Post-plasma catalytic oxidative CO2 reforming of methane over Ni-based catalysts

Catalysis Today 256 (2015) 96–101

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Post-plasma catalytic oxidative CO2 reforming of methane over Ni-based catalysts Kai Li a,b , Jing-Lin Liu a,b , Xiao-Song Li a,b , Xiao-Bing Zhu a,b,∗ , Ai-Min Zhu a,b,∗ a b

Center for Hydrogen Energy and Liquid Fuels, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China Laboratory of Plasma Physical Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China

a r t i c l e

i n f o

Article history: Received 13 December 2014 Received in revised form 3 February 2015 Accepted 2 March 2015 Available online 3 April 2015 Keywords: Plasma Ni catalyst Methane reforming Syngas

a b s t r a c t To seek an efficient route for syngas production from oxidative CO2 reforming of methane (OCRM) via post-plasma catalytic technique, three routes were compared using spark-shade plasma (input power = 106 W, with F1 of 1.36 SLM at CH4 :O2 :CO2 = 1:0.6:0.7) and Ni/CeO2 /Al2 O3 catalyst (catalyst temperature = 800 ◦ C, with or without F2 of 0.52 SLM CH4 ). Compared with Route 1 (plasma only, F1 only), XO2 , XCH4 , CH2 +CO and H2 /CO ratio of Route 2 (plasma + catalyst, F1 only) increased to 100%, 99%, 76% and 1.2, respectively; but XCO2 kept at about 35%, which was close to the thermodynamic-equilibrium values. In Route 3 (plasma + catalyst, F1 + F2), XCO2 increased dramatically to 67%, CH2 +CO and H2 /CO ratio further increased to 86% and 1.5, respectively, though XCH4 decreased to 77%. Both SCO and SH2 arrived at nearly 100%. Assuming that the plasma could supply the heat energy for the subsequent catalytic reaction at 800 ◦ C, syngas energy cost as low as 0.5 eV/molecule and energy efficiency as high as 91% were achieved. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Dry reforming of methane (DRM) via simultaneous conversion of CH4 and CO2 has been proposed to produce syngas (H2 + CO) [1–5]. However, it is strongly endothermic and suffers from high energy cost. Combining the exothermal reaction of partial oxidation, via addition of oxygen, the energy cost of DRM can be lowered. Moreover, the presence of oxygen also inhibits coke formation. This process is referred to as oxidative CO2 reforming of methane (OCRM). Catalytic method is extensively employed for OCRM [6–10]. An alternative technique is non-thermal plasma, in which reactant molecules collided with energetic electrons to produce radicals via excitation and dissociation pathways, followed by radical reactions to form final products. Plasma technique has several merits, including compactness, feed flexibility, durability, and quick response. Recently, non-thermal plasma has been explored for OCRM [11–14]. Rueangjitt et al. [11] reported syngas energy cost of 13 eV/molecule with 81% of CH4 conversion and 49% of CO2 conversion using multi-stage gliding arc discharge at CH4 :O2 :CO2

molar ratio of 1:0.3:0.4. Hwang et al. [12] reported syngas energy cost of 4.7 eV/molecule with 54% of CH4 conversion and 27% of CO2 conversion using arc-jet plasma at CH4 :O2 :CO2 molar ratio of 1:0.4:1. In our previous work using a spark plasma reactor [14], syngas energy cost of 3.4 eV/molecule with 69% of CH4 conversion and 52% of CO2 conversion at CH4 :O2 :CO2 molar ratio of 1:0.2:0.7 was reported. Obviously, high energy cost is the major obstacle to plasma technique. We have designed and reported a unique spark-shade plasma reactor with low energy cost to produce highconcentration syngas [15–17]. On the other hand, the combination of plasma with catalyst is a feasible and effective approach to reduce the energy cost [18–22]. Rafiq et al. [18] combined gliding arc discharge with Ni-based catalyst and reported syngas energy cost of 0.7 eV/molecule with 86% of CH4 conversion and 4% of CO2 conversion at CH4 :O2 :CO2 molar ratio of 1:0.7:0.7. In this paper, in order to further reduce the syngas energy cost and increase methane and CO2 conversions, the spark-shade plasma followed by Ni-based catalyst was employed to seek an efficient route for syngas production from oxidative CO2 reforming of methane.

2. Experimental ∗ Corresponding author at: Laboratory of Plasma Physical Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China. Tel.: +86 411 84706094; fax: +86 411 84706094. E-mail addresses: [email protected] (X.-B. Zhu), [email protected], [email protected] (A.-M. Zhu). http://dx.doi.org/10.1016/j.cattod.2015.03.013 0920-5861/© 2015 Elsevier B.V. All rights reserved.

2.1. Plasma and catalytic reactors The spark-shade plasma reactor is the same as that in our previous paper [15]. A stainless steel rod of 6-mm diameter coaxial

K. Li et al. / Catalysis Today 256 (2015) 96–101

with a quartz tube was used as the high-voltage electrode. A stainless steel rod of 3-mm diameter, rotated around the axis of the quartz tube in a radius of 10 mm at a speed of 400 r/min, was employed as the ground electrode. The axial distance between the two electrodes was fixed at 17 mm. The plasma was powered by a 95 kHz AC high-voltage (0–30 kV) power supply (CTP-2000K, Nanjing Suman electronics Co., China). Following the plasma reactor, a quartz tube of I.D. 17 mm packed with Ni/CeO2 /Al2 O3 catalyst was used as a catalytic reactor and heated by an oven. The experiment was performed at 1 bar. The flow rates were controlled by mass flow controllers. Three routes were conducted for oxidative CO2 reforming of methane in this study, as shown in Fig. 1. In Route 1, only sparkshade plasma was employed; in Route 2, Ni/CeO2 /Al2 O3 catalyst followed the spark-shade plasma; in Route 3, besides the same feeding of F1 as Route 1 and 2, another feeding of F2 was introduced behind the plasma. The others were the same as those in Route 2. Route 3 was designed to raise further CO2 conversion via introducing more CH4 and to avoid the coke issue of plasma reactor in the presence of high-concentration CH4 . The feeding F1 was a flow rate of 1.36 SLM (Standard Liter per Minute) with a CH4 :O2 :CO2 molar ratio of 1:0.6:0.7; the feeding F2 was pure CH4 of 0.52 SLM. As a result, the total flow rate of Route 3 was 1.88 SLM and its CH4 :O2 :CO2 molar ratio was 1:0.3:0.4.

N2 adsorption–desorption isotherms measured with a surface area analyzer (NOVA2200, Quantachrome) at −196 ◦ C. Prior to the measurement, the samples were degassed under vacuum at 200 ◦ C for 5 h. The catalyst after reduction was observed by a transmission electron microscopy (TEM, TECNAI Spirit). 2.3. Analytical methods Two gas chromatographs (Agilent 1790 T and Agilent 6890N) were employed to analyze online gaseous products. Nitrogen was used as an internal standard gas to quantify gaseous O2 , CH4 , CO2 , CO, C2 H2 , C2 H4 and C2 H6 , and helium was used to quantify H2 . The detailed analysis of gaseous products was described in our previous papers [14,23]. Definitions of conversion, conversion rate, carbon-based (Cbased) and hydrogen-based (H-based) selectivity, carbon and hydrogen balance are consistent with those in our previous papers [14], and briefly stated as follows: Conversions of O2 (XO2 ), CH4 (XCH4 ), CO2 (XCO2 ) and total-carbon (XTC ) are:



XO2 =

Ni/CeO2 /Al2 O3 catalyst was prepared by sequential wetness impregnation method. CeO2 /Al2 O3 was prepared by impregnation of ␥-Al2 O3 pellets (particle size: 1–2.5 mm) with Ce(NO3 )3 ·6H2 O aqueous solution, followed by drying at 110 ◦ C for 6 h and calcination in air at 500 ◦ C for 6 h. CeO2 /Al2 O3 was incipiently impregnated overnight at room temperature with Ni(NO3 )2 ·6H2 O aqueous solution, followed by drying at 110 ◦ C for 6 h and calcination in air at 500 ◦ C for 6 h. The prepared catalyst had Ni of 11 wt.% and Ce of 8 wt.%, which was determined by inductively coupled plasmaatomic emission spectroscopy (ICP-AES, Optima 2000DV, Perkin Elmer). In Route 2 and 3, Ni/CeO2 /Al2 O3 catalysts of 6 g were packed into the catalytic reactor and reduced with 5 vol.% H2 /N2 flow of 0.2 SLM for 1 h at 850 ◦ C prior to the reaction test. X-ray diffraction (XRD) patterns were obtained from an Xray diffractometer (D/MAX-2400, Rigaku) using a Cu K␣ radiation at 40 kV and 100 mA in the 2 range from 20◦ to 80◦ . The Brunauer-Emmett-Teller (BET) surface area was determined from

FN2

1−

2

·

in FCH

FN2

·

in FCO



4

out CN out CCO

(E1)

× 100%

(E2)

× 100%

(E3)



2

out CN 2

2

XTC =

out CCH 2

4

1−

× 100%

out CN

FN2





2

2

1−

XCO2 =

COout

·

FOin

 XCH4 =

2.2. Catalyst preparation and characterization

97

in · X in FCH CH4 + FCO · XCO2 4

2

in + F in FCH CO 4

× 100%

(E4)

2

in , F in and F where FOin , FCH N2 denote the flow rates of O2 , CH4 and CO2 2 4 CO2 fed into the reactor, and internal standard N2 gas, respectively. out , C out and C out represent the concentration of O , CH , COout , CCH 2 4 N2 CO2 2 4 CO2 and N2 in the effluent gas, respectively. Conversion rates of O2 (rO2 ), CH4 (rCH4 ) and CO2 (rCO2 ) are:

rO2 = FOin · XO2

(E5)

in rCH4 = FCH · XCH4

(E6)

in rCO2 = FCO · XCO2

(E7)

2

4

2

C-based selectivities of CO (SCO ) and C2 hydrocarbons (C2 H2 + C2 H4 + C2 H6 , SC2 ) and carbon balance (BC ) are:

Spark-shade Products Plasma

F1



SCO =

Route 1 F1

Spark-shade Plasma

Ni/CeO2/Al2O3

Products

FN2 in · X in FCH CH4 + FCO · XCO2 4

 SC2 =

2 · FN2 in · X in FCH CH4 + FCO · XCO2

·

CCout 2

out CN

× 100%

(E8)

× 100%

(E9)



2

BC = SCO + SC2

Route 2 F2 Spark-shade Plasma

2



out CN 2

2

4

F1

·

out CCO

Products Ni/CeO2/Al2O3

(E10)

out and C out represent the concentrations of CO and C where CCO 2 C2 hydrocarbons in the effluent gas, respectively. H ) and C hydrocarH-based selectivities of H2 (SH2 ), H2 O (SH 2 O

bons (SCH ) and hydrogen balance (BH ) are:

2

2

Route 3 Fig. 1. Three routes for oxidative CO2 reforming of methane. F1 = 1.36 SLM (CH4 :O2 :CO2 = 1:0.6:0.7); F2 = 0.52 SLM (CH4 ).

SH2 =

0.5 · FHe in FCH 4

· XCH4

·

out CH 2

out CHe

× 100%

(E11)

98

K. Li et al. / Catalysis Today 256 (2015) 96–101

2O

=

0.5 · FHoutO 2

in FCH 4

 SCH 2

=

· XCH4

3. Results and discussion

× 100%

FN2 in · X 4 · FCH CH4

·

(E12)

2 · CCoutH + 4 · CCoutH + 6 · CCoutH 2

2

2

4

2



6

out CN

× 100%

2

4

(E13)

H BH = SH2 + SH

2O

+ SCH

(E14)

2

out and where FHe denotes the flow rate of internal helium gas. CH 2

out represent the concentration of H and He in the effluent gas, CHe 2 respectively. FHoutO represents the flow rate of H2 O produced, which 2 can be calculated via E15: out CCO

in FHoutO = 2 · (FCO · XCO2 + FOin · XO2 ) − FN2 · 2

2

(E15)

out CN

2

2

Syngas concentration (CH2 +CO ) is calculated from: out /C out ) + F out out FN2 · (CCO He · (CH /CHe ) N

CH2 +CO =

2

2

out /C out ) + (C out /C out ) + (C out /C out ) FN2 · ((CCH N2 N2 N2 CO2 O2 4 out /C out ) + (C out /C out )) + F out + F out out + (CCO He · (CH /CHe ) N N C H O 2

2

2

2

× 100%

2

(E16)

Energy cost of syngas (ECH2 +CO ) and energy efficiency () are defined as: ECH2 +CO =

P out /C out ) + F out out FN2 · (CCO He · (CH /CHe ) N 2

=

out · LHV out F out · (CH H2 + CCO · HVCO ) 2

in · X P + FCH CH4 · LHVCH4

(E17)

2

× 100%

(E18)

4

where P denotes the input power of the plasma measured by a wattmeter; HVCO denotes the heating value of CO; LHVH2 and LHVCH4 denote the lower heating value of H2 and CH4 , respectively. 2.4. Thermodynamic-equilibrium calculation Thermodynamic-equilibrium data was calculated by the HSC Chemistry software (v7.0) using Gibbs free energy minimization method. The CH4 :O2 :CO2 molar ratios of 1:0.6:0.7 and 1:0.3:0.4 in the three routes were calculated at the temperature range of 600–1000 ◦ C and 1 bar. The thermodynamic-equilibrium enthalpy change (H) was calculated via E19. H =



0 nB f Hm (B, g, 298.15 K) −

B



298.15 K T

+ 298.15 K



0 nB Cp,m (B, g) dT

B



q CeO2

˜

¿ Al2O3

ê Ni

NiO

0 nconv f Hm (A, g, 298.15 K) A

A T

+





As shown in Fig. 2, for the catalyst before reduction, the peaks corresponding to CeO2 and NiO can be identified. After reduction at 850 ◦ C, NiO was reduced to metallic nickel with the size of 15.9 nm estimated by Scherrer formula. The metallic nickel could be easily distinguished from the TEM image with average particle size of 18.1 nm, as shown in Fig. 3. Along with the NiO reduction, the CeO2 phase disappeared, which could be attributed to the formation of CeAlO3 phase at 850 ◦ C [24,3]. As shown in Table 1, the BET surface area for Ni/CeO2 /Al2 O3 catalyst before reduction was 157 m2 /g and it decreased to 116 m2 /g after reduction at 850 ◦ C. This was mainly resulted from the destruction of some micropores and reconstruction of mesopores by the sintering process at high temperature. Thermodynamic-equilibrium conversions of reactants, syngas concentration and H2 /CO ratio versus temperature were illustrated in Fig. 4. For the both CH4 :O2 :CO2 molar ratios, XO2 was maintained at 100%, XCH4 , XCO2 , XTC and CH2 +CO increased rapidly and then turned to increase slightly with temperature. At CH4 :O2 :CO2 = 1:0.6:0.7, the temperature increased from 600 to 800 ◦ C, XCH4 , XCO2 , XTC and CH2 +CO increased rapidly from 70%, −10%, 38% and 53% to 100%, 39%, 75% and 76%, respectively. As temperature further increased to 1000 ◦ C, XCO2 and XTC increased slowly to 54% and 81%, respectively; XCH4 and CH2 +CO hardly changed. The H2 /CO ratio reduced gradually from 1.5 to 1.1 with increasing the temperature from 600 to 1000 ◦ C. At CH4 :O2 :CO2 = 1:0.3:0.4, the temperature increased from 600 to 800 ◦ C, XCH4 , XCO2 , XTC and CH2 +CO increased rapidly from 49%, 10%, 39% and 56% to 91%, 90%, 91% and 95%, respectively; as temperature further increased to 1000 ◦ C, they all increased slightly to 99%. Obviously, XCO2 , XTC and CH2 +CO were improved significantly with the addition of more CH4 . The H2 /CO ratio was almost kept at 1.4–1.5 in the temperature range of 600–1000 ◦ C. As shown in Fig. 5, for the both CH4 :O2 :CO2 molar ratios, H increased rapidly and then turned to increase gradually with temperature. The H value of CH4 :O2 :CO2 = 1:0.3:0.4 was higher than that of CH4 :O2 :CO2 = 1:0.6:0.7 in the temperature range of 600–1000 ◦ C. At CH4 :O2 :CO2 = 1:0.6:0.7 and T < 730 ◦ C, H was negative, which means the overall reaction is exothermic. When T > 730 ◦ C, the reaction turned to become endothermic. For CH4 :O2 :CO2 = 1:0.3:0.4, the overall reaction was endothermic when the temperature over 600 ◦ C. Based upon the energy conservation principle, the lowest energy supplied from the plasma, which is the minimum specific energy

0 nunconv Cp,m (A, g) dT A

(E19)

A

where nB denotes the moles of products (H2 , CO and H2 O) in equilibrium; nconv and nunconv represent the converted and A A unconverted moles of reactants (CH4 , CO2 and O2 ), respec0 (A, g, 298.15 K) and  H 0 (B, g, 298.15 K) represent tively. f Hm f m the standard molar enthalpy change of formation for reactions and 0 (A, g) and C 0 (B, g) are the standard products, respectively. Cp,m p,m molar heat capacity for reactants and products, respectively.

Intensity (a.u.)

H SH

ê ê

b

¿

ê

q q

a 20

30

˜

˜

40

q

¿

q

50

2θ (degree)

60

q

70

80

Fig. 2. XRD patterns for Ni/CeO2 /Al2 O3 catalysts (a) before and (b) after reduction.

Conversion & syngas concentration (%)

2.0

100

1.6

60

40 1.2

H2/CO ratio

80

20

0 600

(b)

99

XO2

XCH4

XCO2

XTC

CH2+CO

H2/CO

700

800

T ( C) o

0.8 1000

900

2.0

100

80 1.6

60

40 1.2 20

0 600

XO2

XCH4

XCO2

XTC

CH2+CO

H2/CO

700

800

T ( C) o

H2/CO ratio

(a)

Conversion & syngas concentration (%)

K. Li et al. / Catalysis Today 256 (2015) 96–101

0.8 1000

900

Fig. 4. Thermodynamic-equilibrium conversions, syngas concentration and H2 /CO ratio versus temperature at (a) CH4 :O2 :CO2 = 1:0.6:0.7 and (b) CH4 :O2 :CO2 = 1:0.3:0.4.

150 Fig. 3. (a) TEM image and (b) Ni particle size distribution of Ni/CeO2 /Al2 O3 catalyst after reduction.

CH4:O2:CO2

Catalyst

SBET (m2 /g)

dNi -XRDa (nm)

dNi -TEMa (nm)

Before reduction After reduction

157 116

– 15.9

– 18.1

a dNi -XRD and dNi -TEM represent Ni particle size obtained from XRD and TEM, respectively.

ΔH (kJ/mol)

100

Table 1 BET surface area and Ni particle size of Ni/CeO2 /Al2 O3 catalyst.

1:0.6:0.7 1:0.3:0.4

50

0

input (SEImin ), equals H. At a given flow rate (F) of the feeding, the minimum power Pmin could derive from E20. Pmin = F · SEImin = F · H

(E20)

For the conversions of reactants and syngas concentration, thermodynamic-equilibrium value of 800 ◦ C are very close to the limit and this temperature was selected in this experiment. Accordingly, SEImin at CH4 :O2 :CO2 molar ratios of 1:0.3:0.4 and 1:0.6:0.7 were 75 kJ/mol and 9 kJ/mol, respectively. The latter SEImin was too low to generate the spark-shade plasma, and therefore the former SEImin was selected. The corresponding input power of the plasma was 106 W with the applied voltage of 3.7 kV (peak value) at F = 1.88 SLM.

-50 600

700

800

T ( C) o

900

1000

Fig. 5. Thermodynamic-equilibrium enthalpy change versus temperature.

As shown in Fig. 6a, in Route 1, XO2 , XCH4 , XCO2 and XTC were 95%, 79%, 36% and 61%, respectively. In Route 2, as Ni/CeO2 /Al2 O3 catalyst was loaded, XO2 , XCH4 and XTC increased to 100%, 99% and 74%, respectively. But XCO2 hardly changed. The thermodynamicequilibrium conversions of O2 , CH4 , CO2 and total-carbon at CH4 :O2 :CO2 = 1:0.6:0.7 and 800 ◦ C were 100%, 100%, 39% and 75%,

100

K. Li et al. / Catalysis Today 256 (2015) 96–101

100

Route 1, EXP

Route 2, TE Route 2, EXP

Route 3, TE Route 3, EXP

(a)

60

40

20

0

XCH4

XCO2

XTC

(b)

(R1)

CH4 + CO2 → 2H2 + 2CO

(R2)

the ratios of (2rO2 + rCO2 )/rCH4 are both equal to 1. As shown in Fig. 6b, (2rO2 + rCO2 )/rCH4 of Route 1, Route 2 and Route 3 were 1.75, 1.44 and 1.15, respectively. The ratio of (2rO2 + rCO2 )/rCH4 in Route 3 is close to 1 (theoretical ratio), which means that methane was almost converted via R1 and R2. As shown in Fig. 7, the products mainly consisted of CO, H2 and H2 O. A small amount of C2 hydrocarbons with a molar ratio of C2 H2 :C2 H4 :C2 H6 ≈ 5:3:1 were formed in Route 1, while there were no C2 hydrocarbons in the presence of catalysts (Route 2 and 3). As a result, SCO increased slightly from 94% of Route 1 to nearly H 100% in Route 2 and 3. For Route 1, SH2 and SH were 55% and 42%, 2O respectively. For Route 2, SH2 increased dramatically to 75% and H consequently decreased to 21%. For Route 3, with the addition SH 2O of more methane, SH2 approached 100%. This demonstrated that CH4 and CO2 were efficiently converted to CO and H2 with ∼100% selectivity in Route 3, which supports again that methane in Route 3 was almost converted via R1 and R2. In this experiment, the carbon and hydrogen balances both approached 100%.

1.5

Route 1

1.0

0.5

Route 1

Route 2

Route 3

Fig. 6. (a) Experimental (EXP) and thermodynamic-equilibrium (TE) conversions and (b) experimental (2rO2 + rCO2 )/rCH4 ratios in three routes.

respectively. The experimental results of Route 2 were close to the thermodynamic-equilibrium values. This suggests that the case of Route 2 is under the thermodynamic control. In Route 3, due to the addition of more methane, the CH4 :O2 :CO2 molar ratio changed from 1:0.6:0.7 to 1:0.3:0.4. Compared with Route 2, XCO2 increased dramatically to 67%, XCH4 decreased to 77% due to the increase of methane concentration, and XTC almost stayed unchanged. XO2 still maintained at 100%. The thermodynamic-equilibrium conversions of O2 , CH4 , CO2 and totalcarbon at CH4 :O2 :CO2 = 1:0.3:0.4 and 800 ◦ C were 100%, 91%, 90% and 91%, respectively. Except for O2 conversion, the experimental results of Route 3 were lower obviously than the thermodynamicequilibrium values. It could be ascribed to the high gas hourly space velocity (GHSV) in Route 3, which makes it kinetically controlled. The total flow rate of effluent gas from the plasma reactor was calculated at about 1.9 SLM according to the analysis results of gas chromatograph. In Route 3, the effluent gas from the plasma, together with 0.52 SLM CH4 of addition, flowed into the catalytic reactor. Its GHSV was as high as 23,000 h−1 . It can be deduced that the conversions of CH4 , CO2 and total-carbon in Route 3 would further increase if the GHSV decreases. For CH4 oxidation, an O2 molecule can contribute two oxygen atoms, but a CO2 molecule only provides one oxygen atom. Therefore, the overall ratios of reactant conversion rate can be expressed

C-based selectivity & balance (%)

(2rO2+rCO2)/rCH4

2.0

XO2

CH4 + 1/2O2 → 2H2 + CO

H-based selectivity & balance (%)

Conversion (%)

80

as (2rO2 + rCO2 )/rCH4 . In partial oxidation of methane (R1) and dry reforming of methane (R2),

100

Route 2

Route 3

(a)

80

60

40

20

0

SCO

SC2 Route 1

100

BC

Route 2

Route 3

(b)

80

60

40

20

0

SH2

SH2O H

SC2 H

BH

Fig. 7. (a) C-based selectivity & balance and (b) H-based selectivity & balance of oxidative CO2 reforming of methane in three routes.

K. Li et al. / Catalysis Today 256 (2015) 96–101

Route 2, TE Route 2, EXP

100

H2/CO ratio

80 1.2 60

40 0.8 20

0.4

CH2+CO

H2/CO

that low energy cost and high energy efficiency can be achieved via Route 3, which provides an efficient route of post-plasma catalysis for syngas production from oxidative CO2 reforming of methane.

Route 3, TE Route 3, EXP

Syngas concentration (vol%)

1.6

Route 1, EXP

0

Route 1

Route 2

Route 3

100

1.2

80

0.9

60

0.6

40

0.3

20

Energy efficiency (%)

Energy cost of syngas (eV/molecule)

Fig. 8. Experimental (EXP) and thermodynamic-equilibrium (TE) H2 /CO ratios and syngas concentrations in three routes.

1.5

101

4. Conclusions Spark-shade plasma (input power = 106 W, with F1 of 1.36 SLM at CH4 :O2 :CO2 = 1:0.6:0.7) followed by Ni/CeO2 /Al2 O3 catalyst (catalyst temperature = 800 ◦ C, with or without F2 of 0.52 SLM CH4 ) was studied for syngas production from oxidative CO2 reforming of methane. Due to the presence of Ni/CeO2 /Al2 O3 catalyst, XCH4 , SH2 , CH2 +CO and H2 /CO ratio increased from 79%, 55%, 57% and 0.9 in Route 1 (plasma only, F1 only) to 99%, 75%, 76% and 1.2 in Route 2 (plasma + catalyst, F1 only), respectively; but XCO2 hardly changed, which could be ascribed to the thermodynamic limit. In Route 3 (plasma + catalyst, F1 + F2), XCO2 increased dramatically to 67% and CH2 +CO and H2 /CO ratio further increased to 86% and 1.5, respectively, though XCH4 decreased to 77%. The (2rO2 + rCO2 )/rCH4 ratio of 1.15 is close to the theoretical ratio of 1. Both SCO and SH2 arrived at nearly 100%. Assuming that the plasma could supply the heat energy for the subsequent catalytic reaction at 800 ◦ C, syngas energy cost as low as 0.5 eV/molecule and energy efficiency as high as 91% were achieved, which provides an efficient route of post-plasma catalysis for syngas production from oxidative CO2 reforming of methane. Acknowledgements This work was supported by International Science & Technology Cooperation Program of China (2013DFG60060) and the Fundamental Research Funds for the Central Universities (DUT14RC(3)012). References

0.0

ECH2+CO

η

0

Fig. 9. Syngas energy cost and energy efficiency in three routes.

As shown in Fig. 8, the H2 /CO ratio and CH2 +CO of Route 1 were 0.9 and 57%, respectively. For Route 2, the H2 /CO ratio and CH2 +CO increased to 1.2 and 76%, respectively. For Route 3, the H2 /CO ratio and CH2 +CO further increased to 1.5 and 86%, respectively. Moreover, the H2 /CO ratios of Route 2 and 3 were very close to the thermodynamic-equilibrium values. The CH2 +CO of Route 2 closely approached its thermodynamic-equilibrium value, but that of Route 3 was lower than the thermodynamic-equilibrium value of 95% because it was under kinetic control. As shown in Fig. 9, for Route 1 (plasma only), syngas energy cost and energy efficiency were 1.2 eV/molecule and 57%, respectively. In Route 2, due to the post-plasma catalysis, syngas energy cost reduced to 0.8 eV/molecule and energy efficiency increased to 70%. In Route 3, owing to the contributions from the post-plasma catalysis and another part of methane added behind the plasma, syngas energy cost further decreased to 0.5 eV/molecule and energy efficiency increased dramatically to 91%. It should be noted that the energy cost and the energy efficiency were calculated by E17 and E18, assuming that the plasma could be supply the heat energy for the subsequent catalytic reaction at 800 ◦ C. It was concluded

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