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Ind. Eng. Chem. Res. 1997, 36, 568-573
Pyrolysis Kinetics of Acid-Leached Rice Husk Tzong-Horng Liou, Feg-Wen Chang,* and Jen-Jon Lo Department of Chemical Engineering, National Central University, Chungli, Taiwan 32054, Republic of China
A highly pure mixture of carbon and silica was obtained on pyrolysis rice husk leached with acid at high temperature in a nonoxidizing atmosphere. The product was suitable for use as a starting material in the the manufacture of silicon nitride powder. The effect of treatment including the kind and concentration of acid and the pyrolysis temperature on the constituents of the specimen was presented. Kinetic tests on pyrolysis of rice husk in a nitrogen atmosphere were carried out with a thermal gravimetric analysis (TGA) technique at heating rates 2, 3, and 5 K/min. The results indicated that thermal degradation of rice husk consisted of two distinct pyrolysis stages. The corresponding kinetic parameters including the activation energy were determined. A reasonable pyrolysis mechanism was proposed, which agreed satisfactorily with the experimental results. Introduction Rice husk constitutes the milling byproduct of rice and is a major waste product of the agricultural industry. The rice husk is bulky, creating a problem of storage in terms of an amount increasing annually. Because rice husk burnt in air pollution is serious, this method of disposal is currently impracticable. Therefore, it is necessary to seek a suitable method to solve the disposal problem. The pyrolysis of rice husk is an important industrial process. A large surface area and intimate contact available for a mixture of carbon and silica can be obtained on removing volatile constituents during pyrolysis or coking of rice husk in a nonoxidizing atmosphere. This process has attracted much attention to make a starting material for production of highly pure silicon nitride (Si3N4) powdersan extensively investigated structural ceramic material, which can be applied in engine and turbine construction. As silica and carbon in the pyrolyzed husk are more intimately dispersed than those in a conventional mechanical mixing method, the formation of silicon nitride from rice husk may well proceed more readily than that in a material manufactured from the mechanical mixing procedure. The major constituents of rice husk are organic materials and ash. The organic materials consist of celluloses (containing cellulose and hemicellulose, those in which cellulose is a major component) and lignin. Approximately one-fifth of the ash is obtained on burning rice husk in air. The ash containing 87-97% silica by mass with a small proportion of metallic elements is considered to be the most economical source of silica. Metallic ingredients presented in rice husk are known to have a substantial effect on nitridation kinetics. This situation results in byproducts apparently formed. The temperature of pyrolysis may also affect the nature of the reactants (carbon and silica) and consequently the formation of silicon nitride. A lower pyrolysis temperature yields the products incompletely decomposed, but a higher pyrolysis temperature results in the formation of products of crystalline morphology. The crystalline constituents make nitridation proceed slowly. Therefore, it is preferable to treat rice husk with an optimally acidic solution and the pyrolysis procedure, so as to * To whom correspondence should be addressed. S0888-5885(96)00453-8 CCC: $14.00
diminish effectively impurities and to obtain highly pure silicon nitride powder. In the previous literature, the pyrolysis procedure of rice husk under a nonoxidizing atmosphere is reported (Hanna et al., 1984; James and Rao, 1986; Patel et al., 1987; Wang and Low, 1990). A similar pyrolysis procedure to produce silicon nitride powder is reported by other workers (Cutler, 1974; Motoi, 1977; Rahman and Riley, 1989). Conventional practice involves pyrolysis of rice husk in a flowing stream of nitrogen or argon at a temperature in the range 573-1273 K. The duration of the reaction is of on the order of 0.5-10 h. James and Rao (1986) observed that the major loss of mass during thermal decomposition of rice husk was attributed to production of combustible volatiles. Hanna et al. (1984) found that addition of iron to the rice husk increased the volatile matter evolved during coking but also decreased the amount of residual carbon in the char. Previous authors focused on pyrolysis pretreatment of rice husk. However, inorganic constituents contained in husk can contaminate products and may increase the cost of purification of silicon nitride. The kinetics of pyrolysis of rice husk have received little attention. The aim of this work was to investigate the effect of thermal treatment on pyrolysis of rice husk in order to produce a highly pure mixture of carbon and silica. The rice husk is treated with acid leaching to remove impurities from samples. A thermal gravimetric analysis (TGA) technique was applied to the pyrolysis of rice husk. The pyrolysis kinetics and effect of acidic treatment on the purity of a mixture of carbon and silica are extensively investigated. The nitridation of pyrolyzed husk and the results of physical examination are presented to explain experimental results. The development of pyrolysis not only has the benefit to produce valuable silicon nitride powder (Liou and Chang, 1996) but also may be a potentially attractive method to produce silicon tetrachloride (Chen and Chang, 1991b) and silicon carbide (Lee and Cutler, 1975), as well as use as a source of silicon and silicon dioxide of metallurgical and semiconductor grades. Simultaneously, it solves problems of disposal and pollution created during burning of rice husk. Experimental Section Material Used and Sample Preparation. The raw material was rice husk obtained from a rice mill. The © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 569 Table 1. Amount of Metallic Ingredients in the Rice Huska metallic ingredients as oxides (wt %)
Figure 1. Schematic diagram of the experimental apparatus for pyrolysis of rice husk.
rice husk was washed thoroughly with distilled water to remove adhering soil and clay and then dried in air at 373 K in an oven for 24 h. HCl, H2SO4, and H3PO4 used for acidic treatment were all of laboratory grade (Merck & Co.). The dried rice husk was refluxed with an acidic solution in a glass round-bottomed flask at 373 K within a thermostat for 1 h. After the acidic solution was drained, the husk was washed repeatedly with warm distilled water until the filtrate was free from acid. The leached husk was then dried at 373 K for 24 h. The coking reaction was conducted in a tubular reactor made of quartz. A weighted amount of rice husk was packed into the reactor. Highly pure nitrogen gas (99.999%, San-Fu Chem. Co.) was used as purge to ensure that the environment surrounding the rice husk was nonoxidizing. The reactor was inserted into a furnace at a desired coking temperature for 1 h and then quenched to room temperature. The nitriding reaction was undertaken under a nitrogen atmosphere. The acid-leached and pyrolyzed husk samples were placed in a platinum crucible and inserted into an alumina tube furnace. The temperature was increased with a program of 2 K/min from 1473 to 1723 K. Apparatus and Procedures of Pyrolysis. Fundamental tests of rice husk pyrolyzed in an inert atmosphere provided useful data for process design. Thermogravimetric analysis (TGA) of samples was made (Perkin-Elmer, Model TGA7). A schematic diagram of the experimental system for the pyrolysis of rice husk is shown in Figure 1. A sample of known mass was placed in a platinum disk which was placed on the suspension wire of the balance and enclosed in a platinum-wound microfurnace and quartz tube. A standard thermocouple (chromel-alumel) passed the base of the furnace near the sample material to provide an accurate temperature of the sample during analysis. To the reactor was first introduced nitrogen at a large rate of flow to purge residual oxygen within the system; then the nitrogen was adjusted to the desired rate of flow. The effect of resistance to thermal and mass transfer on pyrolysis was eliminated on placing small amounts of specimen into the platinum sample pan after the sample had been ground to a powder (325 mesh size, ASTM). A sample mass of 4.5 ( 0.2 mg was employed for all experimental runs. A temperature gradient about the sample during pyrolysis was also considered to be effectively decreased on heating the furnace at a small rate of 2, 3, or 5 K/min. When the tests were initiated, the computer continuously recorded the mass and temperature for the entire period.
water rinsed 0.5 N HCl 1.0 N HCl 2.0 N HCl 3.0 N HCl 6.0 N HCl 0.5 N H2SO4 1.0 N H2SO4 2.0 N H2SO4 3.0 N H2SO4 6.0 N H2SO4 0.5 M H3PO4 1.0 M H3PO4 2.0 M H3PO4 3.0 M H3PO4 6.0 M H3PO4 a
Ca
Mg
K
Fe
Na
P
total
1.546 0.715 0.650 0.627 0.560 0.647 0.587 0.590 0.595 0.594 0.573 0.630 0.546 0.536 0.544 0.532
0.468 0.049 0.024 0.018 0.011 0.008 0.046 0.048 0.048 0.047 0.047 0.089 0.066 0.046 0.067 0.040
0.897 0.022 0.011 0.007 0.004 0.002 0.022 0.012 0.013 0.019 0.022 0.059 0.032 0.034 0.037 0.022
0.075 0.010 0.014 0.005 0.021 0.005 0.010 0.010 0.010 0.009 0.009 0.010 0.008 0.007 0.013 0.014
0.355 0.005 0.004 0.002 0.004 0.007 0.007 0.004 0.002 0.005 0.011 0.027 0.008 0.007 0.009 0.006
0.543 0.157 0.056 0.042 0.026 0.004 0.144 0.153 0.147 0.152 0.138 1.098 1.218 1.528 1.877 0.758
3.884 0.958 0.759 0.701 0.626 0.673 0.816 0.817 0.815 0.826 0.803 1.913 1.878 2.158 2.547 1.372
Leached by acid for 1 h and pyrolyzed with N2 flow for 1 h.
The degree of conversion of rice husk, X, is defined here as
X)
W0 - W W0 - W∞
(1)
in which W0, W, and W∞ represented initial, instantaneous, and final masses of sample, respectively. Once three masses had been read from TGA, the conversion was readily obtained according to a simple calculation. Analysis of Metallic Impurities and Organic Elements. Analyses of metallic impurities of untreated and acid-treated samples were performed using an inductively coupled plasma mass spectrometer (Kontron Plasmakon, Model S-35). Rice husk samples were dissolved in a solution of HNO3 and HF before analysis. Elemental analysis was conducted with an Heraeus elemental analyzer. Dried husk was powdered to 80 mesh size (ASTM); this powder was employed in the analysis. Analysis of Physical Properties. X-ray diffraction analysis was undertaken with an X-ray diffractometer (Siemens, Model D-500) with Cu KR radiation. Electron micrographs were obtained with a scanning electron microscope (Hitachi, Model S-520). Infrared spectra in the region 400-4000 cm-1 were recorded with an infrared spectrometer (Digilab, Model FT-40). The samples were burnt to ash at 973 K for 1 h. Results and Discussion Analysis of Metallic Impurities and Organic Elements. The results of analysis of metallic ingredients in water-rinsed and acid-leached samples appear in Table 1. The main metallic impurities present in the samples are calcium, magnesium, potassium, iron, sodium, and phosphorus; the concentrations of calcium are greater than those of other metallic elements. The metals were effectively removed to a substantially decreased concentration on impregnating rice husk with a HCl solution. No effects were obvious from an increased concentration of the acid. The decreased impurities may result from chemical reactions between acid and metals; then the reacted metals are leached from the acidic solution during filtration. The proportions of residual metal elements in the samples treated with H2SO4 are slightly greater than those treated with HCl, but impurities in samples treated with H3PO4
570 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 Table 2. Organic Element Composition of Rice Husk after Pyrolysisa temp (K)
C
unpyrolyzed husk 573 623 673 723 773 973 1173 1273
40.90 42.38 42.41 43.01 41.92 41.87 42.49 40.08 39.45
a
composition (wt %) H O N ash 5.23 4.77 3.22 2.82 2.57 2.39 1.40 0.86 0.88
36.81 31.23 16.41 15.53 13.30 11.31 5.59 3.66 4.34
0.35 0.38 0.40 0.46 0.46 0.34 0.37 0.37 0.26
16.71 21.24 37.56 38.18 41.75 44.09 50.15 55.03 55.07
C/SiO2 (molar ratio) 12.24 9.98 5.65 5.63 5.02 4.75 4.24 3.64 3.58
Leached by 3 N HCl for 1 h and pyrolyzed with N2 flow for 1
h.
exceed those after treatment with HCl or H2SO4. Hence, the treatment of husk with H3PO4 is unsuitable for an acid-leached process. Leaching of rice husk with HCl is better than that with H2SO4 or H3PO4. About 84% of impurities were extracted after treatment of rice husk with 3 N HCl. Pyrolysis of rice husk in a nonoxidizing atmosphere yielded an intimate mixture of carbon and silica, which is the raw material for the manufacture of silicon nitride. The elemental analysis of rice husk pyrolyzed at various temperatures is listed in Table 2. Samples were pyrolyzed at a constant duration (1 h). The major organic elements in rice husk are carbon, hydrogen, oxygen, and nitrogen. The percentage of hydrogen and oxygen decreases with increased pyrolysis temperature; increased content of ash correlated with increased pyrolysis temperature. These effects decreased when the coking temperature was increased from 973 to 1273 K. The rice husk pyrolyzed above 973 K offered the advantage of the ability to produce a carbon and silica with unwanted elements in minimal proportions. A similar conclusion was reported by Chen and Chang (1991a). Hence, a higher pyrolysis temperature is conducive to decomposition of organic matter to carbon. Table 2 also shows that the molar ratio of carbon to silica alters in the coked rice husk from 9.98 to 3.58 as the coking temperature increased from 573 to 1273 K. In the following experiment, rice husk leached with HCl (3 N) for 1 h served as the raw material for pyrolysis reaction. During nitridation, excess carbon is commonly required to increase the rate of reaction (Komeya and Inoue, 1975; Durham et al., 1991; Liou and Chang, 1995); the theoretical molar ratio of C to SiO2 is 2. When rice husk was pyrolyzed at 1173 K for 1 h, a mixture of C and SiO2 in an almost fixed proportion was obtained. It is suitable for use as a starting material in the preparation of silicon nitride powder and was chosen as the starting reactant in our nitridation experiment. Effect of Temperature on Pyrolysis. Figure 2 presents plots of conversion vs temperature of rice husk pyrolyzed in nitrogen at a heating rate of 2, 3, or 5 K/min. This conversion effect appears for samples of rice husk pyrolyzed from 500 to 850 K. The conversion increased because of loss in mass during pyrolysis representing the mass of volatile matter removed. There are clearly two principal stages of reaction distinguished by two significant and distinct variations of conversion for all three heating rates investigated. For a given temperature of pyrolysis, the corresponding conversion at a greater rate of heating is less than that at a small rate of heating. The reason for this phenomenon is that, at a small rate of heating, the duration of retention required for rice husk to reach a given temperature increases, so increasing the conversion.
Figure 2. Effect of different heating rates on the conversion for pyrolysis of rice husk (leached by 3 N HCl for 1 h and pyrolyzed with N2 flow).
Figure 3. Variation of the instantaneous reaction rate with temperature at different heating rates for pyrolysis of rice husk (leached by 3 N HCl for 1 h and pyrolyzed with N2 flow).
From the variation of conversion shown in Figure 2, the conversion increase in the first stage is about 60% when the temperature was varied from 500 to 620 K for all three heating rates. A rapid increase of conversion in the first stage may be due to evolution of volatile matter. This phenomenon may result from dehydration or depolymerization of the original organic constituents that further decomposed to yield volatiles, water, and char. The second stage when the temperature varied from 620 to 850 K is mainly attributed to breakdown of residual organic components (or char) that decompose to yield gases and the other char. When the temperature exceeded 850 K, the organic materials were almost completely decomposed to carbon, and a greater pyrolysis temperature did not affect the loss of mass. A theory with degradation in two or three stages for decomposition of rice husk or cellulose in nitrogen or air was reported by other workers (Antal et al., 1980; Hanna et al., 1984; Hanna and Farag, 1985; James and Rao, 1986; Agrawal, 1988; Chakraverty and Kaleemullah, 1991), with a consistent conclusion that the major loss of mass during degradation is attributed to evolution of volatile matter, with further decomposition to char or tar. Rate of Reaction. The variation of the instantaneous rate of conversion (dX/dt) during pyrolysis of rice husk with temperature is shown in Figure 3. One large maximum rate occurs at a reaction temperature depending on the heating rate. A greater rate of heating shifted the rate curve to a greater range of temperature, and the maximum rate also increased. When the rate
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 571 Table 3. Kinetic Parameters of Rice Husk Pyrolysis first stage X ) 0-0.5 X ) 0.5-0.62 E (kJ/mol) A (s-1) n
Figure 4. Calculated activation energies at different conversions for pyrolysis of rice husk (leached by 3 N HCl for 1 h and pyrolyzed with N2 flow).
of heating was increased, the duration of retention was shorter and the temperature required for organic matter to decompose was also greater, causing the maximum rate curve to shift rightward. Further, with a larger rate of heating, a larger instantaneous thermal energy is provided in the system, this situation also results in a larger instantaneous rate. According to results in Figure 3, the main pyrolysis temperature of the rice husk is clearly in the range 520-630 K, in which there is a maximum rate of pyrolysis occurring from 585 to 605 K for the three rates of heating. Activation Energy. The derivation of pyrolysis kinetic data in the present work follows that of previous investigators (Wendlandt, 1974; Antal et al., 1980; Nishizaki et al., 1980; Petrovic and Zavargo, 1986). The rate of conversion, dX/dt, in thermal decomposition is expressed by
dX ) kf(X) dt
(2)
The reaction rate constant k is expressed according to the Arrhenius equation
(-E RT )
k ) A exp
(3)
A function of conversion independent of temperature, f(X), is expressed as
f(X) ) (1 - X)n
(4)
Substituting eqs 3 and 4 into eq 2 and taking a natural logarithm of the above equation yields
E1 ) ln[A(1 - X) ] (dX dt ) RT
ln
n
(5)
We determined E, A, and n using a relationship between dX/dt and 1/T based on eq 5. The values of T at the same degree of conversion, X, were taken from Figure 2 obtained at varied heating rates. A set of instantaneous rates (dX/dt) is deduced corresponding to these reaction temperatures (1/T) from Figure 3. Thus, a family of parallel straight lines of slope -E/R is obtained. The activation energy (E) corresponding to the selected conversion is thus obtained. The variation of activation energy as a function of conversion is presented in Figure 4. The activation energy for nitrogen pyrolysis of rice husk may be divided into three groups. The average activation energies in the three groups are E ) 143 ( 15 kJ/mol for X ) 0-0.5;
143 4.9 × 109 1.0
205 3.1 × 1014 1.0
second stage X ) 0.62-1.0 116 1.8 × 106 2.0
E ) 205 ( 20 kJ/mol for X ) 0.5-0.62; and E ) 116 ( 15 kJ/mol for X ) 0.62-1.0. The apparent order of reaction (n) and preexponential factor (A) obtained by curve fitting are listed in Table 3. At the conversion 0-0.62 (the first stage), the activation energies appear to have two major groups; hence, the mechanism of thermal degradation of rice husk in the first stage must comprise more than one reaction. There are two components of organic matter (mainly celluloses and lignin) present in the first stage. That there is only one activation energy observed in the second stage (conversion 0.62-1.0) indicates that pyrolysis in this stage may be regarded as being a further decomposition of intermediates (noted as residual organic matter of the first stage). A reasonable assumed mechanism is proposed as follows to describe the pyrolysis process of rice husk. The stoichiometric expression given in eq 6 is accounted for as follows:
Because the main constituents of rice husk are celluloses and lignin, the pyrolysis reaction reflects the two raw organic materials. The pyrolysis process at the first stage includes decomposition or depolymerization of celluloses (C) and lignin (L) into the same intermediates (I) that may be organic material of smaller molecular weight; then both gaseous volatiles (V1 and V2) were released from individual pyrolysis reactants (C and L). There is an overlap of decomposition of celluloses and lignin in the first stage. As the lignin is more difficult to decompose than cellulose, a greater activation energy is observed in the later period of the first stage. In the second stage, the residual organic matter (regarded as intermediates, I) is further pyrolyzed to form other volatile species (V3), tar, and char (P); the latter is composed of a pure mixture of carbon and silica. A similar conclusion is reported by James and Rao (1986) for characteristics of thermal decomposition of rice husk explained on the basis of superposition of decomposition of celluloses and lignin, which are the major organic constituents of rice husk. As rice husk is composed of more than one organic material, this situation leads to a complicated nature of the pyrolysis reaction, with more than one pathway of chemical decomposition observed in the present work. Analysis of Physical Properties. The effects of thermal treatment on the crystalline property of coked husk are presented in Figure 5. The diffraction patterns of husk pyrolyzed at various temperatures (Figure 5ad) illustrate no characteristic feature and indicate a completely amorphous form. These curves have a maximum at 2θ ) 22.5°, characteristic of silica. Chakraverty et al. (1988) found that silica obtained from combustion of acid-treated husk samples at 973 K had an amorphous nature, and they thought that acidtreated husk did not affect the structure of silica. As
572 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997
Figure 5. X-ray diffractogram of rice husk (leached by 3 N HCl for 1 h and pyrolyzed with N2 flow for 1 h): (a) unpyrolyzed husk; (b) pyrolyzed husk at 973 K; (c) pyrolyzed husk at 1173 K; (d) pyrolyzed husk at 1273 K; (e) nitrided husk at a heating rate of 2 K/min from 1473 to 1723 K.
Figure 6. FTIR spectrogram of rice husk (carbon had been removed in air at 973 K for 1 h): (a) pyrolyzed husk ash; (b) nitrided husk at a heating rate of 2 K/min from 1473 to 1723 K.
mentioned earlier, during preparation of silicon nitride powder, the crystalline silica and carbon exhibit a greater temperature of reaction than amorphous reactants. Therefore, pyrolyzed-husk samples in the present investigation are favorable for the preparation of silicon nitride powder. X-ray analysis of the nitrided-husk sample (Figure 5e) reveals that powder produced during nitridation is a highly pure silicon nitride powder. Most silicon nitride is in the R-phase (∼99%). Figure 6 shows IR patterns of both coked and nitrided samples after firing in air at 973 K for 1 h. For an ash sample from acid-leached husk (Figure 6a), the positions of features are the same as those from silica in a commercial grade. Hence, silicon distributed in rice husk exists mainly in the form of silica. A sample of pyrolyzed husk nitrided under N2 (Figure 6b) shows a result distinct from that of a sample of silica ash. On comparison with a silicon nitride powder of commercial grade, the two IR patterns are completely matched. Hence, the product from nitridation of acid-leached and pyrolyzed rice husk is a highly pure silicon nitride powder. Typical scanning electron micrographs that illustrate the morphological variation of rice husk samples and nitridation products appear in Figure 7. Parts a and b of Figure 7 show outer and inner surfaces of rice husk. The well-organized rice husk that contains a corrugated outer epidermis and a thin lamellar inner epidermis is clearly visible. The same morphology is observed by James and Rao (1986). Sharma et al. (1984) reported
Figure 7. Scanning electron micrographs of rice husk: (a) outer epidermis of husk (100×); (b) inner epidermis of husk (100×); (c) carbon-free husk (150×); (d) powder of pyrolyzed husk (1500×); (e) nitrided husk at a heating rate of 2 K/min from 1473 to 1723 K (20000×); (f) nitrided husk at a heating rate of 2 K/min from 1473 to 1723 K (610×).
that silica is mainly localized in both the outer and inner epidermis of rice husk and also fills in spaces between epidermal cells. When pyrolyzed husk is burnt in air at 973 K, Figure 7c shows a morphology similar to that of the original raw husk. The figure also shows that many residual pores are due to carbon removed and become distributed within the ash sample. For rice husk pyrolyzed at 1173 K and ground to 325 mesh size (ASTM), Figure 7d shows a porous surface of the powder. The total cumulative pore volume and average pore diameter are 0.146 mL/g and 16.2 Å (according to
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 573
the N2 adsorption method), respectively. The result indicates that pyrolyzed husk is a highly porous material, which exhibits a large internal surface area. The rice husk might have become broken up during thermal decomposition of organic matter, thus leaving a highly porous structure. Figure 7e shows that uniform sizes of silicon nitride powder are formed when pyrolyzed husk reacted under N2. The shape of the silicon nitride is a hexagonal column that has a mean particle size of 1.3 µm. It is the most nearly perfect crystalline morphology of silicon nitride grain. A similar morphology is observed by Rahman and Riley (1989). Fine white silicon nitride whiskers grow in small proportions on the surface of rice husk tissue (Figure 7f); the ratio of the length to diameter is about 60. Conclusions Pyrolysis of rice husk with appropriate acid leaching at a high temperature and in a nonoxidizing atmosphere yielded an intimate contact within a mixture of carbon and silica. Experimental results revealed that metallic ingredients effectively removed on leaching the rice husk with an acidic solution. A pyrolysis temperature above 973 K was conducive to complete decomposition of organic matter into carbon. Thermogravimetric analysis was used to measure the rate of pyrolysis of rice husk in nitrogen at various heating rates. A twostage pyrolysis mechanism is proposed to account for possible thermal decomposition. The mechanism comprises depolymerization of original materials (celluloses and lignin) to yield gaseous volatiles and intermediates; then these intermediates further decompose to yield other volatiles and char. The analytical results of physical properties provided clear proof that the pyrolyzed husk is suitable to produce highly pure silicon nitride powder. The development of the pyrolysis process has the benefit not only to produce valuable materials, such as Si3N4, SiCl4, SiC, and Si, but also to solve problems of disposal and pollution created during burning of rice husk. Acknowledgment The authors express their thanks to the National Science Council of ROC in Taiwan for its financial support under Project NSC-83-0402-E008-001. Nomenclature A ) preexponential factor, s-1 C ) celluloses E ) activation energy, kJ mol-1 f ) function of conversion H ) rice husk I ) intermediates during pyrolysis of husk k ) pyrolysis rate constant, s-1 L ) lignin n ) reaction order P ) char R ) gas constant ) 8.314 kJ kmol-1 K-1 t ) pyrolysis time, s T ) pyrolysis temperature, K V1, V2, V3 ) volatiles W ) weight of sample at time t, mg W0 ) initial weight of sample, mg W∞ ) final weight of sample, mg X ) conversion of rice husk
Greek Symbol β ) heating rate, K min-1
Literature Cited Agrawal, R. K. Kinetics of Reaction Involved in Pyrolysis of Cellulose I. The Three Reaction Model. Can. J. Chem. Eng. 1988, 66, 403. Antal, M. J.; Friedman, H. L.; Rogers, F. E. Kinetics of Cellulose Pyrolysis in Nitrogen and Steam. Combust. Sci. Technol. 1980, 21, 141. Chakraverty, A.; Kaleemullah, S. Conversion of Rice Husk into Amorphous silica and Combustible Gas. Energy Convers. Manage. 1991, 32 (6), 565. Chakraverty, A.; Mishra, P.; Banerjee, H. D. Investigation of Combustion of Raw and Acid-Leached Rice Husk for Production of Pure Amorphous White Silica. J. Mater. Sci. 1988, 23, 21. Chen, J. M.; Chang, F. W. Rice Husk as a Source of High Purity Carbon/Silica to Produce Silicon Tetrachloride. Proc. Natl. Sci. Counc., Repub. China, Part A: Phys. Sci. Eng. 1991a, 15 (5), 412. Chen, J. M.; Chang, F. W. The Chlorination Kinetics of Rice Husk. Ind. Eng. Chem. Res. 1991b, 30 (10), 2241. Cutler, I. B. Production of Silicon Nitride from Rice Hulls. U.S. Patent 3,855,395, 1974. Durham, S. J. P.; Shanker, K.; Drew, R. A. L. Carbothermal Synthesis of Silicon Nitride: Effect of Reaction Conditions. J. Am. Ceram. Soc. 1991, 74 (1), 31. Hanna, S. B.; Farag, L. M. Kinetic Studies on Thermal Degradation of Treated and Untreated Rice Hulls. Thermochim. Acta 1985, 87, 239. Hanna, S. B.; Farag, L. M.; Mansour, N. A. L. Pyrolysis and Combustion of Treated and Untreated Rice Hulls. Thermochim. Acta 1984, 81, 77. James, J.; Rao, M. S. Silica from Rice Husk Through Thermal Decomposition. Thermochim. Acta 1986, 97, 329. Komeya, K.; Inoue, H. Synthesis of the R-form of Silicon Nitride from Silica. J. Mater. Sci. Lett. 1975, 10, 1243. Lee, J. G.; Cutler, I. B. Formation of Silicon Carbide from Rice Hulls. Ceram. Bull. 1975, 54 (3), 195. Liou, T. H.; Chang, F. W. Kinetics of Carbothermal Reduction and Nitridation of Silicon Dioxide/Carbon Mixture. Ind. Eng. Chem. Res. 1995, 34 (1), 118. Liou, T. H.; Chang, F. W. The Nitridation Kinetics of Pyrolyzed Rice Husk. Ind. Eng. Chem. Res. 1996, 35 (10), 3375. Motoi, S. Silicon Nitride. Jpn. Patent 77,00,799, 1977. Nishizaki, H.; Yoshida, K.; Wang, J. H. Comparative Study of Various Methods for Thermogravimetric Analysis of Polystyrene. J. Appl. Polym. Sci. 1980, 25, 2869. Patel, M.; Karera, A.; Prasanna, P. Effect of Thermal and Chemical Treatments on Carbon and Silica Contents in Rice Husk. J. Mater. Sci. 1987, 22, 2457. Petrovic, Z. S.; Zavargo, Z. Z. Reliability of Methods for Determination of Kinetic Parameters from Thermogravimetry and DSC Measurements. J. Appl. Polym. Sci. 1986, 32, 4353. Rahman, I. A.; Riley, F. L. The Control of Morphology in Silicon Nitride Powder Prepared from Rice Husk. J. Eur. Ceram. Soc. 1989, 5, 11. Sharma, N. K.; Williams, W. S.; Zangvil, A. Formation and Structure of Silicon Carbide Whiskers from Rice Hulls. J. Am. Ceram. Soc. 1984, 67 (11), 715. Wang, N.; Low, M. J. D. Spectroscopic Studies of Carbons. XVIII. The Charring of Rice Hulls. Mater. Chem. Phys. 1990, 26 (2), 117. Wendlandt, W. W. Thermal Methods of Analysis; Wiley-Interscience: New York, 1974.
Received for review July 29, 1996 Revised manuscript received November 27, 1996 Accepted December 2, 1996X IE9604536
X Abstract published in Advance ACS Abstracts, January 15, 1997.
