From Wood to Syngas by Non Thermal Arc
K. Arabi, O. Aubry, A. Khacef, J. M. Cormier
GREMI, Polytech'Orléans, 14 rue d'Issoudun BP 6744, 45067 Orléans, France
Abstract: The present study is devoted to the non thermal plasma steamreforming of wood. The reactor is powered by a generator delivering a sinusoidal
current at 50Hz. The arc is formed between two electrodes made of graphite; one
of them is surrounded by wood. We show the influence of the electrodes gap on
the total flow of production of syngas and on the electrical power. The chemical
and physical diagnostics are performed to determine the efficiency of the process.
Keywords: Non thermal plasma, syngas, production, wood.
1. Introduction
Biomass is considered as a renewable, storable and
transportable energy source which is available in
various forms such as wood, agricultural and forest
residues. Pyrolysis and gasification are efficient
ways for biomass utilization [1- 4] and specially
syngas production. Reduction of greenhouse gas
emission can be improved using syngas and
hydrogen obtained from biomass.
These requirements present a necessity to develop
new types of method for the hydrogen production [4,
5]. The production of hydrogen from biomass is an
interesting and promising option for many
applications. The treatment of biomass provides an
alternative for the hydrogen production. This work is
dedicated to the study of biomass treatment (beech
wood) using a non thermal plasma reactor at low
temperature and at atmospheric pressure.
2. The experimental system
The figure 1 shows the experimental plasma reactor
used in this work. The non thermal plasma reactor
includes a quartz tube (30 mm diameter) containing
two carbon electrodes placed to opposite each other
with a gap between 10 mm and 30 mm; one of them
is surrounded by wood.
The water is introduced through the lower electrode
and is vaporized in plasma region. At startup, the
plasma column is developed only between the
electrodes. Subsequently, due to the wood pyrolysis,
the discharge can move on the surface of the carbon
layer appeared on the wood.
condenser
Wood
Graphite
electrodes
gas
Chemical analysis
(GC, FTIR)
Oscilloscope
Power supply
15kV, 50HZ
Water
Figure 1: Schematic experimental arrangement.
The output gas mixture passes through a refrigerated
coil (T = -15°C) which allows the removal of
condensable vapors, including water. The gas
mixtures analyzed after condensation are called “dry
gas”.
The electrical discharge is powered by a 50 Hz high
voltage step-up transformer with leakage flux
(230 V/20 kV, 155 mA).
The current and voltage waveforms are observed by
means of a digital oscilloscope (Tektronix TDS
460A) using a voltage and current probes (Tepcel
DP25, Langlois PSY30, respectively). The power is
calculated from the current and voltage data.
The figure 2 shows an example of the discharge
obtained from wood treatment.
Figure 2: Photography of the discharge between electrodes.
To determine the produced species, the dry gas is
analyzed using a gas chromatography (GC-Varian
CP 3800) equipped with a thermal conductivity
detector (TCD) and a flame ionization detector
(FID).
We observe that H2, CO and CO2 concentrations are
similar to those obtained from experiment.
3. Results and discussion
However, before concluding, we have to examine
the influence of plasma on water decomposition.
3CO (g) + 3CO2(g) + 9.5H (g) (1)
C H O (s) + 5H O (g)
6
9
4
2
2
rH (298K) = 5758.6 kJ.mol
-1
Waveforms of voltage and current
The figures 3 and 4 show the temporal evolution of
the voltage and current obtained for the two
distances.
U (V)
In table I, the results obtained with biomass
treatment are reported for two values of the
electrodes gap (Dint = 1cm and Dint = 2.5cm).
The observed species are mainly H2, CO, CO2 and
CH4. C2 hydrocarbons species are less than 0.5 %
D int
(cm)
Analysis GC dry gas
Voltage (V)
Influence of electrodes gap
CO2
CO
CH4
1
57.8
17.9
22.5
1.2
5.6
2.5
58.3
19.3
19.7
2.3
13.6
The efficiency of plasma increases with the length of
the plasma column.
Theoretically, the concentration of H2, CO and CO2
are calculated from stoichiometric coefficient using
reaction (1) for the biomass treatment. The following
mixture composition is found as:
H2 = 61% (9.5 / 15.5)
CO = 19% (3 / 15.5)
CO2 = 19 % (3 / 15.5)
2000
0.2
1000
0.1
0
0
-1000
-0.1
-2000
-0.2
-0.3
Figure 3: Waveforms of voltage and current (d=1 cm).
U (V)
Voltage (V)
Moreover, the flow of gas production increases
(from 5.6 L/h to 13.6 L/h) when the length of the
plasma column increases.
0.3
Time (s)
Table I: Species concentration in dry gas from wood and gas
flow rate as a function of electrodes gap.
The concentrations of the produced species are of
the same order for the two distances between
electrodes.
3000
-3000
Produced
gas flow rate
(L/h)
H2
I (A)
Current (A)
The equation (1) of the biomass treatment (beech
wood) is written as:
I (A)
3000
0.3
2000
0.2
1000
0.1
0
0
-1000
-0.1
-2000
-0.2
-3000
-0.3
Current (A)
a. Biomass treatment
Therefore, the reaction (1) of biomass treatment well
describes the experimental results and it is the
dominant process.
Time (s)
Figure 4: Waveforms of voltage and current (d=2.5 cm).
The voltage between the electrodes increases when
the electrodes gap increases.
The electrical current remains at the same RMS
value (155 mA) for the various electrodes distances.
Therefore, an increasing of the voltage corresponds
to an increasing of the plasma resistance.
In both cases (1 and 2.5 cm), the asymmetry of the
voltage is due to an asymmetry of the electrodes.
Waveforms of electrical power
In figure 5, the evolution of the instantaneous
electrical power is illustrated as a function of the
time for the two distances (1 and 2.5 cm).
500
(a)
Power (W)
400
CO2 is generated by oxidation of the graphite
electrodes. The oxidizing species are produced by
water decomposition under the plasma conditions.
(b)
The comparison between the results from pure water
and the wood is done for an electrodes gap of 1cm.
300
200
We note that the gas flow rate in the case of wood is
higher than in pure water. The production of gas is
increased 22 times.
100
0
0
0,01 0,02 0,03 0,04 0,05 0
0,01
0,02
0,03
0,04
0,05
Time (s)
Time (s)
Figure 5: Waveforms of instantaneous electrical power
(a) d=1cm ;(b) d=2.5cm.
A non-periodic variation of power is observed in the
case of large electrodes gaps.
In large gap conditions, erratic movements of the
plasma column are similar to those observed in the
case of Gliding Arcs [6].
The average electrical power is calculated as follow:
P = <i (t)*u (t)>
The values of average power are 74 W and 184 W
for Dint= 1cm and Dint = 2.5cm, respectively.
b. Comparison between wood and water
An experiment with the pure water was performed to
compare the results with the wood treatment and
those obtained from the decomposition of water.
The goal of this study is to assess the contribution of
the decomposition of water in the process of wood
treatment.
The experimental set-up used for pure water is the
same as showed in figure 1 except that the discharge
is done between two carbon electrodes [7, 8].
The results obtained with pure water are given in the
table II:
D int (cm)
1
The species produced from the decomposition of
water are H2 and CO2.
Analysis GC dry gas
H2
CO2
Produced gas flow
rate (L/h)
76.8
23.2
0.25
Power (W)
112
Table II: H2 and CO2 concentration in dry gas from water, gas
flow rate (L/h) and electrical power (W).
In experimental condition of wood treatment, the
contribution of the decomposition of water is not
significant.
Energy cost
From the experiments, the energy cost per kg of H2
produced, ECH2, is calculated from:
EC H 2 
W
mH 2
W is the electrical energy consumed (Wh) to
produce a given mass of H2 (mH2).
The ECH2 evaluates the efficiency of the processes.
The energy cost calculated experimentally from the
decomposition of the pure water is 6460 kWh/kgH2
and 260 kWh/kgH2 from the experiments of the wood
treatment.
The energy cost from the water decomposition is
higher than that obtained with the wood treatment.
The energy cost of the water decomposition using
plasma is high compared to the electrolysis
(56 kWh/kgH2)
The non-thermal plasma treatment is not effective in
the case of the pure water and also that the steam
reforming is dominant in the case of wood.
To evaluate the efficiency of the plasma treatment,
the theoretical value of hydrogen production is
compared to the experimental value.
The hydrogen energy cost obtained from theoretical
reaction (1) from the wood treatment is:
73 kWh/kgH2. The hydrogen energy cost obtained
experimentally is 260 kWh/kgH2.
The experimental value is higher than the theoretical
one. This difference is due to the various losses in
the reactor (thermal conduction and convection).
A complete and a large study remain to be
developed to improve the chemical and physical
efficiency of the plasma reactor.
c. Energy interest of wood plasma treatment
References
The energy produced from the syngas combustion is
compared to the energy obtain from combustion of
wood consumed.
[1] L. Shiguang, X. Shaoping, L. Shuqin, Y. Chen,
L. Qinghua. Fuel Process Technology 85 (2004)
1201
The initial mass of electrode surrounded by beech
wood is: mi = 4.0 g; the total duration of the
experiment is 45min and the consumed mass of
wood is equal 2.5 g.
[2] G. Chen, J. Andries, H. Spliethoff, M. Fang, P.
J. van de Enden. Solar Energy 76 (2004) 345
The heating value of wood is (18.103 kJ/mol), so the
combustion energy of this mass equal to 46 kJ.
[3] H. Qinglan, W. Chang, L. Dingqiang, W. Yao,
L. Dan, L. Guiju. International Journal of
Hydrogen Energy 35 (2010) 8884
The total volume of gas produced, corresponding to
0.75 h, is equal to 7.2L. The composition of gas
mixture is: 58% H2 (46 kJ) and 21% CO (20 kJ).
[4] K. Aasberg-Petersen, J.-H. Bak Hansen, T.S.
Christensen, I. Dybkjaer, P. Seier Christensen,
C. Stub Nielsen, S.E.L. Winter Madsen, J.R.
Rostrup-Nielsen. Applied Catalysis A: General
221 (2001) 379
Taking into account the heating values of H2 and CO
(241 kJ/mol, 283 kJ/mol respectively), we calculate
the combustion energy of the gas mixture contained
in 7.2 L. The energy production is equal to 66 kJ.
[5] J.D. Holladay, J. Hu, D.L. King, Y. Wang.
Catalysis Today 139 (2009) 244
The energy combustion of the wood consumed is
less compared to that obtained with the combustion
of syngas. This results means that the non thermal
plasma can be a way of biomass conversion
improvement.
4. Conclusion
The results obtained in this study present the first
development of the wood treatment using a nonthermal plasma reactor.
The flow rate of the hydrogen production is
increasing with the electrodes gap. Efficiency of
plasma process is increasing with the length of the
plasma column. So "Gliding Discharges" can be
used to produce large volumes of plasma.
The energy produced by the combustion of syngas is
higher than the energy combustion of wood
consumed to obtain the syngas.
This work shows that the biomass treatment using
non thermal plasma could be a promising technique
for syngas production.
[6] E. El Ahmar, C. Met, O. Aubry, A.Khacef, J.M.
Cormier. Chemical Engineering Journal, 116
(2006) 13
[7] O. Aubry, C Met, A. Khacef, J. M Cormier.
Chemical Engineering Journal 106 (2005) 241
[8] J. Luche, O. Aubry, A. Khacef, J.M. Cormier.
Chemical Engineering Journal 149 (2009) 35