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340 Lawrence K. Wang et al.

where Cost is the cost in the month-year a ($), Cost is the cost in the month-year b
a b
($), Index is the CE Equipment Cost Index in the month-year a, and Index is the CE
a b
Equipment Cost Index in the month-year b.
It should be noted that although the CE Equipment Cost Indices (19) are recom-
mended here for Index , and Index , the ENR Cost Indices (20–22), the US EPA Cost
a b
Indices (23), or the US Army Cost Indices (24) can also be adopted for updating the
costs. Wang et al. (21) have shown how mathematical models can be developed for
various cost indices, which, in turn, can be used for forecasting future costs.

5. DESIGN EXAMPLES
Example 1

Emission stream 3 (see Table 1) is to be properly treated. Assume the HAP control
requirement for emission stream 3 is 98% reduction. In this case, the inlet HAP concen-
tration falls outside the operating range of thermal incineration, catalytic incineration,
carbon adsorption, absorption, and condensation; therefore, none of the control devices is
applicable. Note that dilution air could be used to decrease the HAP concentration.
Alternatively, this stream may warrant consideration as a fuel gas stream (25,26).
However, for example purposes, assume that this stream is to be flared. Flares can be used
to control emission streams with high heat contents; hence, flaring can be considered an
option. Assume a steam-assisted elevated flare system as shown in Fig. 1, with a flare tip
diameter of 54 in. The molecular weight of the emission stream is 33.5 lb/lb-mol. The
minimum flare exit velocity for a stable flame is 0.03 ft/s. Determine the following:

1. Required destruction efficiency, DE
2. Supplementary fuel requirements in terms of natural gas flow rate, Q (scfm)
3. Flare gas flow rate, Q (scfm)
flg
4. Flare gas heat content (Btu/scf)
5. Maximum flare gas exit velocity, U (ft/s)
max
6. Flare gas exit velocity, U (ft/s)
flg
7. Steam requirements (lb/min)
Solution
1. Determine the required destruction efficiency using the air emission stream charac-
teristics data (from Table 1):
Expected emission stream flow rate, Q = 30,000 scfm
e
Emission stream temperature, T = 100°F
e
Heat content, h = 180 Btu/scf
e
Mean molecular weight of emission stream, MW = 33.5 lb/lb-mol
e
Flare tip diameter, D = 54 in.
tip
Based on the control requirements for the emission stream, destruction efficiency
(DE) = 98%.
2. Determine the supplemental fuel requirements, using Eq. (1).
Because h is less than 300 Btu/scf, supplementary fuel is needed:
e
h = 180 Btu/scf
e
Q = 30,000 scfm
e
Q = [(300 − h )Q ]/582
f e e