Effects of the Torrefaction Conditions on the Fixed-Bed Pyrolysis of Norway Spruce

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. ■ INTRODUCTIONThe 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 biomass 18−20 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 internal zones of the samples, significant temperature overshoots with res...

Effect of the Torrefaction Temperature on the Structural Properties and Pyrolysis Behavior of Biomass

“…Bio-oil is mainly composed of aldehydes, ketones, acids, esters, phenols, saccharides, and hydrocarbons. When torrefaction temperature was increased, the phenol content in bio-oil noticeably increased because the production of phenolic compounds was mainly caused by the occurrence of ether bond breakage and the reaction of demethyl oxygen in the process of torrefaction (Branca et al 2014). During torrefaction, hemicellulose and cellulose became partly cracked, which resulted in the increase of the relative content of lignin.…”

Composition, Utilization and Economic Assessment of Torrefaction Condensates

“…These compounds are mainly degradation products of hemicelluloses. 16,17 Acetic acid and methanol comprised 80−90 wt % of the compounds in the fractions.…”

Effects of water washing and torrefaction pretreatments on rice husk pyrolysis by microwave heating

“…Therefore, to improve the qualities of solid, liquid and gas products from pyrolysis, the pretreatment of biomass prior to pyrolysis is an alternative and efficient approach (Chen et al, 2014;Ren et al, 2013 Torrefaction, also referred as a mild pyrolysis, is a thermochemical process for pretreatment of biomass at the temperature range between 200 and 300°C under an inner atmosphere and low heating rate (Mei et al, 2015;Ren et al, 2012). It can be seen that most studies mainly focus on the effect of torrefaction on the pyrolysis characteristics of woody biomass (Branca et al, 2014;Ren et al, 2013;Zheng et al, 2013). Only a few studies are performed to investigate the effect of torrefaction on the pyrolysis behavior of rice husk (Chen et al, 2014).…”