The syngas purification step is the most expensive part of an FT complex. It accounts for 60-70% of the total cost in the case of natural gas (simplest option). This cost rises up to 50% more in the case of coal-based FT process, with additional 50% cost increase in the case of biomass feedstock (Zhang, in press). Syngas cleaning is, therefore, considered the biggest challenge to the commercialization of the BTL process.

The presence of impurities in the syngas produced by the gasification step is inevitable. Syngas contains different kinds of contaminants such as particulates, condensable tars, BTX (benzene, toluene and xylenes), alkali compounds, H2S, HCl, NH3 and HCN. The catalysts employed in the FT reactor for the synthesis of the liquid fuels are notoriously sensitive to such impurities, and especially sulphur and nitrogen compounds, which irreversibly poison FT catalysts. Alkaline metals and tars deposit on catalysts and contaminate the products, while particles cause fouling of the reactor. Therefore, extensive cleaning of the syngas is required prior to entering the FT reactor. Moreover, the concentration of inert gases (i. e. CO2, N2, CH4, etc.) must be approximately less than 15 vol.% (Boerrigter et al., 2004). Indicative syngas specifications for FT synthesis are shown in Table 19.1 (Boerrigter et al., 2004).

The first step in all syngas cleaning configurations considered so far is the removal of BTX and larger hydrocarbons, the tars. BTX should be removed upstream the active carbon filters in the syngas cleaning train, as active carbon adsorbs BTX and would therefore require frequent regeneration, reducing process reliability. Tars normally condensate at the typical FT reactor conditions and foul downstream equipment, coat surfaces and enter pores in filter and sorbents. Therefore, tars should be removed to a concentration below condensation point at the operating pressure of the FT reactor (Hamelinck et al., 2004). Three processes can be used for tar removal. Thermal cracking of tars involves high temperatures, 1000-1200°C, and tars are cracked in the absence of a catalyst with the use of

steam or oxygen. However, thermal cracking has low thermal efficiency, requires expensive materials and results in the production of large amounts of soot. Catalytic cracking/reforming of tars in the presence of dolomite/olivine, nickel — based catalysts or alkalis (Wang et al., 2008) overcomes these limitations. Still, this technology is not yet proven and costs are increased due to catalyst consumption (Milne et al., 1998). Alternatively, tars can be removed at a low temperature by advanced scrubbing, using a special organic washing liquid (‘oil’). Such a system has been developed by ECN, who have patented the OLGA tar removal technology (Boerrigter et al., 2004). It should be mentioned that the use of entrained flow gasifiers removes the need for a tar cracking/removal step as the high gasifier operating temperatures (1300-1500°C) yield a tar-free syngas.

After the removal of the tars, other contaminants can be removed from the syngas by either the conventional ‘wet’ low temperature or the ‘dry’ high temperature cleaning. The wet gas cleaning technology is proven and has been well commercialized for large-scale coal gasification systems (Zhang et al., 2007). The general approach involves the quenching of the raw hot gas with water to cool the gas and remove solid particles (e. g. dust, soot, ash) and the volatile alkaline metals (Boerrigter et al., 2004). NH3 is then removed by a water washer along with halides and H2S is removed either by absorption or the Claus process to elementary sulphur. In the final step, the gas passes through a ZnO and active carbon filters, which remove H2S and remaining trace impurities and act as guard beds for the FT catalyst. Although proven, this technology has efficiency penalties and requires additional waste-water treatment. Many research efforts have been focused on the development of dry hot syngas cleaning processes, which appear to be potentially more efficient and cleaner than the proven conventional wet technology (Sharma et al., 2010). Hot gas cleaning consists of candle or ceramic filters for removing solid contaminants and sorbents for fluid contaminants, through which the high temperature of the syngas can be maintained, achieving efficiency benefits and lower operational costs. Dry gas cleaning can be especially advantageous when preceding a reformer or shift reactor, as these processes have high inlet temperatures. However, as aforementioned, the performance and reliability of the filters and sorbents has still to be proven at high temperatures, especially above 400°C, for a commercial implementation of the dry gas cleaning technology. Recent developments and critical review of the different syngas cleaning technologies have been published and can be found in Sharma et al. (2008) and Sharma et al. (2010).

After the gas cleaning train, the biomass-derived syngas has to be conditioned in order to adjust the H2/CO ratio to that required for the FT reactor. Typical conditioning includes steam reforming of methane and light hydrocarbons to CO and H2 over a nickel catalyst, followed by a water gas shift (WGS ) reactor. Finally, as the concentration of inert gases must be kept below 15 vol.% (Boerrigter et al., 2004), CO2 is removed with amine treating. The purified and conditioned synthesis gas is then compressed to the required pressure and is fed to the FT reactor.