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Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept
Energy xxx (2015) 1e7
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept L. Guerra a, J. Gomes a, b, *, J. Puna a, b, J. Rodrigues c Area Departamental de Engenharia Química, ISEL e Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal CERENA, IST e Instituto Superior T ecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c ~o C-13, 2565-641 Torres Vedras, Portugal GSyF, Pol. Ind. Alto do Ameal, Pavilha a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2014 Received in revised form 11 June 2015 Accepted 16 June 2015 Available online xxx
This paper describes preliminary work on the generation of synthesis gas from water electrolysis using graphite electrodes without the separation of the generated gases. This is an innovative process, that has no similar work been done earlier. Preliminary tests allowed to establish correlations between the applied current to the electrolyser and flow rate and composition of the generated syngas, as well as a characterisation of generated carbon nanoparticles. The obtained syngas can further be used to produce synthetic liquid fuels, for example, methane, methanol or DME (dimethyl ether) in a catalytic reactor, in further stages of a present ongoing project, using the ELECTROFUEL® concept. The main competitive advantage of this project lies in the built-in of an innovative technology product, from RE (renewable energy) power in remote locations, for example, islands, villages in mountains as an alternative for energy storage for mobility constraints. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Water electrolysis Syngas Renewable energy Electrofuel Methanol
1. Introduction Production of renewable synthetic fuels, as an alternative to oil derivative products, is a path which deserves more and more attention from the scientific community and also energy players. Nowadays, there is a considerable drive to develop new and cheaper fuels, particularly from renewable sources, that can be used in order to substitute currently used oil derived fuels for several applications, such as for electricity production from synthesis gas (syngas). This research, which is part of a broader project, aims to promote the development of syngas produced by alkaline water electrolysis [1] using carbon/graphite electrodes, without separation of produced gases. Syngas is essentially a mixture of hydrogen, carbon monoxide and carbon dioxide [2]. Syngas can be obtained from fossil fuels (steam reforming, coal gasification) [3] or renewable ones (forest wastes, biomass). It can also be produced from hydrogen obtained by water electrolysis [4]. However, the synthetic fuel produced from renewable hydrogen is still not economically competitive when compared with fossil fuels derivatives.
* Corresponding author. Area Departamental de Engenharia Química, ISEL e Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emídio Navarro, 1959007 Lisboa, Portugal. Tel.: þ351 963902456; fax: þ351 213850991. E-mail address: [email protected] (J. Gomes).
However, a recent discovery [5] revealed that simple alkaline water electrolysis using carbon/graphite electrodes, without separation of gases, can produce syngas in only one step, at an economic cost, making feasible to develop an entire chain of synthetic fuels at a price that can compete with fossil derived fuels. The main goal of the project is to test the economy of synthetic fuels produced from alkaline water electrolysis, using carbon/ graphite electrodes, and powered by renewable energies (mainly wind and solar). To achieve this purpose, a pilot unit of 1 kW generating syngas to produce methanol is under construction. This pilot unit is intended to be integrated in an off-grid module of 2 MWh, primary powered by photovoltaic energy, thus benefiting from low tariff of electricity for producing syngas. This is an innovative process, and no previous similar works have been done so far. Therefore, an explanation of the process is needed: regarding the production of syngas, oxygen is released on the anode from electrolysis, reacts directly with the carbon of the electrode producing CO/CO2, which is mixed with the hydrogen released on the cathode. The final obtained product is syngas, the key raw material to synthetic fuels production. This way, the technology is not particularly complex, as the electrolysers are cheap and quite easy to build, when compared to conventional electrolysers [6], either PEM [7] or alkaline [8,9], resulting in a cheap apparatus.
http://dx.doi.org/10.1016/j.energy.2015.06.048 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
2
L. Guerra et al. / Energy xxx (2015) 1e7
CO2 þ H2 $ CO þ H2O Nomenclature %CLa %CO %CO2 %ConvC %O2 C DME Ec EDS Ep I Qv RE TEM Ti
(5)
The process for producing syngas is less complex and can compete with conventional technologies, such as steam reforming and coal gasification. If renewable energy is used to power the electrolyser, syngas or its derivative products can be a cheap storage alternative process to manage grid intermittences and random effects induced by the source. The renewable energy can then be stored as a liquid (as diesel or gasoline) at a cost that can compete with the current commercial cost of similar oil-based products. Due to its simplicity, the process may become economically feasible in small scale units, where families will be able to satisfy their own energy needs in an off-grid solution, using renewable energies (mainly wind and solar [13]) as a primary source [14]. Energy sustainability in mobility can be foreseen with these renewable fuels, keeping the present mobile platform. They will represent an interim technology before new hydrogen related technologies (fuel cells and storage) reach maturity [15]. Fig. 1 shows the overall concept of ELECTROFUEL®.
percentage of carbon lost in the anode composition of carbon monoxide on the generated syngas composition of carbon dioxide on the generated syngas carbon conversion into CO and CO2 composition of oxygen on the generated syngas concentration dimethyl ether consumed energy Energy Dispersive Spectroscopy electric potential in the electrolyser applied current gas flow rate renewable energy Transmission Electron Microscopy initial temperature of the electrolyte
2. Assumptions and materials
The electrochemical reactions for the syngas production are described as follows. The reaction taking place on the cathode is the one known for the conventional water electrolysis [10]: 2H2O þ 2e / H2 þ 2HO
(1)
The reactions taking place on the anode are: 2C þ O2 $ 2CO
(2)
2CO þ O2 / 2CO2
(3)
4HO / O2 þ 2H2O þ 4e
(4)
Additionally, CO2 conversion occurs on the cathode through reverse water gas shift reaction, as follows [11,12]:
Syngas production was carried out on the prototype shown in Fig. 2. It consists of four parts: the electrolyte vessel, the electrolyser, the power supply and the system to measure the flow rate of the generated syngas. The electrolyte vessel is made of polycarbonate and the electrolyser of teflon. The electrodes are graphite discs, with a diameter of 5 cm and 5 mm thickness, as shown in Fig. 3, with a total free volume for the electrolyte solution of approximately 9.8 cm3. Gas analysis was performed using an Orsat analyser (Strohlein). TEM (Transmission Electron Microscopy) analysis were made using a TEM Hitachi, model H-8100 II, equipped with an EDS (Electron Dispersive Spectroscopy) probe. 3. Methodology The electrolyte solution (sodium hydroxide or potassium hydroxide) feeds the electrolyser from the bottom, then syngas is generated and exits the electrolyser for the top. Then the generated
Fig. 1. ELECTROFUEL® concept.
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
L. Guerra et al. / Energy xxx (2015) 1e7
3
Fig. 5. EDS spectra of collected carbon particles expressed as Carbon Particle Counts (full scale counts: 24) versus signal intensity (keV).
Fig. 2. Electrolyser prototype. 2.5
A
2.0
I (A)
1.5
1.0
0.5
I (A)
0.0 0.1
0.2
0.3
0.4
0.5
0.6
C (M)
Fig. 3. Schematic of the electrolyser. 45
35
40
B
30
35
Qv (mL.min-1)
20
25 20
15 Qv (mL/min)
15
Ec (Wmin.mL-1)
25
30
10
10 Ec (Wmin/mL)
5
5 0
0 0.1
0.2
0.3
0.4
0.5
0.6
C (M)
Fig. 4. TEM image of collected carbon particles.
Fig. 6. Concentration influence on the applied current (A), gas flow rate and consumed energy (B), after a 2-h reaction.
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
L. Guerra et al. / Energy xxx (2015) 1e7
Preliminary tests, conducted on the electrolyser prototype, allowed to establish correlations between the applied current to the electrolyser and the flow rate and composition of the generated syngas. The preliminary tests also included the analysis of carbon nanoparticles resulting from the attack of the graphite electrodes, which remain in the electrolyte solution flowing from the electrolyser. These particles were analyzed by TEM (Transmission Electron Microscopy) showing central amorphous particles surrounded by nanocarbon particles, as depicted in Fig. 4. The chemical composition was analyzed by EDS (Energy Dispersive Spectroscopy), as shown in Fig. 5, showing the presence of carbon, mainly, and traces of alkaline elements such as sodium and potassium, from the electrolyte. 8
35
1.0
30 0.8
25 20
0.6
15
0.4
10 0.2
5 0
0.0 0.1
0.2
0.3
0.4
0.5
0.6
C (M)
Table 1 Concentration influence on the composition and flow rate of the generated syngas, after a 2-h reaction. C (M)
Qv (mL min1)
%CO (%)
%CO2 (%)
%O2 (%)
0.1 0.2 0.3 0.4 0.5 0.6
0.633 1.232 35.294 41.429 38.750 35.294
3.4(±1.0) 6.3(±1.0) 7.1(±1.0) 7.4(±1.0) 6.8(±1.0) 4.6(±1.0)
3.8(±1.0) 2.5(±1.0) 2.2(±1.0) 2.0(±1.0) 2.5(±1.0) 2.6(±1.0)
24.6(±1.0) 16.0(±1.0) 10.6(±1.0) 10.3(±1.0) 11.1(±1.0) 12.6(±1.0)
Data in bold correspond to the determined optimized condition.
30
7
25
6 20
5 4
15
3
%O2 (%)
%CO e %CO2 (%)
1.2
%CLa
% convc
Fig. 8. Concentration influence on the carbon conversion and percentage of carbon lost on the anode (wt%), after a 2-h reaction.
4. Results and discussion
A
40
%CLa (%)
syngas is feed to the system of measuring flow rate or to a gas analyzer, where it is possible to measure its composition. In order to optimize the syngas production process, several tests were carried out varying parameters such as the electrolyte concentration, initial temperature and electric potential applied to the eletrolyser, in three optimization steps. Therefore, it was possible to understand the system behavior, as well as the influences of the studied parameters on the flow rate and composition of the generated syngas, which are the most important outputs of the process. Other outputs were studied, such as applied current, carbon conversion and percentage of carbon lost in the anode during the reaction. The generated syngas passed into an Orsat analyzer, which was used to determine the composition of a gas mixture in terms of carbon monoxide, carbon dioxide and oxygen, with an experimental uncertainty of 1%.
% ConvC (%)
4
10
2
CO
1
CO2
5
O2
0 0.1
0.2
0.3
0.4
0.5
0 0.6
C (M) 22
Raatio H2:CO
B
20 18 16 14 12 10 0.1
0.2
0.3
0.4
0.5
0.6
C (M)
Fig. 7. Concentration influence on the composition of the generated syngas (A) and H2:CO ratio (B), after a 2-h reaction.
Fig. 9. Initial temperature influence on the applied current (A), gas flow rate and consumed energy (B), after a 2-h reaction.
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
L. Guerra et al. / Energy xxx (2015) 1e7
Fig. 10. Initial temperature influence on the composition of the generated syngas (A) and H2:CO ratio (B), after a 2-h reaction.
Further use of these particles as a source of carbon, for instance by steam reforming in the presence of an adequate catalyst, will be tested in later tests of the project. The concentration (C) influence on parameters, such as applied current (I), gas flow rate (Qv), consumed energy (Ec), composition of
5
the generated syngas (%CO, %CO2 and %O2), carbon conversion into CO and CO2 (%ConvC) and percentage of carbon lost in the anode (% CLa), was studied, as shown in Figs. 6e8. Likewise, the composition and flow rate of the generated syngas is shown in Table 1. The initial conditions used were: sodium hydroxide as electrolyte, a twographite electrode electrolyser, temperature of 25 C and an electric potential of 5 V. From Fig. 6, it can be noticed that the applied current, and also the gas flow rate, increases with the increase of sodium hydroxide concentration until 0.4 M, and then starts to decrease for higher concentrations. The consumed energy increases with the increase of sodium hydroxide concentration. Fig. 7 shows that carbon monoxide yield increases with the increase of sodium hydroxide concentration until 0.4 M, and then it starts to decrease for higher concentrations. Other outputs like the yield of oxygen and the H2:CO ratio have the inverse behavior, decreasing while concentration increases, until 0.4 M and then starts to increase. In this process, oxygen is oxidized to carbon monoxide and then to carbon dioxide. Thus, considering the reactions taking place on the anode, the complete oxidation is observed for lower flow rates. This fact is due to a higher time of contact between the bubbles of oxygen and the graphite layer from the electrode. For higher flow rate a higher applied current is achieved, as well as a higher yield of carbon monoxide and a lower yield of oxygen. From Fig. 8 it can be noticed that carbon conversion, and also the percentage of carbon lost on the anode, increases with the increase of sodium hydroxide concentration until 0.4 M, then it starts to decrease for higher concentrations. Although a higher carbon loss on the anode was achieved, carbon conversion, applied current, gas flow rate and yield of carbon monoxide were maximized and the presence of oxygen was minimized. One of the goals in the optimization is, in fact, the minimization of oxygen content on the gas, as its presence deactivates the catalysts used on methanol production process. So, for this reason the optimization was made considering a concentration of sodium hydroxide of 0.4 M. On a the second stage, the initial temperature of the electrolyte (Ti) and its influence on the applied current, gas flow rate,
1.6 41
% convc %CLa
1.4 1.2
%PMa (%)
1.0
31 0.8
%CLa (%)
% Convc (%)
36
0.6
26
0.4 21 0.2 16
0.0 25
30
35
40
45
50
Ti (ºC) Fig. 11. Initial temperature on the carbon conversion and percentage of carbon lost on the anode (wt. %), after a 2-h reaction.
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
6
L. Guerra et al. / Energy xxx (2015) 1e7
Table 2 Initial temperature influence on the composition and flow rate of the generated syngas, after a 2-h reaction. Ti ( C)
Qv (mL min1)
%CO (%)
%CO2 (%)
%O2 (%)
25 30 35 40 45 50
41.429 42.857 44.505 44.505 44.505 44.505
7.4(±1.0) 7.5(±1.0) 7.7(±1.0) 7.5(±1.0) 7.4(±1.0) 7.2(±1.0)
2.0(±1.0) 2.0(±1.0) 2.0(±1.0) 2.0(±1.0) 2.0(±1.0) 2.0(±1.0)
10.3(±1.0) 9.2(±1.0) 8.9(±1.0) 10.0(±1.0) 11.7(±1.0) 12.3(±1.0)
Data in bold correspond to the determined optimized condition.
consumed energy and composition of the generated syngas was studied, as shown in Figs. 9e11. Likewise, the composition and flow rate of the generated syngas is shown in Table 2. In this case, the initial conditions used were the same shown before and with the use of sodium hydroxide in a concentration of 0.4 M. From Fig. 9 it can be noticed that the applied current increases with the increase of initial temperature, showing signs of stability for higher temperatures. Gas flow rate increases with the increase of the initial temperature until stability at 35 C. Also, the consumed energy increases with the increase of initial temperature. Fig. 10 shows that carbon monoxide yield increases with the increase of initial temperature until 35 C and then starts to decrease for higher temperatures. Other outputs like the yield of oxygen and H2:CO ratio have the inverse behavior, decreasing while Fig. 13. Electric potential influence on the composition of the generated syngas (A) and H2:CO ratio (B), after a 2-h reaction.
temperature increases, until 35 C and then started to increase. Considering the reactions occurred on the anode, the partial oxidation is favored at an initial temperature of 35 C. As shown in Fig. 11, carbon conversion, and also the percentage of carbon lost on the anode, increases with the increase of initial temperature until 35 C, and then starts to decrease for higher temperatures. Although, a higher carbon loss on the anode was achieved, carbon conversion, gas flow rate and yield of carbon monoxide were maximized and the content of oxygen was minimized. Therefore, the optimum initial temperature is 35 C. On a third stage, the electric potential in the electrolyser (Ep) and its influence on the applied current, gas flow rate, consumed
Fig. 12. Electric potential influence on the applied current (A), gas flow rate and consumed energy (B), after a 2-h reaction.
Fig. 14. Electric potential on the carbon conversion and percentage of carbon lost (wt. %) on the anode, after a 2-h reaction.
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
L. Guerra et al. / Energy xxx (2015) 1e7 Table 3 Electric potential influence on the composition and flow rate of the generated syngas, after a 2-h reaction. Ep (V)
Qv (mL min1)
%CO (%)
%CO2 (%)
%O2 (%)
3 4 5 6 7 8
4.580 21.429 44.505 46.154 50.000 52.273
3.8(±1.0) 6.0(±1.0) 7.7(±1.0) 6.8(±1.0) 5.7(±1.0) 4.9(±1.0)
2.3(±1.0) 2.2(±1.0) 2.0(±1.0) 2.2(±1.0) 2.3(±1.0) 2.6(±1.0)
12.3(±1.0) 9.8(±1.0) 8.9(±1.0) 10.0(±1.0) 10.8(±1.0) 11.4(±1.0)
7
respectively, and a H2:CO ratio of 10.6. For an applied current of 2.45 A, a gas flow rate of 44.5 mL min1 was achieved, resulting in an energy consumption of 27.6 Wmin mL1. These results were obtained for a sodium hydroxide concentration of 0.4 M, an initial temperature of 35 C and an electric potential of 5 V, using a twographite electrode electrolyser [16].
Acknowledgements
Data in bold correspond to the determined optimized condition.
energy and composition of the generated syngas were studied, as shown in Figs. 12e14. Likewise, the composition and flow rate of the generated syngas is shown in Table 3. In this case, the initial conditions used were the same shown before and with the use of an initial temperature of 35 C. Fig. 12 shows that the applied current increases with the increase of electric potential, showing signs of stability for higher potentials. Gas flow rate and consumed energy increases with the increase of electric potential. As shown in Fig. 13, carbon monoxide yield increases with the increase of electric potential until 5 V and then starts to decrease for higher potentials. Other outputs like yield of oxygen and H2:CO ratio have the inverse behavior, decreasing while potential increases, until 5 V and then started to increase. Considering the reactions occurred on the anode, the partial oxidation is favored at an electric potential of 5 V. Fig. 14 shows that carbon conversion and percentage of carbon lost on the anode increases with the increase of electric potential until 5 V and then starts to decrease for higher potentials. Although a higher carbon loss on the anode, the carbon conversion and yield of carbon monoxide was maximized and the oxygen content was minimized. 5. Conclusions Preliminary tests were conducted on the electrolyser prototype and were able to establish correlations between applied current to the electrolyser, gas flow rate and composition of the generated syngas, as well as carbon conversion, percentage of carbon loss on the anode and a characterisation of generated carbon nanoparticles spread in the electrolyte solution. Electrolyte concentration and electric potential were found to be the factors that have major influence on the composition of the generated syngas. Interestingly enough, the temperature seems to have almost no influence on the syngas composition. A maximum carbon monoxide yield of 7.7(±1.0)%, a yield of carbon dioxide and oxygen of 2.0(±1.0) and 8.9(±1.0) %,
The authors acknowledge the support of COMPETE through ~o de Metanol da Eletro lise da Project N 38940 e SyM, Produça trodos de Grafite. Agua, usando Ele
References [1] Stoji c DL, Mar ceta MP, Sovílj SP, Miljani c SS. Hydrogen generation from water electrolysis e possibilities of energy saving. J Power Sources 2003;118:315e9. [2] Wilhelm D, Simbeck D, Karp A, Dickenson R. Syngas production for gas-toliquids applications: technologies, issues and outlook. Fuel Process Technol 2001;71:139e48. [3] Moulijn JA, Makkee M, Van Diepen A. Chemical process technology. England: Wiley; 2001. [4] Kato T, Kubota M, Kobayashi N, Suzuoki Y. Effective utilization of by-product oxygen from electrolysis hydrogen production. Energy 2005;30:2580e95. s de síntese por [5] Rodrigues J. Portuguese Patent 106779 T: Obtenç~ ao de ga lise alcalina da a gua, 02/13/2013 (in Portuguese). eletro [6] Kreuter W, Hofmann H. Electrolysis: the important energy transformer in a world of sustainable energy. Int J Hydrogen Energy 1998;23:661e6. [7] Degiorgis L, Santarelli M, Calì M. Hydrogen from renewable energy: a pilot plant for thermal production and mobility. J Power Sources 2007;171:237e46. [8] Stucki S, Scherer G, Schlagowski S, Fischer E. PEM water electrolysers: evidence for membrane failure in 100 kW demonstration plants. J Appl Electrochem 1998;28:1041e9. [9] Holladay JD, Hu J, King DL, Wang Y. An overview of hydrogen production technologies. Catal Today 2009;139:244e60. [10] Ni M. Na electrochemical model for syngas production by co-electrolysis of H2O and CO2. J Power Sources 2012;202:209e16. [11] Redissi Y, Bouallou C. Valorization of carbon dioxide by co-electrolysis of CO2/ H2O at high temperature for syngas production. Energy Procedia 2013;37: 6667e78. [12] Becker WL, Braun RJ, Penev M, Melaina M. Production of Fischer-Tropsch liquid fuels from high temperature solid oxide co-electrolysis units. Energy 2012;47:99e115. [13] Dutton A, Bleijs J, Dienhart H, Falchetta M, Hug W, Prischich D, et al. Experience in the design, sizing, economics, and implementation of autonomous wind-powered hydrogen production systems. Int J Hydrogen Energy 2000;25: 705e22. [14] Glockner R, Kloed C, Nyhammer F, Ulleberg O. Wind/hydrogen systems for remote areas. A Norwegian case study. In: 14th World hydrogen energy conference, Montreal, 9e13 June 2002. WHEC; 2002. [15] Koroneos C, Dompros A, Roumbas G, Moussiopoulos N. Life cycle assessment of hydrogen fuel production processes. Int J Hydrogen Energy 2004;29: 1443e50. lise de ~o de g [16] Guerra L. Electro agua para produça as de síntese. MSc Thesis in Chemical Engineering. Lisboa: ISEL; 2015 (in Portuguese).
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept L. Guerra a, J. Gomes a, b, *, J. Puna a, b, J. Rodrigues c Area Departamental de Engenharia Química, ISEL e Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal CERENA, IST e Instituto Superior T ecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c ~o C-13, 2565-641 Torres Vedras, Portugal GSyF, Pol. Ind. Alto do Ameal, Pavilha a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2014 Received in revised form 11 June 2015 Accepted 16 June 2015 Available online xxx
This paper describes preliminary work on the generation of synthesis gas from water electrolysis using graphite electrodes without the separation of the generated gases. This is an innovative process, that has no similar work been done earlier. Preliminary tests allowed to establish correlations between the applied current to the electrolyser and flow rate and composition of the generated syngas, as well as a characterisation of generated carbon nanoparticles. The obtained syngas can further be used to produce synthetic liquid fuels, for example, methane, methanol or DME (dimethyl ether) in a catalytic reactor, in further stages of a present ongoing project, using the ELECTROFUEL® concept. The main competitive advantage of this project lies in the built-in of an innovative technology product, from RE (renewable energy) power in remote locations, for example, islands, villages in mountains as an alternative for energy storage for mobility constraints. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Water electrolysis Syngas Renewable energy Electrofuel Methanol
1. Introduction Production of renewable synthetic fuels, as an alternative to oil derivative products, is a path which deserves more and more attention from the scientific community and also energy players. Nowadays, there is a considerable drive to develop new and cheaper fuels, particularly from renewable sources, that can be used in order to substitute currently used oil derived fuels for several applications, such as for electricity production from synthesis gas (syngas). This research, which is part of a broader project, aims to promote the development of syngas produced by alkaline water electrolysis [1] using carbon/graphite electrodes, without separation of produced gases. Syngas is essentially a mixture of hydrogen, carbon monoxide and carbon dioxide [2]. Syngas can be obtained from fossil fuels (steam reforming, coal gasification) [3] or renewable ones (forest wastes, biomass). It can also be produced from hydrogen obtained by water electrolysis [4]. However, the synthetic fuel produced from renewable hydrogen is still not economically competitive when compared with fossil fuels derivatives.
* Corresponding author. Area Departamental de Engenharia Química, ISEL e Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emídio Navarro, 1959007 Lisboa, Portugal. Tel.: þ351 963902456; fax: þ351 213850991. E-mail address: [email protected] (J. Gomes).
However, a recent discovery [5] revealed that simple alkaline water electrolysis using carbon/graphite electrodes, without separation of gases, can produce syngas in only one step, at an economic cost, making feasible to develop an entire chain of synthetic fuels at a price that can compete with fossil derived fuels. The main goal of the project is to test the economy of synthetic fuels produced from alkaline water electrolysis, using carbon/ graphite electrodes, and powered by renewable energies (mainly wind and solar). To achieve this purpose, a pilot unit of 1 kW generating syngas to produce methanol is under construction. This pilot unit is intended to be integrated in an off-grid module of 2 MWh, primary powered by photovoltaic energy, thus benefiting from low tariff of electricity for producing syngas. This is an innovative process, and no previous similar works have been done so far. Therefore, an explanation of the process is needed: regarding the production of syngas, oxygen is released on the anode from electrolysis, reacts directly with the carbon of the electrode producing CO/CO2, which is mixed with the hydrogen released on the cathode. The final obtained product is syngas, the key raw material to synthetic fuels production. This way, the technology is not particularly complex, as the electrolysers are cheap and quite easy to build, when compared to conventional electrolysers [6], either PEM [7] or alkaline [8,9], resulting in a cheap apparatus.
http://dx.doi.org/10.1016/j.energy.2015.06.048 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
2
L. Guerra et al. / Energy xxx (2015) 1e7
CO2 þ H2 $ CO þ H2O Nomenclature %CLa %CO %CO2 %ConvC %O2 C DME Ec EDS Ep I Qv RE TEM Ti
(5)
The process for producing syngas is less complex and can compete with conventional technologies, such as steam reforming and coal gasification. If renewable energy is used to power the electrolyser, syngas or its derivative products can be a cheap storage alternative process to manage grid intermittences and random effects induced by the source. The renewable energy can then be stored as a liquid (as diesel or gasoline) at a cost that can compete with the current commercial cost of similar oil-based products. Due to its simplicity, the process may become economically feasible in small scale units, where families will be able to satisfy their own energy needs in an off-grid solution, using renewable energies (mainly wind and solar [13]) as a primary source [14]. Energy sustainability in mobility can be foreseen with these renewable fuels, keeping the present mobile platform. They will represent an interim technology before new hydrogen related technologies (fuel cells and storage) reach maturity [15]. Fig. 1 shows the overall concept of ELECTROFUEL®.
percentage of carbon lost in the anode composition of carbon monoxide on the generated syngas composition of carbon dioxide on the generated syngas carbon conversion into CO and CO2 composition of oxygen on the generated syngas concentration dimethyl ether consumed energy Energy Dispersive Spectroscopy electric potential in the electrolyser applied current gas flow rate renewable energy Transmission Electron Microscopy initial temperature of the electrolyte
2. Assumptions and materials
The electrochemical reactions for the syngas production are described as follows. The reaction taking place on the cathode is the one known for the conventional water electrolysis [10]: 2H2O þ 2e / H2 þ 2HO
(1)
The reactions taking place on the anode are: 2C þ O2 $ 2CO
(2)
2CO þ O2 / 2CO2
(3)
4HO / O2 þ 2H2O þ 4e
(4)
Additionally, CO2 conversion occurs on the cathode through reverse water gas shift reaction, as follows [11,12]:
Syngas production was carried out on the prototype shown in Fig. 2. It consists of four parts: the electrolyte vessel, the electrolyser, the power supply and the system to measure the flow rate of the generated syngas. The electrolyte vessel is made of polycarbonate and the electrolyser of teflon. The electrodes are graphite discs, with a diameter of 5 cm and 5 mm thickness, as shown in Fig. 3, with a total free volume for the electrolyte solution of approximately 9.8 cm3. Gas analysis was performed using an Orsat analyser (Strohlein). TEM (Transmission Electron Microscopy) analysis were made using a TEM Hitachi, model H-8100 II, equipped with an EDS (Electron Dispersive Spectroscopy) probe. 3. Methodology The electrolyte solution (sodium hydroxide or potassium hydroxide) feeds the electrolyser from the bottom, then syngas is generated and exits the electrolyser for the top. Then the generated
Fig. 1. ELECTROFUEL® concept.
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L. Guerra et al. / Energy xxx (2015) 1e7
3
Fig. 5. EDS spectra of collected carbon particles expressed as Carbon Particle Counts (full scale counts: 24) versus signal intensity (keV).
Fig. 2. Electrolyser prototype. 2.5
A
2.0
I (A)
1.5
1.0
0.5
I (A)
0.0 0.1
0.2
0.3
0.4
0.5
0.6
C (M)
Fig. 3. Schematic of the electrolyser. 45
35
40
B
30
35
Qv (mL.min-1)
20
25 20
15 Qv (mL/min)
15
Ec (Wmin.mL-1)
25
30
10
10 Ec (Wmin/mL)
5
5 0
0 0.1
0.2
0.3
0.4
0.5
0.6
C (M)
Fig. 4. TEM image of collected carbon particles.
Fig. 6. Concentration influence on the applied current (A), gas flow rate and consumed energy (B), after a 2-h reaction.
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Preliminary tests, conducted on the electrolyser prototype, allowed to establish correlations between the applied current to the electrolyser and the flow rate and composition of the generated syngas. The preliminary tests also included the analysis of carbon nanoparticles resulting from the attack of the graphite electrodes, which remain in the electrolyte solution flowing from the electrolyser. These particles were analyzed by TEM (Transmission Electron Microscopy) showing central amorphous particles surrounded by nanocarbon particles, as depicted in Fig. 4. The chemical composition was analyzed by EDS (Energy Dispersive Spectroscopy), as shown in Fig. 5, showing the presence of carbon, mainly, and traces of alkaline elements such as sodium and potassium, from the electrolyte. 8
35
1.0
30 0.8
25 20
0.6
15
0.4
10 0.2
5 0
0.0 0.1
0.2
0.3
0.4
0.5
0.6
C (M)
Table 1 Concentration influence on the composition and flow rate of the generated syngas, after a 2-h reaction. C (M)
Qv (mL min1)
%CO (%)
%CO2 (%)
%O2 (%)
0.1 0.2 0.3 0.4 0.5 0.6
0.633 1.232 35.294 41.429 38.750 35.294
3.4(±1.0) 6.3(±1.0) 7.1(±1.0) 7.4(±1.0) 6.8(±1.0) 4.6(±1.0)
3.8(±1.0) 2.5(±1.0) 2.2(±1.0) 2.0(±1.0) 2.5(±1.0) 2.6(±1.0)
24.6(±1.0) 16.0(±1.0) 10.6(±1.0) 10.3(±1.0) 11.1(±1.0) 12.6(±1.0)
Data in bold correspond to the determined optimized condition.
30
7
25
6 20
5 4
15
3
%O2 (%)
%CO e %CO2 (%)
1.2
%CLa
% convc
Fig. 8. Concentration influence on the carbon conversion and percentage of carbon lost on the anode (wt%), after a 2-h reaction.
4. Results and discussion
A
40
%CLa (%)
syngas is feed to the system of measuring flow rate or to a gas analyzer, where it is possible to measure its composition. In order to optimize the syngas production process, several tests were carried out varying parameters such as the electrolyte concentration, initial temperature and electric potential applied to the eletrolyser, in three optimization steps. Therefore, it was possible to understand the system behavior, as well as the influences of the studied parameters on the flow rate and composition of the generated syngas, which are the most important outputs of the process. Other outputs were studied, such as applied current, carbon conversion and percentage of carbon lost in the anode during the reaction. The generated syngas passed into an Orsat analyzer, which was used to determine the composition of a gas mixture in terms of carbon monoxide, carbon dioxide and oxygen, with an experimental uncertainty of 1%.
% ConvC (%)
4
10
2
CO
1
CO2
5
O2
0 0.1
0.2
0.3
0.4
0.5
0 0.6
C (M) 22
Raatio H2:CO
B
20 18 16 14 12 10 0.1
0.2
0.3
0.4
0.5
0.6
C (M)
Fig. 7. Concentration influence on the composition of the generated syngas (A) and H2:CO ratio (B), after a 2-h reaction.
Fig. 9. Initial temperature influence on the applied current (A), gas flow rate and consumed energy (B), after a 2-h reaction.
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L. Guerra et al. / Energy xxx (2015) 1e7
Fig. 10. Initial temperature influence on the composition of the generated syngas (A) and H2:CO ratio (B), after a 2-h reaction.
Further use of these particles as a source of carbon, for instance by steam reforming in the presence of an adequate catalyst, will be tested in later tests of the project. The concentration (C) influence on parameters, such as applied current (I), gas flow rate (Qv), consumed energy (Ec), composition of
5
the generated syngas (%CO, %CO2 and %O2), carbon conversion into CO and CO2 (%ConvC) and percentage of carbon lost in the anode (% CLa), was studied, as shown in Figs. 6e8. Likewise, the composition and flow rate of the generated syngas is shown in Table 1. The initial conditions used were: sodium hydroxide as electrolyte, a twographite electrode electrolyser, temperature of 25 C and an electric potential of 5 V. From Fig. 6, it can be noticed that the applied current, and also the gas flow rate, increases with the increase of sodium hydroxide concentration until 0.4 M, and then starts to decrease for higher concentrations. The consumed energy increases with the increase of sodium hydroxide concentration. Fig. 7 shows that carbon monoxide yield increases with the increase of sodium hydroxide concentration until 0.4 M, and then it starts to decrease for higher concentrations. Other outputs like the yield of oxygen and the H2:CO ratio have the inverse behavior, decreasing while concentration increases, until 0.4 M and then starts to increase. In this process, oxygen is oxidized to carbon monoxide and then to carbon dioxide. Thus, considering the reactions taking place on the anode, the complete oxidation is observed for lower flow rates. This fact is due to a higher time of contact between the bubbles of oxygen and the graphite layer from the electrode. For higher flow rate a higher applied current is achieved, as well as a higher yield of carbon monoxide and a lower yield of oxygen. From Fig. 8 it can be noticed that carbon conversion, and also the percentage of carbon lost on the anode, increases with the increase of sodium hydroxide concentration until 0.4 M, then it starts to decrease for higher concentrations. Although a higher carbon loss on the anode was achieved, carbon conversion, applied current, gas flow rate and yield of carbon monoxide were maximized and the presence of oxygen was minimized. One of the goals in the optimization is, in fact, the minimization of oxygen content on the gas, as its presence deactivates the catalysts used on methanol production process. So, for this reason the optimization was made considering a concentration of sodium hydroxide of 0.4 M. On a the second stage, the initial temperature of the electrolyte (Ti) and its influence on the applied current, gas flow rate,
1.6 41
% convc %CLa
1.4 1.2
%PMa (%)
1.0
31 0.8
%CLa (%)
% Convc (%)
36
0.6
26
0.4 21 0.2 16
0.0 25
30
35
40
45
50
Ti (ºC) Fig. 11. Initial temperature on the carbon conversion and percentage of carbon lost on the anode (wt. %), after a 2-h reaction.
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6
L. Guerra et al. / Energy xxx (2015) 1e7
Table 2 Initial temperature influence on the composition and flow rate of the generated syngas, after a 2-h reaction. Ti ( C)
Qv (mL min1)
%CO (%)
%CO2 (%)
%O2 (%)
25 30 35 40 45 50
41.429 42.857 44.505 44.505 44.505 44.505
7.4(±1.0) 7.5(±1.0) 7.7(±1.0) 7.5(±1.0) 7.4(±1.0) 7.2(±1.0)
2.0(±1.0) 2.0(±1.0) 2.0(±1.0) 2.0(±1.0) 2.0(±1.0) 2.0(±1.0)
10.3(±1.0) 9.2(±1.0) 8.9(±1.0) 10.0(±1.0) 11.7(±1.0) 12.3(±1.0)
Data in bold correspond to the determined optimized condition.
consumed energy and composition of the generated syngas was studied, as shown in Figs. 9e11. Likewise, the composition and flow rate of the generated syngas is shown in Table 2. In this case, the initial conditions used were the same shown before and with the use of sodium hydroxide in a concentration of 0.4 M. From Fig. 9 it can be noticed that the applied current increases with the increase of initial temperature, showing signs of stability for higher temperatures. Gas flow rate increases with the increase of the initial temperature until stability at 35 C. Also, the consumed energy increases with the increase of initial temperature. Fig. 10 shows that carbon monoxide yield increases with the increase of initial temperature until 35 C and then starts to decrease for higher temperatures. Other outputs like the yield of oxygen and H2:CO ratio have the inverse behavior, decreasing while Fig. 13. Electric potential influence on the composition of the generated syngas (A) and H2:CO ratio (B), after a 2-h reaction.
temperature increases, until 35 C and then started to increase. Considering the reactions occurred on the anode, the partial oxidation is favored at an initial temperature of 35 C. As shown in Fig. 11, carbon conversion, and also the percentage of carbon lost on the anode, increases with the increase of initial temperature until 35 C, and then starts to decrease for higher temperatures. Although, a higher carbon loss on the anode was achieved, carbon conversion, gas flow rate and yield of carbon monoxide were maximized and the content of oxygen was minimized. Therefore, the optimum initial temperature is 35 C. On a third stage, the electric potential in the electrolyser (Ep) and its influence on the applied current, gas flow rate, consumed
Fig. 12. Electric potential influence on the applied current (A), gas flow rate and consumed energy (B), after a 2-h reaction.
Fig. 14. Electric potential on the carbon conversion and percentage of carbon lost (wt. %) on the anode, after a 2-h reaction.
Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048
L. Guerra et al. / Energy xxx (2015) 1e7 Table 3 Electric potential influence on the composition and flow rate of the generated syngas, after a 2-h reaction. Ep (V)
Qv (mL min1)
%CO (%)
%CO2 (%)
%O2 (%)
3 4 5 6 7 8
4.580 21.429 44.505 46.154 50.000 52.273
3.8(±1.0) 6.0(±1.0) 7.7(±1.0) 6.8(±1.0) 5.7(±1.0) 4.9(±1.0)
2.3(±1.0) 2.2(±1.0) 2.0(±1.0) 2.2(±1.0) 2.3(±1.0) 2.6(±1.0)
12.3(±1.0) 9.8(±1.0) 8.9(±1.0) 10.0(±1.0) 10.8(±1.0) 11.4(±1.0)
7
respectively, and a H2:CO ratio of 10.6. For an applied current of 2.45 A, a gas flow rate of 44.5 mL min1 was achieved, resulting in an energy consumption of 27.6 Wmin mL1. These results were obtained for a sodium hydroxide concentration of 0.4 M, an initial temperature of 35 C and an electric potential of 5 V, using a twographite electrode electrolyser [16].
Acknowledgements
Data in bold correspond to the determined optimized condition.
energy and composition of the generated syngas were studied, as shown in Figs. 12e14. Likewise, the composition and flow rate of the generated syngas is shown in Table 3. In this case, the initial conditions used were the same shown before and with the use of an initial temperature of 35 C. Fig. 12 shows that the applied current increases with the increase of electric potential, showing signs of stability for higher potentials. Gas flow rate and consumed energy increases with the increase of electric potential. As shown in Fig. 13, carbon monoxide yield increases with the increase of electric potential until 5 V and then starts to decrease for higher potentials. Other outputs like yield of oxygen and H2:CO ratio have the inverse behavior, decreasing while potential increases, until 5 V and then started to increase. Considering the reactions occurred on the anode, the partial oxidation is favored at an electric potential of 5 V. Fig. 14 shows that carbon conversion and percentage of carbon lost on the anode increases with the increase of electric potential until 5 V and then starts to decrease for higher potentials. Although a higher carbon loss on the anode, the carbon conversion and yield of carbon monoxide was maximized and the oxygen content was minimized. 5. Conclusions Preliminary tests were conducted on the electrolyser prototype and were able to establish correlations between applied current to the electrolyser, gas flow rate and composition of the generated syngas, as well as carbon conversion, percentage of carbon loss on the anode and a characterisation of generated carbon nanoparticles spread in the electrolyte solution. Electrolyte concentration and electric potential were found to be the factors that have major influence on the composition of the generated syngas. Interestingly enough, the temperature seems to have almost no influence on the syngas composition. A maximum carbon monoxide yield of 7.7(±1.0)%, a yield of carbon dioxide and oxygen of 2.0(±1.0) and 8.9(±1.0) %,
The authors acknowledge the support of COMPETE through ~o de Metanol da Eletro lise da Project N 38940 e SyM, Produça trodos de Grafite. Agua, usando Ele
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Please cite this article in press as: Guerra L, et al., Preliminary study of synthesis gas production from water electrolysis, using the ELECTROFUEL® concept, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.048