| Ammonia vital to human life predominantly rely on the industrial Haber-Bosch（H-B）process via the N2-H2 reactions mediated by catalysts at high reaction temperature and high pressure.The H-B process has been called the most impactful invention of the 20th century and has led to fundamental changes in the way food was produced,and its impact was underscored by the fact that about 50%of the N atoms in humans today originate from it.However,the H-B process needs to consume a lot of fossil energy,and hydrogen mainly comes from methane reforming.The annual energy consumption accounts for 1～2%of the total global energy consumption,and the CO2 emissions account for 1.2%of the total global emissions.The H-B process in China belongs to the high energy consumption and high pollution production industry and its energy consumption ranks first in the chemical industry.In September 2020,Chinese President Xi Jinping said during the United Nations General Assembly that"China will increase its national ownership contribution,CO2 emissions will strive to peak by 2030,and strive to achieve carbon neutralization by 2060."In order to achieve this goal,it was of great significance to explore more mild and environmentally friendly low carbon nitrogen fixation methods with low energy dependence.The nitrogen fertilizer production strategy of"solar fertilizer"can realize the miniaturization and decentralized production of nitrogen fertilizer.Using solar energy,wind energy and other clean renewable energy as energy supply,using nitrogen,oxygen and water in the air as reaction raw materials to produce low concentration nitrogen fertilizer in or near farms can overcome the production defects of traditional nitrogen fertilizer and provide an efficient and economical alternative method for local nitrogen fertilizer production.Among new nitrogen fixation processes,the theoretical energy consumption of non-thermal plasma-assisted nitrogen fixation（NTPNF）is only～0.2 MJ/mol and combine very well with the characteristics of renewable energy processes,and thus allows green synthesis of nitrogen containing compounds in an energy-efficient way,which triggers tremendous interest in the use of plasma for nitrogen fixation.Combined with the current research progress NTPNF can be divided into two categories:gas-gas NTPNF and gas-liquid NTPNF.Among them,the research on gas-gas NTPNF began earlier,but there have been many problems to be solved urgently,such as the decomposition of nitrogen fixation products in plasma,the use of hydrogen as reaction raw material,and the energy efficiency of this technology can not be compared with that of H-B process.In order to overcome the limitations of gas-gas NTPNF,a NTPNF based on gas-liquid interface reaction was proposed at the beginning of the 21st century.This technology can directly use water as raw material to avoid the use of hydrogen.At the same time,nitrogen fixation products can be dissolved in water to avoid its decomposition again.The NTPNF at the gas-liquid interface is highly dependent on the contact area between the plasma and the liquid.At the same time,because of the short life of the active substance in the plasma,the distance between the plasma discharge and the liquid surface is also the key factor affecting the nitrogen fixation effect.At present,almost all gas-liquid NTPNF adopt the reactor design of"Pin-to-Plate".The contact area between plasma and liquid is small.When the afterglow of plasma reaches the liquid level,the active particles in plasma may have been transferred back to the ground state or quenched by other particles,thus limiting the improvement of synthesis rate and energy efficiency.According to the technical characteristics of gas-liquid NTPNF,water falling film dielectric barrier discharge low temperature plasma reactor（WF-DBD）was proposed in this thesis.This is the first application of WF-DBD in the field of nitrogen fixation.The biggest feature of WF-DBD reactor is that a layer of liquid film is covered on the surface of the central electrode.Because the liquid film has a large ratio of surface area to volume,the active particles in the gas phase of the discharge region can be quickly diffused into the solution.At the same time,there is the strongest electric field at the central electrode,and the electron and active particles can maintain high activity when they react with the surface of the liquid film.Finally,the liquidity of the liquid film makes the contact between the plasma and the solution more adequate,which is more beneficial to the transfer of active particles and products from gas phase to liquid phase,and finally improves the rate of nitrogen fixation reaction and the energy utilization ratio.In this thesis,the characteristics of several common plasma-liquid reactors are analyzed and compared.At the same time,according to the technical characteristics of gas-liquid NTPNF,WF-DBD reactor was proposed.Then it was verified by experiments that the reactor can directly use air and water as raw materials and can fix nitrogen at atmospheric pressure and ambient temperature without any catalyst.Under the operating parameters of discharge voltage of 21 k V,liquid temperature of 40 oC and liquid flow rate of 148m L/min,the reactor has good nitrogen fixation effect and good product selectivity.The synthesis rate and energy consumption were 198.3μmol/min and 39.6 MJ/mol,respectively.Compared with the related literature reports,it was found that the liquid membrane dielectric barrier discharge low temperature plasma nitrogen fixation reactor can reach a good level at the same time in the synthesis rate and energy consumption,which is superior to previous gas-liquid NTPNFThen,the structure of WF-DBD reactor was optimized,and the effects of the structure of high voltage electrode,the material of central electrode and the diameter of central electrode on the synthesis rate and energy consumption were investigated,and the variation of electrical parameters and emission spectra with the structure of the reactor was obtained.The experimental results show that the nitrogen fixation reactor with multi-electrode structure can achieve faster synthesis rate and higher energy efficiency,reduce the discharge current and ensure the safe and stable operation of the reactor for a long time.The main results are as follows:（1）.The high voltage electrodes with different structures have great influence on the electrical performance,emission spectrum and nitrogen fixation performance of the reactor.By dividing the high voltage electrode into multiple electrodes,the corona discharge at the edge of the electrode can be induced by the edge effect at both ends of a single electrode,so that the corona discharge at the edge of the electrode can be induced by the edge effect at both ends of a single electrode,thus having lower current and more uniform micro discharge.Under the same discharge voltage,the reactor power with multi-electrode structure was the lowest.On the other hand,compared with sheet electrode and mesh electrode,the discharge region of multi-electrode structure is more concentrated,which improves the energy density of plasma and promotes more micro-discharge.Because of the larger area covered by the electrode,the higher discharge current and the serious heating of the electrode and dielectric layer,a large part of the energy will be emitted into the atmosphere in the form of Joule heat,which will lead to the increase of energy consumption.（2）.The material of the central electrode has little effect on the electrical performance,emission spectrum and nitrogen fixation performance of the reactor,but its hydrophilicity,oxidation resistance and corrosion resistance are of great significance to the long-term stable operation of the reactor.Considering the physical and chemical properties,processing price and influence on nitrogen fixation effect of four kinds of metal materials,the central electrode of stainless steel material was selected for follow-up experiment.（3）.The diameter of the central electrode has a great influence on the plasma volume,the equivalent capacitance of the reactor,the equivalent capacitance of the medium,the equivalent capacitance of the gap and the properties of the liquid film.In this paper,the effects of electrode diameter,discharge voltage and liquid flow rate on the synthesis rate and energy consumption of the reactor were studied by orthogonal experiment.It was found that the effect on the synthesis rate,electrode diameter and discharge voltage was very significant,while for energy consumption,discharge voltage had a very significant effect,and the electrode diameter had a certain effect.Considering the quality of synthesis rate and energy consumption,the optimum condition selected is the discharge voltage of 16 mm,discharge voltage of electrode,the discharge voltage of 21 k V,and the liquid flow rate of 148 m L/min.After optimizing the structure of the reactor,the effects of discharge gas properties such as oxygen content,gas flow rate and Helium content on the nitrogen fixation performance of the reactor were studied.The types,distribution characteristics and concentration variation of active groups produced by different discharge gases during discharge are discussed in detail.The experimental results show that:（1）.Oxygen is an electronegative gas,when added to the discharge system a large number of free electrons will be captured,resulting in a decrease in the number of free electrons,and electrons with lower energy are easier to be captured during discharge.On the whole,electrons with lower energy are more likely to be captured by oxygen than high-energy electrons,so although the number density of free electrons decreases,the population of the generated high-energy electrons is larger,and the average electron temperature of the plasma increases,which is reflected in the higher peak current in the current curve.When nitrogen does not contain oxygen in the discharge gas,nitrate ion and ammonium ion are detected in the product,and the selectivity of nitrate ion is 95.9%.When there is oxygen in the discharge gas,although only a small amount of oxygen（5%）exists in the product,the synthesis rate can be significantly increased and the energy consumption can be reduced by adding a small amount of oxygen.2)After adding He,the gas breakdown voltage of the reactor is reduced,and the number of microdischarge pulse produced in the discharge process decreases gradually with the increase of Helium content,which indicates that the plasma produced by adding Helium is more uniform and filamentous discharge is less.Helium can form high energy metastable state in plasma,and the average life is long.Panning ionization is easy to occur in the reaction,which is beneficial to the production of nitrogen molecular ions.After adding Helium to the discharge system,the selectivity of ammonium ion in the product increased from 4.1%to 31.6%,and the selectivity of nitrate ion decreased.The synthesis rate of the reactor decreases with the increase of Helium ratio,and the energy consumption increases rapidly with the increase of Helium content.The reason is that in the mixture of nitrogen and Helium,Helium itself consumes a large proportion of energy,and only part of it will be used for nitrogen fixation related conversion,which will eventually lead to the decrease of reactor synthesis rate and the increase of energy consumption.Finally,the nitrogen fixation mechanism and reaction path of WF-DBD reactor discharge in air are speculated.The key step of WF-DBD nitrogen fixation is the activation and decomposition of nitrogen to form active nitrogen in plasma environment,including excited molecules,nitrogen atoms and a small amount of nitrogen molecular ions.In the process of nitrogen fixation,nitrous acid and nitric acid are mainly produced by plasma gas phase chemical reaction.Then the nitrous acid and nitric acid in the gas phase are diffused and transferred to the solution,in which nitric acid is oxidized to produce nitric acid under the action of active substances in the solution,and nitric acid is the main final product of nitrogen fixation. |
