Energy & Fuels 2004, 18, 1865-1869
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A Kinetic Study on Biomass Fast Catalytic Pyrolysis Pengmei Lv,* Jie Chang, Tiejun Wang, and Chuangzhi Wu Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 81 Xian Lie Zhong Road, Guangzhou 510070 People’s Republic of China
Noritatsu Tsubaki School of Engineering, Nagoya University, Gofuku, Toyama, Japan Received February 26, 2004
An apparatus for the fast pyrolysis of biomass is designed and set up to simulate the fast heating rate in the fluidized bed. The features of biomass fast catalytic pyrolysis in the apparatus are studied, whereas the kinetics model of biomass catalytic pyrolysis is presented. The results show that both calcined dolomite and a nickel-based catalyst can elevate the hydrogen content greatly. The nickel-based catalyst has a stronger effect, and the hydrogen content is almost doubled. The content of methane (CH4) can be greatly reduced through the use of nickel-based catalysts. Calcined dolomite can also decrease the content of CH4, to a smaller extent. A simplified model is presented, in which the entire process is treated as a single reaction. It assumes that biomass first decomposes to gaseous products, tars, and chars via three competitive reactions and then tars go through a second cracking reaction to produce gases and chars. Through the proposed model, the calculated data fit well with the experimental data obtained from pyrolysis tests of pine sawdust, lignin, and cellulose. The calculated reaction order is in the range of n ) 0.66-1.57. It is concluded that calcined dolomite must be used at a temperature of >800 °C.
Introduction Many studies have been performed to investigate the kinetics of biomass pyrolysis, or catalytic pyrolysis.1-10 Some people have given detailed reviews based on a general literature survey.1-4 Recently, Caballero et al.5 presented a new mathematical model to describe the complexity of lignin decomposition. Di Blasi and Lanzetta6,7 have performed studies on the decomposition kinetics of hemicellulose, wheat, and corn straw. Thermogravimetric analysis (TGA) is the general approach to be used to conduct the process. The great virtue of TGA is its accurate and continuous record of the weight loss of tested samples and of the reaction temperature. However, the results from a low-heating-rate TGA are not suitable to be applied for a fast reaction reactor, such as fluidized-bed or circulating-bed gasifiers, which have been developed maturely in recent years. Therefore, it is valuable to develop an apparatus for the kinetic study
of biomass pyrolysis under fast heating rate. In the study of weight loss dynamics of wood chips, Di Blasi et al.9 applied radiative heating to achieve a fast heating rate (varying over a range of ∼20-120 K/min). Many researchers have proved the usefulness and effectiveness of calcined dolomite and nickel-based steam reforming catalysts in regard to decreasing tar yield and improving gas quality in the process of biomass gasification.11-17 However, little study has involved the detailed catalytic mechanism of these catalysts and their optimum operating conditions for the process of biomass gasification. To achieve a fast heating rate and continuous record of reaction parameters, as well as to explore the detailed effect of calcined dolomite and nickel-based catalysts on the fast pyrolysis of biomass, an experimental apparatus has been developed in this study. Experimental Section
* Author to whom correspondence should be addressed. Telephone: 86-20-87057750. Fax: 86-20-87057789. E-mail:
[email protected]. (1) Antal, M. J. Advances in Solar Energy, Vol. 1; American Solar Energy Society: Boulder, CO, 1982; pp 61-111. (2) Antal, M. J. Advances in Solar Energy, Vol. 2; American Solar Energy Society: New York, 1985, pp 175-255. (3) Di Blasi, C. Prog. Energy Combust. Sci. 1993, 19, 71-104. (4) Antal, M. J.; Va´rhegyi, G., Jr. Ind. Eng. Chem. Res. 1995, 34, 703-717. (5) Caballero, J. A.; Font, R.; Marcilla, A.; Conesa, J. A. Ind. Eng. Chem. Res. 1995, 34, 806-812. (6) Di Blasi, C.; Lanzetta, M. J. Anal. Appl. Pyrolysis 1997, 40-41, 287-303. (7) Lanzetta, M.; Di Blasi, C. J. Anal. Appl. Pyrolysis 1998, 44, 181192. (8) Sharma, A.; Rao, T. R. Bioresource Technol. 1999, 67, 53-59. (9) Di Blasi, C.; Branca, C.; Santoro, A.; Bermudez, R. A. P. J. Anal. Appl. Pyrolysis 2001, 57, 77-90. (10) Conesa, J. A.; Caballero, J. A.; Marcilla, A.; Font, R. Thermochim. Acta 1995, 254, 175-192.
Test Materials and Catalysts. Three types of biomasss pine sawdust, lignin, and celluloseswere used as the test materials. Pine sawdust was obtained from a timber mill in (11) Lv, P. M.; Chang, J.; Wang, T. J.; Fu, Y.; Chen, Y.; Zhu, J. X. Energy Fuels 2004, 18, 228-233. (12) Narva´ez, I.; Orı´o, A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110-2120. (13) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 1535-1543. (14) Caballero, M. A.; Corella, J.; Aznar, M. P.; Gil, J. Ind. Eng. Chem. Res. 2000, 39, 1143-1154. (15) Courson, C.; Makaga, E.; Petit, C.; Kiennemann, A. Catal. Today 2000, 63, 427-437. (16) Aznar, M. P.; Caballero, M. A.; Gil, J.; Martı´n, J. A.; Corella, J. Ind. Eng. Chem. Res. 1998, 37, 2668-2680. (17) Corella, J.; Aznar, M. P.; Gil, J.; Caballero, M. A. Energy Fuels 1999, 13, 1122-1127.
10.1021/ef0400262 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/04/2004
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Lv et al. Table 2. Effect of Catalysts on Gas Composition of Biomass Fast Pyrolysis Gas Composition (dry, inert free, vol %) sample
Figure 1. Schematic diagram of kinetic investigation on biomass catalytic pyrolysis. Legend is as follows: 1, tubular reactor; 2, sample boat; 3, furnace No. 1; 4, furnace No. 2; 5, filled catalyst; 6, unilateral valve; 7, valve No. 1; 8, valve No. 2; 9, U-tube; 10, pressure transmitter; 11, temperature transmitter; and 12, computer. Table 1. Proximate and Ultimate Analysis of Test Materials pine sawdust lignin cellulose higher heating value (kJ/kg) proximate analysis (wt %, dry basis) volatile matter, VM fixed carbon, FC ash ultimate analysis (wt %, dry basis) C H O N S
19500
19137 15750
84.2 15.2 0.5
60.46 26.92 12.62
94.23 5.29 0.48
48.28 7.31 43.88 0.03 0
45.7 4.54 31.17 0.19 5.78
42.45 7.06 49.64 0.05 0.31
Guangzhou City, PRC. Lignin and cellulose were prepared from poplar and rice straw, respectively. The particle size of pine sawdust was 0.3-0.45 mm; the particle size of lignin was 0.125-0.15 mm, and cellulose was ground by a pulverizer into the shape of a floccule. The proximate and ultimate analyses of the test materials are reported in Table 1. Calcined dolomite and nickel-based catalysts were used in the experiments. The dolomite was first crushed into small blocks with an average size of 6 mm and then calcined in air at 900 °C for 4 h. Nickel-based catalysts of Z409R were produced by Qilu PetroChemical Company (Shandong Province, PRC). The original Z409R was annular, with a size of 16 mm (outer diameter) × 6 mm (inner diameter) × 6.0-6.8 mm (thickness) and had a composition of g22 wt % NiO and 6.5 ( 0.3 wt % K2O. To acquire a larger surface area, the nickelbased catalyst was cut into four equal sectors, and the thickness of each sector was the same as that of the original component. Apparatus. Figure 1 shows the apparatus that was used to conduct the tests. Furnace No. 1 contains the tested sample, and the catalysts are filled in furnace No. 2. The temperatures of these two furnaces were controlled independently. A U-tube was installed to hold the gaseous products. A thermocouple was inserted into the middle of the sample to measure the instantaneous temperature. A temperature transmitter and a pressure transmitter were used to convey the temperature and gas-pressure data to the computer. The computer was used to convert the gas-pressure data to a gas volume, as well as to record the temperature variation continuously. At the start of each test, the weighted specific sample was placed in the left end of a quartz tube and the catalysts were placed in the middle of furnace No. 2. Meanwhile, the U-tube was filled with water and valve No. 2 was closed. The electric furnaces then were turned on to preheat the entire quartz tube and valve No. 1 was opened to allow N2 to pass through, to drive out the air that remained in the quartz tube. After the temperature in the quartz tube reached the desired level and the air in it had been cleaned, the sample was pushed quickly from the left end of the tube reactor to the middle of furnace
H2
CO2
C2
pine sawdust lignin cellulose
16.19 17.10 19.28
No Catalyst 15.36 52.16 17.30 26.83 13.38 53.76
CH4
CO
9.71 37.52 7.41
6.58 1.25 6.16
pine sawdust lignin cellulose
Dolomite Catalyst 28.14 12.18 38.40 31.75 12.06 17.94 29.89 12.78 41.85
20.00 37.35 14.75
1.27 0.89 0.74
pine sawdust lignin cellulose
Nickel-Based Catalyst 31.31 1.80 49.83 46.03 0.79 39.03 34.67 1.08 47.36
16.40 14.15 16.83
0.67 0 0.06
No. 1. At the same time, valve No. 1 was closed and valve No. 2 was opened. The test then began and the sample was promptly heated from room temperature to reactor temperature. In this test, the average heating rate of the samples exceeded 1000 °C/min. The produced gaseous product forced the water in the left portion of the U-tube to discharge and enter into a graduate for volume measurement. Also, the information of sample temperature and gas pressure was recorded by the computer continuously through the temperature transmitter and pressure transmitter. When the water level in the left portion of the U-tube remained unchanged, the reaction of biomass catalytic pyrolysis was complete and the test was terminated. Sampling and Gas Analysis. The cool, dry, clean gas was sampled using gas bags and analyzed on a gas chromatograph (Model GC-2010, Shimadzu, Japan) that was fitted with a GScarbon plot column (30 m × 0.530 mm × 3.00 µm) and flame ionization detection (FID) and thermal conductivity detection (TCD) detectors, and standard gas mixtures were used for quantitative calibration.
Results and Discussion Effect of Catalysts on Gas Composition. The effect of calcined dolomite and nickel-based catalysts on gas composition of biomass pyrolysis are investigated with the temperatures for both furnace No. 1 and furnace No. 2 being 700 °C. The experimental results are presented in Table 2. For these three samples, both calcined dolomite and nickel-based catalyst can elevate the hydrogen content greatly. The nickel-based catalyst has a stronger effect, and the hydrogen content is almost doubled. The content of CH4 can be greatly reduced through the use of nickelbased catalysts. Calcined dolomite can also decrease the content of CH4, to a smaller extent. For the catalytic pyrolysis of all the samples, both calcined dolomites and nickel-based catalysts can reduce the content of C2 in their gaseous products greatly. Nickel-based catalysts have the same affect on the content variation of CO and CO2 as calcined dolomite for the pyrolysis of pine sawdust and cellulose. That is, the contents of CO and CO2 are decreased and increased, respectively. In regard to the pyrolysis of lignin, nickel-based catalysts and calcined dolomite exhibit different effects on the content changes of CO and CO2. Using calcined dolomite, the content of CO is greatly reduced, whereas the content of CO2 is changed only slightly. Under the effect of nickel-based catalysts, the content of CO is increased whereas the content of CO2 is greatly decreased. Kinetic Model Construction of Reaction Process. Biomass may vary significantly, in regard to its
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Energy & Fuels, Vol. 18, No. 6, 2004 1867
Figure 2. Schematic diagram of biomass catalytic pyrolysis process in this study.
physical and chemical properties, because of its diverse origins and types. In addition, biomass pyrolysis involves several sequential and parallel reactions, and the difficulty in describing the mechanism of the pyrolysis process is obvious. In practical application, it is hard to use tar or char; therefore, the gas yield is given particular attention in this study. As Figure 2 shows, samples are pyrolyzed in furnace No. 1 to produce gases, char, and tar, and then the gases and tar undergo further reactions in furnace No. 2. This is obviously a complex process and is hard to model. Therefore, a simplified model is presented in this study. This model treats the entire process as a single reaction. It presumes that these three materials follow the same mechanism model. That is, they first decompose to gaseous products, tars, and chars via three competitive reactions, and then the tars undergo a second cracking reaction to produce gases and chars. The three competitive reactions happen simultaneously and independently. The rate of gas formation is determined only by the quantity of biomass, not by the mass of tar or char. Kinetic Calculation and Expression. Based on the experimental results of biomass fast catalytic pyrolysis and a great amount of calculation with the aid of a computer, the gas formation rate is taken as the main expression to describe the pyrolysis process. To simplify the calculation, the reaction degree ∂ is introduced to express the quantity of gas:
Figure 3. Simulation of the temperature curve in the tests.
( )
ln
E d∂/dt ) ln A RT f(∂)
(4)
should be a highly linear equation. After a considerable selection, it has been determined that the experimental data can fit eq 4 very well only when
f(∂) ) (1 - ∂)n
(for n ) 0.5-1)
(5)
The kinetic expression of biomass fast catalytic pyrolysis then can be described:
d∂ E ) A exp (1 - ∂)n dt RT
(
)
(6)
By transforming eq 6, we get
( )( )
(d2∂/dt2)T2 E d∂/dt T2 + ) -n 1 - ∂ dT/dt R (d∂/dt)(dT/dt)
(7)
In eq 7, we make two definitions:
Vt ∂) V0
(1)
where Vt is the volume of gas at time t and V0 is the total gas formed at the end of the reaction. According to the chemical kinetics, the following equation is established:
( )
X)
d∂/dt T2 ∂ - 1 dT/dt
Y)
(d2∂/dt2)T2 (d∂/dt)(dT/dt)
A simpler version of eq 8 then is obtained:
d∂ E f(∂) ) A exp dt RT
(
)
(2) s-1),
where A is the frequency factor (given in units of E the apparent activation energy (given in units of kJ/ mol), and T the temperature (°C). Because sample temperature T varies with the reaction time t, it can be fitted from discrete experimental data (as shown in Figure 3) to be a continuous function of time:
T(t) ) T0[1 - M exp(-Bt)]
(3)
T(t) represents the results of the mathematical approach, and M and B are coefficients that are obtained by computer fitting; T0 is the given heating temperature. Figure 3 shows that eq 3 models the actual T data well. According to the principles of thermoanalysis, f(∂) is determined by the reaction mechanism. However, if f(∂) is selected correctly, the equation
Y ) nX +
E R
(8)
Equation 8 is linear; by applying experimental data to it, n (the slope of the straight line) can be obtained. By substituting n into eqs 5 and 4, the values of A and E can also be determined. The calculation process can be clearly observed in Figure 4a-g, in which the terms “temperature 1” and “temperature 2” denote the temperature in furnace No. 1 and furnace No. 2, respectively. All the hollow diamonds in the figures are calculated results from experimental data, and the dotted lines are fitted linear lines. The calculation results of kinetic parameters are listed in Table 3, which shows that the calculated reaction order (n) is in the range of 0.66-1.57. The linear relation for eq 7 is valid for ∂ ) 0.2-0.9, whereas it is worse for ∂ values beyond this range. This can be explained by the fact that, when ∂ < 0.2, the test just
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Figure 4. Curves of calculation results of eq 7.
starts and the moisture evaporates rapidly to form a large amount of gas. This obviously is a physical process; therefore, it does not apply to the chemical kinetic expression: when ∂ > 0.9, the reaction is approaching completion, which cannot be similarly suited to eq 7. From Table 3, an another important phenomena can be found that, when used with dolomite, the activation energy of the entire process cannot be decreased by a
great amount until the temperature is >800 °C. Therefore, it is concluded that calcined dolomite must be used at a very high temperature, such as 800 °C. Comparison of the results in Table 3 with values from the literature10,18,19 on the kinetics of biomass pyrolysis, it can be found that the activation energy values of current study are much lower. This can be explained by the fact that, in the literature, TGA is an extensively
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Figure 5. Comparison between calculation results and experimental data. Table 3. Calculation Results of Kinetic Parameters catalyst
temp in reactor 2 (°C)
n
E (kJ/mol)
no catalyst dolomite dolomite nickel-based nickel-based nickel-based
700 700 800 600 700 800
0.872 0.910 0.660 0.914 1.038 1.201
24.45 23.64 5.94 10.36 18.58 14.35
no catalyst dolomite dolomite nickel-based
700 700 800 700
Cellulose 0.245-0.826 0.71 0.778 0.251-0.842 174.45 1.383 0.222-0.835 4.92 1.086 0.146-0.722 19.06 1.403
15.12 34.48 8.13 21.97
nickel-based
700
Lignin 0.329-0.917
∂
Pine Sawdust 0.318-0.954 0.246-0.823 0.238-0.790 0.214-0.867 0.122-0.828 0.186-0.878
A (1/s) 4.81 4.23 0.26 1.73 3.48 2.48
0.45 1.570
7.56
used method, in which weight loss is measured; however, in this method, gas formation is measured. Other reasons that may cause much lower values of activation energy may be much higher heating rates, the presence of catalysts, and different materials. Comparison between the Calculation Results and the Experimental Data. After the kinetic parameters are obtained, the theoretical calculation results are available through a mathematical method. Substituting n, A, and E in eq 6 and combining eq 3, an ordinary differential equation is obtained. Applying a fourth-order Runge-Kutta method, this equation can be solved to acquire an approximate solution to eq 6. Figure 5a-c show the comparison between the calcula(18) Va´rhegyi, G., Jr.; Antal, M. J.; Jakab, E.; Szabo´, P. J. Anal. Appl. Pyrolysis 1997, 42, 73-87. (19) Reina, J.; Velo, E.; Puigjaner, L. Ind. Eng. Chem. Res. 1998, 37, 4290-4295.
tion and the experimental data, which indicates a good agreement. This proves the correction and reliability of the selected mechanism model. Conclusions Both calcined dolomite and nickel-based catalyst can elevate the hydrogen content greatly. The nickel-based catalyst has a stronger effect, and the hydrogen content is almost doubled. The content of methane (CH4) can be greatly reduced through the use of nickel-based catalysts. Calcined dolomite can also decrease the content of CH4 to a smaller extent. In the presence of calcined dolomite and nickel-based catalysts, the gas composition of pine sawdust and cellulose exhibits the same variation trend. In regard to the pyrolysis of lignin, calcined dolomite and nickel-based catalysts have a similar effect on the change of H2, CH4, and C2 species, whereas the opposite effect is observed, in regard to the variation of CO and CO2. It is proved that the assumed mechanism model fits the experimental data well. The calculated reaction order is n ) 0.66-1.57, and, when used with dolomite, the activation energy of the entire process cannot be decreased by a great amount until the temperature is >800 °C. Therefore, it is concluded that calcined dolomite must be used at a very high temperature, such as 800 °C. Acknowledgment. The financial support received from the National 863 Program Foundation of China (Project No. 2002AA514020), Guangdong Province Natural Science Foundation (Project No. 010876), and the “One-Hundred-Scientist Program” of the Chinese Academy of Sciences is gratefully appreciated. EF0400262
