Uncovering StructureâReactivity Relationships in Pyrolysis and

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Uncovering Structure–Reactivity Relationships in pyrolysis and gasification of biomass with varying severity of torrefaction Luwei Li, Yuqian Huang, Dongyan Zhang, Anqing Zheng, Zengli Zhao, Mingzhu Xia, and Haibin Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04649 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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ACS Sustainable Chemistry & Engineering

Uncovering Structure–Reactivity Relationships in pyrolysis and gasification of biomass with varying severity of torrefaction

Luwei Li1,2, Yuqian Huang2,3,4,5, Dongyan Zhang1,2, Anqing Zheng∗2,3,4, Zengli Zhao2,3,4, Mingzhu Xia∗1, Haibin Li2,3,4 1. School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, PR China. 2. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. 3. CAS Key Laboratory of Renewable Energy, Guangzhou 510640, PR China. 4. Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China. 5. University of Chinese Academy of Sciences, Beijing 100049, China. ∗

Corresponding author, Tel.:+86 02087057716. Fax: +86 02087057737. E-mail address:

[email protected]

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The objective of this study is to establish the structure-reactivity relationships in pyrolysis and gasification of biomass with varying severity of torrefaction. Pine was torrefied in a bench scale tubular reactor with varying torrefaction temperature (220-300 °C). The structural alterations in torrefied pine and its derived biochar were characterized by solid-state nuclear magnetic resonance spectroscopy (NMR) and Raman spectroscopy, respectively. The effect of torrefaction severity, as well as the resulting structural changes in pine, upon subsequent pyrolysis and gasification reactivity were systematically studied. The experimental results showed that the pyrolysis reactivity of pine was promoted by torrefaction, whereas the gasification reactivity of biochar derived from pine was reduced by torrefaction. The results were mainly attributed to the severe degradation, polycondensation and carbonization of hemicellulose and lignin fractions during torrefaction of pine. The pyrolysis and gasification reactivity of torrefied pine was a strong linear function of its aromaticity or H/C molar ratio. Therefore, H/C molar ratio and aromaticity of pine were good indicators to quantitatively assess the structural alterations of pine during torrefaction and their impacts on the reactivity of subsequent pyrolysis and gasification. These findings provide a simple and efficient method to predict the pyrolysis and gasification reactivity of biomass with varying severity of torrefaction.

Keywords: Torrefaction; Pyrolysis reactivity; Gasification reactivity; Structure alterations


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Introduction

Lignocellulosic biomass is the widely available source of renewable organic carbon that can be used as a potential alternative to fossil fuel for the sustainable production of chemicals, liquid fuels, power and heat1-6. Lignocellulosic biomass is typically considered as carbon neutral, and it usually contains much less sulfur and nitrogen content than fossil fuels. However, despite these advantages, the efficient utilization of lignocellulosic biomass is hampered by its inherent physicochemical properties including structural heterogeneity, high moisture content, hydrophilicity, low bulk density, tenacious fibrous structure and low energy density7. These unfavorable properties of lignocellulosic biomass pose enormous technical, logistical and economic challenges for their collection, storage, transportation, grinding, feeding and fluidization8, 9. Torrefaction is an ideal pretreatment method to address these technical, logistical and economic issues in large-scale sustainable energy solutions10. Torrefaction is a low-temperature pyrolysis process carried out at 200-300 °C under inert atmosphere11-14. The quality of biomass can be improved by torrefaction from these aspects: (1) the composition of biomass can be homogenized by the polycondensation and carbonization of hemicellulose, lignin and cellulose fractions during torrefaction15-17; (2) the moisture of biomass can be reduced to less than 5wt.% by torrefaction; (3) the hygroscopic nature of biomass can be destructed by the loss of surface hydroxyl functional group during torrefaction, thereby yielding hydrophobic material that is resistant to moisture uptake, biological decay and easy to storage18-20; (4) the bulk and energy density of biomass can be effectively improved through various deoxygenation reactions (dehydration, decarboxylation and decarbonylation) during torrefaction, which is beneficial for subsequent transportation and conversion processes. The energy yield of 90% can be achieved with a mass loss of 30% during torrefaction12, 21; (5) The cell wall structure of biomass can be partially destroyed by torrefaction, ACS Paragon Plus Environment


thus making biomass particle more brittle and less fibrous. Torrefaction can reduce the energy demand of grinding by about 70%22, 23; (6) biomass fibers can be shortened and the biomass particles tend to become more spherical by torrefaction, which render torrefied biomass more easily fluidizable, and less prone to undergo arching and bridging in feeding system through suppressing the interlocking and potential coherence between biomass particles24. Due to these merits, torrefaction of biomass has received considerable attentions from academia, industry and governments. Torrefaction coupled with pyrolysis or gasification is a promising technology for obtaining various solid, liquid or gaseous product25-28. Up to date, numerous studies have demonstrated that torrefaction of biomass can exert important impacts on its subsequent pyrolysis or gasification process. Zheng and co-workers found that the crosslinking and charring of biomass during torrefaction can lead to the increase in char yield and decrease in liquid yield during fast pyrolysis17. Chen and couhert demonstrated that torrefaction can significantly improve the gasification performance of biomass. Gasification of torrefied biomass exhibited higher carbon conversion and cold gas efficiency compared with raw biomass29, 30. Cheah and co-workers reported that torrefaction can effectively reduce the formation of tar during gasification31. Fisher and co-workers showed that torrefaction significantly reduced char reactivity during gasification32. The changes in reaction rate and product distribution from pyrolysis or gasification of biomass before and after torrefaction were primarily attributed to the compositional and structural alterations of biomass during torrefaction. Biomass usually contains three major components: hemicellulose, cellulose and lignin33, 34. Each of these components exhibits unique thermal stability and reaction chemistry during torrefaction. The rank order of their thermal stability during torrefaction was hemicellulose < lignin < cellulose. Hemicellulose was the most reactive components35. However, there is a lack of quantitative information on the structural transformations of biomass during


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torrefaction and their influences on subsequent pyrolysis and gasification reactivity. Previous studies demonstrated that the pyrolysis and gasification reactivity of coal were related to its aromaticity or H/C molar ratio36, 37. Hence, in the present study, the H/C molar ratio or aromaticity of biomass is attempted as an indicator of structural alteration of biomass during torrefaction, aiming to predict reactivity of torrefied biomass during pyrolysis/gasification. As shown in Fig.1, Under different torrefaction conditions, biomass will proceed various reactions (such as dehydration, decarboxylation and decarbonylation) with different severity to form crosslinking and charring biomass with varying H/C molar ratio and aromaticity, thus leading to the alterations in the pyrolysis/gasification reactivity of torrefied biomass. The objective of this study is to establish a structure–reactivity relationship in pyrolysis and gasification of biomass with varying severity of torrefaction.

Fig.1 The schematic diagram of this study to establish quantitative structure–reactivity relationships in pyrolysis and gasification of torrefied biomass.

Experimental Section



Torrefaction of pine in a fixed bed reactor Pine was obtained from a local wood processing factory in Guangzhou. Prior to torrefaction, pine was ground and sieved to a particle size of 180-250 µm, and then dried at 105 °C for 12 h. Torrefaction of pine was conducted in a quartz tubular reactor with 12.5cm in length and 5.37cm in internal diameter. The quartz tubular reactor is illustrated schematically in Fig.2. About 3 g pine was put into the quartz container, and the quartz container was placed into the top of the quartz reactor. The reactor was purged with N2 with a flow rate of 300 mL/min for 1h, and then was heated to desired temperature. When the reactor reached the desired torrefaction temperature (220, 240, 260, 280 or 300°C), the quartz container was pushed into the reaction zone (the center of the reactor) and held there for 40 min. After torrefaction, the quartz container was pulled out and cooled with a nitrogen flow for 10 min so that the torrefied biomass can be collected.

Fig. 2 Schematic diagram of tubular reactor. Characterization of torrefied pine The ultimate analysis of raw and torrefied pine was carried out on a Vario EL (Elementar Analysensysteme, Germany). The proximate analysis of raw and torrefied pine was conducted ACS Paragon Plus Environment

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according to Chinese National Standards (GBT 28731-2012). The

13

C NMR spectra of raw and

torrefied pine were obtained on a solid-state NMR spectrometer (AscendTM 300WB, Burker, Germany) with a combination of cross polarization (CP), magic angle sample spinning (MAS) and high-power proton decoupling methods. The quantitative structural changes of pine during torrefaction are obtained from NMR spectra by the methods described in our previous works and literature’s works17, 38. Pyrolysis and Gasification Reactivity The pyrolysis of raw and torrefied pine was conducted in a thermogravimetric analyzer (STA449 F3, NETZSCH, Germany). About 10 mg samples were heated up from 30 to 900°C at a ramp rate of 10 °C/min. High-purity argon was used as carrier gas with a flow rate of 40 ml/min. The comprehensive pyrolysis index (CPI) was introduced to quantitatively assess the pyrolysis reactivity of biomass39-42. CPI was calculated by the following formula42: CPI =

( )

(1)

In the formula, Dmax is the maximum mass loss rate from DTG curve, which is the maximum peak value of DTG curve. Tmax is the corresponding temperature of the maximum mass loss rate. Ti is the initial pyrolysis temperature, which is the first inflection point in TG curve. Tf is the final pyrolysis temperature, and Tf=2Tmax-Ti39. The pyrolysis char of raw and torrefied pine was prepared in the same tubular reactor at a reaction temperature of 700 °C and a residence time of 15 minutes. The CO2 gasification reactivity of biochar was quantitatively evaluated by the same thermogravimetric analyzer (STA449 F3, NETZSCH, Germany). About 10 mg of biochars were heated up from 30 to 700 °C at a ramp rate of 30 °C/min under nitrogen atmosphere, after which the carrier gas was switched to CO2 with a flow rate of 60 ml/min, and the temperature was stayed at 700 °C for 10min, then increased at 10 °C/min to 1100 °C and held there for 40 min43. Carbon conversion ( ), instantaneous gasification reactivity ACS Paragon Plus Environment


( , min-1) and average gasification reactivity( , min-1) were calculated by the following formula, respectively44, 45:

=

= −

=

(2) ⋅

=

(3)

(4)

In the formula, !" means initial weight, ! means weight at time t, and !#$ means the weight of ash left after the thermogravimetric procedure. All experiments were carried out at least two times and averaged to compensate experimental reproducibility.

Results and Discussion The mass yield, ultimate and proximate analyses of torrefied pine.

95

90

Mass Yield / [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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85

80

75

70

65 220

240

260

280

300

o

Temperature / [ C]

Fig.3 The mass yield of torrefied pine as a function of torrefaction temperature (residence time: 40 minutes). The mass yield of torrefied pine is shown in Fig. 3. As the torrefaction temperature increased from 220 to 300 °C, the mass yield of torrefied pine gradually dropped from 95.0 to 69.3%. The main reactions during torrefaction included the devolatilization of three major components. And the


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rank order in weight loss of them during torrefaction was hemicellulose > lignin > cellulose9. The liquid and gasoues products from torrefaction involved water, acetic acid, 1-hydroxy-2-propanone, furfural, phenols, CO2, CO and so on. And water was accounted for about 50~80 wt.% of total products, depending on the torrefaction severity46, 47. The results indicated that torrefaction mainly proceeded deoxygenation reaction (dehydration, decarboxylation and decarbonylation)48. In addition, the polycondensation and carbonization of hemicellulose and lignin fractions was also observed during torrefaction17. Table 1 The Ultimate and Proximate analysis of raw and torrefied pine Torrefaction Temperature / [°C] Feedstocks

Raw 220

240

260

280

300

C

48.78±0.3

49.98±0.1

51.22±0.2

53.02±0.1

54.58±0.1

57.55

Ultimate analysis /

H

6.36±0.05

6.32±0.05

6.24±0.02

6.09±0.05

6.03±0.01

5.85±0.05

[wt.%], daf a

N

0.05±0.01

0.05

0.05±0.01

0.06

0.07±0.01

0.06±0.01

Ob

44.81

43.64

42.49

40.83

39.32

36.54

Vd

84.64±0.08

84.49±0.05

83.50±0.08

80.59±0.30

75.16±0.02

74.46±0.32

Ad

0.50±0.02

0.51

0.66±0.02

0.69±0.06

0.91±0.05

0.99±0.12

FCc

14.86

15.00

15.84

18.72

23.93

24.55

H/C molar ratio

1.56

1.52

1.46

1.38

1.33

1.22

O/C molar ratio

0.69

0.65

0.62

0.58

0.54

0.48

Proximate analysis / [wt.%]

a

daf: dry and ash-free basis

b

O was calculated by difference

c

FC was calculated by difference

d

dry basis

The ultimate analysis of raw and torrefied pine is summarized in Table 1. The carbon content of pine progressively increased with elevating torrefaction temperature, whereas the oxygen and hydrogen content of pine gradually decreased with increasing torrefaction temperature. Therefore, the H/C and O/C molar ratios significantly decreased with elevating torrefaction temperature, ACS Paragon Plus Environment


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indicating that the dehydration was the primary reaction during torrefaction. The proximate analysis of raw and torrefied pine is also given in Table 1. The content of volatile matter moderately decreased with rising torrefaction temperature, whereas the contents of fixed carbon and ash increased. These results supported that pine mainly underwent the devolatilization and charring reactions of biomass components during torrefaction.

Structural characterization of torrefied pine by solid 13C NMR.

12 3 4

5

6 7 7'

14 11 8 9 10 1213

15

300 280 260

240

220 Raw 200

180

160

140

120

100

80

60

40

20

0

Chemical shift / [ppm] Fig.4. Solid 13C NMR spectra of raw and torrefied pine. The color lines are the deconvolution of spectrum of raw pine using Gaussian–Lorentzian fitting. The solid 13C NMR spectra of raw and torrefied pine are graphed in Fig.4. The relative content of different types of carbon, representing by the normalized integration value of the specific signal corresponding to its chemical shift ranges, is tabulated in Table 2. As shown in Table 2, the signals


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at 152.6 were assigned to C-3/5 of etherified syringyl (S) units. Their contents first decreased with increasing torrefaction temperature, and then changed slightly. In contrast, the signals at 147.6 are related to C-3/5 of unetherified S units and C-3/4 in guaiacyl (G) units, and their contents smoothly increased with elevating torrefaction temperature. The results can be explained from two aspects: (1) the β-O-4 bonds in etherified S units were cleavaged and then transformed into unetherified S units; (2) the methoxyls of S units was removed to form G units. The signals at 21.2 and 102 ppm are assigned to methyls and C-1 in hemicellulose, respectively. Their contents significantly declined with increasing torrefaction temperature, implying that the structure of hemicellulose was significantly destroyed during torrefaction. The signals at 88.1 ppm are related to C-4 in crystalline cellulose. Their contents first increased when the torrefaction temperature was less than 280 °C. However, their contents started to drop when the torrefaction temperature reached 280 °C, suggesting that the crystalline degree of cellulose was reduced at this temperature. The results were in line with previous study17. The signals at 104.7 ppm are associated with C-1 in cellulose. Their content grew gradually as increasing torrefaction temperature, indicating that more severe decomposition of hemicellulose and lignin can result in the increase in relative content of cellulose. The signals at 106-160 ppm are mainly assigned to aromatic carbons, while the signals at 60-106 ppm are predominantly related to carbohydrate carbons. The intensity of total aromatic carbons remarkably raised with increasing torrefaction severity, whereas the intensity of total carbohydrate carbons gradually dropped, indicating that the severe cross-linking and charring of pine took place during torrefaction. During torrefaction, hemicellulose underwent severe ring breakage, fragmentation and carbonization reactions, while lignin proceeded depolymerization, side-chain cleavage and polycondensation reactions. Cellulose started to decompose at around 300 oC, and mainly underwent depolymerization reactions to form active cellulose with lower degree of



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polymerization9. Hence, the serve carbonization of hemicellulose and polycondensation of lignin was mainly responsible for the crosslinking and charring of pine during torrefaction. Table 2 The distribution of different types of carbon in raw and torrefied pine from Solid 13C NMR. Signal

Chemical Shift

number

/ [ppm]

Assignment

Content of different types of carbon Torrefaction Temperature / [°C] Raw

220

240

260

280

300

1

152.6

Lignin:S-3(e), S-5(e)

0.57

0.56

0.40

0.32

0.33

0.33

2

147.6

Lignin:S-3(ne),

3.56

3.95

4.52

4.93

4.80

7.27

S-5(ne),G-3(ne,e),G-4(ne,e) 3

138-138.5

Lignins: S-1(e), S-4(e), G-1(e)

0.32

1.83

1.83

2.04

2.62

3.48

4

134-133

Lignins: S-1(ne), S-4(ne), G-1(ne)

1.32

1.02

1.60

2.15

4.55

5.16

5

121

Lignin: G-6

0.66

2.39

1.93

2.66

2.15

5.91

6

114-106

Lignins: G-5, G-6, S-2, S-6

3.00

2.43

3.69

4.47

4.97

4.83

7

104.7

Carbohydrates: Cellulose:C-1

6.85

7.22

7.35

8.17

8.23

8.69

7’

104-101

Hemicelluloses: C-1

2.99

3.71

3.16

2.01

1.45

0.62

8

88.1

Cellulose:C-4(crystalline)

2.43

2.25

2.43

2.59

2.46

1.79

9

82.6

Lignin:Cβ, Cellulose:C-4(amorphous)

11.98

12.75

11.34

10.18

8.51

6.48

10

75.0

Lignin:Cα ,Carbonhydrate:C-2/3/5

20.86

18.54

18.99

19.26

20.04

9.39

11

71.7

Carbonhydrate:C-2/3/5

18.59

18.25

18.27

17.62

16.69

25.60

12

65.5

Cellulose:C-6(crystalline)

16.30

15.56

15.29

14.59

14.46

9.52

13

61.8

Lignin:Cγ, Cellulose:C-6(amorphous)

1.22

1.28

1.19

0.96

0.95

2.90

14

56.1

Lignin:–OCH3

7.95

6.99

6.66

7.03

6.68

7.30

15

21.2

Hemicellulose:–CH3

0.93

0.74

0.71

0.54

0.47

0.22

The aromaticity of raw and torrefied pine is showed in Fig.5. The aromaticity was calculated as the integration value of total signals at 106-160 ppm divided by that at 60-160 ppm. It was clear that the aromaticity of torrefied pine noticeably increased with elevating torrefaction temperature. The aromaticity of raw and torrefied pine as a function of H/C molar ratio is also plotted in Fig.5. It was found that the aromaticity of raw and torrefied pine was linearly dependent on the H/C molar ratio. The results were in accord with those from literatures. Takagi and co-workers declared that there was a relationship between the H/C ratio and aromaticity of biochar or coal36, 49. As mentioned ACS Paragon Plus Environment

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above, the H/C and O/C molar ratio of pine declined with increasing torrefaction severity. Pine mainly underwent dehydration and aromatization reactions during torrefaction, thus causing the decreases in H/C molar ratio and the increases in aromaticity of pine. The H/C molar ratio seems to be a good quantitative indicator for reflecting the torrefaction severity and resulting aromaticity of torrefied pine. 40

35

y=-49.16x+94.55 2 R =0.972

36

30 25

Aromaticity / [%]

Aromaticity / [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 15

32

28

24

10 20

5 16

0

Raw

220 260 280 240 o Torrefaction temperature / [ C]

300

1.20

1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

H/C molar ratio

Fig.5. The aromaticity of raw and torrefied pine as a function of H/C molar ratio. The structure-reactivity relationship in pyrolysis of raw and torrefied pine. The weight loss and weight loss rate (TG/DTG) curves of raw and torrefied pine are presented in Fig.6. The pyrolysis behaviors of pine were significantly influenced by torrefaction. As shown in Fig.6, the masses of pyrolysis residues from pyrolysis of torrefied pine raised with increasing torrefaction temperature. It was found that there were three peaks in the DTG curve of raw pine. The first shoulder peak centered at 280-350 °C was assigned to the decomposition of hemicellulose fraction in raw pine. The highest peak centered at 320-370 °C was related to the degradation of cellulose fraction. And the broad shoulder peak centered at 360-420 °C was associated with the pyrolysis of lignin fraction. All three peaks shifted toward high temperature when torrefaction was applied. As the torrefaction temperature increased from 220 to 300 °C, the intensity of hemicellulose peak gradually declined. The peak almost completely disappeared at the torrefaction



temperature of 300 °C. On the contrary, the intensity of lignin peak obviously increased. The results could be attributed to three reasons: (1) hemicellulose and lignin were significantly decomposed during torrefaction9; (2) hemicellulose exhibited higher rate and extent of decomposition than lignin during torrefaction9; (3) hemicellulose could proceed severe polycondensation and charring reactions to form condensed aromatic structure that was similar to lignin structure9. 0

100

Raw 220 240 260 280 300

80

-5

Weight loss rate/ %/min

Weight Loss / [wt.%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

Raw 220 240 260 280 300

-10

-15

-20

-25

20

100

200

300

400

500

o 600 Temperature / C

700

800

900

-30 150

200

250

300

350

400

o Temperature / C

450

500

Fig.6 TG/DTG analysis of raw and torrefied pine. The maximum weight loss rate (Dmax) and its corresponding temperature (Tmax) were read from the peak values of DTG curves. The initial pyrolysis temperature and final pyrolysis temperature were obtained from TG/DTG curves according to the method described by Xing42. These pyrolysis characteristic parameters of raw and torrefied pine are listed in Table 3. The Dmax of pine first raised from 22.01 to 28.12 %/min when the torrefaction temperature increased from 220 to 280 °C. It dropped to 26.60 %/min when the temperature further increased to 300 °C. The Tmax of pine was obviously enhanced by torrefaction. It changed slightly when torrefaction temperature increased from 220 to 280 °C. The Tmax decreased when torrefaction temperature further increased from 280 to 300 °C. These results could be due to the increases in relative content of cellulose and aromaticity in pine caused by faster decomposition and charring of hemicellulose and lignin during torrefaction. The reduction in Dmax and Tmax at torrefaction temperature of 300 °C could be ascribed


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to the formation of active cellulose with lower degree of polymerization at this temperature. Table 3 The pyrolysis characteristic parameters of raw and torrefied pine. Torrefaction Temperature / [°C] Feedstocks

Raw 220

240

260

280

300

Weight loss at 800 [°C]

78.9

78.3

75.7

72.8

69.9

65.0

Dmax [%/min]

21.95

22.01

24.44

27.34

28.12

26.60

Tmax [°C]

338.7

361.7

357.5

361.7

359.3

349.2

Ti [°C]

157.6

208.9

233.0

245.5

262.2

270.1

CPI [10-4%/(min·°C2)]

1.79

1.99

2.75

3.25

4.03

4.83

The comprehensive pyrolysis index (CPI) was introduced to quantitatively evaluate the pyrolysis reactivity of raw and torrefied pine in this study. Higher CPI indicated that the pyrolysis reaction proceeded more rapidly and easily42. The CPI of raw and torrefied pine is also illustrated in Table 3. The CPI of pine was promoted by torrefaction. And it steadily rose with increasing torrefaction temperature. The CPI as a function of aromaticity or H/C molar ratio is depicted in Fig.7. It should be noted that there was a linear correlation between CPI and aromaticity of raw and torrefied pine. The same phenomenon was also observed between CPI and H/C molar ratio of raw and torrefied pine. The CPI increased monotonically with aromaticity and dropped monotonically with H/C molar ratio. The rank order of devolatilization rates of biomass components during pyrolysis was cellulose >> hemicellulose >> lignin9. The devolatilization rate of torrefied pine was mainly depended upon its content of cellulose. Therefore, the improvement in pyrolysis reactivity of torrefied pine could be also contributed to the increase in relative content of cellulose caused by the more severe degradation of hemicellulose and lingin during torrefaction.



5.6

5.6

4.8

-4 o 2 CPI / [10 %/(min* C )]

y=0.18x-1.53 2 R =0.963

-4 o 2 CPI / [10 %/(min* C )]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4.0

3.2

2.4

y=-9.24x+16.15 2 R =0.986

4.8

4.0

3.2

2.4

1.6

1.6

18

21

24

27

30

33

36

1.20

1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

H/C molar ratio

Aromaticity / [%]

Fig.7 The CPI of raw and torrefied pine as a function of its aromaticity and H/C molar ratio The structure-reactivity relationship in CO2 gasification of biochar derived from raw and torrefied pine. The biochar conversions during CO2 gasification are shown in Fig.8. The biochar conversions were strongly dependent upon torrefaction severity. The conversion of biochar derived from pine torrefied at 220 °C was close to that of raw pine. As the torrefaction temperature further increased from 220 to 300 °C, the conversion of biochar gradually declined. The instantaneous reactivity of biochar is also drawn in Fig.7. When the biochar conversion reached about 74%, the instantaneous reactivity of biochar was obviously controlled by torrefaction severity. The instantaneous reactivity of biochar declined with increasing torrefaction severity. These results were presumably due to the crosslinking and charring of pine during torrefaction. It is well known that the gasification of chars occur only at active sites on which carbon-oxygen complexes are formed50. The active sites in biochar can be ascribed to the structural features such as defects in carbon layer planes and carbon edges/facets, disordered carbon atoms, dangling carbon atoms, heteroatoms (O/S/N) and mineral matter50. the first stage of the gasification reaction involves the formation of carbon-oxygen complexes on active sites by absorption with oxygen atom of CO2. The CO2 molecules subsequently decompose into adsorbed oxygen atoms and CO. Severe torrefaction can lead to significant crosslinking and polyconsensation of pine, thus producing more ordered and condensed structure of biochar, through elimination of active sites and recondensation/coalescence of aromatic ACS Paragon Plus Environment

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rings. 7.5 950

80

raw 220 240 260 280 300

6.0

o

900

Temperature / [ C[ -1 Instantaneous reactivity / [min ]

100

Char Conversion / [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

850

raw 220 240 260 280 300

40

20

0

800

750

700

36

42

48 54 Time / [min]

60

4.5

3.0

1.5

0.0

10

20

30

40 50 60 70 Char Conversion / [%]

80

90

100

Fig.8 The conversion and instantaneous gasification reactivity of biochar derived from raw and torrefied pine In order to clarify the structural alterations of biochar caused by torrefaction, two biochars were selected for comparison of their structure via Raman spectroscopy. The Raman spectra and the deconvolution of spectra into five bands are shown in Fig.9 according to the method mentioned by Sheng51. The band G centered at 1580 cm-1 was assigned to the aromatic layers in the graphite crystallines. The D1, D2, D3 and D4 centered at 1350, 1620, 1530 and 1150 cm-1 are associated with the defects in the highly disordered carbons. The ratios of band area can be used to quantitatively estimate the structural order of biochar. As shown in Fig.8, the ratios of D1/G, D2/G, D3/G and D4/G of biochar from pine torrefied at 280 °C were lower than those of char from raw pine. The G/All exhibited opposite trend. These results suggested that the active sites on biochar surface, such as defects in the highly disordered carbons, were destroyed by the crosslinking and charring of pine during torrefaction, resulting in new graphite crystallines in biochar were formed. The decrease in the number of active sites can reduce the absorption of CO2 on biochar surface. It is concluded that the crosslinking and charring of pine during torrefaction can cause the recondensation/graphitization of biochar and the elimination of active sites in biochar surface, thereby reducing the conversion and reactivity of biochar during CO2 gasification. ACS Paragon Plus Environment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity / [a.u.]


1800

Char from raw pine o Char from pine torrefied at 280 C D4 D1 D3 D1/G G D2/G D2 D3/G D4/G G/All

1600

1400

1200

-1 Raman shift / [cm ]

Raw 4.54 0.90 1.25 0.88 0.13

Page 18 of 25

o

280 C 2.67 0.12 1.17 0.58 0.22

1000

800

Fig.9 The Raman spectra of biochars derived from raw and torrefied pine. The average gasification reactivity of biochar is given in Fig.10. It was evident that the average reactivity of biochar derived from pine torrefied at 220 °C was slightly lower than that of biochar from raw pine. As torrefaction temperature rose from 220 to 300 °C, the average reactivity of biochar gradually decreased. The results were also ascribed to less active sites and more ordered and condensed structure of biochar caused by the crosslinking and charring of pine during torrefaction. The available number of active sites on biochar surface was decreased and therefore the CO2 gasification reactivity of biochar was inhibited. It is worthy of note that the relative content of ash in pine was enhanced by torrefaction. It is well known that the inorganic matters in ash, especially alkali and alkaline earth metals, can make a positive contribution to the gasification reactivity52. Therefore, the actual contribution of torrefaction to the reduction in gasification reactivity was higher than its apparent value.


Page 19 of 25

-1

Average Reactivity / [min ]

4.4

4.3

4.2

4.1

4.0

3.9

Raw

220

260 240 o Temperature / [ C]

280

300

Fig.10 The average gasification reactivity of biochars derived from raw and torrefied pine. 4.5

4.5

y=-0.03x-4.94 2 R =0.984

4.4

-1 Average gasificaiton reactivity / [min ]

-1 Average gasificaiton reactivity / [min ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.3

4.2

4.1

4.0

y=1.27x+2.48 2 R =0.935

4.4

4.3

4.2

4.1

4.0

3.9

3.9 18

20

22

24

26

28

30

Aromaticity / [%]

32

34

36

38

1.20

1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.60

H/C molar ratio

Fig.11 The average gasification reactivity of biochar as a function of aromaticity and H/C molar ratio of raw and torrefied pine. The average gasification reactivity of biochar is plotted against the aromaticity and H/C molar ratio of raw and torrefied pine in Fig.11 to acquire the correlation between the structure of pine and its corresponding gasification reactivity. It can be seen that the average gasification reactivity of biochar was a linear monotonically decreasing function of aromaticity. Conversely, the average gasification reactivity of biochar was a line monotonically increasing function of H/C molar ratio. As shown in Fig.12, during torrefaction, hemicellulose primarily proceeded severe ring breakage, fragmentation and charring reactions to form condensed aromatic structure, while lignin underwent depolymerization (e.g. the cleavage of β-O-4 bonds), side-chain cleavage (demethoxylation) and ACS Paragon Plus Environment


recondensation reactions. Cellulose started to decompose at around 280-300 oC, and mainly underwent depolymerization reactions to form active cellulose with lower degree of polymerization. The rank order in the severity of structural alteration of three components was hemicellulose> lignin >cellulose. Consequently, with increasing torrefaction temperature, pine mainly proceeded dehydration reaction to form more severely crosslinked and charred pine accompanied with its decreased H/C molar ration. It was then pyrolyzed to yield more ordered and condensed biochar with less active sites, leading to the reduction in its gasification reactivity.

Fig.12 The structure-reactivity relationships in gasification of biochars derived from raw and torrefied pine.

Conclusions

The correlation between structure of torrefied pine and its corresponding pyrolysis and gasification reactivity was explored in this study. The structural characterization demonstrated that torrefaction of pine mainly underwent dehydration reactions to form crosslinked and charred pine with ACS Paragon Plus Environment

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decreased H/C and O/C molar ratios. The pyrolysis reactivity of pine was promoted by torrefaction. The result could be attributed to the increase in relative content of cellulose in pine caused by the more severe degradation of hemicellulose and lignin during torrefaction. The gasification reactivity of biochar derived from pine was reduced by torrefaction. The crosslinking and charring of pine during torrefaction can cause the recondensation/graphitization of biochar and the elimination of active sites in biochar surface, thereby reducing the gasification reactivity of biochar. The pyrolysis and gasification reactivity of torrefied pine was a strong linear function of its aromaticity or H/C molar ratio. The structure-reactivity relationships would provide some guidance for the selection of operating conditions of torrefaction and the design and optimization of gasifier/pyrolysis reactor in the integrated process consisted of torrefaction and followed pyrolysis/gasification. In addition, the residence time of torrefaction is another key parameter regarding torrefaction severity, its influences on pyrolysis and gasification reactivity should be further elucidated in the near future.

Acknowledgements The authors acknowledge the Major International (Regional) Joint Research Project of the National Science Foundation of China (Grant 51661145011), the National Natural Science Foundation of China (Grants 21406227, 51376186 and 51776209), the Natural Science Foundation of Guangdong Province, China (Grant 2014A030313672) and the Science and Technology Planning Project of Guangdong Province, China (Grants 2014B020216004 and 2015A020215024) for financial support of this work. References 1.

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Table of Contents (TOC): The quantitative structure-reactivity relationships in pyrolysis and gasification of torrefied biomass were established.


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