Flue Gas Desulfurization

Since the 1970s, LIGHTNIN has supplied mixers ranging in size from 1.5 to 500 hp (1 to 375 kw) to plants all over the world. Our extensive experience enables us to specify the best combination of mixer and impeller for maximum performance in FGD applications.

Mixing in the FGD Process

Mixing is essential in four stages of the FGD process:

• Reagent preparation
• Pre-scrubber and absorber tanks
• Waste slurry separation feed and holding tanks
• Area sumps

Suspension of varying percentages of fly ash, limestone and gypsum is required at all stages. Mixer selection and system specification depend on:

• Particle size distribution
• Percentage of solids in the slurry (typically between 1% and 55% by weight)
• Degree of suspension required
• The ratio of liquid height to vessel diameter

Specifying and Sizing Forced Oxidation Systems

• Grid Systems use side entering mixers for solids suspension and blending, but do not play any part in gas dispersion/mass transfer. The grid increases the power required to generate the same degree of suspension as a system without a grid.
• Lance systems are used for solids suspension, blending, gas dispersion and mass transfer, as well as prevention of pump plugging. The power required to disperse the gas usually determines power requirements. Location and size of the lance is critical, and LIGHTIN specifies the correct parameters to ensure that the system meets dispersion or mass transfer requirements.
• Top Entering Mixers with Sparge Rings or Distribution Pipes are used for gas dispersion and solids suspension. Mixer specifications are determined by tank size and gas rate. Mixer power must be increased as flow resistance increases, due to tank internals.

LIGHTNIN Innovation Improves Performance in FGD Systems

The introduction of the laser-designed LIGHTNIN A310 impeller in the early 1980s significantly increased the efficiency of FGD systems. The A310 produces the same process result as the four-blade pitch turbine impellers that had been the industry standard, but reduces power consumption by approximately 50%.

Research conducted by LIGHTNIN in 1990 led to further performance improvements with the introduction of an innovative "Cluster" system design. LIGHTNIN's "Cluster" design uses multiple, closely-spaced side entering mixers with high-efficiency A312 impellers. The Cluster design develops a strong flow pattern across the tank floor, up the wall and across the liquid surface to improve solid suspension and minimize solid deposits. The center fillet associated with alternative system designs is eliminated. The design also saves money due to lower capital cost, reduced power consumption and simpler maintenance. Numerous Cluster installations around the world have confirmed the benefits of the system design.

Products for FGD Applications

The following mixers are typically used in FGD systems:

• Series 10 and EW mixers for sumps and small tanks.
• Series 500 and Series 780 mixers for large applications.
• Side entering LRES Gear Drive and VSS Belt Drive mixers

Materials of Construction

Due to the high chloride content and abrasive nature of the slurries, the wetted parts of the agitator must be either rubber-covered or of high alloy construction. Top entering mixers typically operate at low speeds and have rubber covered wetted parts. Side entering mixers operate at higher speeds and require materials such as Hastelloy C276, AL6XN, 317L or Zeron 100. The choice of material is based on the pitting resistance and chloride content.

Seals

The seals are a critical component of mixer design in FGD applications. LIGHTNIN recommends the use of mechanical seals on side entering mixers in FGD systems. The location of the seal, enclosure and shut-off mechanism in LIGHTNIN mixers have been designed to maximize seal exposure to the slurry while minimizing erosion.

Need More Information?

Contact your local LIGHTNIN Sales Representative to discuss your FGD application.