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Energy & Fuels 2008, 22, 3944–3948
Extraction of Weakly Reductive and Reductive Coals with Sub- and Supercritical Water Bo Wu, Haoquan Hu,* Shiping Huang, Yunming Fang, Xian Li, and Meng Meng State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, Dalian UniVersity of Technology, 129 Street, Dalian 116012, People’s Republic of China ReceiVed April 24, 2008. ReVised Manuscript ReceiVed August 28, 2008
On a semi-continuous apparatus, a weakly reductive Shenfu-Dongsheng (SD) coal and a reductive Pingshuo (PS) coal were non-isothermally extracted with sub- and supercritical water to explore the differences between the two coals. The effect of the temperature on the extract formation rate, conversion, and product composition under different pressures was investigated. The extraction results of two coal samples indicate that the extract formation rate has a maximum in the studied temperature range between room temperature and 500 °C. The temperature corresponding to the maximum extract formation rate, changing with the pressure, is between 390 and 410 °C. The gas yield, extract yield, and conversion of two coals increase with the increasing pressure. In comparison to PS coal, SD coal has a low temperature corresponding to the maximum extract formation rate under the same pressure. Both coals have a main fraction of asphaltene, but SD coal has a higher fraction of oil than PS coal. The main gas components are CO2, CH4, and H2. The gas from PS coal has a higher CH4 content and lower CO2 content than that from SD coal. The analysis results of the extraction residue indicated that SD coal has a low residue yield and the residue shows a large surface area and small average pore diameter compared to PS coal.
1. Introduction Coal has a great potential to be a substitute for petroleum as liquid fuels and chemical feedstocks. Various coal conversion technologies, such as liquefaction, pyrolysis, and gasification have been developed. In addition to conventional technologies, some new processes,1 such as flash pyrolysis, liquid-phase oxidation at low temperature, and solvent extraction are currently being developed. The supercritical extraction of coal with water for its interesting physical and physicochemical properties2,3 has received much attention.4-11 Two aspects were mainly concentrated on: one is the extraction with pure water, and another is the extraction with water and another solvent. Cheng et al.4 studied in detail the effect of the temperature, water density, and reaction time on gas yield, liquid yield, and product composition during the conversion of a lignite in an autoclave with sub- and supercritical water. They also clarified the role of water in conversion of coal. In the studies on extraction with * To whom correspondence should be addressed. Telephone/Fax: +86411-88993966. E-mail:
[email protected]. (1) Miura, K. Fuel Process. Technol. 2000, 62, 119–135. (2) Jessop, P. G.; Leiter, W. Chemical Synthesis Using Supercritical Fluids; Wiley-VCH: New York, 1999; pp 37-41. (3) Savage, P. E. Chem. ReV. 1999, 99, 603–621. (4) Cheng, L. M.; Zhang, R.; Bi, J. C. Fuel Process. Technol. 2004, 85, 921–932. (5) Hu, H. Q.; Guo, S. C; Hedden, K. Fuel Process. Technol. 1998, 53, 269–277. (6) Adschiri, T.; Sato, T.; Shibuichi, H.; Fang, Z.; Okazaki, S.; Arai, K. Fuel 2000, 79, 243–248. (7) Aida, T. M.; Sato, T.; Sekiguchi, G.; Adschiri, T.; Arai, K. Fuel 2002, 81, 1453–1461. (8) Matsumura, Y.; Nonaka, H.; Yokura, H.; Tsutsumi, A.; Yoshida, K. Fuel 1999, 78, 1049–1056. (9) Luik, H.; Luik, L. Energy Sources 2001, 23, 449–459. (10) Iino, M.; Takanohashi, T.; Li, C. Q. Energy Fuels 2004, 18, 1414– 1418. (11) Kershaw, J. R. Fuel 1997, 76, 453–454.
the mixtures of water and other solvents, the chosen solvent included phenol, formic acid, etc. Adschiri et al.6 tried to examine the effect of formic acid on coal conversion reactions by extraction of Taiheiyo coal with supercritical water/formic acid mixtures. In coal classification, some researchers divided coal into two types: reductive and weakly reductive coals, according to the coalification condition. The weakly reductive coal, formed in inland gathering environment and subjected to a strong oxidative but weakly reductive effect, has usually high-inertinite, lowash, and low-sulfur contents and shows a different performance in conversion compared to reductive coal. China is rich in weakly reductive coal reserve. Especially in the western region of China, there are big coal mines, such as the Shenfu coal mine, which produce weakly reductive coal. Some studies on their reactive characteristics in pyrolysis, gasification, combustion, etc. have been carried out. However, the difference in extraction performances between reductive and weakly reductive coals with sub- and supercritical water is still unknown. The purpose of this work is to study the extraction characteristics of ShenfuDongsheng (SD) coal, a typical weakly reductive coal, in suband supercritical water in comparison to that of Pingshuo (PS) coal, a typical reductive coal. 2. Experimental Section 2.1. Coal Samples. A SD coal from the coal field across Shaanxi Province and Inner Mongolia Autonomic Region and a PS coal from Shanxi Province were used in this study. The former coal was typically classified as a weakly reductive coal, and the latter coal was typically classified as a reductive coal. The analysis results of the samples are shown in Table 1. 2.2. Thermogravimetric (TG) Analysis of Coal Samples. The pyrolysis behaviors of two coal samples were investigated by using a TG analyzer (Mettler Toledo TGA/SDTA851e). About 15 mg of
10.1021/ef8002872 CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
Extraction of PS and SD Coals
Energy & Fuels, Vol. 22, No. 6, 2008 3945
Table 1. Proximate, Ultimate, and Petrographical Analyses of Coal Samples proximate analysis (wt %)
ultimate analysis (wt %, daf)
petrographical analysis (wt %, d)
sample
Mad
Ad
Vdaf
C
H
N
S
Oa
V
I
E
M
PS coal SD coal
2.23 9.80
17.93 4.50
37.19 33.72
80.41 79.53
5.20 4.16
1.38 0.91
1.06 0.48
11.95 14.92
44.6 40.1
42.1 56.8
7.2 0.4
6.1 2.7
a
By difference.
coal sample was placed in a ceramic crucible and pyrolyzed in 60 mL/min N2 flow at a heating rate of 10 °C/min from 25 to 900 °C. 2.3. Extraction of Coal Samples. The extraction of coal was carried out with a non-isothermal procedure in a semi-continuous apparatus.5,12 In each run, about 25 g (daf) of coal sample was placed in a fixed bed reactor heated by an electric oven outside. The water was pumped into an electric heater and then to the reactor by a high-pressure-metering pump. The reactor system and the entering pressurized water were slowly heated from room temperature up to 500 °C. The water flow rate of 6.5 mL/min, the heating rate of 4 °C/min, and the pressure was held constant during the extraction. At different temperature intervals, liquid samples were collected with tetrahydrofuran (THF) to dissolve the extract products stuck on the wall of the condenser after depressurization in time, and the gas product was collected in a gasbag. The extraction residue was obtained from the fixed bed reactor after extraction. 2.4. Analyses of Products. The extract was obtained by removing the water and THF from the liquid sample with a rotary evaporator. Subsequently, the extract was separated into oil (cyclohexane soluble), asphaltene (toluene soluble but cyclohexane insoluble), and pre-asphaltene (THF soluble but toluene insoluble) by Soxhlet extraction with cyclohexane, toluene, and THF, respectively. Some obtained oil fractions were characterized with 1H nuclear magnetic resonance (NMR) on a Bruker Avance II 400 M NMR spectrometer. The extract formation rate at temperature Tj was calculated as the ratio of the amount of extract in the time interval to the coal weight. The composition of gaseous products was analyzed by GC 7890 with a thermal conductive detector. The Fourier transform infrared (FTIR) spectra of the coal samples and extraction residues were measured on an EQUINOX55 spectrometer using the KBr-pellet technique. Nitrogen adsorption of coal samples and extraction residues was performed with an Autosorb-1 adsorption analyzer (Quantachrome Instruments) at -196 °C. Prior to the adsorption measurement, the sample was degassed at 200 °C for at least 6 h. The coal conversion and the yield of extract, gas, and residues are calculated by
Ey (%) ) WE/WC × 100 Gy (%) ) WG/WC × 100 Ry (%) ) [(WR - WA)/WC] × 100 C (%) ) 100 - Ry where WE, WG, WR, and WA are the weight of extract, gas, residue, and ash, respectively, WC is the weight of coal sample, on the dried ash-free basis, and Ey, Gy, Ry, and C are the yield of extract, gas, residue, and coal conversion, respectively, in the coal extraction process, all on the dried ash-free basis.
3. Results and Discussion 3.1. FTIR Analysis of Coal Samples. Figure 1 shows the FTIR spectra of two coal samples. In comparison to PS coal, SD coal has strong adsorption bands of O-H stretching vibrations (3600-3200 cm-1), CdO stretching vibrations (1700-1650 cm-1), aromatic ring vibrations (1650-1550 and 900-650 cm-1), and R-O-R (1300-1200 cm-1). The CdO groups (1700-1650 cm-1) of SD coal are in conjunction with (12) Hu, H. Q.; Zhang, J.; Guo, S. C.; Chen, G. H. Fuel 1999, 78, 645– 651.
Figure 1. FTIR spectra of PS and SD coal samples.
Figure 2. TG/DTG curves of PS and SD coal pyrolysis in N2.
the aromatic ring, while the CdO groups (1750-1700 cm-1) of PS coal are not conjugated with the aromatic ring.13 Because of higher ash content, the FTIR spectrum of PS coal obviously shows strong adsorption bands of kaolinite (3700-3600 cm-1) and silicates (1100-1000 and 550-460 cm-1).14 3.2. TG Analysis of the Coal Sample. TG/DTG curves of two coal samples are shown in Figure 2. It can be seen that the pyrolysis of coal samples is divided into two steps. The first step is below 350 °C, where water is lost with volatilization of some light substances from coal. The second one is at a temperature range between 350 and 600 °C, where the relatively heavy organic substances in coal crack and a deep peak of the weight loss rate appears at about 450 °C. To obtain a high extract yield, coal cracking is necessary; therefore, 500 °C was selected as the ending temperature in sub- and supercritical water extraction. (13) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X. H.; Chan, W. G.; Hajaligol, M. R. Fuel 2004, 83, 1469–1482. (14) Zhuo, Y.; Lemaignen, L.; Chatkazis, I. N.; Reed, G. P.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2000, 14, 1049–1058.
3946 Energy & Fuels, Vol. 22, No. 6, 2008
Figure 3. Relations between the extract formation rate and temperature at different pressures.
In comparison to PS coal, SD coal presents different pyrolysis performances, as shown in Figure 2. Besides higher weight loss, another peak of the weight loss rate of SD coal appears at the temperature between 600 and 700 °C, which is considered to be the second reaction of the pyrolysis product and the mineral matter decomposition. 3.3. Effect of the Temperature and Pressure on the Water Extraction of Coal. Temperature and pressure are the important factors to affect the state of solvent and pyrolysis process of coal. The experiments were carried out with a nonisothermal technique from room temperature up to 500 °C at different pressures to explore their effects on the water extraction of coal. Figure 3 shows the relations between the extract formation rate and temperature at different pressures for two coal samples. As shown in Figure 3, only a little extract can be obtained at the temperature below 300 °C. Extraction of coal mainly takes place in the temperature range between 350 and 450 °C, which is consistent with the results from TG/DTG analysis. With an increase of temperature, the extract formation rate increases and reaches a maximum at a temperature between 390 and 410 °C and then decreases with further raising temperature. It is thought that the phenomenon is the result of two opposite effects of temperature on the pyrolysis of coal and solubility of solvent. At low temperature, a high dielectric constant of water results in low solubility of water to organic substances in coal;15 (15) Weingartner, H.; Franck, E. U. Angew. Chem., Int. Ed. 2005, 44, 2672–2692.
Wu et al.
therefore, little organic substance can be extracted. Moreover, the pyrolysis of coal is still at the beginning, and little light substance is formed. With an increase of the temperature, the dielectric constant of water decreases and its solubility to organic substances accordingly increases. Meanwhile, large molecular substances in the coal sample decompose to form smaller molecular substances, which can be extracted by water. All of these make the extract formation rate increase. However, with a further increasing temperature, the density of water will decrease and its extraction capacity accordingly decreases. At about 390-410 °C, the opposite effects of temperature on the extract formation rate can balance each other and, consequently, a peak value is obtained. Pressure affects the solubility of the substances in the solvent and then the extraction yield. The results at three different pressures (20, 25, and 30 MPa) are presented in Table 2. It can be seen that the extract yield of PS and SD coals increases from 16.1 and 12.5 to 21.3 and 16.3 wt % daf, respectively, with increasing pressure from 20 to 30 MPa. SD coal shows lower extract yield than PS coal. The analyses of extracts at different pressures show that both coals have a main fraction of asphaltene, but SD coal has a higher fraction of oil than PS coal. From the hydrogen distributions of the oil fraction from extraction of two coals at 30 MPa, as shown in Table 3, the oil fraction from SD coal contains a lower content of aliphatic H at carbon atoms bonded to other aliphatic carbon atoms but more content of aliphatic H adjacent to aromatic alkene groups than that from PS coal. At different pressures, the effect of the temperature on the extract formation rate as shown in Figure 3 exhibits a similar trend, but the location and value of the peak are different. In comparison to SD coal, PS coal shows a high temperature corresponding to the maximum formation rate (Tmax) and high peak value of the extract formation rate (EFRmax). When the pressure is increased from 20 to 30 MPa, the Tmax increases from 395 to 408 °C and the EFRmax increases from 5.87 × 10-5 to 20.15 × 10-5 s-1 for PS coal, while the Tmax increases from 390 to 403 °C and the EFRmax increases from 5.43 × 10-5 to 16.35 × 10-5 s-1 for SD coal, respectively. It can also be seen that, in comparison to that at a pressure of 20 MPa, more extract will be formed at a high temperature region at a pressure of 25 and 30 MPa. This indicates that relatively bigger molecules decomposed from coal at higher temperatures can be extracted at higher pressure. For a comparison between extraction and pyrolysis of coal, some experimental data obtained from coal pyrolysis in a vertical fixed bed reactor at different temperature under atmospheric pressure of nitrogen was also presented, as shown in Table 4.16 Obviously, coal pyrolysis shows a lower liquid yield and higher reaction temperature than coal extraction (see Table 2), indicating that sub- and supercritical water extraction can promote coal pyrolysis and obtain a high liquid yield at a relatively low temperature. During the extraction, the gas yield is about 5 wt % for PS coal and 8 wt % for SD coal, as shown in Table 2. The gas yield increases with the increasing pressure, although the increment is very small. In contrast with the extract yield, SD coal shows a higher gas yield than PS coal. Table 5 shows the composition of gas obtained from different temperature ranges at different pressures. It can be seen that neither C2H6 nor C2H4 appears below 325 °C at experimental pressure. With an increase of the temperature, more H2, CH4, and C2 are produced from (16) Huang, S. P.; Hu, H. Q.; Wu, B.; Zhao, Y. P.; Zhu, S. W.; Meng, M. The 2007 International Conference on Coal Science and Technology, Nottingham, U.K., Aug 28-31, 2007.
Extraction of PS and SD Coals
Energy & Fuels, Vol. 22, No. 6, 2008 3947 Table 2. Results of Coal Extraction with Sub- and Supercritical Watera
sample
P (MPa)
C (wt % daf)
Gy (wt % daf)
Ey (wt % daf)
oil in extract (wt %)
asphaltene in extract (wt %)
pre-asphaltene in extract (wt %)
20 25 30 20 25 30
25.7 26.3 27.2 31.1 32.1 33.0
4.8 5.2 5.7 8.2 8.3 8.6
16.1 17.9 21.3 12.5 14.3 16.3
4.88 6.87 4.66 6.27 20.70 11.25
71.96 70.91 57.84 48.24 46.72 34.77
23.16 22.22 37.50 45.49 32.58 53.98
PS coal SD coal
a Fixed conditions: coal sample weight, 25 g (daf); particle size,
