The most important application of absorption for gas control technology is FGD. Because flue gas from power plants contributes about two-thirds of the U.S. emissions, large efforts have been spent on developing an effective control technology. Currently, several hundred FGD systems are in commercial operation. The absorption can be regenerative or nonregenerative as well as wet or dry; in effect, four categories of FGD processes exist. Table 5.20.3 lists the processes in use. The following sections describe the most important FGD processes.

Nonregenerative Systems

Most FGD systems are nonregenerative. The reagents used in the absorber are alkaline compounds that react with SO2. Most well-established technology (see Figure 5.20.5) is based on limestone and lime. Other processes are based on NaOH, Na2CO3, and NH4OH. Since lime and limestone are the most inexpensive and abundant reagents, they are used in about 75% of the installed FGD systems. In this process, the reaction products are CaSO3 and CaSO4 in a sludge form that must be disposed. These systems can remove 90% of the SO2 in the flue gases. Although lime is more reactive than limestone, it is more expensive; therefore, lime is not used as widely as limestone.

The main drawback of lime- and limestone-based FGD systems is scaling and plugging of the column internals. This problem is eliminated in the dual alkali system (see Figure 5.20.6) by absorption with a Na2SO3/Na2SO4 solution which is then sent to a separate vessel where lime or limestone and some NaOH are introduced. The lime or limestone precipitates the sulfite and sulfate and regenerates NaOH for reuse in the absorption column. The major disadvantage of this process is the loss of soluble sodium salts into the sludge which may require further treatment. As a result, this process is not used as widely as the lime and limestone process.



Clean gas to stack

Flue gas

Limestone or Lime —1

Make up water

Make up water

To disposal

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

Limestone or Lime —1

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