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Effect of introducing a steam pipe to n-dodecane decomposition by in-liquid plasma for hydrogen production
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 6 2 4 8 e1 6 2 5 6
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Effect of introducing a steam pipe to n-dodecane decomposition by in-liquid plasma for hydrogen production Ryoya Shiraishi*, Shinfuku Nomura, Hiromichi Toyota, Shinobu Mukasa, Yuki Amano Graduate School of Science and Engineering, Ehime University, 790-8577, Bunkyocho-3, Matsuyama, Japan
article info
abstract
Article history:
A method has been developed to improve the hydrogen production efficiency (HPE) by in-
Received 18 March 2019
liquid plasma n-dodecane decomposition. A thin steam pipe is placed over the plasma
Received in revised form
electrode to recover the thermal energy emitted from the plasma to its surroundings. The
26 April 2019
steam generated by this energy is supplied to the vaporized n-dodecane around the edge
Accepted 29 April 2019
of the plasma to cause a steam reforming reaction (SRR). Water pyrolysis is suppressed
Available online 23 May 2019
by not supplying the steam directly to the plasma. A large amount of CO and a small amount of CO2 were detected in the produced gas. This indicates that a strong SRR has
Keywords:
occurred. The HPE obtained by this method is 0.28 Nm3/kWh, which is two times greater
Hydrogen
than those obtained by previous methods, and similar to or greater than the yield of
n-dodecane
water electrolysis. This result is a major advance in the field of plasma heavy hydro-
In-liquid plasma
carbon decomposition aimed at hydrogen production. HPE is expected to be further
Steam reforming
improved by simply increasing the input power, due to synergy between the heat recovery effect of the steam pipe and the bubble stabilization effect. This indicates that this method has a high potential. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has high energy density (143 kJ/g) and does not emit any greenhouse gases when burned, so it is drawing much attention as a next generation clean energy source [1e4]. Hydrogen is a secondary energy form, which means that it requires energy to produce it. Therefore, much research has been invested in reducing the energy costs for hydrogen production [5e8]. From this viewpoint, decomposition of waste oil is seen as an advantageous method, because it can
simultaneously accomplish two goals: waste oil disposal and hydrogen production. Waste oil primarily consists of heavy hydrocarbons, such as n-dodecane. On other hand, methane steam reforming (SR) is the most common method for industrial hydrogen production and has been researched in detail [9e12]. Recently, some papers reported heavy hydrocarbon decomposition by SR aimed at hydrogen production [13e15]. These studies were done to investigate in detail effects of elements such as the steam carbon ratio (S/C ratio), catalyst bed temperature, and hourly space velocity of the
* Corresponding author. E-mail address: [email protected] (R. Shiraishi). https://doi.org/10.1016/j.ijhydene.2019.04.270 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
19 1
0.13 0.10 0.14
0.28 C12H26 þ H2O
0.01*
25
20 17 14 0 0 0 0 0 8 11 15 0 0 0 0 4.6 70 71 70 35 25 74 82.5 79.3
55
2 1 1 65 75 26 17.5 16.1
Pascuzzi et al., 2016 [35] Randolph U.S. DOE, 2013 [39] Dors et al., 2012 [19] Jasinski et al., 2014 [18] Du et al., 2012 [17] Henriques et al., 2011 [20] Jasinski et al., 2015 [18] Takeda et al., 2017 [15] Martin et al., 2015 [14] Vita et al., 2016 [13] Song et al., 2019 [21] Ganieva et al., 2016 [22] Nomura et al., 2009 [24] Mochtar et al., 2017 [23] 0 0 0 100 0.20e0.23 0.67 0.056 0.70 1.12 0.015 0.25
H2O CH4 þ H2O CH4 þ CO2 CH4 þ H2O C2H5OH þ H2O CH3OH þ Ar C2H5OH þ N2 C12H26 þ H2O Diesel þ H2O C12H26 þ H2O C10H22 Fuel oil C12H26 C12H26 C12H26 þ H2O 5. Our research group In-liquid plasma (with steam reforming)/n-dodecane
4. Plasma/heavy hydrocarbons
3. Steam reforming/heavy hydrocarbons
2. Plasma/methane or alcohols
Water electrolysis Industrial steam reforming Dielectric barrier discharge Microwave plasma with catalyst Laval nozzle arc Microwave (2.45 GHz) plasma Microwave (915 MHz) plasma Steam reforming Steam reforming Steam reforming Dielectric barrier discharge Arc discharge In-liquid microwave (2.45 GHz) plasma In- liquid microwave (2.45 GHz) plasma In-liquid microwave (2.45 GHz) plasma with steam reforming In-liquid radio frequency plasma (27.12 MHz) with steam reforming 1. Commercialized
H2
CO
CO2
Others (Mainly Light CxHy)
Reference Contents of produced gas (%)
HPE (Nm3/kWh) Initial composition Production method Classification
Table 1 e Comparison of various hydrogen production methods with our method. * Not mentioned directly, calculated from experimental conditions and results shown in the original article.
feedstock. The hydrogen composition rate of gas produced in these studies is approximately 70%, which is much higher than that of simple pyrolysis [16]. However, the hydrogen production efficiency (HPE) (g/kWh or Nm3/kWh) for a real machine has not been mentioned explicitly. On the other hand, some methods using plasma for light hydrocarbon or alcohol decomposition for hydrogen production, have achieved high HPE [17e20]. Mizeraczyk et al. summarized the status of the field in an excellent review [18]. Whereas the SR method requires a high temperature of approximately 1000 K and catalysts, the plasma methods do not necessarily require high temperatures. However, there are few studies about heavy hydrocarbon decomposition by plasma methods [21,22]. Moreover, in these examples, the concentration of hydrogen contained in the production gas is 25e35%, and the remainder is light hydrocarbons such as acetylene. In other words, these methods are primarily causing decomposition of heavy hydrocarbons rather than hydrogen production. We have been researching n-dodecane decomposition by in-liquid plasma, aiming for hydrogen production from waste oil [23,24]. In this method, the concentration of hydrogen contained in the production gas is approximately 80%. Furthermore, the processes of vaporization and mist formation are not required by the in-liquid plasma method. Thus, n-dodecane can be decomposed by direct discharge in the liquid stored in a reactor container [25e30]. Therefore, the apparatus is small and simple enough, suitable for a distributed hydrogen production device [24,31e34]. Someday, it may be possible to commercialize a device that decomposes used-cooking oil while refilling the household fuel cell with hydrogen. However, at present, the HPE for ndodecane decomposition is around 0.10 Nm3/kWh [23,24]. This is approximately half that of water electrolysis [35,36]. Therefore, improvement of HPE is necessary. The results of the various studies about hydrogen production introduced above are described in detail in the “Results and Discussion” section and compared with the results of this study (see Table 1). Recently, A. A. Mochtar et al. reported that HPE was improved 1.4 times in n-dodecane decomposition by incorporating the steam reforming reaction (SRR) into an in-liquid plasma reaction field [23]. This was accomplished by generating plasma at the tip of a pipe-type electrode inserted into a decompressed reactor vessel and supplying steam from the lower side of the electrode into the plasma reaction field. It seems that the steam reacts with the vaporized n-dodecane around the plasma. Then, SRR occurs in addition to the inliquid plasma reaction. However, when the steam is supplied into the plasma reaction field directly, a portion of the plasma energy is wasted on water pyrolysis, causing deterioration in HPE. When pure water is directly decomposed by the in-liquid plasma, HPE is very low, 0.02 Nm3/kWh [37]. Furthermore, to vaporize water and transport it to the reaction field, a decompression device is required, resulting in the need for electrical power to drive such a decompression device, further deteriorating the HPE. Furthermore, a basic problem is that a considerable amount of heat energy is wasted due to heat radiation from the plasma to the surroundings. Therefore, to improve the HPE, it is necessary to find solutions to the following two problems:
Present study
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(1) Recover the surplus thermal energy around the edge of the plasma beyond the main plasma and use it for water vaporization and SRR. (2) Conduct reactions at atmospheric pressure, eliminating the need for a decompression device. The purpose of the present research is to resolve these problems and improve HPE by an in-liquid plasma method assisted by SRR. To achieve these goals, a thin pipe was fixed over the plasma electrode, and any surplus energy around the plasma was recovered by this pipe. Furthermore, experiments were conducted at atmospheric pressure.
Experimental method The schematic image of the experimental setup is shown in Fig. 1. A tungsten electrode, 4 mm in diameter, is inserted from the bottom of the reactor vessel and a nickel steam pipe is fixed over the electrode. An electric current of 27.12 MHz from 200 to 500 W is supplied via a matching box and the plasma is generated between the tip of the electrode and the pipe. Once the plasma is generated, the surrounding liquid is continuously vaporized by the plasma heat and bubbles are generated. Because the pipe is very thin, it is completely enveloped in one of these bubbles, insulating it from the surrounding liquid, which also causes it to rise to a very high temperature. For reference, an image of a discharging thin bar in liquid methanol is shown in Fig. 2, where the bar is completely enveloped by the bubble. (This image should ideally be of a discharge in n-dodecane. However, when discharging in n-dodecane, it becomes muddied so it is difficult to
Fig. 2 e Image of a discharging thin bar in liquid methanol.
obtain a clear image.) The very high pipe temperature vaporizes the water when it enters the pipe. This high temperature steam is then supplied to the reaction field via a 1-mm diameter supply hole in the pipe. In this way, thermal energy around the plasma is recovered and put back to use. Therefore, preheating the water is not necessary and room temperature water can be used in this system. Because the steam pipe is easily enveloped by bubbles, the thinner the pipe is the better. However, a too thin pipe melts when the input power is high. Consequently, a steam pipe 3 mm in outer diameter, 2 mm in inner diameter is used when the input power is from 0 to 300 W and 5 mm in outer diameter, 3 mm in inner diameter when the input power is from 350 to 500 W.
Fig. 1 e Schematic of the experimental set up.
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In our research we used n-dodecane, which is a common substance found in waste oil. Because the in-liquid plasma is generated inside bubbles in the liquid, it is difficult to generate it immediately in a substance with a high boiling point. Because the boiling point of ndodecane is high, 216 C, at atmospheric pressure [38], we need some preparation steps. First, the electrode and pipe are immersed in water and all valves except V1 and V2 are closed. The reactor vessel is decompressed by an aspirator to 0.02 MPa, allowing for plasma generation. The aspirator is then turned off and V1 is closed. V3 is then opened slowly, which returns the inner pressure of the reactor vessel to atmospheric pressure. The water supply pump is then turned on and V4 and V5 are opened, allowing water into the pipe. V4 is a flow control valve. In this way, steam is supplied to the reaction field. After the stability of the plasma is confirmed, V6 is opened and n-dodecane is supplied from syringe 1. Because ndodecane has lower specific gravity than water, it floats on top of the water. This condition is maintained for approximately 1 min to raise the temperature of the n-dodecane. Then, V7 is opened and the water is drawn out by syringe 2, lowering the water level. If the capacity of syringe 2 is not sufficient, V8 may be opened and to further drain the water away. Syringe 2 plays the role of level adjuster. By using the syringe 2, the level of the water/n-dodecane interface can be maintained at that of the plasma until the plasma stabilizes. If the plasma is extinguished, it can be rekindled by raising water level again. After the stability of plasma is confirmed at the interface level, the plasma is completely immersed in ndodecane, and continuous decomposition is conducted. Gas generated by the n-dodecane decomposition is collected by water replacement, and the time it takes to collect 150 mL of the gas is measured. The gas composition rate is measured by gas chromatography. The temperature distribution within the pipe is measured by inserting a K-type thermocouple.
Results and discussion Confirmation of high temperature steam supply The temperature distribution inside the pipe as measured by the thermocouple is shown in Fig. 3. The steam supply hole faced upward when the measurement was taken. The flow rate is 1.5 mL/s and the input power is 300 W.
Fig. 3 e Temperature distribution inside the pipe.
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The temperature is around 100 C at a distance of 5e20 mm from the plasma center. This temperature indicates thermal equilibrium between the water, pipe, and surrounding ndodecane. Nearer to the plasma center, the temperature rises dramatically. This is because, as mentioned before, the pipe is insulated from the liquid n-dodecane by the bubbles. Therefore, we confirm that the water is vaporized, and high temperature steam is supplied into the reaction field by installing the thin steam pipe over the plasma electrode.
Flow rate and hole direction effect The effect the flow rate and hole direction has on HPE is shown in Fig. 4. Because the electrical power required to drive the water supply pump is a mere 3 W, it is deemed negligible compared to the input power of 300 W. When no water is supplied, HPE is 0.09 Nm3/kWh. When water is supplied at a rate of 1.5 mL/s, HPE increases 1.7 times, reaching 0.16 Nm3/ kWh. This confirms that a high temperature steam supply improves HPE. We observe no effect of the hole direction on HPE at 1.5 mL/s. At 2.7 mL/s, HPE is maintained when the hole direction faces upward, but when the hole direction is downward, while production still occurs, it decreases to 0.10 Nm3/kWh. This shows that SRR occurs regardless of the hole direction, because the pipe is completely enveloped by the bubble, so the steam and the surrounding gas react regardless of the direction in which the steam is injected. However, when the hole direction is downward, the steam is injected into the plasma reaction field directly, resulting in much of the energy being consumed for water pyrolysis. Consequently, HPE is smaller than 2.7 mL/s when the hole is in the downward direction.
Maximum HPE and its evaluation With the hole faced upward and the water flow rate at 1.5 mL/ s, the power input was increased to 500 W. Under these conditions, HPE reached its maximum value of 0.28 Nm3/kWh. To evaluate this value, we compared it with other research results, as shown in Table 1. Water electrolysis and methane SR have been already commercialized and those HPE are 0.20e0.23 Nm3/kWh in water electrolysis [35] and 0.67 Nm3/ kWh in methane SR [39]. To develop a more efficient method
Fig. 4 e Flow rate and hole direction effect on HPE.
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for hydrogen production, various studies have been conducted. In methane or alcohol decomposition by the plasma method, Jasinski et al. [18] and Du et al. [17] obtained outstandingly high HPE. Especially, the HPE obtained by Du et al. is 1.12 Nm3/kWh, drastically exceeding the 0.67 Nm3/ kWh of methane SR. Hydrogen production from heavy hydrocarbons is still in the developing stages, compared to that from methane or alcohol decomposition. Recently, some studies about hydrogen production from heavy hydrocarbons by SR have been reported [13e15]. These studies investigated in detail the effects of the steam carbon ratio (S/C ratio), catalyst bed temperature, and hourly space velocity of the feedstock. The gas composition in each study is almost same, 70% of hydrogen, 29% of CO and CO2, and 1% of light hydrocarbons such as CH4. Even though it is reported that the energy conversion ratio is 40e50% in these studies, HPE (g/kWh or Nm3/kWh) as a hydrogen production device is not mentioned clearly. As mentioned above, in the case of methane or alcohol decomposition, the plasma method achieves a higher HPE than has been obtained by industrial SR. However, there are few studies about heavy hydrocarbons decomposition by the plasma method. Song et al. reported the dielectric barrier discharge method [21] and Ganieva et al. reported the arc discharge method [22]. However, the concentration of hydrogen contained in the production gas is 25e35%, and the remainder light hydrocarbons such as acetylene. In other words, these methods primarily cause decomposition of heavy hydrocarbons rather than hydrogen production. HPE is very low (around 0.01 Nm3/kWh) or not mentioned. Thus, we have conducted pioneering research on plasma decomposition of n-dodecane for hydrogen production [23,24]. The hydrogen concentration obtained by our inliquid plasma method is approximately 80%, which is much greater than that of other studies about heavy hydrocarbon plasma decomposition. In addition, in heavy hydrocarbon plasma decomposition, only our study mentions HPE clearly. HPE was 0.10e0.14 Nm3/kWh in our previous study. In the present study, we reach 0.28 Nm3/kWh by adding SRR to the in-liquid plasma reaction. This value is twice that of the previous researches. This value is still low compared to those of methane or alcohol decomposition, but this result is a major advance in the field of plasma heavy hydrocarbon decomposition aimed at hydrogen production. It is approaching the level required for practical use. To discuss the results obtained in the present study in more detail, we compared them with those of our previous studies. The experimental conditions and results for the ndodecane decomposition for each study are summarized in Table 2. All these studies are about n-dodecane decomposition by the in-liquid plasma method. In Ref. 24, the experiment was conducted by the pure in-liquid plasma method at atmospheric pressure and yielded 0.17 Nm3/kWh of gas production efficiency (GPE) and 0.13 Nm3/kWh of HPE. In this case, no CO or CO2 were detected. In Ref. 23, the experiments were conducted by a pure in-liquid plasma or in-liquid plasma with SR at the decompressed condition. In this case, HPE is 0.10 Nm3/kWh. In Ref. 23 with steam, HPE increased to 0.14 Nm3/kWh, accompanied by a little CO generation. The reaction formulas, reaction enthalpy, and HPE calculated from the reaction enthalpy, of n-dodecane SR are shown in R1 and R2.
C12H26 þ 12H2O / 12CO þ 25H2 (DН298Κ ¼ þ1867 kJ/mol, R1 HPE ¼ 75 kJ/mol-H2) 12CO þ 12H2O / 12CO2 þ 12H2 (DН298Κ ¼ 495 kJ/mol, HPE ¼ 41.25 kJ /mol-H2)
R2
From these formulas, the generation of CO and CO2 indicates the occurrence of SR, which improved HPE. However, in Ref. 23, the amount of CO is small; HPE is almost the same as that of Ref. 24. As mentioned before, because the steam was supplied to the plasma reaction field directly, it seems that water pyrolysis occurred in Ref. 23, deteriorating HPE. In Refs. 23 and 24, the surplus thermal energy emitted from the plasma is wasted to the surroundings, further deteriorating HPE. In contrast, in the present study, this surplus energy was recovered by the pipe and used for steam formation at atmospheric pressure. Then, this steam was supplied to the reaction field indirectly (to the vaporized n-dodecane rather than to the main plasma). As a result, HPE was drastically improved and accompanied by a large amount of CO and a little CO2 generation. This indicates that HPE was improved by a strong SRR. The maximum GPE 0.50 Nm3/kWh is three times greater than those of previous studies, and the maximum HPE of 0.28 Nm3/kWh is twice as big as those. This result proves that the method suggested in the present study is valid.
Discussion of the composition of the produced gas However, even in the present study, water pyrolysis is not suppressed completely. The hydrogen composition rate in the present study is 55%, which is the lowest value in Table 2. From R1, assuming that all of the 25% CO is generated by SR, we conclude that almost all of the produced hydrogen is produced by SR. However, because SR only occurs by plasma decomposition and gasification of n-dodecane in the present study, a considerable amount of hydrogen should be made by plasma decomposition. Therefore, the composition ratio of CO to hydrogen is obviously large. The oxygen for the CO generation is only supplied from water decomposition under these experimental conditions. Therefore, a larger amount of CO indicates decomposition of more water than necessary. The expected reactions are water pyrolysis (R3), solid carbon burning (R4), and solid carbon SR (R5). In addition, the reaction formula of the n-dodecane ideal plasma decomposition is shown in R6. HPE calculated from the reaction enthalpy is also shown in each formula. H2O / H2 þ 1/2O2 (DН298Κ ¼ þ286 kJ/mol, HPE ¼ 286 kJ/molR3 H2) C þ 1/2O2 / CO (DН298Κ ¼ 111 kJ/mol)
R4
C þ H2 O / H 2 þ CO (DН298Κ ¼ þ176 kJ/mol, HPE ¼ 176 kJ/ R5 mol-H 2 ) C 12 H 26 / 13H 2 þ 12C (DН298Κ ¼ þ352 kJ/mol, HPE ¼ 27 kJ/ R6 mol-H 2 )
10.5
3.9
Mochtar et al., 2017 [23] 3.7 10.7
Nomura et al., 2009 [24] 2 20
6 9
0 0 82.5 0.10 0.12 300 No steam supply (pure in-liquid plasma method)
0 4.6 79.3 0.14 0.17 10.0 From the pipe type plasma electrode
225
101.3 No steam supply (pure in-liquid plasma method)
750
0.17
0.13
74
0
0
4 19 2 24 1.7 16.1 3.1 17.5 1 25 55 0.28 0.50 500 101.3 From the steam pipe fixed over the plasma electrode
C2H4 C2H2 CH4 CO2 CO H2
Contents of produced gas (%)
Max. HPE (Nm3/kWh) Max. GPE (Nm3/kWh) Power input at Max. HPE (W) Pressure (kPa) Steam supply method
Table 2 e Experimental conditions and results for n-dodecane decomposition in each study.
Total hydrocarbon
Present study
Reference
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Because oxygen was not detected in the produced gas, R3 and R4 are summarized by R5. HPE of R5 is very low compared to those of R1 and R6. Thus, the water decomposition not being involved in SR significantly degrades HPE. In this study, water pyrolysis was reduced by avoiding supplying water directly to the plasma. However, that is not enough and suppressing this water decomposition is a topic for future study. On the other hand, in each study shown in Table 2, light hydrocarbons mainly composed of acetylene were detected in the produced gas. The decomposition reaction by inliquid plasma is mainly a thermal decomposition process, and these light hydrocarbons were generated by the incomplete decomposition of n-dodecane due to temperature shortage. The temperature of the in-liquid plasma ranges from 3000 to 4000 K [40e42]. A thermal equilibrium calculation of n-dodecane in this temperature range shows that mainly acetylene is produced in addition to hydrogen (See Supplementary material). A temperature of 4600 K or more is required for complete decomposition. Reference 23 conducted experiments at 10 kPa and obtained a higher hydrogen content and a lower content of light hydrocarbons than those of Ref. 24, which was conducted at atmospheric pressure. This is because, by decompression, the energy supplied per molecule is increased and decomposition of ndodecane proceeded. In the present study, the experiment was conducted by the in-liquid plasma with SR at atmospheric pressure. The ratio of light hydrocarbons to hydrogen is not much different from Ref. [24] (19/55 ¼ 0.35 in the present study, 24/74 ¼ 0.32 in Ref. [24]). This means that incomplete decomposition of n-dodecane occurs in the SR process in the present study. If the decomposition is complete, the ratio of light hydrocarbons to hydrogen should be lower than in Ref. 24. To achieve complete decomposition of n-dodecane, it is sufficient to raise the temperature of the plasma. Alternatively, by introducing catalysts, an almost complete decomposition reaction may occur. However, the cost effectiveness of these methods needs to be examined further.
Fig. 5 e Input power effect on HPE in each study. The conditions in the present study are hole facing upward and a 1.5 mL/s water flow rate. HPE of water electrolysis shown in this figure is 70e80% of 0.28 m3/kWh, which is calculated from the standard enthalpy for the formation of water, 286 kJ/mol [35,36].
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Fig. 6 e Schematic illustration for the mechanism of HPE improvement by the steam pipe heat recovery method.
Input power effect The effect of input power on HPE in this research is shown in Fig. 5. For comparison, the effects in previous studies are also plotted. In Ref. 23, whether with or without steam supply, HPE increases in proportion to the input power in the range from 150 to 225 W, and then peaks beyond 225 W. The maximum HPE in Ref. 23 is 0.14 Nm3/kWh. In addition, when the experiment was conducted with 750 W of input power in Ref. 24, HPE is almost the same as that of Ref. 23. From these results, it is found that HPE cannot be promoted by simply increasing the input power, and the maximum value stays approximately at 0.14 Nm3/kWh in the previous methods. In contrast, in this research with steam, HPE increased linearly in proportion to the input power from 200 to 500 W. The maximum HPE of 0.28 Nm3/kWh was obtained at 500 W. This is 1.3 times greater than that of water electrolysis. On the other hand, without steam, HPE becomes almost the same as that of Refs. 23 and 24, both also without steam. Therefore, because the decomposition ability of plasma in the previous and present research are the same, this proves that heat recovery by the steam pipe works effectively. From the results shown above, a mechanism for HPE improvement has been developed, shown in Fig. 6. In-liquid plasma is plasma generated inside of a bubble in a liquid. When the power input is higher, a stronger plasma is generated, and the heat diffusion into the surrounding liquid is increased, causing generation of bigger bubbles and bubbles that contain the plasma for a longer time. This effect makes the plasma more stable. By this effect, HPE is temporarily improved by increasing the input power (as shown in Fig. 5 of Ref. 23). Supporting this, we found that it was impossible to maintain the plasma at or below 200 W. However, when the
input power is increased, heat radiation to the surrounding liquid is also increased, deteriorating HPE even if more ndodecane is decomposed (from Fig. 6(a) to (b)). In contrast, in this research, steam is supplied by recovery in the steam pipe. By this method, n-dodecane decomposition by plasma and SR are improved simultaneously (from Fig. 6(c) and (d)) because thermal energy that is wasted in the conventional method can be recovered. HPE is increased by employing the bubble stabilizing to this heat recovery and producing a synergistic effect. In this experiment, 500 W is the upper limit due to the capacities of the electrical source and the matching box. However, HPE may be improved even more by simply increasing the input power.
Conclusion To improve HPE by n-dodecane decomposition in an in-liquid plasma, a thin steam pipe was introduced over the plasma. The steam pipe is heated to a high temperature by the envelope bubbles generated around the plasma and internally produces a high temperature steam. This steam was supplied not to the plasma but instead to the surrounding vaporized ndodecane to cause SRR. By avoiding supplying it directly to the plasma, the influence of water pyrolysis was reduced. A large amount of CO and a small amount of CO2 were detected in the produced gas. This indicates that a strong SRR has occurred. HPE obtained by this method is 0.28 Nm3/kWh, which is twice as large as those in previous in-liquid plasmas and similar to or greater than that in water electrolysis. This result is a major advance in the field of plasma heavy hydrocarbon decomposition aimed at hydrogen production. However, the water pyrolysis was not completely suppressed. Further control of
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the water pyrolysis is a future task. In addition, further study is necessary on whether or not to proceed with the complete decomposition of n-dodecane. HPE improves in proportion to the increasing input power. This is due to synergy between the heat recovery effect of the steam pipe and the bubble stabilization effect, which indicates that this method has a high potential.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.270.
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[34] Nomura S, Yamashita H, Toyota H, Mukasa S, Okamura Y. Simultaneous production of hydrogen and carbon nanotubes in a conventional microwave oven. In: Proceedings of 19th international symposium on plasma chemistry; 2009. p. 2e5. [35] Pascuzzi S, Anifantis AS, Blanco I, Mugnozza GS. Electrolyzer performance analysis of an integrated hydrogen power system for greenhouse heating a case study. Sustain Times 2016;8:1e15. https://doi.org/10.3390/su8070629. [36] Turner JA. Sustainable hydrogen production. Science 2004;305:972e4. https://doi.org/10.1126/science.1103197 (80). [37] Shiraishi R, Nomura S, Mukasa S, Nakano R, Kamatoko R. Effect of catalytic electrode and plate for methanol decomposition by in-liquid plasma. Int J Hydrogen Energy 2018;43:4305e10. https://doi.org/10.1016/ j.ijhydene.2018.01.060. [38] Haynes WM. CRC hand book of chemistry and physics. 95th Edition. 95th ed. CRC Press LLC, Boca Raton; n.d. [39] Randolph K. Annual merit review and peer evaluation meeting. U.S. DOE; 2013. [40] Mukasa S, Nomura S, Toyota H, Maehara T, Abe F, Kawashima A. Temperature distributions of radio-frequency plasma in water by spectroscopic analysis. J Appl Phys 2009;106:1e6. https://doi.org/10.1063/1.3264671. [41] Mukasa S, Nomura S, Toyota H, Maehara T, Yamashita H. Internal conditions of a bubble containing radio-frequency plasma in water. Plasma Sources Sci Technol 2011;20:034020. https://doi.org/10.1088/0963-0252/20/3/034020. [42] Mukasa S, Nomura S, Toyota H. Measurement of temperature in sonoplasma. Japanese J Appl Physics, Part 1 Regul Pap Short Notes Rev Pap 2004;43:2833e7. https:// doi.org/10.1143/JJAP.43.2833.
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Effect of introducing a steam pipe to n-dodecane decomposition by in-liquid plasma for hydrogen production Ryoya Shiraishi*, Shinfuku Nomura, Hiromichi Toyota, Shinobu Mukasa, Yuki Amano Graduate School of Science and Engineering, Ehime University, 790-8577, Bunkyocho-3, Matsuyama, Japan
article info
abstract
Article history:
A method has been developed to improve the hydrogen production efficiency (HPE) by in-
Received 18 March 2019
liquid plasma n-dodecane decomposition. A thin steam pipe is placed over the plasma
Received in revised form
electrode to recover the thermal energy emitted from the plasma to its surroundings. The
26 April 2019
steam generated by this energy is supplied to the vaporized n-dodecane around the edge
Accepted 29 April 2019
of the plasma to cause a steam reforming reaction (SRR). Water pyrolysis is suppressed
Available online 23 May 2019
by not supplying the steam directly to the plasma. A large amount of CO and a small amount of CO2 were detected in the produced gas. This indicates that a strong SRR has
Keywords:
occurred. The HPE obtained by this method is 0.28 Nm3/kWh, which is two times greater
Hydrogen
than those obtained by previous methods, and similar to or greater than the yield of
n-dodecane
water electrolysis. This result is a major advance in the field of plasma heavy hydro-
In-liquid plasma
carbon decomposition aimed at hydrogen production. HPE is expected to be further
Steam reforming
improved by simply increasing the input power, due to synergy between the heat recovery effect of the steam pipe and the bubble stabilization effect. This indicates that this method has a high potential. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has high energy density (143 kJ/g) and does not emit any greenhouse gases when burned, so it is drawing much attention as a next generation clean energy source [1e4]. Hydrogen is a secondary energy form, which means that it requires energy to produce it. Therefore, much research has been invested in reducing the energy costs for hydrogen production [5e8]. From this viewpoint, decomposition of waste oil is seen as an advantageous method, because it can
simultaneously accomplish two goals: waste oil disposal and hydrogen production. Waste oil primarily consists of heavy hydrocarbons, such as n-dodecane. On other hand, methane steam reforming (SR) is the most common method for industrial hydrogen production and has been researched in detail [9e12]. Recently, some papers reported heavy hydrocarbon decomposition by SR aimed at hydrogen production [13e15]. These studies were done to investigate in detail effects of elements such as the steam carbon ratio (S/C ratio), catalyst bed temperature, and hourly space velocity of the
* Corresponding author. E-mail address: [email protected] (R. Shiraishi). https://doi.org/10.1016/j.ijhydene.2019.04.270 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
19 1
0.13 0.10 0.14
0.28 C12H26 þ H2O
0.01*
25
20 17 14 0 0 0 0 0 8 11 15 0 0 0 0 4.6 70 71 70 35 25 74 82.5 79.3
55
2 1 1 65 75 26 17.5 16.1
Pascuzzi et al., 2016 [35] Randolph U.S. DOE, 2013 [39] Dors et al., 2012 [19] Jasinski et al., 2014 [18] Du et al., 2012 [17] Henriques et al., 2011 [20] Jasinski et al., 2015 [18] Takeda et al., 2017 [15] Martin et al., 2015 [14] Vita et al., 2016 [13] Song et al., 2019 [21] Ganieva et al., 2016 [22] Nomura et al., 2009 [24] Mochtar et al., 2017 [23] 0 0 0 100 0.20e0.23 0.67 0.056 0.70 1.12 0.015 0.25
H2O CH4 þ H2O CH4 þ CO2 CH4 þ H2O C2H5OH þ H2O CH3OH þ Ar C2H5OH þ N2 C12H26 þ H2O Diesel þ H2O C12H26 þ H2O C10H22 Fuel oil C12H26 C12H26 C12H26 þ H2O 5. Our research group In-liquid plasma (with steam reforming)/n-dodecane
4. Plasma/heavy hydrocarbons
3. Steam reforming/heavy hydrocarbons
2. Plasma/methane or alcohols
Water electrolysis Industrial steam reforming Dielectric barrier discharge Microwave plasma with catalyst Laval nozzle arc Microwave (2.45 GHz) plasma Microwave (915 MHz) plasma Steam reforming Steam reforming Steam reforming Dielectric barrier discharge Arc discharge In-liquid microwave (2.45 GHz) plasma In- liquid microwave (2.45 GHz) plasma In-liquid microwave (2.45 GHz) plasma with steam reforming In-liquid radio frequency plasma (27.12 MHz) with steam reforming 1. Commercialized
H2
CO
CO2
Others (Mainly Light CxHy)
Reference Contents of produced gas (%)
HPE (Nm3/kWh) Initial composition Production method Classification
Table 1 e Comparison of various hydrogen production methods with our method. * Not mentioned directly, calculated from experimental conditions and results shown in the original article.
feedstock. The hydrogen composition rate of gas produced in these studies is approximately 70%, which is much higher than that of simple pyrolysis [16]. However, the hydrogen production efficiency (HPE) (g/kWh or Nm3/kWh) for a real machine has not been mentioned explicitly. On the other hand, some methods using plasma for light hydrocarbon or alcohol decomposition for hydrogen production, have achieved high HPE [17e20]. Mizeraczyk et al. summarized the status of the field in an excellent review [18]. Whereas the SR method requires a high temperature of approximately 1000 K and catalysts, the plasma methods do not necessarily require high temperatures. However, there are few studies about heavy hydrocarbon decomposition by plasma methods [21,22]. Moreover, in these examples, the concentration of hydrogen contained in the production gas is 25e35%, and the remainder is light hydrocarbons such as acetylene. In other words, these methods are primarily causing decomposition of heavy hydrocarbons rather than hydrogen production. We have been researching n-dodecane decomposition by in-liquid plasma, aiming for hydrogen production from waste oil [23,24]. In this method, the concentration of hydrogen contained in the production gas is approximately 80%. Furthermore, the processes of vaporization and mist formation are not required by the in-liquid plasma method. Thus, n-dodecane can be decomposed by direct discharge in the liquid stored in a reactor container [25e30]. Therefore, the apparatus is small and simple enough, suitable for a distributed hydrogen production device [24,31e34]. Someday, it may be possible to commercialize a device that decomposes used-cooking oil while refilling the household fuel cell with hydrogen. However, at present, the HPE for ndodecane decomposition is around 0.10 Nm3/kWh [23,24]. This is approximately half that of water electrolysis [35,36]. Therefore, improvement of HPE is necessary. The results of the various studies about hydrogen production introduced above are described in detail in the “Results and Discussion” section and compared with the results of this study (see Table 1). Recently, A. A. Mochtar et al. reported that HPE was improved 1.4 times in n-dodecane decomposition by incorporating the steam reforming reaction (SRR) into an in-liquid plasma reaction field [23]. This was accomplished by generating plasma at the tip of a pipe-type electrode inserted into a decompressed reactor vessel and supplying steam from the lower side of the electrode into the plasma reaction field. It seems that the steam reacts with the vaporized n-dodecane around the plasma. Then, SRR occurs in addition to the inliquid plasma reaction. However, when the steam is supplied into the plasma reaction field directly, a portion of the plasma energy is wasted on water pyrolysis, causing deterioration in HPE. When pure water is directly decomposed by the in-liquid plasma, HPE is very low, 0.02 Nm3/kWh [37]. Furthermore, to vaporize water and transport it to the reaction field, a decompression device is required, resulting in the need for electrical power to drive such a decompression device, further deteriorating the HPE. Furthermore, a basic problem is that a considerable amount of heat energy is wasted due to heat radiation from the plasma to the surroundings. Therefore, to improve the HPE, it is necessary to find solutions to the following two problems:
Present study
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(1) Recover the surplus thermal energy around the edge of the plasma beyond the main plasma and use it for water vaporization and SRR. (2) Conduct reactions at atmospheric pressure, eliminating the need for a decompression device. The purpose of the present research is to resolve these problems and improve HPE by an in-liquid plasma method assisted by SRR. To achieve these goals, a thin pipe was fixed over the plasma electrode, and any surplus energy around the plasma was recovered by this pipe. Furthermore, experiments were conducted at atmospheric pressure.
Experimental method The schematic image of the experimental setup is shown in Fig. 1. A tungsten electrode, 4 mm in diameter, is inserted from the bottom of the reactor vessel and a nickel steam pipe is fixed over the electrode. An electric current of 27.12 MHz from 200 to 500 W is supplied via a matching box and the plasma is generated between the tip of the electrode and the pipe. Once the plasma is generated, the surrounding liquid is continuously vaporized by the plasma heat and bubbles are generated. Because the pipe is very thin, it is completely enveloped in one of these bubbles, insulating it from the surrounding liquid, which also causes it to rise to a very high temperature. For reference, an image of a discharging thin bar in liquid methanol is shown in Fig. 2, where the bar is completely enveloped by the bubble. (This image should ideally be of a discharge in n-dodecane. However, when discharging in n-dodecane, it becomes muddied so it is difficult to
Fig. 2 e Image of a discharging thin bar in liquid methanol.
obtain a clear image.) The very high pipe temperature vaporizes the water when it enters the pipe. This high temperature steam is then supplied to the reaction field via a 1-mm diameter supply hole in the pipe. In this way, thermal energy around the plasma is recovered and put back to use. Therefore, preheating the water is not necessary and room temperature water can be used in this system. Because the steam pipe is easily enveloped by bubbles, the thinner the pipe is the better. However, a too thin pipe melts when the input power is high. Consequently, a steam pipe 3 mm in outer diameter, 2 mm in inner diameter is used when the input power is from 0 to 300 W and 5 mm in outer diameter, 3 mm in inner diameter when the input power is from 350 to 500 W.
Fig. 1 e Schematic of the experimental set up.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 6 2 4 8 e1 6 2 5 6
In our research we used n-dodecane, which is a common substance found in waste oil. Because the in-liquid plasma is generated inside bubbles in the liquid, it is difficult to generate it immediately in a substance with a high boiling point. Because the boiling point of ndodecane is high, 216 C, at atmospheric pressure [38], we need some preparation steps. First, the electrode and pipe are immersed in water and all valves except V1 and V2 are closed. The reactor vessel is decompressed by an aspirator to 0.02 MPa, allowing for plasma generation. The aspirator is then turned off and V1 is closed. V3 is then opened slowly, which returns the inner pressure of the reactor vessel to atmospheric pressure. The water supply pump is then turned on and V4 and V5 are opened, allowing water into the pipe. V4 is a flow control valve. In this way, steam is supplied to the reaction field. After the stability of the plasma is confirmed, V6 is opened and n-dodecane is supplied from syringe 1. Because ndodecane has lower specific gravity than water, it floats on top of the water. This condition is maintained for approximately 1 min to raise the temperature of the n-dodecane. Then, V7 is opened and the water is drawn out by syringe 2, lowering the water level. If the capacity of syringe 2 is not sufficient, V8 may be opened and to further drain the water away. Syringe 2 plays the role of level adjuster. By using the syringe 2, the level of the water/n-dodecane interface can be maintained at that of the plasma until the plasma stabilizes. If the plasma is extinguished, it can be rekindled by raising water level again. After the stability of plasma is confirmed at the interface level, the plasma is completely immersed in ndodecane, and continuous decomposition is conducted. Gas generated by the n-dodecane decomposition is collected by water replacement, and the time it takes to collect 150 mL of the gas is measured. The gas composition rate is measured by gas chromatography. The temperature distribution within the pipe is measured by inserting a K-type thermocouple.
Results and discussion Confirmation of high temperature steam supply The temperature distribution inside the pipe as measured by the thermocouple is shown in Fig. 3. The steam supply hole faced upward when the measurement was taken. The flow rate is 1.5 mL/s and the input power is 300 W.
Fig. 3 e Temperature distribution inside the pipe.
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The temperature is around 100 C at a distance of 5e20 mm from the plasma center. This temperature indicates thermal equilibrium between the water, pipe, and surrounding ndodecane. Nearer to the plasma center, the temperature rises dramatically. This is because, as mentioned before, the pipe is insulated from the liquid n-dodecane by the bubbles. Therefore, we confirm that the water is vaporized, and high temperature steam is supplied into the reaction field by installing the thin steam pipe over the plasma electrode.
Flow rate and hole direction effect The effect the flow rate and hole direction has on HPE is shown in Fig. 4. Because the electrical power required to drive the water supply pump is a mere 3 W, it is deemed negligible compared to the input power of 300 W. When no water is supplied, HPE is 0.09 Nm3/kWh. When water is supplied at a rate of 1.5 mL/s, HPE increases 1.7 times, reaching 0.16 Nm3/ kWh. This confirms that a high temperature steam supply improves HPE. We observe no effect of the hole direction on HPE at 1.5 mL/s. At 2.7 mL/s, HPE is maintained when the hole direction faces upward, but when the hole direction is downward, while production still occurs, it decreases to 0.10 Nm3/kWh. This shows that SRR occurs regardless of the hole direction, because the pipe is completely enveloped by the bubble, so the steam and the surrounding gas react regardless of the direction in which the steam is injected. However, when the hole direction is downward, the steam is injected into the plasma reaction field directly, resulting in much of the energy being consumed for water pyrolysis. Consequently, HPE is smaller than 2.7 mL/s when the hole is in the downward direction.
Maximum HPE and its evaluation With the hole faced upward and the water flow rate at 1.5 mL/ s, the power input was increased to 500 W. Under these conditions, HPE reached its maximum value of 0.28 Nm3/kWh. To evaluate this value, we compared it with other research results, as shown in Table 1. Water electrolysis and methane SR have been already commercialized and those HPE are 0.20e0.23 Nm3/kWh in water electrolysis [35] and 0.67 Nm3/ kWh in methane SR [39]. To develop a more efficient method
Fig. 4 e Flow rate and hole direction effect on HPE.
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for hydrogen production, various studies have been conducted. In methane or alcohol decomposition by the plasma method, Jasinski et al. [18] and Du et al. [17] obtained outstandingly high HPE. Especially, the HPE obtained by Du et al. is 1.12 Nm3/kWh, drastically exceeding the 0.67 Nm3/ kWh of methane SR. Hydrogen production from heavy hydrocarbons is still in the developing stages, compared to that from methane or alcohol decomposition. Recently, some studies about hydrogen production from heavy hydrocarbons by SR have been reported [13e15]. These studies investigated in detail the effects of the steam carbon ratio (S/C ratio), catalyst bed temperature, and hourly space velocity of the feedstock. The gas composition in each study is almost same, 70% of hydrogen, 29% of CO and CO2, and 1% of light hydrocarbons such as CH4. Even though it is reported that the energy conversion ratio is 40e50% in these studies, HPE (g/kWh or Nm3/kWh) as a hydrogen production device is not mentioned clearly. As mentioned above, in the case of methane or alcohol decomposition, the plasma method achieves a higher HPE than has been obtained by industrial SR. However, there are few studies about heavy hydrocarbons decomposition by the plasma method. Song et al. reported the dielectric barrier discharge method [21] and Ganieva et al. reported the arc discharge method [22]. However, the concentration of hydrogen contained in the production gas is 25e35%, and the remainder light hydrocarbons such as acetylene. In other words, these methods primarily cause decomposition of heavy hydrocarbons rather than hydrogen production. HPE is very low (around 0.01 Nm3/kWh) or not mentioned. Thus, we have conducted pioneering research on plasma decomposition of n-dodecane for hydrogen production [23,24]. The hydrogen concentration obtained by our inliquid plasma method is approximately 80%, which is much greater than that of other studies about heavy hydrocarbon plasma decomposition. In addition, in heavy hydrocarbon plasma decomposition, only our study mentions HPE clearly. HPE was 0.10e0.14 Nm3/kWh in our previous study. In the present study, we reach 0.28 Nm3/kWh by adding SRR to the in-liquid plasma reaction. This value is twice that of the previous researches. This value is still low compared to those of methane or alcohol decomposition, but this result is a major advance in the field of plasma heavy hydrocarbon decomposition aimed at hydrogen production. It is approaching the level required for practical use. To discuss the results obtained in the present study in more detail, we compared them with those of our previous studies. The experimental conditions and results for the ndodecane decomposition for each study are summarized in Table 2. All these studies are about n-dodecane decomposition by the in-liquid plasma method. In Ref. 24, the experiment was conducted by the pure in-liquid plasma method at atmospheric pressure and yielded 0.17 Nm3/kWh of gas production efficiency (GPE) and 0.13 Nm3/kWh of HPE. In this case, no CO or CO2 were detected. In Ref. 23, the experiments were conducted by a pure in-liquid plasma or in-liquid plasma with SR at the decompressed condition. In this case, HPE is 0.10 Nm3/kWh. In Ref. 23 with steam, HPE increased to 0.14 Nm3/kWh, accompanied by a little CO generation. The reaction formulas, reaction enthalpy, and HPE calculated from the reaction enthalpy, of n-dodecane SR are shown in R1 and R2.
C12H26 þ 12H2O / 12CO þ 25H2 (DН298Κ ¼ þ1867 kJ/mol, R1 HPE ¼ 75 kJ/mol-H2) 12CO þ 12H2O / 12CO2 þ 12H2 (DН298Κ ¼ 495 kJ/mol, HPE ¼ 41.25 kJ /mol-H2)
R2
From these formulas, the generation of CO and CO2 indicates the occurrence of SR, which improved HPE. However, in Ref. 23, the amount of CO is small; HPE is almost the same as that of Ref. 24. As mentioned before, because the steam was supplied to the plasma reaction field directly, it seems that water pyrolysis occurred in Ref. 23, deteriorating HPE. In Refs. 23 and 24, the surplus thermal energy emitted from the plasma is wasted to the surroundings, further deteriorating HPE. In contrast, in the present study, this surplus energy was recovered by the pipe and used for steam formation at atmospheric pressure. Then, this steam was supplied to the reaction field indirectly (to the vaporized n-dodecane rather than to the main plasma). As a result, HPE was drastically improved and accompanied by a large amount of CO and a little CO2 generation. This indicates that HPE was improved by a strong SRR. The maximum GPE 0.50 Nm3/kWh is three times greater than those of previous studies, and the maximum HPE of 0.28 Nm3/kWh is twice as big as those. This result proves that the method suggested in the present study is valid.
Discussion of the composition of the produced gas However, even in the present study, water pyrolysis is not suppressed completely. The hydrogen composition rate in the present study is 55%, which is the lowest value in Table 2. From R1, assuming that all of the 25% CO is generated by SR, we conclude that almost all of the produced hydrogen is produced by SR. However, because SR only occurs by plasma decomposition and gasification of n-dodecane in the present study, a considerable amount of hydrogen should be made by plasma decomposition. Therefore, the composition ratio of CO to hydrogen is obviously large. The oxygen for the CO generation is only supplied from water decomposition under these experimental conditions. Therefore, a larger amount of CO indicates decomposition of more water than necessary. The expected reactions are water pyrolysis (R3), solid carbon burning (R4), and solid carbon SR (R5). In addition, the reaction formula of the n-dodecane ideal plasma decomposition is shown in R6. HPE calculated from the reaction enthalpy is also shown in each formula. H2O / H2 þ 1/2O2 (DН298Κ ¼ þ286 kJ/mol, HPE ¼ 286 kJ/molR3 H2) C þ 1/2O2 / CO (DН298Κ ¼ 111 kJ/mol)
R4
C þ H2 O / H 2 þ CO (DН298Κ ¼ þ176 kJ/mol, HPE ¼ 176 kJ/ R5 mol-H 2 ) C 12 H 26 / 13H 2 þ 12C (DН298Κ ¼ þ352 kJ/mol, HPE ¼ 27 kJ/ R6 mol-H 2 )
10.5
3.9
Mochtar et al., 2017 [23] 3.7 10.7
Nomura et al., 2009 [24] 2 20
6 9
0 0 82.5 0.10 0.12 300 No steam supply (pure in-liquid plasma method)
0 4.6 79.3 0.14 0.17 10.0 From the pipe type plasma electrode
225
101.3 No steam supply (pure in-liquid plasma method)
750
0.17
0.13
74
0
0
4 19 2 24 1.7 16.1 3.1 17.5 1 25 55 0.28 0.50 500 101.3 From the steam pipe fixed over the plasma electrode
C2H4 C2H2 CH4 CO2 CO H2
Contents of produced gas (%)
Max. HPE (Nm3/kWh) Max. GPE (Nm3/kWh) Power input at Max. HPE (W) Pressure (kPa) Steam supply method
Table 2 e Experimental conditions and results for n-dodecane decomposition in each study.
Total hydrocarbon
Present study
Reference
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Because oxygen was not detected in the produced gas, R3 and R4 are summarized by R5. HPE of R5 is very low compared to those of R1 and R6. Thus, the water decomposition not being involved in SR significantly degrades HPE. In this study, water pyrolysis was reduced by avoiding supplying water directly to the plasma. However, that is not enough and suppressing this water decomposition is a topic for future study. On the other hand, in each study shown in Table 2, light hydrocarbons mainly composed of acetylene were detected in the produced gas. The decomposition reaction by inliquid plasma is mainly a thermal decomposition process, and these light hydrocarbons were generated by the incomplete decomposition of n-dodecane due to temperature shortage. The temperature of the in-liquid plasma ranges from 3000 to 4000 K [40e42]. A thermal equilibrium calculation of n-dodecane in this temperature range shows that mainly acetylene is produced in addition to hydrogen (See Supplementary material). A temperature of 4600 K or more is required for complete decomposition. Reference 23 conducted experiments at 10 kPa and obtained a higher hydrogen content and a lower content of light hydrocarbons than those of Ref. 24, which was conducted at atmospheric pressure. This is because, by decompression, the energy supplied per molecule is increased and decomposition of ndodecane proceeded. In the present study, the experiment was conducted by the in-liquid plasma with SR at atmospheric pressure. The ratio of light hydrocarbons to hydrogen is not much different from Ref. [24] (19/55 ¼ 0.35 in the present study, 24/74 ¼ 0.32 in Ref. [24]). This means that incomplete decomposition of n-dodecane occurs in the SR process in the present study. If the decomposition is complete, the ratio of light hydrocarbons to hydrogen should be lower than in Ref. 24. To achieve complete decomposition of n-dodecane, it is sufficient to raise the temperature of the plasma. Alternatively, by introducing catalysts, an almost complete decomposition reaction may occur. However, the cost effectiveness of these methods needs to be examined further.
Fig. 5 e Input power effect on HPE in each study. The conditions in the present study are hole facing upward and a 1.5 mL/s water flow rate. HPE of water electrolysis shown in this figure is 70e80% of 0.28 m3/kWh, which is calculated from the standard enthalpy for the formation of water, 286 kJ/mol [35,36].
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Fig. 6 e Schematic illustration for the mechanism of HPE improvement by the steam pipe heat recovery method.
Input power effect The effect of input power on HPE in this research is shown in Fig. 5. For comparison, the effects in previous studies are also plotted. In Ref. 23, whether with or without steam supply, HPE increases in proportion to the input power in the range from 150 to 225 W, and then peaks beyond 225 W. The maximum HPE in Ref. 23 is 0.14 Nm3/kWh. In addition, when the experiment was conducted with 750 W of input power in Ref. 24, HPE is almost the same as that of Ref. 23. From these results, it is found that HPE cannot be promoted by simply increasing the input power, and the maximum value stays approximately at 0.14 Nm3/kWh in the previous methods. In contrast, in this research with steam, HPE increased linearly in proportion to the input power from 200 to 500 W. The maximum HPE of 0.28 Nm3/kWh was obtained at 500 W. This is 1.3 times greater than that of water electrolysis. On the other hand, without steam, HPE becomes almost the same as that of Refs. 23 and 24, both also without steam. Therefore, because the decomposition ability of plasma in the previous and present research are the same, this proves that heat recovery by the steam pipe works effectively. From the results shown above, a mechanism for HPE improvement has been developed, shown in Fig. 6. In-liquid plasma is plasma generated inside of a bubble in a liquid. When the power input is higher, a stronger plasma is generated, and the heat diffusion into the surrounding liquid is increased, causing generation of bigger bubbles and bubbles that contain the plasma for a longer time. This effect makes the plasma more stable. By this effect, HPE is temporarily improved by increasing the input power (as shown in Fig. 5 of Ref. 23). Supporting this, we found that it was impossible to maintain the plasma at or below 200 W. However, when the
input power is increased, heat radiation to the surrounding liquid is also increased, deteriorating HPE even if more ndodecane is decomposed (from Fig. 6(a) to (b)). In contrast, in this research, steam is supplied by recovery in the steam pipe. By this method, n-dodecane decomposition by plasma and SR are improved simultaneously (from Fig. 6(c) and (d)) because thermal energy that is wasted in the conventional method can be recovered. HPE is increased by employing the bubble stabilizing to this heat recovery and producing a synergistic effect. In this experiment, 500 W is the upper limit due to the capacities of the electrical source and the matching box. However, HPE may be improved even more by simply increasing the input power.
Conclusion To improve HPE by n-dodecane decomposition in an in-liquid plasma, a thin steam pipe was introduced over the plasma. The steam pipe is heated to a high temperature by the envelope bubbles generated around the plasma and internally produces a high temperature steam. This steam was supplied not to the plasma but instead to the surrounding vaporized ndodecane to cause SRR. By avoiding supplying it directly to the plasma, the influence of water pyrolysis was reduced. A large amount of CO and a small amount of CO2 were detected in the produced gas. This indicates that a strong SRR has occurred. HPE obtained by this method is 0.28 Nm3/kWh, which is twice as large as those in previous in-liquid plasmas and similar to or greater than that in water electrolysis. This result is a major advance in the field of plasma heavy hydrocarbon decomposition aimed at hydrogen production. However, the water pyrolysis was not completely suppressed. Further control of
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the water pyrolysis is a future task. In addition, further study is necessary on whether or not to proceed with the complete decomposition of n-dodecane. HPE improves in proportion to the increasing input power. This is due to synergy between the heat recovery effect of the steam pipe and the bubble stabilization effect, which indicates that this method has a high potential.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.270.
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