Direct and Indirect Applications of Dielectric Barrier Discharge Plasma to Catalytic Reduction of Nitrogen Oxides from Exhaust Gas

Dielectric barrier discharge （DBD） plasma was utilized to oxidize NO contained in the exhaust gas to NO2, ultimately improve the selective catalytic reduction of nitrogen oxides （NOx）. In the one case, DBD was created directly in the exhaust gas （direct application）, and in the an other case, ozone produced by DBD was injected into the exhaust gas （indirect application）. A comparative study between such direct and indirect applications of DBD plasma was made in terms of the NOx removal efficiency and the energy consumption. The NO2 content in the exhaust gas was changed by the voltage applied to the DBD device （for direct application） or by the amount of ozone added to the exhaust gas （for indirect application）. In both cases, NO was easily oxidized to NO2, and the change in NO2 content largely affected the NOx removal performance of the catalytic reactor placed downstream, where both NO and NO2 were reduced to N2 in the presence of ammonia as the reducing agent. The experiments were primarily concerned with the effect of reaction temperature on the catalytic NOx reduction at various NO2 contents. The direct and indirect applications of DBD were found to remarkably improve the catalytic NOx reduction, especially at low temperatures.     

Study on the Removal of SO2 from Simulated Flue Gas Using Dry Calcium-Spray with DBD Plasma

In this study, lime-hydrate （Ca（OH）2） desulfurizer was treated by plasma with strong ionization discharge of a dielectric barrier. The removal of SO2 from simulated flue gas was investigated. The principles of SO2 removal are discussed. Several factors affecting the efficiency of SO2 removal were studied. They included the ratio of calcium to sulfur （Ca/S）, desulfurizer granularity, residence time of the flue gas, voltage applied to the discharge electrode in the plasma generator, and energy consumption. Experimental results indicate that the increase in Ca/S ratio, the applied voltage and discharge power, the residence time, and the reduction in the desulfurizer granularity all can raise the SO2 removal efficiency. The SO2 removal efficiency was up to 91.3% under the following conditions, namely a primary concentration of SO2 of 2262×10^-6 （v/v） in the emission gas, 21%（v/v） of oxygen, 1.8% （v/v） of water, a Ca/S ratio of 1.48, a residence time of 2.8 s, a 3.4 kV voltage and a 10kHz frequency power applied to the discharge electrodes in the plasma generator, and a flow rate of 100 m^3/h for emission gas.     

Destruction of Gaseous Styrene with a Low-Temperature Plasma Induced by a Tubular Multilayer Dielectric Barrier Discharge

The destruction of gaseous styrene was studied using a low-temperature plasma induced by tubular multilayer dielectric barrier discharge(DBD).The results indicate that the applied voltage,gas flow rate,inlet styrene concentration and reactor configuration play important roles in styrene removal efficiency(η_(styrene)) and energy yield(EY).Values of η_(styrene) and EY reached 96%and 15567 mg/kWh when the applied voltage,gas flow rate,inlet styrene concentration and layers of quartz tubes were set at 10.8 kV,5.0 m/s,229 mg/m~3 and 5 layers,respectively.A qualitative analysis of the byproducts and a detailed discussion of the reaction mechanism are also presented.The results could facilitate industrial applications of the new DBD reactor for waste gas treatment.     

Advanced oxidation technology for H2S odor gas using non-thermal plasma

Non-thermal plasma technology is a new type of odor treatment processing. We deal with H2S from waste gas emission using non-thermal plasma generated by dielectric barrier discharge. On the basis of two criteria, removal efficiency and absolute removal amount, we deeply investigate the changes in electrical parameters and process parameters, and the reaction process of the influence of ozone on H2S gas removal. The experimental results show that H2S removal efficiency is proportional to the voltage, frequency, power, residence time and energy efficiency,while it is inversely proportional to the initial concentration of H2S gas, and ozone concentration. This study lays the foundations of non-thermal plasma technology for further commercial application.     

Achieving Better NOx Removal in Discharge Plasma Reactor by Field Enhancement

Application of plasma chemistry for gas cleaning is gaining prominence in recent years, mainly from an energy efficiency point of view. In this paper we conducted a comparative study of NO/NOx removal using two different types of dielectric barrier discharge electrodes, wire- cylinder reactor, pipe-cylinder reactor. Investigations were first carried out with synthetic gases to obtain the baseline information on the NO/NOx removal with respect to the two geometries studied. Further, experiments were carried out with raw diesel exhaust under loaded condition. A high NOx removal efficiency of 90% was observed for the pipe-cylinder reactor as compared to that of 53.4% for the wire-cylinder reactor. Furthermore, for the same energy consumed per NO molecule （about 73 eV/NO molecule）, the removal efficiency increased from 67% for the wire- cylinder to about 98% for the pipe-cylinder which was quite appreciable.     

Design of a MT-DBD reactor for H_2S control

This study aimed to discuss the removal of hydrogen sulfide(H_2S)with non-thermal plasma produced by a multilayer tubular dielectric barrier discharge reactor,which is useful in the field of plasma environmental applications.We explored the influence of various factors upon H_2S removal efficiency(η_(H_2S))and energy yield(Ey),such as specific energy density(SED),initial concentration,gas flow velocity and the reactor configuration.The study showed that we can achieveη_(H_2S)of 91%and the best Ey of 3100 mg kWh~(-1)when we set the SED,gas flow velocity,initial H_2S concentration and layers of quartz tubes at 33.2 J 1~(-1),8.0 m s~(-1),30 mg m~(-3)and five layers,correspondingly.The average rate constant for the decomposition of hydrogen sulfide was 0.206 gm~(-3)s~(-1).In addition,we also presented the optimized working conditions,byproduct analysis and decomposition mechanism.     

Degradation of Benzene by Using a Silent-Packed Bed Hybrid Discharge Plasma Reactor

In this work,a novel gas phase silent-packed bed hybrid discharge plasma reactor has been proposed,and its ability to control a simulative gas stream containing 240 ppm benzene is experimentally investigated.In order to optimize the geometry of the reactor,the benzene conversion rate and energy yield(EY) were compared for various inner electrode diameters and quartz tube shapes and sizes.In addition,benzene removal efficiency in different discharge regions was qualitatively analyzed and the gas parameter(space velocity) was systematically studied.It has been found that silent-packed bed hybrid discharge plasma reactor can effectively decompose benzene.Benzene removal proved to achieve an optimum value of 60% with a characteristic energy density of 255 J/L in this paper with a 6 mm bolt high-voltage electrode and a 13 mm quartz tube.The optimal space velocity was 188.1 h-1,which resulted in moderate energy yield and removal efficiency.Reaction by-products such as hydroquinone,heptanoic acid,4-nitrocatechol,phenol and 4-phenoxy-phenol were identified by mean of GC-MS.In addition,based on these organic by-products,a benzene destruction pathway was proposed.     

Study on SO2 Removal Efficiency by Nanosecond Rising Edge Pulse DBD Under Different Environmental Conditions

In this paper, an experimental study on SO2 removal by nanosecond rising edge pulse dielectric barrier discharge （DBD） plasma, generated by multi-needle-to-plane electrodes, is carried out. The mechanism of the effect of various factors, such as gap size between dielectric barrier and discharge needles, environmental humidity, and inlet speed of gas flow upon the removal efficiency of air purification is analyzed. The studies show that SOs removal efficiency improves with the increase in the gap size between dielectric barrier and discharge needles in the case of a fixed space between two electrodes, and also improves with the increase in the environmental humidity. For a mixed gas with a fixed concentration, there is an optimal inlet speed of gas flow, which leads to the best removal efficiency.     

Discoloration of Congo Red by Rod-Plate Dielectric Barrier Discharge Processes at Atmospheric Pressure

A dielectric barrier discharge (DBD) reactor with a rod-plate electrode configuration was used for the oxidative decomposition of Congo red dye in an aqueous solution.Plasma was generated in the gas space above the water interface under atmospheric pressure.Discharge characteristics were analyzed by voltage-current waveforms.Effects of applied voltage,initial conductivity,and initial concentration were also analyzed.Congo red discoloration increased with increased applied voltage and decreased conductivity.The initial conductivity significantly influenced the Congo red discoloration.Under the same conditions,the highest discoloration rate was obtained at 25 mg/L.The presence of ferrous ions in the solutions had a substantial positive effect on Fenton dye degradation and flocculation.At an applied voltage of 20 kV,about 100％of dye was degraded after 4 min of Fe2+/DBD treatment.Results showed that adding a certain dosage of hydrogen peroxide to the wastewater could enhance the discoloration rate.Possible pathways of Congo red discoloration by DBD plasma were proposed based on GC/MS,FTIR,and UV-vis spectroscopy analyses.     

Studies on Nitrogen Oxides Removal Using Plasma Assisted Catalytic Reactor

An electric discharge plasma reactor combined with a catalytic reactor was studied for removing nitrogen oxides. To understand the combined process thoroughly, discharge plasma and catalytic process were separately studied first, and then the two processes were combined for the study. The plasma reactor was able to oxidize NO to NO2 well although the oxidation rate decreased with temperature. The plasma reactor alone did not reduce the NOx (NO NO2) level effectively, but the increase in the ratio of NO2 to NO as a result of plasma discharge led to the enhancement of NOx removal efficiency even at lower temperatures over the catalyst surface (V2O5-WO3/TiO2). At a gas temperature of 100℃, the NOx removal efficiency obtained using the combined plasma catalytic process was 88% for an energy input of 36 eV/molecule or 30 J/L     

Degradation of phenol using a combination of granular activated carbon adsorption and bipolar pulse dielectric barrier discharge plasma regeneration

A combined method of granular activated carbon (GAC) adsorption and bipolar pulse dielectric barrier discharge (DBD) plasma regeneration was employed to degrade phenol in water. After being saturated with phenol, the GAC was filled into the DBD reactor driven by bipolar pulse power for regeneration under various operating parameters. The results showed that different peak voltages, air flow rates, and GAC content can affect phenol decomposition and its major degradation intermediates, such as catechol, hydroquinone, and benzoquinone. The higher voltage and air support were conducive to the removal of phenol, and the proper water moisture of the GAC was 20％. The amount of H2O2 on the GAC was quantitatively determined, and its laws of production were similar to phenol elimination. Under the optimized conditions, the elimination of phenol on the GAC was confirmed by Fourier transform infrared spectroscopy,and the total removal of organic carbons achieved 50.4％. Also, a possible degradation mechanism was proposed based on the HPLC analysis. Meanwhile, the regeneration efficiency of the GAC was improved with the discharge treatment time, which attained 88.5％ after 100 min of DBD processing.     

Experiment and Analysis on the Treatment of Gaseous Benzene Using Pulsed Corona Discharge Plasma

An experiment and analysis on removal of gaseous benzene by pulse corona inducedplasma is presented in this article. Important parameters effecting removal efficiency have been investigated, such as pulse peak voltage, pulse frequency, gas inlet concentration, gas flow rate and reactor temperature. The result shows that the removal efficiency increases with the increase in pulse peak voltage, pulse frequency and reactor temperature, but decreases in the rise of gas inlet concentration and gas flow rate. On the condition of Vp = 36 kV, f = 80 Hz, C = 1440 mg/m^3 and Q = 640 ml/min, the largest removal efficiency is 98%. Finally, the reacted products are qualitatively analysed and the reaction processes are deduced in combination with plasma-chemistry theory.     

Regulation characteristics of oxide generation and formaldehyde removal by using volume DBD reactor

Discharge plasmas in air can be accompanied by ultraviolet(UV) radiation and electron impact,which can produce large numbers of reactive species such as hydroxyl radical(OH·),oxygen radical(O·),ozone(O3),and nitrogen oxides(NOx),etc.The composition and dosage of reactive species usually play an important role in the case of volatile organic compounds(VOCs) treatment with the discharge plasmas.In this paper,we propose a volume discharge setup used to purify formaldehyde in air,which is configured by a plate-to-plate dielectric barrier discharge(DBD) channel and excited by an AC high voltage source.The results show that the relative spectral-intensity from DBD cell without formaldehyde is stronger than the case with formaldehyde.The energy efficiency ratios(EERs) of both oxides yield and formaldehyde removal can be regulated by the gas flow velocity in DBD channel,and the most desirable processing effect is the gas flow velocity within the range from2.50 to 3.33 m s-1.Moreover,the EERs of both the generated dosages of oxides(O3 and NO2) and the amount of removed formaldehyde can also be regulated by both of the applied voltage and power density loaded on the DBD cell.Additionally,the EERs of both oxides generation and formaldehyde removal present as a function of normal distribution with increasing the applied power density,and the peak of the function is appeared in the range from 273.5 to 400.0 W l-1.This work clearly demonstrates the regulation characteristic of both the formaldehyde removal and oxides yield by using volume DBD,and it is helpful in the applications of VOCs removal by using discharge plasma.     

Removal of NO and SO2 in Corona Discharge Plasma Reactor with Water Film

In this paper, a novel type of a corona discharge plasma reactor was designed, which consists of needle-plate-combined electrodes, in which a series of needle electrodes are placed in a glass container filled with flue gas, and a plate electrode is immersed in the water. Based on this model, the removal of NO and SO2 was tested experimentally. In addition, the effect of streamer polarity on the reduction of SO2 and NO was investigated in detail. The experimental results show that the corona wind formed between the high-voltage needle electrode and the water by corona discharge enhances the cleaning efficiency of the flue gas because of the presence of water, and the cleaning efficiency will increase with the increase of applied dc voltage within a definite range. The removal efficiency of SO2 up to 98%, and about 85% of NO~ removal under suitable conditions is obtained in our experiments.     

Dielectric Barrier Discharge Plasma-Induced Photocatalysis and Ozonation for the Treatment of Wastewater

The physicochemical processes of dielectric barrier discharge （DBD） such as insitu formation of chemically active species and emission of ultraviolet （UV）/visible light were utilized for the treatment of a simulated wastewater formed with Acid Red 4 as the model organic contaminant. The chemically active species （mostly ozone） produced in the DBD reactor were well distributed in the wastewater using a porous gas diffuser, thereby increasing the gas-liquid contact area. For the purpose of making the best use of the light emission, a titanium oxide-based photocatalyst was incorporated in the wastewater treating system. The experimental parameters chosen were the voltage applied to the DBD reactor, the initial pH of the wastewater, and the concentration of hydrogen peroxide added to the wastewater. The results have clearly shown that the present system capable of degrading organic contaminants in two ways （photocatalysis and ozonation） may be a promising wastewater treatment technology.     

NO reduction using low-temperature SCR assisted by a DBD method

This paper discusses the removal of nitric oxide(NO) with low-temperature selective catalytic reduction driven by a dielectric barrier discharge with ammonia(NH_3) as a reductant. We explored the effects of NH_3, O_2, temperature and water under different applied voltage on NO removal at atmospheric pressure. The results showed that when the gas concentration ration of NH_3/NO was 0.23–0.67, the NO removal efficiency and the energy consumption was acceptable. The NO removal efficiency reached 84% under an applied voltage of 7 kV, 400 ppm NO and 90 ppm NH_3 at a temperature of 150 ℃. Water vapor had a negative effect because NO formation reactions were strengthened and NH_3 was oxidized directly rather than reduced NO molecules. The outlet gas components were observed via Fourier transform infrared spectroscopy for revealing the decomposition process and mechanism.     

Plasma-assisted abatement of SF_6 in a packed bed plasma reactor: understanding the effect of gas composition

The potential impact of SF_6 as a potent greenhouse gas on the global climate is highly attractive.This paper studies the effect of H_2O concentration, SF_6 inlet concentration and pre-heating temperature on SF_6 abatement in a packed bed plasma reactor in terms of the removal efficiency and products selectivity. The results showed that the best performance in SF_6 abatement was obtained at 1% H_2O and 100 °C with 98.7% destruction and remove efficiency(DRE) at 2% SF_6.Higher energy yields was obtained under higher SF_6 inlet concentration. Moreover, the existence of water vapor weakened the micro-discharge and provided H and OH radicals for this system,which showed a close relationship to removal efficiency and products selectivity. Among four sulfur-containing products, SO2 F2 was more stable than SOF_2, SOF_4 and SO_2. Meanwhile, SOF_4 and SO_2 were very susceptible to the above parameters. This article provides a better understanding of SF_6 abatement in a view of both scientific and engineering.     

Novel electrode structure in a DBD reactor applied to the degradation of phenol in aqueous solution

Phenol degradation experimental results are presented in a similar wastewater aqueous solution using a non-thermal plasma reactor in a coaxial dielectric barrier discharge. The novelty of the work is that one of the electrodes of the reactor has the shape of a hollow screw which shows an enhanced efficiency compared with a traditional smooth structure. The experimentation was carried out with gas mixtures of 90% Ar–10% O_2, 80% Ar–20% O_2 and 0% Ar–100% O_2. After one hour of treatment the removal efficiency was 76%, 92%, and 97%, respectively, assessed with a gas chromatographic mass spectrometry technique. For both reactors used, the ozone concentration was measured. The screw electrode required less energy, for all gas mixtures, than the smooth electrode, to maintain the same ozone concentration. On the other hand, it was also observed that in both electrodes the electrical conductivity of the solution changed slightly from~0.0115 S m~(-1) up to ~0.0430 S m~(-1) after one hour of treatment. The advantages of using the hollow screw electrode structure compared with the smooth electrode were:(1) lower typical power consumption,(2) the generation of a uniform plasma throughout the reactor benefiting the phenol degradation,(3) a relatively lower temperature of the aqueous solution during the process, and(4) the plasma generation length is larger.     

Effect of Water Vapor on Toluene Removal in Catalysis-DBD Plasma Reactors

The experiment was carried out in a cylindrical dielectric barrier discharge(DBD)reactor assisted with a catalyst to decompose toluene under different humidity.In order to explore the synergistic effect on removing toluene in the catalysis-DBD reactor,this paper investigated the decomposition efficiency and the energy consumption in the catalysis-DBD and the non-catalyst DBD reactors under different humidity.The results showed that the catalysis-DBD reactor had a better performance than the non-catalysis one at the humidity ratio of 0.4%,and the removal efficiency of toluene could reach 88.6% in the catalysis-DBD reactor,while it was only 59.9% in the non-catalytic reactor.However,there was no significant difference in the removal efficiency of toluene between the two reactors when the humidities were 1.2% and 2.4%.Additionally,the degradation products were also analyzed in order to gain a better understanding of the mechanism of decomposing toluene in a catalysis-DBD reactor.     

Carbon Dioxide Reforming of Methane to Syngas by Thermal Plasma

Experiments were conducted on syngas preparation from dry reforming of methane by carbon dioxide with a DC arc plasma at atmospheric pressure.In all experiments,nitrogen gas was used as the working gas for thermal plasma to generate a high-temperature jet into a horizontal tube reactor.A mixture of methane and carbon dioxide was fed vertically into the jet.In order to obtain a higher conversion rate of methane and carbon dioxide,chemical energy efficiency and fuel production efficiency,parametric screening studies were conducted,in which the volume ratio of carbon dioxide to methane in fed gases and the total flux of fed gases were taken into account.Results showed that carbon dioxide reforming of methane to syngas by thermal plasma exhibited a larger processing capacity,higher conversion of methane and carbon dioxide and higher chemical energy efficiency and fuel production efficiency.In addition,thermodynamic simulation for the reforming process was conducted.Experimental data agreed well with the thermodynamic results,indicating that high thermal efficiency can be achieved with the thermal plasma reforming process.