EPA-650/2-75-040
June 1975
Environmental Protection Technology Series
-------
EPA-650/2-75-040
DIRECT USE OF COAL
IN A FUEL CELL:
FEASIBILITY INVESTIGATION
by
R. D. Weaver, Laura Tietz,
and Daniel Cubicciotti
Stanford Research Institute
Menlo Park, California 94025
Contract No. 68-02-1808
ROAP No 2 IBJV-02 4
Program Element No. 1AB013
EPA Project Officer: Stanley J. Bunas
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
WASHINGTON, D. C. 20460
June 1975
-------
EPA REVIEW NOTICE
This report has been reviewed by the National Environmental Research
Center - Research Triangle Park, Office of Research and Development,
EPA, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These series are:
1. ENVIRONMENTAL HEALTH EFFECTS RESEARCH
2. ENVIRONMENTAL PROTECTION TECHNOLOGY
3. ECOLOGICAL RESEARCH
4. ENVIRONMENTAL MONITORING
5. SOCIOECONOMIC ENVIRONMENTAL STUDIES
6. SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS
9. MISCELLANEOUS
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to
develop and demonstrate instrumentation, equipment and methodology
to repair or prevent environmental degradation from point and non-
point sources of pollution. This work provides the new or improved
technology required for the control and treatment of pollution sources
to meet environmental quality standards.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
Publication No. EPA-650/2-75-040
11
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ABSTRACT
The first phase of a study of the feasibility of using coal to
produce electricity directly in a fuel cell is reported. The cell under
investigation is a molten-carbonate electrolyte cell using a coal anode
and an air cathode. The cell voltage, the polarization of the anode, and
the nature of gaseous products formed were investigated. Electrodes made
from charred coal yielded open-circuit voltages close to one volt. At
975 degrees Kelvin, the activation plus concentration polarization was
about 200 millivolts at current densities of 100 milliamperes per centi-
meter square and larger. At higher temperatures smaller polarizations
were observed. The gaseous anode products were primarily carbon dioxide,
with some carbon monoxide. Current efficiencies were somewhat less than
100 percent. The low values were possibly due to losses of anode gases
by mechanical means. Coal ash added to the molten electrolyte did not
appear to be deleterious to cell operation. Descriptions of possible
future engineering systems and thermodynamic limitations are presented.
This report was submitted in fulfillment of Contract No. 68-02-1808
by Stanford Research Institute under the sponsorship of the Environmental
Protection Agency. Work was completed as of 28 February 1975,
iii
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SERIES NUMBER
DISCLAIMER CLAUSE
This report has been reviewed by the Office of Research and Monitoring,
EPA, and approved for publication. Approval does not necessarily reflect
the views and policies of the Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
IV
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CONTENTS
ABSTRACT iii
LIST OF FIGURES vi
LIST OF TABLES vii
ACKNOWLEDGMENTS viii
I CONCLUSIONS 1
II RECOMMENDATIONS 3
III INTRODUCTION 4
IV EXPERIMENTAL PROCEDURES 12
V EXPERIMENTAL RESULTS 19
VI DISCUSSION OF RESULTS 33
VII EXTENSION OF CONCEPT TO POWER GENERATION 41
REFERENCES 45
APPENDICES
A ANALYTICAL TECHNIQUES, SPECIAL ANALYSES, AND EQUIPMENT . . 47
B RECORD OF EXPERIMENTS 56
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FIGURES
1 Total Coal-to-Electricity Conversion Concept 7
2 Theoretical Cell Voltage Based on CO /CO Equilibrium 9
3 CO Fraction for Various Temperatures and Pressures 11
Calculated for the Equilibrium C + CO = 2 CO
4 Illustration of Cell III 13
5 Current-Voltage Behavior of Graphite at Various Temperatures . . .21
o
6 Current-Voltage Behavior of Peabody Char at 973 K 22
7 Current-Voltage Behavior of Pocahontas Coal . .23
8 Current-Voltage Behavior of I'linois No. 6 Seam Coal 24
9 Gas Composition with Time Behavior for Graphite 26
10 Gas Composition with Time Behavior for Graphite, 27
Ash Contaminated
11 Gas Composition with Time Behavior for Peabody Char 28
12 Gas Composition with Time Behavior for Pocahontas Coal 29
13 Gas Composition with Time Behavior for Illinois No. 6 Seam Coal. .30
14 Flow Chart for Coal-to-Electricity System 42
15 Elution of Gases 49
16 Calibration Curves 50
vi
-------
TABLES
1 Open-Circuit Potentials Observed with Ash-Contaminated
Electrolyte 20
2 Summary of Reaction Product Studies at 973 K 31
3 Elution Times and Estimated Detection Limits for Various Gases 51
4 Record of Experiments Performed 57
vii
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ACKNOWLEDGMENTS
This project was funded by the Environmental Protection Agency,
Control System Laboratory, Research Triangle Park, North Carolina, whose
support is gratefully acknowledged. Particular expression of gratitude
is due Dr. John Smith, Chief, Engineering Analysis Branch, and
Mr. Stan Bunas, Project Officer, both of whom greatly aided this
project by their technical direction.
viii
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I CONCLUSIONS
The results of an initial study to determine the feasibility of the
direct utilization of coal to produce electricity in a high temperature
fuel cell are presented in this report. The system consisted of a molten
carbonate fuel cell with an air cathode and a carbon anode, and the study
concentrated on the anodic behavior of carbon in molten carbonate electro-
lyte, Satisfactory behavior was observed for anodes made from charred
o
coal at temperatures of 975 K and above. The potential efficiency of
production of electricity in the molten carbonate fuel cell system was
estimated to be more than one-and one-half times greater than that of a
steam generator. The system can utilize coal in a nonpolluting fashion
because the molten carbonate retains the pollutants from the coal.
The following conclusions can be made from this study:
• Carbon anodes can be successfully oxidized in a
molten carbonate electrolyte.
• Satisfactory electrochemical behavior can be achieved
at or above 975°K (700°C).
• The open circuit voltage of a cell with a charred
coal anode/molten carbonate electrolyte/air cathode
is about 1.1 v.
• Polarization of the anode is not excessive (200 mV
or less at current densities above 100 mA/cm ) at
o
temperatures of 975 K and higher.
• The presence of ash in the electrolyte does not impair
performance of the cell.
-------
Carbon dioxide was observed as the principal product
even though carbon monoxide is the thermodynamically
favored product.
A rough assessment indicates that the efficiency of
generation of electricity might be as high as 1.65
times that of steam-electrical generators. (This value
is based on a projection of fuel cell characteristics,
see p. 44).
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II RECOMMENDATIONS
The following are recommendations for future work:
The study of the feasibility of the direct use of
coal in a fuel cell should be continued and should
be directed toward the development of an engineered
system.
Studies of factors influencing CO /CO ratios in
product gas streams should be investigated.
Attempts should be made to define conditions yielding
CO /CO ratios of 10:1 or higher.
£t
' Experimental cells should be used that will allow
repeatable and precise correlation between the
quantity of carbon used and the electricity produced.
* Further studies should be made of electrochemical be-
havior at various conditions to allow necessary design
information for later engineering studies.
* Studies of ash on performance should be continued, with
the use of pure carbon (not graphite) in pure electro-
lyte as a control reference.
A study of matching air cathodes for use with the cell
should be carried out in parallel with the above studies.
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Ill INTRODUCTION
Coal is growing in its importance as a primary energy source for the
United States; this importance increases as the total of energy sources
is forecast for the next twenty or more years. Although coal is the most
abundant of energy resources for the United States, the present tech-
nology for its use provides two undesirable and economically and ecolog-
ically unacceptable products: thermal and atmospheric pollution. The
thermal pollution results from inefficiencies, imposed by Carnot limi-
tations to heat engines, in the conversion of the energy content of the
coal to electrical power. At best, as much heat is lost as is converted
to electricity. Atmospheric pollution results when particulate matter
(fly ash), oxides of nitrogen, and sulfur compounds are incorporated
in flue gases.
A conceptual method for the more efficient conversion of the energy
content of coal to electricity has long been known—electrochemical
conversion. The work energy available electrochemically is equal to
^G (the free energy of oxidation) whereas the work energy from heat
engines is /\Hn where /\H is the enthalpy of oxidation and n rs the
theoretical Carnot efficiency.
* These are the standard thermodynamic terms for free-energy change,
enthalpy change, and Carnot efficiency. The relationships are the
familiar AH = AG + TAS and n = Th - Tc/Th. In practice n is about 0.4.
c c
-------
Since ^G is not significantly different from /^H for many reactions and
n is generally about 0.4, the electrochemical energy available can be
twice the energy available from heat engines.
These facts were taken into consideration in early studies of direct
carbon fuel cells in which attempts were made to use carbon (from coal)
in an electrochemical cell. The reaction desired was:
C + 0 -» CO + electricity
Zt £t
in which the oxygen would have been derived from air used as a cathodic
reactant and the carbon would have represented coal as the anodic reac-
tant. In the studies performed between 1900 and 1930, however it was
demonstrated that, at the temperatures at which carbon provides satis-
factory current densities, carbon monoxide rather than carbon dioxide
is the principal product. Thus, full utilization of the energy content
of the coal was not achieved.
The work done in that early period did show, however, that carbonate
salts of the alkali metals in molten form served as chemically compat-
ible electrolytes for the electrochemical cells. More recently, work on
hydrogen and fuel-gas (methane, for example) fuel cells, using the
molten carbonate electrolyte, advanced the air cathode to nearly satis-
factory levels of performance. Thus, two essential parts of the cell
system, the cathode and the electrolyte, had received adequate initial
development by 1970. These developments were summarized by Liebhafsky
1 2
and Cairns and by Trachtenberg and Cole.
The use of carbonate salts to avoid or eliminate air pollution from
the stack gases of coal combustion systems has received recent attention
3
with favorable results. In terms of electrochemical cells for the use
of coal, the use of molten carbonates thus offers the added advantage
that pollutants would be trapped in the cell electrolyte rather than
released to the air.
-------
A concept for taking advantage of these facts was proposed by SRI
as a means of efficiently converting coal to electricity by using fuel
cells. The concept, shown schematically in Figure 1, consists of three
major steps: (1) a coal/air fuel cell, (2) a water-gas shift reactor,
and (3) a hydrogen/air fuel cell. The overall reaction for the entire
process can be represented as:
C + O -.CO + electricity (1)
£ ^
Because of the use of a fused-carbonate electrolyte, the proposed process
for converting the energy of coal to electrical power promises to provide
electrical power in a nonpolluting process.
The research reported here was undertaken to explore the feasibility
of the concept just described. Except for the carbon anode of the carbon/
air fuel cell, the technology for the three subsystems of the concept has
been developed. Therefore, a laboratory study of the carbon anode in a
carbonate electrolyte was undertaken.
The carbon/air fuel cell of interest involves chemical reactions at
the anode (the carbon electrode) and at the cathode (the air electrode)
in an electrolyte of molten alkali carbonates. The cathode reaction can
be written as:
1/2 02 + C02 + 2e~ _ C0=3 (2)
so that the reversible voltage depends on the pressure of O and CO at
£ 2
the cathode. The anode reaction can be most simply represented by:
C + 2CO" = SCO + 4e (3)
o 2
The anode reaction involves three oxidation states of carbon, and the
equilibrium among those three states, namely,
C + CO = 2CO (4)
^
must be considered in calculating the open circuit voltages. At low
temperatures, the equilibrium lies to the left and the gas is mainly
-------
COAL/AIR
FUEL CELL
l
CO
WATER-GAS
SHIFT REACTOR
H2
1
HYDROGEN/AIR
FUEL CELL
REACTIONS
2C + C0_ - 3CO +• 2e~ (anode)
O
1/2 O2 + CO2 + 2e~ - CO 3 (cathode)
2C + CO
1/2 0
SCO (net reaction)
Electricity
J3CO + 3H2O -» 3CO2
[3H2 + 60H" - 6H2O + 6e~ (anode)
1 6e" + 3/2 02 + 3H20 - 60H' (cathode)
I 3H2 + 3/2 O2 - 3H2O (net reaction)
*• Electricity
Overall: C + O2(air) - CO + Electricitv
SA-3547-25
FIGURE 1 TOTAL COAL-TO-ELECTRICITY CONVERSION CONCEPT
-------
CO ; at high temperatures, the gas is CO. Thus, it might appear that one
should consider the anode reaction to be
2C + CO~ = SCO + 2e (5)
*J
at high temperatures and
1/2 C + C0~ = 3/2 CO + 2e~ (6)
at low temperatures. However, it is sufficient to consider one anode
reaction, i.e., Eq. (5), provided the equilibrium Eq. (4) is invoked to
evaluate the partial pressure of CO at the anode.
The cell reaction can, therefore, be written:
2C + C0~ = SCO + 2e (anode)
2e~ + 1/2 0 + CO = CO~ (cathode)
2C + 1/2 0 (cathode) + CO (cathode) = SCO (anode) (7)
The standard free energy change for reaction (7) can be calculated from
4
literature data to be
AG° = 14100 - 62.65 T = -nFE° (8)
o -3
E = -0.3057 + 1.358 x 10 T (9)
Thus, the theoretical cell voltage is
o RT
E = E -~2F~ loS Q (10)
o -5
E = E - 9.92 x 10 T log Q (11)
3
p (CO, anode)
in WhiCh Q = p(CC- , cathode) • P1/Z(0. cathode) <12>
&t £
The theoretical cell voltages calculated are given in Figure 2.
The E° values were calculated from Eq. (9). The equilibrium CO pressures
at the anode are calculated as discussed below. The O and C0 pressures
8
-------
2.0
I I I I
1000
TEMPERATURE
1500
SA-3BA7-1R
FIGURE 2 THEORETICAL CELL VOLTAGE BASED ON CO./CO EQUILIBRIUM
-------
in the cathode were calculated for a mixture of 1/3 CO and 2/3 dry air
at the total cell pressure. The voltage is a nonlinear function of
temperature and pressure because of the shift in the CO /CO equilibrium.
The fraction of CO in the equilibrium vapor is shown in Figure 3.
At low temperatures, the gas is predominantly CO ; at high temperatures,
it is predominantly CO. The ratio of CO to CO is a function of temper-
ature and pressure.
Although open-circuit voltages are a necessary test for proper per-
formance, a more important criterion is voltage behavior while current
is passing because the voltages calculated from thermodynamics will be
modified by the kinetics of the reactions involved. As a practical guide,
2
it was decided that if operation at current densities of 100 mA/cm could
be achieved at cell voltages near those predicted thermodynamically,
feasibility would have been demonstrated.
Carbon can be anodically consumed to produce electricity in two
ways as discussed above. If oxidized to form carbon dioxide, as in
reaction (3), four Faradays of electricity per mole of carbon would be
passed, together with the formation of three moles of carbon dioxide.
If oxidized to form carbon monoxide, as in reaction (5), one Faraday of
electricity would be passed with the formation of 3/2 mole of carbon
monoxide. These relationships provide an additional test of the feasi-
bility.
10
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500
1000
TEMPERATURE — °K
1500
SA-3547-26
FIGURE 3 EQUILIBRIUM C02 FRACTION FOR VARIOUS TEMPERATURES AND PRESSURES
Calculated from the equilibrium C + C02 = 2CO
P2(CO) , . . 8919
11
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IV EXPERIMENTAL PROCEDURES
This section describes the various apparatus and procedures used in
this study.
ELECTROCHEMICAL CELLS
During the course of this study, three cells of different designs
were used. Cells I and II were useful only for measuring cell voltages
because the anodes were not designed for collecting the anode gases.
They were similar to cell III, except that the alumina container was
open to the furnace atmosphere and cell I used a nickel counter electrode
instead of graphite. Cell III was used to obtain almost all the results.
Figure 4 presents a schematic diagram of cell III; its components are
described below.
Working electrode
Anode materials - Graphite, coal char, and charred coals
Dimensions - Graphite, 0.64-cm diameter, 30.5-cm long,
immersed 1 cm in melt
Coals, approximately 2-cm long, 1.5-cm wide, and 0.8-cm thick
Gas atmosphere - He plus CO and CO generated by reaction with
the melt
Counter electrode
Spectrographic graphite rod 0.635 cm diameter x 30.5 cm long
12
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ANODE GASES OUT
1
•MMMM
OUTLET
1
M
c
[
X.
r
\N
/
x^
N. -il
1
I
'
'
't (
/ .
I
/
FLUSHING GAS (He) INLET
— A
*
—
—
•>
L- <_>
' /
IR
s*
/
1C
/
°2 '
^\
!
p
|
|
/ /
NLET GAS (CO2) INLET
"1
J
"I
J
^
r
•> — ^
/
*
/ /
P
HEAT BAFFLES
-------
Counter electrode gas environment
Helium was used up to run III-E, then changed to
CO to prevent cathodic deposition of alkali metals
in all subsequent runs.
Reference electrodes
Two separate electrodes were used, consisting of a gold or
silver wire coil dipping into the melt and flushed with a
slow stream of CO (33%) plus air (67%).
^
The cells were heated in an electrical resistance furnace equipped with
temperature controls actuated by thermocouples.
ELECTROLYTE AND ASH
The electrolyte used was the ternary eutectic 'of sodium, potassium,
and lithium carbonates in a 3:3:4 mole ratio. The carbonates were ACS
grade reagents, used as received without drying before use in the cell.
Studies of ash contaminated electrolytes were made by adding 10 wt % of
fly ash to the electrolyte. Spectrographic analysis of the fly ash is
given in Appendix A.
REFERENCE ELECTRODES
Reference electrodes for use in carbonate melts have been described
5,6 7
by Borucka and Sugiyama and by Appleby and Nicholson. The electrodes
used in the present work were similar. The half cell reaction occurring
was expected to be
2e~ + |02 + C02 = C0~ (13)
The partial pressures of 0 (0.14 atm) and CO (0.33 atm) were kept con-
£» £
stant by a small flow of air-CO mixture through the electrode. Since
£i
the literature mentions both silver and gold as suitable electrodes, one
reference electrode of each was included in the cell. We found no real
difference in the performances of the two reference electrodes, although
performance was often erratic for unknown reasons.
14
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ANODES
Studies of pure carbon in the form of graphite were made using
spectroscopic grade electrodes, 0.635 cm in diameter.
Studies on charred coal were made on samples of four types of coal:
Peabody char, Pocahontas, Decker Main and Illinois No. 6 seam coal.
Analyses are presented in Appendix A. The samples of the already-charred
coal (Peabody char, Western Kentucky coal, No. 9) were used as electrodes
by forming a lump to approximately a rectangular solid, 2-cm long,
1.5-cm wide, and 0.8-cm thick. A hole was then drilled in the end of
the bar, a 0.05-cm diameter platinum wire was placed through the hole,
and both ends of the wire were passed through an Al 0 tube of 0.64 cm
2 3
o.d. When pushed toward the sample of char, the alumina tube tightens
the platinum wire, assuring electrical contact. The alumina tube was
brought to the cold regions of the cell assembly where electrical con-
tact to internal cell circuitry was made through the ends of the
platinum wire.
The coal samples were charred by two techniques. The first in-
o
volved heating the sample in a helium atmosphere to 825 K at a controlled
rate of 3 degrees per minute. The second involved placing a sample of
crushed and sieved coal in a small fused-silica tube. A length of the
0.05-cm-diameter platinum wire, wrapped at the end into a helix, was
centered in the tube before addition of the crushed coal. The tube,
containing the platinum wire and coal was then heated with a torch to
a red heat (estimated 1300 K). The total time for heating was
approximately five minutes. This destructive distillation technique was
stopped when evolution of volatile matter ceased and the carbon had been
heated to a red heat. During the heating, a fused silica rod, placed
over the protruding platinum wire, was pressed against the top of the
coal sample to prevent expansion of the coal. Charred-coal electrodes
15
-------
so prepared slipped easily from the fused silica tube and retained the
dimensions of the tube's diameter.
The graphite electrode was submerged in the electrolyte about two
2
centimeters, which exposed about 4 cm of surface to the melt. The coal
electrodes as formed had a diameter of 0.95 cm and were submerged about
2
2 cm so that about 5 cm of geometric surface was in the melt. The true
electrochemical area was difficult to determine, however, because the
samples were honeycombed by large pores.
GAS AND GAS-FLOW CONTROL SYSTEMS
Commercial helium was used in the anode. Traces of oxygen were
removed by passing the gas over copper metal heated to 725 K, and
moisture was removed by passing the gas through a molecular sieve drying
o
tube (Linde 5A, dried under vacuum at 400 C) at room temperature. The
air-CO mixture was dried over molecular sieve. CO was a commercial
2* ^
dry grade and was used directly from the cylinder.
Flow rates were calibrated by observing soap-bubble movement in a
burette inserted in the line as needed. Flow rates were monitored by
glass-ball flow-meters.
GAS ANALYSES
All gas analyses were performed with a dual-column gas chromatograph,
Details of the equipment are presented in Appendix A. Samples of gases
to be analyzed were extracted using Hamilton Company Gas-Tight syringes.
3
The usual sample size was 1 cm . Calibration curves for CO and CO were
£
checked daily, and results obtained were accurate to within ± 2%.
ELECTROCHEMICAL ANALYSES
A list of the equipment most frequently used in the studies is pre-
sented in Appendix A. Study of the electrochemical performance of the
16
-------
anodes required measurement of anode versus reference potentials, the
current, and the polarization with current. Techniques used for these
measurements are described below.
Potential Measurements
The potential of the anode versus the reference electrode was
monitored with one channel of a strip-chart recorder. Assurance that
source resistance did not introduce errors was achieved by use of the
electrometer or by measurement of resistance by pulse techniques. The
electrometer was used for this purpose by varying its input impedance.
If no change in indicated voltage occurred at input resistances to one
decade below the minimum of the recorder, that is, down from 10 ohms
3
to 10 ohms, then it was accepted that the recorder presented accurate
potential readings. Pulse techniques are described below under polari-
zation measurements.
Polarization Measurements
The voltage behavior of the anode with various levels of current
was studied under steady-state and transient (pulse) conditions. Steady-
state behavior at various levels of current was studied by adjusting the
current to desired levels, either by adjusting the programmable power
supply or by adjusting the potentiostat in the galvanostatic mode. The
potential of the anode versus the reference was recorded on a strip-
chart recorder. An alternative method, and one consuming less'operator
time, was to program the potentiostat in the galvanostatic mode to
follow a linear change in signal with time. This signal was generated
by the waveform generator. At very low sweep rates, requiring 16
minutes to vary the current from zero to the desired upper current limit,
the results were in agreement with the manual steady-state technique.
Since the data obtained in this programmed mode were recorded on an x-y
recorder using the current and voltage analog outputs from the
17
-------
potentiostat, they directly produced the polarization curves.
Measurement of resistance and activation polarization require
fast-response pulse techniques. An oscilloscope was used to monitor
voltage behavior with time when the current was abruptly (less than 3
microseconds) changed from one level to another. Resistance values were
also obtained from the slope of the I-V polarization curves, discussed
above, and by use of an ac resistance bridge.
TYPICAL PROCEDURE
The procedure used to study the anodic behavior of carbon was as
follows. The assembled cell, equipped with all gas and electrical
connections and already containing the correct amount of electrolyte and
the anode under study, was raised from an idling temperature of about
773°K to the temperature desired for the study. Two to four hours were
required to reach and stabilize the temperature. During this time, the
flow rates of gases from the components of the cell were checked, the
chromatograph was readied for analysis, and chromatographic response was
checked against previously established calibration curves. If measure-
ments to correlate gas composition and rate with current were desired, it
was necessary to wait until a steady-state background for the gases was
attained. If the anode had been studied previously, this steady state
was sometimes reached in as little as two to four hours. If the anode
was newly prepared, however, or if the temperature change made.was
extreme (e.g., 1100-1200°K), longer periods were required.
Correlations of gas kind and evolution rate with various current
levels were undertaken after steady-state gas composition had been reached.
After the particular study was completed, the temperature of the cell was
lowered to 773 K. The temperature of the cell was not allowed to cool to
the freezing point of the electrolyte.
18
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V EXPERIMENTAL RESULTS
Studies of the electrochemical oxidation of carbon electrodes in a
molten carbonate electrolyte were made with various carbons and at
various temperatures, both with pure electrolyte and with electrolyte
containing 10% ash. The studies included the voltage of the carbons
relative to a reference electrode, which was essentially equivalent in
potential to the potential expected with an air cathode in a complete
cell. The electrochemical behavior during passage of significant current
was also studied.
POTENTIAL MEASUREMENTS
The voltages observed between the various carbon electrodes and the
reference electrode are presented in Appendix B. The values were often
erratic, but the erratic behavior could generally be traced to the
reference electrode; that is, the voltage between the working anode and
the graphite counter electrode was generally stable even when the anode-
reference voltage was erratic. The last eight runs with cell III (P to
W) seemed to have stable open circuit voltages. These values are
collected in Table 1, together with some of the earlier, erratic values .
CURRENT-VOLTAGE BEHAVIOR
The polarization of the carbon anodes was studied under various
conditions of current drain and sweep rate (rate of change of current)
as well as temperature and kind of carbon. The currents were controlled
by a programmable potentiostat and were recorded versus voltage on an
x-y recorder. Sweep rates were limited by the response of the recorder
to about 200 mA/sec. Representative results are presented in Figures 5
through 8.
-------
Table 1
OPEN-CIRCUIT POTENTIALS OBSERVED WITH
ASH-CONTAMINATED ELECTROLYTE
Anode
Graphite
Peabody char
Pocahontas
Illinois No. 6
seam
Run No .
III-I
III-J
III-K
III-L
III-M
III-N
III-O
III-P
III-Q
III-R
III-S
III-T
III-U
III-V
III-W
Temperature
(°K>
793
973
1168
983
981
973
1169
877
833
1043
971
982
985
974
973
Open-circuit
a
Voltage
(volts)
-0.81
-0.65
-0.47
—
-1.07
-0.74
-0.86
-1.10
-0.96
-1.08
-1.12
-1.08
-1.10
-1.13
-1.09
Theoretical
Voltage
(voits)
-0.95
-0.99
-1.17
-0.99
-0.99
-0.99
-1.17
-0.96
-0.96
-1.04
-0.99
-0.99
, -0.99
-0.97
-0.97
Reference electrode behavior was erratic during Runs III-I through III-o.
Voltages were stable and repeatable during Runs III-P through III-W.
20
-------
> -0.1
-0.8 —,
-0.9
TEMPERATURE SWEEP RATES
A - 973° K, (700° C) 16.7 min/1000 mA
B - 1173°K, (900°C) 3.33 min/1000 mA
C - 773° K. (500°C) 3.33 min/1000 mA
-1.0
I
I
I
I
100 200 300 400
CURRENT — mA
500
600 700
SA-3 547-28
FIGURE 5
CUR RENT-VOLT AGE BEHAVIOR OF GRAPHITE
AT VARIOUS TEMPERATURES
21
-------
SWEEP RATES
A - 0.0833 min/1000 mA
B - 8.33 min/1000 mA
-1.1
FIGURE 6 CURRENT-VOLTAGE BEHAVIOR OF PEABODY CHAR AT 973°K
22
-------
-0.1 —
-t.o
-1.1 -
-1.2
A - 0.0833 min/1000 mA, Before Electrolysis
B — 8.33 min/1000 mA, Before Electrolysis
C - 0.0833 min/1000 mA. After 100 mA Electrolysis
r
D
1
100
1 1
200 300
CURRENT — mA
1
400 500
SA-3547-30
FIGURE 7 CURRENT-VOLTAGE BEHAVIOR OF POCAHONTAS COAL AT 973°K
23
-------
-0.1 —
> Jl* —
CO
A - 0.0833 min/1000 mA, Before Electrolysis
B — 8.33 min/1000 mA, Before Electrolysis
C - 0.0833 min/1000 mA, After 100 mA Electrolysis
D - 0.0833 min/1000 mA, After 300 mA Electrolysis
-1.1
100
200
300 400
CURRENT — mA
FIGURE 8 CURRENT-VOLTAGE BEHAVIOR OF ILLINOIS NO. 6 SEAM COAL AT 973°K
-------
STUDIES OF REACTION PRODUCTS
The rates at which CO and CO were generated by electrolysis were
2
measured as described. Electrolysis corresponded to the anodic
(oxidative) electrode reaction of coal and, therefore, to the discharge
of a coal/air fuel cell. Examples of the variation of concentration of
gases with time for the various samples are presented in Figures 9
through 13. Results of the reaction-product studies are given in
Appendix B. The information on coals is summarized in Table 2.
CELL AUTOPSY
The cell had been maintained continuously at temperatures at or
o
above 773 K for more than five months. During this time, the alumina
cell components were in contact with the fused-carbonate electrolyte.
During the last two months, the electrolyte was contaminated with fly
ash. The cell was disassembled on termination of the study, and the
parts were examined.
On autopsy, a crack was discovered in the outer alumina tube. The
crack was small and coincided exactly with the end of the heating element
in the furnace. The surfaces of the alumina that had been exposed to
the electrolyte showed no evidence of chemical attack or staining.
The effect due to this crack is not known.
Most of the electrolyte could be poured from the cell while the
temperature was above the melting point. The electrolyte that remained
in the cell was dark and was clearly highly contaminated with the fly
ash. Polarographic analysis of the dark electrolyte showed that it
contained approximately 2 wt% of Fe (CO ) and 0.5 wt% of FeCO . The
£33 3
electrolyte that was poured from the cell was white and showed no evidence
of colored particulate matter dispersed within it. The concentration of
reducible species was undetectable in the white samples. It follows,
therefore, that most of the ash was insoluble in the electrolyte and
settled to the bottom of the cell.
25
-------
40
LU
E
UJ
Q
O
20
X
z
UJ
U
o
o
w
Flow Rate
1 CO2 Concentration
HI
cr
00
HI
o
o
TIME — hours
SA-3547-32
FIGURE 9 GAS COMPOSITION WITH TIME BEHAVIOR FOR GRAPHITE
AT 973°K (RUN III-F}
26
-------
50
to
Flow Rate
CO, Compotition
FIGURE 10 GAS COMPOSITION WITH TIME BEHAVIOR FOR GRAPHITE, ASH-CONTAMINATED ELECTROLYTE,
AT 973° K (RUN III-N)
-------
3 4
TIME — hours
FIGURE 11 GAS COMPOSITION WITH TIME BEHAVIOR FOR PEABODY CHAR AT
982°K (RUN III-T)
28
-------
10
CO
SA-3547-35
FIGURE 12
GAS COMPOSITION WITH TIME BEHAVIOR FOR POCAHONTAS COAL AT 985°K,
(RUN Ill-V)
-------
CO
o
TIME
hours
SA-3547-36
FIGURE 13
GAS COMPOSITION WITH TIME BEHAVIOR FOR ILLINOIS NO. 6 SEAM COAL AT 974°K
(RUN III-V)
-------
Table 2. SUMMARY OF REACTION PRODUCT STUDIES
AT 973°K (700°C)
CO
Anode
Graphite, pure electrolyte
Graphite, ash-contaminated
electrolyte
Peabody char
Illinois No. 6 seam coal
* balance helium
x. «>r-f-;«-*-SAw*s*-ioQ rral CU
Run
No.
III-F
III-M&N
III-T
III-T
III-U
III-U
III-V
III-W
Steady-state gas concentration
during electrolysis
fraction*
0.290
0.223
0.161
0.324
0.192
0.445
0.183
0.441
CO
fraction*
< 0.001
0.003
0.037
0.034
0.078
O.O49
0.082
0.048
Ratio
CO /CO
2 —
75
4
10
2
10
2
9
Electrolysis
current
mA
100
100
100
200
100
300
100
300
Current
Efficiency
Cj
95
70
60
75
65
80
70
75
/2.66 x fraction CO + 5.32 x fraction CO \ /gas flow ml,
ated from CE% = 1001 ,m__ __- 'HI fin
/min
-------
Samples of the black residue of electrolyte and ash from the
bottom of the cell were extracted in a Soxhlet extractor with concentrated
hydrochloric acid. A white gel, presumably silica resulting from the ash,
was found as an insoluble constituent.
The samples of charred coal electrodes were weighed before and after
electrolysis studies. The weight loss should be correlatable with the
known quantity of electrochemical charge passed through the electrode.
However, due to attack by the CO dissolved in the electrolyte or by
£t
mechanical loss, the weight losses observed were one order of magnitude
larger than those calculated on the basis of the known charge passed.
32
-------
VI DISCUSSION OF RESULTS
The purpose of the research was to observe the performance of
carbon anodes derived from coal in a carbonate melt to assess the
feasibility of direct utilization of coal electrochemically. The im-
portant factors for that purpose are:
• Cell voltage under current drain
Effect of temperature
• Current efficiency
• Gases produced (CO or CO )
Effect of ash
With the limited funds available it was possible to evaluate only general
trends in cell behavior with respect to these factors.
In general, the carbon anodes behaved surprisingly well, and better
than graphite anodes. The erratic behavior of the reference electrodes
caused the greatest electrochemical difficulty, but there was no time to
spend on improving their performance. The mechanical operation of the
cells was relatively straightforward but could be simplified by better
design. The biggest problem was the small leaks that occurred in the
anode gas-handling system. There was no problem with container materials;
alumina seemed unattacked after hundreds of hours of operation.
33
-------
OPEN CIRCUIT VOLTAGES
The most reliable operation of the reference electrodes was
observed in the last eight runs (III-P to III-W) during the time that the
various charred coals were tested in the ash-contaminated electrolyte.
These open circuit voltages, given earlier in Table 1, were generally
about -1.1 volts, which is about 0.1 volt greater than theoretical for
graphite. The greater value may be due to the fact that the carbon in
charred coal is in a thermodynamically less stable state than graphite.
It is well known that the heat of combustion of coke or amorphous carbon
g
is greater than that of graphite.
o
Theoretical calculations indicated that above about 1000 K the open
circuit voltages should become larger. With graphite in the absence of
ash, the voltages did increase with temperature (see results of cell II
in Appendix B). Because the charred coal anodes showed acceptably small
o
polarizations at about 970 K, they were not studied at higher tempera-
tures.
The voltages observed with graphite before and after ash addition
were not significantly different, although both were uncertain because of
poor reference electrode behavior. Therefore, to a first approximation
at least, the ash added to the electrolyte had no effect on open circuit
voltage, This result tends to be corroborated by the fact that the open
circuit voltages observed in runs with ash present on charred coals
(Runs III-P to III-W) were greater than theoretical.
POLARIZATION STUDIES
Three phenomena tend to reduce cell voltage when current is passed:
activation polarization, concentration polarization, and IR drop due to
cell resistance. These phenomena are influenced in different ways by
sweep rate during polarization and by cell temperature, and they can be
34
-------
modified to different degrees by cell design. For example, activation
polarization reflects a slowness in the chemical reactions occurring at
the electrode surface. It is a strong function of temperature, decreasing
with increased temperature and possibly mitigated by catalysts.
In the present work, the charred coals showed an acceptably small
o
activation polarization at about 1000 K. The resistance or IR drop is
not so sensitive to temperature but is dependent on geometrical factors
in the cell. The cell resistance was about 0.5 ohm in the present
studies, as measured by fast response (oscilloscope) techniques. Polari-
zation due to concentration effects was observed in some cells.
Presumably a difference in concentration of some ion(s) between the
anode and reference electrode was generated by passage of current. This
effect decreased with increase of temperature as expected (because
diffusivities of ions increase with temperature).
The results of polarization studies were illustrated earlier in
Figures 5 to 8 for triangular-sweep voltametry and by the voltages
observed during 100 mA electrolyses reported in Appendix B. The behavior
of graphite is shown in Figure 5. The curve at 1173°K, except for the
erratic reference electrode, is essentially linear with little hysteresis,
which indicates that IR drop is the predominant source of polarization.
The slope of the curve is in accord with the 0.5-ohm cell resistance.
At 973°K the substantially larger slope indicates polarization other
than IR, and the hysteresis of the curve implies that there is signifi-
cant concentration polarization. At 773°K the polarization is severe.
For the charred coals at 973°K, the fast sweep rate curves (curve A
of Figures 6, 7, 8) showed a small degree of hysteresis and a slope
slightly larger than that corresponding to 0.5 ohm. After a correction
for IR drop, the polarization (activation plus concentration) amounted
35
-------
to about 0.08, 0.15, and 0.1 volts at 100 mA for Peabody char,
Pocahontas, and Illinois No. 6 respectively.
The presence of concentration polarization with the charred coals
at 973°K is shown by the shift in the curves (from A to B in Figures
6, 7, and 8) as sweep rate decreased. At the lower sweep rates, the
electrolysis time to reach a given current was greater as was the con-
centration of electrolysis product. Also the curves immediately after
electrolysis (curves C of Figures 7 and 8) are shifted in the same
direction, again indicating the effects of concentration. Some indi-
cation of changing surface character (activation effects) were observed
in the scan taken after electrolysis.
The extent of steady-state concentration polarization did not alter
significantly as a result of extended electrolysis, as shown in Figure 7.
The slow-rate study of concentration polarization taken before electrolysig
(curve B) is essentially the same as the fast rate response, curve C,
taken after several hours of 100-mA electrolysis. Therefore, even
though the anode area may have changed somewhat during the electrolysis,
and the composition of the electrolyte might have been expected to change,
neither seemed to cause a noticeable change in concentration polarization
behavior.
The same behavior is displayed in Figure 8, in which curves A, B,
and C correspond to data taken for Run III-V at 100 mA. The level of
concentration polarization does change with the level of current, as
shown by curve D, which was obtained after a 300-mA electrolysis and
which shows greater polarization than observed at 100 mA.
To relate the observed polarization effects to large scale cell
operation, it is necessary to convert the effect of current to current
densities. That is, the effective electrode areas need to be known.
It was estimated that the geometrical anode surface exposed to the melt
36
-------
2
was about 4 to 5 cm . However, the tips of the electrodes are expected
to have a much greater current density than the sides. It is difficult
to calculate the exact distribution of the current. From consideration
of the geometry and current paths involved, we estimate that the anode
2
surface area was 2 ± 1 cm for purposes of scaling current densities.
CURRENT EFFICIENCY
The efficiency of conversion of the available energy of carbon to
electrical energy is a product of two terms: the voltage efficiency and
the current efficiency. The voltage efficiency depends on the activation,
concentration, and resistance polarization terms, as discussed above.
The current efficiency is a measure of how effectively the material is
being used to produce electricity. As indicated by equations (5) and
(6) in Section III, the quantity of electricity produced (moles of
electrons) per mole of carbon consumed, depends on the course of the
reaction, which, in turn, is indicated by the nature and amount of the
products.
Thus, four Faradays of electricity are produced if a mole of carbon
is oxidized to CO , while only one is produced if CO is the product.
Therefore, an important aspect of this study was to determine the amounts
of CO and CO produced at the anode during electrolysis.
£i
The current efficiencies and relative amounts of CO and CO for all
i "
the runs are shown in Appendix B. The results of some representative
runs were shown in Figures 9 to 13, and the results for the charred coals
were summarized in Table 2.
The first few runs with cell III (A to D) were made with helium in
the cathode compartment. When current efficiencies fell to very low
values, it was realized that the CO of the melt had been depleted and
o
that the melt must have become an electronic conductor by formation of
37
-------
Ikali metal. When CO was passed into the cathode, the current
efficiency rose to essentially 100%.
The measured current efficiencies were very sensitive to small gas
leaks in the anode gas-handling line. In Runs III-L and III-O, the low
current efficiencies were caused by such leaks. In some of the other
runs, the current efficiency was somewhat less than 100% and averaged
about 75%. Low current efficiencies can arise from either mechanical
losses in the cell due to poor design (i.e., leaks in the anode gas
handling system or the escape of bubbles from the bottom of the anode out
into the cathode region rather than into the anode sheath) or from
physicochemical processes (i.e., electronic conduction by the melt, or
chemical shuttle mechanisms in which anode products diffuse to the
cathode and are reduced). Limitation of time and funds prevented us
from exploring the true reason for the less than 100% current efficiency.
The ratio of CO to CO in the anode gas for all runs is shown in
Appendix B. The ratio during electrolysis was always larger than
expected from the equilibrium values shown in Figure 3. The most impor-
tant results pertained to the charred coals, as summarized in Table 2.
o
The equilibrium CO /CO ratio at 975 K is about 0.3 (dependent somewhat
£t
on the total pressure of CO plus CO). The observed values were from 7
£*
to 30 times as large. It is also apparent that the CO /CO ratio increased
iZi
with increased current density, and it was larger for graphite than for
the charred coals. These results indicate that the greater the degree
of irreversibility of the electrochemical oxidation, the larger the
CO /CO ratio. This fact is important for projection to industrial scale
^
operation because it indicates that the greater the current density, the
more effective will be the utilization of the char (i.e., the more com-
plete its combustion).
38
-------
The results also imply that the process of charring the coal may be
altered so as to result in chars that are electrochemically more active
and therefore give higher cell voltages by 10 to 100 mV.
The high CO /CO ratio in the cells studied is in accord with obser-
£
vations in the electrolysis of aluminum. In the aluminum industry,
carbon (Soderberg electrode) is anodically oxidized in a molten salt
(cryolite) at high temperatures. The conditions for that process differ
from the present anodic carbon oxidation in two ways: the temperature
is higher (~ 1300 K), and the melt has a much lower oxide ion activity.
Both of these factors would be expected to result in CO /CO ratios
A
smaller than those in our molten carbonate cells, although the anode
9 10
gas in aluminum electrolysis cells is mainly CO . Thonstad has
A
postulated electrochemical mechanisms on carbon surfaces to explain the
C02 production. Because of the importance of the nature of the gas pro-
duced for operation of the present cells, studies of the mechanism and
methods for optimizing it should be made.
INFLUENCE OF ASH
Ash might be expected to affect the cell by increasing melt viscosity
and increase the resistance by introducing impurities that poison the
carbon surfaces. Activation polarization could also increase by adding
impurities that reduce current efficiency.
In cell III, runs from I to W had 10 wt% fly ash in the electrolyte,
but there were no deleterious effects due to the ash. The performance
of graphite anodes seemed unaffected by the ash. The charred coal anodes
were tested with ash in the electrolyte only; however, their superior
performance, compared with graphite, indicated that the ash was not
harmful. The container and insulators were not corroded after hundreds
39
-------
of hours of use in the presence of ash. The only unresolved aspect was
the possibility that the iron impurity may have been the cause of the
current efficiencies below 100^; however, equally likely was the
possibility of mechanical loss of anode gas. That point needs further
resolution.
The solubility of ash in the molten carbonate is limited, although
unknown at present. The fact that ash constituents seemed to precipitate
from the melt suggests that, in a large scale cell, ash might be removed
by allowing the melt to become saturated so that additional ash simply
falls to the bottom of a cell.
40
-------
VII EXTENSION OF CONCEPT TO POWER GENERATION
The goal of this program is to increase the efficiency of utiliza-
tion of the work energy in coal, compared with steam-electric power
generation, by direct production of electricity. Although the information
required for careful designing is not available, it is instructive to
consider, in a preliminary way, overall systems for using the concept
in power generation.
Although our results have not defined the exact ratio of CO and
CO that would be produced in a large scale cell, the system does produce
electricity. A possible flow chart is shown in Figure 14. As shown,
the coal would be converted into a usable form for electrodes, and that
process would produce gases, the volatile components of coal. The
electrodes would be used in the carbon/air fuel cell, and the gases
would be used either in other fuel cells (using the hydrogen and hydro-
carbons) or as sources of heat. Technology for some of the steps is
already part of industrial practice. Carbon monoxide is converted to
11 12
H by several commercial processes (see Haddeland or Savage or
2 13
Hottel and Howard , and Soderberg electrodes are produced for the
14
aluminum industry (see Mantell).
An alternative concept for the use of coal could avoid the fabri-
cation of electrodes. This approach would use coal of suitable size In
a basket-type electrode suspended in the molten carbonate electrolyte.
In this system, the coal gases, many of which are electrochemically
active, would be consumed directly at the electrode to produce additional
power.
41
-------
MINE
I
Coal
ELECTRODE
FABRICATION
Coal Gas
Carbon Electrodes
CARBON/AIR
FUEL CELL
Useful Gases
CONVERSION
OF GASES
dc Electrical Power
HEAT
SYSTEMS
HYDROGEN/AIR
FUEL CELL
Heat
Mechanical dc Electrical
Power Power
I
POWER
INVERSION
ac POWER
DISTRIBUTION
SYSTEM
ac Electricity
SA-3547-37
FIGURE 14 FLOWCHART FOR COAL-TO-ELECTRICITY SYSTEM
42
-------
The electrochemical cell itself will receive much attention in terms
of design in future engineering studies. The air cathode must be
engineered to be electrochemically compatible, in terms of current
density, with the anode area. The hydrostatic pressure of the liquid
electrolyte must be accommodated in the operation of porous, gas
diffusion, air electrodes. The internal resistance of the electrolyte
must be kept low, and ash removal methods must be incorporated into the
design.
Whatever the final design of the cell, the anode gases must be
collected, and electronic contact to the carbon anode must be made. In
the concept envisioned, a conductive and premolded anode is used. The
thickness of this anode would be determined by mechanical strength and
electrical conductivity requirements. For immediate purposes, an elec-
trode thickness of two to four centimeters is assumed.
The rate at which the electrode must supply carbon is dependent on
the area of the cathode and the current density. For a current density
2
of 100 mA/cm , the total current will be 20 A/cm length of an electrode
immersed to 100 cm. Thus, the electrode must move down into the
electrolyte at a rate of about 1 cm/hour, and each 1-meter length of the
cell would produce 2000 Amperes.
The current density assumed will influence the cell voltage in two
ways: activation and concentration polarization (assumed to be 200 mV
from the present results) and resistive loss. An anode to cathode spacing
of 1 cm would result in a 50-mV IR drop since the melt resistivity is
about 0.5 ohm-cm. The total operating voltage then would be approxi-
mately the 1.1 volts observed for charred coals minus 0.25 (or 0.85 volts)
From the observed current efficiencies and assuming that only CO is
£t
produced, we can expect the efficiency of an operating cell to be at
least 80% (however, values close to 100% can probably be attained).
43
-------
On that basis the cell should produce
96.500 amp sec
0.85 volts x 0.80 x 4 x ' , =73 watt-hours
3,600 sec/hrs
of electricity per gram atom of carbon. In a conventional steam
generator, the efficiency is about 40% of the heat of coal combustion,
or about 44 watt-hours of electricity is generated per gram atom of
carbon. The efficiency of the fuel cell is significantly greater
(1.65 times as great).
Use of the volatile fraction of coal will not be explored in this
discussion. The pyrolysis of coal to produce coal char has been studied
8
in some detail , anJ. use of the gaseous products is already commercial
practice.
The subject of ash buildup and removal will require further inves-
tigation. As a means of evaluating the magnitude of the problem, the
following calculations can be made. In the cell envisioned above, the
3 2
electrolyte was 1-cm thick, so there is 1 cm /cm of electrode, and for
2 3
100 mA/cm about 0.1 cm /min of electrode is consumed. In about two days
of operation, the melt would contain about 10% ash. If the subsequent
ash is insoluble, it is expected that it will precipitate as alkali
metal silico-aluminate. The reaction will remove the alkali metal of
the carbonate at a rate of about 5% per day. For engineering estimates
one can assume that the ash will remove a weight of molten carbonate
from the electrolyte about equal to its own weight. The alkali metal
from that precipitate can probably be recovered by the same process used
15
in the Kellogg coal gasification concept.
44
-------
REFERENCES
1. Liebhafsky, H. A., and E, V. Cairns. Fuel Cells and Fuel Batteries.
New York, John Wiley and Sons, Inc., 1968. Chapter 2.
2. Trachtenberg, Isaac. Electrode Processes in Molten Carbonate Fuel
Cells in Fuel Cell Systems,Advances in Chemistry Series 47.
Washington, D.C., American Chemical Society 1965. Also, Trachtenberg,
Isaac, and David F. Cole. Electrolyte Studies for Molten Carbonate
Fuel Cells in Fuel Cell Systems - II, Advances in Chemistry Series
90. Washington, D.C., American Chemical Society, 1969.
3. Development of a Molten Carbonate Process for Removal of Sulfur
Dioxide from Power Plant Stack Gases. Atomics International
Company. Progress Report No. 4 to Environmental Protection Agency.
July 28, 1971.
4. Kubaschewski, 0., et al. Metallurgical Thermochemistry, 4th ed.
Oxford, Pergamon Press, 1967.
5. Borucka, A., and C. M. Sugiyama. Thermodynamic Evaluation of the
O /CO /Au Gas Electrode in Molten Alkali Carbonates. Electrochim.
Acta. 13:1887, 1968.
6. Borucka, A., and C, M, Sugiyama. Correlation of the Equilibrium
Potentials and Micropolarization Characteristics of the 0 /CO and
CO/CO Gas Electrodes in Molten Carbonates. Electrochim. Acta.
14:871, 1969.
7. Appleby, A. J., and S. Nicholson. The Reduction of Oxygen in Molten
Lithium Carbonate. Electro. Anal. Chem. Interfac. Chem.
53:105, 1974.
8. Lowry, H. H. Chemistry of Coal Utilization, Supplementary Volume.
New York, J. Wiley and Sons, Inc., 1963. p. 497.
9. Thonstad, J. Critical Current Densities in Cryolite-Alumina Melts.
Electrochim. Acta. 1967, 12, 1219.
45
-------
10. Thonstad, J. The Electrode Reaction on the C, CO Electrode in
Cryolite-Alumina Melts. Electrochim. Acta. 15: 1569 and 1581,
1970.
11. Haddeland, G. E. Hydrogen. SRI Process Economics Report No. 32A.
Menlo Park. December 1973.
12. Savage, R. L., et al. A Hydrogen Energy Carrier. Vol. II, Report
of MSA Grant, NGT 44-005-114. 1973.
13. Hottel, H. C., and J. B. Howard. New Energy Technology.
Cambridge, Massachusetts, MIT Press, 1971.
14. Mantell, C. L. Carbon and Graphite Handbook. New York. Inter-
science Publishers, 1968.
15. M. W. Kellogg Company. Commercial Potential for the Kellogg Coal
Gasification Process. R & D Report No. 38, Coal Research. 1968.
46
-------
Appendix A
ANALYTICAL TECHNIQUES, SPECIAL ANALYSES, AND EQUIPMENT
47
-------
Appendix A
ANALYTICAL TECHNIQUES, SPECIAL ANALYSES, AND EQUIPMENT
CHROMATOGRAPHIC ANALYSES
The gas chromatograph used was a Loenco Company Model 160 dual-
column gas chromatograph. After studies of conditions, the following
was established:
Column 1: molecular sieve, 0.635-cm o.d. copper tubing,
183-cm long, packed with 5-A 80/10
carrier flow rate of 65.9 cm /min.
183-cm long, packed with 5-A 80/100 mesh molecular sieve;
• Column 2: poropak Q , 0.635-cm o.d. copper tubing packed
with the Poropak Q material; carrier flow rate of 30.8
cm /min.
• Detector: thermal conductivity.
Detector temperature: 364 K (91 C) .
o o
• Column temperature: 403 K (130 C) .
o o
• Injection port temperature: 403 K (130 C) .
• Carrier gas: helium.
The gases detectable on these columns are indicated in Figure 15.
(S)
As indicated, the Poropak Q (PPQ) column was used to quantitatively
assay CO while the molecular sieve (MS) column was used to assay CO
^2
Typical calibration curves for CO and CO are presented in Figure
£1
16. The curves are presented at attenuation of x 32 (CO) and x 128
(CO ) . That is, the sensitivity for detection of these gases can be
&t
increased 32 and 128 times from the ranges normally used during runs.
Typical elution times and estimated limits of detection are listed in
Table 3.
48
-------
CO,
1.4 1.9 3.7
TIME — minutes
(a) POROPAK Q COLUMN
0 0.8 1.2 1.4 2.0 2.5
TIME — minutes
(b) MOLECULAR SIEVE COLUMN
FIGURE 15 ELUTION OF GASES
SA-3547-38
49
-------
10 —
100 200 300 400
MICROLITERS INJECTED
(a) CARBON DIOXIDE
500
s100
X
H
H
UJ
I
50
10
T
T
I
I
I
I
I
0 100 200 300 400 500 600
MICROLITERS INJECTED
(b) CARBON MONOXIDE
SA-3547-39
FIGURE 16 CALIBRATION CURVES
50
-------
Table 3
ELUTION TIMES AND ESTIMATED DETECTION
LIMITS FOR VARIOUS GASES
t
Gas
Hydrogen
Oxygen
Nitrogen
Methane
Carbon Monoxide
Carbon Dioxide
Water
Porapak Q Column
Elution time
min
-
1.4
1.4
-
1.4
1.9
3.7
Estimated
detection
limit
ml/1000 ml
-
-
-
-
-
0.15
1
Molecular Sieve
Elution time
min
0.8
1.2
1.4
2.0
2.5
30
"
Estimated
detection
limit
ml/1000 ml
10
0.3
0.4
0.05
0.5
200
~
-------
The typical chromatographic procedure was to activate the instru-
ment at the beginning of the day, and, during the next two hours, to
then check the flow rate of the carrier gas in each column. During that
period, and as free time during the day allowed, injections of a check
mix were made to assure constancy of chromatographic response. The
elution times and chromatographic response were remarkably constant for
all gases during the course of this study with the exception of the be-
havior for carbon monoxide. The response to this gas varied from day to
day by as much as ± 10%, whereas the response for all other gases varied
by about 2%. The behavior of CO during a single day, however, was
similar to the constancy of the other gases. Since calibration every
day served to assure accuracy of analysis within 2% error, no further
study of the anomalous behavior of CO was made.
SPECTROGRAPHIC ANALYSIS OF FLY ASH
Fly ash, formed from the combustion of pulverized coal and discharged
from an electrostatic precipitator, was used to contaminate the ternary
carbonate eutectic. The elemental spectrographic analysis of the ash,
made by the American Spectrographic Laboratories, Inc., San Francisco
California, on assay ASL #2154, is given below in weight percent.
Fe
Al
Ca
K
Mg
B
Ti
Na
Li
Zn
Ba
Cr
V
20
15
4
1
0.6
0.6
0.85
0.5
0.1
0.1 (?)
0.06
0.02
0.02
Sr
Mn
Cu
Zr
Ni
Mo
Co
Ga
Y
Sc
Be
Pb
0.02
0.02
0.01
0.03
0.01
0.01
0.005
0.005
0.005
0.004
0.002
<0.005
Si + nondetectables: Balance
52
-------
PROXIMATE ANALYSES OF COAL SAMPLES
Samples of four types of coal were obtained from Commercial Testing
and Engineering Company, Chicago, Illinois. Analyses of these coals, as
provided by Commercial Testing and Engineering, are given below.
PEABODY CHAR
% Moisture
% Ash
% Volatile
% Fixed carbon
Btu/lb
% Sulfur
As received
7.55
13.01
8.92
70.52
100.00
11,396
1.46
Dry basis
14.07
9.65
76.28
100.00
12,327
1.58
POCAHONTAS
% Moisture
% Ash
% Volatile
% Fixed carbon
Btu/lb
% Sulfur
As received
2.33
4.37
25.42
67.88
100.00
14,702
0.41
Dry basis
..
4.47
26.03
69.50
100.00
15,053
0.42
53
-------
ILLINOIS NO. 6 SEAM
% Moisture
% Ash
% Volatile
% Fixed carbon
Btu/lb
% Sulfur
As received
7.07
8.56
32.82
51.55
100.00
12,342
1.84
Dry basis
_.
9.21
35.32
55.47
100.00
13,281
1.98
DECKER MINE
% Moisture
% Ash
% Volatile
% Fixed carbon
Btu/lb
% Sulfur
As received
22.51
5.40
29.42
42.67
100.00
9,513
0.34
Dry basis
__
6.97
37.97
55.06
100.00
12,277
0.44
54
-------
LIST OF PRINCIPAL ELECTROCHEMICAL EQUIPMENT
Three dual-channel strip-chart recorders Mosely 2100B
Potentiostat, Princeton Applied Research Corporation, Model 173
Wave-form generator, Princeton Applied Research Corporation, Model 175
x-y Recorder, Hewlett Packard, Model 7035B
Conductivity bridge, Philips Corporation, Model GM 4249
Electrometer, Keithly, Model 610A
Programmable power supply, New Jersey Electric Company, Model SVC 40-5
Oscilloscope, Tektronix, Model 535A
55
-------
Appendix B
RECORD OF EXPERIMENTS
56
-------
Table 4. RECORD Of EXPERIMENTS PERFORMED
01
-J
I
II
III
A
B
C
D
A
B
C
D
E
F
G
H
A
B
C
D
E
T
G
H
I
j
K
L
O
P
II
f
T
U
V
w
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Graphite
Peabadr"
•""1*
Peabodjr' •
Peabadr"
Peabody- •
Pea body*
Peabody"
Pocahnntas
Pocahontaa
Illinois'"
llltnot«'"
Ash
Preaent
-
-
:
_
-
-
-
.
-
-
-
-
_
_
«
+
,
•
.
4
t
4
f
Temperature
838
733
943
1053
656
865
973
1031
1031
973
1051
1176
773
773
1163
1163
8*1
871
973
1078
1078
1183
1183
793
9T3
11M
M3
9(1
•73
1169
1189
877
833
1040
971
982
9*2
9*5
985
974
973
565
460
670
7«0
383
592
TOO
758
758
TOO
778
9O3
500
500
890
890
608
588
TOO
•OS
•OS
•10
910
520
TOO
•95
•95
T10
701
700
MM
•96
•04
560
770
•»»
709
TO9
T12
T12
T01
TOO
Voltage, Anode
Veraua Reference
Open
Circuit
-0.86
-0.57
-O.74
-1.O3
-1.28
-1.16
-1.15
-1.13
-1.18
-1.10
-1.16
-1.15
-0.65
-0.97
-1.06
-1.45
-1.05
-O.IS
-O.92
-0.26
-0.81
-O.69
-0.47
-1.07
-0.74
-1.10
-0.9«
-1 .08
-1.12
-1.08
-1 .10
-1.13
-1.09
At 100 aA
-0.19
0
-0.52
-0.76
-0.50
-0.78
-0.77
• -0.95
-1.10
-0.84
-0.88
-1.10
-0.08
-O.O4
-0.85
-0.45
-0.1
-0.08
-0.57
•0.3
tO.OS
tO.Oft to -0.55
-0.28 to -0.38
-o.es
-1.10
Too erratic
-0.94
-0.9
-0.7>
-0.55
-1.10
-0.74
-1.13
-O.IS to -0.43
Gaa Composition
Before Electrolysis
Fraction
Co.
.014
,002
.OO3
.OO9
.027
.017
.002
.023
.109
,OO6
.031
.017
.090
.005
,O1O
.168
.172
.021
.OBI
.042
.073
Fraction
CO
.057
*
.009
.114
•
.004
.407
.OO3
.006
.OO2
.477
.
.102
OO6
.004
.030
.037
.010
Ratio
CO./CO
.035
3.0
.150
5.75
.268
2.0O
3.17
».50
.189
1 65
12.0
5.25
2.03
1.14
7.3
During
Fraction
CO,
.314
.003
.OO5
. OO6
.272
.280
.260
.056
.056
.033
.316
.250
.170
.152
.021
.196
.223
.Ot)4
.OHH
.432
. 223
. 174
.233
. 161
.324
.182
.445
.183
.441
Electrolyal*
Fraction
CO
.059
.266
.038
.024
.275
.232
.480
.536
.001
.OO5
.487
.303
•
207
.016
.037
.034
.078
.049
.082
.O4K
Ratio
CO./CO
.051
.019
6.84
2.33
.204
.142
.354
.284
21.0
38.2
74.3
.183
.171
.K41
14. e
4. 3D
9.33
2.46
V.UH
2.23
9.18
Electralyala
Current
•A
100
100
300
100
100
100
100
20
100
50
100
100
1OO
200
100
100
100
100
3OO
100
a 00
100
100
kOO
200
too
3OO
10O
3OO
Current
Efficiency
112
1.2
14
2
93
93
»7,5
5H. 1
52. H
60. 1
105
81 .6
1O4
74 . 9
5.3
62.2
70.3
H.'.>
'2. 0
HM,<*
73 . 2
43.0
AH . U
5S . 7
74 , 4
67 . ft
82 . H
7O. *>
77 , 0
Remarks
Cells I and II were
designed tor anode
volt a tie studies only.
Analyses ui anode
with cells I and II.
Runs III AtoD: Prubatale
interference to perfor-
mance by alkali metals
deposited at cathode.
Huns til E to N:
Reference electrode
crrat ic .
Added ash tu ele^lro-
lyle at Hun I 1 1- 1 .
PrubMblu leak in Mytttum.
Standard procedure
f inml 1 /oil »n ol Hun
I I I -0 .
b«unk (1 1 m ovpifd In
nystvm.
Value* **re less than O.O01
Peahodr char.
•Illlaols lo. 6 aval.
-------
TECHNICAL REPORT DATA
(Please read /imnelions on the reverse before completing)
I. REPORT NO.
EPA-650/2-75-040
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Direct Use of Coal in a Fuel Cell: Feasibility
Investigation
5. REPORT DATE
June 1975
6. PERFORMING ORGANIZATION CODE
7. AUTHORSRobert D. Weaver, Laura Tietz, and
Daniel Cubicciotti
8. PERFORMING ORGANIZATION REPORT NC,
Project PYU-3547
9. PERFORMING OR9ANIZATION NAME AND ADDRESS
Stanford Research Institute
Menlo Park, CA 94025
10. PROGRAM ELEMENT NO.
1AB013; ROAP 21BJV-024
11. CONTRACT/GRANT NO.
68-02-1808
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
NERC-RTP, Control Systems Laboratory
Research Triangle Park, NC 27711
IOD COVERED
13. TYPE OF REPORT AND PERIOD CO
Final; 6/26/74 - 2/28/75
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACTThe rep()rt gives results of the first phase of a study of the feasibility of
using coal to produce electricity directly in a fuel cell that uses a molten-carbonate
electrolyte, a coal anode, and an air cathode. The cell voltage, the polarization of the
anode, and the nature of the gaseous products formed were investigated. Electrodes
made from charred coal yielded open-circuit voltages close to 1 V. At 975 K, the
activation plus concentration polarization was about 200 mV at current densities of
100 mA per sq cm and larger. At higher temperatures smaller polarizations were
observed. The gaseous anode products were primarily CO2, with some CO. Current
efficiencies were somewhat less than 100 percent. The low values were possibly due
to losses of anode gases by mechanical means. Coal ash added to the molten electro-
lyte did not appear to be deleterious to cell operation. Descriptions of possible
future engineering systems and thermodynamic limitations are presented.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C, COSATI Field/Group
Air Pollution
Fuel Cells
Coal
Anodes
Molten Salt
Electrolytes
Carbonates
Cathodes
Sulfur Oxides
Nitrogen Oxides
Dust
Air Pollution Control
Stationary Sources
Air Cathode
Particulates
13 B
10B
2 ID
09A
07D
07B
11G
3. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
64
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
58
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