The final stage of the process to produce synthetic diesel is the Fischer-Tropsch synthesis.

The Fischer-Tropsch synthesis converts syngas into hydrocarbons, which form the basis for gasoline, diesel, jet fuel, and chemicals such as olefins and waxes.

In a Fischer-Tropsch (FTS) reactor, CO is reacted with H₂ using a catalyst, iron or cobalt. Iron ihas a lower price while cobalt has higher efficiency and a longer life time.

The reaction takes place at 200-240 ^oC.

Simplified Fischer-Tropsch (FTS) reactions:

nCO + (n+m/2)H₂ → C_nH_m + nH₂O

CO + 2H₂ → CH₂ + H₂O

Hydrocarbon chains, water and energy are producedfromthe synthesis gasusingthe catalyst.

1)                    2)                 H2                     3)
CO     →        CO                 →                    CH₃ + H₂O
Catalyst          ↓↑                        Catalyst      ↓↑

The development of hydrocarbon chains continues until the chain is being released from the catalyst to form either an alkane or alkene.

The carbon chain will either take up a CH₂ group, or a H₂ group, whereby the chain development is ended. The carbon chain may also terminate the growth without taking up an H₂ group – then an alkene is formed.

For the production of diesel fuel with carbon chain length from 13 to 20, the formation of long carbon chains is desired.

The chains that is longer than 20 atoms can easily be cracked into shorter chains.

In addition to diesel, the FTS also produce other hydrocarbons, such as methane that may be recicrculated into syngas, alcohols that may be used as fuel additives and wax that may be sold as such or be futher refined into diesel.

The Fischer-Tropsch (FTS) process also produces considerable amount of heat that may be used for other purposes.