Geology Reference
In-Depth Information
D.3 Calculation of the chemical exergy of fuels
According to Lozano and Valero (1988), gaseous fuels can be treated as a mixture
of ideal gases and thus their chemical exergy can be calculated as in Eq. (D.1).
b
chgas
=
X
x
i
(b
chi
+ RT
0
lnx
i
)
(D.1)
where x
i
is the molar fraction of the chemical substance i and b
chi
is the standard
chemical exergy of substance i.
The exergy for gaseous fuels can thus be calculated with Eq. (D.1) or with the
general methodology applied to liquid and solid fuels, explained next.
The procedure is based on the general molecular formula of Eq. (D.2).
CH
h
O
o
N
n
S
s
+ W
w
+ Z
z
(D.2)
where W represents the moles of liquid water (moisture) and Z the ashes. Coe
-
cients h, o, n, s, w and z are moles of H, O, N, S, water and ashes contained in
the molecular structure of the fuel, per mole of C content, respectively:
h =
C
12:011
o =
C
12:011
n =
C
12:011
1:008
15:999
14:007
s =
C
12:011
w =
C
12:011
z =
C
12:011
1:000
Note that W and Z apply only to solid fossil fuels and s = 0 in gaseous fuels.
The standard average fuel exergy on a molar basis is then:
32:064
18:015
b
fuel
= b
CH
h
O
o
N
n
S
s
+ wb
0
W
+ zb
Z
(D.3)
And b
i
is calculated as follows:
b
i
= H
f;i
T
0
s
i
X
f
j
j;00
(D.4)
where H
f;i
and s
i
are the fuel's standard enthalpy and entropy, T
0
the standard
ambient temperature, f
j
the elements of the atomic composition vector of the fuel
f = [1;h;o;n;s]
0
, and H
j;00
and
j;00
the enthalpy and the chemical potential of
the elements in the dead state (reference environment).
The atomic composition vector of a gaseous fuel f
j;gas
can be obtained with
Eq. (D.5):
P
i=1
r
j;i
i
d
1
f
j;gas
=
(D.5)
being r
j;i
the number of atoms j contained in component i of the mixture of gases
and
i
the molar composition of substance i in the fuel
23
. The moles of C contained
in the fuel are expressed as d
1
and are calculated as
P
i=1
r
C;i
i
.
23
Gaseous fuels are assumed to contain the following 7 gases: CH
4
, C
2
H
6
, C
3
H
8
, C
4
H
10
, C
5
H
12
,
N
2
and CO
2
.
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