482
I&.
~ n g chem. . process
~ e v 1901, . 20, 482-488
BS.
f = fraction of RTD curve corresponding to first symmetric
bubble holdups separately. It was found that large bubbles have kLa values which agree with extrapolations of the values in the literature; there is no decrease in these values due to decreased interfacial area a. The kL values for the large bubbles appear to be an order of magnitude higher than those calculated from correlations derived for small bubbles. It is conjectured that the violently turbulent state of the gas-liquid interface is the cause for this increase. The data reported here should be applicable to tall, narrow columns in the absence of cocurrent liquit flow. With cocurrent liquid flow a portion of the small bubble population will be transported up the column, giving rise to extra mass transfer. Our results show that kLa’(= kLa/el)is independent of the superficial gas velocity in the range 0.1 and 0.3 m/s. Thus, if one were able to measure, or estimate, the large bubble holdup in the column, the mass transfer coefficient kLa could be easily determined. This suggests a simple scale-up rule for use in practice. Further experimentation with systems having different physical properties (surface tension, viscosity etc.) will be required to study the factors which determine the bubble size distribution and consequently the gas-liquid mass transfer. Acknowledgment The authors would like to express their appreciation to Shell Research B.V. for permitting publication of these results. B. H. Bosman (deceased) and C. N. Garnier performed most of the experimental work reported here and are gratefully acknowledged. Nomenclature a = interfacial area per unit volume of reactor, m-l a’ = interfacial area per unit volume of transported gas, m-l A = interfacial area of tracer-containinggas bubbles going up in plug flow, m2 B = constant of proportionality used in eq 5, s-l/* c = tracer concentration, mol/m3 co = tracer concentration at gas inlet, mol/m3 cL = tracer concentration in the liquid hase, mol/m3 DL = tracer diffusivity in the liquid, mP/s
distribution
fo = value o f f at column inlet H = Henry coefficient Ah = decrease in height during dynamic gas disengagement,
m kL = liquid phase mass transfer coefficient, m/s L = column height, m Q = quantity of tracer, mol t = time, s V = volume of tracer-containing gas (plug flow), m3 V,, = rise velocity of large bubbles, m/s V = superficial gas velocity (usually at column outlet), m/s flw = superficial gas velocity at column inlet, m/s Greek Letters 6 = solubility parameter, ( c a l / ~ m ~ ) ’ / ~ tg = total gas holdup el = transport gas holdup t2 = entrained gas holdup 9 = dynamic viscosity, Ns/m2 p = liquid density, kg/m3 y = interfacial tension, N/m
Literature Cited Akita, K.; Yoshida, F. Ind. Eng. Chem. procesS Des. Dev. 1974, 13, 84. Bach. H. F. Ph.D. Dissertation, Technical University, Munkh, 1977. Calderbank. P. H.; Moo-Young, M. B. Chem. Eng. Scl. 1961, 76. 39. Coppus, J. H. C. Ph.D. Dissertatbn, Technical University, Eindhoven, 1977. Fair, J. R. Chem. Eng. July 3, 1967, 67. Hliis, J. H. Trans. Inst. Chem. Eng. 1974, 52, 1. Hills, J. H. Chem. Eng. J. 1976. 12, 69. Hills, J. H.; Darton, R. C. Trans. Inst. Chem. Eng. 1976, 54, 258. Jekat, H. W.D. DissertatkM, Technical University, Munich, 1975. Kim, S. D.; Baker, C. G.; Bergougnou. M. A. Chem. Eng. Scl. 1977. 32, 1299. KBlbei, H.; Beinhauer, R.; Langemann, H. Chem. Ing. Tech. 1972, 44, 697. MasheBcar, R. A. Brit. Chem. Eng. 1970, 15, 1297. Mashelkar, R. A.; Sharma, M. M. Trans. Inst. Chem. Eng. 1970, 48, 162. Ohki, Y.; Inooe, H. Chem. Eng. Sci. 1976, 25, 1. Prausnitz, J. M. “Molecuiar Theory of Fluid Phase Equilibria”, Prentlce-Hell: Engbwood Cuffs, NJ, 1969. Reith. T. W.D. Diswtatbn, Technical University, Delft, 1988. Towell, G. D.; Strand, C. P.; Ackermann, G. H. A.I.Ch.E. I.Chem.E. symp. Serbs No. 70, 1985. Ueyama, K.; Mlyauchl, T. Kogaku Kogaku Ronbunshu 1977, 3 , 115. Voyer, R. D.; Miller, A. I. Can. J. Chem. Eng. 1968, 46, 335. Wilke, C. R.; Chang, P. C. AIChE J 1955, 7, 264.
Received for review June 3, 1980 Accepted January 15, 1981
Kinetic Investigation of Wood Pyrolysis Franr Thurner and Uzl Mann‘ h p a t f m n t of Chemical Engineerhg, Texas Tech University, Lubbock, Texas 79409
The kinetics of wood pyrolysis into gas, tar, and char was investigated in the range of 300 to 400 O C at atmospheric pressure. An experimental system which facilitates the monitoring of the actual sample temperature, collection of gas and tar, and measurement of the sample weight loss as a function of time was developed. It has been found that, in the range investigated, wood decomposition into gas,tar, and char can be described by three parallel first-order reactions as suggested by Shafizadeh and Chin (1977). The actbation energies for these reactions are 88.6, 112.7 and 106.5 kJ/mol, respectively, and their frequency factors defined on a mass basis are 8.61 X IO6, 2.47 X IO8, and 4.43 X IO7 min-’. The composltion of the pyrolysis products was also analyzed. It was found that the gas consists mainly of carbon dioxide, carbon monoxide, oxygen, and C3+ compounds with trace amounts of methane, ethylene, and acetylene. The tar consists of seven compounds with levoglucosan accounting for m e than half. The char was analyzed by elemental analysis and it was found that its carbon content increases wlth increasing reactor temperature.
Introduction The substantial increase in the price of crude oil in the last few years, as well as the concern over the ultimate 0196-4305/81/1120-0482$01.25/0
availability of fossil fuels, prompted increasing interest in using biomass as renewable resources for chemical feedstocks. Wood and other cellulosic materials represent more 0
1981 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981 483
/
1 4
Wood’
Figure 1. Mechanism of wood pyrolysis suggested by Shafizadeh (1977).
than two-thirds (on a dry basis) of all renewable matters produced on land (Goldstein, 1978). The utilization of these huge resources depends on the development of an economically viable technology for converting cellulosic materials into more conveniently used gaseous and liquid products. One of the most promising ways to do so is pyrolysis-thermal decomposition at high temperature in the absence of oxygen. However, the implementation of such processes depends to a large extent on reliable design of large-scale units, in which the pyrolysis reactor plays an important role. Proper design of such a reactor requires knowledge and understanding of wood pyrolysis kinetics. Many investigators have studied the pyrolysis of wood and other cellulosic materials and reported overall product yields, heat effects, and weight loss at different reactor temperatures. However, very few kinetic data are available on the formation rates of the various pyrolysis products. This is probably due to the wide variety of products obtained in wood pyrolysis, and the limitations of the available experimental methods used to determine kinetic parameters. Shafizadeh and Chin (1977) have proposed a simplified mechanism for wood pyrolysis, shown in Figure 1. The mechanism is based on lumping the different products into three product groups: gas, tar, and char. Thus, the wood decomposition is described by three parallel reactions (reactions 1, 2, and 3), called the primary reactions, whereas the tar decomposes according to two parallel reactions (reactions 4 and 5 ) , referred to as secondary reactions. Experimental verification of the model and determination of the kinetic parameters require simultaneous collection of tar and gas and measurement of wood weight loss as a function of time. No experimental technique capable of providing these data has been described in the literature. The most commonly used techniques employed in pyrolysis kinetic studies are based on thermogravimetric methods. These techniques provide measurements of sample weight loss as a function of either time (see, for example, Kanury, 1972; Roberts and Clough, 1963) or temperature (see, for example, Hileman et al., 1976; Fairbridge et al., 1978), but do not provide the capability to collect the pyrolysis products. Recently, Shafizadeh et al. (1979) used a tube furnance which enabled the measurement of sample weight loss and the amount of tar collected. However, the amount of gas generated was not measured independently. Both the latter method and the thermogravimetric techniques have two major drawbacks. First, measurement of sample weight loss without measuring the amounts of tar and gas generated does not allow the determination of the products distribution. Second, the temperature variation during the sample heating-up period is neglected and the final (steady-state) temperature is taken as the reaction temperature. The variation of the sample temperature is probably the main reason for the wide deviations of the activation energy for the weight loss reaction reported in the literature (Roberts, 1970; Kanury,
1972; Roberts and Clough, 1963; Stamm, 1956). The objective of this work was to investigate the kinetics of wood pyrolysis into gas, tar,and char, to determine the reaction rate parameters, and to identify the composition of the pyrolysis products. For this purpose an experimental system was constructed which facilitated the measurement of actual sample temperature, as well as the mass of gas and tar generated and the sample weight loss as a function of time. The system is simple, inexpensive, and provides accurate kinetic data without the need of sophisticated instruments.
Theory Consider the pyrolysis mechanism suggested by Shdizadeh and Chin (1977) shown schematically in Figure 1. Wood is pyrolyzed into gas, tar,and char according to three parallel reactions (reaction 1,2,3),called primary reactions, and the tar decomposes into gas and char according to two parallel reactions (reaction 4,5), called secondary reactions. Each product in Figure 1 represents a sum of numerous components which are lumped together to simplify the analysis. The composition of each product, especially the distribution between the gas and the tar, depends, among other things, on the conditions under which the products are collected. In principle, the reaction rate constants of these five reactions can be determined by measuring the amount of each product as a function of time. When the tar is removed from the reaction zone the secondary reactions are avoided and the reaction rate constants of the primary reactions can be determined directly from these measurements. Assuming that each primary reaction is first-order, the formation or disappearance rate of each component is given by
(4)
These relations are expressed in terms of mass rather than mole. It is more convenient to express these equations in terms of mass fraction by dividing them by the initial mass of the wood, mw(0), i.e.
(7)
For isothermal pyrolysis, eq 5 to 8 can be solved analytically. The solutions under the initial conditions that ww(0) = 1 and wG(0) = wT(0) = wc(0) = 0 are ww(t) = exp(-kt) (9) kl WG(t) = -[I - exp(-kt)] k
484
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981 k3
w d t ) = -11 k - exp(-kt)]
where k = kl + k2 + k3 is the overall rate constant of wood pyrolysis. Reactions 1, 2, and 3 are independent. Therefore, to determine their rate constants it is necessary to measure the mass fraction of any three products of Figure 1 as a function of time. Experimentally, the mass fraction of gas and tar can be measured directly, but it is impossible to measure the mass fraction of either unreacted wood or char separately, because both are collected together as soild residue. The procedure developed below enables one to determine the reaction rate constants of the primary reactions. Using the overal mass balance, the mass fraction of the residue is related to the mass fractions of gas and tar by w R ( t ) = ww(t) + wc(t) = 1 - w ~ ( t-) w ~ ( t ) (13) At isothermal conditions, a relation between the char mass fraction and the residue mass fraction can be obtained by combining eq 9 to 12 with eq 13 wc(t) -- W C ( t ) =- k3 (14) w ~ ( t+) W T ( ~ ) 1 - W R ( ~ ) k i + k2 Note that eq 14 is independent of time and is also valid for a long reaction time or high conversion. At complete conversion ww(t m) = 0, and from eq 13 w R ( t m) = wc(t a). Thus, eq 14 can be expressed in terms of w R ( t m) by
--
-
wR(t
-
-
--
-*
k3
(15) k l + k2 This relation indicates that k , / ( k , + k2) can be easily determined by measuring the residue mass fraction at complete conversion. Combining eq 14 and 15, the mass fraction of char at any time can be calculated by 1- w R ( t
m)
The wood mass fraction, ww(t), is obtained from the overall mass balance, ww(t) = 1 - wG(t)- wc(t) - wT(t), and eq 16, i.e.
where A w R ( t ) = 1 - w R ( t ) . Equations 14 to 17 are valid only for isothermal pyrolysis. But, isothermal conditions are rarely achieved experimentally due to the heating-up of the sample. In principle, for nonisothermal pyrolysis the kinetic parameters can be estimated by first integrating eq 5 to 8 according to the temperature variation with time and then by nonlinear parameter estimation. This is a difficult and cumbersome procedure. Fortunately, in many cases k3/(kl + k& varies very little over a wide temperature range. This occurs when the activation energy of the char formation reaction (reaction 3) is about the same as the activation energy for the weight loss reaction (reactions 1+ 2). This assumption can be experimentally verified a priori by determining whether wR(t is the same at different reaction temperatures in the range where the pyrolysis reaction is the rate-controlling step. When this assumption is valid, the reaction rate constant of reaction 3 can be calculated by using eq 15
-
h
F
C2
CONDENSER TR T E M P E R A T U R E RECORDER T C TEMPERATURE C O N T R O L L E R
S6 S A W L E BOA'
Figure 2. Schematic diagram of the experimental system.
-LQQLEB
-25r---lii
483-
RFACTION CHAMBER
- -
----330--*--
CONDENSER *,9---
254
TC 2
4E
UITROGEN INLT 3-RIYG SEA,
SAMPLE B3AT
I N S ~ _ ' A TO h
ASSEMBLY
ASBESTOS GASKtT
RESISTANCE HEATER
Figure 3. The pyrolysis reactor.
where the reaction rate constants of reaction 1and 2, kl and k2, can be determined from the experimental composition-time curves of gas and tar by
The mass fraction of unreacted wood, ww(t), is calculated by eq 17. Experimental System and Procedure The kinetic measurements were carried out on a simple experimental system designed and constructed for this investigation. The system facilitated the collection of gas and tar and measurement of residue mass as a function of time. The variation of the wood temperature with time was also monitored. The system provided accurate data without the need of sophisticated instrumentations. A flow diagram of the experimental system is shown in Figure 2. The system is a modification and improvement of the pyrolysis tube used by Shafhadeh et al. (1979). It consists of three sections: a reactor, a gas supply section, and a gas collection section, Details and dimensions of the reactor are given in Figure 3. The reactor consisted of a reaction chamber, a cooler, and a condenser. The reaction chamber was made of a l-in. (25.4 mm), schedule 40 stainless steel tube. This tube was heated by two semi-cylindricalelectrical heating units having a total power of 435 W. The temperature inside the chamber was measured with a stationary chromelalumel thermocouple (TC-1) and controlled by an on-off proportional controller. A sample boat assembly was used to insert the wood sample into the reaction chamber and to withdraw it into the cooler at the end of a run. The sample boat assembly was constructed of a 3/4-in.(19mm), schedule 40 stainless steel tube, the upper half of which was cut off to allow the
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981 485
sample boat to be moved on it. A ceramic combustion boat 90 mm long and 14 mm wide was used as sample boat. Thermocouple TC-2 was embedded in the middle of the sample boat in order to measure the reaction temperature accurately inside the sample. This thermocouple was also used to move the sample boat into and out of the reaction chamber. The temperatures were recorded every 3 s. The time constant of both thermocouples was about 5 s. The cooler was used to maintain the sample at a low temperature before it was inserted into the reaction chamber and to cool it down quickly at the end of a run. The condenser was used to condense and collect the tar. Prior to each run the condenser wall was covered with aluminum foil and its interior space was loosely filled with steel wool. A 1.5411. (38 mm) edge of the aluminum foil was extended into the reaction chamber to assure complete collection of the tar. Both the cooler and the condenser were cooled with tap water (20 "C). The gas leaving the reactor was collected in an 18-L plastic gas collection bag. More details on the construction and operation of the system are provided elsewhere (Thurner et al., 1980). In each run, a known amount of sawdust was charged into the ceramic sample boat and placed in the cooler. Preweighed aluminum foil and steel wool were placed inside the condenser. After the system was checked for leaks, the nitrogen flow started, the heater was turned on, and the controller setpoint was adjusted at the desired reactor temperature. When steady-state conditions were achieved, the gss flow rate was measured and the gas was diverted into the gas collection bag. Then the sample boat was inserted into the center of the reaction chamber. When the desired reaction time expired the sample boat was withdrawn into the cooler. The nitrogen flow was continued for a few more minutes to purge the reactor and to assure that all gaseous products were collected in the gas collection bag. Both the reaction time and the time of gas collection were recorded. The mass of the residue in the sample boat and the mass of tar collected on the aluminum foil and steel wool were determined. The gas collected was analyzed by a gas chromatograph and the amount of gas generated was calculated on the basis of nitrogen balance. In most runs the overall material balance over the wood and pyrolysis products was higher than 95%.
Results and Discussion The pyrolysis of oak sawdust from Missouri was studied in this investigation. The as-received wood particles were slightly cylindrical in shape and their surface-mean size based on sieve analysis was 0.840 mm (see Thurner et al., 1980). Prior to pyrolysis, the particles were dried at 120 "C for 1h and the surface-mean size of the dried particles was 0.615 mm. An elemental analysis performed on the dry sawdust indicated that the wood contained 47.0% C, 5.6% H, 41.8% 0 (weight percent). First, a series of experiments was conducted to determine whether heat or mass transports affected the pyrolysis rate and to identify the range of operating conditions in which the reaction is the rate-controlling step. Only under such conditions the measured rate can be directly used to study the pyrolysis kinetics. These experiments were conducted at 450 "C, at different nitrogen flow rates, with different particle size and different sample thicknesses. It was found that for temperatures below 450 "C, wood particles smaller than 2 mm and nitrogen flow rates higher than 2 mL/s the pyrolysis reaction is the ratecontrolling step. Consequently, in this study the kinetic of wood pyrolysis was determined in the range of 300 to 400 "C, with wood particles of about 1 mm and at a ni-
Figure 4. Measurements at reactor temperature of 354 "C. loot
1400 w
n
RESIDUE A
---m t
-
TAR GAS REACTION TEMPERATURE
-300
5 d
I
XEACTION TIME,mtn
Figure 5. Measurements at reactor temperature of 369 "C.
1;
m
t
in
'00
'0
5
10 15 20 REACTION TlME,min
25
3oo
Figure 6. Measurements at reactor temperature of 392 "C.
trogen flow rate of about 3.5 mL/s (see more details in Thurner et al., 1980). Kinetic Parameters. The kinetic parameters of wood pyrolysis were determined from measurements of the mass fraction of residue, tar, and gas as a function of reaction time at three reactor temperatures: 354,369, and 392 "C (see Figures 4, 5, and 6). The experimental points are marked in the figures, and the drawn curves represent the best visual curve fit of the experimental data. The dashed curve indicates the reaction temperature measured with the thermocouple embedded in the sample (TC-2 in Figure 3). The reaction rate constants kl,k2,and It3 were determined according to eq 18 to 20. The residue mass fraction at complete conversion, wR(t a),measured at reactor temperatures of 329, 354, and 392 "C was 0.299 with a standard deviation of 0.028. This indicated that the assumptions that k 3 / ( k l + k,) is independent of the temperature is valid and the use of these equations is justified. In determining the reaction rate constants from the experimental data, two points were considered. First, Roberts (1970) indicated that the pyrolysis mechanism of cellulosic materials above 300 "C was different from that at lower temperatures. Therefore, in this investigation the kinetic parameters were determined from the experimental data at reaction temperatures above 325 "C. Second, a small amount of the tar was condensed on the aluminum foil edge inside the reaction chamber. This tar was deocomposed at long reaction times according to the secondary reactions. Therefore, the kinetic parameters were deter-
-
486
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981
Table I. Activation Energies and Calculated Frequency Factors frequency factor, activation energy, reaction rate constant min-' kJ/mol 8.607 X l o 5 88.6 k, 2.475 X l o 8 112.7 k* 4.426 X l o 7 1.039 X l o 8 1.481 X l o 8
k3
k, tk, k
90% confidence interval for activation energy, kJ/mol
correlation coefficient
k23.1 + 29.8 t27.2 t 27.4 t27.4
0.96 0.95 0.96 0.96 0.96
106.5 106.5 106.5
looK
I
I
RESICUE A TAR
20
1'5
i5
jG
REACTION TiME,m)n
Figure 8. Comparison of calculated and experimental results at reactor temperature of 354 "C. ~2~
55'
16CC B g Z r*
575
162s
183:
Figure 7. Arrhenius plot of reaction rate constants.
mined between 3 and 10 min to avoid the effect of the secondary reactions. The reaction rate constants kl,k2,and k3 were determined at seven different reaction temperatures in the range of 325 to 385 "C. The Arrhenius plot of these data is shown in Figure 7. The slopes and intercepts were calculated by linear regression. Table I gives the activation energies and frequency factors of the three primary reactions, the overall pyrolysis reaction, and the mass loss reactions (reactions 1 and 2). The activation energy of the mass loss reactions is somewhat lower than the 109 to 139 kJ/mol reported by other investigators (Antal et al., 1979; Roberts, 1970; Kanury, 1972; Roberts and Glough, 1963; Stamm, 1956). The reason for the wide variation in activation energies reported by these investigators is probably the inaccurate measurement of the wood temperature during the pyrolysis. Note that the activation energies of the three primary reactions are comparable. This implies that in the range investigated the distribution of the pyrolysis product does not strongly depend on the pyrolysis temperature. To evaluate whether the kinetic parameters calculated above can predict the yield of the pyrolysis products, we computed the composition-time curves for the various products using these parameters. In principle, it is possible to calculate the cumulative mass fraction of component i as a function of time by
where i stands for W , G, T , and C and dwi(8)/dt are given by eq 9 to 12. In the integration of eq 21, the variation of the kinetic rate constants with time should be taken into consideration. This was done by incorporating the temperature-time curve. However, the calculated curves did not fit well the experimental data, especially in the heating-up period. This is probably due to the different pyrolysis mechanism at low temperatures (Roberts, 1970). In another attempt, we calculated the composition-time curves using the rate constants at the steady-state (final)
Figure 9. Comparison of calculated and experimental results at reactor temperature of 369 "C.
1
:Oah
k T1 I
A
PEAC'IOU
TlME,mr
Figure 10. Comparison of calculated and experimental results at reactor temperature of 392 O C .
reactor temperature. As expected, the calculated conversion in the heating-up period was higher than the experimental conversion. We then decided to check whether the experimental composition-time curves can be predicted by eq 9 to 12 with the kinetic parameters taken at some constant temperature. The problem was to find the proper reaction temperature to be used. We tried to use an integral-mean temperature
T
=
t
st o
T(t9)d0
calculated on the basis of the measured reaction temperature. We found that the best fit was achieved when t was the reaction time for 95 to 97% conversion. The calculated composition-time curves are shown in Figures 8, 9, and 10. The predicted curves agree quite well with the experimental data. Only for long reaction times the experimental values for the tar generated were lower than
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981 487
7 0 7 s
150-
9
$40
~
m t
-
0 a30
I
s$ 0 0
-
lo/
oh
5
b
io
io
io
25
REACTION TIME,mln
'
REACTOR TEMPERATURE pC
Figure 14. Gas composition at complete conversion.
Figure 11. Composition of gas generated at reactor temperature of 354 OC. 50
i40m
BI 3 0 -
.
0
U
m20-
c;
.
1
L
4L ?HYDROGEN
0
300
' 0
5
IO
15
20
25
30
REACTION TIME,min
Figure 15. Elemental analysis of tar.
Figure 12. Composition of gas generated at reactor temperature of 369 "C. - O '
j
ib
i5
io
REACTION TIME,min
320 340 360 380 400 i)EACTOR TEMPERATJREpC
2'5
io
Figure 13. Composition of gas generated at reactor temperature of 392 "C.
calculated. The reason for this is, as explained above, the decomposition of tar condensed in the downstream edge of the reaction chamber. We believe that eq 22 provides a more realistic reaction tempeature than the common practice of using the final reaction temperature. Product Yield and Analysis. Gas. For each run a gas chromatographic analysis of the collected gas was carried out. The nitrogen-free product gas consisted mainly of carbon dioxide, carbon monoxide, oxygen, and C3+ compounds. Trace amounts of acetylene, ethylene, and methane were also detected. Figures 11, 12,and 13 show the gas composition as a function of time a t the three reactor temperatures. The gas composition is expressed in cumulative volume percent which is the composition of the gas collected from the
commencement of the run. Figure 14 shows the cumulative gas composition obtained for complete conversion at different reactor temperatures. Figures 11, 12, and 13 indicate that for each reactor temperature the amount of carbon dioxide and carbon monoxide in the product gas have a maximum which at higher reactor temperatures occurred at shorter reaction times. Determination of the reaction temperatures at which the maximums occurred showed that they appeared between 360 and 390 "C. This corresponds to thermal gravimetric analysis data of wood and cellulosic materials reported by other investigators, indicating maximum weight loss at this temperature range (Shafiiadeh et al., 1979;SERI, 1979;Hileman et al., 1976, Fairbridge et al., 1978). Carbon dioxide and carbon monoxide are probably produced by decarboxylation in this temperature range. A relative large amount of oxygen was measured in the product gas, especially at short reaction times. This is a rather surprising result since previous investigators have not indicated the presence of oxygen in the product gas. In this investigation precautions were made to prevent oxygen penetration into the experimental system, and the oxygen content obtained in a blank run was negligible. However, it should be noted that the total amount of oxygen analyzed in the product gas was only 20 to 30 mg. Perhaps oxygen adsorbed on the sample surface was released during the heating-up period. This point should be further investigated. Tar. Tar obtained from pyrolysis of cellulosic materials consists of a wide variety of compounds. To get a qualitative estimate on its composition two analyses were conducted. First, an elemental analysis of the tars collected at complete conversion at different reactor temperatures were performed. The results are shown in Figure 15. Note that the oxygen content in the tar increases with increasing
488
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981
F7ACTlON NUMBER
Figure 16. Column chromatography of tar.
:,i 0
..t
‘j
3
131
I
1
330 323 340 36C ;tEAC-OR
38C 4GC
‘EMPERATUREPC
Figure 17. Elemental analysis of char.
reactor temperature while hydrogen and carbon contents decrease. Second, a thin layer chromatography analysis was conducted using a silica gel layer irrigated with a 1:l mixture of chloroform and methyl ethyl ketone. A mixture of tars collected a t different reactor temperatures was analyzed. The analysis indicated that the tar consisted of seven components. An attempt to separate these components using a silica gel chromatographic column yielded the fractions shown in Figure 16. Based on the thin layer chromatography analyais fraction 1was a mixture of three compounds and fraction 2 consisted of one compound. The species collected between the two main fractions were a mixture of the remaining three or four compounds. An attempt was made to identify the compound of fraction 2 by IR analysis. The IR spectrum of fraction 2 corresponded to the spectrum obtained for pure levoglucosan. Another indication that fraction 2 was probably levoglucosan was the fact that, like levoglucosan, it was insoluble in methyl ethyl ketone. Char. Very little information is available on the composition of char. An elemental analysis of chars obtained at four different reactor temperatures at complete conversion was performed; the results are shown in Figure 17. Note that the carbon content increases with increasing reactor temperature. Concluding Remarks In this paper we described an experimental method for determining the pyrolysis rate parameters into the three main pyrolysis produck gas, tar,and char. The technique facilitates the monitoring of the actual wood sample tem-
perature, which varies during the experiment. The method provided information on the distribution of the pyrolysis products, which is important in the evaluation and design of pyrolysis processes. Further, because the variation in sample temperature was considered, the rate parameters obtained in this study are more accurate than those reported previously. It is important to note the measurements in this study were carried out between 300 and 400 OC, a range where the pyrolysis reaction is the rate-controlljng step. However, in practice, pyrolysis is usually carried out at higher temperatures. At elevated temperatures the overall pyrolysis rate may depend also on the wood transport properties (e.g., effective thermal and mass diffusivities). The role and magnitude of these parameters should be thoroughly investigated in order to develop understanding of pyrolysis mechanism at practical operating conditions. Understanding of the relationship between the pyrolysis and transport mechanisms, and experimental data on the kinetic and transport properties are key for reliable design of wood pyrolysis processes. Acknowledgment The authors thank Dr. Steven R. Beck for supporting this investigation. Financial support was provided by the US. Department of Energy, Division of Solar energy, Fuels from Biomass Branch, Contract No. DE AC04 79ET20041. Nomenclature A = frequency factor, min-’ E = activation energy, kJ/mol lz, = reaction rate constant for the ith reaction, min-’ m = mass, g t = reaction time, min T = reaction temperature, “C or K w = mass fraction Subscripts C = Char G = Gas R = Residue T = Tar W = Wood
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Received for review July 7, 1980 Accepted February 27, 1981
