Dry Reforming of Methane with Dielectric Barrier Discharge and

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Dry Reforming of Methane with DBD and Ferroelectrics Packed-bed Reactors Wei-Chieh Chung, Kuan-Lun Pan, How-Ming Lee, and Moo Been Chang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef5020555 • Publication Date (Web): 25 Nov 2014 Downloaded from http://pubs.acs.org on December 4, 2014

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Energy & Fuels

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Dry Reforming of Methane with DBD and

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Ferroelectrics Packed-bed Reactors

3 Wei-Chieh Chung1, Kuan-Lun Pan1, How-Ming Lee2, Moo-Been Chang1*

4

1

5

No.300, Jhongda Road, Jhongli City, Taoyuan County 32001, Taiwan

6

7 8

Graduate Institute of Environmental Engineering, National Central University,

2

Physics Division, Institute of Nuclear Energy Research, No. 1000, Wenhua Road, Longtan Township, Taoyuan County 32546, Taiwan

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

*Corresponding author Address: No.300, Jhongda Rd., Chungli, Taoyuan City 32001, Taiwan

24 25 26

E-mail: [email protected] Phone: +886-3-4227151-34663 Fax: +886-3-4226774 1

ACS Paragon Plus Environment

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Dry Reforming of Methane with DBD and

28

Ferroelectrics Packed-bed Reactors

29 Wei-Chieh Chung1, Kuan-Lun Pan1, How-Ming Lee2, Moo-Been Chang1*

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1

31


32

33


2

Physics Division, Institute of Nuclear Energy Research, No. 1000, Wenhua Road,

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Longtan Township, Taoyuan County 32546, Taiwan

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Keywords: Dry reforming of methane, dielectric barrier discharge (DBD), greenhouse

36

gases (GHGs), relaxor ferroelectrics, plasma catalysis, syngas

37

Abstract

38

A hybrid plasma catalytic system consisting of a dielectric barrier discharge (DBD)

39

reactor and relaxor ferroelectrics is investigated for syngas generation via the carbon

40

dioxide reforming of methane. The study tests three kinds of packing materials

41

including two relaxor ferroelectrics BaZr0.75T0.25O3 (BZT, εr = 149), BaFe0.5Nb0.5O3

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(BFN, εr = 2025) and glass beads (εr = 3 – 5). The BFN and BZT packed bed DBD

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reactors achieve higher CO2 and CH4 conversions and better energy efficiencies for 2


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syngas production than the DBD does. On the contrary, the DBD packed with glass

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beads achieves lower conversions and energy efficiencies than the DBD does. In

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terms of packings filled in a DBD reactor, there is a tradeoff between an enhancement

47

of the electric field strength and a reduction of the gas retention time. It is interesting

48

that the conversions are increased with the increase of the dielectric constant of the

49

packings tested in the study. Overall speaking, a relaxor ferroelectric with high

50

dielectric constant is a good candidate for constituting the packed-bed DBDs. The

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finding is beneficial to further development of a plasma-based technique for the dry

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reforming of methane (DRM).

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1. INTRODUCTION

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Various kinds of global warming relief techniques such as applications of

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renewable energy, carbon capture, carbon storage and carbon dioxide reuse and

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utilization are intensively investigated[1]-[5]. Among them, CO2 utilization into energy

57

feedstock is of a high potential to alleviate the global warming. Dry reforming of

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methane with CO2 to form syngas (H2 and CO) as illustrated in Rxn. 1 has received

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much attention due to simultaneous utilization of two most abundant GHGs and the

60

application of syngas generated to produce long-chain hydrocarbons via

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Fischer-Tropsch process[6]. Fischer-Tropsch process can synthesize syngas into higher

62

hydrocarbons, especially alkanes (see Rxn. 2), and, different reactant ratio leads to 3


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different product compositions. For example, H2/CO ratio of 2 is suitable for

64

methanol production while H2/CO ratio of 1 is suitable for acetic acid or methyl

65

formate production[7]. Moreover, H2/CO can be controlled beforehand by altering

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CH4/CO2 mixing ratio in feedings[8],[9]. Su et al. (2014) proved that H2/CO ratio

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increases from 0.4 to 3.3 as CH4/CO2 ratio is increased from 0.33 to 3.0[10]. CO2 + CH4 → 2CO + 2H2

0 △ H298K = 247 kJ/mol

(2n + 1) H2 + n CO → CnH(2n+2) + n H2O

(1) (2)

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Reforming techniques including catalysis and non-thermal plasmas are intensively

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studied during the last decade. For catalysis, noble metal catalysts including Pt, Pd,

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Rh, Ru and Ir have been proved to have good activity and stability, however, fairly

71

high cost makes this technique uneconomical[11]-[15]. Compared to noble metals,

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transition metal oxide catalysts are less expensive. Among them, Ni-based catalysts

73

have been extensively investigated[16]-[20]. Zhu et al. (2011) applied Ni-La/SiO2

74

catalyst for DRM and 93% CO2 and 91% CH4 can be converted with 99% CO and

75

68% H2 selectivity, respectively, when operated at 800oC[21]. Rivas et al. (2010)

76

indicate that LaNiO3/SBA-15 catalyst has good durability, however, the resistivity of

77

coke deposition is still not good enough to be scaled up[22]. The main cause of the

78

catalyst deactivation is coke formation, resulting from CH4 cracking and CO

79

disproportionation[20],[21]. Another problem associated with catalytic reforming is that 4


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the energy input is high due to relatively high operating temperature (typically higher

81

than 700oC)[22].

82

Thermal plasmas have been studied for DRM with a high efficiency. For instance,

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Lan et al. (2007) applied binode thermal plasma for DRM with a power of 8.5kW and

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a gas flow rate of 21.7 L/min and indicated that as high as 85% conversion

85

efficiencies were achieved for both GHGs[23]. Non-thermal plasmas have relatively

86

lower conversion efficiency and treating capacity compared to the thermal plasma.

87

However, non-thermal plasmas can be operated at room temperature, making this

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technique feasible and easy to be applied[24]-[27]. Zhang et al. (2003) applied DBD for

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dry reforming with a power of 100 W and a flow rate of 60 mL/min, the conversions

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of CO2 and CH4 reached 56% and 64%, respectively, and by-products such as ethanol,

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acetic acid and ethane were observed[28].

92

Non-thermal plasmas can be integrated with appropriate catalyst to induce

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synergistic effects between catalyst and non-thermal plasma[29]-[32]. Plasma can

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redistribute metals on catalyst surface to have higher dispersion and smaller clusters,

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resulting in higher coke resistance and higher activity. On the other hand, packing

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catalysts into discharge zone can alter the electric field distribution as well as the

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electron energy distribution functions (EEDFs). If the dielectric constant of a catalyst

98

is high enough, charges can be stored on the catalyst surface, and more 5


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99

microdischarges can be induced between the gaps of catalysts pellets[33]. Moreover,

100

catalysts can adsorb gas molecules or radicals to increase the probability of collisions

101

between electrons and gas molecules.

102

Relaxor ferroelectrics are perovskite-type materials with a structure of ABO3 or

103

A2BO4. If B sites are partially substituted by tri- or higher valence metals, the

104

asymmetric lattice can be polarized under electric fields and results in a high

105

dielectric constant[34],[35]. In the past decades, the catalysts with high dielectric

106

constant have been applied for the flue gas treatment. For instance, BaTiO3 has been

107

proved to enhance the removal efficiency of toluene with plasmas[36].

108

In this study, DBD and ferroelectrics packed-bed DBD reactors are studied for the

109

production of syngas via DRM at room temperature. Two ferroelectrics including

110

BaFexNb1-xO3 and BaZryTi1-yO3 with high dielectric constants are used as packing

111

material, respectively, to enhance the DBD’s performance. Possible mechanisms and

112

synergistic effect are also discussed.

113 114

2. EXPERIMENTAL SECTION

115

2. 1 Preparation of ferroelectrics

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The preparation method of ferroelectric material BaFe0.5Nb0.5O3 used in the study

117

can be referred to Chung et al.[35] with the following procedure: 0.1 mole of NbCl5 6


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was firstly added into C2H5OH to form Nb(OC2H5)5. Next, 0.01 mole of

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Fe(NO3)3.9H2O and 0.06 mole of citric acid anhydrate (CA) were dissolved in

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deionized water, and added to 0.01 mole of Nb(OC2H5)5. When the precursor was

121

completely dissolved, 0.02 mole of Ba(NO3)2 was added into the solution. Ethylene

122

glycol (EG) was also added to the above solution as a stabilizing agent. The precursor

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containing Ba, Fe and Nb was dried in an oven at 120oC for 10 h and then calcined in

124

air at 1000oC for 3 h at a ramping rate of 5oC/min to obtain nanocrystalline

125

BaFe0.5Nb0.5O3 powder. Finally, the ferroelectric material was crushed and sieved

126

with 80 mesh to have particle sizes between 74 – 177 μm. For convenience, in the

127

paper hereafter the BaFe0.5Nb0.5O3 ferroelectric is denoted as BFN.

128

BaZr0.75Ti0.25O3 was prepared by the procedure similar to that reported in

129

literature[36] as following: 0.3 mole of Ti(C4H9O)4 and 0.1 mole of Zr(C3H7O)4 were

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added into n-propanol. Next, 0.04 mole of Ba(C2H3O2)2 and 0.06 mole of CA were

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dissolved in C2H5OH. When the precursor was completely dissolved, EG was added

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to the above solution as a stabilizing agent. The precursor containing Ba, Zr and Ti

133

were dried in an oven at 120oC for 10 h and then calcined in air at 1000 oC for 3 h at a

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ramping rate of 5oC/min. After calcination, the ferroelectric material was crushed and

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sieved with 80 mesh to have particle sizes between 74 – 177 μm. The BaZr0.75Ti0.25O3

136

ferroelectric is hereafter denoted as BZT. 7


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137 138

2.2 Characterization of ferroelectrics

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BZT and BFN were characterized by X-ray diffraction (XRD). The XRD patterns

140

were obtained with a D8AXRD diffractometer using Cu Kα monochromatic X-ray,

141

operated at 40 kV and 40 mA over the scattering angle of 2θ from 5o to 80o with step

142

of 0.05o/sec. The nitrogen adsorption and desorption isotherms of the catalysts were

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carried out at 77 K using a Micromeritics ASAP–2010 Analyzer while the specific

144

surface areas were measured by applying the Brunauer-Emmett-Teller (BET) method

145

to the nitrogen adsorption data within the 0.05 – 0.2 P/P0 range. The morphologies of

146

catalysts were examined by scanning electron microscopy (SEM), and fitted with an

147

INCA

148

semi-quantitatively verify the composition of supported phases. IR spectra were

149

recorded using a NICOLET 6700 plus Fourier transforms infrared (FT–IR)

150

spectrometer. Last, the dielectric constant of the catalyst was measured at a frequency

151

of 20 kHz at room temperature using an LCR meter (HP4284A).

X-sight

energy

dispersive

X-ray

microanalysis

(EDS)

system

to

152 153

2.3 Experimental system

154

The schematic of the experimental setup is shown in Fig. 1. The lab-scale

155

experimental system is mainly composed of four parts: a gas feeding system, an AC 8


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high voltage quasi-pulse power supply, a plasma reactor and an analysis system. The

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DBD reactor consists of a ceramic tube with outer diameter of 20 mm and inner

158

diameter of 15 mm, a spiral stainless steel rod with outer diameter of 3 mm inside the

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ceramic tube as inner electrode, and a stainless steel wire mesh with length of 70 mm

160

wrapped around the ceramic tube as the outer electrode. The total discharge volume is

161

approximately 47.5 cm3. The feeding gases, CO2 (99.99%) and CH4 (99.99%), were

162

provided from gas cylinders and controlled by a set of mass flow controllers. The

163

CH4/CO2 ratio and the total gas flow rates were adjusted at ranges of 0.33 – 3.0 and

164

40 – 120 mL/min, respectively. An AC quasi-pulse power supply (EN Technologies,

165

Genius 2) with the voltage output up to 18 kV and a frequency of 20 – 60 kHz is used.

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In this study, the alternating current voltage was controlled at a range of 12.1 to 13.6

167

kV while the frequency was fixed at 20 kHz. The waveforms of applied voltage,

168

current and power were recorded by an oscilloscope (Tektronix DPO3014) equipped

169

with a high voltage probe (Tektronix P6015A) and a current probe (Tektronix

170

TCPA300). Typical voltage and current waveforms are shown in Fig. 2. The power

171

was measured by the product of the applied voltage and the current as described in eq.

172

3, and energy efficiency (EE), which represents the amount of H2, CO or syngas

173

(H2+CO) produced by one kilowatts, was calculated by eq. 4: 𝑇

P w =

𝑉 𝑡 ∙ 𝐼 𝑡 𝑑𝑡

(3)

0

9


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Energy efficiency, EE

mol kWh

=

[𝐻2 +𝐶𝑂]𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑃𝑡 /3.6

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=

𝑃𝑔 ∙𝑄𝐻 2 +𝐶𝑂 𝑃∙𝑇∙𝑅/3.6

(4)

174

where P is the discharge power (W), t is the total discharge time (s), Pg is the

175

pressure (atm), 𝑄𝐻2 +𝐶𝑂 is the flow rate of H2, CO or total syngas (H2 + CO) (L/s),

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T is the temperature (K) and R is the ideal gas constant (0.082 atm.L/mole.K),

177

respectively. All experimental tests were carried out at atmospheric pressure and room

178

temperature (298.15 K) and eq. 4 can be simplified to eq. 5: EE =

𝑄 6.788𝑃

(5)

179

For plasma-catalysis packed-bed DBD system, 4.2 g of BZT or 4.4 g of BFN pellets

180

were placed into the discharge zone and the height of packing is 20 mm, the volume

181

of packed bed is 13.6 cm3. For better understanding of the dielectric effect, a

182

non-catalytic dielectric material (glass beads) was adopted as a control group. The

183

concentrations of CO2, CH4 and products before and after reactions were analyzed

184

with an on-line gas chromatography (GC, Agilent Technologies 6890N) equipped

185

with a thermal conductivity detector (TCD) and a flame ionization detector (FID).

186

Two column HP-PLOT/Q and 5A molecular sieves were used. The experimental data

187

were taken and recorded when the reactions reached steady-state and the total

188

discharge time is about 30 minutes to avoid arcing. All measurements were conducted

189

twice to ensure the relative deviation is smaller than 10%.The overall conversions

190

(𝑋𝐶𝑂2 and 𝑋𝐶𝐻4 ), selectivities (𝑆𝐶𝑂 , 𝑆𝐻2 and 𝑆𝐶x 𝐻y ), the H2/CO ratio and the carbon 10


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191

Energy & Fuels

balance (CB) are defined as follows:

192 𝐶𝑂2

𝑋𝐶𝑂2 % =

− 𝐶𝑂2 𝐶𝑂2 𝑖𝑛

𝑖𝑛

𝑜𝑢𝑡

(6)

[𝐶𝐻4 ]𝑖𝑛 − [𝐶𝐻4 ]𝑜𝑢𝑡 [𝐶𝐻4 ]𝑖𝑛

𝑋𝐶𝐻4 % =

(7)

𝑆𝐶𝑂 % =

[𝐶𝑂]𝑜𝑢𝑡 [𝐶𝑂2 ]𝑖𝑛 − 𝐶𝑂2 𝑜𝑢𝑡 + [𝐶𝐻4 ]𝑖𝑛 − [𝐶𝐻4 ]𝑜𝑢𝑡

(8)

𝑆𝐻2 % =

[𝐻2 ]𝑜𝑢𝑡 2 ∗ ( 𝐶𝐻4 𝑖𝑛 − 𝐶𝐻4

(9)

𝑆𝐶x 𝐻y % =

𝑥 ∗ 𝐶x 𝐻𝑦 𝐶𝐻4

𝑖𝑛

𝑜𝑢𝑡 )

𝑜𝑢𝑡

− 𝐶𝐻4

(10) 𝑜𝑢𝑡

𝐻2 [𝐻2 ]𝑜𝑢𝑡 = 𝐶𝑂 [𝐶𝑂]𝑜𝑢𝑡 CB % = 193

(11)

[𝐶𝑂]𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 + 𝑥 ∗ [𝐶x 𝐻𝑦 ]𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 [𝐶𝑂2 ]𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 + [𝐶𝐻4 ]𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑

(12)

where [ ]in and [ ]out are the molar flow rates of inflow and outflow, respectively.

194 195

Figure. 1

196

Figure. 2

197 198

3. RESULTS AND DISCUSSION

199

3.1 Characterizations of catalysts

200

The XRD patterns of fresh BZT and BFN are presented in Fig. 3. Both BZT and

201

BFN have characteristic peaks at 30o, 43o, 54o, 63o and 72o, indicating that they have a

202

similar perovskite structure. It is also noted that no significant difference is observed 11


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203

as comparing the XRD patterns before and after discharges. Moreover, the grain size

204

of BFN and BZT are calculated via Scherrer’s equation to be 28.0 nm and 19.3 nm.

205

FT-IR spectra of fresh and used BZT and BFN are presented in Fig. 4, neither C-O

206

nor C-H bonding is observed, showing a little adsorption during reforming. SEM

207

patterns illustrated in Fig. 5 indicate the sizes of the pellets are in the range of 100 –

208

200 nm.

209

As shown in Table. 1, the specific surface areas of fresh BZT and BFN are 9.74 and

210

7.68 m2/g, respectively. Surface areas are slightly reduced after reforming, this may

211

be caused by the redistribution of clusters on the catalyst and valance changing by

212

electron bombardment, the former can restructure the clusters more uniformly while

213

the latter may reduce or oxidize the metals resulting in smaller surface area.

214

EDS results of BZT and BFN are also presented in Table 1.The ratio of Ba/Zr/Ti is

215

about 3.4/2.6/1.0 for BZT and the ratio of Ba/Fe/Nb is 2.2/1.1/1.0 for BFN, which are

216

close to their stoichiometric ratios (i.e., 4/3/1 and 2/1/1, respectively). The dielectric

217

constants (εr) of BZT and BFN are measured as 149 and 2025, respectively, showing

218

both catalysts are relaxor ferroelectrics. Comparing with the glass beads, (εr = 3 – 15),

219

a non-catalytic dielectric commonly used in packed-bed plasmas, the dielectric

220

constants of BZT and BFN are one to two orders of magnitude higher.

221 12


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Energy & Fuels

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Figure. 3

223

Figure. 4

224

Figure. 5

225

Table. 1

226 227

3.2 Effect of applied voltage

228

The effect of applied voltage was evaluated with the applied voltage varying from

229

12.1 to 13.6 kV under the condition of a total gas flow rate of 40 mL/min and a

230

CH4/CO2 feeding ratio of 1. Fig. 6 (a) shows the conversions of CH4 and CO2 increase

231

with the increasing applied voltage. Additionally, CH4 conversion is always higher

232

than that of CO2, which might be associated with the different dissociation energy

233

(4.5 eV for CH4 and 5.5 eV for CO2, respectively).

234

The main products measured including CO, H2, C2H6 and C2H4 are shown in Fig. 6

235

(b). With the increase of applied voltage, it is found that the CO selectivity is slightly

236

decreased from 56.4% to 54.0%, the C2H6 selectivity is notably decreased from 28.8%

237

to 17.0%, while the H2 selectivity is significantly increased from 35.3% to 59.1%. The

238

reactions involved with the CO, H2 C2H6 and C2H4 productions are listed in Rxns. (13)

239

to (23)[45]-[48]. CO2 + e* → CO + O + e*

k = f1(E/N) 13


(13)

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CO + e* → C + O + e*

k = f2(E/N)

(14)

C + O → CO

k = 6.5 x 10-32

(15)

CH4 + e* → CH3 + H + e*

k = f3(E/N)

(16)

CH3 + e* → CH2 + H + e*

k = f4(E/N)

(17)

CH2 + e* → CH + H + e*

k = f5(E/N)

(18)

CH + e* → C + H + e*

k = f6(E/N)

(19)

H + H → H2

k = 6.04 x 10-33(298/Tg)

(20)

CH3 + CH3 → C2H6

k = 4 x 10-10 Tg-0.4

(21)

C2H6 + e* → C2H4 + 2H + e*

k = f7(E/N)

(22)

2CO → CO2 + C

k = 9.6 x 10-18

(23)

240

The insignificant effect of applied voltage on CO selectivity is probably due to the

241

increase of electron-impact dissociations of both CO2 and CO with increasing applied

242

voltage. The cross section of electron impact dissociation for CO2 is 3.3 – 3.6 10-16

243

cm2 for electron energy of 5 – 10 eV. The cross section of CO2 is higher than that of

244

CO (2.4 – 2.6 10-16 cm2), indicating that dissociation of CO2 is always higher than CO

245

dissociation, resulting in higher CO formation at a high applied voltage.

246

For H2 selectivity, as stated in Rxns. (16) – (19), the higher electrons energy, the

247

higher possibility to dissociate CH4 to form hydrogen atoms. H2 mainly comes from

248

recombination of hydrogen atoms, operating at a higher applied voltage may form 14


Page 15 of 45

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Energy & Fuels

249

more hydrogen atoms and then recombine to H2, resulting in higher H2 selectivity. On

250

the other hand, CxHy radicals can rapidly recombine to form ethane, ethylene and

251

other hydrocarbons. The selectivity of ethane is influenced by concentrations of CH3

252

radicals, the higher concentrations of CH3 radicals, and the higher possibility of

253

recombination of CH3 to form C2H6. Higher free electron energy is prone to dissociate

254

CH4 into CH2 or CH instead of CH3, resulting in lower selectivity of C2H6. However,

255

selectivity of C2H4 has a decreasing trend similar to C2H6, indicating that C2H4 mainly

256

comes from C2H6 dissociation.

257

An interesting finding is the tendency of H2/CO ratio. Due to the decrease of CO

258

selectivity and increase of H2 selectivity, H2/CO ratio increases from 0.75 to 1.2 as

259

applied voltage is increased from 13.1 to 13.6 kV as shown in Fig. 6 (c). The feature

260

of adjustable H2/CO ratio is useful and important to the syngas-to-fuels synthesis

261

process.

262

Carbon might be formed by Rxs. (14), (19) and (20), and decreases the carbon

263

balance. The other factor influencing the carbon balance is carbon monoxide

264

disproportion as stated in Rx. (23). However, the carbon monoxide disproportion is

265

not a spontaneous reaction at low temperatures, that is, the most important factors

266

influencing the carbon balance are methane cracking and carbon monoxide

267

dissociation. As shown in Fig. 6 (c), the carbon balance decreases with increasing 15


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

268

applied voltage, indicating that carbon monoxide dissociation may be enhanced,

269

which also results in lower CO selectivity in Fig. 6 (b).

270

The results of energy efficiency are shown in Fig. 6 (d). The energy efficiency

271

increases slightly from 2.91 to 3.19 mol/kWh, with increasing applied voltages as

272

well as CO2/CH4 conversions. The result indicates that higher applied voltage is

273

favorable for syngas production. However, higher applied voltage also results in

274

higher carbon deposition, and may eventually limit the operation. The other obstacle

275

is that higher power input enhances micro-discharges while arcing would take place

276

and damage the reactor.

277 278

Figure. 6

279 280

3.3 Effect of total flow rate

281

The effect of flow rate on the reaction is evaluated with the gas flow rate varying

282

from 40 to 120 mL/min with the applied voltage of 13.1 kV and CH4/CO2 feed ratio

283

of 1. As shown in Fig. 7 (a), CO2 and CH4 conversions decrease from 43.4% and

284

59.2% to 36.2% and 53.4%, respectively, as the flow rate is increased from 40 to 120

285

mL/min. The influences of flow rate on selectivities, H2/CO ratio, and the carbon

286

balance are relatively insignificant as shown in Figs. 7 (b) and (c). Higher flow rate 16


Page 16 of 45

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Energy & Fuels

287

reduces the dissociation probability, resulting in lower conversion efficiency, however,

288

CO and H2 mainly come from CO2 dissociation and H atom recombination, the

289

mechanisms of CO2 and CH4 dissociations are not influenced by increasing flow rate.

290

Although the increase of the gas flow rate decreases the conversions, the energy

291

efficiency can be significantly increased. For instance, as the flow rate is increased 3

292

times, the conversions decrease by 10 – 20% and the syngas energy efficiency

293

increases by 2.66 times. It is supposed that the plasma system tested in the study is

294

better to be used in a high flow rate condition.

295 296

Figure. 7

297 298

3.4 Effect of CH4/CO2feeding ratio

299

The effect of feeding ratio is evaluated with the CH4/CO2 ratio varying from 0.33 to

300

3.0 under the condition of an applied voltage of 13.1 kV and a gas flow rate of 40

301

mL/min and the results are presented in Fig. 8. The conversions increase with

302

increasing CH4/CO2 ratio. It might be due to the influence of gas compositions on

303

EEDFs as well as plasma characteristics.

304

Selectivities of CO, H2, C2H6 and C2H4 are presented in Fig. 8 (b). As the CH4/CO2

305

ratio is increased from 0.33 to 3.0, a significant decrease of the CO selectivity from 17


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

306

66.1% to 36.5% and increases of H2 and C2H6 selectivities are observed (from 41.4%

307

and 12.8% to 53.5% and 22.6%, respectively).

308

As presented in Fig.8 (c), the carbon balance decreases slightly with increasing

309

CH4/CO2 ratio. It is expected that the losses of carbon mainly resulted from the

310

carbon deposition which would be an obstacle needed to be solved in terms of

311

long-term operation for a DBD reactor.

312

The H2/CO ratio is highly dependent on the CH4/CO2 feeding ratio of a feed gas.

313

For instance, as the CH4/CO2 ratio is increased from 0.33 to 3.0, the H2/CO ratio

314

increases from 0.5 to 2.4. The H2/CO ratio is adjustable to fit the Fischer-Tropsch

315

range. Fig. 8 (d) shows that high CH4/CO2 ratio is favorable to the increase of the

316

energy efficiency. Overall speaking, DRM with DBD is suggested to be operated at

317

higher CH4/CO2 ratios for achieving higher conversions, H2 selectivity and energy

318

efficiency.

319 320

Figure. 8

321 322

3.5 Effect of ferroelectric catalysts

323

The discharge powers for various reactor configurations are compared as shown in

324

Fig. 9. For a packed-bed reactor, the current is intensified because of 18


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Energy & Fuels

325

micro-discharges occurring at the gaps between packing particles. The dielectric

326

constant of the packing influences the magnitude of micro-discharges, and reduces the

327

resistance of the reactor, resulting in higher current and power dissipation in reactor.

328

The CO2 and CH4 conversions achieved with different reactors including

329

glass-bead-packed bed, BZT-packed bed, BFN-packed and without packing material

330

are evaluated with the applied voltage ranging from 12.1 to 13.6 kV under the

331

conditions of a CH4/CO2 ratio of 1 and a flow rate of 40 mL/min as shown in Fig. 10.

332

Filling the reactor with powders or pellets can enhance micro-discharge at gaps

333

between powders or pellets, and micro-discharge may promote inelastic

334

electron-molecule collisions, however, the retention time of gas is also reduced due to

335

smaller void volume. Shorter retention time results in lower conversions of CO2 and

336

CH4, indicating that the amplification of micro-discharge is limited for the packing

337

material with a small dielectric constant, such as glass bead. The material with a high

338

dielectric constant including BZT and BFN can store electrons via the electrostatic

339

attraction which is the effect of polarization under electric field, hence,

340

micro-discharge can be enhanced under electric field to promote CO 2 and CH4

341

conversion. With applied voltage of 13.6 kV, the CO2 conversions achieved with

342

BZT- and BFN-packed bed reactors are 52.7% and 55.6%, respectively, both are

343

higher than that achieved with the reactor without packing. Meanwhile, the CH4 19


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

344

conversions achieved with BZT- and BFN-packed bed reactors are 66.4% and 68.4%,

345

respectively. The reactor with BFN-packed bed has increased CH4 and CO2

346

conversion efficiencies by 5.8% (51.0% v.s. 55.6%) and 9.0% (64.6% v.s. 68.4%),

347

respectively.

348

The CO and H2 selectivities achieved with various reactors are presented in Fig. 10

349

(c) and (d). Experimental results show that the selectivities are influenced by the

350

packing material. The H2 selectivities are in the order of BFN > BZT > DBD > Glass,

351

which is the same as the conversions. The CO selectivity is in the order of BFN >

352

BZT > Glass > DBD. As mentioned previously, the influence of the gas flow rate on

353

the selectivity is insignificant (Fig. 7 (b)), and so is the reduction of gas retention time

354

by packing. Therefore, from a physical viewpoint, the packing material redistributed

355

the discharge area and distribution of electron energy. Existence of ferroelectrics in

356

discharge zone enhances the density of free electron, resulting in more radicals

357

including O, H and CHy due to more electron impact excitation including vibrational,

358

rotational and electron excitation. That is, CO and H2 can be formed by other

359

mechanisms, resulting in higher CO and H2 production. Moreover, radicals and

360

reactants may be adsorbed by catalyst and subsequent reaction might take place. For

361

glass bead packed bed, its low dielectric constant indicates that the average electron

20


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Energy & Fuels

362

energy within the packing zone is lower than that packing with ferroelectrics,

363

resulting in lower selectivity.

364

Another phenomenon observed with packing ferroelectrics is the increase of carbon

365

balance. Carbon balance can be a parameter to indicate the severity of coke formation

366

since other carbon-containing products are in trace amount. For reactors packing with

367

glass beads or ferroelectrics, carbon balance increases by 0.8 to 3%. Higher carbon

368

balance is possibly attributed to the conversion of more reactants including CH4, CO2,

369

CO, resulting in more highly active radicals to recombine with carbon atoms and

370

decrease the severity of coke formation.

371

Relaxor ferroelectrics can store charges due to polarization, hence, syngas

372

production can be enhanced due to more electrons with higher energy to cause

373

effective collision. With applied voltage of 13.6 kV, feeding flow rate of 40 mL/min

374

and CH4/CO2 ratio of 1, the energy efficiency achieved with DBD plasma alone,

375

BZT-packed bed reactor and BFN-packed bed reactor are 3.19, 3.81 and 3.83

376

mol/kWh, respectively, as shown in Fig. 11 (a). For four different reactors, the energy

377

efficiencies under the same applied voltage are in the order of BFN > BZT > DBD >

378

Glass. Moreover, Fig 11 (b) shows the influence of power on energy efficiency. The

379

energy efficiencies are in the order of BZT > BFN > DBD > Glass, indicating that

380

packing ferroelectric including BZT and BFN enhances syngas production. To ensure 21


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

381

the synergistic effect under different conditions, we adjust the CH4/CO2 ratio to 3,

382

with the applied voltage of 13.1 kV and flow rate of 40 mL/min, the energy efficiency

383

of DBD plasma alone and BFN-packed bed reactor are 3.90 and 4.61 mol/kWh,

384

respectively. Higher feeding flow rate results in higher energy efficiency (see Fig. 7),

385

however, the effect of ferroelectrics packing under higher feeding flow rate is not

386

evaluated in this study due to the high pressure drop. The energy efficiencies are

387

presented in Table 2, with the comparison with relevant studies. To compare the

388

effect of packing, the feeding flow rate is fixed at 40 mL/min. Table 2 indicates that

389

the energy efficiency depends on not only discharge power but also on other factors

390

such as discharge gap, gas retention time, material of dielectric barrier and waveform

391

of power supply. In this study, we demonstrate that packing the DBD reactor with the

392

material of a higher dielectric constant can promote the CH4 and CO2 conversions and

393

thus enhances energy efficiency.

394 395

Figure. 9

396

Figure. 10

397

Figure. 11

398

Table. 2

399 22


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Energy & Fuels

400

4. CONCLUSIONS

401

This study experimentally evaluated the effect of applied voltage, flow rate and

402

CH4/CO2 feeding ratio on the efficiency of DRM. Results obtained indicate that the

403

higher applied voltage and CH4/CO2 ratio have a positive effect on conversion of CH4

404

and CO2, but the lower carbon balance could be a shortcoming to limit the operation

405

time. Operating the system with a higher flow rate can generate syngas more

406

economically due to better energy efficiency, moreover, H2/CO almost keeps as a

407

constant, and this value can be controlled by adjusting CH4/CO2 ratio.

408

The concept of packing relaxor ferroelectrics to enhance CH4/CO2 reforming by

409

increasing micro-discharges has been proved in the study. The direct evidence of this

410

phenomenon is the increased density of micro-discharges, thus the collisions between

411

high-energy electron and gas molecules are enhanced. Since interactions between

412

ferroelectric and plasma are complicated, more scientific works need to be done.

413

Especially a systematic study on ferroelectrics such as sizes, dielectric losses and

414

shapes is needed for the advancement of the DRM process with plasma.

23


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

415

Figure Captions

416

Figure 1 Experimental setup for DRM reaction

417

Figure 2 Voltage and current waveforms across the discharge; (a) voltage, (b) current.

418

Figure 3 X-ray diffraction patterns of ferroelectric catalysts powder; (a) BZT, (b)

419

BFN.

420

Figure 4 FT-IR spectra of (a) BZT, (b) BFN.

421

Figure 5 SEM photos of ferroelectrics; (a) BZT, (b) BFN

422

Figure 6 Influence of input voltage on DRM reaction with DBD; (a) conversions of

423

CO2 and CH4, (b) selectivities of H2, CO and by-products, (c) ratio of H2/CO and

424

carbon balance, and (d) energy efficiency of syngas generation (the ratio of feeding

425

CH4/CO2 = 1, and the feeding gas flow rate = 40 mL/min)

426

Figure 7 Influence of gas feeding flow rate on DRM reaction with DBD; (a)

427

conversions of CO2 and CH4, (b) selectivities of H2, CO and by-products, (c) ratio of

428

H2/CO and carbon balance, and (d) energy efficiency of syngas generation (the ratio

429

of feeding CH4/CO2 = 1 and the input voltage = 13.1 kV).

430

Figure 8 Influence of CH4/CO2 on DRM reaction with DBD; (a) conversions of CO2

431

and CH4, (b) selectivities of H2, CO and by-products, (c) ratio of H2/CO and carbon 24


Page 24 of 45

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Energy & Fuels

432

balance, and (d) energy efficiency of syngas generation (the feeding gas flow rate =

433

40 and the input voltage = 13.1 kV).

434

Figure 9 Powers of different reactor types at various applied voltage.

435

Figure 10 Influence of ferroelectric catalysts on DRM reaction with DBD; (a) and (b)

436

conversions of CO2 and CH4, (c) and (d) selectivities of CO and H2, (e) carbon

437

balance (the ratio of feeding CH4/CO2 = 1, and the feeding gas flow rate = 40

438

mL/min.

439

Figure 11 Energy efficiencies of different reactors investigated; (a) applied voltage, (b)

440

power (the ratio of feeding CH4/CO2 = 1 and the feeding gas flow rate = 40 mL/min).

25


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

441 442

Fig. 1 Experimental setup for DRM reaction

26


Page 26 of 45

Page 27 of 45

15

(a)

Voltage (kV)

10

5

0

-5

-10

-15 0.00

0.05

0.10

0.15

0.10

0.15

Time (ms)

443

80

(b) 60 40

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

20 0 -20 -40 -60 -80 0.00

444 445

0.05

Time (ms)

Fig. 2 Voltage and current waveforms across the discharge; (a) voltage, (b) current.

27


Energy & Fuels

(a)

● ●

●

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 45

●

●

●

●

● (b)

●

●

20

30

40

50

60

70

80

2 theta (degree)

446 447

Fig. 3 X-ray diffraction patterns of ferroelectric catalysts powder; (a) BZT, (b) BFN.

448

(● perovskite phase)

28


Page 29 of 45

(a)

Transmittance (a. u.)

fresh

used

5000

4000

3000

2000

1000

0

1000

0

-1

Wavenumber (cm )

449

(b) fresh

Transmittance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

used

5000

4000

3000

2000

-1

450 451

Wavenumber (cm ) Fig. 4 FT-IR spectra of (a) BZT, (b) BFN. 29


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

452 (b)

453 454

Fig. 5 SEM photos of ferroelectrics; (a) BZT, (b) BFN.

30


Page 30 of 45

Page 31 of 45

80

80

(a)

70

CO H2

(b)

C2 H 6 C2 H 4

60

Selectivity (%)

Conversion (%)

60

40

20

50 40 30 20

CH4

10

CO2 0

0 12.1

455

12.6

13.1

13.6

12.1

Applied voltage (kV)

80

1.1 60 1.0 40 0.9

H2/CO Carbon balance

0.7

20

0 12.1

456

Energy efficiency (mol/kWh)

(c)

0.8

12.6

13.1

13.1

13.6

4

100

Carbon balance (%)

1.2

12.6


1.3

H2/CO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

H2

(d)

CO Syngas

3

2

1

0

13.6

12.1


12.6

13.1

13.6


457

Fig. 6 Influence of input voltage on DRM reaction with DBD; (a) conversions of CO2

458

and CH4, (b) selectivities of H2, CO and by-products, (c) ratio of H2/CO and carbon

459

balance, and (d) energy efficiency of syngas generation (the ratio of feeding CH4/CO2

460

= 1, and the feeding gas flow rate = 40 mL/min).

31


Energy & Fuels

80

80

(a)

C2H6 C2H4

60

Selectivity (%)

Conversion (%)

60

40

20

40

20

CH4 CO2

0 40

461

80

0

120

40

80

Feed flow rate (mL/min)

100

1.06 60

40 1.04

H2/CO Carbon balance 1.02

Carbon balance (%)

80

20


10

(c)

0 40

80

120


1.08

462

CO H2

(b)

H2/CO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 45

H2

(d)

CO Syngas

8

6

4

2

0

120

40


80

120

Set of feed flow rate (mL/min)

463

Fig. 7 Influence of gas feeding flow rate on DRM reaction with DBD; (a) conversions

464

of CO2 and CH4, (b) selectivities of H2, CO and by-products, (c) ratio of H2/CO and

465

carbon balance, and (d) energy efficiency of syngas generation (the ratio of feeding

466

CH4/CO2 = 1 and the input voltage = 13.1 kV).

32


Page 33 of 45

80

80

(a)

C2H6 C2H4

60

Selectivity (%)

60

Conversion (%)

CO H2

(b)

40

20

40

20

CH4 CO2

0 0

1

2

0

3

0

1

2

CH4/CO2

467

3

CH4/CO2

5

(c) 2 60

40 1

Carbon balance (%)

80

20

H2/CO


100

3

H2/CO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

H2

(d)

CO Syngas

4

3

2

1

Carbon balance 0

468

0

0

0 1

2

0.33

3

0.5

1

2

3

Set of CH4/CO2 ratio

CH4/CO2

469

Fig. 8 Influence of CH4/CO2 on DRM reaction with DBD; (a) conversions of CO2 and

470

CH4, (b) selectivities of H2, CO and by-products, (c) ratio of H2/CO and carbon

471

balance, and (d) energy efficiency of syngas generation (the feeding gas flow rate =

472

40 and the input voltage = 13.1 kV).

33


Energy & Fuels

30

25

20

Power (W)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 45

15

10

BFN packed bed BZT packed bed

5

Glass bead packed bed Plasma alone

0 12.1

473 474

12.6

13.1

13.6


Fig. 9 Powers of different reactor types at various applied voltage.

34


Page 35 of 45

80

60

(b) Conversion of CH4 (%)

Conversion of CO2 (%)

(a) 50

40

30

BFN packed bed BZT packed bed Plasma alone

20

70

60

50

40 20

Glass bead packed bed

0

0 12.1

475

12.6

13.1

13.6

12.1


12.6

13.1

13.6


65

70

(c)

(d) H2 selectivity (%)

60

CO selectivity (%)

60

55

50

40

30 50 10 0

0 12.1

476

12.6

13.1

13.6

12.1


12.6

13.1

13.6


85

80

Carbon balance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(e)

75

70

65

60 0 12.1

477

12.6

13.1

13.6


478

Fig. 10 Influence of ferroelectric catalysts on DRM reaction with DBD; (a) and (b)

479

conversions of CO2 and CH4, (c) and (d) selectivities of CO and H2, (e) carbon

480

balance (the ratio of feeding CH4/CO2 = 1, and the feeding gas flow rate = 40

481

mL/min).

35


Energy & Fuels

5

Plasma alone Glassbead packed bed BZT packed bed BFN packed bed


(a) 4

3

2

1

0 12.1

12.6

13.1

13.6

Set of applied voltage (kV)

482

4.0

(b) Energy efficiency (mol/kWh)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

3.6

3.2

2.8

BFN packed bed BZT packed bed 2.4

Glass bead packed bed Plasma alone

0.0 12

483

16

20

24

28

Power (W)

484

Fig. 11 Energy efficiencies of different reactors investigated; (a) applied voltage, (b)

485

power (the ratio of feeding CH4/CO2 = 1 and the feeding gas flow rate = 40 mL/min).

36


Page 37 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

486

Table Captions

487

Table 1. BET, EDS and LCR results for two catalysts

488

Table 2. Comparisons of energy efficiencies

37


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

Table 1. BET, EDS and LCR results for two catalysts

489

BET (m2/g)

EDS (mol %)

Dielectric

Catalyst Fresh

Used

Ba

Zr

Ti

Fe

Nb

constant

BaZr0.75Ti0.25O3

9.74

9.68

48.44

37.37

14.19

-

-

149

BaFe0.5Nb0.5O3

7.68

7.21

51.26

-

-

25.63

23.11

2025

38


Page 39 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Table 2. Comparisons of energy efficiencies

490

Applied power

Packing material

Power (W)

Q (mL/min)

CO2 conv. (%)

CH4 conv. (%)

CH4/CO2

H2/CO

EE (mol/kWh)

Ref.

AC

–

100

60

43

64

3

3.3

0.76

[28]

AC

–

30

100

53

89

1

–

0.90

[38]

AC

Zeolite A

500

200

39

66

3

3.11

0.43

[39]

AC

Ni/γ-Al2O3

130

30

31

58

1

1.35

0.096

[40]

AC

–

107.4

20

40

48

1

1

0.35

[41]

AC pulse

LaNiO3

–

22.5

21

23

1.5

1.5

1.73

[42]

AC pulse

La2O3/γ-Al2O3

45

80

30

25

14

–

0.83

[43]

AC

Ni/γ-Al2O3

19

50

21

38

1

–

1.17

[44]

AC pulse

–

–

80

55

70

1

1.23

1.12

[26]

AC quasi pulse

–

19

40

47

66

3

2.37

3.90

This study

AC quasi pulse

BFN

22.8

40

51

70

3

1.81

4.61

This study

491

39


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

492

Corresponding author

493

Moo-Been Chang

494

[email protected]

495


496


497

498

Acknowledgement

499

Dr. Shiaw-Huei Chen of INER and Prof. Ta-Chin Wei of CYCU are acknowledged

500

for valuable suggestions and discussions.

501

ABBREVIATIONS

502

DBD, dielectric barrier discharge; BZT, BaZr0.75Ti0.25O3; BFN, BaFe0.5Nb0.5O3;

503

GHGs, Greenhouse Gases; DRM, dry reforming of methane; EEDFs, electron energy

504

distribution functions; BET, Brunauer-Emmett-Teller; SEM, scanning electron

505

microscopy; EDS, energy dispersive X-ray microanalysis; FT-IR, Fourier transforms

506

infrared; EE, energy efficiency; CB, carbon balance.

507 508 40


Page 40 of 45

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509

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Dry Reforming of Methane with Dielectric Barrier Discharge and

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