352
Energy & Fuels 1990,4,352-355
Effect of Acetylation on Caking Properties of Bituminous Coals Hiroyuki Seki, Osamu Ito, and Masashi Iino* Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Katahira, Aoba-ku, Sendai 980, Japan Received December 22, 1989. Revised Manuscript Received May 2, 1990 The effect of acetylation for five bituminous coals on caking properties and extraction yield was examined. The Roga index for all the coals used was decreased by acetylation. The free swelling index (FSI) for the coals with low fluidity was also decreased by acetylation. For Shin-yubari and Zao Zhuang coals, which possess considerably high fluidities, the FSI was slightly increased by acetylation, suggesting the existence of an optimum fluidity for coal swelling. The extraction yield with mixed solvents for the acetylated coals was decreased. ESR and swelling ratio measurements of the heat-treated acetylated coals showed that cross-links were formed during heat treatment, leading to the reduction of caking properties. Introduction Weathering (oxidation) alters chemical and physical properties of coal and consequently affecta coal utilization. Coal oxidation, in general, reduces caking however, the oxidation mechanism and the reasons for the reduction of caking properties are still not well understood. Oxidation can introduce various functional groups such as ether, hydroxyl, and carboxyl groups. Liotta et al.' attributed the reduction of caking properties to the formation of ether groups during oxidation. On the other hand, it is possible that most of the oxygen-related cross-linkings occur during heat treatment by the condensation of the oxygen-functional groups introduced by oxidation.24 Lynch et al.2 reported the inverse correlation between caking parameters such as Gieseler maximum fluidity and maximum dilatation volume and the amount of carbonyl groups for oxidized coals. This suggests that the formation of reactive groups that may cross-link upon heating is responsible for the reduction of caking properties. Painter et al.3 also pointed out the importance of the formation of cross-links by the condensation of phenolic hydroxyl and carboxyl groups, which results in the formation of ester groups. In the previous study, we reported that the degree of loss of aliphatic hydrogens and that of the formation of reactive oxygen functional groups were different among coals.' In addition, we pointed out that oxidation of the coal matrix (residue after mixed-solvent extraction) strongly affected the bulk fluidity and swelling ability.8 In the present study, we examined the effect of acetylation on caking properties, comparing that of phenolic hydroxyl groups which would cross-link upon heating. We used bituminous coals of more than 79% carbon (daf), which seems to have much fewer carboxylic acid groups than phenolic hydroxyl groups. Although coal acetylation is, in general, widely utilized for the determination of hydroxyl groups,S,loace(1)Liotta, R.;Brons, G.; Isaaca, J. Fuel 1983,62, 781. (2)Lynch, B.M.;Lancaeter, L. I.; MacPhee, J. A. Energy Fuels 1988, 2,13. (3)Painter, P. C.;Snyder, R. W.; Pearson, D. E.; Kwong, J. Fuel 1980, 59, 282. (4)Szladow, A. J.; Ignasiak, B. S. Fuel 1976,55,253. (5)Larsen, J. W.;Lee, D.; Schmidt, T.; Grint, A. Fuel 1986,65,595. ( 6 ) Huffman, G.P.; Huggins, F. E.; Dunmyre, G.R.; Pignocco, A. J.; Lin, M. C. Fuel 1985,64,849. (7)Seki, H.; Ito, 0.; Iino, M. Fuel 1990,69, 317. (8)Seki, H.; Ito, 0.;Iino, M. Fuel, in press.
o a a m m p o / 2504-0352$02.50/0
Table I. Characteristics for Raw and Acetylated Coals ultimate anal., wt %, proximate anal. daf wt %, db H N 0 + S VM ash FC samples C PSOC 1376P raw 78.8 5.0 2.6 13.6 29.3 11.2 59.5 acetylated 81.2 4.9 2.0 11.9 29.6 5.9 64.5 PSOC 702 80.4 5.4 1.4 12.8 38.4 6.5 55.1 raw 4.4 acetylated 77.8 5.3 1.4 15.5 PSOC 815 82.8 5.3 2.7 9.2 30.8 11.9 57.3 raw acetylated 82.4 5.2 1.8 10.6 32.9 8.1 59.0 Shin-yubari 85.1 6.0 2.1 6.8 35.7 5.1 59.2 raw 7.9 36.2 4.1 59.7 acetylated 84.0 6.2 1.9 Zao Zhuang raw 87.5 5.2 1.6 5.7 29.5 7.6 62.9 8.3 30.5 6.6 62.9 acetylated 85.0 5.1 1.6
tylation can be used to elucidate the effect of phenolic hydroxyl groups on the caking properties. The concept of "caking components" is one of the ways to understand caking properties. We have recently reported that caking properties have a dependency on the amount of extractable substances;11J2 the larger the amount of extractable substances coals have, the higher their caking properties are. Therefore, we investigated the changes of coal extractability as well as those of caking properties by acetylation. Experimental Section Materials. Five caking bituminous coals were used in this study. The coals were ground to pass a 150-pm (100mesh) sieve and dried at 90 OC under vacuum overnight. The ultimate and proximate analyses of the raw and acetylated coals are listed in Table I. Acetylation of the Coals. Fifteen grams of coal was refluxed for 24 h with 15 mL of acetic anhydride in 250 mL of purified pyridine under nitrogen. After cooling, the reaction mixture was added to 1L of water and filtered. The residue was washed twice with water under ultrasonic irradiation for 30 min to remove the unreacted acetic anhydride and acetic acid and then three times (9)Blom, L.; Edelhausen, L.; van Krevelen, D. W. Fuel 1957,36,135. (10)Baltisberger, R.J.;Patel, K. M.; Woolsey, N. F.; Stenberg, V. I. Fuel 1982,61, 848. (11)Seki, H.; Ito, 0.; Iino, M. Fuel 1989,68, 837. (12)Seki, H.;Kumagai, J.; Mastuda, M.; Ito, 0.; Iino, M. Fuel 1989, 68,978. 0 1990 American Chemical Society
Energy & Fuels, Vol. 4, No. 4, 1990 353
Caking Properties of Bituminous Coals
c-0 in ester
C-H (2923cm.')
4000
3000
2000
loo0
Wavenumber (cm-')
Fmre 1. FTIR spectra for PSOC 815 (a) raw coal, (b) acetylated coal, and (c) difference spectrum (b - a). with 20% aqueous acetone to remove the remaining pyridine. The acetylated sample was finally dried at 90 "C under vacuum overnight. T o check the effect of refluxing in pyridine on the extraction yield, a blank experiment was carried out without acetic anhydride. Extraction of Coal Samples. The extraction a t room temperature was performed as follows; the coal sample (2 g) was extracted with 50 mL of various organic solvents under ultrasonic irradiation for 45 min. Pyridine (Py), N-methyl-2-pyrrolidinone (NMP), Cs2-Py,13 and CS2-NMP14 mixed solvents were used as extraction solvents. After centrifugation, the supernatant was separated by decantation and fresh solvent was added to the residue, which was extracted again. These procedures were repeated until the supernatant became almost colorless (five to seven times). The residue was then washed with acetone three times under ultrasonic irradiation for 10 min and dried a t 90 "C under vacuum over night. Heat T r e a t m e n t of Samples. Raw and acetylated coals (3 g) were heated in an electric furnace a t the rate of 10 "C/min under nitrogen. When the temperature reached 270,320,or 370 "C, the heat-treated samples were immediately taken out of the furnace and cooled to room temperature. Characterization of Coal Samples. FTIR measurements were carried out on a JEOL JIR-100 FTIR spectrometer with diffuse reflectance to investigate the chemical changes in the coal structure by acetylation and by heat treatment. All spectra obtained were for 1 mg of coal/l50 mg of KBr, and the difference spectra were obtained by direct subtraction. T o obtain the degree of cross-links in the coal matrix, the swelling ratio (Q) of the residues was measured with Py for the heat-treated samples by the method described e1~ewhere.l~ ESR spectra of samples were obtained in situ on a JEOL-FE3X ESR spectrometer equipped with a high-temperature-ESR cavity. Using a standard procedure, the sample (20 mg) was placed in a quartz tube and heated a t the rate of 10 "C/min up to 600 "C. The sample tube was degassed to prevent oxidation by air. The electron spin-lattice relaxation time (TJ, which reflects the coal structure's and mobility,16 was evaluated by the continuouswave-saturation method. Caking Properties of Coal Samples. Caking properties of raw coals and acetylated coals were evaluated from Gieseler maximum fluidity, Roga index, and free swelling index (FSI) according to Japanese Industrial Standard JIS-M8801. The Roga index indicates the power of agglomerating a standard anthracite. The FSI indicates the swelling characteristics of coal.
Results and Discussion Changes of Coal Structure by Acetylation. Figure 1 shows an FTIR difference spectrum of acetylated and raw PSOC 815 coals. It can be seen that the peak for (13)Iino, M.; Matauda, M. Fuel 1983,62, 744. (14)Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988,67, 1639. (15) Ito, 0.;Seki, H.; Iino, M. Bull. Chem. Soc. Jprt. 1987,60,2967. (16)Seki, H.;Ito, 0.; Iino, M. Energy Fuels 1988,2, 321.
Table 11. Caking Properties and Extractabilities for Raw and Acetylated Coals extraction yield, wt% max samples fluidity FSI Roga index Py" CS,-NMPb PSOC 1376P raw 0 5.5 70 15.4 35.0 acetylated c 0.5 15 19.3 26.7 PSOC 702 raw 4 4.0 81 32.0 acetylated c 0.5 16 PSOC 815 raw 2620 8.0 82 16.2 46.8 acetylated c 3.5 64 32.7 Shin-yubari raw 39871 5.5 82 20.4 60.6 acetylated c 6.5 71 34.1 40.8 Zao Zhuang raw 30947 4.5 92 65.6 acetylated c 7.5 74 Pyridine. * N-Methyl-2-pyrrolidinone.e Fluidity of acetylated coal could not be measured because of skidding of stirring shaft of Gieseler plastometer.
hydroxyl groups near 3400 cm-' was significantly decreased by acetylation. Also, the peaks at 1768 and 1201 cm-I attributed to C = O and C-O in ester, respectively, and the peaks at 2923 and 1367 cm-I due to the methyl group were increased by acetylation. The larger decrease of hydroxyl groups was observed for the lower rank coals, compared to that for the higher rank coals. However, the FTIR spectra for the acetylated coals showed that a small portion of hydroxyl groups was still remained unchanged for the lower rank coals (PSOC 1376P, 702, and 815), while the broad bands near 3400 cm-I due to hydroxyl groups were not observed for the higher rank coals (Shin-yubari and Zao Zhuang coals), indicating quantitative acetylation. From Table I, the increase of (0+ S)% can be seen by acetylation except for PSOC 1376P. Although the decrease for PSOC 1376P seems to be related to the large decrease of mineral matter (ash in Table I, from 11.2% to 5.9%), we cannot determine what reactions are responsible for it. The volatile matter and the fixed carbon were changed very little by acetylation by estimating on a daf basis. Effect of Acetylation on Caking Properties. Table I1 shows caking properties of raw and acetylated coals. It can be seen that the Roga index for all samples was decreased by acetylation. Especially, lower rank coals such as PSOC 1376P and 815 showed a considerable decrease. Table I1 also shows that the FSI for three PSOC coals was decreased by acetylation. Acetylation is expected to increase caking properties since it removes phenolic hydroxide, which increases caking properties through the formation of cross-links by their condensation to an ether bond. We will discuss the mechanism for the decrease of caking properties of acetylated coals later. On the other hand, the FSI for Shin-yubari and Zao Zhuang coals was slightly increased. In our previous studies, the FSI for Shin-yubari and Zao Zhuang coals also increased with treatments such as solvent extraction" and oxidation7 which were believed to decrease caking properties. The increase in the FSI is due to the reduction of the fluidity of the coals as indicated by Kreulen;17fluidity of these raw coals is too high to confine volatile matter in a plastic mass, and the decrease in fluidity (increase in viscosity) by acetylation (namely, the approach to the optimum fluidity) results in high swelling. Although we tried to measure the fluidity of acetylated coals to confirm the decrease in the (17) Kreulen, D.J. W. Fuel 1950,29,112.
Seki et al.
354 Energy & Fuels, Vol. 4, No. 4, 1990 Table 111. Effect of Acetylation for Shin-yubari Coal on Extraction Yield extraction yield. wt % samples NMPO Pyb CS2-Py CSZ-NMP 39.2 60.6 18.0 20.4 raw 34.8 40.8 34.1 acetylated 35.5 36.6 44.6 34.2 blank' 37.2
I, N-Methyl-2-pyrrolidinone. Pyridine. pyridine for 24 h.
'Sample refluxed in
fluidity, we could not obtain the data since the stirring shaft in the Gieseler plastometer was skidded before 350 "C was reached; the acetylated coal weakly adhered to the stirring shaft with a contraction and a crevice arose between the coal samples and the crucible. Consequently, the stirring shaft ran idle. Effect of Acetylation on Extractability. In order to interpret the changes of caking properties by acetylation, it is important to examine the extractability of the acetylated coals since extractable substances play an important role in caking properties."J2 Extraction yields for acetylated coals by Py and CS,-NMP are also listed in Table 11. Although the extraction yield with Py was increased by acetylation, those with CS2-NMP, which was a better solvent than Py, were decreased as already reported.18 Furthermore, in order to explain the different behavior of extraction yields between Py and CS2-NMP, we investigated the extractability of Shin-yubari coal, as a representative sample, using various solvents. The results are shown in Table 111; the extraction yield with the Py or NMP solvent was increased by acetylation, while the reverse tendency was observed with the mixed solvents. Since Shin-yubari coal is a bituminous coal that does not have as many hydroxyl groups, a considerable increase in the extraction yield with the Py or NMP solvent cannot be attributable only to acetylation. It can be seen that the extraction yield with Py or NMP solvent was increased only by refluxing in Py (blank experiment). Such an increase is reasonable, since extractable substances that are trapped in the coal matrix (three-dimensional macromolecules) for the raw coal are removed from the coal matrix by refluxing. The findings in Table I11 also show that the extraction yields obtained in the blank experiment are similar to those for acetylated coal, suggesting that acetylation is not effective in enhancing the extractability of the bituminous coal. Details about the effects of refluxing will be reported elsewhere.18 Although the decrease of extractable substances (so-called "caking components") is one of the reasons for the reduction in caking properties, the reduction in the caking properties by acetylation is greater than that by extraction, compared to the previous study." Therefore, the reduction in caking properties is primarily due to the introduction of the acetyl groups. Heat Treatment of Acetylated Coals. The behavior of acetylated coals during heat treatment was investigated to understand the reduction in caking properties. In Figure 2, the variations of T1 for raw and acetylated Shin-yubari coals are plotted against the heat-treatment temperature. Different tendencies can be seen between the raw coal and the acetylated coal; at 250-400 "C, the TIof the raw coal shows an increase, which was attributed to the fragmentation process of the coal segment because T1increases with an increase in mobility of the solid ~ 0 a l . l ~ On the other hand, the Tl for the acetylated coal gradually (18) Takanohashi, T.; Iino, M. Energy Fuels, submitted for publication.
Acetylated coal
\
1 0 0 200 300 400 500 600
Temperature ("C) Figure 2. Variations of spin-lattice relaxation time (TI) for raw and acetylated Shin-yubari coals with heat-treatment temperature.
Table IV. Effect of Heat Treatment on Extractabilities and Swelling Ratio ( 8 )for Raw and Acetylated Coals extraction swelling yield,'" ratiob,' samples temp,OC wt % (Q) acetylated PSOC 1376P 19.3 1.71 (1.53) 270 17.2 1.75 1.81 320 14.0 10.5 1.61 370 acetylated PSOC 815 32.7 1.71 (1.32) 29.5 1.62 270 320 28.7 1.59 24.4 1.50 370 acetylated Shin-yubari 34.1 (20.4) 2.20 (1.70) 270 34.3 (27.2) 1.95 (1.86) 320 26.6 (29.6) 1.85 (1.93) 370 22.4 (30.2) 1.69 (2.00) Swelling ratio of a Pyridine extraction at room temperature. the residue after pyridine extraction using pyridine as a swelling solvent. 'The values in parentheses are for the raw coal.
decreased and did not show any increase at the heattreatment-temperature range of 250-400 "C. This suggests that fragmentation is not likely to occur for the acetylated coal, i.e., cross-linking reactions tend to occur in this temperature range. Table IV shows the effects of heat treatment on the extraction yield and swelling ratio for acetylated coals. Heat treatment was performed in the temperature range of 270-370 "C, since a marked difference was observed in the temperature range from ESR measurement (Figure 2). It is clearly seen that the extraction yield for acetylated coals decreased with the heat-treatment temperature, while an increase in the extraction yield is observable for raw Shin-yubari coal. It is well-known that the maximum extraction yield with solvents such as chloroform or pyridine for caking coals appears near the plastic softening point, but the extraction yield for noncaking coals decreases as the temperature increases.lS2l From this aspect, caking coals become noncaking by acetylation. Although the decrease in extraction yield for the acetylated coal is partly due to the volatilization of extractable substances, it was confirmed from thermogravimetric analysis (19) Dryden, I. G.C.; Pankhurst, K. S. Fuel 1955,34, 363.
(20) (21)
Lazarov, L.; Angelova, G. Fuel 1968,47,342. Chen, P.; Yang, P. W. J.; Griffiths, P. R. Fuel 1985, 64, 307.
Energy & Fuels, Vol. 4, No. 4, 1990 355
Caking Properties of Bituminous Coals !70'C
----l/---r--h/
u
17OoC
Lh
I/
'
'11
aliphatic C-H
I ps0c815 4000
3000
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1000
Wavenumber (cm-')
Figure 3. FTIR difference spectra for aetylated PSOC 815 upon
heat treatment.
(TGA) that the total yield of the extract and volatile matter actually decreased. This observation indicates that part of the extractable substances were polymerized during heat treatment. The changes in swelling ratio (Q) for acetylated coals, which reflects a degree of cross-linking,also decreased with the heat-treatment temperature, while Q for the raw Shin-yubari coal increased. These indicate that cross-links in the coal matrix of the acetylated coals developed during heat treatment, in good agreement with the result obtained from ESR measurement. It could be concluded that the reduction of caking properties is due to the development of cross-links during heat treatment. Figure 3 shows the difference spectra for acetylated PSOC 815 upon heating. The bands at 2923,1768,1367, and 1201 cm-', which are attributed to acetyl groups, decreased with heat-treatment temperature, suggesting that the acetyl groups introduced were decomposed during heat treatment as follows: coal-O-COCH3
heating
coal-0'
+ 'COCH,
The coal- radical formed has a high reactivity and will stabilize by abstracting hydrogen, coupling with other radicals in coal, or adding to the The coupling of the coal-0' radical to a radical in the coal and the addition to the coal would result in the development of cross-links (polymerization). The stabilization of the coal-0' radical by abstracting hydrogen from the coal causes the loss of transferable hydrogen in the coal which plays an important role in caking properties. The acetyl groups, therefore, give more deleterious effects on decomposition during pyrolysis than do the hydroxyl groups. These reactions could take place not only in extractable substances or coal matrix themselves but also between both. Thus, the acetylated extractable substances could not work more efficiently as "caking components" than untreated ones. Concluding Remarks The effect of acetylation for bituminous coals on caking properties and extractability was investigated. The agglomeration ability for all coals used in this study was decreased by acetylation, while the swelling ability showed a different tendency among the coals; the FSI for PSOC coals was decreased by acetylation, while that for Shinyubari and Zao Zhuang coals was increased. This suggests that the fluidity of those coals was decreased by acetylation. Acetylation did not enhance extractability when efficient mixed solvents were used. From the swelling ratio (Q) and the extraction yield measurements with the heat treatment, it was found that the formation of cross-links occurred for the acetylated coals during heat treatment. These chemical changes may be responsible for the reduction of caking properties. Acknowledgment. We thank Dr. Takashi Iwasaki of Tohoku Technical Research Institute and Y utaka Takahashi of Kawasaki Steel Co. Ltd. for their help in the ESR and Gieseler fluidity measurements. We are also grateful to Dr. Yasuo Ohtsuka for his help in the TGA measurement. Registry No. Pyridine, 110-86-1;N-methyl-2-pyrrolidinone, 872-50-4. (22) Patei, S. In The Chemistry of Carboxylic Acids and Esters; Intrerscience Publishers: New York, 1969; p 352.
