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Catalytic Hydrogenation of HyperCoal (Ashless Coal) and Reusability of Catalyst Koji Koyano, Toshimasa Takanohashi,* and Ikuo Saito Energy Technology Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan ReceiVed February 16, 2009. ReVised Manuscript ReceiVed May 11, 2009
HyperCoal (HPC) is ashless coal obtained by a mild thermal extraction of coal to remove unextractable, heavy compounds, and minerals. The temperature and duration of HPC hydrogenation was systematically varied with and without solvent in an autoclave under hydrogen pressure. Unlike raw coal, hydrogenation of HPC in the absence of solvent proceeded without coke formation when the reaction was performed for 60 min at 450 °C in 10 MPa hydrogen (initial pressure). The hydrogenation catalyst was recycled five times with no detection of deactivation. Longer reactions at slightly higher temperatures (120 min at 460 °C), with replenishing the hydrogen, afforded a 90 wt % oil (hexane-soluble fraction) yield.
Introduction In recent years, the direct liquefaction of coal has attracted much attention against a backdrop of global increases in crude oil costs. Relative to crude oil, large reserves of coal are more evenly distributed around the globe and may provide a stable supply at a lower price than other fossil fuels. In Japan, the NEDOL process has been shown to convert coal to oil with a 58 wt % yield over 3000 h.1 Pulverized pyrite, which has a lower catalytic activity than molybdenum compounds but is naturally occurring and inexpensive, was employed as the catalyst.2 Although demonstrating the principles of coal liquefaction, the disadvantages of the NEDOL process rendered it impractical for wide-scale implementation. The catalyst can be deactivated through the deposition of coke and ash onto the catalyst surface, and the presence of minerals in the reaction mixture can cause accumulation of solid particles and eventual deterioration of equipment such as liquefaction reactors, separators, and pipelines.3,4 Furthermore, relatively high temperatures and hydrogen pressures were required, leading to a high cost for coal liquefaction. To avoid catalyst deactivation, the coal may be deashed prior to conversion.5,6 Recently, production and utilization technologies of ashless coal (HyperCoal, or HPC) have been developed in Japan7 as a clean coal technology. HPC is produced by thermal extraction of coal at temperatures less than 400 °C in a nitrogen atmosphere using solvents such as 1-methylnaph* Corresponding author. E-mail:
[email protected]. (1) Hirano, K. Fuel Proc. Tech. 2000, 62, 1009. (2) Hirano, K.; Kouzu, M.; Okada, T.; Kobayashi, M.; Ikenaga, N.; Suzuki, T. Fuel 1999, 78, 1867. (3) Matsuoka, K.; Tomita, A.; Shah, N.; Huggins, F. E.; Huffman, G. P. Energy Fuels 2001, 15, 936. (4) Onozaki, M.; Namiki, Y.; Aramaki, T.; Takagi, T.; Kobayashi, M.; Morooka, S. Ind. Eng. Chem. Res. 2000, 39, 2866. (5) Stohl, F. V.; Stephens, H. P. Ind. Eng. Chem. Res. 1987, 26, 2466. (6) Cillo, D. L.; Stiegel, G. J.; Tischer, R. E.; Narain, N. K. Fuel Proc. Tech. 1985, 11, 273. (7) Okuyama, N.; Komatsu, N.; Shigehisa, T.; Kaneko, T.; Tsuruya, S. Fuel Proc. Tech. 2004, 85, 947.
thalene (1-MN)7 and light cycle oil (LCO).8,9 HPC therefore consists of extractable components, free of minerals, and exhibits high fluidity and high calorific value.7 For low-ranking coals such as subbituminous coals and lignites, the extraction yield has been low. However, the use of more polar solvents such as crude methylnaphthalene oil (CMNO) resulted in extraction yields equivalent to those obtained with bituminous coals.10 The polar components of CMNO had a significant effect on extraction yields.11 Additionally, acid pretreatment of coal can enhance the extraction yields12 by releasing cation-bridging cross-links between metal carboxylate groups.13,14 Therefore, the production of HPC has been demonstrated for varieties of coal. HPC gasified rapidly at temperatures lower than 700 °C, and the catalyst was not deactivated.15-17 A similar effect is expected in the direct liquefaction of HPC. A hydrogenation catalyst containing Mo, although partially deactivated by deposition of coke and minerals during coal liquefaction,18 may provide higher oil yields than a Fe catalyst without fouling. The continuous use of catalyst may also be possible if HPC is used in the liquefaction process. Several studies involving the hydrogenation of coal extract produced according to the liquid solvent extraction (LSE) (8) Takanohashi, T.; Sakanishi, K.; Saito, I.; Fujita, M.; Mashimo, K. Fuel 2007, 81, 1463. (9) Yoshida, T.; Takanohashi, T.; Sakanishi, K.; Saito, I.; Fujita, M.; Mashimo, K. Energy Fuels 2002, 16, 1006. (10) Yoshida, T.; Li, C.; Takanohashi, T.; Matsumura, A.; Sato, S.; Saito, I. Fuel Proc. Tech 2004, 86, 61. (11) Kashimura, N.; Takanohashi, T.; Saito, I. Energy Fuels 2006, 20, 2063. (12) Masaki, K.; Yoshida, T.; Li, C.; Takanohashi, T.; Saito, I. Energy Fuels 2004, 18, 995. (13) Li, C.; Takanohashi, T.; Yoshida, Y.; Saito, I.; Aoki, H.; Mashimo, K. Fuel 2004, 83, 727. (14) Li, C.; Takanohashi, T.; Saito, I. Energy Fuels 2004, 18, 97. (15) Wang, J.; Sakanishi, K.; Saito, I.; Takarada, T.; Morishita, K. Energy Fuels 2005, 19, 2114. (16) Sharma, A.; Takanohashi, T.; Morishita, K.; Takarada, T.; Saito, I. Fuel 2008, 87, 491. (17) Sharma, A.; Takanohashi, T.; Saito, I. Fuel 2008, 87, 2686. (18) Zhang, T.; Jacobs, P. D.; Haynes, H. W., Jr.; Swanson, A. J. Fuel 1995, 74, 431.
10.1021/ef900135r CCC: $40.75 2009 American Chemical Society Published on Web 06/08/2009
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Table 1. Element Composition and Ash Content of Gunung Bayan Raw Coal and Its HPC ultimate analysis (daf, wt %) coal HPC
C
H
N
S
0(diff)
ash (db, wt %)
76.4 85.0
5.5 5.6
1.9 1.4
0.9 0.7
15.3 7.3
8.2 0.0
process19 have been reported, and catalyst fouling during hydrogenation of coal extract was studied. Begon et al.20,21 reported that carbonaceous material deposition on fresh catalyst occurred during heating-up, whereas that deposition on the reused catalyst was quite low level. In the present work, HPC was catalytically hydrogenated and its reactivity was compared to that of the corresponding raw coal. The reusability of the catalyst was also evaluated.
Figure 1. Separation procedure for hydrogenation products.
Experimental Section Materials. Gunung Bayan sub-bituminous coal and its corresponding HPC (extract fraction) were used. The extraction yield with 1-MN was 58 wt % on dry-ash-free (daf) coal basis. The samples were ground and sieved less than 150 µm. Commercial NiMo/Al2O3 catalyst C-217 (Mo 7 wt %) was ground to 150-350 µm. Finer particles (under 150 µm) were also employed in several runs. The coal samples and catalyst were dried at 80 °C for 12 h in a vacuum oven. The elemental analysis and ash content of coal samples are shown in Table 1. The ash content of HPC was 0.0 wt % (less than 500 ppm), and the oxygen content also low compared to that of raw coal. Thermogravimetric Analysis. A sample of approximately 10 mg was analyzed using a thermogravimetric analyzer TGA50(Shimadzu). The sample was heated at a rate of 3 °C/min from room temperature to 600 °C under nitrogen flow (50 mL/min). Dynamic Viscoelastic Analysis. Dynamic viscoelastic measurements were carried out using a rheometer ARES 2K-STD (Rheometrics). Sample pellets was prepared by pressing 0.4 g of sample and put between two stainless parallel plates. A strain was applied to the lower plate periodically with a frequency of 1 Hz and amplitude of 0.1%. The sample pellet was heated at a rate of 3 °C/min from 40 to 550 °C. The detailed procedure is described elsewhere.22 Hydrogenation. Approximately 5 g of sample and 0.5 g of catalyst were reacted using an autoclave of 80 mL, with or without 5 g of 1-MN as a solvent. The heating rates were 10 °C/min with the solvent or 5 °C/min without the solvent. The determination procedure of product distribution is shown in Figure 1. The amount of gaseous components (gas) and the amount of hydrogen consumption were determined using system GC composed of DP-281 and two of GC320 with TCD (GL Sciences). The amount of material with a boiling point over 320 °C [HS (320+)] was determined using the Shimadzu TGA-50. The distillation curve of HS was determined by GC-17A with FID. The initial boiling point was approximately 150 °C, and the boiling point corresponding to the peak end of 1-MN in DTG curve was approximately 320 °C. The amount of under 320 °C boiling point (HS (320-)) and light oil (volatile) were given by difference. SEM-EDX. Several catalyst particles were analyzed directly using S-3500NLC (Hitachi Science Systems) with Genesis (EDAX). (19) Kamall, R. Coal Liquefaction, Technology Status Report 010, DTI/ Pub URN 99/1120; Department of Trade and Industry: London, October 1999; p 9. (20) Begon, V.; Warrington, S. B.; Megaritis, A.; Charslay, E. L.; Kandiyoti, R. Fuel 1999, 78, 681. (21) Begon, V.; Megaritis, A.; Lazaro, M.-J.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 1261. (22) Yoshida, T.; Iino, M.; Takanohashi, T.; Kato, K. Fuel 2000, 79, 399.
Figure 2. TG and DTG thermograms for raw coal and HPC.
EDX spectra were obtained by scanning the whole area of one particle in SEM image.
Results and Discussion Difference in Reactivity between Coal and HPC. TG and its derivative curves of raw coal and HPC are shown in Figure 2. The TG curves were greatly different under 400 °C and were similar over 400 °C. The derivative curve of HPC represented three clear peaks at 130, 270, and 440 °C, whereas the curve of raw coal showed only a peak at 440 °C. The content at 130 °C was 3 wt %, due to the residual 1-MN used as the extraction solvent in production of HPC. That at 270 °C was 13 wt %, estimated as the coal-derived oil. The third peak around at 440 °C is attributed to thermal decomposition. Dynamic viscoelastic curves of the raw coal and HPC are shown in Figure 3. For the raw coal, storage elastic modulus (G′) was less changed. A discontinuous change around 460 °C was attributed to fracturing sample. On the other hand, G′ of HPC greatly decreased over 170 °C, with a loss tangent (tan δ ) G′′/G′) greater than 1, indicating that HPC had a high fluidity in heating-up period of reaction. The result suggests that HPC
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Figure 3. The viscoelasticity of raw coal and its corresponding HPC.
Figure 4. The effect of initial hydrogen pressure on the product distribution of the hydrogenation with solvent for the raw coal and its HPC. Reaction conditions: 450 °C, 60 min.
significantly softens and fuses by itself. Therefore, it may work partly as a solvent in hydrogenation, as described later. Figure 4 shows the product distribution after hydrogenation at 450 °C for 60 min for the raw coal and HPC by varying initial hydrogen pressure in the presence of solvent. The weight of HS was not determined because 1-MN was not evaporated, so HS (320-) was indistinguishable from Volatile. The THFI yield of HPC (3 wt %) was smaller than that of raw coal (8 wt %) at 10 MPa. At the lower initial hydrogen pressure of 6 MPa, these percentages increased to 5 and 13 wt %, respectively. The gas yield changed little with varying initial pressure. Their composition were CH4, 8 wt %; C2-C4, 3 wt %; and CO + CO2, 3 wt % from raw coal and CH4, 8 wt %; C2-C4, 3 wt % from HPC. The difference of CO + CO2 yield was attributable to that HPC had lesser carboxyl groups, which had been removed during the extraction. As described above, HPC exhibited a high fluidity, and the formation of THFI was relatively small. Hydrogenation was also carried out in the absence of solvent as a function of temperature.
Koyano et al.
Figure 5. The effect of reaction temperature on the nonsolvent hydrogenation of the raw coal and its HPC. Reaction conditions: 10 MPa (initial pressure), 60 min.
The results are shown in Figure 5. In nonsolvent hydrogenations of raw coal, the significant amount of THFI was formed, 17 wt % at 450 °C and 34 wt % at 460 °C. Sakanishi et al.23 reported that NiMo/Al2O3 catalyst did not show enough activity in such a nonsolvent hydrogenation of raw coal. On the other hand, in nonsolvent hydrogenations of HPC the amount of THFI was quite low, 4 wt %. Additionally, the amount of THFI did not change almost with increasing the temperature, although it greatly increased in the case of raw coal. The decomposition of HPC progressed with increasing the reaction temperature, and the volatile and gas yields increased from 13 and 9 wt % at 450 °C to 20 and 14 wt % at 460 °C, respectively. In the hydrogenation of HPC, the coal molecules were more easily relaxed in initial heating-up period even without solvent, resulting in the similar product distribution to the case with solvent. Recycled Use of Catalyst. To investigate the influence of coke deposition on the activity of catalyst, the recycled use of catalyst including THFI was carried out. Nonsolvent hydrogenations were repeated five times at 450 °C and three times at 460 °C. The results are summarized in Figure 6. At 450 °C, the formation of THFI at the first reaction (fresh catalyst) was 4 wt %. However, after the second use of catalyst, the amount of deposited THFI were below 1 wt %, indicating that accumulation of THFI on the recycled catalyst did not occur under this reaction condition. Additionally, no significant change of product distribution was observed for five uses. Usually, deactivation of catalyst is caused by two factors, coke deposition and ash fouling.18 Suelves et al.24 investigated the hydrogenation of coal extract with tetralin and reported that heavy materials with boiling points of over 450 °C deposited rapidly on catalyst in the initial stage of hydrogenation. These results indicate that HPC was possibly hydrogenated without any coke and that the catalyst can be used repeatedly. Although, at 460 °C, 2% of THFI formed at the second use of catalyst, and after that the formation of coke was also confirmed. Thus, the influence of reaction temperature on the coke formation was significant in the nonsolvent hydrogenation of HPC. SEM-EDX images and spectra of the used catalyst particle are shown in Figure 7. Two samples were compared; (a) after five-times use for HPC, (b) after first-time use for raw coal. EDX spectra were obtained by scanning the whole area of a
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Energy & Fuels, Vol. 23, 2009 3655
Figure 6. Nonsolvent hydrogenation of the HPC using the reused catalyst. Reaction conditions: 10 MPa (initial pressure), 60 min.
Figure 7. SEM-EDX images and spectra of the used catalyst particle from (a) HPC after five-times of sequential use and (b) raw coal after a one-time use. Reaction conditions: no solvent, 10 MPa (cold, initial), 450 °C, 60 min.
Figure 8. The effect of reaction time on the nonsolvent hydrogenation of the HPC. Reaction conditions: 10 MPa (initial pressure), 450 °C.
catalyst particle. After the first-time use for HPC, the color of catalyst became brown due to some adhered materials, and after the five-times use it changed into black. However, as shown in Figure 7, the amount of materials such as Si and Fe was very low because the ash content of HPC is at quite a low level, less than 500 ppm, as described in Experimental Section. On the other hand, after only the first-time use for the raw coal was a lot of coke and ash particles observed around the catalyst particle. The catalyst particle was rugged and some small ash
particles adhered on the surface of catalyst, leading to 1.7 times of C, 13.3 times of Si, and 3.1 times of Fe compared to that for HPC (after five-times use). Therefore, catalyst fouling during hydrogenation of HPC was significantly lower than that of raw coal. (23) Sakanishi, K.; Hasuo, H.; Kishino, M.; Mochida, I. Energy Fuels 1998, 12, 284. (24) Suelves, I.; Lazaro, M.-J.; Begon, V.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2001, 15, 1153.
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Koyano et al.
Table 2. Yield and Element Composition of HI and HI/TI Produced at 450 °C ultimate analysis (db, wt %) fraction reaction time yield (daf, wt %) HI
HI/TI
raw HPC 0 min 60 min 120 min 240 min raw HPC 0 min 60 min 120 min 240 min
58.9 58.5 23.0 14.8 9.0 42.3 35.9 7.0 4.5 2.4
C
H
N
S
0(diff)
82.9 85.4 90.4 91.1 91.9 82.7 85.4 90.0 90.4 90.8
5.5 5.6 5.2 4.8 4.6 5.3 5.1 4.4 4.1 3.7
1.6 1.9 2.0 1.7 1.4 1.7 2.0 2.1 1.9 1.8
0.5 0.3 0.1 0.0 0.0 0.5 0.3 0.3 0.2 0.2
9.5 6.8 2.4 2.4 2.0 9.8 7.2 3.3 3.4 3.4
Effect of Reaction Time. As described above, hydrogenation of HPC proceeded at 450 °C without any coke; however, 20 wt % HI still remained. Wasaka et al.25 reported that prolonging residence time is the most effective factor to increase the oil yield by decomposing asphaltene in the NEDOL process and showed the toluene solubles in coal liquefaction residue converted to oils during further hydrogenation. Thus, the reaction time was changed, and the results are shown in Figure 8. To observe the heavier fraction in HI, toluene insoluble was fractionated as HI/TI. The result of fractionation for the raw HPC (without hydrogenation) is also shown for comparison. THFI greatly decreased during the heating-up period, from 26 wt % for the raw HPC to 2 wt % at 0 min, although HI significantly decreased in the initial 60 min of reaction time, from 61 to 36%. Notably, HI/TI was reduced greatly from 36 to 7 wt %. The ultimate analysis of HI and HI/TI is shown in Table 2. The oxygen in HI/TI decreased rapidly until 60 min. Begon et al.26 reported that a half of the basic OH groups in the coal extract had been released after only 10 min of hydrogenation. Zhang et al.27 investigated the hydrogenation of coal extract using tetralin and reported that heavy fractions reacted rapidly in first 30 min. Therefore, the decomposition of oxygen functionality and solubilization of the heavier fraction seemed to occur in the early stage of hydrogenation. To obtain more light fractions such as HS and volatile, the further hydrogenation of HI can be necessary. As shown in Figure 8, the extension of reaction time to 120 min was effective; HI was reduced to 15 wt % without increasing THFI. Begon et al.26 also observed the decrease of bridgehead carbon by hydrogenation and reported that the hydrogenation required a longer reaction time. The HI yield decreased with only small change on the ultimate analysis shown in Table 2. Therefore, a slower degradation occurred as second-stage reaction. The content of THFI was below 4 wt % up to 120 min, meaning that only initial adhesion took place on the surface of catalyst but further accumulation of coke-like materials did not
occur. However, after 240 min of reaction, THFI increased to 9 wt % and bulky coke materials were obviously observed in the bottom of reactor. There are possibly two reasons, one is due to settling of catalyst. The sedimentation velocity of catalyst may increase with lowering of liquid viscosity during hydrogenation. Such sedimented catalyst can be ineffective for hydrogenation. The other possibility is the acceleration of coke formation because of hydrogen shortage, since 51% of initial hydrogen was consumed after 240 min. The slow increase of C% in HI after 60 min as shown in Table 2 also indicates it. The result at 460 °C is shown in Figure 9. HI decreased rapidly to 20 wt % at 60 min. However, THFI significantly formed already at 120 min, earlier than the case at 450 °C. Therefore, in the reaction at 460 °C, the coking reaction might be accelerated as well as hydrogenation. Inhibition of Coke Deposition. To inhibit the coke deposition during the hydrogenation as much as possible, the effects of fineness of catalyst and resupplying of hydrogen were investigated. The results are shown in Figure 9. By using catalyst with the fine particle of
