Pyrolysis is a form of treatment that chemically decomposes organic materials by heat in the absence of oxygen. Pyrolysis typically occurs under pressure and at operating temperatures above 430 ÁC (800 ÁF). In practice, it is not possible to achieve a completely oxygen-free atmosphere. Because some oxygen is present in any pyrolysis system, a small amount of oxidation occurs. If volatile or semi-volatile materials are present in the waste, thermal desorption will also occur.
Organic materials are transformed into gases, small quantities of liquid, and a solid residue containing carbon and ash. The off-gases may also be treated in a secondary thermal oxidation unit. Particulate removal equipment is also required. Several types of pyrolysis units are available, including the rotary kiln, rotary hearth furnace, and fluidized bed furnace. These units are similar to incinerators except that they operate at lower temperatures and with less air supply.
A molten salt process may also be used for waste pyrolysis. In molten-salt oxidation (MSO), combustible waste is oxidized in a bath of molten salts (at 500–950ÁC). There is no direct flame, and this prevents many of the problems associated with incineration. Shredded solid waste is injected with air under the surface of a molten salt bath. Hot gases rise through the molten salt bath. The salt, being alkaline, scrubs acids from the gases. The heat of the molten salt degrades and melts the waste material. Because the salt bath is liquid, it also removes some particles in the gas. Byproducts are retained in the melt. Gases exiting the salt bath are treated before discharge to the atmosphere. Spent molten salt is tapped from the reactor, cooled, and placed in a landfill.
Limitations and Concerns
The technology requires drying of soil prior to treatment.
Limited performance data are available for systems treating hazardous wastes containing polychlorinated biphenyls (PCBs), dioxins, and other organics. There is concern that systems that destroy chlorinated organic molecules by heat have the potential to create products of incomplete combustion, including dioxins and furans. These compounds are extremely toxic in the parts per trillion range. The MSO process reportedly does not produce dioxins and furans.
The molten salt is usually recycled in the reactor chamber. However, depending on the waste treated (especially inorganics) and the amount of ash, spent molten salt may be hazardous and require special care in disposal.
Pyrolysis is not effective in either destroying or physically separating inorganics from the contaminated medium. Volatile metals may be removed as a result of the higher temperatures associated with the process, but they are not destroyed. Byproducts containing heavy metals may require stabilization before final disposal.
Pyrolysis treats and destroys semi-volatile organic compounds (SVOCs), fuels, and pesticides in soil. The process is applicable for the treatment of organics from refinery wastes, coal tar wastes, creosote-contaminated soils, hydrocarbons, and volatile organic compounds (VOCs). MSO is also being tested for treating mixed radioactive wastes.
Technology Development Status
Pyrolysis is a developed technology. The basic concepts of the process have been validated. However, there are limited performance data on the treatment of hazardous wastes containing PCBs, dioxins, and other organics.
Other Resources and Demonstrations
The U.S. Army Environmental Center (USAEC), in cooperation with the Tennessee Valley Authority (TVA) and Vanguard Research, Incorporated successfully developed and demonstrated the destruction of hazardous and regulated medical wastes using plasma energy pyrolysis. The PEPSTM was successfully demonstrated a ten-ton-per-day capacity fixed/transportable system in Lorton, Virginia. The Virginia Department of Environmental Quality certified PEPSTM as an alternative to incineration with a destruction and removal efficiency (DRE) of 99.99999%. With the concern about dioxin formation, this system may have particular interest.
See http://www.clu-in.org/download/remed/destruct_tech.pdf, p.30 for a description of Molten Metal Pyrolysis for treatment and reduction of hazardous wastes and low-level radioactive materials.