Pyrolysis kinetics and behavior of potassium-impregnated pine wood in TGA and a fixed-bed reactor

Energy Conversion and Management 130 (2016) 184–191

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Pyrolysis kinetics and behavior of potassium-impregnated pine wood in TGA and a fixed-bed reactor Feiqiang Guo, Yuan Liu, Yan Wang, Xiaolei Li, Tiantao Li, Chenglong Guo ⇑ School of Electric Power Engineering, China University of Mining and Technology, 221116 Xuzhou, PR China

a r t i c l e

i n f o

Article history: Received 23 August 2016 Received in revised form 9 October 2016 Accepted 24 October 2016

Keywords: Biomass Potassium Pyrolysis Kinetics Tar yield

a b s t r a c t Potassium is a well-known alkali catalyst in the thermal reactions of biomass. The effect of potassium on the pyrolysis behavior and kinetics of biomass was investigated through TGA and a fixed bed in this study. The addition of potassium reduced initial and peak temperature in TGA curves, promoting the decomposition process of biomass. The effect of potassium on the apparent activation energy varies at different conversion degree (a) and lower apparent activation energy was obtained at the initial stage (a 6 0.3) of the pyrolysis process. The influence of potassium on the volatiles releasing behavior was dependent on the amount of loading potassium. The gas yield was significantly increased with the increasing of potassium concentration when the impregnated potassium was below 0.3 mol/kg. Particularly, the yield of H2 and CO2 was promoted by potassium. Higher potassium content may contribute to the increase in reactivity towards tar molecules, leading to quickly decrease in tar yield. The surface chemical characteristics of char were determined by FTIR spectroscopic method. Above a certain threshold surface concentration of potassium, SEM analysis shows that agglomeration is a potential threat at high temperature, which can block the active sites and decrease the activity during biomass thermal decomposition. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction As a renewable and sustainable resource, biomass utilization has been recognized as a promising solution for the current environmental and energy challenges. Pyrolysis is the fundamental principle of various thermochemical conversion processes of biomass, including combustion, gasification, liquefaction and carbonization, and therefore detailed study on the pyrolysis is important for understanding and improve these processes [1,2]. Generally, combustible gases or bio-oil is the target product through thermochemical conversion processes. Particularly, clean gases have attracted the most attention and have been applied due to its high conversion efficiency and its versatility in accepting a wide range of feedstocks. However, tar is always an unwanted byproduct during the process of gas production, and catalytic pyrolysis has been reported as an effective method to enhance the clean gas production with less tar content [3]. The alkali metals present in biomass have been revealed having effective influence on biomass thermal decomposition. The effect of alkali metals on the biomass pyrolysis has been reported to

⇑ Corresponding author. E-mail address: [email protected] (C. Guo). http://dx.doi.org/10.1016/j.enconman.2016.10.055 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

increase the yield of char, hydrogen, carbon monoxide and reduce the tar yield and pyrolysis starting temperature [4,5]. Up to now, a number of intensive works have been carried out to investigate the catalytic mechanism of alkali on biomass decomposition. It has been reported that even a small amount of alkali metal could change the surface molecules reactivity in crystalline cellulose and significantly influence the product yield [6]. Thus, it can be an effective method to enhance the pyrolysis process of biomass for gaining the desired product by changing the alkali metals concentration in biomass. The effect of alkali metals on the biomass pyrolysis process has been investigated based on the kinetics studied as well using thermogravimetric analysis by some researchers. Saddawi et al. [7] found that an increase in alkali metals concentration clearly caused a decrease in both the activation energies and the pre-exponential factors of willow samples. It was also observed by Lv et al. [8] that the characteristic of biomass pyrolysis and gasification was dependent on its components and alkali metallic species. Alkali metallic species increased the peak pyrolysis value, whereas decreased initial pyrolysis temperature. Thermogravimetric analysis conducted by Wang et al. [9] showed that metallic salts catalyzed the formation of active cellulose strongly and decreased the activation energy of cellulose pyrolysis.

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Nomenclature a da/dt t A Ea T b Ti

maximum mass loss rate (wt% min1) temperature when the weight loss rate is maximum (°C) residue weight (wt%) universal gas constant (8.314 J mol1 K1) differential form of the reaction model

DTGmax Tp WR R f(a)

conversion degree (%) rate of conversion reaction time (s) frequency factor (s1) apparent activation energy (kJ/mol) absolute temperature (K) constant heating rate (K/min) the temperature when conversion reached 5% (°C)

Among the alkali and alkaline earth metals, potassium has considerable effects on the pyrolysis behavior of biomass, which can promote the decomposition process, increase gas and char yields and decrease the tar yield [10,11]. Eom et al. [12] believed that potassium had a distinguished catalytic effect on promoting the formation of low molecular compounds and suppressing the formation of levoglucosan. Zhou et al. [13] found that potassium had significant catalytic influence on the biomass pyrolysis, while this catalytic ability varied with the potassium content. Since the presence of potassium plays an important role in biomass pyrolysis, it is important to clarify the influence of potassium for further application. Thus, a comprehensive study of the potassium catalytic mechanism, together the kinetics, product distribution and char surface variation should be carried out to provide practical support for potassium utilization in biomass thermochemical conversion. The aims of the present work are to examine the influence of potassium on the kinetics and pyrolysis characteristics of biomass pyrolysis using a thermogravimetric analyzer together with a fixed bed reactor. The thermal events taking place during pyrolysis of biomass were identified and the kinetic data were obtained to fit thermogravimetric data. The evolution of biomass as well as product distribution was also investigated to further discover the effect of potassium on biomass pyrolysis system.

2. Material and methods 2.1. Sample preparation The biomass feedstock of pine was from the surrounding areas of Xuzhou, Jiangsu province. The pine sample was crushed to a size range of 150–180 lm and then dried in an atmospheric oven at 105 °C for 24 h before each test. Table 1 summaries the results of ultimate and proximate analyses for the tested pine, which was conducted by an elemental analyzer (Vario MICRO Cube, Elementar). Impregnation was employed to prepare samples with different potassium ion content equably and adequately. The preparation process was as follows: firstly, dried pine (12 g) was immersed in 150 mL KNO3 solution, with K+ concentration of 0.1, 0.3 and 0.5 mol/kg. Then, these three types of samples after impregnation were put in a drying oven for 24 h at 35 °C. After that, the solution was filtered and dried at 105 °C for about 24 h. These samples were

labeled according to the K+ concentration of the solution in which they were impregnated (e.g. 0.3 K-pine).

2.2. Thermogravimetric experiments The pyrolysis characteristics of samples were tested by a thermogravimetric analyzer (Labsys Evolution Setaram, France). In order to avoid diffusion limitations, about 10 mg of sample was heated at heating rates of 20, 30, 40 and 50 K/min from room temperature to 800 °C under nitrogen flow (60 mL/min). Experiments for each test were repeated twice in order to confirm the repeatability of the results.

2.3. Kinetics modeling The kinetic parameters (apparent activation energy and preexponential factor) were determined by differential method based on the Arrhenius equation. Generally, the non-isothermal thermal decomposition of biomass is expressed by the following equation:

da ¼ A expðEa =RTÞf ðaÞ dt

ð1Þ

where da/dt represents the rate of conversion; a denotes to conversion degree in the process (%); t is reaction time (s); A is frequency factor (s1); Ea is apparent activation energy (kJ/mol); R is the universal gas constant; T is the absolute temperature (K); f(a) is a differential form of the reaction model, which is a function of a. For a constant heating rate b (K/min) during the pyrolysis, b = dT/dt, Eq. (1) can be transformed into:

da=dt ¼ ðA=bÞ expðEa =RTÞf ðaÞ

ð2Þ

Flynn-Wall-Ozawa (FWO) method was chosen in this study to estimate the kinetic parameters because it has been successfully applied previously to study solid decomposition [14,15]. The expression of FWO method is given by Eq. (3).

lg b ¼ lg

AE Ea  2:315  0:4567 RgðaÞ RT

ð3Þ

For a constant a, a straight line should be obtained with a slope of E/R if lg b is plotted versus 1/T, and Ea can be calculated.

Table 1 Ultimate and proximate analysis of pine. Sample

Pine

Ultimate analysis (wt%, daf)

Proximate analysis (wt%, db)

C

H

Oa

N

S

Ash

Volatile

Fixed carbon

51.05

5.78

42.29

0.75

0.13

1.3

81.0

17.7

db – Dry basis; daf – Dry and ash free basis. a By difference.

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Mass flowmeter

Sample feeder

Loaded feedstock N2

Temperature controller

Gas sample bag

Electric furnace

Gas washing bottles Micro pump

quartz reactor

Silica gel

Ice cooling Fig. 1. Schematic diagram of experimental system.

80

100

100

80 70

60

80

80

50

340

350

360

370

TG (%)

TG (%)

40

60

40

20

20 K/min 30 K/min 40 K/min 50 K/min

330

60

400 o Temperature ( C)

600

340

350

360

pine 0.1K-pine 0.3K-pine 0.5K-pine

40

20

200

60

100

200

800

300 400 500 o Temperature ( C)

600

700

800

0

0 -5

-1

DTG (% min )

-1

DTG (% min )

-10

20 K/min 30 K/min 40 K/min 50 K/min

-20

-10

pine 0.1K-pine 0.3K-pine 0.5K-pine

-15

-30

-20

-40

-25

200

400

600

800

o

200

400

600

800

o

Temperature ( C) Fig. 2. Influence of temperature on TG and DTG curves for 0.3 K-pine pyrolysis.

2.4. Fixed-bed pyrolysis The combination of TGA and fast pyrolysis methods using a fixed-bed allows a comprehensive and systematic study of biomass

Temperature ( C) Fig. 3. Influence of potassium concentration on TG and DTG curves at b = 30 K/min.

pyrolysis characteristics. Experiments were carried out in the setup shown in Fig. 1, which was composed of a fixed-bed reactor, a gas supply system, an electrically heated furnace and a tar and gas collection and detection system. The fixed-bed quartz reactor employed consists of a porous plate to support the sample with inner diameter of 30 mm.

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four samples (pine, 0.1 K-pine, 0.3 K-pine, 0.5 K-pine) at four heating rates (20, 30, 40, 50 K/min). Table 2 summaries the thermal parameters including the initial temperature (Ti, defined as the temperature when conversion reached 5%), the maximum mass loss rate (DTGmax), the temperature when the weight loss rate is maximum (Tp) and the residue weight at 800 °C (WR). The results showed that the primary decomposition reactions of the samples occurred between 250 and 450 °C and the maximum weight loss rate was reached at 340–385 °C. The heating rate is known to influence both the location of the TGA curves and DTG peaks [16]. The effect of heating rates on pyrolysis behaviors of 0.3 K-pine is depicted in Fig. 2 and similar trend was obtained for other samples. With the increase of heating rate, the curves of TG shifted to the higher temperature. Meanwhile, the values of Tp also shifted to elevated values at higher heating rate, which is attributed to the various decomposition temperatures of biomass components. It is believed that the pyrolysis process of biomass is delayed as a result of the decrease of heat transfer efficiency at higher heating rate [17,18]. In addition, it was apparent that the values of DTGmax increased significantly at higher heating rate, suggesting that higher reaction rate was associated with higher heating rate, which may be due to the heat and mass transfer limitation at lower heating rate. As suggested by Gašparovicˇ et al. [19] and Park et al. [20], a larger instantaneous thermal energy is provided in the system and a longer time is required for the purge gas to reach equilibrium at a lower heating rate, whereas a short reaction time is needed at higher heating rate. The TG and DTG results for un-impregnated and K-impregnated samples are plotted as Fig. 3 and some characteristic parameters of pyrolysis are shown in Table 2. The initial temperature and the peak temperature shifted to lower temperature by impregnating potassium. Meanwhile, the values of DTGmax increased slowly as well after impregnating potassium. The results indicated that potassium present in biomass can promote the decomposition process of biomass, leading to higher reactive activity, which can be called as catalytic effect of potassium [21]. It can also be seen from Fig. 3 and Table 2 that the residue weight of char increased gradually with the increasing potassium content, which was mainly due to the polycondensation reaction facilitated by impregnating potassium [13]. The tendency in this study are partly agree with relevant previous studies at similar conditions [22,23].

About 5 g of pine particles were used in each experimental run. Before the start of each experiment, feedstock was pre-loaded into sample feeder and the reactor was purged with N2 (99.999%) at 200 mL/min flow rate to ensure an inert atmosphere. When the heated furnace temperature reached the desired values (500, 600, 700 or 800 °C), the sample was put into the pyrolysis zone rapidly and heated for around 30 min. The volatile matters released in the form of vapor, including the tar and syngas. The generated tar was collected by both condensation and absorption in a system consisting of four isopropyl alcohol-washing bottles (100 mL) immersed in an ice-water bath (Fig. 1). Following the isopropyl alcoholwashing bottles, one bottle of water and two bottles of silica gel are employed to make sure that the escaping isopropyl alcohol vapor can be absorbed completely. At the end of each test, all of isopropyl alcohol solvent in the four bottles were mixed together and dried by a rotary evaporator at 90 °C. In this study, tar was experimentally defined as the material dissolved in the isopropyl alcohol but not evaporated at 90 °C. Then, the amount of residue tar was weighed and calculated. The syngas was collected by a gas collector bag and was analyzed by a gas chromatograph (SC8000-010) with TCD detectors after each experiment. Different sample chars after pyrolysis were recovered for further analysis. Several repeat runs were conducted under the same conditions to ensure the repeatability of the experiment. 3. Results and discussion 3.1. Thermogravimetric analysis Figs. 2 and 3 illustrate the mass loss (TG) and the rate of mass loss (DTG) curves of typical tests during the pyrolysis process of Table 2 Pyrolysis characteristic parameters determined by TGA. Samples

b (K/min)

Ti (°C)

DTGmax (wt%/min)

TP (°C)

WR (wt%)

Pine

20 30 40 50

252.6 254.7 259.1 264.9

15.55 21.36 30.12 34.80

367.4 367.3 382.2 384.9

24.3 23.9 24.1 22.8

0.1 K-pine

20 30 40 50

250.8 251.7 257.0 261.5

17.54 23.08 33.60 40.33

350.8 352.1 362.2 366.9

25.2 25.7 25.8 23.9

0.3 K-pine

20 30 40 50

247.1 249.9 253.6 257.9

17.80 24.19 35.82 42.02

344.3 344.7 354.6 357.7

29.3 28.1 28.1 26.6

20 30 40 50

245.1 248.6 251.0 255.7

18.17 23.84 37.32 40.37

337.8 341.9 353.6 354.6

29.5 29.9 29.8 30.4

0.5 K-pine

3.2. Kinetic analysis By using Eq. (3), the values of Ea were estimated from the Arrhenius plot of lg b vs. 1/T at the selected conversion degrees (0.2 6 a 6 0.8), as shown in Table 3. Fig. 4 shows the plots used for determination of activated energy at different conversion rates using FWO method. The correlation coefficient (R2) ranged from 92.46 to 99.95 for all cases, which means that the points are fitted well. Fig. 5 intuitively shows the evolution of apparent activation energy with the increase of conversion rate. The apparent activa-

Notes: Ti – the initial temperature (pyrolysis conversion of 5%); DTGmax – the maximum mass loss rate; TP – the temperature when the weight loss rate is maximum; WR – the residue weight at 800 °C. Table 3 The pyrolysis kinetics parameters of different samples.

a 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average

Pine

0.1 K-pine

0.3 K-pine

0.5 K-pine

Ea (kJ/mol)

R2

Ea (kJ/mol)

R2

Ea (kJ/mol)

R2

Ea (kJ/mol)

R2

145.2 149.9 147.3 148.6 146.5 145.9 149.6 147.6

94.85 96.87 97.83 97.67 99.44 99.91 99.65

139.4 147.4 161.3 161.9 167.8 159.5 164.4 157.4

93.69 98.06 98.97 98.41 98.69 96.26 92.46

132.1 145.0 170.6 169.5 167.7 160.9 176.0 160.3

94.39 99.86 99.95 99.90 98.94 97.46 93.13

126.9 142.1 153.4 155.2 158.8 166.7 195.9 157.0

94.52 95.45 95.64 98.41 99.55 99.21 98.30

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0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.7

1.6

0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.7

1.6

1.5

lg

lg

1.5

1.4

1.4

1.3

1.3

0.00147

0.00154

0.00161

0.00168

0.001496

0.00175

0.001564

0.001632

0.001700

0.001768

-1

-1

1/T (K )

1/T (K )

(a) pine

(b) 0.1K-pine

0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.7

1.6

0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.7

1.6

1.5

lg

lg

1.5

1.4

1.4

1.3

1.3

0.00150

0.00156

0.00162

0.00168

0.00174

0.00180

0.001518

0.001587

0.001656

0.001725

0.001794

-1

1/T (K )

-1

1/T (K )

(d) 0.5K-pine

(c) 0.3K-pine Fig. 4. Plots of lg b vs 1/T calculated by FWO method.

200

180

pine 0.1K-pine 0.3K-pine 0.5K-pine

Ea (kJ/mol)

160

140

120 Main reaction stage

Initial stage

100 0.1

0.2

0.3

0.4

0.5

0.6

Termination stage

0.7

0.8

a Fig. 5. a vs Ea estimated by FWO method for different samples.

0.9

tion energy of pine was almost unchanged at different conversion rates. In order to analysis the effect of potassium concentration on apparent activation energy of pine in detail, the pyrolysis process was divided into three stages according to the conversion rate, including initial stage (a 6 0.3), main reaction stage (0.3 < a < 0.7) and termination stage (a P 0.7). The initial stage is mainly correspond to the thermal decomposition of hemicelluloses and the values of Ea decreases with the increase of impregnated potassium content. At a = 0.2, the apparent activation energy decreased from 145.2 kJ/mol to 126.9 kJ/mol with the impregnated potassium concentration increased from 0 to 0.5 mol/kg, indicating that potassium promotes the pyrolysis of hemicellulose. The thermal decomposition of cellulose occurs at the main reaction stage (320–400 °C) [24], corresponding to the range of 0.3 < a < 0.7. At this stage, the addition of potassium in pine resulted in the increase in apparent activation energy compared with original pine, which might be attributed to the improvement of cross-linking reaction as a result of the aromatic and complexity of carbon structure in the presence of potassium. When the conversion increased from 0.7 to 0.8, the apparent

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activation energy sharply increased with the increase of impregnated potassium content. The temperature at this stage was higher than 400 °C, higher content of potassium may lead to more orderly carbon structure and active sites reduced quickly [25], which in turn resulted in higher apparent activation energy.

3.3. Fixed-bed pyrolysis Based on the kinetic analysis of potassium impregnated biomass, the properties of tar, char and gas yield were investigated using a fixed bed reactor to further investigate the effect of potassium ion on the biomass pyrolysis behavior. The products yield

from fast pyrolysis of original and potassium impregnated pine samples are presented in Fig. 6. It can be seen that char yield decreased gradually with the increase of pyrolysis temperature, indicating that higher temperature promoted the releasing of volatiles. Moreover, the results also showed that the char increased gradually at higher potassium impregnated concentration, which means that the impregnation of potassium led to more char yield. The results are in accordance with the analyzed results of Section 3.1. Other authors have also reported similar trends in char yields for other types of biomass such as willow [7] and aspen wood [10]. It was apparent that tar yield decreased with increasing pyrolysis temperature in all cases. For the original pine, the tar yield

300

20

10

0 15

o

500 C o

600 C o

700 C

10

o

800 C

C C o C o C o

200 150 100

0

pine

0.1K-pine

0.3K-pine

0.5K-pine

pine

0.1K-pine

0.3K-pine

60

40

500 C o 600 C o 700 C o 800 C

140 120

CO yield (mL/g)

80

0.5K-pine o

o

500 C o 600 C o 700 C o 800 C

100

H2 yield (mL/g)

o

50

5

0

500 600 700 800

250

Syngas yield (mL/g)

Tar yield (wt.%)

Char yield (wt.%)

30

100 80 60 40

20 20

0

pine

0.1K-pine

0.3K-pine

0

0.5K-pine

50

pine

30

30

20

10

0

0.5K-pine

0.1K-pine

0.3K-pine

0.5K-pine

500 C o 600 C o 700 C o 800 C

25

CO2 yield (mL/g)

CH4 yield (mL/g)

40

0.3K-pine

o

o

500 C o 600 C o 700 C o 800 C

0.1K-pine

20 15 10 5 0

pine

0.1K-pine

0.3K-pine

0.5K-pine

pine

Fig. 6. Tar, char and gas yield under different conditions.

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Fig. 7. SEM images of char samples after pyrolysis. (a) Pine char (800 °C); (b) 0.3 K-pine char (800 °C); (c) 0.5 K-pine char (700 °C); (d) 0.5 K-pine char (800 °C).

Transmittance (%)

0.3K-pine char

-CH3

C-H

Original char

C=C&C=O

C-O

-OH&-NH 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 8. FTIR spectra of pine char and 0.3 K-pine char produced at 700 °C.

decreased from 16.64 to 3.56% in weight when the temperature increased from 500 to 800 °C, representing the effect of temperature on the thermochemical decomposition of tar. In comparison with the original pine, the potassium-impregnated samples yielded less tar. Particularly, the tar yield decreased from 16.35 to 6.89% using 0.5 K-pine at 600 °C. Nevertheless, when the potassium impregnated increased from 0.3 mol/kg to 0.5 mol/kg at higher temperature (700 °C and 800 °C), the tar yield sharply increased. High content of alkali metal can decrease ash fusion point and lead to agglomeration at high temperature [26], and in turn result in the change of volatiles yield, which may be the reason for the sharp tar yield increase.

With the decrease in tar yield for higher potassium impregnated samples, more gas was produced, as can be seen in Fig. 6. Correspondingly, the total gas yield sharply dropped when the potassium impregnated increased to 0.5 mol/kg at high temperature over 700 °C. Four main permanent components (H2, CO, CH4 and CO2) were detected and their yields all increased with increasing temperature. When more potassium was impregnated in biomass, the yields of H2 and CO2 increased gradually, while the yield CO and CH4 decreased. However, for 0.5 K-pine samples, when the temperature was higher than 700 °C, the yield of H2 decreased significantly. Compared with original pine, it was apparent that adding a certain amount of potassium promoted the production of hydrogen and the total gas. It has been reported that alkali metal significantly promotes the release of small-molecule gases even at a very low level of addition [6]. Furthermore, the addition of potassium might promote the reforming of hydrocarbons, which can contribute to H2 yield through reforming reactions as well [27]. Jiang et al. [28] found that the presence of alkali metallic species enhanced the production of H2 and CO2 during the gasification process of biomass, while inhibited the production of CO and CH4. Similar phenomenon was also found by Patwardhan et al. [29] and Wang et al. [30], who studied the effect of alkali metal on the catalytic reforming of bio-oil. The possible agglomeration problem with addition of potassium during biomass pyrolysis may be the reason for the sharp turning of the yield of gas and tar at high temperature and potassium concentration. The residue char samples of pine (800 °C), 0.3 K-pine (800 °C) and 0.5 K-pine (700 °C and 800 °C) were analyzed by the scanning electron microscope (SEM), as shown in Fig. 7. For the original pine char (Fig. 7(a)), it can be observed that no agglomeration happened during pyrolysis process, and good porous structure obtained at 800 °C. For the 0.3 K-pine at 800 °C (Fig. 7(b)), no obvious agglomeration happened as well, and the

F. Guo et al. / Energy Conversion and Management 130 (2016) 184–191

char show good porous structure attached by potassium uniformly. The existence of potassium may catalytically accelerate the pyrolysis of biomass, leading to higher gas releasing and lower tar yield. However, it seems that agglomeration happened on the surface of the residue 0.5 K-pine chars at 700 °C (Fig. 7(c)) and 800 °C (Fig. 7 (d)). The porous structure of these two chars samples was almost disappear and the microstructure occurred great change due to the agglomeration, which may explain why the tar yield increased and gas yield sharply decreased at these two conditions. Thus, it can be concluded that a low level of addition of potassium is favorable to the biomass pyrolysis for high permanent gas releasing, while excessive potassium content is a potential risk of agglomeration when the temperature is higher than 700 °C. The FTIR (Nicolet iS5, USA) spectra of the pyrolysis char of original pine and 0.3 K-pine at 700 °C in the region 4000–500 cm1 is presented in Fig. 8. The peak at 3432 cm1 is assigned to the vibration of hydroxyl group linked with benzene ring. The peaks at 2920 cm1 corresponded to the aliphatic ACH (CH, CH2, CH3) groups. The peak at near 1628 cm1 belongs to C@C/C@O vibration. The band at 1045 cm1 is ascribed to the CAO stretch of OACH3 and CAOH groups. The peak at 703 and 831 cm1 is attributed to the CAH stretching vibration in aromatic structures [31]. As seen from the FTIR spectrums of original pine and 0.3 K-pine chars, the C@C/C@O stretching vibrations absorption bands got intensified with the addition of potassium, denoting that potassium ion may promote the polycondensation reaction for aromatic nucleus. Moreover, the CAH stretching vibration in aromatic structures became much more obvious in the present of potassium, indicating that potassium might improve the aromaticity and stability of pine pyrolysis char. Thus, it can also be speculated that addition of potassium in biomass can promote the H2 releasing and char yield. 4. Conclusion The results obtained from TGA and fixed bed reactor indicated that the addition of potassium significantly influenced the pyrolysis behavior and kinetics of pine. The values of apparent activation energy decreased with the increase in potassium concentration at the initial stage of biomass pyrolysis (a 6 0.3), indicating that potassium has catalytic effect on biomass pyrolysis and results in lower apparent activation energy at the start of pyrolysis process. When the conversion rate was higher than 0.3, the addition of potassium changed the structure of the biomass and resulted in the increase of apparent activation energy. The results from the fixed bed reactor showed that the addition of potassium was attribute to lower tar yield and higher gas yield. The production of H2 and CO2 was enhanced by the presence of potassium. However, excessive concentration of potassium led to agglomeration problem at higher temperature, resulting in higher tar yield and lower gas yield. Consequentially, adding of appropriate potassium had significant promoting effects on biomass pyrolysis and the agglomeration problem can be avoided. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (51406226) and China Postdoctoral Science Foundation (2014M551693). References [1] Abnisa F, Daud WMAW. A review on co-pyrolysis of biomass: an optional technique to obtain a high-grade pyrolysis oil. Energy Convers Manage 2014;87:71–85.

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[2] Balat M, Balat M, Kırtay E, Balat H. Main routes for the thermo-conversion of biomass into fuels and chemicals. Part 1: Pyrolysis systems. Energy Convers Manage 2009;50:3147–57. [3] Wu C, Wang Z, Huang J, Williams PT. Pyrolysis/gasification of cellulose, hemicellulose and lignin for hydrogen production in the presence of various nickel-based catalysts. Fuel 2013;106:697–706. [4] Fahmi R, Bridgwater AV, Darvell LI, Jones JM, Yates N, Thain S, Donnison IS. The effect of alkali metals on combustion and pyrolysis of Lolium and Festuca grasses, switchgrass and willow. Fuel 2007;86:1560–9. [5] Moud H, Andersson KJ, Lanza R, Engvall K. Equilibrium potassium coverage and its effect on a Ni tar reforming catalyst in alkali- and sulfur-laden biomass gasification gases. Appl Catal B-Environ 2016;190:137–46. [6] Shimada N, Kawamoto H, Saka S. Different action of alkali/alkaline earth metal chlorides on cellulose pyrolysis. J Anal Appl Pyrol 2008;81:80–7. [7] Saddawi A, Jones JM, Williams A. Influence of alkali metals on the kinetics of the thermal decomposition of biomass. Fuel Process Technol 2012;104:189–97. [8] Lv D, Xu M, Liu X, Zhan Z, Li Z, Yao H. Effect of cellulose, lignin, alkali and alkaline earth metallic species on biomass pyrolysis and gasification. Fuel Process Technol 2010;91:903–9. [9] Wang S, Liu Q, Liao Y, Luo Z, Cen K. A study on the mechanism research on cellulose pyrolysis under catalysis of metallic salts. Korean J Chem Eng 2007;24:336–40. [10] Veksha A, Zaman W, Layzell DB, Hill JM. Enhancing biochar yield by copyrolysis of bio-oil with biomass: impacts of potassium hydroxide addition and air pretreatment prior to co-pyrolysis. Bioresour Technol 2014;171:88–94. [11] Guo FQ, Don YP, Fan PF, Lv ZC, Yang S, Dong L. Catalytic decomposition of biomass tar compound by calcined coal gangue: a kinetic study. Int J Hydrogen Energy 2016;41:13380–9. [12] Eom IY, Kim JY, Kim TS, Lee SM, Choi D, Choi IG, Choi JW. Effect of essential inorganic metals on primary thermal degradation of lignocellulosic biomass. Bioresour Technol 2012;104:687–94. [13] Zhou LM, Jia YY, Nguyen TH, et al. Hydropyrolsis characteristics and kinetics of potassium-impregnated pine wood. Fuel Process Technol 2013;116:149–57. [14] Slopiecka K, Bartocci P, Fantozzi F. Thermogravimetric analysis and kinetic study of poplar wood pyrolysis. Appl Energy 2012;97:491–7. [15] Chen CX, Ma XQ, He Y. Co-pyrolysis characteristics of microalgae Chlorella vulgaris and coal through TGA. Bioresour Technol 2012;117:264–73. [16] Mabuda AI, Mamphweli NS, Meyer EL. Model free kinetic analysis of biomass/sorbent blends for gasification purposes. Renew Sustain Energy Rev 2016;53:1656–64. [17] Kim SS, Ly HV, Kim J, Choi JH, Woo HC. Thermogravimetric characteristics and pyrolysis kinetics of Alga Sagarssum sp. biomass. Bioresour Technol 2013;139:242–8. [18] Guo FQ, Don YP, Lv ZC, Fan PF, Yang S, Dong L. Pyrolysis kinetics of biomass (herb residue) under isothermal condition in a micro fluidized bed. Energy Convers Manage 2015;93:367–76. ˇ ová Z, Jelemensky´ Lˇ. Kinetic study of wood chips [19] Gašparovicˇ L, Koren decomposition by TGA. Chem Pap 2010;64:174–81. [20] Park YH, Kim J, Kim SS, Park YK. Pyrolysis characteristics and kinetics of oak trees using thermogravimetric analyzer and micro-tubing reactor. Bioresour Technol 2009;100:400–5. [21] Anca-Couce A, Mehrabian R, Scharler R, Obernberger I. Kinetic scheme of biomass pyrolysis considering secondary charring reactions. Energy Convers Manage 2014;87:687–96. [22] Nowakowski DJ, Jones JM. Uncatalysed and potassium-catalysed pyrolysis of the cell-wall constituents of biomass and their model compounds. J Anal Appl Pyrol 2008;83:12–25. [23] Postma RS, Kersten SRA, van Rossum G. Potassium-salt-catalyzed tar reduction during pyrolysis oil gasification. Ind Eng Chem Res 2016;55:7226–30. [24] Muley PD, Henkel C, Abdollahi KK, Marculescu C, Boldor D. A critical comparison of pyrolysis of cellulose, lignin, and pine sawdust using an induction heating reactor. Energy Convers Manage 2016;117:273–80. [25] Chen DY, Zhu XF. Thermal reaction mechanism of biomass and determination of activation energy II. Pyrolysis section. J Fuel Chem Technol 2011;39:670–4. [26] Wang Y, Shao Y, Matovic MD, Whalen JK. Exploring switchgrass and hardwood combustion on excess air and ash fouling/slagging potential: laboratory combustion test and thermogravimetric kinetic analysis. Energy Convers Manage 2015;97:409–19. [27] Kuchonthara P, Vitidsant T, Tsutsumi A. Catalytic effects of potassium on lignin steam gasification with c-Al2O3 as a bed material. Korean J Chem Eng 2008;25:656–62. [28] Jiang L, Hu S, Wang Y, Su S, Sun L, Xu B, He L, Xiang J. Catalytic effects of inherent alkali and alkaline earth metallic species on steam gasification of biomass. Int J Hydrogen Energy 2015;40:15460–9. [29] Patwardhan PR, Satrio JA, Brown RC, Shanks BH. Influence of inorganic salts on the primary pyrolysis products of cellulose. Bioresour Technol 2010;101:4646–55. [30] Wang S, Zhang F, Cai Q, Li X, Zhu L, Wang Q, Luo Z. Catalytic steam reforming of bio-oil model compounds for hydrogen production over coal ash supported Ni catalyst. Int J Hydrogen Energy 2014;39:2018–25. [31] Ke YH, Yang ET, Liu X, Liu CL, Dong WS. Preparation of porous carbons from non-metallic fractions of waste printed circuit boards by chemical and physical activation. New Carbon Mater 2013;28:108–13.