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Radiant flash pyrolysis of cellulose—Evidence for the formation of short life time intermediate liquid species
Journal of Analytical and Applied Pyrolysis 47 (1998) 13 – 31
Radiant flash pyrolysis of cellulose — Evidence for the formation of short life time intermediate liquid species O. Boutin, M. Ferrer, J. Le´de´ * Laboratoire des Sciences du Ge´nie Chimique, CNRS-ENSIC-LSGC, 1, rue Grand6ille, BP 541, F-54001 Nancy Cedex, France Received 17 March 1998
Abstract This paper reports the first results of experiments and modelling of the radiant flash pyrolysis of cellulose. Small samples are exposed to brief flashes of a concentrated radiation, at the focus of an image furnace operating with a 5 kW xenon lamp associated to two elliptical mirrors. The mean heat flux densities may be higher than 107 W m − 2. The microscopic observations of the sample after the flash reveal the presence of short life time liquid species formed for flash durations lower than about 1 s. These products which are liquid in pyrolysis conditions are solid at room temperature, where they show a good stability. They are soluble in water. For longer flashes, they give rise to vapours escaping in the gas phase, while practically no char is formed. These results show that, if in biomass pyrolysis, lignin is known to give rise to a liquid phase, it is also the case for cellulose. A first simple modelling of these experiments is proposed. It relies on heat and mass balances at the sample level, on the Broido–Shafizadeh (BS) model and on experimentally estimated values of some of the optical characteristics of cellulose (reflectivity and absorptivity). Indeed, cellulose is a highly reflecting and weakly absorbing (semi-transparent) material. These properties must be necessarily taken into account in any predictive calculation (only a small fraction of the incoming flux is effectively absorbed by cellulose). The calculated values of the times of beginning and end of the reaction are compared with the results of the experiments. The good agreement confirms that the intermediate products have life times shorter than about 1 s at the reaction temperature, predicted to be close to 750 K. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Biomass; Cellulose; Pyrolysis; Image furnace; Flash experiments; Modelling; Kinetic pathway; Depolymerisation
* Corresponding author. Tel.: + 33 3 83175240; fax: + 33 3 83322975; e-mail: [email protected] 0165-2370/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0165-2370(98)00088-6
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1. Introduction A considerable number of papers have been and continue to be published in the field of the fundamentals of lignocellulosic material thermal conversion with special emphasise on cellulose. The two main objectives of these studies are to enhance the knowledge of the very complicated chemical mechanisms involved and to finally build efficient models for biomass thermochemical processes. However, the elementary chemical pathways occurring in the first moment of the pyrolysis are so fast that the other slower physical factors (mass and heat transfers, hydrodynamics,…) may be rate controlling [1]. The result is that many laboratory methods of investigation are not adapted to the study of the chemical mechanisms involved in flash pyrolysis. It follows that very often the results which are valid for a given experiment are of no use for other situations and that to derive scaling up relationships make no sense. In the same time, the kinetic constants are often derived from temperature values (obtained with thermocouples) which are not representative of the true reacting sample temperature [2–5]. A great number of chemical pathways aimed at representing the primary processes have been published. Most of them consider competitive steps giving rise to char, tar, gases [6 – 8] followed by subsequent consecutive and/or interphase reactions [9]. It has been also suggested to interpret the results of TGA measurements on the basis of two possible activation energies according to the heating rate employed in the experiments [10]. However, according to the authors, the competitive reactions directly concern the starting solid macrobiopolymer, or an ‘active component’ sometimes called ‘active cellulose’ (AC) for cellulose, that would be intermediately produced without mass loss [11]. The model involving the formation of AC is the so called Broido – Shafizadeh (BS) model [12] shown in Fig. 1. However, in a recent review, Antal and Varhegyi [9] suggest that a single step giving rise directly to vapours could explain the experiments made in TGA, the primary step giving rise to AC being superfluous between 523 and 643 K. It has been also noticed by Narayan et al. [2] that the ‘active’ liquid species observed by several authors, for example in ablative pyrolysis [13–17], could be attributed to lignin and not to cellulose pyrolysis. In a recent paper [18], it has been also pointed out that the description of the steps giving rise to AC did not appear necessary for the prediction of the global degradation kinetics of cellulose. As a conclusion, evident controversies appear in the literature as for the possible existence and importance of AC during the primary steps of cellulose and biomass pyrolysis. This is of primary importance from a fundamental point of view (better understanding of the depolymerization mechanisms), but also for process reactors where the interactions
Fig. 1. Cellulose decomposition pathway given in reference [12] and used in the modelling.
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between the particles of the feedstock could be influenced by the presence of a liquid film. It is hence attractive to find suitable experimental devices for the observation and study of this possible AC product that remains currently unknown as it was very recently noticed by Miller and Bellan [19]. The TGA methods widely used for studying biomass pyrolysis do not seem appropriate for easily observing AC, for three main reasons. The heating rates usually imposed on the sample, ranging roughly between 10 − 3 and 10 K s − 1, are probably too low. It is basically impossible to detect a non rate limiting reaction occurring without weight loss. The conditions of mass transfer efficiencies are very poor in such a way that if primary (liquid) products are formed they can not be removed and hence observed, before their local transformation into gaseous compounds resulting from secondary reactions. The laboratory experiments based on the use of a spinning hot disk [13] on which are pressed rods of biomass are much more appropriate for studying the behaviour of biomass in fast pyrolysis conditions (ablative pyrolysis). The heat transfer efficiencies are very high with corresponding mass flow rates of wood pyrolysed of up to 3000 times higher than in TGA [11] and heating rates reaching 105 K s − 1. The mass transfer efficiencies are also very high: the friction between the spinning disk and the surface of the reacting feedstock very rapidly eliminates the primary products thus preventing further local decompositions. However, the available heat flux densities are not accurately known, the reacting interphase cannot be observed and the liquid primary products stay on the hot disk where they decompose rapidly before they can be quenched and recovered for analysis. It is possible to have similar heat flux densities to those in ablative pyrolysis by using concentrated radiation. This can be practically achieved in solar concentrators (solar furnaces) or by using discharge lamps associated with concentrating mirrors (image furnaces). The available heat flux densities are measurable and can exceed 107 W m − 2 inside the focal zone of the furnace. Concentrated radiation has been already used several years ago [20–29] and continues to be used or suggested [8,30,31] to perform biomass thermochemical conversion. However, the fundamental difference with the spinning disk experiments is that, as in TGA, mass transfer efficiencies are very poor. The consequence is that, if primary products are locally formed, they can undergo further degradations before their elimination from the reacting zone. These secondary reactions can be cracking to produce vapours going into the gas phase but also char remaining on the sample in the case of too long exposures or of too small heat flux densities. This is probably one of the reasons why several of these previous experiments were failures. For example, unexpected amounts of char were often obtained while one would expect that because of the cold environment surrounding the focal zone, the great majority of the primary products formed would have been immediately quenched in the gas phase. It is possible to take advantage of these drawbacks by operating under controlled short periods of irradiation of known available heat flux densities, and by observing the surface of the sample which is rapidly quenched as soon as the illumination is stopped. In addition, we can imagine that the products (other than char) remaining on the sample after a short exposure to the radiation are of a much more primary
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Fig. 2. Scheme of the experimental set-up (sizes in 10 − 3 m)
nature than those contained in the usual bio oils resulting from vaporization and cracking processes at the level of the sample and in the gas phase before their final recondensation and possible repolymerisation at the exit of the reactor. The idea of studying flash pyrolysis of cellulose by using flashes of light is not new [20,21]. However, as will be seen later, the duration of the flashes must be very well controlled inside narrow values and for known available fluxes. However, cellulose is a partly reflecting and not opaque material and thus only a fraction of the incident flux is absorbed. Such a phenomenon, which has been often presumed by the authors, has never been quantified. It is probably one of the reasons for the apparent low reactivity of cellulose under concentrated radiation. It could also explain some of the failures observed in the literature. The first purpose of this paper is to show that it is possible to perform the flash pyrolysis of cellulose samples exposed to controlled short flashes of light and to report a few properties of the primary products formed. The second purpose is to compare these results with those derived from a very simple mathematical model taking into account the kinetic pathway reported in Ref. [12] and some of the estimated optical properties of cellulose.
2. Experimental section
2.1. Source of concentrated radiation The experiments were made in an image furnace, the schematic of which is shown in Fig. 2. The light source is an air cooled 5000 W xenon high pressure arc lamp. It is situated at the first focus F1 of a wrap round elliptical mirror M1. The second focus F%1 of M1 is adjusted at the same location as the focus F%2 of a second elliptical mirror M2 in such a way that the image of the arc is formed at its first focus F2. The available thermal power at the focus F2 has been estimated by measuring the rate of temperature increase of a known volume of water darkened with Indian ink
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and contained in a pyrex spherical vessel placed at F2. In these conditions, the measured available power is close to 400 W. However, during flash experiments with cellulose, the power is only 350 W because of a screening effect due to the experimental device supporting the cellulose sample. The cross section of the concentrated beams at the focus F2 determines a circular surface with an estimated diameter between 5 and 6 × 10 − 3 m leading to mean available heat flux densities between 1.8 and 1.2×107 W m − 2. These values are of the same order of magnitude as those achieved in the most powerful solar furnaces and also in ablative pyrolysis performed with a spinning hot disk [13]. The available power at F2 can be easily changed and calibrated by setting a diaphragm at different locations between F%2 and F2. However the diameter of the focus slightly depends on the resulting available power. It can be estimated between 4 and 5× 10 − 3 m for 175 W and between 3.5 and 4× 10 − 3 m for 117 W, leading to mean heat flux densities between 1.4 and 0.9× 107 W m − 2 (for 175 W) and between 1.2 and 0.93× 107 W m − 2 (for 117 W). This diaphragm also supports a pendulum inside which is made an adjustable window which has the shape of an arc of a circle centred on the axis of rotation of the pendulum. The window and the hole in the diaphragm are at the same distance from the axis of rotation and face each other when the pendulum is at rest. At the beginning of the test, the pendulum is lifted in such a way that the radiation is occulted by a side out of the window. It is then dropped from a known position. The light crosses the hole for as long as it faces the window. The duration of the flash is determined by the initial position of the pendulum and the length of the window. A photoelectric cell facing the mirror M2 produces two signals corresponding, respectively, to the beginning and the end of the crossing of the light through the window. The time of the flash is then computed from the time elapsed between these two signals. It is possible to make flashes as short as 0.01 s.
2.2. Preparation of the sample The experiments are made with samples prepared from cellulose powder (Whatman microgranular CC31). Fig. 3 shows a representative microscopic picture of the particles. An aqueous suspension of the powder is regularly spread on a flat thin glass surface and then dried. The thickness of the layers used in the experiments reported in this paper was roughly measured with a caliper gauge having an electronic digital display (accuracy of 10 mm). From 10 measurements made at different locations on the sample (after drying) it is possible to estimate the thickness as 450950 mm.
2.3. Properties of the sample A given quantity of the same cellulose aqueous suspension was introduced into a test tube. After drying, the volume and weight were measured leading to a mass density close to 1100 kg m − 3.
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The heat capacity was measured in a SETARAM differential calorimeter between 304 and 466 K. The values of Cp show a regular increase with temperature according to the equation: Cp =970 + 5.13(T −273)
(1)
Such behaviour, which is well known for polymers, does not seem to have been previously reported for cellulose. The constant values reported in the literature vary to a very large extent (1600 – 2800 J kg − 1 K − 1) but are, however, compatible with those given by equation Eq. (1). Cellulose is not a perfectly absorbing material: when the radiation reaches its surface, a given fraction ar is reflected. It is also a semi transparent medium and hence, the remaining fraction (1−ar) is progressively absorbed, as it crosses the solid, according to a given attenuation law. If the sample is thin enough, only a fraction aa(1 − ar) of the incident radiation is absorbed. Quantitative values of ar and aa and of their variations with the thickness of the sample do not seem to exist in the literature. It is possible to experimentally derive estimated orders of magnitude for these two parameters. A similar experiment as that made for determining the available power at F2 can be made after deposition of a layer of cellulose on the spherical vessel at the side of the incoming radiation from the mirror M2. The experiment has been made with a mean thickness of cellulose near 360 mm (estimated from the mass and area of cellulose deposited on the vessel and intercepting the incoming radiation). From the time (30 s) after which the reaction of pyrolysis begins (observation of the first yellowing of the surface, supposed to occur at 739 K [14]), the mass of cellulose exposed to the radiation (0.932×10 − 3 kg) and Eq. (1) giving Cp, it is possible to calculate the power Pa absorbed by the irradiated cellulose from the beginning of the reaction:
Fig. 3. Microscopic photograph of the cellulose powder used in the experiments (scale: one square represents 80 × 10 − 6 m).
&
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Pa = 0.932 ×10 − 3
739
293
970 +5.13(T − 273) dT 30
19
(2)
In these calibrating experimental conditions, Pa is then calculated to be close to 30 W. At the same time, a fraction of the radiation crosses the layer of cellulose and heats the black water contained in the vessel. The power absorbed by the water is calculated to be approximately 70 W. It can be deduced that among the 400 W arriving on the cellulose surface, about 30 W are absorbed while 70 W leave the sample without absorption and hence 300 W are reflected. The fraction ar is then close to 0.75. Assuming an exponential law for representing the absorption in a semi transparent solid [32] it is possible from this experiment to estimate the variations of the fraction aa as a function of the thickness L of the sample: aa = 1 − exp( − 1000L)
(3)
Finally, assuming a constant value of the reflectivity ar (0.75), the fraction a absorbed at the depth L with respect to the total incident available flux is: a = 0.25[1 −exp( − 1000L)]
(4)
Applied to the case of the cellulose samples used in the flash experiments (L0 =4509 50 mm) one can estimate: 0.0825 a5 0.098. This result shows that less than 10% of the available power is effectively absorbed. Of course, for much smaller thicknesses the values would be much lower still: 2.4, 1.2 and 0.25% for, respectively, 100, 50 and 10 mm samples. Remember that these small sizes are of the same order of magnitude as the mean dimensions of the elementary particles of cellulose used by many authors. This result explains the difficulty in performing flash pyrolysis of pure cellulose particles, even under concentrated radiation.
2.4. Experiments with flashes of light Once the layer of cellulose has been deposited and dried on the glass surface, the sample is settled at the focus F2 and then submitted to a flash of a given duration (generally between approximately 0.1 and 2 s). Several flashes of different periods are made at different locations on the surface, which is then simply observed after its removal from the focal zone. Two times for the flashes are defined. The time tb at which the reaction begins is supposed to correspond to the observation of the first yellowing of the cellulose surface. The time te at which the reaction is finished is supposed to correspond to the moment where the majority of the products have left the area of irradiation.
3. Results of the experiments
3.1. Simple obser6ations For the maximum available power of 350 W, no apparent reaction is visible for periods of flashes less than about 0.21 s. For increasing values of the duration of
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Fig. 4. Microscopic photograph of a sample of cellulose after a short exposure to a high concentrated radiant flux. It shows the agglomeration of particles after they have passed through a liquid phase (same scale as Fig. 3).
the flashes, the microscopic observations reveal the following behaviour. The shape of the cellulose particles change with the disappearance of the fibrillar structure and blunting of the contours exactly as during the phase change from a solid to a liquid. Over a short time, the particles that have passed through this liquid form, keep their individuality (Fig. 3). As the duration of the flashes increases, agglomerations occur between the melted particles, with the formation of large networks (Fig. 4), before the final disappearance (vaporization) of the majority of the products. At the same time, a very low fraction of the products darken with the final formation of char particles remaining on the glass support. Note that the same observations can be made in experiments performed either under argon or air, and either at normal or reduced pressure. Table 1 gives the ranges of the measured values of tb and te for several experiments and for available powers of 350, 175 and 117 W at F2. These results reveal that under our experimental conditions, the life time of these molten species is lower than 1 s. These simple observations clearly show that cellulose subjected to high flux densities passes through a liquid phase that is rapidly transformed into gaseous products and/or small fractions of char according to the period of irradiation. Table 1 Experimental values of tb and te for the reaction for three different available fluxes P
tb (s) te (s)
P= 350 W
P= 175 W
P= 117 W
0.20–0.22 0.90–1.10
0.28–0.33 1.00–1.19
0.44–0.46 1.32–1.50
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These results are in quite good agreement with the Nordin et al. experiments [33] performed with a laser beam and clearly showing the formation of spherical molten nodules. This gives a new clear proof that, as in wood pyrolysis it is well known that lignin gives rise to a liquid phase, it is also the case for cellulose. It should be noted that the same observations can be made after brief contacts of cellulose powder with a red hot surface [11], a logical observation, because of the similar high heat flux densities also available in ablative pyrolysis.
3.2. First estimation of the liquid properties The observations are made at room temperature after the very fast self cooling of the sample that occurs at the end of the flash. Under these conditions, the products are solid although they are liquid at the reaction temperature. They are also water soluble showing that they are not pure cellulose, but other products resulting from the thermal degradation of cellulose. It is, hence, incorrect to speak of molten cellulose. These products are, thus, of an a priori different nature to the bio oils obtained in fast pyrolysis processes and also to the syrups derived from previous solar flash pyrolysis experiments. In both these cases the products are liquid at room temperature. They are obtained after recondensation and/or repolymerization, at the exit of the reactor, of the vapours resulting from probable partial cracking of the primary products. In the present case, the products were recovered immediately before vaporization and are more likely of a primary nature. It is possible to carry out GC/MS analysis of the recovered products and also to perform HPLC analysis on these aqueous solutions. First tests revealed that they contain relatively few species and in any case, much less than is usual in bio oils. They are mainly molecules resulting from the splitting of the cellulose polymer with low degrees of polymerization.
4. Theoretical section In a recent paper Le´de´ et al. [11] has gathered several experimental and theoretical data showing that cellulose has a high affinity to primarily decompose into an intermediate product which is liquid at the reaction temperature. They also showed that such an intermediate had a great ability to rapidly vaporize (after more or less cracking) or repolymerize to give char. The observations reported in the previous section of this paper confirm the existence of such an unstable product, showing clearly that direct sublimation or vaporization from the cellulose solid phase to the gas phase is not occurring under our conditions of high flux. Our clear experimental observations show that the mathematical modelling of the reaction must a priori rely on a kinetic pathway taking into account the formation of intermediate species rather than on a simple solid vapour type reaction. Such a more complete pathway is similar to the BS kinetic model even if Ref. [12] reports lower temperature experiments where a phase change had few chances to occur.
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The theoretical evolution of the different species and particularly of AC has been studied by Diebold [34]. However his mathematical approach does not take into account the problem of heat transfer (outside and inside the sample) and assumes a linear increase of the sample temperature. This is a first approximation, as it has been shown [2 – 5] that the true temperature of a solid particle undergoing endothermic decomposition under the influence of an external heat flux depends strongly on close couplings between heat transfer processes and chemical steps. One of the consequences of such general behaviour is that after the pure heating period and during the reaction period, one can expect a stabilization of the sample temperature, leading to a phenomenon similar to a phase change. This explains the fusion like behaviour observed for biomass, for example [2,15]. This has been modelled in the cases where heat is provided to the solid sample by an external source at a constant [3,4] or increasing temperature [2,5], the exchange being ensured by gas or solid convection (Fourier conditions). It is interesting to consider another practical situation where heat is provided by a given constant external heat flux, as is the case in the present study where the cellulose is submitted to a given radiant flux.
4.1. Basic assumptions of the model We suppose, in a first approximation that the mass densities, heat capacities and optical properties (reflectivity, absorptivity and emissivity) are the same for cellulose, active cellulose and char. The Eqs. (1) and (4) are used to calculate Cp and a, before (pure heating) and during the reaction. Note that the model relies on Eq. (4) and hence takes into account the fact that a decreases when L decreases during the reaction. The kinetic pathway is that of the BS model (Fig. 1) All the elementary chemical steps are irreversible and of first order with chemical rates r1, r2 and r3 defined, respectively, as the mass of cellulose decomposed, of intermediate species transformed into vapour, and gases+ char, per unit time and per unit particle volume. They obey Arrhenius type laws with constant values of the enthalpies DH1, DH2 and DH3. r1 =rA1 exp( − E1/RT)
(5)
r2 =rA2 exp( − E2/RT)
(6)
r3 =rA3 exp( − E3/RT)
(7)
The mass fractions of char and gases (step 3, Fig. 1) are, respectively, 0.35 and 0.65 [12]. The gases and vapours escape freely towards the outside of the sample without any diffusional resistance. The equations are written for an infinite surface slab. As the reaction proceeds, the intermediate species as well as char, are supposed to stay inside the sample where they are homogeneously dispersed without creation or disappearance of any porosity. The mass density remains constant and hence the thickness of the slab decreases as the reaction proceeds.
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The slab initially at room temperature (T0 = 293 K) is suddenly exposed to a given heat flux density q0. Only the fraction a of q0 is absorbed (for heating and chemical reaction). The fraction aq0 of the flux is supposed, in a first approximation, to be immediately and entirely absorbed at the front surface of the sample and then distributed inside the internal parts in such a way that no temperature gradient exists. The radial conduction is neglected (one dimensional problem, perpendicular to the slab surface). The only losses of energy from the sample are supposed to occur by radiation from the reacting mixture (with an emissivity o =0.92 [32]). The losses by convection by the surrounding gas and by conduction by the supporting glass material are neglected.
4.2. Basic equations They rely on mass and energy balance equations.
4.2.1. Mass balances Let us define, at time t, the reduced masses Xi of each of the components (i= C: cellulose; AC: active cellulose [12]; Ch: char; G = gas; V: vapours) with respect to the extent of each of the three reactions X1, X2 and X3 [35] as: XC =mC/m0 =1 − X1
(8)
XAC =mAC/m0 =X1 −X2 −X3
(9)
XCh =mCh/m0 =0.35X3
(10)
XG =mG/m0 =0.65X3
(11)
XV =mV/m0 =X2
(12)
m0 being the initial mass of cellulose defined by the unit surface. The mass balances are: dX1/dt = A1 exp( − E1/RT)(1 − X1)
(13)
dX2/dt = A2 exp( − E2/RT)(X1 −X2 − X3)
(14)
dX3/dt = A3 exp( − E3/RT)(X1 −X2 − X3)
(15)
with the initial conditions: t=0: X1 =X2 =X3 =0
4.2.2. Energy balance It can be written by the unit surface of the slab sample:
(16)
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Table 2 Selected values of the kinetic parameters A and E [11,12] and of the enthalpies DH [4,36,37] of the three elementary steps 1, 2 and 3 (model represented in Fig. 1)
A (s−1) E (J mol−1) DH (J kg−1)
Step 1
Step 2
Step 3
2.8×1019 242 000 40 000
3.2×1014 198 000 418 000
1.3×1010 151 000 418 000
aq0 =os(T 4 −T 40) + rL0
dX1 dX2 dX3 dT DH1 + rL0 DH2 + rL0 DH3 + rLCp dt dt dt dt
(17) with: t= 0: T = T0 =293 K
(18)
Or, after combination with the three mass balances Eqs. (13)–(15):
dT 1 aq0 −os(T 4 − T 40) = − k1(1−X1)DH1 dt (1 −X2 −0.65X3)Cp rL0
n
−(k2DH2 +k3DH3)(X1 −X2 − X3)
(19)
The integration is made by using the method of Runge–Kutta–Merson (5th order with fixed steps of 10 − 4 s). The calculations lead to the variations of T and Xi as a function of time t for several possible values of L0 and q0. The calculations also give the theoretical values of tb (corresponding arbitrarily to XC = 0.99) and of te (corresponding arbitrarily to XAC = 0.01 and XC B 0.01). The kinetic parameters are those given by the BS model [11,12]. The choice of the values of DH1, DH2 and DH3 is difficult. They are taken from the references [4,36,37]. These chemical parameters are given in Table 2.
5. Results of the theoretical model Fig. 5 shows an example of the results obtained with q0 = 1.8× 107 W m − 2 and L0 = 450×10 − 6 m. They show that the fraction of intermediate species passes through a maximum close to 93%. This is a consequence of the fact that under the conditions used in the modelling (the relatively high value of the sample temperature), reaction 1 is much faster than reactions 2 and 3. This means that it might be possible to transform almost all the cellulose into very short life time species (about 0.5 s) before its further decomposition. This result explains the difficulties in experimentally isolating these liquid species. Fig. 5 also shows that the fractions of vapours are much higher than those of gas and char (reaction 2 is much faster than reaction 3). These results are in agreement with our experimental observations (almost no char on the glass at the end of the reaction, for t\ te) and also with the
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conclusions of Diebold [34]. In a first approximation, it is hence possible to say that the pyrolysis occurs according to two reactions in series: formation of short life time liquid species that decompose later into vapours, with only minor fractions of char. Fig. 5 shows that the sample temperature T increases regularly until reaction 2 occurs significantly, followed by a very strong stabilization. This effect corresponding to a liquid vapour type reaction (reaction 3 is negligible) is similar to the observations previously reported for solid fluid type reactions [2–5]. The authors mentioned in that case a fusion-like phenomenon, while now it is possible to speak of a boiling-like phenomenon. In the present case, the stabilization of temperature occurs between 700 and 800 K, quite similar to the fusion temperature of cellulose [14,15,17]. These results also show that the modelling of the reaction must rely on both mass and heat balances, and that theoretically expecting a constant heating rate of the sample is unrealistic. However, from Fig. 5, it is possible to estimate heating rates of about 1.6×103 K s − 1 at the beginning of the reactions, which is similar to the high values postulated by Diebold [34] who also claimed the possibility to reach very high fractions of liquid species and low fractions of char. In Fig. 6 are reproduced the theoretical variations of tb and te as a function of q0 in the case where a is given by Eq. (4). The results calculated in the assumption of a = 1 at any time are also reported (a simplified limit case where all the radiation would be absorbed by the sample). It is clear that very large mistakes can be made
Fig. 5. Theoretical variations of the mass fractions Xi of the species formed in the flash pyrolysis of cellulose, as a function of the time t. The evolution of the sample temperature T is also represented. The calculations have been made with a heat flux density of 1.8 × 107 W m − 2, an initial size of the cellulose sample of 450 × 10 − 6 m and an absorptivity given by Eq. (4).
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Fig. 6. Theoretical variations of tb and te as a function of the available heat flux density q0 (W m − 2) and for an initial size of the sample of 450× 10 − 6 m. The results rely on values of a calculated by Eq. (4) and also on the simple assumption of a= 1. The second curve from the top is obtained with values of enthalpies DH2 and DH3 given in Ref. [19]. The experimental results presented in Table 1 are also reported. The four points given for each power (P =350, 175 and 117 W) result from the uncertainties in the experimental estimations of tb and te, and of the focus diameter (and hence of q0).
if the reflectivity and absorptivity of the cellulose are not taken into account (factors of more than 10 in the times of reactions). The experimental results in Table 1 are also reported. They take into account two types of inaccuracies. The first one is connected with the measurement of tb and te. The second one results from the poor knowledge of the diameters of the focus (and hence of the mean heat flux densities) for each value of the available power. In any case, it is clear that the experiments fit much better with the theoretical values, taking into account the reflecting and absorbing properties of cellulose (Eq. (4)). Supposing a= 1 would have lead to important discrepancies. Fig. 6 also shows a curve corresponding to other values of DH2 and DH3 as given by the recent paper of Miller and Bellan (Ref. [19]) (DH2 =255000 J kg − 1 ; DH3 = − 20000 J kg − 1). It is clear that in spite of the great differences between the values chosen in Table 2, the resulting theoretical values of te (there is of course no influence on tb) are not much changed. Moreover, because of the inaccuracies of the experiments, it is difficult to conclude which values of the enthalpies fit better to the measurements. Fig. 6 also shows that in a logarithmic scale, the variations of tb and te are roughly linear to q0 mainly for a =1, and that the mean life time of the intermediate species which is close to te −tb logically increases as the available flux q0 decreases. It is, however, very short, from a fraction to a few seconds.
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In Fig. 7 the variations of tb and te as a function of the initial thickness L0 of the sample for q0 =1.8 ×107 W m − 2 are reported. These times of reaction appear to be practically independent of L0. These interesting results can be explained by a compensation effect between two opposite phenomena. When L0 increases, a increases (according to Eq. (4) which is almost linear for low values of L0) leading to higher fractions of the flux absorbed, but at the same time the mass of the sample increases leading to a higher heat demand for heating and the reaction. Such behaviour shows that the values of the reaction times are roughly independent of the accuracy of the measurement of L0 and are for example, the same for extreme values of the thickness of the samples used in our experiments (400 and 500 mm). Of course, for a 100% opaque material, the times of reactions would increase with the increase in L0. All these conclusions are only valid for the assumption of no temperature gradients and hence for relatively thin samples.
6. Discussion and conclusions Very simple experiments have shown that it is possible to perform the flash pyrolysis of small samples of pure cellulose by exposures over short periods (0.2–2 s) to concentrated radiation. According to the time of the flash the products evolved can be obtained in the form of a condensed phase remaining on the support, or of species escaping in the gas phase. In any case, the fraction of char appears to be very small.
Fig. 7. Theoretical variations of tb and te as a function of the initial size L0 of the cellulose sample. The calculations have been made with a heat flux density of 1.8 × 107 W m − 2, the absorptivity a being given by Eq. (4).
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The condensed phase observed with a microscope after fast cooling of the sample is liquid at the reaction temperature and solid at room temperature. It is water soluble. The first HPLC measurements show that it contains relatively few species resulting from the depolymerization of cellulose. It is not molten cellulose. It is very stable at room temperature. These products are probably of a different nature than the liquids usually obtained in flash pyrolysis processes. In these usual cases, they are evolved after the recondensation of vapours, while in the present case, they are observed before the initial vaporization step. These results show that, as for fast wood pyrolysis where lignin is well known to give rise to a liquid phase, this is also the case for cellulose. These products could be compared to the active cellulose postulated in the Broido–Shafizadeh model even if these authors had few chances to observe a liquid phase in their low temperature experiments. However, this research does not address if the liquid species observed in ablative pyrolysis of wood result from the pyrolysis of lignin, cellulose or both. In spite of much previous experimental and theoretical evidence, the assumption of the existence of a liquid intermediate product has been often questioned and even recently rejected [38]. The results reported in the present paper bring clear new and definitive proof of its existence during the fast pyrolysis of cellulose. It is, thus, impossible to deny the chemical and physical roles of a thin liquid layer surrounding the particles of biomass during the reaction of fast pyrolysis. Cellulose is a strongly reflecting material. It is also semi-transparent in such a way that only a small fraction of the incoming radiation is used for heating and for the reaction. Such an important phenomena must be necessarily taken into account in any predictive calculations. The first modelling of the experiments has been proposed. It relies on the kinetic model reported in Ref. [12] and on very simple assumptions writing of the heat and mass balances. The derived times of the beginning and end of the reaction compare very favourably with the experimental observations. Both results show that for high heat flux densities, the reaction begins after approximately 0.2 s, while almost no more product is present on the sample after approximately 1.5 s (almost no char formation). The life time of the active intermediate species is in the order of 1 s or lower. The model predicts that they can reach concentrations higher than 90%. However, high accuracies in the measurements of the period of the flashes are necessary in order to be able to effectively observe such high concentrations. The calculations confirm that the reaction occurs at about 750 K. In any case, the definitions of tb and te used in this paper are only first approximations aimed at showing the orders of magnitudes. They are too vague to quantitatively test the kinetic model with high accuracy. For that purpose, further research must include experimental studies as a function of time from the beginning to the end of the reaction. The experiments must also include the determination of mass balances in order to confirm the formation of high yields of liquids. The values of the absorptivity and of the reflectivity of cellulose have been estimated. Future studies must necessarily include much more accurate measurements of these values and of their variations with the size of the sample and the
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progress of the reaction. The optical properties reported in this paper have been measured on pure cellulose at room temperature. There is little chance of their being still valid after the reaction begins: the reflectivity probably becomes lower and absorptivity higher, leading to higher values of a. Future modelling must include these phenomena but also the fact that the fraction aq0 of the available flux is, in reality, progressively absorbed inside the sample. The chemical analysis of the short life time species must be improved. Defining their true nature would bring new data for a better knowledge of the mechanisms of cellulose depolymerization. It would also be useful for understanding the vapour formations that precede the recondensations giving rise to the bio oils in the usual pyrolysis processes. Finally, the presence of a high temperature liquid film on the surface of the reacting biomass can influence the hydrodynamic behaviour of the process reactors. On one hand it can produce favourable lubrication effects between the feedstock and the walls of the reactor, but, on the other hand, it can lead to agglomerations of the particles (mainly during cooling), with subsequent plugging in the case of concentrating beds or in the underflow of a separator. It could also be one of the possible origin of the formation of aerosols.
7. Nomenclature A1, A2, A3 Cp E1, E2, E3 i L, L0 m, m0 mi P Pa q0 r1, r2, r3 R t, tb, te T, T0 Xi X1, X2, X3 a aa ar
Arrhenius factors for reactions 1, 2 and 3 (s−1) heat capacity (J kg−1 K−1) activation energies for the reactions 1, 2 and 3 (J mol−1) C for cellulose; AC for active cellulose; V for vapours; G for gas; Ch for char thickness of the sample at time t and t= 0 (m) mass of the sample per unit surface at time t and t= 0 (kg m−2) mass per unit surface of each of the components at t (kg m−2) available flux (W) power absorbed by the sample until the beginning of a visible reaction (W) available heat flux density (W m−2) rates of the reactions 1, 2 and 3 (kg m−3 s−1) gas constant (8.314 J mol−1 K−1) time; at the beginning of the reaction of cellulose (XC = O.99); when AC has disappeared (XAC = 0.01 and XCB0.01) (s) sample and room temperatures (K) reduced masses extents of the reactions 1, 2 and 3 total fraction of the available flux absorbed at L absorbed fraction of the non reflected flux reflected fraction of the available flux
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DH1, DH2, DH3 enthalpies of the reactions 1, 2 and 3 (J kg−1) o emissivity of the sample r mass density (kg m−3) s Stefan Boltzmann constant (5.67 10−8 W m−2 K−4)
Acknowledgements The authors want to thank ADEME (Agence de l’Environnement et de la Maıˆtrise de l’Energie) who has funded this research under the framework of AGRICE (AGRIculture pour la Chimie et l’Energie) Contract No. 96 01 048. They are also grateful to M. Bouroukba for his valuable help in the measurement of the heat capacity of cellulose with the SETARAM differential calorimeter of the Laboratoire de Thermodynamique des Solides (LTS-ENSIC, Nancy, France).
References [1] J. Le´de´, in: A.V. Bridgwater, D.G.B. Boocock (Eds.), Developments in Thermochemical Biomass Conversion, Blackie, Glasgow, 1997, pp. 104 – 116. [2] R. Narayan, M.J. Antal, Ind. Eng. Chem. Res. 35 (1996) 1711. [3] J. Le´de´, J. Villermaux, Can. J. Chem. Eng. 71 (1993) 209. [4] J. Le´de´, Biomass Bioenergy 7 (1994) 49. [5] J. Le´de´, J. Ther. Anal. 46 (1996) 67. [6] B.M. Wagenaar, W. Prins, W.P.M. Van Swaaij, Fuel Process. Technol. 36 (1993) 291. [7] C. Di Blasi, Chem. Eng. Sci. 51 (1996) 1121. [8] M. Gro¨nli, Thesis, The Norwegian University of Science and Technology, Faculty of Mechanical Engineering, Trondheim, Norway, 1996. [9] M.J. Antal, G. Varhegyi, Ind. Eng. Chem. Res. 34 (1995) 703. [10] I. Milosavljevic, E.M. Suuberg, Ind. Eng. Chem. Res. 34 (1995) 1081. [11] J. Le´de´, J.P. Diebold, G.V.C.Peacocke, J. Piskorz, in: A.V. Bridgwater, D.G.B. Boocock (Eds.), Developments in Thermochemical Biomass Conversion, Blackie, Glasgow, 1997, pp. 27 – 42. [12] A.G.W. Bradbury, Y. Sakai, F. Shafizadeh, J. Appl. Polym. Sci. 23 (1979) 3271. [13] J. Le´de´, J. Panagopoulos, H.Z. Li, J. Villermaux, Fuel 64 (1985) 1514. [14] J. Le´de´, H.Z. Li, J. Villermaux, in: J. Soltes, T.A. Milne (Eds.), Pyrolysis Oils from Biomass. Producing, Analyzing and Upgrading, ACS Symp. Ser. 376, American Chemical Society, Washington DC, 1988. [15] J. Le´de´, H.Z. Li, J. Villermaux, J. Anal. Appl. Pyrolysis 10 (1987) 291. [16] J. Le´de´, H.Z. Li, J. Villermaux, H. Martin, C. Moyne, A. Degiovanni, Int. J. Heat Mass Transf. 29 (1986) 1407. [17] G. Peacocke, Thesis, Aston University, Birmingham, UK, 1994. [18] M. Lanzetta, C. Di Blasi, F. Buonanno, Ind. Eng. Chem. Res. 36 (1997) 542. [19] R.S. Miller, J. Bellan, Combust. Sci. Technol. 126 (1997) 97. [20] K.A. Lincoln, in: J.P. Diebold (Ed.), Proc. of the Specialist’s Workshop on Fast Pyrolysis of Biomass, Solar Energy Research Institute (SERI/CP-622-1096), Copper Mountain, CO, 19 – 22 October 1980. [21] M.W. Hopkins, M.J. Antal, J. Appl. Polym. Sci. 29 (1984) 2163. [22] A. Tabatabaie-Raissi, W.S.L. Mok, M.J. Antal, Ind. Eng. Chem. Res. 28 (1989) 856. [23] L. Hofmann, M.J. Antal, Sol. Energy 33 (1984) 427. [24] M.W. Hopkins, C. Dijenga, M.J. Antal, Sol. Energy 32 (1984) 547.
O. Boutin et al. / J. Anal. Appl. Pyrolysis 47 (1998) 13–31
31
[25] M.J. Antal, L. Hofmann, J.R. Moreira, Sol. Energy 30 (1983) 299. [26] M.J. Antal, ACS Symp. on Thermal Conversion of Solid Wastes and Biomass, Washington DC, September 1979. [27] J. Le´de´, J. Villermaux, C. Roye`re, B. Blouri, G. Flamant, Entropie 10 (1983) 57. [28] J. Le´de´, P. Berthelot, J. Villermaux, A. Rolin, H. Franc¸ois, X. Deglise, Rev. Phys. Appl. 15 (1980) 545. [29] R.W. Taylor, R. Berjoan, J.P. Coutures, Report UCRL-53063, Lawrence Livermore Laboratory, CA, 1980. [30] J. Le´de´, F. Pharabod, Entropie 204 (1997) 47. [31] J.P. Murray, E.A. Fletcher, Energy 19 (1994) 1083. [32] R. Siegel, J.R. Howell, Thermal Radiation Heat Transfer, 3rd edn., Hemisphere Publishing Corporation, Washington, 1992. [33] S.B. Nordin, J.O. Nyren, E.L. Back, Text. Res. (1974) 152. [34] J.P. Diebold, Biomass Bioenergy 7 (1994) 75. [35] J. Villermaux, Ge´nie de la Re´action Chimique. Conception et fonctionnement des Re´acteurs, Lavoisier, Technique et Documentation, Paris, 1993. [36] C. Di Blasi, Ind. Eng. Chem. Res. 35 (1996) 37. [37] R.S. Miller, J. Bellan, Combust. Sci. Technol. 119 (1996) 331 – 373. [38] R.S. Miller, J. Bellan, Energy Fuels 12 (1998) 25 – 40.
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Radiant flash pyrolysis of cellulose — Evidence for the formation of short life time intermediate liquid species O. Boutin, M. Ferrer, J. Le´de´ * Laboratoire des Sciences du Ge´nie Chimique, CNRS-ENSIC-LSGC, 1, rue Grand6ille, BP 541, F-54001 Nancy Cedex, France Received 17 March 1998
Abstract This paper reports the first results of experiments and modelling of the radiant flash pyrolysis of cellulose. Small samples are exposed to brief flashes of a concentrated radiation, at the focus of an image furnace operating with a 5 kW xenon lamp associated to two elliptical mirrors. The mean heat flux densities may be higher than 107 W m − 2. The microscopic observations of the sample after the flash reveal the presence of short life time liquid species formed for flash durations lower than about 1 s. These products which are liquid in pyrolysis conditions are solid at room temperature, where they show a good stability. They are soluble in water. For longer flashes, they give rise to vapours escaping in the gas phase, while practically no char is formed. These results show that, if in biomass pyrolysis, lignin is known to give rise to a liquid phase, it is also the case for cellulose. A first simple modelling of these experiments is proposed. It relies on heat and mass balances at the sample level, on the Broido–Shafizadeh (BS) model and on experimentally estimated values of some of the optical characteristics of cellulose (reflectivity and absorptivity). Indeed, cellulose is a highly reflecting and weakly absorbing (semi-transparent) material. These properties must be necessarily taken into account in any predictive calculation (only a small fraction of the incoming flux is effectively absorbed by cellulose). The calculated values of the times of beginning and end of the reaction are compared with the results of the experiments. The good agreement confirms that the intermediate products have life times shorter than about 1 s at the reaction temperature, predicted to be close to 750 K. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Biomass; Cellulose; Pyrolysis; Image furnace; Flash experiments; Modelling; Kinetic pathway; Depolymerisation
* Corresponding author. Tel.: + 33 3 83175240; fax: + 33 3 83322975; e-mail: [email protected] 0165-2370/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0165-2370(98)00088-6
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1. Introduction A considerable number of papers have been and continue to be published in the field of the fundamentals of lignocellulosic material thermal conversion with special emphasise on cellulose. The two main objectives of these studies are to enhance the knowledge of the very complicated chemical mechanisms involved and to finally build efficient models for biomass thermochemical processes. However, the elementary chemical pathways occurring in the first moment of the pyrolysis are so fast that the other slower physical factors (mass and heat transfers, hydrodynamics,…) may be rate controlling [1]. The result is that many laboratory methods of investigation are not adapted to the study of the chemical mechanisms involved in flash pyrolysis. It follows that very often the results which are valid for a given experiment are of no use for other situations and that to derive scaling up relationships make no sense. In the same time, the kinetic constants are often derived from temperature values (obtained with thermocouples) which are not representative of the true reacting sample temperature [2–5]. A great number of chemical pathways aimed at representing the primary processes have been published. Most of them consider competitive steps giving rise to char, tar, gases [6 – 8] followed by subsequent consecutive and/or interphase reactions [9]. It has been also suggested to interpret the results of TGA measurements on the basis of two possible activation energies according to the heating rate employed in the experiments [10]. However, according to the authors, the competitive reactions directly concern the starting solid macrobiopolymer, or an ‘active component’ sometimes called ‘active cellulose’ (AC) for cellulose, that would be intermediately produced without mass loss [11]. The model involving the formation of AC is the so called Broido – Shafizadeh (BS) model [12] shown in Fig. 1. However, in a recent review, Antal and Varhegyi [9] suggest that a single step giving rise directly to vapours could explain the experiments made in TGA, the primary step giving rise to AC being superfluous between 523 and 643 K. It has been also noticed by Narayan et al. [2] that the ‘active’ liquid species observed by several authors, for example in ablative pyrolysis [13–17], could be attributed to lignin and not to cellulose pyrolysis. In a recent paper [18], it has been also pointed out that the description of the steps giving rise to AC did not appear necessary for the prediction of the global degradation kinetics of cellulose. As a conclusion, evident controversies appear in the literature as for the possible existence and importance of AC during the primary steps of cellulose and biomass pyrolysis. This is of primary importance from a fundamental point of view (better understanding of the depolymerization mechanisms), but also for process reactors where the interactions
Fig. 1. Cellulose decomposition pathway given in reference [12] and used in the modelling.
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between the particles of the feedstock could be influenced by the presence of a liquid film. It is hence attractive to find suitable experimental devices for the observation and study of this possible AC product that remains currently unknown as it was very recently noticed by Miller and Bellan [19]. The TGA methods widely used for studying biomass pyrolysis do not seem appropriate for easily observing AC, for three main reasons. The heating rates usually imposed on the sample, ranging roughly between 10 − 3 and 10 K s − 1, are probably too low. It is basically impossible to detect a non rate limiting reaction occurring without weight loss. The conditions of mass transfer efficiencies are very poor in such a way that if primary (liquid) products are formed they can not be removed and hence observed, before their local transformation into gaseous compounds resulting from secondary reactions. The laboratory experiments based on the use of a spinning hot disk [13] on which are pressed rods of biomass are much more appropriate for studying the behaviour of biomass in fast pyrolysis conditions (ablative pyrolysis). The heat transfer efficiencies are very high with corresponding mass flow rates of wood pyrolysed of up to 3000 times higher than in TGA [11] and heating rates reaching 105 K s − 1. The mass transfer efficiencies are also very high: the friction between the spinning disk and the surface of the reacting feedstock very rapidly eliminates the primary products thus preventing further local decompositions. However, the available heat flux densities are not accurately known, the reacting interphase cannot be observed and the liquid primary products stay on the hot disk where they decompose rapidly before they can be quenched and recovered for analysis. It is possible to have similar heat flux densities to those in ablative pyrolysis by using concentrated radiation. This can be practically achieved in solar concentrators (solar furnaces) or by using discharge lamps associated with concentrating mirrors (image furnaces). The available heat flux densities are measurable and can exceed 107 W m − 2 inside the focal zone of the furnace. Concentrated radiation has been already used several years ago [20–29] and continues to be used or suggested [8,30,31] to perform biomass thermochemical conversion. However, the fundamental difference with the spinning disk experiments is that, as in TGA, mass transfer efficiencies are very poor. The consequence is that, if primary products are locally formed, they can undergo further degradations before their elimination from the reacting zone. These secondary reactions can be cracking to produce vapours going into the gas phase but also char remaining on the sample in the case of too long exposures or of too small heat flux densities. This is probably one of the reasons why several of these previous experiments were failures. For example, unexpected amounts of char were often obtained while one would expect that because of the cold environment surrounding the focal zone, the great majority of the primary products formed would have been immediately quenched in the gas phase. It is possible to take advantage of these drawbacks by operating under controlled short periods of irradiation of known available heat flux densities, and by observing the surface of the sample which is rapidly quenched as soon as the illumination is stopped. In addition, we can imagine that the products (other than char) remaining on the sample after a short exposure to the radiation are of a much more primary
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Fig. 2. Scheme of the experimental set-up (sizes in 10 − 3 m)
nature than those contained in the usual bio oils resulting from vaporization and cracking processes at the level of the sample and in the gas phase before their final recondensation and possible repolymerisation at the exit of the reactor. The idea of studying flash pyrolysis of cellulose by using flashes of light is not new [20,21]. However, as will be seen later, the duration of the flashes must be very well controlled inside narrow values and for known available fluxes. However, cellulose is a partly reflecting and not opaque material and thus only a fraction of the incident flux is absorbed. Such a phenomenon, which has been often presumed by the authors, has never been quantified. It is probably one of the reasons for the apparent low reactivity of cellulose under concentrated radiation. It could also explain some of the failures observed in the literature. The first purpose of this paper is to show that it is possible to perform the flash pyrolysis of cellulose samples exposed to controlled short flashes of light and to report a few properties of the primary products formed. The second purpose is to compare these results with those derived from a very simple mathematical model taking into account the kinetic pathway reported in Ref. [12] and some of the estimated optical properties of cellulose.
2. Experimental section
2.1. Source of concentrated radiation The experiments were made in an image furnace, the schematic of which is shown in Fig. 2. The light source is an air cooled 5000 W xenon high pressure arc lamp. It is situated at the first focus F1 of a wrap round elliptical mirror M1. The second focus F%1 of M1 is adjusted at the same location as the focus F%2 of a second elliptical mirror M2 in such a way that the image of the arc is formed at its first focus F2. The available thermal power at the focus F2 has been estimated by measuring the rate of temperature increase of a known volume of water darkened with Indian ink
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and contained in a pyrex spherical vessel placed at F2. In these conditions, the measured available power is close to 400 W. However, during flash experiments with cellulose, the power is only 350 W because of a screening effect due to the experimental device supporting the cellulose sample. The cross section of the concentrated beams at the focus F2 determines a circular surface with an estimated diameter between 5 and 6 × 10 − 3 m leading to mean available heat flux densities between 1.8 and 1.2×107 W m − 2. These values are of the same order of magnitude as those achieved in the most powerful solar furnaces and also in ablative pyrolysis performed with a spinning hot disk [13]. The available power at F2 can be easily changed and calibrated by setting a diaphragm at different locations between F%2 and F2. However the diameter of the focus slightly depends on the resulting available power. It can be estimated between 4 and 5× 10 − 3 m for 175 W and between 3.5 and 4× 10 − 3 m for 117 W, leading to mean heat flux densities between 1.4 and 0.9× 107 W m − 2 (for 175 W) and between 1.2 and 0.93× 107 W m − 2 (for 117 W). This diaphragm also supports a pendulum inside which is made an adjustable window which has the shape of an arc of a circle centred on the axis of rotation of the pendulum. The window and the hole in the diaphragm are at the same distance from the axis of rotation and face each other when the pendulum is at rest. At the beginning of the test, the pendulum is lifted in such a way that the radiation is occulted by a side out of the window. It is then dropped from a known position. The light crosses the hole for as long as it faces the window. The duration of the flash is determined by the initial position of the pendulum and the length of the window. A photoelectric cell facing the mirror M2 produces two signals corresponding, respectively, to the beginning and the end of the crossing of the light through the window. The time of the flash is then computed from the time elapsed between these two signals. It is possible to make flashes as short as 0.01 s.
2.2. Preparation of the sample The experiments are made with samples prepared from cellulose powder (Whatman microgranular CC31). Fig. 3 shows a representative microscopic picture of the particles. An aqueous suspension of the powder is regularly spread on a flat thin glass surface and then dried. The thickness of the layers used in the experiments reported in this paper was roughly measured with a caliper gauge having an electronic digital display (accuracy of 10 mm). From 10 measurements made at different locations on the sample (after drying) it is possible to estimate the thickness as 450950 mm.
2.3. Properties of the sample A given quantity of the same cellulose aqueous suspension was introduced into a test tube. After drying, the volume and weight were measured leading to a mass density close to 1100 kg m − 3.
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The heat capacity was measured in a SETARAM differential calorimeter between 304 and 466 K. The values of Cp show a regular increase with temperature according to the equation: Cp =970 + 5.13(T −273)
(1)
Such behaviour, which is well known for polymers, does not seem to have been previously reported for cellulose. The constant values reported in the literature vary to a very large extent (1600 – 2800 J kg − 1 K − 1) but are, however, compatible with those given by equation Eq. (1). Cellulose is not a perfectly absorbing material: when the radiation reaches its surface, a given fraction ar is reflected. It is also a semi transparent medium and hence, the remaining fraction (1−ar) is progressively absorbed, as it crosses the solid, according to a given attenuation law. If the sample is thin enough, only a fraction aa(1 − ar) of the incident radiation is absorbed. Quantitative values of ar and aa and of their variations with the thickness of the sample do not seem to exist in the literature. It is possible to experimentally derive estimated orders of magnitude for these two parameters. A similar experiment as that made for determining the available power at F2 can be made after deposition of a layer of cellulose on the spherical vessel at the side of the incoming radiation from the mirror M2. The experiment has been made with a mean thickness of cellulose near 360 mm (estimated from the mass and area of cellulose deposited on the vessel and intercepting the incoming radiation). From the time (30 s) after which the reaction of pyrolysis begins (observation of the first yellowing of the surface, supposed to occur at 739 K [14]), the mass of cellulose exposed to the radiation (0.932×10 − 3 kg) and Eq. (1) giving Cp, it is possible to calculate the power Pa absorbed by the irradiated cellulose from the beginning of the reaction:
Fig. 3. Microscopic photograph of the cellulose powder used in the experiments (scale: one square represents 80 × 10 − 6 m).
&
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Pa = 0.932 ×10 − 3
739
293
970 +5.13(T − 273) dT 30
19
(2)
In these calibrating experimental conditions, Pa is then calculated to be close to 30 W. At the same time, a fraction of the radiation crosses the layer of cellulose and heats the black water contained in the vessel. The power absorbed by the water is calculated to be approximately 70 W. It can be deduced that among the 400 W arriving on the cellulose surface, about 30 W are absorbed while 70 W leave the sample without absorption and hence 300 W are reflected. The fraction ar is then close to 0.75. Assuming an exponential law for representing the absorption in a semi transparent solid [32] it is possible from this experiment to estimate the variations of the fraction aa as a function of the thickness L of the sample: aa = 1 − exp( − 1000L)
(3)
Finally, assuming a constant value of the reflectivity ar (0.75), the fraction a absorbed at the depth L with respect to the total incident available flux is: a = 0.25[1 −exp( − 1000L)]
(4)
Applied to the case of the cellulose samples used in the flash experiments (L0 =4509 50 mm) one can estimate: 0.0825 a5 0.098. This result shows that less than 10% of the available power is effectively absorbed. Of course, for much smaller thicknesses the values would be much lower still: 2.4, 1.2 and 0.25% for, respectively, 100, 50 and 10 mm samples. Remember that these small sizes are of the same order of magnitude as the mean dimensions of the elementary particles of cellulose used by many authors. This result explains the difficulty in performing flash pyrolysis of pure cellulose particles, even under concentrated radiation.
2.4. Experiments with flashes of light Once the layer of cellulose has been deposited and dried on the glass surface, the sample is settled at the focus F2 and then submitted to a flash of a given duration (generally between approximately 0.1 and 2 s). Several flashes of different periods are made at different locations on the surface, which is then simply observed after its removal from the focal zone. Two times for the flashes are defined. The time tb at which the reaction begins is supposed to correspond to the observation of the first yellowing of the cellulose surface. The time te at which the reaction is finished is supposed to correspond to the moment where the majority of the products have left the area of irradiation.
3. Results of the experiments
3.1. Simple obser6ations For the maximum available power of 350 W, no apparent reaction is visible for periods of flashes less than about 0.21 s. For increasing values of the duration of
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Fig. 4. Microscopic photograph of a sample of cellulose after a short exposure to a high concentrated radiant flux. It shows the agglomeration of particles after they have passed through a liquid phase (same scale as Fig. 3).
the flashes, the microscopic observations reveal the following behaviour. The shape of the cellulose particles change with the disappearance of the fibrillar structure and blunting of the contours exactly as during the phase change from a solid to a liquid. Over a short time, the particles that have passed through this liquid form, keep their individuality (Fig. 3). As the duration of the flashes increases, agglomerations occur between the melted particles, with the formation of large networks (Fig. 4), before the final disappearance (vaporization) of the majority of the products. At the same time, a very low fraction of the products darken with the final formation of char particles remaining on the glass support. Note that the same observations can be made in experiments performed either under argon or air, and either at normal or reduced pressure. Table 1 gives the ranges of the measured values of tb and te for several experiments and for available powers of 350, 175 and 117 W at F2. These results reveal that under our experimental conditions, the life time of these molten species is lower than 1 s. These simple observations clearly show that cellulose subjected to high flux densities passes through a liquid phase that is rapidly transformed into gaseous products and/or small fractions of char according to the period of irradiation. Table 1 Experimental values of tb and te for the reaction for three different available fluxes P
tb (s) te (s)
P= 350 W
P= 175 W
P= 117 W
0.20–0.22 0.90–1.10
0.28–0.33 1.00–1.19
0.44–0.46 1.32–1.50
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These results are in quite good agreement with the Nordin et al. experiments [33] performed with a laser beam and clearly showing the formation of spherical molten nodules. This gives a new clear proof that, as in wood pyrolysis it is well known that lignin gives rise to a liquid phase, it is also the case for cellulose. It should be noted that the same observations can be made after brief contacts of cellulose powder with a red hot surface [11], a logical observation, because of the similar high heat flux densities also available in ablative pyrolysis.
3.2. First estimation of the liquid properties The observations are made at room temperature after the very fast self cooling of the sample that occurs at the end of the flash. Under these conditions, the products are solid although they are liquid at the reaction temperature. They are also water soluble showing that they are not pure cellulose, but other products resulting from the thermal degradation of cellulose. It is, hence, incorrect to speak of molten cellulose. These products are, thus, of an a priori different nature to the bio oils obtained in fast pyrolysis processes and also to the syrups derived from previous solar flash pyrolysis experiments. In both these cases the products are liquid at room temperature. They are obtained after recondensation and/or repolymerization, at the exit of the reactor, of the vapours resulting from probable partial cracking of the primary products. In the present case, the products were recovered immediately before vaporization and are more likely of a primary nature. It is possible to carry out GC/MS analysis of the recovered products and also to perform HPLC analysis on these aqueous solutions. First tests revealed that they contain relatively few species and in any case, much less than is usual in bio oils. They are mainly molecules resulting from the splitting of the cellulose polymer with low degrees of polymerization.
4. Theoretical section In a recent paper Le´de´ et al. [11] has gathered several experimental and theoretical data showing that cellulose has a high affinity to primarily decompose into an intermediate product which is liquid at the reaction temperature. They also showed that such an intermediate had a great ability to rapidly vaporize (after more or less cracking) or repolymerize to give char. The observations reported in the previous section of this paper confirm the existence of such an unstable product, showing clearly that direct sublimation or vaporization from the cellulose solid phase to the gas phase is not occurring under our conditions of high flux. Our clear experimental observations show that the mathematical modelling of the reaction must a priori rely on a kinetic pathway taking into account the formation of intermediate species rather than on a simple solid vapour type reaction. Such a more complete pathway is similar to the BS kinetic model even if Ref. [12] reports lower temperature experiments where a phase change had few chances to occur.
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The theoretical evolution of the different species and particularly of AC has been studied by Diebold [34]. However his mathematical approach does not take into account the problem of heat transfer (outside and inside the sample) and assumes a linear increase of the sample temperature. This is a first approximation, as it has been shown [2 – 5] that the true temperature of a solid particle undergoing endothermic decomposition under the influence of an external heat flux depends strongly on close couplings between heat transfer processes and chemical steps. One of the consequences of such general behaviour is that after the pure heating period and during the reaction period, one can expect a stabilization of the sample temperature, leading to a phenomenon similar to a phase change. This explains the fusion like behaviour observed for biomass, for example [2,15]. This has been modelled in the cases where heat is provided to the solid sample by an external source at a constant [3,4] or increasing temperature [2,5], the exchange being ensured by gas or solid convection (Fourier conditions). It is interesting to consider another practical situation where heat is provided by a given constant external heat flux, as is the case in the present study where the cellulose is submitted to a given radiant flux.
4.1. Basic assumptions of the model We suppose, in a first approximation that the mass densities, heat capacities and optical properties (reflectivity, absorptivity and emissivity) are the same for cellulose, active cellulose and char. The Eqs. (1) and (4) are used to calculate Cp and a, before (pure heating) and during the reaction. Note that the model relies on Eq. (4) and hence takes into account the fact that a decreases when L decreases during the reaction. The kinetic pathway is that of the BS model (Fig. 1) All the elementary chemical steps are irreversible and of first order with chemical rates r1, r2 and r3 defined, respectively, as the mass of cellulose decomposed, of intermediate species transformed into vapour, and gases+ char, per unit time and per unit particle volume. They obey Arrhenius type laws with constant values of the enthalpies DH1, DH2 and DH3. r1 =rA1 exp( − E1/RT)
(5)
r2 =rA2 exp( − E2/RT)
(6)
r3 =rA3 exp( − E3/RT)
(7)
The mass fractions of char and gases (step 3, Fig. 1) are, respectively, 0.35 and 0.65 [12]. The gases and vapours escape freely towards the outside of the sample without any diffusional resistance. The equations are written for an infinite surface slab. As the reaction proceeds, the intermediate species as well as char, are supposed to stay inside the sample where they are homogeneously dispersed without creation or disappearance of any porosity. The mass density remains constant and hence the thickness of the slab decreases as the reaction proceeds.
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The slab initially at room temperature (T0 = 293 K) is suddenly exposed to a given heat flux density q0. Only the fraction a of q0 is absorbed (for heating and chemical reaction). The fraction aq0 of the flux is supposed, in a first approximation, to be immediately and entirely absorbed at the front surface of the sample and then distributed inside the internal parts in such a way that no temperature gradient exists. The radial conduction is neglected (one dimensional problem, perpendicular to the slab surface). The only losses of energy from the sample are supposed to occur by radiation from the reacting mixture (with an emissivity o =0.92 [32]). The losses by convection by the surrounding gas and by conduction by the supporting glass material are neglected.
4.2. Basic equations They rely on mass and energy balance equations.
4.2.1. Mass balances Let us define, at time t, the reduced masses Xi of each of the components (i= C: cellulose; AC: active cellulose [12]; Ch: char; G = gas; V: vapours) with respect to the extent of each of the three reactions X1, X2 and X3 [35] as: XC =mC/m0 =1 − X1
(8)
XAC =mAC/m0 =X1 −X2 −X3
(9)
XCh =mCh/m0 =0.35X3
(10)
XG =mG/m0 =0.65X3
(11)
XV =mV/m0 =X2
(12)
m0 being the initial mass of cellulose defined by the unit surface. The mass balances are: dX1/dt = A1 exp( − E1/RT)(1 − X1)
(13)
dX2/dt = A2 exp( − E2/RT)(X1 −X2 − X3)
(14)
dX3/dt = A3 exp( − E3/RT)(X1 −X2 − X3)
(15)
with the initial conditions: t=0: X1 =X2 =X3 =0
4.2.2. Energy balance It can be written by the unit surface of the slab sample:
(16)
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Table 2 Selected values of the kinetic parameters A and E [11,12] and of the enthalpies DH [4,36,37] of the three elementary steps 1, 2 and 3 (model represented in Fig. 1)
A (s−1) E (J mol−1) DH (J kg−1)
Step 1
Step 2
Step 3
2.8×1019 242 000 40 000
3.2×1014 198 000 418 000
1.3×1010 151 000 418 000
aq0 =os(T 4 −T 40) + rL0
dX1 dX2 dX3 dT DH1 + rL0 DH2 + rL0 DH3 + rLCp dt dt dt dt
(17) with: t= 0: T = T0 =293 K
(18)
Or, after combination with the three mass balances Eqs. (13)–(15):
dT 1 aq0 −os(T 4 − T 40) = − k1(1−X1)DH1 dt (1 −X2 −0.65X3)Cp rL0
n
−(k2DH2 +k3DH3)(X1 −X2 − X3)
(19)
The integration is made by using the method of Runge–Kutta–Merson (5th order with fixed steps of 10 − 4 s). The calculations lead to the variations of T and Xi as a function of time t for several possible values of L0 and q0. The calculations also give the theoretical values of tb (corresponding arbitrarily to XC = 0.99) and of te (corresponding arbitrarily to XAC = 0.01 and XC B 0.01). The kinetic parameters are those given by the BS model [11,12]. The choice of the values of DH1, DH2 and DH3 is difficult. They are taken from the references [4,36,37]. These chemical parameters are given in Table 2.
5. Results of the theoretical model Fig. 5 shows an example of the results obtained with q0 = 1.8× 107 W m − 2 and L0 = 450×10 − 6 m. They show that the fraction of intermediate species passes through a maximum close to 93%. This is a consequence of the fact that under the conditions used in the modelling (the relatively high value of the sample temperature), reaction 1 is much faster than reactions 2 and 3. This means that it might be possible to transform almost all the cellulose into very short life time species (about 0.5 s) before its further decomposition. This result explains the difficulties in experimentally isolating these liquid species. Fig. 5 also shows that the fractions of vapours are much higher than those of gas and char (reaction 2 is much faster than reaction 3). These results are in agreement with our experimental observations (almost no char on the glass at the end of the reaction, for t\ te) and also with the
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conclusions of Diebold [34]. In a first approximation, it is hence possible to say that the pyrolysis occurs according to two reactions in series: formation of short life time liquid species that decompose later into vapours, with only minor fractions of char. Fig. 5 shows that the sample temperature T increases regularly until reaction 2 occurs significantly, followed by a very strong stabilization. This effect corresponding to a liquid vapour type reaction (reaction 3 is negligible) is similar to the observations previously reported for solid fluid type reactions [2–5]. The authors mentioned in that case a fusion-like phenomenon, while now it is possible to speak of a boiling-like phenomenon. In the present case, the stabilization of temperature occurs between 700 and 800 K, quite similar to the fusion temperature of cellulose [14,15,17]. These results also show that the modelling of the reaction must rely on both mass and heat balances, and that theoretically expecting a constant heating rate of the sample is unrealistic. However, from Fig. 5, it is possible to estimate heating rates of about 1.6×103 K s − 1 at the beginning of the reactions, which is similar to the high values postulated by Diebold [34] who also claimed the possibility to reach very high fractions of liquid species and low fractions of char. In Fig. 6 are reproduced the theoretical variations of tb and te as a function of q0 in the case where a is given by Eq. (4). The results calculated in the assumption of a = 1 at any time are also reported (a simplified limit case where all the radiation would be absorbed by the sample). It is clear that very large mistakes can be made
Fig. 5. Theoretical variations of the mass fractions Xi of the species formed in the flash pyrolysis of cellulose, as a function of the time t. The evolution of the sample temperature T is also represented. The calculations have been made with a heat flux density of 1.8 × 107 W m − 2, an initial size of the cellulose sample of 450 × 10 − 6 m and an absorptivity given by Eq. (4).
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Fig. 6. Theoretical variations of tb and te as a function of the available heat flux density q0 (W m − 2) and for an initial size of the sample of 450× 10 − 6 m. The results rely on values of a calculated by Eq. (4) and also on the simple assumption of a= 1. The second curve from the top is obtained with values of enthalpies DH2 and DH3 given in Ref. [19]. The experimental results presented in Table 1 are also reported. The four points given for each power (P =350, 175 and 117 W) result from the uncertainties in the experimental estimations of tb and te, and of the focus diameter (and hence of q0).
if the reflectivity and absorptivity of the cellulose are not taken into account (factors of more than 10 in the times of reactions). The experimental results in Table 1 are also reported. They take into account two types of inaccuracies. The first one is connected with the measurement of tb and te. The second one results from the poor knowledge of the diameters of the focus (and hence of the mean heat flux densities) for each value of the available power. In any case, it is clear that the experiments fit much better with the theoretical values, taking into account the reflecting and absorbing properties of cellulose (Eq. (4)). Supposing a= 1 would have lead to important discrepancies. Fig. 6 also shows a curve corresponding to other values of DH2 and DH3 as given by the recent paper of Miller and Bellan (Ref. [19]) (DH2 =255000 J kg − 1 ; DH3 = − 20000 J kg − 1). It is clear that in spite of the great differences between the values chosen in Table 2, the resulting theoretical values of te (there is of course no influence on tb) are not much changed. Moreover, because of the inaccuracies of the experiments, it is difficult to conclude which values of the enthalpies fit better to the measurements. Fig. 6 also shows that in a logarithmic scale, the variations of tb and te are roughly linear to q0 mainly for a =1, and that the mean life time of the intermediate species which is close to te −tb logically increases as the available flux q0 decreases. It is, however, very short, from a fraction to a few seconds.
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In Fig. 7 the variations of tb and te as a function of the initial thickness L0 of the sample for q0 =1.8 ×107 W m − 2 are reported. These times of reaction appear to be practically independent of L0. These interesting results can be explained by a compensation effect between two opposite phenomena. When L0 increases, a increases (according to Eq. (4) which is almost linear for low values of L0) leading to higher fractions of the flux absorbed, but at the same time the mass of the sample increases leading to a higher heat demand for heating and the reaction. Such behaviour shows that the values of the reaction times are roughly independent of the accuracy of the measurement of L0 and are for example, the same for extreme values of the thickness of the samples used in our experiments (400 and 500 mm). Of course, for a 100% opaque material, the times of reactions would increase with the increase in L0. All these conclusions are only valid for the assumption of no temperature gradients and hence for relatively thin samples.
6. Discussion and conclusions Very simple experiments have shown that it is possible to perform the flash pyrolysis of small samples of pure cellulose by exposures over short periods (0.2–2 s) to concentrated radiation. According to the time of the flash the products evolved can be obtained in the form of a condensed phase remaining on the support, or of species escaping in the gas phase. In any case, the fraction of char appears to be very small.
Fig. 7. Theoretical variations of tb and te as a function of the initial size L0 of the cellulose sample. The calculations have been made with a heat flux density of 1.8 × 107 W m − 2, the absorptivity a being given by Eq. (4).
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The condensed phase observed with a microscope after fast cooling of the sample is liquid at the reaction temperature and solid at room temperature. It is water soluble. The first HPLC measurements show that it contains relatively few species resulting from the depolymerization of cellulose. It is not molten cellulose. It is very stable at room temperature. These products are probably of a different nature than the liquids usually obtained in flash pyrolysis processes. In these usual cases, they are evolved after the recondensation of vapours, while in the present case, they are observed before the initial vaporization step. These results show that, as for fast wood pyrolysis where lignin is well known to give rise to a liquid phase, this is also the case for cellulose. These products could be compared to the active cellulose postulated in the Broido–Shafizadeh model even if these authors had few chances to observe a liquid phase in their low temperature experiments. However, this research does not address if the liquid species observed in ablative pyrolysis of wood result from the pyrolysis of lignin, cellulose or both. In spite of much previous experimental and theoretical evidence, the assumption of the existence of a liquid intermediate product has been often questioned and even recently rejected [38]. The results reported in the present paper bring clear new and definitive proof of its existence during the fast pyrolysis of cellulose. It is, thus, impossible to deny the chemical and physical roles of a thin liquid layer surrounding the particles of biomass during the reaction of fast pyrolysis. Cellulose is a strongly reflecting material. It is also semi-transparent in such a way that only a small fraction of the incoming radiation is used for heating and for the reaction. Such an important phenomena must be necessarily taken into account in any predictive calculations. The first modelling of the experiments has been proposed. It relies on the kinetic model reported in Ref. [12] and on very simple assumptions writing of the heat and mass balances. The derived times of the beginning and end of the reaction compare very favourably with the experimental observations. Both results show that for high heat flux densities, the reaction begins after approximately 0.2 s, while almost no more product is present on the sample after approximately 1.5 s (almost no char formation). The life time of the active intermediate species is in the order of 1 s or lower. The model predicts that they can reach concentrations higher than 90%. However, high accuracies in the measurements of the period of the flashes are necessary in order to be able to effectively observe such high concentrations. The calculations confirm that the reaction occurs at about 750 K. In any case, the definitions of tb and te used in this paper are only first approximations aimed at showing the orders of magnitudes. They are too vague to quantitatively test the kinetic model with high accuracy. For that purpose, further research must include experimental studies as a function of time from the beginning to the end of the reaction. The experiments must also include the determination of mass balances in order to confirm the formation of high yields of liquids. The values of the absorptivity and of the reflectivity of cellulose have been estimated. Future studies must necessarily include much more accurate measurements of these values and of their variations with the size of the sample and the
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progress of the reaction. The optical properties reported in this paper have been measured on pure cellulose at room temperature. There is little chance of their being still valid after the reaction begins: the reflectivity probably becomes lower and absorptivity higher, leading to higher values of a. Future modelling must include these phenomena but also the fact that the fraction aq0 of the available flux is, in reality, progressively absorbed inside the sample. The chemical analysis of the short life time species must be improved. Defining their true nature would bring new data for a better knowledge of the mechanisms of cellulose depolymerization. It would also be useful for understanding the vapour formations that precede the recondensations giving rise to the bio oils in the usual pyrolysis processes. Finally, the presence of a high temperature liquid film on the surface of the reacting biomass can influence the hydrodynamic behaviour of the process reactors. On one hand it can produce favourable lubrication effects between the feedstock and the walls of the reactor, but, on the other hand, it can lead to agglomerations of the particles (mainly during cooling), with subsequent plugging in the case of concentrating beds or in the underflow of a separator. It could also be one of the possible origin of the formation of aerosols.
7. Nomenclature A1, A2, A3 Cp E1, E2, E3 i L, L0 m, m0 mi P Pa q0 r1, r2, r3 R t, tb, te T, T0 Xi X1, X2, X3 a aa ar
Arrhenius factors for reactions 1, 2 and 3 (s−1) heat capacity (J kg−1 K−1) activation energies for the reactions 1, 2 and 3 (J mol−1) C for cellulose; AC for active cellulose; V for vapours; G for gas; Ch for char thickness of the sample at time t and t= 0 (m) mass of the sample per unit surface at time t and t= 0 (kg m−2) mass per unit surface of each of the components at t (kg m−2) available flux (W) power absorbed by the sample until the beginning of a visible reaction (W) available heat flux density (W m−2) rates of the reactions 1, 2 and 3 (kg m−3 s−1) gas constant (8.314 J mol−1 K−1) time; at the beginning of the reaction of cellulose (XC = O.99); when AC has disappeared (XAC = 0.01 and XCB0.01) (s) sample and room temperatures (K) reduced masses extents of the reactions 1, 2 and 3 total fraction of the available flux absorbed at L absorbed fraction of the non reflected flux reflected fraction of the available flux
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DH1, DH2, DH3 enthalpies of the reactions 1, 2 and 3 (J kg−1) o emissivity of the sample r mass density (kg m−3) s Stefan Boltzmann constant (5.67 10−8 W m−2 K−4)
Acknowledgements The authors want to thank ADEME (Agence de l’Environnement et de la Maıˆtrise de l’Energie) who has funded this research under the framework of AGRICE (AGRIculture pour la Chimie et l’Energie) Contract No. 96 01 048. They are also grateful to M. Bouroukba for his valuable help in the measurement of the heat capacity of cellulose with the SETARAM differential calorimeter of the Laboratoire de Thermodynamique des Solides (LTS-ENSIC, Nancy, France).
References [1] J. Le´de´, in: A.V. Bridgwater, D.G.B. Boocock (Eds.), Developments in Thermochemical Biomass Conversion, Blackie, Glasgow, 1997, pp. 104 – 116. [2] R. Narayan, M.J. Antal, Ind. Eng. Chem. Res. 35 (1996) 1711. [3] J. Le´de´, J. Villermaux, Can. J. Chem. Eng. 71 (1993) 209. [4] J. Le´de´, Biomass Bioenergy 7 (1994) 49. [5] J. Le´de´, J. Ther. Anal. 46 (1996) 67. [6] B.M. Wagenaar, W. Prins, W.P.M. Van Swaaij, Fuel Process. Technol. 36 (1993) 291. [7] C. Di Blasi, Chem. Eng. Sci. 51 (1996) 1121. [8] M. Gro¨nli, Thesis, The Norwegian University of Science and Technology, Faculty of Mechanical Engineering, Trondheim, Norway, 1996. [9] M.J. Antal, G. Varhegyi, Ind. Eng. Chem. Res. 34 (1995) 703. [10] I. Milosavljevic, E.M. Suuberg, Ind. Eng. Chem. Res. 34 (1995) 1081. [11] J. Le´de´, J.P. Diebold, G.V.C.Peacocke, J. Piskorz, in: A.V. Bridgwater, D.G.B. Boocock (Eds.), Developments in Thermochemical Biomass Conversion, Blackie, Glasgow, 1997, pp. 27 – 42. [12] A.G.W. Bradbury, Y. Sakai, F. Shafizadeh, J. Appl. Polym. Sci. 23 (1979) 3271. [13] J. Le´de´, J. Panagopoulos, H.Z. Li, J. Villermaux, Fuel 64 (1985) 1514. [14] J. Le´de´, H.Z. Li, J. Villermaux, in: J. Soltes, T.A. Milne (Eds.), Pyrolysis Oils from Biomass. Producing, Analyzing and Upgrading, ACS Symp. Ser. 376, American Chemical Society, Washington DC, 1988. [15] J. Le´de´, H.Z. Li, J. Villermaux, J. Anal. Appl. Pyrolysis 10 (1987) 291. [16] J. Le´de´, H.Z. Li, J. Villermaux, H. Martin, C. Moyne, A. Degiovanni, Int. J. Heat Mass Transf. 29 (1986) 1407. [17] G. Peacocke, Thesis, Aston University, Birmingham, UK, 1994. [18] M. Lanzetta, C. Di Blasi, F. Buonanno, Ind. Eng. Chem. Res. 36 (1997) 542. [19] R.S. Miller, J. Bellan, Combust. Sci. Technol. 126 (1997) 97. [20] K.A. Lincoln, in: J.P. Diebold (Ed.), Proc. of the Specialist’s Workshop on Fast Pyrolysis of Biomass, Solar Energy Research Institute (SERI/CP-622-1096), Copper Mountain, CO, 19 – 22 October 1980. [21] M.W. Hopkins, M.J. Antal, J. Appl. Polym. Sci. 29 (1984) 2163. [22] A. Tabatabaie-Raissi, W.S.L. Mok, M.J. Antal, Ind. Eng. Chem. Res. 28 (1989) 856. [23] L. Hofmann, M.J. Antal, Sol. Energy 33 (1984) 427. [24] M.W. Hopkins, C. Dijenga, M.J. Antal, Sol. Energy 32 (1984) 547.
O. Boutin et al. / J. Anal. Appl. Pyrolysis 47 (1998) 13–31
31
[25] M.J. Antal, L. Hofmann, J.R. Moreira, Sol. Energy 30 (1983) 299. [26] M.J. Antal, ACS Symp. on Thermal Conversion of Solid Wastes and Biomass, Washington DC, September 1979. [27] J. Le´de´, J. Villermaux, C. Roye`re, B. Blouri, G. Flamant, Entropie 10 (1983) 57. [28] J. Le´de´, P. Berthelot, J. Villermaux, A. Rolin, H. Franc¸ois, X. Deglise, Rev. Phys. Appl. 15 (1980) 545. [29] R.W. Taylor, R. Berjoan, J.P. Coutures, Report UCRL-53063, Lawrence Livermore Laboratory, CA, 1980. [30] J. Le´de´, F. Pharabod, Entropie 204 (1997) 47. [31] J.P. Murray, E.A. Fletcher, Energy 19 (1994) 1083. [32] R. Siegel, J.R. Howell, Thermal Radiation Heat Transfer, 3rd edn., Hemisphere Publishing Corporation, Washington, 1992. [33] S.B. Nordin, J.O. Nyren, E.L. Back, Text. Res. (1974) 152. [34] J.P. Diebold, Biomass Bioenergy 7 (1994) 75. [35] J. Villermaux, Ge´nie de la Re´action Chimique. Conception et fonctionnement des Re´acteurs, Lavoisier, Technique et Documentation, Paris, 1993. [36] C. Di Blasi, Ind. Eng. Chem. Res. 35 (1996) 37. [37] R.S. Miller, J. Bellan, Combust. Sci. Technol. 119 (1996) 331 – 373. [38] R.S. Miller, J. Bellan, Energy Fuels 12 (1998) 25 – 40.
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