Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 180

0.1 mg/Nm 3 for tar. Interest in fuel cells is rising, especially for the direct
production of electricity from hydrogen through gasification. The limiting
level of tar in the gas fed into a fuel cell that produces electricity directly
from fuel gas is specific to the organic constituents of the gas.

6.2.1.1 Level of Tar Production
The amount of tar in product gas depends on the gasification temperature as
well as on the gasifier design. Typical average tar levels in gases from down-
draft and updraft biomass gasifiers are 1 and 50 g/Nm 3 , respectively
( Table 6.2 ). Average tar levels in product gas from bubbling and circulating
fluidized-bed gasifiers are about 10 g/Nm 3 . For a given gasifier, the amount of
the tar yield (percentage of dry mass of biomass) reduces with temperature.
Actual amount of tar yield depends on a number of factors like gasifier-type
temperature. Figure 6.1 shows the range of tar yield at different temperatures.

6.2.2 Tar Formation
Tar is produced primarily through depolymerization during the pyrolysis
stage of gasification. Biomass (or other feed), when fed into a gasifier, first
undergoes pyrolysis that can begin at a relatively low temperature of 200 C
and complete at 500 C. In this temperature range, the cellulose, hemicellu-
lose, and lignin components of biomass break down into primary tar, which
is also known as wood oil or wood syrup. This contains oxygenates and pri-
mary organic condensable molecules called primary tar (Milne et al., 1998,
p. 13). Char is also produced at this stage. Above 500 C, the primary tar
components start reforming into smaller, lighter noncondensable gases and
into a series of heavier molecules called secondary tar ( Figure 6.2 ). The non-
condensable gases include CO 2 , CO, and H 2 O. At still higher temperatures,
primary tar products are destroyed and tertiary products are produced.

TABLE 6.2

Typical Levels of Tar in Biomass Gasifier by Type

Average Tar Concentration in
Product Gas (g/Nm 3 ) Gasifier Type References

Downdraft 0.01 6 Hasler (1999)

Circulating fluidized bed 1 30 Han and Kim (2008)

Bubbling fluidized bed 1 23 Han and Kim (2008)

Updraft 10 150 Milne and Evans

(1998), p. 15

Entrained flow Negligible