Ind. Eng. Chem. Res. 1988,27, 1181-1185 33DMC5, 562-49-2; 23DMC5, 565-59-3; 2MC6, 591-76-4; 3MC6, 589-34-4; 3EC5,617-78-7;Pt, 7440-06-4; H3C(CHZ)&H3,142-82-5.
Literature Cited Anufriev, D. M.; Kuznetsov, P. N.; Ione, K. G. J. Catal. 1980,65,221. Bourdillon, G. Thesis, University of Poitiers, 1985. Bourdillon, G.; Guisnet, M.; Gueguen, C. Zeolites 1986, 6, 221. Breck, W. C.; Grose, R. W. Adv. Chem. Ser. 1973, 121, 219. Chen, N. Y.; Schlenker, J. L.; Garwood, N. E.; Kokotailo, G. T. J. Catal. 1984,86, 24. Chevalier, F.; Guisnet, M.; Maurel R. In Proceedings of the 6th International Congress on Catalysis, London; Bond, G. C., et al., Eds.; The Chemical Society: London, 1977; pp 478-485. Giannetto, G.; Perot, G.; Guisnet, M. In Catalysis by Acids and Bases; Imelik, B., et al., Eds.; Studies in Surface Science and Catalysis 20; Elsevier: Amsterdam, 1985; pp 265-271. Giannetto, G. E.; Perot, G. R.; Guisnet, M. R. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 481.
1181
Giannetto, G.; Cartraud, P.; Alvarez, F.; Guisnet, M. React. Kinet. Catal. Lett. 1988, in press. Guisnet, M.; Perot, G. In Zeolite: Science and Technology; Ribeiro, F. R., et al., Eds.; Nato Asi Series E 80; Martinus Nijhoff: The Hague, Boston, Lancaster, 1984; pp 397-420. Guisnet, M.; Alvarez, F.; Giannetto, G.; Perot, G. Catal. Today 1987, 1 415-433. Martens, J. A.; Thielen. M.; Jacobs, P. A.; Weitkamp, J. Zeolites 1984, 4, 98. Mirodatos. C.: Barthomeuf. D. J. Catal. 1985.93. 246. Stull, D. R.; Westrum, E. F., Jr.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; Wiley: New-York, 1969; pp 249-252. Weitkamp, J.; Farag, H. Acta Phys. Chem. 1978, 24, 327. Whyte, T. E.; Wu, E. L.; Ken, G. T.; Venuto, P. B. J. Catal. 1971, 20, 88. Received for review September 16, 1987 Revised manuscript received January 4, 1988 Accepted February 10, 1988
PROCESS ENGINEERING AND DESIGN Performance of an Internal Direct-Oxidation Carbon Fuel Cell and Its Evaluation by Graphic Exergy Analysis Nobuyoshi N a k a g a w a and M a s a r u Ishida* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta, Midoriku, Yokohama 227, J a p a n
An internal direct-oxidation carbon fuel cell was operated to evaluate its characteristics. The cell thickness. It had 2.0-cm2 platinum porous electrolyte was a yttria-stabilized zirconia disk of 1.5" electrodes on both of the surfaces. In the fuel chamber, charcoal was put as raw fuel and gasified by oxygen permeated from the air chamber. It was operated a t 1075, 1180, and 1275 K under atmospheric pressure, and its performance was analyzed from the viewpoint of exergy by using the energy-utilization diagram, assuming uniform gas composition in both the air chamber and the fuel chamber. On this diagram, exergy losses caused by electrode polarization, gasification, and ohmic loss were represented. The relation between the electric power obtained and the exergy loss in each subsystem was disclosed by examining the amount and level of the energy transformed. Solid oxide, like stabilized zirconia, is being applied as the electrolyte of high-temperature fuel cells because of its high oxygen ion conductivity (10 S m-l at 1200 K). A power generation system based on this cell may become quite efficient, since heat released from the electrode at high temperature is available for other processes demanding heat, e.g., gasification of solid fuel. Moreover, carbon monoxide which cannot be used in the phospholic acid fuel cell can be applied as fuel. The combination of a coal-gasification unit and a high-temperature solid oxide fuel cell was proposed by Zahradnik et al. in 1965, and some requirements imposed on the gasifier by this coupling were discussed. An internal direct-oxidation carbonaceous fuel cell using the solid oxide (Figure 5) may become a candidate for an efficient power generator in which low-cost fuel, like charcoal or coal, can directly be utilized. However, such a device has not been examined experimentally. Its performance will be affected not only by the polarization at the electrodes but also by the reactivity of the solid fuel in the cell chamber. 0888-5885/88/2627-1181$01.50/0
In this study, a high-temperature solid oxide fuel cell driven by carbon monoxide generated by gasification of charcoal was constructed and operated at 1075,1180, and 1275 K under atmospheric pressure to get the basic data on its performance. The results were analyzed from an energy conversion point of view by using a graphical method (Ishida and Kawamura, 1982). Experimental Section Experimental Apparatus. The fuel cell used in this study is represented in Figure 1. An 8 mol % yttriastabilized zirconia disk of 30-mm diameter and 1.5-mm thickness supplied by Nippon Kagaku Togyo Co., Ltd.was used as the electrolyte. Platinum paste (Tokuriki Co., Ltd., No. 8105) was smeared over 2.0 cm2on both faces and over 0.1 cm2 on the periphery. When this disk was baked at 1375 K for 5 min, the paste was changed to porous electrode. Then BO-mesh platinum gauze with a 0.2-mm-diameter platinum wire was connected to each electrode to serve as a current collector. The electrode chambers were separated from each other by this electrolyte disk with 0 1988 American Chemical Society
1182 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988
&.
A i r Inlet
,-+ A l r Outlet
I
I--
Pyrex Cap
-Pyrex Tube Mulllte Tubes
,Thermocouple
,Pt
Porous Electrode
c
Solid Electrolyte
-1
Mullite Crucible
1
1
1
2
4
6
8
C urrent, 16'A Ceramic Cement
Figure 2. Cell potentials at three temperature levels. 0 251
I
:I'l key
>
StoppeF
I
I
T. K
0 15
e-
Figure 1. Schematic diagram of the apparatus. Table I. Elementary Composition of the Charcoal element mass fraction element mass fraction C 85.21 S 0.18 H 1.20 ash 5.84 N 0.71
g h s ring seal and were kept at atmospheric pressure. The electrode on the periphery was used as the reference. In the cathode chamber, air was blown gently against the electrode, while in the anode chamber, a mullite crucible, with 17 holes of 0.7" diameter at the bottom, filled with charcoal of about 4-mm block was set at 5 mm below the electrode on the top of a mullite tube. This tube was also used to substitute nitrogen (containing oxygen of less than 20 ppm) for air in the fuel chamber while heating at the start of the experiment. The temperature in the vicinity of the disk was measured by a Pt-Pt + 13%Rh thermocouple. The electrode potentials were measured with a digital voltmeter. The current was controlled with a variableresistance box. Resistances between the electrode terminals were measured with a Kohlraush bridge at 800 Hz. The product gas from the fuel chamber was analyzed by TCD gas chromatography (with accuracy of 0.1-0.5%). The elementary analysis of the charcoal used as fuel is given in Table I. Characteristics of the Fuel Cell. The observed potentials of the air and fuel electrodes vs the reference are shown in Figure 2. The measurement was started at 15 h after closing of the circuit. By that time, the fuel chamber had been occupied by CO and COz as shown in Table 11, while the amount of consumed carbon calculated from the current load and the product gas composition was found to be negligibly small. It was confirmed that the potential of the air electrode was scarcely affected by the
i n ( [ am2) Figure 3. Polarization vs logarithm of the current density.
air flow rate. The difference in potentials of both of the electrodes gives the terminal voltage, V. The dashed lines in Figure 2 are obtained by taking only ohmic voltage loss into consideration. In this evaluation, the ohmic resistances between each terminal and the middle of the disk thickness, ra-mand rf-m,are calculated by dividing the whole resistance, ra-f,into two parts, ra-m and rf-m, proportional to the resistances between each terminal and the reference, ra-r and rf-r, as follows: = ra-dra-r/ (ra-r + rf-r)I
(1)
rf-m = ra-f[rf-r/(ra-r +rdl
(2)
ra-m
Since the observed potentials of the air electrode appeared along the dashed lines, Ira-m,the polarization of the air electrode is found to be negligibly small for the current examined. On the other hand, the observed potentials of the fuel electrode deviated far from the dashed lines, Irf-m, especially at 1075 K. The polarization, q, observed in the cell at current I is given by 7 = V, - V - Ira-, (3) where V , is the open-circuit voltage. Since the polarization for the air electrode was negligibly small, 7 in eq 3 is nearly equal to the fuel one, qfi In figure 3, vf is plotted against In i based on qf = [RT/(cunF)I In (i/io) (4)
Table 11. Open-circuit Voltages a n d Product Gas Compositions for t h e R u n in Figure 2 and Comparison of Equilibrium Constant ( K , ) for Reaction R-4with Kp' ( = P c o ~ / P c o zand r ) KP',, (Based on V , ) composition, mole fraction T.K v,. v i. A m-2 co cog NO H, Kp' KP'* KP 1075 1188 1275
0.936 1.05 1.10
130 194 223
40.1 90.3 98.4
56.2 8.0 0.2
3.0 1.5 1.2
0.7 0.2 0.2
0.29 10 484
0.41 23 123
8.11 49.1 156
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1183
r.
-- -.
-. -.
-
-.
r
-----
I
I x c
I
-
0
100
200
300
Operating Time, h
Figure 4. History of resistances at 1188 K.
x - 1
Isothermal Expansion
r"l= current
I
- -Gasification'
-Ce,(-----J
Figure 6. Energy conversion among energy donors and acceptors in the fuel cell system.
Porous Electrodes
Figure 5. Schematic representation of the fuel cell system.
where io is the exchange current density. Similar plots were reported by Archer et al. (1965), Karpachev and Ovchinnikov (1969), and Etsell and Flengas (1971). This Tafel plot gave straight lines at higher values of i, and an and io were obtained. as 1.84 and 9.8 A m-2 at 1275 K, 1.83 and 10.7 A m-2 at 1188 K, and 0.28 and 17.2 A m-2 at 1075 K. Each run was continued for 15 days. Figure 4 shows the and rf-m,inditypical history of the resistances, ra-f,raWm, cating its gradual increase especially at the early stage. System S t r u c t u r e Figure 5 illustrates the scheme of the system. When the circuit is closed, the oxygen passes through the electrolyte from the air chamber to the fuel chamber according to the difference in its potential, with the following reactions taking place at the air electrode and the fuel electrode, respectively:
-
02-+ C0(PCOf) C02(PCOZf) + 2e
03-2)
where Pjaand.Pjf denote the partial pressures of component j in the air chamber and in the fuel chamber, respectively. Summation of R-1 and R-2 yields 1/202(Poe) +
-
CO(PC0f)
CO?(PC02f)
03-3)
Then the open-circuit voltage, V,, is given as V, = [RT/ (4F)I In (P02a/P024
-
c + CO2~PC02,
2CO(PCO,)
Ta
= [RT/(4F)l In
(6)
(P02a/P02h)
V + Ira-f= [RT/(4F)I In (P02h/P021) Vf =
[ R T / ( ~ F ) Iin ~
~
~
(7) ~
(8) / p
During the expansion, heat (Qw) is absorbed from the circumstance at temperature T, producing the following power (W) as shown by the work sink in Figure 6: QW =
W
= IV
= [Jo~RTIIn
+ 12ra-f
(P02h/P021)
(9)
where Jo2 is the oxygen flow rate calculated by I/(4F)and 12ra-fdenotes the ohmic loss. Po21 is kept low by the following combustion of carbon monoxide, donating heat (QR5)to the circumstance:
(5)
where Pozfis the partial pressure of oxygen in equilibrium with Pcofand PcOzf. In the gasification section, C02 generated at the fuel electrode reacts with carbon in the charcoal layer, yielding
co:
for reaction R-4 calculated from the thermodynamic data for each component. Because a small error in the measurement of C02 concentration significantly affects the value of Kp' at high temperatures, Kp' at 1275 K may be considered to be equal to Kp, and the composition of the gas in the fuel chamber is found to be nearly equal to that at equilibrium for the current examined. A t 1075 K, however, Kp' and Kp', are smaller than Kp by an order of magnitude, indicating that the partial pressure of CO in the fuel chamber was smaller than that at equilibrium. On the basis of Figure 5, the energy transformation in this system is illustrated in Figure 6, where processes are represented by circles and energy flows are indicated by arrows. The process of the electric power generation may be expressed by the oxygen expansion from Pozato PoZpBy denoting the partial pressures of oxygen at the air and fuel electrodes by PoZh and P021,the following equations are given:
(R-4)
A part of CO produced is consumed as the reactant of R-2, and the rest leaves the chamber as gas product. Table I1 shows compositions of the product gas for the runs shown in Figure 2. Kp' denotes the ratio Pcof2/Pco2f at the specified current. K p f ,denotes Pco2/Pco2 at the open circuit obtained by substituting the observed values of Vo and Pozainto eq 5. KP is the equilibrium constant
In the gasification subsystem, the endothermic reaction R-4 takes place. So the circumstance at reaction temperature T donates heat for reaction R-4. Energy and Exergy Analysis Energy-Utilization Diagram. The changes in enthalpy and entropy for an energy-donatingprocess (ed) and an energy-accepting process (ea) satisfy the following first and second laws of thermodynamics (Ishida and Kawamura, 1982):
cAHj=
AHed
CASj = AS,d
+ AH,,
=0
(10)
+ AS,,
10
(11)
~
~
1184 Ind. Eng. Chem. Res., Vol. 27, No. 7,1988 Table 111. Fuel-Cell Performance When i = 80 A m-2
,.-
LU,
T,K 1075 1275
Tt, V 0.508 0.127
W , J s-l 6.84 X loT3 1.56 X
mol/mol of c 0.416 0.998
exergy loss, J s-l DOk. easif. 2.26 X 2.62 X lo'' 4.74 X 6.82 X
heat of reaction, J s-l R-5 R-4 -2.34 X lo-' 8.90 X -2.34 X lo-' 1.39 X lo-'
mol/mol of c 0.584 0.002
~~
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1
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ro-c.Cl 2.7 0.5
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,, ~
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