An Experimental Investigation of Heat-Transfer Limitations in the Flash

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Ind. Eng. Chem. Res. 1997, 36, 542-552

An Experimental Investigation of Heat-Transfer Limitations in the Flash Pyrolysis of Cellulose Mario Lanzetta, Colomba Di Blasi,* and Federico Buonanno Dipartimento di Ingegneria Chimica, Universita´ degli Studi Napoli Federico II, P.le V. Tecchio, 80125 Napoli, Italy

A new experimental system is presented to investigate the fast pyrolysis of solid fuels, in the absence of heat- and mass-transfer limitations. It consists of an electrically heated furnace, where a thin layer of powdered solid is exposed, on both sides, to radiative heating. A PID temperature controller is programmed for two different working conditions: the usual constant furnace temperature (A) and a constant sample temperature (B). Cellulose pyrolysis is investigated in the temperature range 523-699 K. It is shown that significant heat-transfer limitations cannot be avoided with the modality A, unless very slow heating rates, as in the classical TGA systems, are applied. In the modality B (global heating rates 19-56 K/s), the independence of the char yields from the sample thickness, for values of this below a critical value, indicates negligible spatial temperature gradients and activity of intraparticle secondary reactions of primary vapors. External heat-transfer limitations, due mainly to endothermic reaction energetics, are also avoided through proper variation in the intensity of the external radiative heat flux. Consequently, conversion occurs under exactly determined temperature conditions. A cold helium flow carries away from the reaction environment volatile products so that the activity of extra-bed secondary reactions is hindered as well. Cellulose weight loss and temperature curves are applied to evaluate the global degradation kinetics and to study the influences of heat- and mass-transfer limitations. Introduction Pyrolysis of lignocellulosic materials is important in the fields of energy recovery, through thermochemical conversion, and fire safety science (Di Blasi (1993)). In general, for convective/radiative heating (Di Blasi (1994, 1996a,b)), pyrolysis can occur in a chemically controlled regime (negligible intraparticle spatial gradients of temperature and no temperature differences between particle and reacting environment) or in a heat-transfercontrolled regime. In the heat-transfer-controlled regime, two further cases (Di Blasi (1996c)) can be observed: (1) internal heat-transfer control (thermally thick regime) with very large spatial gradients along the particle, and (2) external heat-transfer control (thermally thin regime) with no spatial gradients along the particle, though the temperature may continue to change with time, depending on the heat-transfer rate from reactor to particle. Heat-transfer conditions determine the selectivity of primary solid degradation reactions and thus the characteristics of pyrolysis. It is known (Scott et al. (1988)) that slow solid heating rates lead to comparable amounts of char, gas, and liquid products (slow-conventional pyrolysis), whereas solid char formation is almost completely hindered under very fast heating rates (flash pyrolysis) and reduced volatile residence times (Antal et al. (1990)). It should also be mentioned that a special case of flash pyrolysis is achieved in the ablative regime (Lede et al. (1985), Di Blasi (1996d)), where heat transfer from reactor to particle occurs by direct contact. The knowledge of the chemical kinetics of pyrolytic solid degradation is an important step in the optimal design and operation of chemical reactors. However, kinetic investigations, mainly for the fast heating rates of flash pyrolysis, suffer from large limitations in heat * Corresponding author. Telephone: 39-81-7682232. Fax: 39-81-2391800. S0888-5885(96)00551-9 CCC: $14.00

and mass transfer and rather narrow ranges of (low) temperature conditions (Antal (1982, 1985), Varhegyi et al. (1994), Antal and Varhegyi (1995)). In most cases, the sample (reaction) temperature is not known, given the significant differences with the controlled system and the lags in the measuring devices (Narayan and Antal (1996)). Intra- and extraparticle mass-transfer limitations also exert a strong influence on the extent and selectivity of secondary reactions of vapors evolved from primary solid degradation. Indeed, primary pyrolysis volatiles may undergo cracking and repolymerization in the hot charred particle and/or the reacting environment. Most of the studies on the thermal decomposition of organic polymers have been carried out by means of thermogravimetric analysis (TGA). Generally, associated with this methodology, there are several shortcomings (Agrawal and McCluskey (1983)): (1) uncertainties in temperature measurement, (2) slow heating rates (usually below 2 K/s) and (3) difficulties in product recovery, so that the kinetic analyses are based only on weight loss curves. In particular, to avoid increases in the mechanical inertia of the sample pan and to obtain accurate and sensitive weight measurements, the thermocouple is never in contact with the degrading sample. Consequently, the reaction temperature is not exactly known. Tube furnaces, which allow faster heating rates (up to 5-10 K/s), have also been proposed (Bradbury et al. (1979), Thurner and Mann (1981), Agrawal and McCluskey (1983)). In this case, products and weight loss are recorded at selected times, through experiment repetition, to draw the curves of interest. However, this procedure is much more tedious than that of TGA analyses, and again only the furnace temperature is controlled. Therefore, in some cases, the reaction temperature is significantly lower than that of the heating system, usually taken as the reaction temperature (see, for instance, Bradbury et al. (1979)). © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 543

As the rates of solid heating and degradation are made faster, the characteristic times of measuring devices become comparatively too slow, so that accurate continuous monitoring of the variables of interest turns in a rather hard task. These problems are well pointed out in the fast TGA system developed by Antal and coworkers (Tabatabaie-Raissi et al., 1989), where high radiative heat fluxes are applied to pyrolyze cellulose under conditions similar to those of solar environments. Usually, only final product yields, as functions of the final reaction temperature, are obtained from experimental systems characterized by very fast nominal heating rates (up to 10 000 K/s). Also, it should be noted that, given the very high heat-transfer rates, though the temperature of the (inert) heating system is well controlled, significant differences between this and that of the reacting samples and large intraparticle gradients are expected. These systems include screen heaters (for instance, Lewellen et al. (1976), Hajaligol et al. (1982)), shock tubes (Ozturk and Merklin (1995)), and Curiepoint pyrolyzers (Wanzl (1994)). In the first case, a stainless steel mesh screen, suspended between massive brass electrodes, is rapidly heated to the desired temperature. In the shock tube, the propagation of a planar shock, through a low-pressure gas, results in an almost step increase in temperature and pressure. The Curiepoint technique is based on the electric current produced by a conductor introduced in a high-frequency magnetic field. Finally, bench-scale, fluid-bed reactors are also used (for instance, Lipska and Parker (1966), Barooah and Long (1976)) to measure the total condensable and gas yields and reaction time on the dependence of the pyrolysis temperature and to evaluate reaction kinetics. Shortcomings are related to the difficult determination of the yields of char (mixed with the granular solid heat carrier) and, again, to significant heat and mass transfer limitations. In this study, a new experimental system is presented to investigate the pyrolysis of lignocellulosic materials under heating rates significantly faster than those used in TGA systems, while continuous, accurate monitoring of sample temperature and weight is still carried out. The most innovative aspect of the experimental apparatus consists of the control of the sample temperature, after conditions for negligible spatial gradients of temperature are established (thermally thin regime). The externally applied heat flux is used as the adjustable variable. In this way, heat-transfer limitations are eliminated. Furthermore, intra- and extraparticle secondary reaction activity is minimized by reducing the volatile residence times (thin sample size and inert flow gas). Cellulose is chosen as fuel because, notwithstanding the numerous investigations already available (see reviews by Antal (1982, 1985), Antal and Varhegyi (1995), Milosavljevic and Suuberg (1995)), a still large uncertainty is shown on conditions and kinetics of flash pyrolysis. Experimental System An experimental system has been designed and constructed to investigate solid pyrolysis under reduced limitations of heat and mass transport, so that information on the chemical kinetics (semiglobal mechanisms and data) can be obtained. The system allows for the continuous monitoring of sample and system temperature and weight and for the collection of products, lumped into three main classes: solid char, liquid tar,

Figure 1. Schematic of the experimental system: (A) general, (B) cross view of the heating chamber, (C) sample holder.

and gas. The system is a combination of the pyrolysis tube used in previous experiments (Madorsky et al. (1956), Bradbury et al. (1979), Thurner and Mann (1981), Agrawal and McCluskey (1983), Cullis et al. (1983)) and the fast TGA thermoanalyzer developed by Antal and his group (Tabatabaie-Raissi et al., 1989). Parts A-C of Figure 1 schematically represent the main characteristics of the experimental apparatus. It consists of (1) a radiant heating chamber, (2) a quartz reactor, (3) a PID temperature controller, (4) an inert gas feeding system, (5) an acquisition data set (PC and related accessories), (6) a precision balance, and (7) a collection system for condensable and noncondensable products of solid pyrolysis. The heating system is a radiant chamber, manufactured by Research Inc. The heating elements are tubular quartz infrared lamps with a tungsten wire filament that emits radiant energy in proportion to the applied voltage. Four elliptical, polished aluminum, water-cooled reflectors (Figure 1B) focus the highdensity infrared energy, emitted by lamps, onto a cylindrically shaped target area, whose diameter is 6.5 × 10-2 m. The length of the heated zone is 6.02 × 10-2 m, delimited on both sides by two inert zones of about the same length. The radiant flux is uniform over 80% of the chamber length. The maximum temperature,

544 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997

2500 K, is achieved within 3 s, resulting in rather fast reactor heating rates. To avoid interaction between the volatile pyrolysis products and the lamps, a quartz tube (i.d. 6 × 10-2 m and length 20 × 10-2 m), transparent to infrared radiation, is located inside the furnace and used as a reaction chamber. A powdered solid sample is exposed to thermal radiation by means of a 325 stainless steel mesh screen (4.2 × 10-2 m large and 5.5 × 10-2 m long; Figure 1C), whose sides are wrapped on two steel rods. Accurate temperature measurements, under different heating conditions, have been made to define the uniform temperature zone (this corresponds to an extention of about 2.6 × 10-2 m, in the direction of the reactor axis, and 2 × 10-2 m, along the other). The sample is located at the center of the radiant chamber and is radiatively heated on both the upper and bottom sides, this in contact with the sample holder (Figure 1C). Aluminum supports directly connect the sample holder to a precision balance (Mettler-Toledo AT400) with an accuracy of order 0.1 mg and a time constant of 0.2-0.4 s. The temperature of the reacting solid is measured by a thin (0.1 mm bead) chromel-alumel thermocouple in direct physical contact with the powdered sample layer (Figure 1C). As pyrolysis takes place, the thermocouple bead moves according to shrinking rate, and, depending on the final solid residual yield, at the completion of the process, it may touch the surface of the sample holder. The measured temperature coincides with the sample temperature only if the conversion process occurs in a thermally thin regime, that is, if a uniform profile is established along the sample thickness. Therefore, conditions for conversion in a thermally thin regime should be first determined. The procedure applied in this study is based on the well-known dependence of the residual solid yield on the reaction temperature (primary char) and the intraparticle residence times of primary volatile products (secondary char). Indeed, as the temperatures of the primary degradation are increased (Shafizadeh et al. (1979), Bradbury et al. (1979)) or the residence times of primary vapors inside the reacting particle are made shorter (Antal et al. (1990)), the char yields decrease. Thus, for chosen heating conditions, the sample thickness is decreased until the solid residual yield becomes constant. The radiant heater is made to work according to two modalities, here indicated as A and B, which correspond to the emission of a constant flux and to the attainment of a certain temperature with an assigned rate, respectively. The modality A, which is the easier one, also corresponds to a constant temperature of the emitters, after short transients whose duration is successively reduced as the intensity of the radiation is increased. The heating modality B can be implemented to impose a certain time evolution of a temperature, which can be that of the emitters or the specimen, by varying the intensity of the radiant flux. In this study, the controlled variable is the temperature of the sample surface, after sample sizes for conversion in a thermally thin regime are determined. The manipulated variable is the electric voltage applied to the furnace or, in other words, the intensity of the radiative heat flux Qe. In the absence of spatial gradients along the sample layer, the dynamics of cellulose degradation, for the heating modality A, are well described by means of the mathematical model for a thermally thin regime reported by Di Blasi (1996c) (assumptions are listed in the same reference). In particular, the time evolution

of the sample temperature, T, for the computational domain consisting of the half-thickness layer, τ, can be described as:

(Fscs + cgFg)

σS(TL4 - T4) ∂T ) Qr + ∂t V

(1)

where Qr and Fc are dependent on temperature and chemical composition. Also, the emissivity may not be constant, with further effects on the external heattransfer rate. Thus, eq 1 should be formulated and solved in conjunction with chemical species conservation equations (Di Blasi (1996c)). In the modality A, Qe ) σSTL4 is constant and the temperature evolution is thus the result (eq 1) of the effects of both the external (constant) heating and the variations in the enthalpy and properties caused by chemical reactions. From the mathematical point of view, eq 1 and the control equations are applied to determine how Qe should be varied, in order to achieve a chosen sample temperature evolution in the modality B. According to the classical theory of chemical process control (Seborg et al. (1989)), eq 1 is linearized, expressed in terms of deviation variables, transformed in the Laplace domain and coupled to a PID controller. In the experiments, the first step deals with a trial and error procedure applied to the empty sample holder for a first estimate of the PID controller parameters. Then, prior to each run, these are adjusted for the presence of the sample to be pyrolyzed, again with some trial and error tests, so that the maximum heating rate can be achieved, with no deviation from the chosen reaction temperature. A continuous flow of helium is applied to establish an inert environment and to reduce the residence time of vapors inside the reaction chamber. It is distributed at the entrance of the quartz reactor, at a distance of about 6 × 10-2 m from the sample holder. This is an important point. It is generally retained that, under conditions of radiative heating (Tabatabaie-Raissi et al. (1989)), secondary reactions are promptly quenched because, as soon as volatile products are released from the hot reacting solid, they encounter a zone where the temperature is rather low (apart from a narrow gas layer, adjacent to the solid surface, which is heated by conduction). However, it is important to reduce the residence time of volatile pyrolysis products in the radiant chamber, because it is also well known (Di Blasi et al. (1991)) that they can absorb some of the incident radiation and thus attain temperatures sufficiently high for secondary reactions to become active. The continuous flow gives rise to very low concentrations of volatiles in the reaction chamber (for the worst case, that is, the highest temperatures, it is estimated that the ratio between volatiles and inert is about 1/1000 mol), thus keeping gas-phase absorption of radiation to a minimum. Furthermore, helium is preferred to argon, given the higher diffusion coefficient (a factor of about 3) of the high molecular weight volatiles (Varhegyi et al. (1994)). The flow of inert gas, at ambient temperature, is also used to cool the residual char, left after complete solid devolatilization. Usually, two runs are required to measure the weight loss. In the first, the weight loss curve of the degrading sample is obtained. The second run is carried out to record the so-called “blank” curve, which takes into account the effects of spurious weight changes due to temperature effects in the gas phase surrounding the sample while degrading (Tabatabaie-Raissi et al. (1989)).


This curve, obtained with the sample holder left in the position of the first test and with the final char yield, is then used to correct the weight loss curve. These corrections are, at the higher temperatures, on the order of 50% with respect to the measured sample weight. In order to verify the correctness of the procedure, after each run, the final char yield is also weighed and compared with that previously determined. Given the relatively large sample mass (20-8 mg) and the very thin thermocouple, the close contact of this with the sample does not affect the mechanical inertia of the system and simultaneous accurate measurements of temperature and weight are obtained. An acquisition data system, based on a PC and the software LabView (National Instruments), allows for continuous monitoring (0.4-1 s) and storage of temperature and weight loss values. In this study no extensive investigation on the dependence of product yields on reaction conditions has been carried out, as the analysis is focused mainly on the effects of heat-transfer limitations. However, a product collection system is being applied, consisting of a condenser (a 10 × 10-2 m long tube filled in with glass wool) and a plastic bag for gaseous products. Results Thermal degradation of thin cellulosic layers is carried out according to the heating modalities A and B, by varying the intensity of the radiative heat flux and the final temperature of the sample, respectively. The influence of intrabed temperature gradients on the conversion process (apparent kinetics) is investigated, by varying the sample thickness. Conditions leading to a true isothermal process, controlled by (intrinsic) chemical kinetics, are determined. In all cases, commercial cellulose with lengths of the fibers in the range 20-150 µm (Sigma Aldrich), predried for about 10 h at a temperature of 388 K, is degraded. Cellulose is chosen as fuel, given its importance in the field of biomass thermochemical conversion and the still uncertain behavior under flash pyrolysis conditions. The helium flow, at ambient temperature, is equal to 8 × 10-4 m3/ min, which, given a cross section of the quartz reactor of 28 × 10-4 m2, gives rise to velocities of about 30 × 10-2 m/min. No variations in the degradation process are seen with larger mass flow rates (factors of 2-3). Cellulose Degradation under Constant Radiative Heat Flux (Heating Modality A). Tests have been conducted, according to the heating modality A, for about 10 mg of cellulose powder, uniformly distributed along the uniform temperature zone of the sample holder, to give τ ) 50 µm. As will be shown later, this sample thickness corresponds, for moderate reaction temperatures, to conversion in a thermally thin regime. The steady temperature value, Tf, here indicated as the “system temperature”, is used to characterize the different tests and has been varied in the range 543-699 K, through different values of Qe. For the sample thickness given above, the cellulose heating rate (referred to the sample surface and before significant weight loss) varies from about 12 to 54 K/s as the system temperature is varied. As will appear from the experiments, external heat-transfer limitations can hardly be avoided with this heating modality, so that it is not applicable to study the intrinsic kinetics of degradation. However, some results have been obtained which can be of interest from the practical point of view, that is, in relation to pyrolysis units, where biomass particles are exposed to a certain reactor temperature. Also, comparison with previous analyses can be carried out.

Figure 2. Solid mass fraction as function of time (τ ) 50 µm) for different system temperatures (heating modality A).

Figure 3. Time-temperature curves for cellulose (solid lines) and char (dashed lines) (A) for two different system temperatures (Tf ) 577 and 633 K) and τ ) 50 µm (heating modality A) and (B) for two different sample half-thicknesses (τ ) 100 and 44 µm) and Tf ) 633 K (heating modality A).

Examples of the weight loss curves (the solid residual, W, including char and cellulose, expressed as a fraction of the initial sample mass, M0) are reported, for system temperatures in the range 543-622 K, in Figure 2. As expected, preheating times, before significant weight loss, become successively shorter and the amounts of solid, left as charred residual, decrease as the intensity of the radiative heat flux (or the system temperature) is increased. Associated with weight loss curves, the temperature of cellulose (recorded in the course of degradation) and char mass fraction (recorded together with the blank curve) are available. These can be used to get information on the thermal history of the sample. Examples of the (char and cellulose) temperature curves are plotted in Figure 3A,B, for two different system temperatures (Tf ) 577 and 633 K) and τ ) 50 µm (A) and for two different sample sizes (τ ) 100 and 40 µm) and Tf ) 633 K (B). In all cases, a region exists where the sample


temperatures are significantly lower than those of the char. The difference becomes successively larger as the system temperature or the sample thickness is increased. The lower cellulose temperature, measured during the process transients, is due to reaction energetics and, in principle, larger medium thermal capacity. Concerning this last point, it should be noted that the reduction in the effective sample capacity is rather large, given that, even at the lowest temperature, a reduction in the sample weight of about 75% is measured. However, given that, in all cases, for temperatures below 500 K no significant difference is seen between cellulose and char, it can be assumed that the role played by thermal capacity is less important than that of the endothermicity of pyrolysis reactions (tar formation). As expected, the temperature difference becomes successively smaller as time increases, because the reaction process approaches completion and the two media tend to be the same. Also, a plateau in the cellulose time-temperature curve is clearly seen, associated with the onset of high rates of solid degradation, for Tf g 600 K. It becomes successively more evident as the system temperature or the sample thickness is increased. In fact, given the exponential increase of the reaction rate with temperature, the heat requirements by the endothermic degradation also become more stringent, as Tf or the amount of solid to be pyrolyzed (sample thickness) are increased. Under mild heating conditions, (Tf < 580 K), both cellulose and char slowly approach the steady final conditions and a period (whose duration is, at least, 1/3 of the total conversion time) is seen where the two temperatures become equal. The period where the two temperatures are equal is reached only for conversion levels of about 90% for 580 K < Tf e 600 K. Finally, for Tf > 600 K, cellulose always degrades at temperatures significantly lower than those of the system, though they remain almost constant for most of the devolatilization. It should be noted that this behavior has been observed also for very thin sample layers, indicating that external heat-transfer limitations are very high and cannot be overcome under constant heating conditions and temperatures above 600 K, which give rise to rather fast sample heating rates. This behavior was already observed in previous experimental studies (Antal et al. (1980), Bradbury et al. (1979)) and is the thermal lag due to the endothermicity of cellulose degradation (high temperatures), well described in the theories proposed by Di Blasi (1996c) and Narayan and Antal (1996). Convective outflow of volatiles also affects the sample heating rate. As the devolatilization rate (and the flow velocity) increases with temperature, it has been observed that such a contribution becomes successively more important as more severe conversion conditions are considered (Di Blasi (1994, 1996c)). The maximum temperature difference (between char and reacting cellulose), ∆Tm, and two sample temperatures, T1 and T2, corresponding to ∆T ) 50%∆Tm and ∆T ) ∆Tm, respectively, are reported in Figure 4 as functions of the system temperature. The temperature T1, which can be taken as representative of the onset of significant rates of solid degradation, increases almost linearly with the system temperature (apart from the very low values). On the other hand, T2, which can be taken as representative of the temperature of the completion of the degradation process, also shows an almost linear increase with the system temperature for

Figure 4. Maximum temperature difference between cellulose and char, ∆Tm, and cellulose temperature for ∆T ) 50%∆Tm, T1, and for ∆T ) ∆Tm, T2, as functions of the system temperature (τ ) 50 µm, heating modality A).

values of this below 600 K. However, for larger values, the rate of increase is successively reduced and a tendency toward a constant value of about 600 K appears. Apart from low Tf values (e580 K), ∆Tm continuously increase with Tf, at a rate that becomes faster for values of this above 600 K, as a consequence of the trend previously observed in T2. It should be noted that the temperature difference may become as high as 30 K. Results presented in Figure 4 indicate that, for assigned external thermal conditions (radiation), such as in chemical reactors, the effective reaction temperature can hardly be brought above a certain critical level of about 600 K. Increasing the sample heating rate, through reactor temperature, reduces the duration of the first stage of degradation, which occurs below 600 K and on average the reaction temperature increases. However, given the endothermicity of the degradation process and the successively faster reaction rates, the increase in the effective reaction temperature of cellulose is successively reduced and hardly goes above 600 K, with this heating modality. According to eq 1 and extensive computations (Di Blasi (1996c), Narayan and Antal (1996)), an almost constant temperature is attained at the fast heating rates because of a balance between the requirements of endothermic pyrolysis and the external heat supply. Also, the occurrence of pyrolysis during the time-temperature plateau may explain why the thermal conversion of cellulosic materials has been interpreted as a fusion process. In particular, for contact (ablative) pyrolysis of wood (Lede at al. (1985)), a “wood fusion temperature”, equal to 738 K, has been introduced. This value is significantly higher than the limit observed in this study for cellulose, but the different chemical behavior between the two solids and the difference in the external heat-transfer rates (much larger in the case of ablative pyrolysis) may justify the difference. The significant differences between cellulose and system temperature for a large part (low temperatures) or for the whole duration of the degradation process (high temperatures and/or heating rates) make questionable the use of the data (weight loss and product yields) obtained according to the heating modality A for the formulation of kinetic models and the estimation of kinetic data, since conversion occurs under heat-transfer control. Heat-transfer limitations for modality A can be reduced only for slow heating rates, such as those usually applied in the slow TGA systems which, of course, are not directly applicable for flash pyrolysis. Also, very thin sample thickness (or small mass) ap-


Figure 5. Solid mass fraction for solid residence times, ts, equal to 300 s (Tr ) 573 K) and 10 s (Tr ) 623 K) as a function of the sample half-thickness (heating modality B).

Figure 6. Temperature-time curves of the cellulose sample during the initial stage of the process for the heating modality B (τ ) 50 µm).

pears to cause a reduction in the heat-transfer limitations, but this approach is again effective only for relatively slow heating rates. Cellulose Degradation under Controlled Sample Temperature (Heating Modality B). The analysis of the thermal degradation data, obtained through modality A, has shown that it is possible to avoid heattransfer limitations only for very thin samples and slow heating rates (low system temperatures). In the heating modality B the temperature of the sample is the result of a control process. If the proper size of the sample is chosen to avoid intrabed gradients, a proper selection of control parameters allows degradation to be carried out under exactly known temperature conditions. Therefore, some preliminary tests have been conducted to determine the conditions giving rise to a conversion in a thermally thin regime for modality B. It should be pointed out that, in relation to intrabed heat- and masstransfer limitations, the sample thickness, not the sample mass, is the key parameter. Consequently, an important point is to determine, before all, the size of the sample thickness which does not give rise to significant spatial gradients of temperature (primary degradation at different temperatures) and volatile residence times (activity of secondary reactions). As previously illustrated, the char yields are taken as a measure of the importance of these processes. In particular, conditions of char yields not dependent on the sample thickness correspond to negligible effects on primary degradation of intrabed transport phenomena and secondary reactions. Given that intrabed space gradients and secondary reaction rates are expected to increase as heating conditions are made more severe, two reaction temperatures have been considered, 573 and 623 K. Tests have been conducted, according to the heating modality B (examples of the heating curves will be given in the following), by decreasing the thickness of the cellulose layer from about 200 µm. The solid residence times are 300 (low temperature) and 10 s (high temperature). After this time, the power is turned off, and the sample is promptly moved into a cold zone under a continuous helium flow. The solid residual, expressed as a fraction of the initial sample mass, is reported as a function of the sample half-thickness in Figure 5. As expected, it initially decreases with the sample size, because of the increase in the average reaction temperature and the decrease in the volatile residence times, and then tends to a constant value. The attainment of a constant value of the char yield corresponds to conversion in a thermally thin regime.

Indeed, the char yield does not depend on the sample thickness only when solid degradation occurs at the same temperature, independently of the spatial position, that is, no spatial temperature gradients exist. Moreover, these conditions also indicate that intrabed secondary charring reactions do not occur to a significant extent. For cellulose and for the chosen heating conditions, the transition from a thermally thick to a thermally thin regime occurs for sample half-thicknesses slightly increasing with the temperature, that is, τ ) 70 µm (Tr ) 573 K) and τ ) 60 µm (Tr ) 623 K), respectively. Therefore, conversion of samples with half-thicknesses below 60 µm and reaction temperatures below 623 K can be assumed to occur in a thermally thin regime. Examples of the time history of the cellulose temperature, for the initial stage of the process, are shown in Figure 6. These have been obtained for different set points (reaction temperatures) and τ ) 50 µm (about 10 mg of cellulose powder). After the first stage of linear increase (heating rates from 40 to 78 K/s for the temperature range considered), a constant (reaction) temperature, Tr, is more slowly attained (the time needed to attain this value is indicated as heating time, th) and maintained for the whole duration of the process, with no overshoot in practice. The global heating rates, for the attainment of the reaction temperature, vary from about 19 to 56 K/s. For the whole range of reaction temperatures tested, the endothermicity of degradation reactions is well compensated by increased external heat supply. Consequently, no plateau is seen in the timetemperature curve, even when fast degradation rates are achieved. Therefore, compared to the curves of Figure 3, weight loss curves obtained with the heating modality B show shorter degradation times (Figure 7) for comparable values of Tf (system temperature of the modality A) and Tr (the reaction temperature of modality B). The weight loss curves of Figure 7 are obtained under isothermal sample conditions, apart from the very beginning of the process. Figure 8 reports the ratio between the conversion time, tc, and the heating time, th, and the solid mass fraction at t ) th as functions of the reaction temperature. For Tr e 607 K, the conversion time is significantly longer than the sample heating time (factors in the range 1000-16), and the conversion level achieved in relation to the heating period is also small (0.001-22%) (the factors become 5 and 40%, respectively, for Tr ) 623 K). Effects of Intrabed Resistances. In order to understand the influence of intrabed heat- and mass-


Figure 7. Solid mass fraction as a function of time for different sample temperatures (τ ) 50 µm, heating modality B).

Figure 8. Ratio of the conversion to the heating time, tc/th, and solid mass fraction for t ) th, as functions of the reaction temperature (τ ) 50 µm, heating modality B).

Figure 9. Final char mass fraction as a function of temperature for the heating modality B (τ ) 50 and 100 µm) and the heating modality A (τ ) 50 µm). For comparison purposes, literature data are also enclosed.

transfer resistances on the conversion process, the final char yields (the solid residual left on the sample holder) are examined on the dependence of the heating conditions, through Figure 9. The temperature range considered is 523-630 K, where the upper limit is dictated by the accuracy of the weighing instrument, that cannot detect char yields below 1% of the initial cellulose mass. For comparison purposes and in relation to the temperature range of interest, not only data obtained for τ ) 50 µm (modality A and B) and τ ) 100 µm (modality B) but also some literature data on isothermal cellulose degradation are enclosed, obtained through fluid-bed reactors (Lipska and Parker (1966), Barooah and Long (1976)), a vacuum tube furnace (Shafizadeh et al. (1979)), and a vacuum TGA system (Ramiah (1970)) (these data are obtained under known external temperature conditions (modality A)).

The trend shown by Figure 9 is in qualitative agreement with the previous literature on the pyrolysis of cellulose. In other words, the solid residual increases as the effective reaction temperature decreases (from modality B to A) or the cellulose layer is made thicker for the same heating modality (B). Extrapolation of the measured data for values of the char yield below 1% shows that cellulose degradation, under radiative heating, occurs with no char formation for Tr ) 625 K or Tr ) 630 K as the sample layer thickness is increased (modality B) or for Tf ) 640 K (modality A). At first glance, it appears that, for temperatures in the range 523-560 K, the measured char yields are comparable with those given by Lipska and Parker (1966), though, in this case, the particle sizes (762 µm) are much larger than the sample thicknesses used in the present experiments. This is understandable because at slow heating rates (low temperatures) a kinetic regime can be more easily established (Di Blasi (1996c)) or, in other words, the degradation process is so slow that heat transfer limitations do not impact the results (Varhegyi et al. (1994)). In general, in all cases, the char yields measured are lower that those reported in the literature. For instance, at high temperatures, a comparison with the data by Shafizadeh et al. (1979) shows values lower by factors from 3 to 8. Also, it is worth observing that the average sample heating rates, achieved in this study, should be faster than those typical of fluid-bed flash pyrolysis (Scott et al. (1988)), given that, in this case, negligible char yields have been found only for Tf g 900 K (data from this study are not included in Figure 9, because they are available only for Tf g 700 K). On the other hand, negligible char yields have been reported by Lewellen et al. (1976) for very fast heating rates (400-10 000 K/s). It can be understood that these results are due to significant intra- and extraparticle (bed) heat- and mass-transfer effects, given the much larger sample sizes used in previous studies, namely, 595 µm (Scott et al. (1988)), 200 µm (Shafizadeh et al. (1979)), and 420-700 µm (Barooah and Long (1976)), the different external heattransfer mechanisms, or the different solid and volatile residence times. Therefore, the analysis of the weight loss curves, measured in this study, can give information on the kinetic mechanism and data of cellulose flash pyrolysis (modality B, τ ) 50 µm). Another consideration can be made in relation to the effective reaction temperature of modality A (plausibly in the range of temperatures T1-T2, introduced in Figure 4) and the final solid residual yields (Figure 9). It appears that, with the heating modality A and for the range of heating conditions considered in this study, a very small part of the reaction, if any, occurs at sample temperatures above 600 K. On the other hand, for system temperatures above 630 K the char yields are of the order of 1%. Therefore, in terms of primary degradation, significant char formation can be avoided if a temperature of 600 K is achieved within a relatively short time. However, it should be stressed that the limit temperature of 600 K is typical of the experimental conditions achieved in this study (very low heat- and mass-transfer resistances). Global Kinetics. For the heating modality B, the process can be divided into two stages: the first is dynamic, that is, the system or sample temperature increases to the desired isothermal point, and the second is the true isothermal stage, that is, the temperature is


kept at the constant value attained before. In general, the isothermal approach of the pyrolysis process can be considered valid if (1) there is a negligible weight loss in the first stage and (2) the duration of the first unsteady stage is a small fraction of the whole time needed for the thermal degradation of the sample. In fact, in general, one of the disadvantages of the static approach is that there is an excessive weight loss before pyrolysis temperature is reached and, of course, the higher the temperature, the greater the weight loss (Ramiah (1970)). It has been shown (Figures 5 and 8, τ ) 50 µm and heating modality B) that, for large part of the temperature range investigated, the above assumptions are verified. Thus, a pure kinetic, isothermal theory is proposed. Of course, this theory is not applicable for the heating modality A. The kinetics of primary cellulose degradation are described through a semiglobal mechanism: K1

νCCHAR + νGGAS (a1–a2)

CELLULOSE K2 TAR

This is a modified version of the well-known BroidoShafizadeh scheme (Broido and Weinstein (1971), Bradbury et al. (1979)) for cellulose pyrolysis and takes into account the competitive formations of tar (high temperature) and linked char and gas (low temperature). The formation of the “active” cellulose with a reduced degree of polymerization, preceding devolatilization, has been recently shown not to be controlling (Varhegyi et al. (1994)), and thus it has been dropped from the kinetic model. Moreover, though different orders at different stages of the reaction have been suggested (Milosavljevic and Suuberg (1995)), the assumption of first-order kinetics is usually retained valid in engineering applications (Agrawal (1988)). The kinetic equations for the pyrolysis scheme (a1a2) are expressed as:

∂MS ) -KMS ∂t

(2)

∂MT ) K2MS ∂t

(3)

∂MC ) νCK1MS ∂t

(4)

First-order rates are expressed as:

Ki ) Aie-Ei/(RT),

i ) 1, 2

(5)

and, from mass conservation:

K ) K1 + K2

(6)

Integration of eqs 2-4 (the subscript ∞ is used to indicate the final yields) with conditions (Agrawal (1988)):

MS(0) ) M0,

MT(∞) ) MT∞,

MC(∞) ) MC∞

(7)

gives

MS ) M0e-Kt

(8)

K2M0 -Kt e K

(9)

K1νCM0 -Kt e K

(10)

MT ) MT∞ MC ) MC∞ -

The final tar yield, MT∞, can be obtained from eq 9 written for t ) 0 as:

MT∞

K2M0 K

(11)

Furthermore, the total solid mass (char and cellulose), W, can be obtained from eqs 8 and 10 as:

(

W ) W∞ + M0 1 -

)

K1νC -Kt e K

(12)

where W∞ is the final solid residual and can be obtained from eq 10 written for t ) 0:

K1νC W∞ ) M0 K

(13)

An alternative formulation of eq 12 is thus

P)

W - W∞ ) e-Kt M0 - W∞

(12′)

Figure 10 shows the plots of ln(P) versus time, for the weight loss curves reported in Figure 7 (heating modality A, τ ) 50 µm). It should be noted that the final char yields for Tr ) 614 and 623 K are those extrapolated from Figure 9, given their very low values and the limited accuracy of the weighing system. The slope of the straight lines is the kinetic constant, K, for the different temperatures. Clearly, in accordance with Varhegyi et al. (1994), the process of weight loss can be described by a simple first-order reaction. The usual Arrhenius plot (Figure 11) and a leastsquares analysis give the activation energy and the preexponential factor of the global degradation kinetics. The solid line describes the process in the temperature range 523-623 K by one set of kinetic data: E ) 214.5 kJ/mol, A ) 1.2 × 1017 s-1, with a standard deviation equal to 6%. Again, these data are very close to those reported by Varhegyi et al. (1994), who analyzed the slow dynamics of the thermal degradation of small (0.5-2 mg) cellulose samples. It is interesting to note that, notwithstanding the highly different experimental systems and heating conditions, the absence of significant heat- and mass-transfer limitations leads to comparable values of the kinetic data. As expected, the presence of significant intraparticle effects and unmet heat demands (Antal and Varhegyi (1995)) gives rise to a process described by a lower activation energy (for 543 K e Tr e 623 K, E ) 191.8 kJ/mol for τ ) 100 µm against 200.6 kJ/mol for τ ) 50 µm). The kinetic data for the rates of tar and the linked char and gas formation can also be estimated by means of eqs 8 and 15, if the stoichiometric coefficient νC is known. Alternatively eqs 8 and 11 together with the measurements of the final tar yields can be used. Though volatile products have not been collected and thus the final tar and gas yields (and νC) are not known, an estimation of the kinetic data of the two competitive reactions has been made on the basis of literature values for νC. The results obtained for νC ) 0.35 (Bradbury et


Figure 10. Plots of ln(P) versus time (reduction in the scale as in Figure 7) for several reaction temperatures (τ ) 50 µm, heating modality B).

Figure 11. Arrhenius plots for the global degradation rate, K, the reaction rates of tar (K2) and linked char and gas (K1) formation, obtained with the heating modality B for τ ) 50 µm.

al. (1979)) are also enclosed in Figure 11. The activation energy of tar formation is significantly higher (225.1 kJ/ mol, standard deviation 5%) than that of char and gas formation (158.3 kJ/mol, standard deviation 9%), confirming, in agreement with the Broido-Shafizadeh mechanism, that cellulose degradation occurs according two competitive pathways, the low temperatures leading to char formation and the high temperatures to tar formation. The appearance of a single smooth curve of weight loss indicates that the transition from the chardominating to the tar-dominating reaction is gradual (Agrawal (1988)). Again, kinetic data for tar and char formation are in the range of those previously determined (Bradbury et al. (1979), Agrawal (1988), Arseneau (1988), Varhegyi et al. (1994)). In a recent study (Milosavljevic and Suuberg (1995)) a rough classification of global cellulose kinetics has been made on the dependence of the thermal conditions. For rapid heating to temperatures above 600 K, activation energies are seen to be in the range 140-155 kJ/ mol, while slow heating to temperatures below 600 K appears to give rise to larger values (about 218 kJ/mol). Therefore, kinetic constants have also been estimated, taking the reaction temperature of 600 K as the boundary between two different kinetics (curves are not reported to avoid crowding). Again, this interpretation gives data in the same range of those reported in the above reference, that is, E ) 220.5 kJ/mol (A ) 3.47 × 1017 s-1) for 523 K e Tr e 600 K and E ) 143.2 kJ/mol (A ) 3.77 × 1010 s-1) for 600 K e Tr e 623 K. However, standard deviations, equal to 13% and and 10%, respectively, are higher. On the other hand, a comparison between the model predictions through only one set of data (E ) 214.5 kJ/mol, A ) 1.2 × 1017 s-1) and the

Figure 12. Experimental (symbols) and simulated (dashed lines, one set of kinetic data) solid mass fraction versus time for τ ) 50 µm (heating modality B).

experimental weight loss curves shows very good agreement, confirming the validity of the kinetic scheme and data proposed (Figure 12). On the contrary, the evaluation of the difference between the measured solid mass fraction and the predictions, based on the two sets of kinetic constants, shows rather large values at both low and high temperatures. The results presented in this study show that for both the heating modalities A and B, that is, with and without significant heat-transfer limitations, cellulose degradation hardly occurs for temperatures above 600 K. In the first case, the increase in the external heattransfer rate does not change significantly the temperature of the reacting cellulose, which is cooled by the endothermic evolution of volatiles, so that the process is controlled by heat-transfer (Bradbury et al. (1979), Antal and Varhegyi (1995)). In the second case, for high temperatures, the process becomes very fast and complete sample devolatilization is seen for Tr ) 625 K. It is likely that heat- and mass-transfer limitations are responsible for the decrease in the activation energy as the temperature is increased and for weight loss reported at temperatures much higher than it actually occurs (Antal and Varhegyi (1995)). Consequently, the temperatures reported for negligible char formation are also higher. Some considerations are also needed about the initial processes which alter the substrate but cause only minor changes in the weight, described through the formation of “anhydrocellulose” (Broido and Weinstein (1971)) or “active cellulose” (Bradbury et al. (1979)) (loss of water and reduction in the degree of polymerization). The descriptions of these processes do not appear necessary for the prediction of the global degradation kinetics in the kinetic regime, as observed by Varhegyi et al. (1994) and confirmed by this study. However, changes in the physical properties (i.e., melting) can be important in practical situations, where conversion is the result of a strong interaction between chemical and physical processes such as in the case of ablative pyrolysis. Assuming that the rate of active cellulose formation is proportional to 1/ti, where ti is the time needed to attain the maximum rate of weight loss (Bradbury et al. (1979)), the following kinetic data are obtained: E ) 250.5 kJ/ mol, A ) 1.8 × 1021 s-1. These are again comparable to those reported by Bradbury et al. (1979). Conclusions A new experimental system has been developed to investigate the kinetics of the thermal degradation of solids under reduced heat- and mass-transfer limita-


tions. The system, which is based on radiative heat transfer, allows for the continuous monitoring of the sample temperature and weight, while the heating rates are much faster than those usually encountered in thermogravimetric analyses. Experiments have been carried out for cellulose, taken as representative of biomass fuels, with two different heating modalities. The first modality (A) consists of the sample degradation made to occur under known system temperatures, as in most of the previous studies. It is shown that external heat-transfer limitations, which become progressively more important as the reaction conditions are made more severe, cannot be avoided for system temperatures above 600 K. Furthermore, for this and higher values, it is confirmed that most of the pyrolytic degradation occurs for effective reaction temperatures below 600 K, with negligible char formation. As a consequence of reduced mass transport limitations, the char yields are significantly lower than those reported in the literature for temperatures in the range 543-630 K. The second heating modality (B) represents the most innovative aspect of the study. It is based on the control of the sample temperature, using the applied radiative heat flux as the manipulated variable, in order to eliminate external heat-transfer limitations. The process is two-stage. In the first, the sample is rapidly brought to the chosen sample temperature (set point), while in the second the temperature is maintained at the chosen value. The global heating rate has been varied from about 19 to 56 K/s for cellulose setpoints in the range 523-623 K. Sample half-thicknesses below 60 µm guarantee the absence of intrabed temperature gradients and activity of secondary charring reactions. Therefore, the process is made to occur under known thermal conditions and reduced heat- and mass-transfer limitations. Cellulose weight loss curves have been interpreted in terms of a single-step process (consisting of two competitive reactions leading to tar and linked gas and char formation), and the estimated kinetic data are in agreement with previous studies conducted under kinetic control. Acknowledgment The research was funded in part by the European Commission in the framework of the Non Nuclear Energy Programme (JOULE III), Contract JOR3-CT950081. Nomenclature A ) preexponential factor c ) specific heat E ) activation energy M ) mass MC∞ ) final solid char mass M0 ) initial cellulose mass Qe ) radiative heat flux Qr ) enthalpy variation due to chemical reactions S ) heat-exposed surface Tf ) system temperature TL ) lamp temperature Tr ) (final) sample temperature tc ) conversion time th ) heating time ts ) solid residence time V ) sample volume W ) solid residual (including char and cellulose)

) emissivity F ) density σ ) Stephan-Boltzmann constant τ ) bed half-thickness ν ) stoichiometric coefficient Subscripts C ) char g ) gas/vapor phase S ) cellulose T ) tar

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Received for review September 9, 1996 Revised manuscript received November 19, 1996 Accepted November 20, 1996X IE960551R X Abstract published in Advance ACS Abstracts, January 15, 1997.

An Experimental Investigation of Heat-Transfer Limitations in the Flash

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