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Ethanol conversion in a DC atmospheric pressure glow discharge
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 1 ( 2 0 1 6 ) 1 8 3 2 0 e1 8 3 2 8
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Ethanol conversion in a DC atmospheric pressure glow discharge V.I. Arkhipenko a, A.A. Kirillov a,*, L.V. Simonchik a, A.V. Kazak a, A.P. Chernukho b, A.N. Migoun b a b
B.I. Stepanov Institute of Physics of NAS of Belarus, Pr. Nezavisimosti 68, 220072 Minsk, Belarus Private R&D Enterprise, Advanced Research & Technologies, Sovkhoznaya 1, 223058 Leskovka, Belarus
article info
abstract
Article history:
Conversion of ethanol-water mixture into syngas in a DC atmospheric pressure air glow
Received 10 April 2016
discharge with a plasma cathode has been investigated experimentally and theoretically.
Received in revised form
The electric power of discharge was varied from 100 W up to 250 W. Novel diagnostics
11 July 2016
based on the absorption infrared spectroscopy was developed and used to determine the
Accepted 16 August 2016
syngas composition and the conversion parameters. The main components of syngas
Available online 16 September 2016
were: hydrogen, carbon monoxide, methane, ethylene and acetylene. The achieved degree of conversion to hydrogen was about 90%, with hydrogen content in conversion products
Keywords:
being equal to 40%. A method for numerical simulation of the conversion process has been
Atmospheric pressure glow
developed under an assumption that the role of discharge in the conversion process is
discharge
purely thermal in nature. The experimental and numerical data were found to be in a good
Ethanol
agreement.
Conversion
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Syngas Experimental Numerical
Introduction Theoretical and experimental researches have shown that an addition of small amounts of hydrogen and synthesis-gas into the main fuel allows to increase the efficiency of internal combustion engines and to reduce the emission of pollutants [1e3]. However, this also leads to problems with the storing and transporting of hydrogen [4]. The most promising solution is to generate hydrogen directly before the fuel burning. Among possible implementations of hydrogen generators, the plasma systems have occupied a special place [5]. In such generators, the process of fuel conversion could be fully or
partially maintained by plasma [5e8]. The main advantages of technologies based on the use of plasma include the acceptable modes of operation (atmospheric pressure, low gas temperatures, quick start, compact size, etc.). Different types of discharges at atmospheric pressure (corona, spark, barrier, sliding arc of DC, AC and pulse current in different frequency bands) are offered as plasma sources for these purposes [9e14]. In order to optimize the plasma conversion processes, it is needed to understand the kinetics of plasma-chemical processes, the role of which can vary significantly in different discharges. Plasma effect manifests itself in both heating of
* Corresponding author. E-mail address: [email protected] (A.A. Kirillov). http://dx.doi.org/10.1016/j.ijhydene.2016.08.122 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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the gas and generation of chemically active particles during collisions of electrons with molecules. The latter process is important in cold plasmas [15], such as dielectric-barrier discharge [16], corona discharge [17] and spark discharge [18]. In discharges with hot plasmas the effect of gas heating plays the principal role [19,20]. As it was shown in Ref. [21], the use of the standard kinetic schemes, without inclusion of specific plasma processes, is sufficient for simulation of methane and octane reforming in low-current arc discharges. The reforming of ethanol-water mixture in an electric discharge in a gas channel with liquid wall (named ‘tornado’type) and in argon discharge was done in Refs. [22e24]. It was assumed that low temperatures take place in these discharges. Therefore, it was concluded that for the correct description of ethanol reforming it is necessary to include the electronemolecules reactions into the kinetic scheme. In this case, the results of numerical modeling and experimental data are coinciding. Subsequently developed procedure was used to describe the conversion of ethanol in a barrier discharge [25]. A comprehensive experimental and theoretical investigation of the conversion of ethanol into syngas assisted by a DC atmospheric pressure discharge with plasma cathode
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was performed in Ref. [26]. Numerical simulation of conversion kinetics was performed in a one-dimensional approximation using the Konnov's model in accordance with the assumption of thermal nature of the process. On the assumption that heat losses into ambience constituted 30e35%, the calculated data was in a good agreement with the experimental data of the gas mixture composition and the conversion degree into hydrogen. Unlike [26], in this paper we present experimental data for the conversion of ethanol in a range of electric discharge power (100 We250 W) and perform a numerical simulation using a two-dimensional steady-state model which takes into account both the radial diffusion and thermal conductivity. This model allows to abandon the adjustable parameter of heat losses when comparing with the experimental data. A summary of the preliminary results was presented in Ref. [27].
Experimental set-up and measurements The conversion of ethanol is carried out in a plasma-chemical reactor composed of a three-section chamber with three-
Fig. 1 e Schematic of the plasma-chemical reactor (a); (b) photo of the discharge during ethanol conversion (b); (c) photo of the flame at the exit of the reactor (c) [26].
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electrode configuration of the cathodeeanodeeanode [28,29]. Schematic of discharge chamber is shown in Fig. 1(a). In section A, there is the cooled copper cathode 2 (a rod with diameter of 6 mm) and copper anode 3 (a plate with thickness of about 1 mm) with 1e1.5 mm distance in between, where a self-maintained discharge is ignited. Discharge current of 150e200 mA is provided using DC power supply source U1 (1500 V, ballast resistance R1 ~ 1200 U). The airflow rate through section A is about 0.4 l/min. Air output takes place through the hole (2 mm in diameter) in the center of electrode 3 in section C (a quartz tube 5, 10 mm in diameter and 15 mm long) which is located under this hole. The discharge in section A serves as plasma cathode for the non-self-maintained discharge in section C, which is ignited between middle electrode 3 and the second anode 4 using other DC power supply U2 (3000 V, R2 ~ 1000 U). The electrodes in section С are located at a distance of 1.5 cm from each other. The discharge current can vary from 50 mA up to 180 mA. A watereethanol solution (15% water: 85% ethanol) is fed into section B using dispensers through two channels in electrode 4 that provide heating and evaporation of the solution. The watereethanol solution flow is about 1.25 ml/min. Then, the vapors of ethanol and water with additional airflow at the rate of 0.4 l/min is transported through section B into section C, where plasma-chemical processes take place in the plasma of non-self-maintained Atmospheric Pressure Glow Discharge (APGD). The positive column plasma fills almost the whole volume of section C (Fig. 1(b)). Without watereethanol solution and depending on the currents, the voltage drop between electrodes 3 and 4 is in the range of 400e500 V, which is close to the voltage drop in air in normal glow discharge [30]. However, with an injection of the ethanolewater mixture into the reactor, the voltage drop increases almost 3 times. Fig. 1(c) shows a photograph of the burning synthesis-gas from exhaust tube of the reactor. The length of the blue-colored flame reaches 10 cm. To determine the conversion products of the mixture treated in the plasma of reactor we use the diagnostics based on infrared (IR) absorption spectroscopy [31]. For this purpose, we register absorption spectra of exhaust gases from the
YðH2 Þ ¼
number of H atoms in generated hydrogen molecules H2 : number of H atoms in ethanol molecules
reactor using the infrared spectrometer NEXUS (Thermo Nicolet) in the range of 600e4000 cm1 with the resolution of 2 cm1. For this, the 5.7 cm gas cell with germanium windows preheated to 90 C to prevent condensation of water and ethanol vapors is filled with exhaust gases. Spectra of the gas mixture leaving the reactor show the intense vibration-rotational bands belonging to the molecules of CO (2150 cm1), CO2 (740 cm1 and 2350 cm1), H2O (1600 cm1 and 3750 cm1), CH4 (3100 cm1) and C2H2 (750 cm1 and 3300 cm1), C2H4 (950 cm1) and C2H5OH (1050 cm1 and 2950 cm1) (see Fig. 2). Molar fractions of these optically active components in the products of ethanol conversion are determined by comparison
Fig. 2 e Infrared absorption spectrum of the gas mixture at the exit of the plasma reactor for the discharge power of 250 W. of the experimental absorption spectra and the calculated ones using spectral database Hitran [32]. The molar fraction of ethanol is determined using the experimental absorption curve constructed using gas mixtures with known alcohol concentrations. Molar fractions of hydrogen, nitrogen and oxygen are estimated considering the flow rates of ethanol, water, oxygen and nitrogen at the reactor entrance, while molar fractions of IR active components are determined experimentally from IR spectra [31].
Thermodynamic and kinetic analysis of the conversion The mixture injected into the reactor contains 30% of ethanol, 16% of water and 54% of air. We suppose that production of synthesis gas from this mixture occurs by means of partial fuel oxidation. Potentially, the considered mixture possesses its own internal energy. It has adiabatic temperature Tad ¼ 861 K that corresponds to the equilibrium degree of conversion into hydrogen Yad ¼ 52.9%. Here, the degree of ethanol conversion into hydrogen is defined by the following equation:
(1)
To obtain a higher degree of conversion it is necessary to transfer some extra energy to the mixture. Theoretically, the maximum possible conversion degree for the considered mixture is equal to Ymax ¼ 114%. Figs. 3 and 4 show the influence of temperature on the characteristics of equilibrium conversion process. It is obvious that the temperature T ¼ 1100 K is sufficient to obtain the maximum possible equilibrium conversion degree Y ¼ 108% and hydrogen concentration [H2] ¼ 47%. With further increase in temperature all process parameters practically reach saturation. Let us recall that the equilibrium composition is the one that is reached during the infinite settling time. In real conditions, the characteristic time of a process is limited, for
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Fig. 3 e Temperature dependence of the equilibrium composition of the ethanolewatereair mixture.
Fig. 4 e Temperature dependence of the conversion degree of the ethanolewatereair mixture to hydrogen.
example by the residence time of a reaction mixture in the reaction zone (discharge gap). Therefore, the calculated equilibrium composition may not be achieved due to the kinetic constraints (the constants of kinetic processes depend strongly on temperature and are usually increasing exponentially when temperature increases). Fig. 5 shows the numerical calculation results of characteristic times for achieving the equilibrium that depend on the temperature of isothermal conversion in the ethanolewatereair mixture. The calculation is carried out using the kinetic mechanism of A. Konnov [33]. The characteristic times are defined as the time period needed to reach molecular hydrogen concentrations equal to 0.9, 0.95 and 0.99 of the equilibrium concentration [H2]eq respectively. The same diagram shows the characteristic residence time for the conditions in the plasma-chemical reactor presented above. It can be seen that for an effective conversion process the
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Fig. 5 e Temperature dependence of the characteristic time to reach the equilibrium for the ethanolewatereair mixture: 1 e [H2](t) ¼ 0.9 [H2]eq, 2 e [H2](t) ¼ 0.95 [H2]eq, 3 e [H2](t) ¼ 0.99 [H2]eq. temperature needs to be higher than that provided in a thermodynamics process; to be more specific, at least T ¼ 2100e2400 K is needed. Taking into account the fact that temperature in the reactor can sharply drop towards the peripheral zones than, the axis temperature should be even higher. Fig. 6 shows the kinetics of the ethanolewatereair mixture conversion for temperature T ¼ 2000 K. It is evident that the process can be divided into two main stages. The fast exothermic first stage implies an ignition and partial combustion of the initial fuel to form CO, H2, CO2, H2O and products of incomplete fuel conversion (CH4, C2H2, C2H4, …). The initial fuel disappears almost completely by the time 106 s, oxygen e by the time 3 105 s. By this time, the maximum concentrations [H2O] ~ 27% and [CO2] ~ 2% become established. Then, the much slower endothermic second stage of the process begins, when a steam-carbon dioxide conversion
Fig. 6 e Evolution of chemical composition in the process of the ethanolewatereair mixture conversion under isothermal conditions: T ¼ 2000 K.
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of the remaining hydrocarbons takes place. At this stage the remaining hydrocarbons react with water and carbon dioxide to form additional quantities of H2 and CO. The final equilibrium composition of the mixture for the considered temperature is set by the time about 1e2 101 s. At the second stage the concentrations of H2O and CO2 decrease by approximately 4e6 times. It can be seen that the second stage of the process yields the major amount of hydrogen. This means that in the real process, it is unsufficient to ensure simply a sufficiently high temperature for the efficient conversion, but it is needed also to maintain the process long enough.
Numerical simulation For deeper understanding of peculiarities of the conversion process we carried out a series of numerical calculations for the conditions realized in the experiments and presented above. For this purpose, a two-dimensional steady-state model is developed and implemented in the form of a code. Similarly to the chemical model of the process, we use a kinetic mechanism of A. Konnov containing 1207 reversible elementary reactions between 127 neutral chemical components [33]. The mathematical model that describes the flow of a chemically reacting gas in a cylindrical tube includes the energy equation cp ru
k X vT 1 v þ Ci Vi hi rr vx r vr i¼1
!
X k 1 v vT rl þ u_ i hi mi W ¼ 0 r vr vr i¼1 (2)
and k continuity equations of a mixture with k chemical components ru
vCi 1 v ðrrCi Vi Þ ¼ u_ i mi þ vx r vr
ði ¼ 1; …; kÞ:
(3)
The system of 2D differential equations (1) and (2) is supplemented by the continuity equation for the gas flow ruF ¼ const;
(4)
the equation of state for the ideal gas r¼P
m RT
(5)
The numerical procedure is based on the finite difference formulation. We implement a specific numerical method to reduce the order of the differential system to unity: the whole tube is divided into Ngrid (20e30) annular fluid tubes with adaptive cross-sections. The mass flow rate of the gas in every tube is considered to be constant. The use of an explicit onestep scheme over the march variable x allows to reduce the problem of k þ 1 parabolic equations of the second order to the problem of (k þ 1)Ngrid ordinary differential equations. Diffusion and heat flows between fluid tubes are taken from the previous step. After each step, the position and size of the fluid tubes are adjusted considering the changed flow parameters. The role of the discharge is reduced to thermal heating of the gas. A bell-type radial profile of the energy release in the discharge tube is used, with the primary energy deposition in the central parts of the tube.
Results and discussion Table 1 shows values of current IA and voltage UA of the selfmaintained discharge in section A, current IC and voltage UC of the non-self-maintained discharge in chamber C and the total electric power of electrical discharges in the plasmachemical reactor, where ethanol conversion occurs. According to the hypothesis about thermal nature of ethanolewatereair mixture conversion in DC atmospheric pressure discharge, which is taken as an assumption in this article, for the case of the fixed parameters of gas mixture, the conversion parameters should be depending mainly on the total power of the gas discharges in both chambers A and C (see Fig. 1(a)). To perform numerical calculations, it is necessary to set the boundary condition in the energy equation (2). That is the temperature value at the boundary of chamber C, i.e. temperature of its wall. To determine this temperature, a
Table 1 e Parameters of electrical discharges. N
IA, mA
UA, V
IC, mA
UC, V
Total power, W
1 2 3 4 5
150 150 150 200 150
200 200 200 190 380
50 100 150 180 50
1400 1300 1200 1100 1400
100 130 210 235 250
and the condition of a constant pressure across the computational domain Pðx; rÞ ¼ P0 ¼ const:
(6)
A steady turbulent radial velocity profile was used instead of the equation of motion [34]. The notation used in equations (2)e(6) are as follows: x and r e longitudinal and radial coordinates respectively; P, r and u e pressure, density and velocity of the gas respectively; cp e specific heat capacity of the gas; T e gas temperature; Ci e mass fraction of the i-th specie; hi e enthalpy of the i-th specie; l e thermal conductivity of the gas mixture; mi e molecular weight of the i-th specie; u_ i e production rate of the i-th specie; W e volumetric energy input; m e molecular weight of the gas mixture; Vi e diffusion velocity of the i-th specie, F e cross section of the tube.
Fig. 7 e Spectrum of the (0,0) band in OH (A-X).
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Table 2 e Experimental and theoretical data on the products of conversion. Components
Mole fraction, % At the inlet
Fig. 8 e Evolution of the chemical composition of the ethanolewatereair mixture and the temperature in the center of the discharge tube.
tungsten wire of 0.25 mm in diameter is placed in the middle part of chamber C perpendicular to the axis of the discharge. The wire has a dark-red color near the wall of the chamber and a light-yellow color on the axis. It corresponds to temperatures of approximately 1000 K and 3500 K respectively. The temperature of 3500 K in the region near the axis of the discharge was also obtained by the method of relative intensities of rotational lines using the emission spectrum of hydroxyl OH (see Fig. 7). It should be noted, however, that due to the high concentration of water in the discharge region, this method can lead to an overestimated value of temperature [35]. Temperature measurements were done at total discharge power of 250 W. The temperature of 1000 K, the value obtained near the chamber C wall, was used as the boundary and the initial temperature during all calculations. The results of numerical simulation of the conversion process presented below are for the injected power of 250 W. Fig. 8 shows evolution of the chemical composition and temperature in the center of the discharge tube. It is clear that up to x ¼ 0.02 cm the initial composition does not change, and it is
C2H5OH H2O N2 O2 CO CO2 CH4 C2H2 C2H4 H2
30 16 43 11
Products of conversion Experiment
Calculation
0.33 8.6 22.03 0 24.8 1.1 1.3 0.9 0.44 40.5
0 9.3 21.9 0 24.0 1.1 0.46 1.1 0.2 41.5
only the heating of the gas mixture that occurs. When temperature reaches 1300 K, ignition and rapid partial combustion of the mixture is observed. This is accompanied by a sharp rise in the temperature up to 1600 K. After a microsecond, another slow stage of the process begins (see the description of Fig. 6) with a speed increasing at the temperature growth. By the time the discharge in the considered regime is finished, temperature reaches a sufficiently large value of 3300 K, which is close to the experimentally determined and equal to 3500 K. Нigh temperature at the end of the discharge gap results in the dissociation of molecular hydrogen. Later, when the gas flow is outside of a discharge volume a temperature sharply decreases due to a recombination. A dissociation of hydrogen in axial region of discharge is one of important processes resulting in a radial distribution of molecular and atomic hydrogen (see Fig. 9). It should be noticed that there are no unreacted hydrocarbons in the center of the discharge tube at the reactor exit due to the high temperature. At that, the output mixture consists mainly of Н2, CO, N2 and small amounts of H2O and CO2. This indicates that the conversion in this area proceeds quite effectively and the residence time is sufficient to complete the conversion
Fig. 9 e Radial profiles of concentrations of the conversion products at the exit from the discharge gap.
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process. Nevertheless, there is a rather high concentration of unreacted hydrocarbons in the output mixture for the considered regime (see Table 2). This means that the conversion kinetics proceeds differently in the peripheral zones of the discharge. It can be seen in Fig. 9 that there is not enough time in the peripheral zones of the discharge to complete the second stage of the process. Here, the temperature is substantially lower than in the center and this areas are responsible for the
appearance of unreacted hydrocarbons in the output mixture. Also, concentrations of H2O and CO2 are significantly higher in peripheral zone of the discharge than in the center. The composition of the reaction mixture as well as the experimental and theoretical data on the conversion products is presented in Table 2. The experimental data is determined using the IR absorption spectrum of the conversion products, which is shown in Fig. 2 specifically for the case of the applied electrical power of 250 W.
Fig. 10 e IR absorption spectra of the conversion products obtained at the following values of the discharge power: (a) 235 W; (b) 210 W; (c) 160 W; (d) 100 W.
Fig. 11 e Dependence on the discharge power of gas component concentrations at the exit of the reactor and the degree of conversion: experimental (symbol) and calculated (solid curve) data.
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Table 3 e The mole fraction of ethanol at the reactor outlet. Power, W Mole fraction C2H5OH
100 5.4
160 2.3
210 0.39
236 0.37
250 0.33
It can be seen that the experimental and calculated concentrations of conversion products are in a good agreement with each other. A good agreement is also observed for the degree of conversion: 88.5% and 91%. Let us consider the changes in the results of the conversion process occurring with a decrease in electric power of the discharges. The IR absorption spectra shown in Fig. 10 correspond to the conversion products concentrations obtained at four different values of electric power of the discharge. Let us compare these experimental data with the results of numerical simulations. That is reflected in Fig. 11 for the conversion degree and the main products concentrations, where they are presented as functions of the discharge power. It is clearly seen that a reduction in the discharge power leads to a decrease in the main components concentration of the synthesis-gas: CO and H2. The concentrations of the other conversion products increase with the decrease in the deposited energy. A very good agreement between the experimental and theoretical data is observed at the discharge power in the range of 210 We250 W. Discrepancies appear at lower values of the power, presumably due to the overestimated boundary conditions for the temperature since the temperature of 1000 K near the wall of chamber C was measured at the power of 250 W. In addition, at lower powers the discharge becomes constricted and does not fill the entire volume of chamber C. That is different from what is assumed in the model. Therefore, a significant increase in the unreacted alcohol concentration in the mixture at the exit of the reactor can be connected with the appearance of this relatively cold near-wall region (see Table 3). In contrast, in the results of numerical simulations there is no unreacted alcohol. Thus, the comparison of the experimental and theoretical data in this work demonstrates that the use of the kinetic scheme of A. Konnov [33] without inclusion of any specific plasma-chemical reactions involving charged particles allows to calculate the conversion of ethanolewatereair mixtures with a very good accuracy.
Conclusion The conversion of ethanolewatereair mixture in a DC atmospheric pressure discharge with a plasma cathode at electrical power of discharges in the range of 100 We250 W is performed. The concentrations of gas components at the exit of the reactor and the degree of conversion are determined. It is shown that the degree of conversion and molar fractions of the main components (H2 and CO) of synthesis gas decrease while the electric power of discharge reduces. At the same time, molar fractions of the remaining conversion products CO2, CH4, C2H2, C2H4 and H2O increase. Numerical simulations of the conversion are performed based on a two-dimensional stationary model developed in
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this work taking into account the radial diffusion and thermal conductivity. The mechanism proposed by A. Konnov is used as a kinetic model. It is established that the role of discharge is reduced only to thermal heating of the gas. A good agreement between the experimental and theoretical data on the composition of the gas mixture at the exit of the reactor and the degree of conversion to hydrogen is a testament to the adequacy of the theoretical model and confirms the assumption about thermal nature of the conversion in a DC atmospheric pressure discharge.
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2012;38:913e21. http://dx.doi.org/10.1134/ S1063780X1210008X. Levko DS, Tsymbalyuk AN. Ethanol conversion in a barrier discharge. Tech Phys Lett 2015;41(3):228e30. http:// dx.doi.org/10.1134/S1063785015030104. Arkhipenko VI, Kirillov AA, Simonchik LV, Safronau YaA, Chernukho AP, Migoun AN. Atmospheric pressure glow discharge in air used for the ethanol conversion: experiment and modelling. Open Chem 2015;13:292e6. http://dx.doi.org/ 10.1515/chem-2015-0037. Arkhipenko VI, Kirillov AA, Simonchik LV, Paulava AV, Chernukho AP, Migoun AN. Ethanol conversion into syngas assisted by DC atmospheric pressure discharge. High Temp Mater Process 2014;18:195e200. http://dx.doi.org/10.1615/ HighTempMatProc.2015015647. Arkhipenko VI, Callegari Th, Kirillov AA, Safronau YaA, Simonchik LV. Non-self-sustained atmospheric pressure glow discharges maintained by the DC helium glow discharge. IEEE Trans Plasma Sci 2009;37:740e9. http:// dx.doi.org/10.1109/TPS.2009.2012512. Arkhipenko VI, Callegari Th, Safronau YaA, Simonchik LV. Atmospheric pressure air glow discharge in a three-electrode configuration. IEEE Trans Plasma Sci 2009;37:1297e304. http://dx.doi.org/10.1109/TPS.2009.2020905. Arkhipenko VI, Kirillov AA, Simonchik LV, Safronau YaA, Zgirouski SM. Plasma non-equilibrium of the DC normal glow discharges in atmospheric pressure atomic and molecular gases. Eur Phys J D 2012;66:252. http://dx.doi.org/ 10.1140/epjd/e2012-30359-x. Arkhipenko VI, Zgirouski SM, Karoza AG, Kirillov AA, Simonchik LV. Diagnostics of ethanol conversion products by IR absorption spectroscopy. J Appl Spectrosc 2013;80:99e103. http://dx.doi.org/10.1007/s10812-013-9727-2. The HITRAN Database. http://www.cfa.harvard.edu/hitran/ [accessed 05.10.15]. Konnov AA. Development and validation of a detailed reaction mechanism for the combustion of small hydrocarbons. In: 28-th International symposium on combustion. Abstr. Symp. Pap.; 2000 August. p. 317. Edinburg, Scotland. Chernukho A, Migun A, Zhdanok S, Rostaing J, Perrin J. Simulation of perfluoromethane decomposition in an atmospheric-pressure microwave discharge. J Eng Phys Thermophys 2005;78:394e404. http://dx.doi.org/10.1007/ s10891-005-0074-x. Verreycken T, Schram DC, Leys C, Bruggeman P. Spectroscopic study of an atmospheric pressure dc glow discharge with a water electrode in atomic and molecular gases. Plasma Sources Sci Technol 2010;19:045004. http:// dx.doi.org/10.1088/0963-0252/19/4/045004.
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Ethanol conversion in a DC atmospheric pressure glow discharge V.I. Arkhipenko a, A.A. Kirillov a,*, L.V. Simonchik a, A.V. Kazak a, A.P. Chernukho b, A.N. Migoun b a b
B.I. Stepanov Institute of Physics of NAS of Belarus, Pr. Nezavisimosti 68, 220072 Minsk, Belarus Private R&D Enterprise, Advanced Research & Technologies, Sovkhoznaya 1, 223058 Leskovka, Belarus
article info
abstract
Article history:
Conversion of ethanol-water mixture into syngas in a DC atmospheric pressure air glow
Received 10 April 2016
discharge with a plasma cathode has been investigated experimentally and theoretically.
Received in revised form
The electric power of discharge was varied from 100 W up to 250 W. Novel diagnostics
11 July 2016
based on the absorption infrared spectroscopy was developed and used to determine the
Accepted 16 August 2016
syngas composition and the conversion parameters. The main components of syngas
Available online 16 September 2016
were: hydrogen, carbon monoxide, methane, ethylene and acetylene. The achieved degree of conversion to hydrogen was about 90%, with hydrogen content in conversion products
Keywords:
being equal to 40%. A method for numerical simulation of the conversion process has been
Atmospheric pressure glow
developed under an assumption that the role of discharge in the conversion process is
discharge
purely thermal in nature. The experimental and numerical data were found to be in a good
Ethanol
agreement.
Conversion
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Syngas Experimental Numerical
Introduction Theoretical and experimental researches have shown that an addition of small amounts of hydrogen and synthesis-gas into the main fuel allows to increase the efficiency of internal combustion engines and to reduce the emission of pollutants [1e3]. However, this also leads to problems with the storing and transporting of hydrogen [4]. The most promising solution is to generate hydrogen directly before the fuel burning. Among possible implementations of hydrogen generators, the plasma systems have occupied a special place [5]. In such generators, the process of fuel conversion could be fully or
partially maintained by plasma [5e8]. The main advantages of technologies based on the use of plasma include the acceptable modes of operation (atmospheric pressure, low gas temperatures, quick start, compact size, etc.). Different types of discharges at atmospheric pressure (corona, spark, barrier, sliding arc of DC, AC and pulse current in different frequency bands) are offered as plasma sources for these purposes [9e14]. In order to optimize the plasma conversion processes, it is needed to understand the kinetics of plasma-chemical processes, the role of which can vary significantly in different discharges. Plasma effect manifests itself in both heating of
* Corresponding author. E-mail address: [email protected] (A.A. Kirillov). http://dx.doi.org/10.1016/j.ijhydene.2016.08.122 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 1 ( 2 0 1 6 ) 1 8 3 2 0 e1 8 3 2 8
the gas and generation of chemically active particles during collisions of electrons with molecules. The latter process is important in cold plasmas [15], such as dielectric-barrier discharge [16], corona discharge [17] and spark discharge [18]. In discharges with hot plasmas the effect of gas heating plays the principal role [19,20]. As it was shown in Ref. [21], the use of the standard kinetic schemes, without inclusion of specific plasma processes, is sufficient for simulation of methane and octane reforming in low-current arc discharges. The reforming of ethanol-water mixture in an electric discharge in a gas channel with liquid wall (named ‘tornado’type) and in argon discharge was done in Refs. [22e24]. It was assumed that low temperatures take place in these discharges. Therefore, it was concluded that for the correct description of ethanol reforming it is necessary to include the electronemolecules reactions into the kinetic scheme. In this case, the results of numerical modeling and experimental data are coinciding. Subsequently developed procedure was used to describe the conversion of ethanol in a barrier discharge [25]. A comprehensive experimental and theoretical investigation of the conversion of ethanol into syngas assisted by a DC atmospheric pressure discharge with plasma cathode
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was performed in Ref. [26]. Numerical simulation of conversion kinetics was performed in a one-dimensional approximation using the Konnov's model in accordance with the assumption of thermal nature of the process. On the assumption that heat losses into ambience constituted 30e35%, the calculated data was in a good agreement with the experimental data of the gas mixture composition and the conversion degree into hydrogen. Unlike [26], in this paper we present experimental data for the conversion of ethanol in a range of electric discharge power (100 We250 W) and perform a numerical simulation using a two-dimensional steady-state model which takes into account both the radial diffusion and thermal conductivity. This model allows to abandon the adjustable parameter of heat losses when comparing with the experimental data. A summary of the preliminary results was presented in Ref. [27].
Experimental set-up and measurements The conversion of ethanol is carried out in a plasma-chemical reactor composed of a three-section chamber with three-
Fig. 1 e Schematic of the plasma-chemical reactor (a); (b) photo of the discharge during ethanol conversion (b); (c) photo of the flame at the exit of the reactor (c) [26].
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electrode configuration of the cathodeeanodeeanode [28,29]. Schematic of discharge chamber is shown in Fig. 1(a). In section A, there is the cooled copper cathode 2 (a rod with diameter of 6 mm) and copper anode 3 (a plate with thickness of about 1 mm) with 1e1.5 mm distance in between, where a self-maintained discharge is ignited. Discharge current of 150e200 mA is provided using DC power supply source U1 (1500 V, ballast resistance R1 ~ 1200 U). The airflow rate through section A is about 0.4 l/min. Air output takes place through the hole (2 mm in diameter) in the center of electrode 3 in section C (a quartz tube 5, 10 mm in diameter and 15 mm long) which is located under this hole. The discharge in section A serves as plasma cathode for the non-self-maintained discharge in section C, which is ignited between middle electrode 3 and the second anode 4 using other DC power supply U2 (3000 V, R2 ~ 1000 U). The electrodes in section С are located at a distance of 1.5 cm from each other. The discharge current can vary from 50 mA up to 180 mA. A watereethanol solution (15% water: 85% ethanol) is fed into section B using dispensers through two channels in electrode 4 that provide heating and evaporation of the solution. The watereethanol solution flow is about 1.25 ml/min. Then, the vapors of ethanol and water with additional airflow at the rate of 0.4 l/min is transported through section B into section C, where plasma-chemical processes take place in the plasma of non-self-maintained Atmospheric Pressure Glow Discharge (APGD). The positive column plasma fills almost the whole volume of section C (Fig. 1(b)). Without watereethanol solution and depending on the currents, the voltage drop between electrodes 3 and 4 is in the range of 400e500 V, which is close to the voltage drop in air in normal glow discharge [30]. However, with an injection of the ethanolewater mixture into the reactor, the voltage drop increases almost 3 times. Fig. 1(c) shows a photograph of the burning synthesis-gas from exhaust tube of the reactor. The length of the blue-colored flame reaches 10 cm. To determine the conversion products of the mixture treated in the plasma of reactor we use the diagnostics based on infrared (IR) absorption spectroscopy [31]. For this purpose, we register absorption spectra of exhaust gases from the
YðH2 Þ ¼
number of H atoms in generated hydrogen molecules H2 : number of H atoms in ethanol molecules
reactor using the infrared spectrometer NEXUS (Thermo Nicolet) in the range of 600e4000 cm1 with the resolution of 2 cm1. For this, the 5.7 cm gas cell with germanium windows preheated to 90 C to prevent condensation of water and ethanol vapors is filled with exhaust gases. Spectra of the gas mixture leaving the reactor show the intense vibration-rotational bands belonging to the molecules of CO (2150 cm1), CO2 (740 cm1 and 2350 cm1), H2O (1600 cm1 and 3750 cm1), CH4 (3100 cm1) and C2H2 (750 cm1 and 3300 cm1), C2H4 (950 cm1) and C2H5OH (1050 cm1 and 2950 cm1) (see Fig. 2). Molar fractions of these optically active components in the products of ethanol conversion are determined by comparison
Fig. 2 e Infrared absorption spectrum of the gas mixture at the exit of the plasma reactor for the discharge power of 250 W. of the experimental absorption spectra and the calculated ones using spectral database Hitran [32]. The molar fraction of ethanol is determined using the experimental absorption curve constructed using gas mixtures with known alcohol concentrations. Molar fractions of hydrogen, nitrogen and oxygen are estimated considering the flow rates of ethanol, water, oxygen and nitrogen at the reactor entrance, while molar fractions of IR active components are determined experimentally from IR spectra [31].
Thermodynamic and kinetic analysis of the conversion The mixture injected into the reactor contains 30% of ethanol, 16% of water and 54% of air. We suppose that production of synthesis gas from this mixture occurs by means of partial fuel oxidation. Potentially, the considered mixture possesses its own internal energy. It has adiabatic temperature Tad ¼ 861 K that corresponds to the equilibrium degree of conversion into hydrogen Yad ¼ 52.9%. Here, the degree of ethanol conversion into hydrogen is defined by the following equation:
(1)
To obtain a higher degree of conversion it is necessary to transfer some extra energy to the mixture. Theoretically, the maximum possible conversion degree for the considered mixture is equal to Ymax ¼ 114%. Figs. 3 and 4 show the influence of temperature on the characteristics of equilibrium conversion process. It is obvious that the temperature T ¼ 1100 K is sufficient to obtain the maximum possible equilibrium conversion degree Y ¼ 108% and hydrogen concentration [H2] ¼ 47%. With further increase in temperature all process parameters practically reach saturation. Let us recall that the equilibrium composition is the one that is reached during the infinite settling time. In real conditions, the characteristic time of a process is limited, for
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Fig. 3 e Temperature dependence of the equilibrium composition of the ethanolewatereair mixture.
Fig. 4 e Temperature dependence of the conversion degree of the ethanolewatereair mixture to hydrogen.
example by the residence time of a reaction mixture in the reaction zone (discharge gap). Therefore, the calculated equilibrium composition may not be achieved due to the kinetic constraints (the constants of kinetic processes depend strongly on temperature and are usually increasing exponentially when temperature increases). Fig. 5 shows the numerical calculation results of characteristic times for achieving the equilibrium that depend on the temperature of isothermal conversion in the ethanolewatereair mixture. The calculation is carried out using the kinetic mechanism of A. Konnov [33]. The characteristic times are defined as the time period needed to reach molecular hydrogen concentrations equal to 0.9, 0.95 and 0.99 of the equilibrium concentration [H2]eq respectively. The same diagram shows the characteristic residence time for the conditions in the plasma-chemical reactor presented above. It can be seen that for an effective conversion process the
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Fig. 5 e Temperature dependence of the characteristic time to reach the equilibrium for the ethanolewatereair mixture: 1 e [H2](t) ¼ 0.9 [H2]eq, 2 e [H2](t) ¼ 0.95 [H2]eq, 3 e [H2](t) ¼ 0.99 [H2]eq. temperature needs to be higher than that provided in a thermodynamics process; to be more specific, at least T ¼ 2100e2400 K is needed. Taking into account the fact that temperature in the reactor can sharply drop towards the peripheral zones than, the axis temperature should be even higher. Fig. 6 shows the kinetics of the ethanolewatereair mixture conversion for temperature T ¼ 2000 K. It is evident that the process can be divided into two main stages. The fast exothermic first stage implies an ignition and partial combustion of the initial fuel to form CO, H2, CO2, H2O and products of incomplete fuel conversion (CH4, C2H2, C2H4, …). The initial fuel disappears almost completely by the time 106 s, oxygen e by the time 3 105 s. By this time, the maximum concentrations [H2O] ~ 27% and [CO2] ~ 2% become established. Then, the much slower endothermic second stage of the process begins, when a steam-carbon dioxide conversion
Fig. 6 e Evolution of chemical composition in the process of the ethanolewatereair mixture conversion under isothermal conditions: T ¼ 2000 K.
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of the remaining hydrocarbons takes place. At this stage the remaining hydrocarbons react with water and carbon dioxide to form additional quantities of H2 and CO. The final equilibrium composition of the mixture for the considered temperature is set by the time about 1e2 101 s. At the second stage the concentrations of H2O and CO2 decrease by approximately 4e6 times. It can be seen that the second stage of the process yields the major amount of hydrogen. This means that in the real process, it is unsufficient to ensure simply a sufficiently high temperature for the efficient conversion, but it is needed also to maintain the process long enough.
Numerical simulation For deeper understanding of peculiarities of the conversion process we carried out a series of numerical calculations for the conditions realized in the experiments and presented above. For this purpose, a two-dimensional steady-state model is developed and implemented in the form of a code. Similarly to the chemical model of the process, we use a kinetic mechanism of A. Konnov containing 1207 reversible elementary reactions between 127 neutral chemical components [33]. The mathematical model that describes the flow of a chemically reacting gas in a cylindrical tube includes the energy equation cp ru
k X vT 1 v þ Ci Vi hi rr vx r vr i¼1
!
X k 1 v vT rl þ u_ i hi mi W ¼ 0 r vr vr i¼1 (2)
and k continuity equations of a mixture with k chemical components ru
vCi 1 v ðrrCi Vi Þ ¼ u_ i mi þ vx r vr
ði ¼ 1; …; kÞ:
(3)
The system of 2D differential equations (1) and (2) is supplemented by the continuity equation for the gas flow ruF ¼ const;
(4)
the equation of state for the ideal gas r¼P
m RT
(5)
The numerical procedure is based on the finite difference formulation. We implement a specific numerical method to reduce the order of the differential system to unity: the whole tube is divided into Ngrid (20e30) annular fluid tubes with adaptive cross-sections. The mass flow rate of the gas in every tube is considered to be constant. The use of an explicit onestep scheme over the march variable x allows to reduce the problem of k þ 1 parabolic equations of the second order to the problem of (k þ 1)Ngrid ordinary differential equations. Diffusion and heat flows between fluid tubes are taken from the previous step. After each step, the position and size of the fluid tubes are adjusted considering the changed flow parameters. The role of the discharge is reduced to thermal heating of the gas. A bell-type radial profile of the energy release in the discharge tube is used, with the primary energy deposition in the central parts of the tube.
Results and discussion Table 1 shows values of current IA and voltage UA of the selfmaintained discharge in section A, current IC and voltage UC of the non-self-maintained discharge in chamber C and the total electric power of electrical discharges in the plasmachemical reactor, where ethanol conversion occurs. According to the hypothesis about thermal nature of ethanolewatereair mixture conversion in DC atmospheric pressure discharge, which is taken as an assumption in this article, for the case of the fixed parameters of gas mixture, the conversion parameters should be depending mainly on the total power of the gas discharges in both chambers A and C (see Fig. 1(a)). To perform numerical calculations, it is necessary to set the boundary condition in the energy equation (2). That is the temperature value at the boundary of chamber C, i.e. temperature of its wall. To determine this temperature, a
Table 1 e Parameters of electrical discharges. N
IA, mA
UA, V
IC, mA
UC, V
Total power, W
1 2 3 4 5
150 150 150 200 150
200 200 200 190 380
50 100 150 180 50
1400 1300 1200 1100 1400
100 130 210 235 250
and the condition of a constant pressure across the computational domain Pðx; rÞ ¼ P0 ¼ const:
(6)
A steady turbulent radial velocity profile was used instead of the equation of motion [34]. The notation used in equations (2)e(6) are as follows: x and r e longitudinal and radial coordinates respectively; P, r and u e pressure, density and velocity of the gas respectively; cp e specific heat capacity of the gas; T e gas temperature; Ci e mass fraction of the i-th specie; hi e enthalpy of the i-th specie; l e thermal conductivity of the gas mixture; mi e molecular weight of the i-th specie; u_ i e production rate of the i-th specie; W e volumetric energy input; m e molecular weight of the gas mixture; Vi e diffusion velocity of the i-th specie, F e cross section of the tube.
Fig. 7 e Spectrum of the (0,0) band in OH (A-X).
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Table 2 e Experimental and theoretical data on the products of conversion. Components
Mole fraction, % At the inlet
Fig. 8 e Evolution of the chemical composition of the ethanolewatereair mixture and the temperature in the center of the discharge tube.
tungsten wire of 0.25 mm in diameter is placed in the middle part of chamber C perpendicular to the axis of the discharge. The wire has a dark-red color near the wall of the chamber and a light-yellow color on the axis. It corresponds to temperatures of approximately 1000 K and 3500 K respectively. The temperature of 3500 K in the region near the axis of the discharge was also obtained by the method of relative intensities of rotational lines using the emission spectrum of hydroxyl OH (see Fig. 7). It should be noted, however, that due to the high concentration of water in the discharge region, this method can lead to an overestimated value of temperature [35]. Temperature measurements were done at total discharge power of 250 W. The temperature of 1000 K, the value obtained near the chamber C wall, was used as the boundary and the initial temperature during all calculations. The results of numerical simulation of the conversion process presented below are for the injected power of 250 W. Fig. 8 shows evolution of the chemical composition and temperature in the center of the discharge tube. It is clear that up to x ¼ 0.02 cm the initial composition does not change, and it is
C2H5OH H2O N2 O2 CO CO2 CH4 C2H2 C2H4 H2
30 16 43 11
Products of conversion Experiment
Calculation
0.33 8.6 22.03 0 24.8 1.1 1.3 0.9 0.44 40.5
0 9.3 21.9 0 24.0 1.1 0.46 1.1 0.2 41.5
only the heating of the gas mixture that occurs. When temperature reaches 1300 K, ignition and rapid partial combustion of the mixture is observed. This is accompanied by a sharp rise in the temperature up to 1600 K. After a microsecond, another slow stage of the process begins (see the description of Fig. 6) with a speed increasing at the temperature growth. By the time the discharge in the considered regime is finished, temperature reaches a sufficiently large value of 3300 K, which is close to the experimentally determined and equal to 3500 K. Нigh temperature at the end of the discharge gap results in the dissociation of molecular hydrogen. Later, when the gas flow is outside of a discharge volume a temperature sharply decreases due to a recombination. A dissociation of hydrogen in axial region of discharge is one of important processes resulting in a radial distribution of molecular and atomic hydrogen (see Fig. 9). It should be noticed that there are no unreacted hydrocarbons in the center of the discharge tube at the reactor exit due to the high temperature. At that, the output mixture consists mainly of Н2, CO, N2 and small amounts of H2O and CO2. This indicates that the conversion in this area proceeds quite effectively and the residence time is sufficient to complete the conversion
Fig. 9 e Radial profiles of concentrations of the conversion products at the exit from the discharge gap.
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process. Nevertheless, there is a rather high concentration of unreacted hydrocarbons in the output mixture for the considered regime (see Table 2). This means that the conversion kinetics proceeds differently in the peripheral zones of the discharge. It can be seen in Fig. 9 that there is not enough time in the peripheral zones of the discharge to complete the second stage of the process. Here, the temperature is substantially lower than in the center and this areas are responsible for the
appearance of unreacted hydrocarbons in the output mixture. Also, concentrations of H2O and CO2 are significantly higher in peripheral zone of the discharge than in the center. The composition of the reaction mixture as well as the experimental and theoretical data on the conversion products is presented in Table 2. The experimental data is determined using the IR absorption spectrum of the conversion products, which is shown in Fig. 2 specifically for the case of the applied electrical power of 250 W.
Fig. 10 e IR absorption spectra of the conversion products obtained at the following values of the discharge power: (a) 235 W; (b) 210 W; (c) 160 W; (d) 100 W.
Fig. 11 e Dependence on the discharge power of gas component concentrations at the exit of the reactor and the degree of conversion: experimental (symbol) and calculated (solid curve) data.
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Table 3 e The mole fraction of ethanol at the reactor outlet. Power, W Mole fraction C2H5OH
100 5.4
160 2.3
210 0.39
236 0.37
250 0.33
It can be seen that the experimental and calculated concentrations of conversion products are in a good agreement with each other. A good agreement is also observed for the degree of conversion: 88.5% and 91%. Let us consider the changes in the results of the conversion process occurring with a decrease in electric power of the discharges. The IR absorption spectra shown in Fig. 10 correspond to the conversion products concentrations obtained at four different values of electric power of the discharge. Let us compare these experimental data with the results of numerical simulations. That is reflected in Fig. 11 for the conversion degree and the main products concentrations, where they are presented as functions of the discharge power. It is clearly seen that a reduction in the discharge power leads to a decrease in the main components concentration of the synthesis-gas: CO and H2. The concentrations of the other conversion products increase with the decrease in the deposited energy. A very good agreement between the experimental and theoretical data is observed at the discharge power in the range of 210 We250 W. Discrepancies appear at lower values of the power, presumably due to the overestimated boundary conditions for the temperature since the temperature of 1000 K near the wall of chamber C was measured at the power of 250 W. In addition, at lower powers the discharge becomes constricted and does not fill the entire volume of chamber C. That is different from what is assumed in the model. Therefore, a significant increase in the unreacted alcohol concentration in the mixture at the exit of the reactor can be connected with the appearance of this relatively cold near-wall region (see Table 3). In contrast, in the results of numerical simulations there is no unreacted alcohol. Thus, the comparison of the experimental and theoretical data in this work demonstrates that the use of the kinetic scheme of A. Konnov [33] without inclusion of any specific plasma-chemical reactions involving charged particles allows to calculate the conversion of ethanolewatereair mixtures with a very good accuracy.
Conclusion The conversion of ethanolewatereair mixture in a DC atmospheric pressure discharge with a plasma cathode at electrical power of discharges in the range of 100 We250 W is performed. The concentrations of gas components at the exit of the reactor and the degree of conversion are determined. It is shown that the degree of conversion and molar fractions of the main components (H2 and CO) of synthesis gas decrease while the electric power of discharge reduces. At the same time, molar fractions of the remaining conversion products CO2, CH4, C2H2, C2H4 and H2O increase. Numerical simulations of the conversion are performed based on a two-dimensional stationary model developed in
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this work taking into account the radial diffusion and thermal conductivity. The mechanism proposed by A. Konnov is used as a kinetic model. It is established that the role of discharge is reduced only to thermal heating of the gas. A good agreement between the experimental and theoretical data on the composition of the gas mixture at the exit of the reactor and the degree of conversion to hydrogen is a testament to the adequacy of the theoretical model and confirms the assumption about thermal nature of the conversion in a DC atmospheric pressure discharge.
references
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 8 3 2 0 e1 8 3 2 8
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