Ind. Eng. Chem. Res. 1993,32, 2573-2579
2573
Pyrolysis Kinetics of Lignocellulosic Materials Suna Balci, Timur Do~u,'and Hayrettin Yucel Department of Chemical Engineering, Middle East Technical University, Ankara, Turkey
Pyrolysis kinetics of almond and hazelnut shells and beech wood were carried out using a thermogravimetric technique. Experiments were repeated for different final pyrolysis temperatures ranging from 300 to 850 O C . Approximately 90% of the pyrolysis reactions were completed up to 450 O C . The initial values of the activation energy of pyrolysis reaction were found to be around 22 kcal/mol for shells of almond and hazelnut. On the other hand, initial activation energy of beech wood pyrolysis was found as 29.4 kcal/mol. Results indicated that a first-order decomposition in terms of volatile content of the reactant showed good agreement with the data only a t the initial stages of the reaction. The reaction rate constant was found to decrease with reaction extent due to the changes in the chemical and physical structure of the solid. Among several models proposed, a model which predicted an increase of activation energy with reaction extent gave the best agreement with the experimental data.
Introduction As lignocellulosicmaterials are heated in a nonreactive atmosphere,they decompose to various pyrolysis products. Depending on their volatilities, these products can be grouped into three classes: char, gases and tars. Char is a carbon-rich nonvolatile solid residue. Gas-phase products include all lower molecular weight products including water vapor. Usually gas-phase products constitute 2025% of the total products of pyrolysis. Tars, which are in liquid form at room temperature, are found to contain many constituents over a wide range of molecular weights. Usually tar comprises approximately 60-65 % of the total products. Any one of the pyrolysis products could be the most desirable product. Most commercial-scalepyrolysis plants are designed to have only one class as the principal product with at least one of the other classes serving as a fuel source. Pyrolysis of lignocellulosicmaterials such as wood, shells, or stones of fruit is extremely complex. During pyrolysis, numbers of parallel and consecutive reactions take place. Depending upon the temperature and overall conversion level, different reactions take place during pyrolysis and all these reactions are expected to have different rate constants. As a result of this complex network of reactions, the apparent rate constant and activation energy of pyrolysis are expected to change with overall reaction extent. The major components of lignocellulosicmaterials, namely lignin, cellulose, and hemicellulose are expected to react independently, and different bonds break up at different temperatures. This complex network of pyrolysis reactions is generally modeled by an overall reaction mechanism. Agrawal (1988) modeled the kinetics of reactions involved in pyrolysis of cellulose by a threereaction model, according to which cellulose decomposed to tars,chars, and gaseousproducts by first-order reactions. Early work of Lipska and Parker (1966) for the pyrolysis of a-cellulose in a fluidized bed indicated three distinct stages of pyrolysis: an initial rapid decompositionfollowed by zero-order volatilization and first-order char-forming reactions. Bradbury et al. (1979) also proposed a threereaction model. According to their model an initiation reaction leads to the formation of active cellulose, which is further decomposed by two competitive first-order reactions. Thurner and Mann (1981) and Nunn et al. (1985) reported kinetic data for the pyrolysis of wood and sweet
* To whom all correspondence should be addressed.
gum hardwood, respectively. Gaseous, tar,and solid products were determined during the pyrolysis. Agrawal and McCluskey (1983)observed that dilute acid wash pretreatment increased the fractional tar yields. Pretreatment also increased the extent of rapid initial reaction which was followed by a zero-order reaction. Alves and Figueiredo (1988, 19891, Koufopanos et al. (1989),and Blasco et al. (1990) reported thermogravimetric data obtained for different lignocellulosic materials. Effects of temperature, particle size, and heating rate on the pyrolysis rate were investigated by Koufopanos et al. Chan et al. (1984),Antal(1985), and Pyle and Zaror (1984) considered heat-transfer effects on the observed rate of pyrolysis of wood and large biomass particles. Alves and Figueiredo (1989) proposed a reaction scheme for the pyrolysis of pure cellulose and investigated the product composition using the proposed reaction scheme. Samolada and Vasalos (1991) found the activation energies of the evolution of total volatiles and gases for the pyrolysis of wood in a temperature range of 400-500 OC in a fluidized bed reactor. The activation energy values were reported to range between 13.5 and 22.6 kcal/mol for the evolution of total volatiles and gases, respectively. Julien et al. (1991)studied vacuum pyrolysis of cellulose with different heating rates. They observed that major transformation of cellulose took place between 300 and 350 "C. Considering this complex nature of pyrolysis reactions and the changes in the reaction mechanism which take place during pyrolysis, we used a different approach and proposed a deactivation model (Balci, 1992). Deactivation models were commonly used in catalyst deactivation studies. Balci et al. (19871, D o h (1981), and Lee et al. (1980,1981)used this idea to model fluidaolid noncatalytic reactions such as cod gasification and SOZ-CaO reaction. In these models an exponential decrease of solid reactivity with respect to time was proposed. Reactivity was expected to be directly related to the conversion level, and more practical expressions could be obtained if solid reactivity was expressed in terms of solid conversion. In this work, models were proposed for the rate of change of activity with respect to solid reactant conversion in pyrolysis of lignocellulosic materials. As a result, expressions were derived for the variation of rate constant and for the activation energy with respect to overall reaction extent.
OS~S-5SS5/93/2632-2573$04.00/00 1993 American Chemical Society
2574 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 nn ..-I
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Experimental Work Pyrolysisof almond shell, hazelnut shell, and beech wood particles were investigated in flowing nitrogen atmosphere. Experiments were carried out on a Netzsch STA 429 thermogravimetric analyzer (TGA). Experiments were repeated for different final pyrolysistemperatures ranging from 300 to 850 "C. A set of initial experiments were conducted to test the possible effects of intraparticle and external transport limitations on the rate of pyrolysis measured by the TGA apparatus (Balci, 1992). To investigate the effect of heating rate on the carbonization rate, experiments were carried out at heating rates of 5,10,20,50, and 100 "C/min for a final temperature of 450 "C. In these experiments, particle size and flow rate of nitrogen flowing over the sample were kept constant at 0.91 mm (16-20 mesh) and 50 cm3/min,respectively. Results showed that (Figure 1) fractional weight losses are independent of heating rate as long as the carbonization temperature is the same. This result is an indication of negligible heat-transfer limitations on the observed rate in the TGA apparatus. Such effects could be significant in large-scaleunits. To test the effects of intraparticle transport resistances on the observed rate, some experiments were conducted at a constant heating rate (20 "C/min) but with particles having different diameters (Figure 2). Results of these experiments showed a negligible effect of particle size on the carbonization rate. Some initial experiments were also conducted at different flow rates of nitrogen flowing over the solid sample (Figure3). These experiments showed a negligible effect of flow rate on the pyrolysis. As a result of these initial experiments, it was decided to keep the heating rate, nitrogen flow rate, and particle size constant and the effect of temperature on pyrolysis kinetics was investigated in detail. In the rest of the experiments, initial heating
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250 220 20
Time, min
Figure 4. Variation of residual weight fraction of almond shell and pyrolysis temperature with time at different final pyrolysis temperatures.
rate and particle size were kept constant at 20 "C/min and 0.91 mm, respectively. Results of initial experiments indicated that carbonization reactions started a t around 220 "C and most of the carbonization was completed by 450 "C. Therefore, in most of the runs the final carbonization temperature was kept below 450 "C. In order not to go over the predetermined final carbonization temperature, the heating rate was automatically reduced as the final carbonization temperature was approached. Variations of residual weight fraction ( W/WO) of almond shell and hazelnut shell samples are given in Figures 4 and 5. In the same figures, temperature variations with respect to time are also indicated. As shown in Figures 4 and 5,almond shell and hazelnut shell samples gave similar weight loss data. As expected, an increase of decomposition rate was observed with temperature at the initial stages of reaction. Due to the depletion of the solid reactant and decreaseof the reactivity of the remaining solid, pyrolysis reactions slowed down as reaction proceeded. A t 300 OC final carbonization temperature, maximum fractional weight loss reached in thermogravimetric experiments was slightly over 0.3. Increase of reaction time at this temperature did not change this value significantly. It was also observed that 90% of the pyrolysis was completed up to 450 O C . At 450 "C, weight loss values reached for almond shell and hazelnut shell were 64% and 61% ,respectively. Corresponding values reached at 850 OC were 67 ?6 and 66 76,respectively. The differences
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2575 Table I. Elemental Analysis of Raw Materials and Solid Residue Obtained at Different Conditions raw materials almond shell hazelnut shell beech wood solid residue obtained from almond shell at 350 "C final pyrolysis temp at 450 "C final pyrolysis temp solid residue obtained from hazelnut shell at 350 "C final pyrolysis temp at 450 "C final pyrolysis temp solid residue obtained from beech wood shell at 350 "C final pyrolysis temp at 450 "Cfinal pyrolysis temp I O
O
c, %
H,%
N,%
0, %
ash, %
47.63 49.94 47.91
5.71 5.65 5.90
trace 0.27 trace
44.48 42.81 44.71
2.18 1.33 1.46
77.04 84.08
2.64 2.23
0.01 0.29
17.93 10.98
2.38 2.42
79.53 83.56
3.31 2.87
0.24 0.47
15.45 11.51
1.47 1.59
78.28 82.24
3.06 2.57
0.03 0.56
16.98 12.42
1.65 1.71
and Figueiredo (1988) reported results related to the kinetics of this reaction. In some studies, the mechanism for the pyrolysis of wood and its constituents is considered to follow the following steps.
h
solid (lignocellulosicmaterials)
c 300 02OC
11250
Time, min.
Figure 5. Variation of residual weight fraction of hazelnut shell and pyrolysis temperature with time at different final Pyrolysis temperatures. Table 11. Chemical Composition of Almond Shell, Hazelnut Shell, and Beech Wood Samples (Balci, 1992) raw extractive lignin, holocellulose, materials components,wt % wt % (ext free) wt % (ext free) 68.85 almond shell 11.7 31.15 66.50 hazelnut shell 8.3 33.50 79.91 beech wood 8.25 20.09
in maximum weight loss values reached in experiments conducted a t different final pyrolysis temperatures was mainly due to differences in pyrolysis temperatures of different components of the solid reactant. Some pyrolysis experiments were also carried out with beech wood (Balci, 1992). Results of these experiments were analyzed in the following sections. Elemental analysis of the raw shells, beech wood, and the solid residue remained at the end of the pyrolysis at 350 and 450 "C were determined using a Leco CHN 600 elemental analyzer, and results are reported in Table I. Also, lignin and holocellulose contents of beech wood and raw shells were determined. Results reported in Table I1 show that the lignin content of beech wood is less than the lignin content of the shells. Model Development Pyrolysis of lignocellulosic materials involves various reactions due to the complex nature of the solid reactant. In most of the published studies on the kinetics of pyrolysis of lignocellulosic materials, this complex nature of the reaction network was simplified and the net result was represented by a single reaction. Bradbury et al. (19791, Thurner and Mann (19811, Nunn et al. (19851, and Alves
-
tar
\I
char
The assumption of taking three independent reactions for gaseous, liquid, and solid products requires further justification. Gaseous, liquid, and solid products might be the result of a number of consecutive and parallel reactions. Different components are expected to decompose and different bonds are expected to break up a t different temperatures. In the present study, this complex nature of the pyrolysis was modeled with a deactivation model. In the pyrolysis of the lignocellulosicmaterials, solid is considered to consist of a reactive part, W,, which decomposes to gaseous products and to tar under the pyrolysis conditions. Weight of the total reactive part of a solid, Wro, is determined by measuring the maximum weight change at the highest carbonization temperature. The simplest model considered in the analysis assumes a first-order overall decomposition of the reactive part of the solid (Agrawal, 1988). The rate expression based on first-order decomposition of the reactive solid is written in terms of fractional volatile conversion as dx/dt = kapp(l - X) where x is defined as x = (Wr.,- W,)/Wro
(1) (2)
The apparent rate constant values calculated by differential analysis of the pyrolysis data obtained for the almond shell and hazelnut shell are plotted as a function of reciprocal of temperature in Figures 6 and 7,respectively. Up to a conversion level of about 0.1, Arrhenius behavior was observed. For higher conversions, the increase of apparent reaction rate constant with temperature became less than the predictions of the Arrhenius law. Even a decrease of apparent rate constant was observed with an increase in temperature at high conversion levels. Deviations from Arrhenius plots became negligible for temperatures below 270 OC. Initial values of the Arrhenius constants were found in this low conversion range (0-0.1). Results are tabulated in Table 111. The deviation of apparent reaction rate constant from the Arrhenius law is due to the decrease of reactivity of the solid. This tendency is known as activity of solid a. However, as pyrolysis proceeds, chemical and physical
2576 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
constant which is a function of temperature only.
1
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Eaperimental
(4) Mode2 2 The activity of the solid reactant is expected to decrease with the reaction extent due to the changes in chemicaland pore structure of solid. Here, the normalized conversion, 2 , is defined ( z = x/xm&. Here xmar:is the maximum conversion level observed in a run with a fixed final pyrolysis temperature. In this model, the rate of change of activity with respect to normalized conversion was expressed as a function of
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Figure 6. Temperaturedependency of first-orderapparentreaction rate constant for Pyrolysis of almond shell. IO
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Experimental
-daldz = B2" (5) Here B corresponds to a deactivation rate constant. Decrease of activity of solid with conversion was obtained by the integration of eq 5 taking activity to be unity when 2 = 0.
On the other hand, activity is expected to approach zero when dimensionless conversion, 2 , goes to unity. With this simplification, variation of apparent reaction rate constant with conversion was obtained as
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Figure 7. Temperaturedependencyof firat-orderapparentreaction rate constant for pyrolysis of hazelnut shell. Table 111. Initial Values of Arrhenius Constants of the Pyrolysis of Almond Shell, Hazelnut Shell, and Beech Wood Samples E,, cal/mol Ao, m i d
almond shell 22 185 8.22 x 107
hazelnut shell 22 075 8.02 x 107
beech wood 29 400 8.09 X 1Olo
(7)
The implication of this model is a decrease of preexponential factor of pyrolysis rate constant with conversion. In this model activation energy is constant. Model 3 In this model, the deactivation process is considered to be proportional to activity itself. -daldz = Ba (8) Integration of eq 8 yields the apparent reaction rate constant as follows:
kapp= A, exp[-E%(l+ @'Tz)/RTI
(9)
where changes occur within the solid reactant. Decomposition of different components takes place at different temperatures. Therefore, composition of the reactive part of the solid changes while conversion increases. Meanwhile, the active surface area for thermal decomposition also changes due to solid depletion and pore structure variations. All these changes are expected to cause deactivation of solid with the extent of pyrolysis. In a previous study (Balci et al., 1987),it was shown that significant variations occur in the pore structure and surface area of coal by devolatilization at different temperatures. Deactivation of the solid affects the value of apparent reaction rate constant. By considering the influence of deactivation, the apparent reaction rate constant is written in the following form:
kaPp= ak = a(A, exp(-E%/R"))
(3)
where a is the activity of the solid. Activity Relations. Activity of the solid reactant is unity at zero conversion, and it decreaseswith conversion. Its value approaches zero when conversion approaches ita maximum value. Model 1: In the simplest model, change of activity with respect to conversion is assumed to be negligible. Thus, activity is taken as unity throughout the pyrolysis. This condition corresponds to assuming an apparent rate
8' = /3RlE%
(10)
This model predicts an increase of activation energy of pyrolysis with conversion. The deactivation rate constant 0 might also be temperature dependent. By taking a temperature-dependent deactivation rate constant, model 3 is modified.
8' = Bo exp(-EB/RT) (11) In eq 11, E@is the activation energy of the deactivation rate. This modified version of model 3 is called model 3*. Model 4: For the further improvement of Model 3, change of activity is expressed as follows: -daldz = Baz" (12) For this model the variation of apparent rate constant is found as
k,
= A, exp[-EJl+
B'Tz"+')/RTI
(13)
where (14) This model also predicts an increase in activation energy with fractional conversion. For n = 0, its limit becomes the same as the prediction of model 3. If the temperature dependency of deactivation rate constant is considered
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2577 Table IV. Model Eauations of the Amarent Reaction Rate Constant ~
model
model equations
1 2 3 35 4 4*
,k = Ao eW-EdRT) k, = A d 1 - zn+l) exp(-EdRT) k, = A0 exp(-E,(l+ @'Tz)/RT) 8' = BOexp(-E,dRT) k,,, = A0 exp(-E,(l+ j3'Tz*l)/RT) B' = BOexp(-EdRT)
model params of deactivn
no. of model params for deactivn (except E, and Ao)
Table V. Model Constant Values for Almond Shell and Hazelnut Shell Pyrolysis almond shell hazelnut shell 22 185 model 1 E,, caUmol 22 075 Ao, min-l 8.22 x 107 8.02 x 107 model2 n -0.618 -0.407 O.OO0 296 model 3 8' O.OO0 311 0.611 model35 8' 0.565 10 134 E@, cal/mol 9 621 O.OO0 293 8' model4 o.OO0 33 n 0.613 0.650 model4* 0.70 0.62 10 134 ED,cal/mol 9 627 0.90 n 0.89
(eq l l ) , a modified form of model 4 can be formulated (model 4*). Expressions of apparent reaction rate constant for different models are summarized in Table IV together with number of model parameters. Using the experimental apparent reaction rate constant values, all the models were tested. Using a regression analysis, model parameters were evaluated. In this analysis initial activation energies and preexponential factor values which were evaluated using the low-conversion data were kept constant. The deactivation model parameters are then evaluated using all the data points corresponding to the complete range of conversion. The predicted values of model parameters are reported in Table V. Model predictions of apparent reaction rate constants were evaluated for both almond shell and hazelnut shell samples at different final pyrolysis temperatures. The results of modeling study at 400, 350, and 300 "C final pyrolysis temperature are reported together with the experimental values in Figures 8 and 9 for almond shell and hazelnut shell, respectively. In model 1, the change of activity with conversion is neglected. In this model, neither the activation energy for the preexponential factor changes with conversion. As discussed earlier in this paper, this approach gives significant deviations between the experimental and predicted rate constant values for both almond shell and hazelnut shell samples. This is essentially due to changes in chemical composition and physical properties of solid reactant with reaction extent. The two models which gave the best agreement with experimental data obtained with shells were models 2 and 4. In model 2, the decrease of activity with conversion is reflected to the preexponential factor of the rate constant. In this model preexponential factor decreased with an increase in conversion. It has only one adjustable parameter, which is n. The value of n was found as -0.618 for almond shell and -0.407for hazelnut shell. Models 3, 3*, 4, and 4* predicted an increase in apparent activation energy of pyrolysis with conversion. In model 3, a linear increase of activation energy with conversion was predicted. This model predicted underestimated rate constant values. This model was improved by considering the temperature dependency of the deactivation rate constant (model 3*). With this modification better estimates of apparent reaction rate constant values were
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Figure 8. Variation of reaction rate constant for different models for pyrolysis of almond shell at (a) 400, (b)350, and (c) 300 OC final pyrolysis temperatures (heating rate = 20 "C/min).
obtained. Still, however, the agreement with experimental values was not satisfactory. This result indicated that a linear function for the increase of activation energy with conversion was not appropriate. Model 4 gave good agreement with the experimental data. The two parameters of this model are the deactivation rate constant and n. As reported in Table V, the values of these parameters obtained for both almond shell and hazelnut shell were very close to each other. The increase of activation energy of pyrolysis was found to be proportional with 1.63th power of conversion. Inclusion of temperature dependence to the deactivation parameter p' (model 4*) did not give any further improvement to the model. Models 2 and 4 showed similar behavior. They both gave good estimates for both almond and hazelnut shells and in all the temperature ranges investigated. Although it is a one-parameter model, the agreement of predictions of model 2 with the experimental data is quite good. Using the predicted model expression, conversion-time relations were calculated by the numerical integration of eq 1.
2578 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 e
Eaperimrntol
09
.
Time, min
Figure 10. Variation of conversion of almond shell at 350 "Cfinal pyrolysis temperature for different models (heating rate = 20 "C/ min). Exprrimrntal
0 9 t
0.01I
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 I
Conversion
Figure 9. Variation of reaction rate constant for different models for pyrolysis of hazelnut shell at (a) 400, (b)350, and (c) 300 "Cfinal pyrolysis temperatures (heating rate = 20 OC/min).
Experimental and calculated conversion-time values are given in Figures 10 and 11 for almond shell and hazelnut shell pyrolysis at 350 "C final pyrolysis temperatures, respectively. Agreement of models 2 and 4 with the experimental conversion-time data is also seen in these figures (Figures 10 and 11). Conversion data obtained with beech wood were also compared with the predictions of models 2 and 4. With the model parameters obtained for almond shell (Table V), calculated conversion-time relations at 350 "C final carbonization temperature and the experimental conversion-time data obtained with beech wood are shown in Figure 12. As shown in this figure, the experimental data obtained with beech wood showed some deviation at the initial stages of the reaction. Initial rate of carbonization for beech wood was found to be larger than the initial rate for the shells. This behavior is the result of higher holocelluloseand less lignin content of beech wood (Table 11). Lignin is known to show more resistance to pyrolysis than holocellulose (Ramiah, 1970). In fact, analysis of beech wood pyrolysis data showed a higher initial activation energy and preexponential factor for the rate constant (Balci, 1992). For beech wood, initial activation energy was found as 29.4 kcal/mol. The corresponding value of A0 is 8.09 X 1Olomin-l. Analysis of the data showed that among the two best models (models 2 and 4) model 4 showed a much better agreement with the beech wood
Time, min.
Figure 11. Variation of conversion of hazelnut shell at 350 OC final pyrolysis temperature for different models (heating rate = 20 "C/ min).
data (Figure 13). The B value for beech wood is about the same (B = 0.0003)as the value obtained for the shells. On the other hand, increase of activation energy with conversion is found to be more with beech wood. This is reflected in the value of (n + l),which is 1.30for beech wood and is about 1.63 for shells. As a result of this analysis, it is concluded that the firstorder rate constant varies with the extent of pyrolysis during the pyrolysis reactions. This variation is essentially due to the changes in the chemical composition and the physical structure of the solid. This change in the rate constant can be represented by the deactivation models. A deactivation model which predicts an increase in activation energy of pyrolysis (model 4)or a model which predicts a decrease in preexponential factor (model 2)with conversion gave good agreement with the experimental results obtained with the shells. On the other hand, model 4 was shown to be a better model for beech wood. The values of adjustable parameters of model 4 were found to
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2579
*'"I
o,81
WO= initial weight of the pyrolyzing solid, g W , = reactive part of the pyrolyzing solid at any time, g W , = total reactive part of the pyrolyzing solid, g x = volatile conversion x,, = maximum attainable conversion achieved in a run with a fixed pyrolysis temperature z = normalized conversion 8, @' = deactivation rate constants 80 = preexponential factor of deactivation rate constant, min-1
0.9
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Literature Cited
0.4
Agrawal, R. K. Kinetics of Reactions Involved in Pyrolysisof Cellulcae I. The Three Reaction Model. Can. J. Chem.Eng. 1988,66,403412.
0.0 0
2
4
S
6
IO 12 T i m e , min
14
16
IS
20
Figure 12. Variation of conversion of beech wood at 350 OC final pyrolysis temperature and predictions of models 2 and 4 (using the model parameters of almond shell reported in Table V) (heating rate = 20 OC/min). Experimental
---- Model 2 Model 4
-
-Model
I
Agrawal, R. K.; McCluskey, R. J. The Low Pressure Pyrolysis of Newsprint. J. Appl. Polym. Sci. 1983, 27, 367-382. Alves, S. S.; Figueriedo, J. L. Pyrolysis Kinetics of Lignocellulosic Materials by Multistage Isothermal Thermogravimetry. J. Anal. Appl. Pyrolysis 1988, 13, 123-134. Alves, S. S.; Figueiredo, J. L. Interpreting Isothermal Thermogravimetric Data of Complex Reactions: Application to Cellulose Pyrolysis at Low Temperatures. J. Anal. Appl. Pyrolysis 1989, 15,347-355.
Antal, M. J., Jr. Mathematical Modelling of Biomass Pyrolysis Phenomena. Fuel 1985,64, 1483-1486. Balci, S. Kinetics of Activated Carbon Production from Almond Shell, Hazelnut Shell and Beech Wood and Characterization of Products. Ph.D. Dissertation, METU, Ankara, 1992. Balci, S.;Do&, T.; D O ~G.,Structural Variations and a Deactivation Model for Gasification of Coal. Znd. Eng. Chem. Res. 1987,26, 1454-1458.
.-C
Blasco, J. M.; Cordero, T.; Gomez Martin, J. P.; Rodriguez, J. J. A Kinetics Study on Chemical Activation of Holm Oak Wood. J. Anal. Appl. Pyrolysis 1990,18,117-126. Bradbury, A. G. W.; Sakai, Y.;Shafiiadeh, F. A Kinetic Model for Pyrolysis of Cellulose. J. Appl. Polym. Sci. 1979,23,3271-3280. Chan, Wai C. R.; Kelbon, M.; Krieger, B. B. Modelling and Experimental Verification of Physical and Chemical Processes during Pyrolysis of a Large Biomass Particle. Fuel 1985,64,1505-
E
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1
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0.2 0.3
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0.4
I
0.5
1
I
0.6 0.7
A 0.8
0.9
Conversion
Figure 13. Variation of reaction rate constant for models 2 and 4 for pyrolysis of beech wood at 350 OC final pyrolysis temperature (heating rate = 20 OC/min; E , = 29.4 kcal/mol; A0 = 8.09 X 1010 min-1; model 4, @ = 0.0003, n + 1 = 1.3; model 2, n + 1 = 0.45).
be about the same for both almond and hazelnut shells, while there are some differences of model parameter between two different reactants in model 2. In the beech wood pyrolysis, initial activation energy and initial rate are higher. The changes in chemical structure of the solid with reaction extent might cause an increase in activatiton energy of pyrolysis which was predicted by model 4. This approach is consistent with the idea that, during pyrolysis, first the weakest bonds break up giving gaseous and liquid products and as conversion increases breakage of stronger bonds takes place. It is concluded that deactivation models proposed in this work can be successfully used for the pyrolysis of lignocellulosic materials.
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Nomenclature A0 = initial preexponential factor, min-1 a = activity of solid E, = initial activation energy, cal/mol Eb = activation energy of the deactivation rate, cal/mol kaPP= apparent reaction rate constant, min-1 R = universal gas constant, 1.987 cal/(mol.K-l) T = pyrolysis temperature, OC,OK t = pyrolysis time, min W = total weight of the pyrolyzing solid at any time, g
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Thurner, F.; Mann, U. Kinetic Investigation of Wood Pyrolysis. Znd. Eng. Chem. Process Des. Dev. 1981,20,482-488. Received for review September 28, 1992 Revised manuscript received March 30, 1993 Accepted May 26, 1993
