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

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.

Accumulation of liquid hydrocarbons in catalyst pores during cobalt-catalyzed Fischer–Tropsch synthesis

“…7,19 Nevertheless, a constant (mean) α value was assumed here over the entire course of reaction and pore filling, respectively. 7,19 Nevertheless, a constant (mean) α value was assumed here over the entire course of reaction and pore filling, respectively.…”

“…For cobalt as catalyst, the water-gas-shift reaction (CO 2 formation) is negligible. 7,19 However, CH 4 (and other volatile short chain HCs of at least up to a C-number of 10) are irrelevant for pore filling. 21 As a result, partial pressure of H 2 O has here no effect on the kinetics.…”

“…2 lists the values of the intrinsic kinetic parameters of Eq. This model is based on the diffusivity of CO and H 2 in the partially liquid filled pores and the data of the intrinsic kinetics determined by experiments with small particles; CO consumption by CH 4 formation was also considered (details in 7,19 ). The effectiveness factor η pore as a function of pore filling degree was calculated by a complex reaction-pore diffusion model of a single catalyst particle as presented in 7 .…”

Surface Chemistry of Perovskite-Type Electrodes During High Temperature CO2 Electrolysis Investigated by Operando Photoelectron Spectroscopy

“…From a storage and distribution point of view, hydrocarbons offer a couple of advantages over hydrogen such as high volumetric energy density, safe and easy storability, and compatibility with existing fuel infrastructure. 4 , 5 One way out of this dilemma is hydrocarbon formation from H 2 O and CO 2 , which uses electrochemically produced hydrogen together with carbon dioxide via a methanation reaction (CO 2 + 4H 2 → CH 4 + 2H 2 O). This is commonly referred to as a power-to-gas (P2G) process.…”

Formation and Vaporization of Hydrocarbons During Cobalt‐Catalysed Fischer‐Tropsch Synthesis

“…Initially, the pores are empty (syngas filled; HC filling degree F = 0) and internal mass transport limitations can be excluded; the effectiveness factor η may decrease with increasing filling degree , . This effect is included in the model, although – as stated above – not relevant for the conditions of the presented experiments ( T < 200 °C, η = 1 even for F = 1).…”