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Greenhouse gas treatment and H2 production, by warm plasma reforming
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Greenhouse gas treatment and H2 production, by warm plasma reforming J. Pacheco a,*, G. Soria b, M. Pacheco a, R. Valdivia a, F. Ramos a, H. Frı´as a, n a, M. Hidalgo a M. Dura Instituto Nacional de Investigaciones Nucleares, Km 36.5 Carretera Mexico-Toluca, La Marquesa, Ocoyoacac, CP 52750, Mexico b Instituto Tecnol ogico de Toluca, Av. Instituto Tecnologico, S/N, Ex-Rancho la Virgen, Metepec, Mexico a
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abstract
Article history:
Global warming is a worrying problem that has a direct negative impact on climate change,
Received 4 December 2014
so it is necessary for all countries to reduce emissions of greenhouse gases (GHGs) and
Received in revised form
emergent treatment technologies are developing through worldwide. In this paper, a
17 August 2015
relatively new technique is proposed for the GHGs of gases using a warm plasma reactor.
Accepted 18 August 2015
Carbon dioxide (CO2) and methane (CH4) have been identified as the most significant GHGs
Available online xxx
arising from anthropogenic activities, affecting the climatic global change. The technique here used is the so called Dry Reforming, assisted by a warm plasma reactor, including the
Keywords:
reactants treatment, synthesis gas generation, and finally the syngas production (H2 þ CO).
Plasma reforming
The gases CO2 and CH4 are relatively stable compounds with low potential energies. The
Greenhouse gas
dry reforming is an endothermic reaction and external energy must be provided in order to
Synthesis gas
achieve the targets inquired. More recently, plasma gas reforming is emphasized as promising technique for energy saving and environment safe purposes with increasing demand of hydrogen and synthesis gas production. In the case of plasma reforming, high electron energy provides not only radical species, but also the enthalpy required for endothermic reaction. The conversion of hydrocarbon in by-products with high added value is mainly contributed by dissociation and ionization processes. With respect to other techniques of hydrocarbons reforming by plasma discharges, there are enough references related to RF plasma discharges operating at reduced pressures, even if these low pressure plasma could achieve high hydrocarbon conversion and good H2 selectivity; the low H2 production rate and extra energy requirement for vacuum device constrain its practical use, therefore an alternative procedure and system consisting in a novel plasma reactor is here proposed. Warm plasma is environment-friendly and auto-sustainable processes, besides the electric discharge has low specific energy requirement still maintaining enough high temperature (1000e3000 K) to produce excited species, supporting subsequent chemical reactions. Such plasma discharges have significant advantages: Do not require extra cooling systems, since they work with reduced electric current flows and high voltages, avoiding electrodes erosion. Consequently, reactors can be achieved with a simpler outline and high capacity gas treatment. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ52 53297200x12646. E-mail address: [email protected] (J. Pacheco). http://dx.doi.org/10.1016/j.ijhydene.2015.08.062 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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Partial Oxidation Reforming ðPORÞ:
Introduction Current environmental and energetic problems have influenced researchers to find and develop new processes for alternative and renewable energy production. Even nowadays 88% of world commercial energy consumption (excluding photosynthesis and uncontrolled fires) is by combustion processes [1]. Fuel combustion is an exothermic reaction and usually a slow chemical process, whereby a fuel is oxidized leading to energy conversion of the chemical species usually present in the GHG, and delivering toxic exhaust gases, normally composed by NOx, CO and CO2, all of them considered as greenhouse gases (GHG). In recent decades, different laboratories around the world, have investigated reforming processes assisted by several plasma configurations, from thermal to non-thermal ones [2e8]. Plasma researchers have utilized various reforming processes, such as Partial Oxidation Reforming (POR), Dry Reforming (DR), Steam Reforming (SR), and Steam-Oxidative Reforming (SOR) to test plasma reactors capabilities in order to replace traditional combustion process or metal catalysis-based systems. The catalytic process has the disadvantages of restricted operating conditions and problems associated with catalytic converters such as cost, sulfur poisoning and extended start up time [9e11]. Warm plasma is a transitional discharge, which lies between conventional thermal plasma and non-equilibrium plasma conditions, working under moderate power density; but still maintaining enough high gas temperature (1000e3000 K) to produce excited species, supporting subsequent chemical reactions. Such plasma discharges have significant advantages: they do not require extra cooling systems, since they work with reduced electric currents and high voltages, therefore electrode erosion is avoided, and the reactor has simple construction [12]. Warm plasmas are also characterized by high chemical selectivity and have found applications in fuel conversion to syngas production, hydrogen sulfide dissociation, and carbon dioxide dissociation. Gliding arc plasma discharge or jet atmospheric pressure plasmas are additional examples of warm plasma discharges. The ionization process in warm plasma discharges is induced by a strong electric field, producing relatively high-energy electrons as well as excited ions, atoms or molecules, leading in selective chemical transitions in a very effective manner. Because of its high power density, this plasma reactors can be designed for small and large-scale applications, these types of reactors can also work in portable or on-board processes [13]. There are numerous methods to generate syngas (H2 þ CO) [14]. These methods include steam reforming, dry reforming (also called CO2 reforming) and partial oxidation, as shown in the following equations (1)e(3): Steam Reforming ðSRÞ:
Dry Reforming ðDRÞ:
CH4 þ H2 O /3H2 þ CO ðDH ¼ 206 kJ=molÞ
CH4 þ CO2 /2H2 þ 2CO ðDH ¼ 247 kJ=molÞ
(1)
(2)
1 O/2H2 þ CO 2 ðDH ¼ 36 kJ=molÞ
CH4 þ
(3) Reforming reactions (1e3) are not only a process of substance conversion, but a process of energy conversion. The merits of dry reforming (2), compared with the others procedures, include the use of carbon dioxide in the reactants mixture, as is well known, this mixture represents the 94% of the GHG. The focus of this work comprises the research of the warm plasma discharge, applied for GHG reforming; in addition the electrical energy cost is optimized to produce higher GHG conversion and obtaining a high yield and selectivity rates of H2. However, hydrogen as a fuel has significant drawbacks, especially those related with its storage. Although hydrogen's high mass heating value (120 MJ/kg), it has a very low volumetric heating value (11 kJ/l), compared for instance to 16,000 kJ/l for methanol. The specific energy (SE) and energy conversion efficiency (ECE) are concepts treated and evaluated in section Energy consumption analysis. Gasification is the thermochemical conversion of organic matter (i.e. hydrocarbons) into a gaseous product, which could be used directly for combustion or synthesized into fuel or chemicals. The main exhaust gas is the syngas composed by H2 and CO in majority, including lower concentrations of CO2, H2O, CH4, higher hydrocarbons and N2. Dry CO2 reforming can be described as a reforming technique whereby the reaction between carbon dioxide and a hydrocarbon yields a combination of hydrogen and carbon monoxide as is denoted in (2). Given the large emissions of CO2 in some industries such as cement plants, blast furnaces, among others, several authors [6e11] have proposed alternative uses for CO2 reforming, using its conversion to synthetic gas and subsequent transformation to useful energy. The ideal scenarios where DR might be considered, is where the supply of methane is linked with carbon dioxide and there are no arranged facilities for its separation. In Energy consumption analysis energy consumption analysis during reactants conversion is considered, products selectivity and yield rate production are described, and finally some results are summarized. The general idea about DR technique consists in using the GHG into plasma discharge to produce higher energetic synthetic byproducts (H2 þ CO). No other gases (neither inert gas) are used during warm plasma treatment. In (2) it can be seen that the reaction process of CH4 with CO2 is highly endothermic, typically [14] this process is accomplished by introducing the gases in a cylinder containing a fixed volume of catalyst, which is generally composed by Ni/Al2O3 composite. The process of reforming catalysts has been investigated since 1988 [11]. Previously this alloy has received a number of physical chemical and thermal treatments, such that when a whole energetic balance is considered the overall treatment efficiency is affected. To improve this type of reforming assisted by catalysts a combination of plasma technology has been considered by many authors [11,25e27] as a possible solution, however, this technique has shown several limitations among which: its scaling to industrial level, a low capacity
Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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 x x x ( 2 0 1 5 ) 1 e7
production and use of metals such as nickel and chromium for the manufacture of catalysts [28]. The catalysts suffer various phenomena such as poisoning, saturation and deactivation of solid carbon deposition, coupled with the fact that they require ongoing maintenance to exchange the catalyst once your responsiveness begins to decline. Another disadvantage of catalysts using is the cost increasing of the process and furthermore requires large amounts of energy in the manufacturing process, increasing the whole energy consumption of the application, thus the process becomes rather inefficient. Consequently in this paper is not considered the catalyst use and the GHG are treated directly in the warm plasma reactor without using any additional gas or any type of catalyst. With reference to the additional POR and SR procedures, which are alternatives to produce syngas from hydrocarbon fuels, but have some disadvantages, for instance the SR requires an extra energy to crack the H2O molecule, and the POR process, requires the use of catalyst. The real energy needed for such process cannot solely be deduced from thermodynamic calculations, since a significant amount of extra energy will be needed to overcome the reaction energy barrier. This amount of extra energy is not reflected in the thermodynamic enthalpy values. Moreover, these enthalpy values are defined only for a specific reaction, whereas in a plasma conversion reactor not all energy can be directed into a specific reaction in a specific place; then an important fraction of energy is lost due to heat loss, chemical reactions; consequently, there will always be a large difference between the thermodynamic ideally required energy and the real required energy for a process. Besides, the rotationally and vibrationally excited species are generally considered useless for hydrocarbon reforming because of the short lifetimes (several ns) and low threshold energy (<2 eV). It is noted that the bonding energy for most hydrocarbons are generally between 3 and 6 eV [29]. As a result, the decomposition of hydrocarbons is obtained from the electron-impact dissociation and ionization. As expected, a higher electric field leads to higher electron energy, which is more favorable for the electron-impact reactions responsible for hydrocarbon reforming. In other words, the energy consumed by the electron-impact reactions would be reduced while the electric field is increased. It is why; we have selected the DR technique and warm plasma reactors for this application in GHG reforming.
Experimental setup During experimental tests the flux rate of GHG mixture (CH4 þ CO2) is previously measured and homogenized and then introduced to the reactor (See Fig. 1a), which is constructed in double wall stainless steel. The gas mixture enters straight by the bottom side and ascends between the space formed by a double wall having a dual purpose: to maintain the reactor wall at low temperature and to preheat the gas mixture before to be treated by plasma discharge. The plasma discharge is generated between an external electrode and a central conical electrode made of tungsten material; at that point, it moves down to the end of conical electrode and does not detach from it, formerly, the arc column length
3
increases slowly, just rotating by the flow, during a time represented by (1/uc). The arc discharge continues the spiral motion descending along the chamber reactor increasing the column length (during time 1/ud) and a new cycle is recommenced each 25 ms (See Fig. 1b). The plasma jet is expelled down the reactor chamber and flow in a free space or in a container. In our case, the post-chamber elongates the plasma jet (See Fig. 1a), because the atmospheric air surrounding the jet has a shield effect, therefore, the plasma jet is spread out with a very long volume, as it was reported in Ref. [12]. Contrary, when the plasma jet blows directly in air, it has a reduced volume. The residence time tr of the plasma discharge inside the reactor could be calculated by the following relation (4):
tr ¼
h i pðR1 rÞ2 ð[1 Þ þ pðR2 Þ2 ð[2 Þ ðmlÞ flow rateðml=sÞ
(4)
where, R1 the radius of the discharge chamber, R2 the radius of the post-discharge chamber, r the radius of the internal electrode, and [1 and [2 the discharge and post discharge length respectively. As the arc length expands, the heat dissipation increases. This results in a lowered temperature, and the plasma tends to be in nonthermal plasma discharge. The power source consists on a high-frequency series resonant inverter [15] constituted by one ferrite core setup transformer to provide up to 15 kV at high frequencies. Four sequential control pulses (S1 e S4) generated by a microprocessor circuit, trigger the four high-power MOS transistor (M1 e M4) in a specified sequence to command the transformer primary winding. The power supply handles a wide frequency range (5e200 kHz). The duty cycle in each phase is adjusted to 50% providing a soft start in the MOS avoiding redundant power consumption in the plasma discharge. The high frequency transformer transfers the maximum energy towards the plasma discharge; in addition, it also functions as a stabilizer, because a natural negative feedback controls the impedance plasma discharge, when the impedance charge descends, the voltage is automatically adjusted to sustain a stable plasma discharge, and consequently a lower electrical field of about 3 kV/mm is applied between electrodes, as a result, a low-current discharge streaks between the electrodes closest points and pre-ionizes the gas gap, causing the formation of a stronger current to sustain the warm plasma discharge in the GHGs. The gliding discharge on the central electrode is reflected in the electrical signals [12]. The gliding period (Tg) can be determined from the electrical signals, in this case, a Tg of 25 ms is obtained, so the gliding frequency corresponds to 40 Hz. The arc column length is a function of power applied to the discharge, reactor geometry, and nature of the gas to be treated.
Energy consumption analysis Normally in literature, two important parameters are discussed: SE which represents the energy needed to produce a syngas mol according (5) [14,17], and ECE which represents the relation of the output energy of products respect the entrance energy of reactants (6).
Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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(a) (b)
Fig. 1 e Schematic of experimental set up system (a) for dry reforming and electrical discharge period (b).
SE ¼
Pdischarge ðH2 þ COÞproduced
(5)
where Pdischarge represents the plasma discharge energy applied expressed in (kJ) and the denominator ðH2 þ COÞproduced denotes the syngas produced in (mol). The process will have a major performance whereas this rapport stands slight. h i hH2 produced LHVH2 þ hCOproduced ðLHVCO Þ 100 ECE ¼ Pdischarge þ hCH4 converted ðLHVCH4 Þ
(6)
ECE expresses the lower heating value (LHV) of synthesis gas produced in the reforming process divided by the input energy. According (6), the desirable ECE value must be equal to 100%. From Refs. [16e19] next values are found: 1) LHVH2 ¼ 242.056 kJ*mol1; 2) LHVCO ¼ 283.179 kJ*mol1; and 3) LHVCH4 ¼ 802.933 kJ*mol1 @ 298.16 K. Whereas h refers to the moles of the product (H2 þ CO)produced (mol); hCH4 the moles of methane converted (mol); Pdischarge is the input energy of plasma expressed in kJ; LHV the lower heating value of substance in kJ/mol. SE and ECE appear to be good indicators for describing DR process efficiency. By using the same procedure indicated in Ref. [16], which is centered on the instantaneous values intensity of voltage and current waveforms; the root medium square power PRMS can be deduced, and finally the energy consumption can be obtained which correspond to 175 W at a frequency of 140 kHz, and flow rates of 3 LPM for CH4 and 3 LPM for CO2. According these operating conditions, the SE required for CO and H2 (syngas) generation can be calculated from expression 5 and 6. Considering these two important parameters, plus the factors listed in Equations (7)e(13), some numerical results can be obtained and compared with other references [20e22], as Tables 1 and 2 shown in the Results and discussion. Equations (7) and (8) correspond to entering GHG conversion; while (9) and (10) relate product selectivity and 11e12 correspond to syngas yield in DR processes. Anywhere hx
refers to the moles of the product x (converter, produced or feed) in each case. CH4 conversion ð%Þ ¼
hCH4 converted 100 hCH4 feed
(7)
hCO2 converted 100 hCO2 feed
(8)
CO2 conversion ð%Þ ¼
In the case of 100% of hydrogen selectivity (9) means that all hydrogen atoms of the methane molecules are converted into hydrogen molecules and 100% of CO selectivity (10) means that all the carbon reactants are converted to CO. hH2
H2 selectivity ð%Þ ¼
produced
2 hCH4
CO selectivity ð%Þ ¼
(9)
converted
hCOproduced hCH4
converted
þ hCO2
(10) converted
The H2 and CO yields (11, 12) are defined as, the relationship of the H2 produced respect the hCH4 entrance; and the CO produced respect the GHG entrance [21].
H2 yield ð%Þ ¼
hH2
produced
2 hCH4
(11)
feed
Table 1 e Reactants and products on dry reforming process. Substances CO CO2 CH4 H2 O2 N2
Before treatment (% vol.)
After treatment (% vol.)
0 44 50 0 0 6
24 20 26 24 0 6
Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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Table 2 e Energy and conversion parameters obtained by using (5e13). Parameter
Result
SE ECE CH4 conversion CO2 conversion H2 selectivity CO selectivity H2 yield CO yield H2/CO ratio
CO yield ð%Þ ¼
88.6 kJ/mol 91.4% 48% 54% 100% 100% 48% 51% 1
hCOproduced hCH4
feed
þ hCO2
Equation Ec. Ec. Ec. Ec. Ec. Ec. Ec. Ec. Ec.
5 6 7 8 9 10 11 12 13
(12) feed
Finally a relationship of the principal syngas components is obtained by (13). All these equations are used to evaluate the results in next section Relation ðH2 =COÞ ¼
hH2
produced
hCOproduced
(13)
Results and discussion According the experimental set-up (Fig. 1) the exhaust gases were evaluated by using the Gas Analyzer GasBoard3100P® and the results are shown in Fig. 2, where three zones are depicted: The first region (Zone 1) corresponds to the untreated mixture inlet, the Zone 2 relates to the diminution of gases concentration during DR as a consequence of warm plasma application, and finally the Zone 3 corresponds to the recuperation of initial concentration when the plasma is turned off. The GHG volume distribution before treatment is 50% for CH4 and 44% for CO2 the complementary 6% is due to the permanent gas inside reactor as N2 (see Table 1). Giving these results, there is a 24% volume of H2 and 24% volume of CO generation. A remnant 26% of CH4 and 20% of CO2 appears without conversion for the power inserted in warm plasma
Fig. 2 e Warm plasma DR analyzed with Gasboard 3100P.
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discharge. These results confirm that CH4 and CO2 mixture is mainly converted into syngas, which is consistent with the stoichiometry in equation (2). The gas analyzer used is not able to quantify the N2 part, remnant in the reforming process as is estimated in Table 1, where a 6% of the remaining volume is occupied by N2 gas. Due to these limitations, a supplementary analysis method was used to determinate the presence of other minor substances. This technique is the well-known mass spectrometry, which experimental results in 630 samples, are shown in Fig. 3. In the mass spectra shown in Fig. 3, the horizontal axis represents the (m/z) index, while the vertical axis corresponds to the intensity of each of the 630 samples analyzed. The first pulse (m/z ¼ 2) corresponds to the H2 production intensity, at the same time, the evolution in mass 16, indicates a significant decrease of CH4 intensity and a growth in the mass 28 (CO or N2), corresponds to CO generation whose intensity is the difference in magnitude before and after treatment. In 44 mass indicates a CO2 abatement. These results are consistent with Gasboard 3100P analyzer. In this mass spectrum, none important peak in mass 30 and 46 is revealed, therefore there is no NOX formation. Agreeing to Tables 1 and 2, a relationship of 1 in syngas byproducts (H2/CO) is obtained; and an energy yield of 48% for H2 and 51% for CO is attained. A comparison of these results respect similar jobs is shown in Table 3, were some key parameters and competitive performance of this process is attained. The references cited [4,23and24], use a very slow flows (lower than 1 LPM) of gas treated, whereas [25] employs a very high flow 73 LPM, but the power used is very significant (18 kW), giving a SE and ECE of 274 and 57% respectively. Another significant remark is that [25] uses noble gas (Ar) and a preheated Catalyst (1000 C during 30 min) to ameliorate the activation energy during reforming process, but at the same time, these two factors increase the reforming profitable cost. In Table 3 the selectivity and yield rates obtained in this work, are shown in the last raw.
Conclusions Warm plasma fuel reforming technology has several advantages over traditional catalytic or thermal processes including fast start time, high productivity by means of a moderately low electrical energy costs operation. The results show that warm plasma reactor is talented for heavy hydrocarbon fuels reforming having high conversion efficiency. In this work, a high H2 and CO selectivity is obtained (reaching 100%) whereas compared with other references their selectivity is ranging from 30% to 90%, in addition, in this work there is no generation of NOx. In this work, the specific energy and energy conversion efficiency are shown, getting optimal SE and ECE values. With these results it can be argued that DR technique with warm plasma is an emerging procedure with competitive results without the generation of unwanted species. The high power consumption in the case of thermal plasma procedure could become higher than the energy
Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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Fig. 3 e Mass spectra sweep during experimental test.
Table 3 e Some key performance parameters in energy conversion from several authors. Plasma form [Reference] Corona discharge [23] Microwave discharge [24] Gliding arc discharge [4] Thermal plasma jet [25] Warm plasma This work
Flow P (W) Conversion Conversion Selectivity Selectivity H2/CO CH4/CO2 SE ECE (a.u.) rate (LPM) CO2 (%) CH4 (%) CO (%) H2 (%) (a.u.) (kJ/mol) (%) 0.043 0.2 1 73.3 6
46.3 60 190 18 k 175
47.8 68.8 31 64.8 54
62.4 70.8 40 78 48
contained in the reformed products, on the other hand, the use of noble gases raises the cost of the process, moreover the consumption of N2 in the thermal plasma, causes the production of unwanted species such as NOx. In this paper, a practical domestic power supply is used, based in a resonant electronic converter developed by our group, thus assuring the energy consumption with high transfer efficiency. Electric analysis was performed to determine the current, voltage, power and energy required for the GHG treatment and to provide enough energy required to break GHG gas chemical bonds, under atmospheric pressure without using supplementary inert gases. Given his compactness is very adequate for on-board vehicular reforming systems that should be further developed and optimized. Reforming conversion of GHG by warm plasmas offers a promising route to generate higher value products such as syngas at low energetic cost.
Acknowledgements The authors would like to thank for the invaluable support of the LAP laboratory of the ININ. This work was supported under EAOO2 project from ININ.
66.8 75 62 96.8 100
70 50 50 82.85 100
1.2 1.5 0.9 0.8 1
1 1.5 1 0.66 1
1798 307 608 274 88.6
13 47 28 57 91.4
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Greenhouse gas treatment and H2 production, by warm plasma reforming J. Pacheco a,*, G. Soria b, M. Pacheco a, R. Valdivia a, F. Ramos a, H. Frı´as a, n a, M. Hidalgo a M. Dura Instituto Nacional de Investigaciones Nucleares, Km 36.5 Carretera Mexico-Toluca, La Marquesa, Ocoyoacac, CP 52750, Mexico b Instituto Tecnol ogico de Toluca, Av. Instituto Tecnologico, S/N, Ex-Rancho la Virgen, Metepec, Mexico a
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abstract
Article history:
Global warming is a worrying problem that has a direct negative impact on climate change,
Received 4 December 2014
so it is necessary for all countries to reduce emissions of greenhouse gases (GHGs) and
Received in revised form
emergent treatment technologies are developing through worldwide. In this paper, a
17 August 2015
relatively new technique is proposed for the GHGs of gases using a warm plasma reactor.
Accepted 18 August 2015
Carbon dioxide (CO2) and methane (CH4) have been identified as the most significant GHGs
Available online xxx
arising from anthropogenic activities, affecting the climatic global change. The technique here used is the so called Dry Reforming, assisted by a warm plasma reactor, including the
Keywords:
reactants treatment, synthesis gas generation, and finally the syngas production (H2 þ CO).
Plasma reforming
The gases CO2 and CH4 are relatively stable compounds with low potential energies. The
Greenhouse gas
dry reforming is an endothermic reaction and external energy must be provided in order to
Synthesis gas
achieve the targets inquired. More recently, plasma gas reforming is emphasized as promising technique for energy saving and environment safe purposes with increasing demand of hydrogen and synthesis gas production. In the case of plasma reforming, high electron energy provides not only radical species, but also the enthalpy required for endothermic reaction. The conversion of hydrocarbon in by-products with high added value is mainly contributed by dissociation and ionization processes. With respect to other techniques of hydrocarbons reforming by plasma discharges, there are enough references related to RF plasma discharges operating at reduced pressures, even if these low pressure plasma could achieve high hydrocarbon conversion and good H2 selectivity; the low H2 production rate and extra energy requirement for vacuum device constrain its practical use, therefore an alternative procedure and system consisting in a novel plasma reactor is here proposed. Warm plasma is environment-friendly and auto-sustainable processes, besides the electric discharge has low specific energy requirement still maintaining enough high temperature (1000e3000 K) to produce excited species, supporting subsequent chemical reactions. Such plasma discharges have significant advantages: Do not require extra cooling systems, since they work with reduced electric current flows and high voltages, avoiding electrodes erosion. Consequently, reactors can be achieved with a simpler outline and high capacity gas treatment. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ52 53297200x12646. E-mail address: [email protected] (J. Pacheco). http://dx.doi.org/10.1016/j.ijhydene.2015.08.062 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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Partial Oxidation Reforming ðPORÞ:
Introduction Current environmental and energetic problems have influenced researchers to find and develop new processes for alternative and renewable energy production. Even nowadays 88% of world commercial energy consumption (excluding photosynthesis and uncontrolled fires) is by combustion processes [1]. Fuel combustion is an exothermic reaction and usually a slow chemical process, whereby a fuel is oxidized leading to energy conversion of the chemical species usually present in the GHG, and delivering toxic exhaust gases, normally composed by NOx, CO and CO2, all of them considered as greenhouse gases (GHG). In recent decades, different laboratories around the world, have investigated reforming processes assisted by several plasma configurations, from thermal to non-thermal ones [2e8]. Plasma researchers have utilized various reforming processes, such as Partial Oxidation Reforming (POR), Dry Reforming (DR), Steam Reforming (SR), and Steam-Oxidative Reforming (SOR) to test plasma reactors capabilities in order to replace traditional combustion process or metal catalysis-based systems. The catalytic process has the disadvantages of restricted operating conditions and problems associated with catalytic converters such as cost, sulfur poisoning and extended start up time [9e11]. Warm plasma is a transitional discharge, which lies between conventional thermal plasma and non-equilibrium plasma conditions, working under moderate power density; but still maintaining enough high gas temperature (1000e3000 K) to produce excited species, supporting subsequent chemical reactions. Such plasma discharges have significant advantages: they do not require extra cooling systems, since they work with reduced electric currents and high voltages, therefore electrode erosion is avoided, and the reactor has simple construction [12]. Warm plasmas are also characterized by high chemical selectivity and have found applications in fuel conversion to syngas production, hydrogen sulfide dissociation, and carbon dioxide dissociation. Gliding arc plasma discharge or jet atmospheric pressure plasmas are additional examples of warm plasma discharges. The ionization process in warm plasma discharges is induced by a strong electric field, producing relatively high-energy electrons as well as excited ions, atoms or molecules, leading in selective chemical transitions in a very effective manner. Because of its high power density, this plasma reactors can be designed for small and large-scale applications, these types of reactors can also work in portable or on-board processes [13]. There are numerous methods to generate syngas (H2 þ CO) [14]. These methods include steam reforming, dry reforming (also called CO2 reforming) and partial oxidation, as shown in the following equations (1)e(3): Steam Reforming ðSRÞ:
Dry Reforming ðDRÞ:
CH4 þ H2 O /3H2 þ CO ðDH ¼ 206 kJ=molÞ
CH4 þ CO2 /2H2 þ 2CO ðDH ¼ 247 kJ=molÞ
(1)
(2)
1 O/2H2 þ CO 2 ðDH ¼ 36 kJ=molÞ
CH4 þ
(3) Reforming reactions (1e3) are not only a process of substance conversion, but a process of energy conversion. The merits of dry reforming (2), compared with the others procedures, include the use of carbon dioxide in the reactants mixture, as is well known, this mixture represents the 94% of the GHG. The focus of this work comprises the research of the warm plasma discharge, applied for GHG reforming; in addition the electrical energy cost is optimized to produce higher GHG conversion and obtaining a high yield and selectivity rates of H2. However, hydrogen as a fuel has significant drawbacks, especially those related with its storage. Although hydrogen's high mass heating value (120 MJ/kg), it has a very low volumetric heating value (11 kJ/l), compared for instance to 16,000 kJ/l for methanol. The specific energy (SE) and energy conversion efficiency (ECE) are concepts treated and evaluated in section Energy consumption analysis. Gasification is the thermochemical conversion of organic matter (i.e. hydrocarbons) into a gaseous product, which could be used directly for combustion or synthesized into fuel or chemicals. The main exhaust gas is the syngas composed by H2 and CO in majority, including lower concentrations of CO2, H2O, CH4, higher hydrocarbons and N2. Dry CO2 reforming can be described as a reforming technique whereby the reaction between carbon dioxide and a hydrocarbon yields a combination of hydrogen and carbon monoxide as is denoted in (2). Given the large emissions of CO2 in some industries such as cement plants, blast furnaces, among others, several authors [6e11] have proposed alternative uses for CO2 reforming, using its conversion to synthetic gas and subsequent transformation to useful energy. The ideal scenarios where DR might be considered, is where the supply of methane is linked with carbon dioxide and there are no arranged facilities for its separation. In Energy consumption analysis energy consumption analysis during reactants conversion is considered, products selectivity and yield rate production are described, and finally some results are summarized. The general idea about DR technique consists in using the GHG into plasma discharge to produce higher energetic synthetic byproducts (H2 þ CO). No other gases (neither inert gas) are used during warm plasma treatment. In (2) it can be seen that the reaction process of CH4 with CO2 is highly endothermic, typically [14] this process is accomplished by introducing the gases in a cylinder containing a fixed volume of catalyst, which is generally composed by Ni/Al2O3 composite. The process of reforming catalysts has been investigated since 1988 [11]. Previously this alloy has received a number of physical chemical and thermal treatments, such that when a whole energetic balance is considered the overall treatment efficiency is affected. To improve this type of reforming assisted by catalysts a combination of plasma technology has been considered by many authors [11,25e27] as a possible solution, however, this technique has shown several limitations among which: its scaling to industrial level, a low capacity
Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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 x x x ( 2 0 1 5 ) 1 e7
production and use of metals such as nickel and chromium for the manufacture of catalysts [28]. The catalysts suffer various phenomena such as poisoning, saturation and deactivation of solid carbon deposition, coupled with the fact that they require ongoing maintenance to exchange the catalyst once your responsiveness begins to decline. Another disadvantage of catalysts using is the cost increasing of the process and furthermore requires large amounts of energy in the manufacturing process, increasing the whole energy consumption of the application, thus the process becomes rather inefficient. Consequently in this paper is not considered the catalyst use and the GHG are treated directly in the warm plasma reactor without using any additional gas or any type of catalyst. With reference to the additional POR and SR procedures, which are alternatives to produce syngas from hydrocarbon fuels, but have some disadvantages, for instance the SR requires an extra energy to crack the H2O molecule, and the POR process, requires the use of catalyst. The real energy needed for such process cannot solely be deduced from thermodynamic calculations, since a significant amount of extra energy will be needed to overcome the reaction energy barrier. This amount of extra energy is not reflected in the thermodynamic enthalpy values. Moreover, these enthalpy values are defined only for a specific reaction, whereas in a plasma conversion reactor not all energy can be directed into a specific reaction in a specific place; then an important fraction of energy is lost due to heat loss, chemical reactions; consequently, there will always be a large difference between the thermodynamic ideally required energy and the real required energy for a process. Besides, the rotationally and vibrationally excited species are generally considered useless for hydrocarbon reforming because of the short lifetimes (several ns) and low threshold energy (<2 eV). It is noted that the bonding energy for most hydrocarbons are generally between 3 and 6 eV [29]. As a result, the decomposition of hydrocarbons is obtained from the electron-impact dissociation and ionization. As expected, a higher electric field leads to higher electron energy, which is more favorable for the electron-impact reactions responsible for hydrocarbon reforming. In other words, the energy consumed by the electron-impact reactions would be reduced while the electric field is increased. It is why; we have selected the DR technique and warm plasma reactors for this application in GHG reforming.
Experimental setup During experimental tests the flux rate of GHG mixture (CH4 þ CO2) is previously measured and homogenized and then introduced to the reactor (See Fig. 1a), which is constructed in double wall stainless steel. The gas mixture enters straight by the bottom side and ascends between the space formed by a double wall having a dual purpose: to maintain the reactor wall at low temperature and to preheat the gas mixture before to be treated by plasma discharge. The plasma discharge is generated between an external electrode and a central conical electrode made of tungsten material; at that point, it moves down to the end of conical electrode and does not detach from it, formerly, the arc column length
3
increases slowly, just rotating by the flow, during a time represented by (1/uc). The arc discharge continues the spiral motion descending along the chamber reactor increasing the column length (during time 1/ud) and a new cycle is recommenced each 25 ms (See Fig. 1b). The plasma jet is expelled down the reactor chamber and flow in a free space or in a container. In our case, the post-chamber elongates the plasma jet (See Fig. 1a), because the atmospheric air surrounding the jet has a shield effect, therefore, the plasma jet is spread out with a very long volume, as it was reported in Ref. [12]. Contrary, when the plasma jet blows directly in air, it has a reduced volume. The residence time tr of the plasma discharge inside the reactor could be calculated by the following relation (4):
tr ¼
h i pðR1 rÞ2 ð[1 Þ þ pðR2 Þ2 ð[2 Þ ðmlÞ flow rateðml=sÞ
(4)
where, R1 the radius of the discharge chamber, R2 the radius of the post-discharge chamber, r the radius of the internal electrode, and [1 and [2 the discharge and post discharge length respectively. As the arc length expands, the heat dissipation increases. This results in a lowered temperature, and the plasma tends to be in nonthermal plasma discharge. The power source consists on a high-frequency series resonant inverter [15] constituted by one ferrite core setup transformer to provide up to 15 kV at high frequencies. Four sequential control pulses (S1 e S4) generated by a microprocessor circuit, trigger the four high-power MOS transistor (M1 e M4) in a specified sequence to command the transformer primary winding. The power supply handles a wide frequency range (5e200 kHz). The duty cycle in each phase is adjusted to 50% providing a soft start in the MOS avoiding redundant power consumption in the plasma discharge. The high frequency transformer transfers the maximum energy towards the plasma discharge; in addition, it also functions as a stabilizer, because a natural negative feedback controls the impedance plasma discharge, when the impedance charge descends, the voltage is automatically adjusted to sustain a stable plasma discharge, and consequently a lower electrical field of about 3 kV/mm is applied between electrodes, as a result, a low-current discharge streaks between the electrodes closest points and pre-ionizes the gas gap, causing the formation of a stronger current to sustain the warm plasma discharge in the GHGs. The gliding discharge on the central electrode is reflected in the electrical signals [12]. The gliding period (Tg) can be determined from the electrical signals, in this case, a Tg of 25 ms is obtained, so the gliding frequency corresponds to 40 Hz. The arc column length is a function of power applied to the discharge, reactor geometry, and nature of the gas to be treated.
Energy consumption analysis Normally in literature, two important parameters are discussed: SE which represents the energy needed to produce a syngas mol according (5) [14,17], and ECE which represents the relation of the output energy of products respect the entrance energy of reactants (6).
Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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(a) (b)
Fig. 1 e Schematic of experimental set up system (a) for dry reforming and electrical discharge period (b).
SE ¼
Pdischarge ðH2 þ COÞproduced
(5)
where Pdischarge represents the plasma discharge energy applied expressed in (kJ) and the denominator ðH2 þ COÞproduced denotes the syngas produced in (mol). The process will have a major performance whereas this rapport stands slight. h i hH2 produced LHVH2 þ hCOproduced ðLHVCO Þ 100 ECE ¼ Pdischarge þ hCH4 converted ðLHVCH4 Þ
(6)
ECE expresses the lower heating value (LHV) of synthesis gas produced in the reforming process divided by the input energy. According (6), the desirable ECE value must be equal to 100%. From Refs. [16e19] next values are found: 1) LHVH2 ¼ 242.056 kJ*mol1; 2) LHVCO ¼ 283.179 kJ*mol1; and 3) LHVCH4 ¼ 802.933 kJ*mol1 @ 298.16 K. Whereas h refers to the moles of the product (H2 þ CO)produced (mol); hCH4 the moles of methane converted (mol); Pdischarge is the input energy of plasma expressed in kJ; LHV the lower heating value of substance in kJ/mol. SE and ECE appear to be good indicators for describing DR process efficiency. By using the same procedure indicated in Ref. [16], which is centered on the instantaneous values intensity of voltage and current waveforms; the root medium square power PRMS can be deduced, and finally the energy consumption can be obtained which correspond to 175 W at a frequency of 140 kHz, and flow rates of 3 LPM for CH4 and 3 LPM for CO2. According these operating conditions, the SE required for CO and H2 (syngas) generation can be calculated from expression 5 and 6. Considering these two important parameters, plus the factors listed in Equations (7)e(13), some numerical results can be obtained and compared with other references [20e22], as Tables 1 and 2 shown in the Results and discussion. Equations (7) and (8) correspond to entering GHG conversion; while (9) and (10) relate product selectivity and 11e12 correspond to syngas yield in DR processes. Anywhere hx
refers to the moles of the product x (converter, produced or feed) in each case. CH4 conversion ð%Þ ¼
hCH4 converted 100 hCH4 feed
(7)
hCO2 converted 100 hCO2 feed
(8)
CO2 conversion ð%Þ ¼
In the case of 100% of hydrogen selectivity (9) means that all hydrogen atoms of the methane molecules are converted into hydrogen molecules and 100% of CO selectivity (10) means that all the carbon reactants are converted to CO. hH2
H2 selectivity ð%Þ ¼
produced
2 hCH4
CO selectivity ð%Þ ¼
(9)
converted
hCOproduced hCH4
converted
þ hCO2
(10) converted
The H2 and CO yields (11, 12) are defined as, the relationship of the H2 produced respect the hCH4 entrance; and the CO produced respect the GHG entrance [21].
H2 yield ð%Þ ¼
hH2
produced
2 hCH4
(11)
feed
Table 1 e Reactants and products on dry reforming process. Substances CO CO2 CH4 H2 O2 N2
Before treatment (% vol.)
After treatment (% vol.)
0 44 50 0 0 6
24 20 26 24 0 6
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Table 2 e Energy and conversion parameters obtained by using (5e13). Parameter
Result
SE ECE CH4 conversion CO2 conversion H2 selectivity CO selectivity H2 yield CO yield H2/CO ratio
CO yield ð%Þ ¼
88.6 kJ/mol 91.4% 48% 54% 100% 100% 48% 51% 1
hCOproduced hCH4
feed
þ hCO2
Equation Ec. Ec. Ec. Ec. Ec. Ec. Ec. Ec. Ec.
5 6 7 8 9 10 11 12 13
(12) feed
Finally a relationship of the principal syngas components is obtained by (13). All these equations are used to evaluate the results in next section Relation ðH2 =COÞ ¼
hH2
produced
hCOproduced
(13)
Results and discussion According the experimental set-up (Fig. 1) the exhaust gases were evaluated by using the Gas Analyzer GasBoard3100P® and the results are shown in Fig. 2, where three zones are depicted: The first region (Zone 1) corresponds to the untreated mixture inlet, the Zone 2 relates to the diminution of gases concentration during DR as a consequence of warm plasma application, and finally the Zone 3 corresponds to the recuperation of initial concentration when the plasma is turned off. The GHG volume distribution before treatment is 50% for CH4 and 44% for CO2 the complementary 6% is due to the permanent gas inside reactor as N2 (see Table 1). Giving these results, there is a 24% volume of H2 and 24% volume of CO generation. A remnant 26% of CH4 and 20% of CO2 appears without conversion for the power inserted in warm plasma
Fig. 2 e Warm plasma DR analyzed with Gasboard 3100P.
5
discharge. These results confirm that CH4 and CO2 mixture is mainly converted into syngas, which is consistent with the stoichiometry in equation (2). The gas analyzer used is not able to quantify the N2 part, remnant in the reforming process as is estimated in Table 1, where a 6% of the remaining volume is occupied by N2 gas. Due to these limitations, a supplementary analysis method was used to determinate the presence of other minor substances. This technique is the well-known mass spectrometry, which experimental results in 630 samples, are shown in Fig. 3. In the mass spectra shown in Fig. 3, the horizontal axis represents the (m/z) index, while the vertical axis corresponds to the intensity of each of the 630 samples analyzed. The first pulse (m/z ¼ 2) corresponds to the H2 production intensity, at the same time, the evolution in mass 16, indicates a significant decrease of CH4 intensity and a growth in the mass 28 (CO or N2), corresponds to CO generation whose intensity is the difference in magnitude before and after treatment. In 44 mass indicates a CO2 abatement. These results are consistent with Gasboard 3100P analyzer. In this mass spectrum, none important peak in mass 30 and 46 is revealed, therefore there is no NOX formation. Agreeing to Tables 1 and 2, a relationship of 1 in syngas byproducts (H2/CO) is obtained; and an energy yield of 48% for H2 and 51% for CO is attained. A comparison of these results respect similar jobs is shown in Table 3, were some key parameters and competitive performance of this process is attained. The references cited [4,23and24], use a very slow flows (lower than 1 LPM) of gas treated, whereas [25] employs a very high flow 73 LPM, but the power used is very significant (18 kW), giving a SE and ECE of 274 and 57% respectively. Another significant remark is that [25] uses noble gas (Ar) and a preheated Catalyst (1000 C during 30 min) to ameliorate the activation energy during reforming process, but at the same time, these two factors increase the reforming profitable cost. In Table 3 the selectivity and yield rates obtained in this work, are shown in the last raw.
Conclusions Warm plasma fuel reforming technology has several advantages over traditional catalytic or thermal processes including fast start time, high productivity by means of a moderately low electrical energy costs operation. The results show that warm plasma reactor is talented for heavy hydrocarbon fuels reforming having high conversion efficiency. In this work, a high H2 and CO selectivity is obtained (reaching 100%) whereas compared with other references their selectivity is ranging from 30% to 90%, in addition, in this work there is no generation of NOx. In this work, the specific energy and energy conversion efficiency are shown, getting optimal SE and ECE values. With these results it can be argued that DR technique with warm plasma is an emerging procedure with competitive results without the generation of unwanted species. The high power consumption in the case of thermal plasma procedure could become higher than the energy
Please cite this article in press as: Pacheco J, et al., Greenhouse gas treatment and H2 production, by warm plasma reforming, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.08.062
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Fig. 3 e Mass spectra sweep during experimental test.
Table 3 e Some key performance parameters in energy conversion from several authors. Plasma form [Reference] Corona discharge [23] Microwave discharge [24] Gliding arc discharge [4] Thermal plasma jet [25] Warm plasma This work
Flow P (W) Conversion Conversion Selectivity Selectivity H2/CO CH4/CO2 SE ECE (a.u.) rate (LPM) CO2 (%) CH4 (%) CO (%) H2 (%) (a.u.) (kJ/mol) (%) 0.043 0.2 1 73.3 6
46.3 60 190 18 k 175
47.8 68.8 31 64.8 54
62.4 70.8 40 78 48
contained in the reformed products, on the other hand, the use of noble gases raises the cost of the process, moreover the consumption of N2 in the thermal plasma, causes the production of unwanted species such as NOx. In this paper, a practical domestic power supply is used, based in a resonant electronic converter developed by our group, thus assuring the energy consumption with high transfer efficiency. Electric analysis was performed to determine the current, voltage, power and energy required for the GHG treatment and to provide enough energy required to break GHG gas chemical bonds, under atmospheric pressure without using supplementary inert gases. Given his compactness is very adequate for on-board vehicular reforming systems that should be further developed and optimized. Reforming conversion of GHG by warm plasmas offers a promising route to generate higher value products such as syngas at low energetic cost.
Acknowledgements The authors would like to thank for the invaluable support of the LAP laboratory of the ININ. This work was supported under EAOO2 project from ININ.
66.8 75 62 96.8 100
70 50 50 82.85 100
1.2 1.5 0.9 0.8 1
1 1.5 1 0.66 1
1798 307 608 274 88.6
13 47 28 57 91.4
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