Article pubs.acs.org/EF
Effects of the Torrefaction Conditions on the Fixed-Bed Pyrolysis of Norway Spruce C. Branca,† C. Di Blasi,*,‡ A. Galgano,† and M. Broström§ †
Istituto di Ricerche sulla Combustione, C.N.R., P.le V. Tecchio, 80125 Napoli, Italy Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy § Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, S-901 87 Umeå, Sweden ‡
ABSTRACT: Fixed-bed pyrolysis of Norway spruce wood previously subjected to torrefaction at temperatures between 533 and 583 K and retention times between 8 and 25 min was studied. Although the thermal pretreatment always results in an increased production of char at the expense of volatile products, appropriate torrefaction conditions give rise to maximum percentages of anhydrosugars, guaiacols possessing a carbonyl group, and phenols in the liquid fraction. Other carbohydrates (e.g., acetic acid, formic acid, hydroxyacetaldehyde, hydroxypropanone, furfural, and furfuryl alcohol) and the large majority of guaiacols show continuously decreasing values. The percentages of carbon monoxide and carbon dioxide in the gas product remain approximately the same, but that of methane slightly increases. The pyrolysis temperatures of torrefied wood are lower than those of the raw material, mainly because of the partial or complete absence of the exothermic contribution associated with extractives and hemicellulose degradation.
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INTRODUCTION The search for optimal conditions for biomass torrefaction to obtain a highly energetic and high-performing material is currently attracting significant research interest (e.g., see the reviews in refs 1and 2 and the more recent studies in refs 3−8). The effects of torrefaction on combustion or gasification of torrefied biomass have also been investigated in a number of studies, while less attention has been paid to the effect of torrefaction on the pyrolysis process, even though this is also the first stage of gasification and combustion technologies. The investigations have been carried out by means of auger reactors,9 fluidized-bed reactors,10−14 microwave cavities,15 and pyroprobes.16,17 Although in ref 16 a fixed-bed microreactor (5 g mass) was also used, the actual thermal conditions did not reproduce those of fixed-bed pyrolyzers and the pyrolysis zones of updraft and downdraft gasifiers. Hence, the influence of the torrefaction pretreatment on the fixed-bed pyrolysis of wood has not yet been examined. Moreover, these studies have focused exclusively on the changes in the pyrolysis products induced by torrefaction, with findings that are only in partial qualitative agreement, most likely as a result of differences in feedstock properties and torrefaction and pyrolysis conditions. In fact, no consideration has been given to possible changes induced by the torrefaction pretreatment on the pyrolysis dynamics, in particular the thermal aspects. Recent studies of the fixed-bed pyrolysis of lignocellulosic biomass18−20 provide evidence of significant exothermic effects at relatively low temperatures during the decomposition of the more thermally labile components, extractives and hemicelluloses. A second exothermic zone is observed at higher temperatures and is plausibly associated with the decomposition of lignin fractions. On the other hand, the torrefaction of thick wood pieces is also observed to cause, at the more © 2014 American Chemical Society
internal zones of the samples, significant temperature overshoots with respect to the final steady values reached once the reaction process terminates.21−23 Therefore, it can be expected that the partial degradation of the biomass components during torrefaction also induces changes in the amounts of heat released/absorbed during pyrolysis. However, as anticipated above, none of the previous investigations devoted space to the analysis of the possible changes induced by torrefaction on the heating rate and reaction temperature during the pyrolysis process. As a continuation of the analysis of the thermogravimetric behavior and devolatilization kinetics of Norway spruce wood torrefied at several process temperatures and residence times,24 in the present work fixed-bed pyrolysis experiments were conducted at both the bench and laboratory scales. The objective of the study is twofold: to ascertain the influence of the conditions for torrefaction of Norway spruce wood on both the yields/composition of the pyrolysis products and the pyrolysis temperature/heating rate.
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MATERIALS AND METHODS
Stem wood of Norway spruce, a coniferous tree, consisting of 42% cellulose, 26% hemicellulose, 31% lignin, and 1% extractives (as determined according to the standards SCAN-CM 71:09, Tappi T222, and SCAN-CM 49) was used as the feedstock. The samples were from the same batch used in a previous analysis,24 and a comparison is made between raw and torrefied material. The torrefied material was produced in a pilot plant in an inert atmosphere by feeding dry wood chips into an electrically heated rotating drum in which the torrefaction temperature and residence time were controlled. Table Received: June 23, 2014 Revised: August 11, 2014 Published: August 24, 2014 5882
dx.doi.org/10.1021/ef501395b | Energy Fuels 2014, 28, 5882−5891
Energy & Fuels
Article
Table 1. Properties of Raw Spruce [Sample (a)] and Spruce Torrefied at Several Process Temperatures (Tp)/Residence Times (tr) [Samples (b)−(e)] ash [%] fixed carbon [%] volatiles [%] C [%] H [%] O [%] S [%] Cl [%] N [%] energy yield [%] mass yield [%] HHV [MJ/kg] LHV [MJ/kg]
(a) raw wood
(b) 533 K/25 min
(c) 558 K/16.5 min
(d) 583 K/8 min
(e) 583 K/25 min
0.23 14.6 85.4 50.3 6.2 43.2
