Kinetic Study on Bamboo Pyrolysis - American Chemical Society

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Ind. Eng. Chem. Res. 2008, 47, 5710–5722

Kinetic Study on Bamboo Pyrolysis Edward L. K. Mui, W. H. Cheung, Vinci K. C. Lee, and Gordon McKay* Department of Chemical Engineering, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Hong Kong SAR

Bamboo is an indigenous plant to many Asian countries; it is an extremely rapid growing plant, and with planned harvesting it can be produced as a sustainable raw material wood resource. The pyrolysis of bamboo has been studied to understand, model, and control the decomposition process as part of the overall process of producing activated carbons from bamboo. Different heating rates have been tested, and the pyrolysis mechanism has been modeled using the Runge-Kutta mechanism. The model has been applied assuming three-, four-, five-, and six-component reaction models. The best fit model, based on SSE values, is the sixcomponent scheme using a Runge-Kutta solution methodology although the three-component reaction model correlates the data very well too. The basis for the selection of the six-component model is that each of the three main bamboo components, xylan, cellulose, and lignin, has one major decomposition step to volatiles and a minor decomposition to form char. 1. Introduction The trend for material and energy recovery from wastes and sustainable biomass resources has led to an increased interest in the development and understanding of thermal treatment processing of wastes and/or biomass.1,2 Bamboo is a large, woody-grasses member of the family of Bambusoideae encompassing about 1250 species within 75 genera worldwide.4 Found in tropical and subtropical areas over many continents except Europe, major producing nations are China, Thailand, India, Bangladesh, Indonesia, and South Korea with a total annual production of 6 to 7 million tonnes and 36.2 million USD (U.S. dollars) revenue.5 The composition of bamboo differs from species, growing conditions, maturity, or even fraction of harvest.4 The elemental composition of Phyllostachys Pubesescens (moso bamboo), a major bamboo species that contributes to the waste scaffolding generated in Hong Kong is mainly carbon (51.7%) and oxygen (41.7%). Its low sulfur (0.05%) and nitrogen (0.05%) and relatively low ash (0.5-15%) content make it an ideal precursor for activated carbon manufacturing. Wet chemical analysis shows that bamboo consists of lignin, cellulose, pentosan, and a trace amount of ethanol extractives.6,7 Owing to its strong, light, and flexible woody stems, few abundant species such as Phyllostachys Pubesescens (Mao Jue, or moso bamboo), are popular for use in building construction in China dating back to 2000 years ago. 8 In Hong Kong bamboo is widely used in building construction, building facade, wall repairs, decoration, and sign erection.9 Two bamboo species, namely, Bambusa PerVariabius grade A (Kao Jue) and Phyllostachys Pubesescens (Mao Jue, or moso bamboo), are the most popular.9 Only a small number of studies have been conducted to evaluate the possibility of using bamboo in the production of a low-cost active carbon adsorbent even though its ability to grow at a rate of 1-3 m per year makes it a potential sustainable raw material source. Asada et al. 10 studied the production of bamboo charcoal at temperatures between 500 to 1000 °C, and the resulting charcoal exhibited a moderately high surface area up to 490.8 m2/g. Wu et al. 11 prepared activated carbon from * To whom correspondence should be addressed. E-mail: kemckayg@ ust.hk. Tel.: (852) 2358 8412. Fax: (852) 2358 0054.

bamboo via a two-stage activation process, in which raw bamboo was first carbonized at 450 °C for 2 h, followed by steam activation at the temperature ranging from 700 to 880 °C. It was found that the BET surface area increased with temperature with a maximum of 1038 m2/g and the maximum iodine number was reported to be 1095 g/kg. Abe et al.12 reported a similar finding in the study in which activated carbon was first prepared by a process which consisted of a carbonization step at 600 °C and an activation process at 850 °C for various periods of time. The iodine adsorption capacity was up to 1200 mg/g. Mizuta et al. 13 employed a process by heating the bamboo char to 900 °C for 1 h, producing carbon with surface area up to 400 m2/g. Although the results of the bamboo active carbons are encouraging there is no design methodology in the approach to bamboo char and carbon production so far. Consequently, this research presents a detailed study of the charring process and a mechanism and model for bamboo pyrolysis at four heating rates. The fundamental steps are also studied by investigating the pyrolysis of the main constituent components (i.e., xylan, cellulose, and lignin) of bamboo. 2. Theory: Model Development In the gas-solid reaction the rate of conversion dR/dt can be expressed in accordance with the Arrhenius equation:1 dR -E ) A exp (1 - R)n (1) dt RT where A is the pre-exponential factor of the pyrolysis (1/min), E is the activation energy (J/mol), t is the reaction time (min), n is reaction order, R is the fraction of reactants decomposed at time t, and R and T are the universal gas constant (8.314 J/(mol · K)) and absolute temperature (K), respectively. The fractional conversion of reactants R is defined in terms of mass change of the sample:

( )

R)

Wi - W Wi - Wf

(2)

where Wi, W and Wf are the initial, actual, and final mass of the sample, respectively. To determine the kinetic parameters of the pyrolysis under nonisothermal condition at the heating rate β, a mathematical

10.1021/ie070763w CCC: $40.75  2008 American Chemical Society Published on Web 07/04/2008

Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5711

Figure 1. Thermograms of raw bamboo with different particle sizes pyrolyzed under flowing nitrogen at 5 °C/min.

Figure 2. Thermograms of raw bamboo (710-500 µm) with different initial mass pyrolyzed under flowing nitrogen at 5 °C/min.

solution, namely, fourth-order Runge-Kutta Method,14 was applied. Assuming each sample (i.e., bamboo) consists of different components that decomposed to gaseous volatiles independently, the overall decomposition rate is shown as dR ) dt

i

∑ 1

dRi mi,0 dt

(or step-size, h) between the points x0 to xn where xn ) xn-1 + h. They were given by

(3)

where mi is the initial mass fraction of individual component i. Found on the basis of modified Euler’s method, 14 the fourthorder Runge-Kutta method is a widely used numerical solution to the initial value problem as

k′ 01 ) hf(xn, yn)

(5a)

1 1 k′ 02 ) hf xn + h, yn + k01 2 2

(5b)

k′03

(5c)

( ) 1 1 ) hf(x + h, y + k ) 2 2 n

n

02

k′04 ) hf(xn + h, yn + k03)

(5d)

and

dy ) f(x, y) dx

(4a)

h yn+1 ) yn + (k′01 + 2k′02 + 2k′03 + k′ 04) (6) 6 To calculate the fractional conversion of reactants R with respect to reaction time t these four points were redefined as

y(x0) ) y0

(4b)

k′01 ) hf(tn, Rn)

and

It involves a weighted average of the gradients of four points known as k′01, k′02, k′03, and k′04 taken in the uniform interval

(7a)

1 1 k′02 ) hf tn + h, Rn + k′ 01 2 2

(

)

(7b)

5712 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008

1 1 k′03 ) hf tn + h, Rn + k′ 02 2 2

(

)

(7c)

k′04 ) hf(tn + h, Rn + k′03)

(7d)

h Rn+1 ) Rn + (k′01 + 2k′02 + 2k′03 + k′04) (8) 6 If the system temperature is elevated linearly at the heating rate β, the four factors involved in the calculation of R of component i in each sample can be as

(

k′01 ) Ai exp

( (

k′02 ) Ai exp

k′03 ) Ai exp

-Ei R

-Ei R

(

k′04 ) Ai exp

)

-Ei 1 (1 - Rn) R T0 + βtn

(9a)

( [ ) ( )

hk′01 2

[ ( ( ))

hk′02 2

1

1 - Rn +

h T0 + β tn + 2 1

1 - Rn +

h T0 + β tn + 2

)

)] (9b)

)]

deionized water, dried bamboo was crushed and sieved to different fractions of particle size. For the synthetic mixture, each sample was prepared by mixing xylan, cellulose, and lignin by mortar and pestle. The total mass was made up to approximately 1 g. Then the mixture was sealed in a 50 mL plastic bottle, placed over a ball-mill roller, and shaken for eight hours. A thermogravimetric analyzer (Setaram, TGA/DTA 92 Setaram II) was used in the analysis of bamboo/ synthetic mixture decomposition with respect to time. Nitrogen (UHP 99.999%) from a gas cylinder was used as a carrier gas at 180 mL/min. Samples were placed in a platinum crucible and then loaded onto a holder made of alumina and platinum wire that was hung onto a precise balance above. Approximately 10-15 mg of samples were heated to 500 °C at the heating rate of 1 to 20 °C/min and then held isothermal for 10 min. All experiments were performed in duplicate. 4. Results and Discussion

(9c)

-Ei 1 [1 - (Rn + hk′03)] R T0 + β(tn + h) (9d)

where T0 is the initial temperature (K) and β is the heating rate (°C/min). The accuracy of the solution is dependent on the step size h, which means the smaller the size of the intervals the better the accuracy between estimated and exact values. Compared with the solutions by Euler or modified Euler’s methods the result obtained via Runge-Kutta is considerably more accurate.14,15 3. Experimental Section Thermal analysis such as thermogravimetry (TG) involves the continuous, precise monitoring of the mass in terms of time under gas-solid reaction. It is desirable to the study the decomposition of substances like bamboo. Raw bamboo was obtained from the campus of Hong Kong University of Science and Technology. After washing by

Figure 3. Derivative thermogram of raw bamboo (710-500 µm) pyrolysis.

4.1. Thermogram for Raw Bamboo. The proximate chemical compositions of bamboo culms are similar to hardwoods but with higher ash and silica content.7 Three major components were found, namely, xylan, cellulose, and lignin. Elemental analysis in this study shows that raw bamboo consists of moderately high carbon content (48.64%) and low amounts of nitrogen (0.14%), sulfur (0.11%), and hydrogen (6.75%). The pyrolysis of bamboo and its major components has been studied and analyzed in this section. In general the overall thermal pyrolytic decomposition scheme is Bamboo f Volatiles + Char Figures 1 and 2 show the thermograms of bamboo pyrolysis under flowing nitrogen with various bamboo particle sizes and initial masses. As shown, there is little effect of the variation of these two factors. Therefore, the size fraction of bamboo was set to 710-500 µm with an initial mass of 12 mg throughout the study. Furthermore, there is little change in the thermogram between duplicates. Figure 3 is a derivative thermogram of bamboo pyrolysis under flowing nitrogen. It could be seen that the pyrolysis initiated at approximately 220 °C, where the reaction rate increased linearly to 320 °C, then rose sharply to the peak at

Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5713 a

Table 1. Estimated Bamboo Composition in This Study wt % component xylan cellulose lignin a

Scurlock et al.4

this study

24.6 (1.2) 43.3 (1.0) 26.2 (1.0)

24.6 46.7 28.1

Data in parentheses are standard deviations.

approximately 340 °C and went down. The reaction rate curve became flat beyond the temperature 380 °C, revealing a slower decomposition which was attributed to a component such as lignin. Since there has been no previous study on the pyrolytic decomposition of bamboo, it was decided to do a more fundamental study also. This study was based on investigating the pyrolysis of individual major components considered to make up bamboo, namely, xylan, cellulose, and lignin. There are a number of literature sources available relating to the pyrolysis of these three components.16 Owing to the structural difference of each component it is expected that these three components will undergo individual

Figure 4. Arrhenius plot for bamboo pyrolysis at different heating rates.

Figure 5. Arrhenius plot of xylan at different heating rates.

reaction pathways during nonisothermal pyrolysis. It can be seen that the mass fractions of each component vary significantly even in the same species (moso bamboo, Phyllostachys Pubescens), data from Scurlock et al.4 from the same family (Phyllostachys) is considered reasonable as an initial guess. Experimental data in this work found that the lignin content is 28.1%, in other words, the holocellulose (hemicellulose + cellulose) is 71.9%, which agrees closely with both literature4 (67.9%) and calculated data (71.3%, see also Table 1). Compared with hardwoods, bamboo has a similar chemical composition,7 the hemicellulose, cellulose, and lignin contents of woods being 14.1-36.6%, 38-56%, and 17-32.5%, respectively.4,17,21In the present study, the composition from Scurlock4 will be used as the initial estimated bamboo composition (dry and ash free) as shown in Table 1. Four component-based models were proposed in an attempt to obtain the best-fit model that best describes the pyrolysis of raw bamboo. In the theory section, it is found that a numerical solution such as the Runge-Kutta method is able to yield kinetic parameters to correlate experimental data values in a best-fit analysis approach. Throughout this section a fourth-order


Figure 6. Arrhenius plot of cellulose at different heating rates.

Figure 7. Arrhenius plot of lignin at different heating rates. Table 2. Kinetic Parameters of Xylan, Cellulose and Lignin Derived from Arrhenius Plots (see also Figures 5–7) in This Study heating rate (°C/min)

pre-exponential factor, A (1/min)

activation energy, E (kJ/mol)

correlation coefficients

2.03 × 102 3.21 × 103 5.07 × 103 2.28 × 107

81.1 97.2 100.9 137.9

0.950 0.948 0.944 0.992

Cellulose 1 5 10 20

3.63 × 1019 4.43 × 1019 2.33 × 1024 1.37 × 1022

1 5 10 20

9.25 × 10-5 6.37 × 10-4 8.00 × 10-4 5.20 × 10-4


references

Xylan 1 5 10 20

Table 3. Kinetic Parameters of Xylan, Cellulose and Lignin Reported in the Literatures

278.5 293.3 354.4 326.8

0.996 0.990 0.966 0.993

16.8 28.3 30.7 27.7

0.964 0.950 0.947 0.971

Lignin

Runge-Kutta algorithm was adopted to estimate the kinetic parameters for bamboo pyrolysis. Furthermore, the number of reaction order was assumed to be 1 (i.e., n ) 1).16,18,19,28


Xylan

Manya et al.29 Bilbao et al.37

9.66 × 1010-1.6 × 1024 7.94 × 1016 5.01 × 106 4.7 × 1015 590-4.6 × 107

Varhegyi et al.23 Bradbury et al.27 Gronli et al. 28 Rao and Sharma30 Radmandesh et al.39

1.26 1.70 1.00 1.33 2.17

Ferdous et al.20 Varhegyi et al.23 Nunn et al.35 Caballero et al.38

3.3 × 107-1.84 × 109 2-1 × 103 3.39 × 105 1.56 × 18

Williams and Besler18 Varhegyi et al.22

124.-256.9 199 95 194 42.7-86.2

Cellulose × × × × ×

1018 1021 1019 1023 109

240 243 244 282 234-263

Lignin 80-158 34-65 82 52.6


Figure 8. Comparison between experimental data and predicted values (three-component model).

4.2. Estimation of Kinetic Parameters. Figure 4 is an Arrhenius plot of the bamboo pyrolysis at heating rates between 5 to 20 °C/min. The curves have a similar pattern at all heating rates but exhibited a significant decrease in the gradient at temperatures > 350 °C when the heating rate was increased. When this effect was considered together with the increasing rate constant (ln k) at rapid pyrolysis, it reveals that there could be a heat-transfer effect to the particle, which results in a shift of reaction rate and leads to the sudden release of volatiles at higher temperature. On the basis of the thermogravimetric analysis of individual components at different heating rates, the Arrhenius plots of xylan, cellulose, and lignin (Figures 5–7) provide initial guess values for iteration in the model. Table 2 summarizes the kinetic parameters derived from the plots. For comparison some data from literature are tabulated in Table 3. It could be seen that the kinetic parameters derived in this study correlated linearly as nearly all of them have a correlation coefficient (R2) higher than 0.94. Comparing the results with those data from the literature the kinetic parameters reported here agree reasonably well with other researchers’ findings. In particular, with regard to the relationship between heating rates and kinetic parameters (i.e., pre-exponential factor and activation energy) the values derived from the Arrhenius plots in this study increased with heating rate.18–20 It is because at higher heating rates more reactions are triggered simultaneously, leading to a sharp rise in reaction rates, more unstable radicals/intermediates, and lower activation energy. These values were then taken into the kinetic equations as the initial guess values and further optimized. 4.3. Three-Component Model. Assuming that all three components decomposed independently to gaseous volatiles and char, the overall decomposition scheme is

Recalling eq 1, for each component, the reaction rate with respect to time is expressed as

( )

Ei dRi ) Ai exp (1 - Ri) dt RT

(1)

Taking the mass fraction of each component into account, the overall decomposition rate of raw bamboo was the sum of the individual decomposition rates: dR1 dR2 dR3 dRT ) m1 + m2 + m3 (10) dt dt dt dt where mi was the mass fraction of component i, representing xylan, cellulose, and lignin, respectively. Figure 6, 7, 8 demonstrates the predicted values obtained from the threecomponent model with experimental data. The optimized kinetic parameters are shown in Table 4. Experimental results show that there was a lateral shift at peak temperatures following the heating rates. At the heating rate of 1 °C/min, the peak occurred at 300 °C where the maximum was shifted to 340 °C at the heating rate of 20 °C. The three-component model demonstrated sufficient fit between predicted values and experimental data. However, discrepancy is observed at a higher temperature region (400 °C or above) which is attributed to the presence of lignin. It is probably due to the highly aromatic structure of lignin that shifted the degradation over a broad temperature range from 250 to 600 °C.21 Also, secondary reactions between intermediates evolved from primary reactants (e.g., cellulose) are shown to be significant to the overall reaction at the lower temperature regime.3,21 4.4. Four-Component Model. As a result of the effect of lignin throughout the entire pyrolysis process, the overall decomposition scheme was modified to a four-component reaction:

Figure 9 is the comparison between experimental data and values predicted by the four-component model. The optimized kinetic parameters are tabulated in Table 4. It can be seen that at the heating rate of 20 °C/min the SSE of the optimized value was even lower than that of the threecomponent model. Although the SSEs of the four-component

1.30×10 8.50×1012 1.50×1011 1.35× 107

13

component 1a

1.33×10 2.78×1010 8.98× 109 1.35× 107

10

component 1 54.29 198 2510 3.31 × 1014

component 3a

1.69×10 9.53×1015 1.21×1010 1.35× 107

10

component 1b 3.47×10 1.38×1013 6.57×1012 3.14×1012

15

component 2 2.40×10 1.29×103 4.56×105 3.31×104

5

component 3a


1.97×10 2.85×1015 3.48×1015 3.14×1012

13

component 2

pre-exponential factor, A (1/min) component 1 106.0 105.8 102.3 69.5

5.00 × 10 6.49 × 1010 4.94 × 1010 8190

135.0 130.5 135.0 69.5

component 1a

six-component

5.11×10 3.89×1010 5.25×1010 4.18× 107

10

component 3b

five-component

10

component 3b

57.4 60.3 61.0 68.3

component 1

component 1b 96.1 170.0 105.5 90.0

26.6 35.0 34.3 47.4

component 3a

160.5 139.8 135.4 127.4

component 2

25.7 27.8 31.6 34.9

73.3 45.4 49.0 47.7

105.6 100.4 102.1 80.0

SSE

1 5 10 20

2.37× 10 4.62× 107 5.82×1010 1.25×1010

6

1.77×10 8.15× 108 1.40×1010 1.66×1012

10

3.72×10 5.50×1014 1.89×1015 1.12× 108

15

3.67×10 1.37×1015 1.85×1015 1.51×1010

15

45.8 451 3310 3.00

4.75×10 3.96×1010 3.41×1010 1.28×1010

10

76.4 82.2 102.6 102.4

99.6 86.4 110.7 125.0

74.4 149.3 159.2 74.4

160.7 160.5 161.8 103.3

38.2 31.4 48.0 33.1

110.9 109.3 107.9 102.5

0.0005 0.0028 0.0088 0.0098

SSE

0.0024 0.0032 0.0094 0.0279

SSE

0.0016 0.0060 0.0131 0.0121

SSE

0.0006 0.0021 0.0042 0.0169

component 3b

101.6 101.1 100.8 52.5

component 3b

component 3

component 3a


140.1 163.0 163.9 127.4

component 2


131.5 139.6 136.7 127.1

component 2


pre-exponential factor, A (1/min) activation energy, E (kJ/mol) heating rate (°C/min) component 1a component 1b component 2a component 2b component 3a component 3b component 1a component 1b component 2a component 2b component 3a component 3b

1 5 10 20

heating rate (°C/min)

1 5 10 20


6.06 × 10 2.54 × 106 3.59 × 106 9.54 × 106

1 5 10 20 four-component

9.69 54.1 307 1010

1.05 × 10 4.82 × 1014 2.26 × 1014 3.07 × 1012

5

14

component 3

component 2

component 1



three-component

Table 4. Optimized Kinetic Parameters from the Three-, Four-, Five- and Six-Component Models Based on the Runge-Kutta Method



Figure 9. Comparison between experimental data and predicted values (four-component model).

Figure 10. Comparison between experimental data and predicted values (five-component model).

model were not as low as those of the three-component model, the results are comparable, showing that the model is able to provide a reasonable fit to the experimental data. It further suggests a possibility of constructing a better model by introducing an additional component to the overall degradation kinetic equation. 4.5. Five-Component Model. While discrepancy between experimental data and reaction rate in the lower temperature region (300 °C or below) was observed, more reactions were expected to be involved in the decomposition in which component 1 (xylan) was further divided into two subcomponents in an attempt to construct a model that may describe the pyrolysis better. As reported in the literature 22–25 the xylan decomposition is usually a multistage process occurring at

temperatures 200 and 400 °C. Considering the fact of the multireaction at lower temperature, the overall decomposition scheme was modified:

The five-component model only had partial success in achieving lower SSEs (Table 4) compared with the four-


Figure 11. Comparison between experimental data and prediction (six-component model).

component model (5 °C/min and 10 °C/min). It is worthwhile to note that the fit between experimental data and the simulation results at lower temperature region (300 °C or below) in these two runs had improved (Figure 10), revealing that the introduction of additional components for xylan provides a better reaction scheme description. 4.6. Six-Component Model. As with xylan and lignin, the decomposition of cellulose is a multireaction process which cannot be adequately simulated by a simple rate law.21,23,26–29 The popular Broido-Shafizadeh mechanism described the cellulose pyrolysis as a two-stage scheme:

In the report by Varhegyi et al. 23 the reaction scheme was further simplified in which the presence of “active” cellulose was considered insignificant to the overall weight loss. While this fact was taken into account, component 2 (cellulose) was divided into two subcomponents known as components 2a and 2b, giving a six-component decomposition model as:

Table 4 presents the optimized kinetic parameters obtained from the six-component model. The simulated values are presented in Figure 11 with experimental data. Comparing the SSEs between the five- and six-component models, it is found that the latter have lower values at all heating rates, revealing that the presence of the additional cellulose component is close to the actual decomposition phenomenon.

4.7. Kinetic Parameters of Individual Components. Since models for the pyrolysis of bamboo have not been reported in the literature before, it was decided to undertake a fundamental study using the basic chemical constituents of bamboo. It is commonly accepted that three major components are subject to decomposition in bamboo pyrolysis, namely, xylan, cellulose, and lignin.4,7 To accurately evaluate the composition of bamboo, attempts were made to produce a synthetic mixture having the best-fit decomposition profile in comparison with the real bamboo. During the preparation procedure of the mixture, all three components were ground and mixed thoroughly in solid form in accordance to the different mass fractions, starting with the composition data based on the values shown in Table 1. For a range of synthetic mixtures, the composition was varied significantly and the synthetic mixtures were pyrolyzed under identical conditions (heating rate, 5 °C/min; temperature, 105-500 °C; atmosphere, nitrogen). The thermograms in Figure 12 demonstrate the pyrolysis profiles of the three biomass components. Decomposition of xylan initiated at approximately 200 °C and proceeded rapidly in a relatively narrow range between 250 to 290 °C. In the case of cellulose, the reaction started at temperature approximately 310 °C, but the decomposition is more rapid: over 70% of the weight loss occurred at the temperature below 350 °C. Both reactions are in good agreement with literature.23,28,30–32 It is well-known that lignin decomposed over a broad range of temperatures.23,32,33 The starting temperature of lignin decomposition is approximately the same as that of xylan (ca. 200 °C), but the reaction is often slow at temperatures below 500 °C, possibly due to its strong benzene-propane, crosslinked macromolecular structure.21,33,34 From the pyrolysis curve shown in Figure 12, significant decomposition is observed at 250-350 °C at a very low rate (approximately 0.5 wt %/min) and went flat at temperatures beyond 350 °C. It might reveal that lignin was undergoing a slow carbonization process in which tar and other condensable volatiles are involved.35,36 Figure 13 is the thermogram of real bamboo (710-500 µm), initial synthetic mixture, and the best-fit composition values after


Figure 12. Thermograms of three biomass components at the heating rate of 5 °C/min.

Figure 13. Thermograms of bamboo and synthetic mixtures pyrolyzed at 5 °C/min. Table 5. Comparison in Weight Loss Percentages between Bamboo and Synthetic Mixture in Optimized Compositions

Table 6. Kinetic Parameters of Synthetic Mixture at the Heating Rate of 5 °C/min

wt loss %

bamboo synthetic mixture

regions

200-300 °C

300-350 °C

350-500 °C

11.9 14.1

38.4 34.8

3.3 6.1

composition adjustment. From the decomposition profiles one can see that both synthetic mixtures have similar weight loss characteristics in comparison with the real bamboo at the temperature up to approximately 270 °C, where it is believed that all xylan decomposed at that point. When the temperature was further elevated, the other two components (mainly cellulose) started to decompose. It is expected that the cellulose decomposition reached its maximum rate at a temperature below

pre-exponential factor, A (1/min) activation energy, E (kJ/mol) correlation coefficients

200-300 °C

300-350 °C

350-500 °C

2.96

0.92

5.00 × 10-5

70.3 0.997

65.7 0.990

13.6 0.946

400 °C,19,34,36 accompanied by a slow decomposition of lignin at the same time.21,34 Comparing the optimized mixture with real bamboo, a significant difference in the temperature interval 320 to 450 °C is observed. As shown in Figure 13, in the temperature range between 300 and 350 °C, raw bamboo has a decomposition rate of approximately 0.0068/°C, which is 1.3


Figure 14. Arrhenius plot of synthetic mixture at the heating rate of 5 °C/min. Table 7. Summary of SSEs of Different Models model heating rate (°C/min)

threecomponent

fourcomponent

fivecomponent

sixcomponent

1 5 10 20

0.0006 0.0021 0.0042 0.0169

0.0016 0.0060 0.0131 0.0120

0.0024 0.0032 0.0094 0.0279

0.0005 0.0028 0.0088 0.0098

total

0.0238

0.0327

0.0429

0.0219

times higher than the rate of the synthetic mixture (ca. 0.0052/ °C). This is probably due to the purity and specific properties of lignin used in the synthetic mixture which has a high ash content (15.52 wt %) and as a result increased the weight loss in the high temperature regime (400-500 °C). However, the overall thermal characteristics of both remain similar as the weight loss percentage at individual temperature intervals is very close (see Table 5).

4.8. Kinetic Parameters of the Synthetic Mixture. From the Arrhenius plot (Figure 14) the synthetic mixture decomposition is approximately divided into three regions which is consistent with the degradation profiles of pure components. However, overlapping between individual pyrolysis may occur since there can be more than one component undergoing decomposition simultaneously (see also Figure 12), for example, at 200-300 °C, the decomposition was mainly xylan but lignin is also involved to a small extent, because it undergoes slow decomposition over a wide temperature range from 300 to 650 °C. When the system was heated up to 300-350 °C, cellulose decomposition will occur but the lignin continued its slow degradation. Once the system reached 400 °C or above, most xylan and cellulose have decomposed to volatiles and char with lignin. The kinetic parameters derived are shown in Table 6 in accordance with the defined temperature regions. Moreover, the cellulose degradation temperature is much earlier than in wood

Figure 15. Derivative thermograms for bamboo and optimized synthetic mixture at 5 °C/min.


pyrolysis (typically around 380 to 450 °C). In this study, the cellulose pyrolysis occurs from 300 to 350 °C (Figure 12). This may be due to a cellulose of low molecular weight or the presence of metal oxides catalyzing the cellulose decomposition reaction.23,40,44 Figure 15 shows the derivative thermograms for bamboo and the synthetic mixture in the optimized composition at the heating rate of 5 °C/min. It can be seen that the synthetic mixture decomposition pattern exhibits three separate regions and agreed reasonably well with the experimental data of bamboo, except for a lateral shift of peak temperature in the medium temperature regime (300-350 °C) where cellulose is mainly decomposed. The kinetic parameters derived from the Arrhenius plot (Figure 15) in the medium (300-350 °C) and high temperature (400-500 °C) regimes are opposite to the values derived from the optimized Three-component model (Table 4). It can be explained by two possible reasons: 1. Presence of Metal Oxides in the Bamboo Matrix. The presence of metal oxides has been found in bagasse 41,42 and wood;17,43 similarly, they will be present in bamboo. In other pyrolysis systems, it has been shown that these metal oxides are catalytic to the decomposition of biomass.23,40,44 Under their influence the reaction is triggered at the medium temperature regime at a higher reaction rate, giving a sharp peak at approximately 330 °C in the DTG curve (Figure 15). 2. Different Secondary Reactions Due to Different Physical/Chemical Characteristics. Although the proximate composition of bamboo and the synthetic mixture are similar, impurities (particularly in the raw bamboo) may change the nature of volatiles and further affect the secondary reaction pathway to a certain extent. In addition, there is a significant difference in the physical distribution of decomposable components between raw bamboo and the mixture. As three major components (i.e., xylan, cellulose, and lignin) in raw bamboo are closely and uniformly bound with each other, the relatively loose contact between components in the mixture may result in a slightly different volatiles evolution and eventually change the reaction pathway. 5. Conclusion: Model Selection As an important step prior to the production of activated carbon, pyrolysis of bamboo was studied via the technique of thermogravimetry. In the views of SSEs, all four models were shown able to simulate the bamboo pyrolysis satisfactorily (maximum SSE ) 0.0279) at all heating rates from 1 to 20 °C/min (Table 7). It appears that the decomposition could be better represented by three- and six-component models, for slow heating rates (below 10 °C/min) and fast heating rate (20 °C/ min), respectively, since their SSEs are not only the lowest but also comparable especially at lower heating rates (1 and 5 °C/ min). As reported in several studies, biomass component pyrolysis was rarely simulated by the single-step kinetic model because the evolution of intermediates and associated secondary reactions are usually significant to the contribution of overall weight loss. It is, therefore, an appropriate approach to separate each component further to take those key secondary reactions into account. Consequently, either the three- or the six-component model is preferred in describing the bamboo pyrolysis process. Therefore it seems likely that each component has one main dominant decomposition reaction to volatiles (i.e., the threecomponent model) and that each component has a smaller less significant decomposition reaction to char, resulting in a slightly

more enhanced fit to the six-component model. Further study on the intermediate reactions as well as secondary subcomponents should be carried out in an attempt to establish a better fit kinetic model on the basis of comprehensively understanding the overall pyrolysis scheme. Acknowledgment The authors are grateful to the Research Grant Council (RGC) and Green Island Cement (GIC) for providing the financial support for this research project. Nomenclature A ) pre-exponential factor (1/min) E ) activation energy (kJ/mol) h ) step size (min) m ) mass fraction (dimensionless) k ) Arrhenius rate constant (1/min) k′ ) coefficient in Runge-Kutta algorithm R ) universal gas constant (8.314 J/(mol · K)) SSE ) sum-of-squared error (dimensionless) T ) temperature (K) t ) time (min) R ) fraction decomposed (dimensionless), R ) (w0 - w)/(w0 - wf) β ) heating rate (°C/min) w ) weight (mg) Subscripts 0 ) initial f ) final I ) component i T ) total

Literature Cited (1) Skodras, G.; Grammelis, P.; Basinas, P.; Kakaras, E.; Sakellaropoulos, G. Pyrolysis and Combustion Characteristics of Biomass and Wastederived Feedstock. Ind. Eng. Chem. Res. 2006, 45 (11), 3791. (2) Gonzalez, J. F.; Ramiro, A.; Gonzalez-Garcia, C. M.; Ganan, J.; Encinar, J. M.; Sabio, E.; Rubiales, J. Pyrolysis of Almond Shells. Energy Applications of Fractions. Ind. Eng. Chem. Res. 2005, 44 (9), 3003. (3) Antal, M. J., Jr; Allen, S. G.; Dai, X.; Shimizu, B.; Tam, M. S.; Gronli, M. Attainment of the Theoretical Yield of Carbon from Biomass. Ind. Eng. Chem. Res. 2000, 39 (11), 4024. (4) Scurlock, J. M. O.; Dayton, D. C.; Hames, B. Bamboo: An Overlooked Biomass Resource. Biomass Bioenerg. 2000, 19, 229. (5) European Commission; United Nation Food and Agricultural Organisation. Non-wood Forest Products in 15 Tropical Countries - An OVerView; European Commission: Rome, Italy, 2002. (6) Higuchi, H. Biochemical Studies of Lignin Formation, III. Physiologia Plantarum 1957, 10, 633. (7) Higuchi, H. In Concise Encyclopaedia of Wood & Wood-based Materials; Schniewind,A. P., Ed.; Pergamon Press: Cambridge, 1989. (8) Chung, K. F.; Yu, W. K. Mechanical Properties of Structural Bamboo for Bamboo Scaffoldings. Eng. Structures 2002, 24, 429. (9) Wong, P. Y. Safety and Reliability of Bamboo Scaffolding. M.Phil. Thesis, Hong Kong University of Science & Technology, Hong Kong, PRC, 2002. (10) Asada, T.; Ishihara, S.; Yamane, T.; Toba, A.; Yamada, A.; Oikawa, K. Science of Bamboo Charcoal: Study on Carbonizing Temperature of Bamboo Charcoal and Removal Capability of Harmful Gases. J. Health Sci. 2002, 48 (6), 473. (11) Wu, F. C.; Tseng, R. L.; Juang, R. S. Preparation of Activated Carbons from Bamboo and Their Adsorption Abilities for Dyes and Phenol. J. EnViron. Sci. Health, Part A 1999, 34 (9), 1753. (12) Abe, I.; Fukuhara, T.; Maruyama, J.; Tatsumoto, H; Iwasaki, S. Preparation of Carbonaceous Adsorbents for Removal of Chloroform from Drinking Water. Carbon 2001, 39, 1069. (13) Mizuta, K.; Matsumota, T.; Hatate, Y.; Nishihara, K.; Nakanishi, T. Removal of Nitrate-Nitrogen from Drinking Water Using Bamboo Powder Charcoal. Bioresour. Technol. 2004, 95 (3), 255.

5722 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 (14) Steiner, E. The Chemistry Maths Book; Oxford University Press: Oxford, 1996. (15) Guo, J. Development of Effective Adsorbents from Agricultural Waste for Air Pollution Control. Ph.D. Thesis, Nanyang Technological University, Singapore, 1998. (16) Gomez, C. J.; Manya, J. J.; Velo, E.; Puigjaner, L. Further Applications of a Revisited Summative Model for Kinetics of Biomass Pyrolysis. Ind. Eng. Chem. Res. 2004, 43 (4), 901. (17) Sjostrom, E. Wood Chemistry: Fundamentals and Applications; Academic Press: New York, 1981. (18) Williams, P. T.; Besler, S. The Pyrolysis of Rice Husks in A Thermogravimetric Analyser and Static Batch Reactor. Fuel 1993, 72 (2), 151. (19) Antal, M. J., Jr.; Varhegyi, G.; Jakab, E. Cellulose Pyrolysis Kinetics: Revisited. Ind. Eng. Chem. Res. 1998, 37 (4), 1267. (20) Ferdous, D.; Dalai, A. K.; Bej, S. K.; Thring, R. W. Pyrolysis of Lignins: Experimental and Kinetics Studies. Energy Fuels 2002, 16 (6), 1405. (21) Gaur, S.; Reed, T. B. Thermal Data for Natural and Synthetic Fuels; Marcel Dekker: New York, 1998. (22) Varhegyi, G.; Antal, M. J., Jr.; Szekely, T.; Szabo, P. Kinetics of the Thermal Decomposition of Cellulose, Hemicellulose, and Sugar Cane Bagasse. Energy Fuels 1989, 3 (3), 329. (23) Varhegyi, G.; Antal, M. J., Jr.; Jakab, E.; Szabo, P. Kinetic Modelling of Biomass Pyrolysis. J. Anal. Appl. Pyrolysis 1997, 42 (1), 73. (24) Di Blasi, C.; Lanzetta, M. Intrinsic Kinetics of Isothermal Xylan Degradation in Inert Atmosphere. J. Anal. Appl. Pyrolysis 1997, 40-41, 287. (25) Fisher, T.; Hajaligol, M.; Waymack, B.; Kellogg, D. Pyrolysis Behaviour and Kinetics of Biomass Derived Materials. J. Anal. Appl. Pyrolysis 2002, 62 (2), 331. (26) Broido, A.; Nelson, M. A. Char Yield on Pyrolysis of Cellulose. Combust. Flame 1975, 24 (2), 263. (27) Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. Kinetic Model for Pyrolysis of Cellulose. J. Appl. Polym. Sci. 1979, 23 (11), 3271. (28) Gronli, M.; Antal, M. J., Jr; Varhegyi, G. Round-robin Study of Cellulose Pyrolysis Kinetics by Thermogravimetry. Ind. Eng. Chem. Res. 1999, 38 (6), 2238. (29) Manya, J. J.; Velo, E.; Puigjaner, L. Kinetics of Biomass Pyrolysis: A Reformulated Three-parallel-reactions Model. Ind. Eng. Chem. Res. 2003, 42 (3), 434. (30) Rao, T. R.; Sharma, A. Pyrolysis Rates of Biomass Materials. Energy 1998, 23 (11), 973. (31) Capart, R.; Khezami, L.; Burnham, A. K. Assessment of Various Kinetic Models for the Pyrolysis of A Microgranular Cellulose. Thermochim. Acta 2004, 417, 79.

(32) Meszaros, E.; Varhegyi, G.; Jakab, E. Thermogravimetric and Reaction Kinetic Analysis of Biomass Samples from an Energy Plantation. Energy Fuels 2004, 18, 497. (33) Lopez Pasquali, C. E.; Herrera, H. Pyrolysis of Lignin and IR Analysis of Residues. Thermochim. Acta 1997, 293, 39. (34) Yang, H.; Yan, R.; Chen, H.; Zheng, C.; Lee, D. H.; Liang, D. T. In-depth Investigation of Biomass Pyrolysis Based on Three Major Components: Hemicellulose, Cellulose and Lignin. Energy Fuels 2006, 20, 388. (35) Nunn, T. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Product Composition and Kinetics in the Rapid Pyrolysis of Milled Wood Lignin. Ind. Eng. Chem. Process. Des. DeV. 1985, 24, 844. (36) Orfao, J. J. M.; Antunes, F. J. A.; Figueiredo, J. L. Pyrolysis Kinetics of Lignocellulosic Materials - Three Independent Reactions Model. Fuel 1999, 78 (3), 349. (37) Bilbao, R.; Millera, A.; Arauzo, J. Kinetics of Weight Loss by Thermal Decomposition of Xylan and Lignin. Influence of Experimental Conditions. Thermochim. Acta 1989, 143, 137. (38) Caballero, J. A.; Font, R.; Marcilla, A. Kinetic Study of the Secondary Thermal Decomposition of Kraft Lignin. J. Anal. Appl. Pyrolysis 1996, 38, 131. (39) Radmanesh, R.; Courbariaux, Y.; Chaouki, J.; Guy, C. A Unified Lumped Approach in Kinetic Modelling of Biomass Pyrolysis. Fuel 2006, 85, 1211. (40) Muller-Hagedorn, M.; Bockhorn, H. Pyrolytic Behaviour of Different Biomasses (Angiosperms)(Maize Plants, Straws, and Wood) in Low Temperature Pyrolysis. J. Anal. Appl. Pyrolysis 2007, 79, 136. (41) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D. Release of Inorganic Constitutes From Leached Biomass During Thermal Conversion. Energy Fuels 1999, 13 (4), 860. (42) Valix, M.; Cheung, W. H.; McKay, G. Preparation of Activated Carbon Using Low Temperature Carbonization and Physical Activation of High Ash Raw Bagasse for Acid Dye Adsorption. Chemosphere 2004, 56 (5), 493. (43) Senneca, O. Kinetics of Pyrolysis, Combustion and Gasification of Three Biomass Fuels. Fuel Process. Technol. 2007, 88 (1), 87. (44) Yang, H.; Yan, R.; Chen, H.; Zheng, C.; Lee, D. H.; Liang, D. T. Influence of Mineral Matter on Pyrolysis of Palm Oil Wastes. Combust. Flame 2006, 146 (4), 605.

ReceiVed for reView June 3, 2007 ReVised manuscript receiVed March 10, 2008 Accepted April 23, 2008 IE070763W

Kinetic Study on Bamboo Pyrolysis - American Chemical Society

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