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 matter) produced in burning MSW waste. The choice of scrubber type depends on
the pollutants to be controlled and the degree of control required. Dry sorbent
(lime) injection with an electrostatic precipitator (ESP) is used extensively in
Japan for acid gas control, but wet scrubbing is preferred where high metals
control is needed. The ESP/wet scrubber combination appears to be favored in
West Germany for plants started up in the past decade and those expected to
start up in the next several years. In the U.S., the lime spray
absorption/fabric filter system is now frequently being selected for multi-
pollutant control.
4.3.17.10
Acid gas removals of 90% or more have been achieved with a
lime circulating fluid bed or lime spray dryer absorber preceding a fabric
filter or ESP. Wet scrubbing preceded by an ESP is at least as effective as the
systems noted when used to control acid gases. These systems are also effective
for controlling organics and trace heavy metals, with mercury control appearing
to be improved at lower temperatures and when a fabric filter rather than an ESP
is used. Both the ESP and fabric filter can meet current particulate control
requirements, but the fabric filter may have the edge for multi-pollutant
control. More data, especially from commercial units under long-term operation,
are needed to more fully quantify the performance of scrubbers designed to
remove trace organic compounds and trace heavy metals.
4.3.17.11
Although stack gas cleanup is one answer to pollutant
control, minimization of pollutants can also be achieved by careful combustion
control. This has been shown to be true for control of the dioxins
polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans
(PCDF).
4.3.17.12
There can be many reasons for the formation and
destruction of dioxins. Dioxins can enter in the MSW, be created in cold
regions of a furnace, be destroyed in combustion, form in the cooler outlet
sections of the boiler, and "all of the above" (Hasselnis 1988).
4.3.17.13
Vogg, Metzger, and Steiglitz (1987) have found from
extensive laboratory research as well as field tests on an MSW incinerator that
! Formation of PCDD and PCDF takes place at temperatures ranging from 3900F
to 7500F. No effect on dioxins and furans occurs at temperatures below
3900F; a sharp peak in both dioxins and furans occurs at 5700F, and they
are destroyed at 7500F.
! In this temperature range, formation of PCDD and PCDF leveled off after
about 6 h, but in 30 mm about 20% conversion had taken place.
! Oxygen concentration influenced formation of PCDD/DF linearly: zero oxygen
resulted in decomposition or no formation; increasing oxygen levels
resulted in a reduction in the fraction of dioxins and furans having the
more toxic 4-chlorine forms (congeners) and an increase in fraction of less
toxic forms having 6 to 8 chlorine molecules attached.
! Moisture strongly influenced dechlorination, causing formation of the
highly toxic penta and tetra (4- and 5-chlorine) isomers.
4-105
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