Abstract
Increasing emphasis on the de-carbonization of energy production has led researchers to consider carbon-free, renewable, and green fuels such as hydrogen (H2) and ammonia (NH3). While H2 suffers from the major challenges of production difficulties, transportation, and storage, NH3 is plagued with challenges of low flame speeds causing unstable flames, high autoignition temperatures resulting in longer ignition delays, narrow flammability limits, and higher levels of NOx emission. Among the different solutions to overcome these challenges of NH3 combustion, non-equilibrium plasma-based igniters are significant owing to the promotion of localized volumetric ignition kernel development by both thermal and chemical assistance. Computational investigation of plasma-assisted combustion of ammonia-air mixtures in constant volume and constant pressure reactors are conducted, to determine the impact of operating conditions on ignition delays and NOx emissions. A mechanism has been assembled in this work using well-validated plasma reactions of NH3 with O2 and N2, alongside plasma kinetics of air from the literature. Subsequently, the newly developed mechanism was used to investigate the plasma-assisted oxidation of NH3. In particular, the impact of the reduced electric field (E/N), equivalence ratio, pressure, pulse frequency, and energy density on the ignition delays and NOx emission were investigated. A Global Pathway-based Analysis algorithm for plasma-assisted systems (PGPA) is used to analyze the nanosecond pulsed nonequilibrium plasma-assisted combustion of NH3/air mixtures. Firstly, a faster ignition and lower production of NOx are observed in the case of plasma discharges compared to thermal energy deposition, owing to the enhanced production of OH radicals and the early reforming of NH3 to produce N2 and H2 with plasma, respectively. At lower reduced electric fields (E/N), PGPA analyses elucidated the significance of gas heating due to vibrational-translational cycles of NH3 and N2 on the increased reactivity of NH3/air mixtures as compared to ignition at a higher E/N. The fuel-lean mixture is observed to exhibit higher production of NOx than stoichiometric and fuel-rich mixtures, resulting from plasma chemistry involving oxygen radical and electronic excited states of N2. Higher rates of collisional quenching at higher pressures during the inter-pulse gaps are found to result in a lesser amount of electronically excited states of N2 and O2, resulting in lower production of air-bound NOx during the pulses.
Complementing combustion enhancements, the study also considers the role of plasma-assisted systems in gas reforming, thereby imparting specific desired characteristics lacking in the original mixture. For instance, plasma-assisted reforming can be utilized to control emissions by reforming specific emission precursors or by improving the gas reactivity to promote clean combustion. Natural gas associated with oil wells and natural gas fields is a significant source of greenhouse gas emissions and airborne pollutants. Flaring/burning of the associated gas removes greenhouse gases like methane (CH4) and other hydrocarbons. Our study explores the possibility of enhancing the flaring of associated gas mixtures (C1 – C4 alkane mixture) using nanosecond pulsed non-equilibrium plasma discharges. A well-studied conventional combustion chemistry for small alkanes is 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 nonequilibrium plasma-assisted reforming and subsequent combustion of the flare gas mixture diluted with N2 at different conditions. The results indicate an enhanced production of H2 and C2H4 in the reformed gas mixture, owing to the electron-impact dissociations of alkanes and subsequent H-abstractions and recombination reactions, thereby resulting in a mixture of CH4, H2, C2H4, C2H2, and other unsaturated C3. The reformed mixture exhibits significantly high reactivity as exhibited by their increased flame speeds and shorter ignition delays. The reformed mixture is also observed to promote increased CH4 destruction levels and complete flaring, thereby reducing the emissions of CH4 and other hydrocarbons.