6 a05 slurry pump specs

Flue Gas Desulfurization: Technology Overview

Wet FGD Systems

Wet FGD systems utilize a scrubber, or spray tower, to contact an aqueous slurry of alkaline sorbent with the flue gas to absorb acid gases including SO 2. Captured SO 2 is typically removed from the system as calcium sulfate (gypsum), which can be sold commercially, utilizing filtration processes. Wet FGD systems exhibit SO 2 capture efficiencies on the order of 90% to 98%.

Wet FGD systems comprise reagent preparation and delivery, absorber, and solids handling/filtration equipment and are generally more complicated than semi-dry and dry FGD systems. With respect to operating costs, limestone is usually readily available and relatively inexpensive, and reagent stoichiometry is close to 1:1, although parasitic power related to sorbent pumping can be a concern. Thus, wet FGD systems generally exhibit higher capital expenses (CapEx) but lower operating expenses (OpEx) than other FGD technology types and are therefore well suited for higher-sulfur applications at sites with significant anticipated remaining plant life.

Because contact with the liquid sorbent cools flue gas to its saturation point, downstream corrosion of the ductwork/chimney is a concern, and the stack plume may be visible, raising concerns regarding aesthetics. Some plants opt to employ flue gas reheat strategies to mitigate the wet stack concerns. Additionally, based on the system water balance, a liquid discharge may be required, which can introduce wastewater disposal and/or treatment concerns.

Semi-Dry FGD Systems

Semi-dry FGD systems utilize a spray dryer absorber (SDA) to contact the flue gas with an aqueous slurry, similar to wet FGD systems, but the slurry has a higher sorbent concentration and finer spray droplets than wet systems. SO 2 is captured as a solid byproduct and removed from the system. Although various calcium- and sodium-based sorbents can be utilized, lime is typically employed because it is more reactive than limestone and less expensive than sodium-based reagents. Semi-dry FGD systems exhibit slightly lower SO 2 capture efficiency than wet FGD systems, typically on the order of 80% to 95%.

Semi-dry FGD systems require less equipment than wet FGD systems, typically just lime preparation equipment and the spray dryer vessel, although the design of the plant’s PM control unit will need to account for the additional solids loading due to the FGD byproduct. Reagent stoichiometry is generally higher than in wet FGD, potentially as high as 1.4 to 1.6 mols of calcium per mol of SO 2 at the SDA inlet, depending on the application, with lime being more expensive than the limestone typically employed in wet FGD. However, power requirements are typically lower than wet FGD systems. Thus, semi-dry FGD systems exhibit lower CapEx but potentially higher OpEx than wet FGD systems.

Because the flue gas does not cool all the way to its saturation point, wet stack concerns such as downstream corrosion and visible plume are mitigated. Further, because the water in the sorbent slurry is completely evaporated in the SDA, semi-dry FGD systems are inherently zero-liquid discharge (ZLD), alleviating wastewater disposal and/or treatment concerns.

Dry FGD Systems

Dry FGD systems inject powdered sorbent, typically calcium- and sodium-based alkaline reagents, directly into the furnace, the economizer, or downstream ductwork to absorb SO 2 and other acid gases. SO 2 is captured as a solid byproduct and removed from the system. Dry FGD systems exhibit a wide range of SO 2 capture efficiencies based on the combination of the sorbent, injection point, and PM collection device, and can exhibit SO 2 removal efficiencies as high as 90%.

Dry FGD systems require a relatively small amount of equipment compared to wet and semi-dry systems, typically just the reagent storage, delivery, and injection systems. Similar to semi-dry systems, the design of the PM control unit may need to be revised to account for the additional solids loading. Reagent stoichiometry is higher than wet or semi-dry FGD, as high as 3 mols of sorbent per mol of SO 2 in the flue gas for high SO 2 removal efficiency applications. Further, for high SO 2 removal efficiencies, a sodium-based sorbent such as trona, which is more expensive than lime or limestone, is typically required. Thus, dry FGD systems exhibit lower CapEx but higher OpEx when compared to wet and semi-dry FGD systems and are therefore better suited for lower-sulfur applications or at sites with limited anticipated remaining plant life.

Because no water is introduced into the system in dry FGD, wet stack concerns do not exist. Further, the dry FGD system is inherently ZLD.