Production of Hydrogen-Rich Synthesis Gas by Pulsed Atmospheric Plasma Submerged in Mixture of Water with Ethanol
Hydrogen-rich synthesis gas was produced by pulsed dc plasma submerged into ethanol–water mixtures using an original system with a coaxial geometry. The ignition of the discharge is immediately followed by production of hydrogen and after a short time necessary for filling the outlet tubing a flame can be ignited. No auxiliary gas was used for the reforming process. The synthesis gas containing up to 60% of hydrogen was formed, at the outflow rate of 250 sccm at the average power as low as 10 W. The hydrogen production efficiency corresponds to 12 kWh/kg H2.
KeywordsPlasma discharge in liquid Hydrogen production Synthesis gas Ethanol reforming Pulsed plasma
Rapid development of non-thermal (cold) atmospheric plasma sources since last century led to a new field of research and applications of the plasma discharges submerged into liquids. Among many applications of such systems for e.g. coatings or production of nanoparticles and nanotubes a number of recent reports has been devoted to plasma-chemical applications with main focus in the production of hydrogen [1, 2, 3, 4, 5]. It is interesting to note that first studies of water electrolysis and interactions of glow discharges with water solutions were described already in 1887 by Gubkin , followed in 1952 by Davies and Hickling . The submerged arc discharges have been used already since 1930 for an underwater welding . In about two last decades the growing interest in clean energy carriers have dramatically enhanced the development of different methods for production of hydrogen as an environmentally friendly alternative fuel. The utilization of gas discharge plasmas for this purpose, the submerged plasmas in particular, shows very promising results [9, 10, 11, 12]. Plasma-based processes using ethanol with water as the source of hydrogen are of particular interest because of accessible bioethanol as a waste product. Small reactors powered by renewable electric power sources could be very useful in different applications with hydrogen-on-demand.
This paper demonstrates production of a hydrogen-rich synthesis gas H2 + CO, using an original small and low power plasma system submerged in a mixture of water with ethanol. Our systems that are predecessors of the design shown in this paper were already used for generation of the plasma discharges in liquids and were introduced in [13, 14, 15, 16, 17, 18].
Several types of power generators were tested in the system with pure water, the 13.56 and 27.12 MHz rf generators, 2.4 GHz microwave generator, 0.1 µs pulsed dc generator and 9 ns pulsed dc generator with variable repetition frequencies, in order to select the best generation with minimum heating of the solution. In this paper, the results of the experiments performed with 9 kV, 9 ns negative pulses (rise time of 2 ns) at the repetition frequency of 15 kHz, are presented. The repetition frequency of 15 kHz was selected from preliminary experiments, in order to keep the solution temperature below 60 °C and to avoid water vapor in the produced gas. On the other hand, the repetition frequency, representing the average power, needs to provide the high efficiency of the reforming process. The root mean square value of the pulsed current delivered to the plasma from the generator was measured by oscilloscope. The content of hydrogen in the produced gas was measured continually by a HY-AlertaTM detector and measured values were checked by and compared with the gas chromatograph GC-406 (Agilent Technologies) acquired measurements of gas samples using Tedlar® Gas Sampling Bags.
Results and Discussion
The following lines and bands were observed and monitored in the optical emissions from the discharges: hydrogen atomic lines Hα (2–3 transition, λ = 656.3 nm), Hβ (2–4 transition, λ = 486.1 nm) and Hγ (2–5 transition, λ = 434 nm), atomic oxygen lines (407.7, 615.7, 777 nm triplet, 821.2, 844.6, and 926 nm) and CO Ångström system [(0,2), λ = 519.8 nm]. The OH bands were not observed in the spectra with ethanol. However, it should be noted that OH bands were observed in the discharge in pure water.
Hydrogen Production Efficiency
Comparison of energy consumptions reported in production of hydrogen by different methods
Process used for hydrogen production
Natural gas or methane reforming with catalysts (4.5 m3 CH4/kgH2)
Production from coal (7.6 kg coal/kgH2, 1 kg coal ≈ 6.7 kWh)
Production using nuclear or hydro power
Electrolysis of water
Steam-oxidative reforming of bio-ethanol in Laval gliding arc
Gliding arc in methane + water + air
Dielectric barrier discharge in methane + air
Non-thermal arc torch 15 kV 0.2–0.7 A, ethanol + steam
High-power (≤5 kW) microwave atmospheric plasma in methane
High-power microwave plasma in wet ethanol (with steam)
Contact glow in methanol + water
Contact glow in ethanol + water
THIS WORK: nanosecond pulsed discharge in ethanol + water
It has been found that the nanosecond pulsed submerged plasma induces chemical reactions already at very low power with reasonably low heat losses. The plasma reforming of C2H5OH in water produces up to 60% hydrogen content in the resulting synthesis gas. The ignition of the discharge is followed by an instant start of the gas production, as well as the switching-off the power stops the gas production. Experiments indicate very low values of the energy consumption, about 12 kWh/kg hydrogen using short dc pulses with average power as low as 10 W. Moreover, the relevant production of the syngas at these conditions is up to 250 sccm. Keeping the temperature in ethanol mixtures with water roughly below 60 °C was found important to avoid vapor content in the produced gas. This is because of the lower boiling point of ethanol (about 78 °C) than that of water. Close to the ethanol boiling point the total flow rate of the produced gas can raise up to over 1500 sccm.
Obtained results confirm feasibility of using small size low power reactors as the compact bio-ethanol reformers producing hydrogen “on-demand”. These reactors could have a number of applications and they can be powered also from simple renewable energy generators.
Financial support by the J. Gust. Richert Foundation in Sweden and by the European Institute of Innovation and Technology, under the KIC InnoEnergy SynCon project is gratefully acknowledged. The authors are grateful to dr. M. Oestberg from Haldor Topsoe A/S in Lyngby, Denmark for help with GC measurements.
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