Nano-iron carbide synthesized by plasma as catalyst for Fischer-Tropsch synthesis in slurry reactors: The role of iron loading and K, Cu promoters

Nano-iron iron carbides (NFeC), generated by a plasma spray technique, were used as-produced and promoted with potassium (K) and copper (Cu) in Fischer-Tropsch synthesis (FTS) in a 3-phi continuously stirred tank slurry reactor. NFeC particles were initially encapsulated in a carbon matrix to protect them from air-borne oxidation. This matrix was partially removed under reductive conditions as a pre-treatment of subsequent FTS reactions. Cu and K promoters were added to the catalyst by mixing Cu metallic powder and/or K2CO3 powder with iron-oil suspension fed to the plasma. The reactants simulated the composition of synthesis gas produced by urban biomass gasification. At-line gas chromatography of the more volatile products and global liquid product analyses by micro-distillation provided the necessary data for mass balance, conversion and selectivity estimations. Also, an appropriate algorithm successfully estimated the ratio of FTS over water-gas shift (WGS) reactions extents. The results disclosed that this nano-powder, characterized by very low internal porosity, generated a liquid fuel that was lighter than other commercial catalysts under similar reaction severities but at a significantly lower catalyst load (9% of the mass of liquid). Increased catalyst loading in the slurry above a critical limit led to higher product water hold-up and, consequently, higher rates of catalyst deactivation. Conversion, selectivity and the Anderson-Schulz-Flory distribution probability of chain growth alpha were compiled and reported. Tests with Cu- and K-promoted catalysts showed a highly significant decrease of the catalyst deactivation rate, with CO and H-2 conversion increasing respectively to 82% and 44% from 35% and 30% obtained with no-doped catalyst. Moreover, CH4 yield was reduced to 4.7% from 12%, and the WGS rate tripled. Finally, the absence of internal porosity in the catalyst facilitated the definition of process-operating conditions in which surface reaction kinetics was the controlling step. (C) 2013 Elsevier B.V. All rights reserved.