Nanosecond Pulsed Discharge for CO2 Conversion: Kinetic Modeling To Elucidate the Chemistry and Improve the Performance
We study the mechanisms of CO2 conversion in a nanosecond repetitively pulsed (NRP) discharge, by means of a chemical kinetics model. The calculated conversions and energy efficiencies are in reasonable agreement with experimental results over a wide range of specific energy input (SEI) values, and the same applies to the evolution of gas temperature and CO2 conversion as a function of time in the afterglow, indicating that our model provides a realistic picture of the underlying mechanisms in
... ying mechanisms in the NRP discharge, and can be used to identify its limitations, and thus, to suggest further improvements. Our model predicts that vibrational excitation is very important in the NRP discharge, explaining why this type of plasma yields energy efficient CO2 conversion. A significant part of the CO2 dissociation occurs by electronic excitation from the lower vibrational levels towards repulsive electronic states, thus resulting in dissociation. However, vibration-translation (VT) relaxation (depopulating the higher vibrational levels) and CO+O recombination (CO + O + M → CO2 + M), as well as mixing of the converted gas with fresh gas entering the plasma in between the pulses, are limiting factors for the conversion and energy efficiency. Our model predicts that extra cooling, slowing down the rate of VT relaxation and of the above recombination reaction, thus enhancing the contribution of the highest vibrational levels to the overall CO2 dissociation, can further improve the performance of the NRP for energy efficient CO2 conversion. 12-30% for pure CO2 splitting 30 . It is suggested that this type of discharge shows a high degree of non-equilibrium, explaining these high conversions and energy efficiencies 29,30 . Detailed diagnostics experiments in pure CO2 and CO2/H2O mixtures have recently been performed 30,31 , but to our knowledge, no chemical kinetics model has been developed yet, to support the experiments, and to obtain additional insight in the underlying mechanisms, responsible for the high conversions and energy efficiencies. Such a model could be helpful to further improve the performance of NRPs for energy efficient CO2 conversion. Therefore, in this paper we present a detailed study of the CO2 conversion in an NRP discharge, using 0D chemical kinetics modelling, validated by experiments. We have to make quite some assumptions in the 0D model, in order to account for the characteristic features of the NRP discharge, which will be explained in the next section. However, as we want to study the detailed chemistry, including the role of the vibrational kinetics, many different species (and excited levels) must be included in the model, which would lead to excessive calculation times in a more-dimensional model. Therefore, a 0D model is the most appropriate (and currently the only feasible) model to describe the detailed reaction kinetics. This model allows us to elucidate the most important CO2 dissociation mechanisms, pointing towards the role of vibration induced dissociation in energy-efficient CO2 conversion. Furthermore, our model can also pinpoint the limitations, and therefore suggest further improvements for the experiments, as will also be demonstrated. Model description First, we will give a brief description of the 0D model and the chemistry used to describe CO2 splitting, followed by explaining the assumptions made in the 0D approach to describe the NRP discharge.