Hydrogen Production from Methanol Decomposition

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Oct 5, 2015 - (E-mail: [email protected]). Herein, we report a simple and efficient method of hydrogen production from methanol decomposition using a gliding ...

Hydrogen Production from Methanol Decomposition in a Gliding Arc Discharge Plasma with High Processing Capacity Hao Zhang, Fengsen Zhu, Zheng Bo, Kefa Cen, and Xiaodong Li* State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38# Zheda Road, Hangzhou, Zhejiang 310027, P. R. China (E-mail: [email protected]) A gliding arc discharge (GAD) plasma was investigated for hydrogen production from methanol decomposition. A maximum methanol conversion of 87.1% can be achieved. The GAD plasma process has shown significant advantages compared to other typical nonthermal plasmas in terms of processing capacity, hydrogen production rate, and hydrogen energy yields, offering a promising technology for industrial applications.

REPRINTED FROM

Vol.44 No.10

2015 p.1315–1317 CMLTAG October 5, 2015

The Chemical Society of Japan

Received: June 8, 2015 | Accepted: June 17, 2015 | Web Released: June 27, 2015

CL-150563

Hydrogen Production from Methanol Decomposition in a Gliding Arc Discharge Plasma with High Processing Capacity Hao Zhang, Fengsen Zhu, Zheng Bo, Kefa Cen, and Xiaodong Li* State Key Laboratory of Clean Energy Utilization, Zhejiang University, 38# Zheda Road, Hangzhou, Zhejiang 310027, P. R. China (E-mail: [email protected]) Herein, we report a simple and efficient method of hydrogen production from methanol decomposition using a gliding arc discharge (GAD) plasma. A maximum methanol conversion of 87.1% can be achieved. Results have shown that, the GAD plasma process can provide a processing capacity, H2 production rate, and energy yields of several orders of magnitude higher than other typical nonthermal plasmas, offering a promising and flexible route for industrial applications. Hydrogen energy is regarded as one of the most promising regenerable energy sources that will probably play a key role in various specialized areas, particularly portable fuel cells which have been considered as an appealing alternative to traditional battery technology for higher energy density and the added benefits of lower operating costs and almost nil emission in comparison with combustion systems.1 Nevertheless, technical problems with the handling, storage, and transport of H2 limit its widespread use.2 Portable fuel reforming technology can overcome the aforementioned technical difficulties by taking advantages of the high energy density of fuels and providing a H2 source for fuel cells.1b Methanol has been considered as an excellent liquid H2-containing fuel due to its high hydrogen to carbon ratio, easy transportation, low boiling point, ready biodegradation, no sulfur content, etc.3 Conventional hydrogen production strategies based on methanol rely on the use of catalysts including transition or noble metals.4 Nevertheless, rapid loss of catalyst activity, associated with high capital costs, requirement of high temperature, and large equipment size still limit its further applications particularly for on-board hydrogen production systems.4 Nonthermal plasma technology provides an attractive alternative to the conventional catalytic route for methanol reforming at low temperatures. In nonthermal plasmas, the overall gas temperature can be as low as room temperature, whilst the electrons are highly energetic with a typical temperature of 104­ 105 K, which is sufficient to break down most chemical bonds of molecules and produce highly reactive species for the initiation and propagation of chemical reactions. In addition, high reaction rate and fast attainment of steady state in plasma processes allow rapid start-up and shutdown of the process compared to thermal and catalytic processes.5 Different nonthermal plasma systems have been investigated for methanol conversion into hydrogen, such as microhollow cathode discharge (MHCD),1b microwave discharge,6 and dielectric barrier discharge (DBD).7 However, the low power levels of these plasmas make it difficult to achieve high efficient conversion of methanol, particularly at a high gas flow rate, restricting their applications on large-scale industrial processes.8 For instance, Futamura et al. reported a methanol conversion rate of only 8­26% with a feed carrier gas flow rate Chem. Lett. 2015, 44, 1315–1317 | doi:10.1246/cl.150563

of 100 mL min¹1 and input CH3OH molar concentration of only 1% in a DBD assisted methanol decomposition process.9 It this letter, a developed gliding arc discharge (GAD) plasma, which is considered a powerful transient plasma with a relatively high level of energy density, good chemical selectivity, and low energy consumption,5,8 has been investigated for hydrogen production from methanol decomposition. The GAD plasma exhibits typical nonthermal plasma properties with an electron temperature of 1­1.5 eV and gas temperature of 300­ 3000 K,10 while keeping an electron density of up to 1017 cm¹3.5 In addition, compared to other nonthermal plasmas (e.g. DBD and microwave discharge), the GAD reactors operating at atmospheric pressure and ambient temperature have a quite compact and simple design that consists of only two divergent electrodes.5 A schematic diagram of the experimental setup is shown in Figure 1. Two semi-ellipsoidal steel electrodes (60 mm long, 30 wide for each) are fixed in an insulating bracket and symmetrically placed on both sides of a gas nozzle (1.5 mm in inner diameter). The electrodes are powered by an AC 220 V/15 kV (50 Hz) transformer. The arc is initiated at the narrowest gap between the electrodes when the applied voltage reaches the critical value of the gas breakdown, and then the formed arc is pushed by the high-speed gas flow and glides along the electrodes until it extinguishes due to the discontinuity between the arc and the surface of electrode.5 An image of the GAD plasma zone is also shown in Figure 1. Figure 2 and Figure 3 show the effect of input CH3OH concentration on the CH3OH conversion, the selectivities of H2 (S(H2)) and CO (S(CO)), H2/CO molar ratio, and the selectivities of by-products (S(CO2), S(CH4), S(C2H2), and S(C2H4)). It has been demonstrated that N2 facilitates the decomposition of fuels in nonthermal plasmas, particularly for methane and methanol.1b,11 As shown in Figure 2, decreasing input N2 concentration from 95 to 65% leads to an approximately linear

Figure 1. Schematic of the GAD-assisted methanol decomposition system.

© 2015 The Chemical Society of Japan | 1315

Figure 2. The CH3OH conversion, selectivities of H2 and CO, and H2/CO molar ratio as a function of input CH3OH concentration.

Figure 3. The selectivities towards by-products (CO2, CH4, C2H2, and C2H4) as a function of input CH3OH concentration.

Table 1. Comparison of the performance of methanol decomposition processes among different nonthermal plasmas Plasma type

Carrier gas /mL min¹1

Feed CH3OH /%

Power /W

CH3OH conversion /%

S(H2) /%

S(CO) /%

H2/CO

P(H2)a /¯mol s¹1

EY(H2)b /g kW¹1 h¹1

Ref.

MHCD Corona DBD GAD

N2, 10.5­23.7 Ar, 40 N2, 100 N2, 4500­6500

4.7­56.7 20­75 1 5­35

DC, AC, AC, AC,

7.4­47.0 10­80 8­26 44.3­87.1

55­80 ® 12­21.7 13.3­17.9

50­80 ® 11.7­20.0 17.8­22.5

1.5­2.8 ® 1.96­2.70 1.48­1.60

0.22­1.05 ® ¯0.4 69.0­208.7

C2H2 > CH4 > C2H4 µ CO2. Similar results were also obtained by Wang et al. in a microwave plasma-assisted methanol decomposition process with N2 as carrier gas.12 We can see from Figure 2 that both the selectivities of H2 and CO first slightly increase to a peak of 17.9% and 22.5% respectively at a CH3OH concentration of 15%, possibly resulting from the increased retention time of methanol in the plasma zone with decreasing N2 flow rate, and then tend to slightly decline with further rise of CH3OH concentration due to the decrease of specific energy input on reactant species. The H2/CO ratio is weakly dependent on the CH3OH concentration and varies slightly in the range of 1.45 to 1.60. It should be mentioned that, from the stoichiometry of the methanol decomposition reaction, a H2/CO ratio of 2.0 would be expected. The reduction of specific energy input should be responsible for the decrease of the selectivities towards all the byproducts: CO2, CH4, C2H2, and C2H4, as shown in Figure 3. A comparison of the performance of methanol decomposition processes among different nonthermal plasmas is shown in Table 1. We can see that, compared to other nonthermal plasmaassisted methanol decomposition technologies, the GAD plasma

1316 | Chem. Lett. 2015, 44, 1315–1317 | doi:10.1246/cl.150563

provides a significantly higher CH3OH conversion. Although H2 was produced with a relatively low selectivity in comparison to the MHCD and DBD plasmas, the processing capacity (feed flow rate), associated with the H2 production rate and energy yields of H2 in the GAD plasma are all drastically enhanced by several orders of magnitude, all of which are definitely more beneficial to industrial applications. The advantages of the GAD plasma are attributed to the significantly high electron density within the plasma. Previous studies have shown that the electron density in the GAD (1017 cm¹3) is found to be several orders of magnitude higher than that in DBD (1010­1013 cm¹3) and corona discharges (109­1013 cm¹3) due to the high power (e.g. 40 kW per electrode pair13) that the GAD plasma can obtain from the power source.5 The probable predominant chemical reactions proceed in the plasma-assisted methanol decomposition process are schematically shown in Figure 4. The initial dissociation of methanol results in the generation of CH3, CH2, CHOH, CH2OH, CH3O, OH, H, H2O, and H2, followed by complex processes of excitation, dissociation, attachment, ionization, and/or combination, leading to the formation of products.14 The CHOH + H2 channel of methanol dissociation mainly contributes to the formation of the primary gaseous products: H2 and CO. The CHOH intermediate product is very unstable in plasmas and will then readily decompose to generate H2 and CO. CO2 is generated from the reaction of CO with OH (CO + OH ¼ CO2 + H)9 or the Boudouard reaction (2CO ¼ CO2 + C).5 The CH3 + OH and 1CH2 + H2O channels of methanol dissociation are responsible for the formation of other gaseous byproducts (CH4, C2H4, and C2H2) via the hydrogenation, coupling, and dehydrogenation reactions of the CmHn species, as shown in Figure 4.

© 2015 The Chemical Society of Japan

shown significant advantages compared to other typical nonthermal plasmas in terms of processing capacity, hydrogen production rate, and hydrogen energy yields, offering a promising technology for industrial applications. This work is supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120101110099) and the Fundamental Research Funds for the Central Universities (No. 2014FZA4011). Figure 4. Possible prominent reaction channels of methanol decomposition in GAD plasma. Considering that the above-mentioned gaseous products were formed with relatively low selectivity, it is conjectured that the other two channels: CH2OH + H and CH3O + H channels probably dominate in the methanol decomposition process, giving rise to the formation of some liquid alcohols and/or ethers, such as ethanol, ethylene glycol, propanol, and dimethyl ether through the combination reactions of CH3O and CH2OH with CH3 or CH3CH2 etc.14 It should be noted that, these combination reactions are all exothermic, indicating that the formation of these liquid products is thermodynamically favorable. The mean electron energy of the GAD plasma is found to be only around 1­1.5 eV (23­34.5 kcal mol¹1),10,13,15 whereas the activation energy for the dissociation reactions of the alcohols and ethers is thought to be greater than 2.88 eV (66.43 kcal mol¹1).14 Therefore, in the GAD plasma, there are only a limited number of electrons that are energetic enough to break down the produced liquid products. As a consequence, most of the dissociated methanol molecules are converted into the liquid products, rather than H2 and CO, resulting in the low selectivities towards H2 and CO. In addition, the high speed gas flow results in a low retention time of the reactant gases in the plasma zone, and consequently reduce the contact time between the reactant species and energetic electrons, probably leading to a decrease in both the H2 and CO selectivities on the other hand. Further improvement in the energy conversion efficiency for plasma reforming in the GAD can be expected from the optimization of the reactor geometry and plasma power to achieve a higher electron energy as well as longer retention time of reactant in the plasma. For instance, by developing a rotating gliding arc discharge reactor with a 3-dimensional plasma zone, in which the plasma area is largely enlarged and the retention time of reactants is significantly improved,11,16 the efficiency of plasma chemical processes is expected to be considerably enhanced. In addition, a power source with high frequency (e.g. 30­60 kHz) is also found to significantly improve the reactor efficiency and decrease the energy consumption, as reported by some authors.17 In conclusion, this work reported an efficient method of hydrogen production from methanol decomposition using transient GAD discharge plasma. A maximum methanol conversion of 87.1% can be obtained and this method have

Chem. Lett. 2015, 44, 1315–1317 | doi:10.1246/cl.150563

Supporting Information is available electronically on J-STAGE.

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