Cohn, D.R.

Rhee, D.Y.

Surma, J.E.

Titus, C.H.

Wittle, J.K.

Woskov, P.P.

DSpace@MIT

PFC/JA-93-28Diagnostics for a Waste RemediationPlasma Arc FurnaceP. P. Woskov, D. R. Cohn, D. Y. Rhee,C. H. Titus*Plasma Fusion CenterMassachusetts Institute of TechnologyCambridge, Massachusetts 02139J. K. WittleElectro-Pyrolysis, Inc.Wayne, Pennsylvania 19087andJ. E. SurmaBattelle - Pacific Northwest LaboratoryRichland, Washington 99352October 1993*Consultant, T & R Associates, Wayne, Pennslvania 19087Supported by Buried Waste Integrated Demonstration, Office of TechnologyDevelopment, Environmental Demonstration and Waste Management, U. S.Department of Energy through Battelle - Pacific Northwest Laboratory ContractNo. 154462-A-L2.Published in the Proceedings of the 6th International Symposium on Laser-AidedPlasma Diagnostics, Bar Harbor, Maine, October 24-28,1993.Presented at the 6th International Symposium on Laser-Aided Plasma Diagnostics,October 24-28, 1993, Bar Harbor, Maine.Diagnostics for a Waste Remediation Plasma Arc Furnace*P. P. Woskov, D. R. Cohn, D. Y. Rhee, and C. H. Titus**Plasma Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139J. K. WittleElectro-Pyrolysis, Inc. Wayne, PA 19087J. E. SurmaPacific Northwest Laboratory, Richland, WA 99352Abstract: A graphite electrode dc arc plasma furnace is being developed to remediatecontaminated soils in the DOE complex. Real time, continuous diagnostics are neededto monitor and control the entire waste remediation process from input to output.Laboratory feasibility tests have been carried out on an active millimeter-waveradiometer for spatially resolved furnace temperature measurements and a microwavetorch plasma for trace metals measurements in the off-gas stream.1. IntroductionThe development of technologies to remediate hazardous waste sites is agrowing area. The U. S. Department of Energy (DOE) has approximately 4,000contaminated sites covering tens of thousands of acres, as identified in the current DOEEnvironmental Restoration and Waste Management Five-Year Plan [1]. These wastesinclude low-level, high-level, and transuranic radioactive wastes; and soilscontaminated with metals, volatile organic compounds (VOCs), sludges, oils, andcombustibles. The five year plan has stated that DOE must develop new technologiesto assist it in cleaning up these sites in faster, better, safer, and cheaper ways. Thedevelopment of such technologies have broad ranging application to commercial andmunicipal waste sites as well.A "National Laboratory-University-Industry" three party collaboration wasestablished between Pacific Northwest Laboratory (PNL), Massachusetts Institute ofTechnology (MIT), and Electro-Pyrolysis, Inc. (EPI) to develop graphite electrode DCplasma arc technology for the treatment of buried wastes [2,3]. This program nowincludes the development of advanced diagnostic systems to characterize performance,optimize operation, and control the plasma arc furnace. The DC graphite arc furnacetechnology has demonstrated the capability of forming a high quality, nonleachableglass from high melting point (1400-1600'C) waste and soil typical of DOE waste sites.Supported by Buried Waste Integrated Demonstration, Office of TechnologyDevelopment, Environmental Demonstration and Waste Management, U.S.D.O.E.** Consultant, T&R Associates, Wayne, PA-1-In addition, the high temperature region (up to 6000'C) around the DC arc can destroyorganic species and vapors that evolve from the material being processed.Following successful feasibility tests in a 300 kW, 100 pound batch processingMark I furnace, a pilot-scale Mark II furnace with up to 1 MW and 500 pound per hourcontinuous processing capability has been constructed at the MIT Plasma FusionCenter. Fig. 1 shows the major features of Mark II which has internal dimensions of0.9 m diameter by 1.4 m high. It is completely sealed and purged with nitrogen. Aunique feature is the coaxial arrangement of the graphite electrodes with a 15 cmdiameter inner electrode and a 25 cm i.d. by 36 cm o.d. outer electrode. This designallows easy furnace start up by first running an arc between the electrodes to initiallymelt the material being processed and then transferring the arc to the melt for moreefficient treatment. Not shown in Fig. 1 are large 1.0 x 0.7 m rectangular portslocated 900 circumferentially from the furnace feed chamber and glass dischargesection. These ports are a unique feature of Mark II to accommodate diagnostics.Diagnostics are a crucial part of this project since new waste remediationtechnologies will not be acceptable unless they are proven to be an improvement overolder technologies and absolutely safe to the environment. Diagnostics are needed tomonitor the entire waste remediation process from input to output, as well as internalfurnace parameters. In general, accurate, reliable, real time diagnostics are lacking formany of the parameters of interest. Table 1 identifies important parameters of interestand applicable diagnostic techniques. So far we have undertaken laboratory feasibilitytests of the first two diagnostics listed and these are described below.Table 1. WASTE REMEDIATION PROCESS DIAGNOSTICSObectives ApproachSpatially Resolved Furnace Temperature Active Millimeter-Wave RadiometerTrace Metal Content in Off-Gas Microwave Torch SensorOff-Gas Molecular Content Infrared/Millimeter Wave SpectroscopyFeed and Slag Characterization Laser Ablation SpectroscopyParticle Size/Density Laser ScatteringOff-Gas Velocity Laser Doppler ScatteringII. Active Millimeter-Wave RadiometerKnowledge of the internal temperature distribution is a key parameter formonitoring furnace performance and necessary for furnace optimization. However,-2-present temperature measurement instrumentation have a number of short comingswhich make them unreliable, inaccurate, or hazardous for use inside the hot hostileenvironment of a furnace. For example, infrared pyrometers can not view throughsmoky, hazy, and/or particle filled off-gases, their view can be further blocked bydeposits on exposed viewing windows, and knowledge of surface emissivity is requiredto accurately interpret the temperature. Other temperature sensors such asthermocouples, which are directly exposed to the harsh furnace environment, have highfailure rates and require electrical wires being brought into the furnace which wouldrequire additional seals and could be a hazard in electric arc furnace applications.We have tested, and are implementing on Mark II, an active millimeter-waveradiometer which overcomes the limitations of currently available instrumentation forspatially resolved temperature measurements. Millimeter-waves are sufficiently longthat they can penetrate visibly opaque atmospheres such as fog and smoke and areminimally effected by surface deposits on window and mirror surfaces, yet they are stillshort enough for good spatial resolution. Furthermore, a probe beam can be readilysuperimposed onto the radiometer field-of-view to provide a surface reflectionmeasurement. Such a measurement is important to determine the surface emissivitywhich must be known to interpret the hot surface emission as a temperature when thesurrounding surfaces are not in thermal equilibrium [4]. Also, it could provide someinformation on surface deposition chemistry inside the furnace. Millimeter-waves areideal for this probe function because in addition to good propagation inside the furnaceatmosphere, optical surfaces are not required for a spectrally reflected beam.The laboratory test setup for an active millimeter-wave radiometer is illustratedin Figure 2. A 135 GHz heterodyne receiver with a double-sideband-bandwidth (DSB)of 3.0 GHz and a noise temperature of 28,000 K was used. The chopper blades werecovered with a millimeter-wave absorbing material [5] and acted as a room temperatureblack-body reference. With a lock-in amplifier integration time constant of 1.25seconds, temperature deviations from room temperature as low as 10 *C could bedetected. There is no effective upper limit for measurement of temperatures relevant tofurnace operations. An Impatt diode beam was superimposed onto the receiver field-of-view via a low reflectivity (<10%) Teflon beamsplitter. The Impatt diode wasrapidly frequency swept over the receiver bandwidth to average out coherentinterference effects. In this test setup the hot surface emission and reflectionmeasurements were not simultaneous. The emission was measured with the Impattdiode off and chopper on, and the reflection was measured with the Impatt on andchopper off. A liquid nitrogen black-body and a gold mirror were used to absolutelycalibrate the temperature and reflection measurements, respectively.The active millimeter-wave radiometer was tested with one of the aluminabricks, GP Green AD-99, which will line the interior surface of Mark II. A table topoven, Omegalux Model LMF-M525, was used to heat this brick to a high temperatureafter which the oven door was opened and the brick surface was monitored during cool-3-down. The surface emissivity (s) was determined using the assumption e = 1 - R,where R is the measured reflection. The plotted results in Figure 3 show that theemissivity of this brick at 135 GHz is constant at about 0.88 for temperatures of up to750 *C. This is a sufficient deviation from the ideal black-body emissivity of 1.00 thatit must be taken into account for an accurate temperature determination by radiometry.In Mark II much higher temperatures will be achieved, surface deposits couldform, and due to continuous feed the internal surfaces will not be in thermalequilibrium. Therefore, an active radiometer is necessary for temperaturemeasurements. The radiometer being implemented on Mark II has a novel waveguideoptics system which is compatible with the high furnace temperatures and can verticallyscan the interior surface from the melt to the ceiling. Also it will be capable ofsimultaneous emission and emissivity measurements.III. Microwave Torch Trace Metals DiagnosticThe emission of metals in the off-gas stream of waste burning and remediationfurnaces is an important environmental issue. The U. S. Environmental ProtectionAgency (EPA) has regulations that limit the emission of 12 metals (Sb, Ba, Pb, Hg,Ag, TI, As, Cd, Cr, Be, Ni, Se). To this list we must add a number of radioactivemetals because of the ultimate intended application of the Mark II furnace technology.Reliable continuous emission monitors for metals are not currently available.Generally, samples of the off-gas stream must be taken to a laboratory for detailedanalysis. A waste furnace's metals emission is presently controlled by keeping themaximum metals feed rate below that during trial test runs. It would be much moredesirable to be able to monitor metals emission in real time and use such data forfurnace control.We have tested the feasibility of using a microwave torch plasma in the off-gasstream to atomize and excite the trace metals for real time atomic emissionspectroscopy (AES). Plasmas such as inductively coupled plasmas (ICP) andmicrowave induced plasmas (MIP) are routinely used in the laboratory for traceelement analysis [6]. By suitable design of a microwave plasma device and by going tohigh power levels (>0.5 kW) we plan to demonstrate a real time in situ trace metalssensor. Such a sensor would be robust in the off-gas flow stream, would atomizemetals condensed on particulates as well as those in the vapor phase, and could operatecontinuously.The feasibility of this approach to trace metals sensing was tested using theexperimental arrangement shown in Fig. 4. An ASTeX 1.5 kW, 2.45 GHz microwavesystem was used to generate the plasma in a flow of ambient laboratory air or nitrogen.The plasma breakdown occurs 1/4% from the shorted end of a rectangular waveguidewith outside dimensions of 7.6 x 3.8 cm (WR-284 guide). A small 3 mm diameterhole in the shorted end of the waveguide allowed the plasma emission to be viewed by-4-a SPEX Model THR-640, 0.64 m Czerny-Turner spectrometer with a 2400 g/mmgrating and a 512 silicon diode array from Princeton Instruments. Figure 4 shows thesetup for trace mercury measurements. Mercury vapor was introduced into the plasmaby blowing nitrogen or air through a Pyrex glass u-tube with a drop of mercury in thebottom.The trace mercury measurements were calibrated by using a commercialBacharach Instruments Company Mercury Sniffer obtained from the MIT safety office.Fig. 5 shows a typical spectrum at the 253.65 nm transition of neutral mercury innitrogen gas flow of 28 1/min., 0.55 kW of microwave power, and a measured mercuryconcentration of 0.04 mg/m 3 (6 ppb) at point A in Fig. 4. This spectrum was obtainedin 0.17 seconds. The good signal to noise ratio indicates that mercury concentrationsas low as 0.5 ppb could be detected in a nitrogen gas flow. In air flow plasmas thissensitivity degraded to about 3 ppb due to an increase in the UV background. Boththese measured sensitivities are below the EPA threshold limit value of 6 ppb anddemonstrate the potential usefulness of this approach to real time trace metalsmonitoring.Additional tests were carried out with fine iron particulates (147 im dia.) andparticulates obtained in the off-gas stream of earlier Mark I tests. These tests were notquantitative but did demonstrate the robustness of the microwave plasma to thepresence of particulates, the atomization and excitation of the particulates, and goodsensitivity to trace metals (a Mn impurity was clearly detected in a 99% pure ironparticulate). Future work will focus on evolving the microwave device to aconfiguration that can be adapted to the off-gas stream of Mark II and other furnaces.Another area of emphasis will be the development of an in situ calibration technique.References1. USDOE, Environmental Restoration and Waste Management Five Year Plan, DOE IS-00097P, Vol. I & II, United States Department of Energy, Washington, D.C., 1993.2. J. E. Surma, D. R. Cohn, D. L. Smatlak, P. Thomas, P. P. Woskov, C. H. Titus, J. K.Wittle, and R. A. Hamilton, Graphite Electrode DC Arc Technology Development forTreatment of Buried Wastes, PNL-SA-21891, Pacific Northwest Laboratory, Richland,Washington, 1993.3. J. K. Wittle, C. H. Titus, R. A. Hamilton, D. R. Cohn, D. L. Smatlak, P. Thomas, P. P.Woskov, and J. E. Surma, DC Graphite Arc Furnace and Diagnostic Systems for Soils,Hazardous Waste and Hazardous Materials, to be published, 1993.4. F. P. Incropera and D. P. DeWitt, Intoduction to Heat Transfer, 2nd Ed., Chap. 12, JohnWiley & Sons, New York, 1990.5. Eccosorb made by Emerson Cuming.6. M. W. Blades, P. Banks, C. Gill, D. Huang, C. Le Blanc, and D. Liang, IEEE Trans. onPlasma Sci., Vol. 19, pp. 1090-1113, 1991.-5--7--,--G-.sFigure 1. Mark II, graphite electrodedc arc furnace.Emissivity of Alumina Brick at 135 GHz1.Go ". , . . . . . . . . . . . .ZW0 150 300 450 600 750Temperature (*C)Figure 3. Measured emissivity of an aluminafurnace brick using a 135 GHz activeradiometer.ImpattSourceOven Var. BeamsplitterAtten.MillimeterWaveReceiverFurnace ChopperBrick DVMRef. Lock-in Amp.Figure 2. Experimental test setup for anactive millimeter-wave radiometer.plasmaA-A fromt- 25GHzmeter lenainitrogen waveguideair or flow qUartZZalumina or air flowmercury dropFigure 4. Experimental setup for test of amicrowave torch trace metals diagnostic.-6-0.50.8. -- ---------------- - -- -... .......0.7 ---.. -- - -- - -- -. -Three Experimental RunsGP Green AD99 Alumina Brick00 .. .............. .. .............. ____ ....... ............ ..I .......Mercury Detection in Microwave PlasmaNitrogen Plasma550 WattsHg 1 253.65 nm253.8 252.3Wavelength (nm)258.8 249.3 1Figure 5. Detected spectrum in a nitrogen torch plasma having a trace mercuryconcentration of approximately 5 ppb.-7-4.'In0).4-.0)0)588 .-48 -488300200188 -256.7 255.2Mercury concentration - 0.04 mg/m 3lsv

Diagnostics for a waste remediation plasma arc furnace

http://dspace.mit.edu/bitstream/1721.1/95187/1/93ja028_full.pdf

Diagnostics for a waste remediation plasma arc furnace

Abstract

Similar works

Full text

Available Versions

DSpace@MIT