Fact sheet: Pyrolysis

On this page

• Description
• Implementation of the technology
• Recommended analyses for detailed characterization
• Recommended trials for detailed characterization
• Other information recommended for detailed characterization
• Applications
• Applications to sites in northern regions
• Treatment type
• State of technology
• Target contaminants
• Treatment time
• Long-term considerations (following remediation work)
• Secondary by-products and/or metabolites
• Limitations and Undesirable Effects of the Technology
• Complementary technologies that improve treatment effectiveness
• Required secondary treatments
• Application examples
• Performance
• Measures to improve sustainability or promote ecological remediation
• Potential impacts of the application of the technology on human health
• References
• Author and update

Description

Pyrolysis is defined as chemical decomposition of organic materials induced by heat. In contrast to incineration, pyrolysis occurs in the almost complete absence of oxygen (less than stoichiometric quantities of oxygen). This treatment transforms organic materials into gas (syngas), small quantities of liquid, and a solid residue (coke) containing fixed carbon and ash. Pyrolysis treatment does not produce toxic gases from the combustion process; the syngas is generally composed of carbon monoxide, hydrogen, methane and other hydrocarbons. However, gas emissions do require further treatment before being released into the atmosphere. The off-gas treatment process may consist of adsorption through granular activated carbon (GAC), condensation, or thermal oxidation. Throughput rates can vary from less than 5–10 to approximately 50 metric tons per hour depending on the type of soil and treatment unit.

Target contaminants for pyrolysis are semi-volatile organic compounds (SVOCs) and pesticides. The process is applicable for the treatment of a variety of organic derived from petrochemical 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 reactions typically occur under pressure and at temperatures above 430 °C (800 °F). There are various conventional pyrolysis treatments including rotary kiln, rotary hearth furnace, fluidized bed furnace and pyrolysis with molten salt destruction.

Pyrolysis is a marginal technology that is almost never used. Limited information can be found regarding the performance of the technology.

Internet links:

Pyrolysis - Techtree - Centre for Public Environmental Oversight (CPEO)

4.24 Pyrolysis - FRTR Remediation Technologies Screening Matrix and Reference Guide, Version 4.0

Implementation of the technology

• Implementation of this technology may include:
• Physical and chemical characterization of the soil or sediment
• Mobilization of equipment and construction of temporary facilities, site access considerations and site preparation (clearing/grubbing/demolition, topsoil stripping and temporary stockpiling)
• Delivery and installation of thermal desorption system (kiln or furnace)
• Excavation of soils which may include
  • Excavation dewatering
  • Slope stability controls
  • Foundation protection for retained structures (shoring, underpinning, etc.)
• Soil screening and pretreatment to separate and remove (or crush) oversize materials before placement in heating unit
• Pretreatment soil drying (soil moisture content below 1% is required)
• Gas collection and treatment through thermal oxidation, condensation, or adsorption (including removal of dust particles)
• Compliance sampling of treated soil stockpiles and off-site transport or on-site disposal of soil (backfilling of excavations or spreading the soil on site)
• Rotary kilns or furnaces used for pyrolysis that are physically similar to the equipment used during the incineration process but operate at lower temperatures and without oxygen
• Fluidized bed furnace, operated at temperatures up to 430 °C (800 °F), circulates air with waste particles in a heating loop. The turbulence produces a uniform temperature around the pyrolysis chamber and a hot cyclone. The fluidized bed technique completely mixes the contaminated material
• Molten salt destruction (MSD) is a treatment similar to the fluidized bed furnace. The gas passes through the molten salt bath and then needs to be treated using a gas emissions cleanup system before being released to the atmosphere. Acidic by-products of pyrolysis and particulates are retained in the molten salt. After several pyrolytic cycles, the molten salt within the reactor needs to be replaced
• Decommissioning and removal of thermal system (kiln or furnace)
• Surface restoration (planting, grading, paving, etc.)

For the excavation component of the technology, the method relies on traditional/commonly-available civil/earthworks construction equipment and methods. Commercial and transportable units are available for the treatment component. Depending on the soil throughput rates, units may be mounted on one to five trailers.

For all ex situ thermal treatment systems, captured vapours require treatment for the removal of particulates and contaminants. Captured vapours, known as off-gas, include water vapour as well as volatile organic compound (VOC) vapours. Particulate removal equipment may be wet scrubbers or fabric filters, while condensation or adsorption (e.g. through granular activated carbon [GAC]) equipment may be used for contaminant removal. Alternatively, the contaminants in the off-gas may be destroyed in a thermal oxidation system, which can be operated flameless, with a direct flame or with oxidizers in a catalytic environment. A carrier gas or vacuum is used to transport water and VOC vapours to the gas treatment system.

Materials and storage

• Sufficient storage space is required to house the thermal treatment system
• On-site storage is typically limited to small amounts of fuel and lubricant (daily fuelling of excavator is often from a mobile tank) as well as miscellaneous construction site supplies
• Contractor may create temporary stockpiles of contaminated materials pending treatment.
• To protect from rain and minimize soil moisture content, the soil stockpiles and feed equipment need to be covered

Waste and Discharges

• Pyrolysis has limited potential for in situ residuals. Treatment systems may generate solid, liquid and gaseous residuals, but the appropriate storage, management and disposal of treatment residuals are part of proper treatment system operation
• Pyrolysis produces off-gas that requires monitoring and potentially treatment prior to release to the atmosphere
• If molten salt is used in the pyrolysis treatment system, it may require replacement. In addition, the used molten salt may be toxic and requires properly handling and disposal
• Spent sorbent (e.g. activated carbon), if used, will require off-site regeneration or disposal. If a wet gas scrubber is used, the resulting sludge in the wastewater treatment system requires management/removal before discharge
• Other residuals from thermal desorption include treated off-gas, particulates, filters, catalysts, non-contact combustion gases. Thermal desorption treatment of soil with halogenated compounds can generate halogenated acids and off-gases containing dioxins and furans. While halogenated compounds can be removed through incineration, treatment of these compounds should be approached with caution. The treated soil is commonly not considered a residual material although it could be considered as such depending on the contaminant levels remaining in the soil
• Windblown dust can be generated by soil loading. Track-out or stockpiles, for example, may deposit directly on downwind surfaces
• Pyrolysis can result in solid residue (coke) containing fixed carbon and ash and may require stabilization prior to disposal if it contains heavy metals
• Diverted and collected/treated stormwater are typically passed into the local stormwater system. Small quantities of liquid residue are produced from pyrolysis. A wet gas scrubber, if used, requires control and monitoring of the wastewater. If the gas treatment step involves condensation, proper management of the concentrated liquid is required
• This treatment method leads to limited vapour phase emissions from equipment exhaust and the volatilization of contaminants from fresh excavation faces or soil stockpiles
• In specific cases, naturally occurring radon may also be mobilized

Recommended analyses for detailed characterization

Recommended trials for detailed characterization

Other information recommended for detailed characterization

Notes:

Treatability studies are recommended to identify potential gas emissions and the composition of the resulting coke are required.

Applications

• The target contaminants for pyrolysis include SVOCs and pesticides
• Pyrolytic systems may be used to treat organic materials that decompose in the presence of heat
• This technology is applicable to residual and free-phase contamination

Applications to sites in northern regions

Remote sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short work windows. Soils with high water or organic carbon content can reduce the efficiency of ex situ thermal remediation systems through increased energy and time needed for material drying prior to the pyrolytic treatment.

Since it requires large and complex equipment and great energy consumption, pyrolysis is not well adapted for northern and remote environments.

Treatment type

State of technology

Target contaminants

Treatment time

Notes:

Depending on the volume of the soil requiring treatment, the treatment plant may be in operation from weeks to months.

Long-term considerations (following remediation work)

If the treated soil is used as backfill, minor long-term considerations are related to potential changes in geotechnical and/or geochemical properties. 

Secondary by-products and/or metabolites

• Pyrolytic by-products from the combustion of organic compounds are syngas, small quantities of liquid and coke
• The syngas generally contains carbon monoxide, hydrogen, methane and other hydrocarbons
• Decomposition of chlorinated organic compounds by pyrolysis has the potential to produce toxic compounds such as dioxins and furans due to incomplete combustion. Molten salt destruction treatment eliminates the release of dioxins and furans
• When the gas emissions are cooled, liquids condense, producing an oil/tar residue and contaminated water. These oils/tar residues may be toxic and require proper treatment, storage and disposal

Limitations and Undesirable Effects of the Technology

• Limited performance when treating soil contaminated with polychlorinated biphenyls (PCBs) and dioxins
• Volatile metals may be removed as a result of the high temperatures associated with the treatment but are not destroyed
• Molten salt may accumulate toxic compounds and require adequate treatment or disposal
• Coke containing heavy metals may require stabilization before final disposal
• If the treated soil is used as backfill, its geochemical and physical properties could have changed, which in turn, could impact the in situ geochemical conditions
• The limitations to soil excavation in general apply (slope stability considerations and protection, groundwater dewatering, protection of infrastructures, safety measures, etc.)
• Off-gas treatment can be complex. Handling of high-temperature, explosive gases presents risks
• If properly designed and operated, the thermal process has few if any special considerations. Some considerations associated with nuisance or safety include noise, odour, lights and traffic
• At higher temperatures, physical properties of the soil may change and loss of organic matter may inhibit biological activity

Complementary technologies that improve treatment effectiveness

Pyrolytic treatment requires drying of the soil before treatment

Required secondary treatments

• Gas emission quality monitoring and treatment when necessary
• Proper treatment and disposal of the waste products (liquid, coke, ash, etc.)

Application examples

Application example is available at this address:

• Pyrolytic Treatment and Fertility Enhancement of Soils Contaminated with Heavy Hydrocarbons

Performance

Pyrolysis is an emerging technology and performance data are limited. Treatability studies are essential to further refine pyrolysis technology.

Measures to improve sustainability or promote ecological remediation

• Use of renewable energy and energy-efficient machinery (for example, geothermal, wind or solar energy)
• Process optimization to reduce wastes and consumables
• Schedule optimization for resource sharing and fewer days of mobilization
• Assessment of pre-treatment options for feedstock to increase efficiency of thermal treatment system (such as optimum soil moisture content)

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

• FRTR. 2002. Remediation Technology Matrix and Reference Guide, Version 4.0. U.S.A.

Author and update

Composed by : Josée Thibodeau, M.Sc, National Research Council

Updated by : Martin Désilets, B.Sc., National Research Council

Updated Date : March 1, 2008

Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée

Updated Date : March 31, 2018