Environmental Engineering Reference
In-Depth Information
Biomass
Gasiier with
tar removal
Gas cleaning
Gasifying
agent
Product gas,
tar free
Clean syngas
Particulates, N, S,
halogen compounds
Gas cleaning with
tar
removal/conversion
Biomass
Gasiier
Gasifying
agent
Clean syngas
Product
gas
Particulates, (tar),N, S,
halogen compounds
FIGURE 10.15
Methods to reduce tar concentration levels during biomass gasification:
primary methods (top) and secondary methods (bottom).
10.5.3.1 Primary Tar Reduction Methods
Secondary methods would not be
needed in case of application of ideal primary methods (Devi et al., 2003). When gas-
ification process conditions, such as temperature, gasifying medium, stoichiometric
oxygen ratio (
λ
), and residence time, are properly selected, this can reduce the amount
of tars formed during gasification of biomass (Van Paasen and Kiel, 2004). High CC
and diminished tar formation both are favored by a higher gasification temperature
(Gil et al., 1999; Narvaez et al., 1996). For fluidized bed gasification, though, serious
attention must be paid to the temperature in the denser bed zone so as to prevent sin-
tering (Bartels, 2008). High values of
λ
enhance tar destruction, which, however, is at
the expense of gas quality due to increased CO
2
formation in oxidation reactions.
Values in the
0.3 have been reported to be optimal (Devi et al.,
2003). Also, the gasifying medium has an impact on the composition of tars. In flui-
dized bed gasification tests, it has been shown (Gil et al., 1999) that steam addition
leads to an increased share of phenolic species in comparison with the use of air as
gasifying medium. There is also an impact of gasifier pressure on tar formation
and distribution. Entire elimination of phenols was found by Knight (2000) in the case
of pressurized gasification of wood chips.
The gasifier design also has an influence on tar production and destruction. The
addition of a secondary oxidizer (Pan et al., 1999) and, for instance, the process
concept of two-stage FICFB gasification combined with the use of tar reducing
bed material, e.g., olivine and added Ni species (Pfeifer et al., 2004), are a few existing
process design alternatives for enhancing fluidized bed gasification in terms of gas
λ
range of 0.25
-
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