Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 185

where steam cracks the tar, producing simpler and lighter molecules like
H 2 and CO.

C n H x 1 nH 2 O -ð n x = 2 Þ H 2 1 nCO (6.1)

1

ii. Dry tar reforming. The dry reforming reaction takes place when CO 2 is
the gasifying medium instead of steam. Here tar is broken down into H 2
and CO ( Eq. (6.2) ). Dry reforming is more effective than steam reform-
ing especially when dolomite is used as the catalyst (Sutton et al., 2001).
C n H x 1

nCO 2 -ð x = 2 Þ H 2 1 2nCO (6.2)

iii. Thermal cracking. Thermal cracking can reduce tar, but it is not as
attractive as reforming because it requires high (

1100 C) temperature
and produces soot (Dayton, 2002). Because this temperature is higher
than the gas exit temperature for most biomass gasifiers, external heating
or internal heat generation with the addition of oxygen may be neces-
sary. Both options have major energy penalties.
iv. Steam cracking. In steam cracking, the tar is diluted with steam and is
briefly heated in a furnace in the absence of oxygen. The saturated
hydrocarbons are broken down into smaller hydrocarbons.
The following sections elaborate the operating conditions used in in situ
reduction of tar.

.

6.3.1.2 Operating Conditions
Operating parameters that influence tar formation and conversion include
reactor temperature, reactor pressure, gasification medium, equivalence ratio,
and residence time.

Temperature
Reactor operating temperature influences both the quantity and the composi-
tion of tar. The quantity in general decreases with an increase in reaction
temperature, as does the amount of unconverted char. Thus, high-
temperature operation is desirable on both counts. The production of
oxygen-containing compounds like phenol, cresol, and benzofuran reduces
with temperature, especially below 800 C. With increasing temperature, the
amount of 1- and 2-ring aromatics with substituents decreases but that of 3-
and 4-ring aromatics increases. Aromatic compounds without substituents
(e.g., naphthalene and benzene) are favored at high temperatures. The naph-
thalene and benzene content of the gas increases with temperature (Devi
et al., 2003). High temperature also reduces the ammonia content of the gas
and improves the char conversion but has a negative effect of reducing the
product gas' useful heating value.
An increase in the freeboard temperature in a fluidized-bed gasifier can
also reduce the tar in the product gas. A reduction in tar was obtained by