Feasibility study of water plasma jets for combustion promotion

•Effectiveness of water plasma jets (PJs) for combustion was examined.•Water PJs are as effective as oxygen PJs at promoting combustion.•Water PJs are effective because of presence of OH radicals at low specific power.•Water plasma torch is a practical and useful device for combustion promotion.

Plasma-assisted combustion can be used to achieve combustion of fuels that are difficult to utilize, which can help in combating the current energy and environmental problems. However, current plasma-assisted combustion technologies employ oxygen, which is proficient in reactivity but heavily degrades electrical contacts over time. Therefore, we propose the use of water, which is readily available, low cost, and ecofriendly, as feedstock of plasma jets (PJs), by examining the usefulness of water PJs for combustion promotion. Combustion tests for methane (CH4)/air mixtures were conducted using our original experiment apparatus with three types of PJ feedstocks, i.e., Ar seeded with O2, N2, or H2O, coupled with numerical simulations of the reaction mechanism. The results revealed that H2O-seeded PJs, (Ar + H2O) PJs, included O, H, and OH radicals and that for high specific powers to PJs, O and H radicals mainly contribute to combustion promotion, whereas for low specific powers, mainly OH radicals contribute to combustion promotion. At combustion promotion, (Ar + H2O) PJs were almost as effective as O2-seeded PJs, i.e., (Ar + O2) PJs, which agrees with numerical estimations. Moreover, (Ar + H2O) PJs are preferable because they cause very less wear on the electrodes of a plasma torch. Therefore, we proposed that the utilization of water PJs is a useful approach to promote combustion.