The Plasma Hearth Process has the potential to vitrify a wide variety of mixed wastes stored at sites throughout the U.S. Department of Energy complex. Its use could reduce DOE's volume of mixed waste without requiring extensive characterization or pretreatment. In order to make this thermal treatment more acceptable to the public and regulators, DOE's Office of Science and Technology is also developing, in conjunction with the PHP demonstration project, an efficient off-gas system, process controls, and process monitoring.
Plasma technology
PHP is a high-temperature thermal technology that destroys
organic constituents and immobilizes inorganic constituents and radionuclides in a stable
waste form--a glassy slag that will meet the U.S.
Environmental Protection Agency's Land
Disposal Restrictions. PHP is based on plasma arc technology in which a plasma torch
discharges electrical energy to torch gases to increase the temperature beyond what is
normally attained by chemical reaction. The plasma torch produces a transferred arc that
directly contacts the waste material and heats it to a temperature sufficient to
volatilize hazardous organics (typically 3,000 degrees F). Hazardous inorganics and
radioactive metals are melted and incorporated into a stable, glassy slag waste form.
Technology development
The PHP program is split into non-radioactive and radioactive activities. Under non-radioactive testing, a proof-of-principle system (a simplified version of the PHP concept) was designed and built in 1992. The system operates in a batch mode and has the capacity to process two 30-gallon drums per batch. Testing on the proof-of-principle system was completed in fiscal year 1994. A second non-radioactive system, designed at the pilot-scale to be a more complete version of the PHP concept, was constructed at Retech, Inc. in Ukiah, California. Testing is scheduled for FY96.
To test radioactive waste processing, a bench-scale unit was designed, constructed, and installed in the Argonne National Laboratory-West Transient Reactor Test Facility during FY95. Radioactive testing is scheduled to begin in FY96. The bench-scale unit processes up to eight one-gallon cans of material per test. It was built and will be tested in parallel with the non-radioactive pilot-scale unit. The plasma chamber is designed to closely model the pilot-scale plasma chamber, allowing comparisons between operating batches of radioactive and surrogate materials.
The final activity in the PHP demonstration program will be the construction and operation of a production-sized PHP system for treating actual mixed waste from Idaho National Engineering Laboratory. Installed at the Argonne National Laboratory-West Transient Reactor Test facility, the unit will have a 1.2 megawatt torch and a throughput of two 55-gallon drums per hour.
The radioactive field-scale system conceptual design report was completed in FY95; procurement and construction is scheduled for FY96; and radioactive testing is expected to begin in FY97.
Off-gas treatment, process controls, and process monitoring
Public concerns about uncontrolled releases from thermal treatment systems have had a significant impact on the permitting and operation of thermal systems. To address these concerns, the Controlled Emission Demonstration will investigate systems for off-gas treatment, process controls, and process monitoring. All effluent from the plasma processing system will be captured, separated, analyzed for contaminants of concern, and then either released or recycled through the process.
The mixed waste focus area has tasked MSE Technology Applications, the primary contractor of OST's Western Environmental Technology Office in Butte, Montana, to demonstrate a controlled emissions off-gas system to support the Plasma Hearth Process demonstration program. The Controlled Emission Demonstration will use facilities at WETO and at the Science Technology and Research Center, which is operated by Science Applications International Corporation, to develop an effluent treatment system to further reduce emissions to the environment and ensure future regulatory requirements are met.
A decreased effluent stream is the first goal of the project. One way to achieve this decrease is by replacing the natural gas-fired secondary combustion chamber with a flameless oxidizer. Another way is by separating and recycling useable process gases such as nitrogen and oxygen. At the WETO facility, the replacement of a natural gas-fired secondary combustion chamber with the Thermatrix Flameless Oxidizer has the potential to reduce off-gas emissions by 50 percent. The Pall Ceramic Filter will be tested to determine its efficiency in removing particulate from the off-gas.
Another purpose of this demonstration is to determine if gas phase analytical instrumentation can detect very low concentrations of hazardous components in the off-gas. The ultimate goal is to develop real-time or near real-time continuous emissions monitors to control the system. Metals monitors will be developed to detect vapor phase metals, such as mercury and polychlorinated aromatic hydrocarbons. The use of real-time monitors could gain the confidence of regulators and the public and also reduce the cost of retreatment by minimizing the amount of effluent to be retreated.
More information about the Plasma Hearth Process can be found on the Internet at http://www.em.doe.gov/rainmxws/mxws21.html.
