i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 0 9 3 4 e2 0 9 3 8
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Hydrogen production by low-temperature plasma decomposition of liquids* N.A. Bulychev a, M.A. Kazaryan a, A.S. Averyushkin a, A.A. Chernov b, A.L. Gusev c,* a
P.N. Lebedev Physical Institute of RAS, 53 Leninsky Ave., Moscow, 119991, Russia Moscow Physical-Technical Institute, 9 Institutsky per., Dolgoprudny, Moscow reg., 141700, Russia c Scientific Technical Centre “TATA”, 29 Moscow Str., Sarov, Nizhny Novgorod reg., 607181, Russia b
article info
abstract
Article history:
The paper shows, that a low-temperature plasma initiated in liquid media in interelectrode
Received 27 September 2016
discharge gap is able to decompose hydrogen containing organic molecules resulting in
Accepted 28 September 2016
obtaining gaseous products with volume part of hydrogen higher than 90% (up to gas
Available online 29 July 2017
chromatography data). Tentative assessments of energy efficiency, calculated with regard for hydrogen and feedstock heating value and energy consumption, have shown efficiency
Keywords:
factor of 60e70%, depending on the source mixture composition. Theoretical model cal-
Hydrogen production
culations of discharge current and voltage have been performed; the values are in good
Plasma
accordance with experimental data.
Ultrasonic cavitation
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen Electrolysis
Introduction
Experimental
The paper shows that a low-temperature plasma initiated in liquid media in interelectrode discharge gap is able to decompose hydrogen containing organic molecules resulting in obtaining gaseous products with volume part of hydrogen higher than 90% (up to gas chromatography data). Tentative assessments of energy efficiency, calculated with regard for hydrogen and feedstock heating value and energy consumption, have shown efficiency factor of 60e70%, depending on the source mixture composition. Theoretical model calculations of discharge current and voltage have been performed; the values are in good accordance with experimental data.
One of the issues in modern alternative energy of the day is development of methods and technologies of hydrogen production to be used as fuel. The most common used technologies of hydrogen production today are steam reforming of methane and electrolysis. The advantage of methane steam conversion is high value of energy efficiency (60e80%); however it requires bulky and expensive equipment, and it also consumes methane, which is fuel and valuable feedstock for chemical industry itself. Water electrolysis is less expensive in terms of capital investments, but electrolysis process industrial efficiency has
*
This paper is the English version of the paper reviewed and published in Russian in International Scientific Journal for Alternative Energy and Ecology “ISJAEE”, 2013, issue 126, number 05/2, date 03.06.2013. * Corresponding author. E-mail address:
[email protected] (A.L. Gusev). http://dx.doi.org/10.1016/j.ijhydene.2016.09.226 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 0 9 3 4 e2 0 9 3 8
Nomenclature Fig L lcav E U m q v g t l(t) lcav Fion Uth lc Ucth Ufth Ei
Figure Distance between electrodes Size of cavitation area electric field strength, V/m electric potential difference, V mass of electron charge of electron gravitational acceleration, m/c2 time, c Distance, which electron passes at time t, m Time, in which the electron passes the cavitation area, c Energy of ionization, J Threshold potential difference, V Electron free path length, m Threshold potential difference with cavitation, V Threshold potential difference without cavitation, V Electric field strength in the interior area of liquid, V/m Linear dimension of atom, m
Greek letters v electron velocity in electric field, m/c Superscripts and subscripts cav Cavitation ion Ionization th Threshold Abbreviations/Acronyms M Discharge electrodes material
almost attained estimated performance nowadays and it ranks considerably below methane steam conversion in speed and energy efficiency; besides, electrolysis requires preliminary water treatment. Therefore there is a necessity for developing of alternative hydrogen production methods from varying available feedstock. Pilot experiments have allowed finding out that new form of electrical discharge could be in existence in liquid in intensive ultrasonic field above cavitation threshold, characterized by volume glow throughout interelectrode gap and by increasing current-voltage characteristic peculiar to abnormal glow discharge in gas [1]. Such microbubbles' extended interface discharge could be of interest for generating new acousto-plasma-chemical processes, as extended gas-liquid interface leads to diffusion fluxes of reactive species augmentation from plasma into liquid. It is potentially possible to conduct a large number of new chemical reactions [2e4]. Pilot experiments have shown that solid-phase carbonbearing products are derived as a result of liquid hydrocarbons decomposition reaction in acousto-plasma discharge, as well as chemical conversions in liquid phase take place and hydrogen-containing combustible gas is generated. Acoustoplasma technique involves the initiating of an electric discharge in a liquid that goes along with ultrasonic
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power assisted cavitation. In general, the specially designed acoustoplasma reactor includes a reservoir with two immersed electrodes and ultrasonic cavitator. Cavitation bubbles in the liquid phase provide the outstanding characteristics of a discharge and these characteristics have been proven to be governed by the regimes of ultrasonification. In other works, it was demonstrated that ultrasonic cavitation itself is a promising way for modification of properties of solid nano- and microparticles [5e10]. Furthermore, ultrasonic power prevents the secondary agglomeration of nanoparticles being synthesized from metal electrodes in electric discharge. The scheme of experimental setup is shown in Fig. 1. The setup consists of a reaction chamber, where discharge electrodes and an ultrasonic irradiator are introduced, a generator of high-voltage pulses for discharge initiation, a power supply of discharge in liquid, an ultrasonic generator and blocks of control of electric and acoustic characteristics. The chamber is provided with quartz windows for the observation of dynamic processes and registration of optical spectra of visible discharge glow. The ultrasonic generator with a piesoceramic transducer provides the output acoustic power up to 2 kW in the frequency range 27e44 kHz. The parameters of acoustic equipment allow one to implement the intensity of an ultrasonic field in the volume of liquid up to 10 W/cm2 and to vary cavitation regime in a wide range. Optical spectroscopy observations allowed to evaluate the composition of reaction mixture (Fig. 2). The analysis of the gaseous products of reactions in acoustoplasma discharge in liquid media confirmed the optical spectroscopy data and revealed that the main product gas was hydrogen. Results of the gas mixture analysis demonstrate that the decomposition of water in acoustoplasma discharge leads to formation of almost pure hydrogen (98%) (see Fig. 3); decomposition of organic liquids produces gas that also comprises carbon oxides, but their concentration does
Fig. 1 e Scheme of experiment: 1- liquid medium; 2ultrasonic sonotrode; 3-cavitation area.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 0 9 3 4 e2 0 9 3 8
oxygen ionization). Equation of electron motion in electric field with strength E ≡ U/L, where U is electric potential difference, has a very simple form: dv q U ¼ 0 g; dt m L
(1)
where q and m are electronic mass and charge, v is electron velocity in electric field. Considering that electron rests inside the bubble at reference time, Eq. (1) could be written down as follows: nðtÞ ¼ gt:
(2)
Distance l(t), which electron passes at time t, is expressed through formula: gt2 : 2
Fig. 2 e Optical spectra of plasma discharge in water.
lðtÞ ¼
Rel. Unit
In this way electron passes cavitation area length lcav at time tcav, correlated with с lcav by formula:
42
tcav ¼
36
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2lcav =g:
(3)
(4)
Inserting (4) into (2), velocity vcav, which electron gains on passing cavitation area in length lcav overall, could be formulated. If electron kinetic energy ranks over ionization energy of atoms containing liquid Fion, then electrode potential difference level could be considered as threshold for high-voltage discharge ignition, i.e.,
30 24 18 12
mv2cav 3 Fion 2
6
This is potential difference level Uth for discharge ignition equation. Now, from Eqs. (2), (4) and (5):
(5)
0 0:00
0:10
0:20
0:30
0:40
0:50 Time,
Fig. 3 e Gas chromatography analysis of gas mixture composition obtained from ethanol in plasma. Main peak belongs to hydrogen.
not exceed 5e6%, as a major amount of carbon during the decomposition of organic liquids forms solid particles.
Theoretical To improve understanding of this phenomenon, it is necessary to carry out ultimate analysis of this discharge's whence. Ultrasonic field, generated by trembling elastic waveguide, leads to cavitation in this liquid. Cavitation area is characterized by a large number of bubbles generated as a result of liquid's rupturing. There are vapor and gas at high temperature and pressure inside these bubbles during ionization process attended with generation of electrons. It is necessary to determine voltage is lowest value at which discharge phenomena is possible, i.e. there is a fulfillment of conditions, under which electron has time to accumulate energy exceeding proper value, necessary for ionization of generating liquid atoms (in case of water, it is the energy of hydrogen or
Uth ¼
LFion : qlcav
(6)
With interelectrode gap increase threshold value increases, and it decreases with the increase of cavitation area size. The previously considered simple model allows explaining the observed phenomenon qualitatively. As long as a condition for discharge ignition in liquid with no cavitation is sine qua non for electron to gain energy, sufficient for ionization over the mean free path lc, which is generally much less than lcav, sometimes the discharge effect could occur in cavitation area and on its boundaries. And if a distance between electrode and cavitation area ranks over lc, then electron in this area has no intercollision time to gain momentum up to ionization energy, and there is no discharge; that could lead to detectable reduction in total current discharge as compared to discharge with no cavitation phenomenon. Simple formula, connecting potential difference threshold values for discharge ignition in liquid with cavitation (Ucth ) or f without it (Uth ), could be derived from Eq. (6): Ucth ¼
lc f U : lcav th
(7)
Thus, cavitation decreases significantly potential difference values necessary for discharge in fluid.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 0 9 3 4 e2 0 9 3 8
The mean free path at the beginning of Townsend ionization lc from Eq. (7) could be expressed by means of permittivity constant of liquidε, linear dimension of atom а, undergoing ionization, and other medium parameters. To do this let's make use of boundary condition of dielectric density equation at electrode-liquid interface . f eEi ¼ Uth L;
(8)
where Ei is electric field strength in the interior of liquid for values close to threshold potential of discharge inception. On f the other hand, Uth value with no cavitation is determined by Eq. (6), in which lc instead of lcav should be applied. Expression for lc in terms of liquid and electric field characteristics could be deduced: lc ¼
Fion : qEi ε
(9)
Electric field strength in (9) could be expressed in terms of ionization energy of atoms containing liquid Fion, taking into account that at the instant of ionization atom absorbs the following quantity of electric field energy: Fion ¼ E2i a3 48:
(10)
Resultant expression for lc could be derived from Eq. (9) with the aid of Eq. (10):
lc ¼
pffiffiffiffiffiffiffiffiffiffi aFion ffi a: pffiffiffiffiffiffiffi 4e 3q2
(11)
It is evident from Eq. (11) that the mean free path close to discharge ignition threshold values is proportional to atomic size, i.e. is considerably small. Eq. (6) allows deriving some assessed values of potential difference threshold above which discharge in cavitating liquid ignites. For example, water molecules consists of oxygen and hydrogen atoms with identical ionization energy of 13 eV; if cavitation area occupies the whole of interelectrode gap, then its Uth z 13 eV. The obtained value is in good accordance with the experiment. Order of-magnitude value lcav is close to atomic sizes of water molecules. Investigation of reaction gaseous products during acoustoplasma discharge in fluid mediums has allowed finding out that the basic gaseous product is hydrogen. Thus, the possibility of hydrogen production in the breakdown of various liquids by acoustoplasma discharge has been investigated. In addition discharge current and voltage, as well as evolved gas volume have been observed; besides, gas compositional analysis has been carried out by gas chromatography. Discharge current and voltage values are necessary for calculating quantity of energy needed for resolution of source liquid mass identity, as well as for calculating quantity of energy needed for hydrogen unit mass production. Water, alcohols, hydrocarbons and their mixtures have been used as feedstock. In their breakdown solid-phase products are also derived in plasma: carbon nanoparticles and discharge electrodes' materials oxides. Analysis and stoichiometric calculation have revealed that most of carbon and oxygen in source liquid molecules is spent to produce
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these by-products; hence the resulting gas mixture is considerably hydrogen-enriched. Physicochemical mode of operation consists in breaking down of complicated hydrogenous molecules and in their ionization succeeded by posterior recombination to form simple molecules: Н2, Н2О, С, СО2, МОx, where M is plasma electrodes material. Plasma discharge, triggered in reactor between metal and graphitized electrodes, is maintained by specially designed direct or alternating voltage supply, which allows investigating plasma characteristics effect on reaction and chemical composition of its products. Chromatographic analysis of gas mixture reveals that nearly pure hydrogen (98%) is produced in acousto-plasma water breakdown, carbon oxides are also constituents of evolved gas in organic liquid breakdown, but their concentration is no more than 5e6%, since main carbon in organic liquid breakdown is evolved as solid precipitation - colloidal carbon. Calculations of gas mixture quantity, formed in organic liquid breakdown, show that efficiency is heavily dependent on discharge current, as well as on discharge voltage, which could vary in relation to interelectrode gap size in reaction chamber. In experiments discharge current is 4e8 A, discharge voltage is 30e45 V in relation to type of liquid. It is found that use of the lowest quality feedstock is assumed with the use of acousto-plasma method, i.e., there is no need for costly scavenging treatment. A significant advantage is also the lack of toxical and hard recyclable waste by-products of the given synthesis, as well as the fact that gas mixture leaves reactor under slight pressure (0.2e0.3 atm), that simplifies its primary blowing-up. Hydrogenbearing gas could be used as fuel immediately after synthesis, i.e. it doesn't require separation as it contains little more than СО2 admixtures and water vapor in addition to hydrogen. By-product of hydrogen production with the use of acousto-plasma method in organic liquid breakdown is carbon, formed in the shape of agglomerated nanoparticles of different structure and precipitated to the bottom of reaction chamber during the reaction. Scanning and transmission electronic microscopy analysis of these nanoparticles has shown that carbon fibers, nanotubes, plates etc. could be obtained during the reaction [4]. Obtained nanoparticles and their agglomerates could be used also as filling material, colorants, components of composites, etc. Capacity of 100 ml reaction chamber used in pilot experiments when employing oxygen-containing organic compounds as feedstock is 2 L of hydrogen per minute. Electrical energy consumption is about 150 W, source liquid flowrate is about 20e30 ml/min. Tentative assessments of energy efficiency, calculated with regard for hydrogen and feedstock heating value and energy consumption, have shown efficiency factor of 60e70%, depending on the source mixture composition. Theoretical model calculations of discharge current and voltage have been performed; the values are in good accordance with experimental data.
Conclusion Thus, it is shown that acousto-plasma method of hydrogen production has a number of advantages as compared with the
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 2 0 9 3 4 e2 0 9 3 8
most commonly used nowadays steam reforming of methane and electrolysis. At efficiency factor of 60e70%, comparable with steam reforming of methane, the suggested method doesn't require bulky and expensive equipment and it outperforms electrolysis for rate and energy efficiency. The essential advantage of the suggested method is possibility to use wide variety of source materials.
Acknowledgement This work has been supported by RFBR, projects No. 15-0203559, 14-02-92019, 14-02-31515 and 14-02-00602.
references
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[4] Abramov OV, Andrijanov JV, Kisterev EV, Gradov OM, Shehtman AV, Klassen NV, et al. Plasma discharge in cavitating fluids. Eng Phys 2009;(8):34. [5] Ganiev RF, Bulychev NA, Fomin VN, Arutyunov IA, Eisenbach CD, Zubov VP, et al. Effect of mechanical activation on surface modification in aqueous pigment disperse systems. Dokl Chem 2006;407:54e6. [6] Bulychev NA, Kisterev EV, Arutunov IA, Zubov VP. Ultrasonic treatment assisted surface modification of inorganic and organic pigments in aqueous dispersions. J Balk Tribol Assoc 2008;1(14):30e9. [7] Bulychev NA. Conformational changes in polymers adsorbed on titanium and iron oxides. Inorg Mater 2010;46(I. 4):393e8. [8] Bulychev N, Dervaux B, Dirnberger K, Zubov V, Du Prez FE, Eisenbach CD. Structure of adsorption layers of amphiphilic copolymers on inorganic or organic particle surfaces. Macromol Chem Phys 2010;9(211):971e7. [9] Bulychev N, Confortini O, Kopold P, Dirnberger K, Schauer T, Du Prez FE, et al. Application of thermo-responsive poly(methylvinylether) containing copolymers in combination with ultrasonic treatment for pigment surface modification in pigment dispersions. Polymer 2007;48(9):2636e43. [10] Klassen N, Krivko O, Kedrov V, Shmurak S, Kiselev A, Shmyt’ko I, et al. Laser and electric arс synthesis of nanocrystalline scintillators. IEEE Trans Nucl Sci 2010;57(3):1377e81.