Hydrogen Production from Hydrogenated Liquids Compounds by a Nonthermal

Transcription

1 International Journal of Plasma Environmental Science & Technology, Vol., No., SEPTEMBER Hydrogen Production from Hydrogenated Liquids Compounds by a Nonthermal Plasma K. Arabi, O. Aubry, A. Khacef, and J.-M. Cormier GREMI, Polytech Orléans, Université Orléans-CNRS (UMR), France Abstract The study presented is devoted to the plasma steam-reforming of ethanol, methanol, ammonia, and vegetable oil. A particular attention is directed to the ability to use the plasma technology for the upgrading of the biomass products. A stationary discharge is used in order to perform physical diagnostics and also chemical analysis. The arc is formed between two electrodes made of graphite. The Gas Chromatography (GC) and Fourier Transform Infra Red (FTIR) techniques are used to determine the concentrations of various species (H, CO, CO, CH and C ) and to evaluate the efficiency of the process The effects of total flow rate of the inlet liquid mixtures and the nature of the reactants (alcohol, NH, vegetable oil) on the produced species are studied. Keywords Steam reforming, syngas, alcohol, biomass, non-thermal plasma I. INTRODUCTION During the last decade, many studies have shown that H has a great potential as a fuel for electricity generation and transportation purposes. H is a renewable energy source and do not contribute to the green house effect [-]. Several studies have demonstrated the possibility to use catalytic reactors and alcohol as source of H [-9]. Some authors have also performed the possibility to use a plasma reactor for the hydrogen production with high H yields and limited costs from water and hydrocarbons, alcohol or biomass [-]. The steam reforming using non thermal plasma processes is studied many years ago [, 7]. This paper is devoted to a comparative study of syngas (H +CO) production from alcohol (methanol and ethanol) and from others hydrogenated liquids diluted or mixed in water such as vegetable oil or ammonia. We present the H concentrations and the energy costs from these various hydrogen sources. The non thermal plasma used is a laboratory scale experimental device static discharge, also called "Statarc". II. EXPERIMENTAL TECHNIQUES The experimental plasma reactor used in this work is described in a previous paper []. The laboratory-scale experiments are carried out with a liquid mixture injected at atmospheric pressure and room temperature (T ~ 9 K) into a non-thermal plasma reactor (Fig. ). The Non- Thermal Arc (NTA) reactor includes a quartz tube ( mm length and mm inner diameter) containing two electrodes made of graphite. The conical extremities of the two electrodes are settled opposite each other and the electrode gap is mm. The liquid mixtures are injected by using a syringe Corresponding author: Olivier Aubry address: olivier.aubry@univ-orleans.fr Power supply kv, HZ Syringe pump Graphite electrodes condenser discharge Gas chromatography pump, which permits to adjust the total liquid flow rate at the inlet of the reactor from about. to. sccm. The inlet liquid mixture is injected in the upper electrode. The graphite electrodes are heated by the discharge, the liquid is vaporized during its flow in the upper electrode. Because the inlet liquid is entirely vaporized, the moles of the reactants in the inlet liquid or in the inlet gas mixtures have the same values in the liquid phase and in the vapor phase, respectively. Then, the inlet gas mixture is injected in the discharge. The mole fraction ratios (alcohol/water) of the inlet liquid mixtures are between.7 and.9 and between. and.7 for the ethanol and methanol mixtures, respectively. The gas discharge is powered by a Hz highvoltage transformer with leakage flux. AUPEM SEFLI high voltage transformer: primary V, secondary kv, I = ma). The current and voltage waveforms are measured using a PSY Langlois probe and a differential voltage probe (Tepcel, DP), respectively. The signals are recorded on a Tektronix TDS A digital oscilloscope. From the current and voltage data, the power is calculated and the energy cost of production H can be estimated. The discharge is called NTA (Non Thermal Arc) []. Typical plots of voltage and current as functions of the time are given on Fig.. At a maximum of current (about ma), a quasi constant voltage is observed (9 V). Periodic high voltages peak are observed at the zero current transition of the alternating current. FTIR Fig.. Schematic reactor experimental. Presented at the Seventh International Symposium on Non- Thermal/Thermal Plasma Pollution Control Technology & Sustainable Energy, ISNTP-7, in June

2 Arabi et al. voltage (V) time (s) U (V) I (A) current (A) Fig.. Variations of the voltage and current as a function of the time, inlet liquid volume ethanol/water ratio =.. Chemical analyses of the exhaust gas are performed. To quantify the concentrations of the species in the dry gas, the exhaust gas is injected into a cryogenic trap (- C) then in a gas phase chromatography analyzer (GC- Varian CP ). The GC analyzer contains Å Molecular Sielve and Hayesep A columns equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID), respectively. The GC is calibrated with standards of known composition. Thus, H, CO, CO, CH, C H, C H and C H are easily quantified. Infra-Red Transform Fourier (FTIR) analyses are also realized on the wet gas at the outlet of the reactor to measure the non-consumed alcohol and to determine the alcohol conversion rate. III. RESULTS A. Alcohols steam reforming A.. Effects of the total flow rate and inlet alcohol concentration Fig. display the concentrations of the main produced species as a function of the total flow rate of the inlet liquid mixtures. For each alcohol, two concentrations of alcohol (in % liquid volume) in the liquid mixtures are studied. The main produced species are H, CO, CO and CH. C species are also produced but their concentrations are remaining lower than.%. The concentrations of H, CO, CO and CH are highly dependent on the total flow rate of the inlet liquid mixture. The H concentrations slightly decrease when the total flow rate increases for both alcohols and whatever the alcohol concentration in the inlet mixture. [H ] increases when the flow rate of the inlet liquid mixture decreases corresponding to a rise of the residence time in the plasma bulk. We can note that in the methanol mixtures, H is promoted in comparison to ethanol ones. This can be explained by the number of H atom available in the inlet mixture (this is discussed in Influence of the inlet atomic composition section). Moreover, a rise of the alcohol/water ratio (in % liquid volume) leads to a decrease of the H production. The produced CO and CO amounts depend on the inlet alcohol concentration. When the inlet alcohol concentration increases, [CO] increases too. The H (%) CO (%) CO (%) CH (%) methanol/water=. methanol/water=. ethanol/water=. ethanol/water=. Fig.. H, CO, CO and CH concentrations (T = 9 K) vs. the inlet total flow rate. Open symbols: methanol+water mixtures; full symbols: ethanol+water mixtures. Alcohol/water volume ratios: diamond symbols:.; square symbols:..

3 International Journal of Plasma Environmental Science & Technology, Vol., No., SEPTEMBER H /CO methanol/water=. methanol/water=. ethanol/water=. ethanol/water=. Fig.. H /CO vs. the inlet total flow rate. Open symbols: methanol+water mixtures; full symbols: ethanol+water mixtures. Alcohol/water volume ratios: diamond symbols:.; square symbols:.. TABLE I CONVERSION RATE (%) OF THE ALCOHOLS CALCULATED FROM THE PRODUCED WET GAS. TOTAL FLOW RATE OF THE INLET MIXTURE =. SCCM. Inlet mixtures Alcohol conversion rate (%) 9% vol. CH OH + H O 7. 7.% vol. CH OH + H O 79. 7% vol. CH OH + H O..% vol. C H OH + H O..% vol. C H OH + H O 9. 7% vol. CH OH + HO 9. influence of the inlet flow rate on [CO] variations highly depends on the inlet alcohol concentration. For the highest alcohol concentration, a rise of the inlet flow rate leads to a decrease of the outlet [CO]. When the alcohol/water ratio (in % liquid volume) is low, an increase of [CO] is observed in both cases. The variations of CO concentrations are the opposite of CO ones when the flow rate and or the inlet alcohol concentration increase. CO is promoted out of favor with CO when the inlet alcohol/water ratio (in % liquid volume) increases and/or the inlet liquid total flow rate decreases. CH is the main hydrocarbon produced; its maximum concentration in volume is about % in the ethanol mixtures. CH is promoted when the alcohol/water ratios and/or the total flow rate of the inlet liquid mixture increase. In the ethanol mixtures, the hydrocarbon species seem to be produced more easily. This corresponds to an increase of the inlet carbon number. Similar results are obtained for the C species, which are not presented here because their concentrations remain low. For the highest ethanol concentrations, deposits of carbon solid have been observed on the wall of the reactor. The wet gas is quantified by the FTIR technique. CO, CO and the remaining alcohol are measured. In the Table, the degrees of conversion of alcohols as functions of the alcohol concentration in the inlet mixtures (in % liquid volume) are reported. We can observe that the alcohol conversion rate is high for all the studied mixtures: > 7% for the ethanol mixtures and > % for the methanol mixtures. Moreover, when the inlet alcohol concentration increases in the inlet liquid mixture, the conversion rate increases too. In Fig., the H /CO ratio is displayed as functions of the flow rate and the alcohol concentration of the inlet liquid mixture. The H /CO ratio is a parameter to determine the quality of the produced syngas. For example, to use the syngas in fuel cells applications CO must to be avoided. If the syngas is injected in turbines, the amount of the produced CO can be larger. At a given liquid flow rate of the inlet mixture, the H /CO values are not dependent of the nature of the alcohol. Moreover, the inlet alcohol/water volume ratios (in % liquid volume) have effects on the H /CO levels. For the lowest studied alcohol/water ratios, the H /CO ratios have the higher values, with a value of about.. The flow rate of the inlet liquid mixture is an important parameter to determine the H /CO levels. An increase of the flow rate from. to. sccm leads to a decrease of the H /CO ratio from about. to for an alcohol/water inlet volume ratio equals to.. For the highest alcohol/water ratio studied, the H /CO values remain at a about constant level. A.. Influence of the inlet atomic composition In Fig., the main produced species concentrations as functions of the inlet H/C and H/O ratios are reported. The ratios (H/C and H/O) are calculated from the composition of the inlet mixtures (alcohol+water) with the following hypothesis: The moles of the alcohol in the inlet liquid mixture and in the vapor mixture are the same. All the injected liquid mixture in the upper electrode is vaporized. One can observe that the variations of H/C and H/O ratios have antagonists effects on the concentrations of the produced species. An increase of the inlet alcohol concentration leads to a decrease of H/C and a rise of H/O values. H and CO concentrations increase when H/C increases or H/O decreases. CO and CH concentrations decrease for H/C increasing or H/O decreasing. For identical mole fractions of ethanol or methanol in the inlet liquid (alcohol+water), the amount of hydrogen atoms in the inlet mixture is higher in the case of the methanol mixture. To calculate the amount of the inlet H atom, it is necessary to take into account the number of H in the alcohol and the number of H in the water. If in the inlet liquid mixture, there is a moles of alcohol and b moles of water, the available H, C and O atoms in the inlet mixtures are calculated: ac H OH + bh O ach OH + bh O C = a, H = a + b, O = a + b C = a, H = a + b, O = a + b One can note that the amount of H in the inlet mixture is higher in the methanol mixtures. This explains why H can be more produced from steam reforming with methanol. In Fig., we can note that the main parameter to determine the produced H concentration is the amount of H in the inlet mixture in comparison to the number of carbon and oxygen atoms. For a given inlet H/C ratio,

4 Arabi et al. H (%) H (%) CO (%) CO (%) CO (%) CO (%) CH (%) CH (%) inlet H/C..... inlet H/O Fig.. H, CO, CO and CH concentrations as functions of inlet H/C and H/O values. Total flow rate =. sccm. Open symbols: methanol+water mixtures; full symbols: ethanol+water mixtures. H /CO H /CO inlet H/C inlet H/O Fig.. H /CO as functions of inlet H/C and H/O values. Total flow rate =. sccm. Open symbols: methanol+water mixtures; full symbols: ethanol+water mixtures. whatever the injected alcohol (ethanol or methanol), a same amount of H is obtained in the dry gas. A H +CO gas enrichment of the exhaust gas is promoted when the alcohol concentration increases in the inlet mixture. Indeed, for given inlet H/C and H/O ratios and for both alcohols, the concentrations of H, CO and CO in the exhaust dry gas are measured. We observe that an increase of the alcohol concentration in the inlet mixture leads to a decrease of the CO concentration and a rise of the sum H +CO. H +CO grows from.9% to.% when the inlet alcohol concentration (in % liquid volume) increases from 9% to 7%, respectively, in the ethanol mixtures. In the methanol mixtures, the sum H +CO rises from.% to 9.% when the inlet alcohol concentration (in % liquid volume) rises from 9% to 7%, respectively. The nature of the inlet mixtures, for given H/C and H/O values, have not significant effects on the species concentrations of main produced species: H, CO and CO. On the other hand, the hydrocarbons species (CH

5 7 International Journal of Plasma Environmental Science & Technology, Vol., No., SEPTEMBER TABLE II PRODUCED SPECIES CONCENTRATIONS IN DRY GAS FROM AMMONIA AND VEGETABLE OIL MIXTURES Inlet mixtures H/O H/C flow rate (sccm) H (%) CO (%) CO (%) CH (%) % vol. NH + H O Pure Vegetable oil % vol. Vegetable oil + H O NH NH NH CO Fig. 7. Outlet dry gas FT-IR spectrum in NH +H O mixture. is a representative molecule of these species) are promoted in the ethanol mixtures whatever the inlet H/C or H/O values in comparison to the methanol ones. In Fig., the values of the H /CO ratios are reported as functions of the inlet H/O and H/C values. The H /CO ratio increases when H/O decreases and/or H/C increases, corresponding to an increase of the alcohol concentration in the inlet mixture. B. Influence of the reactants nature Experiments with others reactants have been also performed. The liquid mixtures studied are: ammonia in water and a vegetable oil diluted or not in water. In the Table, we report the studied mixtures and the GC concentrations of the main produced species in the outlet dry gas. The main produced dry gas is H balanced by N. Ammonia could be an interesting hydrogen source by reforming in-situ because NH is easily transportable and storable. The H concentration obtained from the ammonia mixture is slightly lower than from alcohol ones for identical flow rate of the inlet liquid mixture and inlet H/O: about -% in alcohols mixtures against % with NH. If NH is injected in the water, a rise of the produced dry gas flow rate ( ml/min) is obtained compared to the produced dry gas in a pure water experiment (.7 ml/min). If pure water is injected in the reactor, only H and CO are produced in the dry gas and their concentrations are 7.% and.%, respectively. Fig. 7 displays a FTIR spectrum from NH in the inlet mixture. Only NH and a very low amount of CO are observed in the outlet dry gas. CO is produced from reactions between the graphite electrodes and the oxygenated species produced in the discharge (H O, OH, O-atom, ). The consumption rate of the graphite electrodes has been calculated in a GREMI internal study []. The consumption rate is about. -7 mol/min, which is a very low rate in comparison to the flow rate of the inlet liquid mixture. The composition of the dry gas is not the same that in the pure water experiments, there is an important effect of NH on the produced gas. Thus, CO is not detected but CO is produced when NH is added to the water. The production of CO and CO in these conditions can be explained by a consumption of the graphite electrodes. One can note that a main advantage of the ammonia mixture is that no hydrocarbon is produced because no carbon atom is injected in the discharge. In the case of the vegetable oil mixtures, we observe that a mixing of the oil vegetable in water does not modify the H concentration in the produced dry gas. Moreover, an opposite effect of the mixing on the H concentrations is observed in the case of the vegetable oil mixtures in comparison to the results obtained from the mixtures with alcohol. When the vegetable oil is mixed in the water, the H concentrations slightly decrease, the concentration of CO highly increases and the CO concentration decreases in comparison to the results obtained with the pure oil liquid. The effects of the water mixed to the vegetable oil on the CO and CO concentrations are antagonists to ones obtained when the alcohol is diluted in water. These results must to be confirmed by complementaries experiments where the flow rate of the injected liquid (oil or oil+water) will be identical to the experiments with NH or the alcohols. Moreover, in the case of the pure vegetable oil some solids products have been observed on the reactor walls. Deposits are also observed as in the experiments with the ethanol mixtures. In the experiments with the ethanol mixtures, the deposits are only observed for the highest ethanol concentrations in the inlet mixture. Nevertheless, the amount of the deposited solid seems to be very low in all the cases. No analyses of the solid phase have been performed

6 Arabi et al. Syringe pump Gas volume gas TABLE III H PRODUCTION ENERGY COSTS, EC, KWH PER KG-H TOTAL FLOW RATE =. SCCM Power supply kv, HZ Column Conductance (A/V *)... Graphite electrodes C. Energy cost condenser discharge water Fig.. Schematic experimental set-up to determine EC H... Xa/Xw=. Xa/Xw=.7. Liquid flow rate (ml/h) Fig. 9. Conductance of the plasma column as a function of the inlet liquid flow rate; in methanol-water mixtures (diamond symbols: mole fraction alcohol to water ratio =.; diamond symbols: mole fraction alcohol to water ratio =.7). From the experiments, the energy cost per kg of produced H, EC H, is calculated. EC H evaluates the efficiency of the processes and is expressed as follows: W EC H m H where W is the electrical energy consumed per hour to produce a given mass of H (m H ). The electrical power is obtained from electrical measurements. Fig. displays the experimental set-up diagram to evaluate the energy cost. The dry gas is injected in a volume containing initially some water. After a given time, a part of the water volume is removed by the gas. For a given volume of produced dry gas, the H concentration is known. Thus, we can determine the mass of produced H and calculate the energy cost. We observe that the nature of the alcohol does not play a role on the amount of the produced dry gas. If the concentration of the alcohol increases in the inlet mixtures, the rate of the production of the dry gas increases too. The amount of the produced dry gas for the lowest alcohol volume in the inlet liquid mixtures is ml/min, from the ethanol mixture or from the methanol mixture. When the ethanol or methanol/water volume ratio is.9, the flow rate of the produced dry gas is 7 ml/min. When the alcohol/water ratio is equal to.7, the flow rate of the produced dry gas is ml/min. In the Table, the EC H are reported for various inlet mixtures. We can note that the EC H is highly dependent on injected reactants for about the same inlet H/O values (mixtures n, and ). In the alcohol mixtures, EC H is n Inlet mixtures H/O H/C % vol. NH + H O 9% vol. CH OH + H O 7.% vol. CH OH + H O 7% vol. CH OH + H O.% vol. C H OH + H O.% vol. C H OH + H O 7% vol. CH OH + H O lower than for the ammonia one. On the other hand, for a given H/O, EC H do not seem to be affected by an increase of the H/C (mixtures n and ). We observe that high H concentration levels can be obtained in various liquids inlet conditions. The energy cost to produce H by our plasma device is in the same order of magnitude that others techniques. For example, Bromberg et al. [9] have obtained an energy cost at about 7 kwh/kg-h using a plasmatron for a steam reforming of ethanol. In the case of the steam reforming from methanol mixtures using a corona discharge, Liu et al. [] have an EC H equals to kwh/kg-h. These values can be compared to the electrolysis to produce H (EC H = kwh/kg-h ). In our experiments, the energy costs can be higher. In the "Statarc" experimental device, there are power losses because electrode voltage drops. Nevertheless, this reactor is interesting because we can to perform easily optical emission spectroscopy diagnostics on this stationary discharge and determine the gas and electronic temperatures. The variations of the injected liquid mixtures parameters, inlet alcohol concentration and/or the flow rate, lead to changes on the consumed power. If ethanol or methanol is added in the inlet liquid mixture, the electrical resistance in the plasma is varied. In Fig. 9, the conductance values as functions of the total flow rate of the inlet liquid mixture are reported. A rise of the alcohol concentration in the inlet mixture leads to a rise of the conductance in the plasma column. For a given inlet alcohol concentration, an increase of the flow rate of the liquid mixture injected leads to a decrease of the conductance. Similar results are obtained when the ethanol is in the inlet mixtures. The power changes like the conductance because the current is constant in this experimental device. IV. CONCLUSION H (%) EC H (kwh/kg-h ) From the above experimental results, an interesting rate of produced H is observed by using a non thermal arc treatment. The CO and CO concentrations are in

7 9 International Journal of Plasma Environmental Science & Technology, Vol., No., SEPTEMBER accordance with usual thermo-chemical techniques. The nature of the inlet reactant and its concentration are studied and the effects of the inlet atomic composition on produced species are presented. The energy cost is relatively high compared to the industrial requirements in the field of hydrogen production. However, the laboratory technology described above will really be of a great interest in understanding the physicochemical properties of non-thermal plasmas. The stationary non-thermal arc is particularly interesting for the direct treatment of hydrogenated liquids and hydrocarbons by plasma. The experimental results can easily be used as a basis for modeling and thereby define the scaling methods to consider industrial applications. [] I. Rusu and J.-M. Cormier, "On a possible mechanism of the methane steam reforming in a gliding arc reactor," Chemical Engineering Journal, vol. 9, pp. -,. [7] I. Rusu and J.-M. Cormier, "Study of a rotarc plasma reactor stability by means of electric discharge frequency analysis," International Journal of Hydrogen Energy, vol., pp. 9-,. [] P. A. Cormier, internal report. [9] L. Bromberg, D. R. Cohn, A. Rabinovich, N. Alexeev, A. Samokhin, K. Hadidi, J. Palaia, and N. Margarit-Bel "Onboard Plasmatron Hydrogen Production for Improved Vehicles," MIT Plasma Science and Fusion Center, JA--,. [] X. Z. Liu, C. J. Liu, and B. Eliasson, "Hydrogen Production from Methanol Using Corona Discharges," Chinese Chemical Letters, vol., pp. -,. REFERENCES [] F. Barbir, "PEM electrolysis for production of hydrogen from renewable energy sources," Solar Energy, vol. 7, pp. -9,. [] S.-H. Jensen, P. H. Larsen, and M. Mogensen, "Hydrogen and synthetic fuel production from renewable energy sources," International Journal of Hydrogen Energy, vol., pp. - 7, 7. [] A. Ajanovic, "On the economics of hydrogen from renewable energy sources as an alternative fuel in transport sector in Austria," International Journal of Hydrogen Energy, vol., pp. -,. [] G. Busca, T. Montanari, C. Resini, G. Ramis, and U. Costantino, "Hydrogen from alcohols: IR and flow reactor studies," Catalysis Today, vol., pp. -, 9. [] M. A. Goula, S. K. Kontou, and P. E. Tsiakaras, "Hydrogen production by ethanol steam reforming over a commercial Pd/[gamma]-Al O catalyst," Applied Catalysis B: Environmental, vol. 9, pp. -,. [] A. J. Vizcaíno, A. Carrero, and J. A. Calles, "Hydrogen production by ethanol steam reforming over Cu-Ni supported catalysts," International Journal of Hydrogen Energy, vol., pp. -, 7. [7] J.-S. Suh, M.-t. Lee, R. Greif, and C. P. Grigoropoulos, "A study of steam methanol reforming in a microreactor," Journal of Power Sources, vol. 7, pp. -, 7. [] C. Horny, L. Kiwi-Minsker, and A. Renken, "Micro-structured string-reactor for autothermal production of hydrogen," Chemical Engineering Journal, vol., pp. -9,. [9] G. A. Deluga, J. R. Salge, L. D. Schmidt, and X. E. Verykios, "Renewable Hydrogen from Ethanol by Autothermal Reforming," Science, vol., pp ,. [] H.-L. Tsai, C.-S. Wang and C.-H. Lee, "Hydrogen production in a thermal plasma hydrogen reformer using ethanol steam reforming," Journal of the Chinese Institute of Engineers, vol., pp. 7-,. [] O. Aubry, C. Met, A. Khacef, and J. M. Cormier, "On the use of a non-thermal plasma reactor for ethanol steam reforming," Chemical Engineering Journal, vol., pp. -7,.. [] T. Paulmier and L. Fulcheri, "Use of non-thermal plasma for hydrocarbon reforming," Chemical Engineering Journal, vol., pp. 9-7,. [] G. Petitpas, J. D. Rollier, A. Darmon, J. Gonzalez-Aguilar, R. Metkemeijer, and L. Fulcheri, "A comparative study of nonthermal plasma assisted reforming technologies," International Journal of Hydrogen Energy, vol., pp. -7, 7. [] J.-D. Rollier, J. Gonzalez-Aguilar, G. Petitpas, A. Darmon, L. Fulcheri, and R. Metkemeijer, "Experimental Study on Gasoline Reforming Assisted by Nonthermal Arc Discharge," Energy & Fuels, vol., pp. -, / 7. [] B. Sarmiento, J. J. Brey, I. G. Viera, A. R. Gonz 疝 ez-elipe, J. Cotrino, and V. J. Rico, "Hydrogen production by reforming of hydrocarbons and alcohols in a dielectric barrier discharge," Journal of Power Sources, vol. 9, pp. -, 7.