CO2 Gasification Kinetics of Biomass Char Derived from High

ARTICLE pubs.acs.org/EF

CO2 Gasification Kinetics of Biomass Char Derived from High-Temperature Rapid Pyrolysis Shuai Yuan, Xue-li Chen,* Jun Li, and Fu-chen Wang Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: The char of three typical biomasses, rice straw char (RS char), chinar leaves char (CL char), and pine sawdust char (PS char), was prepared in a high-frequency furnace, which could efficiently reduce secondary reactions under rapid pyrolysis conditions at 800
1200 °C. The rapid pyrolysis char produced was isothermally gasified in a thermogravimetric analyzer (TGA) under a CO2 atmosphere. Effects of biomass type and pyrolysis temperature on intrinsic carbon structures and morphologic structures of char and further on gasification characteristics of char were investigated using a Raman spectrum analyzer, scanning electron microscopy (SEM), and a surface area and pore size distribution analyzer. Gasification kinetic models were also contrastively discussed under different conditions. Results show that gasification rates decrease with the increasing pyrolysis temperature. Under the morphologic characteristic reserved conditions, morphologic structures present obvious effects on gasification rates. Gasification reactivity of the three biomass chars is in the order of CL char > RS char > PS char. Melting and shrinkage happen during rapid pyrolysis of PS, and the disappearance of the pore and decrease of the specific surface area of PS char lead to the low specific surface area and gasification rates of PS char. Unobvious melting happens to RS char and CL char, and the initial physical structures can be almost reserved, while CL char presents larger porosity and specific surface area, which make its gasification rates higher than those of RS char. In most conditions, the random pore model (RPM) performs well to describe gasification rates of biomass char studied in this work. However, for gasification of PS char at high temperatures, during which high gasification rates can be maintained in high conversion ranges, the modified random pore model (M-RPM) performs better. For gasification of RS char and CL char at low temperatures, during which gasification rates present a sharp decrease and trailing in medium
high conversion ranges, the shifted M-RPM performs better.

1. INTRODUCTION Extensive attention has been attracted by biomass energy for its advantages, including renewability, low pollution, wide distribution, etc. Efficient and clean use of biomass has become an important research field. Pyrolysis, combustion, and gasification are the main methods of biomass thermal conversions for productions of gas, liquid, and solid products. Pyrolysis is not only a biomass thermal conversion method but also an important process of combustion and gasification. Many studies focus on bio-oil production through medium
low temperature rapid pyrolysis.1
10 However, some researchers have found that high-quality syngas can be directly produced through high-temperature rapid pyrolysis of biomass, with the carbon conversions being high and the liquid yields being low;11
15 therefore, it may also be an available method for biomass use. Although carbon retained in char might be low after rapid pyrolysis of biomass at high temperatures, this part of carbon should also be used to increase the efficiency of biomass. Gasification or co-gasification with coal might be good use methods for this part of char. Therefore, research on gasification characteristics of biomass char derived from rapid pyrolysis has important significance. Presently, gasification characteristics of char derived from rapid pyrolysis at high temperatures have not been fully understood. Pyrolysis conditions have important effects on gasification characteristics, and high-temperature rapid pyrolysis char was mostly prepared by the drop-tube furnace in the present r 2011 American Chemical Society

literature. Rapid pyrolysis char of sawdust was prepared in a drop-tube furnace at 1000 and 1400 °C by Fermoso and co-workers, and effects of the pyrolysis temperature, pressure, and CO2 concentration on gasification characteristics were investigated.16 Rapid pyrolysis of sawdust and bagasse was carried out in a drop-tube furnace by Cetin and co-workers, and effects of the pyrolysis temperature, heating rates, and pressure on char structures and further on gasification characteristics were investigated.17,18 A similar reactor was used by Biagini and co-workers to prepare rapid pyrolsis char, and correlations between gasification/combustion characteristics and char structures were discussed. However, the pyrolysis temperatures were relatively low (400
800 °C).19 A fluidized-bed reactor20 and wire-mesh reactor17,18 were also used to prepare rapid pyrolysis char, while the pyrolysis temperatures were lower than 900 and 950 °C, respectively. Moreover, during pyrolysis in a drop-tube furnace and fluidized-bed reactor, fuel samples were continuously fed into the reactor; thus, there are gas-phase products that remained in the reactors. Char derived from the later fed fuel samples may react with CO2 and H2O in gas-phase products in the reactor (called self-gasification).19 Diversities of gas velocity and residence time may lead to the differences of char properties and gasification characteristics. Received: January 10, 2011 Revised: March 30, 2011 Published: March 30, 2011 2314

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Table 1. Contents of Indigenous Elements in Fuel Samples (ppm, dry basis) K

Na

Ca

Mg

Si

RS

1584

CL PS

688 51

101

603

196

3779

59 15

1451 1187

255 44

834 256

In addition, the random pore model (RPM) was found as the most reliable model for describing biomass char gasification characteristics. However, in some conditions, melting and plastic deformations happen during pyrolysis of biomass,17,19,21 together with the catalysis effects of mineral matters, and gasification characteristics may vary significantly. Therefore, whether RPM could perform well on various biomass and pyrolysis conditions is a worthy question to discuss. In this study, a high-frequency furnace was used to carry out rapid pyrolysis of biomass. The reactor could efficiently restrain the secondary reactions between volatiles and char, and nascent char without “self-gasification” conversion during pyrolysis could be obtained. Particle morphological structures were reserved, and effects of biomass type and pyrolysis temperature on morphological structures and intrinsic carbon structures and further on gasification characteristics were investigated. Applicability of RPM was also discussed.

2. MATERIALS AND METHODS 2.1. Fuel Samples. Three typical agricultural and forestry wastes, rice straw (RS), chinar leaves (CL), and pine sawdust (PS), were chosen as the fuel samples. Compositions and particle sizes have been reported in detail elsewhere.22 Contents of indigenous mineral elements (K, Na, Ca, Mg, and Si) in biomass are listed in Table 1. 2.2. Experimental Setup. Rapid pyrolysis was carried out at 800
1200 °C and 1 atm. Morphological structures of char were investigated by scanning electron microscopy (SEM, JSM-6360LV) at 15 kV. Intrinsic carbon structures of char were investigated by a Raman spectrometer (IUVIA REFL). The contents of mineral elements in biomass samples were detected by inductively coupled plasma
atomic emission spectroscopy (ICP
AES, IRIS 1000). The contents of C in char were analyzed in an elemental analyzer (Vario MACRO CHN/ CHNS). Surface areas and pore size distributions of the char were analyzed by an automated surface area and pore size distribution analyzer (Micromeritics ASAP-2020M). N2 adsorption isotherms at 77 K were measured for the relative pressure (P/P0) range from 0.01 to 0.99. Specific surface areas were obtained by the Brunauer
Emmett
Teller (BET) model, and the pore size distributions were obtained by the Barrett
Joyner
Halenda (BJH) model. Gasification experiments of char were carried out in a thermogravimetric analyzer (TGA, CAHN D-110) at 1 atm. Biomass char was heated to the gasification temperature in a N2 atmosphere, and then N2 (1 L/min) was shifted to CO2 (1.2 L/min) for isothermal gasification. Pyrolysis setup and operating conditions have been described in detail elsewhere.22 The feeding time of the fuel sample was 1.5 min each time, and the power supplier was shut down after 2 min when the injection of fuel samples was finished. Because coking happened in some conditions during pyrolysis of biomass, all of the char was crushed and char with a particle size of 56
180 μm was gasified.

3. RESULTS AND DISCUSSION 3.1. Char Yields. Effects of the temperature on the yields of char and char C during rapid pyrolysis of RS, CL, and PS are

Figure 1. (a) Char yields (dry basis) and (b) char C yields during biomass rapid pyrolysis at different temperatures.

shown in Figure 1. It can be found from Figure 1a that, as the pyrolysis temperature increases, the char yields decrease gradually. Char yields from the three biomasses are in the order of RS > CL > PS. Ash contents of the three biomasses express the same order.22 During rapid pyrolysis of biomass under a high temperature, char yields are low; thus, ash accounts for large proportions of char. Therefore, ash contents in biomass may affect their char yields to some extent. The yields of char C were calculated by the following equation: char C (%) = C in char (mg)/C in fuel (mg)  100%. The equation denotes the proportion of carbon retained in char after pyrolysis. It can be found from Figure 1b that the char C yields of the three biomasses decrease with an increasing temperature. As the temperature increases from 800 to 1200 °C, char C yields of RS decrease from 28.4 to 16.0 wt % and char C yields of CL decrease from 20.6 to 9.6 wt %. PS gives much lower char C yields, which decrease from 6.8 wt % at 800 °C to 3.8 wt % at 1200 °C. 3.2. Effect of Char Structures on Gasification Characteristics. Gasification rates of the biomass char produced at different temperatures are shown in Figure 2. The gasification temperature ranges from 850 to 1050 °C. The gasification conversion X and gasification rate r were calculated by the following functions, respectively: X = (m0
mt)/(m0
mash) and r = dX/dt, where m0 is the sample weight at the start of gasification, mt is the sample weight at a gasification time of t, and mash is the weight of ash remaining in char after being completely converted. Gasification rates decrease with the increasing pyrolysis temperature. The increase of the graphitization degree might be the reason. Raman spectra of CL char derived from different 2315

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Figure 2. Gasification rates of soybean cake char and fitting curves of RPM.

temperatures and Raman spectra of the three biomass chars derived at 1000 °C are shown in panels a and b of Figure 3, respectively. Two main peaks appear in the range of the Raman shift between 800 and 2000 cm
1. The peak appearing at approximate 1350 cm
1 is the D band, which is related to the amorphous carbon structures. The peak appearing at approximate 1580 cm
1 is the G band, which is related to the CdC bonds of graphite crystals. The intensities of D and G bands can be denoted by ID and IG, respectively. A valley appears between D and G bands, and its intensity can be denoted by IV. Generally, the G band becomes sharpened as the graphitization degree increases. The uniformity of carbon structures can be reflected by the IV/IG ratio, which decreases with the increasing uniformity of carbon structures.23,24 It can be found from Figure 3a that both ID and IG of CL char decrease with the increasing pyrolysis temperature. IV/IG ratios of CL char at 800 °C, CL char at 1000 °C, and CL char at 1200 °C are 0.516, 0.512, and 0.494, which decrease with the increasing pyrolysis temperature. It proves that the uniformity of the biomass char increases with the increasing pyrolysis temperature. Under the same conditions, gasification rates of CL char are found to be the highest, followed by those of the RS char, and

gasification rates of PS char are the lowest. IV/IG ratios of the three biomass chars are 0.568 (RS char), 0.512 (CL char), and 0.550 (PS char). Thus, the uniformities of the three biomass chars can be ordered as CL char > PS char > RS char. Generally, the gasification rate decreases with the increasing uniformity. However, it can be found from Figure 2b that gasification rates of CL char are the highest, followed by those of RS char, and gasification rates of PS char are the lowest. In other words, among the three biomass chars, gasification rates have no obvious correlations to their uniformities. Large diversities of morphological structures of char derived from different biomasses might be the reason. Figure 4 shows the SEM graphs of the three biomass chars derived from different temperatures. It can be found that initial morphological structures of RS remained after pyrolysis at 800 °C. Slight destruction and melting happen as the temperature increases. In other words, morphological structures of RS char are not seriously affected when the temperature increases from 800 to 1200 °C. A similar phenomenon can be found from those of CL char. Initial structures remained in CL char at 800 °C, and slight melting happens on CL char at 1000 °C and CL char at 1200 °C. However, morphological structures of PS 2316

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char are seriously affected by the increasing pyrolysis temperature. Slight deformations of the particle surface happen during pyrolysis of PS at 800 °C. As the temperature increases to 1000 °C, melting happens to PS char. The melting phenomenon becomes more serious when the temperature increases to 1200 °C. BET specific surface areas and pore distributions of the biomass char derived under different temperatures are shown in Table 2 and Figure 5, respectively. Because only the pores with diameters larger than 1.5 nm can contribute to the CO2 gasification reaction.25 Features of the pore structures (1.7
200 nm) obtained from the N2 adsorption analysis can reflect their effects on CO2 gasification characteristics. It can be found that, as the pyrolysis temperature increases, specific surface areas of the RS char increase gradually, specific surface areas of CL char increase first and then decrease slightly, while specific surface areas of the PS char decrease sharply. From the comparisons of specific surface areas derived from different biomass, it can be found that the specific surface areas of the CL char are the highest, those of the RS char are lower, and specific Table 2. BET Specific Surface Areas of the Biomass Char (m2/g) pyrolysis temperature (°C)

Figure 3. Raman spectra of biomass char.

RS

CL

PS

800

85.5

186.1

47.5

1000

102.6

239.3

13.1

1200

133.9

225.3

8.9

Figure 4. SEM photographs of biomass char derived from different temperatures (1000). 2317

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Figure 5. Pore size distributions of the biomass char.

surface areas of the PS char are the lowest. As melting happens to PS char, leading to the disappearance and decrease of the specific surface area, gasification rates are sharply restrained. From comparisons, it can be found that, although initial morphological structures of both RS and CL are reserved after pyrolysis, CL char presents a higher porosity and specific surface area than RS char. It is the reason that gasification rates of CL char are higher than those of RS char. In the studies by Cetin and co-workers, melting of woody samples was also found during rapid pyrolysis, while melting of herbaceous samples was very slight and just some destruction happened when the temperature was very high.17,18 Biagini and co-workers consider that melting of cell structures and plastic transformation are the reasons for the disappearance of pore structures during rapid pyrolysis of biomass.19,21 However, morphologic structures of PS char in this study are different from those of PS char produced from a drop-tube furnace.17 Although melting also happened during rapid pyrolysis of PS in the drop-tube furnace at 1000 °C, the cell structure with a high inner porosity appeared in PS char. Differently, PS char at 1000 °C in this study as shown in Figure 4 presents a solid structure with poor porosity. The reason might be that, during rapid pyrolysis of PS in a drop-tube furnace, volatiles released from inside of the molten char lead to the formation of bubbles in char. However, the residence time of char in the drop-tube furnace is short, and char can be quickly quenched when it dropped into the char collector. Shrinkage of the molten char can be stopped rapidly, and bubble structures can be reserved. During rapid pyrolysis of PS in the high-frequency furnace of

this study, the residence time of PS char in the crucible is longer than that of the drop-tube furnace, shrinkage of the molten char happened after volatiles completely released, and the solid structure can be formed. In addition, large diversities of char coking characteristics indicate that, although biomasses all constitute lignin, cellulose, and hemicellulose, properties of the components vary with the plants, especially for lignin and hemicellulose.26 It is also the reason that pyrolysis products cannot be predicted just by the additivity rule of products derived from pyrolysis of lignin, cellulose, and hemicellulose separately.27 Indigenous mineral species in biomass may also play important roles on the char gasification rates. During CO2 gasification of biomass char, potassium (K) is the strongest catalyst, followed by sodium (Na), the catalysis effect of calcium (Ca) is much weaker, and magnesium (Mg) performs almost no catalysis effect. Silicon (Si) has a strong inhibition effect on the gasification reaction.28 It can be found from Table 1 that contents of the mineral elements in PS are low, especially for K and Na, which have strong catalysis. Therefore, the catalysis effects of mineral elements on the gasification of PS char should be weak. Zhang and co-workers found that, when the Si content is higher than those of K and Na, the catalysis effects of K and Na can be completely dismissed.28 It can be found from Table 1 that, although the contents of K and Na in RS are 2.3 and 1.7 times those of CL, respectively, the Si content of RS is much higher, which is 4.5 times that of CL. Therefore, the high Si content in RS dismisses the catalysis effects of K and Na and may also be one of the reasons that gasification rates of RS are lower than CL. 2318

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Table 3. Frequency Factors and Apparent Activation Energy char at 800 °C A0 (105 s
1) RS 3.937  102 CL 0.359 PS 8.589  102

Figure 6. Arrhenius curves of biomass char gasification reactions: (a) RS char, (b) CL char, and (c) PS char.

3.3. Gasification Kinetics of Char. RPM is the most successful model to describe biomass char gasification rates. Its function is r = dX/dt = K(1
X)(1
ψ ln(1
X))1/2, where K is the reaction rate constant and ψ is a parameter of the particle structure.29 Fitting results of RPM on gasification rates are shown in the dotted lines of Figure 2. It can be found that RPM performs well in most conditions. Taking gasification rates at a conversion of 0.5 (r0.5) as the basis, ln r0.5 versus 1/T is plotted in Figure 6. It can be found that linear correlations appear between ln r0.5 and 1/T at a gasification temperature lower than 950 °C in most conditions. As the temperature further increases, ln r0.5 slopes to be lower to the straight line. In the study by Fermoso and co-workers, a similar phenomenon was found.16 They consider that the gasification reactions are

char at 1000 °C

char at 1200 °C

Ea (kJ/mol)

A0 (105 s
1)

Ea (kJ/mol)

A0 (105 s
1)

Ea (kJ/mol)

238.3 159.7 248.5

8.464 2.306 2.457  102

202.9 179.7 236.5

5.983 4.798  101 1.229  102

203.4 215.7 232.4

chemically controlled when the gasification temperature is not very high. As the temperature increases to a high enough level, gasification reactions will turn to the diffusion-controlled regime. Therefore, the gasification reaction turned to the diffusioncontrolled regime at 950
1000 °C during gasification of biomass in this study. Frequency factors, A0, and activation energy, Ea, were calculated and listed in Table 3. It can be found that A0 and Ea of RS char and PS char present increasing trends with the increasing pyrolysis temperature, while A0 and Ea of CL char show opposite trends. Although RPM can fit well on gasification rates of biomass char in most conditions, in several conditions, RPM cannot obtain good results. Typical conditions are shown in the dotted lines of Figure 7. In Figure 7a, high gasification rates can be kept in the high conversion range. In this condition, satisfactory results cannot also be obtained by RPM, which is suitable for the conditions where peak values appear at conversion ranges lower than 0.393. To this condition, a modified random pore model (M-RPM) developed by Zhang and co-workers was chosen to fit the gasification rates. The M-RPM is a semiempirical model based on RPM by introducing a conversion term to RPM.28 It can be written as r = dX/dt = K(1
X)(1
ψ ln(1
X))1/2(1 þ (cX)p), where c is a dimensionless constant and p is a dimensionless power law constant. Fitting results of M-RPM are shown in the dotted line of Figure 7a and Table 4. It can be found that better results can be obtained by M-RPM than RPM. In panels b and c of Figure 7, where gasification rates increase in the low conversion range but decrease sharply and present trailing phenomenon, RPM cannot perform well. This phenomenon seems just opposite to the condition of Figure 7a; therefore, the gasification rates of RS char at 1200 °C and CL char at 800 °C might be fitted by changing the conversion term (1 þ (cX)p) of M-RPM from multiplication to division. The shifted M-RPM can be written as r = dX/dt = K(1
X)(1
ψ ln(1
X))1/2/(1 þ (cX)p). Fitting results are shown in the solid lines of panels b and c of Figure 7 and Table 4, and obviously, the shifted M-RPM also performs much better than RPM. With regard to the condition of Figure 7a (PS char), some researchers consider that the increase of the specific area caused by pore collapse and development during gasification is the reason.30,31 Some researchers consider that the increase of active sites caused by alkalis is the reason.32 Wigman and co-workers believe that alkalis might be covered by some inert products during char preparation and, with the increasing conversion of char during gasification, this part of alkalis could be released and leads to the high gasification rates in high conversion ranges.33 It can be found from Figure 4 that serious melting happened to PS char, solid particles with poor porosity were produced, and pore collapse should not be happened during gasification. Therefore, the mechanism proposed by Wigman and co-workers should be more likely in this study. During rapid pyrolysis of PS, particle melting makes some of the alkalis be wrapped and, with the 2319

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Table 4. Fitting Parameters of M-RPM and Shifted M-RPM gasification r0 temperature (°C) (10
3 s
1) PS char at 1000 °C RS char at 1200 °C CL char at 800 °C

1050 950 850

1.68 1.16 1.36

ψ

c

p

R2

50.01 1.08 3.48 0.997 12.60 1.29 2.04 0.996 8.60 1.15 2.02 0.996

4. CONCLUSION (1) Under the morphologic structure reserved condition, gasification rates are obviously affected by surface characteristics and porosity of char. Melting and accumulation happen during rapid pyrolysis of PS, leading to the destruction of pore structures, low specific surface area, and gasification rates of PS char. Initial structures of RS and CL can be reserved during rapid pyrolysis, just some slight destruction happens at the hightemperature conditions. The higher porosity and specific surface area of CL char make its gasification rates higher than those of RS char. (2) RPM performs well in most conditions. However, for gasification of PS char at high temperatures, where high gasification rates can remain for high conversion ranges, M-RPM performs better. For gasification of RS char and CL char at low temperatures, where gasification rates present a sharp decrease and trailing in the medium
high conversion range, the shifted M-RPM performs better. ’ AUTHOR INFORMATION Corresponding Author

*Telephone/Fax: þ86-21-64250734. E-mail: [email protected].

’ ACKNOWLEDGMENT This study was financially supported by the National Basic Research Program of China (2010CB227000), the Shanghai “Technology Innovation Action Plan”, and the Fundamental Research Funds for the Central Universities. ’ REFERENCES

Figure 7. Fitting result comparisons of M-RPM, shifted M-RPM, and RPM on gasification rates of biomass char: (a) PS char at 1000 °C, (b) RS char at 1200 °C, and (c) CL char at 800 °C.

conversion of char, alkalis can be released gradually. It is the reason that, although gasification rates of PS char are low, relatively high gasification rates can remain for high conversion ranges. In the study by Zhang and co-workers, where as much as 14 biomass chars were gasified, gasification rates of all of the wooden biomass chars present a similar feature as Figure 7a (PS char), but this feature has not been found from gasification rates of the herbaceous biomass char.28 The easily happened melting on wooden biomass char (Figure 4) might be the reason. Certin and co-workers also found that melting tends to happen on the wooden biomass char other than the herbaceous biomass char,17,18 but further research should be made to understand what is the reason for this phenomenon.

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