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

Transcription

1 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, 4567 Orléans Cedex 2, France 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 (1-5 ma, 5 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 H 2 (5%), (up to 3%) and no-consumed or C 3 H 8 (depending on the experiment). We should highlight the presence of C 2 -hydrocarbons (C 2 H 2, C 2 H 4, C 2 H 6,) and 2. 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 -72%. The highest mole fractions of H 2,, 2, 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, H 2 and syngas (a mixture of and H 2 ) 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 bioethanol) for energy production could be an effective solution for reducing 2 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 2. Experimental 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 15 C for all the experiments. Total flow rate of the gas mixture is in the range 8-12 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 nonequilibrium 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

3 intensity is produced. In the usual sliding discharges, the plasma thermalization was avoided by using external current limitation 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 Graphite Electrodes Water/Ethanol mixture Fig. 3: Schematic of the direct discharge reactor The plasma reactor was powered by a 5 Hz high voltage step-up transformer with leakage flux (AUPEM SEFLI, 1 kv, 155 ma). The ethanol and water mole fractions ratio of the inlet mixture studied was in the range 72%. The exhaust gas was sampled into two ways: via a 11 C heating line for humid gas analysis, or via the heating line until a -3 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 CP23-P) and Fourier Transform Infra Red spectroscopy (FTIR, Nicolet Magna-IR 55 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 H 2,, 2,, C 3 H 8, C 2 H 5 OH, C 2 H 2, C 2 H 4, C 2 H 6, and H 2 O. The electrical diagnostics were performed by using Tektronix current and voltage probes (TCP22 and P525, respectively). The signals from the probes were recorded on a transient digitizer (Tektronix TDS 334B) and processed in a PC. 3. Results and discussion The steam reforming process of hydrocarbons could be described by the main following reaction: ( n + m ) H n C H m no (1) n + In conventional catalytic technology, this reaction is strongly endothermic ( and kj.mol -1 for and C 3 H 8, respectively) and requires high temperature (7-12 K) to be achieved. As shown in figure 4, thermodynamic calculations demonstrate that increasing temperature promotes the steam reforming reaction. Both and C 3 H 8 steam reforming reactions take place at temperature higher than 6 K. Equilibrium is reached at about 8 K for C 3 H 8 and at 12 K for. A higher temperature is necessary to activate methane.

4 12 1 Conversion rate (%) 8 C 3 H Temperature (K) 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: C m n H m H nc (2) For and C 3 H 8 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 and 2 production. In the following we present the experimental data showing the, C 3 H 8, and C 2 H 5 OH 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 C 3 H 8 -H 2 O mixture. Similar spectra were observed in the case of -H 2 O mixture. Absorbance (u. a.) H 2O C 2H 2 35 C 3H 8 C 3H 8 C 2H 4 C 2H 2 2 C 2H wavenumber (cm -1 ) 1 C 2H Fig. 5: Typical FTIR spectrum of C3H8-O Beside the main products of the steam reforming reaction such as H 2, and, we should highlight the presence of 2,, and C 2 -hydrocarbons. Figures 6a and 6b shows examples of results (main species concentrations) obtained at a flow rate of about 1 L/min for -H 2 O and C 3 H 8 -H 2 O mixtures, respectively.

5 [H 2 ],[ ],[] (%) (a) CH4 [H 2 ],[],[C 3 H 8 ] (%) C3H8 (b) [H 2 O]/[ ] (%) [H 2 O]/[C 3 H 8 ] (%) Fig. 6: Main products of steam reforming of (a), and (b) C 3 H 8. (Flow rate: 1 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 H 2 and amounts were lowered as the initial C 3 H 8 concentration increase (fig 6b) whereas they are constant in the case of (fig 6a). This comportment was demonstrated in a catalytic steam reforming process [6]. Also increasing the H 2 O to C 3 H 8 ratio increases the no-transformed C 3 H 8. For the two hydrocarbons studies, the maximum H 2 concentration obtained is about 5%. The H 2 production is connected to hydrocarbons conversion rates which are calculated by using the reactions (1) and (2). Results are in the range 2-35% for and 7-3% for C 3 H 8 (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 4 to 45 % of the injected gas mixture was treated by the plasma C 3 H 8 Conversion rate Flow rate (L/min) Inlet [C 3 H 8 ] (%) Fig. 7: C 3 H 8 conversion rate Figure 8 shows an example of 2,, and C 2 -hydrocarbons concentrations measured at the outlet of the plasma reactor as a function of the water to hydrocarbon ratio for -H 2 O and C 3 H 8 -H 2 O mixtures, respectively. Results with methane (fig. 8a) show that the secondary species produced ( 2 and C 2 -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 H 2 O to C 3 H 8 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, 2 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

6 acetylene. They demonstrated that the main products in the steam reforming process of hydrocarbons were H 2, and C 2 H 2. Concentrations (%) (a) 2 C2H4 C2H6 C2 Concentrations (%) (b) 2 C2H4 C2 CH [H 2 O]/[ ] (%) [H 2 O]/[C 3 H 8 ] (%) Fig. 8: By- products of steam reforming of (a) and (b) C 3 H 8 The effect of the inlet parameters on the steam reforming and the cracking reactions selectivity was studied. In the case of, for H 2 O to 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..6 Steam reforming selectivity Inlet C 3 H 8 (%) Flow rate (L/min) Fig. 9: Steam reforming selectivity for C 3 H 8 The experiments with ethanol were performed at an average input power of about 7 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 1. The species detected were: H 2,, 2,, C 2 -hydrocarbons, and reactive species (C 2 H 5 OH and H 2 O). The mole fractions of and 2 depended on the inlet composition while those of H 2 remained constant. The highest mole fractions of H 2,, 2, obtained in the "dry gas" study are 72%, 28%, 12%, and 5%, respectively. In that case, the concentration of C 2 -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 C 2 H 5 OH + 3H 2 O H 2 (3) C 2 H 5 OH + H 2 O 2 + 4H 2 (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

7 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. Mole fraction (%) (a) C 2 H 5 OH / H 2 O (%) Mole fraction (%) 1 (b) O C2H5OH C 2 H 5 OH / H 2 O (%) Fig. 1: 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 H 2 (5%), (up to 3%) and no-consumed or C 3 H 8 (depending on the experiment), we should highlight the presence of C 2 -hydrocarbons (C 2 H 2, C 2 H 4, C 2 H 6,) and 2. Propane treatment allows producing a high amount of 2 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 The highest mole fractions of H 2,, 2, obtained are 72%, 28%, 12%, and 5%, respectively. The mole fractions of and 2 depend on the inlet gas composition while those of H 2 concentration remained constant. 5. References [1] F. Auprêtre, C. Decorme, D. Duprez., Catal. Commun., 3, 22, pp [2] G.A. Deluga, J.R. Salge, L.D. Schmidt, X.E. Verykios, Science, 33, 24, pp [3] F. Ouni, A. Khacef, J. M. Cormier, Chem. Eng. Technol, 29(5), 26, pp 1-6. [4] I. Rusu, J.M. Cormier, Chem. Eng, J., 91, 23, pp [5] O. Aubry, C. Met, A. Khacef, J.M. Cormier, Chem. Eng. J., 16(3), 25, pp [6] S. Ayabe, H. Omoto, T. Utaka, R. Kikuchi, K. Sasaki, Y. Teraoka, K. Eguchi, Appl. Catal. A: General 241, 23, pp [7] Y. Sekine, K. Urasaki, S. Kado, M. Matsukata, E. Kikuchi, Energy & Fuels, 18, 24, pp