Thermal Runaway in the Pyrolysis of Some Lignocellulosic Biomasses

Energy Fuels , 2014, 28 (4), pp 2684–2696 ... Publication Date (Web): March 10, 2014 ..... Journal of Analytical and Applied Pyrolysis 2014 110, 130...
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Thermal Runaway in the Pyrolysis of Some Lignocellulosic Biomasses C. Di Blasi,*,† C. Branca,‡ F. E. Sarnataro,† and A. Gallo† †

Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli ″Federico II″, P.le V. Tecchio, 80125 Napoli, Napoli, Italy ‡ Istituto di Ricerche sulla Combustione, C.N.R., P.le V. Tecchio, 80125 Napoli, Napoli, Italy ABSTRACT: The thermal decomposition of a cylindrical fixed bed consisting of agricultural residues (hazelnut shells, olive pomace, straw pellets) or softwood pellets, uniformly heated along the lateral surface, is investigated for heating temperatures in the range 473−800 K, and a comparison is made with results previously obtained for beech wood pellets. Although dependent on the external heating conditions, exothermic reaction heat effects are evident for all the biomasses, giving rise to maximum temperature overshoots of 225 K (hazelnut shells), 170 K (olive pomace), 78 K (straw), and 53 K (softwood pellets) (versus 86 K for beech wood pellets). For the first two materials and mild/moderate heating conditions, the entire bed volume experiences large temperature overshoots, so that the qualitative features of the pyrolytic conversion are those of a thermal runaway. Explanations are given, based on the different chemical and physical properties of the samples, for the different exothermicity level, and its implications in the practical application of torrefaction and pyrolysis are discussed.

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INTRODUCTION The role of the reaction heat in wood pyrolysis is important in practical conversion systems, dominated by the interaction between chemical reactions and heat and mass transfer phenomena, where the energy demand is a key control parameter. Exothermic effects are reported at the sample center during the pyrolysis of thick samples or fixed (packed) beds uniformly heated along the external surface for both pyrolysis1−12 and torrefaction,13−15 in some cases with significant temperature overshoot with respect to the steady value. Although these effects may determine the characteristics of the pyrolysis zone in downdraft and updraft gasifiers and in general the thermal conditions in fixed-bed reactors used for pyrolysis and torrefaction, a systematic investigation has been recently conducted only for beech wood pellets.16,17 Measurements of the thermal field show maximum temperature overshoots around 90 K and the occurrence at the central core of the fixed bed of sequential 1) exothermic, 2) endothermic or neutral, and 3) exothermic effects. Based on the corresponding characteristic temperatures, a plausible association is made with specific chemical processes. However, it is not known if a similar behavior is also typical of other lignocellulosic materials, in particular softwoods and agricultural residues, and if the magnitude of the exothermic effects is, in this case, comparable to that of beech wood. Differences exist between hardwoods and softwoods in the chemical composition deriving from variable contents of the main components (cellulose is around 40−50%, hemicellulose around 28% in softwoods and 35% in hardwoods, and lignin between 23 and 33% for softwoods and 16−25% for hardwoods) and mainly between wood and agricultural residues which present much wider ranges of variation in their chemical composition, not to mention the much higher contents of extractives and ashes.18,19 Furthermore the chemical structure of hemicellulose and lignin also depends on the botanical origin of the biomass. Based on the literature cited in ref 20, hemicelluloses in softwoods are mainly galacto-glucomannan © 2014 American Chemical Society

(about 5−8%) and glucomannan (about 10−15%). Arabinoglucuronoxylans are also present in softwoods (7−8 wt %), whereas in hardwoods acetylglucuronoxylans are present (15− 30 wt %) in association with variable percentages of galactose, arabinose, rhamnose, and methylglucuronic acid units and acetyl groups and small amounts of glucomannan. In herbaceous biomass, such as straw, arabinoxylans are the predominant hemicellulose polysaccarides and those containing galactose and mannose only minor ones. Lignin is a complex macromolecule synthesized by chemical polymerization of three main precursors, p-coumaryl, coniferyl, and sinapyl alcohols, via enzymatically generated radicals.21 These monolignols produce the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) phenylpropanoid lignin units when incorporated into the lignin polymer, units which are linked by several types of C−C or ether bonds. Hardwood lignins are composed of S and G units in varying ratios, softwood lignin is composed of G units and small amounts of H units, and grass lignins include the three units (with H-units still comparatively minor) with also p-hydroxycinnamates (p-coumarates and ferulates), making its structure apparently more complex. It is easily understandable that the differences in the chemical composition affect the primary degradation reactions and the nature of the vapor-phase products which exhibit a different reactivity. Moreover changes in the chemical composition are generally associated with a different morphological structure of the material (for instance hardwoods and softwoods or, more specifically, agricultural residues) which is expected to influence the intraparticle residence times of tar vapors and thus the activity of secondary reactions. In this way the primary and secondary reaction heat effects are also affected and hence vary with the fuel properties. In fact thermogravimetric analysis shows that the different percentages and structure of chemical Received: January 31, 2014 Revised: March 10, 2014 Published: March 10, 2014 2684

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Table 1. Contents of Holocellulose, Lignin, and Extractives of the Various Samples on an Ash Free Basisa holocellulose [%wt]

lignin [%wt]

extractives [%wt]

cellulose [%wt]

hemicellulose [%wt]

78 69 56.8 62.4 70

20 29 33.7 28.7 24.4

2 2 9.5 8.9 5.6

45 45 30.5 17.9 30.5

33 21 28.9 23 28.9

beech softwood hazelnut shells olive pomace straw a

For qualitative comparison the contents of hemicellulose and cellulose as derived from the literature reported in ref 23 are also listed. followed by exposure to a forced air flow at about 393 K for additional 30 min before the experiment). Snapshots of the samples are shown in Figure 1, whereas the chief properties are summarized in Table 3. The size refers to the smallest one, generally the thickness of the particles or the diameter of the pellets. The chief information on these aspects is as follows: the softwood and beech wood pellets show diameters and lengths around 5−6 mm, the straw pellets present a diameter of 8 mm and a length of 6 mm, the hazelnut shells exhibit thickness around 1− 1.5 mm and width up to about 5−6 mm, and finally the average size of olive pomace is around 1.6 mm (the actual size distribution is as follows: 0.28−0.5 mm (7%), 0.5−1 mm (25%), 1−2 (32%), 2−3 mm (36%)). The experimental system is the same already used in previous studies,16,17 so the details are not again given here. The particles/ pellets are packed in a stainless steel mesh, cylindrically shaped holder (40 mm diameter and length) which is instantaneously exposed to an assigned radiative heat flux along its lateral surface. Values examined here are in the range 19.9−43.1 kW/m2, resulting in heating temperatures, Ts, (final temperatures measured for the char bed at a radius of 15 mm), of about 473−800 K. For an assigned heat flux, the differences among the various materials for the temperature Ts are in the order of the experimental uncertainty and so not attributable to differences in the sample properties (in the following results are referred to the Ts value measured for each fuel). The observation times (that is, the duration of the experiments) which, for sufficiently severe conditions corresponds to the attainment of steady temperature profiles and constant yields of solid residues, is limited to a maximum of 3600s for heat fluxes below 21.55 kW/m2 (Ts around 535−540 K) for the agricultural residues and 22.7 kW/m2 (Ts around 567 K) for the two wood pellets.

components influence significantly the rates of biomass weight loss22 while, for conditions of heat and mass transfer control, these are the main factors responsible for the yields and composition of the lumped classes of pyrolysis products,23 but, as already observed, the reaction heat effects have not yet been given extensive consideration. In this study the reaction heat effects are investigated for three agricultural residues (hazelnut shells, olive pomace, straw pellets) and softwood pellets packed in a small scale cylindrical reactor, instantaneously exposed along the lateral surface to a uniform radiative heat flux varied so as to reproduce thermal conditions that span from torrefaction to pyrolysis. The analysis of the spatial temperature profiles is combined with that of the global rates of weight loss and the yields of solid product, using for comparison results already available for beech wood pellets.

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MATERIALS AND METHODS

Samples examined are as follows: softwood pellets (Pellis, now PelliTop, Ausberger HVO, DE), straw pellets, hazelnut shells, olive pomace (a byproduct in the manufacture process of olive oil in mills and consisting of pulp, peel, and pits, also indicated as ″olive husks″ in previous studies of this research group2,23,24) and, for comparison, beech wood pellets (SITTA, IT). The chemical composition, in terms of extractives, lignin, and holocellulose, on an ash-free basis, is summarized in Table 1, where the contents of hemicellulose and cellulose, derived from the literature reported in ref 23, are also listed (these can be useful only for a rough comparison). The lignin content is determined according to the Klason method, while a Soxhtec HT2 apparatus with acetone as solvent is used to evaluate the extractive content23 (holocellulose by difference). As expected, the main difference between beech and softwood pellets consists in a higher lignin content of the latter at the expense of hemicellulose. High lignin contents are also observed for olive pomace and mainly for hazelnut shells whose value is also higher than that of the softwood pellets (33 versus 29%). Moreover, the hazelnut shell composition also presents an amount of hemicellulose similar to that exhibited by beech wood. The lignin and hemicellulose contents of straw are comparable with those of beech wood. Finally, the extractive contents always attain significant values for the agricultural residues. Table 2 reports the

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RESULTS The results obtained for softwood pellets, straw pellets, hazelnut shells, and olive pomace are compared with those previously obtained for beech wood pellets in relation to the yields of solid product, temperature profiles, and global weight loss characteristics. Temperature Overshoots and Yields of Solid Product. The temperature dynamics are qualitatively similar in all cases. The fixed bed, heated along the lateral surface, experiences a 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. According to the schematization previously proposed,16,17 the thermal field consists of 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 smaller. In the first zone, the pyrolysis reactions take place at very close spatial positions and the appearance of reaction heat effects on 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. This zone, for the

Table 2. Proximate Analysis of the Various Samples beech softwood hazelnut shells olive pomace straw

volatile matter [%wt]

fixed carbon [%wt]

ash [%wt]

86.5 85.4 77.7 75.6 79.3

13.1 14.4 21.3 19.2 16.7

0.40 0.26 0.96 5.20 4.04

proximate analysis of the various samples, made according to the ASTM E1131-08 method. The most evident difference between wood and agricultural residues is that, in the latter case, the fixed-carbon content is higher, especially for the hazelnut shells (about 21% versus 13−14%). Experiments are made for randomly packed beds made of pellets or particles, preliminarily predried (oven drying at 373 K for at least 12 h 2685

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Figure 1. Snapshots of the various samples (properties listed in Tables 1−3) used in the pyrolysis experiments: a) softwood pellets, b) straw pellets, c) hazelnut shells, d) olive pomace.

to an increase in the surface area where heterogeneous condensation takes place.17 In fact, as the external heating conditions are made more severe, the temperature gradients along the bed radius increase and the size of the almost uniform central region is progressively reduced, in addition to the increase in the average reaction temperature and heating rate. Furthermore all the materials present temperature overshoots which attain a maximum on dependence of the heating conditions, but quantitative differences are significant. The main features of the thermal field, that is, the maximum temperature overshoot, ΔT, and the maximum temperature, Tmax, achieved by the fixed bed (with the corresponding time, tmax) are reported versus the heating temperature in Figures 2 and 3 for the various samples. The yields of solid product are reported versus Ts and Tmax in Figures 4 and 5, respectively. The maximum temperature overshoot refers to the bed center and is evaluated with respect to the final, steady conditions at the same position. The maximum temperature, Tmax, is attained at the bed center when significantly large overshoots are established, otherwise it coincides with the final steady value of the bed surface. The time tmax approximately represents the conversion time (amounts of volatile mass to be still released around 2−5%16) (the cases corresponding to the maximum solid retention time of 3600 s, at very low heat fluxes, are not plotted). As anticipated, in all cases the temperature overshoot attains a well evident maximum on dependence of the external heating conditions (Figure 2), which can be explained by taking into account the interaction between chemical reactions and heat and mass transfer phenomena.16,17 The first region of

Table 3. Size and Shape of the Pellets/Particles and Mass and Density of the Samples Used in the Fixed Bed Pyrolysis Experimentsa beech softwood hazelnut shells olive pomace straw

size [mm]

shape

m0 [g]

ρb [g/cm3]

5.5 5.5 1−1.5 1.6 8

pellet pellet slab sphere, slab pellet

26 26 18 21 24

0.52 0.52 0.36 0.42 0.48

a

The actual size distribution of olive pomace is as follows: 0.28−0.5 mm (7%), 0.5−1 mm (25%), 1−2 (32%), 2−3 mm (36%).

symmetry condition, does not encounter any inward conductive heat transfer, permitting a better visualization of the reaction heat effects on the temperature profile. It is important to observe that it is also the site where vapors, produced across the more external layer, migrate and condense. Successively, when sufficiently high temperatures are achieved, re-evaporation and pyrolysis of the condensate occur, affecting both heating dynamics and product yields and composition.17 Indeed, the temperature profiles show a plateau around 473 K resulting from water evaporation, whereas the evolution of the organic species is not clearly visible as it occurs over a wide temperature interval also giving rise to condensed-phase products.25−27 The effects of condensation and re-evaporation/pyrolysis of volatile products are enhanced as the thermal conditions are made more severe, owing to the increased temperature gradients between the surface and the core of the bed, and as the size of single fuel elements is diminished, owing 2686

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Figure 2. Temperature overshoot, ΔT, evaluated at the bed center with respect to the corresponding steady temperature versus the heating temperature (steady temperature reached at 15 mm from the bed center), Ts, for the various samples (beech wood results for comparison).

Figure 5. Yield of solid product versus the maximum temperature achieved by the fixed bed, Tmax, for the various samples (beech wood results for comparison).

overshoot curve, at higher heat fluxes, can be attributed to the further decrease in the size of the central almost uniform core (and an increase in the heat loss rate) which is not compensated by an increased amount of heat produced given that complete conversion has already been attained, as confirmed by the leveling of the yields of solid product. The shape of the overshoot curve shows some qualitative differences between wood and straw pellets, on one side, and the other two agricultural residues, on the other. For the first group the initial zone, preceding the maximum, consists of a more or less rapid increase followed by a well evident plateau that is completely absent for the second group. Moreover, for the three agricultural residues, the central zone of maximum values is narrower and the descending part wider. Taking the results obtained for beech wood as a reference, it can be seen that the appearance of a temperature overshoot is displaced at lower temperatures for the agricultural residues (Ts around 500 K (hazelnut shells) or barely higher) and at higher temperatures for the softwood (550 K) (versus 530 K for beech wood), but the conditions of zero overshoot are approximately the same in all cases (Ts values around 800 K). On the whole while the agricultural residues attain their maximum temperature overshoot for Ts around 550−600 K, the two wood varieties exhibit their maxima for Ts around 630−730 K. Moreover while the overshoots observed for the straw pellets are comparable to those of the two wood samples (78 K versus 86 K and 53 K for the beech and the softwood pellets, respectively), much higher values are measured for the hazelnut shells and the olive pomace (225 and 170 K, respectively). Very interesting is also the dependence of Tmax on Ts (Figure 3), which is determined by both reaction heat and external heat flux effects. A first zone is observed with an obvious linear trend, corresponding to zero or very small overshoots, so that Tmax is the final steady temperature attained at the bed surface. Then a second zone appears showing a rapid increase and essentially corresponding to the increasing part of the overshoot curve. Its features (extent and increasing rate) are essentially determined by the thermal conditions causing the beginning of the pyrolysis reactions and the related amount of heat produced. In accordance with the results reported in Figure 2, a very rapid rise to very high values, at relatively mild thermal conditions, is observed for the hazelnut shells and the olive pomace and a slower one for the other fuels. The third zone, which corresponds to the descending part of the overshoot curve, exhibits trends depending on the specific

Figure 3. Maximum temperature, Tmax, reached by the fixed bed, and corresponding time, tmax, versus the heating temperature, Ts, for the various samples (beech wood results for comparison).

Figure 4. Yield of solid product versus the heating temperature, Ts, for the various samples (beech wood results for comparison).

increasing values results from a predominance of the successively higher conversion degree over the successively slightly smaller size of the almost uniform core region of the fixed bed. Indeed devolatilization occurs at a larger extent, as testified by the successively lower yields of solid residue (Figures 4 and 5). The increase in the spatial gradients of temperature is associated with a size diminution of the uniform core and an increase in heat loss rate from the hot core of the bed toward the annular external zone (the ratio between the lateral surface and the volume of the central uniform core increases). Instead the decreasing part of the temperature 2687

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fuel (in other words, on the Tmax value where it begins). For straw and wood pellets, whose exothermicity is moderate, this part of the Tmax curve shows barely increasing values. In fact, despite the reaction heat effects becoming less visible, the successively higher heat fluxes give rise to Tmax values higher than those observed for the second zone of the curve. Instead, for the hazelnut shells and the olive pomace, the increase in the external heat flux is not sufficient to compensate the reduced appearance of reaction exothermicity, so that Tmax initially decreases. Finally, for Ts values of 800 K and above, when temperature overshoots disappear, all the fuels tend to attain the same Tmax value, located at the bed surface. Although experiments at higher heat fluxes have not been made, it is understandable that another linear zone would appear. The time tmax always presents the same qualitative dependence on Ts, that is, it becomes successively shorter as the external heating conditions are made more severe. The actual values are related to the Tmax values and the intrinsic reactivity of each feedstock. However it is worth observing that the very large temperature overshoots observed for the hazelnut shells and the olive pomace do not give rise to trends qualitatively different from those observed for the other fuels. In general, wood conversion requires times longer than those of the agricultural residues. The dependence of the yields of solid product on the heating conditions, shown in Figures 4 and 5, is well-known from previous literature28 with values that are progressively reduced as the thermal conditions are made more severe. The lower yields of solid product and the very rapid decay observed for the agricultural residues, with respect to wood, at low Ts values can be explained by actual reaction temperatures much higher than those established by the external heating conditions, owing to large reaction exothermicity. Then, at high Ts, when temperature deviations, with respect to the external imposed values, diminish the usual trend appears,28 with agricultural residues producing higher yields of solid product (about 32− 33% versus 25−26% for Ts around 800 K). Indeed the higher extractive content and mainly the much large amounts of alkali metal in agricultural residues enhance the formation of char. When the dependence on the maximum temperature is examined, it can be observed that, given temperatures above 573 K, agricultural residues always produce yields of solid products higher than those generated from wood. The contrary occurs for lower Tmax values. Moreover Figure 5 shows that, for the two residues characterized by very large temperature overshoots, the results of the experiments, although made for variable external heat fluxes, are almost entirely localized in the region of Tmax below 573 K, with charred product yields above 70%, and the region of Tmax above 730 K, with charred product yields below 40%. This indicates that the large exothermicity of the pyrolysis reactions makes it difficult to establish thermal conditions giving rise to yields of solid product intermediate between the values reported above. The other samples also show a similar but a less exacerbate behavior. In reality the effects of the pyrolysis temperature on the product yields and heating dynamics depend on the intrinsic reactivity of the material. Also the actual reaction temperature is different from both Ts and Tmax, owing to dynamic conversion and the presence of spatial temperature gradients. Temperature Profiles and Characteristic Values. To better understand the different thermal behavior and the influence of the feedstock nature on the magnitude of the spatial gradients, the temperature profiles measured at several

radial positions can be used. A comparison between the two wood categories can be made through Figures 6A−6C which

Figure 6. A−C - Temperature versus time profiles at several radial positions for the fixed beds made of softwood pellets (solid lines) using the results obtained for the beech wood pellets (dashed lines) for comparison for an external heat flux equal to A) 26.5 kW/m2, B) 28.9 kW/m2, and C) 36.5 kW/m2.

refer to external heat fluxes of 26.5, 29.8, and 36.5 kW/m2. For all the external heat fluxes, apart from deviations attributable to thermocouple positioning, the most external layer of the bed is heated with the same rate up to the same final temperatures for both cases. For the more internal zones, the differences between the two samples are again negligible as long as the temperatures remain below approximately 400−450 K, that is, before significant degradation rates are established. These results can be attributed to the same density of the fixed beds (and pellets) and most likely to negligible differences between their thermal capacities and thermal conductivities. For higher temperatures and the internal part of the fixed beds, independently from the external heat flux, the beech wood pellets exhibit a more rapid temperature increase, reasonably 2688

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kW/m2. Despite the different physical properties, it can be observed that for low temperatures (below 400 K) the heating dynamics are again not significantly different for the two cases (slightly lower rates are, on the average, observed for beech wood presumably due to the higher fixed-bed density). For higher temperatures, the hazelnut shell samples always exhibit a more rapid temperature rise, which occurs at shorter times/ temperatures, and higher maxima. In particular for the lowest heat flux, which results in Ts values around 530 K, the degradation reactions of beech wood are scarcely active as the solid product yield, after a retention time of 3600 s, are around 90% (Figure 4). As a consequence of the very slow decomposition rates, the temperature profiles essentially represent the inert heating of the bed. On the contrary, for the hazelnut shells, the evident temperature overshoots indicate that decomposition reactions already attain significant rates. The relatively slow rates of external heat transfer, resulting from the low external heat flux, give rise to small spatial gradients of temperature. Hence decomposition reactions take place simultaneously along the entire cross section of the bed, which is also, at the same time, the site of large temperature overshoots. At the higher heat fluxes (Figures 7B and 7C) spatial gradients are significant for both samples, but it is evident that the hazelnut shell temperatures exhibit a more rapid increase and exceed by a significant amount their steady final values still along the entire section of the bed. This, on one side, is a further confirmation of the occurrence of the pyrolysis reactions at lower temperatures and, on the other side, a proof of a much more exothermic process (with larger effects in relation to both magnitude and spatial extension), compared with beech wood. Therefore the beginning of the decomposition reactions at lower temperatures, with respect to beech wood, is associated with a larger extension of the central uniform reaction zone (practically the entire reaction volume except for high external heat fluxes (above 33.2 kW/m2)) and reduced heat losses contributing to the attainment of higher maxima. The thermal behavior of the three agricultural residues is qualitatively similar (pyrolysis begins at relatively low temperature and temperature overshoots extend over the entire bed volume for low and moderate external heat fluxes) as shown by the profiles reported in Figure 8 for a heat flux of 23.2 kW/m2. However, in accordance with the results reported in Figure 2, the maximum temperatures are different (they decrease in the

due to exothermic effects during thermal degradation, which begins at slightly lower temperatures. For instance, for the low external heat flux of Figure 6A, all the internal thermocouples report a rapid temperature increase up to values above the final steady value. Conversely the increase in the heating rate and the temperature overshoot with respect to the steady final value, which are displaced at longer times, are barely visible for the softwood. The profiles reported in Figures 6B and 6C confirm that the quantitative differences between the two materials are still present at high temperatures, that is, the temperature overshoots are lower and are detected at longer times in the case of the softwood pellets. Figures 7A−7C compare the thermal profiles for hazelnut shells and beech wood at heat fluxes of 21.5, 23.2, and 26.5

Figure 7. A−C - Temperature versus time profiles at several radial positions for the fixed beds made of hazelnut shells (solid lines) using the results obtained for the beech wood pellets (dashed lines) for comparison for an external heat flux equal to A) 21.5 kW/m2, B) 23.2 kW/m2, and C) 26.5 kW/m2.

Figure 8. Temperature versus time profiles at several radial positions for the fixed beds made of hazelnut shells (solid lines), olive pomace (dashed lines), and straw pellets (dotted lines) for an external heat flux equal to 23.2 kW/m2. 2689

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conditions, giving rise to the passage from one condition to another, depend on the specific feedstock. In general, the overlap between the various processes is much higher for the agricultural residues, especially hazelnut shells and olive pomace, as the reactions tend to occur simultaneously. The characteristic temperatures, as defined in Figures 9A and 9B, are plotted in Figures 10A−10D using the results previously

order for hazelnut shells, olives pomace, and straw pellets). Moreover the time tmax, associated with the maximum temperature, also becomes progressively longer in the same order. At high heating temperatures (not shown), as expected, the effects of condensation and re-evaporation/pyrolysis of volatile products are more evident for the olive pomace, owing to the lower particle sizes. The dynamics of the bed center heating, which are useful to better observe exothermic and endothermic effects of the degradation reactions, also present a qualitatively similar dependence on the intensity of the external heat flux in all cases. An example for the two wood species and the agricultural residues (straw and hazelnut shells) is reported in Figure 9A

Figure 10. Characteristic temperatures (THi, THm, TCm, TLm), as defined through Figures 9A and 9B of the fixed beds made of softwood pellets (A), hazelnut shells (B), olive pomace (C), and straw pellets (D) versus the heating temperature, Ts (beech wood data for comparison).

obtained16 for beech wood for comparison. In all cases these parameters tend to increase with the external heat flux (Ts) and, in accordance with the temperature overshoot curves, they can be defined already at lower Ts for the agricultural residues and only at higher Ts for the softwood, compared with beech wood. The THi values are around 490−500 K, except for the softwood where the corresponding range becomes 500−530 K. The THm range corresponds to 560−630 K for the softwood, 510−600 K for straw, 520−620 for olive pomace, and 520−650 K for hazelnut shells (versus 530−600 K for beech wood). The range of TCm values is approximately the same for all the feedstocks and vary in the range 600−650 K (straw and olive pomace), 630−650 K (wood) and 620−670 K (hazelnut shells). Finally the TLm values vary between 630 and 700 K (straw and olive pomace), 650−730 K (wood), and 630−750 K (hazelnut shells). The variations of the bed center characteristic temperatures can be attributed, in the first place, to the specific properties of each feedstock, but it is worth observing that the actual heating rates experienced by the samples, highly affected by the reaction exothermicity, may also play a role. Although not shown, on one side, there are the two wood samples with maximum values remaining below 2K/s (hHm and hCm) and 4 K/s (hLm) (the straw sample presents similar values except for lower hLm, which remains below 2K/s) and, on the other, olive pomace and hazelnut shell samples with maximum values reaching 6−13 K/s (hHm), 6−7 K/s (hCm), and 7−10 K/s (hLm).

Figure 9. A and B - Profiles of the center temperature, Tc, and the corresponding time derivative, dTc/dt, versus time for the two wood pellets for an external heat flux equal to 29.8 kW/m2 (A) and two agricultural residues (hazelnut shells and olive pomace) for an external heat flux equal to 26.5 kW/m2 (B).

(external heat flux of 29.8 kW/m2) and Figure 9B (external heat flux of 26.5 kW/m2), respectively, where it can be seen that a first maximum is always followed by a well visible minimum and then by another maximum. Characteristic temperatures (THi, THm, TCm, TLm) and heating rates (hHi, hHm, hCm, hLm) can be introduced, corresponding to the beginning of the first peak rate, the minimum and the second peak, to clearly define these sequential events.16,17 Furthermore at very low heating temperatures, no effects of the reaction heats are visible on the temperature profiles, as the reaction rates are very slow (and so are the associated rates of heat release/demand). Then, as the heating temperature is increased, first only one maximum then two maxima appear in the center bed heating rate. For very high heating temperatures, the heating and pyrolysis rates become so fast that it is not any longer possible to clearly distinguish different zones in the heating rate. However the external heating 2690

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Weight Loss Curves. The main features of the global weight loss dynamics during pyrolysis of fixed beds of softwood pellets, exposed to several external heat fluxes, can be observed from Figure 11 where, for comparison purposes, the curves

maximum at intermediate heating conditions (Q = 23.1 kW/ m2). This result can be explained taking into account that, for these conditions, the temperature overshoot is also maximum and extends over the entire cross section of the bed. Hence the simultaneous occurrence of the pyrolysis reactions for the entire bed volume results in a very rapid release of a large amount of volatile products. More precisely, the shape of the rate curve consists of a first, slow stage, corresponding to the release of a small volatile mass, as expected from the application of a relatively low external heat flux, and a second very short stage characterized by a sharp increase followed by a similar sharp decay. The first stage essentially determines the duration of the conversion process (tmax, reported in Figure 3), but the total amount of volatile products released is rather small (total solid mass fraction around 0.8). These process dynamics, which have been observed for the first time in detail in fixed-bed pyrolysis, present qualitative features similar to uncontrolled ignition/combustion29,30 or thermal runaway. In fact, once the fixed-bed reactor reaches a temperature sufficiently high for the degradation process to begin (a relatively slow stage owing to moderate heating resulting in an almost uniform thermal field), the related exothermicity causes a further rapid increase in temperature (Figures 7B, 7C, and 8), well above the heating values, leading to rapid complete conversion. The comparison among the various samples reported in Figure 13 for a heat flux of 23.2 kW/m2 confirms that

Figure 11. Time profiles of solid mass fractions and time derivatives of solid mass fraction for the fixed-bed made of softwood pellets (solid lines), using beech wood results (dashed lines) for comparison, for various external heat fluxes.

obtained for beech wood pellets under the same heating conditions are also included. The duration of the process is progressively reduced, as the heating conditions are made more severe, and the maximum rate, which is initially observed for sample heat-up conditions, occurs when the more external layer of the bed is degraded. These qualitative features are independent from the wood variety but quantitative differences are significant. Despite the same bulk density of the fixed bed and the same size and density of the pellets, the rates of weight loss are lower for the softwood and the decomposition process requires longer times, as already indirectly observed from the temperature measurements. The dependence of the global rate of weight loss of the straw pellets on the external heating conditions is qualitatively similar to that discussed for the wood samples. On the contrary a different trend is shown by the other two residues, characterized by a highly exothermic degradation, as can be seen from Figure 12 (hazelnut shells). The duration of the process is again progressively reduced as the external heat flux is increased, but the rate of volatile release attains its absolute

Figure 13. Time profiles of solid mass fractions and time derivatives of solid mass fraction for the fixed-bed made of various samples and an external heat flux of 23.1 kW/m2.

agricultural residues start to decompose at lower temperatures and, owing to the high exothermicity of the reactions and the attainment of temperatures well above to those imposed by the external heating conditions, their maximum rates of volatile release are higher up to a factor of 10 compared to wood.

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DISCUSSION Experimental results show that the temperature profiles of lignocellulosic biomasses, when pyrolyzed in fixed-bed reactors, present the same qualitative features, but quantitative differences are significant between hardwood and softwood and especially between wood and agricultural residues. In summary softwood decomposition occurs at a slower rate, with respect to beech wood, with temperature overshoots that are lower and appear at higher heating temperatures. Agricultural residues start to degrade at lower temperature and may present very high temperature overshoots (higher than those observed for wood up to factors of 2−4). Moreover, due to low temperature

Figure 12. Time profiles of solid mass fractions and time derivatives of solid mass fraction for the fixed-bed made of hazelnut shells for various external heat fluxes. 2691

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is plausible that the exothermic effects of lignin degradation prevail over those of endothermic degradation of cellulose and the convective cooling caused by the outflow of volatile products,40,41 not to mention that the actual energetics of the latter component depend on the experimental conditions42 and the sample properties.43 At a first glance, the characteristic temperatures observed for the softwoods pellets and agricultural residues do not appear to be exceedingly different from those observed for beech wood, so it can be assumed that, in the first approximation, a similar association can again be made. These issues are discussed, also using the results of previous literature, in the following. The differences observed between the characteristic temperatures of beech and softwood pellets are in agreement with results obtained from thermogravimetric analysis (heating rate 5 K/min).34 For a significant number of species belonging to the hardwood and softwood category the initial degradation temperature is approximately the same (516 K for the hardwoods and 508 K for the softwoods). The main differences are on Tonset and Tshoulder, that is, the temperatures associated with the beginning and the peak top of hemicellulose degradation. The average values of Tonset are 510 K (hardwoods) and 530 K (softwoods), while the average Tshoulder values are 562 and 591 K, respectively. These results indicate that the degradation of the softwood hemicellulose is displaced at temperatures by about 20−30 K higher with respect to the hardwood hemicellulose. The average Tpeak values, essentially associated with the peak in the cellulose decomposition rate, and the average Toffset values, representative of the final tailing zone dominated by lignin decomposition, are the same for the two wood categories. The measured values are 620 and 621 K for Tpeak and 642 and 648 K for Toffset for the hardwoods and the softwoods, respectively. Moreover, a more recent analysis carried out for oxidative conditions and including the simultaneous kinetic evaluation of multiple curves24 also puts into evidence that the main difference between hardwood and softwood devolatilization is attributable to the hemicellulose behavior. Hence it is plausible to assume that the sequence of thermal events, observed in the fixed bed pyrolysis experiments of both hardwood and softwood, can be attributed to the same chemical processes. The shoulder in the ΔT versus Ts curve, observed for the wood samples, can be related to the exothermic degradation of hemicellulose (and extractives) and the zone of maxima to the simultaneous contribution of all the components. The overshoot curve shows that, on the whole, the softwood exothermicity is lower than that of beech wood. Given the comparable physical properties, as confirmed by the almost coincident characteristics of the inert heating stage of the fixed bed, the chemical properties are most likely playing the predominant role. The lower maximum temperature overshoot of the shoulder zone of the softwood (22 K versus 48 K) is attributable to the lower hemicellulose content and/or its different nature, resulting in a reduced exothermicity. Moreover, the absolute maximum of the temperature overshoot is also lower for the softwood (53 K versus 86 K) suggesting that the higher lignin content and/or its properties are not adequate to bring the global exothermicity of the process to that of the beech wood. More precisely the difference between the absolute maximum and that of the shoulder zone is around 36 K and 31 K for the beech and the softwood, respectively, and it can be hypothesized that softwood lignin degradation is also a less exothermic process than that of the beech wood

degradation, the spatial extension of the temperature overshoot zone practically coincides with the entire bed, except for very high heat fluxes. The different thermal conditions which determine the appearance of the temperature overshoots are clearly related to different material reactivity (essentially the initial degradation temperature), whereas the disappearance of such a feature, at rather severe thermal conditions, can be essentially attributed to physical effects, that is, the attainment of very large spatial gradients of temperature across the fixed bed which, for the conditions of this study, is practically independent from the feedstock. The straw pellets behavior closely resembles that of wood, but those of the other two residues are very distant. The presence of two distinct zones in the ΔT versus Ts curves, for wood and straw, corresponding to low and high heating temperature, suggest that two groups of sequential globally exothermic processes occur. On the contrary, for the hazelnut shells and the olive pomace a single zone of much stronger exothermicity is evident, most likely resulting from the simultaneous occurrence of all the degradation reactions. In general the magnitude of the reaction exothermicity depends on the specific feedstock, although it is generally more important for the agricultural residues for both spatial extension and magnitude, and the external heating conditions, so its impact is different in pyrolysis or torrefaction. The first aspect to be discussed concerns the attribution of the exothermic zones, shown by the temperature overshoot and/or the bed center heating rate, to specific chemical processes, taking into account that, given the large sample size, both primary and secondary reactions are involved. In the second place, the different exothermicity of the lignocellulosic materials investigated should be given consideration, taking into account that differences are observed on the content and nature of the chemical components and inorganic matter, the shape and density of the single fuel elements, which influence the bulk density (Table 2) and presumably the porosity and the thermal conductivities of the fixed bed, and the textural properties, all affecting both primary and secondary degradation (and the related reaction heat effects). As already done in previous studies,16,17 the temperatures, measured in correspondence of the peaks and valley of the center bed heating rates, can be useful for the association of these with specific chemical processes. For this scope, it is also worth mentioning that, similar to the behavior exhibited by beech wood, the position of the first maximum in the bed center heating rate corresponds to relatively high values of the mass fraction (and mass loss rate), whereas lower values are detected for the other two points, specially the second peak (amounts of volatile species to be released usually below 2−5 wt % of the total amount). In general, for an assigned heat flux, the characteristic mass fractions are higher for the softwood pellets and lower for the agricultural residues, compared with beech wood, plausibly as a result of a different reactivity. Taking into account31−39 that hardwood hemicellulose typically degrades exothermally in the range 500−600 K, cellulose degrades endothermally in the temperature range 600−650 K, and the large part of the exothermic degradation of lignin takes place between 600 and 750 K, the sequence of a) exothermic, b) endothermic or thermally neutral, and c) exothermic effects observed at the bed center of beech wood pellets were linked16,17 in the order, to the (primary and secondary) degradation of (a) hemicellulose, (b) cellulose and lignin, and (c) lignin. For the endothermic or thermally neutral event b) it 2692

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net separation between the two exothermic reaction zones clearly visible from the bed center heating rate or the temperature overshoot curves, which is less evident or absent for the other two residues. Finally, the straw samples are in the form of pellets, and, given the preliminary milling, it can be hypothesized that the microstructure of the material has been destroyed for a large part so that the intrapellet activity of secondary reactions (and related energetics) is also comparable to that of the wood pellets. The hazelnut shells and the olive pomace show the largest deviations with respect to wood, especially in relation to the global reaction exothermicity. Based on the chemical composition reported in Table 1, it appears that the contents of the lignin and hemicellulose components, which are generally thought to degrade exothermally, are very high. The extractive contents are also high, which have been observed to degrade exothermically, mainly producing char.4 The proximate analysis (Table 2) also shows that these residues own the highest fixed-carbon content, another factor that is directly related to high char production43 and again to more pronounced exothermicity. Therefore the peculiar chemical composition of these residues, specifically the low cellulose content and the high ash content (and/or its composition), may be a key issue in the observed high exothermicity. Moreover the beginning of the degradation process at relatively low temperature is enhanced by the significant rate of heat release, associated with extractive and hemicellulose decomposition, thus causing the simultaneous degradation of the other components over the entire bed volume with process dynamics similar to those of a thermal runaway. It should also be observed that hazelnut shells and olive pomace have not been subjected to pellettization, and this affects the fixed-bed density and permeability and probably the intraparticle activity of secondary reactions. The modification in the fixed-bed density, caused by the physical properties of the single fuel elements, influences the sample heating rate and thus the conversion time during pyrolysis.17 Slow heating rates, induced by high fixed-bed density, also reduce the spatial temperature gradients with an enlargement of the central almost uniform core of the bed, with consequent lower heat losses from this region toward the external annulus (this effect is similar to that caused by the onset of the pyrolysis reactions at low temperatures). As the fixed-bed density also affects the porosity/permeability, the importance of convective cooling associated with the forced nitrogen flow fed at ambient temperature, for the experimental system used in this study, may also vary (for sufficiently high porosity/permeability the cold nitrogen stream flows across both the void space between the bed and the reactor wall and the fixed bed). Indeed it is worth observing that an increase in the gas flow rate is indicated46 as one of the means to avoid large temperature overshoots in the industrial-scale application of mild thermal treatments of wood stacks in order to improve durability, dimensional stability, and decay resistance. The maximum temperature detected for beech wood experiments, where the fixed-bed density is varied as consequence of variations in the size or density of the single fuel elements,17 is reported in Figure 14 for a low (26.5 kW/ m2) and a high (36.5 kW/m2) heat flux. It should be noticed that the highest density value refers to a wood piece of the same size and shape as the fixed bed. For the high heat flux, the maximum temperature is practically constant for fixed-bed densities between 0.25 and 0.50 g/cm3. The same trend is also

lignin. Hence the impact of exothermic reaction effects is lower for the softwood variety in both torrefaction and pyrolysis, as also reported from DSC analysis of beech and spruce wood.44 More complicated is the behavior of agricultural residues in relation to the role played by chemical components in reaction exothermicity. Thermogravimetric curves under oxidative conditions (5 K/min)24 show that the residues of interest here present a peak devolatilization rate which is anticipated (by about 15−20 K) and is lower (by about 35−27%) with respect to beech wood. As a consequence of the peak anticipation, a separation between the shoulder and the peak zones, very clearly shown for beech wood, is not evident despite the similarities in the chemistry of the hemicellulose component. In reality the devolatilization of the cellulose component occurs at lower temperature which hinders the attainment of fast rates (comparable with those obtained for wood), due to the different amount and composition of ashes which also enhance the exothermic formation of char.44,45 Measurements carried out under inert conditions (heating rate of 5 K/min, not shown) confirm these findings, but the reaction process occurs at barely higher temperatures. More precisely degradation begins at temperatures between 503 and 510 K, Tshoulder varies between 553 and 561 K, Tpeak between 596 and 617 K, and Toffset between 630 and 638 K (versus 522 K, 566 K, 622 K, and 645 K, respectively, for beech wood). It is possible to evaluate already at low Ts values the characteristic center bed temperatures for the agricultural residues, due to the anticipation in the degradation process. Moreover, apart from straw, temperature overshoots and profiles testify a highly exothermic process which, as a consequence, also results in significantly higher heating rates contributing, together with ash catalysis, to enhance the overlap between the component degradation rates. Indeed, as wellknown from thermogravimetric analysis,35,38,39 the increase in the heating rate enhances the overlap between the various reaction zones and causes a displacement of the reaction process at higher temperatures. This can, in part, justify the higher characteristic temperatures measured for the hazelnut shells and the olive pomace in relation to the first peak and the valley in the bed center heating rate. Moreover, it is plausible that a larger part of the lignin component degrades simultaneously with the other components at lower temperatures. Accordingly the TLm values, concerning a smaller fraction of this component, are lower (straw and olive pomace) or approximately the same (hazelnut shells) as for wood. In conclusion, although the sequential occurrence of the thermal events at the bed center can still be attributed mainly to the same components previously identified for wood, the respective degradation processes are less separated (especially for hazelnut shells and olive pomace). Among the residues, straw presents the chemical composition and the pyrolytic behavior more similar to those of beech wood. In particular, the characteristic temperatures are also very close to those of beech wood apart from the already discussed beginning of the degradation process at lower temperatures and the slightly lower maximum TLm. It is interesting to observe that the maximum temperature overshoot in the plateau zone is practically the same as for beech wood (50 K versus 48 K), but the absolute maximum is slightly lower (78 K versus 86 K), despite the higher yields of chars, whose formation occurs exothermally. It is possible that the different lignin structure (many monolignols are substituted with acetyl or ferulic acid residues) is partly responsible for this result as well as for the 2693

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retention times from several minutes to about one hour50 (to facilitate the discussion these limits are indicated in Figures 2−5). It is evident that, if as a reference temperature Ts is chosen, the limit value of 573 K gives rise to solid yields below 70% only for the wood pellets. Instead, for the agricultural residues, the yields of charred product are much lower (up to 40% for hazelnut shells or 55% for straw). In fact for Ts values below 573 K, the temperature overshoots are already high with maxima up to 190 or 150 K for the hazelnut shells and the olive pomace (for the other fuels they remain below 15 K (softwood), 37 K (beech), or 40 K (straw)). When as indicative of the thermal conditions, the temperature Tmax is selected, it can be seen that for a value of 573 K, the yields of solid product remain above 70% for all the fuels examined. Furthermore, for this zone, there is a linear dependence on Tmax although with a different slope (higher for the agricultural residues). It has already been pointed out that, with reference to the maximum temperature, owing to the very large temperature overshoots, it is difficult to achieve intermediate conversion between torrefaction and severe pyrolysis. The findings of this study demonstrate that, in contrast with the usual assumption made for fixed-bed pyrolysis, the temperature providing a more meaningful representation of the thermal conditions is the maximum temperature and not the heating temperature. Depending on the size of the bed where temperature overshoots are significant, it can be understood that the solid product (torrefied material or biochar) may present highly nonhomogeneous properties. In conclusion the analysis of these results indicates that the reaction heat effects are more important for the agricultural residues than for wood, and, given their occurrence at relatively low temperatures, they can be important already for the torrefaction process. The temperature overshoots also affect the global rate of weight loss for fixed-bed pyrolysis. However, for wood, their occurrence at relatively severe thermal conditions, when spatial gradients of temperature along the radial direction are high, does not alter the expected qualitative trend with an increase in the rate of mass loss with the external temperature. Also, this same trend is preserved by the straw pellets, possibly owing to the moderate temperature deviations. On the contrary, the other two agricultural residues show an unexpected behavior with maximum rates that are attained at intermediate heating conditions, due to significant temperature overshoots over a large part of the bed volume. Hence, for these two agricultural residues, it is difficult to attain intermediate values not only for yields of solid products (between the low and high values corresponding to torrefaction and severe pyrolysis) but also of the volatile release rate. The sudden release, after an induction period commensurate with the relatively mild heating conditions, of large quantities of volatile products or, in other words, the occurrence of a thermal runaway, is thus a factor that should be given adequate consideration in the design and operation of fixed-bed reactors.

Figure 14. Maximum temperature, Tmax, achieved by the fixed bed made of beech wood pellets/particles of various size and by a beech wood block in the same shape and size as the fixed bed versus the bulk density of the fixed bed or the wood block, ρb.

shown for the low heat flux except for very low bed densities where the maximum temperature shows a rather steep decay. Moreover, for both low and high heat fluxes, the maximum temperature attains larger values for the high density of the wood block. These deviations from the constant value can be attributed to the negligible importance of convective heat losses in the case of the wood block, which is not prone to cold gas filtration (in addition to the effects of higher density already mentioned above). For the case of low heat flux and low bed density, they can be ascribed to the comparatively more important role played by the convective cooling, exerted by the external cold flow, with respect to the external heat flux and the exothermic reaction heat effects. Based on the range of densities reported in Table 3 (0.36−0.52 g/cm3) and the circumstance that the maximum temperatures (and temperature overshoots) are observed for the fuels with the lowest values (hazelnut shells and olive pomace) it can be reasonably assumed that factors other than this property should be responsible for the large reaction exothermicity. The variation in the size of the wood particles in fixed-bed reactors, unless in the pulverized form, has been found not to affect the yields of the lumped classes of products and their composition.17 This finding is explained by observing that the characteristic size of the process, which determines the temperatures of primary degradation and the residence time of vapors in the hot reaction environment, is the bed diameter, higher by several orders of magnitude than the particle size. It is also possible that the activity of secondary reactions is mainly located at the level of the microscopic cellular structure of wood,47−49 which is always preserved for nonpulverized material. However the textural structure of agricultural residues is different (in particular that of the hazelnut shells and the stone fraction of olive pomace) from that of wood, and it is not known whether or how this and the size/shape of the particles affect the activity of secondary reactions. This aspect requires further experimental investigation. Results of this study can be exploited to evaluate the importance of the temperature overshoots in relation to possible unwanted transition from torrefaction to pyrolysis and/or the achievement of pyrolysis conditions much more severe than those actually planned in practical conversion systems. For this scope it is useful to recall that torrefaction, a mild pyrolysis process, is generally established for temperatures around 473−573 K, solid product yields up to 70%, and solid

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CONCLUSIONS The fixed-bed pyrolysis of some agricultural residues (hazelnut shells, olive pomace, and straw) and softwood is investigated in relation to the thermal behavior, specifically the reaction heat effects, and a comparison is made with results already available for beech wood. The external heating conditions are varied so as to achieve near-surface final temperatures in the range 473− 800 K. In all cases significant temperature overshoots (with respect to the final steady values) are measured, whose 2694

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fixed bed in relation to the global exothermicity requires further investigation with the analysis of the pyrolytic behavior of a much larger variety of biomass fuels and a systematic examination of the effects of particle size for fuels other than wood. The fixed-bed pyrolysis results reported in this study put into evidence that reaction heat effects, specifically the exothermicity, may play a role of paramount importance in relation to the actual conversion conditions in practical systems. Some agricultural residues may be very problematic owing to the very high temperature deviations over a large part of the reaction volume (with respect to the imposed heating conditions), with the almost instantaneous release of the large part of the volatile content of the feedstock. In this way, without adequate control devices, the attainment of intermediate yields of solid products is difficult as the exothermic effects of the reaction may easily determine a transition from torrefaction to pyrolysis or to highly severe pyrolysis, reproducing the behavior typical of thermal runaway processes.

magnitude and spatial extension depend on the specific feedstock and the external heating conditions. The curve of the maximum temperature overshoot versus the heating temperature presents a single very high peak in the case of hazelnut shells and olive pomace (225 and 170 K). Also the extension of the spatial zone interested by such phenomena coincides with the entire cross section of the fixed bed but for very severe external heating. It appears that (primary and secondary) decomposition of all the components begins at lower temperatures (compared with beech wood) and occurs simultaneously with the reaction exothermicity giving rise to dynamic features qualitatively similar to those of thermal runaway processes. In fact, after a more or less long induction period, the temperature of the entire bed rises very rapidly with the consequent almost instantaneous release of the volatile matter content. Therefore the maximum global rate of mass loss is attained at intermediate heating temperatures. However, the temporal profile of the bed center temperature always permits the detection of two consecutive exothermic processes separated by an endothermic or thermally neutral process, as observed for beech wood. The degradation of straw also begins at low temperatures, but, similar to wood, the overshoot curve shows two sequential zones of maximum values on dependence of the heating conditions. It is reasonable to assume that, at low temperature, only hemicellulose and extractive degrade, and then, at higher values, the exothermic contribution results from the degradation of all the components. The temperature overshoots are again observed over almost the entire extension of the fixed bed, but the maximum values are moderate and approximately the same as for beech wood (78 versus 86 K). This circumstance permits a net separation between the decomposition zones of the various components and reproduces the usual trends of the global mass loss rate on dependence of the heating temperature. The behavior of softwood pellets is qualitatively similar to that of beech wood, but the appearance of temperature overshoots is displaced at slightly higher heating temperatures and the maximum values are lower (53 versus 86 K), testifying a lower exothermicity. For both wood categories, the beginning of the degradation reactions at relatively high temperatures is associated with the existence of spatial gradients across the bed. In this way the temperature overshoots are essentially observed only along the central core of the bed, with no evident alteration of the qualitative dependence of the global mass loss characteristics on the external heating conditions. The much higher temperature overshoots (and exothermicity) of the hazelnut shells and olive pomace in fixed-bed pyrolysis can be attributed in the first place to their specific chemical composition rich of extractives, hemicellulose and lignin, which degrade exothermally, and ash, which catalyzes exothermic char formation, and presumably to their peculiar textural structure which may favor the intraparticle activity of vapor-phase reactions (and again exothermic formation of secondary char). Also, it is plausible that the quantitative differences observed between the behavior of straw and softwood with respect to beech wood can be attributed mainly to the different content and nature of lignin and hemicellulose, respectively. In fact given the same pretreatments adopted for these feedstocks (milling and pelletization) it is expected that the material textural properties play a secondary role in this issue. However, the importance of the chemical composition and the textural properties of the single fuel elements of the

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AUTHOR INFORMATION

Corresponding Author

*Phone: 39-081-7682232. Fax: 39-081-2391800. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ef500296g | Energy Fuels 2014, 28, 2684−2696