3D phenomenological modeling of plasma-assisted methane reforming

Natural gas contains a significant fraction of methane, a strong greenhouse gas besides being a potent hydrogen carrier. Thus, reforming methane to a more reactive gas mixture could potentially abate the associated greenhouse heating by depleting methane and provide a pathway to generate hydrogen. The present study investigates the non-equilibrium plasmaassisted reforming of methane to produce hydrogen and reactive alkenes using repetitive nanosecond pulse discharges. A detailed gas-phase chemical kinetics mechanism along with plasma reforming kinetics derived from our previous work are used to perform 0D calculations to obtain the energy fractions for various plasma processes. A phenomenological model for the plasma-assisted reforming of methane/nitrogen mixtures is developed by considering the vibrational energy transport equations of both methane and nitrogen separately. The energy fractions involved in various plasma processes, such as ultra-fast gas heating and ultra-fast gas dissociation due to the electron excitation reactions, and slow gas heating due to the relaxation of vibrational excitation modes of methane and nitrogen, are accounted for in our new phenomenological model using energy fractions derived earlier. The newly developed phenomenological model is then used to perform 3D direct numerical simulation (DNS) of methane reforming diluted with 60% nitrogen in a pin-to-pin electrode configuration with a discharge gap of 1 mm. The effect of pulsing on the evolution of reformed mixture kernels is investigated by comparing two cases: a single-pulsed case with a pulse energy of 0.8 mJ, and another case using 4 pulses at 200 kHz, with a per pulse energy of 0.2 mJ. The single-pulsed case was observed to promote kernel separation and higher fractions of reformed products, while the multiple-pulsed case resulted in a more diffused kernel.