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Optimized mixed reforming of biogas with O2 addition in spark-discharge plasma
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Optimized mixed reforming of biogas with O2 addition in spark-discharge plasma Bin Zhu, Xiao-Song Li, Jing-Lin Liu, Ai-Min Zhu* Laboratory of Plasma Physical Chemistry, School of Physics and Optoelectronic Engineering & School of Chemistry, Dalian University of Technology, Dalian 116024, China
article info
abstract
Article history:
Adding O2 into biogas to achieve partial oxidation and CO2 mixed reforming can not only
Received 23 April 2012
increase H2 þ CO concentration, but also reduce energy cost for H2 production. In this
Received in revised form
study, optimized mixed reforming of biogas with O2 addition in spark-discharge plasma
21 August 2012
was pursued in combination with thermodynamic-equilibrium calculation. With respect to
Accepted 21 August 2012
mixed reforming of biogas with O2 addition in spark-discharge plasma, combination
Available online 18 September 2012
coefficients of independent reactions were given to quantitatively evaluate the mixed
Keywords:
experimental results, it can be concluded that the optimal O2/(CH4eCO2) ratio for optimized
Mixed reforming of biogas
mixed reforming of biogas in spark-discharge plasma was about 0.7. When total-carbon
Syngas
conversion was relatively high (>75%), H2 þ CO concentration on wet basis was the high-
extent at various O2/(CH4eCO2) ratios. Compared thermodynamic-equilibrium with
Plasma
est and energy cost for H2 production was the lowest at O2/(CH4eCO2) ¼ 0.7, and their
Partial oxidation
experimental results were closest to their thermodynamic-equilibrium values.
Dry reforming
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Biogas typically refers to a gas produced from the anaerobic digestion or fermentation of biodegradable materials (biomass, municipal waste and sewage, etc.). As a renewable, carbonneutral and widely available energy source, biogas is regarded as a promising alternative to fossil fuels [1e4]. Recently, considerable research efforts have been focused on converting biogas into H2 or H2 þ CO (syngas). Due to its typical composition (60% CH4 þ 40% CO2), biogas is generally converted into syngas via dry reforming reaction which is considered as a direct process for the use of biogas [5e12]. Among the reforming methods, the conventional catalytic method has the disadvantages of the high temperature required and the rapid deactivation of catalysts. A promising alternative is the nonthermal plasma, which is characterized by very high electron
temperature but relatively low gas temperature, offering high reactant conversions at low temperatures without catalyst [5,7,8,16]. However, strongly endothermic dry reforming reaction would inevitably result in the high-energy cost. Furthermore, CH4 in biogas could not be completely converted into H2 þ CO only via the dry reforming reaction due to the CH4/CO2 ratio higher than the stoichiometric ratio. To resolve the above two problems, partial oxidation and CO2 mixed reforming of biogas can be taken as an effective approach. With adding moderate O2 or air into biogas, not only the excess CH4 can be converted directly into H2 þ CO via the partial oxidation reaction, but also energy cost can be reduced because the partial oxidation reaction is exothermic. Therefore, researches on the mixed reforming are becoming more and more active [13e22]. However, most of the works reported on the mixed reforming reaction were conducted with the excessive addition of O2 or
* Corresponding author. E-mail address: [email protected] (A.-M. Zhu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.091
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air. Since the partial oxidation reaction is much more favorable on thermodynamics than the dry reforming reaction, the conversion of CO2 would be restrained intensively in excess of O2 or air [18e22]. That is, the mixed reforming studies reported in the literatures are only nominal and not real. Additionally, for the case of adding air instead of O2, not only H2 þ CO concentration in product gas would be reduced significantly due to large amounts of N2 introduced from excessive addition of air, but also NOx by-products could be formed under plasma. Rafiq et al. reported that only 10% of CO2 conversion and 35% of H2 þ CO concentration were obtained for mixed reforming of biogas in a plasma-assisted gliding arc reactor with excess air addition [22]. In this study, optimized mixed reforming biogas with O2 addition to achieve simultaneously high H2 þ CO concentration and low energy cost in spark-discharge plasma was pursued in combination with thermodynamicequilibrium calculation.
2.
Experimental
2.1.
Thermodynamic-equilibrium calculation
The concentrations and moles of reactants (CH4, CO2 and O2) and products (H2, CO, C2H2 and H2O) in thermodynamic equilibrium for mixed reforming of biogas (CH4/CO2 ¼ 3/2) as a function of temperature at various O2/(CH4eCO2) ratios were calculated by the HSC Chemistry software (v 7.0) using Gibbs free energy minimization routine. The calculations were performed at the temperature range of 550e1000 C and 1 bar. Based on the equilibrium moles of reactants and products, reactant conversions, H2/CO ratios and hydrogen-based (Hbased) selectivity of H2O were also obtained. The thermodynamic-equilibrium energy cost for H2 production ðECTE H2 Þ was calculated using (E1) (CO produced was also taken into account because CO can be converted into H2 by Water Gas Shift reaction): ECTE H2
0:0104$DH eV=molecule ¼ nH2 þCO
(E1)
where nH2 þCO is the moles of H2 þ CO in equilibrium; DH ðkJÞ represents the enthalpy change in equilibrium calculated via (E2), DH ¼
X
nB Df H0m ðB; g; 298:15 KÞ
B
X
nconv Df H0m ðA; g; 298:15 KÞ A
2.2.
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Experimental setup and methods
Fig. 1 shows a diagram of the experimental setup. The detailed plasma reactor configuration and experimental procedures were described in detail in our previous papers [5,16,23]. Briefly, a stainless steel tube (as the high-voltage electrode) and a rotary stainless steel disc (as the ground electrode) with a 9-mm gap were placed into a quartz cylinder to compose the plasma reactor. The top and bottom of the quartz cylinder were both sealed with a metal plate cooled with water. A sinewave high voltage with a frequency of 4.8 kHz was applied to the high-voltage electrode. The total input power (Pinput, W) was measured by a wattmeter installed in the primary side of the transformer. All experiments were performed at 1 bar. CH4 (99.99%), CO2 (99.99%) and O2 (99.99%), controlled by mass flow controllers, were introduced into the plasma reactor through the high-voltage electrode at room temperature as the feed gases. Before and after the experiments, N2 (99.99%) was switched to purge the plasma reactor and the pipelines. As internal standard gases, N2 and He mixed with the effluent gas from the reactor before flowing into the cold trap. N2 was used to quantify O2 and carbon-containing components [5,16,23e26] and He was used to quantify H2 [5]. After gaseliquid separation, the composition of gaseous products was analyzed online by two gas chromatographs (GCs). The first GC (Agilent 1790 T) was equipped with a thermal conductivity detector (TCD) and two columns of TDX-01 and 5 A molecular sieve using H2 as the carrier gas. The TDX-01 column (2 mm I.D. 1.5 m length) was used to separate CO, A molecular sieve column (2 mm I.D. 1 m CH4 and CO2; the 5 length) was used to separate O2, N2 and CH4 after CO2 in sample gas removed by alkali asbestos. The second GC (Agilent 6890 N) was equipped with TCD and flame ionization detector (FID) using N2 as the carrier gas. The TCD was used to detect H2 and He with a carbon molecular sieve 601 column (2 mm I.D. 1 m length) and the FID was used to detect CH4, C2H2, C2H4, C2H6, etc. with a porapak-N column (2 mm I.D. 3 m length). As described in our previous papers [5,16], the conversions of CH4 ðXCH4 Þ; CO2 ðXCO2 Þ; O2 ðXO2 Þ, total-carbon ðXTC Þ and the carbon-based (C-based) selectivities of COðSCO Þ, C2 H2 ðSC2 H2 Þ and the carbon balance (BC) can be briefly stated as follows. The total effluent gas flow rate (Fout, SCCM) can be calculated from the flow rate of N2 ðFN2 Þ and its concentration ðCout N2 Þ in the effluent gas,
A
ZT þ 298:15 K
ZT þ 298:15 K
X
nB C0p;m B; g dT
Fout ¼
B
X
nunconv C0p;m A; g dT A
FN2 Cout N2
(E3)
Based on (E3), XCH4 , XCO2 , XO2 , XTC, SCO, SC2 H2 and BC were given in turn:
A
(E2) Here nB denotes the moles of products (H2, CO, C2H2 and H2O) in and nunconv represent the converted and equilibrium; nconv A A unconverted moles of reactants (CH4, CO2 and O2), respectively. Df H0m ðA; g; 298:15 KÞ and Df H0m ðB; g; 298:15 KÞ represent the standard molar enthalpy change of formation for reactants and products, respectively. C0p;m ðA; gÞ and C0p;m ðB; gÞ are the standard molar heat capacity for reactants and products, respectively.
XCH4 ¼ 1
XCO2 ¼ 1
Fout $Cout CH4 Fin CH4
Fout $Cout CO2 Fin CO2
¼1
FN2 Cout CH $ out4 CN2 Fin CH4
(E4)
¼1
FN2 Cout CO $ out2 Fin CO2 CN2
(E5)
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Fig. 1 e Schematic diagram of the experimental setup.
XO2 ¼ 1
XTC ¼
SCO
Fout $Cout O2 Fin O2
¼1
FN2 Cout O2 $ out Fin O2 CN2
in Fin CH4 $XCH4 þ FCO2 $XCO2 in Fin CH4 þ FCO2
Fout $Cout FN2 Cout CO ¼ in ¼ in $ CO in Cout FCH4 $XCH4 þ Fin $X F $X þ F $X CO CH CO N2 CO2 CH4 CO2 4 2 2
SC2 H2 ¼
2$Fout $Cout C2 H2 Fin CH4 $XCH4
BC ¼ SCO þ SC2 H2
þ Fin CO2 $XCO2
¼
Cout 2$FN2 C2 H 2 $ out in þ FCO2 $XCO2 CN2
Fin CH4 $XCH4
0:5$Fout $Cout C2 H 2
SH C2 H2 ¼
(E7)
out For SH H2 O , the flow rate of H2O produced ðFH2 O Þ should be calculated via (E12) because it was difficult to determine directly.
Fin CH4 $XCH4
¼
0:5$FN2 Cout C H $ 2 2 in FCH4 $XCH4 Cout N2
(E6)
Cout in in FN2 $ CO Fout H2 O ¼ 2 FCO2 $XCO2 þ FO2 $XO2 Cout N2 (E8)
(E9)
(E10)
in in where Fin CH4 , FCO2 and FO2 denote the flow rates of CH4, CO2 and out out out out , O2, respectively; Cout CH4 CCO2 , CO2 , CCO and CC2 H2 represent the concentrations of CH4, CO2, O2, CO and C2H2 in the effluent gas, respectively. In this experiment, the concentrations of and C2H6 ðCout are extremely low C2H4 ðCout C2 H4 Þ C2 H6 Þ out out out ðCC2 H4 =CC2 H2 < 0:1 and Cout C2 H6 =CC2 H2 < 0:05Þ, therefore C2H4 and C2H6 are negligible. Additionally, the H-based selectivities of C2H2 ðSH C2 H2 Þ, H2O Þ and the hydrogen balance (B ) were also calculated in ðSH H H2 O was calculated using (E11): this work. SH C 2 H2
(E11)
(E12)
In (E12), it was assumed that the O atoms in converted CO2 and O2 was equal to those distributed in CO and H2O produced, which was reasonable because no oxygencontaining organic compounds were detected and BC in this work is close to 100%. Therefore, SH H2 O was calculated by (E13): SH H2 O ¼
0:5$Fout H2 O Fin CH4 $XCH4
(E13)
The selectivity of H2 ðSH2 Þ and hydrogen balance (BH) were calculated from (E14) and (E15), respectively. SH2 ¼
0:5$FHe Cout H2 $ out CHe Fin CH4 XCH4
H BH ¼ SH2 þ SH C2 H2 þ SH2 O
(E14)
(E15)
where FHe is the flow rate of the internal standard He gas; Cout H2 and Cout He are the concentrations of H2 and He in the effluent gas, respectively.
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The energy cost for H2 production ðECEXP H2 Þ was calculated via (E16): !
13:95$Pinput out Fout Cout H2 þ CCO
ECEXP H2
eV=molecule
3.
Results and discussion
¼
(E16)
3.1. Mixed reforming of biogas at various O2/(CH4eCO2) ratios in thermodynamic equilibrium Fig. 2 illustrated the dependence of thermodynamicequilibrium conversions of CH4, CO2 and total-carbon, concentrations of H2 þ CO and H2O and H2/CO ratio on temperature for mixed reforming of biogas (CH4/CO2 ¼ 3/2) at various O2/(CH4eCO2) ratios. According to the mixed reforming reaction of biogas (R1), the stoichiometric ratio of
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CH4/CO2/O2 is 3/2/0.5, thus the corresponding ratio of O2/ (CH4eCO2) is 0.5. 3CH4 þ 2CO2 þ 0:5O2 /5CO þ 6H2
(R1)
As shown in Fig. 2a and b, with increasing O2/(CH4eCO2) ratio from 0 to 1, the thermodynamic-equilibrium conversion of CH4 increases but CO2 conversion decreases. At 800 C and O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, XCH4 are 66%, 91%, 95% and 98%, while XCO2 are equal to 99%, 93%, 87% and 75%, respectively. Accordingly, XTC presented a non-monotonic variation with the increase in O2/(CH4eCO2) ratio, as shown in Fig. 2c. It should be noted that the maximum XTC can be achieved at O2/ (CH4eCO2) ¼ 0.5e0.7 in the temperature range of 750e850 C. For example, at 800 C and O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, XTC are 79%, 92%, 92% and 89%, respectively. The equilibrium concentrations of H2 þ CO on wet basis were almost the same at the four ratios of O2/(CH4eCO2) under temperature below 700 C (Fig. 2d). When temperature exceeded 700 C, however, H2 þ CO concentration firstly
Fig. 2 e Thermodynamic-equilibrium conversions of CH4 (a), CO2 (b) and total-carbon (c), thermodynamic-equilibrium concentrations of H2DCO and H2O (d) and H2/CO ratio (e) versus temperature for mixed reforming of biogas (CH4/CO2 [ 3/2) at various O2/(CH4eCO2) ratios (designated as r).
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increased to the maximum at O2/(CH4eCO2) ¼ 0.5 and then turned to decrease with the increase in O2/(CH4eCO2) ratio. The equilibrium concentration of H2O increased monotonically with increasing O2/(CH4eCO2) ratio from 0 to 1. As it can be seen clearly from Fig. 2d, at 800 C and O2/ (CH4eCO2) ¼ 0, 0.5, 0.7 and 1, CH2 þCO are 88%, 95%, 94% and 90%, respectively; while CH2 O are 0.2%, 1%, 3% and 5%, respectively. The thermodynamic-equilibrium results in Fig. 2e indicate that, when O2/(CH4eCO2) ¼ 0, H2/CO ratio increased gradually with increasing temperature from 550 C to 750 C and then remained around 1 when temperature exceeded 750 C. The reason is that, at temperatures lower than 750 C, the dry reforming reaction (R2) could take place along with the reverse reaction of Water Gas Shift (R3), which would result in H2/CO < 1. CO2 þ CH4 /2CO þ 2H2
(R2)
CO2 þ H2 /H2 O þ CO
(R3)
However, (R3) turned to be negligible in the overall reactions due to almost no H2O formed in the products (Fig. 2d) when temperature exceeded 750 C, which consequentially resulted in H2/CO ¼ 1. For the cases of O2/(CH4eCO2) ¼ 0.5, 0.7 and 1, H2/ CO ratios were always greater than 1. This can be explained by the occurrence of the partial oxidation reaction (R4) which is thermodynamically favorable. CH4 þ 1=2O2 /CO þ 2H2
(R4)
Additionally, the thermodynamic-equilibrium results demonstrated that C2H2 could be formed only in excess CH4. For the cases of O2/(CH4eCO2) ¼ 0.5, 0.7 and 1, no C2H2 was formed because CH4 was not in excess. For the case of O2/ (CH4eCO2) ¼ 0, C2H2 began to be formed via pyrolysis of the residual CH4 when the temperature reached above 1000 C. Based on the data of Fig. 2, the thermodynamicequilibrium energy costs for H2 production were calculated using (E1) and (E2). Fig. 3 illustrated the thermodynamicequilibrium energy costs for H2 production as a function of
temperature at various O2/(CH4eCO2) ratios. With increasing O2/(CH4eCO2) ratios at the same temperature, the energy costs for H2 production decreased gradually. As depicted in Fig. 3, at 800 C and O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, ECTE H2 are 0.95, 0.68, 0.60 and 0.48 eV/molecule, respectively. In addition, when O2/(CH4eCO2) ¼ 0, the energy costs for H2 production firstly decreased with increasing temperature from 550 C to 750 C but then turned to increase slightly when temperatures higher than 750 C. The reason is that, H2O concentration decreased gradually with temperature (Fig. 2d) and the endothermic reaction of (R3) became more and more unfavorable compared with (R2). This results in the energy consumed in (R3) became less and less. When temperature exceeded 750 C, (R3) could be negligible and the slight rise of ECH2 should be attributed to the energy consumed in heating gas. For the cases of O2/(CH4eCO2) ¼ 0.5, 0.7 and 1, with increasing temperature, the energy costs for H2 production firstly increased significantly and then rose slowly. This variation of the energy costs for H2 production versus temperature depends on the variation of H2 þ CO and H2O concentrations (Fig. 2d) versus temperature.
3.2. Mixed reforming of biogas at various O2/(CH4eCO2) ratios in the spark-discharge plasma 3.2.1. Reactant conversions, selectivities and mass balance, H2 þ CO concentrations and H2/CO ratios At various O2/(CH4eCO2) ratios and input powers, reactant conversions, C-based selectivities and balance, H-based selectivities and balance, H2 þ CO concentrations on wet basis and H2/CO ratios for mixed reforming of biogas (FCH4 þCO2 ¼ 150 SCCM, CH4/CO2 ¼ 3/2) in the spark-discharge plasma were listed in Table 1. As shown in Table 1, with increasing O2/(CH4eCO2) ratio from 0 to 1 at Pinput ¼ 56 W, XCH4 and XTC increased from 72% and 68% to 79% and 72%, respectively; but XCO2 changed very little. Furthermore, SCO and SH H2 O increased from 68% and 4% to 89% and 20%, respectively; SH2 , SC2 H2 and H2/CO ratio decreased from 81%, 30% and 1.5 to 73%, 9% and 1.1, respectively. In addition, CH2 þCO increased from 76% to 78% with increasing O2/ (CH4eCO2) ratio from 0 to 0.7 at Pinput ¼ 64 W; however, as the O2/(CH4eCO2) ratio further increased to 1, CH2 þCO reduced to 74%. All the data of carbon and hydrogen balances reached 95%e98% in this experiment.
3.2.2.
Combination coefficients of independent reactions
Based on the results listed in Table 1, the overall reaction for mixed reforming of biogas in the spark-discharge plasma can be expressed as (R5): CH4 þ vCO2 CO2 þ vO2 O2 /vCO CO þ vH2 H2 þ vC2 H2 C2 H2 þ vH2 O H2 O (R5)
Fig. 3 e Thermodynamic-equilibrium energy cost for H2 production versus temperature at various O2/(CH4eCO2) ratios (designated as r).
where vCO2 , vO2 , vCO , vH2 , vC2 H2 and vH2 O denote the stoichiometric coefficient of CO2, O2, CO, H2, C2H2 and H2O, respectively; the stoichiometric coefficient of each substance was obtained from that it converted or formed was divided by CH4 converted. Since three reactants (CH4, CO2 and O2) and four products (CO, H2, C2H2 and H2O) included in the overall
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Table 1 e Reactant conversions, C-based selectivities and balance, H-based selectivities and balance, H2 D CO concentration and H2/CO ratio for mixed reforming of biogas ðFCH4 DCO2 [150 SCCM; CH4 =CO2 [3=2Þ at various O2/(CH4eCO2) ratios in the spark-discharge plasma. O2/(CH4eCO2)
0
0.5
0.7
1
Pinput (W)
40
48
56
64
40
48
56
64
40
48
56
64
48
56
64
72
Conversions XCH4 (%) XCO2 (%) XTC (%) XO2 (%)
65 52 60 /
68 57 64 /
72 63 68 /
76 67 72 /
68 52 62 79
72 58 66 80
76 63 71 83
80 67 75 85
69 52 62 80
74 58 68 80
77 63 72 83
80 67 75 85
76 57 68 83
79 62 72 85
81 65 74 87
83 68 77 89
C-based selectivity & balance SCO (%) 61 65 35 32 SC2 H2 (%) 96 97 BC (%)
68 30 98
71 27 98
76 20 96
79 18 97
80 16 96
83 15 98
80 17 97
82 15 97
85 13 98
89 10 99
87 11 98
89 9 98
90 8 98
92 6 98
H-based selectivity SH2 (%) SH C2 H2 (%) SH H2 O (%) BH (%)
81 12 4 97
82 11 3 96
74 8 14 96
77 7 14 98
78 6 12 96
80 6 10 96
72 6 18 96
74 6 17 97
77 5 15 97
79 4 12 95
71 4 22 97
73 4 20 97
75 3 20 98
76 2 20 98
66 1.3
69 1.3
74 1.3
77 1.2
68 1.2
69 1.2
74 1.2
78 1.1
69 1.1
73 1.1
74 1.1
77 1.1
& balance 77 78 13 13 6 6 96 97
H2 þ CO concentration on wet basis & H2/CO ratio 64 67 72 76 CH2 þCO (%) 1.7 1.5 1.5 1.4 H2/CO
reaction (R5), four independent reactions (R2), (R3), (R4) and (R6) can be determined by the Gibbs’ rule of stoichiometry: CO2 þ CH4 /2CO þ 2H2
(R2)
CO2 þ H2 /H2 O þ CO
(R3)
CH4 þ 1=2O2 /CO þ 2H2
(R4)
CH4 /1=2C2 H2 þ 3=2H2
(R6)
Thereby, the overall reaction (R5) can be expressed as the linear combination of independent reactions (R2), (R3), (R4) and (R6): R5 ¼ aR2 þ bR3 þ gR4 þ dR6
(E17)
In Eq. (E17), a, b, g and d are the combination coefficients of independent reactions (R2), (R3), (R4) and (R6), respectively. Accordingly, the combination coefficients a, b, g and d were calculated by Eqs. (E18)e(E21), respectively: a¼1gd
(E18)
b ¼ vH2 O
(E19)
g ¼ 2$vO2
(E20)
d ¼ 2$vC2 H2
(E21)
The combination coefficients a, b, g and d as functions of CH4 conversion at various O2/(CH4eCO2) ratios were illustrated in Fig. 4. As depicted in Fig. 4, at O2/(CH4eCO2) ¼ 0, g ¼ 0. Increasing XCH4 from 65% to 76%, combination coefficient a increased from 0.42 to 0.54, whereas b and d decreased from 0.11 and 0.58 to 0.06 and 0.46, respectively. At O2/(CH4eCO2) ¼ 0.5, increasing XCH4 from 68% to 80%, a increased from 0.21 to
0.34, but b, g and d decreased from 0.30, 0.45 and 0.34 to 0.22, 0.40 and 0.26, respectively. At O2/(CH4eCO2) ¼ 0.7, with increasing XCH4 from 69% to 80%, a increased from 0.14 to 0.30, while b, g and d decreased from 0.36, 0.56 and 0.30 to 0.26, 0.51 and 0.19, respectively. At O2/(CH4eCO2) ¼ 1, increasing XCH4 from 76% to 83%, a increased from 0.07 to 0.18, but b, g and d decreased from 0.43, 0.75 and 0.18 to 0.37, 0.70 and 0.12, respectively. It is worthy to be mentioned that, with the increase in O2/ (CH4eCO2) ratio at the same XCH4 , b and g increased gradually, but a and d decreased. For example, at XCH4 ¼ 75%, when O2/ (CH4eCO2) ratio increased from 0 to 1, b and g increased from 0.06 and 0 to 0.43 and 0.75, respectively; a and d decreased from 0.54 and 0.46 to 0.07 and 0.18, respectively. This means that independent reaction (R4) increasingly predominates over independent reactions (R2) and (R6) with increasing O2/ (CH4eCO2) ratio.
3.3. Comparisons of thermodynamic-equilibrium and experimental results at various O2/(CH4eCO2) ratios The thermodynamic-equilibrium and experimental H-based selectivities of H2O, H2 þ CO concentrations on wet basis and energy costs for H2 production as functions of total-carbon conversion at various O2/(CH4eCO2) ratios were illustrated in Fig. 5. Fig. 5a indicated that both thermodynamic-equilibrium and experimental H-based selectivities of H2O decreased linearly with total-carbon conversion. For the equilibrium line of H2O selectivity, the absolute value of its slope presented a slight decrease with the increase in O2/(CH4eCO2) ratio. Increasing XTC from 55% to 80% at O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, SH H2 O decreased from 8.1%, 13.2%, 15% and 17.6% to 0.1%, 5.8%, 7.9% and 11.1%, respectively. It is worth noting that the experimental value of H2O selectivity was very close to its
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Fig. 4 e Combination coefficients of independent reaction (a) a, (b) b, (c) g and (d) d versus CH4 conversion at various O2/ (CH4eCO2) ratios (designated as r). equilibrium one at O2/(CH4eCO2) ¼ 0, whereas the deviation of experimental results from equilibrium values became more and more significant with the increase in O2/(CH4eCO2) ratio. As shown in Fig. 5b, both thermodynamic-equilibrium and
experimental H2 þ CO concentrations (on wet basis) increased linearly with total-carbon conversion. For the equilibrium line of H2 þ CO concentration, its slope decreased gradually with the increase in O2/(CH4eCO2) ratio. When XTC increased from
Fig. 5 e Thermodynamic-equilibrium (TE) and experimental (EXP) (a) H-based selectivities of H2O, (b) H2 D CO concentrations on wet basis and (c) energy costs for H2 production versus total-carbon conversion at various O2/(CH4eCO2) ratios (designated as r).
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55% to 80% at O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, CH2 þCO increased from 67.2%, 67.5%, 67.3% and 67.2% to 88.8%, 87.1%, 85.9% and 84.7%, respectively. As far as the lines of experimental H2þCO concentration versus total-carbon conversion is concerned, the slopes at O2/(CH4eCO2) ¼ 0.7 and 1 were the largest and smallest, respectively. This indicated that, when total-carbon conversion was relatively high, H2 þ CO concentration on wet basis was the highest at O2/(CH4eCO2) ¼ 0.7 (CH2 þCO ¼ 78% at XTC ¼ 75%), and its experimental results were closest to their thermodynamic-equilibrium values. As depicted in Fig. 5c, with increasing XTC from 55% to 80% at O2/(CH4eCO2) ¼ 0.5, 0.7 and 1, the thermodynamicequilibrium energy costs for H2 production increased slowly from 0.60, 0.46 and 0.27 eV/molecule to 0.65, 0.56 and 0.43 eV/ molecule, respectively. However, when O2/(CH4eCO2) ¼ 0, ECTE H2 firstly decreased gradually to the minimum (0.94 eV/ molecule) with increasing XTC from 55% to 76% and then turned to increase with the further increase in XTC, which is in accordance with the result in Fig. 3. Fig. 5c also illustrated that the thermodynamic-equilibrium energy costs for H2 production exhibited a descending trend with the increase of O2/ (CH4eCO2) ratio at the same total-carbon conversion. In addition, the slope of experimental energy costs for H2 production versus total-carbon conversion at O2/(CH4eCO2) ¼ 0.7 was the lowest. Therefore, when total-carbon conversion was relatively high (>75%), not only the highest experimental H2 þ CO concentration but also the lowest experimental energy cost for H2 production can be achieved at O2/ (CH4eCO2) ¼ 0.7. That is, the optimal O2/(CH4eCO2) ratio for optimized mixed reforming of biogas in spark-discharge plasma was about 0.7. In Fig. 5c, the slope of experimental curve of energy cost is greater than that of thermodynamicequilibrium curve, which may be ascribed that the energy efficiency of the plasma decreased with total-carbon conversion [23]. The experimental energy cost is much higher than the thermodynamic-equilibrium one, which means that the energy efficiency of the plasma is still required to be further improved.
4.
Conclusions
The thermodynamic-equilibrium calculation results for mixed reforming of biogas with O2 addition showed that, the maximum total-carbon conversion and H2 þ CO concentration on wet basis were achieved at O2/(CH4eCO2) ¼ 0.5e0.7 in the temperature range of 750e850 C. The thermodynamicequilibrium energy costs for H2 production decreased gradually with the increase in O2/(CH4eCO2) ratio. At 800 C and O2/ (CH4eCO2) ¼ 0, 0.5, 0.7 and 1, ECTE H2 are 0.95, 0.68, 0.60 and 0.48 eV/molecule, respectively. The experimental results of spark-discharge plasma for mixed reforming of biogas with O2 addition showed that, with increasing O2/(CH4eCO2) ratio from 0 to 1, both CH4 and totalcarbon conversions increased, but CO2 conversion changed very little. All the data of carbon and hydrogen balances reached around 95%e98% in this experiment. With increasing O2/(CH4eCO2) ratio at the same CH4 conversion, the combination coefficients of CO2 reforming reaction (a) and partial oxidation reaction (g) decreased and
16923
increased, respectively. When O2/(CH4eCO2) ratio increasing from 0 to 1 at 75% of CH4 conversion , a decreased from 0.54 to 0.07 and g increased from 0 to 0.75. Compared thermodynamic-equilibrium with experimental results, it can be concluded that the optimal O2/(CH4eCO2) ratio for optimized mixed reforming of biogas in sparkdischarge plasma was about 0.7. When total-carbon conversion was relatively high (>75%), H2 þ CO concentration on wet basis was the highest and energy cost for H2 production was the lowest at O2/(CH4eCO2) ¼ 0.7, and their experimental results were closest to their thermodynamic-equilibrium values.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11079013) and the Fundamental Research Funds for the Central Universities.
references
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Available online at www.sciencedirect.com
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Optimized mixed reforming of biogas with O2 addition in spark-discharge plasma Bin Zhu, Xiao-Song Li, Jing-Lin Liu, Ai-Min Zhu* Laboratory of Plasma Physical Chemistry, School of Physics and Optoelectronic Engineering & School of Chemistry, Dalian University of Technology, Dalian 116024, China
article info
abstract
Article history:
Adding O2 into biogas to achieve partial oxidation and CO2 mixed reforming can not only
Received 23 April 2012
increase H2 þ CO concentration, but also reduce energy cost for H2 production. In this
Received in revised form
study, optimized mixed reforming of biogas with O2 addition in spark-discharge plasma
21 August 2012
was pursued in combination with thermodynamic-equilibrium calculation. With respect to
Accepted 21 August 2012
mixed reforming of biogas with O2 addition in spark-discharge plasma, combination
Available online 18 September 2012
coefficients of independent reactions were given to quantitatively evaluate the mixed
Keywords:
experimental results, it can be concluded that the optimal O2/(CH4eCO2) ratio for optimized
Mixed reforming of biogas
mixed reforming of biogas in spark-discharge plasma was about 0.7. When total-carbon
Syngas
conversion was relatively high (>75%), H2 þ CO concentration on wet basis was the high-
extent at various O2/(CH4eCO2) ratios. Compared thermodynamic-equilibrium with
Plasma
est and energy cost for H2 production was the lowest at O2/(CH4eCO2) ¼ 0.7, and their
Partial oxidation
experimental results were closest to their thermodynamic-equilibrium values.
Dry reforming
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Biogas typically refers to a gas produced from the anaerobic digestion or fermentation of biodegradable materials (biomass, municipal waste and sewage, etc.). As a renewable, carbonneutral and widely available energy source, biogas is regarded as a promising alternative to fossil fuels [1e4]. Recently, considerable research efforts have been focused on converting biogas into H2 or H2 þ CO (syngas). Due to its typical composition (60% CH4 þ 40% CO2), biogas is generally converted into syngas via dry reforming reaction which is considered as a direct process for the use of biogas [5e12]. Among the reforming methods, the conventional catalytic method has the disadvantages of the high temperature required and the rapid deactivation of catalysts. A promising alternative is the nonthermal plasma, which is characterized by very high electron
temperature but relatively low gas temperature, offering high reactant conversions at low temperatures without catalyst [5,7,8,16]. However, strongly endothermic dry reforming reaction would inevitably result in the high-energy cost. Furthermore, CH4 in biogas could not be completely converted into H2 þ CO only via the dry reforming reaction due to the CH4/CO2 ratio higher than the stoichiometric ratio. To resolve the above two problems, partial oxidation and CO2 mixed reforming of biogas can be taken as an effective approach. With adding moderate O2 or air into biogas, not only the excess CH4 can be converted directly into H2 þ CO via the partial oxidation reaction, but also energy cost can be reduced because the partial oxidation reaction is exothermic. Therefore, researches on the mixed reforming are becoming more and more active [13e22]. However, most of the works reported on the mixed reforming reaction were conducted with the excessive addition of O2 or
* Corresponding author. E-mail address: [email protected] (A.-M. Zhu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.08.091
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air. Since the partial oxidation reaction is much more favorable on thermodynamics than the dry reforming reaction, the conversion of CO2 would be restrained intensively in excess of O2 or air [18e22]. That is, the mixed reforming studies reported in the literatures are only nominal and not real. Additionally, for the case of adding air instead of O2, not only H2 þ CO concentration in product gas would be reduced significantly due to large amounts of N2 introduced from excessive addition of air, but also NOx by-products could be formed under plasma. Rafiq et al. reported that only 10% of CO2 conversion and 35% of H2 þ CO concentration were obtained for mixed reforming of biogas in a plasma-assisted gliding arc reactor with excess air addition [22]. In this study, optimized mixed reforming biogas with O2 addition to achieve simultaneously high H2 þ CO concentration and low energy cost in spark-discharge plasma was pursued in combination with thermodynamicequilibrium calculation.
2.
Experimental
2.1.
Thermodynamic-equilibrium calculation
The concentrations and moles of reactants (CH4, CO2 and O2) and products (H2, CO, C2H2 and H2O) in thermodynamic equilibrium for mixed reforming of biogas (CH4/CO2 ¼ 3/2) as a function of temperature at various O2/(CH4eCO2) ratios were calculated by the HSC Chemistry software (v 7.0) using Gibbs free energy minimization routine. The calculations were performed at the temperature range of 550e1000 C and 1 bar. Based on the equilibrium moles of reactants and products, reactant conversions, H2/CO ratios and hydrogen-based (Hbased) selectivity of H2O were also obtained. The thermodynamic-equilibrium energy cost for H2 production ðECTE H2 Þ was calculated using (E1) (CO produced was also taken into account because CO can be converted into H2 by Water Gas Shift reaction): ECTE H2
0:0104$DH eV=molecule ¼ nH2 þCO
(E1)
where nH2 þCO is the moles of H2 þ CO in equilibrium; DH ðkJÞ represents the enthalpy change in equilibrium calculated via (E2), DH ¼
X
nB Df H0m ðB; g; 298:15 KÞ
B
X
nconv Df H0m ðA; g; 298:15 KÞ A
2.2.
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Experimental setup and methods
Fig. 1 shows a diagram of the experimental setup. The detailed plasma reactor configuration and experimental procedures were described in detail in our previous papers [5,16,23]. Briefly, a stainless steel tube (as the high-voltage electrode) and a rotary stainless steel disc (as the ground electrode) with a 9-mm gap were placed into a quartz cylinder to compose the plasma reactor. The top and bottom of the quartz cylinder were both sealed with a metal plate cooled with water. A sinewave high voltage with a frequency of 4.8 kHz was applied to the high-voltage electrode. The total input power (Pinput, W) was measured by a wattmeter installed in the primary side of the transformer. All experiments were performed at 1 bar. CH4 (99.99%), CO2 (99.99%) and O2 (99.99%), controlled by mass flow controllers, were introduced into the plasma reactor through the high-voltage electrode at room temperature as the feed gases. Before and after the experiments, N2 (99.99%) was switched to purge the plasma reactor and the pipelines. As internal standard gases, N2 and He mixed with the effluent gas from the reactor before flowing into the cold trap. N2 was used to quantify O2 and carbon-containing components [5,16,23e26] and He was used to quantify H2 [5]. After gaseliquid separation, the composition of gaseous products was analyzed online by two gas chromatographs (GCs). The first GC (Agilent 1790 T) was equipped with a thermal conductivity detector (TCD) and two columns of TDX-01 and 5 A molecular sieve using H2 as the carrier gas. The TDX-01 column (2 mm I.D. 1.5 m length) was used to separate CO, A molecular sieve column (2 mm I.D. 1 m CH4 and CO2; the 5 length) was used to separate O2, N2 and CH4 after CO2 in sample gas removed by alkali asbestos. The second GC (Agilent 6890 N) was equipped with TCD and flame ionization detector (FID) using N2 as the carrier gas. The TCD was used to detect H2 and He with a carbon molecular sieve 601 column (2 mm I.D. 1 m length) and the FID was used to detect CH4, C2H2, C2H4, C2H6, etc. with a porapak-N column (2 mm I.D. 3 m length). As described in our previous papers [5,16], the conversions of CH4 ðXCH4 Þ; CO2 ðXCO2 Þ; O2 ðXO2 Þ, total-carbon ðXTC Þ and the carbon-based (C-based) selectivities of COðSCO Þ, C2 H2 ðSC2 H2 Þ and the carbon balance (BC) can be briefly stated as follows. The total effluent gas flow rate (Fout, SCCM) can be calculated from the flow rate of N2 ðFN2 Þ and its concentration ðCout N2 Þ in the effluent gas,
A
ZT þ 298:15 K
ZT þ 298:15 K
X
nB C0p;m B; g dT
Fout ¼
B
X
nunconv C0p;m A; g dT A
FN2 Cout N2
(E3)
Based on (E3), XCH4 , XCO2 , XO2 , XTC, SCO, SC2 H2 and BC were given in turn:
A
(E2) Here nB denotes the moles of products (H2, CO, C2H2 and H2O) in and nunconv represent the converted and equilibrium; nconv A A unconverted moles of reactants (CH4, CO2 and O2), respectively. Df H0m ðA; g; 298:15 KÞ and Df H0m ðB; g; 298:15 KÞ represent the standard molar enthalpy change of formation for reactants and products, respectively. C0p;m ðA; gÞ and C0p;m ðB; gÞ are the standard molar heat capacity for reactants and products, respectively.
XCH4 ¼ 1
XCO2 ¼ 1
Fout $Cout CH4 Fin CH4
Fout $Cout CO2 Fin CO2
¼1
FN2 Cout CH $ out4 CN2 Fin CH4
(E4)
¼1
FN2 Cout CO $ out2 Fin CO2 CN2
(E5)
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Fig. 1 e Schematic diagram of the experimental setup.
XO2 ¼ 1
XTC ¼
SCO
Fout $Cout O2 Fin O2
¼1
FN2 Cout O2 $ out Fin O2 CN2
in Fin CH4 $XCH4 þ FCO2 $XCO2 in Fin CH4 þ FCO2
Fout $Cout FN2 Cout CO ¼ in ¼ in $ CO in Cout FCH4 $XCH4 þ Fin $X F $X þ F $X CO CH CO N2 CO2 CH4 CO2 4 2 2
SC2 H2 ¼
2$Fout $Cout C2 H2 Fin CH4 $XCH4
BC ¼ SCO þ SC2 H2
þ Fin CO2 $XCO2
¼
Cout 2$FN2 C2 H 2 $ out in þ FCO2 $XCO2 CN2
Fin CH4 $XCH4
0:5$Fout $Cout C2 H 2
SH C2 H2 ¼
(E7)
out For SH H2 O , the flow rate of H2O produced ðFH2 O Þ should be calculated via (E12) because it was difficult to determine directly.
Fin CH4 $XCH4
¼
0:5$FN2 Cout C H $ 2 2 in FCH4 $XCH4 Cout N2
(E6)
Cout in in FN2 $ CO Fout H2 O ¼ 2 FCO2 $XCO2 þ FO2 $XO2 Cout N2 (E8)
(E9)
(E10)
in in where Fin CH4 , FCO2 and FO2 denote the flow rates of CH4, CO2 and out out out out , O2, respectively; Cout CH4 CCO2 , CO2 , CCO and CC2 H2 represent the concentrations of CH4, CO2, O2, CO and C2H2 in the effluent gas, respectively. In this experiment, the concentrations of and C2H6 ðCout are extremely low C2H4 ðCout C2 H4 Þ C2 H6 Þ out out out ðCC2 H4 =CC2 H2 < 0:1 and Cout C2 H6 =CC2 H2 < 0:05Þ, therefore C2H4 and C2H6 are negligible. Additionally, the H-based selectivities of C2H2 ðSH C2 H2 Þ, H2O Þ and the hydrogen balance (B ) were also calculated in ðSH H H2 O was calculated using (E11): this work. SH C 2 H2
(E11)
(E12)
In (E12), it was assumed that the O atoms in converted CO2 and O2 was equal to those distributed in CO and H2O produced, which was reasonable because no oxygencontaining organic compounds were detected and BC in this work is close to 100%. Therefore, SH H2 O was calculated by (E13): SH H2 O ¼
0:5$Fout H2 O Fin CH4 $XCH4
(E13)
The selectivity of H2 ðSH2 Þ and hydrogen balance (BH) were calculated from (E14) and (E15), respectively. SH2 ¼
0:5$FHe Cout H2 $ out CHe Fin CH4 XCH4
H BH ¼ SH2 þ SH C2 H2 þ SH2 O
(E14)
(E15)
where FHe is the flow rate of the internal standard He gas; Cout H2 and Cout He are the concentrations of H2 and He in the effluent gas, respectively.
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The energy cost for H2 production ðECEXP H2 Þ was calculated via (E16): !
13:95$Pinput out Fout Cout H2 þ CCO
ECEXP H2
eV=molecule
3.
Results and discussion
¼
(E16)
3.1. Mixed reforming of biogas at various O2/(CH4eCO2) ratios in thermodynamic equilibrium Fig. 2 illustrated the dependence of thermodynamicequilibrium conversions of CH4, CO2 and total-carbon, concentrations of H2 þ CO and H2O and H2/CO ratio on temperature for mixed reforming of biogas (CH4/CO2 ¼ 3/2) at various O2/(CH4eCO2) ratios. According to the mixed reforming reaction of biogas (R1), the stoichiometric ratio of
16919
CH4/CO2/O2 is 3/2/0.5, thus the corresponding ratio of O2/ (CH4eCO2) is 0.5. 3CH4 þ 2CO2 þ 0:5O2 /5CO þ 6H2
(R1)
As shown in Fig. 2a and b, with increasing O2/(CH4eCO2) ratio from 0 to 1, the thermodynamic-equilibrium conversion of CH4 increases but CO2 conversion decreases. At 800 C and O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, XCH4 are 66%, 91%, 95% and 98%, while XCO2 are equal to 99%, 93%, 87% and 75%, respectively. Accordingly, XTC presented a non-monotonic variation with the increase in O2/(CH4eCO2) ratio, as shown in Fig. 2c. It should be noted that the maximum XTC can be achieved at O2/ (CH4eCO2) ¼ 0.5e0.7 in the temperature range of 750e850 C. For example, at 800 C and O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, XTC are 79%, 92%, 92% and 89%, respectively. The equilibrium concentrations of H2 þ CO on wet basis were almost the same at the four ratios of O2/(CH4eCO2) under temperature below 700 C (Fig. 2d). When temperature exceeded 700 C, however, H2 þ CO concentration firstly
Fig. 2 e Thermodynamic-equilibrium conversions of CH4 (a), CO2 (b) and total-carbon (c), thermodynamic-equilibrium concentrations of H2DCO and H2O (d) and H2/CO ratio (e) versus temperature for mixed reforming of biogas (CH4/CO2 [ 3/2) at various O2/(CH4eCO2) ratios (designated as r).
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increased to the maximum at O2/(CH4eCO2) ¼ 0.5 and then turned to decrease with the increase in O2/(CH4eCO2) ratio. The equilibrium concentration of H2O increased monotonically with increasing O2/(CH4eCO2) ratio from 0 to 1. As it can be seen clearly from Fig. 2d, at 800 C and O2/ (CH4eCO2) ¼ 0, 0.5, 0.7 and 1, CH2 þCO are 88%, 95%, 94% and 90%, respectively; while CH2 O are 0.2%, 1%, 3% and 5%, respectively. The thermodynamic-equilibrium results in Fig. 2e indicate that, when O2/(CH4eCO2) ¼ 0, H2/CO ratio increased gradually with increasing temperature from 550 C to 750 C and then remained around 1 when temperature exceeded 750 C. The reason is that, at temperatures lower than 750 C, the dry reforming reaction (R2) could take place along with the reverse reaction of Water Gas Shift (R3), which would result in H2/CO < 1. CO2 þ CH4 /2CO þ 2H2
(R2)
CO2 þ H2 /H2 O þ CO
(R3)
However, (R3) turned to be negligible in the overall reactions due to almost no H2O formed in the products (Fig. 2d) when temperature exceeded 750 C, which consequentially resulted in H2/CO ¼ 1. For the cases of O2/(CH4eCO2) ¼ 0.5, 0.7 and 1, H2/ CO ratios were always greater than 1. This can be explained by the occurrence of the partial oxidation reaction (R4) which is thermodynamically favorable. CH4 þ 1=2O2 /CO þ 2H2
(R4)
Additionally, the thermodynamic-equilibrium results demonstrated that C2H2 could be formed only in excess CH4. For the cases of O2/(CH4eCO2) ¼ 0.5, 0.7 and 1, no C2H2 was formed because CH4 was not in excess. For the case of O2/ (CH4eCO2) ¼ 0, C2H2 began to be formed via pyrolysis of the residual CH4 when the temperature reached above 1000 C. Based on the data of Fig. 2, the thermodynamicequilibrium energy costs for H2 production were calculated using (E1) and (E2). Fig. 3 illustrated the thermodynamicequilibrium energy costs for H2 production as a function of
temperature at various O2/(CH4eCO2) ratios. With increasing O2/(CH4eCO2) ratios at the same temperature, the energy costs for H2 production decreased gradually. As depicted in Fig. 3, at 800 C and O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, ECTE H2 are 0.95, 0.68, 0.60 and 0.48 eV/molecule, respectively. In addition, when O2/(CH4eCO2) ¼ 0, the energy costs for H2 production firstly decreased with increasing temperature from 550 C to 750 C but then turned to increase slightly when temperatures higher than 750 C. The reason is that, H2O concentration decreased gradually with temperature (Fig. 2d) and the endothermic reaction of (R3) became more and more unfavorable compared with (R2). This results in the energy consumed in (R3) became less and less. When temperature exceeded 750 C, (R3) could be negligible and the slight rise of ECH2 should be attributed to the energy consumed in heating gas. For the cases of O2/(CH4eCO2) ¼ 0.5, 0.7 and 1, with increasing temperature, the energy costs for H2 production firstly increased significantly and then rose slowly. This variation of the energy costs for H2 production versus temperature depends on the variation of H2 þ CO and H2O concentrations (Fig. 2d) versus temperature.
3.2. Mixed reforming of biogas at various O2/(CH4eCO2) ratios in the spark-discharge plasma 3.2.1. Reactant conversions, selectivities and mass balance, H2 þ CO concentrations and H2/CO ratios At various O2/(CH4eCO2) ratios and input powers, reactant conversions, C-based selectivities and balance, H-based selectivities and balance, H2 þ CO concentrations on wet basis and H2/CO ratios for mixed reforming of biogas (FCH4 þCO2 ¼ 150 SCCM, CH4/CO2 ¼ 3/2) in the spark-discharge plasma were listed in Table 1. As shown in Table 1, with increasing O2/(CH4eCO2) ratio from 0 to 1 at Pinput ¼ 56 W, XCH4 and XTC increased from 72% and 68% to 79% and 72%, respectively; but XCO2 changed very little. Furthermore, SCO and SH H2 O increased from 68% and 4% to 89% and 20%, respectively; SH2 , SC2 H2 and H2/CO ratio decreased from 81%, 30% and 1.5 to 73%, 9% and 1.1, respectively. In addition, CH2 þCO increased from 76% to 78% with increasing O2/ (CH4eCO2) ratio from 0 to 0.7 at Pinput ¼ 64 W; however, as the O2/(CH4eCO2) ratio further increased to 1, CH2 þCO reduced to 74%. All the data of carbon and hydrogen balances reached 95%e98% in this experiment.
3.2.2.
Combination coefficients of independent reactions
Based on the results listed in Table 1, the overall reaction for mixed reforming of biogas in the spark-discharge plasma can be expressed as (R5): CH4 þ vCO2 CO2 þ vO2 O2 /vCO CO þ vH2 H2 þ vC2 H2 C2 H2 þ vH2 O H2 O (R5)
Fig. 3 e Thermodynamic-equilibrium energy cost for H2 production versus temperature at various O2/(CH4eCO2) ratios (designated as r).
where vCO2 , vO2 , vCO , vH2 , vC2 H2 and vH2 O denote the stoichiometric coefficient of CO2, O2, CO, H2, C2H2 and H2O, respectively; the stoichiometric coefficient of each substance was obtained from that it converted or formed was divided by CH4 converted. Since three reactants (CH4, CO2 and O2) and four products (CO, H2, C2H2 and H2O) included in the overall
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Table 1 e Reactant conversions, C-based selectivities and balance, H-based selectivities and balance, H2 D CO concentration and H2/CO ratio for mixed reforming of biogas ðFCH4 DCO2 [150 SCCM; CH4 =CO2 [3=2Þ at various O2/(CH4eCO2) ratios in the spark-discharge plasma. O2/(CH4eCO2)
0
0.5
0.7
1
Pinput (W)
40
48
56
64
40
48
56
64
40
48
56
64
48
56
64
72
Conversions XCH4 (%) XCO2 (%) XTC (%) XO2 (%)
65 52 60 /
68 57 64 /
72 63 68 /
76 67 72 /
68 52 62 79
72 58 66 80
76 63 71 83
80 67 75 85
69 52 62 80
74 58 68 80
77 63 72 83
80 67 75 85
76 57 68 83
79 62 72 85
81 65 74 87
83 68 77 89
C-based selectivity & balance SCO (%) 61 65 35 32 SC2 H2 (%) 96 97 BC (%)
68 30 98
71 27 98
76 20 96
79 18 97
80 16 96
83 15 98
80 17 97
82 15 97
85 13 98
89 10 99
87 11 98
89 9 98
90 8 98
92 6 98
H-based selectivity SH2 (%) SH C2 H2 (%) SH H2 O (%) BH (%)
81 12 4 97
82 11 3 96
74 8 14 96
77 7 14 98
78 6 12 96
80 6 10 96
72 6 18 96
74 6 17 97
77 5 15 97
79 4 12 95
71 4 22 97
73 4 20 97
75 3 20 98
76 2 20 98
66 1.3
69 1.3
74 1.3
77 1.2
68 1.2
69 1.2
74 1.2
78 1.1
69 1.1
73 1.1
74 1.1
77 1.1
& balance 77 78 13 13 6 6 96 97
H2 þ CO concentration on wet basis & H2/CO ratio 64 67 72 76 CH2 þCO (%) 1.7 1.5 1.5 1.4 H2/CO
reaction (R5), four independent reactions (R2), (R3), (R4) and (R6) can be determined by the Gibbs’ rule of stoichiometry: CO2 þ CH4 /2CO þ 2H2
(R2)
CO2 þ H2 /H2 O þ CO
(R3)
CH4 þ 1=2O2 /CO þ 2H2
(R4)
CH4 /1=2C2 H2 þ 3=2H2
(R6)
Thereby, the overall reaction (R5) can be expressed as the linear combination of independent reactions (R2), (R3), (R4) and (R6): R5 ¼ aR2 þ bR3 þ gR4 þ dR6
(E17)
In Eq. (E17), a, b, g and d are the combination coefficients of independent reactions (R2), (R3), (R4) and (R6), respectively. Accordingly, the combination coefficients a, b, g and d were calculated by Eqs. (E18)e(E21), respectively: a¼1gd
(E18)
b ¼ vH2 O
(E19)
g ¼ 2$vO2
(E20)
d ¼ 2$vC2 H2
(E21)
The combination coefficients a, b, g and d as functions of CH4 conversion at various O2/(CH4eCO2) ratios were illustrated in Fig. 4. As depicted in Fig. 4, at O2/(CH4eCO2) ¼ 0, g ¼ 0. Increasing XCH4 from 65% to 76%, combination coefficient a increased from 0.42 to 0.54, whereas b and d decreased from 0.11 and 0.58 to 0.06 and 0.46, respectively. At O2/(CH4eCO2) ¼ 0.5, increasing XCH4 from 68% to 80%, a increased from 0.21 to
0.34, but b, g and d decreased from 0.30, 0.45 and 0.34 to 0.22, 0.40 and 0.26, respectively. At O2/(CH4eCO2) ¼ 0.7, with increasing XCH4 from 69% to 80%, a increased from 0.14 to 0.30, while b, g and d decreased from 0.36, 0.56 and 0.30 to 0.26, 0.51 and 0.19, respectively. At O2/(CH4eCO2) ¼ 1, increasing XCH4 from 76% to 83%, a increased from 0.07 to 0.18, but b, g and d decreased from 0.43, 0.75 and 0.18 to 0.37, 0.70 and 0.12, respectively. It is worthy to be mentioned that, with the increase in O2/ (CH4eCO2) ratio at the same XCH4 , b and g increased gradually, but a and d decreased. For example, at XCH4 ¼ 75%, when O2/ (CH4eCO2) ratio increased from 0 to 1, b and g increased from 0.06 and 0 to 0.43 and 0.75, respectively; a and d decreased from 0.54 and 0.46 to 0.07 and 0.18, respectively. This means that independent reaction (R4) increasingly predominates over independent reactions (R2) and (R6) with increasing O2/ (CH4eCO2) ratio.
3.3. Comparisons of thermodynamic-equilibrium and experimental results at various O2/(CH4eCO2) ratios The thermodynamic-equilibrium and experimental H-based selectivities of H2O, H2 þ CO concentrations on wet basis and energy costs for H2 production as functions of total-carbon conversion at various O2/(CH4eCO2) ratios were illustrated in Fig. 5. Fig. 5a indicated that both thermodynamic-equilibrium and experimental H-based selectivities of H2O decreased linearly with total-carbon conversion. For the equilibrium line of H2O selectivity, the absolute value of its slope presented a slight decrease with the increase in O2/(CH4eCO2) ratio. Increasing XTC from 55% to 80% at O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, SH H2 O decreased from 8.1%, 13.2%, 15% and 17.6% to 0.1%, 5.8%, 7.9% and 11.1%, respectively. It is worth noting that the experimental value of H2O selectivity was very close to its
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Fig. 4 e Combination coefficients of independent reaction (a) a, (b) b, (c) g and (d) d versus CH4 conversion at various O2/ (CH4eCO2) ratios (designated as r). equilibrium one at O2/(CH4eCO2) ¼ 0, whereas the deviation of experimental results from equilibrium values became more and more significant with the increase in O2/(CH4eCO2) ratio. As shown in Fig. 5b, both thermodynamic-equilibrium and
experimental H2 þ CO concentrations (on wet basis) increased linearly with total-carbon conversion. For the equilibrium line of H2 þ CO concentration, its slope decreased gradually with the increase in O2/(CH4eCO2) ratio. When XTC increased from
Fig. 5 e Thermodynamic-equilibrium (TE) and experimental (EXP) (a) H-based selectivities of H2O, (b) H2 D CO concentrations on wet basis and (c) energy costs for H2 production versus total-carbon conversion at various O2/(CH4eCO2) ratios (designated as r).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 9 1 6 e1 6 9 2 4
55% to 80% at O2/(CH4eCO2) ¼ 0, 0.5, 0.7 and 1, CH2 þCO increased from 67.2%, 67.5%, 67.3% and 67.2% to 88.8%, 87.1%, 85.9% and 84.7%, respectively. As far as the lines of experimental H2þCO concentration versus total-carbon conversion is concerned, the slopes at O2/(CH4eCO2) ¼ 0.7 and 1 were the largest and smallest, respectively. This indicated that, when total-carbon conversion was relatively high, H2 þ CO concentration on wet basis was the highest at O2/(CH4eCO2) ¼ 0.7 (CH2 þCO ¼ 78% at XTC ¼ 75%), and its experimental results were closest to their thermodynamic-equilibrium values. As depicted in Fig. 5c, with increasing XTC from 55% to 80% at O2/(CH4eCO2) ¼ 0.5, 0.7 and 1, the thermodynamicequilibrium energy costs for H2 production increased slowly from 0.60, 0.46 and 0.27 eV/molecule to 0.65, 0.56 and 0.43 eV/ molecule, respectively. However, when O2/(CH4eCO2) ¼ 0, ECTE H2 firstly decreased gradually to the minimum (0.94 eV/ molecule) with increasing XTC from 55% to 76% and then turned to increase with the further increase in XTC, which is in accordance with the result in Fig. 3. Fig. 5c also illustrated that the thermodynamic-equilibrium energy costs for H2 production exhibited a descending trend with the increase of O2/ (CH4eCO2) ratio at the same total-carbon conversion. In addition, the slope of experimental energy costs for H2 production versus total-carbon conversion at O2/(CH4eCO2) ¼ 0.7 was the lowest. Therefore, when total-carbon conversion was relatively high (>75%), not only the highest experimental H2 þ CO concentration but also the lowest experimental energy cost for H2 production can be achieved at O2/ (CH4eCO2) ¼ 0.7. That is, the optimal O2/(CH4eCO2) ratio for optimized mixed reforming of biogas in spark-discharge plasma was about 0.7. In Fig. 5c, the slope of experimental curve of energy cost is greater than that of thermodynamicequilibrium curve, which may be ascribed that the energy efficiency of the plasma decreased with total-carbon conversion [23]. The experimental energy cost is much higher than the thermodynamic-equilibrium one, which means that the energy efficiency of the plasma is still required to be further improved.
4.
Conclusions
The thermodynamic-equilibrium calculation results for mixed reforming of biogas with O2 addition showed that, the maximum total-carbon conversion and H2 þ CO concentration on wet basis were achieved at O2/(CH4eCO2) ¼ 0.5e0.7 in the temperature range of 750e850 C. The thermodynamicequilibrium energy costs for H2 production decreased gradually with the increase in O2/(CH4eCO2) ratio. At 800 C and O2/ (CH4eCO2) ¼ 0, 0.5, 0.7 and 1, ECTE H2 are 0.95, 0.68, 0.60 and 0.48 eV/molecule, respectively. The experimental results of spark-discharge plasma for mixed reforming of biogas with O2 addition showed that, with increasing O2/(CH4eCO2) ratio from 0 to 1, both CH4 and totalcarbon conversions increased, but CO2 conversion changed very little. All the data of carbon and hydrogen balances reached around 95%e98% in this experiment. With increasing O2/(CH4eCO2) ratio at the same CH4 conversion, the combination coefficients of CO2 reforming reaction (a) and partial oxidation reaction (g) decreased and
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increased, respectively. When O2/(CH4eCO2) ratio increasing from 0 to 1 at 75% of CH4 conversion , a decreased from 0.54 to 0.07 and g increased from 0 to 0.75. Compared thermodynamic-equilibrium with experimental results, it can be concluded that the optimal O2/(CH4eCO2) ratio for optimized mixed reforming of biogas in sparkdischarge plasma was about 0.7. When total-carbon conversion was relatively high (>75%), H2 þ CO concentration on wet basis was the highest and energy cost for H2 production was the lowest at O2/(CH4eCO2) ¼ 0.7, and their experimental results were closest to their thermodynamic-equilibrium values.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11079013) and the Fundamental Research Funds for the Central Universities.
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