Besides the diesel main product, naphtha, a gasoline fraction of less value is produced as a by-product. Straight-run FT naphtha has a low octane number, is olefinic and has high levels of oxygenates (Gregor and Fullerton, 1989). The chemical composition of two naphtha streams produced via LTFT and HTFT process is summarized in Table 19.7. Currently, the BTL-FT synthetic naphtha is rather sold as a low-cost chemical feedstock and cannot be used as a fuel. Untreated naphtha can also be used as an energy source for the production of heat and power or can be alternatively reformed on-site to synthesis gas and fed to the FT reactor to increase the process yield (Bienert, 2007). In the frame of the EU-funded NICE (New Integrated Combustion System for Future Passenger Car Engines) project, Renault/Regienov and Volkswagen tested naphtha fuels in experimental homogeneous charge compression ignition (HCCI) engines and found significant improvements compared to standard diesel fuel (RENEW, 2008). In this context, although BTL-FT naphtha is not a suitable fuel for conventional engines, it may be advantageous for future power trains like HCCI

and combined combustion system (CCS) being even more efficient and having less emissions. It should, however, be mentioned that the requirements for these future engines are not clear for the time being.

Even though the light FT by-product naphtha is not suitable for application as a fuel in its present form and in conventional gasoline engines, it could be upgraded by an additional isomerization or reforming unit to boost its octane number and fulfil the above, as discussed in Section 19.5.3. It should be noted that the production of finished gasoline blendstock is not yet considered because of the added cost and energy expenditures associated with upgrading naphtha to gasoline with the current technology.