Study On The Destruction Of Volatile Organic Waste Gas With Plasma And Combined Plasma Photolysis (CPP) Technology

Dielectric barrier discharge (DBD) plasma was a mature and feasible technologyfor organic waste gas treatment.Up to now,there have been many reports on DBDplasma used for treating volatile organic compounds (VOCs) as an alternativeapproach.However,their studies mainly based on pollutants destruction under staticstate or low-flow case (gas flow rate in reaction region＜0.1m/s) and on combinedplasma technology,as a result,their results were hard to be used for practical exhauststreatment directly.In this dissertation,we studied VOCs destruction with DBDplasma and combined plasma photolysis under high gas flow rate (gas flow rate inreaction region＞1m/s) in order to give some valuable proposals to commercialavailability.This dissertation was mainly composed of two parts.In Part 1,the destruction of"three benzene"(benzene,toluene,xylene) and isobutyl acetate gas with DBDtechnology was studied and the influence factors were investigated.Moreover,weconducted experiment in DBD system in series and actual engineering applicationresearch and inferred reaction mechanisms as well as feasibility.The results mainlyincluded:(1)The removal efficiency of"three benzene"increased with the increase of inputpower and applied voltage,and decreased with increasing gas flow rate and initialconcentration.The energy yield (Ey) increased with the increase of initialconcentration and applied voltage,while gas flow gas has a relatively complex effecton Ey.Moreover,we found that benzene,toluene and xylene have closed removalefficiency and energy yield with DBD destruction technology under the samecondition.We also carried out"scale-up"experiment on hybrid"three benzene"andthe experimental results showed that"three benzene"removal efficiency was morethan 50% in one DBD and over 89% in three DBD systems in series,when theirconcentrations were in the range of 300-700mg/m³.Treatment cost assessment of perunit volume"three benzene"hybrid waste gas showed that only 2 to 4 yuan RMB wasneeded per 1000m³ waste gases with DBD technology.At last,we pointed out that thefirst thing to consider was to remove solid residues in the wall of tube and aerosolparticles in effluent,before"three benzene"destruction with DBD were actuallyapplied.(2)When gas flow rate was 2.58 m³/h,removal efficiency of isobutyl acetate of 1788 mg/m³ can reach 75.3% at 9 kV.The removal efficiency of isobutyl acetate increasedwith increasing applied voltage and decreasing initial concentration and gas flow rate.Ey increased with the increase of initial concentration and the highest Ey can reach32.9 g/(kW.h) according to our results.We also investigated the decomposition ofactual industrial waste gas containing isobutyl acetate in terms of three aspectsincluding the removal efficiency,treatment cost and feasibility.The results showedthat removal efficiency of isobutyl acetate was above 80% when applied voltage,gasflow rate,and flow rate in DBD reactor region were set at 16 kV,2000m³/h,and5.4m/s,respectively.The treatment cost of isobutyl acetate was 12 yuanRMB/1000m³.At last,the likely reaction mechanisms for the removal of isobutylacetate by DBD were suggested on the basis of byproducts analysis.In order to optimize DBD plasma for waste gas removal,in part 2,a new-typereactor was designed and constructed,nominated combined plasma photolysis (CPP)that simultaneously produced plasma and excimer UV radiation with onehigh-pressure power supply.Destruction principle and characteristic of CPP for wastegas were discussed.In addition,we investigated degradation performance of CPP withVOCs (benzene,styrene and isobutyl acetate) as targets and analyzed influencefactors.Finally,we measured the excimer spectrum from CPP,and the results were asfollows:(1)The elementary study showed that it was unsuitable for Xe/I₂ mixture as excimergas filled in CPP reactor,because wavelength of XeI^* excimer (253nm) was too long.When we utilized ICPP (inner combined plasma photolysis) reactor for benzeneremoval,removal efficiency of benzene was lower.In OCPP (KrI^*/DBD),benzene removal efficiency increased by 15.9% in contrastto DBD,when applied voltage,gas flow rate,and initial concentration of benzenewere set at 9 kV,35 L/min,and 800 mg/m³,respectively.In addition,themineralization degree and CO₂ selectivity in OCPP was higher than that in DBD.Theeffects of various operational parameters including total pressure,ratio of Kr/I₂,andquartz dielectric characteristics on OCPP performance were investigated.At last,theemission spectra of KrI^* excimer and byproducts analysis were suggested.The experimental results also showed that benzene has low removal efficiencywhen decomposed only by excimer UV,which by inference only was a lightionization reagent for decreasing breakdown voltage of gas ionization.On the basis of above results,we concluded the reasons for OCPP superior to DBD from six aspects:the decrease of breakdown voltage,the increasing of microdischarge numbers,improvement of OH radical concentration,the enhancement of energy efficiency,more homogeneous discharge and benzene molecule absorption of UV light.(2)Simulated flowing waste gas containing styrene was decomposed with OCPP(KrI^*/DBD) reactor.The results showed that styrene removal efficiency was up to84.4% in OCPP (Kr 26.6kPa,I₂ 6 mg),and removal efficiency was increased by20.6%,Ey increased by 5.7 g/(kW.h) in contrast to DBD plasma,when appliedvoltage,gas flow rate,and initial concentration of styrene were set at 9 kV,3.26 m³/h,and 1265 mg/m³,respectively.Moreover,we investigated influence factors of styrenedecomposition including pressure of filled Kr and mass of I₂,quartz material,gas flowrate,initial concentration of styrene,as well as reactor configuration.At last,weanalyzed solid depositions on the internal wall of OCPP reactor by a FourierTransformation Infrared Spectrometer (FTIR) and a Gas Chromatography-MassSpectrometry (GC-MS),on the basis of which,the likely reaction mechanisms for theremoval of styrene by OCPP were discussed.(3)In OCPP (Kr 35.9 kPa,Br₂ 1.2kPa),isobutyl acetate removal efficiency increasedby 18.3% in contrast to DBD,when applied voltage,gas flow rate,and initialconcentration of isobutyl acetate were set at 9 kV,2.8 m³/h,and 2500mg/m³,respectively.The results showed that the total pressure and ratio of Kr/Br₂ mixturehave effect on isobutyl acetate destruction.When Kr/Br₂ was in the range of 30-50,good removal efficiency for isobutyl acetate was achieved.The increase of totalpressure and the adding of buffer gas Ar were in favor of isobutyl acetate destructionin OCPP to some extent.Three representative wavelengths at 207 nm (KrBr^*),228nm(KrBr^*) and 289nm (Br^*) were produced from Kr/Br₂ excited by OCPP.The excimerradiation intensity at 207nm was enhanced by the increase of applied voltage and totalpressure of Kr/Br₂ mixture.