Preliminary status note: Thermal biomass conversion technologies ...

2.5. Synthesis of transport fuels using gasification gas
2.5.1. Short description of technology
Syngas produced on a gasification plant can in a subsequent catalytic process be converted into one of
several synthetic fuels. Synthetic diesel can be produced in the Fischer-­‐Tropsch synthesis over iron or
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cobalt catalysts (Dry 1996). Syngas can be converted into methanol over a copper-­‐based catalyst (Wender
1996). The methanol can be dehydrated into dimethyl ether (DME) over a solid acid catalyst (Wender
1996). Mixed higher alcohols can be synthesized over a range of modified methanol and Fischer-­‐Tropsch
synthesis catalysts (Wender 1996). Methanol, DME and higher alcohols can furthermore be converted to
synthetic gasoline over a zeolite catalyst in the methanol to gasoline (MTG) process (Chang & Silvestri
1977). Haldor Topsøe A/S has developed the TIGAS process, where methanol/DME synthesis is combined
with MTG in a single processing loop without isolation of the intermediate methanol/DME (Wender 1996).
An advantage of the synthesis processes is their flexibility. It is possible to vary the conversion and
produce fuel from part of the syngas, while the remaining syngas is used for electricity production (Rostrup-­‐
Nielsen et al. 2007). Thereby gasification plants can be used to even out variations in the output from other
renewable power sources (wind, solar etc.).
It is generally a requirement for the catalytic processes that sulfur is completely removed from the
syngas (Kung 1992, Hamelinck et al. 2004), and the synthetic fuels are therefore essentially sulfur-­‐free,
which is a significant advantage over oil-­‐derived fuels. Alcohols are efficient fuels in gasoline engines, but in
the presence of water methanol has a very limited miscibility with gasoline, which hampers the
introduction of methanol into the existing fuel infrastructure (Keller 1979). The mixed higher alcohols
produced from syngas are easier to integrate into the existing fuel infrastructure, as they have a better
miscibility with gasoline than methanol (Keller 1979). The synthetic gasoline produced from methanol/DME
is fully compatible with the existing infrastructure (Phillips et al. 2011). DME and Fischer-­‐Tropsch diesel are
efficient fuels for use in diesel engines (Dry 2001, Semelsberger 2006). The primary challenge with respect
to DME is that it must be handled under pressure and requires a special fuel delivery system (Semelsberger
et al. 2006). In some applications methanol or DME could however be more directly applicable – for
example as a sulfur-­‐free fuel on board ships.
2.5.2. Global status of technology development
The production of methanol, DME, synthetic gasoline and Fischer-­‐Tropsch diesel from syngas derived from
coal gasification is already practiced commercially. Today methanol is produced in large quantities from
natural gas or coal (Methanol Institute 2012). In recent years the production of DME from coal via
gasification has grown significantly -­‐ especially in China (Larson 2008). A 2500 barrels per day (bpd) plant is
producing synthetic gasoline via methanol from coal gasification is currently operating in China (Phillips et
al. 2011). The production of Fischer-­‐Tropsch fuels from coal has been carried out in South Africa for many
years (Dry 2002). These syngas conversion processes thus represent established commercial technologies.
When moving to biomass applications a processing step that still requires significant research is the
conversion of tar by-­‐products from the gasification into syngas.
The synthesis of mixed higher alcohols is not to the same extent an established technology. For the
synthesis of mixed alcohols several companies (including Haldor Topsøe A/S) operated pilot plants in the
1980’s (Courty et al. 1990). A plant for production of 100 mio. gallons of mixed alcohols per year from