Thermal Decomposition Kinetics of Woods with an Emphasis on ...

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Sep 13, 2013 - content, poor grindability, low calorific value, and low bulk density. .... getting very low mass-loss rates in the entire domain of the reaction. The.

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Thermal Decomposition Kinetics of Woods with an Emphasis on Torrefaction Dhruv Tapasvi,† Roger Khalil,‡ Gábor Várhegyi,*,§ Khanh-Quang Tran,† Morten Grønli,† and Øyvind Skreiberg‡ †

Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway ‡ SINTEF Energy Research, Postboks 4761 Sluppen, NO-7465 Trondheim, Norway § Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, PO Box 17, Budapest, Hungary 1525 ABSTRACT: The pyrolysis kinetics of Norwegian spruce and birch wood was studied to obtain information on the kinetics of torrefaction. Thermogravimetry (TGA) was employed with nine different heating programs, including linear, stepwise, modulated and constant reaction rate (CRR) experiments. The 18 experiments on the 2 feedstocks were evaluated simultaneously via the method of least-squares. Part of the kinetic parameters could be assumed common for both woods without a considerable worsening of the fit quality. This process results in better defined parameters and emphasizes the similarities between the woods. Three pseudo-components were assumed. Two of them were described by distributed activation energy models (DAEMs), while the decomposition of the cellulose pseudo-component was described by a self-accelerating kinetics. In another approach, the three pseudo-components were described by n-order reactions. Both approaches resulted in nearly the same fit quality, but the physical meaning of the model, based on three n-order reactions, was found to be problematic. The reliability of the models was tested by checking how well the experiments with higher heating rates can be described by the kinetic parameters obtained from the evaluation of a narrower subset of 10 experiments with slower heating. A table of data was calculated that may provide guidance about the extent of devolatilization at various temperature−residence time values during wood torrefaction. an earlier work of Di Blasi and Lanzetta16 on xylan kinetics. The same model was used in a recent thermogravimetric analysis− mass spectroscopy (TGA-MS) study by Shang et al.15 Peng et al.12 used a one-component, one-step reaction model for torrefaction with long residence time and a two-component, one-step reaction model for torrefaction with short residence time. Chen and Kuo10 studied the torrefaction of hemicelluloses, cellulose, and lignin separately, using a global onestep reaction model for each. They described the torrefaction process of a biomass material by superimposed kinetics of the three components. The torrefaction kinetics is part of a broader subject: the pyrolysis kinetics of biomass materials. If a kinetic model describes the biomass pyrolysis well, then it can obviously be used for torrefaction kinetics. Moreover, such a model also can describe the pyrolysis behavior of the torrefied wood, if the experimental data used to determine the model parameters include temperature programs where the heating to higher temperatures is preceded by longer residence times in the temperature domain of the torrefaction. This pathway was followed in the present work. Such kinetic descriptions will be presented which describe both the lower- and the highertemperature regions of the wood pyrolysis well. The work is based on TGA experiments, because TGA is a high-precision

1. INTRODUCTION There is a growing interest in lignocellulosic biomass fuels and raw materials, because of climate change problems. However, the widespread use of biomass fuels is frequently hindered by their unfavorable fuel characteristics, such as high moisture content, poor grindability, low calorific value, and low bulk density. Torrefaction is one of the potential solutions to these problems, and it has gained research momentum as a biomass pretreatment process in the last two decades. It results in improved biomass fuel properties, such as reduced moisture content, higher energy density, improved hydrophobic behavior, and less energy consumption during grinding.1−3 Torrefaction is typically conducted at 200−300 °C, at atmospheric pressure, in the absence of oxygen and with particle heating rates below 50 °C/min.4 The lignocellulosic biomass is partially decomposed during the torrefaction, releasing condensable liquids and noncondensable gases into the gas phase.5 Primarily, the xylan-containing hemicellulose polymers decompose because they are the most reactive polymer structures in biomass.6,7 The extractives of the biomass also decompose while the cellulose and lignin are moderately impacted during torrefaction, depending on the feedstock composition and the torrefaction temperature.8 Many studies are available on the production and characterization of torrefaction products. However, fewer works address the torrefaction kinetics.9−15 Most of these studies are based on isothermal experiments. Prins et al.9 and Bates et al.11 employed a one-component, two-step successive reaction model, based on © 2013 American Chemical Society

Received: August 11, 2013 Revised: September 12, 2013 Published: September 13, 2013 6134

dx.doi.org/10.1021/ef4016075 | Energy Fuels 2013, 27, 6134−6145

Energy & Fuels

Article

Table 1. Proximate and Ultimate Analyses of the Samples Proximate Analysisa

a

Ultimate Analysisa

sample

volatile matter

fixed carbon

ash

C

H

O

N

S

HHVb

birch spruce

89.4 86.3

10.4 13.4

0.2 0.2

48.62 50.10

6.34 6.36

44.90 43.52

0.09 0.07

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