22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Conversion of ethanol-water mixture into syngas in a DC atmospheric pressure
glow discharge in air
V.I. Arkhipenko1, A.A. Kirillov1, L.V. Simonchik1, A.V. Paulava1, A.P. Chernukho2 and A.N. Migoun2
1
2
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
Abstract: Conversion of an ethanol-water mixture into syngas in a DC atmospheric
pressure glow discharge with plasma cathode in air is investigated experimentally and
theoretically. Electrical power applied to plasma is in the range of 100-250 W. The main
components of syngas are hydrogen, carbon monoxide, methane, ethylene and acetylene. A
conversion degree in hydrogen of about 90% is achieved. Hydrogen constitutes up to 40%
of syngas composition. Two-dimensional steady-state numerical model of the conversion
processes is developed. A good agreement between the calculated and experimental results
is observed. A purely thermal role of the discharge in the conversion is established.
Keywords: glow discharge, ethanol conversion, hydrogen production, synthetic gas
1. Introduction
Among the possible versions of hydrogen generators,
a special place is occupied by plasma systems, where the
process could be fully or partially maintained by plasma
[1]. The main advantages of technologies based on the use
of plasma include the acceptable modes of operation
(atmospheric pressure, low temperature gas, quick start,
compact size, etc.). Different types of discharges at
atmospheric pressure (corona, spark, barrier, sliding arc of
DC, AC and pulse current of different frequency bands)
are offered as plasma sources for these purposes.
Conversion optimization must be based on the
understanding of plasma-chemical processes, the role of
which can vary significantly in different discharges.
Plasma effect manifests as a heating gas and generating
chemically active particles due to electron collisions with
molecules. The last process is important in cold plasmas,
such as dielectric-barrier discharge, corona discharge and
spark discharge [2]. In discharges with hot plasmas the
effect of gas heating plays the major role. It is shown, for
example, that the use of standard kinetic schemes without
an inclusion of specific plasma processes is sufficient for
simulation of methane and octane reforming in lowcurrent arc discharges [3].
In [4], the comprehensive experimental and theoretical
investigations of a conversion of ethanol-water mixture in
syngas assisted by a DC atmospheric pressure glow
discharge with plasma cathode were fulfilled. Numerical
modelling of conversion kinetics was performed in a onedimensional approximation using Konnov’s model [5]
and with an assumption of thermal nature of the
processes. As it was shown, assuming about 30-35% heat
losses into ambiance, the calculated data is in a good
agreement with the experimental data relative to the gas
mixture composition and the conversion degree in
hydrogen. In this paper, we present the experimental data
for the conversion of ethanol in the range of electrical
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discharge power of 100-250 W and numerical simulations
based on the developed two-dimensional steady-state
model which takes into account radial diffusion and
thermal conductivity.
2. Experimental setup
The conversion of an ethanol-water mixture was
carried out in a plasma-chemical reactor, which represents
a three-piece chamber with a cathode-anode-anode
configuration of electrodes [6]. The schematic of reactor
is shown in Fig. 1, a.
In section A, copper cathode 2 (rod diameter 6 mm)
and copper anode 3 (plate thickness of about 1 mm) are
located at a distance of 1-1.5 mm. The self-sustained glow
discharge at a current of 150-200 mA between these
electrodes is maintained using source U1 (1500 V, ballast
resistance R1 ~ 1200 Ω). The airflow is provided through
a
b
Fig. 1. Schematic of plasma-chemical reactor (a) and its
photo (b) with flame of burning syngas.
section A at a rate of about 0.4 liter per minute. Air output
takes place through the hole (diameter 2 mm) in the center
of electrode 3 into section C (quartz tube 5 with the
diameter of 10 mm and 15 mm long) located under the
hole. The glow discharge in section A is used as a plasma
cathode for non-self-sustained discharge in section C.
This discharge is ignited between electrode 3 and the
second anode 4 with the help of source U2 (3000 V, R2 ~
1000 Ω). Electrode 4 is arranged in section С at a distance
1
of 15 mm from electrode 3. The current value of this
discharge is 130 mA.
The mixture of ethanol (85%) and water (15%) with
an air flow of 0.4 l/min is introduced in section B. The
flow of an ethanol-water mixture is 1.25 ml / min. This
mixture evaporates in section B and with help of
airstream flows into section C, where the non-selfsustained glow discharge operates.
A voltage drop between the electrodes 3 and 4 is 450 500 V in air discharge without the ethanol-water mixture.
It is a typical voltage drop in a normal glow discharge in
atmospheric pressure air at the current of 150-200 mA [7].
However, with the introduction of ethanol-water mixture,
the increase of the voltage drop up to 1500 V is observed.
Accordingly, the total electrical power is about 250 W.
Output of gases from section C occurs through the hole of
2 mm in diameter in anode 4. Figure 1b shows a
photograph of burning syngas at the end of the exit tube.
The length of the flame having a blue color reaches 10
cm.
For determining the conversion products, a diagnostic
technique based on infrared (IR) absorption spectroscopy
is used [8]. For this purpose, we use infrared spectrometer
NEXUS (Thermo Nicolet). The absorption spectra are
recorded in the range of 600–4000 cm-1 with the
resolution of 2 cm-1. The 5.7-cm gas cuvette with
germanium windows is used for the spectroscopic
analysis. This cuvette is preheated up to 90°C before
pumping the exhaust gases from reactor through the
cuvette to prevent condensation of water vapor and
ethanol.
Fig. 2. The infrared absorption spectrum of the
exhaust gases mixture from reactor.
The infrared absorption spectrum of the exhaust gases
mixture from reactor contains the intense vibrationrotational bands of different molecules, namely, CO
(2150 cm-1), CO 2 (740 and 2350 cm-1), H 2 O (1600 and
3750 cm-1), CH 4 (3100 cm-1) and C 2 H 2 (750 and 3300
cm-1), C 2 H 4 (950 cm-1) and C 2 H 5 OH (1050 and 2950 cm1
) (Fig. 2). Mole fractions of optically active components
in the gas mixture after conversion (CO, CO 2 , CH 4 , C 2 H 2
and H 2 O) are determined by comparison of the
2
experimental absorption spectrum and the calculated one
using the base spectral data Hitran [9].
The mole fraction of ethanol is determined using a
calibration dependence obtained by measurements with
gas mixtures containing the known concentration of
ethanol. The mole fraction of hydrogen, nitrogen and
oxygen is calculated with the help of the law of mass
conservation in chemical reactions taking into account the
contents of ethanol, water, oxygen and nitrogen at the
reactor inlet and the mole fractions of the infrared active
components in conversion products. Additionally, the
amount of hydrogen in the exhaust mixture was
determined using effect of its leaking through a palladium
diaphragm. The conversion products concentrations
determined in the described way are presented in Table 1
below.
3. Thermodynamic and kinetic parameters of
conversion
The reaction mixture composition at the reactor inlet
is as follows: 30% ethanol, 16% water and 54% air. The
considered mixture has potentially its own internal
energy. The adiabatic temperature is T ad = 861 K. It
corresponds to the equilibrium degree of conversion in
hydrogen h = 52.9%. Here, the conversion degree of
ethanol to hydrogen is defined by the following ratio:
h ( H2 ) =
number of H - atoms in generated hydrogen molecules H 2
number of H - atoms in ethanol molecules
.(1)
For the achievement of high conversion degrees it is
needed to put additional energy in adiabatic volume with
the mixture. Theoretically, the maximum possible
Fig. 3. Equilibrium composition of ethanol-water-air
mixture against the temperature.
conversion degree of this mixture is equal to
η max = 114.4%. Figure 3 shows that the temperature
influences the characteristics of the equilibrium
conversion process. It is evidential fact, that the
temperature T = 1100°K is enough to obtain the
maximum possible equilibrium conversion degree
CD = 108% and hydrogen concentration [H 2 ] = 47%.
With further increase in temperature, all the process
parameters practically reach saturation.
Let us remind that for the setting of the equilibrium
composition, infinite time is needed. In real conditions,
the characteristic time of the process is limited e.g. by the
residence time of the reaction mixture in the discharge
gap. Therefore, the calculated equilibrium composition
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cannot be achieved due to kinetic restrictions, because the
kinetic processes constants as a rule are strongly
(exponentially) dependent on temperature.
Figure 4 shows the kinetics of the ethanol-water-air
mixture conversion for temperature T = 2000 K. It is
obvious that the process can be divided into two main
stages. The first fast exothermic stage implies ignition and
partial combustion of the fuel to form CO, H 2 , CO 2 , H 2 O
and products of incomplete fuel conversion (CH 4 , C 2 H 2 ,
C 2 H 4 ….). The initial fuel disappears almost completely
by the time 10-6 s, oxygen – 3⋅10-5 s. By this time, the
maximum concentration [H 2 O] ~27% and [CO 2 ] ~2% is
set.
and the continuity equations for k mixture chemical
components
ru
∂Yi 1 ∂
+
ω i µi
( r rYi Vi ) =
∂x r ∂r
Then, the second and the slowest endothermic stage of
the process begins once steam-carbon dioxide conversion
of the remaining hydrocarbons takes place. In this stage,
the remaining hydrocarbons react with water and carbon
dioxide to form an additional amount of H 2 and CO. The
final equilibrium composition of the mixture for the
considered temperature is set by the time ~0.1–0.2 s. In
the second stage, the concentrations of H 2 O and CO 2
decrease by approximately 4–6 times. As it can be seen,
the second stage of the conversion process yields a major
amount of hydrogen. This means that in real process it is
needed not only to provide a sufficiently high
temperature, but also to maintain it during whole time of
process.
4. Model
For further understanding of the conversion process
peculiarities, we carried out a series of numerical
simulations for the experimental conditions. For this
purpose, a two-dimensional steady-state model was
developed and implemented in the form of a code. As a
chemical model of the process, we used a kinetic
mechanism of A. Konnov [5]. The mathematical model
describing the flow of chemically reacting gas in the
cylindrical tube includes the energy equation
k
∂T 1 ∂ 
 1 ∂  ∂T  k
,(2)
0
+
 r r ∑ Yi Vi hi  − r ∂r  r λ ∂r  + ∑ ω i hi µi − W =
x
r
r
∂
∂

 i1
=
 i 1=

cr r u
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(3)
The system of 2D differential equations (2) and (3) is
added by the continuity equation of gas flow
ρ u F = const ,
the equation of ideal gas state
ρ=P
µ
(4)
(5)
RT
and the condition of constant pressure across the
computational domain
(6)
P ( x, r=
) P=
const .
0
A steady turbulent radial velocity profile was used instead
of the motion equation [10].
Here in (2)–(6), x and r are longitudinal and radial
coordinates, respectively; P, r and u – pressure, density
and gas velocity, respectively; c p – specific gas heat
capacity; T – gas temperature; Y i – mass fraction of i-th
specie; h i – enthalpy of i-th specie; λ– thermal
conductivity coefficient of gas mixture; µ i – molecular
weight of i-th specie;
Fig. 4. Evolution of chemical composition in the
process of ethanol-water-air mixture conversion under
isothermal conditions: T = 2000°K.
1,..., k )
(i =
ω i
– production rate of i-th specie;
W – volumetric energy input; µ – molecular weight of
gas mixture; V i – i-th specie diffusion velocity, F – cross
section of a tube.
The numerical procedure was based on the finite
difference formulation. We have implemented a specific
numerical method to reduce the order of the differential
system to unity: the whole tube has been divided into
N grid (20–30) annular fluid tubes with adaptive crosssections. The gas mass flow rate in every tube is
considered to be constant. Use of explicit one-step
scheme over variable x allows reducing the problem from
k+1 parabolic equations of the second order to (k+1)N grid
ordinary differential equations. Diffusion and heat flows
between fluid tubes were taken from the previous step.
After each step, the position and size of the fluid tubes
were adjusted considering the changed flow parameters.
5. Results and discussion
The numerical modeling of the conversion process is
performed at the applied electrical power of 250 W. The
temperature of 1000 K obtained by a thermocouple close
to the chamber wall is used as the boundary and initial
conditions during simulations. Figure 5 shows evolution
of the chemical composition and temperature at the center
of the discharge tube. It is clear that up to z = 0.02 cm the
initial composition does not change, and there is only
heating of the gas mixture. Then, when the temperature
reaches 1300 K, an ignition and rapid partial combustion
of the mixture occur, which leads to the temperature jump
to 1600 K. Then, another slow microsecond stage of the
process begins (see the description of Fig. 4). The
velocity of conversion process increases once the
temperature rises and reaches a sufficiently large value of
3
3300 K. This value is close to the experimental value of
3500 K defined at the discharge axis by the relative
intensities of the rotational band of the hydroxyl emission
spectrum.
Let us note that there are no unreacted hydrocarbons
in the center of the discharge tube at the reactor outlet due
to high temperature, and the output mixture consists
mainly of Н 2 , CO, N 2 and small amount of H 2 O and
CO 2 . This indicates that the conversion in near axis
volume is effective enough, and the residence time is
sufficient to complete the conversion process.
Nevertheless, there is a rather high concentration of
unreacted hydrocarbons in the output mixture for this
regime. This can mean that the conversion kinetics
proceeds differently in the peripheral zones of the
discharge.
products are in good agreement. The satisfactory
agreement takes place for the conversion degrees as well:
88.5% and 91%.
Table 1. Experimental and theoretical data on the
products of conversion
Components
C 2 H 5 OH
H2O
N2
O2
CO
CO 2
CH 4
C2H2
C2H4
H2
Conversion
degree
Fig. 5. The time evolution of chemical composition of
ethanol-water-air mixture and temperature in the
center of the discharge tube.
Fig. 6 shows the radial composition at the end of the
discharge gap. It can be seen that in the peripheral zones
of the discharge, where the temperature is substantially
lower than in the center, there is unsufficient time to
Fig. 6. Radial concentrations profiles of conversion
components.
complete the conversion process. It is evident that the
space close to the tube wall is responsible for the
availability of unreacted hydrocarbons in the exhaust
mixture. Concentrations of H 2 O and CO 2 are also
significantly higher in these peripheral zones of the
discharge, since the temperature is substantially lower
than in the center.
Table 1 shows data on the composition of the inlet
reactive mixture and the experimental and calculated data
on conversion products. It can be seen that the
experimental and calculated concentration of conversion
4
At the
inlet
30
16
43
11
Mole fraction, %
Products of conversion
Experiment
Calculation
0.33
8.6
22.03
0
24.8
1.1
1.3
0.9
0.44
40.5
88.5
0
9.3
21.9
0
24.0
1.1
0.46
1.1
0.2
41.5
91
The conversion products concentrations change when
energy deposition decreases, i.e. the fraction of large
concentration components decreases and the fraction of
lower concentration components increases. Very good
agreement demonstrated above for the discharge power of
250 W is not so satisfactory at lower power, e.g. at 100
W. On one hand, it happens due to the changes in the
temperature boundary conditions close to the tube wall.
On the other hand, as at low power the discharge is more
diffuse, another radial distribution of energy can be
formed which differs from the one used in the model.
Thus, the comparison of the experimental and
simulated data shows that the use of the kinetic Konnov’s
scheme [5] allows good accuracy in calculating the
conversion of ethanol-water-air mixture and performing
optimization of the reactor, even without inclusion of the
specific plasma-chemical reactions involving charged
particles.
6. References
[1] Bromberg L., Cohn D.R. et al. Int. J. Hydrogen
Energy, 26, 1115-1121 (2001)
[2] Chen F., Huang X. et al. Int. J. Hydrogen Energy, 39,
9036-9046 (2014)
[3] Benilov M.S., Naidis G.V. Int. J. Hydrogen Energy,
31, 769-774 (2006)
[4] Arkhipenko V.I., Kirillov A.A. et al. Open Chem., 13,
292–296 (2015)
[5] Konnov A. A. Proceed. of the 28th Intern. Symposium
on Combustion, Abstr. Symp. Pap., 317 (2000)
[6] Arkhipenko V.I., Callegari Th. et al. IEEE Trans.
Plasma Sci., 37, 740–749 (2009)
[7] Arkhipenko V.I., Kirillov A.A. et al. J. Phys. D: Appl.
Phys., 66, 252 (2012)
[8] Arkhipenko V.I., Zgirouski S.M. et al. J. Appl.
Spectrosc., 80, 99-103 (2013)
[9] http://www.cfa.harvard.edu/hitran/
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[10] Chernukho A., Migun A. et al. J. Eng. Phys.
Thermophys., 78,394-404 (2005)
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