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Energy & Fuels 2004, 18, 90-96
Nonisothermal Catalytic Liquefaction of Corn Stalk in Subcritical and Supercritical Water Chuncai Song,† Haoquan Hu,*,† Shengwei Zhu,† Gang Wang,† and Guohua Chen‡ Institute of Coal Chemical Engineering, Dalian University of Technology, 129 Street, Dalian 116012, People’s Republic of China, and Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China Received January 9, 2003. Revised Manuscript Received May 16, 2003
The pyrolysis of corn stalk was performed with thermogravimetry (TG), and its liquefaction was investigated in a semicontinuous apparatus with a nonisothermal fluid extraction technique, both with and without sodium carbonate (Na2CO3) as a catalyst. The results indicated that the main pyrolysis region of corn stalk is ∼500-650 K and the maximum rate occurs at ∼600 K on the differential thermogravimetry (DTG) curve. The presence of a catalyst has an obvious effect on the pyrolysis, especially in the temperature range of 550-650 K. When more than 1.0 wt % of catalyst was added, the DTG curve is altered greatly, from two peaks to one, for the catalyst, which has a greater effect on hemicellulose than on cellulose and lignin. Kinetic analysis shows that the activation energy in the main pyrolysis range (10-70 wt %) varies with different amounts of catalyst addition. The liquefaction conversion of corn stalk with 3 mL/min of water as a solvent at a pressure of 25 MPa, with or without the addition of 1.0 wt % of Na2CO3, is up to 95.7 and 95.4 wt %, respectively. The catalyst mainly improved the yield of bio-oil, from 33.4% without a catalyst, increasing to 47.2% with 1.0 wt % of Na2CO3. The catalyst has a positive effect on the liquefaction at relatively higher temperatures and can increase the yield of liquid product, as well as improve the quality of liquid product. More bio-oil and less gas can be obtained with a catalyst than that without a catalyst. A two-stage main reaction may occur for both the pyrolysis and liquefaction of corn stalk under the experimental conditions.
Introduction Recently, global environmental problems, such as global warming and acid rain, have become a serious issue. Biomass, as a renewable and potential resource (and its effective utilization), has been receiving considerable attention worldwide. Biomass resources are abundant in China, but they have not been used efficiently, especially in regard to the handling of agricultural biomass that is often burned on site, which not only causes environmental pollution but also is a waste of resources. Therefore, in China, it is of great significance to develop new technology to make rational and efficient use of biomass as an alternative energy for fossil fuels or to obtain chemicals. There are many conversion technologies for utilizing biomass, such as direct combustion processes, thermochemical processes, biochemical processes, and agrochemical processes. In thermochemical conversion processes, we may use gasification, pyrolysis, and direct liquefaction. More liquid products can be obtained using liquefaction than by other processes. Therefore, liquefaction can be considered to be one of the most common and effective routes for the conversion of biomass. Since Appell et al.1 liquefied wood chips into heavy oil * Author to whom correspondence should be addressed. E-mail address:
[email protected]. † Dalian University of Technology. ‡ The Hong Kong University of Science and Technology.
at ∼350 °C with high-pressure CO in the presence of sodium carbonate (Na2CO3) as a catalyst, much literature has appeared in the field of biomass liquefaction.2-7 Supercritical fluid extraction, as a relatively new extraction technique for the isolation of analytes from solid samples, is becoming widely used in the conversion of biomass. Various solvents, such as supercritical methanol,8 ethanol,9 acetone,10 tertbutyl alcohol,11 and water,12,13 have been applied in the liquefaction process of biomass. Among the selected solvents, water has received extensive attention because it is nontoxic, (1) Appell, H. R.; Fu, Y. C.; Friedman, S., Yavorsky, P. M.; Wender, I. Bur. Mines Rep. Invest. 1971, 7560. (2) McCartney, J. T. U.S. DOE Report No. LBL-350, Lawrence Berkeley National Laboratory, Berkeley, CA, 1980. (3) Yokoyama, S.; Ogi, T.; Koguchi, K.; Nakamura, E. Liq. Fuels Technol. 1984, 2, 115-163. (4) Minowa, T.; Kondo, T.; Sudirjo, S. T. Biomass Bioenergy 1998, 14, 517-524. (5) Yan, Y. J.; Xu, J.; Li, T. C.; Ren, Z. W. Fuel Process. Technol. 1999, 60, 135-143. (6) Demirbas, A. Energy Convers. Manage. 2000, 41, 633-646. (7) Demirbas, A. Energy Convers. Manage. 2001, 42, 1357-1378. (8) Demirbas, A. Energy Convers. Manage. 2002, 43, 2349-2356. (9) Miller, J. E.; Evans, L.; Littlewolf, A.; Trudell, D. E. Fuel 1999, 78, 1363-1366. (10) Caglar, A.; Demirbas, A. Energy Convers. Manage. 2001, 42, 1095-1104. (11) Goto, M.; Smith, J. M.; McCoy. B. J. Ind. Eng. Chem. Res. 1990, 29, 282-289. (12) Schutt, B. D.; Serrano, B.; Cerro, R. L.; Abraham, M. A. Biomass Bioenergy 2002, 22, 365-375. (13) Watanabe, M.; Inomata, H.; Arai, K. Biomass Bioenergy 2002, 22, 405-410.
10.1021/ef0300141 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/25/2003
Catalytic Liquefaction of Corn Stalk in Water
inexpensive, and easy to recycle; meanwhile, the dry step of biomass is unnecessary when water used as a solvent, which can involve feedstocks with large amount of moisture and therefore reduce the cost of management. Funazukuri et al. tested the liquefaction of lignin sulfonate with subcritical and supercritical water in a batch tube-type bomb reactor and observed that, at 673 K, much-higher oil yields were obtained with supercritical water than with pyrolysis in an argon atmosphere.14 Later, on the basis of the research, the effects of additives such as HCl, Na2CO3, or CO to a water solvent on oil yields were also studied, showing that molecular weight distributions of oils with the additives were similar to that with water, whereas the oil yields and the amount of residual solids were affected by the additives.15 Arai et al.16 proposed a new method to hydrolyze cellulose rapidly in supercritical water to recover glucose, fructose, and oligomers (cellobiose, cellotriose, cellotetraose, etc.). Cellulose decomposition experiments were conducted with a flow-type reactor in the temperature range of 290-400 °C at a pressure of 25 MPa. A high-pressure slurry feeder was developed to feed the cellulose-water slurries, and these procedures showed that hydrolysis product yields (∼75%) in supercritical water were much higher than those in subcritical water, showing that the hydrolysis of cellulose in supercritical water is a promising technology for converting cellulose to the hydrolysis products. Obtaining a deeper and clearer understanding of the mechanism regarding liquefaction is an important issue, because the components of biomass are very complicated and exhibit very different behaviors under different conditions. According to the published literature,1-20 isothermal liquefaction in batch or semibatch reactors has been a focus of attention for most of the investigators. In this work, however, a nonisothermal liquefaction in a semicontinuous apparatus will be performed. Compared to isothermal methods, the nonisothermal technique can give more-detailed information, such as how the yield of gas and liquid and the composition of the products vary with temperature in an individual run,21,22 which proves to be an effective and efficient way to study the process of biomass liquefaction. Until now, we have not found reports in the literature about the liquefaction of biomass with a nonisothermal method in a semibatch-type reactor. The liquefaction process of biomass in a solvent is, in fact, a thermal pyrolysis process in the presence of a (14) Funazukuri, T.; Wakao, N.; Smith, J. M. Fuel 1990, 69, 349353. (15) Funazukuri, T.; Cho, J. S.; Wakao, N. Fuel 1990, 69, 13281329. (16) Sasaki, M.; Kabyemela, B.; Malaluan, R.; Hirose, S.; Takeda, N.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 1998, 13, 261-268. (17) Dote, Y.; Inoue, S.; Ogi, T.; Yokoyama, S. Bioresour. Technol. 1998, 64, 157-160. (18) Kucuk, M. M.; Agirtas, S. Bioresour. Technol. 1999, 69, 141143. (19) Maldas, D.; Shiraishi, N. Biomass Bioenergy 1997, 12, 273279. (20) Wang, D. N.; Czernik, S.; Chornet, E. Energy Fuels 1998, 12, 19-24. (21) Song, C. C.; Hu, H. Q.; Liang, C. H.; Li, C. Presented at the 5th International Symposium on Green Chemistry, Hefei, China, May 23-26, 2002. (22) Song, C. C.; Hu, H. Q.; Wang, G.; Chen, G. H. Presented at the 17th International Symposium on Chemical Reaction Engineering, Hong Kong, China, August 25-28, 2002.
Energy & Fuels, Vol. 18, No. 1, 2004 91 Table 1. Analysis of Corn Stalk parameter
value
composition carbon hydrogen nitrogen oxygen sulfur chlorine H/C atomic ratio O/C atomic ratio ash content benzene/ethanol (2:1) extractable content hemicellulose content cellulose content lignin content
43.83 wt % 5.75 wt % 0.97 wt % 48.94 wt % 0.13 wt % 0.32 wt % 1.57 0.84 7.44 wt % dry 18.58 wt % daf 43.01 wt % daf 22.82 wt % daf 15.59 wt % daf
solvent; therefore, the general pyrolysis characteristics of corn stalk and its main components may give useful information and may be helpful for the further understanding of the process and mechanism in the following liquefaction. In this paper, corn stalk, which is a form of agricultural waste in northeast China, was used to investigate both the pyrolysis by thermogravimetry (TG) with different amounts of Na2CO3 addition and the liquefaction in subcritical and supercritical water with a catalyst (1 wt % Na2CO3) or without any catalyst with a nonisothermal technique. Experimental Section Raw Materials. Crushed and sieved air-dried corn stalk, obtained from a Dalian suburb in northeast China, and the main components of biomass, including microcrystalline cellulose (from Alfa Aesar), low-sulfonate lignin (from Aldrich), and hemicellulose that had been replaced with poly(beta-Dxylopyranose) (xylan) (from Sigma) were used as the test samples in this study. A specific amount of catalyst was dissolved in distilled water and added into the test samples for 2 h and then dried for thermogravimetric analysis. The chemical and elemental compositions of corn stalk are shown in Table 1. Pyrolysis. The pyrolysis of biomass was performed in a thermogravimetric analyzer (Mettler-Toledo model TGA/ SDTA851e). Approximately 6 mg of sample was placed in an Al2O3 ceramic pan. The sample was heated from ambient temperature to 873 K at a constant heating rate (10 K/min). Purified nitrogen at a constant flow rate of 20 mL/min was used as the purge gas to provide an inert atmosphere for pyrolysis and to remove any gaseous and condensable products that evolved, thus minimizing any secondary vapor-phase interactions. The sample weight was recorded as a function of time or temperature. Liquefaction. The liquefaction of biomass was performed in a semicontinuous apparatus with a nonisothermal fluid extraction technique. Details has been provided in our earlier publications.23,24 Briefly, ∼15 g of sample of biomass was placed into a fixed-bed reactor that could be heated by an electrical oven. Water (with 1.0 wt % of Na2CO3 or without catalyst) was fed into the reactor continuously by a high-pressure metering pump. During the experiment, the pressure (25 MPa) and solvent flow rate (3 mL/min) were held constant, whereas the reactor system (including pressurized feed water) was slowly heated from the initial room temperature to the final temperature of 683 K at a constant heating rate (10 K/min). Samples were taken at definite time intervals or at a given temperature value from the outwardly flowing solution that contained (23) Hu, H. Q.; Zhang, J.; Guo, S. C.; Chen, G. H. Fuel 1999, 78, 645-651. (24) Missal, P.; Hu, H. Q.; Guo, S. C. Erdoel, Erdgas, Kohle 1992, 108, 279-283.
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Song et al.
Figure 1. Weight loss and weight loss rate, versus temperature, for Na2CO3 pyrolysis.
Figure 2. TG and DTG curves of corn stalk. solvent, liquid product, and gas. The composition of the gas samples was analyzed using a gas chromatograph (Model GC920, with a thermal conductivity detector). The liquid product was separated into two fractions: one is soluble in tetrahydrofuran (THF) but insoluble in cyclohexane, which was defined as asphaltene (hereafter abbreviated as Asp), whereas the other is soluble in both of the aforementioned solvents and was defined as bio-oil. Residues were dried and weighed to determine the liquefaction conversion.
Results and Discussion Pyrolysis of Biomass. To understand the thermal behavior of Na2CO3, the pyrolysis of Na2CO3 was conducted under the same conditions as those for biomass pyrolysis. Figure 1 shows the weight loss and weight loss rate of Na2CO3 at a heating rate of 10 K/min during the pyrolysis. The figure shows that, except for a moisture weight loss of ∼2% near 373 K, there is no obvious weight loss with further increases in temperature. This means that the Na2CO3 is stable in the temperature range studied in this work, and its effect on the calculation of weight loss in the following experiments can be eliminated by subtracting the quantities of catalyst according to the amount added in the sample. Figure 2 shows thermogravimetry (TG) and differential thermogravimetry (DTG) curves of corn stalk without any catalyst. The main pyrolysis region of corn stalk is observed to be ∼500-650 K. There are two peaks on the DTG curve, at 572 and 614 K, and a long tail is observed at high temperatures.
Figure 3. (a) TG and (b) DTG curves of hemicellulose, cellulose, and lignin.
Comparison of the TG and DTG curves of hemicellulose, cellulose, and lignin, shown in Figure 3, indicates that the first peak on the DTG curve of corn stalk is mainly attributed to hemicellulose, the second peak is attributed to cellulose, and the long tail corresponds to the thermal decomposition of lignin. Figure 4 shows the catalytic effect on the TG and DTG curves with Na2CO3 additions of 0.5, 1.0, 3.0, and 10 wt %. Comparison with the result without catalyst addition indicates that the catalyst has an effect on the pyrolysis of corn stalk at temperatures above ∼470 K, especially in the range of 550-650 K. The catalyst increases the final weight loss of corn stalk. In samples without a catalyst, the weight loss is 62.4 wt %, whereas a weight loss of 69.9 wt % is observed with the addition of 1 wt % of Na2CO3. The catalyst also alters the temperature at which the maximum weight loss rate occurs on the DTG curve, from 620 K with 0.5 wt % of Na2CO3 to 565 K with 10 wt % of Na2CO3. Generally speaking, when the catalyst addition is >1.0 wt %, the shape of the DTG curve is greatly changed, from two peaks to only one peak, meaning that the catalyst has a greater effect on hemicellulose than on the other main fractions of biomass. The detailed mechanism of catalytic effect on the components needs to be studied further. Determination of Kinetic Parameters. An understanding of pyrolysis kinetics is necessary to design a suitable pyrolysis reactor and obtain a better insight of
Catalytic Liquefaction of Corn Stalk in Water
Energy & Fuels, Vol. 18, No. 1, 2004 93
usual thermodynamic meanings.) For a constant heating rate of β,
β)
dT dt
(3)
the combination of eqs 1, 2, and 3 yields the expression
E A E dR A ) exp f(R) ) exp (1 - R)n dT β RT β RT
(
)
(
)
(4)
Through rearrangment and integration, the following equations can be deduced:17
Figure 4. (a) TG and (b) DTG curves of corn stalk with different amounts of Na2CO3 addition.
the effective catalyst or study the mechanisms of pyrolysis or liquefaction of biomass. Kinetics studies, with respect to biomass pyrolysis that involves different reaction models, were performed by many investigators.25-30 The Coats-Redfern method26 has been widely used because it is simple and easy to use with data obtained from the thermogravimetric analyzer. Therefore, in this work, the Coats-Redfern method was applied to calculate the kinetic parameters of biomass pyrolysis. In the pyrolysis reaction of solid material, the rate of decomposition can be expressed by the relation
dR ) kf(R) dt
(1)
where R is the decomposed fraction at time t (R ) (m0 - m)/(m0 - m∞) × 100%) and k is the rate constant given by the expression
(
k ) A exp -
E RT
)
(2)
where A is a pre-exponential factor and E is the activation energy of the reaction. (R and T have their (25) Ferdous, D.; Dalai, A. K.; Bej, S. K.; Thring, R. W. Energy Fuels 2002, 16, 1405-1412. (26) Coats, A. W.; Redfern, J. P. Nature 1964, 201, 68-69. (27) Demirbas, A. Bioresour. Technol. 1998, 66, 247-252. (28) Lanzetta, M.; Di, B. C. J. Anal. Appl. Pyrolysis 1998, 44, 181192.
ln
(
ln
[
) [
- ln(1 - R) T
2
) ln
2
T (1 - n)
(
] [
1 - (1 - R)1-n
)]
AR 2RT E 1βE E RT (for n ) 1) (5)
) ln
)]
AR 2RT E 1βE E RT
(
(for n * 1) (6)
For most values of E and for the temperature range over which the reactions generally occur, the expression ln{AR/(βE)[1 - (2RT/E)]} is sensibly constant. Thus, a plot of either ln[(- ln(1 - R)/T2)] against 1/T for n ) 1 or ln[(1 - (1 - R)1-n/T2(1 - n))] against 1/T for n * 1 should result in a straight line of slope -E/R for the correct value of n. At temperatures 1.0 wt % of catalyst was added, the differential thermogravimetry (DTG) curve is altered greatly, from two peaks to one, showing that the catalyst has a greater effect on hemicellulose than on cellulose and lignin. Kinetic analysis shows that the activation energy in the main pyrolysis range varies with different amounts of catalyst addition. Liquefaction of corn stalk in water shows that the catalyst has a positive effect on the liquefaction at relatively higher temperature and can increase the yield
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Energy & Fuels, Vol. 18, No. 1, 2004
of liquid product, as well as improve the quality of liquid product. With a catalyst, more bio-oil and less gas can be obtained than that without using a catalyst. A twostage main reaction may occur for both the pyrolysis and liquefaction of corn stalk under the experimental conditions. Conversions up to ∼95 wt % (in dry basis) and a 77% liquid product yield of biomass studied can be achieved by liquefaction in water at a pressure of 25 MPa, which
Song et al.
shows that water is an effective and potential solvent or reactant for biomass liquefaction. Acknowledgment. The authors are grateful for financial support by the Specialty Scientific Research Fund for Doctoral Award Unit in Chinese University (No. 2000014115). EF0300141
