By Michael Shiralian, Ph.D., C.Chem.* the opposite end of the flame entering the gas treatment units (see Figure B). In general, at low temperatures, low
Low Temperature Thermal Desorption
Screen and
Process Flow Diagram
conwyor
moisture and low organic content, both
Contaminated Sou
^Co o o o b)
counter current and co-current mechanisms
—^OR^'
can produce similar results. However, at higher temperatures, moisture and organic content, the counter current provides greater flexibility. The co-current mechanism, on
Son RemoHturliadon
the other hand, is more attractive in treat
ing highly contaminated finer particles. Stage 2. The second stage of the LTTD technology is the treatment ofexhaust gases. This involves capturing the dust and fine particulates from the gas stream carrying the
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s
VOCs and exiting the dryer. There are two different mechanisms for
handling the exhaust material. These are known as"cold configuration" and "hot con figuration". In the cold configuration the exhaust material from the dryer enters a high efficiency baghouse where solid particles are discharged before the VOCs are routed to an afterburner for combustion, and final emission at the stack (Figure A). In the hot
•Mictiael Shiralian is an independent practi tioner specializing In site remediation. He is a pioneer scientist in obtaining Certificates ofAp proval and implementing the first mobile LTTD In Ontario. He has been project manager for the design and construction of a LTTD unit, and several remediation projects Involving LTTD in Ontario since 1991.
FIGUREA: Counter current dryer with cold configuration exhaust treatment
FIGURE B: Co-current dryer with hot configuration exhaust treatment system.
system.
configuration, the exhaust material from the dryer enters the afterburner first and then is routed into the baghouse for dust filtration and final emission (Figure B). In this con figuration the fine particles undergo expo sure to high temperatures (>870°C) under an oxygen rich atmosphere. This combus tion condition can effectively remove and/ or destroy the residual organics. Regardless of the type of exhaust treat ment configurations, both dryer and baghouse are operated under slight negative
pressure by the exhaust fan blower in order to prevent leaking and also to facilitate ex haust transport through the system and exit at the stack.
The major disadvantage of hot configu ration, is the cost of cooling down the ex
haust gases exiting the afterburner prior to entry into the baghouse. The afterburner temperature is usually above 870°C. This temperature can destroy all types of known filter material used inside the house
Continued overleaf
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Environmental Science & Engineering, September 1995
For more Information, Gircie reply card No. 125
21