Thermal desorption is a physical separation process, contaminated soils are heated to volatilise water and organic contaminants. A carrier gas or vacuum system transports volatilised water and organics to the gas treatment system. There are generally three methods of thermal desorption:
- Indirect heated: An externally fired rotary dryer volatilises the water and organics from the contaminated media into an inert carrier gas stream, steam can also be used as an indirect heating method.
- Indirect fired: A direct-fired rotary dryer heats an air stream which is in contact with the contaminated soil.
- Direct fired: Fire is applied directly upon the surface of contaminated media.
Thermal desorption processes can be separated into two groups:
Low temperature thermal desorption (LTTD)
In LTTD, soils are heated to between 90°C and 320°C, this system is used for VOC and petroleum removal. Remediated soil retains the majority of its physical properties unless they have been heated at the higher end of the temperature range although many UK soils can be detrimentally affected.
High temperature thermal desorption (HTTD)
HTTD soils are heated to 320°C to 960°C, this system is used for higher chain hydrocarbons, pesticides etc. HTTD systems are often combined with incineration, solidification/stabilisation treatments. Soils are rarely re-useable without significant geotechnical improvement.
Thermal desorption can be used to remediate a wide range of compounds including but not limited to:
- Diesel range organics (DRO)
- Petroleum range organics (PRO)
- Volatile organic compounds (VOCs)
- Semi-volatile organic compounds (SVOCs)
- Poly-aromatic hydrocarbons (PAHs)
- Poly-chlorinated biphenyl's (PCBs)
Excavated soils require soil processing before thermal treatment. A common thermal desorption design is the rotary dryer, rotary dryers are indirect or direct-fired horizontal cylinders. The cylinder is normally inclined and rotated to move the contaminated soil along the cylinder. All thermal desorption systems produce gas which requires treatment to remove water vapour, particulates and contaminants. Particulates can be removed by equipment such as wet scrubbers or filters. Contaminants can be incinerated in a second combustion chamber or removed through condensation and then activated carbon adsorption units. Common equipment used in thermal desorption:
- Soil processing equipment
- Rotary kiln
- Granular activated carbon pods (gas treatment)
- Particulate filters
- Fuel storage for rotary kiln
- Dewatering unit
- Control systems
Advantages and Limitations
- Able to remediate recalcitrant contaminants
- Can cope with a wide range of contaminants and combinations of contaminants
- Not suitable for water logged soils
- Very high fuel costs
- Gas treatment system required
- Larger particle size can cause incomplete desorption
- Clay, silts and high humic content in soils increase required residence times in desorption unit, increasing costs
- Most soils geotechnically poor following treatment
- Not suitable for much of UK made ground and alluvial materials
- Regulatory and public perception challenges