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Pyrolysis Decomposition Kinetics of Cellulose-Based Materials by Constant Heating Rate Micropyrolysis John G. Reynolds* and Alan K. Burnham Lawrence Livermore National Laboratory, University of California, L-369, P.O. Box 808, Livermore, California 94551 Received June 11, 1996. Revised Manuscript Received October 15, 1996X
Fibrous, powdered fibrous, and acid-washed celluloses, newsprint, and paper dunnage were examined by Pyromat micropyrolysis to determine volatile organic compound evolution kinetic parameters. For the cellulose samples, the interpolated Tmax values (temperature of maximum evolution rate for a constant heating rate) indicate the fibrous cellulose is the least reactive. The fibrous cellulose samples have activation energies and frequency factors around 43 kcal/mol and 5 × 1012 s-1. A three-parameter nucleation kinetic model gave the best fits to the reaction profile, which is narrower than a first-order reaction. Newsprint and dunnage were also examined. The interpolated Tmax values indicate that dunnage is more reactive than the newsprint, and both are more reactive than the cellulose samples. Newsprint and paper dunnage have energy distributions that are similar but shifted from each other. Because of the diversity in chemical structure in the papers, the best fits were found using a discrete energy distribution method, which uses parallel first-order reactions. The newsprint has a principal activation energy of 43 kcal/mol and a frequency factor of 5 × 1012 s-1, consistent with cellulose being the predominant component. The dunnage has a principal activation energy of 40 kcal/mol and a frequency factor of 9 × 1011 s-1. Pyrolysis-MS measurements indicate that the difference between the total mass loss and organic evolution profiles is only a few degrees and cannot account for the 15-20 °C difference between decomposition temperatures measured by Pyromat and some TGA results in the literature.
Introduction Hydrothermal pretreatment has been proposed1 as a method for converting municipal solid wastes (MSW) into compatible feeds for gasification.2 In this type of pretreatment, low-grade paper, a major component of MSW, is converted to gas, water-soluble liquids, and water-insoluble residual char,3 which forms a slurry to be fed to the gasifier. An assessment of the pyrolytic decomposition kinetics of the initial feedstock paper dunnage starting material is useful for process design. With hundreds of earlier pyrolysis decomposition studies of cellulose and wood products, it would seem that a new study is not needed. However, the diversity of results in the literature along with the questionable validity of some of the kinetic methods employed caused us to characterize our feedstock by using techniques we have found useful in other applications.4,5 This effort has resulted in the demonstration of what we believe are superior global kinetic models for cellulose and paper pyrolysis. The applicability of these models to * Author to whom correspondence should be addressed [telephone (510) 422-6028; fax (510) 423-4289; e-mail
[email protected]]. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Khan, M. R.; Albert, C.; McKeon, R.; Zang, R.; DeCanio, S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (3), 802-809. (2) McMahon, M. A.; Khan, M. R. Am. Chem. Soc. Symp. Ser. 1992, No. 515, 157-171. (3) Wallman, H. Laboratory Studies of a Hydrothermal Pretreatment Process for Municipal Solid Waste; Lawrence Livermore National Laboratory Report UCRL-ID-120296; April 6, 1995. (4) Braun, R. L.; Burnham, A. K.; Reynolds, J. G.; Clarkson, J. E. Energy Fuels 1991, 5, 192-204. (5) Burnham, A. K.; Braun, R. L.; Coburn, T. T.; Sandvik, E. I.; Curry, D. J.; Schmidt, B. J.; Noble, R. A. Energy Fuels 1996, 10, 4959.
S0887-0624(96)00086-2 CCC: $14.00
hydrothermal pyrolysis is a distinct issue and is discussed in a following paper. Here we report the pyrolysis decomposition kinetics of three pure cellulose materials and two types of paper from constant heating rate data. We have employed both TGA (to a limited extent) and a Pyromat II micropyrolyzer,4 as well as different methods of kinetic analysis. The Pyromat is quite different from the TGA. The Pyromat measures evolution of organic species with a flame ionization detector (FID), while the TGA measures overall weight loss, including water and CO2. The Pyromat uses sample sizes of 1-4 mg, 10 times lower than most TGA studies, and has the thermocouple directly touching the sample. To help understand the differences between the two pyrolysis methods, we have utilized Pyromat-MS to measure single-species evolution. We also have used two kinetic analysis methods not previously used for cellulose or paper: a relatively new nucleation (acceleratory) kinetic model, developed for well-preserved algal kerogens and linear polyolefins,5 and a previously developed parallel reaction model.6 Experimental Section Samples. Paper samples were from high-quality newsprint (LLNL Newsline) and paper dunnage. The paper dunnage came from an industrial packaging company and is composed of tailings from paper mills. The samples were prepared for pyrolysis by cutting into approximately 50-300 µm pieces with an X-acto knife. The cut edges were rough to the 10-20 µm scale of the fibers. Chemical analyses (Integrated Paper (6) Burnham, A. K.; Braun, R. L.; Gregg, H. R.; Samoun, A. M. Energy Fuels 1987, 1, 452-458.
© 1997 American Chemical Society
Decomposition Kinetics of Cellulose-Based Materials Table 1. Chemical Analysis of Paper Samples Used in This Study
c
property
newsprint
dunnage
empirical formula carbohydratea araban xylan mannan galactan glucan total ligninb ashc 525 °C 900 °C moistured
CH1.55O0.67
CH1.66O0.72
0.8 5.5 9.3 1.0 53.9 70.5 20.3
1.3 5.6 10.6 1.9 42.4 61.8 27.5
2.6 2.1 7.2
2.1 1.9 9.8
a TAPPI T249 cm-85. b Klason TAPPI 60(10):143-144 (Oct 77). TAPPI T211 and T413. d TAPPI T413, oven-dried at 105 °C.
Services, Inc., Appleton, WI) of these two samples are given in Table 1. The fibrous cellulose and acid-washed cellulose (chromatography grade) were purchased from J. T. Baker; the powdered cellulose was Whatman CF-11 powdered fibrous cellulose. The fibrous cellulose was a mixture of fibers about 20 µm in diameter by 1 mm long and fiber shards a few micrometers in diameter and 20-30 µm long. Micropyrolysis Tests. The Pyromat II micropyrolyzer has been described previously.4 Samples were pyrolyzed at constant heating rates, using He as the carrier gas (unless specified otherwise). Organic species evolution was measured by FID. Temperature was measured by direct contact of a Type K thermocouple (0.040-in. 304 stainless steel sheath) with the sample. Data were stored and manipulated on a IBM PS/2 Model 70 386 personal computer interfaced with the pyrolysis unit. TGA Analyses. TGA analyses were made with a PerkinElmer TGA-7 in the standard manner using N2 as the sweep gas. Temperatures were calibrated to within 5 °C of magnetic transitions in alumel (163 °C), nickel (354 °C), and Perkalloy (596 °C) measured at heating rates of 1 and 10 °C/min. Sample weights were typically 5 mg. Kinetic Analysis. The method of kinetic analysis using the Pyromat has been described in detail elsewhere.4,5 Kinetics were determined from multiple runs at constant heating rates (nominally)sthree 50 °C/min, one 7 °C/min, and two 1 °C/min runs were performed for each kinetic data set. If Tmax values and profile shapes were not in agreement, more runs at these heating rates were performed. An additional 15 °C/ min run was performed to be included in the best kinetics data set when literature comparisons were made. Rate data were analyzed by using the regression analysis program KINETICS (Lawrence Livermore National Laboratory, Livermore, CA).4-6 For most kinetic analyses, the rate equations are numerically integrated over exact time-temperature history, so deviations from a constant heating rate are of no consequence. (Sometimes noise in the measured temperature gives noise in the calculated rates.) The kinetic parameters used in this study were determined according to the following methods: the discrete distribution (yielding Adiscrete and Ediscrete), the Tmax shift (yielding Aapprox, Eapprox, and σapprox), the modified Friedman (yielding E50%Friedman and A50%Friedman), modified Coats-Redfern (yielding E50%C-R and A50%C-R), the three-parameter nucleation (yielding Enarrow, Anarrow, and m, where m is an acceleration parameter from the rate expression dx/dt ) -kx[1 - 0.99x]m), and nth-order (yielding Enth, Anth, dx/dt ) -kxn) methods. In both cases, x is the fraction unreacted. For m ) 0, the nucleation model reduces to a first-order model, and the reaction profile narrows as m increases. σapprox from the Tmax shift method is a Gaussian distribution parameter for activation energy.4 Pyromat-MS Analyses. A detailed account of the technique is reported elsewhere.7 On a Pyromat II micropyrolyzer
Energy & Fuels, Vol. 11, No. 1, 1997 89 at Lab Instruments, Inc., the FID detector was replaced with a VG Instruments Micromass residual gas analyzer, using a Faraday cup detector. The same crucible and weights were used as in the kinetic determinations. The samples were pyrolyzed from room temperature to 700 °C at the heating rate of 8.5 °C/min. The data were recorded on a Micromass computer and transferred to an ASCII file for work-up. The temperature calibration of this particular apparatus was established by comparing the Tmax of sample AP24 of Green River Formation oil shale to earlier work using calibrated thermocouples. Because the temperature correction may not be constant, the temperatures become progressively less reliable (perhaps as much as 10 °C) as temperature decreases. This is not a serious problem for the objectives of this work, because these results are used only for relative reactivity comparisons.
Results Pyromat Kinetics. Table 2 is a summary of the kinetic parameters found for the cellulose and paper samples by various linear and nonlinear regression analyses. (A complete tabulation of the fits is given in the Supporting Information.) One should keep in mind when examining these data that activation energy alone is not a clear indicator of reactivity when frequency factors differ. It is the activation energy-frequency factor combination that governs reactivity. For simplicity, the interpolated Tmax value at the 25 °C/min heating rate [Tmax (25 °C/min)] is used for indicating relative reactivity, where a higher Tmax value indicates lower reactivity. For the three-parameter nucleation, discrete, and nth-order models, only the parameters from the better of the nucleation and distribution models (based on regression residualssΣ1 is the sum of squares of observed minus calculated normalized rates, and Σ2 is the sum of squares of observed minus calculated fractions reacted) are listed in Table 1. Complete listings of the Friedman and Coats-Redfern results are shown in Tables 6 and 7. (1) Cellulose. The A and E parameters calculated for the fibrous cellulose from the Tmax shift, CoatsRedfern, and Friedman methods are in reasonably good agreement. The sample, however, did produce pyrolysate evolution profiles that are narrower at all heating rates (FWHH of 27-38 °C) than the relevant first-order reaction (FWHH of 40-49 °C), indicating the evolution behavior is probably not first order. Figure 1 shows the data and calculated fits using the three-parameter nucleation, nth-order, and discrete methods for the fibrous cellulose sample. Clearly, from visual examination and regression residuals, the three-parameter model yields the best fit to the data. The activation energy and frequency factor agree well with the corresponding Tmax shift values (Table 2). The kinetic parameters calculated according to the nth-order model also agree well with those calculated by the Tmax shift model, but while the fits are in reasonable agreement with the data in the middle evolution ranges, they have the wrong profile shapes at the beginning and end of evolution. The kinetic parameters calculated according to the discrete method, which assumes parallel firstorder reactions, do not match the corresponding values for the Tmax shift method, for reasons to be discussed later. (7) Burnham, A. K.; Samoun, A. M.; Reynolds, J. G. Characterization of Petroleum Source Rocks by Pyrolysis-Mass Spectrometry Gas Evolution Profiles; Lawrence Livermore National Laboratory Report UCIDID-111012; June 1992.
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Table 2. Summary of Activation Energy Parameters for Selected Cellulose-Based Materials property
fibrous cellulose
powdered cellulose
acid-washed cellulose
dunnage
newsprint
Eapproxa Aapproxb σapproxc Tmaxd (25 °C/min) E50%C-R A50%C-R E50%Friedman A50%Friedman Ediscretee (% of total) Adiscrete Enarrow Anarrow m
43.3 4.1 × 1012 0.0 387.7
43.3 6.4 × 1012 0.0 381.5
47.6 2.2 × 1015 0.0 338.1
39.4 4.0 × 1011 2.1 369.9
43.1 5.6 × 1012 1.2 377.4
41.4 1.2 × 1012 42.8 2.5 × 1012
41.2 1.3 × 1012 43.9 8.2 × 1012
47.2 1.3 × 1015 49.6 1.1 × 1016 49 (68)
37.8 1.2 × 1011 36.9 8.7 × 1010 40 (67)
44.2 1.2 × 1013 42.7 5.6 × 1012 43 (71)
8.1 × 1015
9.2 × 1011
6.6 × 1012
42.3 3.2 × 1012 0.41
43.1 8.7 × 1012 0.43
a E, kcal/mol. b A, s-1. c T, °C; interpolated. distribution.
d
Gaussian distribution parameter for activation energy, in % of Eapprox. e Principal value of
Figure 1. Pyromat pyrolysis profiles for fibrous cellulose at 47.8, 6.7, and 0.94 °C/min heating rates. Kinetic parameters and fits were calculated from the data by the (top) threeparameter nucleation, (middle) nth-order, and (bottom) distribution methods. Σ1 and Σ2 are least-squares residuals for the normalized rates and fractions reacted, respectively. The measured rates are normalized to one in the regression analysis to ensure proper relative weighting. The plots show measured rates as points and calculated rates as lines. They are normalized to the maximum calculated rate (user option). Only 33% of the data points are shown for clarity in this and the next two figures.
Table 2 gives the activation energy and frequency factors for powdered cellulose. As in the case of the fibrous cellulose, the Tmax shift, Coats-Redfern, and Friedman methods are in reasonably good agreement and match the corresponding values for the fibrous cellulose. Figure 2 shows the data and calculated fits using the three-parameter nucleation, nth-order, and discrete methods for the powdered cellulose sample. As in the case for the fibrous cellulose, the best fits come from the three-parameter model, and the nth-order and discrete models fail in the same manner as for the fibrous cellulose. The kinetic parameters of these two materials appear almost identical. In addition, the nucleation parameters, m, from the three-parameter method are similar as well as the reaction order, n, from the nth-order method are similar. However, the interpolated Tmax (25 °C/min) values indicate that fibrous cellulose is less
Figure 2. Pyromat pyrolysis profiles for powdered cellulose at 47.2, 14.4, 6.7, and 0.94 °C/min heating rates. Kinetic parameters and fits were calculated from the data by the (top) three-parameter nucleation, (middle) nth-order, and (bottom) distribution methods.
reactive. Also, there are differences in the symmetry of the evolution profiles, where the powdered cellulose exhibits profiles that are more symmetric than the corresponding profiles of the fibrous cellulose, even though this is hard to see in Figures 1 and 2. Table 2 also shows the activation energy and frequency factors for acid-washed cellulose. Overall, the activation energies and frequency factors are higher than for the fibrous and powdered cellulose, indicating structural differences for the acid-washed cellulose. However, the interpolated Tmax (25 °C/min) value shows the acid-washed cellulose is much more reactive. The parameters for the Tmax shift and Coats-Redfern methods are very close. The profile shapes (included in the Supporting Information) are at least as broad as a firstorder decomposition. The discrete method yielded the best results with a very narrow distribution having the principal activation energy account for 68% of the distribution. The discrete parameters match the parameters from the Friedman analysis very well, confirming the presence of a reactivity distribution. (2) Paper. Figure 3 shows the kinetic parameters for the pyrolysis of the paper dunnage determined from data from four different heating rates from 0.94 to 47
Decomposition Kinetics of Cellulose-Based Materials
Energy & Fuels, Vol. 11, No. 1, 1997 91
Figure 3. Discrete distribution kinetic parameters and calculated fits for (top) paper dunnage and (bottom) newsprint from Pyromat pyrolysis profiles at 46.7, 14.5 (dunnage only), 6.7, and 0.94 °C/min heating rates.
°C/min. The Tmax shift kinetic parameters are given in the inset on the left side of the figures, the discrete distribution of energies is given by the bar graph, and the data and fits generated using the discrete parameters are shown on the right side. The activation energy distribution is dominated by a single energy at 40 kcal/ mol, accounting for the majority of the distribution. Some of the distribution is seen at lower energies than the principal activation energy and some are seen at higher energies. Eapprox and Aapprox agree very well with the principal Ediscrete and Adiscrete. The measured profiles are broader (FWHH of 49-60 °C) than the relevant first-order reactions (FWHH of 41-51 °C), indicating the need for a reactivity distribution. This is reflected in both the Gaussian distribution parameter, σapprox, and the discrete distribution. Lower energy contributions to the distribution correspond to evolution on the lowtemperature side of the prominent maximum seen in the data and fits, particularly at the lower heating rates. This is probably hemicellulose, according to Antal and Varhegyi.8 The high-energy contributions correspond to the broad evolution that appears as tailing in the data and fits. The data presented in Figure 3 come from a much larger set of data to find the best kinetic parameters. These were all done with three heating rates of 0.94, 6.7, and 47 °C/min. For comparisons with literature values, which will be presented below, an additional data set was taken at 15 °C/min and added into the best kinetic parameter data set, producing the results in Figure 3. Figure 3 also shows the kinetic parameters determined for the high-grade newsprint. As for the case of the dunnage, the energy distribution is dominated by a single activation energy that accounts for the majority of the energy distribution. Components to the distribution are also seen at lower and higher energies. Data and fits for the newsprint show similar behavior to the dunnage, with three evolution ranges. These correspond to the three ranges in the energy distribution. (8) Antal, M. J., Jr.; Varhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703-717.
Even though the activation energy distributions are similar in shape, the interpolated Tmax (25 °C/min) values indicate the dunnage is more reactive than the newsprint. The activation energies of the paper dunnage are slightly lower than those for the newsprint. Recalculating the discrete distribution of the dunnage using Adiscrete from the newsprint kinetics yields a principal Ediscrete of 42 kcal/mol (42% of total), which is similar to that of the newsprint. However, the residuals were almost 50% higher. Recalculating the discrete distribution of the newsprint using Adiscrete from the dunnage kinetics yields a principal Ediscrete of 41 kcal/ mol (42% of total), which is similar to that of the dunnage. However, the residuals are over 100% higher. Therefore, the values shown in Figure 3 are the best kinetics parameters. TGA Kinetics. Powdered cellulose decomposition was also examined by TGA at four different heating rates, 0.98, 7.0, 15.0, and 49.8 °C/min. The kinetics were determined from the differential curves by the various analyses methods. Our best determination yielded Eapprox ) 39.9 (2.2) kcal/mol, Aapprox ) 2.4 × 1011 s-1; E50%C-R ) 41.2 (2.1) kcal/mol, A50%C-R ) 5.6 × 1011 s-1; E50%Friedman ) 38.9 (2.6) kcal/mol, and A50%Friedman ) 1.2 × 1011 s-1. Figure 4 compares fits generated from the three-parameter, nth-order, and discrete methods with the experimental data. As for the Pyromat cases above, the three-parameter method gives lower residuals for the TGA decomposition. The agreement with the Tmax shift parameters is much better with the corresponding parameters from the three-parameter method, also. The interpolated Tmax (25 °C/min) value is 392.8 °C, which is over 10 °C higher than the value derived from the Pyromat. We do not consider the TGA temperature measurement to be as accurate, and the primary purpose of the data for this paper is using the reaction profile widths and the differences among the activation energies to discriminate among first-order, nth-order, nucleation, and parallel reaction kinetic models. Pyromat-MS Analyses. To understand the evolution behavior of specific volatile species, the powdered cellulose, paper dunnage, and newsprint were examined
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Figure 4. TGA weight loss profiles for powdered cellulose at 49.8, 15.0, 7.0, and 0.98 °C/min heating rates. Kinetic parameters and fits were calculated from the data by the (top) threeparameter nucleation, (middle) nth-order, and (bottom) distribution methods. Table 3. Tmax Values for Individual Evolving Species by Pyromat-MS mass, m/z
powdered cellulose, °C
paper dunnage, °C
15 18 27 29 31 39 43 44 oxides organics Pyromata TGAa
369 362 364 361 373 366 368 356 359 366 361 371
352 348 350 348 351 347 352 342 345 349 353 nab
b a Interpolated T max at 8.5 °C/min from Tmax shift. Not applicable.
by Pyromat-MS using a single heating rate of 8.5 °C/ min. The masses monitored were m/z 15 (-CH3), 18 (H2O), 27 (-C2H3), 29 (-C2H5, -HCO), 31 (-OCH3 fragments), 39, 43 (-C3H9), and 44 (CO2). In past experiments with fossil fuel materials, m/z 29 was shown to be primarily due to hydrocarbon gases and had an intensity about equal to or less than the intensity for m/z 27.7 For these cellulose-based materials, however, m/z 29 is substantially larger than m/z 27, and the incremental ion intensity can be attributed to the HCO fragment.9 Table 3 shows the Tmax values of these individual species for each of the samples. Tmax values range 356 to 373 °C for cellulose and from 342 to 352 °C for dunnage. For corresponding m/z, dunnage Tmax values are around 16 °C lower than for cellulose. In general, the Tmax values follow the overall pattern for total hydrocarbon evolution seen in the kinetic determinations above show, for a specific m/z, the order is cellulose > dunnage. The oxides are a summation of detector current for m/z 18 and 44. The organics are a summation of (9) Milne, T. A.; Soltys, M. N. In Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier: London, 1985; Chapter 20, p 367.
Figure 5. Pyromat-MS pyrolysis profiles for (top) organics (summation of organic masses monitored) and oxides (water + CO2) evolving from cellulose in absolute intensity, (middle) m/z 29 for dunnage and cellulose in normalized intensity, and (bottom) m/z 15 for dunnage and cellulose in absolute intensity.
detector current for m/z 15, 27, 29, 31, 39, and 43. Figure 5 (top) shows the evolution profiles for both of these summations for cellulose. The organics Tmax value is slightly higher than the oxides Tmax value. In addition, the intensity of the oxides evolution is much higher than for the organics, because most of the tars are condensed prior to the mass spectrometer. Although not shown, dunnage sample exhibits the same behavior. Table 3 also lists the Tmax values interpolated for an 8.5 °C/min heating rate from the kinetic determinations above. For both cellulose and dunnage, the interpolated Pyromat Tmax values are reasonably close to those of the measured Pyromat-MS Tmax values and are considered within experimental error. The interpolated TGA Tmax values are correspondingly higher but are still within the range of the measured values, also. Figure 5 (middle) compares the m/z 29 evolution profiles for each of the samples. These profiles have been normalized for comparison. The m/z 29 signal was selected because of its high intensity and because the other signals, except for m/z 15, follow roughly the same behavior. The dunnage appears to have a broader profile the cellulose, which is distinctively more symmetric and narrower. Both have similar shapes to the corresponding total hydrocarbon evolution profiles in Figures 3 and 4. Figure 5 (bottom) compares the absolute evolution for m/z 15 for both samples. The dunnage profile appears to have three evolution regimes, with the principal maximum about the same as for the other light hydrocarbons (see Table 3). The other evolution ranges are broader and at higher temperatures. These are probably due to secondary reactions such as char decomposition. The profile for cellulose is somewhat different from the dunnage; only two evolution ranges are seen in the profile for cellulose. Even though the principal maxima have different Tmax values for the two samples, the hightemperature peaks seem to come in roughly the same evolution ranges, suggesting structural similarity of the chars at that point.
Decomposition Kinetics of Cellulose-Based Materials
Discussion Fundamentals of Kinetic Analysis. Accurate kinetic parameters require accurate temperature measurements. First, temperature measurement devices must be carefully calibrated over their range of use, and heat and mass transfer resistances must be eliminated. Once accurate temperature measurements are attained, one can deal with kinetic analysis of the chemical reactions. Over the past 10 years, we have developed a set of guidelines to help select an appropriate kinetic model and assess the validity of the kinetic parameters. (1) For nth-order (including n ) 1),10,11 nucleation,5,12 and parallel reaction models,10 the activation energy representing any given measure of constant reaction conversion, including Tmax at a constant heating rate, can be determined by a relatively simple linear regression technique. For a first-order model to be valid, the activation energy calculated from fitting the reaction profile for a single heating rate must equal that derived from the shift with heating rate of Tmax or any other measure of constant conversion. A wide range of heating rates is needed to prevent small temperature errors from causing large errors in the activation energysa 1 °C error per decade of heating rate results in a 1 kcal/mol error in E. (2) For a constant heating rate experiment, a reaction profile broader than that calculated using A and E derived from the shift in Tmax or other measure of constant conversion indicates either a parallel reaction model or an nth-order reaction.10 Because an nth-order reaction has no fundamental basis for complex devolatilization reactions and because most materials with these broad profiles also evolve different products with different kinetics, the parallel reaction model is the sounder approach. Inappropriately fitting a single heating rate experiment to a first-order reaction in this case will yield an activation energy lower than the true average value.11 (3) For a constant heating rate experiment, a reaction profile narrower than that calculated using A and E derived from the shift in Tmax or other measure of constant conversion indicates either an nth-order model with n < 1, a serial reaction model, or a nucleation model.4,5 A reaction order 10 °C/min (high) and those at 1, because there is a compositional variation in products as a function of conversion, suggesting a spectrum of chemical reactions with varying rate parameters. The slight shoulder on the low-temperature side of the reaction profile, the small difference between peak oxide and organic evolution, the complex evolution pattern for methane evolution from dunnage, and the well-known differences in reactivities of lignin, hemicellulose, and cellulose suggest those same arguments apply here. This is not to say that the parallel reaction model is a perfect representation, as has been discussed in detail elsewhere.25,26 (24) Chang, C. Y.; Wu, C. H.; Hwang, J. Y.; Lin, J. P.; Yang, W. F.; Shih, S. M.; Chen, L. W.; Chang, F. W. J. Environ. Eng. ASCE 1996, 299-305. (25) Burnham, A. K.; Oh, M. S.; Crawford, R. W.; Samoun, A. M. Energy Fuels 1989, 3, 42-55.
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Conclusions • The kinetic parameters for the two cellulose materials are essentially the same. The interpolated Tmax (25 °C/min) values indicate a small difference in reactivity. • The kinetic model that gave the best fits to the cellulose data is the three-parameter nucleation method, which indicates cellulose decomposition kinetics are not strictly first order. • Acid-washed cellulose has kinetic parameters that are completely different from those of regular cellulose. The interpolated Tmax (25 °C/min) value indicates a much higher reactivity. • The newsprint and paper dunnage require a distributed reactivity model and exhibited similar activation energy distributions, but they have a difference in principal Ediscrete of 3 kcal/mol. The interpolated Tmax indicates different reactivities. • Non-cellulose components are probably why the paper data requires a reactivity distribution kinetic model, while the pure cellulose data requires a nucleation kinetic model. • Reactivity assessed from kinetic parameters derived from different TGA apparatus differ from the Pyromat and each other, apparently because of temperature calibration. Even so, the narrow profile widths from TGA support the need for a nucleation kinetic model. Acknowledgment. We thank Henrik Wallman of LLNL for providing elemental analyses of the paper samples; Eric Suuberg of Brown University for the powdered cellulose sample, TGA data, and useful discussions; Steve Buckley of LLNL for TGA data; Alain Samoun of Lab Instruments for the Pyromat-MS measurements; and the two reviewers for helpful comments. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-ENG-48. Supporting Information Available: The data input files and kinetic analyses for newsprint, dunnage, fibrous cellulose, powdered cellulose, and acid-washed cellulose; and the DOS computer program (PLOTKIN), which can be used to view plots of the kinetic data and fit. Ordering information is given on any current masthead page. EF960086A (26) Burnham, A. K.; Schmidt, B. J.; Braun, R. L. Org. Geochem. 1995, 23, 931-939.
