Electrodeless discharge in water: Reactive species in liquid and gas phase and energy cost for nitrogen fixation

Abstract

One of the main barriers stopping plasma activated water (PAW) in many practical applications is its high energy cost. On the other hand, a careful review of published literature finds that all the studies reported on the PAW only focus on the reactive species generated in liquid, and there is no measurement of the possible reactive species in the gas phase during the generation of PAW, which might contain high concentration reactive species especially for the cases when gas bubbling is used during the generation of PAW. In this paper, an electrodeless plasma device is reported, where distilled water, tap water, and saline water are used as working liquid while O 2 , N 2 , and air are flowing through the liquid. Not only the reactive species presented in the liquid phase, the reactive species contained in the gas phase were analyzed for the first time in the field of PAW nitrogen fixation. It was found that the total number of moles of reactive species in gas phase is actually several times higher than that in liquid phase. Only when considering the reactive nitrogen species (RNS) in the liquid phase, the lowest energy cost is about 18.97 MJ/mol when air is flowing though the tap water. On the other hand, when both the RNS in liquid phase and in gas phase are included, the lowest energy cost achieved is 5.53 MJ/mol when air is flowing through distilled water, which is only one-seventh of the lowest energy cost reported on PAW. This study helps us shed light on plasma nitrogen fixation by using PAW, which has been greatly underestimated due to the overlooking of gas phase products of PAW in the past.

Falling liquid droplets discharge

“…The Gaussian broadening is mainly due to natural broadening, instrumental broadening, and Doppler broadening, while the Lorentzian broadening is composed of van der Waals broadening and Stark broadening [31]. Natural broadening is typically small in atmospheric pressure plasmas, instrumental broadening is obtained calibration with standard light sources, and Doppler broadening arises from particle thermal motion [32]. The calculation for Doppler broadening is given by:…”

“…The drawback is that it can only produce plasma with dielectric barrier discharge. Luo et al [32] filled two chambers, one above the other, with liquid and connected them to high voltage and ground separately. They perforated the thin wall between the two chambers with many small holes and introduced gas into the lower chamber.…”

Nitrogen fixation and H2O2 production by an atmospheric pressure plasma jet operated in He–H20–N2–O2 gas mixtures

“…When analysing the concentrations of plasma‐generated reactive species measured in the plasma‐treated liquid, it should be noted that not all species are dissolved in the liquid, as shown in a recent work. [ 32 ] Consequently, the energy cost or energy yield of the process might be underestimated as some species are lost in the gas phase and cannot be used in the liquid phase.…”

Plasma for nitrogen fixation by using N2/O2 mixture: Reaction pathway, energy flow, and plasma reactor

“…The presence of water vapor makes the NF reaction more complex. [20][21][22][23][24][25] Huang et al have recently published a review on this topic as well. [26] Therefore, this paper will mainly focus on dry air or different N 2 /O 2 gas mixtures for plasma NF.…”