SYNTHESIS GAS UTILIZATION FOR TRANSPORTATION FUEL PRODUCTION - Biomass as a Sustainable Energy Source for the Future - page 530

50

40

30

20

10

0

470 490 510 530

T (K)
FIGURE 17.2 The FT reaction rate according to the Yates and Satterfield expression as a
function of temperature.

The Yates and Satterfield expression just describes the conversion of syngas by FTS; it
does not say anything about the structure of the obtained products. The most important
products are linear alkanes ( n -paraffins, hydrocarbons with only single C - C bonds),

but also considerable amounts of alkenes (olefins, hydrocarbons with a C C bond)

and branched alkanes can be produced. In addition, small amounts of oxygenated hydro-
carbons such as alcohols are produced. FTS can be considered as a polymerization process
taking place at the surface of a catalyst. By adding more carbon atoms, the chains become
longer and longer (propagation), until the chain growth is terminated (see Figure 17.3a). In
every step, the probability for propagation is

═

α , while the probability for termination is

(1 —

determines the product distribution (see Figure 17.3b). This distribution is named the
Anderson

− α ). This means that the parameter α — denoted as the chain growth probability

Flory distribution, after the proposers of this mechanism. In most
cases, the aim is to produce liquid hydrocarbons, i.e., molecules with a considerable num-
ber of carbon atoms. This means that

- Schulz -

α should be high enough; typical values in practice

are around 0.9. The fact that at high also heavywaxes ( n >20) areproduced is seenas less

of a problem: these can be used invarious applications (e.g., as lubricant and for coating) or
can be easily cracked to shorter molecules. Ways to increase

α

are to operate at lower H 2 :
CO ratio or at lower temperature. The latter has its limitations, since it will also reduce the
reaction rate.

α

17.2.2 Catalyst Aspects

The FTS requires a solid-phase catalyst to enhance the reaction rate. The two main
catalyst types applied are cobalt and iron. Most current processes use cobalt: it is
more active (can be used at a lower temperature) and less prone to sintering, but it