T

Holding tank

i

To disposal

FIG. 5.20.5 Schematic flow diagram of a limestone-based FGD system.

Regenerative Systems

Regenerative processes have higher costs than throw-away processes. However, regenerative processes are chosen when disposal options are limited. Regenerative processes produce a reusable sulfur product. In Japan, where the government mandates FGD, regenerative processes are used almost exclusively.

The Wellman-Lord process is the most well-established regenerative process which uses an aqueous sodium sulfite solution as the solvent. Figure 5.20.7 is a schematic diagram of the Wellman-Lord process. This process consists of the four following subprocesses:

Flue gas pretreatment. In this subprocess, flue gas from an ESP is blown through a venturi prescrubber. The pre-scrubber removes most of the remaining particles and any existing SO3 and HCl, which upset the SO2 absorption chemistry. The prescrubber also cools and humidifies the flue gas. A liquid purge stream from the prescrubber removes the solids and chlorides. SO2 absorption by sodium sulfite solution. In the SO2 absorber tower, the flue gas from the prescrubber contacts the aqueous sodium sulfite, and the SO2 is absorbed and reacted with Na2SO3 in the liquid to form sodium bisulfite. Since excess O2 is always present, some of the Na2SO3 oxidizes to Na2SO4; some of the Na2SO3 reacts with the residual SO3 to form Na2SO4 and sodium bisulfite. The sodium sulfate does not further SO2 absorption. A continuous purge from the bottom of the absorber prevents excessive sulfate buildup. Since the absorber bottom is rich in bisulfite, most of the stream is routed for further processing. Purge treatment. Part of the liquid stream leaving the absorber is sent to the chiller and crystallizer, where the less soluble, sodium sulfate crystals are formed. The slurry is centrifuged, and the solids are dried and discarded. The bisulfite-rich, centrifugal material is returned to the process. Sodium bisulfite regeneration. The remaining part of the liquid stream from the absorber is sent to a heated evap-

FIG. 5.20.6 Schematic diagram of a dual alkali FGD system.

Reheater

Scrubber

Cleaned flue gas to stack

Key:

Dirty flue gas

Water to chemicals makeup tank

Cleaned flue gas to stack

Dirty flue gas

Water to chemicals makeup tank

Concentrated SO2 vapors CW to S recovery

Sodium sulfite/sulfate cake

FIG. 5.20.7 Schematic process flow diagram of the Wellman-Lord SO2 scrubbing and recovery system. (Reprinted from U.S. Environmental Protection Agency (EPA), 1979.)

Concentrated SO2 vapors CW to S recovery

Sodium sulfite/sulfate cake

Blower (fan) Venturi prescrubber Main SO2 scrubber Stack gas reheater Surge tank Evaporator

Chemicals makeup tank

Condenser

Chiller-crystallizer

Centrifuge

Air dryer

Heater

Cyclone

FIG. 5.20.7 Schematic process flow diagram of the Wellman-Lord SO2 scrubbing and recovery system. (Reprinted from U.S. Environmental Protection Agency (EPA), 1979.)

orator and crystallizer, where sodium bisulfite is decomposed to Na2SO3 and SO2. The gas stream contains 85% SO2 and 15% H2O, thus the SO2 can be used as feed stock for producing S or sulfuric acid. To replace the lost Na2SO3, this subprocess adds soda ash (Na2CO3) to make up sodium. The Na2CO3 reacts readily with SO2 in the absorber tower to give sodium sulfite.

Surface condensers are normally shell and tube type and should be set vertically. Vapor should only condense inside the tubes. This arrangement prevents a stagnant zone of inert gas (air) that might blanket the heat transfer surfaces. Figure 5.20.8 shows a typical surface condenser. The feed gas enters the top of the condenser and flows concurrently downward with the condensate.

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