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Pöhlmann, Ferdinand ; Jess, Andreas:
Influence of Syngas Composition on the Kinetics of Fischer–Tropsch Synthesis of using Cobalt as Catalyst.
In: Energy Technology.
Bd. 4
(2016)
Heft 1
.
- S. 55-64.
ISSN 2194-4296
DOI: https://doi.org/10.1002/ente.201500216
Volltext
Abstract
A 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.