Biofuels in Perspective

98 Biofuels
Figure 6.1 Theoretical ASF product distribution for FT-synthesis, with LPG (C3-C4), naphtha (C5-C11),
diesel (C12-C20), light wax (C21-C30), and heavy wax (>C30). Reproduced by permission of ECN.
6.2.2 Feed Gas Specifications
The catalysts used in FT-synthesis are intrinsically very sensitive to small amounts of
impurities. In commercial operation, catalysts are replaced or regenerated after a certain
operational period. The definition of the gas cleaning is therefore based on economic
considerations: investment in gas cleaning versus accepting decreasing production due
to poisoning of the catalyst. Therefore, there are no ‘hard’ data on maximum levels for
impurities in FT feed gas. For each plant the acceptable levels may be different. Rule-ofthumb
specifications are presented in Table 6.1 for known impurities, and for impurities
that might be present in biomass-derived gases. 2,3
A maximum value of less than 1 ppmV is defined for both the sum of the N2-containing
and S-containing compounds. For the halides and alkaline metals a lower level of less than
10 ppbV is assumed. With respect to the organic constituents that are present in biomass
product gases (i.e. tars and BTX), tars in general, there are no limits regarding poisoning
Table 6.1 FT feed gas specifications. Reproduced by
permission of ECN
Impurity Removal level
H2S + COS + CS2
< 1 ppmV
NH3 + HCN < 1 ppmV
HCl + HBr + HF < 10 ppbV
alkaline metals < 10 ppbV
Solids (soot, dust, ash) essentially completely
Organic compoundsα (tars) below dew point
- class 2β (hetero atoms) < 1 ppmV
α organic compounds include also benzene, toluene and xylene (BTX).
β class 2 tars comprise phenol, pyridine, and thiophene.