Pyrolysis is formally defined as chemical decomposition induced in organic materials by heat in the absence of oxygen. In practice, it is not possible to achieve a completely oxygen-free atmosphere; actual pyrolytic systems are operated with less than stoichiometric quantities of oxygen. Because some oxygen will be present in any pyrolytic system, nominal oxidation will occur. If volatile or semivolatile materials are present in the waste, thermal desorption will also occur.
Pyrolysis transforms hazardous organic materials into gaseous components, small quantities of liquid, and a solid residue (coke) containing fixed carbon and ash. Pyrolysis of organic materials produces combustible gases, including carbon monoxide, hydrogen and methane, and other hydrocarbons. If the off-gases are cooled, liquids condense producing an oil/tar residue and contaminated water. Pyrolysis typically occurs under pressure and at operating temperatures above 430 °C (800 °F). The pyrolysis gases require further treatment. The off-gases may be treated in a secondary combustion chamber, flared, and partially condensed. Particulate removal equipment such as fabric filters or wet scrubbers are also required.
Conventional thermal treatment methods, such as rotary kiln, rotary hearth furnace, or fluidized bed furnace, are used for waste pyrolysis. Molten salt process may also be used for waste pyrolysis. These processes are described in the following sections:
The rotary kiln is a refractory-lined, slightly-inclined, rotating cylinder that serves as a heating chamber.
Fluidized Bed Furnace
The circulating fluidized bed uses high-velocity air to circulate and suspend the waste particles in a heating loop and operates at temperatures up to 430 °C (800 °F).
Molten Salt Destruction
Molten-salt destruction is another type of pyrolysis. In molten-salt destruction, a molten salt incinerator uses a molten, turbulent bed of salt, such as sodium carbonate, as a heat transfer and reaction/scrubbing meduim to destroy hazardous materials. Shredded solid waste is injected with air under the surface of the molten salt. Hot gases composed primarily of carbon dioxide, stream, and unreacted air components rise through the molten salt bath, pass throught a secondary reaction zone, and through an off gas cleanup system before discharging to the atmosphere. Other pyrolysis by-products react with the alkaline molten salt to form inorganic products that are retained in the melt. Spent molten salt containing ash is tapped from the reactor, cooled and placed in a landfill.
Pyrolysis is an emerging technology. Although the basic concepts of the process have been validated, the performance data for an emerging technology have not been evaluated according to methods approved by EPA and adhering to EPA quality assurance/quality control standards. Performance data are currently available only for vendors. Also, existing data are limited in scope and quantity/quality and are frequently of a proprietary nature.
The target contaminant groups for pyrolysis are SVOCs and pesticides. The process is applicable for the separation of organics from refinery wastes, coal tar wastes, wood-treating wastes, creosote-contaminated soils, hydrocarbon-contaminated soils, mixed (radioactive and hazardous) wastes, synthetic rubber processing wastes, and paint waste.
Pyrolysis systems may be applicable to a number or organic materials that 'crack' or undergo a chemical decomposition in the presence of heat. Pyrolysis has shown promise in treating organic contaminants in soils and oily sludges. Chemical contaminants for which treatment data exist include PCBs, dioxins, PAHs, and many other organics. 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 are similarly not destroyed.
Factors that may limit the applicability and effectiveness of the process include:
- There are specific feed size and materials handling requirements that impact applicability or cost at specific sites.
- The technology requires drying of the soil to achieve a low soil moisture content (< 1%).
- Highly abrasive feed can potentially damage the processor unit.
- High moisture content increases treatment costs.
- Treated media containing heavy metals may require stabilization.
In addition to identifying soil contaminants and their concentrations, information necessary for engineering thermal systems to specific applications includes soil moisture content and classification (no sieve analysis is necessary), and the soil fusion temperature.
Limited performance data are available for pyrolytic systems treating hazardous wastes containing PCBs, dioxins, and other organics. The quality of this information has not been determined. These data are included as a general indication of the performance of pyrolysis equipment and may not be directly transferrable to a specific Superfund site. Site characterization and treatability studies are essential in further refining and screening the pyrolysis technology.
The overall cost for remediating approximately 18,200 metric tons (20,000 tons) of contaminated media is expected to be approximately $330 per metric ton ($300 per ton).