Analysis of the Physical and Chemical Mechanisms of Potassium

ARTICLE pubs.acs.org/IECR

Analysis of the Physical and Chemical Mechanisms of Potassium Catalysis in the Decomposition Reactions of Wood C. Di Blasi,*,† A. Galgano,‡ and C. Branca‡ † ‡

Dipartimento di Ingegneria Chimica, Universita degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy Istituto di Ricerche sulla Combustione, C.N.R., P.le V. Tecchio, 80125 Napoli, Italy ABSTRACT: Fir wood pyrolysis is carried out for extracted, KOH and KCl impregnated, and impregnated-extracted samples. For Kþ concentrations in wood of 0.14% (dry mass basis), the effects of KOH are quantitatively higher, but both additives produce a significant diminution in the yields of hydroxyacetaldehyde, furfural, and sugars, the decomposition temperature, and the conversion time. Water extraction of KCl impregnated samples is apt to reproduce again the characteristics of extracted wood pyrolysis. Instead, water extraction of KOH impregnated samples only reports the yields of hydroxyacetaldehyde, acetic acid, and hydroxypropanone to the original values whereas differences in the other variables, especially characteristic times and temperatures and levoglucosan yields, remain very large. Therefore, the action of the impregnated Kþ compounds during wood pyrolysis can be attributed to both irreversible modifications in the wood structure during impregnation, whose entity depends on the basicity of the aqueous solution, and intrinsic action of the Kþ ion on the activity of the decomposition reactions.

’ INTRODUCTION Impregnation of alkali compounds in lignocellulosics is pursued to improve the fire retardance performances.1,2 Combustion of solid fuels is essentially a two-stage process, that is, solid devolatilization with flaming ignition of volatile products and heterogeneous oxidation of char. The alkali mechanism of fire retardance consists of a strong reduction in the amount of volatile flammable products, to the advantage of char and water, during the devolatilization stage which is displaced at temperatures so low that gas-phase ignition is hindered. Moreover, the formation of a thick char layer, characterized by a low thermal conductivity, also delays the transfer of heat from the external heated surface to the most internal virgin solid, with a further improvement in the thermal response of the material to fire. However, it should be noted that alkaline additives, in their original state or most likely modified by the thermal treatment undergone during material pyrolysis, remain in the solid-phase charred residue and may catalyze the subsequent oxidation process.3 The good properties of alkali hydroxides (KOH, NaOH) as char promoters have also motivated their large use as chemical activators during the carbonization of carbonaceous precursors for activated carbon preparation.4,5 Fire retardance and chemical activation of lignocellulosic materials require the impregnation of large quantities of alkali compounds as both applications aim at driving the wood pyrolysis toward maximum production of char and water. The addition of small amounts of alkalis (NaOH, KOH, Na2CO3, K2CO3, KC2H3O2, KCl, NaCl) to biomass6-21 essentially reproduce the same qualitative features with results that are dependent on the chemical state and nature of the alkali metal, the concentration, and the mode of addition. Recent results,20,21 which provide a detailed characterization of the condensable pyrolysis products, show that although some major typical compounds are eliminated or highly reduced, the yields of some cyclopentenones and phenols can be increased up to factors of r 2011 American Chemical Society

about 4-6. As these can find applications in various industrial branches, this finding is potentially important for improving the economics of biomass pyrolysis by combining the energetic exploitation of the solid and liquid products with the production of chemicals. However, a slow development is observed of both new flame retardants and biomass pyrolysis for the production of biofuels and chemicals owing to the lack of understanding of the catalysis mechanism of impregnated alkali metal on the decomposition reactions of lignocellulosic materials. In reality, the deliberate addition of alkaline cations to lignocellulosic materials, as a pretreatment before pyrolysis, can be based on either ion exchange22 or physical sorption.6-21 In this case, there is no experimental investigation about the possible modifications induced by the impregnation process even before the alkaline compound could carry out a possible catalytic action on the decomposition reactions. To contribute in this issue, three sets of pyrolysis experiments for fir wood are carried out and analyzed. These consist of samples subjected to (a) water extraction to remove indigenous mineral matter, (b) impregnation, after water extraction, with a strong base (KOH) and a pH-neutral compound (KCl), both including the cation Kþ, and (c) water extraction, after impregnation, to remove the added compounds. Experiments are made by exposing a packed bed to a temperature of 800 K. The main parameters characterizing sample heating/conversion and the gas release rate together with the yields and composition of the main product classes are determined. In this way, results are used to clarify the chemical and physical action of the potassium additives on the decomposition of wood.

Received: October 14, 2010 Accepted: February 22, 2011 Revised: February 4, 2011 Published: March 07, 2011 3864

dx.doi.org/10.1021/ie102092p | Ind. Eng. Chem. Res. 2011, 50, 3864–3873

Industrial & Engineering Chemistry Research

’ EXPERIMENTAL SECTION Similar to previous studies of this research group about the effects of catalysts on pyrolysis characteristics and products,20,21,23-26 fir wood has been selected because its low density permits easy washing and impregnation. Samples from two different batches are used. The first (batch 1) presents contents of lignin (Klason method) of 31 wt %, extractives (Soxhtec HT2 apparatus) of 2.6 wt %, and ash (calcination) of 0.5 wt % (holocellulose, computed by difference, is 66 wt %). The second sample (batch 2) is characterized by lower contents of extractives (1.3 wt %) and lignin (27 wt %) (ash amounts to 0.35 wt % and the holocellulose by difference to 71 wt %). Moreover, the packed bed density is also slightly different with values of 0.21 g/cm3 (batch 1) and 0.19 g/cm3 (batch 2). Pyrolysis has been carried out, combined with characterization of volatile products, of (a) wood and wood subjected to extraction with water, (b) (pre-extracted) wood impregnated with KOH and KCl, and (c) KOH and KCl impregnated wood subjected to water extraction (to remove the additives). The methods of water extraction and impregnation of the two potassium compounds are described below. 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 remove a large part of the alkali compounds, thus avoiding their catalytic action on the conversion process (as shown in the following, the effects of this pretreatment are approximately the same as obtained with micrometer-sized samples). Washed and predried samples are referred to in the following as “extracted” or subjected to “extraction”. Moreover, an effort has been made to improve the water extraction process by repeating three times the procedure already described, in each case after a 18 h period of sample soaking in twice distilled water. Finally, two further treatments (4 h duration each) at 353 K (1.5 L of twice distilled water for 100 g of wood) have been carried out. Samples obtained with this procedure are indicated to have undergone “deep extraction”. Samples do not subjected to any treatment are indicated as “untreated” wood. Impregnated samples are obtained by soaking 200 g of extracted wood in aqueous solution of 1 L for 3 h with stirring. The impregnation solution is prepared by adding 1 L of deionized water to 2 g of KOH or 2.7 g of KCl. These treatments correspond to KOH and KCl contents in wood of 0.2 and 0.27 wt %, respectively, and to a Kþ content of both cases of 0.14 wt % (dry wood basis). These evaluations are made taking into account that, after impregnation, the weight of the sample is approximately doubled and assuming that, after water evaporation, the KOH or KCl content is completely incorporated in the wood structure. Samples obtained following this procedure are indicated as “impregnated” wood. Extraction and deep extraction of impregnated samples are also made, to try to separate the effects caused by the impregnation itself and the presence of KOH or KCl in the sample on the pyrolysis process. Samples treated in this way are indicated as “impregnated-extracted” and “impregnated-deep-extracted” wood, respectively. Drying of wood particles, in all cases and for the various steps of the deep extraction, is made by exposition to ambient air, for about 20 h, and in oven, for about 14 h at a temperature of 333 K. Finally, experiments concerning the action of KOH are made using wood batch 1, and those related to the action of KCl, using wood batch 2.

ARTICLE

The pyrolysis reactor has already been used in previous work of this research group20,21,23-27 but, to facilitate the understanding of the conversion characteristic, a brief description is also provided here. Biomass pyrolysis takes place in a batch system where the core is a cylindrical steel reactor (6.3 cm internal diameter and 45 cm length). Nitrogen, fed through a jacket at the reactor top (internal diameter 8.9 cm), is heated by an electrical furnace and distributed at the bottom by a perforated steel plate, which also supports the bed. Temperature profiles along the reactor axis are measured by seven thermocouples. The lower reactor zone (about 20 cm), at steady conditions, 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 heating temperature, is taken equal to 800 K during the tests. After nitrogen flushing, when steady profiles are established and the desired heating temperature is achieved, the sample is suddenly dropped inside the hot reactor and positioned within the isothermal zone. This feeding modality permits the attainment of sample heating rates and conversion times that are essentially determined by the particle properties and not by the reactor heating time. Also, the relatively small reactor diameter guarantees that radial temperature gradients across the particle bed are very small. The forced nitrogen flow, across the particle bed, is adjusted to give a nominal residence time of gases and vapors along the isothermal reactor zone of about 2 s. This figure is significantly reduced at the pyrolysis conditions owing to the higher pyrolysis temperatures and the addition of quite high mass fluxes of volatile products. Owing to both the relatively short volatile residence times and the countercurrent flow of gas and solid in this reactor configuration, the activity of secondary reactions of the vapor phase products of pyrolysis is highly hindered thereby maximizing the yields of condensable organic products. 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), two wet scrubbers, three cotton wool traps, and a silica gel bed (all connected in series). Gas sampling and analysis are made at selected times allowing the exit volumetric flow rate and mass of each gaseous species to be determined. The analysis of the gaseous and liquid streams is made in accordance with the methods already described.20,21,23-26 Gas analysis is carried out through a gas chromatograph (Perkin-Elmer Auto-System XL) equipped with a thermal conductivity detector and a packed column (Supelco 60-80 Carboxen 1000, 15 ft) with helium as the carrier gas. Analysis of the liquid products is made by means of CG/MS (Focus GC/DSQ, Thermo Electron) with a quadrupole detector and a DB-1701 capillary column (60 m 0.25 mm i.d., 0.25 mm film thickness). Helium 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 13 min. The injector and the GC/MS interface are kept at a constant temperature of 523 and 508 K, respectively. A sample volume of 1 μL (4.5 wt % 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 3865

dx.doi.org/10.1021/ie102092p |Ind. Eng. Chem. Res. 2011, 50, 3864–3873

Industrial & Engineering Chemistry Research 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. 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. 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 Pyrolysis experiments have been made for untreated, extracted, and deep-extracted wood, KOH and KCl impregnated wood, impregnated-extracted, and impregnated-deep-extracted wood. Experimental results deal with the gas release rate (time derivative of the gas mass fraction) and the bed temperature (measured at several distances from the flow distributor) versus time profiles, which are used to extract information about process dynamics. As already done in previous studies of the authors,20,21,27 the following parameters are introduced from the analysis of these profiles: 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 corresponding temperature, Tc, the global devolatilization rate, mtot, the minimum temperature, Tmin, and corresponding time, tmin (temperature values refer to a bed height of 10 cm). The conversion time is computed from the time profile of the amount of gas released and is assumed to coincide with the time when 75 wt % of the total gas has been produced. The global devolatilization rate is defined as the ratio of the mass fraction of volatile products to the conversion time. The yields of char, gas, water, and liquid-phase organics and the composition of the gaseous and liquid products are also discussed. It should be noted that they include the contribution from both wood and possibly the additives. KOH undergoes a molten phase for a temperature of 633 K.28 On the basis of thermodynamic considerations,5 KOH can react with CO2 (at ambient temperature): 2KOH þ 2CO2 S K 2 CO3 þ H2 O or carbon (at temperatures above 900 K) 6KOH þ 2C S 2K þ 3H2 þ 2K 2 CO3 Moreover, for temperatures above 700 K, KOH in the molten state can react with carbon and water vapor:29 4KOH þ C w K 2 O þ K 2 CO3 þ 2H2 (K2O formed during the reaction is regenerated into KOH by reacting with water vapor). Given the heating temperatures of 800 K used in the pyrolysis tests, it appears that KOH and K2CO3 (melting point 1170 K) can be found in the charred residue left at the conclusion of the pyrolysis process. KCl is a relatively stable

ARTICLE

Figure 1. Release rates of gas (dry sample mass basis) as functions of time for the set of experiments dealing with the effects of KOH impregnation (wood batch 1).

Figure 2. Release rates of gas (dry sample mass basis) as functions of time for the set of experiments dealing with the effects of KCl impregnation (wood batch 2).

Figure 3. Temperature versus time profiles, measured at a bed height of 10 cm, for the set of experiments dealing with the effects of KOH impregnation (wood batch 1).

compound, as shown by thermogravimetric analysis,19 with a melting temperature of 1047 K.28 For each case, two experiments are made which show a good reproducibility (average variations on the measured variables between 0 and 3%). The chemical characterization of the liquid phase products 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 the operator. The results obtained from the various pyrolysis tests are summarized in Figures 1and 2 (gas release rate versus time) and 3 and 4 (temperature, at a bed height of 10 cm, versus time) and Tables 1 and 2 (tgm, dYgm, with the percentage of gas 3866



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released, tc, mtot, Tmin, tmin, Tc) and 3 and 4 (yields of char, water, organics, gas, total mass closure, gaseous species (CO2, CO, CH4) and organic compounds (hydroxyacetaldehyde (HAA), acetic acid (AA), hydroxypropanone (HP), levoglucosan (LG), 3-ethyl-2-hydroxy-2-cyclopentenone (3E-2H-2C), furfural (FF), guaiacol (G), 4-methylguaiacol (4-MG), isoeugenol (IE)) for the two additives. The effects are discussed, in the following order, of indigenous alkali metal removal by water extraction, KOH and KCl impregnation, and KOH and KCl removal. To avoid overcrowding, not all the curves obtained from the various experiments are reported in Figures 1-4. Also because, as discussed in the following, in a few cases the differences between curves are negligible (for instance, temperature profiles for the cases of deep-extracted and untreated wood are not reported because variations are very small with respect to that measured for the extracted sample). The trends shown by the temperature profiles

and the gas release rate are always qualitatively the same. Moreover, 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 slow increase toward the initial bed value. 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 heat transport outside the bed by the hot vapors and gases evolved during devolatilization. Sample decomposition begins almost instantaneously, and the maximum rate of gas release also occurs at very short times. This means that the most external layer of the particles is rapidly heated to temperatures sufficiently high for the beginning of the decomposition process. However, soon after intraparticle and/or intrabed spatial gradients become non-negligible and are accompanied by a decline in the gas release rate. The slower rates of lignin decomposition may also be partly responsible for such a trend. Indeed complete conversion requires times significantly longer than those characteristic of the maximum gas release rate. It can also be observed that, as long as pyrolysis takes place, temperatures remain very close to the minimum value. Effects of Water Extraction. The effects of water extraction on the wood pyrolysis characteristics are examined by considering the treatments indicated as extraction and deep extraction using for comparison the results of untreated wood (these effects are described using Figures 1 and 3 and Tables 1 and 3, wood batch 1). Water extraction not only leaves unvaried the main qualitative features of the process dynamics but also introduces very small quantitative changes (these are, however, larger than the typical standard deviations21 that give an estimation of the dispersion of the experiments made with the laboratory-scale reactor). No modification is introduced on the variable Tmin

Figure 4. Temperature versus time profiles, measured at a bed height of 10 cm, for the set of experiments dealing with the effects of KCl impregnation (wood batch 2).

Table 1. 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; and Global Devolatilization Rate, mtot, for the Set of Experiments Dealing with the Effects of KOH Impregnation (Wood Batch 1)a deep-extracted

a

extracted

impregnated-deep-extracted

impregnated-extracted

impregnated

untreated

tgm

[s]

142

139

93

92

73

127

dYgm 103; % gas

[s-1]

0.03; 28.7

0.03; 24.2

0.07; 27.7

0.07; 30.8

0.10; 24.4

0.04; 22.5

Tmin

[K]

561

561

574

579

584

561

tmin

[s]

130

130

110

104

97.0

130

mtot 103 tc

[s-1] [s]

2.2 336

2.2 336

3.5 204

3.8 188

4.0 173

2.2 320

Tc

[K]

674

674

617

616

620

670

Temperatures refer to a bed height of 10 cm.

Table 2. 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, and Global Devolatilization Rate, mtot, for the Set of Experiments Dealing with the Effects of KCl Impregnation (Wood Batch 2)a deep-extracted

a

extracted



impregnated

tgm

[s]

120

118

121

96

90

dYgm 103; % gas

[s-1]

0.04; 25.0

0.04; 22.6

0.04; 25.0

0.05; 20.0

0.07; 23.0

Tmin

[K]

572

572

574

580

592

tmin

[s]

127

127

133

115

101

mtot 103

[s-1]

2.3

2.3

2.2

2.6

3.1

tc Tc

[s] [K]

301 671

301 671

312 672

264 663

220 659

Temperatures refer to a bed height of 10 cm. 3867



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Table 3. Yields of the Main Classes of Pyrolysis Products (Char, Water, Organics, Gas), Total Mass Closure, Gaseous Species (CO2, CO, CH4), and Organic Compounds (Hydroxyacetaldehyde (HAA), Acetic Acid (AA), Hydroxypropanone (HP), Levoglucosan (LG), 3-Ethyl-2-hydroxy-2-cyclopentenone (3E-2H-2C), Furfural (FF), Guaiacol (G), 4-Methylguaiacol (4-MG), Isoeugenol (IE)), Expressed as a Percent of the Initial Dry Sample Mass, for the Set of Experiments Dealing with the Effects of KOH Impregnation (Wood Batch 1) deep-extracted

extracted



impregnated

untreated

char

[% wt]

20.8

22.1

23.6

24.8

25.8

23.1

H2O organics

[% wt] [% wt]

19.8 45.2

19.5 42.6

23 34.7

23 34.8

23.8 30.6

22 38.7

liquids

[% wt]

65.0

62.1

57.7

57.8

54.4

60.7

gas

[% wt]

9.3

10.9

13.7

12.7

15.2

11.4

total

[% wt]

95.1

95.1

95.0

95.3

95.4

95.2

CO2

[% wt]

5.23

6.10

7.82

7.33

8.90

6.44

CO

[% wt]

3.33

3.90

4.95

4.55

5.30

4.05

CH4

[%wt]

0.65

0.77

0.83

0.76

0.85

0.76

HAA AA

[% wt] [% wt]

4.5 2.5

4.5 3.4

4.4 2.8

3.5 2.2

2.1 2.5

6.5 3.7

HP

[%wt]

1.3

1.5

1.4

1.1

1.8

2.8

LG

[% wt]

4.4

4.0

0.9

0.4

0.2

2.0

3E-2H-2C

[% wt]

0.06

0.07

0.15

0.13

0.24

0.10

FF

[% wt]

0.30

0.28

0.19

0.12

0.14

0.27

G

[% wt]

0.35

0.40

0.50

0.50

1.0

0.39

4MG

[% wt]

0.62

0.55

0.40

0.42

0.48

0.60

IE

[% wt]

0.50

0.46

0.48

0.48

0.94

0.49

Table 4. Yields of the Main Classes of Pyrolysis Products (Char, Water, Organics, Gas), Total Mass Closure, Gaseous Species (CO2, CO, CH4), and Organic Compounds (Hydroxyacetaldehyde (HAA), Acetic Acid (AA), Hydroxypropanone (HP), Levoglucosan (LG), 3-Ethyl-2-hydroxy-2-cyclopentenone (3E-2H-2C), Furfural (FF), Guaiacol (G), 4-Methylguaiacol (4-MG), Isoeugenol (IE)), Expressed as a Percent of the Initial Dry Sample Mass, for the Set of Experiments Dealing with the Effects of KCl Impregnation (Wood Batch 2) deep-extracted

extracted



impregnated

char

[% wt]

21.44

21.84

21.5

22.1

25.8

H2O organics

[% wt] [% wt]

20.5 43.5

19.9 43.4

21.7 41.5

22.6 40.3

32.8 23.8

liquids

[% wt]

64.5

63.3

63.3

62.9

56.6

gas

[% wt]

9.8

10.2

10.3

10.7

13.1

total

[% wt]

95.2

95.3

95.1

95.7

95.5

CO2

[% wt]

5.5

5.6

6.0

6.1

7.7

CO

[% wt]

3.5

3.8

3.8

3.7

4.4

CH4

[% wt]

0.72

0.80

0.77

0.75

0.81

HAA AA

[% wt] [% wt]

5.0 3.29

5.5 3.58

5.0 3.34

4.6 3.48

2.9 2.68

HP

[% wt]

2.3

2.5

2.3

2.7

2.1

LG

[% wt]

3.10

3.07

3.06

1.10

0.60

3E-2H-2C

[% wt]

0.055

0.066

0.061

0.160

0.210

FF

[% wt]

0.38

0.38

0.37

0.37

0.27

G

[% wt]

0.37

0.38

0.38

0.37

0.33

4MG

[% wt]

0.51

0.50

0.50

0.48

0.12

IE

[% wt]

0.51

0.50

0.50

0.50

0.08

while, owing to the slightly longer conversion times (336 versus 320 s), extracted wood presents Tc values slightly higher (674 versus 670 K). Also, water extraction slightly reduces the maximum gas release rate. The small modifications in the wood conversion dynamics induced by water extraction are also

confirmed under thermogravimetric conditions. For instance, for extracted beech wood,30 the temperatures for the beginning of the reaction, the hemicellulose decomposition, and the peak rate show a small increase (2, 4, and 9 K) and the enlargement of the entire reaction zone is also quite narrow (7 K). 3868


Industrial & Engineering Chemistry Research Compared with untreated wood, extracted, and deep-extracted wood presents slightly lower yields of char, water, and gas with increased yields of organics. As a consequence of the small variations on the total yields of volatile products and conversion times, the global devolatilization rate remains approximately constant. Gas composition shows successively lower values of CO and CO2. The composition of organic compounds is characterized by a diminution in the yields of HAA (from 6.5 to 4.5 wt %), AA (from 3.7 to 2.5 wt %), and HP (from 2.8 to 1.3 wt %) and an increase in yields of LG (from 2 to 4.4 wt %) (other sugar compounds, such as 2,3anhydro-D-galactosan, 2,3anhydroD-mannosan, and 1,4:3,6dianhydro-R-D-glucopyranose, based on the peak area values, also show factors of increase, on the order of 5, 3, and 2). Water extraction also causes a decrease in the yields of 3E-2H-2C and an increase in the yields of FF. Finally, the influences of the water extraction process on the organic products of lignin decomposition appear to be negligible. Comparison between the results for extracted wood of batch 1 and batch 2 reveal that the characteristic temperatures are slightly higher in the latter case. This can be attributed to the lower packed-bed density which has been shown27 to be the controlling parameter for dynamics of bed heating and conversion. On the other hand, the same trend discussed above between the products of extracted and deep-extracted wood are also shown by the samples of batch 2 with small quantitative differences that can be attributed to variations in the chemical composition. Effects of Impregnation. The influences of the impregnation are discussed using Figures 1 and 3 and Tables 1 and 3 (wood batch 1) and Figures 2 and 4 and Tables 2 and 4 (wood batch 2). In the presence of KOH or KCl, gas (presumably with steam and tar vapors) release is very rapid and terminates much earlier (the maximum rate is about 3.3-1.8 times higher and the conversion time is reduced by factors of about 1.9-1.4 (the effects are stronger for the KOH treatment)). The long tail observed for extracted wood completely disappears, indicating that, as a consequence of the impregnation of the potassium compounds, there is more overlapping among the temperature ranges where decomposition of the three wood components mainly takes place. In particular, lignin decomposition appears to be displaced at lower temperatures. Moreover, the KCl treated sample show the existence of two zones of gas release, characterized by local maxima, that can correspond to the two different zones of weight loss already noted in thermogravimetric analysis of impregnated cellulose.19 The temperature profiles of impregnated wood remain qualitatively similar to that of extracted wood, but values are higher for the minimum and the increasing part. In particular, the minimum temperature, Tmin, is 20 K (KCl) or 23 K (KOH) higher and that measured in correspondence of the conversion time, Tc, is 12 K (KCl) of 54 K (KOH) lower. This means that decomposition of alkali impregnated wood, on the whole, takes place at lower temperatures and, on global terms, is characterized by a lower endothermicity. Given the small concentration of the additives and thus the negligible intrinsic thermal effects associated with the related chemico-physical changes, it can be retained that the latter modification is a consequence of the actions of the additives and the increased activity of the exothermic formation of char with the reduction in the convective cooling associated with the release of gaseous and vapor-phase compounds. Also, the shorter conversion times are responsible for the faster temperature rise in the second part of the profile. The higher Tmin measured for the impregnated samples are associated with an increased rate of exothermic char formation

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that is augmented by about 16.7 wt % (KOH) or 18.1 wt % (KCl). The yields of water also increase, especially for the KCl treatment, at the expense of organic condensable products. The gas yields are higher for the KOH treatment (15 versus 13 wt % of the KCl impregnation) which correspond to higher values of both the CO2 and the CO yields, increased by factors of 1.46 and 1.36 versus 1.37 and 1.16 for KCl. On a total basis, impregnation produces a significant increase in the yields of char, water, and carbon dioxide (about 66 wt % (KCl) and 58 wt % (KOH) versus 47 wt % of extracted wood) with a diminution in the yields of organic products. The major carbohydrates are reduced for both cases, especially LG, that is nearly brought to zero (especially for the KOH treatment) and HAA, reduced by a factor of about 2.1 or 1.9. The effects are less evident for HP, which remains nearly constant, and AA which is barely reduced (factor of 1.4 and 1.3 for KOH and KCl, respectively). The two treatments tend to decrease the yields of FF (with a large increase in the yields of furfuryl alcohol,20,21 confirming the action of the Kþ ion on the secondary decomposition reactions. As already found,20,21 a significant increase in some cyclopentenones is also observed. Thus the yields of 3E-2H-2C, taken as a species representative of this product class, increase by factors of 3.4 (KOH) or 3.2 (KCl). Differences between the two additives are seen in the phenolic compounds. In fact, the species examined are nearly constant (G) or undergo significant decrease (4MG, IE) for the KCl treatment. An increase is observed for the yields of G and IE (factors of 2.5 and 2) and a diminution in the yields of 4-MG (factor of 1.1) for the KOH treatment. These findings suggest a different catalytic action of the two additives on the decomposition of the lignin component. Effects of Additive Removal. Extraction of impregnated samples is carried out to separate the effects of impregnation from catalysis during pyrolysis. These effects are discussed using Figures 1 and 3 and Tables 1 and 3 (wood batch 1) and Figures 2 and 4 and Tables 2 and 4 (wood batch 2). The observation of the gas release rate and temperature profiles for the KOH treated sample show that extraction causes a reduction in the variable dYgm and, in general, in the temperature values. However, the profiles of both variables are still rather distant from those of the extracted sample. In fact, the dYgm value is higher by a factor of 2.3, the conversion time is shorter by a factor of 1.8, and the temperatures Tmin and Tc are different by 18 and -58 K, respectively. An intensification in the extraction process does not appear to modify this situation by an appreciable amount and the differences with both the extracted and the deep-extracted (nonimpregnated) wood are large. Extraction of KCl impregnated samples is not sufficient to eliminate the effects of the additive on the pyrolysis dynamics. The dYgm value is higher by a factor of 1.25, the conversion time is shorter by a factor of 1.1, and Tmin and Tc are both different by 8 K. Instead, deep extraction strongly reduces the differences and, in practice, brings back the temperature and gas release rate profiles to the original (nonimpregnated) values. Although the observation of the temperature and gas release rate profiles clearly show the different role of extraction between KOH and KCl impregnated samples, it is important to establish a criterion to make quantitative comparisons between the two sets of data (deep-extracted and impregnated-deep-extracted samples) for all the process variables. For this scope the standard deviations, |Δσ|%, introduced to evaluate the dispersion of the experimental investigations and previously computed for the alkali-catalyzed pyrolysis of wood,21 are used. More precisely, 3869



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Table 5. Deviations between the Deep-Extracted and the Impregnated-Deep-Extracted Samples, |Δσ|%, for the Set of Experiments Dealing with the Effects of KOH and KCl and Maximum Standard Deviations, |Δσmax|%, Previously Evaluated21 As a Measure of the Maximum Dispersion of the Experimental Tests |Δσmax|%

|Δσ|% [KOH]

|Δσ|% [KCl]

char

1.7

13.5

0.3

H2O

6.5

16.2

5.0

organics

10.8

23.2

5.7

gas tgm

6.8 25.0

47 34.5

8.4 0.8

dYgm 103

8.8

Tmin

0.3

2.3

tmin

9.8

15.4

4.7

mtot 103

4.0

59

4.3

tc

3.7

39

3.6

Tc

0.9

8.5

0.1

CO2 CO

9.0 7.5

49.5 48.6

9.1 8.6

CH4

6.7

28

6.9

HAA

9.8

2.0

0.0

AA

10.0

12.0

2.8

HP

9.9

7.7

LG

7.0

79

1.3

3E-2H-2C

14.3

150

10.9

FF G

10.3 10.8

36.7 43

4MG

11.2

35.5

2.0

IE

15.4

4.0

2.0

133

0.0 0.0

0.0

2.6 2.7

deviations between the deep-extracted and the impregnateddeep-extracted sample below the typical maximum, |Δσmax|%, are assumed to represent restoration to the original values. To facilitate the comparison, the values |Δσmax|% and the actual deviations are summarized in Table 5 for the parameters describing the process dynamics and the products for the treatments based on KOH and KCl. It can be noted that the typical |Δσmax|% values are around 7-15% for the yields of the condensable organic compounds, 2-11% for the product classes, 7-9% for the gas composition, 0.3-0.9 for the characteristic temperatures, 4-25% for the characteristic times, and 9% for the maximum rate of gas release. The comparison between the deep-extracted and the KOH-impregnated-deep-extracted samples show that none of the variables examined are brought back to the original values with the exception of the HAA, AA, and HP yields, which show |Δσ|% values below or comparable with |Δσmax|%. The parameters that show the highest deviations are the characteristic times, mtot, dYgm, and the yields of some products. The other variables present an intermediate behavior, that is, extraction causes some changes that tend to make the behavior of the two samples more similar. All the parameters and yields of products determined for the KCl treatment show deviations that are lower or comparable with the corresponding |Δσmax| %. Therefore it can be assumed that deep extraction reports the wood sample back to the original behavior observed in the absence of the additive.

’ DISCUSSION The results obtained for the yields of the main classes of pyrolysis products for untreated, extracted, and deep-extracted wood are in qualitative agreement with previous results. In particular, the results obtained for the carbohydrate yields are well-described by the mechanism of cellulose pyrolysis indicated as the Waterloo model.31-33 Alkaline cations (potassium and minor amounts of sodium and magnesium) indigenous to all biomass catalyze the cleavage of the monomers making up the natural polymer chain of lignocellulosics, giving rise to the formation of HAA, formaldehyde, HP, methylglyoxal, etc. These reactions concern decarbonylation and fragmentation and, in addition to alkali cations, are also favored by high temperatures. Indeed, formation of HAA is not associated with char formation given that it is abundant only for the conditions of fast pyrolysis. Upon removal of inorganics, depolymerization reactions, occurring at slightly lower temperatures, are favored leading to the formation of LG, anhydrosugars, cellobiosan, and higher oligomers. The depolymerization reactions are thought to proceed through the free chain ends of cellulose,31 so that once a chain reaction is initiated, the whole chain “unzips”. The action of an alkaline cation, with its highly polar field and small size, is suggested to retard the unzipping reaction at the terminal unit of a chain, thus inhibiting levoglucosan production. As expected, the reduction in the activity of fragmentation reactions is more marked for the deep extraction. The activity of secondary reactions, which may cause the decomposition of vapor phase compounds, for instance FF to furfuryl alcohol,6 the cracking of organic vapors to gas,11,13 or their condensation to refractory material9 is also reduced with the removal of indigenous alkali compounds in wood. Results obtained for impregnated wood are also in general agreement with previous literature, in particular the marked effects on the process temperature of the deliberate addition of alkali compounds. For instance, for alkali salt contents of about 15%,7 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 temperatures measured along the bed height in the experiments of this study can be considered as average values between the solid and the gas phase and not the actual particle temperatures and the alkali content is much lower, the observed ranges of variation of temperature at the conversion time, Tc, are comparable. The increase in the yields of char, water, and carbon dioxide, following alkali impregnation in wood, is a result already known in the literature.7-21 In addition, not only formation of sugars from holocellulose decomposition is completely hindered, but the products of fragmentation reactions are also highly diminished. The former effect can be reasonably associated with a high concentration of alkali compounds and, on the other hand, the yields of sugar compounds are known to be affected by trace compounds of alkali metals.32 The latter effect is most likely a consequence of the modifications introduced into the intermediated solid structure by the low-temperature reactions (charring, dehydration, and decarboxylation) catalyzed by the potassium compounds and/or the increased activity of secondary reactions. Indeed, the detailed chemical characterization of the organic products for KOH impregnated wood20,21 shows that the formation of some carbohydrates (cylopentenones) is enhanced, indicating that other reaction paths become active. Under strong alkaline conditions which can resemble the pyrolysis of alkali 3870



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Figure 5. Normalized (with respect to the deep-extracted values) maximum gas release rate, dYgm; conversion time, tc, and corresponding temperature, Tc; and global devolatilization rate, mtot, for the set of experiments dealing with the effects of KOH impregnation (wood batch 1) versus the char yield.

Figure 6. -Normalized (with respect to the deep-extracted values) yields of hydroxyacetaldehyde (HAA), guaiacol (G), levoglucosan (LG), and 3-ethyl-2-hydroxy-2-cyclopentenone (3E-2H-2C) for the set of experiments dealing with the effects of KOH impregnation (wood batch 1) versus the char yield.

impregnated wood, carbohydrates (for instance, HAA, AA, HP) undergo retroaldol cleavage and the cleavage products subsequently produce higher molecular weight condensation products (for instance, cyclopentenones) by aldol reactions.34 The formation of volatile products of lignin decomposition, that is quite insensitive to trace presence of indigenous alkali compounds, is also noticeably increased in the case of KOH impregnation, again as a consequence of the increased reactivity and ether and C-C bonds cleavage caused by basic alkaline agents.35,36 The results obtained about the removal of the additives suggest that, once KOH has been impregnated in wood by physical sorption, either it cannot be removed by water extraction and/or introduces irreversible modifications in the wood structure which profoundly alter the process characteristics. On the contrary, water extraction is an effective method for removing the additive KCl, and it can be speculated that the two processes of KCl impregnation and KCl extraction do not modify the chemico-physical properties of the wood structure. In other words, KOH impregnation modifies the pyrolytic behavior of wood not only for a catalytic action but also for changes in the wood structure whereas the role of KCl appears to be essentially a catalytic one. The impossibility to restore the original behavior of KOH impregnated wood by means of water extraction can be a consequence of both modification in the physical structure of wood, resulting from hydrolytic attack with fiber swelling and matrix deterioration,7 as well as from the presence of Kþ ions that cannot be removed by means of water extraction. Indeed, during the impregnation stage, similar to Naþ,16 Kþ can penetrate into the solid structure and break the intermolecular hydrogen bridges upon swelling, a process enhanced by the high basicity level of the solution. Moreover, the strong base KOH can extract some low molecular compounds in the wood and can react with the chemical components, for instance, through active alcohol groups of cellulose with production of water:16

extracted, extracted, untreated, impregnated-deep-extracted, impregnated-extracted, and impregnated wood. To better show the modifications, some key process variables (Tc, dYgm, mtot, tc) and the yield of some species (HAA, G, LG, and 3E-2H-2C) are plotted versus the char yields for the KOH treatment in Figures 5 and 6. For a better graphic representation, the values of the variables are normalized with respect to those of deep-extracted wood associated with the minimum char yield. As already anticipated, it can be observed that the trends shown by the process variables can be classified into three main groups: (a) the characteristic times and temperatures and the yields of LG (and presumably other sugar compounds) that are left almost unchanged, with respect to the values obtained for impregnated samples, by the extraction process; (b) the yields of products generated from the fragmentation of the holocellulose component, such as AA, HP, and HAA, that the extraction reports back to the original values of the extracted samples (but still below those obtained for untreated sample), and (c) the other parameters that present an intermediate behavior, such as dYgm, mtot, the yields of guaiacol, etc. As for group a, the levoglucosan yields are known to be affected even by trace amounts of alkali metals and by the substrate structure. Two zones of approximately constant values appear: the first of high values is for low Kþ contents and unmodified wood structure (deep-extracted and extracted wood) and the second of low values is for high Kþ contents and modified wood structure (impregnated, impregnated-extracted wood). The intermediate values of the central zone indicate that the potassium content of impregnated-deep-extracted wood is close to that of untreated wood, whereas that of impregnated-extracted samples is barely lower than that of the impregnated sample. The values of Tc and other characteristic temperatures are practically almost independent of the actual Kþ content of the sample, that is they show approximately constant values for the two zones of nonimpregnated and impregnated wood, respectively. It is the sample treatment with a basic alkaline solution (KOH impregnation) which marks a very sharp boundary between the two values: high in the former zone and low in latter. Therefore, it can be understood that this trend is a result of physicochemical modifications of the wood structure which cannot be restored to its original form even though the additive is largely removed. These considerations are also applicable for the characteristic times of the process (for instance, tc) although the boundary between impregnated and nonimpregnated wood is less sharp.

cell-OH þ KOH w ½cell-O- K þ þ H2 O The possible reactions between the hydroxide and the wood components can increase the difficulty in removing the additive by means of water extraction. For the KOH treated samples, it is reasonable that the yield of char increases with the content of alkali metals in the sample. Hence it can be assumed that the K content is proportional to the amount of char produced and increases in the order from deep-

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Industrial & Engineering Chemistry Research More peculiar is the behavior of the yields of HAA (and other products of the holocellulose fragmentation reactions) belonging to group b. Its yield is certainly positively affected by an increase in trace amounts of alkali compounds. Indeed it attains a welldefined peak for untreated wood but it should be noted that the presence of potassium in the untreated sample is not in the form of KOH as the minerals appear as water-soluble salts, mainly oxides, silicates, carbonates, sulfates, chlorides, and phosphates.13 For slightly higher potassium contents (impregnated-deepextracted wood), it lowers to the value of deep-extracted wood. There could be an explanation for this result, taking into account that the yields of HAA are maximized when the char yields are low and, in general, for the conditions of fast pyrolysis (fast heating rates and relatively high particle temperatures). As already pointed out, once wood has been subjected to KOH impregnation, some irreversible modifications of the solid structure always produce reaction temperatures about 60 K lower, which could be retained responsible for the lower, than expected, HAA yields. Therefore, the modifications in the solid structure and the successively higher Kþ contents can be responsible for the decline in the production of HAA (and other products of the fragmentation reactions). As already observed, the variables of group c present an intermediate behavior with values that are constant for the impregnated-extracted zone. The yields of G and other phenolic compounds, obtained for the impregnated sample, are significantly reduced already as a consequence of a mild water extraction process. This means that these compounds are sensitive only to high quantities of Kþ or, more precisely KOH, incorporated in wood and to the changes induced in the wood structure. The same considerations apply to the yields of cyclopentenones and other variables related to the production of volatiles and gas, such as mtot and dYgm. More specifically, the sharpness of the decay, on the left boundary, and the rise, in the impregnated-wood zone, testifies the modification in the solid properties induced by KOH impregnation and the influences of large amount of alkali compounds in wood, respectively. This means that the increased production of gaseous species (especially CO2) and some cyclopentenones and phenolic compounds, observed together with the increase in the char yield, is due to both modifications in the original solid structure and the catalysis exerted by alkali compounds during the decomposition process.

’ CONCLUSIONS Catalytic pyrolysis of biomass, with either acidic or basic additives, can be economically attractive as it combines the usual production of biofuels with specialty chemicals. An experimental study is carried out to investigate whether, in the pyrolysis of wood impregnated with alkali compounds, the effects of the additives are due to a catalytic action, to hydrolytic attack during the impregnation stage resulting in fiber swelling and matrix deterioration, or both. For this scope two alkaline compounds, a strong base (KOH) and a pH-neutral compound (KCl) are considered and three sets of experiments are made for extracted, impregnated, and impregnated-extracted fir wood. Results indicate that impregnation highly modifies the characteristics of the decomposition process with larger modifications for the basic compound. The observed trends are in agreement with previous results. Water extraction is effective for restoring the pyrolysis behavior to that observed for the non impregnated and extracted samples when KCl is used as additive.

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This indicates that this neutral compound does not introduce irreversible changes in the wood structure during the impregnation stage and can be completely removed by water extraction. On the contrary, water extraction cannot, in general, restore the pyrolytic behavior of KOH impregnated wood and it can be postulated that the irreversible changes are enhanced by the basicity level of the additive. In this case, the process variables can be grouped into three main categories. The first includes the process times and temperatures and the yields of LG (and other sugars) that are left almost unchanged, with respect to the impregnated sample values, by water extraction as a consequence of both the variation in the substrate structure and the presence of even trace amounts of Kþ. The second category includes products, such as HAA, HP, AA, whose yields are restored by the water extraction process, possibly owing to the scarce influence of the modification in the substrate structure induced by KOH impregnation on the activity of the fragmentation reactions of the holocellulose component. Finally, the third group include variables, such as the products of lignin decomposition, cyclopentenones, and other variables related to the yields of volatile and solid products, that show an intermediate behavior and appear to be affected by the changes in the substrate structure and high quantities of the additive.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: 39-081-7682232. Fax: 39-081-2391800. E-mail: diblasi@ unina.it.

’ NOMENCLATURE dYgm = maximum gas release rate dYg/dt = gas release rate mtot = global devolatilization rate T = temperature t = time Tc = temperature, at a bed height of 10 cm, corresponding to tc tc = conversion time tgm = time of the maximum gas release rate Tmin = minimum temperature at a bed height of 10 cm tmin = time corresponding to Tmin Acronyms for Chemical Compounds

AA = acetic acid FF = furfural G = guaiacol HAA = hydroxyacetaldehyde HP = hydroxypropanone IE = isoeugenol LG = levoglucosan 3E-2H-2C = 3-ethyl-2-hydroxy-2-cyclopentenone 4-MG = 4-methylguaiacol

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Analysis of the Physical and Chemical Mechanisms of Potassium

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