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242 Lawrence K. Wang et al.

Capital recovery = 0.1628(TCC)
Capital recovery = 0.1628($100,000) = $16, 280 or $16,300
D. Total Indirect Annual Cost
Total indirect annual cost = Overhead + property tax + insurance + adminis-
tration + capital recovery
Total indirect annual cost = $9,770 + $1,000 + $1,000 + $2,000 + $16,300
Total indirect annual cost = $30,100
E. The total annual cost is determined by summation of Annual Direct and Annual
Indirect costs
Total annual costs = Total direct annual costs + Total indirect annual costs
Total annual costs = $21,100 + $30,100 = $51,200

Example 12
What advantages or disadvantages does dry scrubbing versus wet scrubbing technologies
have when these two absorption methods are compared?

Solution
As previously discussed, incineration processes are often found to have some type of dry
scrubbing system treating the exhaust being formed in the process. Halogenated com-
pounds, if present, will produce acid gases as the result of combustion. The scrubber system
(wet or dry) is used to limit the release of such gases into the atmosphere. Both wet and
dry systems use absorption to collect the acid gases present. The dry process will operate
at a lower pressure loss than the wet scrubber system. This has the consequence of reduc-
ing power costs if a dry system is chosen versus a wet scrubbing system. Also, if using a
dry scrubber, the gas exiting the stack will be warm. Additionally, waste product produced
in a dry scrubbing system is collected as a solid.
If a wet scrubber is used, the gas exiting the stack is often reheated. Therefore, in addition
to having higher operating costs (as the result of a higher pressure drop compared to a dry
scrubber), the wet scrubber also may have additional energy costs from this need to warm
the exit gas. Additionally, as the wet scrubber uses slurry as the absorbent, waste product is
also collected in slurry form. This slurry will have a greater total volume than the dry end
product collected in the dry scrubber, and the cost of final disposal may be higher as a result.
At this point of discussion, the advantage/disadvantage comparison indicates that a dry
scrubber will be a more economical air pollution control choice than a wet scrubber.
However, a wet scrubber does not need a downstream collection device, whereas a dry
scrubber does. Also, if SO must be removed from the gas stream, removal efficiency
2
>90% is most often only achieved economically with a wet scrubber. Also, as dry scrub-
bers must be overdosed with absorbent more so than for wet scrubbers, solvent costs will
be higher in dry versus wet scrubbing. (Explained another way, a certain percentage of
absorbent in a dry scrubber is not utilized. Such excess solvent cannot be economically
separated from the final waste product for reuse. As such, unused solvent in a dry scrubber
is a cost that yields no return.)
Example 13
A proposed solution to an air pollution control project is a Venturi scrubber system. A
blank data sheet is given to collect the data necessary for the environmental review pro-
cess to go forward.