A new methodology for the reduction of vibrational kinetics in non-equilibrium microwave plasma: application to CO₂ dissociation

Abstract

Plasma reactor technologies have the potential to enable storage of green renewable electricity into fuels and chemicals. One of the major challenges for the implementation of these technologies is the energy efficiency. Empirical enhancement of plasma reactors performance has proven to be insufficient in this regard. Numerical models are becoming therefore essential to get insight into the process for optimization purposes. The chemistry in non-thermal plasmas is the most challenging and complex part of the model due to the large number of species and reactions involved. The most recent reaction kinetic model for carbon dioxide (CO 2 ) dissociation in non-thermal microwave plasma considers more than one hundred species and thousands of reactions. To enable the implementation of this model into multidimensional simulations, a new reduction methodology to simplify the state-tostate kinetic model is presented. It is based on four key elements; 1) all the asymmetric vibrational levels are lumped within a single group, or fictitious species, CO 2 *, 2) this group follows a nonequilibrium Treanor distribution, 3) an algebraic approximation is used to compute the vibrational temperature from the translational temperature based on the Landau-Teller formula and 4) weighted algebraic expressions are applied, instead of complex differential equations, to calculate the rates of the most influencing reactions; this decreases substantially the calculation time. Using this new approach, the dissociation and vibrational kinetics are captured in a reduced set of 44 reactions among 13 species. The predictions of the reduced kinetic model regarding the concentrations of the heavy species in the afterglow zone are in good agreement with those of the detailed model from which the former was derived. The methodology may also be applied to other state-to-state kinetic models in which interactions of vibrational levels have the largest share in the global set of reactions.

A two-step modelling approach for plasma reactors – experimental validation for CO2 dissociation in surface wave microwave plasma

“…CO 2 and the products of its dissociation comprise the neutral species. Excited states refer to the internal degrees of freedom of CO 2 used in the calculation of the electron temperature and the fictitious species CO* 2 , 22 which groups all levels in the Vibrational and electronically excited states are only included for CO 2 , considering that it is the initial and dominant species in the discharge and therefore it has the highest influence in the electron kinetics. Table 3 lists the excited states along with their energy levels and statistical weights.…”

“…the time at which VT relaxation kicks in and the evolution of T V /T follows the trend described in ref. 22. This characteristic time is approximately 1.6 × 10 −4 s and it is computed at the initial conditions of the discharge from τ VT = (k RV4 n CO 2 ) −1 , where k RV4 is the rate constant of the VT relaxation reaction RV4 and n CO 2 is the population density of CO 2 .…”

“…The work is continuation of our previous published research on the matter. 12,13,22 In the first step, the two-dimensional axisymmetric Argon plasma model, developed in ref. 13, is used to characterize the discharge by identifying the power losses, the deposited power density patterns and the time and length scales as well as by performing a qualitative analysis of the plasma and process variables.…”

“…For this purpose, the methodology for the reduction of vibrational kinetics in non-equilibrium microwave plasma presented in ref. 22 is used. The final model predictions are compared to our previously published experimental results 12 to validate the model and get insight into the performance of our labscale reactor.…”

“…),vib is the contribution of the asymmetric vibrational mode to the molar specific heat at constant pressure (volume) of CO* 2 , R the universal gas constant, E vib the mean vibrational energy, and n i and E i are the population and vibrational energy of the level i, respectively. The reader is referred to ref 22. for the details regarding the calculation of the energies and populations of the vibrational levels grouped within CO* 2 .…”