Lime or Limestone Scrubbing System. Lime or limestone scrubbing
systems illustrated in Figure 23 are the oldest and most common methods of FGD
for power plants. The reactants are less expensive but also less efficient
than those used in the double alkali system because of their solubility.
Precipitated solids and unreacted material are bled from the circuit and
settled. There is a substantial amount of sludge produced by this system.
Plume from Stack. Flue gases leaving a wet scrubber are usually
discharged as a saturated gas. As soon as the gas leaves the scrubber, it
continues to lose heat to the environment, causing condensation. This
condensation may take the form of a steam plume exiting the stack, or may
cause serious condensation within the stack or downstream ducting, which may
lead to corrosion problems. Plume formation can be limited by providing
reheat to the gases; however, the cost of reheat and the cost of corrosion-
resistant construction downstream of the scrubber must be considered in
Waste Disposal. Waste disposal is a major consideration for any
scrubber system. FGD waste disposal alternatives are illustrated in Figure
24. Landfill can be used for dewatered stabilized sludge, but ponds are used
for wet unstable sludge. The possibility of leaching into ground water should
be investigated before a decision on the site and method of sludge disposal is
made. The lengths of time that the disposal site can be used must be
considered as a part of this problem.