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Ind. Eng. Chem. Res. 1997, 36, 3974-3977
RESEARCH NOTES Thermogravimetric Analysis on Global Mass Loss Kinetics of Rice Hull Pyrolysis Hsisheng Teng,* Hung-Chi Lin, and Jui-An Ho Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
Pyrolyses of rice hulls were performed in a thermogravimetric analyzer from room temperature to 1173 K at heating rates of 10, 30, 60, and 100 K/min. The global mass loss during rice hull pyrolysis was modeled by a combination of the volatile evolutions of four independent parallel lumps: one for moisture and the other three for nonmoisture volatiles. The decomposition of each lump was characterized by a single reaction, first-order with respect to the amount of volatile yet to evolve. The moisture lump evolves mainly at low temperatures with an activation energy of 48 kJ/mol. The activation energies for the evolution of the nonmoisture volatile lumps, which are attributed to the decompositions of hemicellulose, cellulose, and lignin, are 154, 200, and 33 kJ/mol, respectively. Excellent agreement between the experimental data and model predictions was found. Introduction
Table 1. Rice Hull Analyses (wt %)
Rice hulls, also known as rice husks, are created as byproducts of the rice milling process. They present a solid waste disposal problem in the Asian and Pacific region. If not disposed of properly, rice hulls provide a refuge for disease-carrying creatures such as mosquitoes, create fire hazards, and constrain the use of a completed landfill site. There have been several technologies proposed to solve this problem (Boateng et al., 1991, 1992; Vempati et al., 1995). Basically, the utilization of rice hulls has been employed in combustion to liberate energy, the controlled combustion to obtain carbon-free ash, and the pyrolysis of hulls to obtain char and liquid and gaseous products (Hamad, 1981). Either combustion or pyrolysis of rice hulls requires the knowledge of volatile evolution behavior to control the processes. However, a better solution from an environmental and economic standpoint is to thermally reprocess the rice hulls into valuable products. Pyrolysis has been widely used for converting solid fossil fuels into liquid and gaseous hydrocarbons and a solid char residue. Coal pyrolysis has been extensively studied (e.g., Howard, 1981; Solomon et al., 1992), but the investigation of rice hull pyrolysis is rarely reported in the open literature. Like all biomass, rice hulls are composed of cellulose, hemicellulose, and lignin (Antal and Varhegyi, 1995; Liou et al., 1997). The kinetics of cellulose and biomass pyrolyses have been explored by numerous investigators in the past two decades (e.g., Broido and Nelson, 1975; Bradbury et al., 1979; Varhegyi et al., 1989, 1994). The critical review by Antal and Varhegyi (1995) has clearly revealed the current state of knowledge in cellulose pyrolysis kinetics. The well-developed thermogravimetric techniques used in cellulose pyrolysis should be applicable to the pyrolysis of rice hulls. The objectives of this study were to determine the kinetic parameters of the global mass loss during rice hull pyrolysis. Global kinetics are of interest in model* To whom correspondence should be addressed. S0888-5885(97)00017-1 CCC: $14.00
carbon nitrogen hydrogen
Ultimate 45 oxygen 0.93 sulfur 5.8
moisture volatile matter
Proximate 1.5 fixed carbon 61 ash
48 0.20
24 13
ing solid hydrocarbon decomposition in many applications in which trying to represent the full complexity of the hydrocarbon degradation process makes no sense (Milosavljevic and Suuberg, 1995). Global pyrolysis kinetics applied to rice hulls are generally intended to predict the overall rate of volatiles release (i.e., mass loss) from the solid. Although various volatile products are released during pyrolysis, global kinetics are looked to as offering a clue to the key mechanistic steps in the overall rice hull breakdown process. Experimental Section The proximate and ultimate analyses of the rice hulls used in this study are shown in Table 1. The contents of C, H, N, O, and S elements in the ultimate analysis were determined by an elemental analyzer (Heraeus, CHN-O-RAPID). The data shown in Table 1 have been normalized to constitute a sum of 100%. The pyrolysis behaviors of rice hulls were studied in a Perkin-Elmer TGA 7 thermogravimetric analyzer (TGA). All TGA experiments were conducted in an inert-gas (helium) environment with a purge flow rate of 40 mL/min. A sample of rice hulls was loaded in a sample pan in the heated zone of the TGA, and the temperature in the vicinity of the sample was measured by a small thermocouple probe (type K). The samples used were in the form of small pieces (1-2 mg/piece) to avoid diffusion limitations. Mass-transfer limitations have been determined to be insignificant in the range of devolatilization rates of interest here; this is confirmed by the fact that the pyrolysis behavior was not affected by changing the gas flow rate and sample size. © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3975
Figure 1. Typical volatile evolution and mass loss curves for rice hull pyrolysis at a heating rate of 100 K/min.
Figure 2. Evolution of volatile products during pyrolysis of rice hulls at a heating rate of 30 K/min. The evolution can be divided into four lumps by curve resolving.
Pyrolysis was carried out by heating the sample from room temperature to 1173 K at a fixed heating rate. This was then followed by rapid cooling. Several heating rates (10, 30, 60, and 100 K/min) were employed in this study to explore the devolatilization kinetics.
Table 2. Ultimate Yields of the Volatile Lumps during Rice Hull Pyrolyses at Different Heating Rates V*1
V*2
V*3
V*4
10 30 60 100
1.5 1.4 1.3 1.3
11 10 12 11
30 31 30 32
19 20 19 18
Results and Discussion The kinetics of volatile evolution during pyrolyses of solid hydrocarbons, including coal and biomass, have been successfully modeled by several independent parallel reactions, first-order with respect to the amount of volatile yet to evolve (Solomon et al., 1988; Suuberg et al., 1979; Antal and Varhegyi, 1995). Since rice hull pyrolysis volatiles are dominated by only a few species, i.e., cellulose, hemicellulose, and lignin, it is likely that pyrolysis can be effectively modeled by dividing the volatile evolution into a few fractions, each of which is represented by a single first-order reaction. It has also been suggested that a thermogravimetric experiment combined with a kinetic analysis could be used to quantitatively determine the amounts of cellulose and hemicellulose present in a biomass sample (Antal and Varhegyi, 1995). Typical volatile evolution and mass loss curves for rice hull pyrolysis at a heating rate of 100 K/min is shown in Figure 1. Four distinct devolatilization peaks can be seen, suggesting that the volatiles in rice hulls could be divided into four noninteracting lumps which evolve by four independent parallel first-order reactions (Antal and Varhegyi, 1995). The first lump is obviously responsible for the moisture retained in the rice hulls, since its evolution mainly occurs below 373 K. The evolution of volatiles from lump i can be represented as
rice hull f volatilei
i ) 1, 2, 3, and 4
with the assumed first-order rate
dVi/dt ) ki(V*i - Vi)
(1)
and the rate constant
ki ) Ai exp(-Ei/RT)
(2)
ultimate volatile yield (wt %, as-received)
heating rate (K/min)
In the above expressions, Vi is the accumulated amount of evolved volatiles from lump i up to time t, V*i is the ultimate yield of volatile i (i.e., at t ) ∞), T is the absolute temperature, R is the gas constant, Ai is the preexponential factor, and Ei is the activation energy for reaction i. If pyrolysis is performed at a constant heating rate, eq 1 can be expressed in the following form:
dVi/dT ) ki(V*i - Vi)/H
(3)
where H is the pyrolysis heating rate. Integration of eq 3 gives
∫0T-(Ai/H) exp(-Ei/RT) dT]}
Vi ) V*i{1 - exp[ and
volatile yield ) V1 + V2 + V3 + V4
(4)
Pyrolysis experiments using four different heating rates, 10, 30, 60, and 100 K/min, were performed for the rice hulls in the present study. In this approach, the volatile yield for pyrolysis at 1173 K was used as the total ultimate yield for the model (approximately 62 wt % on an as-received basis). The fractions of volatiles released by each lump were estimated using the peak-resolution curve shown in Figure 2, and the results are shown in Table 2. At the peak temperature at which volatile evolution reaches a maximum (Tmax), the time derivative of the reaction rate should be equal to zero, i.e.
d2Vi/dt2 ) ki(-dVi/dt) + (V*i - Vi)(dki/dt) ) 0, at T ) Tmax (5)
3976 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 3. Peak Temperatures (Tmax) of the Volatile Lumps during Rice Hull Pyrolyses at Different Heating Rates peak temperature (Tmax, K)
heating rate (K/min)
lump 1
lump 2
lump 3
lump 4
10 30 60 100
351 373 388 400
578 597 610 619
626 643 655 664
626 698 742 795
Table 4. First-Order Kinetic Parameters of the Four-Lump Model for Rice Hull Pyrolysisa lump 1 (min-1)
Ai Ei (kJ/mol) V*i (wt %, as received)
7.0 × 48 1.4
106
lump 2 5.1 × 154 11
1013
lump 3 3.5 × 200 31
1016
lump 4 6.4 × 101 33 19
a The ultimate volatile yield for each lump (V* ) is the mean of i those determined from different heating rates shown in Table 2.
or
d2Vi/dT 2 ) (ki/H)(-dVi/dT) + [(V*i - Vi)/H](dki/dT) ) 0, at T ) Tmax (6) The values of Tmax of the volatile lumps at different heating rates were determined from the peak-resolution curves such as shown in Figure 2, and the results are shown in Table 3. Taking temperature derivatives on both sides of eq 2 gives
dki/dT ) Ai(Ei/RT 2) exp(-Ei/RT)
(7)
Substitution of eqs 3 and 7 into eq 6 gives
ln[H/(Tmax)2] ) ln(AiR/Ei) - Ei/RTmax
(8)
According to eq 8, the first-order kinetic parameters Ei and Ai can be determined from the slope and intercept of a linear plot of ln[H/(Tmax)2] versus 1/Tmax at various heating rates. This method has been employed by previous workers (Kissinger, 1957; Reich, 1964; Sˇ esta´k et al., 1973) to determine the values of the kinetic parameters for pyrolysis. The first-order kinetic parameters for the four-lump model were thus determined, and the results are shown in Table 4. The volatile yield during pyrolysis can be predicted from the kinetic model, using eq 4 and the kinetic parameters in Table 4. The model predictions and the experimental data are compared in Figure 3, showing that this model successfully predicts the global mass loss process of the rice hull pyrolysis. The activation energies for volatile evolution of different lumps vary in the range of 30-200 kJ/mol. The first lump (moisture) evolved with an activation energy of 48 kJ/mol. At pyrolysis temperatures higher than that for moisture evolution, the second and third volatile lumps evolved with higher activation energies (154 and 200 kJ/mol, respectively), and, as the temperature further increased, the activation energy decreased to 33 kJ/mol for the evolution of the fourth lump. It is well-known that thermogravimetric analysis of small samples of whole biomass at low to moderate heating rates usually evidences a distinct evolution (or derivative thermogravimetric, DTG) peak resulting from the decomposition of cellulose and a lower temperature peak associated with hemicellulose pyrolysis (Antal and
Figure 3. Volatile yield during pyrolysis of rice hulls. The symbols are experimental data (9, 100 K/min; ×, 60 K/min; 2, 30 K/min; b, 10 K/min); the solid line curves are predictions from the fourlump model using parameters shown in Table 4.
Varhegyi, 1995; Varhegyi et al., 1989). It has also been reported that the activation energy for the pyrolysis of cellulose present in filter paper and biomass (sugar cane bagasse and wheat straw) lies between 203 and 237 kJ/ mol (Antal and Varhegyi, 1995). The third lump of this rice hull pyrolysis evolved with an activation energy of 200 kJ/mol, which is analogous to the reported values for the pyrolysis of cellulose in biomass. All this evidence indicates that the third lump in rice hull pyrolysis results from the decomposition of cellulose and the second lump is associated with hemicellulose pyrolysis. As for the fourth lump, it is actually not an obvious peak and more like a gently sloping baseline of the evolution curve, as revealed in Figure 1. This lump decomposes slowly over a very broad range of temperatures. According to the interpretation by Evans and Milne (1987a,b), it is likely that most of the evolution from the fourth lump can be attributed to lignin decomposition. Under this circumstance, the ultimate volatile yields for the first, second, third, and fourth lumps (V*i, i ) 1, 2, 3, and 4) represent a rough estimation for the amounts of moisture, hemicellulose, cellulose, and lignin, respectively, present in rice hulls. However, to have a more precise estimation of rice hull composition, the effects of the sample size and heating rate in pyrolysis and the presence of ash in rice hulls need to be taken into account (Antal and Varhegyi, 1995). These aspects will be a subject of additional work. As shown in Figure 3, excellent agreement between the experimental data and model predictions has been found. This was done using the simplest model that would both work and give fundamentally correct magnitudes of kinetic parameters. The proposed model is a compromise between oversimplification (e.g., a singlereaction first-order approach) and unnecessary complexity (e.g., a model with distributed kinetic parameters). The selection of four groups of lumped parameters is well substantiated by the shape of the evolution curve in Figure 1. The relative simplicity gives this model the potential for applications in the design of industrial biomass-pyrolysis facilities.
Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3977
Acknowledgment This research was supported by the National Science Council in Taiwan, through Project NSC 85-2214-E-033003. Literature Cited Antal, M. J.; Varhegyi, G. Cellulose Pyrolysis Kinetics: The Current State of Knowledge. Ind. Eng. Chem. Res. 1995, 34, 703. Boateng, A. A.; Fan, L. T.; Walawender, W. P.; Chee, C. S. Morphological Development of Rice-Hull-Derived Charcoal in a Fluidized-Bed Reactor. Fuel 1991, 70, 995. Boateng, A. A.; Fan, L. T.; Walawender, W. P.; Chee, C. S.; Chern, S. M. Kinetics of Rice Hull Char Burnout in a Bench-Scale Fluidized-Bed Reactor. Chem. Eng. Commun. 1992, 113, 117. Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. A Kinetic Model for Pyrolysis of Cellulose. J. Appl. Polym. Sci. 1979, 23, 3271. Broido, A.; Nelson, M. A. Char Yield on Pyrolysis of Cellulose. Combust. Flame 1975, 24, 263. Evans, R. J.; Milne, T. A. Molecular Characterization of the Pyrolysis of Biomass. 1. Fundamentals. Energy Fuels 1987a, 1, 123. Evans, R. J.; Milne, T. A. Molecular Characterization of the Pyrolysis of Biomass. 1. Applications. Energy Fuels 1987b, 1, 311. Hamad, M. A. Thermal Characteristics of Rice Hulls. J. Chem. Technol. Biotechnol. 1981, 31, 624. Howard, J. B. Fundamentals of Coal Pyrolysis and Hydropyrolysis. In Chemistry of Coal Utilization, Second Supplementary Volume; Elliott, M. A., Ed.; John Wiley and Sons: New York, 1981; Chapter 12, pp 665-784. Kissinger, H. E. Reaction Kinetics in Differential Thermal Analysis. Anal. Chem. 1957, 29, 1702. Liou, T.-H.; Chang, F.-W.; Lo, J.-J. Pyrolysis Kinetics of AcidLeached Rice Husk. Ind. Eng. Chem. Res. 1997, 36, 568.
Milosavljevic, I.; Suuberg, E. M. Cellulose Thermal Decomposition Kinetics: Global Mass Loss Kinetics. Ind. Eng. Chem. Res. 1995, 34, 1081. Reich, L. A Rapid Estimation of Activation Energy from Thermogravimetric Traces. Polym. Lett. 1964, 2, 621. Sˇ esta´k, J.; Sˇ atava, V.; Wendlandt, W. W. The Study of Heterogeneous Processes by Thermal Analysis. Thermochim. Acta 1973, 7, 333. Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. A General Model of Coal Devolatilization. Energy Fuels 1988, 2, 405. Solomon, P. R.; Serio, M. A.; Suuberg, E. M. Coal Pyrolysis: Experiments, Kinetic Rates and Mechanisms. Prog. Energy Combust. Sci. 1992, 18, 133. Suuberg, E. M.; Peters,W. A.; Howard, J. B. Product Compositions and Formation Kinetics in Rapid Pyrolysis of Pulverized Coal. Implication for Combustion. Seventeenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1979; p 177. Varhegyi, G.; Antal, M. J.; Szekely, T.; Szabo, P. Kinetics of the Thermal Decomposition of Cellulose, Hemicellulose, and Sugar Cane Bagasse. Energy Fuels 1989, 3, 329. Varhegyi, G.; Jakab, E.; Antal, M. J. Is the Broido-Shafizadeh Model for Cellulose Pyrolysis True? Energy Fuels 1994, 8, 1345. Vempati, R. K.; Musthyala, S. C.; Mollah, M. Y. A.; Cocke, D. L. Surface Analyses of Pyrolyzed Rice Husk Using Scanning Force Microscopy. Fuel 1995, 74, 1722.
Received for review January 2, 1997 Revised manuscript received May 19, 1997 Accepted May 27, 1997X IE970017Z X Abstract published in Advance ACS Abstracts, July 1, 1997.
