MIL-HDBK-1003/6
Dry Scrubbers. Dry scrubbers using spray dryer absorbers for
6.4.2
removal of sulfur dioxide are illustrated in Figure 21. A finely atomized
alkaline slurry is sprayed into the reaction chamber. Sulfur dioxide is
absorbed as the atomized slurry vaporizes in the hot flue gas stream. The dry
reaction products become entrained in the flue gas stream and either fall into
the scrubber hopper or are passed on to the baghouse. Additional SO removal
2
occurs on the surface of the bags from the carryover of residual alkaline.
The advantages of dry scrubbing are a reduction of water usage, flue gas
reheat requirements, and overall corrosion. Sulfur compounds collected from
the flue gas are discharged from the scrubber in a dry state, and these
compounds are more stable than the products of wet scrubbers. Therefore, the
problem of leaching at the landfill site (if used) will be less when using dry
flue gas desulfurization.
Wet Scrubbers. Wet scrubbers used for flue gas desulfurization
6.4.3
(FGD) utilize a slurry or solution of materials such as limestone, lime,
sodium hydroxide, or sodium carbonate to react with the sulfur oxides in the
flue gas. Wet scrubbers are designed to provide intimate contact between the
gases and the scrubbing liquid. Particulate removal of a wet scrubber is
usually limited. Wet scrubbers may be spray towers, packed towers or plate
towers. Reagent cost is high for the sodium systems so that the systems are
seldom used. Calcium-based scrubbers are generally used because they use
lower cost reagents; the reaction between the sulfur oxides and lime produces
calcium sulfite or calcium sulfate (gypsum) which are of limited solubility,
and plugging of equipment can become a problem. Calcium sulfite salts from
these scrubbers form a sludge which may be difficult to stabilize. Sludge
stabilization ponds may be required for some processes, and land area
requirements may be excessive.
Dual Alkali Scrubber. The dual alkali scrubber is a type of wet
6.4.3.1
scrubber which offers solutions to some of the problems mentioned above.
Sodium salts are recirculated through the scrubber taking advantage of high
solubility and fast reaction rate. The spent sodium-based scrubbing liquor is
then pumped to chemical reaction tanks where it is reacted with lime to
precipitate insoluble calcium sulfate salts, returning soluble sodium salts to
the scrubbing process. The process has a higher capital cost than calcium
scrubbing, but generally a lower operating cost. Operating complexity and
power consumption are increased, however the calcium sulfate salts can be de-
watered more easily. A vacuum filter is often used to de-water the sludge
prior to disposal. In the double alkali FGD system illustrated in Figure 22,
absorption of sulfur dioxide and waste generation are separated. This is
advantageous because there are minimal suspended solids in the absorbent to
contribute to scaling of the scrubber internals and the high concentration of
alkali enhances liquid phase mass transfer. This system, therefore, achieves
high sulfur dioxide removal efficiency at low liquid-to-gas ratios.
106
Previous Page
