Production of Synthetic Fuels and Chemicals from Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 390

syngas produced from biomass or coal because it fetches much less value than
the liquid yield of FTS. So, efforts are made to minimize its production. Water
is a major by-product of FTS, and it has an important effect on the catalysts
used for the synthesis (Dalai and Davis, 2008). Figure 11.5 shows the process
schematically indicating two potential product streams.
There are four major components of a FTS plant:

1. Gasifier
2. Gas cleaning and conditioning unit
3. FTS reactor
4. Product upgrading units.

Of these, a gasifier is the most involved and expensive unit.
One can see from Figure 11.5 that there are two basic modes of FTS
operation:

1. High-temperature Fischer Tropsch (HTFT) process

2. Low-temperature Fischer Tropsch (LTFT) process

350 C to produce

The HTFT process uses iron-based catalysts at 300

gasoline and linear low molecular mass olefins (C3 C11). To maximize gas-

oline production, optimum combination is the use of a fluidized-bed reactor
at 340 C with an iron catalyst (Dry 2002, p. 239). The straight run gasoline
could be about 40%. Some of the remaining products could be oligomerized
to gasoline. Other processes like hydrogenation and isomerization may be
needed to get gasoline of the right octane number.
The LTFT process uses mainly cobalt-based catalysts in 200 240 C

range for the production of high molecular weight linear wax that can be
hydrogenated to diesel. A slurry bed is the preferred reactor for this process.
The straight run yield of diesel for this process is about 20% (Dry, 2002,
p. 240).

LTFTS

Biomass

Clean
gas

Gasifier

Cleaning &
conditioning

Isomerization
etc.

Gasification
medium

HTFTS Gasoline

LTFTS - Low temperature FTS
HTFTS - High temperature FTS

Diesel Hydrocracking

FIGURE 11.5 The FTS process with its product stream.