Operation Characteristics Of A Multiple Type Mcfc

S Kuroe, T Kamo, Hitachi Research Laboratory, Hitachi, Ltd. i -1, Omica-cho 7-chome, Hitachi-shi Ibaraki-ken, 319-12 Japan. E-mail: [email protected] H. Fujimura,

Power and Industrial Systems R&D Division, Hitachi, Ltd. 1-1 Saiwai-cho 3-chome, Hitachi-shi Ibaraki-ken, 317 Japan andT. Kahara Hitachi Works, Hitachi Ltd. 1-1 Saiwai-cho 3-chome, Hitachi-shi Ibaraki-ken, 317 Japan

Multiple type structure of MCFC of which the separator of the cell is divided by fourelement cells has been studied. For the stable operation of this type cell, the effect of gas flow rate and temperature distribution on the cell voltage should be clear.

In orderto clarify these characteristics, a small sized mimic model has been made and tested. The flow rate distribution for the four element cells were varied and cell voltage and temperature distribution were measured for each cell. The dscrease in cell voltage and the increase in maximum temperature became remarkable when the apparent utilization factor for one element cell became over 100%. The calculated results agreed fairly good with test results.

1. Structure and characteristic of a multiple type cell.

Figure 1 shows a schematicof a single cell of multiple type MCFC. A cathode, an anode, a current corrector and a gas channel were divided into fourdivided cells except a electrolyte plate. The reaction gases (for both cathodb and anocfe) flows from the center lattice part toward the peripheral regions, and the flow directions of anode gas and cathode gas are perpendicular to each other. Figure 2 shows a schematic of the multiple type cell used for experiments. The size of the cell was 50 cm x 50 cm, and the area of this cell is one-thirteenth of actual cell(100kW class). We called this multiple type cell as M900 stack. M900 stackhas2cells. They are defined as No. 1 andNo. 2 cellfromthe lowerpartsequentially. And M900 stack has four manifold(A,B,C,D). At each manifold, flow rate of the reaction gas into each divided celI(A,B,C,D) can be controlled independently.

2. Voltage-time characteristics of the multiple type cell

At first, the cell characteristic under equal flow rate to each 4 divided cells has been measured as abasic characteristic of the cell. Only time dependenceof No. 1 cell performance is shown in figure 3, because No. 2 cell showedalmost same tendency.Therefore, we indicate only the result of No. 1 cell in this paper. M900 stack was operated continually about 2,000 hours. The performance change until about 1,500 hours was shown in this figure. As for operating condition, hydrogen/carbon dioxide/steam ratio of anode gas was 72/18/10, and the air/carbon dioxide ratio of cathode gas was 70/30. And the M900 stack was operated in continually under the condition of 150mA/cm2 load(rated load of the cell), 650t operating temperature, 60% fuel gas utilization and 30% oxidant gas utilization. Under various load current condition, cell voltage and internal resistance (alternating current resistance at 1kHz) were measured simultaneously. The alternating current resistance was measured using Milli-ohm meter (HP4231A).

In figure 3, the open circuit voltage showed almost constant value of 1.07 V, and the cell voltage at the rated load showed 0.8 V in early stage, andhas a tendency to fall by gradually. The internal resistance showedalmost the same during the operation, approximately 0.6 Q cm2. The performance decay rate was 0.8% per 1,000 hour.

In figure 4, the relationship between operation temperature and cell performance decay rate is shown, which were obtained by small-sized cells of 100 cm2 class cell. It is clear that the performance decay rate increased with increasing of temperature. The performance decay rate at 650^C was approximately 1 % per 1,000 hours, and this characteristics was also the same as this M900 stack.

3. The effect of the flow rate distribution on the cell performance

Figure 5 shows the influence of the flow rate distribution on cells performance. This figure shows the results when the rate of fuel gas supply to each divided cells were changed, but the total gas flow rate was kept constant, and the fuel utilization at 40%. The case I indicates that the gas flow rate between each divided cells was equal, and the case 2 indicates that the gas flow rate of the divided cell A was increased by 30% than the average, and considerable share was reduced from divided cell B, in other words, the gas flow rate of the divided cell B was considerably reduced. The case 3 indicates that the gas flow rate of divided cells A and B was increased 30% than the average, and flow rate of remaining divided cells C andD were reduced by 30%. Finally, the case 4, the gas flow rate of the divided cell A was reduced by 90% than the average, and remaining divided cells B-D were increased by 30%. The vertical axis of the figure shows a ratio compared with the cell voltage of case 1, i.e., the deviation of the cell performance in the case 2-4 from the uniform gas flow dSstribution(case 1). And horizontal axis was defined by the following equation that represented a standard deviation of fuel gas flow rate of 4 divided cells.

(Deviation of flow rate)= 2 K(Qi/Q-l)}*2]/4 • • • (1)

Where Qi is the flow rate of the fuel gas to each divided cells and Q is the fuel gas flow rate of the average with all the divided cells, respectively. As a result, the performance change of case 4 was extremely large.

In figure 5, it is also indicated the simulated performance change ratio simultaneously, where the open circle plots were experimental results, and the region of slanted line was a simulated one. From this figure, in spite of the performance loss was within 0.6% when the change of the deviation of flow rate was within 20%, it was found, that the performance fall became large suddenly as the inclination with deviation of flow rate surpassed approximately 30%. The measured values were settled in the range that were predicted by the calculation. In this figure, the reason why the calculated values had some range, because there were several cases which had different patterns of flow rate distribution at the same deviation of flow rate.

4. Effect of the cfeviation of flow rate on a temperature distribution in cell

In figure 6, the change of the temperature distribution causedby the deviation of flow rate in four divided cells is shown. In this figure, the isothermal lines were obtained by using interpolation by Lunge Kutta methodusing the 20 points of measured temperature values. From this figure, the temperature in a divided cell A decreased because the fuel flow rate was decreased (The apparent utilization washigh). On the other hand, the temperature increased because the fuel flow rate was increased (The apparent utilization was low) in a divided cell D. These changes can be explained that the electric current was concentrated on a divided cell D by the sufficient gas flow rate and this increased the generation of heat.

5. Conclusion A reducedsize cell was produced, and the effect on the cell performances and the temperature distribution with changing the gas supply condition into four divided cells were examined. And we obtained the following results.

(1) The performance decay rate of the reduction model was agreed with which the performance decay rate of the small-sized cells.

(2) Cell performance deteriorated with increasing the deviation of the flow rate gradually, and the performance fall became remarkable when the apparent utilization with the divided cell exceeded 100%.

Acknowledgment

This work was conduced by the MCFC Research Association. The MCFC Research Association was commissioned to do the woik by NEDO (New Energy and Industrial Technology Development Organization) as a part of the New Sunshine Program of MITI (Ministry of International Trade and Industiy). We appreciate the advice and support of the MCFC R.A., NEDO and MITI.

Current collector-Anode-Electrolyte plate Cathode-Current collector Separator

/Thermocouple (^eristicsolslack ) —/—-J--—r a) Construction

Figure 2 Schematic cross-secSonal view ol one cell in the Multiple Type MCFC.

Figure 1 Schematic of Multiple Type MCFC.

^ -Ce3 size: 2500-I • Number ol cells: 2 I • Tola! electrode | area:900-gb) Characteristics

Quantity and composition olgasinA-Dset independently Temperature distribution measurements (20 points)

Figure 2 Schematic cross-secSonal view ol one cell in the Multiple Type MCFC.

150mA/oii

3 Q Cell voltage 0 Internal resistance

0 0

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