- Email: [email protected]
Bio-ETBE determination in a mixture of gasoline using low level liquid scintillation counting
Journal of Industrial and Engineering Chemistry 49 (2017) 26–29
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
Bio-ETBE determination in a mixture of gasoline using low level liquid scintillation counting Seung-Soo Kima,1, Young-Kwan Limb,1, Jae Hyung Choia,c , Jinsoo Kimd,* , Mohd Roslee Othmane,** a
Department of Chemical Engineering, Kangwon National University, 346 Joongang-ro, Samcheok, Gangwon-do 25913, Republic of Korea Korea Petroleum Quality & Distribution Authority, Chgungcheongbuk-do 28115, Republic of Korea Department of Chemical Engineering, Pukyong National University, 365 Sinseon-ro, Nam-gu, Busan 48513, Republic of Korea d Department of Chemical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi-do 17104, Republic of Korea e School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia b c
A R T I C L E I N F O
Article history: Received 29 November 2016 Received in revised form 6 January 2017 Accepted 17 January 2017 Available online 25 January 2017 Keywords: Bioethanol LSC Radiocarbon dating Bio-ETBE
A B S T R A C T
There is a growing interest in Korean government to add bio-ethyl tert-butyl ether (bio-ETBE) in suboctane gasoline to reduce dependence on fossil fuel based transportation gasoline. The development of bio-ETBE analysis method is an urgent issue to protect illegal circulation of synthetic ETBE because synthetic ETBE and bio-ETBE are chemically indistinguishable even by using chromatography and conventional spectroscopy. This paper communicates the results of bio-ETBE determination from gasoline cocktail using low level liquid scintillation counting (LSC). The counts per minute (CPM) value increased linearly with the concentration of bio-ETBE, showing the correlation coefficient of 0.984. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction The Republic of Korea has embarked upon a long term national strategy for “Green Growth” from 2009 to 2050, which is aimed at promoting eco-friendly new growth engines, enhancing the quality of peoples’ lives and contributing efforts to fight the climate change. Since its inception, the Green Growth initiative reportedly has brought about an increase in the urban green energy projects [1] and improvement of local renewable energy consumption, which has attracted a lot of research in bio-fuels and their upgrading [2–6]. However, the rising energy demand and ongoing dependency on fossil fuel economies consistently posed significant challenges. One of these challenges might stem from the recognition of ethanol as a renewable transport fuel, but the new tax credits which are intended for bio-ethanol might not have served the purpose.
* Corresponding author. Fax: +82 31 204 8114. ** Corresponding author. Fax: +60 4 5996908. E-mail addresses: [email protected] (J. Kim), [email protected] (M.R. Othman). 1 Co-first authors.
Ethanol can be produced from either fermentation of sugar or hydration of ethylene. The ethanol derived from the former method is considered as a bioethanol and is renewable. The latter ethanol (synthetic), however, is derived from ethane that is separated from natural gas and thus, it is not considered renewable. Due to the cheaper synthetic ethanol, there is a growing preference for synthetic ethanol to be blended with suboctane gasoline for transport fuel and regarded as renewable energy. Bioethanol and synthetic ethanol are chemically indistinguishable even by using chromatography and conventional spectroscopy. Because of the inability to identify the differences between the two from the blended fuel, the synthetic ethanol blend would reap the government subsidy and render the government novel effort and motives useless. An effective method to identify whether or not the ethanol comes from a renewable resource is being sought after in order to ensure that the tax credits are issued to the proper recipients. A successful attempt to distinguish bioethanol from synthetic ethanol in a sub-octane gasoline using low level liquid scintillation counter (LSC) was reported [7,8]. There is also a report on determining the level of benzene purity using LSC [9]. LSC makes use of the difference in the properties of the radioactive isotope of carbon (14C) or radiocarbon dating. Bioethanol has 14C
http://dx.doi.org/10.1016/j.jiec.2017.01.020 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
S.-S. Kim et al. / Journal of Industrial and Engineering Chemistry 49 (2017) 26–29
radioactivity originating from atmospheric CO2 which is incorporated into plants by photosynthesis. The synthetic ethanol derives its 14C radioactivity from the complete decay of the fossil for over millions of years. Therefore, there would be less 14C from synthetic ethanol and the distinction of their origins becomes clear by using this technique. Recently there is a growing interest to add bioethanol and ethyl tert-butyl ether (ETBE) in the sub-octane gasoline because it reportedly improves combustion of the gasoline [10–13]. The Ministry of Knowledge Economy has agreed to the proposal that the formula be used as an oxygenate gasoline additive which would reduce the production of smog from automobiles exhausts and dependence of fossil fuel. Should this proposal materialize, determination of the origin of the ethanol would become a challenge because ETBE could cloud the results in the carbon dating exercise [14]. In order to promote the use of bioethanolETBE blended gasoline, while preventing the illegitimate circulation of synthetic ethanol in the transportation fuel market, the development of an accurate and reliable analysis method is urgently required for determining the origin of the fuel component. This paper communicates the results of the approach to identify bio-ETBE in a mixture of gasoline using LSC. Experimental Bio-ETBE was prepared by reacting bioethanol with tertbutanol in the presence of H2SO4. First, a round bottom flask was filled with 100 g (2.17 mol) of bioethanol (Merck Ltd, 99.9%), 161 g (2.17 mol) of tert-butanol, and 500 mL of 10% aqueous H2SO4 solution at room temperature. Then, the reaction mixture was heated and refluxed at 80 C for 6 h. After cooling to room temperature, 300 mL of H2O was added. The organic solvent was extracted with 500 mL of diethyl ether. The sample was washed with 300 mL of H2O twice, dried over anhydrous NaHCO3, filtered, washed with diethyl ether, and concentrated under reduced pressure. Scheme 1 is shown for the synthetic method of Bio-ETBE. The obtained clear liquid Bio-ETBE was characterized by 1 H NMR and GC, showing the purity of 68%. In order to determine a proper scintillation agent for bio-ETBE, various scintillation agents (Optiscint HiSafe, Ultima Gold F, OptiFluor O, Insta Gel Plus, and Optiphase HiSafe 3) were purchased from
27
Scheme 1. Synthetic process of Bio-ETBE.
PerkinElmer and used for cocktail preparation by mixing 10 mL scintillation agent and 10 mL bio-ETBE. LSC (PerkinElmer, WALLAC Quantulus 1220) was used for 14C decay analysis in the bio-ETBE cocktails. Results and discussion For bio-ETBE determination using LSC, organic cocktail was prepared first by mixing scintillation agent with the sample. Scintillation agents are important to prepare cocktails for higher analysis efficiency. Beta particles are emitted from 14C decay in biobased carbonaceous materials, which cause solvent molecules to become excited. Due to the low signal of beta particles, scintillation agents are used to amplify the signals. Various scintillation agents were characterized to analyze bio-ETBE in Fig. 1. LSC spectrums show wider and shifted to higher energy when the efficiency of scintillation agent is higher for 14C decay analysis. Optiscint HiSafe and Ultima Gold F as scintillation agents were the promising candidates because they showed higher spectrum width and energy than others as well as higher analysis efficiency of 14C in bio-ETBE. In analyzing bio-ETBE quantitatively, 10 mL Optiscint HiSafe and 10 mL of sub-octane gasoline were mixed to prepare the cocktails, in which bio-ETBE concentrations were increased from 0.0% to 20.4%. The effect of bio-ETBE concentration on LSC intensity is presented in Fig. 2. The figure shows that the intensity at the energy channel between 150 and 800 increased when higher concentration of bio-ETBE was included in the cocktail. The sample without bio-ETBE shows relatively low energy intensity, replicating the effect of the background noise. Fig. 3 shows the counts per minute (CPM) and spectral quench parameter (SQP) as a function of bio-ETBE concentration. SQP indicates a relative light production from the sample. If the values
0.25 1 2 3 4 5
Intensity
0.20
0.15
0.10
0.05
0.00 0
200
400
600
800
1000
Energy Channel [ - ] Fig. 1. Characteristics of scintillation agent for the higher analysis efficiency of beta emission (b) from 14C decay of bio-ETBE. (1) Optiscint HiSafe, (2) Ultima Gold F, (3) Opti Fluor O, (4) Insta Gel Plus, (5) Optiphase HiSafe 3.
28
S.-S. Kim et al. / Journal of Industrial and Engineering Chemistry 49 (2017) 26–29
0.12
(a)
Intensity
0.10
1 2 3
1 2
0.08 0.06
3
0.04 0.02 0.00 0.12
(b)
0.10
4 5 6
6
0.08
5
0.06
4
0.04 0.02 0.00 0
100
200
300
400
500
600
700
800
900
1000
Energy Channel [ - ] Fig. 2. Bio-ETBE analysis using low-level LSC: (a) Bio-ETBE 0.0% (1), Bio-ETBE 3.4% (2), Bio-ETBE 6.8% (3), and (b) Bio-ETBE 10.2% (4), Bio-ETBE 13.6% (5), Bio-ETBE 20.4% (6).
1000
16
f(x) = 2.66943 + 0.28315x R2 = 0.984
14
900
12
CPM
700
8 6
SQP
800
10
600
4 500
2 0
0
5
10
15
20
400 25
Bio-ETBE concentration [ % ] Fig. 3. Calibration curve depending on the concentration of bio-ETBE with sub-octane gasoline.
of SQP vary from sample to sample, quench correction is needed. CPM value, representing the number of ß emissions per minute, increased linearly from 2.4 to 8.4 with the increasing concentration of bio-ETBE from 0.0% to 20.4%, respectively. The correlation coefficient (R2) between CPM and bio-ETBE concentration was as high as 0.984, indicating that the calibration line fits the data extremely well. The first order polynomial for bio-ETBE concentration in sub-octane gasoline was obtained by least squares method and represented in Eq. (1). For comparison, the equation for bio-ethanol concentration at the same scintillation agent was shown in Eq. (2) from the literature [7].
F(x) = 2.66943 + 0.28315x
(1)
F(x) = 1.44475 + 0.39472x
(2)
The two differing equations suggest that bio-ETBE presence in the gasoline cocktail would require different calibration curves for different bio-fuel detection. The use of equation (1) in determining the concentration of bio-ethanol and vice-versa would yield inaccurate results.
S.-S. Kim et al. / Journal of Industrial and Engineering Chemistry 49 (2017) 26–29
Conclusions There is an inclination to use the previous calibration line/ curve in determining the concentration of bio-ETBE although the formula should be used as an oxygenate gasoline additive in the transportation fuel market. This would yield inaccurate result because the data used for the calibration were based on bioethanol–gasoline cocktail. This communication proposes more developed equation for bio-ETBE concentration determination in the gasoline sample using low level liquid scintillation counting (LSC). The counts per minute (CPM) value, representing the number of ß emissions per minute, increased linearly from 2.4 to 8.4 with the increasing concentration of bio-ETBE from 0.0% to 20.4%. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology
29
(NRF-2014R1A1A4A01008538), Ministry of Science, ICT & Future Planning (no. 2014R1A5A1009799) and Fundamental Research Grant Scheme (FRGS-GSP) of Malaysia. References [1] J.-S. Lee, J.-W. Kim, Cities 54 (2016) 20. [2] H. Lee, Y.M. Kim, I.G. Lee, J.K. Jeon, S.C. Jung, J.D. Chung, W.G. Choi, Y.K. Park, Korean J. Chem. Eng. 33 (12) (2016) 3299. [3] J.H. Choi, S.-S. Kim, H.V. Ly, J. Kim, H.C. Woo, Fuel 193 (2017) 159. [4] T.K. Vo, O.K. Lee, E.Y. Lee, C.H. Kim, J.-W. Seo, J. Kim, S.-S. Kim, Chem. Eng. J. 306 (2016) 763. [5] J.H. Choi, S.-S. Kim, D.J. Suh, E.-J. Jang, K.-I. Min, H.C. Woo, Korean J. Chem. Eng. 33 (9) (2016) 2691. [6] S.-S. Kim, H.V. Ly, B.H. Chun, J.-H. Ko, J. Kim, Korean J. Chem. Eng. 33 (11) (2016) 3128. [7] S.-S. Kim, J. Kim, S.C. Shin, F.A. Agblevor, Chem. Lett. 38 (2009) 850. [8] S. Yunoki, M. Saito, Bioresour. Technol. 100 (2009) 6125. [9] F.G. McCormac, Radiocarbon 34 (1992) 37. [10] M.G. Sneesby, M.O. Dade, R. Datta, T.N. Smith, Ind. Eng. Chem. Res. 36 (1997) 1855. [11] E.W. Menezes, R. Cataluña, Fuel Process. Technol. 89 (2008) 1148. [12] L.M. Aodríguez-Antóne, F. Gutíerrez-Martín, Y. Doce, Fuel 166 (2016) 73. [13] E.W. Menezes, R. Cataluña, D. Samios, R. Silva, Fuel 85 (2006) 2567. [14] Y. Nagawa, S. Yunoki, M. Saito, Radioisotopes 63 (2014) 139.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
Bio-ETBE determination in a mixture of gasoline using low level liquid scintillation counting Seung-Soo Kima,1, Young-Kwan Limb,1, Jae Hyung Choia,c , Jinsoo Kimd,* , Mohd Roslee Othmane,** a
Department of Chemical Engineering, Kangwon National University, 346 Joongang-ro, Samcheok, Gangwon-do 25913, Republic of Korea Korea Petroleum Quality & Distribution Authority, Chgungcheongbuk-do 28115, Republic of Korea Department of Chemical Engineering, Pukyong National University, 365 Sinseon-ro, Nam-gu, Busan 48513, Republic of Korea d Department of Chemical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi-do 17104, Republic of Korea e School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia b c
A R T I C L E I N F O
Article history: Received 29 November 2016 Received in revised form 6 January 2017 Accepted 17 January 2017 Available online 25 January 2017 Keywords: Bioethanol LSC Radiocarbon dating Bio-ETBE
A B S T R A C T
There is a growing interest in Korean government to add bio-ethyl tert-butyl ether (bio-ETBE) in suboctane gasoline to reduce dependence on fossil fuel based transportation gasoline. The development of bio-ETBE analysis method is an urgent issue to protect illegal circulation of synthetic ETBE because synthetic ETBE and bio-ETBE are chemically indistinguishable even by using chromatography and conventional spectroscopy. This paper communicates the results of bio-ETBE determination from gasoline cocktail using low level liquid scintillation counting (LSC). The counts per minute (CPM) value increased linearly with the concentration of bio-ETBE, showing the correlation coefficient of 0.984. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction The Republic of Korea has embarked upon a long term national strategy for “Green Growth” from 2009 to 2050, which is aimed at promoting eco-friendly new growth engines, enhancing the quality of peoples’ lives and contributing efforts to fight the climate change. Since its inception, the Green Growth initiative reportedly has brought about an increase in the urban green energy projects [1] and improvement of local renewable energy consumption, which has attracted a lot of research in bio-fuels and their upgrading [2–6]. However, the rising energy demand and ongoing dependency on fossil fuel economies consistently posed significant challenges. One of these challenges might stem from the recognition of ethanol as a renewable transport fuel, but the new tax credits which are intended for bio-ethanol might not have served the purpose.
* Corresponding author. Fax: +82 31 204 8114. ** Corresponding author. Fax: +60 4 5996908. E-mail addresses: [email protected] (J. Kim), [email protected] (M.R. Othman). 1 Co-first authors.
Ethanol can be produced from either fermentation of sugar or hydration of ethylene. The ethanol derived from the former method is considered as a bioethanol and is renewable. The latter ethanol (synthetic), however, is derived from ethane that is separated from natural gas and thus, it is not considered renewable. Due to the cheaper synthetic ethanol, there is a growing preference for synthetic ethanol to be blended with suboctane gasoline for transport fuel and regarded as renewable energy. Bioethanol and synthetic ethanol are chemically indistinguishable even by using chromatography and conventional spectroscopy. Because of the inability to identify the differences between the two from the blended fuel, the synthetic ethanol blend would reap the government subsidy and render the government novel effort and motives useless. An effective method to identify whether or not the ethanol comes from a renewable resource is being sought after in order to ensure that the tax credits are issued to the proper recipients. A successful attempt to distinguish bioethanol from synthetic ethanol in a sub-octane gasoline using low level liquid scintillation counter (LSC) was reported [7,8]. There is also a report on determining the level of benzene purity using LSC [9]. LSC makes use of the difference in the properties of the radioactive isotope of carbon (14C) or radiocarbon dating. Bioethanol has 14C
http://dx.doi.org/10.1016/j.jiec.2017.01.020 1226-086X/© 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
S.-S. Kim et al. / Journal of Industrial and Engineering Chemistry 49 (2017) 26–29
radioactivity originating from atmospheric CO2 which is incorporated into plants by photosynthesis. The synthetic ethanol derives its 14C radioactivity from the complete decay of the fossil for over millions of years. Therefore, there would be less 14C from synthetic ethanol and the distinction of their origins becomes clear by using this technique. Recently there is a growing interest to add bioethanol and ethyl tert-butyl ether (ETBE) in the sub-octane gasoline because it reportedly improves combustion of the gasoline [10–13]. The Ministry of Knowledge Economy has agreed to the proposal that the formula be used as an oxygenate gasoline additive which would reduce the production of smog from automobiles exhausts and dependence of fossil fuel. Should this proposal materialize, determination of the origin of the ethanol would become a challenge because ETBE could cloud the results in the carbon dating exercise [14]. In order to promote the use of bioethanolETBE blended gasoline, while preventing the illegitimate circulation of synthetic ethanol in the transportation fuel market, the development of an accurate and reliable analysis method is urgently required for determining the origin of the fuel component. This paper communicates the results of the approach to identify bio-ETBE in a mixture of gasoline using LSC. Experimental Bio-ETBE was prepared by reacting bioethanol with tertbutanol in the presence of H2SO4. First, a round bottom flask was filled with 100 g (2.17 mol) of bioethanol (Merck Ltd, 99.9%), 161 g (2.17 mol) of tert-butanol, and 500 mL of 10% aqueous H2SO4 solution at room temperature. Then, the reaction mixture was heated and refluxed at 80 C for 6 h. After cooling to room temperature, 300 mL of H2O was added. The organic solvent was extracted with 500 mL of diethyl ether. The sample was washed with 300 mL of H2O twice, dried over anhydrous NaHCO3, filtered, washed with diethyl ether, and concentrated under reduced pressure. Scheme 1 is shown for the synthetic method of Bio-ETBE. The obtained clear liquid Bio-ETBE was characterized by 1 H NMR and GC, showing the purity of 68%. In order to determine a proper scintillation agent for bio-ETBE, various scintillation agents (Optiscint HiSafe, Ultima Gold F, OptiFluor O, Insta Gel Plus, and Optiphase HiSafe 3) were purchased from
27
Scheme 1. Synthetic process of Bio-ETBE.
PerkinElmer and used for cocktail preparation by mixing 10 mL scintillation agent and 10 mL bio-ETBE. LSC (PerkinElmer, WALLAC Quantulus 1220) was used for 14C decay analysis in the bio-ETBE cocktails. Results and discussion For bio-ETBE determination using LSC, organic cocktail was prepared first by mixing scintillation agent with the sample. Scintillation agents are important to prepare cocktails for higher analysis efficiency. Beta particles are emitted from 14C decay in biobased carbonaceous materials, which cause solvent molecules to become excited. Due to the low signal of beta particles, scintillation agents are used to amplify the signals. Various scintillation agents were characterized to analyze bio-ETBE in Fig. 1. LSC spectrums show wider and shifted to higher energy when the efficiency of scintillation agent is higher for 14C decay analysis. Optiscint HiSafe and Ultima Gold F as scintillation agents were the promising candidates because they showed higher spectrum width and energy than others as well as higher analysis efficiency of 14C in bio-ETBE. In analyzing bio-ETBE quantitatively, 10 mL Optiscint HiSafe and 10 mL of sub-octane gasoline were mixed to prepare the cocktails, in which bio-ETBE concentrations were increased from 0.0% to 20.4%. The effect of bio-ETBE concentration on LSC intensity is presented in Fig. 2. The figure shows that the intensity at the energy channel between 150 and 800 increased when higher concentration of bio-ETBE was included in the cocktail. The sample without bio-ETBE shows relatively low energy intensity, replicating the effect of the background noise. Fig. 3 shows the counts per minute (CPM) and spectral quench parameter (SQP) as a function of bio-ETBE concentration. SQP indicates a relative light production from the sample. If the values
0.25 1 2 3 4 5
Intensity
0.20
0.15
0.10
0.05
0.00 0
200
400
600
800
1000
Energy Channel [ - ] Fig. 1. Characteristics of scintillation agent for the higher analysis efficiency of beta emission (b) from 14C decay of bio-ETBE. (1) Optiscint HiSafe, (2) Ultima Gold F, (3) Opti Fluor O, (4) Insta Gel Plus, (5) Optiphase HiSafe 3.
28
S.-S. Kim et al. / Journal of Industrial and Engineering Chemistry 49 (2017) 26–29
0.12
(a)
Intensity
0.10
1 2 3
1 2
0.08 0.06
3
0.04 0.02 0.00 0.12
(b)
0.10
4 5 6
6
0.08
5
0.06
4
0.04 0.02 0.00 0
100
200
300
400
500
600
700
800
900
1000
Energy Channel [ - ] Fig. 2. Bio-ETBE analysis using low-level LSC: (a) Bio-ETBE 0.0% (1), Bio-ETBE 3.4% (2), Bio-ETBE 6.8% (3), and (b) Bio-ETBE 10.2% (4), Bio-ETBE 13.6% (5), Bio-ETBE 20.4% (6).
1000
16
f(x) = 2.66943 + 0.28315x R2 = 0.984
14
900
12
CPM
700
8 6
SQP
800
10
600
4 500
2 0
0
5
10
15
20
400 25
Bio-ETBE concentration [ % ] Fig. 3. Calibration curve depending on the concentration of bio-ETBE with sub-octane gasoline.
of SQP vary from sample to sample, quench correction is needed. CPM value, representing the number of ß emissions per minute, increased linearly from 2.4 to 8.4 with the increasing concentration of bio-ETBE from 0.0% to 20.4%, respectively. The correlation coefficient (R2) between CPM and bio-ETBE concentration was as high as 0.984, indicating that the calibration line fits the data extremely well. The first order polynomial for bio-ETBE concentration in sub-octane gasoline was obtained by least squares method and represented in Eq. (1). For comparison, the equation for bio-ethanol concentration at the same scintillation agent was shown in Eq. (2) from the literature [7].
F(x) = 2.66943 + 0.28315x
(1)
F(x) = 1.44475 + 0.39472x
(2)
The two differing equations suggest that bio-ETBE presence in the gasoline cocktail would require different calibration curves for different bio-fuel detection. The use of equation (1) in determining the concentration of bio-ethanol and vice-versa would yield inaccurate results.
S.-S. Kim et al. / Journal of Industrial and Engineering Chemistry 49 (2017) 26–29
Conclusions There is an inclination to use the previous calibration line/ curve in determining the concentration of bio-ETBE although the formula should be used as an oxygenate gasoline additive in the transportation fuel market. This would yield inaccurate result because the data used for the calibration were based on bioethanol–gasoline cocktail. This communication proposes more developed equation for bio-ETBE concentration determination in the gasoline sample using low level liquid scintillation counting (LSC). The counts per minute (CPM) value, representing the number of ß emissions per minute, increased linearly from 2.4 to 8.4 with the increasing concentration of bio-ETBE from 0.0% to 20.4%. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology
29
(NRF-2014R1A1A4A01008538), Ministry of Science, ICT & Future Planning (no. 2014R1A5A1009799) and Fundamental Research Grant Scheme (FRGS-GSP) of Malaysia. References [1] J.-S. Lee, J.-W. Kim, Cities 54 (2016) 20. [2] H. Lee, Y.M. Kim, I.G. Lee, J.K. Jeon, S.C. Jung, J.D. Chung, W.G. Choi, Y.K. Park, Korean J. Chem. Eng. 33 (12) (2016) 3299. [3] J.H. Choi, S.-S. Kim, H.V. Ly, J. Kim, H.C. Woo, Fuel 193 (2017) 159. [4] T.K. Vo, O.K. Lee, E.Y. Lee, C.H. Kim, J.-W. Seo, J. Kim, S.-S. Kim, Chem. Eng. J. 306 (2016) 763. [5] J.H. Choi, S.-S. Kim, D.J. Suh, E.-J. Jang, K.-I. Min, H.C. Woo, Korean J. Chem. Eng. 33 (9) (2016) 2691. [6] S.-S. Kim, H.V. Ly, B.H. Chun, J.-H. Ko, J. Kim, Korean J. Chem. Eng. 33 (11) (2016) 3128. [7] S.-S. Kim, J. Kim, S.C. Shin, F.A. Agblevor, Chem. Lett. 38 (2009) 850. [8] S. Yunoki, M. Saito, Bioresour. Technol. 100 (2009) 6125. [9] F.G. McCormac, Radiocarbon 34 (1992) 37. [10] M.G. Sneesby, M.O. Dade, R. Datta, T.N. Smith, Ind. Eng. Chem. Res. 36 (1997) 1855. [11] E.W. Menezes, R. Cataluña, Fuel Process. Technol. 89 (2008) 1148. [12] L.M. Aodríguez-Antóne, F. Gutíerrez-Martín, Y. Doce, Fuel 166 (2016) 73. [13] E.W. Menezes, R. Cataluña, D. Samios, R. Silva, Fuel 85 (2006) 2567. [14] Y. Nagawa, S. Yunoki, M. Saito, Radioisotopes 63 (2014) 139.