Devolatilization and Heterogeneous Combustion of Wood Fast

After collection and milling, in the second set of experiments, heterogeneous combustion of the secondary char is carried out to temperatures ... Seco...
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Ind. Eng. Chem. Res. 2005, 44, 799-810

799

Devolatilization and Heterogeneous Combustion of Wood Fast Pyrolysis Oils Carmen Branca, Colomba Di Blasi,* and Rosario Elefante Dipartimento di Ingegneria Chimica, Universita` degli Studi di Napoli “Federico II” Piazzale V. Tecchio, 80125 Napoli, Italy

Weight loss curves of wood fast pyrolysis oils in air have been measured, under controlled thermal conditions, carrying out two separate sets of experiments. The first, which has a final temperature of 600 K, concerns evaporation/cracking of the oil and secondary char formation, processes associated with sample swelling and solidification. After collection and milling, in the second set of experiments, heterogeneous combustion of the secondary char is carried out to temperatures of 873 K. Although the details of the rate curves appear to be dependent on the commercial process (BTG, Dynamotive, Ensyn, Pyrovac) applied to produce the oil, the same qualitative features are observed in all cases. Secondary char formation and sample modification begin for temperatures of about 460-490 K. Moreover, a conceptual reaction mechanism, consisting of six main zones, can always explain the low-temperature (e600 K) devolatilization characteristics. Similar to primary char produced from wood pyrolysis, secondary char exhibits weight loss curves that present a devolatilization stage and a combustion stage, but with a significantly lower reactivity, as a consequence of a physical structure completely lacking micropore networks. Introduction The liquid products from pyrolysis of lignocellulosic materials (bio-oils) have the potential to be used as a fuel oil substitute in many static applications for heat or electricity generation.1-3 A method that combines fast pyrolysis of biomass to generate bio-oil and catalytic steam reforming of the bio-oil to hydrogen and carbon dioxide has also been proposed.4-8 This is part of an integrated, economically attractive process in which the biomass is also used to produce more valuable materials or chemicals. From a chemical point of view, bio-oils are characterized by the presence of oxygenated compounds, such as furan derivatives, carboxylic acids, alcohols, dihydroxyphenols, and dimethoxyphenols, derived from the decomposition of biomass components (cellulose, hemicellulose, and lignin).9-16 They also contain an appreciable proportion of water (up to 30 wt %) originating from both the moisture content and the decomposition reactions.1,2,17 From the physical point of view, bio-oils are characterized by high mass density, high viscosity, high surface tension, low pH, low heating value (4050% that for hydrocarbon fuels), and a very wide range of boiling points.17 Significant quantities of small char particles have also been observed,17 derived from the abrasion of biomass particles during the pyrolysis process itself, which contribute to an increase in the oil viscosity. Some effort has been made to find a link between the chemical and physical properties.18-20 Apart from accurate chemical and physical characterization, detailed knowledge of the combustion behavior of bio-oil is needed for the development of practical systems.3 However, fundamental studies have been few and have focused on either single-drop dynamics21-24 or classical thermogravimetry.25,26 Combustion takes place according to several stages.21-24 A quiescent blue flame is followed by droplet swelling/distortion and micro-explosions, which are associated with the formation of a * To whom correspondence should be addressed. Tel.: 39081-7682232. Fax: 39-081-2391800. E-mail:[email protected].

cellular network of vapor bubbles that ultimately contracts and results in the formation of coke particles. Coke burning is the final stage of the process. Weight loss curves of bio-oil in air25,26 indicate the existence of three different zones: evaporation, release of light components, and combustion of the solid residue. Although all of these analyses have contributed significantly to the qualitative understanding of the details of bio-oil combustion, the strong interaction between physical and chemical processes21-24 and the absence of accurate temperature control25,26 hinder the analysis of the intrinsic reaction kinetics and reactivity of the samples. In this study, the weight loss dynamics in air of four bio-oils, produced by means of different fast pyrolysis technologies, are examined. The experimental conditions are determined so as to allow the processes to take place under assigned thermal conditions and kinetic control. The chief objectives of this study are concerned with (a) the characterization of the main combustion stages and (b) the quantification of the reactivities of the bio-oils, depending on the fast pyrolysis process, aimed at formulating lumped-species reaction mechanisms that can be incorporated into comprehensive transport models for process design and development. Experimental Details Bio-oil Samples. The bio-oil samples were provided by the PyNe network27 and are the same as already characterized.15,16 They were produced in a rotating cone reactor [Netherlands Biomass Technology Group BV (BTG), Enschede, The Netherlands] and a bubbling fluid-bed reactor (Dynamotive, Vancouver, Canada) from softwood mixtures, in a transported bed reactor (Ensyn, Ottawa, Canada) from hardwood mixtures, and in a vacuum pyrolysis reactor (Pyrovac, Quebec, Canada) from softwood barks. In accordance with fast pyrolysis conditions, reactor temperatures varied between 733 and 798 K, initial moisture contents between 7 and 12 wt % (dry basis), and particle sizes between 1 and 6 mm. Table 1 summarizes the current state of the art in relation to quantitative composition determinations of

10.1021/ie049419e CCC: $30.25 © 2005 American Chemical Society Published on Web 01/12/2005

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Table 1. Yields and Boiling Temperatures of Bio-oil Compounds Tb (K)

BTG (wt %)

Dynamotive (wt %)

Ensyn (wt %)

Pyrovac (wt %)

formaldehyde28 acetaldehyde28 propionaldehyde28 glycolic acid28 glyoxal28 acetone28 methanol28 2-oxobutanoic acid30 ethanol28 MEK28 2-propanol28 (5H)-furan-2-one29 total group I

253.9 294 322 323 324 329.3 337.8 351 351.6 353 355.5 360 300-360

2.63 0.92 0.03 0.57 1.51 0.12 0.91 0.18 0.00 0.20 0.25 0.54 7.86

2.07 1.01 0.05 0.50 1.32 0.15 1.03 0.17 0.09 0.37 0.37 0.62 7.75

0.83 0.68 0.01 0.31 0.84 0.15 0.39 0.13 0.06 0.06 0.00 0.50 3.96

0.76 0.55 0.01 1.07 0.92 0.14 0.07 0.15 0.01 0.23 0.06 0.51 4.48

water28 formic acid28 hydroxyacetaldehyde31 5-hydroxymethylfurfural28 acetic acid28 butanol28 lactic acid30 4-propylguaiacol29 4-aceton guaiacol29 total group II

373.2 373.9 383 388 391.2 390.6 395 399 400 360-400

30.40 0.19 6.65 0.49 3.17 3.15 0.21 0.07 0.10 44.42

21.10 0.13 5.65 0.52 2.46 2.85 0.18 0.14 0.14 33.16

20.30 0.12 3.69 0.23 4.73 1.29 0.09 0.07 0.07 30.59

15.70 0.07 2.54 0.83 2.25 0.80 0.08 0.05 0.11 22.42

propionic acid28 acrylic acid28 hydroxypropanone28 cis-furantetrahydro2,5-dimethoxy28 trans-furantetrahydro2,5-dimethoxy28 4-methyl syringol29 isobutyric acid28 furfuryl alcohol28 3-methyl-2-cyclopentenone28 2-methyl-2-cyclopentenone28 2-ethyl-3-hydroxy-2cyclopentene-1-one 2-hydroxy-2-cyclopentene1-one 2-hydroxy-1-methyl1-cyclopentene-3-one 1-hydroxy-2-butanone28 2-furaldehyde28 methacrylic acid28 N-butyric acid28 2-acetylfuran28 acetoxyacetone28 coniferylaldehyde29 total group III

414.2 412.2 418.7 418.9

0.28 0.14 2.82 0.03

0.33 0.03 3.91 0.05

0.66 0.04 1.63 0.00

0.30 0.08 1.10 0.00

418.9

0.01

0.01

0.01

0.00

419 427 430.1 430.7 431.4 -

0.13 0.07 0.07 0.11 0.06 0.04

0.12 0.19 0.08 0.16 0.17 0.04

0.22 0.06 0.08 0.23 0.14 0.04

0.15 0.18 0.09 0.21 0.53 0.09

-

0.30

0.46

0.06

0.10

-

0.43

0.50

0.32

0.52

433.2 434.7 435.7 436 446 447 448 400-450

0.07 0.35 0.01 1.08 0.03 0.15 0.35 6.51

0.21 0.53 0.01 0.98 0.03 0.22 0.36 8.37

0.09 0.53 0.01 0.93 0.05 0.22 0.09 5.39

0.10 0.30 0.01 1.43 0.03 0.11 0.08 5.39

phenol28 crotonic acid28 valeric acid28 3-hydroxypropanoic acid 5-methyl-2-furaldehyde28 o-cresol28 syringaldehyde28 tiglic acid28 4-methylpentanoic acid28 p-creosol28 m-creosol28 hexanoic acid28 2-ethyl phenol28 guaiacol28 2,5- and 2,4-dimethyl phenol28 4-hydroxybenzaldehyde28 4-methyl guaiacol28 vinylguaiacol29 3,4-dimethyl phenol28 total group IV

455 456 459 460.2 464.3 465.6 471.7 472 475 475.2 477 478 478.2 484 489.6 494.2 497 500 450-500

0.08 0.07 0.05 0.02 0.06 0.02 0.04 0.01 0.01 0.02 0.01 0.05 0.00 0.36 0.02 0.01 0.65 0.10 0.00 1.57

0.10 0.02 0.34 0.00 0.15 0.04 0.00 0.04 0.01 0.04 0.03 0.14 0.00 0.53 0.05 0.01 1.23 0.13 0.01 2.86

0.23 0.05 0.15 0.02 0.08 0.08 1.16 0.00 0.00 0.05 0.04 0.16 0.01 0.21 0.07 0.01 0.23 0.00 0.01 2.54

0.32 0.06 0.34 0.04 0.12 0.13 0.07 0.00 0.02 0.12 0.10 0.16 0.02 0.33 0.22 0.03 0.41 0.07 0.02 2.57

compound

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 801 Table 1. Continued compound 4-ethyl guaiacol29 2-methyl-4-propyl phenol28 1,2-benzendiol28 levulic acid28 benzoic acid28 eugenol28 syringol28 vanillin28 isoeugenol (cis + trans)30 acetosyringone total group V hydroquinone28 levoglucosan31 glucose xilose cellobiosan total group VI

Tb (K)

BTG (wt %)

Dynamotive (wt %)

Ensyn (wt %)

Pyrovac (wt %)

0.12 0.02 0.00 0.12 n.d. 0.42 0.16 0.82 1.41 0.01 3.08

0.37 0.09 0.13 0.11 0.02 0.71 0.14 0.29 2.80 0.00 4.66

0.08 0.01 0.10 0.11 0.00 1.80 0.61 0.08 0.49 0.21 3.49

0.13 0.03 0.91 0.23 0.05 0.24 0.21 0.25 1.18 0.02 3.25

0.17 2.96 0.00 0.00 1.80 4.93

0.30 4.48 0.00 0.14 2.30 7.22

0.33 3.71 0.00 0.00 0.00 4.04

0.47 3.72 0.00 0.00 0.70 4.89

508 513 518 518.7 522.2 526.4 536 538.7 540.5 500-550 558.2 659 >550

bio-oil. The yields of 39 compounds, as quantified in ref 16, and the average values, as obtained from the data reported by the Round Robin testing in Europe,15 for other species and water, are listed (in this case, given about 1 order of magnitude difference, the highest value given for formaldehyde was disregarded). Species have been grouped on the basis of the corresponding boiling temperatures, which, for the most part, were derived from refs 28 and 29, along with a few from refs 30 and 31. In some cases, values refer to subatmospheric pressure [2-oxobutanoic acid, (5H)-furan-2-one, 5-hydroxymethylfurfural, lactic acid, 4-propylguaicol, 4-acetonguaiacol, 4-methylsyringol, coniferyladehyde, syringaldehyde], decomposition was observed before evaporation (glycolic acid, 4-hydroxybenzaldehyde), or values were not available (3-ethyl-3-hydroxy-2-cyclopentene, 2-hydroxy-2-cyclopentene-1-one, 2-hydroxy-1methyl-1-cyclopente-3-one, hydroxypropanoic acid, acetosyringone, glucose, xilose, cellobiosan) (these species have been grouped with those of similar molecular structure). Differences in the properties of the bio-oils, indicated by the name of the production process, stem from variations in the conversion technology, method of liquid collection, and feedstock properties. It has been observed16 that the last two items are the main causes for the very large differences shown by the Pyrovac sample with respect to the other samples. The entire amount of liquid collected was provided by BTG and Dynamotive and presumably by Ensyn whose collection method also includes water scrubbing. For the Pyrovac technology, apart from the use of bark (versus wood), the liquid is collected in two fractions: a low-volatility organic fraction, which was provided for the analyses, and the water and volatile-rich fraction. In this case, much lower amounts of major carbohydrates (hydroxyacetaldehyde, acetic acid, and hydroxypropanone) and furans (in particular, 2-furaldehyde) are observed. On the contrary, the yields of phenolics are roughly 2-4 times higher. In accordance with the different lignin structure, the yields of guiacols are higher for the softwood oils (BTG and Dynamotive samples), whereas the hardwood oils (Ensyn sample) show much higher contents of syringols. Elemental composition, contents of pyrolytic lignin (the water-insoluble fraction of the oil obtained by extraction) and solids, and viscosity are reported in Table 2.15 The solid content is relatively low in all cases,

Table 2. Main Physical Properties, Pyrolytic Lignin Contents, and Elemental Compositions of the Bio-oil Samples15

bio-oil BTG Dynamotive Ensyn Pyrovac

elemental analysis viscosity (wt %) x103 (m2/s) pyrolytic solid content lignin (wt %) 293 K 313 K (wt %) C H N O 0.08 0.10 0.43 1.02

0.10 0.03 1.27 1.71

0.03 0.01 0.21 0.23

8 25 54 44

37.1 44.7 47.2 51.4

7.6 7.2 6.9 7.0

0.1 0.1 0.1 0.3

55.1 48.1 45.6 41.6

although it varies by about 1 order of magnitude between the BTG and Dynamotive samples on one side and the Ensyn and Pyrovac samples on the other. The content of pyrolytic lignin and the viscosity are also much higher for the last two samples. It is plausible that these features are responsible for the highest percent of carbon. Thermogravimetric System and Char Samples. The experimental system (see, for instance, refs 32 and 33) consists of a radiant heating chamber, a quartz reactor, a PID temperature controller, a gas feeding system, an acquisition data set, and an analytical balance. The furnace is a radiant chamber that creates a uniformly heated zone where a quartz reactor is located (Figure 1A). The proper reaction environment is established by a continuous air flow (volumetric flow rate ) 77 mL/s). The liquid is exposed to thermal radiation by means of a semispherical quartz cup with 10-mm internal diameter, 1-mm thickness, and 5-mm height (Figure 1B). The cup is supported by Ni-Cr wires wrapped to steel rods and positioned at the center of the uniformly heated zone of the furnace. The steel support is connected to the precision balance for continuous weight loss measurements. In the case of a solid sample, the quartz cup is substituted by a wire mesh basket. To carry out a process under controlled thermal conditions, the intensity of the applied heat flux is used as the adjustable variable, and the sample temperature is monitored. Following the differences between the measured and desired temperatures, the PID controller sends an electrical signal, I, which regulates the potential difference at the two ends of the furnace lamps through a silicon controlled rectifier (SCR) process. Temperature is measured by means of a chromelalumel thermocouple (0.1-mm bead) previously calibrated with three reference metals (tin, zinc, and tellurium).

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Figure 1. Schematic representation of (A) the reactor with (B) the semispherical quartz cup containing the pyrolysis liquid samples.

Several preliminary tests were made to select the heating rate, the final temperature, the position of the thermocouple, and the sample mass that permit an effective control of the thermal conditions experienced by the sample. A thermocouple in direct contact with the sample and close to the cup bottom has proven to be the most sensible (Figure 1B). Using a heating rate of 5 K/min and a final temperature of 873 K, all of the samples cannot avoid ignition for temperatures higher than 615-675 K, even for an initial mass of 8 mg. However, when measurements are carried out up to high temperatures, i.e., 873 K, the weight loss curves are qualitatively similar to those discussed in refs 25 and 26. Apart from the exothermicity of the combustion reactions, visual observations indicate that serious complications in the temperature control are introduced, at a certain stage, by the physical transformations of the sample, that is, swelling and formation of a solid residue that is indicated here as “secondary char”, to make a distinction with the primary char generated from wood/ biomass pyrolysis. It has been found that an accurate control of the sample temperature is possible only when the weight loss curves are measured by means of two separate experiments. The first experiment is carried out under mild thermal conditions (up to final temperatures of 600 K), so that heterogeneous combustion of char does not take place. Successively, the secondary char is collected, milled as is usually done in thermogravimetric analysis of primary char,34 and then combusted in air. Details about the experimental system and the control procedure can be found in ref 35. The first stage was investigated using samples consisting of 41 mg of liquid. The thermogravimetric program includes a dynamic section, with a heating rate of 5 K/min from ambient conditions to a temperature of 600 K, with an isothermal section at 600 K whose duration is 1500 s. The isothermal part permits a complete devolatilization and avoids, at the same time, any significant activity of heterogeneous combustion reactions. Each test was performed in triplicate, showing good repeatability. Changes in the sample mass (850 mg) do not result in any variation in the measured weight loss characteristics, indicating the absence of significant heat- and mass-transfer limitations. Chars collected after the conclusion of the isothermal section at 600 K were milled to powder (particle sizes below 80 µm) and predried for 10 h at 373 K. Then, a second set of experiments was performed to collect thermogravimetric curves with a heating rate of 5 K/min to a final temperature of 873 K, followed by an isothermal hold for 1100 s. The conditions of the tests (7 mg of secondary char distributed over a surface area of 20 ×

Table 3. Elemental Analysis of Secondary Chars (Generated from Bio-oil Devolatilization) and Primary Chars34 C (wt %) conventional pyrolysis fast pyrolysis BTG Dynamotive Ensyn Pyrovac

H (wt %)

N (wt %)

O (wt %)

Primary Char 79.50 3.29 79.20 3.06

0.43 0.42

20.79 19.80

Secondary Char 72.53 4.81 71.99 4.39 70.93 3.90 71.91 5.09

0.49 0.10 0.35 0.84

24.55 24.58 24.82 22.16

4 mm2) guarantee negligible heat- and mass-transfer limitations, as observed by varying the initial sample mass. Again, each curve was measured in triplicate, and good repeatability was observed. Weight loss curves of primary char34 generated from conventional or fast pyrolysis of beech wood carried out at 800 K are used for comparison. Table 3 reports the elemental compositions of both primary and secondary chars. Primary chars roughly present the same C content, corresponding to 76-77%. Variations between the secondary chars are also small, but in this case, the values are lower (71-72%). Results and Discussion The thermogravimetric characteristics of the weight loss curves are discussed for the first and second sets of experiments. The formation of secondary char, when bio-oil samples are exposed to mild thermal conditions, is illustrated by means of photographs taken at several times, whereas scanning electron microscopy (SEM) is applied to investigate its structure. A comparison with primary chars is also provided. Finally, a conceptual mechanism for bio-oil combustion is formulated. Devolatilization of Fast Pyrolysis Oils. Figure 2A,B reports the mass fraction and the time derivative of the mass fraction (devolatilization rate), respectively, of the different bio-oils versus time, as measured in the first set of experiments (the sample temperature is also shown). Mass loss starts at low temperatures, given the presence of highly volatile compounds in the chemical composition, and a large fraction of volatiles is released at temperatures below 550 K. Also, the solid residue (secondary char) is always a significant percentage of the initial mass. The differential curves indicate quite complex dynamics, with several clearly evident peaks and shoulders whose position and size are affected by the pyrolysis process. To understand whether a relation between the thermogravimetric behavior and chemical composition of the

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Figure 2. (A) Weight loss (Y) and (B) weight loss rate (-dY/dt) curves of fast pyrolysis liquids as functions of time (dynamic section with a heating rate of 5 K/min to 600 K and an isothermal section of 1500 s).

oil samples exists, the weight loss curves are divided into six main regions, following the lumped classes of chemical species introduced in Table 1 on the basis of the boiling temperatures. A characteristic point (with the corresponding temperatures, mass fractions, and devolatilization rate) is then located in each region. An example of the definition of these reaction zones and related parameters is given for the Dynamotive and Ensyn samples in Figure 3A,B, respectively, which reports the first and second time derivatives of the mass fraction versus time. The definition of a characteristic point for each zone is particularly easy when a maximum in the devolatilization rate exists, which can be associated with the presence of at least one reaction step (such as in the case of regions 1, 4, and 5 of Figure 3A and regions 2, 5, and 6 of Figure 3B). A second type of characteristic point, again taken as representative of at least one reaction step, corresponds to the presence of a plateau in the devolatilization rate. This is defined as the position where the second time derivative of the mass fraction is closest to zero (such as in the case of regions 2 and 3 of Figure 3A and regions 1, 3, and 4 of Figure 3B). Table 4 reports the devolatilization parameters for each zone, that is, the temperatures, the mass fractions, and devolatilization rates corresponding to the characteristic points introduced above. The secondary char yields are also reported in the last row (as mass fractions). The amounts of volatiles released for each zone are summarized in Table 5, where, for comparison purposes, those evaluated using the species identified and quantified (Table 1) are also reported. It is evident that, apart from the Ensyn sample, it is not possible to locate a well-defined peak or shoulder in the devolatilization rate for temperatures above 550 K (sixth region).

Figure 3. First and second derivatives of the mass fraction for the (A) Dynamotive and (B) Ensyn samples as functions of time (experimental conditions as in Figure 2). Table 4. Devolatilization Characteristics of the Bio-oils for the Six Temperature Zones as Defined in Figure 3A,B and Secondary Char Yields, Yfa BTG

Dynamotive

Ensyn

Pyrovac

T1 Y1 -(dY/dt)1 × 103 (s-1)

334 0.880 0.555

355 0.856 0.342

342 0.956 0.174

357 0.920 0.194

T2 Y2 -(dY/dt)2 × 103 (s-1)

376 0.630 0.339

372 0.791 0.309

382 0.839 0.283

395 0.826 0.242

T3 Y3 -(dY/dt)3 × 103 (s-1)

415 0.510 0.243

420 0.639 0.239

427 0.712 0.197

415 0.768 0.225

T4 Y4 -(dY/dt)4 × 103 (s-1)

460 0.418 0.160

463 0.533 0.194

490 0.584 0.161

465 0.638 0.267

T5 Y5 -(dY/dt)5 × 103 (s-1)

-

509 0.446 0.176

517 0.529 0.186

536 0.477 0.130

T6 Y6 -(dY/dt)6 × 103 (s-1)

-

-

570 0.442 0.158

-

Yf

0.245

0.299

0.369

0.392

a

Heating rate of 5 K/min and final temperature of 600 K.

Moreover, for the BTG sample, peaks or shoulders are also absent for the fifth zone. It can be observed that the values for the first reaction zone are completely different in terms of the chemical composition and mass loss, presumably because the main constituent, water, starts to evaporate well before the corresponding boiling point is attained. When the entire amount of volatiles released in the first two zones

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Table 5. Fractions of Volatile Products Released along the Six Temperature Zones Defined by Figure 3A,B, as Evaluated by Means of the Measured Weight Loss Curves (WL) and the Chemical Compositions and Boiling Temperatures Reported in Table 1 (BT) BTG

Dynamotive

Ensyn

Pyrovac

stage

∆T (K)

WL

BT

WL

BT

WL

BT

WL

BT

I II III IV V VI I + II I + II + III

300-360 360-400 400-450 450-500 500-550 550-600 300-400 300-450

0.276 0.166 0.131 0.083 0.057 0.042 0.442 0.573

0.079 0.444 0.065 0.016 0.031 0.049 0.522 0.587

0.167 0.133 0.137 0.097 0.089 0.060 0.299 0.436

0.078 0.332 0.084 0.029 0.047 0.072 0.409 0.492

0.089 0.130 0.121 0.094 0.093 0.087 0.219 0.340

0.040 0.306 0.054 0.026 0.035 0.041 0.345 0.399

0.082 0.108 0.126 0.139 0.088 0.048 0.190 0.316

0.045 0.224 0.054 0.026 0.033 0.049 0.269 0.323

is compared, the agreement is barely acceptable for the BTG sample (44% versus 52%), and the values are rather different for the Pyrovac (19% versus 27%), Dynamotive (30% versus 41%), and Ensyn (22% versus 35%) samples. The BTG oil, which has the highest content of water (30 wt %) and volatile compounds of the first (formaldehyde, acetaldehyde, acetone, methanol, ethanol, 2-propanol) and second (simpler carbohydrates, such as formic acid, acetic acid, and hydroxyacetaldehyde, and water) groups, shows the largest weight loss. The peak rate is localized in the first reaction zone. Although lower and displaced to higher temperatures, the peak rate of the Dynamotive oil is also attained in the first region, and the corresponding mass fraction is comparable with that of the BTG sample (0.86 and 0.88, respectively). The Ensyn and Pyrovac samples show lower devolatilization rates. Moreover, the former attains a peak in the second region (for a temperature of 382 K). The third zone corresponds to an almost constant rate of weight loss, with a slightly visible peak only for the Dynamotive oil at 420 K. When the comparison between the volatile yields observed from the thermogravimetric curves and those estimated on the basis of the chemical composition and the boiling temperatures is extended to consider the total contribution up to the third zone (temperatures below 450 K), the agreement is improved and becomes good for all of the samples [57% versus 59% (BTG), 44% versus 49% (Dynamotive), 34% versus 40% (Ensyn), 32% versus 32% (Pyrovac)]. Consequently, in addition to the previous groups, weight loss accounts for the evaporation of a large part of carboxylic acids (propionic, acrylic, butyric) and lighter furans (furfural). In accordance with this analysis, it appears that the first three reaction zones (temperatures below 450 K) can be related mainly to evaporation of water and the low-boiling-point components. Hence, the thermogravimetric behavior can be explained on the basis of the chemical composition. The disagreement between the yields of the three separate zones is most likely due to a wide overlap between the evolution time of the different groups of species, which implicate, at least for the low-temperature behavior, a parallel-reaction mechanism. The comparison between the volatile yields of the different zones cannot be rigorous because of the uncertainty about the quantification of the chemical compounds and the absence of information, even after inclusion of the contribution of pyrolytic lignin, for about 18% (Dynamotive and Pyrovac oils) or 29% (BTG oil) of the total mass. The devolatilization rates of the Pyrovac sample are the lowest and present a wide zone of nearly constant values for temperatures below 450 K. The peak rate, comparable to that of the Ensyn sample, is attained in

the fourth zone for a temperature of 465 K and for a significantly lower mass fraction (0.64 versus 0.84). Again, this behavior can be explained by the chemical composition, as the Pyrovac oil contains the lowest content of water (15 wt %) and highly volatile compounds. In reality, for temperatures above 450 K, the weight loss curves always report higher amounts of volatiles released than those estimated from the chemical composition and the boiling points of the different compounds. Apart from the incomplete chemical characterization of the samples, this is due to the onset of more complicated processes associated with cracking reactions. As for the evaporation process, the fourth zone, including a significant peak in the evaporation rate curve of the Dynamotive and Pyrovac oils at 465463 K, can be attributed to the simpler phenols (phenol, creosols, and guaiacol), whereas the fifth zone, up to 550 K, can be attributed to heavier methoxy- and dimethoxyphenol compounds (eugenol, syringol, vanillin, and isoeugenol). The significant peak at 570 K, observed only for the Ensyn oil, is probably due to the evaporation of lighter polyaromatic hydrocarbon (PAH) compounds, such as acenaphtene and fluorine, which are present in quite high amounts.15 It is well-known that bio-oils are thermally unstable,3,4,12,17 and changes during storage have been observed. Chemical reactions occur to produce oligomers and polymers that have an increased viscosity and a reduced solubility in the bio-oil.36 Apart from the formation of peroxides in the presence of oxygen, it has been reported36 that acids and alcohols react to form esters and water; aldehydes react with water to form hydrates; aldehydes react with alcohols to form hemiacetals, acetals, and water; aldehydes react to form oligomers and resins; aldehydes react with phenolics to form novolak resins and water; and olefins polymerize to form oligomers and polymers. However, the correlation found here between the chemical composition and the rate of weight loss indicates that, for the much more severe thermal conditions of this study, the devolatilization reactions are more important than the reactions occurring in the condensed phase. On the other hand, the latter reactions are responsible for the significant amounts of secondary char generated. The yield of secondary char (Table 4) varies from about 25% to 39% (on a total oil basis), reflecting the same trends as in the carbon content of the oils. In accordance with these results and based on the elemental composition of the samples (Tables 2 and 3), it can be observed that secondary chars retain carbon contents of 48% (BTG and Dynamotive samples) and 55% (Ensyn and Pyrovac samples) referred to the initial (oil) value. The significant energetic content of the secondary char evidences the need to develop effective combustion

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Figure 4. Physical appearance of the Ensyn sample (top and front views) for temperatures of (A, initial conditions) 300, (B) 490, (C) 520, and (D) 550 K during devolatilization (heating rate 5 K/min).

technologies for both the homogeneous and heterogeneous stages of bio-oil combustion. Sample Transformation and Formation of Secondary Char. To clarify the dynamics of sample transformation, numerous tests were made. The radiative heater was turned off at selected times (temperatures), and sample cooling was achieved with rapid exposure to ambient conditions. Then, photographs were taken that indicate acceptable repeatability in the sample modifications. An example of the changes in the physical appearance of the sample is shown by the photographs (top and front view) of Figure 4A-D for the Ensyn bio-oil, corresponding to temperatures of 300, 490, 520, and 550 K, respectively (the corresponding amounts of volatiles released are 0%, 42%, 47.5%, and 53% on a total liquid basis, as reported by the weight loss curve of Figure 2A). Initially (Figure 4A), the liquid occupies only a small part of the quartz cup. A strong increase in the viscosity is most likely associated with the first appearance of bubbles occurring for a temperature of 460 K. Volume changes are still negligible despite the loss of about 36%

of the initial mass (Figure 2A). However, for a temperature of 490 K (Figure 4B), it can been seen that the swelling process is under way as a consequence of the increasing difficulty for the volatile products to leave the melt phase. Swelling is further enhanced by the temperature, as shown in Figure 4C, even though the amount of mass released in the temperature range 490520 K is relatively small (below 6%, Figure 2A). A further increase of 30 K in the temperature, associated with an additional mass release of about 5% (Figure 2A), causes that the entire cup to be occupied by secondary char. Visual observation reveals that the sample is still in a plastic phase. Finally, for a temperature of 600 K, no significant changes are seen in the physical appearance of the sample, independently of the holding time, presumably owing to complete solidification. The process dynamics are qualitatively similar in all cases. The Ensyn and Pyrovac samples are very similar. They also begin to bubble at about the same temperature (460 K). Similarities are likely to come from comparable physical properties, i.e., low water contents (Table 1), large amounts of solid particles and pyrolytic

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Figure 5. SEM images of secondary chars for a temperature of 600 K and a holding time of 200 s (after heating at 5 K/min). Views of the top surface: (A) Dynamotive and (B) Pyrovac oils. Details of the structure: (C) Surface bubble (Ensyn), (D) thickness of a surface fragment (Pyrovac), (E) fragment of the internal structure (Ensyn), and (F) internal burst bubble (Pyrovac).

lignin, and high viscosities (Table 2). Sample modification, associated with the appearance of bubbles, is slightly delayed for the BGT (480 K) and Dynamotive (490 K) samples. Furthermore, swelling occurs to a lower extent, and the resulting char is somewhat more irregular in shape. Additional information on the physical structure of the secondary char can be gained from the SEM images presented in Figure 5A-F for conditions corresponding to a temperature of 600 K with a holding time of 200 s. Images A and B of Figure 5 provide examples of the appearance of the top surface of the solid residue for the Dynamotive and Pyrovac samples, respectively. It is compact, glossy, and brittle with the presence of intact

or burst bubbles. The number of bubbles per unit surface area is the main difference between the different samples and, in accordance with the physical properties of the oils, is highest for the Pyrovac sample. A magnified view of a bubble, developed at the sample surface, is shown in Figure 5C (Ensyn oil). The thickness of the char (Figure 5D, Dynamotive oil) is compact and does not present any micropore network. A channeled structure was always observed, and it is likely to be caused by stresses resulting in breakage. The absence of microporosity is also confirmed by the secondary char fragment, obtained from the internal part of the sample and shown in Figure 5E (Ensyn oil). An example of an internal burst bubble is given in Figure 5F (Pyrovac oil).

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Figure 6. SEM images of primary chars (from a beech wood particles 5-mm thick, injected into a fluidized bed at 800 K). Views of the (A) lateral (parallel to the wood fibers) and (B) cross surfaces. (C-F) Details of the cross surface showing cracks and pores.

For comparison purposes, SEM images are reported in Figure 6A-F for a primary char particle obtained from beech wood (a parallelepiped with a cross section of 5 × 5 mm2 and a length of 10 mm) injected in a fluidized bed at 800 K.37 Some features of the wood structure are preserved for the directions parallel (Figure 6A) and perpendicular (Figure 6B) to the fibers. From the cross section, the formation of wide cracks and more or less integral zones with significant porosity can be seen. Magnified views show that the cracks are up to 100 µm wide (Figure 6C), whereas for the integral part, the sizes of the macropores are about 25-50 µm (Figure 6D,E) and those of micropores about 5-10 µm (Figure 6F). Contrary to the case of very slow heating,38 through which vessel, fiber, and ray characteristics of

wood are preserved, fast heating causes shrinkage and structural failure,37 and the porous structure of resulting char is highly nonhomogeneous.39 Combustion of Secondary Char. Secondary chars, collected after the conclusion of the first set of experiments and after adequate pretreatments, were burned in air in accordance with the procedure already described. Figure 7A,B reports the solid mass fraction and the time derivative of the solid mass fraction, respectively, as functions of time. The weight loss curves of primary chars,34 generated from conventional and fast pyrolysis, are also reported for comparison. From a qualitative point of view, all of the curves present the same shape, with the rate of weight loss going through a maximum after a first region of low combustion rate,

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Table 6. Characteristic Temperatures, Mass Fractions, and Weight Loss Rates of Primary and Secondary Charsa secondary char

Tshoulder Yshoulder -(dY/dt)shoulder × 103 (s-1) Tpeak Ypeak -(dY/dt)peak × 103 (s-1) a

primary char

BTG

Dynamotive

Ensyn

Pyrovac

conventional pyrolysis

fast pyrolysis

665 0.732 0.276 747 0.342 0.532

669 0.692 0.304 747 0.262 0.704

657 0.760 0.255 766 0.224 0.654

646 0.810 0.256 729 0.316 0.915

623 0.892 0.240 699 0.340 1.250

642 0.920 0.353 701 0.350 1.360

Heating rate of 5 K/min and final temperature of 873 K.

Figure 8. Conceptual mechanism of bio-oil devolatilization and combustion. Figure 7. (A) Weight loss (Y) and (B) weight loss rate (-dY/dt) curves of primary (conventional and fast pyrolysis) and secondary chars as functions of time (dynamic section with a heating rate 5 K/min to 873 K and isothermal section of 1100 s).

characterized by a more or less pronounced shoulder. As already discussed for primary chars,34 the first reaction zone is essentially a devolatilization process, whereas the second is combustion. The characteristic parameters of the two reaction zones for both secondary and primary chars are listed in Table 6 (definitions as in ref 34). It can be seen that, in the former case, the first zone is weakly affected by the pyrolysis process, with Tshoulder varying between 646 and 669 K. The variations in the corresponding mass fractions (0.6920.810) and rates [(0.255-0.304) × 10-3 s-1] are also relatively small. The characteristics of the second zone, where a large part of the combustion process takes place, are significantly dependent on the secondary char samples. The maximum combustion rate is attained by the Pyrovac char, followed by the Dynamotive, Ensyn, and BTG chars. The position of the peak (Tpeak) is the same for the BTG and the Dynamotive chars (747 K), whereas it is displaced toward higher temperature (766 K) for the Ensyn char and toward lower temperature (729 K) for the Pyrovac char. The corresponding solid mass fractions (Ypeak) fall between 0.22 (Ensyn) and 0.34 (BTG). From a quantitative point of view, the differences between primary and secondary chars are large, espe-

cially when primary chars generated from fast pyrolysis are used for comparison. As for the first reaction zone, primary chars attain nonnegligible rates of weight loss at higher temperatures, but Tshoulder is lower (differences of about 20-40 K) and the corresponding mass fraction higher. The peak rate is also higher (up to factors of 2.5) and is about 30-60 K. The higher reactivity of primary chars can be explained by taking into account the heterogeneous nature of the combustion reactions and the highly microporous structure of the material (completely absent for secondary char), as found by SEM analysis. Conceptual Reaction Mechanism of Bio-oil Combustion. The analysis reported above allows a conceptual mechanism to be formulated for the low-temperature devolatilization dynamics of the different bio-oils and the subsequent combustion of the solid residue, as shown in Figure 8. It includes eight main reaction steps. For temperatures below 450 K, in accordance with the analysis given above, evaporation of three different classes of compounds, which presumably take place through parallel, independent reactions, should be considered. For higher temperatures, again following the thermogravimetric curves, it can be reasonably assumed that the rate of weight loss is determined by both cracking reactions and evaporation. A seriesreaction mechanism is proposed that also takes into account the onset of condensed-phase reactions leading ultimately to secondary char formation. Finally, thermogravimetric measurements support a two-step global

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process for the final stage of secondary char combustion. However, the exact number of the reaction steps to be used and the selection between parallel- and seriesreaction mechanisms can be determined only by means of accurate modeling, which should examine measurements carried out for several heating rates and should evaluate the kinetic constants for each reaction step. Conclusions The combustion behavior of bio-oils derived from wood by means of different fast pyrolysis technologies (BTG, Dynamotive, Ensyn, Pyrovac) has been investigated by measuring weight loss curves in air under controlled thermal conditions. Two separate sets of experiments were performed, corresponding to (1) devolatilization (evaporation and cracking reactions) and formation of secondary char and (2) heterogeneous combustion of secondary char. A heating rate of 5 K/min was applied, with a final temperature of 600 K in the former case and 873 K in the latter. The rate of weight loss for the first stage always presents three zones characterized by a well-defined peak or shoulder for temperatures in the range of 300450 K. The total amounts of volatiles released are well correlated with the contents of chemical compounds presenting boiling points in the above range. Hence, BTG oil, which has the highest content of water and volatile compounds (among the major contributors formaldehyde, glyoxal, aceton, acetol, acetic acid, butanol, hydroxyacetaldehyde), shows the most significant weight loss. On the other hand, the lowest weight loss occurs for Pyrovac oil, which has the lowest content of water and volatile compounds. Again, for the devolatilization stage and temperatures in the range of 450-600 K, additional reaction zones can be clearly defined (up to a maximum of three for the Ensyn oil). Even though the weight lost by the sample is always higher than that estimated on the basis of a pure evaporation process, the highest amount of volatiles is released by the Pyrovac sample characterized by the highest content of species (essentially simple and complex phenols) with boiling points in this temperature range. Aside from incomplete chemical characterization, the activity of cracking reactions of the heavier compounds should be taken into account. It is also likely that polymerization reactions attain nonneglible rates. A careful examination of the oil samples reveals that, for temperatures of about 460-490 K, depending on the pyrolysis process, the viscosity is highly increased, vapor release becomes difficult, and bubbles appear. These processes are further enhanced at successively higher temperatures and result first in extensive swelling and ultimately in solidification (secondary char). In addition to the chemical composition and, particularly, the percentage of water, the contents of solids and pyrolytic lignin and the viscosity play an important role in the physical appearance of secondary char. The yield of secondary char varies from about 25% to 39% (on a total oil basis) and reflects the carbon content of the oils. Elemental analyses indicate that the secondary char retains about 48-55% of the initial carbon content. Weight loss curves of secondary and primary chars in air are qualitatively similar, with two different reaction zones attributable to devolatilization and combustion. However, secondary chars show significantly lower reactivities. On the basis of SEM analysis, the

absence of any micropore structure (in contrast with the highly porous primary chars) can be considered responsible for such a behavior. Among the secondary chars, the highest reactivity belongs to Pyrovac char and the lowest to BTG char. The experimental data produced by the technique proposed in this study have been especially useful for formulating a conceptual mechanism of bio-oil combustion that proposes a correlation between the chemical composition of the oil and the devolatilization rate. Further experimental work is needed to validate the proposed mechanism or to propose alternatives. A significant contribution in this direction can be given by the characterization of the volatile products generated. Moreover, quantitative information is needed about the dependence of the ratio between volatiles and secondary char generated for the temperatures and surface areas of single-droplet combustion examined by previous analyses,21-24 which try to reproduce conditions typical of practical systems. Acknowledgment The kindness of the bio-oil producers (BTG in The Netherlands and Dynamotive, Ensyn, and Pyrovac in Canada), who made samples available through the European network PyNe, is greatly appreciated. C.D.B. also thanks the PyNe group for many useful discussions on bio-oil composition and properties. Thanks are also due to Mrs. Clelia Zucchini and Mr. Sabato Russo of Instituto di Ricerche sulla Combustione, CNR (Napoli, Italy) for the photographs of the samples and the SEM images of primary and secondary chars. Literature Cited (1) Bridgwater, A. V.; Czernik, S.; Piskorz, J. An overview of fast pyrolysis. In Progress in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackwell Science Ltd.: Oxford, U.K., 2001; Vol. 1, pp 977-997. (2) Bridgwater, A. V.; Czernik, S.; Piskorz, J. The status of biomass fast pyrolysis. In Fast Pyrolysis of Biomass: A Handbook Volume 2; Bridgwater, A. V., Ed.; CPL Press: Newbury, U.K., 2002; pp 1-23. (3) Oasmaa, A.; Meier, D. Analysis, characterization and test methods of fast pyrolysis liquids. In Fast Pyrolysis of Biomass: A Handbook Volume 2; Bridgwater, A. V., Ed.; CPL Press: Newbury, U.K., 2002; pp 23-35. (4) Czernik, S.; Bridgwater, A. V. Overview of application of biomass fast pyrolysis oil. Energy Fuels 2004, 18, 590. (5) Wang, D.; Czernik, S.; Montane, D.; Mann, N.; Chornet, E. Biomass to hydrogen via fast pyrolysis and catalytic steam reforming of the pyrolysis oil or its fractions. Ind. Eng. Chem. Res. 1997, 36, 1507. (6) Wang, D.; Czernik, S.; Chornet, E. Production of hydrogen from biomass by catalytic steam reforming of fast pyrolysis oils. Energy Fuels 1998, 12, 19. (7) Marquevich, M.; Czernik, S.; Chornet, E.; Montane, D. Hydrogen from biomass: Steam reforming of model compounds of fast pyrolysis oil. Energy Fuels 1999, 13, 1160. (8) Czernik, S.; French, R.; Feik, C.; Chornet, E. Hydrogen by catalytic steam reforming of liquid byproducts from biomass thermoconversion processes. Ind. Eng. Chem. Res. 2002, 41, 4209. (9) Evans, R. J.; Milne, T. A. Molecular characterization of the pyrolysis of biomass. 1. Fundamentals. Energy Fuels 1987, 1, 123. (10) Evans, R. J.; Milne, T. A. Molecular characterization of the pyrolysis of biomass. 2. Applications. Energy Fuels 1987, 1, 311. (11) Piskorz, J.; Radlein, D.; Scott, D. S.; Czernik, S. Liquid products from the fast pyrolysis of wood and cellulose. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: London, 1988; pp 557-571.

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Received for review July 2, 2004 Revised manuscript received November 17, 2004 Accepted November 18, 2004 IE049419E