Environmental Engineering Reference
In-Depth Information
Example 3.5 Ethanol combustion in an internal combustion engine (ICE)
Bioethanol produced by fermentation of sugars extracted from sugarcane (see
Chapter 13) is combusted in a test Otto engine. The excess air amount is 5% and
the combustion product gas (flue gas) leaves the engine at 800 K. The brake-specific
fuel consumption (BSFC), defined as fuel consumed divided by the power produced
“
(kWh)
−1
.Theconsumptionrateoffuelis7000g
h
−1
.
at the brake,
”
is 275 g
What is the rate of heat removal (in kW)?
Data
h
o
h
800 K
Þ
−
h
o
kmol
−1
)
kmol
−1
)
f
(kJ
ð
f
(kJ
Compound
C
2
H
5
OH
−
229,757
—
CO
2
−
393,546
22,810
H
2
O (g)
−
241,845
18,005
O
2
0
15,838
N
2
0
15,046
Solution
The reaction equation for stoichiometric ethanol combustion is
C
2
H
5
OH+ a O
2
+3
ð
:
76N
2
Þ!
bCO
2
+cH
2
O+3
:
76aN
2
C balance: 2 = b
H balance: 6 = 2c, so c = 3
O balance: 2a + 1 = 2b + c, so a = 3
Then
C
2
H
5
OH+ 3 O
2
+3
ð
:
76N
2
Þ!
2CO
2
+3H
2
O+11
:
28N
2
The ethanol combustion equation for excess air situations is given by
C
2
H
5
OH+ 3
λ
ð
O
2
+3
:
76N
2
Þ!
2CO
2
+3H
2
O+
ð
λ
−
1
Þ
O
2
+11
:
28
λ
N
2
where
λ
is the air stoichiometry coefficient, in this case
λ
= 1.05.
Thus, the reaction equation is
C
2
H
5
OH+ 3
:
15 O
2
+3
ð
:
76N
2
Þ!
2CO
2
+3H
2
O+0
:
05O
2
+11
:
84N
2
The energy balance is
Q
cv
=
W
cv
+
h
f
+
Δ
h
+3
h
f
+
Δ
h
h
f
+
Δ
h
φ
n
,
fuel in
2
+0
:
05
CO
2
H
2
O
O
2
#
h
0
h
0
Δ
h
Δ
h
+11
:
84
f
+
−
f
+
+3
:
15×0+11
:
84 × 0
N
2
C
2
H
5
OH
prod
:
react
:
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