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Applications for Molten Carbonate Fuel Cells
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0001-8755-2972
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Molten Carbonate Fuel cells are high temperature fuel cells suitable for distributed generation and combined heat and power, and are today being installed on commercial basis in sizes from 100kW to several MW. Novel applications for MCFC which have attracted interest lately are MCFC used for CO2 separation from combustion flue gas, and high temperature electrolysis with reversible fuel cells. In the first application, the intrinsic capability of the MCFC to concentrate CO2 from the cathode to the anode side through the cell reaction is utilized. In the second application, the high operating temperature and relatively simple design of the MCFC is utilized in electrolysis, with the aim to produce a syngas mix which can be further processed into hydrogen of synthetic fuels.

In this thesis, the effect on fuel cell performance of operating a small lab-scale molten carbonate fuel cell in conditions which simulate those that would apply if the fuel cell was used for CO2 separation in combustion flue gas was studied. Such operating conditions are characterized especially by a low CO2 concentration at the cathode compared to normal operating conditions. Sulfur contaminants in fuel gas, especially H2S, are known poisoning agents which cause premature degradation of the MCFC. Furthermore, combustion flue gas often contains sulfur dioxide which, if entering the cathode, causes performance degradation by corrosion and by poisoning of the fuel cell. This makes poisoning by sulfur contaminants of great concern for MCFC development. In this thesis, the effect of sulfur contaminants at both anode and cathode on fuel cell degradation was evaluated in both normal and in low CO2 simulated flue gas conditions.     

The results suggested that the poisoning effect of SO2 at the cathode is similar to that of H2S at the anode, and that it is possibly due to a transfer of sulfur from cathode to anode. Furthermore, in combination with low CO2 conditions at the cathode, SO2 contaminants cause fuel cell poisoning and electrolyte degradation, causing high internal resistance.

By using a small lab-scale MCFC with commercial materials and standard fuel cell operating conditions, the reversible MCFC was demonstrated to be feasible. The electrochemical performance was investigated in both fuel cell (MCFC) and electrolysis cell (MCEC) modes. The separate electrodes were studied in fuel cell and electrolysis modes under different operating conditions. It was shown that the fuel cell exhibited lower polarization in MCEC mode than in MCFC mode, and a high CO2 concentration at the fuel cell anode reduced the polarization in electrolysis mode, which suggested that CO2 is reduced to produce CO or carbonate.

Abstract [sv]

Smältkarbonatbränsleceller (MCFC) är en typ av högtemperaturbränsleceller som är anpassade för kombinerad el- och värmeproduktion i mellan-till stor skala. Idag installeras MCFC på kommersiell basis i storlekar mellan 100kW och flera MW. En ny typ av tillämpning för MCFC som har väckt intresse på senare tid är användandet av MCFC för CO2-avskiljning i kombination med konventionell elproduktion genom förbränning. En annan ny tillämpning är högtemperaturelektrolys genom användandet av reversibla bränsleceller. I det första fallet utnyttjas att CO2 kan koncentreras från katod- till anodsidan, vilket sker genom cellreaktionen för MCFC. I det andra fallet utnyttjas den höga arbetstemperaturen och den relativt enkla cell-designen för att använda reversibla MCFC till elektrolys, med syfte att producera en syngas-blandning som kan förädlas till vätgas eller till syntetiskt bränsle.

I denna avhandling studeras effekten på bränslecellens prestanda genom att operera en MCFC i lab-skala med driftförhållanden som simulerar de som förväntas uppkomma om bränslecellen användes för CO2-avskiljning ur rökgaser från förbränning. Dessa driftförhållanden karaktäriseras av låg CO2-koncentration på katodsidan jämfört med normal drift. Svavelföroreningar i bränsle, speciellt H2S, är kända för att orsaka förgiftning av anoden, vilket i sin tur försämrar bränslecellens prestanda. Dessutom innehåller rökgaser ofta SO2, vilket antas orsaka korrosion och förgiftning av katoden. Detta gör effekten av svavelföroreningar till ett prioriterat ämne för utvecklingen av MCFC. I denna avhandling undersöks effekten av svavelföroreningar på både anod- och katodsidan, i normala driftförhållanden och i förhållanden med låg CO2 som simulerar användandet av rökgaser för CO2-avskiljning. Resultaten tyder på att effekten av förgiftning med SO2 på katoden liknar den med H2S på anoden, och att detta kan vara orsakat av en transport av svavel från katod till anod. Vidare, i kombination med låg CO2 koncentration på katoden så orsakar SO2-föroreningar elektrolytdegradering, vilket orsakar hög inre resistans.

Genom att använda en liten MCFC i lab-skala med kommersiella material och standardförhållanden för MCFC påvisades att reversibla smältkarbonatbränsleceller kan vara ett lovande koncept. Den elektrokemiska prestandan av både cell och separata elektroder undersöktes både som bränslecell (MCFC)och vid elektrolys (MCEC). Resultaten visade att cellen uppvisade lägre polarisation vid elektrolys än som bränslecell, och att ten hög CO2-koncentration på det som är bränslecellens anodsida gav upphov till en minskad elektrodpolarisation, vilket indikerar att CO2 reduceras för att producera CO eller karbonat.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xii, 62 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2014:51
Keyword [en]
Molten Carbonate Fuel Cell (MCFC), poisoning, electrolyte degradation, SO2, Electrochemical Impedance Spectroscopy (EIS), CO2 separation, button cell set-up; reversible molten carbonate fuel cell, high temperature electrolysis.
National Category
Other Chemical Engineering
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-154585ISBN: 978-91-7595-326-7 (print)OAI: oai:DiVA.org:kth-154585DiVA: diva2:757952
Public defence
2014-11-14, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20141028

Available from: 2014-10-28 Created: 2014-10-23 Last updated: 2014-11-03Bibliographically approved
List of papers
1. Effect of sulfur contaminants on MCFC performance
Open this publication in new window or tab >>Effect of sulfur contaminants on MCFC performance
2014 (English)In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 39, no 23, 12242-12250 p.Article in journal (Refereed) Published
Abstract [en]

Molten carbonate fuel cells (MCFC) used as carbon dioxide separation units in integrated fuel cell and conventional power generation can potentially reduce carbon emission from fossil fuel power production. The MCFC can utilize CO2 in combustion flue gas at the cathode as oxidant and concentrate it at the anode through the cell reaction and thereby simplifying capture and storage. However, combustion flue gas often contains sulfur dioxide which, if entering the cathode, causes performance degradation by corrosion and by poisoning of the fuel cell. The effect of contaminating an MCFC with low concentrations of both SO2 at the cathode and H2S at the anode was studied. The poisoning mechanism of SO2 is believed to be that of sulfur transfer through the electrolyte and formation of H2S at the anode. By using a small button cell setup in which the anode and cathode behavior can be studied separately, the anodic poisoning from SO2 in oxidant gas can be directly compared to that of H2S in fuel gas. Measurements were performed with SO2 added to oxidant gas in concentrations up to 24 ppm, both for short-term (90 min) and for long-term (100 h) contaminant exposure. The poisoning effect of H2S was studied for gas compositions with high- and low concentration of H-2 in fuel gas. The H2S was added to the fuel gas stream in concentrations of 1, 2 and 4 ppm. Results show that the effect of SO2 in oxidant gas was significant after 100 h exposure with 8 ppm, and for short-term exposure above 12 ppm. The effect of SO2 was also seen on the anode side, supporting the theory of a sulfur transfer mechanism and H2S poisoning. The effect on anode polarization of H2S in fuel gas was equivalent to that of SO2 in oxidant gas.

Keyword
Molten carbonate fuel cell (MCFC), Performance degradation, SO2, Electrochemical impedance spectroscopy (EIS)
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-150925 (URN)10.1016/j.ijhydene.2014.03.068 (DOI)000340328800045 ()2-s2.0-84904768022 (Scopus ID)
Note

QC 20140912

Available from: 2014-09-12 Created: 2014-09-11 Last updated: 2017-12-05Bibliographically approved
2. Molten carbonate fuel cells for CO2 separation and segregation by retrofitting existing plants - An analysis of feasible operating windows and first experimental findings
Open this publication in new window or tab >>Molten carbonate fuel cells for CO2 separation and segregation by retrofitting existing plants - An analysis of feasible operating windows and first experimental findings
Show others...
2015 (English)In: International Journal of Greenhouse Gas Control, ISSN 1750-5836, E-ISSN 1878-0148, Vol. 35, 120-130 p.Article in journal (Refereed) Published
Abstract [en]

Molten carbonate fuel cells (MCFC) used as active carbon dioxide concentrator units are a promising solution to reduce greenhouse gas (GHG) emissions from traditional combustion plants. The cell reaction transfers carbonate ions from the cathode to the anode and allows the fuel cell to simultaneously produce power and separate CO2 from a stream of flue gas. Carbon dioxide separation is of high interest for use in natural gas combined cycles and coal gas combustion plants, as a large part of anthropogenic CO2 worldwide originates from such installations. The flue gas from these types of combustion technologies typically contains 3-15% CO2, which is in the lower operational range of the MCFC. The aim of this work was to investigate the possibility to retrofit existing power plants with MCFC to reduce the total release of CO2 without necessarily reducing the power output, and to understand which kind of power plant could have the major benefits with an MCFC retrofitting. The performance of lab scale MCFC fed with simulated flue gas was evaluated, and a number of operational parameters, such as utilization factor and cathode humidification were varied to study the effect on fuel cell performance. The results show that it is feasible to operate the MCFC as a CO2 separator for simulated gas turbine flue gas; however, the voltage drop due to low CO2 concentration may restrict the operating window depending on various operating conditions.

Keyword
Molten Carbonate Fuel Cells (MCFC), Carbon capture, single cell, button cell, electrochemical impedance spectroscopy (EIS)
National Category
Other Chemistry Topics
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-154552 (URN)10.1016/j.ijggc.2015.01.012 (DOI)000352328800011 ()2-s2.0-84923337300 (Scopus ID)
Projects
MCFC-CONTEX
Funder
EU, European Research Council, 245171
Note

QC 20150508. Updated from manuscript to article in journal.

Available from: 2014-10-23 Created: 2014-10-23 Last updated: 2017-12-05Bibliographically approved
3. Performance degradation of Molten Carbonate Fuel Cells caused by SO2 in simulated flue gas
Open this publication in new window or tab >>Performance degradation of Molten Carbonate Fuel Cells caused by SO2 in simulated flue gas
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effect on MCFC performance degradation SO2 contaminant at the cathode, in combination with operating the fuel cell with CO2 lean oxidant gas, simulating combustion flue gas, was evaluated. Of special focus was the effect of electrolyte degradation.

 Measurements were performed to test the effect of SO2 in the oxidant gas stream, followed by regeneration with clean gas. A 3cm2 button cell MCFC allowing active electrolyte management by refilling was for 1500h to benchmark the performance degradation without contaminants. In order to study the poisoning effect of SO2 entering the fuel cell gas, the MCFC was operated for 250h with the addition of 18ppm SO2 in the oxidant gas. Electrolyte was added after 1500h of benchmark operation and after 250h of contaminant operation. The addition of 18ppm SO2 greatly accelerated the performance degradation of the fuel cell. Measurements showed that the internal resistance was the single factor which was most affected by the SO2 poisoning, and that the performance degradation after 250h was not reversed by regeneration with clean gas, but with the addition of fresh electrolyte. This led us to conclude that SO2 in oxidant gas leads to an accelerated loss of electrolyte and subsequent decrease in conductivity of the electrolyte, causing loss of performance and meeting end of life criteria after relatively short operational time. Other factors, such as poisoning of the anode and corrosion of cathode side current collectors, were also detected. 

Keyword
Molten Carbonate Fuel Cell, MCFC, CO2 separation, flue gas, SO2 contaminant
National Category
Other Chemistry Topics
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-154584 (URN)
Projects
MCFC CONTEX
Funder
EU, European Research Council, 245171
Note

QS 2014

Available from: 2014-10-23 Created: 2014-10-23 Last updated: 2014-10-28Bibliographically approved
4. Electrochemical performance of reversible molten carbonate fuel cells
Open this publication in new window or tab >>Electrochemical performance of reversible molten carbonate fuel cells
2014 (English)In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 39, no 23, 12323-12329 p.Article in journal (Refereed) Published
Abstract [en]

The electrochemical performance of a state-of-the-art molten carbonate cell was investigated in both fuel cell (MCFC) and electrolysis cell (MCEC) modes by using polarization curves and electrochemical impedance spectroscopy (EIS). The results show that it is feasible to run a reversible molten carbonate fuel cell and that the cell actually exhibits lower polarization in the MCEC mode, at least for the short-term tests undertaken in this study. The Ni hydrogen electrode and the NiO oxygen electrode were also studied in fuel cell and electrolysis cell modes under different operating conditions, including temperatures and gas compositions. The polarization of the Ni hydrogen electrode turned out to be slightly higher in the electrolysis cell mode than in the fuel cell mode at all operating temperatures and water contents. This was probably due to the slightly larger mass-transfer polarization rather than to charge-transfer polarization according to the impedance results. The CO2 content has an important effect on the Ni electrode in electrolysis cell mode. Increasing the CO2 content the Ni electrode exhibits slightly lower polarization in the electrolysis cell mode. The NiO oxygen electrode shows lower polarization loss in the electrolysis cell mode than in the fuel cell mode in the temperature range of 600-675 degrees C. The impedance showed that both charge-transfer and mass-transfer polarization of the NiO electrode are lower in the electrolysis cell than in the fuel cell mode.

Keyword
Molten carbonate electrolysis cell, Ni electrode, NiO electrode, Reversible molten carbonate fuel cell
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:kth:diva-150929 (URN)10.1016/j.ijhydene.2014.02.144 (DOI)000340328800055 ()2-s2.0-84904768004 (Scopus ID)
Note

QC 20150623

Available from: 2014-09-12 Created: 2014-09-11 Last updated: 2017-12-05Bibliographically approved

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