Production of Synthetic Fuels and Chemicals from Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 392

Undesired Products:

Boudouard reaction: 2 CO - C CO 2 (11.14)

1

Water-gas shift reaction: CO 1 H 2 O 2 CO 2 1 H 2 (11.15)

Methanation reaction: CO 1 3H 2 - CH 4 1 H 2 O (11.16)

The FTS reaction is a highly exothermic polymerization reaction. It is
kinetically controlled. It produces a wide spectrum of oxygenated com-
pounds, including alcohols and aliphatic hydrocarbons ranging in carbon
numbers from C 1 C 3 (gases) to C 35 1 (solid waxes). For synthetic fuels, the
desired products are olefinic hydrocarbons in the C 5 C 10 range (Probstein

and Hicks, 2006, p. 128). The upper end of the range favors a gasoline prod-
uct. The selectivity of different hydrocarbons may be predicted on the basis
of a statistical distribution given by the Anderson-Schulz-Flory model.
(Spath and Dayton 2003, p. 95):

2 n 2 1

W n 5 n ð 1

2 αÞ

α (11.17)

where W n is the % weight of a product containing n carbon atoms and α is

the chain growth probability.

11.4.2.4 Catalysts
Catalysts play a pivotal role in the FTS process. Besides enhancing the yield
of desired product or selectivity, it contributes to higher volumetric produc-
tivity of the process, which in turn reduces the volume of the reactor.
Group VIII metals have highest catalytic activity in FTS reaction.
Besides the commonly used iron (Fe) and cobalt (Co) catalysts, several other
catalysts are also used in FTS. Activities of these catalysts vary and they
rank as below (Adesina, 1996):
High activity Ru Fe Ni Co Rh Pd Pt Low activity (11.18)

. . . . . .

It is apparent from here that ruthenium has the highest activity and high
selectivity for products of large molecular mass at lower temperatures, but it
is significantly more expensive (approx. 300,000 times) than iron. The other
choice is Co which is also expensive but to a lesser extent (230 times).
Fe catalysts are less expensive and owing to its WGS activity requires
lower hydrogen to carbon monoxide molar ratio, but it has a stronger ten-
dency to form carbon that deposits on its surface deactivating the catalyst.
Cobalt (Co) catalysts, on the other hand, do not have WGS activity,
which improves carbon conversion and it produces straight chain hydrocar-
bons at a high yield (Spath and Dayton, 2003). This catalyst has a longer
life. The cobalt-based catalyst is only used in the low-temperature FTS as at
higher temperature an excess of methane is produced (Dry, 2002, p. 233).