Synthesis gas production via CO conversion with in-situ CO$_2$ removal for e-fuels synthesis

Climate change is an urgent global issue that has been caused mainly by the extensive use of fossil fuels. Fossil fuels combustion leads to increasing concentrations of the greenhouse gas CO$_2$, ultimately resulting in rising average global temperatures. The effects of climate change are far-reaching, impacting not only the environment but also social and economic systems.

The aviation sector is a major contributor to CO$_2$ emissions, with estimates predicting an increase from 1 Gt in 2018 to up to 1.9 Gt of CO$_2$ by 2050. These data highlight the necessity to move away from fossil fuels and shift towards sustainable alternatives. Synthetic e-fuels are among the discussed options to serve as sustainable fossil fuel substitutes in the aviation sector. E-fuels are typically produced in Power-to-Liquid (PtL) processes using renewable electricity, CO$_2$, and water as feedstocks. By capturing the emitted CO$_2$ from the air and reusing it for the production of e-fuels, a closed carbon cycle is achieved and e-fuels can be considered to be carbon neutral.

A novel PtL route based on the Fischer-Tropsch pathway was developed in the EU project Kerogreen. ... mehrUnlike conventional Fischer-Tropsch-based PtL processes, this route does not employ energy-intensive electrolysis. Instead, captured CO$_2$ is dissociated into CO and O$_2$ using microwave plasma technology. The crucial step of synthesis gas production is realized by means of CO conversion via the Water-Gas Shift (WGS) reaction with in-situ CO$_2$ removal. The synthesis gas is chemically converted into a range of hydrocarbons using the Fischer-Tropsch reaction. A synthetic hydrocarbon product is obtained that can potentially be used as synthetic jet fuel by cracking heavy hydrocarbons into the kerosene range. This thesis explores the possibilities of the synthesis gas production step and its influence on the subsequent kerosene synthesis. Furthermore, the thesis provides a technical solution for this process step in a pilot plant.

Based on engineering and economic considerations, the Sorption-Enhanced Water-Gas Shift (SEWGS) technology was employed for synthesis gas production. In a packed-bed reactor, a WGS catalyst mixed with a material that adsorbs CO$_2$ enables the WGS reaction with simultaneous CO$_2$ separation in a single unit operation. As the sorbent is saturated with CO$_2$ at some point, cyclic operation with reactive adsorption and subsequent desorption phases for regeneration is inevitable.
The choice of SEWGS materials, sorption parameters, and reactor configurations was examined in characterization tests and lab-scale experiments. A Cu/ZnO-Al$_2$O$_3$ catalyst showed the best activity and stability. Calcination of commercial hydrotalcite at 400 °C led to the desired CO$_2$-adsorbing mixed oxide structure and potassium impregnation improved the CO$_2$ adsorption capacity of this material. Experiments recording the breakthrough curves of CO$_2$ in a packed-bed microchannel reactor revealed that the sorption properties were influenced by the prevailing water content during the reactive adsorption as well as the desorption phase length. Furthermore, the reactor packing configuration was found to affect the sorption performance. The presence of catalyst-free zones at the reactor outlet enhanced the CO$_2$ uptake significantly.

In addition to these experimental studies, numerical simulations were carried out to investigate the SEWGS performance using dynamic reactor modeling. A novel graphical simulation approach was created in Matlab Simulink to optimize cyclic process operation and automated plant control. The model allowed for automatic adjustment of cyclic switching times from reactive adsorption to regeneration phases during runtime based on pre-defined parameters. The simulation results highlighted the potential benefits of combining carefully selected operating parameters and process configurations. The experimental and simulation results were used to develop a compact SEWGS module for the Kerogreen pilot plant. This first-of-its-kind demonstration plant aims to feature the full process chain from CO$_2$ to a crude form of synthetic kerosene. Simulation-driven process design was applied to optimize the intended operation procedures of the SEWGS reactor. Therefore, the Simulink model on system scale was employed to quantify the influence of design configurations and key operating parameters.

Finally, an experimental study elucidated the impact of possibly remaining CO$_2$ from the SEWGS step on consecutive steps in the Kerogreen process chain. Different synthesis gas compositions with and without CO$_2$ were used to simulate a possible malfunction of the SEWGS module and investigate the influence of CO$_2$-rich synthesis gas feeds on the product distribution of a Fischer-Tropsch reactor with a subsequent hydrocracking reactor cascade. The experimental results demonstrated that Fischer-Tropsch syncrude upgrade by means of direct hydrocracking is not remarkably influenced by the CO$_2$ content in the synthesis gas feed. However, it was concluded that the diluting effect has to be considered in the reactor design and the process parameters.

Both, the experimental as well as the simulation results, confirmed that the SEWGS technology investigated in this thesis has the necessary potential and technical feasibility for the production of synthesis gas from pure CO. This development marks a significant step in the Kerogreen PtL process chain, towards new possibilities for replacing fossil kerosene and lowering CO$_2$ emissions from the aviation sector.