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Effect of Wet Torrefaction on Thermal Decomposition Behavior of Microalga Chlorella vulgaris ESP-31
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 206 – 211
The 8th International Conference on Applied Energy – ICAE2016
Effect of wet torrefaction on thermal decomposition behavior of microalga Chlorella vulgaris ESP-31 Quang-Vu Bacha, Wei-Hsin Chena, *, Shih-Cheng Lina, Herng-Kuang Sheenb, JoShu Changc a
Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan b Sugar Business Division, Taiwan Sugar Corporation, Tainan 701, Taiwan c Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Abstract In this work, thermal decompositions of wet torrefied microalga during pyrolysis and combustion are investigated. Microalga Chlorella vulgaris ESP-31 was first subjected to wet torrefaction with microwave-assisted heating. Then, the thermal decompositions of the raw and torrefied microalgae are studied by means of a thermogravimetric analyzer in nitrogen and synthetic air to simulate pyrolysis and combustion process, respectively. The results show that carbohydrate and protein are degraded to some extent during WT. In addition, thermal reactivity of lipid is also changed after WT. The changes in the microalgal composition affect the intensity and location of the corresponding peaks during pyrolysis and combustion. Moreover, the char produced from the raw microalga is more reactive and less stable than those from the torrefied microlagae.
© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). responsibility of 8th ICAE Selection under and/or peer-review Peer-review responsibility of theunder scientific committee of the International Conference on Applied Energy.
Keywords: Wet torrefaction; Microalga and biomass; Microwave-assisted heating; Thermogravimetric analysis; Thermal decomposition.
1. Introduction Microalga is one of important sources for third generation biofuels. It is a promising feedstock for the production of bio-oil, bio-ethanol, bio-diesel, bio-hydrogen and bio-methane via several thermochemical and biochemical conversions [1]. Among those, thermochemical conversion processes such as pyrolysis, * Corresponding author. Tel.: +886-6-2004456; fax: +886-6-2389940. E-mail address: [email protected]; [email protected].
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.303
Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
combustion, and gasification of microalgal biomass have been become attractive options for heat and power generation [2, 3]. However, an inherent characteristic of microalga is its very high moisture content, which obviously reduces the conversion efficiency and consumes more energy. In addition, microalga also has high ash content, which may cause some problems such as agglomeration, deposition, corrosion, slagging and fouling for the conversion systems [4]. In order to overcome the aforementioned issues coupled with microalgal biomass, pretreatment is required prior to consequent thermal chemical conversions. WT is a pretreatment method of biomass in hydrothermal media at temperatures within 180-260 °C [5-7], and very suitable to upgrade biomass with high moisture and ash content such as microalga [6, 8]. In the last decades, studies mostly focus on WT with conventional heating method. Although heat integration can be applied at industrial scale, this consumes lot of energy. On the other hand, microwave assisted-heating has some advantages over conventional heating such as rapid heating rate, instantaneous start/stop, more uniform heat distribution, and energy and space saving [9, 10]. Our recent work [11] has studied the effect of WT with microwave-assisted heating on the yield and fuel properties of the solid products from microalga Chlorella vulgaris ESP-31. However, thermal decomposition behaviors of the products were not included. As pyrolysis and combustion are the main applications of the torrefied fuel; it is worth an investigation on the decomposition behaviors of the obtained torrefied microalga during pyrolysis and combustion. 2. Materials and methods 2.1. Microalga production and collection The feedstock used in this study is microalga Chlorella vulgaris ESP-31, which was collected from a fish pond in southern Taiwan [12]. The microalga was cultivated in self-made transparent tubes (length of 200 cm and diameter of 20 cm) for one week, at the National Cheng Kung University. The tube contained 50 L of basal solution as cultivation medium [13]. The microalgal solution was harvested and dehydrated by centrifugal method. Then, the microalga was freeze-dried before stored at -5 °C.
Fig. 1. Microwave-assisted WT setup
207
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Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
2.2. Wet torrefaction setup and procedure Fig. 1 shows a schematic setup for microwave-assisted WT. The system includes a Teflon cylindrical reactor with volume of 625 ml (length=318 mm and inner diameter=50 mm). The reactor was closed at one end and connected to a stainless steel (316L) head at the other end, where a pressure gauge, a thermocouple, and a valve were attached. A modified microwave oven (Tatung TMO-231) was used as microwave generating source. The temperature inside the reactor was monitored and controlled by a temperature controller. An amount of 20 g dried microalga and 100 g distilled water were used for each WT run. The reactor with microalgal and water mixture was closed, sealed, and purged by nitrogen for 10 min to remove air. Thereafter, the reactor was pressurized by compressed nitrogen gas where the gauge pressures in the reactor for WT at 160, 170 and 180 °C were 3, 2.5 and 2 bar, respectively. Then, the reactor was placed in the microwave oven and heated to preset WT temperatures for 10 min, counted from the time when the reactor reaching the preset temperature to the moment when turning off the oven power for cooling. When the reactor was cooled to room temperature, the gaseous products were gradually released and the reactor was opened for product collection. The solid product (i.e., torrefied microalga) was dewatered from the produced mixture by a Hettich EBA12 centrifuge operating at 7000 rpm for 5 min. After separation, the solid product was dried at 105 °C for 24 h and stored in a closed bottle for further analyses. Main fuel properties of the raw and torrefied microalgae used in this study are tabulated in Table 1. Table 1. Fuel characterization of microalgal samples used in this study Torrefaction
Proximate analysis (wt%) a
Ultimate analysis (wt%) b
conditions
Ash
VM
FC
C
H
N
O
Raw
9.02
74.59
16.39
53.01
8.67
3.26
35.05
22.02
160 °C
7.12
73.76
19.12
55.53
8.34
5.80
30.33
23.79
170 °C
7.31
71.76
20.93
55.56
8.08
6.94
29.42
24.23
180 °C
4.38
70.33
25.29
54.59
7.98
9.29
28.14
24.49
HHV b
VM: Volatile matter; FC: Fixed carbon; HHV: Higher heating value (MJ/kg) a
: Dry basis
b
: Dry-ash-free basis
2.3. Thermogravimatric analysis A thermogravimetric analyzer (SDT Q600 TGA, TA Instruments) was employed for studying the thermal decomposition behavior of the torrefied microalga in inert (i.e., pyrolysis) and oxidative (i.e., combustion) atmospheres. In each TGA run, a very small amount of tested sample (about 0.5 mg) was spread in a 150 μL alumina crucible, located inside the TGA reactor. The sample was heated to 105 °C and hold at this temperature for 30 min for drying. Then, the sample was heated to 700 °C at a constant heating rate of 10 °C/min under nitrogen and synthetic air with a constant flow rate of 100 mL/min, applied respectively for pyrolysis and combustion experiments. After the TGA run, thermogravimetric (TG) and differential thermogravimetric (DTG) curves were generated automatically.
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Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
3. Results and discussions 3.1. Pyrolysis behavior Fig. 2 shows TG and DTG curves for the pyrolysis of the raw and torrefied microalgae. The TG curves reveal that the torrefied microalgae are more reactive than the raw microalga at temperatures less than 275 °C. At higher temperatures, the torrefied samples show better thermal stability than the raw one. It can be explained that low thermally stable components in the microalga were already removed during WT, and thus relatively more thermally stable compounds are retained in the torrefied microalgae. In addition, DTG curves provide more detailed information about the pyrolysis of the microalgal samples. It appears that pyrolysis of microalga show two main zones associated with two peaks. The first peak located at lower temperature is attributed to the degradation of carbohydrate and protein, while the second one located at higher temperature is addressed to the decomposition of lipid [14]. In the raw microalga, the first peak is much higher than the second one. After undergoing WT, the first peak is gradually reduced and shifted to lower temperatures with increasing WT temperature. On the other hand, the second peak is intensified and also shifted to lower temperatures with increasing WT severity. In case of WT at 180 °C, the second peak is even higher than the first one. The results indicate that carbohydrate and protein were degraded to some extent during WT, which significantly decreases the intensity of the first peak. In addition, the relocation of the second peak reveals that the thermal reactivity of lipid is also changed after WT. 100
(A)
14
Conversion rate (%/min)
12 Mass fraction
80
60
40 Raw o 160 C o 170 C 180oC
20 100
200
300
(B)
Raw o 160 C o 170 C 180oC
10 8 6 4 2
400
500
Temperature (oC)
600
700
0 100
200
300
400
500
600
700
Temperature (oC)
Fig. 2. TG (A) and DTG (B) curves of raw and torrefied microalgae during pyrolysis
3.2. Combustion behavior Fig. 3 shows TG and DTG curves during combustion of the raw and torrefied microalgae. Similar to the thermal decomposition in inert atmosphere, the TG curves show that the torrefied microalgae are more reactive at temperatures lower than 275 °C but less reactive at higher temperature during combustion, compared with the raw one. The detailed combustion behaviors can be observed from the DTG curves. It is easy to identify three different stages during microalgae combustion: the first stage is between 105 °C and 350 °C, explicating the devolatilization and oxidative reactions of low thermally stable components (e.g., carbohydrate and protein); the second stage is in the range of 350-475 °C for the raw microalga and 350-500 °C for the torrefied microalgae, interpreting the devolatilization and burning
210
Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
of more thermally stable components (e.g., lipid); the third stage is a consequence of the combustion of the produced char at higher temperatures. Again, the peak in the first stage is gradually reduced and shifted to lower temperatures with increasing WT temperature and this can be explained by lower carbohydrate content in the torrefied samples than that in the raw sample. On the other hand, the combustion peaks of the torrefied microalgae in the third stage are broader and lower as well as located at higher temperatures than that of the raw microalga. This indicates that the char produced from the raw microalga is more reactive and less stable than those from the torrefied ones. More important, it can be observed many instantaneous and intensified peaks in the second stage for the torrefied microalgae, while only a minor peak is found for the raw microalga. This phenomenon was also observed in other studies on algal biomass combustion [15, 16]. During WT, several depolymerization reactions would take place [17]. The newly formed compounds are very reactive and likely to recombine into new species via repolymerization reactions. This might explain the peaks occurred in the second stage. 12
100
(A) Conversion rate (%/min)
Mass fraction
80
60
40 Raw o 160 C o 170 C 180oC
20
0 100
200
300
(B)
Raw o 160 C o 170 C 180oC
10 8 6 4 2
400
500
Temperature (oC)
600
700
0 100
200
300
400
500
600
700
Temperature (oC)
Fig. 3. TG (A) and DTG (B) curves of raw and torrefied microalgae during combustion
4. Conclusions The decompositions of raw and wet torrefied microalga during pyrolysis and combustion are thermogravimetrically studied. Two pyrolysis zones and three combustion stages are figured out with identified peaks. The intensity and location of the peaks are strongly affected by WT. The results show that carbohydrate and protein are degraded to some extent during WT and the thermal reactivity of lipid is changed after WT. In addition, the char produced from the raw microalga is more reactive and less stable than those from the torrefied microlagae. Acknowledgements This research received funding in part from the Headquarters of University Advancement at the National Cheng Kung University, which is supported by the Ministry of Education, Taiwan, R.O.C. The authors also acknowledge the financial support from the Ministry of Science and Technology, Taiwan, R.O.C., under the contracts MOST 102-2221-E-006-288-MY3 and MOST 105-2811-E-006-003 for this research.
Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
References [1] M. F. Demirbas, "Biofuels from algae for sustainable development," Applied Energy, vol. 88, pp. 3473-3480, 2011. [2] W.-H. Chen, B.-J. Lin, M.-Y. Huang, and J.-S. Chang, "Thermochemical conversion of microalgal biomass into biofuels: A review," Bioresource Technology, vol. 184, pp. 314-327, 2015. [3] D. López Barreiro, W. Prins, F. Ronsse, and W. Brilman, "Hydrothermal liquefaction (HTL) of microalgae for biofuel production: State of the art review and future prospects," Biomass and Bioenergy, vol. 53, pp. 113-127, 2013. [4] D. J. Lane, M. Zevenhoven, P. J. Ashman, P. J. van Eyk, M. Hupa, R. de Nys, et al., "Algal Biomass: Occurrence of the Main Inorganic Elements and Simulation of Ash Interactions with Bed Material," Energy & Fuels, vol. 28, pp. 4622-4632, 2014. [5] W. Yan, T. C. Acharjee, C. J. Coronella, and V. R. Vásquez, "Thermal pretreatment of lignocellulosic biomass," Environmental Progress & Sustainable Energy, vol. 28, pp. 435-440, 2009. [6] Q.-V. Bach, K.-Q. Tran, R. A. Khalil, Ø. Skreiberg, and G. Seisenbaeva, "Comparative Assessment of Wet Torrefaction," Energy & Fuels, vol. 27, pp. 6743-6753, 2013. [7] Q.-V. Bach and Ø. Skreiberg, "Upgrading biomass fuels via wet torrefaction: A review and comparison with dry torrefaction," Renewable and Sustainable Energy Reviews, vol. 54, pp. 665-677, 2016. [8] Q.-V. Bach, K.-Q. Tran, and Ø. Skreiberg, "Accelerating wet torrefaction rate and ash removal by carbon dioxide addition," Fuel Processing Technology, vol. 140, pp. 297-303, 2015. [9] M.-F. Li, Y. Shen, J.-K. Sun, J. Bian, C.-Z. Chen, and R.-C. Sun, "Wet Torrefaction of Bamboo in Hydrochloric Acid Solution by Microwave Heating," ACS Sustainable Chemistry & Engineering, vol. 3, pp. 2022-2029, 2015. [10] W.-H. Chen, S.-C. Ye, and H.-K. Sheen, "Hydrothermal carbonization of sugarcane bagasse via wet torrefaction in association with microwave heating," Bioresource Technology, vol. 118, pp. 195-203, 2012. [11] Q.-V. Bach, W.-H. Chen, S.-C. Lin, H.-K. Sheen, and J.-S. Chang, "Wet torrefaction of microalga Chlorella vulgaris ESP-31 with microwave-assisted heating," Energy Conversion and Management, 2016. [12] W.-H. Chen, M.-Y. Huang, J.-S. Chang, and C.-Y. Chen, "Torrefaction operation and optimization of microalga residue for energy densification and utilization," Applied Energy, vol. 154, pp. 622-630, 2015. [13] D.-T. Tran, K.-L. Yeh, C.-L. Chen, and J.-S. Chang, "Enzymatic transesterification of microalgal oil from Chlorella vulgaris ESP-31 for biodiesel synthesis using immobilized Burkholderia lipase," Bioresource Technology, vol. 108, pp. 119-127, 2012. [14] W.-H. Chen, Z.-Y. Wu, and J.-S. Chang, "Isothermal and non-isothermal torrefaction characteristics and kinetics of microalga Scenedesmus obliquus CNW-N," Bioresource Technology, vol. 155, pp. 245-251, 2014. [15] S. Wang, X. M. Jiang, X. X. Han, and J. G. Liu, "Combustion Characteristics of Seaweed Biomass. 1. Combustion Characteristics of Enteromorpha clathrata and Sargassum natans," Energy & Fuels, vol. 23, pp. 5173-5178, 2009. [16] L. Sanchez-Silva, D. López-González, A. M. Garcia-Minguillan, and J. L. Valverde, "Pyrolysis, combustion and gasification characteristics of Nannochloropsis gaditana microalgae," Bioresource Technology, vol. 130, pp. 321-331, 2013. [17] E. Sabio, A. Álvarez-Murillo, S. Román, and B. Ledesma, "Conversion of tomato-peel waste into solid fuel by hydrothermal carbonization: Influence of the processing variables," Waste Management, vol. 47, Part A, pp. 122-132, 2016.
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ScienceDirect Energy Procedia 105 (2017) 206 – 211
The 8th International Conference on Applied Energy – ICAE2016
Effect of wet torrefaction on thermal decomposition behavior of microalga Chlorella vulgaris ESP-31 Quang-Vu Bacha, Wei-Hsin Chena, *, Shih-Cheng Lina, Herng-Kuang Sheenb, JoShu Changc a
Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan b Sugar Business Division, Taiwan Sugar Corporation, Tainan 701, Taiwan c Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
Abstract In this work, thermal decompositions of wet torrefied microalga during pyrolysis and combustion are investigated. Microalga Chlorella vulgaris ESP-31 was first subjected to wet torrefaction with microwave-assisted heating. Then, the thermal decompositions of the raw and torrefied microalgae are studied by means of a thermogravimetric analyzer in nitrogen and synthetic air to simulate pyrolysis and combustion process, respectively. The results show that carbohydrate and protein are degraded to some extent during WT. In addition, thermal reactivity of lipid is also changed after WT. The changes in the microalgal composition affect the intensity and location of the corresponding peaks during pyrolysis and combustion. Moreover, the char produced from the raw microalga is more reactive and less stable than those from the torrefied microlagae.
© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). responsibility of 8th ICAE Selection under and/or peer-review Peer-review responsibility of theunder scientific committee of the International Conference on Applied Energy.
Keywords: Wet torrefaction; Microalga and biomass; Microwave-assisted heating; Thermogravimetric analysis; Thermal decomposition.
1. Introduction Microalga is one of important sources for third generation biofuels. It is a promising feedstock for the production of bio-oil, bio-ethanol, bio-diesel, bio-hydrogen and bio-methane via several thermochemical and biochemical conversions [1]. Among those, thermochemical conversion processes such as pyrolysis, * Corresponding author. Tel.: +886-6-2004456; fax: +886-6-2389940. E-mail address: [email protected]; [email protected].
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.303
Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
combustion, and gasification of microalgal biomass have been become attractive options for heat and power generation [2, 3]. However, an inherent characteristic of microalga is its very high moisture content, which obviously reduces the conversion efficiency and consumes more energy. In addition, microalga also has high ash content, which may cause some problems such as agglomeration, deposition, corrosion, slagging and fouling for the conversion systems [4]. In order to overcome the aforementioned issues coupled with microalgal biomass, pretreatment is required prior to consequent thermal chemical conversions. WT is a pretreatment method of biomass in hydrothermal media at temperatures within 180-260 °C [5-7], and very suitable to upgrade biomass with high moisture and ash content such as microalga [6, 8]. In the last decades, studies mostly focus on WT with conventional heating method. Although heat integration can be applied at industrial scale, this consumes lot of energy. On the other hand, microwave assisted-heating has some advantages over conventional heating such as rapid heating rate, instantaneous start/stop, more uniform heat distribution, and energy and space saving [9, 10]. Our recent work [11] has studied the effect of WT with microwave-assisted heating on the yield and fuel properties of the solid products from microalga Chlorella vulgaris ESP-31. However, thermal decomposition behaviors of the products were not included. As pyrolysis and combustion are the main applications of the torrefied fuel; it is worth an investigation on the decomposition behaviors of the obtained torrefied microalga during pyrolysis and combustion. 2. Materials and methods 2.1. Microalga production and collection The feedstock used in this study is microalga Chlorella vulgaris ESP-31, which was collected from a fish pond in southern Taiwan [12]. The microalga was cultivated in self-made transparent tubes (length of 200 cm and diameter of 20 cm) for one week, at the National Cheng Kung University. The tube contained 50 L of basal solution as cultivation medium [13]. The microalgal solution was harvested and dehydrated by centrifugal method. Then, the microalga was freeze-dried before stored at -5 °C.
Fig. 1. Microwave-assisted WT setup
207
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Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
2.2. Wet torrefaction setup and procedure Fig. 1 shows a schematic setup for microwave-assisted WT. The system includes a Teflon cylindrical reactor with volume of 625 ml (length=318 mm and inner diameter=50 mm). The reactor was closed at one end and connected to a stainless steel (316L) head at the other end, where a pressure gauge, a thermocouple, and a valve were attached. A modified microwave oven (Tatung TMO-231) was used as microwave generating source. The temperature inside the reactor was monitored and controlled by a temperature controller. An amount of 20 g dried microalga and 100 g distilled water were used for each WT run. The reactor with microalgal and water mixture was closed, sealed, and purged by nitrogen for 10 min to remove air. Thereafter, the reactor was pressurized by compressed nitrogen gas where the gauge pressures in the reactor for WT at 160, 170 and 180 °C were 3, 2.5 and 2 bar, respectively. Then, the reactor was placed in the microwave oven and heated to preset WT temperatures for 10 min, counted from the time when the reactor reaching the preset temperature to the moment when turning off the oven power for cooling. When the reactor was cooled to room temperature, the gaseous products were gradually released and the reactor was opened for product collection. The solid product (i.e., torrefied microalga) was dewatered from the produced mixture by a Hettich EBA12 centrifuge operating at 7000 rpm for 5 min. After separation, the solid product was dried at 105 °C for 24 h and stored in a closed bottle for further analyses. Main fuel properties of the raw and torrefied microalgae used in this study are tabulated in Table 1. Table 1. Fuel characterization of microalgal samples used in this study Torrefaction
Proximate analysis (wt%) a
Ultimate analysis (wt%) b
conditions
Ash
VM
FC
C
H
N
O
Raw
9.02
74.59
16.39
53.01
8.67
3.26
35.05
22.02
160 °C
7.12
73.76
19.12
55.53
8.34
5.80
30.33
23.79
170 °C
7.31
71.76
20.93
55.56
8.08
6.94
29.42
24.23
180 °C
4.38
70.33
25.29
54.59
7.98
9.29
28.14
24.49
HHV b
VM: Volatile matter; FC: Fixed carbon; HHV: Higher heating value (MJ/kg) a
: Dry basis
b
: Dry-ash-free basis
2.3. Thermogravimatric analysis A thermogravimetric analyzer (SDT Q600 TGA, TA Instruments) was employed for studying the thermal decomposition behavior of the torrefied microalga in inert (i.e., pyrolysis) and oxidative (i.e., combustion) atmospheres. In each TGA run, a very small amount of tested sample (about 0.5 mg) was spread in a 150 μL alumina crucible, located inside the TGA reactor. The sample was heated to 105 °C and hold at this temperature for 30 min for drying. Then, the sample was heated to 700 °C at a constant heating rate of 10 °C/min under nitrogen and synthetic air with a constant flow rate of 100 mL/min, applied respectively for pyrolysis and combustion experiments. After the TGA run, thermogravimetric (TG) and differential thermogravimetric (DTG) curves were generated automatically.
209
Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
3. Results and discussions 3.1. Pyrolysis behavior Fig. 2 shows TG and DTG curves for the pyrolysis of the raw and torrefied microalgae. The TG curves reveal that the torrefied microalgae are more reactive than the raw microalga at temperatures less than 275 °C. At higher temperatures, the torrefied samples show better thermal stability than the raw one. It can be explained that low thermally stable components in the microalga were already removed during WT, and thus relatively more thermally stable compounds are retained in the torrefied microalgae. In addition, DTG curves provide more detailed information about the pyrolysis of the microalgal samples. It appears that pyrolysis of microalga show two main zones associated with two peaks. The first peak located at lower temperature is attributed to the degradation of carbohydrate and protein, while the second one located at higher temperature is addressed to the decomposition of lipid [14]. In the raw microalga, the first peak is much higher than the second one. After undergoing WT, the first peak is gradually reduced and shifted to lower temperatures with increasing WT temperature. On the other hand, the second peak is intensified and also shifted to lower temperatures with increasing WT severity. In case of WT at 180 °C, the second peak is even higher than the first one. The results indicate that carbohydrate and protein were degraded to some extent during WT, which significantly decreases the intensity of the first peak. In addition, the relocation of the second peak reveals that the thermal reactivity of lipid is also changed after WT. 100
(A)
14
Conversion rate (%/min)
12 Mass fraction
80
60
40 Raw o 160 C o 170 C 180oC
20 100
200
300
(B)
Raw o 160 C o 170 C 180oC
10 8 6 4 2
400
500
Temperature (oC)
600
700
0 100
200
300
400
500
600
700
Temperature (oC)
Fig. 2. TG (A) and DTG (B) curves of raw and torrefied microalgae during pyrolysis
3.2. Combustion behavior Fig. 3 shows TG and DTG curves during combustion of the raw and torrefied microalgae. Similar to the thermal decomposition in inert atmosphere, the TG curves show that the torrefied microalgae are more reactive at temperatures lower than 275 °C but less reactive at higher temperature during combustion, compared with the raw one. The detailed combustion behaviors can be observed from the DTG curves. It is easy to identify three different stages during microalgae combustion: the first stage is between 105 °C and 350 °C, explicating the devolatilization and oxidative reactions of low thermally stable components (e.g., carbohydrate and protein); the second stage is in the range of 350-475 °C for the raw microalga and 350-500 °C for the torrefied microalgae, interpreting the devolatilization and burning
210
Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
of more thermally stable components (e.g., lipid); the third stage is a consequence of the combustion of the produced char at higher temperatures. Again, the peak in the first stage is gradually reduced and shifted to lower temperatures with increasing WT temperature and this can be explained by lower carbohydrate content in the torrefied samples than that in the raw sample. On the other hand, the combustion peaks of the torrefied microalgae in the third stage are broader and lower as well as located at higher temperatures than that of the raw microalga. This indicates that the char produced from the raw microalga is more reactive and less stable than those from the torrefied ones. More important, it can be observed many instantaneous and intensified peaks in the second stage for the torrefied microalgae, while only a minor peak is found for the raw microalga. This phenomenon was also observed in other studies on algal biomass combustion [15, 16]. During WT, several depolymerization reactions would take place [17]. The newly formed compounds are very reactive and likely to recombine into new species via repolymerization reactions. This might explain the peaks occurred in the second stage. 12
100
(A) Conversion rate (%/min)
Mass fraction
80
60
40 Raw o 160 C o 170 C 180oC
20
0 100
200
300
(B)
Raw o 160 C o 170 C 180oC
10 8 6 4 2
400
500
Temperature (oC)
600
700
0 100
200
300
400
500
600
700
Temperature (oC)
Fig. 3. TG (A) and DTG (B) curves of raw and torrefied microalgae during combustion
4. Conclusions The decompositions of raw and wet torrefied microalga during pyrolysis and combustion are thermogravimetrically studied. Two pyrolysis zones and three combustion stages are figured out with identified peaks. The intensity and location of the peaks are strongly affected by WT. The results show that carbohydrate and protein are degraded to some extent during WT and the thermal reactivity of lipid is changed after WT. In addition, the char produced from the raw microalga is more reactive and less stable than those from the torrefied microlagae. Acknowledgements This research received funding in part from the Headquarters of University Advancement at the National Cheng Kung University, which is supported by the Ministry of Education, Taiwan, R.O.C. The authors also acknowledge the financial support from the Ministry of Science and Technology, Taiwan, R.O.C., under the contracts MOST 102-2221-E-006-288-MY3 and MOST 105-2811-E-006-003 for this research.
Quang-Vu Bach et al. / Energy Procedia 105 (2017) 206 – 211
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