36-19

THE PLASMA TORCH COMPONENT LIFE EXTENSION
PROGRAM (PTCLEP) SMALL-SCALE PLASMA
FURNACE (SSPF)

Patrick Cashell and Jason M. McClafferty
MSE Technology Applications, Inc.

ABSTRACT

The U.S. Department of Energy’s (DOE) Plasma Torch Component Life Extension Program (PTCLEP) testing under the Mixed Waste Focus Area (MWFA) was conducted by MSE Technology Applications, Inc. (MSE), at the Western Environmental Technology Office (WETO) in Butte, Montana, using the small-scale plasma furnace (SSPF) thermal treatment system.

The FY97 work scope for the PTCLEP focused on evaluating plasma arc torch performance, optimizing plasma torch operating parameters, and designing to extend electrode life. One of the FY97 overall objectives of the PTCLEP was to determine the operating parameters, conditions, torch component configurations, and designs to provide a plasma torch electrode life of 100 hours operating time for a nominal 150-kilowatt (kW) transferred arc plasma torch.

Under the PTCLEP, the electrode life of a Pegasus Refractory Materials, Inc. (PERMA) nominal 150-kW PT4T transferred arc plasma torch that used air as the torch gas was improved to greater than 100 hours of torch operation, performed in 10-12 hour segments.

The increase in electrode life is attributed to several modifications completed by MSE, including upgrades/modifications to the torch cooling system and torch component design modifications, i.e., design modifications to the electrode and vortex generator.

In addition, lessons learned from the PERMA nominal 150-kW PT4T transferred arc plasma torch have been applied to the Retech nominal 500-kW RP-250-T transferred arc plasma torch that used air as the torch gas improving the torch electrode life to greater than 175 hours of torch operation.

However, additional testing and analytical evaluation/analysis is recommended to evaluate the effects on the plasma torch electrode life and to optimize the operation and performance of the plasma arc torch including:

heat transfer (finite element analysis);
fluid dynamics;
cold flow modeling;
development of the 2-D/3-D code of the electrode arc gas flow dynamics;
metallurgical evaluation/analysis of ceramics, alloys, brush platting, and thermal sprays; and
torch gasses, carbon dioxide, nitrogen, oxygen, argon, etc.

BACKGROUND

The major shortcoming of plasma arc torch designs is the failure of the torch electrode and degradation of the water-cooled ram. The electrode in a plasma arc torch is one of the two attachment points for the electric arc that generates the plasma (when operated in the transferred arc configuration, the other arc attachment point is within the material being processed). Failure of the electrode causes waste processing to stop until the electrode is replaced and repairs are made to associated torch components. Past testing has shown that plasma torch electrode life is variable. Electrodes in a nominal 150-kW transferred arc plasma torch that used air as the torch gas typically exhibit a life ranging from 1 to 4 hours. In addition, a nominal 500-kW transferred arc plasma torch that used air as the torch gas typically exhibits a life ranging between 10 and 30 hours.

The Pegasus Refractory Materials, Inc. (PERMA) torch electrode tests, titled Plasma Torch Component Life Extension Program (PTCLEP) testing, utilized different torch electrodes, components, and parameters of differing materials, design, and/or configurations to evaluate their performance in the furnace environment and operation.

The overall objective of the PTCLEP and the need of the thermal treatment and plasma torch industry is to improve the life of the plasma arc torch electrode and provide continuous long-term torch operation that supports a viable plasma waste treatment operation. The thermal treatment and plasma torch industry is aware that, historically, the weak link in thermal treatment systems is the premature failure of the plasma arc torch electrode. Failure of the electrode causes thermal treatment/waste processing to stop and impacts system reliability and availability. A typical waste treatment thermal processing facility operates 24 hours a day, 7 days a week, for 4 to 6 weeks (700 to 1200 hours) with scheduled shutdowns.

Plasma technology can be used to treat heterogeneous nonsorted waste directly or can be used as part of an integrated waste treatment system to stabilize the final waste from other treatment processes. Therefore, extending the plasma torch electrode life will increase the overall system reliability and availability and, as a result, will increase processing time available to treat waste, i.e., system throughput.

The U.S. Department of Energy (DOE) and Lockheed Martin Advanced Environmental Systems are involved in a first-of-a-kind partnership for the hazardous waste cleanup at the Idaho National Environmental Engineering Laboratory (INEEL) Pit-9 site. Pit-9 is a critical DOE program, and success of the program is largely dependent upon the performance of the thermal treatment system.

Furthermore, plasma torch development and improvements in electrode life are directly applicable to several DOE and Department of Defense (DoD) programs including:

The hazardous waste cleanup at the INEEL Pit-9 site
The Advanced Mixed Waste Treatment Program
The Armament Research, Development, and Engineering Center (ARDEC) Plasma Ordnance Demilitarization System (PODS) at the Hawthorne Army Depot in Hawthorne, Nevada
The U.S. Navy Shipboard Plasma Arc Waste Destruction System (PAWDS)
The U.S. Navy Plasma Arc Hazardous Waste Treatment Facility (PAHWTF) program at the Norfolk Naval Base in Norfolk, Virginia
The U.S. Army Corps of Engineers Construction Engineering Research Laboratory (CERL) Transportable Plasma Test Bed Project

Table I shows the current status of the plasma torch technology and identifies the need to improve the plasma arc torch electrode life for the overall plasma thermal treatment systems reliability and availability to be used for a thermal waste treatment processing facility.

Table I. Status of the plasma torch technology.
Plasma Torch Size	Average Continues Torch Operation Hours	Average Segmented Torch Operation Hours	Maximum Continues Torch Operation Hours	Maximum Total Segmented Torch Operation Hours
150-kW PERMA Torch ¹	25 -30	12 - 14	42	100
150-kW Aerotherm Torch	see note ²	see note ²	see note ²	see note ²
150-kW Busek Torch	see note ³	see note ³	see note ³	see note ³
500-kW Retech Torch	(30 - 35) ⁴	(20 - 24) ⁴	(44) ⁴	180
A typical waste treatment thermal processing facility operates 24 hours a day, 7 days a week, for 4 to 6 weeks (700 to 1200 hours) with scheduled shutdowns.
^{1The PERMA nominal 150-kW PT4T transferred arc plasma torch average segmented torch operation hours was not based on electrode failure but was related to standard test conduct and system operation. ^{2The Aerotherm nominal 150-kW segmented arc column transferred/nontransferred plasma torch was a developmental torch undergoing initial evaluation and testing which was designed, fabricated, and acceptance tested under the Alternative Plasma Torch Technologies Development, Testing, and Evaluation Project. ^{3The Busek nominal 150-kW Plasmatron transferred/nontransfered plasma arc torch was a developmental torch undergoing initial evaluation and testing that was designed, fabricated, and acceptance tested under the Alternative Plasma Torch Technologies Development, Testing, and Evaluation Project. Further information on the Busek Torch can be found in Section 17 of this report. ^{4The Retech RT-250-T 500-kW transferred arc plasma torch operational hours was not based on electrode failure but was related to the test conduct and system operation.}}}}

SYSTEM DESCRIPTION

Fig. 1 shows the PERMA PT4T (150-kW nominal) transferred arc plasma torch installed in the small-scale plasma furnace (SSPF) and Fig. 2 shows the process schematic of the SSPF thermal treatment system. The transferred arc plasma torch uses argon as the torch gas for torch ignition after which the torch is switched over and operated on air as the plasma torch gas. The plasma arc is initiated on the conductive copper crucible. Material adjacent to the top of the copper crucible is heated to conducting temperature, then the torch is moved slowly down into the crucible to heat the material, thereby producing a molten bath in the crucible. This creates an electrically conductive molten pool with a temperature of approximately 2,500 to 3,000 °F (estimation dependent upon feedstock material).

Figure 1. PERMA PT4T Transferred Arc Plasma Torch Installed in the Small-scale Plasma Furnace.

Figure 2. Process Schematic of the Small-scale Plasma Furnace System.

Material can then be added to maintain a conductive molten pool by placing it in the manual feeder system and then gradually feeding it into the rotating crucible; solid material falls into the molten pool where it is processed. The main chamber operating gas temperature is approximately 1,200 to 1,600 F while the exit gas temperature is heated to approximately 600 to 800°F.

During the melting process, combustion gases are drawn out through a port in the bottom of the main chamber. The gases are routed through the offgas pipe to the secondary combustion chamber (SCC). The SCC uses a natural gas-fired afterburner to maintain a temperature high enough (>1,400°F) and a 2- to 5-second residence time to ensure complete combustion of residual organic materials. The offgas exits the SCC where a dilution air compressor adds cooling air to the gas stream. The gas stream temperature is cooled to less than 400°F by mixing with the cooling air prior to entering the baghouse. Dry particulate is removed from the gas stream within the baghouse. The offgas exits the baghouse and passes through the offgas fan. The induced draft fan provides the motive force for moving the combustion gases throughout the system and out the exhaust stack to the atmosphere.

During normal operation, there is a slight negative pressure (10 to 30 inches water) vacuum in the main chamber, which is produced by the induced draft offgas fan. This also maintains a slight vacuum on the complete system, which ensures any leaks in gas seals will be directed into the process rather than out into the operational area. If the offgas temperature at the inlet to the baghouse exceeds 400 °F, an automatic shutdown is initiated, i.e., the torch and afterburner trip and the oxygen lance is secured.

When the feedstock is fully processed, the torch is secured, the main chamber is purged, and the slag is allowed to cool/solidify. When the chamber purge is completed, the SSPF chamber will be opened and the slag removed from the crucible. Slag samples are obtained for analysis. The remaining material is containerized, labeled for identification, and stored pending the results of analysis of the sample.

The offgas treatment system meets the requirements established in the State of Montana Air Quality Permit; therefore, a Continuous Emissions Monitoring System is not required on the SSPF system.

Typical process operating conditions of the current SSPF thermal treatment system configuration are:

plasma torch gas (air) mass flow rate » 3–5 standard cubic feet per minute (scfm);
crucible rotation speed » 14–20 rotations per minute;
torch plasma column centerline temperature » 15,000 to 20,000 °F (estimated);
plasma torch power » 100 to 150 kW_e;
plasma torch voltage » 300 to 500 volts;
plasma torch current » 325 amps (nominal);
primary chamber pressure » 10–30 inches in a water column gauge vacuum;
primary chamber oxygen flow rate » 0 to 3 scfm;
molten material temperature » 2,500 to 3,000 °F (estimated dependent upon feedstock);
primary chamber gas temperature » 1,200 to 1,600 °F (nominal);
SCC gas temperature » 1,600 °F (nominal);
afterburner heat input to the SCC » 80,000 British thermal units per hour;
SCC gas residence time of 2 seconds minimum; and
stack gas oxygen maintained at stoichiometry.

PROGRAM OVERVIEW

The FY97 work scope for the PTCLEP focused on evaluating plasma arc torch performance, optimizing the plasma torch operating parameters, and designing to extend electrode life. Increasing the plasma torch electrode life will increase overall system reliability and availability and, as a result, increase processing time available to treat waste, i.e., system throughput.

FY96 SS-TCLED PERMA TORCH TESTING—PERMA NOMINAL 150-kW
PT4T TRANSFERRED ARC PLASMA TORCH

The SSPF Torch Component Life Extension Development (TCLED) Project was initiated on May 7, 1996, and continued through July 31, 1996. The primary objectives for the TCLED tests were to evaluate multiple designs to identify which of the electrode designs and material configurations exhibit the least erosion rate characteristics and were the most durable for furnace operation and environment. The electrode life of a nominal 150-kW transferred arc plasma torch that used air as the torch gas was improved to 19.5 hours.

FY-97 SS-PTCLEP PERMA TORCH TESTING—PERMA NOMINAL 150-kW
PT4T TRANSFERRED ARC PLASMA TORCH

The SSPF PTCLEP test series was initiated on October 30, 1996, and continued through September 30, 1997. The FY97 work scope for the PTCLEP focused on the evaluation of plasma arc torch performance and optimization of the plasma torch operating parameters and design to extend electrode life. Increasing the plasma torch electrode life will increase overall system reliability and availability and, as a result, increase processing time available to treat waste, i.e., system throughput.

PTCLEP TEST OBJECTIVES

The objective of the PTCLEP is to determine the operating parameters, conditions, torch component configurations, and designs to provide a plasma torch electrode life of 100 hours operating time for a nominal 150-kW transferred arc plasma torch and 250 hours operating time for a nominal 500-kW transferred arc plasma torch.

The PTCLEP focused its testing, evaluation, and analysis on the plasma torch performance; optimization of the torch operating parameters and conditions, i.e., the effect of different torch gases; torch gas mass flow and pressure; cooling water flow rate (velocity), pressure, and temperature; and torch component configurations, geometry, designs, and material selection to improve electrode life.

ACCOMPLISHMENTS

Under the PTCLEP, the electrode life of a nominal 150-kW transferred arc plasma torch that used air as the torch gas was improved to 19.5 hours.

The DOE's PTCLEP testing under the Mixed Waste Focus Area (MWFA) was conducted by MSE at the WETO in Butte, Montana, using the SSPF thermal treatment system.

Under the PTCLEP the electrode life of a PERMA nominal 150-kW PT4T transferred arc plasma torch that used air as the torch gas was improved to greater than 100 hours of torch operation, performed in 10-12 hour segments.

The SSPF PTCLEP replication test, SS-PTCLEP-5-11R, was initiated on May 16, 1997, and continued through May 29, 1997, successfully replicating/validating two previous PTCLEP 100-hour electrode tests: 1) SS-PTCLEP-5-10R, April 30 through May 15, 1997, and 2) SS-PTCLEP-5-1, October 30 through December 19, 1996. The replication testing was conducted in 10 separate 12-hour segments.

The PTCLEP test series replication and validation of the 100-hour electrode design and configuration is a significant accomplishment, meets a major program objective, and completes the testing of the PERMA nominal 150-kW PT4T transferred arc plasma torch.

The increase in electrode life is attributed to several modifications MSE has performed, including upgrades/modifications to the torch cooling system and torch component design modifications, i.e., design modifications to the electrode and vortex generator. Increasing plasma torch electrode life increases overall system reliability and availability and, as a result, increases processing time available to treat waste, i.e., system throughput.

CONCLUSIONS AND RECOMMENDATIONS

The PTCLEP work scope focused on evaluating plasma arc torch performance, optimizing plasma torch operating parameters, and designing to extend electrode life. The FY97 overall objective of the PTCLEP was to determine the operating parameters, conditions, torch component configurations, and designs to provide a plasma torch electrode life of 100 hours operating time for a nominal 150-kW transferred arc plasma torch and 250 hours operating time for a 500-kW (nominal) transferred arc plasma torch.

Under the PTCLEP, the electrode life of a PERMA, nominal 150-kW PT4T transferred arc plasma torch that used air as the torch gas was improved to greater than 100 hours of torch operation, performed in 10-12 hour segments. A typical plasma arc of the 100-hour electrode is shown in Fig. 3.

Figure 3. Typical 100-hour PERMA PT4T (150-kW Nominal) Transferred Plasma Arc.

The PTCLEP test series replication/validation of the 100-hour electrode design and configuration is a significant accomplishment, meets a major program objective, and completes the testing of the PERMA PT4T transferred arc (150-kilowatt nominal) plasma torch. Increasing plasma torch electrode life increases overall system reliability and availability and, as a result, increases processing time available to treat waste, i.e., system throughput.

The SSPF PTCLEP duration test, SS-PTCLEP-5-11R, was initiated on May 16, 1997, and continued through May 29, 1997, successfully replicating/validating two previous PTCLEP 100-hour electrode tests: 1) SS-PTCLEP-5-10R, April 30, 1997, through May 15, 1997 and 2) SS-PTCLEP-5-1, October 30, 1996, through December 19, 1996. The replication testing was performed in 10-12 hours segments.

In addition, lessons learned from the PERMA, nominal 150-kW PT4T transferred arc plasma torch have been applied to the Retech nominal 500-kW RP-250-T transferred arc plasma torch that used air as the torch gas improving the torch electrode life to greater than 175 hours of torch operation.

The increase in electrode life is attributed to several modifications MSE completed, including upgrades/modifications to the torch cooling system and torch component design modifications, i.e., design modifications to the electrode and vortex generator.

heat transfer (finite element analysis);
fluid dynamics;
cold flow modeling;
torch component design;
development of the 2-D/3-D code of the electrode arc gas flow dynamics;
metallurgical evaluation/analysis of materials, ceramics, alloys, brush platting, and thermal sprays;
torch gasses, carbon dioxide, nitrogen, oxygen, argon, etc.;
torch gas modulation and;
feedstock processing effects on metals volitility and plasma arc resistance.

ACKNOWLEDGEMENTS

Work was conducted through the DOE-EM Office of Science and Technology at the Western Environmental Technology Office under DOE Contract Number De-AC22-96EW96405.

BACK