Reactivity of Paper Residues Produced by a Hydrothermal

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Energy & Fuels 1997, 11, 98-106

Reactivity of Paper Residues Produced by a Hydrothermal Pretreatment Process for Municipal Solid Wastes John G. Reynolds,* Alan K. Burnham, and P. Henrik Wallman Lawrence Livermore National Laboratory, University of California, L-369, P.O. Box 808, Livermore, California 94551 Received June 11, 1996. Revised Manuscript Received October 17, 1996X

Yields of gases, water, water-soluble organics, and water-insoluble char are reported for hydrothermal pretreatment of paper dunnage at several conditions. Chars are characterized in terms of chemical composition, slurry-forming properties, and pyrolysis properties. Pyrolysis yields, relative reactivities, and kinetic parameters for evolution of volatile organic components from isolated chars were determined by Pyromat micropyrolysis. For both the water-insoluble chars and solvent-extracted chars, pyrolysate yields decreased with increasing pyrolysis severity, and the temperature of maximum evolution rate for a constant heating rate (Tmax) increased. The extracted residues have less low-temperature evolution and narrower evolution profiles overall. Kinetic analysis used the Tmax shift (approximate) and discrete distribution methods. Except for one sample, all of the extracted chars have similar kinetic parameters, i.e., activation energies in the range of 54-56 kcal/mol and frequency factors in the range of 1015 s-1. The exception was from the most severe pretreatment conditions and exhibited much higher activation energies. FTIR analyses of the extracted chars and isolated extracts indicate the structures of the extracted chars are very similar, except for the most severe pretreatment sample. The results are used to assess the suitability and limitations of a parallel reaction model for pyrolysis of paper, especially for modeling hydrothermal pretreatment of gasifier slurry feeds.

Introduction The utilization of municipal solid waste (MSW) materials for energy and chemical production is currently receiving attention.1-5 Paper is a principal component, ranging from 30 to 50 wt % of many MSW streams.2 Knowledge of the behavior of paper in conversion processes is critical for design of and commercializing technology. One such process is mild hydrothermal pretreatment,6,7 which has been practiced for many years7 and consists of heating the paper in the presence of water in a pressurized system. At temperatures of 250-350 °C, the paper is decomposed into gases, liquids, and solids. This decomposition produces a more carbonrich and less water-absorbing solid due to altered pore structure, resulting in a slurry that is more pumpable and has a higher heating value than one from the untreated paper.6 To understand this conversion process better and possibly to predict the distribution of heating value in the various phases as a function of processing conditions, we have been studying the nonisoAbstract published in Advance ACS Abstracts, December 15, 1996. (1) Soltes, E. J., Milne, T. A., Eds. Pyrolysis Oils from Biomass; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988. (2) Milosavljevic, I.; Suuberg, E. M. Ind. Eng. Chem. Res. 1995, 34, 1081-1091. (3) Antal, M. J.; Varhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703707. (4) Cozzani, V.; Nicolella, C.; Petarca, L.; Rovatti, M.; Tognotti, L. Ind. Eng. Chem. Res. 1995, 34, 2006-2020. (5) Kuester, J. L. Thermal Systems for Conversion of Municipal Solid Wastes, Vol. 5, Pyrolytic Conversion: A Technology Status Report; Argonne National Laboratory; ANL/CNSV-TM-120; 1983. (6) Khan, M. R.; Albert, C.; McKeon, R.; Zang, R.; DeCanio, S. Prepr. Pap.sAm. Chem. Sos., Div. Fuel Chem. 1993, 38 (3), 802-809. (7) Bobleter, O. Prog. Polym. Sci. 1994, 19, 797-841. X

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thermal, pyrolysis kinetics of the starting feedstocks and solid materials produced in this hydrothermal processing. Development of a reliable, predictive kinetic model of complex chemical processes is a challenging task. A common problem is that simplifications of the chemical reaction model invalidate its extrapolation outside the range of calibration. In one very challenging application, pyrolysis kinetics have shown great predictive utility in geochemistry and have been used to model generation of petroleum from its insoluble precursor, kerogen.8 Kinetic parameters determined in the laboratory have been extrapolated to time scales of millions of years to predict the location of oil in a source rock or reservoir when an adequate paleothermal history can be estimated for the formation. A key tool in this application is the parallel reaction model with an activation energy distribution. Originally developed for coal pyrolysis,9 it has been refined and more extensively used for petroleum exploration. It has not, however, been applied to pyrolysis of cellulose-rich materials prior to the preceding paper. As in geochemical applications, fundamental questions arise concerning the validity of approximating the complicated set of parallel and serial reactions that occur in a heterogeneous material with a set of independent parallel reactions. If a parallel reaction approach is valid, should the reactivity distribution be realized in the activation energy, frequency factor, or (8) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurence; Springer-Verlag: New York, 1988. (9) Anthony, D. B.; Howard, J. B. AIChE J. 1976, 22, 625-656.

© 1997 American Chemical Society

Reactivity of Paper Residues

both? Further, do kinetic parameters from rapid and convenient open system pyrolysis experiments using milligram-sized samples apply to pyrolysis in the presence of water? These issues were recently addressed in some detail by comparing the predictions of an activation energy distribution kinetic model with pyrolysis properties of hydrous pyrolysis residues.10 As was suspected by indirect means, it was found that the activation energy model works fairly well for a marine kerogen that had eliminated most of its oxygen via geochemical maturation, but it works less well for immature type III kerogen with substantial oxygen content. Dry pyrolysis decomposition kinetics have been used in biomass studies, particularly in the study of cellulose decomposition.2-4 Although a multitude of studies have been performed using weight loss methods such as TGA, controversy still surrounds the methods and interpretation of the kinetic parameters with regard to an appropriate cellulose decomposition model. In the preceding paper,11 we applied pyrolysis kinetic methods developed during geochemical and polymer studies to decomposition of cellulose-based materials. Various models were used to determine kinetic parameters and relative reactivities. Cellulose decomposition was best modeled by a three-parameter nucleation model, which gives narrower reaction profiles than a first-order model, yielding an activation energy of 43 kcal/mol and a frequency factor of 5 × 1012 s-1. In contrast, we found that newsprint decomposition was best modeled by parallel first-order reactions because of its broad evolution profiles, presumably because of the inclusion of other components in the paper, such as hemicellulose and lignins. The main activation energy and frequency factor were the same as those of cellulose. Most pyrolysis kinetics studies in biomass have focused on the reactivity of raw wood and cellulose. Little or no work measured and interpreted the decomposition kinetics of the residual char left after hydrothermal pretreatment. The reactivity determined by the pyrolytic decomposition of the residual chars can be used to understand the pretreatment process, or even better, help optimize the process. This reactivity should correlate with some desired engineering parameter, such as process severity, or some fundamental chemical property, such as product rheology or effects of various feed materials. Initial results12 have already shown some dependence of the slurry properties on reaction severity and chemical changes, suggesting a coalification process occurring during processing. Further, the reactivity and kinetic parameters of the residues can be used to check the validity of kinetic models derived from the original material. Here we report the pyrolysis decomposition kinetic parameters of the residual chars remaining after hydrothermal pretreatment of paper dunnage at various conditions. The measured reactivities and kinetic parameters are related to process conditions and are combined with limited characterization studies to assess (10) Burnham, A. K.; Schmidt, B.; Braun, R. L. Org. Geochem. 1995, 23, 931-939. (11) Reynolds, J. G.; Burnham, A. K. Energy Fuels 1997, 11, 8897. (12) Wallman, P. H. Laboratory Studies of a Hydrothermal Pretreatment Process for Municipal Solid Waste; Lawrence Livermore National Laboratory Report UCRL-ID-120296; April 6, 1995.

Energy & Fuels, Vol. 11, No. 1, 1997 99

the global mechanism of hydrothermal pretreatment. In particular, we assess how well activation-energydistribution kinetic models derived from dry pyrolysis of paper apply to pyrolysis in the presence of water. Experimental Section Chemicals. Methylene chloride (CH2Cl2) and methanol (MeOH) were B&J Brand chromatography grade purchased from Baxter Scientific; potassium bromide (KBr) was infrared grade purchased from Aldrich. Water was deionized. Samples. The paper dunnage came from an industrial packaging company and is composed of tailings from paper mills. The sample was prepared for pyrolysis by cutting into pieces of less than 0.5 mm × 0.5 mm (∼0.2 mg). Chemical analysis of the paper, given in more detail in the preceding article, indicated 42% glucan, 19% other carbohydrates, 27% lignin, and 2% ash. The hydrothermal pretreated samples were prepared by batch autoclave or pilot-plant treatment of the paper dunnage as reported below.13 The chars were the water-insoluble residues isolated from the product mixture by filtration followed by drying in vacuo. Although ash analyses of our specific samples are not available, ash from a similar 310 °C/30 min sample contained 45% each of Al and Si plus minor amounts of many other metals, and a similar 320 °C/ 30 min sample contained 46% Si, 26% Al, 17% Fe, and minor amounts of other metals. Nearly all alkali metals had been extracted by the hydrothermal treatment. The char was then ground into a fine powder with a mortar and pestle. Corresponding extracted chars were prepared from the chars by Soxhlet extraction with 92 vol % CH2Cl2 and 8 vol % methanol for 24 h, dried in vacuo, and reground before analyses. The solvent was removed from the extract with flowing N2 followed by drying in vacuo. Autoclave Pretreatment. The hydrothermal pretreatment was carried out in a 140 mL stainless steel Parr bomb batch autoclave (Autoclave Engineering). The reactor was loaded with the paper dunnage and water under atmospheric pressure. Water to feed ratios were 2 to 1 except for the 275 °C/120 min pretreatment condition. The sample for this run was a composite of five samples from 275 °C/120 min pretreatments at water to feed ratios of 1, 2, 3, 4, and 10 to 1. Isothermal conditions were obtained only approximately because of the 1.5 h heat-up times and slow heat transfer between the oven and reactor. Reaction times were counted from the time the internal temperature reached 10 °C of the desired temperature. Gas yields were determined, assuming ideal gas laws, by allowing the product gas to expand into an evacuated 1 L volume at room temperature and recording the final pressure at ambient temperature. Gas composition was determined by mass spectrometry. The water-soluble organic yield was determined by analyzing the water sample for organic carbon and assuming the composite soluble material had the same stoichiometry as the starting material. Pilot-Plant Pretreatment. The pilot-plant sample was generated by treatment of paper dunnage in a large scale batch reactor (approximately 150 kg). The feed was moistened and then loaded into the reactor. Initial water to feed ratio was 0.5 and at run temperatures was 4.0. The system was heated with steam to 275 °C, which took approximately 3.5 h, with steam condensing to form the aqueous phase. Steam injection continued for 2 h with the reactor kept at 275 °C. Steam injection was then stopped, and the system was vented slowly over 2 h to atmospheric pressure. Further cooling was achieved by a stream of cool N2/air for 4 days. The product was isolated by draining the free flowing water and then (13) Pasternak, A. D.; Richardson, J. H.; Rogers, R. S.; Thorsness, C. B.; Wallman, H.; Richter, G. N.; Wolfenbarger, J. K. MSW to Hydrogen; Livermore National Laboratory Report UCRL-JC-116421; April 1994.

100 Energy & Fuels, Vol. 11, No. 1, 1997 removing the solids out of the reactor. The self-generated pH of the produced water was 3 and was highly buffered. Micropyrolysis Tests. The Pyromat II micropyrolyzer has been described previously.11 Samples were pyrolyzed at constant heating rates, using He as the carrier gas. Volatile organic compound evolution was measured by flame ionization detection. Sample size was 1-4 mg. Temperature was measured by direct contact of a Type K thermocouple (0.040in. 304 stainless steel sheath) with the sample. The thermocouples were calibrated against a Type R primary standard. Data were stored and manipulated on a personal computer interfaced with the pyrolysis unit. Yields. Pyrolysis yields of the chars were determined for each sample by comparison to the yield from Green River oil (AP22) oil shale. This yield has been determined from Fischer Assay14 and Rock-Eval10 analysis to be 88 mg of pyrolysate/g of oil shale. The yield was determined from multiple runs at the heating rate of 24.5 °C/min from 100 to 700 °C. The AP22 standard was run immediately before and after each sample. Individual profiles were normalized to total yield. Kinetic Analysis. The method of kinetic analysis has been described in detail elsewhere.15 Multiple runs at constant heating ratessthree 47 °C/min, one 6.7 °C/min, and two 0.94 °C/min runsswere performed for each kinetic data set. If Tmax values and profile shapes did not agree, more runs were performed. Rate data were analyzed by using the regression analysis program KINETICS.16 The kinetic parameters used in this study were determined primarily according to the Tmax shift method, which is an approximate mathematical method (yielding Aapprox, Eapprox, and σapprox), and by the discrete activation energy distribution method, which is a rigorous mathematical method (yielding Adiscrete and Ediscrete). σapprox is a Gaussian distribution parameter for activation enrgy, but it is not reported here for simplicity. The paper dunnage kinetic results have been reported in detail previously.11 Viscosity Measurements. Slurries were prepared from either the filtered residue without drying or from the dried char by adding back the separated aqueous phase. Shearing of the product using a food processor (Cuisinart) was very important for refluidization. The slurries were processed for several minutes in the food processor, and the product was assumed to be at constant viscosity (no measurements were made to verify this). Slurry rheograms were determined on a Haake RV30 viscometer. This viscometer uses a rotating spindle with a narrow gap between the spindle and the stationary wall. Some samples, particularly thin slurries, showed thixotropic behavior. In these cases, the viscosity was based on the average of the shear rate increase and shear rate decrease curves. Thick samples showed separation upon loading and could not be run at all. Effective viscosity, defined by a forced Newtonian model at a shear rate of 100 s-1, in the range of about 50-500 mPa‚s was the ideal viscosity range for the measurements. Spectroscopic Analyses. The infrared spectra were taken on a Nicolet Impact 400R spectrometer. The extract samples were prepared by coating a KBr window. The hydrothermal residues were powdery enough to use KBr pellets.

Results Process Yields as a Function of Severity. Table 1 shows the total yield distribution of the whole products from the pretreatments. The extracted organic solubles yield was measured by the weight difference of the char before and after extraction with CH2Cl2/methanol and (14) Singleton, M. F.; Koskinas, G. J.; Burnham, A. K.; Raley, J. H. Lawrence Livermore National Laboratory Report UCRL-53273, rev 1; April 12, 1986. (15) Braun, R. L.; Burnham, A. K.; Reynolds, J. G.; Clarkson, J. E. Energy Fuels 1991, 5, 192-204. (16) Burnham, A. K.; Braun, R. L.; Gregg, H. R.; Samoun, A. M. Energy Fuels 1987, 1, 452-458.

Reynolds et al. Table 1. Product Distribution from Hydrothermal Pretreatment of Paper Dunnage

conditions 260 °C/30 min 275 °C/30 min 310 °C/30 min 320 °C/30 min 275 °C/120 min 310 °C/120 min a

water organic gas, water,a solubles, solubles, extracted wt % wt % wt % wt % char, wt % 12 12 17 16 14 17

11 16 18 18 12 21

16 17 17 16 22 17

26 21 20 15 21 16

35 34 28 35 31 29

Water by balance

represents water insoluble/organic solvent-soluble material. The other yields have been reported previously.12 For the 30 min pretreatments from 260 to 310 °C, the overall yields of organic solubles and extracted chars decrease with pretreatment severity at the expense of making incrementally more gas and water solubles. The char product is also less reactive as the pretreatment temperature increases, as measured by Pyromat Tmax values of the 30 min pretreatment chars (see next section). More cracking reactions yielding gas and coke are probably occurring also, because the organic solubles yields are decreasing, while the gas yields are increasing. At the 320 °C pretreatment temperature, this trend is reversed: more extracted char is produced, primarily at the expense of the soluble organic species. This suggests the temperature has been reached at which condensation reactions producing coke-like materials are becoming dominant over cracking reactions producing liquid products. Overend and Chornet17,18 reported an easy and concise method to correlate hydrothermal product properties as a function of process severity. The method adapts a common phenomenological engineering approach using a reaction ordinate, R0 ) t exp[(T - 100)/ 14.75] in their case, which combines time, t (minutes), and temperature, T (°C), to express the severity of a given pretreatment condition. These performance curves are plotted as log10 R0 and are in minutes. Using this correlation, parts a and b of Figure 1 show that for both extracted and unextracted chars, yields of char decrease and measured Tmax values at 24.5 °C/min increase with increasing severity. The extracted samples also exhibit Tmax values that are higher than those of the corresponding unextracted sampless7, 9, 17, and 9 °C for 30 min pretreatment at 260, 275, 310, and 320 °C, respectively. While a quantitative relationship to Overend and Chornet’s work requires that hydrolytic reactions are predominant during pretreatment, the plots are good ways to present our data even if other mechanisms are dominant. Figure 1c also shows the secondary pyrolysate yields from chars and extracted chars measured by the Pyromat at a heating rate of 24.5 °C/min. Trends appear less clear than for the char yields and Tmax values, but overall it appears the secondary pyrolysate yields decrease with increasing severity. The chars produce between 190 and 250 mg of organics/g of char, and the extracted chars produce between 70 and 160 mg of organics/g of char. (17) Overend, R. P.; Chornet, E. Philos. Trans. R. Soc. London A. 1987, 321, 523-536. (18) Jollez, P.; Chornet, E.; Overend, R. P. In Advances in Thermochemical Biomass Conversion; Bridgewater, A. V., Ed.; Blackie: London, 1994; pp 1659-1669.


Figure 1. Behavior of (a) char yields, (b) measured Tmax at 24.5 °C/min heating rate, (c) pyrolysate yields of char and extracted chars, and (d) maximum practical organic content of the slurry (wt %, extrapolated to 1000 mPa‚s) for hydrothermally pretreated paper dunnage chars and extracted paper dunnage chars with respect to pretreatment severity using the Overend and Chornet severity scale.

Figure 2. (a) Volatile organic compound evolution profiles at the heating rate of 47 °C/min of paper dunnage, hydrothermally treated paper dunnage (char), and CH2Cl2/methanol extracted char, all normalized to 1. (b) Absolute evolution profiles at the heating rate of 24.7 °C/min in mg of pyrolysate/g of feed of paper dunnage and extracted chars from hydrothermal pretreatment at 260, 275, 310, and 320 °C for 30 min.

Pyrolysis Behavior. Figure 2a shows the pyrolysis evolution profiles at the 47 °C/min heating rate for paper dunnage, a char from a selected pretreatment, and the corresponding extracted char, all with the maximum evolution normalized to 1.0. The paper dunnage evolves organics in the range of 250-550°C with a single sharp maximum having a Tmax value of 385 °C. A low-


intensity shoulder on the high-temperature side of the maximum and a hint of a shoulder on the low-temperature side are evident. The char has a much wider evolution range, from 100 to past 550 °C, with a much broader single maximum having a Tmax of 433 °C. The char also has a significant amount of low-temperature evolving material, which is not apparent in the paper dunnage. The extracted char behaves similarly to the char, except most of the low-temperature evolving material is absent and the Tmax value is 449 °C, higher than either the paper dunnage or the char. This incremental yield from unextracted chars is most likely due to volatile organic compounds evolving between 100 and 400 °C, and the true char pyrolysate yields are measured from the extracted chars only. This difference between char and extracted char behavior has been seen in the extraction of semivolatile bitumen from source rocks and tar sands.19,20 Figure 2b shows the evolution profiles at the 24.7 °C/ min heating rate of the dunnage and all of the extracted chars for the 30 min pretreatments in absolute evolution of milligrams of organics per gram of feed. The paper dunnage has a total measured yield of 253 mg of organics/g of paper. The extracted chars evolve much less organics and correspond to the low-intensity shoulder seen in the paper dunnage profile. As the severity of the pretreatment increases, the amount of absolute evolution from these chars decreases as the Tmax value increases. Although difficult to see in the figure, the 310 and 320 °C samples overlap except for a slight change in the Tmax. In addition, within the sensitivity range of the determinations, no evolution is seen in temperature ranges higher than what is seen for the paper dunnage. This suggests a selective concentration of the refractory material in the insoluble phase during the main stage of the hydrothermal pretreatment through conversion of the more labile hemicellulose and cellulose, consistent with a parallel reaction model. The 120 min severity pretreatments (not shown) also have this behavior. Kinetic Analyses. Kinetics of the solid samples were determined by linear and nonlinear regression analyses. When examining these data, one must remember that activation energy alone is not a clear indicator of reactivity when frequency factors differ. It is the activation energy-frequency factor combination that permits calculation of reaction rate or rate constants. A convenient measure of relative reactivity is the Tmax at some reference heating rate, either measured or calculated, where a higher Tmax value indicates lower reactivity. For simplicity, we use an interpolated Tmax value at 25 °C/min [Tmax (25 °C/min)] for indicating relative reactivity. Figure 3a shows the kinetic parameters determined previously11 for the pyrolysis of the paper dunnage sample determined from four different heating rates from 0.94 to 47 °C/min. The A and E values from the Tmax shift (approximate) method are given in the inset on the left side of the figure, the discrete distribution of energies is given by the bar graphs, and the data and the fits generated using the discrete parameters are shown in the figure on the right side. The activation (19) Clementz, D. M. A. A. P. G. 1979, 63 (12), 2227. (20) Reynolds, J. G. Proceedings, 1991 Eastern Oil Shale Symposium; Institute of Mining and Minerals Research: Lexington, KY, 1992; IMMR 91 07, pp 7-16.

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Figure 3. Kinetic parameters for volatile organic compound pyrolysis by the discrete distribution and Tmax shift (approximate) methods determined from the pyrolysis of paper dunnage at 47, 15, 6.7, and 0.94 °C/min heating rates, and extracted chars from pretreatment conditions 260, 275, 310, and 320 °C for 30 min and 275 and 310 °C for 120 min; pilot plant sample at 52, 6.7-6.8, and 0.95-0.96 °C/min heating rates. The Tmax (25 °C/min) was interpolated from the data. Only 30% of the data points are shown for clarity. The fits were calculated using the discrete distribution parameters. S1 and S2 are residuals from least-squares analyses.

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 and higher energies than the principal activation energy. The Eapprox and Aapprox agree very well with the principal Ediscrete and Adiscrete. The lower energy contributions to the distribution correspond to evolution on the lowtemperature side of the prominent maximum seen in the data and the fits, particularly at the lower heating

rates. The high energy contributions correspond to the high-temperature shoulder in the data and fits. To understand the reactivity during the pretreatment process as well as the reactivity of the residual solid materials, moisture-free chars from selected hydrothermal pretreatments were examined by pyrolysis to determine the kinetic parameters. It was not possible to obtain good kinetic parameters for the unextracted chars. The activation energies from the Tmax shift


method were all above 60 kcal/mol and appeared to decrease with increasing severity. The material evolving in the low-temperature range indicates volatile liquid hydrocarbon-like species are involved,21 and its presence probably compromises the integrity of the kinetic parameters. Extraction with an organic solvent mixture removes the low-temperature evolving material (see Figure 2a), thereby simplifying the evolution behavior by restricting it to thermal cracking and enabling better kinetic determinations. Figure 3b-g shows the best kinetic parameters determined on the extracted chars produced from several pretreatment conditions. The presentation is the same as for the paper dunnage except that the broad evolution profiles required 2 kcal/mol spacings for valid kinetic determinations. Except for the 310 °C/120 min sample, all of the extracted chars have similar kinetic parameters. For approximate parameters, the activation energies fall between 54 and 56 kcal/mol, while the discrete distributions are broad and the principal Ediscrete values range from 53 to 57 kcal/mol. All but one of the extracted chars have lower Eapprox values than for the unextracted chars, which suggests that the trend of decreasing E with increasing severity observed for unextracted chars is probably due to a decreasing contribution from the extracted organics. The 310 °C/120 min sample has kinetic parameters distinctly different from the rest of the samples. Even upon extraction, the activation energy increased. This may be because of the long reaction time, and the behavior seems to reflect a more coke- or coal-like char. Figure 3h shows the discrete distribution for the pilotplant sample, which appears to most closely resemble that of the 275 °C/30 min sample. The pilot-plant sample was pretreated at 275 °C for >30 min. The exact processing conditions for the pilot-plant sample are not known because of the size and placement of thermocouples and because the residence time at reaction temperature is not well determined due to the long heating and cooling times. Correcting the Tmax (25 °C/ min) value for the char on Figure 1b indicates log10 R0 around 7.55. Using 275 °C as the temperature, this severity value would suggest an effective residence time of 250 min. Comparison with Autogenous Pretreatment. The paper dunnage was also pretreated in the same system at 310 °C for 30 min but without added water and with a vent tube. The CH2Cl2/methanol-extracted char has a similar broad evolution profile as seen in Figure 3 for the hydrothermal treated product and has qualitatively similar kinetic parameterssEapprox ) 54 kcal/mol, Aapprox ) 7.4 × 1014 s-1; principal Ediscrete ) 55 kcal/mol, Adiscrete ) 1.8 × 1015 s-1. However, the interpolated Tmax (25 °C/min) of 437 °C is significantly lower than the corresponding value of 450 °C for the hydrothermal pretreated product, and the extractable yield was dramatically lower (6%). This indicates that hydrothermal pretreatment more effectively converts the paper into liquid products (or prevents coking of liquid products to gas and solid) than the equivalent ambient pressure autogeneous pyrolysis. Similar dif(21) Reynolds, J. G.; King, K. J. Proceedings, 6 UNITAR International Conference on Heavy Oil and Tar Sands; U.S. DOE DE95000188; U.S. GPO: Washington, DC, 1995; Vol. 1, pp 281-291.


ferences between dry and hydrous high-pressure pyrolysis are also observed for kerogen.22 Infrared Analyses of Residual Materials. Although special techniques, such as diffuse reflectance IR,23 are usually required to obtain meaningful spectra for paper and wood samples, the hydrothermal pretreatment denatured the paper dunnage enough that good quality spectra can be attained using KBr pellets. Figure 4 shows the IR spectrum from 400 to 4000 cm-1 of the CH2Cl2/methanol extracted char from the 320 °C/ 30 min pretreatment and compares it with that of the organic-soluble extract after the solvent has been removed. The prominent bands in both spectra assigned from literature values24-26 are a broad band at 3400 cm-1 (O-H on phenol or carboxylic acid) and prominent bands at 1710 cm-1 (CdO of carboxylic acids, probably next to aromatic rings or CdC), 1606 cm-1 (aromatic or CdC), 1453 cm-1 (methyl and methylene bridge), 1380 cm-1 (CH3 on aromatics), 1273, and 1215 cm-1 (CsO, possibly next to aromatic ring). Comparison of the differences between the two spectra show that the extract has more a resolved aliphatic region, 2850-2975 cm-1, probably due to the smaller molecules in the extract, with no evidence of the weak aromatic band at 3040 cm-1. The extract has a stronger carbonyl band at 1710 cm-1, suggesting the acids are fairly small and organic-solvent soluble. The extracted char has a carbonyl band that is slightly shifted to 1697 cm-1, indicating a different carbonyl structure, probably next to a more condensed aromatic structure. The extracted char has a much stronger aromatic band at 1606 cm-1, also suggesting a more condensed structure. The other extracted chars and extracts were also examined by FTIR. The spectra for all of the extracted samples are essentially identical except for the 310 °C/ 120 min sample. The spectra of the extracts are also more or less the same except for the extract of the 310 °C/120 min sample. Differences were observed in the ratio of the 1710 to 1606 cm-1 bands, but because of the baseline variations, this was hard to utilize. The differences between the extracted chars and the corresponding extracts are also essentially the same for all of the samples except for the 310 °C/120 min sample. The spectra of the extracted char and extract from the 310 °C/120 min pretreatment are also shown in Figure 4. Both fractions appear different from the corresponding fractions of the other samples. For the extracted char, the carbonyl band at 1696 cm-1 is just a shoulder and the CdC band was shifted to 1590 cm-1. For the extract, the carbonyl band is split, having bands at 1710 and 1969 cm-1. Elemental Analyses. The C, H, and O composition was determined for the chars (before CH2Cl2/methanol extraction).12 Figure 5 shows the results for the chars on a modified van Krevelen diagram,8,27 which classifies them similar to type III (coaly) geological samples. The paper dunnage has a CH1.66O0.72 relationship. The 30 (22) Lewan, M. D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37 (4), 1643-1649. (23) Fuller, M. P.; Griffiths, P. R. Anal. Chem. 1978, 50 (13), 19061910. (24) Grandmaison, J. L.; Thibault, J.; Kaliaguine, S.; Chantal, P. D. Anal. Chem. 1987, 59, 2153-2157. (25) Meier, D.; Larimer, D. R.; Faix, O. Fuel 1986, 65, 916-921. (26) Dollimore, D.; Hoath, J. M. Thermochim. Acta 1981, 45, 103113. (27) van Krevelan, D. W. Coal; Elsevier: Amersterdam, 1961.


Reynolds et al.

Figure 6. Shear vs shear rate for liquids and solids from hydrothermal pretreatment of paper dunnage at 275 °C for 120 min. Slurry is 50% organic material.

Figure 4. FTIR analyses of CH2Cl2/methanol extracted char and isolated extract produced from the pretreatment of paper dunnage at 320 °C for 30 min and at 310 °C for 120 min.

Figure 5. Modified van Krevelen diagram27 from Tissot and Welte8 including C, H, and O values for unextracted paper dunnage chars from hydrothermal pretreatment at 260, 275, 310, and 320 °C for 30 min and at 275 and 310 °C for 120 min. The other symbols are from textbooks: (b) Green River shales; (O) algal kerogens; (*) lower torsian shales; (9) silurian shales; (f) various type II shales; ()) upper cretaceous coals; (2) lower mannville shales.

min pretreatment chars parallel coal behavior; increasing process temperature is comparable to increasing burial depth (or maturity). The O/C decreases rapidly with increasing pretreatment temperature, while the

H/C decreases at a much slower rate. The 120 min pretreatment chars also show a similar behavior that parallels the coal behavior. This behavior does not fit with what is expected, i.e., further advancement down the 30 min curve, on the basis of the 30 min char results. However, overall grouping and trends for pretreatment severity suggest that hydrothermal pretreatment can be roughly viewed as an artificial coalification process. An analogy has been observed for hydrothermal pretreatment of waste sludge6 and has been suggested previously as occurring for biomass processing in general by various researchers.28 Slurry Formation. The primary reason for hydrothermal pretreatment is to make the waste stream (paper in this case) more pumpable into the gasifier. Because gasifier performance improves with slurry heating value, the slurry should have the maximum organic content that can be pumped practically. As a reference, the organic content of typical pumpable slurries of Pittsburgh No. 8 bituminous coal is 62 wt %, while raw paper dunnage at only 5 wt % produces an unpumpable slurry. A shear stress/shear rate relationship for a hydrothermally pretreated paper dunnage derived slurry of 50 wt % organic content is shown in Figure 6. The rheogram shows Bingham plastic behavior with a yield stress of 17 Pa and a coefficient of rigidity of 180 mPa‚s for the increasing shear curve. The decreasing shear curve shows shear thinning due to the thixotropic characteristic of the slurry. An effective viscosity can be obtained from the rheogram from the slope of a straight line through the origin and the point on the rheogram at 100 s-1. The effective viscosity as a oneparameter model can then be correlated to slurry properties such as solids content. The maximum practical organic content is the content giving a viscosity of 1000 mPa‚s, the maximum practical value for a pumpable slurry. Figure 1d shows the correlation between the maximum practical organic content of the slurry and pretreatment severity. Organic contents were determined by plotting measured effective viscosity at a shear rate of 100 s-1 vs measured organic content of the slurries made for each pretreatment severity and extrapolating to 1000 mPa‚s. Pretreatment allows formation of pumpable slurry concentrations approaching those of coal slurries. The slurries appear to reach a maximum practical organic content of 55 wt % at the 275 °C/120 min pretreatment condition. Higher severities appear not to be more effective in making higher organic content slurries. (28) Elliott, D. C. ACS Symp. Ser. 1988, No. 376, 55-65.



Figure 7. Extrapolated apparent viscosity at 100 s-1 in mPa‚s as a function of Tmax (25°C/min) of extracted chars.

This is further demonstrated in Figure 7, which shows the extrapolated apparent viscosity as a function of the Tmax (25°C/min) of the extracted chars. Clearly, at lower pretreatment severities [as indicated by Tmax (25°C/ min)], as the Tmax increases due to increasing pretreatment severity, the viscosity of the slurry decreases rapidly. However, above the 430°C Tmax range, the viscosity remains roughly constant and ultimately may increase, indicating further pretreatment does not improve pumpability. This is analogous to pyrolytic Tmax of petroleum-generating source rocks where, after a certain time-temperature history, the rocks yield no more oil but continue to show increasing Tmax. Tmax can be used in both cases as a severity indicator that is independent of material balance constraints or a knowledge of the precise thermal history. The pretreatment effects are attributed to changes in chemical composition (different O contents, for example) and differences in the structure of the individual particles. These two factors produce different water retention characteristics of the pores. Bituminous coal produces a good slurry because the internal porosity is low and the physical composition includes few sites for water to adsorb (hydrophobic surface).6 Discussion The objective of this work is to understand how the pyrolysis properties of the residual char relate to various pretreatment steps in MSW utilization. From a simple viewpoint, the results in Figures 1 and 7 suggest that open system pyrolysis Tmax might be a simple and useful way of indirectly measuring process severity, estimating the distribution of products in various phases, and predicting potential slurry viscosity. For example, the very high Tmax of the char from the 310 °C/120 min pretreatment correlates with its distinctive kinetic parameters [E is in the range of higher maturity type III kerogens (coals)29 ] and FTIR spectrum (less carbonyl functionality in the char compared to those of the other samples, suggesting more condensed material). From a more sophisticated viewpoint, the kinetic properties of the residues give us clues about the mechanisms of pyrolysis and the validity of various kinetic schemes for process modeling. Two related questions are whether the parallel reaction model is the best one for modeling paper pyrolysis (hydrothermal, autogeneous, or dry) and whether easily measured open system pyrolysis kinetics are valid for (29) Reynolds, J. G.; Burnham, A. K. Energy Fuels 1993, 7, 610619.

Figure 8. Calculated effect of pretreatment on the pyrolysis profile of extracted chars. Part a uses the Pyromat kinetics as measured on the starting material. Part b accelerates by 100 times the reaction channels having an activation energy of e40 kcal/mol to mimic the effect of water. In both cases, the shift to higher temperatures upon pretreatment is due to selective concentration of the less reactive species in a parallel reaction model.

hydrothermal or autogeneous pyrolysis conditions. We argued in the preceding paper that the parallel reaction model is better than nth-order and diffusion models, but it does not have the key aspect of some lignocellulosic mechanisms that the char is formed by condensations reactions. The key question is, do the chemical moieties that evolve organics in the high-temperature shoulder of paper pyrolysis profiles exist in the original material or are they products of primary reactions? Further, is the same mechanism operative in dry and hydrothermal pyrolysis? Figure 2b is consistent with but not proof of a selective concentration mechanism operating on a system of independent parallel reactions. Increasing severity seems to react away the more labile components, selectively concentrating the refractory components. However, it is also consistent with a mechanism in which the amount and kinetic properties of char formed during primary pyrolysis are similar in hydrothermal and dry conditions. Two bits of information suggest that the mechanism is more complicated than a simple activation-energydistribution parallel reaction model. First, the frequency factor of the chars is substantially higher than that of the dunnage, and the principal activation energies are at least 10 kcal/mol higher than the corresponding, minor, high-energy components in initial paper distribution. Second, at equivalent time and temperature, the Tmax of the hydrothermal pyrolysis residue is significantly higher than that of a residue from autogenous pyrolysis. The best way to test the kinetic model is to compare the measured evolution profiles of the residues with those calculated from the kinetics of the starting material, i.e., partially react the kinetic network numerically according to the pretreatment and then calculate the evolution profile for the Pyromat heating rate. This is shown in Figure 8a. Although the kinetic model predicts the shift of the evolution profile to higher temperature upon pretreatment, the Pyromat kinetic parameters are far to slow. The model predicts that “complete” conversion to char is accomplished only at the most


severe conditions, while the experiment accomplishes the conversion at even the mildest conditions tested. If the reaction channels at 40 kcal and lower are accelerated by multiplying their A by 100, the model calculations agree well with experiments, as shown in Figure 8b. This suggests that hydrolysis dramatically increases the rate of cellulose and hemicellulose destruction, but the residual material is relatively immune to hydrolysis. This would be consistent with both the modest effect of water on oxygen-poor kerogens10 and the substantial effect of water on cellulosic materials.7 Our 100× rate constant is 10 times faster than Bobleter’s7 at 300 °C, but the difference is not too significant given the approximations involved in our autoclave thermal history and Bobleter’s statement that the frequency factor depends on the cellulose source. Now Figures 8b (calculated) and 2b (measured) are almost identical. The ultimate kinetic model for paper (and presumably other heterogeneous lignocellulosic materials) will require a modification of the parallel reaction model. Some modifications have been suggested: (1) allowing A to be a function of E,30 (2) allowing a shift of pyrolysis potential between reaction channels,31 (3) a reaction network in which pseudocomponents undergo serial and competing reactions, some having an activation energy distribution,32 (4) accounting for more complicated crosslinking and devolatilization reactions with a statistical treatment of degredation reactions,33 and (5) incorporating model compound reactions with a Monte Carlo approach.34 (30) Ungerer, P. In Thermal Phenomenon in Sedimentary Basins; Durand, B., Ed.; Technip: Paris, 1984; pp 235-246. (31) Bossata, E.; A˙ gren, G. Geochim. Cosmochim. Acta 1995, 59, 3833-3835. (32) Braun, R. L.; Burnham, A. K. Org. Geochem. 1992, 19, 161172. (33) Serio, M. A.; Charpenay, S.; Bassilakis, R.; Solomon, P. R. Biomass Bioenergy 1994, 7, 107-124. (34) Train, P. M.; Klein, M. T. Ind. Eng. Chem. Res. 1993, 32, 12971303.

Reynolds et al.

Conclusions • Evolution profiles, yields, and FTIR structural examination of extracted chars from hydrothermal pretreatment of the paper dunnage indicate that similar decomposition chemistry is occurring at all but the highest severity conditions. • Pyrolysate yields from the chars and CH2Cl2/ methanol extracted chars are in the range of 70-250 mg of organics/g of char. Pyrolysate yields from the chars decrease with increasing pretreatment severity, and the Tmax values of the extracted chars increase with increasing pretreatment severity. • The activation energies and frequency factors for the CH2Cl2/methanol extracted chars (except for one severity) were found to be in the range of 54-57 kcal/ mol and 1015-1016 s-1, respectively. Both are higher than those for the starting material. The char from the most severe pretreatment condition has much higher activation energies. All samples have very broad distributions about their principal energy values. • While a parallel reaction model accounts for many aspects of dry and hydrothermal pyrolysis, the rate of pyrolysis in hydrothermal conditions is perhaps 100 times faster than in a flowing inert gas. Consequently, kinetic expressions derived from conventional micropyrolysis (Pyromat, TGA, DTA, etc.) cannot be used directly for hydrothermal pyrolysis. Acknowledgment. We thank Ann M. Murray for experimental assistance, Jeffrey H. Richardson for partial support through the LLNL chemistry program office, and Neil Rossmeissl, program manager of the U.S. Department of Energy Hydrogen Program for partial support. 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. EF9600873

Reactivity of Paper Residues Produced by a Hydrothermal

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