Influences of the Chemical State of Alkaline Compounds and the

Feb 10, 2009 - (1) Indigenous or added inorganic matter, depending upon their specific ... It is known that alkali compounds in lignocellulosic fuels ...
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Ind. Eng. Chem. Res. 2009, 48, 3359–3369

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Influences of the Chemical State of Alkaline Compounds and the Nature of Alkali Metal on Wood Pyrolysis Colomba Di Blasi* and Antonio Galgano Dipartimento di Ingegneria Chimica, UniVersita` degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy

Carmen Branca Istituto di Ricerche sulla Combustione, C.N.R., P.le V. Tecchio, 80125 Napoli, Italy

Motivated by the production of compounds of good industrial value, pyrolysis of fir wood catalyzed by alkaline compounds (NaOH, KOH, Na2CO3, K2CO3, KC2H3O2, and NaCl) has been investigated. Catalysts have been impregnated in wood (preliminarily extracted with water) by means of aqueous solutions resulting in concentrations of the K or Na ion in wood of about 0.37-0.41%. Pyrolysis experiments have been done with a fixed-bed reactor preheated at 800 K.The decomposition process is anticipated at lower temperatures with conversion times from 2.5 (NaOH) to 1.7 (NaCl) times shorter. Formation of char, carbon dioxide, and water is favored with total yields between 70 and 61% versus 48% of extracted wood (dry sample mass basis). The yields of carbon monoxide are increased approximately from 4% to 6% while the yields of organic compounds are lowered to 19-29% (versus 43%) with the disappearance of sugar compounds and a strong diminution in other typical products of uncatalyzed wood pyrolysis but hydroxypropanone. However, sodium and potassium hydroxides increase the yields of some minor carbohydrate compounds (1-hydroxy-2butanone, 2-methyl-2cyclopentenone, 3-ethyl-2-hydroxy-2-cyclopentenone, and 3-methyl-2-cyclopentenone) by factors of 4-6 (yields of about 0.8-0.12%). The yields of total phenols are also increased. In particular, NaOH is slightly more effective for the production of guaiacol, cresols, and 4-ethylguaicol (factors of increase between 3 and 4 with yields of 0.3-1.2%), whereas KOH is slightly better for the production of phenol, cis-isoeugenol and transisoeugenol (factor of increase of 2-6 with yields between 0.7 and 0.2%). Finally, the production of furfuryl alcohol can be augmented up to factors of about 15 by potassium hydroxide or carbonate (yields up to 0.6%). Introduction The basic components of biomass are cellulose, hemicellulose, and lignin with small amounts of extractives and inorganic compounds. It is difficult to use these polymers directly as chemical feedstocks without adequate treatments. Pyrolysis is a thermochemical conversion technology apt to convert the biomass into three main classes of products (gas, liquids, char) that, in addition to their energetic exploitation, can be the source of bioproducts such as chemicals and activated carbon. Yields and composition of pyrolysis products are affected by the conversion conditions and the chemicophysical properties of the biomass.1 Indigenous or added inorganic matter, depending upon their specific properties, can modify the activity and selectivity of both primary decomposition reactions, taking place in the solid phase (biomass), and secondary decomposition reactions of organic vapor-phase products. However, while the understanding of the catalytic decomposition of vapor-phase products of biomass pyrolysis can rely on a significant amount of fundamental and applied research, motivated by the development and optimization of the gasification plants, the role of catalysts in primary decomposition has received much less attention apart from the use of additives in very high concentrations to increase the resistance to fire of lignocellulosics.2,3 It is recognized that catalytic pyrolysis can contribute significantly for the development of profitable biorefineries4 by modifications in the decomposition paths which can drive organic products toward specific classes. Moreover, to maximize the yields of specialty chemicals, it is of paramount importance to limit the * Corresponding author. Tel: 39-081-7682232. Fax: 39-081-2391800. E-mail: [email protected].

occurrence of secondary cracking reactions, that is, to establish conditions resembling those of fast pyrolysis (reaction temperatures below 750 K and short residence times of the vaporphase products).5 While the effects of indigenous inorganic matter on the yields of sugars and light oxygenated compounds, generated from decomposition of holocellulose components, have been investigated,6-8 studies about the deliberate addition of various alkaline cations to lignocelulosic materials by means of physical sorption9-21 do not provide a detailed analysis of the qualitative and quantitative modifications in the spectrum of these organic products. It is known that alkali compounds in lignocellulosic fuels always cause an increase in the yields of char, water, and gas with a reduction in the liquid products. A reduction in the yields of 2-furaldehyde9,10,12,20 and an increase in hydroxypropanone9,21 or furfuryl alcohol10 are also reported. In general, the molecular weight of tar is reduced,15 most likely as a consequence of the catalytic action of cations on the secondary cracking reactions.15,17 On the other hand, the char structure puts into evidence the activity of vapor-phase condensation.13 It is also shown that basic alkaline agents increase the thermal reactivity of lignin and cause ether and C-C bonds cleavage yielding volatile phenols.22,23 However, the results by different authors, reviewed by Amen-Chen et al.,24 and those of more recent studies20,21 are not in concord or do not give sufficient information about the enhancement in the production of specific phenols and the total phenolic yield. The differences in the type of alkaline additive, the experimental conditions, and the substrate do not allow a quantitative comparison among the findings of the various studies. The effects associated with

10.1021/ie801468y CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

3360 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 1. Properties25 of the Alkaline Catalysts catalyst

mol wt (kg/kmol)

density (293 K) (kg/m3)

solubility (293 K) (kg/kg of H2O)

melting point (K)

fusion heat (kJ/kg)

boiling point (K)

NaOH KOH Na2CO3 K2CO3 KC2H3O2 NaCl

40 56.1 106 138.2 98.2 58.4

2130 2040 2530 2290 1578 2163

1.09 1.12 0.22 1.10 2.56 0.36

595 633 1127 1170 565 1073

209 149 276 236 516

1651 1593 decomposn decomposn 1738

the nature of the alkali metal (K, Na) or its mode of occurrence in compounds used for physical sorption are not known. Moreover, as anticipated, a detailed characterization of the organic products is not provided, as needed to explore the possibility of catalytic biomass pyrolysis as a conversion technology for biorefinery development. The main aim of this study is to investigate the effects associated with the chemical state of the alkaline compounds and the nature of the alkali metal impregnated in wood on the pyrolysis process (pyrolysis characteristics, yields of the product classes, and chemical composition of the gaseous and organic products). The experiments have been made for comparable concentrations of the alkali metals in wood using six compounds. Wood samples have been preliminarily subjected to hot water extraction, to eliminate a large part of the indigenous alkali metals. Experimental Section Samples examined in this study consist of wood impregnated with the following alkaline compounds: sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), potassium acetate (KC2H3O2), and sodium chloride (NaCl), all purchased from Sigma-Aldrich. Catalyst properties25 are collected in Table 1 (molecular weight, density, water solubility, melting point, heat of fusion, boiling point). Sodium and potassium hydroxides are used as activating agents for the production of activated carbon from carbonaceous precursors (for instance, see ref 26). Sodium and potassium carbonates and sodium chloride are used as fire retardants for lignocellulosics.2,3 Both applications require concentrations of the additives much higher than those potentially useful for catalytic pyrolysis in biorefineries. Finally, potassium acetate has been used as an added catalyst for lignocellulosic pyrolysis20 or carbon oxidation.27 As in the previous studies of this research group about the effects of acidic catalysts on pyrolysis characteristics and products,28-31 fir wood has been used. The wood chemical composition is determined using the Klason method for lignin, which corresponds to 31%, a Soxhtec HT2 apparatus for extractives which are 2.6%, and calcination32 for the ash content which is 0.5% (holocellulose, computed by difference, is 66%). Before impregnation, particles (cubes 5 mm thick) are washed in hot (333 K) twice-distilled water (1 L for 100 g of wood) for 2 h under stirring, which is an effective procedure to remove a large part of alkali compounds, thus avoiding their catalytic action on the conversion process (given the low amounts of alkali metals in wood, water extraction does not cause a measurable variation in the ash content). Washed and predried samples are referred to in the following as “extracted” or subjected to “extraction”. Impregnated samples are obtained by soaking 200 g of extracted wood in an aqueous solution of 1 L for 3 h under stirring. The solution is prepared by adding 1 L of deionized water to a proper amount of catalyst to obtain the desired concentration. For comparison purposes, in all cases, the same

Table 2. Properties33 of the Aqueous Solutions Used for Wood Impregnation and Catalyst Contents in Wood

catalyst

catalyst concn in water (g/L)

pH of the aq solution

catalyst content in wood (wt %)

alkali metal content in wood (wt %)

NaOH KOH Na2CO3 K2CO3 KC2H3O2 NaCl

6.06 5.0 8.03 6.15 8.75 8.85

13.10 12.86 11.50 11.38 8.80 6.95

0.68 0.58 0.90 0.66 1.05 0.95

0.39 0.40 0.39 0.37 0.41 0.37

concentration has been considered of the alkaline metal in the aqueous solution, that is, 3.5 g of alkaline metal per 1 kg of water. The pH values of the aqueous solutions, reported in Table 2, have been evaluated using the dissociation constants.33 As expected, the hydroxides and carbonates are, respectively, strong and weak bases (Na compounds are slightly superior), potassium acetate presents a low basicity and NaCl is nearly neutral. Drying of wood particles is made by exposition to ambient air, for about 20 h, and in oven, for about 14 h at a temperature of 333 K. The dried samples are weighed after the treatment to evaluate the amount of catalyst adsorbed. Results obtained for the various catalysts are summarized in Table 2, which correspond to alkali metal concentrations in wood roughly between 0.37 and 0.41%. It is worth noting that, after 3 h of impregnation, the water content in wood remains constant and corresponds to an increase in the particle weight by about 100-120%. Assuming that, after drying, the entire amount of catalyst in water is impregnated in wood, the metal content is between 0.35 and 0.42% (on dry sample basis) which is nearly the same as the range of experimental values. However, it is not possible to ascertain whether the alkali metal concentration presents gradients along the particle thickness. The characteristics of the pyrolysis reactor (0.063 m internal diameter and 0.45 m length) have been described in the previous work of this research group,28-31 so only the essential information is provided here. Nitrogen, fed through a jacket (internal diameter 0.089 m) at the reactor top, is heated by an electrical furnace and distributed by a perforated steel plate, which also supports the bed. Temperature profiles along the reactor axis are measured by seven thermocouples, with their tips exiting from a protective steel tube, at chosen distances from the flow distributor. The lower reactor zone is isothermal at a temperature determined by a proper set point of the furnace. The temperature along the isothermal region, indicated in the following as reactor (or heating) temperature, Tr, is taken equal to 800 K during the tests. The sample (about 200 g) is placed in the feeder and, after nitrogen flushing, when the desired reactor temperature is achieved, the feeding valve is opened so that it is dropped inside the hot reactor (in about 30 s), resulting in a bed density of about 176 kg/m3. The inlet nitrogen flow at ambient temperature is 8 × 10-3 m3/min, resulting in nominal residence times of vapors along the isothermal reactor zone of 14 s. However, owing to high gradients of temperature in the reactor zone above the isothermal section of the packed bed, the activity of secondary reactions is small.

Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3361

Nitrogen and volatile pyrolysis products pass through a condensation train consisting of two water/ice-cooled condensers (with a catch pot, where the largest fraction of liquids is collected and chemically characterized), a wet scrubber, two cotton wool traps, and a silica gel bed (all connected in series). Gas sampling and analysis are carried out at selected times during the duration of the tests (15-18 samples). Steady global mass balances over the system allow the exit volumetric flow rate and mass of each gaseous species to be determined. The analysis of the gas samples (CO2, CO, CH4, H2, C2H4, C2H6) is carried out through a gas chromatograph (Perkin-Elmer Auto-System XL) equipped with TDC and a packed column (Supelco 60-80 Carboxen 1000, 15 ft) with helium as carrier gas with the same conditions as in previous work.28-31 The liquid products are subjected to chemical analysis within the day of their production. Before analysis, filtration is made with microfilters Millex-Gx of 0.45 µm. Chemical analysis is performed by GC/MS (Focus GC, ThermoElectron) with a quadrupole detector and a DB-1701 capillary column (60 m × 0.25 mm id, 0.25 mm film thickness). Helium (99.9999%) is used as carrier gas with a constant flow of 1.0 mL/min. The oven temperature is programmed from 318 K (4 min) to 508 K at a heating rate of 3 K/min and held at 508 K for 13min.The injector and the GC/MS interface are kept at a constant temperature of 523 and 508 K, respectively. A sample volume of 1 mL (4.5–25% of pyrolysis liquid in acetone) is injected. The MS is operated in electron ionization (EI) mode and a m/z range from 30 to 300 is scanned. Standard mass spectra with 70 eV ionization energy are recorded. Qualitative analysis uses the total ion chromatograms (TICs) obtained from a full scan acquisition method. The identification of the peaks is based on computer matching of the mass spectra with the NIST library or on the retention times of known species injected in the chromatographic column. For the quantitative analysis, Selected Ion Monitoring chromatograms (SIMs), obtained by monitoring only three masses corresponding to the three major fragments of each compound, are employed. Thirtyone chemical compounds are quantified which belong to the classes indicated as major carbohydrates (levoglucosan, hydroxyacetaldehyde, hydroxypropanone, acetic acid), minor carbohydrates (acetoxyacetone, 1-hydroxy-2-butanone, propionic acid, 3-ethyl-2-hydroxy-2-cyclopentenone, 2-methyl-2-cyclopentenone, 3-methyl-2-cyclopentenone), furans (2(5H)-furanone, acetylfuran, 5-methyl-2-furaldehyde, 2-furaldehyde, furfuryl alcohol), and phenols (eugenol, trans-isoeugenol, cis-isoeugenol, 4-acetonguaiacol, vanillin, guaiacol, 4-ethylguaiacol, 4-methylguaiacol, o-,m-,p-cresol, 4-propylguaiacol, phenol, 3,4-dimethylphenol, hydroquinone). Quantification is carried out by means of the internal standard method with fluoranthene as internal standard. For each of the quantified compounds, calibration lines are prepared by injection of at least four standard solutions. For sample injection and compound quantification, the concentrations range is determined by successive approximations until it is within the range of calibration line. For each compound to be quantified at least three injections are made. Finally, the water content of the pyrolysis liquid, collected from the condensation train, is determined by means of Karl Fischer titration (Crison Compact titrator) according to the standard test method ASTM E203-96. The organic fraction is

computed as difference between the measured total liquids and water so determined. Results and Discussion The results on the effects of the alkaline additives on fir wood pyrolysis are discussed by means of the gas release rate and the bed temperature versus time profiles, to gain information about the characteristic times and the actual thermal conditions of the reaction process. Also, the yields of char, gas, water, and condensable organics and the composition of the gaseous and condensable products are presented. The products of wood pyrolysis catalyzed by alkaline compounds include contributions from both substrate and catalysts as a separation between the two groups is not possible. From the catalyst properties listed in Table 1, it can be observed that potassium and sodium hydroxides and potassium acetate undergo a molten phase for temperatures of 565-633 K, which roughly are those for the onset of wood component decomposition.1 The molten phase is expected to improve the contact between reacting wood and catalyst. Other catalysts remain in the solid phase. Moreover, heating NaCl at 10 K/min up to 1073 K does not result in any weight loss.34 Carbonates are relatively stable compounds as their decomposition temperatures are above 1073 K, although, in the presence of carbon, decomposition is reported to occur around 723 K.35 They have also been detected among the solidphase products of potassium acetate decomposition36 and those of the carbonization of carbonaceous material in the presence of KOH and NaOH.26 After melting, potassium acetate decomposes above 663 K giving rise to acetone, CO2, H2O, and a solid residue containing potassium carbonate and carbon. Based on thermodynamic considerations and product analysis, it is suggested26 that, in the production of activated carbon, carbonates can be formed from reactions between the alkali hydroxides and CO2 (already possible at ambient temperature) or carbon (at temperatures above 900 K). Moreover, for temperatures above 700 K, they are also formed from reactions between carbon, alkali hydroxides in the liquid state, and water vapor.37 For each treatment with alkali compounds, a set of two to three experiments were done showing good reproducibility of the results. The case with the best mass closure is discussed in detail but the information gained from the various tests is used to carry out a statistical analysis. The chemical characterization of the liquid-phase organic compounds is made on the basis of at least three chromatographic injections for each compound which are free from evident flaws of the measuring device and/ or the operator. These are also used for statistical analysis. The statistical analysis is made in terms of relative standard deviation, ∆σ(%), to evaluate the dispersion of the experimental observations, for each set of experiments made. One-way ANOVA is applied to statistically correlate the observed effects of the impregnation of wood with various alkaline compounds. In particular, results of ANOVA are given in terms of F/Fcrit. The symbol F is used for the F ratio, that is, the ratio of the systematic variance (mean square between treatments) and the error variance (mean square within each treatment). The symbol Fcrit is used for the minimal cutoff value of a test statistic necessary to reject the null hypothesis (i.e., there are no differences due to treatments in the experimental observations). Hence, it is evident that the higher F/Fcrit is (which should always be higher than unity) the stronger the effects of the alkali metal treatment or, in other words, variations due to errors are negligible with respect those caused by the treatments. Temperatures and Times of Alkali-Catalyzed Pyrolysis. The effects of alkaline additives on the dynamics of the conversion can be seen from Figure 1 (gas release rate versus

3362 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 3. (A) Time of the Maximum Gas Release Rate, tgm, Maximum Gas Release Rate, dYgm, with the Percentage of Gas Released, Conversion Time, tc, and Global Devolatilization Rate, mtot, for the Various Catalysts (Temperatures Refer to a Bed Height of 10 cm); Relative Standard Deviation, ∆σ(%), and Results of ANOVA in Terms of the Ratio F/Fcrit. (B) Minimum Temperature, Tmin, and Corresponding Time, tmin, Conversion Time, tc, and Corresponding Temperature, Tc, Time of the Maximum Rate of Temperature Increase, tm, and Corresponding Temperature, Tm, for the Various Catalysts (Temperatures Refer to a Bed Height of 10 cm); Relative Standard Deviation, ∆σ(%), and Results of ANOVA in Terms of the Ratio F/Fcrit Temperature and Gas Release Rate Profiles (A)

Figure 1. Release rates of gas (dry sample mass basis) as functions of time for various alkaline catalysts.

extraction NaOH KOH Na2CO3 K2CO3 KC2H3O2 NaCl ∆σ(%) F/Fcrit

dYgm (s-1); % gas mtot × 103 (s-1)

tgm (s)

tc (s)

139 60 60 60 60 75 94 10.8-25.0 3.5

336 134 142 143 139 154 200 1.5-3.7 285.5

0.03; 24.2 0.16; 26.9 0.13; 24.8 0.13; 26.6 0.13; 24.8 0.12; 28.5 0.09; 28.0 1.5-8.8; 2.6-10.4 301.8; 0.5

2.2 5.0 4.7 4.7 4.7 4.5 3.4 2.2-4.0 57.2

Temperature and Gas Release Rate Profiles (B) Tmin (K) tmin (s)

Figure 2. Temperature versus time profiles, measured at a bed height of 10 cm, for various alkaline catalysts.

time) and Figure 2 (temperature, at bed height of 10 cm, versus time) for extracted wood and wood treated with various additives (some cases are excluded to avoid overcrowding). Some useful parameters obtained from the analysis of the temperature and gas release rate profiles are reported in Table 3A (the time of the maximum gas release rate, tgm, the maximum gas release rate, dYgm, with the percentage of gas released, the conversion time, tc, and the global devolatilization rate, mtot), Table 3B (the minimum temperature, Tmin, and corresponding time, tmin, the conversion time, tc, and the corresponding temperature, Tc, the time of the maximum rate of temperature increase, tm, and the corresponding temperature, Tm, and rate, hm (temperature values refer to a bed height of 10 cm)), and Figure 3A,B (range of temperatures between tmin and tc and between tc and tm, respectively). The conversion times are evaluated from the gas release rate curves and, to avoid possible uncertainties associated with the final slow zone, assumed to coincide with the time when the production of 75% of the total gas has occurred. The global devolatilization rate is defined as the ratio of the mass fraction of total volatile species produced to the conversion time. The presence of alkaline compounds does not alter the shape of the temperature profiles, although values are quantitatively different. On the contrary, the gas release rate curves (which presumably also resemble those of vapor-phase species release) present both qualitative and quantitative differences with the case of extracted wood. For the treated samples, a well-defined peak is rapidly attained which, after a rather narrow zone of very high values, is followed by a very rapid decay to nearly zero values. Instead, the extracted sample presents a very wide zone of slowly decreasing values after a barely visible maximum. It can be understood that additives highly favor the simultaneous degradation of the three main wood components, which is anticipated at lower temperatures. In particular, the

extraction NaOH KOH Na2CO3 K2CO3 KC2H3O2 NaCl ∆σ(%) F/Fcrit

561 583 593 584 592 596 569 0.1-0.3 150.9

130 80 88 93 82 85 108 3.9-9.8 6.1

tc (s)

Tc (K)

tm (s)

336 134 142 143 139 162 200 1.5-3.7 285.5

674 604 620 612 627 631 636 0.4-0.9 31.4

340 182 205 180 158 208 230 3.5-7.6 34.3

Tm (K) hm (K/s) 688 654 676 657 656 674 674 1.4-2.2 1.5

0.92 1.45 1.33 1.41 1.65 1.18 1.29 5.8-10.9 6.6

absence of a long tail from the gas release rate curves indicates that lignin decomposition, generally characterized by a wider range of reaction temperatures,1 becomes narrower and presents

Figure 3. (A) Range of temperatures between tmin (time of minimum temperature) and tc (conversion time) for the various catalysts (temperature values refer to a bed height of 10 cm). (B) Range of temperatures between tc (conversion time) and tm (time of maximum rate of temperature increase) for the various catalysts (temperature values refer to a bed height of 10 cm).

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a more evident overlap with hemicellulose and cellulose decomposition. The quantitative differences, always large, depend on the specific additive. The temperature versus time profiles along the bed axis, recorded from the time of the batch feed to complete conversion, are qualitatively similar for the various spatial positions. An initial rapid decrease is followed by a successively slow increase toward the initial bed value as shown for a bed height of 10 cm in Figure 2. The first trend can be attributed to the feed preheating from ambient to the pyrolysis temperature, the global endothermic character of the wood decomposition reactions, and the convective transport of heat outside the bed by the hot vapors and gases resulting from the decomposition process. Sample decomposition begins almost instantaneously and the maximum rate of gas release also occurs at very short times; that is, the most external layer of the particles is rapidly heated to temperatures sufficiently high for the beginning of the decomposition process. However, intraparticle and/or intrabed spatial gradients soon after become nonnegligible with a decline in the gas release rate. Indeed, complete conversion requires times much longer than those of the maximum gas release rate (Table 3A). It can also be observed that, as long as pyrolysis takes place, the bed temperatures remain very close to the minimum value, whose position is slightly delayed with respect to that of the maximum gas release rate. Therefore, the increasing trend in the temperature profile, at long times, is essentially representative of the slow heating of the solid residue, left at the conclusion of the degradation process, toward the initial, preheating temperature. In the presence of alkaline additives, conversion times become shorter. More precisely, the shortest conversion times are evaluated for the NOH treatment (134 s versus 336 s) but they are also short and comparable for the treatments with KOH, the two carbonates, and potassium acetate (139-154 s). On global terms, the strong diminution with respect to extracted wood can be quantified by factors of 2.5-2.2. The reduction is less evident for NaCl (200 s). The maximum gas release rate occurs earlier for the treated samples, especially in the case of hydroxides, carbonates, and potassium acetate (60-75 s versus 139 s) and is significantly higher (factors of 4-5) with respect to extracted wood. The percentage of gas released, at the time of the maximum rate, is about 25-28% (versus 25% of extracted wood). The global rate of volatile release is also significantly increased owing to the strong reduction in the conversion time (as shown below, the yield of volatile products are reduced by alkali compounds). From Figure 2, it clearly appears that, for the decreasing part of the temperature versus time profile, impregnated wood presents lower values. The contrary can be observed for the increasing part. In the presence of alkali catalysts, the zone where temperatures undergo small variations, after the rapid initial decay, is narrower while the minimum temperature is slightly higher (569-598 K versus 561 K) and is anticipated (80-108 s versus 130 s) with respect to extracted wood. The temperature values in correspondence of the conversion time are about 604 K (NaOH)-636 K (NaCl) versus 674 K of extracted wood. Moreover, for the second zone of the temperature profile, the maximum heating rate is higher (factors of about 1.3-1.8). It is still recorded when the conversion process is terminated but for slightly lower temperatures (654-676 versus 688 K) and much shorter times (158 versus 230 s versus 340 s). The right boundaries of the temperature ranges represented in Figure 3A, which approximately coincide with those of

complete conversion, are about 38-70 K lower than those of untreated wood. This result and the lower values recorded in the first decreasing part of the profiles indicate that wood decomposition catalyzed by alkaline compounds takes place at lower temperatures. The higher values of the minimum temperatures support the speculation that alkali metals cause a diminution in the global endothermicity of the decomposition process. As discussed in the following section, these additives give rise to an increase in the yields of char. It is known38,39 that char formation is an exothermic process. Moreover, an increase in its yields simultaneously reduces the effects of convective cooling40 due to volatile transport outside the bed (given the low contents in wood, the thermal effects of catalyst transformation are small). In this way, the global endothermicity of the decomposition process is reduced. The higher heating rates (Table 3B) and temperatures of the second part of the temporal profiles (Figure 2), observed for alkali-catalyzed pyrolysis, are the result of the highly reduced tailing zone in the gas (and vapor) release curve, a consequence of the increased overlap between the degradation of holocellulose and lignin (Figure 1). Moreover, as a consequence of the displacement of the decomposition at lower temperatures, the heating of the solid residue takes place over a wider range of lower temperatures (Figure 3B). Table 3A,B also summarizes the results of the statistical analysis. The range of values reported for the relative standard deviation, ∆σ(%), shows the values evaluated for all the sets of experiments carried out for each treatment. It can be observed that it is small for all the characteristic variables except for the time of the maximum gas release, tgm. Indeed, this variable, evaluated from the curves of gas release, is affected by the time period between two successive gas samplings which is only about the 2-3 times shorter. The values of the ratio F/Fcrit are generally well above the unity (3.5-301), indicating that the effects of the treatments are by far higher that the typical errors of the experimental method. The value below the unity (0.5) obtained for the percentage of gas released in correspondence of the maximum release rate (Table 3A) confirms, as already pointed out, that this variable does not depend on the alkali treatment. Also, the relatively small value (1.5) computed for the temperature Tm (Table 3B) indicates that the heating of char is not significantly affected by the type of treatment undergone by wood. The anticipation of the decomposition of lignocellulosic materials at lower temperatures by the presence of alkaline compounds, especially sodium and potassium hydroxides and carbonates, found in this study is in agreement with previous literature.11,18-21 Moreover, as reported for sodium compounds,19 it is also found that, given a class of alkaline compounds based either on K or Na metal, the higher the basicity of the catalyst the lower the temperature of the conversion process. A further result of this study is that the Na ion is more effective than the K ion for lowering the decomposition temperature, but the K treated samples are characterized by a lower endothermicity (higher values of Tmin). The displacement of the wood decomposition process at lower temperatures has been associated with the alkali catalysis of the reactions and/or the consequences of the hydrolytic attack resulting in fiber swelling and solid matrix deterioration during impregnation.11 Alkaline cations, given their small size, can penetrate into the biomass structure and cause the cleavage of the intermolecular hydrogen bridges upon heating and/or swelling, thus displacing the decomposition process at lower temperatures. The possible partial catalytic hydrolysis of wood,

3364 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 Table 4. (A) Yields of the Main Classes of Pyrolysis Products and Gaseous Species, Expressed As Percent of the Initial Dry Sample Mass, and Total Mass Closure for the Various Catalysts; Relative Standard Deviation, ∆σ(%), and Results of ANOVA in Terms of the Ratio F/Fcrit. (B) Yields of Major Carbohydrates, Minor Carbohydrates, Furans and Phenols (Dry Sample Mass Basis) for the Various Catalysts; Relative Standard Deviation, ∆σ(%), and Results of ANOVA in Terms of the Ratio F/Fcrit (A) Yields of the Main Classes of Pyrolysis Products and Gaseous Species

extraction NaOH KOH Na2CO3 K2CO3 KC2H3O2 NaCl ∆σ(%) F/Fcrit

char (wt %)

H2O (wt %)

organics (wt %)

gas (wt %)

total (wt %)

CO2 (wt %)

CO (wt %)

CH4 (wt %)

22.1 29.4 28.6 28.8 27.8 27.0 26.9 0.7-1.7 84.6

19.5 28.5 29.2 28.3 27.0 27.4 24.7 1.1-6.5 15.5

42.6 19.0 20.8 22.0 22.3 24.8 28.7 1.3-10.8 53.1

10.9 19.1 17.3 17.5 16.5 16.8 15.7 0.7-6.8 54.2

95.1 96.0 95.9 96.6 93.6 96.0 96.0 0.1-0.5 -

6.1 11.7 10.3 10.5 9.8 10.0 9.6 1.6-9.0 22.2

3.9 5.8 6.0 5.9 5.7 5.8 4.9 1.7-7.5 12.1

0.77 1.0 0.88 0.94 0.81 0.87 0.93 1.9-6.7 6.9

(B) Yields of Major Carbohydrates, Minor Carbohydrates, Furans and Phenols

extraction NaOH KOH Na2CO3 K2CO3 KC2H3O2 NaCl ∆σ(%) F/Fcrit

organics (wt %)

major carbohydrates (wt %)

minor carbohydrates (wt %)

furans (wt %)

phenols (wt %)

42.6 19.0 20.8 22.0 22.3 24.8 28.7 -

13.51 4.89 6.29 6.87 6.24 7.35 8.20 1.6-8.0 32.2

0.6 1.57 1.43 1.21 1.37 0.93 0.70 1.5-14.3 21.9

0.67 0.85 0.90 0.68 0.89 0.54 0.54 0.2-12.1 10.4

2.10 4.19 3.83 2.89 2.34 2.50 1.70 0.7-10.5 61.8

enhanced by basicity, during the impregnation stage can contribute in the higher activity of the decomposition reactions at lower temperatures. It is also likely that the stronger effects caused by the Na ion on the time and temperatures of catalytic decomposition are due to the lower molecular weight (23 versus 39 kg/kmol of the K ion), which facilitates penetration inside the substrate structure. Yields of Products Classes and Gas Composition. The effects of the alkaline additives on the yields of the product classes of wood pyrolysis can be seen from Table 4A (yields of products, mass closure, and gas composition) and Table 4B (yields of the classes of organic products) again using extracted wood for comparison. These also report the main findings of the statistical analysis. It should be pointed out that for the classes of organic compounds the statistical variables have been evaluated for the yields on a total liquid basis to separate the experimental uncertainty associated with the pyrolysis experiments with that of the GC-MS technique. Results are qualitatively similar in all cases; that is, a significant increase is observed in the char, gas, and water yields, essentially at the expense of organic compounds. Moreover, for the treatments with basic compounds of alkali metals, the yields of major carbohydrates and minor carbohydrates are always reduced and increased, respectively. The yields of phenols are increased except for sodium chloride. The yields of furan compounds show small increments (hydroxides and carbonates) or small diminutions (potassium acetate and sodium chloride). Similar to the effects on characteristic times and temperatures of the decomposition process, hydroxides, carbonates, and potassium acetate exert a much stronger action than sodium chloride. The yields of char, water, and carbon dioxide are highly increased leading to yields between 61 and 70% (versus 48% of extracted wood). NaOH causes the highest variations with high values of both char and water (29.4 and 28.5% versus 22 and 19% of extracted wood, respectively). The yields of char are slightly lower for the other additives (27-29%) while comparable yields of water (27-29%) are observed in all cases except for NaCl which produces lower amounts (25%). In reality, the final solid residue also includes the contribution of

the added catalyst which may have undergone chemical and physical modifications when subjected to heating. Alkaline additives also cause an increase in the gas yields (about 16-19% versus 11% of untreated wood). This is mainly due to CO2 which presents yields roughly equal to 10-12% (versus 6% of untreated wood), while the increase in CO is less evident (about 5-6% versus 4%). Variations on the yields of CH4, H2, and C2 compounds (not shown) are small. The increase in the yields of char, gas and water, induced by alkaline compounds, takes place at the expense of organic compounds. The yields of this product class are lowered to about 19-29% (versus 43% of extracted wood). In accordance with the maximum in the production of char, water, and CO2, the minimum yield of organic products is obtained when NaOH is used as additive. Low values (21-22%) are also obtained for KOH, Na2CO3, and K2CO3, which indicates a noticeable inhibition of the reactions leading to volatile species formation (total mass closure is between 94-96%). The yields of major carbohydrates are highly reduced, especially for the NaOH treatment when they become 2.7 lower (from about 13.5 to 4.9%). Reductions are also quite high for KOH and the two carbonates (factors of 2-2.2) and lower for the other additives (factors of about 1.7-1.8). On the contrary, the yields of minor carbohydrates are increased by about 2.4-2.6 times for hydroxides (from about 0.6 to 1.6-1.4%) and 2-2.3 times for carbonates. The effects are lower for potassium acetate and sodium chloride (increase factors of 1.6 and 1.2, respectively). The decrease in the yields of main carbohydrates and the increase in the yields of minor carbohydrates appear to be directly proportional to the level of basicity of the catalyst and to the decrease of the decomposition temperature. Hydroxides and carbonates also cause a small increase in the yields of furan compounds. The K ion in the form of hydroxide and carbonate is slightly better (increase factors of about 1.34 with yields from about 0.7 to 0.9%) than NaOH (increase factor of 1.27). The other additives induce a diminution (potassium acetate and sodium chloride) or no significant alteration (sodium carbonate). As for phenolic compounds, the highest influences are exerted by the hydroxides

Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3365 Table 5. Yields of the Main Organic Compounds (Dry Sample Mass Basis) for Potassium and Sodium Hydroxide and Carbonate; Relative Standard Deviation, ∆σ(%), and Results of ANOVA in Terms of the Ratio F/Fcrit

major carbohydrates (%) acetic acid hydroxyacetaldehyde hydroxypropanone levoglucosan minor carbohydrates (%) 1-hydroxy-2-butanone 2-methyl-2-cyclopentenone 3-ethyl-2-hydroxy-2-cyclopentenone 3-methyl-2-cyclopentenone acetoxyacetone propionic acid furans (%) 2-acetylfuran 2-furaldehyde 2(5H)-furanone 5-methyl-2-furaldehyde furfuryl alcohol phenols (%) 4-acetonguaiacol 4-ethylguaiacol 4-methylguaiacol eugenol guaiacol cis-isoeugenol trans-isoeugenol vanillin 4-propylguaiacol 3,4-dimethylphenol cresols hydroquinone phenol

extraction

NaOH

KOH

Na2CO3

K2CO3

∆σ(%)

F/Fcrit

13.51 3.40 4.53 1.52 4.0 0.60 0.19 0.03 0.07 0.03 0.08 0.20 0.67 0.03 0.28 0.26 0.06 0.04 2.10 0.06 0.15 0.55 0.11 0.40 0.32 0.14 0.15 0.06 0.01 0.08 0.04 0.03

4.89 1.45 1.52 1.76 0.16 1.57 0.81 0.12 0.23 0.09 0.09 0.23 0.85 0.03 0.12 0.09 0.03 0.58 4.19 0.08 0.06 0.48 0.13 1.20 0.70 0.27 0.11 0.15 0.00 0.29 0.03 0.15

6.29 2.42 1.88 1.89 0.10 1.43 0.51 0.07 0.40 0.12 0.09 0.24 0.90 0.07 0.11 0.14 0.02 0.56 3.83 0.09 0.41 0.31 0.08 1.12 0.72 0.39 0.07 0.13 0.02 0.22 0.09 0.18

6.87 2.36 2.03 2.36 0.12 1.21 0.78 0.08 0.18 0.07 0.10 0.24 0.68 0.05 0.10 0.08 0.04 0.41 2.89 0.07 0.32 0.42 0.08 0.86 0.40 0.17 0.11 0.08 0.01 0.15 0.04 0.08

6.24 2.51 1.80 1.76 0.17 1.37 0.77 0.05 0.16 0.05 0.09 0.25 0.89 0.03 0.10 0.10 0.04 0.62 2.34 0.07 0.18 0.33 0.07 0.72 0.40 0.18 0.07 0.07 0.01 0.10 0.06 0.08

1.6-5.4 3.6-10.0 0.2-9.8 2.0-9.9 1.7-7.0 1.5-11.4 1.7-11.4 4.0-14.8 3.2-14.3 1.5-15.2 1.3-5.6 1.1-14.2 0.2-10.6 0.2-3.1 0.1-10.3 3.2-15.8 2.1-15.9 0.3-8.9 0.7-10.5 2.7-6.5 1.2-12.0 2.3-11.2 2.8-14.0 0.6-10.8 1.9-10.6 6.1-15.4 1.2-15.9 1.5-15.5 3.9-15.5 2.3-10.8 1.6-10.4 3.3-11.3

51.0 26.1 10.2 73.2 159.9 14.4 34.3 6.3 14.6 9.5 26.8 8.6 10.9 150.2 127.9 34.0 26.7 5.9 7.5 56.5 18.9 17.3 13.4 28.7 9.4 4.7 5.8 20.5 24.7 25.2 19.1 18.7

which roughly double the yields from about 2.2 to 4.2 and 3.9% for NaOH and KOH, respectively. Lower factors of increase are observed for sodium carbonate, potassium acetate, and potassium carbonate, whereas NaCl causes a diminution. The results of the statistical analysis (Table 4A,B) again show the good quality of the measurements, as testified by small values of the ranges of the relative standard deviation, ∆σ(%). The high values of the of ratio F/Fcrit (7-85), on the other hand, confirm the dramatic changes introduced by alkali compounds of the yields and composition of the main classes of wood pyrolysis products. The increase in the yields of low-temperature products (char, water, CO2) with an associated diminution in the yields of organic compounds, as a consequence of alkali metal catalysts in the pyrolysis of lignocellulosic materials, is already known in the literature9-21 at least from the qualitative point of view. The present study clarifies the role played by the chemical nature and alkali metal of the added catalyst, with the largest variations caused by hydroxides and then, in the order carbonates, potassium acetate, and sodium chloride. The displacement of the reaction process at lower temperatures, induced by alkali metals, favors the charring and dehydrating reactions, versus fragmentation and depolymerization paths, in the primary decomposition of the holocellulosic fraction. This circumstance combined with the presence of alkali compounds produces a char with a more condensed structure.21 The stabilization of the carbon atoms in crystallites,41 after removal of cross-linking owing to the presence of oxygen in the potassium compounds,42 is retained to be the cause of the minimization in the formation of primary tars. The modified structure of the charred solid, when heated at higher temperatures, undergoes further degradation but with modified reaction paths which lead to the production of organic compounds in lower amounts and with a highly different chemical composition. Moreover, basic additives

catalyze the loss of water during cellulose pyrolysis through the formation of double bonds rather than through ether-type bonds as in the case of acidic catalysts.10 As already noted, alkali metals also enhance dehydration, demethoxylation, decarboxylation, and char formation in lignin pyrolysis,22,23 thus modifying the yields of water, char, and volatile phenolic compounds. Composition of Organic Products. Results summarized in Table 4A,B put into evidence that incorporation of NaCl in wood causes not only a decrease in total yields of organic compounds but also on all the classes of organic compounds. Thus, it can be concluded that this additive is not effective for maximizing the production of specific compounds but its role is essentially that of flame retardant for wood.2,3 The effects of potassium acetate are always lower than those of both potassium and sodium carbonate for the reduction in the production of major carbohydrates and the enhancement in the production of minor carbohydrates and furans. The performances of this catalyst for phenol production are also lower than those of sodium carbonate and barely higher that those of potassium carbonate. Therefore, for the sake of simplicity and clarity in the discussion, the detailed composition of organic products is discussed only for the treatments based on hydroxides and carbonates (again using extracted wood for comparison). A more profound inspection on these aspects can be made by means of Table 5, which reports the yields of single organic products, and Figure 4A-D. It can be observed (Table 5) that levoglucosan yields are practically brought to zero in the presence of alkaline additives. A strong diminution, up to factors of about 5-6 for the KOHtreated samples, has also been evaluated by means of the peak area for the yields of 2,3-anhydro-D-galactosan and 2,3-anhydroD-mannonsan. On the contrary, the peak area of 1,4:3,6dianhydro-R-D-glucopyranose is left almost unchanged. NaOH causes the largest reduction in the light compounds of major

3366 Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009

Figure 4. (A) Yields of hydroxypropanone, 1-hydroxy-2-butanone, and furfuryl alcohol (dry sample mass basis) for treatments with sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. (B) Yields of 2-methyl-2-cyclopentenone, 3-ethyl-2-hydroxy-2-cyclopentenone, and 3-methyl2-cyclopentenone (dry sample mass basis) for treatments with sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. (C) Yields of 4-ethylguaicol, guaiacol, and cis-isoeugenol (dry sample mass basis) for treatments with sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. (D) Yields of trans-isoeugenol, cresols, and phenol (dry sample mass basis) for treatments with sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate.

carbohydrates, especially hydroxyacetaldehyde reduced by a factor of 3 (from about 4.5 to 1.5%) but large reductions are also observed for the other catalysts (1.8-2%). The effects on the production of acetic acid are less evident (from about 3.4 to 1.4-2.5%). Instead, the yields of hydroxypropanone are increased (from about 1.5 to 1.8-2.4%) with the maximum for the Na2CO3 treatment (Figure 4A). The reduction in the production of levoglucosan in the biomass pyrolysis products simply owing to the indigenous alkali compounds or to impregnation with alkali compounds, in this case to improve the fire performance of lignocellulosic materials, is already known in the literature.1-3,6-8 Results of the present investigation also indicate that the formation of light compounds, especially hydroxyacetaldehyde and acetic acid, generally favored by the presence of alkaline cations at least for the amounts naturally present in biomass,6-8 is also strongly hindered. Indeed, a strong diminution in the yields of these compounds is observed for both Na and K, in the form of hydroxides and carbonates, and for potassium acetate. It can be speculated that hydroxyacetaldehyde, which is a major product of wood fast pyrolysis (minimum in the char yields)6 and therefore is favored by high heating rates and reaction temperatures (higher activation energy for the formation reaction compared with other major products1) is not formed. This may be the result of modifications in the material structure introduced by the physical impregnation together with an excessively high alkali metal concentration and/or the lowering of the decomposition temperatures. In relation to the last issue, the mechanism proposed by Kandola et al.2 for the base-catalyzed pyrolysis of cellulose points out the importance of the decomposition temperatures. In fact, it explains that “inorganic bases act to catalyze the degradation so that it occurs at temperatures below

the minimum required for the conformational interconversion necessary for levoglucosan formation, thus reducing volatile fuel formation”. Finally, it is also worth noting that, even for the relative high alkali metal concentration of these experiments and in agreement with other studies,9,21 the production of hydroxypropanone is increased, indicating that its reaction path is independent from those of other major carbohydrates formation. Similarly, the routes for the formation of levoglucosan, 2,3-anhydro-D-galactosan, and 2,3-anhydro-D-mannonsan, hindered by the presence of alkaline compounds, are different from that of 1,4:3,6-dianhydro-R-D-glucopyranose, left almost unvaried by the alkali impregnation. Hydroxides show better performances than carbonates for the maximization of the compounds indicated as minor carbohydrates. In particular, NaOH is more effective for the formation of 1-hydroxy-2-butanone (Figure 4A), used as flavor,43 and 2-methyl-2-cyclopentenone (Figure 4B), used in the synthesis of cyclopentenoids, compounds important for their biological and potentially medicinal properties.43-45 These compounds are roughly increased 4 times (sodium carbonate is also better than KOH for their maximization). On the other hand, KOH is better (Figure 4B) for increasing the yields of 3-ethyl-2-hydroxy-2cyclopentenone, a valuable reaction intermediate for various high value added products,44 and 3-methyl-2-cyclopentenone, used as flavor,44,46 which become 6-4 times higher (a significant increase is also caused by NaOH). The yields of these minor carbohydrates are between 0.8 and 0.12% which, although not high, could be of interest because these species are expensive and difficult to produce.46 These findings are new and require further evaluation in relation to their actual practical importance. However, it is worth noting that an increase in 3,4-dihydroxy2-methyl(4H)pyran-4-one and the appearance of methyl-1,2-

Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3367

cyclopentanedione (not found for untreated wood), which can be classified as minor carbohydrates, are reported21 for wood treated with a potassium carbonate fire retardant, and attributed to alterations in the mechanisms of thermal degradation of carbohydrates. It is possible that intermolecular aldol condensation reactions, catalyzed by bases,46 are responsible for the enhanced formation of this class of compounds. The increase in the furan yields caused especially by potassium hydroxide and carbonate and sodium hydroxide is essentially due to augmented yields of furfuryl alcohol. A maximum of 0.62% is obtained for K2CO3 treated wood versus 0.04% for extracted wood (values of the other catalysts between 0.4 and 0.56%), giving rise to an increase factor up to about 15 (Figure 4A). Other compounds are reduced, especially 2-furaldehyde (yields of approximately 0.1% for hydroxides and carbonates versus 0.28% of extracted wood), or brought to zero such as in the case of 5-hydroxymethylfurfural (evaluations based on the peak area). Furfuryl alcohol is widely used in producing various synthetic fibers, rubbers, resins, and farm chemicals.47 These results are in agreement with previous findings,9,10,12 which attribute the modifications in the distribution of furan compounds to the activity of cracking reaction in the vapor phase catalyzed by alkalis. Results of this study show that K compounds are superior to Na compounds, independently of the basicity level. Among the phenolic compounds (Table 5) the most abundant is guaiacol which attains a maximum of 1.2% (versus 0.4% of extracted wood) for NaOH, followed by KOH (1.1%). NaOH is also the most effective for the production of 4-ethylguaiacol (yields from 0.15 to 0.6%, Figure 4C), 4-propylguaiacol (approximately from 0.06 to 0.15%) and cresols (from about 0.08 to 0.3%, Figure 4D) (KOH presents slightly lower performances). The maximum production of isoeugenol cis (from 0.32 to 0.72%, Figure 4C), isoeugenol trans (from 0.14 to 0.4%, Figure 4D) and phenol (from 0.03 to 0.18%, Figure 4D) is obtained for the KOH treatment, followed by the NaOH treatment. In all cases, the yields of vanillin and 4-methylguaiacol diminish. A diminution in the production of eugenol is also observed apart from the NaOH treatment. The effects on 4-acetonguaiacol are negligible. Other compounds are present in very low quantities (Table 5). Phenolic compounds have a good industrial value48,49 for use as food aromas, pharmaceuticals, or intermediates for chemical synthesis (phenolic compounds can be deoxygenated or converted to methyl aryl ethers) or for commercial phenol formaldehyde resin replacement (thermosetting polymers, such as phenol-formaldehyde resins, are widely used as adhesives in the wood particle board and plywood industry). Therefore, there is significant interest for improving their production from biomass pyrolysis. The increase of phenolic compounds can be attributed to an enhanced activity of the demethoxylation and demethylation paths in the decomposition of the ligninic components.22,23 The increase in the yields of guaiacol (with a diminution in the amounts of 4-methylguaiacol), attributed to the catalysis of the cleavage of the alkyl-aryl CdC bond, and the decrease in the yields of vanillin, resulting from the modification in the chemical structure of the material and the possible disappearance of R-carbonyl groups, are also reported in another study.21 Also, it is known that KOH catalyzes the isomerization of eugenol to isoeugenol.50 Results of the present study evidence that the spectrum of phenolic compounds among wood pyrolysis products is not reduced by the presence of alkaline compounds. Moreover,

although some compounds are diminished or left unaltered, the factors of increase on some specific phenolic are particularly high. Hydroxides present better catalytic properties than carbonates but the metallic ion should be selected on the basis of the desired products. Table 5 also summarizes the results of the statistical analysis. To take into account the experimental uncertainty of the data provided by the GC-MS techniques, the statistical parameters are referred to the yields of the compounds evaluated on a total liquid basis. It can be observed that the maximum values of the relative standard deviation, ∆σ(%), are in general lower than 16%, indicating an acceptable accuracy. The values of the ration F/Fcrit (5-252) are again always very high. Conclusions The pyrolysis of wood catalyzed by alkaline compounds has been experimentally investigated with the scope of clarifying the effects of the nature of the alkali metal and the chemical state of the alkali compound. Catalysts have been impregnated in wood with the aim of modifying mainly the primary decomposition paths, in this way enhancing the production of specific organic compounds. Findings not only confirm some trends already known from previous literature but also provide new useful information about the qualitative and quantitative composition of organic compounds. The displacement of the pyrolysis reactions at lower temperatures, the increased exothermic contribution associated with the enhanced formation of char, and the reduction in the convective cooling resulting from the diminished formation of volatile products, in the presence of alkaline additives, cause a strong reduction in the conversion times. These effects are enhanced by increasing the basicity of the additive and the use of the Na with respect to the K ion. Primary dehydration and charring reactions are catalyzed, leading to total yields of char, carbon dioxide, and water between 1.3 and 1.5 times higher. The yields of carbon monoxide are approximately increased by a factor of 1.5 while the yields of organic compounds are lowered by factors of about 2.3-1.5. The diminution in the organic compounds is mainly due to a strong decay in the most abundant products of uncatalyzed pyrolysis of the holocellulosic components, that is, hydroxyacetaldehyde, acetic acid, 2-furaldehyde, 5-hydroxymethylfurfural, and mainly some sugars (levoglucosan, 2,3-anhydro-Dgalactosan, and 2,3-anhydro-D-mannonsan) whereas other compounds remain roughly unchanged (1,4:3,6-dianhydro-RD-glucopyranose) or are slightly augmented (hydroxypropanone). Sodium chloride does not increase the yields of any organic compound and therefore it essentially behaves as a fire retardant. Only small positive variations are also caused by potassium acetate compared with the other alkaline additives. Indeed sodium and potassium hydroxides and carbonates are apt to maximize the yields of some carbohydrates and furans. Hydroxides show better performances than carbonates. However, although in terms of global parameters (conversion time, devolatilization rate, and yields of product classes) the Na ion causes the largest variations, the selection of sodium- or potassium-based catalysts for lignocellulosic material pyrolysis depends on the specific chemical compound that should be maximized. Some minor carbohydrates are increased by factors of about 4-6 (1-hydroxy-2-butanone and 2-methyl-2-cyclopentenone for NaOH catalysis; 3-ethyl-2-hydroxy-2-cyclopentenone and 3-methyl-2-cyclopentenone) for KOH catalysis. The yields of furfuryl alcohol are also highly increased (up to factors of 15) by the use of potassium hydroxide and carbonate.

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The decomposition of the lignin component is also highly modified by the presence of alkaline compounds. Sodium and potassium hydroxides roughly double the yields of phenolic compounds. NaOH is particularly effective for the production of guaiacol, cresols, and 4-ethylguaicol (factors of increase between 2.5 and 4), whereas KOH favors the production of cisisoeugenol, trans-isoeugenol, and phenol (factors of increase of 2-6). A diminution is always observed in the yields of vanillin and 4-methylguaicol and, apart from NaOH, for eugenol. In general, alkaline compounds highly favor the carbonization, dehydration, decarboxylation, and demethoxylation reactions of wood, leading to a modified carbonaceous structure which is more stable. For the concentrations of additives used in this study (Na or K content in wood of about 0.37-0.41%), not only the primary depolymerization paths leading to levoglucosan and sugar formation are inhibited but also the primary fragmentation and depolymerization paths in the decomposition of the cellulose and hemicellulose fractions of wood. Probably this is also a consequence of the displacement of the decomposition process at too low temperatures for the formation of products such as hydroxyacetaldehyde. The highly modified structure of the dehydrated and charred solid also undergoes reduced devolatilization occurring according to new paths. It can also be understood that the presence of alkaline compounds alters the activity of secondary vapor-phase products, in particular the degradation of carbohydrates and furans. Although the modifications induced on the temperature and products of wood pyrolysis by alkali catalysis have been quantified, the catalysis mechanism is not yet known. Aspects that require further research efforts also concern the role played by the catalyst concentration and the nature of the feedstock on the qualitative and quantitative distribution of pyrolysis products. Moreover, from the point of view of energy recovery from the solid (char) and liquid (organics) products of biomass pyrolysis, it could be very useful the study of the gasification and oxidation reactivity of these product classes. Literature Cited (1) Di Blasi, C. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog. Energy Combust. Sci. 2008, 34, 47. (2) Kandola, B. K.; Horrocks, A. R.; Price, D.; Coleman, G. V. Flameretardant treatments of cellulose and their influence on the mechanism of cellulose pyrolysis. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1996, C36 (4), 721. (3) Kozlowski, R.; Wladyka-Przybylank, M. Natural polymers, wood and lignocellolosic materials. In Fire Retardant Materials; Horrocks, A. R., Price, D., Eds.; Woodhead Publishing Ltd: Cambridge, UK, 2001; Chapter 9, pp 293-317. (4) Fernando, S.; Adhikari, S.; Chandrapal, C.; Murali, N. Biorefineries: current status, challenges, and future direction. Energy Fuels 2006, 20, 1727. (5) Mohan, D.; Pittman, C. U.; Steele, P. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy Fuels 2006, 20, 848. (6) Piskorz, J.; Radlein, D.; Scott, D. S.; Czernik, S. Liquid products from the fast pyrolysis of wood and cellulose. In Research in Thermochemical Biomass ConVersion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: London, 1988; pp 557-571. (7) Antal, M. J.; Varhegyi, G. Cellulose pyrolysis kinetics: the current state of knowledge. Ind. Eng. Chem. Res. 1995, 34, 703. (8) Scott, D. S.; Paterson, L.; Piskorz, J.; Radlein, D. Pretreatment of poplar wood for fast pyrolysis: rate of cation removal. J. Anal. Appl. Pyrolysis 2000, 57, 169. (9) Beaumont, O.; Schwob, Y. Influence of physical and chemical parameters on wood pyrolysis. Ind. Eng. Chem. Process Des. DeV. 1984, 23, 637. (10) Pavlath, A.E.; Gregorski, K. S. Carbohydrate pyrolysis. II. Formation of furfural and furfuryl alcohol during the pyrolysis of selected carbohydrates with acidic and basic catalysts. In Fundamentals of Biomass Thermochemical ConVersion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier: London, 1985; pp 155-163.

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ReceiVed for reView September 30, 2008 ReVised manuscript receiVed December 22, 2008 Accepted January 13, 2009 IE801468Y