CO2 gasification of char from raw and torrefied biomass: Reactivity, kinetics and mechanism analysis

Bioresource Technology 293 (2019) 122087

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CO2 gasification of char from raw and torrefied biomass: Reactivity, kinetics and mechanism analysis Qing Hea, Qinghua Guoa, Lu Dingc, Juntao Weia, Guangsuo Yua,b,

T

⁎

a

Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai 200237, PR China State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, Ningxia University, Yinchuan 750021, PR China c Department of Environmental Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Torrefaction Gasification Pyrolysis Kinetics Biomass char

In this study, the effect of torrefaction on the gasification reactivity of chars from raw and torrefied biomass was investigated. Three torrefaction temperatures and four pyrolysis temperatures were taken into consideration. It was found that the severe torrefaction (300 °C) would reduce the char gasification reactivity by at least 19% according to the normalized gasification rate. Moreover, the reduction of gasification reactivity appeared after the midterm stage. The gasification reaction were further analyzed by nucleation/growth model and model-free method. The activation energy increased by ~80 kJ/mol with conversion, indicating an enhancement of the reaction resistance. Furthermore, 800 °C pyrolysis was found to be a turning point, beyond which the gasification reactivity reduced significantly. These reactivity changes were implied by the bio-char structure evolution and active alkali and alkaline earth metals (AAEMs) contents variations. The research results provide insights into the effect of torrefaction on biomass gasification.

1. Introduction Biomass gasification is one of the promising technologies to effectively utilize biomass-energy resources. Fluidized bed reactor and entrained flow reactor for biomass gasification are developed (Bates et al., 2016; Ku et al., 2019; Moilanen et al., 2009; Okumura et al., 2009). Biomass gasification consists of two processes: pyrolysis followed by the slower reaction of char with CO2 or steam (Czerski et al., 2017). It is

⁎

necessary to separate pyrolysis from char gasification to comprehend the gasification process (Kolb et al., 2016). Moreover, the second process is considered as the rate-controlling step for the whole process of biomass gasification. It was reported that approximately 50–80% of char is unconverted in the single-stage fluidized bed biomass gasifier, and a complex multistage system is proposed (Bates et al., 2016). Therefore, understanding char gasification reactivity and kinetics are essential for the design and modelling of the biomass gasifier.

Corresponding author at: Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai 200237, PR China. E-mail address: [email protected] (G. Yu).

https://doi.org/10.1016/j.biortech.2019.122087 Received 2 August 2019; Received in revised form 26 August 2019; Accepted 27 August 2019 Available online 29 August 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.

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light, mild and severe torrefaction. The biomass was hold at the desired time for 30 min with the carrier gas (N2, ≥99.999%) flow rate of 500 mL/min. Torrefied poplar wood prepared at 200, 250 and 300 °C were denoted as PW200, PW 250 and PW300, respectively. The typical temperature for biomass gasification was lower than 900 °C (Molino et al., 2018). Because higher temperature would lead to ash melting and agglomeration, as well as the rigorous reactor specification requirement. In the present work, the chars were prepared in the fixed bed reactor at 600, 700, 800 and 900 °C, respectively. The specific steps were similar with torrefaction process. The chars were kept for 15 min at the final temperature. Rapid heating of particles could be achieved in the fixed bed reactor, which simulate the char generated in a gasifier experimentally (Ren et al., 2011). Bio-char was named by sample and pyrolysis temperature. For example, the char of PW300 pyrolyzed at 600 °C was denoted as PW300-600P.

Recently, torrefaction, a low temperature thermal conversion that upgrades the fuel propertied of biomass, has been received much attention. Compared to the raw biomass, torrefied biomass with higher energy density is easily ground, stored and transported, making it more suitable for advanced gasification (Tran et al., 2016). Recent researches on the torrefied biomass gasification focus on the optimum torrefaction conditions. Xue et al. concluded that 250 °C was the optimal torrefaction condition for Miscanthus giganteus gasification (Xue et al., 2014b). Similar conclusion for the torrefied wood was reported by Chen et al. (Chen et al., 2011). However, Karlström et al. found that the effect of torrefaction on different biomass was not in any systematic way (Karlström et al., 2015). They observed that torrefaction promoted the reactivity of straw char, while inhibited the reactivity of olive stone char. On the other hand, the bio-char gasification reactivity is directly governed by its structural and chemical properties (Guizani et al., 2016; Gupta et al., 2018). The structure of bio-char are basically determined by the pyrolysis conditions (Tran et al., 2016). It has been reported that pyrolysis would lead to coalescence, ordering and rearrangement of aromatic ring structures (Guizani et al., 2016). These structures in biomass are mainly dependent on the fibre composition, which is changed significantly after torrefaction (He et al., 2019). Besides, the minerals in bio-char is another critical factor affecting the gasification performance (Ding et al., 2017; Dupont et al., 2016). These characteristics are also highly coupled. However, analysis of the relationship between the bio-char gasification reactivity and its physicochemical structure has not been adequately studied, especially for torrefied biomass. The thermogravimetric analyzer (TGA) technique is considered as a proven method to study the gasification kinetics (Bach et al., 2017). Many kinetic models are developed to study the char gasification, including the random pore model, the shrinking core model and etc. (Gupta et al., 2018; Xue et al., 2017). Recently, the nucleation and growth analysis and model-free method are applied to study the gasification kinetics (De Micco et al., 2012; Hu et al., 2019). The nucleation and growth analysis was used to study other gas-solid reactions, such as iron ore pellet reduction and salt decomposition (Hancock & Sharp, 1972; Piotrowski et al., 2007). It has the advantage to explain the reaction controlling steps due to the diffusion behaviors (Hu et al., 2019). Model free method is an integral iso-conversional method, which could determine the variations in activation energy with no risk to select a wrong kinetic model (De Micco et al., 2012; Loy et al., 2018). Consequently, it is meaningful to obtain quantitative knowledge of gasification kinetics based on different methods, which can form useful guidelines for new reactors design. The main object of present work is to study the effect of torrefaction on the char gasification reactivity, as well as the kinetics and mechanism. Poplar wood (PW), an important source of lignocellulosic biomass, was used as the raw material. Three torrefaction temperatures of 200, 250 and 300 °C, and four pyrolysis temperatures of 600, 700, 800 and 900 °C were taken into consideration. Gasification reactivity and kinetics of bio-char was investigated using a TGA with the temperatures ranged from 750 to 900 °C. Besides, the carbon structure of bio-char and the content of active AAEMs were characterized to explain the changes of gasification reactivity. These detailed analyses provide references for the application of torrefied biomass as gasifier feedstock.

2.2. Char-CO2 gasification in TGA The gasification reactivity of samples were carried out by a TGA (NETZSCH STA449 F3) using CO2 as gasification medium. In each run, around 8 mg sample were loaded in the crucible. The samples were heated in inert atmosphere of N2 (20 mL/min) at 25 °C/min. Once the desired temperature (750, 800, 850 and 900 °C) was attained, the gas was switched from the N2 to CO2. The flow rate of CO2 was set as 120 mL/min. The gasification reaction proceeded to completion under isothermal condition. The conversion of the Boudouard reaction was calculated by (Gao et al., 2017; Gupta et al., 2018)

X=

m 0 − mt m 0 − m∞

(1)

where m 0 , mt and m∞ were the initial, instantaneous and final weights during the reaction, respectively. The simplified normalized gasification rate (min−1) K was calculated to evaluate the gasification reactivity of bio-char (Gao et al., 2017)

K = ln4/Δt

(2)

where Δt was the time of carbon conversion between 20% and 80%. 2.3. Characterization analysis of samples 2.3.1. Analysis of raw and torrefied biomass Proximate analysis of the four samples were measured using a muffle furnace (5EMAG6700, CKIC Co. Ltd., China) according to the standard of the Proximate Analysis of Solid Biofuels (GB/T 287312012). Ultimate analysis of samples were carried out in an elemental analyzer (Vario MACRO Cube, Elementar, Germany). The higher heating value (HHV) was tested in an oxygen bomb calorimeter (5EC5508, CKIC Co. Ltd., China). The biomass ash composition was determined by using X-ray fluorescence analyzers (Thermo Scientific, USA) based on GB/T 30725-2014. The thermal stability of raw and torrefied biomass was analyzed using TGA. Specifically, the temperature was raised from room temperature to 800 °C at a rate of 20 °C/min using N2 as purge gas (50 mL/min). The major constituents of lignocellulosic biomass could be identified by the DTG distributions (Eseltine et al., 2013).

2. Materials and methods

2.3.2. Analysis of biomass char Thermofisher DXR Raman spectrometer with a laser wavelength of 455 nm was used to record the Raman spectra. The wavenumber ranges of 800–2000 cm−1 were collected. Considering the heterogeneity of the char particles, 10 particles were randomly selected from each sample. Each Raman spectra was normalized by the maximum intensity and the mean spectra was further determined (Zou et al., 2018). Agilent 725 ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometer) was used to quantify the active AAEMs (water-soluble and NH4Ac-soluble) in bio-chars. The active AAEMs were acquired by

2.1. Sample preparation Poplar wood (PW) was crushed and sieved to 80–120 mesh. A fixed bed reactor was used to prepare the torrefied biomass. The detail schematic diagram for torrefaction as well as the procedures were outlined previously (He et al., 2019). For each experimental run, approximately 10 g of sample was used. Three different torrefaction temperatures of 200, 250 and 300 °C were considered, corresponding to 2

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Table 1 Properties of raw and torrefied biomass. PW

HHV (ad, MJ/kg) 17.95 Proximate analysis (ad, wt%) Moisture 6.26 Volatile 77.04 Fixed carbon 15.74 Ash 3.45 Ultimate analysis (daf, wt%) C 47.14 H 6.62 a 45.68 O N 0.35 S 0.20 H/Cb 1.69 O/Cb 0.73

PW200

PW250

PW300

18.13

19.45

22.36

5.10 76.81 15.86 4.24

3.29 72.58 20.18 5.17

3.37 56.48 34.62 6.50

47.72 6.61 45.09 0.40 0.19 1.66 0.71

50.96 6.42 42.05 0.40 0.17 1.51 0.62

62.19 5.77 31.39 0.49 0.16 1.11 0.38

-dX/dt (%/min)

Samples

0.4

PW PW200 PW250 PW300

0.3

0.2

left shoulder right shoulder

0.1

0.0 100

ad: air-dry basis; daf: day ash-free basis. a By difference. b Atomic ratio.

200

300

400

500

600

700

800

Temperature (°C) Fig. 1. DTG curves of raw and torrefied biomass.

chemical fractionation analysis (Chen et al., 2016). Approximately 0.2 g bio-char was extracted with 1.0 mol/L NH4Ac for 72 h, then the percolate was mixed with ultrapure water to 100 mL. The standard deviation of ICP-OES measurement was less than 3%.

3.2. Gasification reactivity of char 3.2.1. Effect of torrefaction temperature on char gasification In order to evaluate the effect of torrefaction on gasification, the gasification reactivity of raw and torrefied biomass chars pyrolyzed at 800 °C (800P-chars) were compared. Fig. 2 showed isothermal gasification of the bio-chars at the temperatures of 750–900 °C. As can be seen from Fig. 2(a)–(d), the difference between PW-800P and PW200800P was not obvious, suggesting that low-temperature torrefaction had slight effect on gasification. Experimental results from Xue et al. showed that the torrefaction at 230 °C had a minor impact on the properties of biomass (Xue et al., 2014b). When the torrefaction temperature was higher than 250 °C, the values of normalized gasification rate K decreased gradually. In the most critical condition (300 °C), the value of K decreased by 33%, 28%, 23% and 19% with the increasing gasification temperatures, respectively. Although the active cellulose was increased after mild torrefaction, the gasification reactivity of PW250-800P char was lower than that of PW-800 char. The content of lignin with low reactivity increased for PW300, which may be the reason for the lowest gasification reactivity. It was worth to note that the gasification reactivity of the four samples were similar at the initial stage of the reaction (elliptic region in Fig. 2). The difference between raw and torrefied biomass appeared at the midterm stage and became larger with the follow gasification proceeding.

3. Results and discussion 3.1. Characteristics of raw and torrefied materials The properties of raw and torrefied poplar wood were presented in Table 1. After undergoing torrefaction, the higher heating value (HHV) of the wood were higher than that of raw biomass. For example, the HHV values of the biomass torrefied at 250 and 300 °C were increased by 8.4% and 24.6% with respect to the raw biomass, respectively. The linear regression relationship between H/C and O/C was found, suggesting that torrefaction removed hydrogen and oxygen selectively (Ru et al., 2015). Moreover, the value of ΔO/C/ΔH/C (0.602) was lower than 1 with the increasing torrefaction temperature, indicating that the hydrogen was removed faster than oxygen. In addition, the ash composition of raw biomass was analyzed using XRF. The results showed that Ca (31.63%), Si (28.00%) and K (12.34%) were the main elements in the ash. These values and general trends compared well with other findings (Arias et al., 2008; Karlström et al., 2015). Hemi-cellulose, cellulose and lignin were the main constituents in lignocellulosic biomass, which could be identified using TGA based on the differing pyrolytic reactivity (Xue et al., 2014a). The rate of mass loss curves of the raw and torrefied biomass (DTG) were shown in Fig. 1. Theses curves presented a peak with two shoulders. The DTG curve of PW200 showed a shape similar to that of the raw biomass, suggesting that the lower temperature torrefaction had slight effect on the changes of biomass components. The PW250 had a more pronounced peak in DTG curve, indicating that the mild torrefaction resulted in an accumulation of thermally sensitive cellulose (He et al., 2018). Compared with PW or PW250, an obvious right peak appeared for PW300. This can be attributed to the increase of the lignin, which was the most stable component in biomass (Eseltine et al., 2013). Besides, it could be found that the maximum degradation rate of PW300 was lower than that of PW250, which was mainly contributed to the decomposition of cellulose after severe torrefaction (Eseltine et al., 2013; Lu et al., 2013). Generally, the torrefied biomass became more stable, especially after severe torrefaction. For all the investigated samples, rapid pyrolysis were carried out at the temperatures of 600, 700, 800 and 900 °C to obtain the bio-chars. Gasification reactivity as well as the properties of chars were further analyzed.

3.2.2. Effect of pyrolysis temperature on char gasification In order to study the effect of the pyrolysis temperatures on the reactivity of raw and torrefied biomass char, the PW and PW300 were pyrolyzed at 600, 700, 800 and 900 °C, respectively. The corresponding chars were gasified at the temperatures of 750–900 °C, as shown in Fig. 3. As for PW-char, the char gasification reactivity below 800 °C decreased with the increasing pyrolysis temperatures (Fig. 3(a1, b1)). Moreover, the reactivity for PW-900P was significant lower than that of others. When the gasification temperature was higher than 850 °C, the reactivity was independent of pyrolysis temperatures (Fig. 3(c1, d1)). However, the reactivity for PW300-900P was still lower than that of other PW300-chars even at higher gasification temperatures (Fig. 3(c2, d2)). On the other hand, the gasification reactivity for PW300-chars at each temperature were systematically lower than that of PW-char regardless of pyrolysis temperatures. Generally, the gasification reactivity of bio-chars decreased slightly with the increase pyrolysis temperature at the range of 600–800 °C. While the gasification reactivity of bio-char produced at 900 °C reduced significantly, especially for the torrefied biomass. 3

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PW-800P PW200-800P PW250-800P PW300-800P 0.015

-1

0.010

PW

P PW

80

0 20

-8

00

P

P

00 -8

0 25

00

PW

PW

100

P

120

8 030

PW

140

0.0

160

0

5

10

Gasification time (min)

1.0

PW-800P PW200-800P PW250-800P PW300-800P 0.18

-1

0.12

4

6

8

0-

80

0P

0P

80

0-

30

PW

30

35

40

0.4

0.4

0.3 0.2

10

12

0.0

14

0

1

2

Gasification time (min)

3

4

5

6

0P

0P

0.0

80

0P 80 030 W P

0-

P

0P

00

80

-8

PW 30

PW

0 25

P

P

00

00

-8

0 20 PW

0.2

PW -8

P

00

-8

2

PW

PW

25

25

0.1

PW

0

0.6

0.06 0.00

0.2

0P

80

0-

20

0-

0.4

P

PW-800P PW200-800P PW250-800P PW300-800P

-1

0.24

0.6

0.0

(d)

0.8

K (min )

Conversion X

0.8

00

20

K (min )

(c)

15

-8

Gasification time (min)

Conversion X

1.0

0.02 0.00

0.2

PW 25

60

00

0.04

80

40

-8

0.06

0-

20

0.4

0.005 0.000

0.2

0.08

0.6

PW 20

0.4

0

PW-800P PW200-800P PW250-800P PW300-800P

-1

0.020

0.6

0.0

(b)

0.8

K (min )

Conversion X

0.8

1.0

K (min )

(a)

Conversion X

1.0

7

8

Gasification time (min)

Fig. 2. Carbon conversion vs. time for 800P-chars of raw and torrefied biomass at different gasification temperatures (a) 750 °C, (b) 800 °C, (c) 850 °C and (d) 900 °C.

at different gasification temperatures, it showed an inconspicuous model based on Table 2. Thus the isothermal gasification was further analyzed through the model-free method.

3.3. Gasification kinetics analysis 3.3.1. Nucleation and growth analysis The isothermal gasification could be analyzed based on the nucleation and growth model:

X = 1 − exp(−kt m)

(3)

ln(−ln(1 − X )) = ln k + m ln t

(4)

3.3.2. Model-free methods The kinetic expression for gas-solid reaction were generally assumed as:

dX = k (T ) f (X ) dt

where k was the reaction constant related to nucleation frequency and grain growth rate, m was the constant depended on geometry of the reaction system (Hu et al., 2018). The slope m of curve ln(−ln (1 − X)) ~ lnt was used as an indicator of reaction pathways based on different reaction models (Table 2). Moreover, the X was usually limited to values from 0.15 to 0.5, which neither affected appreciably by uncertainty in the initial conditions nor influenced greatly by particle size distribution (Hancock & Sharp, 1972; Hu et al., 2018; Piotrowski et al., 2007). The determined parameters were listed in Table 3. It could be found that the m of torrefied biomass char (PW300-char) was lower than that of raw biomass char (PW-char) under the same conditions. The reaction model indicated by m was not quite the same as that from Vincent et al. They pointed out the gasification was a first order reaction for torrefied chars (Vincent et al., 2014). Moreover, the m increased with gasification temperature, and an instantaneous change was observed. When the temperature was lower than 800 °C, the average value of m was 0.82 and 0.72 for PW-char and PW300-char, respectively. It indicated that gasification may be controlled by heat transfer phenomena (Hancock and Sharp, 1972). However, when the temperature was higher than 850 °C, the average value of m was 1.11 and 1.03, suggesting a phase boundary controlled reaction according to Table 2. Since the values of m varied from ~0.7 to ~1.0 for each sample

(5)

where k (T ) reflected the effect of temperature on reaction, which was given by Arrhenius equation k (T ) = k 0 exp(−Ea/ RT ) ; and f (X ) accounted for the physiochemical properties changes of char with the conversion. Eq. (5) was also used in its integral form, it became:

∫0

X dX f (X )

t

= ∫0 k 0 e−Ea/ RT dt

F (X ) = k 0 e−Ea/ RT t

(6)

Eq. (6) showed the reaction time as a function of temperature under a certain conversion. Taking the logarithm of both sides of Eq. (6)

F (X ) ⎤ E + a ln t = ln ⎡ ⎢ k0 ⎦ ⎥ RT ⎣

(7)

The Ea for isothermal gasification could calculated from the slope of the lnt vs 1/T plots under a given value of X. Eq. (7) allowed to obtain the activation energy at different conversions, even though the F(X) was unknown. This procedure was applied for the conversion ranges of 0.05 to 0.95, as shown in Fig. 4. The value of R2 was greater than 0.95, indicating the result was reliable. Fig. 4(a) showed the effect of torrefaction temperature on Ea. It 4

Bioresource Technology 293 (2019) 122087

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(a1)

0.8

Conversion X

Conversion X

1.0

0.6 0.4

PW-600P PW-800P

0.2

PW-700P PW-900P

(a2)

0.8

Conversion X

Conversion X

0.6 0.4

PW-600P PW-800P

0.2 1.0 0.0

1.0 0.0

0.6 0.4

PW300-600P PW300-800P

0.2 0.0

(b1)

0.8

0

20

40

60

80

100

PW300-700P PW300-900P 120

140

(b2)

0.8 0.6 0.4

PW300-600P PW300-800P

0.2 0.0

160

0

5

10

Gasification time (min)

Conversion X

0.6 0.4

PW-600P PW-800P

0.2 1.0 0.0

Conversion X

1.0

(c1)

0.8

PW-700P PW-900P

0.6 0.4

PW300-600P PW300-800P

0.2 0.0

0

20

25

30

35

40

(d1)

0.8 0.6 0.4

PW-600P PW-800P

0.2

PW-700P PW-900P

1.0 0.0

(c2)

0.8

15

PW300-700P PW300-900P

Gasification time (min)

Conversion X

Conversion X

1.0

PW-700P PW-900P

3

6

9

PW300-700P PW300-900P 12

0.6 0.4

Gasification time (min)

PW300-600P PW300-800P

0.2 0.0

15

(d2)

0.8

0

2

4

PW300-700P PW300-900P 6

8

Gasification time (min)

Fig. 3. Carbon conversion vs. time for PW-char and PW300-char at different gasification temperatures (a) 750 °C, (b) 800 °C, (c) 850 °C and (d) 900 °C.

could be found that the light and mild torrefaction had slight impact on Ea, while the severe torrefaction increased the Ea at high conversion. The increase of Ea for severe torrefied biomass char might be due to the accumulation of lignin, which had the lowest reactivity (Eseltine et al., 2013). Moreover, the Ea increased with the conversion, suggesting the gasification reactivity decreased gradually (Liu et al., 2008; Sun et al., 2004). The gasification agent might first react with the site of weakly linked bonds, followed by that linked with strong bonds (Liu et al., 2008). It can be deduced that the reactive sites decreased with the conversion, especially for the torrefied biomass char. On the other hand, the Ea increased with the increase of pyrolysis temperatures, as shown in Fig. 4(b). The average Ea of PW-600P was 152.5 kJ/mol, while the value of PW-900P was 202.7 kJ/mol. At the late stage of gasification, the Ea of torrefied biomass char pyrolyzed at low-temperature was overlapped with that of high-temperature pyrolysis char of raw biomass. For example, the Ea of PW300-600P was close to that of PW-700P when the conversion was greater than 0.5. Similar feature was found for PW300-700P and PW-800P. However, the value of PW300-800P was lower than that of PW-900P, indicating a significant reduction of reactivity for chars produced at 900 °C. Generally, the Ea was mainly affected by pyrolysis temperature. The value of Ea increased with the increasing pyrolysis temperatures, especially at 900 °C. Compared with PW-600P, the average Ea of PW900P was increased by ~50 kJ/mol. However, the average Ea of PW300-char was increased by ~6 kJ/mol compared with PW-char at different pyrolysis temperatures. Moreover, torrefaction mainly affected the Ea at the late stage of bio-char gasification.

Table 2 Kinetic models for gas-solid reaction. Kinetic model

Equation

First order reaction Phase boundary controlled (contracting sphere) Phase boundary controlled (contracting cylinder) One-dimensional diffusion

m

1 − (1 − X )1/3 = kt

1 1.07

1 − (1 − X )1/2 = kt

1.11

X2 = kt (1 − X )ln(1 − X ) + X = kt

0.62

- ln(1 − X ) = kt

(1 − (1 − X )1/3)2 = kt

0.57 0.54

Three-dimensional diffusion (G-B eq.)

1 − 2/3X − (1 − X )2/3 = kt

0.57

Two-dimensional growth of nuclei

(- ln(1 − X ))1/2 = kt

2

Three-dimensional growth of nuclei

(- ln(1 − X ))2/3 = kt

3

Two-dimensional diffusion Three-dimensional diffusion (Jander eq.)

Table 3 Calculated parameter m under different gasification temperatures. Samples

PW-600P PW-700P PW-800P PW-900P PW300-600P PW300-700P PW300-800P PW300-900P

750 °C

800 °C 2

m

R

0.72 0.74 0.77 0.87 0.63 0.67 0.70 0.72

0.998 0.999 0.999 0.999 0.998 0.998 0.998 0.999

850 °C 2

m

R

0.84 0.86 0.86 0.87 0.77 0.79 0.78 0.72

0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999

900 °C 2

m

R

1.05 1.08 1.10 1.19 0.99 1.01 1.03 0.97

0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999

m

R2

1.04 1.08 1.14 1.22 1.03 1.05 1.09 1.07

0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999

5

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220

PW

240 220 200

160

PW300 600P 700P 800P 900P

(b)

180 160

220

Ea (kJ/mol)

180

Ea (kJ/mol)

Ea (kJ/mol)

200

260

(a)

PW-800P PW200-800P PW250-800P PW300-800P

140

140

120

120

PW300

180 PW

160 140

100

100 0.0

200

600

700

800

900

Pyrolysis temperature

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

80 0.0

1.0

Conversion X

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Conversion X

Fig. 4. Activation energy vs. conversion (a) different torrefaction temperatures (b) different pyrolysis temperatures.

increased from 500 to 800 °C. This could be explained by the increase in the large aromatic ring systems leading to enhance the relative Raman intensity at D band position. When the pyrolysis temperature continued to raise, the relative content of graphite increase significantly, which could be observed at G band position. Thus the net result was that the ID/IG ratios decreased at 900 °C. Zhao et al. also found that ID/IG ratios first increased and then decreased with the increasing pyrolysis temperature (Zhao et al., 2016). Besides, similar trend was observed for IS/ IG, indicating S band structure lasted together with the aromatic structures. The Raman parameters changed significantly with the increasing pyrolysis temperatures, whereas even severe torrefaction had slight effect on these parameters. It suggested that the carbon structure of the nascent chars from raw and torrefied biomass were similar. Moreover, no obvious changes of Raman parameters were observed at the initial stage of gasification. This partially explained the gasification reactivity of raw and torrefied biomass char showed little difference at the early stage (Fig. 2). With the gasification proceeding, the ID/IG of PW-800P char increased faster, indicating its reactive sites were consumed more easily. Furthermore, the increase of graphite content and the decrease of cross-linking structure at 900 °C may be the reason for the significant reduction of 900P-char gasification reactivity (Fig. 3). Apart from the carbon structure of chars, the AAEMs were reported to play a catalytic role in char-CO2 reaction, which could inhibit the graphitization or condensation with the gasification proceeding (Perander et al., 2015). However, not all the chemical speciation of AAEMs in bio-char could work as catalysts. For examples, the formation of AAEMs-aluminosilicate crystals would inhibit gasification (Ellis et al., 2015). The ion-exchanged AAEMs mainly included the watersoluble salt and AAEMs bounded organically in the samples (Chen et al., 2016; Zhao et al., 2016), which could be considered as the active AAEMs. The ion-exchanged K and Ca of PW-char and PW300-char were listed in Table 5. After torrefaction, the content of active AAEMs

3.4. Mechanism study The gasification reactivity of chars was closely related to its physicochemical structure. In the present work, the mechanism of bio-char gasification was discussed from the perspective of the carbon structure evolution and the changes of AAEMs contents. Raman spectra was used to study the char structural evolution, which was well suited to analyze the structural features of bio-chars (Guizani et al., 2017). Raman spectra were further curve-fitted with 10 Gaussian bands, which represented the typical structures in bio-chars (summarized in Table 4) (Wang et al., 2015). Structural features of chars can be evaluated by analyzing the intensities of the major Raman bands. The ratio between D band and G band (ID/IG) reflected the crystalline or graphite-like carbon structures (Xie et al., 2019). S band was considered as the cross-linking structures and substitutional groups (Zhang et al., 2017). The ID/IGR+VL+VR ratio was representative of the ratio between the large aromatic ring systems (≥6 rings) and small aromatic ring systems (3–5 rings) (Zhao et al., 2016). The variations of ID/IG, IS/IG and ID/I(GR+VL+VR) for PW-800P and PW300-800P semi-char with different gasification time, which was obtained from interrupted TGA experiments, were shown in Fig. 5(a). The ratio of ID/IG increased with the gasification proceeding, especially at the final conversion level. The bio-char was far from forming graphite crystals (Xie et al., 2019). Hence this behavior was related to the increase in the larger aromatic structures, suggesting a decrease of char reactivity. The ratio of IS/IG decreased with the conversion. This behavior was related to the decrease of cross-linking structures, indicating that this structures were the active sites in char gasification. In addition, insignificant change in ID/I(GR+VL+VR) was observed during gasification, which revealed that the gasification occurred on all ring systems unselectively. Fig. 5(b) showed the variation of Raman parameter of the PW/PW300-char with the different pyrolysis temperatures. It was found that ID/IG ratios increased when the pyrolysis temperatures

Table 4 Summary of Raman band assignments. Band name

Band position (cm−1)

Description

Band type

GL G GR VL VR D SL S

1700 1580 1540 1465 1380 1320 1230 1180

sp2 sp2 sp2 sp2,sp3 sp2,sp3 sp2 sp2,sp3 sp2,sp3

SR R

1060 960-800

Carbonyl group C]O Graphite; aromatic ring quadrant breathing; alkene C]C Aromatics with 3–5 rings; amorphous carbon structures Methylene or methyl; semi-circle breathing of aromatic rings; amorphous carbon structures Methyl group; semi-circle breathing of aromatic rings; amorphous carbon structures Highly ordered carbonaceous materials; C]C between aromatic rings and aromatics with not less than 6 rings Aryl-alkyl ether; para-aromatics Caromatic–Calkyl; aromatic (aliphatic) ethers; CeC on hydroaromatic rings; hexagonal diamond carbon sp3; CeH on aromatic rings C–H on aromatic tings; benzene (ortho-di-substituted) ring CeC on alkanes and cyclic alkanes; CeH on aromatic rings

6

sp2 sp2,sp3

Bioresource Technology 293 (2019) 122087

Q. He, et al. 1.8

1.8

(a)

PW

(X=0.90)

1.5

(X=0.49)

Band area ratio

Band area ratio

(X=0.23) 1.2

1.5





PW-800P PW300-800P D/G D/(GR+VR+VL) S/G (X) Conversion

0.9

0.6

(b)

PW300 D/G D/(GR+VR+VL) S/G

1.2

0.9

0.6

0.3

0.3 0

4

8

12

16

20

24

28

600

Gasification time (min)

700

800

900

Pyrolysis temperature (°C)

Fig. 5. Raman band area ratios (a) semi-gasification char for PW-800P and PW300-800P, (b) char pyrolyzed at different temperatures.

pathways of char structure evolution changed and the reactivity reduced significantly than that before this temperature.

Table 5 Contents of ion-exchanged K and Ca in chars (mg/g). Samples

K

Ca

PW-600P PW-700P PW-800P PW-900P PW300-600P PW300-700P PW300-800P PW300-900P

6.24 6.76 6.98 6.74 6.00 6.23 6.24 6.02

22.48 22.79 27.41 34.94 20.25 20.44 26.44 24.83

Acknowledgement This work was supported by National Natural Science Foundation of China (21878093). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122087.

decreased in bio-char. It should be noted that the ash content of PW300 was 6.50%, whereas the PW had an ash content of 3.45% (Table 1). Therefore, the decrease of active AAEMs in torrefied biomass char was likely that the reaction between K/Ca and Si/Al, forming the aluminosilicate which could not be removed by the NH4Ac solution. This indicated the torrefied biomass char had a lower gasification reactivity. As for PW300-chars, it was found that the contents of active K/Ca were the highest in char pyrolyzed at 800 °C. The intercalation compounds of AAEMs and carbon were stable up to the temperature of at least 830 °C in an inert atmosphere (Jensen et al., 2000). When the pyrolysis temperature was below 800 °C, much smaller aromatic ring existed in bio-chars (Fig. 5(b)), which may reduce the solubility of active AAEMs. Some researchers also found that the NH4Ac-soluble K increased while the water-soluble K decreased with increasing pyrolysis temperature (Zhao et al., 2016). Besides, the active AAEMs decreased at 900 °C, which was another reason for the significant reduction of PW300-900P char gasification reactivity (Fig. 3). As for PW-chars, the variation of active-AAEMs were similar to the PW300-char, except the active-Ca in PW-900 char. It was reported that the Ca species, regardless of the chemical forms, were much less volatile than K (Okuno et al., 2005). In present work, the content of K begun to decrease whereas the active-Ca was continual increased in PW-900P char. This also explained the gasification reactivity of PW-900P was stronger than that of PW300900P, even at higher gasification temperatures (Fig. 3(c, d)).

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