and t ) 60 min. These results showed that dehydrogenation reactions through aliphatic group losses occurred as the pyrolysis temperatures and times were increased. The dehydrogenation reactions seemed to start at 300 °C, and as the temperature was increased to higher temperatures, a drastic decrease was observed in the aliphatic C-H peak area. Table 4 shows the change in the peak area of aliphatic C-H peaks in the FTIR spectra of the coke samples obtained in the pyrolysis experiments with 10% catalyst added-HCl/HF-treated coal samples. As it is seen in Table 4, addition of the catalysts decreased the intensity of the aliphatic peaks in all of the experiments to values even lower than those of the cokes obtained without addition of any catalysts (Table 3). In all of the cases, as the temperatures and experimental times were increased, the areas of the aliphatic bands also decreased. Losses in the aliphatic structures cause an increase in the tar production, and the radicals formed as the alkyl groups left during pyrolysis were stabilized by forming cross-links in the coke produced. This is in accordance with our swelling results given above.

Energy & Fuels, Vol. 14, No. 4, 2000 825 Table 4. Changes of the Total Aliphatic Peak Area in the Coke Sample Obtained from the Pyrolysis of the HCl/ HF-Treated Coal Samples with Various Catalysts

catalyst

pyrolysis time, min

ZnCl2

0 30 60 0 30 60 0 30 60 0 30 60 0 30 60

CuCl2 NiCl2 CoCl2 Fe2O3‚SO4

peak area (%) for given pyrolysis temp 300 °C 400 °C 500 °C 3.0 1.6 1.1 2.8 1.6 1.4 2.8 1.3 1.1 2.4 1.7 1.4 3.9 4.3 2.2

1.3 1.2 0.7 1.5 0.8 0.4 1.3 0.6 0.3 2.4 0.6 0.6 2.1 0.7 0.6

1.5 0.3 0.5 1.4 0.3 0.6 1.5 1.1 0.4 0.9 0.6 0.3 1.3 0.3 0.4

Table 5. Results of Curve Fitting for the Aromatic C-H Out-of-Plane Bands (600-900 cm-1) of the HCl/ HF-Treated Coal Sample

a

wave number, cm-1

assigna

area, %

900 876 859 844 812 748 724 700

1H 1H 1H 1H 2H, 3H 4H (CH2)n 5H

12.0 8.4 4.1 3.5 35.5 22.2 4.0 10.3

Number of adjacent aromatic hydrogens per ring.

Aromatic Hydrogen. Eight out-of-plane C-H deformation bands were observed by curve fitting in the FTIR spectrum of the HCl/HF-treated coal samples in the 600-900 cm-1 region, Table 5. Peaks due to isolated aromatic hydrogens (1H), two or three adjacent aromatic hydrogens per ring (2H, 3H), and four or five adjacent aromatic hydrogens (4H, 5H) were in the 830-900 cm-1, 770-830 cm-1, and 700-770 cm-1 sections of the spectrum, respectively.27 Figure 9 shows the change in the degree of aromatic condensation26 (one isolated aromatic hydrogen, 1H) in the FTIR spectra of the cokes of HCl/HF-treated coals with the pyrolysis temperature and time. The percent area of the peaks due to one isolated aromatic hydrogen in the spectra of HCl/HF-treated coal and of the coke obtained at 300 °C and 0 min stayed almost constant at 27.9 and 25.6, respectively. This might be considered as the structures containing isolated aromatic hydrogens (1H) in the HCl/HF-treated coal did not change too significantly in the coke obtained at 300 °C and 0 min. At longer times of 30 and 60 min the area percentage increased to 35.6 and 41.1, respectively. When the temperature reached 300 °C (0 min), aromatic condensation reactions did not start, but at extended times it seemed that extensive condensation occurred even at 300 °C. Formation of structures with isolated aromatic hydrogens by intra- and intermolecular radical-transfer reactions at 350 °C was reported.28 In experiments at 400 and 500 °C even at the start of the experiment the area percentages due to isolated aro(27) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993. (28) van Heek, K. H.; Hodek, W. Fuel 1994, 73, 886.

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Figure 9. Change of the degree of aromatic condensation (1H) in the pyrolysis of the HCl/HF-treated coal sample with pyrolysis temperature and time.

Figure 11. Change of the degree of aromatic substitution (4H, 5H) in the pyrolysis of the HCl/HF-treated coal sample with pyrolysis temperature and time. Table 6. Changes in the Degree of Aromatic Condensation (One Isolated Aromatic Hydrogen, 1H) in the FTIR Spectra of the Cokes Obtained in the Pyrolysis Experiments with 10% Catalyst Added to HCl/HF-Treated Coal Samples

catalyst no catalyst ZnCl2 CuCl2 NiCl2

Figure 10. Change of the degree of aromatic substitution (2H, 3H) in the pyrolysis of the HCl/HF-treated coal sample with pyrolysis temperature and time.

matic hydrogens (1H) sharply increased to greater values than those of HCl/HF-treated coal. Aromatic condensation reactions seemed to increase with increasing pyrolysis temperature and time. The change of the percentage of the peak areas in the 770-830 cm-1 (2H, 3H) and 700-770 cm-1 (4H, 5H) regions are presented in Figures 10 and 11, respectively. The results in both Figures 10 and 11 are in accordance with those in Figure 9. As the temperature and time of the pyrolysis experiments increased, the area due to aromatic structures containing (2H, 3H) and (4H, 5H) groups seemed to decrease extensively. The meaning of this is that aromatic structures with a few condensed aromatics tend to produce aromatics with large numbers of condensed rings. The results indicated that the degree of aromatic substitution decreased with increasing temperature and time of the pyrolysis. The decrease of aromatic structures with two or three adjacent aromatic hydrogens per ring can be explained with removal of terminal groups attached to aromatic groups from coal and subsequent stabilization of free radicals by the hydrogen produced in pyrolysis or transformation to aromatic structure with one isolated aromatic hydrogen per ring with condensation of the aromatic.

CoCl2 Fe2O3‚SO4

pyrolysis time, min 0 30 60 0 30 60 0 30 60 0 30 60 0 30 60 0 30 60

peak area (%) for given pyrolysis temp 300 °C 400 °C 500 °C 25.6 35.6 41.1 36.1 39.9 42.9 41.6 38.5 47.1 38.9 39.3 42.5 39.9 29.7 40.3 42.4 25.6 35.7

33.0 39.3 41.6 34.9 35.1 35.1 39.2 41.7 40.1 36.7 35.3 44.5 39.6 42.6 40.0 39.2 40.9 33.0

35.3 37.0 39.5 37.9 44.5 37.7 46.4 38.1 43.0 39.8 37.1 40.1 47.7 39.3 43.7 32.2 39.1 32.2

The change in the degree of aromatic condensation (one isolated aromatic hydrogen, 1H) in the FTIR spectra of the cokes obtained in the pyrolysis experiments with 10% catalyst added to HCl/HF-treated coal samples was given in Table 6. The general conclusion of the data presented in Table 6 is that addition of catalysts increased the areas of bands due to one isolated aromatic hydrogen in all catalysts, temperatures, and times. Areas increased as the time of pyrolysis experiments at 300 and 400 °C was also increased. Although it is not possible to differentiate the effect of any catalyst in the production of such aromatic structures, it seemed that addition of NiCl2 caused higher areas due to one isolated aromatic hydrogen. Areas due to one isolated aromatic hydrogen produced in experiments done at 500 °C did not follow a regular trend with experimental time. Conclusions (1) When ZnCl2 was added to HCl- and HCl/HFtreated coals, conversions observed were lower than


those of the ZnCl2/raw coal mixture. This indicated that neither the mineral matter nor the added chemicals acted catalytically when they are by themselves. But catalysis seemed to be active when both the original minerals and ZnCl2 were present in the pyrolysis system. This indicated that there existed a synergetic effect between the minerals present in the coal and the chemicals added as catalysts. (2) The order of the efficiency of the catalytical chemicals is as follows: ZnCl2 > CoCl2 > NiCl2 > Fe2O3‚ SO4 > CuCl2. The decreasing order in the efficiency for the catalysts was exactly the same as the increasing order of the reduction potentials of the chemicals that were used. It seemed that the lower the reduction potential of the ionic species, the higher the conversion obtained in the pyrolysis and the higher the efficiency of the catalyst. (3) It seemed that when catalysts added to the raw and HCl- and HCl/HF-treated samples the pyrolysis


cokes contained more cross-linked structures. This was probably due to the synergetic effect of the native minerals present in the raw coal and the chemicals added as catalysts. (4) In all of the cases as the temperatures and experimental times were increased the areas of the aliphatic bands also decreased. Losses in the aliphatic structures cause an increase in the tar production and the radicals formed as the alkyl groups left during pyrolysis were stabilized by forming cross-links in the coke produced. This is in accordance with our swelling results. (5) Aromatic condensation reactions seemed to increase with increasing pyrolysis temperature and time in the coke samples obtained from HCl/HF-treated coal. The degree of aromatic substitution decreased with increasing temperature and time of the pyrolysis. EF9901847

Effect of Catalysts on the Pyrolysis of Turkish Zonguldak Bituminous

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