Air Pollution Control Equipment Selection Guide, 2nd Edition

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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).