Research On Mechanism Of Efficient Nitrogen Fixation By Warm Plasmas In Air

Nitrogen is an important element that sustain the living of organism.And nitrogen fixation is the process transforming atmospheric nitrogen into N-compound that can be used by life directly.Non-thermal plasma(NTP),as an artificial nitrogen fixation way under normal pressure and temperature,has attracted wide attention from the fields of physics,chemistry of plasma and energy applications in recent years.Among that,the research of NO_xfixation by atmospheric air glow discharge become more and more popular due to the low price and easy-getting of its raw material-air.The topic of this dissertation is about the mechanism of efficient nitrogen fixation by warm plasmas generated from atmospheric pin-to-pin air DC glow discharge.Main research contents are listed below:1.Gas temperatures(T_g)measurements of glow discharge region:Radial and spatial distribution of gas temperature was obtained by Rayleigh scattering at the center(axial potion)of air glow discharge with a resolution of 26?m.It is found that the amplitude and profile of the gas temperature become bigger and wider as the discharge current or gap distance increase,and they get smaller and narrower as the air flow rate increases.2.Absolute density of main particles in the glow discharge region:Combined with gas temperature,the radial and spatial distribution of absolute density of atomic N&O and NO molecule was obtained by one/two-photon absorption laser-induced fluorescence spectroscopy(LIF/TALIF)for the same region with the same resolution.The‘hill-like'distribution of atomic N&O density was observed and the variation tendency of its amplitude and profile is the same as that of the gas temperature as the current and airflow rate increase.However,the peak density of atomic N falls as the gap distance rises which is opposite to that of atomic O.In between,the absolute density of atomic N(10²⁰ m^-3)is 1?2 order smaller than the peak density of atomic O(10²² m^-3),and the profile of atomic N density is much narrower compared to that of the latter.On the other hand,the spatial distribution of NO density is‘hollow-like'which seems to be the complement of the density of atomic N&O,with the same trend under different discharge conditions as atomic O.3.Calculated reduced electric field(E/N)and energy efficiency of nitrogen fixation by DC air-glow discharge:The radial distribution of reduced electric field under different discharge conditions can be calculated with the help of measured discharge voltages,currents and gas temperatures.As observed,the amplitude rises as the discharge current and air flow rate increases,while it drops as the gap distance increases.And the profile widths are similar as that of gas temperature.The concentration of NO_xin the exhaust was measured by FTIR,which gave a same variation trend under different conditions compared to that measured in the discharge region.Thus,the energy efficiency of nitrogen fixation can be calculated and found to get smaller when the discharge current increases while bigger as the gap distance and especially air flow rate increases.4.Major reactions involved and mechanism of energy transmission behind the nitrogen fixation by air warm plasmas:Main reactions between electrons and particles were listed and analyzed to get the generating and consuming process of atomic N&O and NO particle.And the‘hollow-like'distribution of NO density can be explained as consumption of atomic N&O to from N₂ or NO₂.Furthermore,according to the fraction of reduced electric field in different ranges and the electron energy needed for activation of different particles,atomic N is mainly induced by reduced electric field above 70 Td while atomic O generates mostly in the range of 40?70 Td.And the reason why the amplitude of atomic O is 1?2 order higher than atomic N can be judged from the fraction of these two reduced electric field.Lower ranges(7?40 Td)of reduced electric filed are corresponded to the vibrational state of N₂ and O₂,which appears to be the key to generate NO by Zel'dovich reactions.The conclusion is that energy efficiency can be lifted by increasing the fraction of reduced electric field in the range of 7?40 Td,which means more N2(v)and O2(v),and depressing the reverse reactions of NO.Finally,the best energy efficiency was 265.67 GJ/t N(3.72 MJ/mol correspondingly)under discharge current of 25 m A,air flow rate of 10 slm,gap distance of 5 mm.