integration of solid oxide fuel cells and ... - Ea Energianalyse

3. COMPONENT DESCRIPTION
cell temperature and the free Gibbs energy of the species in the
electrochemical reaction.
K W GS = ṅCO 2 ṅ H2
ṅ CO ṅ H2 O
(3.78)
K W GS = e −∆g
RTav (3.79)
∆g = (g CO2 + g H2 ) − (g CO + g H2 O) (3.80)
One more equation comes from the assumption that the C H 4 is not present
at the outlet (meaning that reforming runs 100%).
The last equation is given by the fuel Utilization factor U f , which is an
input parameter. This factor states how much H 2 is used in the electrochemical
reaction (ṙ Elk ) divided by the much H 2 that would be available
for the electrochemical reaction, if all the C H 4 and CO in the anode inlet
was reformed and water gas shifted into H 2 :
U f =
ṙ Elk
4ṅ C H4,i + ṅ CO,i + ṅ H2,i
(3.81)
Together with the conservation of mass (for each element), this is enough
to determine composition of the outlet gas. Nitrogen and oxygen leaves
the SOFC at the cathode side, whereas all other species leaves on the
anode side.
OC ratio
If the concentration of Carbon atoms at the anode side of the SOFC
becomes too large relative to the concentration of Oxygen atoms on
the anode side, there is a risk that Carbon will deposit in the cell (this
includes the O- and C- content in all the different species at the anode
side). So it is desirable that there is at least twice as much Oxygen as
Carbon [25].
The ratio is controlled by altering the degree of anode recycling, since
more recycling means more Oxygen atoms into the anode side (from CO,
CO 2 , and H 2 O). So it is desired that OC r atio ≧ 2:
ṅ C H4 ,ai + ṅ CO,ai + ṅ CO2 ,ai
OC r atio =
ṅ CO,ai + 2ṅ CO2 ,ai + 2ṅ H2 O,ai + 2ṅ O2 ,ai
(3.82)
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