Ind. Eng. Chem. Res. 2001, 40, 5547-5556
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Kinetics of Primary Product Formation from Wood Pyrolysis Colomba Di Blasi* and Carmen Branca Dipartimento di Ingegneria Chimica, Universita´ degli Studi di Napoli “Federico II”, Napoli, Italy 80125
Weight loss curves of thin layers (150 µm) of beech wood powder, measured for heating rates of 1000 K/min and final temperatures in the range 573-708 K, show final char yields of 37-11%. The process is kinetically controlled and, for the most part, isothermal. A one-step global reaction, with E ) 141.2 ( 15.8 kJ/mol and ln A ) 22.2 ( 2.9 s-1, is a degradation mechanism capable of capturing the main features of the process. The thermogravimetric curves also allow the formation rate constants to be estimated for char and total volatiles (activation energies of 111.7 ( 14.3 and 148.6 ( 17.4 kJ/mol, respectively) and, once integrated byproduct distribution, those for liquids and gases (activation energies of 148 ( 17.2 and 152.7 ( 18.2 kJ/mol, respectively). A comparison is provided with pyrolysis mechanisms available in the literature. Introduction Kinetics of wood pyrolysis are needed for the design of chemical reactors applied for recovering energy and chemicals. The kinetic analysis is complicated by the composite nature of wood, constituted by a mixture of hemicellulose, cellulose, lignin, and extractives, with proportion, chemistry, and reactivity affected by the variety. On an indicative basis, for slow thermogravimetry, primary wood degradation starts at about 500 K;1,2 however, fast rates are attained at about 573 K,3,4 and the process is practically terminated at 700 K.5 At higher temperatures, secondary reactions of primary tar vapors also become active.6,7 Reaction products are usually lumped into three main classes (liquids, char, and gas),8 whose relative amounts and composition are specifically dependent on the conversion unit (for instance, fixed-bed or fluid-bed reactors), but heating rate and reaction temperature certainly are the most important process variables. Weight loss curves of wood, obtained with slow heating rates, show several reaction zones, associated with component decomposition, which attains maximum rates at different temperatures.1-2,9 The related kinetic mechanisms, usually formulated by the use of a single experiment (heating rate), are based either on a onestep reaction or on several parallel reactions (a review is given in ref 2). These mechanisms, properly coupled with the description of transport phenomena, have been applied for fixed-bed reactors,10,11 where the coarse particles and/or the significant external heat transfer limitations establish slow heating rates and low conversion temperatures (slow/conventional pyrolysis). As a constant ratio between the yields of char and volatiles is assumed, only the conversion time can be predicted. Results of kinetic modeling of wood pyrolysis obtained for fast heating rates or isothermal conditions, needed for the development of fast pyrolysis technologies12,13 and, in some cases, for the devolatilization stage of gasifiers and combustors,14 include refs 3, 4, and 1520. Furthermore, the kinetic constants, estimated by means of different literature sources and used in ref 21 * To whom correspondence should be addressed. Phone: 39081-7682232. Fax: 39-081-2391800. E-mail:
[email protected].
to model large particle pyrolysis, are also of interest because capable of predicting22 at least qualitatively the correct behavior of wood pyrolysis. Apart from ref 3, the kinetic constants are based on the assumption of a one stage process, that is, on a simple devolatilization reaction or on three parallel reactions for the formation of the main product classes introduced above, in accordance with the mechanism originally proposed by Shafizadeh and Chin in ref 23. Mechanisms, which require a large set of parameters, have also been proposed, based on the component degradation rates, such as in ref 24. The activation energy of the global reaction presents widely variable values, roughly comprised between 89 and 175 kJ/mol. This can be the result of the different heating conditions established by the experimental devices, which include tube furnaces,4 entrained15 and fluid bed3,18 reactors, screen heaters,16 drop tubes19 and classical thermogravimetry,17,20 the different sample characteristics (size and wood variety), and the mathematical treatment of the experimental data. The following critical points motivate the need for further, more accurate analysis of the intrinsic kinetics of primary wood degradation valid for fast pyrolysis: (a) the narrow range of very low (below 593-598 K) temperatures investigated,17,20 so that only the less stable components degrade,1 (b) the inability of the majority of the mechanisms3,15-18,24 to predict the rates of product formation, which, together with the conversion time, are needed for the formulation of engineering models for reactor optimization and design, (c) the failure of the most complete mechanisms to predict25 quantitatively21 or even qualitatively4 the correct dependence of product yields from pyrolysis on the reaction temperature, (d) the use of high temperature (above 750 K) data in the estimation of kinetic constants,19,20 so that secondary reaction activity is not negligible and, given the extremely fast reaction rates, it is likely9that conversion occurs under heat transfer control (apparent kinetics), and (e) the lack of information on the evolution of the sample temperature in all cases or the use of thick particles/large samples,4,20,24 which again switch the control from chemical reaction to heat and mass transfer. In this study, isothermal weight loss curves of beech
10.1021/ie000997e CCC: $20.00 © 2001 American Chemical Society Published on Web 10/10/2001
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Figure 1. (A and B) Schematic representation of the fast thermogravimetric system: (A) cross section of the furnace and (B) the reactor. (C) Initial transients of the sample temperature and the intensity of the radiative heat flux (electric signal entering the SCR) during thermogravimetric degradation tests (t* is the time corresponding to the attainment of isothermal conditions).
wood have been measured under kinetic control. They, integrated byproduct distribution specifically measured or derived from previous literature, have been used to estimate the kinetic constants of a semiglobal mechanism based on first-order parallel reactions for the formation of the main product classes (char, gas, and liquids). A comparison is also given with the kinetic mechanisms currently available. Experimental Section To determine the kinetics of isothermal pyrolysis of wood, experiments have been made by means of a fast thermogravimetric system and a laboratory scale reactor. Characteristics of the material and methods are given below. Material. Beech (Fagus sylvatica), a hardwood, was used for the tests. The chemical composition10 consists of 20% lignin, 33% hemicellulose, 45% cellulose, and 2% extractives. Prior to thermogravimetric tests, the wood was milled to powder (particle sizes below 80 µm). For the laboratory scale reactor, the particles were slab shaped with thicknesses in the range 100-500 µm. For both cases, samples were predried for 10 h at 373 K. Fast Thermogravimetry. The fast thermogravimetric system presents the same characteristics of that previously used by this laboratory for kinetic analyses.26,27 It consists of (1) a furnace, (2) a quartz reactor, (3) a PID controller and a SCR, (4) a gas feeding system, (5) an acquisition data set (PC and related accessories), and (6) a precision balance. The furnace is a radiant chamber, whose heating elements are tubular quartz infrared lamps with a tungsten wire filament which emits radiant energy in proportion to the applied voltage. Four elliptical, polished aluminum, water-cooled reflectors (Figure 1A) focus the high-density infrared energy, emitted by lamps, onto a cylindrically shaped target area, whose diameter is 6.5 × 10-2 m. The length of the heated zone
is 6.02 × 10-2 m, delimited on both sides by two inert zones of about the same length. To avoid interaction between the volatile pyrolysis products and the lamps, a quartz tube transparent to infrared radiation is located inside the furnace and used as a reaction chamber. Compared with the apparatus used before,26,27 the reactor size and configuration of this study (Figure 1B) allowed a better control of both the reacting atmosphere and the region of uniform radiation. A wood layer was exposed to thermal radiation by means of a 325 stainless steel mesh screen, whose sides were wrapped on two titanium rods (the extension of the uniform heated zone is about 2.6 × 10-2 m, in the axial direction of the reactor, and 2 × 10-2 m, along the other). Aluminum supports connected the sample holder to a precision (0.1 mg) balance. A continuous nitrogen flow at ambient temperature (1 × 10-3 m3/min, resulting in a nominal velocity of 3 × 10-2 m/s) established an inert environment and reduced the residence time of vapors inside the reaction chamber. It also cooled both the gas-phase environment during pyrolysis and the solid residue after complete devolatilization. During the tests, the controlled variable was the sample temperature and the manipulated variable was the electric voltage applied to the furnace or, in other words, the intensity of the radiative heat flux. The temperature of the reacting solid was measured by a thin (75 µm bead) chromel-alumel thermocouple into direct physical contact with wood (Figure 1B). The measured temperature coincides with the actual reaction temperature only if the conversion process occurs in a regime controlled by external heat transfer, that is, if a uniform profile is established along the sample. Therefore, conditions for this conversion regime should be determined first. The procedure applied in this study is based on the dependence of the char yield on the reaction temperature (primary char) and the
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intraparticle residence times of primary volatile products (secondary char). Indeed, as the temperature of the primary degradation increases or the residence times of tar vapors inside the reacting particle become shorter, the char yields decrease. Thus, for chosen heating conditions, the sample thickness should be decreased until the solid residue becomes constant. The limit char residue was determined for a heating rate of 1000 K/min and a final temperature of 708 K and was observed to vary from about 14.5 to 11% (initial dry wood basis) as the wood layer thickness was decreased from 500 to 200 µm. Thinner layers (up to 100 µm) did not give rise to any variation in the amount of char generated. On the basis of this result, thermogravimetric tests were made for a wood layer thickness of 150 µm (a mass of 9 mg). The wood heating rate was 1000 K/min for final temperatures varying from 573 to 708 K. This thickness of the wood layer, being twice the thermocouple size, also permitted accurate measurements of temperature. Indeed, the experiments started with the thermocouple completely immersed in the wood layer, then the temperature achieved during the heating stage (when mass loss and layer shrinkage are negligible) was maintained until complete conversion. Examples of the process dynamics are shown in Figure 1C, which reports the temperature recording (controlled variable) and the corresponding variations in the intensity of the radiative heat flux (manipulated variable), through the electric signal entering the SCR. After the initial transients, no significant variation is shown by the manipulated variable. Hence, the temperature attained, with no direct exposure of the thermocouple to heat radiation, was maintained during the whole conversion time. In addition, possible changes in the radiative properties of the sample did not appear to play any role, probably owing to the moderate temperatures investigated. Temperature measurements showed good reproducibility with maximum deviations always below 2 K. Finally, given the slow velocities, doubling the nitrogen flow did not cause any detectable effect in the control (conversion) process. Three runs were required to measure the weight loss. In the first, the history of the radiant heat flux needed for a certain evolution of the wood temperature was recorded (see Figure 1C). Indeed, notwithstanding the very thin thermocouple, the close contact of this with the sample affected the mechanical inertia of the system and simultaneous accurate measurements of temperature and weight were not possible. Then, the weight loss curve of the degrading sample was obtained. The third run recorded the so-called “blank” curve, which takes into account the effects of weight changes because of buoyancy forces in the gas-phase surrounding the sample while degrading.26 This curve, obtained with the same heating conditions established during wood degradation, was then used to get the correct weight loss curve. Laboratory Scale Reactor. Product yields from the pyrolysis of beech wood were determined by means of a laboratory scale system (Figure 2) including a reactor (6.3 × 10-2 m internal diameter and 45 × 10-2 m length), where the sample was fluidized by nitrogen. This, fed through a jacket (internal diameter 8.9 × 10-2 m) at the reactor top, was heated by an electrical furnace and distributed by a sintered metal plate, which
Figure 2. Schematic representation of the laboratory system for wood pyrolysis: (1) purging nitrogen, (2) fluidizing gas, (3) sample injection valve, (4) isolation valve, (5) gas heating, (6) reactor, (7) furnace, (8) controller, (9) sample, (10) acquisition data set, (11) condensers, (12) liquid pot, (13) demisters, and (14) silica gel bed.
also supported the bed. The residence time of volatiles, defined as in ref 28, was 10 s. Temperature profiles along the reactor axis were measured by seven thermocouples (chromel-alumel type, 500 µm diameter), with their tips exiting from a protective steel tube, at chosen distances from the flow distributor. The lower reactor zone (about 15 × 10-2 m) was isothermal at a temperature determined by a proper set point of the furnace (PID controller), but gradients in the upper part were high. However, this was not a drawback for the process because the thermal conditions were chosen so as to prevent both vapor condensation and secondary degradation. The exit gas stream entered an externally heated pipe equipped with a wire mesh (30 µm) filter in order to separate fine particles. The wood particles (35-40 g) were placed in the feeder, and after nitrogen flushing, when the desired reactor temperature was achieved, the feeding valve was opened, so that they were dropped inside the hot reactor (in about 30 s). Suddenly the temperature in the lower region of the reactor underwent a decrease. Then it started to increase again and slowly returned the initial value. To refer product yields to the actual reaction temperature, a time-integral value was introduced4 with reference to the conversion time. This, indicated as reaction temperature, Tr, is lower than the initial reactor temperatures of 13-30 K. Thus, for initial reactor temperatures in the range 600-750 K, the reaction temperatures are comprised between 587 and 720 K. Nitrogen and volatile pyrolysis products passed through a condensation train consisting of two water cooled condensers (with a catch pot), a wet scrubber, two cotton wool traps, and a silica gel bed (all connected in series). Gas sampling and analysis were carried out at selected times during the duration of the tests (1518 samples). Steady global mass balances over the system allowed the exit volumetric flow rate and mass of each gaseous species to be determined (in particular, also the final total mass indicated as “gas”), given the
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inlet conditions (nitrogen flow rate, temperature, and pressure) and the measurement of species molar fractions at the exit (assumed to be at ambient pressure and temperature). After complete conversion, the power was turned off, the solid residue was left under a nitrogen flow until the reactor temperature lowered to ambient values and then was weighed. All of the condensable products collected and weighed from condensers and traps (organic compounds and product water formed) are indicated here as “liquids”. Results Pyrolysis tests of beech wood were conducted as described above. Weight loss curves (heating rates of 1000 K/min with final temperatures in the range 573708 K), integrated by the distribution of volatile product yields determined in this study (reaction temperatures in the range 587-720 K) and derived from the literature,6,28 were used to estimate the rate constants of a global mechanism.23 Finally, pyrolysis characteristics (kinetic constants and product yields) predicted by the mechanism proposed in this study are compared with literature results. Kinetic Modelling and Global Degradation Rate When wood is slowly heated, the weight loss dynamics show the existence of three main zones associated with the devolatilization of different components (hemicelluloses decompose at 498-598 K, cellulose at 598-648 K, whereas lignin decomposes gradually over the temperature range 523-773 K1). Three main zones also exist, though without a straight separation, in isothermal analyses,3,4,17,20 given that the temperature range usually examined barely spans that of dynamic experiments. Different zones are also shown by the plot of the logarithm of the nondimensional weight loss (Y - YC)/ (Y* - YC) versus time in Figure 3A (char yields comprised between 37 and 11%) representative of a process which, based on the examination of the time needed to attain isothermal conditions t* (Figure 1C), and the corresponding solid mass fractions, Y*, can be treated by an isothermal theory. Indeed t* is roughly comprised between 30 and 40 s, which is only a small fraction of the conversion time (about 10 500 s for Tr ) 573 K and 500 s for the Tr ) 708 K), whereas Y* is comprised between 0.96 and 0.73, indicating that weight loss during dynamic heating is relatively small. In accordance with previous studies,2,9,29-31 the activity of the different zones can be attributed to (I) extractives and the most reactive fractions of hemicellulose, (II) cellulose and part of lignin and hemicellulose, and (III) lignin and small fractions of the other two constituents. For temperatures below 600 K, given the very slow decomposition rates of lignin, only zones I and II can be seen from Figure 3A, whereas for higher temperatures, due to weight loss during heating dynamics, only zones II and III appear. Temperatures well above 700 K are expected to cause the simultaneous occurrence of the different processes,4,32 but a kinetic control is not likely to be established. When the formation rates of the three main product classes should be estimated, the corresponding yields (in particular, liquids and gases) are needed for each reaction stage. As accurate measurements cannot be accomplished for the small sample quantity used in thermogravimetric tests33 and given that laboratory
Figure 3. (A) Logarithm of the nondimensional weight mass fractions as functions of time (eq 3A, beech wood pyrolysis with a heating rate of 1000 K/min and final temperatures in the range 573-708 K). (B) Logarithm of the nondimensional weight mass fractions as functions of time (eq 3a) for the interval where the kinetic constants are estimated (model a). (C) Logarithm of the nondimensional weight mass fractions as functions of time (eq 3b) for the interval where the kinetic constants are estimated (model b).
scale reactors allow only the total final yields to be obtained, there is no alternative to a one-stage mechanism of primary wood degradation. Hence, the mechanism with three parallel reactions for the formation of the main product classes, as originally proposed in ref 23, is also used in this study:
The separate formation of different product classes introduced by the reaction mechanism a1-a3 may be questionable34 from the point of view of analytical chemistry. However, as already shown for cellulose,35
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the comparable activation energies of reactions a1-a3 do not allow the selectivity to be displaced toward only one of the products. For negligible activity of secondary reactions, product distribution from cellulose pyrolysis36 indicates that both char and gas decrease as the reaction temperature is increased (that is, their formation is linked), whereas in wood pyrolysis, both liquid and gas yields continuously increase at the expense of char (for instance, see refs 6 and 37). An indirect confirmation of the linked formation of char and gas also from degradation of holocellulose in biomass is obtained by comparing product yields from untreated and washed straw,38 where it is shown that, in the latter case, lower char yields are associated with lower gas yields. It can be postulated that, during fast pyrolysis of biomass, given that holocellulose is converted mainly into liquids, the other two product classes (gases and char) are mainly due to lignin degradation. Hence, at low temperatures, on a global basis, there is a competition between liquid (holocellulose degradation) and char (lignin degradation) formation, with the former becoming successively more favored. At high temperatures, gas formation rates tend to increase, owing to the faster devolatilization rates of lignin. To estimate the global rate constant, k, for the isothermal process described by reactions a1-a3, two different treatments can be used, though assuming in both cases that only the central part of the weight loss curves, the most important from the quantitative point of view,2,9,29-31 has to be described. That is, the mass conservation equations can be integrated over the entire time duration of the process,4,15,16,18 or specifically over the time corresponding to the central part of the weight loss curve.3,17,20 In this case, a multistep process is implied, consisting of sequential (separated) stages. It can be easily shown (for instance, see ref 39) that the integration of conservation equations leads to the following expression:
P)
Y - Yf ) e-kt Yi - Yf
(3)
where Yi and Yf indicate the initial and final mass fractions over the time interval of interest, respectively. Thus, upon adequate selection of Yi and Yf, eq 3 can be applied for both approaches, which are also examined in this study. The first treatment (one-stage process over the entire time domain) specifies as
(a) Pa )
Y - YC ) e-kt Y* - YC
(3a)
(the corresponding plot is shown in Figure 3A). For the evaluation of the kinetic constants, only the central part of the curve is considered which, as shown in Figure 3B, is linear with good accuracy and corresponds to mass fractions between 0.55 and 0.4 for Tr e 584 K (not shown) and 0.7-0.37 for higher temperatures. In the second approach (one-stage process for the zone II), it is assumed
(b) Pb )
Y - YB ) e-kt YS - YB
(3b)
where YS - YB varies in the range 0.55-0.4 for Tr e 584 K and 0.8-0.5 for higher temperatures. The rela-
Figure 4. Arrhenius plot for the global wood degradation rate, k, as estimated by means of the models a and b. Literature values are also plotted for comparison purposes. Table 1. Kinetic Constants for Wood Pyrolysis: Global Kinetic Constants, k, for the Models a and b and Kinetic Constants for Product Formation (kV, kC, kL, and kG) Estimated for the Model b (a) k (b) k kC kV kG kL
E [kJ/mol]
ln A [s-1]
95.4 ( 4.6 141.2 ( 15.8 111.7 ( 14.3 148.6 ( 17.4 152.7 ( 18.2 148.0 ( 17.2
12.4 ( 0.9 22.2 ( 2.9 15.0 ( 2.7 23.4 ( 3.3 22.2 ( 3.4 23.1 ( 3.2
tively narrow range of the mass fractions, chosen by careful examination of Figure 3A and the aid of the time derivate of the mass fraction curves, is motivated by the need to avoid interferences from the initial fast stage (especially evident for low reaction temperatures) and the slow final stage (especially evident for high reaction temperatures) of the process. Furthermore, it is worth noting that Yf plays a role more important than 1/(Yi Yf) (see eq 3) for the kinetic constants to be estimated for each temperature. Indeed, the former is directly responsible for the estimated values, whereas the latter essentially affects the standard deviations. Figure 3C again shows straight lines of the plots ln (Pb) versus time and the accuracy is again good. The usual Arrhenius plot (Figure 4) and a leastsquares analysis give the activation energy and the preexponential factor of the global degradation kinetics. Two sets of kinetic constants are obtained (Table 1), E ) 95.4 ( 4.6 kJ/mol (model a) and E ) 141.2 ( 15.8 kJ/mol (model b). Figure 4 shows that both sets are comprised in the range of literature values, reported over the temperature interval where derived. The kinetics of model a and refs 3, 4, and 18 are roughly the same, whereas close similarity exists between the model b and refs 19 and 20 (the last for a temperature range of 498-598, forest waste). Differences are high with kinetic models,15,16 which were essentially proposed as correlations for specific experiments. The actual particle temperatures were unknown in ref 15, whereas screen heater measurements16 are affected by strong heat/mass transfer limitations.9 Furthermore, in this apparatus, it is difficult to achieve good control of the sample temperature,19 and the recorded value is strongly affected by thermocouple positioning (differences up to 100-300 K are pointed out19). Finally, the low values of the kinetic constant predicted by the model20 in the high-temperature range (973-1173 K, not shown) can again be due to a lack of kinetic control.
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The standard deviation is smaller for the model a, probably because the parameters (essentially the final char yields) needed in the mathematical treatment are univocally determined from the process characteristics. It is possible that the higher standard deviation observed for the model b is related to the difficulties in separating the different reaction stages for isothermal mass fraction curves (dynamic differential data are known to be more appropriate2,9,29). From the data plotted in Figure 4 for models a and b it is evident that the mathematical treatment of the central part of the same weight loss curves affects significantly the activation energies estimated for the pyrolysis process. At low temperatures, the model a predicts degradation rates faster than those of the model b but fails to predict the fast increase of the wood degradation rate with temperature, which is due mainly to the activity of components in the zone II. Indeed, for the temperature range examined in this study, this zone corresponds to the largest amount of volatiles generated and, with reference to volatile formation, to the highest activation energy.2,9,29-31 Hence, the model b is more appropriate than the model a for predicting the behavior of chemical reactors in practical applications and has been used for the estimation of the kinetic constants for the rates of product formation. For isothermal conditions, it can be shown33,35 that the formation rates of the three product classes introduced through reactions a1-a3 are related to product yields and the global constant, k, by
YG )
kG kL kC , YL ) , YC ) k k k
(4a-4c)
The final char yield is obtained from the weight loss curves. The total volatile yield, YV, and the corresponding kinetic constants, kV, can also be determined as
YV ) 1 - YC, YV )
kV , kV ) kG + kL k
Figure 5. Char yields, expressed as a percent of the initial dry mass, from fast pyrolysis of wood as functions of the reaction temperature, as measured by laboratory scale reactors and fast thermogravimetry (for ref 28, triangles are for maple waste, crosses are for hybrid poplar, and diamonds are for IEA poplar).
Figure 6. Liquid yields, expressed as a percent of the initial dry mass, from fast pyrolysis of wood as functions of the reaction temperature, as measured from laboratory scale reactors (symbols as Figure 5).
(5a-5c)
Furthermore, if the ratio between gas and liquid yields, R, can be measured, final yields of liquid and gas can also be determined as
YG )
YG R 1 YV, YL ) YV, R ) (6a-6c) 1+R 1+R YL
so that the rate constants can be obtained from eqs 4a,b. Rates of Product Formation. To get values for the parameter R, additional tests were carried out with a laboratory scale reactor. Product yields determined in this study, expressed as percent of the initial dry wood mass (db), are reported in Figures 5-7 as functions of the reaction temperature. These figures also show some literature data, obtained3,6,28,40-43 for small particle sizes (below 1700 µm) and relatively low temperatures (below 900 K). From the qualitative point of view, the dependence of product yields on temperature is roughly the same in all cases and reproduces trends already extensively discussed for both fast6,28 and conventional37 pyrolysis of wood. That is, as the temperature is increased, the liquids go through a maximum, whereas the gases continuously increase, and the char yields decrease (for temperatures above 773 K secondary reaction activity becomes important).
Figure 7. Gas yields, expressed as a percent of the initial dry mass, from fast pyrolysis of wood as functions of the reaction temperature, as measured from laboratory scale reactors (symbols as Figure 5).
In quantitative terms, the data of Figures 5-7 show large scatter, which can be explained by the differences in reactor configuration/operation and nature and properties of the feedstocks. Two main groups of studies can be identified. The first group includes this study and those of refs 3 and 40. These have in common the same wood variety (beech particles with sizes comprised between 100 and 700 µm), the temperature range displaced toward low values (523-773 K), the relatively long residence time of volatiles (at least 10 s), batch
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operation, and for this study and ref 40, also the reactor configuration. In these two cases, wood particles are pyrolyzed in a sweeping gas stream, a process characterized by low external heat transfer coefficients.44 Though a sand fluidized bed reactor was used in ref 3, the low temperatures examined (below 673 K) again result in slow external heating rates. Consequently, the quantitative agreement between the data of the first group is good, apart the lowest temperature (573 K) in ref 40, which reports very high char yields (57.5%; and low liquid yields), probably because the experimental time was not sufficiently long for complete devolatilization. Measurements of the second set of studies were made starting from temperatures of at least 673 K, by continuous operation of sand fluidized reactors and for volatile residence times of 0.4-2.5 s, that is, for conditions typically used in fast pyrolysis.13 Feedstock characteristics, that is, wood variety and particle size, however, are different. Indeed, hardwoods,6,28,42 softwoods,43 and wood waste42 have been considered with particle sizes between 250 and 1700 µm. For conventional pyrolysis, it has been found37 that beech, a classical hardwood, produces gas and liquid yields higher (and lower char yields) up to a factor of 5% (with respect to the initial dry mass) than classical softwoods (Douglas fir and redwood). Also, it is well-known that, as the particle size increases, liquid production becomes successively less favored.45 The actual external heat transfer rates established in the experimental devices of refs 6, 28, and 41-43 can also be significantly different. For good mixing of solids, external heat transfer coefficients are very high, up to 1-2 orders of magnitude than those established in the systems of the first group (particle-particle contact against convective heating44,45). However, good mixing is somewhat problematic for biomass materials,46 and segregation may cause that particle conversion takes place above the sand bed, establishing again, in this way, a low external heat transfer rate. It is plausible that this is the main factor responsible for the large quantitative differences between product yields reported by Scott and co-workers6,28 (low char and gas values), on one side, and Agblevor et al.,41 Horne and Williams,42 and Peacocke et al.43 (high char and gas values), on the other. As fast heating is associated with high liquid yields, the data by Scott and co-workers6,28 appear to be the most representative of fast (isothermal) pyrolysis. In addition, these data have been obtained for two hardwoods (poplar and maple) which are expected to present a pyrolytic behavior similar to beech wood, the variety used in the thermogravimetric tests of this study. Another important parameter is the residence time of solids. Though for fast pyrolysis it is longer6,28 than the residence time of volatiles, for low temperatures, particles may be elutriated before complete conversion. Hence char yields may become higher than expected. For temperatures above 720 K, char yields measured by Scott and co-workers6,28 (dashed line of Figure 5) tend to lower values than those of this study and refs 3 and 40. Indeed, the faster external heat transfer rates established in sand fluidized reactors cause that the reaction actually occur at successively higher temperatures as the bed temperature is increased. On the other hand, for the slow heating rates of convective heating, the char yields tend to become constant (dotted line of
Figure 8. Arrhenius plot for the formation rates of char, kC, total volatile, kV, total liquid, kL, and total gas, kG (global kinetic constant estimated according to the model b).
Figure 5) as the reactor conditions are made more severe, because of the reaction going to completion before the particles attain temperatures comparable with those of the reactor. According to these considerations, final char yields from this study and refs 3 and 40 for low temperatures (below 700 K) and from Scott and co-workers6,28 for high temperatures (above 720 K) correspond to complete conversion (solid line of Figure 5) for heating rates which are the fastest currently available in the literature of laboratory scale reactors. Figure 5 also reports the char yields measured through isothermal thermogravimetry (full symbols and dashed-dotted line). Agreement between these and the laboratory scale reactor data is observed only for very low temperatures, when the reaction rates are slow and the process takes place under kinetic control, independently of the conversion unit. For high temperatures, the reactor data show much higher values, as a consequence of extra- and intraparticle heat/mass transfer resistances. However, as shown above, for the estimation of the kinetic constants of volatile product formation, only the ratio, R, between the yields of gas and liquids is required (the total volatile yields are already known from the weight loss curves). The assumption made in this treatment is that R is not strongly affected by heat/mass transfer effects for the relatively narrow range of primary degradation temperatures. Thus, as already shown for the char yields (Figure 5, solid line), the low-temperature data (this study and refs 3 and 40) are combined with the high-temperature data by Scott and co-workers6,28 for the liquid and gas distribution as function of temperature (solid lines of Figures 6-7) in order to get values representative of fast pyrolysis (R varies from about 0.137 to 0.169 for the temperatures of thermogravimetric tests). The Arrhenius plot of Figure 8 and the use of eqs 5a5b allows the kinetic constants to be determined for the global devolatilization reaction (kV) and the char formation rate (kC) (global degradation constant estimated according to model b). As expected, the activation energy of volatile formation is higher (148.6 ( 17.4 kJ/mol) than that of char formation (111.7 ( 14.3 kJ/mol; Table 1); that is, volatile formation is successively more favored as the reaction temperature is increased. Figure 8 also reports the Arrhenius plot for the total gas and liquid formation rates, as estimated by eqs 4a-4b and the use of R values (eqs 6a-6c) as derived from the volatile product distribution shown in Figures 6-7 (solid lines), indicating that the activation energies for the two
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Figure 9. Char yields from wood pyrolysis as functions of the reaction temperature, as predicted by this study and literature mechanisms (experimental data used for kinetic analysis are included, when available).
processes are comparable (148 ( 17.2 and 152.7 ( 18.2 kJ/mol for liquids and gas, respectively; Table 1). A comparison can be made (Figure 9) in terms of the char yields as function of the reaction temperature (YC ) kC/k, eq 4c), as predicted by this study and the literature mechanisms 4,19,21 which include kinetic data for kC. Results from the mechanism,24 obtained by excluding the zero-order reaction of active species formation for the three wood components and using the chemical composition of beech wood, are also examined. For completeness, the experimental data used by the different authors for kinetic analysis are included. In all cases, the measured char yields decrease as the temperature is increased, but values of this study are lower. Only in two cases17,20 and for low temperatures is good agreement observed. The Thurner and Mann mechanism4 does not reproduce the experimental trends even from the qualitative point of view, whereas quantitative differences are high between the other mechanisms. In particular, only this study and the mechanism19 predict a strong temperature dependence of the char yields (higher values in the latter case, probably because derived for a softwood). In other cases,21,24 they remain high (20-30%). Consequently, these mechanism are not applicable for fast pyrolysis, where typical char yields are below 15%, as shown, for instance, by the fluid-bed measurements of Figure 5. A comparison between the distribution of product yields predicted by this study and the literature mechanisms,4,19,21 which account for liquid and gas formation, is made in Figure 10. Differences between mechanisms exist from both the qualitative and the quantitative point of view. This study and the mechanism,21 in qualitative agreement with experimental measurements (Figures 6 and 7), indicate that both liquid and gas yields increase with temperature. The gas yields attain about the same values, but liquid yields are higher for the present study (lower char yield). The other two mechanisms do not give predictions in qualitative agreement with experimental observation. Indeed, the gas4 or the liquid19 yields decrease as the temperature is increased (though, in the latter case, this is hardly evident). It is difficult to explain the differences, but the use of thick particles4,24 clearly gives rise to heat and mass transfer limitations, which appear as high char yields. Other critical aspects are the very narrow temperature
Figure 10. Total liquid and gas yields from wood pyrolysis as functions of the reaction temperature, as predicted by this study and literature mechanisms.
range,4 the evaluation of product yields at high temperature,19 when secondary reaction activity is not negligible, and the absence of temperature measurement/control.4,19 Conclusions In this study, a global mechanism of beech wood pyrolysis is proposed, based on the analysis of isothermal (573-708 K) thermogravimetric curves determined under kinetic control. The kinetic parameters have been estimated for a one-step degradation reaction (E ) 141.2 ( 15.8 kJ/mol) together with those for the separate formation of char and total volatiles (activation energies of 111.7 ( 14.3 and 148.6 ( 17.4 kJ/mol, respectively). Values are in the range of those reported by previous analyses. The one-step reaction mechanism only describes the central part of the weight loss curves, whose dynamics correspond to those of components quantitatively higher. Consequently, it cannot be applied for the prediction of process details shown by slow heating rates/low-temperature experiments, in particular, the dynamics of the low-temperature components (hemicelluloses and extractives) and the slow tail in the weight loss curves corresponding to lignin devolatilization. On the other hand, a global approach is preferred when the formation rates of the three lumped pyrolysis product classes (char, gas, and liquids) are required. Indeed, the global devolatilization rate is already known from thermogravimetric tests and the ratio between the two volatile product classes, obtained by means of a laboratory scale reactor and literature data, has allowed the kinetic constants to be also estimated for the formation of liquids and gases (activation energies of 148 ( 17.2 and 152.7 ( 18.2 kJ/mol, respectively). Despite the simplifications, the kinetic mechanism of wood pyrolysis proposed in this study represents an advancement with respect to the current state of the art. Indeed, to apply detailed computer modeling for the design and development of conversion units, the mathematical description of transport phenomena should be coupled with chemical kinetics able to predict not only the conversion time but also the variation of production distribution with the reaction conditions. The few mechanisms available, which include both these features, do not produce predictions even in qualitative agreement with experimental observation4,19 or qualitative agree-
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ment is observed only for slow pyrolysis,21 where char yields are high and only weakly dependent on temperature. On the contrary, the predictions of the mechanism proposed here reproduce correctly the experimental trends, at least for kinetic control. Nomenclature A ) preexponential factor [s-1] E ) activation energy [kJ/mol] Tr ) reaction/reactor temperature [K] t ) time [s] t* ) time needed to attain the final temperature [s] Y ) total solid mass fraction Yj ) species mass fraction/yield Y* ) solid mass fraction for t ) t* τV ) volatile residence time Subscripts C ) char G ) gas L ) liquid S ) wood V ) total volatiles W ) water
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Received for review November 27, 2000 Revised manuscript received July 30, 2001 Accepted August 15, 2001 IE000997E
