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Oxidative reforming of n-heptane in gliding arc plasma reformer 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 ) 2 2 8 3 1 e2 2 8 4 0
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Oxidative reforming of n-heptane in gliding arc plasma reformer for hydrogen production Baowei Wang*, Yeping Peng, Shumei Yao Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
highlights
graphical abstract
H2 production from n-heptane reforming in gliding arc plasma was investigated. The
maximum
energy
yield
94.5 L (kW$ h)1 and yield 50.1% of H2 were obtained. The N2 (B3Pg), O (1D), Oþ, O play vital role in n-heptane oxidative reforming. The reaction mechanism was proposed based the analysis of OES and GC-MS.
article info
abstract
Article history:
The exploration of novel technologies to reduce the air pollution and greenhouse gas
Received 30 March 2019
emissions has been of great interest. Gliding arc plasma reformer at atmospheric pressure
Received in revised form
has been developed for converting n-heptane to hydrogen. The system has been evaluated
1 July 2019
by H2 yield and energy yield via continuous n-heptane oxidative reforming at room tem-
Accepted 5 July 2019
perature. Effects of some process parameters (discharge gap, input power, residence time,
Available online 26 July 2019
and O/C) have been studied on the reaction performance. The maximum H2 yield and energy yield are 50.1% and 94.5 L (kW h)1. To investigate the role of inert gas (N2, Ar) in the
Keywords:
plasma oxidative reforming system, the performance of C7H16/air, C7H16/N2/O2/Ar and
Hydrogen production
C7H16/O2/Ar have been investigated. The results show that N2 (B3Pg) and Ar* can accelerate
Gliding arc plasma
the formation of active oxygen species (such as Oþ, O (1D) and O). The presence of active
n-heptane reforming
oxygen species promotes the progress of the oxidative reforming reaction. What's more, N2
Electronic excited state
(B3Pg) is also conducive to the direct conversion of n-heptane. The reaction mechanism of hydrogen production from gliding arc plasma oxidative reforming of n-heptane was proposed based on the analysis of the OES and GCeMS. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (B. Wang). https://doi.org/10.1016/j.ijhydene.2019.07.042 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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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 ) 2 2 8 3 1 e2 2 8 4 0
Introduction Hydrogen is one of the most important fuel alternatives in the future because of the process of release energy without air pollution, which is widely used in combustion engines, fuel cells and gas turbines. Hydrogen can be converted from fossil fuels, chemical raw materials (H2S, H2O, etc.) [1e4]. At present, the most important hydrogen production technology in the industry is steam methane reforming (SMR), which accounts for 40% of the world's hydrogen production [5]. However, this is a strong endothermic reaction that requires a lot of heat from the outside. The process emits a large amount of CO2 that is unfavorable to the environment [6]. In addition, the reforming process requires catalysts to enhance the reaction rates. Nickel-based catalysts are currently used in the process, but they are also deactivated due to carbon deposition or metal oxidation [7]. There are a lot of previous investigations have proposed that plasma is an alternative technique to hydrogen production [8e11]. Many chemical reactions can be strengthened by plasma which is a kind of high energy medium. Plasma shows great potential for hydrogen production because of its low energy consumption, fast start-up, low reaction temperature and universality for various hydrogen compounds. At present, the main discharge forms of hydrogen generated by plasma are dielectric barrier discharge (DBD) [12e14], gliding arc discharge [15,16] and corona discharge [17]. DBD is a mild form of discharge with lower electron energy (1e10 eV), resulting in lower yield of hydrogen. In general, DBD is used in conjunction with catalysts to increase hydrogen selectivity and energy efficiency [18]. Most corona discharge reactors consist of asymmetric needle-plate electrodes, which are difficult to use for industrial amplification. Gliding arc plasma is a form of automatic periodic discharge that occurs between at least one pair of electrodes. The plasma arc is generated at the minimum gap between the counter electrodes and rises to the top of the electrode along the gas flow. The arc length increases as the electrode distance increases until it extinguishes, but regenerates at the minimum gap of the electrodes to begin a new cycle. Unlike DBD and corona discharge, gliding arc discharge has the characteristics of both non-thermal plasma and thermal plasma. When the discharge begins to thermal, the arc can be operated in a transient state, but becomes a non-thermal arc during the space-time evolution. The transitional discharge mode of the gliding arc provides high energy efficiency and selectivity of the chemical process while maintaining a high degree of non-equilibrium [19,20]. Gliding arc discharge is more promising in terms of hydrogen production than other plasmas. Reformed hydrocarbon fuel is relatively promising due to the availability of raw materials for hydrogen and syngas production [21e23]. There are many reforming processes to produce hydrogen from hydrocarbon fuel, such as partial oxidative reforming [24,25], dry reforming [26] and trireforming [27]. From the point of view of reaction conditions and product composition, each process has advantages and disadvantages. Plasma technology has been used in fuel reforming processes for many years [28e30]. Most of them focus on the
study of gas fuel, and there are few reports on liquid fuel. Nheptane, a cetane number of about 56, is the primary reference fuel for octane in internal combustion engines. Which have the similar cetane number to conventional diesel fuels [31]. In this work, n-heptane was chosen as an alternative to investigate the effect of hydrogen production from diesel oxidative reforming. Based on the gas product yield and the energy yield of H2, the effects of some process parameters such as discharge gap, input power, residence time and oxygen to carbon molar ratio (O/C) are comprehensively evaluated. The previous study indicated inert gas has great influence on the plasma discharge characteristics [32]. In order to discuss the role of inert gas (N2, Ar) in the plasma oxidative reforming system, the performance of C7H16/air, C7H16/N2/O2/Ar and C7H16/O2/Ar have been investigated experimentally. Besides, the spectroscopic parameters and liquid products in C7H16/air mixtures were investigated based on OES diagnostics and GC-MS analysis.
Experimental Gliding arc plasma oxidative reforming system An overall diagram of the n-heptane oxidative reforming system in gliding arc plasma (GAP) reformer is demonstrated in Fig. 1. The GAP reformer made up of a cylindrical glass cover (inner diameter 45 mm, length 80 mm), a pair of knife-shaped stainless steel electrodes and a nozzle having an inner diameter of 0.1 mm. The discharge gap includes 3.8, 4.3, 5.8 and 6.5 mm which can be adjusted by two knife-shaped stainless steel electrodes. No external heating provide to the GAP reformer which operates at a temperature of 298 K. The high voltage electrode is connected to one of the knife-shaped electrodes, and the other knife-shaped electrode is connected to the ground. The GAP reformer is powered by an AC high voltage power supply with a peak voltage of 30 kV and a variable frequency of 5e20 kHz (CTP-2000K).
Experiment parameters and product analysis The n-heptane was pumped into the preheater at a temperature of 423 K and mixed with air controlled by a mass flow meter (SY-9320B), and then injected into the reactor from the nozzle. The applied voltage and discharge frequency were measured by 100 MHz digital oscilloscope (Tektronix DPO 2012). The product gases are analyzed by gas chromatography (FULI 9790II) equipped with TCD detector (packed column: TDX-01 and capillary column: HP PLOT KCl/Al2O3). The exhaust flow is measured by a wet flow meter. The basic plasma parameters were obtained by emission spectrometer (Maya 2000 pro). The liquid products were analyzed via GCMS. This GCeMS instrument was equipped with a HPINNOWAX chromatographic; the column ion source temperature is 503 K; and the temperature rose from 323 to 513 K with a rate of 10 K/min. The average residence time (t) can be expressed as follows: tðsÞ ¼
Active volume of reactor ðmLÞ 1
Total flow rate ðmL,min Þ
60
(1)
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 ) 2 2 8 3 1 e2 2 8 4 0
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Fig. 1 e Flowchart of experiment device for n-heptane oxidative reforming. The oxygen-carbon molar ratio (O/C) was calculated using Eq. (2). O=C ¼
0:21 2 moles of air 7 moles of C7 H16
(2)
The performance of GAP oxidative reforming system were evaluated by yield (Y) and energy yield (EY) as defined below. YðH2 Þ% ¼
Moles of H2 produced 8 moles of C7 H16 feed
(3)
YðCOÞ% ¼
Moles of CO produced 7 moles of C7 H16 feed
(4)
YðCO2 Þ% ¼
Moles of CO2 produced 7 moles of C7 H16 feed
YðCx Hy Þ% ¼
(5)
x moles of Cx Hy produced 7 moles of C7 H16 feed 1
EYðH2 Þ L,ðkW,hÞ
(6) 1
¼
Flow rate of H2 produced ðmL,min Þ Input power ðWÞ (7)
Results and discussion Hydrogen production by n-heptane plasma oxidative reforming Effect of input power The chemical reaction is basically driven by heat. Especially in the partial oxidative reforming exothermic reaction, the heat involved in the reaction is critical to the distribution and selectivity of the reaction products [33]. In plasma-assisted chemical reactions, particularly in gliding arc plasma reaction systems, the growth and extinction of the arc is controlled by the heat that converted by externally supplied electrical energy. To investigate the effect of input power on plasma oxidative reforming performance and products
distribution, a set of tests were conducted with the input power of 26, 28, 30 and 34 W. The corresponding applied voltage at various input powers of 26, 28, 30 and 34 W were 14.7, 15.8, 17.9 and 20.4 kV, respectively. The minimum voltage value that produces a stable sustained plasma discharge in the experiment is 14.7 kV (breakdown voltage), which is affected by the gas medium and reactor structure. Effect of input power on products yield is shown in Fig. 2. The results show that the main products of the plasma oxidative reforming of n-heptane are CO and H2. The gas hydrocarbons (CH4, C2H6, C2H4, C3H6, C3H8, C4H10, cis-2-butene, trans-2butene, a-butane, and isobutene) can also be detected in the products. The yields of them are much lower than CO and H2. A small amount of CO2 and H2O appears in reaction products. When the input power increased from 26 W to 34 W, the yields of H2 and CO increased significantly, but the yields of byproducts did not change significantly. The reason for the lower yields of H2 and CO at low input power is that the arc cannot slide to the top of the electrode, resulting in a small effective plasma reaction zone [34]. The energy yield of H2 increase as the input power increase from 26 to 30 W, but decreases slightly as the input power increase from 30 to 34 W. The stronger electric field by increasing the applied voltage is conducive to the conversion of n-heptane to CO and H2. However, at a certain electric field strength, n-heptane is almost completely converted. Higher power not only does not effectively increase the conversion rate, but also accelerates the reverse water gas reaction as shown in Eq. (8). The process converts H2 into H2O, causing a decrease in H2 energy yield shown in Fig. 2 (a). As the input power increased, the amount of water vapor observed on the inner wall of the reactor increased. Take into consideration of yield and energy yield of H2, 30 W is suggested as optimal input power for GAP oxidative reforming, the maximum energy yield of H2 is 94.5 L$ (kW$ h)1 and the yields of CO and H2 are 32.9%, 50.1% respectively. CH4 and CO2 are the main by-products, and the yields are 2.89% and 2.0%, respectively. H2 þ CO2 ¼ H2 O þ CO
(8)
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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 ) 2 2 8 3 1 e2 2 8 4 0
Fig. 2 e Effect of input power on yields of gaseous products and energy yield of H2. (Discharge gap ¼ 6.5 mm, O/C ¼ 0.71, t ¼ 24.7 s).
The effect of oxygen-carbon molar ratio The oxygen-carbon molar ratio (O/C) is used as parameter to descript the composition of mixture entering the plasma reformer [35]. As shown in Eq. (2), It is the molar ratio of the total number of oxygen atoms from air to the total carbon atoms from C7H16 in the inlet mixture. The experiments were carried out in the range of 0.48e1.1 to study the effect of O/C by adjusting the content of air and n-heptane, which the other parameters were fixed (discharge gap ¼ 6.5 mm, t ¼ 24.7 s, input power ¼ 30 W). The impact of O/C on products yields on the process of nheptane oxidative reforming is shown in Fig. 3. The highest yields of H2 and CO were obtained at O/C of 0.71. In a plasma system, the O2 dissociation reactions occur under the impact of high-energy electrons, as shown in Eqs. (9) and (10). The active oxygen species (such as Oþ and O) have higher energy levels and promote the oxidative reforming reaction. First, the chain scission and dehydrogenation reaction of n-heptane molecules are carried out under the attack of high-energy electrons and active oxygen. H2 is produced by coupling reaction between H radical [36]. In the range of 0.48e0.71, the yields of H2 and CO increase as the O/C increases. The increase in the amount of active oxygen in the system by increase of O/ C provides more energy for the cleavage of the n-heptane molecule. When the O/C continues to increase, the yields of H2
Fig. 3 e Effect of oxygen-carbon molar ratio on yields of gaseous products. (Discharge gap ¼ 6.5 mm, t ¼ 24.7 s, input power ¼ 30 W).
and CO decrease significantly. However, as the O/C increases, the production of CO2 increases all the time. Similarly, what was directly seen from the discharge, as the O/C increases, the amount of water in the experiment also increases. This is related to oversaturation of the active oxygen species in the system, resulting in continued reaction with H2 and CO to form H2O and CO2. At a lower molar ratio of oxygen to carbon, filamentous carbon deposits appear on the blade electrode, which affects the persistence and stability of the discharge. Therefore, for the n-heptane gliding arc plasma oxidative reforming reaction system, suitable oxygen to carbon molar ratio plays a vital role in the yields of the products and the stable and continuous discharge. e þ O2 / e þ 2O e þ O2 / 2e þ O þ Oþ
(9) (10)
The residence time effect The effect of residence time on products yields at a constant O/C of 0.71 are illustrated in Fig. 4. The corresponding total feed flow rate at different residence time of 14.8, 18.5, 24.7, 37.3 and 46.5 s were 280, 223, 167, 111, and 89 mL min1, respectively. The yields of H2 and CO increased rapidly with increasing residence time from 14.8 to 24.7 s, and then sharply declined with further increasing residence time from 24.7 to
Fig. 4 e Effect of residence time on yields of gases products. (Discharge gap ¼ 6.5 mm, O/C ¼ 0.71, input power ¼ 30 W).
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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 ) 2 2 8 3 1 e2 2 8 4 0
46.5 s. The CO2 yield always increased with increasing residence time, and the trend for CH4 yield was completely opposite. Meanwhile, what was directly seen from the discharge, as the residence time increased, the amount of water vapor in the experiment also increased. By simplifying the reforming reaction into two stages, the tendency of H2 and CO yields associated with residence time can be explained [33]. The first stage, dehydrogenation and coupling reactions between n-heptane and high-energy electrons and active species produce large amounts of H2 and CO. The primary reaction at this stage is the cleavage of hydrocarbon fuel molecules. The alkyl radicals of C1eC7 continue to undergo dehydrogenation reaction. Other side reactions also occur and the reaction rates are much lower than the dehydrogenation reaction. Most of the n-heptane molecules are decomposed in the first stage. Therefore, the reaction rates of the dehydrogenation process of the alkyl radicals rapidly decreases in the second stage. The main products, H2 and CO, continue to react with the oxidizer in the system to form H2O and CO2. The secondary oxidation reaction was enhanced with an increase in residence time. As a result, the yields of H2 and CO decrease with the residence time increases. The productions of H2O and CO2 increase with the residence time increases. CH4, the most stable alkane, has formed by coupling reactions of methyl radicals with H radical in the first stage. However, CH4 was decomposed as the reaction time increases, resulting in a decrease in the yield of CH4.
The effect of discharge gap In the chemical process assisted by gliding arc plasma, the arc length and the effective plasma zone can affect the distribution and yields of the products. The discharge gap of the GAP reformer is directly related to the arc length [37]. To examine the effect of discharge gap on plasma oxidative reforming performance, a set of tests were conducted with the discharge gap of 3.8, 4.3, 5.8 and 6.5 mm. The maximum discharge gap of 6.5 mm was limited by the size of glass reactor. Fig. 5 illustrates the gas products yields under different discharge gap. Yields of H2 and CO increase dramatically as discharge gap increases. However, yields of CH4 and CO2 were almost unaffected by the discharge gap. In this study, the discharge arcs
of different discharge gap can slide toward the top of the blade electrode to obtain the longest arc. On the one hand, the longer arc length can improve the actual discharge power and promote the oxidation reforming reaction. On the other hand, a longer arc can increase the effective plasma zone and generate more radicals. Which increase the probabilities of collision among radicals and contributes to completely dissociate n-heptane. So the yields of H2 and CO significantly increase as the discharge gap increase. In fact, the effect of the discharge gap on plasma oxidative reforming performance should be considered in conjunction with the energy input in the system. Under sufficient energy input conditions, a larger discharge gap facilitates the growth of the arc length and can promote the yields of H2 and CO. In the case of insufficient input power, the arc can only glide in the middle of the counter electrode, resulting in reduction in the effective plasma reaction zone and decrease in energy utilization. In general, one strategy to achieve maximum energy utilization is combination the effects of input energy as well as discharge gap.
The plasma reforming performance comparison among C7H16/air, C7H16/N2/O2/Ar and C7H16/O2/Ar It is reported that inert gas (Ar, He) as a carrier gas for the plasma reactor contribute to the conversion of hydrocarbons [28]. To quantify the effects of inert gas (Ar, N2) in the plasma oxidative system, the plasma oxidative reforming performance of C7H16/air, C7H16/N2/O2/Ar and C7H16/O2/Ar were investigated. The contents of N2, O2, Ar in C7H16/air, C7H16/N2/ O2/Ar and C7H16/O2/Ar as shown in Table 1. The effects of N2 and Ar on the oxidative reforming of nheptane were investigated by adjusting the contents of Ar and N2 (O/C ¼ 0.71, t ¼ 24.7 s). The content of Ar is defined as the volume flow of Ar as a percentage of the total volume flow of Ar, O2 and N2. As presented in Fig. 6, the results show that the oxidative reforming performance for C7H16/N2/O2/Ar (56.0%) is better than C7H16/O2/Ar (46.0%) and C7H16/air (50.1%). The worst performance was gained by C7H16/O2/Ar. A variety of radicals of oxygen molecules and oxygen atoms produced by O2 plasma. The lowest excited oxygen atom O (1D) has high reactivity and can react with hydrocarbons to form syngas at room temperature [38], as shown in Eq. (11) e (17). It has been reported [39,40] that the threshold of dissociation and ionization caused by electronic collision decreases as the number of carbon atoms in the n-alkane molecule increases, as shown in Table 2. The cracking of nalkane mainly includes chain scission reaction and dehydrogenation reaction. The chain scission reaction is more advantageous as the number of carbon atoms in the alkane
Table 1 e Contents of N2, O2, Ar in C7H16/air, C7H16/N2/O2/ Ar and C7H16/O2/Ar.
Fig. 5 e Effect of discharge gap on yields of gases products. (t ¼ 24.7 s, O/C ¼ 0.71, input power ¼ 30 W).
System
O/C
Content of Ar/%
Content of N2/%
Content of O2/%
C7H16/air C7H16/N2/O2/Ar C7H16/O2/Ar
0.71 0.71 0.71
0 10e30 79
79 69e49 0
21 21 21
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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 ) 2 2 8 3 1 e2 2 8 4 0
process is a highly exothermic reaction due to the dissociation energy of O2 is much lower than excitation energy of Ar atom [32]. The extra heat release is not beneficial for the n-heptane oxidative reforming system. Thus, the amount of O (1D) generated by Ar* should be significantly smaller than the O P 3 3 (1D) produced by N2 [ðA3 þ u Þ, n), (B Pg, n), (C Pu, n)] and the direct electron collision dissociation of O2. The energy levels P 3 3 ofN2 ðA3 þ u Þ, N2 (B Pg) and N2 (C Pu) are 6.17 eV, 7.35 eV and 11.0 eV, respectively. They can form various Cx Hy radicals and H radicals by directly dissociating hydrocarbons fuel molecules, as shown in Eq. (21)e(25). For these reasons, it can be clearly stated that the oxidative reforming performance of the C7H16/Ar/O2 is inferior to C7H16/Ar/N2/O2. It also shows that N2 plays an important part in promoting the oxidative reforming performance. Fig. 6 e The yields of gas products of C7H16/air, C7H16/N2/ O2/Ar and C7H16/O2/Ar. (Discharge gap ¼ 6.5 mm, t ¼ 24.7 s, O/C ¼ 0.71, input power ¼ 30 W).
N2 ðA3
X
þ ; B3 Pg ; C3 Pu Þ þ O2 / N2 þ O þ O ð1 DÞ
(18)
þ ; B3 Pg ; C3 Pu Þ þ O/ N2 þ O ð1 DÞ
(19)
u
N2 ðA3
X u
increases. In summary, the various radicals produced by the dissociation of oxygen molecules provide energy for the cracking of n-heptane and promote its conversion to syngas.
Ar* þ O2 /Ar þ O ð3 PÞ þ Oð1 D; 1 SÞ N2 ðA3
O ð DÞ þ CH4 / CH3 , þ ,OH
(11)
O ð1 DÞ þ CH4 / CH2 O þ H2
(12)
N2 ðA3
O ð1 DÞ þ CH4 / CH3 O, þ H,
(13)
N2 ðA3
1
X
(20)
þ ; B3 Pg ; C3 Pu Þ þ CH4 / N2 þ CH3 , þ H,
(21)
þ ; B3 Pg ; C3 Pu Þ þ C2 H6 / N2 þ C2 H4 þH2
(22)
þ ; B3 Pg ; C3 Pu Þ þ C3 H8 / N2 þ C3 H6 þH2
(23)
þ ; B3 Pg ; C3 Pu Þ þ C2 H4 / N2 þ C2 H3 , þ H,
(24)
þ ; B3 Pg ; C3 Pu Þ þ C3 H6 / N2 þ C3 H5 , þ H,
(25)
u
X u
X u
O ð1 DÞ þ CH4 / CH3 OH
(14) N2 ðA3
O ð1 DÞ þ C2 H4 / CH3 , þ HCO, O ð DÞ þ C2 H4 / C2 H3 , þ ,OH
(16)
O ð1 DÞ þ C2 H4 / CH2 , þ CH2 O,
(17)
Previous studies showed that the dominant reaction pathway for atomic oxygen generation is e* þ O2 ¼ e þ O þ O (1D) [41]. In N2/O2, the electronically excited states of N2 P 3 3 [N2 ðA3 þ u , n), (B Pg, n), (C Pu, n)] can be quenched by O2 molecules [42,43], as shown in Eqs. (18) and (19). The existence of electronically excited states of N2 in the system will be described in section OES identification below. Besides, the excited oxygen atoms were also produced by the dissociative quenching of the electronic excited Ar atom (Ar*) by O2 [32], as shown in Eq. (20). The worse performance of C7H16/O2/Ar compare to C7H16/ N2/O2/Ar can be explained by the following reasons. Although the Ar* can also be quenched by O2 to produce O (1D), the
Table 2 e Dissociation and ionization potentials. Molecule CH4 C2H6 C3H8 C4H10
u
(15)
1
Ionization, eV
Dissociation, eV
13.6 11.99 11.51 11.09
9.7 8.7 8.1 7.7
X
N2 ðA3
X u
It is found that the better plasma oxidative performance is achieved by C7H16/N2/O2/Ar compare to C7H16/air. According to report [44], the addition of Ar to the reaction system can increase the electron density and electron temperature. It accelerates the rate of cleavage of n-heptane by accelerating the frequency and efficiency of collisions between electrons and fuel and oxygen molecules. In addition, Penning ionization effect of Ar* atoms can also be considered. The excitation energy of the Ar3P2 is as high as 11.49 eV, and the internal energy can be transferred to the ground state n-heptane molecules [45]. Another reason for Ar/N2/O2/C7H16 to achieve better reforming performance is that additional channel of O2 dissociation by Ar* as mentioned above. It also shows that Ar contributes to the improvement of oxidative reforming performance.
Exploration on the mechanism of oxidative reforming by GAP OES identification The optical emission spectroscopy (OES) has been used to identify the active species formed in the gliding arc plasma. It
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 ) 2 2 8 3 1 e2 2 8 4 0
helps to clarify the conversion mechanism of n-heptane in air plasma. As shown in Fig. 7, the global spectrum of C7H16/air discharge was detected in the wavelength range of 200e1100 nm. The second positive systems (SPS) generated by the N2 (C3Pu) to N2 (B3Pg) transition were detected at the wavelength from 297 to 357 nm, which is the strongest and clearest band in the nitrogen emission spectrum. The energetic states of excited states N2 (C3Pu) and N2 (B3Pg) are 11.0 eV and 7.35 eV, respectively. By observing and analyzing the N2 (C3Pu / B3Pg), the existence of N2 (B3Pg) in the gliding arc plasma oxidative reforming system was proved. N2 plays an important role in the plasma oxidative reforming of n-heptane reaction system. Hydrocarbon fuels can be quenched by the excited state of N2 while generating large amounts of CxHy radicals and H radical. In addition, it is also involved in the production of O (1D) which promotes oxidative reforming performance. The details have been discussed in section The plasma reforming performance comparison among C7H16/ air, C7H16/N2/O2/Ar and C7H16/O2/Ar above. CH (B2S/X2P, 388.31 nm) [46] is considered to be one of the important radical of n-heptane chain scission and dehydrogenation. The CH radical (2.9 eV) were formed by collision of high-energy electrons and n-heptane molecules with alkyl radicals, as shown in Eq. (26)-(28) [47]. At the same time, the Ha line derived from the hydrogen 1Balmer line was also detected at 656 nm [46]. C7 H16 þ e / C6 H13 , þ CH3 , þ e
(26)
CH3 , þ e / CH, þ H2 þ e
(27)
CH2 , þ e / CH, þ H, þ e 3
(28) 3
22837
The radiative lifetime of the metastable O (1D) (1.967 eV) is rather long (~100 s) [49], the emission spectra of O (1D) is generally not observed. Moreover, the energy level of O (1D) is rather low, so the concentration of O (1D) is relatively high in the discharge region. The wavelength of the oxygen atom line in Fig. 7 and the theoretical parameters in Refs. [50,51] are comprehensively analyzed. It can be seen that during the C7H16/air discharge, a large number of excited oxygen atoms between 9.15 and 15.78 eV were generated. The spectra of O (3D - 3P0), O (5P - 5S0), O (3D - 3D0), O (3P - 3S0) appeared at 748.2, 777.4, 823.6 and 844.7 nm, respectively. The chemical activity and internal energy of O are very high, which can break the chemical bonds of almost all organic compounds. It is an effective radical in the process of reforming hydrocarbons by plasma oxidative. In addition, the spectra of Oþ (4P0-4P) and Oþ (4S0-4P) appeared at 416.7 and 488.9 nm, respectively. The transition energy level of Oþ is very high (greater than 22 eV) [52]. It indicates that Oþ has higher potential energy than O, and it can provide the necessary energy source for conversion of stable organic compounds. Generally, the routes of production of Oþ may be as follow: þ 4 3 2 Oþ 2 ðX Pg Þ þ e / O ð SÞ þ O ð PÞ þ e
(32)
Liquid products The liquid products compositions of the n-heptane plasma oxidative reforming were analyzed by GC-MS. The detected organic by-products and GC-MS are presented in Fig. 8. The reaction mechanism in the n-heptane plasma oxidative reforming system can be presumed by these products in the following.
3
The C2 swan bands of (d Pg / a Su, 468e474 nm), (A Pg / X3Pu, 500e515 nm), (A3Pg / X3Pu, 550e590 nm) were detected, which may be generated as follows [48]. CH , þ C, / C2 , þ H,
(29)
C , þ C, / C2 ,
(30)
C2 H , þ C2 H, / C2 , þ C2 H2
(31)
Fig. 7 e OES analysis of n-heptane plasma oxidative reforming system. (Discharge gap ¼ 6.5 mm, O/C ¼ 0.71, t ¼ 24.7 s, input power ¼ 30 W).
Mechanism of n-heptane plasma oxidative reforming According to the analysis above, the conversion mechanism of n-heptane in the plasma oxidative reforming system is shown in Fig. 9. In this study, the conversion mechanism proposed can be divided into two steps: the cracking of nheptane and reforming reactions dominated by alkyl radicals. The processes of the cracking of n-heptane mainly include three pathways: one is collision of accelerated electrons with n-heptane molecules directly shown in Eq. (32), other is reaction between active oxygen species (such as O (1D), Oþ and O) with n-heptane. In addition, the main sources of reactive
Fig. 8 e GCeMS diagram of liquid products in the nheptane plasma oxidative reforming system.
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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 ) 2 2 8 3 1 e2 2 8 4 0
Fig. 9 e Conversion mechanism of n-heptane in the plasma oxidative reforming system.
oxygen species ([O]) are the direct collision with electrons and the quenching reaction with the electronic excited state of nitrogen (N2 (B3Pg)). Another pathway is the direct reaction between N2 (B3Pg) with n-heptane, the cracking of n-heptane largely produces H radical and alkyl radicals. A large amount of H radical combines to form a stable hydrogen molecule. Furthermore, the alkyl radicals can continue to react with the electrons, active oxygen species and N2 (B3Pg). As shown in Fig. 2 (b), gas hydrocarbon by-products (CH4, C2H6, C2H4, C3H6, C3H8, C4H10, Cis-2-butene, Trans-2-butene, a-butane, and isobutene) can be detected. Fig. 8 shows the presence of acetic acid, propionic acid and ether in the liquid products. Generally, the production of olefins were formed by the dehydrogenation reaction of alkyl radicals under electron collision. Furthermore, the olefin is oxidized by the active oxygen species to form a carboxylic acid. Ether is formed by the interaction of two alkyl radicals and active oxygen species. The reaction of the alkyl radicals and the active oxygen species can simultaneously generate CO and H2. When O/C are relatively high, CO and H2 are further oxidized to form CO2 and H2O. Conversely, CO2 and H2O can also generate CO and H2 by electron impact, but the process is relatively difficult.
yield of CO (32.9%) and H2 (50.1%) were obtained. Besides, the performance of C7H16/air, C7H16/N2/O2/Ar and C7H16/O2/Ar were compared in terms of yield of H2. Similarly, the electronic excited states of N2 and excited Ar atom can be quenched by O2 to produce O (1D), which is favorable for hydrogen generation. However, the latter is less efficient for generating reactive atomic oxygen than the former. More importantly, the electronic excited state of N2 also can directly react with hydrocarbons to form H radical and alkyl radicals ($CH3, $C2H5, etc.) and promote the formation of hydrogen. The plasma parameters were obtained by monitoring n-heptane oxidative reforming in the GAP reformer using OES. The spectrum of excited state of N2 (SPS) which facilitates the conversion of n-heptane and formation of reactive oxygen species were detected. CH (B2S/X2P, 388.31 nm), C2 swan bands and Ha line were detected, which are important radicals involved in plasma oxidative of nheptane reforming. The spectra of O and Oþ were observed, which own higher energy and provide the sufficient energy for the conversion of n-heptane. Besides, the liquid products were analyzed by GC-MS and indicate the existence of acetic acid, propionic acid and ether. The mechanism of hydrogen production from gliding arc plasma oxidative reforming of nheptane is proposed based on the diagnosis of the OES and the analysis of GC-MS.
Conclusions In this work, the effects of input power, O/C, residence time and discharge gap on hydrogen production from n-heptane plasma oxidative reforming were investigated. The optimal parameters were 30 W, 0.71, 24.7 s and 6.5 mm, respectively. The maximum energy yield of H2 (94.5 L (kW$ h)1) and the
Acknowledgments This work is financially supported by the National Key Research and Development Program of China (No. 2016YFB0600703).
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 ) 2 2 8 3 1 e2 2 8 4 0
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Oxidative reforming of n-heptane in gliding arc plasma reformer for hydrogen production Baowei Wang*, Yeping Peng, Shumei Yao Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
highlights
graphical abstract
H2 production from n-heptane reforming in gliding arc plasma was investigated. The
maximum
energy
yield
94.5 L (kW$ h)1 and yield 50.1% of H2 were obtained. The N2 (B3Pg), O (1D), Oþ, O play vital role in n-heptane oxidative reforming. The reaction mechanism was proposed based the analysis of OES and GC-MS.
article info
abstract
Article history:
The exploration of novel technologies to reduce the air pollution and greenhouse gas
Received 30 March 2019
emissions has been of great interest. Gliding arc plasma reformer at atmospheric pressure
Received in revised form
has been developed for converting n-heptane to hydrogen. The system has been evaluated
1 July 2019
by H2 yield and energy yield via continuous n-heptane oxidative reforming at room tem-
Accepted 5 July 2019
perature. Effects of some process parameters (discharge gap, input power, residence time,
Available online 26 July 2019
and O/C) have been studied on the reaction performance. The maximum H2 yield and energy yield are 50.1% and 94.5 L (kW h)1. To investigate the role of inert gas (N2, Ar) in the
Keywords:
plasma oxidative reforming system, the performance of C7H16/air, C7H16/N2/O2/Ar and
Hydrogen production
C7H16/O2/Ar have been investigated. The results show that N2 (B3Pg) and Ar* can accelerate
Gliding arc plasma
the formation of active oxygen species (such as Oþ, O (1D) and O). The presence of active
n-heptane reforming
oxygen species promotes the progress of the oxidative reforming reaction. What's more, N2
Electronic excited state
(B3Pg) is also conducive to the direct conversion of n-heptane. The reaction mechanism of hydrogen production from gliding arc plasma oxidative reforming of n-heptane was proposed based on the analysis of the OES and GCeMS. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (B. Wang). https://doi.org/10.1016/j.ijhydene.2019.07.042 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Hydrogen is one of the most important fuel alternatives in the future because of the process of release energy without air pollution, which is widely used in combustion engines, fuel cells and gas turbines. Hydrogen can be converted from fossil fuels, chemical raw materials (H2S, H2O, etc.) [1e4]. At present, the most important hydrogen production technology in the industry is steam methane reforming (SMR), which accounts for 40% of the world's hydrogen production [5]. However, this is a strong endothermic reaction that requires a lot of heat from the outside. The process emits a large amount of CO2 that is unfavorable to the environment [6]. In addition, the reforming process requires catalysts to enhance the reaction rates. Nickel-based catalysts are currently used in the process, but they are also deactivated due to carbon deposition or metal oxidation [7]. There are a lot of previous investigations have proposed that plasma is an alternative technique to hydrogen production [8e11]. Many chemical reactions can be strengthened by plasma which is a kind of high energy medium. Plasma shows great potential for hydrogen production because of its low energy consumption, fast start-up, low reaction temperature and universality for various hydrogen compounds. At present, the main discharge forms of hydrogen generated by plasma are dielectric barrier discharge (DBD) [12e14], gliding arc discharge [15,16] and corona discharge [17]. DBD is a mild form of discharge with lower electron energy (1e10 eV), resulting in lower yield of hydrogen. In general, DBD is used in conjunction with catalysts to increase hydrogen selectivity and energy efficiency [18]. Most corona discharge reactors consist of asymmetric needle-plate electrodes, which are difficult to use for industrial amplification. Gliding arc plasma is a form of automatic periodic discharge that occurs between at least one pair of electrodes. The plasma arc is generated at the minimum gap between the counter electrodes and rises to the top of the electrode along the gas flow. The arc length increases as the electrode distance increases until it extinguishes, but regenerates at the minimum gap of the electrodes to begin a new cycle. Unlike DBD and corona discharge, gliding arc discharge has the characteristics of both non-thermal plasma and thermal plasma. When the discharge begins to thermal, the arc can be operated in a transient state, but becomes a non-thermal arc during the space-time evolution. The transitional discharge mode of the gliding arc provides high energy efficiency and selectivity of the chemical process while maintaining a high degree of non-equilibrium [19,20]. Gliding arc discharge is more promising in terms of hydrogen production than other plasmas. Reformed hydrocarbon fuel is relatively promising due to the availability of raw materials for hydrogen and syngas production [21e23]. There are many reforming processes to produce hydrogen from hydrocarbon fuel, such as partial oxidative reforming [24,25], dry reforming [26] and trireforming [27]. From the point of view of reaction conditions and product composition, each process has advantages and disadvantages. Plasma technology has been used in fuel reforming processes for many years [28e30]. Most of them focus on the
study of gas fuel, and there are few reports on liquid fuel. Nheptane, a cetane number of about 56, is the primary reference fuel for octane in internal combustion engines. Which have the similar cetane number to conventional diesel fuels [31]. In this work, n-heptane was chosen as an alternative to investigate the effect of hydrogen production from diesel oxidative reforming. Based on the gas product yield and the energy yield of H2, the effects of some process parameters such as discharge gap, input power, residence time and oxygen to carbon molar ratio (O/C) are comprehensively evaluated. The previous study indicated inert gas has great influence on the plasma discharge characteristics [32]. In order to discuss the role of inert gas (N2, Ar) in the plasma oxidative reforming system, the performance of C7H16/air, C7H16/N2/O2/Ar and C7H16/O2/Ar have been investigated experimentally. Besides, the spectroscopic parameters and liquid products in C7H16/air mixtures were investigated based on OES diagnostics and GC-MS analysis.
Experimental Gliding arc plasma oxidative reforming system An overall diagram of the n-heptane oxidative reforming system in gliding arc plasma (GAP) reformer is demonstrated in Fig. 1. The GAP reformer made up of a cylindrical glass cover (inner diameter 45 mm, length 80 mm), a pair of knife-shaped stainless steel electrodes and a nozzle having an inner diameter of 0.1 mm. The discharge gap includes 3.8, 4.3, 5.8 and 6.5 mm which can be adjusted by two knife-shaped stainless steel electrodes. No external heating provide to the GAP reformer which operates at a temperature of 298 K. The high voltage electrode is connected to one of the knife-shaped electrodes, and the other knife-shaped electrode is connected to the ground. The GAP reformer is powered by an AC high voltage power supply with a peak voltage of 30 kV and a variable frequency of 5e20 kHz (CTP-2000K).
Experiment parameters and product analysis The n-heptane was pumped into the preheater at a temperature of 423 K and mixed with air controlled by a mass flow meter (SY-9320B), and then injected into the reactor from the nozzle. The applied voltage and discharge frequency were measured by 100 MHz digital oscilloscope (Tektronix DPO 2012). The product gases are analyzed by gas chromatography (FULI 9790II) equipped with TCD detector (packed column: TDX-01 and capillary column: HP PLOT KCl/Al2O3). The exhaust flow is measured by a wet flow meter. The basic plasma parameters were obtained by emission spectrometer (Maya 2000 pro). The liquid products were analyzed via GCMS. This GCeMS instrument was equipped with a HPINNOWAX chromatographic; the column ion source temperature is 503 K; and the temperature rose from 323 to 513 K with a rate of 10 K/min. The average residence time (t) can be expressed as follows: tðsÞ ¼
Active volume of reactor ðmLÞ 1
Total flow rate ðmL,min Þ
60
(1)
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 ) 2 2 8 3 1 e2 2 8 4 0
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Fig. 1 e Flowchart of experiment device for n-heptane oxidative reforming. The oxygen-carbon molar ratio (O/C) was calculated using Eq. (2). O=C ¼
0:21 2 moles of air 7 moles of C7 H16
(2)
The performance of GAP oxidative reforming system were evaluated by yield (Y) and energy yield (EY) as defined below. YðH2 Þ% ¼
Moles of H2 produced 8 moles of C7 H16 feed
(3)
YðCOÞ% ¼
Moles of CO produced 7 moles of C7 H16 feed
(4)
YðCO2 Þ% ¼
Moles of CO2 produced 7 moles of C7 H16 feed
YðCx Hy Þ% ¼
(5)
x moles of Cx Hy produced 7 moles of C7 H16 feed 1
EYðH2 Þ L,ðkW,hÞ
(6) 1
¼
Flow rate of H2 produced ðmL,min Þ Input power ðWÞ (7)
Results and discussion Hydrogen production by n-heptane plasma oxidative reforming Effect of input power The chemical reaction is basically driven by heat. Especially in the partial oxidative reforming exothermic reaction, the heat involved in the reaction is critical to the distribution and selectivity of the reaction products [33]. In plasma-assisted chemical reactions, particularly in gliding arc plasma reaction systems, the growth and extinction of the arc is controlled by the heat that converted by externally supplied electrical energy. To investigate the effect of input power on plasma oxidative reforming performance and products
distribution, a set of tests were conducted with the input power of 26, 28, 30 and 34 W. The corresponding applied voltage at various input powers of 26, 28, 30 and 34 W were 14.7, 15.8, 17.9 and 20.4 kV, respectively. The minimum voltage value that produces a stable sustained plasma discharge in the experiment is 14.7 kV (breakdown voltage), which is affected by the gas medium and reactor structure. Effect of input power on products yield is shown in Fig. 2. The results show that the main products of the plasma oxidative reforming of n-heptane are CO and H2. The gas hydrocarbons (CH4, C2H6, C2H4, C3H6, C3H8, C4H10, cis-2-butene, trans-2butene, a-butane, and isobutene) can also be detected in the products. The yields of them are much lower than CO and H2. A small amount of CO2 and H2O appears in reaction products. When the input power increased from 26 W to 34 W, the yields of H2 and CO increased significantly, but the yields of byproducts did not change significantly. The reason for the lower yields of H2 and CO at low input power is that the arc cannot slide to the top of the electrode, resulting in a small effective plasma reaction zone [34]. The energy yield of H2 increase as the input power increase from 26 to 30 W, but decreases slightly as the input power increase from 30 to 34 W. The stronger electric field by increasing the applied voltage is conducive to the conversion of n-heptane to CO and H2. However, at a certain electric field strength, n-heptane is almost completely converted. Higher power not only does not effectively increase the conversion rate, but also accelerates the reverse water gas reaction as shown in Eq. (8). The process converts H2 into H2O, causing a decrease in H2 energy yield shown in Fig. 2 (a). As the input power increased, the amount of water vapor observed on the inner wall of the reactor increased. Take into consideration of yield and energy yield of H2, 30 W is suggested as optimal input power for GAP oxidative reforming, the maximum energy yield of H2 is 94.5 L$ (kW$ h)1 and the yields of CO and H2 are 32.9%, 50.1% respectively. CH4 and CO2 are the main by-products, and the yields are 2.89% and 2.0%, respectively. H2 þ CO2 ¼ H2 O þ CO
(8)
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Fig. 2 e Effect of input power on yields of gaseous products and energy yield of H2. (Discharge gap ¼ 6.5 mm, O/C ¼ 0.71, t ¼ 24.7 s).
The effect of oxygen-carbon molar ratio The oxygen-carbon molar ratio (O/C) is used as parameter to descript the composition of mixture entering the plasma reformer [35]. As shown in Eq. (2), It is the molar ratio of the total number of oxygen atoms from air to the total carbon atoms from C7H16 in the inlet mixture. The experiments were carried out in the range of 0.48e1.1 to study the effect of O/C by adjusting the content of air and n-heptane, which the other parameters were fixed (discharge gap ¼ 6.5 mm, t ¼ 24.7 s, input power ¼ 30 W). The impact of O/C on products yields on the process of nheptane oxidative reforming is shown in Fig. 3. The highest yields of H2 and CO were obtained at O/C of 0.71. In a plasma system, the O2 dissociation reactions occur under the impact of high-energy electrons, as shown in Eqs. (9) and (10). The active oxygen species (such as Oþ and O) have higher energy levels and promote the oxidative reforming reaction. First, the chain scission and dehydrogenation reaction of n-heptane molecules are carried out under the attack of high-energy electrons and active oxygen. H2 is produced by coupling reaction between H radical [36]. In the range of 0.48e0.71, the yields of H2 and CO increase as the O/C increases. The increase in the amount of active oxygen in the system by increase of O/ C provides more energy for the cleavage of the n-heptane molecule. When the O/C continues to increase, the yields of H2
Fig. 3 e Effect of oxygen-carbon molar ratio on yields of gaseous products. (Discharge gap ¼ 6.5 mm, t ¼ 24.7 s, input power ¼ 30 W).
and CO decrease significantly. However, as the O/C increases, the production of CO2 increases all the time. Similarly, what was directly seen from the discharge, as the O/C increases, the amount of water in the experiment also increases. This is related to oversaturation of the active oxygen species in the system, resulting in continued reaction with H2 and CO to form H2O and CO2. At a lower molar ratio of oxygen to carbon, filamentous carbon deposits appear on the blade electrode, which affects the persistence and stability of the discharge. Therefore, for the n-heptane gliding arc plasma oxidative reforming reaction system, suitable oxygen to carbon molar ratio plays a vital role in the yields of the products and the stable and continuous discharge. e þ O2 / e þ 2O e þ O2 / 2e þ O þ Oþ
(9) (10)
The residence time effect The effect of residence time on products yields at a constant O/C of 0.71 are illustrated in Fig. 4. The corresponding total feed flow rate at different residence time of 14.8, 18.5, 24.7, 37.3 and 46.5 s were 280, 223, 167, 111, and 89 mL min1, respectively. The yields of H2 and CO increased rapidly with increasing residence time from 14.8 to 24.7 s, and then sharply declined with further increasing residence time from 24.7 to
Fig. 4 e Effect of residence time on yields of gases products. (Discharge gap ¼ 6.5 mm, O/C ¼ 0.71, input power ¼ 30 W).
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46.5 s. The CO2 yield always increased with increasing residence time, and the trend for CH4 yield was completely opposite. Meanwhile, what was directly seen from the discharge, as the residence time increased, the amount of water vapor in the experiment also increased. By simplifying the reforming reaction into two stages, the tendency of H2 and CO yields associated with residence time can be explained [33]. The first stage, dehydrogenation and coupling reactions between n-heptane and high-energy electrons and active species produce large amounts of H2 and CO. The primary reaction at this stage is the cleavage of hydrocarbon fuel molecules. The alkyl radicals of C1eC7 continue to undergo dehydrogenation reaction. Other side reactions also occur and the reaction rates are much lower than the dehydrogenation reaction. Most of the n-heptane molecules are decomposed in the first stage. Therefore, the reaction rates of the dehydrogenation process of the alkyl radicals rapidly decreases in the second stage. The main products, H2 and CO, continue to react with the oxidizer in the system to form H2O and CO2. The secondary oxidation reaction was enhanced with an increase in residence time. As a result, the yields of H2 and CO decrease with the residence time increases. The productions of H2O and CO2 increase with the residence time increases. CH4, the most stable alkane, has formed by coupling reactions of methyl radicals with H radical in the first stage. However, CH4 was decomposed as the reaction time increases, resulting in a decrease in the yield of CH4.
The effect of discharge gap In the chemical process assisted by gliding arc plasma, the arc length and the effective plasma zone can affect the distribution and yields of the products. The discharge gap of the GAP reformer is directly related to the arc length [37]. To examine the effect of discharge gap on plasma oxidative reforming performance, a set of tests were conducted with the discharge gap of 3.8, 4.3, 5.8 and 6.5 mm. The maximum discharge gap of 6.5 mm was limited by the size of glass reactor. Fig. 5 illustrates the gas products yields under different discharge gap. Yields of H2 and CO increase dramatically as discharge gap increases. However, yields of CH4 and CO2 were almost unaffected by the discharge gap. In this study, the discharge arcs
of different discharge gap can slide toward the top of the blade electrode to obtain the longest arc. On the one hand, the longer arc length can improve the actual discharge power and promote the oxidation reforming reaction. On the other hand, a longer arc can increase the effective plasma zone and generate more radicals. Which increase the probabilities of collision among radicals and contributes to completely dissociate n-heptane. So the yields of H2 and CO significantly increase as the discharge gap increase. In fact, the effect of the discharge gap on plasma oxidative reforming performance should be considered in conjunction with the energy input in the system. Under sufficient energy input conditions, a larger discharge gap facilitates the growth of the arc length and can promote the yields of H2 and CO. In the case of insufficient input power, the arc can only glide in the middle of the counter electrode, resulting in reduction in the effective plasma reaction zone and decrease in energy utilization. In general, one strategy to achieve maximum energy utilization is combination the effects of input energy as well as discharge gap.
The plasma reforming performance comparison among C7H16/air, C7H16/N2/O2/Ar and C7H16/O2/Ar It is reported that inert gas (Ar, He) as a carrier gas for the plasma reactor contribute to the conversion of hydrocarbons [28]. To quantify the effects of inert gas (Ar, N2) in the plasma oxidative system, the plasma oxidative reforming performance of C7H16/air, C7H16/N2/O2/Ar and C7H16/O2/Ar were investigated. The contents of N2, O2, Ar in C7H16/air, C7H16/N2/ O2/Ar and C7H16/O2/Ar as shown in Table 1. The effects of N2 and Ar on the oxidative reforming of nheptane were investigated by adjusting the contents of Ar and N2 (O/C ¼ 0.71, t ¼ 24.7 s). The content of Ar is defined as the volume flow of Ar as a percentage of the total volume flow of Ar, O2 and N2. As presented in Fig. 6, the results show that the oxidative reforming performance for C7H16/N2/O2/Ar (56.0%) is better than C7H16/O2/Ar (46.0%) and C7H16/air (50.1%). The worst performance was gained by C7H16/O2/Ar. A variety of radicals of oxygen molecules and oxygen atoms produced by O2 plasma. The lowest excited oxygen atom O (1D) has high reactivity and can react with hydrocarbons to form syngas at room temperature [38], as shown in Eq. (11) e (17). It has been reported [39,40] that the threshold of dissociation and ionization caused by electronic collision decreases as the number of carbon atoms in the n-alkane molecule increases, as shown in Table 2. The cracking of nalkane mainly includes chain scission reaction and dehydrogenation reaction. The chain scission reaction is more advantageous as the number of carbon atoms in the alkane
Table 1 e Contents of N2, O2, Ar in C7H16/air, C7H16/N2/O2/ Ar and C7H16/O2/Ar.
Fig. 5 e Effect of discharge gap on yields of gases products. (t ¼ 24.7 s, O/C ¼ 0.71, input power ¼ 30 W).
System
O/C
Content of Ar/%
Content of N2/%
Content of O2/%
C7H16/air C7H16/N2/O2/Ar C7H16/O2/Ar
0.71 0.71 0.71
0 10e30 79
79 69e49 0
21 21 21
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process is a highly exothermic reaction due to the dissociation energy of O2 is much lower than excitation energy of Ar atom [32]. The extra heat release is not beneficial for the n-heptane oxidative reforming system. Thus, the amount of O (1D) generated by Ar* should be significantly smaller than the O P 3 3 (1D) produced by N2 [ðA3 þ u Þ, n), (B Pg, n), (C Pu, n)] and the direct electron collision dissociation of O2. The energy levels P 3 3 ofN2 ðA3 þ u Þ, N2 (B Pg) and N2 (C Pu) are 6.17 eV, 7.35 eV and 11.0 eV, respectively. They can form various Cx Hy radicals and H radicals by directly dissociating hydrocarbons fuel molecules, as shown in Eq. (21)e(25). For these reasons, it can be clearly stated that the oxidative reforming performance of the C7H16/Ar/O2 is inferior to C7H16/Ar/N2/O2. It also shows that N2 plays an important part in promoting the oxidative reforming performance. Fig. 6 e The yields of gas products of C7H16/air, C7H16/N2/ O2/Ar and C7H16/O2/Ar. (Discharge gap ¼ 6.5 mm, t ¼ 24.7 s, O/C ¼ 0.71, input power ¼ 30 W).
N2 ðA3
X
þ ; B3 Pg ; C3 Pu Þ þ O2 / N2 þ O þ O ð1 DÞ
(18)
þ ; B3 Pg ; C3 Pu Þ þ O/ N2 þ O ð1 DÞ
(19)
u
N2 ðA3
X u
increases. In summary, the various radicals produced by the dissociation of oxygen molecules provide energy for the cracking of n-heptane and promote its conversion to syngas.
Ar* þ O2 /Ar þ O ð3 PÞ þ Oð1 D; 1 SÞ N2 ðA3
O ð DÞ þ CH4 / CH3 , þ ,OH
(11)
O ð1 DÞ þ CH4 / CH2 O þ H2
(12)
N2 ðA3
O ð1 DÞ þ CH4 / CH3 O, þ H,
(13)
N2 ðA3
1
X
(20)
þ ; B3 Pg ; C3 Pu Þ þ CH4 / N2 þ CH3 , þ H,
(21)
þ ; B3 Pg ; C3 Pu Þ þ C2 H6 / N2 þ C2 H4 þH2
(22)
þ ; B3 Pg ; C3 Pu Þ þ C3 H8 / N2 þ C3 H6 þH2
(23)
þ ; B3 Pg ; C3 Pu Þ þ C2 H4 / N2 þ C2 H3 , þ H,
(24)
þ ; B3 Pg ; C3 Pu Þ þ C3 H6 / N2 þ C3 H5 , þ H,
(25)
u
X u
X u
O ð1 DÞ þ CH4 / CH3 OH
(14) N2 ðA3
O ð1 DÞ þ C2 H4 / CH3 , þ HCO, O ð DÞ þ C2 H4 / C2 H3 , þ ,OH
(16)
O ð1 DÞ þ C2 H4 / CH2 , þ CH2 O,
(17)
Previous studies showed that the dominant reaction pathway for atomic oxygen generation is e* þ O2 ¼ e þ O þ O (1D) [41]. In N2/O2, the electronically excited states of N2 P 3 3 [N2 ðA3 þ u , n), (B Pg, n), (C Pu, n)] can be quenched by O2 molecules [42,43], as shown in Eqs. (18) and (19). The existence of electronically excited states of N2 in the system will be described in section OES identification below. Besides, the excited oxygen atoms were also produced by the dissociative quenching of the electronic excited Ar atom (Ar*) by O2 [32], as shown in Eq. (20). The worse performance of C7H16/O2/Ar compare to C7H16/ N2/O2/Ar can be explained by the following reasons. Although the Ar* can also be quenched by O2 to produce O (1D), the
Table 2 e Dissociation and ionization potentials. Molecule CH4 C2H6 C3H8 C4H10
u
(15)
1
Ionization, eV
Dissociation, eV
13.6 11.99 11.51 11.09
9.7 8.7 8.1 7.7
X
N2 ðA3
X u
It is found that the better plasma oxidative performance is achieved by C7H16/N2/O2/Ar compare to C7H16/air. According to report [44], the addition of Ar to the reaction system can increase the electron density and electron temperature. It accelerates the rate of cleavage of n-heptane by accelerating the frequency and efficiency of collisions between electrons and fuel and oxygen molecules. In addition, Penning ionization effect of Ar* atoms can also be considered. The excitation energy of the Ar3P2 is as high as 11.49 eV, and the internal energy can be transferred to the ground state n-heptane molecules [45]. Another reason for Ar/N2/O2/C7H16 to achieve better reforming performance is that additional channel of O2 dissociation by Ar* as mentioned above. It also shows that Ar contributes to the improvement of oxidative reforming performance.
Exploration on the mechanism of oxidative reforming by GAP OES identification The optical emission spectroscopy (OES) has been used to identify the active species formed in the gliding arc plasma. It
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 ) 2 2 8 3 1 e2 2 8 4 0
helps to clarify the conversion mechanism of n-heptane in air plasma. As shown in Fig. 7, the global spectrum of C7H16/air discharge was detected in the wavelength range of 200e1100 nm. The second positive systems (SPS) generated by the N2 (C3Pu) to N2 (B3Pg) transition were detected at the wavelength from 297 to 357 nm, which is the strongest and clearest band in the nitrogen emission spectrum. The energetic states of excited states N2 (C3Pu) and N2 (B3Pg) are 11.0 eV and 7.35 eV, respectively. By observing and analyzing the N2 (C3Pu / B3Pg), the existence of N2 (B3Pg) in the gliding arc plasma oxidative reforming system was proved. N2 plays an important role in the plasma oxidative reforming of n-heptane reaction system. Hydrocarbon fuels can be quenched by the excited state of N2 while generating large amounts of CxHy radicals and H radical. In addition, it is also involved in the production of O (1D) which promotes oxidative reforming performance. The details have been discussed in section The plasma reforming performance comparison among C7H16/ air, C7H16/N2/O2/Ar and C7H16/O2/Ar above. CH (B2S/X2P, 388.31 nm) [46] is considered to be one of the important radical of n-heptane chain scission and dehydrogenation. The CH radical (2.9 eV) were formed by collision of high-energy electrons and n-heptane molecules with alkyl radicals, as shown in Eq. (26)-(28) [47]. At the same time, the Ha line derived from the hydrogen 1Balmer line was also detected at 656 nm [46]. C7 H16 þ e / C6 H13 , þ CH3 , þ e
(26)
CH3 , þ e / CH, þ H2 þ e
(27)
CH2 , þ e / CH, þ H, þ e 3
(28) 3
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The radiative lifetime of the metastable O (1D) (1.967 eV) is rather long (~100 s) [49], the emission spectra of O (1D) is generally not observed. Moreover, the energy level of O (1D) is rather low, so the concentration of O (1D) is relatively high in the discharge region. The wavelength of the oxygen atom line in Fig. 7 and the theoretical parameters in Refs. [50,51] are comprehensively analyzed. It can be seen that during the C7H16/air discharge, a large number of excited oxygen atoms between 9.15 and 15.78 eV were generated. The spectra of O (3D - 3P0), O (5P - 5S0), O (3D - 3D0), O (3P - 3S0) appeared at 748.2, 777.4, 823.6 and 844.7 nm, respectively. The chemical activity and internal energy of O are very high, which can break the chemical bonds of almost all organic compounds. It is an effective radical in the process of reforming hydrocarbons by plasma oxidative. In addition, the spectra of Oþ (4P0-4P) and Oþ (4S0-4P) appeared at 416.7 and 488.9 nm, respectively. The transition energy level of Oþ is very high (greater than 22 eV) [52]. It indicates that Oþ has higher potential energy than O, and it can provide the necessary energy source for conversion of stable organic compounds. Generally, the routes of production of Oþ may be as follow: þ 4 3 2 Oþ 2 ðX Pg Þ þ e / O ð SÞ þ O ð PÞ þ e
(32)
Liquid products The liquid products compositions of the n-heptane plasma oxidative reforming were analyzed by GC-MS. The detected organic by-products and GC-MS are presented in Fig. 8. The reaction mechanism in the n-heptane plasma oxidative reforming system can be presumed by these products in the following.
3
The C2 swan bands of (d Pg / a Su, 468e474 nm), (A Pg / X3Pu, 500e515 nm), (A3Pg / X3Pu, 550e590 nm) were detected, which may be generated as follows [48]. CH , þ C, / C2 , þ H,
(29)
C , þ C, / C2 ,
(30)
C2 H , þ C2 H, / C2 , þ C2 H2
(31)
Fig. 7 e OES analysis of n-heptane plasma oxidative reforming system. (Discharge gap ¼ 6.5 mm, O/C ¼ 0.71, t ¼ 24.7 s, input power ¼ 30 W).
Mechanism of n-heptane plasma oxidative reforming According to the analysis above, the conversion mechanism of n-heptane in the plasma oxidative reforming system is shown in Fig. 9. In this study, the conversion mechanism proposed can be divided into two steps: the cracking of nheptane and reforming reactions dominated by alkyl radicals. The processes of the cracking of n-heptane mainly include three pathways: one is collision of accelerated electrons with n-heptane molecules directly shown in Eq. (32), other is reaction between active oxygen species (such as O (1D), Oþ and O) with n-heptane. In addition, the main sources of reactive
Fig. 8 e GCeMS diagram of liquid products in the nheptane plasma oxidative reforming system.
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Fig. 9 e Conversion mechanism of n-heptane in the plasma oxidative reforming system.
oxygen species ([O]) are the direct collision with electrons and the quenching reaction with the electronic excited state of nitrogen (N2 (B3Pg)). Another pathway is the direct reaction between N2 (B3Pg) with n-heptane, the cracking of n-heptane largely produces H radical and alkyl radicals. A large amount of H radical combines to form a stable hydrogen molecule. Furthermore, the alkyl radicals can continue to react with the electrons, active oxygen species and N2 (B3Pg). As shown in Fig. 2 (b), gas hydrocarbon by-products (CH4, C2H6, C2H4, C3H6, C3H8, C4H10, Cis-2-butene, Trans-2-butene, a-butane, and isobutene) can be detected. Fig. 8 shows the presence of acetic acid, propionic acid and ether in the liquid products. Generally, the production of olefins were formed by the dehydrogenation reaction of alkyl radicals under electron collision. Furthermore, the olefin is oxidized by the active oxygen species to form a carboxylic acid. Ether is formed by the interaction of two alkyl radicals and active oxygen species. The reaction of the alkyl radicals and the active oxygen species can simultaneously generate CO and H2. When O/C are relatively high, CO and H2 are further oxidized to form CO2 and H2O. Conversely, CO2 and H2O can also generate CO and H2 by electron impact, but the process is relatively difficult.
yield of CO (32.9%) and H2 (50.1%) were obtained. Besides, the performance of C7H16/air, C7H16/N2/O2/Ar and C7H16/O2/Ar were compared in terms of yield of H2. Similarly, the electronic excited states of N2 and excited Ar atom can be quenched by O2 to produce O (1D), which is favorable for hydrogen generation. However, the latter is less efficient for generating reactive atomic oxygen than the former. More importantly, the electronic excited state of N2 also can directly react with hydrocarbons to form H radical and alkyl radicals ($CH3, $C2H5, etc.) and promote the formation of hydrogen. The plasma parameters were obtained by monitoring n-heptane oxidative reforming in the GAP reformer using OES. The spectrum of excited state of N2 (SPS) which facilitates the conversion of n-heptane and formation of reactive oxygen species were detected. CH (B2S/X2P, 388.31 nm), C2 swan bands and Ha line were detected, which are important radicals involved in plasma oxidative of nheptane reforming. The spectra of O and Oþ were observed, which own higher energy and provide the sufficient energy for the conversion of n-heptane. Besides, the liquid products were analyzed by GC-MS and indicate the existence of acetic acid, propionic acid and ether. The mechanism of hydrogen production from gliding arc plasma oxidative reforming of nheptane is proposed based on the diagnosis of the OES and the analysis of GC-MS.
Conclusions In this work, the effects of input power, O/C, residence time and discharge gap on hydrogen production from n-heptane plasma oxidative reforming were investigated. The optimal parameters were 30 W, 0.71, 24.7 s and 6.5 mm, respectively. The maximum energy yield of H2 (94.5 L (kW$ h)1) and the
Acknowledgments This work is financially supported by the National Key Research and Development Program of China (No. 2016YFB0600703).
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 ) 2 2 8 3 1 e2 2 8 4 0
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