Renewable hydrogen production by alcohols reforming using plasma and plasma-catalytic technologies: challenges and opportunities.

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Renewable Hydrogen Production by Alcohols Reforming Using Plasma and Plasma-Catalytic Technologies: Challenges and Opportunities ChangMing Du,* JianMin Mo, and HongXia Li Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China 1. NOx Adsorber Regeneration 2. Spark Ignition Engines Using HydrogenEnhanced Ultralean Turbo-Charged Operation 3. Hydrogen-Rich Gas-Enhanced Regeneration of Diesel Particulate Filters 4. Ignition Control in Homogeneous Charge Compression Ignition (HCCI) Engines Author Information Corresponding Author Notes Biographies Acknowledgments Nomenclature References

CONTENTS 1. Introduction 2. Plasma Technique 2.1. Plasma-Assisted Alcohols Reforming 2.1.1. Thermal Plasma 2.1.2. Nonthermal Plasma 2.2. Comparison of Reforming Efficiency 2.2.1. Evaluation Methods 2.2.2. Comparison and Analysis 2.3. Mechanism of Plasma-Assisted Reforming 2.4. Factors Influencing Alcohols Reforming 2.4.1. Feedstock Components 2.4.2. Carrier Gas 2.4.3. Input Power 2.4.4. Geometry of the Plasma Reactor 2.4.5. Temperature 2.4.6. Residence Time 2.4.7. Others Factors 3. Plasma-Catalytic Technique 3.1. Types of Catalysts 3.1.1. Noble Metal Catalysts 3.1.2. Non-noble Metal Catalysts 3.2. Comparison and Analysis 3.3. Understanding of the Reforming Mechanism 3.3.1. Interactions between Plasma and Catalyst in the Postplasma Configuration 3.3.2. Interactions between Plasma and Catalyst in the In-Plasma Configuration 3.3.3. Photocatalysis in Plasma-Catalytic Process 4. Models of Alcohols Reforming for Hydrogen Production 5. Discussion 5.1. The Energy Balance of Hydrogen Production from Ethanol 5.2. Comparison of Alcohols Reforming Assisted by Different Plasma Treatment Systems 6. Outlook © XXXX American Chemical Society

A C C C C J J K M N N O P Q Q Q R R S S S S

AH AH AH AH AI AI AI AJ AJ AJ AJ

1. INTRODUCTION Global dependence on fossil fuels for electricity production, transportation, or as reagents for the chemical industry is increasing today. Hydrogen has the potential to provide a clean and affordable energy supply that can minimize dependence on fossil fuels, thereby enhancing the global economy and reducing environmental pollution. Fuel cells that use hydrogen as a fuel to convert chemical energy directly into electrical and thermal energy are considered a replacement for current less-efficient energy-generation technologies. Hydrogen is widely preferred as a fuel for fuel cells due to its energy security, reliability, low operating cost, constant power production, clean emissions, and high electrical efficiency. It is an important energy carrier for power systems, particularly in remote regions. Hydrogen can be produced from a variety of primary sources, such as alcohols, heavy oil, and natural gas. Hydrogen production from alcohols has the following advantages: (1) renewability; (2) high power density; (3) high energy density; and (4) easy storage.1 The mixture of hydrogen-containing gas generated by biofuels can be used directly in the fuel cells. Therefore, there is a potential market for alternative solutions, such as the use of alcohols, for fuel cell technologies. With respect to biofuels, Sweden represents 4% of the petrol market for bioethanol, and Germany is the world leader in biodiesel, with 6% of the diesel market. Biofuels could account for as much as 14% of the fuels used for transportation by 2020.2 Global ethanol production has increased substantially since the oil crisis of the 1970s. The market for ethanol has grown from

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Received: July 12, 2014

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dx.doi.org/10.1021/cr5003744 | Chem. Rev. XXXX, XXX, XXX−XXX

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Table 1. Classification and Properties of Various Plasmas28a properties

thermal plasma (quasi-equilibrium plasma)

nonthermal plasma (nonequilibrium plasma)

temp

Te ≈ Ti ≈ Tg ≤ 2 × 104 K

Te ≫ Ti ≈ Tg ≤ 300...103 K

density

≥1020 m−3

∼1010 m−3

classification:

thermal arc plasma

excitation pressure, bar electron energies, eV electron density, cm−3 breakdown voltage, kV current, A Tmax, K carrier gas

gliding arc discharge

dielectric barrier discharge

corona discharge

glow discharge

atmospheric pressure plasma jet

DC 0.1−100 1−10 1015−1019

DC/AC ∼1 1.4−2.1 >1013

AC/RF ∼1−3 1−30 1012−1015

pulsed DC ∼1 ∼5 109−1013

DC/AC carbon black > C2H2 > CH4 > CO2 ≈ C2H4. Deminsky et al. (Russia)58 discussed several possible plasma applications for hydrogen-rich production from hydrocarbons and ethanol. Both thermal and nonthermal plasmas were used to investigate the relationship between reforming efficiency and various operating parameters. The specific energy requirement (SER) of H2 was 0.35 and ∼0.1 eV mol−1 for methane steam reforming and ethanol steam reforming, respectively. These studies demonstrate that MW discharge is a promising technology in terms of the conversion rate, hydrogen yield, and energy consumption and may provide an efficient plasma environment for hydrogen production during the reforming process. 2.1.2.3. Dielectric Barrier Discharge (DBD). DBD, a wellknown method of nonthermal plasma discharge, consists of a layered electrode structure in which two metal electrodes are separated by a thin layer of dielectric material. It can be used in many industrial processes. DBD is a kind of weakly ionized, but strong nonequilibrium plasma and can generate a significant fraction of active chemical species that favor fuel ignition and reforming processes, although the system requires preheating to reach an elevated initial temperature of 800−1100 K in the fuel reforming process. The DBD plasma provides flexibility with respect to the geometry and operational parameters of the reactors.17c The dielectric layers limit the discharge current, thus avoiding the arc transition and enabling the reactor to operate in either a continuous or a pulsed mode.59 The DBD plasma is a stable, simple, and reliable chemical reactor.60 However, a fuel conversion system using DBD plasma is limited by its low power density and low gas temperature. In addition, the irregular structure of DBD plasmas does not favor uniform gas treatment in a fuel conversion system. One solution to increase input power is to improve the structure of electrode. Experimental research conducted by Hu et al. (China)60 showed that input power of electrode filled with quartz beads was higher than that without the beads. The generation of uniform and stable plasma in DBD reactor can be achieved using RF magnetron sputter to deposit a highly conductive ZnO film on the dielectric layer.61 Previous investigations have demonstrated that increasing plasma power at a constant excitation frequency greatly contributes to the enhancement of electric field, electron density, and gas temperature in DBD, which are found to promote the reforming process.62 It is interesting to note that packing Ni/Al2O3 catalyst into the discharge gap can result in synergistic effects of DBD and catalyst. In the case of dry reforming of methane in DBD, the introduction of Ni/Al2O3 catalyst leads to a decrease in conversion of CH4 and CO2 and an increase in H2 selectivity in comparison with DBD alone.62a

The CSIC Institute in Spain has studied the development and evaluation of a DBD plasma system under different operating conditions. Useful results were obtained from the study of the current−voltage variation (I−V variation) at atmospheric pressure and low temperatures for hydrogen production. Two types of alcohol (methanol and ethanol) have been used by Sarmiento et al. (Spain)17c to investigate the reforming efficiency of syngas production. For both alcohols, 100% alcohol conversion was obtained for relatively high reactant flows. In their work, calculations of the energy consumed in the discharge demonstrated that 1 mol of hydrogen can be produced in their reactor using approximately 0.6 and 0.3 kWh for CH3OH and CH3CH2OH as the fuel, respectively. The reactor was versatile in operation and inexpensive to manufacture. A schematic of the experimental DBD setup is shown in Figure 7.

Figure 7. Experimental DBD setup developed by CSIC: (1) a stainless steel cylinder with controlled roughness in the zone of plasma discharge, (2) entry and circulation of gases, (3) an alumina or quartz tube, (4) a stainless steel electrode, and (5) an external furnace to maintain the reactor at temperature higher than 110 °C. Reprinted with permission from ref 17c. Copyright 2007 Elsevier.

Wang et al.63 at Tianjing University (China) have also studied plasma-assisted steam reforming of ethanol for hydrogen generation under different operating conditions. In their work, a maximum hydrogen yield of 31.8% was obtained at an ethanol conversion of 88.4% under the following optimum operating conditions: vaporization room temperature of 120 °C, ethanol flux of 0.18 mL min−1, water/ethanol ratio of 7.7, and oxygen volume concentration of 13.3%. Moreover, they report detailed information on the ethanol reforming efficiency as a function of the reaction conditions, the vaporization temperature, the ethanol flow rate, the water/ ethanol ratio, and the addition of oxygen. The schematic diagram of the reaction setup is similar to that in Figure 7. Hu et al. (China)60 used a DBD plasma reactor filled with quartz beads to produce hydrogen from hydro-ethanol. A comparative experimental study on different electrode geometries was reported to elucidate the ethanol reforming process. A conversion rate of 45% was obtained from 75% ethanol at an input power of 100 W, a flow rate of 5 mL min−1, and quartz bead size of 2.0 mm. They confirmed that nonthermal DBD had a positive catalytic effect on the generation of hydrogen from hydro-ethanol. Their system was reliable and could operate for several hours. Tanabe et al. (Japan)64 investigated the generation of hydrogen from methanol in a DBD plasma system induced by low power, that is, in the range of 0.27−6.40 Wh (N cm3)−1. They reached G


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Figure 8. Schematic of the corona discharge reactor used by Paterkowski et al. (Poland).19c

Figure 9. Schematic of the pulse corona discharge system for ethanol reforming and details of the reactor. Reprinted with permission from ref 19g. Copyright 2011 Institute of Physics.

destruction, illustrating that the reactor was highly efficient in the destruction of isopropyl alcohol. Carbon dioxide and water vapor were the only products of the destruction process. Paterkowski et al. (Poland)19c also employed a corona discharge reactor to destroy ethanol at 303 K, obtaining 90% destruction at a power supply of 650 kW per active volume (m3) of plasma reactor. Similar to Kalisiak’s work, only harmless carbon dioxide and water vapor were produced. A schematic of their reactor is shown in Figure 8. A corona discharge reactor was also used by Liu et al. (China)19d to study methanol reforming efficiency at room temperature. They observed that the water content of the methanol solution had a significant effect on hydrogen production. Ethylene glycol was the major byproduct in their process. Li et al. (China)19e reported a simple and effective method for hydrogen production from plasma methanol decomposition using AC and DC corona discharges at ambient conditions. A small discharge space is adequate for sufficiently high decomposition of methanol, making corona discharge very effective for methanol decomposition. The highest hydrogen production rate was achieved with AC corona discharges. Shchedrin et al. (Ukraine)19f performed a complex theoretical and experimental investigation of the plasma kinetics of a discharge in a mixture of air and vapors of ethanol and water. The largest hydrogen output was obtained using a 1:1 water:ethanol ratio in the feedstock. Hoang et al. (U.S.)19g demonstrated a successful ethanol reforming technology using energy-efficient nonthermal plasma to produce hydrogen for fuel cell vehicles and mobile power generators. The schematic of their pulse corona discharge system for ethanol reforming and details of the reactor are shown in Figure 9.

a maximum methanol-to-hydrogen conversion of 80% with CO or CO2 as the other major product, regardless of the addition of water. Rico et al. (Poland)65 investigated methanol reforming by combining DBD with catalysts; details of that study are discussed in a subsequent section. 2.1.2.4. Corona Discharge. The corona discharge is a weak plasma that is typically found in regions of high electric field strength near sharp edges, points, or thin wires. Man-made corona discharges are typically constructed with a point-toplane geometry or as a concentric annulus. The strong nonuniformity of the electric field between the two electrodes, which is the driving force igniting a corona discharge, is generated when the characteristic size of the high voltage electrode is less than the distance between the electrodes. A spark can be produced if an additional voltage and current is applied to the plasma reactor. Spark production should be avoided not only to stabilize the discharge but also because it leads to local overheating and nonuniform treatment. Another limitation of the corona discharge is the power input. One way to avoid spark production and increase the power input is to use a pulsed power supply to deliver a high-amplitude voltage with a short rise time. Bychkov et al. (Russia)19a applied a negative corona discharge on an alcohol surface to create and visualize the electro-hydrodynamic phenomena at the interface of the discharge and the liquid (alcohol) surface. They visualized a bursting jet and a liquid column with a charged head. The products from those tests were not further analyzed. The concept and practical implementation of a corona discharge reactor for the destruction of isopropyl alcohol were studied by Kalisiak et al. (Poland).19b They reached a 95% level of H


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than that implied in ethanol steam reforming. A schematic of the experimental reactor is shown in Figure 11.

In summary, these results demonstrate that corona discharge is a useful method for the destructive oxidation of ethanol because of its high electron density and low current density, which can increase the lifetime of the electrodes. Therefore, in addition to the application in hydrogen production, corona discharge can also be used to remove ethanol and other aliphatic alcohols from various gases. 2.1.2.5. Glow Discharge. A glow discharge, a form of nonthermal plasma, is defined by a voltage drop at the electrodes and a temperature difference between the electrodes and the gas. Glow is an incandescent, self-sustaining, continuous DC discharge with a cold cathode that emits electrons into an ion-induced secondary emission. A normal glow discharge should be operated in a limited current range. If the current is too high, transition to an abnormal glow discharge or an arc discharge may occur. Yan et al. (China)21 investigated the glow discharge plasma electrolysis (GDPE) of methanol solutions for hydrogen generation. H2 and HCHO were obtained as the major products of methanol decomposition during the GDPE process. The hydrogen yield of cathodic GDPE was higher than that of anodic GDPE. The experimental results demonstrated that GDPE of methanol solutions is a promising technique for the simultaneous production of hydrogen and formaldehyde. However, the water cooling setup complicated the implement of GDPE. It is important to note that the byproduct formaldehyde is hazardous to human health and the environment. The experimental setup is shown in Figure 10.

Figure 11. Schematic reactor used in the experiments by Aubry et al. Reprinted with permission from ref 66. Copyright 2005 Elsevier.

2.1.2.6. Other Nonthermal Plasma Reactors. Kabashima et al. (Japan)67 investigated the production of hydrogen from methanol using different types of nonthermal plasma reactors under different conditions and observed that the H2 yield was closely related to the components of the carrier gas and the initial water concentration in the feedstock, among other factors. The H2 yield was analyzed as a function of the carrier gas, Ar, N2, air, or O2, which revealed that the H2 yield decreased in the order Ar > N2 > air ≈ O2 under the same conditions. Uhm et al. (Korea)68 developed a highly powerefficient electrolytic hydrogen production method using electrochemical reforming of methanol−water solutions. Experimental data demonstrated that the current efficiency of the methanol electrolysis increases as the current density increases. RF plasmas have been used to study the efficiency of methanol reforming by Hiramastsu et al. (Japan)20c and Milošević et al. (Ukraine).20a Their studies focused primarily on diamond deposition from methanol and the optical emission spectroscopy characterization of ethanol vapor. Chernyak et al. (Ukraine)69 presented the experimental and theoretical reforming process with the dynamic nonthermal plasma liquid system (PLS) based on electric discharge in a gas channel with a liquid wall (DGCLW). Compressed air was introduced along electrodes through the open nozzle ends, producing a stable counter-flow gas channel surrounded by liquid ethanol. The discharge was ignited in the gas channel between the immersed electrodes, where an electric breakdown occurred. As a result, DGCLW permitted liquid fuel burning without initial gasification. The data confirmed that the low-temperature plasmachemical conversion of liquid ethanol into synthesis gas is both possible and efficient. The schematic of the PLS reactor based on DGCLW with air as the carrier gas in water/ethanol is shown in Figure 12. Petitpas et al. (France)7a studied ethanol and E85 reforming assisted by nonthermal arc discharge and observed that a small addition of steam favored the conversion of ethanol. The optimum conditions in their study had an O/C slightly higher than the reaction stoichiometry. Sekine et al. (Japan)22b,70 used a diaphragm discharge with a liquid phase discharge at ambient temperature and atmospheric pressure for hydrogen production from biomass-ethanol. The

Figure 10. Schematic of a glow discharge plasma reactor for hydrogen production from methanol. Reprinted with permission from ref 21. Copyright 2009 Elsevier.

Aubry et al. (France)66 conducted a study of ethanol steam reforming in a nonthermal glow discharge reactor at low temperature and atmosphere pressure. The outlet species, primarily H2, CO, and CO2, were determined and analyzed as functions of the ethanol/water ratio, the electrical power, and the electrical discharge. The authors also investigated the effect of inlet ethanol/water ratio on the variations of voltage and current. Characteristics of the discharge showed that the breakdown potential VB and the input power Pi increased as the ethanol concentration increased. In addition, the rise of the ethanol/water ratio resulted in an increase of electric resistance in the ionized gas phase. From the viewpoint of energy balance, the vaporization of the ethanol/water required more energy I


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Figure 12. Schematic of the PLS reactor with a DGCLW.69

efficiency depends on several factors. Among them, the O/C and S/C are important parameters used to evaluate the product distribution and predict the optimum conditions for reforming. The O/C is the ratio of twice the molar oxygen flow rate to the molar flow rate of the fuel carbon within an ethanol oxidation reaction scheme, as will be shown in eq 2.1. Similar to the O/C, the S/C is the ratio of the molar flow rate of H2O to the molar flow rate of fuel carbon, as will be shown in eq 2.2. In our previous work on bioethanol reforming using the LNAD method,40 we observed that as the O/C increases, the production of CO2 increases; similar results were obtained by Petitpas et al. (France),7a who demonstrated that the mass of CO2 (CH4) increases at a high (low) O/C. Thus, the masses of CH4 and CO2 are important when evaluating changes in syngas production; the product rates of both are included in our previous work.14d,40 We also observed that suitable addition of water favors hydrogen production. The conversion rate (eq 2.3), the H2 yield (eq 2.4), the H2 selectivity (eq 2.5), the SER of H2 (eq 2.6), the input power, the H2 formation rate, and the energy efficiency (eq 2.7) appear to be good indicators of the reforming efficiency of alcohols for hydrogen production. The masses of other gaseous products generated from alcohols, such as CO, CH4, and CO2, are also needed to estimate the product selectivity and the corresponding reforming efficiency of various plasma reactors for hydrogen production, particularly CO2, which is crucial in the estimation of CO oxidation. In addition, some nitrogen oxides in the gaseous products, for example, NO and NO2, may be generated during the plasma-assisted reforming of alcohols. The concentration of nitrogen oxides is another important parameter for the evaluation of environmental pollution in fuel cell applications. In alcohol autothermal reforming, the conversion of both alcohols and water is crucial for the production of hydrogen. In the case of ethanol reforming assisted by GAD, the ethanol conversion rate is calculated as follows:

configuration of their reactor is shown in Figure 13. They reported that this low energy pulsed (LEP) discharge did not

Figure 13. Configuration of a diaphragm discharge reactor.22b

require high temperature or high pressure, thereby avoiding problems such as the need to provide a high temperature environment, catalyst deactivation, and coking. Steam and oxidative reforming of ethanol assisted by a sliding discharge has been developed and studied extensively at GREMI (CNRS-University of Orleans, France). New reactor designs have been proposed to optimize the hydrogen production efficiency using nonthermal plasma processes for steam reforming. 2.2. Comparison of Reforming Efficiency

2.2.1. Evaluation Methods. Gas chromatography (GC) is used to determine gaseous products. In the reforming process, the primary gaseous products are always H2, CO, CO2, CH4, C2H2, C2H4, and C2H6. In addition to gaseous products, liquid products may include trace amounts of HCOOH, CH3COOH, CH3CHO, and CH3COCH3, among others. For example, the production of trace amounts of CH3CHO and CH3COCH3 in the reforming process was observed by Zhu (U.S.),11 Rossi (Brazil),71 and Vasudeva (India) et al.72 The reforming J


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nethanol input − nethanol output

In general, the conversion rate 2.3, the power, the product production rates, and the H2/CO mole ratio as functions of O/ C 2.1, S/C 2.2, and the ethanol flow rate (G) can be obtained directly or measured experimentally. These parameters can be used to directly evaluate the reforming efficiency. Other parameters, such as the SER, the product yield, and the energy efficiency, can be calculated to estimate the effectiveness in terms of the energy transformation, the mass balance, etc. The SER 2.6 can be calculated using power and production rate data. The product yield and selectivity 2.4,2.5 are strongly related to the production rate. The energy efficiency is closely related to the product yield and the consumed power. 2.2.2. Comparison and Analysis. According to the reactive mechanisms, alcohols reforming methods can be divided into steam reforming (SR), partial oxidative reforming (POX), and autothermal reforming (ATR). The primary chemical reaction for steam reforming (including partial steam reforming and complete steam reforming) is as follows:

nethanol input

where nethanol input is the mass of ethanol entering the reforming reactor and nethanol input is the unreacted ethanol mass. The unreacted ethanol mass includes the unreacted ethanol mixed in the exhaust gas and that collected by the condenser or other container and can be measured by gas-phase chromatography. In plasma-assisted steam reforming of alcohols, the conversion rate of water is crucial and can be calculated by the following approximate method: χwater = m water output /m water input

A portion of the water can be converted to liquid products (acid, etc.); the remainder forms gaseous products during the reforming process. In addition, there is a simple method to calculate the approximate conversion of water. First, the H2 yield is defined as follows:

CH3OH + H 2O → CO2 + 3H 2

H 2 yield = n H2 in product /3nethanol input

(2.8)

The maximum hydrogen yield of 100% is represented as a maximum production of 3 mol of hydrogen from 1 mol of ethanol converted. From this equation, if the H2 yield is greater than 1, the water will contribute to the production of the syngas. The higher is this value, the more the water will contribute to the syngas. In the reforming process, the energy efficiency is also an important parameter in the ethanol conversion estimation in terms of energy. The SEI for ethanol and the SER of the products (primarily H2, CO, and H2 + CO) are calculated to evaluate the economic cost in industrial use. Other terms, such as the H2 selectivity (SH2) and the molar fraction of the desirable product, are also significant. The former can be considered as the fraction of H2 (mol) in the output of the total input hydrogen (mol), and the latter can be used to calculate the percentage of hydrogen in the gaseous products. The equations related to the reforming efficiency are as follows:

2nO2 O ratio = C xnCx HyOH

η=

(2.9)

C2H5OH + 3H 2O → 2CO2 + 6H 2 ΔH = +174 kJ mol−1

(2.10)

CH3OH + 0.5O2 → 2H 2 + 3CO2 ΔH = −192 kJ mol−1 C2H5OH + 0.5O2 → 2CO + 3H 2

(2.11)

ΔH = +20 kJ mol−1 (2.12)

C2H5OH + 3O2 → 2CO2 + 3H 2O

ΔH = −552.0 kJ mol−1

(2.2)

(2.13)

(y + 1)nCx HyOHinput 2n H2 in product nH in product

× 100%

× 100% (2.4)

× 100% (2.5)

P (W) × 0.001 R[X] (mol s−1)

n H2 × LHVH2 + nCO × LHVCO nCx HyOHinput × LHVCx HyOH + IPE

(2.14)

Because steam reforming is slightly endothermic and ethanol oxidation to H2 is slightly exothermic, autothermal reforming is ideally obtained by balancing the two reactions.73 Autothermal reforming has the potential to achieve high hydrogen yield and hydrogen selectivity because syngas can be produced from air or H2O. Plasma assistance of the partial oxidation and steam reforming of ethanol then can exploit the extra H2 produced by the water gas shift (WGS) reaction and the heat generated by the total oxidation. Thus, the autothermal reaction is more favorable for hydrogen production than pyrolysis or oxidative reforming. The autothermal reforming of alcohols is described as follows:

(2.3)

2n H2 in product

SER (H2,CO,H2 + CO) =

ΔH = + 256 kJ mol−1

C2H5OH + 1.5O2 → 3H 2 + 2CO2

nCx HyOHinput

H 2 selectivity (%) =

C2H5OH + H 2O → 2CO + 4H 2

(2.1)

(nCx HyOHinput − nCx HyOHoutput)

H 2 yield (%) =

In addition, the process of oxidative reaction (including partial oxidation and complete oxidation) can be described as follows:

ΔH = −1280 kJ mol−1

n H 2O S ratio = C xnCx HyOH χ=

ΔH ° = + 59 kJ mol−1

(2.6)

CH3OH + 0.4H 2O + 0.5O2 → CO2 + 2.4H 2 ΔH 573≌ − 86 kJ mol−1

× 100% (2.7)

(2.15)

C2H5OH + 2H 2O + 0.5O2 → 2CO2 + 5H 2

LHV is defined as the lower heating values of each component, and IPE is defined as the input plasma energy.

ΔHR ≌ − 50 kJ mol−1 K

(2.16)


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Figure 14. Chemical reactions for ethanol reforming.

Table 2. Comparison of the Performances of Different Plasmasa compactness

gliding arc discharge microwave discharge dielectric barrier discharge corona discharge glow discharge radiofrequency arc

++

++

++

+

−

+

+

++

++

++

++

+

excellent reformer, while the resistance time of reactant should be extended further for mixing better and reacting longer high economic cost, a high conversion rate and good H2 selectivity; while it has low H2 production rate and is limited by the extra energy requirement for vacuum device low H2 yield in the products, suitable for combining with catalysts

+

+

+

+

limited by power input

++ −

+ +

+ +

− −

low pressure and conversion rate for reforming process very low pressure, high frequencies; it usually is used to produce diamond

++

+

+

+

short lifetime of electrodes; high conversion rate and low SER

a

stability reliability

reforming efficiency

type

descriptions

“−” means poor. “+” means moderate. “++” means good.

reforming to improve the conversion efficiency and energy efficiency. A comparison of the performances of different plasmas is presented in Table 2. The data in Table 2 indicate that nonthermal GAD plasma is the most suitable for fuel reforming. On the basis of the previous work on plasma-assisted fuel reforming described above, the limitations of DBD are low energy efficiency and low power density due to the limited current and low gas temperature. GAD, which is autooscillating and normally extinguishes and reignites periodically, has a better reforming efficiency than either DBD or corona discharge. This increased efficiency is due to the higher transitional gas temperature of GAD, which can accelerate the kinetics of the reforming reactions, while the plasma radicals reduce the energy barriers and prevent unwanted byproduct (soot) formation.74 Microwave discharge requires a complex and somewhat more expensive power supply that requires a high-frequency generator to initiate high-frequency plasma. The most suitable method for hydrogen production from ethanol reforming is reported as the nonthermal gliding arc discharge, due to its flexibility and high effectiveness. The characteristics of various nonthermal plasmas for hydrogen production from hydrocarbon reforming have been analyzed by Petitpas et al. (France),22a who demonstrated that the plasma process can reform fuels with high conversion efficiency. The plasma process is an important technology for fuel cell applications that should be further developed and optimized.

Cx HyOz + H 2O + O2 → COx + CH + HCs + H 2 + char

(2.17)

CH3CH 2OH 2 + (3 − 2x)H 2O + xO → 2 CO2 + (6 − 2x)H 2

(0 < x < 0.5)

(2.18)

(ΔH = ((3 − 2x)/3) × 207.7 − (x /1.5) × 545.2 kJ mol−1)

Thermal decomposition (or pyrolysis) also plays a key role in the reforming process. The chemical reactions for ethanol reforming are shown in Figure 14. In the case of methanol, the pyrolysis is described by the following: CH3OH (liquid) → CO + 2H 2 ΔG298K = 29.29 kJ mol−1

(2.19)

2CH3OH (liquid) → (CH 2OH) + H 2 ΔG298K = 13.14 kJ mol−1

CH3OH (g) → HCHO (g) + H 2

(2.20)

ΔG298K = 59 kJ mol−1 (2.21)

Plasma fuel reforming studies have primarily focused on the identification of the best discharge method for ethanol L


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Figure 15. Plausible radical reactions for alcohol reforming in plasma reactors.75 The radicals include H• and OH•, among others.

2.3. Mechanism of Plasma-Assisted Reforming

In this section, we discuss several factors and how they relate to the reforming efficiency. We focus on the complex interactions between radicals and alcohols in the plasma-assisted reforming process; the detailed mechanisms are shown in Figure 15. Some of the intermediates produced during the process are not shown in Figure 15 because they do not contribute directly to the end products in the regime under investigation; however, they cannot be ignored in the scheme of the reactions. As shown in Figure 15, ethanol dissociates into three isomeric radicals, CH3CH2O, CH3CHOH, and CH2CH2OH, with comparable probabilities; however, CH3CHOH is always chosen and devoted to C2H5O because of the high rate constants of its chemical reactions. For a better understanding of the reforming mechanism, the electron−molecule−radical reactions for ethanol reforming are shown in Figure 16. The equations in Figures 15 and 16 show that the electrons and radicals both contribute to ethanol reforming. The significant radicals related to the reforming process include O, H, OH, HO2, and CH3 for the production of CH4, H2O, and syngas (CO + H2). The OH and CH3 groups of the ethanol molecules are preferentially decomposed in the plasma-assisted reforming process.76 Initial H2 production is generated via two pathways: fuel and water dissociation by electron impact. In the plasma region, during the first 10−100 μs, the primary sources of atoms and radicals (O, OH, H, CH3, CH2OH, C2H5O, and C2H5) are electron−molecular dissociation reactions of primary components. These reactions are negligible in the lowtemperature plasma but play an important role in the tornado-type electrical discharge.48 The reactions in Figure 16 show that atomic hydrogen is removed from ethanol and the

Figure 16. Electron−molecular reactions for ethanol reforming.75

C−C bonds are subsequently dissociated by electron impact, leading to the formation of H2 and CO. Hydrogen is then generated by the reaction between the radicals of CH2OH, CH3CHOH, or formaldehyde and hydrogen atoms.14b Further hydrogen removal by electron and radical impact leads to the formation of COx and C2 species.53 During the last five decades, the kinetics of gas-phase ethanol oxidation in the absence of plasma has been investigated extensively by Marinov (U.S.),77 Egolfopoulos (U.S.),78 and Li (U.S.) et al.79 Elder et al. (Brazil)80 presented a systematic comparison of current detailed chemical kinetic models and available experimental data related to the kinetics of ethanol. However, the kinetic mechanism of plasma-assisted ethanol reforming remains unclear and has not been systematically evaluated as part of the elucidation of the reforming process. M


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Levko et al. (Ukraine)48,69,81 have proposed mechanisms for both the low-temperature region and the higher gas-temperature region for the tornado-type electrical discharge. Marinov’s77a mechanism and Dagaout’s80 ethanol submechanism have been shown to be more appropriate for describing the chemical reactions in the investigated mixture. Dunphy et al. (Ireland)82 proposed a kinetic mechanism for high-temperature ethanol oxidation. In their study, the temperature investigated was similar to that of Levko’s, that is, approximately 1080− 1660 K. Density functional theory (DFT) was used by Wang et al. (China)83 to study the thermodynamics of ethanol steam reforming under cold plasma conditions. A methanol submechanism was studied by Held et al. (U.S.).84 If active species generated by nonthermal plasma are capable of promoting many cycles of chemical transformation, then the high specific productivity of plasma can be combined with the low energy consumption of traditional catalysts. This potential is why the plasma-catalyst approach has attracted permanent, strong interest in recent decades.13 Generally, the gaseous products of ethanol reforming assisted by plasma are H2, CO, CO2, CH4, C2H2, C2H4, C2H6, and C3H8. Trace amounts of liquid and solid products are also produced in the process, including HCOOH, CH3COOH, CH3CHO, CH3COCH3, and C. Trace liquid products generated in the process were also observed by Zhu (U.S.),11 Rossi (Brazil),71 and Vasudeva (India) et al.72 Sarmiento et al. (Spain)85 reported that a slight excess of CO2 or H2O added to the fuel mixture prevents the production of soot particles because of the direct dissociation of alcohols. The relevant reactions are as follows: CH3CH 2OH + CO2 → 3CO + 3H 2

(2.22)

CH4 + CO2 → 2CO + 2H 2

(2.23)

CH4 + H 2O → CO + 3H 2

(2.24)

CH3OH → HCOH + H 2

Generally, plasma-generated radicals and ions behave in ways similar to catalysts, participating in chain reactions and promoting or accelerating reaction pathways. Therefore, the discharge acting as a catalyst contributes to the generation of active species, gas heating, and fuel conversion into syngas, CxHy, CO2, and coke. The conversion can occur in the discharge region and in the postdischarge region, where the density of electrons and radicals is high. 2.4. Factors Influencing Alcohols Reforming

2.4.1. Feedstock Components. Feedstock components are important in the production of radicals and subsequently influence the reforming efficiency and product distribution. In the reforming process, the emission discharge spectrum, determined by optical emission spectroscopy, can measure the characteristics of bands and lines of various species in the spectrum of the ethanol plasma. The binding energy of the O− H bond in H2O is 497.1 kJ mol−1, which is higher than that of the C−H bond in methanol by 95.18 kJ mol−1. Therefore, methanol is more reactive than water in plasma.21 However, the addition of water is an important parameter in the reforming process. The reactions in Figure 15 show that the generation of the hydroxyl radical is particularly important in alcohol oxidation because it is one of the most powerful oxidizing agents. The active form of the hydrogen atom and the hydroxyl radical can be generated by the hemolytic dissociation of water molecules by plasma. Liu et al. (China)19d studied methanol reforming with corona discharge using DFT, which revealed that the presence of water may promote the formation of CH2OH via the dissociation of water because the generation of OH from water molecules initiates an oxidative dissociation of methanol. Obtaining the maximum hydrogen yield depends on a sufficient water content in the feedstock. One reason is that the oxygen required by the POX alcohols reforming process can be generated by H2O, leading to a higher hydrogen yield in the products. Moreover, the H atoms produced by the water decomposition process can recombine with one another and generate hydrogen gas. Wang et al. (China)63 studied O/C and S/C as functions of the reforming efficiency and demonstrated that the addition of water favored ethanol decomposition and H2 production. The conversion rate increased initially and then decreased slightly with the increasing volume fraction of oxygen. With increasing water, the oxygen radical and the hydroxyl radical from the water molecules become more active in the energy transfer and collide with ethanol molecules to promote their decomposition. As the volume concentration of oxygen increases, oxygen radicals generated from oxygen easily combine with free radicals produced from ethanol decomposition and are converted to hydrocarbon compounds, leading to an increase in the ethanol conversion rate. Radicals generated from the feedstock usually play a crucial role in reforming reactions. The appearance of intermediate species of the C2 and CHx types in plasma-assisted ethanol reforming can be an indication of the formation of solid carbon and higher hydrocarbons because C2 and CHx species are typically intermediate radicals in the formation mechanism of these products. To obtain a higher hydrogen yield, it is important to increase the mass of H radicals, which can be produced from fuel and added water. The dehydrogenation by the interaction of H with C1 and C2 species and the dimerization reactions of the CHx radicals are shown in Table 3.

The pathways for direct ethanol reforming can be divided into dehydrogenation (2.25,2.26),7a,86 dehydration (2.27), decomposition (2.28−2.30)86,87 and coking (2.31), described separately as follows: C2H5OH → C2H4O + H 2

(2.25)

C2H5OH → CH3CH 2O + 0.5H 2

(2.26)

C2H5OH → C2H4O + H 2O

(2.27)

2C2H5OH → C3H6O + CO + 3H 2

(2.28)

C2H5OH → CH4O + CO + H 2

(2.29)

C2H5OH → 0.5CO2 + 1.5CH4

(2.30)

CH3CH 2OH → CO + 3H 2 + C

(2.31)

According to previous studies, the electron−molecule reactions and decomposition of methanol are as follows:21 CH3OH + e → CH3 + OH + e

(2.32)

CH3OH + e → CH 2OH + H + e

(2.33)

CH3OH + e → CH 2 + H 2O + e

(2.34)

CH3OH → trans‐HCOH + H 2

(2.35)

CH3OH → cis‐HCOH + H 2

(2.36)

(2.37)

N


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Table 3. Dehydrogenation of C1 and C2 Species and the Dimerization Reactions of CHx Radicals no.

reaction

(2.38) (2.39) (2.40) (2.41) (2.42) (2.43) (2.44) (2.45) (2.46) (2.47) (2.48) (2.49) (2.50) (2.51) (2.52) (2.53) (2.54) (2.55) (2.56) (2.57) (2.58) (2.59) (2.60) (2.61) (2.62) (2.63)

C2H6 + H → C2H5 + H2 C2H5 + H → 2CH3 C2H5 + H → C2H4 + H2 C2H4 + H → C2H3 + H2 C2H3 + H → C2H2 + H2 C2H2 + H → C2H + H2 CH4 + H → CH3 + H2 CH3 + H → CH2 + H2 CH3 + CH3 → C2H6 CH3 + CH3 → C2H5 + H CH3 + CH3 → C2H4 + H2 CH3 + CH2 → C2H4 + H CH3 + CH → C2H3 + H CH3 + OH → CH2O + H2 CH3 + O → CH2O + H CH3 + H + M → CH4 + M CH3 + CH3 + M → C2H6 + M CH3 + CH2 → C2H4 + H CH3 + C2H4(+M) → n-C3H7(+M) CH3 + C2H2(+M) → C3H5(+M) CH2 + CH2 → C2H4 CH2 + CH2 → C2H2 + H2 CH2 + H → CH + H2 CH + CH4 → C2H4 + H CH + CH2 → C2H2 + H CH + CH → C2H2

rate constant k (cm3 s−1) 2.4 × 10−15T1.5 exp(−3730/T) 6 × 10−11 5 × 10−11 9 × 10−11 exp(−7500/T) 2 × 10−11 1 × 10−10 exp(−14 000/T) 2.2 × 10−20T3 exp(−4045/T) 1 × 10−10 exp(−7600/T) 6 × 10−11 5 × 10−11 exp(−6800/T) 1.7 × 10−8 exp(−16 000/T) 7 × 10−11 5 × 10−11 2.59 × 10−13(T/298)−0.53 exp(−5440/T) 1.4 × 10−10 2.68 × 10−28(T/298)−2.98 exp(−635/T) 1.68 × 10−28(T/298)−7 exp(−1390/T) 1.2 × 10−10 3.5 × 10−13 exp(−3700/T) 6.1 × 10−11 1.7 × 10−12 2 × 10−10 exp(−400/T) 1 × 10−11 exp(900/T) 16 × 10−10 6.6 × 10−11 2 × 10−10

The data in Table 3 indicate that the concentration of C1− C2 radicals is closely related to the product selectivity, particularly the generation of higher hydrocarbons. Although the production of specific radicals to increase the selectivity of hydrogen cannot be controlled, the fundamental mechanisms of plasma-assisted fuel conversion merit further investigation. An optimal feedstock concentration may reduce the cost of hydrogen generation and increase product selectivity. Yanguas-Gil et al. (Spain)53 studied hydrogen production from ethanol with MW surface wave discharge. They reported that the average rotational temperature of the OH* species for pure ethanol was higher than that for a mixture of ethanol and water in the MW plasma reactor. The dissociation energies of C2, CO, CH, and OH species are 6.2, 11.09, 3.46, and 4.39 eV, respectively,93 and thus more energy may be released by the formation and postdecomposition of diatomic species. For example, the excited species CO* and C2* transfer the vibrational and rotational modes of CO and C2 to the rest of the species in the plasma, particularly OH*. This leads to an excess of energy, resulting in higher temperatures and higher degrees of excitation in the pure ethanol. Therefore, the product distribution generated in the reforming process can differ, depending on the addition of water to the feedstock. In general, the components of the feedstock have a significant impact on the reforming efficiency, particularly the gaseous product yield during the reforming process. 2.4.2. Carrier Gas. The carrier gas is closely related to the radical species in alcohols reforming and subsequently influences the generation of the gaseous products. When air is used as the carrier gas, the primary components, oxygen and nitrogen, can also react with radicals and electrons. The electron−molecule reactions of air and the reactions related to the generation of NOx are shown in Table 4. Nitrogen, which

k (1500 K)

k (2500 K)

ref

1.2 × 10−11 6 × 10−11 5 × 10−11 6 × 10−11 2 × 10−11 8.8 × 10−11 5 × 10−11 6.3 × 10−13 6 × 10−11 5.4 × 10−13 3.9 × 10−13 7 × 10−11 5 × 10−11

6.7 × 10−11 6 × 10−11 5 × 10−11 4.5 × 10−11 2 × 10−11 3.7 × 10−11 6.8 × 10−11 4.8 × 10−12 6 × 10−11 3.3 × 10−12 2.8 × 10−11 7 × 10−11 5 × 10−11

1.7 × 10−12 1.5 × 10−10 1.8 × 10−11 1 × 10−10 6.6 × 10−11 2 × 10−10

1.7 × 10−12 1.7 × 10−10 1.4 × 10−11 1 × 10−10 6.6 × 10−11 2 × 10−10

88 88 88 88 88 88 88 88 88 88 88 88 88 89 90 91 92 92 92 92 88 88 88 88 88 88

Table 4. Electron−Molecule Reactions of Air and Reactions Related to the Generation of NOx number

reactions

ref

(2.64) (2.65) (2.66) (2.67) (2.68) (2.69) (2.70) (2.71) (2.72) (2.73) (2.74) (2.75) (2.76) (2.77) (2.78) (2.79) (2.80) (2.81)

N2 + e → N2(∑u+) + e N2 + e → N2(∏g) + e N2 + e → N2(ν) + e N2 + e → N + N + e N2 + e → N2+ + 2e 2N2 + e → N2+ + 2N+ + 3e N2 + e → 2N2+ + 3e O2 + e → O + O + e O2 + e → O2(1Δg) + e O2 + e → O2+ + 2e NO + O → NO2 NO + OH → HNO2; HNO2 + OH → NO2 + H2O NO + HO2 → NO2 + O NO + O3 → NO2 + O2 NO2 + O → NO + O2 N + OH → NO + H N + HO2 → NO + OH N + O2 → NO + O

48, 95 48, 96 48, 97 48, 94 48, 98 98 98 48, 99 48, 100 48, 98 94 94 94 94 94 94 94 94

acts as a third party in the recombination and thermal dissociation reactions, is closely related to the concentration of nitrogen oxide, which is an important parameter that is currently strictly limited in vehicle exhausts. Young et al. (Korea)94 proposed a mathematical model to describe the behavior of the removal of nitrogen oxides (NOx) in a positive pulsed corona discharge reactor. The ozone (O3) produced by the reaction of O radicals with oxygen was demonstrated to play a crucial role in the oxidation of NO to NO2, both theoretically and experimentally. In addition to ozone, O and O


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Figure 17. Infrared absorption spectra of the Ar and ethanol mixture before and after plasma processing for two different [(a) Gethanol = 4.6 sccm and (b) Gethanol = 12.5 sccm] ethanol flow rates from Henriques et al. Reprinted with permission from ref 55. Copyright 2011 Elsevier.

conclusions. The former determined that the conversion of biogas increased by increasing the power supply regardless of changes in the voltage or frequency; however, the latter demonstrated that the density and energy of the electrons related only to the voltage did not change with changes in the power input by adjusting the frequency at constant voltage. In our lab, the input power at a fixed frequency has been investigated as a function of the ethanol conversion and H2 yield to determine the influence of power on the steamoxidative reforming of ethanol. The results obtained in our study were consistent with those of Batiot-Dupeyra et al. Yan et al. (China)21 demonstrated that increasing the voltage has a positive effect on methanol conversion. Futamura et al. (Japan)103 studied the effects of reactor type and voltage properties on methanol conversion, reaction behavior, and product yields for hydrogen production with nonthermal plasma. Considering the influence of the voltage waveform on the reforming process, they reported that the methanol conversion decreased in the order triangle > sine > square with 50-Hz AC power at the same applied voltage levels for a ferroelectric packed-bed reactor (FPR) and a silent discharge reactor (SDR). As mentioned above, the reforming efficiency is also influenced by the power frequency. In a DBD plasma reactor, the input power and energetic electron population increase as the frequency of the supply power approaches the resonant frequency of its load, leading to a higher alcohols reforming efficiency. A more energy-efficient system can be constructed for alcohols reforming with nonthermal plasma if power with higher efficiency energy transfer is available. The increase in plasma power causes an increase in high-electron density, which can enhance the excitation processes induced by electron impact, that in turn produces a positive effect on alcohol conversion. The power is closely related to the density of plasma, and almost all of the energy is deposited into the electrons in the nonthermal plasma. A large quantity of active species (energetic electrons, radicals, etc.) is generated from the reaction of electrons and molecular decomposed alcohols with the assistance of plasma. In the case of ethanol, the well-known reactions occurring between its molecules and electrons include the following:48,83

OH are also important because the O radical is a source of ozone, as shown in Table 4. Pure Ar, N2, or O2 are often considered as carrier gas. Gaseous components in the products are usually influenced by the type of carrier gas employed. Kabashima et al. (Japan)67 demonstrated that the H2 yield under the same conditions with a nonthermal plasma decreased in the order of Ar > N2 > air ≈ O2. This relationship can be described as follows: (1) energy transfers to methanol from Ar more efficiently (1∏01, 11.83 eV) than from N2 (a1∏g, 8.59 eV), leading to a higher H2 yield in Ar carrier gas; (2) the transition from N2 (X1Σ+g ) to N2 (a1∏g) by electric dipole is forbidden, and N2 excitation is expected to be less efficient than that of Ar; and (3) O2 and air are much less effective background gases. O2 and the hydroxyl radical generated from H2O react rapidly with H or H2. The dissipation of energetic electrons by O2 lowers the efficiency of H2 generation. The infrared absorption spectra of the Ar/ ethanol mixture before and after plasma processing for two different ethanol flow rates obtained by Henriques et al. (Portugal)55 are shown in Figure 17. These data demonstrate that the Ar environment enabled complete ethanol decomposition and yielded primarily CO2 and H2O as byproducts. The steam reforming and WGS reactions played a crucial role in the plasma-assisted steam reforming of ethanol. It is important to choose a suitable carrier gas for different applications of plasma reactors. Further studies of the relationship between plasma-assisted alcohols reforming and the carrier gas are needed and are in progress. 2.4.3. Input Power. The input power is a key parameter influencing the reforming efficiency of a plasma reactor system. The input power may be affected by both the instantaneous voltage and the duration of a single period. The discharge power, which has units of watt, can be calculated by the instantaneous voltage and current within a single period as follows: T

P (W) =

∫0 UIt t dt T

where the instantaneous voltage Ut and the instantaneous current It are the functions of time (t), and T is the duration of a single period. Both Batiot-Dupeyra (France)101 and Liu (China) et al.102 have studied the influence of the input power on the performance of the reactor system and obtained conflicting P

C2H5OH + e → CH 2OH + CH3 + e

(2.82)

C2H5OH + e → CH3CHOH + H + e

(2.83)

C2H5OH + e → CH3CH 2O + H + e

(2.84)


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C2H5OH + e → CH3CH 2 + OH + e

(2.85)

C2H5OH + e → CH 2O + CH4 + e

(2.86)

C2H5OH + e → CH3CHO + H 2 + e

(2.87)

C2H5OH + e → CH3CH 2OH+ + 2e

(2.88)

C2H5OH + e → CH3CH 2OH−

(2.89)

to assume that the reactor type has no impact on the product distribution when similar levels of electron energy per volume are injected into the reaction media in the nonthermal discharge mode.103 In the case of other plasma reactors such as the GAD, the rotation of the carrier gas in the GAD can increase both the residence time of the particles generated from the feedstock and the turbulence in the reactive space, which can increase the collision frequency and rapidly regenerate the reaction interface of the fluid pathway. A variety of studies have been performed on the modification of plasma reactor geometry to obtain higher reforming efficiency. Future advances in manufacturing technology will improve the geometry of plasma reactors to achieve higher treatment efficiency and lower overall cost. 2.4.5. Temperature. Among influencing factors, gas temperature is an indicator of the energy of the heavy plasma particles that participate in the excitation reaction of the plasma-generated radicals.104 Radical-based chemistry generally depends strongly on temperature, whereas the rates of electron impact and electron attachment are not strongly dependent on temperature.105 In practice, a low reaction temperature will lead to the formation of various undesirable products, such as acetaldehyde, acetone, and ethylene. Increasing the reaction temperature of the gas mixture may increase ethanol conversion and hydrogen production, accompanied by a corresponding increase in the selectivities of H2, CO, CO2, and CH4 and a decrease in the selectivities of acetaldehyde, acetone, and ethylene.106 Because the gas mixture still reaches a relatively high temperature in the postdischarge region, the conversion is effective after the discharge is quenched.107 Tsymbalyuk et al. (Ukraine)107 attributed the effect of high temperature to the increased rate of electronic−molecular reactions. They also demonstrated that H2/CO increases with temperature, whereas the concentration of hydrocarbons decreases. Tsyganov et al. (Portugal)54a developed a theoretical model to describe ethanol decomposition in microwave plasma. They determined that H2 production increased at temperatures ranging from 0 to 1000 K and was dominant at temperatures of 1000−3500 K. However, a complete decomposition of H2 and C2 could result in carbon formation at temperatures above 3500 K. The equilibrium diagram for ethanol decomposition products is shown in Figure 18. 2.4.6. Residence Time. The residence time is a critical parameter affecting ethanol reforming and hydrogen gener-

The specific energy in the discharge region, the average energy of energetic electrons, and their number increase as the input power increases. The increase in energetic electrons has a positive effect on the production of small molecular species such as hydrogen. H2 generation during the reforming process is primarily determined by the reaction (C2H5OH + H → CH3CH2O + H2), whose rate depends on the number of H atoms generated mostly from the reaction of e-impact dissociation of H2O (H2O + e → OH + H + e). The rate of H atom generation is directly proportional to the deposited discharge power.19f The thickness of the plasma sheath, that is, the discharge plasma volume, increases with the increase in discharge voltage. This indicates that the path of the highenergy electrons within the discharge plasma increases. That is, the opportunity for alcohol molecules to collide with other particles increases. The efficiency of high-energy electrons to decompose fuel molecules then also increases with increasing voltage; however, the voltage cannot increase without limitation because the discharge electrode will melt if the voltage is too high. The plasma-assisted reforming of methanol is easier than that of ethanol, propanol, and butanol because there is no C−C bond in methanol with energy higher than the C−H bond. Sarmiento et al. (Spain)17c also presented the I−V characteristic as a function of time, revealing that increasing the voltage was always beneficial for the conversion of alcohols. To optimize the reaction yield, further studies of the alcohols reforming mechanism are needed to enable control of the plasma-assisted reforming process. 2.4.4. Geometry of the Plasma Reactor. Many studies have been performed to determine the influence of different operating conditions, such as the geometry of the plasma reactor, the voltage and the frequency of the power supply, the carrier gas, and the additions of catalysts. In this section, we focus on the influence of geometry on the reforming efficiency. For example, for DBD discharge reactors, studies have demonstrated that the reaction yield depends on the roughness of the inner stainless steel tube; that is, the yield decreases when using a flat tube in which no roughness has been artificially created by sand blasting.17c Futamura et al. (Japan)103 demonstrated that the reactor type is closely related to the molar ratio of H2 to COx. They used a tubular-type SDR consisting of a stainless-steel rod coated with copper and an encircling glass tube, which was wrapped with aluminum tape (100 mm wide). The other reactor was a coaxial-type FDR consisting of an inner cylindrical electrode and an outer electrode. Different H2/COx ratios were obtained with the different reactors (SDR and FPR); higher-energy electrons were more densely populated in the FPR than in the SDR. Moreover, because the bond dissociation energy of C−O in MeOH (236.2 kJ mol−1) is lower than that of C−H (401.8 kJ mol−1), some of the CO produced was oxidized to CO2, and the CH4/COx ratios of the products decreased more in the FPR than in the SDR. Because the lifetime of the activated electron in the nonthermal plasma is shorter than 10−6 s, it is reasonable

Figure 18. Simplified equilibrium diagram for ethanol decomposition products. Reprinted with permission from ref 54a. Copyright 2013 Elsevier. Q


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Figure 19. Schematic representations and I−V characteristics of different DBD configurations. The first DBD configuration is similar to Figure 7. The second DBD configuration included: (1) a stainless steel cylinder without controlled roughness in the zone of plasma discharge, (2) the inner electrode covered with an alumina or filled with ferroelectric pellets of BaTiO3, (3) inlet and circulation of gases, (4) a stainless steel electrode, and (5) an external furnace to maintain the reactor at temperature high than 110 °C. Reprinted with permission from ref 113. Copyright 2010 American Chemical Society.

3. PLASMA-CATALYTIC TECHNIQUE Hydrogen has been studied extensively as an attractive and environmentally friendly alternative fuel. The production of H2 and CO from liquid feedstock is of interest for many industrial processes but does not represent a straightforward approach for some fuel cell applications because the carbon monoxide fouls the electrode, for example, in proton exchange membrane (PEM) fuel cells. Most of the conventional hydrogen production strategies based on alcohols rely on the single use of catalysts containing transition or noble metals,11,109 but the method has many disadvantages, such as a high device weight and a relatively long transient time. Plasma technology appears to be a good technique for alcohols reforming with a rapid startup, high conversion rate, and good H2 yield but is limited by the high CO concentration of its products. One solution is to combine plasma with a catalyst to take advantage of the high efficiency of plasma technology and the selectivity of a catalyst that can oxidize CO to CO2 in the presence of excess hydrogen using water as an oxidative reactant. The plasma-catalytic technique has the potential to enhance alcohols conversion, increase the selectivity toward the desired products, and reduce the operating temperature of the catalyst, increasing both the energy efficiency and the catalyst stability by reducing fouling, coking, and sintering.110 The combined application of plasma and catalyst to achieve low CO mole fractions has been only minimally addressed in the literature52,85,103 but will be discussed in this Review. Catalysts play a crucial role in the reactivity toward complete conversion of fuels, and the choice

ation. The reforming efficiency can increase with increasing residence time because there is more time for the reaction between various active species and molecules. When the flow rate of the feedstock is high, the particles generated from the feedstock leave the high electric field in a short residence time because of the fast speed, leading to a reduced reaction time, and, subsequently, a decreased conversion rate. If the residence time is too long, however, the reverse reaction would be initiated, resulting in a decrease in the conversion efficiency. Therefore, there is a theoretical optimum fuel flow rate associated with each plasma reactor, particularly in the case of GAD, and the residence time of the alcohol has a significant effect on the conversion rate and the reforming efficiency. 2.4.7. Others Factors. Other factors, such as the electric field, the gas pressure, the ozone concentration, the electron density, and the active species, can also influence the fuel reforming activity. For example, because electron impact gives rise to the first step of the decomposition process and the generation of reactive species, the electron density is an important parameter in plasma-assisted reforming of alcohols for hydrogen production.104 Decreasing ethanol conversions and increasing hydrogen production were obtained with increasing pressure.108 An increasing number of influencing factors have been considered to obtain a more efficient reforming system. Additional analyses of these factors are necessary and are in progress. R


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configurations on the direct decomposition of methanol.113 The schematic representations and I−V characteristics of different DBD configurations are also shown in Figure 19. Bromberg et al. (U.S.)8 investigated ethanol reforming using plasma-catalytic technology in the GEN 3 plasma reactor using a Ni/Al2O3 catalyst. The conversion rate and hydrogen selectivity were improved by the combination of these two technologies. Futamura et al. (Japan)103 studied the reactor type and voltage properties affecting the reforming behavior of methanol in N2 with nonthermal plasma. The energy conversion efficiency increased 40−60% at ∼80% methanol conversion for FPR and SDR. The primary products were H2, CO, and CO2. Schematics of the FPR and the SDR are shown in Figure 20. BaTiO3 pellets were used as the catalyst in this study.

of catalyst should be consistent with maximizing hydrogen selectivity and inhibiting coke formation and CO production. 3.1. Types of Catalysts

3.1.1. Noble Metal Catalysts. Zhu et al. (China)111 studied bioethanol reforming using a plasma-catalytic reactor for hydrogen production at low temperatures and atmospheric pressure without gas diluents or external heating. The plasma used in their work was the DC pulse discharge (corona) plasma. The discharge-generated heat was effectively used for feed vaporization and the WGS reaction. A large amount of the CO in the H2-rich gas from the plasma reforming of ethanol was reduced when a Pt/TiO2 and Pt−Re/TiO2 stacked bed was used as the in situ WGS catalyst with a gas hourly space velocity up to 12 000 cm3 g−1 h−1. In addition to H2 and CO2, only small amounts of CO, CH4, and C2H6 were produced, which is acceptable for H2 fuel cell use after residual CO removal. Both the plasma and the catalyst exhibited good stability toward daily startup and shutdown. Uhm et al. (Korea)68 reported high power savings in electrolytic hydrogen production using electrochemical reforming of methanol−water solutions. They determined that Pt was a more effective electro-catalyst for methanol electrolysis than PtRu based on current efficiency and overvoltage in conjunction with stability against dissolution. In their work, Pt and PtRu black were used as anode catalysts, and Pt black (3 mg cm−2) was used as the cathode catalyst. 3.1.2. Non-noble Metal Catalysts. A pulsed corona discharge reactor serially combined with Al2O3−MnO2 fixedbed catalytic post-treatment has been investigated by Jarrige et al. (France)112 for the treatment of dry air containing isopropyl alcohol at room temperature. Catalytic post-treatment reduced the energy cost of the removal of volatile organic compounds (VOCs) by nonthermal plasma by 2−4-fold. In the decomposition process, a large amount of ozone produced in the discharge reactor is efficiently decomposed, and a high destruction rate of the VOCs was obtained using the catalysts. The conversion of pollutants into CO2 was favored over CO using the plasma-catalyst method. Hazardous byproducts, such as formaldehyde or methyl nitrate, resulting from VOCs degradation in nonthermal plasma were partly or totally removed. Rico et al. (CSIC, Spain)65 employed a DBD operating at atmospheric pressure and reduced temperatures (T < 115 °C) for hydrogen production and preferential CO oxidation (CO-PROX). A copper−manganese oxide catalyst was added for the direct decomposition and steam reforming of methanol. The maximum 100% conversion rate was obtained with a combination of DBD plasma and Cu−Mn catalyst. The outlet reaction products included CH2O, HCOOH, CO, H2, and H2O. A WGS-specific experiment was performed to oxidize CO (CO-PROX) (0.22 mmol min−1) with water (0.22 mmol min−1) in the presence of an excess of H2 (0.44 mmol min−1) to obtain nearly complete oxidation of CO with respect to hydrogen, which was partially reoxidized into water. Their work demonstrated that a synergistic interaction between the plasma and catalyst constitutes an efficient alternative to conventional catalytic reactions for the reforming of methanol and other related processes.65 A schematic representation of the reaction conditions and primary products obtained during the direct decomposition of methanol in the hybrid catalyst-plasma reactor is shown in Figure 19. The experiment was performed with BaTiO3 and Cu−Mn catalysts in the presence of plasma. They also studied the influence of various DBD plasma

Figure 20. Schematics of the FPR (A) and the SDR (B).103

3.2. Comparison and Analysis

Interest in the application of plasma technologies to alcohols reforming to generate hydrogen has increased recently due to its fast ignition, reforming compatibility, and compactness. In general, hydrogen selectivity is lower with plasma technologies than with catalytic technologies. Plasma-catalytic technology combines the advantages of the high product selectivities of catalysts with the fast startup gained from the plasma technique. Commonly used catalysts include Ni, Cu, Mn, Re, and Rt supported on metal oxides such as TiO2, CeO2, Al2O3, and MnO2. BaTiO3 is also a useful catalyst for plasma-assisted alcohols reforming. The relative reforming efficiency of the plasma-catalytic technology as compared to plasma technology alone is shown in Table 5. These results demonstrate that the combination of catalysts increases the conversion rate of alcohols, thereby enhancing the reforming efficiency. The selectivity of H2 may increase during the reforming process with the plasma-catalytic technology, while those of CO and C2Hx decrease to some extent, which is favorable for fuel cell applications. Rico et al. (Spain)65 observed that the production of carbon dioxide decreased during methanol reforming using a DBD-catalytic method. NOx emissions, which are an important component of environmental pollution, are influenced by the catalytic effect S


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Table 5. Reforming Efficiency of Plasma-Catalytic Technology As Compared to Plasma Technology Alonea S (product selectivity) catalyst

discharge type

materials

Ni/Al2O3 Rh/Al2O3 ceramic

GEN 3

Pt/CeO2 Pt/TiO2 Pt−Re/TiO2 Pt/ TiO2+Pt−Re/TiO2 Cu− ZnO/Al2O3 Pd− ZnO/Al2O3 BaTiO3 Cu−Mn/BaTiO3 Ni/Al2O3 Al2O3−MnO2 CuO/ZnO/Al2O3

corona

ethanol, bio-oil, methane, gasoline bioethanol

MW DBD RF corona MW

methanol methanol methanol isopropyl alcohol methanol

a

catalyst

χ (%)

H2

↑

↑

−

−

−

↑

↓

−

↑ − ↓ − − ↓ − − ↑ ○ ↓ ↑ − − − − ↑ ↑ − − − − ↑ ↑ ↑ − ↑ ↑ − − − − modeling and simulation of microwave double absorption on methanol steam reforming for hydrogen production

NOx

surface area

dispersion

↓

↓

−

−

↓

−

↑

↑

CO CO2 C2Hx

ref 8, 23, 114 11 115 65 109b 112 116

“↑” means increase. “↓” means decrease. “○” means uncertain change. “−” means ungiven.

Figure 21. Schematic summary of catalytic phenomena assisted by plasma and plasma-catalytic technologies.

and can be decreased using plasma-catalytic technology. The surface area and metal dispersion of the catalyst increase after the modification of plasma during the reforming process. Overall, the optimum catalyst for alcohols reforming using the plasma-catalytic method is Ni/Al2O3 when the cost and reforming efficiency are considered. The catalyst can be modified by the plasma treatment, resulting in a higher dispersion and surface area of catalyst and a higher reforming

efficiency. Simultaneously, the plasma may have regeneration effects on the catalyst such that their lifetime is extended as compared to catalytic technology alone by avoiding the fouling problem. Thus, interest in plasma-catalytic technology is likely to increase further. T


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Figure 22. Schematic overview of plasma−catalyst hybrid configurations: (a) in-plasma catalysis; (b) postplasma catalysis; and (c) common catalyst insertion method. Reprinted with permission from ref 121. Copyright 2007 Elsevier.

3.3. Understanding of the Reforming Mechanism

material and the catalyst properties influenced by plasmas are discussed in this Review. Last, the plasma light emission triggering photocatalysis is discussed. 3.3.1. Interactions between Plasma and Catalyst in the Postplasma Configuration. The interactions between the plasma and the catalyst in the postplasma configuration are relatively simple because most of the highly reactive plasma species have very short lifetimes. These species vanish before they reach the surface of the catalyst. The plasma provides chemically reactive species for further catalysis or excites hydrocarbon molecules in such a way that they are more easily converted by the catalyst material.27 In addition, plasma stimulation of alcohols reforming allows the catalyst to act more effectively on the intermediate oxidized products and advance the reforming process. Zhu et al. (U.S.)111 reported a plasma-catalytic reactor configuration of the postdischarge catalysis of bioethanol reforming for low-CO-content hydrogen production. Pt and Pt−Re supported on CeO2 and TiO2 were used as catalysts. Under the simulated postplasma conditions, the reaction activity followed the order Pt−Re/TiO2 > Pt/ CeO2 > Pt/TiO2. They insulated the catalyst to maintain the catalyst bed at a higher temperature to increase the WGS reaction rate, while ensuring that the temperature was sufficiently low to maintain a relatively low CO concentration.111 The results obtained by Rico et al. (Spain)65 for methanol reforming with plasma-catalyst technology are shown in Figure 23, which illustrates that different products are generated in a DBD reactor with different configurations. The results in Figure 23 demonstrate that the conversion rate of methanol increases when plasma and catalysts are combined. Comparison of Figure 23c with Figure 23d indicates that the use of Cu−Mn has a small positive effect on methanol conversion and the reduced production of CO. 3.3.2. Interactions between Plasma and Catalyst in the In-Plasma Configuration. With respect to the in-plasma configuration, the plasma and catalyst regions can completely or partially overlap, and the plasma discharge is ignited with the catalyst bed or on the electrodes, which are coated with catalyst material.27 Previous studies65,121 have demonstrated that introducing catalysts into the plasma discharge may affect the type of discharge or induce a shift in the distribution of the accelerated electrons, which influences the production of excited and short-lived reactive plasma species. The catalytic or noncatalytic pellets can remarkably enhance the electric field,

As mentioned above, depending on the catalyst and reaction conditions, alcohol reforming reactions involve several reaction pathways, including dehydration, decomposition, dehydrogenation, and coking. Figure 21 summarizes the influencing factors, the reaction process, the intermediate species, and the end products in ethanol reforming assisted by plasma and plasmacatalytic technologies. The choice of catalyst is essential to obtain a reforming process that avoids carbon deposition on the catalyst surfaces and prevents the generation of unwanted products. Catalysts for ethanol reforming with hydrogen selectivity must be able to (1) dehydrogenate ethanol; (2) break the carbon−carbon bonds of the surface intermediates to produce CO and CH4; and (3) reform these C1 products to generate hydrogen.117 The proper choice of metal and its support is key to obtaining an efficient reforming process. Metal-based catalysts and noble metal-based catalysts are active in the reforming process. The combination of plasma and catalyst has been investigated using various components such as toluene,118 benzene,119 hexane,120 and methane120 with different plasma reactors, including DBD and corona. Plasma catalysis for VOC abatement was recently reviewed by Van Durme et al. (Belgium).121 Jarrige et al. (France)112 combined corona discharge with the catalysts Al2O3−MnO2 and observing that the combination enhanced the formation of CO2 and CO by the catalyst; moreover, the formation of CO was reduced due to its conversion to CO2, demonstrating that further oxidation occurred in the process. Ozone decomposition on MnO2 sites leads to the formation of atomic oxygen, which reacts with residual substances in the effluent downstream of the nonthermal plasma reactor. The catalysts generate additional reactive species without requiring the injection of additional energy and also recover the energy wasted in the production of oxygen atoms that primarily recombine to form O3 during the postdischarge phase, leading to a reduced energy cost of the reforming process. They also found that the decomposition of O3 on the surface of MnO2 leads to the production of absorbed atomic oxygen O*, which reacts with the remaining pollutants on the surface and releases O (3P) atoms through desorption; these atoms then further react with VOCs in the gas phase. On the basis of the combination of plasma and catalyst, the operating configurations can be described as in-plasma catalysis or postplasma catalysis, as shown in Figure 22. Both the change in plasma properties resulting from the introduction of catalyst U


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of thermal catalysis. With respect to methanol reforming for hydrogen production, plasma-treated catalysts can increase methanol conversion as compared to nontreated catalysts.125 The hydrogen selectivity obtained with plasma-treated catalyst increases or remains the same.125 The higher methanol conversion and hydrogen selectivity is attributed to the reduction of the active metal, which is favored by plasma discharges. In the case of catalytic reforming, the reaction activity increases with the surface area of catalysts and decreases with metal particles under the same reaction conditions.126 When catalysts are exposed to plasma electrons, each particle can trap thousands of electrons, and these trapped electrons form a plasma sheath around the particle.127 The electron flow in the plasma discharges exerts a strong repulsive force on the sheath as a result of coulomb repulsions between electrons.127b Under these conditions, the bounds of the precursors or clusters are elongated or distorted and easily split when impacted by energetic electrons and radicals, leading to a high dispersion and small particle size.110c,127a In addition, the smaller particle size and the higher dispersion of plasma-treated catalysts enhance metal−support interaction, and thus suppress coke formation or sintering and improve catalyst stability.128 The most widely accepted mechanism of sintering is particle migration and coalescence.129 The rate of sintering strongly depends on the dispersion, the size, and the amount of metal particles and the metal−support interaction. It is believed that strong metal−support interaction is helpful in avoiding particle migration.129 In the case of Ni/Al2O3 catalyst, there could be several Ni active phases formed on the surface of γ-Al2O3, such as microcrystalline NiO and spinel NiAl2O4. The generation of microcrystalline NiO leads to migration, aggregation, and growth of particles. As a result, the dispersion of active phase rapidly decreases, which is the major reason for the deactivation of supported Ni-based catalyst.128a As compared to microcrystalline NiO, the spinel NiAl2O4 exhibits high dispersion, weak migration, and large active surface area due to its strong interaction with the support.130 These advantages increase the activity of catalyst and inhibit coking and sintering.128a In summary, the properties of catalysts are influenced by the following: (1) the enhancement of the dispersion of the active catalytic components from the discharge; (2) the change of the oxidation state of the catalyst exposed to the discharge; and (3) a specific surface area enhancement or a change in catalytic structure resulting from plasma exposure. The adsorption efficiency and thermal activation of the catalysts can also change when they are combined with plasma technology. The surface modification of the solution-combustion-synthesized Ni/Al2O3 catalyst for aqueous-phase reforming of ethanol was studied by Roy et al. (U.S.).109b They demonstrated that the catalytic activity (in terms of ethanol conversion and H2 yield per g) of the sample increased after plasma modification. Bright-field TEM images of unmodified and modified catalysts after use in the reactor are shown in Figure 24. For the modified powder (Figure 24b), the particle size appeared to be smaller (∼20 nm), with a narrower distribution than the unmodified powder (15−50 nm, Figure 24a). The reason for the variations should be investigated further. 3.3.3. Photocatalysis in Plasma-Catalytic Process. Nonthermal plasma produces a high level of ozone.123,131 As one of the long-lived species formed in the discharge, ozone can stimulate the conversion of VOCs via adsorption or decomposition on the catalyst surfaces, leading to the

Figure 23. Schematic representation of the reaction conditions and primary products obtained during the direct decomposition of methanol in the hybrid catalyst−plasma reactor. Reprinted with permission from ref 65. Copyright 2009 Royal Society of Chemistry.

leading to higher electron energy. As a result, electron-impact reactions increase the effectiveness of alcohols reforming. From the perspective of plasma chemistry, rotationally and vibrationally excited species generated in plasma do not assist alcohol conversion because they have short lifetimes and low threshold energies.27 However, vibrationally excited species can enhance catalytic reactions. Previous research122 showed at low temperatures, where catalytic reactions hardly proceeded, the conversions of reactants were improved by introducing plasma discharges in the volume of catalysts; that is, the temperature for activating the reactions was reduced. One reaction for the effect of plasma discharges on catalytic reactions is that plasmagenerated electrons can excite vibrationally the reactants.122a These vibrationally excited species are absorbed with a lower activation energy that that of ground-state species. In addition, the plasma discharges enhance the absorption intensities of reactants on the active sites of the catalyst surface. Researchers have devoted significant effort to the enhancement of dissociative adsorption by increasing the vibrational energy of reactants because catalytic reactions can be greatly accelerated once dissociative adsorption is enhanced. As VOCs, alcohols (methanol, ethanol, etc.) are decomposed primarily through surface reactions rather than discharge characteristics during the plasma-catalysis reforming process. Plasma-catalytic systems exhibit zero-order kinetics during VOC decomposition, indicating the significance of surface reactions.123 The additional production of stable species, such as ozone, resulting from recombination in the discharge region may also contribute to plasma-assisted fuel reforming. Van Durme et al. (Belgium)121,124 reported that the energy efficiency of the combination of non-thermal plasma with CuOMnO2/TiO2 was substantially higher than that of plasma alone. The behavior of the catalyst can be influenced by plasmainduced surface heating and possibly other unknown effects because of the current flowing across the catalyst surface. Many researchers have used various plasma-treated catalysts to investigate the effect of plasma discharges on the performance V


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can enhance photocatalysis upon exposure to plasma. In the presence of a TiO2 catalyst under UV irradiation, ozone can be decomposed in two different ways:134 (1) As electron acceptor: TiO2 + hν → h+ + e−

(3.1)

O3 + e− → O3•−

(3.2)

•− O•− + O2 3 → O

(3.3)

Figure 24. Bright-field TEM images of unmodified and modified catalysts after use in the reactor. Reprinted with permission from ref 109b. Copyright 2010 Elsevier.

(2) By the adsorption of photons:

generation of strongly oxidizing species, such as atomic oxygen.123,132 Using plasma in combination with a TiO2 photocatalyst can enhance ozone decomposition on the catalyst surface. Under ultraviolet (UV) irradiation of photo energy equal to or higher than the bandgap energy of TiO2, electron− hole pairs are generated in the TiO2 films. The photogenerated hole pairs in the valence band and the electrons in the conduction band diffuse to the TiO2 surface and produce highly energetic hydroxyl radicals and superoxide radical anions that can oxidize organic molecules on the TiO2 surface.133 Plasma can also provide high-energy electrons for TiO2 activation by generating the same electron−hole pairs that are provided in photocatalysis. In addition, plasma-generated electron beams favor desorption of intermediate oxidized products, leading to an increase in the turnover of the catalytic sites.134 Using external UV lamps in plasma−photocatalyst systems is a more efficient way to enhance plasma-photocatalysis. Maciuca et al. (France)134 observed that the ozone residual concentration at the exit of the reactor decreased in the order plasma alone (without catalyst) > plasma with TiO2 catalyst > combined plasma (with TiO2 catalyst) and UV. Their results indicate that UV can play a role in ozone decomposition. Guaitella et al. (France)135 observed that O atoms play a much more important role than ozone in C2H2 oxidation using the plasma/TiO2 combination. The O atoms on the TiO2 surface

Other oxidative species, such as •OH and H2O2, are generated in the synergistic plasma−TiO2 photocatalysis system. These species can enhance alcohol decomposition and achieve higher energy efficiency. One effective way to enhance photocatalytic efficiency is to develop cocatalystmodified photocatalysts, such as noble metals or their oxides (e.g., Pd,136 Pt,137 Ru,138 Ag,139 or Rh140), transition metal oxides (e.g., NiO141), or transition metal sulfides (e.g., MoS2142 and CuS143). A glow discharge plasma treatment was used by Zou et al. (China)144 to modify the impregnation method to prepare Pt/TiO2 photocatalysts using the following steps: impregnation, cold plasma treatment, calcination, and reduction. The activity of modified catalysts for hydrogen generation from water/alcohols was significantly improved. The highresolution TEM images of the two types of catalysts are shown in Figure 25.144,145 The cocatalysts loaded on the photocatalysts provide low activation potential for hydrogen evolution and often serve as active sites for hydrogen production. Therefore, loading proper cocatalysts on semiconductor photocatalysts can greatly improve the activities of the photocatalysts.143 The reaction pathways of ethanol steam reforming over metal catalysts are shown in Figure 26.10 These pathways show the various gaseous products; further work is needed to avoid the generation of undesirable products.

O3 + hν (λ < 310 nm) → O2 + O•

(3.4)

Figure 25. High-resolution TEM images of (a) Pt/TiO2−P and (b) Pt/TiO2−C with speculated metal−support interfaces. Reprinted with permission from ref 144. Copyright 2007 Elsevier. W


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distance when microwaves travel into a sample. Maxwell’s equations are used for small or low-loss dielectric samples. The governing equations simultaneously consider the continuity, the momentum, the energy, the species, and Maxwell’s equations (eqs 4.1−4.11), thus including both the reactants and the catalyst bed in the computation. Equations in nonporous catalyst bed: Continuity equation ⎯⇀ ⎯

∇·(ρV ) = 0

(4.1)

Momentum equations ⎯⇀ ⎯

⎯⇀ ⎯

⎯⇀ ⎯

⎯⇀ ⎯

ρV ·∇V = −∇p + ∇[μ(∇V + (∇V )T )]

(4.2)

Energy equation ⎯⇀ ⎯

ρCpV ·∇T = ∇(k f ∇T ) + Q mw

(4.3)

Species equations ⎯⇀ ⎯

∇·(V ci) = ∇·(D∇ci)

(4.4)

Equations in the porous catalyst bed: Continuity equation ⎯⇀ ⎯

⎯⇀ ⎯

∇·(ργV ) = ∇·(ρVs ) = 0 Figure 26. Reaction pathways that can occur during ethanol steam reforming over metal catalysts. Reprinted with permission from ref 10. Copyright 2005 American Chemical Society.

(4.5)

Momentum equations ⎤ ⎯ ⎡ u ⎯⇀ ⎯ ⎯⇀ ⎯ u ⎯⇀ Vs = −∇p + ⎢ (∇Vs + (∇Vs )T )⎥ ⎦ ⎣γ k

4. MODELS OF ALCOHOLS REFORMING FOR HYDROGEN PRODUCTION Ethanol reforming using catalytic technology has been widely investigated and modeled.146 Using plasma-assisted technology, a plasma kinetic model for an ethanol/water/air mixture in a tornado-type electrical discharge reactor was presented by Levko et al. (Ukraine).48 The nature of the nonthermal conversion, the kinetic mechanism of the nonequilibrium plasma chemical transformations, and the evolution of the hydrogen during the reforming were studied by numerical modeling. Levko et al. (Ukraine)147 also theoretically investigated the plasma kinetics of molecular hydrogen in electrical discharge using an Ar/CH3OH/H2O mixture for the first time. The dependence of H2 on the breakdown field, the discharge power, the solution compound, and the argon pumping rate through the interelectrode gap were studied. On the basis of some assumptions, Petitpas et al. (France)7a simulated the partial oxidation of E85 using a 1D phenomenological model. A perfectly stirred reactor (PSR) with an input heating power equal to the electric power was used to describe the plasma region. The mixture temperature was calculated from the global enthalpy balance using the PSR outflow and the remaining cold gas. A plug flow reactor (PFR) was used to model the postdischarge region. The model was implemented in FORTRAN code using the PSR and SENKIN modules of the CHEMKIN II package. Chen et al. (Taiwan, China)56 modeled and simulated methanol steam reforming for hydrogen production using microwave double absorption. The two different methods that have been developed to describe microwave heating processes are Lambert’s law and Maxwell’s equations. The former is valid for large samples and high-loss dielectric materials because it presents MW power reduction primarily as a function of

(4.6)

Energy equation ⎯⇀ ⎯

ρCpVs ·∇T = ∇(keff ∇T ) + Q mw + Q reaction

(4.7)

Species equations ⎯⇀ ⎯

∇·(Vs ci) = ∇·(γD∇ci) + Ri

(4.8)

Equation of state (nonporous and porous zone) N

p = ρRT ∑ i

1 X i Mi

(4.9)

Maxwell’s equations ⎯⇀ ⎯

⎯⇀ ⎯

∇ × E = −iωμo μr H ⎯⇀ ⎯

⎯⇀ ⎯

∇ × H = iωεoεr E ⎯⇀ ⎯

∇· E = 0 ⎯⇀ ⎯

∇·H = 0 ⎯⇀ ⎯

(4.10)

⇀

V = wink , T = Tin , ci = ci ,in(upstream inflow)

Boundary conditions ⎯⇀ ⎯

∇Vs = ∇T = ∇ci = 0 and p = patm (downstream outflow) (4.11)

The reaction rates of methanol steam reforming (Rst) and methanol decomposition (Rdc) are expressed as follows: R sr = (1 − γ )ρs ·ksr·cCH3OH·c H2O X

(4.12)


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Figure 27. Distributions of hydrogen volumetric concentration in the catalyst bed with microwave powers of (a) 300 W, (b) 400 W, and (c) 500 W. Reprinted with permission from ref 56. Copyright 2011 Elsevier.

R dc = (1 − γ )ρs ·kdc·cCH3OH

where Ni,Nj,Nl are the concentrations of molecules, atoms, and radicals; kij,kiml are the rate constants of the chemical reactions for the corresponding reagents; W,V are the discharge power and discharge cavity volume, respectively; m, ne are the mass and the concentration of electrons, respectively; Qei is the crosssection of the corresponding inelastic process; f(ε) is the electron energy distribution function (EEDF); Mi is the molecular mass; Qi is the transport cross-section of elastic scattering; and q = 1.602 × 10−12 erg/eV. In their calculation, the maximal hydrogen output was achieved for equal amounts of ethanol and water in the solution. The hydrogen output increased linearly with the specific discharge power and reached saturation at high values. Similar to the modeling research mentioned above, the plasma kinetics of an ethanol/water/air mixture in a tornadotype electrical discharge has been studied by Levko et al. (Ukraine).48 Their work clarified the nature of nonthermal conversion and explained the kinetic mechanism of nonequilibrium plasma chemical transformations. The total time of the calculation is divided into two time intervals. The first is the generation of active atoms and radicals in the discharge region, and the second interval is the oxidation of the gas mixture in the postdischarge region resulting from the high gas temperature and the presence of O and OH. The kinetic equation used in their work is as follows:

(4.13)

where ρs is the catalyst density; γ is the void fraction of the catalyst bed; and ksr,kdc are the rate constants of methanol steam reforming and methanol decomposition, respectively. Some of the simulation results showing the concentration contours of methanol and hydrogen at the vertical cross section through the center of the catalyst bed are shown in Figure 27. The results indicate that increasing the microwave power increases the H2 yield. The simulations also demonstrated that the extent of methanol steam reforming depends on the microwave power and temperature distribution and that the preheating of the reactants is an important factor in determining the performance of methanol steam reforming. Plasma-assisted reforming of ethanol in a dynamic plasmaliquid system was modeled by Yukhymenko et al. (Ukraine)69,148 to clarify the nature and explain the kinetic mechanism of nonequilibrium plasma-chemical transformations in the system. In the numerical model, the composition and the concentration of the gas species produced in the PLS reactor with the DGCLW discharge were calculated using a series of kinetic equations with the Boltzmann equation. The full kinetic mechanism in their work included 76 species, 97 electronimpact processes, and 441 reactions. Their model showed that the final transformation for CO and CO2 is related to the WGS reaction, which becomes the main pathway for hydrogen production outside of the discharge region. The WGS reaction can be described as follows:

dNi = Sei = dt ⎡ ⎢∑ ⎢⎣ i

−1

(ΔH = − 41 kJ mol )

CO + H 2O → CO2 + H 2

(4.14)

In Shchedrin’s study, after the detailed analysis of plasmachemical reactions in the air−water−ethanol mixture with DGCLW, 59 components were included in the kinetic equation:

+

⎡ ⎢∑ ⎢⎣ i

2q neNiεei m

∑ 2m

2q neNi m

i

+

Mi

∫0

∫0

∫0

∞

+

⎤ εQ ei(ε)f (ε) dε⎥ ⎦

m,l

⎤ εQ ei(ε)f (ε) dε⎥ ⎦

∞

εQ ei(ε)f (ε) dε

∑ 2m

2q neNi m

∫0

∞

where Ki is the inflow of the molecules of the primary components into the plasma; and (G/V)Ni,kNi are the gas outflow as the result of air pumping and the outflow from the pressure difference between the discharge and the atmosphere, respectively. The gas temperature in the postdischarge region was calculated using the following equation:

εQ ei(ε)f (ε) dε ⎤ ε 2Q i(ε)f (ε) dε⎥ ⎥⎦

dT 1 =− dt ρCp

∑ kijNj+ ∑ kimlNmNl+.... j

∫0

∞

(4.16)

∞

∞

2q neNiεei m

∫0

⎤ ε 2Q i(ε)f (ε) dε⎥ ⎥⎦ i Mi G + ∑ kijNj+ ∑ kijlNN Ni − kNi j l + .... + K i − V j j,l

19f

dNi W 1 ⎡ 2q = Sei = neNiεei ⎢ dt V εei ⎣ m

W 1 ⎡ 2q ⎢ neNiεei V εei ⎣ m

(4.15) Y

∑ Hi(T )·μi i

dNi dt

(4.17)


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where ρ is the gas density; Cp is the average specific gas heat capacity at constant pressure; and Hi,μi are the molar enthalpy and the molar mass of the ith component, respectively. The results of their numerical simulation demonstrated that the nonequilibrium plasma induces a high-temperature process in a relatively low temperature interval. The stimulation increased H2 production in the plasma by ethanol abstraction for 10−100 μs and hydrocarbon abstraction thereafter. In addition, at low ethanol concentrations, the reaction between H2O and hydrogen atoms heated the gas and generated the initial H2. Derakhshesh et al. (Canada)149 proposed a reaction mechanism for the destruction of methanol using nonthermal plasma technology. A mathematical model was developed and used to predict the effluent concentration of the pulsed corona reactor using the mass balance and considering the axial dispersion, the linear velocity, and the pollutant decomposition rate. The order of the methanol decomposition reaction was estimated using this performance model150 and available experimental data. Their analysis resulted in a first-order reaction rate when the concentration of methanol was much lower than the concentration of electrons. Paterkowski et al. (Poland)19c studied the theoretical destructive oxidation of ethanol in a corona discharge reactor. The mathematical model of corona discharge reactor was based on the following assumptions: (1) the operation of plasma reactor was described by the design equation of the flow tubular reactor; (2) under the isothermal condition, the reaction rate only depended on the input power and the conversion degree; (3) the reaction rate was described by the first-order equation related to ethanol concentration; and (4) the process took place at a constant temperature. The overall theoretical basis can be described as follows:

experimental results. The apparent reaction rate constant can be written as below: ⎛ 82.598 ⎞ ⎟ Z = φ(P) = = 3.233· exp⎜ − ⎝ P ⎠

(4.24)

On the basis of the analysis of the previous equations in the corona discharge reactor, the total rate of destructive ethanol oxidation can be described in the following equation: ⎛ −82.598 ⎞ ⎟ · c · (1 − α ) ri = 3.233 exp⎜ − i ⎝ ⎠ 0i P

(4.25)

This equation can be used to design the corona reactor for the purification of the air or industrial waste gases polluted with ethanol. It is worth mentioning that the real reaction rate can be described by the empirical equation under the applied conditions of experiments; that is, inlet ethanol concentration was in the range of 0.0028−0.132 mol/m3, and the space velocity of gases was in the range of 0.15−0.33 m3/h. Previous studies151 demonstrated that ethanol was decomposed via three pathways: dehydrogenation to acetaldehyde, dehydration to ethane, and decomposition to methane and formaldehyde. This mechanism considered H atom abstraction from all distinct sites (CH3, CH2, OH) in the ethanol molecule, and the subsequent reactions of all three isomers of C2H5O. These three pathways can be described as follows: CH3CH 2OH−( + X) → CH 2CH 2OH−( + M) → C2 H4 + OH

(4.26)

CH3CH 2OH−(+ X) → CH3CHOH−( +M, O2 ) → CH3CHO + (H, HO2 )

(4.27)

·

F0v·c0i dαi = ri·dVR

(4.18)

ri = f (P , αi) = φ(P) ·ψ (αi)

(4.19)

ψ (αi) = c0i·(1 − αi)

(4.20)

⎛ B⎞ φ(P) = Z = A ·exp⎜ − ⎟ ⎝ P⎠

(4.21)

CH3CH 2OH−( +X) → CH3CH 2O−( +M) → CH3 + CH 2O

where X = (OH, H). On the basis of the three pathways and other reactions, Saxena et al. (U.S.)152 performed numerical and experimental studies of ethanol combustion. In this mechanism, ethylene concentration was high in ethanol combustion because it was produced not only from the CH2CH2OH pathway but also from the direct decomposition of ethanol. The CH3CH2O pathway contributed to 40% of CH3 radical generation, and the rest of 60% was generated through acetaldehyde pathway. The reactions of CH3 radicals were the primary sources of methane and ethane. Norton et al. (U.S.)153 determined the branching ratios of the C2H4OH/CH3CHOH/CH3CH2O pathways according to the corresponding reactions of methanol154 and the early studies.155 The model was consistent with the results of a series of ethanol oxidation experiments at 1100 K and at atmospheric pressure using the Princeton turbulent flow reactor. In addition, the authors made a comparison between ethanol oxidation chemistry and methanol oxidation chemistry. In both alcohols oxidation, HO2 chemistry played an important role, while the H + O2 chain branching reaction played only a minor role until late alcohols decomposition. Important differences included the origin of OH and H radicals. While OH was generated primarily in reactions of HO2 and H2O2 for both alcohols, C2H4OH decomposition represented an additional significant source of OH radicals for ethanol oxidation. The reactions of CH3 + HO2 in ethanol oxidation not only produced OH

where αi is the degree of ethanol destruction; ri is the reaction rate in the corona discharge reactor; Z is the apparent reaction rate constant, s−1; Ḟ 0v is the gas flow velocity, m3 s−1 or m3 h−1; c0i is the inlet ethanol concentration, mol m−3 or g m−3; d is the outer diameter of the external pipe, m; dVR is the element of the reactor volume; A and B are the parameters of the equation, s−1; and P is the power supplied to the reactor, W. Solving the equations can obtain the Z function and the design equation of the corona discharge reactor:

Z=

1 ⎛ 1 ⎞ ·ln⎜ ⎟ τz′ ⎝ 1 − αi ⎠

⎛ ⎞ Z αi = 1 − exp⎜⎜ − · ·VR ⎟⎟ ⎝ F0v ⎠

(4.28)

(4.22)

(4.23)

where τz′ = VR/Ḟ0v is the total residence time of the gases inside the reactor, s. The parameters of A and B of apparent constant rate equation were calculated using the statistical method for the Z


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content, superior air-to-fuel ratio, and renewability, but the notable toxicity of butanol remains problematic. The energy balance for hydrogen production from bioethanol for fuel cell applications is shown in Figure 28. Carbon dioxide and water are converted into biomass material, which is presented as glucose, via photosynthesis as follows:

radicals, but also resulted in H atoms production via the decomposition of both CH3O and HCO. In contrast, H atom production was independent of HO2 in methanol oxidation. Although the model was successful in predicting flow reactor data in the applied conditions of experiments, any use of the model under different conditions should take into account recalculating falloff rates for pressure-dependent reactions and the branching ratio of the C2H4OH/CH3CHOH/CH3CH2O pathways. These models have contributed to the enhancement of understanding of ethanol reforming. However, in developing kinetic models for numerical predictions of ethanol reforming, relative ranges of the independent variables that exceed those in any particular experiment should ideally be considered.82

photosynthesis

6CO2 + 6H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ C6H12O6 ΔH ° = 2538.5 kJ/mol

(5.1)

5. DISCUSSION 5.1. The Energy Balance of Hydrogen Production from Ethanol

Four types of common alcohols are discussed in this section with respect to hydrogen production. In general, their respective characteristics and advantages/disadvantages for fuel cell applications are described in the following. Among these alcohols, methanol is excellent for use in highcompression engines, although 30% may be converted to formaldehyde during the reforming process. In addition, reforming with methanol occurs at a lower reformer operating temperature than with other liquid fuels, such as hydrocarbon mixtures, which can greatly reduce the heat loss. This is crucial for small engine systems. Methanol reforming can be limited, however, due to a lack of infrastructure, its solubility in water, its toxicity, its lower energy density, and, to some extent, environmental concerns. In addition, the conventional catalytic method for methanol reforming used for hydrogen production has the following disadvantages: (1) the methanol reformers are mostly packed-bed reactors, which may contain hot and cold spots; (2) the potential for problems during cold startups and with transients; and (3) further improvements in the catalytic activity and stability are needed.19d Therefore, a new technology that addresses these problems must be developed to reform methanol for hydrogen production. As another alcohol in common use, ethanol has the following advantages in terms of its chemical-physical characteristics for fuel cell applications: (1) it is a renewable resource that can be generated from agricultural residues and fermentation of surplus crops; (2) it is easy to store and transport; and (3) it decomposes easily in the presence of water to produce hydrogen-rich gas.10,40a However, because of the 35% oxygen content in the ethanol molecule, reduced particles and NOx emissions are generated during the combustion process.1 Fast and efficient renewable fuel reforming is one of the critical steps in producing H2 for fuel cells and the hydrogen economy, and ethanol is now the most available and economical renewable fuel.73 However, ethanol features low flame luminosity and a lower vapor pressure, resulting in difficult cold starts, miscibility with water, toxicity to ecosystems, an increase in exhaust emissions of acetaldehyde, and an increase in vapor pressure (and evaporative emissions) when blending with gasoline. Therefore, ethanol reforming requires further improvements to increase the reforming efficiency and reduce pollutants in emissions, etc. Propanol and butanol are rarely used to produce hydrogen-containing gas for fuel cell applications. Butanol is a good candidate to replace gasoline as a transportation fuel because of its high energy

Figure 28. An energy diagram showing the renewable energy cycle of ethanol autothermal reforming for hydrogen production in fuel cell application. Reprinted with permission from ref 40b. Copyright 2014 Elsevier.

Bioethanol is produced from the fermentation of biomass material, which is a slightly endothermic reaction. With insufficient oxygen, yeasts convert glucose into ethanol and carbon dioxide, which can be expressed as follows: fermentation

C6H12O6 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2C2H5OH + 2CO2 ΔH ° = 69.1 kJ/mol

(5.2)

Subsequently, ethanol is transformed to H2 and CO2 via the autothermal reforming reaction, which is written as follows: C2H5OH + 2H 2O + 0.5O2 → 2CO2 + 5H 2 ΔH ° = −25.7 kJ/mol

(5.3)

The reaction above is somewhat exothermic; that is, theoretically, 1 mol of ethanol reacts with 2 mol of water and 0.5 mol of oxygen, forming 5 mol of hydrogen without applying any external heat. The hydrogen generated from the autothermal reforming reaction then is oxidized into H2O in a fuel cell as follows: 0.5O2 + H 2 → H 2O

ΔH ° = 241.8 kJ/mol

(5.4)

As shown in Figure 28, the energy content of 0.5 mol of glucose is 1270 kJ, while 5 mol of hydrogen, which can be converted from 0.5 mol of glucose via eqs 5.2 and 5.3, ideally provides 1209 kJ of energy in a fuel cell via eq 5.4, indicating that little energy lost occurred during the autothermal reforming of ethanol. AA


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Table 6. Comparison of Alcohols Reforming Assisted by Different Plasma Treatment Systems

AB


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Table 6. continued

AC


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Table 6. continued

AD


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Table 6. continued

5.2. Comparison of Alcohols Reforming Assisted by Different Plasma Treatment Systems

conversion rate indicates how many alcohols have gone through the plasma region and how many C−H links have been broken. The H2 molar fraction is a good indicator of the concentration of hydrogen in the products. The energy efficiency is an important parameter used to estimate the conversion of ethanol in terms of energy. The results of recent studies in the field of plasma technology for alcohols reforming are also provided in Table 6. Some of the values presented in

Table 6 enables a comparison of the reforming of alcohols by different plasma systems and plasma-catalytic systems. The ethanol conversion, the H2 molar fraction, and the energy efficiency, as defined in the above equations, were used to estimate the reforming efficiency of the alcohols for hydrogen production using the different reforming technologies. The AE


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Figure 29. Specific energy requirement versus plasma reactors.75

reported to be one of the best plasma discharges for the reforming process of alcohols with acceptable energy efficiency. Gliding arc, which is a transitional arc, is also an excellent plasma discharge for this purpose. The Plasma Science and Fusion Center (PSFC) group at MIT (Massachusetts Institute of Technology), one of the most advanced institutes in plasmaassisted reforming, has performed an in-depth investigation of nonthermal arc systems for efficient hydrogen production of fuel cell applications. Based on GEN 1 (a thermal plasmatron), low current nonthermal plasmatron fuel converters with fuel injection between the electrode gap (GEN 2) and with fuel injection at different inputs (GEN 3) were developed to test various fuels for hydrogen production. In contrast to GEN 2, in GEN 3, the mixtures of feedstock are injected at ambient temperatures, and a swirl flow rotates and pushes the discharge toward the axis of the reactor. Both the reforming efficiency and the lifetime of electrodes are greatly improved with the plasmatron GEN 3. The combination of GEN 3 and catalyst (Ni/Al2O3) was investigated for fuel reforming.66 Researchers at NASU/NTSU (Spain) have also extensively studied plasmaassisted reforming using the DGCLW reactor. A postdischarge high temperature pyrolytic chamber combined with the pulse DGCLW reactor was found to contribute to the further pyrolysis of ethanol after initial plasma-assisted ethanol reforming. The results demonstrated that the energy efficiency value of this new system was 3 times greater than that of the plasma-only system. Comparing the data in Table 6 reveals that plasma-assisted ethanol reforming is a competitive method of hydrogen production. Future studies to increase the H2 yield and selectively reduce CO in the products are necessary to enhance the reforming efficiency and limit the CO concentration for fuel cell applications. As shown in Table 6, the focus of plasmaassisted fuel reforming studies has mostly been on finding the best discharge for alcohols reforming to improve the conversion efficiency and hydrogen production; however, the combination of plasma and catalysts has not yet been adequately studied. Previous studies8,11,65,109b,115,116,144 have demonstrated that the addition of catalysts favors an increase in reforming efficiency and H2 selectivity. Nonthermal alcohols reforming can achieve a high conversion rate, compactness, and rapid start-up;

this table may be slightly different from those reported by the authors in their papers and proceedings. The results in Table 6 demonstrate that plasma or plasmacatalytic technology can be applied to the generation of diamond (1998, Hiramatsu et al.20c) and syngas (CO + H2). For example, methanol can be used to deposit diamond with low-pressure, radio frequency, inductively coupled plasma.20b,c During the past decade, the production of hydrogen from liquid fuels, such as alcohols, using plasma or plasma-catalytic technology has become increasingly popular due to its numerous advantages. The types of power supply, the reaction temperature, the feedstock components, the carrier gas, the discharge reactors, and the addition of catalysts have been widely researched for this purpose. The simulation and kinetic modeling of alcohols reforming assisted by plasma and plasmacatalytic technologies have also been investigated to achieve a better understanding of the mechanisms of the reforming process. The thermal plasma used in the reforming process is often associated with high temperatures, which can lead to the erosion and evaporation of the electrodes. On the basis of the information presented in Table 6, the advantages of alcohols reforming assisted by nonthermal plasma are as follows: (1) Any supply power (DC or AC) and carrier gas (N2, O2, Ar, air, etc.) can be used to produce plasma. (2) Various alcohols, particularly methanol and ethanol, can be used to generate hydrogen for fuel cell applications. Water and oxygen are often added to the feedstock to SR, POX, and the WGS reaction to generate excess hydrogen. (3) A variety of nonthermal plasma reactors with different geometries, such as MWD, DBD, RF, GAD, arc discharge, and corona, can be used for alcohols reforming. (4) Catalysts using different metals (Ni, Cu, Pt−Re, Pd, etc.) and various supports (ZnO, Al2O3, CuO, TiO2, etc.) can be combined with plasma technology, and this combination usually has a positive effect on the fuel conversion rate and hydrogen yield. (5) The low temperature in the surrounding gas and the heavy particles used in a nonthermal plasma reactor are beneficial for the protection of the electrodes. MWD has been investigated in a number of countries, including Russia, U.S., China, Portugal, and Spain, and is AF


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Figure 30. Conversion rate versus plasma reactors.75

sumption in the production of hydrogen continues to be a great challenge. In general, alcohols reforming using plasma technology are very promising in terms of the reforming capacity, ethanol conversion rate, and molar fraction of H2. The relationship between the SER and the plasma reactors is shown in Figure 29. The data in the figure indicate that the SER varies with plasma type. Some values claimed here may be slightly different from those reported in other papers and proceedings. The data in Figure 29 show that the SER of various plasma reactors decreases in the order DBD ≈ MWD > glow ≈ corona > GAD. The conversion rate as a function of discharge type is shown in Figure 30, which illustrates that good alcohol conversions have been obtained with DBD, corona, and MWD. These discharge types have been widely studied in many countries, for example, U.S., China, Russia, France, Japan, Ukraine, and Poland. In general, plasma and plasma-catalytic technologies have shown excellent behavior for alcohols reforming.

therefore, continued research focusing on plasma-catalytic reforming technologies that combine plasma with conventional catalytic technology will be important for fuel cell applications. Less-expensive catalysts with extended lifetimes will be of great interest to decrease industrial costs for alcohols reforming. The major results presented in Table 6 include the conversion rate, the hydrogen molar fraction in the gaseous products, the energy efficiency ,and the input power because these terms can directly indicate the reforming capacity, the energy cost, and the potential for industrial application. The data in Table 6 demonstrate that alcohols reforming with glow, GAD, and MW discharge yield good hydrogen molar fractions, that is, between 60 and 80 vol % in the gaseous product. The highest energy efficiency, 89.3%, was obtained at Waseda University using a lower energy plasma (LEP) reactor,70a,c,156 which indicates that the energy efficiency is several times higher than that of the direct combustion of alcohols, thereby increasing the degree of effective utilization of energy. A similarly high conversion rate, that is, between 76% and 100%, was achieved by Oklahoma University, with an energy efficiency of 51−66%. The molar fraction of hydrogen, which is an indicator of the concentration and selectivity of hydrogen in the gaseous products generated from the feedstock, varied widely, from 10% to 88.74%. The products generated by alcohols reforming are typically H2, CO, CO2, CH4, C2H2, C2H4, C2H6, trace NOx (mainly NO and NO2), and H2O. The power consumed by the plasma reactors can be influenced by various factors, such as the type of discharge, amount of reformed alcohol, and electrode geometry. The reduction of power consumption to decrease the specific energy con-

6. OUTLOOK The introduction of nonthermal plasma into alcohols reforming is a challenge for experimental research and kinetic analysis but provides opportunities to develop a solution to meet the energy crisis and organic contaminant removal. Media reports have fueled globally increasing expectations for innovative devices and techniques that will contribute to energy and environmental goals. Although plasma reforming of alcohols is in the experimental stage of development, there are signs of its potential to achieve high alcohols conversion and high energy efficiency. A crucial condition for the sustained success of alcohols reforming by plasma and plasma-catalytic technologies AG


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that could negatively impact the performance of the trap, thus enhancing the reliability and longevity of the trap.

is the development of plasma reactors with the following characteristics: good manageability for operation with air as the oxidizer at atmospheric pressure in a practical technology; reproducibility and reliability with regard to the reforming result; and well-characterized plasma parameters for process control and monitoring. Plasma-assisted reforming is economically attractive for operations at low hydrogen production. Plasma reformers are being developed for hydrogen production for a variety of stationary applications, such as distributed, low-pollution electricity generation from fuel cells; hydrogen-refueling gas stations for fuel cell-powered cars; and decentralized hydrogen production for industrial processes.114b Plasma reformers provide many advantages for industrial applications, including rapid startup and fast transient response, and the boost provided by plasma can facilitate partial oxidation reactions with negligible soot production and efficient conversion of alcohols into hydrogen-rich gas. Plasma reformers are capable of reforming a wide range of fuels, including ethanol, diesel, and bio-oils. In addition, the compact size of a plasma reformer facilitates the use of hydrogen production for multiple applications on vehicles. From an operational perspective, the variable parameters (power input, flow rate, gas composition, etc.) of plasma reformers make this technology attractive for applications to meet the dynamic demands of onboard hydrogen production. The major applications related to onboard hydrogen production are described below.

4. Ignition Control in Homogeneous Charge Compression Ignition (HCCI) Engines

Temperature is one of the variables that can be used for controlling the ignition timing in HCCI. The partial oxidation reforming is exothermic, releasing a small fraction of the heating value of the fuel. The heat can be used to control the temperature of the gas that is injected into the engine. The temperature depends on the amount of air that is mixed with the reformate. In addition to controlling the temperature, the plasma reactor can be used to generate fuels of different octane values. Hydrogen has a very high octane value, while the octane value of carbon monoxide is similar to that of methane. Therefore, hydrogen-rich gas can be used to increase the octane value of the fuel. In this method, the hydrogen-rich gas replaces a substantial fraction of the fuel rather than serving as an additive.8 For onboard applications, it is difficult to provide the required water, which could be a major deterrent to the use of the WGS reaction for onboard applications in internal engines. It is necessary to determine the minimum hydrogen additive requirements to meet pollution reduction and efficiency goals. By controlling the electrical and thermodynamic parameters of the plasma, the operation can provide high energy conversion efficiency and chemical selectivity. To further improve the ease of the reforming operation, some of the hydrogen-rich gas is recirculated and mixed with the fuel-air mixture. In this method, the equilibrium of reformate is not changed, but the kinetics of the partial oxidation reaction could be dramatically amplified. The heat from the hydrogen-rich gas is helpful in quickly raising the temperature required for startup. The development of a solar powered plasma reactor may become a direction as it combines the advantages of solar powered systems. Solar energy is in abundance, free, and clean, which does not produce any noise or any pollution to environment. One of the solar energy applications in industries is the photovoltaic (PV) system, which has been used in various domains with the development of economy and society. In China, PV systems are utilized in the city road lighting system such as for street lamps, community lighting, and scenery lighting.168 More and more cities in China have started to replace the conventional street lamps with solar powered street lamps. Solar PV system is also a reliable substitute for power source for plasma process. In the past years, W2 Energy Inc. has developed a microplasma unit powered by solar energy. The reactor is capable of plasma assisted partial oxidization of natural gas, coal, and biomass. Because the reactor portion of the unit would be powered by solar power, the cost of converting these types of deposits to liquid fuel and or syngas would be greatly reduced. Scientists at the University of Minnesota (UM) have successfully developed a promising ethanol reforming reactor using renewable resources instead of methanol and hydrocarbons as the fuel to produce hydrogen for fuel cell applications. They reported that the potential application of this technology, which may lead to the lowest cost of hydrogen production, is related to the power supply of microportable electronics and that the storage of the liquid feedstock would be the primary problem. After mixing through the onboard fuel nozzle, the ethanol/water mixture is oxidized to H2 and CO2 using Rh−CeO2 catalysts in the postsystem. The most

1. NOx Adsorber Regeneration

Hydrogen is a powerful reductant, and the advantages of using hydrogen-rich gas for the regeneration and desulfurization of NOx traps have been researched at ArvinMeritor. In contrast to using diesel fuel, adequate regeneration can be obtained at low exhaust temperatures (150−200 °C). This system was capable of 70% NOx reduction in a Ford F250 truck and 80−90% NOx reduction in a transit bus. In addition, a lower fuel penalty was observed when comparing hydrogen-rich gas regeneration to diesel fuel regeneration. 2. Spark Ignition Engines Using Hydrogen-Enhanced Ultralean Turbo-Charged Operation

Onboard hydrogen production is useful for emission reduction from spark-ignition engines. Hydrogen-rich gas significantly increases flame speed, thereby extending the lean region of engine operation without misfire. The combination of the increased flame speed and the lower flammability limits of hydrogen can stabilize the combustion during lean operation. Hydrogen addition could be used to reduce NOx emissions by facilitating the use of increased exhaust gas recirculation.8 Hydrogen addition can also increase the octane value of the fuel. The increased octane provided by hydrogen-rich gas can be used to increase the compression ratio of the engine or allow turbo-charged operation, thus increasing the efficiency of the engine. 3. Hydrogen-Rich Gas-Enhanced Regeneration of Diesel Particulate Filters

Hydrogen-rich gas can be used for the regeneration of a diesel particulate filter at lower soot levels because hydrogen-rich gas has a wide flammability range and low ignition energy, thereby improving reliable combustion in the dilute environment of an exhaust gas stream. Although this regeneration method may lead to more frequent regeneration, it has the advantage of decreasing the probability of a large uncontrolled regeneration AH


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Figure 31. Mass cycle and energy flow of ethanol plasma reforming for hydrogen production for fuel cell application.

of specific radical species. The chemical reaction process of plasma-assisted alcohols reforming is a complex system, and its dynamic modeling requires further improvement to develop a more detailed and complete simulation. However, in addition to technical details and features, the usefulness of hydrogen production by alcohols reforming using plasma and plasma-catalytic technologies for industrial applications will be demonstrated by addressing the following 10 questions: (1) Is the energy efficiency proven? (2) Is the H2 selectivity acceptable? (3) Is it cost-effective? (4) Are the electrical and thermodynamic parameters of the plasma well-controlled to ensure availability and stability? (5) Are hydrogen storage systems that offer adequate safety developed and available? (6) Can fuel cells for use in a vehicle achieve the durability, efficiency, cost point, and performance needed for the automotive market? (7) Can unreacted organic compounds in the plasma/plasma reactors be utilized and converted into harmless products? (8) Can carbon be captured to provide sufficient environmental protection? (9) Is it necessary to develop a multistage plasma discharge system or establish microcirculation? (10) Are there no simpler alternative solutions? Only if most of these questions can be answered positively will further development of plasma/plasma-catalytic reactors for industrial applications be acceptable. For this purpose, both a comprehensive characterization of plasma and catalysts and the optimization of reactor geometry are indispensable.

promising and competitive process for hydrogen production is the plasma-catalytic system with ethanol and water due to its low cost as compared to other alcohols, flexibility, validity, and convenience without dehydration. They also reported that the energy efficiency of hydrogen production from ethanol is 3 times higher than that of direct combustion. The mass cycle and energy flow of ethanol plasma reforming for hydrogen production for fuel cell applications are shown in Figure 31. Ethanol steam reforming will be applied in fuel cells when the reforming process can achieve a higher hydrogen yield and lower CO concentration in the product. It then will also have a potential industrial market in remote regions with no electric power. As a renewable resource, bioethanol can partly or completely replace fuels for onboard applications to relieve the energy crisis. Bioethanol, which can be generated from plants, is a potential future energy source. Previous studies have demonstrated that plasma-catalytic reforming technology combines the advantages of plasma technology with those of catalytic processes. Therefore, future research focusing on this technology will play a dominant role in fuel cell applications. Less-expensive catalysts with extended lifetimes will be of great interest in the reforming process to decrease the unit cost of hydrogen production. In addition to hydrogen production, the deposition of diamond, nanosized carbon black, or nanocarbon tubes by the plasma-assisted reforming process is also promising. In recent years, efforts have been made to develop a novel reformer with the optimum reactor geometry, electrode material, and discharge characteristics to achieve better ethanol reforming efficiency. Further work to find a more efficient plasma reactor is still needed and is currently underway. Because it would have the advantages of lower power, higher stability, and compactness, the microplasma reactor, which is smaller, lighter, and has higher efficiency, will become the trend in plasma development. In the near future, the focus will become the study of the fundamental mechanisms of plasma-assisted fuel conversion, particularly the primary radical reactions that convert ethanol to hydrogen, to improve hydrogen selectivity by controlling the concentration

AUTHOR INFORMATION Corresponding Author

*Phone/fax: 0086-20-39332690. E-mail: [email protected]. Notes

The authors declare no competing financial interest. AI


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Biographies

ACKNOWLEDGMENTS This project is supported by the National Nature Science Foundation of China (50908237) and Science and Technology New Star in Zhu Jiang Guangzhou City (201312). We would like to give great thanks to Prof. JiePeng Zhang for help. NOMENCLATURE V velocity, m s−1 ρ gas mixture density, kg m−3 p pressure, Pa μ viscosity, Pa s T temperature, K γ porosity, dimensionless Vs seepage velocity, m s−1 keff effective thermal conductivity, W m−1 K−1 Ri reaction rate of species i, mol m−3 s−1 Mi molar mass of species i, kg mol−1 R universal gas constant, 8.314 m3 Pa K−1 mol−1 w velocity, m s−1 μr relative permeability, dimensionless εr complex relative permittivity, dimensionless ω angular frequency Cp gas mixture specific heat, J kg−1 K−1 kf fluid phase thermal conductivity, W m−1 K−1 Qmw energy source term due to microwave heating, J m−3 c molar concentration, mol m−3 D diffusion coefficient, m2 s−1 K catalyst-layer permeability, m−2 Qreaction energy source term due to chemical reaction, J m−3 Xi molar fraction of species i, dimensionless N number of species E electric field intensity, V m−1 μo free space permeability, 4π × 10−7 T m A−1 εo free space permeability, 8.854 × 10−12 F m−1 H magnetic field intensity, A m−1 i species i

ChangMing Du received his Ph.D. in 2006 from ZheJiang University, China, with a thesis titled “Degradation of organic contaminations from gas and liquid phase using gliding arc discharge plasma”. He is currently an Associate Professor with the School of Environmental Science and Engineering, Sun Yat-Sen University, working on plasma science and engineering, and energy and environment. He is skilled in equipment manufacturing and product innovation. He has contributed more than 70 papers and one book (Non-Thermal Arc Plasma Technology and Application), and holds 10 patents in plasma areas.

REFERENCES (1) Fatih Demirbas, M.; Balat, M.; Balat, H. Energy Convers. Manage. 2011, 52, 1815. (2) Potocnik, J. Science 2007, 315, 810. (3) Mussatto, S. I.; Dragone, G.; Guimaraes, P. M. R.; Silva, J. P. A.; Carneiro, L. M.; Roberto, I. C.; Vicente, A.; Domingues, L.; Teixeira, J. A. Biotechnol. Adv. 2010, 28, 817. (4) Farrell, A. E.; Plevin, R. J.; Turner, B. T.; Jones, A. D.; O’Hare, M.; Kammen, D. M. Science 2006, 311, 506. (5) Kim, S.; Dale, B. E. Biomass Bioenergy 2005, 29, 426. (6) (a) Descorme, C.; Aupretre, F.; Duprez, D. Catal. Commun. 2002, 3, 263. (b) Schmidt, L. D.; Salge, J. R.; Deluga, G. A. J. Catal. 2005, 235, 69. (c) de la Piscina, P. R.; Llorca, J.; Homs, N.; Sales, J.; Fierro, J. L. G. J. Catal. 2004, 222, 470. (7) (a) Petitpas, G.; Gonzalez-Aguilar, J.; Darmon, A.; Fulcheri, L. Energy Fuels 2010, 24, 2607. (b) Mallinson, R. G.; Hoang, T.; Zhu, X. L.; Lobban, L. L. Abstr. Pap. Am. Chem. Soc. 2009, 238. (8) Bromberg, L.; Cohn, D. R.; Rabinovich, A.; Alexeev, N.; Samokhin, N.; Hadidi; Kaaja, R. Plasma Science and Fusion Center, JA02-30, 2006. (9) García, E. Y.; Laborde, M. A. Int. J. Hydrogen Energy 1991, 16, 307. (10) Fernando, S.; Haryanto, A.; Murali, N.; Adhikari, S. Energy Fuels 2005, 19, 2098. (11) Zhu, X. L.; Hoang, T.; Lobban, L. L.; Mallinson, R. G. Appl. Catal., B 2010, 94, 311. (12) Descorme, C.; Aupretre, F.; Duprez, D. Ann. Chim.-Sci. Mater. 2001, 26, 93.

JianMin Mo received his B.Sc. in 2013 from Sun Yat-Sen University, and then joined Associate Professor Changming Du’s research group as a graduate student. His research focuses on alcohols reforming using nonthermal arc discharge and microplasma.

HongXia Li received her M.Sc. in 2012 from Sun Yat-Sen University. She focuses her research on ethanol reforming using nonthermal arc discharges. AJ


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Chemical Reviews

Review

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