SYNTHESIS GAS UTILIZATION FOR TRANSPORTATION FUEL PRODUCTION - Biomass as a Sustainable Energy Source for the Future - page 539

DME production. In this section, we will make a concise comparison of the different
routes, based on material available in literature.
Zhang (2010) made a comparison of the different fuels, with an emphasis on the
production efficiency (well-to-tank efficiency), the energy content of the synthe-
sized fuel as a fraction of the energy in the biomass feed. He reported that the
production efficiency is highest for SNG ( 65%). Gassner and Maréchal (2012)

reported an even higher efficiency for SNG ( 70%). They also reported that

the overall efficiency can even reach 90% when additional production of heat
and electricity is considered (polygeneration). In addition to the high efficiency,
SNG has the advantage that it can efficiently be produced in relatively small plants
(<100 MW). The drawback is that it is a gaseous fuel. Zhang (2010) reported a pro-
duction efficiency of approximately 55% for methanol, which has the lowest energy
content among the liquid fuels. Regarding the production efficiency, it seems better
to convert methanol into DME (production efficiency 60%). Moreover, DME can

be applied in diesel engines, which makes it the fuel with the highest well-to-wheel
efficiency (not just taking into account the production efficiency but also the effi-
ciency of the engine it is driving). A disadvantage is that DME is a gaseous fuel
that is hard to mix with standard diesel. Alternatives are to fuel diesel engines
with 100% DME or to use it in a gasoline engine mixed with liquefied petroleum
gas (LPG).
According to Zhang (2010), diesel obtained via FTS has a production efficiency of
only approximately 40%. One reason for the low efficiency for the biomass-based
production of FT fuel compared to, e.g., methanol is that the reaction is more exother-
mal. Moreover, it requires more elaborate separation of the crude product than the
other routes and produces more water: about 25% of the energy losses are due to con-
densation of the formed water. Zhang (2010) also states that the production of a wide
range of hydrocarbons, including alcohols and aldehydes, contributes to the low effi-
ciency. This is of course true when the only product of interest is diesel, but when FTS
is integrated in a biorefinery scheme (see Chapter 15), this is less of a problem. Tock
et al. (2010) report much higher production efficiencies ( 60%) for FTS when not just

FT diesel but the whole FT crude is considered. In their analysis, the production of FT
crude is more efficient and cheaper than that of methanol and DME (both with a pro-
duction efficiency of

53%). Moreover, they point out that the choice of the gasifi-
cation process has a large influence on the overall performance: different synthetic
fuels require different gasification technologies to maximize the efficiency. They
claim that for liquid fuel synthesis, the best choice is directly heated gasification using
a circulating fluidized bed, followed by steam methane reforming. It should be noted
that FTS is a relatively complex process that requires large investment costs. This gives
a large barrier for developing countries to implement this technology (Zinoviev et al.,
2010). For large-scale operation, Haarlemmer et al. (2012) conclude after comparing a
large number of studies that use of FTS for biomass conversion will lead to prices
between 1.00 and 1.40 L −1 of fuel. It will only become economically viable when

crude oil price levels are high or when the environmental benefits of green FTS fuels
are valued (Hamelinck et al., 2004). Van Vliet et al. (2009) report that oil prices should
stay above $75 per barrel to make biofuel production by FTS profitable.

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