Kinetics of low temperature plasma assisted NH₃/H₂ oxidation in a nanosecond-pulsed discharge

Ammonia (NH₃) has been widely recognized as one of the carbon-neutral fuels. However, ammonia combustion suffers low reactivity and high N₂O/NO_x emissions. To overcome these issues, this work reports plasma assisted NH₃/H₂ oxidation and unveils the kinetics of fuel oxidation and N₂O/NO_x formation by combining time-resolved laser diagnostics with plasma modeling. Firstly, we found that the NH₃ consumption is promoted with a H₂ blending ratio of 0.3, due to enhancements of H and OH formation by plasma assisted H₂ dissociation. Secondly, at a high reduced electric field, when the H₂ blending ratio increases, the NH₃ oxidation is promoted due to both the HO₂ formation and strong NO kinetic enhancement via NO-HO₂ and NO₂-H pathways. In the meantime, it is shown that the NO mole fraction also increases with H₂ blending ratio, because the NO formation is enhanced via N(²D)-O₂ pathways, and the DeNO_x chemistry is weakened with less NH₂ production. By contrast, at a lower reduced electric field, when the H₂ blending ratio increases, the decreased N(²D) formation does not produce enough NO to replenish the NO formation drop caused by lower NH₃ concentration. Thirdly, the reduced electric field non-monotonically affects fuel consumption and N₂O/NO_x formation by manipulating electron energy deposition pathways. The NH₃ consumption is maximized with an optimal reduced electric field where N₂* excitation and O₂ dissociation are most efficient. When the reduced electric field deviates from its optimum, the NH₃ consumption decreases due to the discharge energy deposition to either vibrational excitation or dissociation of N₂. The N₂O/NO_x emissions governed by the NH₃ oxidation follow the above NH₃ consumption trend. © 2024 The Combustion Institute.