Modeling of Plasma Reforming of Ethanol into

AIAA 2010-7062

46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 25 - 28 July 2010, Nashville, TN

Modeling of Plasma Reforming of Ethanol into Hydrogen in the Electric Discharge in a Gas Channel with Liquid Wall Anatolij I. Shchedrin1, Dmitry S. Levko2 and Vadym V. Naumov3 Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, 03028 Ukraine and Valeriy Ya. Chernyak4, Vitalij V. Yukhymenko5 and Sergei V. Olszewski6 Faculty of Radio Physics, Taras Shevchenko Kyiv National University, Kyiv 02033 Ukraine

This paper presents the results of the theoretical study of the process of low-temperature plasma-assisted reforming of ethanol into molecular hydrogen in the electric discharge in a gas channel with liquid wall (DGCLW) in the regime where the discharge is ignited by the breakdown with the air injection between tubular electrodes and then it burns in the ethanol-water mixture self-sustainingly without air supply. The numerical modeling clarifies the nature of non-thermal conversion and explains the kinetic mechanism of nonequilibrium plasma-chemical transformations in the gas-liquid system and evolution of hydrogen during the reforming as a function of discharge parameters and ethanol-water ratio in the mixture.

Nomenclature G T p N W Id E/N dt



L V

= = = = = = = = = = =

gas flow rate temperature pressure concentration discharge power discharge current reduced electric field time step time scale length volume

I. Introduction

A

T the present time, hydrogen (H2) objectively is considered as one of the most prospective energy resources for the future that can be economic, renewable, ecologically clean and environmentally safe.1 Among possible physical-chemical technologies for bio-origin H2 production, including steam reforming and partial oxidation of hydrogen-containing bio-fuels,2,3 a low-temperature plasma-assisted reforming of bio-ethanol (biomass-derived ethyl alcohol, C2H5OH or EtOH, in mixture with water) is believed to be a good alternative approach. 4-6 There are various methods of plasma reforming of liquid hydrocarbon fuels by using quasi-equilibrium (thermal) and nonequilibrium (non-thermal) plasmas in arc, corona, microwave, dielectric barrier discharges, etc. 7,8 Each plasma system has its merits and demerits, and even difficult to compare. 9 One of the most promising among them is a low-temperature plasma-fuel processing at the plasma-liquid system (PLS) using the dc discharge in a gas channel 1

DSc, Head of Research Group, Department of Gas Electronics, [email protected] PhD Student, Junior Researcher, [email protected] 3 Senior Research Scientist, [email protected], AIAA Associated Member 4 Professor, Department of Physical Electronics, Plasma Research Lab, [email protected], AIAA Member 5 Junior Researcher, [email protected] 6 Senior Research Scientist, [email protected] 1 American Institute of Aeronautics and Astronautics 2

Copyright © 2010 by A.Shchedrin et al. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

with liquid wall (DGCLW).10 Advantages of this approach are high chemical plasma activity and selectivity of plasma-chemical transformations, providing high productivity and efficiency of conversion in the continuous mode at a relatively small electric power consumption (in comparison, for example, with electrolysis) at a high-voltage / low-current discharging in a flow of atmospheric pressure. Non-equilibrium plasma assists as an energetic catalyst containing electrons and electronically excited atoms and radicals, which easily break chemical bonds (C-H, O-H) and initiate a chain-branching conversion of hydrocarbons that does not occur in usual conditions at ambient temperature. The highly developed plasma-liquid interface with a large surface-to-volume ratio and deep injection of plasma particles into the liquid is also favored to intensification of fuel conversion in the system. At that, due to intensive turbulent heat and mass transfer, the PLS is thermally 'cold'. The main idea of the DGCLW is that it can burn directly in liquid fuels without preliminary gasification. It has already demonstrated its capability in the plasma-supported conversion of ethanol into hydrogen-rich synthesis gas (syngas), followed by plasma-enhanced combustion in experiments with EtOH-air mixtures,11 where the estimated plasma power budget does not exceed a few percent of the heat of combustion. In this paper we report new results of our numerical modeling of the process of nonthermal plasma-assisted reforming of ethanol in the DGCLW-PLS, focusing on kinetic effects of plasma-chemical conversion and production of H2 in the specific case of electric discharging without air supply.

II. Methodology The numerical modeling of the process of plasma-chemical reforming of EtOH was done for a scheme of the PLS with the DGCLW used in experiments12 (Fig. 1). The initial plasma-forming gas (air) was injected into the work liquid (ethanol/water mixture) through the tubular electrodes: cooper rods (3) inserted in dielectric quartz tubes (1) installed one opposite other in the reactor (quartz vessel). The electric discharge was ignited in the gas channel (2) in the gap between the electrodes (3), where a high-voltage breakdown occurred. Such discharge initiation was used because electric breakdown in gas phase is much easy than in liquid phase.13,14 After the ignition the air supply was stopped and dc electric discharge was burning in the appearing gas cavity in the mixture of ethanol-water vapors self-sustainingly.

Figure 1. Schematic of the DGCLW cavity for the EtOH/H 2O processing: 1 are dielectric tubes, 2 is a plasma column, 3 are metallic rod electrodes. Due to heat confinement, the cavity provides good environment for fast ethanol-water vaporization and mixing in the discharge flow. A high degree of ionization in the field between electrodes makes the cavity conductive and stable in discharging. The discharge plasma contains a lot of active atoms and radicals associated with electronimpact dissociation of ethanol and water molecules and dissolved air that actually sustains a chain process of plasma-chemical conversion of hydrocarbons. The gas-discharge products in the form of microbubbles moved through the liquid volume in the reactor, where were collected in the measuring chamber, cooled and analyzed as syngas (at room temperature). The initial gas flow rate, EtOH/H2O ratio and processing time were varied in order to optimize the process. In nominal regimes, the discharge power did not exceed 200 W (currents 50-300 mA). According to measurements11,12 the temperature of the work liquid in the reactor did not exceed the boiling point of aqueous ethanol solution at atmospheric pressure.15 In plasma-chemical kinetic modeling of the PLS-DGCLW, we used the theoretical model16 according to which the process is divided into two stages: I) process in the discharge cavity, and II) process in the reactor volume. The physical model statement uses the next assumptions17: i) electric power during the discharge is averaged over the discharge volume; ii) electric field in the discharge is uniform and does not vary in time and space; iii) discharge plasma in the cavity is homogeneous. It is supposed also that initial air flow does not affect the discharge products 2 American Institute of Aeronautics and Astronautics

after the air supply was stopped and that after the passage of the discharge products from the discharge to the reactor the gas composition in the cavity is fully refreshed. The geometry of the discharge cavity is assumed cylindrical with radius R equal to the radius of the electrode tube and with length L equal to the interelectrode distance. The gas flow rate in the reactor is considered the same as in the discharge and it is kept constant for each simulation run. The computational modeling includes: 1) calculation of the electron energy distribution function by solving the Boltzmann kinetic equation; 2) hydrodynamic modeling in quasi-1D approximation; and 3) chemical kinetic modeling by solving a system of kinetic equations for all kinetically valuable components in the plasma-chemical system. The kinetic mechanism involves 65 species (C2H5OH, N2, O2, H2O, H2, CO, etc.) and includes 92 electronmolecular processes and 446 chemical reactions with a set of corresponding cross-sections and rate constants compiled according to update recommendations of the IUPAC, NASA and NIST databases18 (details are available at A.I. Shchedrin's group Web-site19). For description of plasma-chemical kinetics and calculation of the composition of the plasma-chemical medium under consideration, a following system of kinetic balance equations was used dNi (1)  S ei   k ij N j   k iml N m N l  Pi  kNi  ... , j m ,l dt where Ni, Nj, Nm, Nl are concentrations of neutral and charged molecules, atoms and radicals, kij, kiml are rate constants of chemical reactions between different components, Sei is the source of formation of species in electronmolecular reactions, Pi is the input of primary molecular components in the discharge cavity, kNi is the escape of the discharge products from the discharge cavity caused by the heat expansion. The values Pi in eq. (1) for H2O and C2H5OH (fuel components) are derived by their evaporation, for N2 and O2 (air components) it is derived by their aqueous solubility, 15 for H2 it is derived by its cathodic evolution due to electrolysis. Since the electrodes are located in the liquid, beside the electric discharge, the electrolysis also occurs in the system: hydrogen is liberated on the cathode, and oxygen is liberated on the anode. This effect was taken into account according to the first Faraday's law of electrolysis described by the equation N k ech N A , (2)  Id t V M where N is the number of gas species liberated on the electrodes per unit of time per unit of volume, kech is the electrochemical equivalent for H2 and O2, NA is the Avogadro number, Id is the electric current, and V is the volume. In fact, the radius of electrodes covered by liquid varies with gas flow rate G, but in calculations it is kept constant. The values Sei in eq. (1) are determined as Wei W 1 , (3) S ei  V  ei  Wei   Wi i

i

 2q (4) ne N i  ei   Qei ( ) f ( )d , Wei  m 0  2m 2q (5) ne N i   2 Qi ( ) f ( )d , Wi  Mi m 0 where W is the discharge power; V is the plasma volume; Wei is the specific power consumed in the electronmolecular process of inelastic scattering with the threshold electron energy ei; Qei() is the cross-section of the inelastic process; Wi is the specific power spent for the plasma heating; Qi() is the transport cross-section of electron scattering; m, ne are the mass and concentration of electrons; Mi is the mass of species; q = 1.602x10-12 erg/eV; and f() is the electron energy distribution function (EEDF) normalized as 1/2f()d = 1. The EEDF was calculated by solving the Boltzmann kinetic equation in the standard two-term approximation taking into account the electron-impact excitation, dissociation and ionization processes with primary molecular components including N2, O2, H2O and C2H5OH listed in Table 1. In fact, the ionization processes could be neglected, since the degree of ionization in plasma is small (< 10-5), but they have been included for completeness. It was assumed that the electric field in the discharge is constant, E= 20 kV/cm (reduced electric field E/N  100 Td). Results of the EEDF calculations are shown in Fig. 2. According to these calculations, in the low energy range (with a mean energy of electrons 0.05-2 eV) the EEDF is related to rotational and vibrational excitation of molecules, mainly H2O and C2H5OH; in the high energy range (> 10 eV) the EEDF is related to dissociation and ionization of molecules and characterized by a Maxwellian function, dropping with increasing energy of electrons. In the studied discharge mode of the DGCLW without input air flow, the EEDF has practically no effects of O2 and N2 because their content in the plasma-liquid vapors is relatively small if compared with H2O and C2H5OH.

3 American Institute of Aeronautics and Astronautics

Figure 2. Typical EEDF in the DGCLW in the EtOH/H2O mixture at the breakdown electric field E = 20kV/cm; p = 1 atm and T = 350 K. Table 1. Basic electron-molecular processes used in the EEDF calculations (E/N = 100 Td) N

Process (Threshold energy th, eV)

1 o

O2 + e  O( P) + O + e (6.0 eV)

2

N2 + e  N( S) + N + e (9.8 eV)

3a

O2 + e  O2(a g) + e (0.98 eV)

3b

O2 + e 

4a

N2 + e 

4b

N2 + e 

5

N2 + e 

6

O2 + e 

7

N2 + e 

8

O2 + e 

9

N2 + e 

10

H2 + e  H + H + e (8.9 eV)

11 11 12

O3 + e  O2 + O + e (6.2 eV)

13

H2O + e  OH + H + e (9.51 eV)

14

H2O + e  H2O(v) + e (0.46 eV)

15

H2O + e  H2O + 2e (12.6 eV)

16

C2H5OH + e  CH3 + CH2OH + e (7.5 eV)

17

C2H5OH + e  C2H5 + OH + e (7.5 eV)

18

C2H5OH + e  CH3CHOH + H + e (7.5 eV)

19

C2H5OH + e  C2H5OH + 2e (10.48 eV)

-

3

-

-

4

-

-

-

1

-

O2(b g+) + e-(1.63 eV) N2(A3u+) + e- (6.17 eV) N2(B3g) + e- (7.35 eV) N2(a1g) + e- (8.4 eV) O2(v) + e- (0.19 eV) N2(v) + e- (0.29 eV) O2+ + 2e- (12.1 eV) N2+ + 2e- (15.6 eV) 1

Rate constant ke, cm3s-1

Reference

1.4x10

-9

[20]

1.0x10

-10

[20]

1.8x10

-9

[21]

1.1x10-9

[21]

3.6x10

-10

[22]

3.1x10

-10

[22]

2.1x10

-10

[23]

5.4x10

-9

[24]

1.7x10

-7

[25]

6.2x10

-12

[26]

5.3x10

-13

[26]

3.0x10

-10

[26]

H2 + e  H2(v) + e (0.52 eV)

1.0x10

-9

[26]

H2 + e  H2 + 2e (15.4 eV)

5.0x10-13

-

-

-

-

-

-

+

-

-

-

-

-

-

-

-

+

-

-

-

-

-

-

-

+

-

[26]

5.9x10

-9

[27]

3.6x10

-10

[27]

1.7x10

-9

[27]

1.0x10

-12

[27]

1.0x10

-9

[28]

1.0x10

-9

[28]

1.0x10

-9

[28]

1.0x10

-12

[28]

The nonisothermality of discharge plasma is characterized by two character temperatures: 1) average electronic temperature Te that is determined by the EEDF, i.e., by the applied electric field E/N, and 2) gas kinetic temperature 4 American Institute of Aeronautics and Astronautics

T that is determined by conditions of convective cooling in the gas channel with liquid wall, i.e., by temperature of surrounding liquid (T  350K). In fact, the ratio ΣWi/ΣWei in eq. (3) is very small, so the effect of gas-plasma heating is practically negligible. The static pressure in the system is taken equal to 1 atm. According to our methodology, at the first stage of calculations in the DGCLW cavity, the complete time of the DC discharge burning was divided into the equal time intervals which duration is determined by the cavity residence time, i.e. by the time equal to the ratio of the cavity volume V to the gas flow rate G, τ1 = V/G 10-3 s. During this time the concentrations of all components came to the stationary state due to the balance between the supply and generation of the products in the cavity, on the one side, and the destruction and escape of the products from the cavity, on the other side. Actually, this time is equal to or greater than the character gas diffusion time in the cavity. As was assumed, the every time interval 1 the primary component content in the cavity is refreshed, so the previous period did not influence on the subsequent periods. This allowed us doing the plasma kinetic calculations in the discharge by using the system eq. (1) only one time as all components during the every time interval come to the same values. At the second stage of calculations in the PLS reactor, during the time τ2 = Vw/G 1 s, when the discharge products in the form of gas bubbles (size ~1 mm) entered the liquid volume and floated to the surface, then passed to the chamber where were collected as syngas, the chemical kinetics was also calculated by using the same system eq. (1) but without terms Sei, Pi and kNi. Here, outside the discharge, the charged plasma particles disappear, free atoms and radicals are reduced rapidly, stable molecules and hydrocarbons react poorly, and concentration of EtOH and H2O take values corresponding to the saturated vapor pressure at a given temperature. With the aim of verification of the used model, as a case study, the comparative calculations by the method of simulation of plasma kinetics in the discharge of the DGCLW type16 based on the assumption of averaging of the energy deposited in the discharge over the plasma volume without microdetails of its temporal-spatial structure and by the method of simulation of plasma kinetics in microdischarges17 based on the assumption of multi-channeling of the discharge current in the plasma volume were conducted for the air-water vapor system. It was shown that both approaches give similar results in calculations of the component content and concentrations of major species (N2, O2, NO, H2O, H2) and minor intermediates (O3, NO3, HNO2, etc). In experiments, due to efficient turbulent motion and mixing in the narrow discharge gap, the plasma composition can be nearly uniform across the flow; therefore, in calculations it is reasonable to average energy and assume power density constant in the discharge volume. As for validation of the used model of chemical kinetics of EtOH conversion, it is based on a sub-mechanism of EtOH oxidation taken from the model of Marinov et al.29 that is widely tested in conditions of jet-stirred and flow reactors. Our test calculations have shown quite correct results in predictions for major species (H 2, CO, CO2, H2O) and radicals (H, O, OH, HO2, CH3) but less satisfactory for some derivative hydrocarbons (C2H4, C2H6, etc.). In the whole, it was concluded that the main features of EtOH conversion by the used model are captured well.

III. Results and discussion The numerical modeling and parametric simulations allowed us to provide some insight into peculiarities and regularities of the complex reaction mechanism of non-thermal plasma-chemical conversion of liquid ethanol into hydrogen in the PLS with the DGCLW in different conditions in details. Fig. 3 demonstrates the effect of EtOH reforming as a function of the processing time, i.e. ethanol decomposition and syngas production during the fuel conversion in the DGCLW cavity and in the PLS reactor. The major stable components from the EtOH conversion are molecular H2 and CO/CO2 with minor amounts of CH4, C2H4, C2H6 and H2O. The fractional amount of H2 in the syngas (H2+CO) reaches ~89% and the ratio H2/CO is of ~6 (total syngas amount in output gas-phase products is ~30% by volume). Fig. 4 shows that EtOH conversion increases with discharge current and with EtOH/H2O ratio in the mixture. It indicates that discharge power always promotes the EtOH reforming and enhanced the H2 production. At the fixed discharge current, reducing the input air in the DGCLW causes more dissociation and ionization of fuel fragments, thereby increasing the H2 production but reducing the total conversion efficiency compared to the mode with the constant input air flow. According to calculations, the H2 production initially increases with the input air flow rate to some extent, but it saturates after that because of short residence time of the products in plasma. Actually, it is difficult to set a common tendency since different species have ambiguous behavior. However, with increasing discharge current and with EtOH/H2O dilution, the H2 and CO increase whereas the O2 slightly decreases, and the N2 changes non-monotonically. At that, atoms and radicals: O, H, OH, and CH3 in the discharge grow with current linearly. The output syngas components with the discharge processing in ethanol and water are also different. In the case of excess of ethanol, the H2 and oxides CO/CO2 reach its maximum values; in the case of excess of water – H2, ozone O3 and acids HNO3/HNO2, especially at high discharge currents. In the case of pure water at low currents (Id < 100 mA) it is found that the H2 yield due to the cathodic electrolysis is almost the same as 5 American Institute of Aeronautics and Astronautics

due to the gas discharge if the gas cavity is contacted directly with the metal anode. At that, the summary H 2 yield in the system due to the cathodic evolution and discharge production is practically equal to the H 2 yield in the mode with the liquid anode. In the case of pure ethanol it is found that the polarity of electrodes does not influence on the H2 yield. It is caused by the fact that the amount of H2 produced due to the electrolysis is many times smaller than H2 produced in the discharge, and, therefore, it can be neglected. Fig. 3 (left side) gives results of modeling of dynamics of formation of basic syngas components during the processing of EtOH in the DGCLW-PLS. The data are given for a case of mixture EtOH : H2O = 1:1 at Id = 100 mA. It is seen that during the time period ~10-3s (the residence time in the discharge volume), the concentrations of H2 and other molecular species rapidly grow with the time up to the steady-state level, and the highest value is for H2. Outside the discharge, during the next time period from 10-3s to ~10 s (the residence time in the reactor volume) the concentration curves of molecular components did not reveal dramatic changes. The only noticeable transformation of CO2 and CO at the end of the process is related to the water-gas shift (WGS) reaction: H2O + CO  CO2 + H2 (H = - 41 kJ/mol). It is an exothermic reaction that is thermodynamically favored at low temperatures but its rate depends on the residence time of the mixture in the reactor with some excess of water. At concentration [H 2O] ~1018 cm-3, a character time of the WGS reaction is ~1 s.

Figure 3. Time evolution of H2 and other components of syngas products during the reforming of EtOH/H2O mixture in the DGCLW-PLS.

Figure 4. Concentration of H2 in syngas products after the EtOH reforming in the DGCLW-PLS vs. discharge current at the fixed EtOH/H2O ratio (left) and vs. EtOH/H2O ratio at the fixed current (right). Kinetically, the behavior of H2 concentration curves can be explained by concurrent processes of formation and reduction of H2 in the system. At the first stage of the fuel reforming in the discharge, it relates to the reagent activation by initial electron collisions and by reactions with participation of atoms and radicals generated in plasma. The primary process of formation of H2 during the discharge in ethanol vapors is reaction of H-abstraction producing isomeric ethoxy radicals i-C2H5O, predominantly α-hydroxyethyl CH3CHOH: 6 American Institute of Aeronautics and Astronautics

C2H5OH + H  CH3CHOH + H2 (k = 1x10-14 cm3/s at T = 350 K),

(R1)

Since the concentration of EtOH during the discharge varies weakly with current and temperature T is nearly constant, the rate of H2 production in reaction R1 is proportional to the concentration [H]. The main channel of generation of H atoms in the discharge in ethanol vapors is e-impact dissociation of C2H5OH molecules: C2H5OH + e-  CH3CH2O + H + e- (3.2 eV) ,

(R2a)

which has the same probability as other channels producing hydroxyl OH and methyl CH3 radicals: C2H5OH + e-  CH3CH2 + OH + e- ,

(R2b)

C2H5OH + e  CH3O + CH3 + e ,

(R2c)

-

-

The rate of e-impact dissociation according to eq. (3) is proportional to the specific discharge power W/V, i.e. discharge current Id. Therefore, the rate of H2 production in reaction R1 is proportional to the current. At the same time, the loss of H2 in the discharge also takes place, especially at higher currents, and the main channel is also eimpact dissociation of H2 molecules: H2 + e-  H + H + e- (8.9 eV),

(R3)

which rate linearly depends on the discharge current. Contra versa three-body recombination of H atoms and other reactions occur, giving a channel of reduction of H. Therefore, the concentration of H rapidly (within a time ~10-7s) reaches a level of [H] ~4x1013cm-3; then its formation/reduction goes steady. As a result, the H2 production in the discharge increases with increasing current nonlinearly, having a tendency to saturation: at Id > 100 mA the concentration [H2] is ~3x1018cm-3. During the discharge in water vapors in the mode of liquid anode, the content of H 2 produced by the plasma electrolysis, first decreases with the time due to direct e-impact dissociation, then, this decrease gradually ceases as the H2 production go along. The cathode and anode reactions in liquid phase (charge carrier is H +) are H2O => ½O2 +2H+ + 2e- and 2H+ + 2e- => H2. The main channel of generation of H atoms in gas phase in water vapors is again eimpact dissociation of H2O molecules: Н2О + е- → OH + Н + е- (9.7 eV),

(R4)

which rate is proportional to the discharge current. In this case, the H2 production increases linearly with increasing current in the entire range. In addition to e-impact dissociation of injected/formed molecules, a number of atoms and radicals are produced in the chain reactions: H + OH = H2 + O (R5 k = 4x10-16cm3/s), O + OH = O2 + H (R6 k = 3x10-11cm3/s), OH + H2 = H2O + H (R7 k = 2x10-14cm3/s) and others. The chain process is limited only by reactions of propagation O + OH + M = HO2 + M (R8 k = 1.5x10-32cm6/s), OH + OH + M = H2O2 + M (R9 k = 6x10-31cm6/s) and termination H + OH + M = H2O + M (R10 k = 1.5x10-31cm6/s) in which active atoms and radicals recombine to form less active molecules. In the presence of O2 there is reaction of association H + O2 + M = HO2 + M (R11 k = 4x10-32cm6/s) producing HO2. When hydroperoxy radical HO2 is present in a high enough concentration, it reacts with H as HO2 + H = 2 OH (R12a k = 7x10-11 cm3/s) and HO2 + H = H2O + O (R12b k = 2x10-12cm3/s), consumes O and OH as HO2 + O = O2 + OH (R13 k = 5x10-11cm3/s) and HO2 + OH = H2O +O2 (R14 k = 1.1x10-10cm3/s) and self-recombines HO2 + HO2 = H2O2 + O2 (R15 k = 1.2x10-12cm3/s). Then, hydrogen peroxide H2O2 can react as H2O2 + OH = H2O + HO2 (R16 k = 1.8x10-12cm3/s) and H2O2 + H = H2O + OH (R17 k = 1.0x10-13cm3/s). The production of O atoms results in production of O2 and O3 molecules due to three-body recombination O + O + M = O2 + M (R18 k = 1x10-33cm6/s), O + O2 + M = O3 + M (R19 k = 3x10-34cm6/s) and two-body association O3 + O = 2 O2 (R20 k = 2x10-14cm3/s). Ozone O3 also contributes to the chain process by reactions O3 + H = O2 + OH (R21 k = 3x10-11cm3/s), O3 + OH = HO2 + O2 (R22 k = 1.1x10-13cm3/s). At that, e-impact dissociation of O3 and O2 in the discharge also occurs. In the result, the O formation/reduction goes steady within the limit of [O] ~10 14cm-3. As long as EtOH is in excess in the fuel mixture, O atoms are consumed in reactions with hydrocarbons. It is well known in plasma chemistry that oxidation of hydrocarbons in the presence of O atoms generated by plasma goes via the chain of free-radical reactions producing stable molecular species H2, CO2 and H2O in the end. Indeed, each O atom consumes C2H5OH molecule via H abstraction producing ethoxy radicals i-C2H5O: C2H5OH + O  CH3CHOH + OH (k = 1.3x10-13 cm3/s at T = 350 K)

(R23)

C2H5OH + OH  CH3CHOH + H2O (k = 3x10

(R24)

-12

3

cm /s at T = 350 K)


Radicals i-C2H5O readily oxidize to form acetaldehyde (ethanal) CH 3CHO, which subsequently decomposes via O2-addition and H-abstraction to form derivatives: acetyl radical CH3CO, methoxy radical CH3O, formaldehyde (methanal) CH2O, formyl HCO, etc. producing finally stable species through reactions: CH2O + OH = HCO + H2O (R25 k = 1.4x10-11cm3/s), CH2O + H = HCO + H2 (R26 k = 1x10-13cm3/s), CH2O + O = HCO + OH (R27 k = 4x10-13 cm3/s), HCO + O2 = HO2 + CO (R28 k = 6x10-12cm3/s), HCO + O = OH + CO (R29 k = 5x10-11cm3/s) and HCO + H = CO + H2 (R30 k = 2x10-10cm3/s). All these reactions are thermodynamically favored. The alkyl (ethyl) radicals decompose or oxidize producing ethylene (ethene) C 2H4. Another pathway is recombination of two methyl radicals CH3 giving ethane C2H6, which yields to C2H4 by two successive dehydrogenation steps. Then, C2H4 is consumed via H-abstractions producing acetylene C2H2. In addition, methylene radicals CH2 recombine to form C2H2: CH2 + CH2 = H2 + C2H2 (R31 k = 2x10-11cm3/s) and CH2 + C2H = CH + C2H2 (R32 k = 3x10-11cm3/s). In its turn, acetylene is decomposed via reaction C2H2 + O = C2HO + CO (R33 k = 3 x10-13 cm3/s). Besides, e-impact decomposition of C2H2 as well as other CxHy fragments always takes place. Methane CH4 is formed by reactions with radicals of the methyl group: CH3O + CH3 = CH2O + CH4 (R34 k = 4x10-11cm3/s), HCO + CH3 = CO + CH4 (R35 k = 2x10-10cm3/s) and CH3 + H + M = CH4 + M (R36 k = 2x10-28 cm6/s). And it is consumed giving the same methyl radical: CH4 + OH = CH3 + H2O (R37 k = 2x10-14cm3/s), CH4 + C2H = CH3 + C2H2 (R38 k = 3x10-12cm3/s). Then, CH3 rapidly oxidizes to form hydrogen: CH3 + O = CH2O + H (R39 k = 1.4x10-10cm3/s) and CH3 + O = CO + H2 + H (R40). Fast reactions of H-abstraction work as well: CH3 + OH = CH2O + H2 (R41 k = 1.2x10-12cm3/s), CH2 + OH = CH2O + H (R42 k = 3x10-11cm3/s) and CH2 + H = CH + H2 (R43 k = 3x10-10cm3/s). However, in the presence of H2O the rates of corresponding radical reactions are reduced proportionally to reducing concentrations of radicals. The pathway related to formation of carbon monoxide CO is determined by oxidation of methylene CH2: CH2 + O2 = H2O + CO (R44 k = 4x10-12cm3/s) and CH2 + O = H2 + CO (R45 k = 2x10-10cm3/s). In its turn, the formation of CH2 is determined by reaction of CH3 and OH: CH3 + OH = CH2 + H2O (R46 k = 2x10-12cm3/s). CH3 and OH in the discharge are formed mainly due to direct e-impact decomposition of C2H5OH in reactions R2b and R2c, which rates are proportional to the discharge current. Therefore, the total rate of CO production is proportional to the square discharge current. The main pathway of CO consumption is reaction CO + OH = CO2 + H (R47 k = 1.5x10-13 cm3/s), which rate is also linearly dependent on the current. In addition, recombination of CO and O atoms can contribute to the CO reduction. However, the increase of CO in the system (Fig. 3a, left side) clear shows that the CO production during the ethanol reforming is stronger than the CO consumption. At the second stage of the fuel reforming, outside of discharge, i.e. without electric field, fast reactions proceed between the species produced due to plasma-chemical reactions at the previous stage. At that, concentrations of atoms and radicals H, OH, CH3, etc. rapidly drop down (Fig. 3b, right side) whereupon chemical processes become slower and concentrations of molecular components H2, CO, CO2, CH4, etc take their stationary values (Fig. 3a, left side). Here, with the time, CO transforms to CO2 and H2 by the above mentioned reaction WGS. In considered non-thermal conditions in the PLS reactor, unimolecular decomposition of EtOH has no any effect because it needs substantially higher temperatures (T > 900 K). The effect of the steam reforming (SR) C2H5OH + H2O  2CO + 4H2, (H 298= 256 kJ/mol) here is negligible because of its very low rate: reaction is highly endothermic and requires very high temperatures or heat energy input. The effect of partial oxidation (PO) C 2H5OH +1/2O2  2CO + 3H2, (H 298= 14 kJ/mol) is also negligible due to its low rate in the considered conditions. Concerning the effect of ethanol-water ratio in the mixture (Fig. 4b, right side), we have the argument, EtOH/H2O = x, which varies from 0 to 1, and it is crucial for plasma-chemistry during breakdown in the DGCLW. At x  0.3 the kinetics is determined by e-impact dissociation of H2O (reaction R4), at x > 0.3 it is determined by eimpact dissociation of EtOH (reaction R2). Therefore, at x  0.3 the rate of H2 production in reaction R1 is proportional to the product [EtOH][H2O]; at x > 0.3 it depends as [EtOH]2. Assuming the low of ideal solutions, i.e. [EtOH] x, and [H2O]  (1–x), we found at x  0.3 the rate of R1 is proportional to x(1–x); at x > 0.3 it depends as x2. The first function passes through the maximum at x = 0.5, the second function has no maximum and grows until x = 1. In the result, concentration of H2 increases with x nonlinearly, having a tendency to saturation after x = 0.3. For other components, some of them (CO, C2H6) increase with x and have a smooth maximum at x = 0.3-0.5; other (CO2, CH4) exponentially decrease with x, depending on the proportion of dissolved and evaporated ingredients. The observed increase of H2 (~1019cm-3) at the outlet of the PLS reactor compared with H2 in the discharge is related to condensation of vapors in the accumulation chamber. It is worth to note that in experiments11,12 no serious solid carbon deposition was observed in the PLS during the EtOH reforming in the DGCLW. Our estimations of the carbon balance in the process of the EtOH conversion (by using the ratio of all carbon-containing product moles to the consumed moles of EtOH accounting for stoichiometry)


confirmed that most of carbon in the EtOH was converted into the CO and CO 2. Thus, our calculations have clearly demonstrated the effect of chemical kinetics on the conversion efficiency. Summary Fig. 5 presents comparison of calculated and measured data for the EtOH reforming in the DGCLW. One can see that all major syngas components (H2, CO/CO2) and even minor species (CH4, C2H4) in average values predicted well. This evidences that the kinetic modeling is true and the kinetic mechanism of plasma-chemical process is correct. The obtained numbers correlate well with our earlier data 30,31 and are comparable with results reported by Fulcheri et al.6

Figure 5. Composition of output syngas products after the EtOH reforming in the DGCLW - PLS: calculations vs. experimental measurements.

IV. Conclusion  The numerical plasma-chemical kinetic modeling of low-temperature plasma-assisted conversion of ethanol in the PLS with the DGCLW at different conditions allowed understanding basic peculiarities and regularities of this very complex process in main details. The calculations confirmed that the discharge of the DGCLW type is quite efficient in plasma reforming of ethanol-water solution and generating syngas with high concentration of H2. The promoting plasma catalytic effect is caused by initial e-impact dissociation of EtOH and H2O molecules into H and OH radicals and CxHy fragments followed by the chain-branching reactions of plasma-chemical transformations enhancing the deep decomposition/conversion of fuel and production of hydrogen-rich syngas.  The main components of syngas produced from ethanol in the PLS reactor are hydrogen H2 and oxides CO/CO2, which relative fraction reaches 90%, i.e. many times higher than other hydrocarbons CH 4, C2H2, C2H4, and C2H6.  The composition content of syngas products and the electric power inputs on the ethanol conversion depends on the input gas that forms the plasma in the discharge and on the initial ethanol-water ratio in the mixture.  Electrochemical processes in the DGCLW do not influence the H2 production in the case of ethanol; however, in the case of water the electrolysis can influence the relative yield of H2 in the mode with a liquid anode.  Depending on whether there is or no input airflow, H2 production has extremum or grows with the percentage of ethanol up to the saturation. At the fixed ethanol-water ratio, the H2 yield increases with the discharge power.  The net yield of H2 reaches maximum ~15% when ethanol and water in the mixture are taken in equal amounts. At that, the specific electric energy demand for the syngas/H2 production is twice less than the combustion energy of the same amount of syngas/H2.  The results of kinetic modeling and calculations are in a fairly good agreement with experimental data, at least, for the main syngas components, H2 and CO/CO2, thus explaining a nonequilibrium character of the non-thermal plasma-chemical mechanism of the ethanol conversion in the studied conditions.

Acknowledgments This work was supported in part by the U.S. European Office of Aerospace Research & Development, by the Science & Technology Center in Ukraine, by the Ministry of Science & Education of Ukraine and by the National Academy of Sciences of Ukraine. Authors thank Dr. Julian M. Tishkoff from the U.S. Air Force Office of Scientific Research for helpful discussions and advices in research. 9 American Institute of Aeronautics and Astronautics

References 1

18th World Hydrogen Energy Conf., 17-19 May 2010, Essen, Germany. http://www.whec2010.com 2 Demirbas, A., “Progress and Recent Trends in Biofuels,” Progress in Energy Combust. Sci., Vol. 33, No.1, 2007, pp. 1-18. 3 Holladay, J. D., Hu, J., King, D. L., and Wang, Y., “An Overview of Hydrogen Production Technologies,” Catalysis Today, Vol. 244, No. 4, 2009, pp. 244-260. 4 Demіnsky, M., Jіvotov, V., Potapkіn, B., and Rusanov, V., “Plasma-Assisted Production of Hydrogen from Hydrocarbons,” Pure Appl. Chem., Vol. 74, No. 3, 2002, pp. 413-418. 5 Bromberg, L., Cohn, D. R., Rabinovich, A., Surma, J. E., and Virden, J., “Compact Plasmatron-Boost Hydrogen Generation Technology for Vehicular Applications,” Int. J. Hydrogen Energy, Vol. 24, No. 4, 1999, pp. 341-350. 6 Petitpas, G., Rollier, J.-D., Gonzalez-Aguilar, J., Darmon, A., and Fulcheri L., “Ethanol and E85 Reforming Assisted by a Non-Thermal Arc Discharge,” Energy & Fuels, Vol. 24, No. 4, 2010, pp. 2607-2613. 7 Rusanov, V. D., and Fridman, A. A., Physics of Chemically Active Plasma, Nauka, Moscow, 1984. 8 Fridman, A., Plasma Chemistry, Cambridge University Press, New York, 2008. 9 Petitpas, G., Rollier, J.-D., Darmon, A., Gonzalez-Aguilar, J., Metkemeijer, R., and Fulcheri L., “A Comparative Study of Non-Thermal Plasma Assisted Reforming Technologies,” Int. J. Hydrogen Energy, Vol. 32, No. 14, 2007, pp. 2848-2867. 10 Chernyak, V. Ya., Olszewski, S. V., Yukhymenko, V. V., Solomenko, E. V., Prysiazhnevych, I. V., Naumov, V. V., Levko, D. S., Shchedrin, A. I., Ryabtsev, A. V., Demchina, V. P., Kudryavtsev, V. S., Martysh, E. V., and Verovchuck, M. A., “PlasmaAssisted Reforming of Ethanol in Dynamic Plasma-Liquid System: Experiments and Modeling,” IEEE Trans. Plasma Sci., Vol. 36, No. 6, Dec. 2008, pp. 2933-2939. 11 Yukhymenko, V. V., Chernyak, V. Ya., Naumov, V. V., Veremii, Yu. P., and Zrazhevskij, V. A., “Combustion of Ethanol Air + Mixture Supported by Arc Plasma,” Problems Atom. Sci. Technol., Ser. Plasma Physics, Vol. 13, No. 1, 2007, pp. 142-144. 12 Yukhymenko, V. V., Chernyak, V. Ya., Olshevskii, S. V., Prisyazhnevich, I. V., Zrazhevskii, V. A., Verovchuk, M. O., Solomenko, O. V., Skalny, J. D., Matejcik, S., Demchina, V. P., Kudryavtsev, V. S., and Naumov, V. V., “Plasma Conversion of Ethanol-Water Mixture to Synthesis Gas,” Ukr. J. Phys., Vol. 53, No. 5, 2008, pp. 409-413. 13 Naugol'nykh, K. A., and Roy, N. A., Electric Discharges in Water, Nauka, Moscow, 1971. 14 Raizer, Yu. P., Gas Discharge Physics, Nauka, Moscow, 1987. 15 Lide, D. R., (ed.), CRC Handbook of Chemistry and Physics, CRC Press/Taylor & Francis Group, Boca Raton, 2009. 16 Shchedrin, A. I., Levko, D. S., Chernyak, V. Ya., Yukhymenko, V. V., and Naumov, V. V., “Effect of Air on the Concentration of Molecular Hydrogen in the Conversion of Ethanol by a Nonequilibrium Gas-Discharge Plasma,” JETP Lett., Vol. 88, No. 2, 2008, pp. 99-102. 17 Kalyuzhna, H. G., Levko, D. S., and Shchedrin, A. I., “The Influence of the Parameters of an Atmospheric Pressure Barrier Discharge in Air on the Plasma Kinetics,” Ukr. J. Phys., Vol. 53, No.10, 2008, pp. 957-961. 18 NIST Standard References Databases. [Online] URL: http://www.nist.gov/srd/ 19 A. I. Shchedrin's Group Web site. [Online] URL: http://www.iop.kiev.ua/~plasmachemgroup/ 20 A. V. Phelps Web site. [Online] URL: http://jilawww.colorado.edu/~avp/collision data/ 21 Higgins, K., Noble, C. J., and Burke, P. G., “Low-Energy Electron Scattering by Oxygen Molecules,” J. Phys. B: At. Mol. Opt. Phys., Vol. 27, No. 14, 1994, pp. 3203-3216. 22 Gillan, C. J., Tennyson, J., McLaughlin, B. M., and Burke, P. G., “Low-Energy Electron Impact Excitation of the Nitrogen Molecule: Optically Forbidden Transitions,” J. Phys. B: At. Mol. Opt. Phys., Vol. 29, No. 8, 1996, pp. 3203-3216. 23 Ajello, J. M., “Emission Cross Sections of N2 in the Vacuum Ultraviolet by Electron Impact,” J. Chem. Phys., Vol. 53, No. 3, 1970, pp. 1156-1165. 24 Hake, R. D., and Phelps, A. V., “Momentum-Transfer and Inelastic-Collision Cross Sections for Electrons in O2, CO and CO2 Impact,” Phys. Rev., Vol. 158, No. 1, 1967, pp. 70-84. 25 Vicic, M., Poparic, G., and Belic, D. S., “Large Vibrational Excitation of N 2 by Low-Energy Electrons,” J. Phys. B: At. Mol. Opt. Phys., Vol. 29, No. 6, 1996, pp. 1273-1281. 26 Straub, H. C., Renault, P., Lindsay, B. G., Smith, K. A., and Stebbings, R. F., “Absolute Partial Cress Sections for ElectronImpact Ionization of H2, N2, and O2 from Threshold to 1000 eV,” Phys. Rev. A, Vol. 54, No. 3, 1967, pp. 2146-2153. 27 Itikawa, Y., and Mason, N., “Cross Sections for Electron Collisions with Water Molecules,” J. Phys. Chem. Ref. Data, Vol. 34, No. 1, 2005, pp. 1-22. 28 Rejoub, R., Morton, C. D., Lindsay, B. G., and Stebbings, R. F., “Electron-Impact Ionization of the Simple Alcohols,” J. Chem. Phys., Vol. 118, No. 4, 2003, pp. 1756-1760. 29 Marinov, N. M., “A Detailed Chemical Kinetic Model for High Temperature Ethanol Oxidation,” Int. J. Chem. Kinet, Vol. 31, No. 3, 1999, pp. 183-220. 30 Shchedrin, A. I., Levko, D. S., Ryabtsev, A. V., Chernyak, V. Ya., Yukhymenko, V. V., Ol'shevskiy, S. V., Prisyazhnevich, I. V., Solomenko, E. V., Naumov, V. V., Demchina, V. P., and Kudryavtsev, V. S., “Plasma's Kinetics in Discharge in Mixture of Air, Water and Ethanol Steams and the Questions of Alternative Fuel,” Problems Atom. Sci. Technol., Ser. Plasma Electronics, Vol. 6, No. 4, 2008, pp. 159-162. 31 Chernyak, V. Ya., Olzhevskij, S. V., Yukhymenko, V. V., Prisyzhnevich, I. V., Verovchuk, M. A., Solomenko, E. V., Zrazhevskij, V. A., Naumov, V. V., Shchedrin, A. I., Levko, D. S., Demchina, V. P., and Kudryavzev, V. S., “Study of Plasma Conversion of Ethanol into Syngas in Dynamic Plasma-Liquid Systems,” Proc. 4th Int. Workshop & Exhibit. on Plasma Assisted Combust., 16-19 Sept. 2008, Falls Church, Virginia, ed. by I. Matveev, APT, McLean, 2008, pp. 68-70.
