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Extraction of Taiheiyo coal with supercritical water–HCOOH mixture
Fuel 79 (2000) 243–248 www.elsevier.com/locate/fuel
Extraction of Taiheiyo coal with supercritical water–HCOOH mixture T. Adschiri*, T. Sato, H. Shibuichi, Z. Fang, S. Okazaki, K. Arai Department of Chemical Engineering, Tohoku University, Aoba 07, Aramaki, Aoba-ku, Sendai 980-8579, Japan
Abstract Taiheiyo coals were extracted with supercritical toluene (SC-toluene) (653 K, 20 MPa), supercritical water (SCW) (653 K, 35 MPa) and formic acid (HCOOH)–SCW mixed solvents (653 K, 35 MPa) using a semi-batch type system. The results clearly indicate that the coal conversion ( 1 2 weight of residual coal (daf)/weight of loaded coal (daf)) and the liquid yield ( weight of liquid extracts/weight of loaded coal) in HCOOH–SCW were higher than those in SCW and SC-toluene. A considerable portion of coal is thus converted into light oils probably through hydrolysis and hydrogenation in HCOOH–SCW. We conducted another series of experiments for direct in situ observation of coal conversion in SCW and HCOOH–SCW using a diamond anvil cell (DAC). In HCOOH–SCW, the effective and rapid hydrogenation of coal with HCOOH occurs mainly at the early stage of the reaction. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Supercritical water; Coal conversion; Formic acid
1. Introduction and Supercritical water (SCW) (Tc 647 K Pc 22:1 MPa) is the so-called dense steam and thus can be made miscible with light gases such as H2, CO and O2 [1]. Above 3508C, water becomes miscible with oils and aromatics [2], since the dielectric constant of supercritical water is reduced from 2 to 20, which is similar to that of polar organic solvents at room temperature. We think the miscibility of water with H2, aromatics and oils provides a unique homogeneous reaction atmosphere for coal liquefaction. Since water becomes immiscible with gases and oils by the reduction of temperature and pressure, the process of the recovery of solvent and the separation of products after the liquefaction process would be simplified. In the coal liquefaction in SCW, various unique reactions take place, as described below. Supercritical water can provide a reaction atmosphere for the heterolytic reactions of many types of compounds even without any acid or base catalyst. Townsend et al. [3] studied the reaction of coal related chemicals in SCW at temperatures from 417 to 823 K and water density from 0.1 to 0.21 g/cm 3, and reported that compounds which had a heteroatom combined with the saturated carbon (dibenzylether, benzylphenylether, guaiacol, phenethylphenylether, etc.) were hydrolyzed under those conditions. Antal et al. [4,5] reported that ethylene and propane could be produced * Corresponding author. Tel.: 181-22-217-7246; fax: 181-22-217-7246. E-mail address: [email protected] (T. Adschiri).
through intramolecular dehydration of ethanol and propanol, respectively, in SCW at 668 K and 34.5 MPa. Adschiri et al. [6] reported that the hydrolysis of cellulose occurred rapidly in SCW without any catalysts. All of these results suggest that SCW itself works as an acid or base catalyst. The hydrolysis of coal has been reported in the literature [7–10]. A number of studies have been conducted on SCW extraction of coal. Li et al. [7] and Kershaw et al. [8,9] reported that coal conversion in SCW was higher than that in SC-toluene, and that water-soluble compounds such as alcohol and carboxylic acid were produced in SCW: they attributed this result to the hydrolysis at ether or ester bonds in coal. Adschiri et al. [10] conducted the extraction of Taiheiyo coal using SCW and SC-toluene and reported that coal conversion and liquid yields were higher in the SCW experiment and suggested the effect of hydrolysis as above. Some researchers have studied the hydrogenation of coals through a water–gas shift reaction (WGS reaction: CO 1 H2 O ! CO2 1 H2 ). Penninger [11] compared the reaction of Wyoming coal with a CO–SCW and a N2 – SCW mixture at 723 K and 7.1–10.9 MPa. They reported that the coal conversion (53.9 wt.% daf at 7.1 MPa) and the yields of aromatics and polar compounds were much higher in CO–SCW than in N2 –SCW. They attributed the results to formic acid formation through WGS reaction that is known to be an active hydrogen donor. A similar hydrogenation through the WGS reaction was reported for the extraction of coal with a CO–SCW mixture at 673 K and 21.9 MPa [12], and at 678 K and 14 MPa [13].
0016-2361/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00158-1
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Table 1 Proximate and ultimate analyses of Taiheiyo coal Proximate analysis (wt.% db) Ash VM 11.0 48.6
FC 40.4
Ultimate analysis (wt% daf) C H 75.8 6.26
N 1.27
S 0.25
O (diff.) 16.4
Although the hydrogenation with CO–SCW as previously reported is very attractive for liquefying coal, under these conditions 1:12 . T=Tc . 1:04; 0:99 . P=Pc . 0:32 SCW does not seem to work as an extraction solvent, since the density of SCW at these conditions is low (0.02–0.12 g/cm 3). If the reaction could be conducted in more dense SCW, a higher coal conversion could probably be obtained, because in general the solubility increases with increasing solvent density, and hydrolysis is promoted in high dielectric constant medium, namely in high-density SCW. We studied the catalytic hydrogenation of dibenzothiophene and naphthalene through the WGS reaction in dense SCW (0.42, 0.5 g/cm 3) [14,15], and found that the hydrogenation rate of dibenzothiophene to biphenyl or cyclohexylbenzene, and that of naphthalene to tetralin or decalin is higher in CO–SCW than in H2 –SCW. We also conducted experiments using HCOOH for the hydrodesulfurization of dibenzothiophene. The results showed the higher hydrogenation rate in the HCOOH–SCW atmosphere than that in the H2 –SCW atmosphere. This suggests that active hydrogenating species were formed through the WGS reaction. Melius et al. [16] studied the effect of water density on the activation energy of the WGS reaction by using MO calculation and found that the activation energy decreased with increasing water density. Their study implies
Fig. 1. Diamond anvil cell.
that formic acid might be the intermediate species of the WGS reaction. Despite the fact that HCOOH could be a candidate for an effective hydrogen donor in SCW, the extraction of coal with HCOOH has not been reported so far. As described earlier, for understanding the results of coal extraction with HCOOH–SCW, the effect of hydrolysis should also be taken into account. Thus, the comparison of the results in HCOOH–SCW and SCW is required. For the elucidation of coal liquefaction in SCW, an understanding of the phase in the reaction atmosphere is essential. Recently, we have employed a diamond anvil cell (DAC) for the direct observation of the reactions in SCW [17]. For cellulose decomposition in SCW, dissolution of cellulose particles could be directly observed. The first objective of this study is the direct observation of coal conversion in SCW and HCOOH–SCW with DAC. The second objective is to conduct the coal extraction experiments in SC-toluene, SCW and HCOOH–SCW under a rather dense phase and compare the results. 2. Experiment 2.1. Materials Taiheiyo coal was used in the experiments. Proximate and ultimate analyses of the coal are shown in Table 1. The particle size of the coal used was in the range from 16 to 24 mesh. For the DAC experiment, coal powder having an approximate diameter of 50 mm was used. Solvents used were 10 wt.% formic acid (purity 98%) aqueous solution, distilled water and toluene (purity 99.5%). 2.2. Procedures 2.2.1. Direct observation of coal conversion A DAC was used for the direct in situ observation of coal conversion in SCW and HCOOW–SCW. The DAC equipped with diamond windows used for this experiment had been developed by Bassett et al. [18]. Fig. 1 shows the schematics of the cell. The reactor was a space which was enclosed by upper and lower diamonds and a gasket hole (0.5 mm i.d., thickness: 0.25 mm). A heater and a thermocouple were attached around upper and lower diamonds. The reaction temperature was defined as the average of the two temperatures. Heaters were used for controlling the reaction temperature. During the experiment, argon gas with 1% hydrogen was introduced into the cell to prevent the diamond from oxidation. A piece of ruby for pressure measurement and coal powder were put into the gasket hole on the diamond. The solvent was then introduced into the gasket hole by using a microsyringe. The reactor was pressurized and the Raman spectrum shift of the ruby was measured to find the reaction pressure by the method of Forman et al. [19]. Heating was started and the inside of the reactor was continuously
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The reaction temperature was measured by a K-type thermocouple inserted in the coal bed. The time required to heat the reactor from room temperature to 633 K was extremely short. About 2 min was required for the reactor to reach 653 K from 633 K during which the coal conversion proceeded to some extent. The extraction was conducted for 120 min. The residence time of the extract with the solvent in the reaction was controlled by changing the flow rate of the solvent. The residence time was evaluated by Eq. (1). Fig. 2. Experimental apparatus for the coal extraction.
observed with a microscope (SZH10, Olympus) and was recorded with a CCD camera (KY-F55MD, Olympus) and a video camera. About 2.5 min was taken to heat up the reactor to the reaction temperature (653 K). The reaction pressure was about 85 MPa. After observation for 60 min, the cell was cooled and the pressure was reduced. The inside of the reactor was observed again. 2.2.2. Coal extraction A schematic diagram of the apparatus used for the coal extraction experiment is shown in Fig. 2. About 4 g of coal was loaded in the reactor (8.6 cm 3) which was made of SUS316 stainless steel. The coal bed was sandwiched with 200 mesh screen made of SUS316 to prevent the entrainment of coal particles. After the system was purged with N2, the solvent was fed into the reactor by a HPLC Pump (PUS-8, GL Science), at a feed rate from 0.2 to 14 cm 3/min (for water, HCOOH-water), from 0.05 to 10 cm 3/min (for Toluene). A back pressure regulator (Model 26-1721-24-071, Tescom) was used to control the pressure of the system. After the pressure and the flow rate became stable, the preheating tube and the reactor were submerged in a molten salt bath whose temperature had been controlled at 653 K, and thus the reactor was rapidly heated to the extraction temperature.
Fig. 3. Analysis procedure.
t
V 2 W=rcoal G=rf
1
where V (cm 3) was the volume of the reactor, W (g) was the weight of coal loaded, r coal (g/cm 3) was the density of coal and G (g/s) was the mass flow rate of solvent. Here, r f (g/ cm 3) is the density of fluid in the reactor and was approximated to be that of the pure solvent by using a steam table or data. The extract, along with the solvent, was cooled in a cooling unit. The pressure was reduced by the back pressure regulator and the produced gas was separated from the liquid effluent. The liquid effluent was recovered, and after an extraction time of 120 min, the reactor was taken out of the bath and then cooled. For the HCOOH–SCW experiments, the gases (CO, CO2 and H2) were produced from formic acid and thus the density of the mixed solution in the reactor might be lower than that in SCW atmosphere. In general, the solubility in a solvent decreases with reducing solvent density and thus that of HCOOH–SCW was suppressed to be lower than that of SCW. Thus, we conducted further extraction with only SCW at 653 K and 20 MPa for 30 min for recovery of the products after 120 min of the HCOOH–SCW experiments for comparison of the reactions. The residue was recovered from the reactor and the extract in the line was completely recovered by washing with tetrahydrofuran (THF). The procedure of analysis is shown in Fig. 3. The residue was dried at a reduced pressure of 483 K for 4 h and then weighed. The liquid products were distilled (333 K) and dried (483 K) at a reduced pressure and then weighed. This was defined as the extract. Then, 1 g of the extract was extracted with about 200 g of acetone at room temperature for 3 h, and separated into acetone soluble compounds (AS) and acetone insoluble compounds (AI). AS were further analyzed by gas chromatography–mass spectrometry (GC–MS) (Automass 20, JEOL) and gas chromatography–flame ionization detector (GC–FID) (HP-5890-II, Hewlett–Packard) while AI were weighed after drying. The compounds that were vaporized with water or THF during these procedures were recovered by a cold trap immersed in liquid nitrogen, and analyzed by GC–MS and GC–FID. These compounds and gases produced during the experiment were defined as volatile matter (VM).
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Fig. 4. Coal conversion in SCW at 653 K and 85 MPa.
Ash analysis was conducted for the residue and the extracts as follows. A small amount (0.6 g) of sample was combusted with air (100 cm 3/min) at 1088 K for 4 h in a tubular reactor (quartz glass, 60 × 2 cm2 i.d.) and the resultant ash was weighed. For the SCW and the HCOOH–SCW experiments, a portion of ash was found to be dissolved in the solvents, which affects the conversion of coal and the yield of extracts. By considering the ash content in the parent coal (a 0) and the residue (a ), the coal conversion and the yield were evaluated by the following equations, respectively. Conversion wt:% daf
Wcoal 1 2 a0 2 Wresidue 1 2 a × 100 Wcoal 1 2 a0
Yield wt:% daf
Wi × 100 Wcoal 1 2 a0
2
3
where Wcoal, Wresidue and Wi were the weight of parent coal, residue coal and each constituent of extracts, respectively.
3. Results and discussion 3.1. Direct observation of coal conversion Direct observation of coal conversion with DAC supplies information for the reaction with reaction time, which cannot be obtained in the semi-batch type extraction experiments. Fig. 4 shows the results of the DAC observed in SCW. The large particle was the ruby introduced for measurement of pressure in the reactor and the small particles were coal. Fig. 4(a)–(c) shows that with increasing reaction time, the coal particles gradually become small but retain their particle shape. At 60 min of reaction time, the diameter of coal particles became about 80% of the parent coal. For the HCOOH–SCW experiments, as shown in Fig. 5, a drastic change in the shape of coal particles was observed at 2.5 min of reaction time (during the preheating period), which is followed by a gradual change. This suggests a different reaction atmosphere from SCW. We also observed the solution after cooling, namely at reducing pressure. As shown in Fig. 4(d), in SCW, a lot of spheres were produced which were probably gas bubbles. However, as shown in Fig. 5(d), in HCOOH–SCW, a lot of unsymmetric spheres appeared, which probably indicate an
Fig. 5. Coal conversion in HCOOH–SCW at 653 K and 85 MPa.
T. Adschiri et al. / Fuel 79 (2000) 243–248
Fig. 6. Coal conversion at various residence times at 653 K and 20 MPa (SC-toluene), 35 MPa (SCW, HCOOH–SCW).
oil or liquid phase. By comparing Fig. 5(a) and (d), we found that the volume of the oil phase that appeared after the cooling and reducing pressure (Fig. 5(d)) was as large as the volume of the loaded coal particles (Fig. 5(a)) or rather larger. This indicates a very high conversion of coal to liquid oils in HCOOH–SCW. 3.2. Extraction experiments Fig. 6 shows the results of coal conversion at various residence times. As elucidated in our previous works, the results shown in this figure imply two stages of reaction; that is, the extraction of liquid products and the secondary reaction in the reactor. The slight change of conversion in SC-toluene and SCW suggests that the polymerization or recombination of the liquid products on the residual coals are not significant. However, for HCOOH–SCW, the coal conversion decreased down to 63 wt.% at 210 s residence time from 80 wt.% at 20 s. It is not reasonable that the polymerization takes place more in HCOOH–SCW, which is considered to be a more reactive atmosphere than SC-toluene or SCW. One reason might be the decomposition of HCOOH into H2 and CO2 gases in the preheating zone, especially at a longer residence time, as reported previously [20]. For checking this hypothesis, another series of experiments of HCOOH
Fig. 7. Yield of acetone soluble compounds (AS) at various residence times at 653 K and 20 MPa (SC-toluene), 35 MPa (SCW, HCOOH–SCW).
247
reaction under the same conditions as the coal extraction experiments were conducted. The conversion of HCOOH at 21 s of residence time was less than 30%, but at 210 s of residence time, was almost 100%. Therefore, the results in the experiment with 210 s of residence time are considered to be those obtained in H2 –CO2 –SCW atmosphere. This implies that HCOOH has a higher activity for the hydroliquefaction than H2. For exclusively discussing the extractive reactions for these cases, the intercepts at zero residence time are compared. The coal conversion in SCW was higher than that in SC-toluene. As elucidated in the previous paper, this is probably because of the effect of hydrolysis on the liquefaction of coal. The conversion in HCOOH–SCW was the highest and reached 80 wt.%, which is consistent with the results of DAC experiment. Furthermore, both particle size and morphology of the coal particles changed greatly before and after the HCOOH–SCW experiments, although these changed little for SC-toluene and SCW experiments. These results suggest a significant reaction of HCOOH with coal. Fig. 7 shows the yield of AS with residence time; Fig. 8 is the yield of VM. The change of these yields with residence time is due to the reaction of products in the reactor. Fig. 7 shows that AS yield did not change significantly with residence time under these conditions. However, the yields are different among these three cases. In the SC-toluene experiment, the yield of AS was around several percent. For SCW, it ranged from 20 to 30%. For the case of HCOOH–SCW, it was highest and about 50%. Fig. 9 shows the GC–FID chromatogram of AS obtained at similar residence times. Peaks observed were mostly alkanes or alkenes whose carbon number ranged from 10 to 20. In these chromatograms, the left-hand side shows the lower molecular weight species. For HCOOH–SCW, the product distribution clearly shifted towards the lower molecular weight side compared with SC-toluene and SCW. As mentioned above, AS were about 50 wt.% in HCOOH– SCW. Thus, in HCOOH–SCW, a large amount of AS were obtained, and the AS were lighter components than those found in SCW or SC-toluene. As shown in Fig. 8, the yield of VM slightly increased with the residence time for all cases. This is because of the decomposition of liquid products into volatile matters in the reactor. Although the difference of the yield of VM was not clear, there was a tendency that the yield of VM in SCW was lower than that in SC-toluene and for that in HCOOH–SCW to be lower than in SCW. This is also consistent with the result observed by the DAC experiment. Higher conversion, higher AS yield and lower VM yield in SCW over that obtained in SC-toluene can be explained by considering the contribution of hydrolysis in the liquefaction of coal, as reported in previous work [10]. In the HCOOH–SCW experiments, the coal conversion was 80 wt.% and the AS yield was as high as 49 wt.%. Liquid products consisted of lower molecular weight than that in
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T. Adschiri et al. / Fuel 79 (2000) 243–248
Fig. 8. Yield of volatile matter (VM) at various residence times at 653 K and 20 MPa (SC-toluene), 35 MPa (SCW, HCOOH–SCW).
SCW according to the AS analysis. Also in the DAC experiment, an extremely large amount of oil was observed after reaction in HCOOH–SCW. These results suggest that there is enhanced decomposition and fragmentation in the presence of HCOOH. However, in HCOOH–SCW, the VM yield was lowest among these three cases. Also in the DAC experiments, no gas phase was observed. These surprising results cannot be simply explained by enhanced hydrolysis with HCOOH. At the moment, we think that these results are because of the hydrogenation of coal with HCOOH. Hydrogen capping of radicals suppresses b-scission of radicals, which may be one of the reasons for the lower VM yield. 4. Conclusion We conducted a series of experiments for direct in situ observation of coal conversion in SCW and in HCOOH– SCW using a DAC. In SCW, the coal particles gradually became smaller, however, in HCOOH–SCW, the dramatic change of shape of coal particles was observed at 2.5 min of the reaction time. After cooling at a reducing pressure, in HCOOH–SCW, an oil phase appeared and its volume was as large as that of the loaded coal, which indicates high coal conversion to liquid oils. We have also conducted a study of coal extraction in SCtoluene (653 K, 20 MPa), SCW (653 K, 35 MPa) and HCOOH–SCW (653 K, 35 MPa) with a semi-batch type apparatus. In the extraction with HCOOH–SCW, the coal conversion (80 wt.% daf) and the yield of AS (49 wt.% daf) were much higher and the VM yield was lower than those in SC-toluene or in SCW, which was consistent with the results of the DAC experiment. The product distribution of AS clearly shifted towards lower molecular weight side compared with SC-toluene or SCW. Thus, it has been demonstrated that coal had been converted to the lighter oils probably through hydrolysis and hydrogenation in HCOOH–SCW.
Fig. 9. GC–FID chromatogram of acetone soluble compounds at 653 K, 20 MPa and residence time 10 sec (SC-toluene), 35 MPa and residence time 21 sec (SCW, HCOOH–SCW)).
Acknowledgements The authors wish to express their thanks to the grant in aid for New Energy and Industrial Technology Development Organization (NEDO), Scientific Research on Priority Areas (09450281) and to Idemitsu Kosan Co. Ltd. for supplying Taiheiyo coal.
References [1] Frank EU. Pure Appl Chem 1981;53:1401. [2] Schneider GM. Ber Bunsenges Phys Chem 1972;76:325. [3] Townsend SH, Abraham MA, Huppert GL, Klein MT, Paspep NC. Ind Engng Chem Res 1988;27:143. [4] Xiaodong X, Antal MJ. J Supercritic Fluids 1990;3:228. [5] Ramayya S, Antal MJ. Fuel 1987;66:1364. [6] Adschiri T, Hirose S, Malaluan R, Arai K. J Chem Engng Jpn 1993;26(6):676. [7] Li L, Egiebor NO. Energy and Fuels 1992;6:35. [8] Kershaw JR, Bagnell LJ. Supercritical fluids. ACS Symp Ser, 329, 1987. p. 267. [9] Kershaw JR. Fuel Process Technol 1984;9:235. [10] Adschiri T, Nagashima S, Shibuichi H, Shishido M, Arai K. J Jpn Inst Energy 1996;75(8):750. [11] Penninger JML. Fuel 1989;68:983. [12] Ross DS, Blessing JE, Nguyen QC, Hum GP. Fuel 1986;63:1206. [13] John ST, Chan W, Jackson R, Marshall M. Fuel 1994;73(10):1628. [14] Adschiri T, Shibata R, Sato T, Watanabe M, Arai K. Ind Engng Chem Res 1998;37(7):2634. [15] Okazaki S, Kurosawa S, Adschiri T, Arai K. Proceedings of the 6th Japan–China symposium on coal and C1 chemistry, 1998. p. 192. [16] Melius CF, Bergan NE. 23rd Symposium (International) on Combustion, 217. Pittsburgh, PA: The Combustion Institute, 1990. [17] Arai K, Adschiri T. Fluid Phase Equilibria 1999, in press. [18] Bassett WA, Shen AH, Bucknum M. Rev Sci Instrum 1993;64(8):2340. [19] Forman RA, Pirmarini GJ, Barnett JD, Block S. Science 1972;176:284. [20] Jianli Y, Savage PE. Ind Engng Chem Res 1998;37:2.
Extraction of Taiheiyo coal with supercritical water–HCOOH mixture T. Adschiri*, T. Sato, H. Shibuichi, Z. Fang, S. Okazaki, K. Arai Department of Chemical Engineering, Tohoku University, Aoba 07, Aramaki, Aoba-ku, Sendai 980-8579, Japan
Abstract Taiheiyo coals were extracted with supercritical toluene (SC-toluene) (653 K, 20 MPa), supercritical water (SCW) (653 K, 35 MPa) and formic acid (HCOOH)–SCW mixed solvents (653 K, 35 MPa) using a semi-batch type system. The results clearly indicate that the coal conversion ( 1 2 weight of residual coal (daf)/weight of loaded coal (daf)) and the liquid yield ( weight of liquid extracts/weight of loaded coal) in HCOOH–SCW were higher than those in SCW and SC-toluene. A considerable portion of coal is thus converted into light oils probably through hydrolysis and hydrogenation in HCOOH–SCW. We conducted another series of experiments for direct in situ observation of coal conversion in SCW and HCOOH–SCW using a diamond anvil cell (DAC). In HCOOH–SCW, the effective and rapid hydrogenation of coal with HCOOH occurs mainly at the early stage of the reaction. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Supercritical water; Coal conversion; Formic acid
1. Introduction and Supercritical water (SCW) (Tc 647 K Pc 22:1 MPa) is the so-called dense steam and thus can be made miscible with light gases such as H2, CO and O2 [1]. Above 3508C, water becomes miscible with oils and aromatics [2], since the dielectric constant of supercritical water is reduced from 2 to 20, which is similar to that of polar organic solvents at room temperature. We think the miscibility of water with H2, aromatics and oils provides a unique homogeneous reaction atmosphere for coal liquefaction. Since water becomes immiscible with gases and oils by the reduction of temperature and pressure, the process of the recovery of solvent and the separation of products after the liquefaction process would be simplified. In the coal liquefaction in SCW, various unique reactions take place, as described below. Supercritical water can provide a reaction atmosphere for the heterolytic reactions of many types of compounds even without any acid or base catalyst. Townsend et al. [3] studied the reaction of coal related chemicals in SCW at temperatures from 417 to 823 K and water density from 0.1 to 0.21 g/cm 3, and reported that compounds which had a heteroatom combined with the saturated carbon (dibenzylether, benzylphenylether, guaiacol, phenethylphenylether, etc.) were hydrolyzed under those conditions. Antal et al. [4,5] reported that ethylene and propane could be produced * Corresponding author. Tel.: 181-22-217-7246; fax: 181-22-217-7246. E-mail address: [email protected] (T. Adschiri).
through intramolecular dehydration of ethanol and propanol, respectively, in SCW at 668 K and 34.5 MPa. Adschiri et al. [6] reported that the hydrolysis of cellulose occurred rapidly in SCW without any catalysts. All of these results suggest that SCW itself works as an acid or base catalyst. The hydrolysis of coal has been reported in the literature [7–10]. A number of studies have been conducted on SCW extraction of coal. Li et al. [7] and Kershaw et al. [8,9] reported that coal conversion in SCW was higher than that in SC-toluene, and that water-soluble compounds such as alcohol and carboxylic acid were produced in SCW: they attributed this result to the hydrolysis at ether or ester bonds in coal. Adschiri et al. [10] conducted the extraction of Taiheiyo coal using SCW and SC-toluene and reported that coal conversion and liquid yields were higher in the SCW experiment and suggested the effect of hydrolysis as above. Some researchers have studied the hydrogenation of coals through a water–gas shift reaction (WGS reaction: CO 1 H2 O ! CO2 1 H2 ). Penninger [11] compared the reaction of Wyoming coal with a CO–SCW and a N2 – SCW mixture at 723 K and 7.1–10.9 MPa. They reported that the coal conversion (53.9 wt.% daf at 7.1 MPa) and the yields of aromatics and polar compounds were much higher in CO–SCW than in N2 –SCW. They attributed the results to formic acid formation through WGS reaction that is known to be an active hydrogen donor. A similar hydrogenation through the WGS reaction was reported for the extraction of coal with a CO–SCW mixture at 673 K and 21.9 MPa [12], and at 678 K and 14 MPa [13].
0016-2361/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00158-1
244
T. Adschiri et al. / Fuel 79 (2000) 243–248
Table 1 Proximate and ultimate analyses of Taiheiyo coal Proximate analysis (wt.% db) Ash VM 11.0 48.6
FC 40.4
Ultimate analysis (wt% daf) C H 75.8 6.26
N 1.27
S 0.25
O (diff.) 16.4
Although the hydrogenation with CO–SCW as previously reported is very attractive for liquefying coal, under these conditions 1:12 . T=Tc . 1:04; 0:99 . P=Pc . 0:32 SCW does not seem to work as an extraction solvent, since the density of SCW at these conditions is low (0.02–0.12 g/cm 3). If the reaction could be conducted in more dense SCW, a higher coal conversion could probably be obtained, because in general the solubility increases with increasing solvent density, and hydrolysis is promoted in high dielectric constant medium, namely in high-density SCW. We studied the catalytic hydrogenation of dibenzothiophene and naphthalene through the WGS reaction in dense SCW (0.42, 0.5 g/cm 3) [14,15], and found that the hydrogenation rate of dibenzothiophene to biphenyl or cyclohexylbenzene, and that of naphthalene to tetralin or decalin is higher in CO–SCW than in H2 –SCW. We also conducted experiments using HCOOH for the hydrodesulfurization of dibenzothiophene. The results showed the higher hydrogenation rate in the HCOOH–SCW atmosphere than that in the H2 –SCW atmosphere. This suggests that active hydrogenating species were formed through the WGS reaction. Melius et al. [16] studied the effect of water density on the activation energy of the WGS reaction by using MO calculation and found that the activation energy decreased with increasing water density. Their study implies
Fig. 1. Diamond anvil cell.
that formic acid might be the intermediate species of the WGS reaction. Despite the fact that HCOOH could be a candidate for an effective hydrogen donor in SCW, the extraction of coal with HCOOH has not been reported so far. As described earlier, for understanding the results of coal extraction with HCOOH–SCW, the effect of hydrolysis should also be taken into account. Thus, the comparison of the results in HCOOH–SCW and SCW is required. For the elucidation of coal liquefaction in SCW, an understanding of the phase in the reaction atmosphere is essential. Recently, we have employed a diamond anvil cell (DAC) for the direct observation of the reactions in SCW [17]. For cellulose decomposition in SCW, dissolution of cellulose particles could be directly observed. The first objective of this study is the direct observation of coal conversion in SCW and HCOOH–SCW with DAC. The second objective is to conduct the coal extraction experiments in SC-toluene, SCW and HCOOH–SCW under a rather dense phase and compare the results. 2. Experiment 2.1. Materials Taiheiyo coal was used in the experiments. Proximate and ultimate analyses of the coal are shown in Table 1. The particle size of the coal used was in the range from 16 to 24 mesh. For the DAC experiment, coal powder having an approximate diameter of 50 mm was used. Solvents used were 10 wt.% formic acid (purity 98%) aqueous solution, distilled water and toluene (purity 99.5%). 2.2. Procedures 2.2.1. Direct observation of coal conversion A DAC was used for the direct in situ observation of coal conversion in SCW and HCOOW–SCW. The DAC equipped with diamond windows used for this experiment had been developed by Bassett et al. [18]. Fig. 1 shows the schematics of the cell. The reactor was a space which was enclosed by upper and lower diamonds and a gasket hole (0.5 mm i.d., thickness: 0.25 mm). A heater and a thermocouple were attached around upper and lower diamonds. The reaction temperature was defined as the average of the two temperatures. Heaters were used for controlling the reaction temperature. During the experiment, argon gas with 1% hydrogen was introduced into the cell to prevent the diamond from oxidation. A piece of ruby for pressure measurement and coal powder were put into the gasket hole on the diamond. The solvent was then introduced into the gasket hole by using a microsyringe. The reactor was pressurized and the Raman spectrum shift of the ruby was measured to find the reaction pressure by the method of Forman et al. [19]. Heating was started and the inside of the reactor was continuously
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The reaction temperature was measured by a K-type thermocouple inserted in the coal bed. The time required to heat the reactor from room temperature to 633 K was extremely short. About 2 min was required for the reactor to reach 653 K from 633 K during which the coal conversion proceeded to some extent. The extraction was conducted for 120 min. The residence time of the extract with the solvent in the reaction was controlled by changing the flow rate of the solvent. The residence time was evaluated by Eq. (1). Fig. 2. Experimental apparatus for the coal extraction.
observed with a microscope (SZH10, Olympus) and was recorded with a CCD camera (KY-F55MD, Olympus) and a video camera. About 2.5 min was taken to heat up the reactor to the reaction temperature (653 K). The reaction pressure was about 85 MPa. After observation for 60 min, the cell was cooled and the pressure was reduced. The inside of the reactor was observed again. 2.2.2. Coal extraction A schematic diagram of the apparatus used for the coal extraction experiment is shown in Fig. 2. About 4 g of coal was loaded in the reactor (8.6 cm 3) which was made of SUS316 stainless steel. The coal bed was sandwiched with 200 mesh screen made of SUS316 to prevent the entrainment of coal particles. After the system was purged with N2, the solvent was fed into the reactor by a HPLC Pump (PUS-8, GL Science), at a feed rate from 0.2 to 14 cm 3/min (for water, HCOOH-water), from 0.05 to 10 cm 3/min (for Toluene). A back pressure regulator (Model 26-1721-24-071, Tescom) was used to control the pressure of the system. After the pressure and the flow rate became stable, the preheating tube and the reactor were submerged in a molten salt bath whose temperature had been controlled at 653 K, and thus the reactor was rapidly heated to the extraction temperature.
Fig. 3. Analysis procedure.
t
V 2 W=rcoal G=rf
1
where V (cm 3) was the volume of the reactor, W (g) was the weight of coal loaded, r coal (g/cm 3) was the density of coal and G (g/s) was the mass flow rate of solvent. Here, r f (g/ cm 3) is the density of fluid in the reactor and was approximated to be that of the pure solvent by using a steam table or data. The extract, along with the solvent, was cooled in a cooling unit. The pressure was reduced by the back pressure regulator and the produced gas was separated from the liquid effluent. The liquid effluent was recovered, and after an extraction time of 120 min, the reactor was taken out of the bath and then cooled. For the HCOOH–SCW experiments, the gases (CO, CO2 and H2) were produced from formic acid and thus the density of the mixed solution in the reactor might be lower than that in SCW atmosphere. In general, the solubility in a solvent decreases with reducing solvent density and thus that of HCOOH–SCW was suppressed to be lower than that of SCW. Thus, we conducted further extraction with only SCW at 653 K and 20 MPa for 30 min for recovery of the products after 120 min of the HCOOH–SCW experiments for comparison of the reactions. The residue was recovered from the reactor and the extract in the line was completely recovered by washing with tetrahydrofuran (THF). The procedure of analysis is shown in Fig. 3. The residue was dried at a reduced pressure of 483 K for 4 h and then weighed. The liquid products were distilled (333 K) and dried (483 K) at a reduced pressure and then weighed. This was defined as the extract. Then, 1 g of the extract was extracted with about 200 g of acetone at room temperature for 3 h, and separated into acetone soluble compounds (AS) and acetone insoluble compounds (AI). AS were further analyzed by gas chromatography–mass spectrometry (GC–MS) (Automass 20, JEOL) and gas chromatography–flame ionization detector (GC–FID) (HP-5890-II, Hewlett–Packard) while AI were weighed after drying. The compounds that were vaporized with water or THF during these procedures were recovered by a cold trap immersed in liquid nitrogen, and analyzed by GC–MS and GC–FID. These compounds and gases produced during the experiment were defined as volatile matter (VM).
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Fig. 4. Coal conversion in SCW at 653 K and 85 MPa.
Ash analysis was conducted for the residue and the extracts as follows. A small amount (0.6 g) of sample was combusted with air (100 cm 3/min) at 1088 K for 4 h in a tubular reactor (quartz glass, 60 × 2 cm2 i.d.) and the resultant ash was weighed. For the SCW and the HCOOH–SCW experiments, a portion of ash was found to be dissolved in the solvents, which affects the conversion of coal and the yield of extracts. By considering the ash content in the parent coal (a 0) and the residue (a ), the coal conversion and the yield were evaluated by the following equations, respectively. Conversion wt:% daf
Wcoal 1 2 a0 2 Wresidue 1 2 a × 100 Wcoal 1 2 a0
Yield wt:% daf
Wi × 100 Wcoal 1 2 a0
2
3
where Wcoal, Wresidue and Wi were the weight of parent coal, residue coal and each constituent of extracts, respectively.
3. Results and discussion 3.1. Direct observation of coal conversion Direct observation of coal conversion with DAC supplies information for the reaction with reaction time, which cannot be obtained in the semi-batch type extraction experiments. Fig. 4 shows the results of the DAC observed in SCW. The large particle was the ruby introduced for measurement of pressure in the reactor and the small particles were coal. Fig. 4(a)–(c) shows that with increasing reaction time, the coal particles gradually become small but retain their particle shape. At 60 min of reaction time, the diameter of coal particles became about 80% of the parent coal. For the HCOOH–SCW experiments, as shown in Fig. 5, a drastic change in the shape of coal particles was observed at 2.5 min of reaction time (during the preheating period), which is followed by a gradual change. This suggests a different reaction atmosphere from SCW. We also observed the solution after cooling, namely at reducing pressure. As shown in Fig. 4(d), in SCW, a lot of spheres were produced which were probably gas bubbles. However, as shown in Fig. 5(d), in HCOOH–SCW, a lot of unsymmetric spheres appeared, which probably indicate an
Fig. 5. Coal conversion in HCOOH–SCW at 653 K and 85 MPa.
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Fig. 6. Coal conversion at various residence times at 653 K and 20 MPa (SC-toluene), 35 MPa (SCW, HCOOH–SCW).
oil or liquid phase. By comparing Fig. 5(a) and (d), we found that the volume of the oil phase that appeared after the cooling and reducing pressure (Fig. 5(d)) was as large as the volume of the loaded coal particles (Fig. 5(a)) or rather larger. This indicates a very high conversion of coal to liquid oils in HCOOH–SCW. 3.2. Extraction experiments Fig. 6 shows the results of coal conversion at various residence times. As elucidated in our previous works, the results shown in this figure imply two stages of reaction; that is, the extraction of liquid products and the secondary reaction in the reactor. The slight change of conversion in SC-toluene and SCW suggests that the polymerization or recombination of the liquid products on the residual coals are not significant. However, for HCOOH–SCW, the coal conversion decreased down to 63 wt.% at 210 s residence time from 80 wt.% at 20 s. It is not reasonable that the polymerization takes place more in HCOOH–SCW, which is considered to be a more reactive atmosphere than SC-toluene or SCW. One reason might be the decomposition of HCOOH into H2 and CO2 gases in the preheating zone, especially at a longer residence time, as reported previously [20]. For checking this hypothesis, another series of experiments of HCOOH
Fig. 7. Yield of acetone soluble compounds (AS) at various residence times at 653 K and 20 MPa (SC-toluene), 35 MPa (SCW, HCOOH–SCW).
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reaction under the same conditions as the coal extraction experiments were conducted. The conversion of HCOOH at 21 s of residence time was less than 30%, but at 210 s of residence time, was almost 100%. Therefore, the results in the experiment with 210 s of residence time are considered to be those obtained in H2 –CO2 –SCW atmosphere. This implies that HCOOH has a higher activity for the hydroliquefaction than H2. For exclusively discussing the extractive reactions for these cases, the intercepts at zero residence time are compared. The coal conversion in SCW was higher than that in SC-toluene. As elucidated in the previous paper, this is probably because of the effect of hydrolysis on the liquefaction of coal. The conversion in HCOOH–SCW was the highest and reached 80 wt.%, which is consistent with the results of DAC experiment. Furthermore, both particle size and morphology of the coal particles changed greatly before and after the HCOOH–SCW experiments, although these changed little for SC-toluene and SCW experiments. These results suggest a significant reaction of HCOOH with coal. Fig. 7 shows the yield of AS with residence time; Fig. 8 is the yield of VM. The change of these yields with residence time is due to the reaction of products in the reactor. Fig. 7 shows that AS yield did not change significantly with residence time under these conditions. However, the yields are different among these three cases. In the SC-toluene experiment, the yield of AS was around several percent. For SCW, it ranged from 20 to 30%. For the case of HCOOH–SCW, it was highest and about 50%. Fig. 9 shows the GC–FID chromatogram of AS obtained at similar residence times. Peaks observed were mostly alkanes or alkenes whose carbon number ranged from 10 to 20. In these chromatograms, the left-hand side shows the lower molecular weight species. For HCOOH–SCW, the product distribution clearly shifted towards the lower molecular weight side compared with SC-toluene and SCW. As mentioned above, AS were about 50 wt.% in HCOOH– SCW. Thus, in HCOOH–SCW, a large amount of AS were obtained, and the AS were lighter components than those found in SCW or SC-toluene. As shown in Fig. 8, the yield of VM slightly increased with the residence time for all cases. This is because of the decomposition of liquid products into volatile matters in the reactor. Although the difference of the yield of VM was not clear, there was a tendency that the yield of VM in SCW was lower than that in SC-toluene and for that in HCOOH–SCW to be lower than in SCW. This is also consistent with the result observed by the DAC experiment. Higher conversion, higher AS yield and lower VM yield in SCW over that obtained in SC-toluene can be explained by considering the contribution of hydrolysis in the liquefaction of coal, as reported in previous work [10]. In the HCOOH–SCW experiments, the coal conversion was 80 wt.% and the AS yield was as high as 49 wt.%. Liquid products consisted of lower molecular weight than that in
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Fig. 8. Yield of volatile matter (VM) at various residence times at 653 K and 20 MPa (SC-toluene), 35 MPa (SCW, HCOOH–SCW).
SCW according to the AS analysis. Also in the DAC experiment, an extremely large amount of oil was observed after reaction in HCOOH–SCW. These results suggest that there is enhanced decomposition and fragmentation in the presence of HCOOH. However, in HCOOH–SCW, the VM yield was lowest among these three cases. Also in the DAC experiments, no gas phase was observed. These surprising results cannot be simply explained by enhanced hydrolysis with HCOOH. At the moment, we think that these results are because of the hydrogenation of coal with HCOOH. Hydrogen capping of radicals suppresses b-scission of radicals, which may be one of the reasons for the lower VM yield. 4. Conclusion We conducted a series of experiments for direct in situ observation of coal conversion in SCW and in HCOOH– SCW using a DAC. In SCW, the coal particles gradually became smaller, however, in HCOOH–SCW, the dramatic change of shape of coal particles was observed at 2.5 min of the reaction time. After cooling at a reducing pressure, in HCOOH–SCW, an oil phase appeared and its volume was as large as that of the loaded coal, which indicates high coal conversion to liquid oils. We have also conducted a study of coal extraction in SCtoluene (653 K, 20 MPa), SCW (653 K, 35 MPa) and HCOOH–SCW (653 K, 35 MPa) with a semi-batch type apparatus. In the extraction with HCOOH–SCW, the coal conversion (80 wt.% daf) and the yield of AS (49 wt.% daf) were much higher and the VM yield was lower than those in SC-toluene or in SCW, which was consistent with the results of the DAC experiment. The product distribution of AS clearly shifted towards lower molecular weight side compared with SC-toluene or SCW. Thus, it has been demonstrated that coal had been converted to the lighter oils probably through hydrolysis and hydrogenation in HCOOH–SCW.
Fig. 9. GC–FID chromatogram of acetone soluble compounds at 653 K, 20 MPa and residence time 10 sec (SC-toluene), 35 MPa and residence time 21 sec (SCW, HCOOH–SCW)).
Acknowledgements The authors wish to express their thanks to the grant in aid for New Energy and Industrial Technology Development Organization (NEDO), Scientific Research on Priority Areas (09450281) and to Idemitsu Kosan Co. Ltd. for supplying Taiheiyo coal.
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