Article pubs.acs.org/EF
Role of Pretreatments in the Thermal Runaway of Hazelnut Shell Pyrolysis C. Di Blasi,*,† C. Branca,‡ A. Galgano,‡ and B. Gallo† †
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: This study examines the effects of various pretreatments (size reduction, pelletization, hot water washing, torrefaction) on the pyrolysis of packed beds of hazelnut shells, exposed to moderate heating along the lateral surface, aimed at identifying the controlling or predominant mechanisms for the high reaction exothermicity leading to thermal runaway. As long as the particles preserve their chief structural properties (roughly or finely crushed shells), the process dynamics are not altered by size variation. Instead exothermic effects are highly reduced or almost disappear for milled shells, even in the pelletized form, depending on the actual powder size. Hence, secondary intraparticle reactions, owing to a peculiar scarcely porous microstructure, play a paramount role in the process exothermicity and thermal runaway. The displacement of the beginning of the reaction process at higher temperature, following sample washing, is enhanced by deepening the pretreatment. In this way sample conversion takes place in the presence of large spatial gradients, which hinder the occurrence of thermal runaway. Finally torrefaction, by partial or complete conversion of the more thermally labile chemical components, also eliminates the conditions that trigger and partly sustain the thermal runaway.
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INTRODUCTION The yields and composition of pyrolysis products are largely dependent on the conditions under which heating is performed, in particular, temperature and heating rate.1−4 The properties of the pyrolysis products are also important for the processes of gasification and combustion. For instance, the reactivity of char and tar is highly dependent on the pyrolysis conditions.5−7 Apart from external heating and reactor configuration, the reaction heat also contributes to determine the actual conversion conditions, but these effects are not easily identifiable under practical pyrolysis conditions as heat transfer generally plays a predominant role. However, well documented experimental evidence8−18 shows that pyrolysis/torrefaction is an exothermic process at least at the center of thick samples or fixed (packed) beds uniformly heated along the external surface. Recent studies carried out for bench-scale fixed-bed pyrolysis of beech wood,19,20 based on detailed measurements of the thermal field, show maximum temperature overshoots, with respect to the steady final value, up to 90 K. Moreover, at the central core of the fixed bed, by means of the heating rate, the occurrence is visible of sequential (1) exothermic, (2) endothermic or neutral, and (3) exothermic effects. On the basis of the temperatures characterizing these events, a plausible association is made with specific chemical processes, namely, the primary and secondary decomposition of (1) extractives and hemicellulose, (2) cellulose and a lignin fraction, and (3) residual lignin. A further analysis,21 carried out for softwood species and some agroindustrial residues, not only confirms the existence of exothermic effects during pyrolysis but that they, in some cases (hazelnut shells and olive pomace) and appropriate (moderate) heating, give rise to process dynamics qualitatively similar to those of a thermal runaway. Primary and secondary © 2015 American Chemical Society
decomposition of all the components begins at lower temperatures compared with wood and, after a more or less long induction period, the entire bed is very rapidly heated, with temperature overshoots up to 230 K and the consequent almost instantaneous release of the entire volatile matter content. Therefore, contrary to the usual trends observed in pyrolysis, the maximum global rate of mass loss is attained at intermediate heating temperatures. It is difficult to identify the causes for the transition from pyrolysis to a pyrolytic thermal runaway, although some observations can be made.21 The comparison between the chemicophysical properties of agroindustrial residues and wood demonstrates that the former exhibit higher contents of extractive, hemicellulose and lignin (chemical composition), and fixed carbon and ash/alkali compounds (proximate analysis). Also, residues and wood are expected to have different textural properties that can influence the intraparticle activity of secondary reactions. This study examines the effects of various pretreatments on the bench-scale fixed-bed pyrolysis of hazelnut shells which have been observed21 to behave with a highly exothermic character. The main aim of this analysis is to evaluate the role played by various factors on the thermal runaway and possibly to identify the controlling or predominant mechanisms. The pretreatments investigated concern size reduction and pelletization, to ascertain the role played by intraparticle (or intrapellet) secondary reactions, washing to reduce the alkali content, and thus to modify the initial degradation temperature and reaction selectivity, and torrefaction to reduce the contents Received: January 23, 2015 Revised: March 20, 2015 Published: March 20, 2015 2514
DOI: 10.1021/acs.energyfuels.5b00171 Energy Fuels 2015, 29, 2514−2526
Article
Energy & Fuels
0.28−0.45 (38.4%)). These two samples are identified as roughly milled (RM) and finely milled (FM). Moreover, the FM particles are used to produce pellets (intrinsic density 1.21 g/cm3) that are halved into two 5−6 mm sized parts (thickness and half-width). The particle sizes affect the packing properties, so that the bulk density of the bed is also significantly modified (values from 0.36 to 0.68 g/cm3, corresponding to sample mass in the range 17−32 g), as indicated in Table 1. The FC sample was used for the other pretreatments and for comparison (Table 1). To reduce or to remove the alkali compounds in ashes, water washing was applied, using two different sets of conditions.22 Hot water washing (W) considers a biomass to distilled water ratio of 100 g/L with a temperature of 353 K and a washing time of 2 h under continuous stirring. The sample is then filtered with a 43μm stainless steel mesh sieve. Deep (hot) water washing (DW) consists of three stages: the first and the third one consider a biomass to distilled water ratio of 35 g/L at a temperature of 353 K for 4 h, again under a continuous stirring. The second (intermediate) stage consists of sample soaking in still distilled water, again with a biomass to water ratio of 35 g/L, at ambient temperature, for 15 h. The W and DW pretreatments cause a reduction in the total ash content from about 0.96% to 0.68 and 0.55%, respectively (with reductions of 29 and 43%). Taking into account that only the soluble part is actually lost and that, for the feedstock under study, the total amount of alkali compounds is around 40%,23 it is plausible that these are in large part or completely removed. Moreover, after sample filtration, the washing water is brown, evidence that some organic material is also lost, most likely part of the tannins. Subjected to evaporation, it is found to produce a solid residue amounting to 1.2% (washing) or 3% (deep washing) of the initial sample mass. The last pretreatment is torrefaction24 (To), carried out by means of the packed-bed system already described elsewhere,19−21 which is also used for the pyrolysis experiments. During the experiments the sample is packed in a stainless steel mesh, cylindrically shaped holder (40 mm diameter and length or depth) which is instantaneously exposed to an assigned radiative heat flux along its lateral surface. The size of the holder coincides with the size of the packed bed. This is vertically positioned in the uniformly heated zone of a radiant furnace, that emits radiant energy in proportion to the applied voltage. To avoid interaction between the volatile pyrolysis products and the furnace, a quartz tube (internal diameter 60 mm), transparent to infrared radiation, is located inside the furnace and used as a reaction chamber. Nitrogen, fed at ambient temperature, flows with a nominal velocity of 1.3 cm/s from the top of the quartz reactor to establish the proper reaction environment. Temperatures along the sample 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). Nitrogen and volatile pyrolysis products pass through a condensation train consisting of two water/ice cooled condensers, a wet scrubber, a cotton wool trap, and a silica gel bed (all connected in series). For the torrefaction pretreatment, heat fluxes of 20.9 and 21.5 kW/ m2 are applied, corresponding to heating temperatures, Ts, of 518 and 539 K, respectively (these temperatures are those reached under the heat exposed surface of the torrefied material for a retention time of 1 h). The yields of torrefied product, for the two cases, amount to 90.5% and 80.5%, and the samples are indicated with the acronym To1 and To2, in the order. The standard techniques of chemical analysis may give inaccurate evaluations when applied to torrefied samples, and, on the other hand, the changes induced by this pretreatment are not simply related to a modified chemical composition of the sample but also to different properties of the components.24 However, given the very mild heating conditions (small spatial gradients of temperature) and based on the total amount of volatile generated and the data available from thermogravimetric analysis,25 which report a total volatile content generated from extractive and hemicellulose around 19%, it can be estimated that for the samples To1 and To2 the half and the total content of these components, respectively, has undergone decomposition.
of extractives and hemicellulose, that may promote through exothermic reactions the degradation of the other components. Combined pretreatments, i.e., washing followed by torrefaction, are also investigated.
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MATERIALS AND METHODS
Samples consisted of hazelnut shells of the same batch used before.21 The chemical composition, on ash-free basis, is made of 56.8% holocellulose (30.5% cellulose and 28.9% hemicellulose), 33.7% lignin, and 9.5% extractives, whereas the proximate analysis gives 77.7% volatile matter, 21.3% fixed carbon, and 0.96% ash. Experiments are made for randomly packed beds made of pellets or particles, preliminarily predried (oven drying at 373 K for at least 12 h followed by exposure to a forced air flow at about 393 K for additional 30 min before the experiment). However, it can be expected that the initial moisture content may affect the pyrolytic thermal runaway, depending on the overlap degree between the endothermic evaporation of water and the exothermic degradation of wood. In fact the degradation dynamics of moist wood cylinders8 is highly dependent on both the initial moisture content and the external heating conditions. These aspects need to be considered in successive studies. Different pretreatments are implemented, always starting from predried samples (Table 1). The hazelnut shells that, as received,
Table 1. Size and Bulk Density of the Packed Bed Consisting of Roughly and Finely Crushed (RC and FC) Shells, Roughly and Finely Milled (RM and FM) Shells, Pellets (P) Made of FM Shells, Washed (W) and Deep Washed (DW) FC Shells, Torrefied (To1, To2) FC Shells, and Deep Washed and Torrefied (DWT02) FC Shellsa length [mm]
thickness [mm]
bulk density [g/cm3]
RC FC RM FM P W
