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4.2. SOFC subsystem
The air utilization factor is defined as the fraction of the oxygen input
at the cathode side of the SOFC component which is consumed:
U air = ṅSOFC ,cat,i ,O 2
− ṅ SOFC ,cat,o,O2
ṅ SOFC ,cat,i ,O2
= ṅ14,O 2
− ṅ 15,O2
ṅ 14,O2
(4.1)
4.2.4 Exhaust gas
The air which is not consumed in the SOFC stack (point 15) is split by
SPG2. α SPG2 defines how much of the air is sent to the burner inlet, point
16. The exhaust gas (containing excess fuel) from the SOFC is sent to the
burner fuel inlet point 10.
It is also possible to bypass the pretreatment of the fuel and the SOFC
by adding fuel to the burner directly at point 0 if more heat and less
electricity is wanted. The parameter FuelBP Ratio determines the fraction
of the total fuel input which is sent directly to the burner circumventing
the rest of the SOFC system. This option, though, is only used for a
single simulation, so normally FuelBP Ratio = 0. More details are given
in section 5.3.3 page 130.
λ BURN ,i is the air excess ratio of the burner inlet (point 0, 10 and 16)
defined as:
λ BURN ,i = 2ṅ i ,C H 4
+ 0,5ṅ i ,H2 + 0,5ṅ i ,CO
ṅ i ,O2
(4.2)
λ BURN ,i is set to 1,5 which is common for combustors. This implicitly
sets the value of α SPG2 such that the right amount of air is bypassed the
burner (point 17). The bypassing air is mixed with the exhaust gas from
the burner (point 18) in MIXG2. The gas is returned to the hot side of
GGHEX3 (point 19) before it leaves the SOFC subsystem in point 20.
After some of the energy in the exhaust gas has been utilized in the
absorption cycle, the gas is returned to the SOFC subsystem entering at
point 22. Heat is transferred via GGHEX4 to the inlet air and the exhaust
gas leaves at point 23.
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