Page 228 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 211

Example 5.34: Determine the Combustion Efficiency
The off-gas from a combustion unit was analyzed by an Orsat gas analyzer.
The off-gas (on a dry basis) consisted of 17% CO , 2.5% O , 80% N , and 160
2
2
2
ppmV of CO. Estimate the combustion efficiency of this combustion unit.
Solution:
As mentioned in Chapter 2, 1% = 10,000 ppmV
From Equation (5.50),

17,000
Combustion efficiency = 100%× = 99.06%
17,000 160+

Discussion:
The calculated combustion efficiency is <99.9%. A better mixing, more
excess air, or higher combustion temperature may be needed to raise
the combustion efficiency above 99.9%.

5.8 Low-Temperature Thermal Desorption
5.8.1 Description of the Low-Temperature Thermal Desorption Process

Low-temperature thermal desorption (LTTD), also known as low-tempera-
ture thermal heating, low-temperature thermal volatilization, and thermal
stripping, is an ex situ soil remediation technique. In the low-temperature
thermal desorption process, volatile and semi-volatile COCs are removed
from soil, sediments, or slurries through volatilization that is enhanced by
elevated temperatures. The process is typically operated at temperatures
from 200°F up to 1,000°F. The term low temperature is used to differentiate the
process from incineration. At these lower temperatures, the COCs are physi-
cally driven off from the soil matrix instead of being combusted. The pro-
duced off-gas requires further treatment before being vented to atmosphere.

5.8.2 Design of the Low-Temperature Thermal Desorption Process
There are no set guidelines for design of a low-temperature heating reactor.
The time required to achieve a specific final concentration would depend
mainly on the following factors:

• Temperature inside the reactor: The higher the temperature, the higher
the desorption rate will be and, consequently, the shorter the reten-
tion time.