Energy & Fuels 1989,3,755-760
PNA PVT RPBP TBP
paraffin-naphthene-aromatic pressure-volume-temperature reduced-pressure boiling point true boiling point
Nomenclature %A weight percent aromatics carbon number density (at 15 "C) N number of oils
3
S
wt % X
755
standard deviation weight percent weight fraction
Subscripts a aromatic fraction h hexane n naphthene paraffin P S saturate fraction t total (whole) fraction
Thermogravimetric-Mass Spectrometric Characterization of the Thermal Decomposition of Sunflower Stem Gabor Varhegyi,* Emma Jakab, Ferenc Till, and Tamas Szekely Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, P.O. Box 132, Budapest 1502, Hungary Received April 3, 1989. Revised Manuscript Received August 30, 1989
The thermal decomposition of sunflower stem was studied since (i) sunflower is an important crop with a rapidly growing worldwide production, (ii) sunflower stem is a cheap waste material with interesting thermal properties, and (iii) the results could contribute to the understanding of the decomposition mechanism of other lignocellulosic materials, too. Mass spectra and thermogravimetric data were simultaneously measured at a heating rate of 10 "C/min in argon flow. The thermal decomposition of the untreated sunflower stem showed marked differences from wood and other lignocellulosic materials. The temperatures of the hemicellulose and cellulose decompositions overlapped each other while a separate reaction due to the decomposition of the lignin component appeared at higher temperatures. The net char yield (residue minus mineral content) was high, 25%. Addition of inorganic salts from dilute water solution resulted in only minor changes. Acid washing, as proposed by DeGroot and Shafizadeh for woods, increased the temperature of the differential thermogravimetric peak maximum by 77 "C and changed other characteristics of the decomposition, too. Addition of 0.9% FeS04to the acid-washed samples resulted in noticeable dehydration reactions and catalyzed the cellulose component to decompose at lower temperatures. Decarboxylation reactions around 260 "C, observed by earlier investigators as the first chemical reactions of the decomposition of wood, appeared in all experiments of the present paper.
Introduction The large-scale economic utilization of the huge amounts of lignocellulosic materials produced year-by-year as agricultural by-products and wastes is an unsolved problem. Their direct use as fuels is hindered by the relatively high costs of collection, drying, and transportation. Their processing to commodities of higher value requires the careful evaluation and comparison of their fundamental properties, including the kinetics and mechanism of their thermal behavior and the possibilities to alter the thermal decomposition to obtain more favorable products or product distributions. Sunflower (Helianthus annuus) is a major industrial crop. About 15 million hectares are used for its cultivation all over the world, producing 21 million tons of seeds yearly.' The production has been doubled in the past 15 years. The main producers are the USSR,Argentina, and France. The per capita production is the highest in Argentina and Hungary (about 135 and 85 kg, respectively). The amount of the sunflower stems is roughly twice as
* T o whom correspondence should be addressed.
much.2 In a recent study, Martin et ala2determined the fundamental properties of the sunflower stem. On a dry basis, the stem has a heat of combustion similar to other lignocellulosic materials (15.6 kJ/g), with 8% extractives, 14% lignin, 30% a-cellulose, and 31% hemicellulose. Its sulfur content is about 0.14%. The hydrolysis properties of the sunflower stem were studied by Repka et a1.,3 who reported relatively unfavorable results. In earlier works$-' we studied the thermal decomposition of various lignocellulosicmaterials by the simultaneous ~
(1) FA0 Production %rbook,Vol.
~~
40,198& Food and Agriculture
Organization of the United Nations: Rome, 1987. (2) Martin Martin, A.; Jimhez Alcaide, L.; Ferrer Herranz, J. L. Afinidad 1987,44,133-137. (3)Repka, V. P.; Panaeyuk, V. G.; Lazarenko, N. I.; Panasyuk, L. V.; Sheludko, E. N.; Gorodetzkii, N. I.; Baranovakii, S. A. Vopr. Chim. Chim. Technol. 1972,26,34-37. (4)Varhegyi, G.; Antd, M. J., Jr.; Szekely, T.; Till, F.; Jakab, E. Energy Fuels 1988,2,267-272. (5)Varhegyi, G.;Antal, M. J., Jr.; Szekely, T.; Till, F.; Jakab, E.; Szabo, P. Energy Fuels 1988,2,273-277. (6) Varhegyi, G.; Antal,M. J., Jr.; Szekely, T.; Szabo, P. Energy Fuek 1989,3,329-335. (7)Simkovic, I.; Varhegyi, G.; Antal, M. J., Jr.; Ebringerova, A.; Szekely, T.;Szabo, P. J. Appl. Polym. Sci. 1988,36,721-728.
08874624J 89~25Q3-Q755$Q1.5Q/Q 0 1989 American Chemical Society
756 Energy & Fuels, Vol. 3, No. 6, 1989 application of thermogravimetry and mass spectrometry at moderate temperatures and low heating rates with the hope that such studies would complement the high-temp e r a t u r e pyrolysis data reported i n the literature. The main advantage of the applied approach is the comparison of the rate of the overall mass loss of the sample (DTG) with the simultaneously measured mass spectrometric intensities of the molecular weight products as functions of the sample temperature. The main disadvantage is that the high molecular weight products ( t h e “tar” fraction) c a n n o t be s t u d i e d b y the applied apparatus. In the present study, the thermal decomposition mecha n i s m and the possibilities of i t s alteration b y relatively simple means are studied. A brief survey of the literature of similar lignocellulosic materials and references for more detailed literature reviews have already been
Experimental Section The TG-MS system was built from a Perkin-Elmer TGS-2 thermobalance, a Balzers QMG-511 quadrupole mass spectrometer, and a PDP-11 A heating rate of 10 OC/min, low sample masses (2-3 mg), and an open platinum sample pan were applied. The ambient gas was high-purity argon with a gas flow rate of 140 mL/min. The intensities of 147 mass spectrometric peaks were recorded as a function of the sample temperature. The high molecular weight compounds (the tar fraction) did not reach the ion source and were not analyzed, nor was the residual (“char”) fraction. The sunflower stems were collected in the fields of a large-scale sunflower plantation near Budapest after the mechanized harvesting of the sunflower heads in late October. The ash content of the dry stems was 9.7%, close to the values reporbd in a recent biomass conversion s t u d 9 for sunflowers grown in the USA. The ash was digested by concentrated HCl for atomic absorption analysis and was found to contain 23% (by weight) potassium, 12% calcium, 3% magnesium, and approximately 0.1% iron and sodium. Before the experiments, randomly chosen parts of the stems were homogenized by milling in a ball mill and dried in a desiccator a t room temperature. Part of the powder obtained was acid washed as follows;1oa 10-g sample was placed in 200 mL of 0.1 mol/L HCl and stirred for 4 h a t room temperature. The suspension was filtered and washed with distilled water until neutrality. The acid washing decreased the ash content to 1.0%. Atomic absorption analysis, based on digestion by concentrated HCl, revealed 2% Ca, 1%K, 1% Fe, and 0.5% Mg in the ash, indicating that the acid washing removed the vast majority of the metallic constituents bound to carboxyl groups or being present in the form of inorganic salts. The ash of the acid-washed samples contained higher concentrations of quartz and silicate components which were dissolved for the analysis by a dry alkaline digestion (heating with Na20zto 400 OC) and were found to contain 35% Si, 6% Al, 3% Fe, and a small amount of K. Inorganic salts of analytical purity were added to both the untreated and acid-washed samples. The amount of each catalyst added to the sample was identical with the cellulose experiments described earlier,’ where one cation was added per 100 monomer units of cellulose. Thus mmol of catalyst, dissolved in 2 mL of distilled water, was added to 162 mg of pulverized sunflower stem. This ratio has no direct physical meaning for multicomponent samples, but we needed some standardization to obtain results comparable with the earlier e ~ p e r i m e n t s .Expressed ~~ in weight percentage, the concentrations of the added NaC1, KCl, MgC12, CaCl,, ZnC12, and FeS04 in the samples were 0.36,0.46, 0.59,0.69,0.84, and 0.94%, respectively. In a separate experiment, the concentrations of the calcium, magnesium, and potassium of (8) Varhegyi, G.; Till, F.; Szekely, T. Thermochim. Acta 1986, 102, 115-124. (9) Sealock, L. J., Jr.; Butner, R. S.; Elliot, D. C.; Neuenschwander, G. G. Low-Temperature Conversion of High-Moisture Biomass; Report No. PNL-6726; Pacific Northwest Laboratory, Battelle Memorial Institute: Richland, WA,1988. (10) DeGroot, W.F.;Shafizadeh,F. J. Anal. Appl. Pyrolysis 1984,6, 217-232.
Varhegyi et al.
I
c Figure 1. Thermal decomposition of untreated sunflower stem. Comparison of the DTG curve (-dn/dt, dark solid line) with three characteristic mass spectrometric curves: m/z 15 (compounds containing methyl group($, A), m z 18 (water, O ) , and m/z 44 (carbon dioxide, thin solid line). he curves are scaled to equal height for comparison. 20 0
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400
500
600
?i
Table I. Assignment and Relative Intensities of Mass Spectrometric Ions (Untreated Sunflower Stem) mlz assignmenta 265 O C 31OOC 5OOoC 15 CH3’, methaneb 19 21 224 18 water lo00 lo00 lo00 26 C2H2+,hydrocarbonsb 3 5 32 27 CzH3+,hydrocarbonsb 5 46 3 28 carbon monoxide 238 261 190 29 aldehydes, hydrocarbonsb 51 62 37 19 21 17 30 formaldehyde 31 methyl alcohol, glycolaldehyde 13 11 3 41 C2HO+,hydrocarbonsb 2 3 15 42 C2H20+,hydrocarbonsb 4 7 10 43 aldehydes, hydrocarbonsb 9 16 11 44 carbon dioxide 402 245 159 58 acetone 1 2 60 acetic acid, methyl formate 1 The compound names refer to the probable source of the ions. The + charge sign indicates that the corresponding formula represents a mass spectrometric fragment ion forming from various compounds. The assignment of methane and “hydrocarbons” refers only to the 500 “C spectrum. the original sample were restored by adding a solution of 3.5 mg of CaC12, 4.4 mg of KCl, and 1.2 mg of MgC12 to 90 mg of acidwashed sample and drying it in the usual way. An additional experiment was carried out with the stem of a sunflower grown without fertilizers and any chemicals. The ash content of this stem was 4.5%. Atomic absorption spectroscopy, based on the digestion of the ash by concentrated HCl, showed 21% potassium, 8% calcium, 4% magnesium, and 0.1% iron and sodium in the ash of this stem.
Results and Discussion First the thermal decomposition of the untreated sample is discussed. The effects of the applied treatments are presented in the second half of t h i s section.
DTG Curve and Mass Spectra of the Untreated Sunflower Stem. The DTG curve and three characteristic MS intensities of the untreated sunflower stem are shown i n Figure 1. The DTG curve was different from that of sugar cane bagasse5or a typical wood.*o The main p e a k of the DTG curve of the sunflower stem was narrower, the maximum appeared at a lower temperature, 310 “C, and the hemicellulose and cellulose decompositions did not separate from each other. The low-temperature peak observed in the sugar cane bagasse experiments, however, appeared on the DTG curve of the sunflower stem, i n the form of a “shoulder” around 265 “C. Our DTG curve agrees well in the temperature range 200-340 “C with the
Energy & Fuels, Vol. 3, No. 6,1989 757
Thermal Decomposition of Sunflower Stem DSC curve reported by Martin at a1.2 for sunflower stem decomposition. Three mass spectra of the decomposition of the untreated sample are shown in Table I. The temperatures selected belong to the maximum (310 "C) and two "shoulders" (265 and 500 "C) of the DTG curves. The second column indicates the probable source of the ions.41~ The spectra are background corrected. If the difference between the background level and the highest intensity value was close to the noise level, the corresponding ion was omitted from Table I. (The mathematical criterion for rejection was a signal/noise ratio of less than 10.) The m / z values of the carrier gas (Ar) and the fragments of water ( m / z 16 and 17) were also rejected. The tar fraction (compoundshaving boiling points higher than 200 "C) were lost during the measurement. Its amount could be estimated only in an indirect way, as follows. The quantities of H20, CO, and C02were determined by calibrating the instrument with calcium oxalate monohydrate? The total H20, CO, and C02 released up to 600 "C were found to be 22, 6, and 10% by weight, respectively. The amounts of the other volatile products could not be determined by the applied experimental method. However, the integral of their combined mass spectrometric intensities is in the order of the integral of the CO or C02 intensities. The yield of the solid residue (char plus minerals) was 35% at 600 "C. In this way the amount of the non-detected tar fraction is estimated to be about 15-20%. The First Chemical Events of the Decomposition. The mass spectrometric intensities belonging to water ( m / z 18),carbon monoxide ( m / z 28), carbon dioxide ( m / z 44), and various aldehydes ( m / z 29, 30, 43) had shapes and temperature dependence similar to those of the DTG curve. However, an interesting exception occurred around 265 "C, at the "shoulder" of the DTG curve, where relatively more carbon dioxide and less other products formed than at higher temperatures. (See Figure 1.) This fact supports the results of DeGroot, Pan, Rahman, and Richards," who established that the first significant chemical reaction in woods is the decomposition of the uronic acids in the hemicellulose fraction yielding C02, H20,some methanol, and char or char precursors at about 250 "C. In this way the increased C02 formation may be due to decarboxylation reactions at this temperature. The main fragment of methyl alcohol, CH30+ ( m / z 311, also showed a well-defined partial maximum confirming the observation of DeGroot et al." about the methyl alcohol formation. On the other hand, the contribution of water to the overall mass loss was smaller around 265 "C than at higher temperatures. Decomposition of the Lignin Component. The observed hydrocarbon fragments, CH3+(m/z 15), C2H2+( m / z 26), and C2H3+ ( m / z 27), exhibited two well-separated maxima: a smaller one at 300 "C and higher one at about 500 "C. (See the intensity of CH3+in Figure 1.) These fragments may arise from compounds of type CH3Y and CH2=CHY, where Y stands for an electronegative functional group12as well as from hydrocarbon molecules. A former study13of milled wood lignin samples by the same apparatus showed that the corresponding reactions can be due to the decomposition of the lignin component. The lignin samples, prepared from bamboo, beech, and spruce, (11) DeGroot, W. F.; Pan, W.-P., Rahman, M. D.; Richards, G. N. J. Anal. Appl. Pyrolysis 1988, 13, 221-231. (12) McLafferty, F. W.; Venkataraghavan, R. Mass Spectral Correlations; Advances in Chemistry Series 40; American Chemical Society: Washington, DC, 1982. (13) Faix, 0.; Jakab, E.; Till, F.; Szekely, T. Wood Sci. Technol. 1988, 22,323-334.
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600 C
Figure 2. Thermal decomposition of the pith of the sunflower stem. (Notation: see Figure 1.)
released a high amount of low molecular weight compounds. The weight loss occurred in a wide temperature range between 200 and 800 "C. The intensities of ions CH3+ ( m / z 15), C2H2+( m / z 26), and C2H3+ ( m / z 27) showed sharp peaks between 400 and 500 "C while the other fragments had their peak maxima outside this domain. Since the thermal decomposition of pure cellulose4 and hemicellulose7 samples is practically finished at 400-420 "C at a heating rate of 10 "C/min, the corresponding intensity maxima of the sunflower stem experiments may belong to the decomposition of the lignin component. The parent compounds of these ions arise probably from the numerous methoxy side groups and alkyl bridges of the lignin.I4 The mass spectrometric intensity of the methoxy ion CH30+had a main maximum at 340 "C which is higher than the peak temperature of the DTG curve (310 "C). A plausible interpretation may be to assume some contribution to this fragment by the lignin decomposition. The monomer and dimer structures evolving during the pyrolysis of lignin15J6cannot be detected in our apparatus due to their high boiling points. Under the applied experimental conditions, however, the studied lignin samples produced a higher overall mass of low molecular weight products than tar fra~ti0n.l~ (Note that the applied heating rate was lower than that of the fast pyrolysis studies of lignin decomp~sition'~J~ by several orders of magnitude.) Pith and Skin. The thermal decomposition of the soft inner part of the sunflower stem is shown in Figure 2. Compared to the experiments with whole sunflower stem samples, the carbohydrate decomposition falls into a narrower range and finishes a t about 320 "C. The "shoulder" belonging to early decarboxylation reactions is more pronounced while the peaks of the hydrocarbon fragments are essentially the same. On the other hand, the thin hard skin covering the stems evidenced a smaller decarboxylation shoulder than the whole samples. These differences are probably connected with the different cellulsoe-hemicellulose ratios in the different parts of the stems. Further studies in this direction were out of the scope of the present paper. Addition of Inorganic Salts to the Untreated Sample. The addition of MgC12, NaC1, ZnC12, and FeS04, described in the Experimental Section, led only to slight (14) Sj(istrBm, E. Wood Chemistry, Fundamentals and Applications; Academic Press: New York, 1981. (15) Evans, R. J.; Milne, T. A.; Soltys, M. N. J . Anal. Appl. Pyrolysis 1986,9, 207-236. (16) Faix, 0.; Meier, D.; Grobe, I. J. Anal. Appl. Pyrolysis 1987, 11, 403-416.
758 Energy & Fuels, Vol. 3, No. 6, 1989
Varhegyi et al.
Table 11. Summary of Experiments DTG peak maximum of intensities, nA/mg residue at T, OC -dm/dt, % / s CH3+ H2O co COz 600 OCtb% samplen untreated 310 0.116 1.3 40 10 10 35 (25) u.t. + MgCl, 312 0.108 1.4 42 11 11 37 (26) u.t. + NaCl 316 0.109 1.3 40 10 10 36 (26) u t . + ZnC1, 317 0.113 1.1 40 9 10 36 (25) u.t. + FeSO, 319 0.107 1.1 42 11 10 36 (25) pith 285 0.121 1.0 47 15 14 40 hard skin' 319 0.134 1.4 51 15 10 30 acid washed 387 0.149 1.1 32 10 7 14 (13) a.w. + ZnCl, 385 0.139 0.8 27 8 5 18 (16) a.w. + CaCl, 377 0.158 1.2 30 11 8 15 (13) a.w. + NaCl 373 0.152 0.8 40 14 8 17 (15) a.w. + KCl 1.3 33 362 0.155 11 10 18 (16) a.w. + FeSO, 346 0.099 1.0 23 6 4 18 (16) a.w. + CaCl, + KCl + MgCl, 341 0.155 1.5 43 13 11 28 (18) untreated, grown without chemicals' 334 0.174 1.9 38 21 10 24 (20) 'Abbreviations u.t. and a.w. stand for untreated and acid washed sunflower stem, respectively. The addition of the catalysts indicated in this column is described under Experimental Section. bThe numbers in parentheses express the net char yield (residue yield minus mineral content). cSee text. -dm/dt
Figure 3. Thermal decomposition of NaC1-treated sunflower stem. (Notation: see Figure 1.)
changes in the results. (See Figure 3.) Though the decarboxylation shoulder became less expressive, the main peaks of the DTG and the most important intensity curves did not show considerable changes. However, some smaller intensity curves, e.g., that of the hydrocarbon fragments, diminished in the range of the carbohydrate decomposition, due probably to small changes in the secondary reactions. The temperatures of the DTG peak maximum remained close to that of the untreated sample. The reason of the observed small temperature shifts (up to 9 "C; see Table 11) was not investigated. Effects of Acid Washing. After acid washing, the temperature domain of the carbohydrate decomposition widened and reached the more usual domain of about 250-420 "C observed for other lignocellulosic materials at low heating rate~.~JOTwo separate DTG peaks arose at 310 and 390 "C. As Figure 4 shows, the overall DTG curve evidenced a remarkable similarity to that of the sugar cane bagasse. Using the same arguments as in the case of the bagasse experiments: the first and second DTG peaks of the acid-washed sunflower stem can be associated with the decomposition of the hemicellulose and cellulose, respectively. The low-temperature shoulder of the DTG curve was reduced to an inflection point at 280 "C. However, the ratio of the CO, and HzO curves to the corresponding DTG values is about twice as much in the range 250-280 "C as in the vicinity of the DTG peak maxima. (See Figure 5.) The H,O/DTG ratio of the untreated sample was lower in the domain 250-280 "C than around the DTG peak maximum. The difference in the low-temperature
,
Figure 4. Comparison of DTG curves of untreated sunflower stem (O),acid-washed sunflower stem (A),sunflower stem from a plant grown without chemicals (thin solid line), and sugar cane bagasse (bold solid line).
m .. .& c
Figure 5. Thermal decomposition of acid-washed sunflower stem. (Notation: see Figure 1.)
water production may be due to the high ash content of the untreated sunflower stem (9.7%). One can assume that most carboxyl groups are in salt form in the untreated sample and in acid form after the acid washing. However, we cannot exclude the possibility that the 4-h treatment with hydrochloric acid resulted in a limited hydration, too. The net char yield at 600 "C was reduced from 25% to 13 %. The integrals of the mass spectrometric intensities did not change significantly, indicating that the difference between the char yields of the untreated and acid-washed
Energy & Fuels, Vol. 3, No. 6, 1989 759
Thermal Decomposition of Sunflower Stem -dm/dt VI .a I .d
VI 01 I
C
.>
C
.0
m c
?
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400 C
Figure 6. Kinetic evaluation of the DTG curve of acid-washed sunflower stem. Experimental data (-dmob"/dt,o ) ,calculated data (-dm*c/dt, dark solid line), and the contribution of the partial reactions to -dm*/dt (-dmh/dt and -dm,/dt, thin solid line) are shown.
stem went to the undetected tar fraction. Actually, we have observed more and heavier tar which caused contamination problems in the instrument in all of the experiments with acid-washed samples, but no quantitative determination was carried out. The amounb of the H20, CO, and C02formed up to 600 "C were 21,7, and 10% by weight, respectively. Kinetic Evaluation. The well-separated DTG peaks of the acid-washed sample are suitable for kinetic evaluation. A least-squares curve fitting was applied. The algorithms were described earlier.6J7 The decomposition of the hemicellulose and cellulose components were assumed to be independent of each other. Attempts to include a separate kinetic equation for the low-temperature reactions in the range of 250-280 "C failed. The corresponding least-squares calculations did not lead to a unique solution, due to the small size of the low-temperature shoulder on the DTG curve. Former studies by the authors have also shown that the least-squares evaluation of the DTG and DSC curves can be carried out only in cases when at least the top of the peaks are separated!J8J9 In this way, the reactions of the hemicellulose component were described formally by a single first-order reaction: dah/dt = A h eXp(-Eh/Rr)(l - a h ) (1) Here a h stands for the reacted fraction of the hemicellulose component. The cellulose decomposition was also described by a similar first-order equation. In this latter case, however, the first-order kinetics may be due to a rate-controlling reaction in a reaction network! The overall mass loss rate, -dm/dt, was composed from the curves of the two partial reactions -dm/dt = Ch dah/dt + C, da,/dt (2) where subscripts h and c refer to the hemicellulose and cellulose, respectively, and the unknown constants Ch and C, express the contribution of these components to the overall mass loss. The least-squares evaluation resulted in A h = 4 X E h = 89 kJ/mol, Ch = 29%, A, = 3 X 10" s-l, E, = 172 kJ/mol, and C, = 50%,respectively. The partial curves and the fit obtained are shown in Figure 6. The low Ah and Eh may be due to the fact that the lowand medium-temperature decomposition reactions of the (17)Varhegyi, G. Thermochim. Acta 1979,28, 367-376. (18)Varhegyi, G.;Groma, G.; Lengyel, J. Thermochim. Acta 1979,30, 311-317. (19)Pokol, G.;Varhegyi, G. CRC Crit. Rev. Anal. Chem. 1988, 19, 65-93.
Figure 7. Thermal decomposition of acid-washed sunflower stem after treatment with FeS04solution. (Notation: see Figure 1.)
hemicellulose were described formally by the equation of a single reaction. The values obtained for the cellulose decomposition seem to be reasonable, though they are lower than the corresponding values obtained from the DTG curves of Avicel cellulose and sugar cane bagasse by the same evaluation technique.6 The differences may be due to any sort of inhomogeneity of the cellulose in the present sample. Here the term Ynhomogeneity'' may refer to impurities as well as to morphological differences. If different forms of cellulose decompose at slightly different temperatures, the corresponding DTG peak widens and, as is well-known in the literature on thermal analysis, wider thermoanalytical peaks lead to lower formal A and E values.19 Combined Effects of Acid Washing and Addition of Inorganic Salts. The addition of CaC12, KC1, NaC1, and ZnC1, to acid-washed samples resulted only in minor changes. As Table I1 shows, the ZnC12-catalyzedsample had a less sharp DTG peak [lower (-dmldt),] while the addition of NaCl and KC1 decreased the temperature of the DTG peak maximum by 15 "C. There were small changes in the magnitudes of the mass spectrometric intensities, too. However, the similar bagasse experiments evidenced considerable greater effects of the catalyst^.^^^ There was only one exception; the addition of 0.94% FeS04 changed thoroughly the DTG and the MS curves. (See Figure 7.) In this case the DTG peak temperature decreased by about 40 "C and the shape of the DTG curve indicates more than one reaction step for the decomposition of the cellulose component. The comparison of the DTG and H 2 0 curves in Figure 7 indicates a strong dehydration process in the low-temperature domain. The C02curve evidences a sharp local maximum at 250 "C due to decarboxylation reactions. A small but well-defined furfural curve (not shown in the figure) is also present with a shape identical with that of the H 2 0 curve. Remarks on the Thermal Decomposition of the Cellulose Component. In all of the experiments the highest DTG value belonged to the thermal decomposition of the cellulose component. As the figures and the data of Table I1 show, there is a wide variation in the temperatures of the DTG peak maximum, the lowest and the highest values belonging to 285 and 387 "C, respectively. I t is well-known that the thermal decomposition of the cellulose is highly influenced by the presence of inorganic impurities. The nature of this interaction is assumed to be catalytic. (The inorganic constituents are not bound chemically to the cellulose and those cations that were originally bound to the carboxyl groups of the hemicellulose form probably either organic salts or carbonates
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760 Energy & Fuels, Vol. 3, No. 6, 1989
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Figure 8. Thermal decomposition of acid-washed sunflower stem after restoring the concentrations of Ca, K, and Mg of the untreated stem by adding CaCl,, KC1, and MgC1,. (Notation: see Figure 1.)
as the hemicellulose component decomposes.) The detailed examination of this problem was out of the scope of the present study. We tried to answer only one simple question: can the presence of the cations of the inorganic constituents alone explain the observed high differences in the DTG peak temperatures? For this reason, the concentrations of the calcium, magnesium, and potassium of the original sample were restored in the acid-washed sample by adding chloride salts as it was described under Experimental Section. As Figure 8 shows, the DTG peak maximum temperature of this experiment, 341 "C, was closer to that of the untreated sample (310 "C) than to the corresponding value of the acid-washed sample (387 "C). The differences between the decomposition of this sample and the untreated stem (cf. Figures 8 and 1)may be due to the fact that the addition of salts to the acid-washed samples cannot simulate the real physical and chemical distribution of the inorganic constituents in a natural plant material. Stem from Plant Grown without Chemicals. Present-day sunflower cultivation includes the application of a wide range of chemicals starting with fertilizers before sowing and ending with a drying agent spread on the plants a week before the harvest. It may be interesting to know to what extent the results of the present paper depend on these treatments. For this reason the decomposition of a stem from a plant grown without chemicals was also examined. The results are shown in Figures 4 and 9 and in the last row of the Table 11. The temperature range of the main decomposition was near that of the stems arising from large-scale production. The higher contribution of the carbon dioxide to the weight loss around 265 "C appeared in this case, too. The high-temperature peak of the methyl fragments, however, was less pronounced. The overall picture of the decomposition shows some similarities to that of the experiment described in the previous paragraph. (Cf. Figures 8 and 9.) The observed differences
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Figure 9. Thermal decompositionof the stem of a sunflower plant grown without chemicals. (Notation: see Figure 1.)
between the two untreated sunflower stem samples may be due to their different ash contents. It is hoped that the experience accumulated through the continuous development of this field will provide more specific explanations later. Conclusions Due to its high char yield at 600 "C, the studied sunflower stem seems to be a valuable raw material for charcoal production. If liquefaction or some other thermochemical processing is desired, it may be interesting that a simple low-cost operation (acid washing at room temperature) can diminish the net char yield from 25% to 13%. The marked effects caused by the addition of FeS04 to the acid-washed sample indicate that it is possible to influence the decomposition mechanism by relatively simple means. Sunflower stem may be an interesting model material to obtain information about the decomposition mechanism of lignocellulosic substances in further studies. From this point of view, three of its properties seem to be noticeable: (i) The decomposition properties showed a high dependence on treatments influencing the inorganic material content of the samples. (ii) The appearance of the lowtemperature reactions described by DeGroot et al.ll as the first chemical events in wood indicate that their findings are not limited to the decomposition of woods. (iii) Characteristic reactions of the decomposition of the lignin component occurred in a temperature domain well separated from the temperature of the carbohydrate decomposition in the untreated samples. Acknowledgment. We thank Linda Nemes and Eva Ady for their contribution to the experimental work and the reviewers of Energy & Fuels for their helpful comments. Registry No. HCl, 7647-01-0; NaCl, 7647-145; KC1,7447-40-7; MgCl,, 7786-30-3; CaCl,, 10043-52-4; ZnCl,, 7646-85-7; FeS04, 7720-78-7.
