# Techno-Economic Potential of Plasma-Based CO\({}_{2}\) Splitting in Power-to-Liquid Plants
## 2 State of the Art
### Plasma-Based CO\({}_{2}\) Splitting--A Short Review
\[\eta_{CO_{2}}=\frac{\chi_{eff}\cdot\Delta H^{298.15K}}{SEI} \tag{1}\]  
\[\chi_{eff}=\chi_{CO_{2}}\cdot\frac{\dot{n}_{CO_{2}in}}{\dot{n}_{in}}=\frac{ \dot{n}_{CO_{2}in}-\dot{n}_{CO_{2}out}}{\dot{n}_{CO_{2}in}} \tag{2}\]  
\[SEI=\ P_{e}/\dot{n}_{CO_{2}in} \tag{3}\]  
where \(\dot{n}_{CO_{2}in}\) is the molar stream of CO\({}_{2}\) at the inlet of the reactor, \(\dot{n}_{in}\) is the molar stream at the inlet of the reactor, \(\dot{n}_{CO_{2}out}\) is the molar stream of CO\({}_{2}\) at the outlet of the reactor, and \(P_{e}\) is the electric power input to the reactor. [PERSON] and [PERSON] et al. considered the different plasma technologies for CO\({}_{2}\) splitting in their 2017 study [18]. The paper focused on dielectric barrier discharge, and gliding arc and microwave plasmas. Different studies were summarized and plotted in a conversion-efficiency diagram. Microwave plasmas with both a high conversion and efficiency stood out. Gliding arc plasmas achieve high efficiencies with low conversions. Dielectric barrier discharge (DBD) plasmas perform worse due to the reduced field strength deviating from Microwave plasma (MW) and gliding arc plasma (GA), with a few exceptions. In a comparative study from 2021 by [PERSON] et al., experimental studies on plasma-based CO\({}_{2}\) splitting were considered and compiled [27]. Only studies that achieved comparatively high conversions and/or efficiencies for the individual technologies were included. Minor extensions were made to the original diagram; the adjusted diagram is shown in Figure 2. \begin{table}
\begin{tabular}{c c c c} \hline
**Process** & **Educts** & **Products** & **TRL** \\ \hline CO\({}_{2}\) splitting & CO\({}_{2}\) & CO\({}_{2}\) & 2-3 [18] \\ CO\({}_{2}\) splitting & CO\({}_{2}\), CH\({}_{4}\) & CO, H\({}_{2}\) & 2-4 [19] \\ CO\({}_{2}\) splitting & CO\({}_{2}\), H\({}_{2}\)O & CO, H\({}_{2}\) & 1-2 [18] \\ CH\({}_{4}\) splitting & CH\({}_{4}\) & H\({}_{2}\) & 2-4 [20] \\ Air purification & VOC & CO\({}_{2}\), H\({}_{2}\)O & 6–7 [21] \\ Odor control & Odors/air & Harness connections & 8–9 [22] \\ NH\({}_{3}\) synthesis & N\({}_{2}\), H\({}_{2}\) & NH\({}_{3}\) & 1-2 [23] \\ Tar reforming & Tar & CO, H\({}_{2}\) & 2-3 [24] \\ Methanol splitting & MeOH, H\({}_{2}\)O & H\({}_{2}\) & 1-2 [25] \\ \hline \end{tabular}
\end{table} 
Table 1: Applications of plasma catalysis and their technology readiness levels (TRL) in accordance with [15]. (VOC = volatile organic compounds). [PERSON] et al. conducted experiments with a coaxial DBD reactor, varying a number of parameters, including the flow rate, operating frequency, power, barrier thickness, and duty cycle [38]. By using a burst mode, they were able to achieve an efficiency of 23%, which is comparably high for a DBD reactor, with a conversion of 26%. [PERSON] et al. achieved the highest DBD conversions of 71% by reducing the gap size of the reactor; however, the efficiency of the reactor was only 2.5% [39]. [PERSON] et al. published a techno-economic assessment of CO\({}_{2}\) splitting with a DBD reactor in 2022. Experiments were carried out on a coaxial design with an integrated catalyst-packed bed [34]. The packed bed, the residence time, the specific energy input, and the feed were varied. When varying the feed, pure CO\({}_{2}\) splitting and dry reforming of methane (CO\({}_{2}\) and CH\({}_{4}\) feed) were investigated. For the analysis of the economic feasibility, the net present value (NPV) was determined, which offset incoming and outgoing cash flows. This resulted in only negative values that were largely due to the high electricity costs, which were assumed to be EUR 80/MWh in the study. Among the considered experiments, the studies with CO\({}_{2}\) and CH\({}_{4}\) performed better than the pure CO\({}_{2}\) splitting. For CO\({}_{2}\) splitting, a maximum conversion of \(\chi_{CO_{2}}\approx 75\%\) with an \(SEI=3.5\times 10^{8}\) J/kg\({}_{\rm CO_{2}}\) (\(P=30\) W, \(\dot{V}=2.6\times 10^{-3}\) SLM) was achieved.