- Email: [email protected]
Hydrogen production from a solution plasma process of bio-oil
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Hydrogen production from a solution plasma process of bio-oil Heejin Lee, Young-Kwon Park* School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea
highlights Hydrogen was produced from bio-oil using solution plasma process. Guaiacol, m-cresol, and anisole were used as model bio-oil compounds. Among model bio-oils, m-cresol achieved the highest hydrogen yield. Lignin bio-oil with ethanol produced 1.91L hydrogen from solution plasma process. Carbon black doped with electrode material was formed as byproduct.
article info
abstract
Article history:
This study examined the possibility of hydrogen production using a solution plasma pro-
Received 9 September 2019
cess (SPP). The reactants were lignin model compounds and actual lignin oil. The highest
Received in revised form
amount of hydrogen was generated in SPP using m-cresol. The total amount of gas
30 October 2019
generated by the plasma reaction for 20 min using 23 g of m-cresol was 1.69 L, which
Accepted 23 November 2019
comprised of 65.51% hydrogen and 29.85% CO. Furthermore, a maximum of 1.91 L of
Available online xxx
hydrogen was generated by a reaction between pyrolysis oil and ethanol with a weight ratio of 1:1. The presence of carbon black, a reaction byproduct, was measured by Fourier
Keywords:
transform infrared spectroscopy, which revealed molybdenum trioxide peaks. It was
Solution plasma process (SPP)
confirmed that molybdenum used as an electrode was doped on carbon.
Bio-oil
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Lignin pyrolysis oil Hydrogen
Introduction The development of alternative energy sources is essential for solving the problem of greenhouse gas emissions and the depletion of fossil fuels [1,2]. Hydrogen is expected to be one of the most important energy sources of the future. It is a zeroemission fuel that does not generate carbon dioxide after combustion and has a higher heating value than conventional fossil fuels [3].
The global market for hydrogen energy is expected to expand steadily. By 2050, the hydrogen fuel market may account for approximately 20% of the global energy demand, and the hydrogen demand is expected to increase more than 10 fold [4e6]. Hydrogen technology will increase globally with market needs. Therefore, it is essential to develop new hydrogen energy technology through a range of sources suitable for new markets. Several methods of producing hydrogen energy have been studied. Hydrogen can be obtained from the electrolysis of water, steam reforming of fossil fuels, or partial oxidation, as
* Corresponding author. E-mail address: [email protected] (Y.-K. Park). https://doi.org/10.1016/j.ijhydene.2019.11.185 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
2
international journal of hydrogen energy xxx (xxxx) xxx
well as by the pyrolysis and gasification of biomass [6e8]. Therefore, hydrogen can be produced from all energy sources, such as water, petroleum, coal, natural gas, and combustible waste [9]. On the other hand, most commercial hydrogen production currently uses the steam reforming of petroleum or natural gas [1,9]. Plasma pyrolysis technology can separate carbon and hydrogen easily by the heat source. Taghvaei et al. produced hydrogen from the cracking of hydrocarbons using non ski et al. also examthermal plasma of methane [10]. Jasin ined hydrogen production using microwave plasma with methane and tetrafluoroethane [11]. Many studies on the production of hydrogen using plasma focused on gas phase plasma. Plasma reactions using a liquid source have been evaluated mainly using methanol, ethanol, and dimethyl ether with a high H/C ratio [12,13]. This reaction generates hydrogen through steam reforming and partial oxidation rather than by direct decomposition through plasma [12]. On the other hand, compared to the gas phase reactions, there have been few studies on liquid reaction plasma. Recently, research on solution plasma process (SPP) or liquid phase plasma (LPP) technology, which generates highly active species, including radicals, atoms, and ions from plasma inside a liquid, has attracted considerable attention [14,15]. SPP is mostly applied to nanoparticle synthesis, wastewater treatment, purification, and decomposition of organic compounds [14,16,17]. Chung et al. examined acetaldehyde degradation and hydrogen production by LPP using a photocatalyst in a 10% acetaldehyde solution. As a result, little hydrogen was generated [18]. On the other hand, more research on hydrogen production using SPP technology is needed because SPP technology has a high potential to be applied to a range of liquid sources. Liu et al. reported the degradation of dissolved phenol by SPP for wastewater treatment [19]. Meeprasertsagool et al. obtained short-chain hydrocarbons and hexadecane by the SPP of vegetable oil [20]. Bio-oils obtained from diverse and abundant biomass feedstock have a high value for fuels, but require expensive upgrading processes. Recently, the reforming of bio-oil into engine fuel and hydrogen energy production through gasification were reported [21,22]. Bio-oil is cleaner than biomass because many minerals and metals in biomass are concentrated in the char produced by biomass pyrolysis and are separated from bio-oil [21]. Therefore, bio-oil is a promising feedstock for biohydrogen production. Furthermore, the hydrogen production of bio-oil through SPP can generate hydrogen at room temperature without the formation of secondary pollution sources, such as nitrogen oxides. Solution plasma (SP) has a range of energy sources, such as AC power supply, DC power supply, radio frequency, microwave irradiation, and laser ablation [23,24]. The configuration of the electrode according to each energy source also varies, including glow discharge plasma, dielectric barrier discharge, dual plasma electrolysis, and high-voltage cathodic polarization [23,25]. Selecting an electrode suitable for the reaction is also important. Gold, copper, aluminum, platinum, titanium, tungsten, and carbon are used in various forms, such as rods, wires, and plates [16,26]. The shape and type of electrode act to enhance the activity, and are also used as precursors when synthesizing nanomaterials.
This study assessed the potential for hydrogen production from the SPP of bio-oil, which involved a glow discharge in lignin model compounds and actual pyrolysis oil. The novelty of this work is that hydrogen is produced from bio-oil using solution plasma for the first time. A bipolar pulsed power supply was used to generate the discharge in the oils. In addition, the use of carbon black from liquid phase plasma is also discussed.
Experimental Solution plasma experiment The solution plasma system for hydrogen production is shown in Fig. 1. A quartz reactor with a volume of 50 ml was used to generate the plasma in the liquid material. The reactor consisted of a double tube in which cooling water could flow to maintain 20 C during the plasma reaction. The electrodes were placed on both sides of the reactor. The gap between the electrodes was 0.5 mm. A molybdenum rod was used as the electrode. The length and thickness of the electrode were 150 mm and 2 mm, respectively. The electrode was wrapped with an aluminum tube insulator with the exception of a 2 mm electrode tip in the reactant. The reactive materials were guaiacol (98%, Kanto), anisole (99.7%, Sigma Aldrich), mcresol (99%, Sigma Aldrich), and actual bio-oil. The bio-oil was obtained by the fast pyrolysis of the Kraft lignin (softwood, Sigma Aldrich) at 500 C. The oil was mixed with ethanol because of its high viscosity. The amount of each reactant was 23 g. The reactant was stirred continuously during the reaction using a magnetic bar. The plasma generator with a highfrequency bipolar pulse power supply (NTI-P1000W, Nano Technology) was used. The reaction conditions were as follows: voltage 250 V, frequency 30 kHz, pulse width 4 mS, and electrode distance 0.5 mm. The inside of the reactor was purged with 10 ml/min of nitrogen using mass flow controllers, and the gas generated was collected using a tedlar gas sampling bag. The collected gas product was analyzed by gas chromatography coupled to a thermal conductivity detector (YL 6100, Young Lin Instrument Co., Ltd) with a Carboxen 1000 column.
Fig. 1 e Solution plasma system for hydrogen production.
Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
international journal of hydrogen energy xxx (xxxx) xxx
Liquid material and carbon black analysis The liquid material after the SPP was analyzed by gas chromatography-mass spectrometry (GC-MS, 7820A, Agilent Technologies) using a metal capillary column (UA-5). Carbon produced in the SP reaction was washed several times with acetone, and the remaining reactants were separated using a centrifuge (H1850, Cence). The separated carbon was dried at 110 C for 12 h and calcined at 450 C for 20 min in air. The components of carbon black were measured by Fourier transform infrared (FT-IR, Nicolet iS10, Thermo Scientific) spectroscopy.
Result and discussion SPP of model compounds The solution plasma (SP) reaction was carried out using guaiacol with two functional groups: a phenolic group and methoxy group. Fig. 2 presents the gas composition and amount of gas generated from molybdenum electrodes at reaction times of 5, 10, and 20 min for SPP of guaiacol. As shown in the figure, the gas composition did not change significantly according to the reaction time. The amount of H2, CO, and CH4 generation decreased with increasing reaction time with a concomitant increase in CO2 production, but the variation in gas composition ratio was not as large: 0.21e2.59%. The level of gas production was increased significantly from 0.12 L for 5 min to 1.27 L for 20 min. This reaction did not alter the gas composition ratio due largely to the short reaction time for 20 min or less. More hydrogen is expected to be generated by increasing reaction time. As the reaction time increases, however, the reactants react with the oxidative active species generated by the plasma reaction, which can increase CO2 production [27]. The detailed mechanism of the SP reaction has not been identified clearly [28]. This is because radicals, intermediates, molecules, and materials are determined by the dielectric constant, dielectric strength, band gap, ionization potential, and density of the liquid reactants [26], as well as by the SP reaction conditions, such as voltage and electrode distances. The possible route through which hydrogen can be produced
Fig. 2 e Gas composition and gas production amount according to the reaction time of SPP of guaiacol.
3
are as follows: (i) plasma discharge in the reactant generates primary radicals H∙ and can generate hydrogen through a combination of H∙ and H∙ [29]; and (ii) the reactant can be decomposed directly into OeCH3 or CeCH3 bonds and then into smaller fragments, CH2, CH, and H, to generate hydrogen [30]. To effectively produce hydrogen, variables, such as voltage, current, electrode distances, and discharge time, need to be adjusted according to the type and material of the electrodes. A titanium electrode was evaluated under the same conditions as molybdenum, but the plasma reaction was not maintained because the electrode melted. The SP reaction was performed using guaiacol, anisole, and m-cresol, which are lignin-derived compounds as a reactant. Each reaction time was 20 min. Fig. 3 shows the amount of gas generated in the SP reaction. The total amount of gas produced when guaiacol, m-cresol, and anisole were used was 1.27, 1.69 and 1.68 L, respectively. The SPP of cresol and anisole was higher than the SPP of guaiacol. In the SPP of m-cresol, 1.11 L of H2, which was 65.51% of the total gas volume, was generated and comprised the largest amount. The amount of hydrogen produced through the SPP of anisole was 1.01 L, which was similar to the amount of hydrogen produced by m-cresol. The proportion of CO, CH4, and CO2 in the gas was slightly higher than that of the SPP of m-cresol. After the reaction, the liquid product was analyzed by GC/MS; the results are exhibited in Fig. 4. Because of the short reaction time, the yield of the converted liquid material was up to 1.5% of the total amount of reactants. The most abundant product was benzene; other hydrocarbons, such as toluene and naphthalene, were produced. Other products, such as phenol, 1-methoxy-2-methyl benzene, 1propynyl benzene, and 2-methyl phenol were also detected. The methyl radicals generated from the plasma reaction can be decomposed to CH2, CH, and H free radicals by breaking the OeCH3 or CeCH3 bond [30]. The conversion of reactants was in the order of anisole > m-cresol > guaiacol. The amount of phenol in the reaction between anisole and cresol showed that the demethoxylation reaction was more dominant than the demethylation reaction. The results
Fig. 3 e Gas composition and gas production amount according to the reactants of SPP.
Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
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international journal of hydrogen energy xxx (xxxx) xxx
Fig. 4 e GC/MS analysis of liquid products after SPP of (a) Guaiacol, (b) m-Cresol and (c) Anisole. (1: 1,3-Butadiyne, 2: 1-Buten3-yne, 2-methyl-, 3: 1,4-Cyclohexadiene, 4: Benzene, 5: Toluene, 6: Phenylethyne, 7: Phenol, 8: Benzene, 1-methoxy-2methyl-, 9: Benzene, 1-propynyl-, 10: Phenol, 2-methyl-, 11: Naphthalene, 12: 1,3,5,7-Cyclooctatetraene, 13: Biphenylene).
confirmed that the anisole was a more active material in the SP reaction. Based on these results, the applicability of the bio-oil for the upgrading reaction and for hydrogen production through the SPP was confirmed. The bio-oil upgrading reaction of biooil via SPP is possible at room temperature and atmospheric pressure. Moreover, it is an innovative reaction without the use of a catalyst or hydrogen. Further research on the bio-oil upgrading reaction using SPP is needed.
SPP of bio-oil The SPP was evaluated using organic phase oil from lignin pyrolysis oil. The composition of the kraft lignin oil was 62.45 wt% C, 5.68 wt% H, 30.61 wt% O, 0.56 wt% N and 0.7 wt% S. An organic phase oil reacted with ethanol because of its high viscosity. Plasma reactions were carried out when the weight ratio of ethanol: bio oil was 1:0, 1:1 and 1:4, respectively. Ethanol also contains hydrogen. Fig. 5(a) compares the
Fig. 5 e Gas production amount(a) and gas composition(b) of SPP according to the mixing ratio of ethanol and bio-oil. Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
international journal of hydrogen energy xxx (xxxx) xxx
Table 1 e The amount and yield of carbon black according to the reaction time. Reactant
Reaction time (min)
Carbon black amount (g)
Carbon black Yield (wt.%)
5 10 20
0.01 0.11 0.26
0.04 0.48 1.13
Guaiacol
experimental values obtained by mixing ethanol and bio-oil with the calculated values, assuming that the amount of ethanol is proportional to the amount of gas produced. The total amount of gas generated when pure ethanol was reacted was 2.97 L; the hydrogen amount was 1.89 L. When ethanol: bio oil was reacted at 1:1 and 1:4, the total amount of gas was 3.03 L and 2.30 L, respectively, and the corresponding amount of hydrogen was 1.91 L and 1.41 L. The hydrogen evolution in the SPP of ethanol and bio-oil was higher than that of pure ethanol. This result shows that hydrogen production is possible through the SPP of bio-oil. The composition of the gas (Fig. 5(b)) differed slightly according to the ratio of ethanol and bio-oil. H2, CH4, and CO2 decreased slightly with decreasing ethanol ratio, and the composition of CO increased. The presence of oxygen-containing functional groups in the bio-oil may mean an increase in CO production with increasing biooil ratio in the reactants. Wu et al. reported a high CO yield when the rotating gliding arc plasma was reacted with rapeseed oil [31].
Carbon black Carbon black was produced as a byproduct of SPP of bio-oil. The physical and chemical properties of carbon black varied according to the reaction method and raw materials of SP. Table 1 lists the amount and yield of carbon black according to the reaction time of the SPP of guaiacol. The amount of carbon produced increased with increasing reaction time. The amount of byproduct also increased with increasing activity of SPP. Molybdenum and carbon electrodes were used in the SPP of guaiacol for 20 min. The carbon black generated was measured by FT-IR spectroscopy. Fig. 6 presents the results. Compared with the reaction of the carbon electrode, absorbance peaks at 997 and 844 cm 1 were observed in the
Fig. 6 e FT-IR spectra of carbon black generated from different electrodes.
5
molybdenum electrode. It has been confirmed that these peaks are from molybdenum trioxide. This suggests that Modoped carbon black had been synthesized. Hu et al. reported that PtAu alloy clusters were synthesized from gold and platinum wires as electrodes for SPP. These synthesized PtAu alloy nanoparticles can be used in fuel cells or Li-air batteries [32]. Because the metal electrode is doped on carbon black, it can be applied not only as an adsorbent and conductive material of a secondary battery, but also as a catalyst in many fields.
Conclusion Hydrogen energy can be produced from a range of sources and methods. This study examined hydrogen production through the SPP of bio-oil. Liquid reactants were traditional lignin monomers, such as guaiacol, cresol, anisole, and actual lignin pyrolysis oil. The results showed that the amount of gas evolution increased with increasing reaction time, and the composition ratio of each gas was similar for 5e20 min. In the SPP of the model compounds, the highest hydrogen production was observed in m-cresol. Moreover, hydrogen was generated in the SPP of lignin oil. When pyrolysis oil and ethanol were mixed at 4:1 and reacted for 20 min, 1.41 L of hydrogen was produced. In addition, the use of carbon black produced through the SPP of bio-oil was evaluated. These results highlight the potential of the SPP of bio-oil for hydrogen production. Further work is needed to determine the relationship between the properties of the pyrolysis oil and the hydrogen yield and the reaction route.
Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1B07049487).
references
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Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Hydrogen production from a solution plasma process of bio-oil Heejin Lee, Young-Kwon Park* School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea
highlights Hydrogen was produced from bio-oil using solution plasma process. Guaiacol, m-cresol, and anisole were used as model bio-oil compounds. Among model bio-oils, m-cresol achieved the highest hydrogen yield. Lignin bio-oil with ethanol produced 1.91L hydrogen from solution plasma process. Carbon black doped with electrode material was formed as byproduct.
article info
abstract
Article history:
This study examined the possibility of hydrogen production using a solution plasma pro-
Received 9 September 2019
cess (SPP). The reactants were lignin model compounds and actual lignin oil. The highest
Received in revised form
amount of hydrogen was generated in SPP using m-cresol. The total amount of gas
30 October 2019
generated by the plasma reaction for 20 min using 23 g of m-cresol was 1.69 L, which
Accepted 23 November 2019
comprised of 65.51% hydrogen and 29.85% CO. Furthermore, a maximum of 1.91 L of
Available online xxx
hydrogen was generated by a reaction between pyrolysis oil and ethanol with a weight ratio of 1:1. The presence of carbon black, a reaction byproduct, was measured by Fourier
Keywords:
transform infrared spectroscopy, which revealed molybdenum trioxide peaks. It was
Solution plasma process (SPP)
confirmed that molybdenum used as an electrode was doped on carbon.
Bio-oil
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Lignin pyrolysis oil Hydrogen
Introduction The development of alternative energy sources is essential for solving the problem of greenhouse gas emissions and the depletion of fossil fuels [1,2]. Hydrogen is expected to be one of the most important energy sources of the future. It is a zeroemission fuel that does not generate carbon dioxide after combustion and has a higher heating value than conventional fossil fuels [3].
The global market for hydrogen energy is expected to expand steadily. By 2050, the hydrogen fuel market may account for approximately 20% of the global energy demand, and the hydrogen demand is expected to increase more than 10 fold [4e6]. Hydrogen technology will increase globally with market needs. Therefore, it is essential to develop new hydrogen energy technology through a range of sources suitable for new markets. Several methods of producing hydrogen energy have been studied. Hydrogen can be obtained from the electrolysis of water, steam reforming of fossil fuels, or partial oxidation, as
* Corresponding author. E-mail address: [email protected] (Y.-K. Park). https://doi.org/10.1016/j.ijhydene.2019.11.185 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
2
international journal of hydrogen energy xxx (xxxx) xxx
well as by the pyrolysis and gasification of biomass [6e8]. Therefore, hydrogen can be produced from all energy sources, such as water, petroleum, coal, natural gas, and combustible waste [9]. On the other hand, most commercial hydrogen production currently uses the steam reforming of petroleum or natural gas [1,9]. Plasma pyrolysis technology can separate carbon and hydrogen easily by the heat source. Taghvaei et al. produced hydrogen from the cracking of hydrocarbons using non ski et al. also examthermal plasma of methane [10]. Jasin ined hydrogen production using microwave plasma with methane and tetrafluoroethane [11]. Many studies on the production of hydrogen using plasma focused on gas phase plasma. Plasma reactions using a liquid source have been evaluated mainly using methanol, ethanol, and dimethyl ether with a high H/C ratio [12,13]. This reaction generates hydrogen through steam reforming and partial oxidation rather than by direct decomposition through plasma [12]. On the other hand, compared to the gas phase reactions, there have been few studies on liquid reaction plasma. Recently, research on solution plasma process (SPP) or liquid phase plasma (LPP) technology, which generates highly active species, including radicals, atoms, and ions from plasma inside a liquid, has attracted considerable attention [14,15]. SPP is mostly applied to nanoparticle synthesis, wastewater treatment, purification, and decomposition of organic compounds [14,16,17]. Chung et al. examined acetaldehyde degradation and hydrogen production by LPP using a photocatalyst in a 10% acetaldehyde solution. As a result, little hydrogen was generated [18]. On the other hand, more research on hydrogen production using SPP technology is needed because SPP technology has a high potential to be applied to a range of liquid sources. Liu et al. reported the degradation of dissolved phenol by SPP for wastewater treatment [19]. Meeprasertsagool et al. obtained short-chain hydrocarbons and hexadecane by the SPP of vegetable oil [20]. Bio-oils obtained from diverse and abundant biomass feedstock have a high value for fuels, but require expensive upgrading processes. Recently, the reforming of bio-oil into engine fuel and hydrogen energy production through gasification were reported [21,22]. Bio-oil is cleaner than biomass because many minerals and metals in biomass are concentrated in the char produced by biomass pyrolysis and are separated from bio-oil [21]. Therefore, bio-oil is a promising feedstock for biohydrogen production. Furthermore, the hydrogen production of bio-oil through SPP can generate hydrogen at room temperature without the formation of secondary pollution sources, such as nitrogen oxides. Solution plasma (SP) has a range of energy sources, such as AC power supply, DC power supply, radio frequency, microwave irradiation, and laser ablation [23,24]. The configuration of the electrode according to each energy source also varies, including glow discharge plasma, dielectric barrier discharge, dual plasma electrolysis, and high-voltage cathodic polarization [23,25]. Selecting an electrode suitable for the reaction is also important. Gold, copper, aluminum, platinum, titanium, tungsten, and carbon are used in various forms, such as rods, wires, and plates [16,26]. The shape and type of electrode act to enhance the activity, and are also used as precursors when synthesizing nanomaterials.
This study assessed the potential for hydrogen production from the SPP of bio-oil, which involved a glow discharge in lignin model compounds and actual pyrolysis oil. The novelty of this work is that hydrogen is produced from bio-oil using solution plasma for the first time. A bipolar pulsed power supply was used to generate the discharge in the oils. In addition, the use of carbon black from liquid phase plasma is also discussed.
Experimental Solution plasma experiment The solution plasma system for hydrogen production is shown in Fig. 1. A quartz reactor with a volume of 50 ml was used to generate the plasma in the liquid material. The reactor consisted of a double tube in which cooling water could flow to maintain 20 C during the plasma reaction. The electrodes were placed on both sides of the reactor. The gap between the electrodes was 0.5 mm. A molybdenum rod was used as the electrode. The length and thickness of the electrode were 150 mm and 2 mm, respectively. The electrode was wrapped with an aluminum tube insulator with the exception of a 2 mm electrode tip in the reactant. The reactive materials were guaiacol (98%, Kanto), anisole (99.7%, Sigma Aldrich), mcresol (99%, Sigma Aldrich), and actual bio-oil. The bio-oil was obtained by the fast pyrolysis of the Kraft lignin (softwood, Sigma Aldrich) at 500 C. The oil was mixed with ethanol because of its high viscosity. The amount of each reactant was 23 g. The reactant was stirred continuously during the reaction using a magnetic bar. The plasma generator with a highfrequency bipolar pulse power supply (NTI-P1000W, Nano Technology) was used. The reaction conditions were as follows: voltage 250 V, frequency 30 kHz, pulse width 4 mS, and electrode distance 0.5 mm. The inside of the reactor was purged with 10 ml/min of nitrogen using mass flow controllers, and the gas generated was collected using a tedlar gas sampling bag. The collected gas product was analyzed by gas chromatography coupled to a thermal conductivity detector (YL 6100, Young Lin Instrument Co., Ltd) with a Carboxen 1000 column.
Fig. 1 e Solution plasma system for hydrogen production.
Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
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Liquid material and carbon black analysis The liquid material after the SPP was analyzed by gas chromatography-mass spectrometry (GC-MS, 7820A, Agilent Technologies) using a metal capillary column (UA-5). Carbon produced in the SP reaction was washed several times with acetone, and the remaining reactants were separated using a centrifuge (H1850, Cence). The separated carbon was dried at 110 C for 12 h and calcined at 450 C for 20 min in air. The components of carbon black were measured by Fourier transform infrared (FT-IR, Nicolet iS10, Thermo Scientific) spectroscopy.
Result and discussion SPP of model compounds The solution plasma (SP) reaction was carried out using guaiacol with two functional groups: a phenolic group and methoxy group. Fig. 2 presents the gas composition and amount of gas generated from molybdenum electrodes at reaction times of 5, 10, and 20 min for SPP of guaiacol. As shown in the figure, the gas composition did not change significantly according to the reaction time. The amount of H2, CO, and CH4 generation decreased with increasing reaction time with a concomitant increase in CO2 production, but the variation in gas composition ratio was not as large: 0.21e2.59%. The level of gas production was increased significantly from 0.12 L for 5 min to 1.27 L for 20 min. This reaction did not alter the gas composition ratio due largely to the short reaction time for 20 min or less. More hydrogen is expected to be generated by increasing reaction time. As the reaction time increases, however, the reactants react with the oxidative active species generated by the plasma reaction, which can increase CO2 production [27]. The detailed mechanism of the SP reaction has not been identified clearly [28]. This is because radicals, intermediates, molecules, and materials are determined by the dielectric constant, dielectric strength, band gap, ionization potential, and density of the liquid reactants [26], as well as by the SP reaction conditions, such as voltage and electrode distances. The possible route through which hydrogen can be produced
Fig. 2 e Gas composition and gas production amount according to the reaction time of SPP of guaiacol.
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are as follows: (i) plasma discharge in the reactant generates primary radicals H∙ and can generate hydrogen through a combination of H∙ and H∙ [29]; and (ii) the reactant can be decomposed directly into OeCH3 or CeCH3 bonds and then into smaller fragments, CH2, CH, and H, to generate hydrogen [30]. To effectively produce hydrogen, variables, such as voltage, current, electrode distances, and discharge time, need to be adjusted according to the type and material of the electrodes. A titanium electrode was evaluated under the same conditions as molybdenum, but the plasma reaction was not maintained because the electrode melted. The SP reaction was performed using guaiacol, anisole, and m-cresol, which are lignin-derived compounds as a reactant. Each reaction time was 20 min. Fig. 3 shows the amount of gas generated in the SP reaction. The total amount of gas produced when guaiacol, m-cresol, and anisole were used was 1.27, 1.69 and 1.68 L, respectively. The SPP of cresol and anisole was higher than the SPP of guaiacol. In the SPP of m-cresol, 1.11 L of H2, which was 65.51% of the total gas volume, was generated and comprised the largest amount. The amount of hydrogen produced through the SPP of anisole was 1.01 L, which was similar to the amount of hydrogen produced by m-cresol. The proportion of CO, CH4, and CO2 in the gas was slightly higher than that of the SPP of m-cresol. After the reaction, the liquid product was analyzed by GC/MS; the results are exhibited in Fig. 4. Because of the short reaction time, the yield of the converted liquid material was up to 1.5% of the total amount of reactants. The most abundant product was benzene; other hydrocarbons, such as toluene and naphthalene, were produced. Other products, such as phenol, 1-methoxy-2-methyl benzene, 1propynyl benzene, and 2-methyl phenol were also detected. The methyl radicals generated from the plasma reaction can be decomposed to CH2, CH, and H free radicals by breaking the OeCH3 or CeCH3 bond [30]. The conversion of reactants was in the order of anisole > m-cresol > guaiacol. The amount of phenol in the reaction between anisole and cresol showed that the demethoxylation reaction was more dominant than the demethylation reaction. The results
Fig. 3 e Gas composition and gas production amount according to the reactants of SPP.
Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
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Fig. 4 e GC/MS analysis of liquid products after SPP of (a) Guaiacol, (b) m-Cresol and (c) Anisole. (1: 1,3-Butadiyne, 2: 1-Buten3-yne, 2-methyl-, 3: 1,4-Cyclohexadiene, 4: Benzene, 5: Toluene, 6: Phenylethyne, 7: Phenol, 8: Benzene, 1-methoxy-2methyl-, 9: Benzene, 1-propynyl-, 10: Phenol, 2-methyl-, 11: Naphthalene, 12: 1,3,5,7-Cyclooctatetraene, 13: Biphenylene).
confirmed that the anisole was a more active material in the SP reaction. Based on these results, the applicability of the bio-oil for the upgrading reaction and for hydrogen production through the SPP was confirmed. The bio-oil upgrading reaction of biooil via SPP is possible at room temperature and atmospheric pressure. Moreover, it is an innovative reaction without the use of a catalyst or hydrogen. Further research on the bio-oil upgrading reaction using SPP is needed.
SPP of bio-oil The SPP was evaluated using organic phase oil from lignin pyrolysis oil. The composition of the kraft lignin oil was 62.45 wt% C, 5.68 wt% H, 30.61 wt% O, 0.56 wt% N and 0.7 wt% S. An organic phase oil reacted with ethanol because of its high viscosity. Plasma reactions were carried out when the weight ratio of ethanol: bio oil was 1:0, 1:1 and 1:4, respectively. Ethanol also contains hydrogen. Fig. 5(a) compares the
Fig. 5 e Gas production amount(a) and gas composition(b) of SPP according to the mixing ratio of ethanol and bio-oil. Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185
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Table 1 e The amount and yield of carbon black according to the reaction time. Reactant
Reaction time (min)
Carbon black amount (g)
Carbon black Yield (wt.%)
5 10 20
0.01 0.11 0.26
0.04 0.48 1.13
Guaiacol
experimental values obtained by mixing ethanol and bio-oil with the calculated values, assuming that the amount of ethanol is proportional to the amount of gas produced. The total amount of gas generated when pure ethanol was reacted was 2.97 L; the hydrogen amount was 1.89 L. When ethanol: bio oil was reacted at 1:1 and 1:4, the total amount of gas was 3.03 L and 2.30 L, respectively, and the corresponding amount of hydrogen was 1.91 L and 1.41 L. The hydrogen evolution in the SPP of ethanol and bio-oil was higher than that of pure ethanol. This result shows that hydrogen production is possible through the SPP of bio-oil. The composition of the gas (Fig. 5(b)) differed slightly according to the ratio of ethanol and bio-oil. H2, CH4, and CO2 decreased slightly with decreasing ethanol ratio, and the composition of CO increased. The presence of oxygen-containing functional groups in the bio-oil may mean an increase in CO production with increasing biooil ratio in the reactants. Wu et al. reported a high CO yield when the rotating gliding arc plasma was reacted with rapeseed oil [31].
Carbon black Carbon black was produced as a byproduct of SPP of bio-oil. The physical and chemical properties of carbon black varied according to the reaction method and raw materials of SP. Table 1 lists the amount and yield of carbon black according to the reaction time of the SPP of guaiacol. The amount of carbon produced increased with increasing reaction time. The amount of byproduct also increased with increasing activity of SPP. Molybdenum and carbon electrodes were used in the SPP of guaiacol for 20 min. The carbon black generated was measured by FT-IR spectroscopy. Fig. 6 presents the results. Compared with the reaction of the carbon electrode, absorbance peaks at 997 and 844 cm 1 were observed in the
Fig. 6 e FT-IR spectra of carbon black generated from different electrodes.
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molybdenum electrode. It has been confirmed that these peaks are from molybdenum trioxide. This suggests that Modoped carbon black had been synthesized. Hu et al. reported that PtAu alloy clusters were synthesized from gold and platinum wires as electrodes for SPP. These synthesized PtAu alloy nanoparticles can be used in fuel cells or Li-air batteries [32]. Because the metal electrode is doped on carbon black, it can be applied not only as an adsorbent and conductive material of a secondary battery, but also as a catalyst in many fields.
Conclusion Hydrogen energy can be produced from a range of sources and methods. This study examined hydrogen production through the SPP of bio-oil. Liquid reactants were traditional lignin monomers, such as guaiacol, cresol, anisole, and actual lignin pyrolysis oil. The results showed that the amount of gas evolution increased with increasing reaction time, and the composition ratio of each gas was similar for 5e20 min. In the SPP of the model compounds, the highest hydrogen production was observed in m-cresol. Moreover, hydrogen was generated in the SPP of lignin oil. When pyrolysis oil and ethanol were mixed at 4:1 and reacted for 20 min, 1.41 L of hydrogen was produced. In addition, the use of carbon black produced through the SPP of bio-oil was evaluated. These results highlight the potential of the SPP of bio-oil for hydrogen production. Further work is needed to determine the relationship between the properties of the pyrolysis oil and the hydrogen yield and the reaction route.
Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1B07049487).
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Please cite this article as: Lee H, Park Y-K, Hydrogen production from a solution plasma process of bio-oil, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.185