Biomedical Engineering Reference
In-Depth Information
Example 10.1
A PC-fired boiler is designed to fire bituminous coal with 20% excess air. It was
decided to switch the boiler entirely into (i) torrefied biomass, (ii) raw biomass,
or (iii) lignite firing.
Find the impact on combustion air requirement and flue gas production if the
boiler is to retain the energy input into the furnace. Assume the air to contain
1.3% moisture. Composition and heating values of these fuels are given below:
Composition
Bituminous Coal
Torrefied Biomass
Raw Biomass
C (%)
67.36
54.70
38.49
H (%)
4.58
6.00
4.86
O (%)
5.69
36.40
37.19
N (%)
1.30
0.10
0.25
S (%)
2.08
0
0.00
Ash (%)
8.84
2.80
2.8
Moisture (%)
9.91
0.00
16.4
HHV (MJ/kg)
28.91
21.9
13.97
Solution
a. First we carry out the calculation for raw biomass:
Using Eq. (3.34), we calculate the theoretical dry air needed per kilogram
raw biomass.
M
da
11
:
53
C
34
:
34
3
½
H
2
ð
O
=
8
Þ
5
3
1
5
11
:
53
3
38
:
49
1
34
:
34
3
ð
4
:
86
37
:
19
=
8
Þ
4
:
512 kg air
=
kg raw biomass
5
Actual air with 20% excess air
5
1.2
3
4.512
5
5.41 dry air/kg of raw
biomass.
Moisture in air is 1.3%.
So, the wet air requirement is M
wa
5
(1
1
0.013)
3
5.41
5
5.48 wet air/kg
of raw biomass.
Amount of flue gas product through complete combustion is found from
Eq. (3.36).
Flue gas mass W
c
5
M
wa
2
0
:
2315M
da
1
3
:
66C
1
9H
1
N
1
O
1
2
:
5S
5
:
48
0
:
2315
4
:
512
1
ð
3
:
66
38
:
49
9
4
:
86
5
2
3
3
1
3
0
:
25
37
:
19
Þ=
100
1
1
kg raw biomass
Heating value of biomass is 13.97 MJ/kg. So, for 1 MJ energy input the
furnace needs (5.48/13.97) or 0.392 kg air and produce (6.66/13.97) or
0.476 kg flue gas.
b. Repeating the above computation for coal, we find that 1 kg coal needs
11.22 kg wet air and produces 12.03 kg flue gas and releases 28.91 MJ heat.
So, for 1 MJ energy input in the energy the furnace needs (11.22/28.91)
or 0.39 kg air and produces (12.03/28.91) or 0.416 kg flue gas.
5
6
:
66 kg
=
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