Can post-plasma CH4 injection improve plasma-based dry reforming of methane? A modeling study

Thermal plasma-driven dry reforming of methane (DRM) has gained increased attention in recent years due to its high conversion and energy conversion efficiency (ECE). Recent experimental work investigated the performance of a pure CO₂ plasma with post-plasma CH₄ injection. The rationale behind this strategy is that by utilizing a pure CO₂ plasma, all plasma energy can be used to dissociate CO₂, while CH₄ reforming proceeds post-plasma in the reforming reactor with residual heat, potentially improving the energy efficiency compared to injecting both CO₂ and CH₄ into the plasma. To assess whether post-plasma CH₄ injection indeed improves the DRM performance, we developed a chemical kinetics model describing the post-plasma conversion process. We first validated our model by reproducing the experimental results of the pure CO₂ plasma with post-plasma CH₄ injection. Subsequently, we compared both strategies: injecting only CO₂ inside the plasma while injecting CH₄ post-plasma, vs. classical plasma-based DRM. Our modeling results indicate that below specific energy inputs (SEI) of 220 kJ mol⁻¹, the total conversion slightly improves (ca. 5%) with the first strategy. However, the ECE is slightly lower due to the low H₂ selectivity caused by substantial H₂O formation. The highest conversion and ECE are obtained at SEI values of 240–280 kJ mol⁻¹, where both strategies yield nearly identical results, indicating the limited potential of improving the performance of DRM by pure CO₂ plasma with post-plasma CH₄ injection. Nevertheless, the approach is still very valuable to allow higher CH₄/CO₂ ratios without problems of coke formation within the plasma, and thus, to improve plasma stability and reach higher syngas ratios, which is more useful for further Fischer–Tropsch or methanol synthesis.