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N-decane decomposition by microsecond pulsed DBD plasma in a flow reactor
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 ) 3 5 6 9 e3 5 7 9
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N-decane decomposition by microsecond pulsed DBD plasma in a flow reactor Feilong Song a, Yun Wu a,b,c,*, Shida Xu a, Di Jin a, Min Jia a a
Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi'an, 710038, China Science and Technology on Plasma Dynamics Laboratory, Xi'an Jiaotong University, Xi'an, 710049, China c Institute of Aeroengine, Xi'an Jiaotong University, Xi'an, 710049, China b
article info
abstract
Article history:
N-decane decomposition with plasma to obtain prone detonation gaseous fuel is investi-
Received 1 November 2018
gated by adopting fuel liquid film feeding mode at applied voltage of 10 kV and 15 kV,
Received in revised form
discharge frequency of 300e3000 Hz with interval 100 Hz. The result shows that the
9 December 2018
discharge parameters can regulate the composition and the concentration of the cracked
Accepted 13 December 2018
products. When the composition of the product is unchanged, the concentration and
Available online 5 January 2019
proportion of the components can be increased as the discharge frequency becomes higher. The hydrogen concentration increases 3.4 times and 2.3 times respectively for the
Keywords:
applied voltage of 15 kV and 10 kV as the discharge frequency is increased from 500 to
n-decane
3000 Hz. However, the voltage has no effect on the proportion of the components, and
Decomposition
increasing the voltage only increases the concentration of the species. For selectivity,
Plasma
raising the discharge frequency can greatly increase the species selectivity of hydrogen and
Selectivity
acetylene that are most prone to detonate. The selectivities of hydrogen and acetylene are the highest at 15 kV and 3 kHz, reaching 18.40% and 4.22% respectively, and at this time, the selectivity of ethylene is the lowest, which is 60.4%. Besides, it is confirmed that high selectivity can be achieved at the lower specific energy input causing by low-voltage/highfrequency discharge. Through the analysis of the bond dissociation energy, there is a competitive relationship between the production of ethylene, ethane and pentane which all consume ethyl, and two possible pathways for the production of acetylene are put forward. © 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction The thermal cycle efficiency of rotation detonation engines (RDE) is about 30% higher than that of conventional gas turbine engines. At present, RDE has been an international research hotspot and frontier of military aviation power, and it will greatly reduce the fuel consumption [1e3]. However,
kerosene/air detonation has been the core technology and key problem. The energy required for the direct initiation of kerosene/air mixture is very large and difficult to achieve. The auxiliary detonation effects of mixed hydrogen, supplemental oxygen and preheating methods are also limited [4e6]. By using the method of cracking the kerosene, it can provide more small molecular fuel and active group and increase the chemical reaction rate. It is expected to significantly reduce
* Corresponding author. Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi'an, 710038, China. E-mail address: [email protected] (Y. Wu). https://doi.org/10.1016/j.ijhydene.2018.12.100 0360-3199/© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
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the initiation energy, shorten the initiation time and increase the detonation frequency, so as to provide an effective way to solve the problem of kerosene/air detonation. The existing cracking methods include thermal cracking [7,8], catalytic cracking [9] and microwave plasma cracking [10]. Among these traditional technologies of fuel decomposition, thermal cracking is the most mature, but its development is limited due to its high energy consumption, high reaction temperature, poor selectivity for products and long operating cycle. As for the catalytic cracking, the conventional cracking catalyst such as molecular sieve will face the problems of insufficient activity, easy poisoning and serious carbon deposition in the long term work. Moreover, due to the presence of the catalyst, the operable temperature range is narrow and the response time is long. These problems severely restrict the engineering application of catalytic cracking. Plasma technology uses high-energy electrons to collide with neutral molecules in the fuel, causing dissociation, excitation and even ionization, generating large amounts of active atoms, excited-state particles, ions, electrons and other intermediate products. The increase in the activity of the intermediate particles in the chemical chain reaction ultimately affects the distribution of components in the combustion system and accelerates the combustion chemical kinetics [11e14]. Zhu [15] has applied plasma to the hydrogen/air detonation. Under his experimental conditions, plasma can significantly improve the detonation performance with combustion annulus widths of 4.75 mm: the RDE cannot be successfully detonated without plasma assistance, while high efficiency detonation can be achieved when plasma works. At present, the research progress of plasma assisted combustion is outstanding. Starikovskaya et al. experimentally verifies that atomic O can strengthen the chain reaction in the kinetic characteristics of plasma enhanced combustion [16]. Prof. Ju's team, through a large number of optical measurements and simulation predictions, has proved the dynamic role of active groups such as O, OH and H in the process of fuel oxidation and ignition, and has successfully predicted the reaction channel of plasma ignition and combustion [17,18]. Tsolas et al. [19] have simulated and predicted the process of plasma assisted H2 and C2H2 fuels cracking and oxidation. The results show that the collision quenching of electron excited state of argon (Ar*) and C2H2 fuel molecules is the direct path of low-temperature fuel consumption. In addition, the plasma degradation of gaseous organic pollutants [20], the use of nonequilibrium plasma to synthesize the greenhouse gases into high value fuels [21e23] and biofuels decomposition with plasma for the production of hydrogen rich gases [24] have been carried out a great deal of research, providing a large number of mechanism of plasma decomposition reaction. However, there are few studies on plasma cracking of hydrocarbon species in the kerosene components. Zhang et al. [25] introduces argon gas bubbles into n-dodecane/water mixture and uses needle-plate discharge plasma to crack the fuel at the discharge frequency of 4.6 kHz, and methane or carbon dioxide is mixed with argon to change radical composition to achieve the purpose of selecting the product; Yao et al. [26] uses dielectric barrier discharge plasma to crack gaseous n-decane at the discharge frequency of 250 Hz. The
types of carrier gas are argon, nitrogen and oxygen/nitrogen. The analysis shows that the initial decomposition of n-decane is generated by the dehydrogenation reaction to produce decane free radicals which are further broken bonds to form small molecules; Reddy [27] uses temperature-controlled DBD to crack n-heptane/water mixture, with different discharge frequencies for different carrier gases, 4.8 kHz, 5.5 kHz and 5.2 k Hz for Ar, CH4 and CO2, respectively, to achieve the goal of controlling the short-chain hydrocarbon and oxygenate concentrations in the product. The above experiment is to study the relationship between the type of carrier gases and the products obtained by the plasma cracking fuel at the specific voltage and discharge frequency, and there is no systematic study about the effect of the discharge frequency on the product. Besides, the existing research lacks the analysis of the mechanism. Ju [17] divides the role of plasma into three major mechanisms: thermal enhancement mechanism, kinetic enhancement mechanism, and transport enhancement mechanism. Since the flow reactor is used in this study, the influence of the transport enhancement mechanism is weakened, and at the same time, the microsecond pulse power supply is employed as the plasma power supply, which could produce uniform and stable non-equilibrium plasma, so temperature rise effect on the reaction system can be ignored. Therefore, this experiment focuses more on the kinetic characteristics of the plasma decomposition. In this paper, a representative alternative fuel of kerosene, n-decane, is used as the treated object, and a new cracking method that fuel liquid film is cracked by plasma in the flow reactor is adopted. The distribution and concentration of the products are performed by gas chromatography (Agilent 7890 b). The effect of the discharge frequency on the products distribution under different applied voltages is systematically studied to provide experimental data for kinetic models.
Experimental approach Experiment is conducted using a plasma flowing reactor, as shown in Fig. 1. Liquid n-decane (>99% in purity, Huaxia, Chengdu, China) is placed in a quartz tube with internal diameter of 6 mm to form a liquid column. The liquid column height is kept at 140 mm, so that the distance from the top of the liquid column to the bottom of the discharge plasma is 50 mm. Through the Teflon sleeve, the bottom of the quartz tube is connected with the argon supply pipe of which top has a orifice with an internal diameter of 1.5 mm. Argon (with a purity of 99.999%) flow rate is controlled at 30 sccm using a mass flow controller (DSN-MFC-400 A) with an accuracy of 1% and the gas flows out from the tip of metal nozzle at the bottom of the liquid column to form a large number of bubbles. As the argon bubbles rise, the pressure gradually decreases, eventually resulting in an increasing volume of argon bubbles. When the bubbles rise to the liquid surface, the bubble size is close to the inner diameter of the quartz tube. As a result, many bubbles gather here, and the liquid film is intertwined. With the continuous supply with argon, the liquid n-decane film keeps rising and forms a uniform flow state before reaching the discharge area.
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Fig. 1 e Schematic of the plasma cracking fuel film experimental setup. The core part of the experimental system is the plasma reactor of which details can be found in Refs. [28] and only a brief description will be outlined here. The volume of the plasma region formed by the dielectric barrier discharge is 1200 mm3 with 6 mm discharge gap, as shown in Fig. 2. So, the liquid film flow needs 2.4 s to pass through the plasma area at a flow rate of 30 sccm. This means that, at a discharge frequency of 500e3000 Hz, the flow will experience 1200e7200 discharge pulses. Under all experimental conditions, there are no filament discharge formation. Besides, interestingly, the liquid film disappears after it enters the plasma zone, and no liquid film remains behind the plasma for all experimental conditions, indicating that the n-decane liquid film has been cracked into gaseous substances or vaporized. The reactor is power by a laboratory-made microsecond pulsed power supply of which rise time is 1.0 ms. The pulse frequency can be continuously adjusted from 0 Hz to 5 kHz controlled via a digital pulse external triggering generator (FY3200S). The voltage is regulated by a voltage regulator and the adjustment range is 0e15 kV. In the experiment, a 10 U
Fig. 2 e Scheme of the plasma reactor.
non-inductive resistance is connected between the ground and the low voltage copper electrode. The high-voltage probe (P6015A) and the passive high-resistance probe (RP3500A) with attenuation ratios of 1000:1 and 10:1, respectively, are used to measure the total voltage and the voltage across the resistor. The measured voltage values are recorded by the oscilloscope (Tektronix MDO3024). Based on the voltage values obtained, the discharge current can be calculated by dividing the voltage across the resistance by the resistance value, and then integrating the current and total voltage to obtain energy, resulting in energy deposition of ~83.2 mJ and ~137.7 mJ per pulse for the total voltages 10 kV and 15 kV respectively. Since the voltage across the resistor is three orders lower than the total voltage, the power consumed by the resistor is ignored in calculating the plasma discharge power in this paper. After n-decane liquid film decomposition with plasma, some species among the cracked gas are liquid at room temperature. These species are separated after condensation and the remaining gaseous products are analyzed by the gas chromatograph (Agilent 7890 B). The GC is configured to analyze the permanent gas and the hydrocarbons from C1 to C5. Quantitative analysis of hydrogen, alkanes, alkenes and alkynes containing three carbon atoms and fewer, alkanes and alkenes containing four carbon atoms, n-pentane and isopentane can be performed. Among them, hydrogen is analyzed by a thermal conductivity detector with a detection limit of 200 ppm; hydrocarbons are analyzed by a flame ion detector with a detection limit of 2 ppm. The experiment is performed at two total voltages of 10 kV and 15 kV. At each voltage, the frequency is adjusted from 500 Hz to 3000 Hz with the interval of 100 Hz. There are 32 experimental conditions totally, and all experiments are performed at room temperature. Besides, the cracking experiments are repeated three times for each condition with the product concentration error less than 3%.
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Results and discussions Discharge waveforms The oscillation attenuation of the voltage signal representing the current obviously consists of two parts over one high voltage pulse (HVP), as shown in Fig. 3. The first oscillation decay starts at the rising edge of the HVP, and the second one starts at the moment of the end of the HVP belonging to capacitive discharge. The reasons for the former are analyzed here. Due to the presence of the barrier dielectric, the charge generated during the discharge gradually accumulates on the surface of the dielectric, which means that the equivalent capacitance of the barrier dielectric is charged. The electric field formed by the accumulated charges is opposite to the direction of the applied electric field. Since the voltage of the discharge gap is the applied voltage minus the voltage formed by the accumulated charges, as the discharge continues, the reverse voltage will rise rapidly, causing the discharge gap voltage to drop rapidly. When the effective voltage drops to be insufficient to maintain the discharge, the discharge current rapidly decreases. As a result, the pulse width of the discharge current is very narrow at each breakdown between the dielectrics. After the discharge completed, the charging progress of the equivalent capacitance is ended, and the reverse voltage remains unchanged. However, the voltage of the discharge gap varies with applied voltage. When the applied voltage continues to rise, reaching the required breakdown voltage, the discharge channel is formed again and the second current peak appears. Afterwards, as the loading voltage continues to rise, similar discharge current decays and surges occur again, resulting in multiple current peaks within one pulse of applied voltage. It can be seen from Fig. 3 that the peak value of the discharge current gradually decreases during multiple discharges. This is attributed to the short interval between the two breakdowns. A large amount of discharge residues left by the previous breakdown, such as charged particles, highenergy metastable particles, etc., make the later breakdown more likely to occur.
Fig. 3 e Typical waveforms of voltage and current over one pulse discharge duration.
By reason of charge accumulation in the process of multiple discharges, the second oscillation attenuation occurs by the form of capacitive discharge after the discharge of HVP, and the discharge channel is not formed at this time.
Effect of applied voltage and discharge frequency on gaseous products content The products detected in the experiment include 9 species, which are hydrogen, methane, ethane, ethylene, acetylene, propane, propylene, butane and n-pentane. Other species may also be produced but not detected because their concentrations are below the detection limit of the gas chromatogram. The detected species are not produced under any experimental conditions, as shown in Fig. 4, but are generated in turn as the discharge voltage or frequency increases. The experiment is performed under two loading voltages. When the applied voltage is 10 kV, the cracked gas contains hydrogen, ethylene and propane at the lowest frequency of 500 Hz. As the frequency increases, ethane occurs at 700 Hz, npentane at 800 Hz, n-butane at 1000 Hz, acetylene at 1500 Hz and methane at 1600 Hz. At the voltage of 10 kV, there is no propylene produced in the cracked gas at all discharge frequencies. When the loading voltage is improved to 15 kV, most species have already appeared the lowest frequency, including hydrogen, ethane, ethylene, propane, n-butane, and n-pentane. At the same time, the discharge frequencies corresponding to the generation of acetylene and methane are also greatly reduced, which are 600 Hz and 800 Hz, respectively. In addition, the increase in the applied voltage causes new specie to appear in the product, with propylene detected at the discharge frequency of 1300 Hz. The applied voltage and the discharge frequency have a great influence on the yield of the product. It can be seen from Fig. 5 that the ethylene yield rapidly decreases with the increase of the discharge frequency, but the ethylene yield is always the highest under any experimental conditions. This phenomenon is attributed to that the main precursor for generating ethylene is ethyl radical. On one hand, the ethyl radical can be produced in large amounts by the b-scission reaction of macromolecular hydrocarbons. On the other hand,
Fig. 4 e Schematic diagram of the species present under each experimental condition. The applied voltage without pattern filled is 10 kV, and the applied voltage with filling pattern is 15 kV.
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Fig. 5 e Production rate of (a) C2H4, (b) H2, (c) CH4, C2H6 and C3H8, (d) C2H2, n-C4H10, n-C5H12 and C3H6 in the cracking gas with various discharge frequencies. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
as the discharge frequency increases, the yield of other species increases, as shown in Fig. 5, and at the same time, more new species are produced. The production of other species consumes more ethyl radicals, which leads to the decrease in the production rate of ethylene with the increase of discharge frequency. Fig. 5 also illustrates that the increase in frequency is more beneficial for increasing the species yield when the loading voltage is higher. For example, for hydrogen, increasing the frequency from 500 Hz to 3000 Hz can increase the yield by a factor of 3.4 when the applied voltage is 15 kV, while it increases the yield by 2.3 times at the voltage of 10 kV. In addition, the increase in voltage can also increase the product yield. As can be seen from Fig. 5, when the discharge frequency is higher, increasing the voltage is more conducive to improve the product yield, especially for hydrogen, methane and acetylene. For example, due to the increase in voltage, the hydrogen yield increases by a factor of 2.0 at 3000 Hz, and by a factor of 1.3 at 500 Hz; the methane yield increases 2.0 times at 3000 Hz and 1.6 times at 1600 Hz; for acetylene, the yield increases 2.1 times at 3000 Hz and 1.8 times at 1500 Hz. According to the product yield and the appearance order with each species in the product, the difficulty degree to crack n-decane into each component from the easiest to the hardest is: ethylene, hydrogen, propane, ethane, methane, acetylene, n-butane, n-pentane and propylene. Combined with the above analysis, the higher the applied voltage and the discharge frequency are, the higher the product yield will be. However, under actual circumstances, the higher voltage will make the possibility of creepage or breakdown increase significantly and there will be potential safety hazards. In fact, it is not inclined to select high voltage under industrial condition, and other than that, increasing the frequency is easier to implement. Therefore, in order to
increase the yield of cracking products, it is a good choice to increase the discharge frequency in the case of relatively low voltage. In Fig. 6, the concentration of the components contained in the product is compared between this cracking experiment and the previous cracking experiment [29] in which plasma is used to split liquid n-decane directly at the room temperature. The components involved in the comparison include hydrogen, ethylene, ethane, propane, butane, and pentane, and they are all produced at the same experimental condition in both experiments, for which the applied voltage is 10 kV and the discharge frequency is 3 kHz. The results show that in the experiment of cracking liquid film, the cracking effect is
Fig. 6 e Comparison on the concentration of the product present in both this experiment and the previous experiment [29]. Applied voltage 10 kV and discharge frequency 3000 Hz.
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better, the ethylene concentration drops sharply, and the concentrations of other species increase. In addition to the effects resulting from the modification of the power supply, there are two factors responsible for it. On the one hand, for the liquid film decomposition experiment, the heat generated by the discharge can be more fully used to heat the liquid ndecane to evaporate, and after the liquid fuel becomes gaseous molecule, it will react with the plasma better. However, for the experiment of cracking liquid n-decane, the heat generated by the discharge is dissipated to heat the fuel that is not in the plasma region, so that the heat is not fully utilized. On the other hand, it may be the cause of the carrier gas flow rate. The carrier gas flow rate for this experiment is 30 mL/ min, while in the previous experiment, the carrier gas flow rate is 1 L/min, so at the same discharge frequency, the number of the discharge pulse experienced per unit time by the flow is different. For this experiment, the number of pulses experienced is more, resulting in a better cracking effect. Discharge parameters can not only change the concentration of the cracking product, but also have an effect on the percentage of each component in the product. The effect of discharge voltage and frequency on the volume percentage of each species is shown in Fig. 7. The percentage change curves of each species in the graph all show an upward trend with the increase of the discharge frequency, except for ethylene. In addition, the percentage in the product corresponding to each species is almost unchanged at two different loading voltages. This indicates that the increase in frequency promotes the percentage of the corresponding species on the basis of increasing the yield. However, increasing the voltage only promotes the yield of
the components and has no effect on the percentage of each component in the product. The possible reason for this phenomenon is that: due to the increase in voltage, the electric field strength (E) between discharge gap becomes larger. Based on the formula mv2/2 ¼ eEl (where l, m and e are the electron mean free path, electron mass and electron charge amount, respectively), the electron gains higher energy, and its excitation cross section with the argon atom will increase, leading to generate more excited argon atoms. In turn, more collisional cracking reactions occur with the fuel molecules to obtain more free radicals or precursors required for the production of the product, so that the concentration of each particle in the reaction pool is proportionally increased. As a result, although the concentration of each species increases in the product, the percentage of each substance is not affected by the voltage.
Selectivity and energy consumption analysis Selectivity refers to the proportion of a certain product in the cracking gas. Hydrogen selectivity refers to the proportion of hydrogen atoms contained in hydrogen in all hydrogen atoms contained in gas-phase products formed by the decomposition of n-decane. The heat value of hydrogen reaches 1.4 108 J/kg, which is higher than the heat value of all fossil fuels. And hydrogen is extremely flammable. If a small amount of hydrogen is generated at the outlet of the fuel injector by cracking kerosene, it will play an important role in improving the ignition performance of the engine and expanding the ignition boundary. Therefore, hydrogen selectivity is an important parameter for evaluating the cracking
Fig. 7 e Molar percentage of (a) C2H4, (b) H2, (c) CH4, C2H6 and C2H2, (d) C3H8, n-C4H10and n-C5H12 in the cracking gas with various discharge frequencies. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
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effect. For detonation engines, hydrogen as fuel is much more prone to detonation compared to the other common gaseous fuels at standard conditions. However, the probability of having a detonation in hydrogeneair mixture is not higher than that in other hydrocarbon fuels at elevated initial pressure. In Hoi Dick Ng's research [30], the acetylene-air mixture has a greater likelihood to detonate than hydrogeneair mixture at higher pressure. When gaseous fuel is used as the supply fuel for detonation engines, the characteristic cell size and the critical direct initiation energy are generally considered as two significant parameters to characterize the detonation sensitivity to the given mixture. The smaller the cell size and the critical direct initiation energy are, the more sensitive to detonation the mixture will be. Table 1 and Table 2 show the detonation cell size and the critical ignition energy of some gaseous species. From the table, it can be seen that, when the initial pressure, the initial temperature and the equivalent ratio are ~1 atm, ~293 K and ~1 respectively, the cell size and the critical ignition energy of hydrogen and acetylene are the smallest, and the corresponding parameter for ethylene is ranked third. Therefore, we selected hydrogen, acetylene and ethylene as the three target products to analyze the selectivity. The selectivity of hydrogen, acetylene and ethylene are calculated by the following equations: H2 selectivityð%Þ ¼
Amount of H atoms in the formed H2 Total amount of H atoms in the products 100%
Cx Hy selectivityð%Þ ¼
(1)
Amount of C atoms in the formed Cx Hy Total amount of C atoms in the products 100%
(2)
Here, x and y represent the number of carbon atom and the number of hydrogen atom, respectively, in a hydrocarbons molecule. The decomposition of n-decane is caused by the energy injected into the DBD reactor. Theoretically, the higher the energy injected into the reactor, the more favorable the conversion of n-decane. The specific energy input refers to the energy input per unit argon flow. The higher the specific energy input, the more high energy particles in the carrier gas will be produced to participate in the cracking reaction, thereby increasing the concentration of each component in the product. The specific energy input (SEI) in this study is defined as the input power of overall system divided by the flow rate of the total feed Ar (mL min1).
. SEI J,L1 ¼ 60000 Pinput Fin Ar
(3)
In Formula (3), the amount of input power is calculated by following equation: Z Pinput ðwÞ ¼
UðtÞ
UR ðtÞ dt frequence R
(4)
single cycle
The selectivity of the three target products under various experimental conditions is shown in Fig. 8, and the change regulation is similar to the rule of the volume percentage changing with the variety of the discharge frequency. The change of the loading voltage has an effect on the selectivity of ethylene. When the loading voltage and discharge frequency are high, the selectivity of ethylene is small, and the lowest selectivity of ethylene fell to 60.4%. In conjunction with the previous analysis, increasing the voltage is accompanied by the increase in the ethylene yield, and the percentage in the product remains almost unchanged. The phenomenon of the decrease of ethylene selectivity under high voltage shows that the n-decane molecules are cracked out to produce a large number of carbon atoms of which the proportion used to produce ethylene becomes smaller and the proportion used to generate other substances increases. Under all experimental conditions, the ethylene selectivity at high voltage decreases by an average of 3.9% compared to the low voltage conditions. For acetylene and hydrogen, increasing the discharge frequency can enhance selectivity, and the change of loading voltage has little effect on the two species. However, when the discharge frequency is higher than 2000 Hz and 2700 Hz, respectively, the selectivity of acetylene and hydrogen under high voltage conditions is slightly higher than that of low voltage conditions. In summary, there is no obvious advantage for improving the selectivity by increasing the voltage for the two gaseous fuels which are the most prone to detonation. In this experiment, hydrogen selectivity ranged from 3.62% to 18.40%, and acetylene selectivity varied from 0.76% to 4.22%. The selectivity of the target product is analyzed from the perspective of specific energy input. For the three target products, when the load voltage is increased at the same discharge frequency, the input energy in the unit flow carrier gas increases, causing the selectivity curves to shift horizontally to the right, as shown in Fig. 9. In other words, in order to achieve the same selectivity, more energy needs to be consumed when the high voltage is loaded. For example, at 3000 Hz, the specific energy input increased from 498.9 J/L to 826.1 J/L, about 1.65 times: the selectivity of ethylene decreased from 62.4% to 60.4%; the selectivity of hydrogen
Table 1 e Cell size data for gaseous fuels.
H2 CH4 C2H2 C2H4 C2H6 C3H8 C4H10
Initial pressure/kPa
Initial temperature/K
Equivalence ratio
Cell width/mm
Table number in Ref. [31]
100e101.452 101.3 101.3 92.5e101.3 92.5e101.3 92.5e101.3 101.3
293e300 293e298.15 293e298.15 293e298.15 293e298 293e298.15 293
0.9721e1.0233 1 1e1.3242 0.999537e1.070638 0.99325e1.0958 1e1.2925 1.0088
8.0e15.1 279.6e349.5 4.6e9.2 19.5e33.8 50.0e66.1 46.0e75.2 75.1
10, 13,30, 61 111, 112, 116, 119 120, 135, 154 157, 166, 170, 172, 173, 177 181, 202, 210, 216 182, 201, 211, 221 200
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Table 2 e Critical initiation energy data for gaseous fuels.
H2 CH4 C2H2 C2H4 C2H6 C3H8 C4H10
Initial pressure/kPa
Initial temperature/K
Equivalence ratio
Critical initiation energy/J
Table number in Ref. [31]
101.3 101.3 101.3 101.3 101.3 101.3 101.3
293 293 293 293 293 293 293
1e1.19051 1 1e1.0871 0.9866e1.0844 1.0765e1.157 0.99308e1.124 1.0169e1.13
4248.8e9040.0 88658800.0 4332.0e6780.0 55596.0e60568.0 77368.8e90137.8 215192.7e283114.7 196362.4e275824.0
325, 327, 328, 348 354 361, 362 368, 369, 373 374 375 376
Fig. 8 e Selectivity of (a) C2H4, (b) H2 and C2H2 with various discharge frequencies. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
Fig. 9 e Selectivity of (a) C2H4, (b) H2 and C2H2 with various specific energy input. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
changes from 18.3% to 18.4%; for acetylene, the selectivity increased from 3.97% to 4.22%. And, with the increase in specific energy input, in order to achieve similar selectivity, the difference in specific energy input required under the two loading voltages becomes greater. This fully demonstrates that the choice of low voltage and high frequency has great advantages for the decomposition of longer carbon chain molecules. It can not only get the higher selectivity of the target product, but also does not consume much energy. However, if greater yields are required, it is necessary to choose a higher discharge voltage. Usually, to assess energy efficiency, the sum of the calorific values of all products is divided by the sum of the discharge input energy and the calorific value of the input reactant, and the ratio is called the energy conversion efficiency. Combined with the test conditions currently available in the laboratory, the cracking gaseous products are comprehensively analyzed
in this experiment. In this paper, the energy conversion ratio is defined to indirectly evaluate the energy efficiency, and it refers to the ratio that the total calorific value of all gaseous species detected is divided by the input power. The energy conversion ratio (ECR) is calculated using equations: ECR ¼
LHVH2 Fout H2 þ
P
LHVCx Hy Fout Cx H y
The total input power
(5)
where LHV is the lower heating value (J mol1), F represents the molar flow rate of hydrogen and hydrocarbons in the products (mol s1). Fig. 10 shows the effect of discharge frequency and specific energy consumption on the energy conversion ratio. From the previous analysis, it is known that ethylene is the species with the highest concentration in the product. At the same time, the ethylene selectivity shows a decreasing trend with
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Fig. 10 e Energy conversion ratio with (a) various discharge frequencies, (b) various specific energy input. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
increasing discharge frequency and specific energy input. And this determines the changing trend of the energy conversion ratio. As can be seen from Fig. 10, the initial decline of the ECR is rapid and gradually becomes flat. This is because, as the discharge frequency increases, the yield of hydrogen with the highest calorific value gradually increases during the decrease of the ethylene yield, and the concentrations of other species also greatly increase, resulting in a slowing down in the rate of ECR decline. Fig. 10a confirms that the loading voltage has almost no effect on the ECR, and Fig. 10b further confirms the advantage that, at the same discharge frequency, choosing a low voltage can achieve similar ECR to high voltage, and at the same time the specific energy input is still very low.
Reaction path analysis Based on the changes in the species detected in the experiment and the concentration of each component, some possible reaction paths are constructed, as shown in Fig. 11. Because the bond dissociation energies (BDE) of the CeC bonds in the straight chain alkanes are about 32.6e61.9 kJ/mol
lower than that of the CeH bond, and the b-CeC bond is the weakest in the hydrocarbon molecules, followed by the g-Ce C. It is speculated that the initial reaction of n-decane decomposition is caused by the collision of energetic particles resulting in the break of b-CeC to form ethyl and octyl radicals, and the scission of g-CeC to form heptyl and propyl radicals. The reaction path is divided into three phases, which are respectively represented by red, blue and green in Fig. 11. In the first stage (shown in red), the ethyl radical obtained from the initial cracking reaction undergoes b-CeH scission to form hydrogen atom and ethylene, and the propyl group generates the ethylene and methyl group through the b-CeC scission. Ethyl is easily produced, and propyl radicals also generate ethylene, resulting in the highest ethylene yield in the product. Since hydrogen atom is derived from the b-CeH scission of the ethyl group, a large amount of hydrogen atoms attack the n-decane molecule or the macromolecular chain hydrocarbon radical to generate hydrogen, resulting in the second highest hydrogen yield. In addition, some of the hydrogen atoms and methyl groups react with the remaining reactants to form propane. The first-stage reaction path
Fig. 11 e Reaction paths of n-decane decomposition with discharge plasma.
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explains well that ethylene, hydrogen and propane are present in the product under all experimental conditions, and the concentration of ethylene is the highest, followed by hydrogen and propane. With the increase of the applied voltage and discharge frequency, the free radical pool in the dashed box in Fig. 11 is enlarged, supplementing more ethyl and propyl groups, resulting in the enhancement of the reaction path in the second stage (shown in blue). The excess ethyl and propyl groups are combined with the hydrogen and methyl radicals formed by their own bond-scission, respectively, to generate ethane and butane. At the same time, pentane is formed by the recombination reaction between ethyl and propyl. This also explains that these three species do not appear under all experimental conditions, and they only appear when the discharge frequency and the applied voltage are increased. Since the amount of ethyl and hydrogen atom is higher than that of other free radicals, the relative content of ethane in these three species is relatively high. In general, the BDE of the b-CeC bond of the alkyl group is ~90 kJ/mol, and the BDE of the b-CeH bond is ~140 kJ/mol in the alkyl group. Therefore, the macro alkyl radical is prior to split the b-CeC bond into smaller alkyl groups. The increase in discharge frequency actually increases the number of pulses experienced by the flow so that the free radicals react further before the stable product is generated. In the third phase (in green), the discharge conditions are further enhanced, providing more free radicals in the plasma region. The methyl attack reaction becomes more intense and methyl abstracts hydrogen atoms of other alkyl to produce massive methane. In addition, the formation of 2-butyl provides more pathways for the production of propylene. For the generation of acetylene, there are two paths, one is that ethylene loses one hydrogen atom first to generate a vinyl group, and then the bCeH bond is broken to form acetylene; the other pathway is that the methyl group undergoes twice CeH bond scission to form a CH group, and two CH groups are compounded to generate acetylene. The BDEs required for the initial bondscission reaction of these two pathways are similar and it is considered that the contributions to the yield of acetylene are comparable.
Conclusions In this paper, a volume dielectric barrier discharge cracker powered by microsecond pulse power is used to crack ndecane, a kerosene alternative fuel, to obtain detonable gas. Argon bubbling to produce liquid film is adopted innovatively as the way of fuel feeding to carry out the cracking experiment. The following conclusions are obtained: ethylene concentration in the product decreases with the increase of discharge frequency, but its concentration is still the highest under all experimental conditions. Other product concentrations increase with the discharge frequency rising, accompanied by the appearance of new species. It is considered that the increase of the concentration of nonethylene products and the formation of new species will compete for consumption of ethyl, resulting in the decrease of ethylene concentration with the increase of discharge
frequency. Compared with the previous cracking experiment, due to the large specific surface area of the liquid film, it can be better heated and vaporized by the discharge area, so that the cracking reaction is more effective. The concentration of species except ethylene is higher than that in the previous experiments. It is found that the discharge parameters can not only change the concentration of the product, but also regulate the proportion of each component in the product. In particular, increasing the discharge frequency can promote the concentration of the components, and at the same time, the proportion of the components in the gaseous products can be increased. However, the increase of the applied voltage only promotes the concentration of each component, but has no effect on the proportion of the components. In addition, both the loading voltage and the discharge frequency have selectivity to the target products. The selectivity of ethylene decreases when the loading voltage and discharge frequency increase. However, the selectivity of hydrogen and acetylene increase with increasing the discharge frequency. Besides, the effect of loading voltage on selectivity of these two products is not obvious. In order to analyze the cracking effect from the perspective of energy utilization, this paper introduces the concept of energy conversion ratio (ECR) and specific energy input (SEI) to do auxiliary analysis work. It is found that the ECR decreases with the increase of discharge frequency, and the rate of decline gradually slows down. The reason is that the ethylene concentration in the initial stage decreases significantly with increasing frequency, leading to the rapid decrease of the energy conversion ratio. With the further increase of the frequency, the concentration of hydrogen with the highest calorific value rises as the concentration of ethylene decreases, and the concentrations of other products also greatly increase, which leads to the slower reduction of the ECR. With the same discharge frequency, increasing the loading voltage has no significant effect on the ECR. The increase in SEI and the increase in frequency have the same effect on the ECR, and the ECR decreases as the SEI increases. When the SEI is the same, ECR increases as the voltage rises. However, for the same discharge frequency, the similar ECR can be reached at low applied voltage compared with the higher voltage, and the SEI is also greatly reduced at this time. Therefore, the use of lowvoltage/high-frequency discharge is more advantageous. Based on the detected product and concentration changes, the path and mechanism of n-decane decomposition reaction are inferred. Since the b-CeC bond energy is the lowest, the break of macromolecule to produce ethyl is the most easy to occur, and the b-CeH cleavage of the ethyl group provides hydrogen atom, resulting in the highest concentration of ethylene and hydrogen at each experimental condition. According to the different generation paths of products, the reaction path is divided into three stages. For acetylene, it is presumed that there are two possible paths of generation. One is dehydrogenation of ethyl b-CeH bond to acetylene, the other is that methyl is broken by CeH bond two times to generate CH group, and then two CH groups combine to generate acetylene. The analysis holds that the contribution degree of the two paths to acetylene generation is similar.
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Acknowledgment Research reported in this publication was supported by the National Natural Science Foundation of China (Grant Nos. 91541120, 91641204, 51507187, 51790511).
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Available online at www.sciencedirect.com
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N-decane decomposition by microsecond pulsed DBD plasma in a flow reactor Feilong Song a, Yun Wu a,b,c,*, Shida Xu a, Di Jin a, Min Jia a a
Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi'an, 710038, China Science and Technology on Plasma Dynamics Laboratory, Xi'an Jiaotong University, Xi'an, 710049, China c Institute of Aeroengine, Xi'an Jiaotong University, Xi'an, 710049, China b
article info
abstract
Article history:
N-decane decomposition with plasma to obtain prone detonation gaseous fuel is investi-
Received 1 November 2018
gated by adopting fuel liquid film feeding mode at applied voltage of 10 kV and 15 kV,
Received in revised form
discharge frequency of 300e3000 Hz with interval 100 Hz. The result shows that the
9 December 2018
discharge parameters can regulate the composition and the concentration of the cracked
Accepted 13 December 2018
products. When the composition of the product is unchanged, the concentration and
Available online 5 January 2019
proportion of the components can be increased as the discharge frequency becomes higher. The hydrogen concentration increases 3.4 times and 2.3 times respectively for the
Keywords:
applied voltage of 15 kV and 10 kV as the discharge frequency is increased from 500 to
n-decane
3000 Hz. However, the voltage has no effect on the proportion of the components, and
Decomposition
increasing the voltage only increases the concentration of the species. For selectivity,
Plasma
raising the discharge frequency can greatly increase the species selectivity of hydrogen and
Selectivity
acetylene that are most prone to detonate. The selectivities of hydrogen and acetylene are the highest at 15 kV and 3 kHz, reaching 18.40% and 4.22% respectively, and at this time, the selectivity of ethylene is the lowest, which is 60.4%. Besides, it is confirmed that high selectivity can be achieved at the lower specific energy input causing by low-voltage/highfrequency discharge. Through the analysis of the bond dissociation energy, there is a competitive relationship between the production of ethylene, ethane and pentane which all consume ethyl, and two possible pathways for the production of acetylene are put forward. © 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction The thermal cycle efficiency of rotation detonation engines (RDE) is about 30% higher than that of conventional gas turbine engines. At present, RDE has been an international research hotspot and frontier of military aviation power, and it will greatly reduce the fuel consumption [1e3]. However,
kerosene/air detonation has been the core technology and key problem. The energy required for the direct initiation of kerosene/air mixture is very large and difficult to achieve. The auxiliary detonation effects of mixed hydrogen, supplemental oxygen and preheating methods are also limited [4e6]. By using the method of cracking the kerosene, it can provide more small molecular fuel and active group and increase the chemical reaction rate. It is expected to significantly reduce
* Corresponding author. Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi'an, 710038, China. E-mail address: [email protected] (Y. Wu). https://doi.org/10.1016/j.ijhydene.2018.12.100 0360-3199/© 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
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the initiation energy, shorten the initiation time and increase the detonation frequency, so as to provide an effective way to solve the problem of kerosene/air detonation. The existing cracking methods include thermal cracking [7,8], catalytic cracking [9] and microwave plasma cracking [10]. Among these traditional technologies of fuel decomposition, thermal cracking is the most mature, but its development is limited due to its high energy consumption, high reaction temperature, poor selectivity for products and long operating cycle. As for the catalytic cracking, the conventional cracking catalyst such as molecular sieve will face the problems of insufficient activity, easy poisoning and serious carbon deposition in the long term work. Moreover, due to the presence of the catalyst, the operable temperature range is narrow and the response time is long. These problems severely restrict the engineering application of catalytic cracking. Plasma technology uses high-energy electrons to collide with neutral molecules in the fuel, causing dissociation, excitation and even ionization, generating large amounts of active atoms, excited-state particles, ions, electrons and other intermediate products. The increase in the activity of the intermediate particles in the chemical chain reaction ultimately affects the distribution of components in the combustion system and accelerates the combustion chemical kinetics [11e14]. Zhu [15] has applied plasma to the hydrogen/air detonation. Under his experimental conditions, plasma can significantly improve the detonation performance with combustion annulus widths of 4.75 mm: the RDE cannot be successfully detonated without plasma assistance, while high efficiency detonation can be achieved when plasma works. At present, the research progress of plasma assisted combustion is outstanding. Starikovskaya et al. experimentally verifies that atomic O can strengthen the chain reaction in the kinetic characteristics of plasma enhanced combustion [16]. Prof. Ju's team, through a large number of optical measurements and simulation predictions, has proved the dynamic role of active groups such as O, OH and H in the process of fuel oxidation and ignition, and has successfully predicted the reaction channel of plasma ignition and combustion [17,18]. Tsolas et al. [19] have simulated and predicted the process of plasma assisted H2 and C2H2 fuels cracking and oxidation. The results show that the collision quenching of electron excited state of argon (Ar*) and C2H2 fuel molecules is the direct path of low-temperature fuel consumption. In addition, the plasma degradation of gaseous organic pollutants [20], the use of nonequilibrium plasma to synthesize the greenhouse gases into high value fuels [21e23] and biofuels decomposition with plasma for the production of hydrogen rich gases [24] have been carried out a great deal of research, providing a large number of mechanism of plasma decomposition reaction. However, there are few studies on plasma cracking of hydrocarbon species in the kerosene components. Zhang et al. [25] introduces argon gas bubbles into n-dodecane/water mixture and uses needle-plate discharge plasma to crack the fuel at the discharge frequency of 4.6 kHz, and methane or carbon dioxide is mixed with argon to change radical composition to achieve the purpose of selecting the product; Yao et al. [26] uses dielectric barrier discharge plasma to crack gaseous n-decane at the discharge frequency of 250 Hz. The
types of carrier gas are argon, nitrogen and oxygen/nitrogen. The analysis shows that the initial decomposition of n-decane is generated by the dehydrogenation reaction to produce decane free radicals which are further broken bonds to form small molecules; Reddy [27] uses temperature-controlled DBD to crack n-heptane/water mixture, with different discharge frequencies for different carrier gases, 4.8 kHz, 5.5 kHz and 5.2 k Hz for Ar, CH4 and CO2, respectively, to achieve the goal of controlling the short-chain hydrocarbon and oxygenate concentrations in the product. The above experiment is to study the relationship between the type of carrier gases and the products obtained by the plasma cracking fuel at the specific voltage and discharge frequency, and there is no systematic study about the effect of the discharge frequency on the product. Besides, the existing research lacks the analysis of the mechanism. Ju [17] divides the role of plasma into three major mechanisms: thermal enhancement mechanism, kinetic enhancement mechanism, and transport enhancement mechanism. Since the flow reactor is used in this study, the influence of the transport enhancement mechanism is weakened, and at the same time, the microsecond pulse power supply is employed as the plasma power supply, which could produce uniform and stable non-equilibrium plasma, so temperature rise effect on the reaction system can be ignored. Therefore, this experiment focuses more on the kinetic characteristics of the plasma decomposition. In this paper, a representative alternative fuel of kerosene, n-decane, is used as the treated object, and a new cracking method that fuel liquid film is cracked by plasma in the flow reactor is adopted. The distribution and concentration of the products are performed by gas chromatography (Agilent 7890 b). The effect of the discharge frequency on the products distribution under different applied voltages is systematically studied to provide experimental data for kinetic models.
Experimental approach Experiment is conducted using a plasma flowing reactor, as shown in Fig. 1. Liquid n-decane (>99% in purity, Huaxia, Chengdu, China) is placed in a quartz tube with internal diameter of 6 mm to form a liquid column. The liquid column height is kept at 140 mm, so that the distance from the top of the liquid column to the bottom of the discharge plasma is 50 mm. Through the Teflon sleeve, the bottom of the quartz tube is connected with the argon supply pipe of which top has a orifice with an internal diameter of 1.5 mm. Argon (with a purity of 99.999%) flow rate is controlled at 30 sccm using a mass flow controller (DSN-MFC-400 A) with an accuracy of 1% and the gas flows out from the tip of metal nozzle at the bottom of the liquid column to form a large number of bubbles. As the argon bubbles rise, the pressure gradually decreases, eventually resulting in an increasing volume of argon bubbles. When the bubbles rise to the liquid surface, the bubble size is close to the inner diameter of the quartz tube. As a result, many bubbles gather here, and the liquid film is intertwined. With the continuous supply with argon, the liquid n-decane film keeps rising and forms a uniform flow state before reaching the discharge area.
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Fig. 1 e Schematic of the plasma cracking fuel film experimental setup. The core part of the experimental system is the plasma reactor of which details can be found in Refs. [28] and only a brief description will be outlined here. The volume of the plasma region formed by the dielectric barrier discharge is 1200 mm3 with 6 mm discharge gap, as shown in Fig. 2. So, the liquid film flow needs 2.4 s to pass through the plasma area at a flow rate of 30 sccm. This means that, at a discharge frequency of 500e3000 Hz, the flow will experience 1200e7200 discharge pulses. Under all experimental conditions, there are no filament discharge formation. Besides, interestingly, the liquid film disappears after it enters the plasma zone, and no liquid film remains behind the plasma for all experimental conditions, indicating that the n-decane liquid film has been cracked into gaseous substances or vaporized. The reactor is power by a laboratory-made microsecond pulsed power supply of which rise time is 1.0 ms. The pulse frequency can be continuously adjusted from 0 Hz to 5 kHz controlled via a digital pulse external triggering generator (FY3200S). The voltage is regulated by a voltage regulator and the adjustment range is 0e15 kV. In the experiment, a 10 U
Fig. 2 e Scheme of the plasma reactor.
non-inductive resistance is connected between the ground and the low voltage copper electrode. The high-voltage probe (P6015A) and the passive high-resistance probe (RP3500A) with attenuation ratios of 1000:1 and 10:1, respectively, are used to measure the total voltage and the voltage across the resistor. The measured voltage values are recorded by the oscilloscope (Tektronix MDO3024). Based on the voltage values obtained, the discharge current can be calculated by dividing the voltage across the resistance by the resistance value, and then integrating the current and total voltage to obtain energy, resulting in energy deposition of ~83.2 mJ and ~137.7 mJ per pulse for the total voltages 10 kV and 15 kV respectively. Since the voltage across the resistor is three orders lower than the total voltage, the power consumed by the resistor is ignored in calculating the plasma discharge power in this paper. After n-decane liquid film decomposition with plasma, some species among the cracked gas are liquid at room temperature. These species are separated after condensation and the remaining gaseous products are analyzed by the gas chromatograph (Agilent 7890 B). The GC is configured to analyze the permanent gas and the hydrocarbons from C1 to C5. Quantitative analysis of hydrogen, alkanes, alkenes and alkynes containing three carbon atoms and fewer, alkanes and alkenes containing four carbon atoms, n-pentane and isopentane can be performed. Among them, hydrogen is analyzed by a thermal conductivity detector with a detection limit of 200 ppm; hydrocarbons are analyzed by a flame ion detector with a detection limit of 2 ppm. The experiment is performed at two total voltages of 10 kV and 15 kV. At each voltage, the frequency is adjusted from 500 Hz to 3000 Hz with the interval of 100 Hz. There are 32 experimental conditions totally, and all experiments are performed at room temperature. Besides, the cracking experiments are repeated three times for each condition with the product concentration error less than 3%.
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Results and discussions Discharge waveforms The oscillation attenuation of the voltage signal representing the current obviously consists of two parts over one high voltage pulse (HVP), as shown in Fig. 3. The first oscillation decay starts at the rising edge of the HVP, and the second one starts at the moment of the end of the HVP belonging to capacitive discharge. The reasons for the former are analyzed here. Due to the presence of the barrier dielectric, the charge generated during the discharge gradually accumulates on the surface of the dielectric, which means that the equivalent capacitance of the barrier dielectric is charged. The electric field formed by the accumulated charges is opposite to the direction of the applied electric field. Since the voltage of the discharge gap is the applied voltage minus the voltage formed by the accumulated charges, as the discharge continues, the reverse voltage will rise rapidly, causing the discharge gap voltage to drop rapidly. When the effective voltage drops to be insufficient to maintain the discharge, the discharge current rapidly decreases. As a result, the pulse width of the discharge current is very narrow at each breakdown between the dielectrics. After the discharge completed, the charging progress of the equivalent capacitance is ended, and the reverse voltage remains unchanged. However, the voltage of the discharge gap varies with applied voltage. When the applied voltage continues to rise, reaching the required breakdown voltage, the discharge channel is formed again and the second current peak appears. Afterwards, as the loading voltage continues to rise, similar discharge current decays and surges occur again, resulting in multiple current peaks within one pulse of applied voltage. It can be seen from Fig. 3 that the peak value of the discharge current gradually decreases during multiple discharges. This is attributed to the short interval between the two breakdowns. A large amount of discharge residues left by the previous breakdown, such as charged particles, highenergy metastable particles, etc., make the later breakdown more likely to occur.
Fig. 3 e Typical waveforms of voltage and current over one pulse discharge duration.
By reason of charge accumulation in the process of multiple discharges, the second oscillation attenuation occurs by the form of capacitive discharge after the discharge of HVP, and the discharge channel is not formed at this time.
Effect of applied voltage and discharge frequency on gaseous products content The products detected in the experiment include 9 species, which are hydrogen, methane, ethane, ethylene, acetylene, propane, propylene, butane and n-pentane. Other species may also be produced but not detected because their concentrations are below the detection limit of the gas chromatogram. The detected species are not produced under any experimental conditions, as shown in Fig. 4, but are generated in turn as the discharge voltage or frequency increases. The experiment is performed under two loading voltages. When the applied voltage is 10 kV, the cracked gas contains hydrogen, ethylene and propane at the lowest frequency of 500 Hz. As the frequency increases, ethane occurs at 700 Hz, npentane at 800 Hz, n-butane at 1000 Hz, acetylene at 1500 Hz and methane at 1600 Hz. At the voltage of 10 kV, there is no propylene produced in the cracked gas at all discharge frequencies. When the loading voltage is improved to 15 kV, most species have already appeared the lowest frequency, including hydrogen, ethane, ethylene, propane, n-butane, and n-pentane. At the same time, the discharge frequencies corresponding to the generation of acetylene and methane are also greatly reduced, which are 600 Hz and 800 Hz, respectively. In addition, the increase in the applied voltage causes new specie to appear in the product, with propylene detected at the discharge frequency of 1300 Hz. The applied voltage and the discharge frequency have a great influence on the yield of the product. It can be seen from Fig. 5 that the ethylene yield rapidly decreases with the increase of the discharge frequency, but the ethylene yield is always the highest under any experimental conditions. This phenomenon is attributed to that the main precursor for generating ethylene is ethyl radical. On one hand, the ethyl radical can be produced in large amounts by the b-scission reaction of macromolecular hydrocarbons. On the other hand,
Fig. 4 e Schematic diagram of the species present under each experimental condition. The applied voltage without pattern filled is 10 kV, and the applied voltage with filling pattern is 15 kV.
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Fig. 5 e Production rate of (a) C2H4, (b) H2, (c) CH4, C2H6 and C3H8, (d) C2H2, n-C4H10, n-C5H12 and C3H6 in the cracking gas with various discharge frequencies. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
as the discharge frequency increases, the yield of other species increases, as shown in Fig. 5, and at the same time, more new species are produced. The production of other species consumes more ethyl radicals, which leads to the decrease in the production rate of ethylene with the increase of discharge frequency. Fig. 5 also illustrates that the increase in frequency is more beneficial for increasing the species yield when the loading voltage is higher. For example, for hydrogen, increasing the frequency from 500 Hz to 3000 Hz can increase the yield by a factor of 3.4 when the applied voltage is 15 kV, while it increases the yield by 2.3 times at the voltage of 10 kV. In addition, the increase in voltage can also increase the product yield. As can be seen from Fig. 5, when the discharge frequency is higher, increasing the voltage is more conducive to improve the product yield, especially for hydrogen, methane and acetylene. For example, due to the increase in voltage, the hydrogen yield increases by a factor of 2.0 at 3000 Hz, and by a factor of 1.3 at 500 Hz; the methane yield increases 2.0 times at 3000 Hz and 1.6 times at 1600 Hz; for acetylene, the yield increases 2.1 times at 3000 Hz and 1.8 times at 1500 Hz. According to the product yield and the appearance order with each species in the product, the difficulty degree to crack n-decane into each component from the easiest to the hardest is: ethylene, hydrogen, propane, ethane, methane, acetylene, n-butane, n-pentane and propylene. Combined with the above analysis, the higher the applied voltage and the discharge frequency are, the higher the product yield will be. However, under actual circumstances, the higher voltage will make the possibility of creepage or breakdown increase significantly and there will be potential safety hazards. In fact, it is not inclined to select high voltage under industrial condition, and other than that, increasing the frequency is easier to implement. Therefore, in order to
increase the yield of cracking products, it is a good choice to increase the discharge frequency in the case of relatively low voltage. In Fig. 6, the concentration of the components contained in the product is compared between this cracking experiment and the previous cracking experiment [29] in which plasma is used to split liquid n-decane directly at the room temperature. The components involved in the comparison include hydrogen, ethylene, ethane, propane, butane, and pentane, and they are all produced at the same experimental condition in both experiments, for which the applied voltage is 10 kV and the discharge frequency is 3 kHz. The results show that in the experiment of cracking liquid film, the cracking effect is
Fig. 6 e Comparison on the concentration of the product present in both this experiment and the previous experiment [29]. Applied voltage 10 kV and discharge frequency 3000 Hz.
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better, the ethylene concentration drops sharply, and the concentrations of other species increase. In addition to the effects resulting from the modification of the power supply, there are two factors responsible for it. On the one hand, for the liquid film decomposition experiment, the heat generated by the discharge can be more fully used to heat the liquid ndecane to evaporate, and after the liquid fuel becomes gaseous molecule, it will react with the plasma better. However, for the experiment of cracking liquid n-decane, the heat generated by the discharge is dissipated to heat the fuel that is not in the plasma region, so that the heat is not fully utilized. On the other hand, it may be the cause of the carrier gas flow rate. The carrier gas flow rate for this experiment is 30 mL/ min, while in the previous experiment, the carrier gas flow rate is 1 L/min, so at the same discharge frequency, the number of the discharge pulse experienced per unit time by the flow is different. For this experiment, the number of pulses experienced is more, resulting in a better cracking effect. Discharge parameters can not only change the concentration of the cracking product, but also have an effect on the percentage of each component in the product. The effect of discharge voltage and frequency on the volume percentage of each species is shown in Fig. 7. The percentage change curves of each species in the graph all show an upward trend with the increase of the discharge frequency, except for ethylene. In addition, the percentage in the product corresponding to each species is almost unchanged at two different loading voltages. This indicates that the increase in frequency promotes the percentage of the corresponding species on the basis of increasing the yield. However, increasing the voltage only promotes the yield of
the components and has no effect on the percentage of each component in the product. The possible reason for this phenomenon is that: due to the increase in voltage, the electric field strength (E) between discharge gap becomes larger. Based on the formula mv2/2 ¼ eEl (where l, m and e are the electron mean free path, electron mass and electron charge amount, respectively), the electron gains higher energy, and its excitation cross section with the argon atom will increase, leading to generate more excited argon atoms. In turn, more collisional cracking reactions occur with the fuel molecules to obtain more free radicals or precursors required for the production of the product, so that the concentration of each particle in the reaction pool is proportionally increased. As a result, although the concentration of each species increases in the product, the percentage of each substance is not affected by the voltage.
Selectivity and energy consumption analysis Selectivity refers to the proportion of a certain product in the cracking gas. Hydrogen selectivity refers to the proportion of hydrogen atoms contained in hydrogen in all hydrogen atoms contained in gas-phase products formed by the decomposition of n-decane. The heat value of hydrogen reaches 1.4 108 J/kg, which is higher than the heat value of all fossil fuels. And hydrogen is extremely flammable. If a small amount of hydrogen is generated at the outlet of the fuel injector by cracking kerosene, it will play an important role in improving the ignition performance of the engine and expanding the ignition boundary. Therefore, hydrogen selectivity is an important parameter for evaluating the cracking
Fig. 7 e Molar percentage of (a) C2H4, (b) H2, (c) CH4, C2H6 and C2H2, (d) C3H8, n-C4H10and n-C5H12 in the cracking gas with various discharge frequencies. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
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effect. For detonation engines, hydrogen as fuel is much more prone to detonation compared to the other common gaseous fuels at standard conditions. However, the probability of having a detonation in hydrogeneair mixture is not higher than that in other hydrocarbon fuels at elevated initial pressure. In Hoi Dick Ng's research [30], the acetylene-air mixture has a greater likelihood to detonate than hydrogeneair mixture at higher pressure. When gaseous fuel is used as the supply fuel for detonation engines, the characteristic cell size and the critical direct initiation energy are generally considered as two significant parameters to characterize the detonation sensitivity to the given mixture. The smaller the cell size and the critical direct initiation energy are, the more sensitive to detonation the mixture will be. Table 1 and Table 2 show the detonation cell size and the critical ignition energy of some gaseous species. From the table, it can be seen that, when the initial pressure, the initial temperature and the equivalent ratio are ~1 atm, ~293 K and ~1 respectively, the cell size and the critical ignition energy of hydrogen and acetylene are the smallest, and the corresponding parameter for ethylene is ranked third. Therefore, we selected hydrogen, acetylene and ethylene as the three target products to analyze the selectivity. The selectivity of hydrogen, acetylene and ethylene are calculated by the following equations: H2 selectivityð%Þ ¼
Amount of H atoms in the formed H2 Total amount of H atoms in the products 100%
Cx Hy selectivityð%Þ ¼
(1)
Amount of C atoms in the formed Cx Hy Total amount of C atoms in the products 100%
(2)
Here, x and y represent the number of carbon atom and the number of hydrogen atom, respectively, in a hydrocarbons molecule. The decomposition of n-decane is caused by the energy injected into the DBD reactor. Theoretically, the higher the energy injected into the reactor, the more favorable the conversion of n-decane. The specific energy input refers to the energy input per unit argon flow. The higher the specific energy input, the more high energy particles in the carrier gas will be produced to participate in the cracking reaction, thereby increasing the concentration of each component in the product. The specific energy input (SEI) in this study is defined as the input power of overall system divided by the flow rate of the total feed Ar (mL min1).
. SEI J,L1 ¼ 60000 Pinput Fin Ar
(3)
In Formula (3), the amount of input power is calculated by following equation: Z Pinput ðwÞ ¼
UðtÞ
UR ðtÞ dt frequence R
(4)
single cycle
The selectivity of the three target products under various experimental conditions is shown in Fig. 8, and the change regulation is similar to the rule of the volume percentage changing with the variety of the discharge frequency. The change of the loading voltage has an effect on the selectivity of ethylene. When the loading voltage and discharge frequency are high, the selectivity of ethylene is small, and the lowest selectivity of ethylene fell to 60.4%. In conjunction with the previous analysis, increasing the voltage is accompanied by the increase in the ethylene yield, and the percentage in the product remains almost unchanged. The phenomenon of the decrease of ethylene selectivity under high voltage shows that the n-decane molecules are cracked out to produce a large number of carbon atoms of which the proportion used to produce ethylene becomes smaller and the proportion used to generate other substances increases. Under all experimental conditions, the ethylene selectivity at high voltage decreases by an average of 3.9% compared to the low voltage conditions. For acetylene and hydrogen, increasing the discharge frequency can enhance selectivity, and the change of loading voltage has little effect on the two species. However, when the discharge frequency is higher than 2000 Hz and 2700 Hz, respectively, the selectivity of acetylene and hydrogen under high voltage conditions is slightly higher than that of low voltage conditions. In summary, there is no obvious advantage for improving the selectivity by increasing the voltage for the two gaseous fuels which are the most prone to detonation. In this experiment, hydrogen selectivity ranged from 3.62% to 18.40%, and acetylene selectivity varied from 0.76% to 4.22%. The selectivity of the target product is analyzed from the perspective of specific energy input. For the three target products, when the load voltage is increased at the same discharge frequency, the input energy in the unit flow carrier gas increases, causing the selectivity curves to shift horizontally to the right, as shown in Fig. 9. In other words, in order to achieve the same selectivity, more energy needs to be consumed when the high voltage is loaded. For example, at 3000 Hz, the specific energy input increased from 498.9 J/L to 826.1 J/L, about 1.65 times: the selectivity of ethylene decreased from 62.4% to 60.4%; the selectivity of hydrogen
Table 1 e Cell size data for gaseous fuels.
H2 CH4 C2H2 C2H4 C2H6 C3H8 C4H10
Initial pressure/kPa
Initial temperature/K
Equivalence ratio
Cell width/mm
Table number in Ref. [31]
100e101.452 101.3 101.3 92.5e101.3 92.5e101.3 92.5e101.3 101.3
293e300 293e298.15 293e298.15 293e298.15 293e298 293e298.15 293
0.9721e1.0233 1 1e1.3242 0.999537e1.070638 0.99325e1.0958 1e1.2925 1.0088
8.0e15.1 279.6e349.5 4.6e9.2 19.5e33.8 50.0e66.1 46.0e75.2 75.1
10, 13,30, 61 111, 112, 116, 119 120, 135, 154 157, 166, 170, 172, 173, 177 181, 202, 210, 216 182, 201, 211, 221 200
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Table 2 e Critical initiation energy data for gaseous fuels.
H2 CH4 C2H2 C2H4 C2H6 C3H8 C4H10
Initial pressure/kPa
Initial temperature/K
Equivalence ratio
Critical initiation energy/J
Table number in Ref. [31]
101.3 101.3 101.3 101.3 101.3 101.3 101.3
293 293 293 293 293 293 293
1e1.19051 1 1e1.0871 0.9866e1.0844 1.0765e1.157 0.99308e1.124 1.0169e1.13
4248.8e9040.0 88658800.0 4332.0e6780.0 55596.0e60568.0 77368.8e90137.8 215192.7e283114.7 196362.4e275824.0
325, 327, 328, 348 354 361, 362 368, 369, 373 374 375 376
Fig. 8 e Selectivity of (a) C2H4, (b) H2 and C2H2 with various discharge frequencies. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
Fig. 9 e Selectivity of (a) C2H4, (b) H2 and C2H2 with various specific energy input. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
changes from 18.3% to 18.4%; for acetylene, the selectivity increased from 3.97% to 4.22%. And, with the increase in specific energy input, in order to achieve similar selectivity, the difference in specific energy input required under the two loading voltages becomes greater. This fully demonstrates that the choice of low voltage and high frequency has great advantages for the decomposition of longer carbon chain molecules. It can not only get the higher selectivity of the target product, but also does not consume much energy. However, if greater yields are required, it is necessary to choose a higher discharge voltage. Usually, to assess energy efficiency, the sum of the calorific values of all products is divided by the sum of the discharge input energy and the calorific value of the input reactant, and the ratio is called the energy conversion efficiency. Combined with the test conditions currently available in the laboratory, the cracking gaseous products are comprehensively analyzed
in this experiment. In this paper, the energy conversion ratio is defined to indirectly evaluate the energy efficiency, and it refers to the ratio that the total calorific value of all gaseous species detected is divided by the input power. The energy conversion ratio (ECR) is calculated using equations: ECR ¼
LHVH2 Fout H2 þ
P
LHVCx Hy Fout Cx H y
The total input power
(5)
where LHV is the lower heating value (J mol1), F represents the molar flow rate of hydrogen and hydrocarbons in the products (mol s1). Fig. 10 shows the effect of discharge frequency and specific energy consumption on the energy conversion ratio. From the previous analysis, it is known that ethylene is the species with the highest concentration in the product. At the same time, the ethylene selectivity shows a decreasing trend with
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Fig. 10 e Energy conversion ratio with (a) various discharge frequencies, (b) various specific energy input. Solid points, discharge voltage 10 kV; empty points, discharge voltage 15 kV.
increasing discharge frequency and specific energy input. And this determines the changing trend of the energy conversion ratio. As can be seen from Fig. 10, the initial decline of the ECR is rapid and gradually becomes flat. This is because, as the discharge frequency increases, the yield of hydrogen with the highest calorific value gradually increases during the decrease of the ethylene yield, and the concentrations of other species also greatly increase, resulting in a slowing down in the rate of ECR decline. Fig. 10a confirms that the loading voltage has almost no effect on the ECR, and Fig. 10b further confirms the advantage that, at the same discharge frequency, choosing a low voltage can achieve similar ECR to high voltage, and at the same time the specific energy input is still very low.
Reaction path analysis Based on the changes in the species detected in the experiment and the concentration of each component, some possible reaction paths are constructed, as shown in Fig. 11. Because the bond dissociation energies (BDE) of the CeC bonds in the straight chain alkanes are about 32.6e61.9 kJ/mol
lower than that of the CeH bond, and the b-CeC bond is the weakest in the hydrocarbon molecules, followed by the g-Ce C. It is speculated that the initial reaction of n-decane decomposition is caused by the collision of energetic particles resulting in the break of b-CeC to form ethyl and octyl radicals, and the scission of g-CeC to form heptyl and propyl radicals. The reaction path is divided into three phases, which are respectively represented by red, blue and green in Fig. 11. In the first stage (shown in red), the ethyl radical obtained from the initial cracking reaction undergoes b-CeH scission to form hydrogen atom and ethylene, and the propyl group generates the ethylene and methyl group through the b-CeC scission. Ethyl is easily produced, and propyl radicals also generate ethylene, resulting in the highest ethylene yield in the product. Since hydrogen atom is derived from the b-CeH scission of the ethyl group, a large amount of hydrogen atoms attack the n-decane molecule or the macromolecular chain hydrocarbon radical to generate hydrogen, resulting in the second highest hydrogen yield. In addition, some of the hydrogen atoms and methyl groups react with the remaining reactants to form propane. The first-stage reaction path
Fig. 11 e Reaction paths of n-decane decomposition with discharge plasma.
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explains well that ethylene, hydrogen and propane are present in the product under all experimental conditions, and the concentration of ethylene is the highest, followed by hydrogen and propane. With the increase of the applied voltage and discharge frequency, the free radical pool in the dashed box in Fig. 11 is enlarged, supplementing more ethyl and propyl groups, resulting in the enhancement of the reaction path in the second stage (shown in blue). The excess ethyl and propyl groups are combined with the hydrogen and methyl radicals formed by their own bond-scission, respectively, to generate ethane and butane. At the same time, pentane is formed by the recombination reaction between ethyl and propyl. This also explains that these three species do not appear under all experimental conditions, and they only appear when the discharge frequency and the applied voltage are increased. Since the amount of ethyl and hydrogen atom is higher than that of other free radicals, the relative content of ethane in these three species is relatively high. In general, the BDE of the b-CeC bond of the alkyl group is ~90 kJ/mol, and the BDE of the b-CeH bond is ~140 kJ/mol in the alkyl group. Therefore, the macro alkyl radical is prior to split the b-CeC bond into smaller alkyl groups. The increase in discharge frequency actually increases the number of pulses experienced by the flow so that the free radicals react further before the stable product is generated. In the third phase (in green), the discharge conditions are further enhanced, providing more free radicals in the plasma region. The methyl attack reaction becomes more intense and methyl abstracts hydrogen atoms of other alkyl to produce massive methane. In addition, the formation of 2-butyl provides more pathways for the production of propylene. For the generation of acetylene, there are two paths, one is that ethylene loses one hydrogen atom first to generate a vinyl group, and then the bCeH bond is broken to form acetylene; the other pathway is that the methyl group undergoes twice CeH bond scission to form a CH group, and two CH groups are compounded to generate acetylene. The BDEs required for the initial bondscission reaction of these two pathways are similar and it is considered that the contributions to the yield of acetylene are comparable.
Conclusions In this paper, a volume dielectric barrier discharge cracker powered by microsecond pulse power is used to crack ndecane, a kerosene alternative fuel, to obtain detonable gas. Argon bubbling to produce liquid film is adopted innovatively as the way of fuel feeding to carry out the cracking experiment. The following conclusions are obtained: ethylene concentration in the product decreases with the increase of discharge frequency, but its concentration is still the highest under all experimental conditions. Other product concentrations increase with the discharge frequency rising, accompanied by the appearance of new species. It is considered that the increase of the concentration of nonethylene products and the formation of new species will compete for consumption of ethyl, resulting in the decrease of ethylene concentration with the increase of discharge
frequency. Compared with the previous cracking experiment, due to the large specific surface area of the liquid film, it can be better heated and vaporized by the discharge area, so that the cracking reaction is more effective. The concentration of species except ethylene is higher than that in the previous experiments. It is found that the discharge parameters can not only change the concentration of the product, but also regulate the proportion of each component in the product. In particular, increasing the discharge frequency can promote the concentration of the components, and at the same time, the proportion of the components in the gaseous products can be increased. However, the increase of the applied voltage only promotes the concentration of each component, but has no effect on the proportion of the components. In addition, both the loading voltage and the discharge frequency have selectivity to the target products. The selectivity of ethylene decreases when the loading voltage and discharge frequency increase. However, the selectivity of hydrogen and acetylene increase with increasing the discharge frequency. Besides, the effect of loading voltage on selectivity of these two products is not obvious. In order to analyze the cracking effect from the perspective of energy utilization, this paper introduces the concept of energy conversion ratio (ECR) and specific energy input (SEI) to do auxiliary analysis work. It is found that the ECR decreases with the increase of discharge frequency, and the rate of decline gradually slows down. The reason is that the ethylene concentration in the initial stage decreases significantly with increasing frequency, leading to the rapid decrease of the energy conversion ratio. With the further increase of the frequency, the concentration of hydrogen with the highest calorific value rises as the concentration of ethylene decreases, and the concentrations of other products also greatly increase, which leads to the slower reduction of the ECR. With the same discharge frequency, increasing the loading voltage has no significant effect on the ECR. The increase in SEI and the increase in frequency have the same effect on the ECR, and the ECR decreases as the SEI increases. When the SEI is the same, ECR increases as the voltage rises. However, for the same discharge frequency, the similar ECR can be reached at low applied voltage compared with the higher voltage, and the SEI is also greatly reduced at this time. Therefore, the use of lowvoltage/high-frequency discharge is more advantageous. Based on the detected product and concentration changes, the path and mechanism of n-decane decomposition reaction are inferred. Since the b-CeC bond energy is the lowest, the break of macromolecule to produce ethyl is the most easy to occur, and the b-CeH cleavage of the ethyl group provides hydrogen atom, resulting in the highest concentration of ethylene and hydrogen at each experimental condition. According to the different generation paths of products, the reaction path is divided into three stages. For acetylene, it is presumed that there are two possible paths of generation. One is dehydrogenation of ethyl b-CeH bond to acetylene, the other is that methyl is broken by CeH bond two times to generate CH group, and then two CH groups combine to generate acetylene. The analysis holds that the contribution degree of the two paths to acetylene generation is similar.
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Acknowledgment Research reported in this publication was supported by the National Natural Science Foundation of China (Grant Nos. 91541120, 91641204, 51507187, 51790511).
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