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Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power
G Model JIEC 4952 No. of Pages 12
Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
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Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power Nabil Majd Alawia,b , Jaka Sunarsoc,* , Gia Hung Phama , Ahmed Barifcania , Minh Hoang Nguyena,d, Shaomin Liua a
Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia Petroleum Technology Department, University of Technology, Baghdad, Iraq Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350, Kuching, Sarawak, Malaysia d The University of Da Nang - University of Science and Technology, Da Nang 550000, Vietnam b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 July 2019 Received in revised form 9 January 2020 Accepted 30 January 2020 Available online xxx
Dry reforming of methane (DRM) is an attractive route to convert CH4 and CO2 into syngas (a mixture of CO and H2). In this work, the performance of microwave-assisted DRM at atmospheric pressure (in terms of CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio) is studied as functions of additive gas flow rate, microwave power, CO2 to CH4 inlet supply ratio, and reaction time. Two additive gases were used, i.e., nitrogen and argon. Microwave-assisted DRM experiments in both gases show identical trend of conversions, selectivities, yields, and product ratio. DRM performances in both additive gases atmosphere were stable for up to 8 h. Under the same operating conditions, using Ar as an additive gas, however, led to higher H2 and CO selectivities and yields and thus, higher H2 to CO ratio relative to using N2 as an additive gas. Maximum CH4 and CO2 conversions of 79.35% and 44.82%, H2 and CO selectivities of 50.12% and 58.42%, H2 and CO yields of 39.77% and 32.89%, and H2 to CO ratio of 0.86 were obtained at 700 W and N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively. © 2020 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Additive gases Atmospheric pressure Dry methane reforming Microwave Plasma Syngas production
Introduction
partial oxidation of methane (POM, Eq. (2)), and (3) dry reforming of methane (DRM, Eq. (3)).
Greenhouse gases such as methane (CH4) and carbon dioxide (CO2) are undesirable products of energy and chemicals production from fossil fuel-based resources, which can be used more productively in their concentrated forms as a feedstock for the production of syngas [1–5]. Syngas is a mixture of carbon monoxide (CO) and hydrogen (H2) that can be utilised directly as a fuel for combustion with oxygen (O2) or as a fuel in molten carbonate and solid oxide fuel cells as well as a raw material for the production of chemicals such as ammonia, ethanol, methanol, acetic acid, dimethyl ether, and methyl formate [6–10]. Three different chemical pathways have been established to convert CH4 to syngas, i.e., (1) steam reforming of methane (SRM, Eq. (1)), (2)
CH4 þ H2 O ! CO þ 3H2
ð1Þ
1 CH4 þ O2 ! CO þ 2H2 2
ð2Þ
CH4 þ CO2 ! 2CO þ 2H2
ð3Þ
* Corresponding author. E-mail addresses: [email protected], [email protected] (N.M. Alawi), [email protected], [email protected] (J. Sunarso).
SRM is presently one of the most widely used pathways to obtain syngas (and H2) from CH4 since it generates syngas with the highest H2 to CO ratio of 3 among the three pathways [11]. SRM however is a highly endothermic reaction (DH (298 K) = 206 kJ mol1) that requires temperature above 700 C to activate and self-sustain [12], which translates to high capital and energy investments requirement. POM, on the other hand, is a mildly exothermic reaction (DH (298 K) = 36 kJ mol1) [13]. Although the H2 to CO ratio of POM is approximately two, which is ideal for syngas conversion to liquid
https://doi.org/10.1016/j.jiec.2020.01.032 1226-086X/© 2020 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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fuels and methanol via Fischer–Tropsch (F–T) process, pure O2 is required for POM since the downstream process cannot endure the presence of the excessive amount of nitrogen (N2). Such pure O2 requirement leads to high investment and energy costs [12]. DRM is unique in the sense that it combines two greenhouse gases, CH4, and CO2 to produce syngas with the lowest H2 to CO ratio of 1 [1,9,14,15]. Despite the fact that DRM is an extremely endothermic reaction (DH (298 K) = 247 kJ mol1), the interest to DRM comes mainly from three aspects, i.e., (1) Reduction in the net emission of CH4 and CO2 if the energy for DRM is supplied by non-hydrocarbon source, (2) Lower operating cost compared to SRM and POM, and (3) Increased selectivity towards long chain hydrocarbons during the subsequent F–T process enabled by low H2 to CO ratio [16]. Plasma-induced DRM has recently emerged as an attractive alternative to conventional DRM since it can enhance the reaction performance and suppress the carbon deposition issue with respect to the conventional DRM [15,17]. Although there are two different plasma technologies, i.e., non-thermal (cold) plasma and thermal (hot) plasma upon which the reaction can take place, the former is generally preferred over the latter due to the significantly higher energy consumption in the former case [18,19]. Nonthermal plasma reaction can be induced via glow discharge, corona discharge, silent discharge, dielectric barrier discharge, radio frequency discharge, or microwave discharge [15,19]. By using microwave irradiation, plasma reaction can be carried out in a homogeneous, controlled, and rapid manner [20,21]. Such features essentially enable reproducibility and scale-up of the reaction [22]. In the presence of microwave irradiation, dissociative collisions of molecules occur. This leads to the formation of reactive atoms, ionised gases, free electrons, and free positive and negative ions; which activate the plasma-induced reaction [14,15,18,23,24]. In such microwave-assisted DRM, aside from the microwave power output,
reactor type and design, one of the other important parameters that strongly affects the performance and stability of reaction performance is the type and the flow rate of the additive gas (N2, Ar, He, or H2) used in the reaction [25,26]. In this study, nitrogen and argon were chosen as additive gases since nitrogen and argon are the first and the third most abundant gas in the atmosphere. Plasma generation in N2 and Ar would also require a lower power consumption relative to those performed in He and H2 plasma [27]. Furthermore, they are relatively cheap and safe when used as additives gases compared with the other additives gases. Numerous works are available that report the performance of plasma-induced DRM in terms of CH4 and CO2 conversions, H2 and CO selectivities and yields, and the molar ratio of H2 to CO in Ar and/or N2 atmosphere(s) [6,28–36]. None of these works, however, have evaluated the effect of the additive gas type and flowrate in syngas production from CH4 and CO2 using microwave source. Therefore, in this work, the use of N2 as an additive gas is compared against Ar. DRM performance is also evaluated as a function of microwave power, different CO2 to CH4 molar ratio, and sampling time at an atmospheric pressure. Experimental Experimental setup and runs A commercial microwave reactor system (Alter, SM 1150T, Canada) consisting of 8 main systems, i.e., gas cylinders system, mass flow controllers system (MFC, Alicat Scientific, MCS-Series), gas mixer system, plasma reactor system (microwave generator (power supply SM1150x and magnetron GA4313) and quartz glass tube), microwave generator system, water cooling system, gas sampling system, and gas chromatographic (GC-MSD and GC-TCD)
Fig. 1. Schematic diagram of microwave plasma reactor setup.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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3
Table 1 Different experimental runs for microwave-assisted DRM. Experiment type
CO2 to CH4 ratio
CO2 [L min1]
CH4 [L min1]
N2 [L min1]
Ar [L min1]
TFRa [L min1]
MWPb [W]
Sampling time [min]
Different N2 flow rate
2/1
0.4
0.2
0.5 0.75
–
1.1 1.35
700
20
0.5 0.75
0.7 1.85 2.1 1.1 1.35
700
20
1 1.5 1.5
0.7 1.85 2.1 700 800 900
20
1 1.25 1.5 Different Ar flow rate
2/1
0.4
0.2
–
1.5 Different microwave power
Different CO2 to CH4 molar ratio
2/1
0.4
2/1 2.5/1 3/1
0.4 0.5 0.6
3.5/1 4/1 4.5/1 5/1
0.7 0.8 0.9 1
–
0.2
2.1
–
1.5
1.5
–
0.2 –
1.5
1000 1100 1200 2.1 2.2 2.3
700
20
700
480
2.4 2.5 2.6 2.7
– 1.5 DRM performance stability
2/1
0.4
0.2
2.1 –
a b
1.5
Total flow rates. Microwave power.
Fig. 2. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of N2 flow rate between 0.3 and 1.5 L min1. Microwave-assisted DRM experiments were performed using a constant microwave power of 700 W and a CO2 to CH4 inlet supply ratio of 2 to 1.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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analysis system (Fig. 1) was used. The three gases used in the feeding system were methane (CH4, 99.99%), carbon dioxide (CO2, 99.99%), and nitrogen (N2, 99.99%) or argon (Ar, 99.99%), the flows of which were controlled by mass flow controllers. These gases were passed into the gas mixer to achieve the desired composition of the resultant gas mixture before entering the plasma reactor. Inside the plasma reactor, plasma flame was generated by nitrogen or argon gas to provide the desired gas reaction condition. The quartz reactor has 1.68 mm wall thickness, 25.5 mm outer diameter, and 126 mm length. K-type thermocouples were inserted at several different locations of the experimental apparatus to monitor and control the temperatures during the reaction. The gas sample was first drawn by syringe then injected into the GC detector. The cooler trap was installed outside the plasma reactor to separate the gases products from water, which may be produced as a side product. All experiments were performed at an atmospheric pressure. Then, the gas product sample was collected and analysed by GC, which is connected online with a mass spectroscopy detector. GC can separate and provide the relative amount of different gases species such as CH4, CO2, H2, and CO using thermal conductivity detector (TCD). The molecular weight information, on the other hand, was determined by using the mass spectroscopy detector. The performance of microwave-assisted DRM in the microwave plasma reactor was evaluated in N2 and Ar atmosphere at an atmospheric pressure. Table 1 lists the details of the experimental runs that evaluate the DRM performance in terms of CH4 and CO2 conversions, H2 and CO selectivities and yields, and H2 to CO ratio as functions of N2 and Ar flow rate, microwave power, the molar ratio of CO2 to CH4 as well as the DRM performance stability. Each experimental run was repeated three times to account for variation. In our experimental runs, generally, a small amount of
carbon powder (soot) can be observed to be present in the reactor at the end of the reaction since CO2 cannot be completely converted to syngas. More experimental details were presented elsewhere [37]. The calculations of conversions of CH4 and CO2; selectivities and yields of H2, CO, and CO; and H2/CO ratio were performed using Eqs. (4)–(10): moles of CH4 converted 100 CH4 % conversion ¼ moles of CH4 introduced
ð4Þ
moles of CO2 converted 100 CO2 % conversion ¼ moles of CO2 introduced
ð5Þ
moles of H2 produced 100 H2 % selectivity ¼ 2 moles of CH4 converted
ð6Þ
moles of CO produced CO % selectivity ¼ ½moles of CH4 þ moles of CO2 converted 100 ð7Þ
moles of H2 produced 100 H2 % yield ¼ 2 moles of CH4 introduced
moles of CO produced ½moles of CH4 þ moles of CO2 introduced 100
ð8Þ
CO % yield ¼
ð9Þ
Fig. 3. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of microwave power between 700 and 1200 W. Microwave-assisted DRM experiments were performed using a constant N2 flow rate of 1.5 L min1 and a CO2 to CH4 inlet supply ratio of 2 to 1 (CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively).
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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H2 moles of H2 produced ratio ¼ moles of CO produced CO
5
CH4 + e → CH3 + H + e
(11)
Results and discussion
CH4 + e → CH2 + H + e
(12)
The performance of microwave-assisted DRM in N2 and Ar atmospheres will be discussed separately.
CH4 + e → CH + H + e
(13)
CH4 + e → C + H + e
(14)
CH4 + O → CH3 + OH
(15)
CH4 + OH → CH3 + H2O
(16)
CO2 + e → CO + O + e
(17)
CO2 + e → C + 2O + e
(18)
CO2 + e → CO + O
(19)
O + CO2 → CO + O2
(20)
ð10Þ
DRM performance in N2 atmosphere Fig. 2(a), (b), (c), and (d) displays CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of N2 flow rate between 0.3 and 1.5 L min1. These data were obtained at a constant microwave power of 700 W and constant CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively (Table 1). A five-fold increase in N2 flow rate from 0.3 to 1.5 L min1 translates to a minor reduction in CH4 and CO2 conversions from 88.01 to 79.35% and from 56.69 to 44.82%, respectively (Fig. 2(a)). These trends reproduce the trends reported elsewhere and can be attributed to the reduction in the residence time of the gases in the microwave exposed region [38]. A similar conversion tendency has also been reported previously [14,20,27,29,39–42]. Shorter residence time translates to shorter reaction time and lower frequency of collisions between CH4 and CO2 with energetic species such as electrons, OH, O, and O that lead to dissociations of CH4 and CO2 as represented by possible reactions listed in Eqs. (11)–(16) and Eqs. (17)–(22) for CH4 and CO2, respectively [43].
Fig. 4. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of CO2 to CH4 ratio between 2 and 5. Microwaveassisted DRM experiments were performed using a constant microwave power of 700 W and a constant N2 flow rate of 1.5 L min1.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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Fig. 5. Microwave-assisted DRM performance stability for up to 8 h-duration in N2 atmosphere. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of time between 20 and 480 min. Microwave-assisted DRM experiments were performed using a constant microwave power of 700 W and constant N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively.
CO + O → CO2 + e
(21)
CO + O → CO2
(22) 1
At N2 flow rate between 0.3 and 1.5 L min , CH4 conversion was always higher than CO2 conversion. Such discrepancy can be attributed to the lower bond dissociation energy for CH4 (4.5 eV) relative to that for CO2 (5.5 eV) [20].
H2 and CO selectivities also decrease from 62.94 to 50.12% and from 70.43 to 58.42%, respectively, with an increase in N2 flow rate from 0.3 to 1.5 L min1 (Fig. 2(b)), in accord with the reduction of the selectivities with increasing total gases flow rate observed in other works [14,27,38,39,42]. The decrease in H2 and CO yields over the same flow rate region was more pronounced, from 55.39 to 39.77% and from 47.26 to 32.89%, respectively (Fig. 2(c)). This is consistent with the previous studies [14,27,38,39,42], which show that H2 and CO selectivities decrease with increasing total feed
Table 2 Comparison of DRM performance in N2 atmosphere reported here with the others in the literature. Plasma form
Gliding arc discharge (GAD) DC arc thermal plasma Arc jet plasma (AJP) Cold plasma jet (CPJ) Single-anode thermal plasma jet Binod-anode thermal plasma + Ni/Al2O3 Spark discharge plasma (SDP) AC spark discharge plasma (SDP) kHz spark discharge Microwave discharge plasma a
TFRa [L min1]
CO2/CH4 ratio
Power [W]
Selectivity [%]
Yield [%]
CH4
Conversion [%] CO2
N2
H2
CO
H2
CO
References
1
1/1
182
40.63
29.32
NA
50.25
62.41
20.53
NA
[26]
16 4 16.667 30
1/1 1/1 2/3 3/2
3,400 1000 770 9,600
92.06 50.74 45.68 89.82
88.18 35.55 34.03 80.14
NA NA NA NA
65.16 80.98 78.11 68.60
66.53 78.31 85.41 88.37
60.12 40.51 36.73 62.34
NA NA 33.09 NA
[35] [28] [29] [33]
83.334
3/2
1,440
77.1
62.4
NA
88.6
96.7
86.15
NA
[34]
0.2
1/1
26.6
53.43
58.8
NA
79.2
61.7
42.64
NA
[42]
0.15
1.5/1
45
65.27
55.84
–
62.35
87.63
40.83
NA
[54]
0.15 2.1
2/3 2/1
1,344 700
75.33 79.35
70.78 44.82
– 3.22
82.78 50.12
70.32 58.42
62.57 39.77
NA 32.89
[55] This work
Total flow rates.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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flow rates. This means that with an increase in the total flow rates, the residence time of the molecules of the gas mixture is reduced in the discharge zone, which leads to a reduced probability of collisions of H2 and CO molecules with energetic electrons to break down C–H bond in CH4 molecules and C O bond in CO2 molecules, which then manifests into an observed decrease in the H2 and CO production [43]. H2 to CO ratio decreases from 0.97 to 0.86 with increasing N2 flow rate from 0.3 to 1.5 L min1 (Fig. 2(d)), as is observed elsewhere [20,30,41,44,45]. Such decrease may come from the production of additional CO (and H2O) from reverse water-gas shift reaction (RWGS, Eq. (23)) and reverse Boudouard reaction (C þ CO2 ! 2CO, Eq. (24)). Water and carbon powder presence were observed at the end of DRM experiments. CO2 þ H2 ! CO þ H2 O
ð23Þ
C þ CO2 ! 2CO
ð24Þ
Fig. 3(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of microwave power between 700 and 1200 W. These data were obtained at constant N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively (Table 1). Increase in microwave power from 700 to 1200 W leads to a minor increase in CH4 and CO2 conversions from 79.35 to 89.03% and from 44.82 to 48.41%, respectively (Fig. 3(a)). Previous studies [14,20,27,30,40,44–49] have shown that increasing microwave power imput leads to an increase in CH4 and CO2 conversions. Increasing power generally enhances the electrical field, the
7
electron density, and the gas temperature in the microwave discharge zone, which leads to the enhanced production of more energetic species such as electrons, OH, O and O and more frequent collisions between CH4, CO2, and these species (Eqs. (11)–(22)) [14,20,27,30,40,44–49]. H2 selectivity decreases from 50.12 to 33.08% while CO selectivity increases from 58.42 to 75.04% with an increase in microwave power from 700 to 1200 W (Fig. 3(b)). Such opposite behaviour has been observed in previous works and can be attributed to the higher dissociation energy of CO (11.1 eV) relative to H2 (4.5 eV) as well as the possible subsequent reactions between CO2 and H2 to form H2O via RWGS reaction (Eq. (23)) and between CO2 and C to form CO via reverse Boudouard reaction (Eq. (24)) [20,30]. Likewise, H2 yield decreases from 39.77 to 29.45% while CO yield increases from 32.89 to 46.47% over the same microwave power range increase (Fig. 3(c)). Water and carbon powder presence was again observed at the end of DRM. This result is consistent with previous studies [20,30]. Decreasing H2 yield accompanied by increasing CO yield over an increase in microwave power from 700 to 1200 W translates to a significant reduction in H2 to CO ratio from 0.86 to 0.44 as observed in other works (Fig. 3(d)) [20,30,44,45]. Since microwave power increase affects marginally CH4 and CO2 conversions and affects significantly H2 and CO selectivities, low microwave power is desirable to maintain balanced H2 and CO selectivities. Fig. 4(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of CO2/CH4 inlet supply ratio between 2 and 5. These data were obtained at a constant microwave power of 700 W and a constant N2 flow rate of 1.5 L min1 (Table 1).
Fig. 6. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of Ar flow rate between 0.3 and 1.5 L min1. Microwave-assisted DRM experiments were performed using a constant microwave power of 700 W and a CO2 to CH4 inlet supply ratio of 2 to 1 (CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively).
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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With an increase in CO2 to CH4 ratio from 2 to 5, CH4 conversion increases from 79.35 to 96.31% while CO2 conversion decreases from 44.82 to 21.18% (Fig. 4(a)). Such trends mirror the trends observed in other DRM studies [14,38,50]. The decrease in CO2 conversion happened since the main reaction of CH4 and CO2 under microwave plasma at atmospheric pressure was the dry reforming reaction (Eq. (3)). When the CO2 amount exceeds the reaction stoichiometry of the main reaction, the conversion of CO2 decreases while the conversion of CH4 increases [38,50]. H2 selectivity decreases from 50.12 to 29.83% while CO selectivity increases from 58.42 to 76.77% with an increase in CO2 to CH4 ratio from 2 to 5 (Fig. 4(b)). Increase in CO selectivity comes mainly from the increasing amount of CO2 reactant, which favours the production of CO product via DRM reaction (Eq. (3)) as is observed in other works [38,51–53]. H2 and CO yields nonetheless decrease from 39.77 to 28.72% and from 32.89 to 25.85% over the increase in the same CO2 to CH4 ratio region (Fig. 4(c)). Increasing CO2 to CH4 ratio from 2 to 5 also decreases H2 to CO ratio from 0.86 to 0.36; in agreement with the results of other works [39,50]. The increasing amount of CO2 in the reactor favours the formation of water via RWGS (Eq. (23)). Water was clearly present at the reactor tube at the end of DRM reaction. The stability of DRM performance was also evaluated as a function of time at a constant microwave power of 700 W and constant N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively. Fig. 5(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of time from 20 to 480 min (up to 8 h). The data point was recorded after 20 min-reaction time to ensure that steady-state condition was achieved. Over this time
period, CH4 and CO2 conversions fluctuated marginally around median values of 80 and 44%, respectively (Fig. 5(a)) while H2 and CO selectivities fluctuated around median values of 50 and 59%, respectively (Fig. 5(b)). Likewise, over this same time period, H2 and CO yields also fluctuated slightly around median values of 39.5 and 34.5%, respectively (Fig. 5(c)) while H2 to CO ratio fluctuated around 0.85 (Fig. 5(d)). In essence, stable microwave-assisted DRM performance can be observed for up to 8 h-reaction duration. Comparison between the performance of microwave-assisted DRM performed here in N2 atmosphere against other works in N2 atmosphere is made in Table 2 [26,28,29,33–35,42,54,55]. Sun et al., for example, reported that maximum CO2 and CH4 conversions of 88.18 and 92.06%, respectively, and an H2 yield of 60.16% in a DC arc thermal plasma reactor were obtained at a discharge power of 3400 W, a total flow rate of 16 L min1, and a CO2/CH4 ratio of 1/1 [35]. These highest conversions and yields were achieved at a very high flow rate. Li et al. (2011), on the other hand, reported CO2 and CH4 conversions of 55.84 and 65.27%, respectively, and an H2 yield of 40.35% in an AC spark discharge plasma reactor was obtained at a low discharge power of 45 W and a low total flow rate of 0.15 L min1 [54]. Zhu et al. reported that CO2 and CH4 conversions of 70.78 and 75.33%, respectively, and an H2 yield of 62.78% in a kHz spark discharge reactor were obtained at a relatively high discharge power of 1344 W and a low total flow rate of 0.15 L min1 [55]. Our maximum CO2 and CH4 conversions of 44.82 and 79.35%, respectively, and an H2 yield of 39.77% is comparable with the values reported in these works. The discrepancies between this work and these works reflect the different equipment setup and operating conditions.
Fig. 7. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of microwave power between 700 and 1200 W. Microwave-assisted DRM experiments were performed using a constant Ar flow rate of 1.5 L min1 and a CO2 to CH4 inlet supply ratio of 2 to 1 (CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively).
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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DRM performance in Ar atmosphere Fig. 6(a), (b), (c), and (d) displays CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of Ar flow rate between 0.3 and 1.5 L min1. These data were obtained at a constant microwave power of 700 W and constant CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively (Table 1). Marginal variations in CH4 and CO2 conversions from median values of 77.6 to 76.94% and of 43.85 to 41.11%, respectively, manifest when the Ar flow rate was increased from 0.3 to 1.5 L min-1 (Fig. 6(a)). Marginal fluctuation can also be observed for H2 and CO selectivities (around median values of 61.36% and 68.47%, respectively) (Fig. 6(b)) as well as for H2 and CO yields (around median values of 47.42% and 38.83%, respectively) (Fig. 6(c)). The increase in CO yield over this N2 flow rate region, however, is still slightly larger than the increase in H2 yield, which translates to a minor decrease in H2 to CO ratio from 1.02 to 0.91 over this region (Fig. 6(d)). Such reduction in H2 yield and H2 to CO ratio with increasing flow rate was also observed in another work [39]. Fig. 7(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of microwave power between 700 and 1200 W. These data were obtained at constant Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively (Table 1). A minor increase in CH4 and CO2 conversions from 76.94 to 87.55% and from 41.11 to 52.47% occured as the microwave power rose from 700 to 1200 W (Fig. 7(a)). Such an increase can be attributed to the enhanced generation of energetic species and more frequent collisions between CH4, CO2 [56], and these species represented in Eqs (11)–(22). CH4 conversion was always higher
9
than CO2 conversion as observed earlier when DRM was carried out in an N2 atmosphere. H2 selectivity decreases from 62.87 to 44.79% while CO selectivity increases from 69.62 to 81.93% with an increase in microwave power from 700 to 1200 W (Fig. 7(b)). Such reversed behaviour between H2 and CO reflects the occurrence of RWGS reaction and reverse Boudouard reaction. Likewise, H2 and CO yields show the same reversed behaviour, i.e., H2 yield decreases from 48.37 to 39.21% while CO yields increases from 40.56 to 52.55% (Fig. 7(c)). A concurrent increase in CO yield and decrease in H2 yield with an increase in microwave power from 700 to 1200 W essentially leads to a reduction in H2 to CO ratio from 0.91 to 0.49 over this power region (Fig. 7(d)). The amount of water generated increases with increasing microwave power input, which affects slightly the conversion, selectivity, yield, and H2 to CO ratio. Water was again observed at the end of DRM. Fig. 8(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of CO2/CH4 inlet supply ratio between 2 and 5. These data were obtained at a constant microwave power of 700 W and a constant Ar flow rate of 1.5 L min1 (Table 1). Increasing CO2 to CH4 ratio from 2 to 5 translates to an increase in CH4 conversion from 76.94 to 95.11% and a decrease in CO2 conversion from 41.11 to 18.79% (Fig. 8(a)). The decrease in CO2 conversion occurred since the main reaction of CH4 and CO2 under microwave plasma at atmospheric pressure was the DRM (Eq. (3)). When CO2 amount exceeds the stoichiometry of the main reaction, CO2 conversion decreases while CH4 conversion increases [50]. While H2 selectivity decreases from 62.87 to 37.83% with an increase in CO2 to CH4 ratio from 2 to 5, CO selectivity increases from 69.62 to 90.77% (Fig. 8(b)). As observed above also in an N2
Fig. 8. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of CO2 to CH4 ratio between 2 and 5. Microwaveassisted DRM experiments were performed using a constant microwave power of 700 W and a constant Ar flow rate of 1.5 L min1.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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Fig. 9. Microwave-assisted DRM performance stability for up to 8 h-duration in Ar atmosphere. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of time between 20 and 480 min. Microwave-assisted DRM experiments were performed using a constant microwave power of 700 W and constant Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively.
atmosphere case, enhanced CO selectivity is a result of a higher amount of CO2 reactants, which shifts the equilibrium towards CO product via DRM reaction (Eq. (3)). H2 and CO yields however decrease from 48.37 to 36.44% and from 40.56 to 28.51%, respectively, with an increase in CO2 to CH4 ratio from 2 to 5 (Fig. 8(c)). Increasing CO2 to CH4 ratio from 2 to 5 leads to a reduction in H2 to CO ratio from 0.91 to 0.42 (Fig. 8(d)). The trends observed in Fig. 8(a), (b), (c), and (d) as a function of CO2/CH4 inlet supply ratio
between 2 and 5 in Ar atmosphere reproduce the trends observed in Fig. 4(a), (b), (c), and (d) in N2 atmosphere. The stability of DRM performance was also studied as a function of time at a constant microwave power of 700 W and constant Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively. Fig. 9(a), (b), (c), and (d) displays CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of time from 20 to 480 min (up to 8 h). Minor fluctuations are observed in CH4 and CO2 conversions, with median
Table 3 Comparison of DRM performance in Ar atmosphere reported here with the others in the literature. Plasma form
AC dielectric barrier discharge (DBD) AC atmospheric pressure glow discharge (APGD) DC pulsed plasma Dielectric barrier discharge AC gliding arc discharge (GAD) DC spark discharge (SD) AC dielectric barrier discharge (DBD) Pulsed DC arc discharge Dielectric barrier discharge Microwave discharge plasma
TFRa [L min1]
CO2/CH4 ratio
MWb power [W]
Conversion [%]
Selectivity [%]
Yield [%]
CH4
CO2
H2
CO
H2
CO
References
0.5
1/4
500
64.34
34.63
52.8
23.38
33.79
NA
[57]
2.2
6/4
69.85
60.97
39.91
89.3
72.58
54.44
NA
[58]
0.18 0.03 4
1/1 1/1 1/1
21 71.5 190
33.26 63 12.36
23.55 35 15.93
68.15 88 50.13
65.67 95 47.52
22.44 55.44 6.43
NA NA NA
[31] [49] [6]
0.12 0.0167
2/1 2/1
16 19
48.34 58.53
37/05 33.32
84.25 32.8
100 39.62
40.32 19.02
NA NA
[30] [60]
0.1 0.06 2.1
1/1 1/1 2/1
204 60 700
99.6 15.09 76.94
99.3 3.11 41.11
100 34 62.87
100 40.13 69.62
9.96 5.1 48.37
NA NA 40.56
[36] [27] This work
NA is an abbreviation of not available. a Total flow rates. b Microwave.
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values of 76 and 42%, respectively (Fig. 9(a)). H2 and CO selectivities fluctuated in the same manner, with median values of 63 and 69%, respectively (Fig. 9(b)). H2 and CO yields also varied around median values of 48 and 40% (Fig. 9(c)). Likewise, H2 to CO ratio varied slightly around 0.91 (Fig. 9(d)). As is the case for N2-based DRM studied above, performing microwave-assisted DRM in Ar atmosphere also provides highly stable performance for up to 8 h. Table 3 provides a comparison between the performance of microwave-assisted DRM performed here in Ar atmosphere against other works in Ar atmosphere [6,27,30,31,36,57–60]. Our maximum CO2 and CH4 conversions of 41.11 and 76.94%, respectively, and an H2 yield of 48.37% is comparable with the values reported in these works. Zhou et al. (1998), for instance, reported that the highest CO2 and CH4 conversions of 34.63 and 64.34%, respectively, and an H2 yield of 33.79% in an AC dielectric barrier discharge (DBD) reactor were obtained at a low discharge power of 500 W, a total flow rate of 0.5 L min1, and a CO2/CH4 ratio of 1/4 [57]. Li et al., on the other hand, reported CO2 and CH4 conversions of 39.91 and 60.97%, respectively, and an H2 yield of 54.44% in an AC atmospheric pressure glow discharge (APGD) plasma reactor was obtained at a low discharge power of 69.85 W and a total flow rate of 2.2 L min1 [58]. Allah and Whitehead (2015) reported that low CO2 and CH4 conversions of 15.93 and 12.36%, respectively, and an H2 yield of 6.43% in an AC gliding arc discharge (GAD) plasma reactor were obtained at a relatively low discharge power of 190 W and a total flow rate of 4 L min1 [6]. Different equipment setup and operating conditions are the main contributors between the observed different performances.
11
yields, and H2 to CO ratio, respectively, between DRM carried out in N2 atmosphere against that carried out in Ar atmosphere at a constant microwave power of 700 W, CO2/CH4 ratio of 2/1 and constant N2, Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively. Although DRM in N2 gave slightly higher CH4 and CO2 conversions relative to DRM in Ar (Fig. 10(a)), the latter clearly provided higher H2 and CO selectivities and yields (Fig. 10(b) and (c)), which translates to higher H2 to CO ratio relative to the former (Fig. 10(d)). These results showed that the Ar case gave better performance relative to the N2 case since in N2 case, ammonia and cyanide may be produced as side products (Eqs. (25)–(27)) [26,61]. Moreover, it is also possible that in N2 case, N2 reacts with O2 to form various oxides of nitrogen (Eq. (28)) [62]. In Ar case however, Ar firstly collides with electrons and become excited (Eq. (29)) [63]. Then, the CH4 and CO2 molecules obtain energy from the excited Ar atoms instead of electrons (Eqs. (30) and (31)) [64]. Therefore, Ar does not react with CH4 and CO2 to form any side products [65]. N2 þ 3H2 ! 2NH3
ð25Þ
N2 þ CO ! CN þ NO
ð26Þ
CN þ H2 ! HCN þ H
ð27Þ
DRM performance comparison overview
N2 þ O2 ! 2NO
ð28Þ
Fig. 10(a), (b), (c), and (d) presents a comparison overview on CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO
Ar þ e ! Ar þ e
ð29Þ
Fig. 10. Comparison overview between microwave-assisted DRM performed in N2 atmosphere against that performed in Ar atmosphere. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio. Microwave-assisted DRM experiments were performed using at a constant microwave power of 700 W CO2/ CH4 ratio of 2/1 and constant N2, Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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CH4 þ Ar ! CH3 þ H þ Ar
ð30Þ
CO2 þ Ar ! CO þ O þ Ar
ð31Þ
Conclusions It has been shown that microwave-assisted dry reforming of methane (DRM) in N2 and Ar atmosphere show identical performance trends as a function of additive gas flow rate, microwave power, and CO2 to CH4 inlet supply ratio. For example, with increasing additive gas flow rate from 0.3 to 1.5 L min1, there are slight fluctuations in CH4 and CO2 conversions, H2 and CO selectivities, and H2 and CO yields as well as a minor reduction in H2 to CO ratio. Increase in microwave power from 700 to 1200 W, on the other hand, brought a clear increase in CO selectivity and yield and decrease in H2 selectivity and yield, which translates to a significant reduction in H2 to CO ratio. Furthermore, increasing CO2 to CH4 inlet supply ratio from 2 to 5 led to an apparent reduction in CO2 conversion, H2 selectivity, H2 and CO yields, and H2 to CO ratio and an increase in CH4 conversion and CO selectivity. Despite these identical trends, at a constant microwave power of 700 W, CO2/CH4 ratio of 2/1 and constant additive gas, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively, DRM in Ar gave higher H2 and CO selectivities and yields and higher H2 to CO ratio relative to DRM in N2. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Nabil Majd Alawi would like to acknowledge the Ministry of Higher Education & Scientific Research in Iraq for the sponsorship for his Ph.D. study at Curtin University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
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Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power Nabil Majd Alawia,b , Jaka Sunarsoc,* , Gia Hung Phama , Ahmed Barifcania , Minh Hoang Nguyena,d, Shaomin Liua a
Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845, Australia Petroleum Technology Department, University of Technology, Baghdad, Iraq Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350, Kuching, Sarawak, Malaysia d The University of Da Nang - University of Science and Technology, Da Nang 550000, Vietnam b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 July 2019 Received in revised form 9 January 2020 Accepted 30 January 2020 Available online xxx
Dry reforming of methane (DRM) is an attractive route to convert CH4 and CO2 into syngas (a mixture of CO and H2). In this work, the performance of microwave-assisted DRM at atmospheric pressure (in terms of CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio) is studied as functions of additive gas flow rate, microwave power, CO2 to CH4 inlet supply ratio, and reaction time. Two additive gases were used, i.e., nitrogen and argon. Microwave-assisted DRM experiments in both gases show identical trend of conversions, selectivities, yields, and product ratio. DRM performances in both additive gases atmosphere were stable for up to 8 h. Under the same operating conditions, using Ar as an additive gas, however, led to higher H2 and CO selectivities and yields and thus, higher H2 to CO ratio relative to using N2 as an additive gas. Maximum CH4 and CO2 conversions of 79.35% and 44.82%, H2 and CO selectivities of 50.12% and 58.42%, H2 and CO yields of 39.77% and 32.89%, and H2 to CO ratio of 0.86 were obtained at 700 W and N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively. © 2020 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Additive gases Atmospheric pressure Dry methane reforming Microwave Plasma Syngas production
Introduction
partial oxidation of methane (POM, Eq. (2)), and (3) dry reforming of methane (DRM, Eq. (3)).
Greenhouse gases such as methane (CH4) and carbon dioxide (CO2) are undesirable products of energy and chemicals production from fossil fuel-based resources, which can be used more productively in their concentrated forms as a feedstock for the production of syngas [1–5]. Syngas is a mixture of carbon monoxide (CO) and hydrogen (H2) that can be utilised directly as a fuel for combustion with oxygen (O2) or as a fuel in molten carbonate and solid oxide fuel cells as well as a raw material for the production of chemicals such as ammonia, ethanol, methanol, acetic acid, dimethyl ether, and methyl formate [6–10]. Three different chemical pathways have been established to convert CH4 to syngas, i.e., (1) steam reforming of methane (SRM, Eq. (1)), (2)
CH4 þ H2 O ! CO þ 3H2
ð1Þ
1 CH4 þ O2 ! CO þ 2H2 2
ð2Þ
CH4 þ CO2 ! 2CO þ 2H2
ð3Þ
* Corresponding author. E-mail addresses: [email protected], [email protected] (N.M. Alawi), [email protected], [email protected] (J. Sunarso).
SRM is presently one of the most widely used pathways to obtain syngas (and H2) from CH4 since it generates syngas with the highest H2 to CO ratio of 3 among the three pathways [11]. SRM however is a highly endothermic reaction (DH (298 K) = 206 kJ mol1) that requires temperature above 700 C to activate and self-sustain [12], which translates to high capital and energy investments requirement. POM, on the other hand, is a mildly exothermic reaction (DH (298 K) = 36 kJ mol1) [13]. Although the H2 to CO ratio of POM is approximately two, which is ideal for syngas conversion to liquid
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fuels and methanol via Fischer–Tropsch (F–T) process, pure O2 is required for POM since the downstream process cannot endure the presence of the excessive amount of nitrogen (N2). Such pure O2 requirement leads to high investment and energy costs [12]. DRM is unique in the sense that it combines two greenhouse gases, CH4, and CO2 to produce syngas with the lowest H2 to CO ratio of 1 [1,9,14,15]. Despite the fact that DRM is an extremely endothermic reaction (DH (298 K) = 247 kJ mol1), the interest to DRM comes mainly from three aspects, i.e., (1) Reduction in the net emission of CH4 and CO2 if the energy for DRM is supplied by non-hydrocarbon source, (2) Lower operating cost compared to SRM and POM, and (3) Increased selectivity towards long chain hydrocarbons during the subsequent F–T process enabled by low H2 to CO ratio [16]. Plasma-induced DRM has recently emerged as an attractive alternative to conventional DRM since it can enhance the reaction performance and suppress the carbon deposition issue with respect to the conventional DRM [15,17]. Although there are two different plasma technologies, i.e., non-thermal (cold) plasma and thermal (hot) plasma upon which the reaction can take place, the former is generally preferred over the latter due to the significantly higher energy consumption in the former case [18,19]. Nonthermal plasma reaction can be induced via glow discharge, corona discharge, silent discharge, dielectric barrier discharge, radio frequency discharge, or microwave discharge [15,19]. By using microwave irradiation, plasma reaction can be carried out in a homogeneous, controlled, and rapid manner [20,21]. Such features essentially enable reproducibility and scale-up of the reaction [22]. In the presence of microwave irradiation, dissociative collisions of molecules occur. This leads to the formation of reactive atoms, ionised gases, free electrons, and free positive and negative ions; which activate the plasma-induced reaction [14,15,18,23,24]. In such microwave-assisted DRM, aside from the microwave power output,
reactor type and design, one of the other important parameters that strongly affects the performance and stability of reaction performance is the type and the flow rate of the additive gas (N2, Ar, He, or H2) used in the reaction [25,26]. In this study, nitrogen and argon were chosen as additive gases since nitrogen and argon are the first and the third most abundant gas in the atmosphere. Plasma generation in N2 and Ar would also require a lower power consumption relative to those performed in He and H2 plasma [27]. Furthermore, they are relatively cheap and safe when used as additives gases compared with the other additives gases. Numerous works are available that report the performance of plasma-induced DRM in terms of CH4 and CO2 conversions, H2 and CO selectivities and yields, and the molar ratio of H2 to CO in Ar and/or N2 atmosphere(s) [6,28–36]. None of these works, however, have evaluated the effect of the additive gas type and flowrate in syngas production from CH4 and CO2 using microwave source. Therefore, in this work, the use of N2 as an additive gas is compared against Ar. DRM performance is also evaluated as a function of microwave power, different CO2 to CH4 molar ratio, and sampling time at an atmospheric pressure. Experimental Experimental setup and runs A commercial microwave reactor system (Alter, SM 1150T, Canada) consisting of 8 main systems, i.e., gas cylinders system, mass flow controllers system (MFC, Alicat Scientific, MCS-Series), gas mixer system, plasma reactor system (microwave generator (power supply SM1150x and magnetron GA4313) and quartz glass tube), microwave generator system, water cooling system, gas sampling system, and gas chromatographic (GC-MSD and GC-TCD)
Fig. 1. Schematic diagram of microwave plasma reactor setup.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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3
Table 1 Different experimental runs for microwave-assisted DRM. Experiment type
CO2 to CH4 ratio
CO2 [L min1]
CH4 [L min1]
N2 [L min1]
Ar [L min1]
TFRa [L min1]
MWPb [W]
Sampling time [min]
Different N2 flow rate
2/1
0.4
0.2
0.5 0.75
–
1.1 1.35
700
20
0.5 0.75
0.7 1.85 2.1 1.1 1.35
700
20
1 1.5 1.5
0.7 1.85 2.1 700 800 900
20
1 1.25 1.5 Different Ar flow rate
2/1
0.4
0.2
–
1.5 Different microwave power
Different CO2 to CH4 molar ratio
2/1
0.4
2/1 2.5/1 3/1
0.4 0.5 0.6
3.5/1 4/1 4.5/1 5/1
0.7 0.8 0.9 1
–
0.2
2.1
–
1.5
1.5
–
0.2 –
1.5
1000 1100 1200 2.1 2.2 2.3
700
20
700
480
2.4 2.5 2.6 2.7
– 1.5 DRM performance stability
2/1
0.4
0.2
2.1 –
a b
1.5
Total flow rates. Microwave power.
Fig. 2. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of N2 flow rate between 0.3 and 1.5 L min1. Microwave-assisted DRM experiments were performed using a constant microwave power of 700 W and a CO2 to CH4 inlet supply ratio of 2 to 1.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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analysis system (Fig. 1) was used. The three gases used in the feeding system were methane (CH4, 99.99%), carbon dioxide (CO2, 99.99%), and nitrogen (N2, 99.99%) or argon (Ar, 99.99%), the flows of which were controlled by mass flow controllers. These gases were passed into the gas mixer to achieve the desired composition of the resultant gas mixture before entering the plasma reactor. Inside the plasma reactor, plasma flame was generated by nitrogen or argon gas to provide the desired gas reaction condition. The quartz reactor has 1.68 mm wall thickness, 25.5 mm outer diameter, and 126 mm length. K-type thermocouples were inserted at several different locations of the experimental apparatus to monitor and control the temperatures during the reaction. The gas sample was first drawn by syringe then injected into the GC detector. The cooler trap was installed outside the plasma reactor to separate the gases products from water, which may be produced as a side product. All experiments were performed at an atmospheric pressure. Then, the gas product sample was collected and analysed by GC, which is connected online with a mass spectroscopy detector. GC can separate and provide the relative amount of different gases species such as CH4, CO2, H2, and CO using thermal conductivity detector (TCD). The molecular weight information, on the other hand, was determined by using the mass spectroscopy detector. The performance of microwave-assisted DRM in the microwave plasma reactor was evaluated in N2 and Ar atmosphere at an atmospheric pressure. Table 1 lists the details of the experimental runs that evaluate the DRM performance in terms of CH4 and CO2 conversions, H2 and CO selectivities and yields, and H2 to CO ratio as functions of N2 and Ar flow rate, microwave power, the molar ratio of CO2 to CH4 as well as the DRM performance stability. Each experimental run was repeated three times to account for variation. In our experimental runs, generally, a small amount of
carbon powder (soot) can be observed to be present in the reactor at the end of the reaction since CO2 cannot be completely converted to syngas. More experimental details were presented elsewhere [37]. The calculations of conversions of CH4 and CO2; selectivities and yields of H2, CO, and CO; and H2/CO ratio were performed using Eqs. (4)–(10): moles of CH4 converted 100 CH4 % conversion ¼ moles of CH4 introduced
ð4Þ
moles of CO2 converted 100 CO2 % conversion ¼ moles of CO2 introduced
ð5Þ
moles of H2 produced 100 H2 % selectivity ¼ 2 moles of CH4 converted
ð6Þ
moles of CO produced CO % selectivity ¼ ½moles of CH4 þ moles of CO2 converted 100 ð7Þ
moles of H2 produced 100 H2 % yield ¼ 2 moles of CH4 introduced
moles of CO produced ½moles of CH4 þ moles of CO2 introduced 100
ð8Þ
CO % yield ¼
ð9Þ
Fig. 3. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of microwave power between 700 and 1200 W. Microwave-assisted DRM experiments were performed using a constant N2 flow rate of 1.5 L min1 and a CO2 to CH4 inlet supply ratio of 2 to 1 (CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively).
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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H2 moles of H2 produced ratio ¼ moles of CO produced CO
5
CH4 + e → CH3 + H + e
(11)
Results and discussion
CH4 + e → CH2 + H + e
(12)
The performance of microwave-assisted DRM in N2 and Ar atmospheres will be discussed separately.
CH4 + e → CH + H + e
(13)
CH4 + e → C + H + e
(14)
CH4 + O → CH3 + OH
(15)
CH4 + OH → CH3 + H2O
(16)
CO2 + e → CO + O + e
(17)
CO2 + e → C + 2O + e
(18)
CO2 + e → CO + O
(19)
O + CO2 → CO + O2
(20)
ð10Þ
DRM performance in N2 atmosphere Fig. 2(a), (b), (c), and (d) displays CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of N2 flow rate between 0.3 and 1.5 L min1. These data were obtained at a constant microwave power of 700 W and constant CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively (Table 1). A five-fold increase in N2 flow rate from 0.3 to 1.5 L min1 translates to a minor reduction in CH4 and CO2 conversions from 88.01 to 79.35% and from 56.69 to 44.82%, respectively (Fig. 2(a)). These trends reproduce the trends reported elsewhere and can be attributed to the reduction in the residence time of the gases in the microwave exposed region [38]. A similar conversion tendency has also been reported previously [14,20,27,29,39–42]. Shorter residence time translates to shorter reaction time and lower frequency of collisions between CH4 and CO2 with energetic species such as electrons, OH, O, and O that lead to dissociations of CH4 and CO2 as represented by possible reactions listed in Eqs. (11)–(16) and Eqs. (17)–(22) for CH4 and CO2, respectively [43].
Fig. 4. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of CO2 to CH4 ratio between 2 and 5. Microwaveassisted DRM experiments were performed using a constant microwave power of 700 W and a constant N2 flow rate of 1.5 L min1.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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Fig. 5. Microwave-assisted DRM performance stability for up to 8 h-duration in N2 atmosphere. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of time between 20 and 480 min. Microwave-assisted DRM experiments were performed using a constant microwave power of 700 W and constant N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively.
CO + O → CO2 + e
(21)
CO + O → CO2
(22) 1
At N2 flow rate between 0.3 and 1.5 L min , CH4 conversion was always higher than CO2 conversion. Such discrepancy can be attributed to the lower bond dissociation energy for CH4 (4.5 eV) relative to that for CO2 (5.5 eV) [20].
H2 and CO selectivities also decrease from 62.94 to 50.12% and from 70.43 to 58.42%, respectively, with an increase in N2 flow rate from 0.3 to 1.5 L min1 (Fig. 2(b)), in accord with the reduction of the selectivities with increasing total gases flow rate observed in other works [14,27,38,39,42]. The decrease in H2 and CO yields over the same flow rate region was more pronounced, from 55.39 to 39.77% and from 47.26 to 32.89%, respectively (Fig. 2(c)). This is consistent with the previous studies [14,27,38,39,42], which show that H2 and CO selectivities decrease with increasing total feed
Table 2 Comparison of DRM performance in N2 atmosphere reported here with the others in the literature. Plasma form
Gliding arc discharge (GAD) DC arc thermal plasma Arc jet plasma (AJP) Cold plasma jet (CPJ) Single-anode thermal plasma jet Binod-anode thermal plasma + Ni/Al2O3 Spark discharge plasma (SDP) AC spark discharge plasma (SDP) kHz spark discharge Microwave discharge plasma a
TFRa [L min1]
CO2/CH4 ratio
Power [W]
Selectivity [%]
Yield [%]
CH4
Conversion [%] CO2
N2
H2
CO
H2
CO
References
1
1/1
182
40.63
29.32
NA
50.25
62.41
20.53
NA
[26]
16 4 16.667 30
1/1 1/1 2/3 3/2
3,400 1000 770 9,600
92.06 50.74 45.68 89.82
88.18 35.55 34.03 80.14
NA NA NA NA
65.16 80.98 78.11 68.60
66.53 78.31 85.41 88.37
60.12 40.51 36.73 62.34
NA NA 33.09 NA
[35] [28] [29] [33]
83.334
3/2
1,440
77.1
62.4
NA
88.6
96.7
86.15
NA
[34]
0.2
1/1
26.6
53.43
58.8
NA
79.2
61.7
42.64
NA
[42]
0.15
1.5/1
45
65.27
55.84
–
62.35
87.63
40.83
NA
[54]
0.15 2.1
2/3 2/1
1,344 700
75.33 79.35
70.78 44.82
– 3.22
82.78 50.12
70.32 58.42
62.57 39.77
NA 32.89
[55] This work
Total flow rates.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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flow rates. This means that with an increase in the total flow rates, the residence time of the molecules of the gas mixture is reduced in the discharge zone, which leads to a reduced probability of collisions of H2 and CO molecules with energetic electrons to break down C–H bond in CH4 molecules and C O bond in CO2 molecules, which then manifests into an observed decrease in the H2 and CO production [43]. H2 to CO ratio decreases from 0.97 to 0.86 with increasing N2 flow rate from 0.3 to 1.5 L min1 (Fig. 2(d)), as is observed elsewhere [20,30,41,44,45]. Such decrease may come from the production of additional CO (and H2O) from reverse water-gas shift reaction (RWGS, Eq. (23)) and reverse Boudouard reaction (C þ CO2 ! 2CO, Eq. (24)). Water and carbon powder presence were observed at the end of DRM experiments. CO2 þ H2 ! CO þ H2 O
ð23Þ
C þ CO2 ! 2CO
ð24Þ
Fig. 3(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of microwave power between 700 and 1200 W. These data were obtained at constant N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively (Table 1). Increase in microwave power from 700 to 1200 W leads to a minor increase in CH4 and CO2 conversions from 79.35 to 89.03% and from 44.82 to 48.41%, respectively (Fig. 3(a)). Previous studies [14,20,27,30,40,44–49] have shown that increasing microwave power imput leads to an increase in CH4 and CO2 conversions. Increasing power generally enhances the electrical field, the
7
electron density, and the gas temperature in the microwave discharge zone, which leads to the enhanced production of more energetic species such as electrons, OH, O and O and more frequent collisions between CH4, CO2, and these species (Eqs. (11)–(22)) [14,20,27,30,40,44–49]. H2 selectivity decreases from 50.12 to 33.08% while CO selectivity increases from 58.42 to 75.04% with an increase in microwave power from 700 to 1200 W (Fig. 3(b)). Such opposite behaviour has been observed in previous works and can be attributed to the higher dissociation energy of CO (11.1 eV) relative to H2 (4.5 eV) as well as the possible subsequent reactions between CO2 and H2 to form H2O via RWGS reaction (Eq. (23)) and between CO2 and C to form CO via reverse Boudouard reaction (Eq. (24)) [20,30]. Likewise, H2 yield decreases from 39.77 to 29.45% while CO yield increases from 32.89 to 46.47% over the same microwave power range increase (Fig. 3(c)). Water and carbon powder presence was again observed at the end of DRM. This result is consistent with previous studies [20,30]. Decreasing H2 yield accompanied by increasing CO yield over an increase in microwave power from 700 to 1200 W translates to a significant reduction in H2 to CO ratio from 0.86 to 0.44 as observed in other works (Fig. 3(d)) [20,30,44,45]. Since microwave power increase affects marginally CH4 and CO2 conversions and affects significantly H2 and CO selectivities, low microwave power is desirable to maintain balanced H2 and CO selectivities. Fig. 4(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of CO2/CH4 inlet supply ratio between 2 and 5. These data were obtained at a constant microwave power of 700 W and a constant N2 flow rate of 1.5 L min1 (Table 1).
Fig. 6. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of Ar flow rate between 0.3 and 1.5 L min1. Microwave-assisted DRM experiments were performed using a constant microwave power of 700 W and a CO2 to CH4 inlet supply ratio of 2 to 1 (CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively).
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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With an increase in CO2 to CH4 ratio from 2 to 5, CH4 conversion increases from 79.35 to 96.31% while CO2 conversion decreases from 44.82 to 21.18% (Fig. 4(a)). Such trends mirror the trends observed in other DRM studies [14,38,50]. The decrease in CO2 conversion happened since the main reaction of CH4 and CO2 under microwave plasma at atmospheric pressure was the dry reforming reaction (Eq. (3)). When the CO2 amount exceeds the reaction stoichiometry of the main reaction, the conversion of CO2 decreases while the conversion of CH4 increases [38,50]. H2 selectivity decreases from 50.12 to 29.83% while CO selectivity increases from 58.42 to 76.77% with an increase in CO2 to CH4 ratio from 2 to 5 (Fig. 4(b)). Increase in CO selectivity comes mainly from the increasing amount of CO2 reactant, which favours the production of CO product via DRM reaction (Eq. (3)) as is observed in other works [38,51–53]. H2 and CO yields nonetheless decrease from 39.77 to 28.72% and from 32.89 to 25.85% over the increase in the same CO2 to CH4 ratio region (Fig. 4(c)). Increasing CO2 to CH4 ratio from 2 to 5 also decreases H2 to CO ratio from 0.86 to 0.36; in agreement with the results of other works [39,50]. The increasing amount of CO2 in the reactor favours the formation of water via RWGS (Eq. (23)). Water was clearly present at the reactor tube at the end of DRM reaction. The stability of DRM performance was also evaluated as a function of time at a constant microwave power of 700 W and constant N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively. Fig. 5(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of time from 20 to 480 min (up to 8 h). The data point was recorded after 20 min-reaction time to ensure that steady-state condition was achieved. Over this time
period, CH4 and CO2 conversions fluctuated marginally around median values of 80 and 44%, respectively (Fig. 5(a)) while H2 and CO selectivities fluctuated around median values of 50 and 59%, respectively (Fig. 5(b)). Likewise, over this same time period, H2 and CO yields also fluctuated slightly around median values of 39.5 and 34.5%, respectively (Fig. 5(c)) while H2 to CO ratio fluctuated around 0.85 (Fig. 5(d)). In essence, stable microwave-assisted DRM performance can be observed for up to 8 h-reaction duration. Comparison between the performance of microwave-assisted DRM performed here in N2 atmosphere against other works in N2 atmosphere is made in Table 2 [26,28,29,33–35,42,54,55]. Sun et al., for example, reported that maximum CO2 and CH4 conversions of 88.18 and 92.06%, respectively, and an H2 yield of 60.16% in a DC arc thermal plasma reactor were obtained at a discharge power of 3400 W, a total flow rate of 16 L min1, and a CO2/CH4 ratio of 1/1 [35]. These highest conversions and yields were achieved at a very high flow rate. Li et al. (2011), on the other hand, reported CO2 and CH4 conversions of 55.84 and 65.27%, respectively, and an H2 yield of 40.35% in an AC spark discharge plasma reactor was obtained at a low discharge power of 45 W and a low total flow rate of 0.15 L min1 [54]. Zhu et al. reported that CO2 and CH4 conversions of 70.78 and 75.33%, respectively, and an H2 yield of 62.78% in a kHz spark discharge reactor were obtained at a relatively high discharge power of 1344 W and a low total flow rate of 0.15 L min1 [55]. Our maximum CO2 and CH4 conversions of 44.82 and 79.35%, respectively, and an H2 yield of 39.77% is comparable with the values reported in these works. The discrepancies between this work and these works reflect the different equipment setup and operating conditions.
Fig. 7. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of microwave power between 700 and 1200 W. Microwave-assisted DRM experiments were performed using a constant Ar flow rate of 1.5 L min1 and a CO2 to CH4 inlet supply ratio of 2 to 1 (CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively).
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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DRM performance in Ar atmosphere Fig. 6(a), (b), (c), and (d) displays CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of Ar flow rate between 0.3 and 1.5 L min1. These data were obtained at a constant microwave power of 700 W and constant CO2 and CH4 flow rates of 0.4 and 0.2 L min1, respectively (Table 1). Marginal variations in CH4 and CO2 conversions from median values of 77.6 to 76.94% and of 43.85 to 41.11%, respectively, manifest when the Ar flow rate was increased from 0.3 to 1.5 L min-1 (Fig. 6(a)). Marginal fluctuation can also be observed for H2 and CO selectivities (around median values of 61.36% and 68.47%, respectively) (Fig. 6(b)) as well as for H2 and CO yields (around median values of 47.42% and 38.83%, respectively) (Fig. 6(c)). The increase in CO yield over this N2 flow rate region, however, is still slightly larger than the increase in H2 yield, which translates to a minor decrease in H2 to CO ratio from 1.02 to 0.91 over this region (Fig. 6(d)). Such reduction in H2 yield and H2 to CO ratio with increasing flow rate was also observed in another work [39]. Fig. 7(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of microwave power between 700 and 1200 W. These data were obtained at constant Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively (Table 1). A minor increase in CH4 and CO2 conversions from 76.94 to 87.55% and from 41.11 to 52.47% occured as the microwave power rose from 700 to 1200 W (Fig. 7(a)). Such an increase can be attributed to the enhanced generation of energetic species and more frequent collisions between CH4, CO2 [56], and these species represented in Eqs (11)–(22). CH4 conversion was always higher
9
than CO2 conversion as observed earlier when DRM was carried out in an N2 atmosphere. H2 selectivity decreases from 62.87 to 44.79% while CO selectivity increases from 69.62 to 81.93% with an increase in microwave power from 700 to 1200 W (Fig. 7(b)). Such reversed behaviour between H2 and CO reflects the occurrence of RWGS reaction and reverse Boudouard reaction. Likewise, H2 and CO yields show the same reversed behaviour, i.e., H2 yield decreases from 48.37 to 39.21% while CO yields increases from 40.56 to 52.55% (Fig. 7(c)). A concurrent increase in CO yield and decrease in H2 yield with an increase in microwave power from 700 to 1200 W essentially leads to a reduction in H2 to CO ratio from 0.91 to 0.49 over this power region (Fig. 7(d)). The amount of water generated increases with increasing microwave power input, which affects slightly the conversion, selectivity, yield, and H2 to CO ratio. Water was again observed at the end of DRM. Fig. 8(a), (b), (c), and (d) shows CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of CO2/CH4 inlet supply ratio between 2 and 5. These data were obtained at a constant microwave power of 700 W and a constant Ar flow rate of 1.5 L min1 (Table 1). Increasing CO2 to CH4 ratio from 2 to 5 translates to an increase in CH4 conversion from 76.94 to 95.11% and a decrease in CO2 conversion from 41.11 to 18.79% (Fig. 8(a)). The decrease in CO2 conversion occurred since the main reaction of CH4 and CO2 under microwave plasma at atmospheric pressure was the DRM (Eq. (3)). When CO2 amount exceeds the stoichiometry of the main reaction, CO2 conversion decreases while CH4 conversion increases [50]. While H2 selectivity decreases from 62.87 to 37.83% with an increase in CO2 to CH4 ratio from 2 to 5, CO selectivity increases from 69.62 to 90.77% (Fig. 8(b)). As observed above also in an N2
Fig. 8. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of CO2 to CH4 ratio between 2 and 5. Microwaveassisted DRM experiments were performed using a constant microwave power of 700 W and a constant Ar flow rate of 1.5 L min1.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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Fig. 9. Microwave-assisted DRM performance stability for up to 8 h-duration in Ar atmosphere. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio – as a function of time between 20 and 480 min. Microwave-assisted DRM experiments were performed using a constant microwave power of 700 W and constant Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively.
atmosphere case, enhanced CO selectivity is a result of a higher amount of CO2 reactants, which shifts the equilibrium towards CO product via DRM reaction (Eq. (3)). H2 and CO yields however decrease from 48.37 to 36.44% and from 40.56 to 28.51%, respectively, with an increase in CO2 to CH4 ratio from 2 to 5 (Fig. 8(c)). Increasing CO2 to CH4 ratio from 2 to 5 leads to a reduction in H2 to CO ratio from 0.91 to 0.42 (Fig. 8(d)). The trends observed in Fig. 8(a), (b), (c), and (d) as a function of CO2/CH4 inlet supply ratio
between 2 and 5 in Ar atmosphere reproduce the trends observed in Fig. 4(a), (b), (c), and (d) in N2 atmosphere. The stability of DRM performance was also studied as a function of time at a constant microwave power of 700 W and constant Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively. Fig. 9(a), (b), (c), and (d) displays CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio, respectively, as a function of time from 20 to 480 min (up to 8 h). Minor fluctuations are observed in CH4 and CO2 conversions, with median
Table 3 Comparison of DRM performance in Ar atmosphere reported here with the others in the literature. Plasma form
AC dielectric barrier discharge (DBD) AC atmospheric pressure glow discharge (APGD) DC pulsed plasma Dielectric barrier discharge AC gliding arc discharge (GAD) DC spark discharge (SD) AC dielectric barrier discharge (DBD) Pulsed DC arc discharge Dielectric barrier discharge Microwave discharge plasma
TFRa [L min1]
CO2/CH4 ratio
MWb power [W]
Conversion [%]
Selectivity [%]
Yield [%]
CH4
CO2
H2
CO
H2
CO
References
0.5
1/4
500
64.34
34.63
52.8
23.38
33.79
NA
[57]
2.2
6/4
69.85
60.97
39.91
89.3
72.58
54.44
NA
[58]
0.18 0.03 4
1/1 1/1 1/1
21 71.5 190
33.26 63 12.36
23.55 35 15.93
68.15 88 50.13
65.67 95 47.52
22.44 55.44 6.43
NA NA NA
[31] [49] [6]
0.12 0.0167
2/1 2/1
16 19
48.34 58.53
37/05 33.32
84.25 32.8
100 39.62
40.32 19.02
NA NA
[30] [60]
0.1 0.06 2.1
1/1 1/1 2/1
204 60 700
99.6 15.09 76.94
99.3 3.11 41.11
100 34 62.87
100 40.13 69.62
9.96 5.1 48.37
NA NA 40.56
[36] [27] This work
NA is an abbreviation of not available. a Total flow rates. b Microwave.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
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values of 76 and 42%, respectively (Fig. 9(a)). H2 and CO selectivities fluctuated in the same manner, with median values of 63 and 69%, respectively (Fig. 9(b)). H2 and CO yields also varied around median values of 48 and 40% (Fig. 9(c)). Likewise, H2 to CO ratio varied slightly around 0.91 (Fig. 9(d)). As is the case for N2-based DRM studied above, performing microwave-assisted DRM in Ar atmosphere also provides highly stable performance for up to 8 h. Table 3 provides a comparison between the performance of microwave-assisted DRM performed here in Ar atmosphere against other works in Ar atmosphere [6,27,30,31,36,57–60]. Our maximum CO2 and CH4 conversions of 41.11 and 76.94%, respectively, and an H2 yield of 48.37% is comparable with the values reported in these works. Zhou et al. (1998), for instance, reported that the highest CO2 and CH4 conversions of 34.63 and 64.34%, respectively, and an H2 yield of 33.79% in an AC dielectric barrier discharge (DBD) reactor were obtained at a low discharge power of 500 W, a total flow rate of 0.5 L min1, and a CO2/CH4 ratio of 1/4 [57]. Li et al., on the other hand, reported CO2 and CH4 conversions of 39.91 and 60.97%, respectively, and an H2 yield of 54.44% in an AC atmospheric pressure glow discharge (APGD) plasma reactor was obtained at a low discharge power of 69.85 W and a total flow rate of 2.2 L min1 [58]. Allah and Whitehead (2015) reported that low CO2 and CH4 conversions of 15.93 and 12.36%, respectively, and an H2 yield of 6.43% in an AC gliding arc discharge (GAD) plasma reactor were obtained at a relatively low discharge power of 190 W and a total flow rate of 4 L min1 [6]. Different equipment setup and operating conditions are the main contributors between the observed different performances.
11
yields, and H2 to CO ratio, respectively, between DRM carried out in N2 atmosphere against that carried out in Ar atmosphere at a constant microwave power of 700 W, CO2/CH4 ratio of 2/1 and constant N2, Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively. Although DRM in N2 gave slightly higher CH4 and CO2 conversions relative to DRM in Ar (Fig. 10(a)), the latter clearly provided higher H2 and CO selectivities and yields (Fig. 10(b) and (c)), which translates to higher H2 to CO ratio relative to the former (Fig. 10(d)). These results showed that the Ar case gave better performance relative to the N2 case since in N2 case, ammonia and cyanide may be produced as side products (Eqs. (25)–(27)) [26,61]. Moreover, it is also possible that in N2 case, N2 reacts with O2 to form various oxides of nitrogen (Eq. (28)) [62]. In Ar case however, Ar firstly collides with electrons and become excited (Eq. (29)) [63]. Then, the CH4 and CO2 molecules obtain energy from the excited Ar atoms instead of electrons (Eqs. (30) and (31)) [64]. Therefore, Ar does not react with CH4 and CO2 to form any side products [65]. N2 þ 3H2 ! 2NH3
ð25Þ
N2 þ CO ! CN þ NO
ð26Þ
CN þ H2 ! HCN þ H
ð27Þ
DRM performance comparison overview
N2 þ O2 ! 2NO
ð28Þ
Fig. 10(a), (b), (c), and (d) presents a comparison overview on CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO
Ar þ e ! Ar þ e
ð29Þ
Fig. 10. Comparison overview between microwave-assisted DRM performed in N2 atmosphere against that performed in Ar atmosphere. (a) CH4 and CO2 conversions; (b) H2 and CO selectivities; (c) H2 and CO yields; and (d) H2 to CO ratio. Microwave-assisted DRM experiments were performed using at a constant microwave power of 700 W CO2/ CH4 ratio of 2/1 and constant N2, Ar, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively.
Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032
G Model JIEC 4952 No. of Pages 12
12
N.M. Alawi et al. / Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
CH4 þ Ar ! CH3 þ H þ Ar
ð30Þ
CO2 þ Ar ! CO þ O þ Ar
ð31Þ
Conclusions It has been shown that microwave-assisted dry reforming of methane (DRM) in N2 and Ar atmosphere show identical performance trends as a function of additive gas flow rate, microwave power, and CO2 to CH4 inlet supply ratio. For example, with increasing additive gas flow rate from 0.3 to 1.5 L min1, there are slight fluctuations in CH4 and CO2 conversions, H2 and CO selectivities, and H2 and CO yields as well as a minor reduction in H2 to CO ratio. Increase in microwave power from 700 to 1200 W, on the other hand, brought a clear increase in CO selectivity and yield and decrease in H2 selectivity and yield, which translates to a significant reduction in H2 to CO ratio. Furthermore, increasing CO2 to CH4 inlet supply ratio from 2 to 5 led to an apparent reduction in CO2 conversion, H2 selectivity, H2 and CO yields, and H2 to CO ratio and an increase in CH4 conversion and CO selectivity. Despite these identical trends, at a constant microwave power of 700 W, CO2/CH4 ratio of 2/1 and constant additive gas, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min1, respectively, DRM in Ar gave higher H2 and CO selectivities and yields and higher H2 to CO ratio relative to DRM in N2. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Nabil Majd Alawi would like to acknowledge the Ministry of Higher Education & Scientific Research in Iraq for the sponsorship for his Ph.D. study at Curtin University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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Please cite this article in press as: N.M. Alawi, et al., Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power, J. Ind. Eng. Chem. (2020), https://doi.org/10.1016/j.jiec.2020.01.032