This article was downloaded by: [134.117.10.200] On: 31 December 2014, At: 08:19 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
International Journal of Phytoremediation Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bijp20
Effect of Pyrolysis Temperature on Chemical and Surface Properties of Biochar of Rapeseed (Brassica napus L.) a
Dilek Angın & Sevgi Şensöz
b
a
Department of Food Engineering, Faculty of Engineering , Sakarya University , Esentepe Campus , Sakarya , Turkey b
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Department of Chemical Engineering, Faculty of Engineering and Architecture , Eskisehir Osmangazi University , Meselik-Eskisehir , Turkey Accepted author version posted online: 29 Oct 2013.Published online: 06 Jan 2014.
To cite this article: Dilek Angın & Sevgi Şensöz (2014) Effect of Pyrolysis Temperature on Chemical and Surface Properties of Biochar of Rapeseed (Brassica napus L.), International Journal of Phytoremediation, 16:7-8, 684-693, DOI: 10.1080/15226514.2013.856842 To link to this article: http://dx.doi.org/10.1080/15226514.2013.856842
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International Journal of Phytoremediation, 16:684–693, 2014 C Taylor & Francis Group, LLC Copyright ISSN: 1522-6514 print / 1549-7879 online DOI: 10.1080/15226514.2013.856842
EFFECT OF PYROLYSIS TEMPERATURE ON CHEMICAL AND SURFACE PROPERTIES OF BIOCHAR OF RAPESEED (BRASSICA NAPUS L.) Dilek Angın1 and Sevgi S¸ens¨oz2
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1
Department of Food Engineering, Faculty of Engineering, Sakarya University, Esentepe Campus, Sakarya, Turkey 2 Department of Chemical Engineering, Faculty of Engineering and Architecture, Eskisehir Osmangazi University, Meselik-Eskisehir, Turkey The biochar is an important carbon-rich product that is generated from biomass sources through pyrolysis. Biochar (charcoal) can be both used directly as a potential source of solid biofuels and as soil amendments for barren lands. The aim of this study was investigate influence of pyrolysis temperature on the physicochemical properties and structure of biochar. The biochars were produced by pyrolysis of rapeseed (Brassica napus L.) using a fixed-bed reactor at different pyrolysis temperatures (400–700◦ C). The produced biochars were characterized by proximate and elemental analysis, Brunauer–Emmett–Teller (BET) surface area, particle size distributions, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy. The results showed that both chemical and surface properties of the biochars were significantly affected by the pyrolysis temperature. Aromatic hydrocarbons, hydroxyl and carbonyl compounds were the majority components of the biochar. The biochar obtained at 700◦ C had a high fixed carbon content (66.16%) as well as a high heating value, and therefore it could be used as solid fuel, precursor in the activated carbons manufacture (specific surface area until 25.38 m2 g−1), or to obtain category-A briquettes. KEY WORDS: rapeseed, pyrolysis, biochar, characterization
INTRODUCTION Biomass is becoming the most popular raw material among new renewable energy sources day after day. Biomass sources including wood and wood wastes, energy crops, aquatic plants, agricultural crops and their waste by-products, and municipal and animal wastes can be considered as potential sources of fuels and chemical feed stocks (Shuping et al. 2010). Biomass can be treated in numerous ways to produce gases, liquids or solids, but one of the technologies that has the best industrial perspectives is pyrolysis (thermal decomposition in absence of oxygen or with a low concentration of oxygen without affecting the process to a large extent) since the process conditions can be optimized to maximize the yields of liquid, solid (char or biochar) or gases products. The pyrolysis process has
Address correspondence to D. Angin, Department of Food Engineering, Sakarya University, Esentepe Campus, 54187 Sakarya, Turkey. E-mail: [email protected] 684
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been practiced for centuries for production of biochar from biomass and requires relatively slow reaction at very low temperatures to maximize biochar yield (Amutio et al. 2012). Biochar derived from biomass is defined as a carbonaceous residue from pyrolysis, including natural fires under limited oxygen. The formation of biochar from biomass is complex and remains unclear. Biochar has recently received much attention, even though its structure is still uncertain (Kim et al. 2012). Biochar can be used directly as a replacement for pulverized coal, but its application to soil exhibits much more value (Zheng et al. 2010). The application of biochar to the soil can improve its fertility and crop production, with the positive effect of mitigating the rising concentration of atmospheric carbon dioxide (Amutio et al. 2012; Lehmann et al. 2006). Nowadays, biochar are recognized as an environmental-friendly adsorbent to abate organic pollutants (Chen and Chen 2009). Also, the biochar can be used in the preparation of activated carbon when its pore structure and surface area are appropriate (Yaman 2004). Evaluating the value of biochar in an agricultural context is now the subject of considerable research interest, as it has been shown to improve the productivity of soil and can enhance crop yields (Hossain et al. 2011). Properties of biochar are decisively affected, not only by properties of precursor material, but also by pyrolysis operating conditions used, mainly the heating rate, temperature and residence time at this temperature (Angın 2013). Also, the applications and functions of biochar are highly depending on its physicochemical properties such as volatile matter, fixed carbon, ash content, elemental composition, higher heating value (HHV) and surface area. Because biochar can be produced of various biomass sources under different processing conditions, it is therefore very important to characterize their physicochemical properties before use (Yao et al. 2011). Several papers have been published about the characterization of biochars from different biomass. Recent studies have highlighted the benefits of biochar technologies. Particularly with respect to carbon sequestration and soil management of biochar (Hossain et al. 2011; S´anchez et al. 2009; Tsui and Roy 2008; Galinato et al. 2011; McHenry 2009; Zheng et al. 2010). Some kinds of biomass have special importance with respect to be energy crop. Rapeseed is one of the most valuable energy crops. Although, it can be used in the production of vegetable oil, the most important usage of rapeseed is the bio-oil or bio-fuel that can be produced from its pyrolysis. There is a lot of study on the characterization of bio-oil obtained from pyrolysis of rapeseed (Brassica napus L.) in the literature (Haykırı-Ac¸ma et al. 2006; Ucar and Ozkan 2008; S´anchez et al. 2009; S¸ens¨oz et al. 2000a,b), but studies on characterization of biochar are very limited (Ucar and Ozkan 2008; S´anchez et al. 2009). The purpose of the present study was to investigate the effects of the pyrolysis temperatures, on the pore structure, and physicochemical properties of biochar produced from rapeseed (Brassica napus L.). Ultimate and proximate analyses were carried out and calorific values, surface areas and chemical compositions of the biochars were investigated.
MATERIALS AND METHODS Sample Preparation Rapeseed (Brassica napus L.) used in the experiments were obtained from the city of Erzurum located in east Anatolia. Seeds of rape was firstly separated and cleaned from other flora (such as straw stalk and leaf) then stored on dry floor at ambient temperature. Finally,
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Table 1 Proximate and ultimate analysis of rapeseed (S¸ens¨oz et al. 2000a)
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Characteristics Proximate analysis (dry, wt%) Volatile Matter Ash Fixed Carbon∗ Ultimate analysis (wt%) C H N S O∗ H/C O/C Emprical formula Higher Heating Value (MJ kg−1) ∗ By
Rapeseed
86.04 5.73 8.23 52.25 8.06 3.91 n.d.∗∗ 35.78 1.85 0.51 CH1.85 O0.51 N0.06 28.36
difference. ∗∗ Not detection.
rapeseed was ground in a Retsch mill to obtain a grain size lower 2 mm. Rapeseed sample was stored in glass jars at room temperature under dry conditions. The main characteristics of the rapeseed are given in Table 1 (S¸ens¨oz et al. 2000a). Pyrolysis Experiments Pyrolysis experiments were performed with 20 g of biomass samples (rapeseed) in a stainless steel (#316) fixed-bed reactor with a length of 104 mm and an internal diameter of 70 mm. The reactor was heated externally by an electric furnace, with the temperature being controlled by thermocouple inside the bed. The thermocouple was connected to a proportional controlling unit which is capable of maintaining the oven temperature within an accuracy range of ±5◦ C and installed onto a control panel. The biochars in this study were obtained at 400, 450, 500, 550 and 700◦ C pyrolysis temperatures and at heating rate of 7◦ C min−1. Pyrolysis experiment system was described in detail in previous studies (S¸ens¨oz et al. 2000a,b; S¸ens¨oz and Angın 2008). Characterization of Biochars Proximate analyses were determined according to the ASTM 3174 and ASTM 3175. Fixed carbon content was determined by difference. All the results of proximate analyses were expressed as the average of three experiments by ±standard deviations. Ultimate analyses were performed on a CARLO ERBA model EA 1108 Elemental Analyzer with ±0.4% accuracy (Carlo Erba Instruments). Oxygen was determined by difference. Surface areas and pore volumes of the biochars were determined as by the application of the Brunauer–Emmett–Teller (BET) and t-plot analysis software available with Micromeritics Gemini V instrument with ±0.5% accuracy. Chemical functional groups were determined by Fourier transform infrared spectra (FTIR) using SHIMADZU IR Prestige 21. pH values were measured by adding biochar to de-ionized water in a mass ratio of 1:20. The solution was then hand shaken and allowed to stand for 5 min before measuring the pH with a pH meter Hanna Instruments pH 211 with ±0.01 accuracy (Inyang et al. 2010). Higher heating
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Table 2 Properties of rapeseed biochars produced at different pyrolysis temperatures
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Pyrolysis temperature (◦ C)
Proximate analysis (dry, wt%) Volatile Matter Ash Fixed Carbon∗ Ultimate analysis (wt%) C H N O∗ H/C O/C Higher Heating Value (MJ kg−1) pH ∗ By
400
450
500
550
700
25.14 ± 0.63 19.28 ± 0.48 55.58 ± 0.83
16.18 ± 0.57 21.40 ± 0.51 62.42 ± 0.91
13.62 ± 0.38 23.35 ± 0.55 63.03 ± 0.88
10.42 ± 0.22 24.94 ± 0.64 64.64 ± 0.97
7.76 ± 0.21 26.08 ± 0.72 66.16 ± 0.95
57.95 3.43 5.43 33.16 0.71 0.43 28.15 8.33
59.77 2.36 5.12 32.75 0.47 0.41 28.89 8.44
61.98 1.92 4.32 31.78 0.37 0.38 29.22 8.66
67.29 1.75 4.75 26.21 0.31 0.29 30.03 10.70
70.41 1.24 3.24 25.11 0.21 0.27 30.47 10.84
difference.
values (HHV) were determined by GALLENKAMP Auto Adiabatic Bomb Calorimeter according to the ASTM D240. Surface morphologies were visualized by scanning electron microscopy (SEM) using JEOL-JSM-6060LV.
RESULTS AND DISCUSSION Properties of Biochars The effect of pyrolysis temperature on the ultimate and proximate analysis, pH values and higher heating values (HHV) of biochars are given in Table 2. As it is seen from Table 2, the volatile matter content of biochars decreases from 25.14% to 7.76% as the pyrolysis temperature was raised from 400 to 700◦ C. The ash content of the biochars was quite high and ash content increased with increasing pyrolysis temperature. The ash content is a measure of the non-volatile matter and non-combustible component of the biochar. Increased ash content occurred due to the reduction in the contents of other elements like carbon, nitrogen, hydrogen and oxygen. Hence, as pyrolysis proceeded with an increase in temperature, the ash content increased (Maiti et al. 2006). As the pyrolysis temperature increased from 400 to 700◦ C, the fixed carbon content of the biochars increased and fixed carbon content as high as 66.16% was achieved for biochar pyrolysis at 700◦ C in this study. This was to be expected because the increased devolatilization during pyrolysis resulted in biochar to be predominantly carbon. In comparison with raw biomass (rapeseed), a decrease in volatile matter (86.04%) and an increase in fixed carbon (8.23%) content were observed for biochars, as expected. The presence of volatile matter in biochars shows incomplete thermal degradation during pyrolysis. The ash content of biochars is also found higher than the raw biomass due to the mineral matter form ash content in biochar after pyrolysis. Generally, these findings are compatible with the results given in the literature (Chen et al. 2012; Hossain et al. 2011; Maiti et al. 2006; Katyal et al. 2003; Guo and Lua 1998).
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The pH values of the biochars increased with increasing pyrolysis temperature. Measurements of the pH of the biochars were alkaline (8.33–10.84), which are similar to the reported values other biochars produced at high temperature (Hossain et al. 2011; Jindarom et al. 2007; Yao et al. 2011; Maiti et al. 2006). These values indicate the basic characteristic of the biochar since all biochars are positively charge in the solution (Yao et al. 2011). The higher heating values (HHV) of the biochars increased with an increase the pyrolysis temperature. This agrees with the increase in fixed carbon content or biochar quality. Higher heating values indicate the biochar’s potential to be used as fuel. The higher heating value of the biochar obtained at 700◦ C are much higher (30.47 MJ kg−1) than other solid fuels, as soft coal (29 MJ kg−1) and lignite (20 MJ kg−1) (Amutio et al. 2012). The higher heating values of the biochars were similar in comparison with that of other biochars such as those derived from cotton stalk, Miscanthus, eucalyptus, cherry stones (Chen et al. 2012; Melligan et al. 2012; Kim et al. 2013; Gonz´alez et al. 2003). Ultimate Analysis In ultimate analyses (Table 2), with an increase in the pyrolysis temperature from 400 to 700◦ C, the carbon content of the biochars increased from 57.95 to 70.41 wt.%; however, the hydrogen content decreased from 3.43 to 1.24 wt.%, and oxygen content decreased from 33.16 to 25.11 wt.% for heating rate of 7◦ C min−1. This observation indicates that increasing the pyrolysis temperature increases the degree of carbonization of the biochars become progressively more aromatic. The elemental analysis results of biochars also reflect the higher conversion at 700◦ C (Chen et al. 2012; Maiti et al. 2006). As a consequence of the process of pyrolysis, the carbon content of biochars increased with regard to the original biomass (52.25 wt.%), along with a deoxygenation as a result of loss of functional groups during the pyrolysis process. Losses in hydrogen and oxygen correspond to the scission of weaker bonds within the structure of biochars favored by higher temperature. These were also a decrease in the hydrogen content, probably due to the great proportion of hydrogen compounds in the volatile matter (S´anchez et al. 2009). Nitrogen contents of biochars decreased from 5.43 to 3.24 wt.% with increased pyrolysis temperature. Also, sulfur in raw material (rapeseed) was below the detection limits of the instruments. Therefore, this biochar could be used in fuel applications and activated carbon production. This trend in the elemental analysis were found similar in biochar produced from other biomass sample such as wheat grains, cotton stalk, pitch pine (Sanna et al. 2011; Chen et al. 2012; Kim et al. 2012). Figure 1 illustrates the van Krevelan diagram for rapeseed and its biochars obtained at different temperatures. The decreasing H/C following O/C ratios at increasing temperature suggests a growing aromaticity of the biochar (Mui et al. 2010). The O/C ratios of the biochars were the lowest at 700◦ C, while the O/C ratios of biochars the highest at 400◦ C indicating that the biochars produced at high temperature are poorer in oxygen (Fu et al. 2011). FTIR Analysis The FTIR analysis of rapeseed and biochars prepared at different pyrolysis temperatures are presented in Figure 2. The broad band corresponding to vibration of O H C bending vibration at 1644 cm−1 are present only in groups at around 2900 and C
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2 Rapeseed
Atomic H/C ratio
1,5
1 400 °C
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0,5 550 °C 700 °C
450 °C 500 °C
0 0,2
0,3
0,4
0,5
0,6
0,7
0,8
Atomic O/C ratio Figure 1 Van Krevelan diagram for rapeseed and its biochars obtained at different temperature.
raw material (rapeseed). The bands at about 2700 and 3400 cm−1 correspond to aliphatic CH3 asymmetric and symmetric stretching vibration respectively. These bands are absent in samples heated at 400◦ C (450 except) and above (Hossain et al. 2011). The bands in the region of 1250–1800 cm−1 (alkanes and aromatics) completely disappeared with increase in the pyrolysis temperature. Similarly, the intensities at 900–1250 cm−1 (C O C) were eliminated from 400 to 500◦ C and disappeared at 550 and 700◦ C. These peaks are due to the presence of primary, secondary and tetriary alcohols, phenols, ethers and esters showing the C O stretching and O H deformation vibrations (Chen and Chen 2009). The decrease in the intensities of the above described bands showed that the surface concentration of the acidic functional groups has sufficiently decreased (Haider et al. 2011). However, there is an alteration in aromatic structure of biochar samples in comparison with biomass samples. The peaks between 600 and 900 cm−1 corresponding to an aromatic C H stretching vibration that indicates the presence of adjacent aromatic hydrogen in biochar sample is not seen in biomass sample. BET Surface Areas and Pore Volumes The effect of pyrolysis temperature on the BET surface areas and micro- and mesopore volumes of the biochars are shown in Table 3. The devolatilization of biomass materials developed porosity in the biochars, resulting in particles with an essentially micro-macropore structure. BET surface areas were generally low for biochars produced at all pyrolysis temperatures, but did slightly increase while increasing pyrolysis temperature up to 550◦ C (25.38 m2 g−1), and decreased thereafter. Structural ordering and micropore coalescence are found to be responsible for the decrease in the value of the surface area observed at 700◦ C, resulting in a thermal deactivation of the biochar. The same trends are observed in
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Carboxyl and Carbonyl
Alkanes and Aromatics
Alcohols, Phenols, Ethers,
700 °C
Transmittance, (%)
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550 °C
500 °C
450 °C
2187,58
400 °C 1413,82
Aliphatics
1043,49
651,94
Rapeseed
4000
3500
3000
2500
2000
1750
1500
1250
1000
500
-1
Wavenumber, (cm ) Figure 2 FTIR spectra of rapeseed and biochars obtained at different pyrolysis temperatures.
the micropore volume and area data (Guerrero et al. 2005). Micropore volume and area of biochars increased with the pyrolysis temperature (550◦ C) and reached a maximum of 0.0397 cm3 g−1 and 18,31 m2 g−1, respectively. At the temperature of 700◦ C, the surface area and total pore volume were lowered to 22.86 m2 g−1 and 0.0437 cm3 g−1, respectively. In table 3, it can be seen that an increase of the temperature does not improve the textural properties of the biochars. This fact has been observed by other authors (Gonz´ales et al. 2005), and it can be due to a sintering effect, following by shrinkage of the biochar and realignment of the biochar structure which leads to a decrease of the mean-size and volume of the pores. Therefore, the optimal conditions of the pyrolysis process for biochar production would be at temperature until 550◦ C. Due to the high temperature, the porous structure cracked and the pores might be partially blocked as a result of the softening and melting of the rapeseed constituents, which could to a low surface area (Liu et al. 2010). It is observed that the BET surface area is low compared to other biochars obtained other
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Table 3 Effect of pyrolysis temperature on surface areas and pore volumes of rapeseed biochars Pyrolysis temperature (◦ C)
SBET (m2/g)
Smicro (m2/g)
Smeso (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Vmicro (%)
Smicro (%)
400 450 500 550 700
14.73 19.87 20.12 25.38 22.86
9.46 12.77 12.93 18.31 16.19
5.27 7.10 7.19 7.07 6.67
0.0278 0.0376 0.0393 0.0480 0.0437
0.0197 0.0273 0.0291 0.0397 0.0322
0.0081 0.0098 0.0102 0.0083 0.0115
70.86 73.94 74.05 82.71 73.68
64.22 64.27 64.26 72.14 70.82
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biomass materials such as maize stalk, almond shell, pitch pine (Fu et al. 2011; Gonz´ales et al. 2005; Kim et al. 2012). However, the surface area found is quite similar to the one reported by other authors (Amutio et al. 2012; Liu et al. 2010; Hu et al. 2008). Scanning Electron Microscopy Scanning electron microscopy (SEM) has been especially used to evaluate the structural variations in biochar particles after different thermal treatments. SEM images indicated that the surface structures of rapeseed biochars obtained at different pyrolysis temperatures were very similar (Figure S1) and that amorphous and heterogeneous structures are dominated. The irregular shape solids containing pores with different sizes were observed as depicted in Figure S1. When the pyrolysis temperature increases to 550◦ C, an increase in the solid porosity can be found. This results support the increase in the BET surface area as a function of temperature (Jindarom et al. 2007). But, at a higher pyrolysis temperature of 600◦ C, the biochars had lower BET surface areas due to the shrinkage of chars at postsoftening and swelling temperatures, resulting in narrowing or closing pores. The pore structures of the biochars were identified a distortion at high temperature (700◦ C) (Guo and Lua 1998). The vesicles on the surface of biochars (at low temperatures) resulted from the release of volatile gas contained in the soften biomass matrix during the pyrolysis process. The presence of vesicles indicates that volatile components were formed and released. And, the vesicles occurred through a melt phase of cellular components. From a phenomenological point of view, a gradual release of different volatile compounds occurs as the temperature increases during devolatilization (Liu et al. 2010; Guerrero et al. 2005). CONCLUSION In this study, the effect of pyrolysis temperature on the characteristics of biochars prepared from rapeseed was investigated. Pyrolysis studies indicated that it can be achieved a more valuable and functional product (biochar) from rapeseed (Brassica napus L.). As the pyrolysis temperature increased from 400 to 700◦ C, the volatile matter, hydrogen, and oxygen contents of the biochar decreased, but the fixed carbon, ash and carbon content, pH, surface area and higher heating value increased. This was to be expected because increased devolatilization during pyrolysis resulted in the biochar to be predominantly carbon. At pyrolysis temperature of 700◦ C, most of the volatile matter had been removed. As a result, the rapeseed biochars can be effectively used as a raw material for the preparation of
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activated carbon. Biochars can also be assessed in fuel application and water purification processes.
SUPPLEMENTAL MATERIAL Supplemental data for this article can be accessed on the publisher’s website.
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International Journal of Phytoremediation Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bijp20
Effect of Pyrolysis Temperature on Chemical and Surface Properties of Biochar of Rapeseed (Brassica napus L.) a
Dilek Angın & Sevgi Şensöz
b
a
Department of Food Engineering, Faculty of Engineering , Sakarya University , Esentepe Campus , Sakarya , Turkey b
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Department of Chemical Engineering, Faculty of Engineering and Architecture , Eskisehir Osmangazi University , Meselik-Eskisehir , Turkey Accepted author version posted online: 29 Oct 2013.Published online: 06 Jan 2014.
To cite this article: Dilek Angın & Sevgi Şensöz (2014) Effect of Pyrolysis Temperature on Chemical and Surface Properties of Biochar of Rapeseed (Brassica napus L.), International Journal of Phytoremediation, 16:7-8, 684-693, DOI: 10.1080/15226514.2013.856842 To link to this article: http://dx.doi.org/10.1080/15226514.2013.856842
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International Journal of Phytoremediation, 16:684–693, 2014 C Taylor & Francis Group, LLC Copyright ISSN: 1522-6514 print / 1549-7879 online DOI: 10.1080/15226514.2013.856842
EFFECT OF PYROLYSIS TEMPERATURE ON CHEMICAL AND SURFACE PROPERTIES OF BIOCHAR OF RAPESEED (BRASSICA NAPUS L.) Dilek Angın1 and Sevgi S¸ens¨oz2
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1
Department of Food Engineering, Faculty of Engineering, Sakarya University, Esentepe Campus, Sakarya, Turkey 2 Department of Chemical Engineering, Faculty of Engineering and Architecture, Eskisehir Osmangazi University, Meselik-Eskisehir, Turkey The biochar is an important carbon-rich product that is generated from biomass sources through pyrolysis. Biochar (charcoal) can be both used directly as a potential source of solid biofuels and as soil amendments for barren lands. The aim of this study was investigate influence of pyrolysis temperature on the physicochemical properties and structure of biochar. The biochars were produced by pyrolysis of rapeseed (Brassica napus L.) using a fixed-bed reactor at different pyrolysis temperatures (400–700◦ C). The produced biochars were characterized by proximate and elemental analysis, Brunauer–Emmett–Teller (BET) surface area, particle size distributions, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy. The results showed that both chemical and surface properties of the biochars were significantly affected by the pyrolysis temperature. Aromatic hydrocarbons, hydroxyl and carbonyl compounds were the majority components of the biochar. The biochar obtained at 700◦ C had a high fixed carbon content (66.16%) as well as a high heating value, and therefore it could be used as solid fuel, precursor in the activated carbons manufacture (specific surface area until 25.38 m2 g−1), or to obtain category-A briquettes. KEY WORDS: rapeseed, pyrolysis, biochar, characterization
INTRODUCTION Biomass is becoming the most popular raw material among new renewable energy sources day after day. Biomass sources including wood and wood wastes, energy crops, aquatic plants, agricultural crops and their waste by-products, and municipal and animal wastes can be considered as potential sources of fuels and chemical feed stocks (Shuping et al. 2010). Biomass can be treated in numerous ways to produce gases, liquids or solids, but one of the technologies that has the best industrial perspectives is pyrolysis (thermal decomposition in absence of oxygen or with a low concentration of oxygen without affecting the process to a large extent) since the process conditions can be optimized to maximize the yields of liquid, solid (char or biochar) or gases products. The pyrolysis process has
Address correspondence to D. Angin, Department of Food Engineering, Sakarya University, Esentepe Campus, 54187 Sakarya, Turkey. E-mail: [email protected] 684
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been practiced for centuries for production of biochar from biomass and requires relatively slow reaction at very low temperatures to maximize biochar yield (Amutio et al. 2012). Biochar derived from biomass is defined as a carbonaceous residue from pyrolysis, including natural fires under limited oxygen. The formation of biochar from biomass is complex and remains unclear. Biochar has recently received much attention, even though its structure is still uncertain (Kim et al. 2012). Biochar can be used directly as a replacement for pulverized coal, but its application to soil exhibits much more value (Zheng et al. 2010). The application of biochar to the soil can improve its fertility and crop production, with the positive effect of mitigating the rising concentration of atmospheric carbon dioxide (Amutio et al. 2012; Lehmann et al. 2006). Nowadays, biochar are recognized as an environmental-friendly adsorbent to abate organic pollutants (Chen and Chen 2009). Also, the biochar can be used in the preparation of activated carbon when its pore structure and surface area are appropriate (Yaman 2004). Evaluating the value of biochar in an agricultural context is now the subject of considerable research interest, as it has been shown to improve the productivity of soil and can enhance crop yields (Hossain et al. 2011). Properties of biochar are decisively affected, not only by properties of precursor material, but also by pyrolysis operating conditions used, mainly the heating rate, temperature and residence time at this temperature (Angın 2013). Also, the applications and functions of biochar are highly depending on its physicochemical properties such as volatile matter, fixed carbon, ash content, elemental composition, higher heating value (HHV) and surface area. Because biochar can be produced of various biomass sources under different processing conditions, it is therefore very important to characterize their physicochemical properties before use (Yao et al. 2011). Several papers have been published about the characterization of biochars from different biomass. Recent studies have highlighted the benefits of biochar technologies. Particularly with respect to carbon sequestration and soil management of biochar (Hossain et al. 2011; S´anchez et al. 2009; Tsui and Roy 2008; Galinato et al. 2011; McHenry 2009; Zheng et al. 2010). Some kinds of biomass have special importance with respect to be energy crop. Rapeseed is one of the most valuable energy crops. Although, it can be used in the production of vegetable oil, the most important usage of rapeseed is the bio-oil or bio-fuel that can be produced from its pyrolysis. There is a lot of study on the characterization of bio-oil obtained from pyrolysis of rapeseed (Brassica napus L.) in the literature (Haykırı-Ac¸ma et al. 2006; Ucar and Ozkan 2008; S´anchez et al. 2009; S¸ens¨oz et al. 2000a,b), but studies on characterization of biochar are very limited (Ucar and Ozkan 2008; S´anchez et al. 2009). The purpose of the present study was to investigate the effects of the pyrolysis temperatures, on the pore structure, and physicochemical properties of biochar produced from rapeseed (Brassica napus L.). Ultimate and proximate analyses were carried out and calorific values, surface areas and chemical compositions of the biochars were investigated.
MATERIALS AND METHODS Sample Preparation Rapeseed (Brassica napus L.) used in the experiments were obtained from the city of Erzurum located in east Anatolia. Seeds of rape was firstly separated and cleaned from other flora (such as straw stalk and leaf) then stored on dry floor at ambient temperature. Finally,
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Table 1 Proximate and ultimate analysis of rapeseed (S¸ens¨oz et al. 2000a)
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Characteristics Proximate analysis (dry, wt%) Volatile Matter Ash Fixed Carbon∗ Ultimate analysis (wt%) C H N S O∗ H/C O/C Emprical formula Higher Heating Value (MJ kg−1) ∗ By
Rapeseed
86.04 5.73 8.23 52.25 8.06 3.91 n.d.∗∗ 35.78 1.85 0.51 CH1.85 O0.51 N0.06 28.36
difference. ∗∗ Not detection.
rapeseed was ground in a Retsch mill to obtain a grain size lower 2 mm. Rapeseed sample was stored in glass jars at room temperature under dry conditions. The main characteristics of the rapeseed are given in Table 1 (S¸ens¨oz et al. 2000a). Pyrolysis Experiments Pyrolysis experiments were performed with 20 g of biomass samples (rapeseed) in a stainless steel (#316) fixed-bed reactor with a length of 104 mm and an internal diameter of 70 mm. The reactor was heated externally by an electric furnace, with the temperature being controlled by thermocouple inside the bed. The thermocouple was connected to a proportional controlling unit which is capable of maintaining the oven temperature within an accuracy range of ±5◦ C and installed onto a control panel. The biochars in this study were obtained at 400, 450, 500, 550 and 700◦ C pyrolysis temperatures and at heating rate of 7◦ C min−1. Pyrolysis experiment system was described in detail in previous studies (S¸ens¨oz et al. 2000a,b; S¸ens¨oz and Angın 2008). Characterization of Biochars Proximate analyses were determined according to the ASTM 3174 and ASTM 3175. Fixed carbon content was determined by difference. All the results of proximate analyses were expressed as the average of three experiments by ±standard deviations. Ultimate analyses were performed on a CARLO ERBA model EA 1108 Elemental Analyzer with ±0.4% accuracy (Carlo Erba Instruments). Oxygen was determined by difference. Surface areas and pore volumes of the biochars were determined as by the application of the Brunauer–Emmett–Teller (BET) and t-plot analysis software available with Micromeritics Gemini V instrument with ±0.5% accuracy. Chemical functional groups were determined by Fourier transform infrared spectra (FTIR) using SHIMADZU IR Prestige 21. pH values were measured by adding biochar to de-ionized water in a mass ratio of 1:20. The solution was then hand shaken and allowed to stand for 5 min before measuring the pH with a pH meter Hanna Instruments pH 211 with ±0.01 accuracy (Inyang et al. 2010). Higher heating
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Table 2 Properties of rapeseed biochars produced at different pyrolysis temperatures
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Pyrolysis temperature (◦ C)
Proximate analysis (dry, wt%) Volatile Matter Ash Fixed Carbon∗ Ultimate analysis (wt%) C H N O∗ H/C O/C Higher Heating Value (MJ kg−1) pH ∗ By
400
450
500
550
700
25.14 ± 0.63 19.28 ± 0.48 55.58 ± 0.83
16.18 ± 0.57 21.40 ± 0.51 62.42 ± 0.91
13.62 ± 0.38 23.35 ± 0.55 63.03 ± 0.88
10.42 ± 0.22 24.94 ± 0.64 64.64 ± 0.97
7.76 ± 0.21 26.08 ± 0.72 66.16 ± 0.95
57.95 3.43 5.43 33.16 0.71 0.43 28.15 8.33
59.77 2.36 5.12 32.75 0.47 0.41 28.89 8.44
61.98 1.92 4.32 31.78 0.37 0.38 29.22 8.66
67.29 1.75 4.75 26.21 0.31 0.29 30.03 10.70
70.41 1.24 3.24 25.11 0.21 0.27 30.47 10.84
difference.
values (HHV) were determined by GALLENKAMP Auto Adiabatic Bomb Calorimeter according to the ASTM D240. Surface morphologies were visualized by scanning electron microscopy (SEM) using JEOL-JSM-6060LV.
RESULTS AND DISCUSSION Properties of Biochars The effect of pyrolysis temperature on the ultimate and proximate analysis, pH values and higher heating values (HHV) of biochars are given in Table 2. As it is seen from Table 2, the volatile matter content of biochars decreases from 25.14% to 7.76% as the pyrolysis temperature was raised from 400 to 700◦ C. The ash content of the biochars was quite high and ash content increased with increasing pyrolysis temperature. The ash content is a measure of the non-volatile matter and non-combustible component of the biochar. Increased ash content occurred due to the reduction in the contents of other elements like carbon, nitrogen, hydrogen and oxygen. Hence, as pyrolysis proceeded with an increase in temperature, the ash content increased (Maiti et al. 2006). As the pyrolysis temperature increased from 400 to 700◦ C, the fixed carbon content of the biochars increased and fixed carbon content as high as 66.16% was achieved for biochar pyrolysis at 700◦ C in this study. This was to be expected because the increased devolatilization during pyrolysis resulted in biochar to be predominantly carbon. In comparison with raw biomass (rapeseed), a decrease in volatile matter (86.04%) and an increase in fixed carbon (8.23%) content were observed for biochars, as expected. The presence of volatile matter in biochars shows incomplete thermal degradation during pyrolysis. The ash content of biochars is also found higher than the raw biomass due to the mineral matter form ash content in biochar after pyrolysis. Generally, these findings are compatible with the results given in the literature (Chen et al. 2012; Hossain et al. 2011; Maiti et al. 2006; Katyal et al. 2003; Guo and Lua 1998).
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The pH values of the biochars increased with increasing pyrolysis temperature. Measurements of the pH of the biochars were alkaline (8.33–10.84), which are similar to the reported values other biochars produced at high temperature (Hossain et al. 2011; Jindarom et al. 2007; Yao et al. 2011; Maiti et al. 2006). These values indicate the basic characteristic of the biochar since all biochars are positively charge in the solution (Yao et al. 2011). The higher heating values (HHV) of the biochars increased with an increase the pyrolysis temperature. This agrees with the increase in fixed carbon content or biochar quality. Higher heating values indicate the biochar’s potential to be used as fuel. The higher heating value of the biochar obtained at 700◦ C are much higher (30.47 MJ kg−1) than other solid fuels, as soft coal (29 MJ kg−1) and lignite (20 MJ kg−1) (Amutio et al. 2012). The higher heating values of the biochars were similar in comparison with that of other biochars such as those derived from cotton stalk, Miscanthus, eucalyptus, cherry stones (Chen et al. 2012; Melligan et al. 2012; Kim et al. 2013; Gonz´alez et al. 2003). Ultimate Analysis In ultimate analyses (Table 2), with an increase in the pyrolysis temperature from 400 to 700◦ C, the carbon content of the biochars increased from 57.95 to 70.41 wt.%; however, the hydrogen content decreased from 3.43 to 1.24 wt.%, and oxygen content decreased from 33.16 to 25.11 wt.% for heating rate of 7◦ C min−1. This observation indicates that increasing the pyrolysis temperature increases the degree of carbonization of the biochars become progressively more aromatic. The elemental analysis results of biochars also reflect the higher conversion at 700◦ C (Chen et al. 2012; Maiti et al. 2006). As a consequence of the process of pyrolysis, the carbon content of biochars increased with regard to the original biomass (52.25 wt.%), along with a deoxygenation as a result of loss of functional groups during the pyrolysis process. Losses in hydrogen and oxygen correspond to the scission of weaker bonds within the structure of biochars favored by higher temperature. These were also a decrease in the hydrogen content, probably due to the great proportion of hydrogen compounds in the volatile matter (S´anchez et al. 2009). Nitrogen contents of biochars decreased from 5.43 to 3.24 wt.% with increased pyrolysis temperature. Also, sulfur in raw material (rapeseed) was below the detection limits of the instruments. Therefore, this biochar could be used in fuel applications and activated carbon production. This trend in the elemental analysis were found similar in biochar produced from other biomass sample such as wheat grains, cotton stalk, pitch pine (Sanna et al. 2011; Chen et al. 2012; Kim et al. 2012). Figure 1 illustrates the van Krevelan diagram for rapeseed and its biochars obtained at different temperatures. The decreasing H/C following O/C ratios at increasing temperature suggests a growing aromaticity of the biochar (Mui et al. 2010). The O/C ratios of the biochars were the lowest at 700◦ C, while the O/C ratios of biochars the highest at 400◦ C indicating that the biochars produced at high temperature are poorer in oxygen (Fu et al. 2011). FTIR Analysis The FTIR analysis of rapeseed and biochars prepared at different pyrolysis temperatures are presented in Figure 2. The broad band corresponding to vibration of O H C bending vibration at 1644 cm−1 are present only in groups at around 2900 and C
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2 Rapeseed
Atomic H/C ratio
1,5
1 400 °C
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0,5 550 °C 700 °C
450 °C 500 °C
0 0,2
0,3
0,4
0,5
0,6
0,7
0,8
Atomic O/C ratio Figure 1 Van Krevelan diagram for rapeseed and its biochars obtained at different temperature.
raw material (rapeseed). The bands at about 2700 and 3400 cm−1 correspond to aliphatic CH3 asymmetric and symmetric stretching vibration respectively. These bands are absent in samples heated at 400◦ C (450 except) and above (Hossain et al. 2011). The bands in the region of 1250–1800 cm−1 (alkanes and aromatics) completely disappeared with increase in the pyrolysis temperature. Similarly, the intensities at 900–1250 cm−1 (C O C) were eliminated from 400 to 500◦ C and disappeared at 550 and 700◦ C. These peaks are due to the presence of primary, secondary and tetriary alcohols, phenols, ethers and esters showing the C O stretching and O H deformation vibrations (Chen and Chen 2009). The decrease in the intensities of the above described bands showed that the surface concentration of the acidic functional groups has sufficiently decreased (Haider et al. 2011). However, there is an alteration in aromatic structure of biochar samples in comparison with biomass samples. The peaks between 600 and 900 cm−1 corresponding to an aromatic C H stretching vibration that indicates the presence of adjacent aromatic hydrogen in biochar sample is not seen in biomass sample. BET Surface Areas and Pore Volumes The effect of pyrolysis temperature on the BET surface areas and micro- and mesopore volumes of the biochars are shown in Table 3. The devolatilization of biomass materials developed porosity in the biochars, resulting in particles with an essentially micro-macropore structure. BET surface areas were generally low for biochars produced at all pyrolysis temperatures, but did slightly increase while increasing pyrolysis temperature up to 550◦ C (25.38 m2 g−1), and decreased thereafter. Structural ordering and micropore coalescence are found to be responsible for the decrease in the value of the surface area observed at 700◦ C, resulting in a thermal deactivation of the biochar. The same trends are observed in
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Carboxyl and Carbonyl
Alkanes and Aromatics
Alcohols, Phenols, Ethers,
700 °C
Transmittance, (%)
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550 °C
500 °C
450 °C
2187,58
400 °C 1413,82
Aliphatics
1043,49
651,94
Rapeseed
4000
3500
3000
2500
2000
1750
1500
1250
1000
500
-1
Wavenumber, (cm ) Figure 2 FTIR spectra of rapeseed and biochars obtained at different pyrolysis temperatures.
the micropore volume and area data (Guerrero et al. 2005). Micropore volume and area of biochars increased with the pyrolysis temperature (550◦ C) and reached a maximum of 0.0397 cm3 g−1 and 18,31 m2 g−1, respectively. At the temperature of 700◦ C, the surface area and total pore volume were lowered to 22.86 m2 g−1 and 0.0437 cm3 g−1, respectively. In table 3, it can be seen that an increase of the temperature does not improve the textural properties of the biochars. This fact has been observed by other authors (Gonz´ales et al. 2005), and it can be due to a sintering effect, following by shrinkage of the biochar and realignment of the biochar structure which leads to a decrease of the mean-size and volume of the pores. Therefore, the optimal conditions of the pyrolysis process for biochar production would be at temperature until 550◦ C. Due to the high temperature, the porous structure cracked and the pores might be partially blocked as a result of the softening and melting of the rapeseed constituents, which could to a low surface area (Liu et al. 2010). It is observed that the BET surface area is low compared to other biochars obtained other
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Table 3 Effect of pyrolysis temperature on surface areas and pore volumes of rapeseed biochars Pyrolysis temperature (◦ C)
SBET (m2/g)
Smicro (m2/g)
Smeso (m2/g)
Vtotal (cm3/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
Vmicro (%)
Smicro (%)
400 450 500 550 700
14.73 19.87 20.12 25.38 22.86
9.46 12.77 12.93 18.31 16.19
5.27 7.10 7.19 7.07 6.67
0.0278 0.0376 0.0393 0.0480 0.0437
0.0197 0.0273 0.0291 0.0397 0.0322
0.0081 0.0098 0.0102 0.0083 0.0115
70.86 73.94 74.05 82.71 73.68
64.22 64.27 64.26 72.14 70.82
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biomass materials such as maize stalk, almond shell, pitch pine (Fu et al. 2011; Gonz´ales et al. 2005; Kim et al. 2012). However, the surface area found is quite similar to the one reported by other authors (Amutio et al. 2012; Liu et al. 2010; Hu et al. 2008). Scanning Electron Microscopy Scanning electron microscopy (SEM) has been especially used to evaluate the structural variations in biochar particles after different thermal treatments. SEM images indicated that the surface structures of rapeseed biochars obtained at different pyrolysis temperatures were very similar (Figure S1) and that amorphous and heterogeneous structures are dominated. The irregular shape solids containing pores with different sizes were observed as depicted in Figure S1. When the pyrolysis temperature increases to 550◦ C, an increase in the solid porosity can be found. This results support the increase in the BET surface area as a function of temperature (Jindarom et al. 2007). But, at a higher pyrolysis temperature of 600◦ C, the biochars had lower BET surface areas due to the shrinkage of chars at postsoftening and swelling temperatures, resulting in narrowing or closing pores. The pore structures of the biochars were identified a distortion at high temperature (700◦ C) (Guo and Lua 1998). The vesicles on the surface of biochars (at low temperatures) resulted from the release of volatile gas contained in the soften biomass matrix during the pyrolysis process. The presence of vesicles indicates that volatile components were formed and released. And, the vesicles occurred through a melt phase of cellular components. From a phenomenological point of view, a gradual release of different volatile compounds occurs as the temperature increases during devolatilization (Liu et al. 2010; Guerrero et al. 2005). CONCLUSION In this study, the effect of pyrolysis temperature on the characteristics of biochars prepared from rapeseed was investigated. Pyrolysis studies indicated that it can be achieved a more valuable and functional product (biochar) from rapeseed (Brassica napus L.). As the pyrolysis temperature increased from 400 to 700◦ C, the volatile matter, hydrogen, and oxygen contents of the biochar decreased, but the fixed carbon, ash and carbon content, pH, surface area and higher heating value increased. This was to be expected because increased devolatilization during pyrolysis resulted in the biochar to be predominantly carbon. At pyrolysis temperature of 700◦ C, most of the volatile matter had been removed. As a result, the rapeseed biochars can be effectively used as a raw material for the preparation of
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activated carbon. Biochars can also be assessed in fuel application and water purification processes.
SUPPLEMENTAL MATERIAL Supplemental data for this article can be accessed on the publisher’s website.
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