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Biofuels and Biomass
Comparative study on pyrolysis of wet and dry torrefied beech wood and wheat straw Jie Jian, Zhimin Lu, Shunchun Yao, Xin Li, and Weifeng Song Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04501 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019
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Comparative study on pyrolysis of wet and dry torrefied
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beech wood and wheat straw
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Jie Jian1, Zhimin Lu1,*, Shunchun Yao1, Xin Li1, Weifeng Song2
4 5 6 7 8
1 School
of Electric Power, Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, South China University of Technology, No. 381 Wushan Road, Tianhe District, Guangzhou 510640, China 2Zhongshan
9 10
Thermal Power Generation Co.Ltd, No.18, Pu Nan Road, Huangpu Town, Zhongshan City, 528429 *
[email protected] Abstract
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This study compares the influences of wet and dry torrefaction on pyrolysis
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behaviors of beech wood and wheat straw. These two types of torrefaction were
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compared at similar mass loss of samples. FTIR and NMR were employed to
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investigate the chemical evolution during torrefaction pretreatment. Meanwhile TGA
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and Py-GC/MS were used for studying the kinetic features and product distribution of
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the subsequent pyrolysis. Analysis of the chemical composition indicated the
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disparate evolution direction of wet and dry torrefied biomass, leading to increased
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and decreased oxygen-containing functional groups respectively. And the chemical
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change of the wheat straw was more significant than that of beech wood. TGA results
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showed that the wet torrefaction increased while the dry torrefaction decreased the
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volatile content, resulting in lower and higher char yield, respectively. Bio-oil quality
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was improved by both types of torrefaction despite some divergences in product
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distribution.
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Key words: Dry/wet torrefaction; Py-GC/MS; NMR, TGA. 1
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1. Introduction
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With the population and economy growth, global energy demand is expected to
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increase by 37% in 2040 1. Then environmental sustainability and energy security are
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two major emerging issues in the world, which can only be addressed through the
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employment of clean fuels and the diversification in the energy resources. The
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biomass energy with its sustainability and the carbon neutral nature 2,3 could be one of
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the most promising solution 4. Biomass is capable to be used in energy applications
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such as the production of heat, power and transportation fuels 5.
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However some unfavorable properties of biomass such as its particularly high
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oxygen content, high moisture content, heterogeneity, low calorific value, hydrophilic
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nature, and poor conversion efficiency are still impeding the large-scale utilization of
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biofuel 6. To solve these problems torrefaction pretreatment has been proposed.
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Torrefaction usually refers to a mild pyrolysis step in an inert atmosphere, including
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dry torrefaction (DT) and wet torrefaction (WT). DT means biomass is heated to 200
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to 300 oC in an oxygen-deficient atmosphere with an appropriate residence time (0.5
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to 2 h) 7. WT is defined as hydrothermal pretreatment in hot compressed water at
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temperature within 180 to 260 oC 8. During both types of torrefaction, biomass starts
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to decompose and releases different kinds of volatile substance. For DT, the mass
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yield is about 70% with 90% of the energy retained 9 while the mass and energy yield
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for WT are around 41-90% and 80-95%, respectively
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torrefaction methods have been reviewed in articles 7,11–13.
10.
The effects of these two
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In recent years, scholars have started to conduct comparative studies on these
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two thermal pretreatment methods. The reactions of WT are different from DT as
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cellulose and lignin can be partially dissolved in water
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medium of water causes the biomass polymers partially transform into aqueous phase
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and therefore inducing a tremendous mass loss 15. However, Reza et al. reported that
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lignin might be unaffected by WT unlike experiencing a structural transition in DT 16.
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It has been suggested that the fuel properties of WT biomass were superior to those of 2
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Dependence upon the
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DT biomass. Processed at similar temperature, WT biomass not only has higher mass
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and energy density than DT biomass 15,17, but also has improved the pelletability 15,18,
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grindability and hydrophobicity 17. Besides these fuel properties, WT is capable to
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handle extremely wet feedstocks and considerably wash away the ash, which are the
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major hindrances to DT application 19. However, WT is also confronted with some
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challenges, for instance reactor corrosion, precipitation and deposition of the
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inorganic salts released during WT process of biomass, and the handling of produced
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aqueous residues 18. As WT is operated at elevated pressures, safety is a demanding
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issue 17.
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As to the effects of DT and WT on the pyrolysis of the treated biomass, the work
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from Bach et al. 20 revealed that DT had unpronounced effect on the activation energy
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of cellulose and lignin in subsequent pyrolysis while WT increased the activation
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energy. Ash content is another major discrepancy, for WT considerably removes ash
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content including alkali metals which are highly influential in pyrolysis while DT
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might slightly concentrate those elements
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lower char yield in pyrolysis than with DT but the char quality is upgraded due to the
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ash removal 22. WT corncobs produce less carbonaceous residues than raw corncobs,
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while DT corncobs yield much more residues owing to increased content of acid
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insoluble lignin
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pyrolysis vapors 22,23. These findings have proved the existing discrepancy between
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pyrolysis behaviors of DT and WT. However, so far the understanding to the
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discrepancy and similarity of their effects on the chemical composition as well as the
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pyrolysis product distribution has remained limited, especially for woody biomass.
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Filling the current research gaps is necessary for optimizing the pretreatment process.
23.
21.
As a result, corn stalk with WT has
Meanwhile WT is reported to enhance the sugar content in
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In this study, beech wood and wheat straw were employed as feedstocks,
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representing forestry woody biomass and common agricultural waste, to investigate
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the influences of DT and WT pretreatment on biomass pyrolysis. The relevant
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properties and pyrolysis behaviors were compared. Fourier transform infrared (FTIR)
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spectrometer together with Nuclear Magnetic Resonance (NMR) were used to identify 3
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the transition in the chemical composition. Thermogravimetric Analysis (TGA) was
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conducted to analyze the thermal degradation behavior from kinetic perspective.
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Additionally the pyrolysis oils from raw and treated samples were analyzed via
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pyroprobe
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(Py-GC/MS).
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2. Experimental
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2.1 Materials
analyzer
coupled
with
gas
chromatography/mass
spectrometry
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Beech wood and wheat straw were milled into powder form with sieved size of
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less than 0.3 mm. The proximate and ultimate analyses of the feedstock are given in
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Table 1. Straw clearly contained much more ash as well as higher concentration of
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alkali and alkali earth metals than beech wood.
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2.2 Dry and wet torrefaction
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DT was conducted with a three-zone electric tube oven 24. Beech wood and straw
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samples were torrefied at 290 °C for 30 min. Samples were held at ambient
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temperature before the oven was heated to set temperature. Before starting the
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reaction, 30 L of nitrogen was purged into the oven, and the nitrogen was kept at a
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flow rate of 1 L/min throughout the experiment to maintain an inner atmosphere
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inside the oven. Meanwhile WT was carried out by using a 250 ml autoclave reactor
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(Model SLM250, Beijing Shi Ji Sen Lang Experimental Instrument Co., Ltd., Beijing,
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China) 3. Feedstocks were subjected to the hot compressed water at 200 oC for 10 min.
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In each run of WT, approximately 5 g of feedstock was mixed with 100 ml deionized
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water in the reactor cylinder, and was manually stirred for homogeneous mixing. A
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magnetic stirrer was as well added into the reactor, and was kept stirring at a rate of
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500 rpm during the experiment. The reactor was heated up to set temperature at a
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heating rate of 7 to 10 oC/min. Although the mentioned residence time was defined as
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the duration of isothermal period at set temperature, the exact reaction time was still 4
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largely determined by preheating and cooling time. Afterwards the pretreated biomass
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was washed with deionized water. And then the WT samples were dried at 105 oC for
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6 h. The dry and wet torrefied samples were addressed with prefix “D” and “W”
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respectively.
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Different temperature and residence time were applied to DT and WT in order to
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achieve similar torrefaction mass yield, a key parameter, on each type of feedstock.
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With the above mentioned operation conditions, the mass yields of D-beech, W-beech,
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D-straw and W-straw were 68.4%, 62.8%, 55.4% and 53.5%, respectively.
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2.3 Product characterization
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Proximate analysis and ultimate analyses were obtained according to National
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Standard in China GB/ T 28731-2012 and General rules for elemental analyzer DL/T
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568-2013, respectively. Proximate analysis of solid products was performed using a
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muffle furnace. Sulfur was measured by an infrared sulfur analyzer (Sande SDS350,
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China) whilst other elements were measured using an element analyzer (Kaiyuan
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5E-CHN2200, China). Higher heating value (HHV) was then calculated based on the
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following Equation 2-1 25. (2-1)
124 125
KBr-tabletting based FTIR (Nicole, Model iS10) and solid-state
13C
CP/MAS
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NMR (BRUKER AVANCE III HD 400, Germany) were employed to analyze the
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chemical composition of the feedstocks. Solid-state
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out at a
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the magic angle of 90 ° and at a frequency of 5 kHz. The pulse length and acquisition
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time were set to be 6 μs and 2 s, respectively.
13C
13C-NMR
analysis was carried
frequency of 75.5 MHz at room temperature. The samples were spun at
5
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2.4 TGA
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The pyrolysis kinetics of raw and treated samples were measured with a
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thermogravimetric analyzer (NETZSCH STA 449F1). Approximately 2.5 mg sample
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was heated from ambient temperature to 800 °C at a heating rate of 20 oC/min with
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nitrogen at a flow rate of 100 mL/min. Tests were conducted in duplicate and the
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absolute difference on mass loss between replicates was always below 0.5 wt%.
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2.5 Py-GC/MS
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The fast pyrolysis was conducted in a pyroprobe analyzer (CDS 5200) coupled
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with GC/MS (Agilent 7890BGC, 5977AMS). The compositions of the volatiles from
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fast pyrolysis of raw and treated samples were determined. About 1 mg of sample was
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loaded in the pyrolysis quartz tube in nitrogen atmosphere. The flash heating rate was
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set as 10 °C/ms and the pyrolysis temperature was set to be 500 °C with a residence
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time of 30 s. Afterward the volatiles were detected by GC/MS. The injector
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temperature of GC was kept at 300 °C and the chromatographic separation was
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performed with a HP-5 MS (30 m×250 µm×0.25 µm). Column temperature was
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programmed from 50 °C for 2.25 min, then raised to 110 °C at a rate of 4 °C/min.
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Thereafter the temperature of the chromatographic column rose from 110 °C to 280
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°C with a heating rate of 10 °C/min. At last, the column temperature was held at 280
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°C for 5 min. Carrier gas was helium; at a flow rate of 1 mL/min; volume injected 1
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μL; split ratio 1:100; MS conditions: ionization energy 70 eV; mass scan range
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33–500 amu. On the basis of NIST 14 library, the yields of the compounds were 6
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calculated.
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3. Results and discussion
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3.1 Proximate and ultimate analysis
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Pictures of raw samples as well as DT and WT samples are shown in Fig 1. The
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samples lost their natural color and turned dark after torrefaction. Although the mass
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losses of DT and WT samples were close, the DT samples were of apparently darker
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exterior, which indicated severer dehydration and carbonization reactions due to
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higher treatment temperature.
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To evaluate the fuel property, the proximate and elemental analysis results are
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presented in Table 2. After torrefaction pretreatment, the fixed carbon content and
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HHV considerably increased while volatile and moisture content dropped, except for
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W-straw whose volatile content was even slightly higher than the raw feedstock. One
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major difference between DT and WT was that the dry process clearly enriched the
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ash content through the “concentration effect” 26 while the wet process decreased it by
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washing away. Compared with other samples the variation of D-straw was
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particularly pronounced as its fixed carbon and volatile contents were drastically
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different from the raw straw. The fixed carbon content of D-straw significantly rose
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from 7.8% to 41.0% while the volatile decreased from 80% to 46.6%. One possible
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explanation was the high ash content in D-straw, which strongly promoted the
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charring and crosslinking during the DT pretreatment and resulted in more char
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formation in subsequent proximate analysis 27. 7
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The elemental analysis suggested that the carbon and nitrogen content were
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increased after torrefaction. As for hydrogen and oxygen content, DT reduced these
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elements but WT slightly increased them. Further analysis through the Van Krevelen
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diagram is illustrated in Fig 2. Despite having similar mass loss, it can be seen that the
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fuel property of WT samples did not changed too much while the H/C and O/C ratio
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were considerably reduced after DT. And the effect of DT on straw was exceptionally
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pronounced as the D-straw laid on the lower left quarter of the diagram implying a
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fuel property resembling low-rank coal
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contained much more hemicellulose 29 which, degrading at around 225 oC 30, was the
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least thermal stable component in biomass. During DT hemicellulose considerably
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degraded and then went through carbonization making straw more susceptible to
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torrefaction pretreatment.
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3.2 FTIR analysis
28.
Compared with beech wood, straw
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The FTIR spectra of straw and beech wood with different pretreatments are
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presented in Fig 3a and Fig 3b revealing the evolution of functional groups. It can be
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seen that raw and treated straw samples had similar spectra, but differed in absorbance
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intensity. For D-straw in Fig 3a, the intensity of most peaks were clearly reduced. The
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peaks at 3410 and 2920 cm-1 correspond to O-H vibration and C–H stretching
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respectively. Their decline indicated the existence of intensive dehydration
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demethoxylation reaction 33 as well as the dissociation of side chains and methoxyls 34.
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Associated with carboxylic acid groups of hemicellulose, the peak at 1735cm-1 8
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31,
and
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disappeared after DT as hemicellulose considerably decomposed. The peaks between
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1400 and 1600 cm-1 were mainly methoxy group vibration of complicated lignin
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functional groups 35,36 ketone groups of hemicelluloses 15 as well as C-H deformation
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of cellulose
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indicating the intense thermal degradation of lignin and holocellulose. Marked
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decrease was also observed at the peaks situated around 1041 - 1160 cm-1 mainly
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corresponding to C=O and C-OR stretching, which indicated a substantial decline of
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polysaccharides 33 and alcohol groups from lignin 37. However, the benzene peak at
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around 1635 cm-1 representing C=C skeleton vibration in lignin
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distinct. Comparatively other aromatic peaks such as 900 cm-1 and 1380 cm-1
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corresponding to aromatic carbon hydrogen bonds and aromatic acids were weakened.
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Hence the lignin derived aromatic structure was more stable compared to other
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constituents.
31.
These peaks appeared to be significantly less obvious after DT
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was even more
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However in terms of WT, a disparate effect on straw was seen. The majority of
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peaks were strengthened when comparing the WT straw to raw straw. W-straw
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showed slightly strengthened peaks at 3410, 2920 cm-1 and complex peaks from 1400
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to 1600 cm-1 while the largest increase was observed at C-OH peaks at around 1100
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cm-1. This observation was in line with the elemental analysis in Table 2 that
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hydrogen and oxygen content appeared to be more abundant in WT samples. Similar
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trend that the O-H and C-H functional groups were increased by WT pretreatment
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was reported by Yao et al.
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phenomenon occurred only when treatment temperature is low (
