Hydrogen Production by Non Thermal Plasma Steam Reforming of alkanes and ethanol

A. Khacef, F. Ouni, E. El Ahmar, O. Aubry, and J. M. Cormier

GREMI-Polytech'Orléans, 14 rue d'Issoudun, BP 6744, 45067 Orléans Cedex 2, France e-mail: ahmed.khacef@univ-orleans.fr

Abstract The performance of methane, propane and ethanol steam reforming reactions was

investigated in non-thermal plasmas at atmospheric pressure. Plasma reactors were evaluated by means of parameters such as conversion efficiencies and product selectivity.

The methane and propane experiments were conducted in a new sliding discharge reactor (SDR) powered by a three-channel power supply (100-500 mA, 50 Hz). Results showed that hydrocarbons conversion, steam reforming and cracking selectivity, and hydrogen production depend on the nature of the hydrocarbon, the inlet steam to carbon ratio, the gas flow, and the supplied power. The main products of the plasma treatment are H2 (50%), CO (up to 30%) and no-consumed CH4 or C3H8 (depending on the experiment). We should highlight the presence of C2-hydrocarbons (C2H2, C2H4, C2H6,) and CO2.

In the case of ethanol, the experiments were conducted in a direct discharge at atmospheric pressure with a liquid ethanol/water mixture heated by graphite electrodes. The ethanol and water mole fractions ratio of the inlet mixture was in the range 0-72%. The highest mole fractions of H2, CO, CO2, CH4 obtained in that study are 72%, 28%, 12%, and 5%, respectively.

1. Introduction The evolution of the fossil energy resources reveals a great interest for prospecting new

energy vectors. Hydrogen is supposed to have an important role in the future worldwide energy vector supply and environmental safe technologies. Traditionally, H2 and syngas (a mixture of CO and H2) were produced by chemical processes from methane (the main component of natural gas). The use of ethanol obtained by fermentation of surplus or agricultural residues (or bio-ethanol) for energy production could be an effective solution for reducing CO2 emission and preserving the fossil energy resources [1, 2]. However, these processes require extreme operating conditions (high temperature and pressure) and suffer from the rapid deactivation of catalyst. Due to their compactness, efficiency and energetic low cost, non-thermal plasma (NTP) reformers appear as an alternative solution to the catalytic conventional technologies for hydrogen production.

In this study, the performance of methane, propane and ethanol steam reforming reactions was investigated in non-thermal plasmas at atmospheric pressure. In all experiments, the performances of plasma reactors were evaluated by means of parameters such as conversion and product selectivity.

2. Experimental

2. 1. Methane and propane experiments The methane and propane experiments were conducted in a new sliding discharge reactor

(SDR). The gases are mixed before injection in a heated line and then in the SDR. The gas temperature was fixed at about 150°C for all the experiments. Total flow rate of the gas mixture is in the range 80-120 L/min.

The SDR consist on three copper anodes arranged around a single tungsten cathode (Figure 1). The system was described in detail previously [3] and is briefly described here for clarity.

Fig. 1: Schematic of the SDR and discharge produced

Discharges are ignited between electrodes and then pushed by the gas flow. A magnet was inserted in the reactor in order to produce a rotating effect in the discharge region. The discharge column is a plasma string, with a visible diameter less than one millimetre that slides in the gas flow and the magnetic field region. As shown in the photograph of the figure 1, the plasma string performs a helix movement and looks like a wrapped wire around the cathode.

The SDR was powered using a three-channel power supply device [4]. Typical voltage and current waveforms for one of the three discharges are shown in figure 2. For clarity, the high voltage is plotted as a negative signal. As can be seen from this figure, the discharge behaviour is not definitely periodic due to the instability in the growing discharge.

Fig. 2: Current and voltage waveforms

In this type of reactors, plasma can sweep a large part of the inlet gas and maintains its non-equilibrium behaviour. The force acting on the lengthening discharge column is proportional to the product between the current and the magnetic-field strength. This force produces a rapid lengthening of the discharge column. Due to the magnetic field, a self-limitation of the current

intensity is produced. In the usual sliding discharges, the plasma thermalization was avoided by using external current limitation.

2. 2. Ethanol experiments The experiments were conducted at atmospheric pressure with a liquid ethanol/water

mixture heated by graphite electrodes in a direct discharge plasma reactor [5]. Figure 3 show a schematic of the reactor.

Gas Out GraphiteElectrodes

Water/Ethanolmixture

Gas Out GraphiteElectrodes

Water/Ethanolmixture

Gas Out GraphiteElectrodes

Water/Ethanolmixture

Fig. 3: Schematic of the direct discharge reactor

The plasma reactor was powered by a 50 Hz high voltage step-up transformer with leakage flux (AUPEM SEFLI, 10 kV, 155 mA).

The ethanol and water mole fractions ratio of the inlet mixture studied was in the range 0–72%. The exhaust gas was sampled into two ways: via a 110°C heating line for humid gas analysis, or via the heating line until a -30°C cryogenic trap for the desiccated gas.

For all the experiments, the outlet gases were analysed online and quantified using two techniques: micro-gas chromatography (µGC, Varian CP2003-P) and Fourier Transform Infra Red spectroscopy (FTIR, Nicolet Magna-IR 550 series II). The µGC analyser was equipped with Molsieve 5Å and PoraPlot Q columns and the detection was assured by thermal conductivity detector (TCD) calibrated with standards of known composition. Depending on the experiment, the gas components identified were H2, CO, CO2, CH4, C3H8, C2H5OH, C2H2, C2H4, C2H6, and H2O.

The electrical diagnostics were performed by using Tektronix current and voltage probes (TCP202 and P5205, respectively). The signals from the probes were recorded on a transient digitizer (Tektronix TDS 3034B) and processed in a PC.

3. Results and discussion

The steam reforming process of hydrocarbons could be described by the main following reaction:

( ) nCOH2mnOnHHC 22mn ++→+ (1)

In conventional catalytic technology, this reaction is strongly endothermic (+206.16 and +497.72 kJ.mol-1 for CH4 and C3H8, respectively) and requires high temperature (700 - 1200 K) to be achieved. As shown in figure 4, thermodynamic calculations demonstrate that increasing temperature promotes the steam reforming reaction. Both CH4 and C3H8 steam reforming reactions take place at temperature higher than 600 K. Equilibrium is reached at about 800 K for C3H8 and at 1200 K for CH4. A higher temperature is necessary to activate methane.

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400 650 900 1150 1400Temperature (K)

Con

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rate

(%)

CH4

C3H8

Fig. 4: Conversion rate as a function of temperature (thermodynamic calculations)

By checking the stoichiometric conditions for each alkane, to be transformed, methane requires much less water than propane. In the case of methane the equilibrium is attain for a water/methane ratio equal to 1. Whereas for propane to be attained equilibrium requires water to propane ratio equal to 3. Therefore, the higher the number of carbon is, the higher the energy required to evaporate water. This condition promotes the use of methane as a source for hydrogen production.

The steam reforming reaction suffered from competitiveness with the cracking reaction described by the main following reaction:

nCH2mHC 2mn +→ (2)

For CH4 and C3H8 cracking reaction using conventional catalytic scheme, coke deposition is observed [6] even when reaction is carried out in the region expected from the equilibrium to be carbon free.

In plasma process, carbon deposit on the walls of the reactor and on electrodes is a serious problem decreasing the system efficiency [3, 7]. However, this reaction allows obtaining great purity hydrogen and avoids CO and CO2 production.

In the following we present the experimental data showing the CH4, C3H8, and C2H5OH conversion, the steam reforming and cracking selectivity, and hydrogen production as a function of parameters such as: inlet steam to carbon ratio and gas flow.

Figure 5 shows an example of FTIR spectrum obtained after plasma treatment of C3H8-H2O mixture. Similar spectra were observed in the case of CH4-H2O mixture.

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5001000150020002500300035004000wavenumber (cm-1)

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orba

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(u. a

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H2O C2H2CH4

C3H8

CH4

CO2 CO C2H4

C3H8

CH4

C2H4

C2H2

C2H2

CO2

Fig. 5: Typical FTIR spectrum of C3H8-H2O

Beside the main products of the steam reforming reaction such as H2, and CO, we should highlight the presence of CO2, CH4, and C2-hydrocarbons. Figures 6a and 6b shows examples of results (main species concentrations) obtained at a flow rate of about 100 L/min for CH4-H2O and C3H8-H2O mixtures, respectively.

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0 2 4 6 8 10[H2O]/[CH4] (%)

[H2]

,[CH

4],[C

O] (

%)

H2CH4CO

(a)

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[H2O]/[C3H8] (%)

[H2]

,[CO

],[C 3

H8]

(%)

H2COC3H8

(b)

Fig. 6: Main products of steam reforming of (a) CH4, and (b) C3H8. (Flow rate: 100 L/min).

One clearly notes that in the sliding discharge reactor, the propane results are completely different than the methane results. At an equivalent flow rate, the H2 and CO amounts were lowered as the initial C3H8 concentration increase (fig 6b) whereas they are constant in the case of CH4 (fig 6a). This comportment was demonstrated in a catalytic steam reforming process [6]. Also increasing the H2O to C3H8 ratio increases the no-transformed C3H8.

For the two hydrocarbons studies, the maximum H2 concentration obtained is about 50%. The H2 production is connected to hydrocarbons conversion rates which are calculated by using the reactions (1) and (2). Results are in the range 20-35% for CH4 and 7-30% for C3H8 (depending on flow rate and inlet concentrations figure 7). These low conversion rates were explained previously [3, 4] and are attributed to the design of the plasma reactor itself. We demonstrated that only 40 to 45 % of the injected gas mixture was treated by the plasma.

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8 Con

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Inlet [C3H8] (%)

Flow rate (L/min)

0.25-0.3

0.2-0.25

0.15-0.2

0.1-0.15

0.05-0.1

0-0.05

Fig. 7: C3H8 conversion rate

Figure 8 shows an example of CO2, CH4, and C2-hydrocarbons concentrations measured at the outlet of the plasma reactor as a function of the water to hydrocarbon ratio for CH4-H2O and C3H8-H2O mixtures, respectively. Results with methane (fig. 8a) show that the secondary species produced (CO2 and C2-hydrocarbon) exhibit concentrations lower than 1% and seems to be constant when varying the inlet amount. When the propane was used (fig. 8b), the situation is completely different. The concentrations of these species are higher (up to 6%) and depend strongly on the H2O to C3H8 ratio and total flow rate. It seems that the conversion of propane is initiated by the decomposition into lighter hydrocarbons, and then the steam reforming reaction occurs with the decomposed products. In plasma discharge reactor, CO2 production could be prevented under high steam to propane ratio (fig. 8b). These results could be compared to those of Sekine et al [7] who suggested a reaction scheme for the transformation of methane into

acetylene. They demonstrated that the main products in the steam reforming process of hydrocarbons were H2, CO and C2H2.

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0 2 4 6 8 10[H2O]/[CH4] (%)

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cent

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ns (%

)

CO2C2H4C2H6C2H2

(a)

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6

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0 5 10 15 20[H2O]/[C3H8] (%)

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cent

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ns (%

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CO2C2H4C2H2CH4

(b)

Fig. 8: By- products of steam reforming of (a) CH4 and (b) C3H8

The effect of the inlet parameters on the steam reforming and the cracking reactions selectivity was studied. In the case of CH4, for H2O to CH4 ratio lower than 2.5, only the steam reforming reaction takes place. The cracking reaction appears for the inlet ratio higher than 2.5. An example of the results for the propane is given in figure 9.

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Flow rate (L/min)

0.5-0.6

0.4-0.5

0.3-0.4

0.2-0.3

0.1-0.2

0-0.1

Fig. 9: Steam reforming selectivity for C3H8

The experiments with ethanol were performed at an average input power of about 70 W calculated from voltage and current measurements. The chemical analysis was performed in "dry gas" and in "wet gas". An example of results is displayed in figure 10. The species detected were: H2, CO, CO2, CH4, C2-hydrocarbons, and reactive species (C2H5OH and H2O). The mole fractions of CO and CO2 depended on the inlet composition while those of H2 remained constant. The highest mole fractions of H2, CO, CO2, CH4 obtained in the "dry gas" study are 72%, 28%, 12%, and 5%, respectively. In that case, the concentration of C2-hydrocarbon species stays below 5%. Comparison of these chemical analysis show that the mole fraction of the no-condensed species are two times greater in the case of "dry gas" than in the case of "wet gas".

The energy balance was estimated and the species mole fractions are expressed in terms of progress variables of the two global reactions

C2H5OH + 3H2O → 2CO2 + 6H2 (3)

C2H5OH + H2O → 2CO + 4H2 (4)

Chemical interpretation of the results in term of reactions (3) and (4) showed that parallel reactions could explain the conversion of the inlet ethanol and water. For the lowest inlet ethanol

mole fractions, the progress values of the two reactions were nearly the same. For the highest ethanol mole fraction, the steam reforming reaction becomes negligible.

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Mol

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H2COCO2

(a)

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C2H5OH / H2O (%)

Mol

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H2OC2H5OHH2COCO2

(b)

Fig. 10: Main products of steam reforming of ethanol. (a) "Dry gas" and (b) "Wet gas"

4. Conclusion An experimental investigation on the steam reforming of methane, propane, and ethanol was

carried out by non-thermal plasma (sliding discharge and direct discharge) at atmospheric pressure.

Results showed that hydrocarbons conversion, steam reforming selectivity, cracking selectivity, and hydrogen production depend on the nature of the hydrocarbon, the inlet steam to carbon ratio, and the gas flow rate. In all studied cases methane steam reforming selectivity was higher than the propane steam reforming. The methane conversion rate was higher than those of propane in the same experimental conditions. Besides the main products of the plasma treatment such as H2 (50%), CO (up to 30%) and no-consumed CH4 or C3H8 (depending on the experiment), we should highlight the presence of C2-hydrocarbons (C2H2, C2H4, C2H6,) and CO2. Propane treatment allows producing a high amount of CO2 and light hydrocarbons (concentration up to 6%) compared to 1% obtained in the case of methane.

In the case of ethanol, the experiments were conducted in a direct discharge at atmospheric pressure with a liquid ethanol/water mixture heated by the electrodes. The ethanol and water mole fractions ratio of the inlet mixture studied was in the range from 0-0.72. The highest mole fractions of H2, CO, CO2, CH4 obtained are 72%, 28%, 12%, and 5%, respectively. The mole fractions of CO and CO2 depend on the inlet gas composition while those of H2 concentration remained constant.

5. References [1] F. Auprêtre, C. Decorme, D. Duprez., Catal. Commun., 3, 2002, pp 263-267. [2] G.A. Deluga, J.R. Salge, L.D. Schmidt, X.E. Verykios, Science, 303, 2004, pp 993-997. [3] F. Ouni, A. Khacef, J. M. Cormier, Chem. Eng. Technol, 29(5), 2006, pp 1-6. [4] I. Rusu, J.M. Cormier, Chem. Eng, J., 91, 2003, pp 23-31 [5] O. Aubry, C. Met, A. Khacef, J.M. Cormier, Chem. Eng. J., 106(3), 2005, pp 241-247 [6] S. Ayabe, H. Omoto, T. Utaka, R. Kikuchi, K. Sasaki, Y. Teraoka, K. Eguchi, Appl. Catal.

A: General 241, 2003, pp 261–269 [7] Y. Sekine, K. Urasaki, S. Kado, M. Matsukata, E. Kikuchi, Energy & Fuels, 18, 2004, pp

455-459.