Page 209 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

186 Biomass Gasification, Pyrolysis and Torrefaction

Narva ´ez et al. (1996) by injecting secondary air into the freeboard. This may
be due to increased combustion in the freeboard. Raising the temperature
through secondary air injection in the freeboard may have a negative impact
on heating value.
Reactor Pressure
With increasing pressure, the amount of tar decreases, but the fraction of
PAH increases (Knight, 2000).
Residence Time
Residence time has a nominal effect on tar yield in a fluidized-bed gasifier.
Kinoshita et al. (1994) noted that with increasing gas residence time (bed
height/superficial gas velocity), the yield of oxygenated compounds and 1-
and 2-ring compounds (benzene and naphthalene excepted) decreased, but
the yield of 3- and 4-ring compounds increased.
Gasification Medium
Four mediums—air, steam, carbon dioxide, and steam oxygen mixture that
are typically used for gasification—may have different effects on tar forma-
tion and conversion. The ratio of fuel to gasification medium is an important
parameter that influences the product of gasification, including tar. This
parameter is expressed differently for different mediums. For example, for
air gasification, the parameter is the equivalence ratio (ER); for steam gasifi-
cation, it is the steam-to-biomass ratio (S/B); and for steam oxygen gasifi-
cation, it is the gasifying ratio (Table 6.4). An example of the range of tar
production for three gasification mediums for typical values of their charac-
teristic parameters is given in Table 6.5.
In general, the yield of tar in steam gasification is greater than that in
steam oxygen gasification. Of these, air gasification is the lowest tar pro-
ducer (Gil et al., 1999). The tar yield in a system depends on the amount of
gasifying medium per unit biomass gasified.
Gasification in air: Both yield and concentration of tar in the product gas
decreases with an increase in the ER. Higher ER (see Section 8.6.2 for a defi-
nition) allows greater amounts of oxygen to react with the volatiles in the
flaming pyrolysis zone (Figure 6.4). Above an ER of 0.27, phenols are nearly
all converted and less tar is formed (Kinoshita et al., 1994). This decrease is
greater at higher temperatures. At a higher ER, the fraction of PAH, benzene,
naphthalene, and other 3- and 4-ring aromatics increases in the product gas.
While higher ER reduces the tar, it reduces the quality of the gas as well. The
heating value of the gas is reduced because of nitrogen dilution from air.
Gasification in steam: When steam reacts with biomass to produce H 2
(Eq. (6.3)), the tar-reforming reaction reduces the tar.
C n H x 1 nH 2 O-ðn 1 x=2ÞH 2 1 nCO (6.3)