Low-Temperature Thermal Desorption System: 420D
Process Description
Low Temperature Thermal Desorption (LTTD) is a proven technology that vaporizes contaminants from soils through the application of heat and then destroys these contaminants in downstream air pollution control devices. The LTTD unit described below consists of a counter-current rotary desorber and pulse jet baghouse mounted on the Primary Treatment Unit (PTU) and a thermal oxidizer, quench, and acid gas scrubber mounted on a second trailer. MSR refers to the air pollution control system trailer as the Secondary Treatment Unit (STU).
Detailed Description
Contaminated soil is fed into the 3 cubic yard feed hopper by means of a front-end loader. The soil is conveyed over a weigh scale and enters the "cool end" of the rotary drum. As the contaminated soil travels through the drum it is contacted with hot combustion gases flowing in the opposite direction. This counter-current flow of soil and hot combustion gases heats the soil and reduces the gas temperature to approximately 500°F. As the soil is heated, contaminants in the soil are volatilized and enter the gas stream. The rotary drum is equipped with speed, slope and temperature controls to provide a variable soil retention time of 6 to 12 minutes. The unit can achieve soil discharge temperatures as high as 900°F.
The contaminated and dust laden air stream exiting the desorber is routed to a pulsejet baghouse equipped with over 900 square feet of P84 filter cloth. Dust fines collected in the baghouse are conveyed to the rotary discharge auger. The particulate-free gases exiting the baghouse are then routed to the air pollution control system housed on the STU. The STU booster fan draws the gas stream into a thermal oxidizer where the combination of high temperature and residence time converts virtually all of the organic contaminants to carbon dioxide (CO2), water vapor (H2O) and inorganic acids (HCL). The oxidizer is designed to provide a 2-second residence time at an oxidizer discharge temperature of 1800°F.
When treating chlorinated compounds the oxidizer exhaust gases are routed to a quench duct that reduces the gas temperature to below 400°F. This rapid quench (milliseconds) minimizes the potential for dioxin formation. The cooled gas flows into a packed-bed scrubber where HCL formed in the oxidizer is neutralized with a caustic sodium hydroxide (NaOH) solution. Spent scrubber water containing mainly salts is used as dust control on processed soil thereby minimizing waste discharges.
The hot soils exiting the thermal desorber pass into the discharge auger where they are mixed with dust fines removed in the baghouse. The auger is equipped with water spray nozzles that cool and rehydrate the soil using the water described above. The treated soil is stockpiled using a front-end loader and allowed to cool before sampling.
System Specs
Rotary Thermal Desorber Drum
- Maximum Process Rate: 15 Tons/hr.
- Average Process Rate: 8 - 12 Tons/hr.
- Dryer Size: 4' I.D. x 20'
- Dryer Drive-Variable Speed: 3 to 6 RPM
- Soil Dwell Time: 4 to 6 Min.
- Maximum Soil Discharge Temp.: 800 °F
- Exhaust Gas Exit Temp.: 425 °F Ave.
- Exhaust Gas Volume: 3,000 SCFM Ave.
Fabric Filter Baghouse
- Number of Bags: 78
- Air to Cloth Ratio: 4:1
- Gas Inlet Temperature Max: 500 °F
- Cleaning Method: Pulse Jet
- Pulse Jet: 4" W.C.
Thermal Oxidizer
- Operating Temperature: 1,400 – 1,800 °F
- Oxidizer Gas Dwell Time: 1 – 2 Seconds
- Maximum Burner Capacity: 14 mmBTU/hr.
- Shell Material: Carbon Steel
- Outside Diameter: 10' O.D.
- Inside Diameter (I.D. Refractory): 8' 6" I.D.
- Exhaust Gas Volume: 100,000 – 170,000 ACFM
- Oxidizer Refractory Type: 2,100 °F – 6" Pyroblock
Quench
- Maximum Inlet Temperature: 1,900 °F
- Cooling Method: Direct water contact
- Water Consumption: 20 – 35 gpm
- Outlet Temperature: 250 – 425 °F