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High-Pressure Coal Pyrolysis in a Drop Tube Furnace Koichi Matsuoka, Zhi-xin Ma, Hiroyuki Akiho, Zhan-guo Zhang, and Akira Tomita* Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai, 980-8577 Japan
Thomas H. Fletcher Chemical Engineering Department, Advanced Combustion Engineering Research Center, Brigham Young University, Provo, Utah 84602
Marek A. Wo´jtowicz Advanced Fuel Research, Inc., 87 Church Street, East Hartford, Connecticut 06108-3728
Stephen Niksa Niksa Energy Associates, 1745 Terrace Drive, Belmont, California 94002 Received December 24, 2002
To obtain useful and reliable pyrolysis data under high pressures, continuous pyrolysis experiments were carried out using a drop tube furnace. To ascertain the reliability of data, a mass balance during pyrolysis was carefully checked. The pyrolysis data obtained in this equipment was compared with previous results, and it was found that the weight loss observed in the drop tube furnace was somewhat larger than those obtained with other apparatuses. The experimental results were compared with those predicted by three pyrolysis models. Although discrepancies between predictions and experimental results were observed under a certain condition, the agreement between the experimental weight losses with three predictions was fairly good at 800 °C and 1 MPa with all the models. Product distribution was also reasonably predicted with the Flashchain model. It is desirable to further improve the models by comparing their predictions with more extensive and reliable experimental data.
Introduction In most coal utilization processes, pyrolysis reaction takes place prior to the main reaction. Therefore, the understanding of pyrolysis behavior is important to develop new coal utilization technologies. Many pyrolysis studies have been carried out under atmospheric pressure.1-8 However, a recent trend in commercial process development is the operation under high pressures. Many pyrolysis studies under high pressures thus have been carried out extensively.9-22 In many of these * Corresponding author. Tel: +81-22-217-5625. Fax: +81-22-2175626. E-mail:
[email protected]. (1) Kobayashi, H.; Howard, J. B.; Sarofim, A. F. Proc. Combust. Inst. 1977, 16, 411-425. (2) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process. Des. Dev. 1978, 17, 37-46. (3) Scaroni, A. W.; Walker, P. L., Jr.; Essenhigh, R. H. Fuel 1981, 60, 71-76. (4) Doolan, K. R.; Mackie, J. C.; Tyler, R. J. Fuel 1987, 66, 572578. (5) Fletcher, T. H. Combust. Flame 1989, 78, 223-236. (6) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Bassilakis, R.; Gravel, D.; Baillargeon, M.; Baudais, F.; Vail, G. Energy Fuels 1990, 4, 319-333. (7) Niksa, S.; Chen, J. C. Energy Fuels 1992, 6, 254-264. (8) Mae, K.; Inoue, S.; Miura, K. Energy Fuels 1996, 10, 364-370.
studies, the yields of all pyrolysis products were not reported. Furthermore, the results are not consistent with each other even if the pyrolysis conditions, such as sample coal, gas atmosphere, temperature, pressure and others, were kept constant.23-27 This is mainly because different experimental equipment was em(9) Anthony, D. B.; Howard, J. B.; Hottel, H. C.; Meissner, H. P. Fuel 1976, 55, 121-128. (10) Anthony, D. B.; Howard, J. B. AIChE J. 1976, 22, 625-656. (11) Howard, J. B. In Chemistry of Coal Utilization, 2nd Suppl. Vol.; Elliott, M. A., Ed.; John Wiley & Sons: New York, 1981; pp 665-784. (12) Tatterson, D. F.; Robinson, K. K.; Marker, T. L.; Guercio, R. Ind. Eng. Chem. Res. 1988, 27, 1606-1613. (13) Gibbns, J.; Kandiyoti, R. Energy Fuels 1989, 3, 670-677. (14) Karcz, A.; Poranda, S. Fuel 1995, 74, 806-809. (15) Ma, Z.; Zhu, Z.; Zhang, C.; Jin, H. Fuel Process. Technol. 1994, 38, 99-109. (16) Arendt, P.; van Heek, K. Fuel 1981, 60, 779-787. (17) Griffin, T. P.; Howard, J. B.; Peters, W. A. Fuel 1994, 73, 591601. (18) Soneda, Y.; Makino, M.; Xu, W.-C. J. Jpn. Inst. Energy 1998, 77, 906-908. (19) Xu, W.-C.; Soneda, Y.; Makino, M. Proc. 35th Conf. Coal Sci. (Japan) 1998, 295-298. (20) Cor, J.; Manton, N.; Mul, G.; Eckstrom, D.; Olson, W.; Malhotra, R.; Niksa, S. Energy Fuels 2000, 14, 692-700. (21) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Fuel 1980, 59, 405-412. (22) Strugnell, B.; Patrick, J. W. Fuel 1995, 74, 481-486.
10.1021/ef020298+ CCC: $25.00 © 2003 American Chemical Society Published on Web 06/24/2003
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Figure 1. Schematic diagram of a drop tube furnace reactor.
ployed in respective studies and their characteristics are quite different from each other. To obtain data that is useful for the simulation of pyrolysis, gasification, and combustion processes, reliable data should be accumulated under conditions that closely simulate the reactions in a large-scale reactor. Although it is quite difficult to simulate actual conditions by laboratory-scale equipment, a drop tube furnace would be one type of equipment suitable for this purpose.23 In the present study, we carried out continuous pyrolysis experiments under high pressures using a drop tube furnace. The purpose of the present study is multifold. The first objective is to ascertain the reliability of data from various viewpoints. Particularly, we want to get a good mass balance during pyrolysis. Second, to check the difference between pyrolysis data obtained in previous studies, the effect of reactor type was discussed. For this purpose we used Taiheiyo coal, with which many pyrolysis data have already been accumulated using a thermogravimetric balance (TGA),28a Curie-point pyrolyzer,24 a radiant coal flow reactor,25 and others.26,27 Finally, the present experimental results were compared with those predicted by several models.29-34 Since few reliable high-pressure pyrolysis data are available so far, it is important to further improve the models by comparison with more extensive experimental data. (23) Xu, W.-C.; Matsuoka, K.; Akiho, H.; Kumagai, M.; Tomita, A. Fuel 2003, 82, 677-685. (24) Kamo, T.; Furuya, T.; Yamamoto, Y.; Miki, K.; Sato, Y. Proc. 32nd Conf. Coal Sci. (Japan) 1995, 212-215. (25) Niksa, S. Report of BRAIN-C program NEDO-C-9739, 1998. (26) Miura, K.; Morozumi, F. Proc. 6th Japan-China Symp. Coal C1 Chem., Zao 1998, 41-44. (27) Katalambula, H.; Takeda, S. Energy Fuels 2002, 16, 428-435. (28) Wo´jtowicz, M. A.; Bassilakis. S.; Charpenay, S.; Serio, M. A. Proc. 6th Japan-China Symp. Coal C1 Chemistry, Zao 1998, 244-247. (29) Niksa, S.; Kerstein, A. R. Energy Fuels 1991, 5, 647-665. (30) Niksa, S. Energy Fuels 1991, 5, 665-673. (31) Serio, M. A.; Hanblem, D. G.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987, 1, 138-152. (32) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Desphande, G. V. Energy Fuels 1988, 2, 405-422. (33) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1990, 4, 54-60. (34) Grant, D. M.; Pugmire, R. J.; Fletcher, T. H.; Kerstein, A. R. Energy Fuels 1989, 3, 175-186.
Experimental Section Apparatus and Procedure. The apparatus used throughout this study was a drop tube furnace (DTF), and its schematic diagram is shown in Figure 1. The pyrolyzer tube is made of Incoloy 800, and the size was 7.6 mm i.d. and 1830 mm long. The heating zone consists of three sections, the temperature of which was separately controlled by three furnaces to ensure an isothermal region being more than 1000 mm. The pyrolysis temperature was set at 600, 700, 800, and 850 °C. Temperature inside the reactor was calibrated in the following manner. After the temperature reached the predetermined temperature, a thermocouple inserted into the center of reactor tube was moved from the top to the bottom of the reactor, and thereby the temperature profile was determined under He flowing condition at atmospheric pressure. The particle temperature history was estimated using this temperature profile according to the method described by Solomon et al.35 The method is based on the calculation of heat balance between particle, gas, and reactor wall. Coal samples with a size between 75 and 150 µm was used for pyrolysis after being dried in vacuo at 100 °C for 3 h. Coal powders were fed through a screw feeder at a rate of 0.1 g/min with He as a carrier gas. The gas flow rate was 3.5 L(STP)/ min. The pressure was either 0.3, 1.0, or 3.0 MPa. All the char was collected in a char pot, and the tarry materials were collected in a tar trap installed in a high-pressure section. After each experiment, the tar trap was washed with tetrahydrofuran and then the tar was recovered by evaporating the solvent. The weight loss (wt % on the daf basis) was determined by three independent manners to check the reliability of data.
WL1 ) 100 × (Wcoal - Wchar)/Wcoal WL2 ) Ygas + Ytar + Yliquid + Ycoke WL3 ) 100 × (1 - Acoal/Achar)/(1 - Acoal) Wcoal and Wchar are the weight of coal and char, Ygas, Ytar, Yliquid, and Ycoke are the yields of gas, tar, liquid, and coke (wt % on a daf basis), and Acoal and Achar are the weight fraction of ash in coal and char, respectively. Gas, liquid, coke, and char are defined in the present study as follows. Gas is defined as (35) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Markham, J. R. Fuel 1986, 65, 182-194.
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Table 1. Proximate and Ultimate Analysis of Coals proximate analysis (wt %, dry)
ultimate analysis (wt %, daf)
coal
volatile matter
fixed carbon
ash
C
H
N
S+O (diff.)
Taiheiyo Adaro Berau
46.9 45.4 46.6
42.5 53.0 50.5
10.6 1.6 2.9
76.1 70.6 68.4
6.4 4.8 4.7
1.4 1.2 1.6
16.1 23.4 25.3
hydrocarbons from C1 to C4, CO, CO2, H2, and H2O. Note that NH3, HCN, and H2S are not determined in this study. Liquid consists of BTX (benzene, toluene, and xylene), PCXN (phenol, cresol, xylenol, and naphthalenes), and aliphatic hydrocarbons with carbon numbers of 5 and 6. Tar is the component accumulated in the tar trap, and char is the carbonaceous residue collected in the char pot. Coked deposit is that deposited on the reactor wall, and a part of it was weighed after being scratched off the wall and the rest remaining on the wall was quantified by burning with air accompanied by COX analysis. Two types of gas chromatography were used to analyze gas and liquid products. A Microsensor gas analyzer (model 200; Aera) has two columns inside. One was packed with MS-5A for analyzing CH4, CO, and H2; another was packed with Poraplot Q for C2H2 + C2H4, C2H6, C3H6, C3H8, CO2, and H2O analysis. Gas chromatography (G3800; Yanaco) was equipped with Porapak P columns and a flame ionization detector, and it was used for gases heavier than C3 and hydrocarbon liquids up to methyl naphthalene. It should be noted that the water analysis by GC was not reliable enough. The gas yield on a daf basis is thus somewhat uncertain. Therefore, in some cases, the product distribution data will be presented on a carbon weight basis. Selection of Coal Samples for Pyrolysis. In the initial stage of this project, we frequently encountered plugging troubles because of the small diameter of the reactor. To find suitable coals for this experiment, we carried out a sort of “combinatorial pyrolysis” using small amounts of coal samples. About 120 mg samples of 12 coals were set in 12 tiny quartz bottles, and all of them were placed in the isothermal region of the reactor. These coals were heated to 800 °C under He atmosphere, either at 0.1 or 1.0 MPa. Unfortunately, rapid heating was impossible in this system, and the heating rate was set at 25 °C/min. After holding at 800 °C for 10 min, they were allowed to cool and then the appearance of char was checked. The effect of coal type on the caking behavior was much larger than the effect of pressure. Six coals, Blair Athol, Datong, Taiheiyo, Pasir, Adaro, and Berau coals showed noncaking behavior, and they were tested in the DTF. However, a stable operation was achieved only with Taiheiyo, Adaro, and Berau coals. The ultimate and proximate analyses of these three coals are presented in Table 1.
Results Pyrolysis Products Yields. A stable operation is a key issue for the present experiment. A stable operation is a key issue for the present experiment. Figure 2 shows one example of outlet gas flow rate and the yields of selected products during a course of experiment. The production rates of other products are similarly stable. Although there are some fluctuations, the result between 30 and 150 min can be used to calculate the average product yield under this condition. After confirming the stability, we determined the average product yields for each experiment. To check the repeatability, experiments under the same conditions were repeated several times in some cases, and the average yields are presented in this paper. The total weight loss data on
Figure 2. An example of the variation of outlet gas flow rate and the yields of CH4, CO, benzene, and naphthalene. Table 2. Weight Loss in the Pyrolysis of Taiheiyo Coal Determined by Three Independent Methods weight loss (wt %, daf)
temperature (°C)
He pressure (MPa)
WL1
WL2
WL3
600 700 800 850 800 800 800
1.0 1.0 1.0 1.0 0.3 1.0 3.0
42 51 55 56 57 55 50
44 48 53 55 53 53 51
39 51 54 55 57 54 47
the pyrolysis of Taiheiyo coal are presented in Table 2. As described in the Experimental Section, weight loss values were independently evaluated by three methods. The agreement is satisfactory and the difference between two weight losses was less than 3% except for some cases. There are some problems in evaluating WL2: (1) the recovery of deposited coke and tarry materials was not always complete; (2) there may be some unanalyzed products, such as NH3 and HCN; and (3) the accuracy of H2O quantification was rather poor. Since the agreement between WL1 and WL3 is satisfactory, we mainly use WL1 values in the discussion below. Gas, Liquid, and Char Yields. The pyrolysis products are roughly divided into five groups as defined in the Experimental Section. The yields of these groups are presented in Table 3. The closure ranged between 97% and 102%. The total yield in the last column should be in principle 100%, but because of experimental errors it is a little larger or smaller than 100%. The accuracy of the total yield here is equal to the absolute value of (100 - WL1 + WL2). In other words, the deviation from 100% in Table 3 is equal to the difference between WL1 and WL2 in Table 2. The extent of agreement gives some idea about the reliability of the present data.
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Table 3. Material Balance in High-Pressure Pyrolysis yield (wt %, on daf basis) coal
temperature (°C)
He pressure (MPa)
char
tar
gas
liquid
deposited carbon
total
Taiheiyo Taiheiyo Taiheiyo Taiheiyo Taiheiyo Taiheiyo Taiheiyo Taiheiyo Adaro Berau
600 700 800 850 800 800 800 800 800 800
1.0 1.0 1.0 1.0 0.3 1.0 3.0 1.0 1.0 1.0
58 49 45 44 43 45 50 45 48 47
22 17 13 10 16 13 9 13 10 7
17 27 33 37 32 33 34 33 36 39
5 4 5 2 4 5 5 5 3 2
0 0 2 6 1 2 3 2 4 7
102 97 98 99 96 98 101 98 101 102
Table 4. Analysis of Tar Obtained in the Pyrolysis under He of 1.0 MPa ultimate analysis (wt %, daf) coal
temperature (°C)
C
H
N
S+O (diff.)
Taiheiyo Taiheiyo Taiheiyo Adaro Berau
700 800 850 800 800
80.6 82.5 86.0 88.3 86.2
5.9 5.0 5.1 5.5 5.8
1.1 1.3 1.0 1.3 1.6
12.4 11.2 8.9 4.9 6.4
Table 3 indicates that the gas yield monotonically increases with temperature, while the yields of tar and char decrease with increasing temperature. The liquid yield does not change significantly until 800 °C, and decreases when the temperature increases to 850 °C. It is logical that the coke yield increased with temperature. The ratio of experimental volatile matter yield to volatile matter in proximate analysis (TVM/PVM) has been known to be dependent on pyrolysis temperature. The ratios of TVM/PVM in the present experiments are 0.8, 1.0, 1.1, and 1.1 at 600, 700, 800, and 850 °C, respectively, and these are in good agreement with Xu’s values.36 The results on the pressure effect are listed in the middle part of Table 3. With the increase of pressure, char yield increased, and on the contrary, tar yield decreased. The changes of gas and liquid yields did not change significantly with changing pressure. The reason for the high char yield is that the vapor pressure of the tar and liquids is less than the experimental pressure, and hence they remain in the solid.16,17,21 The longer gas residence time at higher pressure is another reason for higher yield of deposited carbon. The product yields from three coals are shown in the bottom part of Table 3. Although the char yield or total weight loss is similar, there are some differences in the yields of tar, gas, liquid, and deposited carbon. With decreasing coal rank, tar and liquid yield decreased, whereas gas and coke yield increased. This trend is exactly the same as observed under atmospheric pressure.37 Elemental Analysis of Pyrolysis Products. The analysis of the tar obtained in this study is shown in Table 4. Increase of pyrolysis temperature led to the increase of carbon content in tar accompanied by the decrease of oxygen content. The carbon contents in tars from other coals are somewhat higher than from Taiheiyo coal. (36) Xu, W.-C.; Tomita, A. Fuel 1987, 66, 632-636. (37) Xu, W.-C.; Tomita, A. Fuel 1987, 66, 627-631.
Figure 3. Effect of temperature on the product distribution in the pyrolysis of Taiheiyo coal.
Table 5 shows the analysis of char. With increasing pyrolysis temperature, volatile matter in char naturally decreased. In parallel, the carbon content increased and hydrogen content decreased. This corresponds to the decrease of the H/C atomic ratio from 0.47 (600 °C) to 0.25 (800 °C). This suggests the development of carbon structure and an increase in aromaticity with severity of pyrolysis. There are some differences in remaining volatile matter in the three chars even though the pyrolysis was performed under the same conditions. On the contrary, the effect of coal type on the ultimate analysis is not remarkable. Yields of Gas and Light Hydrocarbon Liquid. Not only the amount of total volatile matter, but also the yield of each product is of interest, since only a few systematic studies on the product selectivity during high-pressure pyrolysis have been reported.17,20 In this section, the yield of each product is presented on a carbon weight basis. The data are calculated using the analytical data of tar and char shown in Tables 4 and 5. The reason we calculated on a carbon basis is that these are more reliable, because we are free from the uncertainty derived from the uncertain water analysis. The results are presented in Figures 3-5 in a graphical form.
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Table 5. Analysis of Char Obtained in the Pyrolysis under He of 1.0 MPa proximate analysis (wt %, dry)
ultimate analysis (wt %, daf)
coal
temperature (°C)
volatile matter
fixed carbon
ash
C
H
N
S+O (diff.)
Taiheiyo Taiheiyo Taiheiyo Taiheiyo Adaro Berau
600 700 800 850 800 800
26 19 8 5 16 10
58 60 69 72 81 84
16 21 23 23 3 6
82.5 83.2 85.7 91.7 89.1 85.6
3.2 2.8 2.2 1.9 2.2 2.1
1.6 1.9 1.9 1.6 1.6 2.0
12.7 12.1 10.2 4.8 7.1 10.3
Table 6. Comparison of Weight Loss Obtained with Different Reactorsa reactor
sampleb
gas
residence time (s)
temperature (°C)
weight loss (wt %, daf)
reference
Curie point pyrolyzer batch free fall reactor radiant coal flow reactor continuous free fall reactor
5 mg 25 mg 0.5 mg/min 100 mg/min
He He Ar He
∼5 10 0.25 ∼3
800c 800 800c 800
48 53 49 55
[24] [26] [25] this work
a Pressure: ∼1.0 MPa; heating rate: 1000-10000 K/s. b Sample weight or feeding rate; particle size: mostly
