Experimental Analysis of Reaction Heat Effects during Beech Wood

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Experimental Analysis of Reaction Heat Effects during Beech Wood Pyrolysis C. Di Blasi,*,† C. Branca,‡ F. Masotta,† and E. De Biase† †

Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy ‡ Istituto di Ricerche sulla Combustione, C.N.R., P.le V. Tecchio, 80125 Napoli, Italy ABSTRACT: The pyrolytic behavior of a cylindrical packed bed of beech wood pellets, uniformly heated along the lateral surface, is studied. As the heating conditions are varied, the trends shown by the mass loss rate and the yields of the lumped product classes are in agreement with previous literature. Moreover, detailed measurements of the thermal field permit, for the first time, the observation at the central core of the bed, of sequential (a) exothermic, (b) endothermic, and (c) exothermic effects. The examination of the corresponding temperature ranges indicates that they are linked, in the order, to the (primary and secondary) degradation of (a) hemicellulose, (b) cellulose and lignin, and (c) lignin. The dependence on the heating conditions is also explained of the center temperature overshoot (with respect to the steady value) which attains maximum values around 90 K.

1. INTRODUCTION Evaluations of the “heat” or enthalpy of wood and biomass pyrolysis, which have important implications from the practical side, are complicated because simple calculations based on the enthalpies of formation of the starting material and those of the process products are impractical.1 On the other hand, the literature points out widely different values ranging from exothermic to endothermic, as already discussed in the review presented by Roberts2 in the 1970s. A significant number of experiments has been carried out with very small sample mass of wood or biomass in order to eliminate the influence of physical processes. However, measurements are highly dependent on the experimental conditions; in particular, the factors affecting the extent of secondary reactions of tar vapors, with results that are not only quantitatively but also qualitatively different among various authors.3−10 From a conceptual point of view, the large differences in the reaction heats can be easily explained by observing that pyrolysis of lignocelulosic materials, and their components consist of a significant number of primary and secondary reactions whose activity mainly depends on reaction conditions (temperature, pressure, flow rate), biomass chemical composition, nature of chemical components, and presence of catalytically active ash.11 Therefore, changes in the reaction mechanism and/or extent of the various reactions give rise to consequent modifications in the global energetics of the process. Mok and Antal12 in their differential scanning calorimeter (DSC) analysis of cellulose and levoglucosan pyrolysis establish some important points in relation to process energetics. Formation of char and gas is an exothermic process, whereas levoglucosan (tar) formation and evaporation are endothermic processes. Secondary reactions are exothermic, as evidenced by varying the extent of their activity with modifications in the gas flow rate and pressure. Thus, high pressure and low flow rate reduce the heat of pyrolysis and increase the yields of char © XXXX American Chemical Society

which is also produced from the activity of secondary tar reactions both in the gas and the condensed phase.13 More precisely the endothermic heat of pyrolysis linearly decreases as the char yield increases corresponding to exothermic formation of this product of about −3600 J/g (of char formed). Successive studies, based on TG-DSC analysis, have confirmed this important finding, although with some quantitative differences, as shown by the heat of pyrolysis versus the char yield reported in Figure 1. The exothermic formation of char is evaluated as −2000 J/g for cellulose (with reaction endothermicity estimated as 538 J/g of volatiles evolved),1 −3525 or −3827 J/g for beech or spruce wood (with corresponding heat evolved from volatile production of 936 and 1277 J/g),7 and −940 J/g for artichoke thistle9 (this is evaluated from the linear regression of the total reaction

Figure 1. Pyrolysis reaction heat versus the mass fraction of char produced as evaluated1,7,9,12 for various materials. Received: January 29, 2013 Revised: April 17, 2013

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dx.doi.org/10.1021/ef4001709 | Energy Fuels XXXX, XXX, XXX−XXX

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enthalpy versus the char yield, which also provides a reaction heat of about 270 J/g of volatiles formed). The role of the reaction heat is important in practical conversion systems, dominated by the interaction between chemical reactions and heat and mass transfer phenomena. In reality, these effects may be critical for processes where a careful control of the reaction temperature is required to obtain products with improved properties, such as in torrefaction,14 or in high yields, such as in catalytic pyrolysis.15−17 Early studies, dated at the end of 1800 and the beginning of 1900, carefully reviewed by Antal and Gronli,18 already evidenced the exothermicity of wood carbonization above 553 K, evaluated the contribution as about 6% of the HHV of wood (a figure around 1000 J/g, that compares well with those more recently reported) and, in some cases, indicated that, without exothermicity, the process would not be practical. Exothermic effects are clearly observed at the sample center during the pyrolysis of thick samples/beds uniformly heated along the external surface19−33 in some cases with significant temperature overshoot with respect to the steady value. However, a systematic study is not available providing a quantitative dependence of exothermic effects on the heating conditions. Also, the various authors do not concord about the causes of these effects. In this study, a macro-TGA system with a fast heating furnace is applied to study the effects of the heating conditions on the conversion of thick packed beds of beech wood pellets, uniformly irradiated along the lateral surface. The actual heating and conversion conditions of the bed reproduce those of the pyrolysis zone of downdraft and updraft gasifiers and, in general, of fixed-bed reactors used in pyrolysis and torrefaction. The main scope of the study is to produce a comprehensive information about reaction heat effects during pyrolysis, for conditions of practical interest, and possibly to associate such effects with a sound sequence of chemical events.

Figure 2. (A) Schematic of the experimental system: (1) furnace, (2) quartz reactor, (3) sample and ballast, (4) precision balance, (5) damping device, (6) thermocouples, (7) acquisition data set, (8, 9) water/ice condensers, (10) cotton trap, (11) silica gel bed. (B) Top section of the packed bed. understood that, due to the resistance offered by the porous bed, the stream will partly flow across the external annular space, giving rise to variable residence time of volatile pyrolysis products. Also, due to contact of the cold nitrogen with the hot bed surface and mixing with hot volatile pyrolysis products, the temperature of the gaseous stream may increase significantly with respect to the ambient value. Temperature and residence time affect the extent of secondary degradation reactions. A nominal (ambient conditions and absence of sample) gas velocity of 1.3 cm/s has been employed for all the tests, which corresponds to volatile residence times along the heated section of about 5 s. This is a nominal time, often used in pyrolysis, but actual times are shorter owing to the effects of temperature variation, sample volume and mass addition caused by volatile pyrolysis products. 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. The first set of measurements, in addition to providing the details of volatile species release, can also be used to determine the conversion time. This is defined as the time when 95% of the total volatile mass is released. Temperatures along the particle radius, r, at the median section are continuously monitored (0.5 mm bead chromel-alumel thermocouples) at five positions, starting from the center (r = 0, 0.5, 1, 1.5, and 1.9 cm). The composition of the gas is analyzed, at selected times, through a gaschromatograph equipped with a TCD and a packed column. 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. The liquids are collected through a condenser train, consisting of water/ice cooled traps, cotton wool demisters, and a silica gel bed. All the condensable products collected and weighed from the traps (organic compounds (tars) and product water formed) are indicated as “liquids”. 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.”

2. EXPERIMENTAL SECTION The system applied for the pyrolysis of packed beds of beech wood pellets reproduces that used in previous experiments23−27 and is shown schematically in Figure 2A. The furnace, manufactured by Research Inc., presents four tubular quartz infrared lamps with a tungsten wire filament that emits radiant energy in proportion to the applied voltage. Elliptical, polished aluminum, water-cooled reflectors focus the high-density infrared energy, emitted by lamps, onto a cylindrical-shaped target area (diameter 6.5 cm). The furnace, equipped with a PID controller and a transducer (SCR), allows a constant radiative heat flux to be emitted. To avoid interaction between the volatile pyrolysis products and the lamps, a quartz tube (internal diameter 6 cm), transparent to infrared radiation, is located inside the furnace and used as a reaction chamber. A cylindrical packed bed (diameter and height of 4 cm) is vertically positioned in the uniformly heated zone of the reactor, through a suspension system and a damping device,34 which is connected to a precision balance. The bed holder is made by a stainless steel wire mesh (400 μm) basket. A metallic (brass) cylinder (about 167 g with length 2 cm and diameter 3.5 cm) is hung on the bottom of the sample (at about 5 cm distance) and works as ballast, to facilitate alignment and stabilization. The sample is exposed to a constant radiative heat flux along the lateral surface. For each chosen radiation intensity, steady temperatures of the radiant heater are achieved within a few seconds (maximum heating rates of about 750 K/s, evaluated by observing that the maximum temperature of the radiant elements, 2500 K, is achieved in about 3.3 s),35 but given the thick sample, pyrolysis takes place under heat transfer control. Nitrogen flows from the top of the quartz reactor to establish the proper reaction environment and to reduce the extra pellet residence time of volatile pyrolysis products. However, it can be B


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The material consists of beech wood pellets (chemical composition: 2% extractives, 20% lignin, 45% cellulose, 33% hemicellulose, and 0.2% ash)36 prepared in the shape of disks with 0.5 cm thickness and diameter. They are randomly packed inside the sample holder as shown in Figure 2B (top section of the bed), as these conditions are more likely to be established in practical applications. The initial moisture content (about 7%) is eliminated by oven drying at 373 K for at least 12 h followed by exposition to a forced air flow at about 393 K for additional 30 min before the experiment. However, the sample may acquire some moisture during the time interval required for positioning in the quartz reactor and establishing the inert reaction environment. The packed bed (sample mass 28 g) presents a bulk density of 0.56 g/cm3 (versus a density of 1.04 g/cm3 of the pellets) corresponding to a void fraction of 0.46. Experiments are perfectly reproducible in relation to weight loss characteristics and yields of products (typical mss closure around 97− 99%). The temperature recording may be affected by thermocouple positioning and shrinkage of the bed following thermal decomposition. Deviations, evaluated over a set of five experiments at an intermediate external heat flux, vary from a maximum of ±5 K, for the more external thermocouple, to a minimum of ±2 K for the sample center. Measurements also indicate that, due to the uniform heating along the lateral surface, the temperature distribution is symmetric with respect to the bed axis. Temperature measurements at different bed heights have not been made because of difficulties deriving from bed shrinkage especially at high heat fluxes. However, given that the sample is positioned in the zone of uniform heating of the furnace (80% of the total length35), gradients may only be induced by variations in the convective heat exchange and are expected to be small. Finally, it should be observed that, due to random packing, the temperature measurements, at the various radial positions, cannot be attributed to single pellets but are representative of the average bed values. Experiments have been made by varying the intensity of the external radiative heat flux in the range 20−45 kW/m2, which corresponds to steady temperatures of the bed at r = 1.9 cm approximately between 510 and 815 K. However, as the more external thermocouple is prone to the largest experimental uncertainty, also due to possible contact with the sample holder, the steady temperature recorded at r = 1.5 cm and indicated in the following as heating temperature, Ts, is used to identify the heating conditions. For the range of heat fluxes, it varies between 500 and 806 K. Finally, the observation times coincide with the conversion times for Ts values of 600 K and above, whereas, for lower values, they are limited to 3600 s.

Figure 3. Time profiles of solid mass fractions and time derivatives of solid mass fraction for the packed bed of beech wood pellets exposed to several radiation intensities corresponding to heating temperatures, Ts, indicated in the plot (external heat fluxes in the range 20−45 kW/ m2).

Figure 4. Yields of the lumped product classes from the pyrolysis of the packed bed of beech wood pellets as functions of the heating temperature, Ts (external heat fluxes in the range 20−45 kW/m2).

detected in correspondence of the enlargement of the reaction zone to a large part of the bed (low Ts) or the rapid degradation of a surface layer (high Ts). The maximum liquid yields (around 60 wt %) are obtained for heating temperatures above 730 K and maintained up to the maximum value investigated. 3.2. Dynamics of the Thermal Field. The changes in the thermal field of the bed associated with successively higher external heat fluxes are shown by means of the time profiles of the temperature, at several radial positions, the time derivative of the bed center temperature, the solid mass fraction, and the corresponding rate of weight loss in Figures 5, 6, 7, and 8. In accordance with previous experimental analysis,19−33 the most peculiar features of the temperature dynamics are shown at the bed center. The cylindrical sample, heated along the lateral surface, experiences continuous inward transfer of heat, along the radial coordinate, essentially due to conduction. The center corresponds to a symmetric (adiabatic) condition, which allows the effects of the reaction energetics to become more evident. In fact, the thermal field of the packed bed can be schematically represented by an external zone in the shape of a hollow cylinder, where temperature gradients are high and an internal cylindrical core, where spatial variations are much lower. As the central zone of the bed approaches the condition of a perfectly mixed reactor, interesting information on the process dynamics can be obtained by observing the temporal profiles of the

3. RESULTS AND DISCUSSION As anticipated, experiments have been made by exposing the packed bed at various heat fluxes (20−45 kW/m2), which result in heating temperatures, Ts, of 501−806 K. Temperatures, at several spatial positions, weight loss curves, yields of the lumped classes of pyrolysis products (char, gas, condensables) and yields of gaseous species have been determined. 3.1. Weight Loss and Product Yields. The main characteristics of the pyrolysis process are summarized through Figures 3 and 4, which report the integral and differential weight loss curves versus time, at various heating conditions, and the product yields versus the heating temperature. The results reproduce trends already known from previous literature11,37 but are useful to understand how the reaction occurrence and related energetic aspects are linked to the thermal field to be discussed. As expected, the process duration is progressively reduced together with the yields of solid product as the heating conditions are made more severe. Given the large characteristic size (bed diameter), the rates of weight loss do not permit the identification of the various reaction zones reported in thermal analysis. Instead, as detailed in the following, they are strictly related to heat and mass transfer phenomena. In particular, the maximum rate of weight loss is C


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Figure 5. Temperature profiles at several radial positions and weight loss characteristics (A) and profiles of the center temperature, Tc, and the corresponding time derivative, dTc/dt, (B) versus time for the packed bed at a heating temperature, Ts, equal to 565 K (external heat flux equal to 21.5 kW/m2).



Figure 8. Temperature profiles at several radial positions and weight loss characteristics (A) and profiles of the center temperature, Tc, and the corresponding time derivative, dTc/dt, (B) versus time for the packed bed at a heating temperature, Ts, equal to 800 K(external heat flux equal to 43.1 kW/m2). D


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center temperature, Tc, and the corresponding time derivative, dTc/dt. In all cases, convective heat losses should also be taken into account due to both the forced nitrogen flow, important even for the steady temperature profiles of the char bed, and the release of volatile pyrolysis products. The sizes of the two spatial zones highly depend on the intensity of the external heat flux and the properties of the bed. In the first zone, the different pyrolysis reactions take place at very close spatial positions. The appearance of reaction energetics effects through the temperature profiles is hindered because (1) endothermic and exothermic effects tend to counteract each other and/or (2) the effects of heat transfer predominate over those of reaction heat. Instead, as already observed, spatial gradients are much smaller in the central cylindrical region of the sample, which is heated more slowly than the outer zones. The achievement of conditions of an almost perfectly mixed reactor for this zone which, for the symmetry condition, does not encounter any inward conductive heat transfer, permits a better visualization of the reaction heat effects on the temperature profile. In other words, in this zone, the temperature profile is almost devoid of heat conduction effects. For heating temperatures below 550 K, only heat transfer effects are evident from the temperature measurements although degradation occurs at some extent, as indicated by the yields of products reported in Figure 3. It can be speculated that pyrolysis is occurring at so low rate that the effects of the related reaction heats do not explicitly appear on the temperature profiles or that the reactions mainly responsible for such effects are not yet active. Exothermic effects start to become visible on the measured temperature profiles for Ts = 560 K and present qualitative features that are well represented by the example shown in Figure 5 (Ts = 565 K, external heat flux equal to 21.5 kW/m2). All the thermocouples show an almost linear increase followed by a more rapid rise and an overshoot with respect to the steady values, except for the more external position. For this, the increase is much more rapid and the maximum is barely visible. The attainment of maximum temperatures at about the same time for radial coordinates between 0 and 1.5 cm testifies a wide extension of the degradation zone. Indeed, as a consequence of the low external heating temperatures, the spatial gradients are small and the size of the almost uniform central core, previously introduced, approximately coincides with the entire bed. The examination of the time derivative of the bed center temperature shows a first zone (temperatures below 500 K) of inert heating and possible evaporation of the small moisture content where a maximum, due to rapid surface heating, is followed by a decrease caused by the formation of an insulating char layer. More interesting is the zone for temperatures above 500 K, characterized by another local maximum that precedes the attainment of the maximum temperature. This increase in the heating rate can be associated with the onset of exothermic degradation reactions that, for the mild thermal conditions of Figure 5, take place for temperatures approximately between 500 and 550 K. The beginning of the zone of maximum heating rate and the maximum are used as characteristic points, in accordance with the indications reported in the plot (THi, hHi, tHi, THm, hHm, tHm). It can also be seen that the time of the maximum heating rate, tHm, is slightly anticipated with respect to the peak in the mass loss rate (and the maximum temperature). For this time, the amount of released volatiles is about 40% of the total value. Consequently, at least for very

low heating temperatures, the evident exothermic effects of chemical reactions are associated with high mass loss rates. These qualitative features in the process dynamics, in particular, those of the bed center, are maintained for heating temperatures up to 600 K. As the external heating conditions are made more severe, spatial gradients of temperature tend to increase so that the effects of the reaction energetics remain visible over a successively thin core of the bed although the temperature overshoot increases. An example for intermediate heating temperatures is shown in Figure 6 for Ts = 645 K (external heat flux equal to 26.5 kW/m2). The more external zone of the bed is heated very rapidly, whereas for the more internal zone, after an almost linear increase where chemical reactions are not active, the temperature shows some changes in the rising rate to attain a maximum well above the final steady value. More precisely, the examination of the bed center temperature and heating rate shows more complex dynamics than the case previously discussed. Apart from the similarity in the qualitative features for temperatures below 500 K, for higher temperatures, a first maximum is again observed (temperatures of about 500− 600 K), followed by a well visible minimum (temperatures around 640 K) and then by another maximum (temperatures of 673 K). Characteristic temperatures (THi, THm, TCm, TLm), heating rates (hHi, hHm, hCm, hLm), and times (tHi, tHm, tCm, tLm) are again defined, as indicated in the plot (the beginning of the first peak rate, the first peak, the plateau, and the second peak). It can be assumed that, at the center of the bed, peaks or valleys in the heating rate can be associated with exothermic reactions or endothermic reactions or, in the latter case, with the termination of exothermic processes. The weight loss rate provides important information in relation to the amounts of volatile products released in correspondence of the characteristic points in the center bed heating rate, in particular the peaks. For the time tHm (first peak), the amount of volatile products released is around 68% of the total yield (Figure 6A) (versus 40% of the case previously discussed, Figure 5A). Thus, exothermic effects are again observed when mass loss rates are still high. Instead, for the time tLm (second peak rate), the rate of mass loss is very small. Upon a further increase in the external heating temperature, apart from the obvious increase of the spatial gradients and the reduction in the size of the central zone, the temperature overshoots start to decrease (Figure 7, Ts = 730 K and external heat flux equal to 33.1 kW/m2) to finally almost disappear (Figure 8, Ts = 800 K and external heat flux equal to 43.1 kW/ m2). In particular, the thermal field of Figure 8 shows a barely visible maximum only at the bed center, but an acceleration in the heating rate is still clearly evident. The features in the profiles of Tc and dTc/dt remain qualitatively similar to those already presented for the case of Ts = 645 K; that is, for temperatures above 500 K, they testify the existence of two exothermic zones separated by an endothermic zone. From the quantitative point of view, the second peak in the heating rate becomes higher than the first one, plausibly resulting from a higher exothermicity of the related chemical reactions (although, hLm, observed for Ts = 800 K, is lower than that observed for Ts = 730 K). The characteristic temperatures also remain approximately in the same ranges individuated. The times tHm are still associated with significant mass loss rates, but the amount of volatile products already released becomes higher (about 80 and 85% of the total yields for the two heating E


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temperatures examined). Finally, the times tLm are again associated with the tailing zone of the mass loss rate. It should be observed that none of the previous investigations about the pyrolysis of thick wood samples/beds provides such detailed information about the heating dynamics of the sample center. Moreover, an univocal and sound explanation for the observed exothermic effects is not available from previous literature although several hypotheses have been made such as (a) formation of char from the decomposition of the chief chemical components of lignocellulosic fuels, supported by the findings summarized in Figure 1; (b) structural ordering of char in a more stable form;31 (c) degradation of the hemicellulose32 or the lignin22−28,30 component; and (d) activity of secondary reactions of tar conversion (cracking and polymerization) justified by the exothermicity of this process.2,12 On the other hand, the plateau in the temperature profile is generally attributed to cellulose or holocellulose degradation. The shape of the center temperature profiles obtained in this study indicates that, independently from the reactions leading to the formation of the lumped product classes, the logical sequence of exothermic char formation at low temperature and endothermic volatile (tar) formation at high temperature is not valid as a clear alternation between exothermic and endothermic zones is shown. In other words, the dynamics observed in this study cannot be described by the one-stage, three-step mechanism simply consisting of the competitive formation of the lumped classes of pyrolysis products (gas, char, condensables), that has been extensively used in pyrolysis modeling (for instance, see refs 38−42.). It is also unlikely that char rearrangement can be associated with the strong exothermic effects usually observed for thick samples. In their model for the pyrolysis of thick particles, Park and co-workers31 assume that primary degradation of wood takes place according to competitive reactions leading to endothermic formation of tar and a thermally neutral formation of gas and an intermediate solid. It is then postulated that the exothermic complete transformation of the intermediate solid into char is the origin of the rapid temperature rise. However, this mechanism is not supported by any specific experimental evidence, and the devolatilization of chars produced at different thermal conditions, studied in a thermogravimetric system with accurate temperature measurements,43,44 do not show any evident exothermicity. 3.3. Characteristic Temperatures and Heating Rates. A novel explanation of the sequential thermal effects observed in this study can be based on the energetics of the (primary and secondary) degradation of the main wood components, instead of the degradation of either the hemicellulose or the lignin for the exothermic effects as previously hypothesized. To assign the characteristic zones of the heating rate of the bed center to the degradation reactions of each component, the characteristic temperatures, defined through Figures 5B, 6B, 7B, and 8B, are plotted versus the heating temperature in Figure 9. It can be also useful to recall that xylan, a model compound for hardwood hemicellulose, typically degrades in the range 500− 600 K,45 as also confirmed for the pseudocomponent hemicellulose by kinetic models of beech wood pyrolysis.46,47 Cellulose typically degrades in the temperature range 600−650 K,48 whereas lignin gradually degrades for temperatures of 523−773 K although the large part of the process takes place between 600 and 750 K.47 Figure 9 shows that THi is practically independent of the heating temperature and assumes a value

Figure 9. Characteristic temperatures (THi, THm, TCm, TLm), as defined through Figures 5B, 6B, 7B, and 8B, and maximum temperature, Tmax, of the packed bed versus the heating temperature, Ts.

around 510 K. The temperature in correspondence of the first peak in the heating rate, THM, after a slight increase tends to a constant value around 600 K for heating temperatures of 600 K and above. Based on the range of decomposition temperatures reported, it can be reasonably concluded that the first exothermic contribution is essentially due to hemicellulose degradation.4,5,49,50 Moreover, the range of THi and THm values supports the speculation that both the first and second stage of hemicellulose degradation45 occurs exothermally, in contrast with the hypothesis that only the second reaction stage is exothermic.33 The temperature TCm also shows an initial barely visible increase (heating temperature of 600 K) to approach a constant value around 650 K. The second exothermic zone is characterized by peak temperatures, TLm, that after an initial increase from about 650 K reach a constant value around 730 K. Again, based on the ranges of reaction temperatures given, it can be speculated that, over the range of TCm values, the degradation of both cellulose and lignin occurs simultaneously resulting in a process that is globally endothermic, most likely resulting from a predominance of the global endothermicity of cellulose degradation,2,4,5,8,45,50 also enhanced by convective cooling due to the large production of volatile species,51−53 over the exothermic degradation of lignin.2,4,5,22,45,50,54 Then, over the range of TLm values, the sole exothermic degradation of the residual lignin fraction takes place. It can be understood that the decomposition of the three components tends to overlap as the heating temperature (and consequently the heating rate) is increased making more difficult the attribution of variations in the heating characteristics to specific chemical processes and introducing some scatter in the measured variables (e.g., the hLm values of Figures 7B and 8B). The association of the sequence of thermal events observed at the bed center with the degradation of the main wood components, as hypothesized, is also supported by the corresponding mass loss rates. Indeed, high values are observed for the time of the first maximum, tHm, as the degradation of cellulose, close to that of hemicellulose, mainly gives volatile products. The amount of volatile mass released, at this time, decreases as the heating temperature is increased because of the reduction in the size of the uniform core of the bed despite of the decrease in the char yields (Figures 3 and 4). In other words, exothermic/endothermic effects are not associated with successively lower mass loss rates as the heating conditions are made more severe. In fact the reduction in the mass loss rate is F


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tends to merge so that the attribution of characteristic temperatures (and heating rates) is not any longer meaning full. In reality, as already observed, even for the higher heating temperatures examined in this study, this effect combined with the enhanced bed shrinkage is responsible for the significant scatter observed for the characteristic temperatures and heating rates (Figures 7B, 8B, 9, and 10). Figure 9 also reports the maximum temperature attained during the decomposition process, Tmax, versus the heating temperature. It, for Ts below 550 K, coincides with the value attained at steady conditions by the char bed. For higher Ts values, the maximum value is attained during the conversion dynamics as a results of exothermic reactions. Consequently, a linear trend is observed on Ts for the first region, while the second presents a very steep increase followed by a milder one, in accordance with the dependence of the temperature overshoot on the heating conditions. 3.4. Temperature Overshoot. In Figure 11, the char yields and the temperature overshoots at the sample center are

simply due to geometrical effects given that the mass of the thin central core represents a successively smaller percentage of the total mass. For instance, for a 4-cm thick cylinder, the 1-cm thick outer layer contains about 76% of the total mass against a contribution of about 24% for the inner cylinder with a 2 cm diameter. Moreover, the mass loss rates are small in correspondence of the second peak because they are pertinent only to a small fraction of the lignin component and, in addition, this component mainly produces char. Another consideration should be made about the positive or negative reaction heats of wood components degradation in relation to the assumptions made in this work, given that there is some inconsistence in previous literature about this issue. It is well-known that a large percentage of char is produced from hemicellulose and lignin degradation, independently from the primary or secondary origin. This may justify the observed exothermicity for the degradation of these two components, which is in agreement with the fact that char formation is associated with heat production as indicated by various authors (Figure 1). Moreover, cellulose degradation mainly produces volatile products according to a process that is not only generally retained to be globally endothermic but also causing convective cooling, both hindering temperature rise. The heating rates reached at the bed center, in correspondence of the characteristic points introduced, are reported versus the heating temperature in Figure 10. As

Figure 11. Char yields and temperature overshoots, ΔTc, at the sample center versus the steady temperature reached at the same position, Tcs (the overshoot is evaluated with reference to the temperature reported on the abscissa).

reported versus the steady temperature reached at the same position (the overshoot is evaluated with reference to the temperature reported on the abscissa). In accordance with the results shown by the thermal fields, maximum overshoots are attained at intermediate heating conditions, a feature that can be explained taking into account heat transfer and reaction effects. It has already been observed that the size of the central zone, where the effects of reaction energetics become visible, is progressively reduced as the heating conditions become more severe. This means that the total amount of heat generated in this region, owing to the reduction in the amount of mass pyrolyzed, tends to decrease as the heating temperature is increased. Also, the dispersion of the heat generated, that is, the conduction from the hotter central core of the bed toward the outer zone, is more favored due to the increase in the ratio between the external surface and the volume of the central region. Thus, the change in the physical conditions (heat transfer) would tend to decrease the temperature overshoot as the heating temperature is increased. On the other hand, the extent of the degradation reactions and related energetic effects are favored by successively higher heating temperatures until complete conversion is achieved. However, the increase in the external heating temperature above a certain limit (typically 750 K) does not modify significantly the actual reaction temperature because pyrolysis reactions go to completion before the

Figure 10. Characteristic heating rates (hHi, hHm, hCm, hLm), as defined through Figures 5B, 6B, 7B, and 8B, and heating rate at r = 1.9 cm, hmax, of the packed bed versus the heating temperature, Ts.

expected, they increase with the heating temperature, also due to the successively higher conversion of the two components, which degrade exothermally. The heating rates are within the ranges of those typical of thermal analysis except for the maxima of the heating rates, hHm and hLm, (approximately in the ranges 0.2−3 K/s and 0.8−5 K/s, respectively). These maxima, attained for the higher heating temperatures, are above those typically established in thermal analysis but still much slower than the nominal rates of fast pyrolysis. Thus, an acceptable resolution is still shown by the thermal profiles in relation to the sequence of chemical events discussed. The surface layer is heated much more rapidly, as indicated by the maximum values, hmax, detected by the thermocouple positioned at r = 1.9 cm, which vary from about 5 to 20 K/s for the range of heating temperatures investigated. The profiles of the bed center heating rates show the presence of three reaction zones even for heating temperatures above 800 K (experiments not discussed in this study), but component degradation clearly G


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4. CONCLUSIONS The pyrolytic behavior of cylindrically shaped packed beds of beech wood pellets exposed to a radiative heating along the lateral surface has been investigated through measurements of the thermal field, the weight loss, and the product distribution. The trends shown by the characteristic variables as the external heat flux is varied are in agreement with previous results. Moreover, the central zone of the bed has proved to be apt to provide detailed information on the energetic aspects of wood pyrolysis, which give rise to temperature overshoots up to about 90 K. The thermal field across the radial direction of the packed bed shows spatial gradients that are, however, small for the internal core where the temperature evolution, owing to the symmetry (adiabatic) condition, is essentially determined by the effects of the reaction heats. Although the size of such a zone is progressively reduced as the external conditions are made more severe and the control shift from external to internal heat transfer rate, the time derivative of center temperature (center bed heating rate), after a zone not related to chemical reactions, always shows the same peculiar features. Given heating temperatures above 600 K, two local maxima appear separated by a plateau zone. Only the first maximum is observed for heating temperatures between 550 and 600 K, whereas, for even lower values, no effect is detected, which can be attributed to reaction energetics. For the first time these effects are associated with a sound sequence of chemical events. The beginning of the first zone of rapid increase in the center bed heating rate is observed for temperatures around 510 K, independently from the heating temperature, with the local maximum attained for temperatures that increase from about 550 K (heating temperatures around 550 K) to about 600 K (heating temperatures of 600 K and above). This range of temperature corresponds with a good approximation with that typical of hardwood hemicellulose degradation, indicating that for the conditions of this study the related reactions take place exothermally. The plateau is observed for temperatures around 630−650 K, which approximately correspond to the simultaneous degradation of cellulose and lignin. It can be presumably attributed to the predominance of both the global reaction endothermicity and convective cooling deriving from the decomposition of the former over the exothermic effects originated from the decomposition of the latter. The temperatures corresponding to the second peak increase with the heating temperature from about 650 K to about 730 K (heating temperatures above 700 K) and can be reasonably associated with the exothermic degradation of the residual fraction of lignin. The successive reduction in the size of the central bed core, where the effects of reaction energetics are evident, on one hand, and the successively higher conversion of the wood chemical components that then reaches completion, on the other hand, justify the attainment of a maximum temperature overshoot on dependence of the heating conditions. The experimental analysis presented in this study clearly demonstrates that reaction heat effects can be important for the conditions encountered in both torrefaction and pyrolysis of wood pellets in fixed-bed reactors, but there are numerous aspects that still require further research efforts. The role of primary and secondary reactions on the observed changes in the bed heating rates (and temperatures) should be assessed. The modifications in the temperature dynamics induced by changes in the wood variety and mainly the biomass fuel (agroindustrial residues and energy crops) should be carefully

heating process is terminated. Hence, the thermal effects due to chemical reactions tend to become constant. The data of Figure 11 show a first zone of increasing overshoot values with the heating temperature, as a consequence of the predominance of an increased conversion of the hemicellulose component with respect to a reduction in the size of the almost uniform central core of the bed. More precisely, a rapid increase is initially followed by a plateau (steady center temperatures between 530 and 600 K), that can be associated with the decomposition of hemicellulose (maximum overshoot around 40 K). The overshoot increases with the heating temperature because of the increase in the conversion level of the hemicellulose and becomes almost constant when complete conversion is attained. Indeed, at temperatures below 600 K, the degradation of the other two components is not characterized by high rates. Then, for temperatures above 600 K, a second zone of rapid rise leads to the attainment of a maximum (around 90 K) that is the result of the exothermic degradation of hemicellulose and lignin over the globally endothermic contribution of cellulose degradation. Given that for these conditions the yields of char tend to become constant, it can be assumed that conversion is around the maximum value and a further change in the external heating temperature does not modify the reaction activity, at least in relation to pellet degradation. The second zone of descending values in Figure 11 results from a successively more predominant effect of the reduction in the core size with respect to the constant contribution of the reaction energetics. For heating temperatures above 800 K, the temperature overshoot goes to zero because, though the center temperature profile still shows the existence of two zones of rapid heating, the total amount of heat generated is small and is promptly transmitted to the adjacent zones. In this work, it is difficult to attribute the reaction heat effects to either the activity of primary or secondary reactions, as the properties of the wood pellets (relatively large size and high density) and the size of the bed suggest that both are likely to occur. Based on the quantity of secondary char deposited on the reactor and ballast surfaces, extra-pellet secondary reactions do not seem to occur significantly for temperatures below 750 K, owing to the large cold nitrogen flow rate, which acts as a diluent and quenching agent. It should also be observed that, at higher temperatures, char deposition on the reactor wall may reduce the heat flux arriving at the sample surface up to factors of 30%, as evaluated55,56 for the conversion of wood cylinders of the same size as the packed bed considered here. More complicated is to evaluate the activity of intrapellet secondary reactions of tar products. It is known6 that the apparent mass density of the solid has importance in relation to product yields and exothermicity, as it indirectly controls the vapor-phase concentration of volatile products and pressure. The tar concentration exerts a strong effect, more important than pressure, on the yield of char and thus on the extent of secondary reactions. It can be understood that intrapellet concentrations of tar vapors are higher than those in the void fraction of the bed where the flow rate of nitrogen is rather high. Moreover, inside the pellet a more intimate and extensive (small pores and wider surfaces) contact is established for the tar vapors with char, which catalyzes secondary reactions by lowering their activation temperature.57−60 It is also expected that the reactivity of tars is affected by the temperature of formation and/or the chemical component that is degraded at that temperature; so, it is likely that the tars produced from the three components degrade with a different rate. H


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examined. The investigation of the component behavior could be another topic worthy of analysis, although it might not be straightforward to extrapolate the results to practical fuels. Indeed, apart from the modifications introduced in the chemical properties by the separation process and interactions among components in wood, their thermal properties (i.e., thermal conductivity) may also be significantly different, thus modifying the heating and conversion conditions of the bed. Finally, quantitative evaluations of the reaction heats should be made. This will require the formulation of multistep pyrolysis mechanisms for the components and the development of transport models for the packed bed.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 39-081-7682232. Fax:39-081-2391800. E-mail: diblasi@ unina.it. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.D.B. thanks Prof. M. J. Antal (University of Hawaii) for interesting discussions on the subject of this paper. Thanks are also due to V. Lombardi and P. Ciappa (University of Napoli Federico II) for their help in the execution of several experiments.

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NOMENCLATURE h = heating rate [K/s] r = radius of the packed bed [cm] T = temperature [K] Tc = temperatures of the bed center (r = 0, median section) [K] Tcs = steady value of the temperature Tc [K] Ts = steady temperature of the bed at r = 1.5 cm (median section) [K] t = time [s] Y = bed mass fraction ΔH = pyrolysis heat [J/g] ΔTc = difference between the maximum temperature and the corresponding steady value at the bed center (median section) [K]

Subscripts

Cm = plateau in dTc/dt for Tc > 500 K Hi = beginning of the first maximum in dTc/dt for Tc > 500 K Hm = first maximum in dTc/dt for Tc > 500 K Lm = second maximum in dTc/dt for Tc > 500 K max = maximum

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Experimental Analysis of Reaction Heat Effects during Beech Wood

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