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1、Microwave destruction of trichloroethylene in a H2O-O2-Ar plasmaYoungsam Ko1,GoSu Yang1,Daniel P.Y.Chang2 and Ian M.Kennedy31 Department of Mechanical Engineering,Chonbuk National University,Korea2 Department of Civil and Environmental Engineering,3 Department of Mechanical and Aeronautical Engineer
2、ing,University of California DavisSupported by California ARBBackgroundAs of 1997,solvent usage increased to roughly to 6.5 X 106 tpyToxic air contaminants (TAC)may produce toxic by-products while undergoing capture and treatment.Chlorinated solvents,chlorofluorocarbons and sulfur hexafluoride fall
3、into such a category because of their higher activation energies and low biodegradability under oxidative conditionsAn increase of about 67%(16 X 106)tpy has occurred in the emissions of HCFCs,PFCs,and SF6 during the decade of the 1990sAlthough increases in emissions have occurred,concentrations of
4、emitted streams may actually be decreasing because of improved production methods or capture of these pollutantsUC DavisBackgroundControl costs generally increase as concentrations decrease,e.g.,the cost of catalytic incineration of VOCs increases from about$5,000/ton to$50,000/ton for a compound su
5、ch as benzene as its concentration drops from 100 ppmv to 10 ppmControl costs for NOx range from roughly less than about$1000/ton for advanced burner technologies to over$5,000/ton for exhaust gas treatmentsDevelopment of efficient control strategies for low concentrations of these compounds while a
6、voiding NOx formation is highly desirableAdsorption is an efficient and economical method for moderate to low concentration streams.Nevertheless,adsorbates(such as VOCs)must be removed periodically after they saturate the adsorbents(e.g.,activated carbon or zeolites)and require further treatmentUC D
7、avisSorbent regenerationConventional regeneration methods:steam and hot gas-first needs steam source,leaves wet bed;second requires high flow rates,large energy demand to heat whole systemMicrowave regeneration utilizes dielectric heating which eliminates many of the above drawbacksUC DavisAdvantage
8、s of microwave processing of VOCsProduction of high ionization levels and molecular dissociation without excess heating of the contained gasConstruction of reaction vessels which are simple,free from contamination and less subject to damage because of the absence of internal electrodesProduction of
9、little electrical interferenceAbsence of high voltages which can be easily contacted by operating personnel,i.e.,absence of shock hazardousPotentially lower power consumptionThe ability to tolerate high concentrations of water-maybe less NOxUC DavisExperimentsMicrowave sourceContinuous microwave gen
10、erator(low ripple magnetron,1.5 KW,2450 MHz;Gerling Laboratory)and a plasma tuner(ASTEX)generate a steam plasmaMain components of the system consisted of a plasma reactor(AX 7200),a plasma tuner,microwave generator and microwave wave-guideContinuous microwave power from the magnetron conducted throu
11、gh a waveguide to the plasma torch.Forward and reflected powers from the plasma torch maximized and minimized by adjusting the tuning stubs on the plasma tuner.Forward power maintained typically at 600W and the reflected power was maintained below about 100 W.UC DavisExperimentsThe plasma reactor co
12、nsisted of a 1/4 O.D.ceramic tube through which the mixture to be reacted was passed,and an outer quartz tube housingUC DavisApparatusUC DavisPlasma torch and tunersUC DavisExperimentsArgon,oxygen and steam were used to generate plasmaAr was utilized as a basic carrier and reference gasO2 provided s
13、toichiometric oxidation requirementssteam provided a reactive atmosphere containing additional hydrogenSteam was generated by a coiled-tubing heater and was carried by Ar gasTCE added to flowUC DavisSampling and analysisEffluent gas from plasma reactor passed through a coiled water concentrator and
14、an Erlenmeyer flask,in which the most of the steam was condensed.A second back-up trap was used to condense the remaining water vaporGas samples were collected on an adsorbent bed of Carbotrap C.A gas sampling loop and switching valve were used to inject gas samples to the GC from a by-pass line exi
15、ting the second trap.Liquid samples were also collected from the first and the second traps.Reactor effluents were analyzed with an on-line GC equipped with a TCD detector(TCE)and by GC/MS(adsorbent tubes-TCE and other by-products),and a specific ion meter for chloride.UC DavisResultsConditionsThe D
16、RE for TCE was evaluated in the microwave system for a series of microwave powers at a condition of 30%steam with an O2 flow rate of 0.5 lpm and an argon flow rate of 6 lpm.The concentration of TCE in the input flow to the plasma was 1700 ppm.The small amount of O2 was added to ensure that sufficien
17、t O2 was available in the system to complete the oxidation of TCE.UC DavisDestruction efficiency of TCEDRE and Cl ion concentration in post plasma gases at different operating powersUC DavisEffect of steam concentration on DREDRE at 600 W of input microwave power and 1700 ppm TCE with varying steam
18、concentrationsUC DavisThermodynamicsDechlorination of these compounds is thermodynamically favored by reaction in a reducing H2 or water environment.Barat and Bozzelli showed that an overall reaction of the formCCl4+2H2O=4HCl+CO2exhibited large equilibrium constantsT,K Kp3003.8 x 10438007.6 x 1025UC
19、 DavisConclusionsSteam-microwave system capable of complete destruction of TCEModerate power needed(400-600W)Presence of steam improves efficiencyNo dioxins or furans formedFurther studies of kinetics of electron reactions with chlorinated HCs desirableCheaper,half wave rectified system possibleUC Davis