Influences of the Heat-Treatment Temperature and Inorganic Matter

ARTICLE pubs.acs.org/EF

Influences of the Heat-Treatment Temperature and Inorganic Matter on Combustion Characteristics of Cornstalk Biochars Mingzi Xu and Changdong Sheng* School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: The present work was addressed to understand the contributions of the heat-treatment temperature and inorganic catalysis to the combustion characteristics of biochar. A cornstalk sample was leached by water and HCl to partially and fully remove active inorganic elements, respectively. The raw and leached samples were heat-treated at the temperatures of 200
900 °C. The resulting biochars were subjected to X-ray diffractometry and Raman spectroscopy analyses for structural characterization and thermogravimetric analysis for combustion characteristics, particularly, for char reactivity. It was found that the heat-treatment temperature has a significant effect on the evolution of the biochar structure, while inorganic matter has little influence. For lowtemperature (200
400 °C) biochars, the heat-treatment temperature has a considerable influence on the devolatilizaiton and a marginal effect on char combustion behaviors, while the inorganic species, particularly, water-soluble species, have considerable influence on the behaviors of both stages. For high-temperature (500
900 °C) biochars, the heat-treatment temperature and inorganic matter are both important to char reactivity. The high contents of inorganic species of cornstalk resulted in the catalytic effect of inorganic species dominating over the influence of the carbonaceous structure ordering on char reactivity. With active inorganic species removed by leaching, char reactivity of biochars turned to decreasing monotonically with an increase of the treatment temperature.

1. INTRODUCTION Biomass has advantages of being CO2-neutral, having low sulfur and nitrogen contents and high volatile content, etc. and is regarded as a sustainable energy resource, promising to substitute fossil fuels for power generation. However, biomass has low bulk energy density and is highly moist and non-friable. These can cause undesired problems associated with the direct use of biomass as feedstock for energy conversion systems. To overcome these disadvantages and the associated problems, many technologies have been made for pretreating and upgrading biomass materials,1 among them heat treatment in the absence of oxygen is an effective approach. Heat treatment can be carried out under various conditions, aimed at different products and applications. The treatment conducted at 200
300 °C is commonly called torrefaction,2 while that at higher temperatures is pyrolysis. During torrefaction, biomass undergoes actually slow and mild pyrolysis and is partially decomposed to give a solid product, which has the quality superior to raw biomass for direct combustion or co-firing in power plants. Pyrolysis generally results in more severe decomposition to yield gas, oil, and char in varying proportions depending upon the temperature and heating rate.3 As a result, it is virtually a versatile technology in terms of upgrading biomass to high-quality fuels or value-added products.4 Pyrolysis at a slow heating rate can be used for carbonization or charcoal production,5 while fast pylolysis at lower temperatures may be aimed at producing bio-oil6 and bioslurry7 and at higher temperatures mainly for producing fuel gas. Regardless of the applications, the solid product or byproduct of pyrolysis, biochar, can be used as fuel because of its high energy content and good quality.7
10 For example, the residual biochar from fast pyrolysis is often used as supplemental fuel to provide thermal energy for the pyrolysis process.4 r 2011 American Chemical Society

When biochar is used as fuel for combustion, its combustion characteristics are essential data for the design and operation of combustors. The combustion characteristics are affected by the physicochemical structure of biochar.3,11 Moreover, biomass materials generally contain a considerable amount of inorganic matter, particularly, alkali element K and alkaline earth elements Ca and Mg. During pyrolysis, the inorganic matter is concentrated and undergoes complex transformations in biochar,12,13 while active species, such as K-containing species, may be released into the gas phase.13
16 Therefore, the amount, composition, and behaviors of inorganic matter also have significant effects on biochar combustion characteristics. Both the physicochemical structure and inorganic matter properties of biochar are dependent upon parent biomass and, particularly, the heattreatment conditions. The heat-treatment or pyrolysis conditions crucial for the properties of resulting biochar include temperature, heating rate, residence time, etc.3,5 Of these parameters, the temperature is generally the key determinant.3,5 It determines the evolution of the biochar morphological and crystalline structure and also the transformations of inorganic matter. Extensive works have been performed to investigate the impacts of the heat-treatment temperature (HTT) on the structure of biochar,17
22 the catalytic effect of inorganic elements,23
28 and consequently, the combustion characteristics and reactivity of biochar.29
33 However, because both the biochar structure and the inorganic Special Issue: 2011 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: August 1, 2011 Revised: October 1, 2011 Published: October 03, 2011 209

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Table 1. Properties of Raw and Leached Cornstalks raw

water-leached

HCl-leached

cornstalk

cornstalk

cornstalk

Table 2. Contents of Inorganic Elements in the Samples (mg/kg of Dry Matter) and the Leaching of the Elements (Percentage of the Total Amount on Dry Matter)

proximate analysis (%, ad)a moisture ash

9.02

7.59

6.51

7.00

2.77

1.72

67.57

79.57

81.60

C

46.82

45.29

45.23

H N

5.13 1.04

5.23 0.41

5.19 0.41

volatile matter

raw cornstalk

major elements

ultimate analysis (%, ad)

mass loss after leaching (%, dry

23

27

matter) a

ad denotes on an air-dried basis.

HCl-leached

cornstalk

cornstalk

content

content

leaching

content

leaching

(mg/kg)

(mg/kg)

(%)

(mg/kg)

(%)

Si

6273

6896

15.6

6662

22.7

Al

131

142

17.1

105

41.8

Fe Ca

143 1796

138 918

25.9 60.7

78 11

60.1 99.6

Mg

1066

271

80.5

18

98.8

Na

45

14

75.3

15

75.3

20277

513

98.1

24

99.9

K Ti

behaviors evolve with heat treatment and their influences on biochar combustion are coupled, their contributions to biochar combustion characteristics are not yet fully understood. The present work was addressed to understand the contributions of structure evolution and inorganic catalysis to the combustion characteristics of biochar, particularly, to the reactivity of char combustion. Cornstalk was used as raw material, which was subjected to water leaching for partial removal and HCl leaching for full removal of active inorganic elements. Considering the wide range of treatment temperatures in the applications of pyrolysis, the raw and leached samples were treated at the temperatures of 200
900 °C. The resulting biochars were subjected to structural characterization and combustion characteristic analysis to systematically investigate the influences of HTT and inorganic matter on combustion characteristics of biochar.

water-leached

P

9

11

11.5

9

32.6

777

119

88.2

93

91.3

the furnace tube. After the sample was purged in N2 at room temperature for 40 min, it was heated at a heating rate of 10 °C/min to a preset temperature, held for 1 h, and then cooled to room temperature in N2. Through weighing the sample before and after the treatment and simultaneously measuring the moisture content, the yield of biochar under the treatment was also determined. All three samples were pyrolyzed under various HTTs from 200 to 900 °C. 2.3. Biochar Structure Characterization. To investigate the impact of HTT on the structural evolution, the obtained biochars were subjected to the analysis of X-ray diffractometry (XRD) for crystalline structure and Raman spectroscopy for surface carbon structure. The biochars as well as the untreated samples were analyzed using a Bruker D8 ADVANCE powder diffractometer with the characteristic Cu Kα radiation and operating conditions of 35 kV and 40 mA. The XRD patterns were used to represent the structure evolution of organic materials, i.e., cellulose crystalline and carbonaceous microcrystallites. Additionally, the identification of inorganic crystalline phases was performed with the JADE 5.0 software package (MDI, Livermore, CA) and diffraction database of PDF2-2004 to study the transformations of main inorganic species. Raman spectroscopic analysis was performed with a Jobin Yvon Labram HR800 spectrometer using a 514.5 nm exciting line of a Spectra Physics Ar laser. The laser power at the biochar particle surface was controlled at ∼1 mW. The laser spot diameter reaching the sample was ∼1 μm. Therefore, the Raman microprobe actually acquired the average information of a large number of randomly distributed microcrystallites. Nevertheless, considering the heterogeneous nature of biochar particles, five to eight particles were randomly chosen and analyzed for each sample. The spectra were recorded in the wavenumber range of 800
2000 cm
1. Because of a strong fluorescence influence on the analysis of low-temperature biochars, only the biochars generated at high HTTs (500
900 °C) were analyzed. The spectra were curve-fitted with Origin7.5/Peak Fitting Module. Each spectrum was resolved into three Lorentzian bands (G band at ∼1590 cm
1, D band at ∼1350 cm
1, and D2 band at ∼1200 cm
1) and one Gaussian band (D3 band at ∼1500 cm
1), following Zaida et al.34 for cellulose chars. The band area ratios of the D band to the G band (ID/IG) and the G band to the total integrated area (IG/Iall) were calculated. ID/IG was used as a representative to reflect the evolution of the char microstructure, especially the defects and imperfect structure, while IG/Iall was used mainly to reflect the variation of the G band intensity. 2.4. Combustion Characteristic Analysis. Combustion characteristics were analyzed by a Setaram SETSYS thermogravimetric

2. EXPERIMENTAL SECTION 2.1. Sample and Its Leaching. An agricultural residue, cornstalk, was used as raw material, which was ground and sieved to a size of 800 °C biochars, the variations of ID/IG and IG/Iall ratios imply the aging and ordering of the carbonaceous structure with an increase of the HTT. Figure 6a shows RC biochars having slight higher average ID/IG ratios than WLC and HLC biochars. This might imply a slightly more aged biochar formed from RC, as also indicated by the XRD analysis. However, it cannot be confirmed because the difference is within the standard deviations. 3.4. Combustion Profiles of the Biochars. Combustion profiles of the biochars generated from RC, WLC, and HLC under various HTTs are shown in Figure 7. It can be found that the combustion behaviors of the biochars generated from the three samples showed some common trends of varying with the HTT. For 200, 250, and 300 °C biochars, their combustion profiles all presented obviously the devolatilization and char 213

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Figure 7. Combustion profiles of the biochars of raw and water- and HCl-leached cornstalks. The values in the legends indicate the HTTs.

the elements of Ca, Mg, and K left after water leaching having no catalytic effect on devolatilization. In contrast, the behaviors at the char combustion stage were considerably different for the biochars generated at all temperatures, representing as the char combustion stage of HLC biochars occurring at higher temperatures. Considering no significant differences in biochar chemical structure observed by the XRD and Raman analyses, it means that the elements of Ca, Mg, and K left after water leaching did have a catalytic effect on char combustion. When the DTG curves of the biochars generated from RC and the leached samples are compared at the same temperature, Figure 7 shows that the behaviors at both the devolatilization and char combustion stages are significantly different. For the

combustion stages. It is common characteristics observed for torrefied biomass materials.48,49 For 350 and 400 °C biochars, the profiles also presented the two-stage feature, although the devolatilization peaks were not apparent because most the volatile matter had already released during the treatment. For high-temperature (500
900 °C) biochars, only the char combustion stage was observed, implying the complete decomposition during the treatment. When comparing the combustion profiles of WLC and HLC biochars produced at the same temperature, panels b and c of Figure 7 show that the behaviors at the devolatilization stage are very similar for each pair of the low-temperature (200
400 °C) biochars, implying no structural difference of the biochars and 214

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As described above, the biochars generated at the temperatures of 200
400 °C underwent devolatilization and char combustion stages during the combustion process in TGA. The ignition is due to the release and combustion of volatile matter left in the biochar after the treatment. Certainly, higher HTT means severer decomposition during the treatment and less volatile matter left in the biochars, resulting in the increase of the biochar ignition temperature. This explained the variation of the ignition temperatures of the low-temperature biochars with the HTT. As for the difference in the ignition temperatures between the biochars of raw and leached samples, it is believed to be caused by the catalysis of inorganic species rather than the structure difference because the XRD analysis (Figure 4) and the biochar yield measurement (Figure 3) suggested that biochars generated at the same temperature have a similar structure. The data in Figure 8 indicate that K-containing species, mainly KCl, had a considerable catalytic effect on devolatilization and, consequently, ignition of low-temperature biochars, while Ca, Mg, and K left after water leaching had a marginal catalytic role. For the biochars generated at high temperatures (500
900 °C), the decomposition of biomass materials was nearly completed during the treatment. As a result, burning the biochars in the TGA only had the char combustion stage (Figure 7). In this case, the ignition depends upon the combustion reactivity of the biochar. It is well-known that the combustion reactivity is generally dependent upon the char structure and inorganic matter catalysis. The XRD and Raman analyses illustrated that the biochars treated at the same temperature have the same structure, which turned ordered and, consequently, less reactive with an increase of the treatment severity. It means that, if only the structure were dominant, the ignition temperatures of the biochars from RC, WLC, and HLC should be the same and increase with the HTT. However, the nearly constant ignition temperature of RC biochars and the different ignition temperatures between the three types of biochars imply that the catalysis played a significant role in combustion reactivity. Figure 9 shows the PRTs of the char combustion stage varying with the HTT. It can be seen that, for the low-temperature biochars, the PRTs are nearly constant for each type of biochar. By comparison, the PRTs of HLC biochars were considerably higher, while the PRTs of RC biochars were slightly lower than those of WLC biochars. The constant PRT for the biochars of each sample is expected because, as shown in Figure 9, the PRTs of these biochars are all higher than the corresponding HTTs. It means that thermal annealing occurred simultaneously along with char oxidation. In this case, char combustion is most likely to depend upon the reaction temperature rather than the HTT. The differences in the PRT between three types of biochars reflect the catalytic effect of inorganic species on char reactivity. A high inorganic content of RC biochars means high reactivity and low PRT of char combustion, while the removal of all of the active inorganic elements determines HLC biochars having the lowest reactivity. Slightly higher PRTs of WLC biochars than those of RC biochars indicates that Ca, Mg, and K left after water leaching also had a considerable catalytic effect on char combustion reactivity of low-temperature biochars. For the high-temperature biochars, although it is still held that the PRTs of HLC biochars > WLC biochars > RC biochars, the trends of the PRTs varying with the HTT changed significantly. Figure 9 shows that the PRTs of WLC and HLC biochars gradually increase with an increase of the HTT from 500 to 900 °C, while that of RC biochars first decreases for 600 °C

Figure 8. Ignition temperatures of biochars vary with the HTT.

Figure 9. PRTs of the char combustion stage vary with the HTT. The “(2nd)” denotes the second peak of the char combustion stage.

low-temperature biochars, those from RC always decomposed at lower temperatures, implying a significant catalytic effect of inorganic species on devolatilization. At the char combustion stage, all RC biochars burned at much lower temperatures, indicating that inorganic species also had a strong catalytic effect on char reactivity. 3.5. Combustion Characteristics of the Biochars. To further investigate the influence of HTT on biochar combustion characteristics, two characteristic parameters, the ignition temperature and the peak rate temperature (PRT) of the char combustion stage, were derived from the DTG curves. The obtained parameters are plotted against the HTT in Figures 8 and 9, in which the biochars of RC, WLC, and HLC are compared to illustrate the influence of inorganic matter on ignition and char combustion. The ignition temperature is a crucial parameter for characterizing combustion behaviors and for minimizing fire and explosion when using biochar as fuel. Figure 8 shows that, for RC biochars, the ignition temperature increased gradually with an increase of the HTT from 200 to 400 °C and then kept nearly constant at ∼445 °C with a further increase of the HTT. For WLC and HLC biochars, the ignition temperatures first increased slightly with an increase of the HTT from 200 to 250 °C, then increased greatly from 250 to 400 °C, and increased linearly with a further increase of the HTT. When the same temperature biochars were compared, RC biochars had the lowest ignition temperatures; the low-temperature biochars of WLC and HLC had similar ignition temperatures, while the high-temperature biochars from HLC ignited at higher temperatures than those from WLC (Figure 8). 215

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biochar and then is nearly kept constant. Figure 9 shows that the PRTs of all biochars are below the corresponding HTTs, implying no further thermal annealing during the main process of char combustion. For HLC biochars, because of no active inorganic species, char reactivity only depends upon the biochar structure. Therefore, its PRT increasing with the HTT is expected. For WLC biochars, although the inorganic species left still have a catalytic role, the PRT increasing with the HTT implies that the structure is the dominant factor, while the catalytic effect only lowers the PRTs. For RC biochars, the same structural evolution is also believed to have an impact on char combustion reactivity. However, the variation of the PRT demonstrates that the catalytic effect played the predominant role. Figure 7a shows that 400
700 °C RC biochars present two apparent peaks of char oxidation. Even though the second peak was considered, it did not change the trend for the hightemperature biochars (Figure 9). Two char combustion peaks were also observed by Munir et al.50 and Darvell et al.51 in the TGA analyses some biomass chars. They attributed the phenomenon to two types of char reacting, but it supposed to result from two types of chars formed from parent biomass materials with a different nature (i.e., soft fleshy and shell). However, the results in Figure 7 suggest that the occurrence of two peaks may not be caused by different materials in RC, otherwise the structures of their chars should evolve with the heat-treatment severity, which should have been observed in structure characterization, such as Raman analysis. Figures 7a and 9 indicate that the temperatures of the two peaks almost did not change with the HTT. It implies no evolution of the reactivities of the two types of char if the two types of char existed. This could not be explained by the structure evolution. Therefore, most likely, the two peaks were caused by different reactivities because of two types of catalytic effects of the inorganic species. The elemental analysis indicated that RC has a high content of alkali element K and considerable contents of alkaline earth elements

Ca and Mg. They may have different performances of catalyzing char oxidation.24,25,27,28 Moreover, catalytic behaviors of both K and Ca change at the temperatures of 500
700 °C25,27,28 because of their transformation behaviors. XRD analysis (Figure 4) showed the transformations of K-containing species with the HTT, as well as the presence of multi-mineral species. Nevertheless, from the current study, it is not possible to clarify the roles of these mechanisms and the cause of the two peaks. Considering the complexity and importance of the catalytic effect on char combustion, it is worthy of further investigation. 3.6. Combustion Reactivity and Kinetics of High-Temperature (500
900 °C) Biochars. Because char combustion is the key stage of biochar conversion, it is essential to obtain char reactivity and kinetics for describing biochar combustion. For biochars generated at the temperatures lower than 400 °C, further devolatilizaton during the combustion process complicates the determination of their reactivity and kinetics.52 Moreover, the char combustion stage is very likely to take place at the temperature higher than their HTTs. As a result, simultaneous thermal annealing as well as further devolatilization can cause the evolution of the carbonaceous structure and, consequently, char reactivity, implying that it is impossible to obtain real intrinsic kinetics of the low-temperature biochars by non-isothermal TGA analysis. Therefore, no attempt was made to derive the kinetics for lower temperature biochars. Only the biochars generated at the temperatures higher than 400 °C were considered to determine char combustion reactivity and kinetics. The combustion reactivity indexes of RC, WLC, and HLC biochars generated at 500
900 °C are plotted as a function of the HTT in Figure 10. The general trends can be seen that the reactivity indexes of WLC and HLC biochars increased linearly with an increase of the HTT, implying the decrease of the global combustion reactivity with treatment severity. In contrast, the index of RC biochars increased slightly with an increase of the HTT from 500 to 800 °C and then decreased for 900 °C biochar. The reactivity index represented the similar behaviors to the PRT for char reactivity. However, the index uses the temperature at 20% conversion, which can avoid the difficulty of determining the PRT when having double peaks and also make the comparison of reactivity between different biochars easier. Additionally, 20% char conversion is reached generally at the temperature much lower than the PRT, which can also avoid the impact of possible thermal annealing on char reactivity determination and comparison. Therefore, it is commonly used to represent biochar reactivity.11,29 The variation of the reactivity indexes shown in Figure 10 reflects the influences of HTT and inorganic matter catalysis on char reactivity. Increasing the HTT did decrease char reactivity, shown by the leaching biochars, while the strong catalytic role of inorganic matter in RC biochars resulted in very high char

Figure 10. Variations of char reactivity indexes of biochars with the HTT.

Table 3. Kinetic Parameters of Char Combustion of High-Temperature Biochars RC biochar

WLC biochar
1

E (kJ/mol)

k0 (min )

E (kJ/mol)

k0 (min
1)

113.4

2.70  107

106.5

2.81  106

119.2

7

4.54  10

139.0

3.38  108

6.85  10 /2.49  10 6.58  106

111.7 129.6

6

6.02  10 7.48  107

137.5 154.2

1.36  106 7.38  106

6.03  105

148.0

8.16  108

169.1

4.84  109

treatment temperature (°C)

E (kJ/mol)

k0 (min )

500

71.4/135.6

3.97  104/2.36  109

103.6/130.6

2.21  10 /1.71  10

700 800

98.1/105.2 97.9

900

82.7

600

HLC biochar

7 6

9 7

216


1

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combustion reactivity nearly unchanged with treatment severity, which is very much beneficial to biochar use as a fuel. The derived kinetic parameters of char combustion of hightemperature biochars are summarized in Table 3, in which the parameters for 500
700 °C RC biochars were fitted for two peaks and the data were presented in the order of low- and hightemperature peaks. It can be seen that the activation energies of WLC and HLC biochars increase with the increase of the HTT, reflecting more ordered biochar having lower reactivity to oxidation and higher activation energy. The activation energies of RC biochar generally decrease with the HTT, particularly, for the high-temperature peak, implying the strong catalytic effect and transformation of inorganic species on the reactivity. In general, the activation energies of the same temperature biochars are in the order of HLC biochar > WLC biochar > RC biochar. This also represents the catalytic effect on biochar combustion reactivity.

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4. CONCLUSION In the present work, cornstalk materials were heat-treated at the temperatures of 200
900 °C to generate biochar samples for investigating the evolutions of the biochar structure and combustion characteristics with the treatment temperature. Water and HCl leaching was applied to partially and fully remove active inorganic species in raw material, enabling the understanding of the importance of inorganic matter to the combustion of biochars. It was found that heat treatment has a significant effect on the evolution of the biochar structure, while the inorganic matter has little influence. For the low-temperature (200
400 °C) biochars, heat treatment has a considerable influence on biochar devolatilization behaviors and a marginal effect on char combustion behaviors, while the inorganic species, particularly, watersoluble species, have a considerable influence on both the devolatilization and char combustion. For the high-temperature (500
900 °C) biochars, both the HTT and inorganic matter have significant effects on char reactivity. Because of the high contents of inorganic species, the catalytic effect of inorganic species dominates over the influence of the carbonaceous structure aging on char reactivity, leading to high char oxidation reactivity even for severely treated biochars. With the active inorganic species, both water-soluble and acid-soluble, removed by leaching, the char reactivity of biochars decreases monotonically with an increase of the treatment temperature, reflecting the fact that the structure evolution does play an important role in char reactivity. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: +86-25-83790317. E-mail: [email protected].

’ REFERENCES (1) Maciejewska, A.; Veringa, H.; Sanders, J.; Peteves, S. D. Co-firing of Biomass with Coal: Constrains and Roles of Biomass Pre-treatment; Institute for Energy, Directorate General Joint Research Centre (DG JRC), European Commission: Petten, The Netherlands, 2006; EUR 22461 EN. (2) Bergman, P. C. A.; Boersma, A. R.; Zwart, R. W. H.; Kiel, J. H. A. Development of Torrefaction for Biomass Co-firing in Existing Coal-Fired Power Station; Energy Research Center of the Netherlands (ECN): The Netherlands, 2005; ECN-C-05-013. (3) Di Blasi, C. Prog. Energy Combust. Sci. 2009, 35, 121–140. (4) Bridgwater, A. V. Chem. Eng. J. 2003, 91, 87–102. 217

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Energy & Fuels

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