Page 224 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 207

Organic compounds generally contain heating values. These organic com-
pounds can also serve as energy sources for combustion. The higher the
organic concentration in a waste stream, the higher the heat content is and
the lower the requirement of auxiliary fuel would be. If the energy content
in a waste stream is greater than 4,000 Btu/lb, it can sustain burning without
supplementary fuel. If the heating value of a compound is not available, the
following Dulong’s formula can be used:


Heatingvalue (inBtu/lb) = 145.4C + 620 H − O   + 41S (5.47)

 8 
where C, H, O, and S are the percentages by weight of these elements in
the compound.
To ensure a more complete combustion, excess air should be provided
in addition to the stoichiometric amount for combustion of the COCs.
Combustion temperature should be high enough to achieve the required
destruction efficiency. The higher the combustion temperature, the shorter
the required residence time would be for the specified destruction efficiency.
The combustion temperature (T in °F) can be estimated by [10]:

NHV
T = 60 + 1 EA 7.5 10 )( NHV ] ) (5.48)
−
+
4
×
(0.325)[1 ( + ( ) ×
where NHV is the net heating value in Btu/lb and EA is the excess air in percent.
Example 5.30: Determine the Energy Content of a Waste Sample
Several leaky USTs, which previously stored xylenes, C H (CH ) , were
6
3 2
4
removed. The excavated soil was stockpiled on site and contains an average
xylene concentration of 1,500 mg/kg. The volume of the stockpile is 500 m .
3
The excavated soil needs to be treated before final disposal. Direct incinera-
tion of the soil is considered as a remedial alternative. Assuming the original
organic content of the native soil is negligible, show that the heating value of
the soil containing 1,500 mg/kg xylenes is very low.
Solution:
(a) The carbon content in the soil containing 1,500 mg/kg xylenes
(C H ) =
8
10
Mass of C in one mole of xylenes
1,500 ×
MW of xylenes
12 8×
= 1,500 × = 0.136%
(128 110)×+ ×