- Email: [email protected]
SOFC stack performance under high fuel utilization
i n t e r n a t 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 4 0 ( 2 0 1 5 ) 1 1 2 8 e1 1 3 6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
SOFC stack performance under high fuel utilization Qingping Fang a,*, Ludger Blum a, Roland Peters a, Murat Peksen a, Peter Batfalsky b, Detlef Stolten a,c a
Forschungszentrum Ju¨lich GmbH, Institute of Energy and Climate Research (IEK-3), Germany Forschungszentrum Ju¨lich GmbH, Central Institute for Engineering, Electronics and Analytics (ZEA-1), Germany c Chair for Fuel Cells, RWTH Aachen University, Germany b
article info
abstract
Article history:
Based on previous long-term SOFC stack tests, two short stacks (one F20 and one F10
Received 18 August 2014
design) were tested in order to investigate stack performance under high fuel utilization
Received in revised form
(>40%) and possibly also high current densities (>0.5 Acm2). The F20-design stack was still
15 November 2014
operated with relatively mild current densities (0.5 Acm2), but with high fuel utilization
Accepted 17 November 2014
of up to 90% with 10% pre-reformed liquefied natural gas (LNG). The F10-design stack was
Keywords:
current densities of up to 1.5 Acm2. Preliminary analysis shows that both F10- and F20-
SOFC
design stacks can be operated smoothly at a fuel utilization of ~85% in the temperature
operated with 20% humidified H2, but with high fuel utilization of up to 90% and high
ASC stack
range of 750~800 C, although an increase in concentration polarization can already be
Fuel utilization
observed at the fuel utilization of ~80%. Operation with fuel utilization of 90% led to local
Temperature distribution
oxidation of cells at a similar position in both stacks. Based on the calculations with a 1D model, such an effect was assumed to be due to the variation in fuel utilization caused by the temperature gradient in the cell. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFC) combine the major advantages of high energy conversion efficiency and fuel flexibility (e.g. hydrogen, natural gas, biogas, etc.) without using precious metal catalysts. During fuel cell operation, excess air is normally supplied for cooling purposes, while excess fuel has to be supplied to avoid anode oxidation, since the oxygen partial pressure increases by converting the fuel into steam (and CO2 for fuels containing hydrocarbons). The amount of excess fuel needs to be kept as low as possible, because the electrical efficiency of fuel cells is proportional to the fuel utilization uf,
which is defined as the ratio of consumed fuel and total fuel supplied, where the consumed fuel includes theoretically the consumption of both the electrochemical reaction and possible leakages. In most cases, the leakages are unknown and not considered. Fuel utilization is an important parameter for evaluating fuel cell performance. Especially for fuel cell stacks containing multi-layers and large cells, fuel utilization is not only determined by the electrochemical properties of the anode, but is also dependent on the transport and distribution of the fuel inside the stack. A homogeneous distribution of the fuel requires a proper design of the flow field and gas manifold, which is also one of the critical tasks for most SOFC stack developers and manufacturers. In order to benefit from the high
* Corresponding author. Forschungszentrum Ju¨lich GmbH, Institute of Energy and Climate Research (IEK-3), Wilhelm-Johnen-Straße, D52428 Ju¨lich, Germany. Tel.: þ49 2461 61 1573; fax: þ49 2461 61 6695. E-mail address: [email protected] (Q. Fang). http://dx.doi.org/10.1016/j.ijhydene.2014.11.094 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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efficiency of SOFC systems, the stacks need to be operated with higher fuel utilization of about 80% [1]. Currently, most SOFC stack developers claim a nominal single-pass fuel utilization of 60~85% with short or large stacks [2e9]. Even 94% fuel utilization was reported recently with a 6-layer short stack [10]. However, information about the stability of the reported stacks under high fuel utilization of >70% is generally very limited. Focusing on anode-supported cell (ASC) technology, Forschungszentrum Ju¨lich has demonstrated long-term performance of the F10-design stacks, but mostly still with a relatively low fuel utilization of 40% at 0.5 Acm2 with humidified hydrogen as fuel [11,12]. Recently test results were presented for a 2.5 kW F20-design stack operated at 0.5 Acm2 and 70%~80% fuel utilization with 10% pre-reformed liquefied natural gas (LNG) at around 750 C (TStack) [13]. The stack was operated stably up to 80% fuel utilization. Both F10 and F20 are robust stack designs based on 2.5 mm thick Crofer 22 APU with a similar construction. The cell size is 10 cm 10 cm (active area ~80 cm2) for the F10 design and 20 cm 20 cm (active area ~360 cm2) for the F20 design. The major stack components of the F design are shown in Fig. 1. Each repeating unit consists of one interconnector and one frame, together with the cell and Ni mesh. Under normal operating conditions, the F design stacks are operated in counter-flow mode. In order to further investigate stack behavior under high fuel utilization and possibly also high current densities, two short stacks (F10 and F20 design) were tested under different conditions. After the tests, both stacks were disassembled for post mortem analysis.
cells based on Ni/8YSZ (8 mol% yttria-stabilized zirconia) with an LSCF cathode (La0.58Sr0.4Co0.2Fe0.8O3-d, in-house production) and 8YSZ electrolyte. A CGO barrier layer was screenprinted between the electrolyte and cathode. A Ni mesh was welded onto the interconnector as the anode contact, creating at the same time the gas channels. Ceramic glass was applied by dispensing for all bonding inside the stack. Shaped mica gaskets were used as flat sealing between the stack and adaptor plate in the furnace. The upper index Y3 in the stack number FY32005-12 is an internal notation, indicating the third variation/evolution in F20 design. In the following text, Y3 will be ignored for simplification. The differences between the two stacks, besides the cell size, are as follows:
Experimental
An overview of the designs of the two stacks is shown in Table 1. The different designs of the stacks were intended for separate scientific purposes, but not for the investigation of fuel utilization. However, the differences at cathode side should have no influence on the results under high fuel utilization. The possible minor effect from the thickness of the porous anode substrate was neglected in this work.
Stack design Both short stacks (stack number F1004-39 and FY32005-12) consisted of interconnects made of Crofer 22 APU and ASC
- F1004-39 had four cells, while F2005-12 had five. The number of cells or layers was limited by the test benches, in view of the precision of the mass flow controllers (MFC). - All cells in both stacks had similar functional layers. However, the thickness of the anode substrate in F2005-12 was greater than that of F1004-39 (1500 mm vs. 600 mm). - The cathode side of the interconnector in F1004-39 was coated with an MCF protective layer by atmospheric plasma spraying (APS), while F2005-12 was coated with manganese oxide by wet power spraying (WPS). - In both stacks, a perovskite cathode contact layer was applied between the cathode and interconnector by WPS. Two different types of contact layers developed at Forschungszentrum Ju¨lich, i.e. LSCF and LCC10 [14], were applied in F1004-39 and F2005-12, respectively.
Test conditions The joining processes were carried out in the furnace at 850 C with a clamping weight of 100 kg and 650 kg for F1004-39 and
Table 1 e Designs of the two stacks (F1004-39 and F200512).
Fig. 1 e Major stack components of F-design stacks.
No. of cells Active cell area Cell type Thickness of anode substrate Anode contact Anode Electrolyte Barrier layer Cathode Cathode contact layer Protective coating Sealant in stack
F1004-39
F2005-12
4 80 cm2 ASC IIIb (Ju¨lich) 600 mm Ni mesh Ni/YSZ 8YSZ CGO LSCF LSCF (WPS) MCF (APS) Glass H (Ju¨lich)
5 360 cm2 ASC Ib (Ju¨lich) 1500 mm
LCC10 (WPS) MnOx (WPS)
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Fig. 2 e Positions of the thermocouples in stack F2005-12 (left) and F1004-39 (right).
F2005-12, respectively. After reduction at 800 C by increasing the hydrogen concentration stepwise, the stacks were directly characterized and further tested. For stack F1004-39, the air flux was preheated by an electrical gas heater. The hydrogen fuel was humidified by mixing with steam from an electrical evaporator. For stack F2005-12, both air and fuel fluxes were preheated through a recuperative heat exchanger, and then heated to the working temperature (close to furnace temperature) via electrical gas heaters. F2005-12 was mostly operated with 10% pre-reformed LNG, since the internal reforming of methane without pre-reforming can lead to the formation of volatile nickel hydroxide and therefore faster anode degradation [13,15]. The fuel composition after 10% pre-reforming of LNG with a starting steam-to-carbon ratio (S/C) of 2 was simulated with a mixture of LNG, H2 and H2O. Because there is no CO2 pipeline in the laboratory, the small amount of CO2 after pre-
reforming, assuming complete conversion of CO through the water gas shift reaction, was replaced by steam. The S/C ratio was then 2.1 instead of 2. For simplicity of calculation, the S/C ratio and fuel utilization were calculated relative to 100% CH4. The deviations from the actual calculated values relative to LNG are less than 0.6%. The temperatures of the stack at different positions were measured by thermocouples as shown in Fig. 2. The holes for the thermocouples were all 10 mm deep inside the metallic plates. In order to measure the in-plane temperature profile in F2005-12, seven thermocouples were placed along the direction of gas flow in the middle layer of the stack. The thermocouples were positioned in such a way that the temperatures could be measured inside the manifold region, near the edges of the cell and near the center of the cell. Another five thermocouples were placed in the same layer, but at the opposite side, which delivered similar results. In F1004-39, there were
Fig. 3 e Operation of F2005-12 with high fuel utilization at 700 C and 750 C (furnace temperature).
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Fig. 4 e Comparison of the OCVs (left) and temperature profile (right) of F2005-12 at 130 h (after conditioning; blue), 355 h (after 90% uf operation at 700 C; red) and 620 h (after 90% uf operation at 750 C; green). (H2: 25.1 slm, air: 85 slm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
only two thermocouples in the thicker middle layer of the stack, which measured the temperatures near the edges of the cell. Both stacks were cooled down with forming gas (5% H2 in Ar) and then disassembled for visual inspection.
Results and discussion Stack F2005-12 F2005-12 was mainly tested under different fuel utilization by keeping the current density and air-to-fuel ratio (l) at 0.4 Acm2 and 2.9, respectively. The variation of fuel utilization was first carried out at the furnace temperature of 700 C, and later at 750 C. After a short test of 90% fuel utilization at 700 C (0.4 slm H2, 1.3 slm LNG, S/C~2.1), all cells still showed more than 1.22 V under OCV with dry H2, indicating no generation of detectable leakages in the stack due to operation at high fuel utilization. The test at 750 C started with 80% fuel utilization using 10% humidified H2. LNG was then slowly introduced into the stack by simultaneously reducing H2 and increasing the amount of steam step by step. As shown in Fig. 3, the stack was operated for about 100 h with 90% uf. A continuous drop of the voltage in cell 2 (from the bottom) was observed after about 25 h of operation. By decreasing the fuel utilization from 90% to 80%, cell 2 was stabilized, but at a lower level compared to the other cells. The electrical efficiency of
the stack under different fuel utilization is also shown in Fig. 3. Subsequent OCV measurements under forming gas and dry H2 showed that not only cell 2 but also cell 1 had leakages due to 90% uf operation. A comparison of OCVs and temperature profiles at 130 h (after conditioning), 355 h (after 90% uf operation at 700 C) and 620 h (after 90% uf operation at 750 C) is shown in Fig. 4, where overheating near the fuel outlet side due to leakages from cell 1 and cell 2 after 90% uf operation at 750 C can be also observed. Post mortem analysis of the stack showed that both cell 1 and cell 2 were oxidized at the fuel outlet side, as shown in Fig. 5.
Stack F1004-39 F1004-39 was first tested only with 20% humidified H2 at 750 C (furnace), but at different current densities. The current density was increased stepwise from 0.5 Acm2 to 1.5 Acm2. The air-to-fuel ratio l was always kept at 2.5 (i.e. oxygen utilization of 40%) during the tests. The stack performance with 0.5 Acm2 and 1.5 Acm2 is shown in Figs. 6 and 7, respectively. The large gap between cell 1 (bottom) and the other cells was assumed to be due to the poorer contact between the cell and interconnect in this layer, which was already observed before starting the fuel utilization tests. The calculated areaspecific resistances (ASRs) of the cells at four different
Fig. 5 e Oxidation of cell 1 (left) and cell 2 (right) near fuel outlet in F2005-12 after 90% uf operation at 750 C.
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Fig. 6 e Variation of fuel utilization in stack F1004-39 with 0.5 Acm¡2 at 750 C (furnace). current densities are shown in Fig. 8. The ASR was calculated from the difference between the average Nernst voltage and the measured voltage at a given current density. The average Nernst voltage is calculated according to the gas compositions at the inlet and outlet of the stack. Note that the decrease of the ASRs at higher current densities was purely an effect of the rising temperature, since the air-to-fuel ratio was kept constant at 2.5 for all current densities. The increased temperature gradient (T6 versus T5 in Fig. 8) with increasing current density was another indication of insufficient air
cooling. Under current testing conditions, stack F1004-39 showed similar behavior under different current densities. The ASRs of all cells remained almost constant up to a fuel utilization of 60%~70%. A slight increase in ASRs was observed for all cells at a fuel utilization of 80~85%, and subsequently concentration polarization became dominant. A comparison of the OCVs under dry hydrogen after operation at each current density is shown in Fig. 9, indicating that cell 1 and cell 3 were not gas-tight after 90% uf operation at 1 Acm2. This could also explain the relatively faster
Fig. 7 e Variation of fuel utilization in stack F1004-39 with 1.5 Acm¡2 at 750 C (furnace).
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Fig. 8 e ASRs of the cells in F1004-39 with different fuel utilization and current densities at 750 C (furnace).
increase of ASRs in cell 1 and cell 3 under 80% fuel utilization with 1.25 Acm2 and 1.5 Acm2, as shown in Fig. 8. The stack was disassembled after testing for visual inspection. Fig. 10 shows the anode sides of cell 1 and cell 3 during post mortem analysis, where the typical cell morphology after redox can be seen near the fuel outlet side. Compared to the severe oxidation shown in Fig. 5, the leakages in F1004-39 were relatively small, so that the cells could still be reduced during OCV, or during the cooling process with forming gas. It is even possible that the cells were reduced under load with lower fuel utilization.
Discussion Under current stack design and testing conditions, anode oxidation due to local fuel starvation during operation with
90% fuel utilization occurred in both the tested F10 and F20 stacks, which was not (or less) influenced by cell size, current density and type of fuels. The occurrence of oxidation near the fuel outlet side can be easily understood, since the fuel is consumed from the fuel inlet to the outlet side. In the case of operation with LNG at 0.4 Acm2 and 90% fuel utilization, the nominal oxygen partial pressure at the fuel outlet side is 3.2 1016 bar, which is close to the Ni oxidation potential of 1.0 1015 bar at 750 C. Even small inhomogeneities of the temperature and fuel distribution could lead to deviations of the local oxygen partial pressure. The fact that all the observed oxidation occurred in the middle part of the cell near the fuel outlet could either be an effect of the fuel distribution in-plane, or an effect of the temperature distribution, which was then correlated to the current distribution.
Fig. 9 e Comparison of OCVs of F1004-39 after different operation conditions (blue: BOL, 800 C; red: after 40% uf operation with 0.5 Acm¡2 at 750 C; green: after 90% uf operation with 0.5 Acm¡2 at 750 C; violet: after 90% uf operation with 1 Acm¡2 at 750 C; cyan: after 80% uf operation with 1.5 Acm¡2 at 750 C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 10 e Traces of oxidation in cell 1 (top) and cell 3 (bottom) in stack F1004-39 after operation with high fuel utilization and current densities at 750 C (furnace).
Effect of in-plane fuel distribution For standard operating conditions, the flow distribution on the anode side of an F20 design was modeled using a 3D CFD code. For reasons of symmetry, only half of the layer was calculated. In order to keep the model to a reasonable size, the nickel mesh was replaced by a simplified mesh structure with the same flow properties. A validation of this method is given in Ref. [16].
As can be seen from Fig. 11, the difference in flow velocity from the left side to the middle of the cell is very uniform. In particular, no drop in the velocity (i.e. mass flow) of the fuel is visible towards the middle of the cell opposite the outlet channel. Also described in Ref. [16] is the flow distribution in a 36layer stack. This deviates less than ±1%. In a 5-layer stack it is even more relaxed.
Effect of in-plane temperature distribution
Fig. 11 e Distribution of flow in-plane on the anode side of an F20 design: velocity vectors colored by velocity magnitude in m/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
As mentioned above, the holes for the thermocouples in the stacks were 10 mm deep inside the metallic parts. Therefore only temperatures near the edges of the cells along the fuel flow direction were measured. At the moment, it is possible to drill deeper holes of 40 mm for thermocouples, but these are still not deep enough to measure the temperature in the center of the cell. Great efforts have been made to reveal the distribution of the temperature, thermal gradient, as well as thermal stress and strain inside the cells and stacks by coupled 3D CFD and FEM analysis [17], where the temperature distribution transverse to the flow direction was also presented. Based on the CFD calculation, a temperature difference of 10~30 C was estimated along the edge of the cell at the fuel outlet side (i.e. between b and d in Fig. 5) depending on operating conditions. The effect of this temperature difference on fuel utilization was evaluated by a simple 1D model under defined conditions. Assuming a uniform air and fuel flow in the cell and maintaining a constant voltage of 750 mV, the variations of
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Acknowledgments The authors would like to thank all colleagues from the FOB department for carrying out the laboratory work. Cooperation with all colleagues from other departments on materials development, stack design and manufacture, and post-test analysis is also gratefully acknowledged.
references
Fig. 12 e Calculated differences in temperature, current density and H2 concentration in an F20 cell caused by a 20 C difference in the fuel outlet (air inlet) temperature based on a 1D model.
current density, temperature and concentration of H2 along ab and cd in Fig. 5 were calculated using the 1D model, taking the different temperatures at the fuel outlet (i.e. temperatures at b and d in Fig. 5) into account. The heat generated from the fuel cell reaction was not considered for the preliminary calculation. Fig. 12 shows the calculated results along a-b (blue lines) and c-d (green lines) in Fig. 5 when temperatures at b and d are 620 C and 600 C, respectively. Only with a temperature difference of 20 C did the resulting changes in current density lead to a 10% variation in fuel utilization (i.e. 95% vs. 85%) at the fuel outlet. In the case of operation with nominal fuel utilization of 90%, such variation could easily lead to local oxidation.
Conclusions Two stacks of different designs (F1004-39 and F2005-12) were tested in a furnace environment under high fuel utilization of up to 90% with 20% humidified hydrogen or 10% pre-reformed LNG, and showed basically similar behavior under current stack design and testing conditions. Both stacks could be operated smoothly up to a fuel utilization of ~85% in the temperature range of 750~800 C, although the increase of concentration polarization can already be observed at a fuel utilization of ~80%. Operation with 90% fuel utilization introduced a much higher contribution of concentration polarization and a high risk of fuel starvation. Local oxidations of the cells due to fuel starvation during operation with 90% fuel utilization were observed in both stacks at a similar position (i.e. in the middle of the cell near the fuel outlet side), regardless of fuel and current density applied. Preliminary calculations based on a simple 1D model show that local fuel utilization is strongly influenced by the temperature distribution. Even a temperature difference of 20 C could lead to 10% variation in fuel utilization at the fuel outlet. Therefore the fuel utilization is limited to 85% for safe operation of Fdesign stacks under current stack design and testing conditions.
[1] Geyer J, Kohlmu¨ller H, Landes H, Stu¨bner R. Investigations into the kinetics of the Ni-YSZ-cermet-anode of a solid oxide fuel cell. In: Proceedings of the 5th international symposium on solid oxide fuel cells (SOFC-V). Pennington, NJ: The Electrochemical Society; 1997. p. 585e94. € ger K. Fuel cell seminar 2010. Paper: DEM41-3. October [2] Fo 19e21, 2010. San Antonio, TX. [3] Borglum Brian P, Ghezel-Ayagh Hossein. Development of solid oxide fuel cells at versa power systems and fuel cell energy. ECS Trans 2013;57(1):61e6. [4] http://www.sunfire.de/en/produkte/stacks/mk200-sofcstack. [5] http://www.elcogen.com/en/sofc-stack/technical-data. € nicke Andreas, Wunderlich Christian, [6] Reuber Sebastian, Po Michaelis Alexander. Eneramic power generator e a reliable and cycleable 100W SOFC-system. ECS Trans 2013;57(1):161e9. [7] Megel Stefan, Dosch Christian, Rothe Stefan, Kusnezoff Mihails, Trofimenko Nikolai, Sauchuk Viktar, et al. CFY-stacks for use in stationary SOFC and SOEC applications. ECS Trans 2013;57(1):89e98. [8] Komatsu Y, Brus G, Kimijima S, Szmyd JS. The effect of overpotentials on the transient response of the 300W SOFC cell stack voltage. Appl Energy 2014;115:352e9. [9] Mugikura Yoshihiro, Yasumoto Kenji, Morita Hiroshi, Yoshikawa Masahiro, Yamamoto Tohru. Performance evaluation technology for long term durability and reliability of SOFCs. ECS Trans 2013;57(1):649e56. [10] Wuillemin Z, Ceschini S, Antonetti Y, Beetschen C, Modena S, Montinaro D, et al. High-performance SOFC stacks tested under different reformate compositions. Proc 11th Eur SOFC SOE Forum 2014. [11] de Haart LGJ, Vinke IC. Long-term operation of planar type SOFC stacks. ECS Trans 2011;35(1):187e94. [12] Blum L, Packbier U, Vinke IC, de Haart LGJ. Long-term testing of SOFC stacks at forschungszentrum Ju¨lich. Fuel Cells 2013;13(4):646e53. [13] Fang Q, Blum L, Batfalsky P, Menzler NH, Packbier U, Stolten D. Durability test and degradation behavior of a 2.5 kW SOFC stack with internal reforming of LNG. Int J Hydrogen Energy 2013;38(36):16344e53. [14] Haanappel VAC, Vinke IC, de Haart LGJ, Stolten D. Evaluation of the electrical contact area at the SOFC cathode” in 18th world hydrogen energy conference 2010-WHEC 2010. In: Stolten Detlef, Grube Thomas, editors. Parallel Sessions Book 1: Fuel Cell Basics/Fuel Infrastructures. Essen: Forschungszentrum Ju¨lich GmbH, Zentralbibliothek, Verlag; 2010. p. 255. WHEC, May 16e21 2010. [15] Gubner A, Landes H, Metzger J, Seeg H, Stu¨bner R. Investigations into the degradation of the cermet anode of a solid oxide fuel cell. In: Stimming U, Singhal SC, Tagawa H, Lehnert W, editors. Proceedings of the 5th international symposium on solid oxide fuel cells (SOFC-V). Pennington, NJ: The Electrochemical Society; 1997. p. 844.
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[16] Peksen M. A coupled 3D thermofluid-thermomechanical analysis of a planar type production scale SOFC stack. Int J Hydrogen Energy 2011;(6):11914e28. [17] Peksen M. 3D thermomechanical behaviour of solid oxide fuel cells operating in different environments. Int J Hydrogen Energy 2013;38(30):13408e18.
Glossary APS: atmospheric plasma spraying ASC: anode-supported cell ASR: area-specific resistance BOL: beginning of life
CGO: cerium gadolinium oxide Crofer 22 APU: ferritic chromium steel from ThyssenKrupp LNG: liquefied natural gas LSCF: lanthanum strontium cobalt ferrite MCF: manganese cobalt ferrite MFC: mass flow controller OCV: open circuit voltage S/C: steam-to-carbon ratio slm: standard liter per minute uf: fuel utilization WPS: wet powder spraying YSZ: Yttria-stabilized zirconia
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
SOFC stack performance under high fuel utilization Qingping Fang a,*, Ludger Blum a, Roland Peters a, Murat Peksen a, Peter Batfalsky b, Detlef Stolten a,c a
Forschungszentrum Ju¨lich GmbH, Institute of Energy and Climate Research (IEK-3), Germany Forschungszentrum Ju¨lich GmbH, Central Institute for Engineering, Electronics and Analytics (ZEA-1), Germany c Chair for Fuel Cells, RWTH Aachen University, Germany b
article info
abstract
Article history:
Based on previous long-term SOFC stack tests, two short stacks (one F20 and one F10
Received 18 August 2014
design) were tested in order to investigate stack performance under high fuel utilization
Received in revised form
(>40%) and possibly also high current densities (>0.5 Acm2). The F20-design stack was still
15 November 2014
operated with relatively mild current densities (0.5 Acm2), but with high fuel utilization
Accepted 17 November 2014
of up to 90% with 10% pre-reformed liquefied natural gas (LNG). The F10-design stack was
Keywords:
current densities of up to 1.5 Acm2. Preliminary analysis shows that both F10- and F20-
SOFC
design stacks can be operated smoothly at a fuel utilization of ~85% in the temperature
operated with 20% humidified H2, but with high fuel utilization of up to 90% and high
ASC stack
range of 750~800 C, although an increase in concentration polarization can already be
Fuel utilization
observed at the fuel utilization of ~80%. Operation with fuel utilization of 90% led to local
Temperature distribution
oxidation of cells at a similar position in both stacks. Based on the calculations with a 1D model, such an effect was assumed to be due to the variation in fuel utilization caused by the temperature gradient in the cell. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFC) combine the major advantages of high energy conversion efficiency and fuel flexibility (e.g. hydrogen, natural gas, biogas, etc.) without using precious metal catalysts. During fuel cell operation, excess air is normally supplied for cooling purposes, while excess fuel has to be supplied to avoid anode oxidation, since the oxygen partial pressure increases by converting the fuel into steam (and CO2 for fuels containing hydrocarbons). The amount of excess fuel needs to be kept as low as possible, because the electrical efficiency of fuel cells is proportional to the fuel utilization uf,
which is defined as the ratio of consumed fuel and total fuel supplied, where the consumed fuel includes theoretically the consumption of both the electrochemical reaction and possible leakages. In most cases, the leakages are unknown and not considered. Fuel utilization is an important parameter for evaluating fuel cell performance. Especially for fuel cell stacks containing multi-layers and large cells, fuel utilization is not only determined by the electrochemical properties of the anode, but is also dependent on the transport and distribution of the fuel inside the stack. A homogeneous distribution of the fuel requires a proper design of the flow field and gas manifold, which is also one of the critical tasks for most SOFC stack developers and manufacturers. In order to benefit from the high
* Corresponding author. Forschungszentrum Ju¨lich GmbH, Institute of Energy and Climate Research (IEK-3), Wilhelm-Johnen-Straße, D52428 Ju¨lich, Germany. Tel.: þ49 2461 61 1573; fax: þ49 2461 61 6695. E-mail address: [email protected] (Q. Fang). http://dx.doi.org/10.1016/j.ijhydene.2014.11.094 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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efficiency of SOFC systems, the stacks need to be operated with higher fuel utilization of about 80% [1]. Currently, most SOFC stack developers claim a nominal single-pass fuel utilization of 60~85% with short or large stacks [2e9]. Even 94% fuel utilization was reported recently with a 6-layer short stack [10]. However, information about the stability of the reported stacks under high fuel utilization of >70% is generally very limited. Focusing on anode-supported cell (ASC) technology, Forschungszentrum Ju¨lich has demonstrated long-term performance of the F10-design stacks, but mostly still with a relatively low fuel utilization of 40% at 0.5 Acm2 with humidified hydrogen as fuel [11,12]. Recently test results were presented for a 2.5 kW F20-design stack operated at 0.5 Acm2 and 70%~80% fuel utilization with 10% pre-reformed liquefied natural gas (LNG) at around 750 C (TStack) [13]. The stack was operated stably up to 80% fuel utilization. Both F10 and F20 are robust stack designs based on 2.5 mm thick Crofer 22 APU with a similar construction. The cell size is 10 cm 10 cm (active area ~80 cm2) for the F10 design and 20 cm 20 cm (active area ~360 cm2) for the F20 design. The major stack components of the F design are shown in Fig. 1. Each repeating unit consists of one interconnector and one frame, together with the cell and Ni mesh. Under normal operating conditions, the F design stacks are operated in counter-flow mode. In order to further investigate stack behavior under high fuel utilization and possibly also high current densities, two short stacks (F10 and F20 design) were tested under different conditions. After the tests, both stacks were disassembled for post mortem analysis.
cells based on Ni/8YSZ (8 mol% yttria-stabilized zirconia) with an LSCF cathode (La0.58Sr0.4Co0.2Fe0.8O3-d, in-house production) and 8YSZ electrolyte. A CGO barrier layer was screenprinted between the electrolyte and cathode. A Ni mesh was welded onto the interconnector as the anode contact, creating at the same time the gas channels. Ceramic glass was applied by dispensing for all bonding inside the stack. Shaped mica gaskets were used as flat sealing between the stack and adaptor plate in the furnace. The upper index Y3 in the stack number FY32005-12 is an internal notation, indicating the third variation/evolution in F20 design. In the following text, Y3 will be ignored for simplification. The differences between the two stacks, besides the cell size, are as follows:
Experimental
An overview of the designs of the two stacks is shown in Table 1. The different designs of the stacks were intended for separate scientific purposes, but not for the investigation of fuel utilization. However, the differences at cathode side should have no influence on the results under high fuel utilization. The possible minor effect from the thickness of the porous anode substrate was neglected in this work.
Stack design Both short stacks (stack number F1004-39 and FY32005-12) consisted of interconnects made of Crofer 22 APU and ASC
- F1004-39 had four cells, while F2005-12 had five. The number of cells or layers was limited by the test benches, in view of the precision of the mass flow controllers (MFC). - All cells in both stacks had similar functional layers. However, the thickness of the anode substrate in F2005-12 was greater than that of F1004-39 (1500 mm vs. 600 mm). - The cathode side of the interconnector in F1004-39 was coated with an MCF protective layer by atmospheric plasma spraying (APS), while F2005-12 was coated with manganese oxide by wet power spraying (WPS). - In both stacks, a perovskite cathode contact layer was applied between the cathode and interconnector by WPS. Two different types of contact layers developed at Forschungszentrum Ju¨lich, i.e. LSCF and LCC10 [14], were applied in F1004-39 and F2005-12, respectively.
Test conditions The joining processes were carried out in the furnace at 850 C with a clamping weight of 100 kg and 650 kg for F1004-39 and
Table 1 e Designs of the two stacks (F1004-39 and F200512).
Fig. 1 e Major stack components of F-design stacks.
No. of cells Active cell area Cell type Thickness of anode substrate Anode contact Anode Electrolyte Barrier layer Cathode Cathode contact layer Protective coating Sealant in stack
F1004-39
F2005-12
4 80 cm2 ASC IIIb (Ju¨lich) 600 mm Ni mesh Ni/YSZ 8YSZ CGO LSCF LSCF (WPS) MCF (APS) Glass H (Ju¨lich)
5 360 cm2 ASC Ib (Ju¨lich) 1500 mm
LCC10 (WPS) MnOx (WPS)
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Fig. 2 e Positions of the thermocouples in stack F2005-12 (left) and F1004-39 (right).
F2005-12, respectively. After reduction at 800 C by increasing the hydrogen concentration stepwise, the stacks were directly characterized and further tested. For stack F1004-39, the air flux was preheated by an electrical gas heater. The hydrogen fuel was humidified by mixing with steam from an electrical evaporator. For stack F2005-12, both air and fuel fluxes were preheated through a recuperative heat exchanger, and then heated to the working temperature (close to furnace temperature) via electrical gas heaters. F2005-12 was mostly operated with 10% pre-reformed LNG, since the internal reforming of methane without pre-reforming can lead to the formation of volatile nickel hydroxide and therefore faster anode degradation [13,15]. The fuel composition after 10% pre-reforming of LNG with a starting steam-to-carbon ratio (S/C) of 2 was simulated with a mixture of LNG, H2 and H2O. Because there is no CO2 pipeline in the laboratory, the small amount of CO2 after pre-
reforming, assuming complete conversion of CO through the water gas shift reaction, was replaced by steam. The S/C ratio was then 2.1 instead of 2. For simplicity of calculation, the S/C ratio and fuel utilization were calculated relative to 100% CH4. The deviations from the actual calculated values relative to LNG are less than 0.6%. The temperatures of the stack at different positions were measured by thermocouples as shown in Fig. 2. The holes for the thermocouples were all 10 mm deep inside the metallic plates. In order to measure the in-plane temperature profile in F2005-12, seven thermocouples were placed along the direction of gas flow in the middle layer of the stack. The thermocouples were positioned in such a way that the temperatures could be measured inside the manifold region, near the edges of the cell and near the center of the cell. Another five thermocouples were placed in the same layer, but at the opposite side, which delivered similar results. In F1004-39, there were
Fig. 3 e Operation of F2005-12 with high fuel utilization at 700 C and 750 C (furnace temperature).
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Fig. 4 e Comparison of the OCVs (left) and temperature profile (right) of F2005-12 at 130 h (after conditioning; blue), 355 h (after 90% uf operation at 700 C; red) and 620 h (after 90% uf operation at 750 C; green). (H2: 25.1 slm, air: 85 slm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
only two thermocouples in the thicker middle layer of the stack, which measured the temperatures near the edges of the cell. Both stacks were cooled down with forming gas (5% H2 in Ar) and then disassembled for visual inspection.
Results and discussion Stack F2005-12 F2005-12 was mainly tested under different fuel utilization by keeping the current density and air-to-fuel ratio (l) at 0.4 Acm2 and 2.9, respectively. The variation of fuel utilization was first carried out at the furnace temperature of 700 C, and later at 750 C. After a short test of 90% fuel utilization at 700 C (0.4 slm H2, 1.3 slm LNG, S/C~2.1), all cells still showed more than 1.22 V under OCV with dry H2, indicating no generation of detectable leakages in the stack due to operation at high fuel utilization. The test at 750 C started with 80% fuel utilization using 10% humidified H2. LNG was then slowly introduced into the stack by simultaneously reducing H2 and increasing the amount of steam step by step. As shown in Fig. 3, the stack was operated for about 100 h with 90% uf. A continuous drop of the voltage in cell 2 (from the bottom) was observed after about 25 h of operation. By decreasing the fuel utilization from 90% to 80%, cell 2 was stabilized, but at a lower level compared to the other cells. The electrical efficiency of
the stack under different fuel utilization is also shown in Fig. 3. Subsequent OCV measurements under forming gas and dry H2 showed that not only cell 2 but also cell 1 had leakages due to 90% uf operation. A comparison of OCVs and temperature profiles at 130 h (after conditioning), 355 h (after 90% uf operation at 700 C) and 620 h (after 90% uf operation at 750 C) is shown in Fig. 4, where overheating near the fuel outlet side due to leakages from cell 1 and cell 2 after 90% uf operation at 750 C can be also observed. Post mortem analysis of the stack showed that both cell 1 and cell 2 were oxidized at the fuel outlet side, as shown in Fig. 5.
Stack F1004-39 F1004-39 was first tested only with 20% humidified H2 at 750 C (furnace), but at different current densities. The current density was increased stepwise from 0.5 Acm2 to 1.5 Acm2. The air-to-fuel ratio l was always kept at 2.5 (i.e. oxygen utilization of 40%) during the tests. The stack performance with 0.5 Acm2 and 1.5 Acm2 is shown in Figs. 6 and 7, respectively. The large gap between cell 1 (bottom) and the other cells was assumed to be due to the poorer contact between the cell and interconnect in this layer, which was already observed before starting the fuel utilization tests. The calculated areaspecific resistances (ASRs) of the cells at four different
Fig. 5 e Oxidation of cell 1 (left) and cell 2 (right) near fuel outlet in F2005-12 after 90% uf operation at 750 C.
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Fig. 6 e Variation of fuel utilization in stack F1004-39 with 0.5 Acm¡2 at 750 C (furnace). current densities are shown in Fig. 8. The ASR was calculated from the difference between the average Nernst voltage and the measured voltage at a given current density. The average Nernst voltage is calculated according to the gas compositions at the inlet and outlet of the stack. Note that the decrease of the ASRs at higher current densities was purely an effect of the rising temperature, since the air-to-fuel ratio was kept constant at 2.5 for all current densities. The increased temperature gradient (T6 versus T5 in Fig. 8) with increasing current density was another indication of insufficient air
cooling. Under current testing conditions, stack F1004-39 showed similar behavior under different current densities. The ASRs of all cells remained almost constant up to a fuel utilization of 60%~70%. A slight increase in ASRs was observed for all cells at a fuel utilization of 80~85%, and subsequently concentration polarization became dominant. A comparison of the OCVs under dry hydrogen after operation at each current density is shown in Fig. 9, indicating that cell 1 and cell 3 were not gas-tight after 90% uf operation at 1 Acm2. This could also explain the relatively faster
Fig. 7 e Variation of fuel utilization in stack F1004-39 with 1.5 Acm¡2 at 750 C (furnace).
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Fig. 8 e ASRs of the cells in F1004-39 with different fuel utilization and current densities at 750 C (furnace).
increase of ASRs in cell 1 and cell 3 under 80% fuel utilization with 1.25 Acm2 and 1.5 Acm2, as shown in Fig. 8. The stack was disassembled after testing for visual inspection. Fig. 10 shows the anode sides of cell 1 and cell 3 during post mortem analysis, where the typical cell morphology after redox can be seen near the fuel outlet side. Compared to the severe oxidation shown in Fig. 5, the leakages in F1004-39 were relatively small, so that the cells could still be reduced during OCV, or during the cooling process with forming gas. It is even possible that the cells were reduced under load with lower fuel utilization.
Discussion Under current stack design and testing conditions, anode oxidation due to local fuel starvation during operation with
90% fuel utilization occurred in both the tested F10 and F20 stacks, which was not (or less) influenced by cell size, current density and type of fuels. The occurrence of oxidation near the fuel outlet side can be easily understood, since the fuel is consumed from the fuel inlet to the outlet side. In the case of operation with LNG at 0.4 Acm2 and 90% fuel utilization, the nominal oxygen partial pressure at the fuel outlet side is 3.2 1016 bar, which is close to the Ni oxidation potential of 1.0 1015 bar at 750 C. Even small inhomogeneities of the temperature and fuel distribution could lead to deviations of the local oxygen partial pressure. The fact that all the observed oxidation occurred in the middle part of the cell near the fuel outlet could either be an effect of the fuel distribution in-plane, or an effect of the temperature distribution, which was then correlated to the current distribution.
Fig. 9 e Comparison of OCVs of F1004-39 after different operation conditions (blue: BOL, 800 C; red: after 40% uf operation with 0.5 Acm¡2 at 750 C; green: after 90% uf operation with 0.5 Acm¡2 at 750 C; violet: after 90% uf operation with 1 Acm¡2 at 750 C; cyan: after 80% uf operation with 1.5 Acm¡2 at 750 C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 10 e Traces of oxidation in cell 1 (top) and cell 3 (bottom) in stack F1004-39 after operation with high fuel utilization and current densities at 750 C (furnace).
Effect of in-plane fuel distribution For standard operating conditions, the flow distribution on the anode side of an F20 design was modeled using a 3D CFD code. For reasons of symmetry, only half of the layer was calculated. In order to keep the model to a reasonable size, the nickel mesh was replaced by a simplified mesh structure with the same flow properties. A validation of this method is given in Ref. [16].
As can be seen from Fig. 11, the difference in flow velocity from the left side to the middle of the cell is very uniform. In particular, no drop in the velocity (i.e. mass flow) of the fuel is visible towards the middle of the cell opposite the outlet channel. Also described in Ref. [16] is the flow distribution in a 36layer stack. This deviates less than ±1%. In a 5-layer stack it is even more relaxed.
Effect of in-plane temperature distribution
Fig. 11 e Distribution of flow in-plane on the anode side of an F20 design: velocity vectors colored by velocity magnitude in m/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
As mentioned above, the holes for the thermocouples in the stacks were 10 mm deep inside the metallic parts. Therefore only temperatures near the edges of the cells along the fuel flow direction were measured. At the moment, it is possible to drill deeper holes of 40 mm for thermocouples, but these are still not deep enough to measure the temperature in the center of the cell. Great efforts have been made to reveal the distribution of the temperature, thermal gradient, as well as thermal stress and strain inside the cells and stacks by coupled 3D CFD and FEM analysis [17], where the temperature distribution transverse to the flow direction was also presented. Based on the CFD calculation, a temperature difference of 10~30 C was estimated along the edge of the cell at the fuel outlet side (i.e. between b and d in Fig. 5) depending on operating conditions. The effect of this temperature difference on fuel utilization was evaluated by a simple 1D model under defined conditions. Assuming a uniform air and fuel flow in the cell and maintaining a constant voltage of 750 mV, the variations of
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Acknowledgments The authors would like to thank all colleagues from the FOB department for carrying out the laboratory work. Cooperation with all colleagues from other departments on materials development, stack design and manufacture, and post-test analysis is also gratefully acknowledged.
references
Fig. 12 e Calculated differences in temperature, current density and H2 concentration in an F20 cell caused by a 20 C difference in the fuel outlet (air inlet) temperature based on a 1D model.
current density, temperature and concentration of H2 along ab and cd in Fig. 5 were calculated using the 1D model, taking the different temperatures at the fuel outlet (i.e. temperatures at b and d in Fig. 5) into account. The heat generated from the fuel cell reaction was not considered for the preliminary calculation. Fig. 12 shows the calculated results along a-b (blue lines) and c-d (green lines) in Fig. 5 when temperatures at b and d are 620 C and 600 C, respectively. Only with a temperature difference of 20 C did the resulting changes in current density lead to a 10% variation in fuel utilization (i.e. 95% vs. 85%) at the fuel outlet. In the case of operation with nominal fuel utilization of 90%, such variation could easily lead to local oxidation.
Conclusions Two stacks of different designs (F1004-39 and F2005-12) were tested in a furnace environment under high fuel utilization of up to 90% with 20% humidified hydrogen or 10% pre-reformed LNG, and showed basically similar behavior under current stack design and testing conditions. Both stacks could be operated smoothly up to a fuel utilization of ~85% in the temperature range of 750~800 C, although the increase of concentration polarization can already be observed at a fuel utilization of ~80%. Operation with 90% fuel utilization introduced a much higher contribution of concentration polarization and a high risk of fuel starvation. Local oxidations of the cells due to fuel starvation during operation with 90% fuel utilization were observed in both stacks at a similar position (i.e. in the middle of the cell near the fuel outlet side), regardless of fuel and current density applied. Preliminary calculations based on a simple 1D model show that local fuel utilization is strongly influenced by the temperature distribution. Even a temperature difference of 20 C could lead to 10% variation in fuel utilization at the fuel outlet. Therefore the fuel utilization is limited to 85% for safe operation of Fdesign stacks under current stack design and testing conditions.
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[16] Peksen M. A coupled 3D thermofluid-thermomechanical analysis of a planar type production scale SOFC stack. Int J Hydrogen Energy 2011;(6):11914e28. [17] Peksen M. 3D thermomechanical behaviour of solid oxide fuel cells operating in different environments. Int J Hydrogen Energy 2013;38(30):13408e18.
Glossary APS: atmospheric plasma spraying ASC: anode-supported cell ASR: area-specific resistance BOL: beginning of life
CGO: cerium gadolinium oxide Crofer 22 APU: ferritic chromium steel from ThyssenKrupp LNG: liquefied natural gas LSCF: lanthanum strontium cobalt ferrite MCF: manganese cobalt ferrite MFC: mass flow controller OCV: open circuit voltage S/C: steam-to-carbon ratio slm: standard liter per minute uf: fuel utilization WPS: wet powder spraying YSZ: Yttria-stabilized zirconia