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Thermogravimetric kinetic study of the pyrolysis of almond shells and almond shells impregnated with CoCl2
JournaI of Analytical and Applied Pyrolysis, 21(1991)
249
249-264
Elsevier Science Publishers B.V., Amsterdam
Thermogravimetric kinetic study of the pyrolysis of almond shells and almond shells impregnated with CoCl, R. Font *, A. Marcilla, E. Verdti and J. Devesa Chemical Engineering Department,
University of Alicante, Apartado 99, Alicante (Spain)
(Received June 6, 1990; accepted in final form May 7, 1991)
ABSTRACT A kinetic study of the pyrolysis of almond shells, both with and without impregnation with CoCl,, has been carried out by thermogravimetric analysis (TGA). The experimental data have been discussed, assuming a scheme of two reactions as a simplification of the complex network of reactions. The corresponding kinetic parameters (apparent activation energy and pre-exponential factor), assuming first-order kinetics for both processes, have been calculated within the 200-410 o C temperature range, for almond shells without impregnation, and between HO-420°C for almond shells impregnated with CoCl,. When pyrolysing almond shells without impregnation and assuming an overall reaction, the results agree with those obtained from other techniques (Analytical Equipment Eyroprobe 100 and Fluidized Bed Reactor pyrolysis) and also compare well with other results for pyrolysis of cellulose reported in the literature. In the pyrolysis of almond shells impregnated with CoCl,, the results obtained compare well with those from the Fluidized Bed Reactor and have been useful for explaining the differences between the kinetic data deduced from the Eluidized Bed Reactor and the Pyroprobe 100. Almond shells; cobalt chloride; kinetic study; pyrolysis; thermogravimetric
analysis.
INTRODUCTION
Thermal decomposition of lignocellulosic materials occurs via complex mechanisms, which are greatly influenced by heat and mass transfer processes. There is a wide range in the literature of kinetic parameters for these types of processes, both in activation energy and rate constants. This fact is probably due to the large variety of raw materials investigated, the different pyrolysis techniques and different operating conditions used by researchers. Thermogravimetric analysis (TGA) is a technique commonly used for the study of the thermal decomposition of materials. Isothermal data are normally affected by temperature gradients as a result of the rapid heating of the sample up to the set temperature. In the non-isothermal mode these temperature gradients must be lower, since the temperature is increased slowly with time [1,2]. 01652370/91/$03.50
Q 1991 - Elsevier Science Publishers B.V. All rights reserved
250
Several articles have been found in the literature where isothermal or static TGA has been used to study biomass pyrolysis [3-71, as well as similar studies using non-isothermal or dynamic TGA [8-171. In this paper, we study the kinetics of pyrolysis of almond shells and almond shells impregnated with CoCl, using both techniques. CoCl, was chosen as primary catalyst after a screening of the different acid and basic catalysts. Acid catalysts such as CoCl, favour dehydration reactions, leading to the formation of 2-furaldehyde [18,19]. Present results are compared with those reported in the literature and with those obtained elsewhere [20], working with the same biomass and catalyst in a Fluidized Bed Reactor and in a Pyroprobe 100.
EXPERIMENTAL
Almond shells were washed, dried, crushed and sieved. The fraction of 0.500-0.297 mm was selected for the experiments. Almond shells impregnated with CoCl, were prepared by mixing a known amount of almond shells and a solution of known concentration of the catalyst. This mixture was dried in a “Rotavapor B&&i”, in a 60 mm Hg vacuum, and agitated for about 20 h. Finally, the sample was dried at 110 o C for 20 h. The amount of catalyst was determined spectrophotometrically, in accordance with the method described elsewhere [21]. The percentage of impregnation was 14.1% (g of CoCl,/total g). A Mettler TG50 thermobalance was used for the kinetic study. In the dynamic experiments, the operating conditions were the following: nitrogen flow rate, 200 cm3/min; initial temperature, 100°C; final temperature, 800 o C, for 5 min; heating rate 5-45 o C/mm. The composition of the almond shells was: 37% Kiirschner cellulose, 32% pentosans (i.e. hemicellulose), 27% Klasen lignin, 2% extractable matter (extracted with ethanol-benzene) and 0.1% ash. The analytical methods used are described in refs. 22, 23, 24 and 25.
RESULTS AND DISCUSSION
Almond shells without catalyst The decomposition of almond shells was studied at six different heating rates: 5, 10, 15, 20, 30 and 45 OC/mm. Curves similar to that shown in Fig. 1 (10 OC/mm) have been obtained. It can be observed that the pyrolysis of almond shells starts at around 200” C and is practically completed at 425 OC. The DTG curve shows two peaks, one centred at around 310 OC and the other sharper peak centred at around 368 OC. These results agree with
251
100
mcl
300
400
W”
bca
Fig. 1. TG and DTG curves, at 10 OC/min
7””
ROO
heating rate, for almond shells.
those obtained in the pyrolysis of almond shells at increasing temperatures (17 o C/min up to 400 o C and 8 o C/min up to 710 o C) in a Fluidized Bed Reactor.’ It was observed that up to 350” C no significant amount of volatiles were obtained, the major compounds being CO, and H,O. At temperatures higher than 350” C, the volatiles evolved from the sample, increasing the yield of tars, organic liquids, CO and CH, [18]. This fact may be explained in the following terms. From the two processes (peaks) observed in Fig. 1, the first one (310” C) may be produced mainly by decomposition of the lighter fraction of the almond shells (i.e. hemicellulose), leading to a loss of water, the production of a distillate, CO, and some CO. The second process could correspond to decomposition of cellulose, leading to the evolution of tars, hydrocarbons and organic liquids. At the same time, charring of the lignin fraction is taking place, overlapping with the two processes mentioned, since the pyrolysis of this compound takes place in a wide temperature range [7,8,26]. Figure 2 shows the TG and DTG curves, obtained at 10 o C/mm, corresponding to the fractions of hemicellulose, cellulose and lignin obtained from almond shells. It can be observed that the cellulose shows only one
Fig. 2. TG and DTG curves, at 10 o C/min from almond shells.
heating rate, for holocellulose,
cellulose
and lignin
peak, which clearly corresponds to the second peak of the almond shells (it appears at lower temperatures, i.e. 340°C than that of the almond shells, i.e. 368 o C, probably as a consequence of the interactions of this fraction with the hemicellulose and lignin fractions in the almond shells). The residue obtained is around 8.7%. Lignin shows a very wide peak centered at around 36O”C, leaving a high residue (i.e. 39.2%). On the other hand, the holocellulose fraction shows two peaks, one at 280” C and the other at 335 OC, corresponding to the two peaks shown by the almond shells. The residue obtained for this fraction was around 12.1%. These facts clearly show that the first peak in the almond shells corresponds to the decomposition of the hemicellulose and part of the lignin, whereas the second peak corresponds to the decomposition of the cellulose and another part of the lignin fraction. These results are in good agreement with those reported by Varhegyi and Antal [8]. Table 1 shows the temperature intervals corresponding to the two processes (represented by T,-T, and T2-T3,respectively), as well as the temperature where the maximum weight loss per unit time is produced for both processes ), for each run (see Fig. 1 for the meanings of T,,T, , T3, (TWX1 and TMA.W
253 TABLE 1 Temperature ranges and temperatures of maximum weight loss for the pyrolysis of almond shells without impregnation Heating rate ( o C/mm)
&)
;FGt
&)
5 10 15 20 30 45
210 216 217 215 215 220
293 310 313 314 316 323
330 340 340 345 350 350
;zG
&)
360 368 373 378 379 379
400 420 410 415 425 440
T MAX1 and TMAXZ ). It can be observed that a shift of TMAxl and TMAxz towards higher temperatures is produced when increasing the heating rate. These shifts are due to the combined effects of the heat transfer at the different heating rates and the kinetic decomposition. These results agree with those obtained by other authors [16,17]. Almond shells impregnated with CoCI, Six experiments at 5, 10, 15, 20, 30 and 45 o C/min heating rate have been carried out. Figure 3 shows the weight loss rate vs. temperature for the experiment at lO”C/min heating rate and Fig. 4 that corresponding to 45 o C/min heating rate. In the DTG curve corresponding to the experiment
n
\ -.“004
-
-.mR
_
dM/dt (w/s)
DTG
IOU
3cm
500
7wJ
T('C) Fig. 3. TG and DTG curves, at 10 o C/min heating rate, for almond shells impregnated with CoCl,. Fig. 4. TG and DTG curves, at 45 o C/min heating rate, for almond shells impregnated with Co&.
254 TABLE 2 Temperature ranges and temperatures of maximum weight loss for the pyrolysis of almond shells impregnated with CoCl, Heating rate ( o C/min)
r, (“C)
5 10 15 20 30 45
150 155 150 150 155 155
(TzC) 265 240 248 230 232 230
295 310 310 308 305 310
&) 333 340 339 340 337 335
410 405 410 420 420 440
carried out at 10 o C/mm, a broad peak centered at around 240 OC can be observed. This peak shows two small peaks which overlap, one at around 190” C (which can be observed in the experiments at up to 20” C/min heating rate) and the other at around 340 OC. On the other hand, the curve corresponding to the experiment at 45 “C/mm shows a broad peak at 230 OC and a small peak at around 340 OC. The small peak at around 190 OC (Fig. 3) has disappeared. The peak at around 340 OC is observed to decrease progressively with an increase in the heating rate, as can be observed in Figs. 3 and 4. Above 44O”C, the weight loss continues at lower rates although another peak can be observed in the DTG curves at temperatures around 500 o C, in all experiments as the temperature is increased. Table 2 shows the temperature intervals where the two peaks appear (represented by T,-T, and T,-T,, respectively), as well as the temperature where the higher weight loss rate is produced for each peak ( TMAxl and T MAxz). These results show that at low heating rates (10 OC/mm) a loss of approximately two molecules of water per molecule of CoCl, can be observed, even when the catalyst is dried at 100’ C. The shaded zone in the experiment at lO”C/min (Fig. 3) roughly corresponds to the hydration water present in the catalyst. This water loss at higher heating rates cannot be observed, since it overlaps with the peak corresponding to the pyrolysis of almond shells. On the other hand, as the heating rate is increased, the first peak is progressively clearer, whereas the second peak is less marked. These results show the very different behaviour of the almond shells impregnated with CoCl, as compared with the non-impregnated almond shells. KINETIC
STUDY
Kinetic model
The global process of thermal decomposition can be written as: B+aR+bV
of a homogeneous
material 0)
255
where B is the fraction of biomass, R the fraction of residue and V the fraction of volatiles present at time t, with respect to the initial amount of biomass. a and b are average coefficients of yield, expressed as grams of V or R formed per gram of B reacted. The processes of heat transfer and diffusion of the products formed may influence the overall apparent kinetics of the process. Neglecting such effects and considering first-order kinetics, we can write: -dB/dt
= kB
(4
In the TGA, it is not possible to determine the residue formed, since at any time the magnitude measured is: W=B+R
(3)
It can be demonstrated that the first-order kinetics expressed by eqn. (2) is equivalent to: dW/dt=
-k(W-
W,)
(4
where k is the same constant as in eqn. (2), W, is the final fraction of residue obtained, and W - W, is the fraction of biomass susceptible of decomposition. From the experiments carried out (and as previously noted) the decomposition of the almond shells apparently takes place by means of at least two processes, as can be seen in Figs. 1, 3 and 4. Due to this, we consider that the almond shells are formed of two independent fractions A and C, that decompose at different temperatures. Consequently, the following two reactions can be admitted: AkA\aR
+ bV
C%a’R’+
(5)
b’V’
(6)
Applying eqn. (4) to each of the reactions (assuming first-order kinetics) we can write: dW,/dt = -kA( dW,/dt=
(7)
W, - W,,)
-k,(W,-
W,,)
(8)
In the case of dynamic TGA, we have to consider the variation of the rate constant with temperature by means of Arrhenius-type equations. For the first process we can write: k, = k,,
exp( - E,/RT)
(9)
From eqns. (7) and (9): dW,/dt
= kOA exp( -E,/RT)(
Multiplying
W, - W,,)
and dividing eqn. (10) by exp(E,/RT,),
(10) where TM is an
256
intermediate temperature tween the pre-exponential dW,/dt=&
(considered to decrease the large interaction factor and the activation energy), we obtain:
exp(-%A%)
exp[--G/Nl/T-
I/%)](W,-
be-
W&J
We can write: T= &+m(t-t,)
(12)
where To is the initial temperature (K), m is the heating rate (K/s), t, is the initial time (s) and t the time (s). Considering that for the fraction C we would obtain a similar expression, and that, dW/dt
= dW,/dt
+ dWc/dt
(13)
we can finally write: dW/dt
= - k,,
exp( - EA/RTM >
xexp(-E,/R{l/[T,+m(t-t,)l - k,,
exP( - WRTh4
xexP(-&/R{V[T,
-l/L))(W,-
WA,)
1 + m(t-
to>] - VT,})(W,
- W,,)
(14)
This equation allows the kinetic parameters corresponding to the two processes to be calculated, by fitting the experimental data of time and weight loss rate at that time. In addition, the following data must be known: the final mass fraction ( W,), the initial temperature (To), the intermediate temperature considered (TM), the initial time (to) and the heating rate (m). For this purpose, we have used the Simplex flexible optimization method, integrating eqn. (14) by a Runge-Kutta algorithm of fourth order. In this way, we have optimized the following parameters: apparent activation energies corresponding to the first and second processes ( EA and EC respectively); pre-exponential factors corresponding to the first and second to processes (k,, and koc, respectively), and the weight loss corresponding to the the first process (WA, - WA,). Thus the weight loss corresponding second process is defined, since: w,,
- w,cc = 0 - W,)
The objective
function
OF = c [(dB/dt)”
- 06,
- W4,)
(15)
(OF) used has been:
- (dB,‘dt)‘]*
06)
where superscripts e and c represent experimental and calculated values respectively. We have used the values of dB/dt for the calculation of the kinetic parameters, since they allow a better separation of the two processes and a better fitting than the values of B.
10
Lignin
10
Holocellulose
10
7.504
7.898
6.954
0.391
0.121
0.087
0.333 0.336 0.333 0.313 0.308 0.296
5.204 5.071 5.210 4.713 5.042 4.971
5 10 15 20 30 45
Cellulose
Fraction of residue W& (W)
Sample amount (g.103)
Heating rate (“C/min)
k,,
1.50E7
5.10E7 2.8487 2.91E7 4.74E7 2.99E7 1.58E7
(s-l)
k OA
= 7.27E6 s-l,
98.3
112.0 107.8 106.2 108.3 104.5 100.3
(kJ/mol)
EA &03K
230.6
256.8
242.1 243.3 225.3 229.1 215.3 203.6
(kJ/mol)
EC
E, = 109.9 kJ/mol
46.1E- 3
9.05E - 3 11.2E-3 16.5E- 3 17.7E- 3 23.9E- 3 29.3E- 3
(s-l)
k
Kinetic parameters obtained in the decomposition of almond shells
TABLE 3
9.4OE17
1.46E20
5.10E17 7.71E17 2.99E16 6.10E16 4.84E15 5.34El4
k oc
(s - ‘)
&OK
101.7E-4
85.2E - 4
4.27E - 4 5.07E - 4 7.26E - 4 6.91E-4 8.72E - 4 lO.OE - 4
(s-l)
k
WA0 - WA,
0.350
0.359 0.346 0.339 0.375 0.363 0.363
(W)
WC0 - WC,
0.299
0.318 0.309 0.322 0.307 0.328 0.340
(W
8.7
15.7
3.2
7.3 5.8 5.5 7.4 6:O 6.2
(%)
Variation coefficient
258
0.00
dM/dt tmg/s)
-0.01
-0.02
260
310
360
't10
T ('C)
Fig. 5. Experimental () and calculated (almond shells at 30 o C/min heating rate.
- -)
DTG curves for the experiment with
Results and kinetic parameters for almond shells pyrolysis without impregnation Table 3 shows the values of qm considered for fitting the data of each experiment as well as the weight losses calculated for both processes ( W,, W,,) and ( W,, - I&,), and the optimized kinetic parameters obtained at the different heating rates, indicating in all cases the variation coefficients. The values of these coefficients are very low, especially if we consider that we are comparing the experimental and calculated values of the weight loss rate. Figure 5 shows, as an example, the curve (experimental (solid line) and calculated (dotted line)) for the experiment at 30” C/mm. A very good agreement can be observed. In Table 3 the kinetic parameters for both processes are also presented, as well as the rate constants calculated at 600 K. We can observe that the trend is for the rate constants to increase when increasing the heating rate. This indicates that the decomposition process takes place by a complex scheme of reactions. Nevertheless, the simplification assumed, taking into account only two parallel reactions, means an approximation to the real process and leads to values of kinetic constants of the same order. It is also possible that the variation of the kinetic constant values may be explained by bearing in mind that the overall process is exothermic, as can be seen from a DTA of the almond shells [19]. Thus, when increasing the heating rate, a higher temperature difference would exist between the actual sample temperature and the nominal temperature of the equipment. Table 3 also shows the kinetic parameters deduced for holocellulose, cellulose and lignin obtained from almond shells in a similar way. On the other hand, the rate constants of the cellulose and of the second peak corresponding to the cellulose in the holocellulose, are higher than those obtained for the second peak of the almond shells, probably due to the interaction between the different frac-
Sample amount (g.103)
5.410 5.440 5.825 5.322 5.289 5.289
Heating rate (“C/mm)
5 10 15 20 30 45
0.502 0.505 0.509 0.502 0.504 0.507
Fraction of residue y:, (W)
5.14 42.9 46.1 30.2 37.1 32.8
EA (kJ/mol) l.OlE-1 9.85El 3.26E2 4.75 8.01El 2.10El
k OA
(s-l) 3.59E-2 19OE-2 3.33E-2 l.l5E-2 4.90E - 2 3.02E - 2
(s-l)
k ACTOK
28.5 46.2 48.2 67.6 37.8 105.6
$/mot) 3.54E-1 2.50El 3.20El 2.2383 1.16El 6.61E6
$l)
Kinetic parameters obtained in the decomposition of almond shells impregnated with CoCl,
TABLE 4
1.21E-3 2.49E2.15E-3 2.998 6.19E 4.75E -
(s-l)
k%W
3 3 3
3
WA0 - WA,
0.033 0.148 0.189 0.323 0.332 0.380
(W)
W,
- WC,
0.465 0.347 0.302 0.175 0.164 0.113
(%)
1614 18.1 14.7 21.7 7.2 19.9
(W
Variation coefficient
260
tions in the almond shells. The activation energy and the pre-exponential factor of the first peak of the holocellulose (corresponding to the hemicellulose) are similar to the corresponding values in the almond shells pyrolysis. The kinetic parameters obtained for lignin DTG are in agreement with the fact that this material decomposes in a broad range of temperatures. It can be noted that the residue from this fraction is much higher than those of cellulose and holocellulose, showing that the contribution of the lignin decomposition to the overall process is much smaller than that of cellulose and hemicellulose. Results and kinetic parameters
for the almond shells impregnated
with CoCl,
Assuming that the process of decomposition in the presence of CoCl, takes place following a reaction scheme similar to that discussed before, we can apply the same equations previously deduced (eqns. (14) and (16)). Table 4 shows the experimental values of final mass fraction (q:,), the weight losses corresponding to both peaks (( W,, - W,,) and (IV& - I&,)) and the kinetic parameters optimized for the thermal decomposition of the almond shells impregnated with CoCl,, at the different heating rates used, indicating in all cases the variation coefficients. In this case, the fittings are poorer than in the previous case (variation coefficients around 20%), indicating a poorer correlation with the suggested model. We can observe that the values of W,, - W,, and W,, - Wc, (shown in Table 4) are different for each test, showing also that the real process cannot be simplified to two reactions. of the rate constants Figure 6 shows the variation of the logarithms at obtained for the almond shells, both with and without impregnation, rl
p8
).
Lnk
PrnoPnosE
.
-2 Lnk -4
-4 -6 -6
-8 -10
-8
-12 -10 1.5
1.6
1.8
1.7 l/1.103
(K-l1
1.9
1
1.2
1.4 lfT.103
1.6
1.8
2.0
(K-l)
Fig. 6. Variation of k, and kc vs. l/T, for experiments at lO”C/min ) and almond shells impregnated with CoCl 2 (- almond shells (-
heating rate, with -).
Fig. 7. Variation of the overall decomposition rate constant determined by TGA, overall decomposition (0) and total gases (0) constants determined in the F’yroprobe 100, and total gases (A) and total liquids (A) constants determined in a Fluidized Bed Reactor for almond shells.
261
lO”C/min heating rate, vs. the inverse of the temperature. It can be observed for both processes that the rate constants corresponding to the almond shells impregnated with CoCl, are higher than those corresponding to almond shells with no impregnation up to temperatures around 345 o C, showing the effect of the CoCl,. On the other hand, a very low activation energy for the rate constants of almond shells impregnated with CoCl, can be observed, as compared with almond shells without impregnation.
COMPARISON LITERATURE
OF RESULTS PARAMETERS
WITH
OTHER
EXPERIMENTAL
SYSTEMS
AND
Figure 7 shows the logarithms of the rate constants obtained with three different instruments used in this and previous studies [20]: TGA, Ryroprobe 100 and a Fluidized Bed Reactor. In the Fluidized Bed Reactor, the sample at room temperature is poured on a hot fluidized sand bed at the set temperature. In the Ryroprobe 100 analyser, a small sample is introduced in a small tube and remains at 200 OC for a long period before pyrolysis at the set temperature, which is reached quickly. Samples of almond shells with the same particle diameter range have been used in all three instruments. The rate constants for almond she& without impregnation, in the Pyroprobe and the Fluidized Bed Reactor, can be found elsewhere [20]. In the case of rate constants obtained by TGA, an overall constant has been obtained by adjusting the data to an equation similar to eqn. (14), but adapted to only one process (averaging, consequently, the two peaks and considering only one). The values obtained for the apparent activation energy and pre-exponential factor were 63.5 kJ/mol and 841 s-l at S”C/min and 67.2 kJ/mol and 6053 s-l at 45”C/min, respectively. As can be seen, the data obtained in the three experimental systems compare acceptably well. Figure 8 shows the logarithms of the rate constants obtained for almond shells impregnated with CoCl,, in the Pyroprobe 100 and in the Fluidized Bed Reactor [20], and in TGA. The data of the Fluidized Bed Reactor correspond to the evolution of liquids, the yield of which is much greater than the gas yield. (As was observed in ref. 20, the rate constants for the gas evolution are similar to those corresponding to liquid evolution, although the induction period is less. This also confirms that the real process cannot be simplified.) As in the previous case, the constants corresponding to the TGA data have been obtained by considering only one peak in the fitting of eqn. (14). The Fluidized Bed Reactor and Pyroprobe 100 data are not similar. There is an important effect that can explain the differences between the kinetic constants obtained in these two cases. In the Fluidized Bed Reactor, the catalyst (impregnating the almond shells) retains its water of hydration, whereas in the Pyroprobe 100, the sample has lost this water
262
Lnk
-5
-10
1.2 I/T
.103
(K-l)
1.4
1.6 ,IT IO3
1.8
2.0
(K-l1
Fig. 8. Variation of the overall decomposition rate constant determined by TGA, and overall decomposition (0) and total gases (0) constants determined in the Pyroprobe 100 and total liquids (0) constants determined in a Fluidized Bed Reactor for almond shells impregnated with CoCl,. Fig. 9. Comparison of the rate constants for pyrolysis of cellulose and other lignocellulosic materials (if specified) as obtained by the following authors: A, Hajaligol et al. (quoted in ref. 27); B, Tabataie-Raissi et al. [27]; C, Simmons and Lee (quoted in ref. 27); D, Bradbury et al. (quoted in ref. 27); E, present work, hemicellulose from the holocellulose fraction of almond shells (lO°C/min); F, present work, hemicellulose from almond shells (lO°C/min); G, present work, cellulose from holocellulose fraction of almond shells (10 o C/mm); H, present work, cellulose from cellulose fraction of almond shells (lO°C/min); I, present work, cellulose from almond shells (10” C/mm); J, present work, almond shells (lO°C/min); K, present work, lignin from almond shells (10 o C/mm); L, Tabataie-Raissi et al. (tar, [27]); M, Agrawal (carbon and gas [28]); N, Tabataie-Raissi et al. (carbon and gas [27]); 0, Agrawal (tar [28]); P, Ahmed (quoted in ref. 27).
because it remains for half an hour at 200°C before the pyrolysis takes place. The loss of two molecules of water per mol of CoCl, is endothermic, as can be observed from a DTA test shown elsewhere [20]. From Fig. 3, we observe that the loss of the hydration water coincides with the first part of the decomposition process. In Fig. 8 we observe that the kinetic constants obtained for the Fluidized Bed Reactor compare well with those from the TGA tests presented in this paper. This fact confirms that the hydration water of the CoCl, has a great influence on the pyrolysis carried out in the Fluidized Bed Reactor. Figure 9 shows the rate constant reported elsewhere [27,28] for cellulose, as compared with those obtained in the present work for cellulose, lignin and almond shells without impregnation. A general tendency for the rate constants to vary can be observed.
263 CONCLUSIONS
From an analysis of the weight loss rate vs. time data in TGA tests, it has been deduced that the decomposition of the almond shells can be explained by means of two independent processes. When pyrolysing almond shells without impregnation, the two processes considered correspond roughly to the hemicellulose and cellulose respectively. Lignin is decomposed in a broad temperature range, and its contribution to the overall process is smaller than that of hemicellulose and cellulose. Assuming an overall decomposition process with a first-order reaction, the kinetic constants obtained from three different instruments (TGA, Pyroprobe 100 and Fluidized Bed Reactor) compare quite well. An extrapolation of the kinetic data obtained from TG and DTG could be useful in the design of pyrolysers, including operations at high heating rates. As far as the pyrolysis of almond shells impregnated with CoCl, - 2H,O is concerned, the experimental data fit the model with two reactions less well than in the case of non-impregnated almond shells. The considerable influence of the hydration water of the CoCl, on the pyrolysis carried out in the TGA test and in the Fluidized Bed Reactor has been proved.
ACKNOWLEDGEMENT
We are grateful to Professor M. Rubio (University of Murcia, Spain) for his assistance in the chemical analysis of the almond shells.
REFERENCES 1 W.W. Wendlandt, Thermal Methods of Analysis, John Wiley, New York, 1974. 2 J. Sestak and V. Satava, Thermochim. Acta, 7 (1973) 333. 3 F.L. Browne and W.K. Tang, Effects of various chemicals on TGA of Ponderosa pine, Forest Products Laboratory, Paper 6, Madison, WI, 1963. 4 P.K. Chattejee and C.W. Conrad, Text. Res. J., 36 (1966) 487. 5 P.S. Maa and R.C. Bailie, 84th National Meeting AIChE, Atlanta, GA, 1978. 6 Z.L. Pyle and C.A. Zaror, Chem. Eng. Sci., 39(l) (1984) 147. 7 S.S. Alves and J.L. Figueiredo, J. Anal. Appl. Pyrolysis, 13 (1988) 123. 8 G. Varhegyi and J. Antal, Jr., Energy Fuels, 2 (1986) 273. 9 K. Akita and M.J. Kase, Polym. Sci., Part A-l, S(4) (1967) 833. 10 C.H. Mack and D.J. Donaldson, Text. Res. J., 37 (1967) 1063. 11 D.F. Arsenau, Can. J. Chem., 49 (1971) 632. 12 J.C. Leu, Ph.D. Thesis, University of Oklahoma, 1975. 13 D.Q. Tran and C. Rai, Pyrolytic Gasification of Bark, AIChE Symp. Ser., 75(184) (1979) 41. 14 M.J. Antal, Jr., H.L. Friedman and F.E. Rogers, Combust. Sci. Technol., 21 (1980) 141. 15 D.L. Urban and M.J. Antal, Jr., Fuel, 61 (1982) 799. 16 R. Bilbao, J. Arauzo and A. Millera, Thermochim. Acta, 120 (1987) 121.
264 17 R. Bilbao, J. Arauzo and A. Millera, Thermochim. Acta, 120 (1987) 133. 18 R. Font, A. Marcilla, E. Verdu and J. Devesa, Ind. Eng. Chem. Prod. Res. Dev., 25 (1986) 491. 19 R. Font, A. MarciIla, E. Verdu and J. Devesa, Can. J. Chem. Eng., 68 (1990) 312. 20 R. Font, A. Marcilla, E. Verdi and J. Devesa, Ind. Eng. Chem. Res., 29 (1990) 1846. 21 F.D. Snell and C.T. Snell, Calorimetric Methods of Analysis Including Photometric Methods, Vol. IIA, D. Van Nostrand Co., Princeton, NJ, 1959, p. 294. 22 B.L. Browning, Methods of Wood Chemistry, J. Wiley, New York, 1965. 23 ASTM D1105-56, Standard Method for Preparation of Extractive Free Wood, 1972. 24 ASTM D1787, Standard Test Method for Pentosans in CeIIulose, 1972. 25 TAPPI T223 05-78, Pentosans in Wood and Pulp, 1972. 26 F. Shafizadeh, Industrial Pyrolysis of Cellulosic Materials, Appl. Polym. Symp., 28 (1975) 153. 27 A. Tabataie-Raissi, S.L. Wiham and M.J. Antal, Jr., Ind. Eng. Chem. Res., 28 (1989) 856. 28 RR. Agrawal, Can. J. Chem. Eng., 66 (1988) 403.
249
249-264
Elsevier Science Publishers B.V., Amsterdam
Thermogravimetric kinetic study of the pyrolysis of almond shells and almond shells impregnated with CoCl, R. Font *, A. Marcilla, E. Verdti and J. Devesa Chemical Engineering Department,
University of Alicante, Apartado 99, Alicante (Spain)
(Received June 6, 1990; accepted in final form May 7, 1991)
ABSTRACT A kinetic study of the pyrolysis of almond shells, both with and without impregnation with CoCl,, has been carried out by thermogravimetric analysis (TGA). The experimental data have been discussed, assuming a scheme of two reactions as a simplification of the complex network of reactions. The corresponding kinetic parameters (apparent activation energy and pre-exponential factor), assuming first-order kinetics for both processes, have been calculated within the 200-410 o C temperature range, for almond shells without impregnation, and between HO-420°C for almond shells impregnated with CoCl,. When pyrolysing almond shells without impregnation and assuming an overall reaction, the results agree with those obtained from other techniques (Analytical Equipment Eyroprobe 100 and Fluidized Bed Reactor pyrolysis) and also compare well with other results for pyrolysis of cellulose reported in the literature. In the pyrolysis of almond shells impregnated with CoCl,, the results obtained compare well with those from the Fluidized Bed Reactor and have been useful for explaining the differences between the kinetic data deduced from the Eluidized Bed Reactor and the Pyroprobe 100. Almond shells; cobalt chloride; kinetic study; pyrolysis; thermogravimetric
analysis.
INTRODUCTION
Thermal decomposition of lignocellulosic materials occurs via complex mechanisms, which are greatly influenced by heat and mass transfer processes. There is a wide range in the literature of kinetic parameters for these types of processes, both in activation energy and rate constants. This fact is probably due to the large variety of raw materials investigated, the different pyrolysis techniques and different operating conditions used by researchers. Thermogravimetric analysis (TGA) is a technique commonly used for the study of the thermal decomposition of materials. Isothermal data are normally affected by temperature gradients as a result of the rapid heating of the sample up to the set temperature. In the non-isothermal mode these temperature gradients must be lower, since the temperature is increased slowly with time [1,2]. 01652370/91/$03.50
Q 1991 - Elsevier Science Publishers B.V. All rights reserved
250
Several articles have been found in the literature where isothermal or static TGA has been used to study biomass pyrolysis [3-71, as well as similar studies using non-isothermal or dynamic TGA [8-171. In this paper, we study the kinetics of pyrolysis of almond shells and almond shells impregnated with CoCl, using both techniques. CoCl, was chosen as primary catalyst after a screening of the different acid and basic catalysts. Acid catalysts such as CoCl, favour dehydration reactions, leading to the formation of 2-furaldehyde [18,19]. Present results are compared with those reported in the literature and with those obtained elsewhere [20], working with the same biomass and catalyst in a Fluidized Bed Reactor and in a Pyroprobe 100.
EXPERIMENTAL
Almond shells were washed, dried, crushed and sieved. The fraction of 0.500-0.297 mm was selected for the experiments. Almond shells impregnated with CoCl, were prepared by mixing a known amount of almond shells and a solution of known concentration of the catalyst. This mixture was dried in a “Rotavapor B&&i”, in a 60 mm Hg vacuum, and agitated for about 20 h. Finally, the sample was dried at 110 o C for 20 h. The amount of catalyst was determined spectrophotometrically, in accordance with the method described elsewhere [21]. The percentage of impregnation was 14.1% (g of CoCl,/total g). A Mettler TG50 thermobalance was used for the kinetic study. In the dynamic experiments, the operating conditions were the following: nitrogen flow rate, 200 cm3/min; initial temperature, 100°C; final temperature, 800 o C, for 5 min; heating rate 5-45 o C/mm. The composition of the almond shells was: 37% Kiirschner cellulose, 32% pentosans (i.e. hemicellulose), 27% Klasen lignin, 2% extractable matter (extracted with ethanol-benzene) and 0.1% ash. The analytical methods used are described in refs. 22, 23, 24 and 25.
RESULTS AND DISCUSSION
Almond shells without catalyst The decomposition of almond shells was studied at six different heating rates: 5, 10, 15, 20, 30 and 45 OC/mm. Curves similar to that shown in Fig. 1 (10 OC/mm) have been obtained. It can be observed that the pyrolysis of almond shells starts at around 200” C and is practically completed at 425 OC. The DTG curve shows two peaks, one centred at around 310 OC and the other sharper peak centred at around 368 OC. These results agree with
251
100
mcl
300
400
W”
bca
Fig. 1. TG and DTG curves, at 10 OC/min
7””
ROO
heating rate, for almond shells.
those obtained in the pyrolysis of almond shells at increasing temperatures (17 o C/min up to 400 o C and 8 o C/min up to 710 o C) in a Fluidized Bed Reactor.’ It was observed that up to 350” C no significant amount of volatiles were obtained, the major compounds being CO, and H,O. At temperatures higher than 350” C, the volatiles evolved from the sample, increasing the yield of tars, organic liquids, CO and CH, [18]. This fact may be explained in the following terms. From the two processes (peaks) observed in Fig. 1, the first one (310” C) may be produced mainly by decomposition of the lighter fraction of the almond shells (i.e. hemicellulose), leading to a loss of water, the production of a distillate, CO, and some CO. The second process could correspond to decomposition of cellulose, leading to the evolution of tars, hydrocarbons and organic liquids. At the same time, charring of the lignin fraction is taking place, overlapping with the two processes mentioned, since the pyrolysis of this compound takes place in a wide temperature range [7,8,26]. Figure 2 shows the TG and DTG curves, obtained at 10 o C/mm, corresponding to the fractions of hemicellulose, cellulose and lignin obtained from almond shells. It can be observed that the cellulose shows only one
Fig. 2. TG and DTG curves, at 10 o C/min from almond shells.
heating rate, for holocellulose,
cellulose
and lignin
peak, which clearly corresponds to the second peak of the almond shells (it appears at lower temperatures, i.e. 340°C than that of the almond shells, i.e. 368 o C, probably as a consequence of the interactions of this fraction with the hemicellulose and lignin fractions in the almond shells). The residue obtained is around 8.7%. Lignin shows a very wide peak centered at around 36O”C, leaving a high residue (i.e. 39.2%). On the other hand, the holocellulose fraction shows two peaks, one at 280” C and the other at 335 OC, corresponding to the two peaks shown by the almond shells. The residue obtained for this fraction was around 12.1%. These facts clearly show that the first peak in the almond shells corresponds to the decomposition of the hemicellulose and part of the lignin, whereas the second peak corresponds to the decomposition of the cellulose and another part of the lignin fraction. These results are in good agreement with those reported by Varhegyi and Antal [8]. Table 1 shows the temperature intervals corresponding to the two processes (represented by T,-T, and T2-T3,respectively), as well as the temperature where the maximum weight loss per unit time is produced for both processes ), for each run (see Fig. 1 for the meanings of T,,T, , T3, (TWX1 and TMA.W
253 TABLE 1 Temperature ranges and temperatures of maximum weight loss for the pyrolysis of almond shells without impregnation Heating rate ( o C/mm)
&)
;FGt
&)
5 10 15 20 30 45
210 216 217 215 215 220
293 310 313 314 316 323
330 340 340 345 350 350
;zG
&)
360 368 373 378 379 379
400 420 410 415 425 440
T MAX1 and TMAXZ ). It can be observed that a shift of TMAxl and TMAxz towards higher temperatures is produced when increasing the heating rate. These shifts are due to the combined effects of the heat transfer at the different heating rates and the kinetic decomposition. These results agree with those obtained by other authors [16,17]. Almond shells impregnated with CoCI, Six experiments at 5, 10, 15, 20, 30 and 45 o C/min heating rate have been carried out. Figure 3 shows the weight loss rate vs. temperature for the experiment at lO”C/min heating rate and Fig. 4 that corresponding to 45 o C/min heating rate. In the DTG curve corresponding to the experiment
n
\ -.“004
-
-.mR
_
dM/dt (w/s)
DTG
IOU
3cm
500
7wJ
T('C) Fig. 3. TG and DTG curves, at 10 o C/min heating rate, for almond shells impregnated with CoCl,. Fig. 4. TG and DTG curves, at 45 o C/min heating rate, for almond shells impregnated with Co&.
254 TABLE 2 Temperature ranges and temperatures of maximum weight loss for the pyrolysis of almond shells impregnated with CoCl, Heating rate ( o C/min)
r, (“C)
5 10 15 20 30 45
150 155 150 150 155 155
(TzC) 265 240 248 230 232 230
295 310 310 308 305 310
&) 333 340 339 340 337 335
410 405 410 420 420 440
carried out at 10 o C/mm, a broad peak centered at around 240 OC can be observed. This peak shows two small peaks which overlap, one at around 190” C (which can be observed in the experiments at up to 20” C/min heating rate) and the other at around 340 OC. On the other hand, the curve corresponding to the experiment at 45 “C/mm shows a broad peak at 230 OC and a small peak at around 340 OC. The small peak at around 190 OC (Fig. 3) has disappeared. The peak at around 340 OC is observed to decrease progressively with an increase in the heating rate, as can be observed in Figs. 3 and 4. Above 44O”C, the weight loss continues at lower rates although another peak can be observed in the DTG curves at temperatures around 500 o C, in all experiments as the temperature is increased. Table 2 shows the temperature intervals where the two peaks appear (represented by T,-T, and T,-T,, respectively), as well as the temperature where the higher weight loss rate is produced for each peak ( TMAxl and T MAxz). These results show that at low heating rates (10 OC/mm) a loss of approximately two molecules of water per molecule of CoCl, can be observed, even when the catalyst is dried at 100’ C. The shaded zone in the experiment at lO”C/min (Fig. 3) roughly corresponds to the hydration water present in the catalyst. This water loss at higher heating rates cannot be observed, since it overlaps with the peak corresponding to the pyrolysis of almond shells. On the other hand, as the heating rate is increased, the first peak is progressively clearer, whereas the second peak is less marked. These results show the very different behaviour of the almond shells impregnated with CoCl, as compared with the non-impregnated almond shells. KINETIC
STUDY
Kinetic model
The global process of thermal decomposition can be written as: B+aR+bV
of a homogeneous
material 0)
255
where B is the fraction of biomass, R the fraction of residue and V the fraction of volatiles present at time t, with respect to the initial amount of biomass. a and b are average coefficients of yield, expressed as grams of V or R formed per gram of B reacted. The processes of heat transfer and diffusion of the products formed may influence the overall apparent kinetics of the process. Neglecting such effects and considering first-order kinetics, we can write: -dB/dt
= kB
(4
In the TGA, it is not possible to determine the residue formed, since at any time the magnitude measured is: W=B+R
(3)
It can be demonstrated that the first-order kinetics expressed by eqn. (2) is equivalent to: dW/dt=
-k(W-
W,)
(4
where k is the same constant as in eqn. (2), W, is the final fraction of residue obtained, and W - W, is the fraction of biomass susceptible of decomposition. From the experiments carried out (and as previously noted) the decomposition of the almond shells apparently takes place by means of at least two processes, as can be seen in Figs. 1, 3 and 4. Due to this, we consider that the almond shells are formed of two independent fractions A and C, that decompose at different temperatures. Consequently, the following two reactions can be admitted: AkA\aR
+ bV
C%a’R’+
(5)
b’V’
(6)
Applying eqn. (4) to each of the reactions (assuming first-order kinetics) we can write: dW,/dt = -kA( dW,/dt=
(7)
W, - W,,)
-k,(W,-
W,,)
(8)
In the case of dynamic TGA, we have to consider the variation of the rate constant with temperature by means of Arrhenius-type equations. For the first process we can write: k, = k,,
exp( - E,/RT)
(9)
From eqns. (7) and (9): dW,/dt
= kOA exp( -E,/RT)(
Multiplying
W, - W,,)
and dividing eqn. (10) by exp(E,/RT,),
(10) where TM is an
256
intermediate temperature tween the pre-exponential dW,/dt=&
(considered to decrease the large interaction factor and the activation energy), we obtain:
exp(-%A%)
exp[--G/Nl/T-
I/%)](W,-
be-
W&J
We can write: T= &+m(t-t,)
(12)
where To is the initial temperature (K), m is the heating rate (K/s), t, is the initial time (s) and t the time (s). Considering that for the fraction C we would obtain a similar expression, and that, dW/dt
= dW,/dt
+ dWc/dt
(13)
we can finally write: dW/dt
= - k,,
exp( - EA/RTM >
xexp(-E,/R{l/[T,+m(t-t,)l - k,,
exP( - WRTh4
xexP(-&/R{V[T,
-l/L))(W,-
WA,)
1 + m(t-
to>] - VT,})(W,
- W,,)
(14)
This equation allows the kinetic parameters corresponding to the two processes to be calculated, by fitting the experimental data of time and weight loss rate at that time. In addition, the following data must be known: the final mass fraction ( W,), the initial temperature (To), the intermediate temperature considered (TM), the initial time (to) and the heating rate (m). For this purpose, we have used the Simplex flexible optimization method, integrating eqn. (14) by a Runge-Kutta algorithm of fourth order. In this way, we have optimized the following parameters: apparent activation energies corresponding to the first and second processes ( EA and EC respectively); pre-exponential factors corresponding to the first and second to processes (k,, and koc, respectively), and the weight loss corresponding to the the first process (WA, - WA,). Thus the weight loss corresponding second process is defined, since: w,,
- w,cc = 0 - W,)
The objective
function
OF = c [(dB/dt)”
- 06,
- W4,)
(15)
(OF) used has been:
- (dB,‘dt)‘]*
06)
where superscripts e and c represent experimental and calculated values respectively. We have used the values of dB/dt for the calculation of the kinetic parameters, since they allow a better separation of the two processes and a better fitting than the values of B.
10
Lignin
10
Holocellulose
10
7.504
7.898
6.954
0.391
0.121
0.087
0.333 0.336 0.333 0.313 0.308 0.296
5.204 5.071 5.210 4.713 5.042 4.971
5 10 15 20 30 45
Cellulose
Fraction of residue W& (W)
Sample amount (g.103)
Heating rate (“C/min)
k,,
1.50E7
5.10E7 2.8487 2.91E7 4.74E7 2.99E7 1.58E7
(s-l)
k OA
= 7.27E6 s-l,
98.3
112.0 107.8 106.2 108.3 104.5 100.3
(kJ/mol)
EA &03K
230.6
256.8
242.1 243.3 225.3 229.1 215.3 203.6
(kJ/mol)
EC
E, = 109.9 kJ/mol
46.1E- 3
9.05E - 3 11.2E-3 16.5E- 3 17.7E- 3 23.9E- 3 29.3E- 3
(s-l)
k
Kinetic parameters obtained in the decomposition of almond shells
TABLE 3
9.4OE17
1.46E20
5.10E17 7.71E17 2.99E16 6.10E16 4.84E15 5.34El4
k oc
(s - ‘)
&OK
101.7E-4
85.2E - 4
4.27E - 4 5.07E - 4 7.26E - 4 6.91E-4 8.72E - 4 lO.OE - 4
(s-l)
k
WA0 - WA,
0.350
0.359 0.346 0.339 0.375 0.363 0.363
(W)
WC0 - WC,
0.299
0.318 0.309 0.322 0.307 0.328 0.340
(W
8.7
15.7
3.2
7.3 5.8 5.5 7.4 6:O 6.2
(%)
Variation coefficient
258
0.00
dM/dt tmg/s)
-0.01
-0.02
260
310
360
't10
T ('C)
Fig. 5. Experimental () and calculated (almond shells at 30 o C/min heating rate.
- -)
DTG curves for the experiment with
Results and kinetic parameters for almond shells pyrolysis without impregnation Table 3 shows the values of qm considered for fitting the data of each experiment as well as the weight losses calculated for both processes ( W,, W,,) and ( W,, - I&,), and the optimized kinetic parameters obtained at the different heating rates, indicating in all cases the variation coefficients. The values of these coefficients are very low, especially if we consider that we are comparing the experimental and calculated values of the weight loss rate. Figure 5 shows, as an example, the curve (experimental (solid line) and calculated (dotted line)) for the experiment at 30” C/mm. A very good agreement can be observed. In Table 3 the kinetic parameters for both processes are also presented, as well as the rate constants calculated at 600 K. We can observe that the trend is for the rate constants to increase when increasing the heating rate. This indicates that the decomposition process takes place by a complex scheme of reactions. Nevertheless, the simplification assumed, taking into account only two parallel reactions, means an approximation to the real process and leads to values of kinetic constants of the same order. It is also possible that the variation of the kinetic constant values may be explained by bearing in mind that the overall process is exothermic, as can be seen from a DTA of the almond shells [19]. Thus, when increasing the heating rate, a higher temperature difference would exist between the actual sample temperature and the nominal temperature of the equipment. Table 3 also shows the kinetic parameters deduced for holocellulose, cellulose and lignin obtained from almond shells in a similar way. On the other hand, the rate constants of the cellulose and of the second peak corresponding to the cellulose in the holocellulose, are higher than those obtained for the second peak of the almond shells, probably due to the interaction between the different frac-
Sample amount (g.103)
5.410 5.440 5.825 5.322 5.289 5.289
Heating rate (“C/mm)
5 10 15 20 30 45
0.502 0.505 0.509 0.502 0.504 0.507
Fraction of residue y:, (W)
5.14 42.9 46.1 30.2 37.1 32.8
EA (kJ/mol) l.OlE-1 9.85El 3.26E2 4.75 8.01El 2.10El
k OA
(s-l) 3.59E-2 19OE-2 3.33E-2 l.l5E-2 4.90E - 2 3.02E - 2
(s-l)
k ACTOK
28.5 46.2 48.2 67.6 37.8 105.6
$/mot) 3.54E-1 2.50El 3.20El 2.2383 1.16El 6.61E6
$l)
Kinetic parameters obtained in the decomposition of almond shells impregnated with CoCl,
TABLE 4
1.21E-3 2.49E2.15E-3 2.998 6.19E 4.75E -
(s-l)
k%W
3 3 3
3
WA0 - WA,
0.033 0.148 0.189 0.323 0.332 0.380
(W)
W,
- WC,
0.465 0.347 0.302 0.175 0.164 0.113
(%)
1614 18.1 14.7 21.7 7.2 19.9
(W
Variation coefficient
260
tions in the almond shells. The activation energy and the pre-exponential factor of the first peak of the holocellulose (corresponding to the hemicellulose) are similar to the corresponding values in the almond shells pyrolysis. The kinetic parameters obtained for lignin DTG are in agreement with the fact that this material decomposes in a broad range of temperatures. It can be noted that the residue from this fraction is much higher than those of cellulose and holocellulose, showing that the contribution of the lignin decomposition to the overall process is much smaller than that of cellulose and hemicellulose. Results and kinetic parameters
for the almond shells impregnated
with CoCl,
Assuming that the process of decomposition in the presence of CoCl, takes place following a reaction scheme similar to that discussed before, we can apply the same equations previously deduced (eqns. (14) and (16)). Table 4 shows the experimental values of final mass fraction (q:,), the weight losses corresponding to both peaks (( W,, - W,,) and (IV& - I&,)) and the kinetic parameters optimized for the thermal decomposition of the almond shells impregnated with CoCl,, at the different heating rates used, indicating in all cases the variation coefficients. In this case, the fittings are poorer than in the previous case (variation coefficients around 20%), indicating a poorer correlation with the suggested model. We can observe that the values of W,, - W,, and W,, - Wc, (shown in Table 4) are different for each test, showing also that the real process cannot be simplified to two reactions. of the rate constants Figure 6 shows the variation of the logarithms at obtained for the almond shells, both with and without impregnation, rl
p8
).
Lnk
PrnoPnosE
.
-2 Lnk -4
-4 -6 -6
-8 -10
-8
-12 -10 1.5
1.6
1.8
1.7 l/1.103
(K-l1
1.9
1
1.2
1.4 lfT.103
1.6
1.8
2.0
(K-l)
Fig. 6. Variation of k, and kc vs. l/T, for experiments at lO”C/min ) and almond shells impregnated with CoCl 2 (- almond shells (-
heating rate, with -).
Fig. 7. Variation of the overall decomposition rate constant determined by TGA, overall decomposition (0) and total gases (0) constants determined in the F’yroprobe 100, and total gases (A) and total liquids (A) constants determined in a Fluidized Bed Reactor for almond shells.
261
lO”C/min heating rate, vs. the inverse of the temperature. It can be observed for both processes that the rate constants corresponding to the almond shells impregnated with CoCl, are higher than those corresponding to almond shells with no impregnation up to temperatures around 345 o C, showing the effect of the CoCl,. On the other hand, a very low activation energy for the rate constants of almond shells impregnated with CoCl, can be observed, as compared with almond shells without impregnation.
COMPARISON LITERATURE
OF RESULTS PARAMETERS
WITH
OTHER
EXPERIMENTAL
SYSTEMS
AND
Figure 7 shows the logarithms of the rate constants obtained with three different instruments used in this and previous studies [20]: TGA, Ryroprobe 100 and a Fluidized Bed Reactor. In the Fluidized Bed Reactor, the sample at room temperature is poured on a hot fluidized sand bed at the set temperature. In the Ryroprobe 100 analyser, a small sample is introduced in a small tube and remains at 200 OC for a long period before pyrolysis at the set temperature, which is reached quickly. Samples of almond shells with the same particle diameter range have been used in all three instruments. The rate constants for almond she& without impregnation, in the Pyroprobe and the Fluidized Bed Reactor, can be found elsewhere [20]. In the case of rate constants obtained by TGA, an overall constant has been obtained by adjusting the data to an equation similar to eqn. (14), but adapted to only one process (averaging, consequently, the two peaks and considering only one). The values obtained for the apparent activation energy and pre-exponential factor were 63.5 kJ/mol and 841 s-l at S”C/min and 67.2 kJ/mol and 6053 s-l at 45”C/min, respectively. As can be seen, the data obtained in the three experimental systems compare acceptably well. Figure 8 shows the logarithms of the rate constants obtained for almond shells impregnated with CoCl,, in the Pyroprobe 100 and in the Fluidized Bed Reactor [20], and in TGA. The data of the Fluidized Bed Reactor correspond to the evolution of liquids, the yield of which is much greater than the gas yield. (As was observed in ref. 20, the rate constants for the gas evolution are similar to those corresponding to liquid evolution, although the induction period is less. This also confirms that the real process cannot be simplified.) As in the previous case, the constants corresponding to the TGA data have been obtained by considering only one peak in the fitting of eqn. (14). The Fluidized Bed Reactor and Pyroprobe 100 data are not similar. There is an important effect that can explain the differences between the kinetic constants obtained in these two cases. In the Fluidized Bed Reactor, the catalyst (impregnating the almond shells) retains its water of hydration, whereas in the Pyroprobe 100, the sample has lost this water
262
Lnk
-5
-10
1.2 I/T
.103
(K-l)
1.4
1.6 ,IT IO3
1.8
2.0
(K-l1
Fig. 8. Variation of the overall decomposition rate constant determined by TGA, and overall decomposition (0) and total gases (0) constants determined in the Pyroprobe 100 and total liquids (0) constants determined in a Fluidized Bed Reactor for almond shells impregnated with CoCl,. Fig. 9. Comparison of the rate constants for pyrolysis of cellulose and other lignocellulosic materials (if specified) as obtained by the following authors: A, Hajaligol et al. (quoted in ref. 27); B, Tabataie-Raissi et al. [27]; C, Simmons and Lee (quoted in ref. 27); D, Bradbury et al. (quoted in ref. 27); E, present work, hemicellulose from the holocellulose fraction of almond shells (lO°C/min); F, present work, hemicellulose from almond shells (lO°C/min); G, present work, cellulose from holocellulose fraction of almond shells (10 o C/mm); H, present work, cellulose from cellulose fraction of almond shells (lO°C/min); I, present work, cellulose from almond shells (10” C/mm); J, present work, almond shells (lO°C/min); K, present work, lignin from almond shells (10 o C/mm); L, Tabataie-Raissi et al. (tar, [27]); M, Agrawal (carbon and gas [28]); N, Tabataie-Raissi et al. (carbon and gas [27]); 0, Agrawal (tar [28]); P, Ahmed (quoted in ref. 27).
because it remains for half an hour at 200°C before the pyrolysis takes place. The loss of two molecules of water per mol of CoCl, is endothermic, as can be observed from a DTA test shown elsewhere [20]. From Fig. 3, we observe that the loss of the hydration water coincides with the first part of the decomposition process. In Fig. 8 we observe that the kinetic constants obtained for the Fluidized Bed Reactor compare well with those from the TGA tests presented in this paper. This fact confirms that the hydration water of the CoCl, has a great influence on the pyrolysis carried out in the Fluidized Bed Reactor. Figure 9 shows the rate constant reported elsewhere [27,28] for cellulose, as compared with those obtained in the present work for cellulose, lignin and almond shells without impregnation. A general tendency for the rate constants to vary can be observed.
263 CONCLUSIONS
From an analysis of the weight loss rate vs. time data in TGA tests, it has been deduced that the decomposition of the almond shells can be explained by means of two independent processes. When pyrolysing almond shells without impregnation, the two processes considered correspond roughly to the hemicellulose and cellulose respectively. Lignin is decomposed in a broad temperature range, and its contribution to the overall process is smaller than that of hemicellulose and cellulose. Assuming an overall decomposition process with a first-order reaction, the kinetic constants obtained from three different instruments (TGA, Pyroprobe 100 and Fluidized Bed Reactor) compare quite well. An extrapolation of the kinetic data obtained from TG and DTG could be useful in the design of pyrolysers, including operations at high heating rates. As far as the pyrolysis of almond shells impregnated with CoCl, - 2H,O is concerned, the experimental data fit the model with two reactions less well than in the case of non-impregnated almond shells. The considerable influence of the hydration water of the CoCl, on the pyrolysis carried out in the TGA test and in the Fluidized Bed Reactor has been proved.
ACKNOWLEDGEMENT
We are grateful to Professor M. Rubio (University of Murcia, Spain) for his assistance in the chemical analysis of the almond shells.
REFERENCES 1 W.W. Wendlandt, Thermal Methods of Analysis, John Wiley, New York, 1974. 2 J. Sestak and V. Satava, Thermochim. Acta, 7 (1973) 333. 3 F.L. Browne and W.K. Tang, Effects of various chemicals on TGA of Ponderosa pine, Forest Products Laboratory, Paper 6, Madison, WI, 1963. 4 P.K. Chattejee and C.W. Conrad, Text. Res. J., 36 (1966) 487. 5 P.S. Maa and R.C. Bailie, 84th National Meeting AIChE, Atlanta, GA, 1978. 6 Z.L. Pyle and C.A. Zaror, Chem. Eng. Sci., 39(l) (1984) 147. 7 S.S. Alves and J.L. Figueiredo, J. Anal. Appl. Pyrolysis, 13 (1988) 123. 8 G. Varhegyi and J. Antal, Jr., Energy Fuels, 2 (1986) 273. 9 K. Akita and M.J. Kase, Polym. Sci., Part A-l, S(4) (1967) 833. 10 C.H. Mack and D.J. Donaldson, Text. Res. J., 37 (1967) 1063. 11 D.F. Arsenau, Can. J. Chem., 49 (1971) 632. 12 J.C. Leu, Ph.D. Thesis, University of Oklahoma, 1975. 13 D.Q. Tran and C. Rai, Pyrolytic Gasification of Bark, AIChE Symp. Ser., 75(184) (1979) 41. 14 M.J. Antal, Jr., H.L. Friedman and F.E. Rogers, Combust. Sci. Technol., 21 (1980) 141. 15 D.L. Urban and M.J. Antal, Jr., Fuel, 61 (1982) 799. 16 R. Bilbao, J. Arauzo and A. Millera, Thermochim. Acta, 120 (1987) 121.
264 17 R. Bilbao, J. Arauzo and A. Millera, Thermochim. Acta, 120 (1987) 133. 18 R. Font, A. Marcilla, E. Verdu and J. Devesa, Ind. Eng. Chem. Prod. Res. Dev., 25 (1986) 491. 19 R. Font, A. MarciIla, E. Verdu and J. Devesa, Can. J. Chem. Eng., 68 (1990) 312. 20 R. Font, A. Marcilla, E. Verdi and J. Devesa, Ind. Eng. Chem. Res., 29 (1990) 1846. 21 F.D. Snell and C.T. Snell, Calorimetric Methods of Analysis Including Photometric Methods, Vol. IIA, D. Van Nostrand Co., Princeton, NJ, 1959, p. 294. 22 B.L. Browning, Methods of Wood Chemistry, J. Wiley, New York, 1965. 23 ASTM D1105-56, Standard Method for Preparation of Extractive Free Wood, 1972. 24 ASTM D1787, Standard Test Method for Pentosans in CeIIulose, 1972. 25 TAPPI T223 05-78, Pentosans in Wood and Pulp, 1972. 26 F. Shafizadeh, Industrial Pyrolysis of Cellulosic Materials, Appl. Polym. Symp., 28 (1975) 153. 27 A. Tabataie-Raissi, S.L. Wiham and M.J. Antal, Jr., Ind. Eng. Chem. Res., 28 (1989) 856. 28 RR. Agrawal, Can. J. Chem. Eng., 66 (1988) 403.