Page 304 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

280 Biomass Gasification, Pyrolysis and Torrefaction

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.
3
The heating value of the gas is therefore relatively low (4 6 MJ/m ). When
pure oxygen from an air-separation unit is used, the heating value is higher,
3
in the range 12 28 MJ/m , but a large amount of energy (B2.18 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.

Steam
Superheated steam is used as a gasification medium either alone, with air, or
with oxygen. It contributes to the generation of hydrogen.
(8.15)
C 1 H 2 O-CO 1 H 2
The quantity of steam, M fh , is known from the steam-to-carbon (S/C)
molar ratio.
M f C
Steam flow-rate; M fh 5 18 ðS=CÞ kg steam=kg fuel (8.16)
12
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,
3
and 280 N m /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%