Biomedical Engineering Reference
In-Depth Information
Equilibrium calculations can show that as the O/C ratio in the feed increases,
CH
4
, CO, and hydrogen in the product decreases but CO
2
and H
2
O in the prod-
uct increases. Beyond a O/C ratio of 1.0, hardly any CH
4
is produced.
When air is the gasification medium, as is the case for 70% of all gasi-
fiers (Ciferno and Marano, 2002), the nitrogen in it dilutes the product gas.
The heating value of the gas is therefore relatively low (4
6 MJ/m
3
). When
pure oxygen from an air-separation unit is used, the heating value is higher,
in the range 12
28 MJ/m
3
, but a large amount of energy (
2.18 MJ/kg O
2
)
is spent in separating the oxygen from the air (Grezin and Zakharov, 1988).
Either atmospheric air or oxygen from an air-separation unit can meet the
oxygen requirement of a gasifier.
B
Steam
Superheated steam is used as a gasification medium either alone, with air, or
with oxygen. It contributes to the generation of hydrogen.
C
H
2
O
-
CO
H
2
(8.15)
1
1
The quantity of steam, M
fh
, is known from the steam-to-carbon (S/C)
molar ratio.
18
M
f
C
Steam flow-rate
;
M
fh
5
12
ð
S
=
C
Þ
kg steam
=
kg fuel
(8.16)
where M
f
is the fuel feed rate and C is the carbon mass fraction in the fuel.
The S/C mole ratio has an important influence on product composition as
the ER has. Both hydrogen and CO increase with an increasing S/C ratio for
a given temperature and O/C molar ratio. The production of these two gases
increases with decreasing pressure, decreasing oxygen, and decreasing S/C
ratio. However, there is only a marginal gain in increasing the S/C molar
ratio above 2
3, as the excess steam simply leaves the gasifier unreacted
(Probstein and Hicks, 2006, p. 119). So a value in the range of 2.0
2.5 can
give a reasonable starting value.
Example 8.1
A moving-bed gasifier 4 m in diameter operates at 25 bar of pressure and con-
sumes 750 kg/min (dry-ash-free basis) of bituminous coal, 1930 kg/min of steam,
and 280 N m
3
/min of oxygen to produce a product gas that contains 1000 N m
3
of syngas (a mixture of H
2
and CO). The mean gasifier temperature is 1000
C.
The volumetric composition of the product gas is:
CO
2
: 32%
H
2
S: 0.4%
CO: 15.2%
H
2
: 42.3%
CH
4
: 8.6%
C
2
H
4
: 0.8%
N
2
: 0.7%
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