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Energy & Fuels 2007, 21, 1038-1041
Prediction of Thermal Swelling Behavior on Rapid Heating Using Basic Analytical Data Koh Kidena* Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Toru Yamashita Idemitsu Kosan Co. Ltd., 3-1 Nakasode, Sodegaura, Chiba 299-0267, Japan
Akemitsu Akimoto Japan Coal Energy Center, 3-14-10 Mita, Minato, Tokyo 108-0073, Japan ReceiVed August 2, 2006. ReVised Manuscript ReceiVed December 4, 2006
Thermal swelling behavior of coal particles during rapid heating was predicted with basic analytical data of the original coal sample. Plastic property dominates thermal swelling of coal particle; therefore, quantitative correlation between elemental analysis and data relating plastic phenomena enabled the prediction of the thermal swelling ratio of coal particles. The data relating plastic phenomena were parameters derived from thermogravimetirc analysis (TGA). By using the TGA data of 55 kinds of coal samples, multiple linear regression analysis was performed to extract several basic analytical parameters for the prediction. As a result, H/C, O/C, and the amount of volatile matter (VM) employed to the prediction. Then, the correlation between thermal swelling ratio during rapid heating and these basic parameters was established. As an additional parameter, the average size of aromatic ring in coal improved the simulation.
Introduction As one of the clean coal technologies with high efficiency, coal gasification has been attracting considerable attention. Recently, several types of coal gasifier are being developed with intensive investigation on the shape of the gasifier. The behavior of coal particles in the furnace is also one of the research topics given attention for the development of a well-organized coal gasification system. In a furnace treating circulated coal particles, the density of each coal particle is notable because it strongly affects the particle trajectories, which determined the reaction behavior of coal particles in the gasifier. Understanding the changes in morphology of the coal particles is useful for modeling the gasification reaction. Morphologic changes of coal particle by heat are believed to originate in the formation of bubbles inside on the basis of the microscopic observation of coal particles during heating.1-13 (1) Shibaoka, M.; Thomas, C. G.; Young, B. C.; Oka, N.; Matsuoka, H.; Tamaru, K.; Murayama, T. Proc. Int. Conf. Coal Sci. 1985, 665-668. (2) Liu, G.; Wu, H.; Gupta, R. P.; Lucas, J. A.; Tate, A. G.; Wall, T. F. Fuel 2000, 79, 627-633. (3) Yu, J.; Strezov, V.; Lucas, J.; Wall, T. Fuel 2003, 82, 1977-1987. (4) Jones, R. B.; Morley, C.; Mccourt, C. B. Proc. Int. Conf. Coal Sci. 1985, 669-672. (5) Zygourakis, K. Energy Fuels 1993, 7, 33-41. (6) Bend, S. L.; Edwards, I. A. S.; Marsh, H. Fuel 1992, 71, 493-501. (7) Lightman, P.; Street, P. J. Fuel 1968, 47, 7-28. (8) Bailey, J. G.; Tate, A.; Diessel, C. F. K.; Wall, T. F. Fuel 1990, 69, 225-239. (9) Liu, G.; Wu, H.; Benfell, K. E.; Lucas, J. A.; Wall, T. F. Proc. 16th Pittsburgh Coal Conf. 1999, 817-826. (10) Wu, H.; Bryant, G.; Benfell, K.; Wall, T. Energy Fuels 2000, 14, 282-290.
A mechanistic view of the thermal swelling of coal particles that is accepted widely can be summarized as follows.1,14,15 When the coal particle is heated, its surface becomes plastic while devolatilization occurs from both inside and outside the particle. At this time, if temperature ranges showing plastic property overlaps those occurring in the formation of volatile matter, the coal particle makes a bubble like a rubber balloon. Especially for the case where the heating rate is very high, both temperature ranges tend to overlap. Therefore, the thermal swelling behavior of coal particles depends on both the coal type and the heating conditions including temperature, pressure, and heating rate. Two of the factors (particle size and heating rate) were investigated previously.5,15-18 The results indicated that smaller particles and higher heating rates tend to increase the possibility of the formation of bubble-like coal particles during heating. As for the effect of the coal type on thermal swelling behavior, the plasticity of coal must be considered. Plastic phenomena are seen only in the most bituminous coals, and (11) Jones, R. B.; McCourt, C. B.; Morley, C.; King, K. Fuel 1985, 64, 1460-1467. (12) Tsai, C.-Y., Scaroni, A. W. Fuel 1987, 66, 200-206. (13) Sun, C. L.; Xiong, Y. Q.; Liu, Q. X.; Zhang, M. Y. Fuel 1997, 76, 639-644. (14) Hamilton, L. H. Fuel 1981, 60, 909-913. (15) Zygourakis, K. Prep. Pap. Am. Chem. Soc. DiV. Fuel Chem. 1988, 33, 951-959. (16) Hanson, S.; Patrick, J. W.; Walker, A. Fuel 2002, 81, 531-537. (17) Griffin, T. P.; Howard, J. B.; Peters, W. A. Energy Fuels 1993, 7, 297-305. (18) Gibbins, J. R.; Jacobs, J.; Man, C. K.; Pendlebury, K. J. Prep. Pap. Am. Chem. Soc. DiV. Fuel Chem. 1992, 37, 1930-1936.
10.1021/ef060355p CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
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Table 1. Various Analytical Data of Coal Samples id C %
H/C
O/C
VM
Rmax
Rmax/WL
χb
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
0.839 0.832 0.832 0.859 0.868 0.758 0.859 0.884 0.948 0.869 0.920 0.867 0.926 0.738 0.882 0.872 0.896 0.710 0.738 0.802 0.714 0.743 0.814 0.746 0.679 0.653 0.783 0.783 0.683 0.750 0.677 0.774 0.708 0.765 0.778 0.803 0.664 0.836 0.803 0.769 0.680 0.712 0.706 0.686 0.729 0.669 0.693 0.625 0.698 0.784 0.701 0.691 0.615 0.690 0.666
0.213 0.200 0.200 0.195 0.158 0.161 0.153 0.134 0.131 0.134 0.117 0.109 0.086 0.106 0.096 0.103 0.098 0.107 0.102 0.091 0.094 0.098 0.090 0.091 0.099 0.091 0.087 0.087 0.093 0.084 0.086 0.081 0.084 0.080 0.071 0.069 0.076 0.062 0.063 0.059 0.061 0.049 0.049 0.041 0.036 0.046 0.037 0.045 0.038 0.027 0.036 0.026 0.035 0.030 0.014
46.5 46.0 46.0 45.5 41.5 50.0 34.1 44.5 45.7 38.9 43.6 43.0 37.9 36.8 39.7 43.4 38.9 29.6 32.3 34.7 32.0 31.6 36.3 31.9 29.7 32.9 32.2 32.2 29.3 34.1 28.3 33.7 30.3 32.3 29.4 34.0 24.7 34.2 34.4 34.3 27.0 22.9 25.0 26.5 26.4 23.6 23.4 23.5 18.6 33.6 20.4 20.6 16.3 18.2 19.1
0.626 0.641 0.559 0.718 0.862 0.693 0.452 0.844 1.101 0.635 1.241 0.997 1.177 0.735 0.892 0.871 1.228 0.453 0.789 0.846 0.758 0.760 0.988 0.613 0.503 0.754 0.891 0.644 0.485 0.817 0.586 0.885 0.764 1.060 0.710 0.888 0.349 1.036 0.909 0.990 0.643 0.547 0.590 0.580 1.074 0.522 0.627 0.485 0.330 0.889 0.425 0.459 0.315 0.402 0.330
1.311 1.333 1.257 1.519 1.945 1.483 1.451 2.082 2.142 1.678 2.502 2.125 2.587 2.166 2.382 2.040 2.723 1.549 2.076 2.470 2.125 2.031 2.558 1.969 1.583 2.180 2.381 2.100 1.599 2.131 1.809 2.548 2.180 2.856 2.274 2.523 1.513 2.612 2.510 2.762 2.025 2.134 2.105 2.101 2.698 2.076 2.383 1.859 1.796 2.497 1.906 2.030 1.594 2.067 1.630
nd nd 0.36 nd nd nd 0.34 0.38 nd 0.30 nd nd 0.23 nd nd nd nd 0.34 nd 0.30 nd nd nd nd nd nd nd 0.43 nd nd nd 0.48 nd nd 0.31 nd 0.38 nd nd nd nd nd nd nd nd nd nd nd 0.45 nd nd nd nd nd nd
73.1 73.3 73.3 73.5 76.3 77.2 77.4 77.8 78.7 78.8 79.4 80.5 80.7 81.0 81.1 81.2 81.2 81.4 81.7 81.9 82.1 82.1 82.3 82.3 82.7 82.7 82.7 82.7 82.9 83.4 83.4 83.4 83.7 83.8 84.0 84.1 84.4 84.7 84.9 85.7 85.9 86.6 86.7 87.6 88.1 88.1 88.1 88.3 88.3 88.4 88.9 89.1 89.6 90.0 90.6
Csw(800) Csw(1500) nd nd 0.90 nd nd nd 1.11 1.27 nd 1.15 nd nd 1.80 nd nd nd nd 1.00 nd 2.21 nd nd nd nd nd nd nd 1.64 nd nd nd 1.45 nd nd 1.61 nd 1.17 nd nd nd nd nd nd nd nd nd nd nd 1.34 nd nd nd nd nd nd
nd nd 0.86 nd nd nd 1.06 1.42 nd 1.15 nd nd 1.80 nd nd nd nd 0.92 nd 1.51 nd nd nd nd nd nd nd 1.45 nd nd nd 1.14 nd nd 1.52 nd 1.07 nd nd nd nd nd nd nd nd nd nd nd 1.23 nd nd nd nd nd nd
the magnitude of the plasticity varies with the coal type. However, the heating rate during gasification is apparently higher than those treated in the discussion of coal plasticity. Under gasification conditions, a wider range (including subbituminous coals) of coals show plasticity. It may be due to the higher heating rate. General indices of plasticity are only for coal having the plastic property. In order to obtain important elements for describing coal plasticity, we investigated the key factors for determining the plastic property of coal using several kinds of coal samples.19 The resulting factors are applicable for every sample to discuss plasticity. On the basis of the the suggested results, this paper treats the prediction of swelling behavior using basic analytical data by considering the plastic property of coal. Experimental Section Samples. Twelve kinds of coals with different plastic properties were selected from Japanese coal banks (standard coals listed by the BRAIN-C project) as sample coals. These coals are mined from (19) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1998, 12, 782787.
various countries (Japan, China, Indonesia, Australia, South Africa, United States, and Canada). For the basic examination concerning the correlation between plastic property and basic analytical parameters, 55 kinds of coal samples including 18 coking coals (coal with some plasticity) were used in discussion (Table 1). Thermogravimetric Analysis (TGA). Pyrolytic behavior of coal samples was examined with a thermobalance (TGA-50H, Shimadzu Co. Ltd.). The powdered coal sample (under 100 mesh, 10 mg) was put into a platinum cell and being loaded into the electric furnace of the thermobalance. Inside the electric furnace was purged with nitrogen; then the coal sample was heated up to 1273 K at the rate of 3 K/min under 50 mL/min of nitrogen stream. Weight decrease was observed around 600 K as the devolatilization. Observation of Thermal Swelling. A drop tube furnace was applied for the observation of thermal swelling behavior of the coal particle. The electric furnace had an effective length of 800 mm. A tube (42 mm i.d.) was equipped at the center of the furnace, where the coal particle was going down. Size-regulated (45-75 µm) coal particles were supplied into the tube from a particlefeeding machine with the flow of nitrogen. The outlet of the particles was cooled with water. The feeding rate was 10 g/h, and the accompanying gas was nitrogen. The temperature of each particle was not measured, but it was simulated by analysis of the heat history to confirm that the temperature reached its target value. Heating rate was calculated as 1140-2600 K/min. In this experiment, each coal particle was dropped as an isolated one, not to be coalesced. The treated set of coal particles was analyzed by laser diffraction to determine particle size distribution. The mean diameter of particles was used for the calculation of thermal swelling ratio (Csw): volume increasing before and after heat treatment. Two points of temperatures, 800 and 1500 °C, were selected as representative heat treatment temperatures that the coal particle experiences in the gasifier. Solid-State NMR Analysis. 13C NMR spectra of the coal sample were recorded on a Chemagnetics CMX-300 spectrometer with a 13C resonance frequency of 75.55 MHz. The measurement for the quantitative analysis of carbon nuclei was performed using a singlepulse excitation (SPE) method with 83 kHz of proton decoupling below 10.5 kHz of magic angle spinning (MAS). Some measurements were performed with the dipolar dephasing method. The sample was put into a vessel with 5 mm diameter. The weight of the loaded sample was 80-120 mg. The pulse width was 1.5 µs as 45 degree pulse. The number of data accumulation was 3000. The pulse delay time for each accumulation was 20 s or more, depending on the spin-lattice relaxation time of carbon nuclei (TC1 ). Recorded data were electronically treated with a software (GRAMS/ 32 on personal computer) to obtain the structural parameter of coal.
Results and Discussion Parameter Derived from TGA. Pyrolytic behavior of coal was observed with TGA. When the coal sample was heated under inert atmosphere, the weight of coal decreased monotonously due to devolatilization and pyrolysis. In many cases, monotonous weight decrease derives a single peak for rate of weight decrease. Thus, a maximum rate of volatilization (Rmax) and a temperature at the maximum rate of volatilization (Tmax) were determined as characteristic parameters for coal pyrolysis. Lower ranked coal samples tend to have lower Tmax than higher ranked coals, while the magnitude of Rmax depends on the coal type. In our previous research on the appearance of coal plasticity,19 a good correlation between a value showing plastic property and an analytical parameter derived from TGA (Rmax/ WL) was indicated. The parameter Rmax/WL corresponds to the value that Rmax was normalized by the total weight loss (WL) up to 1273 K. If Rmax depended simply on the total amount of volatile matter, Rmax/WL would be constant. However, experimental results indicated that larger Rmax/WL was shown in more plastic coal. A scientific meaning of Rmax/WL is an index of
1040 Energy & Fuels, Vol. 21, No. 2, 2007
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Figure 3. Simulated results of Csw at (a) 800 °C and (b) 1500 °C with H/C, O/C, and VM. Table 2. Simulated Results of Thermal Swelling Ratio (Csw) at 800 and 1500 °C
Figure 1. Plots of TGA parameter (Rmax/WL) against carbon content for 55 kinds of coal.
Csw(800) ) const. + a H/C + b O/C + c VM R2 ) 0.5936 const. a b c
0.394 1.296 -10.01 0.0300
Csw(1500) ) const. + a H/C + b O/C + c VM R2 ) 0.8472 const. a b c
Figure 2. Correlation between TGA parameter (Rmax/WL) and thermal swelling ratio (Csw) at (a) 800 °C and (b) 1500 °C.
volatility at Tmax; concretely, the larger the Rmax/WL is, the higher the volatility at Tmax is. Therefore, concentrated formation of volatile matter is favorable to the appearance of coal plasticity. This concept also fits well a tendency for the formation of cenospheric structure of coal particle during heating: by considering the mechanism of the formation of cenosphere, this concept is understandable. Thus, the relationship between the above parameter (Rmax/WL) was investigated using many coal samples to apply the relationship for establishing a simulation formula of thermal swelling behavior. Plots of Rmax/ WL against carbon content in coal for 55 kinds of coal samples are shown in Figure 1. They tend to have a maximum at the middle ranked coals within each coal samples tested. Considering the fact that the middle ranked coals show higher plasticity, the parameter Rmax/WL is possible to be correlated with a degree of coal plasticity. A tendency for the formation of cenospheric particle, which deeply relates to plastic phenomena, could be expected from volatilization behavior like those tested here. Description of TGA Parameter with General Analytical Parameters. Before simulating thermal swelling behavior from the general analytical parameter of coal, the correlation between thermal swelling ratio and TGA parameter (Rmax/WL as mentioned above) was examined. By using the correlation between TGA parameter and plastic property of coal, simulation of the TGA parameter from general analytical parameters was examined since the TGA test was able to be performed with many coals in an easy way and in a short time. For the coal samples whose thermal swelling behavior was observed, a correlation between Csw and the TGA parameter was determined. Figure 2 indicates the results. The thermal swelling data were obtained at both 800 °C and 1500 °C as the target temperature
-0.406 2.443 -7.430 0.0147
of the coal particle. Both plots showed very good correlation. Therefore, if one collects Rmax/WL data for a coal sample, its Csw could be estimated with the measured values. However, a better way is the use of general analytical data of the coal samples, such as elemental composition and volatile matter/ fixed carbon content. Thus, multiple linear regression analysis of the Rmax/WL was conducted with the general analytical parameters such as C %, H %, N %, S %, O %, H/C, O/C, VM, FC, and ash. Here, since H/C and O/C are strongly correlated with H %, C %, and O %, these data were not contained simultaneously in the multiple linear regression analysis. The analysis was run on a software named TinyQ.20 With this software, unrelated parameters can be excluded in the simulation of the correlation between Rmax/WL and the parameters. As a result, H/C, O/C, and VM were very effective to simulate Rmax/WL values. Then, these three parameters were applied to the multiple linear regression analysis of Csw. The analytical results of Csw at both 800 and 1500 °C with TinyQ software are summarized in Table 2 and in Figure 3. Csw values were well simulated with H/C, O/C, and VM, all of which are quite general analytical parameters of coal samples. In the comparison of two sets of Csw values, those at 1500 °C were fitted better: concerning the correlation factor R, R2 ) 0.85. In order to achieve more precise simulation of the Csw value, another analytical parameter was tested to be introduced into the multiple linear regression analysis. On the basis of mechanism for the formation of balloon-like particles, properties of the shell of the coal particle should be closely correlated. The shell of the particle consists of alignments of polyaromatic sheets. Therefore, the size of aromatic ring would affect a property of the wall of the particle. In this study, an average size of unit aromatic nuclei was taken as an additional parameter (20) http://homepage.mac.com/eigom/TinyQ/TQ.html.
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Energy & Fuels, Vol. 21, No. 2, 2007 1041
Table 3. Simulated Results of Thermal Swelling Ratio (Csw) at 800 and 1500 °C with the Addition of an NMR Parameter Csw(800) ) const. + a H/C + b O/C + c VM + d χb R2 ) 0.6282 const. a b c d
1.358 0.473 -10.28 0.0343 -1.186
Csw(1500) ) const. + a H/C + b O/C + c VM + d χb R2 ) 0.8763 const. a b c d
0.253 1.881 -7.619 0.0178 -0.812
for the simulation. It was evaluated by NMR data. The averagesize of unit aromatic nuclei is correlating with a ratio of bridgehead aromatic carbons to total aromatic carbons,21 which was defined as χb by Solum et al.21 In the present study, χb was applied as an additional parameter for the simulation of thermal swelling ratio. The simulated results are shown in Table 3 and in Figure 4. The difference was small, but the thermal swelling ratio was fitted much better with four parameters: R2 ) 0.63 and 0.88 for Csw(800) and Csw(1500), respectively.
Figure 4. Simulated results of Csw at (a) 800 °C and (b) 1500 °C with H/C, O/C, VM, and χb.
correlation between a parameter from thermogravimetry and the thermal swelling ratio, several general analytical parameters were extracted. H/C, O/C, and the amount of volatile matter were the candidates of the simulation of thermal swelling ratio. A better result was obtained by adding a parameter concerning aromatic ring size. Acknowledgment. This research was performed as a part of the BRAIN-C project organized by the Center for Coal Utilization, Japan (now, JCOAL) and NEDO. The author greatly acknowledge the financial support as well as other support. EF060355P
Conclusion Thermal swelling ratio during rapid heating of coal particles was simulated with basic analytical parameters. By using a
(21) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193.
