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TABs on Contaminated Sites
Contaminated Sites Program - Federal Sites
This is one in a series of Technical Assistance Bulletins (TABs) prepared by Environment Canada-Ontario Region for Federal Facilities operating in Ontario.
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TAB #13: Soil Remediation: Low Temperature Thermal Desorption
DESCRIPTION:
Low Temperature Thermal Desorption (LTTD) is an Ex-Situ means of physically separating volatile and semi-volatile organic contaminants from soils. Contaminated soil is placed in a chamber, and heated to volatilize the associated hydrocarbons. The exhaust gases, containing the hydrocarbons are consumed in an oxidizing unit, and are subsequently cooled and passed through a baghouse to remove particulate matter. The treated soil can then be returned to the original excavation site as clean backfill. This innovative means of remediating soil is a cost effective and environmentally sound alternative to many of the currently practiced methods of handling hydrocarbon contaminated soil.
HYDROCARBON CONTAMINATED SOIL
The problems associated with hydrocarbon contaminated soil have received great attention in recent times as landfill sites have become stock-piled with contaminated soil, increasing the risk of groundwater and surface water contamination.
Until recently, the most commonly used method to treat hydrocarbon contaminated soil was disposal at a landfill site. This method however, does not remediate the soil, it merely relocates the problem to a public or private site. From an environmental stand point, this method is not a viable alternative for dealing with contaminated soils. Various levels of government are currently prohibiting the practice of land filling as a means of disposing contaminated soil. It is becoming increasingly costly to dispose of contaminated soil due to high tipping fees.
Low Temperature Thermal Desorption (LTTD) is an innovative process of remediating contaminated soil, sediments, and sludges. It incorporates sound environmental practices which are consistent with the objectives of the Greening of Government and the Office of Environmental Stewardship.
THE LOW TEMPERATURE THERMAL DESORPTION PROCESS
The following is a basic description of the Low Temperature Thermal Desorption Process technology. This technology has been used at a number of Federal sites, and has proved to be successful at cleaning hydrocarbon contaminated soil.
The steps involved in the process (Figure 1) are as follows:
- Soils contaminated with hydrocarbons are excavated and delivered to the thermal desorption unit. The unit can be located either on site or at permanent locations.
- Soils to be processed are loaded into a feed hopper and then directed to a conveyer which operates at a controlled rate. The conveyer leads into a closed chamber or dryer where the soil is then heated to volatilize the associated hydrocarbons. The dryer temperature and processing time are set according to the type and concentration of petroleum product in the soil, the level of moisture in the soil, and the soil type.
- The hydrocarbons and dust particulates in the gases from the dryer are directed, first into the thermal oxidizer, and then to the baghouse, for removal.
- The recycled soil is then returned to the site of origin or further processed on site for reuse in a variety of ways.
- On site testing equipment permits the continuous monitoring of the system exhaust, as well as the sampling of contaminants in the solids and gaseous material, and in the water by-product. This comprehensive site testing procedure ensures that remediation is undertaken in accordance with all applicable environmental regulations.
Figure 1. The Low Temperature Thermal Desorption Process.
PERFORMANCE FACTORS
Several Factors affect the performance capabilities of the Thermal Desorption Process. The following is a brief summary of these variables.
Equipment Operating Parameters
- Treatment temperature (This factor is a function of the moisture content of the soil, the heat capacity of the soil, the particle size of the soil, and the heat transfer and mixing characteristics of the thermal device.).
- Soil residence time in thermal desorption unit.
- Oxygen content of the purge gas.
- Type of heating method.
- Mode of operation.
Contaminant Characteristics
Vapour pressure and the concentration of hydro-carbons in the soil are the two key contaminant properties that affect performance of the thermal desorber.
- Vapour pressure influences the rate at which a contaminant is thermally desorbed
- The maximum concentration of hydrocarbons in the soil that can be treated by a thermal desorption device depends on the gas flow through the device, the oxygen content of the purge gas, and the type of hydrocarbon product present.
Soil Characteristics
Particle size can influence performance because of pre-treatment requirements to crush or screen soil. Size limits depend on mechanical clearances in conveyer systems and heat transfer considerations
The Low Temperature Thermal Desorption technology can be utilized as a mobile soil remediation solution to satisfy on-site clean up. The cost of operating the system can range from $50/tonne to $80/tonne, depending upon the soil type, moisture content and the level of contamination.
| Comparison of Features of Thermal Desorption and Offgas Treatment Systems |
| Characteristics |
Rotary Dryer |
Asphalt Plant |
Thermal Screw |
Conveyor Furnace |
| Estimated number of systems |
40-60 |
100-150 |
18-22 |
1 |
| Estimated number of contractors |
20-30 |
No estimate |
9 |
-- |
| Mobility |
Fixed and mobile |
Fixed |
Mobile |
Mobile |
| Typical size (tons) |
500-25000 |
0-10000 |
500-5000 |
500-5000 |
| Soil throughput (tons/hours) |
10-50 |
25-100 |
3-15 |
5-10 |
| Maximum soil feed size (inches) |
2-3 |
2-3 |
1-2 |
1-2 |
| Heat transfer method |
Shell rotation and lifters |
Shell rotation and lifters |
Auger |
Soil agitator |
| Discharge soil temperature (°F) |
300-600 (a) |
300-600 |
300-500 (c) |
300-800 |
| |
600-1200 (b) |
|
600-900 (d) |
|
|
|
|
1000-1600 (e) |
|
| Soil residence time (minutes) |
3-7 |
3-7 |
30-70 |
3-10 |
| Thermal Desorber Exhaust Gas Temp. (°F) |
500-850 (a) |
500-850 |
300 |
1000-1200 |
|
800-1000 (b) |
|
|
|
| Gas/solids flow |
Co-current or counter-current |
Co-current or counter-current |
Not applicable |
Counter-current |
| Atmosphere |
Oxidative |
Oxidative |
Inert |
Oxidative |
| Afterburner temperature (°F) |
1200-1800 |
1400-1800 (f) |
Generally not used |
1400-1800 |
| Maximum thermal duty (MM Btu/hr) (g) |
10-100 |
50-100 |
7-10 |
10 |
| Heatup time from cold condition (hours) |
0.5-1.0 |
0.5-1.0 |
Not reported |
0.5-1.0 |
| Cool down time from hot condition (hours) |
1.0-2.0 |
1.0-2.0 |
Not reported |
Not reported |
| Total Petroleum Hydrocarbons |
|
|
|
|
| Initial concentration (mg/kg) |
800-35000 |
Not reported |
60-50000 |
5000 |
| Final concentration (mg/kg) |
<10-300 |
Not reported |
ND-5500 |
<10.0 |
| Removal efficiency (%) |
95.0-99.9 |
Not reported |
64-99 |
>99.9 |
| BTEX |
|
|
|
|
| Initial concentration (mg/kg) |
NR |
Not reported |
155 |
Not reported |
| Final concentration (mg/kg) |
<1.0 |
Not reported |
<1.0 |
<0.01 |
| Removal efficiency (%) |
HR |
Not reported |
>99 |
Not reported |
(a) Carbon steel materials of construction
(b) Alloy materials of construction
(c) Hot oil heat transfer system
(d) Molten salt heat transfer system
(e) Electrically heated system
(f) Not used on all systems
(g) Total duty of thermal desorber plus afterburner
Duplicated from: Troxler, W. et. Al. (June-July 1992) Thermal Desorption. "Now You're Cooking Hot Technology: How it Works, What it Costs, Where's it Going?" Soils, June-July, pg.30.
ADVANTAGES OF THE LTTD PROCESS
- The facility provides for the complete remediation of the soils and destruction of hydrocarbons; removing the threat to water and air.
- It provides a reliable and cost effective option to landfill disposal.
- The equipment can process a wide variety of soil types.
- Hydrocarbons are not released into the atmosphere.
- The exact level of concentration of contaminants of interest is defined immediately following treatment.
- Allows for the re-use of the remediated soil in a variety of ways.
- Removal and destruction of hydrocarbon contamination eliminates future liability and potential environmental problems.
Sources
Troxler, W. et.al. (1992). Thermal Desorption. Now You're Cooking Hot Technology: How it Works What it Costs, Where's it Going? Soils, June-July pg. 30 - 47.
U.S. EPA (1990). Engineering Bulletin Low Temperature Thermal Desorption Treatment.
This document is available for download as an Adobe Acrobat file (.pdf). If you do not have the Adobe Acrobat Reader, you can use the link below to obtain it, at no cost, from Adobe's Web site.
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For further information please contact:
- Environment Canada
Ontario Region - Environmental Protection Branch
Environmental Contaminants &
Nuclear Programs Division
4905 Dufferin Street
Downsview, ON M3H 5T4
Telephone: (416) 739-4826
Fax: (416) 739-4405
Our TABs can be found on the Internet at:
http://www.on.ec.gc.ca/pollution/ecnpd/
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