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Partial oxidation of diesel fuel by plasma – Kinetic aspects of the reaction
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Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction Duy Khoe Dinh a,b, Hee Seok Kang b, Sungkwon Jo b, Dae Hoon Lee a,b,*, Young-Hoon Song a,b a b
University of Science and Technology (UST), 217 Gajeong-ro Yuseong-gu, Daejeon 305-350, Republic of Korea Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
Plasma reforming of fuel has recently been studied for possible applications in hydrogen
Received 19 May 2017
generation and plasma assisted combustion. However, the kinetic aspects of plasma
Received in revised form
reforming have not been well understood yet, especially for the reforming of liquid fuels. In
1 July 2017
this study, a kinetic model was derived for the analysis of diesel partial oxidation assisted
Accepted 20 July 2017
by plasma. Using a rotating arc as the plasma source, the effects of the oxygen-to-fuel ratio,
Available online xxx
specific energy input, temperature, and residence time on the reaction kinetics were investigated. A derived reaction model successfully describes the changes in the selectivity
Keywords:
towards hydrogen relative to the amount of energy provided for the generation of plasma
Plasma reforming
and the oxygen-to-fuel ratio in the reforming process.
Diesel
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Oxygen-to-carbon ratio
Introduction In the chemical industry, synthesis gas is an intermediate step in chemical processes such as the FischereTropsch process. The production of synthesis gas has also been observed in the biomass [1,2]. However, nowadays, synthesis gas is not just an intermediate step in chemical processes. The use of hydrogen and synthesis gas in the energy and environmental fields has been attracting attention as a tentative solution towards clean combustion and CO2 free energy carriers [3,4]. Here, applications for the clean combustion include both fuel side and after-treatment processes. For example, hydrogen (synthesis gas) itself can be a fuel or fuel additive to remove or reduce the emission of hazardous gases and CO2 [5,6]. Hydrogen can also be used as a reducing agent for de-NOx processes [7,8,32].
Reformed hydrocarbon fuel is potential because of an easily accessible feedstock for the production of hydrogen and synthesis gas [6,9]. Regardless of the type of reaction, such as steam reforming [6,10], dry reforming [6,11], and tri-reforming [6,12], the reforming process can be based on catalytic reactions. Each process has its own advantages and drawbacks in view of the product composition and degradation of catalyst [6]. Besides catalytic reactions, plasma techniques can also be a tentative tool for these reforming processes [6,13,30,31]. Although plasma processes consume the electric power for generation of the arc discharge, the plasma reforming process has many beneficial aspects: a fast start-up, ease of control, coking insensitivity, and the use of a rather small reactor scale [28e30,33]. Therefore, this technique would facilitate its prospective use in on-board applications, in which power
* Corresponding author. Korean Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea. E-mail address: [email protected] (D.H. Lee). http://dx.doi.org/10.1016/j.ijhydene.2017.07.164 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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consumption should be within the amount available from a battery. Therefore, the development of plasma reforming aimed at on-board applications should maximize the fuel conversion and hydrogen selectivity while the power consumption should be minimized. Plasma assisted fuel reforming process has been studied for years [13,14,32,33]. However, almost studies has reported on reforming gas-phase fuel (e.g. methane), studies on liquid fuel (e.g. octane and diesel) has been still limited. For example, some studies reported on effects of the O2/C ratio, power condition, and pressure on the reforming process of octane and diesel fuel [14e23]. In addition, a kinetic model was also derived for auto thermal reforming of n-Octane assisted by a non-thermal arc, which simulated that the non-equilibrium products were created in short time right after an ignition by the arc process [18]. However, studies on the kinetic characteristics of the reforming process of liquid fuel should be investigated further for a full understanding. Based on the aforementioned backgrounds, this study is focused on the description of the detailed kinetic characteristics of diesel fuel plasma reforming in relation to the fuel conversion rate and hydrogen selectivity.
Materials and methods Reaction assessment The partial oxidation of diesel fuel by plasma was investigated in this study. The commercial diesel fuel from a gas station was used as fuel. Since an exact composition of this fuel is not available, the chemical composition of the fuel considered for an analytical convenience is that of surrogate fuel, C12H23 [24,25]. The fuel conversion rate is defined as a ratio of reacted fuel to the amount of the initial fuel. However, analysis of the remnant fuel is not possible for the reasons mentioned above. Thus, a new formula for the calculation of the fuel conversion rate is introduced based on the measurable species in the reaction, Eq. (1) [15]. Xf ¼
nCO2 þ nCO þ x 12nf
P
nCx Hy
(1)
where the value of x for the hydrocarbon species ranges from 1 to 4. The selectivity of hydrogen is defined as the ratio of generated hydrogen relative to the maximum available hydrogen from the converted fuel (e.g. (23/2) mole of fuel) for the surrogate composition C12H23. It can be expressed as Eq. (2) [17]. 1 molðproduced H2 Þ 100ð%Þ SELðH2 Þ ¼ 23 molðC12 H23 Þ 2
(2)
Experimental apparatus A rotating arc was employed for this reforming process. Rotating arcs, called non-thermal arcs, can generate arcs with relatively low current levels of tens to hundreds of mA, resulting in temperatures ranging from 1000 to 3000 K, much
lower than typical thermal arcs. A high voltage electrode having a conical shape and a cylindrical ground electrode constitutes a rotating arc reactor. The diesel fuel is supplied through 4 small holes with a diameter of 0.5 mm located on the surface of the high-voltage electrode. The influx of air flow is directed tangentially to generate a swirling flow within the reactor. Along with the swirling reactant flow, the discharging arc is convected downstream with an expanded length. A detail design of the reactor can be found in the previous literature [26]. To sample the gas composition along the reactor length, five small tubes are inserted in the reactor for gas sampling of the reforming product at different positions. With this system, the effect of the reaction time can be investigated. Small holes are located in the sampling tube inside the reactor to get the averaged gas composition along the radial direction. To guarantee the frozen state of the sampled product, a quenching device whose volume is filled with alumina beads of diameter less than 1 mm is placed at each sampling line. So, the gas passes through the sampling line without further reactions. One thermocouple is also placed 50 mm downstream from the tip of the high voltage electrode to measure the product temperature. An AC power supply with a frequency of 10 (kHz) is used to impose a high voltage across the electrodes to initiate the discharge process. Power consumption for the generation of the rotating arc is measured via an oscilloscope using a high voltage probe (1000:1) and a current probe. A schematic of the overall experimental set-up is shown in Fig. 1. The fuel supply line is connected to the plenum inside the high voltage electrode. From the plenum, the fuel is injected into the reactor through small holes located in the circumference of the electrode, near the ignition point (e.g. the smallest gap between the high voltage electrode and the ground). The fuel supply is controlled by a syringe pump with a designated flow rate. The air flow for the reaction is controlled with a mass flow controller (MFC). The products are analyzed by the gas chromatography (GC) by measuring H2, CO, CO2, CO2, and N2 with TCD (Carboxen1010) and hydrocarbons with a carbon number up to four by FID (HP-AL/KCL) [17].
Reaction model and the simplified global kinetic Under the assumption that the surrogate of diesel is C12H23, the global reaction of the reforming process can be described with Eq. (3), a is the O2/C ratio. Plasma
C12 H23 þ 12aðO2 þ 3:76N2 Þ ! Products
(3)
Under the partial oxidation condition (e.g. O2/C ¼ 0.5), Eq. (3) can be rewritten as Eq. (4). Plasma
C12 H23 þ 6ðO2 þ 3:76N2 Þ ! Products
(4)
Basically, the partial oxidation reaction is driven by the thermal activation. The O2/C ratio determines the relative amount of heat released during the course of the reaction. In the case of the plasma driven the reforming reaction, the electric power is converted to thermal energy and transferred to the reactant gases through the arc process. The amount of
Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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Fig. 1 e Experiment model: (1) thermal couple, (2) the high voltage electrode, (3) oscilloscope, (4) the ground, (5) quenching system, (6) sampling line, (7) vale, (8) pump.
electric power that is delivered to the unit volume of reactant flow serves as another parameter towards the thermal activation. The oxidation of diesel fuel is basically a reaction between a liquid phase (e.g. diesel) and a gas phase (e.g. air). It includes essential steps such as fuel vaporization, mixing of gaseous fuel and oxidant, and the gas-phase reaction. In the rotating arc technique, liquid-phase fuel is injected through the holes located close to the tip of the high-voltage electrode, which has a function like a nozzle. Fuel is atomized by the nozzle and atomizing air flow. A fast vaporization and mixing then take place due to the high-temperature environment within the reactor and the discharge-induced turbulence, which is evidenced in our previous publication by a blue flame of gasphase reactions [17]. Thus, the effect of evaporation rate on the fuel oxidation rate can be ignored and a simplified onestep global reaction model of a homogeneous reaction of gas-phase can be proposed for diesel fuel oxidation modeling. Namely, a simplified one-step global reaction of diesel oxidation is proposed based on the assumptions: (i) the instantaneous reaction rate is proportional to the fuel concentration with first order and to the oxygen concentration with order of 12a [27], (ii) the instantaneous reaction rate increases linearly with the energy input (Pin). Actually, these assumptions are unreasonable to exactly express complicated mechanisms of diesel oxidation. However, these assumptions are valid to model the simplified one-step global reaction of diesel oxidation to predict main changes in fuel conversion as well as syngas selectivity. Based on the above assumptions, the reaction rate of fuel (wf ) can be expressed with Eq. (5). dCf ¼ kf Cf C12a wf ¼ o2 Pin dt
(5)
where Cf and Co2 are the concentration of the fuel and the oxidizer respectively, kf is the specific reaction-rate constant, and Pin is the real power consumption. According to the Arrhenius law [27], the reaction-rate constant only depends on the temperature, as shown in Eq. (6). Ea kf ¼ Aexp RT
(6)
where A and Ea (activation energy) are constants of the elementary reaction, R ¼ 8.3144 (J K1 mol1) is the ideal gas constant, and T is the gas temperature in the reaction chamber. Actually, the gas temperature within the reaction chamber change along the reactor length. However, in this simplified kinetic model, the temperature at gate 1 (50 mm far away from the high voltage electrode tip) is assumed as the representative temperature for the gas temperature in the reaction chamber. The temperature at gate 1 measured by a thermocouple is used because almost fuel is quickly oxidized in the zone from the high voltage electrode to gate 1, which was pointed out by experimental data in Fig. 3b. Because of this assumption, the temperature at gate 1 is used for modeling of global reaction as a representative value of the reaction. Considering a molar based expression, Eq. (5) can be rewritten as Eq. (7). 12a dnf 1 ¼ kf ð12aÞ12a n12aþ1 Pin f V dt
(7)
where V denotes the reaction volume. Integrating Eq. (7), Eq. (8) is obtained. # 12a 1 1 12a 12a 12a Pin t ¼ k f V nf ðtÞ12a nf ð0Þ12a
"
1
(8)
Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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here nf(0), and nf(t) are the moles of fuel at initial time and t is the reaction time which can be expressed as a ratio of the reaction volume (V) to the total volumetric flow rate (Q), as shown in Eq. (9). t¼
V LAr ¼ Q Q
(9)
where Ar is the cross section area of the reaction chamber; L is the reactor length. With an assumption that the reaction started at nozzle, L is defined as distance from the atomized fuel point to the sampling point. The total volumetric flow rate Q is a function of the total molar flow rate (F), total pressure (P), gas temperature (T), and the ideal gas constant (R), as shown in Eq. (10). Q¼ Fig. 2 e Fuel conversion rate at different O2/C ratios and SEIs.
FRT P
(10)
By combination Eq. (9) and Eq. (10), Eq. (11) is obtained: t¼
LAr P FRT
(11)
The concentration of oxygen at initial time (CO2 ð0Þ) can be calculated with Eq. (12). 12a
nf ð0Þ ¼ CO2 ð0Þ V
(12)
Using Eq. (11) and Eq. (12) to rewrite Eq. (8) in terms of the fuel conversion rate (Xf ), Eq. (13) is obtained: "
# 12a LAr P 12a SEI 1 ¼ kf CO2 ð0Þ 12a RT 1 Xf 1
(13)
where CO2 ð0Þ depends on the initial temperature (Tin) as per Eq. (14) (ideal gas equation), P and T are the gas pressure and the temperature inside the plasma reactor, respectively. CO2 ð0Þ ¼
P RTin
(14)
The specific energy input (SEI) can be calculated with Eq. (15). SEI ¼ Pin F
fuel
þ Fair
(15)
here Ffuel and Fair denote the fuel and air flow rates, respectively. The units of SEI are J/mole, as used in Eq. (13). All parameters in Eq. (13) can be calculated using the results of the fuel conversion rate measured experimentally. Strictly, 100% conversion does not imply the complete conversion of fuel, since the surrogate fuel is not an exact expression of diesel fuel. Moreover, even if assuming that all the fuel has been converted to hydrogen, the selectivity could be also much smaller than 100% because of the same reason. However, these definitions are sufficient to provide insight into the relative effectiveness of the reaction model.
Results and discussion Fig. 3 e (a) Product concentration and fuel conversion rate at different reactor lengths, total flow rate of 6.875 lpm, O2/C ¼ 0.5 and Pin ¼ 55 W, (b) Fuel conversion rate and hydrogen selectivity at different reactor lengths, O2/C ¼ 0.5, Pin ¼ 55 W.
Effect of specific energy input and oxygen-to-fuel ratio Chemical reactions are basically governed by the thermal activation processes. In exothermic reactions like reforming
Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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processes, the heat released in the course of the reaction is crucial for the sustenance of the reaction. On the other hand, in plasma driven reactions, and particularly in arc processes, the electric power provided to generate plasma is converted to heat by the arc process. Thus, in a plasma reforming process such as that in this study, the amount of heat and the subsequent reforming chemistry can be parameterized by SEI (electric power per unit reactant flow) and the oxygen-to-fuel ratio (e.g. O2/C, relative heat of reaction). An experiment was designed to compare the parametric effects of the O2/C ratio and SEI on the fuel conversion. The samples were obtained at a fixed position (gate 3), the length for the reaction volume was about 180 mm (e.g. L ¼ 0.18 m), and the air flow rate was 10 L per minute (L/min). The impact of both the oxygen-to-carbon ratio and the specific energy input on the fuel conversion is shown in Fig. 2. At low O2/C ratios, close to the stoichiometry for the partial oxidation, the role of SEI is easily distinguishable. Higher SEI values result in higher fuel conversions. However, under a high O2/C ratio conditions, the role of the specific energy input is less obvious as little difference is observed among the tested cases at different SEIs. Between the two parameters, the O2/C ratio was found to be more important for achieving high conversion rates. At an O2/C ratio of 1.1, the fuel conversion rate was over 80% for all of the tested SEI values.
The residence time effect The residence time can be defined as the time of reaction or the time from the beginning of the reaction to the point where the reaction product is sampled. If the residence time is infinite, the product can reaches the equilibrium state, which only depends on the temperature. However, under real conditions, increasing the residence time will result in the reforming product being close to the equilibrium state. Theoretically, the fuel conversion rate will increase with a longer residence time until it reaches the equilibrium value. To have a better understanding of the fuel conversion as well as the product concentration, an experiment was designed to observe the changes of fuel conversion and product concentration according to the reactor length or the reaction time. Fig. 3a showed that the syngas concentration and fuel conversion rate does not always increase according to the residence time. The concentration of syngas reduced after a peak while fuel conversion rate kept a constant value with increase in time scale. To figure out the relation of fuel conversion rate and syngas selectivity, the changes of fuel conversion rate and hydrogen selectivity (a representative for changes of syngas) were investigated at different total flow rates as shown in Fig. 3b. The results indicate that a rapid fuel conversion at the initial stage slows down with time. The conversion rate for each total flowrate exhibits its own maximum value after which the conversion reaches a plateau. This is possibly because, at certain O2/C ratios, there may be a deficiency of fuel or oxidizer, which would retard or stop the progress of the reaction. Heat losses and a reduced heat of reaction in the case of low O2/C ratios could also decelerate the reaction rate. Regarding the hydrogen selectivity, initially, and up to a certain time, the selectivity increases steadily; however, after
5
a certain time or at long residence times, the hydrogen selectivity decreases with time. The specific time scale of the maximum hydrogen selectivity is denoted as t and close to that of fuel conversion. The trend of the hydrogen selectivity in relation to the reaction time scale can be explained by simplifying the reforming reaction into two stages. In the first stage, the reaction between the fuel and oxidizer predominates over those involving other intermediate species. The initial steps of fuel cracking are the main reaction at this stage, which promotes hydrogen generation or hydrogen generation in the course of the dehydrogenation of C1 and C2 species. Other reactions can also be observed but the overall reaction is driven by the above steps. Therefore, hydrogen generation increases with the residence time. In the next stage, most of the fuel is decomposed and when there is no more fuel conversion, then there is no more hydrogen generation. At this stage, synthesis gases such as H2 and CO can react with any remaining oxidizer to produce H2O and CO2. Hence, in this second stage, the hydrogen selectivity decreases with the residence time. To have a better understanding about the kinetic mechanism of diesel fuel reforming, a simplified model was developed to predict the generation of hydrogen in the first stage. The kinetic characteristics of the second stage are largely different from those in the first one, as they involve reactions of intermediate species with remaining oxidizer, which will not be discussed in this study. The time required to achieve the maximum hydrogen selectivity is considered as an important parameter for the optimization of the reactor design. This allows the removal of any redundant part in the reactor because the position (distance) at which the maximum hydrogen selectivity is achieved is the optimal length of the reactor. The experimental results revealed that the time scale for the maximum hydrogen selectivity is less affected by the total flow rate. Accordingly, at an O2/C ratio of 0.5, it is about 0.45 s, the position of the maximum hydrogen selectivity moves downstream from the high voltage electrode with an increase of the total flow rate, Fig. 4. In thi s study, the maximum hydrogen selectivity was obtained at gate 5 at a total flow rate of about 9.625 (L/min). The results also reveal a linear relationship between the optimal reactor length (Lopt) and the total volume flow rate (Q), Eq. (16). Lopt ¼
t Q Ar
(16)
Actually, the time scale for the maximum hydrogen selectivity steadily increased in the tested range of total flow rates. This is due to lower specific energy inputs, Fig. 5. However, hydrogen selectivity slightly decreases with the increasing total flow rate or decreasing SEI. The results reveal that hydrogen generation is not strongly dependent on the conversion step of diesel fuel. In fact, the first step of diesel conversion requires a high activation energy, but hydrogen can be produced in each step of the dehydrogenation of diesel and intermediate hydrocarbon species. Further, low thermal activation conditions or low SEI values result in the retardation of hydrogen oxidation. As a result, the time required for the maximum hydrogen selectivity slightly increases inversely with the SEI value.
Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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1
12a 1
½1Xf kf ¼ 12a LAr P RT 12a SEI CO2 ð0Þ
Fig. 4 e Hydrogen selectivity at different residence times and different SEIs, O2/C ¼ 0.5. The time scale for the maximum hydrogen generation is about 0.45 s (t ¼ 0.45 s).
A series of experiments were designed to derive an empirical model for the reaction rate expression. Fixed experimental parameters included the initial gas temperature (e.g. 300 K), sample position (e.g. gate 3), and air flow rate (e.g. 10 L/min). These conditions were designed to guarantee incomplete fuel conversion at the position of sampling. Some main constants in Eq. (17) were calculated: The initial concentration of the oxidizer (Co2 ð0Þz9:375 mol=m3 ), the term LAr P z0:000682 mole because of an open reactor, L and Ar (e.g. RT 0.18 m and 0.000284 m2, respectively), and T (set 900 K). While all the above parameters were fixed, the SEI and O2/C ratio were studied in the range of 200e400 J/L and 0.5e1.1, respectively. With the above experimental conditions, Eqs. (13) and (17) can be rewritten as Eqs. (18) and (19). "
# 1 12a 12a 1 ¼ kf ½9:375 0:000682 12a SEI 1 Xf 1
Kinetic model The kinetic expression for the reforming process The specific reaction-rate constant (kf) strongly depends on the temperature. As mentioned above, for the reforming of diesel fuel by a rotating arc, the temperature of reaction mainly depends on the specific energy input, heat of reaction, and heat loss. In this study, based on the temperature measurements, a new kinetic model was proposed to simulate the fuel conversion rate and hydrogen selectivity. In the reaction model of this reforming process as per Eq. (13), the reaction-rate constant can be expressed as Eq. (17).
Fig. 5 e The time scale for the maximum hydrogen selectivity and the maximum hydrogen selectivity at different SEIs, O2/C ¼ 0.5.
(17)
kf ¼
1
½1Xf ½9:37512a 0:000682 12a SEI 12a
(18)
(19)
SEI was mainly controlled by adjustment of the supplied electric power. The temperature was measured 50 mm away from the high voltage electrode, the representative of the temperature in the reactor chamber. The product composition was measured at gate 3, which is 150 mm away from the tip of the high voltage electrode. The correlation between the reaction-rate constant (Eq. (19)) and temperature was obtained, Fig. 6. A linear correlation between ln(kf) and 1/T was evaluated for each O2/C ratio. The values of A and Ea in Eq. (6) can be estimated from the experimental results. Here A and Ea are constants that depend on the nature of the elementary reaction and the O2/C ratio. Using the curve fitting method, the values of A and Ea were estimated at different O2/C ratios, as summarized in Table 1.
Fig. 6 e Correlation between kf and the temperature.
Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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i h SELðH2 Þ ¼ SELðH2 ÞðmaximumÞ 1
Table 1 e Coefficients for Eq. (6). O2/C 0.5 0.6 0.7 0.9 1.1
A
Ea ¼ B*R (J/mole)
43.26 1.365Eþ8 1.01Eþ08 6.793Eþ21 1.33Eþ22
1.03Eþ5 2.07Eþ5 2.125Eþ5 4Eþ5 3.665Eþ5
#
12a LAr P Ea 12a SEI 12a 1 ¼ Ae RT CO2 ð0Þ RT 1 Xf 1
(20)
With the proposed kinetic model, the hydrogen generation rate can be predicted, under the consideration that fuel conversion and hydrogen selectivity have the same behavior up to a certain time on the stream. That is the fuel conversion rate (Xf ) identical to the hydrogen generation rate ðhH2 Þ which is defined by Eq. (21).
hH2 ¼
nH2 SELðH2 Þ ¼ nH2 ðmaxÞ SELðH2 ÞðmaxÞ
1=6
1 12402 T
1 þ 801260:3 e
SEI L
(24)
According to the above definition, the maximum hydrogen generation is the selectivity of hydrogen at the point of fuel totally converted, which can be calculated by Eq. (25).
The results show that, as the O2/C ratio increases, the rate coefficient becomes more sensitive to the temperature. At high O2/C ratios, a small change in the temperature leads to a large change of the fuel conversion rate constant. Based on the above results, a complete general expression for the prediction of the fuel conversion rate is obtained as Eq. (20). "
(21)
where nH2 and SEL(H2) are the molar concentration and selectivity of hydrogen, respectively, and nH2(max) and SEL(H2)(max) are the maximum molar concentration and selectivity of hydrogen generation at the point when the fuel is totally converted, respectively. Thus, Eq. (20) can be rewritten based on the hydrogen generation rate, Eq. (22). # 12a LAr P 1 Ea 12a SEI 12a 1 ¼ Ae RT CO2 ð0Þ RT 1 hH2
"
SELðH2 ÞðmaximumÞ ¼
SELðH2 Þ Xf
(25)
Although the detailed mechanism of the partial oxidation of diesel fuel is complicated, longer reaction times ensure higher conversions of fuel and hydrogen selectivity. Strictly speaking, a higher hydrogen generation rate does not imply higher hydrogen selectivity because the maximum hydrogen selectivity depends on the O2/C ratio. However at O2/C of 0.5, the fuel conversion rate and hydrogen selectivity present a strong linear correlation at all SEI conditions. In addition, the maximum hydrogen generation is about 23% for all SEI conditions, Fig. 7. Based on the aforementioned reasoning, in order to predict the hydrogen selectivity in a plasma reactor, one more assumption is proposed: (iii) the maximum hydrogen selectivity only depends on the O2/C ratio. In the case of an O2/C ratio of 0.5, the maximum hydrogen selectivity is approximate 23%. Based on the above assumption, the selectivity of hydrogen can be modeled as per Eq. (26). 2 316 3 1 4 4 55
SELðH2 Þ ¼ ð23%Þ 1 12402 1 þ 801260:3 e T SEI L 2
(26) The experimental data were compared to the predicted values of hydrogen selectivity through the proposed model. A comparison of the different SEI conditions and different residence times is given in Fig. 8. The results show that the proposed model can predict the hydrogen selectivity with satisfactory accuracy.
(22) Eq. (22) implies that the rate of hydrogen generation depends on the initial concentration of oxidant (CO2 ð0Þ), reaction temperature, SEI, O2/C ratio, and residence time (e.g. reactor dimension or the thermal density of reaction volume).
The kinetic model for the partial oxidation (O2/C ¼ 0.5) In the case of an O2/C ratio of 0.5, Eq. (22) can be rephrased as Eq. (23). "
1
# 12402 T
6 1 ¼ 801260:3 e 1 hH2
SEI L
(23)
here the reactor length can be interpreted as the reaction volume or thermal density of the reactor. The hydrogen selectivity can be expressed as Eq. (24), derived from Eqs. (21) and (23).
Fig. 7 e The maximum hydrogen generation derived from Eq. (25) at different SEIs, O2/C ¼ 0.5.
Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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references
Fig. 8 e Hydrogen selectivity during the reaction time at different SEIs, O2/C ¼ 0.5.
Conclusions The kinetic aspects of diesel reforming by a rotating arc were investigated with a particular focus on the effect of the residence time and specific energy input. The generation of hydrogen in the reforming process was simplified as a twostage reaction. During the first stage, plasma power or thermal activation plays the main role in the activation of fuel for the production of hydrogen. Later, in the second stage, fuel conversion reaches its maximum value but the amount of oxygen or the O2/C ratio controls the successive oxidation of hydrogen. The O2/C ratio itself likely controls the amount of heat of reaction; however, at the O2/C ratio corresponding to the partial oxidation by plasma, the amount of heat of reaction is not that high compared to that from combustion, and the role of thermal energy converted from the electric power plays a more significant role than the heat released by the oxidation reaction. Based on the fact that the characteristics of fuel conversion and hydrogen generation are similar in terms of the residence time, the hydrogen selectivity can be also predicted well by the proposed kinetic model. In addition, the time scale for the maximum hydrogen selectivity can be used for the optimization of the plasma reactor length using Eq. (16), regardless of the total flow rate.
Acknowledgements The study was done by the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of Republic of Korea (CRC-14-1-KRICT) and C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016M3D3A1A01913261).
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Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction Duy Khoe Dinh a,b, Hee Seok Kang b, Sungkwon Jo b, Dae Hoon Lee a,b,*, Young-Hoon Song a,b a b
University of Science and Technology (UST), 217 Gajeong-ro Yuseong-gu, Daejeon 305-350, Republic of Korea Korea Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea
article info
abstract
Article history:
Plasma reforming of fuel has recently been studied for possible applications in hydrogen
Received 19 May 2017
generation and plasma assisted combustion. However, the kinetic aspects of plasma
Received in revised form
reforming have not been well understood yet, especially for the reforming of liquid fuels. In
1 July 2017
this study, a kinetic model was derived for the analysis of diesel partial oxidation assisted
Accepted 20 July 2017
by plasma. Using a rotating arc as the plasma source, the effects of the oxygen-to-fuel ratio,
Available online xxx
specific energy input, temperature, and residence time on the reaction kinetics were investigated. A derived reaction model successfully describes the changes in the selectivity
Keywords:
towards hydrogen relative to the amount of energy provided for the generation of plasma
Plasma reforming
and the oxygen-to-fuel ratio in the reforming process.
Diesel
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Oxygen-to-carbon ratio
Introduction In the chemical industry, synthesis gas is an intermediate step in chemical processes such as the FischereTropsch process. The production of synthesis gas has also been observed in the biomass [1,2]. However, nowadays, synthesis gas is not just an intermediate step in chemical processes. The use of hydrogen and synthesis gas in the energy and environmental fields has been attracting attention as a tentative solution towards clean combustion and CO2 free energy carriers [3,4]. Here, applications for the clean combustion include both fuel side and after-treatment processes. For example, hydrogen (synthesis gas) itself can be a fuel or fuel additive to remove or reduce the emission of hazardous gases and CO2 [5,6]. Hydrogen can also be used as a reducing agent for de-NOx processes [7,8,32].
Reformed hydrocarbon fuel is potential because of an easily accessible feedstock for the production of hydrogen and synthesis gas [6,9]. Regardless of the type of reaction, such as steam reforming [6,10], dry reforming [6,11], and tri-reforming [6,12], the reforming process can be based on catalytic reactions. Each process has its own advantages and drawbacks in view of the product composition and degradation of catalyst [6]. Besides catalytic reactions, plasma techniques can also be a tentative tool for these reforming processes [6,13,30,31]. Although plasma processes consume the electric power for generation of the arc discharge, the plasma reforming process has many beneficial aspects: a fast start-up, ease of control, coking insensitivity, and the use of a rather small reactor scale [28e30,33]. Therefore, this technique would facilitate its prospective use in on-board applications, in which power
* Corresponding author. Korean Institute of Machinery and Materials, 156 Gajeongbuk-ro, Yuseong-gu, Daejeon 34103, Republic of Korea. E-mail address: [email protected] (D.H. Lee). http://dx.doi.org/10.1016/j.ijhydene.2017.07.164 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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consumption should be within the amount available from a battery. Therefore, the development of plasma reforming aimed at on-board applications should maximize the fuel conversion and hydrogen selectivity while the power consumption should be minimized. Plasma assisted fuel reforming process has been studied for years [13,14,32,33]. However, almost studies has reported on reforming gas-phase fuel (e.g. methane), studies on liquid fuel (e.g. octane and diesel) has been still limited. For example, some studies reported on effects of the O2/C ratio, power condition, and pressure on the reforming process of octane and diesel fuel [14e23]. In addition, a kinetic model was also derived for auto thermal reforming of n-Octane assisted by a non-thermal arc, which simulated that the non-equilibrium products were created in short time right after an ignition by the arc process [18]. However, studies on the kinetic characteristics of the reforming process of liquid fuel should be investigated further for a full understanding. Based on the aforementioned backgrounds, this study is focused on the description of the detailed kinetic characteristics of diesel fuel plasma reforming in relation to the fuel conversion rate and hydrogen selectivity.
Materials and methods Reaction assessment The partial oxidation of diesel fuel by plasma was investigated in this study. The commercial diesel fuel from a gas station was used as fuel. Since an exact composition of this fuel is not available, the chemical composition of the fuel considered for an analytical convenience is that of surrogate fuel, C12H23 [24,25]. The fuel conversion rate is defined as a ratio of reacted fuel to the amount of the initial fuel. However, analysis of the remnant fuel is not possible for the reasons mentioned above. Thus, a new formula for the calculation of the fuel conversion rate is introduced based on the measurable species in the reaction, Eq. (1) [15]. Xf ¼
nCO2 þ nCO þ x 12nf
P
nCx Hy
(1)
where the value of x for the hydrocarbon species ranges from 1 to 4. The selectivity of hydrogen is defined as the ratio of generated hydrogen relative to the maximum available hydrogen from the converted fuel (e.g. (23/2) mole of fuel) for the surrogate composition C12H23. It can be expressed as Eq. (2) [17]. 1 molðproduced H2 Þ 100ð%Þ SELðH2 Þ ¼ 23 molðC12 H23 Þ 2
(2)
Experimental apparatus A rotating arc was employed for this reforming process. Rotating arcs, called non-thermal arcs, can generate arcs with relatively low current levels of tens to hundreds of mA, resulting in temperatures ranging from 1000 to 3000 K, much
lower than typical thermal arcs. A high voltage electrode having a conical shape and a cylindrical ground electrode constitutes a rotating arc reactor. The diesel fuel is supplied through 4 small holes with a diameter of 0.5 mm located on the surface of the high-voltage electrode. The influx of air flow is directed tangentially to generate a swirling flow within the reactor. Along with the swirling reactant flow, the discharging arc is convected downstream with an expanded length. A detail design of the reactor can be found in the previous literature [26]. To sample the gas composition along the reactor length, five small tubes are inserted in the reactor for gas sampling of the reforming product at different positions. With this system, the effect of the reaction time can be investigated. Small holes are located in the sampling tube inside the reactor to get the averaged gas composition along the radial direction. To guarantee the frozen state of the sampled product, a quenching device whose volume is filled with alumina beads of diameter less than 1 mm is placed at each sampling line. So, the gas passes through the sampling line without further reactions. One thermocouple is also placed 50 mm downstream from the tip of the high voltage electrode to measure the product temperature. An AC power supply with a frequency of 10 (kHz) is used to impose a high voltage across the electrodes to initiate the discharge process. Power consumption for the generation of the rotating arc is measured via an oscilloscope using a high voltage probe (1000:1) and a current probe. A schematic of the overall experimental set-up is shown in Fig. 1. The fuel supply line is connected to the plenum inside the high voltage electrode. From the plenum, the fuel is injected into the reactor through small holes located in the circumference of the electrode, near the ignition point (e.g. the smallest gap between the high voltage electrode and the ground). The fuel supply is controlled by a syringe pump with a designated flow rate. The air flow for the reaction is controlled with a mass flow controller (MFC). The products are analyzed by the gas chromatography (GC) by measuring H2, CO, CO2, CO2, and N2 with TCD (Carboxen1010) and hydrocarbons with a carbon number up to four by FID (HP-AL/KCL) [17].
Reaction model and the simplified global kinetic Under the assumption that the surrogate of diesel is C12H23, the global reaction of the reforming process can be described with Eq. (3), a is the O2/C ratio. Plasma
C12 H23 þ 12aðO2 þ 3:76N2 Þ ! Products
(3)
Under the partial oxidation condition (e.g. O2/C ¼ 0.5), Eq. (3) can be rewritten as Eq. (4). Plasma
C12 H23 þ 6ðO2 þ 3:76N2 Þ ! Products
(4)
Basically, the partial oxidation reaction is driven by the thermal activation. The O2/C ratio determines the relative amount of heat released during the course of the reaction. In the case of the plasma driven the reforming reaction, the electric power is converted to thermal energy and transferred to the reactant gases through the arc process. The amount of
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Fig. 1 e Experiment model: (1) thermal couple, (2) the high voltage electrode, (3) oscilloscope, (4) the ground, (5) quenching system, (6) sampling line, (7) vale, (8) pump.
electric power that is delivered to the unit volume of reactant flow serves as another parameter towards the thermal activation. The oxidation of diesel fuel is basically a reaction between a liquid phase (e.g. diesel) and a gas phase (e.g. air). It includes essential steps such as fuel vaporization, mixing of gaseous fuel and oxidant, and the gas-phase reaction. In the rotating arc technique, liquid-phase fuel is injected through the holes located close to the tip of the high-voltage electrode, which has a function like a nozzle. Fuel is atomized by the nozzle and atomizing air flow. A fast vaporization and mixing then take place due to the high-temperature environment within the reactor and the discharge-induced turbulence, which is evidenced in our previous publication by a blue flame of gasphase reactions [17]. Thus, the effect of evaporation rate on the fuel oxidation rate can be ignored and a simplified onestep global reaction model of a homogeneous reaction of gas-phase can be proposed for diesel fuel oxidation modeling. Namely, a simplified one-step global reaction of diesel oxidation is proposed based on the assumptions: (i) the instantaneous reaction rate is proportional to the fuel concentration with first order and to the oxygen concentration with order of 12a [27], (ii) the instantaneous reaction rate increases linearly with the energy input (Pin). Actually, these assumptions are unreasonable to exactly express complicated mechanisms of diesel oxidation. However, these assumptions are valid to model the simplified one-step global reaction of diesel oxidation to predict main changes in fuel conversion as well as syngas selectivity. Based on the above assumptions, the reaction rate of fuel (wf ) can be expressed with Eq. (5). dCf ¼ kf Cf C12a wf ¼ o2 Pin dt
(5)
where Cf and Co2 are the concentration of the fuel and the oxidizer respectively, kf is the specific reaction-rate constant, and Pin is the real power consumption. According to the Arrhenius law [27], the reaction-rate constant only depends on the temperature, as shown in Eq. (6). Ea kf ¼ Aexp RT
(6)
where A and Ea (activation energy) are constants of the elementary reaction, R ¼ 8.3144 (J K1 mol1) is the ideal gas constant, and T is the gas temperature in the reaction chamber. Actually, the gas temperature within the reaction chamber change along the reactor length. However, in this simplified kinetic model, the temperature at gate 1 (50 mm far away from the high voltage electrode tip) is assumed as the representative temperature for the gas temperature in the reaction chamber. The temperature at gate 1 measured by a thermocouple is used because almost fuel is quickly oxidized in the zone from the high voltage electrode to gate 1, which was pointed out by experimental data in Fig. 3b. Because of this assumption, the temperature at gate 1 is used for modeling of global reaction as a representative value of the reaction. Considering a molar based expression, Eq. (5) can be rewritten as Eq. (7). 12a dnf 1 ¼ kf ð12aÞ12a n12aþ1 Pin f V dt
(7)
where V denotes the reaction volume. Integrating Eq. (7), Eq. (8) is obtained. # 12a 1 1 12a 12a 12a Pin t ¼ k f V nf ðtÞ12a nf ð0Þ12a
"
1
(8)
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here nf(0), and nf(t) are the moles of fuel at initial time and t is the reaction time which can be expressed as a ratio of the reaction volume (V) to the total volumetric flow rate (Q), as shown in Eq. (9). t¼
V LAr ¼ Q Q
(9)
where Ar is the cross section area of the reaction chamber; L is the reactor length. With an assumption that the reaction started at nozzle, L is defined as distance from the atomized fuel point to the sampling point. The total volumetric flow rate Q is a function of the total molar flow rate (F), total pressure (P), gas temperature (T), and the ideal gas constant (R), as shown in Eq. (10). Q¼ Fig. 2 e Fuel conversion rate at different O2/C ratios and SEIs.
FRT P
(10)
By combination Eq. (9) and Eq. (10), Eq. (11) is obtained: t¼
LAr P FRT
(11)
The concentration of oxygen at initial time (CO2 ð0Þ) can be calculated with Eq. (12). 12a
nf ð0Þ ¼ CO2 ð0Þ V
(12)
Using Eq. (11) and Eq. (12) to rewrite Eq. (8) in terms of the fuel conversion rate (Xf ), Eq. (13) is obtained: "
# 12a LAr P 12a SEI 1 ¼ kf CO2 ð0Þ 12a RT 1 Xf 1
(13)
where CO2 ð0Þ depends on the initial temperature (Tin) as per Eq. (14) (ideal gas equation), P and T are the gas pressure and the temperature inside the plasma reactor, respectively. CO2 ð0Þ ¼
P RTin
(14)
The specific energy input (SEI) can be calculated with Eq. (15). SEI ¼ Pin F
fuel
þ Fair
(15)
here Ffuel and Fair denote the fuel and air flow rates, respectively. The units of SEI are J/mole, as used in Eq. (13). All parameters in Eq. (13) can be calculated using the results of the fuel conversion rate measured experimentally. Strictly, 100% conversion does not imply the complete conversion of fuel, since the surrogate fuel is not an exact expression of diesel fuel. Moreover, even if assuming that all the fuel has been converted to hydrogen, the selectivity could be also much smaller than 100% because of the same reason. However, these definitions are sufficient to provide insight into the relative effectiveness of the reaction model.
Results and discussion Fig. 3 e (a) Product concentration and fuel conversion rate at different reactor lengths, total flow rate of 6.875 lpm, O2/C ¼ 0.5 and Pin ¼ 55 W, (b) Fuel conversion rate and hydrogen selectivity at different reactor lengths, O2/C ¼ 0.5, Pin ¼ 55 W.
Effect of specific energy input and oxygen-to-fuel ratio Chemical reactions are basically governed by the thermal activation processes. In exothermic reactions like reforming
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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 x x x ( 2 0 1 7 ) 1 e9
processes, the heat released in the course of the reaction is crucial for the sustenance of the reaction. On the other hand, in plasma driven reactions, and particularly in arc processes, the electric power provided to generate plasma is converted to heat by the arc process. Thus, in a plasma reforming process such as that in this study, the amount of heat and the subsequent reforming chemistry can be parameterized by SEI (electric power per unit reactant flow) and the oxygen-to-fuel ratio (e.g. O2/C, relative heat of reaction). An experiment was designed to compare the parametric effects of the O2/C ratio and SEI on the fuel conversion. The samples were obtained at a fixed position (gate 3), the length for the reaction volume was about 180 mm (e.g. L ¼ 0.18 m), and the air flow rate was 10 L per minute (L/min). The impact of both the oxygen-to-carbon ratio and the specific energy input on the fuel conversion is shown in Fig. 2. At low O2/C ratios, close to the stoichiometry for the partial oxidation, the role of SEI is easily distinguishable. Higher SEI values result in higher fuel conversions. However, under a high O2/C ratio conditions, the role of the specific energy input is less obvious as little difference is observed among the tested cases at different SEIs. Between the two parameters, the O2/C ratio was found to be more important for achieving high conversion rates. At an O2/C ratio of 1.1, the fuel conversion rate was over 80% for all of the tested SEI values.
The residence time effect The residence time can be defined as the time of reaction or the time from the beginning of the reaction to the point where the reaction product is sampled. If the residence time is infinite, the product can reaches the equilibrium state, which only depends on the temperature. However, under real conditions, increasing the residence time will result in the reforming product being close to the equilibrium state. Theoretically, the fuel conversion rate will increase with a longer residence time until it reaches the equilibrium value. To have a better understanding of the fuel conversion as well as the product concentration, an experiment was designed to observe the changes of fuel conversion and product concentration according to the reactor length or the reaction time. Fig. 3a showed that the syngas concentration and fuel conversion rate does not always increase according to the residence time. The concentration of syngas reduced after a peak while fuel conversion rate kept a constant value with increase in time scale. To figure out the relation of fuel conversion rate and syngas selectivity, the changes of fuel conversion rate and hydrogen selectivity (a representative for changes of syngas) were investigated at different total flow rates as shown in Fig. 3b. The results indicate that a rapid fuel conversion at the initial stage slows down with time. The conversion rate for each total flowrate exhibits its own maximum value after which the conversion reaches a plateau. This is possibly because, at certain O2/C ratios, there may be a deficiency of fuel or oxidizer, which would retard or stop the progress of the reaction. Heat losses and a reduced heat of reaction in the case of low O2/C ratios could also decelerate the reaction rate. Regarding the hydrogen selectivity, initially, and up to a certain time, the selectivity increases steadily; however, after
5
a certain time or at long residence times, the hydrogen selectivity decreases with time. The specific time scale of the maximum hydrogen selectivity is denoted as t and close to that of fuel conversion. The trend of the hydrogen selectivity in relation to the reaction time scale can be explained by simplifying the reforming reaction into two stages. In the first stage, the reaction between the fuel and oxidizer predominates over those involving other intermediate species. The initial steps of fuel cracking are the main reaction at this stage, which promotes hydrogen generation or hydrogen generation in the course of the dehydrogenation of C1 and C2 species. Other reactions can also be observed but the overall reaction is driven by the above steps. Therefore, hydrogen generation increases with the residence time. In the next stage, most of the fuel is decomposed and when there is no more fuel conversion, then there is no more hydrogen generation. At this stage, synthesis gases such as H2 and CO can react with any remaining oxidizer to produce H2O and CO2. Hence, in this second stage, the hydrogen selectivity decreases with the residence time. To have a better understanding about the kinetic mechanism of diesel fuel reforming, a simplified model was developed to predict the generation of hydrogen in the first stage. The kinetic characteristics of the second stage are largely different from those in the first one, as they involve reactions of intermediate species with remaining oxidizer, which will not be discussed in this study. The time required to achieve the maximum hydrogen selectivity is considered as an important parameter for the optimization of the reactor design. This allows the removal of any redundant part in the reactor because the position (distance) at which the maximum hydrogen selectivity is achieved is the optimal length of the reactor. The experimental results revealed that the time scale for the maximum hydrogen selectivity is less affected by the total flow rate. Accordingly, at an O2/C ratio of 0.5, it is about 0.45 s, the position of the maximum hydrogen selectivity moves downstream from the high voltage electrode with an increase of the total flow rate, Fig. 4. In thi s study, the maximum hydrogen selectivity was obtained at gate 5 at a total flow rate of about 9.625 (L/min). The results also reveal a linear relationship between the optimal reactor length (Lopt) and the total volume flow rate (Q), Eq. (16). Lopt ¼
t Q Ar
(16)
Actually, the time scale for the maximum hydrogen selectivity steadily increased in the tested range of total flow rates. This is due to lower specific energy inputs, Fig. 5. However, hydrogen selectivity slightly decreases with the increasing total flow rate or decreasing SEI. The results reveal that hydrogen generation is not strongly dependent on the conversion step of diesel fuel. In fact, the first step of diesel conversion requires a high activation energy, but hydrogen can be produced in each step of the dehydrogenation of diesel and intermediate hydrocarbon species. Further, low thermal activation conditions or low SEI values result in the retardation of hydrogen oxidation. As a result, the time required for the maximum hydrogen selectivity slightly increases inversely with the SEI value.
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1
12a 1
½1Xf kf ¼ 12a LAr P RT 12a SEI CO2 ð0Þ
Fig. 4 e Hydrogen selectivity at different residence times and different SEIs, O2/C ¼ 0.5. The time scale for the maximum hydrogen generation is about 0.45 s (t ¼ 0.45 s).
A series of experiments were designed to derive an empirical model for the reaction rate expression. Fixed experimental parameters included the initial gas temperature (e.g. 300 K), sample position (e.g. gate 3), and air flow rate (e.g. 10 L/min). These conditions were designed to guarantee incomplete fuel conversion at the position of sampling. Some main constants in Eq. (17) were calculated: The initial concentration of the oxidizer (Co2 ð0Þz9:375 mol=m3 ), the term LAr P z0:000682 mole because of an open reactor, L and Ar (e.g. RT 0.18 m and 0.000284 m2, respectively), and T (set 900 K). While all the above parameters were fixed, the SEI and O2/C ratio were studied in the range of 200e400 J/L and 0.5e1.1, respectively. With the above experimental conditions, Eqs. (13) and (17) can be rewritten as Eqs. (18) and (19). "
# 1 12a 12a 1 ¼ kf ½9:375 0:000682 12a SEI 1 Xf 1
Kinetic model The kinetic expression for the reforming process The specific reaction-rate constant (kf) strongly depends on the temperature. As mentioned above, for the reforming of diesel fuel by a rotating arc, the temperature of reaction mainly depends on the specific energy input, heat of reaction, and heat loss. In this study, based on the temperature measurements, a new kinetic model was proposed to simulate the fuel conversion rate and hydrogen selectivity. In the reaction model of this reforming process as per Eq. (13), the reaction-rate constant can be expressed as Eq. (17).
Fig. 5 e The time scale for the maximum hydrogen selectivity and the maximum hydrogen selectivity at different SEIs, O2/C ¼ 0.5.
(17)
kf ¼
1
½1Xf ½9:37512a 0:000682 12a SEI 12a
(18)
(19)
SEI was mainly controlled by adjustment of the supplied electric power. The temperature was measured 50 mm away from the high voltage electrode, the representative of the temperature in the reactor chamber. The product composition was measured at gate 3, which is 150 mm away from the tip of the high voltage electrode. The correlation between the reaction-rate constant (Eq. (19)) and temperature was obtained, Fig. 6. A linear correlation between ln(kf) and 1/T was evaluated for each O2/C ratio. The values of A and Ea in Eq. (6) can be estimated from the experimental results. Here A and Ea are constants that depend on the nature of the elementary reaction and the O2/C ratio. Using the curve fitting method, the values of A and Ea were estimated at different O2/C ratios, as summarized in Table 1.
Fig. 6 e Correlation between kf and the temperature.
Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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i h SELðH2 Þ ¼ SELðH2 ÞðmaximumÞ 1
Table 1 e Coefficients for Eq. (6). O2/C 0.5 0.6 0.7 0.9 1.1
A
Ea ¼ B*R (J/mole)
43.26 1.365Eþ8 1.01Eþ08 6.793Eþ21 1.33Eþ22
1.03Eþ5 2.07Eþ5 2.125Eþ5 4Eþ5 3.665Eþ5
#
12a LAr P Ea 12a SEI 12a 1 ¼ Ae RT CO2 ð0Þ RT 1 Xf 1
(20)
With the proposed kinetic model, the hydrogen generation rate can be predicted, under the consideration that fuel conversion and hydrogen selectivity have the same behavior up to a certain time on the stream. That is the fuel conversion rate (Xf ) identical to the hydrogen generation rate ðhH2 Þ which is defined by Eq. (21).
hH2 ¼
nH2 SELðH2 Þ ¼ nH2 ðmaxÞ SELðH2 ÞðmaxÞ
1=6
1 12402 T
1 þ 801260:3 e
SEI L
(24)
According to the above definition, the maximum hydrogen generation is the selectivity of hydrogen at the point of fuel totally converted, which can be calculated by Eq. (25).
The results show that, as the O2/C ratio increases, the rate coefficient becomes more sensitive to the temperature. At high O2/C ratios, a small change in the temperature leads to a large change of the fuel conversion rate constant. Based on the above results, a complete general expression for the prediction of the fuel conversion rate is obtained as Eq. (20). "
(21)
where nH2 and SEL(H2) are the molar concentration and selectivity of hydrogen, respectively, and nH2(max) and SEL(H2)(max) are the maximum molar concentration and selectivity of hydrogen generation at the point when the fuel is totally converted, respectively. Thus, Eq. (20) can be rewritten based on the hydrogen generation rate, Eq. (22). # 12a LAr P 1 Ea 12a SEI 12a 1 ¼ Ae RT CO2 ð0Þ RT 1 hH2
"
SELðH2 ÞðmaximumÞ ¼
SELðH2 Þ Xf
(25)
Although the detailed mechanism of the partial oxidation of diesel fuel is complicated, longer reaction times ensure higher conversions of fuel and hydrogen selectivity. Strictly speaking, a higher hydrogen generation rate does not imply higher hydrogen selectivity because the maximum hydrogen selectivity depends on the O2/C ratio. However at O2/C of 0.5, the fuel conversion rate and hydrogen selectivity present a strong linear correlation at all SEI conditions. In addition, the maximum hydrogen generation is about 23% for all SEI conditions, Fig. 7. Based on the aforementioned reasoning, in order to predict the hydrogen selectivity in a plasma reactor, one more assumption is proposed: (iii) the maximum hydrogen selectivity only depends on the O2/C ratio. In the case of an O2/C ratio of 0.5, the maximum hydrogen selectivity is approximate 23%. Based on the above assumption, the selectivity of hydrogen can be modeled as per Eq. (26). 2 316 3 1 4 4 55
SELðH2 Þ ¼ ð23%Þ 1 12402 1 þ 801260:3 e T SEI L 2
(26) The experimental data were compared to the predicted values of hydrogen selectivity through the proposed model. A comparison of the different SEI conditions and different residence times is given in Fig. 8. The results show that the proposed model can predict the hydrogen selectivity with satisfactory accuracy.
(22) Eq. (22) implies that the rate of hydrogen generation depends on the initial concentration of oxidant (CO2 ð0Þ), reaction temperature, SEI, O2/C ratio, and residence time (e.g. reactor dimension or the thermal density of reaction volume).
The kinetic model for the partial oxidation (O2/C ¼ 0.5) In the case of an O2/C ratio of 0.5, Eq. (22) can be rephrased as Eq. (23). "
1
# 12402 T
6 1 ¼ 801260:3 e 1 hH2
SEI L
(23)
here the reactor length can be interpreted as the reaction volume or thermal density of the reactor. The hydrogen selectivity can be expressed as Eq. (24), derived from Eqs. (21) and (23).
Fig. 7 e The maximum hydrogen generation derived from Eq. (25) at different SEIs, O2/C ¼ 0.5.
Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164
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references
Fig. 8 e Hydrogen selectivity during the reaction time at different SEIs, O2/C ¼ 0.5.
Conclusions The kinetic aspects of diesel reforming by a rotating arc were investigated with a particular focus on the effect of the residence time and specific energy input. The generation of hydrogen in the reforming process was simplified as a twostage reaction. During the first stage, plasma power or thermal activation plays the main role in the activation of fuel for the production of hydrogen. Later, in the second stage, fuel conversion reaches its maximum value but the amount of oxygen or the O2/C ratio controls the successive oxidation of hydrogen. The O2/C ratio itself likely controls the amount of heat of reaction; however, at the O2/C ratio corresponding to the partial oxidation by plasma, the amount of heat of reaction is not that high compared to that from combustion, and the role of thermal energy converted from the electric power plays a more significant role than the heat released by the oxidation reaction. Based on the fact that the characteristics of fuel conversion and hydrogen generation are similar in terms of the residence time, the hydrogen selectivity can be also predicted well by the proposed kinetic model. In addition, the time scale for the maximum hydrogen selectivity can be used for the optimization of the plasma reactor length using Eq. (16), regardless of the total flow rate.
Acknowledgements The study was done by the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of Republic of Korea (CRC-14-1-KRICT) and C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2016M3D3A1A01913261).
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Please cite this article in press as: Dinh DK, et al., Partial oxidation of diesel fuel by plasma e Kinetic aspects of the reaction, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.07.164