Direct Flame Fuel Cell Performance Using a Multi-element Diffusion Flame Burner. Y. Q. Wang, Y. X. Shi, X. K. Yu, N. S. Cai, and S. Q.

Transcription

1 / ecst The Electrochemical Society Direct Flame Fuel Cell Performance Using a Multi-element Diffusion Flame Burner Y. Q. Wang, Y. X. Shi, X. K. Yu, N. S. Cai, and S. Q. Li Key laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing , China. A direct flame fuel cell setup was designed and built in this work based on a multi-element diffusion flame burner. The direct combination of the flat flame and the solid oxide fuel cell in a simple, no-chamber setup is implemented. Flame temperature measurements were made using a fine-wire S-type thermocouple to characterize the burner performance. The MEDB was proven to provide uniform, ~1D conditions above the surface of the burner, with temperature variations of less than ±2% in the transverse direction (parallel to the burner surface). At heights above the flame front ( 10 mm), the axial temperature was also approximately constant within 45 mm. Direct flame fuel cell experiments were performed using an anode-supported solid oxide fuel cell (SOFC) based on button cell geometry under different operation conditions. The cell performance reached power density of up to 400 W/m 2, which was dependent on the flame condition and the original performance of the SOFC. Introduction Fuel cell is a device that directly converts chemical energy from a fuel into electricity through an electrochemical reaction and represents one of the cleanest, most efficient technologies for electricity generation (1). There are five major types of fuel cells and among them the solid oxide fuel cells (SOFCs) are emerging as one of the most promising fuel cells for their high efficiency and fuel flexibility (2). Convectional SOFCs are operated in a dual-chamber setup, in which the fuel and oxidizer are separated in two chambers with the help of sealant. However, the thermal expansion mismatch between the sealant and the electrodes remains a problem for this kind of configuration. Another configuration which has been proposed recently is the single-chamber solid oxide fuel cell (SC-SOFC) (3-6). In this configuration, both electrodes are exposed to the same fueloxidizer atmosphere, and the chemical potential gradient is achieved by the different catalytic selectivity of the anode and the cathode. Nevertheless, the anode and cathode materials are required to have high selectivity for the fuel and oxidizer, thus raising the manufacturing cost. Direct flame fuel cell (DFFC) is a kind of novel fuel cell, which combines the flame and the solid oxide fuel cell (SOFC) in a no-chamber setup. The flame consumes the oxygen at the anode side and provides the reacting heat for keeping the SOFC operating temperature (usually 923 K to 1073 K). The cathode is exposed to the air to maintain the gas concentration gradient between the different electrodes. Compared to the conventional dual-chamber and single-chamber SOFC, DFFC is advantageous in fuel-flexibility, sealing, simple setup and rapid start-up (7). 279

2 In the implementation of the DFFC setup, the selection of the burner type is of great importance since the temperature and composition atmosphere generated by the flame has a great influence on the performance and the stability of the SOFC. In the previous studies of DFFC, various kinds of burners were used. Horiuchi et al. put the SOFC into the fuel-rich flame of n-butane, kerosine, paraffin wax, and wood (7). A Bunsen-type burner was used in their research, which results typical cone-shaped flame and yields the non-uniform distribution of the temperature and the gas components at the DFFC anode. Wang et al. reported a solid oxide fuel cell operated on a fuel-rich ethanol flame and Sun et al. used a methanol flame instead, both with the problem of the flame non-uniformity (8, 9). The non-uniform distribution of the flame temperature will lead to additional thermal stresses to the SOFC and thus decrease the stability of the SOFC. To improve the flame uniformity, Kronemayer et al. used the McKenna-type flat flame burner to study the effects of different operating conditions on fuel cell performance (10, 11). Steady and uniform flame was provided for the fuel cell in these studies, but the adjustment range of fuel/air equivalence ratio and the fuel velocity were both limited due to the restriction of premixed flame. In this study, a multi-element diffusion flame burner (MEDB, a Hencken burner) was applied in a direct flame fuel cell system. The MEDB is a non-premixed, flat flame diffusion burner consisting of an array of the capillary needles set in a honeycomb matrix and is widely used in studies of particle combustion and coal pyrolysis (12-16). The MEDB is advantageous due to its unique arrangement, in which the fuel flows through individually sealed tubes and the oxidizer flows through the surrounding channels in the honeycomb. Consequently, fuel/air mixing occurs external to the body of the burner, eliminating the risk of flashback which is a main potential problem of the premixed flame. The flame provided by the Hencken burner consists of a large number of small diffusion flames, thus contributing to the uniformity of the flame. Wooldridge used the MEDB to investigate the gas-phase combustion synthesis of silica particles and demonstrated the burner used in their experiment can provide uniform, ~1D conditions above the surface of the burner (16). In our study, the burner is used to provide fuel-rich multi-element diffusion flame for DFFC. Its ability for ensuring the flame uniformity is demonstrated. Then the direct flame fuel cell experiments were performed under different operation conditions. Fuel Cell Experimental An anode-supported SOFC button cell made by SICCAS (Shanghai Institute of Ceramics Chinese Academy of Sciences) was used in the tests. It consists of a Ni/YSZ anode-support layer (680 μm), a Ni/ScSZ anode-active interlayer (15 μm), a ScSZ electrolyte layer (20 μm), and a lanthanum strontium manganate (LSM)/ScSZ cathode layer (15 μm) (17). The diameter of cathode layer is 13 mm and diameters of other layers are all 26 mm. The anode-support layer and active layer were prepared by mixing nickel oxide (NiO) powder (Inco Ltd., Canada) with 8mol% YSZ powder (Tosoh, Japan) and with ScSZ powder (Zr0.89Sc0.1Ce0.01O2 x, Daiichi Kigenso Kagaku Kogyo, Japan) respectively. 280

3 The powders were mixed at 50 wt% NiO and 50 wt% stabilized zirconia (YSZ or ScSZ). The electrolyte substrate was a dense film of ScSZ powder. Before testing, silver paste was reticulated on the anode and cathode surface by screen-printing for current collection. MEDB Setup The schematic of the top view of the multi-element diffusion flame burner is shown in Fig.1. The outlet of the burner has a dimension of 55 mm by 55 mm square. The burner is consisted of an array of capillary tubes set in a honeycomb matrix. There are about 232 capillary tubes evenly distributed inside the matrix. The inner and outer diameters of the tube are 1.2 mm and 1.5 mm respectively. The fuel passes through the inside of the tubes, while the oxidizer flows through the honeycomb on the outside of the capillary tubes. The gases were supplied using mass flow controllers. A circulating cooling water system was used to cool the burner. A round glass shade was added outside the burner to help improve flame stability and release the interference of the external environment. Figure 1. Schematic of the top view of the multi-element diffusion burner DFFC Characterization The schematic of the experimental setup is shown in Fig. 2. The fuel cell was located above the flame with the anode facing the flame front and the cathode exposed to ambient air. To avoid the diffuse of anode gas into the cathode side, the fuel cell was held by a metal disk (~150 mm). The metal disk was mounted to a x-y adjustable stage with a resolution of 1 mm to adjust the distance D between the SOFC and the burner. Insulation materials were added on the metal disk to minimize the heat loss from the cathode side. Figure 2. Schematic of experimental setup 281

4 The temperature of the fuel cell and the flame was measured by a K-type thermocouple and an S-type thermocouple separately. All thermocouple measurements were corrected for radiation effects (18), with an estimated uncertainty of ±10%, which is primarily because of uncertainties in the physical properties of the thermocouple and the flow (e.g. bead shape, temperature dependent emissivity, etc.). The composition of the flame was measured by a gas chromatograph (AutoSystem XL, Perkin Elmer, USA). And the IM6ex Electrochemical Workstation (Zahner-Elektrik GmbH, Germany) was used to acquire polarization curves. Burner Characteristics Results and Discussion The temperature field of CH 4 /N 2 /O 2 flame was measured to characterize the burner performance. All experiments were conducted at atmospheric pressure. In the experiments, CH 4 was used as fuel and the mixture of O 2 and N 2 was used as the oxidizer. The gas flow rates and the corresponding equivalence ratio are shown in Table I. The changing of the equivalence ratio was realized by the adjustment of the CH 4 flow rate and the N 2 flow rate was tuned to guarantee the flame stability for each equivalence ratio. The velocity of the reactants can be estimated based on the measured total volumetric flow rates and the known dimensions of the fuel tubes and oxidizer channels. For typical b operating conditions (e.g., =1.2 ), the velocity of the fuel gases was v fuel 0.17m/s and the velocity of the oxidizer gases was vo 2 /N m/s. The corresponding Reynolds numbers for the gases are Refuel 12 and ReO 2/N 182. Hence, a flame front consisting 2 of ~232 small laminar diffusion flamelets above the fuel tubes were formed at the burner outlet. TABLE I. Experimental Conditions for the Measurement of Burner Characteristics. Φ CH 4 (L/min) O 2 (L/min) N 2 (L/min) 1.2 a b b b a Operating Conditions Used to Measure the Transverse Temperature Profiles. b Operation Conditions Used to Measure the Axial Temperature Profiles. The flame front exists about 5 mm above the surface of the burner and in several millimeters above it the temperature gradient is quite steep. Above this region (about 10 mm above the burner outlet), the temperature profiles in both the vertical and horizontal directions show good uniformity. Fig.3 shows the measured temperature for an equivalence ratio of =1.2 in the x-y plane parallel to the surface of the burner at different heights. It can be seen that the flame temperature varies less than 2% from the mean value when the measuring point moves 20mm in x direction. The flame temperature distribution as a function of height above the burner for three different equivalences is shown in Fig. 4. It indicates that the temperature remains approximately constant from z 10mm to greater than z 55mm above the burner surface. It also demonstrates that the same quality of performance can be achieved over a range of burner 282

5 operation conditions. The average temperature for =1.1 is higher than that for =0.95 and =1.2, which is consistent with the well-known fact that the adiabatic temperature peaks at a slightly rich equivalence ratio. Figure 3. Transverse temperature profiles at different heights above the burner surface. All measurements were taken at y 0mm. Figure 4. Axial temperature profiles for different equivalence ratios. All measurements were taken at x 0mm, y 0mm. As mentioned above, the MEDB used in our study is demonstrated to have the ability to provide a steady and uniform temperature field for the DFFC operation. It should be noted that the uniformity can be assured in both the vertical and horizontal directions, which means that the MEDB is advantageous than the burners used in previous studies since the flame environment provided by the MEDB will lead to less thermal stresses to the SOFC in the startup and operation period. 283

6 DFFC Performance The DFFC experiments were performed using the anode-supported SOFCs. Since the fabrication process is not exactly the same for different button cells, the original performances of the cells were characterized in the dual-chamber setup under a standard operation condition for high-temperature SOFCs, as shown in Table II. Two different kinds of cell buttons with different performances were used in our experiments and their original performances were shown in Fig. 5. TABLE II. Standard Operation Condition for High-temperature SOFCs. T (K) H 2 (ml/min) Air (ml/min) Figure 5. Original performances of the SOFCs used in the study. The SOFCs were then used in the DFFC experiments to characterize its performance under different flame conditions which were implemented by adjusting the gas flow rates while maintaining the same equivalence ratio =1.2. The flow rates of the fuel and oxidizer are shown in Table III and the SOFC performances are shown in Fig The measured cell temperature and the flame species concentration near the anode are shown in Table IV. TABLE III. Operation Conditions for DFFC Experiments. CH 4 (L/min) O 2 (L/min) N 2 (L/min) Case Case Case Case

7 (a) (b) Figure 6. Polarization curves under DFFC operations (a) Test SOFC-1 (b) Test SOFC-2 Figure 7. Polarization curves and power density curves of DFFC for different gas flow rates Figure 8. EIS curves for different gas flow rates 285

8 TABLE IV. Flame Species Concentration and Cell Temperature for Different Gas Flow Rates. H 2 CO CO 2 H 2 O N 2 SOFC (%) (%) (%) (%) (%) Temperature (K) Case Case Case By comparing Fig. 5 and Fig. 6 we can see that the DFFC performance is dependent on the original performance of the SOFC used in the experiments. Although the exact values are not the same for different SOFCs, the changing trend of the DFFC performance remains the same. More specifically, when turning up the gas rates, the cell performance firstly increases and then decreases. It should be noted that the maximum limited current density acquired in DFFC operation conditions for different gas flow rates of =1.2 can both reach about 1/3 of that acquired in the dual-chamber setup under a standard operation condition. This indicates that the DFFC performance is comparable to the traditional dual-chamber SOFC. It should also be noted that the DFFC performance may reach a higher value once a SOFC with better original performance is applied. Fig. 7 shows the polarization curves and power density curves of DFFC for different gas flow rates. The OCV reaches 0.9 V in Case 1 and experiences a slight decrease to about 0.85 V when the gas flow rates increase. The maximum power density acquired here is 400 W/m 2, with the limited current density reaching 1700 A/m 2. From Table IV we can see that the cell temperature increases as the gas flow rates increases, ranging from 993 K to 1129 K, which is the typical temperature for SOFC operating. The flame species concentration is shown in Table IV. The exhaust gases of rich flames consist of a mixture of N 2, CO 2, H 2 O, CO, and H 2, while CH 4 and O 2 were nearly totally consumed at the flame front. Since the equivalence ratio remains the same for the different cases here, the gas concentrations varies a little due to the different mixing ratios of the oxidizer. A Novel DFFC Configuration From results shown in the previous section it can be seen that the MEDB used in our study is able to provide a uniform temperature field in both the vertical and horizontal directions. In this study, we have taken advantage of its uniformity in vertical direction and reduced the thermal stresses to the SOFC, thus ensuring the DFFC stability. However, based on the unique flame uniformity in the horizontal direction, a novel DFFC configuration can be proposed to make better use of H 2 and CO as well as the heat produced by the flame. The schematic structure of the newly proposed DFFC configuration is illustrated in Fig. 9. The MEDB is combined with a microtubular SOFC stack in this configuration. Recent years, due to the rapid starting and higher volume power density, the microtubular SOFC has been rapidly developed (19, 20). In this novel DFFC configuration, the outer wall of the single cell is the anode side which was inserted directly into the fuel-rich flame while the inside of the SOFC is the cathode with the air flowing through it. Since the flame of the MEDB has been demonstrated to provide a uniform-temperature region 286

9 extending more than 45 mm in z direction, the whole microtubular SOFC stack can be assumed to be placed in a uniform flame region. A DFFC system with the new configuration may fully utilize the heat and fuel produced by the flame, thus increasing the efficiency of the system. Microtubular SOFC stacks 40 Flame (a) (b) Figure 9. Structure of a novel DFFC configuration (a) single cell (b) a 3D view Conclusions The multi-element diffusion burner has been demonstrated as a valuable facility to provide fuel-rich flame for the direct flame fuel cell system. A direct flame fuel cell based on a MEDB which yielded steady and uniform flame was designed and built. The inherent flow characteristics of the multi-element diffusion burner make it an excellent tool to form a flat flame consisting of some small diffusion flamelets, resulting in a region where the flame condition is approximately uniform in both the transverse and axial directions. The flame temperature varies less than 2% from the mean value when the measuring point moves 20mm in x direction and it remains approximately constant from z 10mm to greater than z 55mm above the burner surface. The DFFC experiments were conducted under different gas flow rates by combining the MEDB and an anode-supported button cell. The performance of DFFC is dependent on the original performance of the SOFC used in the experiments. The maximum limited current density acquired in DFFC operation conditions for different gas flow rates of =1.2 can reach about 1/3 of that acquired in the dual-chamber setup when fueled with sufficient H 2. The cell temperature increases as the gas flow rates increases, peaking at 1129 K, which is the applicable operation temperature for high-temperature SOFC. The maximum power density acquired here is 400 W/m 2, with the limited current density reaching 1700 A/m 2. A novel DFFC configuration which is based on the MEDB and the microtubular SOFC stack was proposed taking advantage of the flame uniformity in the horizontal 287

10 direction. By making more use of the heat and fuel produced by the flame, a DFFC system with the new configuration will be an effective approach for heat and power cogeneration. Acknowledgements The authors acknowledge the Project supported by National Natural Science Foundation of China. We gratefully acknowledge the insightful discussions and offers of button cells used in experiments from Prof. Shaorong Wang in Shanghai Institute of Ceramics Chinese Academy of Sciences (SICCAS), China. References 1. B.C.H. Steele and A. Heinzel, Nature, 414, 345 (2001). 2. S. Kakac, A. Pramuanjaroenkij, and X. Y. Zhou. Int. J. Hydrogen Energy, 32, 761 (2007). 3. T. Hibino and A. Hashimoto, Science, 288, 2031 (2000). 4. Z.P. Shao, C. Kwak, and S.M. Haile, Solid State Ionics, 175, 39 (2004). 5. T. Suzuki, P. Jasinski, V. Petrovsky, H. U. Anderson, and F. Dogan, J. Electrochem. Soc., 151, 1473 (2004). 6. T. W. Napporn, X. Jacques-Bedard,F. Morin, and M. Meunier, J. Electrochem. Soc., 151, A2088 (2004). 7. M. Horiuchi, S. Suganuma, and M. Watanabe, J. Electrochem. Soc., 151, A1402 (2004). 8. K. Wang, R. Ran, Y. Hao, et al., J. Power Sources, 177, 33 (2008). 9. L. L. Sun, Y. Hao, C. Zhang, et al., Int. J. Hydrogen Energy, 35, 7971 (2010). 10. H. Kronemayer, D. Barzan, M. Horiuchi, et al., J. Power Sources, 166, 120 (2007). 11. M. Vogler, D. Barzan, H. Kronemayer, et al., ECS Trans., 7, 555 (2007). 12. J. L. Ma, T. H. Fletcher, and B. W. Webb, Energy Fuels, 9, 802 (1995). 13. J. J. Murphy and C. R. Shaddix, Combust. Flame, 144, 710 (2006). 14. P. J. van Eyk, P. J. Ashman, Z. T. Alwahabi, and G. J. Nathan, Combust. Flame, 155, 529 (2008). 15. C. R. Shaddix and A. Molina, Proc. Combust. Inst., 32, 2091 (2009). 16. M. S. Wooldridge, P. V. Torek, M. T. Donovan, D. L. Hall, and T. A. Miller, Combust. Flame, 131, 98 (2002). 17. Y.X. Shi, N.S. Cai, C. Li, C. Bao, E. Croiset, J.Q. Qian, Q. Hu, and S.R. Wang, J. Power Sources, 172, 235 (2007). 18. W. M. Pitts, E. B. Braun, R. D. Peacock, et al., ASTM Spec. Tech. Publ., 1427, 3 (2002). 19. N. M. Sammes and Y. Du, R. Bove, J. Power Sources, 145, 428 (2005). 20. M. Lockett, M. J. H. Simmons, and K. Kendall, J. Power Sources, 131, 243 (2004). 288