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A comparative kinetic study on the pyrolysis of three different wood species
J. Anal. Appl. Pyrolysis 68 /69 (2003) 231 /249 www.elsevier.com/locate/jaap
A comparative kinetic study on the pyrolysis of three different wood species M. Mu¨ller-Hagedorn a, H. Bockhorn a,*, L. Krebs b, U. Mu¨ller b a
Institut fu ¨ r Chemische Technik, Universita ¨ t Karlsruhe (TH), Kaiserstrasse 12, Geb. 11.23, 76128 Karlsruhe, Germany b Institut fu ¨ r Kern- und Energietechnik, Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Accepted 28 March 2003
Abstract The catalytic effect of pH-neutral inorganic salts on the pyrolysis temperature and on the product distribution was studied by fractionated pyrolysis followed by GC/MS and GC/FID and by thermogravimetric analysis (TGA) of cold-water-washed hornbeam wood. Sodium and potassium chloride have a remarkable effect on the pyrolysis temperature and on the product distribution, whereas calcium chloride only changes the low temperature degradation of hornbeam wood and the product distribution is nearly unchanged compared with waterwashed hornbeam wood. All studied potassium salts (KCl, KHCO3, and K2SO4) decrease the amount of levoglucosan the order of magnitude being dependent on the anion: chloride has a more pronounced effect than sulphate, and sulphate a more pronounced effect than bicarbonate. The thermal degradation of three different wood species (hornbeam, walnut and scots pine) was investigated by analysis of thermogravimetric/mass spectrometric pyrolysis. Commonly used model substances for the main components of wood, like xylan, pure cellulose or filter pulp, were found to be unreliable for the evaluation of formal kinetic parameters that are able to describe the pyrolysis of wood. A method for the individual evaluation of formal kinetic parameters for the main components of wood was used, that uses specific ion fragments from lignin degradation products to study the lignin degradation. Coniferous lignin is thermally more stable than deciduous lignin, and the latter produces smaller char yields. The differences in wood species mainly result in different degradation rates for the lignin and for the early stages of the hemicellulose degradation.
* Corresponding author. E-mail address: [email protected] (H. Bockhorn). 0165-2370/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-2370(03)00065-2
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# 2003 Elsevier Science B.V. All rights reserved. Keywords: Thermal degradation; Pyrolysis; Wood; Lignin; Hemicellulose; Cellulose; Inorganic salts
1. Introduction To decrease the amount of CO2 emission from energy conversion, biomass as a renewable and CO2-neutral resource has gained great interest. Especially wood and agricultural residues like straw are widely distributed and easily accessible at relatively low costs. Of these lignocellulosic materials wood is favourably used because of its higher density (higher energy content per volume), lower amount of ash, and of its very low amount of nitrogen. In principle, there are two ways to release heat and energy from wood: direct combustion or thermochemical conversion into gases and liquids, that can be used in gas turbines or diesel engines. Also, upgrading of wood to quality fuels such as methanol or hydrogen, or production of fine chemicals is a research topic. In all these cases, the knowledge of the kinetics of the devolatilisation of wood is essential, because pyrolysis is always the first step in any gasification or combustion process. However, the mechanisms and the kinetic data for wood pyrolysis are still unknown to a large extent because of the complexity and the varying physical and chemical properties of wood. Wood as a major representative of biomass consists mainly of cellulose, hemicellulose, and lignin. Its thermal decomposition as performed in thermogravimetric analysis (TGA) of small samples reveals two decomposition regimes, which are attributed to the decomposition of cellulose and hemicellulose [1]. The peak in the decomposition rate at lower temperatures can be associated with pyrolysis of hemicellulose and the peak at higher temperatures is associated with cellulose decomposition. A peak due to lignin decomposition can not be observed. Therefore, the integral result of the merged peaks does not promote a reliable analysis of the decomposition kinetics of the major constituents. Assuming that cellulose, hemicellulose and lignin decompose independently, pretreatment in conjunction with separation of the major components may allow investigating into the decomposition of the components. However, commonly applied separation techniques to reduce the complex structure of wood include extensive depolymerisation and structural changes, which may lead to kinetic parameters that do not represent the decomposition of wood. In recent studies [2 /5], it was found that using model substances (such as xylan, Avicel cellulose, filter pulp, or Klason lignin) as substitute compounds can lead to errors for wood pyrolysis when described with formal kinetic parameters obtained from these substitute compounds. From earlier studies [6 /15], it is well known that the addition of inorganic salts to wood samples result in a wide variety of changes in the pyrolysis process. The main intent of the early studies (e.g. [9,14]) was, to investigate the fire-retardant characteristics of organic and inorganic chemicals for the pyrolysis and combustion of wood. Today, the influence of inorganic salts on the pyrolysis of wood is studied
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with multiple aims: e.g. to increase the yield of charcoal, to develop analytical pyrolysis as a quantitative method for the analysis of pulps [10], to dispose heavy metal containing wood waste [16], to increase the yield of valuable fine chemicals from biomass pyrolysis, and to provide a model that comprises the influence of inorganic salts (ash) on the pyrolysis kinetics. The objective of this study is to investigate the influence wood’s naturally occurring inorganic salts on the temperature of pyrolysis and on the product distribution, and to determine the decomposition kinetics of the three wood species, hornbeam (Carpinus betulus), walnut (Juglans regia ), and scots pine (Pinus sylvestris ) by incorporating the thermal degradation kinetics of the main components (lignin, hemicellulose, and cellulose). The kinetic measurements were performed by online thermogravimetry/mass spectrometry (TG/MS) [17] and by isothermal measurements [18,19]. The comparison of the thermal degradation behaviour of the main components of the different wood species allows to identify differences and similarities of the main components during pyrolysis. Also, a better understanding of the relationship between the chemical structure and the pyrolysis behaviour of the individual components can be achieved.
2. Experimental 2.1. Material The wood species used in this study are hornbeam (C. betulus), walnut (J. regia ), and scots pine (P. sylvestris ), each obtained from a single log from the region of Karlsruhe. The bark free wood was ground to a size range less than 250 mm. The composition of the material is given in Table 1. Prior to the experiments, the samples were dried 3 h at 105 8C. Each wood sample was also washed with cold-water to
Table 1 Chemical composition of the wood speciesa
Ash (500 8C)b Hot-water extractc Ligninc Hemicellulosec Xylose Mannose Cellulosec a b c
Hornbeam
Walnut
Scots pine
0.7 7.7 20.1 23.3 20.0 0.6 48.9
0.7 4.1 25.9 22.1 17.2 1.4 47.8
0.3 2.7 26.7 20.8 5.0 13.0 49.8
All values in weight percent. Percentage on dry wood. Percentage on extracted dry wood (cyclohexane, ethanol) and cold-water-washed wood.
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reduce the amount of inorganic salts and treated with dilute sulphuric acid (4 wt.%) at 100 8C to eliminate the hemicellulose. The samples were doped by dissolving the analytically pure metal salts in distilled water and adsorbing this dissolution to the cold-water-washed wood. The resultant slurry was thoroughly stirred and dried at 90 8C with stirring at times to prevent crystallisation. The cation content was measured by heating 3 g of wood for 5 h at 500 8C in air and dissolving the ash with 10 ml of concentrated hydrochloric acid. Then, it was diluted to exactly 50 ml with double-distilled water. The quantitative measurements were carried out using atomic adsorption spectrometry (Perkin / Elmer AAS 4100).
2.2. Fractionated pyrolysis To investigate the pyrolysis products, fractionated pyrolysis under dynamic conditions was carried out. A sample of about 100 mg was heated up with 5 8C min 1 in a helium-purged (100 ml min 1) oven. The volatile pyrolysis products were collected in five cold traps. After pyrolysis, the condensate in the cold traps was solved in methanol and analysed with GC/MS (Finnigan MAT ITD 800) [19] and GC/FID (Varian 3400). The chromatographic parameters for GC/MS were: CP-Sil 8 CB (Chrompack) column (column length: 50 m; I.D.: 0.32 mm; film thickness 0.12 mm); injector temperature 250 8C; oven temperature program: 40 8C at 5 min constant, 10 8C min 1 to 280 8C at 20 min constant. Those for the GC/FID apparatus were: HP-5 (Hewlett /Packard) column (column length: 30 m; I.D.: 0.53 mm; film thickness 0.88 mm); injector temperature 250 8C; detector temperature 300 8C, and the oven temperature program was the same as for GC/MS.
2.3. Thermogravimetric and isothermal apparatus Thermogravimetric experiments were carried out with a thermobalance (DuPont 951 thermogravimetric analyser) coupled to a quadrupole mass spectrometer (Balzers-QMG 420). Pure helium with a flow rate of 100 ml min 1 and sample sizes of about 25 /27 mg were used. A detailed description of the experimental set-up is given in [17]. The isothermal measurements were performed in a closed-loop-type reactor with isothermal and homogeneous conditions with MS online gas analysis (Balzers QMG 421). The thermal degradation was investigated in helium at 0.1 MPa with a flow of 25 ml min 1. Sample sizes of 30 mg were used. A detailed description of this experimental set-up is given elsewhere [18,19]. As shown in [20] these sample sizes should be small enough to prevent heattransfer effects in isothermal and dynamic experiments.
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Fig. 1. TG and DTG curves of chemically treated hornbeam wood; heating rate b /10 8C min 1. Bold lines and filled symbols represent TG curves, thin lines and open symbols represent DTG curves. /' /, untreated; /m /, water-washed; /j /, acid-washed.
3. Results and discussion 3.1. Thermal decomposition of wood Fig. 1 shows the measured TG curves and the negative first derivatives of the thermogravimetric (DTG) curves of differently treated hornbeam wood (heating rate b / 10 8C min 1). The DTG curve of untreated and water-washed wood reveals two decomposition regimes: The peak at higher temperatures is mainly due to the decomposition of the cellulose and the shoulder, at lower temperatures, can be attributed to the decomposition of the hemicellulose. A peak due to the lignin degradation cannot be observed. The mineral salts, present in native wood, were largely removed by washing the wood with cold-water. In Table 2, the cation content of untreated and water-washed wood is given. The TG curve of water-washed hornbeam wood is shifted to higher temperatures compared with the TG curve of untreated wood. Within each species, the inorganic salt content mainly depends on the growing location of the tree. It is known that the ash content of coniferous trees Table 2 Cation content of untreated and cold-water-washed wood mass percentage on dry wood Cation
Na K Mg Ca Fe
Hornbeam
Walnut
Scots pine
Untreated
Washed
Untreated
Washed
Untreated
Washed
0.0003 0.145 0.021 0.180 0.0013
B/0.0001 0.004 0.015 0.091 0.0002
0.0025 0.184 0.083 0.048 0.0022
B/0.0001 0.002 0.053 0.048 0.0008
0.0019 0.010 0.015 0.061 0.0028
B/0.0001 0.0002 0.008 0.045 0.0006
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is usually smaller than the ash content of deciduous trees [21]. Untreated coniferous wood often decomposes at higher temperature ranges in TG measurements [22]. In the case of scots pine, the water washing hardly shifted the TG curve, whereas the walnut and the hornbeam curve were shifted to higher temperatures by as much as 10/15 8C (heating rate 10 8C min 1). As a consequence of this significant effect of inorganic salts, water washing is essential for the comparison of the decomposition of different wood species and also for the comparison of the same wood species from different locations. For pinewood, a distinction of heartwood and sapwood was made. The chemical analysis with regard to the composition and the amount of metal salts resulted only in very small differences. The measured TG curves were similar (also the ones of untreated wood) within experimental error. The different amount of secondary metabolites (e.g. resinous materials, phenolics (except lignin), stilbenes, and quinones) deposited in the heartwood showed no influence on thermochemical behaviour. To identify the volatile pyrolysis products, fractionated pyrolysis experiments in the temperature region from 100 to 500 8C were carried out. The GC/MS analysis of the collected volatile products gave about 50 different main products; the most abundant peak in all samples is acetic acid. The product peaks were assigned by reference compounds and by literature data (retention times and mass spectra) [23 / 27]. Apart from the products that can be attributed to the cellulose and hemicellulose pyrolysis, about 20 main monomer lignin products (hornbeam 24, walnut 22, scots pine 12) were identified. They were classified into catechol, guaiacol, and syringol derivatives. As expected, there were no syringol derivatives found in pine wood pyrolysis vapours. In Fig. 2 the GC/FID spectra of water-washed hornbeam pyrolysis products are shown. Product peaks with retention times (RT) lower than
Fig. 2. GC/FID chromatrogram of the volatile pyrolysis products of water-washed hornbeam wood. Numbers refer to Table 3.
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Table 3 Volatile pyrolysis products of cold-water-washed hornbeam wood Number
Compound
RT (s)
MW
Referencea
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Butanedial 2- or 3-Pentenoic acid methyl ester 2-Hydroxy-butanedial 2-Furaldehyde 2-Furfuryl alcohol (5H)-Furan-2-one Dihydro-methyl-furanone C6H8O2 5-Methyl-2-furaldehyde 4-Hydroxy-5,6-dihydro-(2H)-pyran-2-one Guaiacol Tetrahydro-4-hydroxy-pyran-2-one 4-Methyl-guaiacol 3-Methoxy-catechol Anhydro-pento-furanose 4-Vinyl-guaiacol Syringol Unknown 4-Methyl-syringol Levoglucosan 4-Vinyl-syringol 4-Propenyl-syringol (trans) Syringyl acetone Sinapaldehyde
147 165 192 260 320 425 436 490 497 545 652 660 758 834 870 880 912 941 992 1068 1085 1184 1234 1370
86 114 102 96 98 84 98 112 110 114 124 116 138 140 144 150 154 172 168 162 180 194 210 208
Lit(MS) Lit(MS) Lit(MS) R R R Lit(MS) R Lit(MS) R (MS) Lit(RT/MS) Lit(RT/MS) Lit(MS) Lit(RT/MS) Lit(RT/MS) Lit(MS) Lit(RT/MS) R Lit(RT/MS) Lit(RT/MS) Lit(RT/MS) Lit(RT/MS)
Originb
H H/C H/C H H/C H/C H/C H/(C?) L H L L H L L H L C L L L L
a
R, assignment confirmed by reference compound; Lit(MS), assignment according [23 /27] based on mass spectra; Lit(RT/MS), assignment according [23 /27] based on mass spectra and RT; (MS), assignment according mass spectra. b H, mainly hemicellulose derived product; C, cellulose derived product; L, lignin derived product.
140 s (acetic acid, hydroxyacetone) overlap with the solvent and cannot be seen in this figure. Selected peaks are numbered and identified in Table 3.
3.2. Effect of inorganic salts Inorganic salts influence the pyrolysis temperature and the composition of the products [6 /16]. In Fig. 3, the DTG curves (heating rate b / 10 8C min 1) of doped hornbeam wood with 0.5 wt.% cation content and chlorine as anion are shown. It can be seen that the potassium ion, one of the most abundant ions that naturally occur in native wood, and as well the sodium ion have a strong influence on the pyrolysis temperature, and the DTG curve is shifted to lower temperatures. In the case of calcium doped wood the shoulder at lower temperatures is more pronounced and the degradation of the hemicellulose starts even at lower temperatures than in the case of sodium and potassium doped wood. The peak at about 360 8C, which can
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Fig. 3. DTG curves of cation-doped hornbeam wood; 0.5 wt.% cation; anion: chloride; heating rate b / 10 8C min 1. /j /, water-washed; /m /, Na doped; /' /, K doped; /" / Ca2 doped.
be mainly attributed to the cellulose decomposition, is hardly shifted, compared with water-washed wood. Not only the cations are responsible for the decrease in pyrolysis temperature as can be seen in Fig. 4. Potassiumchloride exhibits the strongest effect. Compared with water-washed wood, potassium bicarbonate doped wood shows no difference in pyrolysis until 300 8C. In Table 4 the relative distribution of the pyrolysis products from doped hornbeam wood compared with water-washed wood is shown. The components 3, 6, 10 and 12 are mainly of hemicellulose origin, the components 4, 5, 7 and 8 are of hemicellulose and cellulose origin, and the component 20 is only produced during cellulose decomposition. The calcium treated sample showed a similar product distribution as water-washed wood. Only the amount of component 8 increases
Fig. 4. DTG curves of anion-doped hornbeam wood; 0.5 wt.% potassium; heating rate b /10 8C min 1. /j /, water-washed; /m /, Cl doped; /" /, SO2 doped; /' /, HCO3 doped. 4
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Table 4 Product distribution of differently treated hornbeam wood (component numbering refers to Table 3) Treatment
Washed CaCl2 NaCl KCl K2SO4 KHCO3
Peak area of componenta 3
6
10
12
4
5
7
8
20
16 145 0.92 0.17 0.23 0.19 0.61
11 235 0.54 1.68 2.15 3.39 1.40
71 938 0.70 0.32 0.41 0.42 0.61
13 200 0.41 3.54 4.85 4.83 1.72
31 737 0.65 0.56 1.05 1.98 0.76
7073 0.37 1.21 1.98 4.20 1.21
18 582 1.27 1.12 3.03 3.98 1.32
2321 7.46 0.31 0.45 0.73 0.31
71 869 1.13 0.27 0.18 0.35 0.50
a In the case of water-washed wood the absolute peak area in mVs for each component is given; in the case of the inorganic salt doped samples, the relative peak area compared with washed wood is presented.
intensively, especially compared with the other inorganic salts, where this component is nearly not present. The alkaline metals have all in common that the yield of levoglucosan (component 20) is reduced to less than half. In the case of the components that are originating from hemicellulose, component 3 and 10 are decreasing while component 6 and 12 are increasing, in the presence of alkaline metal salts. Although the DTG curves of NaCl and KCl-treated samples are similar, a difference in the product distribution can be noticed, especially for component 12, 4 and 7. The sulphate has a more pronounced effect on the studied components than the bicarbonate. The yield of levoglucosan in the presence of inorganic salts during wood pyrolysis has been studied earlier [10,12]. Despite of the fact, that different wood species (thermochemical pulp produced from Norway spruce) and different pyrolysis conditions (pyrolysis /GC/MS at 620 8C) were applied [10], they also observed, that the calcium ion has only a minor effect on the product distribution and that sodium prevents the formation of anhydrosugar-type compounds and increases the relative abundance of low molecular weight compounds (such as aldehydes, ketones and furanones). The influence of different anions (bicarbonate and sulphate) with sodium as cation was also studied, but contrarily to the results presented here with potassium as cation, the bicarbonate had a more pronounced effect than the sulphate. The influence of cations on the yield of levoglucosan studied for milled cottonwood sapwood under vacuum pyrolysis conditions at about 300 8C showed, that the potassium ion and the lithium ion drastically reduce the yield of levoglucosan, while calcium only has a small reducing effect [12]. Although different materials at different experimental conditions were examined, the observed trends are the same: cations and anions are responsible for the pyrolysis temperature and as well for the product distribution. A mechanistic explanation for the observed effects cannot be given, because even the pyrolysis mechanism of levoglucosan formation from pure cellulose is discussed contradictionary by [13,14]. To circumvent the disadvantage of inorganic salts and to minimise the ratio of inorganic salts, only formal kinetic parameters obtained from cold-water-washed wood should be used.
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3.3. Evaluation of formal kinetic parameters for wood pyrolysis In TG-experiments with low heating rates, the resultant DTG curve for wood pyrolysis exhibits a peak with a shoulder (Fig. 1). Depending on the type of wood, this shoulder is more or less pronounced. With the assumption, that the three main components (hemicellulose, cellulose, and lignin) of wood decompose independently, at least three parallel reactions have to be considered. For chemical controlled reactions, Eq. (1) has to be solved. da dt
X i
ci ×
dai dt
X
ci × ki (T)×(1 ai )ni
(1)
i
where a is the degree of conversion, which is defined as a /(m0 /m )/(m0 /m) with m0 being the initial mass, m the actual mass, and m the final mass, k is the rate coefficient, n the apparent reaction order, and c corresponds to the amount of volatiles produces by the ith component of a unit mass of sample. Using an Arrhenius-type of temperature dependency for the rate coefficient k (T ) /k0 exp( / Ea/RT) with Ea as apparent activation energy, and introducing the heating rate b / dT /dt, equation 1 is transformed into Eq. (2). Ea;i da X k ci × 0;i e RT ×(1 ai )ni dT b i
(2)
If no further assumptions are made, at least 12 parameters (i /3) have to be determined from one single TG curve. To circumvent this disadvantage, model substances (such as xylan for the hemicellulose of deciduous trees, Avicel cellulose or filter pulp for the cellulose, and Klason lignin for the lignin component of wood) were used to evaluate the formal kinetic parameters individually. However, as already noted in [2 /5], this can lead to errors, when describing wood pyrolysis with formal kinetic parameters obtained from these model substances. In Fig. 5, the mass-loss curves for model substances are shown. The two commercially available xylans (from beechwood, Sigma, and from birchwood, Fluka) exhibit different degradation curves and different char yields (35 wt.% for birch, and 40 wt.% for beech xylan at 500 8C, b /10 8C min 1). These high char yields indicate that the thermal degradation of wood’s hemicellulose is different to the studied xylans as can be concluded from the following consideration: TG experiments of water-washed hornbeam wood yielded about 12 wt.% char (Fig. 5). Hornbeam wood consists of about 50 wt.% cellulose, 20 wt.% lignin, and 30 wt.% hemicellulose (including the hot-water extract). Pure cellulose normally gives at least 2 wt.%, e.g. [2,5] char and according to [15] milled wood lignin (MWL) at least 30 wt.% char (about 65 wt.% char for Klason lignin [28]). This consideration results in a minimum char yield of about 7 wt.% for the cellulose and lignin fraction. Therefore, the char contribution of the hemicellulose has to be less than 5 wt.%, resulting in an maximum char yield for hemicellulose of about 17 wt.% (or 22 wt.% if it is believed that the hot-water extract decomposes without residue).
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Fig. 5. TG mass-loss curves of model substances; heating rate b /10 8C min 1. / /, water-washed hornbeam; /m /, xylan (beech); /j /, xylan (birch); /^ /, cellulose (Fluka); /2 /, Avicel PH-105 [2]; /\ /, Whatman #42 [2].
As already observed in [2], pure cellulose samples are not alike. The inflection points of the pure cellulose samples (Fig. 5) are ranging between 340 and 373 8C (b /10 8C min 1). This very wide temperature range is not suitable for an accurate description of wood’s cellulose degradation. Therefore, the use of a special cellulose sample for the description would be somewhat arbitrary. In the following sections, a method that allows the individual evaluation of formal kinetic parameters for the main components of wood is presented. 3.3.1. Hemicellulose degradation The hemicellulose can be eliminated to a large extent by treating the wood sample with hot dilute sulphuric acid. The weight-loss curve for the hemicellulose can be obtained by subtracting the TG curve of acid-washed wood from the TG curve of water-washed wood taking into account the weight loss for the acid treatment. This contains the hemicellulose part plus the hot-water extract given in Table 1. The resulting curves (Fig. 6) for all three wood species show two decomposition steps, that were used separately for the evaluation of the overall kinetic parameters. For this equation 2 (with i /1 and c /1) is integrated with a fourth-order Runge /Kutta method for given sets of parameters Ea, k0, and n . The minimum of squares of deviation between the degree of conversion from the integrated rate expression and the measured degree of conversion is directly searched [17]. The results for the two degradation steps (hemicellulose 1 and hemicellulose 2) are given in Table 5. As seen from Fig. 6, the first step of the hemicellullose degradation is nearly identical for the two deciduous trees, whereas the coniferous tree shows a different behaviour. The hemicellulose of deciduous woods consists mainly of xylan whereas coniferous woods contain large amounts of mannan (Table 1). In the case of the studied deciduous trees, only the mass ratio of the first step of the hemicellulose degradation to the second degradation step differs. In all three wood species, the second degradation step of the hemicellulose is nearly identical. The mass ratio that
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Fig. 6. Calculated TG and DTG curves for hemicellulose weight-loss obtained by subtraction of the curves of water-washed and acid-washed samples; heating rate b /5 8C min 1. Bold lines and filled symbols represent TG curves, thin lines and open symbols represent DTG curves. /m /, hornbeam; /' /, scots pine; /j /, walnut.
Table 5 Mean values (b /2, 5, and 10 8C min 1) and 95% confidence intervals in brackets for the formal kinetic parameters calculated from dynamic experiments for the three wood species Ea (kJ mol 1)
Log k0(min 1)
n
Hornbeam Hemicellulose 1 Hemicellulose 2 Lignin Lignin (isothermal) Cellulose 1 Cellulose 2
163 (9/2) 257 (9/5) 99 (9/2) 105 (9/1) 93 (9/8) 181 (9/8)
14.6 (9/0.2) 20.4 (9/0.3) 7.8 (9/0.2) 8.2 (9/0.1) 8.5 (9/0.7) 14.5 (9/0.7)
1.7 0.8 1.3 1.4 0.8 0.9
(9/0.1) (9/0.1) (9/0.1) (9/0.1) (9/0.1) (9/0.1)
Walnut Hemicellulose 1 Hemicellulose 2 Lignin Cellulose 1 Cellulose 2
175 (9/3) 262 (9/1) 95 (9/6) 99 (9/7) 183 (9/5)
15.8 (9/0.3) 20.9 (9/0.2) 7.4 (9/0.5) 9.3 (9/0.7) 14.7 (9/0.5)
1.8 0.8 1.4 0.9 0.8
(9/0.1) (9/0.1) (9/0.1) (9/0.1) (9/0.1)
Scots pine Hemicellulose 1 Hemicellulose 2 Lignin Cellulose 1a Cellulose 2a
101 (9/1.2) 262 (9/1) 93 (9/4) 93 181
8.1 (9/0.1) 20.9 (9/0.1) 7.1 (9/0.4) 8.5 14.5
0.7 (9/0.1) 0.8 (9/0.1) 1.4 (9/0.1) 0.8 0.9
Additionally, the results of the isothermal measurements for hornbeam lignin are given. a The experimental curves are described within experimental error with parameters calculated for hornbeam wood.
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is degraded during the first and second hemicellulose step cannot be attributed to a weight proportion of a special sugar nor to a group of sugars belonging to the hemicellulose. The higher thermal stability of pine hemicellulose is possibly due to the different hemicellulose structure. The char yield for the hemicellulose is found to be largely independent of the heating rate applied (2 /10 8C min 1) and it is determined to be 11.0, 10.2 and 10.1 wt.% for the hemicellulose of hornbeam, walnut, and scots pine, respectively. 3.3.2. Lignin degradation Because lignin cannot be separated from wood without dramatically changing the chemical structure, nor was it the intent to use model substances, ion fragments of lignin-derived products were used to study the lignin degradation by means of the evolution of monomers. The GC/MS analysis of the gaseous pyrolysis products showed that the monomer lignin products give a different ion fragmentation than the products originating from the degradation of cellulose and hemicellulose, especially in the mass region from 105 to 212 amu. Forty-two ion fragments (hornbeam) from this region can be used to identify the lignin degradation for hornbeam wood (39 for walnut and 25 for pine). The degree of conversion a can be calculated with the determined ion fragments by using equation 3. T
g
x X C
a(T) T 100 T 500 C x X
g
T 100 C
Il;i (T)Ml;i (T)dT
i 1
(3) Il;i (T)Ml;i (T)dT
i 1
where x is the number of found lignin ion fragments, Il,i is the ion current of the ith lignin ion fragment, and Ml,i is the molecular mass of the ith lignin ion fragment. By using the same method for the evaluation of the overall formal kinetic parameters as described for the hemicellulose degradation, the results given in Table 5 were obtained. Additionally, isothermal experiments were performed with hornbeam wood to validate the kinetic parameters from dynamic measurements. The main advantage of isothermal measurements is that changes in the mechanism are detectable because decomposition rates are obtained for single temperatures, contrarily to dynamic measurements where only one value for the total temperature range is obtained. Because the residue is also a function of temperature up to 320 8C, only measurements in the temperature range from 320 to 390 8C were used to study the lignin degradation by means of specific lignin ion fragments. The rate coefficients were determined as described in [18] and they are summarised in an Arrhenius-plot in Fig. 7. From the Arrhenius-plot, an apparent activation energy and a preexponential factor are obtained by linear regression (see Table 5). From these measurements no indication of a change in the mechanism of the evolution of lignin-
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Fig. 7. Arrhenius-plot of the degradation of lignin from water-washed hornbeam wood.
derived monomers during wood pyrolysis is obvious (Fig. 8) and the apparent order of reaction n is equal at 1.4. The formal kinetic parameters obtained from dynamic experiments are in good agreement with the ones obtained from isothermal measurements. Both experiments resulted in an apparent order of reaction that is approximately 1.3. The fractionated pyrolysis experiments (for the two deciduous woods) showed that the product distribution of lignin monomers changes with temperature. At lower temperatures (100 /290 8C, b /5 8C min 1), mainly the high molecular weight syringol derivatives (sinapaldehyde and 4-propenylsyringol) were found, whereas at high temperatures (350 /400 8C, b /5 8C min 1) catecol, 3-methoxycatecol, syringol 4-methylsyringol, and coniferaldehyde and only very little of the ones mentioned before were found. Guaiacol derivatives mainly evolved at middle and high temperatures. The lignin degradation can, therefore, be considered as a set of several parallel reactions, resulting in overall reaction orders higher than one.
Fig. 8. Apparent order of reaction of the degradation of lignin from water-washed hornbeam wood.
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Isothermal experiments at 900 8C in a steam-helium atmosphere with milled spruce wood showed that the evolution of major pyrolysis products from lignin varies with reaction time. The evolution of low-molecular-weight species like methane, formaldehyde and methanol coincidences with the evolution of major lignin derived monomers [29]. Although, the lignin degradation mechanism at high temperatures may be different from these observed for low heating rates in dynamic experiments, the observation of parallel lignin degradation reactions is supported according to [29]. Contrarily to E. Jakab et al. [15], but in accordance with Domburg et al. [30], the release of monomeric products is dependent on the lignin type. The coniferous lignin is thermally more stable than the deciduous lignins of hornbeam and walnut (see Table 5). The char residue for the lignin degradation was calculated from dynamic measurements of acid-washed wood, assuming that the char residue for the cellulose degradation is identical for all three wood species, because of its identical chemical structure. The char residues are found to be 32.3, 32.3 and 36.9 wt.% for hornbeam, walnut, and scots pine, respectively. In [15] TG/MS experiments with MWL from 16 different plants or trees were carried out. Although no kinetic parameters are given, the final char yields at 850 8C for a heating rate of 20 8C min 1 are listed. Lignin obtained from spruce wood (Picea abies ) gives 35.3 wt.% and lignin from walnut shells (J. regia ) gives 30.6 wt.% char. In spite of slightly different woods or part of the trees, the char residues presented in this paper are in good agreement with [15].
3.3.3. Cellulose degradation The curve representing the cellulose degradation is obtained by subtracting the measured lignin curve from the curve of acid-washed wood using the degradable amounts. The resulting weight-loss curve shows a minor mass loss of 5 /10 wt.% in the low temperature range (200 /270 8C for b /5 8C min 1) and a main degradation step at high temperatures (280 /390 8C, b /5 8C min 1) for all three wood species. The degradation at lower temperatures is labelled cellulose 1 and the degradation at higher temperatures is labelled cellulose 2. The calculated formal kinetic parameters for the three different wood species are given in Table 5. Due to the incomplete separation of the two degradation steps of cellulose and the very low amount of cellulose 1 (about 3 /4 wt.% for whole wood), the calculated formal kinetic parameters for cellulose 1 are not very suitable for further kinetic discussion. It is possible that this curve belongs to other main components of wood; likely to the hemicellulose caused by incomplete solution of the hemicellulose. However, fractionated pyrolysis experiments with acid-washed hornbeam wood (heating rate b /5 8C min 1) indicate that this portion (cellulose 1) belongs to the cellulose degradation, because of the levoglucosan found in the first cold trap (temperature range 100 /290 8C). The parameters for cellulose 2 lie within literaturegiven parameters for pure cellulose (log k0 (min 1) /14/20, Ea /190 /250 kJ mol 1, with a first-order kinetic model) [1,2,31], but as mentioned already, these parameters cover a broad temperature range.
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The char residue for the cellulose degradation is determined to 3 wt.%. For cellulose model substances char yields were found to be in the range of 2 /10 wt.%, depending on the chemical nature of the substrates and on the heating rate [1]. 3.3.4. Combination of the determined formal kinetic parameters The measured dynamic TG-mass-loss curves of water-washed wood (with heating rates ranging from 2 to 10 8C min 1) are very well described with the five given formal kinetic parameters from the thermal decomposition of the single components (Figs. 9 /11). The degradable mass fractions used for applying the rate expression of the single components are given in Table 6. Contrarily to Groenli [32], who modelled the pyrolysis of different wood species with three to five parallel reactions, the formal kinetic parameters presented here were evaluated without the assumption of first order kinetics and they were obtained from the individual components of wood.
4. Conclusion Inorganic salts have a strong influence on the temperature of pyrolysis as well as on the product distribution. The alkaline metal chlorides studied, strongly decrease the temperature of pyrolysis, whereas calcium chloride mainly influences the low temperature degradation regime. The influence on the pyrolysis temperature is also dependent on the anion. The influence is increasing in the following row: bicarbonate B/sulfate B/chloride. In the case of the product distribution, this trend can only be observed for the yield of levoglucosan. In the presence of calcium chloride a C6H8O2-component was found in high abundance, but it could hardly be observed without calcium. In order to compare the pyrolysis behaviour of different wood species, the inorganic salts naturally occurring should be removed. A different
Fig. 9. Comparison of calculated curves (solid lines) from the presented parameters of hornbeam wood with the measured ones (symbols) at (m) 2 8C min 1, (j) 5 8C min 1 and (') 10 8C min 1. Bold solid lines and filled symbols represent weight-loss curves, thin solid lines and open symbols represent DTG curves.
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Fig. 10. Comparison of calculated curves (solid lines) from the presented parameters of walnut wood with the measured ones (symbols) at (m) 2 8C min 1, (j) 5 8C min 1 and (') 10 8C min 1. Bold solid lines and filled symbols represent weight-loss curves, thin solid lines and open symbols represent DTG curves.
Fig. 11. Comparison of calculated curves (solid lines) from the presented parameters of scots pine wood with the measured ones (symbols) at (m) 2 8C min 1, (j) 5 8C min 1 and (') 10 8C min 1. Bold solid lines and filled symbols represent weight-loss curves, thin solid lines and open symbols represent DTG curves.
Table 6 Degradable mass fractions in percent used for the calculation of the residual curves
Hemicellulose 1 Hemicellulose 2 Lignin Cellulose 1 Cellulose 2
Hornbeam
Walnut
Scots pine
24.5 6.9 15.3 4.3 49.1
17.0 10.0 20.0 3.7 49.3
14.7 9.8 19.5 3.9 45.1
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inorganic salt content or different anions or cations will result in different TG curves even for the same wood species. Formal kinetic parameters obtained from model substances are not very reliable if they are adopted to the description of wood pyrolysis. Pure, ash-free celluloses and xylans from different manufacturers manifest remarkably different pyrolysis temperatures. The chemical structure of MWL is thought to be similar to that of lignin in native wood, but the higher thermochemical stability of coniferous lignins could not be observed with MWL in [15]. The char residues of lignin obtained in this study are similar compared with MWL. From the degradation curves of the main components of wood, the shoulder of the DTG peak (Fig. 1) at lower temperatures can be mainly attributed to the hemicellulose degradation, but lignin, cellulose, and the second step of the hemicellulose degradation contribute to the peak at higher temperatures. The presented formal kinetic parameters are suitable for the description of the pyrolysis of the studied wood species over the investigated heating rate range. The differences in wood species are mainly due to the different thermochemical behaviour of lignin degradation and that of the first step of the hemicellulose degradation.
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M.J. Antal, Jr, G. Varhegyi, Ind. Eng. Chem. Res. 34 (1995) 703. M.J. Antal, Jr, G. Varhegyi, E. Jakab, Ind. Eng. Chem. Res. 37 (1998) 1267. V. Cozzani, A. Lucchesi, G. Stoppato, G. Maschio, Can. J. Chem. Eng. 75 (1997) 127. C.A. Koufopanos, G. Maschio, A. Lucchesi, Can. J. Chem. Eng. 67 (1989) 75. F. Shafizadeh, J. Anal. Appl. Pyrol. 3 (1982) 283. R. Font, A. Marcilla, E. Verdu´, E.J. Devesa, J. Anal. Appl. Pyrol. 21 (1991) 249. G. Varhegyi, M.J. Antal, Jr, T. Szekely, F. Till, E. Jakab, Energy Fuels 2 (1988) 267. G. Varhegyi, M.J. Antal, Jr, T. Szekely, F. Till, E. Jakab, P. Szabo, Energy Fuels 2 (1988) 273. F.L. Browne, W.K. Tang, Effect of Various Chemicals on Thermogravimetric Analysis of Ponderosa Pine, FPL 6, Forest Products Laboratory, US Department of Agriculture, Forest Service, Madison, WI, 1963. M. Kleen, G. Gellerstedt, J. Anal. Appl. Pyrol. 35 (1995) 15. J. Piskorz, D. Radlein, D.S. Scott, S. Czernik, J. Anal. Appl. Pyrol. 16 (1989) 127. G.N. Richards, G. Zheng, J. Anal. Appl. Pyrol. 21 (1991) 133. M.J. Antal, Jr, Biomass pyrolysis: a review of the literature, in: K.W. Boer, J.A. Duffie (Eds.), Advances in Solar Energy, vol. 1, American Solar Energy Society, Boulder, CO, 1982, pp. 61 /111. M.J. Antal, Jr, Biomass pyrolysis: a review of the literature, in: K.W. Boer, J.A. Duffie (Eds.), Advances in Solar Energy, vol. 2, American Solar Energy Society, Boulder, CO, 1985, pp. 175 /255. E. Jakab, O. Faix, F. Till, J. Anal. Appl. Pyrol. 40 /41 (1997) 171. L. Helsen, E. Van den Bulck, J. Anal. Appl. Pyrol. 53 (2000) 51. H. Bockhorn, A. Hornung, U. Hornung, S. Teepe, J. Weichmann, Combust. Sci. Technol. 116/117 (1996) 129. H. Bockhorn, A. Hornung, U. Hornung, J. Anal. Appl. Pyrol. 50 (1999) 77. H. Bockhorn, A. Hornung, U. Hornung, D. Schawaller, J. Anal. Appl. Pyrol. 48 (1999) 93. H. Bockhorn, A. Hornung, U. Hornung, P. Jakobsto¨er, J. Anal. Appl. Pyrol. 49 (1999) 53. H.F.J. Wenzl, The Chemical Technology of Wood, Academic Press, New York, 1970, p. 92. S. Gaur, T.B. Reed, Thermal Data for Natural and Synthetic Fuels, Marcel Dekker, New York, 1998, p. 102.
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O. Faix, I. Fortmann, J. Bremer, D. Meier, Holz als Roh- Werkstoff 49 (1991) 213. O. Faix, I. Fortmann, J. Bremer, D. Meier, Holz als Roh- Werkstoff 49 (1991) 299. O. Faix, D. Meier, I. Fortmann, Holz als Roh- Werkstoff 48 (1990) 281. O. Faix, D. Meier, I. Fortmann, Holz als Roh- Werkstoff 48 (1990) 351. A.D. Pouwels, G.B. Eijkel, J.J. Boon, J. Anal. Appl. Pyrol. 14 (1989) 237. M.G. Groenli, G. Varhegyi, C. Di Blasi, Ind. Eng. Chem. Res. 41 (2002) 4201. R.J. Evans, T.A. Milne, M.N. Soltys, J. Anal. Appl. Pyrol. 9 (1986) 207. G. Domburg, G. Rossinskaya, V. Sergeeva, in: Thermal Analysis, Proceedings of International Conference, 1975, p. 211. [31] M. Groenli, J.A. Antal, Jr, G. Varhegyi, Ind. Eng. Chem. Res. 38 (1999) 2238. [32] M. Groenli, A theoretical and experimental study of the thermal degradation of biomass, Ph.D. thesis, The Norwegian University of Science and Technology, Trondheim, Norway, 1996, p. 42.
A comparative kinetic study on the pyrolysis of three different wood species M. Mu¨ller-Hagedorn a, H. Bockhorn a,*, L. Krebs b, U. Mu¨ller b a
Institut fu ¨ r Chemische Technik, Universita ¨ t Karlsruhe (TH), Kaiserstrasse 12, Geb. 11.23, 76128 Karlsruhe, Germany b Institut fu ¨ r Kern- und Energietechnik, Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Accepted 28 March 2003
Abstract The catalytic effect of pH-neutral inorganic salts on the pyrolysis temperature and on the product distribution was studied by fractionated pyrolysis followed by GC/MS and GC/FID and by thermogravimetric analysis (TGA) of cold-water-washed hornbeam wood. Sodium and potassium chloride have a remarkable effect on the pyrolysis temperature and on the product distribution, whereas calcium chloride only changes the low temperature degradation of hornbeam wood and the product distribution is nearly unchanged compared with waterwashed hornbeam wood. All studied potassium salts (KCl, KHCO3, and K2SO4) decrease the amount of levoglucosan the order of magnitude being dependent on the anion: chloride has a more pronounced effect than sulphate, and sulphate a more pronounced effect than bicarbonate. The thermal degradation of three different wood species (hornbeam, walnut and scots pine) was investigated by analysis of thermogravimetric/mass spectrometric pyrolysis. Commonly used model substances for the main components of wood, like xylan, pure cellulose or filter pulp, were found to be unreliable for the evaluation of formal kinetic parameters that are able to describe the pyrolysis of wood. A method for the individual evaluation of formal kinetic parameters for the main components of wood was used, that uses specific ion fragments from lignin degradation products to study the lignin degradation. Coniferous lignin is thermally more stable than deciduous lignin, and the latter produces smaller char yields. The differences in wood species mainly result in different degradation rates for the lignin and for the early stages of the hemicellulose degradation.
* Corresponding author. E-mail address: [email protected] (H. Bockhorn). 0165-2370/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-2370(03)00065-2
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# 2003 Elsevier Science B.V. All rights reserved. Keywords: Thermal degradation; Pyrolysis; Wood; Lignin; Hemicellulose; Cellulose; Inorganic salts
1. Introduction To decrease the amount of CO2 emission from energy conversion, biomass as a renewable and CO2-neutral resource has gained great interest. Especially wood and agricultural residues like straw are widely distributed and easily accessible at relatively low costs. Of these lignocellulosic materials wood is favourably used because of its higher density (higher energy content per volume), lower amount of ash, and of its very low amount of nitrogen. In principle, there are two ways to release heat and energy from wood: direct combustion or thermochemical conversion into gases and liquids, that can be used in gas turbines or diesel engines. Also, upgrading of wood to quality fuels such as methanol or hydrogen, or production of fine chemicals is a research topic. In all these cases, the knowledge of the kinetics of the devolatilisation of wood is essential, because pyrolysis is always the first step in any gasification or combustion process. However, the mechanisms and the kinetic data for wood pyrolysis are still unknown to a large extent because of the complexity and the varying physical and chemical properties of wood. Wood as a major representative of biomass consists mainly of cellulose, hemicellulose, and lignin. Its thermal decomposition as performed in thermogravimetric analysis (TGA) of small samples reveals two decomposition regimes, which are attributed to the decomposition of cellulose and hemicellulose [1]. The peak in the decomposition rate at lower temperatures can be associated with pyrolysis of hemicellulose and the peak at higher temperatures is associated with cellulose decomposition. A peak due to lignin decomposition can not be observed. Therefore, the integral result of the merged peaks does not promote a reliable analysis of the decomposition kinetics of the major constituents. Assuming that cellulose, hemicellulose and lignin decompose independently, pretreatment in conjunction with separation of the major components may allow investigating into the decomposition of the components. However, commonly applied separation techniques to reduce the complex structure of wood include extensive depolymerisation and structural changes, which may lead to kinetic parameters that do not represent the decomposition of wood. In recent studies [2 /5], it was found that using model substances (such as xylan, Avicel cellulose, filter pulp, or Klason lignin) as substitute compounds can lead to errors for wood pyrolysis when described with formal kinetic parameters obtained from these substitute compounds. From earlier studies [6 /15], it is well known that the addition of inorganic salts to wood samples result in a wide variety of changes in the pyrolysis process. The main intent of the early studies (e.g. [9,14]) was, to investigate the fire-retardant characteristics of organic and inorganic chemicals for the pyrolysis and combustion of wood. Today, the influence of inorganic salts on the pyrolysis of wood is studied
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with multiple aims: e.g. to increase the yield of charcoal, to develop analytical pyrolysis as a quantitative method for the analysis of pulps [10], to dispose heavy metal containing wood waste [16], to increase the yield of valuable fine chemicals from biomass pyrolysis, and to provide a model that comprises the influence of inorganic salts (ash) on the pyrolysis kinetics. The objective of this study is to investigate the influence wood’s naturally occurring inorganic salts on the temperature of pyrolysis and on the product distribution, and to determine the decomposition kinetics of the three wood species, hornbeam (Carpinus betulus), walnut (Juglans regia ), and scots pine (Pinus sylvestris ) by incorporating the thermal degradation kinetics of the main components (lignin, hemicellulose, and cellulose). The kinetic measurements were performed by online thermogravimetry/mass spectrometry (TG/MS) [17] and by isothermal measurements [18,19]. The comparison of the thermal degradation behaviour of the main components of the different wood species allows to identify differences and similarities of the main components during pyrolysis. Also, a better understanding of the relationship between the chemical structure and the pyrolysis behaviour of the individual components can be achieved.
2. Experimental 2.1. Material The wood species used in this study are hornbeam (C. betulus), walnut (J. regia ), and scots pine (P. sylvestris ), each obtained from a single log from the region of Karlsruhe. The bark free wood was ground to a size range less than 250 mm. The composition of the material is given in Table 1. Prior to the experiments, the samples were dried 3 h at 105 8C. Each wood sample was also washed with cold-water to
Table 1 Chemical composition of the wood speciesa
Ash (500 8C)b Hot-water extractc Ligninc Hemicellulosec Xylose Mannose Cellulosec a b c
Hornbeam
Walnut
Scots pine
0.7 7.7 20.1 23.3 20.0 0.6 48.9
0.7 4.1 25.9 22.1 17.2 1.4 47.8
0.3 2.7 26.7 20.8 5.0 13.0 49.8
All values in weight percent. Percentage on dry wood. Percentage on extracted dry wood (cyclohexane, ethanol) and cold-water-washed wood.
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reduce the amount of inorganic salts and treated with dilute sulphuric acid (4 wt.%) at 100 8C to eliminate the hemicellulose. The samples were doped by dissolving the analytically pure metal salts in distilled water and adsorbing this dissolution to the cold-water-washed wood. The resultant slurry was thoroughly stirred and dried at 90 8C with stirring at times to prevent crystallisation. The cation content was measured by heating 3 g of wood for 5 h at 500 8C in air and dissolving the ash with 10 ml of concentrated hydrochloric acid. Then, it was diluted to exactly 50 ml with double-distilled water. The quantitative measurements were carried out using atomic adsorption spectrometry (Perkin / Elmer AAS 4100).
2.2. Fractionated pyrolysis To investigate the pyrolysis products, fractionated pyrolysis under dynamic conditions was carried out. A sample of about 100 mg was heated up with 5 8C min 1 in a helium-purged (100 ml min 1) oven. The volatile pyrolysis products were collected in five cold traps. After pyrolysis, the condensate in the cold traps was solved in methanol and analysed with GC/MS (Finnigan MAT ITD 800) [19] and GC/FID (Varian 3400). The chromatographic parameters for GC/MS were: CP-Sil 8 CB (Chrompack) column (column length: 50 m; I.D.: 0.32 mm; film thickness 0.12 mm); injector temperature 250 8C; oven temperature program: 40 8C at 5 min constant, 10 8C min 1 to 280 8C at 20 min constant. Those for the GC/FID apparatus were: HP-5 (Hewlett /Packard) column (column length: 30 m; I.D.: 0.53 mm; film thickness 0.88 mm); injector temperature 250 8C; detector temperature 300 8C, and the oven temperature program was the same as for GC/MS.
2.3. Thermogravimetric and isothermal apparatus Thermogravimetric experiments were carried out with a thermobalance (DuPont 951 thermogravimetric analyser) coupled to a quadrupole mass spectrometer (Balzers-QMG 420). Pure helium with a flow rate of 100 ml min 1 and sample sizes of about 25 /27 mg were used. A detailed description of the experimental set-up is given in [17]. The isothermal measurements were performed in a closed-loop-type reactor with isothermal and homogeneous conditions with MS online gas analysis (Balzers QMG 421). The thermal degradation was investigated in helium at 0.1 MPa with a flow of 25 ml min 1. Sample sizes of 30 mg were used. A detailed description of this experimental set-up is given elsewhere [18,19]. As shown in [20] these sample sizes should be small enough to prevent heattransfer effects in isothermal and dynamic experiments.
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Fig. 1. TG and DTG curves of chemically treated hornbeam wood; heating rate b /10 8C min 1. Bold lines and filled symbols represent TG curves, thin lines and open symbols represent DTG curves. /' /, untreated; /m /, water-washed; /j /, acid-washed.
3. Results and discussion 3.1. Thermal decomposition of wood Fig. 1 shows the measured TG curves and the negative first derivatives of the thermogravimetric (DTG) curves of differently treated hornbeam wood (heating rate b / 10 8C min 1). The DTG curve of untreated and water-washed wood reveals two decomposition regimes: The peak at higher temperatures is mainly due to the decomposition of the cellulose and the shoulder, at lower temperatures, can be attributed to the decomposition of the hemicellulose. A peak due to the lignin degradation cannot be observed. The mineral salts, present in native wood, were largely removed by washing the wood with cold-water. In Table 2, the cation content of untreated and water-washed wood is given. The TG curve of water-washed hornbeam wood is shifted to higher temperatures compared with the TG curve of untreated wood. Within each species, the inorganic salt content mainly depends on the growing location of the tree. It is known that the ash content of coniferous trees Table 2 Cation content of untreated and cold-water-washed wood mass percentage on dry wood Cation
Na K Mg Ca Fe
Hornbeam
Walnut
Scots pine
Untreated
Washed
Untreated
Washed
Untreated
Washed
0.0003 0.145 0.021 0.180 0.0013
B/0.0001 0.004 0.015 0.091 0.0002
0.0025 0.184 0.083 0.048 0.0022
B/0.0001 0.002 0.053 0.048 0.0008
0.0019 0.010 0.015 0.061 0.0028
B/0.0001 0.0002 0.008 0.045 0.0006
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is usually smaller than the ash content of deciduous trees [21]. Untreated coniferous wood often decomposes at higher temperature ranges in TG measurements [22]. In the case of scots pine, the water washing hardly shifted the TG curve, whereas the walnut and the hornbeam curve were shifted to higher temperatures by as much as 10/15 8C (heating rate 10 8C min 1). As a consequence of this significant effect of inorganic salts, water washing is essential for the comparison of the decomposition of different wood species and also for the comparison of the same wood species from different locations. For pinewood, a distinction of heartwood and sapwood was made. The chemical analysis with regard to the composition and the amount of metal salts resulted only in very small differences. The measured TG curves were similar (also the ones of untreated wood) within experimental error. The different amount of secondary metabolites (e.g. resinous materials, phenolics (except lignin), stilbenes, and quinones) deposited in the heartwood showed no influence on thermochemical behaviour. To identify the volatile pyrolysis products, fractionated pyrolysis experiments in the temperature region from 100 to 500 8C were carried out. The GC/MS analysis of the collected volatile products gave about 50 different main products; the most abundant peak in all samples is acetic acid. The product peaks were assigned by reference compounds and by literature data (retention times and mass spectra) [23 / 27]. Apart from the products that can be attributed to the cellulose and hemicellulose pyrolysis, about 20 main monomer lignin products (hornbeam 24, walnut 22, scots pine 12) were identified. They were classified into catechol, guaiacol, and syringol derivatives. As expected, there were no syringol derivatives found in pine wood pyrolysis vapours. In Fig. 2 the GC/FID spectra of water-washed hornbeam pyrolysis products are shown. Product peaks with retention times (RT) lower than
Fig. 2. GC/FID chromatrogram of the volatile pyrolysis products of water-washed hornbeam wood. Numbers refer to Table 3.
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Table 3 Volatile pyrolysis products of cold-water-washed hornbeam wood Number
Compound
RT (s)
MW
Referencea
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Butanedial 2- or 3-Pentenoic acid methyl ester 2-Hydroxy-butanedial 2-Furaldehyde 2-Furfuryl alcohol (5H)-Furan-2-one Dihydro-methyl-furanone C6H8O2 5-Methyl-2-furaldehyde 4-Hydroxy-5,6-dihydro-(2H)-pyran-2-one Guaiacol Tetrahydro-4-hydroxy-pyran-2-one 4-Methyl-guaiacol 3-Methoxy-catechol Anhydro-pento-furanose 4-Vinyl-guaiacol Syringol Unknown 4-Methyl-syringol Levoglucosan 4-Vinyl-syringol 4-Propenyl-syringol (trans) Syringyl acetone Sinapaldehyde
147 165 192 260 320 425 436 490 497 545 652 660 758 834 870 880 912 941 992 1068 1085 1184 1234 1370
86 114 102 96 98 84 98 112 110 114 124 116 138 140 144 150 154 172 168 162 180 194 210 208
Lit(MS) Lit(MS) Lit(MS) R R R Lit(MS) R Lit(MS) R (MS) Lit(RT/MS) Lit(RT/MS) Lit(MS) Lit(RT/MS) Lit(RT/MS) Lit(MS) Lit(RT/MS) R Lit(RT/MS) Lit(RT/MS) Lit(RT/MS) Lit(RT/MS)
Originb
H H/C H/C H H/C H/C H/C H/(C?) L H L L H L L H L C L L L L
a
R, assignment confirmed by reference compound; Lit(MS), assignment according [23 /27] based on mass spectra; Lit(RT/MS), assignment according [23 /27] based on mass spectra and RT; (MS), assignment according mass spectra. b H, mainly hemicellulose derived product; C, cellulose derived product; L, lignin derived product.
140 s (acetic acid, hydroxyacetone) overlap with the solvent and cannot be seen in this figure. Selected peaks are numbered and identified in Table 3.
3.2. Effect of inorganic salts Inorganic salts influence the pyrolysis temperature and the composition of the products [6 /16]. In Fig. 3, the DTG curves (heating rate b / 10 8C min 1) of doped hornbeam wood with 0.5 wt.% cation content and chlorine as anion are shown. It can be seen that the potassium ion, one of the most abundant ions that naturally occur in native wood, and as well the sodium ion have a strong influence on the pyrolysis temperature, and the DTG curve is shifted to lower temperatures. In the case of calcium doped wood the shoulder at lower temperatures is more pronounced and the degradation of the hemicellulose starts even at lower temperatures than in the case of sodium and potassium doped wood. The peak at about 360 8C, which can
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Fig. 3. DTG curves of cation-doped hornbeam wood; 0.5 wt.% cation; anion: chloride; heating rate b / 10 8C min 1. /j /, water-washed; /m /, Na doped; /' /, K doped; /" / Ca2 doped.
be mainly attributed to the cellulose decomposition, is hardly shifted, compared with water-washed wood. Not only the cations are responsible for the decrease in pyrolysis temperature as can be seen in Fig. 4. Potassiumchloride exhibits the strongest effect. Compared with water-washed wood, potassium bicarbonate doped wood shows no difference in pyrolysis until 300 8C. In Table 4 the relative distribution of the pyrolysis products from doped hornbeam wood compared with water-washed wood is shown. The components 3, 6, 10 and 12 are mainly of hemicellulose origin, the components 4, 5, 7 and 8 are of hemicellulose and cellulose origin, and the component 20 is only produced during cellulose decomposition. The calcium treated sample showed a similar product distribution as water-washed wood. Only the amount of component 8 increases
Fig. 4. DTG curves of anion-doped hornbeam wood; 0.5 wt.% potassium; heating rate b /10 8C min 1. /j /, water-washed; /m /, Cl doped; /" /, SO2 doped; /' /, HCO3 doped. 4
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Table 4 Product distribution of differently treated hornbeam wood (component numbering refers to Table 3) Treatment
Washed CaCl2 NaCl KCl K2SO4 KHCO3
Peak area of componenta 3
6
10
12
4
5
7
8
20
16 145 0.92 0.17 0.23 0.19 0.61
11 235 0.54 1.68 2.15 3.39 1.40
71 938 0.70 0.32 0.41 0.42 0.61
13 200 0.41 3.54 4.85 4.83 1.72
31 737 0.65 0.56 1.05 1.98 0.76
7073 0.37 1.21 1.98 4.20 1.21
18 582 1.27 1.12 3.03 3.98 1.32
2321 7.46 0.31 0.45 0.73 0.31
71 869 1.13 0.27 0.18 0.35 0.50
a In the case of water-washed wood the absolute peak area in mVs for each component is given; in the case of the inorganic salt doped samples, the relative peak area compared with washed wood is presented.
intensively, especially compared with the other inorganic salts, where this component is nearly not present. The alkaline metals have all in common that the yield of levoglucosan (component 20) is reduced to less than half. In the case of the components that are originating from hemicellulose, component 3 and 10 are decreasing while component 6 and 12 are increasing, in the presence of alkaline metal salts. Although the DTG curves of NaCl and KCl-treated samples are similar, a difference in the product distribution can be noticed, especially for component 12, 4 and 7. The sulphate has a more pronounced effect on the studied components than the bicarbonate. The yield of levoglucosan in the presence of inorganic salts during wood pyrolysis has been studied earlier [10,12]. Despite of the fact, that different wood species (thermochemical pulp produced from Norway spruce) and different pyrolysis conditions (pyrolysis /GC/MS at 620 8C) were applied [10], they also observed, that the calcium ion has only a minor effect on the product distribution and that sodium prevents the formation of anhydrosugar-type compounds and increases the relative abundance of low molecular weight compounds (such as aldehydes, ketones and furanones). The influence of different anions (bicarbonate and sulphate) with sodium as cation was also studied, but contrarily to the results presented here with potassium as cation, the bicarbonate had a more pronounced effect than the sulphate. The influence of cations on the yield of levoglucosan studied for milled cottonwood sapwood under vacuum pyrolysis conditions at about 300 8C showed, that the potassium ion and the lithium ion drastically reduce the yield of levoglucosan, while calcium only has a small reducing effect [12]. Although different materials at different experimental conditions were examined, the observed trends are the same: cations and anions are responsible for the pyrolysis temperature and as well for the product distribution. A mechanistic explanation for the observed effects cannot be given, because even the pyrolysis mechanism of levoglucosan formation from pure cellulose is discussed contradictionary by [13,14]. To circumvent the disadvantage of inorganic salts and to minimise the ratio of inorganic salts, only formal kinetic parameters obtained from cold-water-washed wood should be used.
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3.3. Evaluation of formal kinetic parameters for wood pyrolysis In TG-experiments with low heating rates, the resultant DTG curve for wood pyrolysis exhibits a peak with a shoulder (Fig. 1). Depending on the type of wood, this shoulder is more or less pronounced. With the assumption, that the three main components (hemicellulose, cellulose, and lignin) of wood decompose independently, at least three parallel reactions have to be considered. For chemical controlled reactions, Eq. (1) has to be solved. da dt
X i
ci ×
dai dt
X
ci × ki (T)×(1 ai )ni
(1)
i
where a is the degree of conversion, which is defined as a /(m0 /m )/(m0 /m) with m0 being the initial mass, m the actual mass, and m the final mass, k is the rate coefficient, n the apparent reaction order, and c corresponds to the amount of volatiles produces by the ith component of a unit mass of sample. Using an Arrhenius-type of temperature dependency for the rate coefficient k (T ) /k0 exp( / Ea/RT) with Ea as apparent activation energy, and introducing the heating rate b / dT /dt, equation 1 is transformed into Eq. (2). Ea;i da X k ci × 0;i e RT ×(1 ai )ni dT b i
(2)
If no further assumptions are made, at least 12 parameters (i /3) have to be determined from one single TG curve. To circumvent this disadvantage, model substances (such as xylan for the hemicellulose of deciduous trees, Avicel cellulose or filter pulp for the cellulose, and Klason lignin for the lignin component of wood) were used to evaluate the formal kinetic parameters individually. However, as already noted in [2 /5], this can lead to errors, when describing wood pyrolysis with formal kinetic parameters obtained from these model substances. In Fig. 5, the mass-loss curves for model substances are shown. The two commercially available xylans (from beechwood, Sigma, and from birchwood, Fluka) exhibit different degradation curves and different char yields (35 wt.% for birch, and 40 wt.% for beech xylan at 500 8C, b /10 8C min 1). These high char yields indicate that the thermal degradation of wood’s hemicellulose is different to the studied xylans as can be concluded from the following consideration: TG experiments of water-washed hornbeam wood yielded about 12 wt.% char (Fig. 5). Hornbeam wood consists of about 50 wt.% cellulose, 20 wt.% lignin, and 30 wt.% hemicellulose (including the hot-water extract). Pure cellulose normally gives at least 2 wt.%, e.g. [2,5] char and according to [15] milled wood lignin (MWL) at least 30 wt.% char (about 65 wt.% char for Klason lignin [28]). This consideration results in a minimum char yield of about 7 wt.% for the cellulose and lignin fraction. Therefore, the char contribution of the hemicellulose has to be less than 5 wt.%, resulting in an maximum char yield for hemicellulose of about 17 wt.% (or 22 wt.% if it is believed that the hot-water extract decomposes without residue).
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Fig. 5. TG mass-loss curves of model substances; heating rate b /10 8C min 1. / /, water-washed hornbeam; /m /, xylan (beech); /j /, xylan (birch); /^ /, cellulose (Fluka); /2 /, Avicel PH-105 [2]; /\ /, Whatman #42 [2].
As already observed in [2], pure cellulose samples are not alike. The inflection points of the pure cellulose samples (Fig. 5) are ranging between 340 and 373 8C (b /10 8C min 1). This very wide temperature range is not suitable for an accurate description of wood’s cellulose degradation. Therefore, the use of a special cellulose sample for the description would be somewhat arbitrary. In the following sections, a method that allows the individual evaluation of formal kinetic parameters for the main components of wood is presented. 3.3.1. Hemicellulose degradation The hemicellulose can be eliminated to a large extent by treating the wood sample with hot dilute sulphuric acid. The weight-loss curve for the hemicellulose can be obtained by subtracting the TG curve of acid-washed wood from the TG curve of water-washed wood taking into account the weight loss for the acid treatment. This contains the hemicellulose part plus the hot-water extract given in Table 1. The resulting curves (Fig. 6) for all three wood species show two decomposition steps, that were used separately for the evaluation of the overall kinetic parameters. For this equation 2 (with i /1 and c /1) is integrated with a fourth-order Runge /Kutta method for given sets of parameters Ea, k0, and n . The minimum of squares of deviation between the degree of conversion from the integrated rate expression and the measured degree of conversion is directly searched [17]. The results for the two degradation steps (hemicellulose 1 and hemicellulose 2) are given in Table 5. As seen from Fig. 6, the first step of the hemicellullose degradation is nearly identical for the two deciduous trees, whereas the coniferous tree shows a different behaviour. The hemicellulose of deciduous woods consists mainly of xylan whereas coniferous woods contain large amounts of mannan (Table 1). In the case of the studied deciduous trees, only the mass ratio of the first step of the hemicellulose degradation to the second degradation step differs. In all three wood species, the second degradation step of the hemicellulose is nearly identical. The mass ratio that
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Fig. 6. Calculated TG and DTG curves for hemicellulose weight-loss obtained by subtraction of the curves of water-washed and acid-washed samples; heating rate b /5 8C min 1. Bold lines and filled symbols represent TG curves, thin lines and open symbols represent DTG curves. /m /, hornbeam; /' /, scots pine; /j /, walnut.
Table 5 Mean values (b /2, 5, and 10 8C min 1) and 95% confidence intervals in brackets for the formal kinetic parameters calculated from dynamic experiments for the three wood species Ea (kJ mol 1)
Log k0(min 1)
n
Hornbeam Hemicellulose 1 Hemicellulose 2 Lignin Lignin (isothermal) Cellulose 1 Cellulose 2
163 (9/2) 257 (9/5) 99 (9/2) 105 (9/1) 93 (9/8) 181 (9/8)
14.6 (9/0.2) 20.4 (9/0.3) 7.8 (9/0.2) 8.2 (9/0.1) 8.5 (9/0.7) 14.5 (9/0.7)
1.7 0.8 1.3 1.4 0.8 0.9
(9/0.1) (9/0.1) (9/0.1) (9/0.1) (9/0.1) (9/0.1)
Walnut Hemicellulose 1 Hemicellulose 2 Lignin Cellulose 1 Cellulose 2
175 (9/3) 262 (9/1) 95 (9/6) 99 (9/7) 183 (9/5)
15.8 (9/0.3) 20.9 (9/0.2) 7.4 (9/0.5) 9.3 (9/0.7) 14.7 (9/0.5)
1.8 0.8 1.4 0.9 0.8
(9/0.1) (9/0.1) (9/0.1) (9/0.1) (9/0.1)
Scots pine Hemicellulose 1 Hemicellulose 2 Lignin Cellulose 1a Cellulose 2a
101 (9/1.2) 262 (9/1) 93 (9/4) 93 181
8.1 (9/0.1) 20.9 (9/0.1) 7.1 (9/0.4) 8.5 14.5
0.7 (9/0.1) 0.8 (9/0.1) 1.4 (9/0.1) 0.8 0.9
Additionally, the results of the isothermal measurements for hornbeam lignin are given. a The experimental curves are described within experimental error with parameters calculated for hornbeam wood.
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is degraded during the first and second hemicellulose step cannot be attributed to a weight proportion of a special sugar nor to a group of sugars belonging to the hemicellulose. The higher thermal stability of pine hemicellulose is possibly due to the different hemicellulose structure. The char yield for the hemicellulose is found to be largely independent of the heating rate applied (2 /10 8C min 1) and it is determined to be 11.0, 10.2 and 10.1 wt.% for the hemicellulose of hornbeam, walnut, and scots pine, respectively. 3.3.2. Lignin degradation Because lignin cannot be separated from wood without dramatically changing the chemical structure, nor was it the intent to use model substances, ion fragments of lignin-derived products were used to study the lignin degradation by means of the evolution of monomers. The GC/MS analysis of the gaseous pyrolysis products showed that the monomer lignin products give a different ion fragmentation than the products originating from the degradation of cellulose and hemicellulose, especially in the mass region from 105 to 212 amu. Forty-two ion fragments (hornbeam) from this region can be used to identify the lignin degradation for hornbeam wood (39 for walnut and 25 for pine). The degree of conversion a can be calculated with the determined ion fragments by using equation 3. T
g
x X C
a(T) T 100 T 500 C x X
g
T 100 C
Il;i (T)Ml;i (T)dT
i 1
(3) Il;i (T)Ml;i (T)dT
i 1
where x is the number of found lignin ion fragments, Il,i is the ion current of the ith lignin ion fragment, and Ml,i is the molecular mass of the ith lignin ion fragment. By using the same method for the evaluation of the overall formal kinetic parameters as described for the hemicellulose degradation, the results given in Table 5 were obtained. Additionally, isothermal experiments were performed with hornbeam wood to validate the kinetic parameters from dynamic measurements. The main advantage of isothermal measurements is that changes in the mechanism are detectable because decomposition rates are obtained for single temperatures, contrarily to dynamic measurements where only one value for the total temperature range is obtained. Because the residue is also a function of temperature up to 320 8C, only measurements in the temperature range from 320 to 390 8C were used to study the lignin degradation by means of specific lignin ion fragments. The rate coefficients were determined as described in [18] and they are summarised in an Arrhenius-plot in Fig. 7. From the Arrhenius-plot, an apparent activation energy and a preexponential factor are obtained by linear regression (see Table 5). From these measurements no indication of a change in the mechanism of the evolution of lignin-
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Fig. 7. Arrhenius-plot of the degradation of lignin from water-washed hornbeam wood.
derived monomers during wood pyrolysis is obvious (Fig. 8) and the apparent order of reaction n is equal at 1.4. The formal kinetic parameters obtained from dynamic experiments are in good agreement with the ones obtained from isothermal measurements. Both experiments resulted in an apparent order of reaction that is approximately 1.3. The fractionated pyrolysis experiments (for the two deciduous woods) showed that the product distribution of lignin monomers changes with temperature. At lower temperatures (100 /290 8C, b /5 8C min 1), mainly the high molecular weight syringol derivatives (sinapaldehyde and 4-propenylsyringol) were found, whereas at high temperatures (350 /400 8C, b /5 8C min 1) catecol, 3-methoxycatecol, syringol 4-methylsyringol, and coniferaldehyde and only very little of the ones mentioned before were found. Guaiacol derivatives mainly evolved at middle and high temperatures. The lignin degradation can, therefore, be considered as a set of several parallel reactions, resulting in overall reaction orders higher than one.
Fig. 8. Apparent order of reaction of the degradation of lignin from water-washed hornbeam wood.
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Isothermal experiments at 900 8C in a steam-helium atmosphere with milled spruce wood showed that the evolution of major pyrolysis products from lignin varies with reaction time. The evolution of low-molecular-weight species like methane, formaldehyde and methanol coincidences with the evolution of major lignin derived monomers [29]. Although, the lignin degradation mechanism at high temperatures may be different from these observed for low heating rates in dynamic experiments, the observation of parallel lignin degradation reactions is supported according to [29]. Contrarily to E. Jakab et al. [15], but in accordance with Domburg et al. [30], the release of monomeric products is dependent on the lignin type. The coniferous lignin is thermally more stable than the deciduous lignins of hornbeam and walnut (see Table 5). The char residue for the lignin degradation was calculated from dynamic measurements of acid-washed wood, assuming that the char residue for the cellulose degradation is identical for all three wood species, because of its identical chemical structure. The char residues are found to be 32.3, 32.3 and 36.9 wt.% for hornbeam, walnut, and scots pine, respectively. In [15] TG/MS experiments with MWL from 16 different plants or trees were carried out. Although no kinetic parameters are given, the final char yields at 850 8C for a heating rate of 20 8C min 1 are listed. Lignin obtained from spruce wood (Picea abies ) gives 35.3 wt.% and lignin from walnut shells (J. regia ) gives 30.6 wt.% char. In spite of slightly different woods or part of the trees, the char residues presented in this paper are in good agreement with [15].
3.3.3. Cellulose degradation The curve representing the cellulose degradation is obtained by subtracting the measured lignin curve from the curve of acid-washed wood using the degradable amounts. The resulting weight-loss curve shows a minor mass loss of 5 /10 wt.% in the low temperature range (200 /270 8C for b /5 8C min 1) and a main degradation step at high temperatures (280 /390 8C, b /5 8C min 1) for all three wood species. The degradation at lower temperatures is labelled cellulose 1 and the degradation at higher temperatures is labelled cellulose 2. The calculated formal kinetic parameters for the three different wood species are given in Table 5. Due to the incomplete separation of the two degradation steps of cellulose and the very low amount of cellulose 1 (about 3 /4 wt.% for whole wood), the calculated formal kinetic parameters for cellulose 1 are not very suitable for further kinetic discussion. It is possible that this curve belongs to other main components of wood; likely to the hemicellulose caused by incomplete solution of the hemicellulose. However, fractionated pyrolysis experiments with acid-washed hornbeam wood (heating rate b /5 8C min 1) indicate that this portion (cellulose 1) belongs to the cellulose degradation, because of the levoglucosan found in the first cold trap (temperature range 100 /290 8C). The parameters for cellulose 2 lie within literaturegiven parameters for pure cellulose (log k0 (min 1) /14/20, Ea /190 /250 kJ mol 1, with a first-order kinetic model) [1,2,31], but as mentioned already, these parameters cover a broad temperature range.
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The char residue for the cellulose degradation is determined to 3 wt.%. For cellulose model substances char yields were found to be in the range of 2 /10 wt.%, depending on the chemical nature of the substrates and on the heating rate [1]. 3.3.4. Combination of the determined formal kinetic parameters The measured dynamic TG-mass-loss curves of water-washed wood (with heating rates ranging from 2 to 10 8C min 1) are very well described with the five given formal kinetic parameters from the thermal decomposition of the single components (Figs. 9 /11). The degradable mass fractions used for applying the rate expression of the single components are given in Table 6. Contrarily to Groenli [32], who modelled the pyrolysis of different wood species with three to five parallel reactions, the formal kinetic parameters presented here were evaluated without the assumption of first order kinetics and they were obtained from the individual components of wood.
4. Conclusion Inorganic salts have a strong influence on the temperature of pyrolysis as well as on the product distribution. The alkaline metal chlorides studied, strongly decrease the temperature of pyrolysis, whereas calcium chloride mainly influences the low temperature degradation regime. The influence on the pyrolysis temperature is also dependent on the anion. The influence is increasing in the following row: bicarbonate B/sulfate B/chloride. In the case of the product distribution, this trend can only be observed for the yield of levoglucosan. In the presence of calcium chloride a C6H8O2-component was found in high abundance, but it could hardly be observed without calcium. In order to compare the pyrolysis behaviour of different wood species, the inorganic salts naturally occurring should be removed. A different
Fig. 9. Comparison of calculated curves (solid lines) from the presented parameters of hornbeam wood with the measured ones (symbols) at (m) 2 8C min 1, (j) 5 8C min 1 and (') 10 8C min 1. Bold solid lines and filled symbols represent weight-loss curves, thin solid lines and open symbols represent DTG curves.
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Fig. 10. Comparison of calculated curves (solid lines) from the presented parameters of walnut wood with the measured ones (symbols) at (m) 2 8C min 1, (j) 5 8C min 1 and (') 10 8C min 1. Bold solid lines and filled symbols represent weight-loss curves, thin solid lines and open symbols represent DTG curves.
Fig. 11. Comparison of calculated curves (solid lines) from the presented parameters of scots pine wood with the measured ones (symbols) at (m) 2 8C min 1, (j) 5 8C min 1 and (') 10 8C min 1. Bold solid lines and filled symbols represent weight-loss curves, thin solid lines and open symbols represent DTG curves.
Table 6 Degradable mass fractions in percent used for the calculation of the residual curves
Hemicellulose 1 Hemicellulose 2 Lignin Cellulose 1 Cellulose 2
Hornbeam
Walnut
Scots pine
24.5 6.9 15.3 4.3 49.1
17.0 10.0 20.0 3.7 49.3
14.7 9.8 19.5 3.9 45.1
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inorganic salt content or different anions or cations will result in different TG curves even for the same wood species. Formal kinetic parameters obtained from model substances are not very reliable if they are adopted to the description of wood pyrolysis. Pure, ash-free celluloses and xylans from different manufacturers manifest remarkably different pyrolysis temperatures. The chemical structure of MWL is thought to be similar to that of lignin in native wood, but the higher thermochemical stability of coniferous lignins could not be observed with MWL in [15]. The char residues of lignin obtained in this study are similar compared with MWL. From the degradation curves of the main components of wood, the shoulder of the DTG peak (Fig. 1) at lower temperatures can be mainly attributed to the hemicellulose degradation, but lignin, cellulose, and the second step of the hemicellulose degradation contribute to the peak at higher temperatures. The presented formal kinetic parameters are suitable for the description of the pyrolysis of the studied wood species over the investigated heating rate range. The differences in wood species are mainly due to the different thermochemical behaviour of lignin degradation and that of the first step of the hemicellulose degradation.
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