Metal combustion is currently under discussion as a possible basis for a closed energy loop. One potential metal with several benefits for such a process is lithium. While the reaction products in conventional combustion processes are gaseous, the reaction products of lithium combustion are solid (Li2CO3, Li2O) and, hence, easy to capture and to recycle. The current paper describes the lay-out and optimization of a 100 MWth lithium slag tap furnace by computational fluid dynamics (CFD) using CO2 as oxidizer for the lithium. ANSYS Fluent has been extended by two lithium combustion models developed by the authors. The first reference model is one-step model directly converting Li to Li2CO3, neglecting the intermediate species Li2O. The second extended model is a two-step model considering Li2O as intermediate species. Simulations were carried out using a fixed geometry of the slag tab, varying the injection angle of gas and lithium spray and the CO2-Li ratio with respect to the lithium conversion level and lithium product capture efficiency. The simulations show that a high capture efficiency of lithium combustion products is possible when a large injection angle is used. The conversion level is highly dependent on injection angle, CO2-Li ratio and the Li combustion model used. While the conversion level of the reference model is inherently limited and lies between 84 and 87.6%, the extended model predicts significantly higher conversion levels in the order of 96.7–99.2% which would be needed for industrial application.