Biomedical Engineering Reference
In-Depth Information
The net gasification efficiency can be written as:
net energy in the product gas
η
net
5
(8.42)
ð
total energy input to the gasifer
2
credits
Þ
Example 8.5
In most steam-fed gasifiers, a large amount of steam remains unutilized. For the
given problem, find the amount of unutilized steam. Also find the cold-gas and
net gasification efficiency of a fixed-bed gasifier that uses steam and oxygen to
gasify grape wastes (HHV
21,800 kJ/kg). The product gas composition (mass
5
basis) is:
CO: 31.8%
H
2
: 3.1%
CO
2
: 38.2%
CH
4
: 1.2%
C
3
H
8
: 0.9%
N
2
:1%
H
2
O: 44.8%
The HHV of the product gas is 8.78 MJ/kg.
The ultimate and proximate analyses of the biomass are as given in
Table 8.10
.
The total fuel feed rate is 25 kg/s; the oxygen feed rate is 5.3 kg/s. The steam is fed
into the gasifier at a rate of 27 kg/s at 180
C and 5 bar of pressure. The product
contains dry gas, condensable moisture, and tar. The tar production rate is 1.3 kg/s
and is analyzed to contain 85% carbon and 15% hydrogen by weight. The heating
value of the tar is 42,000 kJ/kg. The oxygen is produced from air using an oxygen-
separation unit
that consumes 4000 kJ of energy/kg of
the oxygen produced
(assume full conversion of char).
Find the amount of product gas produced and the fraction of steam that
remains unutilized.
Solution
Hydrocarbon hydrogen from the ultimate analysis is 5.83
(1
0.04)
5.6%.
3
2
5
Additional hydrogen also in the moisture is 0.04
(2/18)
0.44%. Thus, the total
3
5
hydrogen, on an as-received basis, is 5.6
0.44
6.04%. The feed rate of the
1
5
total hydrogen through the fuel is 25
1.51 kg/s.
A mass balance between input and output helps determine the production
rate of the gas. Output equals input, so:
Product
6.04/100
3
5
ash
tar and oil
fuel
oxygen
steam
1
1
5
1
1
Product
(25
0.042)
1.3
25
5.3
27
1
3
1
5
1
1
54.95 kg/s
The product contains gas, the composition of which was given previously
(M
gas
), as well as the condensate, M
cond
. To find the gas, we carry out a carbon
balance from its measured composition.
Product
5
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