Influence of Syngas Composition on the Kinetics of Fischer–Tropsch Synthesis of using Cobalt as Catalyst

Abstract

AbstractA promising method for the utilization of CO2 (e.g., captured from the flue gases of gas‐ or coal‐based power plants) is the production of liquid hydrocarbons from CO2 and renewable H2 (power to liquid, PTL). This is a three‐step process and consists of water electrolysis, reverse water–gas shift (RWGS), and Fischer–Tropsch synthesis (FTS). Here, the syngas for the FTS always contains CO2 owing to the incomplete conversion of CO2 in the RWGS reactor because of thermodynamic constraints. Therefore, the influence of not only the main reactants CO and H2 but also CO2 on the kinetics of FTS using a homemade cobalt catalyst was studied. Moreover, under effective conditions (i.e., with particles of millimeter size, as used in fixed‐bed reactors), the FTS is affected by internal mass‐transport limitations, which lead to an increased H2/CO ratio inside the particle, which has an impact on the local reaction rate and selectivity. Therefore, the effect of the H2/CO ratio was studied in a broad range of 0.5 to 40 at temperatures of 210 to 230 °C at a total pressure of approximately 3 MPa. With increasing H2/CO ratio and a surplus of H2, the methane selectivity rises and the selectivity to higher hydrocarbons decreases. As long as a certain (very low) amount of CO is present, CO2 behaves like an inert component. However, for particle sizes of several millimeters and pronounced pore diffusion limitations, the CO concentration decreases towards the particle center and a core region free of CO is formed. At H2/CO ratios >10, CO2 is also converted (but practically solely to methane). The intrinsic kinetic parameters of the reaction rates were evaluated by using Langmuir–Hinshelwood‐type rate expressions. The selectivities were also described by a model from Vervloet et al.1 The used models are in good agreement with the experimental results.