As discussed in the previous paragraphs, Fe and Co are the industrially relevant catalysts that are currently commercially used in FT, with the choice of catalyst depending primarily on the target product (waxes vs. gasoline and olefins) and the feedstock. Cobalt is the catalyst of choice for GTL processes, using natural gas as feedstock and a H2/CO syngas molar ratio of 2, while Fe is used for CTL processes with a low-hydrogen content syngas. Few studies have investigated in depth the type of catalysts suitable for the BTL-FT process, starting from biomass feedstock (Escalona et al, 2009; Jun et al., 2004; Lapidus et al., 1994; van Steen and Clayes,

2008) . It is of crucial importance to explore the differences between GTL and CTL on the one hand and BTL on the other, in order to successfully implement the FT reaction in the BTL process. Both configurations currently investigated for the BTL process (full conversion and once through FT, see Section 19.2) require high overall and per pass CO conversion and high C5+ selectivity. As cobalt is more active than iron, cobalt has been so far used as the catalyst of choice for economic and exergetic evaluations of the BTL process. However, as analysed in an excellent recent review by van Steen (van Steen and Clayes, 2008), it is debatable whether this is truly the optimal choice of catalyst for the BTL process. van Steen argues that although Fe catalysts can operate with a lower hydrogen content syngas such as that from biomass gasification, a WGS reactor after gasification might be required for both cobalt and iron catalysts in order to obtain a good productivity. Since cobalt yields a higher productivity at high conversion levels, it seems to be the catalyst of choice for BTL synthesis of linear, heavier hydrocarbons if clean syngas is available. However, given that biomass syngas contains several poisons for FT catalysts, such as sulphur-, chloride — and nitrogen — containing compounds, and keeping in view the fact that Fe catalysts are reported to be more resistant to sulphur (van Steen and Clayes, 2008) and ammonia poisoning (Koizumi et al., 2004), the financial risk of operating the FT reactor with an iron-based catalyst seems to be lower. In real operation, deviations from design conditions are inevitable and contamination of the syngas entering the FT reactor is possible. In such case, iron catalysts would be less severely affected than the cobalt ones. Even in the case that the catalyst should be replaced, the much lower cost of iron compared to cobalt offers obvious economical advantages.

Wrapping up, both cobalt and iron catalysts should be considered as options for the FT reactor in the BTL process. A number of scenarios for the BTL process should be developed with both type of catalysts, while the overall process design should be coupled with catalyst developments in both cases in order to clearly prove the superiority of the one catalyst system to the other for commercial application.