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Ind. Eng. Chem. Res. 1992,31, 1162-1166
Formation of Charcoal from Biomass in a Sealed Reactor William Shu-Lai Mok and Michael J e r r y Antal, Jr.* Hawaii Natural Energy Institute and the Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii 96822
P i r o s k a Szabo, Gabor Varhegyi, and Borbala Zelei Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budapest 1502, Hungary
Samples of cellulose, hemicellulose, lignin, and nine species of whole biomass were pyrolyzed in sealed reactors. Very high charcoal yields (e.g., 40% from cellulose, 48% from Eucalyptus gurnrnifera) were obtained. Higher sample loading (sample mass per unit reactor volume) increased charcoal yield and the associated exothermic heat release and lowered the reaction onset temperature. These effects were induced by the vapor-phase concentrations of the volatile products, and not the system pressure. Addition of water catalyzed the reaction and increased the char yield. These observations suggest that charcoal formation is autocatalyzed by water, an initial pyrolysis product. When whole biomass was used as a feedstock, higher charcoal yields were obtained from species with high lignin and/or low hemicellulose content.
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Introduction According to the stoichiometric equation C6HI0O5 6C + 5H,O, the theoretical yield of carbon which can be realized from cellulose is 44% on a mass fraction basis. Because biomass charcoal contains some hydrogen and oxygen, and because biomass contains lignin (which is rich in carbon relative to cellulose) and other components, it is not possible to calculate an exact value for the theoretical yield of charcoal from biomass. Considering representative ranges of biomass and charcoal compositions, we estimate the theoretical yield of a high-quality charcoal from biomass to be between 44 and 55% (Antal et al., 1991). Prior to 1991, the highest yield of charcoal reported in the literature was 38% (Antal et al., 1990). In 1991 this laboratory reported yields of a very high quality charcoal from Eucalyptus and Kiawe wood ranging from 43 to 47% (Antal et al., 1991). These yields were obtained in a pilot plant having a throughput of about 15 kg/h of moist wood logs. The pilot plant achieves near-theoretical yields by heating the wood logs to a temperature of about 400 "C over a 90-150-min period. Pyrolysis in the partially sealed reactor occurs at a self-generated elevated pressure (Antal et al., 1991). Although earlier workers (Fitzer et al., 1971; Mok and Antal, 1983; Capart et al., 1988,Antal et al., 1990) have explored the effects of elevated pressure on charcoal formation from biomass, no rationale for the dramatic improvement in yield realized by the pilot plant is known. The goal of this work was to detail the effects of sample loading (or apparent mass density within the reactor, which indirectly controls the vapor-phase concentrations of the volatile products and total reactor pressure) and moisture on the charcoal yield and heat of reaction in a sealed vessel. A variety of biomass substrates was surveyed to determine if certain species are particularly well suited for charcoal manufacture. Finally, substrates at different stages of carbonizationwere analyzed by Fourier transform infrared (FTIR) spectroscopy and compared with literature data to determine if the chemistry occurring in a closed retort is substantially different from that taking place in conventional charcoal kilns. Because it is not difficult to obtain a high yield of a low-quality charcoal produced by incomplete pyrolysis, the definition of "high-quality charcoal" underpins this work. Charcoal obtained in high yield from our pilot plant has a typical calorific value ranging from 30 OOO to 33OOO kJ/kg (13OOO to 14OOO Btu/lb), a volatile matter content between 21 and 23%, a fixed carbon content of 75%, and an ash
content of 1-3% (Antal et al., 1991). According to most standards, a charcoal with these characteristics is judged to be very high quality and is well suited for cooking and heating applications (Antal et al., 1990, 1991). The experimental conditions emphasized in this work were chosen to be somewhat more severe than those realized in the pilot plant, which should ensure complete pyrolysis, thereby resulting in somewhat lower yields of high-quality charcoal with very low volatile matter content and very high fixed carbon content. Thus the peak temperature employed in this work was 450-500 "C, whereas temperatures in the pilot plant typically do not exceed 400 "C. Similarly, the pilot plant operates at a much lower pressure than that realized in the sealed reactors used in this work. Unfortunately, the charcoal samples produced in the experiments reported here were so small that a proximate analysis of their compositions could not be conducted. Experimental Apparatus and Procedures Two classes of experiments were conducted. In the first, a Setaram (France) differential scanning calorimeter (DSC) was used to measure the heat of pyrolysis within sealed crucibles. The stainless steel crucibles had an internal volume of 0.15 mL and could withstand 100 bar at 500 "C. Accurate measurement of char yield was not feasible in these experiments because of the permanent crucible seal. A linear heating rate of 5 "C/min to 450 "C (Avicel cellulose) and 500 "C (all other substrates) was used. These temperature limits were chosen on the basis of the pyrolytic heat demand as measured by the DSC, which became negligible at these temperatures, thereby indicating that pyrolysis was complete. The DSC was calibrated by a Joule effect cell from 60 to 600 "C and by measurement of the heat of fusion of lead. The response curve, which specified the calibration coefficient (mW/V) as a function of temperature, was generated by fitting a sixth-degree polynomial through all the calibration points. Raw digitized DSC data were treated as follows. The heat flow data (dq/dt) was converted to milliwatt units by applying the appropriate temperature-dependent calibration coefficient. A reference (empty crucible) data set of heat flow vs temperature was subtracted from the experimental curve. Boundary points were determined by averaging the dq/dt data within the "beginning" and "ending" i n t e ~ a l of s the difference curve. These inte~als, during which presumably no chemical reaction occurred, where identified visually, and typically ranged between
0888-5885/92/2631-ll62$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 1163 5
Table I. Summative Analysis of Nine Biomass Species Studied in This Work, Together with the Charcoal Yield (All Data in % on Dry Weight Basis) hemicharcoal cellulose cellulose lignin samde yield Eucalyptus gummifera 16 38 37 47.5 Eucalyptus saligna 15 45.2 45 25 sugar cane bagasse 17 17 44.4 36 sweet sorghum 18 41.4 36 16 Luecaena hybrid KX-3 17 41.2 43 25 23 sweet gum 19 40.6 40 23 silver maple 22 40.3 40 21 Populus deltoides 38.7 39 26 18 energy cane 37 15 38.0
80-120 and 470-500 "C (for cellulose,the "ending" interval was lower, between 400 and 450 "C). A linear baseline was then constructed by interpolating between the boundary points of the difference curve. After baseline correction, key characteristics, including extrapolated onset temperature, peak temperature, half-width (width at half-height), peak symmetry, and peak area (heat of reaction), were obtained numerically. For the second class of experiments, larger reusable reactors were employed to permit removal of charcoal. The reactor was a Hastelloy C-276 tube capped at both ends, with an internal volume of 1.3 mL. Small-diameter (0.8 mm) tubing connected the reactor, placed inside a furnace, to a pressure transducer outside. Silicone oil was used to fill the void volume inside the transducer and the watercooled length of the connecting tubing outside the furnace. In this way, dead volume with uncertain temperature, where condensation could occur, was minimized to less than 0.03 mL. In any case, control experiments with and without the pressure transducer attachment evidenced no change (within experimental precision) in the char yield from cellulose. The furnace temperature was controlled by an isothermal regulator. The system required approximately 20 min to reach 450 "C; thereafter each experiment was held at 450 "C for 10 more minutes. Pressures between 3 and 14 MPa were realized in this reactor at 450 "C. After 10 min at 450 "C the pressure in the vessel largely ceased rising, indicating that pyrolysis was complete. To compare the closed reactor results with conventional char formation in an open vessel, thermogravimetric studies were cmied out on a Perkin-Elmer TGS-2/System 4 thermobalance. For these experiments, the (-2 mg) samples were housed in an open platinum pan purged continuously with high-purity, dry argon (140 mL/min) and heated linearly at 5 "C/min. FTIR spectra of the charcoal product from the reusable reactor were registered on a Perkin-Elmer Model 1710 instrument using a DTGS detector, a Perkin-Elmer DRIFT accessory, and microsampling. Cellulose chars were ground and measured in powder form (undiluted by KBr). All spectra were analyzed from 4000 to 450 cm-' with 50 scans at 2-cm-' resolution. Microcrystalline (Avicel PH105) cellulose was used for a parametric study of the effect of mass loading and water addition on charcoal formation. Depending on the batch and the location (Honolulu or Budapest), the cellulose samples contained varying amounts of moisture (from 1 to 7%). All values of cellulose weights and charcoal yields are given on an oven-dry basis. Xylan from larchwood (Sigma Chemical Company) and Indulin ATR-CK1 (Westvaco Chemical Division) were used as hemicellulose and lignin substrates. Nine plant species, selected by pyrolysis-mass-spometric studies (Agblevor et al., 1989) to be characteristic of a wide spectrum of biomass material,
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Figure 1. DSC traces showing the effects of mass loading on cellulose and xylan pyrolysis in closed crucibles and a DTG plot (dotted line) of a cellulose pyrolysis experiment in an open crucible. BOO
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Figure 2. Effects of mass loading on the heat of reaction (A)and char yield ( 0 )of cellulose pyrolysis in closed crucibles. The range of slopes indicates a 90% confidence interval. Also shown in the insert are the effects of mass loading on the peak temperature ( 0 ) and half-width (A)of the dq/dt curves of the corresponding experiments.
were examined as feedstocks for charcoal manufacture. Summative analyses of the whole biomass species, as determined by quantitative saccharification (Mok and And, 1992) are given in Table I.
Results and Discussion Effect of Mass Loading. Attention was directed first to the effect of loading (sample mass contained in the DSC crucible),which determines the vapor-phase concentrations of the volatile products and the total system pressure. In Figure 1, the mass-normalized DSC traces from a series of experiments with varying sample sizes are displayed. In all cases, the main reaction (peak) is exothermic. As loading (mg of cellulose per mL of crucible volume) increases, peak temperature and peak width (at half-height) decrease, while the exothermic heat of reaction (peak area) increases. These changes, shown in Figure 2, indicate that as loading increases, the reaction occurs more rapidly at a lower temperature with a higher heat release. The decreases in peak temperature and width are particularly dramatic when compared with the reaction at atmospheric pressure with rapid purging of volatiles, as shown by the mass loss rate curve (dotted line in Figure 1).
When working with thermoanalytic instruments, it is important to verify that the observed shifts in peak temperature, width, and area are in fact due to chemical
1164 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 -
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Figure 3. DSC traces contrasting the effects of mass loading vs the effects of absolute sample mass on cellulose pyrolysis in closed crucibles, and the effect of increased system pressure by initial charging of the crucible with COP
changes. Otherwise, it could be argued that the effect wa9" merely an artifact of changes in the local sample temperature and/or heat transfer, induced by autoheating as the absolute amount of heat release increases. To ascertain that mass loading (mass per unit reactor volume rather than absolute sample mass) plays the key role, an experiment was conducted using a crucible with reduced volume (the volume was halved by the inclusion of a stainless steel insert). The result, shown as the dashed line in Figure 3, more closely resembles a previous experiment with equal mass loading (on its left) rather than an earlier run with equal absolute mass (on its right). This clearly demonstrates that mass loading exerta the dominant effect. The shift between the two runs with equal mass loading can be explained by the heat capacity of the stainless steel insert. The insert caused a larger thermal lag between the true sample and measured temperatures, thereby delaying the appearance of the corresponding peak. Char yield (from the reusable reactor) as a function of mass loading is also plotted in Figure 2. Because a small amount of char occasionally adhered to the wall of the reactor, the char measurements were somewhat scattered. Nevertheless a slight trend of increasing char yield with loading can be seen. Statistically (Freund, 1972), the probability that a positive correlation exists between loading and yield is better than 93%. The slope of the regression line remains positive within a 90% confidence interval. As indicated, the char yield from the closed reactor experiments ranged from 36 to 40%. In comparison, the char yield from an experiment which left the crucible open to argon at 1-atm pressure was 22%. When measured by thermogravimetric analysis (TGA) with a high flow of Ar purge gas, the yield was only 6%. This confiims that char formation can be increased remarkably by pyrolysis within a sealed reactor. Since the loading factor from the DSC experiments did not exactly match those employed for measurements of char yields with the closed reactor, the heat of reaction vs yield data were correlated by their respective regression lines (see Figure 2). The results, shown as a bold line in Figure 4, follow the same trend reported almost a decade ago using a semibatch, flow reactor (Mok and Antal, 1983). It is noteworthy that when the exotherm from the main reaction exceeds 500 J/g, enough energy is released to heat the sample from 25 to 400 OC (assuming a heat capacity of 1.38 J/(g°C)). Earlier it was noted that increased loading raises both the total system pressure and the vapor-phase concen-
Figure 4. Comparison of the heat of reaction of cellulose pyrolysis vs char yield data obtained from the present study and those reported by Mok and Antal (1983). 4
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Figure 5. DSC traces showing the effect of moisture (by water addition) on cellulose pyrolysis with a loading of -67 mg/mL. Also shown in the insert is the effect of moisture on the char yield from cellulose pyrolysis with similar loading. The range of slopes indicates a 90% confidence interval.
trations of the volatile pyrolysis products. To distinguish the effects of these two factors, experiments were conducted with the addition of dry ice into the crucible or reactor with the substrate to increase the system pressure, but not vapor-phase concentrations of the volatile produds (other than COP). In Figure 3, the dq/dt data of a COz-addedexperiment (dotted line) are shown. Compared with a previous run without COz at a very similar mass loading, the peak from the COz-added experiment was slightly taller and narrower, but the heat of reaction was actually reduced slightly even though the total pressure was increased. Addition of COz also reduced the charcoal yield by about 10%. It can therefore be concluded that vapor-phase concentration of the volatile product, rather than the total system pressure, exerta the dominant effect on increasing char yield and heat of reaction. Effect of Moisture Content. Results from our pilot plant (Antal et al., 1991) indicate that moisture content affects both the quantity and quality of charcoal formed from biomass. Water is a major product of the early stages of cellulose pyrolysis. These observations naturally led to the speculation that water is an autocatalytic agent in charcoal formation. To test this hypothesis, experiments were conducted to explore the effects of additional moisture. Figure 5 displays the results of a series of DSC experiments with the same amount of cellulose (about 10 mg) but varying amounts of added water. Water appeared to
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1165
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Figure 6. DSC traces of charcoal formation from woody species in closed crucibles.
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Figure 7. DSC traces of charcoal formation from herbaceous species in closed crucibles.
catalyze char formation by shifting the reaction peak toward lower temperatures. While the exotherm might be expected to increase if water catalyzes charcoal formation, the endothermic heat requirement also increases because of the additional heat capacity of water. The data given in Figure 5 indicate a systematic lowering of peak height with the addition of water. There was no discernable trend relating moisture and heat of reaction. The peak area measurements were further complicated by the long tail observed with high-moisture experiments, which introduced a high degree of uncertainty into the placement of the baseline. Similar effeds of decreasing peak height and temperature by water addition were observed in another pair of DSC runs with 15-mg samples. Char yields were also increased by the addition of moisture, as shown in Figure 5. Despite the scatter, the probability that a positive correlation exists between the two is better than 97%. The slope of the regression line remains positive with a 90% confidence interval. Effect of Biomass Substrate. The effects of mass loading and additional moisture on charcoal formation from hemicellulose are shown in Figure 1. The trends were similar to those observed with cellulose. Lignin, on the other hand, gave flat featurelesa DSC curves which are not displayed. Peaks and valleys, if present, were indistinguishable from baseline shifts. These observations suggest that the lignin charring reaction occurs slowly over a wide span of temperature. DSC traces of six woody and three herbaceous biomass samples are shown in Figures 6 and 7 (respectively). All the samples exhibited two major exothermic peaks. Their heights, relative to each other, varied systematically according to the relative amounts of hemicellulose and cellulose present in a species. This correlation permitted the
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Figure 8. DRIFT spectra of residues at various stages of cellulose carbonization in a sealed reactor (weight lost a t A, 5 % ; B, 43%; C, 58%; D, 64%).
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unequivocal assignment of the first peak to hemicellulose and the second to cellulose. As expected, no distinctive feature was found which would correspond with the pyrolysis of lignin. Besides the peak-height differences discussed earlier, the species within the woody and herbaceous categories had similar dq/dt behaviors. The exotherms all peaked within narrow ranges of temperature (for woody species, hemicellulose peak at 273-285 "C, cellulose peak at 312-332 "C; for herbaceous species, 244-246 and 291-299 OC, respectively). As the curves show, there was little variation among the combined peak widths. From an engineering perspective, these data indicate that a charcoal manufacturing reactor can be used to process a wide variety of feedstocks without any need to change the thermal parameters (e.g., input power, cutoff temperature, etc.). Table I also lists char yields from each of the species examined, showing a range from 38 to 48%. On the basis of our earlier survey (Antal et al., 1990),these yields are the highest reported in the literature. There is a positive correlation (with 99% probability) between char yield and lignin content and a negative correlation (with 96% probability) between char yield and the hemicellulose content. However, since the lignin and hemicellulose contents are themselves correlated with each other, the data do not reveal which of the two components exerta the dominant effect. No correlation between char yield and cellulose content was observed. Analysis of Charcoal Formation by FTIR. In order to follow the main chemical changes occurring in the substrate during pyrolysis in a sealed vessel, independent cellulose charring experiments were carried out and interrupted at different stages of carbonization. Diffusereflectance FTIR spectra of the solid product are shown in Figure 8. The main chemical changes observed in the spectra are dehydration (3700-2400 cm-l), carbonyl group formation and subsequent elimination (1710cm-l), pyranose ring opening (889cm-l), and decomposition of aliphatic (2950-2900cm-l) and formation of aromatic (1600 and 900-700 cm-l) char units. All these observations are in good agreement with literature data (Fitzer et al., 1971; Sekiguchi and SWizadeh, 1984) available on atmospheric cellulose pyrolysis in an inert environment. Spectra of cellulose charcoal samples taken from open and sealed reactors were also compared. These spectra were practically identical, indicating that the main chemical constituents of the two charcoals are effectively the same. The vacuum pyrolysis residue of Populus deltoides (Pakdel et al., 1989) gave a surprisingly similar spectrum, although its carbonyl content appeared to be a little higher.
1166 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992
Conclusions Very high yields of charcoal (40% from cellulose, 48% from Eucalyptus gummifera) can be obtained in sealed reactors. In the absence of oxygen, the pyrolytic charcoal formation reaction in a sealed vessel is exothermic. Under certain conditions, the reaction releases enough heat to raise the solid substrate from room temperature to reaction temperature. As loading (sample mass per unit reactor volume) increases, the char yield and exothermic heat of reaction increases, and the reaction onset temperature decreases. These effects are induced by the high concentration of vapor products in the reactor and not the total pressure within the reactor. Addition of water also increases the charcoal yield and lowers the reaction onset temperature, but has no significant effect on the overall heat of reaction. It appears that charcoal formation is autocatalyzed by product water. Higher yields of charcoal can be obtained when species with high lignin and/or low hemicellulose content are used as the feedstock. However, the same reaction temperature-time profile can be employed to obtain high charcoal yields from any biomass feedstock: no modification in operational procedures is necessitated by changing feedstocks. Charcoals prepared in a sealed reactor are chemically the same as those prepared in conventional reactors. The charcoal composition appears to be insensitive to the feedstock identity. Acknowledgment
This joint research program was funded by the National Science Foundation (Grant INT-8914934),the Hungarian OTKA Foundation, and the Coral Industries Endowment. We thank Mr. Ferencs Till (HAS)for assistance and advice and Bonnie Thompson (NSF) and Dr. Maria Burka (NSF) for their interest in this work. We also thank Dr. M. P.
Dudukovic (Associate Editor, Industrial & Engineering Chemistry Research) and three anonymous reviewers for their constructive comments on this paper. Registry No. COz, 124-38-9; H20, 7732-18-5; Avicel PH105, 9004-34-6; xylan, 9014-63-5; Indulin ATR-CK1,8068-05-1; hemicellulose, 9034-32-6; lignin, 9005-53-2.
Literature Cited Agblevor, F. A.; Evans, R. J.; Milne, T. A.; And, M. J. Multivariate Data Analysis of Biomass Feedstocks Using Pyrolysis Mass Spectrometry. Presented at the International Chemical Congress of Pacific Basin Societies, December 1989. Antal, M. J.; Mok, W. S.; Varhegyi, G.; Szekely, T., Review of Methods for Improving the Yield of Charcoal from Biomass. Energy Fuels 1990,4, 221-225. Antal, M. J.; DeAlmeida, C.; Mok, W. S.; Sinha, S. A New Technology for Manufacturing Charcoal from Biomass. Proceedings of the IGT Conference on Energy from Biomass and Wastes XV, Washington, DC; Institute of Gas Technology: Chicago, 1991. Capart, R.; Falk, L.; Gelus, M. Pyrolysis of wood macrocylinders under pressure: application of a simple mathematical model. Appl. Energy, 1988, 30 (l),1-13. Fitzer, E.; Mueller, K.; Schaefer, W. The Chemistry of the Pyrolytic Conversion of Organic Compounds to Carbon. In Chemistry and Physics of Carbon;Walker, P. L., Ed.; Marcel Dekker: New York, 1971; Vol. 7. Freund, J. E. Mathematical Statistics, 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1972, p 382. Mok, W. S.; Antal, M. J. Effects of Pressure on Biomass Pyrolysis. 11. Heats of Reactions of Cellulose Pyrolysis. Thermochim. Acta 1983,68, 165-186. Mok, W. S.-L.; Antal, M. J., Jr. Uncatalyzed Solvolysis of Whole Biomass Hemicellulose by Hot Compressed Liquid Water. Ind. Eng. Chem. Res. 1992, preceding paper in this issue. Pakdel, H.; Grandmaison, J. L.; Roy, C. Analysis of Wood Vacuum Pyrolysis Solid Residue by Diffuse Reflectance Infrared Fourier Transform Spectrometry. Can. J. Chem. 1989,67, 310-314. Sekiguchi, Y.; Shafizadeh, F. The Effect of Inorganic Additives on the Formation, Composition and Combustion of Cellulose Char. J. Appl. Polym. Sci. 1984,29, 1267-1286.
Received for reuiew July 15, 1991 Revised manuscript received November 27, 1991 Accepted December 31, 1991
