Page 212 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 6 Tar Production and Destruction 189

Carbon deposition deactivates dolomite, which, being less expensive, may be
discarded.

Olivine
Olivine is a magnesium iron silicate mineral (Mg, Fe 2 ) SiO 4 that comes in
3
sizes (100 400 μm) and density ranges (2500 2900 kg/m ) similar to those
of sand. Thus, it is conveniently used with sand in a fluidized-bed gasifier.
The catalytic activity of olivine is comparable to that of calcined dolomite.
When using olivine, Mastellone and Arena (2008) noted a complete destruc-
tion of tar from a fluidized-bed gasifier for plastic wastes, while Rapagna `
et al. (2000) obtained a 90% reduction in a biomass-fed unit.

Alkali
Alkali metal catalysts are premixed with biomass before they are fed into the
gasifier. Some of them are more effective than others. For example, the order
of effectiveness of some alkali catalysts can be shown as follows:
K 2 CO 3 . Na 2 CO 3 . ðNa 3 HðCO 3 Þ 3 2H 2 OÞ . Na 2 B 4 O 7 3 10H 2 O (6.7)
2
Unlike dolomite, alkali catalysts can reduce methane in the product gas,
but it is difficult to recover them after use. Furthermore, alkali cannot be
used as a secondary catalyst. Its use in a fluidized bed makes the unit prone
to agglomeration (Mettanant et al., 2009).

Nickel
Many commercial nickel catalysts are available in the market for reduction
of tar as well as methane in the product gas. They contain various amounts
of nickel. For example, catalyst R-67-7H of Haldor Topsøe has 12 14% Ni
on an Mg/Al 2 O 3 support (Sutton et al., 2001). Nickel catalysts are highly
effective and work best in the secondary reactor. Use of dolomite or alkali as
the primary catalyst and nickel as the secondary catalyst has been success-
fully demonstrated for tar and methane reduction. Catalyst activity is influ-
enced by temperature, space time, particle size, and composition of the gas
atmosphere. The optimum operating temperature for a nickel catalyst in a
downstream fluidized bed is 780 C (Sutton et al., 2001). Steam-reforming

nickel catalysts for heavy hydrocarbons are effective for reduction of tar
while nickel catalysts for light hydrocarbons are effective for methane reduc-
tion. Deactivation due to carbon deposition and particle growth is a problem
for nickel-reforming catalysts.

Char
Char, a carbonaceous product of pyrolysis, also catalyzes tar reforming when
used in the secondary reactor. Chembukulam et al. (1981) obtained a nearly
total reduction in tar with this. As it is a major gasification element, char is