The Oxides of Carbon (Chap. 12.6)
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Carbon forms 2 oxides, CO and CO2
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Combining these reactions gives:
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Features of these reactions:
–
–
–
–
Slope of (i) almost zero (same number of moles of gas)
Slope of (ii) is negative (1 more mole gas on the right)
Slope of (iii) is positive (1 more mole of gas on the left)
Much more heat from making CO2 (394,100/223,400 x 2 = 3.5
times per mole of carbon)
– CO is more stable at high temperature
1
2
The Oxides of Carbon (Chap. 12.6)
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The Boudouard reaction is very important in processes that
use carbon as a reductant, such as the blast furnace
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Only occurs in the presence of carbon
Does not appear on the Ellingham diagram
Very endothermic process
Goes further to the right as temperature is increased
In the reduction of FeO with carbon, the direct reaction is
very slow because of limited contact between 2 solids
The reaction mechanism in the presence of carbon goes
between 2 reactions feeding each other:
FeO + CO → Fe + CO2
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CO2 + C → 2CO
3
The Oxides of Carbon (Chap. 12.6)
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The Boudouard reaction as
written has both CO and CO2 in
their standard states so the
total pressure is 2 atm.
We will determine what the PCO
and PCO2 are for a total pressure
of 1 atm over a range of
temperatures
The Boudouard reaction is the
difference between (i) and (ii)
Crossing point on Ellingham
diagram
4
The Oxides of Carbon (Chap. 12.6)

Calculate the free energy change for (i) when CO2 is not in
its standard state, i.e. 0.5 atm
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The line will swing around becoming more negative as PCO2
is reduced
Repeat the procedure for reaction (ii) for the free energy
change when CO is not in its standard state
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5
The Oxides of Carbon (Chap. 12.6)

For reaction (ii):
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So then the difference of the 2 reactions will be the
Boudouard reaction when PCO and PCO2 are both 0.5

This is point c on the diagram
Repeat for other combinations that PCO + PCO2 = 1.0
Temperature will change from a, b, c to d
Plot on the following page
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6
The Oxides of Carbon (Chap. 12.6)

This shows the equilibrium
PCO for:

Must be in the presence or
excess of carbon
In a relatively narrow T
range 100% CO2 to
100%CO
CO is the stable carbon
oxide at high temperature
All these conditions are
reducing, PO2 very low
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7
Implications of the Ellingham Diagram –
More Stable Oxides
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The Ellingham diagram for oxides provides the basis for
metals production and refining
Metals at the bottom have more stable oxides
– The oxides are chemically suitable as refractory materials for
metal containment , e.g. Al2O3, MgO
– Also means that more energy must be provided to reduce them
– The point of crossing the CO line may be greater than 2000˚C
– Problems of containment and the energy cost

It takes 220 MJ/kg to make Al, and only about 20 MJ/kg to
make Fe, most of this is due differences in ΔH˚ and the
lower ore grade of bauxite (Al2O3-containing) ore
– Greater cost and CO2 emissions to make Al
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Implications of the Ellingham Diagram –
Less Stable Oxides – Carbon Reduction
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Metals near the top have oxides that are easily reduced by
carbon or hydrogen, e.g. Fe or Zn
Carbon is abundant in coal reserves
Carbon from fossil fuels contributes to Global Warming
– Carbon from biomass (e.g. wood charcoal) does not, but not
widely available option
– Carbon Capture and Storage (CCS) is being developed in
Alberta and Europe
– No solution to this problem yet
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Implications of the Ellingham Diagram –
Less Stable Oxides – H2 Reduction

Hydrogen for reduction processes primarily comes from
natural gas, methane, CH4
– Various ways of “cracking” this gas to produce mixtures of CO
and H2, such as steam reforming
CH 4 + H 2O → CO + 3H 2
– Still produces some CO2 when used for reduction, but there is
less Global Warming Potential from this route
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Natural Gas is not widely distributed, so this is mainly an
option for gas-rich countries
Hydrogen from electrolysis of water is requires large
amounts of energy because ΔH˚ of water is very negative
– Hydrogen economy will likely not materialize
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Implications of the Ellingham Diagram –
Reduction of Iron Oxides by Carbon
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Iron has a sequence of oxides:
– Hematite Fe2O3
– Magnetite Fe3O4
– Wustite FeO
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Most iron ores are hematite, but there are some magnetites
The solid state reduction of carbon in contact with one of
the oxides is extremely slow because the reaction only
occurs at the point of contact
As discussed earlier, the Boudouard reaction produces CO
for the reduction reaction and then consumes the CO2
product of the reduction reaction:
FeO + CO → Fe + CO2
CO2 + C → 2CO
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Implications of the Ellingham Diagram –
Reduction of Iron Oxides by Carbon

Consider the iron oxides on the Ellingham diagram, highest
oxides are also highest on the diagram
– Highest ones are easiest to reduce
– Highest ones require lower CO/CO2 gas strength
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See Ellingham diagram on next page because it has all the
iron oxides (not the same as the text)
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Reduction of
Iron Oxides

Note sequential
reduction of iron oxides
– Hematite Fe2O3
– Magnetite Fe3O4
– Wustite FeO
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Note CO/CO2 for each
reaction
Difficult to read these
accurately
Recall the Boudouard
reaction also changes
the CO/CO2 in contact
with carbon
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Implications of the Ellingham Diagram –
Reduction of Iron Oxides by Carbon
14
Implications of the Ellingham Diagram –
Reduction of Iron Oxides by Carbon
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Reduction of Fe2O3 does not appear on diagram, since so
easy to do with low CO/CO2 ratio
Wustite is the most difficult, and requires the strongest gas,
highest CO/CO2 ratio
Wustite does not exist in equilibrium below 600˚C
Need to have high temperature for the Boudouard reaction
to renew the CO for reduction
At 900˚C 70% of the CO can be used to reduce Fe3O4
At 900˚C 30% of the CO can be used to reduce FeO
This has a major impact on the way that the blast furnace
operates to make liquid iron from iron ore
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Implications of the Ellingham Diagram –
The Iron Blast Furnace
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The Iron Blast Furnace
Coke (essentially C) and
Fe2O3 charged in alternate
layers
Blast of air through
raceway
Liquid metal (carbonsaturated) and slag tapped
Top gas contains N2 and
CO/CO2 ~ 1
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Implications of the Ellingham Diagram –
The Iron Blast Furnace
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Tuyere pressure and
flow push coke away
from the tuyeres to form
a raceway
Preheated air injected
into raceway with
excess of hot coke,
>2000˚C
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Implications of the Ellingham Diagram –
The Iron Blast Furnace
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O2 reacts with C to form
CO2, but then encounters
more coke for Boudouard
to occur
Excess of coke ensures
Boudouard reaction
continues
Pure CO and N2 leave
raceway region
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Implications of the Ellingham Diagram –
The Iron Blast Furnace
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CO from the raceway
comes up and encounters
alternate layers of FeO and
C from the charging
pattern
FeO left after the other
oxides are easily reduced
higher up in the furnace
Gas alternates between
CO/CO2 = 70/30 at FeO and
CO/CO2 = 100/0 at coke
Reactions are fast so
equilibrium is virtually
reached at each layer
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