Page 221 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

198 Biomass Gasification, Pyrolysis and Torrefaction

of the tar to raise its temperature, which is favorable for thermal cracking.
Thermal decomposition of biomass tars in electric arc plasma is another
option. This is a relatively simple process, but it produces gas with a
lower energy content.
2. Catalytic cracking is commercially used in many plants for the removal
of tar and other undesired elements from product gas. It generally
involves passing the dirty gas over catalysts. The main chemical reactions
taking place in a catalytic reactor are represented by Eq. (6.5) in the pres-
ence of steam (steam reforming) and Eq. (6.6) in the presence of CO 2
(dry reforming). The main reactions for tar conversion are endothermic in
nature. So, a certain amount of combustion reaction is allowed in the
reactor by adding air.

Nonmetallic catalysts include less-expensive disposable catalysts: dolo-
mite, zeolite, calcite, and so on. They can be used as bed materials in a fluid-
ized bed through which tar-laden gas is passed at a temperature of
750 900 C. Attrition and deactivation of the catalyst are a problem

(Lammars et al., 1997). A proprietary nonmetallic catalyst, D34, has been
used with success in a fluidized bed at 800 C followed by a wet scrubber

(Knoef, 2005, p. 153).
Metallic catalysts include Ni, Ni/Mo, Ni/Co/Mo, NiO, Pt, and Ru on sup-
ports like silica alumina and zeolite (Aznar et al., 1997). Some of them are
used in the petrochemical industry and are readily available. A blend of Ni/
Co/Mo converts NH 3 along with tars. Catalysts deactivate during tar crack-
ing and so need reactivation. Typically, the catalysts are placed in a fixed or
fluidized bed. Tar-laden gas is passed through at a temperature of
800 900 C.

Dolomite (calcined) and olivine sand are very effective in in situ reduc-
tion in tar cracking. This type of catalytic cracking takes place in the typical
temperature of fluidized bed. A good improvement in the gas yield and tar
reduction is noted when catalytic bed materials were used.