Page 301 - Materials Chemistry, Second Edition

284 Practical Design Calculations for Groundwater and Soil Remediation

(b) Use Equation (7.15) to determine the heating value of the off-gas:

Heatingvalue (inBtu/scf)
= 18,210 Btu/lb × (6.47 × 10 −4 lb/scf)= 11.8 Btu/scf

(c) Use Equation (7.16) to convert the heating value into Btu/lb:

Heatingvalue of an airstreamcontainingbenzene (inBtu/lb)
= 11.8 Btu/scf ÷ 0.0739 lb/scf =160Btu/lb

Discussion:
The calculated heating value, 11.8 Btu/scf or 160 Btu/lb, is similar to
the value of 13 Btu/scf or 176 Btu/lb for 25% LELs of typical VOCs.

When dilution is required, the volumetric flow rate of the dilution air can
be found as [1]:

 
Q dilution =  H w − 1  Q w (7.18)
 H i 

where
Q dilution = required dilution air, scfm
Q w = waste air stream to be treated, scfm
H = heat content of the waste air stream, Btu/scf (or Btu/lb)
w
H = heat content of the desired influent entering the treatment sys-
i
tem, Btu/scf (or Btu/lb)

Example 7.9: Determine the Requirement of the Dilution Air
An off-gas stream (Q = 200 scfm) is to be treated by direct incineration. The
heating value of the off-gas is 300 Btu/lb. The insurance policy limits the
COC concentration in the influent air to the thermal oxidizer to ≤25% of its
LEL. Determine the required dilution air flow rate.

Solution:
Use 176 Btu/lb as the heating value that corresponds to 25% LEL. The
dilution air flow rate can be determined by using Equation (7.18) as:

 H   300 
=
Q dilution =  w − 1 Q w =  − 1(200) 141 scfm


 H i   176 