393
11.2 Thermodynamic Modelling
At 1 bar, 15 °C, 60 % relative humidity, air contains a 0.0064 mass fraction of water.
Air composition then becomes
Henceforth we assume this air composition with detailed simulations.
Fuel is a mixture of carbon and hydrogen, spread over several chemical compounds. O 2 in air is consumed by combustion with generation of H 2 O and CO 2 . The
ratio between the Ar and N 2 fractions in the air remains constant. Therefore, it is
customary to combine the Ar with the N 2 and denote it then as N 2 from air. We so
note the air composition as
The properties of the relevant gases are listed in Table 11.3, with N 2 from air (Source:
[1]). The heat capacity C p is the integral value, which means that the enthalpy difference between 0 °C and the temperature indicated is this value of C p , multiplied with
the temperature difference.
For air follows
This results in R = 288.2 J/kgK; C p = 1004.2, 1045.0, 1139.2 J/kgK, at 0, 500 ,
1500 °C. The gas constant is somewhat higher than for dry air, which is 287.1 J/
kgK. The heat capacity increases with temperature. For N 2 and O 2 , C p /R = γ/(1 − γ)
is about 3.5 at low temperature (0 °C) and 4 at high temperature. These values are
4–4.3 and 5–6.3 for respectively H 2 O and CO 2 . So C p /R increases with temperature
and increases also by the generation of combustion products. For air in the range
0–500 °C, C p is about 1025 J/kgK. The corresponding value of γ is about 1.39. So,
we may assume this value of γ, together with R = 288 J/kgK for hand calculations
with compressors.
The chemical combustion equations are
0.7505N 0.2299 O 0.0127Ar 0.0064 H O 0.0005 CO
2
2
2
2
+
+
+
+
.
0.7632 N 0.2299 O 0.0064 H O 0.0005 CO
2
2
2
2
+
+
+
.
R
R
R
R
R
N
O
H O
CO
=
+
+
+
0 7632
0 2299
0 0064
0 0005
2
2
2
2
.
.
.
.
,
C
C p
C p
C p
C p
p
N
O
H O
C O
=
+
+
+
0 7632
0 2299
0 0064
0 0005
2
2
2
2
.
.
.
.
.
C+O
CO
2
2
→
,
R (J/
kgK)
C p (J/kgK)
C p /R
0 °C 500 °C 1500 °C 0 °C 1500 °C
N 2
295.31 1026 1057
1150
3.47 3.89
O 2
259.83 908.4 979.1
1071
3.50 4.12
H 2 O 461.52 1858 1976
2302
4.03 4.99
CO 2 188.92 820.5 1016
1195
4.34 6.33
Table 11.3 Gas constant
and heat capacity of
gases relevant with
combustion
11.2 Thermodynamic Modelling
At 1 bar, 15 °C, 60 % relative humidity, air contains a 0.0064 mass fraction of water.
Air composition then becomes
Henceforth we assume this air composition with detailed simulations.
Fuel is a mixture of carbon and hydrogen, spread over several chemical compounds. O 2 in air is consumed by combustion with generation of H 2 O and CO 2 . The
ratio between the Ar and N 2 fractions in the air remains constant. Therefore, it is
customary to combine the Ar with the N 2 and denote it then as N 2 from air. We so
note the air composition as
The properties of the relevant gases are listed in Table 11.3, with N 2 from air (Source:
[1]). The heat capacity C p is the integral value, which means that the enthalpy difference between 0 °C and the temperature indicated is this value of C p , multiplied with
the temperature difference.
For air follows
This results in R = 288.2 J/kgK; C p = 1004.2, 1045.0, 1139.2 J/kgK, at 0, 500 ,
1500 °C. The gas constant is somewhat higher than for dry air, which is 287.1 J/
kgK. The heat capacity increases with temperature. For N 2 and O 2 , C p /R = γ/(1 − γ)
is about 3.5 at low temperature (0 °C) and 4 at high temperature. These values are
4–4.3 and 5–6.3 for respectively H 2 O and CO 2 . So C p /R increases with temperature
and increases also by the generation of combustion products. For air in the range
0–500 °C, C p is about 1025 J/kgK. The corresponding value of γ is about 1.39. So,
we may assume this value of γ, together with R = 288 J/kgK for hand calculations
with compressors.
The chemical combustion equations are
0.7505N 0.2299 O 0.0127Ar 0.0064 H O 0.0005 CO
2
2
2
2
+
+
+
+
.
0.7632 N 0.2299 O 0.0064 H O 0.0005 CO
2
2
2
2
+
+
+
.
R
R
R
R
R
N
O
H O
CO
=
+
+
+
0 7632
0 2299
0 0064
0 0005
2
2
2
2
.
.
.
.
,
C
C p
C p
C p
C p
p
N
O
H O
C O
=
+
+
+
0 7632
0 2299
0 0064
0 0005
2
2
2
2
.
.
.
.
.
C+O
CO
2
2
→
,
R (J/
kgK)
C p (J/kgK)
C p /R
0 °C 500 °C 1500 °C 0 °C 1500 °C
N 2
295.31 1026 1057
1150
3.47 3.89
O 2
259.83 908.4 979.1
1071
3.50 4.12
H 2 O 461.52 1858 1976
2302
4.03 4.99
CO 2 188.92 820.5 1016
1195
4.34 6.33
Table 11.3 Gas constant
and heat capacity of
gases relevant with
combustion