|By Nat Sekhar, Senior Consultant, CH2M HILL|
Power plant operators are striving for zero liquid discharge (ZLD) to meet existing and potential new regulations. Because wet and semi-dry flue gas desulfurization (FGD) systems consume significant amounts of water in a power plant, design features and operating modes on of the FGD systems offer opportunities to reduce water consumption and meet regulatory requirements.
A typical limestone-based wet FGD (WFGD) system producing commercial grade gypsum in a 600-MWe plant that burns a high-sulfur bituminous coal—such as Illinois No. 6 with 3% sulfur content—consumes 500 to 600 gpm of water for evaporation, at full load. A lime based semi-dry FGD (DFGD) system producing a dry disposable end product in a similar plant burning sub-bituminous coal—such as Wyoming Powder River Basin (PRB) coal with 0.4% sulfur content—consumes 300 to 400 gpm of water at full load.
FGD systems can use process water, such as that from cooling tower blow-down, that otherwise require treatment as waste water prior to disposal for many internal processes. Uses of such water in a WFGD system include grinding limestone in a wet ball mill system, serving as seal water for pumps, and when mixed with good-quality water, washing mist eliminators, and in a DFGD system, diluting slaked lime slurry.
The quantity of wastewater from a WFGD system is determined by the chlorine content of coal and the chloride concentration of recirculating slurry. The chloride concentration and the operating pH of the system determine the material of construction selection for the absorber vessel and other wetted system components. The typical materials of construction for chloride concentrations are listed below:
- 0 to 8,000 ppm, 317 LMN (S31726) or equivalent
- 8,000 to 15,000 ppm, super duplex 255 (S32550) or equivalent
- 15,000 to 25,000 ppm, 6% molybdenum stainless steel AL6XN (N08367) or equivalent
- 25,000 to 40,000 ppm, C276 alloy or equivalent
- 0 to 40,000 and above, flake-glass and rubber lining, Stebbins tile
Typically the WFGD wastewater bleed rate decreases as absorber chloride concentration increases. Bleed rates from a WFGD system on a 600-MWe plant burning different types of coals with varying sulfur and chlorine contents are shown in Table 1.
Higher chloride concentrations not only require the use of more expensive materials; they also have process chemistry implications. A higher chloride concentration tends to reduce the rate of limestone dissolution and therefore pH recovery rate, mass transfer, and the sulfur dioxide removal rate. It may also affect gypsum crystallization, crystal habitat, and filtration characteristics.
Often second- or third- generation FGD systems—systems built in the late seventies and early eighties—have flake glass or rubber-lining on carbon steel as the material of construction in absorber vessels. In these systems, the chloride concentration can be significantly increased to reduce the wastewater bleed, thereby facilitating a more closed-loop- and a closer-to-ZLD operation. The increased chloride concentrations will reduce the size and costs, both capital and operating, of the wastewater treatment system and any final ZLD process, such as evaporation and crystallization.
In FGD systems built in later years, alloys such as SS317LMN replaced flake glass and rubber lining. For these systems, cladding the low alloys with higher alloys such as C276 or lining with flake glass or rubber are options worth considering to reduce the bleed rate. Factors to consider include surface conditions of the current material, time required for the modification, and compatibility of other wetted components in the system.
Because they are inherently ZLD systems, DFGD systems do not produce any wastewater. In addition, they can use wastewater from other plant operations as the make-up water source, potentially reducing waste water treatment costs for the existing plant operations. Any dissolved solids in these wastewater streams will tend to retard the rate of evaporation of lime slurry droplets, thereby extending the active mass transfer period and facilitating sulfur dioxide removal.
In summary, FGD systems, both wet and semi-dry, can play an important role in meeting ZLD operation. They also present some relatively inexpensive options to consider for reducing wastewater bleed and the cost of achieving ZLD. A site-specific study may be required to identify the options and optimize the economics.
Power Engineerng Issue Archives
View Power Generation Articles on PennEnergy.com