Environmental Engineering Reference
In-Depth Information
The carbon capture reaction in molten carbonate, combines Equations 8.3.2 and
8.3.3:
900
◦
C
CO
2
(gas)
→
C (solid)
+
O
2
(gas)
T
≤
(8.3.4A)
950
◦
C
CO
2
(gas)
→
CO (gas)
+
1
/
2O
2
(gas)
T
≥
(8.3.4B)
The electrolysis of carbon capture in molten carbonates can occur at lower experimen-
tal electrolysis potentials than the unit activity potentials calculated in Figure 8.3.3. A
constant influx of carbon dioxide to the cell maintains a low concentration of Li
2
O, in
accord with reaction 23. The activity ratio,
, of the carbonate reactant to the oxide
product in the electrolysis chamber, when high, decreases the cell potentials with the
Nernst concentration variation of the potential in accord with Equation 8.3.2, as:
E
CO2
/
X
(T)
E
CO2
/
X
(T)
=
−
0
.
0592 V
·
T(K)
/
(n
·
298 K)
·
log(
);
n
=
4or2,forX
=
C
solid
or CO product
(8.3.5)
For example, from Equation 8.3.5, the expected cell potential at 950
◦
C for the reduc-
tion to the CO product is E
CO2
/
CO
=
1.17 V
−
(0.243 V/2)
·
4
=
0.68 V, with a high
10,000 carbonate/oxide ratio in the electrolysis chamber. As seen in the Fig-
ure 8.3.3 photograph, CO
2
is captured in 750
◦
CLi
2
CO
3
as solid carbon by reduction
at the cathode at low electrolysis potential. The carbon formed in the electrolysis in
molten Li
2
CO
3
at 750
◦
C is in quantitative accord with the 4 e- reduction of Equa-
tion 8.3.2, as determined by (i) mass, at constant 1.25 A for both 0.05 and 0.5 A/cm
2
(large and small electrode) electrolyses (the carbon is washed in a sonicator, and dried
at 90
◦
C), by (ii) ignition (furnace combustion at 950
◦
C) and by (iii) volumetric analy-
sis in which KIO
3
is added to the carbon, converted to CO
2
and I
2
in hot phosphoric
acid (5C
=
4KH
2
PO
4
), the liberated I
2
is
dissolved in 0.05 M KI and titrated with thiosulfate using a starch indicator. We also
observe the transition to the carbon monoxide product with increasing temperature.
Specifically, while at 750
◦
C the molar ratio of solid carbon to CO-gas formed is 20:1,
at 850
◦
in molten Li
2
CO
3
, the product ratio is a 2:1, at 900
◦
C, the ratio is 0.5:1, and
at 950
◦
C the gas is the sole product. Hence, in accord with Figure 8.2.2, switching
between the C or CO product is temperature programmable.
We have replaced Pt, with Ni, nickel alloys (inconel and monel), Ti and carbon,
and each are effective carbon capture cathode materials. Solid carbon deposits on each
of these cathodes at similar overpotential in 750
◦
C molten Li
2
CO
3
. For the anode, both
platinum and nickel are effective, while titanium corrodes under anodic bias in molten
Li
2
CO
3
. As seen in the right side of Figure 8.3.3, electrolysis anodic overpotentials
in Li
2
CO
3
electrolysis are comparable, but larger than cathodic overpotentials, and
current densities of over 1 A cm
−
2
can be sustained. Unlike other fuel cells, carbonate
fuel cells are resistant to poisoning effects, (Sunmacher, 2007) and are effective with
a wide range of fuels, and this appears to be the same for the case in the reverse
mode (to capture carbon, rather than to generate electricity). Molten Li
2
CO
3
remains
transparent and sustains stable electrolysis currents after extended (hours/days) carbon
capture over a wide range of electrolysis current densities and temperatures.
+
4KIO
3
+
4H
3
PO
4
→
5CO
2
+
2I
2
+
2H
2
O
+
Search WWH ::
Custom Search