Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Effect of Torrefaction on the Structure and Pyrolysis Behavior of Lignin Gongxin Dai, Qun Zou, Shurong Wang,* Yuan Zhao, Lingjun Zhu, and Qunxing Huang* State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38, Hangzhou, Zhejiang 310027, People’s Republic of China ABSTRACT: The influences of torrefaction on the structure and pyrolysis behavior of lignin were investigated in this study. After torrefaction, the oxygen content of lignin was reduced, which led to the increase of its high heating value. The main reactions occurring during torrefaction were the cleavage of aryl ether linkages, demethoxylation, and dissociation of the aliphatic side chain, as indicated by Fourier transform infrared spectroscopy analysis. The distributed activation energy model with double combined Gaussian functions was adopted to analyze the pyrolysis kinetics of lignin. It was found that torrefaction increased the activation energy for degradation and condensation reactions. The devolatilization contribution of condensation reactions increased as well after torrefaction. Torrefaction also had a great effect on the pyrolytic product distribution of lignin. The yields of non-methoxylated phenols and phenols without a propyl side chain increased after torrefaction. lignin. Wang et al.10 observed that the propyl branches of lignin were dissociated during torrefaction through 13C CP/MAS NMR. However, the functional group signals of cellulose and hemicellulose might disturb that of lignin when performing studies on the entire biomass. Now, there have been some studies on individual lignin. Wen et al.11 investigated the influence of torrefaction (200−300 °C) on the structure of milled wood lignin isolated from bamboo by various structural characterization techniques, including Fourier transform infrared spectroscopy (FTIR) and solution-state NMR techniques [13C NMR, two-dimensional heteronuclear single-quantum coherence (2D-HSQC), and 31P NMR]. The results showed that reactions including fragmentation, depolymerization, condensation, and demethoxylation occurred together with the cleavages of the main linkages in lignin during torrefaction. However, the influence of torrefaction on the following pyrolysis process was not taken into consideration in this study. Mahadevan et al.12 studied the effect of mild pyrolysis (150−225 °C) on the structure and pyrolysis behavior of lignin. They found that condensation and demethoxylation reactions occurred at a higher torrefaction temperature and more phenolic compounds and char yielded from the pyrolysis of lignin after torrefaction, whereas only mild torrefaction was considered in their study, and the preferable torrefaction conditions (such as 250 °C for fast growing polar and white pine10,13 and 275 °C for bamboo11) were missing. Therefore, it is still necessary to perform detailed studies to explore the evolution of the lignin structure during torrefaction at the whole temperature range of 200−300 °C and its influence on the subsequent pyrolysis.
1. INTRODUCTION The tremendous consumption of fossil fuels in the last few decades has given rise to severe energy and environmental issues. The substitution of conventional fossils by renewable energy, such as biomass energy, is attracting more and more attention from academia and industry. Pyrolysis, the thermal degradation of organics in an inert atmosphere, is a promising platform to produce liquid fuels and value-added chemicals from biomass.1 Lignin, one of the major components of biomass, occupies 18−35% of biomass.2 It is a three-dimensional, complex polymer composed of three basic units named p-hydroxyphenyl (P), guaiacyl (G), and syringyl (S), which are linked with each other through ether linkages (α−O−4, β−O−4) and C−C bonds (5−5, β−5, and β−β). Lignin is the main byproduct of the conventional pulp and paper industry and lignocellulose-toethanol industry.3 However, these lignins are not suitable as the feedstock of pyrolysis as a result of their high moisture content. Removal of water in advance through thermal pretreatment is necessary. Torrefaction, also called mild pyrolysis, is a common thermal pretreatment technology for biomass performed at 200−300 °C. Besides the drying of biomass, torrefaction can remarkably improve the quality of feedstock by enhancing its mass and energy density,4 grindability,5 and hydrophobicity.6 During torrefaction, lignin will be partially decomposed, because it is mainly degraded in the temperature range of 160−800 °C.7 Chen et al.8 found that the mass of lignin reduced by 7.4% after torrefaction at 300 °C for 1 h. The knowledge of the structural change of lignin during torrefaction is important, because it will significantly affect the following pyrolysis process. According to the structural characterization of the raw and torrefied biomass, some information about the evolution of the lignin structure during torrefaction has been obtained. Neupane et al.9 studied the structure changes of pinewood by solid-state 13C crosspolarization magic angle spinning nuclear magnetic resonance (13C CP/MAS NMR) and found that torrefaction mainly led to the cleavage of aryl ether linkages and demethoxylation of © XXXX American Chemical Society
Special Issue: 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 9, 2017 Revised: November 23, 2017 Published: November 28, 2017 A
DOI: 10.1021/acs.energyfuels.7b03038 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels In this study, commercial organosolv lignin was used as the model compound of lignin. FTIR analysis was first employed to study the evolution of the lignin structure during torrefaction. Then, kinetic analysis, thermogravimetry−Fourier transform infrared spectroscopy (TG−FTIR), and pyrolysis equipped with gas chromatography−mass spectrometry (Py−GC/MS) were performed to explore the changes of the pyrolysis behaviors after torrefaction, thus building the influence mechanism of torrefaction on the pyrolysis behaviors of lignin.
where α is the solid conversion rate, β is the heating rate, E is the activation energy, A and R are the pre-exponential factor and universal gas constant, respectively, and f(E) is the distribution function of activation energy, as shown in eq 4
2. MATERIALS AND METHODS
where E01 and E02 are the mean activation energies for degradation and condensation reactions, respectively, σ1 and σ2 are their corresponding standard deviations, and ω is the weight factor, which is introduced to assess the devolatilization contributions for the two reactions. The pyrolytic product distributions of the raw and torrefied lignin were analyzed by Py−GC/MS. About 0.5 mg of sample was loaded in a micropyrolyzer (PY-3030D, Frontier Laboratories, Japan) and pyrolyzed at 600 °C for 30 s. The pyrolytic products were swept into GC/MS (Trace DSQII, Thermo Scientific) for analysis by helium (99.999%), and the injector temperature was 250 °C. The GC oven was heated from 40 °C (1 min hold) to 240 °C (20 min hold) at a heating rate of 5 °C min−1. The split ratio and temperature of the GC/ MS interface were 1:30 and 250 °C, respectively. MS was operated in electron ionization (EI) mode at 70 eV. All of the pyrolytic products were identified according to the National Institute of Standards and Technology (NIST) MS library. Py−GC/MS was repeated 3 times, while the elemental analysis was repeated twice. The standard deviations were provided in this study.
f (E) = ω
⎡ (E − E )2 ⎤ 1 01 ⎥ + (1 − ω) exp⎢− σ1 2π 2σ12 ⎦ ⎣ 1
σ2
2.1. Materials. Organosolv lignin from beechwood (Sigma-Aldrich, St. Louis, MO, U.S.A.) was used to represent lignin and was first dried at 60 °C until the mass remained constant. Then, it was torrefied in a tube furnace. The volatiles released during torrefaction were swept away by pure nitrogen (99.99%). In each experiment, about 4 g of lignin sample was loaded into the tube furnace for 30 min when the temperature reached the set temperature (200, 225, 250, 275, and 300 °C). L60, L200, L225, L250, L275, and L300 were used on behalf of the raw and torrefied lignin. 2.2. Structural Characterization. A vario MICRO elemental analyzer (Elementar Analysensysteme, Hanau, Germany) was adopted to perform the elemental analysis for the raw and torrefied lignin. The high heating values (HHVs) of samples were determined on the basis of the method proposed by Demirbaş,14 as shown in eq 1
HHV = (33.5[C] + 142.3[H] − 15.4[O] − 14.5[N])/100
⎡ (E − E )2 ⎤ 02 ⎥ exp⎢− 2π 2σ2 2 ⎦ ⎣
(4)
(1)
3. RESULTS AND DISCUSSION 3.1. Basic Characteristics. Torrefaction resulted in certain decreases in the mass of lignin. The mass yields were 96.03% for L200, 94.16% for L225, 90.64% for L250, 85.24% for L275, and 81.30% for L300. Torrefaction also changed the elemental composition of lignin. Both the mole ratios of H/C and O/C reduced with the elevated torrefaction temperature, as shown in Figure 1. There was a linear relation between the changes of the
where [C], [H], [O], and [N] are the C, H, O, and N mass contents in biomass. The energy yield was calculated using eq 2 energy yield =
m i HHVi m0 HHV0
(2)
where m0 and mi are the masses of the raw and torrefied lignin, respectively, and HHV0 and HHVi are their corresponding high heating values. The changes of the lignin structure after torrefaction were characterized by FTIR (Nicolet 5700, Thermo Fisher Scientific Corporation, Waltham, MA, U.S.A.). A total of 1 mg of lignin sample was mixed with KBr at a ratio of 1:100 (w/w) and was fully ground. The FTIR spectra were recorded over the wavenumber range of 400− 4000 cm−1 with the resolution of 4 cm−1, and each spectrum was accumulated from 36 scans. The relative intensity (Iv/Im) was used to characterize the evolution of typical functional groups during torrefaction, where Iv is the vibration intensity of each functional group and Im is the maximum vibration intensity among all samples for a given functional group. 2.3. Pyrolysis Behavior Analysis of the Raw and Torrified Samples. The kinetic analysis was performed in a thermogravimetric analyzer (STA 449F5, Netzsch, Germany). The samples were heated from 25 to 800 °C with a heating rate of 20 °C min−1. The released volatiles were swept into the equipped FTIR spectrometer (VERTEX 70, Bruker, Germany) by pure nitrogen (99.99%) with a flow rate of 40 mL min−1. The spectra were recorded between 4000 and 400 cm−1 with a resolution of 4 cm−1 and a scan rate of 32 min−1. Then, the distributed activation energy model with double combined Gaussian functions (DG-DAEM) was used to further analyze the effect of torrefaction on the devolatilization process of lignin, as shown in eq 3. The double Gaussian functions were corresponding to the degradation reactions, leading to the direct volatilization of tar, and the condensation reactions, leading to the formation of char and small-molecule gases15,16
α=1−
∫0
∞
⎛ A exp⎜− ⎝ β
∫T
T
0
⎞ e−E / RT dT ⎟f (E) dE ⎠
Figure 1. Variations in O/C, H/C, and HHV of torrefied lignin.
H/C and O/C ratios with the slope (ΔH/C/ΔO/C) of 1.85. The release of CO2 and CO during torrefaction made the elimination of oxygen faster than that caused by pure dehydration (ΔH/C/ΔO/C = 2). The value of ΔH/C/ΔO/C for lignin was larger than that for cellulose (1.55)17 and hemicellulose (1.65).16 This might result from the release of CH4 and CH3OH produced through demethoxylation during lignin torrefaction. With the removal of oxygen, the content of C−C and C−H bonds in lignin was enhanced, while that of C− O and O−H bonds was reduced. As indicated by Ben et al.,18 the former had greater ability to release energy. As a result, the
(3) B
DOI: 10.1021/acs.energyfuels.7b03038 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 2. FTIR spectra of the raw and torrefied lignin.
The bands at 1150 and 1116 cm−1 are assigned to the C−H inplane deformations of guaiacyl units and C−H in-plane deformations of syringyl units, respectively. Their reduction illustrated the degradation of guaiacyl and syringyl units.11 As for the intensity decrease of the peak at 1214 cm−1 (C−C, C− O, and CO stretching), it was mainly ascribed to the condensation reactions during torrefaction.11,12 3.3. Kinetic Analysis. On the basis of the aforementioned analysis, kinetic analysis was performed to characterize the variation of lignin pyrolysis behavior after torrefaction. The thermogravimetry (TG) result was shown in Table 1.
HHV of lignin after torrefaction rose continuously from 25.83 ± 0.04 MJ/kg for L60 to 27.40 ± 0.02 MJ/kg for L300. However, as a result of the more severe trend of the decrease in the mass yield than that of the increase in HHV, the energy yield of torrefaction reduced. The corresponding energy yields for lignin after torrefaction at 200−300 °C were 97.47, 95.91, 93.26, 89.11, and 86.22%, respectively. 3.2. Structural Analysis of the Raw and Torrefied Lignin. The structure evolution of lignin during torrefaction was studied by FTIR, as shown in Figure 2. The band assignment was mainly according to refs 12 and 19. The 3460 cm−1 band is ascribed to the stretching vibration of −OH. Its intensity decreased sharply with the increasing temperature, which indicated the existence of dehydration during lignin torrefaction. The bands at 2934 and 1460 cm−1 are attributed to the C−H stretch and C−H asymmetric deformations in CH3 or CH2 groups, respectively, which exist in the methoxyl or aliphatic side chain in lignin, while the band at 2842 cm−1 is corresponding to C−H vibration in CH3 of methoxyl. The decrease in their intensities suggested the dissociation of the propyl side chains and methoxyls during torrefaction. The peak at 1712 cm−1 represents the unconjugated CO groups. It increased with the processing of torrefaction, which was possibly due to the dehydration reactions. The signals at 1513 and 1427 cm−1 correspond to the aromatic skeletal vibrations and the aromatic skeletal vibrations combined with C−H in-plane deformation, respectively. Their intensities reduced as the torrefaction temperature increased, especially after high-temperature torrefaction. This was also confirmed by Mahadevan et al.12 and Wen et al.11 Wen et al.11 ascribed it to the degradation of the aromatic structures. The 1370 cm−1 band is the characteristic for the aliphatic C−H stretch in CH3 (not in OCH3) and phenolic OH. It decreased with the elevating torrefaction temperature. A study performed by Wang et al.20 showed that demethoxylation of lignin could occur through the dissociation of entire methoxyl or methyl in methoxyl, in which the latter was more kinetically favorable. Thus, the existence of demethoxylation reactions during torrefaction had a positive effect on the intensity change of the 1370 cm−1 peak. However, the dissociation of aliphatic side branches would lower its intensity. The decrease of the intensities of the syringyl ring breathing with C−O stretching and guaiacyl ring breathing with C−O stretching (1330 and 1271 cm−1, respectively) implied the cleavage of aryl ether linkages,12 which could also be proven by the intensity change of C−O deformation at Cα and aliphatic ether (1030 cm−1).21
Table 1. TG and Differential Thermogravimetry (DTG) Analyses of the Raw and Torrefied Lignin Ti (°C)a Tm (°C)b (dm/dT)max (%/°C)c Ychar (%)d Y′char (%)e
L60
L200
L225
L250
L275
L300
252.7 379.4 0.36
269.4 381.5 0.37
287.0 386.6 0.37
314.3 389.0 0.39
327.0 387.8 0.41
335.3 396.3 0.38
36.7 36.7
38.0 36.5
39.4 37.1
40.8 37.0
44.1 37.6
47.8 38.9
a The temperature corresponding to the mass loss of 5%. bThe temperature corresponding to the maximum mass loss rate. cThe maximum mass loss rate. dThe char yield at 800 °C. eThe nomalized char yield at 800 °C. Y′char = Ychar × mass yield.
According to the structure analysis, the thermal unstable structures, such as aryl ether linkages, propyl side chains, etc., were cleaved during torrefaction. This largely enhanced the thermal stability of lignin and increased the initial decomposition temperature (Ti). The temperature corresponding to the maximum mass loss rate (Tm) described the degradation of the lignin main structure. Its increase might be due to the condensation reaction occurring during torrefaction, which enhanced the difficulty for the main decomposition process. The condensation reaction during torrefaction also increased the proportion of polymerization and cross-linking reactions in the following pyrolysis process and largely enhanced the char yield at 800 °C (Ychar). The maximum mass loss rate ((dm/ dT)max) showed an increase after torrefaction at a lower temperature (
