Effects of Potassium Hydroxide Impregnation on ... - ACS Publications

Energy & Fuels 2009, 23, 1045–1054

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Effects of Potassium Hydroxide Impregnation on Wood Pyrolysis Colomba Di Blasi,*,† Antonio Galgano,† and Carmen Branca‡ Dipartimento di Ingegneria Chimica, UniVersita` degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy, and Istituto di Ricerche sulla Combustione, C.N.R., P.le V. Tecchio, 80125 Napoli, Italy ReceiVed September 30, 2008. ReVised Manuscript ReceiVed December 12, 2008

The pyrolysis of a packed bed of fir wood particles, after impregnation with KOH, is investigated. For a heating temperature of 800 K, maximum variations in the pyrolysis characteristics are observed for KOH concentrations in wood below 1%. Decomposition temperatures become lower (35-70 K), and conversion times are rapidly more than halved. Also, the char, water, and gas yields increase (factors of 1.4, 1.6, and 1.7, respectively) at the expense of liquid-phase organic products. Levoglucosan presents a very steep decay, whereas the diminution in hydroxyacetaldehyde and acid acetic is much slower, and a wide zone of approximately constant values appears for hydroxypropanone. However, small quantities of KOH in wood (about 0.2-0.6%) are apt to increase the yields of furfuryl alcohol up to a factor 15 and some carbohydrates (3-ethyl-2-hydroxy2-cyclopentenone, 3-methyl-2-cyclopentenone, 1-hydroxy-2-butanone) and phenols (phenol, cresols, hydroquinone, guaiacol, isoeugenol-trans, isoeugenol-cis, 4-acetonguaiacol, 4-ethylguaiacol) up to factors of 2-6. Higher KOH concentrations cause a further increase in the yields of char and CO2 associated with a decay to very small yields of all of the organic compounds. Finally, increasing the heating temperature from 600 to 900 K (KOH concentration in wood of about 0.6%) essentially favors devolatilization and cracking of vapor-phase organic products with conversion times roughly reduced by a factor of 6.

Introduction Reaction paths and products of lignocellulosic fuels pyrolysis can be altered with catalysts or pretreatments that modify the inorganic contents and/or the chemical structure of the substrate, so favoring the development of new and high-valued products from renewable sources1,2 or advanced formulations for fire retardancy.3-5 The effects of alkali and alkali-earth metal ions have received significant attention also because they are inherently present in biomass materials or used in pulping processes as reagents. Antal and Varhegyi6 and Scott et al.7 provide a critical overview of the work carried out on the effects of cations (potassium, calcium, and minor amounts of sodium and magnesium) indigenous to all biomass on the pyrolysis products. The existence of two main competitive pathways is pointed out. Conditions that lead to the selective formation of levoglucosan from cellulose realize very low yields of hydroxyacetaldehyde, and vice versa. Temperature plays only a minor role in the * To whom correspondence should be addressed. Telephone: +39-0817682232. Fax: +39-081-2391800. E-mail: [email protected]. † Universita ` degli Studi di Napoli “Federico II”. ‡ Istituto di Ricerche sulla Combustione. (1) Encinar, J. M.; Beltran, F. J.; Ramiro, A.; Gonzalez, J. F. Ind. Eng. Chem. Res. 1997, 36, 4176–4183. (2) Mohan, D.; Pittman, C. U.; Steele, P. Energy Fuels 2006, 20, 848– 889. (3) LeVan, S. L. Chemistry of fire retardancy. In The Chemistry of Solid Wood; Advances in Chemistry Series 207; Rowell, R. M., Ed.; American Chemical Society: Washington, DC, 1984; Chapter 14, pp 531-574. (4) Chen, Y.; Frendi, A.; Tewari, S. S.; Sibulkin, M. Combust. Flame 1991, 84, 121–140. (5) Kandola, B. K.; Horrocks, A. R.; Price, D.; Coleman, G. V. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1996, C36, 721–794. (6) Antal, M. J.; Varhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703– 717. (7) Scott, D. S.; Paterson, L.; Piskorz, J.; Radlein, D. J. Anal. Appl. Pyrolysis 2000, 57, 169–176.

productivity of the two pathways. The presence of trace amounts (ppm) of salts and metal ions determines the predominance of one product over the other: hydroxyacetaldehyde is produced in very high amounts from untreated cellulose or wood under fast heating rates and relatively high temperatures. Conversely, the production of levoglucosan is highly favored when the substrate is subjected to pretreatments apt to reduce the ash content by about 90% (for instance, mild acid washing). Studies on the effects of added alkaline compounds on the pyrolysis of lignocellulosic material are also available.8-22 They are not extensive because the effects are not systematically investigated of the additive concentration in the sample, nature of the alkali metal, and operating variables. A detailed analysis of the organic products, which is needed to evaluate the potential applicability of catalytic pyrolysis as a route for specialty chemicals production, is also lacking. Nevertheless, some qualitative features are always well evident. Alkali compounds cause an increase in the yields of char and gas and a reduction in the liquid products with an enhancement in the activity of (8) Pan, W. P.; Richard, G. N. J. Anal. Appl. Pyrolysis 1989, 6, 117– 126. (9) Beaumont, O.; Schwob, Y. Ind. Eng. Chem. Process Des. DeV. 1984, 23, 637–641. (10) Font, R.; Marcilla, A.; Verdu, E.; Devesa, J. Ind. Eng. Prod. Res. DeV. 1986, 25, 491–496. (11) Pavlath, A. E.; Gregorski, K. S. In Fundamentals of Biomass Thermochemical ConVersion; Overend R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier: London, U.K., 1985; pp 155-163. (12) Zaror, C. A.; Hutchings, I. S.; Pyle, D. L.; Stiles, N.; Kandiyoti, R. Fuel 1985, 64, 990–994. (13) Antal, M. J.; Mok, W. S. L.; Varhegyi, G.; Szekely, T. Energy Fuels 1990, 4, 221–225. (14) Jakab, E.; Faix, O.; Till, F. J. Anal. Appl. Pyrolysis 1997, 40-41, 171–194. (15) Krieger-Brockett, B. Res. Chem. Intermed. 1994, 20, 39–49. (16) Kleen, M.; Gellerstedt, G. J. Anal. Appl. Pyrolysis 1995, 35, 15– 41.

10.1021/ef800827q CCC: $40.75  2009 American Chemical Society Published on Web 01/15/2009

1046 Energy & Fuels, Vol. 23, 2009

dehydration, demethoxylation, decarboxylation, and charring reactions. The lower molecular weight of tar,18 associated with the increased yields of gas, and condensed fragments in the char structure13 suggest that not only primary reactions but secondary reactions are also affected. Moreover, the weight loss curves are shifted toward lower temperatures, and the peaks of the volatile release rate are diminished.12,19-21 This brief survey about the state of the art clearly shows that a systematic analysis is not available for the effects of alkali metal impregnation on the pyrolysis of wood, especially the composition of condensable organic products. In this study, fir wood particles have been preliminarily subjected to water extraction, to partially remove the indigenous mineral matter, and then impregnated with aqueous solutions containing variable concentrations of potassium hydroxide. Pyrolysis has been carried out with a packed-bed reactor for heating temperatures in the range 600-900 K. Results comprise a detailed characterization of pyrolysis products and an evaluation of the process variables, such as rates of gaseous species formation, reaction temperatures, and conversion times. Experimental Section Sample Preparation. Samples examined in this study consist of fir wood impregnated with KOH. The wood chemical composition is determined using the Klason method for lignin, which corresponds to 31%, a Soxhtec HT2 apparatus for extractives,23 which are 2.6% (holocellulose, computed by difference, is 66%), and calcination24 for the ash content, which is 0.5%. 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 and with stirring, which is an effective procedure to eliminate alkali compounds from ash,24 thus avoiding their catalytic action on the conversion process. Washed and predried samples are referred to in the following as “extracted”. Impregnated samples are obtained by soaking 200 g of extracted wood particles in KOH aqueous solutions of 1 L for 3 h with stirring. The solution is prepared by adding 1 L of deionized water to a proper amount of additive to obtain the desired concentration. Particle drying 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 weighted after the treatment to evaluate the amount of potassium hydroxide adsorbed. The results of wood impregnation at ambient temperature for a period of 3 h are shown in Table 1 (amount of potassium hydroxide in wood versus amount of potassium hydroxide in aqueous solution) for a range of KOH concentration in wood of about 0-8% (dry wood basis). As expected, the amount of catalyst in wood increases with its concentration in the aqueous solution, but the impregnation process appears to be enhanced at high concentrations. It is plausible that an increase in the basicity of the aqueous solution with the amount of dissolved KOH causes a successively larger swelling of the wood particles, which affects the extent of wetting and, in this way, the amount of KOH incorporated in the sample. Indeed, while (17) Ravendraan, K.; Ganesh, A.; Khilar, K. C. Fuel 1995, 74, 1812– 1822. (18) Nik-Azar, M.; Hajaligol, M. R.; Sohrabi, M.; Dabir, B. Fuel Process. Technol. 1991, 51, 7–17. (19) Szabo, P.; Varhegyi, G.; Till, F.; Faix, O. J. Anal. Appl. Pyrolysis 1996, 36, 179–190. (20) Wang, J.; Zhang, M.; Chen, M.; Min, F.; Zhang, S.; Ren, Z.; Yan, Y. Thermochim. Acta 2006, 444, 110–114. (21) Nowakowski, D. J.; Jones, J. M.; Brydson, R. K.; Ross, A. B. Fuel 2007, 86, 2389–2402. (22) Dobele, G.; Urbanovich, I.; Zhurin, A.; Kampars, V.; Meier, D. J. Anal. Appl. Pyrolysis 2007, 79, 47–51. (23) Di Blasi, C.; Signorelli, G.; Di Russo, C.; Rea, G. Ind. Eng. Chem. Res. 1999, 38, 2216–224. (24) Di Blasi, C.; Branca, C.; D’Errico, G. Thermochim. Acta 2000, 364, 133–142.

Di Blasi et al. Table 1. Amount of Potassium Hydroxide in Wood versus Amount of Potassium Hydroxide in Aqueous Solution (Dry Wood Basis) KOH in aqueous solution [g/L]

KOH in wood [wt %]

1 2 5 10 20 40

0.1 0.2 0.58 1.28 3.1 7.7

for KOH contents in the aqueous solutions below 5 g/L the amount of water retained by wood is comprised in the range 100-120%, the figure increases to 160% for KOH contents of 40 g/L. Moreover, it is also possible that the gradients of the alkaline compound in the liquid phase, which affect the impregnation process, vary with the amount of KOH dissolved. Pyrolysis Tests and Chemical Characterization of Products. The characteristics of the pyrolysis reactor (0.063 m internal diameter and 0.45 m length) have already been described elsewhere,25-28 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 varied between 600 and 900 K in 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. Nitrogen and volatile pyrolysis products pass through a condensation train consisting of two water 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 (PerkinElmer Auto-System XL), equipped with TDC and a packed column (Supelco 60-80 Carboxen 1000, 15 ft) with helium as carrier gas. The liquid products are subjected to chemical analysis, within 1 day after their production, by means of GC/MS (Focus GC, Thermo Electron) with a quadrupole detector and a DB-1701 capillary column (60 m × 0.25 mm i.d., 0.25 mm film thickness). Gaschromatographic conditions are the same as in previous work.25-28 Thirty-one 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, isoeugenol-trans, isoeugenol-cis, 4-acetonguaiacol, vanillin, guaiacol, 4-ethylguaiacol, 4-methylguaiacol, o-m-p-cresol, 4-propylguaiacol, phenol, 3,4-dimethylphenol, hydroquinone). Fi(25) Di Blasi, C.; Branca, C.; Galgano, A. Ind. Eng. Chem. Res. 2007, 46, 430–438. (26) Di Blasi, C.; Branca, C.; Galgano, A. Polym. Degrad. Stab. 2007, 92, 752–764. (27) Di Blasi, C.; Branca, C.; Galgano, A. Polym. Degrad. Stab. 2008, 93, 335–346. (28) Di Blasi, C.; Branca, C.; Galgano, A. Energy Fuels 2008, 22, 663– 670.

Effects of KOH Impregnation on Wood Pyrolysis

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nally, 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 the difference between the measured total liquids and water so determined. Transformations of Potassium Hydroxide upon Heating. The products of wood pyrolysis catalyzed by KOH include contributions from both substrate and catalyst. As a separation between the two groups is not possible, a few considerations about the transformations undergone by potassium hydroxide when subjected to heating can be useful. KOH presents a fusion temperature of 633-653 K, with a fusion heat of 149.3 kJ/kg, and a boiling temperature of 1593-1600 K with an evaporation heat of 2302 kJ/kg.29 Hence, apart from the case of a heating temperature of 600 K, KOH is expected to undergo a molten phase during the pyrolysis experiments. It may interact with wood and the products of wood pyrolysis with possible modifications in the chemical state of potassium. Using results of biomass carbonization catalyzed by KOH and thermodynamic considerations,30 it appears that formation of potassium carbonate can occur at ambient temperature (a1) or higher (903 K) values (a2):

2KOH + CO2 S K2CO3 + H2O, ∆h ) -1258 kJ/kg (a1) 6KOH + 2C S 2K + 6H + 2K2CO3,∆h ) 797 kJ/kg (a2) It is also postulated31 that KOH dehydration (a3) and formation of potassium carbonate (a4) are thermodynamically possible at 573 K and ambient temperature, respectively:

2KOH w K2O + H2O, ∆h ) 2240 kJ/kg

(a3)

K2O + CO2 w K2CO3,∆h ) -4168 /kg

(a4)

K,32

it can be proposed that Moreover, for temperatures above 700 carbon reacts with molten-phase potassium hydroxide and water vapor according to:

4KOH + C w K2O + K2CO3 + 2H2,∆h ) 893 kJ/kg (a5) with K2O subsequently regenerated into KOH by reacting with water vapor. Release of CO and CO2 at high temperatures (above 1073 K) can be attributed to the decomposition of potassium carbonate.30 Although this is a relatively stable compound, which has a melting temperature of 1164 K,29 decomposition may take place at temperatures much lower than the melting point in the presence of carbon (about 723 K33). When the temperature exceeds 973 K, metallic potassium is formed due to the possible reduction by hydrogen (a6) and carbon (a7):31

K2O + H2 w 2K + H2O, ∆h ) 1263 kJ/kg

(a6)

K2O + C w 2K + CO, ∆h ) 3013 kJ/kg

(a7)

The presence of K, K2O, K2CO3, and KOH in the activated chars34 supports the possible occurrence of reactions a1-a7, but the quantitative extent and the relative importance of each reaction is not yet known. Heating temperatures, applied for the pyrolysis tests of this study, are below 900 K so that not all of the reactions a1-a7 may actually be relevant. The production of CO2 and steam from the decomposi(29) Perry’s Chemical Engineering Handbook, 6th ed., international student ed.; Perry, R. H., Green, D. W., Maloney, J. O., Eds.; McGrawHill Book Co.: New York, 1984; Chapter 3, p 19. (30) Lillo-Rodenas, M. A.; Marco-Lozar, J. P.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2003, 41, 267–275. (31) Otowa, T.; Tanibata, R.; Itoh, M. Gas Sep. Purif. 1993, 7, 241– 249. (32) Ishida, M.; Toida, M.; Shimizu, T.; Takenaka, S.; Otsuka, K. Ind. Eng. Chem. Res. 2004, 43, 7204–7206. (33) Mims, C. A.; Rose, K. D.; Melchior, M. T. J. Am. Chem. Soc. 1982, 104, 6886–6887. (34) Qiao, W.; Yoon, S. H.; Mochida, I. Energy Fuels 2006, 20, 1680– 1684.

tion of the lignocellulosic substrate starts at temperatures much lower than for coals, so that it can be expected that reaction a1 may become active. Other possible processes are described by reactions a3, a4, and a5. Studies on the release of alkali metal during biomass pyrolysis (for instance, see ref 35) indicate that potassium release occurs only for temperatures above 973 K. Therefore, it can be assumed that, for the conditions of this study, all of the metallic potassium is retained by the char. It is worth noting that potassium compounds could be important, in terms of catalytic action, for the successive high-temperature gasification/oxidation reactions of the char. In principle, the energetics aspects of the process (for reactions a1-a7 evaluated by means of the formation heats36) should consider the occurrence of reactions a3, a4 (reaction a1 is the sum of reactions a3 and a4), and a5 and the endothermic contribution of the melting process, but precise evaluations are not possible because the extent of each reaction cannot be foreseen.

Results The gas release rate and the bed temperature (measured at several bed heights) versus time profiles are used to extract information about process dynamics. More precisely, changes caused by KOH are evaluated in the actual reaction temperatures and characteristic times of the process. The yields of char, gas, water, and liquid-phase organics and the composition of the gaseous and liquid products are also discussed in detail. The amount of catalyst in wood is varied (from 0 to 7.7% on a dry wood basis) for a heating temperature of 800 K. The effects of the heating temperature are also examined (600, 800, and 900 K) given a KOH content in wood of 0.58%. Times and Temperatures of Potassium-Catalyzed Pyrolysis. The time profiles of the gas release rate, evaluated at the reactor exit, and temperature, measured at a bed height of 10 cm (from the flow distributor), reported in Figure 1A,B, provide information on the process dynamics as the KOH concentration in wood is varied (heating temperature 800 K). From the qualitative point of view, the temperature versus time profiles are the same at each position of the packed bed height. The first decreasing part is essentially due to the addition of cold particles in the hot (preheated) reactor, the global endothermicity of the decomposition process, and the convective cooling associated with the flow of gaseous and vapor-phase products outside the bed. As long as decomposition reactions are underway, the temperatures hardly move from the region of minimum values. Indeed, the release of gaseous species (and most likely vapor-phase products) essentially occurs during the time period of high variation in the bed temperature, well before steady values are approached. Consequently, the second part of the temperature profile essentially represents the slow heating of the charred solid residues toward the initial preheating value. From the quantitative point of view, temperatures of the impregnated particles are lower for the decreasing part and higher for the increasing part when compared to those of extracted wood. Also, minimum temperatures are always higher and increase with the KOH concentration. The gas release rates show a very rapid process that takes much shorter times for the KOH treated samples. Moreover, the long tail observed for extracted wood completely disappears, indicating that, as a consequence of KOH impregnation, the decomposition of the three main wood components takes place over more overlapping temperature intervals. (35) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280–1285. (36) International Critical Tables of Numerical Data, Physics, Chemistry, and Technology; Washburn, E. W., Ed.; McGraw-Hill Book Co.: New York, 1929; Vol. 5, pp 176-180.


Figure 1. (A) Release rates of gas (dry sample mass basis) as functions of time for several KOH concentrations in wood and a heating temperature of 800 K. (B) Temperature versus time profiles, measured at a bed height of 10 cm, for several KOH concentrations in wood and a heating temperature of 800 K.

Figure 2. Conversion time, tc, and corresponding temperature, Tc, minimum temperature, Tmin, and maximum rate of gas release, dYgm, as functions of KOH concentration in wood for a heating temperature of 800 K (temperatures refer to a bed height of 10 cm).

The gas release rate and temperature profiles of Figure 1A,B permit the introduction of some global parameters useful for a quantitative understanding of the modifications introduced by KOH on the pyrolysis characteristics. From Figure 1A, tgm, the time of the maximum gas release rate, dYgm (with the percentage of gas released), is defined. Another useful parameter is the conversion time, tc, which is time when the production of 75% of the total gas has occurred, and the corresponding temperature, Tc. From Figure 1B, the minimum temperature, Tmin, and corresponding time, tmin, can be defined. A derivative of Figure 1B (not shown) yields the time of the maximum rate of temperature increase, tm, and the corresponding temperature, Tm. Finally, the global devolatilization rate, mtot, is defined as the ratio of the mass fraction of total volatile species produced to the conversion time. The dependence of these parameters on the KOH concentration can be seen in Table 2 and Figure 2. The characteristic times present a strong decrease for KOH concentrations in wood below 1% and then remain approximately constant (maximum decrease factor of about 2.5). The times of maximum gas release rate are also reduced up to a factor of about 2.3, while the maximum in the gas release

Di Blasi et al.

Figure 3. (A) Release rates of gas (dry sample mass basis) as functions of time for heating temperatures of 600, 800, and 900 K and a KOH concentration in wood of 0.58%. (B) Temperature versus time profiles, measured at a bed height of 10 cm, for heating temperatures of 600, 800, and 900 K and a KOH concentration in wood of 0.58%.

rate is up to about 4.3 times higher than that of extracted wood (the corresponding amount of gas released is always around 23-28% of the total value). Despite the continuous diminution in the yields of volatile species production, the simultaneous and stronger diminution in the characteristic process time approximately doubles the global devolatilization rate for KOH concentrations up to about 1%. The minimum temperature is always positioned at times slightly longer than those of the maximum gas release rate, confirming that conversion occurs while temperatures are much lower than the initial bed temperature. The values of the Tc parameter approximately represent the reaction temperatures. These become successively lower as the KOH concentration is increased and are from about 35 to 70 K lower than those of extracted wood. This result and the lower values recorded in the first decreasing part of the temporal profiles clearly indicate that wood decomposition catalyzed by KOH takes place at lower temperatures. The higher values of the minimum temperatures, comprised in the range 580-598 K (versus 561 K of extracted wood), also indicate that KOH causes a reduction in the global endothermicity of the decomposition process. It should be noted that KOH indirectly alters the energetics of wood decomposition through modifications in the activity of the exothermic charring reactions versus the endothermic tarry reactions37,38 and the extent of convective cooling associated with the release of gaseous and vapor-phase compounds (as shown in the following, char formation is enhanced). The direct contribution of KOH transformations should also be taken into account, although for low concentrations it is certainly negligible. The more rapid rise of the temperature versus time profiles (Figure 1B), observed (37) Mok, W. S. L.; Antal, M. J. Thermochim. Acta 1983, 68, 165– 186. (38) Milosavljevic, I.; Oja, V.; Suuberg, E. M. Ind. Eng. Chem. Res. 1996, 35, 653–1091.



Table 2. 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, as Functions of KOH Concentration in Wood for a Heating Temperature of 800 K, and 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 (Temperatures Refer to a Bed Height of 10 cm) tgm [s] extraction KOH-0.1% KOH-0.2% KOH-0.58% KOH-1.28% KOH-3.1% KOH-7.7%

139 79 73 60 60 60 60 Tmin [K]

extraction KOH-0.1% KOH-0.2% KOH-0.58% KOH-1.28% KOH-3.1% KOH-7.7%

561 580 584 593 598 594 596

dYgm [s-1]; % gas

tc [s]

mtot × 103 [s-1]

0.03; 24.2 0.08; 27.6 0.10; 24.4 0.13; 24.8 0.14; 25.7 0.15; 23.0 0.15; 23.7 tmin [s]

336 201 173 142 131 135 135

2.2 3.4 4.0 4.7 4.9 4.7 4.4

130 109 97 88 79 86 103

Table 3. Time of the Maximum Gas Release Rate, tgm, Maximum Gas Release Rate, dYgm, with the Percentage of Gas Released, 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, and Global Devolatilization Rate, mtot, as Functions of the Heating Temperature for KOH Concentration in Wood of 0.58% (Temperatures Refer to a Bed Height of 10 cm) tgm [s] dYgm [s-1]; % gas Tmin tmin tc Tc tm Tm mtot × 103 [s-1]

600 K

800 K

900 K

552 0.03; 52.2 446 231 659 646 543 598 0.9

60 0.13; 24.8 593 88 142 620 205 676 4.7

36 0.21; 21.2 657 77 109 668 146 710 6.7

for KOH-catalyzed pyrolysis, is the result of the highly reduced tailing zone in the gas release curves (Figure 1A), a consequence of the enhanced overlap between the degradation of holocellulose and lignin (the values of the Tm parameter are slightly lower in accordance with the displacement of the decomposition reactions at lower temperatures). Moreover, the heating of the char, in the second part of the temperature curve, may also be affected by alterations in the physical properties of the sample caused by the presence of potassium compounds and/or induced modifications in the solid structure. The influences of the heating temperature (600-900 K) on the wood decomposition dynamics have been examined for a KOH concentration of 0.58%. Results in terms of the global parameters previously introduced are summarized in Table 3, whereas Figure 3A,B reports the temporal profiles of the gas release rate and the bed temperature at a distance of 10 cm from the flow distributor. The conversion times are highly reduced as the heating temperature is increased from 600 to 800 K (factors of 4.6). However, a further increase from 800 to 900 K only introduces a reduction by a factor of about 1.3. The same qualitative trends are also shown by the maximum gas release rate and the corresponding amount of gas released. On the whole, the global rate of volatile release is increased by about 7 times within the range of heating temperatures considered. This produces a reduction in the vapor residence time across the packed bed, thus also affecting the activity of secondary reactions.

tc [s]

Tc [K]

tm [s]

Tm [K]

336 201 173 142 131 135 135

674 639 620 620 617 603 602

340 236 224 205 194 193 182

688 677 676 676 667 648 630

To better explain these results, it is useful to examine the thermal history undergone by the packed bed and the rate of gas release (Figure 3A,B). While the temperature profiles show the same qualitative trends for the heating temperatures of 800 and 900 K, for the case of 600 K, a maximum appears in the second part of the curve, which exceeds the heating value of about 50 K. This peculiar behavior has already been observed for wood pyrolysis catalyzed by acids and low heating temperatures.27 The very mild thermal conditions cause a diminution in the radial gradients across the bed, so that the processes of holocellulose and lignin decomposition, which, at ambient pressure, are globally endothermic and exothermic, respectively, take place over a rather wide spatial zone in a sequential mode. In this way, as testified also by the gas release rate profile, a wide zone of low temperatures, where slow endothermic holocellulose decomposition predominates, is followed by a rapid rise toward a maximum value as a consequence of the onset of exothermic lignin decomposition. As compared to the case of high heating temperatures, the maximum dYgm is attained at times much longer than those of Tmin (552 versus 231 s), the corresponding amount of gas released is roughly doubled, and Tc is nearly coincident with the maximum temperature (and higher than the corresponding value obtained for the experiment with a heating temperature of 800 K). Sample temperatures exceeding the heating value of 600 K also lead to KOH melting, thus reducing the differences with the cases of high heating temperatures. It should be noted that 600 K is a too low heating temperature for a significant conversion of extracted wood, while, for a value of 650 K, the bed temperatures do not show any maximum.27 In other words, the displacement of wood decomposition process at lower temperature, as a consequence of KOH impregnation, is responsible for a wider isothermal zone across the bed so that process energetics (i.e., the sequential occurrence of endothermic and exothermic processes) can be more clearly seen (this result also indicates that, although the temperature ranges of holocellulose and lignin decomposition become closer, they are not coincident). Product Yields and Composition. The yields of the main classes of pyrolysis products, that is, char, permanent gases, organics, and water, expressed as percent of the initial sample mass, are reported as functions of the KOH concentration in wood in Figure 4. The yields of the gaseous species (CO, CO2, CH4) together with the ratio of the CO to CO2 yield and the main classes of organic products (carbohydrates and furans are generated from the decomposition of holocellulose, whereas the


Figure 4. Yields of the main classes of pyrolysis products, expressed as percent of the initial dry sample mass, as functions of the KOH concentration in wood for a heating temperature of 800 K.

Figure 5. Yields of the main gaseous species, expressed as percent of the initial dry sample mass, and ratio of CO2 to CO yields as functions of KOH concentration in wood for a heating temperature of 800 K.

Figure 6. Yields of major carbohydrates, minor carbohydrates, furans, and phenols (dry sample mass basis) as functions of KOH concentration in wood for a heating temperature of 800 K.

hydroxy-phenolics and guaiacols are derived from the lignin building blocks (classes defined in the Experimental Section)) are reported as a function of KOH concentration in wood in Figures 5 and 6, respectively. As the KOH concentration is increased, the amounts of char and water produced increase at the expense of the liquid-phase organic products (Figure 4). As already observed for the global parameters, maximum variations on the product yields are caused by low KOH concentrations (range 0-1%). For this range, organic products are reduced by a factor of about 2.7 (from about 43% to 16%) to the advantage of char (probably including K compounds, such as K, K2CO3, KO2, KOH) and water, augmented by factors of about 1.3 (from 22% to 30%) and 1.6 (from 19% to 30%), respectively. The yields of gas also increase from about 11% to 18%. For KOH concentrations in wood above 1%, a further increase in the yields of gas (from about 18% to 22%) and char (from about 30% to 38%) takes place at the expense of liquid-phase organic products (lowered to about 8% for KOH concentrations around 8%), whereas the yields of water remain practically unchanged.

Di Blasi et al.

Figure 7. Ratio of the noncombustible to combustible volatile products, Rv, yields of char and water, Ywc, yields of flammable volatile products, Yf, and ratio of CO2 to the total gas (dry sample mass basis) as functions of the KOH concentration in wood for a heating temperature of 800 K.

The yields of the main gaseous species (Figure 5) show that carbon dioxide is always the most abundant product, followed by CO, CH4, and traces of H2 and C2-hydrocarbons (not shown). Similar to the results already discussed for the main classes of products, all of the yields present a rapid increase (for instance, for CO2 up to factors of about 2) for low catalyst concentration. Next, while the yields of CO2 still increase significantly, other gaseous species remain roughly constant. In accordance with these trends, the ratio of the CO2 to CO yields also continuously increases from about 1.6 up to 2.5. Similar to the total yields of organics, major carbohydrates attain their maximum for extracted wood (Figure 6). The other classes of compounds present a narrow zone with a maximum for small values of KOH concentrations (0.2-0.6%). Moreover, for KOH concentrations above 1%, while the yields of furans have already attained their minimum and remain nearly at zero values, the diminution in the production of major carbohydrates, minor carbohydrates, and phenols is slower. As KOH in significant concentrations can be used as flame retardant for lignocellulosics,3-5 the results of this study can also be useful to get some information about the flammability of volatile products of wood pyrolysis. Results, summarized in Figure 7, show that the sum of water and char yields, Ywc, thanks to the treatment applied, can be increased from about 40% to about 68% with a strong diminution in the yields of volatile flammable compounds, Yf, already for catalyst concentrations below 1%. The ratio of the noncombustible versus combustible volatile species, Rv, highly increases, attaining a maximum of about 3 for the highest KOH concentrations investigated here. Finally, on a total gas basis, the contribution of CO2 is about 60% for KOH concentrations below 1% and then continuously increases up to about 67%. All of these results show the effectiveness of KOH as a flame retardant additive, which is accomplished through modifications in the amounts and composition of volatile pyrolysis products and the displacement of the production of flammable volatile species at temperatures too low for gas-phase flaming ignition. However, the presence of K compounds enhances the reactivity of char.39 Details about the organic compounds are given in Figure 8A-E. The yield of levoglucosan (Figure 8A) presents a very steep decay to nearly zero values at very low KOH concentrations (the same trend is also observed for 2,3anhydro-Dgalactosan and 2.3-anhydro-D-mannosan, based on peak area evaluations). Furthermore, both hydroxyacetaldehyde and acetic acid also continuously decrease as the KOH concentration in wood is increased. Instead, hydroxypropanone presents a wide (39) Shafizadeh, F.; Bradbury, A. G.; De Groot, W. F.; Aanerud, W. Ind. Eng. Chem. Prod. Res. DeV. 1982, 21, 97–101.



Figure 8. (A-E) Yields of organic compounds (dry sample mass basis) as functions of KOH concentration in wood for a heating temperature of 800 K.

zone (up to KOH concentrations of about 1.3%) of roughly constant values slightly higher than the value obtained for extracted wood. All of the compounds of minor carbohydrates (Figure 8B) peak at low KOH concentrations (0.1-0.6%). It is worth noting that, although these are minor compounds of wood pyrolysis, the maximum yields are increased, as a consequence of the catalytic action of KOH, up to factors of 2 (acetoxyacetone and 2-methyl-2-cyclopentenone), 3 (1-hydroxy-2-butanone), or 4-6 (3-methyl-2-cyclopentenone, 3-ethyl-2-hydroxy-2-cyclopentenone). Potential applications of these products are in sector of aromas and as reaction intermediates for various high value added products,40-42 in particular for the synthesis of cylopentenoids, which exhibit interesting biological and medicinal properties.43 Incorporation of KOH in wood also maximizes the yields of some furan compounds (Figure 8C), such as furfuryl alcohol and 2-acetylfuran, up to factors of 15 and 2, respectively. On the other hand, other compounds (2-furaldehyde, 5-methyl-2furaldehyde, 2(5H)-furanone) are rapidly brought to zero (production of 5-hydroxymethylfurfural is also brought to zero, as appears from evaluations of the peak area). Furfuryl alcohol (40) http://old.iupac.org/publications/pac/1986/5806x0869.pdf. (41) http://www.leffingwell.com/burnt.htm. (42) http://ref.daum.net/item/706594. (43) Miller, J. A.; Pugh, A. W.; Ullah, G. M.; Welsh, G. M. Tetrahedron Lett. 2001, 42, 955–959.

is widely used in producing various synthetic fibers, rubbers, resins, and farm chemicals.44 Some products of lignin pyrolysis (Figure 8D,E), that is, phenol, cresols, hydroquinone, 4-propyl-guaiacol, isoeugenolcis, isoeugenol-trans, 4-acetonguaicol, guiacol, and 4-ethylyguaiacol, also peak at KOH concentrations of about 0.2-0.6% again with quite high increase factors (2-6) and yields between 0.1-1.1% (the most abundant products are guaiacol (1.1%), isoeugenol-cis (0.7%), 4-ethylguaiacol (0.4%), and isoeugenoltrans (0.4%)). However, eugenol, vanillin, and 4-methylguaiacol encounter a continuous diminution. Phenolic compounds have a good industrial value45-47 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). (44) Chen, X.; Li, H.; Luo, H.; Qiao, M. Appl. Catal., A 2002, 233, 13–20. (45) Pakdel, H.; Amen-Chen, C.; Roy, C. Can. J. Chem. Eng. 1997, 75, 121–126. (46) Murwanashyaka, J. N.; Pakdel, H.; Roy, C. In Fast Pyrolysis of Biomass: A Handbook; Bridgwater, A. V., Ed.; CPL Press: Newbury, U.K., 2002; Vol. 2, pp 407-418. (47) Amen-Chen, C.; Pakdel, H.; Roy, C. Bioresour. Technol. 2001, 79, 277–299.


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Table 4. Yields of the Main Classes of Pyrolysis Products and of the Main Gaseous Species, Expressed as Percent of the Initial Dry Sample Mass, as Functions of the Heating Temperature for a KOH Concentration in Wood of 0.58% total CO2 char H2O organics gas CO CH4 [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] 600 K 800 K 900 K

36.8 28.6 26.6

27.3 29.2 28.5

19.7 20.8 24.0

13.1 17.3 20.5

96.9 95.9 99.0

8.1 10.3 11.7

4.8 6.0 7.0

0.2 0.9 1.4

Table 5. Yields of the Main Organic Compounds (Dry Sample Mass Basis) as Functions of the Heating Temperature for a KOH Concentration in Wood of 0.58% temperature [K] 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 [%] 3,4-dimethylphenol 4-acetonguaiacol 4-ethylguaiacol 4-methylguaiacol 4-propylguaiacol cresols eugenol guaiacol hydroquinone isoeugenol-cis isoeugenol-trans phenol vanillin

600

800

900

5.09 1.92 1.50 1.59 0.08 0.71 0.36 0.04 0.19 0.04 0.08 0.18 0.82 0.06 0.07 0.13 0.02 0.54 2.47 0.01 0.08 0.26 0.31 0.07 0.08 0.05 0.89 0.06 0.41 0.14 0.05 0.06

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 0.02 0.09 0.41 0.31 0.13 0.22 0.08 1.12 0.09 0.72 0.39 0.18 0.07

5.12 1.72 1.76 1.42 0.22 1.00 0.40 0.08 0.30 0.10 0.12 0.28 0.96 0.06 0.14 0.17 0.02 0.57 4.18 0.02 0.10 0.34 0.38 0.12 0.33 0.12 1.05 0.10 0.86 0.40 0.24 0.12

To investigate the effects of the heating temperature on the pyrolysis of wood catalyzed by KOH, tests have been made for 600, 800, and 900 K (KOH content in wood of 0.58%). Results are summarized in Tables 4 and 5 in terms of yields of the main product classes and detailed composition of gas and organics. It can be seen that, as the thermal conditions during conversion are made more severe, the amount of char produced is reduced (a factor of about 1.4) with a significant increase in the gas (a factor of about 1.6) and organics (a factor of about 1.2) yields, whereas variations on the water yields are small. These results can be attributed to an increased activity of primary devolatilization reactions, testified by the higher yields of liquidphase organic products, and secondary reactions, as indicated by the higher gas yields; that is, the increase in the organic products is lower than expected because of the simultaneous enhancement of the activity of cracking reactions. The occurrence of secondary reactions is also supported by the presence, at the intermediate temperature, of a maximum yield of some compounds (acetic acid, hydroxyacetaldehyde, 1-hydroxy-2butanone, etc.). The yields of minor carbohydrates and phenols, maximized by KOH impregnation, are slightly decreased and increased, respectively, as the heating temperature is increased from 800 to 900 K, while those of furfuryl alcohol are practically insensitive to the range of heating temperatures examined. In

general, the variations on the organic compounds are relatively small. This can be explained by the control exerted by heat transfer from the reactor wall to the packed bed (the characteristic size is the bed diameter) in the conversion process and mainly by the successively reduced residence time of the primary vapors in the hot reaction zone. Indeed, a rapid decay of the temperature above the bed is established. On the other hand, the separation between endothermic and exothermic effects for low heating temperatures (Figure 4A,B) also contributes in reducing the differences due to external heating. Discussion To explain the role played by KOH in the distribution of wood decomposition products, it should be observed that there are two main effects: (a) modification of the reaction temperature (thermal effects) and (b) modification of primary and secondary reaction paths and activity (chemical effects). The two effects also influence each other. The displacement at lower temperatures of lignocellolosic material decomposition observed for KOH impregnated samples is in qualitative agreement with previous results reported in the presence of deliberately added alkaline compounds.12,19-22 Moreover, a monotonic decrease in the temperature at which significant weight loss commences with increased alkali (sodium and potassium) salt loading is also reported in the thermogravimetric analysis of catalytic pyrolysis of lignocellulosic fuels12,13 and pyrolysis of carbohydrates catalyzed by NaOH.11 In particular, the weight loss curves12 are shifted toward lower temperatures, indicating that increased alkali salt impregnation enhances decomposition and increases weight loss rates at lower temperatures. For maximum salt contents of about 15%, an anticipation is evaluated of about 30-40 and 70-80 K for the beginning and the peak of the weight loss rate, respectively. Although the reaction temperatures evaluated in this study are average (between the gas/vapor stream and the particle surfaces) values and not the actual particle temperatures, it is worth noting that the observed range of variation (approximately 35-70 K) is comparable to that already reported.12 The modifications in the reaction temperature have been attributed to alkali catalysis of the reactions and/or to the hydrolytic attack resulting in fiber swelling and solid matrix deterioration during impregnation.12,20 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 chemical effects consist of a modification in the primary decomposition paths of lignin, hemicellulose, and cellulose and an enhancement in the activity of secondary reactions. From the analysis of the yields of products, it appears that the incorporation of KOH in wood, even in relatively low amounts, introduces drastic changes in the slate of products: char, water, and gas (especially CO2) formation is highly enhanced at the expense of organic compounds. The increase in the yields of char, following KOH incorporation in wood, is a result already known in the field of active carbon preparation and is also reported by the studies on the alkali-catalyzed decomposition of wood (KOH,10 NaOH,15 and K or Na carbonates12). The displacement of the reaction process at lower temperatures enhances the dehydrating, carboxylating, and charring reactions of holocellulose, which also produces a char with a more condensed structure.22 In the primary decomposition of the lignin fraction, alkali metals also favor dehydration, demethoxylation, decarboxylation, demethylation, and char formation,14,16 again


resulting in a char structure modified with respect to nonimpregnated wood. The stabilization of the carbon atoms in crystallites,48 after removal of cross-linking due to the presence of oxygen in the potassium compounds,49 is retained to be the cause of the minimization in the formation of primary tars and the enhancement in the condensation of the solid matrix.50 The increased production of CO2 associated with the presence of alkali hydroxides is also reported by other studies.4,8 It can be attributed8 to the pyrolytic decarboxylation of hemicellulose uronates and parts of the cellulose chain. Moreover, for high KOH concentrations, a predominance of the low-temperature charring reactions versus the high-temperature devolatilization reactions51 and/or the decomposition of potassium carbonate30 may also play an important role. 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. The displacement of the reaction process at temperatures too low for the formation of levoglucosan and, presumably, other liquid-phase organic products is also indicated as an important feature of the base-catalyzed decomposition of cellulose.5 As for the primary decomposition of cellulose and hemicellulose, the detailed composition of organic products shows that the high-temperature and alkali-metal-trace dominated paths6,7 of depolymerization by transglycosylation (formation of levoglucosan, anhydrosugars, cellobiosan, and higher oligomers), on one side, and decarbonylation (CO) and fragmentation (formation of hydroxyacataldehyde, formaldehyde, acetol, and methylglyoxal) reactions, on the other, are profoundly altered. As expected, in agreement with the strong inhibition caused by trace amounts of K ions on the high-temperature paths of cellulose depolymerization, the former path disappears. However, impregnation of extracted wood with KOH, even in very low amounts, always produces yields of hydroxyacetaldehyde much lower than those measured for the extracted samples. On the other hand, a significant increase in char formation is also caused by small KOH concentration. This is in contrast with the formation of hydroxyacetaldehyde, which is maximized for the conditions of fast pyrolysis when char production in minimized. In other words, either the chemical state of the K ion (initially in the form of KOH) combined with the impregnation procedure or the significant diminution in the decomposition temperatures does not reproduce the beneficial effects of alkali metals on the activity of fragmentation reactions. Thus, contrary to the experimental evidence gained comparing the results obtained for demineralized or partial demineralized samples and untreated samples, the yields of hydroxyacetaldehyde become lower. The yields of other typical products of fragmentation reactions, acetic acid and hydroxypropanone, are less affected, but a continuous decrease or a stabilization around constant values, respectively, is observed, at least for moderate KOH concentrations. This means that the action of KOH changes not only with the compound classes but also among compounds of the same (48) Moreno-Castilla, C.; Carrasco-Marin, F.; Lopez-Ramon, M. V.; Alvarez-Merino, M. A. Carbon 2001, 39, 1415–1420. (49) Marsh, H.; Yan, D. S.; O’Grady, T. M.; Wennerberg, A. Carbon 1984, 22, 603–611. (50) Molina-Sabio, M.; Rodriguez-Reinoso, F. Colloids Surf., A: Physicochem. Eng. Aspects 2004, 241, 15–25. (51) Piskorz, J.; Radlein, D.; Scott, D. S.; Czernik, S. In Research in Thermochemical Biomass ConVersion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: London and New York, 1988; pp 557571.


class. In conclusion, KOH impregnation eliminates the depolymerization paths in holocellulose pyrolysis and also highly reduces the activity of the fragmentation and decarbonylation reactions. At this stage, it is difficult to ascertain whether reaction paths other than fragmentation and decarbonylation become important for the further degradation of the condensed and stable charred intermediate generated at low temperature from KOH impregnated wood. Thanks to a detailed quantitative characterization of the organic fraction, this study has shown significant enhancement in the production of some minor carbohydrates not reported by previous investigations. It should be observed that, under strong alkaline conditions that can resemble the condition of KOH-catalyzed pyrolysis, carbohydrates undergo retroaldol cleavage and that the cleavage products subsequently produce higher molecular weight condensation products by aldol reactions.52 Aldol condensation is considered to be a relevant process for the production of large organic compounds from aqueous-phase catalytic processes of biomass conversion because various species containing carbonyl groups can be formed.53 The augmented production of furfuryl alcohol associated with the decay in the production of other furan compounds, in particular 2-furaldehyde, is in agreement with previous findings.10,11 This is generally attributed to the activity of secondary vapor phase reactions enhanced by the presence of potassium compounds. It is also known54 that, in a highly alkaline medium, 2-furaldehyde can react with itself in an oxidoreduction reaction (Cannizaro reaction) to give a higher oxidation product, the 2-furoic acid, and a lower oxidation product, the furfuryl alcohol. Complete devolatilization of the lignin component at low temperatures, as testified by the gas release curves, and alkali catalysis in the decomposition of the original structure are responsible for the noticeable increase in the yields of volatile phenolic products. The increase in the yields of guaiacol (with a diminution in the amounts of methyl guiacol), 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 a-carbonyl groups, are also reported22 for wood pyrolysis in the presence of potassium carbonate. Also, the significant increase in the amount of isoeugenol observed in this study can be attributed to the isomerization of eugenol (whose yields are diminished), a reaction catalyzed by KOH.55 Conclusions Catalytic pyrolysis of fir wood has been experimentally investigated by means of a packed-bed reactor by varying the KOH concentration in wood (from 0 to 8%) and the heating temperature (600-900 K). Samples have been prepared by impregnation of pre-extracted wood particles with aqueous solutions of KOH. In agreement with previous results, it has been observed that the higher is the amount of KOH, the lower are the decomposition temperatures (variations approximately in the range 35-70 K). Moreover, again in agreement with previous results, the production of char, water, and CO2 is enhanced (from 47% to 81%) at the expense of organic liquid(52) Schwarzinger, C. J. Anal. Appl. Pyrolysis 2003, 68-69, 137–149. (53) Huber, G. W.; Dumesic, J. A. Catal. Today 2006, 111, 119–132. (54) Fakhfakh, N.; Cognet, P.; Cabassud, M.; Lucchese, Y.; Dias de Los Rios, M. Chem. Eng. Process 2008, 47, 349–362. (55) Cerveny, L.; Krejcikova, A.; Marhaul, A.; Rozicka, V. Kinet. Catal. Lett. 1987, 33, 471–476.


phase products (from about 43% to 8%), with a small increase in the yields of other gaseous species (from 5% to 7%). Modifications in the reaction temperatures and the sharing between char and volatile products highly shorten the characteristic process times, increasing the global devolatilization rate. The decline in the yields of organic products is associated with that of the major compounds typically produced from uncatalyzed pyrolysis of wood, that is, levoglucosan, hydroxyacetaldehyde, and acetic acid (weak positive effects on the formation of hydroxypropanone). The trends shown by these products indicate that wood pyrolysis catalyzed by KOH does not occur, at least for the holocellulose fraction, according to the well-known Waterloo model6,7 where alkali metal traces play a controlling role. Indeed, although the immediate disappearance of levoglucosan (and other sugars) among the reaction products was expected, the highest concentrations of other main compounds are observed for the extracted sample. Therefore, in the presence of KOH, introduced by physical sorption, not only the depolymerization path is completely eliminated but the competitive decarbonylation and fragmentation reactions are also progressively inhibited. However, a range of small KOH concentrations in wood catalyzes the production of some minor carbohydrates (2-methyl-2-cyclopentenone, 1-hydroxy-2-butanone, 3-methyl-2-cyclopentenone, 3-ethyl-2hydroxy-2cyclopentenone) and furfuryl alcohol, which are highly augmented. In terms of new chemical paths, it is possible that KOH and related compounds formed during pyrolysis not only catalyze secondary vapor-phase decomposition of organic products but also retro aldol cleavage of some carbohydrates followed by the formation of higher molecular weight condensation products by aldol reactions.

Di Blasi et al.

Measurements of the gas release rate show that the decomposition temperature of the three main wood components overlaps at a larger extent; that is, the wide temperature range typical of lignin pyrolysis becomes narrower and is also displaced toward that of cellulose and hemicellulose decomposition. Moreover, again for a range of moderate KOH concentrations in wood (about 0.15-0.6%), the total amount of volatile phenols generated from the decomposition of this fraction is roughly doubled with some compounds (phenol, cresols, isoeugenol-trans, guaiacol, 4-ethylguaiacol), increased by factors of 3-6. It can be understood that this is the result of an increase in the thermal reactivity of lignin and the cleavage of ether and C-C bonds. Modifications introduced in the structure of the solid, on the other hand, do not allow the formation of other phenolic compounds such as vanillin and 4-methylguaiacol. Given the importance in various industrial branches of the products, whose yields can be highly increased by the selection of appropriate KOH concentrations in wood, the results of this study are also important from the practical side. After extraction of the more valuable chemicals, the remaining oil fraction and char can be exploited for energetic applications. In this way, not only the economic aspects of the process can be improved but the conversion process avoids the presence of byproduct (gas could be used to partly contribute in the heat supply to the endothermic pyrolysis process). Although this study has provided quantitative information on the products, temperatures, and times of wood pyrolysis catalyzed by KOH, the mode of action of this metallic compound on the pyrolysis mechanism is still unclear. Further investigations, also including a characterization of the char and its inorganic content, should be carried out to improve the understanding of these aspects. EF800827Q

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