Radiative Pyrolysis of Single Moist Wood Particles - American

Ind. Eng. Chem. Res. 2000, 39, 873-882

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Radiative Pyrolysis of Single Moist Wood Particles Colomba Di Blasi,* Elier Gonzalez Hernandez, and Antonio Santoro Dipartimento di Ingegneria Chimica, Universita´ degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy

Radiative pyrolysis of thermally thick beech wood has been investigated through a comparison between dry and moist [11% dry basis (db)] particles, for heat fluxes in the range 27.5-80 kW/ m2. The initial moisture content has also been varied from 0 to 50% (db) for two radiative fluxes, 27.5 and 49 kW/m2, corresponding to slow and fast external heat-transfer rates, as steady surface temperatures are about 625 and 800 K, respectively. For very slow heating, moisture evaporation precedes wood pyrolysis. As the external heating conditions are made more severe and/or the initial moisture content is increased, the two processes take place simultaneously, associated with the propagation of separate fronts along the particle radius. Spatial gradients also increase, while apparent weight loss kinetics from a single-peak rate turn into a two-peak rate. The conversion times increase almost linearly with the initial moisture content, but differences in primary product (char, gas, and liquids) yields and gas composition are negligible. Introduction The analysis of the thermal response of moist wood is an important issue in thermochemical conversion technologies. The moisture content, always defined in this study on a dry basis (db), acceptable for fuel handling and processing depends on the specific application. It should be below 20% for storage with low material losses and low mould growth, while requirements for wood densification are between 10% and 14%.1 For low solid and gaseous emissions in combustion, feedstocks should not exceed moisture contents of 30%.2 Indeed, the air supply rates need to be increased with the moisture level, to draw off water vapors from the combustion chamber, and at the same time the temperatures become lower. For fixed-bed concurrent (downdraft) air gasification, a moisture content below 25% is necessary to produce a low-tar gas.3 When the initial moisture content is increased, heating rates may become so slow that incompletely devolatilized char may enter the reduction zone and tars may be produced there and remain unconverted in the gas. In principle, very high moisture contents can be afforded by fixed-bed countercurrent (updraft) and fluid-bed gasification (70100%),4 but in practical applications the limit is always below 50%. Finally, the high particle heating rates required by fast pyrolysis technologies impose initial moisture contents below 10%.5 The thermal behavior of moist wood at bench/laboratory scale has been examined by several studies. For wood heated radiatively in air, the presence of moisture increases the ignition time, the total ignition energy, and the minimum intensity for both spontaneous and piloted ignition.6 During burning7 moisture delays the temperature rise of the wet wood and cools the pyrolyzing region, through convective transport of water vapors. The increase in the initial moisture content has been shown8,9 to promote charring reactions and to lower the liquid (oil) yields in the fluid-bed pyrolysis of small-sized wood particles. Few pyrolysis experiments have been conducted for thick wood particles, and a * Corresponding author. Tel: 39-081-7682232. Fax: 39-0812391800. E-mail: [email protected].

plateau in the temperature profiles, for temperatures near the normal water boiling point, has been observed in all cases.10-13 However, no systematic study has been conducted, and there is no clear indication in relation to product distribution, devolatilization rates, and conversion times. Also, process dynamics in terms of sample weight loss are not known, to clarify the interactions between moisture evaporation, transport phenomena, and activity of degradation reactions. On the other hand, though several investigations are available on the pyrolysis of dry wood, product yields have been measured only for a few temperatures.11,13 In this study, experimental results (temperature and weight loss dynamics, product distribution, and gas analysis) are presented of the pyrolysis of thick wood particles for several initial moisture levels. The particle size is representative of fixed-bed reactor requirements, and external heating is varied to reproduce countercurrent (maximum temperatures in the pyrolysis section of 725-850 K14) and concurrent (temperatures in the flaming pyrolysis region up to 1000 K15) gasification conditions. Experimental Section The experimental system applied for single-particle pyrolysis is essentially the same as that presented in ref 16. Hence, only the main features are briefly illustrated here. The heating system is a fast (heating rates of about 750 K/s) radiant furnace, which accommodates a quartz reactor (6 × 10-2 m diameter). The furnace, equipped with PID and a transducer (SCR), allows a constant radiative heat flux to be emitted. A wood cylinder is vertically positioned in the uniformly heated zone of the reactor, through a suspension system, which is connected to a precision balance (Mettler PM2500). A metallic (brass) cylinder is hung on the bottom of the sample and works as a ballast, to facilitate alignment and stabilization. Different values of the emitted radiation result in different sample temperatures and heating conditions. A nitrogen flow is applied at the top of the quartz reactor in order to reduce the extra-particle residence time of volatile pyrolysis products and to establish the

10.1021/ie990720i CCC: $19.00 © 2000 American Chemical Society Published on Web 03/02/2000

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Ind. Eng. Chem. Res., Vol. 39, No. 4, 2000

proper reaction environment. On the basis of preliminary tests, a nominal (ambient conditions and absence of sample) gas velocity of 0.0125 m/s has been considered for all of the tests, which corresponds to residence times of volatiles along the heated section of about 6 s. The actual residence times are much shorter, given the gas expansion at the pyrolysis temperatures and the presence of the sample, which highly reduces the void section of the reactor. Also, the inert flow is established and maintained for 1200 s in order to remove the air from the reaction chamber, before the test begins. Two separate tests are made, one for the continuous recording of the sample weight loss and another for temperature measurement, product collection, and gas analysis. Temperatures along the particle radius, r, at the median section are continuously monitored (0.5 × 10-3 m bead chromel-alumel thermocouples) at five positions starting from the center. They are allocated into holes previously drilled so that direct exposition to radiation is completely prevented. If, as a consequence of structural failure, any of them, in particular that located below the surface, become exposed to radiation, this condition is checked a posteriori, at the completion of the test, when the char residue is taken out from the reactor. Furthermore, the measurements are highly different from those recorded by nonexposed thermocouples, so that the test is disregarded. The composition of the gas is analyzed, at selected times, through a gas chromatograph equipped with a thermal conductivity detector and a packed column (Supelco carboxen 1000). These measurements are also applied to evaluate the yields of noncondensable gaseous components (indicated as “gas”), through integration of the concentration of each species over the time of the experiments.16 After complete conversion, the power is turned off and the solid residue is left under a nitrogen flow until its temperature lowers to ambient values. This residue is indicated as “char”. The liquids are collected through a condenser train, consisting of watercooled traps, cotton wool demisters, and a silica gel bed. All of the condensable products collected and weighed from the traps (organic compounds and product water formed) are indicated as “liquids”. Results Beech wood cylinders with 4 × 10-2 m diameter and length have been pyrolyzed. Fuel characteristics have been determined with the same techniques as those described in ref 16. Chemical composition consists of 45% cellulose, 30% hemicellulose, 27% lignin, 3% extractives, and 0.5% ash. Elemental analysis (percent total mass including ash and a moisture level of 11%) reports 46.23% C, 6.5% H, 0.07% N, 0.03% S, and 47.7% O. The grain direction has been chosen parallel to the longitudinal axis, and samples have been prepared from the same tree, to keep as low as possible variations in the physical properties between the tests. A first set of tests has been carried out with wood predried at 373 K for 8-10 h (oven dry), with an initial moisture content, U, equal to 0%, and with samples exposed for 2 years or more at ambient temperature (ambient dry), with U ) 11%. The applied radiation intensity, Q, has been varied from 27.5 to 80 kW/m2, with steady temperatures at the particle centerline, Tr, varying from about 600 to 950 K. Samples undergo structural changes (especially for Q g 40 kW/m2) with significant shrinkage and crack formation, which are not exactly reproducible and

give rise to differences in the steady temperatures of 10-20 K (given a certain radiation intensity). These effects are completely negligible on the weight loss dynamics, which show an almost exact reproducibility. A second set of experiments has been made for low (Q ) 27.5 kW/m2 and Tr values of 600 K) and high (Q ) 49 kW/m2 and Tr values of 800 K) radiation intensities, by varying the initial moisture content from 0 to 50%. These two heating conditions can be considered representative of countercurrent4,14 and concurrent4,10,15 fixedbed gasification. Moisture levels above 11% are achieved, after predrying, from wetting in water. As was already observed for oak,7 water absorption causes changes in the cross section of the sample which becomes slightly elliptical (for U ) 50% the two axes are 4.4 × 10-2 and 4.15 × 10-2 m) while the length is not significantly affected. The density also varies from 728 kg/m3 for U ) 0% to 911 kg/m3 for U ) 50%. Visual observation of the sample sections, for wetting times longer than 25 h, shows a uniform moisture distribution (the lowest forced levels examined in this study, 27-29%, are achieved after about 30 h). Moisture absorption takes place mainly along the wood fiber direction, and the wetting curve consists of two distinct regions. In the first region the moisture content increases rapidly up to values of about 55% (after 60 h); in the second region there is a very slow increase (about 240 h for moisture contents of 65%). Weight loss curves, radial profiles of temperature, gas composition, and yields of the main product classes are measured. The solid mass fraction, Y, is defined in relation to the total initial mass (including moisture). The time derivative of the solid mass fraction, after sign change, is indicated as the global volatile formation rate, dYV/dt. Integration of the gas species conservation equations, through the use of discrete points corresponding to sampling and analysis, also allows the total mass fraction of gas produced (again with respect to the initial total mass), YG, to be obtained as a function of time. Then the time dependence of the total liquid mass fraction, including condensable pyrolysis products and initial moisture, is obtained as YL(t) ) Y(t) - YG(t). The measured yields of char, gas, and liquids and of gas species are given on a moisture-free (mf) basis, as is usually done in pyrolysis studies.5 Influences of Heating Conditions on Process Dynamics. Comparison between the degradation behavior of oven- and ambient-dry wood for high external radiation intensities (Q ) 49 kW/m2) can be made through Figure 1A,B. These report the time profiles of temperatures at several radial positions and the time profiles of the sample mass fraction, the liquid and gas mass fractions, and the global volatile formation rate. As expected, the radial temperature gradients are large and the presence of moisture results in the appearance of a plateau, clearly visible especially for the more internal regions, in the time-temperature profiles when conditions close to the boiling point of water are attained (373 K). It is a consequence of a balance between inward heat conduction, on the one side, and outflow convective heat transport and endothermicity of moisture evaporation, on the other side, and has also been reported by previous studies (for instance, refs 10-13). As long as there is moisture to be evaporated, not only does the moist region undergo slow heating rates but also the temperature rise in the external and already dried zone is slowed. For instance, the maximum time derivative

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Figure 1. (A) Time-temperature profiles at several radial positions for wood exposed to Q ) 49 kW/m2 and initial moisture contents of 0 (dashed lines) and 11% (solid lines). (B) Time profiles of mass fractions and volatile formation rate for wood exposed to Q ) 49 kW/m2 and initial moisture contents of 0 (dashed lines) and 11% (solid lines).

of temperature, as indicated by the most external thermocouple, varies from about 3.1 K/s for U ) 0 to 2.4 K/s for U ) 11%. Hence, heating is globally slower for the moist particle. The temperature profiles show the existence of two main regions. The first (2 × 10-2 m e r e 0.5 e 10-2 m) is essentially determined by heat-transfer effects and the second (0.5 × 10-2 m e r e 0) by reaction energetics. For both the dry and moist particles, temperature dynamics for the more external region indicates slightly slower heating rates as the distance from the external surface increases. This is due to an increase in the internal heat-transfer resistance, while no significant variation can be directly related to pyrolysis reaction energetics. For the inner core of the particle, in the final stage of wood conversion, the endothermic degradation of holocellulose (cellulose and hemicellulose components) is completed before the slower and exothermic degradation of lignin, in accordance with thermogravimetric curves reported for the single wood components.17 Indeed, there is an almost flat region, corresponding to about 650 K, which is likely to be a consequence of endothermic holocellulose degradation. The subsequent exothermic lignin degradation gives rise to local maxima in the time derivative of temperature (3.7 and 3.3 K/s for the dry and moist cases, respectively) higher than those observed just below the irradiated surface. Given that holocellulose contributes for about 75% in the chemical composition of beech wood, its initial degradation temperature as reported by the centerline thermocouple can be considered representative of the pyrolysis temperature, Tp. Indeed, lignin degradation takes place over a wide temperature interval,17 and its exothermicity does not allow a clear and univocal definition of a reaction

temperature. The time needed to attain the temperature Tp can be considered as the characteristic heating time, th, and can be used to evaluate the average heating rate as β ) (Tp - T0)/th, where T0 is the initial temperature. It varies from about 0.52 K/s for U ) 0% to 0.32 K/s for U ) 11%, whereas Tp is about the same. The existence of two different temperature dynamics, corresponding to the degradation of the external layer and the inner core of the particle, respectively, has been observed in previous studies (for instance, refs 18-20) dealing with dry wood cylinders or spheres uniformly heated along the external surface. The most recent analysis20 gives the same explanation as above for the temperature dynamics. Apart from very short times and because of lowtemperature water vapor release, the devolatilization rates are slower and the process duration is longer for the moist sample (Figure 1B). The devolatilization rate curve shows a first peak at relatively short times, associated with the degradation of a thin layer, beneath the heat-exposed sample surface. Successively, the volatile release rate decreases, because of internal heattransfer resistance. Then a region of constant values (U ) 0%) or a new maximum (U ) 11%) is attained, both conditions corresponding to the enlargement of the reaction front to the whole particle (heat-up conditions). It should be noted that the global devolatilization rate has already undergone a low value region, when sequential degradation of the two main wood components takes place. Indeed, it regards only a small fraction of the total solid volume (and mass). For instance, for dry wood, from an initial mass of about 36.6 × 10-3 kg a reduction to about 2.3 × 10-3 kg is observed when the reaction front reaches r ) 0.5 × 10-2 m and temperature evolution is determined by reaction energetics. Therefore, though the existence of two different zones is evidenced by the shape of temperature profiles, it does not affect weight loss characteristics, at least for high external temperatures. It can be assumed that moisture evaporation is completed when, after an almost constant value region (temperatures of about 373 K), the most internal thermocouple records a significant increase in the temperature rise (as indicated by a maximum in the second time derivative). The corresponding time can be estimated as the drying time, td. For the case under examination it is 700 s and the total mass loss is about 55%. Therefore, as is also shown by the temperature profiles, at high heat fluxes, moisture evaporation and wood pyrolysis take place simultaneously, though at different positions along the particle radius. The last part of the process is concerned with dry wood pyrolysis, and the sudden increase in the rate of heat transfer gives rise to a second local maximum devolatilization rate, which, however, is lower than that of the dry sample in the final stage. A large part of the volatiles released are condensable products, i.e., water and organic compounds (tars). The charred residue is about 25% of the initial total mass. The main degradation characteristics of oven- and ambient-dry wood for low external heating rates (Q ) 27.5 kW/m2) are shown through the time-temperature profiles, measured at several radial positions (Figure 2A), and the time profiles of the sample mass fraction, the liquid and gas mass fractions, and the global volatile formation rate (Figure 2B). Given the slow heating rates associated with a low radiation intensity, spatial tem-

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Figure 3. Time profiles of the solid mass fractions (total mass basis) for wood exposed to several radiation intensities and initial moisture contents of 0 (dashed lines) and 11% (solid lines).

Figure 2. (A) Time-temperature profiles at several radial positions for wood exposed to Q ) 27.5 kW/m2 and initial moisture contents of 0 (dashed lines) and 11% (solid lines). (B) Time profiles of mass fractions and volatile formation rate for wood exposed to Q ) 27.5 kW/m2 and initial moisture contents of 0 (dashed lines) and 11% (solid lines).

Figure 4. Time profiles of the global volatile formation rate for wood exposed to several radiation intensities and initial moisture contents of 0 (dashed lines) and 11% (solid lines).

perature gradients are small for both cases. The process is significantly slower than the case discussed above, but the main characteristics of the temperature dynamics remain unaltered. Again, the presence of moisture results in the appearance of a plateau for temperatures roughly corresponding to the normal boiling point of water, and two main regions exist, the first dominated by heat-transfer effects and the second by reaction energetics. The boundary between the two regions can be assumed to coincide with x ) 1 × 10-2 m. Compared with the case of Q ) 49 kW/m2, the size of the second region is larger, because for the dry particle it regards about 9 × 10-3 kg (against 2.3 × 10-3 kg). The plateau in the more internal temperature profiles due to holocellulose degradation is barely visible and is associated with lower Tp values (588 and 593 K for the dry and moist cases, respectively). However, given the larger size of the inner zone, exothermic lignin degradation gives rise to local maxima in the temperature profiles. Again the presence of moisture results in slower average heating rates (about 0.2 K/s for U ) 0% and 0.13 K/s for U ) 11%). For the drying time, equal to about 1270 s, the mass loss represents only 13% of the total mass, and given that an initial moisture content of 11% db corresponds to about 10% of the total mass, it can be understood that, for low external temperatures and slow heating rates, moisture evaporation takes place, almost completely, before solid pyrolysis. Again, the higher devolatilization rates observed at very short times for U ) 11% are due to the low-temperature water vapor release. For long times, the rate of volatile formation is slower and the duration of the devolatilization process longer. For both cases the global volatile formation rate slowly

increases with time, attains a maximum when temperatures high enough for degradation (above 525 K) are achieved along the whole particle radius, and then decreases to zero, indicating the completion of the pyrolysis process. Water and tars are again the main products. A representation of the weight loss dynamics for different external heating conditions (Q values in the range 27.5-80 kW/m2) is shown through the time profiles of mass fraction (Figure 3) and examples of the global volatile formation rate for oven- and ambientdry wood (Figure 4). For U ) 0%, the apparent kinetics of weight loss can be described as a one-stage process, for Q e 40 kW/m2. Then a second high value region appears, so that the process is globally two stage. The maximum in the volatile formation rate increases significantly with the external heat flux at low values of this. Indeed, because the temperature gradients are small, the size of the degrading region is large and maximum degradation rates are attained for heat-up conditions. For moderate external heat-transfer rates (Q ) 36-40 kW/m2), the maximum degradation rate is still attained when the reaction front enlarges to the whole particle, with values remaining roughly constant. Hence, the effects of higher degradation rates (higher temperatures) are in some way counteracted by the reduced size of the degrading region when heat-up conditions are attained (a successively thicker char zone exists). The larger spatial temperature gradients indicate that internal heat transfer is becoming successively more important, but fast surface degradation is still associated with a relatively thin region, unable to cause high volatile release rates. For high heat fluxes, as is already observed for the case discussed through Figure 1A,B, two regions appear in the weight loss character-


Figure 5. Conversion time, tc, drying time, td, and total mass fraction at the drying time, YVd, as functions of the radiative heat flux for dry and moist (U ) 11%) wood.

Figure 6. Pyrolysis temperature, Tp, and average particle heating rate, β, as functions of the radiative heat flux for dry and moist (U ) 11%) wood.

istics and are due to surface degradation and particle heatup, respectively. At low heat fluxes (e31 kW/m2) and U ) 11% (Figures 4 and 2B), the volatile formation rate initially presents a wide region of low values, due essentially to moisture evaporation. Then a maximum appears because of wood pyrolysis. As the external heat flux is increased, coupling between moisture evaporation and wood pyrolysis is enhanced and the volatile formation rate in the first region becomes progressively higher. For Q g 36 kW/ m2, the first region turns into a peak which, for Q g 49 kW/m2, overcomes the second. Thus, at high heat fluxes, weight loss characteristics are qualitatively similar for the dry and moist particles, though in the latter case the process is always slower. Heating Rate, Pyrolysis Temperature, and Characteristic Times. Other information on the pyrolysis of wood on the dependence of the external heating conditions for U ) 0% and 11% can be obtained from Figures5 and 6. Figure 5 reports the drying time, td, the conversion time, tc (the time when the global volatile formation rates reduces to 1/10 of the maximum value), and the volatile mass fraction measured in correspondence to the drying time, YVd. Figure 6 reports the pyrolysis temperature, Tp, and the average heating rate, β. It should be noted that other definitions of the conversion times, such as the times when 95% of the total gas mass has been released16 or when the centerline thermocouple attains a local maximum (termination of the exothermic degradation of the residue lignin), give rise to variations always below 5%. For all of the variables reported in Figures 5 and 6, values of Q of about 40 kW/m2 separate a region of strong dependence from a region of weak dependence. The particle surface

temperature achieved for steady state is maximum and is comprised of the range 600-750 K for the first region and 750-950 K for the second. Conversion times become successively shorter by increasing Q and, as was already observed, moist particles take longer to degrade (the difference between the two conversion times is roughly constant). For U ) 11%, the difference between the conversion and the drying time is initially high and then approaches a constant value. Indeed, at low external heat fluxes, the time needed to attain temperatures able to cause moisture evaporation (about 373 K) is significantly shorter than those needed for the temperatures of wood degradation (580-650 K). As the external heat flux is increased, associated with the larger spatial temperature gradients, the difference between the times needed to achieve moisture evaporation and wood pyrolysis is reduced and so is the difference between tc and td. The YVd curve shows values increasing with Q, confirming that coupling between drying and pyrolysis becomes successively stronger. However, the variation is again particularly high for Q e 40 kW/m2. The pyrolysis temperature increases with the applied radiative intensity from about 585 to 650 K. It becomes practically independent of Q for values greater than 40 kW/m2. This finding can be partly attributed to the sample heating conditions which are only weakly affected by external heat fluxes, for values above 40 kW/ m2. In particular, the Tp value of 650 K is comparable with that reported (673 K) in ref 10 for birch cylinders submerged in a downdraft gasifier with a gas temperature of 1173 K. In addition, it is known that cellulosic materials degrade in a narrow temperature range and complete wood carbonization occurs for temperatures below 700 K.21 This explanation seems to be corroborated also by values of Tp not affected by the presence of moisture at high Q values. On the other hand, at low Q values, Tp values are slightly higher for moist wood. Because the characteristic sample size is about 1 order of magnitude higher than those typically employed in fast pyrolysis5 and the conversion times are always longer than 300 s, conversion can be classified as conventional pyrolysis. Furthermore, the average heating rates roughly increase from 0.13 to 0.73 K/s with the external heat flux (against values of 100-1000 K/s for fast pyrolysis23,24). Again a region of rapid increase is followed by a tendency toward constant values, and moisture evaporation makes slower the heating process, especially for the higher radiation intensities. Indeed, while the pyrolysis temperature is about the same for the two cases and is practically independent of the heating conditions, the longer heating time plays a role successively more important as the thermal conditions are made more severe. The results presented through Figures 3-6 can be used to assess the relative importance of different processes in the pyrolysis of thick wood through the introduction of characteristic numbers as shown in Table 1 for oven-dry samples (differences with the case of ambient-dry samples are small). An evaluation of the importance of internal heat conduction time versus external heat-transfer time, based on the external radiation intensity, can be made by means of the Biot number Bi ) (ωQR)/(λ∆T) and the use of average values for the medium properties (ω ) 0.8, λ ) 1.4 × 10-1 W/mK, and temperature difference, ∆T, referring to the

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Table 1. Average Devolatilization Rate (db), k, and Characteristic Numbers, Bi and Th, for Several Applied Radiation Intensities and U ) 0 Q [kW/m2]

k × 103 [s-1]

Bi ) ωQR/λ∆T

Th ) kFscsR2/λ

27.5 31 33 36 40 49 69 79

0.29 0.42 0.54 0.67 0.81 1.07 1.44 1.51

10.42 10.80 11.28 12.04 12.63 15.40 21.30 24.50

0.44 0.65 0.83 1.03 1.25 1.65 2.20 2.30

pyrolysis temperature). As shown in Table 1, estimates lead to Bi values varying between about 10 and 25 for the range of heating conditions considered, indicating that the internal heat transfer is controlling and also becomes successively more important as Q is increased. The characteristic time associated with the internal heat transfer can be compared with the characteristic time of chemical reaction, through the thermal Thiele number Th ) (kFscsR2)/λ) (Fs ) 450 kg/m3, cs ) 1.25 kJ/ (kg K) and, k, the average devolatilization rate, defined as the ratio between the mass fraction of volatiles (db) and the conversion time). Computed values of Th (Table 1) vary from 0.44 to 2.3, and it can be understood that applied heat fluxes of about 40 kW/m2 roughly separate a region where the chemical reaction time is slow (sample temperatures are hardly sufficient for wood degradation) from another where the internal heat transfer becomes relatively more important. Finally, it should be noted that particle shrinkage, mainly along the radial direction, also occurs as degradation takes place. For the range of heat fluxes considered, the particle diameter is roughly reduced by factors of 15% and 20% for U ) 0% and 11%, respectively, though evaluations are difficult for Q g 40 kW/m2, because of significant structural failure. Reduction in the particle diameter causes faster internal heattransfer rates.22 Product Yields. Product yields (char, liquids, and gas) for U ) 0% and 11% are reported, on a mf basis, as functions of the radiation intensity in Figure 7. The mass closure is very good (95.5-99%), though some scatter in the data is present, because of structural failure of the samples. The differences between ovenand ambient-dry particles are small and not clearly detectable, being on the same order of magnitude as the errors in the mass balance closure. Again two regions are shown with product distribution highly dependent on the heating conditions for external heat fluxes below 40 kW/m2. When the external heat flux is increased, the final char yield decreases, as a result of a competition between charring and devolatilization reactions, which become successively more favored17 with larger liquid and gas yields. Maximum liquid yields attain about 55% of the initial dry wood mass. The high solid residue measured at low heat fluxes (temperatures) may be due to sample temperatures not sufficiently high to cause complete degradation of the ligninic components. On the other hand, it is well-known that the chemical composition of char is dependent on the reaction temperature;26 in particular, the C content increases as more severe thermal conditions are applied. Also, char conversion (for instance, combustion) is preceded by a rapid rate of nonoxidative weight loss,27 corresponding to further devolatilization caused by temperatures higher than those of char formation.

Figure 7. Char, gas, and liquid yields, expressed as a percent of the initial moisture-free (mf) mass, as functions of the radiative heat flux for dry (unfilled symbols) and moist (11%, filled symbols) wood.

Secondary reaction activity is considered to be negligible for temperatures below 750 K,5,23,24 independent of the reactor configuration. The kinetics of secondary reactions currently available also confirms the above statement as indicated, for instance, by the numerical simulations of single-particle pyrolysis.25 Hence, product distribution is essentially the result of the selectivity of primary wood decomposition reactions. For radiative heat fluxes in the range 45-80 kW/m2, only a slight decrease in the char and liquid yields is seen (Figure 7), with a consequent increase in the gas production. Chars tend to become constant (minimum values of about 23% wt, mf), because the variation in the actual wood degradation temperature (in relation to the external heating temperature) becomes very low. For the experimental conditions of this study, the activity of secondary degradation reactions is very small also for temperatures above 750 K, given that, after the attainment of a maximum, the liquid yields undergo barely visible variations. In principle, it is possible that secondary reactions are active at a certain extent within the pores of the sample. However, at high applied radiation intensities, structural failures occur as soon as a significant amount of volatiles are formed. Consequently, intraparticle residence times are very short because of both the high outflow velocities and the negligible resistance to mass transfer, while extraparticle residence times are also short, because of the forced nitrogen flow. In addition, there is also another important reason for a negligible activity of secondary reactions, that is, the low temperatures of volatiles compared with the values reported by thermocouples in strict contact with the solid phase. Indeed, measurements have been carried out of the volatile temperature by means of a thermocouple positioned below the bottom surface of the sample, just out of the irradiated zone. Because nitrogen is fed at ambient temperature, for Q ) 40-80 kW/m2 (sample temperatures above 750 K) maximum (steady) volatile temperatures are in the range 450-550 K. These temperatures, presumably attained by volatile pyrolysis products, as soon as they escape the hot charred region, are too low for any significant activity of secondary tar degradation. Hence, from the one side, the liquid curve does not show any appearance of significant secondary reaction effects and, on the other side, the relatively high (forced) velocities of the gas at a temperature much lower than that of the solid support and explain the dominant role of primary decomposition reactions, respectively, in the product distribution measured in this study.


Figure 8. Time profiles of the solid mass fraction and the global volatile formation rate for wood exposed to Q ) 27.5 kW/m2 and several initial moisture contents.

The pyrolysis gas consists mainly of CO2 (the largest contribution comprised between 8 and 10.5%, mf basis), CO (values increasing from about 3 to 6%, mf), CH4 (values in the range 0.5-1%, mf), and lower amounts (below 0.2%, mf) of H2 and C2 hydrocarbons. At low temperatures, the evolution of CO and CO2 (and water vapor) is due mainly to the degradation of hemicellulose (decomposition temperature range 498-598 K under thermogravimetric conditions) and the activity of the first path in cellulose degradation (temperatures below 573 K), leading to gas and char formation.28 As the temperature increases, the formation of tar vapor from cellulose becomes predominant, while also lignin degradation begins (decomposition temperature range 523773 K, with the fastest rates attained for values of 583693 K, again for thermogravimetric conditions). The last process is largely responsible for char formation and also for the evolution of CO2, CO, CH4, and H229 (the last two species at high temperatures). The dependence of char yields on the applied heat flux (temperature) is in agreement with previous studies conducted for large particles (for instance, refs 11, 13, and 19) and, from the qualitative point of view, also for small particles undergoing fluid-bed pyrolysis.23,24 As for the volatile product distribution, it should be noted that there is an important difference between this study and previous literature. As was already observed, in this configuration secondary reaction activity is negligible. In other studies, the one-dimensional heating, a wood cylinder heated on the top surface,11,13 the slow external heating rates,20 or the reactor configuration23 allow residence times sufficiently long for tar degradation at temperatures above 750 K. Consequently, the decrease in the liquid yield (and the corresponding increase in the gas) with temperatures is comparatively much larger. The qualitative dependence is, however, the same. Influences of High Moisture Contents. Variation in the process dynamics with the moisture content for low and high external heat fluxes (27.5 and 49 kW/m2, respectively) can be observed from the time profiles of the total mass fraction and the global rate of volatile formation reported in Figures 8 and 9. For both heating conditions, as the initial moisture content is increased, the temperature plateau, caused by endothermic moisture evaporation, is enhanced, that is, its time and space extension increases. Also, the wood heating rates become successively slower, because of both the successively higher amount of water to be heated and evaporated and the higher amount of heat convected out from the particle. In addition, because of the successively

Figure 9. Time profiles of the solid mass fraction and the global volatile formation rate for wood exposed to Q ) 49 kW/m2 and several initial moisture contents.

Figure 10. Conversion time, tc, drying time, td, and total mass fraction at the drying time, YVd, as functions of the initial moisture content for Q ) 27.5 kW/m2 (solid lines) and Q ) 49 kW/m2 (dashed lines).

longer times needed for complete drying, temperatures high enough for degradation to begin are reached in the more external particle layer also for the low heat flux case. Hence, when the initial moisture content is increased, moisture evaporation and wood pyrolysis tend to become successively more coupled. Weight loss dynamics of low-temperature wood pyrolysis shows a slow change from a one-stage global process (U ) 0%) to a two-stage process (U > 0%). The first stage, related to moisture evaporation, presents global moisture release rates increasing with the moisture content. Then, for U ) 29% and above, a slow rate region of increasing extension appears, resulting from moisture evaporation and degradation of the more external particle layer. Finally, degradation of the already dried wood takes place. The maximum global degradation rate for this stage decreases as the moisture level is increased because of a successively reduced size of the volume particle where drying precedes pyrolysis. As was already discussed, for high external heat fluxes, drying and pyrolysis take place simultaneously. The apparent devolatilization kinetics is always characterized by two peaks, which decrease and are separated by a successively larger region of low values as the initial moisture content is increased. Figure 10 reports the conversion times, tc, the drying times, td, and the total mass fraction for the time of complete drying, YVd (definitions as in Figure 5), as functions of the initial moisture content for Q ) 27.5 and 49 kW/m2. Independent of the external heating conditions, the characteristic process times increase almost linearly with U, though the slope is higher for the lower heat flux. It is plausible that this is due to the low temperatures attained in this case, barely

880


Figure 11. Pyrolysis temperature, Tp, and average particle heating rate, β, as functions of the initial moisture content for Q ) 27.5 kW/m2 (solid lines) and Q ) 49 kW/m2 (dashed lines). Table 2. Average Devolatilization Rate (db), k, and Characteristic Numbers, Th and H, for Several Initial Moisture Contents and Q ) 49 kW/m2 U [%]

k × 103 [s-1]

Th ) kFscsR2/λ

H ) βFscsR/ωQ

0 11 27 47

1.07 0.78 0.61 0.47

1.65 1.20 0.94 0.72

0.15 0.11 0.08 0.06

sufficient for wood degradation to take place. Also, the difference (tc - td) remains roughly constant. The volatile mass fraction released at the drying time also presents a linear dependence on the moisture content, confirming the increased coupling between the two processes, though this effect is again slightly more important for the lower heat flux. Figure 11 reports the pyrolysis temperature, Tp, and the average heating rate, β, as functions of the initial moisture content for Q ) 27.5 and 49 kW/m2. For both cases, the pyrolysis temperature tends to become slightly higher (barely visible for the high heat flux when Tp is about 650 K), presumably because the successively longer times required for the evaporation process allow higher temperatures to be attained in the final process stage, when Tp is measured. Indeed, as was already observed from the comparison between the oven- and ambient-dry wood, the particle heating rate is significantly slowed by the presence of moisture. For both heat fluxes, β is almost halved as U increases from 0 to 50%. Reductions in the particle diameter (with respect to the initial value of the moist particle) correspond to about 20% for all of the moisture contents examined. The influence of moisture evaporation on thick wood pyrolysis can again be evaluated by means of characteristic numbers. However, because definitions are based on average property values and moisture influences appear mainly as a strong increase in the conversion (heating) times, instead of the Bi number, another number is introduced as H ) βFscsR/ωQ, that is, a ratio between the characteristic external and internal heating times. The values of Th and H (Table 2, Q ) 49 kW/m2) confirm that, by increasing the moisture level, wood degradation becomes successively more difficult (with values of Th below 1 for U ) 47%) and internal heat transfer slower (H is more than halved as U varies from 0 to 47%). Product yields, reported in Figure 12 as functions of the initial moisture content, remain almost unvaried for both of the two heating conditions considered (negligible variations are also seen in the gas composition). In general, char yields slightly increase at the expense of liquid yields, as a consequence of the reduction in the

Figure 12. Char, gas, and liquid yields, mf basis, from beech wood pyrolysis as functions of the initial moisture content for Q ) 27.5 and 49 kW/m2.

particle heating rate. These effects are less evident for the low heat flux case, because of the weaker coupling between moisture evaporation and wood pyrolysis. The increase in the process characteristic times with the initial moisture content has already been observed in previous experimental studies10,11 and also theoretically predicted.30,31 The dependence of product yields on the initial moisture content for thick wood pyrolysis has been examined, at a certain extent, only in ref 11. A minimum in the char yield for moist particles was found at intermediate heating rates (heat fluxes). It was attributed to a reduction in the secondary char formation (tar polymerization). As was already observed, secondary reaction activity is negligible in the experimental system used here. Hence, the results of the two studies cannot be compared. Conclusions and Further Developments The pyrolytic behavior of thick wood has been investigated for conditions of negligible secondary reaction activity, by varying the applied radiation intensity and the initial moisture content. New process features in relation to both issues have been observed. The average heating rates undergone by the particle, for external radiation intensities comprised between 27.5 and 80 kW/m2, are those of conventional pyrolysis (for instance, 0.17-0.75 K/s for dry wood). The actual pyrolysis temperature only increases in a narrow range of about 585-650 K. Moreover, variations with the external heating conditions are observed only for heat fluxes of 27.5-40 kW/m2 (steady particle temperatures comprised between 600 and 750 K). In this region, internal heat-transfer rates are always slower than external heat-transfer rates (Biot numbers comprised between about 10 and 13). Moreover, temperatures attained during the unsteady heating are barely sufficient for wood degradation, so that the characteristic pyrolysis times and those of inward heat transfer are roughly comparable (thermal Thiele numbers comprised between 0.5 and 1.2). Both gas and liquid yields increase with the reaction temperature, at the expense of char. For dry wood, the maximum liquid yield is about 55% (mf). Though temperature profiles show the existence of two different zones, the outer particle layer dominated essentially by heat-transfer processes and the inner core by reaction energetics, global devolatilization kinetics is a one-stage process. Increasing the external heating rate (heat fluxes above 40 kW/m2 and particle surface temperatures above 750 K) only causes faster average particle heating


rates and shorter conversion times. Given the constant pyrolysis temperature, variations in primary product distribution are small. The increased internal versus external heat-transfer resistance gives rise to a twostage devolatilization process (Biot numbers increase from about 13 to 25), with pyrolysis rates also becoming faster than internal heat-transfer rates (thermal Thiele numbers increasing from about 1.2 to 2.3). The influence of the initial moisture content on the pyrolysis characteristics of thick particles has been examined at low and high external heat-transfer rates (externally applied heat fluxes of 27.5 and 49 kW/m2). For low external heat fluxes (slow particle heating rates) moisture evaporation and wood pyrolysis are sequential processes. As the external heat flux is increased and spatial temperature gradients become significant, the two processes occur simultaneously, though localized at different stations along the particle radius. From the qualitative point of view, the same behavior is observed when the initial moisture level is increased (for a given external heat flux). In accordance, global weight loss characteristics also show a transition from a one-stage process (low moisture contents and/or low heat fluxes) to a two-stage process. The influence of moisture evaporation is small on the actual pyrolysis temperature and consequently also on primary reaction selectivity and product yields. On the contrary, convective cooling is enhanced. This, together with the endothermicity of moisture evaporation, causes a linear increase of the conversion time with the initial moisture content. It is worth noting that this effect is already important at low moisture levels (10-11%, db). The lowest temperature investigated in this study (600 K) represents a superior limit for countercurrent gasification, in that the reactor exit temperatures are in the range 400-600 K4,14 and, given the slow heating rates, the different processes (drying, pyrolysis, gasification, and combustion) are truly stratified along the reactor axis. Successive experimental analyses should examine lower external temperatures, to simulate single wood particle drying in the upper section of updraft gasifiers. On the other hand, all of the higher temperature results discussed in this paper are applicable for the faster external heating rates established in the “flaming pyrolysis” section of concurrent gasifiers.10,15 However, it should be kept in mind that the present findings are related to intraparticle processes. The presence of steam in the gas/vapor mixture evolved from wood degradation in fixed-bed gasifiers, where extraparticle volatile residence times are clearly nonnegligible, is important in relation to char gasification, tar reforming, and equilibrium of the water-gas shift reaction. Wood pyrolysis modeling can give significant contributions for process design and development. Indeed, detailed transport models have been developed (for instance, unsteady two-dimensional models such as that of ref 32), but there are two critical points, which still deserve further developments/improvements: (1) extensive model validation, through experimental data produced under exactly known conditions (properties of feedstock and heating conditions) and (2) determination of correct input data (especially property values, kinetic constants, and heat- and mass-transfer coefficients). The first point is partially addressed in this study, through measurements of the main variables of interest for thick wood pyrolysis. As for the second point, it has been

shown22,33 that the few semiglobal mechanisms of wood pyrolysis currently available are unable to predict the dependence of primary product yields on temperature even from the qualitative point of view. Therefore, in the first place, reliable kinetic mechanisms should be made available for coupling the chemistry of wood pyrolysis with the description of transport phenomena, and then model validation can be accomplished. Acknowledgment The research was funded in part by the European Commission through the project ALFA N.5.0075.9. Nomenclature cs ) heat capacity [kJ/(kg K)] dYV/dt ) global volatile formation rate [s-1] k ) (YL + YG)/tc ) average devolatilization rate (db) [s-1] Q ) applied radiative heat flux [kW/m2] R ) sample radius [m] r ) radial coordinate [m] T ) temperature [K] Tr ) steady temperature at the particle centerline [K] Tp ) pyrolysis temperature [K] T0 ) initial temperature [K] t ) time [s] tc ) conversion time [s] td ) drying time [s] th ) heating time [s] U ) initial moisture content (db) [kg/kg] Y ) total mass fraction YG ) gas mass fraction YL ) liquid mass fraction YVd ) volatile mass fraction for t ) td β ) (Tp - T0)/th ) average particle heating rate [K/s] ∆T ) Tp - T0 ) temperature difference [K] λ ) thermal conductivity [W/mK] Fs ) solid density [kg/m3] ω ) surface emissivity

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Received for review September 30, 1999 Revised manuscript received January 10, 2000 Accepted January 11, 2000 IE990720I

Radiative Pyrolysis of Single Moist Wood Particles - American

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