A maximum value of less than 1 ppmV is defined for both the sum of the nitrogen-containing and sulphur-containing compounds. Although for low-pressure methanol process a levels as low as 0.1 ppmV of total sulphur is required. For the halides and alkaline metals a lower level of less than 10 ppbV is assumed (ssee Table 2). Solids must be removed essentially completely, as they foul the system and may obstruct fixed-bed reactors. A number of other possible impurities, such as organics acids (e.g. formic and acetic acid) and unsaturated and higher boiling hydrocarbons, are normally included in purity specifications for synthesis processes because they are already undesirable in upstream process stages of biosyngas production (e.g. compression steps). However, organic compounds present must be below theirdewpoint at pressure of the gas application to prevent condensation and fouling in the system [20]. For organic compounds with S or N hetero atoms (e.g. thiophene and pyridine) the additional specification applies that they need to be removed below ppmV level, as they are intrinsically poisonous for the catalyst.