205
22
Energy Recovery
22.1 DEVICE TYPE
Packed towers, tray towers, and even spray towers can be used for energy recov-
ery through the direct contact of a heat-absorbing liquid (usually water) with a
hot gas stream. These devices may also simultaneously remove soluble gaseous
pollutants. In these systems the rate of heat transfer is controlled by the differ-
ence in temperature between the hot gas and absorbing liquid. The amount of
heat absorbed is controlled by the mass of liquid that passes through the device.
Devices that are strictly counterow, such as a packed or tray tower, exhibit higher
heat transfer efciency than devices such as a spray tower that may entrain liquid
upward and therefore reduce the differential in temperature required for optimum
heat transfer. With proper design, however, spray towers can be effectively used
for heat recovery.
22.2 TYPICAL APPLICATIONS AND USES
Most wet scrubbers applied to hot sources inherently transfer heat. As mentioned in
other chapters, the initial heat transfer may be used to adiabatically saturate the gas
stream. Thereafter, more heat can be removed through the sensible cooling of the
gas stream. As the gases mix with the liquid, both the latent heat (the heat removed
as the water vapor in the gas stream changes phase to liquid water) and the sensible
heat are recovered. This is quite different from what occurs with surface (nondirect
contact) type heat exchangers such as economizers or tubular heat exchangers. Those
devices recover only sensible heat.
Given the preceding, the best application sources for direct-contact heat recovery
therefore are those devices that emit high humidity (lots of condensable water vapor
from which the latent heat can be recovered) and hot gases. Such sources are boiler ue
gas stacks wherein the hot gases can rst be adiabatically saturated, sources that emit
lots of water vapor or steam (such as food processing or paper making), thermal oxidiz-
ers (particularly ones that oxidize wet fuels), steam vents, and similar hot wet sources.
The recovered heat may be used, for example, to preheat boiler feed water. It may
be used to provide supplemental heat for a building or loading dock. In other appli-
cations it can be used to thaw frozen products. More exotic uses harness the heat to
drive electric generator systems, which in effect are like driving an air-conditioner
backward. Sometimes the recovered heat is used to preheat the wastewater outfall
from a plant so that further biological destruction can more favorably occur. In all
of these, the economics must be investigated. As fossil fuel costs rise, the econom-
ics favor heat recovery. Conversely, low fossil fuel costs can make the use of heat
recovery only marginally benecial. Each application must be evaluated based upon
its own merit (or lack thereof).

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