Environmental Engineering Reference
In-Depth Information
MgCl
2
boiling point of 1412
◦
C, the MgCl
2
electrolysis potential decreases from 3.07 V
to 1.86 V. This decrease provides a theoretical basis for significant, non-CO
2
emitting,
non-fossil fuel consuming processes for the generation of chlorine and magnesium, to
be delineated in Section 8.3.4, and occurring at high solar efficiency analogous to the
similar CO
2
STEP
process.
In Section 8.3.2 the
STEP
process will be derived for the efficient solar
removal/recycling of CO
2
. In addition, thermodynamic calculation of metal and chlo-
ride electrolysis rest potentials identifies electrolytic processes which are consistent with
endothermic processes for the formation of iron, chlorine, aluminum, lithium, sodium
and magnesium, via CO
2
-free pathways. As shown, the conversion and replacement
of the conventional, aqueous, industrial alkali-chlor process, with an anhydrous elec-
trosynthesis, results in a redox potential with a calculated decrease of 1.1 V from 25
◦
C
to 1000
◦
C.
As seen in the top right of Figure 8.2.1, the calculated electrochemical reduction
of metal oxides can exhibit a sharp, smooth decrease in redox potential over a wide
range of phase changes. These endothermic process provide an opportunity for the
replacement of conventional industrial processes by the
STEP
formation of these met-
als. In 2008, industrial electrolytic processes consumed
5% of the world's electricity,
including for aluminum (3%), chlorine (1%), and lithium, magnesium and sodium
production. This 5% of the global 19
∼
10
12
kWh of electrical production, is equiva-
×
10
8
metric tons of CO
2
(Pellegrino, 2000). The iron and steel
industry accounts for a quarter of industrial direct CO
2
emissions. Currently, iron is
predominantly formed through the reduction of hematite with carbon, emitting CO
2
:
lent to the emission of 6
×
Fe
2
O
3
+
3C
+
3
/
2O
2
→
2Fe
+
3CO
2
(8.2.13)
A non-CO
2
emitting alternative is provided by the
STEP
driven electrolysis of Fe
2
O
3
:
E
◦
=
Fe
2
O
3
→
2Fe
+
3
/
2O
2
1
.
28 V
(8.2.14)
As seen in the top right of Figure 8.2.1, the calculated iron-generating electrolysis
potential drops 0.5 V (a 38% drop) from 25
◦
C to 1000
◦
C, and as with the CO
2
ana-
logue, will be expected to decrease more rapidly with non-unit activity conditions, as
will be delineated in a future study. Conventional industrial processes for these metals
and chlorine, along with CO
2
emitted from power and transportation, are responsi-
ble for the majority of anthropogenic CO
2
release. The
STEP
process, to efficiently
recover carbon dioxide and in lieu of these industrial processes, can provide a transition
beyond the fossil fuel-electric grid economy.
The top left of Figure 8.2.1 includes calculated thermoneutral potentials for CO
2
and water-splitting reactions. At ambient temperature, the difference between E
th
and
E
T
does not indicate an additional heat requirement for electrolysis, as this heat is
available via heat exchange with the ambient environment. At ambient temperature,
E
tn
−
E
T
for CO
2
or water is respectively 0.13 and 0.25 V, is calculated (not shown) as
0.15
0.3 V for each of the chlorides.
We find that molten electrolytes present several fundamental advantages com-
pared to solid oxides for CO
2
electrolysis. (i) Molten carbonate electrolyzer provides
10
3
±
0.1 V for Al
2
O
3
and Fe
2
O
3
, and 0.28
±
to 10
6
times higher concentration of reactant at the cathode surface than a solid
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