Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 187

TABLE 6.4 Gasification Mediums and Characteristic Parameters

Medium Parameter

Air ER ratio of air used to stoichiometric air

5

Steam Steam-to-biomass (S/B) ratio

Carbon dioxide CO 2 -to-biomass ratio

Steam and oxygen Gasifying ratio (GR): (steam O 2 )-to-biomass ratio

1

TABLE 6.5 Effect of Gasification Medium on Characteristics of Tar

Production

Operating
Condition

Tar Yield (g/
Nm 3 )

LHV (MJ/
Nm 3 dry)

Tar Yield
(g/kg BM daf ) Medium

Steam S/B 0.9 30 80 12.7 13.3 70

5

Steam and
oxygen

GR 0.9,

H 2 O/O 2 5

4 30 12.5 13.0 8 40

5

3

Air ER 0.3, 2 20 4.5 6.5 6 30

5

H/C 2.2

5

Source: Data compiled from Gil et al. (1999).

A large reduction in tar yield was seen over an S/B ratio range of

0.5 2.5 (Herguido et al., 1992). Further reduction is possible in the presence

of catalyst, which encourages the tar-reforming reaction (Garc´a et al., 1999).
Gasification in a steam oxygen mixture: The addition of oxygen with
steam further improves tar reduction. Additionally, it provides the heat
needed to make the gasification reaction autothermal. When one uses oxygen
along with steam, the mass ratio of (steam oxygen) to biomass, known as

the gasification ratio (GR), is used to characterize this reaction. The tar yield
reduces with an increase in the gasifying ratio. For example, an 85% reduc-
tion in tar is obtained when the GR is increased from 0.7 to 1.2 (Aznar et al.,
1997). Light tars are produced at a low GR.
Gasification in carbon dioxide: The tar may be reformed on the catalyst
surface in a carbon dioxide medium. Such a reaction is called dry reforming
and is shown in Eq. (6.4) (Sutton et al., 2001).
C n H x 1 nCO 2 -

1

2nCO

1 ð x =

2 Þ H 2 (6.4)

The effect of gasifying agents on tar reduction or tar yield is compared in
Table 6.5 (Gil et al., 1999).