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Gas evolution and the mechanism of cellulose pyrolysis
Fuel 80 (2001) 1757±1763
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Gas evolution and the mechanism of cellulose pyrolysis J.L. Banyasz*, S. Li, J. Lyons-Hart, K.H. Shafer Research Development and Engineering, Philip Morris USA, P.O. Box 26583, Richmond, VA 23261-6583, USA Received 15 June 2000; revised 29 January 2001; accepted 29 January 2001
Abstract The real time evolution kinetics of formaldehyde, hydroxyacetaldehyde, CO and CO2 during the pyrolysis of cellulose, Whatman 41, were studied in a fast evolved gas-FTIR apparatus (EGA). The samples were subjected to rapid exponential temperature increases ranging from 400 to 8008C within about one minute. A total of ten compounds were simultaneously detected in the gas phase by FTIR. Four of these: formaldehyde, hydroxyacetaldehyde, CO, and CO2, were studied in detail as a function of time. The yields of formaldehyde, hydroxyacetaldehyde and CO were found to approximately double with heating rate over the range of the experimental temperature pro®les while that of CO2 decreased slightly. The kinetics of formaldehyde and CO formation were analyzed in terms of two competing ®rst order reactions. The rate constants for the formation of formaldehyde and CO were found to have activation energies of 47 kcal/mole each while the competing reactions had activation energies of 35 kcal/mole in both cases. The case of hydroxyacetaldehyde was found to be more complex, with the same initial reactions as were found for formaldehyde and CO but requiring a third reaction step subsequent to the 47 kcal/mole reaction. The kinetics for CO2 were consistent with a single ®rst order reaction with an activation energy of 35 kcal/mole. The results indicate that the formation reactions of formaldehyde, hydroxyacetaldehyde, CO and CO2 exhibit identical rate limiting steps that involve the major pyrolytic pathways of cellulose. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cellulose pyrolysis; Concurrent reactions; Evolution kinetics; Hydroxyacetaldehyde
1. Introduction
2. Experimental
The impetus for this work was the observation that the quantity of formaldehyde produced during the pyrolysis of tobacco is a function of heating rate. The same effect was observed with cellulose which was subsequently used as a model system. An EGA-FTIR system was assembled in order to monitor the real-time kinetics of formaldehyde evolution at fast heating rates. The spectrometer allowed for the detection of about ten gas phase compounds. The evolution kinetics of four of these, formaldehyde, hydroxyacetaldehyde, CO and CO2, were studied in detail. It was determined that the formation of the gases is closely coupled to the major pyrolytic pathways of cellulose. The gases appear to be formed from rapid secondary reactions subsequent to the rate limiting cellulose decomposition processes. They were, thus, used as indicators or probes for the pyrolysis mechanism of cellulose. This approach complements the gravimetric techniques that are commonly used to study cellulose pyrolysis.
The apparatus and the experimental procedures are described in detail elsewhere [1]. Only the salient points are summarized here. Apparatus. The pyrolysis experiments were carried out in an evolved gas analysis-FTIR apparatus assembled inhouse. The heart of the apparatus is a tube furnace equipped with a sliding heater block mounted on a rail. The carrier gas was nitrogen for all experiments. The ¯ow rate was controlled by a MKS mass ¯ow controller (MKS 1259B, MKS Instruments, Inc., Burlington, MA). A gold coated linear gas ¯ow cell with a volume of 50 ml and a path length of one meter was mounted in the sample compartment of the spectrometer. Two fresh Cambridge ®lter pads were placed at the inlet of the cell to protect it from particulate matter. The weight gain of the pads was used to monitor condensable tar generation during the pyrolysis experiments. The spectrometer, a Brucker IFS 66/s, was selected for its fast scanning capability at reasonable spectral sensitivity. It was set to collect 500 scans at a resolution of one wave number with a sampling frequency of 200 kHz. This resulted in a temporal resolution of 0.08 s/scan for a collection time of 40 s.
* Corresponding author. Tel.: 11-804-274-5594; fax: 11-804-274-2468. E-mail address: [email protected] (J.L. Banyasz).
0016-2361/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0016- 236 1( 01) 00060-6
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J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
Sample. The samples in all cases were strips of Whatman No. 41 ®lter paper. The dimensions were about 10 £ 15 mm with a thickness of 0.22 mm. The sample weight was about 20 mg. Procedure. The sample strips were inserted into the quartz tube to form a single layer against the wall taking care to place the sample into the same position each time. Quartz wool was used at both ends of the sample to keep it in position. One thermocouple was placed between the sample and the wall of the quartz tube. Another was placed in contact with the opposite side of the sample. Once the sample was in position the gas ¯ow was started and the heater block, positioned downstream from the sample, was turned on. When the heater block reached the desired temperature it was moved over the sample to initiate the experiment. Temperature pro®les. The temperature pro®les were exponential in all cases as described by the equation t T t T0 1 T1 2 T0 1 2 exp 2 1 t where t stands for time in seconds. The initial temperature, T0, was about 258C in all cases. The end point, T1, was varied from 380 to 7508C by controlling the heater block temperature. The time constant, t , ranged from 12 to 18 s. The temperature rise was complete within one minute in all cases. Overall, the heating rates were quite high ranging from initial values of 20±608C/s to about 28C/s at 40 s. The sample surface in contact with the quartz tube was in all cases subjected to higher temperatures than the opposite surface, which was in contact with the carrier gas. Gradients as large as 20±408C were observed across the sample. The rate equations were evaluated for both the high and low temperature pro®les in all cases. The resulting rate constants were then averaged. 3. Results Each experiment yielded a temporal evolution pro®le for the gas being monitored as well as the thermal pro®les for the hotter and cooler sample surfaces. The thermal evolution pro®les for the gas were calculated from this data. A typical calculated evolution pro®le for formaldehyde versus the average sample temperature is shown in Fig. 1. The kinetic analyzes are based on the peak areas and the peak temperatures. The peak area is proportional to the yield of a compound. The peak temperature, shown in Fig. 1 as Tm, is the temperature at which the rate of formation of a species X is at its maximum, i.e. d dX 0 2 dT dt TTm The peak temperatures increased, as expected, with heating rate in each case. For formaldehyde, CO and
Fig. 1. Typical thermal evolution pro®le for formaldehyde Tm is the peak temperature.
hydroxyacetaldehyde the peak areas increased with heating rate roughly doubling over the experimental range. The yield of CO2 appeared to decrease slightly with heating rate. However, the decrease was too small to resolve from the scatter. Consequently, it was assumed that the yield of CO2 is independent of heating rate. Three different mechanisms are required to account for the kinetic data. The behavior of CO2 is adequately described by a single ®rst order reaction (see Mechanism 1). The effect of heating rate on the yields of formaldehyde and CO requires the incorporation of a competing reaction step into the mechanism (see Mechanism 2). The activation parameters are such that k1 . k2 at high temperature while the reverse is true at low temperature. Thus, the faster the heating rate, the less time the system lingers at low temperatures where the competing reaction preferentially converts the precursor to the competing product. The behavior of hydroxyacetaldehyde was found to be more complex. The addition of a third reaction step was required to account for it (see Mechanism 3). The rate laws for Mechanisms 1±3 have been derived in detail elsewhere [2]. Only a summary is presented here. The non-isothermal rate equation for Mechanism 1 is
Mechanism 1.
Mechanism 2.
J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
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Mechanism 3.
given by Z
dCO2 P0 k texp2 k tdt dt
3
where P 0 is the initial precursor concentration and k(t) is the time dependent time rate constant de®ned by Ea 0 k t k exp 2 4 RT t R, Ea and k 0 are the gas constant, the activation energy and the frequency factor, respectively. T(t) is the time dependent temperature de®ned by Eq. (1). Eq. (3) may be transformed to dCO2 Ea 0 0 P k exp 2 dt RT Z Ea dT 0 5 exp 2k t exp 2 RT T1 2 T Differentiating the right-hand side of Eq. (5) with respect to temperature and, recalling Eq. (2), setting the result to zero, then clearing and taking the logarithm of both sides yields ! tTm2 Ea E ln ln 6 1 a T1 2 Tm RTm Rk0 The values of the activation energy and frequency factor can be obtained from the slope and intercept of a plot of the left hand side of Eq. (6) versus 1/T1. For Mechanism 2 the analog to Eq. (6) is given by " ! ! ! Ea;1 Ea;1 tTm2 0 2 ln k1 exp 2 ln T 1 2 Tm R RTm !# E 1 k20 exp 2 a;2 7 RTm
ln
Unlike Eq. (6), Eq. (7) does not yield a graphical solution but must be solved simultaneously with the equation for the yield Z Z Area P0 k1 texp 2 k1 t 1 k2 tdt dt 8 Evaluation of Eq. (8) requires knowledge of the initial precursor concentration P 0. The values of P 0 were estimated from the yield data as follows. Since the yields
of formaldehyde, hydroxyacetaldehyde and CO all increase with heating rate, it seems reasonable to assume that at some very fast heating rate the precursors would be completely converted to products to the exclusion of the competing product. Under these conditions the peak area would be proportional to the value of P 0. It also stands to reason that, given the same heating interval, the higher the value of T1 the higher the heating rate. Graphs of the logarithm of peak area versus 1/T1 were found to give straight lines. Thus, the exponentiated intercepts of the graphs were taken as the values of P 0. Eq. (7) was rearranged to
T1 2 Tm Ea;1 Ea;1 E 0 ln 2 k1 exp 2 ln k20 2 a;2 9 2 RT RT RtTm m m Eq. (9) gives the relationship between the rate constants k1 and k2 in terms of the experimentally determined kinetic parameter Tm and the variables T1 and t which de®ne the heating rate. Estimates of the frequency factor and activation energy for k1 are inserted into Eq. (9) to generate the corresponding values for k2. This procedure yields pairs of rate constants that are constrained to be consistent with the peak temperature data. The rate constant pairs are then inserted into Eq. (8) to determine whether they are consistent with the yield data. Since Eq. (8) has no closed form solution it was solved numerically using Mathcade. The trial and error procedure was repeated until a rate constant pair was found that adequately gave back the yields when inserted into Eq. (8). It should be noted that this method of solution does not yield unique values for the rate constants but rather a range of values that is consistent with the data within experimental error. The initial guesses for the values of Ea,1 and k10 are limited by the fact that not all pairs yield real values for the left hand side of Eq. (9). Test calculations showed that the uncertainty in the activation energies is 3±4 kcal/mole and that in the frequency factors is about half an order of magnitude. The yields of hydroxyacetaldehyde are well represented by Eq. (8) with the same rate constants as for CO. However, the peak temperatures are on average 138C higher than those rate constants predict. This suggests the presence of an additional step. For Mechanism 3 the yield is determined by k1 and k2 but the peak temperature is shifted to higher values by k3.
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J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
Table 1 Average rate constants K 0 (s 21) CO2 Formaldehyde CO Hydroxyacetaldehyde
1.1 £ 10 ± ± ±
11
Ea (kcal/mole)
k10 (s 21)
Ea,1 (kcal/mole)
k20 (s 21)
Ea,2 (kcal/mole)
k30 (s 21)
Ea,3 (kcal/mole)
35.4 ± ± ±
± 1.7 £ 10 14 7.1 £ 10 13 7.1 £ 10 13
± 47.3 47.4 47.5
± 4.5 £ 10 10 5.3 £ 10 10 5.3 £ 10 10
± 35.3 35.4 35.4
± ± ± 8.8 £ 10 11
± ± ± 39.0
The rate equation for hydroxyacetaldehyde is given by Z Z Z dHAA k3 tP0 exp 2 k3 tdt k1 texp k3 t dt
The kinetics of evolution of the four species studied were evaluated independently with no a priori assumptions about possible relationships between them. Reaction Mechanisms 1±3 are minimum mechanisms necessary to account for the kinetic data for each compound. Nevertheless, perusal of
Table 1 reveals that for formaldehyde, CO and hydroxyacetaldehyde the activation parameters for k1 are, within experimental error, identical. The same is true for k2. These results are consistent with the conclusion that the three compounds derive from the same pathway with rate constant k1 in competition with another pathway characterized by rate constant k2. Since the activation parameters for CO2 production are identical, within experimental error, to those of k2 for the other three compounds, CO2 is likely one of the competing products. These observations are summarized by the following mechanism (see Mechanism 4). Pathway one is, in all probability, a major pyrolysis pathway for cellulose producing intermediates that undergo subsequent reactions to yield hydroxyacetaldehyde, formaldehyde, CO and, presumably, numerous other gas phase products. It is the rate determining step for formaldehyde and CO production. This picture is consistent with the conclusion of VaÂrhegyi et al. [3] that gas phase products are generated by the secondary cracking of intermediates. Our experimental observations support the conclusion that formaldehyde is the product of secondary reactions. When the sample was moved upstream in the heated zone of the tube furnace so as to increase the residence time in the hot zone of products exiting the sample matrix, an increase was observed in the concentration of formaldehyde. This can only be explained if formaldehyde is produced, at least in part, from volatile intermediates after they have left the sample matrix.
Fig. 2. Calculation of formaldehyde yield.
Fig. 3. Calculation of CO yield.
2 k1 t 2 k2 tdtdt
(10)
Eq. (10) was solved numerically. The same values were used for k1 and k2 as for CO while the value of k3 was varied. The hydroxyacetaldehyde peaks were calculated and the peak temperatures determined graphically. The value of k3 was varied until agreement was reached between the observed and calculated values of Tm. The rate constants determined for the hotter and cooler sample surfaces were averaged. The average rate constants are shown in Table 1. The predicted formaldehyde and CO yields, calculated using the rate constants in Table 1 in conjunction with the averaged sample temperature pro®les, are compared to the experimental values in Figs. 2 and 3, respectively. The mean relative error of prediction for formaldehyde is 3% while that for CO is 5%. 4. Discussions
J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
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Mechanism 4.
Unimolecular decomposition reactions have frequency factors in the range of 10 12 ±10 14 s 21. The activation parameters of k1 and, perhaps, k3 are, thus, suggestive of unimolecular decompositions. Such reactions are consistent with the suggestion of Piskorz et al. [4] that hydroxyacetaldehyde is formed via glucose ring cracking reactions. Levoglucosan/tar is the probable predominant product of pathway 2 in Mechanism 4. Piskorz et al. [4,5] have proposed a pyrolysis mechanism that involves an initial reduction of the degree of polymerization of cellulose in competition with dehydration reactions that ultimately lead to char. Once the degree of polymerization is reduced to about 200, a second competition sets in between ring fragmentation leading to hydroxyacetaldehyde and transglycosylation leading to levoglucosan. Richards [6] has also concluded that hydroxyacetaldehyde and levoglucosan are competing products. Our experiments provide no real-time kinetic data as to the formation of levoglucosan/tar. They do, however, provide yield data based on the weight gains of the ®lter pads at the entrance to the gas cell. It was demonstrated that the tars condensed on the ®lter pads contain levoglucosan. Assuming that the weight gain of the pads is proportional to the quantity of levoglucosan/tar generated, one may write the following equation for the normalized weight gain Z Z wt:gain k2 texp 2 k1 t 1 k2 tdt 11
The source term P 0 does not appear in Eq. (11) due to the normalization. Eq. (11) corresponds to Eq. (8) in the previous section. Eq. (7) is of no help here because no kinetic data are available. The procedure used was to initially insert into Eq. (11) the values of k1 and k2 obtained for hydroxyacetaldehyde and adjust them until a good ®t was obtained to the tar yield data. Small adjustments suf®ced to obtain that agreement. The ®nal values of the rate constants are given by 46; 800 21 k1 1:0 £ 1014 exp 2 s and RT 236; 100 21 k2 4:0 £ 1010 exp s RT The agreement with the rate constant values obtained for the gas phase species, see Table 1, is good. The calculated versus the observed tar yields are shown in Fig. 4. The results are consistent with the conclusion that levoglucosan/tar is the predominant competing species in Mechanism 4. Suuberg and coworkers [7±10] have shown that the rate limiting step for weight loss from cellulose involves the vaporization of cellulose tar followed by its diffusion out of the sample matrix. The rate limiting process may be written as shown by Mechanism 5. If the initial phase change equilibrium is fast compared to the diffusion step, the overall rate constant for weight loss may be written kwt:loss Kvap kD ; Pvap kD
12
Kvap is the equilibrium constant of the vaporization step which is thermodynamically equivalent to Pvap, the vapor pressure of tar above the liquid. The activation energy for weight loss is Ea;wt:loss DHvap 1 Ea;D
13
The international critical tables [11] give the following equation for the temperature dependence of the diffusivity
Fig. 4. Calculation of tar yield.
Mechanism 5.
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J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
Mechanism 6.
of organic vapors into gases m T P0 D D0 T0 P
14
where D0 is the diffusion coef®cient at 273 K and one atmosphere and T0 and P0 are 273 K and one atmosphere, respectively. The constant m is either 1.75 or 2.0 depending upon the vapor. Eq. (14), graphed in the Arrhenius form, yields an average value of 2.0 kcal/mole for the activation energy of diffusion over the range of 200±5008C. Suuberg et al. [7] obtained a value of 33.7 kcal/mole for the heat of vaporization of cellulose tar. The activation energy for weight loss should, thus, be 35.7 kcal/mole which is in excellent agreement with the value of 35.4 kcal/mole obtained for Ea,2. Both the hydroxyacetaldehyde and levoglucosan pathways led to weight loss from cellulose. The overall normalized weight loss is given by Z weight 100 exp 2 k1 t 1 k2 tdt 15 However, the hydroxyacetaldehyde pathway contributes signi®cantly to weight loss only at very high heating rates. Calculations with Eq. (15) show that it contributes only 5% to the overall weight loss at a heating rate of 108C/min. Its contribution increases only to 10% at 1008C/min. At ordinary heating rates the contribution of the acetaldehyde pathway may safely be neglected reducing Eq. (15) to Z E weight 100 exp 2k20 2 a;2 dt 16 RT t The Sha®zadeh mechanism, the most widely accepted mechanism for weight loss from cellulose, consists of
three coupled ®rst order steps [12]. There is an initial step, involving no weight loss, which leads to activated cellulose the nature of which is not well understood. This is followed by a pair of competing reactions which both lead to weight loss (see Mechanism 6). A high energy reaction produces volatiles by which are meant volatile tars including levoglucosan. A low energy step produces char and gases such as carbon dioxide and water. Bradbury, Sakai and Sha®zadeh [12] determined the rate constants from weight loss measurements at isothermal conditions. They showed that the activation step may be neglected at temperatures above 3008C. They assumed that 65% of the mass produced by the char forming pathway is lost to gas. The values of the two remaining rate constants were found to be 47; 300 21 14 s and kV 3:2 £ 10 exp 2 RT 36; 600 21 10 kC 1:3 £ 10 exp 2 s RT Given the Sha®zadeh mechanism and the assumptions of Bradbury et al., the non-isothermal rate law for weight loss is given by $ Z weight 100 exp 2 kV t 1 kC tdt 1 0:35
Z
Z
kC texp 2 kV t 1 kC tdt
% 17
A number of apparent contradictions are obvious. Based on a comparison of activation parameters, k1 and k2 should be equivalent to kV and kC, respectively. However, there is disagreement with regard to the reaction products. Our results indicate that the high activation energy reaction leads to hydroxyacetaldehyde whereas Bradbury et al. conclude that it yields levoglucosan and the volatile tars. The low activation energy step yields levoglucosan/tar based on our results while it leads to char and gases according to Bradbury et al.
Mechanism 7.
J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
The explanation for the discrepancies lies in the limitations of the experiments. Bradbury et al. based their kinetic determinations on weight loss data only. Their assignment of products to the reaction steps can, thus, be no more than assumptions. Our experiments, on the other hand, yield no information with regard to charring. Mechanism 7 is consistent with both sets of results. The dotted arrows and italics refer to reactions and species that have not been observed here. The initial stages of the reaction involve the activation of cellulose which, quite possibly, consists of a reduction in its degree of polymerization. The activation step is in competition with dehydration reactions that ultimately lead to charring. The initial stages of the reaction are not apparent in our data. The depolymerization is presumably too rapid to be observed in our experimental regime. Once the degree of polymerization is suf®ciently reduced, the competition between transglycosylation and ring fragmentation is initiated. There is some indirect evidence to support the initial stage of the scheme as shown above [1]. The addition of the char promoter KHCO3 to cellulose reduces both tar and formaldehyde. This is explained if KHCO3 promotes dehydration and the formation of char 1. The opposite has also been shown. The presence of water, which presumably inhibits the dehydration reaction, increases both tar and formaldehyde. Miura et al. have shown that the levoglucosan yield from the pyrolysis of wood increases with heating rate initially but begins to fall off when the heating rate exceeds 1.38C/s [13]. Though it is uncertain to what extent data obtained from wood can be applied to pure cellulose, it is worth pointing out that the behavior is consistent with the above mechanism. Initially, an increase in heating rate favors depolymerization over dehydration and charring but as the heating rate continues to increase ring fragmentation eventually becomes preferable to transglycosylation. Suuberg and coworkers have shown that if the tar vapors cannot escape the cellulose matrix rapidly enough they undergo charring. This leads to char2. Mok and Antal [14] determined that char1 and char2 could be distinguished based on morphology. The charring observed by Bradbury et al. probably involves char2. The authors point out that a reduction in sample size leads to less char. This can only be due to the mass transfer effects demonstrated by Suuberg and coworkers. Tar is thus the likely co-product to the charring observed by Bradbury et al. The behavior of CO2 is interesting. It is usually shown as a product derived from char. Based on our results there are two possible sources for CO2. It can either form by decarboxylation during the vaporization of tar or by a rapid secondary process from char2. The second possibility is more likely. The predictions of Eqs. (16) and (17) for weight loss were compared to experimental data. Fig. 5 shows the experimental weight loss curve for Avicell at a constant heating rate of 208C/min [15]. The circles show the predicted
1763
Fig. 5. Weight loss curve for Avicel at 208C per minute; solid line: TGA data Ref. [15]; squares: Eq. (16); circles: Eq. (17).
weight loss curve calculated using Eq. (17) with the rate constants of Bradbury et al. The squares were calculated using Eq. (16) with k2 4:0 £ 1010 exp 2 36; 100=RT, the value determined from the tar yield data. The two predicted curves are in reasonable agreement showing that, except for charring, the Sha®zadeh mechanism and Mechanism 7 are equivalent in terms of weight loss. The two curves can be made to essentially overlap at temperatures below about 4008C with minor adjustments to the rate constants. Since Eq. (16) neglects charring, its predictions approach 100% weight loss for the higher temperatures.
References [1] Li S, Lyons-Hart J, Banyasz JL, Shafer KH. Fuel 2001;80(12). [2] Banyasz JL, Li S, Lyons-Hart J, Shafer KH. J Anal Appl Pyrol 2001;57:223±48. [3] VaÂrhegyi G, Szabo P, Shu-Lai Mok W, Antal MJ. J Anal Appl Pyrol 1993;26:159±74. [4] Piskorz J, Radlein D, Scott DS. J Anal Appl Pyrol 1986;9:121±37. [5] Radlein D, Piskorz J, Scott DS, Biomass for Energy, Industry and Environment, 6th E. C. Conference, 1992. p. 643±9. [6] Richards GN. J Anal Appl Pyrol 1987;10:251±5. [7] Suuberg EM, Milosavljevic I, Oja V, Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996. p. 1515±21. [8] Milosavljevic I, Oja V, Suuberg EM. Ind Engng Chem Res 1996;35:653±62. [9] Milosavljevic I, Suuberg EM. Ind Engng Chem Res 1995;34:1081± 91. [10] Oja V, Suuberg EM. Anal Chem 1997;69:4619±26. [11] Washburn EW, editor. International critical tables, vol. V. New York: McGraw-Hill, 1929. p. 62. [12] Bradbury AGW, Sakai Y, Sha®zadeh F. J Appl Polym Sci 1979;23:3271±80. [13] Miura M, Tanaka S, Ando K. Kagaku Kogaku Ronbunshu 1995;21:843±6. [14] Mok WS-L, Antal Jr. MJ. Thermochim Acta 1983;68:165±86. [15] Waymack B, unpublished data.
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Gas evolution and the mechanism of cellulose pyrolysis J.L. Banyasz*, S. Li, J. Lyons-Hart, K.H. Shafer Research Development and Engineering, Philip Morris USA, P.O. Box 26583, Richmond, VA 23261-6583, USA Received 15 June 2000; revised 29 January 2001; accepted 29 January 2001
Abstract The real time evolution kinetics of formaldehyde, hydroxyacetaldehyde, CO and CO2 during the pyrolysis of cellulose, Whatman 41, were studied in a fast evolved gas-FTIR apparatus (EGA). The samples were subjected to rapid exponential temperature increases ranging from 400 to 8008C within about one minute. A total of ten compounds were simultaneously detected in the gas phase by FTIR. Four of these: formaldehyde, hydroxyacetaldehyde, CO, and CO2, were studied in detail as a function of time. The yields of formaldehyde, hydroxyacetaldehyde and CO were found to approximately double with heating rate over the range of the experimental temperature pro®les while that of CO2 decreased slightly. The kinetics of formaldehyde and CO formation were analyzed in terms of two competing ®rst order reactions. The rate constants for the formation of formaldehyde and CO were found to have activation energies of 47 kcal/mole each while the competing reactions had activation energies of 35 kcal/mole in both cases. The case of hydroxyacetaldehyde was found to be more complex, with the same initial reactions as were found for formaldehyde and CO but requiring a third reaction step subsequent to the 47 kcal/mole reaction. The kinetics for CO2 were consistent with a single ®rst order reaction with an activation energy of 35 kcal/mole. The results indicate that the formation reactions of formaldehyde, hydroxyacetaldehyde, CO and CO2 exhibit identical rate limiting steps that involve the major pyrolytic pathways of cellulose. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cellulose pyrolysis; Concurrent reactions; Evolution kinetics; Hydroxyacetaldehyde
1. Introduction
2. Experimental
The impetus for this work was the observation that the quantity of formaldehyde produced during the pyrolysis of tobacco is a function of heating rate. The same effect was observed with cellulose which was subsequently used as a model system. An EGA-FTIR system was assembled in order to monitor the real-time kinetics of formaldehyde evolution at fast heating rates. The spectrometer allowed for the detection of about ten gas phase compounds. The evolution kinetics of four of these, formaldehyde, hydroxyacetaldehyde, CO and CO2, were studied in detail. It was determined that the formation of the gases is closely coupled to the major pyrolytic pathways of cellulose. The gases appear to be formed from rapid secondary reactions subsequent to the rate limiting cellulose decomposition processes. They were, thus, used as indicators or probes for the pyrolysis mechanism of cellulose. This approach complements the gravimetric techniques that are commonly used to study cellulose pyrolysis.
The apparatus and the experimental procedures are described in detail elsewhere [1]. Only the salient points are summarized here. Apparatus. The pyrolysis experiments were carried out in an evolved gas analysis-FTIR apparatus assembled inhouse. The heart of the apparatus is a tube furnace equipped with a sliding heater block mounted on a rail. The carrier gas was nitrogen for all experiments. The ¯ow rate was controlled by a MKS mass ¯ow controller (MKS 1259B, MKS Instruments, Inc., Burlington, MA). A gold coated linear gas ¯ow cell with a volume of 50 ml and a path length of one meter was mounted in the sample compartment of the spectrometer. Two fresh Cambridge ®lter pads were placed at the inlet of the cell to protect it from particulate matter. The weight gain of the pads was used to monitor condensable tar generation during the pyrolysis experiments. The spectrometer, a Brucker IFS 66/s, was selected for its fast scanning capability at reasonable spectral sensitivity. It was set to collect 500 scans at a resolution of one wave number with a sampling frequency of 200 kHz. This resulted in a temporal resolution of 0.08 s/scan for a collection time of 40 s.
* Corresponding author. Tel.: 11-804-274-5594; fax: 11-804-274-2468. E-mail address: [email protected] (J.L. Banyasz).
0016-2361/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0016- 236 1( 01) 00060-6
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J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
Sample. The samples in all cases were strips of Whatman No. 41 ®lter paper. The dimensions were about 10 £ 15 mm with a thickness of 0.22 mm. The sample weight was about 20 mg. Procedure. The sample strips were inserted into the quartz tube to form a single layer against the wall taking care to place the sample into the same position each time. Quartz wool was used at both ends of the sample to keep it in position. One thermocouple was placed between the sample and the wall of the quartz tube. Another was placed in contact with the opposite side of the sample. Once the sample was in position the gas ¯ow was started and the heater block, positioned downstream from the sample, was turned on. When the heater block reached the desired temperature it was moved over the sample to initiate the experiment. Temperature pro®les. The temperature pro®les were exponential in all cases as described by the equation t T t T0 1 T1 2 T0 1 2 exp 2 1 t where t stands for time in seconds. The initial temperature, T0, was about 258C in all cases. The end point, T1, was varied from 380 to 7508C by controlling the heater block temperature. The time constant, t , ranged from 12 to 18 s. The temperature rise was complete within one minute in all cases. Overall, the heating rates were quite high ranging from initial values of 20±608C/s to about 28C/s at 40 s. The sample surface in contact with the quartz tube was in all cases subjected to higher temperatures than the opposite surface, which was in contact with the carrier gas. Gradients as large as 20±408C were observed across the sample. The rate equations were evaluated for both the high and low temperature pro®les in all cases. The resulting rate constants were then averaged. 3. Results Each experiment yielded a temporal evolution pro®le for the gas being monitored as well as the thermal pro®les for the hotter and cooler sample surfaces. The thermal evolution pro®les for the gas were calculated from this data. A typical calculated evolution pro®le for formaldehyde versus the average sample temperature is shown in Fig. 1. The kinetic analyzes are based on the peak areas and the peak temperatures. The peak area is proportional to the yield of a compound. The peak temperature, shown in Fig. 1 as Tm, is the temperature at which the rate of formation of a species X is at its maximum, i.e. d dX 0 2 dT dt TTm The peak temperatures increased, as expected, with heating rate in each case. For formaldehyde, CO and
Fig. 1. Typical thermal evolution pro®le for formaldehyde Tm is the peak temperature.
hydroxyacetaldehyde the peak areas increased with heating rate roughly doubling over the experimental range. The yield of CO2 appeared to decrease slightly with heating rate. However, the decrease was too small to resolve from the scatter. Consequently, it was assumed that the yield of CO2 is independent of heating rate. Three different mechanisms are required to account for the kinetic data. The behavior of CO2 is adequately described by a single ®rst order reaction (see Mechanism 1). The effect of heating rate on the yields of formaldehyde and CO requires the incorporation of a competing reaction step into the mechanism (see Mechanism 2). The activation parameters are such that k1 . k2 at high temperature while the reverse is true at low temperature. Thus, the faster the heating rate, the less time the system lingers at low temperatures where the competing reaction preferentially converts the precursor to the competing product. The behavior of hydroxyacetaldehyde was found to be more complex. The addition of a third reaction step was required to account for it (see Mechanism 3). The rate laws for Mechanisms 1±3 have been derived in detail elsewhere [2]. Only a summary is presented here. The non-isothermal rate equation for Mechanism 1 is
Mechanism 1.
Mechanism 2.
J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
1759
Mechanism 3.
given by Z
dCO2 P0 k texp2 k tdt dt
3
where P 0 is the initial precursor concentration and k(t) is the time dependent time rate constant de®ned by Ea 0 k t k exp 2 4 RT t R, Ea and k 0 are the gas constant, the activation energy and the frequency factor, respectively. T(t) is the time dependent temperature de®ned by Eq. (1). Eq. (3) may be transformed to dCO2 Ea 0 0 P k exp 2 dt RT Z Ea dT 0 5 exp 2k t exp 2 RT T1 2 T Differentiating the right-hand side of Eq. (5) with respect to temperature and, recalling Eq. (2), setting the result to zero, then clearing and taking the logarithm of both sides yields ! tTm2 Ea E ln ln 6 1 a T1 2 Tm RTm Rk0 The values of the activation energy and frequency factor can be obtained from the slope and intercept of a plot of the left hand side of Eq. (6) versus 1/T1. For Mechanism 2 the analog to Eq. (6) is given by " ! ! ! Ea;1 Ea;1 tTm2 0 2 ln k1 exp 2 ln T 1 2 Tm R RTm !# E 1 k20 exp 2 a;2 7 RTm
ln
Unlike Eq. (6), Eq. (7) does not yield a graphical solution but must be solved simultaneously with the equation for the yield Z Z Area P0 k1 texp 2 k1 t 1 k2 tdt dt 8 Evaluation of Eq. (8) requires knowledge of the initial precursor concentration P 0. The values of P 0 were estimated from the yield data as follows. Since the yields
of formaldehyde, hydroxyacetaldehyde and CO all increase with heating rate, it seems reasonable to assume that at some very fast heating rate the precursors would be completely converted to products to the exclusion of the competing product. Under these conditions the peak area would be proportional to the value of P 0. It also stands to reason that, given the same heating interval, the higher the value of T1 the higher the heating rate. Graphs of the logarithm of peak area versus 1/T1 were found to give straight lines. Thus, the exponentiated intercepts of the graphs were taken as the values of P 0. Eq. (7) was rearranged to
T1 2 Tm Ea;1 Ea;1 E 0 ln 2 k1 exp 2 ln k20 2 a;2 9 2 RT RT RtTm m m Eq. (9) gives the relationship between the rate constants k1 and k2 in terms of the experimentally determined kinetic parameter Tm and the variables T1 and t which de®ne the heating rate. Estimates of the frequency factor and activation energy for k1 are inserted into Eq. (9) to generate the corresponding values for k2. This procedure yields pairs of rate constants that are constrained to be consistent with the peak temperature data. The rate constant pairs are then inserted into Eq. (8) to determine whether they are consistent with the yield data. Since Eq. (8) has no closed form solution it was solved numerically using Mathcade. The trial and error procedure was repeated until a rate constant pair was found that adequately gave back the yields when inserted into Eq. (8). It should be noted that this method of solution does not yield unique values for the rate constants but rather a range of values that is consistent with the data within experimental error. The initial guesses for the values of Ea,1 and k10 are limited by the fact that not all pairs yield real values for the left hand side of Eq. (9). Test calculations showed that the uncertainty in the activation energies is 3±4 kcal/mole and that in the frequency factors is about half an order of magnitude. The yields of hydroxyacetaldehyde are well represented by Eq. (8) with the same rate constants as for CO. However, the peak temperatures are on average 138C higher than those rate constants predict. This suggests the presence of an additional step. For Mechanism 3 the yield is determined by k1 and k2 but the peak temperature is shifted to higher values by k3.
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J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
Table 1 Average rate constants K 0 (s 21) CO2 Formaldehyde CO Hydroxyacetaldehyde
1.1 £ 10 ± ± ±
11
Ea (kcal/mole)
k10 (s 21)
Ea,1 (kcal/mole)
k20 (s 21)
Ea,2 (kcal/mole)
k30 (s 21)
Ea,3 (kcal/mole)
35.4 ± ± ±
± 1.7 £ 10 14 7.1 £ 10 13 7.1 £ 10 13
± 47.3 47.4 47.5
± 4.5 £ 10 10 5.3 £ 10 10 5.3 £ 10 10
± 35.3 35.4 35.4
± ± ± 8.8 £ 10 11
± ± ± 39.0
The rate equation for hydroxyacetaldehyde is given by Z Z Z dHAA k3 tP0 exp 2 k3 tdt k1 texp k3 t dt
The kinetics of evolution of the four species studied were evaluated independently with no a priori assumptions about possible relationships between them. Reaction Mechanisms 1±3 are minimum mechanisms necessary to account for the kinetic data for each compound. Nevertheless, perusal of
Table 1 reveals that for formaldehyde, CO and hydroxyacetaldehyde the activation parameters for k1 are, within experimental error, identical. The same is true for k2. These results are consistent with the conclusion that the three compounds derive from the same pathway with rate constant k1 in competition with another pathway characterized by rate constant k2. Since the activation parameters for CO2 production are identical, within experimental error, to those of k2 for the other three compounds, CO2 is likely one of the competing products. These observations are summarized by the following mechanism (see Mechanism 4). Pathway one is, in all probability, a major pyrolysis pathway for cellulose producing intermediates that undergo subsequent reactions to yield hydroxyacetaldehyde, formaldehyde, CO and, presumably, numerous other gas phase products. It is the rate determining step for formaldehyde and CO production. This picture is consistent with the conclusion of VaÂrhegyi et al. [3] that gas phase products are generated by the secondary cracking of intermediates. Our experimental observations support the conclusion that formaldehyde is the product of secondary reactions. When the sample was moved upstream in the heated zone of the tube furnace so as to increase the residence time in the hot zone of products exiting the sample matrix, an increase was observed in the concentration of formaldehyde. This can only be explained if formaldehyde is produced, at least in part, from volatile intermediates after they have left the sample matrix.
Fig. 2. Calculation of formaldehyde yield.
Fig. 3. Calculation of CO yield.
2 k1 t 2 k2 tdtdt
(10)
Eq. (10) was solved numerically. The same values were used for k1 and k2 as for CO while the value of k3 was varied. The hydroxyacetaldehyde peaks were calculated and the peak temperatures determined graphically. The value of k3 was varied until agreement was reached between the observed and calculated values of Tm. The rate constants determined for the hotter and cooler sample surfaces were averaged. The average rate constants are shown in Table 1. The predicted formaldehyde and CO yields, calculated using the rate constants in Table 1 in conjunction with the averaged sample temperature pro®les, are compared to the experimental values in Figs. 2 and 3, respectively. The mean relative error of prediction for formaldehyde is 3% while that for CO is 5%. 4. Discussions
J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
1761
Mechanism 4.
Unimolecular decomposition reactions have frequency factors in the range of 10 12 ±10 14 s 21. The activation parameters of k1 and, perhaps, k3 are, thus, suggestive of unimolecular decompositions. Such reactions are consistent with the suggestion of Piskorz et al. [4] that hydroxyacetaldehyde is formed via glucose ring cracking reactions. Levoglucosan/tar is the probable predominant product of pathway 2 in Mechanism 4. Piskorz et al. [4,5] have proposed a pyrolysis mechanism that involves an initial reduction of the degree of polymerization of cellulose in competition with dehydration reactions that ultimately lead to char. Once the degree of polymerization is reduced to about 200, a second competition sets in between ring fragmentation leading to hydroxyacetaldehyde and transglycosylation leading to levoglucosan. Richards [6] has also concluded that hydroxyacetaldehyde and levoglucosan are competing products. Our experiments provide no real-time kinetic data as to the formation of levoglucosan/tar. They do, however, provide yield data based on the weight gains of the ®lter pads at the entrance to the gas cell. It was demonstrated that the tars condensed on the ®lter pads contain levoglucosan. Assuming that the weight gain of the pads is proportional to the quantity of levoglucosan/tar generated, one may write the following equation for the normalized weight gain Z Z wt:gain k2 texp 2 k1 t 1 k2 tdt 11
The source term P 0 does not appear in Eq. (11) due to the normalization. Eq. (11) corresponds to Eq. (8) in the previous section. Eq. (7) is of no help here because no kinetic data are available. The procedure used was to initially insert into Eq. (11) the values of k1 and k2 obtained for hydroxyacetaldehyde and adjust them until a good ®t was obtained to the tar yield data. Small adjustments suf®ced to obtain that agreement. The ®nal values of the rate constants are given by 46; 800 21 k1 1:0 £ 1014 exp 2 s and RT 236; 100 21 k2 4:0 £ 1010 exp s RT The agreement with the rate constant values obtained for the gas phase species, see Table 1, is good. The calculated versus the observed tar yields are shown in Fig. 4. The results are consistent with the conclusion that levoglucosan/tar is the predominant competing species in Mechanism 4. Suuberg and coworkers [7±10] have shown that the rate limiting step for weight loss from cellulose involves the vaporization of cellulose tar followed by its diffusion out of the sample matrix. The rate limiting process may be written as shown by Mechanism 5. If the initial phase change equilibrium is fast compared to the diffusion step, the overall rate constant for weight loss may be written kwt:loss Kvap kD ; Pvap kD
12
Kvap is the equilibrium constant of the vaporization step which is thermodynamically equivalent to Pvap, the vapor pressure of tar above the liquid. The activation energy for weight loss is Ea;wt:loss DHvap 1 Ea;D
13
The international critical tables [11] give the following equation for the temperature dependence of the diffusivity
Fig. 4. Calculation of tar yield.
Mechanism 5.
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J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
Mechanism 6.
of organic vapors into gases m T P0 D D0 T0 P
14
where D0 is the diffusion coef®cient at 273 K and one atmosphere and T0 and P0 are 273 K and one atmosphere, respectively. The constant m is either 1.75 or 2.0 depending upon the vapor. Eq. (14), graphed in the Arrhenius form, yields an average value of 2.0 kcal/mole for the activation energy of diffusion over the range of 200±5008C. Suuberg et al. [7] obtained a value of 33.7 kcal/mole for the heat of vaporization of cellulose tar. The activation energy for weight loss should, thus, be 35.7 kcal/mole which is in excellent agreement with the value of 35.4 kcal/mole obtained for Ea,2. Both the hydroxyacetaldehyde and levoglucosan pathways led to weight loss from cellulose. The overall normalized weight loss is given by Z weight 100 exp 2 k1 t 1 k2 tdt 15 However, the hydroxyacetaldehyde pathway contributes signi®cantly to weight loss only at very high heating rates. Calculations with Eq. (15) show that it contributes only 5% to the overall weight loss at a heating rate of 108C/min. Its contribution increases only to 10% at 1008C/min. At ordinary heating rates the contribution of the acetaldehyde pathway may safely be neglected reducing Eq. (15) to Z E weight 100 exp 2k20 2 a;2 dt 16 RT t The Sha®zadeh mechanism, the most widely accepted mechanism for weight loss from cellulose, consists of
three coupled ®rst order steps [12]. There is an initial step, involving no weight loss, which leads to activated cellulose the nature of which is not well understood. This is followed by a pair of competing reactions which both lead to weight loss (see Mechanism 6). A high energy reaction produces volatiles by which are meant volatile tars including levoglucosan. A low energy step produces char and gases such as carbon dioxide and water. Bradbury, Sakai and Sha®zadeh [12] determined the rate constants from weight loss measurements at isothermal conditions. They showed that the activation step may be neglected at temperatures above 3008C. They assumed that 65% of the mass produced by the char forming pathway is lost to gas. The values of the two remaining rate constants were found to be 47; 300 21 14 s and kV 3:2 £ 10 exp 2 RT 36; 600 21 10 kC 1:3 £ 10 exp 2 s RT Given the Sha®zadeh mechanism and the assumptions of Bradbury et al., the non-isothermal rate law for weight loss is given by $ Z weight 100 exp 2 kV t 1 kC tdt 1 0:35
Z
Z
kC texp 2 kV t 1 kC tdt
% 17
A number of apparent contradictions are obvious. Based on a comparison of activation parameters, k1 and k2 should be equivalent to kV and kC, respectively. However, there is disagreement with regard to the reaction products. Our results indicate that the high activation energy reaction leads to hydroxyacetaldehyde whereas Bradbury et al. conclude that it yields levoglucosan and the volatile tars. The low activation energy step yields levoglucosan/tar based on our results while it leads to char and gases according to Bradbury et al.
Mechanism 7.
J.L. Banyasz et al. / Fuel 80 (2001) 1757±1763
The explanation for the discrepancies lies in the limitations of the experiments. Bradbury et al. based their kinetic determinations on weight loss data only. Their assignment of products to the reaction steps can, thus, be no more than assumptions. Our experiments, on the other hand, yield no information with regard to charring. Mechanism 7 is consistent with both sets of results. The dotted arrows and italics refer to reactions and species that have not been observed here. The initial stages of the reaction involve the activation of cellulose which, quite possibly, consists of a reduction in its degree of polymerization. The activation step is in competition with dehydration reactions that ultimately lead to charring. The initial stages of the reaction are not apparent in our data. The depolymerization is presumably too rapid to be observed in our experimental regime. Once the degree of polymerization is suf®ciently reduced, the competition between transglycosylation and ring fragmentation is initiated. There is some indirect evidence to support the initial stage of the scheme as shown above [1]. The addition of the char promoter KHCO3 to cellulose reduces both tar and formaldehyde. This is explained if KHCO3 promotes dehydration and the formation of char 1. The opposite has also been shown. The presence of water, which presumably inhibits the dehydration reaction, increases both tar and formaldehyde. Miura et al. have shown that the levoglucosan yield from the pyrolysis of wood increases with heating rate initially but begins to fall off when the heating rate exceeds 1.38C/s [13]. Though it is uncertain to what extent data obtained from wood can be applied to pure cellulose, it is worth pointing out that the behavior is consistent with the above mechanism. Initially, an increase in heating rate favors depolymerization over dehydration and charring but as the heating rate continues to increase ring fragmentation eventually becomes preferable to transglycosylation. Suuberg and coworkers have shown that if the tar vapors cannot escape the cellulose matrix rapidly enough they undergo charring. This leads to char2. Mok and Antal [14] determined that char1 and char2 could be distinguished based on morphology. The charring observed by Bradbury et al. probably involves char2. The authors point out that a reduction in sample size leads to less char. This can only be due to the mass transfer effects demonstrated by Suuberg and coworkers. Tar is thus the likely co-product to the charring observed by Bradbury et al. The behavior of CO2 is interesting. It is usually shown as a product derived from char. Based on our results there are two possible sources for CO2. It can either form by decarboxylation during the vaporization of tar or by a rapid secondary process from char2. The second possibility is more likely. The predictions of Eqs. (16) and (17) for weight loss were compared to experimental data. Fig. 5 shows the experimental weight loss curve for Avicell at a constant heating rate of 208C/min [15]. The circles show the predicted
1763
Fig. 5. Weight loss curve for Avicel at 208C per minute; solid line: TGA data Ref. [15]; squares: Eq. (16); circles: Eq. (17).
weight loss curve calculated using Eq. (17) with the rate constants of Bradbury et al. The squares were calculated using Eq. (16) with k2 4:0 £ 1010 exp 2 36; 100=RT, the value determined from the tar yield data. The two predicted curves are in reasonable agreement showing that, except for charring, the Sha®zadeh mechanism and Mechanism 7 are equivalent in terms of weight loss. The two curves can be made to essentially overlap at temperatures below about 4008C with minor adjustments to the rate constants. Since Eq. (16) neglects charring, its predictions approach 100% weight loss for the higher temperatures.
References [1] Li S, Lyons-Hart J, Banyasz JL, Shafer KH. Fuel 2001;80(12). [2] Banyasz JL, Li S, Lyons-Hart J, Shafer KH. J Anal Appl Pyrol 2001;57:223±48. [3] VaÂrhegyi G, Szabo P, Shu-Lai Mok W, Antal MJ. J Anal Appl Pyrol 1993;26:159±74. [4] Piskorz J, Radlein D, Scott DS. J Anal Appl Pyrol 1986;9:121±37. [5] Radlein D, Piskorz J, Scott DS, Biomass for Energy, Industry and Environment, 6th E. C. Conference, 1992. p. 643±9. [6] Richards GN. J Anal Appl Pyrol 1987;10:251±5. [7] Suuberg EM, Milosavljevic I, Oja V, Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996. p. 1515±21. [8] Milosavljevic I, Oja V, Suuberg EM. Ind Engng Chem Res 1996;35:653±62. [9] Milosavljevic I, Suuberg EM. Ind Engng Chem Res 1995;34:1081± 91. [10] Oja V, Suuberg EM. Anal Chem 1997;69:4619±26. [11] Washburn EW, editor. International critical tables, vol. V. New York: McGraw-Hill, 1929. p. 62. [12] Bradbury AGW, Sakai Y, Sha®zadeh F. J Appl Polym Sci 1979;23:3271±80. [13] Miura M, Tanaka S, Ando K. Kagaku Kogaku Ronbunshu 1995;21:843±6. [14] Mok WS-L, Antal Jr. MJ. Thermochim Acta 1983;68:165±86. [15] Waymack B, unpublished data.