Page 298 - Materials Chemistry, Second Edition

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Incineration of MSW 269
Clean gas

Scrubbing
sprays

Dirty gas

FIGURE 9.13 Gravity spray tower
Scrubbing liquor and scrubber for washing acid gases (U.S.
recovered dust EPA, EPA/625/6-829/024, 1990).

A quicklime or limestone solution can also be formulated to absorb the SO . The reaction with
2
quicklime is
SO CaO → CaSO (s) (9.17)
2
3
The reaction with limestone is

SO CaCO → CaSO (s) CO 2 (9.18)
2
3
4
Lime materials can be injected directly into the scrubber or added to the combustion chamber. If
limestone is injected into the furnace, it quickly reacts to form quicklime:

CaCO → CaO CO 2 (9.19)
3
Both calcium sulfite and calcium sulfate are solids of low solubility that can be separated in grav-
ity settling tanks. The calcium salts formed create a new problem, however, of disposal of enormous
quantities of solid waste, actually a high water content slurry. In some facilities, the flue gas desul-
furization sludge is simply stored in ponds on company property. There is much ongoing research
in the utilization of this sludge. Applications under study include use as an agricultural amendment
(Carlson and Adriano, 1993), for reclamation of coal mine spoils (Stehouwer et al., 1995), roadway
construction, and for use in building panels (drywall) (EPRI, 1995). Based on the discussions
above, flue gases may contain heavy metals and other inorganic pollutants; treatment of the scrub-
bing medium must consider these contaminants as well. At a MSW incinerator in Germany,
Reimann (1995) used a combination of lime and trimercaptotriazine in a two-stage process to
remove mercury, other heavy metals, and salts from the scrubbing medium. The treated effluent met
Germany’s stringent discharge requirements.