SYNTHESIS GAS UTILIZATION FOR TRANSPORTATION FUEL PRODUCTION - Biomass as a Sustainable Energy Source for the Future - page 538

Nowadays, mainly, a Cu/ZnO/Al 2 O 3 catalyst is used. This catalyst enables meth-
anol production with a very high selectivity, greater than 99%. Since the methanol
synthesis reaction is quite exothermal, heat removal is an important issue in the design
of a suitable reactor. The most widely used reactor type is an adiabatic reactor con-
sisting of a single catalyst bed (Fiedler et al. 2011) to which the syngas is supplied in a
distributed way: by injecting cold syngas at several points along the reactor, the reac-
tion is quenched. This leads to a sawtooth-shaped temperature profile. An alternative
reactor configuration is the quasi-isothermal reactor, which is very similar to the mul-
titubular reactor employed in FTS. Many parallel tubes filled with catalyst are placed
in a column filled with boiling water providing the cooling.
Methanol can be blended with gasoline, but its use is limited because of concerns
about corrosion of some metals and its toxicity and solubility in water. An alternative
is not to use methanol for combustion engines, but to convert it onboard of a car into
hydrogen that can be used in a fuel cell. The conversion of methanol to hydrogen
can be efficiently carried out using steam reforming at a relatively low temperature
of 500 600 K, making it more efficient than using other liquid transportation fuels

to generate hydrogen (Brown, 2001). An alternative is the use of direct methanol fuel
cells, but these still have a low efficiency.
Instead of using methanol directly as a fuel, it can also be converted into DME or
into gasoline. DME, which is a gaseous compound, can be used in direct DME fuel
cells and in diesel engines. This might be attractive, since DME combines high diesel
engine performance (cetane number 55) with low emissions (Semelsberger et al.,
2006). DME is produced by dehydration of methanol:

−

mol −1 2CH 3 OH ⇄ CH 3 − O − CH 3 +H 2 O Δ r H = − 23 : 4kJ ð RX : 17 : 5 Þ

Various acidic catalysts can be used for this reaction, such as -alumina, aluminum

silicate, or zinc chloride. The reaction is typically carried out in the gas phase in a fixed
bed reactor, at a temperature around 500

γ

20 bar.
Instead of having DME as the final product of the process, it is also possible to pro-
duce a mixture of gasoline-range hydrocarbons; this is the so-called methanol-to-
gasoline (MTG) process. DME is first converted by deoxygenating into a mixture of
light olefins (C 2 −

- 600 K and a pressure around 10 -

C 5 ), which is finally converted into a mixture of C 6+ alkanes and ole-
fins and aromatic compounds. The gasoline synthesis, which uses DME as the input,
typically takes place in parallel fixed bed reactors containing a ZSM-5 catalyst, oper-
ated at 600 700 K and approximately 20 bar (Moulijn et al., 2013). When the prices of

olefins (used in, e.g., plastic production) are high compared to that of gasoline, it ismore
attractive to have olefins as the final product: methanol to olefins (MTO). At the time of
writing (summer 2014), MTO is indeed economically more attractive than MTG.

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17.5 COMPARISON OF THE DIFFERENT OPTIONS

In the previous sections, we have treated the main routes for synthesizing synthetic
fuels from syngas: the FTS, SNG production, methanol production, and briefly