Energy & Fuels 1988,2, 273-217
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Simultaneous Thermogravimetric-Mass Spectrometric Studies of the Thermal Decomposition of Biopolymers. 2. Sugar Cane Bagasse in the Presence and Absence of Cata1ysts Gabor Varhegyit and Michael J. Antal, Jr.* Department of Mechanical Engineering and the Hawaii Natural Energy Institute, University of Hawaii, Honolulu, Hawaii 96822
Tamas Szekely, Ferenc Till, Emma Jakab, and Piroska Szabo Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budapest 1502, Hungary Received September 28, 1987. Revised Manuscript Received December 23, 1987 I
Differential thermogravimetric (DTG) curves are compared with simultaneously measured mass spectrometric intensities, giving the rates of evolution of low molecular weight products formed during the pyrolysis of sugar cane bagasse a t a low heating rate (10 "C/min). The three major DTG peaks observed during the experiments resulted from the decomposition of the carbohydrate components of the bagasse, supporting the concept of the approximate additivity of the thermochemical properties of the biopolymer constituents of lignocellulosic materials. A thermal pretreatment of 2 h at 260 OC destroyed the hemicellulose component of the bagasse but did not enhance the char yield. Treatment of the samples with dilute solutions of inorganic salts (MgCl,, NaC1, FeS04, and ZnCl,) decreased the amount of detected low molecular weight organic products. Taking into account the time dependence of the mass spectrometric intensities and the DTG curves, this decrease can be attributed to a decrease in secondary cracking of the primary pyrolysis products. We suppose that the addition of inorganic salts facilitates the escape of vapors from the solid matrix by swelling the fibrous structure of the bagasse. NaC1, FeS04, and ZnClz have different effects on the various steps of the decomposition. They all increase the char yield, but only ZnCl, catalyzes the dehydration reactions.
Introduction Sugar cane bagasse is a typical example of an agricultural byproduct that is available in abundant quantities. In Hawaii alone over 1million tons of bagasse is produced annually. Its composition is similar to that of other lignocellulosic materials: it contains, on a dry basis, approximately 40% cellulose, 32% hemicelluloses, 20% lignin, 6% extractives, and 2% ash.' At present bagasse is burned on site in the sugar mills to generate power and heat. However, there is the possibility of producing more valuable products from bagasse: charcoals suitable for household, chemical, and metallurgical processes, light gases, methanol, syrups, adhesives, and others. At the present time the production of these higher value products has been restricted by the lack of specificity of the pyrolytic processes that form them. The goal of this work was to identify catalysts that favorably influenced the yields of these high-value products, especially charcoal. Due to the historic role of wood and other lignocellulosic materials in the development of civilization, a large body of literature exists concerning their thermochemical properties, and several recent reviews are The influence of inorganic salts on the pyrolysis of biopolymers has also been extensively studied."I Recently DeGroot and Shafizadehl investigated the influence of the exchangeable cations on the carbonization of cottonwood and Douglas fir. They compared the DTG curves of untreated, acid-washed,and calcium- and potassium-exchangedsam*Towhom all correspondence should be addressed. 'Visiting scientist from the Hungarian Academy of Sciences.
ples. They also carefully analyzed the resulting chars. They observed different effects of the same additives on woods and on pure cellulose. The primary sites for ion exchange were supposed to be glucuronic acids in the hemicellulose component. In this paper we study the moderate-temperature decomposition of bagasse by measuring the evolution of low molecular weight products as a function of time during low heating rate thermogravimetric experiments. The tar fraction was not detected by the mass spectrometric apparatus. Our choice of Catalysts and heating programs follows the considerations outlined in part 1of this series? A recent paper by Milne and EvansQdescribes complementary mass spectrometric studies of biopolymer pyrolysis at high heating rates. (1) Lipinsky, E. S.In Hydrolysis of Cellulose: Mechanism of Enzymatic and Acidic Catalysis; Advances in Chemistry 181; American Chemical Society: Washington, DC, 1979, pp 1-23. (2) Shafiiadeh, F. J.Anal. Appl. Pyrolysis 1982, 3, 283-305. (3) Milne, T.A. In A Survey of Biomass Gasification; Reed, T. B., Ed.; Noyee Data: Park Ridge, NJ, 1981; Vol. 2, Chapter 5. (4)Soltes, E. J.; Elder, T. J. In Organic Chemicals from Biomass; Goldstein, I. S., Ed.; CRC Boca Raton, FL; 1981; pp 64-99. (5) h t a l , M. J., Jr. In Advances in Solar Energy; Boer, K. W., Duffie, J. A., Ede.; American Solar Energy Society: Newark, DE,1982; Vol. 1, pp 61-111. (6) Ana, M.J., Jr. In Aduances in Solar Energy; Boer, K. W., Duffie, J. A., Eds.; Plenum: New York, 1985; Vol. 2, pp 175-255. (7) DeGroot, W. F.; Shafizadeh, F. J. Anal. Appl. Pyrolysis 1984, 6, 217-232. (8) Part 1: Varhegyi, G.; htal, J. M., Jr.; Szekely, T.; Till, F.; Jakab, E. Energy Fuels preceding paper in this issue. (9) Evans, R. J.; Milne, T. A. Energy Fuels 1987,1, 123-137.
0 1988 American Chemical Society
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274 Energy & Fuels, Vol. 2, No. 3, 1988
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Figure 1. DTG curves of untreated (dark solid line) and pre-
heated (light solid line) sugar cane bagasse, pure Avicel cellulose ( 0 )and a hemicellulose (4-methyl-D-glumrono-D-xylan (A). The scale factors of the cellulose and hemicellulose curves were arbitrarily chosen.
Experimental Section The instrumentationand the experimental procedures used in this work were described in part 1.4 The amount of each catalyst added to the sample was identical with the Avicel experiments of part 1: where one cation was added per 100 monomer unita of the cellulose. Thus a sample of 1.62 g of bagasse received lo-' mol of catalyst dissolved in 20 mL of distilled water. The resulting pastes were thoroughly stirred, dried at room temperature in a desiccator, and used for experiments the next day. Although natural plant materials contain many types of monomer units (anhydroglucose,D-xylose,D-mannose,D-glucose,phenylpropane, and other unita), the magnitude of the molecular weight of these unita is similar. Hence in a very rough approximation,the catalyst concentration correaponded to about one cation per 100 monomer units here too. The bagawe was obtained in pelletized form from the Hamauka Sugar Co., Honolulu, HI. Prior to the experiments, the bagasse pellets were ground in a ball mill and dried in a desiccator at room temperature. A related hemicellulose model compound, 4methyl-D-glucurono-D-xylan, was obtained from the Institute of Chemistry of the Slovakian Academy of Sciences. This xylan was prepared from beech sawdust.10 Its thermal decomposition will be described in detail in a subsequent paper." Here we employ its DTG curve for comparison. As in part 1of this series: the mass spectrometric intensities are scaled to the size of the DTG curves to observe differences between the shape and temperature domains of the curves. In spite of its obvious heterogeneity, replicate experiments revealed both the TG amd MS curves obtained from the bagasse to be surprisingly reproducible. For example, the full scale intensity of the major ions varied by only a few percent. Results a n d Discussion Thermal Decomposition of Pure Bagasse. As displayed in Figure 1,pyrolysis of bagasse at 10 "C/min resulted in three DTG peaks: a small one at 240 "C and two major peaks at 310 and 370 "C. Shafizadeh and McGinnid2 hypothesized that the thermal decomposition of a lignocellulosic material may represent a superposition of the decomposition behavior of its individual components. The lignin component undergoes a slow charring process while the carbohydrates break down more rapidly to provide volatile products. As Figure 1shows, our experiments support this vie^.'^'^ The DTG curve of xylan ~~
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(10) Ebringerova,A.; Kramar, A.; Domansky, R.;Holzforschong 1969, 23, 89-92 --- - -. (11) Simkovic, I.; et al. J. Appl. PoZym. Sci., in press. (12) Shafiiadeh, F.; McGinnis, G. D. Carbohydr. Res. 1971, 16, 27.1-277. - .- - . .. (13) Shafizadeh, F. J. Anal. Appl. Pyrolysis 1982,3, 283-305. (14) Windig, W.; M e ~ l e a rH. , L. C. A d . C h m . 1984,56,2297-2303.
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Figure 2. Comparison of the DTG curve (dark solid line) and
the four highest intensity curves [mlz 18 (water;light solid line), m/z 28 (carbon monoxide; o),m/z 29 (variousaldehydes;O),and m/z 44 (carbon dioxide; A)] of untreated bagasse. The intensity curves were scaled to equal height for comparison.
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Figure 3. Comparison of the DTG curve (dark solid line) and the intensity curves of m/z 15 (compounds containing methyl group(s); light solid line), m / z 30 (formaldehyde; o ) , m/z 31 (methylalcohol and glycoaldehyde;o),and m/z 43 (acetaldehyde
and pyruvaldehyde; A) of untreated bagasse.
evidenced a double peak near the temperature domain of the first and second bagasse peaks; hence, these peaks are probably due to the hemicellulosic components of the bagasse. Although it could be argued that one of these peaks results from lignin pyrolysis, the mass spectra of these peaks contained no significant ions with mlz values characteristic of lignin pyr01ysis.l~ Instead, only ions shared in common with cellulose were observed. The DTG peak of the Avicel cellulose enjoyed a surprising similarity in shape and width to the third DTG peak of the bagasse. The small differences in the peak temperatures may be due to the mineral (ash) content of the bagasse as well as the obvious differences in the physical structure of the samples. The tailing part of the Bagasse DTG curve (above 400 "C) reflects the slower charring reaction over a broad temperature region of the lignin component. The mass spectra of the bagasse's three carbohydrate peaks were similar, indicating that the same products evolve in the thermal decomposition of the hemicellulose and cellulose components. The eight highest MS intensity curves, plotted with an enlargement to full scale, are shown in Figures 2 and 3. The H 2 0 evolution closely fits the DTG curve until about 400 "C. The other MS intensities shown in Figures 2 and 3 evidence some alterations from the DTG curve. Nevertheless, the ratios of these mass spectrometric intensities and the corresponding DTG values do not change by more than a factor of 2 in the temperature range of the carbohydrate decomposition. (15)Evans, R. J.; Milne, T. A.; Soltys,M. N. J.Anal. ProZysis 1986, 9, 207-236.
Energy & Fuels, Vol. 2, No. 3, 1988 275
Thermal Decomposition of Biopolymers
Table I. Assignment and Intensity Maxima of the Mass Spectrometric Ions" max intens (ratio to the DTG peak height) bagasse with catalyst bagasse assignt std preheated MgC1, NaCl FeS04 ZnClz CH,+ 2.6 (15.0) 3.0 (19.0) 1.4 (10.0) 1.6 (13.0) 1.2 (11.0) 1.1 (11.01 water 61.0 (350.0) 69.0 (427.0) 38.0 (267.0) 38.0 (307.0) 26.0 (238.0) 27.0 (252.0) 0.9 (4.9) 0.7 (4.4) 0.2 (1.5) 0.1 (0.8) - (-) - (-) C2H2+ 1.2 (7.1) 1.1 (6.7) 0.5 (4.3) 0.3 (2.1) 0.1 (1.0) - (-) C2%+ carbon monoxide 30.0 (170.0) 32.0 (195.0) 15.0 (105.0) 20.0 (163.0) 9.6 (88.0) 10.0 (98.0) 2.1 (20.0) aldehydes 7.9 (45.0) 6.8 (42.0) 3.9 (31.0) 3.3 (23.0) 2.0 (19.0) formaldehyde 2.4 (14.0) 2.0 (12.0) 1.3 (10.9) 1.3 (8.8) 0.9 (8.0) 0.8 (7.9) methyl alcohol, glycoaldehyde 2.0 (12.0) 2.4 (15.0) 0.9 (6.4) 0.5 (5.1) 0.6 (5.2) 1.0 (7.8) 2.4 (13.0) 2.4 (15.0) 0.7 (4.8) acetaldehyde, pyruvaldehyde 0.4 (3.7) 0.4 (3.6) - (-1 17.0 (99.0) carbon dioxide 10.0 (83.0) 5.6 (51.0) 18.0 (112.0) 9.0 (61.0) 7.5 (70.0) chloro compounds 0.1 (0.6) 0.5 (4.6) - (-) - (-) - (-1 - (-) 0.5 (2.7) 0.3 (1.8) 0.1 (0.6) CH2CHCO' - (-) - (-) - (-) acetone 0.5 (2.8) 0.4 (2.2) 0.1 (0.7) 0.3 (2.1) 0.7 (0.7) - (-) 0.1 (0.7) 0.3 (1.3) 0.04 (0.3) - (-) - (-1 - (-1 C4H402+ 2-furaldehyde 0.1 (0.6) - (-1 - 6) - (-1 - (3 - (-1 ~
m/z 15 18 26 27 28 29 30 31 43 44 50 55 58 84
96
DTG max, %/s char a t 450 OC,b wt %
0.18 21
0.16 20
0.14 22
o.1e 29
0.11 26
0.11 29
increased ZnClz 1.3 (18.0) 32.0 (450.0) 0.1 (1.1) 0.1 (0.9) 6.0 (85.0) 1.3 (18.0) 0.6 (7.8) 0.4 (5.5) 0.3 (3.5) 5.7 (80.0) 0.6 (8.5)
- (-)
0.3 (0.5)
- (3
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"The compound names in column 2 refer to the probable source of the ions. The "+" charge sign indicates that the corresponding formula represents a mass spectrometric fragment ion. The intensities are normalized with the initial sample mass and are given in nA/mg of starting material. Listed in parentheses are the ratios of the highest values of the MS intensity curves to the DTG peak heights (wt %/s). The last two rows contain the DTG peak maxima and the char yields. *Includes residual weight of catalyst.
(Only two of the smaller observed intensities, m / e 58 and m / e 84, showed greater variations: they contributed about three times more to the weight loss at 370 "C than at 310 "C.) Altogether, this behavior suggests a considerable similarity in the thermal decomposition of the hemicellulose and cellulose components. On the basis of a calibration using calcium oxalate monohydrate*the amount of HzO, CO, and COzevolved from the bagasse was estimated to be about 22,13, and 10% by weight. No calibration could be made for the many minor components detected by the MS and mentioned in the literature, but a very rough estimate of their overall magnitude can be made by summing their mass spectrometric intensities and observing that this sum lies between the intensities of CO and COD The amount of the carbonaceous residue (char) was 21%;hence, we estimate the nondetected tar fraction to be about 20%. Effect of a Thermal Pretreatment. An isothermal pretreatment of 2 h a t 260 "C was followed by the usual 10 "C/min temperature ramp. As shown in Figure 1, the first and second DTG peaks practically disappeared, indicating that the majority of the hemicellulose components decomposed during the pretreatment and the lignin may have been modified. The remaining DTG peak is dominated by cellulose decomposition superimposedon the slow charring of the lignin and the hemicellulose residue. The weight loss during the pretreatment was 26%. This value is close to the amount of hemicellulose in the bagasse (32%). If we assume a 20% char yield from the hemicellulose, the weight loss during pretreatment is exactly accounted for by the pyrolysis of the hemicellulose. As shown in Figure 4, the mass spectrometric intensity curves were in good correspondence with the DTG curve, indicating the same reaction chemistry during the entire thermal decomposition of the cellulose component. The numerical values of the corresponding intensity maxima, listed in Table I are close to those of the untreated sample, showing that the thermal pretreatment did not lead to significant changes in the decomposition of the cellulose component of the bagasse. For a comparison with the decomposition of Avicel cellulose,8 the data of Table I can be used. This comparison shows that the decomposition of the cellulose
Figure 4. Comparison of the DTG curve (dark solid line) and the four highest intensity curves [m/z 18 (light solid line), m / z 28 (n),m/z 29 (O),and m / z 44 (A)]of bagasse after a pretreatment of 2 h a t 260 "C.
Figure 5. Comparison of the DTG curves of the untreated (dark solid line), MgC12-treated (light solid line), and NaC1-treated (0) samples.
component of the bagasse is similar to the decomposition of the NaC1-treated Avicel cellulose. The influence of the mineral content of the plant material on the decomposition serves as a plausible explanation for this similarity. Effect of MgC12. As Figure 5 shows, the addition of MgC1, slightly changed the shape of the DTG curve: the peaks broadened and became less distinct. The temperature domain of the decomposition and the char yield were
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276 Energy & Fuels, Vol. 2, No. 3, 1988
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Figure 6. Comparison of the DTG curve (dark solid line) and the four highest intensity curves [m/z 18 (light solid line), m/z 28 (a),m/z 29 (O),and m/z 44 (A)]of MgC12-treatedbagasse.
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Figure 8. Comparison of the DTG curves of untreated (dark solid line), FeS04-treated(o),and ZnC12-treatedlight solid line and A) samples. The second ZnCl&reatedsample received three times more catalyst.
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Figure 7. Comparison of the DTG curve (dark solid line) and the four highest intensity curves [ m / z 18 (light solid line), m / z 28 (c]), m/z 29 (O), and m/z 44 (A)] of NaC1-treated bagasse.
Figure 9. Comparison of the DTG curve (dark solid line) and the four highest intensity curves [m/z 18 (light solid line), m/z 28 (c]), m / z 29 (O),and m / z 44 (A)]of FeS04-treatedbagasse.
practically unchanged. The mass spectrometric intensities parallel the DTG curve as before. (See Figure 6.) However, the products detected by the MS studies evidenced a considerably lower contribution to the weight loss, as shown by the data in Table I. This behavior indicates that the primary chemical processes, responsible for the overall rate of the weight loss, remained practically unchanged. Any catalytic influence on these processes would have led to some changes in the decomposition temperature and char yield. The decrease of the products detected by the MS studies shows that the secondary reactions producing low molecular weight products from the primary, high molecular weight products were less intense in the presence of MgC1,. The simplest explanation is to suppose that the MgC12 changes the physical microstructure of the bagasse in such a way as to facilitate the escape of the primary pyrolysis products from the solid sample. Effect of NaCI. As Figure 5 shows, the shapes of the DTG curves of the MgC1,- and NaCl-treated samples were similar,but with NaCl the cellulose DTG peak height was decreased and the peak ended a t a slightly lower temperature. The smaller area of the cellulose peak indicates that the observed higher char yield (29%) from the bagasse is probably due to a higher contribution from its cellulose component. The relative intensities (see Table I and Figure 7) were higher than those for the MgC1, treatment but considerably smaller than the untreated sample intensities. As an explanation, we suppore that the NaCl treatment influences the decomposition in two ways: it catalyzes a cellulose reaction pathway leading to increased char yield and, simultaneously, enhances the escape of the primary high molecular weight products from the sample. Effect of FeSO1. The addition of FeS04 slightly increased the second DTG peak at 300 "C and considerably
decreased the third peak (see Figure 8). The small hemicellulose DTG peak was transformed into a smoothly rising curve from 170 to 250 OC. Note that in the corresponding Avicel experiments the FeS04 treatment shifted the whole DTG curve from 360 to 300 OC. (We suppose that this shift reflected catalysis of the unzipping reactions.) Hence we can assume that the increase of the second DTG peak arose from a portion of the cellulose component decomposing at lower temperature in the absence of FeS04. The fact that a part of the cellulose DTG peak remained uninfluenced by the FeS04 may be due to the physical and chemical inhomogeneity in the microstrcuture of the plant material: we cannot expect a uniform uptake of the catalyst. Regarding the relative heights of the intensity curves (Table I), a considerable decrease can be observed in the FeS04-catalyzedresults relative to those for the untreated sample. The time dependence of the curves did not show marked effects. The water and the carbon dioxide evolution paralleled the DTG curve while the other intensity curves contributed less to the weight loss between 250 and 350 OC. (See Figure 9.) The lower water release together with the good agreement between the water ion intensity and the DTG curves indicate that no special dehydration reaction was catalyzed by FeS04. Effect of ZnCl,. As Figure 8 shows, the ZnCl, pretreatment decreased the height of the third DTG peak, shifted the second peak from 320 to 270 O C , and transformed the first little peak into a smoothly increasing curve. Below 320 OC the weight loss was dominated by a high rate of water release (see Figure 10). Hence the processes below 320 OC are probably dominated by dehydration reactions. From the corresponding Avicel experiment: we can assume that part of the cellulose component
Energy & Fuels, Vol. 2, No. 3, 1988 277
Thermal Decomposition of Biopolymers
A
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Figure 10. Comparison of the DTG curve (dark solid l i e ) and the four highest intensity curves [m/z 18 (light solid line), and the four highest intensity curves [m/z 18 (light solid line), m / z 28 (n),m / z 29 (O), and m / z 44 (A)] of ZnC12-treatedbagasse. also decomposed below 320 OC.
The relative intensities of the third DTG peak (see Table I) were close to the corresponding values of the FeSO, experiment, suggesting that the cellulose decomposition process near 360 "C was similar to the FeS04 experiment. When the experiment was repeated with an increased (3X) ZnClz concentration, a strong decrease of the third DTG peak was observed while the other parts of the DTG curve did not change. The MS intensities were ako similar to the first ZnClz experiment, except for a higher contribution of evolved water to the weight loss around 360 "C (see Table I). The char yield increased to 35%. We suppose that the increased amount of ZnClz catalyzed a reaction of type cellulose char + HzO around 360 "C, too. It may be interesting to note that a significant MS ion intensity was observed at m / e 50 in the ZnClz experiments. This m / e value is characteristic of CHaCl and other chlorinated organic compounds, suggesting that part of the ZnC1, chemically reacted during pyrolysis. We suppose that the ZnClz partly hydrolyzed with the product water and the HC1 produced by hydrolysis participated in addition reactions.
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Conclusions Three DTG peaks were observed during the thermal decomposition of bagasse. The two lower temperature peaks are due to the decomposition of the hemicellulose components of the bagasse, while the high-temperature peak is due to cellulose decomposition. The slowly charring lignin content of the bagasse seemed only to contribute to smoothly increasing base lines of the DTG and
mass spectrometric intensity curves. A thermal pretreatment of 2 h at 260 "C almost entirely eliminated the hemicellulosic components of the bagasse. Unexpectedly, the thermal pretreatment had no effect on the ultimate char yield. The highest MS intensity curve belonged to water evolution. A good fit was observed between the DTG and the water evolution curves in all experiments, except the ZnClz-treated samples. This behavior suggests a considerable similarity between the thermal decomposition of the hemicellulose and cellulose components. The other mass spectrometric intensities contributed in varying degrees to the weight loss of the hemicellulose and cellulose components. The addition of MgClz to the sample did not significantly change the DTG curve and the char yield. However, the observed MS intensities decreased, indicating a decrease in the secondary decomposition of the high molecular weight primary products. This decrease appeared in all of the catalytic experiments of the present study, so we may assume that the addition of ionic salts facilitate the escape of the evolving vapors from the solid matrix, possibly by swelling the fibrous structure of the bagasse. (The swelling may be caused by an increased amount of absorbed water in the presence of absorbed ionic salts.) The other additives (NaC1, FeSO,, and ZnC12) altered the course of pyrolysis of the carbohydrate components of the bagasse to pathways that produced more char, in accordance with the literature. In the case of NaC1, the increased char formation was due to a change in the decomposition behavior of the cellulose component. FeS04-catalyzedcellulose exhibited degradation pathways that overlapped the decomposition of the hemicellulose component. The ZnClz increased the char yield via dehydration reactions. The diversity of effects produced by the simple inorganic compounds used in the present study leads the authors to conclude that the catalytic thermochemical processing of biopolymers is a promising field which merits further research.
Acknowledgment. This work was supported by the Hungarian Academy of Sciences and the National Science Foundation under Grant INT85-04282. We thank Bonnie Thompson and Gerson Sher (NSF) for their continuing interest in this work and the reviewers for their many helpful comments. Registry No. MgC12, 7786-30-3; NaC1, 7647-14-5; FeS04, 7720-78-7;ZnC12,7646-85-7;CH20,50-00-0;CO, 630-08-0; CH30H, 67-56-1;HOCH&HO, 141-46-8; HSCCHO, 75-07-0; H&COCHO, 78-98-8; C02, 124-38-9;HSCCOCHS, 67-64-1; HzO, 7732-18-5; 2-furaldehyde, 98-01-1.
