Hydrogen production from dimethyl ether and bioethanol for fuel cell applications

Hydrogen production from dimethyl ether and bioethanol for fuel cell applications

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y 33 (2008) 3026– 3030 Available at www.sciencedirect.com jou...

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ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

33 (2008) 3026– 3030

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Hydrogen production from dimethyl ether and bioethanol for fuel cell applications Sukhe D. Badmaeva, Pavel V. Snytnikova,b, a

Boreskov Institute of Catalysis, Pr. Akademika Lavrentieva, 5, Novosibirsk, 630090, Russia Novosibirsk State University, Pirogova St., 2 Novosibirsk, 630090, Russia

b

ar t ic l e i n f o

abs tra ct

Article history:

Dimethyl ether (DME) and water–ethanol mixtures are promising and attractive sources of

Received 23 August 2007

hydrogen for fuel cell applications. Copper-cerium oxide systems have been studied as the

Received in revised form

catalysts for DME steam reforming (SR) to hydrogen-rich gas mixtures. Hydrogen and

7 February 2008

carbon dioxide were the main reaction products; CO content was insignificant. For CO

Accepted 7 February 2008

removal to 10 ppm, the level tolerated by low-temperature fuel cells in the hydrogen-rich

Available online 18 April 2008

feed gas—the reaction of preferential CO methanation in the presence of CO2 has been

Keywords: Dimethyl ether steam reforming Copper-cerium oxide Preferential (selective) CO methanation Nickel-cerium oxide

used. Nickel-cerium oxide system was used as the catalyst. SR of water–ethanol mixtures was performed using supported Rh catalyst. Reaction products of ethanol SR at 400 1C contained H2, CH4 and CO2. As the temperature increased to 600–700 1C, the CH4 concentration decreased considerably. The obtained hydrogen-rich gas mixture can be used as a feed for high-temperature solid-oxide fuel cells. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Bioethanol steam reforming

reserved.

Rh/ZrO2

1.

Introduction

Currently, fuel cells are considered as an alternative environmentally sound source of electric power. The most of fuel cells are fed by pure hydrogen or hydrogen-rich gas mixtures produced by catalytic conversion of hydrocarbons. Dimethyl ether (DME) and water–ethanol mixtures (bioethanol) are among the most promising primary sources of hydrogen. DME is synthetic, while bioethanol is a renewable material. It is interesting to demonstrate the difference (as well as possible similarity) in the reactions of DME steam reforming (SR) and ethanol SR (temperature range, product distribution). Presentation of these data in a common paper seems reasonable and important for determination of the most promising areas for fuel cells application, as well as selection of appropriate sources for hydrogen-rich gas production.

Similarly to methanol, DME converts readily and selectively to hydrogen-rich gas at low temperature (250–350 1C). Favorably compared to methanol, DME is corrosion-safe and nontoxic. Direct synthesis of DME is more cost-efficient than methanol synthesis. DME is similar to LPG by physicochemical properties and safe for transportation and storing. DME has been recognized recently as a substitute diesel fuel that will obviously stimulate respective infrastructure development. Preferably, hydrogen-rich gas is produced from DME by the reaction of catalytic SR—a two-step process including DME hydration to methanol and methanol steam reforming to hydrogen-rich gas. The bifunctional catalyst or mechanical mixtures of two catalysts—a solid-acid for DME hydration and a copper-containing oxide for methanol steam reforming—are usually used for DME SR [1,2]. In the first part of this paper we report on the performance of

Corresponding author at: Boreskov Institute of Catalysis, Pr. Akademika Lavrentieva, 5, Novosibirsk, 630090, Russia. Tel.: +7 383 330 97 89; fax: +7 383 330 80 56. E-mail address: [email protected] (P.V. Snytnikov). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.02.016

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highly active bifunctional CuO2CeO2 =g-Al2 O3 catalyst for DME SR. Prior fuelling low-temperature fuel cells, the hydrogen-rich gas produced by DME SR must be cleaned up from CO. In recent years, intensive researches on the catalyst and process for efficient CO removal from hydrogen-rich gas mixtures have been performed worldwide. Preferential CO methanation in the presence of CO2 is one of the promising methods for CO removal from hydrogen-rich gas mixtures produced by hydrocarbon conversion processes. The second part of this work presents the catalytic performance of recently developed nickel-cerium oxide systems for preferential CO methanation in the presence of CO2 with respect to CO removal to the level of 10 ppm in the hydrogen-rich gas. Production of hydrogen-rich gas from water–ethanol mixtures is actively studied now. Bioethanol is a 12–14 wt% aqueous solution of ethanol resulting from biochemical conversion of woodworking and agricultural waste products. Being a renewable resource, bioethanol compares favorably to natural gas and petroleum-refined fuels. Available literature data prove catalytic steam reforming to be the most efficient process for syngas production from bioethanol. Complicating points are endothermic character of the reaction and catalyst coking due to side-reactions. Bioethanol steam reforming is run in a reactor with supported cobalt, rhodium, nickel and copper– nickel catalysts to produce CH4, H2, CO, CO2, acetaldehyde and ethylene [3,4]. Formation of acetaldehyde and ethylene can be suppressed by selecting the proper catalyst and reaction conditions. This paper reports on the results of water–ethanol mixtures SR with highly active Rh/ZrO2-based catalyst.

All catalytic experiments were performed in a quartz flow reactor at atmospheric pressure. The temperature was measured by means of a chromel/alumel thermocouple inserted into the center of the catalyst bed. DME SR was performed with the use of a reaction mixture of composition 20 wt% DME, 60 wt% H2O, 20 wt% N2; temperature 250–370 1C and GHSV ¼ 10 000 h1 . Bioethanol SR was performed with the use of a reaction mixture of composition 16 vol% C2H5OH, 64 vol% H2O, 20 vol% N2; temperature 250–700 1C and GHSV ¼ 2400 h1 . Note that nitrogen was used as an internal standard for chromatographic determination of the reagent and product concentrations. Prior experiments, the catalysts were reduced in a flow of 5 vol% H2 þ 95 vol% N2 at 350 1C for 1 h. The inlet and outlet concentrations in DME SR and bioethanol SR were measured by two gas chromatographs ‘‘Tsvet-5’’ (Russia) fitted with thermal conductivity detectors. The reaction of preferential methanation of CO was studied using hydrogen-rich mixture of composition 1 vol% CO, 60 vol% H2, 10 vol% H2O, 20 vol% CO2 and He-balance, temperature 200–340 1C, GHSV ¼ 40 000 h1 . No catalyst pretreatment was applied prior the experiments. The inlet and outlet concentrations of the reagents and reaction products in the experiments on preferential CO methanation in hydrogen-rich mixtures were registered by means of a ‘‘Kristall2000’’ chromatograph (Russia) equipped with thermal conductivity detector and flame-ionization detector. Detection threshold for CO, CO2, CH4 and other gaseous hydrocarbons was 1 ppm. Selectivity ðSCH4 Þ in the reaction of CO methanation in hydrogen-rich gas mixtures was calculated as

2.

SCH4 ¼

Experimental

The CuO2CeO2 =g-Al2 O3 catalysts were synthesized by treating g-alumina (SBET ¼ 200 m2 =g, Vpore ¼ 0:7 cm3 =g, diameter of granules is 0.25–0.5 mm) with solutions of copper and cerium salts taken at the given ratio. The samples were dried at 100 1C and calcined at 450 1C for 3 h. In earlier studies [5,6], DME SR over a series of catalysts containing 10–20 wt% CuO–CeO2, at the Cu/Ce weight ratio varied from 1/1 to 4/1, have been investigated. The catalyst of composition 10 wt% CuO–2.5 wt% CeO2 =g-Al2 O3 appeared to be the most active one. Further, we report results exactly for this catalyst. Note that the catalyst demonstrated stable operation during more than 100 h. The 10 wt% Ni=CeO2x catalyst was prepared by impregnation method. For this purpose, a ceria powder, produced by calcination of cerium nitrate at 400 1C in air for 2 h, was impregnated to incipient wetness by a solution of nickel salt, and dried at 110 1C in air for 2 h. Then the nickel-cerium sample was reduced in He þ H2 flow at 400 1C for 2 h. Specific surface area of Ni=CeO2x was 70 m2 =g. The obtained catalyst powders were pressed to tablets; the latter were crushed to smaller pieces. The catalyst particles of size 0.25–0.50 mm were used for catalytic experiments. The Rh/ZrO2 catalyst was prepared by impregnating zirconia with a solution of RhCl3. The impregnated sample was dried at 115 1C for 12 h and then air-calcined at 600 1C for 4 h. Supported Rh/ZrO2 catalyst contained 2 wt% Rh and has specific surface area of 68 m2 =g.

out Fin CO  FCO  100%, out FCH4

out out where Fin CO is the inlet molar flow of CO; FCO and FCH4 are the outlet molar flows of CO and CH4.

3.

Results and discussion

3.1.

DME steam reforming

Fig. 1 demonstrates the temperature dependencies of the DME conversion and of the outlet H2, CO2 and CO concentrations in DME SR over the catalyst of composition 10 wt% CuO–2.5 wt% CeO2 =g-Al2 O3 . Fig. 1 presents also the temperature dependencies of respective equilibrium values calculated in the assumption that DME hydration, methanol SR and reverse water–gas shift (RWGS) reactions proceed in the system. The main reaction products at temperatures 250–370 1C were H2, CO2 and CO. The CO concentration in the hydrogen-rich gas was considerably lower, than the calculated equilibrium value. Traces of methanol were detected in the reaction products ðo5  102 vol%Þ. The reaction of methane formation, which occurred only at temperatures 350–370 1C, was neglected. As Fig. 1 shows, the DME conversion and the outlet H2, CO2 and CO concentrations increased with increasing temperature. The DME conversion attained the equilibrium value of 100% at 350 1C. At this temperature, the maximum hydrogen

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80

60

H2

60 40

40 CO2

20

CO

0 250

290 330 Temperature, °C

20 0

370

Fig. 1 – The temperature dependencies of the DME conversion and of the outlet H2, CO2 and CO concentrations for the reaction of DME SR over the 10 wt% CuO–2.5 wt% CeO2 =c-Al2 O3 catalyst. Experimental conditions: GHSV ¼ 10 000 h1 ; inlet reaction mixture: 20 vol% DME, 60 vol% H2O, 20 vol% N2. Points—experiment, dotted lines—calculated equilibrium values.

productivity amounted to 0.61 mole H2 =gcat =h was reached. The H2 and CO2 concentrations exceeded respective equilibrium values, while the CO concentration was much below the equilibrium value. This is most likely due to the fact that CO2 is a primary product of DME SR, whereas CO is formed by reverse water–gas shift reaction which does not reach equilibrium during experiment. As the temperature increases up to 370 1C, the DME conversion and the CO2 and CO concentrations remain the same; the H2 concentration decreases below the equilibrium value due to methane formation by the reaction of carbon oxides hydrogenation. Note that the presence of both copper oxide and ceria is of key importance for getting the active and stable catalysts for DME SR. Special experiments showed that no DME SR to hydrogen-rich gas proceeds on g-Al2 O3 and CeO2 =g-Al2 O3 at temperature up to 400 1C. The CuO=g-Al2 O3 catalysts are unstable under the DME SR conditions; the hydrogen productivity of these catalysts decreases with time and becomes half as much just in 3 h on-stream. As suggested earlier [6], the activity and stability of bifunctional CuO2CeO2 =g-Al2 O3 catalyst for DME SR are most likely associated with the g-Al2 O3 acidic centers and with the formation of an oxide copper–cerium solid solution, which are responsible for, respectively, DME hydration reaction and methanol SR reaction. Essential advantage is that DME SR over these bifunctional catalysts produces hydrogen-rich gas containing p1 vol% CO, that can be used directly for fuelling, for example, high-temperature polymer electrolyte membrane fuel cells (HT PEM FC) which tolerate up to 5 vol% of CO in the feed gas. For fuelling low-temperature PEM FC, it is needed only to clean up the hydrogen-rich gas from CO to the level of 10 ppm. Since no individual WGS stage, that is necessary in most other hydrogen production processes, is involved here, the miniaturization of the fuel processors for hydrogen production by DME SR is quite feasible.

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Preferential CO methanation

We have found recently that some ceria-supported transition metals are highly active for preferential CO methanation at temperatures above 200 1C. The Ni/CeO2 catalysts were found to be active and highly selective for the reaction of CO methanation in CO2 excess. Fig. 2 illustrates the data for catalyst 10 wt% Ni/CeO2. Clearly, the outlet CO concentration o10 ppm is reached at temperature 260–320 1C and selectivity 50290%. As the temperature increases further, the selectivity gradually decreases due to progressing CO2 methanation reaction. Note that the catalyst required no pretreatment procedures. Exposed to air after experiments, the nickel–ceria system was non-pyrophoric and kept activity—no decrease in catalyst activity and selectivity was observed in subsequent experiments. The results obtained prove that Ni/CeO2 is active and selective catalyst for the reaction of CO methanation in hydrogen-rich gas mixtures. It allows reducing CO concentration to o10 ppm within a wide temperature interval at high space velocity and selectivity X50%.

10000 1000 100 10 1

200

240

320 280 Temperature, °C

360

100

3

80 2 60 1

40

CH4 concentration, vol. %

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3.2.

СО concentration, ppm

100

Selectivity, %

100

DME conversion, %

Concentration, vol.%

3028

0

20 200

240

280 320 Temperature, °C

360

Fig. 2 – Temperature dependencies of the outlet CO and CH4 concentrations and selectivity for the reaction of selective CO methanation in H2 excess over catalyst 10 wt% Ni/CeO2. Experimental conditions: inlet reaction mixture: 1 vol% CO, 60 vol% H2, 10 vol% H2O, 20 vol% CO2 and He-balance. GHSV ¼ 40 000 h1 .

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So, preferential CO methanation seems to be quite promising for efficient CO removal from hydrogen-rich gas mixtures containing CO2. Combination of DME SR and preferential CO methanation shows promises for the development of compact fuel processors for hydrogen production for low-temperature fuel cell applications.

The results obtained show that it is preferable to use the Rh/ZrO2 catalyst at temperatures above 600–700 1C. The obtained hydrogen-rich gas mixture can be used as a lowNOx-emission fuel for gas turbines, internal combustion engines and solid-oxide-fuel-cell-based power plants [7].

3.3.

4.

Bioethanol steam reforming

Fig. 3 shows the temperature dependencies of the H2, CO2, CO and CH4 concentrations in bioethanol steam reforming over the 2 wt% Rh/ZrO2 catalyst. Fig. 3 presents also the temperature dependencies of the equilibrium H2, CO2, CO and CH4 concentrations. The latter were calculated on the assumption that the ethanol SR, RWGS and CO methanation reactions proceed in the system: In experiment, 100% conversion of ethanol was reached on Rh/ZrO2 at 300–700 1C that agrees completely with the calculated equilibrium conversion of ethanol in this temperature range. However, as Fig. 3 shows, product distribution varies considerably with increasing temperature. At 300–400 1C, the main products of ethanol SR on Rh/ZrO2 were H2, CO2 and CH4. As the temperature increases to 700 1C, the H2 concentration significantly increases to 45 vol%, while methane concentration falls down to 1 vol%. This occurs most likely due to methane steam reforming which proceeds in the system. At a temperature of 500  C carbon monoxide appears; its concentration reaches 8–9 vol% as the temperature increases to 700 1C. As Fig. 3 shows, starting from 400 1C, the H2, CO2, CO and CH4 concentrations were similar to the calculated equilibrium values. It should be noted that Rh/ZrO2 demonstrated good stability and kept activity during 4100 h on-stream.

Conclusions

It was shown that bifunctional CuO–CeO2/Al2O3 system is novel and efficient catalyst for hydrogen production from DME. Hydrogen and carbon dioxide were the main reaction products; CO content was close to 1 vol%. Such a hydrogenrich gas mixture can be used directly for fuelling hightemperature PEMFC, which demonstrate stable operation with the feed gas containing up to 5 vol% CO. For CO removal to 10 ppm, the level tolerated by lowtemperature PEMFC in the hydrogen-rich feed gas, the reaction of preferential CO methanation in the presence of CO2 have been used. Ni=CeO2x oxide system was used as the catalyst. The results obtained prove that Ni/CeO2 is active and selective catalyst for the reaction of CO methanation in hydrogen-rich gas mixtures. It allows reducing CO concentration to o10 ppm within a wide temperature interval at a high space velocity. Preferential CO methanation is a promising method for efficient CO removal from hydrogen-rich gas mixtures containing CO2. Combination of DME SR and preferential CO methanation shows promises for the development of compact fuel processors for hydrogen production for low-temperature fuel cell applications. Steam reforming of water–ethanol mixtures was performed using Rh/ZrO2 catalyst. Reaction products of ethanol steam reforming at 400 1C contained H2, CH4 and CO2. As the temperature increased to 600–700 1C, the CH4 concentration decreased considerably. The obtained hydrogen-rich gas mixture can be used as a feed for high-temperature solidoxide fuel cells.

50 Concentration, vol.%

Acknowledgments 40

H2

We are grateful to Dr. G.G. Volkova and Dr. Yu.I. Amosov for catalyst preparation. The work is supported partially by Grants BRHE Y4-C-08-12, ‘‘Global Energy’’ Foundation.

30 20

R E F E R E N C E S

CO2

10 0

CO

CH4

400

450

500

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Temperature, °C Fig. 3 – The temperature dependence of the H2, CO2, CO and CH4 concentrations for the reaction of ethanol SR over catalyst 2 wt% Rh/ZrO2. Experimental conditions: inlet reaction mixture: 16 vol% C2H5OH, 64 vol% H2O, 20 vol% N2. GHSV ¼ 2400 h1 . Points—experiment, dotted lines—calculated equilibrium values.

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[6] Volkova GG, Badmaev SD, Belyaev VD, Plyasova LM, Budneva AA, Paukshtis EA, et al. Bifunctional catalysts for hydrogen production from dimethyl ether. Stud Sur Sci Catal 2007;167:445–50.

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[7] Galvita VV, Belyaev VD, Frumin AV, Demin AK, Tsiakaras PE, Sobyanin VA. Performance of a SOFC fed by ethanol reforming products. Solid State Ionics 2002;152:551–4.