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APPLIED CHEMISTRY Gasification of Cellulose, Xylan, and Lignin Mixtures in Supercritical Water Takuya Yoshida*,† and Yukihiko Matsumura‡ Department of Chemical System Engineering and Environmental Science Center, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
We examined the gasification of cellulose, xylan, and lignin mixtures in supercritical water at 623 K and 25 MPa. Our results indicate that the lignin content significantly affects the amount and composition of the product gas. Thus, we surmised that cellulose or xylan is likely to function as a hydrogen donor to lignin. A set of equations developed to estimate the amount and composition of the product gas accurately predicted the actual results using only the lignin fraction as a parameter. This confirmed the importance of the lignin fraction effect on supercritical water gasification characteristics. Introduction Supercritical water gasification is a promising technology for the gasification of biomass with a high moisture content. The use of water as a reaction medium obviates the need to dry the feedstock and allows for a high reaction rate. Many researchers have investigated supercritical water gasification. In the earliest studies, conducted to acquire a fundamental understanding of supercritical water gasification, Yu et al.1 successfully gasified glucose in supercritical water, and Xu et al.2 succeeded in gasifying a 1.2 M glucose solution using carbonaceous catalysts. In Japan, Minowa et al.3-5 gasified cellulose in hot compressed water (473-673 K, 8-22 MPa) with a nickel catalyst. Their carbon gasification ratio reached 70%, demonstrating the potential feasibility of producing hydrogen from biomass. Funazukuri et al.6 and Yokoyama et al.7 decomposed lignin in supercritical water (at 673 K) and obtained conversion to gas of up to 15 wt %.6 Yokoyama et al. also carried out the experiment in supercritical methanol and reported that methanol inhibited char formation by acting as a hydrogen donor. In experiments to gasify actual biomass in supercritical water, Xu et al.8 achieved a carbon gasification ratio as high as 98% by gasifying sewage sludge and poplar wood sawdust mixed in corn starch paste at 923 K and 28 MPa. Antal et al.9 reported the possibility of supercritical water gasification based on their trial for continuous operation of a flow reactor. Elliot et al.10-15 successfully decomposed cheese whey and other compounds using a nickel catalyst. To develop a model for * Author to whom correspondence should be addressed. Tel.: +81-3-5841-2994. Fax: +81-3-3813-7294. E-mail:
[email protected]. † Department of Chemical System Engineering. ‡ Environmental Science Center. Present affiliation: Department of Mechanical System Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima shi, Hiroshima 739-8527.
supercritical water gasification, it is essential to understand the interactions among the three main components of biomass, i.e., cellulose, hemicellulose, and lignin. However, this kind of research has not yet been carried out. Studies on the interactions of biomass components will help to clarify the reaction mechanism occurring in biomass gasification and predict the product gas components. This study was conducted to elucidate the interactions among the three main components of biomass and to develop a method for estimating the gas product from biomass gasification. Apparatus and Experimental Procedures Figure 1 shows the experimental setup employed in this work. The reactor was a batch-type device made of 316 stainless steel tubing with an outer diameter of 9.53 mm and an inner diameter of 6.53 mm. A total of 0.1 g of reactant mixture and 0.04 g of nickel catalyst (Ni5132P, Engelhard) were well mixed and loaded into the reactor with water. The amount of water was determined by a preliminary test so that a partial pressure of water of 25 MPa was attained. Helium gas was also loaded to replace air in the reactor. The reaction was initiated by immersing the reactor in a molten salt bath at the reaction temperature. The reaction temperature was reached inside the reactor in 100 s. After 20 min, the reactor was taken out of the molten salt bath and immediately immersed in a water bath to terminate the reaction. The reactor temperature reached ambient temperature in 60 s. A longer reaction time was also attempted, but the results were the same as those from the 20-min reaction. The temperature and pressure in the reactor were measured with a type-K thermocouple and a pressure gauge attached to the reactor, respectively. Product gas was collected in a bottle through a gas sampling system, as shown in Figure 1, by the following procedure. After the reactor had been cooled, it was joined to the end of the sampling system. The system was evacuated through valve 2. When the
10.1021/ie0101590 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001
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Figure 1. Experimental apparatus (a) and gas sampling system (b).
weighted by the weight fraction of each component
Table 1. Analysis Data of Lignin component
wt %
C H N S ash
47.0 5.2 0.2 2.9 17.0
pressure in the system reached a proper pressure (around -750 mmHg), valve 2 was closed. Then, gas collection was started by opening valve 1. The amount of product gas was calculated by measuring the pressure increase in the gas sampling system upon introduction of the product gas. Changing the flask, and thus the total volume of the sampling system, allowed us to attain the proper pressure difference regardless of the amount of product gas. The gaseous products were analyzed using a gas chromatograph (Shimazu, GC14B) equipped with flame ionization and thermal conductivity detectors. The column used was of the type Shincarbon T (Shimazu). Hydrogen analysis was performed using nitrogen carrier gas with a column temperature starting at 343 K, followed by a 10 K/min ramp to 363 K, and an 18-min hold at 363 K. Helium carrier was used for analysis of other gases with the following temperature differences: 3 min at 343 K, followed by a 10 K/min ramp to 423 K, another ramp at 25 K/min to 573 K, and a 23-min hold at 573 K. Liquid and solid products were also formed and collected, but this paper reports only the experimental results for the gas product. In this work, the reaction temperature was 673 K, and the pressure rose to around 26-29 MPa depending on the reaction. The reactants used in this experiment were cellulose (cellulose powder MN100, Machery Nagel), xylan (Sigma Chemical Co.) as a model material for hemicellulose, and lignin (Kanto Chemicals). Table 1 shows the results of a CHNS analysis of the composition and ash content of the lignin. Results and Discussion Gasification Properties. Figures 2 and 3 show the compositions of the product gases from the gasification of cellulose-lignin and xylan-lignin mixtures, respectively. The yield of C3H8 was negligible. If there is no interaction between cellulose and lignin (and xylan), the gasification behavior of mixtures can be predicted as an average of the results of single-component gasification
Nwa ) NcXc + NxXx + NlXl
(1)
where Nwa is the weighted average; Nc, Nx, and Nl are values for the single-component gasifications of cellulose, xylan, and lignin, respectively; and Xc, Xx, and Xl are the weight fractions in the mixture of cellulose, xylan, and lignin, respectively. In the case of binary mixtures, eq 1 indicates a linear dependence of Nwa on the weight fraction of a component. Comparisons of the experimental values and weighted averages are shown in Figures 2 and 3. In the results for H2 and CH4, there are large differences between the experimental values and weighted averages. The ratio of the experimental value to Nwa reaches a minimum around a cellulose fraction of 0.5. On the other hand, the experimental value agrees much better with Nwa for CO2 production than for the production of H2 and CH4. Thus, whereas the interaction between cellulose and lignin has little effect on the production of CO2, it causes significant decreases in the production of H2 and CH4. Minowa et al.3 suggested that the methanation reaction
CO + 3H2 f H2O + CH4
(2)
plays an important role in cellulose gasification in hot compressed water. In our results, we surmise that the decreased H2 production inhibited the methanation reaction. Figures 2d and 3d show the total numbers of hydrogen atoms in the product gas. The shapes of the curves shown in Figures 2d and 3d resemble those for CH4 (Figures 2c and 3c). This similarity also supports the inhibition of methanation, if other main methane production routes do not exist. The decrease in H2 production might be associated with the hydrogenation of lignin, a phenomenon reported by several researchers. Meier et al.16 conducted hydropyrolysis of lignin with Ni and other catalysts in a temperature range of 635-723 K. Connors et al.17 and Kudsy et al.18 proposed a hydrogenation mechanism of lignin model compounds. Methane and phenolic compounds were the product materials. Figure 4 compares experimental results with equilibrium values of gas compositions from several cellulose-lignin mixtures. The equilibrium values were calculated with STANJAN19 using the compositions of the product gas and the amounts of water loaded into
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Figure 2. Product gas from cellulose-lignin mixtures and weighted average (broken line): (a) H2, (b) CO2, (c) CH4, and (d) H atom amount in the product gas.
Figure 3. Product gas from xylan-lignin mixtures and weighted average (broken line): (a) H2, (b) CO2, (c) CH4, and (d) H atom amount in the product gas.
the reactor. Mixtures containing large amounts of lignin show lower hydrogen contents than their respective equilibrium values, suggesting that only small amount of cellulose and xylan are decomposed to hydrogen. Instead, some kind of intermediate from cellulose and xylan decomposition might rapidly react with lignin. This reaction is not a simple hydrogenation of lignin because there is no indication of methane production resulting from the hydrogenation of lignin.
Figure 5 shows the results of cellulose-xylan mixture gasification, including weighted averages. Unlike the cellulose-lignin and xylan-lignin mixtures, the experimental results do not differ from the weighted averages, indicating an absence of any conspicuous interaction between cellulose and xylan. The experimental results confirm that the lignin content alone expresses the interactions among cellulose, xylan, and lignin. To develop equations for
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Figure 4. Experimental results and equilibrium gas content of cellulose-lignin mixtures with cellulose:lignin ratios of (a) 0:1, (b) 1:3, (c) 1:1, (d) 3:1, and (e) 1:0. Table 2. Gasification Results for Cellulose-Xylan-Lignin Mixtures reactant components cellulose:xylan:lignin 4:1:1 1:4:1 1:1:4 1:1:1
gas product [mmol/(g of reactant)] H2
CO2
CH4
C2H6
C3H8
NCG
5.33 3.64 1.69 2.52
9.51 9.24 6.62 8.49
1.12 1.03 0.90 0.87
0.21 0.19 0.09 0.14
0.08 0.08 NA 0.06
0.31 0.29 0.20 0.26
Table 3. Parameters and Correlation Coefficients of Estimate Equations fitting parameter NCG NH/C NO/C NMR
a1
a2
a3
a4
r2
-4.78 -2.99 3.40 7.46
13.98 6.24 -10.90 -28.40
-16.33 -5.75 12.66 35.84
7.13 2.50 -5.16 -14.90
0.97 0.94 0.96 0.91
Expression of Interactions as a Function of Lignin Content. As parameters to calculate the amount and composition of product gas in this work, we chose the carbon gasification efficiency (CG), the ratio of hydrogen atoms to carbon atoms in the gas phase (H/ C), the ratio of oxygen atoms to carbon atoms in the gas phase (O/C), and a parameter showing the H atom distribution, MR, defined as
MR )
nH2 nH2 + 2nCH4
(3)
An interaction coefficient γj was introduced to express the interaction between lignin and the other compounds. This coefficient is defined by the ratio of the experimental value to the corresponding value of Nwa
γj )
Figure 5. Product gas from cellulose-xylan mixtures and weighted average (broken line): (a) H2, (b) CO2, and (c) CH4.
Nj,ex Nj,wa
(4)
where j represents CG, H/C, O/C, or MR. To express the γj value as a function of the lignin fraction, the following equation was applied 4
product gas estimation, three-component mixtures were also gasified. The results are shown in Table 2.
γj ) 1 +
ai,jXli ∑ i)1
(5)
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5473 Table 4. Experimental Data Used in Estimate Calculation of Product Gas NCG NH/C NO/C NMR
cellulose
xylan
lignin
0.743 2.233 1.216 0.456
0.691 2.163 1.157 0.301
0.086 1.413 1.638 0.523
The coefficients in eq 5 for the ai,j parameters were determined by the least-squares method using a total of 16 data points (3 for single components, 9 for binary mixtures and 4 for tertiary mixtures) and are shown in Table 3. The high r2 value indicates the successful application of eq 5 to express the interaction coefficients. Estimation of Gas Product Using Fitting Parameter. Assuming that the product gas contains only H2, CH4, and CO2, we can calculate gas production using the equations and parameters developed above (Table 3) and the experimental results for single components shown in Table 4. The estimate procedure is shown below. First calculate the numbers of atoms (C, H, O) in the product gas.
nC ) γCGNCG,wawXC
(6)
nH ) γH/CNH/C,wanC
(7)
nO ) γO/CNO/C,wanC
(8)
Next, calculate the amount of each gas in the product gas.
γMRNMR,wanH n H2 ) 2 nH - nH2 2 nCH4 ) 2 nCO2 ) nC - nCH4
(9)
Figure 7. CO2 product estimation vs experimental results.
Figure 8. CH4 product estimation vs experimental results.
pressed by the lignin fraction in the reactant alone, indicating the importance of the lignin fraction effect on supercritical water gasification characteristics. Conclusion
(10) (11)
Figures 6-8 show the agreement between the estimated values thus calculated and the experimental values. In the high-yield region, the estimated values tend to be larger than the experimental ones. This is because the product gas is assumed to contain only H2, CH4, and CO2. In the actual system, C2H6 and C3H8 are also observed. Correlation coefficients reach 0.989 (H2), 0.976 (CO2), and 0.992 (CH4), showing good agreement. Although each fitting parameter itself has no meaning, we can confirm that the interactions among the three main components of biomass were successfully ex-
Cellulose, xylan, and lignin mixtures were gasified in supercritical water at 673 K, 25 MPa, and a reaction time of 20 min. The following results were obtained. 1. A decrease of gas production was observed for the mixtures containing lignin. 2. The reaction of intermediates from cellulose and xylan with lignin resulted in a decrease in H2 production. 3. The interaction between lignin and other components was expressed as a function of the lignin fraction, and equations were developed to estimate the amount and composition of the product gas. The calculated results from the equations agree well with the results from actual gasification experiments. Acknowledgment This study was supported by the Proposal Based New Industry Creation Type Technology R&D Promotion Program for the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Nomenclature
Figure 6. H2 product estimation vs experimental results.
a ) coefficient in eq 4 N ) value representing the carbon gasification efficiency, the H/C ratio in the gas phase, the O/C ratio in the gas phase, or the H2/(H2+CH4) ratio w ) weight of reactant X ) weight fraction γ ) interaction parameter
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Subscripts C ) carbon atom c ) gasification of pure cellulose CG ) carbon gasification efficiency CH4 ) methane CO2 ) carbon dioxide ex ) experimental H ) hydrogen atom H/C ) ratio of hydrogen to carbon molar amount in the gas phase l ) gasification of pure lignin MR ) molar ratio O ) oxygen atom O/C ) ratio of oxygen to carbon molar amount in the gas phase x ) gasification of pure xylan wa ) average weighted by the weight fraction of each compound
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(9) Antal, M. J.; Allen, S. G.; Schulman, D.; Xu, X. D.; Divilio, R. J. Biomass Gasification in Supercritical Water. Ind. Eng. Chem. Res. 2000, 39, 4040. (10) Sealock, L. J.; Elliott, D. C.; Baker, E. G.; Butner, R. S. Chemical Procesing in High-Pressure Aqueous Environments. 1. Historical Pespective and Continuing Development. Ind. Eng. Chem. Res. 1993, 32, 1535. (11) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Chemical Processing in High-Pressure Aqueous Environments. 2. Development of Catalysts for Gasification. Ind. Eng. Chem. Res. 1993, 32, 1542. (12) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Chemical Processing in High-Pressure Aqueous Environments. 3. Batch Reactor Process Development Experiments for Organics Destruction. Ind. Eng. Chem. Res. 1994, 33, 558. (13) Elliott, D. C.; Phelps, M. R.; Sealock, L. J.; Baker, E. G. Chemical Processing in High-Pressure Aqueous Environments. 4. Continuous-Flow Reactor Process Development Experiments for Organics Destruction. Ind. Eng. Chem. Res. 1994, 33, 566. (14) Roy, C.; Pakdel, H.; Zhang, H. G.; Elliott, D. C. Characterization and Catylytic Gasification of the Aqueous Byproduct from Vacuum Pyrolysis of Biomass Can. J. Chem. Eng. 1994, 72, 98. (15) Elliott, D. C.; Sealock, L. J.; Chemical processing in highpressure aqueous environments: Low-temperature catalytic gasification. Trans. Ind. Chem. Eng. A 1996, 74, 563. (16) Meier, D.; Ante, R.; Faix, O. Catalytic Hydropyrolysis of Lignin: Influence Reaction Conditions on the Formation and Composition of Liquid Products. Bioresour. Technol. 1992, 40, 171. (17) Connors, W. J.; Johanson, L. N.; Sarkanen, K. V.; Winslow, P. Thermal Degradation of Kraft Lignin in Tetralin. Holzforschung 1980, 34, 29. (18) Kudsy, M.; Kumazawa, H.; Sada, E. Pyrolysis of Kraft Lignin in Molten ZnCl2-KCl Media with Tetralin Vapor Addition. Can. J. Chem. Eng. 1995, 73, 411. (19) Reynolds, W. C. STANJAN Chemical Equilibrium Solver, version 3.90 IBM-PC; Stanford University: Stanford, CA, 1987.
Received for review February 20, 2001 Revised manuscript received August 1, 2001 Accepted August 15, 2001 IE0101590
