Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 188

Biomass

20 C

Drying

100 C
500-700 C

Pyrolysis

Air Air

Combustion

1000-1400 C

Gasification

Temperature (C)

Gas Gas

FIGURE 6.4 Tar generation is downdraft gasifier. The tar produced passes through the highest
temperature zone in a downdraft gasifier, and as such it is easily cracked.

6.3.1.3 Tar Reduction by Catalysts in Fluidized-Bed Gasifiers
Catalysts accelerate the two main chemical reactions of tar reduction. In a
steam-reforming reaction, we have

Catalyst

C n H x 1 nH 2 O ! ð n x = 2 Þ H 2 1 nCO (6.5)

1

In a dry-reforming reaction, we have

Catalyst

nCO 2 !

ð = Þ

2nCO (6.6)
Catalysts can facilitate tar reduction reactions either in the primary reac-
tor (gasifier) or downstream in a secondary reactor. Three main types of cat-
alysts used are dolomite, alkali metal, and nickel. Olivine, and char have
also found successful use as catalysts for tar reduction. Effects of these cata-
lysts are detailed below.

C n H x 1 x 2 H 2 1

Dolomite
Dolomite (MgCO 3 , CaCO 3 ) is relatively inexpensive and is readily available.
It is more active if calcined and used downstream in the postgasification sec-
ondary reactor at above 800 C (Sutton et al., 2001). The reforming reaction
of tar on a dolomite surface occurs at a higher rate with CO 2 ( Eq. (6.6) ) than
with steam ( Eq. (6.5) ). Under proper conditions, it can entirely convert the
tar but cannot convert methane if that is to be avoided for syngas production.