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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 plasma-assisted 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. 

Natural gas associated with oil wells and natural gas fields is a significant source of greenhouse gas emissions and airborne pollutants. Flaring of the associated gas removes greenhouse gases like methane and other hydrocarbons. The present study explores the possibility of enhancing the flaring of associated gas mixtures (C1 – C4 alkane mixture) using nanosecond pulsed non-equilibrium plasma discharges. Starting with a detailed chemistry for C0 – C4 hydrocarbons (Aramco mechanism 3.0 – 589 species), systematic reductions are performed to obtain a smaller reduced mechanism (156 species) yet retaining the relevant kinetics of C1 – C4 alkanes at atmospheric pressure and varying equivalence ratios (φ = 0.5 – 2.0). This conventional combustion chemistry for small alkanes is then coupled with the plasma kinetics of CH4, C2H6, C3H8, and N2, including electron-impact excitations, dissociations, and ionization reactions. The newly developed plasma-based flare gas chemistry is then utilized to investigate repetitively pulsed non-equilibrium plasma-assisted reforming and subsequent combustion of the flare gas mixture diluted with N2 at different conditions. The results indicate an enhanced production of hydrogen, ethylene and other species in the reformed gas mixture, owing to the electron-impact dissociation pathways and subsequent H-abstractions and recombination reactions, thereby resulting in a mixture of CH4, H2, C2H4, C2H2, and other unsaturated C3 species. The reformed mixture shows an enhanced reactivity as exhibited by their shorter ignition delays. The reformed mixture is also observed to undergo increased methane destruction and higher equilibrium temperatures compared to the original mixture as the gas temperature increases, thereby exhibiting a potential for reducing the unburnt emissions of methane and other hydrocarbons. 

Global Pathway Selection/Analysis (GPSA) algorithm helps in analyzing the chemical kinetics of complex combustion systems by identifying important global reaction pathways that connects a source and a sink species. The present work aims to extend the application of GPSA to plasma assisted combustion systems in order to identify the dominant global pathways that govern the plasma and combustion kinetics at various conditions. The reaction cycles involving the excitation of nitrogen to its vibrational and electronic states and the subsequent de-excitation to its ground state are found to control the reactivity of plasma assisted systems. Provisions are made in the GPSA algorithm to capture the dominant reaction pathways and cycles of plasma assisted combustion (i.e., p-GPSA). Further, the analysis of plasma assisted ammonia combustion are presented as an example, which includes the results obtained using both the traditional path flux analysis and p-GPSA. The dominant pathways for the plasma assisted combustion of ammonia are identified along with the dominant excitation--de-excitation loops and their importance are ascertained and verified using path flux analysis.