The Potential of the Solid Oxide Electrolyser for the Production of Synthetic Fuels

8/3/2019 The Potential of the Solid Oxide Electrolyser for the Production of Synthetic Fuels

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THE POTENTIAL OF THE SOLID OXIDE ELECTROLYSER FOR THEPRODUCTION OF SYNTHETIC FUELS

S. H. JENSEN, A. HAUCH, and M. MOGENSENMaterials Research Department, Ris National Laboratory, DK-4000 Roskilde, Denmark

Improper energy storage systems as well as inefficient conversion technologies are major barriers for a wider application of renewable energy such as wind, photovoltaic andhydropower. Reversible Solid Oxide Cells (SOC) that can be used both as Solid Oxide FuelCells (SOFC) and as Solid Oxide Electrolyser Cells (SOEC), have the potential to become acost effective way to solve the conversion problem: SOEC can split H 2O into H2 and O2.When need is H2 can be utilized in a SOFC with high efficiency. SOEC has also the potentialof splitting carbon dioxide into carbon monoxide and oxygen. Hence electrolysis using aSOEC on a mixture of steam and CO2 produces syngas, which can be used for synthetic fuel

production. Here estimations on production price of H2, CH4 using high temperatureelectrolysis of steam and CO2 are investigated.

1. Introduction.The principle of a SOC is sketched in figure 1. The cell basically consists of three differentlayers. The middle layer (white) is an oxide ion-conducting electrolyte that is gastight. The

topmost layer is the positive electrode and the down most is the negative electrode. Theelectrodes are porous, electron and oxide ion conducting in order to get the gasses into thereaction sites and to get a high three phase boundary where the three species (gas molecules,oxide ions and electrons) can meet and react. A cell voltage is established over the electrodeswhen gasses with different oxygen partial pressures are fed to the electrodes as described by

Nernst equation.1

2H2

O2

2O2-

2H2O

2e-

+

-2H2

O2 +

-

2O2-

2H2O

2e-

Figure 1: Working principles of a Solid Oxide Cell (SOC).

Figure 1a shows SOC in fuel cell mode, where H2 is fed to the cell and reacts with oxide ionsto form H2O. The reaction continues as long as electrons are allowed to pass through the light

bulb in the external circuit and gasses are fed to the electrodes. Figure 1b shows SOC in

electrolysis mode where the reaction is the reverse as in fuel cell mode. Here electrons are


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forced to the negative electrode by an external voltage supply, indicated as a windmill. Thisforces oxide ions (taken from H2O) to migrate through the electrolyte from the negativeelectrode to the positive.

During the last two decades intensive research in SOFC has taken place due to a potential ofhigh electricity efficiency (60%) as compared to ordinary gas turbine power plants (30%-

40%). SOEC were under development as an interesting alternative to ordinary alkalineelectrolysis during the 1980'es. The discussions focussed on the use of heat from solarconcentrators or waste heat from power stations for this purpose2. Due to the low energy pricethis development was stopped around 1990, but is now revived by the renewed interest inhydrogen in general and the higher oil prices in particular.

2. Theoretical background

An important advantage of the high working temperature of SOEC compared to that ofalkaline electrolysers is the possibility of obtaining a substantial part of the free energyrequired for water splitting in the form of high temperature heat, and thus the electrolysis is

performed with lower electricity consumption, see figure 2a.

The thermodynamical trends for steam splitting are also valid for CO2 splitting into CO andO2 as shown in figure 2b. This means that high temperature (850C -1000C) electrolysis of

steam and CO2 provides an extremely efficient way of converting electricity into synthesisgas, which may be converted into synthetic fuel using an appropriate catalyst, which may bestored in large quantities in caverns.

Besides the thermodynamic argument for high temperature electrolysis the kinetics of a SOCgets increasingly better with temperature, the internal polarisation resistance following anexponential decreasing Arrhenius expression.3 SOC kinetics for different temperatures is

presented in Figure 3.

Energy demand for H2O electrolysis

0

50

100

150

200

250

300

278 478 678 878 1078 1278 1478

Temperature (K)

Energydemand(kJ/mole)

H2

Ol

iquid

H2O gaseous

Energy demand for CO2 electrolysis

0

50

100

150

200

250

300

278 478 678 878 1078 1278 1478

Temperature (K)

Energydemand(kJ/mole)

Total energy demand

Electric energy demand

Heat demand

Figure 2: Thermodynamics of steam and carbon dioxide electrolysis. Both steam electrolysis(figure 2a) and CO2 electrolysis (figure 2b) gets more endothermic with temperature. This

provides an opportunity to utilise the inevitably produced Joule heat due to the passage of

electrical current through the cell and hence reduce the overall electricity consumption and

in turn the production price.


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Electrolyser mode

(H2O H2 + O2)

Fuel cell mode

(H2O H2 + O2)

0

200

400

600

800

1000

1200

1400

1600

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

i [A/cm2]

Celvoltage[mV]

1000 C

850 C

Figure 3: Kinetics of a SOC working as an electrolyser cell (negative current densities i) and

as a fuel cell (positive current densities i) The tested SOC is a DK-SOFC 2nd generation

cell4,5 with a working area of 16 cm2. The internal resistance (slope of the curve) is almost asgood in electrolyser mode as in fuel cell mode.

Degradation and passivation is a problem of the tested SOC in electrolysis mode so far.Another type of SOEC has proven high sustainability: (1000

oC, i = 0.3 A/cm

2for 1000 h)

without degradation.6

It should be noted that the kinetics is far slower for these cells than forthe DK-SOFC 2nd generation cell that we tested. The degradation of DK-SOFC 2nd generationcells tested in fuel cell mode is far less than in electrolysis mode. Long-term test (1 year) infuel cell mode has shown limited degradation.4 Investigations into the reason for thedegradation of the DK-cells in electrolysis mode are in progress.

2. System studies

A heat exchanger is used in order to save expenses for heating the feed gas to workingtemperature of the SOC as shown in figure 4.

Nickele-

H2+

CO

O2

O2-

H2O+

CO2

650C 400C

-+

CH4+

H2O

Heat exchanging

Heat exchangingH2O+

CO2

O2 O2

25C

25C

25C

Figure 4: Sketch of a SOEC system for CH4 production by electrolysis of steam and CO2. CO2

and H2O are fed through the heat exchanger to the cell. Here it is split into H2 and CO

(syngas) and O2. On the way out, the synthesis gas is catalysed into CH4 and H2O using a Ni-

catalyst.


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Demineralised water and CO2 are fed through the heat exchanger to the cell. Here it is splitinto H2, CO and O2 where O2 has migrated trough the gastight electrolyte. On the way out, H2and CO is catalysed into CH4 and H2O, if a Ni-catalyst is used. This exothermal process gives

off heat that is used for steam rising in the inlet part of the heat exchanger. Other types offuels can easily be made from the produced syngas, by simply inserting another catalyst.

The negative electrode of a SOC is made of Ni, which is the preferred CH4 catalyst, but at 0.1Mpa and 650 oC only small amounts of CO and H2 is catalysed into H2O and CH4.

7 Therefore

a Ni catalyst is placed downstream in the heat exchanger, where the temperature is lower. Ifthe pressure is increased, more CH4 can be produced at the negative electrode.

8 Theexothermic formation of CH4 from syngas produces heat that can be utilised within thesystem. If the heat is produced at the negative electrode, the heat can be converted into freeenergy in the form of H2 and CO. If the heat is produced downstream at the catalyst the heatcan be used for steam rising.

3. Economical assumptions

Table 1 gives the economical assumptions to make an estimate of synthetic fuel productionprice using the SOEC kinetics at 1000oC as presented in figure 3 (electrolyser mode).

Assuming CO2 is captured from air, the produced synthetic fuel will be CO2 neutral. For this

reason the CO2 cost per tonne is taken to 20 US$, since the technology for wind-scrubbingis quite new. The Gibbs free energy to capture CO2 from air is only 0.50 GJ/tonne equivalentto 1.8 US$ per tonne assuming an electricity price of 1.3 US/kWh

Table 1. Input assumptions for calculation of synthetic fuel production costs. Etn is thermo

neutral potential where the produced joule heat in the cell equals the heat consumed by the

electrolysis process and steam rising.

Fuel cell stack 2100 US$/m2 cell area

Investment cost 6300 US$/m2 cell area

Interest rate 5%

Depreciation time 10 years.

Operation time 5 years

Water cost 2.3 US$/m3

CO2 cost 20 US$/tonne

Electricity price 1.3US/kWh (3.6US$/GJ)

Cell temperature 1000 C

Cell voltage 1.48 V (Etn)

H2O utilization 71%Energy loss in Heat Exchanger 5%

4. Results of economical calculation

Using Table 1 as input for a calculation of production prices for H2 and CH4 we found: H2:4.8 US$/GJ equivalent to a crude oil price of 29 US$/barrel using the higher heating value.


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CH4: 7.8 US$/GJ equivalent to a crude oil price of 48 US$/barrel crude oil using the higherheating value.9,10

Figure 5 shows the parts of H2 and CH4 production price given the assumptions in table 1. Itis seen that electricity costs for electrolysis and steam rising is the largest part of the

production price. Hence the main dependence of production price is that of electricity price.

Note that these production prices are calculated at Etn, which means that the efficiency *

is98%for H2 production and 76% for CH4 production!

H2

2%

3%

19%66%

10%

CH4

1%1%

17%

58%

9%14%

Loss in heat exchanger

Reversed osmosis water

Investment depriciation

Electricity for electrolysis

Electricity for evaporation

CO2 capture

Figure 5: Parts of H2 and CH4 production price. It is seen that electricity costs dominates

investment cost, losses in heat exchanger and water costs.

In Figure 6 the H2 and CH4 production price dependency of the electricity price is shown.

Since the reversible SOC, can work both, as a SOEC and SOFC only small investment cost isto be included when converting the fuel back to electricity. Hence the resulting bufferedelectricity price can be estimated to 4.8 US$/GJ/0.6 = 8 US$/GJ or 2.9 US/kWh. It should be

noted that this calculation does not include costs for storage or distribution of H2, which willof course increase the buffered electricity price.

*o

25 C/

oH El = , whereElis the electric energy used to form 1 mol H2.Elis calculated as the sum of losses in

the heat exhanger, evaporation electricity and reaction electricity.


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0

20

40

60

80

100

120

140

160

0 1 2 3 4 5

Electricity cost [US/kWh]

equiv.crudeoilprice[US$/barrel]

H2

CH4H2 (low temp. electrolysis)

Figure 6: H2 and CH4 production price given as equivalent crude oil price vs. electricity

price. For comparison is presented the estimated H2 production price estimation fromalkaline electrolysis.11

5. Conclusion

If low electricity is available as for instance Norway or Iceland, there is a potential forsynthetic fuel production at prices that are comparable with todays prices for conventional

fuel. Better thermodynamical conditions due to the high working temperature gives SOEC animportant advantage compared to low temperature alkaline electrolysis. Using a catalystdownstream in the heat exchanger for CH4 production makes it possible to save expenses forsteam rising, which is a significant part of the production price.

6. Acknowledgement

Thanks to NEFP and the entire Ris National Laboratory SOFC group for assistance in thiswork.

7. Reference List

1. Atkins P.W.Physical Chemistry. Oxford University Press, Oxford (UK) (2002).

2. Erdle, E., Gross, J. & Meyringer, V. Possibilities for Hydrogen Production byCombination of a Solar Thermal Central Receiver System and High TemperatureElectrolysis of Steam. Becker, M. 2, 726-736. 1986. Heidelberg, Springer-Verlag.

Solar Thermal Central Receiver Systems, Proceedings of the Third InternationalWorkshop June 23 - 27, 1986, Konstanz, Federal Republic of Germany.

3. Hendriksen, P. V., Koch, S., Mogensen, M., Liu, Y. L. & Larsen, P. H. Breakdown oflosses in thin electrolyte SOFCs. Singhai, S. C. and Dokiva, M. 1147-1157. 2003.

Pennington, NJ, 2003, Electrochemical Society. Proceedings. 8. International


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symposium on solid oxide fuel cells (SOFC VIII); Electrochemical Society 203.Meeting, Paris (FR), 27 Apr - 2 May 2003.

4. Christiansen, N. et al. Status of SOFC development at Haldor Topse / Ris.Huijsmans, J. 34-41. 2001. Oberrohrdorf (CH), European Fuel Cell Forum.Proceedings. Vol. 1. 5. European solid oxide fuel cell forum, Lucerne (CH), 1-5 Jul

2002.

5. Hansen, K. K., Larsen, P. H., Liu, Y. L., Kindl, B. & Mogensen, M. Development ofthin-electrolyte solid oxide fuel cells. Huijsmans, J. 875-882. 2002. Oberrohrdorf(CH), European Fuel Cell Forum. Proceedings. Vol. 2. 5. European solid oxide fuelcell forum, Lucerne (CH), 1-5 Jul 2002.

6. Dnitz, W., Erdle, E. & Streicher, R. High temperature electrochemical technology forhydrogen production and power generation. 1990. Amsterdam (NL), Elsevier.

Electrochemical Hydrogen Technologies. Electrochemical production and combustion

of hydrogen. Wendt, H.

7. Rostrup, J. R. Catalytic Steam Reforming. Springer-Verlag, Berlin, Heidelberg, NewYork, Tokyo (1984).

8. Rostrup, J. R. Steam Reforming Catalysts. Danish technical Press, Copenhagen,Denmark (1975).

9. Jensen, S. H. & Mogensen, M. Perspectives of High temperature Electrolysis usingSOEC. World Energy Congress(19th.). 2004. 5th Floor, Regency House, 1-4 WarwickStreet, London W1B 5LT, UK, World Energy Council.

10. Jensen, S. H., Hgh, J. V. T., Barfod, R. & Mogensen, M. High temperatureelectrolysis of steam and carbon dioxide. Snderberg Petersen, L. and Larsen, H.

Energy technologies for Post Kyoto targets in the medium term., 204-215. 2003. RisNational Laboratory (DK), Systems Analysis Department; Ris National Laboratory(DK). Information Service Department. Ris-R-1405(EN). Ris international energyconference. 19-5-2003.

11. Hallgeir, . Hydrogen as an Energy Carrier for Commercial use. Opening page. 1999.Nordic workshop on hydrogen in electrochemical energy conversion. Geilo, Norway,Norsk Hydro Electrolysers A/S

The Potential of the Solid Oxide Electrolyser for the Production of Synthetic Fuels

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