(37a) Impact of Low Levels of Ammonia in Syngas on the Fischer-Tropsch Synthesis Performance of Cobalt and Iron Catalysts in Fixed-Bed Operation

The potential for liquid fuel production using the Fischer-Tropsch synthesis with biomass as the carbon source is confronted by a variety of practical issues.  Among these is the ability to cost‑effectively reduce a variety of potential catalyst poisons to acceptably low levels within the capital constraints of a modest capacity biomass-based plant.  As noted by Tsakoumis et al. in connection with the deactivation cobalt-based FT synthesis catalysts, “Little is known on the effect of nitrogen containing compounds and the available information is mainly found in patents”.¹  We have undertaken an investigation into the effects of ammonia on both cobalt and iron FT synthesis catalysts operating under fixed-bed conditions at ammonia levels between  6 ppmv and 300  ppbv.  Conversion performance after exposing the Fe catalyst to a steady 6 ppmv ammonia in the syngas remains unaffected relative to the performance in an ammonia-free gas.  Conversion performance is stable over several hundred hours of operation without detectable deactivation.  In contrast, two different cobalt catalysts both exhibit considerable sensitivity to the presence of ammonia in the syngas.  The loss of conversion performance is a linear function of the total ammonia exposure until a N:Surface Cobalt ratio near 0.15 is reached, at which point half of the catalyst’s activity is lost.  The impact of each adsorbed N entity is quantitatively similar to the impact of S observed during H₂S exposure.  Unlike the effect of S, the impact of N entities on cobalt is self limiting.  Further exposure beyond N:Surface Cobalt = 0.15 results in no further change in performance.  Altered performance persists unchanged for over 100 hours after removal of ammonia from the syngas stream.  While the measured α value of the cobalt catalysts is unaffected, the relative selectivity to olefins and alcohols increases.

1)       Tsakoumis, Nikolaos E.; Rønning, Magnus; Borg, Øyvind; Rytter, Erling; Holmen, Anders; Deactivation of cobalt based Fischer-Tropsch Catalysts: A Review; Catalysis Today 154 (2010) 162-182