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222 Lawrence K. Wang et al.

F = 1.81 × 10 −4 (Q ) (P) (HRS) (23)
p e,a
where F is the fan power requirement (kWh/yr), Q is the actual emission stream flow
p e,a
rate (acfm), P is the system pressure drop (in. H O), and HRS is the system operating
2
hours (h/yr).
The cost of scrubbing liquor (most likely water) is given by

WR = 0.6 (Q ) HRS (31)
e,a
where WR is the water consumption (gal/yr).
A general assumption of needed operator labor is 2 h for each 8-h shift. Management
labor costs are assumed to be 15% that of operator labor costs. Labor costs are presented
in Table 7. Maintenance is normally estimated in industry as requiring 1 h from each 8-h
shift. This cost is also provided in Table 7. Materials required for normal maintenance
are assumed to equal cost of maintenance.
It should be noted that the cost of the wastewater generated by a Venturi scrubber is
potentially quite high. Although not discussed here, such cost should be included when
considering the use of a Venturi scrubber for an air pollution control project. Table 7
also includes such indirect costs as property taxes, capital recovery, administrative
costs, and so forth. Table 10 presents both the US Environmental Protection Agency’s
conservative control efficiencies (CE) and the typical actual control efficiencies (CE) of
a wet scrubber for removal of various hazardous air emissions. Either a fabric filter system
or a four-field electrostatic precipitator is used as the pretreatment to the wet scrubber.

3. DRY SCRUBBERS

3.1. Dry Absorbents
A dry chemical absorbent scrubber will almost always use either a calcium- or
sodium-based absorbent. These absorbents are classified as alkali absorbents, which
have excellent to good absorbent properties for most of the acid gases as well as for
some organic air pollutants. Additionally, the dry alkali absorbent most commonly
used is slaked lime or Ca(OH) . In some instances, dry (powdered) activated carbon
2
is also added to the dry absorbent. This is done so that the dry (or semidry) scrubber
will also be able to remove heavy metals and/or trace (and often very toxic) organic
pollutants. After the dry scrubbing, the solids (used and unused absorbent as well as
particulate matter) are accumulated at the bottom of the scrubber tower or possibly
directed to a baghouse (or other particulate collector). The removal efficiency of a wet
scrubber is primarily dependent on the acid/alkali ratio used in the scrubber as well
as outlet air temperature.

3.2. Dry Scrubbing Systems
Three dry scrubbers commonly found in air pollution control operations in industry are
presented (15) in Fig. 6A,B. They are dry–dry, semi–dry, and spray dryer absorber systems.

3.2.1. Dry–Dry Systems
When injecting hydrated lime or pulverized limestone directly into a furnace (or
other combustion chamber) or into the ducting downstream from a combustion process,